Optical filter and spectroscopy system employing the filter

It is described an optical filter (100) configured to operate for Brillouin scattering and/or low-frequency Raman scattering spectroscopy applications and comprising: an input (IN) for a first radiation (RB) having a first input linear polarization (pB1) and a second radiation (RR) having a second input linear polarization parallel (pR1) to said first input linear polarization (pB1); a birefringent medium (1) configured to reciprocally rotate the first (pB1) and second (pR1) input polarizations to obtain that the first radiation (RB) has a first output polarization (pB2) and the second radiation (RR) has a second output polarization (pR2) different from said first output polarization (pB2). The filter further includes a polarizer (2; 7, 20) coupled to said birefringent medium (1) to transmit said first radiation (RB) having the first output polarization (pB2) and reject said second radiation (RR) having the second output polarization (pR2).