A High Peak Power and High Beam Quality Sub-Nanosecond Nd:YVO₄ Laser System at 1 kHz Repetition Rate without SRS Process

Abstract

We present a compact sub-nanosecond diode-end-pumped Nd:YVO₄ laser system running at 1 kHz. A maximum output energy of 65.4 mJ without significant stimulated Raman scattering (SRS) process was obtained with a pulse duration of 600 ps, corresponding to a pulse peak power of 109 MW. Laser pulses from this system had good beam quality, where M² < 1.6, and the excellent signal to noise ratio was more than 42 dB. By frequency doubling with an LBO crystal, 532 nm green light with an average power of 40.5 W and a power stability of 0.28% was achieved. The diode-end-pumped pump power limitation on a high peak power amplifier caused by the SRS process and thermal fracture in bulk Nd:YVO₄ crystal is also analyzed.

1. Introduction

Compact diode-pumped solid state lasers with high-energy sub-nanosecond pulses at kHz repetition rates have a variety of applications, such as laser ranging [1,2], materials processing [3], nonlinear optical conversion [4,5] and Mid-IR optical parametric processes [6,7,8,9,10], etc.

The scaling of energy from sub-nanosecond oscillators is strongly limited by optical damage and thermal effect. Laser radiation with high beam quality, short pulse duration, high repetition rate, and high intensity can be obtained with the Master Oscillator Power Amplifier (MOPA) approach [11,12]. Both Yb³⁺-doped lasers [13] and Nd³⁺-doped lasers [14] can use the MOPA approach to achieve high energy sub-nanosecond pulses near 1 µm. Yb³⁺-doped lasers have wide gain bandwidth, high Stokes efficiency, and high saturation fluence and achieve pulse energy with hundreds of mJ. In Yb³⁺-doped lasers, cryogenic cooling is needed to acquire a four-level system, which will increase complexity of laser system [15,16]. Nd³⁺-doped lasers with four-level systems operating in room temperature have higher gain and simpler structure, making them more attractive.

Several diode-pumped solid-state sub-nanosecond laser systems with different Nd³⁺-doped materials geometry have been reported recently. A Nd³⁺-doped amplifier in bounce geometry has higher small-signal gain to amplify sub-nanosecond oscillator with low energy and high repetition. A sub-nanosecond laser using a side-pumped Nd:YVO₄ bounce amplifier operating at 1–10 kHz was demonstrated with 577 ps duration and 545 µJ energy (1 MW peak power) in 2010 [17]. In a 2011 study, a sub-nanosecond Nd:YAG laser system with bounce geometry has been reported, the output energy of which was 0.8 mJ with peak power of 9.35 MW [18]. However, due to the small volume of the active medium, the pulse energy of a sub-nanosecond laser using bounce geometry is less than 1 mJ. In comparison, Nd³⁺-doped lasers based on bulk crystal may acquire high pulse energy at the kHz level [19,20,21]. A sub-nanosecond single frequency MOPA laser system operating at 500 Hz was reported, which generated pulses with a width of 830 ps, energy of 13 mJ, peak power of 15.7 MW, and M² < 1.4 [22]. A Nd:YAG laser system based on the MOPA approach operating at 300 Hz and using selectable 350–600 ps nominally flat temporal pulse shape has been reported, which generate pulses with 127 mJ energy and maximum peak power of 400 MW [23], but beam quality M² were not measured. As is already known, beam quality always gets worse due to thermally induced birefringence in Nd:YAG crystal [21,24]. Laser systems based on end-pumped bulk Nd:YVO₄ crystal have better beam quality because of natural birefringence architecture, which can alleviate thermally induced birefringence [25,26,27]. At the same time, Nd:YVO₄ is a self-excited Raman scattering crystal [28,29,30]. Stimulated Raman scattering (SRS) is a third-order nonlinear optical effect, which must be taken into consideration in high peak power amplification. Through the SRS process in YVO₄ crystal, 1064 nm fundamental wave will be converted to Raman light, which consists of different Stokes waves, such as first Stokes wave $\lambda$ = 1176 nm. Raman light brings in many disadvantages, for example, Raman light consumes amplified fundamental pulse energy and reduces the amplification efficiency. Morever, Raman light with high peak power may enter into the end-pumped laser diode and leads to damage of the laser diode. Because of this, the SRS process should be averted to improve amplification efficiency and keep optical components safety.

