30

CHAPTER 2

CHARACTERIZATION AND ANALYSIS OF OOK AND

DPSK OCDMA SYSTEMS

Section 2.1 discusses the optimization of OSNR by properly

selecting the power level and spectral characteristics of optical source for the

given SSMF fiber attenuation and dispersion characteristics are discussed.

Considering the fiber dispersions such as chromatic dispersion and

polarization mode dispersion and nonlinearities of the fiber such as self phase

modulation (SPM), cross phase modulation (XPM) and four wave mixing

(FWM), the techniques to overcome these effects, EDFA fiber amplifiers and

DCF compensation are included for the desired performance of the OCDMA

system. Dispersion compensation techniques for the single mode fiber are

suggested, which leads to the optimum channel and source conditions to

maximize the OSNR.

The section 2.2 and 2.3 present the issues for mitigation of MAI,

noise, E/D using Super Structured Fiber Bragg Grating (SSFBG), detection

and thresholding schemes for OOK OCDMA and DPSK OCDMA

respectively. Optical and decision thresholding techniques and the

corresponding receiver structures to optimize the BER performance with

varying number of active users and the complexity involved are discussed.

The DPSK OCDMA system with balanced detection and OOK OCDMA

system with power detection are compared. The superior BER performance,

higher receiver sensitivity, merits and demerits of DPSK OCDMA are

presented.

31

2.1 OPTICAL SOURCE AND FIBER CHARACTERIZATION

Dispersion and attenuation are the signal degradation effects

observed in all optical fibers. Dispersion is compensated using dispersion

compensating fiber (DCF) and attenuation is equalized by the erbium doped

fiber amplifiers (EDFA). Various qualitative analyses are carried out in the

simulation for the applications of OOK and DPSK OCDMA systems.

2.1.1 Optical Source and Parameter Selection

Optical fiber communication systems often use semiconductor

optical sources, light emitting diodes (LED) and semiconductor laser diodes

(LD) because of the several inherent advantages offered by them. Some of the

advantages are compact size, high efficiency, good reliability, right

wavelength range, small emissive area compatible with fiber core dimensions

and possibility of direct modulation at relatively high frequencies. The

semiconductor materials with direct band gap energies such as In GaAsP, In P

and GaAs are used for making optical sources. LED and LD are the suitable

optical sources since they have adequate output power for wide range of

applications and the output can be directly modulated resulting in high

efficiency. Surface emitting LEDs operate as Lamberdian source with beam

divergence of 120 in each direction. Edge emitting LEDs have a divergence

of only about 30. The spectral emission width of 40 nm of LED make it

unsuitable for high bit rate system due to the high dispersion caused in the

fiber. In spite of a relatively low output power and a low bandwidth of LEDs

compared to lasers, LEDs are useful for low cost applications with data

transmission at a bit rate of less than 10Mb/s over a few kilometers. Multiple

quantum well (MQW) structures emit light at different wavelengths resulting

in broader spectrum up to 500 nm and are useful for local area WDM

networks. On the other hand laser diodes are coherent light source used to

32

couple sufficiently high power into multimode fiber (MMF) or single mode

fiber (SMF). They are used for 100 Gb/s and more data rate applications.

Narrow spectral emission width, high coupling efficiency and high output

power level are the advantages of LD for high speed application.

Bin Ni and James S. Lehnert (2005) analyzed the performance of

an incoherent temporal spreading OCDMA system with broadband light

sources. Dividing the impact of the non ideal light sources on the system

performances, they had shown that the optimal range for the spreading code

length when the available optical bandwidth and data rate are fixed. Thermal

noise effects on the source were not analyzed. Georg Clarici (2007) presented

the analytical model of laser diode for high speed applications. Simulation of

single mode lasers reproduced all key characteristics for the analysis. Other

types of laser diodes were not analyzed for compression.

Martin Rochette et al (2005) evaluated the upper limit of the

spectral efficiency of OCDMA systems with coherent sources. Spectral

efficiency of 2.24x10-2

b/s/Hz was achieved with a maximum BER of 10-10

in

the direct sequence and phase encoded OCDMA systems. The maximum

spectral efficiency of OCDMA systems with coherent sources was at least a

factor of 5 higher than the OCDMA systems with incoherent sources.

However, the spectral efficiency of systems with incoherent sources

decreased with increasing number of users.

Broader spectral width reduces the spectral efficiency and at higher

power levels the fiber nonlinearities degrade the system performance. To

improve the system performance it is necessary to optimize the source

characteristics.

33

The coherent light source is selected so as to support high data rates

and to provide high spectral efficiency. LD operating in continuous wave is

hereafter referred to as (CW) laser and is the suitable optical source for

OCDMA. The CW laser with narrow spectral emission width of 1nm, line

width of 0.1 MHz and emission power level of 0 dBm at 1551.7 nm is shown

in Figure 2.1.

