Experimental phase control of a 100 laser beam array with quasi-reinforcement learning of a neural network in an error reduction loop.

An innovative scheme is proposed for the dynamic phase control of a laser beam array. It is based on a simple neural network included in a phase correction loop that predicts the complex field array from the intensity of the induced scattered pattern through a phase intensity transformer made of a diffuser. A crucial feature is the use of a kind of reinforcement learning approach for the neural network training which takes account of the iterated corrections. Experiments on a proof-of-concept system demonstrated the high performance and scalability of the scheme with an array of up to 100 laser beams and a phase setting at λ/30.

Shaping of amplified beam from a highly multimode Yb-doped fiber using transmission matrix.

The transmission matrix of an ytterbium doped multimode fiber with gain was measured. It was shown to vary owing to the pump power level. Amplified beam focusing, beam steering and shaping were demonstrated using the measured matrix for input wavefront shaping, with an efficiency similar to the case of a passive fiber. The impact of weak gain saturation was lastly investigated.

Space-time adaptive control of femtosecond pulses amplified in a multimode fiber.

Ultrashort light pulse transport and amplification in a 1.3 m long step-index multimode fiber with gain and with weak coupling has been investigated. An adaptive shaping of the input wavefront, only based on the output intensity pattern, has led to an amplified pulse focused both in space (1/32) and in time (1/10) despite a strong modal group delay dispersion. Optimization of the input owing to the two-photon detection of the amplified signal permitted to excite the fastest and more intense principal mode of the fiber and to get an output pulse duration limited by group velocity dispersion.

Efficient phase-locking of 37 fiber amplifiers by phase-intensity mapping in an optimization loop.

Coherent combination of laser beams from 37 fiber amplifiers in a tiled aperture configuration has been achieved thanks to an innovative iterative process. The high efficiency as well as the speed of the phase control demonstrated the relevance of the method for phase locking of a large array of fiber lasers.

Publisher's Note: Observation of Geometric Parametric Instability Induced by the Periodic Spatial Self-Imaging of Multimode Waves [Phys. Rev. Lett. 116, 183901 (2016)].

This corrects the article DOI: 10.1103/PhysRevLett.116.183901.

Observation of Geometric Parametric Instability Induced by the Periodic Spatial Self-Imaging of Multimode Waves.

Spatiotemporal mode coupling in highly multimode physical systems permits new routes for exploring complex instabilities and forming coherent wave structures. We present here the first experimental demonstration of multiple geometric parametric instability sidebands, generated in the frequency domain through resonant space-time coupling, owing to the natural periodic spatial self-imaging of a multimode quasi-continuous-wave beam in a standard graded-index multimode fiber. The input beam was launched in the fiber by means of an amplified microchip laser emitting sub-ns pulses at 1064 nm. The experimentally observed frequency spacing among sidebands agrees well with analytical predictions and numerical simulations. The first-order peaks are located at the considerably large detuning of 123.5 THz from the pump. These results open the remarkable possibility to convert a near-infrared laser directly into a broad spectral range spanning visible and infrared wavelengths, by means of a single resonant parametric nonlinear effect occurring in the normal dispersion regime. As further evidence of our strong space-time coupling regime, we observed the striking effect that all of the different sideband peaks were carried by a well-defined and stable bell-shaped spatial profile.

One shot endoscopic polarization measurement device based on a spectrally encoded polarization states generator.

We report a novel technique for polarimetric characterization of samples through a flexible fiber endoscope, with a single shot measurement per pixel. The sample is simultaneously probed with a large diversity of polarization states, and both degree of polarization and linear retardance are determined thanks to specific processing of data. The probe polarization states are spectrally encoded on the 10 nm bandwidth of the source. The key component of the endoscope is a 3 m long specially designed optical fiber which consists of the optimized concatenation of highly birefringent fiber pieces. For a proof of principle, different calibrated or manufactured samples were successfully characterized. The proposed technique is attractive in view of reducing the measurement time of polarimetric images, in endoscopic applications.

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.

Experimental demonstration of passive coherent combining of fiber lasers by phase contrast filtering.

We report experiments on a new laser architecture involving phase contrast filtering to coherently combine an array of fiber lasers. We demonstrate that the new technique yields a more stable phase-locking than standard methods using only amplitude filtering. A spectral analysis of the output beams shows that the new scheme generates more resonant frequencies common to the coupled lasers. This property can enhance the combining efficiency when the number of lasers to be coupled is large.

Bragg-scattering conversion at telecom wavelengths towards the photon counting regime.

We experimentally study Bragg-scattering four-wave mixing in a highly nonlinear fiber at telecom wavelengths using photon counters. We explore the polarization dependence of this process with a continuous wave signal in the macroscopic and attenuated regime, with a wavelength shift of 23 nm. Our measurements of mean photon numbers per second under various pump polarization configurations agree well with the theoretical and numerical predictions based on classical models. We discuss the impact of noise under these different polarization configurations.

Soliton triads ensemble in frequency conversion: from inverse scattering theory to experimental observation.

We consider the spectral theory of three-wave interactions to predict the initiation, formation and dynamics of an ensemble of bright-dark-bright soliton triads in frequency conversion processes. Spatial observation of non-interacting triads ensemble in a KTP crystal confirms theoretical prediction and numerical simulations.

