Phase stabilization for long baseline interferometry of incoherent optical sources

Abstract

The maximum baseline, and therefore resolution, of optical astronomical interferometers is limited by attenuation and phase noise within the optical path between the apertures and beam combiner, as well as the practical challenges of constructing optical delay lines more than a few hundred meters in length. We implement off-band phase stabilization on two fiber optic links of 85 km, creating a total baseline of 170 km. We show that the system is able to effectively phase stabilize signals from an incoherent pseudo-thermal source with a bandwidth of 11.2 nm. We are able to reduce the phase noise of a fiber based interferometer with two 85 km arms by 4-5 orders of magnitude between 1 and 100 Hz such that we could resolve an applied phase shift sweep of 0.16 cycles per second with continuous measurement. We show that, with phase stabilization active, the interferometer is able to recover both first-order and second-order photon correlations. These results demonstrate the feasibility of this technique for long-baseline optical and quantum astronomical interferometers. The present results are limited by chromatic dispersion within the fiber, which can be mitigated using dispersion compensating modules.

Figures (5)

• Figure 1: Functional System Diagram. DWDM: dense wavelength demultiplexors (CH31 separating), PD: photodetector, SNSPD: superconducting nanowire single-photon detector, ODL: optical delay line, AOM: acousto-optic modulator. This is the model for how the interferometer would be constructed at a full scale. The system under test placed both receiver A and B next to each other and the two 85 km fibers are fibers that run next to each other around the city of Perth (Boorloo), Western Australia, Australia. The blue fibers are our thermal source collected from the receivers, the red fibers are the stabilizing laser, from an X15 at 1552.51nm, and the green fiber is where the signals are combined (multi-wavelength).
• Figure 2: PSD of link with and without phase stab. Red curves are the phase difference measured between the two stabilizing signals without phase stabilization system (dotted) and with phase stabilization (solid). The blue curves are the phase of one of the thermal signals without phase stabilization (dotted) and with phase stabilization (solid). The green curve is derived from Equation \ref{['eq:stab_limit']}. The thermal signal here is from a coherent source (a laser with 10 kHz linewidth).
• Figure 3: Measured intensity over time without changing the phase offset. Measuring the number of counts in both thermal signals (with a coherent source) over time to observe the drift in the system.
• Figure 4: Photon counts and correlations over time with a coherent source. Top plot is first order interference [0,1] and [1,0] states. Bottom plot is second order interference [1,1] state.
• Figure 5: Photon counts and correlations over time with an incoherent source. Top plot is first order interference [0,1] and [1,0] states. Bottom plot is second order interference [1,1] state.