Semiconductor Optical Amplifiers

Several new products based on Semiconductor Optical Amplifiers (SOAs) made a splash at the recent OFC conference (see Vendors Unveil Amplifier Advances). The technology has been under significant development over the last 10 years, and it seems as if the day of the SOA may at last be arriving.In this report by Stephen Montgomery, the president of ElectroniCast Corp., we take a look at some of the reasons why SOAs are attracting so much attention – specifically focusing on three key applications of the technology.The first and most obvious application of SOAs is Optical Amplification. Although the prevalence of Erbium Doped-Fiber Amplifiers (EDFAs) over recent years has restricted growth in this area, there are now niche applications where SOAs are proving a useful alternative. For example, recent products have been aimed at power and pre-amplification (see Kamelian Launches First Products and Genoa Releases Two Amps) and metro applications (see Corning Intros Amplifiers).SOAs are by no means a one-trick pony – they can also be used as all-optical switching elements and wavelength converters (see Interest Grows in Wavelength Conversion). An inherent advantage with these applications is that, in general, the optical losses of the switching/conversion process are neatly compensated for by the inherent amplification capability of SOAs.The predicted market for these three applications of SOAs is certainly significant – expected to rise to a total world consumption of over $250 million in 2005, according to an ElectroniCast forecast (see Report Sees SOA Market Growth). Although wavelength converters take the smallest share of this total, they are expected to be the largest growth area, with a 75 percent annual increase in consumption anticipated for the first five years of this decade. Table 1 below has the full details.Table 1: SOA Global Consumption Value Forecast

Read on to discover how all of this SOA technology actually works and what challenges still lie ahead. Here's a hyperlinked summary of the report:Technical Basics
An overview of what SOAs are, what they do, and how they do itResearch History
A brief rundown of the background of research and development that has brought SOAs to their current situationOptical Amplification
The merits and drawbacks of SOAs against other optical amplifier technologiesOptical Switching
The design of SOA-based optical switchesWavelength Conversion
The use of nonlinear effects to convert wavelengths— Introduction by Craig Williamson, Associate Editor, Light Reading
http://www.lightreading.comNext: Technical Basics

Semiconductor Optical Amplifiers (SOAs) are similar in construction to conventional semiconductor lasers (see Laser Basics). They consist of a layer of semiconductor material known as the "active region," sandwiched in between two other layers of semiconductor of a different composition.An electrical current is passed through the device and serves to excite electrons in the active region. When photons (light) travel through the active region it can cause these electrons to lose some of their extra energy in the form of more photons that match the wavelength of the initial ones. Therefore, an optical signal passing through the active region is amplified and is said to have experienced "gain."Both edges (or "facets") of the SOA are designed to have very low reflectivity so that there are no unwanted reflections of the signal within the semiconductor itself. This is the main difference from regular lasers that have reflective facets in order to build up the intensity of light within the semiconductor material.The semiconductor layers that sandwich the active region are designed to help guide the light through the device. This is achieved through a difference in refractive index from the active layer, in much the same way as the refractive index difference between an optical fiber's core and its cladding help to guide light (see Optical Fiber).SOAs are attractive for gain over a wide range of wavelengths and can be designed to amplify in either the 1300 or 1550 nanometer transmission windows. Their compactness and semiconductor construction make them relatively simple to integrate with other devices, and they also are potentially lower in cost than optical fiber amplifiers (see Optical Amplification) for certain applications. Significant use of SOAs as amplifiers is anticipated for metro and access applications in the future where some of these benefits over EDFAs are most readily realized.SOA devices from companies such as Alcatel SA (NYSE: ALA; Paris: CGEP:PA), Kamelian Ltd., and Opto Speed SA have been commercially available for the last couple of years. More novel devices are always in the pipeline, such as the Linear Optical Amplifier (LOA) from Genoa Corp. (see Part 4 – Optical Amplification), which is slated for release this year.Below are some typical specifications of SOAs that were commercially available as of last year. You can see that wavelength range is not a problem, and gain in any one device can be obtained over a band of wavelengths spreading up to 30nm, which is a reasonable bandwidth. Polarization sensitivity can be a significant issue for SOAs, however, as the varying polarization of light through standard optical fiber sees varying amounts of amplification, which can lead to noise problems.Table 2: Typical Performance Specifications of Commercially Available SOAs (2001)

