An apparatus includes a spectrometer receiving an optical input signal that includes an input optical spectrum. The spectrometer includes a fiber laser cavity pumped by a first optical replica of the optical input signal that generates stimulated Brillouin scattering traveling in a direction opposite to a direction of the optical input signal. The first optical replica of the optical input signal excites at least one lasing mode in the fiber laser cavity. The at least one lasing mode respectively includes at least one lasing mode frequency. The at least one lasing mode frequency is onset by a respective Brillouin frequency shift from the respective at least one input frequency. The spectrometer also includes an optical heterodyne receiver. The optical heterodyne receiver generates the electrical output signal. The spectrometer outputs a measurement of the input optical spectrum based on the respective Brillouin frequency shift and the at least one input frequency.