Simulated results demonstrate the impact of detector noise on the design of a broadband wavefront coded optical
system. We conclude that noise must be included as a first-order design constraint in cubic phase-encoded
systems.
Sandia has designed and prototyped a monocular for the use in a head-mounted system. The all-reflective design approach utilized freeform and aspheric surfaces to surpass the performance available from more conventional reflective designs. The prototyped design demonstrated and validated the design approach, mirror fabrication process, and alignment of the system. The system exhibited a magnification of 6.6× , a field-of-view of 4.5 deg, and an excellent image quality.
Sandia has developed an optical design for wearable binoculars utilizing freeform surfaces and switchable mirrors. The
goals of the effort included a design lightweight enough to be worn by the user while providing a useful field of view and
magnification as well as non-mechanical switching between normal and zoomed vision. Sandia’s approach is a four
mirror, off-axis system taking advantage of the weight savings and chromatic performance of a reflective system. The
system incorporates an electrochromic mirror on the final surface before the eye allowing the user to switch between
viewing modes. Results from a prototype of a monocular version with 6.6x magnification will be presented. The
individual mirrors, including three off-axis aspheres and one true freeform, were fabricated using a diamond-turning
based process. A slow-slide servo process was used for the freeform element. Surface roughness and form measurement
of the freeform mirror will be presented as well as the expected impact on performance. The alignment and assembly
procedure will be reviewed as well as the measured optical performance of the prototype. In parallel to the optical
design work, development of an electrochromic mirror has provided a working device with faster switching than current
state of the art. Switchable absorbers have been demonstrated with switching times less than 0.5 seconds. The
deposition process and characterization of these devices will be presented. Finally, details of an updated optical design
with additional freeform surfaces will be presented as well as plans for integrating the electrochromic mirror into the
system.
The angular sensitivity of guided mode resonant filters (GMRF) is well known. While at times useful for angle tuning of
the response, this sensitivity can also be a major detriment as angular changes of tenths of a degree can shift the
wavelength response in a narrow bandwidth device by an amount greater than the width of the resonance peak. We
identify geometries where the resonance is more angularly stable, demonstrating high reflectivity at the design
wavelength for several degrees in both azimuth and inclination angular directions with virtually no change in lineshape of the response. The investigation of GMRFs in both classical and conical mounts through simulation using rigorous coupled wave analysis reveals that there are preferred mounts for greater angular tolerance. We simulate a grating at telecom wavelengths using a design that we have previously fabricated. The identical grating placed in different mounts can exhibit angular tolerances that differ by well over an order of magnitude (60x). The most commonly used classical mount has a much more sensitive angular tolerance than does the conical mount. The lineshape of the resonant response shows only negligible changes across the angular band. The angular band for the sample grating is simulated to be several degrees in the conical mount as opposed to a tenth of a degree in the classical mount. We could thus expand the application space for narrow-band GMRFs into areas where angular tolerance cannot be controlled to the degree that we have believed required in the past.
Designing and integrating micro-optical components into atom and ion traps are enabling steps toward miniaturizing
trap dimensions in quantum computation applications. The micro-optic must have a high numerical aperture for precise
illumination of the ion and should not introduce scatter. Due to the extreme optical efficiency requirements in trapped
ion and atom-based quantum information processing, even slight losses from integrated micro-optics are detrimental.
We have designed and fabricated aspheric micro-lenses through grayscale transfer into a fused silica in an effort to
realize increased transmissive efficiency and decreased scatter compared to an equivalent diffractive optical element.
The fabricated grayscale lens profile matched the desired lens profile well, and the measured and predicted optical
performances were in good agreement. The pattern was transferred via coupled plasma reactive-ion etching smoothly
into the fused silica with a RMS roughness ~ 35 nm. The micro-lens had a diameter of 88 um and 14.2 um sag, with an
as-designed focal length of 149 um and spot diameter of 2.6 um. The maximum measured efficiency was ~80% (86% of
theoretical, possibly due to rms roughness). This realized efficiency is superior to the equivalent diffractive lens
efficiency, designed to the same use parameters. The grayscale approach demonstrated an increase in collection
efficiency, at the desired optical focal length, providing the potential for further refinement.
