A probe of a Raman spectroscopy system has a wavelength and/or amplitude referencing system for determining a wavelength of the excitation signal. Preferably, this referencing system is near an output aperture, through which the excitation signal is transmitted to the sample. In this way, any birefringence or polarization dependent loss (PDL) that may be introduced by optical elements in the system can be compensated for since the wavelength reference system will detect the effect or impact of these elements.