The linear electro-optic effect in lithium niobate is capable of realizing a variety of polarization transformations, including TE-to-TM and left-to-right circular polarization conversion. While most LiNbO<sub>3</sub> components are designed for operation in the third telecommunications window around 1.55 μm wavelength, current interest in quantum information processing and atomic physics, where the wavelengths of interest are in the visible and near-infrared, has placed new demands on endless polarization control devices. At short wavelengths, traditional Ti-diffused LiNbO<sub>3</sub> waveguides suffer from photorefractive degradation at optical power levels well below 1 mW. Proton exchanged waveguides have much higher power handling capability but can only guide light polarized parallel to the optic axis, and therefore are not applicable to polarization control. Zinc oxide diffusion is an alternative waveguide fabrication technology that guides both e- and o-waves with much higher power handling capability than Ti:LiNbO<sub>3</sub> waveguides. ZnO:LiNbO<sub>3</sub> waveguides exhibit a highly circular mode field with lower anisotropy than Ti-diffused waveguides. We report on the modeling, fabrication and testing of a polarization controller in ZnO-doped, x-cut lithium niobate operating at a wavelength of 780 nm.
This paper reports on the development of thin film lithium niobate (TFLN™) electro-optic devices at SRICO.
TFLN™ is formed on various substrates using a layer transfer process called crystal ion slicing. In the ion slicing
process, light ions such as helium and hydrogen are implanted at a depth in a bulk seed wafer as determined by the
implant energy. After wafer bonding to a suitable handle substrate, the implanted seed wafer is separated (sliced) at the
implant depth using a wet etching or thermal splitting step. After annealing and polishing of the slice surface, the
transferred film is bulk quality, retaining all the favorable properties of the bulk seed crystal. Ion slicing technology
opens up a vast design space to produce lithium niobate electro-optic devices that were not possible using bulk
substrates or physically deposited films. For broadband electro-optic modulation, TFLN™ is formed on RF friendly
substrates to achieve impedance matched operation at up to 100 GHz or more. For narrowband RF filtering functions,
a quasi-phase matched modulator is presented that incorporates domain engineering to implement periodic inversion
of electro-optic phase. The thinness of the ferroelectric films makes it possible to in situ program the domains, and thus
the filter response, using only few tens of applied volts. A planar poled prism optical beam steering device is also
presented that is suitable for optically switched true time delay architectures. Commercial applications of the TFLN™
device technologies include high bandwidth fiber optic links, cellular antenna remoting, photonic microwave signal
processing, optical switching and phased arrayed radar.
This paper reports on the design, fabrication and testing of quasi-phase-matched (QPM) lithium niobate electro-optic
modulators optimized for the 40-60 GHz frequency range. The device used a single-drive, coplanar-waveguide (cpw)
electrode structure that provided a good balance between impedance and RF loss, and a DC Vπ.L product of
approximately 10 V.cm. Ferroelectric domain engineering enabled push-pull operation with a single drive, while
achieving low chirp. A custom developed pulsed poling process was used to fabricate periodic domain QPM structures
in lithium niobate. QPM periods were in the range of 3 mm to 4.5 mm, depending on the design frequency. The pulse
method enabled precise domain definition with a minimum of overpoling. Low-loss diffused optical waveguides were
fabricated by an annealed proton exchange (APE) process. By operating in both co-propagating and counter-propagating
modes, the QPM devices can be used to implement dual band RF bandpass filters simultaneously covering both 10-20
GHz and 40-60 GHz frequency bands. Arrays of QPM device structures demonstrated in this work form the basis for a
reconfigurable RF photonic filter. The RF photonic QPM technology enables efficient concurrent antenna remoting and
filtering functionality. Applications of the technology include fiber radio for cellular access and finite impulse response
filters for wideband electronic warfare receivers.
