1 Introduction

Lutetium oxyorthosilicate, Lu2SiO5 (LSO) is well-known to be used for scintillator applications. In recent years, many studies have been conducted on rare earth doped LSO as laser material. e.g., Yb:LSO [1–5], Tm:LSO [6], Ho:LSO [7, 8], and Dy:LSO [9]. One of the main advantages is that this monoclinic biaxial crystal has strong natural birefringence which overwhelms the thermally induced stress birefringence [8]. Tkachuk et al. [10] reported a spectral-luminescence investigation of Nd:LSO more than two decades ago, but until very recently the CW laser performance of Nd:LSO was not demonstrated [11]. Soon after, Cong et al. [12] performed passively mode-locking laser operations of Nd:LSO crystal with a semiconductor saturable absorption mirror, and achieved 12.3 ps mode-locked pulse with 148.3 MHz repetition rate. A certain structural disorder due to the two different occupation sites with low symmetry for Nd3+ ion in LSO crystal broadens the absorption and emission bands [10, 11], which are favorable for diode pumping and ultrashort pulse laser generation.

For the 4F3/2→4I11/2 transition of Nd:LSO, the emission cross-sections at about 1.08 μm are only 1/5 of the value for Nd:YAG crystal (2.8×10−19 cm2), and 1/18 of the value for Nd:YVO4 crystal (10.7×10−19 cm2). The fluorescence lifetime of Nd:LSO crystal is measured to be 220–230 μs, which is similar to that of Nd:YAG crystal, but much longer than for Nd:YVO4 crystal (110 μs). Its small emission cross-section led to high CW and mode-locking thresholds [11, 12], but is favorable for Q-switching operation because the emission cross-section and fluorescence lifetime determine the energy storage ability. For the 4F3/2→4I13/2 transition, Nd:LSO has an emission peak at 1357 nm, which has promising applications in laser-based metrology, laser Doppler velocimeter [13], and distributed fiber sensor [14], etc. Furthermore, by frequency doubling of nonlinear optical crystal, the generated 679 nm coherent emission is desirable in many fields, such as optical information processing, optical communication with plastic fibers [15–17], pumping of Cr3+:LiSAF [18, 19], and repumping of strontium optical clock [20, 21]. These special wavelength lasers are only obtained in Nd:YAG by suppressing 1319 and 1338 nm emissions which also correspond to the 4F3/2→4I13/2 transition, but have larger emission cross-sections than that at 1357 nm [22–24]. Unfortunately, up to now, there is no report on those themes. In this paper, actively Q-switched Nd:LSO laser at about 1.08 μm, and CW and actively Q-switched Nd:LSO laser at 1357 nm were demonstrated for the first time, to the best of our knowledge.

2 Experimental setup

A schematic diagram of the laser setup is shown in Fig. 1. Through a focusing system (N.A. = 0.22), the pump laser from a fiber-coupled 808 nm laser diode (LD) was delivered into the Nd:LSO crystal (3×3×7 mm3, 0.5 at.%, cut along the b-axis) with a pump spot radius of 0.256 mm. Both surfaces of the laser crystal were anti-reflection coated at 808 and 1075 nm. The laser cavity consisted of a concave input mirror M1 with a curvature radius of 200 mm coated for high transmission at 808 nm and high reflectivity at 1075 nm, and a flat output coupler (OC) M2 with a certain transmission at 1075 nm. The actively Q-switched performance was carried out using an acousto-optic Q-switch. To remove heat produced in the crystals, the Nd:LSO was wrapped with indium foil and mounted in a temperature-controlled copper block. In the experiments, the output power was measured by a power meter (EPM 2000, Molectron Inc.), the laser spectrum was recorded using an spectrometer (HR4000CG-UV-NIR, Ocean Optics Inc.) which has a resolution of 0.75 nm, and the pulse profile was recorded by a DPO7104 digital oscilloscope (1 GHz bandwidth and 20.0 Gs/s sampling rate, Tektronix Inc.).