Figure 2.1 Optical Emission Spectrum of CW Laser

The emission spectrum is measured at resolution bandwidth of

0.01 nm and all polarization set to zero degrees. It is quite suitable for

OCDMA applications and to overcome MAI. After phase modulation using

LiNbO3 - MZM the power level is reduced to 3.290 dBm, which is taken as

the transmitted power for the user. However in the simulation, the power level

is adjusted using variable optical attenuators and EDFA; operating

frequencies are varied in the range from 1540 to 1560 nm for multiple users.

34

2.1.2 Optical Fiber and Parameter Selection

Optical fiber exhibits the least and uniform attenuation in the wide

range of spectrum used for optical communication.

The parameters of the standard single mode fiber (SSMF) used in

all the simulation are as follows

Attenuation of 0.2 dB/km

Wavelength range of 1400 to 1600 nm

Dispersion of 17 ps/nm/km

Dispersion slope of 0.08 ps/nm2/km,

Differential group delay of 3 ps/km and

Allowable nonlinear shift of 5 mrad

SSMF offers a wide bandwidth and data rate in range of tens of

THz. However the possible transmission data rate is reduced due to the

limitations of electronics. Under these conditions, the pulse spreading due to

dispersion mechanisms and fiber nonlinearities degrade the system

performance in the upper band. Chromatic dispersion varies due to the

exponentially decreasing nature of refractive index of the fiber core with

increasing wavelength of operation. Higher wavelength components travel

faster along the fiber leading to pulse spreading in the time domain. In the

waveguide dispersion, the effective refractive index of the fiber varies and the

effective refractive index is inversely proportional to the wavelength of

operation. Because of the material and waveguide dispersion effects the total

dispersion varies from 1 to 20ps/nm-km with zero dispersion around

1310 nm. The fiber nonlinear terms causes frequency mixing as four wave

mixing (FWM), self phase modulation (SPM), cross phase modulation

35

(CPM). In the presence of large electric field, this becomes significant and

generates a new wave introducing cross talk. Stimulated Brillouin Scattering

(SBS) depends on the source power and line width. By proper selection of

optical coherent source characteristics, fiber channel characteristics, data rate

and launched power; the fiber dispersion and nonlinear effects are minimized

within the tolerable level for the given fiber span.

Distributed nonlinearities of optical fibers and OSNR limit the

communication capacity. It was analyzed by Andreas D. Ellis et al (2010) and

suggested techniques to improve the capacity. The techniques are

(i) compensation of intra-channel nonlinearity either through link design or

signal processing, (ii) optimization of OSNR through careful link design and

phase sensitive amplifiers with spacing optimization. However capacity

demands in access network cannot be met with WDM or dense WDM

techniques. Ansgar Steinkamp and Edgar Voges, (2007) analyzed the

influence of polarization dependent losses (PDL) on the statistics of PMD and

statistical interdependencies between first and second order PMD. However,

the effects of PDL are minimal for short haul OCDMA.

All optical data format conversion to and from DPSK were

proposed and numerically demonstrated by Jian Wang et al (2008).

Multicasting, multi channel and ultrahigh speed (160 GB/s) format

conversions were also demonstrated by simulation. However, the deviation to

the actual application was not discussed.Chromatic dispersion limited by the

hybrid ASK-DPSK modulation format were studied by Jian Zhao et el (2007)

for enhancing the transmission reach, also shown that electronic equalization

of ASK and DPSK separately did not improve the CD tolerance of ASK-

DPSK signal. Other modulation formats were not discussed. Comparative

study of the Shannon channel capacity was presented for dispersion free,

36

constant dispersion and variable dispersion of fiber by Jau Tang (2006).

Different approaches were only approximated to the input power levels.

Hofmann et al (2008) worked on WDM PON for higher bandwidth

of operation in the 1550 nm range with a vertical cavity surface emitting laser

arrays. Channel bandwidth of 10 Gb/s over 20 km of SSMF was

demonstrated. However, Laser arrays and maximum bandwidth of 80 Gb/s

were the limiting factors that are not addressed in detail.

Optical signal to noise ratio is to be optimized for the link losses

and distortion using EDFA and DCF. The characteristics and the number of

EDFA and DCF are selected based on the gain and dispersion compensation

respectively for the desired link length of SSMF. Using dispersion shifted

fiber (DSF) the dispersion can be made zero at 1550 nm with suitable

negative or positive dispersion. The same effect is obtained with dispersion

flattened fiber at 1550 nm but provides uniformly constant dispersion from

1300 to 1550 nm.

