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 LiNbO3 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 LiNbO3 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:LiNbO3 waveguides. ZnO:LiNbO3 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.
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.
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.
This paper reports on a novel optical linearized directional coupler modulator in stoichiometric lithium niobate (SLN). The linearized design has important applications in analog and RF communications systems where fiber optic link performance depends critically on the spurious-free dynamic range of the modulator. Newly available SLN has several distinct advantages over the congruently grown crystals commonly used for high speed integrated optic devices, including higher electrooptic coefficient and better ferroelectric properties. The higher electrooptic coefficient yields lower drive voltage, while the enhanced ferroelectric properties enable better velocity-matched electrode structures using domain inverted waveguides. This paper addresses the operation of the linearized directional coupler design, and the critical advantages of the SLN substrate for implementing high-speed operation using velocity-matching.
SRICO has developed a revolutionary approach to physiological status monitoring using state-of-the-art optical chip technology. The company’s patent pending Photrode is a photonic electrode that uses unique optical voltage sensing technology to measure and monitor electrophysiological parameters. The optical-based monitoring system enables dry-contact measurements of EEG and ECG signals that require no surface preparation or conductive gel and non-contact measurements of ECG signals through the clothing. The Photrode applies high performance optical integrated circuit technology, that has been successfully implemented in military & commercial aerospace, missile, and communications applications for sensing and signal transmission. SRICO’s award winning Photrode represents a new paradigm for the measurement of biopotentials in a reliable, convenient, and non-intrusive manner. Photrode technology has significant applications on the battlefield for rapid triage to determine the brain dead from those with viable brain function. An ECG may be obtained over the clothing without any direct skin contact. Such applications would enable the combat medic to receive timely medical information and to make important decisions regarding identification, location, triage priority and treatment of casualties. Other applications for the Photrode include anesthesia awareness monitoring, sleep medicine, mobile medical monitoring for space flight, emergency patient care, functional magnetic resonance imaging, various biopotential signal acquisition (EMG, EOG), and routine neuro and cardio diagnostics.
This paper describes a paradigm shift in the technology for sensing electro-physiological signals. In recent years, SRICO has been developing small lithium niobate photonic electrodes, otherwise called "Photrodes” for measuring EEG and ECG signals. These extrinsic fiber-optic sensing devices exploit the extremely high electrical input impedance of Mach-Zehnder Intensity (MZI) electro-optic modulators to detect microvolt and millivolt physiological signals. Voltage levels associated with electrocardiograms are typically on the order of several millivolts, and such signals can be detected by capacitive pickup through clothing, i.e., the Photrode may be used in a non-contact mode. Electroencephalogram signals, which typically have an amplitude of several microvolts, require direct contact with the skin. However, this contact may be dry, eliminating the need for conductive gels. The electrical bandwidth of this photonic electrode system stretches from below 0.1 Hz to many tens of kHz and is constrained mainly by the signal processing electronics, not by the Photrode itself. The paper will describe the design and performance of Photrode systems and the challenging aspects of this new technology.