Fig. 1
figure 1

Schematic diagram of the laser setup

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In the case of 1357 nm laser operation, M1 was a 250 mm radius of curvature concave mirror with anti-reflection coating at 808 nm and highly reflecting at 1.3 μm, M2 was a plane mirror with a transmission of T=4.8% at 1.36 μm. Both M1 and M2 had high-transmission at 1075 and 1079 nm to suppress the most possible transition, 4F3/2→4I11/2. The lasing wavelength was examined with a wavemeter (WaveCheck IR, APE Inc.).

3 4F3/2→4I11/2 transition

3.1 CW operation

At first, we examined the CW laser operating characteristics of Nd:LSO using a simple plano-concave cavity with a length of ∼14 mm. In order to find the effective operation, four flat mirrors with various transmittances (1.6, 5, 10, and 13% at 1064 nm) were used as the output coupler (OC). Figure 2 shows the variation of output power with absorbed pump power for these lasers. The maximum output power of 1.09 W was obtained at an absorbed pump power of 3.86 W with the 1.6% OC, giving an optical conversion efficiency of 28.2% and a slope efficiency of 30.9%. Tentatively, the slope efficiencies decrease with increasing output coupling. This might be caused by effects of inhomogeneous spectral broadening [25] and/or increased upconversion losses at higher inversion densities.

Fig. 2
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Variation of output power with absorbed pump power for 1075 and 1079 nm dual-wavelength CW operation

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A typical spectrum of the output laser is shown in Fig. 3. All these lasers had dual-wavelength except that only 1075 nm line appeared near the threshold. For example, with 1.6% OC the thresholds of 1075 and 1079 nm were 0.32 and 0.46 W, respectively. Different optical centers corresponding to the location of Nd3+ in different cationic positions account for the two output wavelengths. From the level list given by Tkachuk et al. [10], we found that 1075 nm laser corresponds to both type I and II optical sites of Nd:LSO, and 1079 nm output corresponds to the type I site. Using a Glan–Taylor polarizer, we found that the two wavelength components were both linearly polarized and their polarized directions were vertical to each other, which will be favorable for further frequency conversions by nonlinear optical crystals, including generating 537.5, 539.5, and 538.5 nm green lasers by frequency doubling or sum-frequency, and generating 1 THz coherent emission by difference frequency.

Fig. 3
figure 3

Typical spectrum of Nd:LSO laser

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3.2 AO Q-switched operation

Lengthening the cavity to ∼50 mm and inserting an acousto-optical Q-switch, AO Q-switched Nd:LSO laser performance was obtained with a 10% OC. Considering that the radiative lifetime of the upper level is 220 μs [11], we set the pulse repetition rate to be 5 kHz. For the sake of contrast, experiments with 10 kHz were also carried out. From Figs. 4, 5 and 6, it can be seen that the results with 5 kHz were better than those with 10 kHz except for average output power and efficiency. At a certain pulse repetition rate, the amount of energy stored in the gain media went up as the pumped power increased, so did the average output power and the pulse energy. And the pulse width decreased accordingly. When the AO Q-switch operated at 5 kHz, the maximum average output power of 244 mW with a slope efficiency of 24.5%, the minimum pulse width of 60.8 ns, the largest pulse energy of 48.8 μJ, and the highest peak power of 0.8 kW were achieved at the maximum absorbed pump power of 3.86 W. In the case of 10 kHz, they were 297 mW, 14.2%, 96.5 ns, 29.7 μJ, and 0.3 kW, respectively. The insert of Fig. 5 presents the pulse profile with the width of 60.8 ns. It reveals that the two pulses of the dual wavelengths should be simultaneous since there is no satellite pulse before or after the single pulse.

Fig. 4
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Variation of average output power with absorbed pump power for 1075 and 1079 nm dual-wavelength AO Q-switched operation

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Fig. 5
figure 5

Variation of pulse width versus absorbed pump power for 1075 and 1079 nm dual-wavelength AO Q-switched operation. Insert: Pulse profile with the width of 60.8 ns

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Fig. 6
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Variation of pulse energy versus absorbed pump power for 1075 and 1079 nm dual-wavelength AO Q-switched operation

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4 4F3/2→4I13/2 transition

4.1 CW operation

For the 1357 nm operation, under an absorbed pump power of 3.41 W, a maximum output power of 234 mW with a slope efficiency of 13.8% and an optical conversion efficiency of 6.9% was obtained, as shown in Fig. 7. The output laser was also linearly polarized. As noted in Sect. 2, the Nd:LSO crystal was anti-reflection coated for 1075 nm, not for 1357 nm. This might have influenced the output power and the efficiencies.

