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.

Diffraction gratings generating orders with selective states of polarization.

We propose specially designed double anisotropic polarization diffraction gratings capable of producing a selective number of diffraction orders and with selective different states of polarization. Different polarization diffraction gratings are demonstrated, including linear polarization with horizontal, vertical and ± 45° orientations, and circular R and L polarization outputs. When illuminated with an arbitrary state of polarization, the system acts as a complete polarimeter where the intensities of the diffraction orders allow measurement of the Stokes parameters with a single shot. Experimental proof-of-concept is presented using a parallel-aligned liquid crystal display operating in a double pass architecture.

Nondiffracting vector beams where the charge and the polarization state vary with propagation distance.

We generate nondiffracting vector beams where the charge and the polarization state vary with the propagation distance. We use reflective geometry where a parallel-aligned spatial light modulator is used to spatially modulate two orthogonal linear polarizations. We encode spiral phases with equal charge but with opposite signs onto the two polarization directions to encode a vector beam and add two axicon phases. Both the charge and the phase shift between the two axicons can be varied along the focus line. We provide experimental results that demonstrate both features.

Generation of integer and fractional vector beams with q-plates encoded onto a spatial light modulator.

We generate programmable vector beams with arbitrary q-plates encoded using a spatial light modulator system. Consequently, we can analyze new and exotic q-plate designs without the difficulty of fabricating individual plates. We show experimental results for positive and negative integer and new fractional vector beam values.

Performance of a q-plate tunable retarder in reflection for the switchable generation of both first- and second-order vector beams.

We examine the performance of a tunable liquid crystal q-plate in a reflective geometry. When the device is tuned to a half-wave retardance, it operates as a q-plate with twice the value (2q) by adding a quarter-wave retarder between the mirror and the q-plate. However, when the device is tuned to a quarter-wave retardance, it acts as the original q-plate without the retarder. Experimental results are shown. Using an input tunable polarization state generator, the system allows the switchable production of all states on both the first- and second-order Poincaré spheres.

Nondiffracting Bessel beams with polarization state that varies with propagation distance.

We generate nondiffracting Bessel beams whose polarization state varies with propagation distance. We use a reflective geometry where a single parallel-aligned spatial light modulator device is used to spatially modulate two orthogonal linear polarizations with two axicon phase profiles. Then, by adding an extra phase retardation radial profile between these linear states, we are able to modulate the state of polarization along the line focus of the axicon. We provide experimental results that demonstrate the polarization axial control with zero-order and higher order Bessel beams.

Analysis of a segmented q-plate tunable retarder for the generation of first-order vector beams.

In this work we study a prototype q-plate segmented tunable liquid crystal retarder device. It shows a large modulation range (5π rad for a wavelength of 633 nm and near 2π for 1550 nm) and a large clear aperture of one inch diameter. We analyze the operation of the q-plate in terms of Jones matrices and provide different matrix decompositions useful for its analysis, including the polarization transformations, the effect of the tunable phase shift, and the effect of quantization levels (the device is segmented in 12 angular sectors). We also show a very simple and robust optical system capable of generating all polarization states on the first-order Poincaré sphere. An optical polarization rotator and a linear retarder are used in a geometry that allows the generation of all states in the zero-order Poincaré sphere simply by tuning two retardance parameters. We then use this system with the q-plate device to directly map an input arbitrary state of polarization to a corresponding first-order vectorial beam. This optical system would be more practical for high speed and programmable generation of vector beams than other systems reported so far. Experimental results are presented.

Vector Beam Polarization State Spectrum Analyzer.

We present a proof of concept for a vector beam polarization state spectrum analyzer based on the combination of a polarization diffraction grating (PDG) and an encoded harmonic q-plate grating (QPG). As a result, a two-dimensional polarization diffraction grating is formed that generates six different q-plate channels with topological charges from -3 to +3 in the horizontal direction, and each is split in the vertical direction into the six polarization channels at the cardinal points of the corresponding higher-order Poincaré sphere. Consequently, 36 different channels are generated in parallel. This special polarization diffractive element is experimentally demonstrated using a single phase-only spatial light modulator in a reflective optical architecture. Finally, we show that this system can be used as a vector beam polarization state spectrum analyzer, where both the topological charge and the state of polarization of an input vector beam can be simultaneously determined in a single experiment. We expect that these results would be useful for applications in optical communications.