Ce3+:LiCaAlF6 crystal for high-gain orhigh-peak-power amplification of ultraviolet femtosecond pulses and newpotential ultraviolet gain medium: Ce3+:LiSr0.8Ca0.2AlF6

To develop high-peak-power ultrashort pulse laser systems in the ultraviolet region, a large Ce³⁺:LiCaAlF₆ (Ce:LiCAF) crystal, a tunable ultraviolet laser medium with large saturation fluence and broad gain spectrum width, was grown successfully with a diameter of more than 70 mm. To demonstrate high small signal gain, a four-pass confocal amplifier with 60 dB gain and 54 μJ output energy was constructed. Chirped pulse amplification (CPA) in the ultraviolet region was demonstrated using Ce:LiCAF for higher energy extraction. A modified bow-tie-style four-pass amplifier pumped by 100-mJ 266-nm 10-Hz pulses from a Q-switched Nd:YAG laser had 370-times gain and delivered 6-mJ 290-nm pulses. After dispersion compensation, the output pulses can be compressed down to 115 fs. This is the first ultraviolet, all-solid-state high-peak-power CPA laser system using ultraviolet gain media, and this demonstration shows further scalability of the Ce:LiCAF CPA system. Additionally, a new gain medium, Ce³⁺ :LiSr_0.8Ca_0.2AlF₆, with longer fluorescence lifetime and sufficient gain spectrum width over 18 nm was grown to upgrade this system as a candidate for a final power amplifier gain module     

Broadband amplification of 800-nm pulses with a combination of negatively and positively chirped pulse amplification

It is experimentally proved that successive amplification of negatively and positively chirped laser pulses (NPCPA) counteracts the gain narrowing effect typical in chirped pulse amplification (CPA) lasers. The scheme is robust and easy to adopt to even petawatt (PW) level high power laser systems. As a demonstration, a multi-terawatt (TW) Ti:sapphire laser system was modified to the NPCPA. The bandwidth of the 150 mJ output pulses exceeds 50 nm without any additional spectral correction, which is 30% broader than those currently available from conventional CPA lasers. Moreover, the NPCPA scheme gives an opportunity to increase an intensity temporal contrast without any compromise in pulse energy.     

Femtosecond self- and cross-phase modulation in semiconductor laseramplifiers

We present detailed derivation of our new model for femtosecond pulse amplification in semiconductor laser amplifiers. The various dynamic nonlinear terms of gain compression and associated self-phase modulation are derived semiphenomenologically, and are discussed physically. Included are the effects of carrier depletion, carrier heating and spectral hole-burning, as well as linear and two photon absorption and the instantaneous nonlinear index. Additionally, we account for dynamically changing gain curvature and slope. We apply the theory to strong signal cross-phase-cross-gain modulation experiments with ~500 fs pulses in a broad area GaAs amplifier and show that the model accurately describes the observed complex phenomena. We also present experimental results on single beam strong signal amplification in two different quantum-well amplifiers using 150-200 fs duration pulses. For such pulse lengths, carrier heating becomes an integrating nonlinearity and its self-phase modulation is similar to that due to carrier depletion. Additionally, since the pulse spectrum is broad, the gain slope and curvature shift and narrow it. The resultant spectral distortions are very different than observed (and modeled) earlier for the ~500 fs pulses. The model is again able to correctly describe the evolution of these ultrashort pulses, indicating that it remains valid, even though pulse durations approach the intraband relaxation time     

Subpicosecond pulse compression in the VUV region by induced-phasemodulation in Xe

The feasibility of subpicosecond pulse compression in the VUV region using the induced phase modulation in Xe is theoretically studied by numerically solving equations for the amplitude and phase of a weak VUV signal pulse in the presence of an intense pump pulse at the visible wavelength. Substantial pulse compression can be realized with compact pulse compressors when the pump pulse width is decreased. When a 9.6 ps pump pulse at 403 nm with the peak intensity of 1 GW/cm² is assumed, the signal pulse at 157 nm is compressed from 12 ps to 260 fs by the pulse compressor, with a grating-mirror separation of <2.5 m. Influences on the pulse compression of the timing between two incident pulses and the dynamic change in nonlinear susceptibility are also described     

