Rose-Hulman Institute of Technology has 35 years of graduating optical scientists and engineers. The change from the Applied Optics degree program to the Optical Engineering degree program occurred in 2003, which paved the way for ABET accreditation. The past and present program reinforces the idea that we are educating our students in optics applications to deal with real-world problems and practice the profession of optical engineers. The RHIT program has been at the forefront of developing project-based learning since its inception in 1983. Since the name changed, the optical engineering program (OE) has completed three cycles of ABET accreditation. We are continuously improving our OE curriculum to meet the current and future needs of the industry and cutting-edge research. To lead the ABET review, Rose-Hulman initiated the dialogue with SPIE to be the lead society for optical engineering in 2004. This paper will discuss several steps taken to develop a world-class OE program with state-of-the-art laboratory facilities for undergraduate optical engineering education.
We conduct a five-week research boot camp each summer for King Abdullah University of Science and Technology (KAUST) STEM students. Undergraduates carry out independent research regardless of their discipline. Each boot camp contains sessions on overview of topics relevant to thin film optics as well as skills needed for conducting research. The key learning goal of the challenge optics problem is to demonstrate the applicability of research skills across modeling, research design, fabrication process, and testing exercise. Here we present our findings from the past five boot camps, assessments of the students using responses in the workshop evaluations, and their research outcomes. We also share a curriculum aimed at broader adoption.
As part of the hands-on instruction for the Silicon Photonics course, students make Models, create layout, and fabricate optical devices using electron-beam lithography (EBL) and nano-pattern generating system (NPGS). Devices are written with a JOEL-100 SEM on a resist coated SiN or SOI substrate. The developed patterns are etched with RIE and tested with one of two available probe stations depending on I/O design (grating coupling or edge coupling). We’ll discuss the resulting passive and active devices made by students for this course.
In Silicon photonics, there is an ever-growing need for optical switches capable of high-speed operation, needed for applications such as optical computing, microwave photonics, and LiDAR. One potential candidate for a highspeed optical switch consists of two PIN junctions connected in series, reverse-biased. When one PIN junction is fed with an oscillating optical input at some frequency and the other with a constant threshold input, the output displays bistable hysteresis, ideal for an optical binary switch. In this research, PSpice and MATLAB are used to develop mathematical models of the circuit and simulate the operation of the Symmetric PIN, or S-PIN switch at various frequencies. While the work done so far has been highly theoretical, measurements have been carried out on an optoelectronic prototype of the S-PIN in order to observe the basic operation of the device. Additionally, a model of the S-PIN developed in MATLAB agrees closely with simulations of the switch in PSpice. In the physical circuit, the presence of parasitic capacitance and inductance in the PIN junctions cause the shape of the output hysteresis to deteriorate as the running frequency is increased. Simulations show that if parasitic capacitance is minimized, the switch can operate in the GHz range. One method explored to mitigate this capacitance is impedance matching, which has shown to improve output bistability to some extent. Results from modeling indicate that the use of components with low capacitance has the most potential to allow the S-PIN to operate at higher frequencies competitive with modern electronic switches.
One of the major challenges in integrated silicon photonics is the light coupling into and out of photonic circuits. Prism coupling, among waveguide coupling methods in the silicon photonics is a rather complex and costly one mostly due to the requirement of a prism with index of refraction higher than that of the effective index of the propagating mode in the waveguide. Surface plasmon polariton (SPP) allows the deployment of a fused silica prism instead of the high index prism, as indicated by our simulation using Lumerical FDTD software and Otto configuration of the SPP excitation scheme. Using the dispersion diagram the surface plasmon angle is determined to be 44◦ for the TM-mode of the incident beam. This is followed by examining the relevant parameters affecting the light coupling efficiency to a silicon waveguide of input dimensions of 220 750 nm. In Lumerical FDTD solver, two-dimensional simulation of plane waves of p-polarization enters the optical system at surface plasmon angle. This model reveals a coupling efficiency of 54% which at the very least is better than that of traditional prism coupling methods. For fabrication purposes, we could impose geometrical constraints on airgap height, making it a function of the waveguide and metal (silver in this case) strip thickness. Furthermore, we will show and discuss the effect of choice of the thickness for the Si waveguide, airgap, and silver layer on coupling efficiency from our mathematical model and Lumerical FDTD simulations.
Photonic Bandgap Crystals (PBGs) have begun to see use in optical communication networks and photonic device coupling due to their high, selective transmission bandgaps and novel optical filtering properties. Understanding how PBGs can be designed for varying purposes is crucial in continuing to find practical applications of these crystals. PBGs are typically periodic as the math is well understood and the transmission or reflection of periodic optical structures is generally well-known, even in closed form. Crystal lattices in position space can be shown to correlate to an “inverse lattice” in momentum space through Fourier analysis. With further description, we can express any form of crystal as a sum of its fundamental, periodic lattices. For simplicity, we analyze only 1-dimensional structures but generate the groundwork for descriptions in higher dimensions. This description of aperiodic lattices allows us to extract information about effective lattice periods and effective lattice wave numbers supported by the crystal lattice. Using the Transfer Matrix Method (TMM) we further examine the optical properties of aperiodic structures by generating dispersion diagrams of general structures. By comparing the results of the TMM and the results of the effective lattice parameters, we seek to determine the efficacy of the claim that disordered structures are sums of ordered structures. To do so, we will apply novel disordered functions such as the logistic map to generate layer thicknesses of an aperiodic crystal and compare results from both the TMM and Lumerical FDTD simulations.