2.1.2.1 Compensation using fiber Bragg gratings for Single Pulse

Dispersion not only broadens the signal pulse but also reduces the

peak power as shown the Figure 2.2 (a-d). For the bit sequence

0000000100000000, the bit rate of 40 Gb/s and launched power of 0 dBm into

fiber are considered for the analysis. The Gaussian pulse with peak power of

0 dBm at 199 ps and width of the pulse being 55 ps at -60 dBm is launched

into the fiber as shown in the Figure 2.2 (a). The peak power of -26 dBm

centered at 1552.83 nm with the spectral width of 10 nm at -60 dBm of the

spectrum is shown in Figure 2.2 (b). Figure 2.2 (c) shows the dispersion

effects experimented with 10kms length of SSMF. The compensated signal

using FBG is shown in Figure 2.2 (e) and 2.2(f). Launched spectrum itself has

37

the power level of signal 26 dBm as in Figure 2.2(b). The power loss of

2dBm shown in the spectrum is observed after 10 kms of transmission as in

Figure 2(d) for the fiber attenuation of 0.2 dB/km spectral loss was not

compensated by the FBG as in Figure 2.2(f). Launched spectrum itself has the

power level of signal 26dBm as in Figure 2.2(b). The power loss of 2 dBm

shown in the spectrum is observed after 10kms of transmission as in

Figure 2.2(d) for the fiber attenuation of 0.2dB/km.spectral loss was not

compensated by the FBG as in Figure 2.2(f).

(a) Waveform (b) Spectrum

Figure 2.2 (a-b) Launched pulse in time and frequency

38

(c) Waveform (d) Spectrum

Figure 2.2 (c-d) Dispersion effects for 10 km of fiber in time and

frequency

(e) Waveform (f) Spectrum

Figure 2.2 (e-f) Dispersion compensation using FBG

39

FBG has many disadvantages compared to DCF in terms of

complexity involved in making the gratings and stability of operation. Further

the FBG introduces on an average 20dBm of noise in the compensated signal.

DCF characteristics are incorporated during the fiber fabrication process with

the required dispersion coefficient and attenuation for a given length of the

fiber. In the following discussions of the sections of 2.1 the spectral

characteristics not provided. Wavelength drift is not reflected in the spectrum

due to dispersion. Due to the time shift introduced by FBG, the dispersion

compensation left shifts the pulse.

Bette et al (2008) demonstrated the wavelength dependencies of

CD and differential group delay (DGD) to the fiber birefringence value and

derived the gratings. However, did not provide the DGD modeling as it was

complex.

2.1.2.2 Compensation using fiber Bragg Gratings for multiple pulses

Scheme for enhancing the thermal sensitivity of the FBG was

discussed by Budiman Dabarsyah et al (2007). Tuning of group velocity

dispersion (GVD) dispersion slope and wavelength of operation were done by

controlling the temperature distribution of the uniform fiber Bragg grating

(FBG). Dispersion slope was tuned from -8.88 to-24.44 ps/nm2 with the

center wavelength at 1554.5 nm. However, the requirement of circulator and

the changes of GVD as dispersion slope is tuned are the disadvantages of the

scheme.

Similar analysis but with the bit Sequence 0010000110010001 at

the bit rate of 40 Gb/s and 0dBm of power level is carried out. Figures 2.3(a),

2.3(b) and 2.3(c) show the launched, distorted and recovered pulse sequence

after compensation respectively.

40

Figure 2.3 (a) Launched pulse into the fiber

Figure 2.3 (b) Distorted pulse after 10 km of fiber length

41

Figure 2.3 (c) Recovered pulse at the end of 10 km

It is observed that the bits are not properly detectable. Worst

situation occur for 100 km of fiber even with periodically compensating for

dispersion using FBG. Therefore using dispersion compensation unit (DCU)

dispersion compensation is effectively carried out.

2.1.2.3 Compensation using DCF for multiple pulses

To compensate for the attenuation and dispersion so as to maintain

the required OSNR and PCR at the receiver front end, an erbium doped fiber

amplifier (EDFA) and dispersion compensating fiber (DCF) are used. Due to

the performance limitations, single EDFA and DCF cannot serve the purpose.

Therefore the following parameters of EDFA and DCF are selected for the

link with two spans each of 50 kms. Each EDFA in Figure 2.4 provides a gain

of 11.5 dB and with a total gain of 23 dB. The parameters of DCF are as

follows

42

Length of the DCF is 5kms

Attenuation is 0.3dB/km

Dispersion Compensation is 850ps/nm/km

DGD is 0.2 ps/km

Each Span of 50 km consists of one DCF and one EDFA as shown

in Figure 2.4 Two such DCF are used in this scheme and provides a total

dispersion of 170 ps/nm/km.

Figure 2.4 Symmetrical Dispersion Compensation for a span

of 50 km

Optical signal to noise ratio is to be optimized for the link losses

and distortion using EDFA and DCF. The characteristics and the number of

EDFA and DCF are selected based on the gain and dispersion compensation

respectively for the desired link length of SSMF. Using dispersion shifted

fiber, the dispersion can be made zero at 1550 nm with suitable negative or

positive dispersion. The same are the effect with dispersion flattened fiber at

1550 nm but provides uniformly constant dispersion from 1300 to 1550 nm.

For the bit sequence 0010101101001000 at the data rate of 10 Gb/s

and with power level of 10dBm is carried out. The launched pulse into the

fiber is shown in Figure 2.5, the broadened pulse is compensated using DCF

and amplified using EDFA after10 km with overlapping for consecutive 1s is

shown in Figure 2.6. The recovered pulse with compensation and

amplification after the link span of 10 km is given in Figure 2.7.

