EDFAs pump up the power again and again

Dhrupad A. Trivedi

Toby Strite

Gerlas van den Hoven

Erbium-doped fiber amplifiers (EDFAs) are an enabling technology for WDM optical networks. Their ability to simultaneously amplify multiple wavelengths provides significant performance and cost advantages over electronic regeneration. EDFAs are used in networks to boost transmitted power (booster amplifier), amplify signals in transit to compensate for losses sustained in the fiber (line amplifier), or amplify signals before a receiver (preamplifier). In reality, the devices are highly system-specific, making the definitions somewhat blurred.

A simple single-stage EDFA consists of an erbium-doped fiber spool (see Fig. 1). The fiber is optically pumped by 980- and/or 1480-nm lasers, the light from which is coupled into the 1550-nm signal fiber by an optical multiplexer (also known as a wavelength-division multiplexer or a WDM). The pump wavelengths are readily absorbed by erbium ions embedded in silica, raising them to an excited state. Amplification occurs when, stimulated by a nearby 1550-nm signal photon, an excited erbium ion relaxes back to the ground state, producing a second, identical 1550-nm photon.

The most critical performance parameters of an EDFA are its amplified 1550-nm output power (typically stated in dBm) and its noise figure (stated in dB). Output power is primarily determined by total pump power and the amplifier internal loss. The noise figure is defined as the ratio of the signal-to-noise ratio at the input to the signal-to-noise ratio at the output. Details of the physical phenomenon determining noise figure can be found in several textbooks.1,2

A single-stage amplifier typically has one or two pump lasers but can have more if polarization- or wavelength-pump-combining is implemented for higher power. When the pump radiation propagates in the same direction as the 1550-nm signal, the amplifier is co-pumped; counter-pumping denotes the case when the pump laser propagates against the signal. For a single pump, a co-pumping 980-nm laser minimizes the noise figure (suitable for a preamplifier) while a counter-pumped 1480-nm architecture optimizes output power at some expense to the noise figure (suitable for a booster amplifier).

Enhanced single-stage designs

Single-stage designs can be enhanced. An isolator can be used at the input and/or output to prevent pump-laser emission or amplified spontaneous emission (ASE) from the erbium-doped fiber "leaking" into the transmission path. Optical taps can be included to provide information about signal spectra at the input and output sides. Their feedback can be used to control pump-laser biases for tuning output power, monitoring amplifier performance, or simply to trigger alarms. In addition, a reflection monitor is sometimes placed at the output to observe backwards-propagating optical signals arising from reflections (see Fig. 2).

Amplifiers are commonly referred to by their saturated output power-for example, a 19-dBm amplifier. This nomenclature can be misleading, however, because it is difficult to achieve this specification for an extremely low input power. The noise figure of an EDFA typically increases with increasing input power.

Single-stage optical amplifiers are suited for a wide range of applications such as single-channel amplifiers, simple WDM amplifiers, and low-cost amplifiers. Using high-power pump lasers or combined pump laser schemes allows such amplifiers to deliver output powers >20 dBm. However, single-stage amplifiers cannot meet the requirements of all telecommunications architectures, leading to increasing demand for multiple-stage EDFAs.

Multistage amplifiers

Unless special consideration is given, the output of an EDFA mimics the erbium-doped fiber gain spectrum and, as a result, the gain per channel can vary by several dB as a function of wavelength across the transmission band. Designs transmitting up to 16 channels at 200-GHz spacing require only the red C-band (1540-1563 nm), where the erbium response is quite flat. However, as more channels are added, the entire C-band (1528-1563 nm), and even the L-band (1570-1610 nm), is being used.

Over this expanded range the erbium gain spectrum varies considerably. This imbalance becomes acute in long-haul systems that cascade many EDFAs. The most common countermeasure is the insertion of filters to attenuate wavelengths with higher gain, thereby equalizing the signal. Typically, gain-flattening filters introduce a wavelength-dependent loss up to 8 dB at selected wavelengths.

The most common technologies for attenuation are thin-film filters and fiber gratings. Filters must be carefully matched to the amplifier architecture to achieve EDFAs with <1dB gain variation in commercial quantities. Gain flatness is usually specified at a particular set of operating conditions such as input power levels and wavelengths.

In single-stage amplifiers, gain flattening can occur at the amplifier output, which results in lost output power and wasted pump power, or by altering the wavelength spectrum entering the amplifier, with the disadvantage of further attenuating an already weak signal before any amplification. Gain-flattened amplifiers are almost always multistage because all gain-flattening options for single-stage amplifiers are unattractive.

