What is Raman optical amplifier?

Raman optical amplifier is an important part of dense wavelength division multiplexing communication system. In many nonlinear optical media, the scattering of pump light with a shorter wavelength causes a small part of the incident power to be transferred to another beam whose frequency is shifted down. The amount of frequency shift down is determined by the vibration mode of the medium. This process is called the pulling Mann effect.

Here is the content list:
Who is Raman?
What is the Raman effect?
The application of Raman effect in life.

Who is Raman?
Raman (1888-1970), an Indian physicist. The study discovered the Raman effect in the light scattering region and won the Nobel Prize in Physics in 1930. He died in 1970 at the age of 82. He won the Nobel Prize in Physics in 1930 in recognition of his research and discovery of the law of light named after him.

On the afternoon of February 28, 1928, Raman made a very beautiful and decisive experiment using monochromatic light as the light source. Look at his scattered light, the area visible to the naked eye from the beam splitter is a blue and green light, two or more sharp bright lines. Each line has a corresponding variable incident scattered radiation. In general, the frequency becomes lower than the scattered rays, and occasionally the frequency of the scattered rays is higher than the incident rays, but the intensity is even weaker. The news of Raman’s discovery of abnormal scattering spread all over the world and caused a strong reaction. Many laboratories have repeated, confirmed, and developed his results.

Due to the discovery of the Raman effect, more and more scientists have joined the study of the Raman effect, and finally developed a Raman optical amplifier based on the principle of the Raman effect.

What is the Raman effect?
The phenomenon of light scattering has a special effect, similar to the Compton effect of X-ray scattering. The frequency of light changes after scattering. “Raman scattering” refers to a certain frequency of the laser-irradiated to the sample surface, molecules, and photons transfer energy substance, generating vibrational state (e.g., distortions atoms and swing, the swing and vibration) chemically) different from the manner and extent to occur. Changes and then scatters light of different frequencies. The frequency change is determined by the characteristics of the scattering material. Different types of atomic groups vibrate in unique ways, so they can generate scattered light with a specific frequency difference from the incident light. This spectrum is called the “fingerprint spectrum” and can be tracked. This principle determines the type of molecules that make up a substance. This was discovered by Raman in 1928 when he studied the scattering process of light. Raman spectroscopy is the result of the superposition of the vibrational energy or rotational energy of the molecule and the photon energy when the incident photon collides with the molecule. Therefore, Raman spectroscopy, as a supplement to infrared spectroscopy, is a powerful weapon for studying molecular structure.

The application of Raman effect in life
The discovery of the Raman effect has promoted the development of our lives to a certain extent and has a wide range of applications in real life. For example, the Raman optical amplifier based on the Raman effect is a typical representative.

Raman optical amplifier is an optical amplifier based on the Raman effect. The Raman active medium is usually an optical fiber, but it can also be a crystal, a waveguide structure in a photonic integrated circuit, a gas, or a liquid medium. The signal light that is in the same direction or opposite to the pump light is amplified, and its wavelength is usually tens of nanometers smaller than the pump light. For the quartz fiber, when the frequency of the pump light and the signal light is detuned at 1-15 THz, the peak gain is obtained. The Raman optical amplifier depends on the composition of the fiber core.

When used in communication systems, Raman optical amplifiers can be compared with erbium-doped fiber amplifiers. Compared with the latter, their features include:

Raman optical amplifiers can work in different wavelength regions, as long as there is a suitable pump light source. Raman optical amplifiers require high pump power (to improve the safety of the laser) and high pump brightness, and can also generate high output power. The noise figure of the Raman optical amplifier is very small. In other words, they transfer pump noise to signal light more directly than laser amplifiers. If the pump light is polarized, the Raman gain also depends on the polarization state. This effect is generally undesirable but can be suppressed by using two polarization-coupled pump diodes or pump depolarizers.

The Raman optical amplifier is pumped by continuous light in a diode laser. If pump pulses in the same direction are used, ultra-short pulses can also be effectively amplified. However, the group velocity mismatch can greatly limit the effective interaction length, especially when the pulse length is less than 1 ps.

The fiber in the Raman optical amplifier does not need to be doped with rare-earth ions. In theory, ordinary single-mode fiber meets the conditions, but in practical applications, Raman optical amplifiers are more suitable for transmission fibers (see distributed amplifiers). However, some special fibers can increase the Raman gain, because certain doping (such as germanium) can increase the Raman cross-section, or simply because the effective mode area is small. These fibers are suitable for Raman optical amplifiers, which are just a small section of fiber used in the amplification process.

At last
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Post time: 12-06-2021
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