Pulse compression and fiber delivery of 45 fs Fourier transform limited pulses at 830 nm.

A specific scheme is used for fiber delivery of ultrashort pulses using conventional elements. Starting from a standard femtosecond Ti:Al(2)O(3) oscillator (150 fs @ 830 nm), perfectly compressed ultrashort pulses with a duration of 45 fs are produced at the output of a standard two meter long single-mode fiber. The setup allows compensating independently and simultaneously second and third orders of chromatic dispersion as well as management of self-phase modulation in the fiber. It includes an optimized dispersion compensation line made of the assembly of diffraction gratings and prisms. The unsurpassed performances of the device are experimentally and numerically highlighted. Fiber delivery of sub-30 fs multinanojoule pulses is discussed.

Velocity-locked solitary waves in quadratic media.

We demonstrate experimentally the existence of three-wave resonant interaction solitary triplets in quadratic media. Stable velocity-locked bright-dark-bright spatial solitary triplets, determined by the balance between the energy exchange rates and the velocity mismatch between the interacting waves, are excited in a KTP crystal.

Coherence properties of two fiber lasers coupled by mutual injection.

Phase locking of two fiber lasers is obtained by mutual injection. The coherence properties of this composite laser are analyzed. In the most general case, the radiations at the two output mirrors of the laser are not mutually coherent. The cross-correlation of the two emitted beams shows that an extra-cavity delay line is required to get stable and high visibility interference fringes. Such a laser configuration is attractive for coherent beam combining in the far field provided the multiple output beams are properly time delayed.

Frequency generation and solitonic decay in three wave interactions.

We consider experimentally three-wave resonant nonlinear interactions of fields propagating in nonlinear media. We investigate the spatial dynamics of two diffractionless beams at frequency omega1, omega2 which mix to generate a field at the sum frequency omega3. If the generated field at omega3 can sustain a soliton, it decays into solitons at omega1, omega2. We report the experimental evidence of the transition from steady frequency wave generation to solitonic decay in nonlinear optics.

Fiber-optic device for endoscopic polarization imaging.

Polarization images were obtained using a flexible optical fiber device. The endoscopic technique is based on the real-time compensation of the birefringence of the standard fiber by means of a miniaturized Faraday rotator. Based on a Jones formalism analysis, specific signal processing allows one to extract both the phase retardation and the degree of polarization of the light backscattered by a remote target.

Discrete focusing in an optical fiber with a two-dimensional square array of coupled waveguides.

We report the experimental demonstration of "discrete focusing" inside a 60-cm-long optical fiber made of a 2D square array of coupled waveguides. The suitable input amplitude and phase distributions are imposed by using a spatial light modulator. Thus we demonstrate that focusing in a single core at the output by discrete propagation is possible despite some amount of transverse heterogeneities of the waveguide array.

[Aspirin in decompression sickness]. / Place de l'aspirine dans le traitement médicamenteux de l'accident de désaturation.

OBJECTIVE: We have performed a survey on the use of aspirin in decompression sickness (DCS) treatment in French hyperbaric centers'. We also conducted a review of literature to determine if aspirin was beneficial to treat human victims of DCS. METHODS: Prospective observational study investigating French hyperbaric centers' prescription of aspirin to DCS' divers victims. The question we asked by mail or phone to French hyperbaric centers was: Do you give some aspirin to a diver with DCS if this treatment has not been given yet (on the site of accident). RESULTS: A large majority of French hyperbaric centers (77.5%) consider aspirin in DCS treatment. However this practice is not consensual. There is no evidence from the literature to support the efficiency of aspirin in DCS. CONCLUSIONS: Although aspirin is widely used for DCS treatment in France, more research is needed to determine if aspirin is useful.

Shaping the light amplified in a multimode fiber.

Propagation of light in multimode optical fibers usually gives a spatial and temporal randomization of the transmitted field similar to the propagation through scattering media. Randomization still applies when scattering or multimode propagation occurs in gain media. We demonstrate that appropriate structuration of the input beam wavefront can shape the light amplified by a rare-earth-doped multimode fiber. Profiling of the wavefront was achieved by a deformable mirror in combination with an iterative optimization process. We present experimental results and simulations showing the shaping of a single sharp spot at different places in the output cross-section of an ytterbium-doped fiber amplifier. Cleaning and narrowing of the amplifier far-field pattern was realized as well. Tailoring the wavefront to shape the amplified light can also serve to improve the effective gain. The shaping approach still works under gain saturation, showing the robustness of the method. Modeling and experiments attest that the shaping is effective even with a highly multimode fiber amplifier carrying up to 127 modes.

Novel active Q-switched fiber laser based on electrostatically actuated micro-mirror system.

It has come to the attention of the Optical Society of America that this article should not have been submitted owing to its substantial replication, without appropriate attribution, of significant elements found in the following previously published material: A. Crunteanu, D. Bouyge, D. Sabourdy, P. Blondy, V. Couderc, L. Grossard, P. H. Ploger and A. Barthelemy, "Deformable micro-electromechanical mirror integration in a fibre laser Q-switched system," J. Opt. A: Pure Appl. Opt. 8 S347-S351 (2006).