Integrated multi-dithering controller for adaptive optics

Effective compensation of phase noise in laser communication calls for fast, real-time, adaptive wavefront control. We present an analog, continuous-time, high-speed VLSI (Very Large Scale Integration) controller implementing multi-dithering perturbative gradient descent optimization of a direct measure,of optical performance. The system,applies parallel sinusoidal perturbations to the wavefront over a range of frequencies, and performs parallel synchronous detection of the metric signal to derive the gradient components over each frequency band. The system operates over a wide range of frequencies, supporting applications of model-free adaptive optics extending from compensation,of slow atmospheric turbulence to compensation,of fast random,phase fluctuations in the actuators and laser amplifiers. The system has been teste d as a phase controller for a multiple laser beam wavefront,propagating through a highly turbulent medium.,The results indicate a compensation,bandwidth,exceeding,300 kHz matching,the turbulence bandwidth. I. I NTRODUCTION Optical communications between a single source, N -transducer transmitting array and a single receiver (Fig. 1) can be significantly impaired due to phase noise introduced in the s ystem. Typical examples,of sources that alter the phase of each channel and lead to non-coherent detection of the combined signal at the receiver are atmospheric turbulence [1]-[4], p hase noise of the laser source itself, as well as phase noise intro duced by the fiber amplifiers [5]. These sources can be charact erized by a highly non-linear and time-varying behavior, featuring bandwidths that can exceed a few MHz. The need for high-speed adaptive controllers for phase correction is therefore app arent, and significant efforts in the optical engineering co mmunity have been directed towards increasing bandwidth and precision of adaptive optics control systems, e.g. [6], [7]. Analog Very Large Scale Integrated (VLSI) circuits have received a lot of attention as a candidate means,of implementing adaptive controllers due to their advantages compared to software and discrete-component solutions: low-power consumption, compact size, faster computation and control of multiple channels in a smaller area. Effective compensation of scintil lation caused by atmospheric turbulence has been demonstrated [7], [8] by using analog VLSI chips implementing the stochastic parallel gradient descent (SPGD) optimization technique for bandwidths,up to several hundreds of Hz. A challenging task has been to opt the adaptation speeds of such controllers, by possibly implementing different algorithms in VLSI, in order to be able to compensate,higher bandwidth,sources of phase distortion. A proposed alternative to the SPGD algorithm has been sinusoidal multi-dithering, so far demonstrated using only discr ete- component hardware [9], [10]. The distinctive difference between the two methods lies in the choice of the dithers superimposed to the controlling signals of the phases; for SPGD, a small-amplitude binary random sequence is applied to each control variable, whereas in sinusoidal multi-dithering the dithers are sinu soidal signals of distinct frequencies, different for each variable. Due