Opto-electronic hybrid integraiton using a silica-based planar lightwave circuit (PLC) platform is an attractive way to realize the various kinds of opto-electronic components required for future photonic networks. This paper briefly introduces the concept and basic techniques used for PLC hybrid integration, and describes recent advances in this field. We also report on several high-performance optical devices that we recently developed using this technology.

We report on a hybrid integrated metro ring node subsystem on a chip that consists of an array of four independent reconfigurable optical add-drop circuits, each with power monitoring and automatic load balancing, and supporting shared and dedicated protection protocols in two-fiber metro ring optical networks. The four-channel metro ring node chip has polymeric optical waveguiding circuitry, thermally actuated with heaters consisting of resistive strips of metal. Photodiode arrays are flip-chip mounted on top of 45° mirrors cut in the waveguides of optical power taps. The mirrors are fabricated by Excimer laser ablation of the polymer followed by smoothing and metalization. The non-integrated implementation of a metro ring node uses 48 discrete elements, namely 8 1×2 switches, 8 2×2 switches, 8 VOAs, 12 taps, and 12 photodiodes. The proposed integrated solution is an exemplary embodiment of the benefits of optoelectronic integration as it provides, when compared to the discrete solution, significant cost reduction, space savings, lower electrical power consumption, higher reliability (fewer devices, runs cooler), and fewer board-level fiber interconnects.

We demonstrate various types of ultrafast all-optical signal processing with Symmetric-Mach-Zehnder (SMZ) all-optical switches incorporating semiconductor optical amplifiers. By using a hybrid-integrated SMZ switch, error-free operations of 336-Gb/s demultiplexing, 42-Gb/s pulse regeneration, and 42-Gb/s wavelength conversion have been achieved. We have also verified the capability of higher bit rate operation by showing 84-Gb/s pulse regeneration and 168-Gb/s wavelength conversion with the variants of the SMZ switch.

This paper reviews some of the key enabling technologies for present and future WDM communications systems. This review concentrates mainly on advances in material growth, lasers, modulators, and fast photodetectors as the basis modern communication systems. Emphasis is placed on integration of components, the need for reconfigurable switching elements, and mass production of a generic intelligent transceiver.

The Sampled-Grating Distributed-Bragg-Reflector laser(SGDBR) provides wide tunability (>40nm), and high output power (>10mW). Driven by the demand for network reconfigurability and ease of implementation, the SGDBR has moved from the research lab to be commercially viable in the marketplace. The SGDBR is most often implemented using an offset-quantum well epitaxial structure in which the quantum wells are etched off in the passive sections. Alternatively, quantum well intermixing has been used recently to achieve the same goal - resulting in improved optical gain and the potential for multiple bandgaps along the device structure. These epitaxial "platforms" provide the basis for more exotic opto-electronic device functionality exhibiting low chirp for digital applications and enhanced linearity for analog applications. This talk will cover state-of-the-art opto-electronic devices based on the SGDBR platform including: integrated Mach-Zehnder modulators, and integrated electro-absorption modulators.

In this paper we report that the electro-absorption modulators can also enable long haul and ultra-long haul transmission in the wavelength-division multiplexed fiber optic networks, with potentially much smaller equipment size and at much lower cost. The error-free transmission distance can be as long as 2,000 km using standard single-mode fiber for both non-return-to-zero and return-to-zero data formats, and can be even longer with forward-error correction technology. Sufficient optical signal-to-noise ratios are maintained and wide-open eye diagrams are observed. No significant path penalties attributable to chirp are observed. A low cost return-to-zero transmitter, simple in architecture and small in physical size, is demonstrated by using only one electro-absorption modulator. These results indicate the maturity, particularly the chirp manageability, of the electro-absorption modulator technology for metro and long haul as well as ultra-long haul applications. Therefore, large-scale opto-electronics integration in wavelength-division multiplexing equipment becomes a promising reality.

In silicon based photonic circuits, optical modulation is usually performed via the plasma dispersion effect, which is a relatively slow process. Until recently, most reserachers utilized Silicon on Insulator (SOI) waveguides with cross secitonal dimensions of the order of 5 microns. This limits the speed of devices based on the plasma dispersion effect due to the finite transit time of charge carriers. Consequently moving to smaller dimensions will increase device speed, as well as providing other advantages of closer packaging density, smaller bend radius, and cost effective fabrication. As a result, the trend in recent years has been a move to smaller waveguides, of the order of 1 micron in cross sectional dimensions. However, coupling light to such small waveguides is relatively inefficient. In the literature, the problem of coupling optical fibers to thin semiconductor waveguides has not been solved sufficiently well to obtain both high coupling efficiency and good fabrication tolerances, due to large difference between the fiber and the waveguide in both dimensions and refractive indices. In this paper, we discuss both the desing of small waveguide modulators (of the order of ~1 micron) together with a novel theoretical solution to the coupling problem. An example of coupling light to a thin silicon waveguide is given, as well as a discussion of a number of modulator design issues.

