Mark A. Ealey, MEMBER SPIE
Litton/Itek Optical Systems
10 Maguire Road
Lexington, Massachusetts 021 73-31 99
Technically, the sensing of phase errors and the use of this feedback signal by a closed loop servo control system that nullifies those errors is the distinguishing factor between adaptive and active optic systems. For the purposes of this special section, the term "active optics" has more to do with the phase conjugate of Gaskill's Law, which is a Fourier integral that describes the ratio of abstracts received to manuscripts accepted for publication.
This paper discusses the application of active structures technology to the control of precision structures for future space-based astrophysics observatories. The state of the art in active structures is reviewed, and technology developments applicable to large optical systems are discussed.
The design and performance of an active optic figure control system for a spaceborne relay mirror application is described. Development centered on minimizing overall system weight while meeting a prescribed set of figure control requirements. The optic, an Ultra Low Expansion ULE, lightweight, 17 in. hexagonal mirror, uses a combination of moment and force actuators to control the optical figure. Modeling of the mirror to determine the placement of the actuators that minimizes the rms aberrations at low spatial frequencies is discussed. It is shown that the actuator configuration chosen is capable of significantly correcting all primary Zernike terms up to trefoil (i.e., focus, tilt, power, coma, astigmatism, and trefoil) with the exception of spherical. Preliminary "manual" closed loop operation of the system has demonstrated that the static optical figure of 2.5 waves peak-to-valley and 0.4 wave rms can be corrected to 0.5 wave peak-to-valley and 0.1 wave rms.
An active structural element for use in precision control of large space structures is described. The active member is intended to replace a passive strut in a truss-like structure. It incorporates an eddy current displacement sensor and an actuator that is either piezoelectric (PZT) or electrostrictive (PMN). The design of the device is summarized. Performance
of separate PZT and PMN actuators is compared for several properties relevant to submicrometer control of precision structures.
The structural aspects of developing a flat laser radar beam steering mirror for optical discrimination of an array of spaceborne targets at 10.6 p.m are discussed. Results of trade studies to determine feasible design concepts capable of meeting the system performance goals during steering are included. Aperture diameters between 0.5 and 2 m were studied for three principal structures: (1) a thin glass sandwich coupled to adeep backup structure, (2) a full-depth beryllium sandwich, and (3) a full depth
graphite epoxy sandwich. The results of computer simulations of the structural performance of each of these concepts are presented, and fabrication efforts in support of these concepts are described. These studies demonstrate that lightweight, dimensionally stable flat mirrors can be fabricated with present technology. Aluminum-clad graphite/epoxy mirrors were successfully manufactured within 0.5 pm rms of a flat surface, and initial test results indicate dimensional stability after several thermal cycles. A low cost, uncomplicated method to manufacture low thermal expansion aluminum-clad graphite/epoxy tubes as backup structural members for large space-based optics was also developed. Further investigations are warranted to determine the dynamic tensile strength of
glass, to develop thermal distortion compensation methods for metal mirrors, and to improve the dimensional and temporal stability of clad graphite/
The profusion of lasers and related sensors has created new demands for line-of-sight stability in precision pointing devices. Meeting these needs for space-based devices on platforms that vibrate because of moving parts of satellites and structural bending modes presents a problem that needs a technically creative solution. Operational scenarios indude
earth observation, surveillance, laser communication, and high-energy lasers. These applications often require the best optical quality available. The difficulty of this design issue is compounded by the friction-induced errors of beam-steering components that have bearings. There is a series of fine-steering mirrors now available that meets these needs in space, aircraft, and land vehicles, as well as stationary environments. These mirrors are based on reactionless concepts that cancel steering forces and
torques at the point where they are generated. Also important is the use of highly linear actuators, sensors, and suspensions. Suspensions are made to avoid bending the mirror, which is designed to be stiff and light for achieving high bandwidth. Technological progress has been made in developing high-bandwidth fine-steering mirrors by solving critical design issues associated with wavefront quality, positioning accuracy, and quick and fast operation. These issues affect the design of the suspension, actuators, mirror, sensors, and reaction cancellation components. This paper explains some of the techniques we use to achieve 1/20 wave surface
accuracy over a 7 in. mirror surface, positioning accuracy of 0.02% of the travel range, 5000 rad/s2 acceleration, and control bandwidths up to 4000 Hz. Although individual mirror designs cannot meet all of these performance parameters simultaneously, any of these can be met and all of these can be approached in various combinations. Tests on these mirrors, when operating in a tracking configuration with a low-noise optical sensor, show that they can achieve line-of-sight stability as low as 10 nrad.
