Blue 489-nm picosecond pulses generated by intracavity frequency doubling in a passively mode-locked optically pumped semiconductor disk laser

We report the generation of blue 489-nm picosecond laser pulses by intracavity second-harmonic generation in a mode-locked optically pumped InGaAs vertical-external-cavity surface-emitting laser. Mode locking achieved by a semiconductor saturable absorber mirror generated 5.8-ps-long sech²-shaped pulses at an emission wavelength of 978 nm and a repetition rate of 1.88 GHz. Intracavity frequency doubling in a 5-mm-long lithium triborate crystal generated blue picosecond pulses with a spectral width of 0.15 nm and an average output power of up to 6 mW.     

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     

Influence of gas circulation on stimulated Raman scattering and amplification of ultrashort laser pulses in methane

Applying gas recirculation in a high pressure cell, laser pulses of 1 ps at 400 nm and with a repetition rate of 1 kHz were frequency shifted by stimulated Raman scattering and amplification in methane gas at high pressure. We studied the influence of gas recirculation on the conversion efficiencies into the Stokes and anti-Stokes components as well as on their spatial distributions and spectral shapes using generator and generator-amplifier arrangements. For high pump energies, recirculation in the generator cell decreases conversion efficiency into the first Stokes component whereas it increases conversion into higher Stokes and anti-Stokes components. It results in a significantly improved spatial characteristics of the frequency-shifted radiation, however, is accompanied by a substantial spectral broadening. Using gas recirculation in the generator-amplifier arrangement we achieved a conversion efficiency into the first Stokes component of about 50% with highly improved spatial and spectral characteristics.     

A picosecond CuBr laser

A novel method for the pulse shortening of the 578 nm spectral line from a CuBr laser is proposed in this paper. By amplifying a seeded picosecond dye laser through the active medium of a CuBr laser, nearly Fourier-transform-limited picosecond pulses of the CuBr laser have been obtained and the output characteristics are also investigated in detail.     

Tunable Ti: Sapphire regenerative amplifier for femtosecond Chirped-Pulse Amplification

We describe a tunable Ti:Sapphire regenerative amplifier which is used to amplify 120 fs pulses from a self-mode-locked Ti:Sapphire laser to energies in the range of 7–12 mJ from 760 nm to 855 nm. We have used three sets of cavity mirrors in the regenerative amplifier to vary the output wavelength of the laser.On leave from Institute of Laser Engineering, Osaka University, 2-6, Yamada-oka, Osaka 565, Japan (Fax: +81-6/877-4799)     

Compact Single-Stage Femtosecond Multipass Ti：Sapphire Amplifier at 1 kHz with High Beam Quality

A compact femtosecond Ti：sapphire amplifier system is reported using single-stage multipass configuration with high beam quality. A high dispersion glass stretcher and a pair of double prisms for compression are introduced based on broadband femtosecond seed pulses. The non-grating-based pulse stretcher and compressor are advantageous to increase high beam quality and to reduce the high-order dispersion. A Gaussian filter is used to reduce the gain narrowing effect in amplification. The compact femtosecond Ti：sapphire nine-pass amplifier delivers pulses with a duration of 26 fs and an energy of 800μJ at 7mJ pumping pulses energy at I kHz. The 1-kHz femtosecond amplifier with high beam quality and high stability is very suitable for ultrafast physics research applications, such as attosecond science, ultra-precision micromachining, and THz wave generation.     

Efficient compression of femtosecond pulses by stimulated Raman scattering

Femtosecond pulses at 496 nm were Raman-shifted in methane with 20% efficiency. The pulse duration could be reduced up to 6.5 times from 560 fs at the fundamental to 85 fs at the Stokes frequency (580nm), which is the shortest pulse duration generated in this way. It was shown experimentally that chirped-pulse Raman scattering avoids the limitations arising from self-phase modulation.     