43

Figure 2.5 The launched pulse into the fiber, clearly distinguishable

at 0 dBm

Figure 2.6 Broadened pulse with overlapping after10 km of fiber

44

The pulse is not distinguishable at 0 dBm as seen from

Figure 2.7 and even at a higher power level (5 dB) too it is barely

distinguishable.

Figure 2.7 Recovered pulse at the end of 10 km

With successive 1s and at higher bit rate, the compensation using

FBG fails to recover the launched pulse as shown in Figures 2.3 (b) and 2.3

(c). There is a difference in power level of 1 dBm. The peak to peak noise

levels were reduced from -12 dBm to -6 dBm as shown in Figures 2.6 and 2.7.

DCF and EDFA for each of the two spans are called as dispersion

compensation unit (DCU). Using the DCU at the transmitting end of the fiber

is pre compensation, at the end of the fiber is post compensation and at the

mid of the total length of the link is symmetrical compensation. Simulation at

different locations of DCU in the fiber is shown in Figure 2.8 for two power

levels and data rates.

45

Figure 2.8 Comparison of eye diagrams for different data rates and

power levels

For each DCU and fiber spans of 50 km which forms a subsystem,

the parameters and the responses are analyzed for the bit sequence

0010101101001000 with 10 Gb/s. The DCF parameters are: 5 km of length,

0.3dB/km of attenuation and dispersion compensation of 170 ps/nm/km with

dispersion slope of 0.11 ps/nm2/km. DGD is taken as 0.2 ps/km. The

parameters of EDFA in the DCU are: gain of 14 dB, noise figure of 4 dB, and

wide noise bandwidth of 13 THz. The launched pulse after modulation is

shown in Figure 2.9 (a). Compensated pulse after the first and second span by

the DCU is shown in Figure 2.9 (b) and 2.9 (c) respectively.

46

Figure 2.9 (a) Gaussian pulse launched into the fiber with 8 dBm of

power

Figure 2.9 (b) Compensated pulses after the first Span

47

Figure 2.9 (c) Compensated pulse after the second Span

The effect of total dispersion, power received and error

performance of the system is obtained and the results are highlighted.

The pulse with 10 dBm of power level at 1552.524 nm (0.01 nm

Resolution bandwidth) is shown in Figure 2.10(a) for 10 Gb/s data rate.

48

Figure 2.10 (a) Spectrum of the Launched optical Pulse

Each span of 50 km of fiber with dispersion compensation is

carried out using DCU. The modulated spectrum and its power level is shown

in Figure 2.10 (b), Figure 2.10 (c) shows the optical spectrum after MZM and

50 km of fiber. Figure 2.10 (d) and Figure 2.10 (e) shows the eye diagram

along with Q factor variation and BER performance respectively. Maximum

Q factor of 10.345, at the decision instant 0.524 of bit period is achieved. The

BER performance after DCU leads to improved power penalty.

49

Figure 2.10(b) Spectrum after MZM

Figure 2.10(c) Optical Spectrum after MZM and 50 km of fiber

50

Figure 2.10 (d) The eye diagram and Q factor variation

Figure 2.10 (e) BER Performance after DCU

51

Therefore for bit rate of more than 10 Gb/s and with the received

power of less than 0 dBm, the symmetrical dispersion compensation leads to

the best BER as shown in Figure 2.8.

2.1.2.4 Minimizing the Effects of Nonlinearities

The fiber nonlinear terms cause frequency mixing as four wave

mixing (FWM), self phase modulation (SPM) and cross phase modulation

(CPM). In the presence of large electric field, this becomes significant and

generates a new wave introducing cross talk. Stimulated Brillouin Scattering

(SBS) depends on the source power and line width. By proper selection of

optical coherent source characteristics and fiber channel characteristics, the

nonlinear effects are minimized within the tolerable level for the given fiber

span.

Analytical expressions for phase and amplitude noises for phase

modulated optical systems due to inter channel four wave mixing (IFWM)

were derived by Alan Pak Tao Lau et al (2008). However, the analysis was

not focused for multiple channel access network applications. Similar analysis

was done by Alper Demir (2007).

Rene-Jean Essiambre et al (2010) described the method to estimate

the capacity limits of optical fiber networks. The sources of noise, Kerr

nonlinearity and mitigation of impairments were described and compared the

capacity limitations. However, the channel capacity for access networks was

not specifically addressed.

PMD monitoring for phase modulated signal using DGD generated

interferometer filter was demonstrated by Yang et al (2008). 0 to 100 ps of

DGD with 20 dB of radio frequency power variation in 20 Gb/s NRZ DPSK

52

system was derived, which was insensitive to 0 to 640 ps/nm of CD.

However, PMD monitoring technique in access network was not discussed.

The fiber nonlinearities and dispersion related issues are studied to

select the power level required for the system. As long as the optical power

within an optical fiber is small, the fiber can be treated as linear medium.

