PTB >> Optical Scanning Using Fast Steering Mirrors

Targeting, surveillance, and free space laser-based communication systems for aircraft and spacecraft require systems for line of sight (LOS) stabilization. These LOS systems are required to compensate for the influences of vehicle motion, vibration, and drift so that images are not blurred. Such systems are often required to simultaneously manage both large angle, low frequency, and small angle, high frequency corrections.

Fast steering mirrors (FSMs), broadly defined as a glass or metal mirror mounted to a flexure support system that may be moved independent of the natural frequency of the spring/mass system to direct a laser beam or other light source, can be used as a complimentary LOS correction device to a system that also tilts a large mirror or the entire optical payload. In this event, the accuracy and bandwidth of the larger mechanism can be significantly degraded and refined LOS corrections are relegated to the FSM. Thus, the overall performance can be improved and the cost of bearings, motors, and angle sensors for movement of a large mirror or the entire optical payload can be reduced.

More On FSMs
Opto-mechanical engineers are faced with the challenge of developing a mirror design that compromises neither the mechanical nor optical requirements of a given FSM application. Optical requirements are generally expressed in terms of aperture size and wavefront error. Aperture size is a function of the size of the beam that must be directed plus a reasonable allowance for minor manufacture (such as edge roll-off) and misalignment error stack-ups within the entire product. Wavefront error tolerance relates to degradation of the beam due to imperfections in the surface flatness of the mirror under both static and dynamic conditions.

Although it is tempting to make the mirror oversized in thickness and aperture in order to eliminate concerns about meeting the optical requirements, the addition of mass and moments of inertia run counter to the goals for the actuator design and the associated control system. In such cases, a sufficient design for the mirror can be determined by finite element analysis (FEA).

Glass or metal mirrors can be used in the FSM design. Glass mirrors are inexpensive and can be very high quality but lack the design versatility of metal mirrors. In many cases mirror materials with an exceptional stiffness/mass ratio, such as beryllium or silicon carbide, can be used. These materials have a specific stiffness (Young’s modulus/density) that is four to six times greater than common optical glass materials, such as BK7 and fused silica, and common structural metals, like aluminum, magnesium, steel, and titanium.

In addition to the type of mirror used, its mounting to the flexure support system is extremely critical. Use of a glass mirror typically requires applying an adhesive to the back and/or sides of the mirror for attachment to a metal structure that is then attached to the flexure and actuator system. Dissimilar coefficients of expansion among mirror and support materials can result in mirror figure distortion upon changes in temperature. Also, the use of adhesives often results in dimensional creep and reduction of the natural frequency of the suspended mirror system.

On the other hand, metal mirrors are readily designed for direct fastening (no adhesives) to the flexure and actuator system. The metal mirror may also be designed with integrally machined flexures, light-weighted to increase the structural efficiency and other features not attainable with glass mirrors.

Control Systems
FSMs are typically used in the context of a closed-loop feedback servo control system. A sensor or sensing system identifies errors in the trajectory of the light beam and reports the error back to the control system. That, in turn, commands the actuators on the FSM until the error is reduced to tolerable limits. The repeatability of the flexure is often exploited and a profile of the elastic response can be stored and replayed in open-loop mode to reduce the dynamic range of closedloop error correction.

Flexure Systems
The design of the flexure system, which sustains the means of support for the mirror and allocates compliance and constraint of motion among the six degrees of freedom, is critical to virtual pivot placement and toward achieving smooth, repeatable, and uncoupled motion. Innovative design can result in low-power consumption and an expanded range of motion while retaining high rigidity in the non-compliant degrees of freedom.

Figure 1: Diagram of flexure-mounted mirrors.

Figure 1 illustrates three different types of flexure-mounted mirrors. Figure 1a shows a flexure-mounted mirror used as a single-axis hinge in order to allow tilt in only one direction. Two such devices can be used in tandem to provide two orthogonal axes of tilt in a scanning or beam stabilization mode. If the beam is collimated, the resultant translation of the mirror in addition to tilt is not a significant issue.

Two flexures are used in Figure 1b inorder to only allow translation along the axis of the mirror. This type of design can be used to rapidly modulate an interferometer cavity for FFT waveform analysis. The mirror depicted in Figure 1c is mounted to a two-axis flexure system with a center pivot flexure that allows only two axes of tilt adjustment. This design is ideal for a number of beam stabilization and scanning applications, including free space laser telecom systems.

Range & Type of Motion
The required range and type of motion is a critical consideration in the design of flexure-mounted mirrors. Generally, flexures can only be used over a limited range of motion compared to linear or rotary bearings. Reducing flexure spring rate and employing innovative actuator interfaces can, however, extend range of motion. An additional consideration is the selection of the actuator type and how it attaches to the moving and fixed portions of the support structure. The relationship of the actuator(s) to the virtual pivot of the flexure system is vital to achieving optimal actuator response as well as optimizing the range and fluidity of motion.

Spring rate in both the compliant and non-compliant directions must be considered simultaneously. An ideal FSM flexure system would have the following attributes:

• Very low stiffness to resist the desired directions of motion
• Very high stiffness in the constrained directions of motion
• Controlled location of virtual pivot of flexure system
• Near-perfect elastic response in the desired directions of motion
• Low mechanical hysteresis
• Very high resistance to metal creep and fatigue
• Compatible coefficient of thermal expansion with the mirror
and its mount

In general, attempts are made to conserve energy and reduce actuator force requirements in minimizing the spring rate of the flexure in the compliant directions. This minimizes the holding force that the actuator must express to maintain the mirror at a commanded position against the resistance of the flexure. Conversely, it is desirable to maintain very high spring rates for constrained degrees of freedom. These principles can be seen in the three flexure-supported mirror diagrams shown in Figure 1. For each case, the mirror will move readily in the desired directions but is resistant to motion in the constrained directions. The appropriate balance of spring rate and range of motion for all of the six degrees of freedom must be accommodated in the flexure design
process.

