A single-longitudinal-mode 1521 nm pulse microchip laser Q-switched by a Co2+:MgAl2O4 saturable absorber was demonstrated in an Er:Yb:YAl3(BO3)4 crystal. The influence of the waist radius of pump beam at 976 nm on the laser performance was investigated. At an incident pump power of 6.54 W and pump beam waist radius of 60 μm, a 1521.4 nm single-longitudinal-mode pulse laser with average output power of 434 mW, energy of 16.5 μJ, repetition frequency of 26.3 kHz and width of 2.9 ns was obtained. The result shows that caused by the mode selection of the saturable absorber and large cavity losses, a single-longitudinal-mode 1.55 μm pulse microchip laser can also be realized in the Er:Yb:YAl3(BO3)4 crystal.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Due to the advantages of eye-safe, excellent transparency in atmosphere, as well as high sensitivity for the room-temperature Ge and InGaAs photodiodes, a single-longitudinal-mode pulse laser around 1.55 μm with high repetition frequency, narrow pulse width, and excellent beam quality has great potential applications in fields such as lidar, laser ranging, high-resolution spectroscopy, and quantum information [1,2].
A microchip laser can be fabricated by directly depositing the dielectric films of laser cavity mirrors onto the both polished faces of a flat-flat laser gain medium . As the most compact and simple solid-state laser, it is favorable for realizing the single-longitudinal-mode laser operation [3,4]. Compared with the widely investigated Nd3+ microchip laser operating around 1.0 μm , the investigation about the single-longitudinal-mode 1.55 μm microchip laser operation based on Er3+/Yb3+ co-doped material is rare up to now. Due to the broad full width at half maximum (FWHM>20 nm ) of the 1535 nm gain band of the Er:Yb:phosphate glass, which is the well-known commercial 1.55 μm laser gain medium, an efficient single-longitudinal-mode 1.55 μm microchip laser oscillation is difficult to be realized. Continuous-wave (cw) 1535 nm single-longitudinal-mode microchip lasers with maximum output powers of about 3 and 25 mW have been reported in 330 and 200 μm thick Er:Yb:phosphate glasses, respectively [5,6]. Then, based on a thermally bonded Er:Yb:glass/Co2+:MgAl2O4 microchip laser with cavity length of 2.19 mm, a single-longitudinal-mode 1535.2 nm pulse laser with energy of 40 μJ, repetition frequency of 0.735 kHz, and width of 3.8 ns has been demonstrated . Recently, benefited from the narrow gain bandwidth around 1537 nm with FWHM of about 1.6 nm for the Er:Yb:Lu2Si2O7 crystal, a cw 1537 nm single-longitudinal-mode microchip laser with a maximum output power of 440 mW and slope efficiency of 12% has also been demonstrated . Furthermore, it is important for the practical applications to realize the stable single-longitudinal-mode operation for every pulse in a train.
Er:Yb:YAl3(BO3)4 (Er:Yb:YAB) crystal has been considered as an excellent 1.55 μm laser material [2,9–11]. 1550 nm cw laser with a maximum output power of 2.05 W and slope efficiency of 39.8%, as well as 1521 nm passively Q-switched pulse laser with energy of about 10 μJ, repetition frequency of 77 kHz, and width of 7 ns have been realized in the crystal, respectively . By using a 0.3-mm-thick uncoated fused silica as an etalon placed in a plano-concave cavity, a cw single-longitudinal-mode 1550 nm laser with maximum output power of 400 mW and slope efficiency of 11.8% has also been obtained . However, due to the large FWHMs (larger than 15 and 7 nm, respectively [10,12]) around 1551 and 1521 nm gain bands of the Er:Yb:YAB crystal, the single-longitudinal-mode 1.55 μm microchip laser has not been realized till now. In this work, by designing the laser cavity, a high performance single-longitudinal-mode passively Q-switched 1521 nm pulse microchip laser is firstly demonstrated in the Er:Yb:YAB crystal.
