Glass light pipes are fabricated using femtosecond laser irradiation followed by etching and thermal processing for minimizing sidewall scattering loss due to surface roughness. Turning mirrors and combiners, or splitters, are demonstrated. A critical step is assembly of the light pipe for coupling to a source or detector, which requires alignment and attachment, typically to a substrate. A combination of such structures has been realized to aid in assembly and optical transmission efficiency. Coupling from a distributed source through a lens array and into a light pipe array has been pursued for a waveguiding solar concentrators. The opposite propagation direction enables LED coupling and light distribution. Light pipes are fabricated with large cross-sectional areas, up to several square millimeters, compared to optical fibers. For glass-to-air cladding, the numerical aperture is substantially larger than for most optical fibers, thus enabling low loss transmission for high etendue sources. Instead of coating with a lower index material, as with optical fibers, a holding structure is desired to maximize the angular range for total internal reflection. We discuss the issues related to surface scattering and losses due to the cladding and light pipe mechanical support for LED lighting and solar applications.
A high-level design method using Fresnel reflections, waveguide mode theory, and wave interference principles was used to develop a parametric model of the grating coupler, and this model is tested and analyzed using a rigorous coupled mode analysis engine contained within the FIMMWAVE software package.
Glass waveguides are fabricated using laser processing techniques that have low optical loss with >90% optical throughput. Advanced light pipes are demonstrated, including angled facets for turning mirrors used for lens-to-light pipe coupling, tapers that increase the concentration, and couplers for combining the outputs from multiple lens array elements. Because they are fabricated from glass, these light pipes can support large optical concentrations and propagate broadband solar over long distances with minimal loss and degradation compared to polymer waveguides. Applications include waveguiding solar concentrators using multi-junction PV cells, solar thermal applications and remoting solar energy, such as for daylighting. Ray trace simulations are used to estimate the surface smoothness required to achieve low loss. Optical measurements for fabricated light pipes are reported for use in waveguiding solar concentrator architectures.
New applications for light pipes include waveguiding solar concentrators. For practical applications, achieving high optical transmission is critical so low-absorption glass is preferred over other materials, and the fabrication approach must show promise for scalability and low manufacturing costs. We present results for fabricated fused silica light pipes using femtosecond laser irradiation followed by chemical etching. After compensating for Fresnel losses and averaging over incident angles (in air) from 0° to 25°, transmission efficiencies of 96% and higher were measured for light pipes up to 20mm in length and 1mm2 cross-sectional area. The feasibility of creating glass light pipes with advanced geometries such as angled facets, tapering of the cross-section along the length, and combiners with micron-scale precision is also demonstrated. Tapered light pipes with concentrating factors up to 7x were fabricated, as well as cascaded structures with 45°-angled facets to couple light from multiple lens array elements into a common light pipe.
A low-loss, high-speed optical phased array (OPA) has been designed and fabricated. Two different platforms have been utilized in combination to leverage electro-optic (EO) tuning. A lithium niobate (LiNbO<sub>3</sub>) optical phased array was fabricated and used in conjunction with a silicon nitride (Si<sub>3</sub>N<sub>4</sub>) 8x8 waveguide array that condenses the output pitch and utilizes the Triplex<sup>TM</sup> waveguide technology. This OPA allows for the non-mechanical beam steering (NMBS) of 1550 nm light on an edge coupled optic platform and takes advantage of the high electro-optic coefficient and high speed capability of LiNbO<sub>3</sub> for electro-optic phase tuning. This coupled OPA has an overall insertion loss of ~3.5 dB which is advantageous to silicon-on-insulator OPAs that have shown overall insertion losses of ~14 dB. To characterize and tune this device, a 3 lens imaging system was employed to produce both near- and far- field intensity patterns of the output of the OPA on a static image plane. At the image plane, a high resolution infrared camera was used to observe the resulting intensity pattern. The control software for tuning the OPA reads the intensity incident at a specified position on the detector array, and has a PWM interface to drive the electro-optic phase controls. Beam steering was accomplished using an iterative tuning algorithm.
Integrated optic notch filters are key building blocks for higher-order spectral filter responses and have been demonstrated in many technology platforms from dielectrics (such as Si<sub>3</sub>N<sub>4</sub>) to semiconductors (Si photonics). Photonic-assisted RF processing applications for notch filters include identifying and filtering out high-amplitude, narrowband signals that may be interfering with the desired signal, including undesired frequencies detected in radar and free-space optical links. The fundamental tradeoffs for bandwidth and rejection depth as a function of the roundtrip loss and coupling coefficient are investigated along with the resulting spectral phase response for minimum-phase and maximum-phase responses compared to the critical coupling condition and integration within a Mach Zehnder interferometer. Based on a full width at half maximum criterion, it is shown that maximum-phase responses offer the smallest bandwidths for a given roundtrip loss. Then, a new role for passive notch filters in combination with high-speed electro-optic phase modulation is explored around narrowband phase-to-amplitude demodulation using a single ring operating on one sideband. Applications may include microwave processing and instantaneous frequency measurement (IFM) for radar, space and defense applications.
