Digital mobile fronthaul scheme based on Diff-Delta-Sigma modulation and OCT Precoding

Xiongwei Yang, Jianjun Yu, Jianyu Long, Chen Wang, yi wei, Feng zhao, Wen Zhou, and KAIHUI WANG

DOI: 10.1364/OL.534068 Received 27 Jun 2024; Accepted 31 Jul 2024; Posted 31 Jul 2024  View: PDF

Abstract: In this letter, we experimentally demonstrate digital mobile fronthaul (MFH) for 65536 quadrature amplitude modulation (65536-QAM) signals based on a Diff-delta-sigma modulation (D-DSM) scheme with orthogonal circulant matrix transform (OCT) precoding. By combining the D-DSM scheme with OCT precoding, we successfully solved the problem of uneven distribution of in-band quantization noise (IBN) while bringing about a quantization SNR gain of about 1.5 dB. In addition, we compare two signal combination schemes, gray coding and power superposition, in the D-DSM scheme. The results show that the power superposition scheme can achieve similar performance to gray coding with lower computational complexity. At the same time, the power superposition scheme is less affected by the saturation effect. Therefore, the D-DSM solution combined with OCT precoding and power superposition provides an effective solution for future 6G digital mobile fronthaul.

Spontaneous breaking of time translation symmetry in a system without periodic external driving

Timofey Sergeev, Alexander Zyablovsky, E. Andrianov, and Yurii Lozovik

DOI: 10.1364/OL.527514 Received 16 Apr 2024; Accepted 30 Jul 2024; Posted 31 Jul 2024  View: PDF

Abstract: It is known that the spontaneous time translation symmetry breaking can occur in systems periodically driven at a certain period. We predict a spontaneous breaking of time translation symmetry in an atom-cavity system without external driving, in which a time scale is determined by the time of light bypass of the resonator. We demonstrate that there is a parameter range, in which a system state returns to its initial state only after two bypasses of the resonator. We believe that the predicted phenomenon opens a way to a new direction in the time crystal field.

Computational optical sectioning via near-field multi-slice ptychography

Ziyang Hu, Yiqian Zhang, and Andrew Maiden

DOI: 10.1364/OL.529190 Received 16 May 2024; Accepted 30 Jul 2024; Posted 31 Jul 2024  View: PDF

Abstract: We introduce a method for computational sectioning of optically thick samples based on a combination of near-field and multi-slice ptychography. The method enables large field-of-view 3D phase imaging of samples an order of magnitude thicker than the depth of field of brightfield microscopy. Axial resolution for these thick samples is maintained in the presence of multiple scattering, revealing complex structure beyond the depth of field limit. In this letter, we describe the new approach and demonstrate its effectiveness using a range of samples with diverse thicknesses and optical properties.

A First-Principle Approach to X-ray Active Optics: Design and Verification

qin diao, han dong, Jun Han, qin xiaobo, Fugui Yang, Le Kang, Ming Li, Weifan Sheng, and Xiaowei Zhang

DOI: 10.1364/OL.533342 Received 21 Jun 2024; Accepted 29 Jul 2024; Posted 30 Jul 2024  View: PDF

Abstract: This paper presents the first-principle design approach for X-ray active optics. The feature of deterministic modulation enables the simulation-modulation cycle in place of the measurement-modulation feedback loops used in traditional active optics. We apply an X-ray mirror with localized thermal elastic deformation to validate the idea. Our experiments have demonstrated that the new active optics have the potential to outperform the accuracy of surface-shape metrology instruments.

Deep Learning Facilitated Superhigh-Resolution Recognition of Structured Light Ellipticities

Ruitao Wu, Kaiyuan Li, Luping Du, and Xiaocong Yuan

DOI: 10.1364/OL.528796 Received 30 Apr 2024; Accepted 29 Jul 2024; Posted 30 Jul 2024  View: PDF

Abstract: The Elliptical Beams (EBs), an essential family of structured light, have been investigated theoretically due to their intriguing mathematical properties. However, their practical application has been significantly limited due to the inability to determine all their physical quantities, particularly the ellipticity factor, a unique parameter for EBs of different families. In this paper, to our knowledge, we proposed the first high-accuracy approach that can effectively distinguish EBs with an ellipticity factor difference of 0.01, equivalent to 99.9% field similarities. The method is based on a Transformer deep learning (DL) network, and the accuracy has reached 99% for two distinct families of exemplified EBs. To prove that the high performance of this model can dramatically extend the practical aspect of EBs, we used EBs as information carriers in free space optical communication for an image transmission task, and an error bit rate as low as 0.22% is achieved. Advancing the path of such a DL approach will facilitate the research of EBs for many practical applications like optical imaging, optical sensing, quantum-related systems, etc.

Multifrequency nonlinear Schrödinger equation

David Castello-Lurbe, Enrique Silvestre, and Miguel Andrés

DOI: 10.1364/OL.528926 Received 01 May 2024; Accepted 29 Jul 2024; Posted 30 Jul 2024  View: PDF

Abstract: The multifrequency character of nonlinearity dispersion is often dismissed because, in principle, it increases the computational load exceedingly rendering an impractical modeling and, typically, nonlinearities barely depend on frequency. Nonetheless, nonlinearity dispersion has recently enabled a solution to a long-standing challenge in optics. To explore the potential of this research avenue on solid theoretical grounds, we derive a propagation equation accounting for multifrequency nonlinearities rigorously that maintains the computational advantages of conventional models.

Second Harmonic Generation of 2D Materials Excited by Laguerre Gaussian Beam

Yuwei Shan, Dong Sun, and Jin Luo Cheng

DOI: 10.1364/OL.530997 Received 29 May 2024; Accepted 29 Jul 2024; Posted 30 Jul 2024  View: PDF

Abstract: We theoretically study second harmonic generation(SHG) of 2D materials excited by a Laguerre Gaussian(LG) beam at normal incidence, and provide a methodto distinguish SHG induced by electric dipole (ED) interactionand SHG induced by electric quadrupole andmagnetic dipole (EQ-MD) interaction by their differentdependence on the LG beam parameters, including thespot radius v0 and the order of orbital angular momentum(OAM) m. In an approximation of neglecting reflectionand taking beam radius to infinity, the intensityof ED induced SHG is proportional to Fm/v20with Fm = 2−2|m|(2|m|)!/(π(|m|!)2), while the EQ-MD inducedone is proportional to (4|m| + 2)Fm/v40. In-planeisotropic substrate can strongly affect the signal amplitudebut slightly change the v0 and m dependence. Ourresults provide an all-optical way to detect the OAM bySHG, as well as a theoretical basis for studying EQ-MDinduced SHG by LG beams.

Fast nonlinear Fourier transform algorithm for reconstruction of optical data from a nonlinear spectra of Manakov system

Sergey Medvedev, Irina Vaseva, and Mikhail Fedoruk

DOI: 10.1364/OL.531316 Received 29 May 2024; Accepted 29 Jul 2024; Posted 30 Jul 2024  View: PDF

Abstract: We propose a high precision algorithm for solving the three-component Gelfand-Levitan-Marchenko (GLME) equation associated with Manakov system, which describes the behavior of light waves through the optical fibers. The algorithm generalizes the High-Order Generalized Toeplitz Inner-Bordering method for solving the two-component GLME associated with the nonlinear Schrodinger equation. Numerical experiments have shown that the proposed algorithm makes it possible to increase the accuracy of solving the GLME associated with Manakov system up to the sixth order.

High diffraction efficiency complex amplitude modulation technique by using antiwave functions

Juan Martinez-Carranza and Tomasz Kozacki

DOI: 10.1364/OL.529878 Received 13 May 2024; Accepted 29 Jul 2024; Posted 30 Jul 2024  View: PDF

Abstract: Complex amplitude modulation (CAM) is a single step technique that codes amplitude and phase of a diffracted optical field into a real function. Loading this function on a phase-only spatial light modulator enables reconstruction of three-dimensional objects. However, objects reconstructed with this technique have low brightness. This is because the informative diffraction order has a low diffraction efficiency. This work shows a novel solution that overcomes this limitation in CAM. This is done by the employment of antiwave complex exponential functions that allow modifying the spectrum of the diffracted field. Proper configuration of these functions enables transferring energy across the diffraction spectrum to the informative diffraction order. Hence, diffraction efficiency of this order increases, and thus, brightness of reconstructed image significantly improves. Simulations and experiments prove the efficiency of the proposed CAM method.

Deep-learning-assisted optical endoscopy for accurately measuring 3D profiles and surface roughness on groove sidewalls of precision components

Xiangyu Zhao, Jinsong Zhang, renlong zhu, Yijun Die, Zhengqiong Dong, Lei Nie, Shiyuan Liu, and Jinlong Zhu

DOI: 10.1364/OL.531552 Received 30 May 2024; Accepted 28 Jul 2024; Posted 30 Jul 2024  View: PDF

Abstract: The accurate measurement of surface three-dimensional (3D) profile and roughness on groove sidewalls of components is of great significance to diverse fields such as precision manufacturing, machining processes, energy transportation, medical equipment, and semiconductor industry. However, conventional optical measurement methods struggle to measure surface profiles on the sidewall of a small groove. Here, we present a deep-learning-assisted optical endoscopy, which consists of a microprism-based interferometer, an optical path compensation device, and a convolutional neural network (CNN), for the accurate measurement of surface 3D profile and roughness on the sidewall of a small groove. We have demonstrated that the optical endoscope can achieve a measurement accuracy of 2.64 nm for the 3D profile on a groove sidewall. Moreover, we have demonstrated that the CNN-based single-image super-resolution (SISR) technique could improve the measurement accuracy of surface roughness by over 30%. Our system can be utilized in cases where the width of the groove is only 1 mm and beyond, limited only by the size of the microprism and the working distance of the objective used in our system.

