We demonstrate the high-repetition rate acousto-optic Q-switching of a dual-rod Nd:YVO4 laser. Two thermally bonded composite YVO4-Nd:YVO4-YVO4 rod crystals were used as the gain medium to reduce the serious thermal effect. The pump mode and its influence on the laser mode were analyzed, and the effective pump radius was controlled by tuning the spectrum of laser diodes with adjustment on the temperature of laser diodes. Three different configurations of cavity optics were designed and the output performance using these cavities was investigated. In Q-switching operation, 73.2 W TEM00 mode average power at 650 kHz was obtained. The stable Q-switching range was from 80 kHz to 650 kHz with the pulse duration increasing from 17.5 ns to 80 ns. In CW operation, 78 W TEM00 mode and 93 W multi-mode output power was achieved with different cavity designs, corresponding to the optical-optical efficiency of 46.5% and 52.2% respectively.
©2009 Optical Society of America
High-repetition-rate high-power lasers are important devices for various applications, such as industrial processing, scientific research, medical treatment, military applications and so on [1–7]. Q-switched lasers, especially acousto-optically (AO) Q-switched lasers, take the advantage of high repetition rate and high peak power of pulses with stable pulse energy and low temporal jitter.
To achieve the high repetition rate operation in Q-switching, laser materials with large emission cross-section (such as Nd:YVO4, Nd:GdVO4) are chosen, with intense pumping, to get high gain. For high repetition rate Q-switching operation, the grazing incidence lasers with high doped concentration are often used [8–14] in which, however, the output power are always low with relatively poor beam quality. Laser resonators with end-pumped geometry are a common and useful pump configuration to achieve high repetition rate with relatively high power [15–17]. Our group has reported a 2.2 MHz grazing incidence AO Q-switched laser resonator with 10W output , while a 850 kHz AO Q-switched dual-end-pumped laser resonator with 36 W output was also reported .
There are two main ways to get higher power laser output. The master-oscillator-power-amplifier (MOPA) configuration can be used to amplify the high-repetition-rate seed laser to high power level, which, however, leads to a complex and large system. Laser resonator with multiple rods especially two rods in the resonant cavity is another way to get high power output. However, most of the reported results were operated in continuous wave (CW) operation [18–21]. S. Lee reported an AO Q-switched two-rod Nd:YAG laser with 17 W output at 10 kHz, but the optical-optical efficiency was only 3.5% . Y. F. Chen et al demonstrated a dual-rod Nd:YVO4 laser, in which 25 W of average power at a pulse-repetition rate of 100 kHz and 0.9 mJ pulse energy at a pulse-repetition frequency (PRF) of 10 kHz were obtained . N. Hodgson et al.reported a dual-rod Nd:YVO4 periodic resonator with 45 W average power output at the highest repetition of 100 kHz, and the optical-optical efficiency was 45% .
All the Q-switched dual-rod lasers mentioned above were all operated at the repetition rate less than 100 kHz and they all had relatively low average power output. The repetition rate is limited by the relatively poor gain which can be enhanced using high power intense pumping, but the serious thermal effect brought in by intense pumping often yields to multi-mode output instead of TEM00 mode. In order to achieve higher repetition rate, higher gain should be provided and the resonator cavity should be designed for TEM00 output under high power intense pumping. In this work, we reported a high-power high-repetition-rate dual-rod AO Q-switched composite Nd:YVO4 laser. The composite Nd:YVO4 was used to reduce the thermal effect under intense dual end pumping. The effective pump radius and its influence on the laser mode were analyzed. In addition, The output performances under three cavity configurations were investigated, and 73.2 W average power TEM00 mode was achieved at 650 kHz corresponding to the pulse duration of 80 ns. To the best of our knowledge, this is the highest repetition rate achieved in the multi-rod laser resonator. In CW operation, 78 W TEM00 mode and 93 W multi mode output power were achieved with different cavity optics design, corresponding to the optical-optical efficiency of 46.5% and 52.2% respectively.
2. Experimental setup
The experimental setup of the dual-rod AO Q-switched resonator was shown in Fig. 1.
