Enhancement in the Gain of EDFA in Fiber Optic Communication

Table of contents

Abstract:

1 Introduction:

2 RELATED WORKS

3 PROPOSED TECHNIQUE

4 RESULTS AND DISCUSSIONS

5 CONCLUSION AND FUTURE SCOPE

6 REFERENCES

Abstract:

Fiber amplifiers are crucial and fastgrowing field in the communication system The studyof the this field show that the formulation procedures of lasers generation and amplifier amplification displays aproblematical process due to the factors affecting and changing amplifier and laser significances in a dynamic way. Gain, noise figure, wavelength, power flatness and power output are directly affected by any element or parameter inside the amplifier configuration. The design parameters such as: erbium ions concentration, EDF length, isolators, wavelength division multiplexing (WDM) position, pump power position,circulators, pump directions, all of these elements and factors are affecting directly the amplifier output. EDFA is an amplifier that is best used because of its low loss and high gain. For communication, there are two windows 1530-1560nm(C-band) and 1560-1610nm (L-band).

1 Introduction:

The most recent information indicates that multimedia and high-capacity wavelength division multiplexed (WDM) networks need high bandwidths. The optical fiber is the only medium that offers such a huge bandwidth by means of a better performance. In the early days, optical fiber could not be used for commercial applications because it had a very high attenuation of up to 1000 dB∕km. But currently, various optical fibers are available with low losses (0.2 dB∕km) which can be efficiently utilized in various multiterabit and bandwidth efficient applications. For efficient utilization of bandwidth, dense wavelength division multiplexing is a technique which allows parallel transmission of various optical channels at different frequencies on a single fiber. Optical amplifiers are devices which can amplify the optical signal directly without electrical to optical and optical to electrical conversion. Their operation is the same as that of a lasing device. EDFA is a well known example .Here the doping of silica core is done with Er3+. It could be pumped effectively at wavelengths of 980nm or 1480nm and displays gain in the 1550 nm region.

Erbium Doped Fibre Amplifier: A particular attraction of EDFAs is its large gain bandwidth, which is typically tens of nanometers. This is more than enough to amplify data channels with the highest data rates without introducing any effects of gain reduction. The advent of EDFA has enabled the optical signals in an optical fiber to be amplified directly in high bit rate systems even beyond Terabits. For long distance communication, EDFAs with high pumping power and larger link length are available these days. So, more and more researchers are exploiting use of EDFAs in WDM systems in order to improve their performance. EDFA can be used to amplify signal in two bands of wavelengths in the third transmission window. The wavelength range 1525 nm to 1565 nm is known as the C-band or the conventional band and the second band from 1568 nm to 1610 nm is known as the L-band or the long band. The EDFA consists of three basic components: length of erbium doped fiber, pump laser and wavelength selective coupler to combine the signal and pump wavelengths The optimum fiber length used depends upon the pump power, input signal power, amount of erbium doping and pumping.

EDFA can be employed in wavelength-division multiplexing (WDM) system, especially DWDM system. WDM is a technique of sending signals of several different wavelengths of light into the Fiber simultaneously 5. In optic fiber communications, WDM is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths (colors) of laser light. This technique enables bidirectional communications over one strand of fiber, as well as multiplication of capacity. A WDM system uses a multiplexer at the transmitter and a demultiplexer at the receiver to split them apart. WDM systems are divided into three different wavelength patterns –normal (WDM), coarse (CWDM) and dense (DWDM).

The RUNGE-KUTTA method with fourth order accuracy is the numerical method used to calculate the input pump power and gain for EDFA. To analyze the optical parameter, we use the dynamics of EDFA with the numerical method based on RUNGE-KUTTA method with fourth order accuracy.

The EDFA is modeled with a set of three differential equations as a three level laser system. The fourth order RK-method is

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These differential equations from EDFA can be modeled with a numerical model, which describe the EDFA dynamics and require to determine the gain and loss spectra .

Gain flattening means achieving a spectrally uniform gain bandwidth. We have seen in EDFA gain spectrum the gain fluctuates between 1530 and 1560nm.There should no fluctuations in the gain spectrum. It is very difficult to work with this type of amplifier whose gain spectrum is not uniform. We need to have a flat gain over its range of operating wavelengths. This characteristic of EDFA is called Gain flattening.

The information signal of interest in fiber optic communication system with EDFA is mainly depends on pump power, pump wavelength, Er-ion concentration, Er-doping radius, fiber length etc termed as input parameters. The performance evaluation parameters of the fiber optic communication systems are gain, noise figure, degradation, radiation tolerance, spectral burning etc termed as output parameters.

