The invention relates to a multi-pass etalon-based optical filter in which input light once reflected from the etalon is returned back to the etalon for a second reflection to enhance etalon contrast. The external mirror may be tilted relative to the etalon so that two planes of incidence are orthogonal. The multi-pass etalon-based optical filter may be used to clean scattered light from an excitation wavelength in Raman and Brillouin spectroscopy.