Optical parametric oscillators for high pulse energy and high average power operation based on large aperture periodically poled KTP and RTA

We report on optical parametric oscillators (OPOs) based on large aperture periodically poled KTiOPO₄ (PPKTP) and RbTiOAsO₄ (PPRTA) pumped with high pulse energy and high average power Q-switched solid-state lasers. The OPOs were pumped with 1064-nm pulses of a diode-pumped Nd:YVO₄ laser at 20 kHz repetition rate. The emitted signal wavelengths were 1.72 μm and 1.58 μm and the idler wavelengths were 2.79 μm and 3.26 μm, respectively. Pumping the PPKTP OPO with 7.2 W and the PPRTA OPO with 8 W average power, 2 W and 1.3 W total OPO output powers were generated. Two-dimensional measurements of the total OPO output power, the signal wavelength and the signal bandwidth in dependence on the crystal location indicated a good uniformity of the quasiphasematching structure over the entire 3-mm-thick crystals. This allowed pumping with larger pump beams and therefore with pulse energies of tens of millijoules. Pumping with different flash-lamp-pumped lasers, good OPO performance and high output pulse energies could be achieved for all pump lasers. Maximum input pulse energies of 56 mJ gave output pulse energies of as much as 18 mJ. The temperature tuning behaviors of both OPOs were measured, showing excellent agreement with calculated temperature tuning curves. New equations for temperature dispersion in RTA are presented. These results show that large-aperture PPKTP and PPRTA crystals are well suited for tunable nanosecond OPO operation with multi-watt average pump power and several tens of millijoules pump pulse energies. Received: 7 September 2001 / Published online: 7 November 2001     

High-power picosecond LiB3O5 optical parametric oscillators tunable in the blue spectral range

The paper reports on an experimental investigation and numerical analysis of noncritically and critically phasematched LiB₃O₅ (LBO) optical parametric oscillators (OPOs) synchronously pumped by the third harmonic of a cw diode-pumped mode-locked Nd:YVO₄ oscillator–amplifier system. The laser system generates 9.0 W of 355-nm mode-locked radiation with a pulse duration of 7.5 ps and a repetition rate of 84 MHz. The LBO OPO, synchronously pumped by the 355-nm pulses, generates a signal wave tunable in the blue spectral range 457–479 nm. With a power of up to 5.0 W at 462 nm and 1.7 W at 1535 nm the conversion efficiency is 74%. The OPO is characterized experimentally by measuring the output power (and its dependence on the pump power, the transmission of the output coupler and the resonator length) and the pulse properties (such as pulse duration and spectral width). Also the beam quality of the resonant and nonresonant waves is investigated. The measured results are compared with the predictions of a numerical analysis for Gaussian laser and OPO beams. In addition to the blue-signal output visible-red 629-nm radiation is generated by sum-frequency mixing of the 1.535-μm infrared idler wave with the residual 1.064-μm laser radiation. A power of 1.25 W of 1.535-μm idler radiation and 5.7 W of 1.064-μm laser light generated a red 629-nm output power of 2.25 W. Received: 2 February 2000 / Revised version: 28 July 2000 / Published online: 22 November 2000     

Optical parametric processes and broadly tunable femtosecond sources

The basic concepts of optical parametric processes and recent results on broadly tunable high-repetition rate femtosecond Optical Parametric Oscillators (fs OPO) are reviewed. Ti:Sapphire-Laser pumped KTP, KTA, and CTA fs OPOs and intracavity-doubled fs KTP OPO in particular are discussed.     

