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Abstract
Passively Q-switched Er:Yb:LuAl3(BO3)4 pulse microlasers were investigated at a low repetition frequency of 10–200 Hz. End-pumped by a 975.6 nm quasi-continuous-wave laser diode with pump pulse width of 0.5 ms and period of 10 ms, a stable 1522 nm pulse microlaser with single pulse energy of 48.3 μJ, duration of 1.9 ns, repetition frequency of 100 Hz, peak output power of 25.4 kW and beam quality factor less than 1.2 was realized at a pump beam waist diameter of 260 μm. This eye-safe passively Q-switched pulse microlaser with high peak output power and narrow duration can be used in the portable laser rangefinder.
© 2021 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Eye-safe 1.5–1.6 μm laser rangefinder has become an indispensable device widely used in many fields, such as telemetry, metrology and military [1,2]. The portable laser rangefinder requires a compact, cheap and reliable 1.5–1.6 μm solid-state pulse microlaser as a detection beam. For measuring a target in a distance of kilometer level accurately, the pulse laser should have a high peak output power of tens of kilowatts, narrow duration of a few nanoseconds, and excellent output beam quality. Benefited from the advantages of short cavity length and compact structure, passively Q-switched microlaser is favorable for this application [3]. At present, the commercial Er:Yb:phosphate glass has been used as the gain medium of the 1.5–1.6 μm passively Q-switched microlaser [1,2,4]. Passively Q-switched Er:Yb:glass microlaser with peak output power higher than 30 kW and duration less than 7 ns has been widely used as the detection beam of eye-safe laser rangefinder [5,6]. However, due to the serious thermal effect caused by the low thermal conductivity (about 0.8 Wm-1K-1) of the phosphate glass, the repetition frequency of the passively Q-switched Er:Yb:glass microlaser is generally limited to be 10 Hz [5,6]. High repetition frequency can realize a high scanning speed and increase the data amount of the received signals, which can improve the measuring accuracy of the rangefinder. Therefore, it is useful to explore a high peak power 1.5–1.6 μm microlaser with a higher repetition frequency.
Er:Yb:RAl3(BO3)4 (Er:Yb:RAB, R = Y, Gd and Lu) crystals have been demonstrated as excellent gain media for the 1.5–1.6 μm laser, due to the high thermal conductivity (about 6–7 Wm-1K-1) and Yb3+→Er3+ energy transfer efficiency (higher than 90%) [7–9]. A 1550 nm continuous-wave (cw) microlaser with output power of 2.05 W and slope efficiency of 39.8% has been realized in an Er:Yb:YAB crystal [10]. Using a Co:Mg0.4Al2.4O crystal as saturable absorber, a 1521 nm passively Q-switched pulse microlaser with pulse energy of 6.1 μJ, duration of 12.6 ns, repetition frequency of 69 kHz has been obtained in an Er:Yb:LuAB crystal [9]. Based on an optical contact bonded Co:MgAl2O/Er:Yb:GdAB component, a 1522 nm passively Q-switched pulse monolithic laser with pulse energy of 15 μJ, duration of 4 ns, repetition frequency of 21 kHz, and peak output power of 3.75 kW has been obtained under cw pumping [11]. When the bonded component was pumped by a quasi-cw laser diode (LD), the pulse energy increased to 27 μJ and duration decreased to 2.8 ns [11]. Therefore, the peak output power of the monolithic laser was close to 10 kW, which is the highest value reported in the passively Q-switched Er:Yb:RAB 1.5–1.6 μm lasers. However, the above pulse laser can only be obtained close to laser threshold because the higher incident power leads to the damage of the gain medium and the repetition frequency was not reported [11]. Recently, a 544 kHz high-repetition-frequency passively Q-switched Er:Yb:YAB microlaser with pulse duration of 8.3 ns and energy of 3.9 μJ has also been demonstrated [12]. However, the output peak power is only 0.47 kW. Therefore, up to now, the investigations on the Er:Yb:RAB crystals mainly focus on their pulse performance operated at high repetition frequency of tens to hundreds of kHz, and the obtained peak output power is lower than 10 kW. In this work, the passively Q-switched Er:Yb:LuAB pulse microlasers operated at a low repetition frequency of 10–200 Hz are explored. The effects of the pump beam waist diameter, pump pulse width and period of the quasi-cw LD on performance of the pulse microlaser are investigated in detail.