The composite neodymium doped yttrium vanadate (Nd:YVO4) was used as the gain medium. The Nd:YVO4 crystal was an efficient four-level laser crystal when being pumped at 808 nm (4I9/2→4 F5/2) and lasing at 1064 nm (4F3/2→4I9/2). The large stimulated emission cross section of Nd:YVO4 (σ 21=25×10-23m2) which is five times higher than that of Nd:YAG, corresponds to the higher gain which is beneficial to the high repetition rate operation over 100 kHz even if partially offset by the shorter upper-level life time (τf≈90ms), and the large stimulated emission cross section also leads to the faster release of energy storage and faster building up of the pulses to achieve narrower pulse duration. Meanwhile, The relatively wide absorption band around the absorption peak of 808.6 nm yields to efficient absorption. For the high gain under high-repetition-rate Q-switching operation pumping with the high intensity pumping is required, which, however, increases the thermal gradient and the mechanical stress in Nd:YVO4 whose thermal fracture limit is only about 53MPa . It has been shown that the pump power fracture limit of Nd:YVO4 is inversely proportional to the doping concentration . Therefore, an a-cut low doping concentration of 0.3 at.% Nd:YVO4 was used to achieve a relatively uniform absorption and reduce the thermal gradient and the mechanical stress to prevent thermal fracture, while the length of the doped crystal should be relatively increased for absorbing completely. The dimension of the doped bulk crystal was 3×3×16 mm3. Meanwhile, thermally bonded composite crystal was used to reduce the thermal gradients and the mechanical stress [27–28], and thus reduce the serious end effect caused by the thermal deformation of the crystal ends in end pumped geometry. The two undoped YVO4 caps were with dimension of 3×3×2 mm3. Both end faces of the composite crystal were dichroic AR coated at 1064 nm and 808 nm with transmissivity higher than 99.9%and 98% respectively. The composite Nd:YVO4 crystal was wrapped with 0.2 mm indium foil and placed in copper heat sinks using water-cooling.
The dual-end pumped geometry was chosen to enhance the pump power. The fiber-coupled laser diode module of Jenoptik JOLD-45-CPXF-1L was used in the experiment as the pump source. The coupling fiber of the LD module was with numerical aperture of 0.22 and core diameter of 400 µm. A aberration-free lens group was used to image the pump mode from the fiber onto the Nd:YVO4 crystal, and the radius of the pump waist spot in the crystal was about ω 0=0.4mmcorresponding to themagnification of the coupling system of 1:2. The temperature of the laser diode was controlled by the thermoelectric cooling module (TECM), and the center wavelength of the laser diode could be temperature-tuned by adjusting the temperature of the LD with TECM. The optical spectrum of LD was monitored by a Agilent 86140B series optical spectrum analyzer (OSA) with the resolution of 0.06 nm. All the pumping-coupling modules were with the identical configuration. The dichroic mirrors were AR coated at 808 nm and HR coated at 1064 nm for lasers at the incidence angle of 45°with the transmissivity more than 98% and reflectivity higher than 99.9% respectively. The NEOS 33041-20-1.5-I crystal quartz acousto-optic Q switch module was inserted into the resonator cavity between the two bulk crystals for intracavity Q-switching. The radio frequency power of the Q switch was more than 20 W at the radio frequency of 40.68MHz, and the loss modulation was higher than 95% when the polarization of the laser was perpendicular to the direction of propagation of the acoustic wave in the Q-switch. The external trigger signal at high repetition rate for the Q-switch drive was provided by a high precision digital signal generator. A symmetric planar-planar resonator was used, and the output coupler (OC) had the transmissivity of 60%.
The output power of the oscillator was measured by the OPHIR FL250A-LP1-DIF power meter, and a Spiricon M2-200s laser beam analyzer was used to measure the beam quality. In Q-switching operation, the pulse signal was detected with a high-speed silicon photoelectric detector as well as an Agilent infinium oscilloscope with 1.5 GHz bandwidth.
The configuration of the cavity optics and partial results were listed in Table 1.
3. Experimental results and discussion
The mode size of the oscillator can be calculated with the ABCD Matrix method. The gain medium can be taken as a thermal lens with focal length of 
In Eq. (1) the thermal expansion term is neglected because of the using of the undoped end-caps can make the end-face curvature negligible. Kc=5.5 Wm-1K-1 is the average thermal conductivity where the anisotropy is neglected since the difference between thermal conductivity along a axis and c axis is relatively small [9, 30]. dn/dT=3.9×10-6 K-1 is the refractive index temperature coefficient, ξ=0.28 is the fractional thermal loading which, compared to the general quantum defect coefficient as ξ=0.24, takes into consideration the additional heat generation induced by non-radiative decay, for the case of high intensity pumping and Q-switching. P0 is the pump power, ωeff is the effective radius of the pump mode in the crystal, η=1-exp(αeffL), where L is the length of the doped-crystal, and αeff is the effective absorption coefficient which is an averaged absorption coefficient measured in the experiment, considering the average of the both absorption coefficients along a axis and c axis, and also averaged on the absorption of the emitting spectrum of LD.