Experimental Demonstration of Ge2Sb2Te5 Loaded Reconfigurable 1D Plasmonic Metasurface Perfect Absorber for Near-IR Wavelength Regime

SAGAR VERMA, Mandeep Jangra, Arnab Datta, and Sachin Srivastava

DOI: 10.1364/OL.532638 Received 13 Jun 2024; Accepted 28 Jul 2024; Posted 30 Jul 2024  View: PDF

Abstract: Inclusion of a phase change material such as germanium-antimony-telluride (Ge2Sb2Te5 or GST) enhances the performance of plasmonic metasurface absorbers (PMAs). 1-D Plasmonic metasurfaces (PMs) support the excitation of surface plasmon modes for the normal incidence of transverse magnetically (TM) polarized light. The 1-D PMAs absorb incident light because of their confinement in the groove region which is possible because of the surface plasmon modes excited at the metal-dielectric interface. A thin layer of phase change material enhances the absorption of incident light because of the increasing strength of the confined electromagnetic field in the vicinity of the PMA. We developed a GST loaded, low cost, 1-D PMA for the absorption of near infrared (NIR) light (740-920 nm). The PMA was fabricated using an Ag coated 1-D patterned polycarbonate which was obtained from a commercial digital versatile disc (DVD). A 1-D PMA of 1 cm2 size obtained from a DVD was coated with a GST layer of 8 nm thickness to achieve the maximum absorption of 99.56% for the hexagonal closed packed (h.c.p.) crystalline state of the GST loaded layer. Control experiments were performed for different temperatures and different thicknesses of the GST layer for achieving an optimal performance nearing perfect absorption. Electric and magnetic field profiles were simulated for the normal incidence of TM polarized light to understand the underlying physics of light-matter interaction with the PMA. Such a PMA can be used to develop various cost-effective optical devices, such as optical sensors, optical filters, photodetectors, and heat absorbing photonic windows in NIR wavelength regime.

Single-shot spatial light interference microscopy for dynamic monitoring of membrane fluctuations

Weinan Huang, Youwei Deng, Yueshu Feng, Binglin Shen, Jiaqing Guo, Rui Hu, Junle Qu, and Liwei Liu

DOI: 10.1364/OL.534784 Received 05 Jul 2024; Accepted 28 Jul 2024; Posted 31 Jul 2024  View: PDF

Abstract: Single-shot spatial light interference microscopy (SS-SLIM) with a pair of non-polarizing beam splitters is proposed for substantially enhancing the speed and efficiency of conventional SLIM systems. Traditional methods are limited by the need for multiple-frame serial modulation and acquisition by spatial light modulators and detectors. Our approach integrates non-polarizing beam splitters to simultaneously capture four phase-shifted intensity images, increasing imaging speed by at least four-fold while maintaining high quality. This capability is crucial for effectively monitoring the dynamic fluctuations of red blood cell membranes. Furthermore, the potential applications of the SS-SLIM system in biomedical research are demonstrated, particularly in scenarios requiring high temporal resolution and label-free imaging.

Enhancement of Brillouin nonlinearities with coupled resonator optical waveguide

Mingyu Xu, Peng Lei, Yunhui Bai, Zhangyuan Chen, and Xiaopeng Xie

DOI: 10.1364/OL.533979 Received 27 Jun 2024; Accepted 27 Jul 2024; Posted 30 Jul 2024  View: PDF

Abstract: Stimulated Brillouin scattering (SBS) is a nonlinear optical phenomenon mediated from the coupling of photons and phonons. It has found applications in various realms, yet the acousto-optic interaction strength remains relatively weak. Enhancing SBS with resonant structures could be a promising solution, but this method faces strict constraints in operation bandwidth. Here, we present the first demonstration of broadband enhancement of Brillouin nonlinearities by a suspended coupled resonator optical waveguide (CROW) on SOI platform. By comprehensively balancing Brillouin gain and operation bandwidth, a 3-fold enhancement for the Brillouin gain coefficient (G_B) and a broad operation bandwidth of over 80 GHz has been achieved. Furthermore, this 1.1-mm device shows a forward Brillouin gain coefficient of 2,422 m^(-1)W^(-1) and a high mechanical quality factor (Q_m) of 1,060. This approach marks a pivotal advancement towards wide bandwidth, low energy consumption and compact integrated nonlinear photonic devices, with potential applications in tunable microwave photonic filters and phonon-based non-reciprocal devices.

Dual microwave frequency modulation of the VCSEL injection current for CPT-based atomic clock

Mariya Vaskovskaya, Dmitriy Chuchelov, Eugene Tsygankov, Vitaliy Vassiliev, Sergey Zibrov, and Vladimir Velichansky

DOI: 10.1364/OL.532983 Received 17 Jun 2024; Accepted 27 Jul 2024; Posted 31 Jul 2024  View: PDF

Abstract: Previously, we have proposed a method to control the emission spectrum of the vertical-cavity surface-emitting laser with the synchronized modulation of the injection current at single and doubled frequencies. In this work, the above method is used to improve the metrological characteristics of the CPT resonance in $^{87}$Rb. The dual-frequency modulation allows to reduce the carrier power and suppress the light shift of the resonance frequency, while it is unattainable with single-frequency modulation.In addition, a higher resonance contrast is achieved by equalizing the powers of the resonant components of the spectrum.

Fiber-coupled 2 mL vacuum-gap Fabry-Pérot reference cavity for portable laser stabilization

Charles McLemore, Naijun Jin, Megan Kelleher, Yizhi Luo, Dahyeon Lee, Yifan Liu, Takuma Nakamura, David Mason, Peter Rakich, Scott Diddams, and Franklyn Quinlan

DOI: 10.1364/OL.531169 Received 28 May 2024; Accepted 27 Jul 2024; Posted 30 Jul 2024  View: PDF

Abstract: Vacuum-gap Fabry-Perot cavities are indispensable for the realization of frequency-stable lasers, with applications across a diverse range of scientific and industrial pursuits. However, making these cavity-based laser stabilization systems compact, portable, and rugged enough for use outside of controlled laboratory conditions has proven difficult. Here, we present a fiber-coupled 1396 nm laser stabilization system requiring no free-space optics or alignment, built for a portable strontium optical lattice clock. Based on a 2 mL vacuum-gap Fabry-Perot cavity, this system demonstrates thermal noise-limited performance and 1x10-14 fractional frequency instability. Fiber-integrated optical components have been instrumental in both advancing the field of optics and leveraging those advances across disciplines to facilitate other fields of study. This portable system represents a major step towards making the frequency stability of cavity-based systems broadly accessible.

A concept for power scaling by harmonic beam coaxial combination in intra-cavity frequency doubling laser

Ao-Nan Zhang, YunPing Wang, Zhenling Li, Zeng Rang, Ke Liu, and Xiao-Jun Wang

DOI: 10.1364/OL.531047 Received 29 May 2024; Accepted 26 Jul 2024; Posted 30 Jul 2024  View: PDF

Abstract: A new method of harmonic beam coaxial combination (HBCC) from two intra-cavity frequency doubling branches was demonstrated. Firstly, two identical nanosecond (ns) 532 nm green laser with high power and good beam quality were created. Each green laser was constructed of an intra-cavity frequency doubling branch based on laser diode (LD) end-pumped acousto-optical (AO) Q-switched 1064 nm Nd:YVO4 laser in LiB3O5 (LBO) nonlinear crystal. Each branch generated about 45 W green output at 50 kHz pulse repetition rate (PRR) with diffraction limited beam quality. The first green beam was injected into the LBO crystal in the second branch. Then, the HBCC was performed. Consequently, an 83 W combined green output power at 532 nm was obtained with a combination efficiency of 92.2 %. The PRR of the HBCC pulse was doubled to be 100 kHz, with a pulse width of about 22 ns, corresponding to a single pulse energy of 0.83 mJ and a peak power of 37.73 kW. The combined beam quality factor was measured to be M_x^2=1.80 in the x direction and M_y^2=1.71 in the y direction, respectively. Moreover, much more beams could also be combined with this method for further scaling the green power.

Magneto-optical metasurfaces for high-Q perfect absorption with quasi-bound states in the continuum

Haosen Zhang, Guo Ping Wang, and Kedi Wu

DOI: 10.1364/OL.536497 Received 11 Jul 2024; Accepted 26 Jul 2024; Posted 30 Jul 2024  View: PDF

Abstract: We present a novel magneto-optical (MO) metasurface composed of a bismuth iron garnet (BIG) nanocube array, designed to achieve near-perfect absorption through quasi-bound states in the continuum (QBICs). This metasurface supports a stable QBIC mode induced by MO-induced permittivity terms that break the symmetry of the permittivity tensors, corresponding to a longitudinal electric dipole (ED) mode. By integrating graphene to introduce material loss, the absorption reaches 99.6% at a wavelength of 1512.3 nm with a Q factor of 9440, despite monolayer graphene’s inherent absorption being only 2.3%. The inherent transverse ED background mode, with high reflection and low-Q, helps decrease the radiative loss of the QBIC mode, allowing the structure to surpass the 50% absorption limit. This approach offers a simplified pathway for designing high-Q metasurface perfect absorbers, with potential applications in optical switches and modulators.

Electro-optical effect in homeoplanar structures of ferroelectric and ferrielectric liquid crystals

Evgeny Pozhidaev, Aleksandra Zhukovich-Gordeeva, Artemy Kuznetsov, and Timofey Tkachenko

DOI: 10.1364/OL.530473 Received 28 May 2024; Accepted 26 Jul 2024; Posted 26 Jul 2024  View: PDF

Abstract: A clearly expressed effect of the unpolarized light electro-optical modulation by a homeoplanar structures of a smectic C* ferroelectric liquid crystal (FLC) and a ferrielectric liquid crystal (FerriLC) was discovered and investigated for the first time. This effect of electrically controlled light scattering is insensitive to the applied voltage sign, as for polymer-dispersed nematic liquid crystals (PDLCs) but the electro-optical modulation frequency reaches the kilohertz range. Occurrence conditions and essential features of the effect as well as its physical origin are discussed in the manuscript.

Zeeman Torque Sampling of Intense THz Magnetic-Field in CoFe

chun geng, Yichen Su, Deyin Kong, Zehao Yang, Jianghao Li, Jiahua Cai, Fei Dai, Cheng Song, and Xiaojun Wu

DOI: 10.1364/OL.532571 Received 18 Jun 2024; Accepted 25 Jul 2024; Posted 26 Jul 2024  View: PDF

Abstract: Free-space strong-field terahertz (THz) electromagnetic pulses have emerged as a potent tool for non-equilibrium quantum state control. However, these applications predominantly rely on electric field components, with limited utilization of magnetic field components. Traditional electro-optical sampling technique may encounter constraints due to the field strength saturation effect when detecting strong-field THz pulses. Here we have identified intense THz-induced Zeeman Torque signals in CoFe, and successfully detected the THz magnetic field components. Through variations in the type of ferromagnetic materials and the thickness of ferromagnetic films, we further refined the detection of THz magnetic field components. Our research revealed that a 15-nm CoFe thin film is more responsive in measuring magnetic field components.

