SOAs provide multiple network design options

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Bart Verbeek, Kees Vreeburg, Hans Naus, Willem van den Brink, Leda Lunardi, and Alexey Turukhin

The semiconductor optical amplifier now allows engineers to exploit unique features in solving local optical-power problems in a cost-effective way. Its fast response can be used for switching and/or signal blocking applications.

The potential size and cost advantages of producing semiconductor optical components have kept researchers intrigued with semiconductor optical amplifiers (SOAs) since the late 1980s. Recently, breakthroughs in performance and price have made SOAs viable for deployment in the network for a variety of amplification and switching applications.

Once thought of only as a possible low-cost alternative for erbium-doped fiber amplifiers (EDFAs), SOAs are now recognized for their unique set of operating properties and features. While EDFAs remain the more effective solution for high-end, long-haul WDM amplification, SOAs are a versatile, small-footprint, low-power-consumption component that can be used in a variety of applications. SOAs can operate in every fiber wavelength window (1310 nm, 1400 nm, and S-, C-, and L-bands) and they can react very quickly to incoming signals, so they can be used in switching applications and for very high bit-rate systems.

SOAs can also be used in Mach-Zehnder interferometers for wavelength conversion, optical time-division multiplexing (OTDM), optical demultiplexing, DWDM medium distance (such as metro) applications, and optical crossconnects. The SOA is the only amplifier that can be integrated as a functional element in photonic integrated optical chips—an important technology for miniaturization and compactness.

Several established companies as well as startup firms are now offering SOA products. The potential cost benefit of semiconductor manufacturing had not been realized because of low volume production, which was primarily done for research needs. However, with increased competition and more widespread use of SOAs anticipated, prices have been reduced by up to 50% compared to last year.

SOA TECHNOLOGY
An SOA is a semiconductor diode chip in which indium phosphide (InP) is used as substrate material and indium gallium arsenide phosphide (InGaAsP) is the active material. Electron-hole pairs are injected into the active layer, then recombine, emitting light. SOAs are fabricated using the same processing technology as that used for semiconductor lasers (see Fig. 1). This process has been honed over many years and with high-volume production, so it is now well understood and highly reliable.

An SOA is essentially a laser that is operating under threshold. Whereas the laser requires some internal or external reflection for feedback, the SOA is a single-pass device. The input signal is usually coupled via a lens to the chip waveguide and the amplified signal exits the chip through another lens and into the fiber.

Because of the high single-pass gain (>30 dB) of the active layer, reflections at the facets of the device are minimized (<10-4). This eliminates optical feedback, which causes gain ripple. Optical isolators are often used to reject back-reflected light from the system. If optical isolators are not used, then the SOA can be used as a bidirectional amplifier.

The gain wavelength can be tailored from approximately 1.0 to 1.7 µm by varying the composition of the active InGaAsP material. This results in amplification of light in a wavelength region ranging from 1.28 up to 1.65 µm. The wavelength-dependent gain curve of a SOA is parabolic in shape (see Fig. 2).

It should be noted that the wavelength at the maximum gain and the optical bandwidth of the SOA are design parameters offering flexibility in applications. An optical bandwidth—the wavelength span in which the gain decreases less than 3 dB with respect to the maximum gain—of up to 100 nm can be achieved.

PERFORMANCE PARAMETERS
The electro-optical performance of SOAs has greatly improved because of progress in design and device fabrication. Key parameters of the SOA include the following:

Gain. Gain should be matched with the requirements of the application. Fiber-to-fiber-gain values up to 30 dB have been achieved. Because of the nature of the semiconductor material and generally asymmetric waveguide structure, SOA gain is, without an optimized design, polarization dependent. Optimized designs, such as those with an active square waveguide or strained layers in a bulk or quantum-well-based active region, achieve both a high gain (25 dB) and low polarization dependence (0.5 dB). Thus, by design, two versions of an SOA can be made: a polarization-dependent as well as a polarization-independent SOA.

Output saturation power. Defined as the power for which the gain is 3 dB compressed from its maximum value, output saturation power is typically 10 to 13 dBm. In certain applications the output saturation power becomes more important than the gain.

Noise figure (NF). The SOA adds some amplified spontaneous emission to the amplified signal. The fundamental lower limit for the noise figure of an SOA is the same as an EDFA (3 dB), however, the short carrier lifetime and the coupling loss (input side only) of a SOA increases the noise figure (NF) to typically 6 to 9 dB.

Switching. By changing the bias current of the SOA, the device can be either absorbing (low current) or amplifying (high current). The high extinction ratio of up to 50 dB and the fast response time (ns) allows the SOA to be used as an optical gate in routing and/or packet switching.

Gain Clamping. If an SOA operates in the nonlinear regime (such as under saturation) the fast gain dynamics of the SOA may cause crosstalk between WDM channels. The carrier lifetime of about 200 ps is in the same order as the bit rate for high-speed systems, where the gain for each bit depends on the previous bit sequence.

