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We demonstrate, for the first time to our knowledge, active Q-switching in self-adaptive, reciprocal, closed-loop, diode-side-pumped Nd:YAG slab laser. Pulse energy of 19 mJ with 9.5 ns pulse duration, corresponding to 2 MW of peak power with near-diffraction-limited beam quality was achieved.
© 2014 Optical Society of America
1. Introduction
Phase-conjugate mirror (PCM) concept [1–3], utilizing four-wave mixing (FWM) phenomenon, seems to be one of the most effective methods to efficiently extract energy from an active medium along with excellent beam quality. The FWM mechanism has been realized in several ways in solid-state lasers in recent thirty years [4–18]. Intense excitation of gain media leads to thermo-optical effects, asymmetric temperature and gain gradients. These effects result in degradation of output beam quality. In case of side-pumped lasers, the phenomenon of severe, vertical, astigmatic thermal lensing can be partly mitigated by application of cylindrical lenses. However, the horizontal profile of inversion and wavefront distortions require a different approach. It can be compensated by means of intracavity adaptive optics or by exploiting the PCM concept. Two groups of self-adaptive cavities, where PCM is formed inside an active medium, were studied in the past: non-reciprocal resonators [7–11], where the direction of beam propagation in the cavity is enforced by additional, nonreciprocal transmission elements (NRTE), resulting in a quasi-ring resonator, and reciprocal ones [12–18], where the two beams inside the cavity counter-propagate, creating standing waves. The most spectacular result was demonstrated in [11], where 100-mJ energy, diffraction-limited, single frequency beam in the self-starting, self-Q-switched, diode-side-pumped Nd:YAG laser was reported. In the case of a reciprocal cavity, open-loop resonators were used as a rule. In this case, the cavity does not have any closed-loop feedback, thus the generation threshold is therefore relatively high. Moreover, the demonstrated efficiencies of these types of self-adaptive lasers are not higher than 15% and a certain spectral selection can be achieved.
In 2012 our group presented a novel scheme of self-adaptive, reciprocal, closed-loop, resonator with FWM inside diode-side-pumped Nd:YAG slab. The output energy above 250 mJ in free running regime with optical efficiency of 30% was achieved [17]. In this paper we demonstrate, for the first time to our knowledge, the possibility of an active Q-switching utilizing this specific type of cavity.
2. Laser setup
In the experiment, a 1% at. doped Nd:YAG slab with dimensions of 4×4×35 mm3 was used. Pump-side-facet was coated with AR@808 nm layer, whereas both 4×4 mm3 facets were AR@1064 nm coated and beveled under 2° angle in order to avoid parasitic lasing. The medium was placed in a copper mount. There was no thermal management system applied. The laser head was cooled by means of natural, external convection. The resonator mirrors M1, M2, M3, M4 had HR@1064 nm coatings. The resonator length was approximately Lrez = 100 cm. A single, fast-axis-collimated, 2D diode laser array, fabricated by DILAS, was used. The pump module was able to deliver up to 2.6 kW of peak power in a single pulse. Beam caustic was formed by a single cylindrical lens with focal length f = 50 mm. The resulting excited volume inside the gain medium was 0.21×4.3×1.48 mm3. The pump pulse duration was 0.2 ms. Due to the intensive pumping over relatively small area, the pulse repetition frequency was set to 2 Hz, avoiding possible thermal fracture of the gain medium and damaging the coatings. Glan-Taylor polarizer along with BBO-made Pockels cell (6×6×50 mm2) were applied for experiments in Q-switching operation. The self-adaptive, reciprocal, closed-loop resonator scheme is depicted in Fig. 1. The ”closed-loop” cavity was formed by the first M1 mirror, two folding mirrors M2 and M3, and a rear, M4 mirror. During one round-trip, the standing wave passes through the active medium four times, intersecting at a small angle Θ, which is determined by the proper alignment of the M2 and M3 folding mirrors. The out-coupling mirror (1st diffraction order of gain grating in active medium) is formed in the beam-intersection region during linear generation development in the cavity. More, specific details can be found in [17].
Fig. 1 The self-adaptive, reciprocal resonator scheme with diffractive output (red, dashed line): M1, M2, M3, M4 – highly-reflective, folding mirros; HCL – horizontal, cylindrical lens, PC – Pockels cell, GT – Glan-Taylor polarizer, Θ – beam intersection angle (20 mrad).