We have fabricated biaxial hyperbolic metamaterials (BHMMs) using layered structures consisting of titanium dioxide (TiO2) and copper (Cu). In order to enable the biaxial property, an oblique angle deposition (OAD) technique is applied to deposit the dielectric layer. We have characterized the biaxial hyperbolic dispersion using variable angle spectroscopic ellipsometry (VASE) measurements in the wavelength range 400 nm to 900 nm. A noticeable difference be- tween in-plane permittivity components of the fabricated BHMM is observed to be 0.13 at 633 nm. The experimental characterization results have been in good agreement with the predictions of effective medium approximation (EMA) with an MSE of 18.
We will discuss laboratory experiments in silicon-photonics developed for senior and graduate level students. The learning objectives for this laboratory includes basic microfabrication techniques in cleanroom, understanding design and process flow, fabrication and testing of silicon waveguides and Mach-Zehnder interferometers on a silicon-on-insulator (SOI) wafer. We will also discuss a typical team project outcomes and examples of experimental work and report by student group. Additionally, we will comment on student’s learning and skill set acquired through this introductory hands-on project-based silicon photonics class.
A multilayer hyperbolic metamaterial (HMM), fabricated from alternating thin films of metal and dielectric, displays a hyperbolic, anisotropic dispersion relation due to the coupling of excited surface plasmons. The design, fabrication, and characterization of an HMM based on TiO2 / Cu alternating layers with a metal-to-dielectric fill factor of 67% is presented. The layers were deposited onto glass and silicon substrates using physical vapor deposition (PVD) with an electron beam evaporator and then characterized using ellipsometry. According to the effective medium theory, this design shows an epsilon-near-zero (ENZ) line near the Helium-Neon wavelength of 633 nm. Our experimental measurements are in good agreement with the theoretical predictions.
Disordered one-dimensional photonic bandgap (PBG) structures could prove useful in designing broadband reflectors capable of filtering chosen polarizations of incoming light. By capitalizing on the similarities between defects and disorder, it is possible to construct a 1D PBG structure such that the layers are non-uniform but the structure can retain its most novel properties. This is done by allowing the thickness of the layers in the structure to deviate uniformly around an average thickness by a preselected amount of deviation. A mathematical model using the Transfer Matrix Method for simulation has been previously constructed by this group. This model has been verified using FDTD simulation as well. The PBG structure was then fabricated consisting of TiO2 deposited by electron-beam physical vapor deposition (e-beam PVD) first at normal incidence and then at a 70o oblique angle. This pattern was repeated to create six bilayers of TiO2 films. This alternating pattern gives rise to the novel structure of a PBG structure by creating a repeating pattern of amorphous and biaxial, columnar, birefringent TiO2 ,which is analogous to using two different materials. Through testing using a polarizer, analyzer, and HeNe laser with a wavelength of 632.8 nm, it has been found that the sample does in fact match well with the expected theoretical results and acts as a broadband reflector for the TM polarization designed for a 70º incidence angle. The average layer thickness of the fabricated TiO2 PBG is 22.7 nm.
In this work, we demonstrated a new method for coupling light, using prism with a small index of refraction and surface plasmon polaritons (SPP), into a crystalline silicon (Si) waveguides and performed simulation work using Lumerical FDTD Solutions. The designed structure is comprised of a dielectric prism, air-gap, metal (Ag) film, Si and silicon dioxide (SiO2). The system follows the Otto configuration for the excitation of SPP which includes a fused silica prism and a 100 nm layer of silver metal sputtered on top of SiO2 with an air-gap between the prism and the metal film. A 0.75×100 μm (height×width) silicon waveguide with tapered coupler is located on the same buried oxide along the silver layer for optical input channel. A p-polarized (TM mode) light with an incident angle of 44° at the wavelength of 1550 nm is incident at the interface of the fused silica/air-gap to excite the SPP. The 2D simulation shows a coupling efficiency of 54% which reveals the potential for application of this I/O coupling method in silicon photonics. For proof of concept, we fabricated and characterized the materials layout described above on an SOI substrate. For the Si structure, a tapered coupler and waveguide is fabricated using a XeF2 dry-etch and lift-off for the metal structure. Also, the experimental setup is suggested to locate the prism on the right position of the wafer and measure the output light from the waveguide by butt coupling.
This is an introductory course in silicon photonics for senior and graduate students. Topics covered in this course include: basic properties of silicon waveguides; the essentials of SOI waveguide fabrication; dispersion and propagation loss in Si waveguides; analysis and design of various passive silicon devices; modeling and characterization of silicon modulators; integrated photonic circuit; and layouts for E-beam lithography. This course also includes a laboratory to enhance student’s learning. Students fabricate a thermo-optic MZI modulator in the cleanroom and then characterize its performance. We will discuss experimental work by student groups and comment on student’s learning in this class.
