Véronique Jolivet

Beam Shaping of Single-Mode and Multimode Fiber Amplifier Arrays for Propagation Through Atmospheric Turbulence

We report experimental results and theoretical analysis of coherent beam combining with active phase control fiber beam shaping. An original optical configuration for target-in-the-loop single-mode fiber amplifier coherent combining through turbulence is presented, with a lambda/15 residual phase error. The experimental results and theoretical analysis demonstrate that detection subsystem aperture reduction is paramount to lower sensitivity to backward turbulence when using a detector in the laser emitter plane. In this configuration, coherent combining is achieved on a remote scattering surface with sole compensation of the onward turbulence. We also present a numerical model capable of assessing the combining efficiency in the case of high-power multimode large-mode-area (LMA) fiber amplifiers. Preliminary theoretical investigations point out that multiple-transverse-mode combining can result in severe wavefront distortion. In the case of multimode LMA fibers, control of the transverse modes phase relationship has to be achieved to preserve combining efficiency.

Coherent beam combination of narrow-linewidth 1.5 μm fiber amplifiers in a long-pulse regime.

We report what we believe to be the first experimental demonstration of coherent beam combining of two fiber amplifiers in a 100 ns pulse regime using a signal leak between the pulses. Pulses of ∼100 W stimulated-Brillouin-scattering limited peak power are combined with 95% efficiency, a residual phase error of λ/27, and no significant beam quality degradation.

Multifilament-core fibers for high energy pulse amplification at 1.5 μm with excellent beam quality

We report on what we believe to be the first demonstration of an erbium-ytterbium-doped multifilament-core (MFC) fiber for single-mode amplification of narrow linewidth high peak power pulses. A master-oscillator-power-fiber-amplifier laser source has been demonstrated using a 37-filament MFC fiber in the last amplification stage. Pulses with 750 microJ (940 W peak power) and laser linewidth<1 MHz have beam generated with M2 approximately 1.3. This value is close to the theoretical value M2 approximately 1.5.

Coherent beam combining of fiber amplifier arrays and application to laser beam propagation through turbulent atmosphere

Coherent beam combining of fiber amplifier arrays is a promising way to increase power of fiber lasers, and overcome the physical limitations to fiber laser power scaling. We performed the coherent combining of fiber amplifier arrays using active control of the phase of each amplifier. The phase fluctuations in the fiber amplifiers have been measured and their effect on the beam combining process stability evaluated. We extended the coherent beam combining technique to perform wavefront shaping, in order to deliver a high brightness beam after turbulent atmospheric propagation. We present experimental results exhibiting the capability of the modulation multiplexing technique that we implemented to compensate phase fluctuations due to turbulent atmospheric propagation on the laser beam path. Moreover, and for the first time to our knowledge, we demonstrate automatic coherent combining of fiber amplifiers on a diffuse surface, after propagation through turbulent atmosphere, without any external turbulence measurement subsystem.

Brightness Scaling Based on 1.55 μm Fiber Amplifiers Coherent Combining

Abstract Modulation multiplexing and beam cleanup are efficient techniques to coherently combine multiple fiber laser sources and achieve high power and very good beam quality. Using modulation multiplexing, three fiber amplifiers are coherently combined with a residual phase error of λ/30. Using beam cleanup by stimulated Brillouin scattering in a multimode gradient-index fiber within a ring cavity, coherent combining of two fiber amplifiers with 50% efficiency and 99% slope is achieved. Further potential of these techniques is also discussed.

Er-Yb-doped LMA fiber structures for high energy amplification of narrow linewidth pulses at 1.5μm

A high-energy (0.6 mJ), narrow-linewidth fiber MOPA is built using Erbium-Ytterbium doped large-mode-area fibers. Pedestal and multifilament core fiber structures are compared. The multifilament fiber shows the best beam quality (M2 ~1.9) and stability.

High Brightness 1.5 μm Pulsed Fiber Laser for Lidar: From Fibers to Systems

Abstract In this article, we discuss the designs and performances of narrow-linewidth erbium-ytterbium fiber MOPA for coherent detection lidars. We report on pedestal and multifilament core erbium-ytterbium large mode area fibers specially designed to generate high energy and good beam quality. Pulse energy of 220 μJ with M2 < 1.4 are obtained with all-fiber three amplification stages. An additional amplifier stage generates 600 μJ with M2 < 2.4. We present the first results of pulsed fiber lidar wake vortices detection using a coherent detection lidar. Maturation of this technology leads to compact all-fiber lasers. Several commercial laser sources for lidars are finally described.

Coherent combining of low-peak-power pulsed fiber amplifiers with 80-ns pulse duration

We investigate the feasibility of pulsed fiber amplifier coherent combining. Therefore, we characterize phase fluctuations in low-peak-power pulsed fiber amplifiers using two different interferometric techniques. These measurements reveal that for low peak-powers, phase fluctuations remain moderate during the pulses. Noticeable phase fluctuations occurring between the pulses can be perfectly controlled using classical continuous-wave-efficient combining techniques. Results of such realization combining two low-peak-power pulsed fiber amplifiers, using classical frequency-tagging coherent combining techniques, are presented. Phase difference measurement is performed between pulses using a small signal leak from the common master oscillator. For the first time to our knowledge, successful coherent combining of two low-peak- power pulsed fiber amplifiers is thereby demonstrated.

Theoretical analysis and quantitative measurements of fiber amplifier coherent combining on a remote surface through turbulence

Coherent beam combining of fiber amplifier arrays is an efficient way to overcome the physical limitations to fiber laser power scaling. Moreover, coherent combining techniques involving active phase control of the laser emitters offer the largest versatility, as they can also be used for complex purposes such as beam steering, wavefront shaping or atmospheric turbulence compensation. We reported last year the first experimental demonstration of coherent combining of fiber amplifiers on a remote scattering surface, after propagation through turbulent atmosphere, using the backscattered signal. These results were achieved with a frequency-tagging technique, and appropriate spatial filtering to lower sensitivity to backward turbulence, and compensate only for onward turbulent propagation. We present now experimental measurements of turbulence strength and resulting residual phase error. With turbulence compensation using the backscattered signal for phase control, this error is λ/15 rms. We also present the theoretical analysis of this experiment, emphasizing how limiting the aperture and not the field of view of the phase difference measurement subsystem reduces sensitivity to backward turbulence, without decreasing the optical flux on the detector.

Laser beam combining by beam cleanup in a gradient index Brillouin ring cavity

We propose and investigate a new beam combining setup using stimulated-Brillouin-scattering in a multimode gradient-index-fiber within a ring cavity. 50% efficiency and 99% slope are achieved with 2 ErYb doped fiber amplifiers without external control.