Patent ID: 7848662

Claim:
A delay interferometer comprising: a half mirror configured to split an optical signal into first and second split beams of light which travel on first and second optical paths, respectively; a first reflector disposed on the first optical path, the first reflector being configured to reflect the first split beam of light toward the half mirror; a second reflector disposed on the second optical path, the second reflector being configured to reflect the second split beam of light toward the half mirror; and at least one phase compensator disposed between the half mirror and at least one of the first and second reflectors, the at least one phase compensator comprising a medium that exhibits thermooptic effect and has temperature dependency of refractive index, wherein the half mirror is configured to couple the first and second split beams of light which have traveled from the first and second reflectors, respectively, so as to generate at least first and second coupled beams of light, and wherein the medium has a thickness t that is given by: 0={ dn 0 /dT ( L+t )}+{1/ L dL/dT L}+{ 1/ t dt/dT (1− n ) t}−{dn/dT t}=L{dn 0/ dT+α 1 }+t{dn 0 /dT+α 2 (1− n )− dn/dT} (n≈1) where n 0 is the refractive index of air, n is the refractive index of the medium, dn 0 /dT is the temperature coefficient of refractive index of air, dn/dT is the temperature coefficient of refractive index of the medium, L is the mechanical path length difference between the first and second optical paths, t is the thickness of the medium, T is the temperature, α 1 is the coefficient of linear thermal expansion of the container, and α 2 is the coefficient of linear thermal expansion of the medium.