L-band erbium-doped fiber amplifier incorporating an inline fibergrating laser

Wide-band erbium-doped fiber amplifiers (EDFAs) having an inherently flat gain spectrum in the long-wavelength-band (L-band) between 1570 nm and 1600 nm have recently been realized. The L-band amplification has been achieved by simply using a longer length of EDF than in the conventional-wavelength-band (C-band) EDFAs. In the paper, we investigate the improvement of the gain flatness in an L-band EDFA co-pumped with the high-power pump light at 1.48 μm and the medium-power auxiliary pump light at 1.55-1.565 μm. We also propose a novel L-band EDFA incorporating an inline fiber grating laser (FGL) operating at the auxiliary pump wavelength, and demonstrate that the proposed EDFA has better gain flatness as well as the gain clamping characteristics     

Seeding of an eleven femtosecond optical parametric chirped pulse amplifier and its Nd/sup 3+/ picosecond pump laser from a single broadband Ti:Sapphire oscillator

We demonstrate direct simultaneous seeding of a few-cycle optical parametric chirped pulse amplifier (OPCPA) in the 700-1000-nm spectral range, and of a Nd:YLF amplifier emitting 30-ps pulses at 1053 nm by use of a chirped-mirror 6-fs Ti:sapphire oscillator. This approach of employing a single master oscillator to drive two power amplifiers simplifies the pump laser design and is applied to eliminate the timing jitter between the seed and the pump pulses in the OPCPA chain. We show that 10 mJ fundamental picosecond pump pulses with the intensity contrast in excess of 10/sup 4/ relative to the nanosecond Q-switched background can be achieved with the seed intensity available in the edge of the oscillator spectrum around 1053 nm. Cross-correlation measurements between the picosecond pump and femtosecond oscillator pulses reveal no traceable timing jitter between the OPCPA pump and seed pulses. The estimated long-term jitter of 0.3 ps is attributed to the thermal expansion of the cavity of the Nd:YLF regenerative amplifier.     

Development of high-power OPCPA laser at 1064 and 780 nm

A compact 10-TW/100-fs level ultrashort-pulse and ultra-intense laser system at 1064 nm based on optical parametric chirped pulse amplification (OPCPA) scheme is described, at which the pump and seed for the optical parametric amplification (OPA) process is optically synchronized. We investigated the output stability and the conversion efficiency of the system. Moreover, a design toward higher peak power output is given and an optically synchronized amplifier based on the concept of OPCPA at 800 nm is preliminarily explored.     

VUV laser in the Lyman band of molecular hydrogen pumped by fstitanium-sapphire laser pulses

We report lasing at 160 nm in the Lyman band of molecular hydrogen. The laser is pumped by 200 mJ/150 fs pulses from the ATLAS titanium-sapphire laser at our institute. The pump pulses are focused at an angle of incidence of 60° onto a 9-cm-long gold target to a line focus, generating traveling-wave excitation. With 80 mbar of hydrogen in the target chamber we measure an average gain of 1.1 cm^-1 and achieve a total gain-length product of 10. The evaluation of the far-field pattern shows that the beam originates from a region with an electron density of 5×10¹⁵ cm^-3. A simple model of the H₂ laser is presented which explains the main part of our observations and supports a pump mechanism of photoelectron pumping     

Characteristics of a saturated 18.9-nm tabletop laser operating at 5-Hz repetition rate

We report the characteristics of a saturated high-repetition rate Ni-like Mo laser at 18.9 nm. This table-top soft X-ray laser was pumped at a 5-Hz repetition rate by 8-ps 1-J optical laser pulses impinging at grazing incidence into a precreated Mo plasma. The variation of the laser output intensity as a function of the grazing incidence angle of the main pump beam is reported. The maximum laser output intensity was observed for an angle of 20/spl deg/, at which we measured a small signal gain of 65 cm/sup -1/ and a gain-length product g/spl times/l>15. Spatial coherence measurements resulting from a Young's double-slit interference experiment show the equivalent incoherent source diameter is about 11 /spl mu/m. The peak spectral brightness is estimated to be of the order of 1/spl times/10/sup 24/ photons s/sup -1/ mm/sup -2/ mrad/sup -2/ within 0.01% spectral bandwidth. This type of practical, small scale, high-repetition soft X-ray laser is of interest for many applications.     