Conductive polymers with high solids loading (> 40wt.%) are challenging to pattern to single micron feature sizes and require unique techniques or templates to mold the material. The development of a conductive polymer optical tag is discussed for identifying the presence of hydrofluoric acid (HF) and leverages free standing silicon fins as a template utilizing deep reactive ion etching (DRIE) techniques will be discussed. This work is aimed towards a future flexible conductive polymer tag to be transferred via adhesive or epoxy for a novel sensor surface. The advantage to this technique over wafer thinning is a higher throughput of device manufacture without damage to the silicon fins or polymer due to chemical-mechanical interactions or added protective layers. The gratings consist of a high spatial frequency (1.15 μm pitch) grating consisting of lines of conductive polymer and lines of silicon which are free standing. A novel running bond pattern aims to minimize the intrinsic stress and allows the conductive polymer to infiltrate without distorting the template. The polymer conductivity mechanism has been designed to break down under a chemical binding to fluorine; changing its conductivity upon exposure, and results in a change in the polarization response. The use of the polarization response makes the signal more robust to intensity fluctuations in the background or interrogation system. Additionally, the use of optical interrogation allows for standoff detection in instances where hazardous conditions may be present. Examples of material and device responses will be shown and directions for further investigation are discussed.
Resonant subwavelength gratings have been designed and fabricated as wavelength-specific reflectors for application as
a rotary position encoder utilizing ebeam based photolithography. The first grating design used a two-dimensional
layout to provide polarization insensitivity with separate layers for the grating and waveguide. The resulting devices had
excellent pattern fidelity and the resonance peaks and widths closely matched the expected results. Unfortunately, the
gratings were particularly angle sensitive and etch depth errors led to shifts in the center wavelength of the resonances.
A second design iteration resulted in a double grating period to reduce the angle sensitivity as well as different materials
and geometry; the grating and waveguide being the same layer. The inclusion of etch stop layers provided more accurate
etch depths; however, the tolerance to changes in the grating duty cycle was much tighter. Results from these devices
show the effects of small errors in the pattern fidelity. The fabrication process flows for both iterations of devices will be
reviewed as well as the performance of the fabricated devices. A discussion of the relative merits of the various design
choices provides insight into the importance of fabrication considerations during the design stage.
We report on the progress of an optical tag designed to indicate the presence of HF. The approach we followed uses a
high spatial frequency grating consisting of lines of conductive polymer. The conductive polymer has been designed to
be sensitive to HF; changing its conductivity upon exposure. This material change results in a change in the polarization
response of the grating which can be read out remotely using optical techniques. The use of a polarization response
makes the signal more robust to intensity fluctuations in the background or interrogation system. Additionally, the use
of optical interrogation allows for standoff detection in instances where hazardous conditions may be present. A review
of the material development work will be presented as well as the device fabrication efforts. Examples of material and
device responses will be shown and directions for further investigation discussed.
Polarimetric imaging captures the polarization state of light from all the points of a scene.
Snapshot polarimetric imaging collects the Stokes' parameters spatial distribution
simultaneously. We will discuss state-of-the-art achievements and some fundamental
diffraction limitations in polarimetric imaging with an array of micro-components. We will
also look at the natural vision system of the mantis shrimp, with many of these same sensing
abilities. The evolved and exquisite vision system possesses a recently-discovered circular
polarization capability. This comprehensive polarization vision may enable
imaging/communicating advantages in the underwater environment as well as more general
turbid environments such as smoke and fog.
