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The utilize of light as communication methods can date back to the distant past if we define the optical communication in broaden way(1). It started with the fire becon or smoke signal and reflecting mirrors. Moreover, in early 1880 Alexander Graham Bell discovers the transmission of speech using a light beam(2). However the modern fibre-optic communication started around 1970s when GaAs semiconductor laser was invented, since then it took many year of research, experiment and development until the first transatlantic cable using optical transmission. The TAT-8 cable was deployed between USA and EROPE in December 1988. Then legendary Russian writer Isaac Asimov made his comment. "Welcome everyone to this historic trans-Atlantic crossing, this maiden voyage across the sea one beam of light. Our worlds has grown small, and this cable, which can carry 40,000 calls at one time is a sign of the voracious demand for communication today .."(3)
The first and second generation light wave system was available is early and middle of 1980's, where the GaSa semiconductor lasers being used in the systems. It operated in wavelength region near 800 nm and 1.3 Âµm. Moreover the second generation systems manage to minimize the fibre loss below 1 dB/km with data rate of 1.7 GB/s and repeater spacing of 50km were available. In year of 1990 the fourth generation light wave system was invented with the use of erbium-doped fibre amplifiers (EDFA) and wavelength-d.1 Project aim and Objectives
1.2 The General Communication System
Optical fibre communication system concept is similar to any basic communication system. A block diagram of basic communication and optical communication system is shown in figure 1, the function of which is to express the signal from the information source over the transmission medium to the destination. The figure 2 in below shows the optical fibre communication system. In optical communication systems information source provide the electrical signal to transmitter then through the optical source in modulate to lightwave carrier. The main difference between these two communications was the transmission medium. In the basic communication systems used twisted pair cables, coaxial cable and the ionosphere, were in optical communication systems used only one medium to send information known as fibre-optic cables.
Figure 1 communication System (2)
Figure 2 Fibre Communication System(2)
1.3 Summary of the report
The chapter 1 describe the basic principals about communication systems and a general introduction to optical communication. Following the next chapter illustrate the characteristic of an optical fibre including attenuation and the dispersion.
2.0 Optical Fibre
This report discuss high data rate modulation format for DWDM systems. This requires the detail knowledge about type of optical fibre. Before discussing the system performance in modulation formats it is necessary to talk about the propagation and the properties of the fibre.
This chapter illustrate the basic principles of optical fibre propagation and different impairments which limit the transmission distance and bit rate.
1.2 Light propagation in optical fibre
The transmission of a light via a dielectric waveguide was first investigated in beginning of the twentieth century by "Hondros" and "Debye". The simple form of optical fibre presents as, consists of a cylindrical core of class covered by the much lower refractive index of glass known as the cladding. The cladding support the waveguide structure by reducing the radiation loss caused by the surrounded air (2). The geometric approaches limit the understanding of the light propagation in the fibre; hence the ray theory was the best example to explain the light propagation along the fibre more clearly than any other ways.
The refractive index of the dielectric medium conceders the propagation of light in the medium the refractive index of the medium can be define as the ratio of light in vacuum to the velocity of light in the medium(2). More over it is the same principle as the refraction can see when look in to the pond. The water in the pond, it has a higher refractive index than the air. When look in the pond through the shallow angle can see the reflection of the surrounding area, more over if look at straight down at the water can able to see the bottom of the pond. The figure 3 below shown the phenomenon theory above described.
Figure 3 Refraction of light ray (5)
Figure 2.1 shows interface between two refractive index of n1 and n2. The angle of incidence is the angle between the incident ray and the normal to the interface between the two media denoted by Î¸1 and the angle of refraction is denoted by Î¸2. The relationship between the angles and refractive indices are given by the Snell's Law, which state that:
n1 sin Î¸1 = n2 sin Î¸2 (2.1)
When the angle of incidence increases, angle of refraction also increases. If n1 greater than n2, then there will be a instant Î¸2 =Ï€/2. In case of higher values of Î¸1 there is no refracted ray, where by all the light energy from the incident ray will be reflected and this phenomenon know as total internal reflection (TIR) (6).