The high bandwidth steering mirror (HBSM) prototype is the product of a research program established to develop a high-bandwidth, low angular range, two-dimensional beam steerer frequently found in optomechanical pointing, acquisition, and tracking systems. This research centered around the optimization of a beam-steering mechanism composed of mirror, restoring flexure, actuators, position sensors, and encompassing housing. Various design trade-offs and manufacturing issues involved in building the prototype are discussed, and the performance data are presented. The resulting HBSM design allows integration with a simple closed-loop control scheme. The mirror/controller has a closedloop bandwidth of 10 kHz and 10 peak-to-peak stroke (mirror normal) at low frequencies. This increased bandwidth yields excellent disturbance rejection in the 10 to 1000 Hz frequency band and enables the generation of faster scan patterns.
The 1 0 m diameter Keck telescope consists of 36 1 .8 m off-axis hyperbolic hexagonal mirror segments (glass ceramic substrates). To produce the aspheric segments in a timely fashion, shears and moments are introduced at the segment periphery, in its circular shape, to bend the mirror into the reverse of the desired shape. A true sphere is polished into the segment, after which the loads are removed and the desired optical prescription obtained. The segment is cut to the hexagonal shape and a central hole is core-drilled partially through its back. The process involves theoretical prediction of the loads required to bend the surface using a detailed finite element model. Very good agreement is found between test and theory such that the deformed surface can be achieved after as little as one iteration, accurate to the 0.1 p.m level. After the segments are cut, their shapes will warp slightly due to residual stress levels measured in the blanks. Using the same model, a first-order analysis of the springing effect is presented, and further correlation to test data is made. Results allow anticipation of the presumed spring prior to polish of the segment, minimizing the range of adjustment and force application for the corrective
devices used to warp the segments to their final shape.
For the past 1 5 years, Litton/ltek Optical Systems has fabricated precision ferroelectric actuators for use in active and adaptive optical components. With the advent of lead magnesium niobate (PMN), such devices attained revolutionary performance improvements. A thorough understanding of the electrostrictive nature of the material has been developed and used to optimize the material properties. Finite element models have been developed to maximize transducer response and establish safe operating bounds. An overview of the multilayer actuator fabrication process is provided, with emphasis on in-process controls and qualification blocks. A patented Standardized Electrodisplacive Transducer (SELECT) method of manufacture has been developed to improve manufacturing yields, sensitivity uniformity, and low voltage response. Several SELECT configurations have been established for a variety of positioning applications. A summary of the SELECT actuator configurations is given for numerous applications, supported by experimental data.
The function of a control system is to convert an operator cornmand into an electrical signal, to convert the electrical signal into a mechanical response, and to verify that the mechanical response achieves and maintains the desired level. A vital element in this interface is the electromechanical actuator. This paper examines the requirements of control actuators and illustrates the unique capabilities offerroelectric devices to meet these requirements. When substantial energy is required from an actuator, stacks of ferroelectric disks are shown to be suitable. They develop high force, respond extremely fast, can be proportionately charged, have almost infinite resolution, require no holding current to maintain displacement, and are neither influenced by nor produce magnetic fields. They have no mechanically wearing components and are not prone to corrosion. Reliability to 1O cycles has been established. Problems are discussed; hysteresis is shown to represent a difficulty in control systems.
