Simulation of Dispersion Compensation in transmission systems Based chirp fiber bragg grating and symmetrical compensation method
H. Taslim1 and S. Shahi2*
1Biophotonics and laser in dentistry research center (BLDRC), Islamic Azad University, Khorasgan (Isfahan) Branch, Arghavanieh, Isfahan, Iran.
2*
Abstract. One of the basic causes of function limitations in optical transmission is dispersion that is the phenomenon in which the phase velocity of a wave depends on its frequency. Waveguides as an optical fiber are highly dispersive due to their geometry for optical frequencies widely used in modern telecommunications systems. The rate at which data can be transported on a single fiber is limited by pulse broadening due to chromatic/material dispersion among other phenomena as well as group velocity, waveguide and modal dispersion. Hence, in thispaper, we introduce the Chirp Fiber Bragg Grating as a key component and dispersion compensation fibers (DCFs) that both employed in symmetrical compensation method for compensate the dispertion in WDM system. By simulating transmission system can find the best numerical quantity of parameters for optimum outputs and its effect on improving gained information in the receiver that compare by different compensation components.
Keywords: Chirp Fiber Bragg Grating, Dispersion Compensation, Symmetrical Dispersion Compensation method.
- Introduction
Over the past 20 years, two major product revolutions have taken place due to the growth of the photonics, opto-electronics and the fiber optic communication industries.
The fiber optic communication industry has literally revolutionized the telecommunication industry by providing higher performance, more reliable telecommunication links with ever decreasing bandwidth cost [1].
Waveguide optical fiber is a di-electric cylinder in which the information on optic waves transfer. The main structure of optical fiber includes core & clad. The clad guides the light in core using total internal reflection method passing the fiber. Light will be affected by some factors such as attenuation, destruction or dispersion. This paper focus on dispersion in optical waveguides as a fiber and simulation of random dispersion compensation. According to the kind of fibers, there are two kinds of dispersion:
Mode dispersion and chromatic dispersion.
Telecommunication systems modulates the intensity of a light source in order to transmit information. Information is modulated and sent as a series of pulses representing binary encoded data. Data can be transmitted with few errors, as long as these pulses travel through the fiber without changing their shape. But usually, as they travel along the fiber, the pulses start to spread, loosing their original shape and overlap each other and so becoming indistinguishable at the receiver input. Dispersion is the general term applied to this cause. Dispersion was initially a problem when multimode step-index fiber, the first optical fibers, were introduced. Multimode graded-index fiber improved the situation, but even when they are well graded there are severe limitations to the information capacity of multimode fibers. It was single-mode fiber that eliminated multipath dispersion and left only intramodal dispersion and polarization mode dispersion to be dealt with by engineers. Intramodal dispersion may occur in multimode and single-mode index fibers, and represents the fact that different colors or wavelengths travel at different speeds, even within the same mode. Pulse broadening due to material dispersion results from the wavelength-dependent index of refraction in the bulk material, which makes up the fiber, as seen in Figure 1.[2].
Figure 1. Refractive index of pure fused silica[2].
The use of erbium doped fiber amplifiers (EDFA) in optical communication systems has made chromatic dispersion, the most significant limitation for the transmission performance since EDFAs compensate for he transmission losses. dispersion compensating fibers (DCF) and bulk type dispersion compensators have been widely used to compensate for the chromatic dispersion. In addition, the amount of compensation is wavelength dependent and can only be perfectly achieved in a relatively narrow band. Chirped fiber Bragg grating (FBG) has also been proposed recently for chromatic dispersion compensation and is recognized as a perfect alternative do DCF. Recently, symmetrical compensation is also suggested to random dispersion compensation fibers that more introduce in next parts[3].
