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Atmospheric Turbulence Compensation and Coherent Beam Combining over a 7 km Propagation Path Using a FiberArray System with 21 Sub-apertures Thomas Weyrauch1, Mikhail Vorontsov1,2, Vladimir Ovchinnikov2, Ernst Polnau1, Guimin Wu2, Thomas Ryan2, and Morris Maynard2 1
Intelligent Optics Laboratory, School of Engineering, University of Dayton, 300 College Park, Dayton, OH 45431, USA 2 Optonicus, 711 E. Monument Ave. Ste 101 Dayton, OH 45402, USA
[email protected])
Abstract: Piston phase control based on SPGD maximization of the target-return optical power resulted in compensation of turbulence-induced wavefront distortions in coherent beam combining experiments over a near horizontal path under various turbulence conditions. OCIS codes: (140.3298) Laser beam combining; (010.1080) Active or adaptive optics; (010.1300) Atmospheric propagation
1. Introduction Coherent tiled-aperture fiber collimator array systems are a promising approach for power-scaling of fiber lasers [1]. These systems use a single-frequency laser as master oscillator and a multichannel power amplifier array based on single-mode fiber elements (MOPA system). In directed energy (beam projection) applications considered here, coherent beam combining requires control of the piston phases of the transmitted beamlets at the fiber collimator sub apertures so that the beamlets constructively interfere at a remotely located target. Electronically controllable electro-optic phase shifters are integrated into all MOPA channels for this purpose. In the ideal case, successful phasing of Nsub beamlets leads to an Nsub-fold increase of the combined beam’s peak intensity, if compared with incoherently combined beamlets [2]. Random phase differences between beamlets at the target are the result of phase noise, thermal drifts, and vibrations within the fiber-based MOPA system on one hand and wavefront phase distortions caused by turbulent air in the beam projection system’s atmospheric propagation path on the other hand. The so-called target-in-the-loop phase control attempts to compensate for both fiber-based and atmospheric phase errors. In this paper, we assume a target considerably smaller than the diffraction-limited size of the combined beam at the target plane (unresolved target). In this case, the power of the target-return light collected by an optical receiver can be used as performance metric, JR, which depends on the beamlets’ phase differences at the target. Iterative maximization of JR by the phase controller leads to minimization of phase differences between beamlets at the target. 2. Experimental setup A notional schematic of the experimental setup is shown in Figure 1. The light from an NP Photonics Rock fiber laser with integrated amplifier (wavelength = 1064 nm) is split up into 24 channels (21 used, not all shown in Figure 1) through a 1×3 splitter and three 1×8 splitters, which are integrated with the phase shifter from EOSPACE. The output fibers of the phase shifters are connected to the amplifier modules from IPG Photonics. The fiber-optical outputs of this multi-channel MOPA system are connected to a fiber collimator array comprising 21 subapertures arranged in three clusters on a hexagonal grid. The collimator aperture diameter is 33 mm and the distance to the next neighbor is 37 mm. Each fiber collimator uses a fiber positioner that allows electronically controllable adjustment of the lateral fiber position through bimorph piezoelectric actuators and thus provides beam steering capabilities for each beamlet. This sub-aperture tip/tilt control is used to optimally overlap the beamlets’ footprint at the target distances [3], to compensate residual alignment errors between the fiber collimators, and to mitigate the impact of turbulence-induced wavefront distortions in the transmitted combined beam. The transmitter/receiver system with the MOPA system is located near a window, which gives access to a 7-km nearly horizontal atmospheric propagation path to the target. The target consists of a cat’s eye retro-reflector with a semi-transparent mirror in the focal plane of a lens. The reflected light (about 90%) returns to the transmitter/receiver array and is collected, in part, by the receiver apertures. The retro-reflecting target allows experiments at low (eye safe) power levels. The light that is transmitted by the cat’s eye’s semi-transparent mirror (~10%) passes through an interference bandpass filter and is incident to a photodiode that determines the power of the light received within the receiver aperture (target-plane power-in-the-bucket metric, JT). The cat’s eye target is placed behind a hole in a cardboard screen (hole diameters varied from 14 to 23 mm). A short-wave infrared camera
PW2E.3.pdf
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placed just outside the beam path images cardboard screen and thus records the irradiance distribution around the target.