Novel Multiwavelength L-Band Brillouin–Erbium Fiber Laser Utilizing Double-Pass Brillouin Pump Preamplified Technique

We experimentally demonstrate successful operation of an enhanced multiwavelength L-band Brillouin-erbium fiber laser and analyze its performance under various operating conditions. This scheme utilizes double-pass amplification technique to preamplify the Brillouin pump (BP) power within the laser cavity before entering the single-mode fiber. Owing to this double-pass pre-amplification within the erbium gain medium, the proposed laser structure is able to operate at low pumping power and exhibits a low-threshold power of 15.9 mW. Moreover, the double-pass pre-amplification of BP is able to shift the unstable operation of the laser to a higher pump power, enabling us to generate high-power laser signals. We experimentally show that the proposed novel setup can produce up to 30 channels at 40 and 0.035 mW of 1480 nm pump and BP powers, respectively. An obvious suppressant for unstable self-lasing cavity modes because of the effect of homogenous saturation of the erbium-doped fiber gain due to the high intensity of BP is observed in the laser cavity configuration.     

High-power mode-locked external cavity semiconductor laser usinginverse bow-tie semiconductor optical amplifiers

This paper presents experimental results of using an inverse bow-tie gain guided semiconductor optical amplifier (SOA) as the optical gain element in a high-power external cavity semiconductor laser. An average output power of 700 mW is demonstrated in continuous-wave (CW) operation while 400 mW of average power is obtained in both passive and hybrid mode-locked operation, with subsequent optical amplification in an identical SOA. The mode-locked laser operates at a repetition rate of 1.062 GHz, owing to the interplay between the gain and saturable absorber dynamics. Optical pulses are generated with a temporal duration of 5 ps, which implies a pulse energy of 376 pJ, and a peak power of 60 W. Further reduction of the optical pulsewidth to 1.3 ps is also achieved by using dispersion compensation techniques. These results show the promise of novel SOA devices for use as gain elements in external cavity semiconductor lasers. The generated output pulse characteristics from mode-locked operation is sufficient for use in novel three-dimensional data storage applications, and in large-scale commercial printing and marking applications     

Picosecond pulse amplification in AlGaAs flared amplifiers

Amplification of ultrashort pulses in a semiconductor flared amplifier is analyzed by using a two dimensional (2-D) time-domain BPM (beam propagation method) model including the effects of gain saturation, finite-gain bandwidth, self-phase modulation, index dispersion, carrier heating, and gain relaxation between successive pulses induced by carrier diffusion and recombination. The paper includes a presentation of the model and of the numerical resolution method, detailed analysis and discussion of the spatio-temporal pulse distortions in the case of picosecond and subpicosecond pulses and an evaluation of the device performance. The performance of the flared amplifier is evaluated considering the temporal length, the time-bandwidth product, the far field, the output energy as a function of the input energy, the injected current, and the repetition rate. While subpicosecond pulse amplification leads to strong distortions and to very low total gain, the flared amplifier is well adapted to the amplification of picosecond pulses at low-repetition rates     

PPMgLN-Based High-Power Optical Parametric Oscillator Pumped by Yb $^{{bm 3}{bm +}}$-Doped Fiber Amplifier Incorporates Active Pulse Shaping

We report a periodically poled magnesium-oxide-doped lithium niobate (PPMgLN) based optical parametric oscillator (OPO) pumped by a diode-seeded, linearly polarized, high-power, pulsed, ytterbium fiber master oscillator power amplifier (MOPA). Using adaptive pulse shaping of the seed laser (using an external modulator), we demonstrate a reduction in the impact of dynamic gain saturation and optical Kerr/Raman nonlinearities within the fiber MOPA, obtaining shaped signal and idler pulses at the OPO output and reduced spectral bandwidths. A maximum average output power of 26.5 W was obtained from the MOPA at 1062 nm. An output power as high as 11 W from the OPO at an overall slope efficiency of 67% was achieved, with 2.7 W of output power obtained at a wavelength of 3.5 mum. Our experiments were pump-power-limited and considerable scope remains for further power scaling of such OPOs using this approach.     