- Chirp Fiber Bragg Grating
Photosensitivity of optical fiber was discovered at the Canadian cammunication research center in 1978 by Ken Hill [4]. Fiber Bragg gratings are spectral filters fabricated within segments of optical fiber. Like any other diffraction grating, they rely on the Bragg Effect (constructive interference from a diffraction grating) to reflect light over a narrow wavelength range and transmit all other wavelengths, but they also can be designed to have more complex spectral responses. Many uses exist for FBGs in today’s fiber communications systems that although they are complex to design, but we will briefly describe their properties. When an optical fiber is exposed to ultraviolet light, the fiber’s refractive index is changed; if the fiber is then heated or annealed for a few hours, the index changes can become permanent which this phenomenon is called photosensitivity. In germanium-doped single-mode fibers, the index differences between 10-3and 10-5 have been obtained. Using this effect, periodic diffraction gratings can be written in the core of an optical fiber. This was first achieved by interference between light propagating along the fiber and its own reflection from the fiber endface; this is known as the internal writing technique and the resulting gratings are known as Hill gratings. Gratings can also be written in the fiber core by irradiating the fiber through a phase mask with a periodic structure. These techniques can be used to write FBGs in the fiber core; such gratings reflect light in a narrow bandwidth centered around the Bragg wavelength, , which is given by
(2.1)
Where (Fig 2) is the spatial period, or pitch of the periodic index variations and is the effective refractive index for light propagating in the fiber core [5-6].
Figure 2. Schematic illustration of a fiber grating. Dark and light shaded regions within the fiber core show periodic variations of the refractive index[6].
One of the most interesting Bragg grating structures with immediate applications in telecommunications is the chirped Bragg grating. This type of grating has a monotonically varying period, as illustrated schematically in Fig 3 [7].
Chirped fiber gratings have a relatively broad stop band and were proposed for dispersion compensation as early as 1987. The optical period in a chirped grating is not constant but changes over its length. Since the Bragg wavelength also varies along the grating length, different frequency components of an incident optical pulse are reflected at different points, depending on where the Bragg condition is satisfied locally. In essence, the stop band of a chirped fiber grating results from over lapping of many mini stop bands, each shifted as the Bragg wavelength shifts along the grating. The resulting stop band can be as wide as a few nanometers. It is easy to understand the operation of a chirped fiber grating from Fig 4, where the low-frequency components of a pulse are delayed more because of increasing optical period (and the Bragg wavelength). This situation corresponds to anomalous GVD. The same grating can also provide normal GVD if it is flipped (or if the light is incident from the right). Thus, the optical period of the grating should decrease for it to provide normal GVD. From this simple picture, the dispersion parameter of a chirped grating of length can be determined by using the relation , where is the round-trip time inside the grating and is the difference in the Bragg wavelengths at the two ends of the grating. Since , the grating dispersion is given by a remarkably simple expression,
(2-2)
Some changes happen in the chirp fiber Bragg Grating during the grating as the Grating periodicity changes along the shaft, different wavelengths are reflected by different grating and consequently they delay in different time interval. Final effect of a compaction is in incident pulse which can be suitable for compensation of random dispersion that stored in transmission link. The most important advantages of chirp fiber Bragg grating are internal loss, nonlinear effects and low cost [8].
Figure 3. Schematic diagram of a chirped grating with an aperiodic pitch. For forward-propagating light as shown, long wavelengths travel further into the grating than shorter wavelengths before being reflected. (b) Schematic diagram of a cascade of several gratings with increasing period that are used to simulate long, chirped gratings[7].
Figure 4. Dispersion compensation by a linearly chirped fiber grating: (a) index profile n(z) along the grating length; (b) reflection of low and high frequencies at different locations within the grating because of variations in the Bragg wavelength[8].
The chief problem in FBG function is Fresnel reflection which is both reflection and refraction that happens to the light where it transfers from one one medium to another with different refractive index (Fig 5).
Figure 5. FBG spectral behaviour[6].
3. Dispersion Compensation Fibers (DCFs)
In recent years, there has been considerable work on dispersion-compensating fibers (DCFs), which are being used extensively for upgrading the installed 1310 nm optimized optical fiber links for operation at 1550 nm.
