Diode-pumped thin-disk Yb:YAG regenerative amplifier

Received: 18 June 1997     

SOA-based actively mode-locked fiber ring laser by forward injecting an external pulse train

An actively mode-locked fiber ring laser based on cross-gain modulation (XGM) in a semiconductor optical amplifier (SOA) is demonstrated to operate stably with a simple configuration. By forward injecting an easily-generated external pulse train, the mode-locked fiber laser can generate an optical-pulse sequence with pulsewidth about 6 ps and average output power about 7.9 mW. The output pulses show an ultra-low RMS jitter about 70.7 fs measured by a RF spectrum analyzer. The use of the proposed forward-injection configuration can realize the repetition-rate tunability from 1 to 15 GHz for the generated optical-pulse sequences. By employing a wavelength-tunable optical band-pass filter in the laser cavity, the operation wavelength of the designed SOA-based actively mode-locked fiber laser can be tuned continuously in a wide span between 1528 and 1565 nm. The parameters of external-injection optical pulses are studied experimentally to optimize the mode-locked fiber laser.     

Experimental study of pulse compression in a side-pumped Yb-doped double-clad mode-locked fiber laser

We have developed a side-pumped passively mode-locked Yb-doped double-clad fiber laser emitting sub-picosecond pulses at around 1.05 μm. Mode locking is achieved through a polarization additive-pulse mode-locking technique and compression is obtained with a grating pair inserted in the cavity. We have investigated the compression properties of this laser. High-energy pulses are emitted and different behaviors are observed, such as sideband generation, secant-like or Gaussian-like pulse emission. Received: 7 May 2001 / Revised version: 13 November 2001 / Published online: 17 January 2002     

Superluminal pulse propagation in an erbium fiber laser

It is well known that saturating gain media can give rise to superluminal pulse propagation velocity. In most mode-locked lasers the effect is unmeasurably small and has not yet been directly demonstrated. We present experiments on the initial transient of an erbium fiber laser that show a dynamic shift in the propagation velocity to a value larger than the medium’s linear group velocity as the pulse builds up. Received: 14 December 1998 / Revised version: 21 May 1999 / Published online: 25 August 1999     

Colliding-pulse harmonically mode-locked fiber laser

A Fabry–Perot semiconductor saturable absorber has been implemented in a colliding pulse configuration for stabilizing a harmonically mode-locked fiber laser. An environmentally stable CPM design is shown to enhance supermode suppression. Received: 24 October 2000 / Revised version: 28 November 2000 / Published online: 9 February 2001     

Directly diode-pumped Colquiriite regenerative amplifiers

The Colquiriite crystals are attractive materials for directly diode-pumped femtosecond oscillators and amplifiers. Cr:LiSAF, Cr:LiSGAF and Cr:LiCAF are for the first time directly compared for their use in regenerative amplifiers both experimentally and theoretically. A maximum pulse energy of 10 μJ was obtained at an absorbed pump power of 1.1 W. A review of the work is given including a Cr:LiSAF seed oscillator, outlining specific needs for seeding.     

Femtosecond thin-disk Yb:KYW regenerative amplifier

We report a compact thin-disk Yb:KYW regenerative amplifier system. Two different concepts are investigated to obtain either subpicosecond pulses with up to 160 μJ or a pulse energy of 20 μJ with a pulse width of about 300 fs. The first concept uses intra-cavity group-velocity dispersion compensation with Gires–Tournois interferometer mirrors to avoid pulse stretching during amplification. The onset of nonlinear effects in this concept inhibits the generation of shorter pulse durations at this energy level. Shorter pulses can be achieved with the second concept, which is based on dispersive pulse stretching during amplification and uses pulse compression after amplification with a grating compressor. Repetition rates up to 45 kHz are demonstrated.This revised version was published online in May 2005. The Article Category was removed.This revised version was published online in August 2005 with a corrected cover date.     

2-GHz passive harmonically mode-locked Yb-doped double-clad fiber laser

We report passive harmonic mode locking of a high-power Yb-doped double-clad fiber laser. 680-fs, 48-pJ pulses are emitted at the repetition rate of 2.13 GHz, while the free spectral range of the cavity is 23.4 MHz. Results indicate a supermode suppression of more than 25 dB.     