The prism-based electro-optic beam deflector is a well-known technology dating back several decades. The primary
factor that has inhibited its wide-spread application is the need for high control voltages - typically around 1,000V per
degree of scanning for a device fabricated in bulk lithium niobate. We have used crystal ion slicing of lithium niobate to
realize a beam deflector with an order-of-magnitude higher deflection sensitivity. We have demonstrated 1x5 switching
of near-infrared light with a voltage swing of only +/-75V. While the optimal design of bulk deflectors is well
established, the thin-film geometry requires careful consideration of the crucial factors of light coupling efficiency and
control of beam divergence. This paper will discuss design issues for integrated 1xN switches based on this technology
and their application to implementing a practical true time delay module for phased array systems.
Photonic methods for electric field sensing have been demonstrated across the electromagnetic spectrum from near-DC to millimeter waves, and at field strengths from microvolts-per-meter to megavolts-per-meter. The advantages of the photonic approach include a high degree of electrical isolation, wide bandwidth, minimum perturbation of the incident field, and the ability to operate in harsh environments. <p> </p>Aerospace applications of this technology span a wide range of frequencies and field strengths. They include, at the high-frequency/high-field end, measurement of high-power electromagnetic pulses, and at the low-frequency/low-field end, in-flight monitoring of electrophysiological signals. The demands of these applications continue to spur the development of novel materials and device structures to achieve increased sensitivity, wider bandwidth, and greater high-field measurement capability. <p> </p>This paper will discuss several new directions in photonic electric field sensing technology for defense applications. The first is the use of crystal ion slicing to prepare high-quality, single-crystal electro-optic thin films on low-dielectricconstant, RF-friendly substrates. The second is the use of two-dimensional photonic crystal structures to enhance the electro-optic response through slow-light propagation effects. The third is the use of ferroelectric relaxor materials with extremely high electro-optic coefficients.
We report on photonic crystal electro-optic devices formed in engineered thin film lithium niobate (TFLN™) substrates.
Photonic crystal devices previously formed in bulk diffused lithium niobate waveguides have been limited in performance by the depth and aspect ratio of the photonic crystal features. We have overcome this limitation by implementing enhanced etching processes in combination with bulk thin film layer transfer techniques. Photonic crystal
lattices have been formed that consist of hexagonal or square arrays of holes. Various device configurations have been
explored, including Fabry Perot resonators with integrated photonic crystal mirrors and coupled resonator structures. Both theoretical and experimental efforts have shown that device optical performance hinges on the fidelity and sidewall profiles of the etched photonic crystal lattice features. With this technology, very compact photonic crystal sensors on the order of 10 μm x 10 μm in size have been fabricated that have comparable performance to a conventional 2 cm long bulk substrate device. The photonic crystal device technology will have broad application as a compact and minimally invasive probe for sensing any of a multitude of physical parameters, including electrical, radiation, thermal and chemical.
Pyroelectric thermal detectors are excellent candidates for detection of broadband radiation. Such detectors utilize
permanently poled ferroelectric single crystal lithium tantalate to generate a charge as the crystal heats up by absorbing
radiation. The charge, which results in a current output when connected to an external electrical circuit, is directly
proportional to the rate of change of temperature of the crystal. The fundamental approach toward enhancing pyroelectric
detector response is to form the pyroelectric material into a thin film. An elegant approach for producing bulk quality
thin films of pyroelectric materials is by crystal ion slicing. In this paper, we report on the formation of thin film lithium
tantalate (TFLT™) pyroelectric detector devices using the ion slicing process. The devices incorporate films less than 9
microns thin and feature apertures as large as 5 mm in diameter. To make functional detectors, ion sliced films were
transferred to ceramic carriers in TO-type can test packages. Test results have shown improvement in room temperature
detectivity about 20 times higher than the state-of-the-art lithium tantalate pyroelectric detectors.
