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Abstract
An injection-seeded Q-switched Er:YAG laser with a double-corner-cube-retroreflector (DCCR) ring cavity has been demonstrated for the first time. The output coupling tuning range of the DCCR ring cavity was calculated to be 2.4% ∼ 97.5%. By tuning the output coupling, the injection-seeded Q-switched Er:YAG laser outputs a 1645 nm single-frequency laser pulse with optimum efficiency at pump powers of 23.2, 24.2, 25.2, and 26.2 W. The corresponding measured output coupling ratios are 1.6, 2.4, 2.4, and 2.4%, respectively. The pulse repetition rate is 143 Hz, and the pulse width varies from 100 ns to 200 ns as the pump power changes. In designing this laser, a new cavity length adjustment configuration of a double optical wedge was employed, achieving a maximum adjustment length of ΔL = 3.62 μm; the procedures for injecting the seed laser and detecting the resonance signal were analyzed, proving the feasibility of applying a DCCR ring cavity to an injection-seeded Q-switched laser.
© 2021 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Injection-seeded Q-switched lasers can output a single-frequency laser pulse. As laser transmitters, they are widely applied in long-range coherent Doppler wind LIDARs and meteorological research [1–5]. At present, there have been many studies on injection-seeded Q-switched lasers. Most of those studies were focused on new operating wavelengths [6], new active media [7,8], higher output energy [9,10], higher pulse repetition frequencies [11], and better frequency stability [12,13]. On the other hand, in engineering applications, laser efficiency is a more important indicator. However, there have been only a few reports about the efficiency of injection-seeded Q-switched lasers. To date, the reported methods to improve the efficiency of outputting single-frequency pulse laser are: optimizing the resonator cavity structure, employing an efficient pump source [9], and using an optimized gain medium [9,14]. However, the laser efficiency varies with the output coupling ratio, and the optimum output coupling ratio varies with pump power [15]. Therefore, those methods cannot maintain the laser at optimum efficiency when the pump power is changed. To achieve optimum laser efficiency at different pump powers, an output coupling ratio tunable cavity is necessary. However, injection-seeded Q-switched lasers with this cavity design have not been reported.
Within the past five years, an output coupling tunable double-corner-cube-retroreflector (DCCR) ring cavity (see Fig. 1) has been proposed [16–18]. It utilizes the polarization evolution of a corner-cube-retroreflector (CCR) to tune the output coupling ratio and has a maximum tuning range of 0% ∼ 97.53% [16]. In addition, it has the advantage of anti-maladjustment compared to traditional ring cavities. Therefore, the DCCR ring cavity is a good choice to optimize the efficiency of an injection-seeded Q-switched laser.
Nevertheless, owing to two obstacles, the DCCR ring cavity has not been reported in injection-seeded Q-switched lasers. One obstacle is adjusting cavity length, which is a necessary requirement for injection-seeded Q-switched lasers [19]. Because of the irregular shape and enormous weight of the CCR, the cavity length of a DCCR ring cavity cannot be adjusted by driving the CCR (cavity reflective mirror) with a piezoelectric (PZT) transducer as in the traditional ring cavity. The other obstacle is how to inject the seed laser and detect the resonance signal. For the complex polarization evolution in a DCCR ring cavity [16], these two operations cannot be realized easily as in a traditional ring cavity; the support of a polarization state calculation is required. In order to overcome these two obstacles, a new cavity length adjustment configuration using a double optical wedge was designed, and the polarization evolution of the injected seed laser in the DCCR ring cavity was calculated and will be discussed. This work proves that the DCCR ring cavity is feasible for use in injection-seeded Q-switched lasers.
In this paper, an injection-seeded Q-switched Er:YAG laser with an output coupling ratio tunable DCCR ring cavity is reported for the first time. This design can keep the laser operating at optimum efficiency regardless of pump power by tuning the output coupling ratio. The characteristics of output coupling ratio tuning in DCCR ring cavity are calculated and analyzed, and the tuning range is 2.4% ∼ 97.5%. A double optical wedge configuration was designed to adjust the cavity length and achieved a maximum adjustment length of ΔL = 3.62 μm and an adjustment frequency of 143 Hz. The polarization evolution of the seed laser in the DCCR ring cavity is calculated and discussed. Based on this design and calculation, an injection-seeded Q-switched Er:YAG laser with the DCCR ring cavity was set up that outputs a 1645 nm single-frequency pulse with a 143 Hz pulse repetition rate (PRF) at pump powers of 23.2, 24.2, 25.2, and 26.2 W. The pulse widths vary from 100 ns to 200 ns as the pump power changes. By tuning the output coupling ratio, the laser achieves optimum efficiency at those pump powers when the corresponding output coupling ratios are 1.6, 2.4, 2.4 and 2.4%, respectively.