When the power level is high, the impacts of nonlinear effects are to be

considered. Two parameters contributing to this are refractive index related

parameters and scattering related impairments. Some nonlinear effects occur

in multi channel WDM systems where interaction of signals at different

wavelengths is possible.

SPM and XPM affect the phase of signals and cause spectral

broadening, which in turn leads to increases in dispersion penalties. SBS and

SRS provide gains to some channels by depleting power from other channels.

The nonlinear interaction depends on the transmission length and effective

area of the fiber. SPM is a significant consideration in designing 10 Gb/s

systems, and it restricts the maximum channel power to below a 10 dBm.

XPM becomes an important consideration when the channel spacing is tens of

GHz. FWM efficiency depends on signal power and dispersion, as well as

channel separation. If the channel is close to the zero dispersion wavelength

of the fiber, considerably high power can be transferred to FWM components.

Using unequal channel spacing can also reduce effect of FWM. These

findings are confirming with the results of earlier work done in analyzing the

optical fiber characteristics.

Dispersion plays a key role in reducing the effects of nonlinearities.

However, dispersion itself can cause intersymbol interference. In the

following example, the effects of dispersion compensation on system

53

performance in a high power regime where nonlinearities are active are

considered. Two different versions are considered for analysis.

In the first version, the system residual dispersion is 0, whereas it is

800 ps/nm in the second version. The transmission link contains 5 spans and

the bit rate is 10 Gb/s. The dispersion at the end of 100 km SMF is

1700 ps/nm/km and its effective area is 72 square microns. The dispersion of

DCF is -80 ps/nm/km. A 20 km DCF is used for the first version to totally

compensate the dispersion. For the second version, an 18 km DCF is used to

leave some residual dispersion after each span and this add 800 ps/nm total

residual dispersion to the system. The effective area of DCF is 30 square

microns. The loss in SMF and DCF are compensated by an EDFA with 25 dB

of gain for the first version and with 24.4 dB gain for the second version. The

Noise figure of the EDFA is 4 dB. It is found that after about 10 dBm average

power, SPM becomes a limiting effect. Figure 2.11 shows the eye diagrams

for two different residual dispersion values and three different signal powers.

when system residual dispersion is a) 0, b) 800 ps/nm.

Experimented with 10 kms, 50 kms and 120 kms of fiber, hence it

is stated in the respective places. In the entire situation, the DCF is at the

middle (half way between the Tx and Rx) of the fiber link distance. The best

performance are obtained with DCF in the Middle (Symmetrical).This is

compared in the Figures 2.8 for 8 to -16 dBm of power levels and for 2.5 Gb/s

and 10Gb/s.

54

Figure 2.11 Eye diagrams of the received signal for several received

signal powers when system residual dispersion is

a) 0, b) 800 ps/nm for one channel system

This simulation shows that effect of SPM is reduced by incomplete

compensation of the dispersion. It is observed that the power increase results

in closure of the eye in the case of zero residual dispersion.

For multi-channel system with 8 channels, the first channel is at

193.1 THz (1552.524 nm) and the channels are separated by 100 GHz. SMF

and DCF parameters are the same as in the previous example. To get more

accurate results, nonlinear phase shift parameter of the fibers is set to a lower

55

value (3 mrad). Simulation results are shown in Figure 2.12. The eye

diagrams of the received signal for several signal powers with residual

dispersion are 0 and 800 ps/nm respectively.

Figure 2.12 Eye diagrams of the received signal for several signal power

levels. a:0ps/nm of residual dispersion and b:800 ps/nm of

residual dispersion for multi-channel system

For an eight-channel system, the threshold power is approximately

10 dBm per channel. In this simulation, both SPM and XPM affect the system

performance. The simulation also shows that nonlinear effects are reduced by

local dispersion and better performance is obtained with nonzero residual

dispersion.

56

2.2 OOK OCDMA SYSTEM

The simplest technique of simulating the OOK OCDMA system

consists in changing the signal power between two levels, one of which is set

to zero and is often called on-off keying (OOK) to reflect the on-off nature is

the resulting optical signal. Most digital lightwave systems employ OOK.

OOK is identical with the modulation scheme commonly used for incoherent

intensity modulation/direct detection (IM/DD) digital lightwave systems. For

IM/DD systems, such unintentional phase changes are not seen by the

detector, as the detector responds only to the optical power. The situation is

entirely different in the case of coherent systems, where the detector response

depends on the phase of the received signal. The implementation of OOK

format for coherent systems requires the phase of the signal to remain nearly

constant. This is achieved by operating the semiconductor laser continuously

at a constant current and modulating its output by using an external

modulator.

Derivation of non linear equation for electronic dispersion

compensation with OOK modulation using direct detection was carried out by

Gilad Katz et al (2007). Suitability of the scheme for OCDMA system was

derived. However, the same for DPSK and other modulations were not done.