A hybrid optical integrator is a recirculating loop that performs oversampling typically for an analog input using the crossgain modulation (XGM) in a semiconductor optical amplifier (SOA). The modulated input signal changes the gain of the loop through XGM and thus modifies the loop accumulation. This paper presents hybrid optical integrator for an all-optical analog-to-digital converter based on silicon photonics platform. The device consists of silicon waveguides of dimension 220 × 500 nm (thick × width) and approximately 2,800 μm total loop length, input and output grating couplers for 1550 nm signal, directional couplers, and external components (SOA, optical isolator and band-pass filter). All devices are designed for fabrication on the SOI wafer using E-Beam lithography. A theoretical model for the system is developed and simulated with Lumerical software and Matlab that accounts for critical design parameters such as the loop length, coupling coefficient, and gain in the SOA. The 3-dB couplers were designed and loop length was minimized to reduce the propagation loss through the silicon waveguide and increase the sampling frequency. From the simulation, total loss of silicon photonics device at one round trip is approximately 27 dB. The system is characterized for square waves at Giga- Hertz input frequencies. Simulation results show excellent agreement with the theoretical model of leaky integrator. The operation frequency of the integrator can be increased from MHz up to GHz by replacing fiber cable to silicon waveguide, leading to a free-spectral range of approximately 4.17 GHz.
A one-dimensional, single-material polarizing photonic bandgap structure is designed and fabricated using e-beam PVD and oblique angle deposition technique. In order to obtain high- and low-index layers, we deposited alternate layers of titanium dioxide (TiO2) at deposition angles of 0° and 70°on top of a fused silica substrate. This approach is chosen since at deposition angle of zero degree, deposited TiO2 using e-beam PVD, show a negligible birefringence while the obliquely deposited TiO2 acts as a biaxial material with significant birefringent behavior. As a result, deposition of a bilayer film at two angles is analogous to using two different materials with the advantage of simplifying fabrication and modeling this polarizing device. The bandgap of the bilayer structure is modeled in a way that only a specific wavelength with certain polarization, p polarization, could pass through while the s polarization is reflected. For modeling we used Transfer Matrix Method and numerical FDTD analysis to simulate behavior of the 1D photonic band gap structure. The simulations produce better than 98% reflection for s polarization and almost no reflection for p polarization for the center wavelength of 632.8 nm. The fabricated device shows 94% reflection for s polarization and less than 6% reflection for p polarization at the red HeNe laser wavelength at an incident angle of 70°. The results demonstrate that a 1D multi-layer photonic crystal, fabricated from a single material, can be designed to selectively reflect or transmit p or s polarization of an incident light beam.
The Accreditation Board for Engineering and Technology (ABET) recently revised their published list of programspecific criteria. Beginning during the 2014-15 cycle, all engineering programs which include “optical” and “photonic” in their titles must meet new specific criteria to receive ABET accreditation. One portion of the wording under the curriculum section states “The curriculum must prepare students to have knowledge of and appropriate laboratory experience in: geometrical optics, physical optics, optical materials, and optical and/or photonic devices and systems.” Last year, the Rose-Hulman optical engineering curriculum committee revised our baccalaureate degree program. A portion of this effort sought to improve alignment to the ABET program-specific criteria. Here we review the outcomes of this effort, including our documented continuous improvement process, the realignment of our existing courses along the four tracks laid out by ABET, and the introduction of new undergraduate courses to improve student learning.
A novel photonic analog-to-binary converter based on the first-order asynchronous delta-sigma modulation (ADSM) has been theoretically investigated and experimentally demonstrated. A fiber-optic prototype ADSM system is constructed and characterized. Delta-sigma modulation is a straightforward approach to A/D conversion because in this case an external clocking is not required and demodulation can be simply performed via a low-pass filtering process. To improve signal-to-noise ratio and thus system ENOB, a non-interferometric optical implementation has been constructed. The ADSM is comprised of three photonic devices: an inverted output photonic leaky integrator, bistable quantizer, and positive corrective feedback. The photonic integrator which is a recirculating loop performs the oversampling of an analog input using the cross-gain modulation in an SOA. We will show that the photonic ADSM produces an inverted non-return-to-zero (NRZ) pulse-density modulated output describing an input analog signal. This fiber-optic ADSM converts up to 7.6 MHz analog input at about 30 MS/s and effective ENOB of 6.
A novel photonic analog-to-digital converter (ADC) based on asynchronous delta-sigma modulation (ADSM) has been investigated. The architecture utilizes an optical leaky integrator, optoelectronic bistable quantizer, and positive corrective feedback for a non-interferometric optical implementation of ADSM. The principles of the proposed 1st –order ADSM are mathematically modeled and simulated.
This paper presents a hybrid opto-electronic asynchronous delta-sigma modulator,
implemented in the form of a fiber-optic Analog-to-Digital converter (ADC). This architecture
was chosen for its independence of an external clock and ease of demodulation through a single
low-pass filter stage. The fiber-optic prototype consists of an input laser (wavelength λ1) which
is modulated with an input RF signal, a high-speed comparator circuit working as bi-stable
quantizer, and a fiber-optic loop that includes a SOA and a band-pass filter that act as a leaky
integrator. The fiber-optic loop acts as a fiber-ring resonator (FRR), and defines the resonance
wavelength λ2 of the system. The gain within this loop is modified through cross-gain
modulation (XGM) by the input wavelength λ1, and thus achieves the desired modulation effect.
