In this paper Microrobotic laser steering for minimally invasive surgery, a new mirror design is proposed for laser steering application.
Integrated with present surgical tools, this device is 6 millimeters in diameter and 16 millimeters in length and is able to focus and steer a fiber-based laser beam at high speed (1.2 kilohertz bandwidth) over a range of more than ±10 degrees in both of two axes.
The author mentioned that a three-mirror design can actually be made smaller than a two-mirror design, if the same range of motion is desired for each.
Also, for scanning applications with forward looking, three-mirror design is recommended. On the other hand, a two-mirror design would be perfectly acceptable in terms of miniaturization and range of motion.
Because the distance that the laser travels through the three-mirror design changes very little as the mirror angles change, the focal plane is nearly symmetric about the neutral configuration. This is not the case for the two-mirror designs; the distance that the laser travels between the mirrors changes drastically with changing mirror angles. This yields the deformed focal plane that is contracted for large negative values of the first-mirror rotation angle.