K-band operation of asymmetric Fabry-Perot modulators

The authors report preliminary high-frequency, small-signal optical measurements of asymmetric multiple-quantum well (MQW) Fabry-Perot electrooptic modulators which indicate that the electrical bandwidth for these devices is about 15 GHz at the onset of saturation, and as high as 21 GHz at low optical intensity-higher than any other measurements published to date. The modulators, 30 μm×30 μm in size, are integrated with on-chip microwave probe pads. The authors detail the fabrication process developed to achieve these high operating frequencies and predict from device models the maximum RC-limited operating frequencies for these devices     

Nonsymmetric Mach-Zehnder interferometers used as low-drive-voltagemodulators

Optical communications systems require fast optical modulators which are compatible with existing microwave-frequency drive circuits and which have low voltage requirements. The authors have designed and demonstrated a novel type of integrated optical modulator: a nonsymmetric Mach-Zehnder interferometer, which has both low switching voltage and efficient coupling to microwave-frequency drive circuits. In 2-cm-long devices, made in z-cut LiNbO₃, switching voltages of 3.2 V and better than 20-dB extinction ratio were measured at λ=1.3 μm. Device performance in the nonsymmetric devices was independent of the asymmetry, which indicates that devices of this kind are tolerant of fabrication variations     

Semiconductor photonic functional devices

Recent advances in semiconductor photonic functional devices based on the nonlinearities of laser diodes (LDs) are reviewed. Because the current research is driven primarily by the needs of optical fiber communications systems, most devices are made of InGaAsP and operate at wavelengths of between 1.3 and 1.55 μm, the wavelength window in which optical fibers have their most favorable properties. Minimum loss is at 1.55 μm, and zero dispersion is at 1.3 μm. AlGaAs devices operating at around 0.85 μm have also been reported. Two types of optical bistability-absorptive and dispersive-are explained, and devices based on them are described. Switching characteristics of bistable devices are discussed. Tunable wavelength converters and filters are also examined. Applications to all-optical communication and to optical switching systems are considered. Directions for future development are indicated     

Electro-optic modulation in a chopped quantum-well electrontransfer structure

The authors demonstrate the first electron-transfer Mach-Zehnder modulators to operate in the 1.55 μm wavelength range. The use of chopped quantum wells provides a wide operating bandwidth compatible with Er-doped fibre amplified systems (>40 nm about 1.54 μm) for electrorefractive waveguide devices. In this wavelength range, obtained voltage-length products are for the 180° optical phase shift V_π×L between 2.7 and 4.0 Vmm     

Optical harmonic mixers

Mixing of optical signals in a waveguide via the second-order nonlinear effect in semiconductors was studied theoretically and experimentally. The InP-based material system was the main focus of the investigations. Both the up-conversion leading to second-harmonic generation and the down-conversion leading to optical rectification were analyzed and experimentally verified. Experiments in nonlinear optical waveguide structures at 1.55 μm wavelength have shown the existence of both the quadratic second-harmonic signals as well as the linearly dependent optical rectification signals. Quantitative estimates confirmed a good agreement between the theory and the experiment. Some interesting applications for high-speed optical data processing required in transparent optical telecommunication systems have been identified. Most attractive functions proposed to be implemented with these types of devices are, for example, flexible-to-bit-rate all-optical clock extraction in a transparent high-performance optical telecommunication network or accurate and fast measurement of optical frequency for optical synchronization needs in wavelength-division-multiplexing systems     

Silicon optical modulators at 1.3-μm based on free-carrierabsorption

Silicon optical waveguide modulators, appropriate for operation in the 1.3-1.55 μm wavelength region, have been fabricated and their performance characterized at the wavelength of 1.3 μm. The modulator structures consist of p-i-n diodes integrated with silicon waveguides; device operation is based on free-carrier absorption. Modulation depths of -6.2 dB and response times less than 50 ns have been measured. Experimental results are compared with the p-i-n diode theory. It is argued that the device is suitable for integration with silicon electronics and silicon optoelectronic devices. The response times measured for the current devices may be improved by reducing the transverse dimensions of the p-i-n structure     

Monolithic integration of multiple quantum well DFB lasers and electroabsorption modulators

Advanced fibre optics telecommunication systems rely on high performance components amongst which photonic integrated circuits (PICs) play a major role. In particular, there has been a growing need for low chirp optical sources, such as externally modulated distributed feedback (DFB) lasers. In this paper, the various monolithic integration schemes of multiple quantum well DFB lasers and electro-absorption modulators are reviewed and typical applications of these devices are briefly presented.     

