A 2090 nm injection-seeded Q-switched Ho:YAG ceramic laser pumped by a 1.91 μm fiber-coupled LD is demonstrated in this paper. Single-frequency operation of Q-switched Ho:YAG ceramic laser is achieved by injection seeding technique. The maximum output energy of the single-frequency Q-switched Ho:YAG ceramic laser is 14.76 mJ, with a pulse width of 121.6 ns and a repetition rate of 200 Hz. The half-width of the pulse spectrum measured by heterodyne technique is 3.84 MHz. The fluctuation of the center frequency of the pulsed laser is 1.49 MHz (RMS) in 1 hour. As far as we know, this is the first time to report a single-frequency, injection-seeded Ho:YAG ceramic pulsed laser.
© 2016 Optical Society of America
Single-frequency solid-state lasers emitting in 2 μm region are useful for a variety of scientific and technical applications, such as Doppler lidars, differential absorption lidars and coherent imaging lidars and so on. In a lidar system, a stable pulsed laser with high energy and narrow linewidth is required. In order to meet these requirements, a common method is injecting a stable single-frequency seed laser into a high-energy Q-switched slave laser [1–3]. ‘Ramp-hold-fire’ (RHF) technique is an effective method to obtain the stable injection-seeded single-frequency Q-switched pulses. In 2009, Bai et al. described a high repetition rate operation of an injection-seeded Ho:YLF laser . In 2010, Gao et al. reported a single-frequency Q-switched Tm:YAG laser injection-seeded by a monolithic nonplanar ring oscillator (NPRO) . In 2013, T. Y. Dai et al. reported a Tm:YLF laser pumped single-frequency Q-switched injection-seeded Ho: YAlO3 laser .
In the last decade, resonantly pumped Ho:YAG laser has attracted much attention due to its low quantum defect, elimination of energy transfer and high efficiency. Ho:YAG crystals have been widely used to obtain continuous-wave (CW) and Q-switched operation in 2μm wavelength. In 2003, a 50 mJ Q-switched 2.09 μm holmium laser resonantly pumped by a diode-pumped 1.9 μm solid-state thulium laser was reported by P. A. Budni et al. . In 2012, single-frequency, Q-switched Ho:YAG laser injection-seeded by a Tm,Ho:YAG seed laser was demonstrated by T.Y.Dai et al. . In summary, these Ho systems are pumped by diode-pumped Tm-doped bulk or fiber lasers, leading to bulky setups and poor overall efficiencies . The 1.91 μm laser diodes (LD) have achieved rapid development over these years and become ideal direct pump sources of Ho:YAG lasers for their enhanced pump absorption, high conversion efficiency, relaxed temperature control requirements, and reduced system cooling complexity. However, there are few reports on Ho-doped lasers directly pumped by 1.9 μm LD.
Recently, more and more scientists are interested in Ho:YAG ceramics, whose thermos-mechanical properties are as good as the Ho:YAG single crystals. Moreover, Ho:YAG ceramics have many advantages compared with Ho:YAG single crystals, such as ease of fabrication, short fabrication time, low cost, mass production, and feasibility of large size . Holmium doped YAG transparent ceramic lasers at 2 μm region have been studied in recent years. In 2014, Lei Wang et al. reported a Q-switched Ho:YAG ceramic laser with an output energy of 9.6 mJ at a repetition rate of 200 Hz . In 2016, Yan Li et al. reported a Q-switched Ho:YAG ceramic MOPA system with output energy of 34 mJ and pulse width of 54 ns at 200 Hz . However, high-energy single-frequency Ho:YAG ceramic laser hasn’t been reported yet.
In this paper, we demonstrate a single-frequency Q-switched injection-seeded Ho:YAG ceramic laser directly pumped by a 1.91 μm LD. A 2.09 μm single-frequency Ho:YAG NPRO with the output power of 140 mW is used as the seed laser. To obtain the stable injection-seeded single-frequency Q-switched pulse, the ‘ramp-hold-fire’ injection seeding technique is adopted . A 2.09 μm single-frequency Q-switched Ho:YAG ceramic laser with a maximum output energy of 14.76 mJ and a corresponding pulse width of 121.6 ns at a pulse repetition rate of 200 Hz is obtained. The M2 factors of the Q-switched injection-seeded laser are measured to be 1.16 and 1.15 in x and y directions, respectively. The half-width of the symmetric spectrum is determined to be about 3.84 MHz by the heterodyne beating measurement. As far as we know, this is the first time to achieve single-frequency Q-switched Ho:YAG ceramic laser.
2. Experimental setup
The experimental setup of the injection-seeded Ho:YAG ceramic laser is demonstrated in Fig. 1.The injection-seeded Q-switched Ho: YAG ceramic laser consists of four parts: the CW seed laser, the Q-switched slave laser, the electronic controlling system for the injection seeding, and the heterodyne beating system.