We are developing a highly miniaturized trapped ion clock to probe the 12.6 GHz hyperfine transition in the
171Yb+ ion. The clock development is being funded by the Integrated Micro Primary Atomic Clock
Technology (IMPACT) program from DARPA where the stated goals are to develop a clock that consumes
50 mW of power, has a size of 5 cm3, and has a long-term frequency stability of 10-14 at one month. One of
the significant challenges will be to develop miniature single-frequency lasers at 369 nm and 935 nm and the
optical systems to deliver light to the ions and to collect ion fluorescence on a detector.
We describe the design of pixelated filter arrays for hyperspectral monitoring of CO2 and H2O absorption in the
midwave infrared (centered at 4.25μm and 5.15μm, respectively) using resonant subwavelength gratings (RSGs), also
called guided-mode resonant filters (GMRFs). For each gas, a hyperspectral filter array of very narrowband filters is
designed that spans the absorption band on a single substrate. A pixelated geometry allows for direct registration of
filter pixels to focal plane array (FPA) sensor pixels and for non-scanning data collection. The design process for
narrowband, low-sideband reflective and transmissive filters within fabrication limitations will be discussed.
We present the design, fabrication, and characterization of a pixelated, hyperspectral arrayed component for Focal
Plane Array (FPA) integration in the Long-Wave IR. This device contains tens of pixels within a single super-pixel
which is tiled across the extent of the FPA. Each spectral pixel maps to a single FPA pixel with a spectral FWHM
of 200nm. With this arrayed approach, remote sensing data may be accumulated with a non-scanning, "snapshot"
imaging system.
This technology is flexible with respect to individual pixel center wavelength and to pixel position within the array.
Moreover, the entire pixel area has a single wavelength response, not the integrated linear response of a graded
cavity thickness design. These requirements bar tilted, linear array technologies where the cavity length
monotonically increases across the device.
Resonant subwavelength gratings have proven to be excellent devices for producing narrow resonances useful for
filtering applications. In this paper we discuss the use of RSGs in a rotary position encoder intended for use in harsh
environments. To avoid problems with routing electrical signals to the encoder, a single fiber optic connection is used to
address the device with multiplexed wavelengths corresponding to position bits. Each wavelength has a corresponding
RSG that is patterned in the appropriate position locations. A demonstration device utilizing RSGs with TiO2 and SiO2
films on a silicon substrate will be presented. The design and modeling effort provided several RSGs with resonances
addressable by a single tunable laser source. Since multimode fiber is used to route the optical signals, the gratings were
designed to be polarization insensitive. Additionally, the individual RSGs accommodate significant wavelength shifts to
simplify the integration of the encoder system. The fabrication of the devices was based on electron beam lithography
and details of this work will be presented. Measurements of the individual RSGs as well as a demonstration of the
determination of rotary position using these gratings will be shown.
In this work, we describe the most recent progress towards the device modeling, fabrication, testing and system
integration of active resonant subwavelength grating (RSG) devices. Passive RSG devices have been a subject of
interest in subwavelength-structured surfaces (SWS) in recent years due to their narrow spectral response and high
quality filtering performance. Modulating the bias voltage of interdigitated metal electrodes over an electrooptic thin
film material enables the RSG components to act as actively tunable high-speed optical filters. The filter characteristics
of the device can be engineered using the geometry of the device grating and underlying materials.
Using electron beam lithography and specialized etch techniques, we have fabricated interdigitated metal electrodes on
an insulating layer and BaTiO3 thin film on sapphire substrate. With bias voltages of up to 100V, spectral red shifts of
several nanometers are measured, as well as significant changes in the reflected and transmitted signal intensities around
the 1.55um wavelength.
Due to their small size and lack of moving parts, these devices are attractive for high speed spectral sensing applications.
We will discuss the most recent device testing results as well as comment on the system integration aspects of this
project.
