A high-energy, single-frequency, injection-seeded, Q-switched Er:YAG laser oscillating at 1645 nm is demonstrated. For obtaining high output energy, a double-crystals-end-pumping architecture is utilized. The maximum output energies of single-frequency pulses are 12.84 mJ, 16.87 mJ, and 20.3 mJ at pulse repetition rates of 500 Hz, 300 Hz and 200 Hz, respectively. Correspondingly, the pulse widths are 162 ns, 125 ns, and 110 ns, respectively. The half-width of the pulse spectrum at the pulse repetition rate of 200 Hz is 4.59 MHz, measured by using the heterodyne technique, which is 1.14 times transform limited. To the best of our knowledge, 20.3 mJ is the highest energy obtained from a single-frequency, injection-seeded Er:YAG laser.
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Eye-safe single-frequency solid-state lasers emitting at the wavelength around 1.6 μm are widely applied in fields as Doppler lidars for wind measurement and differential absorption lidars for methane detection [1–5]. For these applications, a high energy pulse laser as transmitter could guarantee the detection range. So a high energy pulsed laser with narrow linewidth is essential to meet the requirements. An effective method to obtain single-frequency pulse output is a Q-switched slave laser injection-seeded by a single-longitudinal-mode (SLM) continuous-wave (CW) master laser. In 2013, Y. Zheng et al. demonstrated a 1645 nm Er:YAG nonplanar ring oscillator (NPRO) resonantly pumped by a 1470 nm laser diode (LD), which can be used as a master laser . In 2014, R. Wang et al. presented a resonantly pumped 1645 nm injection-seeded single frequency Er:YAG laser, with a single-frequency pulse energy of 4.75 mJ and a pulse width of 336 ns at a PRF of 200 Hz . In 2014, Y. Deng et al. presented a single-frequency, Q-switched Er:YAG laser dual-end-pumped by two 1532 nm LDs . The maximum output pulse energy of 3.5 mJ was obtained with a pulse width of 195 ns at a PRF of 100 Hz. In 2015, B. Q. Yao et al. reported a double LDs end pumped single-frequency, injection-seeded Er:YAG laser based on a bow-tie ring slave laser with a NPRO laser as a seed laser . The single-frequency pulse energy was 2.9 mJ, corresponding to a pulse duration of 160 ns at a PRF of 100 Hz. In 2018, C. Q. Gao et al. demonstrated an injection-seeded Q-switched Er:YAG laser pumped by a 1470 nm LD . The pulse energy of 10.1 mJ was obtained with a pulse duration of 205 ns at a PRF of 200 Hz.
With the technical improvement, many groups devote to obtain high power/pulse energy lasers required for lidars and other applications. In 2005, D. Garbuzova et al. studied the resonant diode pumping of a 1.6 μm Er3+ doped solid-state laser . In the same year, they obtained 0.93 J pulse energy at a repetition rate of 1 Hz with resonantly pumping of a 1.6 μm Er3+ doped bulk solid-state laser . In 2006, D. Y. Shen et al presented a high power Er:YAG laser that was in-band pumped by a high power fiber laser operating at 1532 nm . The Er:YAG laser produced 60.3 W of CW output at 1645.3 nm for the incident pump power of 82 W. In the Q-switched mode operation, a slightly modified resonator configuration incorporating an electro-optic Q-switch produced pulses of ~4 mJ energy and ~100 ns duration at a repetition rate of 1 kHz for an incident pump power of 16.8 W. In 2008, I. S. Moskalev et al. demonstrated a high power, highly efficient single frequency oscillation of Er:YAG fiber-bulk hybrid laser at 1645 nm in actively and passively Q-switched operation modes . The slope efficiencies in the actively and passively Q-switched operation reached 75% and 20%, respectively, with the output powers in the narrow-linewidth and single longitudinal mode regimes of operation. In 2014, M. J. Wang et al. reported a polarized, narrow-linewidth Er:YAG laser operating at 1645 nm, in-band pumped by a 1532 nm fiber-coupled LD . A maximum polarized continuous wave output power of 8.45 W was obtained, with a full width at half-maximum of 0.13 nm. For Q-switched operation, pulse energy of 12 mJ at 100 Hz pulse repetition frequency and 95 ns pulse duration was yielded. In 2015, P. H. Tang et al. reported a stable and wavelength-locked Q-switched narrow-linewidth Er:YAG laser with compact cavity structure, utilizing a volume Bragg grating (VBG) as a wavelength selector and a pump input mirror simultaneously .
