Fiber Raman Amplifier, What is Fiber Raman Amplifier

Fiber Raman Amplifier, What is Fiber Raman Amplifier

With the rapid growth of communication business demand, the requirements for the capacity and non-relay transmission distance of optical fiber transmission systems are getting higher and higher. The rate and bandwidth of dense wavelength division multiplexing (DWDM) communication systems are constantly improving. DWDM systems based on 10Gbit/s or even higher rates will inevitably become the mainstream optical transmission system. Erbium-doped fiber amplifiers (EDFA) can no longer fully meet the requirements of the development of optical communication systems due to their limitations such as gain flatness and noise. Compared with erbium-doped fiber amplifiers, fiber Raman amplifiers have the advantages of larger gain bandwidth, flexible gain spectrum, good temperature stability and low amplifier spontaneous radiation noise. Fiber Raman amplifiers are the only devices that can amplify in the spectrum of 1292~1660nm. In addition, the Raman scattering effect exists on all types of optical fibers and has good compatibility with various optical fiber systems, including various fiber links that have been laid and newly built. Fiber Raman amplifiers, new large effective area transmission fibers, high spectral efficiency modulation codes and forward error correction technology are known as the four key technologies for modern large-capacity and long-distance optical fiber transmission.

1. Working principle and performance of fiber Raman amplifier

(1) Stimulated Raman Scattering (SRS)

Stimulated Raman scattering is produced by the coupling of the photoelectric field of a strong laser with the electron excitation in atoms, the vibration in molecules, or the lattice in crystals. It has strong stimulated characteristics, that is, it has similar characteristics to the stimulated light emission in lasers: strong directionality and high scattering intensity.

(2) Working principle of fiber Raman amplifier

The working principle of the fiber Raman amplifier is based on the stimulated Raman scattering effect in quartz optical fiber. In form, the weak signal within the Raman gain bandwidth of the pump light is transmitted in the optical fiber at the same time as the strong pump light wave, so that the weak signal light is amplified. The schematic diagram of its working principle is as follows:

In RFA, an incident pump photon transfers part of its energy through nonlinear scattering of the optical fiber, generating low-frequency Stokes photons, while the remaining energy is absorbed by the medium in the form of molecular vibration (optical phonons), completing the transition between vibrational states. The Stokes frequency shift Vr=Vp-Vs is determined by the molecular vibration energy level, and its value determines the frequency range of SRS, where Vp is the frequency of the pump light and Vs is the frequency of the signal light. For amorphous quartz optical fiber, its molecular vibration energy levels are fused together to form an energy band, so that the signal light can be amplified through SRS within a wider frequency difference Vp-Vs range (40THz).

Raman fiber amplifiers are significantly different from erbium-doped fiber amplifiers:

(1) Theoretically, as long as there is a suitable Raman pump source, the signal of any wavelength in the fiber window can be amplified, so it has a very wide gain spectrum;

(2) The transmission fiber itself can be used as the gain medium. This feature enables the fiber Raman amplifier to form a distributed amplification of the optical signal, realizing long-distance relay-free transmission and remote pumping. It is especially suitable for occasions such as submarine optical cable communications where it is inconvenient to establish relay stations.

(3) The signal gain flatness can be dynamically adjusted by adjusting the power of each pump;

(4) It has a low equivalent noise index. This feature can greatly reduce the system noise index when used in combination with a conventional erbium-doped fiber amplifier.

(3) Performance analysis of fiber Raman amplifiers

The performance of fiber Raman amplifier determines that it will play a key role in future high-speed, large-capacity fiber communication systems. Table 1 compares the main features and performance indicators of fiber Raman amplifiers with semiconductor optical amplifiers (SOA) and erbium-doped fiber amplifiers (EDFA):

2. Classification of Fiber Raman Amplifiers

(1) Distributed Raman Fiber Amplifier (LRA)

Distributed Raman amplifiers are based on the stimulated Raman scattering (SRS) effect of optical fibers. They generally use reverse pumping, which is implemented as follows: a high-power continuously operating laser is injected into the transmission optical fiber from the output end of the optical fiber span. The transmission direction of the pump light is opposite to that of the signal light. The wavelength of the pump laser is about 100nm shorter than the signal light. The high-power optical field pumps the component substances in the optical fiber to produce virtual excited states; electrons transition from these virtual excited states to the ground state, thereby achieving the gain of the optical signal. The transmission fiber of the distributed Raman amplifier is itself a gain medium, and the signal is amplified while being transmitted in the optical fiber, making the equivalent noise index of the Raman amplifier negative. Low-noise-factor distributed Raman amplifiers can effectively overcome the influence of nonlinear effects such as four-wave mixing and improve the optical signal-to-noise ratio (OSNR) of the system.

(2) Discrete Raman Fiber Amplifier (DRA)

The amplification medium used in discrete Raman amplifiers is usually dispersion-compensating fiber or highly nonlinear fiber, such as DCF fiber or tellurium-based fiber. At present, the Raman gain coefficient of DCF fiber is about 10 times higher than that of SMF, and it can also form a dispersion compensation module (DCM) as a Raman gain medium. Tellurium-based fiber has a Raman gain coefficient 16 times higher than that of quartz fiber, with a peak value of 55W/km.

3. Application and progress of fiber Raman amplifiers

At present, the development of distributed fiber Raman amplifiers is very fast. Most of the optical amplifiers used in many long-distance, ultra-large capacity dense wavelength division multiplexing optical communication systems (DWDM) abroad are distributed fiber Raman amplifiers, which can not only make full use of fiber resources and reduce costs, but also reduce the light density in the gain medium, so as to reduce the degradation of system performance caused by four-wave mixing and crosstalk between channels due to nonlinear effects. However, the gain of Raman amplifiers is low (no more than 16dB when used in actual lines), and although EDFA is not as good as Raman amplifiers in terms of noise index, its small signal gain can exceed 30dB, so a hybrid amplifier combining Raman amplifiers with EDFA is an ideal application form.

The 980nm pumped EDFA amplifies the C band, and the 1497nm Raman pump source is responsible for the L band amplification. Due to the superposition of the gain spectrum, there are three gain peaks near 1535 (generated by EDFA), 1560 (generated by superposition) and 1600nm (generated by Raman amplification), with a size of 1.5~2dB, and two valleys of about 0dB appear near 1540 and 1560. After using GFF, all signal gains are controlled at about 0dB, thus achieving 80nm bandwidth, 256×10Gbit/s×11000km transmission.

4. Current problems

In the process of in-depth research on FRA, the selection and configuration of the pump source and the control of noise are all urgent issues to be solved. Among them, the dispersion characteristics of the optical fiber will cause interference between the front and rear codes in the transmission, that is, inter-code interference, which limits the transmission code rate and transmission distance. In view of the large dispersion problem of the G652 single-mode optical fiber currently laid on the transmission line, DCF optical fiber can be used as the dispersion compensation and dispersion slope compensation part of the G652 optical fiber to form a compensation FRA.

In addition to complex and difficult engineering design, in order to obtain the ideal gain effect, distributed Raman amplifiers often use amplifiers exceeding 1W (>30dBm). Therefore, the optical transmission system has high requirements for the quality of the fiber connectors and fiber joints near the Raman amplifier to minimize the side effects of reflection and loss on the Raman gain mechanism. At the same time, in order to prevent the high-energy laser from causing harm to engineering maintenance personnel, automatic optical power shutdown (ALS) and special personnel training are indispensable.