Multimodal Biomedical Imaging Systems - Biomedical Optical Imaging Technologies: Design and Applications - page 330

9.3.4.2 Scanning Systems

An efficient beam-scanning system with scan mirrors should have the following
characteristics: good duty cycle, stationary pivot point, high scan speed, and the
capability of scan rotation. A good duty cycle requires that all nonuseful scanning
be minimized. A stationary pivot point is required, and it should be at the entrance
pupil or imaged at the entrance pupil plane of the objective lens so that the
beam at the entrance pupil remains constantly in position and purely rotates,
achieving the smallest focused spot it can achieve and maintaining a constant
illumination. When the beam scans at the pupil plane, the objective lens works in
a telecentric mode and converts the change of the scan angle at the pupil plane to
the movement of the focused spot in the object plane. A high scanning speed is
another essential requirement for OCT imaging, especially for in vivo applications.
It is desired that the scanning direction be rotatable to obtain the best possible
image quality and time resolution with respect to the specimen shape or other
characteristics [ 34 ].
A typical scanning system utilizes two high-speed scan mirrors driven by
galvanometer motors or a combination of a galvanometer scanner and an acousto-
optical deflector or a polygon scanner. The two scanners mounted on mutually
perpendicular axes are either close-coupled or optical-coupled through a relay
system and scan in orthogonal directions, producing a rectilinear raster scan. In
a close-coupled scanning system, the two scan mirrors are positioned as closely
as possible, with the entrance pupil of the objective lens or its conjugate plane
located at the midpoint between them. This configuration is relatively simple and
compact. However, this arrangement only achieves approximate telecentricity, the
closer the two mirrors, the smaller the telecentric error. Another problem with close-
coupled mirrors is the beam shift on the second mirror. When the mirrors scan,
the beam moves around the pupil plane, instead of being stationary. Therefore, the
intensity of the image may vary across the field; the resolution and contrast may
be reduced because it is possible that only part of the pupil is effectively used for
illumination and detection during scanning [ 34 ]. In an optical-coupled scanning
system, a relay system is employed to image one mirror onto the other so that
both mirrors can be exactly at the conjugate planes of the entrance pupil of the
objective lens. The beam position is always fixed at both scanning planes because
of the conjugate condition; therefore, there is no beam movement at the entrance
pupil during the scanning process. This configuration meets all of the requirements
of a good scanning system: good duty cycle, stationary pivot point, high speed, and
rotatable scanning direction.
The microelectromechanical system (MEMS) scanner is a device that has
emerged in recent years to provide the beam scan in OCT imaging [ 36 , 37 ].
A MEMS scanner is a silicon device where the mirror is connected to small flexures,
allowing it to oscillate. One-dimensional and two-dimensional MEMS mirrors have
been developed and used in OCT imaging. The MEMS scanner offers a number of
advantages: small size, fast speed, high accuracy, and repeatability. However, the