High‐energy high‐power near‐diffraction‐limited 1064 and 532 nm picosecond Nd:YAG laser

Electronics LettersVolume 56, Issue 7 p. 339-342 PhotonicsFree Access High-energy high-power near-diffraction-limited 1064 and 532 nm picosecond Nd:YAG laser A.F. Kornev, Lasers and Optical Systems Co. Ltd., Russian FederationSearch for more papers by this authorR.V. Balmashnov, Corresponding Author roman.balmashnov@mail.ru ITMO University, Russian FederationSearch for more papers by this authorE.A. Viktorov, ITMO University, Russian FederationSearch for more papers by this authorA.S. Davtian, Lasers and Optical Systems Co. Ltd., Russian FederationSearch for more papers by this authorV.V. Koval, orcid.org/0000-0003-0997-6782 ITMO University, Russian FederationSearch for more papers by this authorA.M. Makarov, ITMO University, Russian FederationSearch for more papers by this authorI.G. Kuchma, Lasers and Optical Systems Co. Ltd., Russian FederationSearch for more papers by this author A.F. Kornev, Lasers and Optical Systems Co. Ltd., Russian FederationSearch for more papers by this authorR.V. Balmashnov, Corresponding Author roman.balmashnov@mail.ru ITMO University, Russian FederationSearch for more papers by this authorE.A. Viktorov, ITMO University, Russian FederationSearch for more papers by this authorA.S. Davtian, Lasers and Optical Systems Co. Ltd., Russian FederationSearch for more papers by this authorV.V. Koval, orcid.org/0000-0003-0997-6782 ITMO University, Russian FederationSearch for more papers by this authorA.M. Makarov, ITMO University, Russian FederationSearch for more papers by this authorI.G. Kuchma, Lasers and Optical Systems Co. Ltd., Russian FederationSearch for more papers by this author First published: 01 March 2020 https://doi.org/10.1049/el.2019.3173Citations: 2AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinked InRedditWechat Abstract The authors describe a diode-pumped Nd:YAG laser that produces 920 mJ output energy pulses with a pulse duration of 76 ps at 200 Hz pulse repetition rate. Second-harmonic generation with a pulse energy of 730 mJ and pulse duration of 63 ps was obtained by using LiB3O5 crystal with II-type phase matching. The output beam divergence was about 1.5 × DL (the diffraction limit) and 1.9 × DL for 1064 and 532 nm wavelengths, respectively. Introduction Picosecond solid-state lasers with high output pulse energy and high repetition rate are attractive laser sources in various fields of science and engineering: precise material processing [1], non-linear optics [2], medicine [3], plasma diagnostics [4] and satellite laser ranging [5]. Nowadays, liquid-cooled short-pulse duration Nd:YAG lasers are characterised by output pulse energies of not more than 0.5 J at few hundred hertz pulse repetition rate. An Nd:YAG diode-pumped picosecond laser system producing 190 mJ pulses with a duration of 470 ps at 100 Hz repetition rate and M2 factor of 1.8 was demonstrated in [6]. In [7], the pulses from Nd:YVO4 master oscillator (MO) were amplified by a regenerative amplifier up to mJ level and then amplified in diode-pumped Nd:YAG power amplifier. It resulted in 130 mJ output energy pulses with the pulse duration of 64 ps and the pulse repetition rate of 300 Hz. We have previously described a Nd:YAG laser system with the output pulse energy of 0.43 J and pulse duration of 100 ps at the pulse repetition rate of 200 Hz [8]. It is known that achieving high-energy picosecond pulses is a significant challenge because of the low damage threshold of dielectric optical coatings at picosecond pulse durations and the effect of small-scale self-focusing. The damage of dielectric optical coatings and the bulk damage of optical materials lead to deterioration of laser output characteristics or its failure. The laser-induced damage threshold fluence is proportional to the square root of the pulse duration for the range of 40–1 ns, and is nearly constant for the range of 10–40 ps [9]. In order to avoid optical damage in a laser operating with 50–100 ps pulse duration, the fluence should not exceed 1 J/cm2 [10]. Another problem, small-scale self-focusing, can lead to the formation of numerous filamentary tracks in the laser medium volume [11, 12]. These tracks can be a source of intensity spiking caused by the diffraction and interference effects. It leads to the gradual failure of the laser elements and the laser system as a whole. In order to avoid the small-scale self-focusing, it is necessary to eliminate external factors such as diffraction and interference spikes, non-uniform distribution of the amplified beam and population inversion. The probability of small-scale self-focusing appearing in high-energy laser amplifiers can be reduced by