The effective radius of pump mode ωeff is a very important parameter for the mode size calculation. Figure 2 shows the pump model of the dual-end-pumped geometry. For the fiber coupled end-pumped laser, the pump distribution in the x-y plane is an N-order super-Gaussian function in radial, and the pump power is absorbed in an exponential function along the longitudinal direction z. Therefore, the pump intensity distribution in the active medium can be represented as Eq. (2), when the crystal is only pumped from left
where ωp(z) is the radius of the pump mode at position z, and N is the order of super-Gaussian function which was measured as N~4 in our experiment using a knife-edgemethod. αeff is the effective absorption coefficient of the pump laser in the active medium, and the constant C 0 was determined by Eq. (3)
Suppose the waist radius of the pump mode is ω p0 and the waist is located at a distance from the end face of the Nd:YVO4 crystal of z 0, i.e., ω p0=ωp(z 0). The radius of pump mode at location z can be represented as
In Eq. (4) θp is the the far-field divergence (half angle) of the pump mode in the crystal.
For the symmetric dual-end pumped geometry, the pump intensity distribution in the active medium can be represented as Eq. (5),
where I←P (x,y,z) is pump intensity in the crystal when only beiing pumped from right with a similar expression as I→P (x,y,z). The absorbed pump distribution per unit volume in the active medium can be represented as Eq. (6),
Then the effective radius of pump mode in the crystal can be defined as Eq. (7),
From Eq. (2) and (4), we can see that the ωeff can be adjusted by the z 0 or αeff for a fixed pump source and fixed pump coupler. Figure 3 shows that ωeff varying with z0 under different αeff. Because of the relatively wide absorption band, we can adjust the temperature of the LD to tune the pump spectrum, in other words, can adjust the effective absorption coefficient to get different ωeff. Since the temperature of the LD can be shifted continuously, the ωeff can be adjusted continuously and conveniently. The wavelength tuning coefficient of the LD was about 0.3 nm/°Cas measured. We measured the effective pump wavelength and the corresponding absorption coefficient varying with the LD temperature at the full output power of 45 W, as shown in Fig. 4, where the effective pump wavelength is defined by λeff=∫ρ(λ′)(λ′)dλ′/∫ρ(λ′)dλ′, and where ρ(λ′) is the power spectral density of the LD. The effective pump wavelength was measured with an optical spectrum analyzer and an integrating-sphere photometer.
The configurations of the three cavity optics were listed in Table 1. The temperature of the LD was controlled at 9.5 °C for C1, 12 °C for C2 and 15 °C for C3, corresponding to the calculated effective pump radius of 0.4 mm, 0.45 mm and 0.5 mm respectively. Figure 5 shows the calculated mode size of TEM00 mode varying with the pump power under the three configurations of cavity optics, and the gray region shows the work region of the laser resonator. The very weak thermal effect of Q switch was not considered in the calculation despite of its existence in fact. Comparing with the focal length of thermal lens in the gain material of about 90 mm (P 0=90 W and ωeff=0.5 mm), the thermal lens in the Q-switch with focal length over 1 m can be neglected It should be noted that, unless otherwise stated, the pump power mentioned is the total pump power, and all the four LD have the output power of the same value.
For the high power pump laser, it is worthwhile to mention that the size ratio ωl/ωeff between signal size and pump size exerts a significant influence on the beam quality, since the phase aberration always occurs in the wing of the pump region where temperature-gradient-induced thermal stress are higher. Therefore, in high power pumped laser, the waist radius of the pump beam should be larger than the radius of laser mode in the crystal to prevent this aberration, which would degrade the beam quality . But the mode size of the pump beam should be appropriate, since if the mode size of the pump beam is much larger than that of the laser mode, the higher-order laser mode can get enough gain for oscillation in the cavity. This will introduce in the mode competition and yields to a mixture of the TEM00 mode and higher-order mode or even higher order mode output. For the fiber-coupled pumped laser, the pump mode is circularly symmetrical, and the higher-order cavity modes can be described as Laguerre-Gaussian mode TEM0n approximately, and always has n=1 since the mode with n>1 corresponds to larger diffraction loss (geometric- and thermally induced- diffraction loss) and poorer gain which can not oscillate. The mode radius ratio of Laguerre-Gaussian mode between TEM00 mode and TEMmn mode is
Therefore, from Eq. (9), we have
In order to get a TEM00 mode output, with Eq. (10), the effective radius of the pump radius should satisfy the following condition
For the design of high beam quality oscillator, the size ratio of ωl/ωeff was always designed between 0.7–0.8 to afford enough gain and prevent thermal aberration for the TEM00 mode, suppressing the oscillation of the high order mode, especially the TEM01 mode.
