1. Introduction

High average power, narrow pulse width and high beam quality Q-switched diode-pumped solid-state lasers are widely used in nonlinear optics, laser radar, material processing and material preparation, et al [1–3]. Nanosecond pulse of high peak power laser is small damage and high precision in the machining process. At the same time, nanosecond pulse laser is processing efficiency and high speed. And the master oscillator power amplifier (MOPA) structure is one of the most effective technology used in the solid-state lasers to get high power, narrow pulse and high beam quality as well.

In recent year, great effort has been made to develop such lasers. Among nanosecond laser crystal, Nd:YVO₄ crystal is proved to be the most effective. First, it has a large stimulated emission cross section which is five times higher than Nd:YAG. Second, it has low lasing threshold and high slope efficiency. Third, it has a high absorption coefficient combined with high gain which is advantageous for compact structure of lasers. Besides, Nd:YVO₄ is naturally birefringent and laser output is linearly polarized along the extraordinary π-direction. The polarized output has the advantage that it avoids undesirable thermally induced birefringence. Since, K. Du et al. invent the partially end-pumped slab laser (Innoslab) [4], which is also proved to be useful for the slab amplifier. Combining the advantages of Nd:YVO₄ crystal and Innoslab structure, many excellent results are obtained for nanosecond amplifier. In 2007, Zhe Ma et al. reported 0.288mJ of pulse energy at a repetition of 100kHz with a pulse width of 15 ns [5]. The same year, they obtained the pulse energy of 3.8mJ and pulse width of 5 ns at a repetition of 1kHz [6]. But, the major drawback of Nd:YVO₄ is its poor thermo-mechanical properties, which limits its output power and beam quality. In order to overcome the problem, direct-pumped is adopted. Under this technique, the quantum defect can be reduced significantly, and the heat generated in the laser crystal is diminished. In 2015, Xin Zhang et al. obtained the pulse energy of 2.57mJ at a repetition of 30kHz [7]. The same year, Sihan Sang et al. reported 3.07mJ of pulse energy at a repetition of 30kHz [8].

In this paper, we reported a compact multi-fold Nd:YVO₄ slab amplifier with two plane mirrors. 8.4mJ, 10kHz, 3.6ns laser output was achieved under the pump power of 330W, and the corresponding optical-to-optical efficiency was 29.8%. The M² factors in the horizontal direction and vertical direction were 1.48 and 1.39, respectively.

2. Experimental setup and design of the amplifier

The experiment arrangement was shown in Fig. 1. The laser diode stack (Jenoptik SB13028) consisted of six bars with the central wavelength fixed at 880nm by adjusting the cooling water temperature. The emission from each diode laser bar was individually collimated by a micro lens, which was coupled into a beam shaping system. Specifically, the beam shaping system was composed of four cylindrical lenses, a rectangular waveguide and a battery of lenses. The beam shaping led to a pump power loss of ~12%. By the shaping system, a homogeneous pumping line of ~0.4mm × 14mm coupled into the central of Nd:YVO₄ crystal with the size of 14mm × 10mm × 1mm. In a high-power solid-state laser system, laser gain media with low doped levels had better performance than those with high doped levels. The low absorption coefficient of a low-doped-level laser crystal could uniform the distribution of the pumping beam and mitigate the thermal problem, which was very important for a high power diode-pumped solid-state laser. Moreover, it could also tolerate very high incident pump power. Thus, Nd:YVO₄ crystal with a Nd-doped level as low as 0.3 at.% was used as the laser gain medium. The Nd:YVO₄ crystal was a-cut with the c axis along the 14mm direction. Indium foil was used for uniform thermal contact and cooling. The laser crystal was mounted between two water-cooled copper heat sinks with two large faces 14 mm × 10 mm. The short distance between the pumped gain volume and the large cooled mounting surfaces allowed for efficient heat removal. The heat conduction inside the laser crystal was quasi one dimensional in vertical direction, which established a homogeneous cylindrical thermal lens and avoided depolarization by birefringence. The two 14 mm × 1 mm surfaces were polished and antireflection coated for the pump light and the laser light. Parasitic lasing was suppressed by the grinding of the mounting and free surfaces. Temperature of LD stack and laser crystal were controlled by cooling circulating water.

