High-Energy Injection-Seeded Single-Frequency Er:YAG Laser at 1645 nm Pumped by a 1532 nm Fiber Laser

Abstract

A single-frequency, Q-switched Er:YAG laser, pumped by a 1532 nm fiber laser, has been demonstrated. At the pulse repetition frequency (PRF) of 200 Hz, the maximum single-frequency laser of 5.5 mJ is attained, and, correspondingly, the pulse width is 212 ns. Using the heterodyne technique, the single-frequency laser spectrum’s full width at half maximum is determined to be 2.73 MHz. The experimental results show that the single-frequency laser has excellent beam quality factors (M²) of 1.18 and 1.21.

1. Introduction

Single-frequency solid-state lasers, with the emission wavelength around 1.6 μm, are widely used in various applications, such as the differential absorption lidar for methane detection and Doppler coherent lidar for wind measurements [1,2,3,4,5]. In these applications, laser sources with high pulsed energy, excellent beam quality, and narrow linewidth are key to meeting requirements. The methods to obtain the pulsed single-frequency laser include seed-injection, amplifying a continuously single-frequency laser after the modulation [6], and using a twisted-mode pulsed laser [7]. Compared with the seed-injection method, the other ways fail to achieve the Fourier transform limit linewidth laser output and have a lower laser energy, which cannot guarantee long-term frequency stability. A Q-switched slave laser, injection-seeded by the single-longitudinal-mode (SLM) continuous-wave (CW) seed laser, is an efficient way to provide single-frequency pulsed laser [8,9,10,11].

In 2014, Deng et al. reported a diode-pumped, injection-seeded Q-switched Er:YAG laser with a ‘four-mirror ring cavity’ structure, operating at room temperature. Under a repetition rate of 100 Hz, the maximum single-frequency laser energy was 3.5 mJ with a pulse of 195 ns [12]. In 2018, a seed-injection Q-switched Er:YAG laser, operating at 1645 nm, was reported by Gao et al. The pumping source was a 1470 nm fiber-coupled laser diode (LD). The highest laser energy of 10.1 mJ with 205 ns of pulse width at a repetition rate of 200 Hz was achieved [13]. In 2019, Zhang et al. demonstrated a 1645 nm single-frequency Er:YAG laser that was injection-seeded by a non-planar ring resonator (NPRO) single-longitudinal-mode laser and pumped by a 1470 nm LD. At a PRF of 1 KHz, the maximum single-frequency laser energy was 5.52 mJ and the pulse width was 500 ns [14].

With technical improvements, a fiber laser with a central wavelength of 1532 nm is more suitable as the direct pumping source for the Er:YAG laser, due to the higher pump absorption and conversion efficiency, better beam quality, fewer stringent temperature control requirements, and more simplified cooling systems compared to the laser diodes. In 2020, a single-frequency Er:YAG laser with a double gain medium pumped by a 1532 nm fiber laser was reported, and the maximum 18.3 W average power at 3 KHz operation was obtained. The beam quality M² factors were 1.51 and 1.54 in the x and y directions, and the fluctuation of the single-frequency laser energy within 30 min was less than 1.23% [15]. In 2020, Zhang et al. reported a maximum pulsed energy of 1.8 mJ for an injection-seeded Er:YAG laser pumped by a home-made 1532 nm fiber laser in a double-corner-cube-retroreflector (DCCR) ring cavity, with a tunable output coupling ratio [16]. In 2023, Fan et al. reported an injection-seeded, single-frequency Q-switched Er:YAG laser operating in a DCCR resonator. A single-frequency Er:YAG laser energy of 1.34 mJ was reached, with a pulse width of 482 ns. And the beam qualities M² of the 1.34 mJ single-frequency pulsed laser were measured to be 1.2 and 1.24 along the horizontal (x) and vertical (y) directions, respectively, with the 90/10 knife-edge method [17]. To summarize, by adopting the 1532 nm fiber laser as the pumping source, the single-frequency pulsed laser’s beam qualities are significantly improved. In

Table 1. Single-frequency Er:YAG laser.

References	Years	Gain Medium	Pumping Source	Beam Quality
[18]	2019	double Er:YAG	1470 nm LD + 1532 nm fiber laser	1.27, 1.30
[4]	2020	Er:YAG cemamics	1470 nm LD + 1532 nm fiber laser	1.51, 1.55
[19]	2020	double Er:YAG	1470 nm LD	1.37, 1.09
[20]	2022	Er:YAG	1470 nm LD + 1532 nm fiber laser	\
[17]	2023	Er:YAG	both 1532 nm fiber laser	1.2, 1.24
This paper	2024	Er:YAG	both 1532 nm fiber laser	1.18, 1.21

, we compare single-frequency Er:YAG lasers with different pumping sources proposed in the past five years.

