By leveraging advanced wafer processing and flip-chip bonding techniques, we have succeeded in hybrid integrating a
myriad of active optical components, including photodetectors and laser diodes, with our planar lightwave circuit (PLC)
platform. We have combined hybrid integration of active components with monolithic integration of other critical
functions, such as diffraction gratings, on-chip mirrors, mode-converters, and thermo-optic elements. Further process
development has led to the integration of polarization controlling functionality. Most recently, all these technological
advancements have been combined to create large-scale planar lightwave circuits that comprise hundreds of optical
elements integrated on chips less than a square inch in size.
We present innovations in Planar Lightwave Circuits (PLCs) that make them ideally suited for use in advanced defense
and aerospace applications. We discuss PLCs that contain no micro-optic components, no moving parts, pose no spark
or fire hazard, are extremely small and lightweight, and are capable of transporting and processing a range of optical
signals with exceptionally high performance. This PLC platform is designed for on-chip integration of active
components such as lasers and detectors, along with transimpedance amplifiers and other electronics. These active
components are hybridly integrated with our silica-on-silicon PLCs using fully-automated robotics and image
recognition technology. This PLC approach has been successfully applied to the design and fabrication of multi-channel
transceivers for aerospace applications. The chips contain hybrid DFB lasers and high-efficiency detectors, each capable
of running over 10 Gb/s, with mixed digital and analog traffic multiplexed to a single optical fiber. This highlyintegrated
functionality is combined onto a silicon chip smaller than 4 x 10 mm, weighing < 5 grams. These chip-based
transceivers have been measured to withstand harsh g-forces, including sinusoidal vibrations with amplitude of 20 g
acceleration, followed by mechanical shock of 500 g acceleration. The components operate over a wide range of
temperatures, with no device failures after extreme temperature cycling through a range of > 125 degC, and more than
2,000 hours operating at 95 degC ambient air temperature. We believe that these recent advancements in planar
lightwave circuits are poised to revolutionize optical communications and interconnects in the aerospace and defense
We report on recent progress in simulations, physical layout, fabrication and hybridization of planar grating-based transceivers for passive optical networks (PONs). Until recently, PON transceivers have been manufactured using bulk micro-optical components. Today, advancements in modeling and simulation techniques has made it possible to design complex elements in the same silica-on silicon PLC platform and create an alternative platform for manufacturing of bi-directional transceivers. In our chips we simulated an integrated chip that monolithically combined planar reflective gratings and cascaded Mach-Zehnder interferometers. We used a combination of the finite element method and beam propagation method to model cascaded interferometers with enhanced coupling coefficients. Our simulations show that low-diffraction order planar reflective gratings, designed for small incidence and reflection angles, possess the required dispersion strength to meet the PON specifications. Subsequently, we created structures for passive alignment and hybridized photodetectors and lasers. We believe that advancements in simulation of planar lightwave circuits with embedded planar reflective gratings will result in displacement of the thin-film filters (TFFs) technology in many applications that require a high degree of monolithic and hybrid integration.
The deployment of Passive Optical Networks (PON) for Fiber-to-the-Home (FTTH) applications currently represents
the fastest growing sector of the telecommunication industry. Traditionally, FTTH transceivers have been
manufactured using commodity bulk optics subcomponents, such as thin film filters (TFFs), micro-optic collimating
lenses, TO-packaged lasers, and photodetectors. Assembling these subcomponents into a single housing requires active
alignment and labor-intensive techniques. Today, the majority of cost reducing strategies using bulk subcomponents
has been implemented making future reductions in the price of manufacturing FTTH transceivers unlikely. Future
success of large scale deployments of FTTH depends on further cost reductions of transceivers. Realizing the necessity
of a radically new packaging approach for assembly of photonic components and interconnects, we designed a novel
way of hybridizing active and passive elements into a planar lightwave circuit (PLC) platform. In our approach, all the
filtering components were monolithically integrated into the chip using advancements in planar reflective gratings.
Subsequently, active components were passively hybridized with the chip using fully-automated high-capacity flip-chip
bonders. In this approach, the assembly of the transceiver package required no active alignment and was readily
suitable for large-scale production. This paper describes the monolithic integration of filters and hybridization of active
components in both silica-on-silicon and silicon-on-insulator PLCs.
