Multiple-wavelength pumping overcomes increased channel counts
New DWDM systems demand increased EDFA output power. One technique that combines several pump wavelengths can provide this increase effectively.
Erbium-doped fiber amplifiers (EDFAs) are generally recognized as the enabling technology behind the widespread implementation of dense wavelength-division multiplexing (DWDM) in long-haul telecommunications systems. Ironically, the success of DWDM in providing multiple channels has now put pressure for improvement on the amplification systems.
Increasing system capacity through wavelength-division multiplexing (WDM), a process of transmitting various wavelengths, or channels, simultaneously through fiber, meets the definition of DWDM when eight or more of these channels are packed together with 0.8-nm channel separation. The rapid commercialization of DWDM technologies has propelled 16-channel DWDM systems away from state-of-the-art and into the mainstream, as bandwidth demand continues to escalate and channel counts increase.
Amplifiers are used in the long-haul network to refresh a signal once the transmission losses total approximately 25 dB. These losses depend upon many factors, including the number of WDM channels, the output power of the optical amplifier, the amplifier noise figure, the transmission bit rate, dispersion, and other fiber-related losses.
EDFAs developed as an alternative means to amplify signals along an optical fiber. Previously, electronic signal regeneration was employed to compensate for fiber attenuation and dispersion in an optical network, two problems that limited the reach of an optical circuit. The regenerators were required every 40 km (25 mi). They converted the modulated laser light to an electrical signal, electronically amplified and regenerated that signal using high-speed integrated circuits, and then converted the signal back into an optical signal using a local semiconductor laser.
Every signal, multiplexed or not, needed its own regeneration. The regenerator system was the limitation to exploiting the large fiber bandwidth both from a cost and capacity perspective. An 8-channel system would have required eight regenerators at $75,000 each.
One of the first commercial applications for EDFAs was in transoceanic systems. The market continued to demand new capacity, but the cost of laying new cabling was very high. Regenerators were the bottleneck to increasing channel counts on the existing fibers by using DWDM.
Unlike electro-optic regenerators, EDFAs are transparent to multiwavelength systems. They can also be located three times as far apart as regenerators. They reduce the number of maintenance points, they operate independently of format or bit rate, and they support very high bandwidth. Additionally, they simplify the transmission network and represent another step toward the all-optical telecommunications network.
How does an optical amplifier work?
A typical optical amplifier (see Fig. 1) uses light at one wavelength (either 980 or 1480 nm) as the energy source to amplify light at a second wavelength (1550 nm). Both the transmission signal at 1550 nm and the pump signal propagate through a section of erbium-doped optical fiber. The erbium-doped fiber acts as a storage medium for the transfer of the energy from the pump to the signal.
Either pump wavelength can be used because erbium has many absorption points. The one at 980 nm decays non-radiatively to the long-lived, meta-stable state. This state is also reached by pumping at 1480 nm, a second absorption frequency of erbium. Radiative emission is triggered from this meta-stable state by the transmission signal at 1550 nm, providing amplification to that signal (see Fig. 2).
Pump modules are commercially available at both wavelengths. Devices at 980 nm have been used more frequently for terrestrial systems, while 1480-nm technology has been used in transoceanic networks. Vendors have done the necessary lifetime tests on the 1480-nm pump modules for the transoceanic systems. Modules at 980 nm appear more efficient, yet the energy must degrade through two excitation states, which results in a loss of some of the energy.
The distance between optical amplifiers is directly related to the output power of the pump laser. The more pump power that is available to the EDFA, the farther apart the EDFAs can be spaced, which reduces installation and maintenance costs for the entire transmission system. Typical commercial laser pumps produce around 165 mW of output power at 980 nm. The commercial lasers at 1480 nm produce about 125 mW. Although configurations vary depending upon system design, EDFAs are generally spaced at 80 km.
A regenerator is still required to refresh the signal, but not nearly as frequently--usually only every 700 km. System designers are working to increase that distance, as these devices are still very costly. Cost becomes even more of an issue as maturing DWDM technologies facilitate channel counts to 16 and above.
As noted previously, most EDFAs can handle much more pump power than a commercial diode laser delivers. Historically, this difference has not mattered much. The EDFA provided such a clear benefit over the electronic repeater that additional performance improvements were not worth pursuing. The mismatch was not an issue. Implementing DWDM has begun to cause equipment manufacturers to reexamine that mismatch.
Each active channel in a DWDM system needs some of the power from the optical amplifier to amplify that signal. The more channels there are, the more power is required to send the multiple signals the same distance. Or conversely, the more channels that are active in the system, the closer the amplifiers need to be if the pump power from the laser remains unchanged.