Raman Amplifiers: Raman scattering occurs in any silica glass which means if we inject an optical beam (pump) in an optical fiber, then a signal passing through that fiber will be amplified if its frequency is around the shifted frequency of the pump. This is called Stock shift, which is a round 13 GHz (equivalent to about 100 nm) from the pump propagating beam frequency assuming that its wavelength is 1450 nm 3. That means the signal will be amplified if its wavelength is 1550 nm. Raman amplifiers are based on this phenomenon. There are two main Raman Amplifiers RAs distributed and discrete or lumped like EDFA's. In distributed types, amplification occurs all along the fiber between say two stations with the pump placed either near the transmitter in which case it is called forward pumping or near the receiver in which case it is called backward pumping. So in distributed RA, the fiber itself is acting as an amplifier which is of great advantage. What is exciting in RAs is the use of DCF to compensate for chromatic dispersion and loss . This can be done by increasing the pump power. But if the number of cascaded amplifiers increases, then gain fluctuation might occur. One way to deal with this situation is to optimize the multispan.

Semiconductor Optical Amplifiers: Output-level control that accepts a wide range of input power and delivers constant output power is essential for in-line optical amplifiers, optical burst and packet systems and in all optical regeneration and reshaping (2R). Semiconductor optical amplifiers SOA's can meet this demand . The reason for this is because SOA has short carrier lifetime of about several tenths to several hundreds of picoseconds compared to several hundred microseconds to several milliseconds in EDFA's. To control the output level of SOA's, external light injection can be used. It was found out that even if the level of the input signal changed by 13.5-18.5 dB at 1530-1560 nm modulated at 10 Gb/s, the output level remained constant at + 10 dBm. This method of level control is used in photonic networks.

2 RELATED WORKS

In 2017, Belloui Bouzid et al. 27 presented the comparative investigation of the design experiment, programing and simulations analysis of EDFA of double pass using three different categories. Experiment, Optisys and Matlab types were used to investigate and analyses the gain and noise figure of EDFA double pass. Two EDFAs and two pumps were used to get the maximum Gain of 41 dB at input power of -40 dBm. They also compared different input power levels to observe the effects on output Gain such that -40 dBm, -20 dBm and 0 dBm. It was observed that maximum Gain was obtained in case of least input power (-40 dBm) and least Gain was attained for 0 dBm input power. Similarly least noise figure was seen in case of least input power.

In 2014 S Sugumaran et al. 38 proposed the wavelength division multiplexed system by usig the simulation software optiwave optisystem. Work demonstrated was considered the effects of the channel spacing in the WDM system and their effects along with the nonlinear effect such as four wave missing. It was perceived that in the WDM systems, FWM deteriorate the performance of the system very efficiently. Moreover, these effects were seen by changing the length of the optical fiber. It was concluded that use of uneven frequency spacing in the WDM systems can lower the effects of four wave mixing. In 2010 R. S. Kaler et a.l 39 presented a single mode fiber-28 based system that was able to carry bit rate of 10 Gbps. Different phase shift keying with return to zero was incorporated in the system due to their features to tolerate dispersion effects. It was perceived that at the lower data rates such as 10 Gbps, there were less performance deteriorating effects due to dispersion and self phase modulation. As there was increase in the data rate such that it approaches to 40 Gbps, effects of dispersion become worse. Tolerance of Rz-Dpsk to SPM was disturbed at the data rate of 40 Gbps.

In 2005 Lee et al. 35 proposed the diverse arrangements of hybrid optical amplifiers in terms of the total gain achieved and emergence of noise figure. Proposed HOA was realized through the use of erbium fiber and Raman fiber. Where, the dispersion compensation fiber in the system was used as the Raman fiber to amplify the signals. For saving the cost of the system, recycling of optical pump was used. Results revealed that competence of the system augmented by giving recycled signal to erbium fiber (EDF) and residual pumping to fiber Raman amplifier (FRA). DCF was investigated for diverse wavelengths of pump sources. However, the experimentation was restricted to single channel classification only, as well as not deliberated the Gain flatness. Gain was required to studied in the system by taking more number of channels. In 2004 Zimmerman et al. 34 presented and elaborated the theoretical study of diverse gain flattening techniques and in addition to this, compared these methods. A range of configurations of amplifiers are studied such as hybrid Al-co-doped with Al/P- co-doped EDFAs, Raman-EDFA HOA. Also they have studied the GFFs (gain flatting filters) for better gain and even power to every WDM channels. It was clear from the results that the gain equalized filter obtained the high-quality gain as well as also gave superior flatness over array of wavelengths On the other hand, the system was projected by not including the integration of costly modules such as multiple pumps, filters.

3 PROPOSED TECHNIQUE

To complete the proposed work, use of a commercial simulation package Optiwave OptisystemTM is taken into use. This package permits us to realize the proposed system, to test and to analyze in modern optical transmission layer.

Due to the high rise in population which use internet, prolonged reach distance communication is required. To fulfill these demands, optical amplifiers are required. EDFA is perfect candidate to use in C-band wavelength window based communication. However it suffers from low Gain and high cost. To overcome this issue, C-band EDFA amplifier is proposed with high gain and less noise figure by incorporating the two fiber bragg gratings (FBGs) for amplified spontaneous noise reinjection. Maximum ASE is emerged at 1565 nm at -55 dBm carrier powers. Maximum gain is found out to be 48.16 dB with noise figure of 5.29 dBm at optimal physical parameters of the EDF, pump wavelength pump power and input power. Only single stage EDFA amplifier and single pump is employs to get maximum Gain.

