1520 and 1560 nm acousto-optic Q-switched pulse lasers with high peak power and narrow width were respectively realized in Er:Yb:RAl3(BO3)4 (R = Y and Lu) crystals end-pumped by a 970 nm diode laser. For Er:Yb:LuAl3(BO3)4 crystal, 1520 nm laser with 350 μJ energy, 32 ns width and 10.9 kW peak power, and 1560 nm laser with 520 μJ energy, 67 ns width and 7.8 kW peak power were respectively obtained at pulse repetition frequency of 1 kHz. For Er:Yb:YAl3(BO3)4 crystal, 1520 nm laser with 210 μJ energy, 45 ns width and 4.7 kW peak power, and 1560 nm laser with 380 μJ energy, 102 ns width and 3.7 kW peak power were respectively obtained at pulse repetition frequency of 1 kHz. Pulse performances of 1520 and 1560 nm lasers were compared and the narrower pulse width of 1520 nm laser was ascribed to the higher stimulated emission cross-section.
© 2013 OSA
High-peak-power and short-duration eye-safe 1.5-1.6 μm solid-state Q-switched pulse lasers with high repetition frequency are required for some applications, such as range finding, lidar, and telecommunications [1–3]. At present, based on Er3+ and Yb3+ codoped materials, 1.5-1.6 μm pulse lasers have been realized by Q-switched techniques [1–5]. Compared with passively Q-switched pulse laser, actively Q-switched one has smaller interpulse time jittering and more stable pulse repetition frequency . Therefore, actively Q-switched pulse laser may be more attractive for some applications.
Different methods of the actively Q-switching by using the acousto-optic modulator , electro-optic modulator  and frustrated total internal reflection (FTIR) shutter  have been adopted in the pulse laser operation of Er3+ and Yb3+ codoped phosphate glass. However, due to the limitation of low thermal conductivity of glass host, high-peak-power 1.5-1.6 μm actively Q-switched pulse laser with high repetition frequency is difficult to be obtained in the Er3+ and Yb3+ codoped phosphate glass [3, 5]. Er3+ and Yb3+ codoped YAG and YVO4 crystals with high thermal conductivity have also been investigated as gain media of 1.5-1.6 μm laser [5, 8, 9]. However, strong reverse energy transfer from Er3+ to Yb3+ in combination with intense upconversion fluorescence of Er3+ in these crystals will reduce the population of upper laser level 4I13/2, and then make the operation efficiency and output power of 1.5-1.6 μm lasers low. Consequently, high-peak-power 1.5-1.6 μm pulse laser with high repetition frequency has not been obtained in the Er:Yb:YAG and Er:Yb:YVO4 crystals till now [5, 8].
Er:Yb:RAl3(BO3)4 (Er:Yb:RAB, R = Y and Lu) crystals have been demonstrated as excellent gain media of 1.5-1.6 μm laser due to their good thermal conductivity and efficient laser operation efficiency (up to 35% for continuous-wave laser) [10–12]. Passively mode-locked and Q-switched 1.5-1.6 μm pulse laser operations have also been realized in the crystals [13–15]. By using an acousto-optic modulator, 1560 nm actively Q-switched laser with peak power of about 5 kW and pulse width of 110 ns at pulse repetition frequency (PRF) of 1 kHz in an Er:Yb:LuAB crystal has been reported . In this work, 1520 nm acousto-optic Q-switched laser with higher peak power and narrower pulse width was realized in the crystals by using a specially designed output mirror. Furthermore, pulse performances of 1520 and 1560 nm lasers were compared under similar experimental conditions.
2. Laser experimental arrangement
End-pumped linear resonator was adopted in the experiment and the schematic diagram of experimental setup is depicted in Fig. 1. An Er(1.1 at.%):Yb(25 at.%):YAB and an Er(1.1 at.%):Yb(24.1 at.%):LuAB crystal chips were used as gain media, respectively. Both the polished chips are c-cut and about 0.7 mm thick. A 970 nm fiber-coupled diode laser (800 μm diameter core) from Coherent Inc. was used as the pump source. After passing a simple telescopic lens system (TLS), the pump beam was focused to a spot with waist radius of about 220 µm in the chip. The uncoated crystal chip was attached on an aluminum slab with heat-conducting adhesive and there is a hole in the center of the slab to permit the passing of the pump and fundamental laser beams. For making the laser more compact, no other device was used to control the temperature of the chip, which is only cooled by the natural air. In order to reduce the influence of pump-induced thermal load on laser performance and avoid the fracture of the chip at high pump power, the diode laser was operated in quasi-continuous-wave (quasi-cw) mode. Pump pulse width was 2 ms and pulse period was 100 ms. About 80% of incident pump power was absorbed by each chip. Laser resonator was consisted of a flat input mirror (IM) and a curved output mirror (OM) with a radius curvature of 100 mm. The resonator length was fixed at about 45 mm.