Comparison of optical millimetre-wave system concepts with regardto chromatic dispersion

The author reports the impact of chromatic dispersion in optical millimetre-wave systems operating in the 1.55 μm wavelength window. Experimental and theoretical results confirm the dramatical millimetre-power degradation in conventional amplitude modulated systems using for example, external optical modulators. In comparison, by use of a similar setup a self-heterodyne-based system concept has been investigated showing negligible dependence on the chromatic dispersion     

Linearity of double heterostructure electroabsorptive waveguidemodulators

The linearity of electrooptical waveguide modulators based on the Franz-Keldysh effect with respect to the voltage-transmission characteristic has been investigated experimentally and theoretically. For a GaAs-AlGaAs double heterostructure modulator the values of the signal-to-noise ratio due to anharmonic and intermodulation distortion have been found to be small enough to suffice the requirements for analog optical transmission in multichannel broad-band fiber network systems compatible to existent cable networks. The results are also applicable to materials in the bandgap region of the optimum fiber transmission wavelengths around 1.3 μm and 1.5 μm. The advantages in comparison with directly modulated semiconductor lasers will be discussed     

Interferometric quantum well modulators with gain

Details on the design, fabrication and properties of the first interferometric intensity modulators with monolithically integrated optical gain are presented. These guided-wave devices make use of two sets of InP-based quantum well heterostructures that are contained in a single base wafer and individually designed for enhanced electrorefraction and amplification at 1.55 μm. The unique quantum well electrooptic properties as well as the fabrication techniques and waveguide design issues that determine final device performance are discussed     

Field-induced waveguides and their application to modulators

The fabrication and characterization of field-induced waveguides (FIGs) as well as guide/antiguide modulators which utilize the FIG concept are presented. Both theoretical and experimental results promise that a relatively strongly confined, low-loss optical waveguide can be achieved. By applying the appropriate bias to the modulator, a guiding or antiguiding situation can be created which corresponds to on- and off-states, respectively. An optical bandwidth from 1 to 1.55 μm, and a propagation loss of 1 dB at 1.3 μm has been achieved     

Scalable etched-pillar, AlAs-oxide defined vertical cavity lasers

Etched-pillar, bottom-emitting vertical cavity lasers have been fabricated using lateral oxidation of AlAs. The devices have threshold currents as low as 315 μA for a 4 μm×4 μm, three quantum well active region. Using the data and a numerical model the authors extract excess optical and carrier losses and an effective surface recombination velocity for the devices. The data show that size dependent optical scattering persists as the lasers are scaled to smaller sizes, but lateral carrier leakage is suppressed, allowing for scaling of lasers to small sizes to achieve lower threshold currents     

A new flip-chip bonding technique using micromachined conductivepolymer bumps

Using micromachining techniques with thick photoresists, a new conductive polymer flip-chip bonding technique that achieves both a low processing temperature and a high bumping alignment resolution has been developed in this work. By the use of UV-based photolithography with thick photoresists, molds for the flip-chip bumps have been patterned, filled with conductive polymers, and then removed, leaving molded conductive polymer bumps. After flip-chip bonding with the bumps, the contact resistances measured for 25 μm-high bumps with 300 μm×300 μm area and 400 μm×400 μm area were 35 mΩ and 12 mΩ respectively. The conductive polymer flip-chip bonding technique developed in this work shows a very low contact resistance, simple processing steps, a high bumping alignment resolution (<±5 μm), and a lower bonding temperature (~170°C). This new bonding technique has high potential to replace conventional flip-chip bonding technique for sensor and actuator systems, bio/chemical μ-TAS, optical MEMS, OE-MCM's, and electronic system applications     

High-speed bulk InGaAsP-InP electroabsorption modulators withbandwidth in excess of 20 GHz

Bulk InGaAsP-InP electroabsorption optical modulators with high extinction ratio, low drive voltage, and high modulation bandwidth at λ=1.3 μm are reported. The devices have a tapered-fiber-to-modulator-to-tapered-fiber extinction ratio greater than 20 dB at a drive voltage of <5 V. Very low capacitance modulators (<0.2 pF) were fabricated using SiO₂ bonding pad isolation, resulting in a measured electrical modulation bandwidth in excess of 20 GHz     