Rapid infrared wavelength access with a picosecond PPLN OPO synchronously pumped by a mode-locked diode laser

We theoretically and experimentally investigate wavelength tuning of synchronously pumped optical parametric oscillators (OPOs) on changing the cavity length or the pump-repetition rate. Conditions for rapid and wide-range wavelength access are derived. Using an OPO pumped directly by a mode-locked diode-laser master-oscillator power-amplifier (MOPA) system, an all-electronically controlled access to near- and mid-infrared wavelengths is demonstrated. The singly (signal) resonant OPO is based on periodically poled lithium niobate (PPLN) and emits 8 ps idler pulses at a repetition rate of 2.5 GHz in the wavelength range 1986 to 2348 nm (signal: 1530 to 1737 nm). Wavelength tuning over 114 nm (signal) and 189 nm (idler) is achieved solely by electronically varying the repetition rate of the diode-laser oscillator over 720 kHz. By controlling the repetition rate with a programmable driver, an arbitrary emission sequence of the OPO on two wavelength channels is generated, with access times as short as 10 μs. 11 OPO wavelengths equally spaced in the range 1627–1689 nm (signal) or 2054–2154 nm (idler) could be addressed. Received: 6 September 2000 / Revised version: 16 March 2001 / Published online: 23 May 2001     

Thermal effects in singly resonant continuous-wave optical parametric oscillators

Stability and tuning characteristics of continuous-wave optical parametric oscillators (CW OPOs) are affected by various thermal effects arising from optical absorption in nonlinear crystals. In this paper, we present an experimental study of such effects in a singly resonant CW OPO. The OPO operates in the 3-μm mid-infrared region and it is based on a MgO-doped periodically poled lithium niobate crystal. We focus our study on two thermally induced phenomena that have been recently reported to exist in singly resonant CW OPOs: optical bi-stability and thermal self-locking. Thermal self-locking effect, which is known to alter the stability and tuning properties of doubly and triply resonant CW OPOs, is shown to be also of importance in singly resonant OPOs. We report the stability and tuning characteristics of a thermally loaded OPO and discuss a simple temperature-tuning method that can be used to scan the OPO idler frequency continuously over several THz.     

Numerical analysis of the spatial behaviour of nanosecond optical parametric oscillators of beta-barium borate

We report on a numerical analysis of the temporal and spatial beam properties of nanosecond optical parametric oscillators (OPOs). The analysis is performed for a 355-nm-pumped critically phase-matched OPO of beta-barium borate. The calculations provide detailed information on the dependence of the OPO beam quality (measured by the quality factor M ²) on pump energy. An important result is the strong increase of the M ² value for pump energies exceeding 1–2 times the energy at threshold. Furthermore, a temporal analysis of single OPO pulses indicates that the M ² value strongly increases during the first few nanoseconds of the OPO oscillation. This increase is understood by considering the temporal dynamics of the spatial profiles of the OPO signal beam and the depleted pump radiation. Received: 1 April 1999 / Revised version: 26 July 1999 / Published online: 20 October 1999     

Ti:Sapphire-pumped femtosecond optical parametric oscillators based on KTiOPO4 and RTiOAsO4

The design, configuration and operation of Ti:Sapphire-laser-pumped femtosecond Optical Parametric Oscillators (OPOs) using KTP and RTA are described. The results presented include configurations capable of producing transform-limited pulses shorter than 40 fs and operating with pump powers as low as 50 mW. Tunability within the 1–3 µn spectral region is demonstrated using pump-tuning alone.     

Self-stabilizing additive-pulse mode-locking due to thermo-optical non-linearity in an optical fiber

16 , 1671 (1991)] described a Nd:YLF APM laser which somehow automatically adjusted the relative resonator phase. We reproduce this behavior and analyse its origin. Thermal effects due to the light power guided in the fiber affect the effective fiber length, which in turn influences the phase and thus the power level; hence a closed servo loop results. We demonstrate this explanation to be correct in quantitative terms. Consequences arise for other systems involving fiber-optic loops or interferometers. Received: 7 April 1997/Revised version: 15 July 1997     

A 4.8-W M^2 = 4.6 Continuous-Wave Intracavity Sum-Frequency Diode-Pumped Solid-State Yellow Laser

A yellow continuous wave with beam quality M^2= 4.6 and output power of 4.8 W at 589nm is generated by intracavity sum-frequency mixing of 1064 nm and 1319 nm radiations of a Nd：YAG laser. To achieve high beam quality at high power, thermally near-unstable flat-flat resonators with two-rod birefringence compensation are designed to obtain the large fundamental mode size inside the Nd：YAG rods and the same beam width inside the KTP crystal. The optimal intracavity power ratio of both 1064nm and 1319nm beams is also considered. The output power fluctuation of the yellow laser remains less than 5% in four hours.     

