Phase-continuous and frequency-variable light source

An external resonator type phase-continuous and frequency-variable light source having an external resonator which includes a first beam splitter, a reflecting mirror, and a diffraction grating, wherein an output beam from an end face with an antireflection film of a laser diode of the light source is split by the first beam splitter so as to input the beam to the reflection mirror, and a reflected beam from the reflection mirror is transmitted through the first beam splitter so as to input the reflected beam to the diffraction grating, the phase continuous and frequency variable light source comprising:

• a second beam splitter for splitting a forwards-directed beam from the laser diode into an outputted beam and a reflected beam;
• a first beam detecting circuit for detecting an optical input of one of the beams from the second beam splitter and transforming the detected optical input into an electric output;
• a second beam detecting circuit for detecting an optical input of a transmitted beam from the first beam splitter and transforming the detected optical input into an electric output;
• and
• a control circuit for controlling a phase condition of the light source by sliding the reflecting mirror in parallel with a longitudinal direction of the external resonator so as to change a length of the external resonator in a manner such that a ratio of the electric outputs from the first and second beam detecting circuits is fixed to a predetermined value.