1. Introduction

Actively Q-switched technique is more beneficial to realizing a stable pulse laser operation with smaller time jitter and higher pulse energy than the passively Q-switched one. Therefore, taking the advantages of excellent transparency in atmosphere and high sensitivity for the room-temperature Ge photodiodes into account, eye-safe 1.55 μm actively Q-switched pulse laser can be used in many applications [1,2], such as lidar, satellite laser ranging, three-dimensional imaging, and environmental sensing.

Acousto-optic Q-switching is one of the main methods to realize the actively Q-switched pulse laser operation. However, there have been few researches on the acousto-optic Q-switched 1.55 μm pulse laser based on Er³⁺/Yb³⁺ co-doped gain media pumped by continuous-wave (CW) diode lasers around 976 nm up to now [3–5]. Using Er:Yb:phosphate glass as a gain medium, a linearly polarized 1535 nm acousto-optic Q-switched pulse laser with an energy of 12 μJ and repetition frequency of 0.5 kHz was obtained [3]. Due to the low thermal conductivity of glass [1], a pulse laser with high energy and high repetition frequency is difficult to be realized in the Er:Yb:phosphate glass. Then, Er³⁺/Yb³⁺ co-doped crystals with a higher thermal conductivity may be more favorable as gain media for generating an actively Q-switched pulse laser with higher energy and higher repetition frequency. Pulse energy of an acousto-optic Q-switched Er:Yb:YVO₄ laser at 1602.6 nm with π-polarization reached to 190 μJ at a repetition frequency of 0.5 kHz [4]. However, the slope efficiency of the Er:Yb:YVO₄ laser (lower than 5%) is limited by the strong upconversion loss and inefficient energy transfer from Yb³⁺ to Er³⁺. Furthermore, an unpolarized 1522 nm acousto-optic Q-switched pulse laser with an energy of 136 μJ and repetition frequency of 1 kHz was realized in an Er:Yb:GdAl₃(BO₃)₄ crystal [5]. Consequently, it is interesting to exploit an efficient acousto-optic Q-switched 1.55 μm pulse laser with linear polarization, which is more favorable for some applications, such as lidar, quantum physics, and non-linear frequency conversion [6,7].

Er:Yb:LaMgB₅O₁₀ (Er:Yb:LMB) crystal belongs to the monoclinic system and is a biaxial crystal. Its thermal conductivity is 5.0 Wm⁻¹K⁻¹ at room temperature and fluorescence lifetime of the upper laser level ⁴I_13/2 is 538 µs [8]. A CW 1566 nm E//Z polarized laser with a maximum output power of 610 mW and slope efficiency of 23% has been demonstrated in the crystal [9]. In this work, polarization switching with wavelength changing of output laser was observed in the Er:Yb:LMB crystal by using the natural birefringence of crystalline quartz, and the pulse characteristics of an acousto-optic Q-switched Er:Yb:LMB laser were investigated.

2. Laser experimental arrangement

An end-pumped linear plano-concave resonator was adopted and is shown in Fig. 1. A 2.0-mm-thick, X-cut uncoated Er(0.68 at.%):Yb(7.51 at.%):LMB crystal with a cross section of 5 × 5 mm² was used as a gain medium. The Er:Yb:LMB side facing the pump beam was closely contacted with a 1.0-mm-thick sapphire crystal also having a cross section of 5 × 5 mm², which was used as a heat sink to remove the pump-induced heating from the gain medium and then improve the output laser performance [10]. Both the Er:Yb:LMB and sapphire crystals were enclosed in a copper chamber, which was cooled by water at about 20 °C. There is a hole with radius of about 1 mm in the center of front and back walls of the chamber for the passing of laser beams. A CW 976 nm fiber-coupled diode laser with core diameter of 100 μm and numerical aperture of 0.15 from Dilas Inc. was used as a pump source. After passing a telescopic lens system (TLS) consisted of two convex lenses with same focal length of 45 mm, a pump beam with a waist diameter of about 100 µm was focused into the Er:Yb:LMB crystal. A thin film was deposited on the outside surface of the sapphire crystal to obtain a 90% transmission at 976 nm and 99.8% reflectivity in 1.5–1.6 μm, which was used as an input mirror (IM). An output mirror (OM) with a radius curvature of 50 mm and transmission of 4.0% in 1.5–1.6 μm was used. In order to realize the actively Q-switched pulse operation, an acousto-optic modulator (AOM) (Gooch & Housego Co.), in which an a-cut crystalline quartz with a thickness of 20 mm was used as the acousto-optical material, was inserted into the resonator and placed as close as possible to the Er:Yb:LMB crystal. The quartz was antireflection coated in 1.5–1.6 μm and driven at 80 MHz center frequency with radio-frequency power of 10 W. Duty cycle of the AOM was kept at 20%. The length of laser resonator was close to 50 mm. Pulse profile was measured by a 2 GHz InGaAs photodiode connected to a digital oscilloscope with a bandwidth of 1 GHz (DSO6102A, Agilent). Spectrum of output laser was recorded by a monochromator (Triax550, Jobin-Yvon) associated with a TE-cooled Ge detector.