The application of in-motion optical sensor measurements was investigated for inspecting livestock soundness as a means of animal well-being. An optical sensor-based platform was used to collect in-motion, weight-related information. Eight steers, weighing between 680 and 1134 kg, were evaluated twice. Six of the 8 steers were used for further evaluation and analysis. Hoof impacts caused plate flexion that was optically sensed. Observed kinetic differences between animals’ strides at a walking or running/trotting gait with significant force distributions of animals’ hoof impacts allowed for observation of real-time, biometric patterns. Overall, optical sensor-based measurements identified hoof differences between and within animals in motion that may allow for diagnosis of musculoskeletal unsoundness without visual evaluation.
A hybrid annealed proton exchange (APE) waveguide with a vertically integrated arsenic trisulfide (As<sub>2</sub>S<sub>3</sub>) waveguide on a lithium niobate (LiNbO<sub>3</sub>) substrate is used to create an optical phased array (OPA) that allows for the non-mechanical steering of 1550 nm light on an integrated optic platform. The high electro-optic coefficient of the x-cut y-propagating LiNbO<sub>3</sub> (r33 = 30.8 pm/V) is utilized by electrode structures fabricated on the LiNbO<sub>3</sub> substrate to create a low-power, lowloss beam steering with high-speed bandwidths, capable of 10 GHz and larger as demonstrated by commercial LiNbO<sub>3</sub> modulators. The As<sub>2</sub>S<sub>3</sub> waveguide is introduced because of its high refractive index, which leads to a highly confined optical mode. Design and fabrication are presented for a large full width steering angle of 34°, representing an order of magnitude improvement over low-confinement, diffused LiNbO<sub>3</sub> waveguides and two orders of magnitude improvement over other reported LiNbO<sub>3</sub> OPAs.
A hybrid platform is proposed for optical processing with capabilities beyond the standard use for spatial filter coefficients. Time domain operations are explored utilizing electro-optical components to generate and steer an optical beam across the digital micro-mirror device (DMD). An input signal in the form of a collimated optical beam is scanned through an imaging optic across the DMD to achieve 1-D operations. Coefficient “bit depth” is determined by the mirror pattern in the local spot zone giving a ratio of ‘on’ state to ‘off’ state area. The reflected signal returns along the original path through the imaging lens and is read by a receiver. Design feasibility is presented along with initial experimental results.
The ability to effectively characterize Fresnel lenses over large areas is essential to verifying their system performance and efficiency for concentrating photovoltaics and solar thermal systems. Under high concentration, it becomes challenging to perform detailed spatial and spectral measurements under full sun conditions. We have developed a method to characterize large Fresnel lenses with unknown optical qualities for concentrating solar applications. Our Lens Characterization Unit (LCU) analyzes the resultant pattern of an incident laser beam which may be scanned across the lens. Using the LCU, we can evaluate the portion of refracted light that is concentrated on the receiver area at each incidence point.
Solar concentrating photovoltaic systems have the potential to reduce total cost and achieve higher efficiency by replacing a large solar cell surface with cheaper optical devices, in which a large area of light can be efficiently collected and concentrated to a small optical device and guided to an array of co-located photovoltaic cells with high optical efficiency. We present an experimental demonstration for a lens-to-channel waveguide solar concentrator using a commercially-available Fresnel lens array. In this work, a 60 mm by 60 mm lens to channel waveguide system is used for demonstration. A separate, aluminum-coated 45° coupler is fabricated and attached to the waveguide to improve the coupling efficiency and to avoid any inherent decoupling loss. The fabrication details and component performance of the prototype device are discussed.
The design, fabrication and measurement of a cylindrical fiber coil structure is presented that has applications for compact fiber-optic amplifiers. A multimode fiber is used as a surrogate for a dual clad, rare-earth doped fiber for coil fabrication and optical testing. A ray trace algorithm, written in Python, was used to simulate the behavior of light travelling along the waveguide path. An in-house fabrication method was developed using 3D printed parts designed in SolidWorks and assembled with Arduino-controlled stepper motors for coil winding. Ultraviolet-cured epoxy was used to bind the coils into a rigid cylinder. Bend losses are introduced by the coil, and a measurement of the losses for two coil lengths was obtained experimentally. The measurements confirm that bend losses through a multimode fiber, representative of pump light propagating in a dual-clad rare-earth doped fiber, are relatively wavelength independent over a large spectral range and that higher order modes are extinguished quickly while lower order modes transmit through the windings with relatively low loss.