spatial filters in the laser amplifier system [13]; increasing the diameter of amplified beam between the different amplification stages [13]; amplification of beams with significant geometric divergence [11]; using relay optics [14]; amplification of circularly polarised radiation [15]. In this Letter, we report on a high-energy diode-pumped picosecond Nd:YAG laser based on 'seed laser – RA (regenerative amplifier) – PA (power amplifier)' scheme. We used a laser rod with a larger diameter in the output amplification stage compared to the laser discussed in [8]. This allowed us to achieve a significantly higher output pulse energy of 0.92 J. Master oscillator The MO design is similar to that reported in [8]. We used passively Q-switched pulsed-diode-pumped microchip laser (BATOP GmbH) as a picosecond seed laser source. It generated near-Fourier-transform-limited 80 ps pulses with energy of ∼100 nJ at 1064 nm, which were coupled to a single-mode polarisation-maintaining fibre with two outputs fibre optic splitter. One output was used for optical synchronisation of Pockels cells, and the other for seeding the RA cavity and providing the necessary time delay. The RA had a ring cavity with polarising output coupler based on a Pockels cell and a polariser. Two Nd:YAG laser rods of the RA were longitudinally pumped by two 808 nm 50 W fibre-coupled laser diode modules. Thermally induced birefringence in two identically pumped laser rods was passively compensated with 90° quartz rotator. Residual birefringence and transmittance of the output polariser led to output pre-pulses with an interpulse period equal to the resonator round-trip time, and the pulse picker was used in order to get rid of the pre-pulses [16]. The MO generated 4.4 mJ energy pulses at 200 Hz pulse repetition rate with 0.3% RMS deviation, 20% optical efficiency and the beam quality factor M2 = 1.3. The high stability of the output pulse shape was achieved due to using of microchip laser [17]. It is more stable to external influences compared to other sources. It is worth emphasising that the RA does not introduce noticeable distortions in the pulse shape. Power amplifier The optical scheme of PA is shown on Fig. 1. The PA consisted of two amplifying stages based on Ø15 × 140 mm and Ø10 × 140 mm Nd:YAG laser rods doped with 0.1 at.% and 0.2 at.% Nd3+ ions, respectively. Each rod was placed in a pump module and side-pumped by three LD arrays (FL-AA05-3 × 10-6000-806(Q), Focuslight Technologies Inc.) The peak power of each LD array was 6.5 kW, and the pump pulse repetition rate was 200 Hz with 270 µs pulse duration. The design of the pump module as described in [18]. Fig 1Open in figure viewerPowerPoint Optical scheme of PA (overall dimensions 1100 × 600 × 225 mm3) At the PA input, the radiation of MO passed a beam forming optical system, consisting of the beam reducing telescope and the beam expander. The beam reducing telescope was used to obtain far-field pattern at a short beam propagation length. Then the beam expander was used to match the central part of the near-Gaussian MO beam to the aperture of the PA in order to provide near flat-top intensity distribution of the amplified radiation. The energy encircled within the aperture of the amplifier was about 30% of the total MO energy. The intensity at the edge of the aperture was 0.7 of the intensity at the centre. Faraday isolator was placed between the beam reducing telescope and the beam expander to protect the MO from residual depolarised radiation. A 45° Faraday rotator was placed in front of the flat mirror in order to compensate for thermally induced birefringence. Without this compensation, the depolarised component energy was 25% of total energy after the first pass through the amplifier. Due to the compensation, the fraction of the depolarised component was reduced down to 1.5% after the second pass of the amplifier. The quarter-wave plate changed the state of polarisation at the PA input into circular. It allowed decreasing the probability of the small-scale self-focusing [15]. Combination of the quarter-wave plate and the output polariser helped to get rid of the parasitic lasing on the flat mirror and the optical surfaces of the beam forming optical system. Vacuum relay optics with the spatial filters was used to image the rods into each other in order to eliminate diffraction effects. It provided a more accurate compensation of the thermally induced birefringence and lensing. Non-stationary low-order wavefront distortions were compensated by an adaptive compensator proposed in [19]. Experimental results Small-signal gain of 4.5 and 14.1 was obtained in Ø15 × 140 mm and Ø10 × 140 mm laser rods, respectively. The magnitude of the input signal was adjusted using a half-wave plate and a polariser (Fig. 1). The experimental dependence of the output pulse energy on the input PA energy is shown in Fig. 2. 