Compact sub-100-fs Nd:silicate laser

We present a compact Nd:silicate laser pumped by a single 1-W high-brightness commercial laser diode that generates pulses as short as 88 fs (nearly Fourier limited) when passively mode-locked with a saturable absorber mirror. We also tested a prismless cavity setup employing a single Gires-Tournois mirror, yielding 100-fs pulses. These setups are significantly simpler and more compact than those reported previously for short pulse Nd:silicate lasers.     

13-W picosecond Nd:GdVO4 regenerative amplifier with 200-kHz repetition rate

We report a diode-pumped picosecond Nd:GdVO₄ regenerative amplifier system designed for micro-machining applications. Nd:GdVO₄ was chosen as gain material because it offers – in comparison to the commonly used Nd:YVO₄ – improved thermal properties and a larger gain bandwidth. At the maximum repetition rate of 200 kHz, the system generated 6.8-ps-long pulses with a pulse energy of 65 μJ, which corresponds to an average output power of 13 W. At 100 kHz, the pulse energy increased to 123 μJ, whereas the average power of 12.3 W remained almost identical. The broadening of the pulses due to gain narrowing was investigated in dependence on the number of cavity round trips and on the repetition rate. At 200 kHz, the injected 5.3-ps-long pulses broadened slightly to about 6.8 ps.     

Pulse Compression Based on Laser-Induced Optical Breakdown in Suspension

Pulse compression based on laser-induced optical breakdown in suspension is investigated. The physical mechanism behind it is analyzed theoretically and validated in the Q-switched Nd：YAG laser system. A 12-ns pump pulse is suppressed to 5ns With good fidelity in the front edge and sharp steepness in the trailing edge. The HT-270, which has a small gain coefficient and absorption coefficient, is used as a solvent, and therefore the disturbance induced by stimulated Brillouin scattering and absorption are minimized and the transmittivity is enhanced.     

High efficiency and high energy parametric wavelength conversion using a large aperture periodically poled MgO:LiNbO3

We have demonstrated an efficient high energy 2 μm laser generation with a 36 mm long large aperture 5 mol% MgO-doped periodically poled LiNbO₃ (PPMgLN) nonlinear optical crystal. A high power Q-switched Nd:YAG laser (1.064 μm) was used to pump the quasi-phase matched (QPM) optical parametric oscillator (OPO). A total output energy of 186 mJ with 58% slope efficiency was obtained in two separate beams at 2 μm.     

Optical and nonlinear optical characteristics of the Ge and GaAs nanoparticle suspensions prepared by laser ablation

The laser ablation of Ge and GaAs targets placed in water and ethanol was carried out using the fundamental radiation of nanosecond Nd:YLF laser. The results of preparation and the optical and nonlinear optical characterization of the Ge and GaAs nanoparticle suspensions are presented. The considerable shift of the band gap energy of the nanoparticles compared to the bulk semiconductors was observed. The distribution of nanoparticle sizes was estimated in the range of 1.5-10 nm on the basis of the TEM and spectral measurements. The nonlinear refractive indices and nonlinear absorption coefficients of Ge and GaAs nanoparticles were defined by the z-scan technique using second harmonic radiation of picosecond Nd:YAG laser (λ = 532 nm).     

