Christopher A. Schuetz

Full spectrum millimeter-wave modulation

In recent years, the development of new lithium niobate electro-optic modulator designs and material processing techniques have contributed to support the increasing need for faster optical networks by considerably extending the operational bandwidth of modulators. In an effort to provide higher bandwidths for future generations of networks, we have developed a lithium niobate electro-optic phase modulator based on a coplanar waveguide ridged structure that operates up to 300 GHz. By thinning the lithium niobate substrate down to less than 39 µm, we are able to eliminate substrate modes and observe optical sidebands over the full millimeter-wave spectrum.

Radiometric Millimeter-wave detection via optical upconversion and carrier suppression

We report a novel technique for radiometric detection of radiation in the millimeter-wave regime based on optical modulation. Millimeter-wave energy modulated onto an optical carrier is detected using a low-bandwidth photodetector with optical filtering to suppress the carrier. Using this technique, we have achieved noise-equivalent powers as low as 20 pW/spl radic/Hz for a 44-GHz narrow-band signal using chopping techniques. Such detection was implemented using commercially available components and without RF or optical amplification. Further improvements in sensitivity are expected as the setup is optimized for operation in the frequency band of interest.

Subwavelength imaging by a flat cylindrical lens using optimized negative refraction

We experimentally demonstrate subwavelength imaging by a “flat cylindrical” lens using negative refraction. A two-dimensional photonic crystal whose dispersion at the second band provides group velocity opposite to the phase velocity for electromagnetic waves is employed to realize the flat lens, and the working frequency is chosen so that the effective refractive index is approximately equal to −1.0. Experiment demonstrated the imaging of a point source in both amplitude and phase in the millimeter-wave regime. By measuring the field distributions in the object plane and image plane, we observed amplification of evanescent waves and subwavelength size image. The image of two incoherent sources with subwavelength distance showed two resolvable spots, which served to further verify subwavelength resolution.

Development of Electro-Optic Phase Modulator for 94 GHz Imaging System

This paper presents the latest improvements in design and fabrication of ridged LiNbO3 optical phase modulators operating in the W-band for millimeter-wave imaging applications at 94 GHz. It describes the design conditions for effective index matching between optical carrier at 1550 nm and millimeter-wave signal, as well as impedance matching and propagation losses reduction. The impacts of different geometric parameters on the device performance are discussed. In addition, the paper reports the diverse material processing techniques for device fabrication. Characterization of the fabricated devices shows conversion efficiency as high as 1.045 W- 1 at 94 GHz with a 7 V dc half-wave voltage and a 3.7 dB optical insertion loss.

Experimental demonstration of negative refraction imaging in both amplitude and phase

We studied a two-dimensional square-lattice photonic crystal with all-angle negative refraction at its first band. Using this photonic crystal, we designed and fabricated a flat lens functioning as a cylindrical lens by increasing the vertical dimension of the photonic crystal. Two-dimensional finite-difference time-domain simulation validated negative refraction imaging. To perform the experiment, a microwave imaging system was built based on a vector network analyzer. Field distributions were acquired by scanning the imaging plane and object plane. The experiment demonstrated negative refraction imaging in both amplitude and phase, and obtained an image with feature size, 0.77lambda0.

Three-dimensional photonic crystal flat lens by full 3D negative refraction

We present the experimental demonstration of imaging by all-angle negative refraction in a 3D photonic crystal flat lens at microwave frequencies. The flat lens is made of a body-centered cubic photonic crystal (PhC) whose dispersion at the third band results in group velocity opposite to phase velocity for electromagnetic waves. We fabricated the photonic crystal following a layer-by-layer process. A microwave imaging system was established based on a vector network analyzer, where two dipoles work as the source and the detector separately. By scanning the volume around the lens with the detector dipole, we captured the image of the dipole source in both amplitude and phase. The image of two incoherent sources separated by 0.44lambda showed two resolvable spots, which served to verify sub-wavelength resolution.