SPM-induced spectral compression of picosecond pulses in a single-mode Yb-doped fiber amplifier

In a fiber amplifier, spectral compression due to self-phase modulation is demonstrated for ultrashort pulses. We report the generation of near-transform-limited picosecond pulses with peak powers of several kW at a repetition rate of 74 MHz and diffraction-limited beam quality in a Yb-doped fiber amplifier when seeding with a negative chirped pulse. Received: 17 September 2001 / Revised version: 22 November 2001 / Published online: 17 January 2002     

Sub-20 fs pulse generation from the mirror dispersion controlled Cr:LiSGaF and Cr:LiSAF lasers

Received: 21 March 1997/Revised Version: 9 May 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     

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     

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.     

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.     

High average power ultra-fast fiber chirped pulse amplification system

We report on a high-gain diode-pumped ytterbium-doped fiber-amplifier system delivering pulse energies in the 100-μJ range at high repetition rates (32 kHz) with nearly-diffraction-limited beam quality (M²∼1.7) at a 1060-nm center wavelength. Femtosecond seed-laser pulses are stretched in a commercially available single-mode fiber and compressed after amplification to subpicosecond pulse duration. In a multimode Yb-doped fiber amplifier we have demonstrated average powers of up to 22 W and single-pulse energies of up to 130 μJ. Received: 16 August 2000 / Revised version: 4 September 2000 / Published online: 8 November 2000     

Harmonically mode-locked laser stabilized by semiconductor saturable absorber modulated with the residual pump

Stabilization of a passive harmonically mode-locked laser is demonstrated by directly modulating a 980-nm pump diode laser used both to provide gain and to optically modulate an intracavity saturable absorber. The results indicate a reduction of the undesired laser modes by more than 55 dB. The timing jitter was decreased by 9 dB. Received: 3 April 2002 / Revised version: 30 April 2002 / Published online: 12 July 2002     

Diode-pumped, chirped-pulse Yb:S-FAP regenerative amplifier for laser-Compton X-ray generation

We present a diode-pumped, chirped-pulse Yb:S-FAP regenerative amplifier. This regenerative amplifier was developed as a first amplifier in an all-solid-state Yb:S-FAP laser system for laser-Compton X-ray generation. The amplifier delivers pulse energies above 24 mJ at a repetition rate of 50 Hz. Pulse compression reduces pulse widths to approximately 2.0 ps.     

Harmonic Mode-Locked Ytterbium-Doped Fibre Ring Laser

We demonstrate a harmonic mode-locked ytterbium-doped fibre ring laser, which consists of a polarization-sensitive isolator, two polarization controllers, two 976nm laser diodes as the pump source and a two-segment ytterbium-doped fibre. Utilizing an additive pulse mode-locked technique based on nonlinear polarization evolution, the ytterbium-doped fibre laser can operate in mode-locked state by adjusting the position of polarization controllers. The cavity fundamental repetition rate is 23.78 MHz. We also observe the second- and third-harmonic mode locking in the normal dispersion region, and their repetition rates are 47.66 MHz and 71.56 MHz, respectively. Over-driving of the saturable absorber in the harmonic mode-locking pulse is analysed and discussed in detail.     

Multimode to single-mode conversion of a Yb-doped fiber amplifier with a photorefractive Rh:BaTiO3 crystal

We present a technique to convert a multimode-transverse beam obtained from a multimode large core fiber amplifier into a single-mode beam by means of two-wave mixing in an infrared-sensitive Rh:BaTiO₃ crystal. Received: 26 March 2001 / Revised version: 27 March 2001 / Published online: 27 April 2001     

Additive pulse mode-locked Nd: YAG laser

Additive pulse mode locking applied to lamppumped Nd: YAG lasers results in an attractive source of picosecond pulses at 1.06 m or 1.32 m with average powers at the Watt level. We provide detailed information on construction and operation and give data on performance. A modified active stabilization scheme allows not only improved stability of operation but also insight into the dynamics of pulse formation.