Publications

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We present a filter consisting of cascaded ring resonators with integrated SOAs. The filter demonstrates an extinction ratio ≥30 dB, a free spectral range of 56 GHz and a FWHM bandwidth of 3 GHz. © 2011 Optical Society of America.

We present the first complete simulation of the dynamic range and noise of InGaAsP multi-ring channel-drop filters with internal SOAs. The results show gain saturation, and spontaneous emission noise limit the dynamic range. © 2011 Optical Society of America.

We demonstrate an optical gate architecture with optical isolation between input and output using interconnected PD-EAMs to perform AND and NOT functions. Waveforms for 10 Gbps AND and 40 Gbps NOT gates are shown. © 2010 Optical Society of America.

We present the bandwidth enhancement of an EAM monolithically integrated with two mutually injection-locked lasers. An improvement in the modulation efficiency and bandwidth are shown with mutual injection locking.

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We present a photonic integrated circuit (PIC) composed of two strongly coupled lasers. This PIC utilizes the dynamics of mutual injection locking to increase the relaxation resonance frequency from 3 GHz to beyond 30 GHz.

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This report summarizes a 3-year LDRD program at Sandia National Laboratories exploring mutual injection locking of composite-cavity lasers for enhanced modulation responses. The program focused on developing a fundamental understanding of the frequency enhancement previously demonstrated for optically injection locked lasers. This was then applied to the development of a theoretical description of strongly coupled laser microsystems. This understanding was validated experimentally with a novel 'photonic lab bench on a chip'.

Most common ionizing radiation detectors typically rely on one of two general methods: collection of charge generated by the radiation, or collection of light produced by recombination of excited species. Substantial efforts have been made to improve the performance of materials used in these types of detectors, e.g. to raise the operating temperature, to improve the energy resolution, timing or tracking ability. However, regardless of the material used, all these detectors are limited in performance by statistical variation in the collection efficiency, for charge or photons. We examine three alternative schemes for detecting ionizing radiation that do not rely on traditional direct collection of the carriers or photons produced by the radiation. The first method detects refractive index changes in a resonator structure. The second looks at alternative means to sense the chemical changes caused by radiation on a scintillator-type material. The final method examines the possibilities of sensing the perturbation caused by radiation on the transmission of a RF transmission line structure. Aspects of the feasibility of each approach are examined and recommendations made for further work.

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We present the bandwidth enhancement of an EAM monolithically integrated with two mutually injection-locked lasers. An improvement in the modulation efficiency and bandwidth are shown with mutual injection locking.

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We demonstrate an optical gate architecture using electro-absorption modulator/photodiode pairs to perform AND and NOT functions. Optical bandwidth for both gates reach 40 GHz. Also shown are AND gate waveforms at 40 Gbps.

Monolithic photonic integrated circuits (PICs) have a long history reaching back more than 40 years. During that time, and particularly in the past 15 years, the technology has matured and the application space grown to span sophisticated tunable diode lasers, 40 Gb/s electrical-to-optical signal converters with complex data formats, wavelength multiplexors and routers, as well as chemical/biological sensors. Most of this activity has centered in recent years on optical circuits built on either Silicon or InP substrates. This talk will review the three classes of PIC and highlight the unique strengths, and weaknesses, of PICs based on Silicon and InP substrates. Examples will be provided from recent R&D activity.

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The goal of this project is to fabricate a four-state pixelated subwavelength optical device that enables mid-wave infrared (MWIR) or long-wave infrared (LWIR) snapshot polarimetric imaging. The polarization information can help to classify imaged materials and identify objects of interest for numerous remote sensing and military applications. While traditional, sequential polarimetric imaging produces scenes with polarization information through a series of assembled images, snapshot polarimetric imaging collects the spatial distribution of all four Stokes parameters simultaneously. In this way any noise due to scene movement from one frame to the next is eliminated. We fabricated several arrays of subwavelength components for MWIR polarization imaging applications. Each pixel unit of the array consists of four elements. These elements are micropolarizers with three or four different polarizing axis orientations. The fourth element sometimes has a micro birefringent waveplate on the top of one of the micropolarizers. The linear micropolarizers were fabricated by patterning nano-scale metallic grids on a transparent substrate. A large area birefringent waveplate was fabricated by deeply etching a subwavelength structure into a dielectric substrate. The principle of making linear micropolarizers for long wavelengths is based upon strong anisotropic absorption of light in the nano-metallic grid structures. The nano-metallic grid structures are patterned with different orientations; therefore, the micropolarizers have different polarization axes. The birefringent waveplate is a deeply etched dielectric one-dimensional subwavelength grating; therefore two orthogonally polarized waves have different phase delays. Finally, in this project, we investigated the near field and diffractive effects of the subwavelength element apertures upon detection. The fabricated pixelated polarizers had a measured extinction ratios larger than 100:1 for pixel sizes in the order of 15 {micro}m by 15 {micro}m that exceed by 7 times previously reported devices. The fabricated birefringent diffractive waveplates had a total variation of phase delay rms of 9.41 degrees with an average delay of 80.6 degrees across the MWIR spectral region. We found that diffraction effects change the requirement for separation between focal plane arrays (FPA) micropolarizer arrays and birefringent waveplates arrays, originally in the order of hundreds of microns (which are the typical substrate thickness) to a few microns or less. This new requirement leads us to propose new approaches to fabricate these devices.