Figure 4 Propagation of light by total reflection (7)
The geometry concern with launching a light ray in to the optical fibre is shown in figure 5. Where clearly illustrate the refractive index of the core and claddings are n1 and n2 respectively. It's clearly indicating the ray I1 propagate with the angle of Î¸a in to the air-core interface before transmission to the core-cladding interface. Hence the ray will enter to the core cladding interface at an angle of Î¸c and will not be totally internally reflected. Where the next ray I2 has an angle greater than Î¸a is refracted in to cladding and then eventually lost by radiation. The both situation illustrate the figure 5. Therefore Î¸a is the maximum angle to the axis at which light may enter the fibre in order to propagate. This is referred to as the angle of acceptance of the fibre (2).
Figure 5 Angle of acceptance Î¸a when launching light in to the fibre (7)
2.2 Single Mode Fibre
The main intend to manufacturing of single mode fibre (SMF) was to eliminate the inter-modal dispersion found in the multi mode fibre. Single mode which has only one mode of light propagating through it, unlike multimode has many modes to transmitted through the fibre where each mode travel at different velocities and reaching the receiver at different time which lead to the inter-model dispersion. This resulted in poor signal quality at the receiver end and the limited the transmission distance. To overcome this substance graded index fibre design as the solution but didn't eliminate it completely.
Figure 6 Cross section of a Single Mode Fibre (8)
SMF has relatively small core diameter (8 to 10Âµm) comparing the multimode fibre. Due to relatively narrow diameter, which only one mode will propagates typically 1310 or 1550nm and also carries higher bandwidth than the multimode which requires a light source with a narrow spectral width. It gives a higher transmission rate up to 50 times more distance than the multimode. Because of it large carrying capacity and low intrinsic loss, SMF preferred for longer distance and the higher bandwidth application such as DWDM.
2.3 Linear and Non-linear effect of the Fibre
The heart of the optical communication is the optical fibre. When an optical fibre transmitting over a fibre, it's suffers from linear and nonlinear degrading. Those are the properties of the fibre. Optical loss or attenuation and chromatic dispersion are linear degrading effects; hence SPM (self-phase modulation), XPM (cross-phase modulation) and FWM (four wave mixing) are the nonlinear effect and also known as "kerr" effects (2).
2.3.1 Optical loss
Optical loss is an important parameter found in optical fibre. When optical signal is propagated over fibre is lost due to material absorption and Rayleigh scattering. The expression of fibre loss is given as following:
PT = P0 exp (-Î±L) (2.2.1)
Where Î± is called the attenuation constant; P0 is the optical signal power at the input of the fibre of length L; and PT is the transmitted power. Usually fibre optic loss is expressed in decibel per unit length (dB/km) following
Modern optical fibres exhibit attenuation constant, below 0.2 dB/km near 1.550 Âµm wavelength window. This is the window with the lowest attenuation, thus used mainly in today's optical communication systems. Rayleigh is a fundamental mechanism arising from random density fluctuation frozen into the fused silica during manufacture and it dominant in the short wavelengths. The solid line represent the typical shape observed in the 90's, the dashed represent the actual shape in figure 7 (1, 2, 9).
Figure 7 Measured loss profile of the SMF (10)
The wavelength selection of the optical communication is most important since it depend on the attenuation, dispersion and the other nonlinear properties of the fibre.
2.3.2 Chromatic Dispersion
Chromatic dispersion (CD) or intramodal dispersion is occurs in both single mode and the multimode fibre. It's occur when different colours of light travelling through the fibre with different speed. Hence this it has different velocities and they are arriving with the different time at the end of the fibre. This delay difference is called the differential group delay (DGD). This phenomenon leads the pulse broadening of the transmission light pulses and illustrate in figure 8. This phenomenon is indirectly affecting to the communication systems by reducing the data carrying capacity.
Figure 8 Pulse broadening due to chromatic dispersion (11)
Material dispersion and Waveguide dispersion are the two effects contribute to the total number of dispersion in the fibre. Material dispersion is the frequency dependent of the refractive index of the fibre and it's unable to change: hence waveguide dispersion depends on the refractive index profile of the fibre and it can e engineered to allow manufacture to design fibres with specific dispersion profile.
The maximum allowable dispersion Î”tmax is given in relation with the transmission but rate which given in equation (2.2.3).
This assures that the ISI will be reduced for this amount of pulse spread. More over it give server distortion to the pulse at the receiver(12). For the SMF standard pulse spread due to CD can be illustrate as,
Î”tmax = D(Î») Î”Î» L (2.2.4)
Where, D(Î») = Chromatic dispersion factor (ps/nm.km)
Î”Î» = Spectral width of the transmitter output (nm)
Î» = wavelength (nm)
L = fibre length (km)
The total transmission bit rate due to chromatic dispersion can be derived by above equation given below.
