The importance of wide-bandwidth photodiodes/photoreceivers
As demands on sonet/stm networks grow, the capabilities of components must keep pace. InGaAs photodiodes and OEICs will prove critical to enhanced performance.
Abhay M. Joshi and Douglas Malchow
Discovery Semiconductors Inc.
Synchronous Optical Network/Synchronous Transfer Mode (sonet/stm) standards have been developed over the last decade to enable global compatibility of fiber transmission systems and the advantages of "drop and insert" flexibility. American National Standards Institute SONET and CCITT (Consultative Committee on International Telegraph and Telephone) Synchronous Digital Hierarchy (SDH)-based fiber-optic transmission systems serve as the principal long-haul and interoffice infrastructure for virtually all new and upgraded telecommunications networks.
sonet/sdh will allow capacity expansion from current OC-48 rates (2.5 Gbits/sec) to OC-192 (10 Gbits/sec) and OC-768 (40 Gbits/sec) via changes in either the transmission terminal equipment or, in many cases, only selected subsystems, such as high-speed laser and photodiode/photoreceiver components. Both of these standards (and others) provide specifications for the performance of optoelectronic components functioning at 1300- and 1550-nm wavelengths and at speeds from 2.5 to 10 and even 40 Gbits/sec. These specifications include reliability standards for the generation of components now entering communications networks as well as for future components that offer higher speeds and more reliability. For example, optoelectronic integration using optoelectronic integrated circuits (OEICs) will enable the rapid upgrade of sonet/sdh networks with reliable, cost-effective front-end modules. Advances in photodiode/photoreceiver components illustrate how the transition to more robust sonet/sdh transmission will be attained.
A threat to SONET?
Recently, there has been a great push in the telecommunications industry to simplify networks, especially SONET-based infrastructure, to facilitate data communications across the World Wide Web. SONET, mainly designed for voice or audio communications, requires multiplexing using electronic circuitry before it reaches the optical layers, i.e., mainly the optical transmitter and optical receiver. The optical networks of the future will eliminate this entire layer of multiplexers in a traditional SONET-based network, thus simplifying the networks.
Although optical networking has tremendous promise, a hybrid system of SONET and optical networks seems most likely to evolve. Regardless of what form or shape the network takes, certain key optoelectronic components (e.g., lasers, photodiodes, and amplifiers) that enable the optical layers in a network have to be developed for the new wide-bandwidth applications. InGaAs photodiodes with bandwidths of DC to 50 GHz and InGaAs photoreceivers starting with a bandwidth of 10 MHz to 10 GHz are examples of such advances (see Fig. 1).
The AC characteristics expected from a photodiode for 40-Gbit/sec SONET are wide bandwidth with a very low ripple factor, which requires skillful microwave packaging of the photodiodes and photoreceivers. Photo 1 shows a fiber pigtailed microwave package with an RF connector, while Figure 2 exhibits a frequency response plot of a 40-GHz microwave packaged photodiode. The flat response with less than +/-1-dB variation plus a smooth roll-off are both necessary to produce a clear eye diagram as shown in Figure 1.
OEICs for telecom systems
Another important optoelectronic component advance for wide-bandwidth applications will be the development of an OEIC incorporating the photodiode, preamplifier, and follow-on amplifier stages for future telecommunications systems such as OC-768 data transmission.
The combination of optical, microwave, and digital functions on the same chip is a technology that has significant potential. Such OEICs will find use in commercial applications such as Ethernet fiber local area networks and optical communications systems such as SONET, all-optical network, Integrated Services Digital Network, telephony, and digital cable TV. Important interservice military applications for such OEICs include optically fed phased array systems and optically controlled microwave networks for airborne and spaceborne systems.
These applications require components with wide bandwidth, reproducibility, and low cost. While the maturity of monolithic microwave integrated circuit (MMIC) technology makes it feasible to combine optical components such as photodetectors and front-end receivers, the demonstrations of integrated optical receivers reported so far have mainly been at frequencies lower than 10 GHz. Although high-speed positive-intrinsic-negative (PIN) photodetectors and narrowband MMIC amplifiers with frequency response in excess of 20 GHz are commercially available, wideband amplifiers with frequency and gain performance necessary for photoreceivers above 2.5 Gbits/sec are currently difficult and expensive to manufacture. Integrated photoreceivers with frequency performance in the 40-Gbit/sec range have only recently been demonstrated, and truly monolithic photoreceiver OEIC chips operating at any frequency are not commercially available.
Classically, high-speed InGaAs detectors and GaAs amplifiers have been manufactured as discrete components and subsequently wirebonded together using hybrid technology. While this practice is sufficient for most applications up to 2.5 Gbits/sec, the hybrid approach has several drawbacks, including increased labor, low yield, low reliability, and increased parasitic losses. These problems become worse at 10 Gbits/sec or higher frequencies.
However, totally integrated InGaAs detector/ InP p-HEMT amplifier OEICs for fiber-optic applications are now under development (see Photo 2). This approach will not only eliminate the drawbacks of the hybrid method, but also produce higher-speed transistors (fT greater than 100 GHz) owing to the use of InP as the active material. This research should lead to the commercial manufacture of OEICs for the 10- and 40-Gbit/sec standards.
OEIC manufacturing challenges
To develop photoreceivers with 10- and 40-Gbit/sec bandwidth, a wideband amplifier technology must be in place. Technology improvements are needed in the following areas:
The wideband amplifier for the proposed photoreceiver must be based on device technologies with high fT (100 GHz or higher), and field-effect transistor (FET) capacitances (CFET) lower than 50 fF. InP HEMT (high-electron mobility transistor) technology is the highest-frequency technology available that is compatible with MMIC fabrication approaches. For a 40-Gbit/sec amplifier, reducing CFET below 50 fF can optimize InP HEMT technology.
Gate-leakage current can severely degrade the noise performance of a photoreceiver. That is particularly true of HEMTs whose Schottky contacts exhibit poor characteristics. Thus, reducing the gate-leakage currents of HEMTs will result in low- noise amplifiers and thus improve the sensitivity of the photoreceivers.
The design of high-speed photoreceivers requires a compromise between bandwidth and noise. In addition, the open loop gain of the amplifier must be maximized while the photodiode and p-HEMT capacitances have to be minimized. The open loop gain of the amplifier is maximized by using InP HEMT technology and a maximum single stage gain of more than 10 can be easily achieved. Therefore, open-loop gains well in excess of 100 using three-stage amplifiers should be achievable.
Using these three technology improvements, high-sensitivity, high-voltage gain, wide-bandwidth photoreceivers will be manufactured.
Wide-bandwidth photodiodes and photoreceivers are key components of the upcoming sonet/stm standards of OC-192 and OC-768. For the new and better OC-192 and OC-768 systems to become a reality, new technology in OEIC/MMIC chip fabrication and microwave packaging has to be developed and inserted. u
Acknowledgments
The authors would like to thank their colleagues Dan Mohr and Xinde Wang for helping prepare this article.
Abhay M. Joshi is president and Douglas Malchow is director of marketing at Discovery Semiconductors Inc. (Cranbury, NJ). The Web site address is www.chipsat.com.