Spectrum Of Erbium Doped Fiber Amplifier Computer Science Essay

Optical Amplifier can lessen the effects of dispersion and attenuation allowing improved performance of long-haul optical systems. With the demand for longer transmission lengths, optical amplifiers have become an essential component in long-haul fiber optic systems.

In optical fiber network, amplifiers are used to regenerate an optical signal, amplify and then retransmitting an optical signal. In long-haul optical systems, the there are many amplifiers are needed to prevent the output of signal seriously attenuated. In order reduce the cost; the amount of amplifiers can be reduced by increase the spacing between them. Current spacing of Erbium Doped Fiber Amplifier is in the range of 80km to 100km.

The gain spectrum of Erbium Doped Fiber Amplifier is not inherently flat. For single channel systems, the gain variation is not a problem. However, in optical fiber network the channel increases, the transmission problem arises. The gain flatness is importance for erbium-doped fiber amplifiers’ (EDFA’s) wavelength division multiplexing (WDM) which is important technique for long haul optical transmission link system. They typically present gain peaking at about 1530 nm and the useful gain bandwidth may be reduced to less than 10 nm.

There are one major problem of EDFA which is amplified spontaneous emission (ASE) generated by stimulation emission during amplification of input signals. The amplified spontaneous emission (ASE) is background noise. This noise signal being amplified with the input signal when after go through another amplifier.

The output signal power increases will decrease the spacing of the EDFA. The output signal power can be optimized by maximize the gain. Moreover, the gain is the parameter of the doping concentration and doping profile of the erbium doped fiber, length of the fiber, windows wavelength of input signal, input powers and the pump power. But the length of the fiber is the spacing of repeater where is one of the project outcome of this project.

OptiSystem is a design and simulation software for fiber optics application. OptiSystem enables users to simulate/design next generation optical networks, current optical networks, SONET/SDH ring networks, amplifiers, receivers, transmitters. This software has many analysis tools eye diagrams, BER, Q-Factor, Signal chirp, polarization state, constellation diagrams, signal power, gain, Noise Figure, OSNR, data monitors, report generation, and more.

Objectives of Project

This project simulates and optimizes the performance parameters of EDFA. The objectives of this project are outlined as below:

to increase output signal power .

to flatten the gain of Erbium Doped Fiber Amplifier.

to reduce the Amplified Spontaneous Emission (ASE) of Erbium Doped Fiber Amplifier .

Problem Statement

In order to have a high capacity transmission system, there are several ways such as by using wave-dense multiplexing(WDM) technique, increase the power of output transmission system, reduce the losses in transmission system and etc. By increase the number of optical amplifier in long haul transmission system is able to increase the output power of transmission system but this will increase the costs. Thus, the better solution is to optimize the performance of optical amplifier.

The output signal power is mainly affected by gain of the EDFA. The gain is affected by many factors which are the doping concentration and doping profile of the erbium doped fiber, length of the fiber, wavelength windows input signal, input powers and the pump power. The increase the input signal power certainly will increase the output signal power but the gain decreases. The wavelength of input signal of EDFA can be in 1530nm to 1565nm (C-band) or 1570nm to 1605nm (L-band). The gain of C band is greater than L-band but the absorption is also large.

Amplified Spontaneous Emission Noise (ASE) of Erbium Doped Fiber Amplifier (EDFA) amplified together with the input signal. To obtain high output power, the Erbium Doped Fiber Amplifier (EDFA) need high optical pump power and high inversion. Thus, the higher inversion applied in EDFA, the higher the ASE noise. In the others word, Noise Figure will increase with the gain of EDFA.

Gain flatness is a function of inversion level. Typically 40% to 60% inversion leads to broadest gain with lowest ripple. The gain at the L band is flatter than C band but the C band has lower gain. The use of gain flattening filter can be a solution of gain flatness problem but it will decrease output signal power.

Scope of the Project

This project only involves the optimization of Erbium Doped Fiber Amplifier. The other types of optical fiber amplifiers are not involved. This project focuses on the simulation using software optisystem and does not involve any hardware. In addition, reduction of the noise covered in this project which covers Amplified Spontaneous Emission noise only and does not covers any others’ noises such as thermal noise, human noise and etc. The optimization of Erbium Doped Fiber output signal power is included in this project. This project is focus on the long haul transmission system where using the single mode fiber only.

Project Outcomes

In the end, The Erbium Doped Fiber Amplifier simulated with increased gain flatness. The Erbium Doped Fiber Amplifier simulated with high output power and the spacing of repeaters were increased but the Erbium Doped Fiber Amplifier simulated have small amount of Amplified Spontaneous Emission.

