Generation of high-quality 10-femtosecond laser pulses with intracavity spatial and spectral control

Spatial and spectral control, using an intracavity capillary and a slit, is applied to improve the output pulse quality of a Ti:sapphire laser. Satellite-free 10-fs optical pulses with a smooth spectral and spatial profile have been generated. Employing a root-mean-square formalism for pulse characterization, spatial, spectral and temporal intensity distributions are analyzed for laser pulses with a duration as short as three to four optical cycles. Received: 11 June 2001 / Published online: 18 July 2001     

Optical limiting in cobalt-doped polyvinylpyrrolidone

The results of optical limiting investigations of cobalt-doped polyvinylpyrrolidone solutions are presented. The optical limiting studies have shown that this process is due to self-defocusing at the wavelength of 1064 nm and also due to reverse saturable absorption and self-defocusing at the wavelength of 532 nm. The results of measurements on the non-linear optical characteristics of organometallic complexes are presented. Received: 10 July 2001 / Revised version: 15 October 2001 / Published online: 29 November 2001     

Femtosecond pulse shaping by an evolutionary algorithm with feedback

Received: 20 September 1997     

Focusing dependence of the efficiency of a singly resonant optical parametric oscillator

Received: 15 February 1998/Revised version: 6 April 1998     

Generation of subpicosecond infrared pulses tunable between 5.2 μm and 18 μm at a repetition rate of 76 MHz

2 (2 mm thick) and GaSe (1 mm thick) crystals are applied as nonlinear media. GaSe has a larger tuning range and is more efficient in the whole spectral range than AgGaS₂. The average IR power reaches up to 2.0 mW at 8.5 μm when the GaSe crystal is used, and up to 1.3 mW at the same wavelength when AgGaS₂ is applied for the frequency conversion. Received: 17 February 1997/Revised version: 14 July 1997     

Phase-matching for generation of few high order harmonics in a semi-infinite gas cell

Small phase-mismatch between the fundamental and high order polarization in the cut-off region, allowing macroscopic phase-matching to be satisfied for these harmonics can be used for generation of extreme ultraviolet radiation with high flux, high coherence and few harmonic orders in a semi-infinite argon gas cell.     

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     

Laser beam homogenization: Modeling and comparison with experimental results

Beam shaping and homogenization techniques are essential to optimize a large number of laser-material processing applications and laser-material interaction studies. In this paper, we present an innovative approach for modeling laser beam homogenization by means of the integration method. The numerical results are compared with experimental data, and the influence of the measurement technique is discussed. The enhancement of the homogenization capability using an asymmetric divider is also presented and discussed.     

Second-harmonic generation in Zn-diffused periodically poled LiNbO3 channel waveguides

In this work second-harmonic generation by quasi-phase matching (QPM) in Zn-diffused periodically poled lithium niobate channel waveguides is presented. A stable TM?TE conversion by QPM has been found. The results are in good accordance with theoretical estimations obtained by the phase-matching condition, either for the polarisation character of the second-harmonic wave as well as for the spectral range, taking into account the periodicity of the domains. Received: 16 May 2001 / Revised version: 7 September 2001 / Published online: 30 October 2001     

Seven-terawatt Ti:sapphire laser system operating at 50 Hz with high beam quality for laser Compton femtosecond X-ray generation

We present a 7-TW Ti:sapphire laser system operating at 50 Hz for laser Compton femtosecond X-ray generation. This laser system delivers 8.4 W of average output power at a repetition rate of 50 Hz with a pulse width of 24 fs. It demonstrates successful management using a dynamically stable resonator in the regenerative amplifier and compensation for thermal lensing by a convex mirror in a ring-type four-pass power amplifier. We also present the results of closed-loop corrections for distorted wavefronts of amplified and compressed laser pulses, using an adaptive optical system consisting of a Shack–Hartmann-type wavefront sensor and a deformable mirror. This closed-loop correction results in dramatic improvements, reducing wavefront distortions below 0.05 λ rms. Received: 31 October 2002 / Revised version: 3 March 2003 / Published online: 5 May 2003 RID="*" ID="*"Corresponding author. Fax: +81-298/61-3349, E-mail: ito@festa.or.jp     

Diffractive effects in singly resonant continuous-wave parametric oscillators

I /k_P=0.33and0.50. It is found that the ratio of pump depletion to maximal depletion as a function of the ratio of pump power to threshold power agrees with the plane-wave prediction to within 5%, for a wide range of focusing conditions. The observed trends are explained as resulting from intensity- and phase-dependent mechanisms. Received: 19 January 1998/Revised version: 13 March 1998     

Second-harmonic generation of 405-nm light using periodically poled KTiOPO4 pumped by external-cavity laser diode with double grating feedback

A frequency-doubled laser diode system for generation of blue–UV light is described. The system is based on an external-cavity high-power laser diode with double feedback from the zeroth and the first orders of a diffraction grating. Light at 405 nm is generated in a single-pass configuration using periodically poled KTiOPO₄. We show that the double grating feedback improves the second harmonic conversion efficiency by several orders of magnitude as compared to the freely running laser. The conversion efficiency may be improved further such that higher second-harmonic powers may be generated.     

