1 Introduction

Passively Q-switched solid-state lasers while combined with diode laser pumping are able to deliver laser pulses with high repetition rate, high peak power and moderate average power. Due to their compactness, relatively low cost and ease of use, a lot of attention has been given to the development of this type of lasers.

The laser crystal plays one of the key roles in the development of solid-state lasers. So far, the most commonly used gain materials are Nd3+-doped crystals such as Nd:YAG and Nd:YVO4. Compared with other Nd3+-doped crystals, Nd:KGW has many outstanding properties. It can achieve high Nd3+-doping level which results in low-threshold and high-efficient laser operation at low pump levels [1, 2]. Its wide absorption band alleviates the constraint on the wavelength stability of the pump lasers [3, 4]. Self-stimulated Raman scattering could also be achieved within Nd:KGW such that multi-wavelength operation in the visible range is achievable via frequency multiplication [5–7]. Owing to those characteristics, Nd:KGW is a good candidate for developing compact and efficient lasers.

Many works done with Nd:KGW mainly focus on the development of passively Q-switched lasers [8–12] pumped around 808 nm because of the larger absorption coefficient and wide absorption range (nearly 12 nm in that region) to that pump. However, the maximum average output power and single-pulse energy reported so far for passively Q-switched Nd:KGW laser were limited at 234 mW [8] and 12 μJ [10], respectively, because of the poor thermal and mechanical properties of KGW host. Compared with the pump at 808 nm, a pump source at 877 nm introduces less thermal issue because of its higher quantum defect ratio. Therefore, improved output power could be achieved for the pump at 877 nm of Nd:KGW in comparison with the pump at 808 nm, at similar absorbed pump powers. Even though the absorption coefficient of Nd:KGW at 877 nm is only half of that at 808 nm, a longer crystal could be used to achieve relatively high absorption efficiency. In the meantime, a longer crystal could also reduce the thermal gradient within it. So, it is possible to achieve higher output power for Nd:KGW laser by using 877 nm pump source [13].

In this work, we report on a stable passively Q-switched diode-pumped Nd:KGW/Cr4+:YAG laser operating at pump wavelength of 877 nm. To the best of our knowledge, it is the first time that the passively Q-switched Nd:KGW laser pumped at 877 nm has been demonstrated. We obtain as high as 1.6 W of average output power as well as a maximum pulse energy of 25.3 μJ, which to our knowledge are both the highest achieved for the passively Q-switched Nd:KGW lasers combined with good beam quality (M 2 ≤ 1.22). The dependence of the operational parameters on the pump power is experimentally investigated.

2 Description of experiment

In order to obtain stable Q-switched operation and ensure TEM00 output, we have carefully designed a V-type resonator, which ensures the mode size inside both laser crystal and Cr4+:YAG not to be considerably influenced by the change in thermal lens. The laser cavity is shown in Fig. 1, which consists of three mirrors: a flat end mirror M 1 with high transmittance at 800–920 nm and high reflectivity at 1020–1080 nm, a folded concave mirror M 2 with its radius ρ = 100 mm and high reflectivity at 1067 nm and a flat output coupler M 3 with different output couplings (the transmissions are T = 2, 8 and 15 %, respectively) at lasing wavelength of 1067 nm. The length between M 1 and M 2 is 130 mm, and the length between M 2 and M 3 is 65 mm. Using this resonator, the laser mode size inside the Nd:KGW crystal is about 300 µm in diameter, which matches with the pump size very well.

Fig. 1
figure 1

Experimental setup of passively Q-switched Nd:KGW laser

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Our experiment employs a trapezoid shape Ng-cut Nd:KGW crystal doped with 3 at.% Nd3+ ions with dimensions of 3 × 3×10 mm. Both end faces of the crystal are AR coated at both lasing and pump wavelength and also cut with 1° angle along N m direction in order to prevent parasitic lasing. The laser crystal is then wrapped within an indium foil and mounted into a water-cooled copper block to dissipate most of the heat. A TEC cooled fiber-coupled diode laser (M1F2S22-885.2-40C-SS2.1 made by DILAS Inc.), with 200 μm core diameter and N.A. of 0.22, is used to end pump the laser crystal. The diode emission is centered at 877 nm with spectral bandwidth (FWHM) of <3 nm at operating temperature of 18 °C controlled by TEC. The pump radiation is collimated and refocused into the laser crystal with a 1:1 optical coupler; the beam radius after the coupler is measured to be 130 µm. The saturable absorber Cr4+:YAG with an initial transmission of either T 0 = 98 % or T 0 = 95 % at 1067 nm is placed near the output coupler.

