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     

Investigation of the spatial beam quality of pulsed ns-OPOs

This paper reports on an experimental investigation of the spatial beam quality of a pulsed nanosecond optical parametric oscillator (OPO). The OPO is a 355 nm pumped type-I BBO-OPO. The spatial quality of the OPO output is characterized by the M²-parameter in dependence of other experimental parameters such as resonator length, pump-beam diameter, pump-pulse duration and pump energy. The results obtained indicate, that an appropriate choice of these parameters substantially improves the spatial beam quality of the OPO output. Received: 19 November 2002 / Revised version: 3 February 2003 / Published online: 5 May 2003 RID="*" ID="*"Corresponding author. Fax: +49-631/205-3906, E-mail: anstett@rhrk.uni-kl.de     

Efficient, single frequency, high repetition rate, PPLN OPO pumped by a prelase Q-switched diode-pumped Nd:YAG laser

Single axial mode operation (<200 MHz optical bandwidth) of a high repetition rate periodically poled lithium niobate optical parametric oscillator (OPO) has been obtained at signal wavelengths between 1.46 μm and 1.64 μm. OPO signal slope efficiencies of 35% have been measured for repetition rates of 5–20 kHz. Single mode operation required spectral narrowing of both the pump laser and the OPO. A simple technique of prelase Q-switching was implemented to reduce the optical bandwidth of the cw diode-pumped Nd:YAG pump laser to <1 GHz. A single intracavity étalon was then sufficient to ensure single frequency oscillation of the OPO signal. The OPO output was stable with a smooth spatial profile and an M ² value of 1.3. Received: 29 September 1999 / Published online: 27 January 2000     

Dye laser pumped, continuous-wave KTP optical parametric oscillators

4 (KTP) optical parametric oscillators (OPOs) with pump and idler resonant cavities. With a linear two-mirror cavity the pump power at threshold was 70 mW. The single-frequency signal and idler output wavelengths were tuned in the range of 1025 to 1040 nm and 1250 to 1380 nm by tuning the dye laser in the range of 565 to 588 nm. With a dual three-mirror cavity the threshold was 135 mW. Pumped by 500 mW of 578 nm radiation the 1040 nm single-frequency signal wave output power was 84 mW. Power and frequency stable operation with a spectral bandwidth of less than 9 MHz was obtained by piezo-electrically locking the length of the pump resonant cavity to the dye laser wavelength. Similar performance was achieved by placing the idler resonant OPO inside the resonator of the dye laser. With this system power stable and single-frequency operation was achieved with a spectral bandwidth of less than 11 MHz for the idler wave. Received: 3 February 1998/Revised version: 9 March 1998     

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     

High-peak power multi-wavelength picosecond pulses generated from a BaWO4 Raman-seeded optical parametric amplifier

We report the generation of high-peak power multi-wavelength picosecond laser pulses using optical parametric amplification (OPA) in BBO seeded with pulses generated in a 5-mm length BaWO₄ crystal by stimulated Raman scattering of 18-ps laser pulses at 532 nm. The maximum output energy of the amplified first-Stokes component at 559.7 nm was about 1.76 mJ. The corresponding maximum peak power, pulse duration and spectral line width were measured to be 117.3 MW, 15 ps and 18.0 cm⁻¹, respectively. The multi-wavelength picosecond laser pulses were in the visible and near infrared ranges. Using this Raman-seeded OPA technique, the beam quality of the stimulated Raman scattering pulses can be improved.     

Beam divergence effects on high power optical parametric oscillation

The beam divergence effects of the input pump laser on a high power nanosecond optical parametric oscillator （OPO） have been numerically simulated. The OPO conversion efficiency is affected due to the angular deviation of real laser beams from ideal phase matching conditions. Our theoretical model is based on the decomposition of the Gaussian beam and assumes each component has a single deviation angle and thus a particular wave vector mismatch. We take into account the variable intensity profile in the spatial and temporal domains of the Gaussian beam, the pump depletion effects for large-signal processes as well as the oscillatory effects of the three waves. Two nonlinear crystals β-BaB2O4 （BBO） and LiB305 （LBO） have been investigated in detail. The results indicate that the degree of beam divergence strongly influences the maximum pump intensity, optimum crystal length and OPO conversion efficiency. The impact of beam divergence is much more severe in the case of critical phase-matching for BBO than in the case of non-critical phase-matching for LBO. The results provide a way to choose the optimum parameters for a high power ns OPO such as the nonlinear material, the crystal length and the pump intensity, etc. Good agreement is obtained with our experimental results.     

Experimental investigation and numerical simulation of the influence of resonator-length detuning on the output power,pulse duration and spectral width of a cw mode-locked picosecond optical parametric oscillator

We report on a cw mode-locked non-critically phase matched KTP optical parametric oscillator synchronously pumped by a picosecond Ti:Sapphire laser. High average signal output power of up to 950 mW over a large tuning range has been achieved. For this OPO the influence of resonator-length detuning on the output power, pulse duration and spectral bandwidth has been investigated. The measured data are in good agreement with the results of a numerical simulation using a split-step Fourier method which considers the group-velocity mismatch, the group-velocity dispersion and the self-phase modulation. The numerical simulation also describes the measured strong pump depletion and its influence on the OPO output and efficiency.     