Methodology

The basic information of optical fiber amplifier was obtained from the reference book and from internet sources. The information obtained is the basic configuration of optical amplifier in optical transmission link, advantages and disadvantages of Erbium Doped Fiber Amplifier, the Amplified Spontaneous Emission Noise (ASE) and etc. Second methods of data and information collections are from journal, magazine and reference books. The data and information is more advance such as the effect of ASE noise, analysis of gain flatness of EDFA, the relationship between ASE noise and pump power and etc.

Thesis Structure

Chapter 1: In this chapter, the objectives of the project were introduced. In overview of the project will introduce the basic idea of project and some basic knowledge related in this project. The problems and the expected outcome of this project were also stated.

Chapter 2: In this chapter, the research to background related of this project will be discussed. The concept structure of future work of this project will be come out by literature review. How to solve the problem stated in chapter 1 by applying the correct theory were explained in details.

Chapter 3: The procedures of solution to the stated problems in this project were explained in this chapter. The procedures that have been chose and the choice of procedures have to state in this chapter. The problems of problems’ analysis and collection of were included.

Chapter 4: The results of this project were show in this chapter. The results include simulation of the transmission link and analysis of the performance transmission link. The discussions of the results also have to include.

Chapter 5: The objective, discuss the findings and conclude the limitation of this project are being justified in this chapter. Some recommendations on how to improve the project were discuss at the end of this chapter.

CHAPTER 2

LITERATURE REVIEW

This chapter discussed all the useful theory and data about this project. The literature review was referring the journal, article, reference books and data sheet. These sources were collected from the library, internet and IEEE library website.

2.1 Introduction

Optical fiber has three main types of property which are dispersion, absorption, and scattering. These properties have cause attenuation, power losses, output power decrease where bring disadvantages to the long haul transmission. Dispersion occurs when the light travelling down a fiber optic cable “spreads out,” becomes longer in wavelength and eventually dissipates. Absorption is resulting by the impurities such as hydroxyl ions where will cause the optical power dissipated as heat power. Scattering is another major mechanism of attenuation in optical fiber. It happens when the light change direction or diffuse where caused by the light striking the small particles or the in-homogeneity of transmitting material.

Attenuation, a reduction in the transmitted power, has long been a problem for the fiber optics community. The increase in data loss over the length of a fiber has hindered widespread use of fiber as a means of communication. However, researchers have categorized three main sources of this loss: absorption, scattering, and, though it is not commonly studied in this category, dispersion.

With the demand for longer transmission lengths, optical amplifiers have become an essential component in long-haul fiber optic systems. An optical amplifier is a device that amplifies an optical signal directly, without the need to first convert it to an electrical signal. By Optical amplifier, the capacity of optical transmission system is increases. Stimulated emission in the amplifier’s gain medium will cause amplification of incoming light.

Semiconductor optical amplifiers (SOAs), erbium doped fiber amplifiers (EDFAs), and Raman optical amplifiers lessen the effects of dispersion and attenuation allowing improved performance of long-haul optical systems. Optical amplifier Semiconductor optical amplifiers (SOAs), erbium doped fiber amplifiers (EDFAs), and Raman optical amplifiers lessen the effects of dispersion and attenuation allowing improved performance of long-haul optical systems.

There are many transmission windows (wavelength bands) where show at table 2.1. Each effect that contributes to attenuation and dispersion depends on the optical wavelength. Raman Amplifier have broadest band among the optical amplifiers where from O band to U band. Whereas erbium doped fiber amplifiers mostly work on C band or L band.

Band

Description

Wavelength Range

O band

original

1260 to 1360 nm

E band

extended

1360 to 1460 nm

S band

short wavelengths

1460 to 1530 nm

C band

conventional (“erbium window”)

1530 to 1565 nm

L band

long wavelengths

1565 to 1625 nm

U band

Ultra-Long wavelengths

1625 to 1675 nm

Table 2.1 transmission windows (wavelength bands) [1].

2.2 Erbium Doped Fiber Amplifier

The invention of the EDFA in the late eighties was one of the major events in the history of optical communications. It provided new life to the optical fibre transmission window centred at 1550 nm and the consequent research into technologies that allow high bit-rate transmission over long distances. Erbium Doped Fiber Amplifier has cores doped with atoms of an element that light from an external laser can excite to a state in which stimulated emission can occur. Pump light from the external laser steadily illuminates one or both ends of the fiber and is guided along the fiber length to excite the atoms in the core.

The basic configuration for incorporating the EDFA in an optical fibre link is shown in Figure 2.1. The signals and pump are combined through a WDM coupler and launched into an erbium-doped fibre. The amplified output signals can be transmitted through 60-100km before further amplification is required.

Figure 2.1 Basic configuration for the incorporation of an EDFA in an optical fiber link.