On off keying (OOK) modulation format is used in OCDMA

system for payload data with power detection. The 10 Gb/s data rate through

100 kms of the fiber for OCDMA is very much affected by the attenuation

and dispersion mechanisms of the fiber. The OOK OCDMA system is OSNR

sensitive due to the power detection process at front end of the receiver.

Hence the compensation for the fiber impairments is carried out to optimize

system performance in terms of receiver sensitivity and BER.

57

OCDMA was the powerful alternative to TDMA and WDMA in

fiber-to-the home (FTTH) Systems. Ken-ichi Kitayama et al (2006)

demonstrated the OCDMA system architecture and its operation principle,

code design, optical E/D using a long SSFBG. OCDMA over WDM PON was

proposed. However, improvement towards MAI was not discussed.

Wei-Ren Peng et al (2006) proposed frequency overlapping multi

group scheme for a passive all optical fast frequency hopped (OFFH)

OCDMA system based on FBG array with higher utilization of spectrum.

Users were assigned the codes and divided into several groups with group

interleaving. The interleaving of frequency allocations of different groups

made the groups less correlated, and hence lowering the MAI. However,

Gaussian profile of grating was chosen by Wei-Ren-Peng et al (2006) and did

not suggest an optimum profile for power efficiency and MAI.

2.2.1 OOK OCDMA Schematic Description

Optical laser diode operating at 0dBm power at 1550 nm is used to

modulate the PRBS data using an intensity modulator followed by encoding

using SSFBG. The encoded data from all the users are launched into the fiber

through a star coupler. Other than the desired user, all the remaining users

random data are to contribute for multiple access interference. The

OOK-OCDMA signal gets attenuated at the rate of 0.2dB/km in the SSMF

and also undergoes a dispersion of 17 ps/nm/km. To compensate for this, the

parameters of DCF and EDFA are selected to ensure the required OSNR at

the front end of the receiver. The threshold level is to set at slightly higher

level due to the noise and interference of multiple users. When an optical

threshold is used, the gain and dispersion of the threshold devices are adjusted

to the required optimum value due to the fact that the optical threshold

58

removes the noise, MAI and beat noise to the required extent that the BER is

maintained.

The theoretical block diagram of OOK OCDMA with power

detection system is shown in Figure 2.13 and the electrical domain receiver

structure is shown in Figure 2.14.

Figure 2.13 OOK OCDMA system model

Figure 2.14 Electrical Domain Receiver Structure

Laser diode operating at 1552.5244 nm corresponding to 193.1 THz

with output power of 1 dBm, line width of 0.1 MHz, without polarization

reference and with zero phase generates the optical signal. The data is

generated using an user defined code generator and precoded using one bit

delay precoder to get the DPSK data and applied to the LiNb Mach-Zehnder

modulator (MZM), and the modulated spectrum is shown in Figure 2.15 (b).

The modulated signal is then encoded with a defined code in the encoder, for

the signature of the user which is an 8 bit code of run length 2. This encoded

output corresponds to the desired user. Other 7 such users operating at

59

1554.4564 to 1549.8522 nm with channel spacing of 0.645 nm (0.08 THz

spacing). The 8 user encoded outputs are combined through a star coupler

then to the fiber. The fiber lengths are varied from 50 kms to 600 kms to

study the OSNR obtained at the input of the photodetector. The general

parameters of the system are i) sample rate = 1.3648 THz, ii) number of

samples = 4096, iii) samples per bit = 32, with a sequence length = 128 bits

Encoded signal from the star coupler has the following

characteristics and are shown in the Table.2.3. Single mode optical fiber

(SMF) of length 100 kms with attenuation of 0.22 dB/km, dispersion of

4.46 ps/nm/km and dispersion slope of 0.09 ps/nm2/km operating within the

performance limit at 1200 nm to1700 nm is used in our experiments. Group

velocity dispersion and third order dispersion are considered.

Josep segarra et al (2007) proposed an all optical metro-access

network using WDM/TDM architecture for PON based on optical burst

switching. In the analysis it was shown that the design of optical network

complicated the issues for the QoS. . Method for monitoring of simultaneous

optical signal to noise ratio (OSNR) and chromatic dispersion (CD) in 40

Gb/s WDM systems were proposed and demonstrated by Lamia Baker-

Meflah et al (2007). 20dB of dynamic range of OSNR was measured with

1dB of accuracy for the OSNR values of less than 20dB. The suitability of the

scheme for access network was not discussed.

2.2.2 Multiple Access Interference Analysis

Incoherent system of our proposed model uses 2-D coding schemes

to provide better correlation performance and improved power and bandwidth

efficiency in the asynchronous mode of operation. As analyzed by Xu Wang

and Ken-ichi Kitayama (2004), the received optical field at the input of the

60

(2.3) T

{ } )( . 0

++ = C m

i i d C t n P P T Z

photodetector of the desired user with m interfering signal (0 < m K-1) is

given in equation 2.1, where K is the total number of users in the system.