A method of treatment is discussed whereby the disk stack is connected in binary groups with separate positive connection to each of the groups. A common constant voltage power supply is switched to selected groups so that only zero volts or maximum volts are applied. In this manner hysteresis is eliminated, the power supply is simplified, and a repeatable, almost linear response is obtained. An alternative approach using a hybrid stack composed of two soft ferroelectric and one hard ferroelectric segments is shown to be capable of linearity and repeatability within 1%. Some problems remaining with these approaches are discussed. A very fast, precise beam steering mirror system is described that employs a split stack assembly. Its performance is described. Fundamental frequency limitations are shown to exist in stacked disk actuators. For example, an 18,000 Hz limit applies to a 25 mm long actuator. Brief reference is made to future developments and to the benefits that may be expected from them.
There is a growing development activity around electromechanically active components and their applications. Piezoelectric ceramics, the first of these, are being supplanted by electrostrictive materials where high strain, high stress, and low hysteresis are required. This paper summarizes the key parameters and highlights some of the areas of application in optics. The intention is to provide the optical engineer with first-order device and application information. For the first time, mechanical control on the order of wavelengths is possible. Some nonoptical uses are addressed in order to suggest possible analogies to the optics field.
Agile steering of a helium-neon laser beam (X = 632.8 nm) has been demonstrated using a complementary pair of 5-cm-aperture binary optic microlens arrays in the Galilean telescope geometry. Segmentation of a collimated input beam by illuminating approximately 60,000 /5 microlenses of 200-jim diameter and parabolic phase profile results in nearly aberration free beam steering over an 1 1° field of view for 100-
pm lateral displacements of one array relative to the other. Wavefront quality and steering efficiency of the deflected beam has been measured as a function of steering angle and is compared to a simple theoretical
Advances in spatial light modulator (SLM) technology have resuited in an active search for signal processing algorithms for use with SLMs in incoherent electro-optical systems. This paper presents an effective means of generating tracking algorithms for locating small targets with a single detector using FM reticle theory. A simple "barrel transformation"
of a spinning FM reticle pattern to an SLM is shown to allow tracking systems to be constructed with solid state devices and avoids the need for moving parts. Also, SLMs allow the FM patterns to change over time, allowing adaptive interrogation of targets in a particular region of a system's field of view.
The wavefront error across a laser aperture is often the limiting factor in laser resonator design and performance. For long-pulse, sidepumped lasers, such as some nuclear-reactor-pumped or flashlamp-pumped lasers, significant transverse index gradients can develop and couple to the laser resonator modes. To design a laser resonator for efficient power
extraction, a spatially and temporally resolved determination of the index variation is required. High-speed photography using a fastframing camera with a modification of the Hartmann technique is applied to measure the index field and the resulting wavefront error in the gain region of a pulsed,
nuclear-reactor-pumped laser. With moderate initial gas pressures, the index fields and the wavefront errors are found to have roughly parabolic shapes except near the side walls. These results are in reasonable agreement with the predictions of a gasdynamic model, which describes the gas motion induced by the spatial nonuniformity of the pumping.
A new technique for fully automatic diffraction fringe measurement in pointwise analysis of speckle photographs is presented. The fringe orientation and spacing are initially estimated with the help of a 1 D fast Fourier transform (FFT). A 2-D convolution filter is then applied to enhance the estimated image. A fringe pattern with high signal-to-noise ratio is achieved, which makes feasible precise determination of the displacement components. The halo effect is also optimally eliminated in a new way by a halo-division technique. High reliability and accurate determination of displacement components are achieved over a wide range of fringe densities,
with the computation time comparing favorably with those for the 2-D autocorrelation method and the iterative 2-D FFT method.
A new kind of amplitude-encoded phase-only filter (AE POF) is presented. The interference of the Fourier phase value with a tilted plane reference wave encodes the pure Fourier phase information of the object wave and results in an off-axis phase-only filter (OA POF). After the binarization step the filter function is recorded directly into the recording medium in final size using a laser scanner. Comparing them with the conventional matched filter, the conventional phase-only filter, and the AE
POF, as well as their binary versions, we discuss the performances of OA POFs (OA BPOFs), present computer simulations, and demonstrate experimental results.