2. Experimental arrangement
Experimental setup of the passively Q-switched Er:Yb:LuAB pulse microlaser is depicted in Fig. 1. The pump source was a fiber-coupled LD from BWT Beijing Ltd. (105 μm core diameter and 0.22 numerical aperture) with central wavelength stabilized at 975.6 nm. The LD can be operated in quasi-cw mode with continuously adjustable pulse width and period. A c-cut, 1.57-mm-thick Er(1.5 at.%):Yb(12 at.%):LuAB crystal with a cross section of 3 × 3 mm2 was used as a gain medium. Figure 2(a) shows the room-temperature absorption coefficient spectrum of the c-cut crystal in 900–1050 nm. The peak absorption coefficient is 28.8 cm-1 at wavelength of 975 nm and the full width at half-maximum (FWHM) of this absorption band is 16 nm. By using a telescopic lens system (TLS) consisting of two convex lenses, the pump beam was focused into the crystal. Different waist diameters of the pump beam in the crystal can be obtained by changing the imaging ratio of the two convex lenses with different focal lengths. A 1.55-mm-thickness uncoated Co2+:MgAl2O4 crystal with a cross section of 3 × 3 mm2 was used as a saturable absorber. Figure 2(b) shows the transmission spectrum of the Co2+:MgAl2O4 crystal in 1400–1750 nm. The transmission of the Co2+:MgAl2O4 crystal at 1700-1750 nm, which is in the spectral range with the absence of Co2+ absorption, is measured to be about 84%. Then, the initial transmissions around 1600 and 1520 nm of the Co2+:MgAl2O4 crystal caused by the Co2+ absorption are estimated to be 96% and 89%, respectively. An input mirror (IM) film with 90% transmission around 975 nm and 99.8% reflectivity between 1.5–1.6 μm, as well as an output mirror (OM) film with 15% transmission between 1.5–1.6 μm were deposited onto the surfaces of two sapphire crystals with same cross section of 3 × 3 mm2, respectively. According to the optimization methodology of the passively Q-switched pulse lasers [13], the adoption of the saturable absorber with the lower initial transmission and OM with the higher transmission is more favorable for the generation of the high-energy pulse laser. At present, the above Co2+:MgAl2O4 saturable absorber has the lowest initial transmission and the OM has the highest transmission in our lab. All the crystals were optically contacted and then mounted in a Cu-chamber cooled by the nature air. The sapphire crystal with a high thermal conductivity of about 40 Wm-1K-1 can also be used as an efficient heat sink to reduce the thermal effect of the Er:Yb:LuAB crystal. There is a hole with diameter of about 1.5 mm in the center of the chamber for the passing of the laser beams. The cavity length was 3.12 mm. Incident pump power was measured by a PM100D power meter associated with a S314C thermal power head from Thorlabs Inc. Pulse energy was recorded by a Centauri energy meter associated with a PE9-C pyroelectric energy sensor from Ophir-Spiricon Inc. Pulse profile was measured by a 5 GHz InGaAs photodiode (DET08C, Thorlabs) connected to a digital oscilloscope with a bandwidth of 1 GHz (DSO6102A, Agilent). Laser spectrum was recorded by a monochromator (Triax 550, Jobin-Yvon) associated with a TE-cooled Ge detector. The spatial profile of the laser beam was recorded with a Pyrocam III camera from Ophir-Spiricon Inc.
Fig. 1. Experimental setup of the passively Q-switched Er:Yb:LuAB pulse microlaser end-pumped by a 975.6 nm quasi-cw LD
Fig. 2. (a) Room-temperature absorption spectrum in 900–1050 nm of a 1.57 mm-thickness c-cut Er:Yb:LuAB crystal. (b) Transmission spectrum in 1400–1750 nm of an uncoated 1.55 mm-thickness Co2+:MgAl2O4 crystal.