The stable single-frequency Ho:YAG NPRO is used as the seed laser, which provides up to 140 mW single-longitudinal-mode output at 2090.3 nm. The line width of the seed laser is measured by beating two similar seed lasers, and the result shows that the line width is less than 9.5 kHz. The seed laser is separated into two parts by a fiber beam splitter: one is for injection seeding (85 mW), and the other is set as the reference signal of the heterodyne beating system. Both of them are collimated by fiber port collimating lenses (CFC-5X-C, THORLABS).
For coherent Doppler lidar applications, the laser transmitter should have a relatively long pulse for a Fourier-transform-limited linewidth on the order of a few megahertz to enhance the velocity accuracy . Moreover, to avoid the damage to the optical components, long cavity should be adopted for its relatively long pulse width and low peak power of the laser pulse. Thus, the laser is configured as a folded cavity with a total resonator length of 2.2 meters.
The Q-switched Ho:YAG ceramic slave oscillator contains three curved mirrors (M1,M3, M5) and two flat mirrors (M2,M4). M1 is a plano-concave mirror with a radius of curvature of 750 mm. It is coated for high transmission at 1.91 μm and high reflectivity at 2.09 μm. M2 and M4 are flat mirrors coated for high transmission at 1.91 μm and high reflectivity at 2.09 μm. M3 is a plano-concave mirror with a coating of high transmission at 1.91 μm and high reflectivity at the 2.09 μm and its radius of curvature is 1000 mm. There is a piezoelectric actuator (PZT) mounted upon M3. The output coupler, M5 with a radius of curvature of 750mm is coated for anti-reflection at 1.91 μm and 40% transmission at 2.09 μm. The physical lengths of arm M1-M2, M2-M3, M3-M4 and M4-M5 are 260 mm, 790 mm, 500 mm and 740 mm, respectively. A 40-mm-long Brewster-cut acousto-optic Q-switch (Gooch & Housego Ltd.) with a radio frequency of 40.68 MHz is inserted into the slave resonator to achieve Q-switched operation.
The pump source is a fiber-coupled 1.91 μm LD (BrightLock Ultra-500, QPC Lasers) with maximum output power of 31.3 W. The full width at half-maximum (FWHM) of the spectral is less than 3 nm. The fiber core diameter of the 1.91 um LD is 600 μm and the fiber core numerical aperture is 0.22. The maximum absorption cross section is at the wavelength of 1907.8 nm . Thus, the center of the emission wavelength of the 1.91 μm LD is tuned to 1907.5 nm for optimal absorption by limiting its temperature to 29.5 °C. F1 is an aspherical mirror with an equivalent focal length of 30 mm and it is coated for high transmission at 1908 nm. It is employed to collimate the pump beam. F2, which is used to focus the collimated pump beam, is a plano-convex mirror with a coating of high transmission at 1.91 μm and an equivalent focal length of 50 mm. M6, a 45° dichroic mirror, has an anti-reflection coating at 1908 nm and high reflection coating at 2090 nm. It is employed to avoid the optical feedback caused by the Q-switched pulse. M7 is a 45° flat mirror with a coating of high reflectivity at 1.91 μm, which is employed to fold the collimated pump beam.
Two kinds of Ho:YAG ceramics are employed as the gain medium in our experiments: one is a 1.0 at. % Ho:YAG ceramic with a length of 20 mm, the other is a 0.8 at. % Ho:YAG ceramic with a length of 25 mm. The Ho:YAG ceramics are wrapped in indium foil, clamped in cooper heat sinks and maintained at 18 °C by a thermoelectric cooler (TEC). The gain medium is positioned at the waist of the 1.05 mm TEM00 mode diameter to achieve the mode matching between the pump beam and the resonant mode.
In order to make the slave laser resonantly match with the seed laser, some optical elements are used in the experiment. As shown in Fig. 1, half-wave plate 1 is used to adjust the polarization of the seed laser. The Faraday isolator is used to protect the seed laser from the feedback of the slave laser. The 85 mW collimated seed laser is injected into the slave laser from the first diffraction order of the acoustic-optic modulator (AOM) when the RF power of AOM is applied. The seed laser beam is spatially mode-matched to the slave laser by using two flat mirrors M8 and M10 (both highly reflecting at 2090 nm). We use the ‘ramp-hold-fire’ technique to obtain the stable injection-seeded single-frequency Q-switched pulses. An active controlling system is designed to make the seed laser and the slave laser oscillate at the same frequency . A digitized voltage ramp is amplified to drive the PZT. Detector1, a photodiode, is used to observe the seed laser wavelength resonance signal in the slave laser cavity during the move of the PZT. When the slave cavity is resonating with the frequency of the seed laser, a signal peak is distinguished by the controlling system. The ramp voltage goes back to compensate for the inertial motion of the PZT and then the system keeps the voltage constant, to make sure the length of the laser cavity maintained. Finally, a latch signal is given to the AOM driver to fire the Q-switched pulse. During this process, the length of the laser cavity is adjusted to be resonant at the seed laser frequency, and the slave laser cavity is flooded with seed laser photons. Then the single-frequency Q-switched pulse is obtained.