In this work, we present fabrication and measurement results of an As<sub>2</sub>S<sub>3</sub>-on-LiNbO<sub>3 </sub>ring resonator waveguide and
sidewall grating cavity waveguide. The nonlinear tuning capability is demonstrated on a fabricated ring resonator
waveguide by injecting the signal-pump optical power into the device and observing the nonlinear phase shift. The
nonlinear tunability of our hybrid As<sub>2</sub>S<sup>3</sup>-on-LiNbO<sub>3</sub> grating cavity waveguide is numerically analyzed.
In this work, we present fabrication and measurement of sidewall Bragg gratings in chalcogenide arsenic tri-sulfide (As2S3) on titanium-diffused lithium niobate (Ti:LiNbO3) channel waveguides. The transfer matrix method was used to analyze the temporal and spectral response of the sidewall gratings in the mid-infrared. The waveguide sidewall Bragg gratings were fabricated by electron-beam lithography (EBL), metal liftoff and subsequent reactive-ion etching (RIE). Insertion loss of the mid-infrared Ti:LiNbO3 optical waveguides were measured at ~2 dB and the propagation loss was estimated to be 0.45 dB/cm. Configuration of an optical low-coherence interferometer that is capable of characterizing the mid-infrared sidewall grating-based devices was experimentally implemented and preliminary results from fiber Bragg gratings are presented.
The initial prototype design and characterization of a small-scale concentrating solar thermal system will be presented. A large Fresnel lens, with an area of 0.77 square meters on a two-axis tracking mount, is used to concentrate the solar energy and focus it to an area of a few square centimeters. The system design and the first prototype testing will be discussed. By leveraging concentration to achieve the maximum solar-to-thermal energy efficiency and low cost, the system is unique compared to currently available residential hot water systems.
The first experimental demonstration results will be presented for a novel, two-dimensional waveguiding solar concentrator consisting of a primary concentrator (a microlens array) and a secondary concentrator (tapered multimode waveguides). The microlens array collects the incident sun light and focuses it onto a turning mirror. The turning mirror couples the light into a tapered multimode waveguide, which alleviates connection, cooling and uniformity issues associated with conventional solar concentrating systems. Therefore, a large area of light can be efficiently concentrated to a small waveguide cross-section and guided to an array of co-located photovoltaic cells with high optical efficiency. To achieve the maximum coupling efficiency of the light to the waveguide, the design of the turning mirror and waveguides are optimized to avoid any inherent decoupling loss in the subsequent waveguide propagation. Experimental results indicate that a 38 mm diameter lens with a multimode waveguide that is 3 mm x 3 mm x 10 cm, using only total internal reflection surfaces, can achieve 126x concentration with 62.8% optical efficiency. We will present details on the experimental device characterization. A critical requirement for this design is maintaining low waveguide propagation losses, which as we demonstrate can be less than 0.1 dB/cm. Considering 100% TIR coupling and the use of antireflection layers, the theoretical efficiency limit for this particular system is ~88%.
The NLFM waveform resulting from a tunable integrated optical ring resonator is simulated and compared with the well
known tan-FM waveform. The metrics of interest are the first sidelobe levels and FWHM times of the autocorrelation,
as these directly relate to the long-range performance and fine range resolution of a LADAR system, and should ideally
be as small as possible. Through simulation, the sidelobe level of the autocorrelation of an NLFM waveform generated
by a series of tunable integrated optical ring resonators is shown to be lower than the autocorrelation sidelobe level of an
equivalent optimized tan-FM waveform with an equal FWHM time. A proof of concept experiment employing
thermally tunable Silicon Nitride integrated optical ring resonator is shown to generate NLFM chirped waveforms with
frequency chirps of 28 kHz.
The integration of a higher index chalcogenide strip as a guiding layer on top of diffused lithium niobate waveguides is
presented. The mode transfer to upper cladding through a 2D taper structure is discussed theoretically. A review of the
fabricated waveguide structures and the results are also provided. A slight modification to current taper design is
proposed to further improve the bend radius and create much smaller ring resonators. Future perspectives for devices and
their potential impacts on integrated optics are also discussed.
