1. Introduction

Solid-state lasers can be passively mode-locked by means of a saturable absorber, which allows the generation of ultra-short pulses close to the limit set by the gain-bandwidth of the active material. However, because of the long upper-state lifetime of the active medium, the laser can be driven into the regime of simultaneous passive Q-switching and mode-locking (QML), in which the mode-locked pulses lie underneath a Q-switched envelope with a typical repetition rate in the kilohertz range [1]. The latter situation is usually undesired because of the highly unstable behavior of the pulse train, which is affected by large amplitude and repetition rate fluctuations. Nevertheless QML provides a simple way to increase by orders of magnitude the pulse energy in comparison to the continuous wave (cw) mode-locking regime. This opportunity can be conveniently exploited by incorporating in the cavity an acousto-optic Q-switch (AOQS) operating at frequencies of several kilohertz. This device being active in nature can provide the stability in QML pulses, the repetition rate of which is fixed by an external modulating signal source. Moreover the use of a CW pump allows a longer pulse build-up time than a pulsed pump because of a lower gain in the active medium. It is thus possible to exploit in an effective way the pulse-shortening process provided by the saturable absorber during several round-trips in the laser cavity and obtain pulses having a duration which can be as short as in the pure CW mode-locking regime. Specially in case of nonlinear mirror (NLM) mode-locked laser with Nd:YVO4 as active medium, where the output power scaling is limited by the laser crystal parameters and the stability of the NLM passive mode-locking with high intracavity average power, active Q-switching by an acousto-optic modulator can lead to a stable QML regime of operation with considerable enhancement in peak power. This technology can find application in many fields like nonlinear optics or materials micro structuring.

Here we report the first demonstration of an efficient and stable multi-kHz repetition rate Nd:YVO₄ laser, actively Q-switched by an acousto-optic modulator and passively mode-locked by NLM. The NLM [2–4] as a passive mode-locker has some distinct advantages over other usual mode-locking techniques because of its simplicity, robustness, and faster response. Furthermore, it can be exploited for laser mode-locking in a wide spectral range. In NLM systems, a frequency doubling nonlinear crystal (NLC) is incorporated in the laser cavity and placed near a dichroic mirror used in place of the usual output coupler. The dichroic mirror partially reflects the fundamental wave (FW) but totally reflects the second harmonic (SH) beam. The FW generates SH in its first pass and if the SH beam experiences a proper phase shift with respect to the FW beam, the SH power is almost totally reconverted into FW during the second pass through the NLC. As the second harmonic generation (SHG) is a second order nonlinear optical process, the NLC along with the dichroic mirror behaves as a NLM having an intensity-dependent reflection coefficient. Under this condition the laser losses decrease with an increase in the peak power of the FW beam and so the behavior is the one of a fast saturable absorber. Although the loss modulation required for mode-locking is maximum for the perfect phase-matching between the FW and the SH wave in the SH generating crystal, it can not ascertain the stability of the cw mode-locking. Earlier we have observed that a little bit of wave vector mismatch between the FW and SH beam in the SH generating crystal can lead to a stable cw mode-locking [5]. NLM mode-locking has some distinct advantages over Kerr lens or additive pulse mode-locking [6,7]: it has sufficient loss modulation at lower power and it does not require interferometric control of cavity length. NLM operation assisted by an active mode-locker has been observed to obtain stable ≈25-ps pulses with ≈9-µJ energy in quasi-CW diode-pumped Nd:YAG and Nd:YAP lasers [8]. Owing to the relatively large gain and the fast build-up time, shorter pulse durations were not allowed, even if NLM mode-locked Nd:YAG lasers can generate pulses as short as 10 ps [4]. Mani et al reported cw NLM operation along with an active mode-locker to obtain pulse width of 12ps and pulse energy of 7-µJ in a flash lamp pumped Nd:YAG laser [9]. Here we use the NLM for mode-locking a Nd:YVO₄ laser pumped by a cw fiber-coupled diode array, and exploit the AOQS to operate the system in the QML regime for increasing the pulse energy as well as to stabilize the system significantly. In fact, using CW-pumping and AOQS repetition rates higher than the inverse of the laser transition fluorescence lifetime, i.e., >10 kHz, the laser gain is conveniently reduced, allowing a slower build-up time during which pulses nearly as short as in pure CW NLM operation [8] can be generated.

2. Experiment

The schematic of the laser configuration is shown in Fig. 1. A 4×4×8 mm³, a-cut, Nd:YVO₄ crystal, having Nd³⁺ concentration of 0.5%, is continuously end-pumped at 808 nm by a fiber-coupled laser diode array (Bright Solutions, Italy) of maximum output power 15 W. The pump beam coming out from the fiber is imaged on the crystal through the rear mirror (RM) using two lenses of focal length 15 mm and 12 mm. The rear face of the RM is anti-reflection coated at 808 nm and has high reflectivity (>99.5%) at 1064 nm on the other side. The Nd:YVO₄ crystal is anti-reflection coated on both its parallel faces for wavelengths 1064 nm and 808 nm and is tilted by an angle of 2° to reduce the effect of undesired reflections inside the cavity. Two concave mirrors M₁ and M₂ of radius of curvature 500 mm and 250 mm respectively are used to focus the beam onto the NLC, placed before the output coupler, which has a reflectivity of ≈100% at 532 nm and 78 % at 1064 nm. A 15 mm-long type-I (θ=90.0° and φ=11.6°) cut LiB₃O₅ (LBO) (Eksma, Lithuania) crystal, anti-reflection coated on both faces at the wavelengths of 1064 nm and 532 nm, is placed very close to the dichroic output coupler. An AOQS (Neos Technologies, USA) with faces cut for Brewster angle at 1064 nm, is inserted in the cavity. The AOQS is driven by a radio frequency signal of 27.2 MHz with a modulation in the frequency range 0-50 kHz. First the laser is optimized for stable Q-switched operation and then the LBO crystal is inserted to get the passive mode-locking. This stable operation depends on the cavity configuration, the pump power, the Q-switch modulation frequency and the nonlinear crystal orientation. The starting laser resonator (including thermal lensing) was configured near the middle of the stability diagram, for TEM₀₀ operation with maximum output power. Then we looked for the optimum Q-switching repetition rate, and also slightly adjusted the NLM crystal orientation, its position and the length of cavity arm that contains the NLC, in order to optimize the stability of QML operation. The different arms of the Z-shaped cavity of length 81.7 cm are optimized to be 250 mm, 405 mm and 162 mm respectively for stable operation.

