Precise control of the center wavelength of a wavelength-selective single-mode-fiber-thin-film directional coupler

The coupled power and the center wavelength of a wavelength-selective single-mode-fiber-thin-film coupler are accurately determined. The center wavelength depends on the fiber-film spacing. The required remaining cladding thickness of the fiber is accurately obtained from the loss-calibration curve of the half-coupler. The thickness of the film is controlled by spinning speed to match the propagation constants of the fiber and the film. Coupling fiber was used for efficient coupling. Experiments were also carried out for a wavelength filter, and the results almost agree with theoretical values. Shifting of the center wavelength with the condition of fire polishing of the half-coupler is shown.