 

Fig. 1 Experimental setup of a CW 976 nm-diode-pumped acousto-optic Q-switched Er:Yb:LMB 1.55 μm pulse laser.

Download Full Size | PDF

3. Results and discussion

When the AOM was inserted into the laser resonator and the radio-frequency signal was not supplied, CW laser operation of the Er:Yb:LMB crystal was realized. By precisely tilting the alignment of the OM, a 1568 nm laser with polarization parallel to Z optical indicatrix axis and a 1556 nm laser with polarization parallel to Y optical indicatrix axis can be easily realized, respectively. The polarized gain cross-section σ_g can be calculated by ${\sigma }_g\left(\lambda \right)=\beta {\sigma }_{em}\left(\lambda \right)-\left(1-\beta \right){\sigma }_{abs}\left(\lambda \right)$ based on polarized absorption cross-section σ_abs and emission cross-section σ_em. Here, the inversion parameter β is the ratio of the number of Er³⁺ ions in the upper laser level to the total number of Er³⁺ ions. The calculated results show that when β is close to 0.4, the peak wavelengths with the maximum gain cross-sections of the LMB crystal are 1568 and 1556 nm for the E//Z and E//Y polarizations, respectively [9]. Output powers of both lasers as functions of absorbed pump power and the spectra of output lasers at an absorbed pump power of 4.0 W are shown in Figs. 2(a) and 2(b). For the 1568 nm laser, the maximum output power was 500 mW at an absorbed pump power of 4.0 W. The slope efficiency was 16% and the threshold power was about 0.85 W. For the 1556 nm laser, the maximum output power was 400 mW at an absorbed pump power of 4.0 W. The slope efficiency was 13.5% and the threshold power was about 1.0 W. The polarization states and output wavelengths of both lasers were kept unchanged at various pump powers. The tilting angle, which is defined as the included angle between the alignments of the OMs corresponding to the maximum output powers of the 1568 and 1556 nm lasers [11], was measured to be about 3.8 mrad in the vertical direction. This direction was parallel to the Y axis of the Er:Yb:LMB crystal and perpendicular to the optical axis of the crystalline quartz. Furthermore, only unstable and low power (about dozens of milliwatts) orthogonally polarized dual-wavelength laser at 1568 and 1556 nm was observed by adjusting the tilting angle of the OM. When the AOM was removed from the cavity, a CW laser at 1568 nm with E//Z polarization emitted steadily, which is also shown in Figs. 2(a) and 2(b). The maximum output power of the laser was 590 mW at an absorbed pump power of 4.0 W. The slope efficiency was 18% and the threshold power was about 0.75 W. Output laser performances were close to those reported previously for this crystal [9]. In this case, by adjusting the tilting angle of the OM, the 1556 nm laser with E//Y polarization was difficult to be observed. Therefore, the polarization switching at different wavelengths observed in the Er:Yb:LMB laser may be originated from the natural birefringence of the crystalline quartz [11–13], which is used as the acousto-optic material in the AOM. When the OM is titled, the path of the fundamental laser in the quartz may be divided into two paths of the ordinary (E//Y polarization) and extraordinary (E//Z polarization) lasers due to the birefringence of the quartz. The cavity losses for the ordinary and extraordinary lasers are different. Therefore, when the alignment of the OM is slightly changed, the polarized laser with lower cavity loss is realized.

 

Fig. 2 (a) CW output power realized in an Er:Yb:LMB crystal as a function of absorbed pump power when the resonator was with or without a quartz crystal. (b) Spectra of CW Er:Yb:LMB lasers at an absorbed pumped power of 4.0 W.

Download Full Size | PDF

When the AOM was inserted into the resonator and the radio-frequency signal was switched off periodically, 1568 and 1556 nm acousto-optic Q-switched pulse lasers with different polarizations were also realized in the Er:Yb:LMB crystal, respectively, by precisely tilting the alignment of the OM. The tilting angle was the same as that in the CW lasers. For various pump power used in this work, each polarized pulse laser can also be steadily obtained.