2. DCCR ring cavity
The schematic of the DCCR ring cavity is shown in Fig. 1. CCR1 and CCR2 are the reflectors of the DCCR ring cavity. Their bottom surfaces are parallel to each other. M1 and M2 are two output couplers with a coating that has high-reflectivity at the pumping wavelength, high-reflectivity to s-polarized laser light, and high-transmission to p-polarized laser light at the oscillating light wavelength. The red lines represent the oscillating light path. The region between the two bottom surfaces (solid red line in Fig. 1) is perpendicular to the CCRs’ bottom surfaces. The gain medium is placed between M1 and M2. The pump laser is reflected into the gain medium by M1, and the unabsorbed pump laser light is reflected out of the cavity by M2.
The tunability of the output coupling ratio in a DCCR ring cavity comes from the polarization evolution [16]. Polarization evolution refers to the change in the polarization state of oscillating light between the end and beginning of a cycle; in a DCCR ring cavity, it is due to the phase shift that is caused by total internal reflection in the CCRs. For the DCCR ring cavity in Fig. 1, the polarization state of oscillating light following M1 is pure p-polarization. Because of polarization evolution, after a clockwise cycle, the polarization state of the oscillating light traveling back to M1 produces an s-polarized component. After undergoing interference at M1, the s-polarized component is reflected out of the cavity to become the s-polarized output laser beam; while the p-polarized component passes though M1 to start a new cycle. In this way, the output coupling ratio of M1 is the ratio of the s-polarized component at M1 to the total clockwise oscillating light (the sum of s- and p- polarized components). The output coupling ratio of M2 can be obtained by a similar method from the counterclockwise oscillating light path. By calculation, the output coupling ratios of M1 and M2 are equal to each other [see Fig. 2(a)].
Fig. 2. (a) Theoretical calculation results of output coupling ratio and seed laser injection ratio, (b) reflection order of CCR1 and CCR2 (a different reflection order produces a different calculation result [16]).
In addition, a λ/2 plate (see Fig. 1) is inserted into the oscillating light path to tune the output coupling ratio. As a phase retarder, this λ/2 plate can change the polarization evolution result [16], i. e., the ratio of s- and p-polarized component. Thus, by rotating the λ/2 plate, the output coupling ratio will be tuned. The tuning result is shown in Fig. 2(a).
Figure 2(a) shows the output coupling ratios varying with the λ/2 plate angle θ. The blue “○” and red “×” curves represent the output coupling ratios of M1 and M2, respectively. The output coupling ratio tuning range is 2.4% ∼ 97.5%, and the tuning cycle is 90°. Obviously, at a given θ, the output coupling ratios of M1 and M2 are equal to each other. This is the same as in a traditional ring cavity and is also a necessary requirement for applying a ring cavity to the injection-seeded Q-switched laser. Figure 2(b) shows the reflection order of CCR1 and CCR2, which are both 1→3→2. A different reflection order would cause a different tuning result [16]. If the reflection orders are not as shown in Fig. 2(b), the tuning range will not be 2.4% ∼ 97.5%.
Next, we will discuss the design of the cavity length adjustment configuration and analyze the process of injecting the seed laser and detecting the resonance signal for this cavity.
3. Double optical wedge configuration
An injection-seeded Q-switched laser requires an adjustable cavity length [19]. When using a traditional ring cavity, the adjustment is realized by a PZT mounted on the cavity mirror [20–23]. However, this method is not suitable for a DCCR ring cavity, because the CCR has an irregular shape and weighs hundreds of grams. In this case, a new cavity length adjustment configuration consisting of a double optical wedge is proposed as shown in Fig. 3.
