Coherent beam combining (CBC) by active phase control is an efficient technique to power scale fiber laser sources emitting in the near-infrared, between 1 and 2 μm, up to the multi-kilowatt level. Interestingly, it has been demonstrated by our team that CBC could also be used to power scale mid-infrared sources, frequency converters, generating a wavelength between 3 and 5 μm. We present our latest results on coherent combining of continuous-wave highefficiency mid-infrared sources: optical parametric oscillators (OPOs) and detail the difficulties encountered to achieve this combining, as well as the main limitations to efficient operation of CBC in this case. In a second part of this talk, we also present recent results on coherent combining of seven 1.5-μm fiber lasers through active phase control, using frequency-tagging, and operating efficiently on a remote target. A testbed has been designed to combine these 7 lasers on a remote surface, with phase-locking operating through analysis of the optical signal backscattered by the target, in a so-called target-in-the-loop (TIL) experiment. In such TIL configuration, CBC mitigates both laser-amplification-induced and atmospheric turbulence-induced phase fluctuations simultaneously. CBC demonstrated proper operation outdoors, on a target located up to 1 km from the laser and the results from this experimental campaign will be described.
KEYWORDS: Fiber lasers, Target detection, Turbulence, Laser development, Signal detection, Signal analyzers, Near field optics, Near field, Laser beam propagation, Fiber amplifiers
Coherent beam combining (CBC) by active phase control is an efficient way to power scale fiber amplifiers. Most often, CBC operates from measuring the phase differences between the lasers at their outputs, hence resulting in efficient combination of the laser beams in the very near-field. We developed a laser testbed coherently combining seven 1.5-µm fiber lasers through active phase control, using frequency-tagging to assess the phase fluctuations to be compensated for. The testbed can operate in a target-in-the-loop (TIL) configuration, with a detection sub-system designed to analyse the optical signal back-scattered by a remote target, in order to achieve coherent combining on the target rather than at the output of the lasers. In this paper, we present the testbed and its components, as well as the results obtained in direct coherent combining, operated at the output of the lasers, during the preliminary tests of the setup. Then, we present the results of the outdoor experimental campaign where the testbed is operated in a TIL-CBC configuration. Measurement of TIL-CBC efficiency when distance to the target is progressively increased from 15 meters to 1 km is detailed. As the experimental campaign took place in hot weather, with a close to the ground horizontal path of propagation for the laser beams, very strong turbulence conditions were encountered. However, efficient atmospheric turbulence compensation was demonstrated, confirming that TIL-CBC can be achieved, even under such detrimental turbulence conditions.
Some applications like range finding, optical counter measures or engine ignition, require lasers capable of delivering high repetition rate bursts of nanosecond pulses with hundreds of microjoules to a few millijoules in terms of energy per pulse. We have developed such a diode pumped Yb:YAG micro-laser with an oscillator comprised of a 2-mm long 10% at. doped Yb:YAG crystal followed by a Cr:YAG passive Q-switch with an initial transmittance of 85 %. The laser planoconcave cavity is 5-cm long. This oscillator emits 250 μJ to 300 μJ per pulse, with a 3 – 5 ns pulse duration, with an intra-burst pulse repetition frequency that can be tuned continuously from 1 kHz to 20 kHz by increasing the pump power. The pumping diode laser is operated in quasi continuous wave regime, emitting 1-ms to 10-ms long pulses with up to 20 W peak power This qcw pumping results in the emission of a burst of pulses at high repetition rate for the duration of this pump long pulse. These pump pulses, and consequently the bursts of nanosecond pulses, are repeated at very low frequency, between 1 Hz and 5 Hz, so that the average power to handle doesn’t require active cooling. This oscillator is then amplified to the millijoule level in a second 3-mm long Yb:YAG crystal pumped by a synchronous qcw emitting diode laser.
Engine ignition using a laser requires very high peak power levels, that can be produced by solid-state lasers such as Yb:YAG passively Q-switched lasers. We developed high repetition rate diode pumped Yb:YAG micro-lasers to study the effect of cumulated pulses on the engine ignition process. The Yb:YAG laser oscillator is pumped by a 5-Hz quasi-continuous wave diode laser emitting 3-ms long pump pulses with up to 20 W peak power. It’s passively Q-switched using a Cr:YAG crystal. Various Yb:YAG dopant concentrations and crystal length have been tested and different initial transmittance values for the Cr:YAG crystal have been compared. As a result of quasi-continuous wave pumping and passive Q-switching, bursts of short pulses are emitted at the 5-Hz repetition frequency of the long pump pulses. The control of the intra-burst repetition rate is achieved through tuning the pump power between a few watts and 20 W. The energy per pulse ranges from 250 μJ to 300 μJ, with a lower than 5 ns pulse duration. The intra-burst repetition rate can go up to 20 kHz. An amplifying stage comprised of one single Yb:YAG crystal is added after this laser oscillator.
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