3. Results and discussion
Output energy and pulse width of the Q-switched Ho:YAG ceramic laser at different PRF without injection seeding are shown in Fig. 2. For the 1.0 at. % Ho:YAG ceramic, the maximum output energy is up to 17.85 mJ with a pulse width of 115.1 ns at 200 Hz. When 0.8 at. % Ho:YAG ceramic is used, the maximum output energy is 17.4 mJ at 200 Hz.
After careful calculation, the diameter of the resonant mode waist is calculated to be 1.05 mm. The diameter of the pump beam waist is estimated to be about 1 mm. Thus, when Ho:YAG ceramic is placed at the TEM00 mode waist of the resonator, spatial mode-matching is well realized. The Rayleigh length of the focused pump beam is calculated to be only about 6.7 mm. In consideration of short pump beam Rayleigh length and the absorption efficiency of the pump power, the shorter ceramic with higher Ho-doping concentration (20 mm, 1.0 at. % Ho:YAG ceramic) is more suitable as the gain medium. Although the 1.0 at. % Ho:YAG ceramic suffers more energy transfer up-conversion (ETU) loss and more heat generation than the 0.8 at. % Ho:YAG ceramic, the 1.0 at. % Ho:YAG ceramic with a length of 20 mm performs better in Q-switched mode.
The output energy and pulse width of injection-seeded single-frequency laser are shown in Fig. 3(a). Single-frequency Q-switched pulse with the maximum output energy of 14.76 mJ and a pulse width of 121.6 ns is obtained. The single-frequency output energy is recorded in the duration of 1 hour and the standard deviation of the single-frequency output energy is 0.21 mJ. The pulse repetition rate of the laser is 200 Hz.
The build-up time is also studied as shown in Fig. 3(b). With the same pump power, the build-up time of the injection-seeded laser is shorter. It is because the pulse is built up from the seed laser rather than the spontaneous radiation. The difference between the build-up time with and without injection-seeding decreases when the pump power increases.
The beam quality of the single-frequency pulse with the output energy of 14.76 mJ is measured by using an infrared PY-III camera. Figure 4 shows the measured beam diameter at different positions along the beam propagation. By fitting the measured data with a hyperbolic curve, the M2 factors are calculated to be 1.16 and 1.15 in x and y directions, respectively.
The pulse spectrum of the single-frequency Q-switched Ho:YAG ceramic laser is measured with the heterodyne technique. The single-frequency Q-switched laser beam and the seed laser are mixed. As shown in Fig. 1, a fraction of the seed laser beam (about 55 mW) and a fraction of the Q-switched laser beam are used for the heterodyne beating measurement. The laser beams are mixed by M11, which is a quartz mirror without coating and then focused into an InGaAs PIN photodiode. The mixed signal is recorded by a digital oscilloscope (Tektronix, TDS5052B). When the pump power is 31.3 W, the typical heterodyne beating signal is recorded and digitized as shown in Fig. 5(a). An analysis of the beating waveform by a fast-Fourier transform technique provides the spectral intensity profile, as shown in Fig. 5(b).
The center frequency of 48.16 MHz shows the difference between the frequencies of the seed laser and the Q-switched laser. The FWHM of the symmetric spectrum is about 3.84 MHz. Therefore the single-frequency Q-switched Ho:YAG ceramic laser is nearly transform-limited with the simultaneously recorded pulse width of 121.6 ns. The fluctuation of the center frequency of the laser pulse with injection-seeding is measured by a Labview software written by ourselves. The root mean square (RMS) is 1.49 MHz.
In summary, we report a single-frequency Q-switched Ho:YAG ceramic laser pumped by a 1.91 μm fiber-coupled LD. A fiber-coupled Ho:YAG NPRO operating in single frequency at 2090.2912 nm is used as the seed laser. The output power of the seed laser is about 140 mW. The maximum output energy of the single-frequency Ho:YAG ceramic is 14.76 mJ at 200 Hz and a corresponding pulse width is 121.6 ns. The M2 factors of the injection seeded laser are measured to be 1.16 and 1.15 in x and y directions, respectively. By using the heterodyne technique, the pulse spectrum is measured to be 3.84 MHz, which is 1.05 times transform limited and the fluctuation of the center frequency of the pulses is 1.49 MHz (RMS) in 1 hour. To the best of our knowledge, this is the first time that we have achieved a single-frequency, injection-seeded Ho:YAG ceramic pulsed laser. The experimental results show Ho:YAG ceramic is a promising gain medium for achieving 2 μm stable high-energy narrow-linewidth single-frequency laser pulses. This stable high-energy single-frequency Ho:YAG ceramic laser operating at 2090 nm is a potential candidate as an optical source for differential absorption lidar (DIAL) and Doppler wind lidar.
National Natural Science Foundation of China (NSFC) (61378021, 61627821).
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