Next: Research History

Much of the European SOA effort has been encouraged by the European Community's ESPRIT and RACE program funding, focused on telecommunication applications in the early to mid 1990s. Substantial SOA work continued at Alcatel SA (NYSE: ALA; Paris: CGEP:PA), and SOA components and subsystems remain a part of the company’s product offerings.Other European developers included Uniphase/Philips, which later became JDS Uniphase Inc. (Nasdaq: JDSU; Toronto: JDU); Siemens AG (NYSE: SI; Frankfurt: SIE); and Ericsson AB (Nasdaq: ERICY).The Siemens Central Corporate Laboratory in Munich had a substantial SOA research program proceeding through the early 1990s, but this was deemphasized in late 1995 to focus research efforts on shorter-term commercial products. More recent European startups include Opto Speed SA of Switzerland and Kamelian Ltd. of Scotland.North American SOA development was strongly supported by the U.S. Defense Advanced Research Projects Agency (DARPA), aimed mainly at use in defense-oriented fiber optic links, satellite lightwave communication, and optical computers. Bell Labs continues to lead SOA R&D efforts. Within the last couple of years some new North American companies pursuing SOA components and subsystems have emerged, including Genoa Corp., Agility Communications Inc., Axon Photonics Inc., and Optical Crossing Inc.Japanese SOA development has been mainly aimed at keeping current in semiconductor technology, rather than developing commercial products. Some of the Japanese developers include NEC Corp. (Nasdaq: NIPNY), Mitsubishi Electric & Electronics USA Inc., and NTT Communications Corp.Next: Optical Amplification

Amplifiers in WDM networks need to be broadband and provide uniform gain with low noise. The advantages of the SOA as a line amplifier include the reliability, compactness, and integration potential noted previously, plus high power output and a broad choice of operating wavelength (SOAs can be designed to amplify anywhere from 600 nm to 2000 nm). There has also been the perception (as is typical with semiconductor devices) that, with the expected high volume production of SOAs, their prices should drop precipitously.As a line amplifier, the SOA faces competition from the Erbium Doped-Fiber Amplifiers (EDFAs) in the 1500-1600nm band and from Praseodymium Doped Fiber Amplifiers (PDFAs) and Raman Amplification in the 1300-1700nm band.In the 1500nm band the EDFA has much higher gain than the SOA (30-40dB small signal gain, double pumped, versus less than 30dB for SOAs), lower noise, and the acceptance advantage of several years of high-reliability performance in substantial quantities in telecommunication fiber networks. Also, with maturity, volume production, and strong competition, the 1500-1700nm band EDFA gain block price has dropped rapidly over the past four years and will continue to decline. The EDFA gain block is now less expensive than a 1500-1700nm band SOA, on a dollars-per-decibel basis.In the 1300-1500nm band, the competitive threat from fiber amplifiers is much less. The PDFA (which works on principles similar to the EDFA's but with praseodymium ions rather than erbium ions giving the amplification) has not achieved high volume production and so is currently more expensive than the EDFA. Also, the power conversion efficiency of the PDFA is much lower than the SOA's. Consequently, the SOA line amplifier has its strongest potential in the 1300-1500nm band for optical communication.Long term, the SOA is projected to provide much higher power output, with better power conversion efficiency, compared to optical fiber amplifiers. In general, linear Wavelength Division Multiplexing (WDM) systems based on SOAs have moderate capacities and short reach. However, their performance and potential cost advantage may be adequate for metro applications.One of the major problems experienced by conventional SOAs is crosstalk between wavelengths, which can be excessive even at the widest ITU spacing (1.6 nm; 200 GHz). This is an inherent problem, caused by the short lifetime of the carriers in the semiconductor materials. This carrier lifetime is only a few nanoseconds, similar to the period of two beating signals separated by typical ITU grid spacings. These beat signals modulate the carriers and generate sidebands, which constitutes four-wave mixing (FWM – see Nonlinear Effects). In the EDFA, in contrast, the lifetime of the erbium ions is much longer (around 10 milliseconds) so they are much less susceptible to this type of modulation.A major factor in the magnitude of this FWM problem is the level of saturation of the amplifier output power, and so crosstalk can be substantially reduced by splitting the input power and dividing it among a number of SOAs, then recombining the outputs. However, this results in low optical signal-to-noise ratio. Whether this is economically feasible depends upon the cost of the individual SOAs and the cost of fabricating them in an integrated assembly. Alcatel SA (NYSE: ALA; Paris: CGEP:PA) is a leading producer of SOA array subassemblies.Another trick is to demultiplex the wavelengths and individually amplify each wavelength or a small group of wavelengths in separate SOAs. The outputs of the SOAs are then multiplexed back into a single fiber.Bell Labs has demonstrated the use of an SOA as an in-line amplifier by using a wavelength modulation technique that maintains a stable channel power in the SOA. The technique makes use of a dual-input, single-output Mach-Zehnder lithium niobate modulator. Using this scheme, researchers were able to transmit 10-Gbit/s modulated WDM signals over 500 km of nonzero dispersion-shifted fiber (NZDSF – see Advanced Fiber Types) with encouraging results.Yet another approach is to use a so-called gain-clamped SOA, which increases the saturation power of the device. The indium phosphide-based linear optical amplifier (LOA) by Genoa Corp., as pictured in the figure above, uses a "cross cavity" chip design. The chip has an active waveguide gain region, and the input and output fibers are aligned to this waveguide. The novel part of the design is an integrated vertical laser that serves to stabilize the gain of the amplifier. This makes it possible to use the SOA to amplify multiple channels without excessive crosstalk problems. Genoa’s LOA chip is expected to be commercially available in 2002.

