Raman amplification enhances system operation
Optical amplification of signals in fiber transmission is a critical part of high-capacity, dense wavelength-division-multiplexed (DWDM) long-haul transmission systems. Without amplification, fundamental Rayleigh background losses in the fiber would require expensive full electrical regeneration of all optical signals after distances on the order of a hundred or hundreds of kilometers, depending on the overall configuration. Optical amplifiers boost the power of all DWDM signals at periodic distances, typically of the order of 100 km, and thereby extend regeneration distances by at least an order of magnitude.
Optical amplification based on Raman gain in fibers was one of the earliest optical amplification methods investigated.1 It is based on a stimulated Raman scattering in the core of the transmission fiber and therefore does not require the insertion of a special gain medium. In the Raman amplification process, a high-power pump source is injected into the fiber, providing gain over a relatively broad range of wavelengths determined by the molecular-vibration frequencies of the fused silica fiber core medium. For instance, a 1450-nm pump source provides gain at the common 1540-1560-nm transmission band near the minimum fiber transmission loss wavelength (see Fig. 1).
The Raman gain depends strongly on the optical power density of the pump light in the core because Raman is a nonlinear effect. Experiments show that the useful Raman gain in typical transmission fibers requires pump power levels greater than 0.5 W. In contrast, Er-doped fiber amplifiers (EDFAs) operating near the 1550-nm transmission window provide similar gains to Raman amplification at less than 100-mW pump power. As a result of their more-efficient optical gain, EDFAs quickly captured the optical amplifier market.
IMPROVING THE GAIN
Recent developments have prompted the rebirth of Raman amplification in optical fiber communications. First, pump power levels at 0.5 W or higher are now commercially available. In addition, Raman amplification has several features that augment Er amplification and improve overall system performance.
As opposed to Er amplification, Raman amplification is a distributed amplification process that occurs over a substantial part of the overall transmission fiber. For example, a 1450-nm Raman pump injected into a typical 100-km fiber span provides distributed gain over approximately 40 km, as determined by the pump attenuation in the fiber.
This distributed gain provides two main benefits. One is a method of boosting the power along the length of the span without physically inserting an amplifier mid-span. Physical mid-span amplifiers carry the associated cost of constructing and maintaining additional physical amplifier locations. Raman amplification can be viewed as an optical amplifier some tens of kilometers back into the fiber that is remotely pumped by the Raman pump source.
The second benefit of distributed pumping is that the Raman process results in much lower local power levels as compared to EDFAs. These lower power levels reduce fiber nonlinearities such as four-wave mixing between channels, which are highly sensitive to signal power levels. Fiber nonlinearities currently restrict the power levels to which signals may be boosted by EDFAs. Overall, Raman amplification results in an improved system signal-to-noise ratio (SNR), permitting higher-speed 40Gbit/s transmission and/or longer repeater spans-thus reducing system cost.
Another benefit of Raman amplification may be to provide gain at wavelength transmission windows not currently covered by Er amplification. Raman amplification can provide gain from 1300 to 1500 nm or wider.
RAMAN PUMP SOURCES
A critical aspect of implementing distributed Raman amplification in optical networks is the availability of high-power pump laser sources. Currently three different technologies are being considered for this application: the Raman fiber laser, single-mode pump laser diodes combined by an optical multiplexer, and flared waveguide laser diodes (see Fig. 2).
In a Raman fiber laser, a cladding-pumped fiber laser is pumped by several multimode diode lasers. The fiber laser output at a wavelength of ~1000 nm pumps a cascaded Raman resonator that shifts the fiber laser wavelength to the desired wavelength in the 1450-nm region. The cascaded Raman resonator is constructed with low-loss fiber Bragg gratings, so conversion efficiency is high. Up to 1.5 W are currently available for commercial applications. The use of multiple discrete pump diodes allows for reliability improvement through redundancy.
One main advantage of this architecture is that it can be scaled to very high output power with the addition of more pump lasers. In addition, the Raman fiber laser can be designed to operate at any wavelength currently of interest for network applications, making it attractive for amplifying signals outside of the standard Er band, including in the 1.3-µm region.
