Abstract: The present invention relates to tunable spectral filters. A wide-band/high-resolution tunable spectral filter uses a single transmissive diffraction element in a double pass configuration. The double pass is provided and separated laterally by a retro-reflective mirror working with a half wave plate, which reduces effectively polarization-dependent loss (PDL). The transmissive diffraction element in low dispersion configuration provides a wide-band operation. An anamorphic system is disposed for increasing filter spectral resolution to a desirable level. The continuous tuning of a selected spectral portion from input port over a wide band is accomplished by rotating the retro-reflective mirror.