Figure 1. Notional schematic of the experimental setup.
Control of the tiled beam wavefront during beam projection onto the remote target was based on maximization of the power of the target-return signal, JR, received by the receiver apertures, which are placed next to the transmitter array. Each receiver aperture couples the received light into a multimode fiber. The power of the received light is determined by a photo-sensor setup that collects the light from all receiver fibers. The phase and tip/tilt controllers utilize an iterative stochastic parallel gradient descent (SPGD) algorithm for performance metric optimization [3, 4]. The phase controller operates at iteration rates of up to 250,000 s-1 and implements a “delayed SPGD algorithm” that accounts for optical roundtrip propagation delays, which is necessary because each iteration is shorter than a photon’s travel time from a transmitter to the target and back to a receiver [5, 6].
Figure 2. Sample temporal evolution of the target-plane power-in-the-bucket metric, JT, with the phase controller on (upper curve) and off (lower curve).
3. Sample results Coherent beam combining experiments were performed over a wide range of turbulence conditions. Sample results for the evolution of the measured target-plane power-in-the-bucket metric, JT, are shown in Figure 2 for two control system operation conditions. The upper curve was recorded with the phase locking controller operating, i.e., the controller was maximizing the received metric, JR. The lower curve was obtained while the controller was stopped, which resulted in random phase differences between beamlets. Here phase control improved the average value of JT (denoted as JT) by a factor of 16.3. This is close to the value of 18.5 expected for the improvement in JT under vacuum propagation conditions when perfect phase locking is compared to incoherent combining (note that the
PW2E.3.pdf
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value of 18.5 is derived for 7-km propagation and a target size of 23 mm). The improvement of the average performance metric JR determined by the receiver system was close to the value obtained for JT in all experiments. Examples of averaged irradiance distributions at the target plane obtained by imaging the cardboard screen around the cat’s eye target are shown in Figure 3(a,b). Both distributions were obtained by averaging 250 frames. If phase locking control is off, the average beam footprint is larger than the shown detail with 17.8 cm side length. The combined beam is thus nearly invisible in Figure 3(a).Turning phase locking on results in a much higher irradiance at the cat’s eye target and the surrounding area, as shown in Figure 3(b). The beam width is close to the diffraction limited value of about 6 cm. Figure 3(c) shows cross sections through both irradiance distributions. The irradiance at the actual target – the 23 mm diameter retro-reflector – cannot be determined with the camera.
Figure 3. Target plane irradiance distribution with the phase-locking controller off (a) and on (b) averaged over 250 frames. The cat’s eye retro-reflector is behind the hole visible in the center. The dashed ring has a diameter of 8 cm; the diffraction-limited beam diameter is about 6 cm. Cross sections through the irradiance distributions are shown in (c).
4. References [1] A. Brignon, ed., Coherent Laser Beam Combining (Wiley-VCH, Weinheim, 2013). [2] M. A. Vorontsov and S. L. Lachinova, Laser beam projection with adaptive array of fiber collimators. I. Basic considerations for analysis, J. Opt. Soc. Am. A 25, 1949–1959 (2008). [3] M. A. Vorontsov, T. Weyrauch, L. A. Beresnev, G. W. Carhart, L. Liu, and K. Aschenbach, Adaptive array of phase-locked fiber collimators: Analysis and experimental demonstration, IEEE J. Sel. Top. Quantum Electron. 15, 269–280 (2009). [4] M. A. Vorontsov, G. W. Carhart, and J. C. Ricklin, Adaptive phase-distortion correction based on parallel gradient-descent optimization, Opt. Lett. 22, 907–909 (1997). [5] M. Vorontsov, T. Weyrauch, S. Lachinova, T. Ryan, A. Deck, M. Gatz, V. Paramonov, and G. Carhart, Coherent Beam Combining and Atmospheric Compensation with Adaptive Fiber Array Systems, in Coherent Laser Beam Combining, A. Brignon, ed. (Wiley-VCH, Weinheim, 2013), chap. 6, pp. 167–191. [6] T. Weyrauch, M. A. Vorontsov, G. W. Carhart, L. A. Beresnev, A. P. Rostov, E. E. Polnau, and J. J. Liu, Experimental demonstration of coherent beam combining over a 7 km propagation path, Opt. Lett. 36, 4455–4457 (2011).