Subwavelength diffractive features etched into a substrate lead to form birefringence that can be utilized to produce polarization sensitive elements such as waveplates. Using etched features allows for the development of pixilated devices to be used in conjunction with focal plane arrays in polarimetric imaging systems. Typically, the main drawback from using diffractive devices is their high sensitivity to wavelength. Taking advantage of the dispersion of the form birefringence, diffractive waveplates with good achromatic characteristics can be designed. We will report on diffractive waveplates designed for minimal phase retardation error across the 2-5 micron spectral regime. The required fabrication processes of the sub-wavelength feature sizes will be discussed as well as the achromatic performance and transmission efficiency of final devices. Previous work in this area has produced good results over a subset of this wavelength band, but designing for this extended band is particularly challenging. In addition, the effect of the finite size of the apertures of the pixilated devices is of particular interest since they are designed to be used in conjunction with a detector array. The influence of small aperture sizes will also be investigated.

We report here on an effort to design and fabricate a polarization splitter that utilizes form-birefringence to disperse an input beam as a function of polarization content as well as wavelength spectrum. Our approach is unique in the polarization beam splitting geometry and the potential for tailoring the polarized beams' phase fronts to correct aberrations or add focusing power. A first cut design could be realized with a chirped duty cycle grating at a single etch depth. However, this approach presents a considerable fabrication obstacle since etch depths are a strong function of feature size, or grating period. We fabricated a period of 1.0 micron form-birefringent component, with a nominal depth of 1.7 microns, in GaAs using a CAIBE system with a 2-inch ion beam source diameter. The gas flows, ion energy, and sample temperature were all optimized to yield the desired etch profile.

This diffractive optical element (DOE) LDRD is divided into two tasks. In Task 1, we develop two new DOE technologies: (1) a broad wavelength band effective anti-reflection (AR) structure and (2) a design tool to encode dispersion and polarization information into a unique diffraction pattern. In Task 2, we model, design, and fabricate a subwavelength polarization splitter. The first technology is an anti-reflective (AR) layer that may be etched into the DOE surface. For many wavelengths of interest, transmissive silicon DOEs are ideal. However, a significant portion of light (30% from each surface) is lost due to Fresnel reflection. To address this issue, we investigate a subwavelength, surface relief structure that acts as an effective AR coating. The second DOE component technology in Task 1 is a design tool to determine the optimal DOE surface relief structure that can encode the light's degree of dispersion and polarization into a unique spatial pattern. Many signals of interest have unique spatial, temporal, spectral, and polarization signatures. The ability to disperse the signal into a unique diffraction pattern would result in improved signal detection sensitivity with a simultaneous reduction in false alarm. Task 2 of this LDRD project is to investigate the modeling, design, and fabrication of subwavelength birefringent devices for polarimetric spectral sensing and imaging applications. Polarimetric spectral sensing measures the spectrum of the light and polarization state of light at each wavelength simultaneously. The capability to obtain both polarization and spectral information can help develop target/object signature and identify the target/object for several applications in NP&MC and national security.

Optical waveguide propagation loss due to sidewall roughness, material impurity and inhomogeneity has been the focus of many studies in fabricating planar lightwave circuits (PLC's)1,2,3 In this work, experiments were carried out to identify the best fabrication process for reducing propagation loss in single mode waveguides comprised of silicon nitride core and silicon dioxide cladding material. Sidewall roughness measurements were taken during the fabrication of waveguide devices for various processing conditions. Several fabrication techniques were explored to reduce the sidewall roughness and absorption in the waveguides. Improvements in waveguide quality were established by direct measurement of waveguide propagation loss. The lowest linear waveguide loss measured in these buried channel waveguides was 0.1 dB/cm at a wavelength of 1550 nm. This low propagation loss along with the large refractive index contrast between silicon nitride and silicon dioxide enables high density integration of photonic devices and small PLC's for a variety of applications in photonic sensing and communications.