Speaker
Description
We present a fiber-optic link that simultaneously delivers DC power
and Manchester-coded slow-control data to a multi-junction GaAs
optical power converter (OPC) operating in a cryogenic environment.
Using a single 808 nm multimode fiber, the link supplies up to 160 mW
of regulated electrical power while transmitting data at 10 kb/s,
with the OPC immersed in liquid nitrogen (LN2) at 77 K.
The recovered data programs slow-control registers of a custom
cryogenic line-driver ASIC, validating the approach for galvanically
isolated, EMI-immune services to in-cryostat front-end electronics in
noble-liquid TPCs (e.g. DarkSide-20k) and noble-liquid calorimeters
proposed for future colliders (e.g. FCC-ee/ALLEGRO).
Summary (500 words)
Cryogenic detector readout electronics are increasingly used across particle physics and quantum sensing. In such systems, every conductor that crosses the cryostat boundary acts as a thermal short, a noise injector, and a source of ground loops. Replacing copper feedthroughs with optical fiber eliminates the thermal load, provides galvanic isolation, and confers immunity to electromagnetic interference. Power-over-fiber (PoF) systems, in which a high-power laser drives an optical power converter (OPC), are an established alternative for delivering bias and supply power to cold electronics [1,2]. To further reduce copper interconnect counts, we transmit slow-control data over the same fiber by modulating the laser drive current and recovering the resulting ripple from the OPC output. This avoids the cost and complexity of wavelength-division multiplexing while preserving available output power.
The system architecture and test setup are shown in Fig. 1(a) and (b); transmitter and receiver schematics are given in Fig. 2. The clock and data streams are combined by an XOR gate into a Manchester-coded waveform. The DC-balanced encoding lets the modulation be superimposed on the laser drive without shifting the average optical power. A linear regulator converts the modulated reference voltage into a laser drive current with an optical modulation depth of ~0.4%. At the cold end, the OPC produces both the DC supply for the cryogenic ASIC and a small AC ripple (~40 mVpp) that an AC-coupled, two-stage amplifier with continuous-time linear equalization (CTLE) restores to rail-to-rail logic levels. The entire receiver chain — OPC, decoupling, and op-amps — is immersed in LN2. It uses AD8605 CMOS op-amps (characterized down to ~6 K) and C0G/NP0 ceramic capacitors to keep the CTLE response stable from 300 K to 77 K.
Both the laser (Broadcom AFBR-POL2120) and OPC (Broadcom AFBR-POC206L) operate at ~808 nm; the OPC produces ~0.9 W electrical for 1.5 W optical input (peak datasheet efficiency ~60%). At our 462 mW operating point, end-to-end OPC efficiencies of 34.6% (300 K) and 34.0% (77 K) were measured: the higher cryogenic open-circuit voltage compensates for reduced short-circuit current and increased output impedance [2].
Eye diagrams of the recovered 10 kb/s data at both temperatures are shown in Fig. 3. Both eyes are open with rail-to-rail ΔV; Gaussian fits to the threshold-crossing histograms give σ ≈ 7% of UI at 300 K and ≈13% of UI at 77 K, with σ_rising ≈ σ_falling at both temperatures. The ~2× increase at 77 K is consistent with reduced signal swing at the receiver input as the OPC output impedance increases. The recovered data was used to program slow-control registers of a custom cryogenic line-driver ASIC [3]. The 77 K demonstration is directly applicable to liquid-argon and liquid-xenon TPCs for dark-matter and rare-event searches — e.g. to bias and slow-control distribution for the ~700 in-LAr SiPM photo-detector units of DarkSide-20k at LNGS — and to charge readout in next-generation noble-liquid TPCs. The ~87 K operating point is also relevant to noble-liquid calorimeters proposed for FCC-ee (e.g. ALLEGRO), where cold preamplifiers immersed in LAr have been proposed.