Interferometric wavefront sensing with a single diode using spatial light modulation.

We present a new technique for the fine alignment sensing of optical interferometers. Unlike conventional wavefront sensing systems, which use multielement photodiodes, this approach works with a single-element photodiode, in combination with a spatial light modulator (SLM) and digitally enhanced heterodyne interferometry. As all signals pass through a single photodetection and analog path, the technique exhibits high common-mode rejection to low frequency errors present in conventional systems. By changing the modulation pattern on the SLM, the technique can also be extended to sensing higher-order wavefront errors. In this paper, we demonstrate the technique experimentally and compare performance with a conventional heterodyne wavefront sensing system. This may improve and simplify alignment systems in space-based interferometers such as the planned LISA gravitational wave detector and provide a way to optimize the power in laser cavities not possible with the traditional segmented diode approach.

Experimental demonstration of impedance match locking and control for coupled resonators.

We describe and verify the dynamic behavior of a novel technique to optimize and actively control the optical impedance matching condition of a coupled resonator system. The technique employs radio frequency modulation and demodulation to interrogate the reflection amplitude response of the coupled cavity system. The sign and magnitude of the demodulated signal is used in a closed loop feedback system which controls the coupling condition of a three-mirror resonator. This was done by actuating on the spacing between two of mirrors, effectively using the pair as a variable reflectivity compound mirror. We propose that this technique can be used for controlling the signal bandwidth of next-generation gravitational wave detectors, as well as optimizing circulating optical carrier power in the instrument.

Picometer level displacement metrology with digitally enhanced heterodyne interferometry.

Digitally enhanced heterodyne interferometry is a laser metrology technique employing pseudo-random codes phase modulated onto an optical carrier. We present the first characterization of the technique's displacement sensitivity. The displacement of an optical cavity was measured using digitally enhanced heterodyne interferometry and compared to a simultaneous readout based on conventional Pound-Drever-Hall locking. The techniques agreed to within 5 pm/ radicalHz at 1 Hz, providing an upper bound to the displacement noise of digitally enhanced heterodyne interferometry. These measurements employed a real-time signal extraction system implemented on a field programmable gate array, suitable for closed-loop control applications. We discuss the applicability of digitally enhanced heterodyne interferometry for lock acquisition of advanced gravitational wave detectors.

Using active resonator impedance matching for shot-noise limited, cavity enhanced amplitude modulated laser absorption spectroscopy.

We introduce a closed-loop feedback technique to actively control the coupling condition of an optical cavity, by employing amplitude modulation of the interrogating laser. We show that active impedance matching of the cavity facilitates optimal shot-noise sensing performance in a cavity enhanced system, while its control error signal can be used for intra-cavity absorption or loss signal extraction. We present the first demonstration of this technique with a fiber ring cavity, and achieved shot-noise limited loss sensitivity. We also briefly discuss further use of impedance matching control as a tool for other applications.