In this paper, we present a Sub-Nanosecond Nd:YVO₄ MOPA system consisting of a master and a series of amplifiers. By suppressing the SRS process during amplification, we obtain 65.4 mJ energy at 1064 nm with 600 ps and peak power above 100 MW at 1 kHz. The M² is less than 1.6 and the signal to noise ratio can be up to 42 dB. We also analyze the influence of SRS on working parameters in a high peak power Nd:YVO₄ amplifier. After frequency doubling, 40.5 mJ 532 nm green light is achieved with conversion efficiency of 61.8% and one hour power stability of 0.28% (RMS).

2. Experiment Setup

Figure 1 shows the layout of the laser system, which comprises one master oscillator, one four-pass high gain pre-amplifier, and three stages of main amplifier.

The master oscillator is a commercial industrial-grade sub-nanosecond Nd:YVO₄ Q-switched laser (Picolo AOT, InnoLas Laser, Munich, Germany), providing 1064 nm light, with 55 µJ, 450 ps at 1 kHz. Pulses from the Nd:YVO₄ Q-switched laser pass through an optical isolator composed of a PBS, an FR, and a HWP to avoid unwanted reflected light or even optical damage to the master oscillator. Before they are injected into the pre-amplifier, laser pulses are aligned and expanded to 2.0 mm with a beam expander (BE1), which suits the pump mode of the pre-amplifier. The pre-amplifier is an off-axis four-pass diode-end-pumped Nd:YVO₄ amplifier, where the Nd:YVO₄ crystal is 4 × 4 × 10 mm³, a-cut, 0.3%-doped, and has double-end-wedged cut at 2°. Both sides of the Nd:YVO₄ have been coated to transmission of 1064 nm and 808 nm. The crystal is end-pumped by a 500 W quasi-CW (QCW) fiber-coupled diode (E15F8S22-808.3-500Q-IS39, DILAS, Mainz, Germany), with pump cycle 100 µs at 808 nm. Light from the pump diode is coupled into an 800 µm core fiber, the output of which is focused into Nd:YVO₄ crystal with a spot diameter of 2.4 mm. A plane–concave lens (Lens1) is used for compensating thermal lens of Nd:YVO₄. To achieve compact architecture, a prism is used to reflect light and decrease off-axis angle to keep uniformity of amplified beam. A TFP, an FR, and a HWP are used as an isolator to output amplified pulses.

The pulses are further amplified in a main amplifier which composes of three stages, as illustrated in Figure 1. The main amplifier with single-pass form has high optical efficiency. In consideration of suppression of SRS effect and thermal fracture in gain medium, each amplifier stage has two identical sub-amplifiers. The gain medium in the main amplifier is 0.2 at.% doped Nd:YVO₄ crystals with the same length of 15 mm and double-end-wedged cut at 2°, but have different apertures: crystals at stage 1 are 5 × 5 mm², crystals at stage 2 are 6 × 6 mm², and crystals at stage 3 are 7 × 7 mm². Both sides of the Nd:YVO₄ have been coated to transmission of 1064 nm and 808 nm. Each sub-amplifier was pumped by 50 mJ, 100 µs (10% duty cycle) pulses provided by a 500W QCW fiber coupled diode (E15F8S22-808.3-500Q-IS39, DILAS, Mainz, Germany) operating at 808 nm. Coupling ratios of the pump beam at each amplifier stage are 1:3.7/1:5/1:5.5. Beam expanders (BE2–BE4) are used to mode matching of different stages for good beam quality. Focal length of plane–concave lens (Lens2–Lens4) used for compensating thermal lens of every stage’s Nd:YVO₄ crystals are 500 mm, 800 mm, and 1000 mm respectively. A 9 × 9 × 20 mm³ type I LBO ($\theta$ = 90°, $\varphi$ = 10.8°) crystal is used for the extra-cavity frequency doubling.