The Figure 2.2 shows amplification in the erbium-doped fiber amplifier. Small quantities of erbium are present in the fiber core. When light excites the erbium atoms, a weak signal in the erbium amplification band guided along the fiber core stimulates emission, and the signal grows in strength along the length of the fiber.

Figure 2.2 Amplification in an erbium-doped fiber amplifier[1].

An EDFA is an optical fiber of which the core is doped with the rare-earth element Erbium. By exciting the Erbium ions to higher energy levels, we can achieve amplification of signals at wavelengths interesting for optical communication, i.e. around 1550 nm. The energy levels are not very sharp, which leads to a relatively large gain bandwidth. In order to excite the Er3+-ions, we send a beam of light, which we call the pump, into the fiber. If the pump is at wavelength 980 nm, Er3+ will rise from the ground state L1 to the higher L3, as illustrated in Figure 2.3. However, the ions will rapidly decay to energy level L2 without producing photons. The lifetime in L3 is approximately 1 μs. Pumping with 1480 nm light will excite the ions directly to state L2. Relaxation from L2 to L1 will occur after approximately 10 ms, producing photons in the wavelength band 1520-1570 nm. This is called spontaneous emission.

Figure 2.3 Energy levels of Er3+ ions in EDFA [3].

Spontaneous emission has no correlation with the signal, but is distributed over the entire bandwidth of the L2-L1 transition and can travel in backward as well as forward propagation direction. Hence, it is noise. Obviously, outcome 2 is the desired behaviour of an amplifier and can be achieved by pumping the fiber until population inversion occurs: when the number of ions in state L2 exceeds the number of ions in the ground state, the probability of outcome 2 is higher than that of 1 and 3.

However, spontaneous emission is always present and will, like the signal, experience amplification as it propagates through the fiber. This is termed amplified spontaneous emission or ASE. While the ASE level is independent of the signal at low signal powers, high signal powers will deplete the population inversion faster than the pump can maintain it, and the ASE level increases.

The pump light must be specific wavelengths in order to stimulate emission from the erbium atoms. Standard pumps are semiconductor lasers that emit 980 or 1480 nm. Each wavelength has its own advantages.

Figure 2.4 shows the overall structure of an erbium-fiber amplifier, omitting the details inside the fiber. The input signal enters from the left (in this example, a single optical channel at 1550nm). It passes through an optical isolator, which blocks light from going back toward the light source, and filter, which transmits the signal wavelength but blocks the wavelength of the pump laser. Then the signal enters the erbium-doped amplifying fiber. Light from the pump laser is coupled into the other end of the erbium-doped fiber to excite erbium atoms, which amplify the signal passing through the loop of fiber. Then the amplified signal is separated from the pump at a wavelength-selective coupler on the right, and exits through another optical isolator into the next leg of the fiber-optic system.

The pump light must be specific wavelengths in order to stimulate emission from the erbium atoms. Standard pumps are semiconductor lasers that emit 980 or 1480 nm. Each wavelength has its own advantages.

Figure 2.4 Erbium doped fiber amplifier[1].

It work typically works at wavelength from 1530nm to 1605nm, where silica fibers have their lowest loss. Over last decade long haul system has evolved from single channel low bit rate to multichannel system at data rate as high as 40 Gbits. Thus, the erbium doped fiber amplifier is widely used in long haul optical transmission link [1] [3]. The amplification band of EDFA is 1530 to 1610nm and the pump length is 980nm or 1480nm [4]. EDFA has 20 to 30dB gain where is considered high for long haul transmission system. The pump power of the EDFA is low where from 10mW to 5W whereas one of the famous optical amplifiers which is Raman amplifier need 500mW to 5W pump power to perform amplification. The Noise Figure of the EDFA is 3.5 to 6.5dB [4].

2.3 Gain of Erbium Doped Fiber Amplifier

The gain of an EDFA is the ratio of the output signal power (Pout) to the input signal power (Pin) where the equation is:

The gain of the amplifier is depends on the doping concentration and doping profile of the erbium doped fiber, length of the fiber, windows wavelength of input signal, input powers and the pump power. Thus the parameters to optimize the Gain of EDFA were shown as above.

Figure 2.5 Gain and absorption in typical erbium-doped fiber[1]

There are two band input wavelengths for the EDFA which are C band (1530 to 1565 nm) and L band (1565 to 1625nm) [1]. For C band, the gain is high but it can use several meters of optical fiber only. From the Figure 2.5, the gain is high at short wavelengths, but it is offset by high absorption. For L band, the gain is not large but the absorption also small. It can be use 30 meters or more of the optical fiber [1] [5]. Thus, C band is not suitable for long haul transmission link whereas L band is suitable but C band and L band can be used simultaneously [2].

Figure 2.6 Variation of gain with EDFA length for different values of pump powers. For a given pump power there is an optimum length for achieving maximum gain [1].