 

Fig. 1 Schematic setup of the Nd:YVO₄ Innoslab amplifier.

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In traditional Innoslab amplifier, a stable-unstable hybrid cavity was used. The hybrid amplifier cavity was a stable one in the vertical direction of the gain volume, the laser mode was reproduced by the thermal lens. In the horizontal direction, no thermal lens shaped the amplified beam, but the amplifier cavity did. The terms stable direction and unstable direction were used synonymously with vertical direction and horizontal direction. As two cylindrical mirrors M₁₁ (R1 was positive) and M₂₂ (R2 was negative) formed an off-axis positive confocal unstable cavity in horizontal direction, the beam expansion by a constant factor or magnification M (M = R1/|R2|>1) at each round trip through the cavity. The increase in beam cross section by the beam expansion in the unstable direction balanced the power increase at amplification and kept the intensity roughly constant. It was a key feature of the concept, because it yielded a constant saturation and therefore efficient operation while simultaneously keeping the intensity evenly away from damage thresholds and allowing for small nonlinearity. The shortcoming of the traditional Innoslab mode-matching was that it needed to replace the cavity mirrors to chang the magnification M. This made alignment was very complicated.

In our experiment, the pulse energy of the Q-switched seed laser was 0.4mJ and the central wavelength was at 1064 nm. The pulse duration was about 3.4ns at the pulse repetition frequency of 10 kHz with the beam quality factors of M²<1.3 in two orthogonal directions. The two cylindrical mirrors were replaced by two plane mirrors M3 and M4 for mode-matching. Two cylindrical telescopes transformed the output beam of the Q-switched seed in horizontal direction and vertical direction, respectively. The beam waists were not overlapped and their separation was adjustable. All of the horizontal cylindrical lenses HCL1, HCL3 and vertical cylindrical lenses VCL2, VCL4 were coated the films antireflection at 1064nm to let the seed beam going through. The 45° plane mirrors M1 and M2 were coated the films high reflection at 1064nm to reflect the seed beam. The input plane mirror M3 was coated the films antireflection at 880nm to let the pump beam going through and coated the films high reflection at 1064nm to reflect the amplified beam. The output plane mirror M4 was coated the films high reflection at 1064nm to reflect the amplified beam and cut at one edge to let the amplified beam go out. M2 was very close to M4. There was a little wedge angle β [9] between M3 and M4 to avoid the parasitic oscillation and to suppress the amplified spontaneous emission (ASE). The cavity length between M3 and M4 was ~20mm.

In the horizontal direction, the seed was shaped by HCL1 (R1 = 250mm) and HCL3 (R3 = 150mm), which made the beam waist W_in-x locate at certain distance (d) away from the edge of M2 (shown in Fig. 1). According to the propagation properties of the Gaussian beam (shown in Fig. 2), the divergence half angle was defined as Eq. (1):

 

Fig. 2 Schematic of unfold amplification in X-Z plane.

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Where λ was the wavelength of the seed. The magnification factor M on every roundtrip was defined as Eq. (2):