Frame-rate Adaptive Fractionally-Spaced Equalization Enabled High-throughput Optical Camera Communication

Shiwen Chen, Meng Xiang, Gai Zhou, Jilong Li, Songnian Fu, and Yuwen Qin

DOI: 10.1364/OL.529945 Received 14 May 2024; Accepted 25 Jul 2024; Posted 30 Jul 2024  View: PDF

Abstract: Optical camera communication (OCC) has garnered worldwide research attentions, due to its immunity to electromagnetic interference (EMI) and efficient utilization of spectrum resources. However, the limited bandwidth of OCC system and the timing offset of camera result in low system throughput. To enhance the OCC throughput, we propose and experimentally demonstrate a frame-rate adaptive fractionally-spaced equalization algorithm (FA-FSE) for the joint mitigation of severe inter-symbol interference (ISI) and timing offset arising in OCC. Experimental results validate its correct and power-efficient function, leading to a record aggregated throughput of 250.96 kbit/s, when the 8-level pulse amplitude modulation (PAM-8) signals are independently modulated to 8 chip-on-board light emitting diode (COB-LED) light strips, while simultaneously received by a smartphone 10 cm away.

Compact and efficient photonic lanterns through multi-stage tapering

Lijie Hou, Linbo Yang, Zhiqun Yang, Yaping Liu, Zhanhua Huang, and Lin Zhang

DOI: 10.1364/OL.524265 Received 20 Mar 2024; Accepted 25 Jul 2024; Posted 30 Jul 2024  View: PDF

Abstract: Photonic lanterns (PLs) have been recently used in mode-division multiplexed systems with a low insertion loss, a low mode-dependent loss and wide bandwidth. However, crosstalk (XT) performance of the PLs requires to be further enhanced within a short taper length. In this Letter, a multi-stage cascaded scheme for short PLs is proposed to further improve the performance on losses and XT. The XT of the optimized 6-mode 3-stage PL is below - .4 dB, while the total length is only 6 cm. As far as we know, it’s the first time to quantitatively optimize a 3-stage tapered PL, achieving a compact structure and an excellent performance. Besides, we experimentally verify the process feasibility of the 3-stage tapering.

Cascaded partitioned phase modulation for cross-connection of orbital angular momentum mode and polarization multiplexing channels

Qingji Zeng, Bowei Zhang, Shu Chen, Haisheng Wu, Zhibin Wu, Huapeng Ye, Xinxing Zhou, Ze Dong, Junmin Liu, Dianyuan Fan, and Shuqing Chen

DOI: 10.1364/OL.528496 Received 29 Apr 2024; Accepted 25 Jul 2024; Posted 30 Jul 2024  View: PDF

Abstract: Multi-dimensional orbital angular momentum (OAM) mode multiplexing provides a promising route for enlarging communication capacity and establishing comprehensive networks. While multi-dimensional multiplexing has gained advancements, the cross-connection of these multiplexed channels, especially involving modes and polarizations, remains challenges due to the needs for multi-mode interconversion and on-demand polarization control. Herein, we propose an OAM mode-polarization cross-transformation solution via cascaded partitioned phase modulation, which enables the divergently separated OAM modes to be independently phase-imposed within distinct spatial regions, leading to the synergistic conversion operation of mode and polarization channels. In demonstrations, we implemented the cross-connection of three OAM modes and two polarization multiplexed channels, achieving the mode purity exceed 0.951 and the polarization contrast up to 0.947. Consequently, 1.2 Tbit/s quadrature phase shift keying signals were successfully exchanged, yielding the bit-error-rates close to 10^-6. Incorporating with increased partitioned phase treatments, this approach shows promise in accommodating massive mode-polarization multiplexed channels, which holds the potential to augment networking capability of large-scale OAM mode multiplexing communication networks.

Room temperature VCSEL operation on InGaN/GaN thin film platform with surface grating

Wai Yuen Fu, Zhongqi Wang, Yuk Cheung, and H. Choi

DOI: 10.1364/OL.534465 Received 02 Jul 2024; Accepted 25 Jul 2024; Posted 25 Jul 2024  View: PDF

Abstract: Vertical-cavity surface-emitting lasers (VCSELs) are pivotal in various applications ranging from data communication to sensing technologies. This study introduces a novel VCSEL design featuring a top surface grating reflector on a thin film substrate, aimed at improving lasing performance while reducing fabrication costs. We fabricated the proposed VCSEL with the surface grating for proof-of-concept and characterized its performance through micro-photoluminescence measurements. The laser demonstrated room temperature lasing at 436.2 nm with a Q factor of 4600 and a lasing threshold of 5.5 kW/cm² under optical pumping. The implementation of the surface grating reflector was instrumental in facilitating vertical lasing, significantly improving surface reflectivity compared to conventional flat GaN/air interfaces. This innovative design holds significant promise for the development of cost-effective and high-efficiency VCSELs, with potential applications extending to photonic integrated circuits and LiDAR systems.

Experimental investigation of high-speed WDM- visible light communication using blue, green and red InGaN μLEDs

Tingwei Lu, Yurong Dai, Wenan Guo, Shuhui Wang, Tzu-Yi LEE, Shouqiang Lai, Zhong Chen, Hao-chung Kuo, and Tingzhu Wu

DOI: 10.1364/OL.521996 Received 20 Feb 2024; Accepted 25 Jul 2024; Posted 26 Jul 2024  View: PDF

Abstract: Micro-light-emitting diodes (μLEDs) emitting in the red, green, and blue spectral regions hold significant promise for applications in displays and visible light communication (VLC). This study substantiates the viability of a wavelength division multiplexing (WDM)-VLC system using InGaN blue, green, and red μLED arrays. The tri-color devices exhibited notable color stability and high modulation bandwidth due to the weakly polarized electric field in the blue and green semipolar devices and the stress-optimized structure in the red device. The aggregated transmission rate reached 11.14 Gbps. Moreover, the tri-color μLED devices exhibited a wide color gamut, encompassing 119.4% of the NTSC and 89.2% of the Rec. 2020 standards, affirming the potential of InGaN RGB μLEDs for comprehensive applications in full-color display and WDM-VLC systems.

Seamless Rate Adaptation for Wide SNR Range in FSO Systems Based on Rate Compatible Modulation

Tao Shu, Yang Zou, Qirun Fan, Tianjin Mei, Xiaoxiao Dai, Chen Liu, Mengfan Cheng, Lei Deng, Qi Yang, and Deming Liu

DOI: 10.1364/OL.523993 Received 19 Mar 2024; Accepted 25 Jul 2024; Posted 26 Jul 2024  View: PDF

Abstract: We propose and experimentally demonstrate a large-range and seamless rate-adaptive free-space optical (FSO) scheme based on rate compatible modulation. It utilizes a mapping method through weighted summation, enabling the even increase of bit energy. This allows the system to seamlessly adjust the throughput. In addition, the handover mechanism based on rate check and acknowledgment signals ensures a wide coverage of signal-to-noise ratio (SNR) fluctuation. It's worth noting that this scheme does not require any changes to the mapping matrix and decoding algorithm, making it easier to implement in real-world systems. Simulation proves the superiority of the proposed approach in terms of coding rates coverage applying a fixed mapping matrix compared to traditional adaptive modulation and coding scheme. Experimental demonstration over a 50-m FSO link verifies that the SNR dynamic range of this scheme is >15dB, and that seamless rate adjustment between 8.7Gb/s to 77.8Gb/s can be achieved, revealing huge potential for the massive connections in future smart cities.

Ultrahigh-channel-count OAM mode conversion utilizing a hybrid few-mode fiber configuration

Chengliang Zhu, Xinyue Meng, Zengxin Qu, Huan Zhang, Tonglei Cheng, and Yong Zhao

DOI: 10.1364/OL.528139 Received 23 Apr 2024; Accepted 25 Jul 2024; Posted 26 Jul 2024  View: PDF

Abstract: We propose and demonstrate a hybrid few-mode fiber configuration (HFMFC) that enables ultrahigh-channel-count orbital angular momentum (OAM) mode conversion. The HFMFC consists of periodically twisted graded-index few-mode fiber segments and a step-index few-mode fiber segment. Our proposed HFMFC-based multichannel OAM mode converter (OAM-MC) offers an exceptionally high channel count in a wide bandwidth, with customizable channel spacing down to 50 GHz (0.4 nm), achieved through optimization of the structural parameters of the HFMFC. By employing this methodology, we have successfully demonstrated 10, 32, 117, and 3 channel OAM mode conversions covering the entire C+L band, representing the highest performance among all reported fiber-based multichannel OAM-MCs to date, to the best of our knowledge. The suggested ultrahigh-channel-count OAM-MC exhibits promising potential for applications in various fields such as OAM fiber communication, OAM holography, OAM information processing, and OAM metrology.

112-GBaud mode-division-multiplexing IM-DD transmission beyond net 6.4 terabit/s over weakly coupled FMF for optical interconnections

Yu Yang, Gang Qiao, Honglin Ji, Zhaopeng Xu, Tonghui Ji, Qi Wu, Shangcheng Wang, Lulu Liu, chengbin long, Mingqing Zuo, Jiarui Zhang, Lei Shen, Jie Luo, Weisheng Hu, and Juhao Li

DOI: 10.1364/OL.528473 Received 26 Apr 2024; Accepted 25 Jul 2024; Posted 26 Jul 2024  View: PDF

Abstract: Weakly coupled mode-division-multiplexing (MDM) systems based on intensity modulation and direct detection (IM-DD) are a good candidate to further improve the capacity of short-reach optical interconnections. However, restrained by the modal crosstalk of the transmission link and the reception of degenerate mode groups (DMG) utilizing bandwidth-limited multimode photodetector (PD), high-speed MDM IM-DD has encountered a capacity bottleneck. In this letter, we demonstrate a high-speed weakly coupled MDM IM-DD transmission system utilizing a degenerate mode diversity receiver adopting high-bandwidth single-mode PDs over multiple-ring-core (MRC) few-mode fiber (FMF) and low-crosstalk mode multiplexer/demultiplexer. An MDM IM-DD transmission with four DMGs and eight wavelengths is experimentally demonstrated with 112-GBaud 4-level pulse-amplitude modulation (PAM4) and probabilistically shaped PAM8 (PS-PAM8) per lane over 200-m weakly coupled MRC-FMF. To the best of our knowledge, this is the first experimental demonstration of the MDM IM-DD transmission system with up to 112-GBaud baud rate and beyond 6.4 Tb/s net rate. Meanwhile, the experimental results show that the proposed MDM IM-DD transmission link has a superior performance only adopting a low-complexity feedforward equalizer (FFE), making it promising for high-speed optical interconnections.