An additional consideration is the virtual pivot of the flexure design. All flexure designs have a virtual pivot point, axis, or surface. In many cases it is desirable to locate the virtual pivot at the plane of the mirror surface.

Actuator Choices
The type, performance characteristics, and number of actuators used in an FSM design is highly application specific. Voice coil and piezoelectric (PZT) type actuators are commonly usedto operate flexure-supported
FSMs.

By modulating the amplitude, frequency, and direction of the current flowing through the coil, a precisely metered push or pull effect can be realized. The coil and magnet must be guided by bearings in order to maintain alignment over the range of travel. Voice coil actuators have the advantages of large travel excursion, moderate frequency response, and the potential for finely metered increments of motion. Voice coils can also be designed so that the coil and magnet are curved about a virtual pivot point or axis. In this manner, expanded ranges of tilt can be achieved.

PZTs typically consist of laminated stacks of piezoelectric material encased in a steel cylinder. By application of a modulated high-voltage signal to the PZT, small increments of motion result. When compared to voice coils, PZT actuators can produce tremendous force in a smaller package at much greater frequency response. However, PZTs suffer from very limited range of travel, hysteresis, and they must be mechanically preloaded in compression to prevent delamination and to provide a restorative spring force. A combination of high frequency, high load, and small tilts or translations favors the use of a PZT to actuate the FSM. Large ranges of excursion and low load favor the voice coil actuator.

Sample Applications
Polygon Cross-Scan Error Correction:
The following application describes a single-axis FSM (see Figure 2) used to correct for cross-scan error introduced by a rotating polygon in the context of a flat field photographic print engine. This product is used to expose computer-to-plate materials employed in offset printing. The FSM is required to compensate for tilt errors on the order of 0.10 arc-second at rates of 20 kHz and small errors in the flatness of the polygon facets. Minute twists in one facet relative to the next facet result in beam trajectory errors and undesirable “banding” artifacts in the photographic copy.

Figure 2: A single axle fast steering mirror
mounted in laser conditioning/focusing objective.

In this case the small angular range of motion and extreme stiffness required suggest the use of a small single-axis flexure- mounted mirror. By using a diamond- machined metal mirror, it is possible to integrally machine the flexures into the mirror substrate. The small range of motion and high frequency response favors the selection of a PZT-type linear actuator.

The selection of a metal mirror substrate allows the mirror to be hard mounted directly to the top of the PZT. Peak accelerations exceed 200 g’s. The rotating polygon is mounted on an extremely accurate self-acting air bearing motor system. The final result is a highly responsive, compact, and reliable FSM module with a repeatable tilting motion about a virtual hinge axis defined by two integrally machined pivot flexures.


Free Space Laser Telecom:
Free space laser telecom systems entail propagation of modulated laser signals across open atmosphere or the vacuum of space rather than through fiber optic cable. In effect, identical bandwidth compared to fiber cable transmission, at the same laser wavelength, can be achieved.

Demanding requirements for acquisition scanning and signal lock in such systems have turned renewed attention to flexure-suspended FSMs. The requirements for low cost and moderate angles of excursion suggest the use of a two-axis flexure-supported FSM driven by voice coil actuators. Figure 3 shows a two-axis tilt FSM with the mirror constructed of diamond machined metal with integral hard fastening. A schematic of a free laser telecom link is shown in Figure 4. In this illustration, a common-path laser emitter and receiver are positioned opposite each other, often at large separation distances. The FSM within each module compensates for any environmental disturbance that results in a shift in the optimal coalignment of the two systems.

Since the dynamic response characteristics are not particularly demanding, a sufficient design can be developed without the use of exotic mirror materials, light-weighting of the mirror, or ultrahigh performance actuators. Systematic engineering analysis results in a spring suspension system that remains exceptionally stiff in three axes of translation and one axis of rotation. The remaining two degrees of freedom, reserved for tilt of the mirror surface, are designed to have a very low spring rate and are essentially uncoupled from one another.

Figure 4: Schematic diagram of a free laser telecom link.

The virtual pivot of rotation is maintained as close to the center of the mirror aperture as possible. Two pairs of voice coils, operating in a push/pull manner for each tilt axis, act directly upon the mirror mount and are suspended with the mirror on the flexure system. The voice coil magnets are fixed mounted in the base of the FSM.

By maintaining very low, yet highly elastic spring rate in the desired compliant directions, the energy required to hold the mirror at a given location for extended periods of time is not excessive. By balancing the mirror, its mount, and the actuator coils such that the center-of-gravity of the mirror coincides with the virtual pivot of tilt, the FSM may be mounted in universal orientations without significant change in mirror angle relative to reference datums.

This article was written by Michael Sweeney, Gerald Rynkowski, Mehrdad Ketabchi, and Robert Crowley of Axsys Technologies Imaging Systems, located in Rochester Hills, Michigan. For questions regarding this article, please contact George Murray at gmurray@axsys.com. Visit Axsys Technologies online at www.axsys.com.