Category Archives: Optical Amplifiers

Optical Amplifiers – a Way to Improve the Signal Quality

An optical amplifier is a fiber optic device that can amplify optical signals directly without the need to convert it to an electrical signal. Optical amplifiers can increase the strength of the optical signal and allow the signals transmit through the cable over long distances. The emergence of optical amplifiers brings great improvement to networking and communication system.

Why Do We Need Optical Amplifiers?

There are three reasons. One reason is that the optical to electrical and electrical to optical conversions requires high-speed electronics and costs too much. The other reason is that signals carried by a fiber will be attenuated because no fiber material is absolutely transparent. Then repeaters must be used in the optical fibers which maybe longer than 100 kilometers. And the last one is that the signal-to-noise ratio of detected signals is too low and bit error rate becomes too high. So the most efficient and best optical method is an optical amplifier. Optical amplifiers could avoid the need for E-O and O-E conversions. It’s beneficial to compensate for attenuation losses during signals transmitting over long distances.

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Types of Optical Amplifiers

There are three types of optical amplifiers: rare earth doped fiber amplifiers (the most popular one- Erbium Doped Fiber Amplifiers), semiconductor optical amplifiers, Raman amplifiers. But the question is which one is suitable for your specific transmission system. By the following comparison you may find the answer.

Erbium doped fiber amplifiers (EDFAs): they have high pump power utilization. Amplifiers can directly and simutaneously amplify a wide wavelength band (>80nm) in the 1550nm region with a relatively flat gain. And EDFAs have low noise, which is suitable for long haul applications. However, compared with other two kinds of amplifiers, EDFAs can’t be integrated with other semiconductor devices and can obtain small signal gain only in a specific frequency band.

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Semiconductor optical amplifiers: they are of small size and electrically pumped. Its high optical nonlinearity is good for all optical signal processing like optical switching and wavelength conversion. SOA provides the possibility for gain in different wavelength regions. The gain reacts rapidly to changes of pump or signal power and the changes of gain cause phase changes which can distort the signals. Compared with EDFA, SOA has higher noise, lower gain, moderate polarization dependence and high nonlinearity with fast transient time. However, it’s less expensive than EDFA and can be integrated with semiconductor lasers, modulators, etc.

Raman amplifiers: they gain exists in every fiber, which provides a cost-effective means of upgrading from the terminal ends. Furthermore, the gain is nonresonant so that it’s available over a wide transparent region of fiber. And the gain spectrum can be tailored by adjusting the pump wavelength. Compared with EDFA, Raman amplifiers have relatively poor pumping efficiency at lower signal powers and require a longer gain fiber.

The main purpose of this article is to give you suggestions to obtain a high transmission capacity over long distances. Optical amplifiers are quite important elements in wavelength division multiplexing (WDM) system. You may apply DWDM (dense wavelength division multiplexing) or CWDM (coarse wavelength division multiplexing). And these optical amplifiers could be used in both DWDM and CWDM transmission system. So you don’t need to take this as a problem for imrpoving your signal quality.

Variable Gain EDFA – Large Dynamic Gain Range & Better Noise Figure Performance

In order to support different span lengths between Optical Amplifiers in a transmission link, as well as to provide for aging of the link, it is desirable for an optical amplifier to support as large as possible dynamic gain range, i.e. to allow the gain to be dynamically configured over a wide range. For a single channel optical amplifier, this can be easily be achieved, and most single stage amplifiers support a large dynamic gain range. For WDM multi-channel amplifiers the situation is much more complicated due to the need to maintain a flat gain for all gain values.

Thus, most low-end WDM amplifiers are designed as fixed gain amplifier, i.e. they provide flat gain for only a given pre-designed gain value. To support different gain values, system designers may either use a number of fixed gain amplifiers with different pre-set gain values, or place a Variable Optical Attenuator (VOA) before the fixed gain amplifier. The former solution requires different part numbers for the various amplifiers, and therefore complicates operational issues such as inventory control and sparing, while the latter solution leads to a large deterioration in OSNR, and therefore is not suitable for links longer than 200 to 300 km. These drawbacks can be addressed with a Variable Gain EDFA.

Variable Gain EDFA is typically designed to have flat gain at the top of the required gain range, using an appropriate Gain Flatness Filter (GFF), while a VOA is used to attenuate all the channels uniformly in order to achieve a range of gain values. If the VOA is placed at the EDFA input, then a large deterioration in Noise Figure (NF) will occur. On the other hand, if the VOA is placed at the EDFA output then high pump power will be required, thus increasing the cost of the EDFA amplifier. Therefore it is necessary to place the VOA between two amplification sub-stages. The figure below shows a basic design of WDM Variable Gain EDFA, where the VOA is placed between two gain sub-stages in order to improve NF.

Variable Gain EDFA Design

The two gain stages are typically pumped by a single pump where the pump power is split between the stages, and the control loop controls the gain of the entire EDFA amplifier (both gain sub-stages and VOA).

By carefully designing the Erbium-Doped Fiber (EDF) length and designated pump power for each sub-stage, it is possible to achieve very good NF performance over a range of gain values, with only a moderate increase in total pump power compared to a fixed gain EDFA. The typical NF performance of a variable gain EDFA is shown in the figure below as compared to a comparable fixed gain EDFA with a VOA placed beforehand.

NF/Gain Comparison between Fixed and Variable Gain EDFA

As can be seen, the NF increases only slightly at the low gain range, representing a significant performance advantage compared to the alternative fixed gain EDFA with a VOA placed before. The variable gain EDFA provides much better NF performance over the entire gain range.

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