A new electrical parallel-to-serial converter (PSC) is proposed for optical communication networks. The PSC uses a simple circuit scheme that markedly reduces the fall time of a ordinary MSM-PD without degrading either its sensitivity or ease of fabrication. An InP-based 4:1 PSC is shown to convert 4-ch parallel electrical signals into a serial 10-Gbit/s 4-bit electrical signal when MSM-PDs in the PSC are optically triggered. It has several advantages including support of burst signal input, low driving voltage, compactness, ease of fabrication, and low power consumption. The electrical PSC is used in a new photonic PSC. By effectively combining electrical multiplexing with optical multiplexing, the photonic PSC can generate a 40-Gbit/s 16-bit optical packet from 16-ch parallel electrical signals with frame rates of 40 MHz. The electrical PSC is also applied to a label comparator for bypass/drop self-routing of optical packets. The label comparator, consisting of the 4:1 PSC and an electroabsorption modulator (EAM), properly processed 10-Gbit/s burst optical packets with no preamble even when a local address was changed at 40 MHz.

Photonic crystals exhibit many unique properties that are particularly useful for producing the next generation of photonic devices. These artificially micro structured materials contain periodically modulated dielectric constant on a length scale comparable to the wavelength of light. It has been realized that the lack of fabrication technology and the lack of photonic materials significantly slow down the technology advancement in this area. In this paper, we will present our research progresses in forming unique photonic polymers and their optoelectronic applications.

We apply the Finite-Difference Time-Domain algorithm to the problem of calculating modal loss in microstructured optical fibers. We use periodic boundary conditions in the longitudinal direction to isolate a mode that decays through transverse perfectly-matched layer boundaries. The loss coefficient is extracted by a direct monitoring of the energy enclosed in the simulation domain. Predictions of the method are compared to results from beam propagation and multipole approaches, and finally the tool is applied to fibers containing elliptical air holes.

In this paper a tunable optic system is presented for use in various optical systems. In contrast to most of the tunable optical components which are composed of a waveguide, an electro-optical layer, and electrodes of different materials. The new system consists of a TiO₂ waveguide with ZnO as a functional layer on top. The TiO₂ layer acts as a high index waveguide, the ZnO system consists of a ZnO:Al/ZnO/ZnO:Al sandwich structure. The ZnO film is used as an electro-optical cladding for the TiO₂ waveguide while the two ZnO:Al films act as transparent electrodes. Applying a voltage results in a shift of the effective refractive index of the waveguide because of the electro-optical effect of the ZnO. The TiO₂ film is deposited on SiO₂ by a PECVD-process from a metal organic precursor CpTiCh (cyclopentadienyl-cycloheptatrienyl-titanium). ZnO and ZnO:Al are rf-sputtered from a Zn target and ZnO:Al target, respectively. While both ZnO layers are c-oriented polycrystals, the TiO₂ grows in a nanocrystalline formation without any texture. The configuration of the high index material TiO₂ in combination with the transparent and electro-optical ZnO layer allows the use in integrated optical subsystems such as active couplers or active micro ring resonators. The system is designed for a wavelength of 1550 nm.

We describe design of 2.5Gb/s CMOS optical transceiver array using 0.35 mm CMOS technology. The transceiver array consists of Laser Diode (LD) driver and limiting amplifier with current decision circuit. CMOS LD driver offers the capability of independent DC and modulation current adjustments. The DC component used to pre-bias the LD is adjustable at a range of 0~30 mA. Optical receiver consists of current decision circuit with voltage output buffer. Optical receiver allows wide dynamic range of input current. The decision circuit uses positive feedback from the cross-gate connection of two NMOSs to increase the gain of the decision element, while the output buffer convert the output of the decision circuit into a logic signal(i.e., 0 or 3.3V). With this design technique, we have succeeded in developing a CMOS optical transceiver array with high performance.