3. Free-running experiments
The output energy vs. incident pump energy curve is shown in Fig. 2.
Fig. 2 The energetic characteristic of free-running Nd:YAG laser with self-adaptive, reciprocal, closed-loop cavity.
The output energy of 120 mJ with slope efficiency of 37% was obtained. It corresponds to 86% of energy achieved in the system with linear cavity with the same length (100 cm). The near-diffraction-limited, linearly-polarized beam with M2 < 1.3 parameter and beam divergence ∼1 mrad was obtained. The polarization was forced by the gain-grating diffraction efficiency, which is higher for s-polarization, as well as the folding mirrors M2 and M3 have higher losses for p-polarization. We demonstrate, for the first time to our knowledge, active Q-switching in self-adaptive, reciprocal, closed-loop, diode-side-pumped Nd:YAG slab laser. Pulse energy of 19 mJ with 9.5 ns pulse duration, corresponding to 2 MW of peak power with near-diffraction-limited beam quality was achieved. Due to the power supply limitation (Imin = 100 A) we were unable to measure correctly the beginning of the energetic characteristic, hence it was extrapolated.
4. Active Q-switching experiments
A BBO-made Pockels cell (ϕ5 mm of clear aperture) and polarizer were introduced to the cavity (see Fig. 1). The initial gain-length-product per pass at the generation threshold was calculated to be g·l = 3.4. The energetic characteristic was measured and is depicted in Fig. 3.
Fig. 3 The pulse energy and pulse width vs. pump energy curves of the actively Q-switched Nd:YAG laser with self-adaptive, reciprocal, closed-loop cavity: red curve – free-running operation; black curve – Q-switched operation.
The Pockels cell and the polarizer introduced a significant amount of parasitic losses (it consists of two BBO crystals and two windows; the transmission at the wavelength of 1064 nm is ∼94%) and diminished the diffraction efficiency of gain grating (about 50% increase in threshold and 30% drop in slope efficiency). For pump energy of 242 mJ, a single pulse generation with pulse energy of 18.3 mJ and pulse width of 9.5 ns was achieved. This corresponds to 68% of energy obtained in free-running operation and 1.93 MW of pulse peak power. Further increase of pump energy led to so high gain that the system generated the beam out of an open-loop cavity (mirror M4 covered; no feedback) and electro-optic modulator (with quarter-wave voltage applied) was not able to break the generation. Because of that, the experiments were carried out below aforementioned pump level, where only single pulse, without the free-running generation in the background, was generated. The temporal shape of the output pulse is depicted in Fig. 4.
Fig. 4 The temporal pulse shape of actively Q-switched Nd:YAG laser with self-adaptive, reciprocal cavity.
The amplitude modulation indicates on generation of several longitudinal resonator modes. The near-diffraction-limited, linearly-polarized beam with M2 < 1.3 parameter and beam divergence ∼1 mrad was obtained (M2 = 1.4 in free-running regime, respectively). The beam intensity profile registered in far-field with Thorlabs CCD Beam Profiler is depicted in Fig. 5.
Fig. 5 The beam intensity profile of the actively Q-switched Nd:YAG laser with self-adaptive, reciprocal cavity, registered in far-field (rulers in μm).
5. Conclusions
We demonstrate experimentally, first to our knowledge, the feasibility of active Q-switching in self-adaptive, closed-loop Nd:YAG slab laser, with reciprocal, diffractive output cavity. Single pulse with energy of 18.3 mJ and width of 9.5 ns for 242 mJ of pump energy was achieved. It corresponds to almost 2 MW of peak power. A near-diffraction-limited, linearly-polarized output, with beam quality parameter M2 < 1.3 was obtained. We believe that proper thermal management of the gain medium and along with an application of Pockels cell with larger clear aperture will provide higher output pulse energy. The solution we came up with in our research on self-adaptive, closed-loop reciprocal cavity, enables efficient generation of short pulses with excellent beam quality from a Nd:YAG laser, diode-side-pumped by a single, 2D laser diode array. We intend to elaborate a new, water-cooled laser head in the near future, which will enable us to run the system with higher duty factors.
Acknowledgments
This research has been supported by Polish National Science Center under Project No. NCN2012/05/B/ST7/00088.
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