A 1-kHz Single Frequency Nd:YAG Ring Laser by Injection Seeding

Xue-Zhe Cao(曹雪喆)$^{1}$, Pei-Lin Li(李沛霖)$^{2}$, Zai-Yuan Wang(王在渊)$^{1}$, Qiang Liu(柳强)$^{1\hyperlink{s}{**}}$

doi:10.1088/0256-307X/36/12/124201

⇦Chinese Physics Letters, 2019, Vol. 36, No. 12, Article code 124201⇨ A 1-kHz Single Frequency Nd:YAG Ring Laser by Injection Seeding * Xue-Zhe Cao (曹雪喆)1, Pei-Lin Li (李沛霖)2, Zai-Yuan Wang (王在渊)1, Qiang Liu (柳强)1** Affiliations 1State Key Laboratory of Precision Measurement Technology and Instruments, Department of Precision Instrument, Tsinghua University, Beijing 100084 2Tianjin Research Institute for Advanced Equipment, Tsinghua University, Tianjin 300300 Received 10 September 2019, online 25 November 2019 ^*Supported by the National Key Research and Development Program of China under Grant No 2017YFB1104500, and the National Natural Science Foundation of China under Grant No 61875100. ^**Correspondence author. Email: qiangliu@mail.tsinghua.edu.cn Citation Text: Cao X Z, Li P L, Wang Z Y and Liu Q 2019 Chin. Phys. Lett. 36 124201 Abstract We report on an injection seeded 1 kHz single frequency pulsed Nd:YAG ring laser with pulse energy of 5.2 mJ and pulse width of 9.9 ns. The ramp-fire technique is used to maintain single frequency operation and the cavity length is modulated by an intracavity RbTiOPO$_{4}$ (RTP) crystal. The frequency stability (rms) of the output pulse is 1.99 MHz over 1 min and the linewidth is 64 MHz. DOI:10.1088/0256-307X/36/12/124201 PACS:42.60.-v, 42.60.Fc, 42.60.Gd, 42.79.Qx © 2019 Chinese Physics Society Article Text High-power/energy single frequency lasers are widely used in applications such as interferometric gravitational wave detection,[1,2] global observations of greenhouse gases,[3,4] and Doppler wind lidars.[5,6] Injection seeding is an effective method for single mode selection in high-peak power Q-switched lasers.[7,8] The most commonly used technique to achieve single frequency operation in injection seeded lasers is the ramp-fire technique,[9] which involves sweeping the optical path length of the slave laser and triggering the Q-switch when the slave laser is in resonance with the seed laser. The intracavity phase modulation approach is based on the electro-optical crystal and was first demonstrated by Nicolaescu et al. for linewidth narrowing in an Alexandrite laser.[10] Because the electro-optical crystal has no mechanical ringing effect and its response speed is fast, this phase modulation approach is especially appropriate for high-repetition rate operation of injection seeded lasers at hundreds to thousands of hertz. In 2014, Zhang et al. reported an injection seeded Nd:YAG laser with pulse energy of 4.8 mJ and pulse duration of 25 ns at the repetition rate of 400 Hz. A lithium niobate (LiNbO$_{3}$) crystal was used as the phase modulator for cavity length sweeping.[11] In 2015, they successfully demonstrated the injection seeding using a RbTiOPO$_{4}$ (RTP) crystal at the same repetition rate of 400 Hz.[12] The output energy of the laser was 9.9 mJ and the pulse duration was 16 ns. They also reported 900 Hz operation of the laser in 2016 with similar energy and pulse duration.[13] Chunang et al. reported an injection seeded end-pumped Nd:YVO$_4$ master oscillator power amplifier (MOPA), which was operated at 4 kHz.[14] However, no detailed design of the laser was described. The higher repetition rate operation of the laser transmitter in a lidar allows us to obtain a higher signal-to-noise rate (SNR); i.e., a better detection accuracy.[15] In this Letter, we demonstrate high-repetition operation of a single frequency Nd:YAG ring laser at 1 kHz by injection seeding with an RTP phase modulator. The maximum laser output energy is 5.2 mJ with a pulse duration of 9.9 ns. The output pulse also exhibits good beam quality, excellent frequency stability and narrow linewidth. Figure 1 depicts the experimental setup of the laser, which consists mainly of a seed laser, a Q-switched slave ring laser and control circuits.