To relax temperature control requirements and reduce the upper level effects, some groups attempt to design double-crystals-end-pumping architecture. In 2014, a diode-pumped Er:YAG laser at 1617 nm was demonstrated by Z. Z. Yu et al . In Q-switched operation, the maximum pulse energy of 7.82 mJ was generated with the pulse duration of about 80 ns at a PRF of 500 Hz. In 2016, they achieved higher energy linearly polarized operation of an Er:YAG laser at 1617 nm, resonantly pumped by 1470 nm LDs . Polarized output with pulse energy of 20.5 mJ and pulse width of 52 ns at a repetition rate of 50 Hz was obtained. However, single-frequency high energy Q-switched Er:YAG laser in a double-crystals-end-pumping architecture hasn’t been reported yet.
In this paper, we demonstrate a high energy single-frequency Q-switched injection-seeded Er:YAG laser. The single-longitudinal-mode CW seed laser is a 1.6 μm Er:YAG NPRO pumped by a 1470 nm LD. The 1645 nm single-frequency Q-switched Er:YAG laser with a maximum output energy of 20.3 mJ is achieved, corresponding to the pulse width of 110 ns at a PRF of 200 Hz. The M2 factors of the Q-switched injection-seeded laser are measured to be 1.27 and 1.30 in x and y directions, respectively. The full width at half maximum (FWHM) of the symmetric spectrum is 4.59 MHz, which is 1.14 times transform limited. To the best of our knowledge, this is the highest energy obtained from a single-frequency, injection-seeded Er:YAG laser and this is also the first time to realize the single-frequency operation in a double-crystals-end-pumping architecture.
2. Experimental setup
The schematic diagram of the experimental setup of the injection-seeded single-frequency Q-switched Er:YAG laser is shown in Fig. 1. The whole system is mainly composed of three parts: a CW single-longitudinal-mode master oscillator (seed laser), a Q-switched slave laser, and a heterodyne detection system.
2.1 Seed laser
The seed laser is a stable CW single-longitudinal-mode Er:YAG NPRO resonantly pumped by a 1470 nm LD. The front end face of the monolithic NPRO is coated with high-transmission coating at 1470 nm, and 18.7% output coupling coating of the s-polarized beam at 1645 nm. The collimated pump beam is focused into the NPRO by a convex lens L1 with a focal length of 100 mm. The seed laser could provide about 500 mW single-frequency CW output power at 1645.278 nm, with a root mean square (RMS) of the wavelength 0.614 pm in 30 minutes. An isolator (ISO) is used to prevent the feedback damage from the Q-switched slave laser and protect the pumping source. A polarizing beam splitter (PBS) separates the seed laser into two parts: one part is for injection seeding, and the other part is used as a reference beam of the heterodyne beating system. A half-wave plate (HWP1) is utilized to adjust the intensity of the reference seed laser and the injection seed laser. The polarization of the injection part of the seed laser could be changed by rotating HWP2. A convex lens L2 with a focal length of 750 mm is placed in front of the HWP2 to focus the injection seed beam to ensure successful injection seeding operation. HWP3 is inserted to adjust the polarization of the reference part of the seed laser for the heterodyne detection.
2.2 Pump sources
Generally, Er:YAG can be pumped by using laser diodes or Er-fiber lasers. The latter has a distinctive advantage since its bandwidth is much narrower than absorption features of Erbium at room temperature, which could improve the output efficiency of the laser. For high power scaling, more powerful and efficient pump sources are required. In the last few years various groups began to use laser diodes as primary pump sources for Er:YAG. Due to narrow absorption range for Er:YAG at the wavelength around 1532 nm, volumetric Bragg gratings (VBGs) are attached to individual diode arrays to accomplish high performance spectral narrowing and stabilization. However, there was a penalty for narrowing linewidth that the output power would be dropped . Besides, the beam quality of 1532 nm LD is worse than 1532 nm fiber laser, which would influence the overall efficiency. Compared to 1532 nm LDs, the absorption range near the 1470 nm is wider, which is more suitable for using 1470 nm LDs as pump sources. Furthermore, 1470 nm LDs can achieve bigger output power. Therefore, a 1532 nm fiber laser and a 1470 nm LD are utilized as the pump sources to increase the output power of our laser system.