Figure 2.7 Variation of gain with pump power for different lengths of erbium doped fiber [1].

The Figure 2.6 shows that the gain increases first by increasing the length of the doped fiber and then start to decrease after reached maximum for given input pump power [2]. This happens because as the pump propagates through the doped fiber it gets absorbed and thus its power reduced. After propagating certain distance, its power is too small to create population inversion, and thus after this length, the doped fiber would start to absorb the signal rather than amplify it. The Figure 2.7 shows the gain increases with pump power for a given length of the doped fiber [2]. The pump power increases and the population inversion created more and more in the certain length of doped fiber given.

2.4 Gain Flatness

One of the disadvantages of the EDFA is the gain is not inherently flat but the gain flatness is importance for erbium-doped fiber amplifiers’ (EDFA’s) wavelength division multiplexing (WDM) which is important technique for long haul optical transmission link system. The shape of the gain spectrum depends on the operating gain level, or, in other words, on the population inversion [6]. The gain of the L band is flatter than gain of C band where can be observed at Figure 2.5 and Figure 2.8. The gain can be flattening by using the gain flattening filter but this filter will decrease the output signal power.

There are two categories of gain flattening techniques which are passive and active. The filtering method is in category of passive. The disadvantages of the filtering method are increase the noise and reduce the output power of EDFA [10].

Figure 2.8 Calculated gain for C- band and L- band amplifier [1]

There is one of techniques is hybrid of Optical Amplifier. Hybrid amplifier can be in series or parallel configuration. The series configuration of hybrid two EDFA is shown as Figure 2.9 [8]. The parallel configuration of C-Band EDFA and L-Band Raman is shown as Figure 2.10 [9].

Figure 2.9 Hybrid Series configuration [8]

Figure 2.10 Hybrid parallel configurations [9]

2.5 Amplified Spontaneous Emission

Optical amplifier is an analogue device, it amplify any input noise that come with the input signal. The background noise called amplified spontaneous emission (ASE) also generated. The ASE is generated by stimulation emission during amplification of input signal. The light that start stimulated emission in a laser is emitted spontaneously when an excited atom releases its excess energy without outside stimulation. A laser bounces this light back and forth through the laser cavity to amplify it by stimulated emission. Fiber amplifiers lack resonator mirrors, so they do not build up a laser beam in the same way. However, spontaneous emission that occurs within the fiber can be amplified if it is guided along the fiber, creating background noise.

Figure 2.11 Amplified Spontaneous Emission Noise [5].

Amplified spontaneous emission is spread across the whole operating range of a fiber amplifier, as shown in Figure 2.9. The power is much lower than at the amplified wavelength, shown as peaks in Figure 2.9 [1]. However it remains in the background and can be amplified in successive amplifiers.

Figure 2.12 Simulation and Experimental characteristic of ASE versus wavelength (EDFA) [7]

Figure 2.8 shows the simulated and experimental backward ASE profiles obtained at pump power of 109 mW. The simulated backward ASE levels are observed to be higher than the experimental backward ASE levels. Such observation is also noted for forward ASE profiles. As the pump power decreases, the discrepancy between the simulated ASE profiles and the experimental ASE profiles changes in a similar pattern. Generally, the simulated ASE level is about 13 dB higher than the experimental ASE levels at high pump power.

The noise figure NF is a measure of how much noise the amplifier adds to the signal.

The definition is:

Where, SNR is the signal-to-noise ratio.

Due to ASE, the SNRout at the amplifier output is less than that at the input, SNRin. If the signal is much stronger than the noise, the noise figure can be written as equation below [2]:

Where;

PASE is the ASE noise power,

h is Planck’s constant,

ν is the frequency of the light ,

G is Gain of EDFA

Δνsp is the bandwidth of the noise (i.e. the bandwidth of the EDFA).

2.6 Typical Specification of EDFA

There are many Erbium Doped Fiber Amplifier sell by different vendors. Each vendor gave different specification to the EDFAs. Usually, vendor would not sell only one type of EDFA. The vendor will sell different type such as Booster amplifier, in-line amplifier, pre-amplifier and gain flatten optical amplifier. Those types of optical amplifiers have different characteristics in gain, noise figure and gain flatness.

There is a typical specification for a commercial EDFA which shown in Table 2.2 [2].

Wavelength range (C-band)

1530 to 1565

nm

Wavelength range (L-band)

1570 to 1605

nm

Total output power

+23

dBm

Total input power range

-26 to +10

dBm

Gain

30

dB

Gain Flatness

0.8

dB

Noise figure Pin = 0dBm (C-band)

5.5

dB

Noise figure Pin = 0dBm (L-band)

6.5

dB

Polarization-dependent gain

0.4

dB

Polarization mode dispersion

0.5

ps

Transient settling time

50

µs

Transient overshoot/undershoot

0.5

dB

Power consumption

20

W

Table 2.2 Typical Specifications of Commercially Available EDFAs [2].