Investigation of Q degradation in low-loss Si3N4 from heterogeneous laser integration

Joel Guo, Chao Xiang, Warren Jin, Jon Peters, Mingxiao Li, Theodore Morin, Yu Xia, and John Bowers

DOI: 10.1364/OL.530161 Received 15 May 2024; Accepted 25 Jul 2024; Posted 26 Jul 2024  View: PDF

Abstract: High-performance, high-volume-manufacturing Si3N4 photonics requires extremely low waveguide losses augmented with heterogeneously integrated lasers for applications beyond traditional markets of high-capacity interconnects. State-of-the-art quality factors (Q) over 200 million at 1550 nm have been shown previously; however, maintaining high Qs throughout laser fabrication has not been shown. Here, Si3N4 resonator intrinsic Qs over 100 million are demonstrated on a fully integrated heterogeneous laser platform. Qi is measured throughout laser processing steps, showing degradation down to 50 million from dry etching, metal evaporation, and ion implant steps, and controllable recovery to over 100 million from annealing at 250C - 350C.

Polarizability inversion suspension for nonlinear optical limiting with low limiting threshold

Jian Huang, Yuangang Lu, Yang Liu, Biao Dong, and Chongjun He

DOI: 10.1364/OL.527878 Received 19 Apr 2024; Accepted 24 Jul 2024; Posted 25 Jul 2024  View: PDF

Abstract: We propose a novel nonlinear optical limiting (NOL) method with low limiting threshold based on light intensity-controlled polarizability inversion suspension (PIS). This suspension has negative polarizability under weak light, allowing stable propagation of weak light with low-loss. Nevertheless, the suspension reverses into positive polarizability due to the optical Kerr effect under strong light, resulting in enhanced scattering that rapidly attenuates the intense light. In a proof-of-concept experiment, PS (polystyrene)-CS₂-CCl₄ suspension is used as the example suspension. We experimentally verify the NOL performance of serval samples. Among them, 4 g/L PS-CS₂-CCl₄ suspension with a volume ratio of 0.15 has the best optical limiting effect, with a high limiting capacity coefficient of 0.48 and a very low limiting threshold of 14.80 kW/cm², which is an order magnitude lower than that of most common NOL materials. Therefore, the proposed method provides a new promising approach to achieve NOL of continuous wave laser with low limiting threshold.

Manipulation and applications of ultrafast all-optical switching based on transient absorption and dispersion in KTN crystals

Xiao Zhu, Shan Lin, Xinze Li, Yiyang Xie, Jiacheng Cao, Wei-Long Liu, Hao Tian, Qingxin Yang, and Peng Jin

DOI: 10.1364/OL.524657 Received 29 Mar 2024; Accepted 24 Jul 2024; Posted 26 Jul 2024  View: PDF

Abstract: Potassium tantalate niobate (KTN) represents a noteworthy category of optical crystals known for their superior nonlinear optical properties. In this study, we conducted measurements of femtosecond time resolved transient absorption (TA) spectra in KTa0.57Nb0.43O3 crystals. Notably, a rapid and pronounced "plateau" phase, ~1.5 ps in duration, was detected at the onset of the TA kinetics, and succeeded by two distinct decay components, exhibiting lifetimes of ~140 ps and over 10 ns, respectively. We attribute these observations to a decay process involving two-photon absorption, dispersion characteristics and excited state absorption. Based on this unique TA characteristic of KTN crystals, an all-optical switching strategy was proposed and utilized to measure the ultrafast lasing dynamics of single-crystal CH3NH3PbBr3 nanowires. This polarization-independent TA gate approach offers an adjustable gate width combining ps and ns time scales, and introduces a versatile tool for advanced optical applications.

Al/Zr-based multilayer mirrors with record-breaking reflectivity

Vladimir Polkovnikov, Nikolay Chkhalo, Sergey Garakhin, Nikolay Salashchenko, and Sergey Zuev

DOI: 10.1364/OL.534480 Received 02 Jul 2024; Accepted 24 Jul 2024; Posted 26 Jul 2024  View: PDF

Abstract: The paper reports on a new Zr/Be/Si/Al multilayer structure that provides record reflectances of up to 67% and a spectral resolution of Δλ=0.63 nm (λ/Δλ≈27) in the spectral range 17–20 nm. It is shown that the structure has a high temporal stability of EUV optical characteristics. This fact makes the structure promising for future missions to study the solar corona.

Solid-state transverse Anderson localized fiber laser

Brian Topper, Cody Bassett, Alexander Neumann, Matthew Tuggle, Thomas Hawkins, John Ballato, and Arash Mafi

DOI: 10.1364/OL.530145 Received 14 May 2024; Accepted 24 Jul 2024; Posted 24 Jul 2024  View: PDF

Abstract: For the first time, an all-solid transverse Anderson localizing optical fiber laser is demonstrated. A combination of the molten core and stack-and-draw fiber fabrication techniques is used to produce a 112 um core diameter fiber that is a random array of Yb-doped high index and passive low index regions. A localized channel first assists in the guidance of amplified spontaneous emission before stimulating laser action, which occurs in the same channel via mixed Anderson localization and step index wave-guiding. Threshold behavior and lasing is monitored with changing output power slopes, beam profiling, spectral content, fluorescence clamping, and temporal intensity. The average output power is stable, while the laser wavelength hops between 1066 nm and 1088 nm. Lasing is highly directional along the fiber axis. The all-solid state nature of this demonstration paves the way for a new type of device for basic and applied sciences including imaging, quantum optics, and statistical physics.

Ultrafast laser oblique incidence for glass butt welding

Hong Shen, Chenyun Tian, and Xingyu Wei

DOI: 10.1364/OL.532096 Received 06 Jun 2024; Accepted 24 Jul 2024; Posted 24 Jul 2024  View: PDF

Abstract: Glass lap welding restricts the connection state of glass by vertically incident ultrafast laser on the interface to be welded, which cannot meet the increasingly flexible welding needs. In this letter, glass butt welding is achieved by ultrafast laser oblique incidence, expanding the applicability of ultrafast laser glass welding. Furthermore, the propagation path of the laser beam after oblique incidence into glass is solved based on geometric optics, and combined with the observation of the growth process of the molten pool in glass through a high-speed camera, the mechanism of glass butt welding is elucidated. Finally, the influence of laser single-pulse energy, glass tilt angle, and defocus amount on welding strength is investigated, achieving welding with a maximum shear force of 79N.

Strong coupling of epsilon-near-zero mode to chiral plasmon

Xiabing Zhu, Shu Chen, and Tinghui Xiao

DOI: 10.1364/OL.533057 Received 19 Jun 2024; Accepted 24 Jul 2024; Posted 24 Jul 2024  View: PDF

Abstract: Reconfigurable chiroptical effect is highly desirable for spin photonics, chiral spectroscopy, and photocatalysis due to its merits for dynamic and broadband applications. The coupling of an epsilon-near-zero (ENZ) mode to a chiral plasmon is expected to enable active and effective manipulation of the chiroptical effect, but remains unexplored. Here we, for the first time, propose and demonstrate the strong coupling of an ENZ mode to a chiral plasmon by using a hybrid system composed of two identical vertically placed gold nanorods and an in-between ENZ film. An analytical three-oscillator model combined with numerical simulations is established to study the coupling mechanism, which predicts a Rabi splitting up to 240 meV with an ENZ film thickness of 60 nm in circular dichroism.

Iterative outliers detection and refinement rule of compensation for phase aberrations in digital holographic microscopy

Cuifeng Xu, You Mo, Ziyue Huang, Jun Ma, and Jing Ling

DOI: 10.1364/OL.531182 Received 29 May 2024; Accepted 23 Jul 2024; Posted 24 Jul 2024  View: PDF

Abstract: We propose a robust and accurate compensation method for phase aberrations based on iterative outliers detection and refinement (ODR) rule. This method does not require additional step to select the known flat region by manually or image segmentation. Based on the proposed method, the phase aberration in regions of specimen can be detected and refined iteratively. Then, the least squares fitting can be carried out to estimate the coefficients of Zernike polynomials and obtain the accurate phase aberration finally. Computer simulations and real experiments validate the feasibility and effectiveness, and the results show that the proposed method is robust to noise and has superior accuracy even when the specimen occupies half of the field of view.

Low-loss Compact Chalcogenide Microresonators for Efficient Stimulated Brillouin Lasers

Yufei Li, Di Xia, Huanjie Cheng, liyang luo, Linyi Wang, Siqing Zeng, Shuixian Yang, Linze Li, Baile Chen, Bin Zhang, and Zhaohui Li

DOI: 10.1364/OL.534233 Received 28 Jun 2024; Accepted 23 Jul 2024; Posted 23 Jul 2024  View: PDF

Abstract: Chalcogenide glasses (ChGs) possess a high elasto-optic coefficient, making them ideal for applications in microwave photonics and narrow linewidth lasers based on stimulated Brillouin scattering (SBS). However, current As2S3-based integrated devices suffer from poor stability and low laser-induced damage threshold, and the planar ChG devices feature limited quality factors. In this letter, we propose and demonstrate a high-quality integrated GeSbS ChG Brillouin photonic device. By introducing Euler bending structures, we suppress high-order optical modes and reduce propagation losses in finger-shaped GeSbS microresonator, resulting a compact footprint of 3.8 mm^2 and a high intrinsic quality factor of 5.19 x 10^6. The combination of GeSbS material's high Brillouin gain and the resonator's high quality factor enables the generation of stimulated Brillouin lasers with a low threshold of 0.96 mW and a fundamental linewidth of 58 Hz. Moreover, cascaded stimulated Brillouin lasers up to the 7th order can be realized, yielding microwave beat frequencies up to 40 GHz.

