Comparison of macro-tip/tilt and mesoscale position beam-steering transducers for free-space optical communications using a quadrant photodiode sensor

Abstract

The National Aeronautics and Space Administration (NASA) plans to develop optical communication terminals for future spacecraft, especially in support of high data rate science missions and manned exploration of Mars. Future, very long-range missions, such as the Realistic Interstellar Explorer (RISE) 1 , will need optical downlink communications to enable even very low data rates. For all of these applications, very fine pointing and tracking is also required, with accuracies on the order of ± 1 µrad or less and peak-to-peak ranges of ± 10 mrad or more. For these applications, it will also be necessary to implement very compact, lightweight and low-power precision beam-steering technologies. Although current commercial-off-the-shelf devices, such as macro-scale piezo-driven tip/tilt actuators exist, which approach mission requirements, they are too large, heavy, and power consuming for projected spacecraft mass and power budgets. The Johns Hopkins University Applied Physics Laboratory (JHU/APL) has adopted a different approach to beam-steering in collaboration with the National Institute of Standards and Technology (NIST). We are testing and planning to eventually package a highly accurate large dynamic range meso-scale position transducer under development at NIST. In this paper we will describe a generic package design of an optical communications terminal incorporating the NIST prototype beam-steerer. We will also show test results comparing the performance of the NIST prototype meso-scale position beam-steerer to a commercial macro-tip/tilt actuator using a quad-cell tracking sensor.

Design of Cognitive Decision Making Controller for Autonomous Online Adaptive Beam Steering in Free Space Optical Communication System

“…The position errors are measured using OPD and the results are briefed. It is shown that a robust FPGA implementation of the tracking controller will give good beam stabilization [11]. Wen-Fang Xie et al have proposed neural network based adaptive controller for piezoelectric actuators with unknown hysteresis.…”

“…In the experimental setup, the devices are mounted on the vibration damped optical breadboards [11]. A serial pseudo random binary sequence (PRBS) generator circuit is developed in the FPGA and is given as input to the optical modulator at the Asynchronous Transfer Mode (ATM) rate of 155Mbps as explained in [17] for Pulse Position Modulation (PPM).…”

“…Voltage generated by each quadrant is proportional to optical energy illuminating its surface. The outputs of the OPD are connected to the mono-pulse arithmetic circuit (MPAC) where the beam spot spatial displacement errors V Ex and V Ey along the x and y-channel (2D Plane) are measured as the relative output voltage changes [11,14] by…”

“…(8). The reference signal V Ref is measured by the algebraic sum of signals from all the quadrant of OPD [11,14] as in Eq. (9).…”

“…Therefore, designing a control system for beam aligning, tracking and positioning to significantly improve the beam stability on the receiver plane becomes significant. The first and most important approach in FSOC is beam steering which mitigates the beam wandering (temporal distortions) [1,10,11] and is addressed in this paper. The complexities of controller design for such process increases since most of the process are inherently nonlinear thereby making the control of non-linear system non-trivial.…”