2. Experimental arrangement
The experimental setup is depicted in Fig. 1. A c-cut, 1.52-mm-thick Er(1.5 at.%):Yb(12 at.%):YAB crystal with cross section of 3 × 3 mm2 was used as a gain medium. The pump source was a fiber-coupled diode laser from Dilas (105 μm core diameter and 0.15 numerical aperture) with central wavelength stabilized at 975.4 nm and FWHM of 1.0 nm for the emission band. The divergence (full angle) of the pump beam was 17.3°. By using a telescopic lens system (TLS) consisting of two convex lenses, the pump beam was focused into the crystal. Different waist radii of the pump beam in the crystal can be obtained by changing the imaging ratio of the two convex lenses with different focal lengths. An input mirror (IM) film with 90% transmission around 975 nm and 99.8% reflectivity between 1.5 and 1.6 μm was directly deposited onto the input surface of the Er:Yb:YAB crystal. In order to reduce the thermal effect of the gain medium, a polished 1.26-mm-thick sapphire crystal with cross section of 3 × 3 mm2 was used as a heat sink and optically contacted with the input surface of the Er:Yb:YAB crystal. The incident pump power was measured after the sapphire crystal. Therefore, the effect of the transmission and reflection of the sapphire crystal on the pump power was taken into account. A dielectric film with 10% transmission between 1.5 and 1.6 μm was directly deposited onto the output surface of another polished sapphire crystal with the same size, which was used as an output mirror (OM) for the cw laser operation. For the pulse laser operation, a 1.08-mm-thick Co2+:MgAl2O4 crystal with initial transmission of 96% was used as a saturable absorber, and an output mirror (OM) film with 10% transmission between 1.5 and 1.6 μm was also directly deposited onto the output surface of the Co2+:MgAl2O4 crystal. Because this coated Co2+:MgAl2O4 crystal was the only one available in our lab, the initial transmission of 96% and the output mirror transmission of 10% were not optimized at present. The using of the OM film with a high transmission of 10% was favorable for realizing a stable single-longitudinal-mode pulse laser operation with higher energy and narrower width. Then, all the crystals were mounted into a copper holder and the total laser cavity length was about 2.6-2.8 mm. The holder was cooled by water at about 20 °C. There was a hole with radius of about 1 mm in the center of the holder to permit the passing of the laser beams. The position of the pump beam waist inside the gain medium was optimized by adjusting the holder to achieve the best laser performance during the experiment. The parallelism of all the crystals was better than 20 arc seconds. Therefore, the resonator can be free from the alignment and adjustment. Output power was measured by a PM100D power meter associated with a S314C thermal power head from Thorlabs Inc. Laser spectrum was recorded by a monochromator (Triax 550, Jobin-Yvon) with a spectral resolution of 0.02 nm and an integral time of 0.3 s associated with a TE-cooled Ge detector. The spatial profile of the laser beam was recorded with a Pyrocam III camera from Ophir Optronics Ltd. 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).
3. Results and discussion
When the OM made of sapphire crystal was used, a cw 1.55 μm microchip laser was realized. Figure 2(a) shows the dependence of the cw output power on incident pump power for the Er:Yb:YAB microchip laser, when the waist radius of the pump beam was 60 μm. A cw 1521 nm laser with the maximum output power of 1.03 W and slope efficiency of 25% was obtained at an incident pump power of 6.54 W, which was limited by the available output power of the used LD in our lab. Above values are less than those (2.05 W and 39.8%) reported previously in the Er:Yb:YAB microchip laser with OM transmission of 2.5% . The insets of the Fig. 2(a) show the laser spectra recorded at the incident pump powers of 6.54 and 3.4 W, respectively. The multi-longitudinal-mode 1521 nm laser oscillations were always observed, due to the broad FWHM of about 7 nm at this gain band of the Er:Yb:YAB crystal . The separation between the adjacent longitudinal modes was about 0.5 nm, and the FWHM of a longitudinal mode was 0.07 nm. Because the mode separation was about 25 times of the monochromator resolution (0.02 nm), the adjacent longitudinal modes can be clearly identified in this work. The theoretical mode separation Δλ caused by the crystal etalon effect can be estimated by Δλ = λ2/2nL . Here λ is the laser wavelength, n is refractive index of the crystal at λ, and L is the crystal length. Then, for the etalon effect associated to the 1.52-mm-thick Er:Yb:YAB and 1.26-mm-thick sapphire crystals with similar refractive indexes of about 1.75 , Δλ were calculated to be 0.44 and 0.52 nm, respectively. Therefore, the longitudinal mode characteristic of the microchip laser may be mainly attributed to the etalon effect of the sapphire crystal.
A convex lens with a focal length of 100 mm was used to focus the generated fundamental laser beam and then the spatial profile of the focused beam was recorded with a Pyrocam III camera. Figure 2(b) shows the spatial profile and quality factor M2 of the fundamental laser beam at an incident pump power of 6.54 W. A near circular symmetric laser beam was always observed for various pump powers. At an incident pump power of 6.54 W, M2 was close to 2.4. Due to the reduction of the thermal effect and the oscillating longitudinal mode number, the beam quality of the fundamental laser was improved with the decrement of the pump power. When the incident pump power was reduced to 3.4 W, M2 was less than 1.2, as shown in the inset of Fig. 2(b).
