Matthew R. Konkol

Thin LiNbO 3 on insulator electro-optic modulator

This Letter presents a method for the fabrication and integration of a thin LiNbO3 substrate with a Si handle wafer. An inverted ridge structure guides a single optical mode in an electro-optic modulator fabricated on a mechanically thinned substrate. To define an optical waveguide, a ridge structure is first patterned on a 500 μm thick X-cut LiNbO3 wafer; then a low dielectric constant adhesive layer is used to bond the etched LiNbO3 to Si. The LiNbO3 is mechanically thinned to 4 μm, and planar electrodes are patterned. Experimental results demonstrating modulation with a V(π)L of 7.1 V-cm were shown, optical loss was low enough, and film quality high enough, to enable an interaction length of 0.8 cm.

High-Power Photodiode-Integrated-Connected Array Antenna

We present a novel optical feeding technique to achieve efficient excitation of an ultrawideband-connected array (CA) antenna. The passive fiber optic feed allows for preservation of the theoretical bandwidth and low profile of elementary connected dipole elements. In order to improve effective radiated power, high-power charge compensated modified unitravelling carrier photodiodes are integrated into an antenna array for the first time. Circuit and full-wave simulations, which include all required antenna and feed components, are conducted for the optimization of the arrays performance. A 9 × 12 element CA is populated with a 1-D array of four photodiode-integrated active elements to demonstrate the concept. The optically fed array is confirmed experimentally to have a 3-dB bandwidth of approximately 7–17 GHz, in good agreement with simulations.

Optically Upconverted, Spatially Coherent Phased-Array-Antenna Feed Networks for Beam-Space MIMO in 5G Cellular Communications

The densification of cellular networks and their soon-to-increase operational frequency is forcing new topological considerations in 5G networks. In particular, networks that enable extreme spatial discrimination are being considered as a means to significantly increase data capacity by realizing spatial-spectral channels that offer frequency reuse without co-channel or adjacent-channel interference. In this paper, we present a new approach to realizing such a capability based on optically upconverted, spatially coherent phased-array feed networks. The details of our approach, presented herein, include the design and initial demonstration of both transmit (Tx) and receive (Rx) array systems that are used in tandem to form a down-/up-link for the purpose of characterizing both array and link performance. Design parameters and initial link characterization results are presented.

Optically addressed ultra-wideband connected array antenna

Modern RF antenna systems are being asked to address many simultaneous and pressing challenges, e.g., wide operational bandwidth, dynamic gain patterns, and conformal profiles. One way to address these problems is to develop a low profile and ultra-wideband (UWB) phased array antenna. However, the design, fabrication, and integration of such an array using an all-RF feed is exceedingly difficult. Thus, in this paper we present a novel optical feeding technique to achieve efficient excitation of a UWB connected array (CA) antenna. By feeding the array optically, preservation of the theoretical bandwidth and low-profile of elementary connected dipole elements is enabled. Coupling of light to a photodiode merely requires enough space to firmly secure a fiber ferrule, allowing for the population of more densely packed arrays and wider operational bandwidth. Additionally, the optical feeding of the array can provide low noise excitation of the radiating elements, which supports high fidelity beam steering of independent signals over the arrays ultra-wide bandwidth. Currently all of these abilities are unattainable by conventional electronic feeding networks. Previously the main limiting factor for the realization of such an optical system was the low power handling capability of the photodiode at the antenna excitation point. Recently, however, modified uni-travelling carrier (MUTC) photodiodes, flip-chip bonded to high-thermal conductivity aluminum nitride (AlN), have been able to achieve output powers of over 1 W at 10 GHz under CW operation, and over 10 W using pulsed power modulation. A prototype MUTC photodiode-integrated antenna array on AlN with direct fiber feed to each antenna element is discussed and demonstrated that can provide 5-20 GHz bandwidth and a size, weight, and power (SWaP) superior to conventional electronic phased array systems.

Progress towards dual vertical slot modulator for millimeter wave photonics

Dual vertical slot modulators leverage the field enhancement provided by the continuity of the normal electric flux density across a boundary between two dielectrics to increase modal confinement and overlap for the propagating optical and RF waves. This effect is achieved by aligning a conventional silicon-based optical slot waveguide with a titanium dioxide RF slot. The TiO2 has an optical refractive index lower than silicon, but a significantly higher index in the RF regime. The dual slot design confines both the optical and RF modes to the same void between the silicon ribs of the optical slot waveguide. To obtain modulation of the optical signal, the void is filled with an organic electro optic material (OEOM), which offers a high optical non-linearity. The optical and RF refractive index of the OEOM is lower than silicon and can be deposited through spin processing. This design causes an extremely large mode overlap between the optical field and the RF field within the non-linear OEOM material which can result in a device with a low Vπ and a high operational bandwidth. We present work towards achieving various prototypes of the proposed device, and we discuss the fabrication challenges inherent to its design.

Photonic Tightly Coupled Array

We present the first demonstration of a tightly coupled array (TCA) excited by high-power photodiodes directly integrated onto the antenna substrate. As a complex electrical feed network is not required to feed the radiating elements, the design can realize ultra-wide bandwidth while improving upon the size, weight, and power of conventional electronic-based arrays. Circuit and full-wave simulations are conducted for optimization of array performance and compared against all-electronic TCA designs within the literature. A prototype

Dual slot modulator for millimeter wave photonics

9\times 12

High-Power, Aperture Coupled Photonic Antenna

element TCA with four photodiode-integrated active elements is characterized to validate the design process. The photonic array exhibits high radiation efficiency between 5 and 20 GHz, and a maximum effective isotropic radiated power of 25 dBm at 13 GHz is measured in the far field.

Millimeter-wave photonic tightly coupled array for 5G applications

Silicon slot waveguides leverage the field enhancement provided by the continuity of normal electric flux density across a dielectric boundary to confine an optical mode to a void between two proximal silicon strips. Silicon-organic hybrid slot modulators make use of this mode profile by infiltrating the slot region with a non-linear organic electro-optic material (OEOM) for modulation. The dual slot modulator takes this idea a step further by similarly confining a propagating RF mode to the same slot region to increase modal overlap for improved modulation efficiency. This effect is achieved by aligning a titanium dioxide RF slot along a conventional silicon slot waveguide. The TiO2 has an optical refractive index lower than silicon, but a significantly higher index in the RF regime. As a result of the large modal overlap and high electro-optic activity of the OEOM this design can produce measured phase modulated VπL of less than 1.40 V•cm. Furthermore, as the modulator operates without the introduction of a doping scheme it can potentially realize high operational bandwidth and low loss. We present work towards achieving various working prototypes of the proposed device and progress towards high frequency operation.

Low profile RF feed networks using RF photonics

A high-power charge-compensated modified uni-travelling carrier photodetector is directly integrated into an aperture coupled patch antenna. The coupling technique offers not only good isolation between feed and radiating patch substrates but also wide operational bandwidth. The antenna is developed to operate at 22 GHz with 3-dB relative bandwidth of ~20%, over which the measured effective isotropic radiated power approaches 31 dBm. The photonic antenna is integrated with a lightweight, low form factor fiber-optic feed that demonstrates potential for future wireless communications applications. The antenna’s electrical and radiation characteristics are observed to be in good agreement with simulations.