Coherent beam combination of seven 1.5 µm fiber amplifiers through up to 1 km atmospheric turbulence: near- and far-field experimental analysis
Bastien Rouzé, Laurent Lombard, Hermance Jacqmin, Anasthase Liméry, Anne Durécu, Pierre Bourdon
Abstract
A laser testbed based on active coherent beam combination (CBC) of seven 1.5 µm, 3 W fiber amplifiers was developed for applications requiring high power such as power density deposition on targets or free space laser communication. For the first time to our knowledge, the frequency-tagging locking of optical coherence by single-detector electronic-frequency tagging technique was implemented in the field in real atmospheric turbulence conditions in a target-in-the-loop configuration. Successful combination was achieved after horizontal propagation of 311 m and 1 km, at 1.5 m above the ground, while the estimated average turbulence strength was <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:msubsup> <mml:mi>C</mml:mi> <mml:mi>n</mml:mi> <mml:mn>2</mml:mn> </mml:msubsup> <mml:mo>∼</mml:mo> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:msup> <mml:mn>4.10</mml:mn> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mo>−</mml:mo> <mml:mn>14</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> <mml:mspace width="thickmathspace"/> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:msup> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi mathvariant="normal">m</mml:mi> </mml:mrow> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mo>−</mml:mo> <mml:mn>2</mml:mn> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mo>/</mml:mo> </mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> . We present the CBC laser bench and an embedded near-field interferometer called PISTIL (PISton and TILt) able to measure the relative phase shift of each emitter. We show that this measurement can provide information on relative turbulence-induced phase variation of the combined laser beams. In particular, the far-field beam envelope wandering can be estimated through this diagnosis. Results are supported by an analytical model and confirmed by numerical post-analysis of measured far-field interference. This additional interferometer may improve CBC beam pointing through turbulence.