Search

Segmented, or slow-wave electrodes have emerged as an index-matching solution to improve bandwidth of traveling-wave Mach Zehnder and phase modulators on the thin-film lithium niobate on insulator platform. However, these devices require the use of a quartz handle or substrate removal, adding cost and additional processing. In this work, a high-speed dual-output electro-optic intensity modulator in the thin-film silicon nitride and lithium niobate material system that uses segmented electrodes for RF and optical index matching is presented. The device uses a silicon handle and does not require substrate removal. A silicon handle allows the use of larger wafer sizes to increase yield, and lends itself to processing in established silicon foundries that may not have the capability to process a quartz or fused silica wafer. The modulator has an interaction region of 10 mm, shows a DC half wave voltage of 3.75 V, an ultra-high extinction ratio of roughly 45 dB consistent with previous work, and a fiber-to-fiber insertion loss of 7.47 dB with a 95 GHz 3 dB bandwidth., Comment: 4 pages, 3 figures

As mobile beam-bandwidth-product requirements accelerate, millimeter-wave (mmW) bands have been opened to telecommunications networks to enable wider channel bandwidths, while Massive Multiple-Input Multiple-Output (mMIMO) technology has been implemented to concurrently address multiple devices at the same frequency from a single base station. Such space-division multiplexing can be combined with spectral multiplexing to enable a very large number of concurrent users, but currently is implemented through computationally intensive digital beamforming networks. We show that a radio-frequency (RF) photonic receiver system, previously shown to be capable of sorting signals into respective spatial-spectral ‘bins’ is further capable, through an injection-locked tunable optical local oscillator (TOLO), of recovering the data upon each signal in the RF scene. The TOLO is combined in free-space with an up-converted optical sideband and the combined optical field impinges upon an array of photodetectors, each corresponding to separate points in k-space, defined by unique combinations of angle-of-arrival (AoA) and carrier frequency. Using this free-space LO insertion, we demonstrate simultaneous recovery of multiple spatially co-located data streams with resilience to interference.

We present a tightly coupled array antenna excited by high-power photodiodes for millimeter wave transmit operation. The design includes thinned photodiode substrates for improved bandwidth performance at high frequency combined with the inherently lightweight, low-loss, and direct feeding architecture afforded by the photonic approach. Circuit and full wave simulations are conducted for array design and compared against the traditional electronic arrays in the literature. A prototype $1 \times 3$ photonic array is developed and characterized to validate the design which achieves a 3 dB ( $VSWR ) radiated power bandwidth of 4.6:1 across 13–60 GHz and maintains a 3 dB bandwidth near 4:1 across scan angles up to 30°. The fabricated array exhibits a 3 dB radiated power bandwidth of 1.8:1 across 30–55 GHz and a maximum measured effective isotropic radiated power of 19.4 dBm at 40 GHz.

Electro-optic modulators are used in a wide variety of photonic systems, and their operation at the 1064 nm wavelength is notable for its applications such as frequency comb generation and optical interconnects. This work demonstrates the first (to the best of our knowledge) thin film lithium niobate electro-optic modulator operating at a wavelength of 1064 nm. The modulator was fabricated from crystal-ion-sliced lithium niobate wafers procured from NanoLN. Electron beam lithography was used to define the waveguides, and laser lithography was used to define the electrodes, forming a ridge waveguide in the lithium niobate, with a width of 900 nm and an etch depth of 90 nm, by inductively coupled plasma etching. The modulator was characterized using a polarization rotation technique, in which two modes of the waveguide were excited and both were shown to exhibit phase modulation. This modulator exhibits a half-wave voltage of 2.73 volts, with a 7 mm long interaction region and 1.91 V-cm modulation efficiency.

As a lightweight and low power alternative to the fiber-based RF-photonic components, photonic integrated circuits (PICs) are well suited for aerospace applications and, in particular, for free-space communications and remote sensing/imaging. In this work we present the design, fabrication, and space radiation testing of high-speed PICs as a means to elevate their technology readiness level (TRL) for avionics. Our efforts focus on improving the efficiency and bandwidth of thin-film lithium niobate and silicon-based devices as well as studying their envrionmental stability for aerospace applications. These devices will be compared between pre- and post-radiation silicon photonic devices for a space environment of near earth orbit.

