KEYWORDS: Diffusers, Free space optics, Atmospheric optics, Radio optics, Wireless communications, Atmospheric turbulence, Speckle, Signal to noise ratio, Binary data, Data modeling
Wireless communication systems that employ free-space optical links in place of radio/microwave technologies carry substantial benefits in terms of data throughput, network security and design efficiency. Along with these advantages comes the challenge of counteracting signal degradation caused by atmospheric turbulence in free-space environments. A fully coherent laser source experiences random phase delays along its traversing path in turbulent conditions forming a speckle pattern and lowering the received signal-to-noise ratio upon detection. Preliminary research has shown that receiver-side speckle contrast may be significantly reduced and signal-to-noise ratio increased accordingly through the use of a partially coherent light source. While dynamic diffusers and adaptive optics solutions have been proven effective, they also add expense and complexity to a system that relies on accessibility and robustness for successful implementation. A custom Hadamard diffractive matrix design is used to statically induce partial coherence in a transmitted beam to increase signal-to-noise ratio for experimental turbulence scenarios. Atmospheric phase screens are generated using an open-source software package and subsequently loaded into a spatial light modulator using nematic liquid crystals to modulate the phase.
Waveguide (WG) photonic-bridge taper modules are designed for symmetric planar coupling between silicon WGs and single-mode fibers (SMFs) to minimize photonic chip and packaging footprint requirements with improving broadband functionality. Micromachined fabrication and evaluation of polymer WG tapers utilizing high-resolution focused ion beam (FIB) milling is performed and presented. Polymer etch rates utilizing the FIB and optimal methods for milling polymer tapers are identified for three-dimensional patterning. Polymer WG tapers with low sidewall roughness are manufactured utilizing FIB milling and optically tested for fabrication loss. FIB platforms utilize a focused beam of ions (Ga+) to etch submicron patterns into substrates. Fabricating low-loss polymer WG taper prototypes with the FIB before moving on to mass-production techniques provides theoretical understanding of the polymer taper and its feasibility for connectorization devices between silicon WGs and SMFs.
Polymer waveguides (PWGs) are used within photonic interconnects as inexpensive and versatile substitutes for traditional optical fibers. The PWGs are typically aligned to silica-based optical fibers for coupling. An epoxide elastomer is then applied and cured at the interface for index matching and rigid attachment. Self-written waveguides (SWWs) are proposed as an alternative to further reduce connection insertion loss (IL) and alleviate marginal misalignment issues. Elastomer material is deposited after the initial alignment, and SWWs are formed by injecting ultraviolet (UV) light into the fiber or waveguide. The coupled UV light cures a channel between the two differing structures. A suitable cladding layer can be applied after development. Such factors as longitudinal gap distance, UV cure time, input power level, polymer material selection and choice of solvent affect the resulting SWWs. Experimental data are compared between purely index-matched samples and those with SWWs at the fiber-PWG interface. It is shown that < 1 dB IL per connection can be achieved by either method and results indicate lowest potential losses associated with a fine-tuned self-writing process. Successfully fabricated SWWs reduce overall processing time and enable an effectively continuous low-loss rigid interconnect.
Efficient modulation of electrical signals onto an optical carrier remains the main challenge in full implementation of microwave photonic links (MPLs) for applications such as antenna remoting and wireless access networks. Current MPLs utilize Mach-Zehnder Interferometers (MZI) with sinusoidal transfer function as electro-optic modulators causing nonlinear distortions in the link. Recently ring resonator modulators (RRM) consisting of a ring resonator coupled to a base waveguide attracted interest to enhance linearity, reduce the size and power consumption in MPLs. Fabrication of a RRM is more challenging than the MZI not only in fabrication process but also in designing and optimization steps. Although RRM can be analyzed theoretically for MPLs, physical structures need to be designed and optimized utilizing simulation techniques in both optical and microwave regimes with consideration of specific material properties. Designing and optimization steps are conducted utilizing full-wave simulation software package and RRM function analyzed in both passive and active forms and confirmed through theoretical analysis. It is shown that RRM can be completely designed and analyzed utilizing full-wave simulation techniques and as a result linearity effect of the modulator on MPLs can be studied and optimized. The material nonlinearity response can be determined computationally and included in modulator design and readily adaptable for analyzing other materials such as silicon or structures where theoretical analysis is not easily achieved.