In this paper, we describe progress towards a multi-color spectrometer and radiometer based upon an active resonant subwavelength grating (RSG). This active RSG component acts as a tunable high-speed optical filter that allows device miniaturization and ruggedization not realizable using current sensors with conventional bulk optics. Furthermore, the geometrical characteristics of the device allow for inherently high speed operation. Because of the small critical dimensions of the RSG devices, the fabrication of these sensors can prove challenging. However, we utilize the state-of-the-art capabilities at Sandia National Laboratories to realize these subwavelength grating devices. This work also leverages previous work on passive RSG devices with greater than 98% efficiency and ~1nm FWHM.
Rigorous coupled wave analysis has been utilized to design RSG devices with PLZT, PMN-PT and BaTiO3 electrooptic thin films on sapphire substrates. The simulated interdigitated electrode configuration achieves field strengths around 3×107 V/m. This translates to an increase in the refractive index of 0.05 with a 40V bias potential resulting in a 90% contrast of the modulated optical signal. We have fabricated several active RSG devices on selected electro-optic materials and we discuss the latest experimental results on these devices with variable electrostatic bias and a tunable wavelength source around 1.5μm. Finally, we present the proposed data acquisition hardware and system integration plans.
Research into the use of multiframe superresolution has led to the development of algorithms for providing
images with enhanced resolution using several lower resolution copies. An integral component of these
algorithms is the determination of the registration of each of the low resolution images to a reference
image. Without this information, no resolution enhancement can be attained. We have endeavored to find
a suitable method for registering severely undersampled images by comparing several approaches. To test
the algorithms, an ideal image is input to a simulated image formation program, creating several
undersampled images with known geometric transformations. The registration algorithms are then applied
to the set of low resolution images and the estimated registration parameters compared to the actual values.
This investigation is limited to monochromatic images (extension to color images is not difficult) and only
considers global geometric transformations. Each registration approach will be reviewed and evaluated
with respect to the accuracy of the estimated registration parameters as well as the computational
complexity required. In addition, the effects of image content, specifically spatial frequency content, as
well as the immunity of the registration algorithms to noise will be discussed.
Conductive polymers have become an extremely useful class of materials for many optical applications.
We have developed an electrochemical growth method for depositing highly conductive (~100 S/cm)
polypyrrole. Additionally, we have adapted advanced fabrication methods for use with the polypyrrole
resulting in gratings with submicron features. This conductive polymer micro-wire grid provides an optical
polarizer with unique properties. When the polymer is exposed to ionizing radiation, its conductivity is
affected and the polarization properties of the device, specifically the extinction ratio, change in a
corresponding manner. This change in polarization properties can be determined by optically interrogating
the device, possibly from a remote location. The result is a passive radiation-sensitive sensor with very low
optical visibility. The ability to interrogate the device from a safe standoff distance provides a device
useful in potentially dangerous environments. Also, the passive nature of the device make it applicable in
applications where external power is not available. We will review the polymer deposition, fabrication
methods and device design and modeling. The characterization of the polymer's sensitivity to ionizing
radiation and optical testing of infrared polarizers before and after irradiation will also be presented. These
experimental results will highlight the usefulness of the conductive infrared polarizer to many security and
monitoring applications.
Polarimetric imaging applications at the 2-to-5-µm or midwave infrared range use large pixel-count focal plane arrays (FPAs) with small pixel sizes. We report on the design, fabrication, and characterization of micropolarizers for the 2-to-5-µm regime. These micropolarizers will be used in conjunction with a FPA and will be in the near field of the imaging device. The pixel pitches for some commercial FPAs are small enough that the finite apertures of the polarizing devices may significantly affect their performance, because their aperture dimension varies between three and five waves. We are interested in understanding the effect on the extinction ratio due to variations in the edge terminations of a polarizer with a small aperture. Edge terminations are the spaces between the first and last wire with the perimeter of the aperture of the polarizer. To verify these effects, we fabricated micropolarizers with apertures of 5 to 20 µm and with termination edge spaces of one-quarter and three-quarters of the wiregrids period. The devices measured extinction ratios from 50:1 for the smallest aperture to 200:1 for the largest. Simulations and measurements show that the extinction ratio is larger for the smaller termination edge spacing.
