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
industries.
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.
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.
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.
KEYWORDS: Photonic integrated circuits, Optics manufacturing, Manufacturing, Optical filters, Bridges, Transceivers, Semiconductor lasers, Signal detection, Fiber to the x, Video
In the quest for cost-effective bi-directional transceivers for the fibre-to-the-premise (FTTP) applications, this article describes the development of such devices manufactured on the Planar Lightwave Circuit (PLC) platform. The transceiver consists of the optical head, which receives, filters and detects the optical signals and the electronics, which processes these signals according to specific protocols. In this article, the realization of the optical head (known as a diplexer or triplexer) will be the main focus, with key results, such as the manufacture of the hybridization platform, the design and performance of the optical filters and the hybridization on the laser and photodiodes, presented. The challenges faced in the development will be described as well as the next steps to achieving the final product.
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.
This paper presents theoretical and experimental results detailing the design and performance of arrayed waveguide grating (AWG) demultiplexers fabricated in silicon-on- insulator (SOI). The SOI waveguide is inherently multimode because of the high refractive index difference between Si and SiO2, although appropriate tailoring of the rib width to height ratio can be used to make single mode rib waveguides. This single mode condition cannot be met in the input and output combiner sections, which can therefore support many higher order modes. Modeling results demonstrate that coupling from a single mode ridge waveguide to the fundamental slab mode is typically two orders of magnitude larger than the coupling to higher modes. Hence the effect of multimode combiners on performance should be minimal. We also present calculations of bending losses which indicate that with a Si thickness of 1.5 micrometers , single mode rib waveguides can be made with radii of curvature as low as 200 micrometers . Such waveguides can also be made with zero birefringence. AWG devices were fabricated with 8 channels centered around (lambda) equals 1550 nm, and chip sizes less than 5 X 5 mm. The performance of these devices is compared with our modeling results.
For Si-based photonic integrated circuits (PICs), photodiodes with good responsivity at 1.3 micrometer and 1.55 micrometer wavelengths made of Si-based materials are highly desirable. Previously, work has been reported using epitaxial SiGe planar multiple quantum wells (MQWs) on Si substrates. Since the high lattice mismatch limits the maximum Ge concentration and SiGe layer thickness, responsivity at 1.55 micrometer was limited. Under appropriate growth conditions, strained SiGe QW's grow with periodic thickness variations along the surface plane. Ge tends to migrate towards the thickness maxima. This increase in local Ge concentration and the reduced quantum confinement at the coherent wave crest produces strained QW's with significantly lower band-gaps compared to planar QW's with the same nominal composition. In this paper, we report the first MSM SiGe waveguide photodetectors fabricated using coherent wave growth mode with a band gap below 800 meV. The heterostructures were grown on a SOI substrate by an ultra- high vacuum chemical vapor deposition (UHVCVD) system. The 2 micrometer thick Si/SiGe/Si on oxide structure provides waveguiding for the detector structures and permits effective fiber coupling. Preliminary measurements have demonstrated internal responsivities of approximately 1 A/W at 1.3 micrometer wavelength and 0.1 A/W at 1.55 micrometer wavelength for a 240 micrometer long device.
The simulation and performance of dual-wavelength demultiplexers fabricated in SiGe are presented. The device design is a symmetric directional coupler optimized for 1.3/1.55 micrometer demultiplexing. Modeling results using the Beam Propagation Method are presented as a means of examining the fabrication tolerances and design considerations of the devices. Initially the duplexers were fabricated with a Si.97Ge.03 core using chemical etching, and displayed crosstalk of -19 dB and -15 dB in the 1.3 and 1.55 micrometer wavelength channels, respectively. This performance was enhanced by thermal tuning, resulting in isolation of -21 dB and -18 dB. The high degree of strain in pseudomorphic SiGe layers results in highly birefringent waveguides. This characteristic restricts effective duplexer operation to a single polarization, but suggests that the same devices can be modified to act as polarization splitters for a chosen wavelength. Modeling of the devices in this configuration is also presented. The simple device design used for demultiplexers and splitters was chosen to evaluate the potential for fabrication of SiGe-based optoelectronic devices using standard VLSI processing relying on the local oxidation of silicon (LOCOS). A second set of devices was successfully fabricated using LOCOS and effectively separated 1.15 and 1.3 micrometer wavelength signals. This performance is compared to similar devices that were wet-etched.
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