Abstract
BOOST (BOOst Symmetry Test) is a proposed space mission to search for Lorentz invariance violations and aims to improve the Kennedy-Thorndike parameter constraint by two orders of magnitude. The mission consists of comparing two optical frequency references of different nature, an optical cavity and a hyperfine transition in molecular iodine, in a low Earth orbit. Naturally, the stability of the frequency references at the orbit period of 5400 s (f=0.18 mHz) is essential for the mission success. Here we present our experimental efforts to achieve the required fractional frequency stability of 7.4×10-14 Hz -1/2 at 0.18 mHz (in units of the square root of the power spectral density), using a high-finesse optical cavity. We have demonstrated a frequency stability of (9±3)×10-14 Hz -1/2 at 0.18 mHz, which corresponds to an Allan deviation of 10-14 at 5400 s. A thorough noise source breakdown is presented, which allows us to identify the critical aspects to consider for a future space-qualified optical cavity for BOOST. The major noise contributor at sub-milli-Hertz frequency was related to intensity fluctuations, followed by thermal noise and beam pointing. Other noise sources had a negligible effect on the frequency stability, including temperature fluctuations, which were strongly attenuated by a five-layer thermal shield.
Highlights
Optical cavities are essential to many high sensitivity measuring techniques and experiments where lasers with high frequency stability are needed
An 8.7 cm cubic optical cavity based on the National Physical Laboratory (NPL) design [32] has been chosen as frequency reference
The frequency and intensity stabilization control loops are implemented in a field programmable gate array (FPGA) together with 16-bit analog-to-digital converters (ADCs) and digital-to-analog converters (DACs) operating at a sampling frequency of 650 kHz
Summary
Optical cavities are essential to many high sensitivity measuring techniques and experiments where lasers with high frequency stability are needed. BOOST consists of comparing two highly stable frequency references based on completely different mechanisms, i.e., an optical resonator and a hyperfine transition of molecular iodine [24,25] in a low-Earth orbit. The frequency stability is required at f0 = 1/5400 s = 0.18 mHz. In Eq (1) we have considered αKT = 7.5 × 10−10, which is about two orders of magnitude better than current values from ground experiments [19]. The requirements of other cavities working in the low frequency range, e.g., the LRI [27,28], LISA [29] and the generation of gravity field missions [30,31], are in the 10−10 Hz−1/2 levels at 0.18 mHz, i.e., 1000 times less demanding.
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