In C band EDFA, prominent and premier technique of amplified spontaneous noise was used by utilizing forward and backward ASE to get the improved gain and less noise figure. This process was done through the narrowband Fiber Bragg Gratings (FBG) or fiber reflectors mirrors.

In this work, a conventional band erbium doped fiber amplifier is proposed with high gain and less noise figure by incorporating the two fiber bragg gratings (FBGs) for amplified spontaneous noise reinjection. Maximum ASE is emerged at 1565 nm for the at -55 dBm carrier powers. Maximum gain is found out to be 48.16 dB with noise figure of 5.29 dBm at optimal physical parameters of the EDF, pump wavelength pump power and input power. Only single stage EDFA amplifier and single pump is employs to get maximum Gain.In this work, a single pumped C band erbium doped fiber amplifiers is proposed using two fiber bragg gratings with amplified spontaneous reinjection as depicted in Figure 4.1. Speed of the operation is fixed to 10 Gbps from binary data bits generator. A continuous wave laser at -55 dBm power is incorporated in the system which is acting as C-band source. Laser signal is passed through optical isolator to prevent optical source from the back flowing optical intensity due to ASE. Laser signal fed to optical co-propagating pump coupler at 1490 nm wavelength is also coupled to this module.

Here, FBG 1 is acting as the reflector of backscattered ASE signal at 1565 nm and combined with pump . We have chosen the 1565 nm wavelength to be reflected because of maximum intensity at this point. An erbium doped fiber with 200 ms metastable lifetime is taken. Various physical parameters are varied such as input power, and pump power. Simulation parameters are shown in Table 4.1 to clear the factors that are considered for the proposed work.

FBG 2 is employed to reflect the forward scattered amplified spontaneous noise. Finally signal passed through the isolator and gain of signal is accessed by dual port WDM analyzer.

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Table 1: Simulation parameters of the demonstrated work Simulation setup of proposed EDFA with ASE reinjection

4 RESULTS AND DISCUSSIONS

Results are discussed in following steps:

1 Effect of different physical parameters of EDF on C-band EDFA

In this research work, we emphasised on the Gain enhancement of the C band erbium doped amplifier with the use of amplified spontaneous emission reinjection through FBGs. To accomplish the work, Optiwave optisystem is considered which provide extensive library and good simulation environment. Effect of various physical parameters of erbium doped fiber such as length of the EDF fiber, launched power, pump power, forward/backward ASE power emergence are investigated in terms of Gain, noise figure. Optical spectrum analyzer is a depicter which is incorporated in the system to represents carriers and their power with respect to frequency or wavelength. OSA is important to check faults, carrier power and noise power. Figure 1(a) represents the OSA diagram after laser source and Figure1 (b) shows the OSA diagram after optical amplifier in the final output at -55 dBm. Similarly Figure 1 (c) and Figure 1 (d) shows the OSA diagram after optical amplifier in the final output at -20 dBm and 0 dBm respectively.

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Fig 1(b)

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Fig 1(c)

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Fig 1(d)

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Fig 1(e)

Forward and backward optical spectrum of ASE noise is shown in Figure 1 (e) and (f) respectively. Amplified spontaneous emission noise is a prominent power degrading issue in the erbium doped fiber amplifiers. However, in this work, the use of ASE has been done to enhance the Gain through the FBGs. This is done by re-injecting the ASE in the EDF fiber with the combined power of pump. It is seen that there are two type of ASEs in the system. One is forward ASE and second is backward ASE. Figure1 (e) (f) represents the output of optical spectrum analyzer to depict the backward/forward ASE.

It is percived from Figure 1 (f) that maximum backward ASE is emerged at the wavelength of 1565 nm and in Figure 1 (e), again the maximum forward noted ASE is near about 1565 nm. Thus, in proposed work, ASE at 1565 nm is reinjected through the two FBGs in the EDF fiber.

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Fig 1(f) : Forward ASE

First and foremost, effect of pump power is analysed on C-band EDFA. A tunable laser at wavelength 1570 nm and pump wavelength of 1490 nm is incorporated in the system with 10 MHz laser linewidth. Pump power is varied from 200 mW to 100 mW and iterated in the EDF fiber which connected with single pumping. Results are analyzed in terms of output power and readings are noted from wavelength division multiplexed analyzer. Figure 2 depicts the performance of the system at varied levels of pump power in terms of Gain at different input power levels. Results revealed that there is increase in output power (Gain) with the increase in pump power. While input power is also changed for three different levels one by one such as -55 dBm, -20 dBm and 0 dBm. It is observed that lower input power levels and high pump power is optimal for getting high Gain in the system. Highest Gain value of 48.16 dB is achieved at input power level of -55 dBm with noise figure of 5.29 dB as shown in Table 5.1. So, it is recommended to use -55 dBm due to highest Gain.

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