This is to certify that this thesis entitled "EFFECT OF XPM AND FWM ON THE PERFORMANCE OF M-ary WSK-DWDM TRANSMISSION SYSTEM" is the result of my study for partial fulfillment of M.Sc. High data rates as well as long spans between amplifiers in a chain require high optical power per channel to meet signal-to-noise ratio (SNR) requirements. For DWDM systems with long repeaterless spans, the simultaneous requirements of high emitted power and low dispersion fibers lead to the generation of new waves by four-wave mixing (FWM) which limits the allowable power input to the fiber and this limits the transmission distance.
In this thesis work, performance analysis is performed for a DWDM system with M-ary walength shift keying (M-WSK) modulation taking into account the effect of Xl'M &FWM in an SMF. Analysis is performed to find out the expression for the signal photocurrent, shot noise due to signal and FWM effect, and beat noise components due to signal and FWM sailing during the photodetection process. Performance results are evaluated in terms of BER for different fiber length, input power and number of DWDM channels.
Significant amount of coding gain as well as increase in allowable input power is obtained as a result of rate-l/2 convolutional coding compared to the system without coding. The results of this thesis will find application in the design of a DWDM system in the presence of FWM.
INTRODUCTION I
- Types of optical communication
- Types of optical fibers
- Optical Sources
- Optical Detectors II
- Objectives of this thesis
- Contribution of this Work
- Organization of the Thesis
An optical communication system in which the carrier is an optical carrier from the optical frequency of the electromagnetic spectrum. This is the first function of the data conversion unit and a specific encoder is used for this. The third function of the data conversion unit is to reformat the electrical format of the data.
Thus, a buffer can also serve to reshape the electricity from the logic input signal. The main function of a photodetector is to convert an optical information signal back into an electrical signal {photocurrent). The photodetector in today's fiber optic communication systems is a semiconductor photodiode (PD). This miniature device is usually manufactured together with electrical circuitry to form an integrated package that provides power supply connections and signal amplification. These carriers are separated from the influence of the electric field in the inner region and represent a reverse diode current that can be amplified.
An analysis is performed for the BER performance of the system with XPM & FWM and extended to a system with convolution coding. In Chapter 4 we perform numerical calculations to evaluate the performance results of the systems described in Chapter 3.
MULTIPLEXING SCHEME AND NONLINEARITIES 15
Multichannel Communication systems
- Wavelength Division Multiplexing (WDM) 15
- Optical Time-division multiplexing (0-TDM)
- Optical"CDMA (O-CDMA)
- Sub carrier multiplexing (SCM)
WDM is expected to be one of the methods of choice for future ultra-high bandwidth multichannel systems. As previously explained, WDM makes use of a significant portion of the available fiber bandwidth by allowing the simultaneous transmission of many independent signals over a single fiber, each signal being at a different wavelength. Routing and detection of these signals can be achieved independently, with the wavelength defining the communication path by acting as a source, destination or routing signature address.
Al and transmitting data simultaneously in the opposite direction at a wavelength of 1.2• This means that information can be transmitted in both directions simultaneously. CWDM and DWDM technology are based on the reasonable concept of using multiple wavelengths of light in a single fiber, but the two technologies differ in the separation of wavelengths, the number of channels, and the ability to amplify signals in the optical space. Before the relatively recent lTU standardization of the term, a common understanding of thick WDM meant two (or possibly more) multiplexed signals on a single fiber, where one signal was in the l550-nm band and the other in the 13I0-nm band .
EDF As can amplify any optical signal within their operating range, regardless of the modulated bit rate. EDFAs therefore allow a single-channel optical link to be upgraded in bit rate by replacing only equipment at the ends of the link, while retaining the existing EDF A or series of EDF As along a long-distance route. An alternative strategy to increase the bit rate of digital fiber optic systems beyond the bandwidth capabilities of the drive electronics is known as optical time division multiplexing (aTDM).” Figure 2.3 shows a block diagram of an aTOM system that has demonstrated a transmission rate of 16 Gbps over 8 krn.
