Photodiode vendors sharpen skills
Although classified as a technically mature product, InGaAs PIN photodiodes continue to find increased usage in diverse high-speed telecommunications and data communications networks
JAY LIEBOWITZ
EPITAXX INC.
The growth and segmentation of the long-wavelength fiber-optic transmission market has created distinct needs for improving the development and manufacture of indium-gallium-arsenide, positive-intrinsic-negative photodiodes. This product is technologically mature: Shipments run at more than two million units a year.
With product maturity, manufacturing skills and management principles have evolved differently from those used for emitter diodes. In addition, the cross-fertilization of detector product lines across fiber-optic segments gives photodiode manufacturers the ability to offer more specialized products than those provided by emitter/detector suppliers.
The emergence of manufacturers that specialize in detectors holds formidable benefits for fiber-optic market growth. Product supplies are plentiful, and costs are low. Moreover, detector manufacturers can use their manufacturing and segment experiences to develop customized detector solutions for specific receiver problems. Such capabilities make possible the realization of innovative and economical fiber-optic devices.
The combination of more segments and larger volumes has created challenges for opto-electronic component suppliers. Traditionally, a company that manufactures emitters has also produced detectors. The growth and segmentation of demand for detectors has, however, enabled device manufacturers to concentrate on 1.3- and 1.55-micron photodiodes without providing complementary emitter diodes.
The development and manufacture of InGaAs detectors has evolved into a niche market that is separate from InGaAsP laser and light-emitting-diode markets. This separation has occurred because of two factors--the larger volume for detectors and the experience that detector manufacturers have accumulated from serving the diverse segments of the fiber-optic communications industry.
Greater volumes have helped bring down detector prices; increased segmentation has inspired product innovation and reduced design cycle times. For fiber-optic equipment manufacturers that are working closely with detector companies, these benefits have increased the value and competitiveness of their receiver products.
The InGaAs PIN photodiode has become the most commonly used opto-electronic diode in fiber-optic communications at 1.3 or 1.55 microns. In data communications, the InGaAs detector is paired with surface-emitting light-emitting diodes in transceivers for 100-megabit-per-second fiber distributed data interface, 155-Mbit/sec asynchronous transfer mode and 200-Mbit/sec enterprise system connection network applications.
In telecommunications, this diode is used as a back facet monitor in laser-diode packages. Furthermore, the PIN-field-effect transistor receiver employed in asynchronous applications to 140 Mbits/sec, the 155-Mbit/sec synchronous optical network PIN transimpedance amplifier and assorted digital receivers that work to several hundred Mbits/sec all use InGaAs detectors. Other common communications applications include undersea repeaters, monitor diodes in fiber amplifiers and amplitude-modulated cable-TV optical receivers.
Moreover, applications for the InGaAs PIN photodiode include uses in test and measurement instruments. Optical power meters, optical-loss test sets, optical time-domain reflectometers, splice-loss test equipment and optical fiber identifiers incorporate this detector.
The volumes that have resulted from these applications have made the InGaAs PIN detector more widely used than any other long-wavelength diode. According to many market research studies, detector unit consumption is three times that of both the light-emitting diode and the laser diode, and 100 times that of avalanche photodiodes and germanium PIN diodes.
Detector volumes
Opto-electronic suppliers who have experienced increased demand for and greater segmentation of InGaAs detectors have, in the process, been able to take advantage of the differences between InGaAs PIN photodiodes and long-wavelength-emitter diodes. The InGaAs PIN diode is easier to make than an emitter diode, and it has reached maturity in its technical evolution; new designs for emitter diodes are still appearing on the market.
Furthermore, because the same PIN design can be used for applications in different segments, volumes for a given design are higher and product life cycles are longer than those for complementary emitter diodes.
These product differences have enabled detector companies to increase business in different ways from those performed by emitter diode companies, even through both serve the same 1.3- and 1.55-micron fiber-optic market. Developing and manufacturing a more mature semiconductor product with higher volumes and longer life cycles mean that detector companies can place more emphasis on improving production techniques and increasing manufacturing capacity. At the same time, these detector companies can still invest in new designs and processes.
The intricacies of InGaAs detector manufacturing management can be analyzed by exploring the various manufacturing steps. For example, in producing epitaxial InP wafers, manufacturers have fabricated structures that meet the requisite performance for PIN photodiodes. Having established these standard structures, manufacturers have set as their next challenges the attainment of greater uniformity and the reduction of costs.