From Table 2.2, the gain, noise figure Pin = 0dBm (C-band) and gain flatness were the reference of the expected result in this project.

2.7 OPTISYSTEM (Software Used)

OptiSystem is a famous optical communication system simulation package for the design, testing, and optimization of virtually any type of optical link in the physical layer of a broad spectrum of optical networks, from long-haul systems to LANs and MANs. A system level simulator based on the realistic modeling of fiber-optic communication systems, OptiSystem possesses a powerful new simulation environment and a truly hierarchical definition of components and systems. Its capabilities can be easily expanded with the addition of user components and seamless interfaces to a range of widely used tools. There are many Models and Tools in Optisystem. The description of basic and relevant models or tools is show as below:

Erbium Doped Fiber

Description:

This model enables large and small input signal analysis, considering 980 nm or 1480 nm as wavelength pump. The numerical solution of the rate and propagation equations, assuming a two-level Er system, enables to design the amplifier in a co- or counter-propagating pump scheme. ASE is included in the simulations and fiber specifications such as geometrical parameters (for example, core radius, Er doping radius, Er metastable lifetime) as well as and absorption and emission characterization (cross-section, fiber loss) are required as input files.

EDFA

Description:

The EDFA 1.0 designs Er-doped fiber amplifiers considering numerical solutions of the rate and the propagation equations under stationary conditions. The model includes amplified spontaneous emission (ASE) as observed in the amplifier Erbium Doped Fiber 1.0, however this module enables to the user selecting forward and/or backward pump as well as the pump power values.

Trapezoidal Optical

Description:

This model is an optical filter with a trapezoidal frequency transfer function.

The transfer function is of the form:

Where:

H (f) is the filter transfer function, α is the parameter Insertion loss.

HGauss(f) is the filter transfer function and

f is the frequency.

Optical Filter Analyzer

Description:

This model can extract the frequency response of an optical component by comparing a reference optical signal before and after the calculation.

Optical source

There are 6 types of optical source:

CW Laser

Pump Laser

White Light Source

Laser Measured

Laser Rate Equations

LED

Multiplexer and Demultiplexer

The WDM need the using of multiplexer and demultiplexer. In the Optisys, multiplexer and demultiplexer is a ideal multiplexer and demultiplexer.

CHAPTER 3

RESEARCH METHODOLOGY

Chapter three explained the project methodology, approached taken and a closer look how the project was being conducted.

3.1 Understanding the project

There are three objectives of this project which are to increases the output signal power of the Erbium Doped fiber Amplifier (EDFA), to flatten the gain of the Erbium Doped Fiber Amplifier and reduce the Amplified Spontaneous Emission (ASE) of Erbium Doped Fiber Amplifier.

Literature review on the three objectives to finds out the methods to achieve the three objectives. The methods used in literature review are reviewing website’s information, reviewing journal and references book. The method of reviewing website’s information is for obtain the basic introduction on the fiber optic and Erbium Doped Fiber Amplifier. The second methods of reviewing journal and references books are to find out the methods of optimization the amplifier, solutions of problems and etc.

3.2 The Transmission Link

The Figure 3.1 is the transmissions link that going to use in the project. The circuit is Erbium Doped Fiber Amplifier in multi-channels Wave Division Multiplexing (WDM) system. The WDM system used because the purpose of this project is to optimize the optical long haul transmission link system and the WDM is the necessary system in the long haul transmission link system.

There are 16 multiplexed signals which each channel spacing is 1nm. Most of the long haul transmission system such as Fiber to the Home system is using 16 or 32 channels in transmitter. At the receiver side, there are 16 channels output de-multiplexed by WDM de-multiplexer.

Figure 3.1 The hybrid EDFAs will be used in this project

3.3 Output power of EDFA

The methods used to optimize the output power are by choosing the suitable parameters. The parameters are the input power and length of fiber. The input power selected is -23dBm for each channel (there are total 16 channels). Lastly, choosing C Band wavelength is one of the methods as C Band has larger gain property than L Band.

The gain of Erbium Doped Fiber amplifier is function of length of Erbium Doped Fiber. The erbium doped fiber length is set between 1m and 20m. The length of erbium doped fiber as the parameter to optimize EDFA’s gain because the EDFA’s gain increase dramatically with the length of erbium doped fiber.

The pump power was not chosen as the parameter to optimize the gain because pump power used as parameter for gain flatness of the EDFA.