Perfect correlation vortices

Xiaofei Li, Sajjad Bashiri, Yuan Ma, Chunhao Liang, Yang Jiancai, Sergey Ponomarenko, and Zhiheng Xu

DOI: 10.1364/OL.529970 Received 13 May 2024; Accepted 23 Jul 2024; Posted 24 Jul 2024  View: PDF

Abstract: We introduce perfect correlation vortices and show thatthe degree of coherence of any such vortex at the sourceis nearly statistically homogeneous and independent ofthe topological charge of the vortex. We demonstratethat while slowly diffracting in free space, perfect correlationvortices maintain their "perfect" vortex structure;they are capable of preserving said structure even instrong atmospheric turbulence. Structural resilience todiffraction and turbulence sets the discovered perfectvortices apart from their coherent cousins and makesthem suitable for free space optical communications.

Fully Automated OCT-based Tissue Screening System

Shaohua Pi, RAZIEH GANJEE, Lingyun Wang, Riley Arbuckle, Chengcheng Zhao, JOSÉ-ALAIN SAHEL, Bingjie Wang, and Yuanyuan Chen

DOI: 10.1364/OL.530281 Received 17 May 2024; Accepted 23 Jul 2024; Posted 23 Jul 2024  View: PDF

Abstract: This study introduces a groundbreaking optical coherence tomography (OCT) imaging system dedicated for high-throughput screening applications using ex vivo tissue culture. Leveraging OCT’s non-invasive, high-resolution capabilities, the system is equipped with a custom-designed motorized platform and tissue detection ability for automated, successive imaging across samples. Transformer-based deep learning segmentation algorithms further ensure robust, consistent, and efficient readouts meeting the standards for screening assays. Validated using retinal explant cultures from a mouse model of retinal degeneration, the system provides robust, rapid, reliable, unbiased, and comprehensive readouts of tissue response to treatments. This fully automated OCT-based system marks a significant advancement in tissue screening, promising to transform drug discovery, as well as other relevant research fields.

Space-time dispersive symmetric Pearcey-Pearcey wave packets in the fractional Schrödinger equation

Liping Zhang, Xiao Zhang, Peixin Yu, Xiao-Lu Ge, Chidao Chen, Zhongsheng Man, and Dongmei Deng

DOI: 10.1364/OL.531702 Received 31 May 2024; Accepted 23 Jul 2024; Posted 23 Jul 2024  View: PDF

Abstract: This study presents an investigation into the propagation characteristics of a symmetric Pearcey-Pearcey space-time (SPPST) wave packet in a dispersive medium for the first time, in an optical system based on the fractional Schrödinger equation. Subsequently, the influence of the dispersion (normal and abnormal dispersion) on the SPPST packet is analyzed comprehensively. By manipulating the parameters of the SPPST wave packet, it is possible to control its shape and propagation dynamics. Simultaneously, the study delves into the effects of the combination of the dispersion and the second-order chirp on the evolution of SPPST wave packets and the associated intensity with these wave packets. Studying self-focusing wave packets with spatiotemporal symmetry provides new theoretical support for the development of quantum optics and optical communication.

Helically twisted nonlinear photonic crystals

Chen Yu, Shan Liu, tianxiang xu, Ruwei Zhao, and Yan Sheng

DOI: 10.1364/OL.532151 Received 05 Jun 2024; Accepted 23 Jul 2024; Posted 23 Jul 2024  View: PDF

Abstract: Nonlinear photonic crystals with a helical structure inthe second-order nonlinear coefficient (χ(2)) are fabricated using infrared femtosecond laser poling in ferroelectric Sr0.61Ba0.39Nb2O6 crystals. The quasi-orbitalangular momentum of the helical χ(2)structure can beimprinted on the interacting photons during nonlinearoptical processes, allowing the topological charge of thegenerated photons at new frequencies to be controlled.Here we study the case of a double helix nonlinear photonic structure for the generation of a second harmonicvortex beam from a Gaussian pump beam without phasesingularity. The conservation law for orbital angular momentum in the second harmonic process is also verified,with the topological charge of the pump photons beingfully compensated by the double helix structure. Theflexible control of light carrying orbital angular momentum at new frequencies will find important applicationsin both classical and quantum photonics, such as nonlinear wavefront shaping and multidimensional entanglement of photons.

Study of the interference fringes-caustic region interaction in a topological Young’s interferometer

elizabeth Saldivia-Gomez, Jessica Martinez, Fabian Camilo Cubillos Morales, Israel Julián Macías, Patricia Martinez Vara, and Gabriel Martinez-Niconoff

DOI: 10.1364/OL.533273 Received 20 Jun 2024; Accepted 23 Jul 2024; Posted 26 Jul 2024  View: PDF

Abstract: Herein, an analysis of the optical field emerging from a topological Young’s interferometer is conducted. The interferometer consists of two 3D slit shape curves and is studied by projecting it onto a trihedral reference system. From the projection, Airy, Pearcey, and cusped type beams emerge. The optical field of these beams is organized around its caustic region. The interference between these types of beams presents interesting physical properties, which can be derived from the interaction between the interference fringeswith the caustic regions. One property of the interaction is the irradiance flow which induces a long-distance interaction between the caustic regions. Another property is the bending of the interference fringes toward the caustic regions, which acts as a sink. Due to the adiabatic features of the caustic regions, the interaction between the fringes-caustic and caustic irradiance is studied using a predator-prey model which leads to a logistic-type differential equation with non-linear harvesting. The stability analysis ofthis equation is in good agreement with the theoretical and experimental results.

FPGA Implementation of Power-lite Volterra-inspired Neural Network Equalizer in 180-Gb/s Net Bitrate IMDD short-reach optical system

wenqing jiang, xiaokai guan, luyao Huang, Yongxin Xu, Yunhao Xie, Weisheng Hu, and Lilin Yi

DOI: 10.1364/OL.533564 Received 25 Jun 2024; Accepted 21 Jul 2024; Posted 25 Jul 2024  View: PDF

Abstract: A white-box power-lite Volterra-inspired neural network (VINN) equalizer is proposed to solve the problem of complexity discontinuity in Volterra nonlinear equalizer (VNLE). By adjusting the granularity of the solution space, it conserves computational resources while maintaining nonlinear compensation capability. The performance of VINN is verified on a field-programmable gate array (FPGA) in a short-reach intensity modulation and direct detection (IMDD) system and a 240-Gb/s real-time signal processing rate is achieved. Under the 25% overhead soft-decision forward error correction (SD-FEC) bit error rate (BER) threshold, we realize a record net rate of up to 180 Gb/s based on FPGA.

Optical fiber integrated WGM cylindrical cavity resonator

Yongli Mei, Dongning Wang, Qiaoben Wang, and Yan Zhang

DOI: 10.1364/OL.528812 Received 30 Apr 2024; Accepted 21 Jul 2024; Posted 25 Jul 2024  View: PDF

Abstract: Whispering gallery mode resonators are usually discrete optical devices, which have integration difficulties with optical fiber system. Here we present a new type of optical fiber whispering gallery mode resonator based on cylindrical cavity, which is located in the multimode fiber core and fabricated by femtosecond laser micromachining together with fast hydrofluoric acid etching techniques. When light traveling in the fiber core is tangent to the cylindrical cavity wall, it is coupled into the cavity and circulates along the cavity wall to excite whispering gallery mode resonance before being coupled out to the same tangential path and continuing propagation in the fiber core. The device is fully integrated into optical fiber, simple in fabrication, convenient in operation and low in cost and has a good quality factor (Q) of 1.06×104. The device enriches the family of whispering gallery mode resonator and is expected to have promising applications in photonics.

Untrained physics-driven aberration retrieval network

Shuo Li, Bin Wang, and Xiaofei Wang

DOI: 10.1364/OL.523377 Received 11 Mar 2024; Accepted 21 Jul 2024; Posted 23 Jul 2024  View: PDF

Abstract: In the field of the coherent diffraction imaging, phase retrieval is essential for correcting the aberration of an optic system. For estimating aberration from intensity, conventional methods rely on the neural networks whoseperformance is limited by the training datasets. In this Letter, we propose an untrained physics-driven aberration retrieval network (uPD-ARNet). It only uses one intensity image and iterates in a self-supervised way.This model consists of two parts: an untrained neural network and a forward physical model for the diffraction of the light field. This physical model can adjust the output of the untrained neural network, which cancharacterize the inverse process from the intensity to the aberration. Moreover, to obtain the anti-noise ability, we further introduce a learnable variable to handle the intensity image with noise. The numerical experiments support that our method is superior to other conventional methods for aberration retrieval.

Resolving ambiguities in phase correction term for optical field encoding

Antoine Rouxel, Olivier Gauthier-Lafaye, and Antoine Monmayrant

DOI: 10.1364/OL.533058 Received 17 Jun 2024; Accepted 21 Jul 2024; Posted 23 Jul 2024  View: PDF

Abstract: This article addresses ambiguities regarding the existence and definition of a phase correction term in phase and amplitude optical field encoding techniques. We present a generalized mixed Fourier-Taylor series expansion that is valid for any phase-wrapping interval. Our theoretical analysis and numerical validations confirm that maintaining consistency within a given phase-wrapping convention ensures equivalent results and reconciles previously conflicting interpretations.

Fiber-integrated quantum microscopy system for cells

Xinyi Li, Shihai Wei, Mingxuan Chen, Qiang Xu, Bo Jing, and Hai-Zhi Song

DOI: 10.1364/OL.527524 Received 26 Apr 2024; Accepted 21 Jul 2024; Posted 23 Jul 2024  View: PDF

Abstract: Quantum entanglement serves as an essential resource across various fields, including quantum communication, quantum computing, and quantum precision measurement. Quantum microscope, as one of significant applications in quantum precision measurement, could bring revolutionary advancements in both signal-to-noise ratio (SNR) and spatial resolution of imaging. Here, we present a quantum microscopy system that relies on a fully fiber-integrated high-performance energy-time entangled light source operating within the near-infrared II window. Complemented by tailored real-time data acquisition and processing software, we successfully demonstrate the quantum imaging of a standard target, achieving a SNR of 131.51±6.74 and a spatial resolution of 4.75±0.27 µm. Furthermore, we showcase quantum imaging of cancer cell, unveiling the potential of quantum entanglement in biomedical applications. Our fiber-integrated quantum microscope, characterized by high imaging SNR, instantaneous image capture and analysis capabilities, marks an important step towards the practical application in life sciences.

