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Chapter 4.4.2 - Optical Amplifiers and Regenerators

4.4.2 Optical Amplifiers and Regenerators

4.4.2.1 Amplifiers

In Chapter 2 (Section 2.17), we described the physics and technology of optical amplification. Amplification is required to overcome signal losses in the fiber and other components (depending on application and fiber, span loss is in the range of 23 to 15 dB). The key three types of optical amplifiers were the fiber (OFA), the Raman, and the semiconductor (SOA). Each of them has distinct benefits and limitations so that no one type by itself is currently suitable for all applications and for the complete spectrum from 1,250 to 1,650 nm (see Figs. 2.66 and 2.81). As such, each amplifier must be evaluated on its own merits and should be applied where it makes more sense in technical and cost terms.

OFAs (EDFA, YEDFA, TDFA, etc.) have been suitable and largely been deployed in fiber networks. There is a continuous effort to improve them in terms of increased gain, bandwidth, and functionality. However, although YEDFAs (yttrium-erbium doped fiber amplifiers) have extended the EDFA range in the C-band and the L-band, they still have a limited range (1,530–1,620 nm). Table 4.1 lists the optical channels (frequency and wavelength) in the extended L-band. Thus, EDFA or YEDFA is optimized for power restoration with minimal added noise of the depleted optical signal in the 1,550-nm wavelength band.

EDFAs are applicable as booster optical amplifiers in DWDM and CATV, as low-noise (5 dB typical) low power consumption (<3.5 W) compact optical preamplifiers in optical cross-connects and in metropolitan networks. EDFA preamplifiers have a relatively small package size (approximately 7 x 9 x 1.2 cm3) so that they can be installed on a board. The complete package includes the pump laser, couplers, isolators, filters, doped fiber, and multiplexers. As amplifier technology evolves, level of integration and functionality will increase and package size will decrease, increasing performance and reducing the cost of the system.

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As optical fiber with very low OH-content becomes available, the complete window 1,250 nm to 1,650 nm will soon be available in DWDM networks. Therefore, it is logical that other solutions must be used to complement YEDFAs. Such amplifiers are the Raman amplifier and parametric fiber amplifiers.

Raman amplification is deployed solely in fiber networks and at repeater sites. It also has been deployed in conjunction with EDFA/YEDFAs to improve transmission performance. That is, to enhance gain flatness and improve the optical signal to noise ratio. These improvements reduce the need for dynamic gain equalization and enable higher channel density, faster transmission bit rates (10 and 40 Gb/s), and better gain response and hence longer distance between regeneration sites. However, the OFA gain is not flat over the amplified spectral range. Excessive OFA gain ripple may be reduced with inverse filters (filters with transfer function compensating for EDFA gain) (Fig. 4.14).

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Figure 4.14 Inverse filters reduce EDFAs gain ripple.

Similar to EDFAs, Raman amplifiers are compact devices. The Raman pump may also be incorporated into the EDFA package, when both a preamplifier EDFA and a Raman amplifier are used in synergy (see Section 2.17). A Raman amplifier package also contains pumps, taps, isolators, couplers, thin-film filters, and combiners.

Because Raman amplification evolves over the existing single-mode fiber used for transmission, the Raman amplifier gain varies with fiber type (from about 15 to 23 dB). For example, although the gain may be 20 dB in one fiber type of manufacturer X, it may be 15 dB in another type of manufacturer Y. As discussed in Chapter 2, Raman amplification may also have pump(s) in co-propagating, counter-propagating, or amphi-propagating (both) directions with respect to the signal.

During the propagation of pump-light an amplification evolution takes place as well as other nonlinear effects. We separate them into two groups, those related to gain and those that degrade the quality of the signal manifested by added noise.

First, there are at least three mechanisms that take place during this evolution: signal loss, amplifier gain, and pump loss (~0.05 dB/km higher than signal loss). As such, the amplifier gain is not uniform along the fiber span but it is stronger at the input to the span (in the copropagating case) or at the output of the span and going backwards (counterpropagating) (Fig. 4.15).

Second, because the pump light is strong (compare hundreds of mW of the pump with few mW of the signal), the signal is subject to nonlinearities due to Raman pump as well as to noise sources. Noise and increased nonlinearity need to be minimized for optimum optical signal-to-noise ratio (OSNR). In either case, fiber amplifiers should not have an SBS threshold less than 3 dBm.

The nonlinearity for a Raman-pumped fiber span is provided by the integral

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Figure 4.15 Counter-propagating Raman amplification.

The nonlinearity for the unpumped span is:

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where P(z) is the Raman power evolution function, a is the loss, γ is a function of the signal power, and L is the length of the span.

From these two relationships, the ratio of nonlinearity increase due to Raman pumping is

04_04_DWDM-12.jpg

The noise sources are mainly due to broadband spontaneous Raman scattering noise and narrow-band double (in both directions) Rayleigh scattering noise (Fig. 4.16).

