1. Introduction

Passively Q-switched microchip laser is composed of a thin laser gain medium bonded to a saturable absorber. It has been considered to be the most compact and simple diode-pumped solid-state pulse laser [1,2]. In the microchip laser, the cavity mirror films are deposited directly onto the input surface of the gain medium and output surface of the saturable absorber, respectively. Benefited from the short cavity length, it is favorable for realizing a single-longitudinal-mode pulse laser with narrow pulse duration and excellent beam quality. In the past thirty years, several kinds of passively Q-switched pulse microchip lasers operating at various wavelengths have been developed, such as 1.0–1.1 µm for Yb:YAG, Nd:YAG and Nd:LaSc₃(BO₃)₄ [2–5], 1.34 µm for Nd:YVO₄ [6], 1.85 µm for Tm:KLu(WO₄)₂ [7], and 2.06 µm for Tm:Ho:KLu(WO₄)₂ [8].

Eye-safe 1.55 µm laser has high transparency in the atmosphere, and is located in the sensitive region of room-temperature Ge and InGaAs photodiodes. Therefore, a compact 1.55 µm passively Q-switched microchip laser with narrow pulse duration, high output peak power, and excellent beam quality has great application potential in some fields, such as lidar, laser ranging, high-resolution spectroscopy, and quantum information [9,10]. Due to the large absorption cross section around 0.97 µm of Yb³⁺ ions and high Yb³⁺ doping concentration, Er³⁺/Yb³⁺ co-doped materials are ideal gain media for realizing the 1.55 µm microchip lasers pumped by the commercial 0.97 µm diode laser [9,10]. However, to the best of our knowledge, 1.55 µm passively Q-switched microchip lasers operating in single longitudinal mode have only been realized in Er:Yb:phosphate glass and Er:Yb:YAl₃(BO₃)₄ (Er:Yb:YAB) crystal up to now [11–13]. Moreover, the large full width at half maximums (FWHMs) of gain bands for the above laser media (larger than 20 and 7 nm for the Er:Yb:phosphate glass around 1535 nm and the Er:Yb:YAB crystal around 1521 nm [13], respectively) may increase the difficulty of realizing the single-longitudinal-mode laser oscillation [1].

Er:Yb:Lu₂Si₂O₇ (Er:Yb:LPS) crystal can be grown by the Czochralski method, and has a high thermal conductivity of about 9.46–14.28 Wm^-1K^-1 [14,15]. The FWHM of gain band around 1537 nm for the crystal is only 1.6 nm, which is favorable for realizing the single-longitudinal-mode laser operation [15]. A continuous-wave (cw) 1537 nm single-longitudinal-mode microchip laser with a maximum output power of 440 mW has been demonstrated in a 1.2-mm-thick Er:Yb:LPS crystal [15]. Furthermore, the crystal has a long fluorescence lifetime (about 8.68 ms) of the upper laser level ⁴I_13/2 [14], which indicates that it has a high energy storage capacity and can realize a high-energy pulse laser. In a linear plano-concave cavity, a 1537 nm multi-longitudinal-mode passively Q-switched pulse laser with energy of 45.5 µJ, repetition frequency of 1.32 kHz, and duration of 25 ns has been obtained in the crystal [16]. In this work, a 1537 nm single-longitudinal-mode passively Q-switched pulse microchip laser with high energy, short duration, and high output peak power is firstly demonstrated in the Er:Yb:LPS crystal.

2. Experimental arrangement

The experimental setup of passively Q-switched Er:Yb:LPS pulse microchip laser is depicted in Fig. 1. The pump source was a fiber-coupled diode laser from Dilas (100 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. A Y-cut, 2.4-mm-thick Er(0.5 at.%):Yb(5.0 at.%):LPS crystal with cross section of 3 × 3 mm² was used as a gain medium. The spectral investigation has shown that the absorption cross sections at 975.4 nm for the E//X and E//Z polarizations of the crystal are larger than that for the E//Y polarization [14]. Here, X, Y, and Z are the optical indicatrix axes and named in order of decreasing of refractive index. Then, for the same thickness, the Y-cut crystal can absorb more incident pump power, which will enhance the output laser performance. The absorption coefficient of the Y-cut Er:Yb:LPS crystal at 975.4 nm is 7 cm^-1 [15], and then about 80% incident pump power can be absorbed by the crystal in a single pass. By using a telescopic lens system (TLS) consisting of two convex lenses, the pump beam was collimated and then focused into the gain medium. An input mirror (IM) film with 90% transmission around 975 nm and 99.8% reflectivity between 1.5–1.6 µm was directly deposited onto the input surface of the Er:Yb:LPS crystal. A 1.2-mm-thickness uncoated Co²⁺:MgAl₂O₄ crystal with cross section of 3 × 3 mm² was used as a saturable absorber. The initial transmission of the Co²⁺:MgAl₂O₄ crystal is 95%. Two output mirrors (OMs) with 3.0% and 10% transmissions between 1.5–1.6 µm, which are made of the sapphire crystals with size of 3×3×1 mm³, were used to investigate the pulse performance for different OM transmissions. The OM film was directly deposited onto the output surface of the sapphire crystal. All the above crystals were polished to achieve a flatness less than one-quarter wave at 633 nm and parallelism better than 20 arc sec. All the crystals were optically contacted and then mounted in a copper chamber cooled by nature air. 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 4.6 mm.

 

Fig. 1. Experimental setup for the passively Q-switched Er:Yb:LPS pulse microchip laser.

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3. Results and discussion

When the pump beam diameter and OM transmission were 200 µm and 3.0%, respectively, the dependence of average output power of the passively Q-switched Er:Yb:LPS pulse microchip laser on absorbed pump power was investigated, as shown in Fig. 2(a). When the absorbed pump power was 3.4 W, the maximum average output power was 50 mW, and no damage of the saturable absorber was observed. The threshold power was about 1.6 W. When the Co²⁺:MgAl₂O₄ saturable absorber was removed from the cavity, 190 mW cw output power was obtained at the pump power of 3.4 W. Then, the conversion efficiency from the cw to the Q-switched operation regime was about 26%. Spectrum of the microchip laser was recorded by a monochromator (Triax 550, Jobin-Yvon) with a spectral resolution of 0.02 nm associated with a Ge detector. For all the pump powers adopted in this work, only one longitudinal mode with a FWHM of about 0.04 nm was observed in the spectrum. For the sake of brevity, only the spectra in 1535–1539 nm recorded at an absorbed pump power of 3.4 W are shown in Fig. 2(b). The spectra are the superimposition of four scans in the same wavelength range. Laser wavelength was centered at 1537 nm and the wavelength drift may be caused by the thermal fluctuation of the gain medium. The refractive indexes of the Er:Yb:LPS, Co²⁺:MgAl₂O₄ and sapphire crystals are 1.7, 1.7 and 1.75 between 1.5–1.6 µm, respectively [15,17]. Therefore, the interface reflection caused by the difference of the refractive indexes between them can be neglected and then the coupled cavity effect may be eliminated [1]. The theoretical separation Δλ between two adjacent longitudinal modes caused by the etalon effect can be estimated by Δλ=λ²/2nL [18]. Here, λ is the laser wavelength of 1537 nm, n is average refractive index of the above crystals at λ (about 1.7), and L is the cavity length of 4.6 mm. Then, the theoretical mode separation was calculated to be 0.15 nm, which is larger than the spectral resolution of the used monochromator. Combined with the fact that no additional longitudinal mode was observed at the mode separation of about 0.15 nm and their integer multiples, the stable single-longitudinal-mode pulse operation can be demonstrated in the passively Q-switched Er:Yb:LPS microchip laser. The realization of the single-longitudinal-mode pulse laser may be caused by the overall effects of narrow gain band around 1537 nm of the Er:Yb:LPS crystal, high cavity loss, and mode selection of the saturable absorber [19].

 

Fig. 2. (a) Dependence of average output power on absorbed pump power for the Er:Yb:LPS microchip laser at an OM transmission of 3.0% and a pump beam diameter of 200 µm. (b) Spectra in 1535–1539 nm of the Er:Yb:LPS pulse microchip laser at an absorbed pump power of 3.4 W. The spectra are the superimposition of four scans in the same wavelength range. The inset shows the enlarged spectra in 1536.8–1537.2 nm.

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Pulse profile of the microchip laser was measured by a 5 GHz InGaAs photodiode (DET08C, Thorlabs) connected to a digital oscilloscope with a bandwidth of 1 GHz (DSO6102A, Agilent). Figure 3(a) shows the dependences of repetition frequency and energy of the Er:Yb:LPS pulse microchip laser on absorbed pump power. At an absorbed pump power of 3.4 W, the repetition frequency was 5.9 kHz and the pulse energy was about 8.3 µJ, as shown in Fig. 3(b). It can be seen that the pulse laser operation was stable and the intensity instability in the pulse train was less than ±4%, which may be caused by the single-longitudinal-mode laser oscillation. When the absorbed pump power was reduced to 1.67 W, the repetition frequency decreased to 1.15 kHz. For all the pump powers adopted in this work, the pulse duration was kept at about 5.7 ± 0.3 ns, as shown in Fig. 3(c). The theoretical minimum duration t_w obtained from a Q-switched pulse laser can be roughly estimated from the following formula [20,21]:

 

Fig. 3. Pulse performance of the 1537 nm single-longitudinal-mode passively Q-switched Er:Yb:LPS microchip laser at an OM transmission of 3.0% and a pump beam diameter of 200 µm. (a) Dependences of repetition frequency and pulse energy on absorbed pump power. (b) Repetition frequency at an absorbed pump power of 3.4 W. (c) Pulse duration at an absorbed pump power of 3.4 W.

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Using a convex lens with a focal length of 10 cm to focus the output laser beam, spatial profiles of the focused beam at different distances from the focusing lens were recorded with a Pyrocam III camera (Ophir Optronics). The beam radius of output laser was calculated by the 4-sigma method. At an absorbed pump power of 3.4 W, the beam quality factors M_x² and M_y² for the horizontal and perpendicular directions of the pulse microchip laser were fitted to be 1.33 and 1.29, respectively, as shown in Fig. 4(a). Figure 4(b) shows 2D and 3D images of the transversal profile of the unfocused output beam. It can be seen that the output beam was nearly circularly symmetric and close to TEM₀₀ transverse mode. Output laser was not completely linearly polarized, which may be originated from the similar gain cross sections around 1537 nm for the E//X and E//Z polarizations of the Er:Yb:LPS crystal, as shown in Fig. 4(c). Furthermore, there was also only one longitudinal mode at the same wavelength observed in the polarized laser spectra for the E//X and E//Z directions.

 

Fig. 4. (a) Squared beam radius ω² of the Er:Yb:LPS pulse microchip laser as a function of the distance from the focusing lens at an absorbed pump power of 3.4 W. (b) 2D and 3D images of the transversal profile of the unfocused output beam. (c) Polarized gain cross section spectra at an inversion parameter β of 0.5 in 1530–1580 nm for the E//X and E//Z polarizations of the Er:Yb:LPS crystal.

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By using the convex lenses with different focal lengths in the TLS, the pump beams with different waist diameters in the Er:Yb:LPS crystal can be obtained. In this work, the pulse characteristics of the microchip laser were investigated at the pump beam diameters of 200, 300 and 420 µm, as well as at OM transmissions of 3.0% and 10%. Experimental results obtained at an absorbed pump power of 3.4 W are summarized in Table 1. Pulse laser at a pump beam diameter of 420 µm and an OM transmission of 10% cannot be realized, because the laser threshold was beyond 3.4 W in this case. All the lasers were operated in single longitudinal mode and beam quality factors were less than 1.3. With the increment of pump beam diameter and OM transmission, repetition frequency and duration decreased, while the energy and output peak power increased. For the pump beam diameter of 420 µm and OM transmission of 3.0%, a 1537 nm single-longitudinal-mode passively Q-switched pulse microchip laser with energy of 25.8 µJ, duration of 4.3 ns, repetition frequency of 0.89 kHz, and output peak power of 6.0 kW was obtained. The pulse profile of the microchip laser at a pump beam diameter of 420 µm and an OM transmission of 3.0% is shown in Fig. 5. Although the obtained pulse energy was lower than that (45.5 µJ) of the 1537 nm multi-longitudinal-mode Er:Yb:LPS laser realized in a plano-concave cavity [16], the pulse duration of the microchip laser greatly reduced to 4.3 ns (25 ns for the plano-concave cavity), due to the shortening of the cavity length. Then, the output peak power of the microchip laser was increased by about 3.3 times.

 

Fig. 5. Pulse profile of the 1537 nm single-longitudinal-mode passively Q-switched Er:Yb:LPS microchip laser at an OM transmission of 3.0%, a pump beam diameter of 420 µm, and an absorbed pump power of 3.4 W. (a) Repetition frequency. (b) Pulse duration.

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Table 1. Experimental results of the 1537 nm single-longitudinal-mode Er:Yb:LPS pulse microchip laser for different pump beam diameters and OM transmissions at an absorbed pump power of 3.4 W.

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Table 2 lists some important characteristics of the single-longitudinal-mode passively Q-switched Er:Yb:LPS, Er:Yb:YAB and Er:Yb:phosphate glass pulse microchip lasers. The Er:Yb:YAB crystal has been investigated as an efficient 1.55 µm laser medium, and a 1521 nm pulse microchip laser with energy of 16.5 µJ and repetition frequency of 26.3 kHz has been obtained. However, the short fluorescence lifetime of the upper laser level ⁴I_13/2 of the Er:Yb:YAB crystal limits its energy storage capacity and then the pulse energy. Due to the low thermal conductivity and broad gain band around 1535 nm of the Er:Yb:phosphate glass, the single-longitudinal-mode pulse laser can only operate at a low average output power [11], which also limits the improvement of its pulse laser performance. Benefited from the high thermal conductivity, long fluorescence lifetime, and narrow gain band around 1537 nm, the Er:Yb:LPS crystal is an excellent gain medium for realizing the single-longitudinal-mode passively Q-switched microchip laser at 1.55 µm. By further optimizing the cavity parameters, the pulse performance of the Er:Yb:LPS microchip laser can be improved.

Table 2. Comparation of the some important characteristics of the single-longitudinal-mode passively Q-switched Er³⁺/Yb³⁺ pulse microchip lasers.

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4. Conclusion

A 1537 nm single-longitudinal-mode passively Q-switched pulse microchip laser was successfully demonstrated in the Er:Yb:LPS crystal. Compared with the other investigated Er³⁺/Yb³⁺ co-doped materials, the Er:Yb:LPS crystal may be more suitable as a gain medium for single-longitudinal-mode passively Q-switched microchip laser at 1.55 µm with high pulse energy.