Optical Fiber Amplifiers

Transcription

1 Optical Fiber Amplifiers Yousif Ahmed Omer 1 and Dr. Hala Eldaw Idris 2 1,2 Department of communication Faculty of Engineering, AL-Neelain University, Khartoum, Sudan Publishing Date: June 15, 2016 Abstract In fiber optic systems to transmit signals over long distances, it is necessary to compensable for the loss of attenuation within the fiber. We need to amplify these indicators to be sent long distances by amplifiers. Semiconductor devices can be Convert optical signals into electric powered signals, amplified and convert the signal back in an optical signals. Nevertheless, this procedure has several drawbacks such as noise produced after each conversion in the analog signals. Once we use optical amplifiers in amplification processes, many of these disadvantages will fade away. This paper reviews optical fiber amplifiers such as Erbium doped fiber amplifiers EDFAs, various kinds of Raman amplifiers RAs, Thulium doped fiber amplifiers, Tellurite-based fiber Raman amplifiers, semiconductor optical amplifiers and fiber optic parametric amplifiers, principle of procedure and construction. As these amplifiers are being used for optical fiber communication projects so we shall go through their main characteristics which are amplifier gain and span length, wavelength bandwidth and noise. Keywords: Optical Fiber Amplifiers, Semiconductor Optical Amplifiers, EDFA. 1. Introduction Fiber optic technology is now favorite techniques for use in marketing communications systems to distinguish those several advantages, including: large bandwidth [nearly 50 terabits per second (Tb/s)], low power requirement, low signal attenuation (as low as 0.2 db/km), low signal distortion, low material usage, small space requirement, and low cost. As the optical transmission a long along a standard single mode fiber SSMF, it gets attenuated along the fiber and if the data speed is high enough (> 10 Gb/s), it gets altered due to chromatic and polarization dispersions. To countertop attenuation optical fiber amplifiers OFA's are used. Introducing OFA's into the system causes additional problems such as amplified impulsive emission ASE noise which accumulates as the quantity of OFA's that the signal goes through increases. The bandwidth of optical fibers is absolutely great if the S-band (short nm), C- band (central nm), and L-band (long nm) are utilized proficiently. So optical fiber amplifiers must be designed to amplify the signal along the fiber, the greater the gain, the more duration distance between amplifiers as provided that the signal is not distorted due to high optical power. To employ this great bandwidth, compacted wavelength division multiplexing DWDM is utilized, but each type of optical fiber amplifier has different bandwidth. Due to spectral dispersion in optical fibers, dispersion compensating fibers DCF's are are generally used to counter this impact even if one channel (wavelength) is used. For rates of speed of more than 10 GB/s such as 40, 80,160 and 320 GB/s more elaborate type of spectral dispersion reimbursement is used. There are elements that need to be taken into concern such as polarization mode dispersion PMD, inter channel funnel due to controlled Raman scattering SRS between signals, pattern dependent and independent and inter channel cross-phase modulation XPM. These kinds of problems are are popular among all type of OFA s. Type of amplifiers is determined by type application used such as long distance under the sea or terrestrial, short distance with a lot of adddrop locations such as in metro projects. 2. Optical Amplifiers An optical amplifier is a device which amplifies the optical signal directly without ever 28

2 changing it to electricity. The light itself is amplified. An optical amplifiers used in wavelength division multiplexing (WDM), each wavelength would need to be separated before being amplified electronically, and then recombined before being retransmitted. Thus, to eliminate the need for optical multiplexers and DE multiplexer in amplifiers, optical amplifiers should enhance the power of optical signals without converting first into electrical signals [3]. Figure 1: A Simple WDM Optical Network An optical amplifier is characterized by: 1. Gain: the ratio of output power to input power (in db). 2. Gain efficiency: gain as a function of input power (db/mw). 3. Gain bandwidth: range of wavelengths over which the amplifier is effective. 4. Gain saturation: maximum output power, beyond which no amplification is reached. 5. Noise: undesired signal due to physical processing in amplifier. Optical amplifier can be divided into two classes: Semiconductor optical amplifiers (Traveling Wave Amplifier (TWA) and Fabry-Perot Amplifier (FPA) and Optical fiber amplifiers (Erbium Doped Fiber Amplifiers and Raman Amplifiers) 2.1 Semiconductor optical amplifiers (SOAs) Semiconductor optical amplifiers are fundamentally laser diodes, with or without end mirrors which may have that contain fiber attached to both ends. They amplify any optical signal that comes from either fiber and transmit an amplified version of the signal out from the second fiber. SOAs are generally constructed in a tiny package and they work for 1310 nm and 1550 nm systems. In addition, they transmit bidirectional, making the reduced scale the device an advantage over other optical amplifiers [4]. However, the downsides of SOAs include high coupling loss and a higher noise figure. 29