The Observation of Mode-splitting in Asymmetrical Photonic molecules via Whispering gallery modes

Jianwei Wang, jiapeng Sun, Yu Zhang, Zhihai Liu, and Hanyang Li

DOI: 10.1364/OL.527543 Received 16 Apr 2024; Accepted 21 Jul 2024; Posted 23 Jul 2024  View: PDF

Abstract: This letter investigates the mode-splitting via whispering gallery modes (WGM) for the asymmetrical photonic molecules (PMs), composed of size-mismatched dual-microspheres. The characteristic of asymmetrical PMs was numerically analyzed, specifically focusing on the separation and the difference in intensity of splitting peaks. The splitting-spectra exhibited a redshift, and the separation of two splitting peaks reached maximum for symmetrical PMs, with a minimal difference of intensity also obtained. It’s observed that the opposite direction of the splitting peaks for the same PMs due to different coupling points with the tapered fiber. It can apply for selecting similar-sized microparticles and recognizing PMs in optical devices.

Efficient multiphoton microscopy with picosecond laser pulses

Katarzyna Kunio, Grzegorz Sobon, and Jakub Boguslawski

DOI: 10.1364/OL.533227 Received 03 Jul 2024; Accepted 20 Jul 2024; Posted 23 Jul 2024  View: PDF

Abstract: Multiphoton microscopes employ femtosecond lasers as light sources because the high peak power of the ultrashort pulse allows for multiphoton excitation of fluorescence in the examined sample. However, such short pulses are susceptible to broadening in a microscope’s highly dispersive optical elements and require careful dispersion management, otherwise decreasing excitation efficiency. Here, we have developed a 10 nJ Yb:fiber picosecond laser with an integrated pulse picker unit and evaluated its performance in multiphoton microscopy. Our results show that performance comparable to femtosecond pulses can be obtained with picosecond pulses only by reducing the pulse repetition rate and that such pulses are significantly less prone to the effect of chromatic dispersion. These findings proved that the temporal compression of fiber lasers is not always efficient, and it can be omitted by using a smaller and easier-to-use all-fiber setup.

Observation of off-axis solitary waves propagating along a specific trajectory in photorefractive crystals

Qingying Quan, Shengdi Lian, Zihan Liu, Chen Hechong, Bo Yan, and Dongmei Deng

DOI: 10.1364/OL.532244 Received 06 Jun 2024; Accepted 20 Jul 2024; Posted 23 Jul 2024  View: PDF

Abstract: We report the propagation dynamics of Swallowtail beams within a photorefractive crystal. In the nonlinear regime, the self-accelerating and secondary self-focusing features of the Swallowtail beams are influenced, and a solitary wave is generated. The main lobe energy of the Swallowtail beams is guided to a specific inclined trajectory, leading to a stable solitary wave. And we control the output position of the solitary wave by changing the launch angle. Our results are supported by the corresponding experiments. In addition, we demonstrate that Gaussian beam can be effectively guided in swallowtail optical waveguide structures. Our research represents an interesting interaction between the Swallowtail beams and nonlinear medium, which may find potential applications in photonic integrated devices and optical information transmission.

On-axis full-field swept-source optical coherence tomography for murine retinal imaging

Ratheesh Meleppat, Denise Valente, Soohyun Lee, Ravi Jonnal, Nathan Doble, and Robert Zawadzki

DOI: 10.1364/OL.531116 Received 29 May 2024; Accepted 19 Jul 2024; Posted 25 Jul 2024  View: PDF

Abstract: A full-field swept-source optical coherence tomography (FF-SS-OCT) for in vivo murine retinal imaging is demonstrated. The on-axis FF-SS-OCT system was built in a Mach-Zehnder interferometer configuration employing a tunable laser source with an adjustable sweep rate and sweep range in conjunction with a fast 2D-CMOS camera. A large field retinal (coherent) illumination was accomplished using an imaging interface comprised of a short-focal length imaging lens and a contact lens. The magnification between the camera and retina (spatial sampling) was appropriately chosen to record the microscopic structural features of the retina in the image. A pupil stop was employed in the detection path to reject unwanted backscattering from the mouse eye and other sources and limit aberrations distorting the retinal images. In vivo mouse retinal imaging was performed at a sweep rate of 150 Hz to acquire volumes unaffected by the system vibrations, which predominated at lower sweep frequencies. Operating the FF-SS-OCT at this speed yielded an effective axial scan rate of 20 million A-scans/s and a field of view of 28.5° x 14.25°. High-quality retinal B-scans and enface images of the retina were obtained with the SS-FF-OCT, revealing all major retinal layers and vascular plexuses.

Unlocking superior performance of broadband powerful mid-IR optical parametric amplifiers with a BaGa2GeS6 crystal pumped at 1.24 μm

Ekaterina Migal, Dina Suleimanova, Dmitrii Badikov, and Fedor Potemkin

DOI: 10.1364/OL.533745 Received 24 Jun 2024; Accepted 18 Jul 2024; Posted 23 Jul 2024  View: PDF

Abstract: We report on the development of a tunable (1.5 – 6.5 µm) femtosecond optical parametric amplifier (OPA) based on a novel BaGa2GeS6 (BGGS) crystal with a Cr:Forsterite pumping laser. Total conversion efficiency as high as 28% is achieved in a robust two-stage set-up resulting in generation of a 340-µJ 1.67-µm signal and a 100-µJ 4.65-µm idler pulses. The achieved efficiency surpasses considerably previously published one in a similar OPA configuration with other non-oxide nonlinear crystals demonstrating the potential of a novel BGGS material. A 5-optical-cycles 94-fs 6-µm idler pulses are demonstrated with a propriate dispersion compensation by Ge and GaAs plates. An experimental estimate is given for the effective nonlinearity of a BGGS material, which for our nonlinear process reaches 19.5 pm/V for Type II phase matching. The crystal is additionally tested as a final amplifier in a high-energy OPA, where total output reaches 1.2 mJ with more than 40% conversion efficiency. The demonstrated high nonlinearity, high damage threshold and chemical stability of the polished surface make BGGS crystal an ideal candidate for the development of a high energy OPAs with multi millijoule pumping lasers.

Optical scattering measurement of highly reflective coatings with cavity ring-down technique

Bincheng Li, Xinyi Zhang, zhe yang, Jing Wang, Yanling Han, TAO LI, Hao Cui, and binxing zhao

DOI: 10.1364/OL.533503 Received 21 Jun 2024; Accepted 18 Jul 2024; Posted 24 Jul 2024  View: PDF

Abstract: Cavity ring-down (CRD) is employed for the first time to precisely measure optical scattering of highly reflective (HR) optics with measurement sensitivity greatly enhanced via power trapping inside the ring-down cavity. The scattering measurement accuracy is significantly improved by calibrating the photo-detector for scattering measurement with the low transmittance of the cavity mirror or test HR mirror, which is accurately measured also by CRD. The influence of environmental stray light (such as the probe light scattered by optics and mechanical parts outside the ring-down cavity) and other background noises on the scattering measurement is greatly eliminated by the temporal behavior of the scattering CRD signal. A scattering measurement sensitivity of 4.0×10-13 is experimentally achieved with a laser with output power of 12 mW.

A Novel Multiple-Signal-Joint-Processing Based Guard Interval Shortening Method for CS-NFDM System

Jianqing He, Li Jianping, Yuwen Qin, Xinkuo Yu, Tao Huang, Jianbo Zhang, Gai Zhou, and Songnian Fu

DOI: 10.1364/OL.530504 Received 20 May 2024; Accepted 17 Jul 2024; Posted 19 Jul 2024  View: PDF

Abstract: With aiming to further improve the spectral efficiency (SE) of continuous spectrum modulated nonlinear frequency division multiplexing (CS-NFDM) system, we propose a novel multiple-signal-joint-processing (MSJP) based guard interval (GI) shortening method. In this method, multiple NFDM time-domain signals are jointly-processed as a whole to carry out nonlinear Fourier transform and inverse nonlinear Fourier transform (NFT-INFT) operations. These operations can fuse the multiple NFDM time-domain signals together, which is equivalent to the corresponding inverse process of fiber transmission. Experimental results show that the normalized SE of the proposed method can reach 0.99 with approaching the limit value of 1, and obtain 2.33dB Q2-factor improvement compared with pre-dispersion compensation (PDC) method under the same GI of 0.03ns in 80km SSMF transmission of 46GHz signal bandwidth. Furthermore, in comparison with the PDC method, the proposed method can achieve 32.86% normalized SE improvement in the 1120km SSMF transmission of 32GHz signal bandwidth under the SD-FEC of 2.4E-2.

Large-Scale-Adaptive Fringe Projection 3D Measurement

Xiaojie Zhang, Yupei Miao, Qijian Tang, Zewei Cai, Zhipeng Chen, anbang liang, yu yin, Xiang Peng, and Xiaoli Liu

DOI: 10.1364/OL.532887 Received 17 Jun 2024; Accepted 17 Jul 2024; Posted 19 Jul 2024  View: PDF

Abstract: The fringe projection profilometry (FPP) faces significant challenges regarding calibration difficulty and stitching error accumulation when operating across scenes ranging from tens to hundreds of meters. This letter presents a calibration-free 3D measurement method by integrating binocular vision of FPP scanner with wide field-of-view vision that constructs global benchmarks to unify local 3D scanning and global 3D stitching, which is adaptive to arbitrarily large-scale scenes. A posterior global optimization model is then established to determine the reconstruction parameters and stitching poses simultaneously at each scanning node with adaptively distributed benchmarks. Consequently, the integrated vision measurement system not only eliminates the large-scale pre-calibration and stitching error accumulation but also overcomes system structural instability during moving measurement. With the proposed method, we achieved 3D measurements with an accuracy of 0.25mm and a density of 0.5mm for over 50-meter-long scenes.