3. Experimental Results and Discussions

3.1. Pre-Amplifier

A regenerative amplifier has also been used as the pre-amplifier. The regenerative amplifier with Nd:YAG as gain medium could suppress the ASE effect through loss of control by the pockels cell, and achieved higher energy of about 10 mJ. However, the time signal to noise ratio after regenerative amplifier was only 23 dB due to leakage pre-pulses from the pockels cell. The time signal to noise ratio after further amplification decreased to only 10 dB, and the pre-pulse energy was amplified to 7.7 mJ, which could consume energy stored in gain medium, as shown in Figure 2. A pulse picker after regenerative amplification based on electro-optic switches can eliminate pre-pulses and improve the time signal to noise ratio to 40 dB [25], but this will increase system complexity. In order to improve time signal to noise ratio and decrease system complexity, we designed an off-axis four-pass diode-end-pumped Nd:YVO₄ amplification architecture as the pre-amplifier, which could provide high gain with excellent time signal to noise ratio. With an input energy of 55 µJ and 28.6 mJ pump energy into the pre-amplifier stage, up to 4.5 mJ pulse energy at 1 kHz was acheived, and the gain of the pre-amplifier stage could reach 81. With further amplification, the time signal to noise ratio after main amplification was still excellent, which was measured by a photodetector (DET10A, Thorlabs), as shown in Figure 3. To increase the dynamic range of the time signal to noise ratio, the photodetector was overexposed to get the pre-pulse amplitude (2 mV), as shown in Figure 3a. Then the main pulse was reduced to 1/100 with a filter. The amplitude of the main pulse was 385 mV within the linear range of the photodetector. The time signal to noise ratio was better than 42 dB.

3.2. Main Amplifier and Suppression of SRS effect

In the Nd:YVO₄ crystal of the main amplifier, the maximum peak power of the amplified pulse can reach about 100 MW, which can easily bring about stimulated Raman scattering (SRS). In the progress of SRS, the Stokes wave of Raman light has power gain $exp\left(G\right)$. The exponent G is given by

$$G=g_{R}IL$$

(1)

where $g_R$ is the Raman gain, I is the amplified fundamental pulse intensity, and L is the length of the nonlinear Raman medium. The first Stokes wavelength beams will appear when G is 25–30 [31]. The Raman gain of $g_R$ of YVO₄ crystal is 16.13 cm/GW [32]. So when

$$IL\le 1.55\mathrm{GW}/\mathrm{cm}$$

(2)

the Raman gain in YVO₄ crystal will be below the threshold of SRS progress, and YVO₄ crystal cannot produce the Stokes wave. In our experiment, a 1064 nm laser with intensity of 2.65 GW/cm² was once injected into a Nd:YVO₄ crystal with a length of 1.5 cm, which brought about the first Stokes wave ($\lambda$ = 1175.84 nm), as shown in Figure 4. The Raman shift of Nd:YVO₄ was 890 cm⁻¹, which was almost the same as that of YVO₄ crystal. After passing through Nd:YVO₄ crystal, the energy loss of fundamental 1064 nm laser reached to 20%. Therefore, the generation and accumulation of Raman light must be prevented in pulse amplification. However, this will affect the working parameters of the Nd:YVO₄ amplifier.

The laser intensity continues to increase as the amplification based on the Nd:YVO₄ amplifier increases. To avoid the SRS process, Equation (2) must be satisfied in a high peak power amplifier.

$${\int }_0^{L}I\left(l\right)dl\le 1.55\mathrm{GW}/\mathrm{cm}$$

(3)

What is more, if

$$I_{max}L\le 1.55\mathrm{GW}/\mathrm{cm}$$

(4)

