Single-pass Small-signal Gain Research Articles

Compact all-solid-state femtosecond Yb amplifier using a high-brightness multi-watt tapered laser diode

In this work, we present a novel amplification module based on a high-brilliance tapered pump laser diode. A monolithic distributed Bragg grating permits to stabilize the diode wavelength, that was kept at 978.5 nm with a full-width at half-maximum bandwidth <0.2 nm during our experiments. Such a compact and reliable single laser source achieves a maximum output power of about 6.5 W, in a linearly polarized beam with high spatial quality (M2 < 1.5). This allows to optimize the performance of compact amplification modules designed to maximize the gain-per-Watt of absorbed pump power and the optical-to-optical extraction efficiency. To assess the potential of such pump unit, we designed a single-pass and double-pass amplification module based on a 3-mm-long 9.5%-doped Yb:KYW crystal, seeded by a either a cw or a femtosecond oscillator providing a maximum average power of ∼500 mW with output wavelength tunable between 1025 nm and 1035 nm. By tightly focusing the pump beam and carefully optimizing the mode-matching between the almost TEM00 pump and seed beam we achieved a single pass small signal gain G0 > 7 at about 3.5 W absorbed pump power and a maximum output power of 2.2 W, corresponding to an optical-to-optical extraction efficiency close to 50%. The excellent beam quality of the seeder was very well preserved during amplification, resulting in a measured Mx2× My2 <1.1 both in single and double pass.

Lamp-pumped eight-pass neodymium glass laser amplifier with high beam quality

This study presents a lamp-pumped Nd:glass laser system capable of achieving joule level output energy at 1-Hz repetition rate with high beam quality. The 1.8 J of energy in a 5 ns pulse was achieved with 1.5 mJ injected energy. The laser system includes a beam shaping module and an eight-pass, lamp-pumped amplifier. The laser amplifier demonstrated long-term energy stability of 1.76% PV and 0.29% RMS when operating at 1.8 J for 30 minutes. In a longer period of operation, no damage or obvious beam degradation was found. A uniform gain distribution with a single-pass small-signal gain of 3.89 was acquired with the four-lamp-pumping method, and thermally induced wavefront aberration was mitigated. Finally, a high beam quality was obtained using a homemade optically addressed liquid crystal light valve. After compensation, the near-field modulation decreased from 1.38 to 1.21, meanwhile, the beam contrast decreased from 6.54 to 4.71%. In terms of the far-field quality, a 90% far-field energy concentration was 2.5 times the diffraction limit.

LD end-pumped joule-level square-rod Nd:glass laser amplifier with high efficiency

This paper presents a laser diode (LD) end-pumped square-rod Nd:glass laser system capable of achieving joule-level output energy at 1-Hz repetition rate. The laser system generated 966 mJ of energy in a 5-ns pulse for a pumping energy of 9.66 J at 802 nm, corresponding to an optical-to-optical conversion efficiency of 10%, which is higher than those of similar existing systems. A single-pass small-signal gain of 2.54 and pulse energy stabilities of 2.5% (peak-valley) and 0.37% (root-mean-square) were achieved by eight-pass amplification. The output energy of 966 mJ was generated, and the near-field modulation of 90% beam aperture was 1.37; further, the 82.5% far-field energy concentration was 2.5 times higher than the diffraction limit. The measured single-pass thermally induced wavefront aberration (1.15λ) was similar to that calculated using the thermal model. The beam quality could be further improved using adaptive phase corrector technology.

1.5 J high-beam-quality Nd:LuAG ceramic active mirror laser amplifier

A 1.5 J Nd:LuAG ceramic active mirror laser amplifier with a high beam quality is demonstrated in which a 0.8% (atomic fraction) Nd-doped Nd:LuAG ceramic disk with a diameter of 64 mm and a thickness of 5.5 mm is used as a laser gain medium. A maximum single-pass small-signal gain of 2.59 is measured when the pump energy is 11.5 J, with an injected seed energy of 0.4 J; a maximum output energy of 1.5 J is obtained at the repetition rate of 10 Hz. A far-field beam spot 1.25 times the diffraction limit (DL) is achieved by using a stimulated Brillouin scattering phase conjugation mirror (SBS-PCM) for wavefront correction.

Off-axis eight-pass neodymium glass laser amplifier with high efficiency and excellent energy stability.

A new relay-imaged off-axial eight-pass laser amplifier with several joules energy and 1Hz repetition rate was demonstrated. The extraction efficiency and pulse-to-pulse energy stability were greatly improved. Under the single-pass small-signal gain of 3.6, a net gain of 900 and an effective extraction efficiency of 42.4% in the beam aperture were realized. Pulse-to-pulse energy stability of 0.83% (peak-valley) and 0.17% (root-mean-square) was achieved by the significant saturation of eight-pass amplification, which, to the best of our knowledge, is the best energy stability in several-joules-class amplifiers. The far-field quality was 2.52 times the diffraction limit, and the near-field modulation of the 90% beam aperture was 1.28. No parasitic oscillations or pencil beams were observed. Moreover, another key feature of the proposed amplifier was the ability to remarkably improve the pulse contrast with a unique design.

High-gain amplification in Yb:CaF2 crystals pumped by a high-brightness Yb-doped 976 nm fiber laser

We report on high single-pass gain in Yb:CaF2 crystal longitudinally pumped with a 40 W high-brightness fiber laser source based on an ytterbium-doped ultra-large core photonic crystal rod-type fiber operating at 976 nm. A single-pass small-signal gain of 3.2 has been achieved in a 6 % Yb-doped 10-mm-long CaF2 crystal at room temperature, outperforming any CW-diode-pumped scheme and paving the way towards very promising innovative lasers and amplifiers schemes merging the Yb-doped solid-state and fiber technologies.