Fig. 1. Experimental setup of the 1 kHz injection seeded single frequency Nd:YAG ring laser.

The seed laser was a continuous-wave monolithic unidirectional single frequency Nd:YAG ring laser[16] with a full width at half maximum (FWHM) linewidth of about 1 kHz and a frequency drift of 1 MHz/min. A Faraday isolator (FI) was used to eliminate the feedback from the Q-switched slave laser. The spatial mode matching between the seed laser and the slave cavity was carried out by careful adjustment of a coupling lens with $f = 400$ mm. In addition, a $\lambda$/4 plate and two $\lambda$/2 plates were used to control the polarization state of the seed laser. The resonator of the slave laser consists of four 45$^{\circ}$ flat mirrors, as shown in Fig. 1. Its cavity length was 340 mm, corresponding to a free spectral range (FSR) of about 880 MHz. The ring configuration resonator has the advantage of being free from spatial hole burning effect.[17] The slave laser was injected by the seed laser with an injection power of about 80 mW through the output coupler mirror with a transmissivity of 50%, leading to a high injection efficiency. Laser-diode (LD) end-pumped rod laser is the most commonly used design to achieve efficient high-repetition rate lasers. The laser gain medium was a Nd:YAG crystal in dimensions of $4\times 4\times 40$ mm$^{3}$ and a doping concentration of 0.8 at.%. The pump light from a fiber-coupled LD (nLight P4-110-0885-0.5-A-R01) at 885 nm was focused by two lenses into the Nd:YAG crystal. The LD was operated at 1 kHz with a pump width of 250 µs. Both the crystal and the LD were mounted on water-cooled heat sinks for heat removal.

Fig. 2. The 10-µs 3-kV linear ramp voltage, the interference signal and the laser pulse.

Single-frequency operation was achieved by the ramp-fire technique with a $5\times 5\times 40$ mm$^{3}$ RTP intracavity phase modulator (PM). The signal of the ramp voltage produced by the PM driver, the interference signal detected by the photodiode (PD), and the laser pulse as a function of time are shown in Fig. 2. The ramp-fire circuit was synchronized with the 1 kHz pump signal in width of 250 µs. At the time of 240 µs within each pump pulse, the PM driver was triggered by the ramp-fire circuit to produce a 10-µs 3-kV linear ramp voltage, sweeping the ring resonator through 2.5 FSR. We choose the second peak of the detected interference signal to trigger the Q-switch, consisting of a pair of RTP crystals and a polarizing beamsplitter (PBS). Because of the fast ramp voltage with a speed of 4 µs/FSR, the maximum jump of pulse emission time was reduced to 4 µs, which improved the timing stability of the laser. Figure 3 shows the output energy and the pulse duration of the laser as a function of the pump energy. In free running regime, the laser produced output in both clockwise (cw) and counterclockwise (ccw) directions. In our laser, the ccw wave had a greater loss, so the output energy in the cw direction grew with the increase of pump energy, while the energy in the ccw direction remained constant. When injection was seeded, the ccw wave was completely suppressed. Unidirectional operation was obtained and the energy was equal to the total energy when unseeded. The slope efficiency was about 25%. The maximal output energy was 5.23 mJ with a pulse width of 9.9 ns when the pump energy was 26.8 mJ, leading to an optical-to-optical efficiency of 19.5%.

Fig. 3. (a) Output energy and (b) pulse duration of the 1 kHz injection seeded laser versus the pump energy with the produced laser output in both clockwise (cw) and counterclockwise (ccw) directions.

