Interference filter module

An interference filter module comprising:

• a first optical fiber collimator and a second optical fiber collimator, which have a fore-and-aft direction as a direction of an optical axis and are arranged at a front end and a rear end of a casing so as to be located on said optical axis and opposed to each other;
• and
• 2n interference filters arranged inside said casing so as to be located on said optical axis where n is a natural number,
• wherein,
• said casing includes:
• a main body portion having a cylindrical shape;
• and
• 2n filter holding portions, each being configured to hold a corresponding one of said interference filters, which are to be mounted into said main body portion,
• each of said interference filters includes a substrate and an interference film laminated on said substrate,
• when k is a natural number equal to or smaller than n, two interference filters including a k-th interference filter when counted from said front end toward said rear end of said casing and a k-th interference filter when counted from said rear end toward said front end of said casing are determined as a k-th set among said 2n interference filters,
• said two interference filters of said k-th set are respectively accommodated in one of said filter holding portions, which is a k-th filter holding portion when counted from said front end toward said rear end of said casing, and in another one of said filter holding portions, which is a k-th filter holding portion when counted from said rear end toward said front end of said casing,
• said two filter holding portions corresponding to said k-th filter holding portion when counted from said front end of said casing and said k-th filter holding portion when counted from said rear end of said casing have rotation axes in directions orthogonal to said fore-and-aft direction and are held rotatably in said casing, and said rotation axes of said two filter holding portions are orthogonal to each other, and
• said two interference filters of said k-th set are arranged so that film surfaces of said interference films of so as to reduce the divergence-angle dependence of a wavelength selectively transmitted by the interference filter module.