Louis Desbiens

Modeling the photodegradation of large mode area Yb-doped fiber power amplifiers

Photodarkening is presently a major concern for the long term reliability and efficiency of high power Yb-doped fiber lasers and amplifiers. This phenomenon has been associated with the formation of color centers in the fiber core of single-clad and large mode area Yb-doped fibers. However, its origin is still not well understood and to date no comprehensive model that could predict the lifetime of Yb-doped fiber-based devices has been put forward. A semi-empirical approach seems at the moment the best way to gain a better understanding of the growth behavior of photo-induced losses in Yb-doped fibers in the presence of both photodarkening and photobleaching processes. A rate equation describing the activation and deactivation of color centers involving stretched exponential functions has been developed. For this approach to be effective and reliable, a minimum of parameters is used, four to describe photodarkening and three for photobleaching. A large mode area Yb-doped fiber fabricated at INO using the MCVD process has been characterized. By properly choosing the initial pumping conditions, each parameter of the stretched exponential functions has been measured separately from the others. The model has then been used to simulate the power decay from a 1 kW, 10 ns-pulse, 100 kHz Yd-doped LMA fiber power amplifier. We show that the photodarkening behavior predicted by the model is in good agreement with the experimental results over more than 6000 hours. Such a model is general in its application but the stretched exponential parameters are unique to the type of fiber tested. The model will be a useful characterization tool for developing photodarkening-resistant fibers and for evaluating the lifetime of Yb-doped fiber-based devices affected by photodegradation.

Material micromachining using bursts of high repetition rate picosecond pulses from a fiber laser source

In this paper, we demonstrate the benefits of using bursts of picosecond pulses for material micromachining and compare the results with those obtained when using a nanosecond source with similar pulse energy, pulse width and pulse shape. The picosecond laser source used for the experiments was delivering 60-ps pulses at a repetition rate of 1.8 GHz, grouped within arbitrarily-shaped bursts having a width that could be varied from 2.5 to 40 ns. The laser output central wavelength was at 1064 nm and the output beam M2 value was below 1.15. Micro-milling experiments were performed on silicon for two levels of energy per burst and with different burst amplitude profiles. We show that the maximum material removal efficiency and the surface quality can be increased by more than 25% when using bursts of picosecond pulses with respect to nanosecond pulses with similar energy per pulse. Effect of shaping the burst envelope of the picosecond laser on the maximum material removal efficiency is also presented.

Relations between phosphorus/aluminum concentration ratio and photodarkening rate and loss in Yb-doped silica fibers

The relations between dopant concentrations (phosphorus and aluminum) and photodarkening rate, excess loss, and activation energies in ytterbium-doped silica fibers are experimentally investigated. It is shown that increasing the concentration of phosphorus from 0.2 to 2.5 mol% in phosphorus/aluminum codoped fiber cores decreases the photodarkening excess loss by a factor of 8 and the photodarkening rate by a factor of 10. Moreover, the effective number of ytterbium ions involved in the photodarkening process increases from 4 to more than 6 for tested phosphorus/aluminum concentration ratios varying from 0.1 to 1 respectively. In contrast, increasing the aluminum concentration from 2 to 5 mol% for a fixed phosphorus concentration of 0.2 mol% has negligible effect on the initial photodarkening rate or the effective number of ytterbium ions involved in the process, but still decreases the photodarkening excess loss by a factor of 5. Those results suggest photodarkening activation energies of 5.2 eV for ytterbium/aluminum-codoped silica fibers and more than 7.8 eV for ytterbium/phosphorus/aluminum-codoped silica fibers. The net improvement in photodegradation of fiber amplifiers based on such phosphorus and aluminum codoping is measured experimentally and numerically simulated. The output power loss of 1064-nm ytterbium-doped LMA fiber amplifiers with phosphorus/aluminum ratios of 0.1 and 0.6 is reduced after 10 000 hours from 17% to less than 2%, respectively. Better understanding of the effects of phosphorus and aluminum on photodarkening will help to design reliable and efficient ytterbium-doped fiber amplifiers.

All-fiber, high power, rugged ultrashort-pulse laser source at 1550 nm

We present here the architecture of an all-fiber, high-power FCPA source emitting at 1550 nm. This system generates sub-300 fs pulses at a repetition rate of 22 MHz and with an average output power of 1.5 W after pulse compression. The power amplifier consists of a polarization-maintaining Er:Yb doped LMA fiber which results in a beam quality factor M2 < 1.2. The seed laser pulses are stretched to 240 ps using dispersion-shifted fiber before being amplified and compressed using a bulk compressor based on a diffraction grating pair. The output power of the source is not limited by the onset of detrimental nonlinear effects such as self-phase modulation or stimulated Raman scattering since the accumulated nonlinear phase-shift in the power amplifier is well below π rad. Maximum output power is rather limited by the available pump power; a likely five-fold increase, given actual state-of-the-art technology, would thus yield a laser source that may serve as a substitute for widespread solid-state lasers in various fields such as laser machining, biophotonics and nonlinear optics.

Yb-doped large mode area fibers with depressed clad and dopant confinement

Large mode area fibers with depressed-index cladding layer and confinement of rare-earth dopants can provide effective suppression of high-order modes. A polarization-maintaining Yb-doped double-clad fiber with 35/250 μm core/clad diameter has been fabricated from conventional methods according to this design. The fiber which has an effective mode area close to 500 μm2 yields near diffraction-limited output with beam quality factor M2 close to 1.1 when tested as a power amplifier with a coherent seed light source. Beam pointing measurements provide further evidence for near single-mode behavior as the pointing fluctuations are shown to be negligible once the fiber is coiled to a given diameter.