Fig. 7
figure 7

Variation of output power with absorbed pump power for 1357 nm Nd:LSO CW laser

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In 2010, Kaminskii et al. [26] obtained a 1358.5 nm laser with Nd:YSO, an isostructural analog of Nd:LSO. In their experiment, 1078.2 nm lasers generated with the increasing of pump power. In the present experiment, 1.07 μm laser emission was not observed over the whole pump level. For Nd:YSO, the ratio between the emission cross-section of 4F3/2→4I11/2 and that of 4F3/2→4I13/2 is 6:1 [10]. While for Nd:LSO, the ratio is only 3:1 [11]. So in the 1.3 μm operation, 1.07 μm oscillating can be suppressed more easily for Nd:LSO crystal. The frequency doubling of 1357 nm at ∼679 nm may pump a Cr:LiSAF laser more effectively than 671 nm generated from a frequency-doubled Nd:YVO4 laser [27], because of the bigger absorption cross-section of Cr:LiSAF at ∼679 nm.

4.2 AO Q-switched operation

Utilizing an AO Q-switch, we realized 1357 nm pulse laser operation of Nd:LSO, as shown in Figs. 8, 9 and 10. It can be seen from Fig. 4 and Fig. 8, that the 1357 nm AO Q-switched laser had relatively lower average output power than 1075 and 1079 nm dual-wavelength laser. When the pulse repetition rate was set to 5 kHz, the average output power, the slope efficiency, the pulse width, and the pulse energy were measured to be 114 mW, 6.3%, 267.9 ns, and 22.8 μJ, respectively. In the case of 10 kHz, they were 138 mW, 7.2%, 285.2 ns, and 13.8 μJ, respectively.

Fig. 8
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Variation of average output power with absorbed pump power for 1357 nm Nd:LSO AO Q-switched laser

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Fig. 9
figure 9

Variation of pulse width versus absorbed pump power for 1357 nm Nd:LSO AO Q-switched laser. Insert: pulse profile with a width of 267.9 ns

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Fig. 10
figure 10

Variation of pulse energy versus absorbed pump power for 1357 nm Nd:LSO AO Q-switched laser

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5 Conclusions

In conclusion, LD pumped Q-switched Nd:LSO lasers at 4F3/2→4I11/2 and 4F3/2→4I13/2 transitions were reported. A 5 kHz AO Q-switched 1075 and 1079 nm dual-wavelength laser with an average output power of 244 mW, a pulse width of 60.8 ns, and a pulse energy of 48.8 μJ was obtained at an absorbed pump power of 3.86 W. In the case of 1357 nm operation, the maximal CW output power was 234 mW, corresponding to a slope efficiency of 13.8% and an optical conversion efficiency of 6.9%. 267.9 ns AO Q-switched pulse output with an average power of 114 mW was achieved at a pulse repetition rate of 5 kHz. All outputs in these experiments are linearly polarized, which avoids undesirable thermal induced birefringence and is also helpful for further nonlinear optical frequency conversion. If a 811 nm LD is used as the pump source instead of the present 808 nm LD, the absorption peak of Nd:LSO crystal will be aimed at accurately, the absorption efficiency of pump power can be elevated to 80% [11] from the present 35%, and the output powers of 4F3/2→4I11/2 and 4F3/2→4I13/2 transitions, hopefully, are to increase greatly. In that condition, assisting with a more detailed optimization of cavity, better pulse performance of Nd:LSO crystal can be obtained undoubtedly. Nevertheless, present results have shown that Nd:LSO crystal is a very promising laser material. Owning to its special spectral property and good pulse characteristic, Nd:LSO has potential applications in many fields, such as frequency up-conversion, THz generation, fiber communication, pumping of Cr3+:LiSAF, repumping of strontium optical clock, etc.