Increasing board-to-board and chip-to-chip computational data rates beyond 12.5 Gbs will require the use of single-mode polymer waveguides (WGs) that have high bandwidths and are able to be wavelength division multiplexed. Laser direct writing (LDW) of polymer WGs provides a scalable and reconfigurable maskless procedure compared to common photolithography fabrication. LDW of straights and radial curves are readily achieved using predefined drive commands of the two-axis direct drive linear stage system. Using the laser direct write process for advanced WG structures requires stage-drive programming techniques that account for specified polymer material exposure durations. Creating advanced structures such as WG S-bends into single-mode polymer WG builds provides designers with the ability to affect pitch control, optical coupling, and reduce footprint requirements. Fabrication of single-mode polymer WG segmented radial arcs is achieved through a smooth radial arc user-programmed defined mathematical algorithm. Cosine and raised-sine S-bends are realized through a segmentation method where the optimal incremental step length and bend dimensions are controlled to achieve minimal structure loss. Laser direct written S-bends are compared with previously published photolithographic S-bend results using theoretical bend loss models. Fabrication results show that LDW is a viable method in the fabrication of advanced polymer WG structures.
Development of passive and active polymer based optical materials for high data rate waveguide routing and interconnects has gained increased attention because of their excellent properties such as low absorption, cost savings, and ease in fabrication. However, optical polymers are typically limited in the range of their refraction indices. Combining polymeric and inorganic optical materials provides advantages for as development of nano-composites with higher refractive indices with the possibility of being used as an active optical component. In this paper a new composite material is proposed based on polymer-metal oxide nano-composites for use as optical wave guiding structures and components. PDMS (Polydimethylsiloxane) is utilized for the polymer portion while the inorganic material is titanium dioxide. Refraction indices as high as 1.74 have been reported using these composites. For PDMS-TiO2 hybrids, the higher the ratio of titanium dioxide to PDMS, the higher the resulting refractive index. The index of refraction as a function of the PDMS:TiO2 ratio is reported with an emphasis on use as optical waveguide devices. Absorption spectrum of the nano-composites is measured showing low absorption at 850 nm and high absorption in the UV regime for direct UV laser/light curing. Prototype multimode waveguides are fabricated using soft imprint embossing that is compatible with the low viscosity nano-composite material. Cross dimensional shape and profile show the potential for full scale development utilizing the material set.
Multimode step index polymer waveguides achieve high-speed, (<10 Gb/s) low bit-error-rates for onboard and
embedded circuit applications. Using several multimode waveguides in parallel enables overall capacity to reach beyond
100 Gb/s, but the intrinsic bandwidth limitations due to intermodal dispersion limit the data transmission rates within
multimode waveguides. Single mode waveguides, where intermodal dispersion is not present, have the potential to
further improve data transmission rates. Single mode waveguide size is significantly less than their multimode
counterparts allowing for greater density of channels leading to higher bandwidth capacity per layer. Challenges in
implementation of embedded single mode waveguides within printed circuit boards involves mass production fabrication
techniques to create precision dimensional waveguides, precision alignment tolerances necessary to launch a mode, and
effective coupling between adjoining waveguides and devices. An emerging need in which single mode waveguides can
be utilized is providing low loss fan out techniques and coupling between on-chip transceiver devices containing Si
waveguide structures to traditional single mode optical fiber. A polymer waveguide bridge for Si to glass optical fibers
can be implemented using silicone polymers at 1310 nm. Fabricated and measured prototype devices with modeling and
simulation analysis are reported for a 12 member 1-D tapered PWG. Recommendations and designs are generated with
performance factors such as numerical aperture and alignment tolerances.
Embedding polymer optical waveguides (WGs) into printed circuit boards (PCBs) for intra-board or board-to-board high
speed data communications requires polymer materials that are compatible and inert when exposed to common PCB
manufacturing processes. Ensuring both WG functionality after chemical exposure and maintaining PCB manufacturing
integrities within the production process is crucial for successful implementation. The PCB manufacturing flow is
analyzed to expose major requirements that would be required for the successful implementation of polymer materials
for embedded WG development. Chemical testing and analysis were performed on Dow Corning ® OE-4140 UV-Cured
Optical Elastomer Core and Dow Corning® OE-4141 UV-Cured Optical Elastomer Cladding which are designed for low
loss embedded optical WGs. Contamination testing was conducted to demonstrate polymer compatibility in both cured
and uncured form. Various PCB chemicals were treated with uncured polymer material and tested for effective
contamination. Fully polymerized multimode WGs were fabricated and exposed to PCB chemicals at temperatures and
durations comparable to PCB manufacturing conditions. Chemical analysis shows that the chosen polymer is compatible
and inert with most common PCB manufacturing processes.
