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.
We have demonstrated a new amplifier topology for coplanar-grid detectors that provides true differential readout with a single, fully-differential, charge-sensitive preamplifier. A prototype, multi-detector system with adjustable gain for each detector has been demonstrated using this fully differential approach. In its initial implementation using general-purpose amplifier chips, this system produces comparable noise performance to the traditional two-amplifier readout with specialized, charge-sensitive preamplifiers. In lieu of a differential gain to correct for electron trapping, the circuit uses a differential sampling scheme. This method enables a symmetrical photopeak to be obtained, but introduces some undesired filtering that limits energy resolution.
We have used room temperature photoluminescence to correlate radiation detector performance with Cadmium Zinc Telluride (CdZnTe) material quality in previous work. Enhancements in the experimental apparatus and the analysis software now allow us to achieve higher spatial resolution along with PL lineshape analysis. We have examined several CdZnTe crystals previously characterized as radiation detectors with the new apparatus and report on the results of this analysis method are presented along with experimental results.
We discuss two simple, computationally efficient methods of estimating pulse height spectra for semiconductor detectors, one based on analytical techniques and the other involving Monte Carlo simulation. The former method gives rapid insight into the impact of major material parameters on detector performance, while the latter yields reasonable realistic spectra incorporating all major effects. We use both techniques in conjunction with a simulated annealing algorithm to extract electron and hole transport parameters from measured spectra.
Cadmium zinc telluride (CZT) gamma-ray detectors show great potential in medical and nuclear material imaging applications. These imagers rely on pixellated arrays of CZT for their operation. Systematic and random material variation among the pixels can introduce noise into the system and makes data analysis more complicated. Photo induced transient current spectroscopy (PICTS), and low temperature photoluminescence have been employed to analyze 4 by 4 pixellated arrays and to determine material variation among the pixels. Particular pixels that have proven to perform well or poorly have been studied in detail to try and identify the origin of this performance variation. This paper reports preliminary results and comments on future analysis.
More recent Russian grown single crystals of Cd<SUB>0.8</SUB>Zn<SUB>0.2</SUB>Te (CZT) were evaluated using proton induced x ray emission (PIXE), x ray diffraction (XRD), photoluminescence (PL), infra red (IR) transmission microscopy, leakage current measurements and response to nuclear radiation. Whereas in the past the Russian grown samples were not acceptable for gamma ray detectors application, the present samples had a somewhat better crystallinity and a higher resistivity, and did even show distinct photopeaks for an <SUP>241</SUP>Am spectrum. Differences in the material properties between various Russian (n- and p-type) and U.S. (n-type) CZT are described.
We have applied several techniques, including photoluminescence, proton-induced x-ray emission, photocurrent, and alpha particle response mapping, for mapping micron- and millimeter-scale variations in cadmium zinc telluride. We have correlated the degree of inhomogeneity determined by these techniques with performance of gamma-ray spectrometers fabricated from the material.
This course explains the underlying physics, device structures, fabrication technology and applications of devices based on the linear electro-optic and second-order nonlinear optical effects in lithium niobate. Practical calculations using modern modeling and simulation tools will be emphasized.