1. Introduction

Diode-laser (LD) pumped solid state lasers at the wavelength around 1.3 μm have been found to have many applications such as in fiber communication, medical treatment, frequency conversion, and scientific research, etc. [1 –3]. The laser near 1.3 μm coincides well with the low dispersion and low-loss spectrum of silica fiber, which makes it very suitable for fiber communication. A 671-nm laser, through frequency-doubling of a 1.342 μm laser, can be used in laser color displaying and is an effective pumping source for a Cr:LiSAF crystal [2]. The 1.3 μm laser has ten times higher absorption in water and one-third extinction in blood compared to the 1.064 μm laser, so it is an ideal laser source for surgical processes [3]. Many applications of a 1.3 μm laser, especially frequency-conversion, benefit much from a short pulse width with high peak power. The common way to obtain a pulse laser source in the nanosecond (ns) region at 1.3 μm is $Q$-switching in an LD-pumped neodymium-doped crystal laser. Among them, $\mathrm{Nd}\text{:}{\mathrm{YVO}}_4$ is a favorable gain medium due to its large stimulated emission cross section ($6\times {10}^{-19}{\mathrm{cm}}^2$) at 1.342 μm. There have been many reports about passively $Q$-switched lasers at 1.3 μm using a saturable absorber such as V:YAG [4 –6], $\mathrm{Co}\text{:}{\mathrm{LaMgAl}}_{11}{\mathrm{O}}_{19}$ [7,8], or a semiconductor saturable absorber [9,10]. A maximum 1.02 W output power 1.34 μm passively $Q$-switched $\mathrm{Nd}\text{:}{\mathrm{LuYVO}}_4$ laser was demonstrated, and the maximum peak power was 820 W at 22.4 kHz with the pulse width of 21 ns [5]. Lai et al. reported a 360 mW LD-pumped 1.3 μm passively $Q$-switched $\mathrm{Nd}\text{:}{\mathrm{YVO}}_4$ laser with a pulse width of 90 ns and PRR of 770 kHz, corresponding to a peak power of 5.2 W [10]. However, the output power of the above passively $Q$-switched lasers is usually quite low, and it is difficult to control the PRR and the pulse width. As for actively $Q$-switched lasers, Lu et al. demonstrated a 13.7 W acousto-optical (AO) $Q$-switched 1.34 μm $\mathrm{Nd}\text{:}{\mathrm{YVO}}_4$ laser at 100 kHz. The shortest pulse width of their laser was 19 ns at 10 kHz, where the corresponding output power and the peak power were 6.3 W and 30 kW, respectively. The laser beam quality factor $M^2$ was about 2.5 at high output power in their experiment [2]. A 1.6 W AO $Q$-switched $\mathrm{Nd}\text{:}{\mathrm{YVO}}_4$ laser with pulse width as short as 6.5 ns at 10 kHz has been reported by Zhao et al., and the maximum peak power of their laser was 24.3 kW, but the laser beam quality factor was not mentioned [11]. However, the pulse width of an AO $Q$-switched laser is proportional to the cavity length, and reducing the cavity length usually leads to a smaller fundamental mode in the gain medium, which is detrimental for a higher output power with good laser beam quality. Also, when the $Q$-switched lasers are operated at high PRR, the gain for each pulse available is reduced, leading to longer time for the pulse to build up as well as longer pulse width and lower peak power. The efficiency for the frequency conversion will also drop due to the low peak power at high PRR [12].

A way to achieve a high power few ns pulse laser with the pulse width independent of gain and PRR is the cavity dumping (CD) approach using an electro-optic device. The pulse width for a CD laser is determined by the optical length of the cavity, and it has been used in a neodymium-doped crystal laser at 1064 nm [12,13]. Using this method, McDonough et al. demonstrated as high as a 47 W cavity-dumped $Q$-switched ${\mathrm{TEM}}_{00}$ 1064 nm $\mathrm{Nd}\text{:}{\mathrm{YVO}}_4$ oscillator with a pulse width of 6 ns [12]. However, there has been no report about an LD-pumped CD laser at 1.3 μm, to the best of our knowledge.