{ })cos(2)cos(2)(1

1 11

ij

m

i

m

ij

jiidi

m

i

idC PPPPPPTtE +++=

= +==

(2.1)

= Data + MAI + PBN + SBN

where Pd and Pi are data decoded and interfering power respectively. The

values of the terms Pi is not fixed, however a random variable fluctuating

around its average Pi leading to MAI. First term is the data signal, second

term is MAI, third term is primary beat noise (PBN) and the fourth term is

secondary beat noise (SBN); TC is the chip duration and is the

responsivity of the photo detector. Assuming the bandwidth of photo detector

is larger than the frequency difference between the incoming signals and for

smaller numbers of users the interference

di PP /= (2.2)

where, is such that m

61

and n(t) is the photo detector noise included for the completeness of analysis.

The average received signal Z is scaled by TC and approximated as

dC

PmT

Z)1(

)(+=

(2.4)

Assuming that the MAI and receiver noise both have Gaussian distributions,

the error probabilities are derived as

=

0

)(

22

1))(0/1(

mDPerfcmP de (2.5)

+=

1

)1(

22

1))(1/0(

DmPerfcmP de (2.6)

where 0 < D < (1+ m ) is the decision threshold; 0 and 1 are the total noise

variance with mark 0 or 1 respectively.

1,

2222

1 sthMAI ++= (2.7)

0,

2222

0 sthMAI ++= (2.8)

where 2MAI ,2

th and 2

s are the MAI, thermal, and shot noise variances,

respectively. Therefore, setting the optimum threshold level is done (i) by

selecting the large RL and nominal bandwidth to minimize the thermal noise

and (ii) by selecting PIN photodiode for suitable responsivity and TIA to

minimize the shot noise.

62

2.2.3 Optical Thresholder

The decoding process at the receiver generates autocorrelation

peaks from the designated user, and lesser dB level cross correlation peaks

from the non designated users. Optical thresholder (OT) is used to suppress

the MAI and the arrangement consists of the following three components

(i) Highly nonlinear fiber,

(ii) Dispersion compensated EDFA (DC-EDFA) and

(iii) Long pass filter to allow the desired wavelength range.

Functional schematic of OT is shown in Figure 2.15.

Figure 2.15 Optical Thresholder

DC-EDFA compensates for the dispersion and signal loss. One of

the decoder as the desired user matching the code of the corresponding

encoder in the presence of the selected number of interferers is applied to the

optical thresholder.The nonlinear optical thresholder utilizes 500 m of highly

nonlinear fiber (HNLF) with zero dispersion at 1553 nm. The dispersion of

0.19 ps/nm/km at 1550 nm, dispersion slope of 0.026 ps/nm2/km at 1550 nm,

effective area of 10 m2

and non linear coefficient of 20/(W.km) are the other

parameters selected for HNLF. The HNLF shifts spectral power into longer

and shorter wavelengths due to self phase modulation (SPM) and other fiber

63

nonlinearities. The longer wavelengths are passed to the receiver through the

long pass filter.

At the receiver, a power contrast ratio (PCR) of 25dB was

measured between desired user and all the other interferers. The power

measured at the input of the thresholder is the total received power in dBm. In

the thresholder, the DC-EDFA average output power is set to 14 dBm for

single user and the power penalty arises from pulse broadening, residual

dispersion in the encoder and decoder. As the interfering users are added, the

average output power of the thresholder is increased from the DC-EDFA to

keep desired users average power into the HNLF constant. As the number of

users increased, the thresholder output increases from 14dBm depending on

number of users. The HLNF used here is polarization independent.

The measurement emulates adaptive threshold detection consisting

of a DC-EDFA, HNLF and a long pass filter, where DC-EDFA adjust its

pump power level to optimize the threshold detection of the desired user

signal while suppressing the interfering users signal. Each interfering user

contributes a different amount of system penalty due to the differing MAI.

Signal attenuation and dispersion leads to power loss and limits the

transmission distance, OSNR and receiver sensitivity. Electronic

amplification needs optical to Electrical and Electrical to optical conversion

(O-E-O conversion). Electronic amplification depends on the bit rate and

modulation format and hence electronic amplification is neither optically nor

electrically transparent.

All optical amplifiers are used as signal regenerators where loss is

the limitation and single amplifier is be used for multiple channels and

independent of modulation formats. Gain, bandwidth, gain flatness, noise

64

figure, maximum output power, coupling loss, pumping efficiency,

polarization dependence and cross talk are the design parameters of an optical

amplifier and are optimally selected. Gain spectrum of two level systems is

Gaussian slope with peak at a particular wave length.

Optical amplifiers impair the detection of phase modulated signals

due to the interaction of signal and amplifier noise through the Kerr effect as

described by Alan Pak Tao Lau and Joseph M. Kahn (2007). However, the

interplay of chromatic dispersion (CD) and the Kerr effect on signal design

and detection were not investigated. Polarization independent optical

demultiplexing of 160 Gb/s optical TDM based data on cross-phase

modulation (XPM) induced wavelength shifting in highly nonlinear fiber

(HNLF). This has been experimentally demonstrated by Jie Li et al (2008).