Transient collisional excitation X-ray lasers with 1-ps tabletopdrivers

Recent results are presented for 1-ps driven X-ray laser amplification in Ne-like and Ni-like transient collisional excitation research at the Lawrence Livermore National Laboratory. Plasma formation, ionization and collisional excitation are optimized using two laser pulses of 600- and 1-ps duration at tabletop energies of typically 5 J or less in each beam. Gain of 35 cm^-1 and higher has been measured on the 147 Å 4d→4pJ=0→1 transition of Ni-like Pd and is a direct consequence of the nonstationary population in, version produced by the high intensity picosecond pulse. We report the characterization of the X-ray laser properties including the transient gain lifetime and beam divergence for different Ne-like and Ni-like X-ray lasers     

Trends in chirped pulse optical parametric amplification

Since the proof-of-principle demonstration of optical parametric amplifier to efficiently amplify chirped pulses in 1992, optical parametric chirped pulse amplification (OPCPA) became a widely recognized and rapidly developing technique for high-power femtosecond pulse generation. In the meantime, we are witnessing an exciting progress in the development of powerful and ultrashort pulse laser systems that employ chirped pulse parametric amplifiers. These systems cover a broad class of femtosecond lasers, with output power ranging from a few gigawatts to hundreds of terawatts, with a potential of generating few-optical-cycle pulses at the petawatt power level. In this paper, we discuss the main issues of optical parametric chirped pulse amplification and overview recent progress in the field.     

Enhanced polymer ablation rates using high-repetition-rateultraviolet lasers

Etch rates (μm/pulse) for glycol-modified polyethylene terephthalate (PETG) under pulsed UV (255 nm) laser processing are measured as a function of pulse repetition frequency in the range 0.7-15 kHz. Materials removal rates (μm/s) scale approximately linearly with pulse repetition frequency at a fluence of 0.59 J/cm², and there appears to be no attenuation of the ablating laser beam by the ejected material plume for pulse rates up to 15 kHz. The instantaneous etch rate for pulses in a sequence increases markedly (~40%) for long pulse sequences (>100 pulses) at high PRF (15 kHz), an effect which can be used to increase machining rates while operating at a moderate laser fluence     

Ultrafast semiconductor-based fiber laser sources

In this paper, a novel ring laser platform is presented that uses a single active element, a semiconductor optical amplifier (SOA), to provide both gain and gain modulation in the optical cavity. Gain modulation is achieved by an externally introduced optical pulsed signal. This signal periodically saturates the amplifier gain and forces the ring laser to mode lock. Using this laser platform, we demonstrate picosecond pulsetrain generation at repetition rates up to 40 GHz, either in single or multiwavelength operation mode. In particular, using rational harmonic mode locking, 2.5-ps pulses were obtained up to a 40-GHz repetition rate, while output pulses and output power were constant over a 20-nm tuning range. In addition, a multiwavelength optical signal was obtained using the same laser platform with the addition of a Fabry-Pe/spl acute/rot filter for comb generation. Multiwavelength oscillation is possible due to the broad gain spectrum of the SOA used and its inhomogeneous line broadening. To this end, 48 oscillating wavelengths were obtained at the laser output, with 50-GHz line spacing. Combining both modes of operation, it was possible to mode lock the oscillating multiwavelength signal and to obtain at the output ten wavelength channels, simultaneously mode locked at a 30-GHz repetition rate. The mode-locked channels are temporarily synchronized and exhibit almost identical spectral and time characteristics.     

Experiments and simulations of short-pulse laser-pumped extreme ultraviolet lasers

Recent experimental work on the development of extreme ultraviolet lasers undertaken using as the pumping source the VULCAN laser at the Rutherford Appleton Laboratory is compared to detailed simulations. It is shown that short duration (/spl sim/picosecond) pumping can produce X-ray laser pulses of a few picosecond duration and that measurement of the emission from the plasma can give an estimate of the duration of the gain coefficient. The Ehybrid fluid and atomic physics code developed at the University of York is used to simulate X-ray laser gain and plasma emission. Two postprocessors to the Ehybrid code are utilized: 1) to raytrace the X-ray laser beam amplification and refraction and 2) to calculate the radiation emission in the kiloelectronvolt photon energy range. The raytracing and spectral simulations are compared, respectively, to measured X-ray laser output and the output of two diagnostics recording transverse X-ray emission. The pumping laser energy absorbed in the plasma is examined by comparing the simulations to experimental results. It is shown that at high pumping irradiance (>10/sup 15/ Wcm/sup -2/), fast electrons are produced by parametric processes in the preformed long scalelength plasmas. These fast electrons do not pump the population inversion and so pumping efficiency is reduced at high irradiance.