All channels, each with a different code, can be transmitted over the same fiber and demliltip1exed asynchronously. Code Division Multiple Access (CDMA) is a form of multiplexing (not a modulation scheme) and multiple access method that does not divide the channel by time (as in TDMA) or frequency (as in FDMA), but instead encodes data with a special code associated with each channel, and uses the constructive interference properties of special codes to perform multiplexing. Provided that the user signals are well synchronized in terms of symbol timing, the orthogonality of the spreading sequences ensures that there is no mutual interference between users.
This means that it concerns the number of users that can be accommodated on a particular channel. When the bandwidth of the information becomes higher, such as more than several GHz, and the transmission distance is very long, such as more than hundreds of kilometers, the DSB scheme will not work if it still only uses a simple photon detector to detect. This is because the dispersion of the fiber will give a very different delay to + if and - if due to the large frequency difference between them.
Fiber Nonlinearities 24
- Cross Phase Modulation (XPM)
- Four Wave Mixing (FWM)
Subcarrier demodulation is the optical filtering of each subcarrier to avoid the effect of fiber dispersion on the dual sideband modulation format. The optical spectra of these systems are very similar to the spectrum shown in Figure a. Optical Single Sided Band (OSSB) SCM Spectrum The lower sideband in the DSB spectrum is removed by means such as an optical filter or special modulation methods.
Cross-phase modulation (XPM) is very similar to SPM, except that it involves two light pulses, while SPM only requires one pulse. In multi-channel WDM systems, all other interfering channels also modulate the refractive index of the channel in question, and thus its phase. In XPM, two pulses pass through the fiber, each changing the refractive index as the optical power varies.
Four-wave mixing (FWM) is a nonlinear process in optical fibers where typically three signal frequencies combine and produce different mixing products. It arises from the weak dependence of the refractive index of the fiber on the intensity of the optical wave propagating along the fiber via the third-order nonlinear sensitivity. If there are three signal waves. From the table above, we can easily see that for each element in the last 6 rows, there is an additional repeat.
To eliminate this problem of calculating the total lights produced by FWM we developed an equation which gives the desired number of FWM lights without any error. If we put this value in equation no. 1, we get the total number of FWM lights produced as 24 which verifies the validity of our equation. The equation is also verified for different numbers of channels by a program coded in C++.
The FWM signal power for the general case of the WSK -WDM system can be found as [20]-. Very low signal-to-noise ratio for satellite communications due to high path loss and limited downlink transmission power. To optimize the use of the debugging capacity of a given code, soft decision is always a good solution.
Convolution Coding
PERFORMANCE ANALYSIS 31
System Model
The difference of the two photocurrents is applied to the amplifier followed by an equalizer. The equalizer is required to equalize the pulse shape distortion caused by the photodetector capacitance and due to the input resistance and capacitance of the amplifier. Receiver design is simple and less expensive due to the absence of the sophisticated wideband IF circuits.
As a result, the 'mark' and 'gap' appear at the two output ports on the OFD. In conventional WDM system, the channels are typically equally spaced, and this choice causes a large number of FWM products in the system bandwidth that can decrease at the channel frequencies. The total number of FWM products can be calculated from the following equation. If we assume that the input signals are not depleted by the generation of mixing products, the effect of the new optical signal generated at frequency fijk leaving the fiber is given by [20].
A multi-channel system, the nonlinear phase shift of the signal at the center wavelength A; described by [23],. The bit error rate (BER) performance, sensitivities and penalties are calculated as functions of the respective receiver and input parameter for the OOK and M"ary WSK-DWDM direct detection schemes. The nominal values of the parameters used during the calculation are given in the table 4.1.
First, the bit error rate performance was calculated for different encoded and uncoded systems, wavelength and optical bandwidth as parameters and plotted against the average signal input power per channel. Keeping the other parameters constant, we compare the performance of the system with and without encryption (convolution). The graph shows that with a constant channel, more input power is required for an uncoded system than for an encoded system.
It is observed that the WSK-WDM system suffers almost the same amount of power penalty for lower values of input power at constant BER. But at higher input power values, the amount of power penalty increases as channel spacing increases. Significant amount of coding gain as well as increase in allowed input power are obtained due to rate-1I2 convolution coding compared to the system without coding.
Taher, "Minimization of the effect of cross-phase modulation in a WDM optical transmission system", M.Sc.