Now, in many countries, the tens of millions kilometers of CSFs already exist in the underground ducts operating at . One could increase the transmission capacity by operating these fibers at 1550 nm and using WDM techniques in optical amplifiers. But, there will be significant residual (positive) dispersion, too. On the other hand, replacing these fibers by DSFs would involve huge costs. Hence, in recent years, there has been considerable work in upgrading the installed 1310 nm optimized optical fiber links for operation at 1550 nm. This is achieved by developing fibers with very large negative dispersion coefficients, a few hundred meters to a kilometer, which can be used to compensate for dispersion over tens of kilometers of the fiber in the link. Compensation of dispersion at a wavelength around 1550 nm in a 1310 nm optimized single-mode fiber can be achieved by specially designed fibers whose dispersion coefficient (D) is negative and large at 1550 nm. These types of fibers are known as DCFs. Since the DCF has to be added on to an existing fiber optic limit, it would increase the total loss of the system and, hence, would propose problems in detection at the end. The length of the DCF required for compensation of dispersion can be reduced by having fibers with very large negative dispersion coefficients. Thus, there has been considerable research effort to achieve DCFs with very large (negative) dispersion coefficients [9]. In next parts, we consider some simulation results for reduce the dispersion in communication links.
4. Simulation results
The experimental setup that designed in figure 6 by optisystem software is contain of Pseudo-Random Bit Sequence Generator, CW Laser, NRZ Pulse Generator, Mach-Zehnder Modulator, Optical Fiber, Optical Time Domain Visualizer, Optical Amplifier, Fiber Bragg Grating, Optical Power Meter, Eye Diagram Analyzer and Photodetector PIN for propose the transmitter external modulated laser by using optical and electrical visualizers. The following parameters of fiber and grating in table 1 are used also to better simulation of transfer system
Figure 6. Simulation of total transfer system.
Table1. Fiber Parameters
Name / ValueLength / 10 Km
Attenuation / 0.2 dB/Km
Dispersion / 17 ps/nm/Km
Dispersion Slope / 0.05 ps/nm2/Km
Table 2. FBG Parameters
Name / ValueFrequency / 193.1 THz
Effective Index / 1.45
Length / 5 mm
Chirp Function / Linear
Linear Parameter / 0.0001 m
Apodization Function / Tanh
Tanh Parameter / 2
In addition, we emloyed CW laser diode in frequency of 193.1THz and 1mW power which is modulated by a Pseudo random bit sequence generator in 10Gbits/s by a Mach-Zehnder modulator with extinction ratio of 30dB. The Erbiume amplifier in this system has gain around 6dB and can be ignored without any destractive noise which is used only for dispersion compensation and nonlinear effects in designed system.
By simulating of transferring system (Fig 6) and depicting the Eye diagram of this system, it can be understood that the system has a lot of dispersion and attenuations (Figs 7- 9).
Figure 7. Eye diagram for the related design with dispersion.
Figure 8. Out put power results from the optical spectrum Analyzer.
Figure 9. Electrical Power Meter Visulizer
The output figures demonstrated clear dispersion factor that after crossing the signal of fiber, causes disforming the primary information. As a result, a tool should be used to recover the dispersion and renew the primary signal shape.
Firstly, the chirp Bragg Grating is used to recover dispersion and the system is simulated as illustrated in Figure 10.
Figure 10. Simulated system by chirp fiber Bragg grating.
By employing the chirp fiber Bragg Grating in this system, the output results of dispersion compensation will be like as Figure 11 that the pulse figure renewed greatly but the peak power of signal is lower than figure 8 that is a problem in system.
Figure 11. Eye diagram for the design by using chirp grating.
In next design, we replaced the system by symmetrical dispersion compensation fiber in transmission system that shown in Figure 17. Summerize description on Pre, Post and symmetrical compensation by DCF and symmetrical compensation configurations are shown in Figs. 12, 13 and 14.
Figure 12. Dispersion pre-compensation.
Figure 13. Dispersion post-compensation.
In post compensation case, DCF is placed after SMF whereas in precompensation case it is inserted before SMF. In symmetrical compensation case, fiber placement follows the sequence of DCF, SMF, SMF, DCF.
Figure 14. Dispersion symmetrical compensation.
Simulation results based Q factor of received signal verses transmitted signal power for these three schemes at 2.5 and 10 Gb/s bit rates are shown in Figs. 15 and 16. These figures are obtained by sweeping the power from –12 dBm to 12 dBm and using the software’s Graph Builder. From these figures, we can receive the best performance is obtained by using symmetrical dispersion compensation. The worst case is dispersion pre-compensation. Post-compensation gives a result as well as symmetrical compensation. This results confirmed by the eye diagrams given in Fig. 16 [10,11].