Design of a high gain single stage and single pass Nd:YVO_4 passive picosecond amplifier

A detailed comparison of the influence of pumping wavelength and crystal doping concentration on the performance of Nd:YVO4 amplifiers is developed through theoretical analysis. The effect of energy transfer upconversion and the strong temperature dependence of the emission cross section were taken into account. This study showed the interest of 808&#xA0;nm pumping with low doping concentration crystals and the importance of the crystal temperature for the design of a high gain amplifier. Using these conclusions, we built a picosecond Nd:YVO4 master oscillator power amplifier reaching 10&#xA0;W output power for only 50&#xA0;mW of seed at 200&#xA0;kHz in a single pass, single stage configuration. With a pulse duration of 22&#xA0;ps, it corresponds to an output pulse energy of 50&#xA0;&#x3BC;J and to a peak power of 2.3&#xA0;MW. With a same setup, a single pass small signal gain over 45&#xA0;dB has been measured, and near 50% extraction efficiency was reached for a seed power of 3.5&#xA0;W. The influence of the Nd:YVO4 amplifier temperature on the output power was also studied for different levels of gain saturation.

210-$\mu$J Picosecond Pulses From a Quasi-CW Nd:YVO$_{4}$ Grazing-Incidence Two-Stage Slab Amplifier Package

A single-pass two-stage Nd:YVO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> amplifier was used to increase the energy of a 7.7-ps seed pulse from < 1 nJ to 210 muJ (11-ps time duration), maintaining the input spatial quality of the pulse. The laser-diode side-pumped grazing-incidence Nd:YVO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> slab amplifier was optimized for minimum amplified spontaneous emission noise and for maximum energy extraction, with single-pass small-signal gain of 59 dB. Diffraction-limited operation up to 1 kHz is possible. Conversion efficiency of 75 % at 532 nm and 40% at 266 nm was observed.

Nd:YAG split-disk laser amplifier for 10 J output energy

A prototype split-disk amplifier consisted of four Nd:YAG ceramics and single crystals was demonstrated with a 3.7cm square clear aperture. A single-pass small-signal gain of 2.0 and a total stored energy of 11.1J in the eight YAG disks were obtained. The maximum output energy of 10.4J with the YAG disk amplifier was achieved in a single-shot operation. A near-field pattern of 31mm×31mm square with super Gaussian shape was measured.

Efficient high-gain laser amplification from a low-gain amplifier by use of self-imaging multipass geometry

We characterize a self-imaging multipass amplifier scheme that provides both high extraction efficiency and overall gain. A diode-pumped slab amplifier with a single-pass small-signal gain of 2.5 is used in a 16-pass mode to amplify an input pulse from 50 muJ to 50 mJ, extracting approximately 22% of the stored energy. A stimulated Brillouin-scattering phase-conjugate mirror provides isolation from amplified spontaneous emission, prevents gain depletion, and also ensures good beam quality. The system can be operated from 10 Hz to in excess of 450 Hz, with modest changes in the beam quality and energy. The scheme has the potential to be scaled to higher-energy and higher-power systems.

Design and performance of a high-average-power flashlamp-pumped Ti:sapphire laser and amplifier

Design, performance and characterization of a high average-power flashlamp-pumped Ti:sapphire oscillator and amplifier are presented. The extensive design study, which covers the whole excitation process of the laser, focused on efficient laser operation at high average powers but also took into consideration the practicability of the laser device. For the first time, pumplight-induced absorptions in Ti:sapphire and their avoidance by appropriate filtering were investigated. The laser performance could be improved, firstly, due to the proper design of the discharge circuit, which resulted in an increased spectral overlap of the flashlamp-emission with the Ti:sapphire absorption. Secondly, the insight into the consequences of the flashlamp plasma reabsorption in conjunction with a small-spectral-band-transmitting pump-light filter led to a simple, experimentally confirmed transfer-efficiency reflector model and, based on this, to an improved reflector shape. As a result of the study, average powers of up to 220 W at a total laser efficiency of 2.25% have been achieved, Also, a single-pass small-signal gain of 3.1 and a total stored energy in the laser rod of 690 mJ at 100-Hz repetition rate could be demonstrated. This significant improvement over former flashlamp-pumped Ti:sapphire lasers and amplifiers for the first time opens up the possibility of femtosecond-pulse amplification to high average powers at 100 Hz repetition rate.

Y-branch waveguide glass laser and amplifier

A Y-branch channel waveguide laser operating near 1057 nm was fabricated by electric-field-assisted ion exchange in Nd-doped silicate glass. The overall length was 24 mm. Optical pumping was performed with a cw Ti:sapphire laser. Mirrors were bonded to the polished waveguide facets. The slope efficiency was 5.1% when a 4%-transmitting output coupler was used. Threshold was reached at 26-mW absorbed pump power. The device exhibited a single-pass small-signal gain of 0.034 dB/mW when operated as an amplifier. The 3-dB splitting loss of the Y-branch structure was overcome when the absorbed pump power was approximately 85 mW.

High-gain small-bore cooled CO2amplifier

A single-pass small-signal gain of 20 dB/m has been obtained in a 5-mm-diameter flowing He:N 2 :CO 2 amplifier. This gain was achieved by optimizing the conditions of operation at -60°C wall temperature.