Yves Taillon

A simple model describing both self-mode locking and sustained self-pulsing in ytterbium-doped ring fiber lasers

This paper presents a model describing self-mode locking (SML) and sustained self-pulsing (SSP) in a unidirectional ring-fiber laser using only the interaction between the saturated population inversion and the optical signal circulating in the laser cavity. Gain saturation alone is used to explain SML behavior close to laser threshold and the sharp transition to an SSP regime. The model also describes self-pulsing inhibition to the profit of SML for a sufficiently high pump power. Experimental results agree with most model predictions, but the overestimation of the self-pulsing threshold suggests that the phenomenon is furthered by an effect other than gain saturation.

Material micromachining using a pulsed fiber laser platform with fine temporal nanosecond pulse shaping capability

We report on recent advances in laser material processing using a novel pulsed fiber laser platform providing pulse shape agility at the nanosecond time scale and at high repetition rates. The pulse shapes can be programmed with a time resolution of 2.5 ns and with an amplitude resolution of 10 bits. Depending on the desired laser performances, the pulses are generated either by directly modulating the drive current of a seed laser diode or by modulating the output of a seed laser diode operated in CW with electro-optic modulators. The pulses are amplified in an amplifier chain in a MOPA configuration. Advanced polarization maintaining LMA fiber designs enable output energy per pulse up to 60 μJ at 1064 nm at a repetition rate of 200 kHz with excellent beam quality (M2< 1.1) and narrow line widths suitable for efficient frequency conversion. Micro-milling experiments were carried out with stainless steel, in which processing microstructures of a few tens of microns in size usually represents a challenge, and aluminum, whose thermal conductivity is about 20 times higher than stainless steel. The results obtained with two metals having very different thermal properties using different pulse shapes with durations varying between 3 ns and 80 ns demonstrate the benefits of using lasers offering flexible pulse durations and controllable pulse intensity profiles for rapidly optimizing a process in different applications while using the same laser with respect to conventional methods based on pulsed laser with fixed pulse shapes. Numerous applications are envisioned in a near future, like the micromachining of multi-layered structures, in particular when working with the harmonics of the laser.

Micro-milling process improvement using an agile pulse-shaping fiber laser

We demonstrate the usefulness of INOs pulse-shaping fiber laser platform to rapidly develop complex laser micromachining processes. The versatility of such laser sources allows for straightforward control of the emitting energy envelop on the nanosecond timescale to create multi-amplitude level pulses and/or multi-pulse regimes. The pulses are amplified in an amplifier chain in a MOPA configuration that delivers output energy per pulse up to 60 μJ at 1064 nm at a repetition rate of 200 kHz with excellent beam quality (M2 < 1.1) and narrow line widths suitable for efficient frequency conversion. Also, their pulse-on-demand and pulse-to-pulse shape selection capability at high repetition rates makes those agile laser sources suitable for the implementation of high-throughput complex laser processing. Micro-milling experiments were carried out on two metals, aluminum and stainless steel, having very different thermal properties. For aluminum, our results show that the material removal efficiency depends strongly on the pulse shape, especially near the ablation threshold, and can be maximized to develop efficient laser micro-milling processes. But, the material removal efficiency is not always correlated with a good surface quality. However, the roughness of the milled surface can be improved by removing a few layers of material using another type of pulse shape. The agility of INOs fiber laser enables the implementation of a fast laser process including two steps employing different pulse characteristics for maximizing the material removal rate and obtaining a good surface quality at the same time. A comparison of material removal efficiency with stainless steel, well known to be difficult to mill on the micron scale, is also presented.

Yb-doped LMA triple-clad fiber laser

The ytterbium-doped large mode area triple-clad fiber design allows for a high concentration of ytterbium in the fiber core which is difficult to achieve with a standard double-clad design. The novelty of the triple-clad fiber design consists in adding to the double-clad fiber design, a first clad next to its core. This first clad offers a better control of the core effective area. With this design a low numerical aperture is achievable (~0.06) for highly rare earth doped large mode area fiber. A 33-μm core ytterbium doped fiber has been fabricated using MCVD and solution doping processes. Selective doping and optimized first clad thickness have been used in the triple-clad design to obtain a nearly bending insensitive and nearly diffraction-limited fiber output. The fiber has been tested in a free-running laser configuration and its slope efficiency is 84% with a laser threshold of 1.4 W. A maximum output power of 26 W at 1070 nm has been achieved for a launched pump power of 34 W at 976 nm. The mode-field diameter has been measured at 18 μm and the output beam M2 quality factor is below 1.1. Both output power and beam quality were not significantly affected by fiber bending with loops diameter as small as 2.5 cm. The optical performance of the triple-clad fiber design is robust to mechanical stress and well suited for building very compact high power fiber lasers and amplifier sources.

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.

Bending insensitive highly Yb-doped LMA triple-clad fiber for nearly diffraction-limited laser output

The new highly rare-earth doped triple-clad fiber design comprises a first clad next to the core of the well-known double-clad design. The added clad allows to reduce and to better control the core effective numerical aperture for achieving a highly doped large mode area amplifying fiber with a very low numerical aperture (~0.07). The triple-clad design is optimized to obtain a nearly bending insensitive fiber output while keeping excellent beam quality through proper ytterbium doping. The high ytterbium concentration allows for very high gain from a short (~1 m) fiber length which, in many applications, is required to prevent the onset of nonlinear effects such as stimulated Brillouin scattering. A polarization-maintaining 22-μm core Yb-doped triple-clad fiber was first tested. A laser slope efficiency of up to 86% with a polarization extinction ratio exceeding 24 dB and a M2 output beam quality factor below 1.1, for both laser and amplifier configurations, have been measured. Moreover, beam quality and output power were not significantly affected when coiling the fiber down to a 1.2 cm diameter, thus showing the optical robustness of the triple clad fiber design and offering the opportunity to build very compact high power fiber amplifiers and laser sources.

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.

Practical design of double-clad ytterbium-doped fiber amplifiers using Giles parameters

We present a simple and accurate method for measuring the Giles parameters of a double-clad ytterbium-doped fiber. The characterization is performed by cut-back on the doped fiber under constant pumping. Using nonlinear curve-fitting of the amplified spontaneous emission (ASE) power-density spectra, along with iterative solution of the photon balance model, we compute both the small-signal gain at complete population inversion and the small-signal absorption of the fiber. The method successfully predicts the extraction efficiency of an amplifier operating at 1064 nm. The ratio between the signal power and the out-of-band ASE power at the output of the amplifier is also accurately predicted by introducing spurious feedback from the fiber facets in the photon balance model. This work shows that a fiber facet reflectivity of a few thousandths of a percent (-40 to -50) dB can significantly enhance the out-of-band ASE power.

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.