In order to realize actively Q-switched operation, an acousto-optic modulator (AOM) (Gooch & Housego Co.) antireflection coated at 1.5-1.6 μm and driven at 80 MHz center frequency with radio-frequency power of 10 W was inserted between the chip and OM. Duty cycle of the AOM was fixed at 15% and PRF can be tuned continuously. By the good alignment of the AOM, lasing can be effectively prohibited when the radio-frequency signal is supplied and Q-switching takes place when the radio-frequency signal is switched off. Pulse profile was measured by a 2 GHz InGaAs photodiode connected to a digital oscilloscope with bandwidths of 1 GHz (DSO6102A, Agilent).
3. Results and discussion
Because the gain spectra of Er:Yb:RAB crystals are broad and have some discrete gain peaks [10, 11], laser oscillations at different wavelengths can be realized simultaneously in the case of high cavity gain when the cavity mirrors with flat transmission curves in the range of 1.5-1.6 μm were used [11, 17]. Multi-wavelength oscillation accompanied by the generation of multi-pulse in Q-switched Er:Yb:RAB lasers will seriously debase the performances of output lasers and limit their applications . In this work, in order to realize the laser operation at single wavelength of 1520 nm and suppress the oscillations at other gain peak wavelengths in the Er:Yb:RAB crystals, transmission curve of OM was specially designed in the range of 1.5-1.6 μm . Transmittance of OM at 1520 nm was about 2.6% and those at the other gain peak wavelengths of 1550, 1580 and 1600 nm were 3.8%, 5.0% and 6.5%, respectively. For IM, transmittance at the pump wavelength of 970 nm was about 90% and reflectance at 1.5-1.6 μm was higher than 99.8%.
By using the above cavity mirrors, actively Q-switched pulse laser operation at single wavelength of 1520 nm was realized in the Er:Yb:RAB crystals. As an example, output spectrum of the acousto-optic Q-switched Er:Yb:LuAB laser at PRF of 1 kHz and absorbed pump power of 13.7 W is shown in Fig. 2, which was recorded with a monochromator (Triax550, Jobin-Yvon) associated with a TE-cooled Ge detector (DSS-G025T, Jobin-Yvon). To our knowledge, 1520 nm is the shortest laser oscillating wavelength realized in Er3+ doped crystalline materials at present. Er3+ doped fiber (EDF) has received a great deal of interest as 1.5-1.6 μm laser medium and broad lasing tunable range from 1450 to 1548 nm has been demonstrated in EDF . However, actively Q-switching technique of EDF laser is complex and some deleterious effects , such as fiber nonlinearity and gain saturation, etc., will degrade the output performances of pulse laser. Therefore, crystalline laser may be more suitable for generating Q-switched pulses. For Er3+ doped crystals, the shortest oscillating wavelength reported previously in actively Q-switched laser is 1560 nm in Er:Yb:RAB crystals . Furthermore, in Er3+ doped YAG and YVO4 crystals, the shortest oscillating wavelength realized in actively Q-switched lasers are 1617 and 1603 nm, respectively [5, 21]. Compared with these laser wavelengths, 1520 nm is closer to the sensitive peak wavelength of Ge and InGaAs photodiodes at room temperature . For example, sensitivities at 1560, 1603 and 1617 nm for lower-cost Ge photodiode are roughly estimated to be about 85%, 65% and 50% of that at 1520 nm, respectively . Therefore, 1520 nm actively-Q switched pulse laser may be more useful for some applications required high measuring accuracy. At absorbed pump power of 13.7 W, pulse energy of the Q-switched Er:Yb:RAB lasers versus PRF is also shown in Fig. 2. When PRF was 1 kHz, the maximum pulse energies of 350 and 210 μJ were obtained in Er:Yb:LuAB and Er:Yb:YAB crystals, respectively.
Figure 3 shows pulse width of the Q-switched Er:Yb:RAB lasers versus PRF at absorbed pump power of 13.7 W. When PRF decreased from 30 to 1 kHz, pulse width reduced from 88 to 32 ns and from 94 to 45 ns for Er:Yb:LuAB and Er:Yb:YAB crystals, respectively. The temporal pulse profiles of the Q-switched Er:Yb:RAB lasers at PRF of 1 kHz are also shown in Fig. 3. Peak power of the Q-switched Er:Yb:RAB pulse lasers versus PRF at absorbed pump power of 13.7 W is shown in Fig. 4. Maximum peak powers of 10.9 and 4.7 kW were obtained at PRF of 1 kHz in Er:Yb:LuAB and Er:Yb:YAB crystals, respectively. Spatial profiles of output beams of the Q-switched Er:Yb:RAB lasers at PRF of 1 kHz were recorded with a Pyrocam III camera (Ophir Optronics Ltd.), which are also shown in Fig. 4. The quality factors M2 of output beams of the Q-switched Er:Yb:LuAB and Er:Yb:YAB lasers were estimated to be about 2.4 and 2.1, respectively, when PRF was 1 kHz and absorbed pump power was 13.7 W.