It is clear from the above eq (2.2.5) that the parameter which alter the bit rate of the signal due to chromatic dispersion are operating wavelength, the distance and spectral width. It prove that the characteristic of the chromatic dispersion can be investigate by varying the distance, bit rate and spectral width(12). As mention above chromatic dispersion arise due to material and waveguide dispersion: hence the both total chromatic dispersion can be expressed as
D(Î»)chrom = D(Î»)mat + D(Î»)wave (2.2.6)
Figure 9 Chromatic dispersion profile with both chromatic and waveguide dispersion
The dispersion for the SMF fibre operating at 1550nm is nearly 17ps/nm-km, but for calculation result the default value of 16.75ps/nm-km was used later for the investigation. In summery too much dispersion in a system is lead to system penalty and poor quality of the signal and also having zero dispersion in DWDM systems leads to system impairments from fibre non-linearities such as FWM. Dispersion compensating fibre (DCF), external modulation and soliton transmission are the suitable steps to manage the chromatic dispersion (11).
2.3.3 Self -Phase modulation (SPM) and Cross-Phase Modulation (XPM)
SPM and XPM are two most important nonlinear affects which originate from the intensity dependence of the refractive index.
SPM refers to the self induced phase shift experience by an optical field during its propagation in optical fibre. In DWDM systems SPM become crucial reach limiting nonlinear effects. It can be mitigation by lowering the optical power at the expanse of increased noise or the dispersion management, because dispersion can partly mitigate the SPM effect.
XPM refers to the nonlinear optical effect, when one wavelength of light can affect the phase of another co-propagating wavelength of light through the optical Karr effect. In contrast to other nonlinear effects XPM effect not involve in power transfer between signals. Increasing the wavelength spacing can lead the XPM reducing in the practical WDM systems, although XPM is negligible in standard SMF in 1550 nm with 0.8 channel spacing (10).
2.3.4 Four-Wave Mixing
Four-wave mixing (FWM) is a phenomenon which must avoid in the DWDM transmission systems and cannot be illuminating by optically or electrically. FWM increases with the length of the fibre. The process is occurring when two or more wavelength propagating over the same fibre and satisfies the phase matching condition. A number of new frequencies are generated due to interaction of two or more frequencies in FWM. If the propagating frequencies are similar therefore there are only two new wavelengths generated in the fibre, as shown in the figure 10. This phenomenon also known as the partially degenerate four-wave-mixing (PDFWM) (13).
Figure 10 Additional Frequencies generated through FWM (7)
In DWDM context FWM cause the cross talk between the channels (10). Nevertheless this process requires phase matching condition, this means the efficiency of FWM depends on dispersion and channel spacing. Hence high dispersion fibre and larger channel spacing are beneficial in DWDM systems.
The chapter 2 provide the reader a clear understanding about the optical fibres and how light propagating over the fibre via total internal reflection. Gradually this chapter details, about type of fibre and the reason to opted SMF. At the late discussion narrator focus into linear and non-linear degrading which are the properties of the fibre and how it will affect to the DWDM systems.
3.0 Wavelength Division Multiplexing
As we know over the last decades fibre optics became the infrastructure of the communication. Using the Time Division Multiplexing (TDM) technology, carriers routinely transmit information at 2.4 Gb/s on a single fibre. Then deploying some equipment it quadruples the rate to 10 Gb/s but the revolution of the bandwidth applications and the explosive growth of the internet it created capacity demand which exceed the traditional TDM links. Then to meet the specific demand Wavelength division multiplexing (WDM) technology introduced and it is become the backbone of the modern optical communication technology. The concept of this technology is to transmit number of different wavelength optical signals in parallel on a single optical fibre (9). The WDM begin in the late 1980s using two widely spaced wavelengths in the 1310nm and 1550 regions, called as the wideband WDM. Beginning of the second generation WDM in 1990s, introduced the narrowband WDM in which two to eight channels were used with 1550nm region using 400GHz between each channel. By the mid 1990s, dense WDM (DWDM) systems were emerging with 16 to 40 channels with spacing from 100 to 200 GHz. Once again in late 1990s developers extend the DWDM systems with 64-160 parallel channels, densely packed at 50 and even 25 GHz intervals named ultra DWDM. Figure 11 below illustrates the evolution of the DWDM since where it began.
