KEYWORDS: Error analysis, Wavefronts, Fiber amplifiers, Near field, Beam analyzers, Fiber lasers, Near field optics, Monte Carlo methods, Data modeling, Laser optics
We perform sensitivity analyses on two different array configurations of coherently combined fiber amplifiers to study
the impact of residual phase errors onto the combining efficiency. The arrays studied are: a square of 16 fibers and a
hexagon of 19 fibers. For the hexagon, two different shaped wavefronts are studied. In this method a global analysisis
performed: we modify simultaneously all the phase errors using numerical space filling designs. Then, the construction
of a metamodel makes it possible to investigate more precisely the role of each fiber and specially the role of interactions
between fibers onto the combination with less runs than classical approaches. Results exhibit different behaviors and
specially interactions between fibers with respect to the array configurations and with respect to the two different shaped
wavefronts. Finally, we demonstrate that we can study arrays of more than 100 fibers.
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.
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 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.
Vibration measurement using coherent laser radar (LADAR) is a promising way to identify air targets at long range. Laser vibrometers can remotely measure the velocity of micrometric displacements and thus exhibit the target surface vibration frequencies. Some of these frequencies are modal frequencies, which result from the target structure. They define a unique signature and allow target identification to be performed. As vibration amplitudes are not reliable, we choose to consider only frequency positions.
In this article, we explain an "extended identification" method which takes into account cumulative signatures in space and time to improve global system identification performance. Using a nearest neighbor classifier and a suitable metric taking into account a simple off-line processing of measured data, the recognition algorithm leads to good identification rates and very low rejection rates for a nine class problem. We show a strong improvement of the identification rate thanks to the "extended identification" method.
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