The proposed architecture has been constructed and characterized at a sampling rate of 15.4
MS/s processing input analog signals in the range of dc-3 MHz with a signal-to-noise ratio of 36
dB and an effective number of bits of 5.7.
The delta sigma modulator (DSM) is a device which transforms the amplitude information of an analog input signal
to the duty cycle and frequency of a binary output. This device, typically employed in oversampled analog-to-digital
converters, is based on a feedback loop which includes at least one integrator and one quantizer in the forward path. In
this paper, a novel photonic second-order DSM is proposed and experimentally demonstrated. The system is composed
of two inverted leaky integrator and one electro-optic quantizer. The maximum input frequency is around 2 MHz,
limited by the fiber length of the accumulator and feedback loops, and the quantizer rise/fall times. The system is
characterized at different input frequencies and waveforms (sinusoidal and saw tooth) to analyze the modulator
performance and linearity. The binary output is acquired, processed and demodulated using a personal computer, in
order to reconstruct the input analog signal. The reported fiber-optic DSM is very promising for future integration
increasing the operation frequency up to GHz range.
A wideband photonic RF vector modulator with novel architecture is presented and demonstrated with capability of
continuous amplitude modulation and 0°-360° phase shifting. In-phase and quadrature-phase components of the output
signal are used to produce 360° continuous phase shifting and optical attenuation is used to control the signal amplitude.
A novel 8-tap all-optical transverse-filter implementing a Hilbert transform is proposed and demonstrated to produce 90° quadrature phase-shift for the broadband RF signal. Experimental apparatus and results for continuous vector modulation
will be presented for the frequency range of 1-6 GHz.
A novel optical bistable device based on an electrically coupled semiconductor optical amplifier (SOA) and a bipolar
juncture transistor (BJT) is proposed and experimentally demonstrated. The measured switching time is about 0.9-1.0 us,
mainly limited by the electrical capacitance of the SOA and the parasitic inductance of the electrical connections.
However, the effects of parasitic components can be reduced employing current electronic-photonic integration circuits
(EPIC). Numerical simulations confirm that for capacitance values in tens of femtofarads switching speed can reach tens
of GHz.
This paper presents a novel optical leaky integrator for an all-optical A/D converter based-on sigma-delta modulation.
The device consists of two main components: a fiber ring resonator (FRR) and a wavelength converter. The FRR
comprises a SOA and an optical filter that defines the resonance wavelength λ2. The modulated input signal, at
wavelength λ1, changes the gain of the loop through cross-gain modulation (XGM) and thus modifies the loop
accumulation. A theoretical model for the system is developed that accounts for critical design parameters such as the
loop coupling ratio, length, and XGM in the SOA. The system is characterized for square input signals ranging 0.5-
5MHz. The integrator time constant is adjusted between 5% and 25% of the input signal period through modifications in
the loop coupling ratio and the SOA driving-current. Experimental results show excellent agreements with the numerical
simulations. Due to the length of the fiber-loop, the operation frequency of the integrator is limited to the MHz range.
However, the operating frequency can be increased up to hundreds of MHz by shrinking the components' optical fibers,
or up to GHz range, by using current photonic integration technologies.
A novel optical inverted bistable switch based on a nonlinear fiber ring resonator (FRR), which contains a
semiconductor optical amplifier (SOA) in the loop, have been analyzed and experimentally demonstrated. The optical
bistability phenomenon is obtained due to the combined nonlinear effects of the transmission characteristics of the
resonator and the SOA's gain property. A complete theoretical analysis and supporting simulations are presented. A
working prototype is build using commercial optical components. Experimental results show switching speed in tens of
MHz with rising and falling times lower than 10 ns. Limitations in switching speed are caused by the length of the fiber
loop. Therefore, improvements in operating frequencies can be increased up to GHz range if the length of the loop is
reduced to the order of centimeters.
A novel optical switching technique based on two electrically coupled SOAs (S-SOAD) operating in reverse-bias mode
is proposed and experimentally demonstrated. The device produces electrical and optical outputs with both non-inverted
and inverted hysteresis behavior. The S-SOAD operates on the basis of two optoelectronic effects. The first is an
electrical bistability resulting from the connection of two p-i-n structures in series. The second is the quantum-confined
Stark effect in the multi-quantum-well structure of the SOA which is responsible for the optical bistability. This effect
causes an electro-absorption modulation of the transmitted light based on the switching voltages across the p-i-n
structures.
Experiments show optical switching at MHz frequencies and rise/fall times lower than 1.1 us, limited mainly by the
electrical capacitance of the SOAs and the parasitic inductance of the connecting wires. The electrical effects can be
mitigated by photonic integrated circuit manufacturing. Predictions of the device performance at high frequencies are
based on a proposed hybrid optoelectronic model of the S-SOAD. In this model each SOA is modeled as a set of parallel
and series electronic components including optical parameters such as the responsivity and optical transmission of the
SOA. System simulations confirm that for capacitance values in tens of femto-Farads the switching-speed increases up
to GHz range. Results for the bistability behavior and switching time as a function of the electrical and optical
parameters will be presented.