Figure 11 Evolution of DWDM
Increases of number of channels which result of a density in DWDM technology have had a dramatic impact of the carrying capacity of the fibre. It's started in 1995 when 10 Gb/s systems were demonstrated, since than this technology developed communication system which has carrying capacity of 100 Gb/s and beyond.
Due to the widespread development of the SMF has encouraged the investigation of WDM on this transmission medium. The development of the WDM in SMF can be can be categorise in to Corse WDM (CWDM) and the Dense (DWDM). This two concepts use the same multiple-channel principle on a SMF, they differ in the channel spacing with they engage. CWDM uses wider channel spacing than DWDM; hence provide significantly fewer channels than the DWDM. According to the International Telecommunication Union (ITU) standards, recommendation for the CWDM channel architecture define in 1271nm and 1611 grid include 18 wavelengths with 20nm channel spacing although can reach distance of 40 to 80 km (9). The more detail about the factor will not be discuss as it is beyond the scope of this project.
3.1 Dense Wavelength Division Multiplexing (DWDM)
DWDM optical technology used to increase bandwidth over existing fibre backbone. Although it has been a known technology for several years, its early application was restricted to providing widely separated. As mention above DWDM originally concerned with optical signals multiplexed in the 1550nm wavelength region using number of channel with 20nm spacing between each of them, also it achieved much higher distance than the CWDM systems. After invention of the Erbium-Dope Fibre Amplifier (EDFA) DWDM technology became much more popular and low cost communication architecture system, because capability of the EDFA can transmit the wavelength to much higher distance without regenerating and therefore it reduce cost of the system which applied before.
Figure 12 Block schematic of a dense wavelength division multiplexing systems (7)
The Figure 12 above shows block schematics of a DWDM system where a large number of channels N, each utilize a single wavelength are multiplexed into a SMF transmission medium. EDFA deployment required for long-haul DWDM systems to prevent any optical signal power losses caused by the multiplexers and the other passive optical devises.
Moreover comparing with the CWDM transmission, DWDM systems are used narrow band width optical filter in the de-multiplexing process due to the narrow band width channel spacing requirement. Furthermore DWDM transmitters required temperature control laser sources to stabilize to the emitted signal wavelength from the each transmitter and also the large number of channel consumes a much higher power level (9).
3.1.1 Erbium Doped Fibre Amplifier
When talk about the DWDM systems, it is more important to spend little extra time on EDFA because it became the booster of the DWDM systems. Advent of the EDFA energizes the commercial development of DWDM systems by providing a way to amplify all the wavelengths at the same time. This optical amplification operation was done by process known as doping the Erbium ions into the core of the fibre. Optical pump lasers are use to pump high level energy in to the especial fibres, energizing the Erbium ions which then boost the weaker optical signal when passing through. Due to the atomic structure of the Erbium compound provide the amplification to the broad spectral range required for densely packed wavelengths operating in 1550nm region by optically boosting the DWDM signals. Instead of using the multiple regenerators, which required converting the optical signal to electronic to amplify the signal and then convert back again to optical signals. But the EDFA directly amplifies the optical signals: hence the composite optical signal travel up to 600kms without generation and up to 120kms between amplifiers in a commercially available terrestrial, DWDM systems (14). The figure 13 below shows the process where EDFA involve in DWDM systems.
Figure 13 Erbium-Doped Fibre Amplifier
3.2 Techniques for Multiplexing and De-Multiplexing
The following segment describes some of the technique that are used in the multiplexing and de-multiplexing of many wavelengths.
3.2.1 Multiplexing and De-Multiplexing Using A Prism
The simple way of multiplexing and de-multiplexing wavelength can be done using a prism shown in figure 12. It shows the de-multiplexing process of a multiple wavelengths exiting from the fibre cable. The first lenses receive the multiple lengths and diverges them to become parallel and incident on the prism surface. Then prism refracted the each component of the light when exits from the prism. This makes a rainbow effect when wavelengths are spreading from the prism. The spread wavelengths are refracted by different angles from the next wavelengths and this angle known as the angle of refraction. This angle depends of the several condition of the prism there are wavelength, apex angle and the refraction index. The second lens focuses each wavelength to the designated input via a fibre optic assembly. Using the same component can be used the reversed (multiplex) process by multiplex different wavelengths into one fibre assembly. Therefore this devise can use as bi-directional (7, 15).