There will be no significant Raman light. $I_{max}$ is the maximum intensity in the amplification process. So the crystal length L in an amplifier based on Nd:YVO₄ is limited to the maximum amplified intensity for averting the SRS process. At the same time, crystal length L also affects the absorption efficiency of pump light of end-pumped Nd:YVO₄ crystal. To obtain enough absorption efficiency, absorption coefficient $\alpha$ and crystal length L in an end-pumped configuration should satisfy Equation (5) [33].

$$\alpha L=4\sim 5$$

(5)

However, absorption coefficient $\alpha$ is an important factor of crystal thermal fracture. According to Reference [33,34], thermal fracture limits pump power for end-pumped Nd:YVO₄ crystal. During the QCW pump operation, the generally accepted way to avoid thermal fracture is pumping the crystal with an average power less than thermal fracture pump power $P_{av}$, which is less than the fracture pump power $P_{cw}$ of CW pump operation. The maximum pump power at QCW operation is given by

$$P_{av}=\eta P_{cw}=\eta \frac{1}{\alpha }\frac{4\pi R}{\xi }$$

(6)

$\eta$ is the correction factor, which depends on QCW pump duty cycle and repetition, considering a QCW pump duty cycle of 10% in Nd:YVO₄ crystal, $\eta \approx 0.6$ [34]. R is thermal shock parameter depending on the mechanical and thermal properties of the host material. $\xi$ is the fractional thermal loading. When Nd:YVO₄ crystal is pumped with 808 nm, $\frac{4\pi R}{\xi }$ is about 250 W/cm [33]. Based on Equation (6), we can see that thermal fracture pump power $P_{av}$ strongly depends on $\alpha$. Lowering the dopant concentration can decrease the absorption coefficient and increase the fracture-limited pump power. But in high peak power amplifier, suppression of the SRS process and pump light absorption efficiency also limit the min absorption coefficient. With Equations (4)–(6), to avert the SRS process, when operating the QCW pump, the thermal fracture pump power $P_{av}$ depends on the pulse intensity and can be given by:

$$P_{av}\simeq 100\frac{\eta }{I_{max}}$$

(7)

Figure 5 shows the relationship between the maximum pump power and the maximum laser intensity at QCW pump duty cycle of 10% and 1 kHz level for Nd:YVO₄ amplifier, which is described as Equation (7). It can be seen that, the higher amplified intensity is the lower the maximum pump power will be. When the amplified intensity is 1 GW/cm², the maximum pump power approaches 60 W. But when the amplified intensity increases to 4 GW/cm², the maximum pump power decreases to 15 W. At high peak power amplifier, the diode-end-pumped power for bulk Nd:YVO₄ is limited to suppression of the SRS process and thermal fracture of bulk Nd:YVO₄ crystal. Therefore, the amplification ability of an end-pumped Nd:YVO₄ amplifier is limited to the laser intensity.

In the main amplifier, the absorption efficiency of Nd:YVO₄ (0.2%, 1.5 cm) with absorption coefficient was 2.5 cm⁻¹ and could reach 95%. Based on Equation (6), the calculated maximum average pump power was about 60 W at 1 kHz with QCW pump duty cycle of 10%, so each sub-amplifier pumped by 50 mJ pulses can work safely.

Table 1. Parameters of the sub-ns amplification stages.

Stage	Epump [mJ]	Eout [mJ]	Dout [mm]	ηo−o [%]	Imax [GW/cm2]	Lcrystal [cm]	Imax ∗ Lcrystal [GW/cm]
Seed	-	0.055	-	-	-	-	-
Pre-amplifier	28.6	4.5	2.0	16.7	0.32	1.0	0.32
Main amplifier 1	100	22.8	2.6	18.3	0.72	1.5	1.08
Main amplifier 2	100	44.9	3.4	22.1	0.82	1.5	1.23
Main amplifier 3	100	65.4	4.0	20.5	0.87	1.5	1.31

shows parameters of the picosecond amplification stages. With an input energy of 4.5 mJ and a total of 300 mJ pump energy into the main amplifier, we achieved maximum output energy of 65.4 mJ, corresponding to an optical efficiency of 20.3%. The spot size in different stages were gradually increased to ensure the laser intensity was below 1 GW/cm² and reduce the Raman gain. At different amplified stages, $I_{max}*L_{crystal}$ in each stage had the maximum value of 1.31 and below the SRS threshold of 1.55. In addition, catadioptric mirrors between the two amplifiers are anti-reflective coated to reduce the accumulation of Raman light. Via these measures, we achieved tens of mJ energy sub-ns pulse through a Nd:YVO₄ amplifier without the SRS process, as shown in Figure 6. There is no Raman light at a wavelength of 1176 nm (first Stokes wave) and wavelength 1313 nm (second Stokes wave) in the maximum laser output.

