Integrated optoelectronic receivers improve lightwave performance at reduced cost

To achieve the parts and cost savings of volume production, optoelectronic receivers and related devices are being researched and developed for migration to commercial integrated circuits

raymond milano

vitesse semiconductor inc.

To achieve cost-effective optical data transfer systems, integrated optoelectronic integrated circuits (ICs) provide the volume packaging and manufacturing technology needed to lower costs, parts count and circuit board area, and optimize optical coupling.

The cost and the performance of the data link are determined, in large part, by the cost and performance of the selected optical devices. These factors must also be evaluated for the mechanical hardware used to mount and align the devices to the optical fiber.

Consequently, the fabrication of the lightwave and optoelectronic parts and circuits onto an IC mandates a detailed analysis of a serial fiber-optic transmission data link (see Fig. 1). The major elements of this link include a serializer and a deserializer for parallel-to-serial-to-parallel data conversion, a transmitter that drives the optical source, a laser or light-emitting diode (LED) light source and a lens mounted in an optical subassembly, an optical detector and lens in an optical subassembly, a trans impedance amplifier for converting the photogenerated current to a voltage, and a postamplifier for increasing the transimpedance amplifier output signals to standard logic levels. Both the serializer and deserializer usually contain clock synthesis and recovery circuits. The key link parameters are data rate, bit-error rate, transmitter output power range, receiver sensitivity, connector/coupling losses, fiber loss, duty-cycle distortion and jitter.

To accommodate moderate and high-speed optical links, singlemode fiber is used in the more-demanding long- distance telecommunications links due to the fiber`s low loss and dispersion characteristics. Multimode fiber is used in less-demanding data communications applications where the link lengths are shorter than 500 meters.

The optical source used in most high-bandwidth links is a laser diode. This diode can be modulated at high bit rates and generates several milliwatts of optical power. In general, LEDs serve as light sources for data rates below 200 megabits per second. Even though they do not possess the spectral purity or output power of laser diodes, they cost less.

Conversion of the received optical signals is accomplished using a detector and a transimpedance amplifier. Most existing systems use a PIN detector made of either silicon or indium gallium arsenide. High-bit-rate systems usually employ an avalanche photodiode. The photodiode detector contains an internal gain mechanism for signal amplification, requires a high bias voltage and costs more than the PIN detector.

The optical subassembly for a laser or a detector uses a metal housing that is machined to accept a TO-style package; TO-46 packages are common for detectors and TO-56 are standard for compact-disk (CD) lasers (see Fig. 2). The lens is needed to interface the optical device to the fiber, and a ceramic-lined ferrule is used to hold the end of the fiber. Typically, an active alignment is employed to maximize the coupling between the lens and optical device.

The cost of the optical subassembly, including the lens and active alignment, is $10 to $30. This price is acceptable in high-performance telecommunications systems where the cost of optical devices is greater than $1000. However, in data communications systems, the cost of an optical device (laser, LED or detector) ranges from $2 to $10. The cost differential presents a barrier to continued growth of data transfer applications for fiber optics technology. Moreover, the size of the subassembly limits how closely the subsequent circuits can be mounted. Size limitations impact data rates greater than 500 Mbits/sec because the effects of lead capacitance and coupled noise limit the device`s bandwidth and sensitivity.

Integrated optoelectronics, the combination of optical and electrical devices on a single substrate, is a technology that can solve discrete component cost and performance problems. After a 15-year research and development phase, this technology is finding its way into commercial optoelectronic products.

Two approaches are currently available to fabricate an integrated optoelectronic receiver. One fabrication process combines the unit processes needed for the production of a high-performance optical detector and the analog integrated circuitry. The second process implements the hybrid integration of the detector and associated circuits.

Monolithic approach

The first, or monolithic, approach is the object of considerable research activity. Several circuits have been demonstrated that require the use of the epitaxy process for the fabrication of the optical detector, usually a PIN structure, and the use of standard IC processing for the circuitry. At present, the additional high-temperature wafer processing for the epilayers and the etching steps needed to isolate the optical device have resulted in low yields.

An alternative approach, based on the use of a metal-semiconductor-metal (MSM) detector has yielded the manufacture of integrated receivers in high volume. The detector is constructed from back-to-back Schottky barrier diodes. The overall structure is easy to integrate and requires no additional high-temperature processing. Because of the use of Schottky barriers, the detector accepts wide bandgap materials such as GaAs.

The MSM detector uses an interdigitated metal structure to define the active area (see Fig. 3). Incident optical radiation is absorbed in the regions between the electrodes; typical electrode spacing is 1 to 2 microns. Device responsivity is limited by the amount of surface area covered by the electrode structure. However, the use of a semi-insulating region requires a detector bias voltage of a few volts to ensure the collection of all the photogenerated carriers. This factor becomes important for circuit integration because the available bias voltage is limited to the supply voltage, typically 5V. In addition, the MSM detector has a low capacitance, approximately 50% of that of an equivalent-sized PIN detector.

Several advantages result from integrating a high-performance trans impedance amplifier and an MSM detector to form a fiber-optic receiver. The additional capacitance of the board traces and package leads that is present in the discrete component implementation is eliminated. Therefore, the detector achieves a higher bandwidth. The low capacitance of the MSM is exploited in two ways. First, a larger detector area is used than is commonly available with other detectors, thereby simplifying the coupling of the fiber to the detector. Second, the trans impedance value of the amplifier can be increased without sacrificing bandwidth. This characteristic produces lower noise and improved sensitivity. Detectors with a 100- to 500-micron diameter are being used to implement receivers with 250- to 1063-MH¥bandwidths.

The integrated receiver effects other advantages, such as a reduction in board space and the external components required for the receiver function, and an improved sensitivity from the reduction in coupled noise. Test costs are also reduced because the detector and the amplifier are tested at the same time rather than individually, before and after assembly. Moreover, only a single part must be inventoried.

The monolithic optoelectronic receiver containing an MSM detector is adequate in systems where the optical radiation can be absorbed by the substrate material. This radiation corresponds to wavelengths of 600 to 865 nanometers for GaAs. Therefore, the GaAs receiver is suitable for systems that use GaAs/GaAlAs LEDs and CD lasers.

Hybrid integrated receivers can be constructed for use in long-wavelength (1300-nm) optical systems. A hybrid optoelectronic circuit is realized by mounting a long-wavelength detector, such as an InGaAs PIN detector die, directly onto the GaAs amplifier die. This approach retains most of the advantages of the monolithic approach, including minimizing the capacitance at the amplifier input, and requires a single package for both the detector and the amplifier. The die-on-die attachment can be accomplished using silver glass epoxy without affecting the reliability of the assembly.

The performance of commercially available integrated optoelectronic receivers is currently limited to bandwidths of approximately 1 GH¥and sensitivities of better than -22 dBm. These devices contain a single detector and amplifier. Recent research and development work supported by the Advanced Research Projects Agency has led to the demonstration of integrated arrays of optoelectronic receivers. One such array contains 32 parallel channels, where each channel comprises an MSM detector, a transimpedance amplifier and a postamplifier. Each channel has a bandwidth of 1 GHz, and the chip is powered by a single 3V supply. u

Raymond Milano is analog product line director at Vitesse Semiconductor Inc. in Camarillo, CA.