There are three solutions to getting more power into the optical amplifier:
using higher-powered lasers as a single source
pumping the amplifier with two laser sources separated by polarization state or by wavelength
pumping the amplifier using two, three, four, or six lasers, each separated by a predetermined wavelength, combined with a multiplexer module.
Diode lasers are generally considered "high-powered" if they emit more than 100 mW of power. However, researchers have pumped EDFAs with a variety of more powerful lasers in an effort to define more clearly the performance range of the amplifiers. These lasers, including a Nd:YAG/TiSapphire combination, an argon-dye laser pairing, and even a fiber laser, can be tuned to any of the absorption points along the spectrum appropriate for pumping an EDFA. These lasers have produced at least 1W of output power, about six times the output of a diode laser, without saturating the EDFA.
But even though more laser power is needed, none of these lasers is suitable for a telecommunications environment. They are expensive, large, and can be difficult to operate. Their conversion efficiency is low, and they don`t age predictably. Finally, they have not been qualified for a telecommunications environment. These qualities run contrary to the needs of the telecommunications industry, which requires inexpensive, small, reliable, and efficient laser sources.
Diode laser manufacturers are developing more powerful lasers. Currently, unpackaged versions emit as much as 300 mW; when assembled into butterfly packages, the output will decline to 250 mW. While this is twice the output of commercial devices, it will be insufficient to meet the needs of a 16-channel DWDM system.
Dual-pump laser devices
Laser light can be combined from two lasers at the same wavelength if the sources are each in different states of polarization. Then, the coupler that combines the light must preserve the state of polarization, something not usually considered in telecommunications systems. Output powers in the range of 20 dBm have been demonstrated using two laser sources at 980 nm. Some of the initial output power of the lasers is lost, however, in the process of polarizing the sources.
Amplifiers that use a hybrid of 980- and 1480-nm lasers are also available commercially. These systems emit output power also in the range of 20 dBm. Their advantages include a reduction of pump crosstalk, and they are more efficient as measured by the ratio of output power to power consumed. But these systems offer no laser redundancy and are limited to just two laser sources.
A third method that gets more power from an optical amplifier involves using the output from up to six lasers, combining them through a multiplexer device and using the combined output to pump the amplifier. The first barrier to using many lasers is crosstalk between the lasers, so they must be of different wavelengths, separated by a small amount, similar to the way that lasers are separated in a DWDM system.
The multiplexer device provides the key to overcoming this barrier. The operation of one such device, called the WaveCombiner, illustrates this concept. The device employs patented fused biconic taper couplers to multiplex up to six laser sources into one common fiber. At 1480 nm, the lasers can be spaced 10 to 15 nm apart (see Fig. 3), as the absorption window is 80 nm wide. At 980 nm, the lasers must be only 2 or 3 nm apart, since the absorption window is only 15 nm wide. The device must have low insertion loss (less than 0.8 dB for a 4-channel device), so that the maximum amount of laser power can excite the amplifier. Low polarization-dependent loss is critical, and typical values are less than 0.1 dB.
Using currently available laser sources, this multiplexer device can provide up to six times the amount of pump power for additional amplification, or one or more of the lasers can be configured to provide redundancy for the system. Additionally, multiplexing lasers to increase pump power can be achieved in a small package.
Technical considerations related to multiwavelength pumping must be addressed for this technique to provide optimal performance. For example, the EDFA is configured with isolators on either side of the EDFA. The first isolator prevents backreflection of the carrier wavelengths; the second one keeps any unabsorbed pump wavelengths from continuing with the signal. The second isolator must have a broad-enough range to accommodate the spectral width of the multiple lasers.
Another consideration concerns the coupler that combines the pump wavelengths with the signal wavelengths; it too must be spectrally broad enough to accommodate both the range of the pump lasers and the range of the transmission lasers.
Higher EDFA output has also been achieved by using longer fibers within the EDFA, by enlarging the core and by doping the core with other materials, including ytterbium. All of these solutions increase the output of the EDFA, but not nearly as much as multiwavelength pumping. Additionally, each of these solutions can accommodate multiwavelength pumping to achieve even higher outputs.
Offsetting the impact
DWDM technology has led to the accelerated deployment of multiple-channel fiber-optic telecommunications systems. As channel count increases, the span between EDFAs decreases, resulting in the need to install more costly amplifiers and potential maintenance points.
The most straightforward solution to offsetting the impact of WDM is to increase the power of the amplifier itself. Doing so increases the range of the amplification, thereby reducing the number of amplifiers needed and the potential maintenance sites within a system. An effective method of pumping additional power into the EDFA is to multiplex up to six lasers into the amplification system, using couplers that employ a cascading, fused biconic taper technology. This approach, used by itself, can increase results in significantly higher EDFA power; used with other alternative technologies, the power can be increased even more. u
Joseph C. Chon is director of engineering at AFO (Fremont, CA).