Aircraft can be equipped with a number of radios which need to be operated simultaneously and in the full-duplex mode. To prevent interference required a combination of antenna separation, frequency separation, transmitter power control, and specific design considerations. Interference cancellation systems often use a common antenna, a common Low Noise Amplifier, and signal splitting to each receiver. Onboard transmitter signal would be sampled, delayed, adjusted in amplitude equal to the unwanted received signal (the transmitted signal), adjusted to 180 degrees out of phase with the unwanted received signal; and injected prior to the LNA, thus canceling the unwanted transmitter signal. The time delay can be accomplished by using coaxial cable or by digitizing the transmit signal spectrum. An optical alternative would convert the RF spectrum to the optical domain, use fiber or polymeric waveguide for time delay, and then convert back to the RF domain for injection into the receiver system to accomplish the cancellation. The optical system would have to process the RF signal(s) without creating distortion, and provide sufficient flexibility to allow the system to be readily adjusted to optimize performance. The optical system, installed at the receiver, would consist of a transmitter, a programmable delay line and a receiver. An attractive implementation would be a single chip to incorporate the optical generation and switching functions to redirect the optical signal into a selected optical path. The optical paths might be polymer waveguides patterned onto a PCB. With newly developed optoelectronic integration technologies, the combination of sources, detectors, waveguide switches and amplifiers becomes a realistic possibility. The result is a rugged system with low cost and high performance. This paper describes these optical technologies and the optical interference cancellation implementation approach.

We have designed and demonstrated a low cost microwave-photonic phased-array antenna with a novel, simplified 16-channel optical true-time-delay array generator for microwave beamformign/steering. The true-time-delay array generator is built with simple optical path duplication architecture eliminating the need for optical switches, 1xN splitters, multiple lasers or Wavelength Division Multiplexing devices. Therefore, the cost is reduced by orders of magnitude as compared with other designs in the literature. To make a low cost demonstration, we have first fabricated the true-time-delay generator using free-space optics having mechanical displacemnt which can provide continuous time delays and power distribution for the array in the optical domain. A commercial 3 GHz laser transmitter and 16 photodetectors are used for optical-microwave link/transformation. The phased-array antenna has demonstrated a continuous scan/steering from -45° to 45° with reduced side lobes using a Dolph-Tschbyscheff array power distribution. As such, it provides an immediate solution, for the first time, to fabricate a realistic prototype photonic phased array antenna with a realistic price. We are also working on a long term solution which is to miniaturize the true-time-delay array generator by using optical MEMS technology and photonic bandgap structures so that our future true-time-delay array generator can be built in chip scale.

In this paper we report on recent improvements in phase noise and frequency stability of a 10 GHz opto-electronic oscillator. In our OEO loop, the high Q elements (the optical fiber and the narrow bandpass microwave filter) are thermally stabilized using resistive heaters and temperature controllers, keeping their temperature above ambient. The thermally stabilized free running OEO demonstrates a short-term frequency stability of 0.02 ppm (over several hours) and frequency vs. temperature slope of −0.1 ppm/°C (compared to -8.3 ppm/°C for non thermally stabilized OEO). We obtained an exceptional spectral purity with phase noise level of -143 dBc/Hz at 10 kHz of offset frequency. We also describe the multi-loop configuration that reduces dramatically the spurious level at offset frequencies related to the loop round trip harmonic frequency. The multi-loop configuration has stronger mode selectivity due to interference between signals having different cavity lengths. A drop of the spurious level below −90 dBc was demonstrated. The effect of the oscillator aging on the frequency stability was studied as well by recording the oscillator frequency (in a chamber) over several weeks. We observed reversal in aging direction with logarithmic behavior of A ln(B t+1)-C ln(D t+1), where t is the time and A, B, C, D are constants. Initially, in the first several days, the positive aging dominates. However, later the negative aging mechanism dominates. We have concluded that the long-term aging behavioral model is consistent with the experimental results.

Electroabsorption modulators (EAM) have been widely used in externally modulated RF link. In the conventional intensity modulation format, the RF efficiency of such link is critically dependent on the optical saturation of the EAM and the fiber dispersion. In an effort to minimize the dependence on these two factors, we have conducted an experimental study of an optical mixer that employs two heterodyned lasers to generate an optical local oscillator (LO). In contrast to the conventional schemes where both laser beams are launched into the same EAM, the EAM in the present scheme modulates only one of the laser beams. In this way, improvement in RF gain is obtained since it is not limited by the optical saturation of the EAM. The resulting optical spectrum shows that this scheme is equivalent to the optical single side band transmission. For sub-octave bandwidth applications, the dispersion effect in the fiber can be ignored.