Remote detection of radiation is a difficult problem due to the 1/r2 fall-off. Recent advances in polymer
research and nanoscale fabrication methods along with advances in optical polarimetric remote sensing
systems suggest a solution. The basic device uses a micro-wiregrid infrared polarizer fabricated in
conductive polymer. When the polymer is exposed to hard radiation, its conductivity will be affected and the
polarization properties of the device will change in a corresponding manner. This change in polarization
properties can be determined by optically interrogating the device, possibly from a remote location. We will
report on the development of a radiation-sensitive passive dosimeter polymer with very low optical visibility.
Progress on material development, lithographic fabrication and optical characterization will be presented.
We report on the design, fabrication, and simulation of a four-state pixelated subwavelength optical device that
enables mid-wave infrared (MWIR) or long-wave infrared (LWIR) snapshot polarimetric imaging. The
polarization information can help to classify imaged materials and identify objects of interest for remote sensing
and military applications. The fabricated pixelated polarizers have measured extinction ratios larger than 100:1
for pixel sizes greater than 9 microns by 9 microns, with transmitted signals greater than 50%. That exceeds, by
7 times, previously reported device extinction ratios for 15 micron by 15 micron pixels.
Traditionally, sequential polarimetric imaging sensors produce scenes with polarization information through a
series of assembled images. Snapshot polarimetric imaging collects the spatial distribution of all four Stokes'
parameters simultaneously. In this way any noise due to scene movement from one frame to the next is
eliminated.
In this paper, we will quantify near-field and diffractive effects of the finite pixel apertures upon detection. We
have designed and built an experimental setup that models a pixel within a focal plane array (FPA) to measure
crosstalk from adjacent gold wiregrid micropolarizers. This configuration simulates a snapshot polarization
imaging device where the two substrates are stacked; micropolarizer array substrate on top of an FPA. Modeling
and measured data indicate crosstalk between the adjacent pixels up to a few microns behind the polarizer plane.
Crosstalk between adjacent pixels increases uncertainty in the measured polarization states in a scene of interest.
Measured and simulated data confirm that the extinction ratio of a micropolarizer pixel in a super-cell will be
reduced by 17% when moving the FPA from 0.5 microns to 1.0 microns away from the polarizer. These
changes in extinction ratio are significant since typical glue separation is on the order of 10 microns.
We have numerically and experimentally determined the effect of crosstalk from adjacent gold wiregrid
micropolarizer pixels in a midwave infrared (MWIR) focal plane array (FPA). Fabrication of a snapshot
polarization-imaging device involves gluing a micropolarizer array substrate on top of an FPA. We evaluated
several arrays of super-cells of four pixelated polarizers by modeling the near fields behind the devices. Each
polarizer in the super-cell is oriented to allow solving three Stokes parameters by themselves or four Stokes
parameters in conjunction with a birefringent waveplate. In addition, we fabricated sets of super-cells for
determining optimum polarizer-FPA separation. Modeling and empirical data indicate cross talk between the
adjacent pixels at several microns after crossing the polarizer plane. Cross talk between adjacent pixels increases
uncertainty in the measured polarization states of a scene of interest. Data shows that the extinction ratio will
decrease by 17% when moving the FPA from 0.5 &mgr;m to 1.0 &mgr;m away from the polarizer. These changes in
extinction ratio are important given that typical glue separation is approximately 10 &mgr;m.
We present the design and initial fabrication for a wavelength-agile, high-speed modulator that enables a long-term vision for the THz Scannerless Range Imaging (SRI) sensor. This modulator takes the place of the currently utilized SRI micro-channel plate which is limited to photocathode sensitive wavelengths (primarily in the visible and near-IR regimes).