Integrating polymer optical waveguides (WGs) for board-to-board high speed data communications require prototyping
samples for proof-of-concept studies before moving to large scale production. A laser direct writing (LDW) method is
shown as a cost savings alternative to photolithographic prototyping large substrate samples. The LDW setup consists of
a 3-axis high-precision motion platform with a commercially available UV laser diode coupled to a lens-capped single
mode fiber. The correlation between writing parameters and the resulting waveguide dimensions is discussed
theoretically and confirmed experimentally with Dow Corning® OE-4140 UV-Cured Optical Elastomer Core and Dow
Corning® OE-4141 UV-Cured Optical Elastomer Cladding for both multimode and single-mode feasibility. Laser
written waveguide radial bends and crossings are also evaluated to show manufacturing capabilities for advanced
prototyping designs. Polymer waveguides fabricated with the LDW method are experimentally validated with losses
comparable to polymer waveguides manufactured with the photolithographic process (< 0.05 dB/cm).
An intensity modulation direct detection RF photonic link using a dual output Mach-Zehnder modulator and a
balanced detection scheme has been modeled and simulated. Validation of the model was performed by comparing the
optical third-order intercept point, spur free dynamic range, and the gain results to actual provided industry measured
output metrics. The model is highly accurate and provides the basis for demonstrating the RF photonics links
performance as a function of varied input parameters such as power levels, detector performance, and varied fiber
lengths. This allows the system designer to analyze performance parameters that are not possible in a laboratory
environment. In addition the designer can analyze the performance of new and improved link designs without having to
incur significant fabrication or manufacturing costs associated with prototypes. The link architecture and specific
implementation challenges particular to the link are discussed. Performance comparisons are shown between the models
to the theoretical calculations as well as to collected experimental data.
Optical-electrical printed wiring boards were fabricated featuring mechanical transfer (MT)-compatible interconnections for out-of-plane optical signal routing with an average optical link loss of 10.7 dB. Commercially available components were integrated into an optical layer for out-of-plane optical routing, including light turning devices that feature spherical micro lens arrays, a total internal reflection mirror, and alignment slots compatible with standard MT connectors. The feasibility of the optical-electrical printed wiring board is discussed in detail to demonstrate its compatibility with common printed circuit board manufacturing processes. The optical-electrical printed wiring board prototypes survived thermal cycling (−40°C to 85°C) and humidity exposure (95% humidity) showing an overall degradation of <3 dB of optical performance. Operational failure (>18 dB ) occurred after environmental aging life testing at 110°C for 216 h.
The SPIE/OSA Student Chapter at Michigan Technological University have developed demonstrations
and workshops for science and engineering outreach. The practical approach to holography promotes the
study of photonic related sciences in high school and college-aged students. An introduction to laser
safety, optical laboratory practices, and basic laser coherence theory is given in order to first introduce the
participants to the science behind the holograms. The students are then able to create a hologram of an
item of their choice, personalizing the experience. By engaging directly, the students are able to see how
the theory is applied and also enforces a higher level of attention from them so no mistakes are made in
their hologram. Throughout the course participants gain an appreciation for photonics by learning how
holograms operate and are constructed through hands on creation of their own holograms. This paper
reviews the procedures and methods used in the demonstrations and workshop while examining the
overall student experience.
With the ever-increasing demand for board-to-board optical data communications, the correlation between
waveguide surface end roughness and coupling losses must be thoroughly investigated. This study
measures end roughness of siloxane polymer optical waveguides in terms of optical coupling losses.
Siloxane Polymers from Dow Corning were used to fabricate 50 x 50 μm rectangular waveguides through
photolithographic processes. Edge roughness was controlled through various grades of fiber-optic polishing
films and then measured using interferometric microscopy (IFM). Controlled lab results are compared with
industrial polishing techniques that are consistent with mass-production methods. Electromagnetic
modeling revealed correlations between experimental and theoretical results.