Recent deployments of fiber-to-the-home (FTTH) represent the fastest growing sector of the telecommunication
industry. The emergence of the silicon-on-insulator (SOI) photonics presents an opportunity to exploit the wide
availability of silicon foundries and high-quality low-cost substrates for addressing the FTTH market. We have now
demonstrated that a monolithically integrated FTTH demultiplexer can be built using the SOI platform. The SOI filter
comprises a monolithically integrated planar reflective grating and a multi-stage Mach-Zehnder interferometer that were
fabricated using a CMOS-compatible SOI process with the core thickness of 3.0 ?m and optically insulating layer of
silica with a thickness of 0.375 ?m. The Mach-Zehnder interferometer was used to coarsely separate the 1310 nm
channel from 1490 and 1550 nm channels. Subsequently, a planar reflective grating was used to demultiplex the 1490
and 1550 nm channels. The manufactured device showed the 1-dB bandwidth of 110 nm for the 1310 nm channel. For
the 1490 nm and 1550 nm channels, the 1-dB bandwidth was measured to be 30 nm. The adjacent channel isolation
between the 1490 nm and 1550 nm channels was better than 32 dB. The optical isolation between the 1310 nm and
1490 and 1550 nm channels was better than 45 dB. Applications of the planar reflective gratings in the FTTH networks are discussed.
Recent progress in the development of planar reflective gratings has resulted in the demonstration of multiplexers, comb filters, interleavers, power monitors, and receivers for long-haul and metro-area networks. Until recently, all of these devices were based on a single-grating architecture. We have now successfully designed, fabricated, and tested optical chips that are composed of cascaded planar reflective gratings. The chips have been realized in both additive and subtractive dispersion configurations. The versatility of cascaded gratings was utilized to produce a variety of optical responses, including single-mode transmission of wide bands (> 100 nm) with simultaneous demultiplexing of narrow optical channels with Gaussian and box-like responses. We have further demonstrated that cascaded gratings can be used to suppress optical noise and improve isolation. The devices were fabricated using a standard silica-on-silicon process with a refractive index contrast of 0.82% and have a remarkably small footprint of less than 0.3 sq. cm. We discuss the potential for tailoring of cascaded planar reflective gratings for applications in biophotonics, spectroscopy, and telecommunications.
Optical add/drop multiplexers (OADMs) have emerged as the key enabling components for building long-haul and metro-area networks. The wide-spread deployment of OADMs in the access market will depend on the availability of cost-effective integrated solutions. We have successfully fabricated a fully-integrated OADM based on planar reflective gratings. The device uses a combination of two grating elements arranged in a subtractive dispersion configuration. The first grating demultiplexes a 300-nm-wide band and drops optical channels at 1490 nm and 1550 nm, commonly used by service providers to send information to the end user. The second grating completely counter-balances the dispersion properties of the first grating and ultimately yields zero dispersion in the output waveguide. Such a configuration allows the transmission of optical signals though the OADM in an ultra-wide band spanning 1250 to 1410 nm. This ultra-wide 'through' band is a critical step allowing the use of low-cost lasers, without temperature stabilization, for sending data to a service provider. The OADM was manufactured using an industry standard silica-on-silicon process which was augmented with grating facet formation and metallization. In spite of using low refractive index contrast waveguides (0.82%), the device had a remarkably low footprint of only 0.25 square centimeters. Applications of the OADM in access market networks is discussed.
A new approach for constructing devices of various free spectral ranges (FSRs) is described. We show that devices with different FSRs can be built around the same aberration-free architecture based on elliptical grating facets. Elliptical facets, combined with double astigmatic point design, are demonstrated to lead to dramatic improvements in reflective grating performance compared to traditional flat facet designs. A discussion on the proper selection of the grating order for devices with various FSRs is given. The proposed theory was applied to manufacture devices with various FSRs. A standard silica-on-silicon process was used to fabricate interleavers with narrow FSR of 0.8 and 1.6 nm. Subsequently, we show how the above methodology can be used to scale the reflective grating design to devices with wide FSR. We applied the theory to produce coarse wavelength division multiplexing filters with FSR in excess of 500 nm. The filters exhibited insertion losses of 2.5 dB and polarization dependent losses of less than 0.2 dB. Applications of wide FSR devices in metro edge and access networks are discussed.