3.4 Amplified Spontaneous Noise of EDFA

Since the Amplified Spontaneous Emission noise (ASE noise) is function of the gain, the noise figure was optimized by adjusting the gain of EDFA. This method will limit the gain of the EDFA but it is more practical than the filtering method. The filtering method requires an optical filter to filter the noise but it is difficult and expensive to fabricate an optical filter [3].

The amplified spontaneous emission noise of EDFA was been observed and measured by noise figure of the EDFA system. The equation below [2] shows that noise figure is function of ASE noise and gain.

Where;

PASE is the ASE noise power,

h is Planck’s constant,

ν is the frequency of the light ,

G is Gain of EDFA

Δνsp is the bandwidth of the noise (i.e. the bandwidth of the EDFA).

3.5 Gain Flattening

In order to flattening the gain of Erbium Doped Fiber Amplifier, the hybrid EDFAs is one of the methods. The Hybrid circuit was shown in Figure 3.1.

Hybrid EDFAs spilt two ranges of wavelength which are 1550nm to 1557nm and 1558nm to 1565nm. These two ranges of wavelength used because the gain is optimum at the both ranges of wavelength as shown in Figure 3.2.

The gains of the two wavelength ranges wavelength have different property of gain. Thus, the gain of the two EDFAs adjusted to have flatter gain simultaneously. Lastly, both EDFAs were being adjusted to have similar gain as each other and combine them. Dual pump used in this project because dual pump have ability that control the gain of EDFA.

Figure 3.2 The inherent gain flatness for each wavelength graph

3.6 Measurement of the result

The results of this project will be analyzed by using dual port WDM analyzer. Dual port WDM analyzer is able to measure gain of EDFA, ratio of maximum and minimum gain and noise figure of the EDFA. The BER analyzer or eye diagram was used to measure the BER rate of each wavelength at receiver. The BER rate determines whether the EDFA designed is applicable in optical transmission system.

START

Literature

Review

Reduction of Amplified

Spontaneous Emission (ASE)

Gain

Flattening

Optimization of

The

Output Power

Change

Methods

SIMULATION

Reduction of Amplified

Spontaneous Emission (ASE)

Optimization of

The

Output Power

Gain

Flattening

NO

Result

Analysis

YES

Conclusion

END

Figure 3.3 The Flow Chart

CHAPTER 4

RESULT AND DISCUSSION

Chapter four are focusing on the documentation of all the data and final result of this project. This chapter, the result from each simulation will be shown and there are discussions about the result.

4.1 Spacing of EDFA

The length of optical fiber was being varied to investigate the spacing between the booster amplifier and in-line amplifier. Assume that the input power is 0dBm and the gain of the booster amplifier is 5dB. The input power of the transmission system has been assumed as 0dBm because the maximum transmitter power of the long haul transmission system is 13dBm. The length of optical fiber was being set into sweep mode iteration from 110km to150km as shown as the Figure 4.1.

Figure 4.1 Sweep mode

The input power and the gain of the amplifier (Booster amplifier) have been set as below:

Input Power = 0dBm

Booster Amplifier = 5dB

Figure 4.2 The spacing length between booster amplifier and the in-line amplifier.

Result of the simulation:

Table 4.1 Length of Optical Fiber versus output signal with 5dB gain of Booster amplifier

The result of Table 4.1 determined that the maximum length of spacing between Booster amplifier and the first inline amplifier by knowing the minimum input of the EDFA. The minimum input power EDFA is inversely proportional to the output power.

4.2 Gain Optimization

The purpose of gain optimization simulation is to investigate and study the suitable parameters for the circuit. According to literature review, the gain were affected by the input power, length of Erbium Doped fiber, pump power and wavelength.

4.2.1 Wavelength input signal

The gain of amplifier were varied with the signal wavelength. The transmitter source signal fixed to -23dBm because there are approximate 3dB to 2 dB at the WDM Multiplexer. Thus the input power is about -25dBm. The wavelength of transmitter from 1530nm to 1565nm which is called C-Band wavelength. The circuit of Figure 4.3 used dual pump because this will increase the gain flatness of the EDFA. There are 32 channels where the wavelength of the transmitter starts from 1550nm to 1565nm. The spacing in between each channel is 1nm.

Figure 4.3 Simulation of the wavelength input signal from 1530nm to 1565nm

Result of the simulation:

Table 4.2 Wavelength versus gain (nominal)

Figure 4.4 Wavelengths versus gain (nominal) Graph

The Gain flatness of the EDFA can be observed as seen in Figure 4.4. Since the method to flatten the gain of EDFA is Hybrid (parallel) two EDFAs that have different gain respect to wavelength. Thus, the wavelengths used are 1550nm to 1557nm and 1558 to 1565nm. The gain for the two frequency range being chosen because both of them have enough flatness of gain and in high gain. The gain of the results inserted is the total gain also known as nominal gain where measured by WDM Dual port analyzer as in Figure 4.5.

