Photonic crystal fiber

High harmonic generation (HHG) of intense infrared laser radiation (Ferray et al., J. Phys. B: At. Mol. Opt. Phys. 21:L31, 1988; McPherson et al., J. Opt. Soc. Am. B 4:595, 1987) enables coherent vacuum-UV (VUV) to soft-X-ray sources. In the usual setup, energetic femtosecond laser pulses are strongly focused into a gas jet, restricting the interaction length to the Rayleigh range of the focus. The average photon flux is limited by the low conversion efficiency and the low average power of the complex laser amplifier systems (Keller, Nature 424:831, 2003; Südmeyer et al., Nat. Photonics 2:599, 2008; Röser et al., Opt. Lett. 30:2754, 2005; Eidam et al., IEEE J. Sel. Top. Quantum Electron. 15:187, 2009) which typically operate at kilohertz repetition rates. This represents a severe limitation for many experiments using the harmonic radiation in fields such as metrology or high-resolution imaging. Driving HHG with novel high-power diode-pumped multi-megahertz laser systems has the potential to significantly increase the average photon flux. However, the higher average power comes at the expense of lower pulse energies because the repetition rate is increased by more than a thousand times, and efficient HHG is not possible in the usual geometry. So far, two promising techniques for HHG at lower pulse energies were developed: external build-up cavities (Gohle et al., Nature 436:234, 2005; Jones et al., Phys. Rev. Lett. 94:193, 2005) and resonant field enhancement in nanostructured targets (Kim et al., Nature 453:757, 2008). Here we present a third technique, which has advantages in terms of ease of HHG light extraction, transverse beam quality, and the possibility to substantially increase conversion efficiency by phase-matching (Paul et al., Nature 421:51, 2003; Ren et al., Opt. Express 16:17052, 2008; Serebryannikov et al., Phys. Rev. E (Stat. Nonlinear Soft Matter Phys.) 70:66611, 2004; Serebryannikov et al., Opt. Lett. 33:977, 2008; Zhang et al., Nat. Phys. 3:270, 2007). The interaction between the laser pulses and the gas occurs in a Kagome-type Hollow-Core Photonic Crystal Fiber (HC-PCF) (Benabid et al., Science 298:399, 2002), which reduces the detection threshold for HHG to only 200 nJ. This novel type of fiber guides nearly all of the light in the hollow core (Couny et al., Science 318:1118, 2007), preventing damage even at intensities required for HHG. Our fiber guided 30-fs pulses with a pulse energy of more than 10 μJ, which is more than five times higher than for any other photonic crystal fiber (Hensley et al., Conference on Lasers and Electro-Optics (CLEO), IEEE Press, New York, 2008).

A review of optical fiber sensing demonstrations based on photonic crystal fibers is presented. The text is organized in five main sections: the first three deal with sensing approaches relying on fiber Bragg gratings, long-period gratings and interferometric structures; the fourth one reports applications of these fibers for gas and liquid sensing; finally, the last section focuses on the exploitation of nonlinear effects in photonic crystal fibers for sensing.

We present an overview of photonic textile displays woven on a Jacquard loom, using newly discovered polymer photonic bandgap fibers that have the ability to change color and appearance when illuminated with ambient or transmitted light. The photonic fiber can be thin (smaller than 300 microns in diameter) and highly flexible, which makes it possible to weave in the weft on a computerized Jacquard loom and develop intricate double weave structures together with a secondary weft yarn. We demonstrate how photonic crystal fibers enable a variety of color and structural patterns on the textile, and how dynamic imagery can be created by balancing the ambient and emitted radiation. Finally, a possible application in security ware for low visibility conditions is described as an example.

Conventional and scanning near-field optical microscopy techniques are cross referenced to femtosecond nonlinear-optical measurements and finite-element numerical simulations to visualize and analyze a strong confinement of electromagnetic radiation in guided modes of a photonic-crystal fiber with only a few air holes surrounding the fiber core. A nonlinear coefficient of about 120 W−1 km−1 is achieved at the wavelength of 670 nm for a fused-silica fiber with a full hexagonal cycle of closely packed air holes around the fiber core. The removal of a single element from this array of air holes is shown to frustrate field confinement in guided modes, leading to mode leakage.

Photonic crystals are wavelength-scale periodic structures built from dielectrics with different refractive indexes As standard 2D photonic crystals are fabricated by lithographic methods, but in this case only planar structure can be obtained. We have adapted stack and draw technique that is usually used for photonic crystal fiber fabrication to develop volume 2D photonic crystals. Technology allows fabrication of high contrast structures with air holes as well as low contrast solid-all structures where air holes are replaced with glass micro rods of refractive index. Use of soft glasses with a high difference in refractive index allows development of a structure where partial photonic band gap exists. The proposed method offers possibility of fabrication volume 2D photonic crystal with a diameter in the order of 1 mm and height of a few mm. Large area photonic crystals are very attractive as new optical material named 'photonic glass' with built-in photonic bandgap functionality. Preliminary fabrication test were performed for two pairs of soft glasses NC21/F2 and SK222/Zr3. The considered glasses are thermally matched and are synthesized in-house except of F2 glass (standard Schott glass). Obtained structures are regular with some defects on the borders between intermediate performs. Some glass diffusion is observed between Zr3 and SK222 glasses. With this technique a 2D photonic crystal with a hexagonal lattice was fabricated with a pair of soft glasses SK222 and Zr3. Microrod diameter is 749nm and lattice constant 1110 nm. Photonic crystal consists of 166421 elements (425 elements on diagonal) and its total surface is about field ~0,178mm2.