Optical cooling of a Yb-doped alumino-phosphosilicate fiber in air by −250 mK

Chun-Wei Chen, Bailey Meeham, Thomas Hawkins, John Ballato, Peter Dragic, Tommy Boilard, Martin Bernier, and Michel Digonnet

DOI: 10.1364/OL.533925 Received 27 Jun 2024; Accepted 17 Jul 2024; Posted 19 Jul 2024  View: PDF

Abstract: Recent progress in the fabrication of Yb-doped silicate fibers with low concentration quenching and low background absorption loss has led to the demonstration of anti-Stokes-fluorescence cooling in several aluminosilicate compositions. This breakthrough is critical to combat deleterious thermal effects due to the quantum defect in fiber lasers and amplifiers. Since cooling efficiencies remain low (1–2.7%), it is paramount to engineer compositions that improve this metric. We report a silica fiber with a core glass heavily doped with aluminum and phosphorus that sets a few new records. This few-mode fiber (16-µm core) was cooled in air by −0.25 K from room temperature with ~0.5 W of 1040-nm power. The measured cooling efficiency is 3.3% at low pump power, and 2.8% at the power that produced maximum cooling. The critical quenching concentration inferred from the measured dependence of cooling on pump power and careful calibration of the pump absorption and saturation is 79 wt.%. The inferred background absorption loss is 15 dB/km. Together with the fiber’s average Yb concentration of 4.2 wt.%, these metrics rank among the best reported in a silica glass.

Binning method for artifact-free time-tag based correlation function calculations

Oscar Urquidi, Johanna Brazard, and Takuji Adachi

DOI: 10.1364/OL.532069 Received 05 Jun 2024; Accepted 16 Jul 2024; Posted 19 Jul 2024  View: PDF

Abstract: Correlation functions are nowadays routinely computed using time-tagged photon information instead of a hardware autocorrelator. The algorithm developed by Laurence et al. [1] is a powerful example. Despite its ease of implementation and fast computation process, it presents a prevalent noisy feature at the short time-lag range when computed on commonly used logarithmically spaced bins. We identified that the mismatch between the edges of generated bins and acquisition frequency results in an aliasing artifact, creating a noise-like feature in the correlation function. We introduce a binning method that effectively eliminates this artifact and improves the accuracy of the autocorrelation at the short time-lag range. Applying the binning method herein can be particularly crucial when one extracts photophysical processes from fluorescence correlation spectroscopy or the diffusion coefficient of nanoparticles from dynamic light scattering at the time range below 10-5 s lag time.

Broadly tunable continuous-wave Tm,Ho:SrF2 and Tm,Ho:BaF2 lasers

Karel Veselsky, Pavel Loiko, Kirill Eremeev, Abdelmjid Benayad, Alain BRAUD, Jan Sulc, Helena Jelinkova, and Patrice Camy

DOI: 10.1364/OL.532598 Received 13 Jun 2024; Accepted 16 Jul 2024; Posted 16 Jul 2024  View: PDF

Abstract: We report on the first room temperature continuous-wave laser operation of Tm3+,Ho3+-codoped barium and strontium fluoride crystals at ∼2.1 µm. The 3 at.% Tm, 0.5 at.% Ho:BaF2 laser generated 160 mW at 2073 nm with a slope efficiency of 31.0% and a laser threshold of 43 mW. Continuous wavelength tuning of this laser from 2010 nm to 2090 nm (tuning range: 80 nm) was demonstrated. The spectroscopic properties of Tm,Ho:SrF2 and Tm,Ho:BaF2 crystals were also determined, showing enhanced Tm3+ → Ho3+ energy transfer in rare-earth clusters. For Ho3+ ions in BaF2, the stimulated-emission cross-section is 0.40×10 20 cm2 at 2044 nm and the thermal equilibrium luminescence lifetime is as long as 14.1 ms.

Dual-chirp-based photonic THz-ISAC system with adaptive frequency synchronization

Zhidong Lyu, Lu Zhang, zuomin yang, Qiuzhuo Deng, Xing Fang, Changming Zhang, oskars ozolins, Xiaodan Pang, and Xianbin Yu

DOI: 10.1364/OL.530911 Received 27 May 2024; Accepted 16 Jul 2024; Posted 17 Jul 2024  View: PDF

Abstract: Recent advancements have brought significant attention to photonic terahertz (THz) integrated sensing and communication (ISAC) systems. In this work, we present an adaptive frequency offset (FO) compensation method for dual-chirp-based ISAC waveforms, using the fractional Fourier transform (FrFT) method. The proposed scheme can enable frequency synchronization without a need of training preambles and exhibit robustness against system noise. We validate this approach through an experimental demonstration in a 300 GHz photonic THz-ISAC system with 20 Gbps quadrature phase shift keying (QPSK) data transmission and 1.5 cm range resolution. The experiment successfully compensates for frequency offsets ranging from -5 GHz to 5 GHz, achieving an estimation error of less than 0.08% and a chirp-pilot power overhead of 0.5%.

Bulk photovoltaic effect in ferroelectric nematic liquid crystals

Faheem Hassan, Donghao Yang, lotfi saadaoui, Yu Wang, Irena Drevensek-Olenik, ZIYANG QIU, JINGBIN SHAO, YIMING ZHANG, Shaohua Gao, yigang Li, Xinzheng Zhang, and Jingjun Xu

DOI: 10.1364/OL.527568 Received 17 Apr 2024; Accepted 16 Jul 2024; Posted 23 Jul 2024  View: PDF

Abstract: The bulk photovoltaic (BPV) effect in ferroelectric liquid crystals is of increasing scientific interest owing to its great potential for light-energy conversion. The ferroelectric nematic phase exhibits a huge spontaneous polarization that can be aligned to a preferred direction. In this letter, we investigate the tensorial properties of the BPV effect in the planarly aligned ferroelectric nematic phase of the liquid crystalline material RM734. A steady-state short-circuit photocurrent of ⁓160 pA and an open-circuit photovoltage of ⁓50 mV were observed in a cell with a thickness of 5.5 microns under the illumination of ultraviolet light without any bias voltage. Based on the photocurrent measurements in different electrode configurations, the nonzero elements of the BPV tensor were obtained. The BPV effect is attributed to the combination of the spontaneous polarization and the asymmetric distribution of photoinduced charge carriers. This study not only provides an understanding of the bulk PV mechanism in soft ferroelectrics but also promises a wide range unprecedented benefits for light harvesting to engineer marketable photovoltaic devices.

Nonlinear Up-converted Thermal Emission through Difference Frequency Generation

Ruixin Ma, HENGZHE YAN, Zhihao Zhou, Yijia Yu, and Wenjie Wan

DOI: 10.1364/OL.529620 Received 08 May 2024; Accepted 13 Jul 2024; Posted 16 Jul 2024  View: PDF

Abstract: Thermal radiation management is of critical importance in energy, sensing, and heat transfer. According to Planck's law, objects at room temperature predominantly emit thermal radiation within the mid and far-infrared bands. Here we demonstrated the upconversion of blackbody radiation via the difference frequency process in a nonlinear second-order medium, translating the mid-and far-infrared bands into the easy-detectable visible band. This nonlinear broad-spectrum upconversion is facilitated by the random quasi-phase-matching technique in a disordered medium. Furthermore, we show the temperature measurement of thermal spots using such nonlinear thermal radiation. This scheme paves the way for applications in thermal management and sensing.

Experimental photon addition and subtraction in multi-mode and entangled optical fields

Kishore Thapliyal, Jan Perina, Ondrej Haderka, Vaclav Michalek, and Radek Machulka

DOI: 10.1364/OL.532242 Received 17 Jun 2024; Accepted 13 Jul 2024; Posted 16 Jul 2024  View: PDF

Abstract: Multiple photon addition and subtraction applied to multi-mode thermal and sub-Poissonian fields as well as twin beams is mutually compared using one experimental setup. Twin beams with tight spatial correlations detected by an intensified CCD camera with high spatial resolution are used to prepare the initial fields. Up to three photons are added or subtracted to arrive at the nonclassical and non-Gaussian states. Only the photon-subtracted thermal states remain classical, though their mean photon numbers contra-intuitively increase. In general, the experimental photon-added states exhibit greater nonclassicality and non-Gaussianity than the comparable photon-subtracted states. Once photons are added or subtracted in twin beams, both processes result in comparable properties of the obtained states owing to twin-beam photon pairing.

Single-cycle, 643-mW average power THz source based on tilted pulse front in lithium niobate

Tim Vogel, Samira Mansourzadeh Ashkani, and Clara Saraceno

DOI: 10.1364/OL.532219 Received 06 Jun 2024; Accepted 11 Jul 2024; Posted 11 Jul 2024  View: PDF

Abstract: We present, to the best of our knowledge, the highest average power from a laser-driven single-cycle THz source demonstrated so far, using optical rectification in the titled pulse-front geometry in cryogenically cooled lithium niobate, pumped by a commercially available 500 W ultrafast thin-disk Yb-amplifier. We study repetition rate dependent effects in our setup at 100 kHz and 40 kHz at this high average power, revealing different optimal fluence conditions for efficient conversion. The demonstrated sources with multi-100 mW average power at these high repetition rates combine high THz pulse energies and high repetition rate and is thus ideally suited for nonlinear THz spectroscopy experiments with significantly reduced measurement times. The presented result is a first benchmark for high average power THz time domain spectroscopy systems for nonlinear spectroscopy, driven by very high average power ultrafast Yb lasers.

Single-carrier 220-Gbit/s Sub-THz Wireless Transmission over 214 m using Photonics-based System

Keisuke Maekawa, Toki Yoshioka, Tomoya Nakashita, Takahiro Ohara, and Tadao Nagatsuma

DOI: 10.1364/OL.527593 Received 24 Apr 2024; Accepted 09 Jul 2024; Posted 18 Jul 2024  View: PDF

Abstract: We have demonstrated, for the first time, a single-carrier 220-Gbit/s wireless link over 214-m distance within hard-decision forward error correction (HD-FEC) limit using a 300-GHz-band photonics-based system incorporated with the on-line digital signal processing (DSP). The approach to obtaining these results was to optimize the transmission system by using a low-noised two-tone laser system to transmitter and receiver, and to apply on the unique Fresnel region of the antenna to achieve longer distances. These research results are expected to serve as a low-cost and basic communication system technology for promising backhaul wireless communications in the 6th generation era.