In this paper, a 1645 nm single-frequency seed-injection Er:YAG pulsed laser is presented. The pumping source of both the single-longitudinal-mode seed and Q-switched slave laser is the fiber laser, at a central wavelength of 1532 nm. To achieve high pulse energy, a ‘bow-tie’ structure ring resonator is employed as the slave laser. The maximum output energy is up to 5.5 mJ under a pump power of 40 W at a PRF of 200 Hz, and, correspondingly, the pulse width is 212 ns. The measured M² factors of the single-frequency pulsed laser are 1.18 and 1.21, which is near the diffraction limit. And the full width at half maximum (FWHM) of the single-frequency laser is 2.73 MHz.

2. Experimental Setup

Figure 1 displays the schematic diagram of the seed-injection, single-frequency Q-switched Er:YAG laser. The seed laser is a stable single-longitudinal-mode NPRO Er:YAG laser oscillating at 1645.1 nm. A high-transmission coating at 1532 nm and 5% output coupling coating of the s-polarized beam at 1645 nm are applied to the front end face of NPRO. NPRO is water-cooled to maintain a temperature of 15 °C and is resonantly pumped by a home-made fiber laser operating at 1532 nm with a maximum pump power of 6.8 W. Reflected by the 45° mirrors M1 and M2, the pumping beam is focused into the NPRO by the convex lenses L1 and L2, which have focal lengths of 10 mm and 200 mm and are coated for anti-reflection at 1.5–1.7 µm. Then, the polarized beam splitter (PBS) divides the seed laser into two parts. The first part (1 W) is injected into the Q-switched slave laser, while the second part (100 mW) is used as the reference light for the heterodyne beating system. Before the PBS, a half-wave plate (λ/2-1) at 1.6 µm is inserted to adjust the intensity of the reference light and the injection seed laser. To adjust the polarization of the injection seed laser, the half-wave plate (λ/2-2) is utilized. The injection seed laser is focused using a convex lens f3, which has a focal length of 200 mm and is coated for high transmission at 1.5–1.7 µm, to match the mode of the slave laser in the center of the Er:YAG crystal. Due to the susceptibility of the fiber laser to feedback from the pulsed slave laser, an isolator (ISO) is used to prevent damage.

The Er:YAG crystal in the slave laser has dimensions of 1.7 mm × 5.5 mm (cross-section) × 50 mm (length), and its doping concentration is 0.5 at.%. Both the end faces of the crystal are polished and coated with high-transmission coating at 1.5–1.7 µm (T > 99.9%). The Er:YAG gain medium is wrapped in indium foil and mounted in a copper heat sink to maintain the temperature at 15 °C using a thermal electric cooler (TEC). The Er:YAG crystal is resonantly pumped by the 1532 nm fiber laser, which has a maximum pump power of 40 W. The pump beam is initially collimated by a convex lens (f4) with a focal length of 10 mm and then focused into the Er:YAG crystal by another convex lens (f5) with a focal length of 160 mm. Both lenses, f4 and f5, have a high-transmission coating at 1.5–1.7 µm. The radius of the pump beam in the center of the Er:YAG crystal is approximately 300 µm. In addition, the unabsorbed pump power is dumped from M8 to prevent feedback into the fiber laser.

The slave laser is a ring-cavity structure with four mirrors (incident angle 20°). A longer resonator length is also present, with a total ring-cavity length of approximately 500 mm. In Doppler coherent lidar, a larger resonator length leads to a greater pulse width, which could enhance the velocity accuracy. Three flat mirrors (M7, M8, and M9) and a curved mirror (OC) make up the ring cavity. The length of the slave laser is adjusted by the PZT, fixed upon the flat mirror M9. The output mirror (OC) has a 15% transmittance at 1.6 µm and a 300 mm radius of curvature. Moreover, the pulse operation is achieved by using a Q-switch with a radio frequency of 40 MHz.

The beam of the seed laser is focused by lens f3 to match the mode of the slave laser in the center of Er:YAG crystal. Then, the seed laser is injected into the slave oscillator by the output mirror (OC). The resonance signal is reflected by M10 and detected with an InGaAs photodiode (PD1). The PZT is driven by the ramp signal, a triangle wave at a repetition of 200 Hz. An active control system is designed to realize the ‘Ramp-Hold-Fire’ (RHF) injection-seeding technique [21]. The technique of RHF requires scanning the length of the slave laser and obtaining the resonance signal by detecting the transmitted laser. When the cavity mode fully resonates with the frequency of seed laser, the resonance signal reaches its peak. At this point, opening the Q-switch can achieve the seed-injection. And the injection-seeded Q-switched pulses are triggered by applying a latch signal to the Q-switch once the resonance signal has been detected. The heterodyne beating measurement system is employed to evaluate the injection-seeding results and measure the linewidth of the single-frequency pulse. The second component of the seed laser is combined with the single-frequency pulse beam in a photodiode. A digital oscilloscope (Tektronix, TDS5052B, Harbin, China) is used to record the mixed signal, which is subsequently examined to determine the linewidth of the single-frequency pulse.

3. Experimental Results and Discussion

The seed laser delivers a maximum 1.1 W single-longitudinal-mode laser with a pump power of 6.8 W, resulting in a slope efficiency of 23.67%. The results of NPRO laser characteristics are shown in Figure 2. In Figure 2a, the seed laser operates in single longitudinal mode (the green line represents the sawtooth-wave signal, and the red line represents the laser signal). The controller of the Fabry–Perot interferometer is used to generate a highly stable, low-noise voltage ramp signal. The ramp signal is used to scan the interval between two mirrors within the interferometer to measure the signal of the seed laser. It demonstrates that the maximum 1.1 W output laser operates in SLM at a central wavelength of 1645.1 nm. And the fitted M² factor in the x direction is 1.26. Moreover, the linewidth of the NPRO seed laser is approximately 40 KHz, measured with the delayed self-heterodyne laser linewidth method [22].

The free-running operation of the Er:YAG slave laser is firstly investigated. The measured maximum output power is 6.03 W at a maximum pump power of 40 W, while the threshold pump power is 20.5 W. The maximum pulse energy is about 6.0 mJ at a PRF of 200 Hz, and, correspondingly, the pulse width is 203 ns.

Figure 3a shows the output pulse energy and pulse width under the condition of seed-injection, at a PRF of 200 Hz. The results show that the maximum output energy of the single-frequency laser is 5.5 mJ, and, correspondingly, the pulse width is 212 ns. Figure 4 depicts the energy fluctuation of the single-frequency Er:YAG laser, at a PRF of 200 Hz. The single-frequency output energy is recorded during the 14 h period, at the maximum laser energy 5.5 mJ. As indicated in Figure 4, the mean energy is around 5.5 mJ, with a standard deviation of 0.104 mJ, and the RMS is about 1.9%.

The comparison results of the build-up time with and without the seed-injection are shown in Figure 3b. As shown in Figure 3b, the injection-seeded laser has a shorter build-up period than the non-injection-seeded laser. This phenomenon is primarily due to the fact that the pulse of the single-frequency Q-switched Er:YAG laser is built up from the injected seed beam rather than spontaneous radiation. The density of the inversion population is low at a low pump power, resulting in a protracted pulse establishment time. As the incident pump power increases, the time of pulse establishment gradually shortens due to the stronger spontaneous radiation and stimulated emission.

To measure the linewidth of the 5.5 mJ single-frequency pulsed laser, the reference light is frequency-shifted by the acoustic-optic modulator (AOM), with an offset of 100 MHz, as shown in Figure 2. Figure 5a shows the beating signal waveform of the 5.5 mJ single-frequency pulsed laser and the reference light. The digital oscilloscope records the beating signal with the heterodyne beating technique. Figure 5b shows the Fast Fourier Transform (FFT) spectrum of the heterodyne signal. The FFT spectrum has a central frequency of about 100 MHz and an FWHM of 2.73 MHz, making it 1.31 times transform-limited.

The beam radius along the beam propagation direction is shown in Figure 6. By fitting the Gaussian curve to the measured data, the M² factors are evaluated as 1.18 for the x direction and 1.21 for the y direction.

4. Conclusions

In summary, we have reported a single-frequency, Q-switched Er:YAG laser with a maximum energy of 5.5 mJ and a pulse width of 212 ns. By adopting the 1532 nm fiber laser as the pumping source, excellent beam quality factors (M²) of 1.18 and 1.21 have been measured in the x, y directions. The energy fluctuation of the single-frequency laser has been recorded in 14 h, with a standard deviation of 0.104 mJ, and the full width at half maximum of the 200 Hz pulse spectrum is measured to be 2.73 MHz.