L.M.B. Hickey

Power Scaling of Single-Frequency Ytterbium-Doped Fiber Master-Oscillator Power-Amplifier Sources up to 500 W

We discuss continuous-wave single-frequency master- oscillator power-amplifier (MOPA) sources based on ytterbium- doped fibers (YDFs) with particular attention to their recent advances and our experimental results. This includes a 402-W plane-polarized MOPA source and a 511-W random-polarized MOPA source. In these MOPAs, the final-stage high-power amplifier operates with high efficiency of 70%-80%, and a high gain of over 20 dB in a near diffraction-limited beam. We report at least 7-dB enhancement of the stimulated Brillouin scattering (SBS) threshold for the 402-W polarization-maintaining (PM) YDF. In fact, we did not see any sign of SBS even at the highest output power. We eventually observed SBS appearance at around 500 W for the non-PM YDF. The observed SBS strengths were far weaker than expected in theory, unless we allowed for the Brillouin gain reduction from thermal Brillouin gain broadening induced by the quantum-defect heating.

Erbium:Ytterbium Codoped Large-Core Fiber Laser With 297-W Continuous-Wave Output Power

We have demonstrated a high-power and high-efficiency erbium:ytterbium (Er:Yb) codoped fiber laser that produces 297 W of continuous-wave output at 1567 nm. The slope efficiency with respect to the launched pump power changed from 40% to 19% at higher output power due to the onset of Yb colasing at 1067 nm. However, the Yb colasing was essential for the suppression of catastrophic pulsation at high pump powers that otherwise results if the Yb-band gain is allowed to build up. Spectroscopic characteristics of the fiber and the impact of the Yb colasing on the 1567-nm slope efficiency are also discussed.

High average power, high repetition rate, picosecond pulsed fiber master oscillator power amplifier source seeded by a gain-switched laser diode at 1060 nm

We demonstrate a pulsed ytterbium-doped fiber master-oscillator power amplifier source at 1060 nm producing over 300 W of average power in 20-ps pulses at 1-GHz repetition rate. The pulses generated by a gain-switched diode were compressed by a chirped fiber Bragg grating and amplified without any distortion with excellent spectral quality. This fiber master oscillator power amplifier system offers versatility and potential for further power scaling.

TI:SAPPHIRE PLANAR WAVEGUIDE LASER GROWN BY PULSED LASER DEPOSITION

We document the lasing performance of a waveguiding layer of Ti:sapphire, of ~12-mum thickness, grown by pulsed laser deposition from a 0.12-wt.% Ti(2)O(3) Ti:sapphire single-crystal target onto an undoped z-cut sapphire substrate. Lasing around 800 nm is observed when the waveguide layer is pumped by an argon-ion laser running on all-blue-green lines, with an absorbed power threshold of 0.56 W, with high-reflectivity (R>98%) mirrors. With a 5% pump duty cycle and a T=35% output coupler, a slope efficiency of 26% with respect to absorbed power is obtained, giving quasi-cw output powers in excess of 350 mW.

A new design approach for fiber DFB lasers with improved efficiency

The standard distributed feedback (DFB) laser optimization method is critically investigated and a new design approach based on the effective cavity length is presented. By applying this method in an erbium-ytterbium co-doped fiber, the pump-to-signal conversion ratio is increased by 40% for the same total device length and pumping conditions. The laser with the proposed design is produced and the theoretical results are verified by the experimental work.

Growth of Ti:sapphire single crystal thin films by pulsed laser deposition

This paper documents the growth of single crystal Ti:sapphire thin films, typically 10µm thick, on undoped sapphire substrates using Pulsed Laser Deposition from a Ti:sapphire single crystal target with a doping level of 0.1 % wt Ti2O3. These thin films are shown to have very high crystal quality using Ion Beam Channelling and X-Ray Diffraction Techniques. The degree of titanium incorporation into the films is investigated using Inductively Coupled Plasma Mass Spectrometry and Particle Induced X-ray Emission. These techniques show that levels of up to 0.08% wt Ti2O3 are present in the deposited layers.

321 W average power, 1 GHz, 20 ps, 1060 nm pulsed fiber MOPA source

Pulses from a gain-switched laser diode were amplified in a fiber MOPA system to produce in excess of 320 W of average power in 20 ps pulses at 1 GHz repetition rate at 1060 nm.

Kilowatt-class single-frequency fiber sources

We discuss the dramatic development of high-power fiber laser technology in recent years and the prospects of kilowattclass single-frequency fiber sources. We describe experimental results from an ytterbium-doped fiber-based multihundred-watt single-frequency, single-mode, plane-polarized master-oscillator power amplifier (MOPA) operating at 1060 nm and a similar source with 0.5 kW of output power, albeit with a degraded beam quality (M2 = 1.6) and not linearly polarized. Experiments and simulations aimed at predicting the Brillouin limit of single-frequency system with a thermally broadened Brillouin gain are presented. These suggest that single-frequency MOPAs with over 1 kW of output power are possible. In addition, the power scalability of a simple single-strand fiber laser to 10 kW is discussed.

Fiber DFB lasers with ultimate efficiency

A novel method for designing distributed feedback (DFB) lasers with ultimate efficiency is presented. The method is based on the calculation of the optimum signal distribution for a given pumping scheme and derivation of the grating profile that sustains this distribution so that at every point in the cavity pump-to-signal conversion efficiency is maximum. Theory applied experimentally in Er-Yb co-doped fiber resulting in a 57% increase in the efficiency with 10% shorter device length compared with standard optimized co-pumped designs.

Experimentally verified modeling of erbium-ytterbium co-doped DFB fiber lasers

For the first time, the simulation results of fiber distributed feedback (DFB) lasers are compared against experimental data in this paper. The pump source, active medium, and grating are all modeled and simulated to predict actual laser characteristics. Simple characterization methods are illustrated for the measurement of model parameters. Large loss at the pump wavelength is observed, attributed to the lifetime quenching of Yb ions, and included in the model as a critical parameter. DFB lasers with two different apodization profiles successfully simulated with the same set of model parameters.