Frequency stability characterization of a broadband fiber Fabry-Pérot interferometer

Abstract

An optical etalon illuminated by a white light source provides a broadband comb-like spectrum that can be employed as a calibration source for astronomical spectrographs in radial velocity (RV) surveys for extrasolar planets. For this application the frequency stability of the etalon is critical, as its transmission spectrum is susceptible to frequency fluctuations due to changes in cavity temperature, optical power and input polarization. In this paper we present a laser frequency comb measurement technique to characterize the frequency stability of a custom-designed fiber Fabry-Pérot interferometer (FFP). Simultaneously probing the stability of two etalon resonance modes, we assess both the absolute stability of the etalon and the long-term stability of the cavity dispersion. We measure mode positions with MHz precision, which corresponds to splitting the FFP resonances by a part in 500 and to RV precision of ≈ 1 m s −1 . We address limiting systematic effects, including the presence of parasitic etalons, that need to be overcome to push the metrology of this system to the equivalent RV precision of 10 cm s −1 . Our results demonstrate a means to characterize environmentally-driven perturbations of etalon resonance modes across broad spectral bandwidths, as well as motivate the benefits and challenges of FFPs as spectrograph calibrators.

Frequency stability of the mode spectrum of broad bandwidth Fabry-Pérot interferometers

“…Such a hypothesis is difficult to model or verify experimentally, but it would be informative if revealed in other FP cavities of similar construction that are in use or intended for similar application. We note that in a previous experiment [30] we did recover the expected dispersion stability ∆ω 1064 /∆ω 1319 at the 10 −3 level in a fiber Fabry-Pérot using the measurement technique and data analysis pipeline we employ here. Though note for the fiber Fabry-Pérot, the fiber is single mode at all wavelengths, thus ensuring ideal spatial overlap of all wavelengths.…”

“…To characterize the frequency stability of the FP cavities we monitor the frequencies of multiple cavity resonances relative to a self-referenced optical frequency comb. Full details of this technique are presented in [30]; here we give a brief summary while noting differences from our previous work. A near infrared, self-referenced, octave spanning LFC [31] calibrates the scanning of continuous wave (CW) lasers at 780 nm, 1064 nm and 1319 nm with the NIR etalon (780 nm, 532 nm and 660 nm with the visible etalon, the latter two being the 1064 and 1319 nm laser light frequency doubled in nonlinear crystals) that are transmitted through the FP to simultaneously measure and track the frequencies of the resonance modes in real time.…”

“…As described in greater detail in [30], our electronic mixing scheme provides a calibration 'tick' each time the CW laser moves 62.5 MHz, or at precisely f rep /4. The known frequency spacing of the calibration ticks gives discrete points that map acquisition time to relative frequency.…”

Effect of Phase Noise on the Frequency Calibration of a Tunable Laser by Heterodyne Signal Filtering

“…For example, in light detection and ranging (Li-DAR) systems based on frequency modulated continuous wave (FMCW), the instantaneous optical frequency of the swept laser should be monitored for passive linearization of the tuning speed [1], [2]. In tunable laser spectroscopy, the frequency should be calibrated in order to record the spectral features correctly [3], [4], [5]. Conventional spectrometers such as Fabry-Perot etalon and dispersion gratings, have difficulties in fulfilling both high spectral resolution and wide measuring range.…”

“…Recently, spectral dynamics analysis based on optical heterodyne method has shown great power in offering high spectral resolution [6], [7]. Meanwhile, in these heterodyning spectrometers, the frequency combs have been used as the local oscillators (LOs) to heterodyne with the laser under test (LUT) [3], [4], [8], allowing for wideband and high precision frequency calibration.…”