Low cost germanium photodetectors for sensing applications in the 900-1600 nm spectral region have been developed. By varying the Ge substrate resistivity as well as device area, photodetector properties such as reverse leakage current, capacitance, and shunt resistance have been engineered. Low leakage current devices of various sizes up to 1 cm2 have been fabricated and have consistently exhibited exceptionally high shunt resistances and excellent linearity. Over 5000 hours of active stress testing have left the ultra-low leakage currents unchanged. These data were measured in accordance with Telcordia 468-CORE requirements at 85°C, 125°C and 175°C. The results indicate that these mesa photodetectors meet telecommunication industry requirements for reliability. These devices are comparable to commercially available Ge photodetectors, and can be readily substituted for more complex InGaAs photo-detectors in applications such as laser monitor diodes.
Germanium (Ge) photodetectors are fabricated by growing epitaxial III-V compounds on Ge substrates and by in-situ formation of the PN junction by MOVPE. After material growth, Ge photodetectors are mesa-etched using conventional optoelectronic device processing techniques. By varying the Ge substrate resistivity and the device area, Ge photodetector properties such as reverse leakage current, capacitance, and shunt resistance have been engineered. Such devices have demonstrated leakage currents below 50(mu) A/cm2 at -0.1 V bias. For optoelectronic applications that require high temperature operation, high shunt resistance detectors exhibit leakage currents below (mu) A/cm2 at 80 degree(s)C. Low capacitance devices have measured as little as 275 pF at 0V bias for a 1 mm diameter detector. High shunt resistance devices are a low cost alternative to conventional InGaAs photodiodes in applications such as laser monitor diodes.
Fly-by-light has been a research and development area for the last 10 years, and much progress has been made in many areas and applications. Developments in fly-by-light component technologies such as vertical cavity surface emitting lasers (VCSELs) will be discussed to include the application of these types of laser to high-speed fiber- optic transmitters. Data will include digital transmission performance characteristics, the effects of temperature changes on the laser output and transmitter performance, and gigabit per second operation. A comparison will be made between typical edge emitter operation and VCSEL operation over similar temperature ranges. Both packaging and connectorization of individual transmitters and transmitter arrays will be discussed. Other VCSEL applications will be discussed including the application to Fiber Channel data links and networks. Fiber Channel data links can provide much larger data rates than current fly-by-light systems. The usefulness of redundant ring architectures and DWDM schemes will be discussed for both enhanced network reliability and speed.
Recent developments in nonlinear optical polymer materials and devices combined with epitaxial liftoff (ELO) and grafting of semiconductor materials are leading to dramatic new possibilities in devices for photonic signal processing. For example, the development of new device architectures is leading to electro-optic modulators that have halfwave voltages of approximately 1V. Applications include very large bandwidth (greater than 100 GHz) electro-optic modulators and high speed (less than 1 ns) switches for programmable optical delay lines for use in phased array systems. Also, with the increase in operating frequency and angular scan resolution, the delay length accuracy can reach magnitudes of micrometers for millimeter wave frequencies. With micro fabrication methods, integrated delay line/switch networks can achieve superior delay performance with a single integrated optic chip that is compact, light weight, and has low optical insertion loss. The use of ELO allows electronic device driver circuits to be integrated with the polymer chip to provide further miniaturization. Also, ELO methods can be used to fabricate very high speed metal-semiconductor-metal (MSM) photodetectors for optical signal detection and monitoring. Here ELO methods can find applications in the fabrication of multispectral detectors and focal plane arrays. Yet other applications include very high speed analog-to-digital converters.
We report calculations of the collection current of interdigitated InGaAs metal-semiconductor-metal photodetectors. We show how interdigital spacing and thickness of the semiconductor layer influence the collection current. Both front and back illumination of devices carried on thin film membranes by means of epitaxial liftoff are examined.