Figure 3(a) is the schematic of the double optical wedge configuration. It consists of two identical “L-shaped” optical wedges placed opposite to each other. The transducers PZT1 and PZT2 are bonded to the top and bottom of the two optical wedges to adjust the vertical displacements Δh1 and Δh2, respectively. The light inputs horizontally from the side of one optical wedge and outputs horizontally from the side of another optical wedge. Thus, the total optical path length L in this configuration is the sum of the optical paths in the optical wedges and in the air gap. Since the refractive indexes in these two areas are different, L can be adjusted by changing the length ratio of the optical wedge area and air gap area by PZT1 and PZT2. The relationship between the total optical path adjusting length ΔL and the PZT displacements Δh1 and Δh2 can be calculated as:
Fig. 4. (a) Schematic of seed laser injection and resonance signal detection, (b) resonance signals detected in experiment.
Figure 3(b) shows the double optical wedge used in our experiments. It has an overall size of 5 mm × 5 mm × 7 mm, a wedge angle α = 30°, and a refractive index n = 1.4429 (fused silica at 1645 nm). The PZT bonded to optical wedge1 has a maximum displacement of 9 μm. Calculated by the above equations (Δh1 = 9 μm, Δh2 = 0 μm), the configuration can produce a maximum adjustment length ΔL = 3.62 μm, which exceeds the requirement of 1.645 μm for an injection-seeded Q-switched Er:YAG laser. The jitter frequency of the PZT is 143 Hz.
4. Seed laser injection and resonance signal detection
For a ring cavity, the seed laser is generally injected into cavity from the output coupler [20,24,25], and the same principle applies to the DCCR ring cavity. However, because of the complex polarization state, the process of injecting the seed laser into a DCCR ring cavity is different from that in a traditional ring cavity.
In this experiment, the seed laser was injected from M1 [see Fig. 4(a)]. M1 has a coating that reflects s-polarized laser light and transmits p-polarized laser light. Hence, in order to obtain efficient injection, the polarization state of the injected seed laser is required to be pure s-polarization. Otherwise, the p-polarized component in the seed laser will transmit through M1 to become a loss in the injection process. The injected seed laser travels counterclockwise in the DCCR ring cavity cycle by cycle. However, it is not really injected seed laser until it passes through M2 in the first cycle [see green arrows in Fig. 4(a)]. The polarization state of the seed laser propagating from M1 to M2 in the first cycle is different from that in subsequent cycles; thus, the seed laser in this optical path does not participate in the processes of generating a resonance signal and longitudinal-mode competition in the DCCR ring cavity, which are the two purposes of injecting the seed laser [21]. These two operations are actually accomplished in subsequent cycles [see orange arrows in Fig. 4(a)]. In this way, the ratio of seed laser passing through M2 in the first cycle and the total seed laser (injected from M1) is the seed laser injection ratio.
The seed laser passing through M2 in first cycle is pure p-polarized and comes from the polarization evolution of the s-polarized seed laser injected from M1. Therefore, the evolution ratio is the injection ratio. By calculation, the evolution ratio is equal to the output coupling ratio of M2 and M1. This means that the seed laser injection ratio is equal to the output coupling ratio of the DCCR ring cavity. This result is same as that in a traditional ring cavity. Furthermore, the injection ratio can also be tuned by the λ/2 plate, and the tuning result is in phase with the output coupling ratio M2 and M1, which is shown as the black curve in Fig. 2(a). In short, the seed laser injection result is same as that of a traditional ring cavity, but the injection process is different.
The resonance signal is generated in the subsequent cycles of the seed laser and outputted by M3 in Fig. 4(a). M3 has a coating of 45° sight-reflectivity at the seed laser wavelength (also the oscillating light wavelength). Influenced by M2 and M1 (reflecting s-polarized laser and transmitting p-polarized laser), the polarization state of the seed laser in each cycle (not including the first cycle) is constant [16]. Therefore, the polarization state of the seed laser at M3 remains constant in each cycle (not including the first cycle). In this case, the DCCR ring cavity with a double optical wedge is equivalent to a Fabry-Pérot interferometer, and M3 is the resonance signal output mirror. With the cavity length adjusted by the double optical wedge, M3 will output a resonance signal when the cavity length is an integer multiple of the seed laser wavelength. The detected resonance signals in this experiment are shown in Fig. 4(b).
Figure 4(a) shows the process of injecting the seed laser and detecting the resonance signal in the DCCR ring cavity. The orange arrow between M1 and M2 represents the really injected seed laser. Figure 4(b) shows the resonance signals detected in this experiment. The green curve is the driving voltage of the PZT bonded to optical wedge1; the red peaks are the resonance signals. Since the PZT jitter frequency is 143 Hz, the adjustment frequency of the cavity length is also 143 Hz.
