Abstract: An improved band pass interferometer for use as a high resolution wavelength selection unit comprising, as main elements, an input optical port (optical fiber) together with a fiber optic collimator for generating a narrow incoming collimated beam, two plane-parallel highly reflective surfaces with low reflection losses, one being totally reflective, (i.e. very little intensity of the light beam incident thereon should pass through the reflective surface,) the other being partially reflective, (i.e., a portion of a reflected light beam, more specifically its intensity, incident thereon should pass through the partially reflective surface becoming an output beam,) which splits the incoming narrow incoming collimated beam into a finite number of output beams, an optical medium located between the reflective surfaces, and a beam focusing element which collects all the output beams and focuses them into an output optical port (optical fiber). There is also provided a refractive index adjuster, such as, e.g.