Fig. 1: Spectrometer based on a cryogenic high-finesse cavity.
From: Cavity-enhanced spectroscopy in the deep cryogenic regime for quantum sensing and metrology
Left: illustration of the cryogenic part of the set-up. The entire cavity, including the mirror position actuator, is placed inside a cryogenic copper vacuum chamber (yellow) that can be cooled to 4 K to ensure the highly homogeneous thermodynamic equilibrium of the entire gas sample. The outer copper layers (orange), kept at approximately 40 K, block room-temperature thermal radiation. The entire system is placed inside a room-temperature vacuum system (grey) to maintain cryogenic conditions. The inner cryogenic and outer room-temperature vacuum chambers are independent; hence, the cryogenic chamber can be independently filled with H2 gas. The inner cryogenic vacuum chamber is suspended on thin titanium rods (not shown here) to eliminate thermal links and attenuate vibrations. The cryogenic mirror position actuator31 is shown in detail in the small inset below the vacuum chamber. Right: simplified scheme of the experimental set-up. We use an OPO to access the H2 fundamental-band transition with an idler beam operating at 2.2 μm. The idler beam is tightly locked to the cryogenic high-finesse cavity via a Pound–Drever–Hall lock33. The absolute frequency after idler frequency-doubling is determined by referring to an optical frequency comb at 1.1 μm. The length of the cryogenic cavity is actively stabilized by referring to the optical frequency comb. Both the comb repetition rate and the offset frequency are stabilized to the radio frequency from the hydrogen maser (H maser), which is integrated with the local representation of UTC at the Astrogeodynamic Observatory in Borowiec43. The plot in the right bottom corner shows a typical ring-down event acquired at 4 K, where τ and \({\mathcal{F}}\) are the photon lifetime and cavity finesse, respectively. AOS, Astrogeodynamic Observatory; PBS, polarizing beam splitter; PD, photodetector; PDH, Pound–Drever–Hall; SHG, second-harmonic generation; ECDL, external-cavity diode laser; AOM, acousto-optic modulator; EOM, electro-optic modulator.