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Next: Optical Switching

In order to handle the increased data throughput of WDM systems, aggregate line rates exceeding terabit levels are becoming commonplace. Crossconnects are required at intermediate network nodes to route these large amounts of traffic toward their destinations.The conventional operation of crossconnects involves the conversion of optical signals to the electrical domain where switching takes place, before conversion back into optical signals for onward transmission through the network. The evolution of optical networks is to use Optical Crossconnects (OXCs) that remove this inefficient optical-electrical-optical (OEO) conversion process.OXCs can transparently route wavelengths optically from one fiber to another. This enables the management of wavelengths at the optical level, which in turn decreases the cost and complications of electrical devices. The most popular technology in optical switching is micro-electro-mechanical systems (MEMS), which involve tiny mirrors being moved around to redirect light from fiber to fiber.SOAs are another option for optical switching and can be used in a variety of ways. By altering the amount of electrical current applied to an SOA, the amount of gain experienced by a signal can be modulated. A simple optical switch can be realized by splitting a signal into two separate waveguides that each lead to an SOA. By applying current to one SOA and not the other, the signal can be made to progress to only one of the outputs.SOAs can provide very fast (nanosecond) switching times, and due to their intrinsic amplification they can recover losses inherent in the switching process. There are downsides too, however, including the addition of relatively high levels of noise to signals and the high cost of initial products. The table below summarizes the various advantages and disadvantages of SOAs as optical switches. Table 3: Advantages and Disadvantages of SOAs as Optical Switching Elements

Several SOA gate switches can be cascaded to create large-scale optical crossconnects. Researchers have fabricated low-loss, monolithically-integrated switch matrices containing several SOA gates. Hybrid integration of SOA gate arrays and silica-based planar lightwave circuits is being pursued by companies including Alcatel SA (NYSE: ALA; Paris: CGEP:PA), NTT Communications Corp., and Kamelian Ltd. These units use silicon-based passive circuits and submounts with indium phosphide SOA arrays to perform the switching.Alcatel began shipping SOA-based optical crossconnect switches in 1997 for use in the OPEN test bed, a project within the RACE European communication program. The Alcatel 4x4 switch was monolithically integrated in indium phosphide. Alcatel also makes SOAs that use Distributed Bragg Reflector (DBR) technology in the passive sections, transforming the devices into gain-clamped SOAs. These devices are designed for applications where polarization insensitivity is required.Optical transparency developments will necessitate the implementation of Reconfigurable Optical Add/Drop Multiplexing (ROADM) units, such as the one shown above. This will allow network operators to remotely provision the addition and removal of wavelengths. Kamelian’s ROADM design is based on passive waveguides that distribute and demultiplex/multiplex signals while SOAs perform the optical switching function through a varying drive current. To minimize crosstalk and achieve stabilized gain across all channels (as discussed in Part 4 – Optical Amplification) each wavelength is amplified by a separate SOA.Related stories on Light Reading:

Next: Wavelength Conversion

Wavelength Division Multiplexing (WDM) has transformed a single fiber into a conduit for dozens to hundreds of individual signals. This is a significant change from a system where there is a one-to-one correlation between the fiber and the signal it carries. WDM means that optical switching in its most basic form is the switching of wavelengths within a fiber, as described in Part 5: Optical Switching.In the situation where two identical wavelengths from different sources need to be switched on to the same fiber, there is the need to convert the wavelength of at least one of the signals. Currently this is again achieved through an optical-electrical-optical conversion in the form of a transponder. However, with the advent of all-optical switching, all-optical wavelength converters would be the more efficient and elegant solution, allowing fully integrated optical crossconnect/wavelength conversion boxes (see Interest Grows in Wavelength Conversion).SOA-based wavelength converters are commercially available as qualified telecommunication products from a variety of companies. Alcatel’s SOA wavelength converter combines two SOA chips in a Mach-Zehnder interferometer structure that uses one of the Nonlinear Effects – cross-phase modulation (XPM) – to give the wavelength conversion.The input wavelength travels through the SOA, and through XPM it modulates a constant signal at the output wavelength that is separately injected into the SOA. The output is this new wavelength that now has the same modulation as the input wavelength.In addition to wavelength conversion, this device can also achieve reshaping and reamplification of the signal – so-called optical 2R regeneration. The SOA is monolithically fabricated on InGaAsP material. Alcatel has also designed an integrated SOA array/AWG-based wavelength selector capable of selecting one or more of 16 channels, spaced 100 GHz apart with zero decibel loss.A recent startup, Luxcore Networks Inc., presented their LambdaXchange add/drop system (illustrated above) at ElectroniCast’s Monterey 2001 Fiber Optics Conference. While providing the raw optical switching through the use of MEMS, this system employs a wavelength converter subsystem based on SOAs. The cross-phase modulation effect is again used in conjunction with SOAs and a tunable laser to give the flexibility of wavelength selection.Related stories on Light Reading: