A high power, dual-end-pumped Nd:YVO4 laser with a MOPA configuration was stably Q-switched at repetition rate up to 500 kHz. The thermally bonded Nd:YVO4 crystal was used in our experiment. In acousto-optically Q-switching operation at repetition rate of 500 kHz, 35 W average power was produced by the oscillator, with optical-optical efficiency of 41.7%. 108 W average power was obtained by a master oscillator power amplifier (MOPA) configuration including two amplifier stages, corresponding to the total optical-optical efficiency of 42.2%. The pulse duration was 48 ns, with a stability of pulse peak value <2.5%. The beam quality was better than two-times diffraction-limit (M 2 x=1.99, M 2 y=1.76).
© 2008 Optical Society of America
High repetition rate diode-pumped solid state lasers (DPSSL) are attractive in a variety of applications, such as material processing, remote sensing, laser radar, medicine and so on [1, 2]. The high repetition rate (>100 kHz) operating can be achieved by Q-switching, especially actively Q-switching for its stable pulse energy and low temporal jitter at high repetition rates. However, the pulse duration becomes excessively lengthened for materials with long upper-state lifetime, like Nd:YAG and Nd:YLF. Nd:YVO4 is an important laser crystal owing to its large stimulated emission cross section , wide pumping wavelength bandwidth  and short upper-state life-time . Therefore, Nd:YVO4 is widely used for high repetition rate Q-switched operating in DPSSL.
In 2003, García-López et al. reported a 500 kHz acousto-optics (AO) Q-switching slab Nd:YVO4 laser with a grazing incidence cavity geometry. The average power is 16.4 W and the pulse duration is 30 ns . In 2005, Minassian et al. demonstrated a 400 kHz AO Q-switching slab Nd:GdVO4 laser with averge power of 101 W based on a diode-side pumped bounce MOPA geometry, and the pulse duration is 20 ns [7, 8]. In 2006, Omatsu et al. configured a Nd doped mixed gadolinium yttrium vanadate bounce laser at repetition rate of 650 kHz with a pulse-width of less than 40 ns, and the average power is 17 W . The stability of pulse in Ref. – are not reported. In 2007, F. He et al. reported a 1 MHz AO Q-switching slab Nd:YVO4 laser with a grazing incidence geometry, and the stability of pulse peak value <15% (RMS) . Y. Wang et al. demonstrated a 1 MHz repetition rate low power gain-switched Nd:YAG microchip laser with 32 ns pulse duration, and the interpulse timing jitter and peak-peak instability were measured to be around 5% and 6% .
The above-mentioned high repetition rate AO Q-switching lasers were all operated with grazing incidence slab configuration. In this work, a dual-end-pumped configuration with fiber-coupled diode was configured, since with this pumped geometry, high degree of spatial overlap between pump volume and laser mode can be offered, and high pump power intensity can be obtained for high repetition operation. The thermally bonded Nd:YVO4 crystals were used to reduce the thermal effect. An AO Q-switching oscillator at repetition rate up to 500 kHz was configured, and the MOPA configuration of two amplifier stages was used. 108 W average power, 500 kHz stable pulse output was obtained, to our knowledge which is the highest average power stable pulse output with repetition rate up to 500 kHz of AO Q-switching.
2. Experimental setup
The experimental arrangement of the Q-switching Nd:YVO4 laser with the MOPA configuration is shown in Fig. 1. The MOPA configuration was built up with the oscillator stage and two amplifier stages.
The thermally bonded Nd:YVO4 crystals were used to reduce thermal effect of high intensity pumping. The bonded Nd:YVO4 crystal consisted of two un-doped YVO4 end caps and a a-cut, 16 mm long, <0.5 at. % doped Nd:YVO4 laser crystal. This scheme of using end cap and low doping concentration can reduce the thermal loading density. This in turn, reduces the likelihood of thermal fracture in the crystals for high intensity pumping and leads to a reduction in the loss of pump power due to energy-transfer upconversion (ETU).The laser crystal was wrapped with indium foil and placed in copper heat sinks. Both surfaces of the boding crystal were coated with AR films at 808 nm and 1064.3 nm with the reflectivity of 1064 nm smaller than 0.1%.
