A system for sensing a characteristic of a sample may include a tunable source configured to emit optical radiation that varies over a wavelength range at a first frequency and a reference source configured to emit optical radiation that varies in amplitude at a second frequency. A science detector may be configured to detect the optical radiation from the tunable source and the reference source after interaction with the sample and generate a science signal. A number of lock-in amplifiers may be respectively configured to generate components of the science signal that are present at the first and second frequencies. A processor may be configured to determine a characteristic of the sample based on the components of the science signal that are present at the first and second frequencies.