To characterize laser performance, the output power is measured by a FieldMax-TO/PM30 power meter (made by Coherent Inc.) while the pulse temporal behavior is recorded by the combination of a 1 GHz MSO4101 digital oscilloscope (from Tektronix Inc.) and a DET10AM detector (from Thorlabs Inc.). A Lasercam HR CCD (made by Coherent Inc.) is also used to measure the spatial profile.

3 Results and discussion

3.1 CW operation

The comparative experimental investigation of CW lasing performance for this Nd:KGW pumped by 877 and 808 nm diode is made firstly by using the same V-type resonator. Since Nd:KGW has a narrow absorption band around 877 nm, the absorption efficiency for the crystal used here is measured to be around 85 % for 877 nm pump and nearly 100 % for 808 nm pump. A TEC cooled fiber-coupled diode laser (G808-3WF-4L2B-TG made by RealLight Inc., 105 μm fiber core diameter, N.A = 0.22, emitting at 808 nm) is collimated and refocused into the laser crystal with a measured beam radius of 140 µm by a 1:2 optical coupler.

The output power versus absorbed pump power is shown in Fig. 2. The output power increases approximately linearly with the absorbed pump power at 877 nm pump, and the highest slope efficiency is ~51 % at T = 8 %. During the whole range of pump power, polarization of the laser is linear and along with the N m -axis, with the measured polarization ratio to be more than 130:1. At 808 nm pump, the slope efficiency appears at T = 2 % with the maximum value of ~42 %, which approaches the highest efficiency achieved so far [10, 12]. The latter is, however, still lower than the efficiency obtained here at 877 nm pump. It therefore confirms the advantage of using 877 nm pump for the Nd:KGW lasers.

Fig. 2
figure 2

CW output power at 1067 nm versus absorbed pump power for Nd:KGW laser at different output couplings at a 877 nm pump and b 808 nm pump

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The output power of the laser at 808 nm pump begins to drop when the pump power increases to P = 2.95 W for T = 2 %, P = 2.58 W for T = 8 % and P = 2.15 W for T = 15 %, respectively. This is because the cavity becomes unstable in the vertical direction, along which speckles gradually appear in the output beam as increasing the 808 nm pump power. Good beam quality is maintained with the output power up to 2.02 W at 877 nm pump.

Figure 3 shows the obtained focal length for thermal lens in the horizontal (N m ) and vertical (N p ) directions by measuring the divergence angle of the output laser beam at the output coupling mirror (T = 8 %). More serious thermal lens effect is observed at 808 nm pump compared with 877 nm pump. The average increasing speed of the laser mode volume in the laser crystal (the mode volume increased per incident pump power) is calculated to be nearly two times larger at 808 nm pump than at 877 nm pump. This gives one explanation on the lower slop efficiency of 808 nm pump as compared to that of 877 nm pump, which is due to the faster pump-laser mode mismatch for the case of 808 nm pump.

Fig. 3
figure 3

Measurements of the thermal lens at 808 and 877 nm diode pump

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3.2 Q-switched operation

Figure 4 presents the average output power versus incident pump power for different output couplings (T = 2, 8 and 15 %) and saturable absorber initial transmissions (T 0 = 98 % and T 0 = 95 %) with different pump wavelengths (877 and 808 nm). The output power increases almost linearly with the increasing incident pump power at 877 nm pump. A maximum average output power of 1.643 W is achieved for T = 8 % and T 0 = 98 % at the pump power of 5.22 W. It is worth mentioning that it is the maximum output power that has been obtained for the passively Q-switched Nd:KGW laser. The polarization of the Q-switched pulse has been confirmed to be linear and parallel to the Nm-axis. At 808 nm pump, serious thermal lens effect leads to a larger mode inside the saturable absorber, which makes it more difficult for the saturable absorber to be bleached. This explains why the Q-switched pulse cannot be realized at high incident pump power [14]. The main reason for lacking of a curve for T = 15 % and T 0 = 95 % in Fig. 4d is that the high round trip optical loss decreases the intra-cavity energy density.

Fig. 4
figure 4

Laser output power at 1067 nm versus incident pump power for Nd:KGW laser with different output couplers with T 0 = 98 % at a 877 nm pump and b 808 nm pump and with T 0 = 95 % at c 877 nm pump and d 808 nm pump

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The dependence of Q-switched pulse energy and pulse width on the incident pump power at 877 nm pump is illustrated in Fig. 5. The maximum pulse energy of 25.3 μJ is obtained at T 0 = 95 % and T = 8 %, while the frequency repletion rate is 59 kHz and pulse width is 89 ns at pump power of 5.22 W. The maximum deviation of pulse width, measured for 10 laser shots, is about (±) 10 % of the average value. As a comparison, we also achieve maximum pulse energy of 20 µJ with T 0 = 95 %, T = 8 % and incident pump power of 2.15 W at 808 nm pump, which is also the highest pulse energy that has been obtained so far for Nd:KGW passively Q-switched laser at 808 nm pump.