Sum and difference frequency generation of white light continuum with the ps pulses of Nd:YAG laser in a thick BBO crystal

The white light continuum (WLC) generated in water/D₂O mixture by pumping with the fundamental of ps Nd⁺³:YAG laser has been used as a variable frequency source for the sum frequency generation as well as for its amplification. 35 ps long pulses with 8 mJ energy at 1064 nm were mixed collinearly with the WLC generated by the same laser beam in a 20 mm thick BBO crystal. The obtained tunable output has been identified as the sum frequency between the fundamental and a portion of the WLC with the required phase matching. Theoretical simulations are also given along with a few initial experiments to use this combination for the difference frequency generation (optical parametric amplification) under non-collinear geometry.     

Synchronously pumped femtosecond optical parametric oscillator of congruent and stoichiometric MgO-doped periodically poled lithium niobate

A synchronously pumped femtosecond optical parametric oscillator based on congruent MgO-doped periodically poled lithium niobate (c-MgO:PPLN) is reported. The system, operating at room temperature, was pumped by a mode-locked Ti:sapphire laser. The wavelengths of the signal and idler waves were tuned from 870 nm to 1.54 μm and 1.58 to 5.67 μm, respectively, by changing the pump wavelength, the grating period or the cavity length. Pumped by 1.1 W of 755 nm laser radiation, the OPO generated 310 mW of 1080 nm radiation. This signal output corresponds to a total conversion efficiency of 50%. Without dispersion compensation the OPO generated phase-modulated signal pulses of 200 fs duration. Besides the OPO of c-MgO:PPLN, an OPO of stoichiometric (s) MgO:PPLN was investigated. Because of the reduced sensitivity to photorefractive damage, both crystals allowed efficient OPO operation at room temperature. Received: 19 August 2002 / Revised version: 11 December 2002 / Published online: 19 March 2003 RID="*" ID="*"Corresponding author. Fax: +49-631/205-3906, E-mail: andres@physik.uni-kl.de     

All-solid-state cw mode-locked picosecond KTiOAsO4 (KTA) optical parametric oscillator

4 (KTA) optical parametric oscillator (OPO) synchronously pumped by a cw diode-pumped mode-locked Nd:YVO₄ oscillator–amplifier system. The laser system (pumped by 84 W of cw 808-nm diode radiation) generates 7-ps-long pulses at 1.064 μm with a repetition rate of 83.4 MHz and an average power of 29 W. The OPO, synchronously pumped by the 1.064-μm laser pulses, consists of a 15-mm-long KTA crystal (cut for type II noncritical phase-matching) in a folded signal resonant linear resonator. The average powers of the 1.54-μm signal radiation and the 3.47-μm idler radiation are 14.6 W and 6.4 W, respectively. The total OPO output of 21 W corresponds to an internal efficiency of 75%. The experimental investigations include measurements of the OPO output power (and its dependence on the pump power, the transmission of the output coupler, and the resonator length) and of the pulse properties (such as pulse duration and spectral width). The measured results are in good agreement with the predictions of a numerical analysis based on a split-step Fourier method. Received: 4 May 1998     

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.     

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     

Efficient generation of tunable VUV laser radiation below 205 nm by SFM in BBO

Design and performance characteristics of a tunable dye laser system for sum frequency mixing (SFM) in a BBO crystal are presented. The system is composed of two tunable pulsed dye lasers pumped synchronously by the second harmonic of a commercial Nd:YAG laser. The radiation produced by the first dye laser is frequency doubled by second-harmonic generation (SHG) in KDP and subsequently mixed by SFM in BBO with the light of the second dye laser. The interest was focussed on generation of tunable laser radiation below 205 nm with high output power and long-time wavelength stability. High conversion efficiencies enable output energies of 100 J (20 kW) at 196 nm using only moderate Nd:YAG pump energies of 67 mJ. Altogether, a laser system with very good specifications for analytical application in the near VUV spectral region is reported.     