Figure 14 Multiplexing and de-multiplexing of wavelength using a prism (7)
3.2.2 Multiplexing and De-Multiplexing Using A Diffraction Grating
Another technology based on the principle of diffraction uses a diffraction grating diffracting (15). When the light source is incident on a diffraction grating, each wavelength is diffracted to different angle to keep a different point in space. Then the diffracted wavelength are divert in to the lens where it focus the individual wavelength in to the fibres as shown in figure 13. The separate wavelengths can be combined into the same output port or single mixed input may be split into multiple outputs, one per wavelength. This device also used as a bi-directional device (15).
Figure 15 Multiplexing and de-multiplexing of wavelengths using a diffraction grating(7)
4.0 40-Gb/s Optical Modulation Format
Bit rate-distance or capacity of the system is a figure-of-merit of the lightwave system. To increase the capacity of the lightwave system the possible solution is increase the data rate of the channel with tighter channel space. 10 Gb/s and 40 Gb/s DWDM system become the next generation lightwave systems. In these DWDM systems linear and non linear impairments become severe. Optical modulation formats are the possible solution to defend from these impairments: hence a modulation format with narrow optical spectrum can enable closer channel spacing and tolerate more CD distortion and also modulation format with multiple signal levels will be more efficient than binary (4).
There are many modulation formats in the scope of the researching area, taking advantage of modulation and de-modulation. Most of the modulation s are binary signalling e.g. Duo-Binary, VSB/ SSB, RZ, Phase Shift Keying (PSK) and the others are multi level signalling e.g. Differential Quadra Phase Shift Keying (DQPSK) and M-PAM. It is not a possible task to cover up all the modulation in this report. However will detail and compare four modulations which often used in recent years. NRZ, CS-RZ, RZ-DPSK and RZ-DQPSK modulation will cover in detail in the next section. Although this not a inclusive showing of advance optical modulation, however the mechanism and the result in this report under each modulation format are still valuable can be used to extend for the future investigation. The following section will illustrate the generation and characteristic of the chosen advance modulation formats
4.1 Generation and Characteristic of the Modulation Formats
4.1.1 Non Return to Zero (NRZ) modulation format.
Non return to zero (NRZ) modulation has been the most dominant modulation format for several years in optical communication systems and widely used at 10 Gb/s DWDM systems. There are many reasons to use this modulation format early days of the fibre optic communication system. It's required less electrical bandwidth for the both transmitter and the receiver comparing with the RZ modulation format. Also comparing with the PSK, it's not sensitive to laser phase noise. The one last thing was simplest design for the transmitter and the receiver. However comparing with the recent designed modulation format NRZ may not be the best choice for high capacity DWDM systems. Nevertheless, the simplicity and the historic performance NRZ would be the good purpose of the comparison.
The block diagram of the NRZ transmitter shown in the figure 16 below, where the electrical signal is modulated with an external intensity modulator. The intensity modulator can be use either Mach-Zehnder type or electro-absorption type. It converts the electrical data signal to optical signal with the same data rate. The optical pulse width of digital "1" is equal to the inverse of data rate. A simple photo diode or an optical receiver can be used to detect the signal at the receiver. This method known as the direct detection (DD) and could be used for the other modulation formats. Comparing to the other modulation formats NRZ modulation has the most compact spectrum, but it's not imply that NRZ optical signal has a superior resistance to the residual chromatic dispersion in an amplified fibre system with dispersion compensation (16).
Figure 16 NRZ modulation scheme: (a) block diagram of NRZ transmitter (b) waveform for intensity (C) phase (17)
4.1.2 Carrier-Suppressed Return to Zero (CS-RZ)
Carrier-Suppressed Return to Zero (CS-RZ) is a pseudo-multilevel modulation format and was proposed by Miyamoto (18). When designing the CS-RZ modulation format transmitter required an additional clock and also optical signal has a Ï€ phase shift between adjacent bits when comparing with the NRZ modulation format. The figure 17 showed the schematic of the CS-RZ transmitter.