Fig. 4. (a) The temporal pulse shapes of the laser output when the salve laser is (1) injection seeded and (2) in free running, (b) the 1 kHz output pulse train.

Figure 4 shows the temporal pulse shapes of the 1 kHz injection seeded laser for seeded and unseeded operation and the pulse train. As is expected, the pulse shape was smooth when the laser was injection seeded and distorted when unseeded, indicating single frequency operation and multi-frequency operation, respectively. The peak intensity of pulse (2) is lower than that of pulse (1) because only the pulse of the clockwise direction of the ring oscillator was measured. In addition, the build-up time was about 30 ns when injection seeded, which was shorter than that when unseeded. A wavelength meter (HighFinesse WSU-2) was used to measure the frequency stability of the output pulse and the results are shown in Fig. 5. The rms value of the frequency jitter was 1.99 MHz over 1 min.

Fig. 5. The frequency stability of the 1 kHz output pulse over 1 min.

Fig. 6. The linewidth of the 1 kHz output pulse at 532 nm measured by the Fabry–Pérot interferometer.

Fig. 7. The beam quality of the 1 kHz output pulse.

The linewidth of the single frequency output pulse was measured using a scanning Fabry–Pérot interferometer (FPI) with an FSR of 1.5 GHz and a finesse of 1500. The available wavelength range for the FPI was from 488 nm to 545 nm, so the output pulse was first converted to 532 nm by a KTP crystal. Figure 6 shows the measured results of linewidth at 532 nm where more than 20 pulses in each peak were detected. The linewidth at 532 nm was 91 MHz, corresponding to 64 MHz at 1064 nm. Since the pulse width was 9.9 ns, a 1.4-time Fourier-transform-limited linewidth was obtained. We measured the beam quality by the 90/10 knife edge method. As shown in Fig. 7, the beam quality factors $M^{2}$ were 1.17 and 1.22 in the $x$ and $y$ directions, respectively. In summary, we have demonstrated high-repetition rate operation of a single frequency Q-switched Nd:YAG laser by injection seeding at 1 kHz. The frequency was stabilized by the fast ramp-fire technique with an RTP crystal as the intracavity phase modulator. The laser produced 5.2 mJ pulse energy with a pulse duration of 9.9 ns. The frequency jitter (rms value) was about 2 MHz over 1 min, and the linewidth was 64 MHz. For higher-repetition-rate operation of injection seeded lasers at several kHz or even up to 10 kHz, the pump width is much shorter. The high-speed phase modulation approach demonstrated in this study is especially suitable for obtaining high frequency stability, energy stability and timing stability in injection seeded lasers in these cases. References Injection-locked single-frequency laser with an output power of 220 WHigh power single frequency solid state master oscillator power amplifier for gravitational wave detectionSpace-borne remote sensing of CO2, CH4, and N2O by integrated path differential absorption lidar: a sensitivity analysisFrequency Stabilization of Pulsed Injection-Seeded OPO Based on Optical Heterodyne TechniqueWind lidar for atmospheric researchSPIE ProceedingsInjection locking and mode selection in TEA-CO2laser oscillatorsSingle axial mode operation of aQ-switched Nd:YAG oscillator by injection seedingFast resonance-detection technique for single-frequency operation of injection-seeded Nd:YAG lasersInjection seeded single-frequency pulsed Nd:YAG laser resonated by an intracavity phase modulatorStable seeder-injected Nd:YAG pulsed laser using a RbTiOPO4 phase modulator900 Hz Single-Frequency Pulsed Nd:YAG Laser Based on RTP Phase ModulatorSPIE ProceedingsConductively cooled 1-kHz single-frequency Nd:YAG laser for remote sensingMonolithic, unidirectional single-mode Nd:YAG ring laserSingle‐frequency traveling‐wave Nd:YAG laser

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