Figure 7 shows the spectra of the oscillator and amplifying systems. This sub-nanosecond oscillator had several longitudinal modea. The main longitudinal mode got more amplification, and, after pulse amplification, spectrum bandwidth became narrower. Narrower spectrum caused by gain narrowing would increase pulse width. A change of pulse width after amplification was shown in Figure 8. The pulse from the oscillator and amplifying systems was measured by a 8 GHz analog oscilloscope and an InGaAs photodiode (UPD-70-IR2-P, ALPHALAS). The pulse width of the oscillator is 450 ps. The pulse width of amplifying systems was 600 ps, corresponding to 109 MW pulse peak power.

The beam images of this amplifying system are shown in Figure 9. The beam images were measured with a commercial CCD setting at a position which was about 1 m away from each amplifier. The laser beam of pre-amplifier had good Gaussian distribution. With the increment of pulse energy in the main amplifier, beam center intensity gradually increased due to self-focusing, which could lead to little beam deterioration, as illustrated in Figure 10. The output beam of the oscillator was near diffraction limitation with ${\mathrm{M}}_x^2$ × ${\mathrm{M}}_y^2$ = 1.28 × 1.20. The M² for the horizontal and vertical axes via the amplifying system were 1.50 and 1.57, respectively. The results show the amplifier system still kept good beam quality using diode-end-pumped Nd:YVO₄ amplifier architecture when running at 1 kHz.

A 9 × 9 × 20 mm³ type I LBO ($\theta$ = 90°, $\varphi$ = 10.8°) crystal was used for extra-cavity frequency doubling. Maximum pulse 532 nm green light energy of 40.5 mJ was obtained at the maximum pump energy. The conversion efficiency was 61.8% and corresponding green light power was 40.5 W. The one-hour power stability has thus been tested, and 0.28% RMS was achieved, as shown in Figure 11. Figure 12 shows beam quality of green light were 1.26 and 1.25 for the horizontal and vertical axes. The beam distribution is also showed as the color image insets in Figure 12. The beam quality of green light was better than that of fundamental light, which is because the nonlinear process during frequency doubling can filter out the stray light with low peak power.

3.3. Conclusions

In conclusion, we have demonstrated a compact and efficient sub-nanosecond diode-end-pumped Nd:YVO₄ laser system. An off-axis four-pass diode-end-pumped Nd:YVO₄ amplification architecture was designed as a pre-amplifier. The main amplifier is composed of three stages, which include two single pass end-pumped Nd:YVO₄ sub-amplifiers. A maximum output energy of 65.4 mJ at 1 kHz without the stimulated Raman scattering (SRS) process was obtained with pulse duration of 600 ps, corresponding to 109 MW peak power. Good beam quality was obtained with M² < 1.6, and the excellent signal to noise ratio was more than 42 dB. Using LBO crystal, 40.5 W of 532 nm green light and a 0.28% power stability (RMS) were achieved. We also analyzed the influence of the SRS process on working parameters of diode-end-pumped bulk Nd:YVO₄ amplifier. At the high peak power amplifier, the limitation of diode-end-pumped pump power of bulk Nd:YVO₄ crystal due to suppression of the SRS process and thermal fracture. Our work offers an approach to estimate the maximum amplified ability of end-pumped Nd:YVO₄ crystal for high peak power laser amplification. This end-pumped Nd:YVO₄ sub-nanosecond laser system with high energy and good beam quality has simple and compact structure, which makes this laser an attractive light source for remote laser ranging.