Genoa Corporation, a next-generation optical semiconductor company in Fremont, CA, announced what the company claims to be the first single-chip, linear optical amplifier (LOA) -- a small, cost-effective semiconductor device capable of amplifying light in optical communications networks.
The LOA represents the first semiconductor-based optical or "photonic" amplifier able to simultaneously amplify dozens of different wavelengths of light without distortion, even if those wavelengths are unpredictably switched in and out of the communications path. The tiny chip, which, when packaged, is about the size of a sugar cube, is expected to find wide application in optical network equipment ranging from optical cross-connects, high-speed routers, optical add-drop multiplexers, transponders, and DWDM systems, particularly in metropolitan applications.
Just like the silicon transistor, the linear optical amplifier is manufactured on a semiconductor chip -- in this case Indium Phosphide, using a planar manufacturing process. Planar fabrication ultimately will allow more than one optical amplifier -- and later other integrated optical devices -- to be simultaneously manufactured on the same chip. Such manufacturing techniques resulted in the revolutionary integrated circuit, and are a requirement to the ultimate attainment of the much anticipated optical IC.
The LOA is capable of the crosstalk-free amplification of lightpaths containing dozens or more channels encoded in different wavelengths of light. It is also free of distortion when used in a switching environment, where the number of channels carried on the lightpath from moment to moment is unpredictable.
These two characteristics are important to equipment designers and service providers. High-speed optical communications networks gain their capacity by the sharing of single fiber-optic strands by dozens -- and increasingly hundreds -- of different channels encoded by wavelength, a methodology called "dense wavelength division multiplexing," or "DWDM." Consequently, for any optical amplifier to be useful in all but limited single-channel applications, it must be able to amplify DWDM signals without crosstalk. That is, the amplifier's output must be an exact -- or "linear" -- copy of its input. Crosstalk is a concept familiar to cell phone users whose reception is occasionally garbled by the mixture of other calls. It can similarly garble optical communications.
Additionally, practical networking applications, particularly in the fast-growing metropolitan networks, need to be able to quickly switch different wavelengths in or out of single fibers without impacting the fidelity of the communications -- a task which previous technologies are struggling to accommodate.
The linear optical amplifier meets both these requirements, as well as being 1/100th the size of current technology.
Amplifiers take small signals and boost them -- higher current, higher voltage, or in the case of optical communications networks, brighter pulses of light. They have always been crucial because, in part, the further signals have to travel -- whether electronic or photonic -- the weaker they get. Additionally, traveling through even the most carefully constructed networking equipment causes losses -- especially with light -- that must be made up.
In the optical network, which typically has been used for inter-city, intercontinental "long-haul" applications, signals have to be amplified every 50 to 80 kilometers, because the fibers that carry the photons gradually absorb them. Such amplification has traditionally been provided by a device called an "erbium-doped fiber amplifier" (EDFA). The EDFA is a complex hybrid optical and electronics module -- about the size of a videocassette -- that costs anywhere from $5,000 to 30,000. One EDFA is required for each fiber needing amplification.
Like the LOA, the EDFA can amplify DWDM signals without crosstalk. However, unlike the LOA, it cannot reliably be used in most switching applications, or designs where the engineer cannot closely control the total power of the signals -- precisely the types of requirements needed in metropolitan optical network equipment. Indeed, in "metro" networking equipment, such limitations and the workarounds they create, combined with the EDFA's large size and extreme cost, make its use impractical. And yet, amplifiers will be of crucial importance to metro networking equipment.
The linear optical amplifier is a single-chip, semiconductor-based amplifier that is to the EDFA-type optical amplifier what the transistor was to the vacuum tube. Like the transistor, it is tiny; a single LOA chip is about 1 millimeter square. Even when packaged in its typical enclosure, it is about the size of a sugar cube. By contrast, the EDFA is 100 times larger.
Semiconductor-based optical amplifiers amplify light as it passes through a crystal such as Indium Phosphide according to the same quantum-mechanical principles that make a transistor work. However, such light amplifiers have an inherent defect: introducing a second signal into the semiconductor while it is amplifying another wavelength will cause the amplified version of each signal to take on some of the characteristics of the other. This is crosstalk.
Crosstalk between multiple wavelengths occurs because each signal passing through the semiconductor varies, or "modulates" the amplifier gain - that is, the amount of amplification - as seen by the other signals. The net result is that each signal takes on some of the characteristics of the others. In optical communications, crosstalk renders the output signals unusable.
The linear optical amplifier avoids this problem by pumping a virtually unlimited supply of photons -- the particles that make up light -- into the heart of the amplifier, using a laser. Acting like a ballast, the laser perfectly counteracts the tendency of the semiconductor to react adversely to "gusts" of photons -- the incoming signal pulses -- and, ultimately, keeps the gain of the amplifier perfectly constant, or "linear." In a linear amplifier, output signals are absolute, faithful copies (within required design limits) of the original inputs.
What is unique about the Genoa linear optical amplifier is that the ballast laser is built right into the chip itself. Using a process invented by Genoa's founders, the amplifier and the ballast laser are manufactured simultaneously as one monolithic structure in Indium Phosphide. In actual operation, the multiple-wavelength signals to be amplified pass horizontally through the chip, directly through the path of the laser, which is busily pumping photons of light vertically in the same chip. As previously described, the ballast photons precisely counteract signal-induced variations in the gain of the amplifier, making it linear.
Genoa's LOA chip, with its intrinsic vertical-cavity, surface-emitting laser, is a unique, new type of semiconductor structure. It is the subject of numerous pending and granted patents. The technology is expected by Genoa to ultimately be viewed as an enabling technology for true optical ICs.
Customer tests of the new Genoa Linear Optical Amplifier are currently underway and commercial samples will be available later this year. Pricing has not been set. First products will be available in an industry standard 14-pin butterfly package.
About Genoa Corporation:
Genoa is a next-generation optical semiconductor company that designs and manufactures single-chip linear optical amplifiers. For more information, visit www.genoa.com.