For the 1568 nm pulse laser with E//Z polarization, the average output power and pulse energy as functions of pulse repetition frequency at an absorbed pump power of 4.0 W are shown in Fig. 3. As seen from the inset of Fig. 3, more longitudinal modes can be observed than those in the CW regime with the quartz, which may be caused by the higher gain in the Q-switched regime. At the repetition frequency of 0.5 kHz, the average output power was 72 mW and pulse energy was 144 μJ. This value is far higher than that (about 12 μJ) obtained in the acousto-optic Q-switched Er:Yb:phosphate laser [3], and comparable to those of the acousto-optic Q-switched Er:Yb:YVO₄ laser (190 μJ) and Er:Yb:GdAl₃(BO₃)₄ laser (136 μJ) [4,5]. When the repetition frequency was increased to 60 kHz, the average output power was increased to 274 mW and pulse energy was reduced to 4.57 μJ. Figures 4(a) and 4(b) show the pulse widths of the 1568 nm laser for different repetition frequencies and the pulse train at 0.5 kHz when the absorbed pump power was 4.0 W. It can be seen from Fig. 4(b) that the pulse was stable and pulse-to-pulse amplitude fluctuation was less than about ± 5%. When the repetition frequency was lower than 10 kHz, the width of the 1568 nm pulse laser was kept at about 300 ns. Then, at the repetition frequency of 0.5 kHz, the peak output power was estimated to be about 0.48 kW. With the increment of repetition frequency, pulse width was increased gradually and the width was about 350 ns at repetition frequency of 60 kHz.

 

Fig. 3 Average output power and pulse energy of the acousto-optic Q-switched Er:Yb:LMB pulse laser at 1568 nm as functions of pulse repetition frequency at an absorbed pump power of 4.0 W. The output laser spectrum at repetition frequency of 0.5 kHz is shown in the inset.

Download Full Size | PDF

 

Fig. 4 (a) Pulse width of the acousto-optic Q-switched Er:Yb:LMB pulse laser at 1568 nm as a function of pulse repetition frequency at an absorbed pump power of 4.0 W. (b) Pulse train at repetition frequency of 0.5 kHz and absorbed pump power of 4.0 W.

Download Full Size | PDF

For the 1556 nm pulse laser with E//Y polarization, the average output power, pulse energy and pulse width as functions of pulse repetition frequency also at the absorbed pump power of 4.0 W are shown in Figs. 5(a) and 5(b). At the repetition frequency of 0.5 kHz, the average output power was 84 mW and pulse energy was 168 μJ. This value is higher than the 144 μJ of the 1568 nm pulse laser. It may be caused by the higher diffraction efficiency of the used AOM in this polarization direction. At the repetition frequency of 60 kHz, the average output power was increased to 316 mW and pulse energy was reduced to 5.27 μJ. At repetition frequency of 0.5 kHz, the pulse width was about 270 ns and the peak output power was estimated to be about 0.62 kW. At repetition frequency of 60 kHz, the pulse width was increased to about 330 ns.

 

Fig. 5 (a) Average output power and pulse energy of the acousto-optic Q-switched Er:Yb:LMB pulse laser at 1558 nm as functions of pulse repetition frequency at an absorbed pump power of 4.0 W. The output laser spectrum at repetition frequency of 0.5 kHz is also shown in the inset. (b) Pulse width as a function of pulse repetition frequency at an absorbed pump power of 4.0 W.

Download Full Size | PDF

Furthermore, another OM with a radius curvature of 50 mm and transmission of 1.5% in 1.5–1.6 μm was also used to investigate the acousto-optic Q-switched pulse characteristics of the Er:Yb:LMB crystal. Similarly, by precisely tilting the alignment of the OM, the 1568 and 1556 nm pulse lasers with different polarizations were also observed. When the repetition frequency was 0.5 kHz and absorbed pump power was 4.0 W, the average output power, pulse energy and pulse width of the 1568 nm pulse laser with E//Z polarization were 65 mW, 130 μJ and 315 ns, respectively. For the 1556 nm pulse laser with E//Y polarization, above parameters were 79 mW, 158 μJ and 290 ns, respectively. For the brevity of clarity, the parameters of the acousto-optic Q-switched Er:Yb:LMB pulse lasers for different OM transmissions T at an absorbed pump power of 4.0 W and repetition frequency of 0.5 kHz are listed in Table 1.

Table 1. Parameters of the acousto-optic Q-switched Er:Yb:LMB pulse lasers for different OM transmissions T at an absorbed pump power of 4.0 W and repetition frequency of 0.5 kHz.

View Table

4. Conclusion

By using the natural birefringence of the crystalline quartz, a 1568 nm laser with E//Z polarization and a 1556 nm laser with E//Y polarization can be realized respectively in both CW and acousto-optic Q-switched pulse Er:Yb:LMB lasers by precisely tilting the alignment of the OM. The 1568 and 1556 nm pulse lasers with the orthogonal polarization and similar pulse characteristics may be used in lidar.