Metallo-organic growth methods have been instrumental in this move, as has been the refinement of these methods for the growth of larger diameter wafers. The adoption of metal-organic chemical vapor deposition and its use on large wafers have enabled detector manufacturers to achieve economies common to the monolithic semiconductor industry.
InGaAs PIN photodiodes should be at least 0.8 ampere/watt, although for certain applications, this specification should be higher. In addition, the detector`s other main parameters--capacitance and dark current--are determined mainly by the size of the diode`s photosensitive area, which is typically dictated by the optical design specified by the market.
In packaging emitter diodes, the manufacturer`s main concern is the alignment of the emitting surface to the fiber to which the diode is coupled. Issues of orthogonal and parallel placement of the chip to the mounting carrier or post, centering of the post to the header, and concentricity of the lens to the header weigh more heavily than other issues. In general, detector manufacturers invoke similar specifications, but pay more attention to the speed at which the diode can be mounted, wirebonded and capped. Such latitude exists because the task of aligning light from a fiber into a detector`s area provides wider tolerances than does the chore of coupling the beam from an emitting surface or facet into the fiber`s narrow core.
Segmentation influence
In addition to larger volumes, industry segmentation has also abetted InGaAs PIN detector specialization. The fiber-optic detector manufacturer has usually amassed skills that have enabled the generation of distinctive products that differ markedly from those produced by traditional emitter/detector suppliers. In the main, these latter suppliers have concentrated on serving a narrow set of industry niches.
Segmentation has fostered detector specialization. By participating in several segments simultaneously, the detector manufacturer can learn skills that benefit the segments for which the new abilities were not originally intended. This cross-fertilization of product lines enables the detector manufacturer to develop products increasingly more diverse and customized than the products a more narrow supplier can create.
When data communications product volumes began to grow in the early 1990s, the cost reductions achieved for InGaAs detectors helped reduce transceiver prices to levels that made sales of fiber-optic local area network equipment possible. The volumes being realized for data communications product sales spurred further cost reduction efforts, especially in the area of packaging.
The acceptance of the data communications market for transceivers inspired the development of precision packaging techniques for aligning LED and photodiode chips inside optical subassemblies. The engineering that opto-electronic suppliers applied to these optical subassemblies led to increased coupling efficiencies and lower pricing. These two benefits, in turn, proved useful to the telecommunications market, which is applying these solutions into fiber-in-the-loop networks.
InGaAs detector
In data communications applications, transceiver manufacturers are exploring the use of short wavelength components at data rates in the 1-gigabit-per-second range. The InGaAs PIN photodiode is one component these companies are exploring for receiving 850-nanometer signals, despite its reduced responsivity at this wavelength. The InGaAs detector is of interest, however, because it can be biased with a readily available 5-volt supply; the silicon detector typically used at 850 nm needs a higher bias to operate at high speeds.
To make the responsivity of InGaAs sufficient for use in these applications, engineers are using processing techniques developed for test and measurement applications. These techniques, which involve modification of the diode`s cap layer, were developed to permit use of a single detector for measuring short- and long-wavelength optical signals. The resulting threefold increase in 850-nm responsivity enables the high-speed transceiver manufacturer to use the same basic design.
New FITL architectures are calling for the use of single fiber systems in which one terminal transmits at 1550 nm and another simultaneously transmits at 1300 nm. In this system, the signal from one terminal`s laser must not reflect back into the detector located at the same terminal. This constraint on crosstalk exists so that the signal from the other terminal is clearly received.
Detector manufacturers have been able to meet these wavelength isolation requirements by employing techniques they had developed for non-optical communication, free-space applications. These techniques are based on the use of monolithic layers that serve as passbands: The layers allow transmission of the desired wavelength, but absorb wavelengths outside the band. The way in which the manufacturer produces the specific layers dictates the wavelengths over which the passband exists.
The advantage of the monolithic solution is that the cost of the passband is spread over the thousands of photodiodes produced on a wafer. This approach is less expensive than the cost of inserting a discrete filter into an optical module. The lower cost, as well as the resulting simplicity of manufacture and improved module reliability, helps increase the likelihood for greater adoption of this FITL architecture. Based on these results, additional examples of segment cross-fertilization should contribute to the advancement of InGaAs PIN photodiodes.
Fiber-optic receiver manufacturers have benefited from the variety of InGaAs detectors available. Detector options exist for diameter, speed and spectral response. Additional choices exist for optical sensitivities and saturation power levels. This range of alternatives has given receiver designers more methods to satisfy and to exceed their product design goals. q
Jay Liebowit¥is director of marketing at Epitaxx Inc. in West Trenton, NJ.