Optical microscope and tapered fiber coupling apparatus for a dilution refrigerator

Abstract

We have developed a system for tapered fiber measurements of optomechanical resonators inside a dilution refrigerator, which is compatible with both on- and off-chip devices. Our apparatus features full three-dimensional control of the taper-resonator coupling conditions enabling critical coupling, with an overall fiber transmission efficiency of up to 70%. Notably, our design incorporates an optical microscope system consisting of a coherent bundle of 37,000 optical fibers for real-time imaging of the experiment at a resolution of ∼1 μm. We present cryogenic optical and optomechanical measurements of resonators coupled to tapered fibers at temperatures as low as 9 mK.

Optomechanics and thermometry of cryogenic silica microresonators

“…The taper is epoxied and mounted to a large Invar block, which minimizes the effects of thermal contractions. A homebuilt lowtemperature microscope [24] enables real-time in situ imaging of the taper-resonator system with a resolution of ∼1 µm (Figure 2(b)). We inject light from a tunable diode laser housed at room temperature into the tapered fiber and detect the transmitted light with a fast photodetector.…”

“…We passively cool the taper and bottle resonators using a dilution refrigerator with a base temperature of 9 mK [24]. To facilitate taper-resonator coupling at cryogenic temperatures, we have built an optomechanical coupling system within the inner vacuum can (IVC), on the mixing chamber plate, as pictured in Figure 2(a).…”

“…For relatively low injected optical powers (P in ≤ 250 nW), the optical resonances can be measured without heating the mixing chamber thermometer and have Q o of 10 6 -10 7 , which are consistent with their room-temperature quality factors. At larger injected powers, light lost from the taper (∼30%) heats the cryogenic environment [24]. Furthermore, the high Q o , and correspondingly long photon lifetime, in the cavity leads to an increased absorption of laser photons in the silica.…”

“…The linewidth of the resonance is given by κ = κ 0 + κ ex where κ 0 is the intrinsic decay rate and κ ex is the rate at which photons are exchanged between the bottle and tapered fiber. Our setup allows us to sensitively control the relative strengths of these intrinsic and extrinsic decay rates [24]. For the measurements that follow, the resonator is operated in the slightly-undercoupled regime (κ ex < ∼ κ 0 ).…”

“…Our system uses a dilution refrigerator for its lower achievable base temperature and, in contrast to the system presented in Ref. 21, uses the highly-efficient tapered-fiber coupling method [22,23], which is compatible with both bulk and on-chip optomechanical systems [24,25].…”