Method and apparatus for employing a light source and heterodyne interferometer for obtaining information representing the microstructure of a medium at various depths therein

An apparatus for obtaining tomographic information, comprising:

• i) a light source for producing a low coherence light beam,
• ii) an optical system for splitting said low coherence light beam, which has been produced by said light source, into a first light beam and a second light beam, which travel along first and second optical paths, and combining said first light beam and said second light beam with each other at a position at which an optical path difference between said first optical path and said second optical path is larger than a coherence length of said low coherence light beam, to produce a combined light beam, said first and second optical paths being different paths each beginning at a position at which said low coherence light beam is split into said first and second light beams and ending at a position at which said first and second light beams are combined,
• iii) a frequency shifter for shifting an original frequency of said first light beam to a second frequency, which is slightly different from the original frequency of said first light beam,
• iv) light guide means for guiding the combined light beam to a position in a vicinity of a medium, which has light scattering properties and tomographic information of which is to be obtained,
• v) an optical member for splitting said combined light beam, which has been guided to the position in the vicinity of the medium, into a third light beam, which travels reversely to a direction of travel of said combined light beam, and a fourth light beam, which continues to travel along the direction of travel of said combined light beam and is thus irradiated onto the medium, said optical member thereafter combining a backward scattered light beam, which comes from the medium when said fourth light beam is irradiated onto the medium, and said third light beam with each other,
• vi) a photodetector for detecting an intensity of an interference light beam obtained from interference of:
• a) one of a component of said first light beam and a component of said second light beam constituting said third light beam and which has passed through a longer of said first and second optical paths, and
• b) one of a second component of said first light beam and a second component of said second light beam constituting said backward scattered light beam and which has passed through a shorter of said first and second optical paths,
• vii) an optical heterodyne detection means for detecting an intensity of said backward scattered light beam in accordance with the intensity of said interference light beam, which has been detected by said photodetector, to obtain information representing a microstructure of the medium at a predetermined depth in the medium, and
• viii) an optical path length modulating means disposed in either one of the first and second optical paths for modulating a length of the one of the first and second optical paths in which said optical path length modulating means is disposed to obtain information representing the microstructure at an arbitrary depth in the medium.