A MEMS-based interferometric sensor is produced using the multi-user MEMS processing standard (MUMPS)
micromirrors, movable by thermal actuation. The interferometer is comprised of gold reflection surfaces, polysilicon
thermal actuators, hinges, latches and thin film polarization beam splitters. A polysilicon film of 3.5 microns reflects and
transmits incident polarized light from an external laser source coupled to a multi-mode optical fiber. The input beam is
shaped to a diameter of 10 to 20 microns for incidence upon the 100 micron mirrors. Losses in the optical path include
diffraction effects from etch holes created in the manufacturing process, surface roughness of both gold and polysilicon
layers, and misalignment of micro-scale optical components. Numerous optical paths on the chip vary by length, number
of reflections, and mirror subsystems employed. Subsystems include thermal actuator batteries producing lateral
position displacement, angularly tunable mirrors, double reflection surfaces, and static vertical mirrors. All mirror
systems are raised via manual stimulation using two micron, residue-free probe tips and some may be aligned using
electrical signals causing resistive heating in thermal actuators. The characterization of thermal actuator batteries
includes maximum displacement, deflection, and frequency response that coincides with theoretical thermodynamic
simulations using finite-element analysis. Maximum deflection of 35 microns at 400 mW input electrical power is
shown for three types of actuator batteries as is deflection dependent frequency response data for electrical input signals
up to 10 kHz.
KEYWORDS: Chemical species, Diffraction, Optical engineering, Near field optics, Energy efficiency, Zone plates, Lens design, Particles, Physics, Near field
A simple focusing device is proposed for de Broglie matter waves—a diffractive lens, based on the optical effect of diffractive multifocal focusing of radiation. This matter-wave lens consists of two coaxial circular apertures in which the second aperture of smaller diameter is located where the Fresnel number of the first aperture is unity. It is shown that diffraction of a de Broglie matter wave by a system of two pinholes on an optical axis exhibits the multifocal focusing effect of matter waves in the near-field (Fresnel) zone, which creates a very intense, spatially localized beam of atoms. Theoretical predictions for the focusing efficiency of a neutral atomic beam by the diffractive lens are as follows. The spot diameter is ~0.1 µm, the ratio of focal and incident intensities is ~15, the focal length of the diffractive lens is in the range ~0.13 to 6 cm, the focusing depth is in the range ~15 to 30 cm, and the energy transmitting efficiency is ~30 %. For the relatively large diameters of the pinholes, 5.0 µm, the proposed configuration acts as a matter-wave lens with a large focal length and a long focusing depth.
KEYWORDS: Diffraction, Lens design, Chemical species, Near field optics, Near field, Particles, Information operations, Physics, Interferometers, Optical properties
Diffraction of a de Broglie matter wave by a system of two pinholes on an optical axis exhibits the multifocal focusing effect of matter waves in the near-field (Fresnel zone). The focusing, defocusing and refocusing phenomenon results from periodic phase changes at singular points, which are observed for the even Fresnel numbers, where the intensity is zero and the phase is undefined. As an on-axis observation point passes through a singular point, the nature of the wave in the neighborhood of the axis changes from that of a diverging wave to that of a converging wave, i.e., the wave refocuses. The amplitudes of the oscillations in the intensity and phase of the de Broglie wave depend on the ratio of the radii of the two apertures in the system. This effect of diffractive multifocal focusing of de Broglie waves can be used for designing a diffractive lens for matter-wave beams.
We report on two types of wavelength conversion techniques that are based on gain saturation effect in semiconductor
optical amplifier (SOA) and erbium doped fiber amplifier (EDFA). In these amplifiers the gain saturation occurs when
the optical density at the gain medium is high enough to result in depletion of the population inversion by stimulated
emission. In each case, the fiber ring laser is assembled using a variable fiber coupler, a narrowband optical filter and the
gain medium. For external input power values higher than the determined threshold value of the ring resonator, the gain
will be saturated. Because the wavelength of the external laser is different from the oscillating wavelength of the ring
resonator, the optical power at the output of the resonator is drastically decreased (low-state). On the other hand, when
the input of the external laser is below the threshold value the output power of the resonator increases (high-state). In our
experiment the operating wavelengths of the ring resonators are 1314 nm and 1553 nm for the SOA and EDFA
respectively. The input signal is modulated around the threshold value for frequencies of 20 MHz and 1 MHz and
resonator lengths of around 8 m and 16 m for the SOA and EDFA cases respectively. Both systems exhibit high contrast
modulation of 41 dB and 33 dB at the output port for the low/high states of the SOA and EDFA ring lasers respectively.
UV light from an Excimer laser operating at 248 nm with fluence of 800 mJ/cm2 per pulse at 30 Hz repetition rate is used for writing process of a fiber Bragg Grating (FBG). Multiple FBGs with different center wavelength and reflectivity are produced by varying strain and exposure time on a single strand of a SMF where 1552.75 nm is the center wavelength of the unstretched FBG. The in-situ measurement of the reflectivity, bandwidth and center wavelength of each Bragg grating will be described. We will also present an RF technique for measuring the separation between two adjacent FBGs.