Recent advances in the integrated electronic circuit industry have spurred efforts to develop technologies that efficiently integrate optics and electronics on a single Complementary Metal Oxide Semiconductor (CMOS) chip. Such CMOS technologies can significantly increase circuit functionality and performance at low fabrication and system cost, thereby accelerating the trend of significant growth in this area. The new functionality could include optical based sensors, image processing, and intelligent optical read heads for faster and more efficient data sorting and searching. The reliability of such monolithic CMOS based functions would be drastically improved relative to their bulk optic counterparts. In the optical telecommunications industry, short haul fiber links would benefit from low cost, silicon CMOS based photoreceivers. One of the primary challenges facing the designers in implementing CMOS based optoelectronic circuits is opto-electrical conversion efficiency. The poor optical responsivity of silicon leads to a bottleneck in the optical to electrical conversion for CMOS based photodetectors. This can be compensated in part through more efficient receiver electronics. Efforts have been made to provide mixed signal circuit design to analyze CMOS based high performance, low noise, integrated receiver circuits. This paper evaluates the performance analysis of five types of photoreceiver configurations that were designed for specific applications.
A compact, low loss, optical tap technology is critical for the incorporation of optical interconnects into mainstream semiconductor processes. Previously, we introduced a vertical multimode interference effect based tap device that had the potential for very high speed optical to electrical performance in compact, rectangular device geometries, and in CMOS compatible processes. In this work, we report on the fabrication and test of various ridge guide based optical tap structures on silicon substrates. The experimental results compared well with simulations. Optimized guide to substrate optical coupling was varied from 2 dB to 3 dB by insertion of a spacer layer. Substrate-isolated (passive) tap structure loss of less than 0.6 dB at 840nm wavelength was measured. Based on the device geometry, this result indicated actual tap excess losses (energy fraction not collected by the substrate) to be less than 0.3 dB. Overall, the test results confirm the low excess loss, multimode interference operation of the new tap design, and pave the way for integration of the tap structures with CMOS photo detector devices.
A compact, low loss, optical tap technology is critical for the incorporation of optical interconnects into mainstream CMOS processes. A recently introduced multimode interference effect based device has the potential for very high speed performance in a compact geometry and in a CMOS compatible process. For this work, 2-D and 3-D device simulations confirm a low excess optical loss on order of 0.1 dB, and a nominal 40% (2.2 dB) optical coupling into the CMOS circuitry over a wide range of guide to substrate distances. Simulated devices are on the order of 25micrometers in length and as narrow as 1 um. High temperature, hybrid polymer materials used for commercial CMOS inter-metal dielectric layers are targeted for tap fabrication and are incorporated into the models. Low cost, silicon CMOS based processing makes the new tap technology especially suitable for computer multi-chip module and board level interconnects, as well as for metro fiber to the home and desk telecommunications applications.
As silicon CMOS circuit technology is scaled beyond the GHz range, both chipmakers and board makers face increasingly difficult challenges in implementing high speed metal interconnects. Metal traces are limited in density-speed performance due to the skin effect, electrical conductivity, and cross talk. Optical based interconnects have higher available bandwidth by virtue of the extremely high carrier frequencies of optical signals (> 100 THz). For this work, an effort has been made to determine an optimal optical tap receiver design for integration with commercial CMOS processes. Candidate waveguide tap technologies were considered in terms of optical loss, bandwidth, economy, and CMOS process compatibility. A new device, which is based on a variation of the multimode interference effect, has been found to be especially promising. BeamProp simulation results show nearly zero excess optical loss for the design, and up to 70% coupling into a 25 micrometer traveling wave CMOS photodetector device. Single-mode waveguides make the design readily compatible with wavelength multiplexing/demultiplexing elements. Polymer waveguide materials are targeted for fabrication due to planarization properties, low cost, broad index control, and poling abilities for modulation/tuning functions. Low cost, silicon CMOS based processing makes the new tap technology especially suitable for computer chip and board level interconnects, as well as metro fiber-to-the- home/desk telecommunications applications.