Fiber mode scrambler experiments for the Subaru Infrared Doppler Instrument (IRD)

We report the results of fiber mode scrambler experiments for the Infra-Red Doppler instrument (IRD) on the Subaru 8.2-m telescope. IRD is a near infrared, high-precision radial velocity (RV) instrument to search for exoplanets around M dwarfs. It is a fiber-fed, high-resolution (R~70000) spectrograph with an Echelle grating and a state-of-the art laser frequency comb. Expected precision of RV measurements is 1m/s. To achieve 1m/s accuracy, we must reduce modal noise, which is intensity instability of light at the end of multimode fibers. Modal noise is caused by interference of finite number of propagating modes of light. This noise can cause false RV signals, which reduce the accuracy of RV measurements. A mode scrambler is a mechanism to reduce modal noise. However, the best mode scrambler system at near infrared wavelengths is still unknown. Thus, we tested many kinds of mode scramblers, various length fibers, a double scrambler, and octagonal fibers, as static scramblers. We also tested dynamic scramblers, which make output uniform by moving optical fibers dynamically. We report the effects of these mode scramblers.