Holographic photopolymerization combined to microfluidics for the fabrication of lab-in-lab microdevices and complex 3D micro-objects.

We show here brand-new possibilities of lab-in-lab fabrication while combining holographic photopolymerization and microfluidics. One shot real-time 3D-printing can produce 3D architectured microchannels, or free-standing complex micro-objects eventually in flow. The methodology is very versatile and can be applied to e.g., acrylate resins or hydrogels.

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.

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.

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.

Temporal synchronization and spectral combining of pulses from fiber lasers Q-switched by independent MEMS micro-mirrors.

We present what we believe to be the first demonstration of spectral combining of multiple fiber lasers Q-switched by independent micro-electro-mechanical system (MEMS). By correlating the actuation of the individual MEMS devices, the associated Q-switched lasers can be operated in either synchronous or asynchronous modes in such a way that their overall combined output may result in high energy emission pulses or in laser emission with higher pulse repetition rate. In a proof-of-principle experiment, we demonstrate the combination of four individual Q-switched lasers (each of them operating at 20 kHz repetition rate) leading to a final laser system generating pulses with a repetition rate of 80 kHz.

Actively mode-locked fiber laser using a deformable micromirror.

We present what we believe to be the first fiber laser system that is actively mode-locked by a deformable micromirror. The micromirror device is placed within the laser cavity and performs a dual function of modulator and end-cavity mirror. The mode-locked laser provides ~1-ns-long pulses with 20 nJ/pulse energy at 5 MHz repetition rates.

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.

Demonstration of a true single-shot 100 GHz-bandwidth optical oscilloscope at 1053-1064 nm.

We demonstrate an innovating design to validate and to optimize the real-time performance of an all-optical oscilloscope at 1053-1064 nm. A unique broadband pulse is generated by means of frequency beats and of proper optical-shaping, which helps us to evidence a signal bandwidth of 100 GHz and a dynamics range in excess of 25 dB. Gain-narrowing and dispersion effects due to the replication of the input pulse are shown to be the first limitations in the broadband capabilities.

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.

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.

Tunable high-repetition-rate fiber laser for the generation of pulse trains and packets.

We report on a mode-locked erbium-doped fiber laser suited to the generation of high-repetition-rate pulse trains or packets. The two-arm interferometer resonator performs a spectral filtering leading to the emission of short pulse packets with a high repetition rate, adjustable up to 200 GHz. A continuous pulse train with a repetition rate of up to 6.9 GHz was obtained by adding a recirculation loop inside the cavity to fill the spaces between all pulse packets.

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).

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

Coherent combining is demonstrated in a clad-pumped Yb-doped double-core fiber laser. A slope efficiency of more than 70% is achieved with 96% of the total output power in the fundamental mode of one of the two cores. This high combining efficiency is obtained when both cores are coupled via a biconical fused taper in a Michelson interferometer configuration.

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.