Athermal interferometer

An interferometer comprising:

• a first port for launching an input beam of light;
• a beamsplitter comprising first and second transparent blocks with a partially reflective surface therebetween, for splitting the input beam of light into first and second sub-beams, and for directing the first and second sub-beams along first and second optical paths through the first and second transparent blocks, respectively, said first and second sub-beams traveling through an unequal amount of solid transparent block causing an optical path mismatch;
• a first reflective surface positioned in the first optical path at a desired distance from the separation point defining a first optical path length, the first reflective surface for redirecting at least a portion of the first sub-beam for interference with the second sub-beam;
• spacer means defining a gap between the first transparent block and the first reflective surface;
• tuning plate means positioned in the first optical path within the gap;
• a second reflective surface positioned in the second optical path at a desired distance from the separation point defining a second optical path length, the second reflective surface for redirecting at least a portion of the second sub-beam for interference with the first sub-beam forming first and second output beams;
• fixed plate means positioned in the second optical path between the second transparent block and the second reflective surface;
• a second port for outputting the first output beam;
• and
• a third port for outputting the second output beam;
• wherein the tuning plate means is oriented so that the fixed plate means and the tuning plate means compensate for the optical path mismatch;
• and
• wherein thermal expansion of the spacer means compensates for thermal expansion of the tuning plate means, which along with thermal expansion of the fixed plate means ensure that a desired optical path length difference between the first and second optical path lengths does not change over a practical range of temperatures.