Recently there has been a great deal of interest in the growth of dilute nitride quaternary alloys, such as InGaNAs, on GaAs substrates for the fabrication of GaAs-based components and optoelectronic integrated circuits. The addition of indium to the binary compound GaAs produces a ternary with a lower bandgap and larger lattice constant. The incorporation of nitrogen in this ternary further decreases the bandgap while reducing the lattice constant. This makes it possible to grow material lattice-matched to a GaAs substrate but with a narrower bandgap offering the possibility of growing materials suitable for opto-electronic devices on a GaAs substrate while operating at wavelengths used in long-distance optical communications. These devices can then be integrated with mature GaAs device technologies (MESFET, HBT) in photoreceivers and receivers/transmitters for improved functionality and reliability,
lower cost, reduced size, etc.
We have fabricated metal-semiconductor-metal (MSM) photodetectors on 1-μm thick In .1Ga.9N.03As.97 epilayers, a composition that results in a bandgap in the 1.3 μm region. We report on the DC characteristics, frequency dependence and wavelength dependence of the photoresponse. The results are compared to MSMs fabricated on GaAs. The temporal response is not as fast as that of GaAs MSMs and may be related to low carrier mobility. This shortcoming has been reported as the cause for the lower-than-expected efficiency of solar cells fabricated using this quarternary. The effect of growth conditions and thermal processing on detector characteristics such as bandwidth and dark current were investigated. The challenges associated with the use of InGaNAs in photodetectors (such as defects, response speed, requirement for thermal anneal) will be discussed.
Optoelectronic (OE) switching is a promising approach for routing signals in fiber optic networks. Recently, the integration of a 4 X 4 MSM array with optical surface waveguides has been reported. This technique greatly simplifies the packaging of an OE switch. The on-chip polyimide optical waveguides perform the optical signal distribution to a matrix of MSMs which are responsible for the switching operation itself. Photoresponse bandwidths exceeding 4 GHz have been demonstrated. Another important characteristic of a switch is the switching speed since it determines the reconfiguration time. Mechanical and thermal optical waveguide switches offer switching speeds of the order of milliseconds which is sufficient for network traffic management but too slow for packet switching. We report measurements on the switching characteristics of a 4 X 4 optoelectronic switch performed in both the frequency and time domain. In the time domain, the individual crosspoints exhibit a rise time of 3 ns. However, a sizeable overshoot and ringing settles only after 35 ns. This constitutes the reconfiguration time at present. This is confirmed by measurements in the frequency domain of the electrical transmission from control line to output line. The 3-dB switching bandwidth is a few hundred megahertz. The 35 ns reconfiguration time indicates that it is already suitable for packet switching in a 10 Mb/s network. Switching speed measurements on individual MSMs suggests that modifications to the switch circuit could improve the switching time. The switch could also find application as a component in the wavelength conversion circuit of a WDM fiber optic network.
In this paper, a high-speed 4 X 4 optoelectronic switch with on-chip optical signal distribution is presented. The switching matrix integrates both metal-semiconductor-metal photodetectors and polyimide waveguides onto a monolithic structure. The bias-switched photodetectors are used to select which incident optical signals are to be detected and converted to electrical signals for distribution. The polyimide waveguides provide on-wafer distribution of the input optical signals to the detectors. This switch had an isolation of less than 48 dB at 1.0 GHz, a crosstalk of greater than -26 dB at 0.8 GHz, and a bandwidth between 4.3 to 6.5 GHz depending on the particular crosspoint being measured. This optoelectronic switch shows improved performance over a wider bandwidth compared to the 3 X 3 optoelectronic switch presented earlier. The current switching implementation is also easier to package because it does not require the precise alignment to discrete fibers over the photodetectors for optical distribution. Fabrication details, circuit configuration, and performance for the 4 X 4 optoelectronic switch will be discussed in greater detail in these proceedings.
Reliable, low cost, compact optical components are essential to the continuing development of fiber-based communications systems. Polymers offer the potential for cheap, low temperature processing of optical components, coupled with low optical losses, and good size and index matching with optical fibers. This paper describes the use of polyimide materials for the fabrication of passive optical devices and on-chip integrated optical interconnections. A variety of basic waveguiding structures, including bends and splitters have been demonstrated, with dimensions suitable for both multimode and single mode applications. The polyimides were patterned by standard photolithography and losses were typically 0.5-1.0 dB/cm. Multimode polyimide ridge waveguides have been integrated with metal-semiconductor- metal photodetectors. Efficient coupling of the light from the guide into the detector was achieved via gaps in the underlying optical buffer layer. The integrated waveguide/detectors have been fabricated into hybrid and monolithic 4 X 4 optoelectronic switches, which offer high bandwidth and low cross-talk. This technology has also been applied to the fabrication of integrated optical/electrical transmit/receive modules for use in optically controlled phased array antenna systems.
Optical fibers offer the wide bandwidth, low losses and low interference required in broadband network applications. Currently, routing the signals to their destination is done by converting the incoming optical signals to an electrical form, carrying out the switching function using electronic circuitry then reconverting to light for the next transmission stage. Recently, we have reported a 3 by 3 optoelectronic switch which combines the functions of conversion and switching. This matrix monolithically integrates metal-semiconductor-metal (MSM) detectors with amplifiers. Very good isolation and crosstalk characterize this switch matrix, but the packaging requires the alignment of nine fibers, the square of the number of inputs, to the various detector crosspoints. In this presentation, we report the fabrication and evaluation of 4 by 4 optoelectronic switching matrices integrating MSM detectors with polyimide waveguides which perform the optical signal distribution on the wafer. These waveguides were fabricated on top of the semiconductor using a photolithographic process. The detector electrodes were formed using a transparent ITO film to maximize the responsivity. The incoming light is distributed using the 'tap' approach, which is more compact than the Y-branching configuration. Two 2 by 4 monolithic arrays were assembled on an alumina circuit using microwave hybrid circuit technology. The bandwidth of the assembled switch exceeds 1 GHz and with improved circuit design, should approach the 5 - 10 GHz bandwidth of the individual MSMs. A similar switch is based on a 4 by 4 monolithic array. The isolation is typically better than 35 dB. These characteristics are compared to the performance of the 3 by 3 OEIC switch and another 4 by 4 switch array assembled using four GaAs MESFET SP4T switches.