2.3 Slave oscillator and heterodyne detection system
The slave oscillator includes two Er:YAG rods (Er:YAG-1 and Er:YAG-2) as the gain medium to improve the efficiency. Both rods have a low doping concentration of 0.25 at. % to attenuate the loss derived from the energy transfer upconversion (ETU). Both end faces of the two rods are polished and coated with high-transmission coating at pumping wavelengths and lasing wavelength. Both Er:YAG rods are wrapped in indium foil and mounted in the copper heat sinks. A thermal electric cooler (TEC) is utilized to maintain the temperature at 18 °C. Er:YAG-1 (ϕ4 mm × 60 mm) is resonantly pumped by a narrow linewidth 1532 nm fiber laser (the spectral width is about 0.2 nm) with a maximum output power of 21.7 W. The fiber core diameter is 200 μm and the numerical aperture is 0.22. The pump beam from the fiber laser is focused into Er:YAG-1 by lenses L3 and L4, whose focal lengths are 400 mm and −100 mm, respectively. The pump beam size in the Er:YAG-1 is about 820 μm. The pump source of Er:YAG-2 is a fiber-coupled 1470 nm LD with a maximum output power of 46 W and a spectral width of about 7 nm. The fiber core diameter of the LD is 200 μm and the numerical aperture is 0.22. Compared to the fiber laser, the LD has shorter Rayleigh length due to its poor beam quality. Thus, Er:YAG-2 has a diameter of 4 mm but a shorter length of 50 mm. The pump beam from the LD is focused into Er:YAG-2 by lenses L5 and L6, whose focal lengths are 30 mm and 120 mm, respectively. The pump beam size in the Er:YAG-2 is about 900 μm.
The pulse width of a laser is influenced by its cavity length. Long laser pulse width contributes to low peak power, which can alleviate the damage to the optical components when high pulse energy is obtained. The total length of the cavity is 1250 mm. Since the fiber laser is sensitive to the feedback from the Q-switched slave laser, the slave oscillator is designed to a standing wave cavity to prevent the feedback damage. M1, M2, and M3 are dichroic plane mirrors which are coated with high reflectivity at 1645 nm and high transmission at 1532 nm. M4 is the end mirror with a radius of curvature of 600 mm, coated for high reflectivity at 1645 nm and high-transmission at 1532 nm. M5 and M6 are dichroic plane mirrors which are coated with high reflectivity at 1645 nm and high transmission at 1470 nm. The output mirror M7 has a radius of curvature of 800 mm with a transmittance of 30% at 1645 nm. The beam sizes of the laser mode on Er:YAG-1 and Er:YAG-2 calculated with the ABCD method are comparable with the mode of the pump beam in the center of Er:YAG rods. An acoustic-optic modulator (AOM) with a 50-MHz frequency shift is inserted into the slave cavity to achieve the Q-switched operation. To obtain stable single-frequency Q-switched pulses, an active programmable logic control system is designed based on the ‘Ramp–Fire’ injection-seeding technique [20,21]. A piezoelectric transducer (PZT) is mounted upon M3 to change the cavity length of the slave oscillator. The seed laser is injected into the slave oscillator by a plane mirror M8 with high reflectivity at 1645 nm via the first-order diffraction of the AOM. The active control system drives the PZT by an amplified digitized voltage ramp to sweep the resonance signal, which is detected by a photodiode (PD1). A locking signal is given to the AOM driver to fire the injection-seeded single-frequency Q-switched pulse when PD1 detects the peak resonance signal, which means the seed laser is resonant with the slave laser. The output pulse width is measured with another photodiode (PD2) and recorded by a digital oscilloscope (Tektronix, TDS5025B). The beating signal of the seed laser and slave pulsed laser is mixed into a high-speed photodiode (PD3) through an uncoated plane mirror M9 and recorded by the digital oscilloscope.
3. Results and discussion
Since 1532 nm fiber laser has better wavelength stability than 1470 nm LD, the pump power of the 1470 nm LD is set at the maximum power 46 W and the 1645 nm output power is measured when the pump power of the 1532 nm fiber laser is gradually increased to the maximum of 21.7 W. This operation can avoid the effect of the wavelength shift of the LD caused by current increasing. At CW operation of the slave laser, the maximum output of 9.2 W is obtained at the total incident pump power of 67.7 W, with corresponding slope efficiency and optical-to-optical efficiency of 43.6% and 13.6%, respectively.
For lidar applications, high repetition rate is required to improve the scanning speed. Increasing the PRF of the transmitter is an effective way to improve the detection capability of the lidar system. To meet different application requirements, laser output performances at different repetition rates are studied. Figure 2 describes the Q-switched operation without injection-seeding. The maximum output energy of 14.4 mJ, 19.7 mJ, 22.3 mJ are achieved at pulse repetition rates of 500 Hz, 300 Hz and 200 Hz, respectively. The corresponding pulse widths are 168 ns, 127 ns, 109 ns from 500 Hz to 200 Hz.
