Richard D. Martin

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.

Passive millimeter wave imaging using a distributed aperture and optical upconversion

We report on our initial results of passive, real-time imaging in the Q-band using a distributed aperture and optical upconversion. The basis of operation is collection of incident mmW radiation by the distributed aperture, as embodied by an array of horn antennas, which is then amplified and upconverted to optical frequencies using commercially available electro-optic modulators. The non-linear mixing of the modulators creates sidebands containing the mmW signal with both amplitude and phase preserved. These signals are relaunched in the optical domain with a homothetic mapping of the antenna array. The optical carrier is stripped via dielectric stack filters and imagery is synthesized from the sidebands using the Fourier transform properties of a simple lens. This imagery is collected using a standard nearinfrared camera with post-processing to enhance the signal of interest and reduce noise. Details of operation and presentation of sample imagery is presented herein.

Millimeter wave imaging with engineered point spread functions

Abstract. We demonstrate experimentally the ability to shape the point spread function of a distributed-aperture millimeter-wave imaging system by modifying its aperture phase. We consider distributions on a regular hexagonal array and a nonredundant array. We also show how to exploit this capability to perform low-resolution analog image processing. A preliminary investigation of system performance reveals nonuniformity in amplitude response across the array is a major contributor to deviations from predicted point spread functions (PSFs).

Sparse aperture millimeter-wave imaging using optical detection and correlation techniques

For many applications, the usefulness of millimeter-wave imagers is limited by the large aperture sizes required to obtain images of sufficient resolution. Sparse aperture techniques could open up wider range of applications by mitigating the volume requirements of high resolution imagers. In previous proceedings, we have presented an approach towards the realization of millimeter-wave, sparse-aperture imagers using optical techniques. By using electro-optic modulators to upconvert received millimeter-wave fields onto an optical carrier, such fields can be readily captured, routed, and processed using optical techniques. Such techniques could provide significant advantages over traditional heterodyne techniques. Herein, we present progress towards the physical realization of such an imager. Specifically, we discuss the implementation challenges that must be addressed to create such an imager and present in further detail the numerous advantages such an approach will yield. We also present results obtained from a working prototype system and show that these results are in good agreement with theoretical performance models.

Multilayer liquid crystal polymer based RF frontend module for millimeter wave imaging

In this paper we proposed an integrated RF front-end module by using multilayer liquid crystal polymer (LCP) substrates in millimeter wave frequency (mmW) range. The module consists of a linearly tapered slot antenna (LTSA), low noise amplifiers (LNA), as well as various passive components, including transmission lines, waveguides, filters, and their transitions. The active LNAs and their related bias circuits are integrated with RF circuits through wire-bonding process to achieve a gain of 12 dB centered at 94GHz. The fabricated devices are characterized and compared with simulated results, showing good agreement. The LTSA has a wide impedance bandwidth from 43 to 90 GHz and high radiation gain. The compact module holds tremendous promise in the high-performance applications, such as mmW imaging, and many other high density RF systems.

Nonmechanical beam steering using optical phased arrays

Beam steering is an enabling technology for establishment of ad hoc communication links, directed energy for infrared countermeasures, and other in-theater defense applications. The development of nonmechanical beam steering techniques is driven by requirements for low size, weight, and power, and high slew rate, among others. The predominant beam steering technology currently in use relies on gimbal mounts, which are relatively large, heavy, and slow, and furthermore create drag on the airframes to which they are mounted. Nonmechanical techniques for beam steering are currently being introduced or refined, such as those based on liquid crystal spatial light modulators; however, drawbacks inherent to some of these approaches include narrow field of regard, low speed operation, and low optical efficiency. An attractive method that we explore is based on optical phased arrays, which has the potential to overcome the aforementioned issues associated with other mechanical and nonmechanical beam steering techniques. The optical array phase locks a number of coherent optical emitters in addition to applying arbitrary phase profiles across the array, thereby synthesizing beam shapes that can be steered and utilized for a diverse range of applications.

Far field millimeter-wave imaging via optical upconversion

Millimeter-wave imaging has the unique potential to penetrate through poor weather and atmospheric conditions and create a high-resolution image. In pursuit of this goal, we have implemented a far-field imaging system that is based on optical upconversion techniques. Our imaging system is passive, in which all native blackbody radiation that is emitted from the object being scanned is detected by a Cassegrain antenna on a rotating gimbal mount. The signal received by the Cassegrain is passed to an optical modulator which transfers the radiation onto sidebands of a near-infrared optical carrier frequency. The signal is then passed to a low-frequency photodetector that converts remaining sideband energy to a photocurrent. Even though optical upconversion can produce loss, our system demonstrates low noise equivalent powers (NEP) due to the low-noise of the photodetection process. Herein, we present our experimental results and images obtained by using the far-field scanning system, which was assembled with commercially available components. In addition, we detail efforts to increase the resolution of the image and to compact the imaging system as a whole.

Slot-Coupled Directional Filters in Multilayer LCP Substrates at 95 GHz

In this paper, traveling-wave directional filters (DFs) in multilayer liquid crystal polymer substrates were proposed at 95 GHz, consisting of two terminating microstrip lines (MSLs) in the top circuit layer, a shared ground and coupling slots in the second layer, and MSL loop resonators in the bottom layer. By using asymmetric phase topology for the loop, the proposed DFs address higher directivity and Q-factor over the traditional symmetric traveling-wave DFs. The single-loop DF was demonstrated with a 3-dB passband width of 4.8% and an insertion loss of 4.6 dB at 94 GHz. To improve the directivity, particularly the insertion loss, two identical DFs were cascaded in series in the direction of the terminating lines. A bandwidth of 8% and a low insertion loss of 2.6 dB can be obtained with a phase delay of 360° between the two DFs. Limited by the practical application, the proposed DFs were fabricated and demonstrated in hybrid substrates, which slightly increases the insertion loss. The measured insertion loss of the single-loop and double-loop DFs is 5.2 and 3.1 dB at 95 GHz, respectively, showing good agreement with the simulated data.

Progress toward a video-rate, passive millimeter-wave imager for brownout mitigation

Currently, brownout is the single largest contributor to military rotary-wing losses. Millimeter-wave radiation penetrates these dust clouds effectively, thus millimeter-wave imaging could provide pilots with valuable situational awareness during hover, takeoff, and landing operations. Herein, we detail efforts towards a passive, video-rate imager for use as a brownout mitigation tool. The imager presented herein uses a distributed-aperture, optically-upconverted architecture that provides real-time, video-rate imagery with minimal size and weight. Specifically, we detail phenomenology measurements in brownout environments, show developments in enabling component technologies, and present results from a 30-element aperiodic array imager that has recently been fabricated.

94 GHz millimetre-wave imaging system implementing optical upconversion

Millimeter wave (mmW) imaging is continually being researched for its applicability in all weather imaging. While previous accounts of our imaging system utilized Q-band frequencies (33-50 GHz), we have implemented a system that now achieves far-field imaging at W-band frequencies (75-110 GHz). Our mmW imaging approach is unique due to the fact that optical upconversion is used as the method of detection. Optical modulators are not commercially available at W-band frequencies; therefore, we have designed our own optical modulator that functions at this frequency range. Imaging at higher frequencies increases our overall resolution two to three times over what was achieved at Q-band frequencies with our system. Herein, we present imaging results obtained using this novel detector setup, as well as key imager metrics that have been experimentally validated.