3. Results and discussion
Pulse performance of the passively Q-switched Er:Yb:LuAB microlaser was firstly investigated at a pump beam waist diameter of 200 μm. The LD was operated in quasi-cw mode with pump pulse period of 10 ms and pulse width of 0.3 ms, which is close to the fluorescence lifetime (about 0.31 ms [14]) of the 4I13/2 upper laser level of the Er:Yb:LuAB crystal. Figure 3(a) shows the dependence of single pulse energy of the microlaser on peak incident pump power. When the peak incident pump power is slightly higher than the laser oscillating threshold and between 19.8 and 22.1 W, single pulse laser operation can be observed in a pump pulse width, as shown in the inset of Fig. 3(a). Single pulse energy increases with the increment of pump power. Figure 3(b) shows the single pulse energy recorded in a measured time of one minute when the peak incident pump power was 22.1 W. It can be seen that pulse lase operation is very stable and the average pulse energy is about 40.2 μJ with a fluctuation lower than 2.7%. Pulse train profiles were also recorded at oscilloscope scanning speeds of 10 and 200 ms/div, respectively. The stable pulse laser operation with a repetition frequency of 100 Hz can always be observed. However, when the peak incident pump power was further increased to 22.4 W, the second pulse was appeared in a pump pulse width and single pulse laser oscillation cannot be maintained. The time interval between the two pulses generated in the same pump pulse is about 64.1 μs, corresponding to a repetition frequency of 15.6 kHz. Then, the average pulse energy decreased to 38.5 μJ and the stability of the pulse laser became poor.
Fig. 3. Pulse performance of the passively Q-switched Er:Yb:LuAB microlaser at a pump beam diameter of 200 μm when the LD was operated in quasi-cw mode with pump pulse width of 0.3 ms and pulse period of 10 ms. (a) Dependence of single pulse energy on peak incident pump power. The insets show the pulse profiles recorded in a pump pulse width for different pump powers. (b) Stability of the single pulse energy in a measured time of one minute when the peak incident pump power was 22.1 W. (c) Short pulse train profile at an oscilloscope scanning speed of 10 ms/div. (d) Long pulse train profile at an oscilloscope scanning speed of 200 ms/div.
When the pump pulse period of the quasi-cw LD was fixed at 10 ms, the effect of pump pulse width on pulse performance of the microlaser was further investigated. In the experiment, the highest peak incident pump power was adopted while a single pulse laser was realized in a pump pulse width. The single pulse energies and peak incident pump powers for pump pulse widths from 0.2 to 1.0 ms are shown in Fig. 4(a). For a shorter pump pulse width, pulse laser operation could not be realized because the peak incident pump threshold was higher than the emission power of the used LD. It can be seen from Fig. 4(a) that with the increment of pump pulse width from 0.2 to 1.0 ms, pulse energy decreases from 42.5 to 31.5 μJ, which may be caused by the increment of the thermal effect. Laser spectra for different pump pulse widths are shown in Fig. 4(b). Laser oscillating wavelength is located at 1598 nm when the pump pulse width is between 0.2 and 0.8 ms, while laser wavelength is blue-shifted to 1522 nm when the pump pulse width is increased to 1.0 ms. This phenomenon can also be explained by the increment of the thermal effect of the crystal, because the previous investigation has shown that the gain peak wavelength in 1.5–1.6 μm of the Er:Yb:RAB crystal is blue-shifted with the increment of the crystal temperature [15]. As mentioned above, the initial transmissions around 1600 and 1520 nm of the used Co2+:MgAl2O4 crystal are about 96% and 89%, respectively. The lower initial transmission will make the pulse duration shorter [13]. Therefore, the duration of the 1598 nm pulse laser for pump pulse width in 0.2–0.8 ms is 4.5 ns, while the one of the 1522 nm pulse laser for pump pulse width of 1.0 ms is shortened to 2.0 ns, as shown in Figs. 4(c) and (d). Due to the shortening of the pulse duration, the peak output power of the 1522 nm pulse laser increases to about 15.8 kW, and is higher than those of the 1598 nm pulse lasers with higher pulse energies. As an example, the peak output power is 9.4 kW for the pump pulse width of 0.2 ms. At the same time, due to the lower initial transmission, the peak incident pump power of the 1522 nm pulse laser is also increased, as shown in Fig. 4(a).
Fig. 4. Pulse performance of the passively Q-switched Er:Yb:LuAB microlaser at a pump beam diameter of 200 μm when the LD was operated in quasi-cw mode with a pump pulse period of 10 ms. (a) Single pulse energies and peak incident pump powers for different pump pulse widths of the quasi-cw LD. (b) Laser spectra for different pump pulse widths. (c) Single pulse profile for pump pulse width from 0.2 to 0.8 ms. (d) Single pulse profile for pump pulse width of 1.0 ms.