In the injection-seeded mode, the output pulse energy and pulse width of injection-seeded single-frequency pulses as a function of the incident pump power at different pulse repetition rates are presented in Fig. 3. The power of the master laser coupled in the slave cavity are about 250 mW at pulse repetition rate of 500 Hz to detect the resonance signal. A lower power with 50 mW can seed the slave laser at pulse repetition rate of 200 Hz. The maximum output energy of single-frequency pulses achieve 12.84 mJ, 16.87 mJ, 20.3 mJ at pulse repetition rates of 500 Hz, 300 Hz and 200 Hz, respectively. Correspondingly, the pulse widths are 162 ns, 125 ns, 110 ns, respectively.
At the highest pump power of 67.7 W, the single output pulse temporal profiles of the Er:YAG laser without and with injection-seeding are presented in Figs. 4(a) and 4(b), respectively. After the seed injection, the pulse waveform became smoother.
The build-up time of the laser at PRF of 200 Hz as a function of the incident pump power is shown in Fig. 5. The build-up time of the laser in the injection-seeded mode is shorter than the laser without injection-seeding. This phenomenon is mainly caused by that the pulse of the single-frequency Q-switched Er:YAG laser is built up from the injected seed beam rather than the spontaneous radiation. The density of inversion population is low at the low pump power, thus the time of pulse establishment is long. When the incident pump power increases, the time of pulse establishment is shortened gradually owing to stronger spontaneous radiation and stimulated emission.
Figure 6(a) depicts the beating signal waveform of the single-frequency pulse recorded from the digital oscilloscope with the output energy of 20.3 mJ at a PRF of 200 Hz. The Fast Fourier Transform (FFT) spectrum of the heterodyne signal is shown in Fig. 6(b). The center frequency of the FFT spectrum is 30.45 MHz and the FWHM is 4.59 MHz, which is 1.14 times transform limited. Generally, when the CW single-longitudinal-mode seed laser is injected into the slave laser from the first diffraction order of the AOM, the frequency of the injected laser should be shifted by about 50 MHz, close to the radio frequency of the AOM. However, due to the double-crystals-end-pumping architecture adopted in this experiment, the coupling operation of the injected seed laser and the slave laser is more difficult than normal cavity. Coupling between modes can be computed by integrating the amplitude over a plane perpendicular to the direction of propagation . Coupling coefficients, Cc, are defined as:Eq. (1) are the electric fields of the seed laser and the slave laser, respectively. The laser field signal is influenced by many factors, such as differences of spot radii, longitudinal waist positions, transverse waist positions and coupling angles. The differences of spot radii, longitudinal and transverse waist positions can be improved by adjusting the coupling lenses in the experiment. We assume beams of the seed laser and the slave laser are both in Gaussian distributions, with same beam radii and waist positions, both transversely and longitudinally. Equation (1) can be simplified as:
At the injection-seeded Q-switched laser operating at the highest output level of 20.3 mJ, the fluctuation of the output energy is measured in a duration of 30 minutes and the results are shown in Fig. 7, from which we could see that the mean is 20.32 mJ and the standard deviation of the single-frequency output energy is 0.124 mJ.
Figure 8 shows the measured beam diameter at different positions along the beam propagation by fitting the data with a hyperbolic curve. The beam quality of the single-frequency pulse with the output energy of 20.3 mJ is measured by using a Pyrocam III camera (Spiricon Inc.) with the 4Σ technique. The M2 factors are 1.27 in x direction and 1.30 in y direction, respectively.
In summary, a high-energy single-frequency injection-seeded Q-switched Er:YAG laser with a double-crystals-end-pumping architecture is demonstrated. A single-frequency Er:YAG NPRO oscillating at 1645.278 nm is used as the seed laser. The output energy of single-frequency pulses are 12.84 mJ, 16.87 mJ, 20.3 mJ at pulse repetition rates of 500 Hz, 300 Hz and 200 Hz, respectively. Correspondingly, the pulse widths are 162 ns, 125 ns, 110 ns, respectively. The measured pulse spectrum of the single-frequency Q-switched Er:YAG laser at 200 Hz is 4.59 MHz by using the heterodyne technique, which is 1.14 times transform limited. The fluctuation at the highest output level at the PRF of 200Hz is measured in a duration of 30 minutes and the standard deviation is 0.124 mJ. The M2 factors of the injection seeded laser are 1.27 and 1.30 in x and y directions, respectively. To the best of our knowledge, this is the highest energy achieved from a single-frequency, injection-seeded Er:YAG laser and this is also the first time to realize the single-frequency operation with a two-crystals-end-pumping architecture. Compared with a master oscillator and power amplifier (MOPA) system, such a scheme can acquire high energy single-frequency Er:YAG laser pulses with improved optical efficiency and avoid the complex system design.
National Key Research and Development Program of China (2017YFB0405203); National Natural Science Foundation of China (NSFC) (61627821).
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