Where N was the number of the roundtrips and Lc was the length of the cavity. The red line in Fig. 2 represented the diameter of the amplifier beam in the crystal and the seed beam. The beam radius Wx(4), Wx(5), Wx(6) and Wx(7) were omitted in Fig. 2. The beam radius of the first passage in the crystal W_x(1) was changed by adjusting the distance from W_in-x to the crystal. By adjusting the HCL1 and HCL3, the W_in-x was changed, consequently, the divergence half angle θ was changed, finally, the magnification factor M was changed. In our experiment, the W_in-x = 0.08 mm and the W_x(1) = 0.55mm, therefore, the beam radius Wx(N) and magnification factor M were calculated in Tab. 1. The total length of beam covering the pump line was $2*{\displaystyle \sum _{N=1}^{8}W{}_x(N)}=$ 13.5mm <14mm, which approached the crystal width 14mm, and nearly all the pump area could be used. The increase in beam cross section by beam expansion in the horizontal direction balanced the power increase at amplification, the same as the traditional Innoslab amplifier. The difference was that M was a constant in the traditional Innoslab amplifier, but was a variable in our experiment. Compared with traditional Innoslab amplifier, it had two advantages [10]. One was the magnification factor M could be tuned continuously when the cavity scheme was fixed, which making optimization of the intensity in the crystal very flexible. The other one was with plane cavity mirrors, there was no astigmatism caused by the cavity mirrors during amplification.

Table 1. Related parameters of amplification

View Table

In the vertical direction, the seed was shaped by VCL2 (R2 = 150mm) and VCL4 (R4 = 100mm), which made the beam waist W_in-y locate at the edge of M2 (shown in Fig. 1). Figure 3 showed the equivalent cavity with thermal lens in Y-Z plane. The laser mode was reproduced by the thermal lens. At the pump power of 330W, the focal length of the thermally induced lens was~137mm, and the diameter of laser mode on the M4 was 0.27mm.

 

Fig. 3 Equivalent cavity with thermal lens in Y-Z plane.

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3.Experimental results and discuss

The amplifier’s performance under the pulse repetition frequency of 10kHz was measured with the seed’s pulse energy of 0.4mJ. The measured diameters of the shaped seed laser were 1.01 mm × 0.31 mm at the entrance of the amplifier. After eight passed through the crystal, the beam diameter in the horizontal direction was expanded to ~2.25mm, but remained roughly unchanged in the vertical direction. A maximum output pulse energy 8.4mJ was obtained at the pump power 330W (shown in Fig. 4). Considering 88% of the coupling efficiency and 95% of the absorbed efficiency, the optical-to-optical efficiency was 29.8%. At the maximum pump power, the small signal gain-length product g₀l = 2.3<3 [11] was calculated, which mean there was no ASE. The result was in agreement with the experimental result.

 

Fig. 4 The output pulse energy versus the pump power.

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Figure 5 showed the waveform of the pulse before and after the amplification, and no remarkable change could be observed on the pulse shape.

 

Fig. 5 The pulse waveform of (a) the seed; (b) the amplifier output.

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To measure the beam quality, a CCD camera was used to measure the laser spot widths at different positions behind an f = 350mm spherical lens. As the M² was defined [12] by , where d₀ was the beam diameter of beam waist; z was the distance to the beam waist; d_(z) was the beam diameters at a distance z from the beam waist; $\lambda$ was the wavelength of laser, the squared beam diameters at different positions in both directions at output pulse energy of 8.4mJ were obtained with the CCD, and were fitted as shown in Fig. 6. The M² factors in the horizontal direction and vertical direction were 1.48 and 1.39, respectively. The far-field distribution of the shaped amplified laser was shown in Fig. 7.

 

Fig. 6 The beam qualities of two directions (a)the horizontal direction; (b)the vertical direction.

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Fig. 7 The far-field distribution of the shaped amplified laser.

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4. Conclusion

An efficient multi-fold Nd:YVO₄ Innoslab amplifier was demonstrated. Unlike the traditional Innoslab amplifier, the amplifier cavity comprised of two plane mirrors rather than two cylindrical mirrors. The mode overlapping was discussed in horizontal direction and so as the mode-matching in vertical direction. A maximum pulse energy 8.4mJ with the pulse repetition frequency of 10kHz was achieved at the pump power of 330W. The optical-to-optical efficiency was 29.8%.The M² factors in the horizontal direction and vertical direction were 1.48 and 1.39, respectively. And no remarkable change of the pulse waveform or ASE was observed in the experiment.