A Radio-over-Fiber system with simplified Base Station (BS) is proposed in which a single chip DBR Reflective Electro-absorption Modulator (REAM) serves both as an optical transceiver and as a mixer at the BS. It enables full duplex optical transmission for base band and RF band services simultaneously due to good isolation between uplink and downlink at the same chip. Grating structure is incorporated into the EA modulator for the sake of system design. It also improves yield and efficiency of high-speed devices.

Wavelength-controlled true-time delay modules based on the dispersive hologram-waveguide are presented here to provide continuous beam-scanning for a X-band phased-array antenna system. The true-time delay modules operating in the 1550nm region were fabricated with continuously tunable time delays from 5ps to 64ps. All-optical wavelength conversion in the semiconductor optical amplifiers was proposed in the system to extend the beam-scanning scope from one dimension to two dimensions. The wavelength-controlled time delays were measured across the x-band (8-12GHz) in the experiment.

Superposed WDM fiber Bragg gratings (FBGs) provide a novel method to realize a large number of very short time-delay steps in applications such as photonics-based beam-forming for multi-beam phased arrays. In this paper, a systematic design method for superposd FBGs is presented. This provides informaiton for the key requirements of grating length determination, grating apodiziation function selection and channel spacing control, and the selection of a suitable method to obtain a linear time-delay profile of the designed superposed FBG. Grating length versus maximum number of superposed gratings for ps time-delays under different apodizations is presented. The effects of grating apodization on wavelength channel spacing of the superposed gratings is also given, and an optimization is described to minimize the wavelength step so as to maximize the nubmer of channels in a given optical spectral width. Finally, a technique for time-delay profile linearization of WDM superposed FBGs with a large number of ps delay steps is presented. Using these design procedures, results are given for a 32-superposed WDM FBG unit which shows 1 ps delay step and 1.7% standard deviation from linearity onthe time-delay profile, while maintaining >99% reflectivity for all grating channels and keeping the total refractive index chagne inteh fiber below 0.01.

We show that asymmetric dual cavity filters, comprising one cavity of Ta₂O₅ (H) and the other of SiO₂ (L), can be designed so that the asymmetry in the transmission intensity between the two peaks is sensitive enough to detect H/L ratio changes down to 1.0001. This accuracy is necessary for precise uniformity determinations for 100 GHz filter production. We show that interpretation of asymmetry changes is still valid despite small absorption or scatter losses due to either of the materials, and also despite small layer thickness errors which have been optically compensated by the optical monitoring system during deposition of the filter. We illustrate the use of asymmetry in adjusting the H/L ratio within 1.0001 at larger and smaller radii away from the optical monitor band on an 8" diameter disc, and also for checking azimuthal variations. Combining the asymmetry data with the wavelength shift data enabled the individual H and L calibrations to be deconvoluted from the multilayer data. This technique is far more accurate than measuring individual material calibration runs, and also takes account of multilayer effects such as interfacial mixing and process sequencing. Ion beam sputtering process parameters were varied to maximize the uniformity and increase the yield for cost-reduction of 100 GHz filters, and the application to 50 GHz filter yield enhancement is discussed.

Wavelength division multiplexing (WDM) and demultiplexing (WDDM) devices are considered to be one of the key elements in optical networks. WDM device by using conventional, such as thin film filter and AWG based devices, can only be used as one device either multiplexing or demultiplexing. Here we propose a novel structure for WDM device, which is based on free-space diffraction-grating multiplexer/demultiplexer technology. By using multi-deck fiber arrays one device can function as both multiplexers and demultiplexers simutaneouly. In this paper we will give the structure and working principle for such device. We will also discuss the optical design issues. Finally we will present our experimental results. The devices can be used for bi-directional transmissions and optical transceivers in optical networks.

Data bit rate, 1dB pass band and device dimensions are the key properties of dense wavelength division multiplexer (DWDM) devices. For blazed grating based DWDM structure, analysis shows that the bit rate is limited by the pulse broadening introduced by the grating. To reduce the pulse broadening, the output fiber array channel-to-channel pitch must be reduced. By decreasing the output channel pitch, 1dB pass band can be increased and the device dimensions can be shrunk. In this paper, we propose an ion-exchanged glass waveguide chip to be inserted into the WDM device in order to reduce the output channel spacing. To fabricate the low loss fiber compatible waveguides, two-step K⁺-Na⁺ and Ag⁺-Na⁺ ion-exchange process using BK7 glass as the substrate has been investigated. The optimal process conditions have been found. The waveguides fabricated by this two-step process have a propagation loss of 0.3dB/cm, a coupling loss to single mode fiber of 0.4dB and a polarization dependent loss of 0.1dB for 5cm long waveguides. The design of a 48 channel DWDM with this glass waveguide chip is given.