Bismuth triborate (BiB3O6) optical parametric oscillators

We report the phase matching of parametric frequency conversion in the nonlinear material BiB₃O₆ (BiBO) and on an investigation of optical parametric oscillators (OPOs) of this new crystal. Based on the calculation of collinear type I and type II phase matching within the refractive-index planes, the most favorable directions for phase matching are identified for OPOs pumped by the fundamental or the harmonics of 1064-nm Nd-doped lasers. Based on these results, pulsed 532-nm-pumped ns OPOs are realized. The pump source is either a Q-switched high repetition rate (10 kHz) Nd:YVO₄ laser (with a pulse energy of 24 J) or a low repetition rate (10 Hz), high pulse energy (120 mJ) Nd:YAG laser system. The BiBO OPO pumped by the Nd:YVO₄ laser showed a very low threshold of 0.047 J/cm². At an average pump power of 2.4 W the total OPO output power was 630 mW. By changing the phase-matching angle within the yz plane from 0 to 11.6° the signal wavelength was tuned from 735 nm to 970 nm, while the spectral width changed from 0.2 nm to 1.4 nm. By pumping the OPO with the Nd:YAG laser, the OPO had a threshold of 0.12 J/cm², a steep slope (59%) and a high total efficiency (of up to 48%). Due to divergence broadening the spectral width changes from 8.5 nm at 800 nm to 70 nm near degeneracy. The properties of BiBO determined from the experimental results are compared with those of well-known nonlinear materials such as BBO, LBO and KTP. PACS 42.65.-k; 42.65.Yj; 42.70.-a; 42.70.Mp     

Comparison of eye-safe KTA OPOs pumped by Nd:YVO4 and Nd:YLF lasers

An eye-safe KTA OPO pumped by a Nd:YLF laser is demonstrated and a comparison with that pumped by a Nd:YVO₄ laser is performed. Although the slope efficiency of the continuous-wave free-running Nd:YLF laser is lower than that of the Nd:YVO₄ laser, the performance of KTA OPOs pumped by the Q-switched Nd:YLF laser is better, especially at lower repetition rates. The slope efficiency of KTA OPO pumped by a Nd:YLF laser is 14.6% at 30 kHz and 11.04% at 10 kHz. The better energy storage ability of Nd:YLF makes it an excellent laser medium in IOPOs.     

Comparison between c-cut and a-cut Nd:YVO4 lasers passively Q-switched with a Cr4+:YAG saturable absorber

Comparison between c-cut and a-cut Nd:YVO₄ microchip lasers passively Q-switched with a Cr⁴⁺:YAG saturable absorber is experimentally made. The lower emission cross section of the c-cut Nd:YVO₄ crystal can enhance the passive Q-switching effect to produce a peak power 10 times higher than that obtained with the a-cut crystal. The experimental result further reveals that a c-cut Nd:YVO₄ crystal is a very convenient material for short-pulse (sub-nanosecond) and high-peak-power (>10 kW) lasers. Received:10December2001/Revisedversion:22January2002 / Published online: 14 March 2002     

Characteristics of Nd:YGG Laser Operating at 4F3/2-4I9/2

The laser properties of the Nd：YGG crystal are investigated. The absorption spectrum from 500 to 90Onto and emission spectrum from 850 to 1400nm of Nd：YGG are measured. As much as 1.35 W output power of fundamental laser operating at 935 and 938nm with a slope efficiency of 15.7% and 105mW output power of frequency doubled blue laser are successfully obtained.     

High energy and short pulses generation by Nd:yttrium aluminum garnet laser mode-locked using frequency-doubling nonlinear mirror

The generation of laser pulses with energies of >40 mJ at 25 Hz and durations variable from 15 ps to 45 ps using an Nd:yttrium aluminum garnet laser mode-locked with a Stankov nonlinear mirror is demonstrated. This laser is used to pump an optical parametric generator-amplifier, which is tunable in the visible spectral range.     

Characterization of biaxial crystals for tangentially phase-matched frequency conversion

Ultraviolet radiation has been generated by tangentially phase-matched sum-frequency mixing in biaxial L-arginine phosphate (LAP) crystal for the first time using Nd:YAG output at 1064 nm and Rh 6G dye laser output at 560 nm as the two input sources. Characterization has also been made of such a cheap, biaxial crystal for its possible use in devices for tangentially phase-matched short wavelength generation. If the crystal is of proper cut, thickness and quality so that its maximum capability can be exploited it can replace the potassium dihydrogen phosphate (KDP) group of crystals for various applications.