An Ultra-Broadband Spatially Dispersed Regenerative Amplifier Free from Spatial Chirp

An ultra-broadband Ti：sapphire regenerative amplifier based on spatially dispersed amplification is demonstrated experimentally. Departing from previous reports, a new design of the cavity gets the amplified pulse free from spatial chirp. Utilizing this new regenerative amplifier, chirped pulses with bandwidth （FWHM） of about 80 nm are obtained, and the bandwidth is limited only by that of the incident seed pulses.     

Table-top KrF amplifier delivering 270 fs output pulses with over 9 W average power at 300 Hz

Applying the combination of a solid-state Ti:Sa laser system and a newly developed wide-aperture, discharge-pumped KrF amplifier, output pulses with over 9 W average power at 300 Hz have been achieved in a single output beam. The frequency-tripled seed pulses of the Ti:Sa system – delivering approximately 10 μJ energy at 248 nm – were amplified to over 30 mJ using a 3-pass off-axis amplification scheme. The optical set-up has been fitted to the amplifier’s parameters, and stored-energy measurements were carried out with different parameters in order to optimize the operational conditions of the device for the highest average power. Received: 10 June 2002 / Published online: 25 September 2002 RID="*" ID="*"Corresponding author. Fax: +49-551/503-599, E-mail: jbekesi@llg.gwdg.de     

Efficient generation of 200 mJ nanosecond pulses at 100 Hz repetition rate from a cryogenic cooled Yb:YAG MOPA system

A diode-pumped Master Oscillator Power Amplifier (MOPA) laser system based on cryogenic cooled Yb:YAG has been designed, developed and its output performance characterised. The laser system consists of a fibre oscillator, an active mirror regenerative amplifier and a four pass main amplifier. 2.4 mJ, 10 ns, 100 Hz seed pulses from the fibre oscillator/regenerative amplifier arrangement were amplified up to pulse energies of over 200 mJ by using the four pass main amplifier arrangement. As a further study we have obtained an increased slope efficiency of 40% and an optical-to-optical efficiency of 30% using a pinhole vacuum spatial filter/image relay for laser mode control. With 1.8 mJ input seed pulses, output pulse energies of around 150 mJ were achieved.     

Enhancement of Gain in L-Band Bismuth-Based Erbium-Doped Fibre Amplifier Using an Un-pumped EDF and Midway Isolator

A hybrid L-band erbium-doped fibre amplifier (EDFA) with enhanced gain characteristic is demonstrated without a significant noise figure penalty. It uses a backward C-band amplified stimulated emission from both the ends of a bismuth-based EDFA system to pump an unpumped erbium-doped fibre (EDF) for gain enhancing. The maximum gain enhancement of 4.0dB is obtained at wavelength 1604nm with EDF length of 20m. The gain spectrum is reasonably fiat in this amplifier compared with the amplifier without an EDF. The gain varies from 27.4 dB to 30.2 dB at wavelength region 1564-1608 nm with incorporation of 20 m EDF. Noise figure also varies from 6.0 to 7.TdB at this wavelength region.     

Mode competition in multi-core fiber amplifier

3D beam propagation numerical code and a newly developed optical mode solver were implemented for studies on the recently experimentally realized Yb fiber amplifier with 7-core hexagonal structure. It was found that the launched seven single-frequency beams matched with the cores converge to a wave field with permanently diminishing space beating patterns, but the result is sensitive to phases of separate beams. Namely, at a spread of phases of individual beams within a few tenths of radian, the convergence of the wave field to the in-phase mode is observed, while at phases random spread on the order of π the out-of-phase mode is dominating. Such behavior is observed at strong gain saturation in the cores. Both effects take place even if the guiding index of the cores fluctuates within a limited range. The mode solver predicts that the in-phase mode has the minimal small signal gain, and the gain of the out-of-phase mode is maximal one. This fact proves that there is no strict correlation between modal gains and amplification in strong saturated multi-core fiber. The smaller core index step significantly improves the convergence of the amplified radiation to the in-phase mode. An approach to the waveguide structure tailoring is described which assures the single in-phase mode stability.     