Similarly, if the span loss is Lf, then the noise figure of the fiber span (without pump) is defined as

04_04_DWDM-13.jpg

Then, the noise figure for the Raman pumped fiber span is:

04_04_DWDM-14.jpg

where h is Planck's constant, ν is the frequency, Pnoiseis the noise power in the electrical filter bandwidth (here we assume that the signal has been received and converted to electrical), and GRis the Raman gain.

Based on these two noise figures, an improvement noise figure is calculated from

04_04_DWDM-15.jpg

As also discussed in Chapter 2, several Raman pumps are used to amplify a wider spectral range. In this case, the above relationships become more complex and a gain ripple effect must be considered (see Fig. 2.78), with a typical average variation of 0.1 to 0.5 dB. When Raman and EDFA are used in synergy, both Raman ripple and EDFA ripple may be designed so that an almost flat gain is realized. Figure 4.17 illustrates the flat gain region in the C-band for various channel densities.

04_04_DWDM-16.jpg

Figure 4.16 A. Model of Raman Noise and Double Rayleigh Scattering, and B. their contributions to the optical signal.

Based on the preceding, Raman amplification and EDFA amplification are not as straightforward but require many parameters to be considered to calculate the optimum strengths of forward and backward pumping for an amplifier with the desired net gain and optimum noise characteristics. However, which parameters should be considered most greatly depend on application and engineering rules. For example, Raman scattering noise is independent of signal power but is proportional to bit rate (the Raman noise bandwidth at 10 Gb/s in SMF is ~12.5 GHz) in contrast to Rayleigh double scattering, which is proportional to signal power and independent of bit rate.

Table 4.2 provides a qualitative comparison between the EDFA and Raman amplifiers.

SOAs have not been as popular in power amplification as EDFA and Raman amplifiers because of lower gain, nonlinear phenomena, and channel cross-talk (here we should distinguish between interchannel and intrachannel cross-talk, the latter being more dominant). However, SOAs present an excellent match with materials that are used for many optical solid-state devices. Therefore, as the level of optical integration increases, SOAs will play a key role in post- and preamplification. For example, SOAs may be integrated with laser pumps, filters, and optical waveguides to produce compact, small, and inexpensive amplifiers, the power of which may then be boosted with EDFA and/or Raman amplifiers. Or they can be similarly integrated with receivers and demodulators. In addition, SOAs may be the main amplifier in coarse WDM applications such as access and small metropolitan networks.

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Figure 4.17 Optical amplifier flat gain region in C-band.

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4.4.2.2 Regenerators

The role of regenerators is to recondition the received weak optical signal; remove noise, jitter and distortions; and amplify the signal (Fig. 4.18). However, this function is good for a maximum fiber span, and therefore it must repeat itself, hence repeaters (Fig. 4.19). Nevertheless, noise is cumulative, and therefore the number of repeaters in a path may be very large. Currently, the common practice (based on amplifiers, fiber, and other parameters) is to have up to seven or eight repeaters in a path of about 4,000 km (see Fig. 4.5). Regenerators that perform reshaping, retiming, and amplification (reconditioning) are known as 3R regenerators. Currently, however, 3R regenerators are opaque, and if they also include functions such as add-drop, performance monitoring, and wavelength translators, they are known as transponders.

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Figure 4.18 Model of an optical regenerator.

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Figure 4.19 Signal regeneration action.

Because of maintenance and cost aspects of opaque regenerators and transponders, the trend is that they be all-optical (transparent). Thus, a simple EDFA and Raman amplification stage (including isolation and filters) that simultaneously amplifies many channels is known as a 1R regenerator (i.e., retiming and reshaping are not included). However, sophisticated techniques using nonlinear fibers have demonstrated that 2R all-optical (amplification and reshaping) is feasible as well as cost-effective. If 2R optical solutions include coherent optical oscillators for retiming, then 3R is also feasible; ongoing work will soon bring forth 3R cost-effective solutions.

Since DWDM regenerators pass through an enormous amount of data, it is desirable that they be redundant; that is, they are duplicated so that when one regenerator degrades or fails, the other continues to provide uninterrupted regeneration functionality. In general, a regenerator consists of three stages: a preamplifier (with proper isolators, couplers, power monitors, and filtering); an equalizer (where an add-drop multiplexer may also be included); and a booster amplifier with filtering and isolation (Fig. 4.20). Clearly, other configurations may also be used. For example, the preamplifier may consist of a Raman-pumped DCF followed by an EDFA or Raman booster amplifier so that both amplification and pulse compression due to dispersion, reshaping, and amplification are accomplished.

In addition to EDFA and Raman, regeneration may also be achieved using the nonlinearity properties of SOAs (such as cross-phase modulation) in a Mach-Zehnder interferometric arrangement (see Chapter 2, optical amplifiers) and using dispersion shifted fiber (DSF). Current research has demonstrated that 2R as well as 3R regeneration is possible for 10, 40, and 80 Gb/s signals and for optical paths exceeding 10,000 km (using multiple regenerators in cascade), thus promising compact devices that may revolutionize the regeneration process in optical DWDM transmission.

 

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