Patent ID: 8488433

Claim:
A modulated signal detecting apparatus for detecting a modulated signal, comprising: a wavelength-variable laser which emits laser light and which is capable of varying oscillation wavelengths; an optical divider which splits the laser light emitted from the wavelength-variable laser into signal light and reference light; a reference light mirror which is arranged on an optical path of the reference light; an interference unit which causes an interference between the signal light that has been modulated and the reference light that has been reflected by the reference light mirror; an interference light detecting unit which detects interference light created by the interference unit and which outputs an interference light detection signal; and a wavelength control signal generating unit which generates an oscillation wavelength control signal for controlling an oscillation wavelength of the wavelength-variable laser based on the interference light detection signal, wherein when ld 0 denotes an initial optical path length that is a length of an optical path, along which the signal light travels, as converted using a refractive index in vacuum, lm 0 denotes an initial optical path length that is a length of an optical path, along which the reference light travels, as converted using a refractive index in vacuum, Δl denotes a fluctuation width of a difference in optical path lengths between the signal light and the reference light as converted using a refractive index in vacuum, λ 0 denotes a central oscillation wavelength of the wavelength-variable laser, and Δλ denotes an oscillation wavelength variable range of the wavelength-variable laser, then an initial optical path difference |ld 0 −lm 0 | satisfies Expression (1) below:  ld 0 - l ⁢ ⁢ m 0  > Δ ⁢ ⁢ l Δλ ⁢ λ o . ( 1 )