Design and performance of a distributed aperture millimeter-wave imaging system using optical upconversion

Passive imaging using millimeter waves (mmWs) has many advantages and applications in the defense and security markets. All terrestrial bodies emit mmW radiation and these wavelengths are able to penetrate smoke, blowing dust or sand, fog/clouds/marine layers, and even clothing. One primary obstacle to imaging in this spectrum is that longer wavelengths require larger apertures to achieve the resolutions typically desired in surveillance applications. As a result, lens-based focal plane systems tend to require large aperture optics, which severely limit the minimum achievable volume and weight of such systems. To overcome this limitation, a distributed aperture detection scheme is used in which the effective aperture size can be increased without the associated volumetric increase in imager size. However, such systems typically require high frequency (~ 30 - 300 GHz) signal routing and down conversion as well as large correlator banks. Herein, we describe an alternate approach to distributed aperture mmW imaging using optical upconversion of the mmW signal onto an optical carrier. This conversion serves, in essence, to scale the mmW sparse aperture array signals onto a complementary optical array. The optical side bands are subsequently stripped from the optical carrier and optically recombined to provide a real-time snapshot of the mmW signal. In this paper, the design tradeoffs of resolution, bandwidth, number of elements, and field of view inherent in this type of system will be discussed. We also will present the performance of a 30 element distributed aperture proof of concept imaging system operating at 35 GHz.

Optical upconversion techniques for high-sensitivity millimetre-wave detection

Millimeter-wave radiation has the unique ability to penetrate atmospheric obscurations such as smoke, fog, and light rain while maintaining the capability for high-resolution imaging. However, suitable technologies for creating high-sensitivity, large pixel-count detectors are a limiting factor in the implementation of such systems. To this end, we present a technique for detecting millimeter-wave radiation based on optical upconversion that promises both high sensitivity and scalability to large pixel arrays. High-speed optical modulation is used to transfer millimeter-wave radiation onto the sidebands of a near-infrared optical carrier frequency. Optical filtering techniques are subsequently used to suppress light at the carrier frequency. The resultant signal is passed to a low-frequency photodetector, which converts the remaining sideband energy to a photocurrent proportional to the incident millimeter wave energy at the modulator input. Utilizing the low noise powers of such photodetectors, high sensitivities may be obtained even accounting for the relatively high signal losses associated with optical upconversion. Since optical upconversion inherently preserves both phase and amplitude information and fiber optics may readily be used for low-loss routing of the modulated signal, such an approach offers promise for high-resolution synthetic aperture imaging. Alternatively, since each of the required components may be fabricated in III-V materials using planar semiconductor processing techniques, integration of multi-pixel arrays is feasible. Herein, we present experimental results obtained using a baseline detector assembled from commercially available fiber-optic components as well as efforts to integrate the desired functionality into a single GaAs substrate. An initial noise equivalent power (NEP) of the proposed detector has been demonstrated at sub-nanowatt levels, with improvements to sub-picowatt NEPs anticipated as the setup is optimized.

Experimental demonstration of self-collimation in low-index-contrast photonic crystals in the millimeter-wave regime

In this paper, we present the theoretical and experimental results for self-collimation in low-index-contrast photonic crystals (PhCs) in the millimeter-wave (MMW) region of the electromagnetic spectrum. The design of the PhCs is based on their equifrequency contours and the two-dimensional finite-difference time-domain simulation results. In the experiments, the MMW PhCs are fabricated in Rexolite slabs by a CNC micro-milling system. A MMW imaging system is built based on a vector network analyzer. The input source is launched either through a waveguide or a monopole, while the field distribution is acquired by scanning a monopole antenna over the surface of the photonic crystal to detect the profile of the evanescent waves. In both cases, we have observed and characterized the self-collimation effect for both the amplitude and phase of the propagating electromagnetic wave in low-index-contrast photonic crystals.

Passive 77 GHz millimeter-wave sensor based on optical upconversion

A passive millimeter-wave (mmW) sensor operating at a frequency of 77 GHz is built and characterized. The sensor is a single pixel sensor that raster scans to create an image. Optical upconversion is used to convert the incident mmW signal into an optical signal for detection. Components were picked to be representative of a single element in a distributed aperture system. The performance of the system is analyzed, and the noise equivalent temperature difference is found to be 0.5 K (for a 1 s integration time) with a diffraction limited resolution of ~8 mrad. Representative images are shown that demonstrate the phenomenology associated with this spectrum.