Figure 4.5 Dual Port WDM analyzer and Nominal gain

The calculation of nominal gain as below:

Where: Gλn = gain of the wavelength λn

N = total channel

From Table 4.2, there are 16 channels with different wavelength respectively. The nominal gain can be calculated as below:

4.2.2 Input Power Signal

The input signal of the EDFA will affect the gain, gain flatness and Noise Figure. Since the method to obtain a flat gain is hybrid of two EDFA , thus the simulaton were done in two different wavelength range which are 1550nm to 1557nm and 1558 to 1565nm.

Figure 4.6 EDFA for frequency range from 1550nm to 1557nm

Figure 4.7 EDFA for frequency range from 1558nm to 1565nm

Results:

Table 4.3 Input power versus gain (1550nm to 1557nm)

Figure 4.8 Input power versus gain (1550nm to 1557nm) Graph

Table 4.4 Input power versus gain (1558nm to 1565nm)

Figure 4.9 Input power versus gain (1558nm to 1565nm) Graph

From the result, the increased in the total gain is decreased with input power. Since the transmitter transmitted power is 0 dBm which was being set at previous simulation, thus the maximum input power is 0 dBm where the losses being counted.

Since the typical input power is -26dBm to 10dBm. Thus, the input power set to approximate -25dBm because the smaller the input power of EDFA, the longer the length of spacing between EDFAs. Moreover, the smaller the input power the higher the gain but the gain flatness will be increase.

The transmitter power of -23dBm is being simulated because the WDM Mux consist of about 2.7 dB losses. Based from the simulation result of Table 4.1, the circuit was able to have 130km to 140km spacing between the booster amplifiers with the input power of -23dBm. Thus, the spacing of the Optical amplifier increased from 80km to 130km.

Figure 4.10 WDM Multiplxer losses=2.7dBm

4.2.3 Length of Erbium Doped Fiber

The gain of the EDFA is function of the length of the Erbium Doped Fiber. The simulation had been done by setting the length of erbium doped fiber in the sweep mode. The length of fiber was varied from 0 meter to 20 meter.

Results:

Table 4.5 Length of Erbium Doped Fiber versus Gain

Figure 4.11 Length of Erbium Doped Fiber versus Gain Graph

The graph in Figure 4.11 shows that the length of the erbium fiber in Erbium Doped Fiber Amplifier is incresed with the total gain(nominal gain). Moreover, the gain increased in small value when the length of the Erbium Doped fiber increased from 10 meter to 20 meter. Since the Erbium Doped fiber is expensive, the length of erbium doped fiber suitable to use is 10 meter because of the cost and gain consideration.

4.2.4 Pump Power

The pump power will also determine the gain of the EDFA. The maximum pump that can subtained by the optical fiber is 1 watt. Thus this simulation had been done by changing the pump to sweep mode and varies it from 0 to 1 Watt.

Results:

Table 4.6 Pump Power versus Gain

Figure 4.12 Pump Power versus Gain Graph

Because the pump power increase the dramatically from 0 to 200mW. Thus, the pump power suggested used in the range of 0 to 200mW. The second simulation being done to investigate the pump power characteristic in EDFA of 200mW.

Results:

Table 4.7 Pump Power versus Gain

Figure 4.13 Pump Power versus Gain Graph

The maximum gain occurs at 200mWatt pump power. The 200mW pump power has been suggested to use in this project.

4.3 Gain Flatness Optimization

The gain flatness can be optimized in many methods. The methods used in this project is Hybrid of two EDFAs. The parallel hybrid being used in this project which is easier to optimize the gain flatness. Thus, the simulation for two operating wavelength for EDFA being done which are 1550nm to 1557nm and 1558nm to 1565nm. The EDFA designed in dual pump because dual pump is easier to control the dynamic gain.

The Gain Flatness can be measured by ratio maximum and minimum gain from the Dual Port WDM Analyzer. The larger the ratio, the lower the gain flatness. The ratio maximum and minimum gain from wavelength of 1535nm to 1565nm is shown as Figure 4.14:

Figure 4.14 Ratio of maximum and minimum gain Graph

The gain flatness can be calculated by equation below:

From the Figure 4.14, The gain flatness can be calculated as below:

= ± 4.5

4.3.1 Simulation for EDFA of frequency range from 1550nm to 1557nm.

This simulation will only operated at wavelength of 1550nm to 1557nm. There 8 channels where each channel spacing is 1nm. This simulation is to investigate the flat gain at the operating wavelength. The length of the Erbium Doped Fiber set to sweep mode where varied from 10 meter to 20 meter. This is to provide the different gain for the EDFA.