Advancements in laser diode technology have lead to the utilization of lasers in pico-projector
systems. These proposed devices would need to take full advantage of the coherent source, while still
providing full RGB color. Previous work has been completed analyzing 532 nm green diodes in
conjunction with hadamard-matrix diffusers for reducing the disruptive speckle noise caused by
coherent sources. In addition to the speckle, many small form factor laser diodes have aspheric or nonsymmetric
beam shapes upon excitation. Non-circular beam shapes are difficult, if not impossible, to
collimate properly. Collimation and beam symmetry is vital in order to keep the bulk of the sources
power in the lower order diffractive modes that form from beam shaping optics such as diffusers and
micro-lens arrays. Because of these requirements and unique diode structure, generic spherical lenses
will not suffice, leading to new techniques for beam collimation. This paper looks at creating the proper
beam shape through the use of multimode fiber optics. The output wave from the fiber is then passed
through a diffuser. Speckle contrast measurements will be analyzed for both rotating and stationary
instances of the diffuser. In addition, other techniques for creating more functional beam shapes will be
evaluated in comparison with a fiber optic setup.
The Paulding Mystery Lights are a purportedly unexplained optical phenomenon, occurring nightly, deep in the woods
of Michigan's Upper Peninsula. The Michigan Tech Student Chapter of the SPIE initiated a project in 2008 to
understand the cause of the Paulding Lights. Previous investigations by skeptics attributed the lights to headlights
without explicitly identifying a source location. Our team applied a number of straightforward techniques to identify
and then verify the source location of the Paulding Light. Beginning with observation through a telescope, the team
moved to using tools such as detailed topographical maps and more common tools such as Google Street View to
identify a candidate source location. The candidate source location was then validated by first recreating the light using a
vehicle parked in that location. Additional verification was achieved by examining the correlation between the
occurrence of the light and the passing of cars at the source location. A spectrometer was also used to compare the
visible spectrum of the light to automotive headlamps. Our findings, presented here, indicate that the source of the
Paulding light is automobile traffic on a stretch of road about 7 km from the viewing location.
In order to maintain the overall optical performance in a step index rectangular waveguide, the complex index of
refraction of the core and cladding material must be maintained throughout the cycle of the lithographic fabrication
process. The percentage of the core and cladding material that is cured and the irradiance that cure took place directly
affects the complex index of refraction of these materials. Siloxanes produced by Dow Corning have been selected to
meet the requirements for embedded waveguides for circuit board applications due to their optical performance
characteristics and their compatibility with current manufacturing techniques. The required total dose for a 50 μm thick
layer of siloxane is 1200 mJ at an irradiance of 30 mW/cm2. In order to utilize lower irradiance levels the total dose of
the ultraviolet exposure must be characterized and calibrated. By measuring the changes in the absorption peaks of the
materials using transmission data from ellipsometric techniques it is possible to define the percentage cure of the
siloxane from different curing profiles. Ellipsometric techniques were also utilized to measure the complex refractive
index of the materials cured using different profiles. It was found that the total dose required for a complete cure and the
complex refractive index of these materials drastically changes with different irradiances and the profile for the total
dose compared to the curing of the siloxane materials at all irradiances is logarithmic.
In order to characterize and optimize the overall link budget for an optical communication channel, the absorption loss
of the waveguides must be well known, stable, and minimized. Research and characterization has been performed to
ascertain the impact of the use of halogen vs. halogen free FR-4 circuit boards. Halogen is utilized within glass resin
epoxy circuit boards as a flame retardant. An analysis of rectangular multi-mode polymer waveguide structures, with a
fixed core dimension of 50 μm × 50 μm, was done to characterize the effects of the halogen FR-4 on the absorption
loss. Thermal cycling times were varied in order to determine the relationship between heating of the polymer
material, halogen diffusion into the optical cladding and core layer, and optical losses.