Efficient multi-frequency generation of ultrashort light pulses using stimulated Raman scattering and optical parametric amplification

Optical parametric amplification of multi-frequency seed pulses generated in a mixture of compressed hydrogen and methane by stimulated Raman scattering of 1 ps, 1 kHz laser pulses at 395.8 nm has been studied. Efficient generation of spectrally narrow ultrashort pulses with a spatial distribution close to the Gaussian profile of the pump beam was obtained in the visible and near infrared ranges.     

High-order stimulated electronic Raman scattering from lithium vapor

An intense radiation at 395.0 nm has been observed when lithium vapor is optically pumped in a heat pipe with a pulsed dye laser whose output wavelength is tuned near the Li 2s–4s two-photon resonance transition. The radiation is emitted in the direction along the pump laser beam. It is proposed that the 395.0 nm radiation is mainly generated through three-photon excitation and one-Raman-photon scattering followed by two-cascade (spontaneous) emission. The overall reaction mechanism can be described by a parametric six-wave mixing process. The quantum efficiency of the observed process is estimated to be of the order of 2 × 10^–6.This work was partially supported by the USC Faculty Research and Innovation Fund     

Generation of high-order rotational lines by four-wave Raman mixing using a high-power picosecond Ti:Sapphire laser

More than thirty rotational lines equally spaced by 587 cm^–1 are generated simultaneously in the vicinity of the fundamental line by four-wave Raman mixing using a high-power picosecond Ti:Sapphire laser as a pump source and hydrogen as a Raman medium. Since the wavelength of this multifrequency laser emission extends from the near-infrared to the near-ultraviolet, it can be utilized as a tunable light source for picosecond spectroscopy. Because of the wide spectral bandwidth available, this procedure has great potential for the generation of ultrashort laser pulses by mode-locking these emission lines.     

Suppression of stimulated hyer-Raman scattering in lithium vapor

The stimulated hyper-Raman scattering (SHRS) in lithium vapor has been observed in both the forward and the backward directions as the laser was tuned near the 2S–3S and 2S–4S two-photon resonances. It is found that in the forward direction the SHRS associated with the 2P excitation near the 2S–3S or the 2S–4S two-photon resonance is completely suppressed, but the SHRS associated with the 3P excitation near the 2S–4S two-photon resonance can be observed. A condition under which the forward SHRS takes place, given by Malakyan [3], has been examined. A reasonable agreement between the theory and the experiment is obtained. Near the 2S–3S two-photon resonance, instead of the expected emission _2P–2S corresponding to the 2P–2S transition, an emission at the 2_P–_3S–2P frequency, which is close to the _2P–2S, has been observed in the backward direction. The process generating this emission could be considered as an inverse SHRS process induced by the 3S–2P spontaneous emission. For the SHRS emission a shift of the observed frequency from the calculated frequency has been found. In order to explain the shift, the Stark effect has been calculated. The results show that the shift can not be attributed to the Stark effect.     

Intracavity frequency-tripling of actively mode-locked diode-pumped Nd:YAG laser

We report the first diode-pumped solid-state laser operating in cw-mode-locked regime and simultaneously achieving intracavity frequency-tripling. This laser provide UV picosecond pulses (λ=355 nm) of 10 ps duration with 0.5 mW average power at 150 MHz repetition rate. A different set of adjustments gave rise to a Q-switched mode-locked regime. Trains of hundred UV pulses of 60 ps duration and 4 W peak power were produced in this latter case at 50 kHz repetition rate. Received: 12 October 1998 / Revised version: 12 December 1998 / Published online: 26 May 1999     

Intense tunable picosecond pulses generated by parametric amplification in β-BaB2O4

Amplified spontaneous emission of a dye cell is parametrically amplified in barium betaborate (BBO) crystals. Pumping the system by the third harmonic of a Nd:YAG laser a tuning range from 3500 to 25080 cm^–1 is accessible with energy conversion efficiencies of several percent. Properties of ultrashort light pulses in the visible and UV spectral range are presented. A spectral width of 10 cm^–1 and a pulse duration of approximately 20 ps are measured.     