The aim of this work was to demonstrate the fabrication and characterization of erbium-doped optical waveguide
amplifiers in X-cut Y-propagating lithium niobate (LiNbO<sub>3</sub>) by erbium (Er) and titanium (Ti) co-diffusion. Optical
small-signal internal gains up to +0.6 dB/cm at 1531 nm were measured for the transverse electric (TE) and magnetic
(TM) modes by optical pumping at 1488 nm with a coupled optical pump power of 95 mW in four different optical
waveguide amplifier lengths. The Er and Ti co-diffusion process has shown adequate internal gain efficiency in dB/mW
and a beneficial path for the development of LiNbO<sub>3</sub>-based integrated optical devices.
An analysis revealing a simple closed form solution for the poles of a polarization-coupled ring resonator is presented for
the first time. The resonant frequencies are easily calculated as well as the pole magnitudes based on the relative phase
and polarization-dependent ring coupling. Applications include wavelength-division multiplexing (WDM) filters with
new pole magnitude tuning capability and analysis of polarization dependence in birefringent fiber-based and integrated-optic
This paper reports on recent advances made in real-time intruder detection for an intrusion system utilizing a phasesensitive
optical time-domain reflectometer developed at Texas A&M University. The system uses light pulses from a
highly coherent laser to interrogate an optical fiber. The Rayleigh backscattered light is detected, and real-time
processing of the received signal is implemented using an FPGA-based system. Signatures from a single human on foot
and automobile have been obtained, and are comparable to results obtained with previous signal processing techniques.
Individual footsteps are clearly identified for the single human intruder. With the introduction of real-time signal
processing, the system can be run continuously, only triggering intrusions when they are detected. These recent
advancements allow us to process intruder signatures more effectively. With these advancements, this technology is a
prime candidate for low-cost perimeter monitoring of high-value and high-security targets such as nuclear power plants,
military bases, and national borders.
Fabrication results are reported for making optical gratings in chalcogenide glasses. Two approaches are being
investigated: 1) thermal nanoimprinting of the glass using a mold master, and 2) direct patterning of the
chalcogenide glass using a combination of electron beam lithography (EBL) and reactive ion etching (RIE).
Preliminary results are presented for the nanoimprint and EBL experiments. The nanoimprint results show very
Recent work on vertically-integrated electro-optic and chalcogenide waveguides is reviewed. By integrating As<sub>2</sub>S<sub>3</sub>
chalcogenide glass with Ti-diffused lithium-niobate (LiNbO<sub>3</sub>) waveguides, we are able to dramatically increase the
integration density for this electro-optic waveguide platform. As<sub>2</sub>S<sub>3</sub> offers a higher refractive index compared to the
LiNbO<sub>3</sub> and consequently enables low-loss, small-bend-radii structures that are crucial for making high density optical
circuits. As<sub>2</sub>S<sub>3</sub> waveguides are patterned on top of LiNbO<sub>3</sub> waveguides. Adiabatic tapers are used to couple light from
the LiNbO<sub>3</sub> waveguide into the As<sub>2</sub>S<sub>3</sub> waveguide. Bend radii that are less than a hundred microns are enabled using this
approach, compared to LiNbO<sub>3</sub> waveguide bend radii that are typically on the order of a centimeter. The design and
fabrication are discussed along with experimental results for s-bend structures.
Using a phase-sensitive optical time-domain reflectometer developed at Texas A&M University, this paper reports on
recent advances in intruder detection and classificatoin for long perimeters or borders. The system uses light pulses from
a narrow linewidth CW laser with low frequency drift to interrogate an optical fiber. The backscattered light is detected,
and real-time processing of the received signal is performed. Signatures from single and multiple humans on foot,
nearby vehicle traffic on a road, construction-like vehicle activity, and animals have been obtained. Individual footsteps
are clearly identified and the cadence readily observed. Time-frequency plots are used to compare the signatures. The
detected signal contains information regarding the weight of the intruder as well. An adult weighing around 60kg may
produce several π-radian shifts in the optical phase, which is detected by the system. While distances up to 20km have
been monitored in previous remote field tests, we report measurements on a local test site with a total fiber length of
12km. A 3-mm diameter fiber cable is buried at a depth of 20-46 cm over a distance of 44m, with a 2km spool of fiber
attached prior to the buried fiber and a 10km fiber spool connected in series after the buried section. Recent advances in
data acquisition and signal processing allow us to avoid false alarms due to drifts in the laser center frequency and
greatly improve the probability of detection. With these advancements, this technology is prime for low-cost perimeter
monitoring of high-value and high-security installations such as nuclear power plants and military bases as well as
This paper establishes a procedure for increasing the sensitivity of measurements in integrated ring resonators beyond
what has been previously accomplished. This is achieved by a high-frequency phase modulation lock to the ring cavities.