The dual-end-pumped configuration was used. The diode laser modules of Jenoptik JOLD-45-CPXF-1L by fiber-coupled were applied as the pumping source in the experiment. The coupling fiber was with a core diameter of 400µm and a numerical aperture (NA) of 0.22. The temperature of the laser diode was controlled by temperature control unit to obtain the best absorption. The pump laser at 808 nm was focused into the crystal by a coupling-lenses system. The mode matching coefficient can be defined by 
where ζp and ςl are the normalized intensity distribution of pump mode and cavity mode respectively. Optimal pumping mode with highest mode matching coefficient can be obtained by adjusting the imaging ratio of the coupling-lenses and the location of the pumping waist spot in the crystal. The radial distribution of the pump mode was measured as a 4-order super-Gaussian function, with the focused spot radius of ~400µm. The pump spot was focused into the doped Nd:YVO4 about 2.3 mm, corresponding to a mode matching coefficient of ~90% provided from Eq. (1). The dichroic mirrors were antireflection coated at 808 nm and high reflection coated at 1064nm for lights at the incidence angle of 45 degree. An acousto-optic Q-switch of quartz crystal was used for intracavity Q-switching operation.
A planar-planar cavity was used in the oscillator, and the transmissivity of the output-coupler (OC) mirror is 38%. The cavity length of L 1 and L 2 is very important. To obtain stable pulse with narrow pulse width at high repetition rate, the high pumping power density is essential, which, however, induces heavy thermal lensing effect for a relatively high temperature dependence of refractive index (dn/dT≈8.5×10-6 K -1). Therefore, the thermal-stabilized oscillator was optimally designed with appropriate cavity length. With the theory of resonant cavity , the cavity length of L 1=115 mm and L 2=85 mm were optimized. The laser mode size varying with pump power is shown in Fig. 2, from which we can see that the oscillator cavity worked in the thermal-stabilized state with the full pump power of 84 W.
Two single-pass amplifier stages were used for power amplification. The pumping/crystal modules in amplifier stages were all the same as that of the oscillator. In order to obtain a simple and compact configuration, the isolator and coupler were not used. The thermal lens of the 1-st amplifier maps the spot sizes of signal beams from the crystal of oscillator to the crystal of the 2-nd amplifier. The mode matching between pump volume and laser mode in amplifiers can be realized with optimal length of L 12=24 cm and L 23=28 cm.
3. Experimental results and discussion
The Nd:YVO4 laser was AO Q-switched at repetition rate up to 500 kHz. Figure 3 shows the output power and extraction efficiency varying with pump power. The output power of the oscillator was as high as 35 W, corresponding to the optical-optical efficiency of 41.7%. The extraction efficiency of the first single-pass amplifier (Amplifier 1) was 34.1% compared with the extraction efficiency as high as 51.2% of the second single-pass amplifier (Amplifier 2), since higher extraction efficiency can be obtained with higher signal laser intensity. The total output power was 108 W, corresponding to the total optical-optical efficiency of 42.2% with 256 W total pumping power. The stability of the average power was less than ±0.5%.
The beam quality factors were measured with a Spiricon M2-200 laser beam analyzer. The M 2 factors of the oscillator were measured as M 2 x=1.55, M 2 y=1.60. Associated with the thermally induced high-order phase aberration terms after being amplified by the two amplifier stages , the M 2 factors degenerated as M 2 x=1.99, M 2 y=1.76 (see Fig. 4), i.e., the beam quality was better than 2-times diffraction-limit. Figure 5 shows the spatial distribution of laser intensity on far-field.
At the repetition rate of 500 kHz, the pulse duration of the oscillator was 51 ns (FWHM). After the amplification of two amplifier stages, the pulse duration of the output laser was 48 ns (FWHM), with a stability of pulse peak value <2.5% (RMS), compared with the pulse stability of <6.5% and <3.8% in the oscillator and the first amplifier respectively. The energy of each pulse was 216µJ with pulse peak power of 4.5 kW. Figure 6 shows the oscilloscope traces of the stable pulse series.
When the repetition rate was enhanced up to 550 kHz, the stability of pulse peak value became greater than 15% (RMS). Further increasing the repetition rate up to 600 kHz, the phenomenon of pulses missing occurred, which is due to insufficient gain for repetition rate up to 550 kHz.
In CW operation, 117Wstable power output was obtained corresponding to the total optical-optical efficiency of 45.7%.
We have presented a 108 W, 500 kHz AO Q-switching Nd:YVO4 laser with the MOPA configuration, with the pulse duration of less than 50 ns, pulse peak power of 4.5 kW, and the stability of pulse peak value of less than 2.5%. By applying the bonded Nd:YVO4 crystals and optimal pumping mode, the global optical-optical efficiency was 42.2%, and the beam quality was better than 2-times diffraction-limit. A compact and simple MOPA laser was obtained without isolators and couplers. Higher repetition rate and higher optical-optical efficiency can be obtained applying with higher pumping power density or with appropriately larger doping concentration crystal.
The research was supported in part by the National Natural Science Foundation of China (Grant No. 60778014), and the Program for New Century Excellent Talents in University.
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