Several alternatives can be used to mitigate or avoid this crosstalk: operate the SOA in the linear regime and increase the saturation output power; load the SOA with multiple channels to statistically average the incoming power; use a high-power reservoir channel to minimize the relative power fluctuations; or use a gain-clamped SOA where an internal laser structure is incorporated.

In the latter case, the laser output power, which has its wavelength outside the operational window of the SOA, serves as ballast to achieve a linear SOA response. A drawback of this structure is that the SOA gain is set at the laser threshold condition and, therefore, is fixed (see Fig. 3). Furthermore, the relaxation oscillation of the internal laser may limit the speed of operation of the SOA in a high-speed optical link (see "Validating the SOA," p. 20).

SOA APPLICATIONS
SOAs can be used in many application areas, from the access to the long-haul markets. All of the following applications employ one or more unique properties of an SOA.

In-line amplification. Demand is generated by the need to compensate for losses and (re)amplification of the signals. SOA performance meets the requirements of many applications in metro and access networks. It has been demonstrated in various system experiments, using 40 channels or more, that SOAs can be successfully employed in DWDM systems for 10 and 40 Gbit/s transmission.

Switching. As optical networks penetrate into metropolitan and access areas, the need for switches increases. With the advantages of a very fast (ns) response time and a high extinction ratio, the SOA can also be used as a switch and, unlike other switches, it can amplify the signal when needed.

Loss compensation. The SOA is a compact, inexpensive solution for compensating for losses caused by other components (such as a splitter) throughout the system. SOAs have generated particular interest for use inside optical crossconnects.

Booster amplification in transmission. A polarization-maintaining SOA can be used for booster amplification applications. For example, a system with a continuous-wave laser with a lithium niobate (LiNbO3) pulse/data generator/modulator (such as for 40 Gbit/s) could have a polarization-maintaining SOA to boost the optical power of the pulse train.

Pre-amplification. An SOA installed prior to the receiver will amplify the signal and improve signal sensitivity. This is a particular advantage for 1310-nm systems where no fiber amplifiers exist, or for systems where an avalanche photodiode is not attractive (because of cost and nonlinear response) or is unavailable (as in 40 Gbit/s systems).

Wavelength conversion. When SOAs are used in the arms of a Mach Zehnder interferometer (ideally integrated into a single chip), wavelength conversion has been demonstrated, including extinction ratio improvements (compared to direct gain modulation), over the full C-band at a speed of 40 Gbit/s and higher.

OTDM. SOAs are effective in terahertz optical asymmetrical demultiplexer (TOAD) applications to demultiplex data streams (for example, a 10-Gbit/s data stream out of a 40-Gbit/s stream or 40-Gbit/s data stream out of a 160-Gbit/s stream) without the use of direct receiver technology.

Other potential applications that have been identified and demonstrated, including mid-span spectral inversion in which the four-wave mixing effect in SOAs is used to compensate dispersion effects in signals. Also, because SOAs have a broad gain profile, they can be used as an amplifier for very short pulses. This wide-gain bandwidth makes an SOA a natural candidate for a "white" source component in every optical telecom wavelength window.

Bart Verbeek is R&D director, Kees Vreeburg is senior development engineer, Hans Naus is an R&D scientist, and Willem van den Brink is product line manager at JDS Uniphase, Prof. Holstlann 4, Eindhoven 5656 AA, The Netherlands. Leda Lunardi is a senior scientist and Alexey Turukhin is staff scientist at JDS Uniphase, Eatontown, NJ,. Willem van den Brink can be reached at willem.vandenblink@jdsu.com.

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VALIDATING THE SOA
To demonstrate the feasibility of SOAs in DWDM applications, we conducted an experiment using 40 wavelengths between 1530 and 1560 nm, spaced by 100 GHz (a). The channels are externally modulated with a 231-1 PRBS 10 Gbit/s nonreturn-to-zero (NRZ) data stream before amplification by the SOA.

The full channel loading is shown in the optical spectrum after the SOA (b). Good eye openings for all channels have been obtained; for example, in channel 40 (1545.32 nm), ER = 9 dB and total input power into SOA = -8 dBm (-24 dBm per channel) (c). The individual channel performance was evaluated by Q-factor analysis by measuring the bit- error rate versus decision threshold, with -25 dBm received power (d). The average Q-value was 18.5 dB, with the best value of 19.2 dB for 1533.47 nm and the worst of 18 dB for 1550.12 nm at -8 dBm total input power to the SOA (-4 dBm per channel).

To measure the impact of interchannel crosstalk, four channels adjacent to 1545.32 nm were turned off. The optical spectrum was not significantly impacted, which is confirmed by the bit-error-rate and a Q-factor of 18.5 dB. This system experiment proves that standard—for example, non-gain-clamped—SOAs can be used for WDM applications and that the performance is not limited by crosstalk.

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