Spectral properties and numerical modelling of a critically phase-matched nanosecond LiB3O5 Optical Parametric Oscillator

We report on the spectral properties and numerical modelling of a singly resonant Optical Parametric Oscillator (OPO) of Lithium-triBOrate (LBO). The OPO is pumped by the second, third or fourth harmonic of an injection-seeded,Q-switched Nd:YAG laser. The measured OPO parameters are the tuning range, the threshold, the spectral linewidth, the efficiency and the output power. For the LBO-OPO critical type-I phase-matching (xy-plane) provides a wide tuning range and optimum values of the effective nonlinear coefficient. Pumped, for example, by the 355nm third harmonic the spectral range extends from 414 nm to 2.47 µm. With a 15 mm long crystal the OPO generates a total output-pulse energy of 77 mJ with an efficiency of 45%. For 266 nm pump radiation the signal wave is in the near ultraviolet at 307–325 nm. If pumped at 532 nm the OPO generates simultaneously two pairs of signal and idler waves in the infrared (0.707–2.15 µm). The pulse-energy fluctuations of the two pairs are correlated. If, however, the OPO is injection-seeded at one of the signal waves the two wavelength pairs are anti-correlated. The observed wavelength tuning as well as the measured spectral line-widths, threshold energies and efficiencies are in agreement with the values predicted by computer simulation.     

A Narrow Bandwidth BBO Optical Parametric Oscillator Injection－seeded by a Tunable Pulsed Ti：Sapphire Laser

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Spectroscopic diagnostics of chemical processes: applications of tunable optical parametric oscillators

Optical parametric oscillator (OPO) and amplifier (OPA) devices are useful for spectroscopic sensing of chemical processes in laboratory, industrial, and environmental settings. This is particularly true of nanosecond-pulsed, continuously tunable OPO/OPA systems, for which we survey a variety of instrumental strategies, together with actual spectroscopic measurements. The relative merits of OPO wavelength control by intracavity gratings and by injection seeding are considered. A major innovation comprises an OPO with a ring cavity based on periodically poled lithium niobate (PPLN) and injection-seeded by a single-mode tunable diode laser (TDL). Active cavity control by an ‘intensity dip’ method yields an optical bandwidth ≤0.005 cm^-1 (150 MHz), which compares favourably with the performance of advanced grating-tuned OPO/OPA systems. A novel adaptation of this TDL-seeded PPLN OPO employs a compact, inexpensive multimode pump laser, with which it is still possible to obtain continuously tunable single-mode signal output. Cavity ringdown (CRD) spectroscopy also figures prominently, with infrared (IR) CRD spectra from both grating-scanned and TDL-seeded OPOs reported. Finally, a tunable ultraviolet (UV) source, combining a TDL-seeded passive-cavity OPO and a sum-frequency generation stage, is developed for measurements of time-resolved IR-UV double resonance spectra of acetylene and UV laser-induced fluorescence spectra of nitric oxide. Received: 28 March 2000 / Published online: 13 September 2000     

High-repetition-rate eye-safe optical parametric oscillator intracavity pumped by a diode-pumped Q-switched Nd:YVO<Subscript>4</Subscript> laser

A high-repetition-rate eye-safe optical parametric oscillator (OPO), using a non-critically phase-matched KTP crystal intracavity pumped by an acousto-optically (AO) Q-switchedNd:YVO₄ laser, is experimentally demonstrated. It is found that the average OPO signal power at 1573 nm can be efficiently increased by increasing the pulse repetition rate. Moreover, the intracavity OPO process effectively shortens the pulse width so that it is in the range 5∼8 ns for pulse repetition rates of 10 to 80 kHz. As a result of the relatively short pulse, the peak power at 1573 nm is higher than 2 kW at a pulse repetition rate of 80 kHz. Received: 10 July 2002 / Published online: 26 February 2003 RID="*" ID="*"Corresponding author. Fax: +886-35/729-134, E-mail: yfchen@cc.nctu.edu.tw     

Optimization of optical parametric chirped pulse amplification with different pump wavelengths

Optical parametric chirped pulse amplification with different pump wavelengths was investigated using LBO crystal, at signal central wavelength of 800 nm. According to our theoretical simulation, when pump wavelength is 492.5 nm, there is a maximal gain bandwidth of 190 nm centered at 805 nm in optimal noncollinear angle using LBO. Presently, pump wavelength of 492.5 nm can be obtained from second harmonic generation of a Yb:Sr₅(PO₄)₃F laser. The broad gain bandwidth can completely support ∼6 fs with a spectral centre of seed pulse at 800 nm. The deviation from optimal noncollinear angle can be compensated by accurately tuning crystal angle for phase matching. The gain spectrum with pump wavelength of 492.5 nm is much better than those with pump wavelengths of 400, 526.5 and 532 nm, at signal centre of 800 nm.     

High power and high beam quality CW green beam generated by diode-side-pumped intracavity frequency doubled Nd:YAG laser

We report a stable high power and high beam quality diode-side-pumped CW green laser from intracavity frequency doubled Nd:YAG laser with LBO crystal. By using a advanced resonator, a large fundamental mode size in the laser crystal and a tight focus in the nonlinear crystal could be obtained simultaneously, which are favorable for high power and high beam quality CW green laser generation. The green laser delivered a maximum 532 nm output power of 40 W. The corresponding optical-to-optical conversion efficiency and electrical-to-optical conversion efficiency were 8.6% and 5.0%, respectively. Under 532 nm output power of 34 W, the beam quality factor was measured to be 1.6.