In order to compare the pulse performances of 1520 and 1560 lasers under similar experimental conditions, the specially designed cavity with 2.5% OM transmittance at 1560 nm, which have been used in previous experiment , was also adopted in this work and the resonator length was also fixed at about 45 mm. Pulse energy and width of the Q-switched Er:Yb:RAB lasers at 1560 nm versus PRF at absorbed pump power of 13.7 W are shown in Figs. 5 and 6, respectively. When PRF was 1 kHz, 520 μJ energy with width of 67 ns and 380 μJ energy with width of 102 ns were obtained in Er:Yb:LuAB and Er:Yb:YAB crystals, respectively. Compared with those reported in previous investigation , the reduction of pulse energy and width in this work is caused by the shortening of the resonator length (the resonator length was 100 mm used in previous experiment .). Due to the narrowing of pulse width, the obtained 1560 nm maximum peak powers at PRF of 1 kHz increased to be about 7.8 and 3.7 kW in Er:Yb:LuAB and Er:Yb:YAB crystals, respectively, as shown in Fig. 7. The quality factors M2 of output beams of the Q-switched Er:Yb:LuAB and Er:Yb:YAB 1560 nm lasers were estimated to be about 2.1 and 1.9, respectively, when PRF was 1 kHz and absorbed pump power was 13.7 W.
It can be found from Figs. 3 and 6 that for both the crystals, pulse width of 1520 nm laser is narrower than that of 1560 nm. Theoretically, the minimum possible pulse width tw of a Q-switched laser can be estimated by [6, 23]:10, 11]. As an example, the stimulated emission cross-sections at 1520 and 1560 nm in Er:Yb:LuAB crystal for α-polarization are 1.85 × 10−20 and 0.65 × 10−20 cm2, respectively . Therefore, pulse width of 1520 nm Q-switched laser become narrower. However, due to the higher reabsorption loss caused by the larger absorption cross-section at 1520 nm [10, 11], the net gain at 1520 nm laser is lower than that at 1560 nm. Then, pulse energy obtained in 1520 nm Q-switched laser is lower than that in 1560 nm laser, which can be seen in Figs. 2 and 5 for both the crystals. Compared with those of Er:Yb:YAB crystal, narrower pulse width and higher energy obtained in Er:Yb:LuAB crystal are originated from its higher round-trip small-signal gain and crystal optical quality [11, 16]. Some parameters of diode-end-pumped actively Q-switched Er:Yb:LuAB, Er:Yb:phosphate glass, and Er:Yb:YVO4 pulse lasers at 1.5-1.6 μm are listed in Table 1. It can be seen that compared with those of Er:Yb:phosphate glass and Er:Yb:YVO4 crystal, a high-frequency 1520 nm pulse laser with narrower width and higher peak power can be realized in the Er:Yb:LuAB crystal. To our knowledge, 10.9 kW peak power is the highest value reported in a 970 nm diode-end-pumped actively Q-switched 1.5-1.6 μm laser till now. Pulse width of 38 ns has been obtained in the Er:Yb:glass laser with the cavity length of 20 mm . In our experiment, the cavity length was limited at about 45 mm by the volume of the metal housing of AOM. Therefore, if the used AOM is more compact in the future, a 1520 nm pulse laser with narrower width less than 32 ns and then higher peak power can be expected in the Er:Yb:LuAB crystal.
Compared with other Er3+ and Yb3+ codoped materials, higher pulse laser performances can be realized in Er:Yb:RAB crystals. However, owing to the nature of incongruent melting, RAB crystals can only be grown by the flux method which required a long growth period (about 1 month). The optical quality of the crystals was affected by the fluctuation of the crystal growth temperature and composition. Some defects, such as impurity, dislocation and twinning, were observed in the Er:Yb:RAB crystals. The variation of crystal optical quality causes difficulties in the accurate measurement of some performance parameters, such as energy transfer rate, upconversion coefficient, thermal conductivity, damage threshold, and thermal focal length, etc., which may result in the lacking of a laser energetic model that describes this crystal till now.
Acousto-optic Q-switched pulse lasers at single wavelength of 1520 nm were demonstrated for Er:Yb:YAB and Er:Yb:LuAB crystals in a 970 nm diode-end-pumped hemispherical cavity with a specially designed output mirror. When absorbed pump power was 13.7 W and PRF was 1 kHz, 1520 nm Q-switched pulse lasers with 350 μJ energy, 32 ns width, 10.9 kW peak power, and with 210 μJ energy, 45 ns width, 4.7 kW peak power were achieved in Er:Yb:LuAB and Er:Yb:YAB crystals, respectively. Compared with that of 1560 nm pulse laser obtained under similar experimental conditions, pulse width of 1520 nm laser become narrower, which may be caused by the larger stimulated emission cross-section. The experimental results shown that Er:Yb:RAB crystal is one kind of excellent gain media of actively Q-switched 1.5-1.6 μm pulse laser, and 1520 nm may be a more suitable laser wavelength for obtaining the pulse laser with higher peak power and narrower pulse width.
This work has been supported by the National Natural Science Foundation of China (grant 91122033), Chunmiao Project of Haixi Institute of Chinese Academy of Sciences (CMZX-2013-005), and the Knowledge Innovation Program of the Chinese Academy of Sciences (grant KJCX2-EW-H03-01).
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