More-complex architectures

In WDM systems, optical amplifiers may also need mid-stage access where the signal is fed into an external device between the amplifier stages. Reasons for doing this include monitoring, add/drop, and dispersion compensation. Because additional losses up to 10 dB are introduced in the amplification path, the amplifier design has to be optimized for those losses. Clearly, such requirements cannot be met in a single-stage amplifier.

When multiple pumps are needed, the most common configuration is a 980-nm pump laser co-pumping the first-stage amplifier and a 1480-nm laser counter-pumping the second. A significant loss element-a gain-flattening filter, add/drop module, or dispersion-compensation module-is situated between the stages. Mid-stage access can be located before or after the gain flattening or between additional stages. The gain spectrum of a multistage gain-flattened EDFA indicates that gain equalization is possible using a multistage amplifier design and gain-flattening filters (see Fig. 3).

EDFAs comprise passive components, erbium-doped fiber, and pump lasers. Passive components are chosen to meet optical and environmental specifications; erbium-doped fiber is selected to meet optical power, gain, and noise figure requirements.

Pump lasers are a key influence on the price and performance of optical amplifiers. The availability of two pump wavelengths offers EDFA designers a vital degree of freedom (see table on p. 20). The 980-nm wavelength is favored for low-noise amplification while 1480 nm is less costly. Despite higher-rated power from 980-nm lasers, one 1480-nm pump can provide more amplification because less power is wasted converting a 1480-nm photon to 1550 nm.

Also, the flatter erbium absorption spectrum around 1480 nm generally eliminates the need for wavelength stabilization, while 980-nm pumps for complex EDFAs must incorporate external fiber-Bragg grating stabilization to avoid the undesirable effects of even small wavelength changes. On the other hand, the superior efficiency of 980-nm laser chips leads to reduced power consumption and heat generation, which are critical parameters in the central-office environments common in metropolitan network architectures.

Further reductions in power consumption and heat load achieved by eliminating Peltier coolers within the pump laser packages are highly attractive for certain applications. Uncooled 1480-nm pumps producing ~40 mW have been available for some time. Uncooled FBG or otherwise wavelength-stabilized 980-nm pumps are also available and should see growing demand as their markets mature.

Raman amplification

One of the exciting new developments has been the increasing use of Raman amplification in optical communication systems. Raman amplification is achieved via a second-order nonlinear effect in the transmission fiber to provide additional signal gain without significant noise degradation. For example, to achieve gain in the 1550-nm transmission band, pumping is necessary at ~1455 nm.

Raman gain is a second-order effect, and thus the required single-mode pump powers can be as high as 500-700 mW. Two basic approaches for delivering such powers exist: polarization-combined diode lasers similar to conventional 1480-nm pumps and an elaborate wavelength-shifted fiber laser. At present, the diode-based approach is a more straightforward solution for lower power, while the fiber-laser approach scales more readily for high-power applications.

In an optical network, Raman amplification can be used as the only amplification mechanism or in conjunction with EDFAs. Future systems are likely to use Raman gain to provide additional design margins and to potentially reduce, but not eliminate, the EDFA requirements.

Gain equalization and new bands

Another trend in optical amplifiers-and systems in general-is the move toward dynamic gain equalization. Several technologies are being considered, and the primary idea is to dynamically control the intensity level of each channel to correct for gain/loss inequalities in the network.

This correction is particularly important as channel counts and bit rates rise concurrently with the functionality demanded of optical communication networks. A common requirement in this category is fast amplifier response so that dropped or added channels do not introduce power distortions at other wavelengths.

Furthermore, with increasing demand for bandwidth, amplifiers have evolved to operate in new wavelength regimes. The L-band transmission window-from 1570 to 1610 nm-has opened up new bandwidth possibilities for increased channel counts. EDFAs are already being deployed to address these markets. Future systems will widen the transmission window further through transmission fiber technologies that allow long-distance transmission over the entire wavelength range of the fiber.

EDFA technology has already evolved to accommodate multiple channels, span several wavelength windows, and provide features such as gain-transient suppression. As future network architectures are introduced incorporating Raman amplification and additional wavelength windows, the demands placed upon EDFAs will only grow.

REFERENCES

1. Emmanuel Desurvire, Erbium-doped fiber amplifiers, John Wiley and Sons (1995).

2. P. C. Becker, N. A. Olsson, and J. R. Simpson, Erbium-doped fiber amplifiers-fundamentals and technology, Academic Press (1999).

FIGURE 3. Multistage amplifiers may have mid-stage access points where the signal is fed into an external device. Gain spectrum of a multistaged optical amplifier with and without gain-flattening filters shows that gain equalization is possible.