An alternative approach for microlithography light source

A hybrid master oscillator–power amplifier (hybrid MOPA) scheme is proposed as a microlithography light source. The seed pulses are generated in the visible spectral range—where the necessary spectral purity is more easily achieved—and after frequency conversion are amplified in an excimer amplifier. The new concept enables us to decrease the bandwidth considerably, approaching the theoretical limit posed by the uncertainty relation. The feasibility of the new approach is demonstrated by a dye/excimer MOPA system, generating deep ultraviolet DUV (248-nm) pulses of 0.2-pm bandwidth.     

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     

Gain depletion due to amplified spontaneous emission in multi-pass laser amplifiers

Calculations of the effects of amplified spontaneous emission (ASE) in laser amplifiers designed to amplify sub-picosecond pulses are presented. A one-dimensional code is used with some simple approximations to account for aspect ratio variation in cases of cylindrical symmetry. Gain depletion due to ASE is compared for various multi-pass geometries. Optimisation for short pulse amplification of a telescopic, beam-expanding, 4-pass amplifier is considered.On leave from Electrotechnical Laboratory, Tsukuba, Ibaraki, Japan     

Design and characterization of a near-diffraction-limited femtosecond 100-TW 10-Hz high-intensity laser system

We present the design and characterization of a femtosecond high-intensity laser system emitting a near-diffraction-limited beam. This system was dimensioned in order to reach intensities in excess of 10²⁰ W/cm² at a high repetition rate for ultrahigh-field physics experiments. We describe the improvements that were added to a conventional chirp pulse amplification configuration in order to decrease the deleterious effects of gain narrowing, gain shifting, thermal focusing in the amplifier stages, and spatial degradation due to multipass amplification processes. Received: 4 January 2002 / Published online: 14 May 2002     

High-beam-quality, 5.1 J, 108 Hz diode-pumped Nd:YAG rod oscillator-amplifier laser system

A high efficiency, high beam quality diode-pumped Nd:YAG master oscillator power-amplifier (MOPA) laser with six amplifier stages is demonstrated. The oscillator with two-rod birefringence compensation was designed as a thermally determined near hemispherical resonator, which presents a pulse energy of 223 mJ with a beam quality value of M² = 1.29 at a repetition rate of 108 Hz. The MOPA system delivers a pulse energy of 5.1 J with a pulse width of 230 μs, a M² factor of 3.6 and an optical-to-optical efficiency of 38.5%. To the best of our knowledge, this is the highest pulse energy for a diode-pumped Nd:YAG rod laser operation with a high beam quality and a pulse width of hundreds of microseconds at a repetition rate of over 100 Hz.     

Liquid-cooled ceramic Nd:YAG split-disk amplifier for high-average-power laser

A liquid-cooled ceramic Nd:YAG split-disk laser amplifier has been designed for a high-average-power laser system. The maximum average output power of 17.1 W (1.71 J at 10 Hz) and the beam size of far-field pattern with 1.5 times diffraction limit was obtained. The combination of liquid cooling for disk laser material and wavefront compensation with a phase conjugation mirror was demonstrated. The wavefront degradation by liquid cooling was observed and measured with Michelson interferometer and far-field patterns.     

High-energy, spatially flat-top green pump laser by beam homogenization for petawatt scale Ti:sapphire laser systems

We have demonstrated that we can homogenize the spatial profile of a high-energy green laser pulse used for pumping a petawatt scale Ti:sapphire amplifier. The second harmonic of a high-energy, large aperture Nd:glass laser system generates laser emission at a green wavelength with 75 J single pulse energy. Using a diffractive optical element for beam homogenization, we have obtained a highly spatially uniform flat-top second harmonic profile.     

Amplification of kW peak power femtosecond pulses in single quantum well InGaAs tapered amplifiers

The amplification of ps and fs pulses with peak powers of up to 4.5 kW has been investigated in a single quantum well InGaAs tapered amplifier. The pulses with durations of 100 fs or 2 ps were generated by a modelocked titanium-sapphire laser. The amplified pulses indicate strong gain saturation and carrier generation due to photon absorption in the laser active region which causes a temporal broadening of the amplified pulses as well as modifications of the optical spectrum. The gain recovery time was measured by a pump-probe experiment. The experimental results are analyzed with respect to the sub-ps gain dynamics which is described by a relaxation time approximation.     