Fig. 5
figure 5

Pulse energy versus incident pump power for Nd:KGW laser with different output couplers with a T 0 = 98 % and c T 0 = 95 % and pulse width versus incident pump power for Nd:KGW laser with different output couplers with b T 0 = 98 % and d T 0 = 95 % at 877 nm pump

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In our experiment, the pulse repetition rate is approximately proportional to the pump power at low pump power level and the saturation is observed at high pump power level (i.e., pump power is larger than 3.5 W at 877 nm pump). The linearly increased average output power and the saturated repetition rate result in the linearly increased pulse energy in Fig. 5, which is the same as the results observed in [10, 12, 15, 16]. Pumping at 877 nm alleviates the thermal deposition inside the gain medium, which guarantees the linear increase in the average output power. Meanwhile, the reason for the saturation of repetition rate at high pump power is due to the bleach of the saturable absorber.

The oscilloscope trace of pulse train from Q-switched Nd:KGW laser at the highest pulse energy at 877 nm pump is given in Fig. 6a. Figure 6b shows the temporal profile of Q-switched pulse with 89 ns pulse width. The delay between the emission of one Q-switched pulse and the next exhibits a jitter of as much as 17 %.

Fig. 6
figure 6

Oscilloscope trace of Q-switched Nd:KGW laser with an output mirror of transmittance T = 8 % and saturable absorber initial transmittance T 0 = 95 % at pump power of 5.22 W. a A train of pulses at 59 kHz; b a typical pulse of 89.0 ns

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The spatial profile of output beam is recorded with Lasercam HR CCD. The output beam is focused with a lens of 500 mm focal length. Its radius along two orthogonal directions (N p and N m directions with reference to Ng-cut Nd:KGW) at the highest pulse energy (E = 25.3 µJ) is measured by CCD. Figure 7 shows the M 2 curve fitted with a hyperbolic function. The inset presents the intensity distribution of the laser beam. Both symmetric distribution of spatial profile and relatively low M 2 value demonstrate that the laser is operating with a nearly Gaussian mode.

Fig. 7
figure 7

Beam quality and spatial beam profile of Q-switched Nd:KGW laser with an output mirror of transmittance T = 8 % at pump power of 5.22 W

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Due to its low heat conductivity (~3 Wm−1 K−1), Nd:KGW is restricted to operate at high pump power, which therefore limits the output power of both CW and Q-switched operation. Up until now, the dependence of the thermal diffusivity, thermal conductivity, thermal linear expansion and refractive indices on temperature for Nd:KGW has not been clearly understood. All of these parameters would definitely influence the stability of Nd:KGW lasers. Thermally induced cracks of Nd:KGW with 808 nm pump were observed in many experiments. A damage threshold of 1 kW cm−2 for 8 at.% Nd3+:KGW was found in Ref. [17]. The experiment in Ref. [10] indicated the fracture at pump power density of 2 kW cm−2 for 4.1 at.% Nd3+:KGW, and Yumashev [18] also observed the fracture at pump power density of 1.7 kW cm−2 for 3 at.% Nd3+:KGW. However, with the use of 877 nm diode laser to pump a 3 at.% Nd3+:KGW, we do not observe strong thermally induced stresses or cracks in the crystal at pump power up to 5.22 W with the corresponding pump power density reaching as high as 16 kW cm−2 (which is calculated by using pump power divided by pump beam area in the crystal). Therefore, diode laser operating at 877 nm could be a very promising pump source for achieving high-power Nd:KGW lasers.

4 Conclusions

A comparative study of CW and passively Q-switched operation of Nd3+:KGW/Cr4+:YAG laser end pumped at 877 nm and 808 nm is demonstrated for the first time. The performance of the laser with three output couplers with different transmittances (T = 2, 8 and 15 %) and two saturable absorbers with different initial transmittances (T 0 = 98 % and 95 %) is experimentally studied. For the optimum case of T = 8 % at 877 nm pump power of 5.22 W, the maximum output power of 1.60 W with the corresponding pulse energy of 16 μJ is achieved, when a saturable absorber with 98 % of initial transmission is used. Highest pulse energy of 25.3 µJ with a corresponding output power of 1.49 W is obtained by employing a saturable absorber with 95 % of initial transmission. The slop efficiency, output power and pulse energy are all the best results that have been obtained for CW and passively Q-switched Nd:KGW lasers. Within the whole range of operation, the laser also exhibits nearly TEM00 output with good beam quality. Our experiments indicate that 877 nm diode laser is a potential pump source for Nd:KGW lasers.