Significant improvement of n-contact performance and wall plug efficiency of AlGaN-based deep UV LEDs by atomic layer etching

Zhiyuan Liu, Tingang Liu, Haicheng Cao, Zixian Jiang, Na Xiao, Glen Isaac Maciel García, Yi Lu, xiao tang, and Xiaohang Li

DOI: 10.1364/OL.530719 Received 22 May 2024; Accepted 04 Jul 2024; Posted 05 Jul 2024  View: PDF

Abstract: The reactive ion etching (RIE) process is needed to fabricate deep ultraviolet (DUV) LEDs. However, the n-contact performance deteriorates when the high-Al n-AlGaN surface undergoes RIE, leading to decreased LED performance. In this study, we employed atomic layer etching (ALE) technology to eliminate surface damage generated during the mesa etching process, thus enhancing the n-Al₀.₆₅Ga₀.₃₅N ohmic contact. The improved contact performance reduced LED operation voltage and mitigated device heat generation. It was observed that DUV LEDs treated with 200 cycles of ALE showed a reduction in operating voltage from 8.3 V to 5.2 V at 10 mA, with a knee voltage of 4.95 V. The peak wall plug efficiency (WPE) was approximately 1.74 times that of reference devices. The X-ray photoelectron spectroscopy (XPS) analysis revealed that ALE removed the surface damage layer induced by plasma etching, eliminating surface nitrogen vacancies and increasing surface electron concentration. Consequently, it facilitated better ohmic contact formation on n-Al₀.₆₅Ga₀.₃₅N. This study demonstrates that ALE technology achieves etching with minor surface damage and is suitable for use in III-Nitride materials and devices to remove surface defects and contaminations, leading to improved device performance.

Ultrasmall-sized light-emitting diodes fabricated by ion implantation based on GaN epitaxial wafers with fully activated or unactivated p-GaN

Kui Pan, Kaixin Zhang, Yang Li, Qi Li, Yijian Zhou, Tianxi Yang, Chang Lin, Jie Sun, yongai zhang, xiongtu zhou, Jianda Shao, Tailiang Guo, and Qun Yan

DOI: 10.1364/OL.528884 Received 30 Apr 2024; Accepted 03 Jul 2024; Posted 25 Jul 2024  View: PDF

Abstract: A key challenge in realizing ultra-high-resolution displays is the efficient preparation of ultrasmall-sized light-emitting diodes (LED). Today, GaN-based LED are mainly prepared through dry etching processes. However, it is difficult to achieve efficient and controllable etching of ultrasmall-sized LED with high aspect ratios, and LED sidewalls will appear after etching, which will have a negative impact on the device itself. Herein, a method for preparing ultrasmall-sized LED based on GaN epitaxial wafers is reported (on two types of wafers, i.e. with p-GaN fully activated and unactivated). ions are injected into the intentionally exposed areas on the two types of wafers to achieve device isolation. The area under the micro/nano-sized protective masks (0.5, 0.8, 1, 3, 5, 7, 9, and 10 μm wide Ni/Au stripes) are the LED lighting areas. The LED on the p-GaN unactivated wafer (UAW) requires further activation. The Ni/Au mask not only serves as the p-electrode of LED, but also Ni as a hydrogen (H) removing metal covering the surface of p-GaN UAM that can desorb H from Mg element in the film at relatively low temperatures, thereby achieving the selective activation of LED lighting areas. Optoelectronic characterization shows that micro/nano-sized LED arrays with individual-pixel control were successfully fabricated on the two types of wafers. It is expected that the demonstrated method will provide a new way towards realizing ultra-high-resolution displays. Analyzing the changes in the current flowing through LED (before and after selective activation) on the injected p-GaN UAW, it is believed that depositing H removing metal on p-GaN UAW could possibly realize the device array through the selective activation only (i.e. without the need of ion implantation), offering a completely new insight.

4×4 differential index modulation for optical OFDM

Zhen Wang, hui Wang, Qihan Tang, Yue Zhang, Minghua Cao, and Qingbin Peng

DOI: 10.1364/OL.530280 Received 15 May 2024; Accepted 02 Jul 2024; Posted 03 Jul 2024  View: PDF

Abstract: In this letter, we propose and demonstrate a 4 × 4 differential index modulation (DIM) scheme for opticalOFDM (OOFDM) systems. The 4 × 4 DIM scheme avoids complex channel estimation by performing differentialindex modulation in the time-frequency domain, with the key aspect lying in the design of the time-frequency dispersion matrix that jointly indexes and constellation symbols. Moreover, we design a deep learning-based DIMFormer detector for the high decoding complexity problem of maximum likelihood (ML) detection. Experimental results show that the 4×4 DIM in OOFDM systems eliminates the need for complex channel estimation, and the loss of signal-to-noise ratio (SNR) is no more than 1 dB compared to conventional index modulation. The designed DIMFormer detector reduces the computational complexity by 38.98% as well as the time complexity by 99% compared to ML.

A 2.7 μm Backward Wave Optical Parametric Oscillator Source for CO₂ Spectroscopy

Adrian Vågberg, Martin Brunzell, Max Widarsson, Patrick Mutter, Andrius Zukauskas, Fredrik Laurell, and Valdas Pasiskevicius

DOI: 10.1364/OL.531038 Received 28 May 2024; Accepted 01 Jul 2024; Posted 02 Jul 2024  View: PDF

Abstract: In this work, a novel 2.7 μm source used for CO₂ and H₂O vapor spectroscopy using the backward propagating wave of a backward wave optical parametric oscillator (BWOPO) is demonstrated for the first time. The unique properties of BWOPOs eliminate the need for additional spectral narrowing or wavelength stabilization, enabling the use of a multi-longitudinal mode Q-switched pump laser centered around 1030 nm. A full characterization of the source is presented, revealing a central output at 2712 nm, showcasing temperature tuning of -1.77 GHz/K, and achieving an output pulse energy of 2.3 μJ. Novel methods are introduced for measuring the linewidth and wavelength stability using the ambient laboratory air. These approaches demonstrate a narrow output of 43 pm and establish an upper limit of stability at 65 MHz, with no active means of stabilization. These findings underscore the potential of BWOPOs as a robust platform for future DIAL systems.

Influence of the Atmosphere on the Phosphorescence of a Brominated Diphenylphosphine Oxide--Ethyl Naphthaleneimide Dyad

Marvin Malchau, Paul Reichstein, Werner Reichstein, Yoav Eichen, and Lothar Kador

DOI: 10.1364/OL.525175 Received 02 Apr 2024; Accepted 28 Jun 2024; Posted 05 Jul 2024  View: PDF

Abstract: The molecular dyad diphenylphosphine-ethyl bromine naphthaleneimide (Br-DPPENI) emits strong fluorescence and phosphorescence. We studied the intensity and lifetime of the phosphorescence of Br-DPPENI embedded in PMMA films in atmospheres of air, He, Ar, N$_2$, and O$_2$ as a function of pressure between vacuum and 1 bar. The experiment was performed in the frequency domain with a two-channel lock-in amplifier and the data were analyzed with the polar-plot or phasor technique. Reversible shortening of the lifetime due to triplet-triplet annihilation was found in the presence of atmospheric or pure oxygen. With small modulation frequencies, an additional slow component of the phosphorescence dynamics is observed, which is ascribed to the diffusion of oxygen into the sample films.

Photonic-Assisted W-band Flexible Integrated Sensing and Communication System for Fiber-Wireless Network Based on CE-LFM-OFDM

Fangb Wu, Boyu Dong, Ping Du, Mingji Dong, Xin Miao, junhao zhao, Zhongya Li, Wangwei Shen, Jianyang Shi, Nan Chi, and Junwen Zhang

DOI: 10.1364/OL.528335 Received 06 May 2024; Accepted 13 Jun 2024; Posted 16 Jul 2024  View: PDF

Abstract: We have experimentally demonstrated a constant envelope linear frequency modulated orthogonal frequency division multiplexing (CE-LFM-OFDM) signal, by employing orthogonal frequency division multiplexing (OFDM) signal to phase modulate the linear frequency modulation (LFM) carrier. The experimental verification was conducted in the photonic-based integrated sensing and communication (ISAC) system working at 94.5 GHz. In our system, 10-km optical fiber and 1-m free space transmission are incorporated, achieving seamless fiber-wireless networks. As a result, we achieved data rates ranging from 8 Gbit/s to 15.4 Gbit/s, and range resolution ranging from 1.5 cm to 7.5 cm respectively.

Simulation of diffraction and scattering using the Wigner Distribution Function

Emilie Pietersoone, Jean Michel Letang, Simon Rit, and Max Langer

DOI: 10.1364/OL.523608 Received 19 Mar 2024; Accepted 23 Apr 2024; Posted 23 Apr 2024  View: PDF

Abstract: X-ray phase-contrast imaging enhances soft tissue visualization by leveraging the phase shift of X-rays passing through materials. It permits to minimize radiation exposure due to high contrast, as well as high resolution imaging limited by the wavelength of the X-rays. Phase retrieval extracts the phase shift computationally, but simulated images fail to recreate low-frequency noise observed in experimental images. To this end, we propose a new method to simulate phase contrast images using the Wigner Distribution Function. This permits the simulation of wave and particle effects simultaneously and simulates images photon by photon. Here, we give a first demonstration of the method by simulating the Gaussian double-slit experiment. It has the potential for realistic simulation of low-dose imaging.

The threshold of stochastic self-focusing from the Poisson property of extreme-event statistics

Aleksei Zheltikov

DOI: 10.1364/OL.517922 Received 05 Jan 2024; Accepted 10 Apr 2024; Posted 10 Jun 2024  View: PDF

Abstract: Statistics of self-focusing events induced by a stochastic laser driver is shown to converge, in the large-sample-size limit, to a generalized Poisson distribution whose mean is given by the exponent of the respective extreme-value statistics. For a given ratio of the laser peak power to the self-focusing threshold Pcr, the mean number of self-focusing counts in a large sample of laser pulses is shown to depend on the number of pulses in the sample, N, and the signal-to-noise ratio of laser pulses, a. We derive a closed-form solution for the threshold of stochastic self-focusing, which, unlike its deterministic counterpart, Pcr, includes an explicit, physically transparent dependence on the sample size N and the signal-to-noise ratio a. We show that the parameter N_a=exp⁡(a^2⁄2) provides an important benchmark, setting a borderline between the deterministic and stochastic regimes of self-focusing. When the number of laser pulses in a sample becomes comparable to N_a, self-focusing can no longer be viewed as deterministic even for high-signal-to-noise laser beams.