In this paper, we present an LD in-band pumped high peak power 4.7 ns pulse CD ${\mathrm{TEM}}_{00}$ 1342 nm $\mathrm{Nd}\text{:}{\mathrm{YVO}}_4$ laser for the first time as far as we know. A comparison with the conventional $Q$-switching mode was also done. The maximum average output power of 3.2 W for the CD 1342 nm laser was obtained at an absorbed pump power of 26 W with a PRR of 10 kHz and a pulse width of about 4.7 ns, corresponding to a peak power of 68 kW and an optical-optical efficiency of 12.3%, respectively. The maximum pulse energy of 0.55 mJ was obtained at 2 kHz, the peak power was up to 117 kW. This was the highest peak power of LD-pumped lasers at 1.3 μm with an active device, to the best of our knowledge. The pulse width in CD operation remained as $4.7\pm 0.1\mathrm{ns}$ with the PRR varying from 2 to 10 kHz, while it increased from 101 ns at 2 kHz to 128 ns at 10 kHz in the conventional $Q$-switched mode. As far as we know, this was the shortest pulse width in the ns region with an active $Q$-switch device.

2. Experimental Setup

The experimental scheme is depicted in Fig. 1. An a-cut composite ${\mathrm{YVO}}_4/\mathrm{Nd}\text{:}{\mathrm{YVO}}_4/{\mathrm{YVO}}_4$ crystal with the dimensions of $3\mathrm{mm}\times 3\mathrm{mm}\times 21\mathrm{mm}$ was selected, and both end sides contained a 3-mm long pure ${\mathrm{YVO}}_4$ section that could serve as a heat sink for the gain medium to reduce the thermal effect on the end face of the laser crystal. The 15-mm long section of $\mathrm{Nd}\text{:}{\mathrm{YVO}}_4$ with 0.5% Nd-doped concentration was adopted as the gain medium and more than 90% pump power at 880 nm could be absorbed by it. Both of the end sides of the laser crystal were antireflection (AR) coated at 1342, 880, and 1064 nm, and they were wedge cut at 2°. An 880 nm CW fiber-coupled diode laser (LD, DILAS) with a maximum output power of 150 W was used as the pump source. The core radius and the numerical aperture of the fiber were 200 μm and 0.22, respectively. Compared with 808 nm LD pumping, the 880 nm in-band pumping reduces the quantum defect by 13.5%, which could obviously reduce the thermal effect of the laser crystal, resulting in the improvement of laser performance. The pump beam from the fiber was imaged into the crystal by coupler lenses with a radius of 400 μm in the center of the crystal. The laser crystal and the pump LD were both conduction-cooled with copper heat sinks cooled by circulating water, maintaining the water temperature at 20°C. M1–M3 were all plane mirrors, AR coated at 880 and 1064 nm and high-reflection (HR) coated at 1342 nm. A BBO pockels cell (PC) was adopted as the CD or conventional $Q$-switch due to its low insertion losses, high damage threshold, and low piezoelectric ringing. A high voltage (HV) driver with PRR range from 1 Hz to 10 kHz was used to supply a HV pulse for the PC. A quarter-wave plate (QWP) for 1342 nm was inserted in the cavity near the PC and its axis was oriented at 45° to the $s$ and $p$ polarizations, rotating the incident $s$ polarization 90° in the double passes. The thin film polarizer (TFP) provided high reflectivity for the $s$ polarization and transmittance of 96% for the $p$-polarized light. The cavity was symmetrical, with a length of 521 mm. When the BBO PC was inserted into the cavity, the cavity length was slightly increased to 531 mm to keep the laser mode unchanged in the laser crystal.

 

Fig. 1. Experimental scheme of CD 1342 nm $\mathrm{Nd}\text{:}{\mathrm{YVO}}_4$ laser.

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3. Experimental Results

We firstly studied the laser performance in CD operation mode with the PRR of the HV driver varied from 2 to 10 kHz. Under the absorbed pump power of 26 W, the duration of the HV driver was adjusted carefully at each PRR to provide the highest pulse energy as well as the maximum average output power. Figure 2 shows the average output power and the pulse energy at different PRR for CD operation. As shown in Fig. 2, the average output power continuously increases with the increasing PRR, but it tends to be saturated at high PRR, which attributed to the reduced gain and longer cavity power buildup time so as to produce more cavity loss as the PRR increased [12]. The limited range of the PRR of our HV driver prevented us from increasing the PRR of the laser. During our experiment, a maximum average output power of 3.2 W was obtained at the absorbed pump power of 26 W at 10 kHz, corresponding to the optical-optical efficiency of 12.3%. Also, the CD 1342 nm $\mathrm{Nd}\text{:}{\mathrm{YVO}}_4$ laser delivered a maximum pulse energy of 0.55 mJ at 2 kHz.

 

Fig. 2. Average output power and the pulse energy for CD operation of the 1342 nm laser under an absorbed pump power of 26 W at different PRRs.