This paper describes a novel A/D converter called "Binary Delta-Sigma Modulator" (BDSM) which operates only with nonnegative signal with positive feedback and binary threshold. This important modification to the conventional delta-sigma modulator makes the high-speed (>100GHz) all-optical implementation possible. It has also the capability to modify its own sampling frequency as well as its input dynamic range. This adaptive feature helps designers to optimize the system performance under highly noisy environment and also manage the power consumption of the A/D converters.
KEYWORDS: Modulators, Bandpass filters, Phased arrays, Antennas, Modulation, Electronic filtering, Interference (communication), Signal processing, Quantization, Signal to noise ratio
In this paper we introduce and analyze a multiple-RF-beam beamformer in receive mode utilizing the principle of space-time delta-sigma modulation. This principle is based on sampling input signals in both time and space and converting the sampled signals into a digital format by delta-sigma conversion. Noise shaping is achieved in 2D frequency domain. We show that the modulator can receive signals of narrow and wide bandwidths with steering capability, can receive multiple beams, and establish tradeoffs between sampling in time and in space. The ability of the modulator to trade off between time and space provides an effective way to sample high frequency RF signals without down conversion. In addition, a space-time delta-sigma modulator has better performance than a solely temporal delta-sigma modulator (for the same filter order), as is typically used in communication systems to digitize the down-converted analog signals.
An optical beamformer capable of controlling a phased array antenna in receive/transmit mode for multiple simultaneous independent rf beams is proposed. The processor can be programmed to sweep the antenna aperture following an independent angular sequence for each rf beam. A two-beam two-channel version of the beamformer has been experimentally demonstrated. The optical beamformer processes two rf beams and it is based on a ternary array of three delay lines. Measurements are performed for both receive and transmit modes and for rf signals between 0.5 and 1.5 GHz. We present beampattern results showing that two independent beams can be steered simultaneously. In the transmit mode both rf beams are characterized for a broadside target position. In the receive mode the beamformer performance is characterized by detecting two rf beams independently.
We propose a novel architecture for an optical programmable dispersion matrix (PDM) able to process simultaneous independent RF beams. The proposal is demonstrated by processing the transmission of two independent RF signals in two-channels with a resolution of 3-bit. The optical beamformer processes two independent RF-beams, for eight different angular directions, and it is based on a binary array of three delay lines. Each delay line is composed of four fiber Bragg gratings whose center wavelengths are channels 30 to 33 of the ITU grid. Beampatterns are characterized in transmit mode for two simultaneous RF beams in 0.6-1.5 GHz frequency range and azimuth angles from 0° to 70°.
We present a new architecture for a programmable disperions matrix for optical beamforming. A 2-channel prototype is constructed to demonstrate beam steering for angles of ±70° in the receive/transmit mode in RF0.1-1 GHz frequency range.
We demonstrate an optical two-channel receive beamformer at 1310 nm using fiber Bragg grating arrays for producing true time delay. The antenna beam patterns are measured for broadside targets at several RF frequencies ranging from 40 MHz to 1 GHz. These results compare favorably with theoretical calculations.
Over the past four years, 2000-2003, undergraduates have participated in summer research at Rose-Hulman Institute of Technology (RHIT) carrying out a wide range of optics projects.
The optics educational programs at Rose-Hulman Institute of Technology have progressed and evolved over the past twenty years. Beginning with a modest undergraduate area minor in applied optics we now offer bachelors and masters degree programs in optical engineering. Distinctive elements of the current optical engineering programs including courses and curricula will be discussed.
Rose-Hulman has a history of meeting the need for optical engineers via the Applied Optics Programs since 1983. We have changed our degree program to Optical Engineering and will seek ABET accreditation. The present paper will deal with the step taken to accomplish this and define the mission of the degree program.
We propose and demonstrate a novel architecture for a programmable dispersion matrix (PDM) able to process two simultaneous independent RF beams with a resolution of 3 bits. The processor takes advantage of wavelength division multiplexing (WDM) encoding. In our two-channel approach, four adjacent ITU optical WDM channels are used as optical carriers. The odd and even optical carriers are independently modulated with each received RF-beam using electro-optic modulators. Thus, each pair of optical carriers directs RF information of one beam. The PDM is programmed to introduce independent true-time delays for even and odd optical channels. The PDM is based on three optical delay lines, arranged in a binary configuration. Each delay line consists of four fiber-Bragg gratings (FBG), with each FBG having a center-wavelength at one of the optical WDM channels. Beampatterns are characterized in receive mode for RF signals in 0.5-1.5 GHz frequency range obtained for target angles of 0° to 70°. The measurement results demonstrate the simultaneous processing of two received RF-beams and the squint free nature of the beamformer.
In this paper we implement and characterize a two-channel optical programmable beamformer. The system is designed to achieve four-bit resolution. The architecture of the programmable dispersion matrix is based on an array of four delay-lines each having two spliced fiber Bragg gratings. We have experimentally investigated the optical signal processing performance of the optical beamformer in receive and transmit modes. Beampatterns for RF range 40 - 200 MHz are presented along with the theoretical calculations. The main lobe of the beampattern is shown to be independent of frequency for several target positions thus demonstrating a “squint-free” characteristic of this optical processor.
