Agnès Desfarges-Berthelemot

Efficient coherent combining of widely tunable fiber lasers

We have experimentally demonstrated coherent combining of 2 and then 4 fiber lasers, with respectively 99% and 95% combining efficiency. The combining method investigated here is based on a multi-arm resonator of interferometric configuration. In spite of its interferometric nature, the multi-arm laser operates without significant power fluctuations, even in an unprotected environment. This occurs when the arm length difference is large enough to introduce spectral modulations of period smaller than the laser bandwidth. We have also experimentally shown that the combining method is compatible with wavelength tuning. A Mach- Zehnder Fiber Laser was tuned over a wide spectral range of 60nm. Theoretically then, we confirm that the combining method can be scaled to a large number of lasers without decreasing the combining efficiency.

Passive phase locking of an array of four fiber amplifiers by an all-optical feedback loop

We report the passive phase locking of an array of four fiber amplifiers in a unidirectional ring cavity. The feedback loop consists of a single-mode fiber that filters intracavity the far-field pattern of the four emitted beams. The pointing of the laser output can be managed by the intracavity filtering.

Design of microstructured single-mode fiber combining large mode area and high rare earth ion concentration

We propose a new design of microstructured fiber combining large doped area (500 μm2), high rare earth concentration and single mode propagation despite the high core refractive index (nSi + 0.01). Actually, original guiding properties, based on a total internal reflection guidance regime modified by coupling between core and resonant cladding modes (close to the ARROW model) ensure single mode propagation. Moreover spectral properties which are largely governed by characteristics of high index cladding rods can be adjusted by properly choosing diameter and refractive index of the rods.

Coherent combining in an Yb-doped double-core fiber laser

We have experimentally demonstrated coherent combining in an Yb doped double core fiber laser. More than 85% of the total output power is combined on the fundamental mode of one of the two cores. This efficient power combining is confirmed by the expected modulated spectrum of the emitted radiation. Finally, this combining method can be scaled with a good efficiency to a larger number of cores with a good efficiency leading to a high power single mode emission.

Iterative reconstruction of thermally induced phase distortion in a Nd3+:YVO4 laser

The thermal distortions in a continuous-wave end-pumped Nd:YVO4 laser are experimentally investigated by use of an external TEM00 probe beam (TEM standing for transverse electric magnetic). After a round-trip of this probe through the active medium, three irradiance profiles are recorded along its propagation axis. The distorted phase pattern of this probe beam is then reconstructed from these profiles by means of a Gerchberg-Saxton-like iterative algorithm. To identify the contribution of each kind of aberration in the phase profile, we have computed a Zernike polynomial expansion, and have shown that the main terms constituting the aberration are (in order) defocusing (optical path difference (OPD) 0.7 λ FWHM (full width at half-maximum)), spherical aberration (OPD 0.3 λ FWHM), and first-order astigmatism (OPD of ±0.14 λp FWHM). Moreover, all higher-order terms result in a significant phase peak (λ/4 FWHM over 400 µm).

Ytterbium-doped fibre laser Q-switched by a cantilever-type micro-mirror

We present an Ytterbium fibre laser operating in the Q-switch regime by using a Micro- Opto- Electro- Mechanical System (MOEMS) of novel design. The cantilever-type micro-mirror is designed to generate short laser pulses with duration between 20 ns and 100 ns at repetition rates ranging from a few kilohertz up to 800 kHz. The bent profile of this new type of MOEMS ensures a high modulation rate of the laser cavity losses while keeping a high actuating frequency.

Coherent combining of 49 laser beams from a multiple core optical fiber by a spatial light modulator

We report co-phasing of 49 beams from a multicore fiber based on a spatial light modulator using a simple feedback loop control. A specific fiber of 7x7 Germanium doped cores was fabricated. The power carried by the far field main lobe of the phased array, in the continuous wave regime, amounts to 96% of its theoretical value. Femtoseconde pulses were also phase-locked with the same device configuration.

Highly efficient phase locking of four diode pumped Nd:YAG laser beams

Abstract We describe the mutual phase-locking of four Nd:YAG beams longitudinally end-pumped by four fiber-coupled diode lasers. Mutual phase-locking is performed by two types of diffractive components located inside a telescopic Fabry–Perot resonator. The control of the phase-locking is shown and the different spatial eigenmodes connected with the pumping geometry in the resonator are theoretically and experimentally studied. 1.2 W is obtained in a single mode stable pattern with 30% optical-to-optical conversion efficiency in the continuous wave regime.

Coherent beam combining with an ultrafast multicore Yb-doped fiber amplifier.

Active coherent beam combination using a 7-non-coupled core, polarization maintaining, air-clad, Yb-doped fiber is demonstrated as a monolithic and compact power-scaling concept for ultrafast fiber lasers. A microlens array matched to the multicore fiber and an active phase controller composed of a spatial light modulator applying a stochastic parallel gradient descent algorithm are utilized to perform coherent combining in the tiled aperture geometry. The mitigation of nonlinear effects at a pulse energy of 8.9 µJ and duration of 860 fs is experimentally verified at a repetition rate of 100 kHz. The experimental combining efficiency results in a far field central lobe carrying 49% of the total power, compared to an ideal value of 76%. This efficiency is primarily limited by group delay differences between cores which is identified as the main drawback of the system. Minimizing these group delay issues, e.g. by using short and straight rod-type multicore fibers, should allow a practical power scaling solution for femtosecond fiber systems.

Kerr self-cleaning of pulsed beam in an ytterbium doped multimode fiber

We experimentally demonstrate that Kerr spatial self-cleaning of a pulsed beam can be obtained in an amplifying multimode optical fiber. An input peak power of 500 W only was sufficient to produce a quasi-single-mode emission from the double-clad ytterbium doped multimode fiber (YMMF) with non-parabolic refractive index profile. We compare the self-cleaning behavior observed in the same fiber with loss and with gain. Laser gain introduces new opportunities to achieve spatial self-cleaning of light in multimode fibers at a relatively low power threshold.