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For comparison, we also performed the conventional $Q$-switched operation with the same laser system, in which just M3 was replaced by an output coupler mirror with a transmittance of 10% at 1342 nm. The average output power and pulse energy versus PRR from 2 to 10 kHz for conventional $Q$-switched operation under the same absorbed pump power of 26 W is shown in Fig. 3. The maximum output power was 3.3 W at 10 kHz, corresponding to a pulse energy of 0.33 mJ and an optical-optical efficiency of 12.7%, respectively, and the maximum pulse energy was 0.56 mJ at 2 kHz. Though the trends of the average output power and the single pulse energy for both CD and conventional $Q$-switched operations at different PRR are similar due to the same pockels cell (with the same insertion loss) used in the experiment, their pulse width and peak power are quite different, as described below.

 

Fig. 3. Average output power and pulse energy for conventional $Q$-switched operation of a 1342 nm laser under absorbed pump power of 26 W at different PRRs.

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The pulse width was measured with an ultrafast photodetector (ALPHALAS GMBH, UPD-40-UVIR-P, rise time of $<40\mathrm{ps}$) that connected to a digital oscilloscope (Agilent, DSO 80804B, bandwidth of 8 GHz). Figures 4(a) and 4(b) depict typical pulse waveforms in CD operation and conventional $Q$-switched operation at 10 kHz, respectively. In this case, the CD pulse width was only about 4.7 ns, while it was as long as 128 ns in conventional $Q$-switched operation, which was almost twenty-seven times longer than the former.

 

Fig. 4. Typical oscilloscope trace of a single pulse: (a) CD operation and (b) the conventional $Q$-switched operation.

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The pulse width at different PRRs for CD operation and conventional $Q$-switched operation is shown in Fig. 5. For CD operation, although the duration time for the pulse to build up become longer with the increase of PRR due to the less stored energy for each pulse, the dumped pulse width remained constant at about $4.7\pm 0.1\mathrm{ns}$. For the conventional $Q$-switched operation, the pulse width increased from 101 ns at 2 kHz to 128 ns at 10 kHz, due to the reduced gain available for each pulse to build up at a high repetition rate. It was easy to calculate the peak power of 2.6 kW at 10 kHz and 5.5 kW at 2 kHz for the conventional $Q$-switched mode. The peak power for the CD mode was up to 68 kW at 10 kHz and 117 kW at 2 kHz, respectively, which was twenty-six and twenty-one times higher than that in conventional $Q$-switched mode, respectively. By the way, because the pulse energy and pulse width are dependent on cavity losses, the peak power may increase slightly once the reflectivity of the output mirror is carefully optimized in the conventional $Q$-switch operation, but it would be still much lower than that in the CD operation. Obviously, the CD mode laser gives a great advantage for obtaining a high peak power few ns pulse 1.3 μm laser.

 

Fig. 5. Pulse width versus PRR for CD operation and conventional $Q$-switched operation.

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The beam quality of the 3.2 W CD laser at 10 kHz was measured with a pyroelectric infrared camera (Spiricon, $M^2$-200-12-D Camera serial). Figure 6 shows the 2D and 3D far-field intensity distribution at full laser output power, which exhibits a perfect Gaussian distribution. The beam quality factors were $M_x^2=1.26$ along the tangential direction and $M_y^2=1.23$ along the sagittal direction, respectively, corresponding to an average value of $M^2<1.25$. This indicated that the laser was in ${\mathrm{TEM}}_{00}$ mode.

 

Fig. 6. Intensity distribution of the laser beam with the average output power of 3.2 W in CD operation at 10 kHz: (a) 2D beam profile and (b) 3D beam profile.

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4. Conclusion

In summary, we have demonstrated an in-band pumped high peak power 4.7 ns pulse ${\mathrm{TEM}}_{00}$ 1342 nm $\mathrm{Nd}\text{:}{\mathrm{YVO}}_4$ laser based on the cavity dumping operation. At different PRRs from 2 to 10 kHz, the cavity-dumped pulse width remained constant at $4.7\pm 0.1\mathrm{ns}$. The maximum average output power of 3.2 W was obtained at 10 kHz. The maximum pulse energy was 0.55 mJ at 2 kHz, corresponding to a peak power as high as 117 kW. This was the shortest ns pulse width and the highest peak power for LD-pumped high beam quality 1.3 μm lasers with an active $Q$-switch device, as far as we know. The short pulse width and high peak power will draw much more interest for applications such as frequency-conversion, remote sensing, and micromachining.