A prototyped fiber Fabry-Perot cavity is used for comparison of the method to a similar cavity. The Pound-Drever-Hall
(PDH) method is chosen as a proven, ultra-sensitive method with the exploration of a much higher frequency modulation
than has been previously discussed to overcome comparatively low finesse of the ring resonator cavities. The high
frequency facilitates the use of the same modulation signal to separately probe the phase information of different,
integrated ring resonators with quality factors of 5.6 x10<sup>5</sup> and 2.4 x10<sup>5</sup>.
The large free spectral range of small cavities and low finesse provide a challenge to sensing and locking the stability of
diode lasers due to the small dynamic range and signal-to-noise ratios (S/N). This can be offset by a calculated increase
in modulation frequency using the PDH approach. A distributed feedback (DFB) laser is locked to a ring resonator
cavity to demonstrate this sensitivity. This approach using integrated ring resonators is measured to have a refractive
index resolution of 1.9x10<sup>-8</sup> that can be compared to other fiber and integrated sensors.
The relationship between the signal-to-noise ratio and dynamic frequency range of the cavity error signal is explored
with an algorithm to optimize this relationship. The free spectral range and the loss of the cavity provide input
parameters to this relationship to determine the optimum S/N and range of the respective cavities used for locking and
sensing. The purpose is to show how future contributions to the measurements and experiments of micro-cavities,
specifically ring resonators, is well-served by the PDH method with high-frequency modulation.
A planar waveguide design is presented that integrates As<sub>2</sub>S<sub>3</sub> chalcogenide glass with Ti-diffused lithium-niobate (LN)
waveguides to increase functionality. As<sub>2</sub>S<sub>3</sub> is a higher index material that is introduced to create small bend radii
structures that are crucial for making high density optical circuits. As<sub>2</sub>S<sub>3</sub> waveguide patterns are aligned on top of the
straight LN waveguide. Power launched into LN is coupled to the higher index As<sub>2</sub>S<sub>3</sub> cladding structure and propagates
through a tight bend before coupling back to the LN waveguide. Experimental results are given for s-bends of 2.48cm
minimum radius of curvature.
A planar waveguide design is studied for the mid-infrared wavelengths from 2.5μm to 4.5μm by simulation. Lithium-niobate and silicon are integrated to increase functionality. Silicon as a higher index material is introduced on behalf of its transparency to the infrared spectrum. Power transfer from a buried lithium-niobate waveguide to silicon is simulated using field mode matching software for several wavelengths and taper dimensions. In addition, bend losses and mode mismatch losses to a quantum cascade laser are evaluated by simulation.
Optical time-frequency processing requires a combination of high-speed, quadratic phase modulators and dispersive
delay lines. The latter is typically achieved using optical fibers, but can be compactly implemented and tunable using
dispersive filters. Time scaling, either dilation or compression, can be achieved with these building blocks. While basic
time scaling followed by direct detection has been demonstrated, we focus on cascading time-scale operations for
potential signal processing applications and implementations using integrated-optic platforms. For cascaded operations,
both the phase and amplitude of the scaled output must be correct. Time scaling is studied analytically and by
simulations. Practical implementation issues are addressed such as the time aperture limits imposed by using sinusoidal
phase modulation to approximate the desired quadratic response. The chirp and dispersion relationships are given for
"factor of one half" and "factor of two" time scaling. The evolution of the signal's time support at intermediate points in
the time-scaling operation is shown to be a critical parameter for practical implementations. Two optical time-scaling
architectures are studied, and one is clearly better in this respect. Furthermore, a special case arises for a Gaussian input
pulse whereby the number of elements needed to realize the time scaling can reduced by a factor of two. Applications
for cascaded time scaling operations are discussed, including optical wavelet processing and photonic-assisted analog-to-digital
conversion. By using the time-scale operation in the optical domain to mimic the discrete-time downsampling
operation, we show that physical scaling of the optical filters between subsequent decomposition levels is not required.
A fast, non-interferometric measurement technique that allows the frequency-dependent delay and amplitude responses to be measured is presented. For a single amplitude and relative phase measurement at a fixed optical wavelength, the measurement time is on the order of a microsecond. RF modulation up to 2.7 GHz can be accommodated. A modified technique using frequency modulation is described to overcome non-idealities in the phase measurement. Results are presented for a fiber Bragg grating and an acetylene gas cell with swept-wavelength laser tuning at a rate of 40 nm/s.