We report the results of an experimental study on near- threshold gain mechanism in optically pumped GaN epilayers and GaN/AlGaN separate confinement heterostructures (SCHs) over the temperature range of 10 to 300 K. We show that in GaN epilayers the near-threshold gain mechanism is inelastic exciton-exciton scattering for temperatures below approximately 150 K, whereas at elevated temperatures an electron-hole plasma is the dominant gain mechanism. An analysis of the relative shift between the spontaneous emission and lasing peaks in SCH samples, combined with the temperature dependence of the lasing threshold, reveals that exciton-exciton scattering is the dominant gain mechanism leading to low-threshold ultraviolet lasing in the GaN/AlGaN SCH over the entire temperature range studied. Strongly polarized (TE:TM > 300:1) lasing peaks were observed in a wavelength range of 358 - 367 nm. We found that high finesse lasing modes originated from self-formed microcavities in the AlGaN and GaN layers. The lasing threshold was measured to be as low as 15 kW/cm<SUP>2</SUP> at 10 K and 105 kW/cm<SUP>2</SUP> at room temperature. Based on our results we suggest ways for the realization of GaN-active-medium UV laser diodes.
Stimulated Emission and Pump-Probe studies were performed in GaN, InGaN, and AlGaN epilayers as well as GaN/AlGaN separate confinement heterostructures. We show that in GaN epilayers the near-threshold gain mechanism is inelastic exciton-exciton scattering for temperatures below approximately 150 K, whereas at elevated temperatures electron-hole plasma is the dominant gain mechanism. An analysis of the relative shift between the spontaneous emission and lasing peaks in SCH samples, combined with the temperature dependence of the lasing threshold, reveals that exciton-exciton scattering is the dominant gain mechanisms leading to low-threshold UV lasing in the GaN/AlGaN SCH over the entire temperature range studied. We further performed optical pumping of AlGaN epilayers at different temperatures. Stimulated emission has been observed in Al<SUB>x</SUB>Ga<SUB>1-x</SUB>N thin films for Al concentrations as high as x = 0.26, with a resultant stimulated emission wavelength as low as 328 nm at room temperature. This result indicated that AlGaN-based structures are suitable not only for use in deep-UV detectors, but also as a potential source of deep-UV laser radiation. The interband optical transitions in GaN and InGaN have also been studied at 10 K and room temperature using nondegenerate nanosecond optical pump-probe techniques. At low temperatures, strong, well- resolved features were seen in the absorption and reflection spectra corresponding to the 1s A and B exciton transitions. Broadening and decrease in intensity of these features were studied as the function of excitation pump density. We found that values of induced transparency and induced absorption are extremely large in GaN epilayers. The pump-probe results in GaN epilayers were directly compared to ones obtained from InGaN films. Significant differences in near-bandedge absorption changes were clearly observed between the two materials.
Edge and surface-emitted stimulated emission (SE) in optically pumped GaN thin films was studied in the temperature range of 20 K to 700 K. The single-crystal GaN films used in this work were grown by MOCVD on sapphire and 6H-SiC substrates. We have observed that the SE peak shifts from 360 nm at 20 K to 412 nm at 700 K, which is the highest temperature at which SE has been reported for this material. The temperature sensitivity of the SE threshold was studied over the entire temperature range. The characteristics temperature was found to be about 170 K over the temperature range of 300 K to 700 K for samples grown on both sapphire and SiC substrates. The energy position of the SE and spontaneous emission peaks were shown to shift linearly to longer wavelengths with increasing temperature and empirical expressions for this shift are given. We demonstrate that the energy separation between the spontaneous and SE peaks gradually increases from 90 meV at 300 K to 200 meV at 700 K indicating that an electron-hole plasma is responsible for the SE mechanisms above room temperature (RT). We demonstrate that the surface-emitted SE in GaN epilayers comes from cracks, burn spots, and other imperfections, and is due to the scattering centers and, under strong optical excitation, become points of origin for burning of the sample surface. This study shows that GaN has an extremely low temperature sensitivity compared to other semiconductors and is suitable for the development of light emitting devices that can operate significantly above RT.