Figure 4.15 EDFA for frequency range from 1550nm to 1557nm

Results:

Table 4.8 The gain flatness varies with gain(1550nm to 1557nm)

Figure 4.16 The gain flatness varies with gain graph

The Gain Flatness increased when the gain increases. But, the gain cannot be set too high as it will increase the Noise of the EDFA. The gain suggested in this project is 36.119dB and gain flatness is about ±0.71dB.

4.3.2 Simulation for EDFA of frequency range from 1558nm to 1565nm.

This simulation will only operate at wavelength of 1558nm to 1565nm. There 8 channels where each channel spacing is 1nm. This simulation is same as previous simulation where is to investigate the flat gain at the operating wavelength. The length of the Erbium Doped Fiber set to sweep mode where varied from 10 meter to 20 meter.

Figure 4.17 EDFA for frequency range from 1558nm to 1565nm

Results

Table 4.9 The gain flatness varies with gain (1558nm to 1565nm)

Figure 4.18 The gain flatness varies with gain graph

The Figure 4.16 shows that the gain increases with the length of the erbium doped fiber until 17meter of the erbium doped fiber. The suggested gain used in this EDFA circuit is 35.5012dB.

4.4 Final Simulation of The Hybrid EDFA

This simulation is the hybrid of two EDFAs. This simulation was to measure the gain, noise figure and gain flatness. Moreover, the quality of output power being investigated by using the eye diagram. The minimum BER measured had been made sure that below Ã-10-9.

Figure 4.19 Hybrid EDFA

Results:

Figure 4.20 Nominal gain, gain ratio max/min and Maximum NF

Table 4.10 Nominal gain, gain ratio max/min and Maximum NF

Wavelength/ nm

The Min BER From Eye Diagram

Eye Diagram

Min BER

1550

8.92873 x 10-139

1551

8.35385 x 10-207

1552

1.15412 x 10-163

1553

8.4381 x 10-139

1554

3.78135x 10-135

1555

3.27086x 10-150

1556

5.20888 x 10-115

1557

5.1161 x 10-147

1558

2.0591 x 10-150

1559

1.59684 x 10-120

1560

1.0206 x 10-202

1561

3.01029 x 10-153

1562

3.81049 x 10-140

1563

3.1592 x 10-125

1564

3.1592 x 10-125

1565

7.65416 x 10-153

Table 4.11 Eye Diagram and BER of each wavelength

The WDM Multiplexer (1 x 2) and WDM Demultiplexer (2 x 1) used to split and combine the two wavelength ranges which are 1550nm to 1557nm and 1558nm to 1565nm. The minimum BER all achieved below Ã-10-9 and this proves that the EDFA is applicable. The eye patterns in the result show that the signals are not distorted too much.

The Noise Figure refers to the maximum value because the large Noise figure values provide high noise of the EDFA. The gain flatness of the EDFA is as below:

= ±0.6953dB

CHAPTER 5

CONCLUSION & RECOMMENDATION

This Chapter will focus on the conclusion of the information contained in the report. This Chapter determine whether the objective is achieved.

5.1 Conclusion

In this report, the theory of the Erbium Doped Fiber Amplifier was described. Theory of parameters that influence the EDFA has been explained in detail. The nominal gain of the EDFA in this project is 36.3523dB. The gain flatness of the EDFA designed is ±0.695285dB and the maximum noise figure of the EDFA is 4.00012dB.

The typical gain of the EDFA is 30dB. However, the typical noise figure and gain flatness of an EDFA are 5.5dB and ±0.8dB. The EDFA designed in this project has been optimized from the typical specification as mentioned above. The gain was optimized 30 percent from the typical gain of EDFA. Whereas, the noise figure of EDFA optimized to 37.50 percent. The gain flatness was optimized in 15.06 percent.

5.2 Recommendation

The Erbium Doped Fiber Amplifier designed in this project can be optimized by further research in future.

Optisystem (Amplifier Edition) was being recommended to use as the simulator to optimize the Erbium Doped Fiber Amplifier. The simulator Optisystem (Amplifier Edition) is more professional in optimization of the Optical Amplifier.

The gain flatness of Erbium Doped Fiber Amplifier can be optimized by using Automatic Gain Control. The automatic gain control system is a feedback system which able adjusting the gain of amplifier. Thus this will maintain the gain flatness of the Erbium Doped Fiber Amplifier. This can be simulated by using the Simulator of Optisystem (Amplifier Edition). The Automatic Gain Control system is an electronic device thus it can be done in hardware.

The Optical Amplifier, EDFA can be designed with its own core diameter or Numerical Aperture to get the optimum performance in practical. The core diameter of Erbium Doped Fiber is function of Gain and Noise figure.

The designed EDFA was done by simulation; this project was recommended fabricated to hardware.