Figure 17 CS-RZ modulation scheme: (a) block diagram of CS-RZ transmitter (b) wave form of intensity (C) phase (17)
In the generation of a CS-RZ optical signal it's requires two MZ modulator as in figure 16, the first modulator encode the NRZ data. Then the generated NRZ optical signal is modulated by the second MZ to generate CS-RZ optical signal. The second MZ modulator operates at minimum transmission point and driven by a sinusoidal clock and the data rate is half compare to the input data rate of the electrical signal. CS-RZ has a better tolerance to fibre nonlinearity and higher receiver sensitivity than NRZ in recent research (17). In addition, carrier suppression reduces the efficiency of FWM in WDM systems.
4.1.3 Return to Zero Differential Phase Shift Keying RZ-DPSK
In order to improve the tolerance to nonlinear distortion and to achieve a longer distance transmission, RZ-DPSK has been proposed in year 2002. As a result of this propose a demonstration has been taken place in excess long-haul experimental demonstrations of DPSK transmission at rate of between 10 and 170.6 Gb/s before 2005 (9). Since then it's been the most often used modulation format in DWDM systems. This modulation format has a advance Tx design comparing to the NRZ and CS-RZ, where precoder take place after the input data signal and precode the date with a 1 bit delay. The block diagram of the RZ-DPSK transmitter is shown in figure 18 below.
Figure 18 RZ-DPSK modulation scheme: (a) block diagram of RZ-DPSK transmitter (b) wave form of intensity (C) phase (17)
First, the phase modulator generates the conventional NRZ-DPSK optical signal and then the generated optical signal is modulated by clock signal with half the data rate of the input signal. Because of the additional bit-synchronized intensity modulation, sometimes it called as the intensity modulated DPSK. This modulation required different receiver, which did not apply for the other modulation. One bit delay Mach-Zehnder Interferometer (MZI) or Delay Interferometer can used to design the DPSK optical receiver. When choosing the Interferometer's need to be considers weather design is frequency or wavelength reference. If the system is frequency reference MZI can use to implement to design the Rx, if not the system is wavelength reference, Delay interferometer (DI) is the ideal at the most cases. The figure 19 blow is shown the block diagram of the RZ-DPSK receiver.
Figure 19 RZ-DPSK transmitter
The one bit delay DI is used to correlate each bit with its neighbour and make the phase-to-intensity conversion. There are two outputs of DI, constructive port or destructive port. When two constructive bits are in phase, it added constructively to the DI and result in a high signal level, otherwise if it is a phase difference between the two bits they cancel with each other in the DI and result in a low signal level. The both constructive and destructive port is used in the practical DPSK receiver. The main advantage when using the DPSK, it has a 3 dB sensitivity than NRZ (17).
4.1.4 Return to Zero Differential Quadrature Phase Shift Keying RZ-DQPSK
RZ-DQPSK is a four level modulation format which recently make attention in optical communication researches today. It transmits four phase shifts (0, +Ï€/2, -Ï€/2, Ï€) at a symbol rate of half the aggregate bit-rate (17). The design stage of the RZ-DQPSK transmitter is shown in figure 20 below.
Figure 20 Figure 21 RZ-DQPSK modulation scheme: (a) block diagram of RZ-DQPSK transmitter (b) wave form of intensity (C) phase (17)
The design process of the transmitter contain two phase modulators and none intensity modulators. This transmitter design is the most complex design comparing with the all the other modulation which discussed above. Firstly the generated data precode by DPSK precoder and supply to the two phase modulator. Then the converted optical signal added to the intensity modulator where driven by the sinusoidal clock and the data rate is quarter compare to the input data rate of the electrical signal. The structure of this TX design can get the advantage of the near perfect Ï€ phase shift produced by the intensity modulator. Comparing with the RZ-DPSK spectrum, RZ-DQPSK has a more compact spectrum in frequency. The compact spectrum is beneficial for achieve high spectral efficiency in DWDM systems and also high tolerance to the CD. Moreover RZ-DQPSK also have a -3 dB Rx sensitivity compare to the NRZ.
The modulation scheme has a more complex receiver similar to the DPSK receiver. The block diagram illustrates the designing process of the RZ-DQPSK receiver. The receiver signal split into two paths, time delay with (2/bit rate) and phase shift of Ï€/4 is used. Then the output of the individual component feed in the star coupler and later feed aging in to the balance receiver
Figure 22 RZ-DQPSK receiver
5.0 Optical Nonlinearity Tolerance in Different modulation formats.
5.1 Nonlinearity tolerance to the 10 Gb/s systems
Figure 23 fibre input power vs Q penalty