0.92 J output pulse energy with stability (RMS) < 0.9% at 200 Hz pulse repetition rate was obtained with the input signal of about 1 mJ. Fig 2Open in figure viewerPowerPoint Experimental and calculated (Esat = 0.4 J/cm2) dependences of the 1064 nm laser output pulse energy on the input pulse energy Numerical analysis was done using Frantz–Nodvik equation [20]. It is necessary to take into account the lifetime of the terminate 4I11/2 level in the four-level Nd:YAG medium which is comparable to the picoseconds pulse duration. As it was reported in [8], the Frantz–Nodvik approach for the calculation of the saturated amplification of 100 ps pulses in Nd:YAG laser system was in a good agreement with experimental data if saturation fluence was assumed to be about 0.4 J/cm2. For this saturation fluence, the stored energy in Ø15 × 140 mm and Ø10 × 140 mm Nd:YAG lase rods was estimated as 1.1 and 0.9 J, respectively. The calculated curve in Fig. 2 predicts laser pulses with more than 1 J energy. The possible output pulse energy, however, is limited by the effect of self-focusing of circularly polarised beam. With the output pulse energy increase, the appearance of filamentation in Ø15 × 140 mm laser rod was observed using Schlieren imaging technique [21]. The filaments appeared when the output energy value was about 1 J (Fig. 3a). This level of the output pulse energy corresponds to the B-integral value of 2.6 [22]. Multiple filamentation was observed at the output pulse energy of 1.05 J (Fig. 3b). After experiments finished, small-scale self-focusing damage tracks in the volume of Ø15 × 140 mm laser rod were not detected. There was no sign of filamentation for the output energy of 920 mJ (B-integral 2.4). Further experiments with the laser amplifier were carried out at the output energy not higher than 920 mJ. Fig 3Open in figure viewerPowerPoint Schlieren image of Ø15 × 140 mm laser rod a The beginning of filamentation at the output pulse energy of about 1 J (B integral 2.6) b Multiple filamentation at the output pulse energy of 1.05 J (B integral 2.7) The near-field distribution of 1064 nm output was close to flat-top, as shown in Fig. 4a. The far-field distribution of 1064 nm output is shown in Fig. 4b. The beam divergence was 0.11 mrad at the 0.5 energy level which corresponds to 1.5 the diffraction-limit divergence angle. Fig 4Open in figure viewerPowerPoint Cross-sectional images of the 1064 nm beam profile for near-field and far-field a Near-field beam profile b Far-field beam profile Second-harmonic generation (SHG) was obtained using an LBO (LiB3O5) crystal (dimensions 17 × 17 × 7 mm3, orientation: θ = 20.9°, φ = 90°) with type-II phase matching (o–e–o). The maximum SHG efficiency was 79% with the maximum 532 nm pulse energy of 730 mJ and 1.5% RMS (Fig. 5a). Only one energy meter was used to measure the depletion of the fundamental harmonic during the SHG efficiency measurements. It allowed to avoid the calibration error. The effect of pulse shortening from 76 ps at 1064 nm to 63 ps at 532 nm was observed. The beam divergence was of 0.07 mrad at 532 nm wavelength at the 0.5 energy level which corresponds to the divergence angle of 1.9 times the diffraction limit. Fig 5Open in figure viewerPowerPoint SHG using an LBO (LiB3O5) crystal a SHG efficiency and the 532 nm pulse energy versus the 1064 nm output pulse energy b Dependence of the 1064 and 532 nm output pulse energy on the temperature of LBO crystal The SHG efficiency could be adjusted in the range of 3–79% by changing the LBO crystal temperature. Fig. 5b shows the temperature dependence of the output pulse energy. Conclusion We demonstrated picoseconds diode-pumped Nd:YAG laser with the maximum output energy of 920 mJ at 1064 nm wavelength with the pulse duration of 76 ps at 200 Hz pulse repetition rate. The achieved output energy was only limited by the effect of small-scale self-focusing. The maximum output pulse energy at 532 nm was 730 mJ with SHG efficiency of 79%, and the pulse duration of 63 ps. The beam divergence was 1.5 × DL at 1064 nm wavelength and 1.9 × DL at 532 nm wavelength at the 0.5 energy level. To the best of our knowledge, the parameters above have never been previously reported for picosecond Nd:YAG diode-pumped lasers with liquid cooling. The two wavelengths of operation, the short-pulse duration, the high output pulse energy and good beam quality make this laser a useful tool for various applications. Acknowledgments Government of Russian Federation (grant no. 08-08). 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