So-called "free-space" laser communication systems working near the surface of the Earth must operate in the
presence of atmospheric turbulence. The effects of the atmospheric turbulence on the laser beam which are
relevant to optical communications are a broadening of the laser footprint, random jitter of the laser beam, and
high spatial frequency intensity fluctuations referred to as scintillation. The overall goal of our program is to
improve performance and extend the range of optical communications systems by exploring the use of adaptive
optics and channel coding. To better model the performance of a real system operating in the real world, we have
developed an outdoor turbulence-measurement and monitoring system. In this paper we describe an atmospheric
turbulence monitoring system for three kilometers, partially over water path. The laser transmitter operates
at 808 nm with a source power of 2mW. The receiver consists of relay optics, a Hartmann wave front sensor
(WFS), and a CCD camera. The WFS is used to monitor atmospheric turbulence-induced phase aberrations,
and the camera is used for both conventional imaging studies and measurements of anisoplanatic effects. In this
paper we describe this system and present some preliminary results obtained from the measurements.
In order to produce future generation micro-projection systems that can function independently or in conjunction with
small personal communication devices lasers are the preferred source of illumination over LEDs. Lasers are much
brighter and require less power than LEDs allowing for larger screen sizes and smaller battery requirements. In order to
illuminate the spatial light modulator used in the micro-projection system micro-optical elements are used within the
illumination path of a projector system to shape the RGB laser sources. Due to the diffractive nature of the optical
elements and the high coherence of the lasers a speckle pattern is produced in the final image reducing the overall image
quality. Diffractive diffusers are typically used in combination with micro-lens arrays in a time varying fashion in an
attempt to reduce the spatial coherence of the laser sources and hence reduce the speckle. Previous reduction techniques
have successfully used pseudo-random hadamard matrix diffusers to greatly minimize the contrast of the pattern. This
paper examines new reduction methods involving diffractive diffusing elements and presents measurements in the
reduction of speckle contrast for each technique.
The IR antenna-pair coupled micro-bolometers has demonstrated its unique power response features compared to the single antenna coupled micro-bolometers. The response pattern is determined by that of the single antenna and an interference oscillation term of the antenna-pair with respective to the angle of incidence of the radiation field, and can be steered by shifting the location of the bolometer. This paper explores the potential application of antenna-pair coupled detector in beam synthesis. It describes an array configuration based upon these micro-bolometers, and discusses the corresponding coherent data processing method for the purpose of obtaining response pattern narrowing effects from such an array. This directional gain enhancement, together with the beam steering control, could potentially lead to an array capable of providing a novel IR lensless imaging technique.
The accuracy of laser designators and range finders must be precise in order to be effective. If the operator only believes he or she is designating a target the results can be catastrophic. Range measurements for navigation may not be correct, potentially causing a collision. Reverse positioning inaccuracy may result in ordnance being misdelivered. If laser designation is incorrect, the target may be missed. Therefore in order to measure the laser designators and range finders performances and capabilities a Laser Rangefinder/Designator Beam Metrology System (LRDBMS) for beam characteristic analysis through environmental and atmospheric conditions was designed and implemented. The LRDBMS is in place at the Night Vision Electro-Optics (NVEO) Outdoor Laser Target Range located at the Naval Surface Warfare Center (NSWC) Crane Division. Atmospheric modeling software was evaluated and chosen in order to predict what is being measured by the LRDBMS.
This paper will describe multi-sensor modeling and how its use is applicable throughout the acquisition process. Since the acquisition process and the consequential modeling and simulation efforts are results of the developed Operational Requirements Document (ORD), the development and essential information that should be included within the ORD will also be addressed. In addition, currently available modeling tools along with their individual tradeoffs and the assumptions that can be made in order to facilitate modeling where design or operational parameters do not exist will be discussed.
As integrated electro-optical sensor payloads (multi- sensors) comprised of infrared imagers, visible imagers, and lasers advance in performance, the tests and testing methods must also advance in order to fully evaluate them. Future operational requirements will require integrated sensor payloads to perform missions at further ranges and with increased targeting accuracy. In order to meet these requirements sensors will require advanced imaging algorithms, advanced tracking capability, high-powered lasers, and high-resolution imagers. To meet the U.S. Navy's testing requirements of such multi-sensors, the test and evaluation group in the Night Vision and Chemical Biological Warfare Department at NAVSEA Crane is developing automated testing methods, and improved tests to evaluate imaging algorithms, and procuring advanced testing hardware to measure high resolution imagers and line of sight stabilization of targeting systems. This paper addresses: descriptions of the multi-sensor payloads tested, testing methods used and under development, and the different types of testing hardware and specific payload tests that are being developed and used at NAVSEA Crane.
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