An apparatus for optically shifting the frequency of an input signal beam includes a first Raman medium that receives an input signal beam, a first pump beam, and a first reference beam to responsively generate an intermediate signal beam comprising a Raman sideband of the first Raman medium. A second Raman medium is optically coupled in series with the first Raman medium. The second Raman medium receives the intermediate signal beam, a second pump beam, and a second reference beam to responsively generate an output signal beam comprising a Raman sideband of the second Raman medium. The generated output signal represents the input signal that is shifted in frequency by a frequency shift that corresponds to the frequency difference between the first and second reference beams.