Unified Optical Scanning Technology

Chapter 4.5.4 - Scanner Devices and Techniques: The Fast-Steering Mirror

4.5.4 The Fast-Steering Mirror

A relatively new availability of low-inertia vibrational scanner is the two-axis fast-steering mirror [New,Ball,Ber,Swe].The typical configuration is that of a single flat mirror mounted through a flexure suspension nominally parallel to a flat base, so that the mirror may pivot about a central point near its surface. Four electrodynamic voice-coil type actuators are mounted in perpendicular axis pairs between the edges of the mirror and the base. The actuator pairs are driven in push-pull to nutate the mirror about its quadrature axes, against the restoring force of the flexure suspension, rendering two-axis angular scan.

Commercially available devices exhibit interesting characteristics. Although the scan angle is currently relatively low (to ±3° mechanical), mirror sizes can be substantive (25-mm round to 147-mm square). They are available in materials such as aluminum, beryllium, or glass. As in the closed-loop galvanometer, mirror position sensing with controlled feedback is incorporated to normalize the bandpass and to provide high positional accuracy. Typical characteristics (see, e.g., [Ball] Mdl. 3B) are: beryllium mirror size, 70-mm square; mechanical scan angle, ±1.5° (6° total, optical); resolution finesse, 1-μrad rms; bandwidth, 1000 Hz (3db); acceleration, 1000 rad/s2; xy cross-coupling <0.1%. The overall mirror assembly size is 5 in. by 5 in. by 2½ in. thick, and its mass is 485 g (1.07 lb). Another typical device (see, e.g., [Swe] two-axis FSM) has mirror size, 35-mm round; mechanical scan angle, ±3° (12° total, optical); bandwidth to 1000 Hz; control system, type 2 position servo.

Complementing the limited angular deflection is their significant mirror size. The first sample unit achieves the substantive scanned resolution (Equation 3-5) of 7000 elements/scan in the visible-near-IR wavelength region, assuming for simplicity a = 1 and λ = 1 μm, Θ = 0.1 radian, and D = 70mm uniformly illuminated. The round mirror of the second unit calls for a = 1.25 (Table 3.1). Although half the width, it executes twice the scan angle, yielding the substantive resolution of 5600 elements per scan at the same wavelength. Considering their utility and compactness, this technology might be viewed as a contender in the pursuit of 'agile beam steering," which is achieved by novel substantively nonmechanical means. Agile beam steering is developed and reviewed comprehensively in Section 4.10.

 

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