The new component is an active Resonant Subwavelength Grating (RSG). An RSG functions as an extremely narrow wavelength and angular band reflector, or mode selector. Theoretical studies predict that the infinite, laterally-extended RSG can reflect 100% of the resonant light while transmitting the balance of the other wavelengths. Previous experimental realization of these remarkable predictions has been impacted primarily by fabrication challenges. Even so, we have demonstrated large-area (1.0mm) passive RSG reflectivity as high as 100.2%, normalized to deposited gold. In this work, we transform the passive RSG design into an active laser-line modulator.
Conductive polymers have become an extremely useful class of materials for many optical applications.
Additionally, advanced fabrication methods have led to the development of metal based micro-wiregrid
polarizers utilizing submicron features. Adapting these fabrication approaches for use with polymer
materials leads to optical polarizers with unique properties. The patterning of conductive polymers with the
small features required for wiregrid polarizers leads to several challenges. First, the deposition of the
polymer must provide a layer thick enough to provide a polarizer with a useful extinction ratio that also has
high conductivity and environmental stability. Two deposition approaches have been investigated, spin
coating and electrochemical growth, and results of this work will be presented. Also, the polymers
considered here are not compatible with basic photoresist processes. Various tactics have been examined to
overcome this difficulty including the use of hard bakes of the polymer, protective overcoats and patterned
growth. The adaptations required for successfully patterning the polymer will be reviewed. Finally,
fabricated devices will be shown and their optical characterization presented.
Subwavelength diffractive features etched into a substrate lead to form birefringence that can produce polarization sensitive elements such as wave plates. Using etched features allows for the development of pixelated devices to be used in conjunction with focal plane arrays in polarimetric imaging systems. Form birefringence exhibits dispersion that can be advantageous to the design of wave plates with an achromatic response. Taking advantage of this dispersion, diffractive wave plates with good achromatic characteristics can be designed for the 2- to 5-µm spectral region. Previous work in this area has produced good results over a subset of this wavelength band, but designing for this extended band is particularly challenging. The fabrication processes for the subwavelength features will be discussed and fabricated devices with a measured average phase retardation of 80.6 deg and rms variation of 9.41 deg will be presented.
Subwavelength diffractive features etched into a substrate lead to form birefringence that can be utilized to produce polarization sensitive elements such as waveplates. Using etched features allows for the development of pixilated devices to be used in conjunction with focal plane arrays in polarimetric imaging systems. Typically, the main drawback from using diffractive devices is their high sensitivity to wavelength. Taking advantage of the dispersion of the form birefringence, diffractive waveplates with good achromatic characteristics can be designed. We will report on diffractive waveplates designed for minimal phase retardation error across the 2-5 micron spectral regime. The required fabrication processes of the sub-wavelength feature sizes will be discussed as well as the achromatic performance and transmission efficiency of final devices. Previous work in this area has produced good results over a subset of this wavelength band, but designing for this extended band is particularly challenging. In addition, the effect of the finite size of the apertures of the pixilated devices is of particular interest since they are designed to be used in conjunction with a detector array. The influence of small aperture sizes will also be investigated.
Polarimetric imaging applications at the 2 to 5 μm or Mid-Wave Infrared (MWIR) range use large pixel-count focal plane arrays (FPA) with small pixel size. This project is centered in designing, fabricating and testing micropolarizers that work in that wavelength regime and intended for that type of FPAs. The micro-polarizers will be used in conjunction with a FPA in snapshot mode and will be in the near field of the imaging device. The pixel pitches for some commercial FPAs are small enough that the finite apertures of the polarizing devices may significantly affect their performance given that their aperture size varies between 3 and 5 waves. We are interested in understanding the effect on extinction ratio due to variations in the edge terminations of a polarizer with a small aperture. Edge terminations are the spaces between the first or last wire with the perimeter of the aperture of the polarizer. While this parameter has negligible effects on a larger polarizer, it will be significant for apertures that are about 3 to 5 waves. We will present data that indicates significant variation in performance due to edge terminations.
