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fiber optics1
ISP Network

Optical Amplifiers Essential in Long-Distance Fiber Optic Links


Optical Amplifiers Essential in Long-Distance Fiber Optic Links

FS Official 2015-01-04

In today’s metro and long-haul fiber optic networks, optical amplifiers together with other compensation modules have been playing a significant role in ensuring an uninterrupted communications link. Whether in DWDM systems or CWDM systems, optical amplifiers like EDFAs and Raman amplifiers are necessary equipment.

In order to transmit the optical signals over long distances, it is necessary to compensate the attenuation losses within the fiber. Optical amplifier is one of the devices designed for this purpose, providing the data capacity for now and future in communication networks. Unlike fiber optic repeaters which need to firstly convert the light beam to an electronic signal to amplify it, optical amplifiers directly amplify an optical signal.

Basic Properties of Optical Amplifiers

Gain is the amount by which the input optical signal is amplified. The gain value is typically measured in dB, such as 5dB or 10dB. Some amplifiers are designed to operate at a single pre-set gain, while others support a range of operating gain values. Besides gain, an optical amplifier is also characterized by the range of supported input and output optical powers (dBm or mWatt). In particular, a key specification of the amplifier is the maximum output power which can be supported, also referred to as saturated output power. This parameter is often critical in determining the amplifiers’ cost. Noise is another property. All optical amplifiers introduce noise during amplification, so the output signal is always noisier than the input signal. The noise performance of it is characterized by its noise figure (NF). Generally, the NF should be kept as low as possible.

How Many Approaches Can Be Used to Amplify Optical Signals?

There are mainly three types of optical amplifiers that are used in long-distance optical transmission, though the technologies for amplification of optical signals are not limited to them. The most common types are erbium doped fiber amplifier (EDFA), semiconductor optical amplifier (SOA) and fiber Raman amplifier (FRA). There is also hybrid optical amplifier (HOA) which combines more than one amplification methods.

EDFA-Typical Example of Doped Fiber Amplifiers

Erbium doped fiber amplifier (EDFA) is the most common example of doped fiber amplifiers that use a length of doped optical fiber as gain medium to amplify an optical signal. The principle of EDFA is by boosting an optical signal via a waveguide. The input signal and a relatively higher-powered light beam (the pump laser) are mixed into the fiber doped with erbium ions. Since the erbium ions are excited by the pump laser to their high-energy states, when the input signal meets the excited erbium ions, the erbium ions give up some of their energy to the signal to be amplified and return to their lower-energy state. EDFA is the most deployed fiber amplifier because its amplification window, which is the usable wavelengths for gain, coincide with the C-band (1525-1565 nm) and L-band (1570-1610 nm) wavelengths, which is known for dense wavelength division multiplexing (DWDM) systems.

EDFA schematic setup Figure 1: EDFA schematic setup.

Intensify Signal by Raman Amplification

Fiber Raman amplifiers (FRAs) booster optical signals by Raman amplification. The gain medium is a length of fiber, which can be a part of the transmission fiber or a dedicated, shorter-length fiber of small core. An FRA operates at all-band wavelengths (1280-1650 nm). The signal wavelength and the pump wavelength are multiplexed into the fiber to achieve gain. The pump power required for Raman amplification is higher than that required by EDFA. Raman gain exists in every fiber. Therefore, it allows for cost-effective upgrading from the terminal ends. For links longer than 120-200 km, usually Raman amplifiers are used to complement the EDFA for transmission expansion. Raman amplifiers have been suggested often for applications in CWDM networks, where the available wavelengths in C- and L-bands are limited.

Raman amplifier schematic setup Figure 2: Raman amplifier schematic setup.

Semiconductor Optical Amplifiers

Semiconductor optical amplifiers (SOAs) use semiconductor to provide gain medium. It is very similar to a laser diode, but the end mirrors are replaced by anti-reflection design, such as coatings and titled waveguide. The SOA is electrically pumped and the gain value is up to 30dB. The main advantage of SOA is that all four types of nonlinear operations (cross gain modulation, cross phase modulation, wavelength conversion and four wave mixing) can be conducted, and it can be run with a low power laser. However, the performance is still not comparable with EDFA since the SOA has defects like higher noise, lower gain and moderate polarization dependence.