Second-order optical non-linearity of proton exchanged lithium tantalate waveguides

A detailed correlation between the fabrication conditions, crystallographic phase state of H_xLi_1-xTaO₃ waveguides and second-order optical non-linearity has been investigated by using reflected SHG measurements from the polished waveguide end face. The non-linearity, strongly reduced after the initial proton exchange, is found to be restored and even increased after annealing. However, this apparent increase in the non-linearity is accompanied by a strong degradation of the quality of the SHG reflected beam in the region of the initial as-exchanged waveguide due to beam scattering. The high temperature proton exchange technique has been shown to produce high-quality α-phase waveguides with essentially undegraded non-linear optical properties. There is no phase transition when the α-phase waveguides are fabricated by direct exchange. This phase presents the same crystalline structure as that of LiTaO₃ and maintains the excellent non-linear properties of the bulk material. The results obtained are important for the design, fabrication and optimization of guided-wave non-linear optical devices in LiTaO₃. Received: 21 May 2001 / Published online: 23 October 2001     

Two-photon photoconductivity in SiC photodiodes and its application to autocorrelation measurements of femtosecond optical pulses

Received: 18 June 1997     

Low-threshold mid-infrared optical parametric oscillation in periodically poled LiNbO3 synchronously pumped by a Ti:sapphire laser

We report the generation of tunable high-repetition-rate optical pulses in the mid-infrared using synchronously pumped parametric oscillation in periodically poled LiNbO₃ (PPLN). Using a Kerr-lens-mode-locked Ti:sapphire laser as the pump source and a PPLN crystal incorporating grating periods of 21.0–22.4 μm, we have achieved wavelength conversion in the -–4 6μm spectral range in the mid-infrared. The use of a semi-monolithic cavity design and hemispherical focusing has permitted pulse generation in the strong idler absorption region of PPLN, resulting in a simple, compact, all-solid-state configuration with a pump power threshold as low as 17 mW and mid-infrared idler powers of up to 64 mW at 9% extraction efficiency. Signal output powers of up to 280 mW at 35% extraction efficiency are available over the -–1.004 1.140μm spectral range at 80.5 MHz and pulse repetition rates at harmonics of the fundamental frequency up to 322 MHz have also been obtained. Received: 5 December 2000 / Revised version: 23 January 2001 / Published online: 27 April 2001     

Reverse saturable absorption of a novel rhodamine-B cation fulleride salt at 532 nm

A novel rhodamine-B cation fulleride salt was synthesized by metathetical reaction. Its tetrahydrofuran solution was prepared and the fluence-dependent transmission was measured with 10-ns 532-nm laser pulses in a collimated optical setup. With increases in the input fluence, its transmittance decreased significantly, which indicated reverse saturable absorption behavior. For comparison, the input fluence-dependent transmittance of fullerene C₆₀ and rhodamine-B solution were also measured. The nonlinear optical limiting effect of the fulleride salt in tetrahydrofuran is slightly lower than that of C₆₀ in toluene and the explanations are provided. Received: 25 August 2000 / Accepted: 6 November 2000 / Published online: 3 April 2001     

Phase-modulation-induced two-wave mixing in a temporal-nonlocal medium

Received: 27 October 1997/Revised version: 11 December 1997     

Phase-modulation effects in degenerate four-wave-mixing

Received: 14 August 1997     

Measuring the temporal intensity of ultrashort laser pulses by triple correlation

Received: 13 May 1997/Revised version: 1 August 1997     

100-W average-power, high-energy nanosecond fiber amplifier

We report on the fiber-based amplification of a Q-switched Nd:YAG thin-disk laser. At repetition rates between 3 and 50 kHz output powers up to 100 W are generated. Pulse energies up to 4 mJ, with diffraction-limited beam quality, are generated in a 30-μm Yb-doped large-mode-area fiber, furthermore pulse energies up to 8 mJ are achieved from a multimode fiber amplifier. Received: 8 May 2002 / Revised version: 10 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +49-3641/65-7680, E-mail: Jens.Limpert@uni-jena.de