1. Technical Field
This disclosure relates to a multiple beam source for a multi-beam interferometer and to a multi-beam interferometer itself, typically but not exclusively for use in Optical Coherence Tomography.
2. Description of the Related Art
Internal medical examinations are typically carried out using an endoscope in which the eye or a CCD (charge coupled device) camera images the view relayed from the distal end of a shaft of a probe, thereby viewing the surface of the tissue adjacent to the probe end. It is often desirable to obtain a cross-sectional image from within the tissue, rather than just the surface. Optical Coherence Tomography (OCT) has been proposed as a technique that can provide such a capability.
OCT is based on the use of interferometry, where light in the measurement arm of an interferometer is passed to the object to be examined and a portion is scattered back to the interferometer. Light in the reference arm is passed to a mirror at a known distance and a reference beam is reflected back. The scattered measurement beam and the reflected reference beam are combined and the interference between the two beams is detected and used to provide data about the examined object.
Thus, optical coherence tomography uses interferometry and the coherence properties of light to obtain depth-resolved images within a scattering medium, providing penetration and resolution which cannot be achieved using confocal microscopy alone. Clinically useful cross-sectional images of the retina and epithelial tissues have been obtained to a depth of 2-3 mm.
At any given time, a single beam of light is restricted to being in sharp focus only over a certain range—the so-called depth of focus or Rayleigh range. In the PCT patent application published as WO2006/054116, it has been proposed to transmit multiple beams with different focal depths so as to spread this range out and to produce depth scans with an increased depth of field.
Where the light for the multiple beams is provided from a common source, as is most convenient, such as a laser, then optical means such as an amplitude beam splitter may be provided to generate a plurality of beams. In WO2006/054116, a “rattle plate” is disclosed, which comprises a parallel-sided glass plate positioned at a slight angle to the perpendicular relative to the incoming light beam.
Referring to the two major faces of the plate as the “entry” face and the “exit” face, the exit face is completely covered with a partially reflective coating, whereas the majority of the entry face, save for the area in which the incoming beam is incident, is covered in a high efficiency reflective coating. Accordingly, the incident beam passes into the rattle plate through the entry face and then alternately bounces off the reflective coverings on the exit and entry faces. Each reflection off the partially reflective covering on the exit face allows a fraction of the light beam to escape, thereby generating discrete beams of gradually reducing power. An opaque plate is provided to block those beams over the number required.