Interferometric imaging using Si3N4 photonic integrated circuits for a SPIDER imager.

This paper reports design, fabrication, and experimental demonstration of a silicon nitride photonic integrated circuit (PIC). The PIC is capable of conducting one-dimensional interferometric imaging with twelve baselines near λ = 1100-1600 nm. The PIC consists of twelve waveguide pairs, each leading to a multi-mode interferometer (MMI) that forms broadband interference fringes or each corresponding pair of the waveguides. Then an 18 channel arrayed waveguide grating (AWG) separates the combined signal into 18 signals of different wavelengths. A total of 103 sets of fringes are collected by the detector array at the output of the PIC. We keep the optical path difference (OPD) of each interferometer baseline to within 1 µm to maximize the visibility of the interference measurement. We also constructed a testbed to utilize the PIC for two-dimension complex visibility measurement with various targets. The experiment shows reconstructed images in good agreement with theoretical predictions.

Coherent beam combining using a 2D internally sensed optical phased array.

Coherent combination of multiple lasers using an optical phased array (OPA) is an effective way to scale optical intensity in the far field beyond the capabilities of single fiber lasers. Using an actively phase locked, internally sensed, 2D OPA we demonstrate over 95% fringe visibility of the interfered beam, λ/120 RMS output phase stability over a 5 Hz bandwidth, and quadratic scaling of intensity in the far field using three emitters. This paper presents a new internally sensed OPA architecture that employs a modified version of digitally enhanced heterodyne interferometry (DEHI) based on code division multiplexing to measure and control the phase of each emitter. This internally sensed architecture can be implemented with no freespace components, offering improved robustness to shock and vibration exhibited by all-fiber devices. To demonstrate the concept, a single laser is split into three channels/emitters, each independently controlled using separate electro-optic modulators. The output phase of each channel is measured using DEHI to sense the small fraction of light that is reflected back into the fiber at the OPA's glass-air interface. The relative phase between emitters is used to derive the control signals needed to stabilize their relative path lengths and maintain coherent combination in the far field.

Laser link acquisition demonstration for the GRACE Follow-On mission.

We experimentally demonstrate an inter-satellite laser link acquisition scheme for GRACE Follow-On. In this strategy, dedicated acquisition sensors are not required-instead we use the photodetectors and signal processing hardware already required for science operation. To establish the laser link, a search over five degrees of freedom must be conducted (± 3 mrad in pitch/yaw for each laser beam, and ± 1 GHz for the frequency difference between the two lasers). This search is combined with a FFT-based peak detection algorithm run on each satellite to find the heterodyne beat note resulting when the two beams are interfered. We experimentally demonstrate the two stages of our acquisition strategy: a ± 3 mrad commissioning scan and a ± 300 µrad reacquisition scan. The commissioning scan enables each beam to be pointed at the other satellite to within 142 µrad of its best alignment point with a frequency difference between lasers of less than 20 MHz. Scanning over the 4 alignment degrees of freedom in our commissioning scan takes 214 seconds, and when combined with sweeping the laser frequency difference at a rate of 88 kHz/s, the entire commissioning sequence completes within 6.3 hours. The reacquisition sequence takes 7 seconds to complete, and optimizes the alignment between beams to allow a smooth transition to differential wavefront sensing-based auto-alignment.

Internally sensed optical phased array.

Extending phased array techniques to optical frequencies is challenging because of the considerably smaller wavelengths and the difficulty of stabilizing the optical path lengths of multiple emitters to this level of precision. This is especially true under real-world conditions where thermal and vibrational disturbances cause path length variations that are considerable in relation to the wavelength. Earlier attempts have relied on an external mechanism to sense and compensate for any unwanted variations in the outgoing beams. Here we propose and demonstrate a method that does not rely on any external components. The method combines a pseudo-random noise phase modulation scheme together with conventional heterodyne interferometry to simultaneously measure phase variations between emitters. This information is then used to control the relative phases between the emitters and compensate for any unwanted disturbance. Experimental results are presented that support the viability of this design.

Laser frequency noise immunity in multiplexed displacement sensing.

Digitally enhanced interferometry (DI) can be used to distinguish between interferometric signals and simultaneously monitor in-line object displacements with subnanometer sensitivity. In contrast to conventional interferometry-where these signals interfere with each other and degrade performance-we experimentally show that by using DI, each of these signals can be isolated and measured at the same time. We present what we believe to be the first demonstration of DI's signal multiplexing capabilities, showing simultaneous length sensing of three sections of an optical fiber. The cross talk between length measurements was less than 2.6×10(-3) with a displacement noise floor of 200 pm/√Hz, which corresponds to a strain sensitivity of less than 80 picostrain(pϵ) in each sensor. We also enhance our system's displacement sensitivity at low frequencies by combining information from multiple lengths to suppress errors due to laser frequency noise.

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.