A stable and inexpensive wavelength reference for precise wavelength calibration of radial velocity spectrographs

ABSTRACT We present a stable, inexpensive wavelength reference, based on a white-light interferometer for the use oncurrent and future (arrays of) di raction-limited radial velocity (RV) spectrographs. The primary aim of usingan interferometer is to obtain a dense sinusoidal wavelength reference with spectral coverage between 450-650nm.Its basic setup consists of an unbalanced ber Mach-Zehnder interferometer (FMZI) that creates an interferencepattern in the spectral domain due to superposition of phase delayed light, set by a xed optical path-lengthdi erence (OPD). To achieve long-term stability, the interferometer is actively locked to a stable atomic line. Thesystem operates in closed-loop using a thermo-optic modulator as the phase feedback component. We conductedstability measurements by superimposing the wavelength reference with thorium-argon (ThAr) emission linesand found the di erential RMS shift to be  5ms 1 within 30 minute bins in an experiment lasting 5 hours.Keywords: Wavelength scale reference, phase locking, radial velocity, ber Mach-Zehnder interferometer