Based on the measured waist radius of the focused fundamental laser beam and the ABCD law of Gaussian beam propagation , the waist radius ωl of the fundamental laser in the Er:Yb:YAB crystal can be roughly estimated. For the different pump beam waist radii ωp of 35, 60, and 75 μm used in this work, ωl were estimated to be 47, 54, and 50 μm, respectively. The experimental results of the Er:Yb:YAB cw microchip laser for different pump beam waist radii are shown in Table 1. It can be seen that the highest laser performance was realized at the pump beam waist radius of 60 μm, in this case the best mode overlap efficiency (ωl/ωp)2 of 0.81 between the pump and fundamental laser beams can be realized .
When the OM made of sapphire crystal was replaced by the Co2+:MgAl2O4 crystal, a passively Q-switched 1.55 μm pulse microchip laser was realized. The dependence of the average output power of the pulse laser on incident pump power is shown in Fig. 3(a), when the waist radius of the pump beam was 60 μm. The waist radius of the generated fundamental laser beam inside the saturable absorber was estimated to be about 45 μm. At an incident pump power of 6.54 W, the maximum average output power was 434 mW and slope efficiency was 16.2%. It is worth noting that only one longitudinal mode centered at 1521.4 nm with a FWHM of about 0.05 nm was always observed in the spectrum for the various pump powers, as shown in the inset of Fig. 3(a) at an incident pump power of 6.54 W. Because the integral time of the monochromator was 0.3 s, several pulses were recorded in the spectrum, which indicates that the spectrum of the different single pulses in the train was the same, and the stable single-longitudinal-mode operation for every pulse in a train was realized. The realization of the single-longitudinal-mode pulse laser was caused by the large cavity losses and mode selection of the saturable absorber . M2 of the single-longitudinal-mode pulse microchip laser was improved to 1.2 at an incident pump power of 6.54 W, as shown in Fig. 3(b).
The repetition frequency and width of the 1521.4 nm pulse laser at an incident pump power of 6.54 W were measured to be 26.3 kHz and 2.9 ns, respectively, as shown in Figs. 4(a) and 4(b). The pulse laser operation was stable and the amplitude variation between various pulses was generally kept within 5%. It can be seen from Fig. 5 that when the incident pump power was reduced to 4.2 W, the repetition frequency was decreased to 4.1 kHz and the pulse energy was kept at about 16.5 μJ, which is higher than that (about 10 μJ) of the passively Q-switched Er:Yb:YAB pulse microchip laser reported previously . Due to the shorter fluorescence lifetime (0.33 ms ) of the 4I13/2 upper laser level of the Er:Yb:YAB crystal than that (about 7 ms ) of the Er:Yb:phosphate glass, the pulse energy obtained in the Er:Yb:YAB microchip laser is lower than that (40 μJ) in the Er:Yb:glass microchip laser, while its pulse repetition frequency is far higher than that (0.735 kHz) in the Er:Yb:glass microchip laser . The pulse width was only slightly changed with the incident pump power and kept at about 3.0 ns. Then, the peak output power of the single-longitudinal-mode 1521.4 nm passively Q-switched Er:Yb:YAB pulse microchip laser was estimated to be about 5.5 kW.
The experimental results of the single-longitudinal-mode passively Q-switched Er:Yb:YAB pulse microchip laser for different pump beam waist radii are shown in Table 2. The highest laser performance was also realized at the pump beam waist radius of 60 μm. Based on a special synchronic accumulation method rather than the direct detection method using a high energy single-pulse laser, a prototype of an eye-safe laser rangefinder with detection distance up to 6.7 km has been successfully built by J. Mlynczak et.al . In this prototype, a 1535 nm Er:Yb:glass pulse microchip laser with energy lower than 20 μJ, width of 10-12 ns, repetition frequency of 1-4 kHz, and peak power of 1.1-1.6 kW was used as the detection beam. Therefore, the single-longitudinal-mode pulse microchip laser realized in this work may be also used as a kind of excellent detection beam applied in the laser rangefinder.
Caused by the mode selection of the Co2+:MgAl2O4 saturable absorber and large cavity losses, a single-longitudinal-mode 1521.4 nm passively Q-switched Er:Yb:YAB pulse microchip laser has been realized, although the FWHM of 1521 nm gain band is larger than 7 nm. The laser performance can be further improved by optimizing the overlap of the pump and generated fundamental laser beams.
Ministry of Science and Technology of the People's Republic of China (2016YFB0701002); Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20000000).
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