5. Experimental setup
The previous analysis and design confirmed that the DCCR ring cavity is feasible for use in an injection-seeded Q-switched laser. Therefore, an injection-seeded Q-switched Er:YAG laser with a DCCR ring cavity was designed and set up.
Figure 5 shows the schematic of the injection-seeded Q-switched Er:YAG laser with a DCCR ring cavity. The DCCR ring cavity consisted of two CCRs (CCR1 and CCR2) placed at a distance of 200 mm. The oscillating light path between two CCRs included the up-light path and the down-light path. The Er:YAG crystal, output couplers (M1 and M2), and λ/2 plate2 were placed in the up-light path; the Q-switch, double optical wedge configuration, and signal mirror M3 were placed in the down-light path. To obtain a stable cavity, a compensating lens L2 with a focal length of 100 mm was placed in the symmetric position of the Er:YAG, between the Q-switch and the double optical wedge. Compared with the traditional ring cavity for injection-seeded lasers, the DCCR ring cavity contains more devices, causing some additional intra-cavity losses. However, these devices are all coated with high-transmission coatings so that the laser threshold is not significantly affected. In our experiments, the injection-seeded Er:YAG laser with a bow-tie ring cavity has a threshold of 19.6 W, and that of a DCCR ring cavity with same cavity length is 21.1 W.
The seed laser was an Er:YAG non-planar ring oscillator (NPRO) pumped by a 1532 nm laser, and emitting a 1645 nm CW single-frequency output. After passing through the λ/2 plate1, isolator, and lens L1, the seed laser was injected into the DCCR ring cavity from output coupler M1. Then, after adjusting the cavity length with the double optical wedge, the resonance signal of the seed laser was reflected out by M3 and was fed into the control system by a photodiode. Finally, at the moment when the resonance signal appeared, the control system turned on the Q-switch, causing the DCCR ring cavity to output a 1645 nm single-frequency pulse from M2. In an operation cycle, the sequential step of control system is adjusting cavity length → detecting resonance signal → turning on Q-switch. The frequency at which the cavity length is adjusted is also the PRF. In this experiment, the PRF was 143 Hz.
According to the output coupling ratio tuning results in Fig. 2(a), the efficiency of outputting a 1645 nm single-frequency pulse can be optimized by rotating the λ/2 plate2 in Fig. 5.
6. Results and discussion
The step angle of the rotating λ/2 plate2 in Fig. 5 is 5°. In this experiment, we measured the pulse energy of the 1645 nm laser from M2 at λ/2 plate angles θ of 70°, 75°, 80°, and 85°, with corresponding measured output coupling ratios of 9.3%, 2.4%, 1.6%, and 7.1%, respectively (see Table 1). The experimental results are shown in Fig. 6.
Table 1. Measured Output Coupling Ratios at Different λ/2 Plate Angles
Figures 6(a), 6(b), 6(c), and 6(d) show the laser pulse energy from M2 at output coupling ratios of 9.3%, 2.4%, 1.6%, and 7.1%, respectively. The black square curve and red circle curve represent the pulse energy of the laser under injection-seeded and free-running conditions, respectively. Obviously, the pulse energy in the injection-seeded situation is higher than that in the free-running state. Furthermore, the build-up time of those laser pulses was also measured. As shown in Fig. 7, the pulse build-up time in the injection-seeded state is significantly shorter than that in the free-running state. These two experimental results confirm that the injection-seeded Q-switched laser with a DCCR ring cavity has been successfully demonstrated.
The temporal profiles of the laser pulse in both the free-running and injection-seeded states are presented in Fig. 8(a) and 8(b), respectively, at a pump power of 25.2 W and an output coupling of 7.1% (θ = 80°). In the injection-seeded case, the pulse waveform is smoother, and the build-up time is shorter. The pulse widths of the single-frequency pulse at different output couplings are shown in Figs. 9(a), 9(b), 9(c), and 9(d). They vary from 100 ns to 200 ns as the pump power changes.
Fig. 8. Temporal profiles of laser pulses. (a) the pulse in the free-running state, (b) the pulse in the injection-seeded state
In addition, the laser efficiency while outputting 1645 nm single-frequency pulse varies with the output coupling ratio. We have analyzed the single-frequency pulse energy in Fig. 6 (pulse energy in the injection-seeded state). The results are shown in Fig. 10.