By changing the pump pulse period of the used quasi-cw LD, the passively Q-switched Er:Yb:LuAB microlasers operated in a repetition frequency of 10–200 Hz were also demonstrated. Some experimental results are listed in Table 1. At a pump pulse width of 0.3 ms, single pulse energy decreases from 45.5 to 37.6 μJ when the pump pulse period is decreased from 100 to 5 ms, corresponding to the increment of the repetition frequency from 10 to 200 Hz. It is worth noting that the laser wavelength is 1522 nm and pulse duration is 2.0 ns for the pulse laser with repetition frequency of 200 Hz when the pump pulse width is 0.3 ms. Therefore, a 1522 nm passively Q-switched Er:Yb:LuAB microlaser with peak output power of 18.8 kW, duration of 2.0 ns and repetition frequency of 200 Hz can be obtained at a pump beam waist diameter of 200 μm. When the pump pulse period was further decreased, pump pulse width was also needed to be reduced to far less than 0.3 ms for reducing the thermal effect and avoiding the damage of the gain medium. Then, a higher peak incident pump power will be required and exceed the emission power of the used LD.
Table 1. Pulse performance of the passively Q-switched Er:Yb:LuAB microlasers for different pump conditions. ωp: waist diameter of pump beam; tp: pump pulse width; T: pump pulse period; Pin: peak incident pump power; f: repetition frequency; E: single pulse energy; tw: pulse duration; Ppeak: peak output power; λl: laser wavelength
Pulse performance of the Er:Yb:LuAB microlaser was also investigated at another pump beam waist diameter of 260 μm, in order to increase the pulse energy of the microlaser. When the pump pulse width was 0.3 ms, pulse laser operation cannot be realized because the peak incident pump threshold was higher than the emission power of the used LD. Then, the pump pulse width of 0.5 ms was adopted. Experimental results are also listed in Table 1. When the repetition frequency is 100 Hz, a stable 1522 nm pulse laser with energy of 48.3 μJ can be realized at a peak incident pump power of 24.6 W and the fluctuation of the pulse energy is lower than 2.5% in a measured time of two minutes, as shown in Fig. 5(a). Pulse duration shown in Fig. 5(b) is 1.9 ns and then the peak output power can reach up to 25.4 kW, which is about 2.5 times of that (close to 10 kW [11]) reported previously in the Er:Yb:GdAB microlaser. The realized peak output power is also close to those (higher than 30 kW) obtained in the passively Q-switched Er:Yb:glass pulse microlasers [5,6]. However, the realized repetition frequency of 100 Hz is an order of magnitude higher than that (generally 10 Hz) of the Er:Yb:glass pulse microlaser. Then, when this passively Q-switched Er:Yb:LuAB microlaser is used as the detection beam, the measuring accuracy of the rangefinder can be effectively improved. The insets of Fig. 5(a) show 2D and 3D images of the transversal profile of the unfocused output pulse beam. It can be seen that the output beam was nearly circularly symmetric and close to TEM00 transverse mode. By fitting the waist radii of the focused pulse beam recorded in different positions, the quality factor M2 of the pulse laser is estimated to be less than 1.2.
Fig. 5. Pulse performance of the passively Q-switched Er:Yb:LuAB microlaser at a pump beam waist diameter of 260 μm when the LD was operated in quasi-cw mode with pump pulse width of 0.5 ms and pulse period of 10 ms. (a) Stability of the single pulse energy in a measured time of two minutes when the peak incident pump power was 24.6 W. The insets show 2D and 3D images of the transversal profile of the unfocused output pulse beam. (b) Single pulse profile at a peak incident pump power of 24.6 W.
4. Conclusion
Passively Q-switched Er:Yb:LuAB pulse microlasers operated in a low repetition frequency of 10–200 Hz were investigated for the first time. A stable 1522 nm pulse microlaser with high output peak power, narrow duration and excellent beam quality was successfully demonstrated at 100 Hz. This eye-safe pulse microlaser can be used as a detection beam for the portable laser rangefinder with measuring distance of kilometer level.
Funding
Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China (2021ZR119, 2021ZZ118); Scientific Instrument Developing Project of the Chinese Academy of Sciences (YZLY202001).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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