A novel processing technique to fabricate planar InGaAsP/InP electroabsorption waveguide modulators

A novel processing technique has been developed to fabricate planar electroabsorption waveguide modulators in compound semiconductor heterostructures. The lateral confinement of light is achieved by introducing controllable, reproducible, and stable stresses into semiconductor heterostructures using WNi surface stressor stripes, which also serve as electrodes for the waveguide modulators. Self-aligned helium implantation is employed to achieve electrical isolation using the Stressors as the templates for the ion masks. An increase as large as 33000 times has been obtained in the dc resistance between the neighboring waveguide modulators 25 μm apart. Propagation loss of 1.7 dB/cm is observed in the photoelastic waveguides at a wavelength of 1.53 μm following the He implantation. A post-implant thermal annealing at 310°C for 40 min increases the dc resistance between the neighboring devices to the maximum value, and at the same time reduces the optical loss to its value before ion implantation (less than 1 dB/cm). Using a combination of the photoelastic effect and helium implantation, planar InGaAsP/InP Franz-Keldysh-effect waveguide modulators 430 μm long with a 10 dB extinction ratio at 3 V for the TM mode have been fabricated. Planar electroabsorption quantum-confined Stark effect waveguide modulators have also been demonstrated. This planar device processing technique may prove valuable in future photonic integrated circuit technology.     

Components for optical communications systems: A review

Current trends in research and development of components for optical communication are reviewed. Emphasis is placed on active components for fiber-optic systems which have undergone recent major advances. Basic properties of optical fibers and recent technological improvements in splices, connectors, and source/detector-fiber couplers are presented first. This background information serves as a basis for describing recent developments in optical sources (e.g., device reliability, LED's and laser diodes) and photodetectors. Developments in both the 0.8-0.9-µm and 1.0-1.7-µm wavelength regions are covered. Also surveyed are results of research in areas of potential interest for optical communications: novel fiber-optic components, integrated optics (sources and modulators/switches), novel device fabrication methods, and integration of optical components.     

Micromachining for optical and optoelectronic systems

Micromachining technology opens up many new opportunities for optical and optoelectronic systems. It offers unprecedented capabilities in extending the functionality of optical devices and the miniaturization of optical systems. Movable structures, microactuators, and microoptical elements can be monolithically integrated on the same substrate using batch processing technologies. In this paper, we review the recent advances in this fast-emerging field. The basic bulk- and surface-micromachining technologies applicable to optical systems are reviewed. The free-space microoptical bench and the concept of optical prealignment are introduced. Examples of micromachined optical devices are described, including optical switches with low loss and high contract ratio, low-cost modulators, micromechanical scanners, and the XYZ micropositioners with large travel distance and fine positioning accuracy. Monolithically integrated systems such as single-chip optical disk pickup heads and a femtosecond autocorrelator have also been demonstrated     

应用于数字和模拟光通信的高速电吸收光调制器

An electro-absorption （EA） modulator is one of key components for optical fiber communications due to the high speed, small size, low voltage and integration ability with other semiconductor devices. A 40 Gb/s InGaAsP/InP multiple-quantum-well （MQW） EA modulator monolithically integrated with a semiconductor optical amplifier （SOA） was fabricated for digital communications. The modulator capacitance was reduced to obtain 40 GHz bandwidth, and the SOA section helped reduce the insertion loss from 18 dB to 3 dB. InGaAlAs/InP MQW EA modulators have also been fabricated and characterized for analog optical fiber communications. A low driving voltage of 2.7 V and high spurious free dynamic range of 107 dB·Hz^2/3 were estimated by static and dynamic measurements.     

Optical lithography stalls X-rays

The revival in optical lithography resulting from continuing advances in photoresists, phase-shifting masks, high-numerical-aperture step-and-repeat optical systems, multilevel-resist processing, and top-surface imaging techniques is discussed. Optical lithography is being used to make advanced IC chips, with 0.35 μm geometries in research, 0.5 μm in production. Ultraviolet (UV) light in the 200-400 nm range is the predominant system for IC manufacturing technology. Deep-UV lithography is not yet accepted for production processes, mostly due to the lack of commercially available positive and negative-tone photoresist systems for deep-UV wavelengths. In addition, negative-tone resists are temperature-sensitive and therefore hard to handle in a manufacturing environment, extensive gas-handling facilities are required for deep-UV excimer laser sources, and optical components have to be replaced often because the intense laser energy devitrifies lenses quickly