SOA schematic setup Figure 3: SOA schematic setup.

Comparison of Optical Amplifiers

The features of different types of optical amplifiers are generally different, including the operating wavelengths, the gain bandwidth, the pump power and pump wavelengths, etc. The table below is a brief comparison of three common optical amplifiers.

Table: Comparison of Optical Amplifiers

Feature EDFA Raman SOA
Wavelength of operation 1525 to 1565 nm, 1570 to 1610 nm Any, depend on pump wavelength and power Any (but limited to less than 50nm bandwidth)
Gain value Up to 40 dB 10-15 dB Up to 30 dB
Gain bandwidth 10 to 40 nm 20 to 50 nm 20 to 50 nm
Noise Low Very low Low
Pumping directions Unidirectional and bidirectional Unidirectional and bidirectional Unidirectional and bidirectional
Polarization sensitivity 0 dB 0 dB Less than few mW
Optical pump wavelength 980, 1400 to 1500 nm Stoke shift below signal /
Optical pump power 20 to 50 mW 100 to 500 mW, 1W /

Optical Amplifiers and Long Distance Applications

For applications in long-distance fiber optic links, it is necessary to choose suitable devices to ensure the continuity of the communication. Optical amplifiers are one of the necessities in such long-distance links.

The Maximum Distance Without Amplification

Ideally an optical link can go as far as 80-100 km without using any optical amplifiers or repeaters. But in practical, due to the wavelength used, the bit rate, the fiber quality, the splices and the in-between connections, optical amplifiers are often deployed in distances shorter than that. For example, a system of bit rate around 10 Gb/s could run up to 100 km without amplification or compensation. However, if you wish to transmit 40Gb/s with a non-compensated and unamplified link, then the maximum distance will be limited to about 20 km.

Which Optical Amplifier to Choose?

During the design stage of a long-distance fiber optic link, the link budget will be calculated. At that time, the transceiver input power and the receiver sensitivity will be taken into consideration, thus the gain needed (pre-set or variable) and the supported input and output optical powers of the amplifiers are added in the calculation. That is to say, the basic properties of an optical amplifier are factors that should be considered in selection. Since a fiber link can be single channel system or WDM system, there are single channel amplifiers and WDM multi-channel amplifiers.

And for long-haul lines, more than one amplifier and compensation device might be used simultaneously. For example, when the dispersion compensating modules (DCMs) are used to correct signal distortion due to Chromatic Dispersion (CD) of the transmission fiber, or when the variable optical attenuators (VOAs) are used to achieve a range of gain values, or when the WDM transponder (OEO) is used for mode conversion such as multimode to single-mode fiber, or dual fiber to single fiber conversion and wavelength conversion, amplification before and after these processes could be required to keep the good performance. Also optical line protection (OLP) modules are required for monitoring the optical power.

How to Manage These Devices Efficiently?

If so many devices are deployed at the same time in long-distance applications, how to manage them efficiently? Have you thought of holding these devices in a chassis as a whole? And we have made it real, in a neat and flexible way. The FMT series include both 1U size optical amplifiers and half-U size OEO, EDFA, OLP, DCM and VOA plug-in cards. The plug-in card design enables the centralized management of all devices and flexible replacement, also minimizes the utilized space. A 4U plug-in type chassis can hold up to 16 devices of different types.

FMT 1U chassis with OEO, EDFA, DCM and OLP plug-in cards Figure 4: FMT 1U chassis with OEO, EDFA, DCM and OLP plug-in cards.

Summary

For the sake of better performance in long-distance links, amplification and compensation modules are essential in these applications. Modern technologies have achieved various types of optical amplifiers and other devices which can be used for very complicated systems. And the ease of managing these devices is also required at the same time. When choosing appropriate equipment for your long link system, it is important to consider how to maintain them and upgrade them in the future as well.

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OTN Solution Team
Senior Telecommunication Engineers

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