Properties of an ultrashort-pumped near-IR high gain dye amplifier

An ultra-short-pumped optical dye amplifier operating in the near-IR (720 nm) has been developed. We present an experimental study of the input–output characteristics of this simple device in a traveling wave collinear configuration. In essence, the combined effects of the ultrafast nature of the anisotropy induced by the exciting laser pulse along with the short transit time across the length of the cell allow for optimal output conditions given the number of molecules in the active volume. Two distinct amplifying regimes have been observed depending on photon density. Typical gain values of 10⁴ of the narrow bandwidth (∼9 nm) subpicosecond output signal were measured. The 8-mm² beam cross section enables this photon amplifier to be utilized in time-gated imaging applications. Received: 12 December 2001 / Revised version: 13 May 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +1-819/564-5442, E-mail: Daniel.Houde@Usherbrooke.ca     

A versatile dye-laser generator-amplifier system for intense,tunable picosecond pulse generation

Passively mode-locked ruby-laser pulses are used to generate nearly diffractionlimited picosecond light pulses in a dye cell by longitudinally amplified spontaneous emission. The output pulses are amplified in three longitudinally pumped dye cells, then spectrally filtered with a grating spectrometer and finally reamplified in a fourth dye amplifier in order to generate intense frequency tunable picosecond light pulses.     

Effect of the spiral phase element on the radial-polarization (0, 1) LG beam

Radially-polarized beams can be strongly amplified without significant birefringent-induced aberrations. However, radially-polarized beam is a high-order beam, and therefore has to be transformed into a fundamental Gaussian beam for reduction the beam-propagation factor M². In effort to transform the radially-polarized beam to a nearly-Gaussian beam, we consider effect of a spiral phase element (SPE) on the Laguerre-Gaussian (LG) (0, 1)^∗ beam with radial polarization, and compare this with the case when the input beam is a LG (0, 1)^∗ beam with spiral phase and uniform or random polarization. The LG (0, 1)^∗ beam with radial polarization, despite its identity in intensity profile to the beam with spiral phase, has distinctly different properties when interacting with the SPE. With the SPE and spatial filter, we transformed the radially-polarized (0, 1)^∗ mode with M² = 2.8 to a nearly-Gaussian beam with M² = 1.7. Measured transformation efficiency was 50%, and the beam brightness P/(M²)² was practically unchanged. The SPE affects polarization state of the radially-polarized beam, leading to appearance of spin angular momentum in the beam center at the far-field.     

Highly Stable, Diode-Pumped Nd：YLF Regenerative Amplifier

We present the design and experimental results for a diode pumped Nd：YLF regenerative amplifier applied to amplify a nanosecond laser pulse. Numerical simulation shows that the maximum output energy and the best stability can be obtained when the regenerative amplifier operates in a saturated mode for all pulse duration and temporal profiles. Using extra post-pulse is a good method to decrease the square-pulse distortion caused by gain saturation effect. The amplifier shows output energy of 4.2mJ with a total energy gain of more than 10^7 and output energy stability of better than 1% rms. When extra post-pulse is added, square-pulse distortion is decreased from 1.33 to 1.17 for the amplifier that is seeded with an optical pulse of 3ns.     

Dependence of spectrum on pump-signal angle in BBO-I noncollinear optical-parametric chirped-pulse amplification

A BBO-I noncollinear optical-parametric chirped-pulse amplifier pumped by a Q-switched and frequency-doubled Nd:YAG laser has been demonstrated. At an optimal pump-signal angle, the temporally stretched chirped signal pulses with bandwidth of 36 nm (FWHM) were amplified without any distortion in spectrum. The gain bandwidth was very sensitive to the pump-signal angle. Variation of this angle by about 1.5 mrad may significantly reduce the amplified bandwidth and result in a significant distortion in the amplified spectrum. Received: 5 October 2000 / Revised version: 13 June 2001 / Published online: 19 September 2001     

Self phase modulation in a femtosecond pulse amplifier with subsequent compression

The pulses from a colliding-pulse mode-locked dye laser (100 fs, 20 pJ) are amplified to 0.2 mJ and self phase modulated in an excimer laser pumped dye amplifier. Suitable chirp compensation leads to nearly bandwidth-limited pulses of about 50 fs duration.