20-Hz operation of an eye-safe cascade Raman laser with a Ba(NO3)2 crystal

Operation of a 1.598-μm eye-safe third-Stokes Raman laser with a Ba(NO₃)₂ crystal pumped by a 1.064-μm Nd:YAG laser is described. We observed a substantial decrease in the output energy during the first 50 s of the continuous operation at 20 Hz. The energy drop is ∼76% of the initial third-Stokes output. We confirmed negative thermal lensing and thermally induced birefringence in the crystal. With a concave cavity mirror at a matched curvature to the thermal lensing, we obtained an output energy of 11 mJ at 20 Hz. TEM₀₀ output was also obtained with a smaller pump-beam diameter with a highest conversion efficiency of 15.5% for a pumping power of only 45 MW/cm² (0.9 J/cm²). Received: 20 November 2001 / Revised version: 20 February 2002 / Published online: 2 May 2002     

Tunable UV-radiation by stimulated Raman scattering in hydrogen

We observed tunable UV radiation down to 175 nm by SRS of the output of a frequency-doubled dye laser in H₂. The high power output of the 4th anti-Stokes to 5th Stokes line generated by the frequency doubled dye laser and the 8th anti-Stokes to 3rd Stokes line generated by the undoubled dye laser represents a broadly tunable coherent radiation source between 189 nm and 2064 nm.     

Diode-pumped Cr:LiCAF fs regenerative amplifier system seeded by an Er-doped mode-locked fiber laser

A fs regenerative amplifier based on Cr:LiCAF is demonstrated for the first time. With direct diode pumping, 8 μJ of pulse energy are obtained directly from the amplifier. When seeded by an Er-doped fiber laser, the amplified seed pulses are compressed down to 252 fs, limited by residual net third-order dispersion of the compression gratings and intracavity elements. Pulse broadening due to second- and third-order dispersion is theoretically investigated and compared to experimental results. Dispersion generated by the geometrical cavity arrangement is measured experimentally. Received: 19 November 2001 / Revised version: 28 January 2002 / Published online: 14 March 2002     

4-W and 23-ps pulses from a lamp-pumped Nd:YAG laser passively mode-locked by polarization switching in a KTP crystal

Passive mode-locking of a cw lamp-pumped Nd:YAG laser using nonlinear polarization switching in a type-II SHG crystal is reported. Light pulses with more than 5 W of average power and pulse duration shorter than 25 ps have been obtained at 1064 nm. Received: 29 January 1999 / Revised version: 24 March 1999 / Published online: 1 July 1999     

Phase conjugation in potassium vapour using subnanosecond laser pulses

Phase conjugated waves (PCW) were generated for the first time by degenerate four-wave mixing with subnanosecond laser pulses in potassium vapour near the D ₁ transition at 7699 Å. The light source was a long cavity grazing incidence dye laser. The pulses had a duration of 700–800 ps and a linewidth of 0.2 Å. The experiments were performed in a temperature range of 400–550° C. A maximum reflectivity of 57% has been observed together with temporal pulse shortening. PCW reflectivity has been measured as a function of the temperature, and the backward pump intensity.     

Walk-off and phase-compensated resonantly enhanced frequency-doubling of picosecond pulses using type II nonlinear crystal

We propose enhanced frequency-doubling inside an external ring-cavity using type II nonlinear crystal. A KTP type II twin-crystal device is implemented for compensation of both walk-off and phase-shift between ordinary and extraordinary fundamental waves. Starting from an 850 mW diode-pumped actively mode-locked Nd:YAG laser at 1.064 μm with 100 MHz repetition rate and 25 ps pulse duration, we performed 54% harmonic conversion efficiency in the green. Received: 31 August 1998 / Revised version: 10 February 1999 / Published online: 19 May 1999     

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.