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Introduction to Optical Amplifier
Alan   utmel.com   2021-10-21 17:37:26

Topics covered in this post:

What is an optical amplifier?

Optical amplifier classification

Optical amplifier functions

The development direction of the optical amplifier


What is an optical amplifier?

The optical amplifier is to amplify the optical signal. Before that, the amplification of the transmission signal was to realize the photoelectric conversion and the electro-optical conversion, that is, the O/E/O conversion. With the optical amplifier, the optical signal can be amplified directly.  Since its commercialization in the 1990s, optical amplifiers have profoundly changed the status quo of the optical fiber communications industry. The successful development and industrialization of optical amplifiers is a very important achievement in optical fiber communication technology, which has greatly promoted the development of optical multiplexing technology, optical soliton communication, and all-optical networks.

Optical amplifiers have a wide range of applications in optical fiber communications, consumer electronics, power systems, commercial advertising, medicine, and life sciences. In recent years, with the rapid development of information and communication technology, the research and development of optical fiber amplifiers have further expanded the gain bandwidth and pushed the optical fiber communication system to the direction of high speed, large capacity, and long-distance. Due to the unique performance of optical fiber amplifiers, it has a wide range of applications in DWDM transmission systems, optical fiber CATV, and optical fiber access networks.

optical fiber amplifier.jpg

optical fiber amplifier

In the optical fiber communication system, the optical amplifier can be used as the power booster amplifier of the transmitter to increase the transmission power. It can also be used as the preamplifier of the receiver to improve the receiving sensitivity, extend the transmission distance and realize all-optical communication. Optical amplifiers can be used not only in long-distance trunk systems but also in optical fiber distribution networks, especially in WDM systems, capable of simultaneous amplification of multiple channels.

Optical amplifier classification

At present, there are three main types of optical amplifier technologies: rare earth-doped optical amplifiers (such as erbium-doped fiber amplifiers EDFA, praseodymium-doped fiber amplifiers PDFA, niobium-doped fiber amplifiers NDFA, etc.); semiconductor optical amplifiers SOA; non-linear optical amplifiers (such as Raman amplifiers FRA, Brillouin amplifier, etc.).

(1) Optical fiber amplifier

It is to dope rare-earth ions (such as erbium, praseodymium, thulium, etc.) in the optical fiber as laser-active materials. The gain bandwidth of each dopant is different. The gain band of the erbium-doped fiber amplifier is wide, covering the S, C, and L bands. The gain band of the thulium-doped fiber amplifier is S-band; the gain band of praseodymium-doped fiber amplifier is around 1310nm.

(2) Raman optical amplifier

It is an optical amplifier made by using the Raman scattering effect, that is after a high-power laser is injected into the fiber, the non-linear effect of Raman scattering will occur. In the process of continuous scattering, the energy is transferred to the signal light, so that the signal light is amplified. Therefore, it is not difficult to understand that Raman amplification is a distributed amplification process. Its working bandwidth can be said to be very wide, almost unlimited. This optical amplifier has begun to be commercialized, but it is quite expensive.

(3) Semiconductor optical amplifier

Its working principle is similar to that of a semiconductor laser. Its working bandwidth is very wide. However, the gain range is slightly smaller, making it more difficult to manufacture. Although this optical amplifier has been practical, the output is small. In a WDM optical transmission system that uses an optical amplifier in its transmission path, a monitoring signal channel is used to monitor and control the operation of the amplifier and spectrally separate from data transmission can be multiplexed with data.

Optical amplifier functions

An optical fiber amplifier is an optical device that can amplify the power of optical signals. It is used in optical fiber communication lines to achieve signal amplification. According to its position and function in the optical fiber line, it is generally divided into three types: relay amplification, pre-amplification, and power amplification. Compared with the traditional semiconductor laser amplifier (SOA), OFA does not need to go through complicated processes such as photoelectric conversion, electro-optical conversion, and signal regeneration. It can directly perform all-optical amplification of the signal. It has good "transparency" and is especially suitable for long distances.

Optical amplifier structure.jpg

Optical amplifier structure

To expand the capacity of communication lines while minimizing the cost, optical fiber amplification is one of the preferred options. When a wavelength division multiplexed optical signal is transmitted in an optical fiber, there is inevitably a certain loss and dispersion. Loss leads to a reduction in the energy of the optical signal, and dispersion causes the optical pulse to expand. Therefore, a repeater needs to be set every certain distance to continue to transmit after amplifying and regenerating the signal.

The conventional method to solve this problem is to use an optical/electrical/optical repeater. Its working principle is to first convert the received weak optical signal into an electrical signal through a PIN or APD, and then amplify, equalize, regenerate in order to obtain a good performance electrical signal, and finally through the semiconductor laser (LtD) to complete the electrical/optical conversion, and then send it to the lower section of the optical fiber.

Schematic diagram of optical amplifier.jpg

Schematic diagram of the optical amplifier

This kind of optical/electrical/optical conversion and the processing method can no longer meet the requirements of modern telecommunication transmission. Since wavelength division multiplexing is a multi-wavelength transmission on a single fiber core, to perform electrical regenerative relaying, each wavelength must be performed one by one. This complicates the electrical relay equipment, and the transmission distance is limited by attenuation, and the cost is higher. The optical fiber amplifier can amplify the fuse time of all wavelengths in the band, that is, use the same one. One amplifier provides gain to multiple channels, and the gain is not affected by signal polarization. Crosstalk will not occur in high-speed, multi-channel transmission systems, and pulse distortion will not occur in high-speed transmission systems. Therefore, the fiber amplifier is a key component of the wavelength division multiplexing system. So far, almost all WDM systems, whether they are test systems or commercial systems, use optical fiber amplifiers.

F1TH (fiber to the home), FTTO (fiber to the office), FTTB (fiber to the building), FTTC (fiber to the roadside), and other methods have appeared in the optical fiber access network. Among them, FTTH is the most widely used, and the difficulty is an optical fiber. There are too many terminal branches. For passive networks, after several branches, the optical power received by the user is very low (the optical power drops by 3dB for every doubling of the branch), making the terminal unable to work. After adopting the optical fiber amplifier, the emitted power increases. After multiple branches, the user end can still receive normally, so the realization of FTTH will become possible. Therefore, the emergence and development of optical fiber amplifiers have overcome the biggest obstacle of high-speed transmission lease distance transmission-the limitation of optical power budget and is an important milestone in the history of optical communication.

The development direction of the optical amplifier

Due to the development of ultra-high-speed, large-capacity, and long-distance optical fiber communication systems, new requirements have been put forward for optical fiber amplifiers, which are key components in the field of optical fiber communication, in terms of power, bandwidth, and gain flatness. Therefore, in the future optical fiber communication network Among them, the development direction of optical fiber amplifiers mainly includes the following aspects:

(1) EDFA develops from C-Band to L-Band;

(2) Broad-spectrum, high-power fiber Raman amplifier;

(3) Use the locally flat EDFA in series with the fiber Raman amplifier to obtain an ultra-wideband flat gain amplifier;

(4) Develop strain-compensated non-polarization, monolithic integrated, optical laterally connected semiconductor optical amplifier optical switches;

(5) Research and develop fiber amplifiers with dynamic gain flattening technology;

(6) Miniaturized and integrated optical fiber amplifier.

With the continuous breakthrough of new materials and new technologies, it will not be a dream for the optical fiber amplifier to obtain an ultra-wideband bandwidth of 300nm in the wavelength range of 1292~1660nm. The Tbit/s DWDM optical network transmission system will surely be realized.

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