Didier Goular

Coherent beam combining of two femtosecond fiber chirped-pulse amplifiers

We demonstrate coherent beam combining of two femtosecond fiber chirped-pulse amplifiers seeded by a common oscillator. Using a feedback loop based on an electro-optic phase modulator, an average power of 7.2 W before compression is obtained with a combining efficiency of 90%. The spatial and temporal qualities of the oscillator are retained, with a recombined pulse width of 325 fs. This experiment opens up a way to scale the peak/average power of ultrafast fiber sources.

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.

Pulsed 1.5-

In this paper, we present the development of an axial aircraft wake vortex light detection and ranging (LIDAR) sensor, working in Mie scattering regime, based on pulsed 1.5-mu m high-brightness large-core fiber amplifier. An end-to-end Doppler heterodyne LIDAR simulator is used for the LIDAR design. The simulation includes the observation geometry, the wake vortex velocity image, the scanning pattern, the LIDAR instrument, the wind turbulence outside the vortex, and the signal processing. An innovative high-brightness pulsed 1.5-mum laser source is described, based on a master oscillator power fiber amplifier (MOPFA) architecture with a large-core fiber. The obtained beam quality is excellent (M 2 = 1.3), and achieved pulsed energy is 120 muJ with a pulse repetition frequency of 12 kHz and a pulse duration of 800 ns. A Doppler heterodyne LIDAR is developed based on this laser source with a high-isolation free-space circulator. The LIDAR includes a real-time display of the wind field. Wind dispersion is postprocessed. Field tests carried out at Orly airport in April 2008 are reported. Axial aircraft wake vortex signatures have been successfully observed and acquired at a range of 1.2 km with axial resolution of 75 m for the first time with fiber laser source.

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We experimentally investigate the impact of spectral phase mismatch on the coherent beam combining of two femtosecond fiber chirped-pulse amplifiers. By measuring the differential spectral phase, both linear and nonlinear contributions are identified. An accumulated nonlinear phase as high as 6 rad has been measured, for which a combination efficiency of 91% can be obtained by symmetrizing the pump and injection powers. This also allows us to quantitatively separate the spatial and temporal contributions of the nonperfect combining efficiency.

m LIDAR for Axial Aircraft Wake Vortex Detection Based on High-Brightness Large-Core Fiber Amplifier

We report on a coherent wind lidar built with two coherently-beam-combined fiber amplifiers. The lidar performances of the combined-amplifier and the single-amplifier are compared using two criterions: carrier-to-noise ratio and wind speed noise floor. In both cases, lidar performances are not degraded with a combined source and are close to the theoretical optimum. Combined sources are well suited to improve coherent wind lidar accuracy, range, and integration time.

Impact of spectral phase mismatch on femtosecond coherent beam combining systems

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.

Eyesafe coherent detection wind lidar based on a beam-combined pulsed laser source

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.

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

A pulsed fibre lidar based on 1.5 mum fibre technology has been demonstrated for wake vortex monitoring on airport sites. The wake vortex cores position resolution is plusmn2 m, the error on circulation 10%.

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

Coherent beam combining (CBC) by active phase control could be useful for power scaling fiber-laser-pumped optical frequency converters like OPOs. However, a phase modulator operating at the frequency-converted wavelength is needed, which is non standard component. Fortunately, nonlinear conversion processes rely on a phase-matching condition correlating, not only the wave vectors of the coupled waves, but also their phases. This paper demonstrates that, using this phase correlation for indirect control of the phase, coherent combining of optical frequency converters is feasible using standard all-fibered electro-optic modulators. For the sake of demonstration, this new technique is experimentally applied twice for continuous wave second-harmonic-generator (SHG) combination: i) combining 2 SHG of 1.55-μm erbium-doped fiber amplifiers in PPLN crystals generating 775-nm beams; ii) combining 2 SHG of 1.064-μm ytterbium-doped fiber amplifiers in LBO crystals generating 532-nm beams. Excellent CBC efficiency is achieved on the harmonic waves in both these experiments, with λ/20 and λ/30 residual phase error respectively. In the second experiment, I/Q phase detection is added on fundamental and harmonic waves to measure their phase variations simultaneously. These measurements confirm the theoretical expectations and formulae of correlation between the phases of the fundamental and harmonic waves. Unexpectedly, in both experiments, when harmonic waves are phase-locked, a residual phase difference remains between the fundamen tal waves. Measurements of the spectrum of these residual phase differences locate them above 50 Hz, revealing that they most probably originate in fast-varying optical path differences induced by turbulence and acoustic-waves on the experimental breadboard.