Arbitrarily-shaped bursts of picosecond pulses from a fiber laser source for high-throughput applications

Increasing the ablation efficiency of picosecond laser sources can be performed by bunching pulses in bursts1 and benefit from heat accumulation effects2-5 in the target. Pulsed fiber lasers are well suited for such a regime of operation, as the single pulse energy in a fiber is limited by the onset of nonlinear effects (SPM, SRS). Increasing the number of pulses to form a burst of pulses allows for average power scaling of picosecond fiber lasers. We are presenting in this paper a high-power fiber laser emitting arbitrarily-shaped bursts of picosecond pulses at 20 W of average output power. Burst duration can be varied from 2.5 ns to 80 ns. The burst repetition rate is externally triggered and can be varied from 100 kHz to 1 MHz. The single pulse duration is 60 ps and the repetition rate within a burst is 1.8 GHz. The output beam is linearly polarized (PER > 20 dB) and its M2 value is smaller than 1.15. The laser source has a tunable central wavelength around 1064 nm and a spectral linewidth compatible with frequency conversion. Conversion efficiency higher than 60% has been obtained at 10 W of 1064-nm output power.

High power, fiber-based chirped-pulse amplification system at 1550 nm with two amplifiers

We report on the experimental demonstration of a fiber-based chirped-pulse amplification system using a dispersion shifted fiber as stretcher, an electro-optic modulator for frequency division, two erbium-doped fiber amplifiers (a single mode and a large mode area) and a grating pair pulse compressor. We obtained 500-nJ pulses at a repetition rate of 1 MHz. Pulse duration was under 500 fs, which is in part due to the fine control of the beam polarization maintained throughout the system.

High-power fiber amplifier using a depressed-clad Yb-doped LMA fiber with low photodarkening

A 225-μJ polarization maintaining ytterbium-doped large-mode-area multiclad fiber was designed and fabricated with an effective mode area of 450 μm2 and a photodarkening maximum excess loss of ~1 dB/m at 1064 nm. The fiber index profile is based on a depressed-clad to obtain a diffraction-limited output. Optimization for low photodarkening and high conversion efficiency while maintaining a good control on the cores refractive index profile has been achieved by adjusting the ytterbium/phosphorus/aluminum concentrations in the fiber core. Concentration ratios of phosphorus/aluminum from 0.12 to 1.25 were experimentally investigated in terms of photodarkening rate and excess loss. Within this range, the photodarkening excess loss was observed to decrease by a factor of 8. The large-mode-area fiber was used in a 10-ns pulse amplifier at 1064 nm with a repetition rate of 100 kHz and 0.5-nm bandwidth. The diffraction-limited output has a measured M2 value of 1.04 when the fiber is coiled to a diameter of 12 cm. The fiber amplifier slope efficiency is 70% with a polarization extinction ratio greater than 23 dB. It is shown how the phosphorus/aluminum ratio reduces photodarkening, and how a depressed-clad design improves higher-order mode filtering for reliable, efficient, and compact ytterbium-doped fiber amplifiers.

Optimization of signal gain and core composition for low photodegradation in Yb-doped fiber amplifiers

Photodarkening and photobleaching processes affect the level of photodegradation of Yb-doped fibers. Characterization and modeling of each process is crucial to understand how to optimize the operating conditions of fiber amplifiers and lasers to obtain acceptable output power degradation. We show that photobleaching is a key factor in the modeling and simulation of a 10-ns pulsed Yb-doped LMA fiber amplifier. Each parameter of the model was separately determined from induced excess loss measurements under selective pump and wavelength excitations. The model was used to simulate accurately the measured fiber amplifier degradation. Optimized fiber length and gain were calculated to improve the output power stability over time and increase the fiber lifetime. Furthermore, eight fibers have been fabricated with various Yb, Al, and P content using the MCVD process to optimize the core composition. The level of photodarkening in each fiber was evaluated by measuring separately rate coefficient and excess loss. It was found that all fibers followed a similar inversion-dependent rate while the maximum excess loss was dependent on the ratios [Al]/[Yb] and [P]/[Yb]. The proposed model allows for rapid evaluation and optimization of fiber parameters and operation conditions to assist Yb-doped laser system design in achieving the desired performance with low photodegradation.

Nonlinear compression for generation of high energy ultrashort pulses using an Yb-doped large mode area tapered fiber

Nonlinear compression for generation of high energy ultrashort pulses using an Yb-doped large mode area tapered fiber is reported. Single-stage amplifier gain larger than 43 dB is achieved, with energy of seed pulses (35 ps, 200 kHz) boosted up to 50 μJ at the amplifier output. Spectral broadening induced by self-phase modulation is shown to take place advantageously along the larger end of the counter-pumped active tapered fiber, where the mode area scales beyond 1000 μm2. Pulse durations as short as 1 ps and peak powers exceeding 16 MW are demonstrated thereafter using a chirped volume Bragg grating as a dispersive compressor. Efficient suppression of higher-order modes in the large mode area tapered fiber yields diffraction-limited output (M2 < 1.2) for optimal pulse compression.