 

Fig. 1. Laser layout: LDA, laser diode array; L₁ and L₂ focusing lenses; RM, rear mirror; AOQS, acousto-optic Q-switch; M₁ and M₂, folding mirrors; NLC, nonlinear crystal; DOC, dichroic output coupler.

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3. Results and discussion

We restricted the pump power to 10 W corresponding to the condition of best stability for the pure CW mode-locking regime [5]. The stable QML operation is found to occur for repetition rates higher than 35 kHz (typically, in the range 35 kHz -50 kHz). The oscilloscope recording of QML pulse trains in longer time scale with the active Q-switch on and off is presented in Fig. 2, which gives a clear evidence of marked improvement in stability. Figure 3. shows the output pulse train for a modulation frequency of 38 kHz, monitored by a silicon photodiode (UDT-HS-040, USA) of rise/fall time 0.5 ns and a 500-MHz (Tektronix TDS 3054B, USA) oscilloscope. The train is made up of 20 mode-locked pulses (train envelope FWHM) separated by 5.6 ns corresponding to the cavity round trip time. The average output power is 2.44 W, and the energy in peak pulse is ≈3.07 µJ. We measured the pulse width by the technique of optical second-order auto-correlation, employing a 3mm long BBO crystal in a noncollinear geometry; the result is shown in Fig. 4: by assuming a Gaussian profile, the estimated pulse width is 9 ps.

 

Fig. 2. QML pulse trains in longer time scale (a) when active Q-switch is on and (b) when it is off

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Fig. 3. Oscilloscope trace of a QML pulse train with active Q-switch on.

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Fig. 4. Pulse autocorrelation trace. Dots: experimental data. Continuous line: Gaussian fit.

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Fig. 5. Variation of Q-switch pulse width and average output power with repetition rate

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The rms value of the fluctuations in the autocorrelation trace, which is observed in the time scale of several minutes, is around ±8%, corresponding to amplitude fluctuations ≈±4% (rms) of the pulse intensity at 1064 nm. We have already predicted the pulse width of a cw nonlinear mirror mode-locked laser and it has been found to be consistent with the experimental value [10]. The measured Q-switch pulse width and the average power for different values of the repetition rate are depicted by solid curve in Fig. 5. The calculated values for the same are shown by the dashed curves. The transition from stable QML to pure Q-switch mode occurs as the frequency of modulation is decreased from 35kHz to 20kHz. When the repetition rate is increased above 50 kHz, the QML envelope becomes unstable. Since for a pump power of 10W and cavity length of 81.7cm, the NLM passive Q-switching repetition rate (‘natural’ rep rate) (although unstable, with large jitter) is about 120kHz. Active Q-switch should give the best performance when its modulation frequency is matched with this natural frequency. The driver used for our Q-switch can be modulated in the range 1kHz-50kHz. We observed in our experiment that the actively Q-switched NLM laser performs optimally when the modulation frequency is adjusted to the one third of such “natural” repetition rate. We tried to take some data at 60kHz (half of natural repetition-rate), but the driver was not performing well. In future, it would be desirable the realization of an electronic feed-back loop sensing directly the natural repetition rate and driving the Q-switch driver for optimum stability. It is also observed that the actively Q-switched and passively mode-locked laser runs in optimum situation for the pump power between 9.0W and 10.0W. For pump power less than 9.0W the Q-switching is not effective enough where as for the pump power greater than 10.0W the laser tends to go to pure Q-switching regime.

The peak power calculated from measured average power and pulse trace corresponding to the Q-switch repetition rate of 38 kHz under stable QML operation is over 341 kW, which is 224 times greater than the peak power attainable under cw mode-locking. The measured beam quality parameter (M²) of the output beam is 1.29 in the horizontal direction, and 1.38 in the vertical direction.

4. Conclusions

In conclusion, we have built a stable and efficient Nd:YVO₄ laser, actively Q-switched by an acousto-optic modulator and passively mode-locked by a NLM. The difficulty in stable operation of a simultaneous passive Q-switching and mode-locking is avoided. The active Q-switching is proved to be efficient for generating short 9-ps pulses with high peak power of more than 341 kW for a pump power of 10 W as well as in stabilizing the QML operation. For medium average power application and where the laser output power scaling is limited by laser crystal parameter and the mode-locking stability, actively Q-switched and passively mode-locked regime of operation is proved to be a stable and efficient source. The high peak power available from such sources might be useful for applications in materials micro-structuring and efficient nonlinear optical processes like pumping a low threshold optical parametric oscillator.