3 Figure 2: Semiconductor Optical Amplifier 2.2 Optical Fiber Amplifier (EDFA) A great EDFA is an optic fiber of which the core is doped with the rare-earth element Erbium. By exciting the Erbium ions to higher energy levels, we can perform hyperbole of signals at wavelengths interesting for optical communication, i. e. around 1550 nm. The energy levels are not very sharpened, which causes a relatively large gain bandwidth [5]. The active dietary fiber is pumped with light from two laser diodes (bidirectional pumping), although unidirectional pumping in the forwards or backward direction (co directional and counter-directional pumping) is really common. Figure 3: Erbium-doped fiber amplifier 3.1 Booster Amplifier Used to boost power into transmission fiber, it had low noise figure and high psat. also used in cable TV systems. 30

4 3.2 In line Amplifier Figure 4: Booster Amplifier Used to increase transmission link (30-70 km), it had high gain and high Psat. 3.3 Pre-amplifier Figure 5: In Line Amplifier Used to boost power into receiver sensitivity, it had Low noise figure and high gain. Figure 6: Pre-Amplifier 4. Amplifier Noise All amplifiers degrade the signal-to-noise ratio (SNR) of the amplified signal because of spontaneous emission that adds noise to the 31 signal during its amplification. The SNR degradation is quantified through a parameter Fn, called the amplifier noise figure in analogy with the electronic amplifiers and defined as [6].

5 Fn = (SNR) in/ (SNR) out In EDFA the impact of ASE is quantified through the noise figure Fn given by Fn = 2nsp. The spontaneous emission factor nsp depends on the relative populations N1 and N2 of the ground and excited states as nsp = N2/ (N2 N1). Since EDFAs operate on the basis of a three-level pumping scheme, N1 equal 0 and nsp more than 1. Thus, the noise figure of EDFAs is expected to be larger than the ideal value of 3 db [7]. The noise figure Fn of SOAs is larger than the minimum value of 3 db for several reasons. The dominant contribution comes from the spontaneous-emission factor nsp. The noise figure of an optical amplifier is particularly important for analog systems. The signal-tonoise ratios of analog systems are often so demanding that noise figures of less than 5 db must be measured reliably. Another important purpose of the noise figure is to characterize the amount of ASE produced by the amplifier, because the ASE tends to accumulate in the communication system. The noise figure of an optical amplifier can be measured electrically or optically. Electrical noise figure measurements are often thought of as being closer to reality. However, they are complicated and usually suffer from a lack of accuracy [8]. 5. Amplifiers Comparison 6. Result and Discussion SOAs are not a good as EDFAs for use as amplifiers. They are being used for other applications: in switches and wavelength converter devices. SOAs introduce severe crosstalk when they are being used in WDM systems. Gains and output powers attainable with EDFAs are higher. Coupling losses and the polarization-dependent losses are also lower with EDFAs. Anticipated to the higher insight coupling loss, SOAs have higher noise figures relative to EDFAs. The SOA requires very high 32 quality antireflective Coatings on its facets (reflectivity of less than 10-4) which is not easy to achieve. 7. Conclusion Optical amplifiers perform a critical function in modern optical networks, enabling the transmitting of numerous terabits of data over long distances of up to thousands of kilometers. In this paper we have discussed the need for optical amplifiers in optical networks, their

6 important properties and functions, and the various systems used to implement optical amplifiers. We now have shown two types of optical amplifier to replace the Optical to Electrical electronic systems. Increase the bandwidth of EDFAs offer great to amplify data channels with highest data rates without the associated with narrow, also there is absolutely no distortion at a high bit rate. SOAs can produce severe crosstalk when amplified multiple optical channels. This makes them unsuitable for use as amplifiers in WDM systems, but gives them the ability to be involved wavelength switches and common sense gates as simple as it is the situation in the optical networking systems. References [1] WDM system, available: optics. [2] Borella, Michael S.; Jue, Jason P.; Banerjee, Dhritiman; Ramamurthy, Byrav; and Mukherjee, Biswanath, "Optical Components for WDM Light wave Networks" (1997). [3] Borella, Michael S.; Jue, Jason P.; Banerjee, Dhritiman; Ramamurthy, Byrav; and Mukherjee, Biswanath, "Optical Components for WDM Light wave Networks" (1997), CSE Journal Articles. Paper73. [4] L. Gue and M.J. Connelly.Signal induced birefringence and dichroism in a tensilestrained bulk Semiconductor optical ampli_er and its application to wavelength conversion IEEE journal of lightwave Tech.vol.23, pp , December [5] Erlend Leirset, Gain and Noise Figure of Erbium-Doped Fiber Amplifier, September 5, [6] R. E. Ziemer, principles of communication; system modulation and noise, Wiley, Newyork, [7] Fiber optic communication systems, Third edition, A John Wiley Sons, INC., Publication. 33