Arrays of interdigitated metal-semiconductor-metal photodetectors carried on thin semiconductor membranes have been transferred and grafted by epitaxial liftoff techniques to non-conducting AlN host substrates for advanced multichip module development purposes. Progress includes the introduction of full surface passivation by thin polyimide films resulting in dramatically reduced dark currents for GaAs photodetectors. Device modeling by means of a computer simulation has suggested new interdigitated electrode configurations which can only be applied to these types of devices carried on thin film membranes. Integration of InGaAs photodetector devices on the same host substrate with transimpedance amplifiers provides an intermediate development prototype of the first stage of a high speed multichip module receiver product for fiber optic telecommunications applications. The use of integrated circuit-sized membranes to carry small discrete devices, broad-area inverted photodetectors, OEICs, focal plane arrays, and multi-spectral sensors continues to provide a manageable path to automated packaging using commercial off-the-shelf hardware.
Novel interdigitated metal-semiconductor-metal structures offer new approaches for the development of broad-area, high-speed photodetectors to be used in optical free space communications and light detection and ranging applications. Inherent advantages include: lower capacitance than typical p-i-n structures, a wide dynamic range, and ease of fabrication. We have constructed broad area metal- semiconductor-metal photodetectors (MSM-PDs) by means of epitaxial liftoff and grafting technologies. Two computer models have been used to examine the effects of design parameters on the performance of broad-area, high-speed MSM- PD devices. The first model indicates that inverting the membrane so that the electrodes are placed between the non- conducting host substrate and the semiconductor material improves the signal-to-noise ration of the device, expanding its dynamic range. This model suggests that processing of the backside of the semiconductor material with antireflection coatings further improves device performance. Carrier collection behavior described by the second model suggests new electrode configurations for improved high speed operation which can only be applied to an inverted MSM-PD carried on a thin film membrane. A number of different fully passivated large area MSM-PD configurations have been fabricated and tested. Initial dark current data are compared favorably to published results.
An eight-channel dense wavelength division multiplexing (DWDM) system has been constructed and tested for applications to video, audio, and data transmission. The system uses DFB laser sources with wavelengths separated by 200 GHz and centered around the 1550 nm silica fiber transmission window. After transmission through the fiber, the demultiplexing of the signal requires that the optical wavelengths be separated while maintaining high isolation between channels, preferably with low insertion loss. Eight- channel DWDM demulitplexers are currently available with better than 20 dB isolation using one of several different approaches: planar arrayed waveguide gratings, bulk optical gratings, and multiple dielectric filters. For this system we are using a planar arrayed waveguide grating DWDM demultiplexer based on integrated optics fabrication techniques. The optical insertion loss ranged from 5 to 7 dB. Individual laser wavelengths were tuned to each respective demultiplexer channel by temperature tuning of the DFB laser. The overall system design requires that each wavelength carry 10 uncompressed video channel sat 8-bit coding, or 8 video channels of 10-bit coding. The data transmission rate is currently 1.2 Gbit/s per wavelength for an effective bandwidth of 9.6 Gbit/s. Results from our testing have shown channel separation of 1.6 nm with an isolation exceeding 30 dB. Bit error rates are less than 10-12 per wavelength channel. With 8-bit video, a total of 80 channels can be transmitted simultaneously with this approach. We conclude that a system of this design is well suited for the simultaneous transmission of multi- channel video, audio, and data, and will be a very appropriate system for multi-media applications. The system is also scaleable to even higher bit rates per wavelength and to a larger number of wavelengths. Thus, systems carrying more than 1000 uncompressed video channels simultaneously appear feasible.
Epitaxial liftoff and grating of multiple thin integrated circuit-sized semiconductor films each containing over a hundred discrete devices confirms that multichip modules can be produced using this technology. the use of epitaxial liftoff membranes containing electronic and optoelectronic circuits and devices will provide (1) highly improved thermal management, (2) much higher density packaging, and (3) applications polylithically interconnecting electronic mixed-signal and optoelectronic devices. Bond strength data of thin epitaxial membranes grafted onto high thermal conductivity substrates quantifies for the first time to our knowledge the adhesion quality of van der Waals forces. In addition, the handling of very small discrete devices by means of integrated circuit-sized thin films permits the use of commercial off-the-shelf hardware in the development of an automated manufacturing assembly tool. This enabling technology will provide a manufacturing path to continue to commercialize new models of high performance optoelectronic modules with advanced features including higher data rates and increased function.
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