We demonstrate the effects of the applied field orientation during the writing period in electrical fixing. The dependency of the diffraction efficiency on the direction of the applied electric field while writing a hologram is studied. Our experiments show that during writing, when the electric field is applied opposite to the c-axis the grating can be successfully revealed with both positive and negative dc voltages. However, when the grating is written with a field parallel to the c-axis, the grating can only be revealed with a field applied in the opposite direction. This situation generates the largest diffraction efficiency of nearly 78%. Theoretical analyses of these observed phenomena are performed.
We demonstrate an optical switch with a holographic Mach-Zehnder interferometer in a bulk photorefractive crystal. We perform theoretical studies of the grating recording conditions, required crystal orientation, and applied voltage for this photorefractive optical switch. We experimentally demonstrated the switch in a LiNbO3:Fe crystal cut in the shape of an elongated hexagon using the 514 nm line from an argon laser. A modulation depth of 75% was obtained experimentally.
This paper describes implemented methods for characterizing a 5-bit programmable dispersion matrix (PDM) that is built to control a two-channel Receive beamformer in the 1550 nm region. The architecture of the PDM, is based on an array of 5 delay lines each having two spliced fiber Bragg gratings. Phase measurements for 32 possible delay configurations of the PMD are presented. Beam-patterns of this Receive beamformer at RF frequencies of 0.2, 0.5 and 1 GHz agrees well with the theoretical calculations. The main lobe of the beam pattern is shown to be independent of frequency for several target positions thus demonstrating a 'squint-free' characteristic of this optical processor.
A single sideband suppressed-carrier (SSB-SC) optical modulator is demonstrated. The sideband is suppressed by means of a fiber Mach-Zehnder interferometer with amplitude electro-optical modulators in each branch. The attenuation of the carrier is achieved by proper biasing of the integrated modulators. The proposal was demonstrated by modulating at 285 MHz. Suppression of 30.8 dB in the carrier and 26.3 dB in the sideband were obtained.
We perform double sideband and single sideband suppressed- carrier modulators using a Mach-Zehnder interferometer in free space. Two bulk LiNbO3 crystals are used to modulate the optical beam at 1319 nm in both branches of the interferometer. We present experimental results of the optical spectrum using heterodyne measurement technique. These results show maximum carrier and sideband suppression of 24 dB and 12 dB respectively for 1 GHz modulation frequency.
We present a simple scheme for implementation of optical single sideband suppressed carrier (OSSB-SC) modulation based on the use of standard MZM and passive fiber optic components. The function of the proposed modulator is also theoretically predicted. A Sagnac interferometer structure is used for sideband suppression. The light propagating in one direction is orthogonally linearly polarized to the oppositely propagating light. The orthogonal polarization is implemented by a non-reciprocal optical element. The optical carrier is attenuated by appropriately setting the Mach-Zender Modulator (MZM) at quadrature bias point. We will experimentally demonstrate sideband suppression and carrier attenuation up to 28 dB by using all commercially available components.
Binary fiber Bragg grating arrays are capable of providing a true-time-delay capability needed for beamforming. The grating arrays consists of a series of fibers connected by circulators and switches. Each fiber contains tow or more fiber Bragg gratings on a fiber may become smaller than the grating length. Alterative architectures that use ternary switching have been proposed that show promise in reducing the constraints on grating spacing, but require carful control of grating position. Component-level simulation was carried out to compare alterative architectures and to evaluate effects of tolerances in grating placement, component imperfections, connector losses, and similar impairments whose overall consequences are difficult to predict. Fiber Bragg gratings, channel balancers, switches, and other beamfomer components were modeled. Beamforming arrays were simulated that focus tow antennas, and allow beamscanning over a +/- 70 degrees angle in thirty-two discrete steps. The baseband bandwidth was assumed to exceed 1 GHz. Binary and ternary array structures were compared. The PHOTOSS photonics system simulator provided the simulation engine. Beam patterns were calculated for a beam sweeping past a broadside target, and for a beam sweeping past targets at other angles. Results of simulation have been used to inform decisions as physical systems are realized.
In this paper we discuss the implementation of a project oriented laboratory course in optics education. Through our discussion we show the relevance and necessity for such a course. The course is tailored not only to make the students to learn concepts, but to build a product from scratch to completion. Emphasis is also made on the design, performance, marketing and aesthetics of the product.
A shear interferometer using wave mixing and angular multiplexing techniques in a BaTiO3 crystal is investigated. By varying the angle between the pump beam and the probe beam, two separate holograms in the form of two index gratings are formed in the crystal. Each grating has a different magnitude and direction in the k-space. Reconstruction is attempted with both the same wavelength as that of the writing beams and with a slightly different wavelength. During the reconstruction process the crystal orientation is adjusted such that the phase-matching condition is satisfied simultaneously for both gratings. As a result of the reconstruction two sheared wavefronts are obtained. A theoretical description, results of theoretical investigations and an experimental configuration of this interferometer are presented.