As a second option, multiple single-mode laser diodes can be combined with low-loss DWDM couplers. The operating wavelength of the individual diodes are stabilized by fiber Bragg gratings, ensuring optimal matching to the multiplexers and stable operation as diode temperature varies.
Devices that offer up to four combined wavelengths are now commercially available and offer more than 350 mW in a single-mode fiber. This multiwavelength approach is attractive because it offers a wider Raman gain bandwidth than a single wavelength, and it also allows for dynamic control of the gain shape by individually adjusting the power of each diode.
The third technology available for distributed Raman amplification is the flared waveguide diode laser. Light from the single-mode region in the back of the chip diffracts freely in the flared region, reducing the optical intensity at the facet and allowing higher output powers than conventional single-stripe technology. The high-power, diffraction-limited output can be coupled into a single-mode fiber with high efficiency. As with standard single-mode diode lasers, this device can be stabilized with fiber Bragg gratings if wavelength control is desired. Development efforts are ongoing but are expected to produce telecommunications-grade devices.
The ability of Raman amplification to improve the performance of an optical network link was measured in a testbed simulating a typical transmission link. The transmission source is a single-frequency distributed-feedback laser diode (DFB) operating at 1555 nm, externally modulated by a LiNbO3 Mach-Zehnder modulator at 2.488 Gbit/s (OC-48).
The modulated signal power is injected into the link and the launched power controlled by a variable attenuator. After attenuation, the signal is transmitted along an 86-km span of standard single-mode fiber (SMF28) and detected by a receiver comprising an optical preamplifier and avalanche photodiode.
The Raman pump power is injected into the link with a fused fiber WDM coupler at the output end. The pump light propagates in the opposite direction to the signal channels. This counter-propagating geometry reduces the pump-mediated crosstalk between channels because of the averaging of the noise over the long interaction length in the fiber. The pump laser is a fiber Raman laser with up to 1.5 W of output power at 1455 nm.
Without Raman pumping, a bit-error rate (BER) of 10-9 is measured at -39 dBm actual input power to the receiver. With 0.5-W Raman pump power injected into the transmission link, the input attenuation is increased by approximately 6 dB while maintaining a 10-9 BER, resulting in an effective received power of -45 dBm (see Fig. 3). A 7.5-dB system margin improvement is measured at 1-W pump power.
The relatively high power levels for Raman amplification may raise concerns about power handling of components. None of these experiments showed significant component degradation, including degradation of the WDM pump coupler.
The system margin improvement resulting from Raman amplification can be used to lower the signal launch power, to increase the span length, to increase the number of WDM channels (at a fixed total launch power), to increase the bit rate of the link, or some combination of these. As new fiber infrastructure and higher data rate systems are deployed, another motivation for using distributed Raman gain will be to increase the distance between repeaters.
The transmission experiments described above illustrate the benefits of Raman amplification in a typical system. Additional system experiments have shown that the system margin improvement benefits are a function of Raman pump power, transmission fiber type, and receiver sensitivity.2-10
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Robert Waarts is research section manager, Vince Dominic is staff scientist, David Giltner is staff engineer, and David Mehuys is engineering manager at SDL Inc., 80 Rose Orchard Way, San Jose, CA 95134. Robert Waarts can be reached at 408-943-4247 or email@example.com.
FIGURE 1. The Raman gain spectra
and gain slope for single-mode fiber SMF28 depends on pumping wavelength, shown here at 1450 nm.
FIGURE 2. Three types of high-power pump sources for Raman amplification-each with advantages and limitations-are the Raman fiber laser (top), single-mode pump laser diodes combined by an optical multiplexer (middle), and flared waveguide laser diodes (bottom).
FIGURE 3. Raman amplification to improve network performance was measured in a testbed comprising a single-frequency externally modulated source, a transmission fiber, receiver, and Raman pump fiber laser (SDL RL30) with up to 1.5 W of output power at 1455 nm. BER was measured as a function of effective received power, with and without 0.5-W Raman pump light. (Effective received power is the power that would be received in the absence of Raman gain.)