GaAs optoelectronic integrated circuits are having an increasing impact on the development of lightwave communication systems and very high sped digital and analog signal processors. However, cost-effective integration of complex electronic and optoelectronic circuits used in high speed networks or backplane interconnects still presents a significant technological challenge and substantial efforts are being devoted to the development of practical, low-loss integration processes. Photodetector/waveguide integration is a key aspect of the successful packaging of optoelectronic devices. In this work, metal-semiconductor- metal (MSM) photodetectors were integrated with polyimide ridge waveguides, and the processing parameters were varied for optimum performance. The absolute responsivities of the integrated MSM/polyimide waveguide structures were typically 0.5 A/W, and 3 dB bandwidths of 4-6 GHz were measured. Series of 1 X 2 and 1 X 4 photodetector arrays were interconnected, demonstrating a uniform division of the optical signal between the detectors. Results indicate that these arrays offer the potential to fabricate optoelectronic switches that can be used in a variety of high speed and broadband communication systems.
The integration of MSM photodetector arrays with polyimide ridge waveguides is demonstrated. MSM detectors consisting of two Schottky interdigitated electrodes were fabricated singly and in arrays of two or four, on semi-insulating GaAs substrates. Following deposition and patterning of an SiO2 buffer layer, polyimide ridge waveguides were fabricated on top by spin coating and photolithography. The guides were multimode, with widths from 10 to 50 micrometer, allowing for ease of coupling from an optical fiber. Light from the waveguides was coupled through gaps in the SiO2 buffer layer into the photodetectors. Transparent indium tin oxide (ITO) Schottky electrodes were employed to maximize absorption of light in the detector region. The end-to-end responsivities of the integrated MSM devices were typically 0.1 to 0.16 A/W. Bandwidths were 1 to 1.7 GHz; however these values could be increased substantially by optimization of the etch conditions used in the detector fabrication. Losses due to butt coupling to the multimode waveguides were around 1.5 dB. Division of the input signal between sets of two and four detectors has been demonstrated using a series of optical taps fabricated in an overlying polyimide ridge waveguide. Results indicate that polyimide waveguides could be a practical means of monolithically integrating optical functions such as signal routing and power division on complex optoelectronic integrated circuits (OEICs).
With the increasing number of optical networks comes a growing demand for hardware to allow management of interconnections. One of the important elements in a network is the cross-point switch. In this presentation, we describe a GaAs 3 multiplied by 3 optoelectronic switch based on a monolithic optoelectronic integrated circuit which combines a receiver array of MSM photodetectors and three transimpedance amplifiers providing gain for the three output channels. The 3 multiplied by 3 matrix of photodetectors acts at the switching element. The three electrical output channels from this receiver are amplified further by MIC circuits using chip amplifiers. This restores the signal to a level sufficient to drive semiconductor lasers thereby converting the electrical signal back to an optical signal for use as an optical- optical router. A critical step in the switch construction is the delivery of the optical signals to the photodetectors. A special mount was designed and fabricated to support and align the 9 fibers in front of their respective detectors. The switch was evaluated in terms of the responsivity, the isolation and the cross-talk. The overall responsivity exceeds 20 A/W with a bandwidth of 400 MHz, limited by the speed of the detectors. The isolation varies between 33 and 55 dB and depends on the device selected and on the bias condition of the detector in the off-state. The operation of the switch was demonstrated using three television signals.
Metal-semiconductor-metal photodetectors have been fabricated on undoped epitaxial GaAs material with gold and indium-tin-oxide interdigitated contacts. In both cases, various electrode configurations were laid out with combinations of finger spacings and finger widths ranging from 1 to 3 micrometers and detector cross-sections of 25 X 25, 50 X 50 and 100 X 100 micrometers 2. Frequency response measurements were carried out up to 20 GHz using a high-speed electro-optic modulator combined with a DC-operated laser diode and an ultrafast photodetector for system calibration. This frequency domain technique ensures accurate measurement of true analog bandwidth compared to time-domain techniques which can easily lead to an overestimation of photodetector bandwidth. Photodetector responsivity has been plotted as a function of bias voltage. We note that for similar devices, Ti/Pt/Au contact MSMs require lower bias voltages before they reach their saturation bandwidth than ITO contact MSMs. For a 100 X 100 micrometers 2 ITO MSM with a 2 micrometers finger width and a 2 micrometers finger spacing, the 3 dB bandwidth was measured to be 4 GHz at 10 V bias. By comparison, similar gold contact MSMs exhibit 3 dB bandwidths in excess of 12 GHz. The difference in speed is partly explained by the higher device parasitics of the ITO MSMs, as confirmed by S11 measurements made on both types of device. The S11 data was also used to extract the MSM equivalent circuit parameters for a high-frequency MSM model. Similar measurements on other electrode configurations show that as expected, the speed of ITO MSMs become considerably higher as device size is decreased, until the limit where transmit-time effects start to dominate the overall performance.
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