This article presents a linearized microwave photonic link by means of two parallel Mach-Zehnder modulators (MZMs) and a polarization beam combiner (PBC). The proposed link exhibits significant suppression in third-order nonlinearity, which results in fifth order limited performance. Experiments are conducted and measured results demonstrate 17-dB improvement in spurious-free dynamic range (SFDR), resulting in a fifth order limited SFDR of 124 dB/Hz4/5. An Erbium Doped Fiber Amplifier (EDFA) and a high-power Modified Uni-Traveling Carrier (MUTC) photodiode are used to realize a link gain of 6 dB at 60 mA photocurrent for operation at 1 GHz. LTE signals with various instantaneous bandwidths centered at 1 GHz are delivered by the linearized link where spectral regrowth is dramatically suppressed.

A Kalman filter is applied to the k-space tomography imaging modality for state-space prediction of radio frequency (RF) sources. By seeding the Kaczmarz computed tomography algorithm with Kalman filtered predictions, spatial-spectral reconstruction of the RF environment is accelerated. This technique enables an N/M decrease in reconstruction time, where M is the standard number of iterations required to fully reconstruct the RF image and N is the number of iterations required when the algorithm is seeded with a correct Kalman filter input.

In this manuscript, we demonstrate the first phase modulator using titanium dioxide as a strip loaded waveguide on thin-film lithium niobate. The amorphous titanium dioxide was fabricated using RF reactive sputtering followed by a dielectric lift-off process. Two gold electrodes were deposited next to the waveguide by using the same process. This process eliminates the need for plasma etching or high-temperature vapor deposition methods, opening the door to fabrication on sensitive material systems. A propagation loss of 0.156 dB/mm and V π of 3.2 V at 1550 nm were experimentally measured and reported.

The ability to both spatially and spectrally demultiplex wireless transmitters enables communication networks with higher spectral and energy efficiency. In practice, demultiplexing requires sub-millisecond latency to map the dynamics of the user space in real-time. Here, we present a system architecture, referred to as k-space imaging, which channelizes the radio frequency signals both spatially and spectrally through optical beamforming, where the latency is limited only by the speed of light traversing the optical components of the receiver. In this architecture, a phased antenna array samples radio signals, which are then coupled into electro-optic modulators (EOM) that coherently up-convert these signals to the optical domain, preserving their relative phases. The received signals, now optical sidebands, are transmitted in optical fibers of varying path lengths, which act as true time delays that yield frequency-dependent optical phases. The output facets of the optical fibers form a two-dimensional optical phased array in an arrangement preserving the phases generated by the angle of arrival (AoA) and the time-delay phases. Directing the beams emanating from the fibers through an optical lens produces a two-dimensional Fourier transform of the optical field at the fiber array. Accordingly, the optical beam formed at the back focal plane of the lens is steered based upon the phases, providing the angle of arrival and instantaneous frequency measurement (IFM), with latency determined by the speed of light over the optical path length. We present a numerical evaluation and experimental demonstration of this passive AoA- and frequency-detection capability.

To investigate the viability of on-chip grating couplers as illuminators, we present a III-V detector illuminated with a bi-layer silicon nitride grating coupler. A responsivity of 0.233 A/W was obtained, corresponding to a measured fiber-to-detector efficiency of 40%.

This paper presents an overview and application of thin-film-lithium-niobate (TFLN) modulators that have low drive voltages, i.e., Vπ. Such modulators are critical components for realizing high-speed operation in a modern telecommunication networks, wireless communications, and RF-Photonic applications. Recent developments in crystalion- slice TFLN have enabled a new class of electro-optic modulators that have a tighter mode confinement, compact footprint, ultra-high bandwidth, and low modulating voltages. However, lithium-niobate suffers from difficult microstructuring in comparison to silicon-based materials, since it can have an etch resistance greater than many metal-based hard masks. To overcome this challenge, a hybrid material system combining the electro-optic properties of TFLN with the ultra-low propagation loss of silicon nitride has been developed. In this work, we demonstrate an integrated hybrid phase modulator, based on a silicon-nitride strip loaded waveguide on a TFLN material platform, which provides tight optical mode confinement, without the need to etch lithium-niobate. As a result, the drive electrodes can be placed closer to the optical waveguide thereby resulting in a strong RF and optical mode overlap. A 2.4 cm long phase modulator and Mach-Zehnder modulator with a demonstrated Vπ of 1.5 V and 0.875 V, respectively in DC are presented along with other candidate and demonstrated devices, such as multimode interference coupler, micro-ring, and racetrack resonator.

