Wavelength multiplexer/demultiplexer

A wavelength multiplexer/demultiplexer comprising:

• a first collimator including a first optical waveguide and a collimating lens optically coupled to an end portion of the first optical waveguide;
• M-number (M is an integer of 2 or more) of second collimators each including a second optical waveguide and a collimating lens optically coupled to an end portion of the second optical waveguide;
• M-number of wavelength selective filters configured to transmit optical signals in mutually differing transmission wavelength bands, and reflect optical signals in wavelength bands other than each transmission wavelength band;
• and
• a base plate including a placement surface on which the M-number of wavelength selective filters are placed,
• wherein an optical path connecting the first collimator and a first second collimator among the M-number of second collimators is configured to pass through a first wavelength selective filter of the M-number of wavelength selective filters,
• wherein an optical path connecting an mth (m=1, . . . , M−1) wavelength selective filter among the M-number of wavelength selective filters and an (m+1)th second collimator among the M-number of second collimators is configured to pass through an (m+1)th wavelength selective filter among the M-number of wavelength selective filters,
• wherein each of the M-number of wavelength selective filters includes a substrate having optical transparency and a multilayer film, and is fixed to the placement surface by a cured adhesive, the substrate includes a first main surface and a second main surface opposite to each other in a first direction, and a bottom surface connecting the first main surface and the second main surface and facing the placement surface, and the multilayer film is formed on the first main surface and is configured to transmit an optical signal in a specific transmission wavelength band and reflect an optical signal in a wavelength band other than the specific transmission wavelength band,
• wherein the cured adhesive is in contact with the bottom surface and is in non-contact with the multilayer film in at least one wavelength selective filter of the M-number of wavelength selective filters,
• wherein each of the M-number of wavelength selective filters includes an antireflection film formed on the second main surface and configured to prevent an optical signal from being reflected by the second main surface, and
• wherein the cured adhesive is in contact with a surface of the antireflection film facing the placement surface in the at least one wavelength selective filter.