Fig. 10. The single-frequency pulse energy at different output coupling ratios and different pump powers
Figure 10 shows the variation of the single-frequency pulse energy as a function of the λ/2 plate angle θ (i.e. the output coupling ratio) when the pump power is 23.2, 24.2, 25.2, and 26.2 W, respectively. At a pump power of 26.2 W (pink curve), the pulse energy is 1.13 mJ at 7.1% output coupling ratio (θ = 85°), and increases to 1.79 mJ when the output coupling ratio is tuned to 1.6% (θ = 80°), while the optimum output coupling that yields a maximum pulse energy of 1.84 mJ is about 2.4% (θ = 75°). Obviously, under a given pump power, the pulse energy varies with the output coupling ratio; that is, the laser efficiency varies with the output coupling ratio. The other three curves (blue, red, and black curves) show similar results. In addition, the optimum output coupling ratio varies with pump power; it is 1.6% at a pump power of 23.2 W and 2.4% at pump powers of 24.2, 25.2, and 26.2 W.
These experimental results demonstrate that the output coupling ratio can significantly affect the efficiency of the single-frequency pulse laser. Consequently, an output coupling ratio tunable DCCR ring cavity is valuable for injection-seeded Q-switched lasers. It enables the injection-seeded Q-switched laser to maintain optimum efficiency regardless of pump power. In addition, it also makes the ring cavity design much easier. For a traditional ring cavity, the output coupling ratio is constant, which requires designing several output couplers to optimize laser efficiency and is therefore expensive; but these are not needed in the DCCR ring cavity.
One noteworthy fact is that the step angle for rotating the λ/2 plate should be as small as possible. Otherwise, a large step angle would result in poor output coupling ratio tuning accuracy, especially for the high-slope part of the curve in Fig. 2(a), which could result in missing the optimum output coupling. Unfortunately, the 5° step angle in this experiment seems to have been too large so that the output coupling ratio tuning accuracy is unsatisfactory. In terms of the variation trend of the four curves in Fig. 10, the optimum output coupling ratio at pump powers of 24.2, 25.2, and 26.2 W may not all be 2.4% (θ = 75°). Thus, employing a λ/2 plate with smaller step angle would have been a better choice. In addition, although injection-seeded Q-switched Er:YAG lasers realize optimum efficiency by tuning the output coupling ratio, only a 1.6% ∼ 9.3% tuning range is utilized. On the other hand, the DCCR ring cavity has a tuning range of 2.4% ∼ 97.8%, so it will present greater efficiency optimization capability in the injection-seeded lasers with high output coupling ratios.
7. Conclusions
In summary, an injection-seeded Q-switched laser with an output coupling ratio tunable DCCR ring cavity has been demonstrated that can maintain optimum efficiency at different pump powers by tuning the output coupling ratio. In order to realize this laser, the property of output coupling ratio tuning in DCCR ring cavity was calculated and analyzed, obtaining a tuning range of 2.43% ∼ 97.53%. Then a double optical wedge was designed to adjust the DCCR ring cavity length, resulting in a 143 Hz adjustment frequency and a maximum adjusting length ΔL = 3.62 μm. The seed laser injection and resonance signal detection in the DCCR ring cavity were also calculated and discussed, the results show that the seed laser injection ratio is equal to the output coupling ratio of the DCCR ring cavity, and the resonance signal was detected experimentally. Based on these preliminary studies, an experiment with an injection-seeded Q-switched Er:YAG laser with a DCCR ring cavity was set up, and this laser outputted a 1645nm single-frequency pulse with a 143 Hz PRF at pump powers of 23.2, 24.2, 25.2, and 26.2 W. The pulse width varied from 100 ns to 200 ns as the pump power changed. By tuning the output coupling ratio of the DCCR ring cavity, the efficiency was changed significantly. The optimum output coupling for maximum pulse energy was about 1.6% for a pump power of 23.2 W and 2.4% for pump powers of 24.2, 25.2, and 26.2 W. Therefore, the DCCR ring cavity is of great value for injection-seeded Q-switched lasers.
Acknowledgments
Thanks Wei Zhang, Tianliang Jiao and Xiaofan Jiang for designing the control system for this work.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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