A small footprint, low voltage and wide bandwidth electro-optic modulator is critical for applications ranging from optical communications to analog photonic links, and the integration of thin-film lithium niobate with photonic integrated circuit (PIC) compatible materials remains paramount. Here, a hybrid silicon nitride and lithium niobate folded electro-optic Mach Zehnder modulator (MZM) which incorporates a waveguide crossing and 3 dB multimode interference (MMI) couplers for splitting and combining light is reported. This modulator has an effective interaction region length of 10 mm and shows a DC half wave voltage of roughly 4.0 V, or a modulation efficiency (Vπ ·L) of roughly 4 V·cm. Furthermore, the device demonstrates a power extinction ratio of roughly 23 dB and shows .08 dB/GHz optical sideband power roll-off with index matching fluid up to 110 GHz, with a 3-dB bandwidth of 37.5 GHz.

In this paper, directional filters (DFs) are first proposed to improve the stability of multichip modules (MCMs), and a new integration approach between microstrip line on liquid crystal polymer (LCP) substrate and electrooptic (EO) modulator on lithium niobate substrate is presented, together enabling an EO front-end receiving MCM at 95 GHz for passive millimeter-wave (mmW) imaging with large distributed aperture. The DFs can efficiently pick the spectrum of interest with low reflection and drop the undesired out-of-band power to the additional ports, thus reducing the possibility of oscillation. With the above and some other integration and packaging technologies, the multilayer LCP-based MCM achieves a high gain of more than 63 dB, a 3-dB bandwidth of 4.7 GHz centered at 95 GHz, and a low noise figure of less than 6 dB within this spectrum, which was characterized using both radio frequency and optical measurement approaches. The packaged MCM has been successfully utilized in passive mmW imaging and may find lots of applications in other wireless systems.

We present the use of a novel optical modulator architecture to achieve simultaneous radiofrequency (RF) multi-user beamforming and data modulation. Optical vector modulators enable control of the complex state of a transmitted wave by directly and independently controlling both amplitude and phase. Operating in the optical domain affords extremely wide bandwidth and negligible dispersion in such modulators. A demonstration experiment is presented based on a 2-element transmit array, where the vector modulators were assembled from pairs of commercial phase and amplitude modulators. The results of the experiment show independent data directed into two different spatial beams simultaneously, using the same RF carrier.

The coherent optical processing technique, referred to as k -space tomography, enables simultaneous detection of radio waves’ frequency and angle of arrival. As with the majority of signals, the spatial and spectral distribution of RF signals in the electromagnetic environment is sparse in nature. This inherent characteristic provides an ideal opportunity to merge this imaging modality with compressive sensing techniques. Compressive sensing has set a foundation for data acquisition of sparse signals from a small amount of data. By adapting specific aspects from this methodology, and applying them to k -space tomography, compressive k -space tomography is developed. Fundamental performance limits of this compressive sensing application are defined through numerical simulations, and these responses are subsequently utilized in experimental spatial-spectrum utilization mapping for impending 5G bands between 28 and 40 GHz. As a result, reconstruction data size and processing time are reduced consistently by a factor of over 100, without loss of information, for sufficiently sparse RF scenes.

A novel photonic beam-space beam-former concept is reviewed and an additional theory addressing precise sampling of desired beam locations for linear arrays is presented. A new, power efficient, method of apodizing an input array's beam pattern via a weighted distribution of the master feed laser to each RF-to-optical up-conversion stage behind the array elements is then presented along with results from several apodization experiments. The experimental results are shown to produce effective, low-sidelobe responses, while also reducing the required prime input power to the system until an artificial side-lobe floor is reached. The root cause of the side-lobe floor is discovered to be due to phase and amplitude errors introduced within the photonic components currently used. Finally, key findings and design considerations are presented with respect to the usage of photonic beam-space sampling on large arrays and several suggestions for performance improvement in future system designs are discussed.

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 $9\times 12$ 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.

Since their conception in the early 1900s, radar systems have mostly used a single RF beam to probe the environment in search of objects of interest. Traditionally, this is accomplished by scanning the beam over a defined region and using time of flight to determine the range of objects located within the beam. As such, a key figure of merit is the spatial extent, or angular divergence, of the RF beam because it defines the lateral (or cross-range) resolution of detectable objects, while the operational bandwidth (BW) of the radar determines the range resolution of detectable objects. In combination, these two metrics often define the overall capability of the radar system.

A 2D distributed-aperture imaging system paired with a newly developed imaging modality, k-space tomography, is presented for detection of radio waves’ frequency and angle of arrival in azimuthal and elevation directions through coherent optical processing. This technique uses fiber-length dispersion in conjunction with a distributed antenna array to provide unique spatio-temporally encoded charge-coupled device (CCD)-captured interferograms, from which a computational tomographic reconstruction of the RF signal environment at newly allocated 5G bands can be obtained.

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.