We report the observation of intensity fluctuations in a semilinear photorefractive oscillator, consisting of a BaTiO3 crystal as the gain medium, and a multimode helium-neon laser as its pump source. Depending on the beam geometry and intensity, the periodical fluctuations range on a timescale from a few seconds to several minutes. The behavior is suspected to be caused by a competition of gratings due to the external cavity, crystal-ordinary mirror assembly, the self-pumped phase conjugate mirror due to total internal reflection.
A photorefractive dynamic Schlieren (PDS) system is designed to monitor the photothermal damage-threshold of ZrSiO2. The PDS is calibrated by observing the fringe movement in a Twyman-Green interferometer (TGI). While the phase changes due to optical path difference (OPD) of about (gamma) /10 are detectable by TGI, the PDS is capable of revealing intensity changes due to OPD of less than (gamma) /20. In this experiment a Nd:YAG laser is the source for photothermal phase change while a HeCd provides the Schlieren field.
Optical phase conjugation and two-wavelength recording in a photorefractive crystal BaTiO3 is used for real-time lateral-shear interferometry. Dispersion in uniaxial BaTiO3 crystal is used to provide perfect phase matching for two gratings of different spatial frequencies. Theoretical predictions of possible geometries are experimentally verified. The magnification effect and modulation of diffraction efficiency due to recording-reconstruction with different wavelengths are also presented.
A HeCd laser and a BaTiO3 crystal were used to obtain a self-pumped phase conjugate beam. This effect is applied to perform an optical tracking filter. A modified liquid crystal television is used to imprint an input image on the laser beam. The input image presents moving and stationary elements of a scenery. The light transmitting through the crystal reveals only the moving elements, whereas the phase conjugate beam only carries the stationary portions of the input image. Therefore, a real time novelty and monotony filter has been realized. There is a lower speed limitation which depends on the response time of the crystal. The high speed limitation is dictated by the electronics. Due to the single-beam requirement of self-pumped phase conjugation, the imaging optical system is simple, making the experimental apparatus compact and versatile. The self-aligning property of self-pumped phase conjugation allows a wide range of incident angles without changing the system performance.
Photorefractive crystals are being used as a means of capturing the spatial information of two coherent light sources interacting within them. Typical recording procedures, important crystal parameters, as well as holographic applications are described.
The holographic storage capacity of an azo dye-doped polymer is demonstrated using a self- referenced interferometer in a two-wave mixing arrangement. The system possesses the ability to store absorption gratings and has the peculiar property of being able to record the complete polarization of the incident light field. The film is used to store a wavefront which is compared to its displaced version at a later time. Diffraction efficiencies of up to 10% have been observed. A theoretical description and experimental results of the interferometer are presented as well as a description of the dye/polymer system in various polarization configurations.
A closed loop digital/optical pattern recognition system is presented, using a photorefractive correlator. A personal computer is used to control an optical cross-correlator, consisting of a 4f-Fourier processor with a BaTiO3 crystal in its Fourier plane. A test pattern is inserted into the optical system, via a modified liquid crystal television. The test pattern is stored in the photorefractive crystal, using a two-wave mixing technique. Several master patterns are compared to the test pattern using the optical cross-correlation technique. The output signal of the optical cross-correlator is detected by a CCD camera, and is transferred to the PC for evaluation. The spatial resolution of the optical system is examined both theoretically and experimentally.
A coherent optical processor is constructed using two-wave mixing in a BaTiO3 crystal. This system is capable of performing autocorrelation of an input image in two successive steps. Theoretical considerations of the system are successfully compared with the experimental results. Influences of the mixing angle and intensity ratios on the result are discussed.
Coherent two-wave coupling in a photorefractive crystal (PRC) BaTiO3 is used to detect the presence of impurities (phase objects) in transparent liquids. Glass pieces of 100 microndiameter are easily detectable with a
reliability of better than 96%.
We investigated a new concept for phase conjugate one-pass distortion correction. The distorted input is imaged into a BaTiO3 crystal. A phase conjugate is produced through a self-pumped phase conjugation technique. The nonlinear material acts as a time averaging filter. Furthermore for a time-varying distorter the self pumped phase conjugator removes distortion in real time. This technique requires only one pass through the distorting medium. 1.
We present a holographic interferometer using a photorefractive crystal SBN:60. A point source is recorded inside the crystal via two wave-mixing. By displacing the crystal laterally or radially a tunable Young''s interferometer or a tunable Michelson interferometer is constructed respectively. An advantage of this device is the elimination of errors due to hologram developing and repositioning processes. The interferometer is capable of producing sinusoidal gratings and zone plates. 1.
This paper gives a brief overview of neural networks noting the advantages of the optical implementation. The Hopfield model mainly used as an associative memory is introduced. Planar holograms have been used to defme the interconnections between neurons in optical systems but volume holograms with photorefractive crystals offer much greater potential and density for these interconnections1. Because of the lack of suitable optical material light modulators have been used to simulate neurons in the neural plane. A simple one-layer network using photorefractive material will be presented along with some preliminary results.
A method for increasing the visibility of Young fringes in Laser Speckle Velocimetry of up to 20 is proposed utilizing optical autocorrelation with photorefractive crystal BaTiO3. 1.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.