We report here on an effort to design and fabricate a polarization splitter that utilizes form-birefringence to disperse an input beam as a function of polarization content as well as wavelength spectrum. Our approach is unique in the polarization beam splitting geometry and the potential for tailoring the polarized beams' phase fronts to correct aberrations or add focusing power. A first cut design could be realized with a chirped duty cycle grating at a single etch depth. However, this approach presents a considerable fabrication obstacle since etch depths are a strong function of feature size, or grating period. We fabricated a period of 1.0 micron form-birefringent component, with a nominal depth of 1.7 microns, in GaAs using a CAIBE system with a 2-inch ion beam source diameter. The gas flows, ion energy, and sample temperature were all optimized to yield the desired etch profile.
This paper presents the passively aligned Wavesetter (PAWS) locker: a micro-optic subassembly for use as an internal wavelength locker. As the wavelength spacing in dense wavelength division multiplexing (WDM) decreases, the performance demands placed upon source lasers increase. The required wavelength stability has led to the use of external wavelength lockers utilizing air-spaced, thermally stabilized etalons. However, package constraints are forcing the integration of the wavelength locker directly into the laser module. These etalons require active tuning be done during installation of the wavelength locker as well as active temperature control (air-spaced etalons are typically too large for laser packages). A unique locking technique will be introduced that does not require an active alignment or active temperature compensation. Using the principles of phase shifting interferometry, a locking signal is derived without the inherent inflection points present in the signal of an etalon. The theoretical background of PAWS locker will be discussed as well as practical considerations for its implementation. Empirical results will be presented including wavelength accuracy, alignment sensitivity and thermal performance.
Guided mode resonant filters (GMRFs) are potentially useful as wavelength selective devices for DWDM telecom and sensor applications. These elements consist ofa high frequency grating deposited on an optical waveguide. In this presentation we investigate several approaches to modeling the characteristics of finite aperture GMRFs. Experimental GMRFs were fabricated with photoresist gratings with different grating characteristics and apertures were deposited on planar waveguides with specific refractive indices and thickness in sol-gel materials. Good agreement between our model prediction and experimental data for the reflection efficiency and spectral bandwidth were found. Limiting the aperture of GMRF below a few millimeters reduces the diffraction efficiency and increases the spectral bandwidth.
Planar optical systems provide an efficient way to integrate multiple optical components into a compact optical substrate. Both lithographically formed surface relief gratings and holographically formed volume gratings have been used to implement diffractive optical elements in planar optic systems. These systems and sub-assemblies are useful for local distribution of optical signals in optical interconnect and other information processing applications. However when signals are propagated over distance greater than 5 - 10 cm alignment, beam diffraction effects, and substrate uniformity become major issues that limit signal fidelity. One approach to solving this problem is to combine free-space planar optic local distribution systems with fiber optic waveguides for long distance signal transfer. This extension provides a framework for integrating data communication and network environments to realize new forms of distributed information processing architectures. Additional signal capacity and effective data transfer rates can be obtained by incorporating wavelength multiplexing and de-multiplexing techniques in the interconnect system. However, diffractive type planar optical elements typically require high spatial frequency gratings that are sensitive to the polarization state of the incident optical beam. This property can have significant impact on the bit error rate of signals transmitted through hybrid fiber/planar optical distribution systems. In this paper we compare the polarization properties of surface relief and volume gratings for planar optic wavelength demultiplexing operations.
The performance of optical interconnects is directly related to the characteristics of the electronic interface between the optical interconnect components and the electronic processing elements. In this paper we examine this issue and determine limits on the power and bandwidth of optical interconnects with low threshold VCSEL sources. For a 0.8 micrometers CMOS process the effects of coupling parasitics and detector capacitance limit signal bandwidth below 500 MHz. The effects of device geometry, fanout, and line length are also studied and show that with existing optoelectronic devices optical interconnects are competitive with electrical connections at the board level, but not below this packaging level.
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