Tunable filter

A variable wavelength filter, comprising:

• an input optical fiber;
• a diffraction grating that disperses input light from the input optical fiber;
• a variable mirror that has a reflective surface, wherein
• an angle of the reflective surface is adjustable,
• the variable mirror reflects the input light dispersed by the diffraction grating,
• the input light reflected by the variable mirror passes through a normal optical path,
• the input light that passes through the normal optical path has a wavelength band defined based on the angle of the reflective surface, and
• the defined wavelength band has a center wavelength corresponding to the angle of the reflective surface;
• an output optical fiber that outputs a portion of the input light that has passed through the normal optical path;
• an optical detector disposed on a propagation path of the input light from the input optical fiber to the output optical fiber, wherein the optical detector detects a light intensity of a portion of the input light;
• and
• a wavelength selection reflector disposed on the normal optical path, wherein
• the wavelength selection reflector selectively reflects a portion of the light of the defined wavelength band reflected by the variable mirror and that passed through the normal optical path toward the output optical fiber, and
• the optical detector receives the light of the wavelength band separated from the center wavelength that:
• is not reflected by the wavelength selection reflector;
• and
• propagates on a downstream side in a progression direction of the input light to the wavelength selection reflector.