In the cavity optics of C1 and C2, the condition of Eq. (11) can be approximately satisfied, and the near-diffracted-limit laser output was obtained. Figure 6 shows the measurement result of the beam quality (Fig. 6(a)) and spatial form of the laser on far field (Fig. 6(b)) in C2 at the work region. In the experiment, by tuning the temperature of the laser diodes (and hence the effective pump wavelength), the effective radius of the absorbed pump distribution in the gain material could be adjusted. This would in turn affect the beam quality both directly and indirectly: directly via the mode overlap; and indirectly by changing the thermal lens focal length and hence the cavity mode radius. Figure 7 shows the beam quality of C2 varying with tuning the temperature of the laser diodes. For C3, the radius of the TEM00 mode was about 0.22 nm, and the effective radius of the pump mode was about 0.5 mm which is still larger than that of the TEM01 mode, then the TEM00 mode and TEM01 mode can both oscillate, and the degenerate mode output was obtained as well (M2=2.9@C3, see Fig. 9(b)). Figure 8 shows the output power varying with the pump power. 78.5 W and 75 W near TEM00 mode (M2=1.35@C1 & M2=1.38@C2) were obtained in the continuous wave (CW) operation for C1 and C2 respectively, corresponding to the optical-optical efficiency of 46.5% and 44.3%. Comparing with C2, C1 had better thermal stability since the work region of C1 was located at the center of the second stable region. 93 W multi mode output was obtained in C3, corresponding to the optical-optical efficiency and slope efficiency of 52.2% and 55.8%. The measured beam quality of high-repletion-rate Q-switching operation was the same as that of the CW operation, which is because that, for the cavities design, the work regions all location at or near the center of the second stable range, in other words, the laser cavity was with good thermal stability, and difference of the heat loading fraction between CW and high-repletion-rate Q-switching is so slightly that can not affect the beam quality.
Figure 9 shows the pulse overlapping in Q-switching operation for TEM00 mode(Fig. 9(a)) and multimode(Fig. 9(b)), from which we can see that multi-mode operation can seriously affect the stability of the Q-switching operation at high repetition rate. This is because for multimode operation, the population inversion is allocated to every order of transverse mode, and when operated at high repetition rate the gain allocated to every order of transverse mode is no longer high enough to maintain the stable Q-switching operation, thus the gain competition between transversemodeswill result in the uncertain building-up time of pulse. Moreover, the loss modulation introduce by the Q switching will induce the transverse mode competition which leads to the unstable transverse modes output, thus intensify the instability of Q-switching. Thus in turn, the Q-switching operation under multi-mode at high repetition rate is with serous temporal jitter. Figure 10 shows the output power and pulse duration varying with the PRF for C1 and C2 respectively. Standard deviation jitters were used to describe the pulse instability in this paper, and the instability of pulse energy was shown in Table 1 at different PRFs and different cavity designs. The stable range of Q-switching for C1 is from 30 kHz to 350 kHz, and for C2 this range is from 80 kHz to 650 kHz, which is due to the larger mode volume in C2 compared with that in C1 corresponds to higher gain which is beneficial to the high repetition rate operation. In C1, the average output increased from 56 W to 76.2 W while the PRF increased from 30 kHz to 350 kHz. Figure 11 shows the pulse waveform at 80 kHz (Fig. 11(a)), 650 kHz (Fig. 11(b)) and 700 kHz (Fig. 11(c)) in C2. At 80 kHz, the sub-pulses was observed associated with the main pulse because of the overly high gain at low PRF. In contrast, missing pulses were observed at 700kHz, because of the lack of gain at very high repetition rate operation. The pulse duration increased from 17.5 ns (FWHM) to 80 ns when the PRF increased from 80 kHz to 650 kHz, and the output power was increased from 65.5Wto 73.2W. The pulse energy was as high as 1.6 mJ at 30 kHz for C1 and 0.82 mJ at 80 kHz for C2, corresponding to the pulse peak power as high as 114.3 kW and 46.7 kW, respectively. The pulse instability was summarized in Table 1, in which the Max. PRF refers to the highest PRF can be achieved with stable Q-switching operation.
In summary, we have demonstrated a high-repetition-rate dual-rod acousto-optics Q-switched composite Nd:YVO4 laser. The laser cavity was formed by a planar-planar symmetric cavity with two composite 0.3-at.% Nd:YVO4 bulk crystals used as the gain medium to avoid thermally induced fracture and reduce the serious thermal effect. We analyzed the influence of the LD temperature-spectrum on the pump mode in the crystal, and three different cavities were designed and the output performance under these cavities were also investigated. Two of the cavity configurations were with the TEM00 mode output with output power of 78.5 and 75 W CW output. In Q-switching operation, the stable PRF range for the above two cavities was 30 kHz–350 kHz and 80 kHz–650 kHz respectively. In the third cavity configuration, 93 W multi-mode average power was achieved corresponding with the optical-optical efficiency of 52.2%.
The research was supported by the National Natural Science Foundation of China (No. 50721004 and No. 60978032).
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