Raman Optical Amplifier

RAMAN OPTICAL AMPLIFIERS

Raman optical amplifiers differ in principle from EDFAs or conventional lasers in that they utilize stimulated Raman scattering (SRS) to create optical gain. Initially, SRS was considered too detrimental to high channel count DWDM systems. Figure 1 shows the typical transmit spectrum of a six channel DWDM system in the 1550 nm window. Notice that all six wavelengths have approximately the same amplitude.

Figure 1 – DWDM Transmit Spectrum with Six Wavelengths

By applying SRS the wavelengths, it is obvious that the noise background has increased, making the amplitudes of the six wavelengths different. The lower wavelengths have a smaller amplitude than the upper wavelengths. The SRS effectively robbed energy from the lower wavelength and fed that energy to the upper wavelength.

Figure 2 – Received Spectrum After SRS is on a Long Fiber

A Raman optical amplifier is little more that a high-power pump laser, and a WDM or directional coupler. The optical amplification occurs in the transmission fiber itself, distributed along the transmission path. Optical signals are amplified up to 10 dB in the network optical fiber. The Raman optical amplifiers have a wide gain bandwidth (up to 10 nm). They can use any installed transmission optical fiber. Consequently, they reduce the effective span loss to improve noise performance by boosting the optical signal in transit. They can be combined with EDFAs to expand optical gain flattened bandwidth. Figure 3 shows the topology of a typical Raman optical amplifier. The pump laser and circulator comprise the two key elements of the Raman optical amplifier. The pump laser, in this case, has a wavelength of 1535 nm. The circulator provides a convenient means of injecting light backwards in to the transmission path with minimal optical loss.

Figure 3 – Typical Raman Amplifier Configuration

Figure 4 illustrates the optical spectrum of a forward-pumped Raman optical amplifier. The pump laser is injected at the transmit end rather than the receive end as shown in Figure 3. The pump laser has a wavelength of 1535 nm; the amplitude is much larger than the data signals.

Figure 4 – Example of Raman Amplifier — Transmitted Spectrum

As before, applying SRS makes the amplitude of the six data signals much stronger. The energy from the 1535 nm pump laser is redistributed to the six data signals.

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Sumber : Fiber Optic Info