However, achieving polarization independency was difficult.

Lijie Qiao et al (2007) described the scheme for maintaining

constant output signal power in the presence of amplified spontaneous

emission (ASE) noise for EDFA. The model was suitable for both single and

multi channel operation for an input power range of 25 dB and a gain of 0 to

37 dB to operate the EDFA. Other than this range was not highlighted. Lee H.

et al (2002) demonstrated an optical thresholder based on a short length of

holey fiber to achieve enhanced code recognition quality in 255 chip

320 Gchip/s SSFBG based OCDMA code/decode system. The nonlinear

thresholder was based on band pass filtering of spectrally broadened

components generated by self-phase modulation (SPM) in an 8.7 m length of

highly nonlinear holey fiber. However, it was not demonstrated for multi-user

OCDMA system.

65

Performance of digital receivers with fixed and adaptive decision

threshold were compared by Benjamin Puttnam et al (2008) in response to

gain transients arising from network operation of gain-clamped EDFA. The

advantages of adaptive decision threshold were given and the complexities

involved were not analyzed.

Anoma D. McCoy et al (2007) studied the semiconductor optical

amplifier (SOA) based noise suppression for SAC-OCDMA system. The

system had the limitation on optical filtering and not suitable for SAC-

OCDMA applications. Thus, forcing the use of erbium doped fiber amplifier

(EDFA) and noise suppression blocks.

Waldimar Amaya et al (2008) presented a time spreading OCDMA

system including non-perfect time gating and optical thresholding for OOK

and DPSK modulation cases. Complexity involved was not analyzed.

The carrier density fluctuations in semiconductor optical amplifier

(SOA) imposed penalties on (i) PSK signals due to nonlinear phase noise and

(ii) OOK signals due to self gain modulation. Francesco Vacondio (2010)

proposed a scheme for equalization of impairments. Other amplifiers were not

discussed. Satoshi Yoshima et al (2010) proposed a novel 10 Gb/s based PON

over OCDMA system to realize full capacity of optical access network. The

system was demonstrated by using multi level PSK, SSFBG, encoder/multi

port decoder and burst mode receiver. However fiber dispersion and

nonlinearities were not addressed in the analysis.

As shown in Figure 2.16, the maximum of EDFA gain of 42 dB

occurs at 1530 nm with the flat gain region being 1540 nm to 1560 nm region.

This feature of EDFA is useful for multiple channel amplification. The power

conversion efficiency is 95.5% at 1480nm pumping and 63.2% at 980 nm

66

pumping for the signal at 1550 nm. Optimum length for doping is selected for

amplification through population inversion as to the pumping signal gets

absorbed along the length of the fiber. The dynamic range of operation is

determined by the input signal power, output signal power and pump power

for the required gain.

Wavelength (m)

Figure 2.16 EDFA Gain variation with Wavelength of operation

Figure 2.17 shows the gain variation with the input power. In the

experiment, the EDFA gain of 14.5 dB is always achieved for the input power

of 1mW (0dBm). Also for 1mW of input power, 52 mW of maximum output

power can be obtained as shown in Figure 2.18. Figure 2.17 and 2.18 are

plotted for 100mW of pump power. Gain up to 30 dB is achieved with the

pump power of 40 mW as seen in the Figure 2.19. The amplified spontaneous

emission (ASE) noise is directly proportional to gain and bandwidth and also

the input signal. ASE is within 1dB in the operating wavelength and it is

removed by properly selecting the gain and bandwidth and a filter is used to

remove the noise outside the bandwidth. The ASE noise interferes with the

detection process in the photo detector and contributes to the output

introducing beat noise.

67

Figure 2.17 EDFA Gain variation with Input Power

Figure 2.18 EDFA Output vs Input Power

Figure 2.19 EDFA Gain variation with Pump Power

68

The Noise figure is given by SNRin/ SNRout, and it is lesser with

1480 nm pumping than the 980nm pumping. Optical signal to noise ratio

(OSNR) is maintained by properly choosing the design parameters for the

given bit rates. The merits of semiconductor optical amplifier (SOA) are

compactness, integration with optoelectronic components, functional

applications and broad choice of operating wavelength. Input power was

maintained constant at 400 mW for plotting the response. The pump power is

varied from 1to 200 mW.

The suitability of EDFA is seen by comparing the values with that

of semiconductor optical amplifier given in the Table 2.1.

Table 2.1 Comparison of Optical Amplifiers: EDFA and SOA

Features EDFA SOA

Typical Maximum

internal gain

30 dB-50 dB 30 dB

Typical Maximum insertion loss 0.1dB-2dB 6 dB-10 dB

Polarization sensitive Not sensitive to

polarization

Sensitive to

polarization

Pump source Optical Electrical

Optical bandwidth 40 nm 30 nm

Maximum output power 23 dBm 20 dBm

Typical intrinsic noise 3-5 dB 7-12 db

2.2.4 OSNR Sensitivity

In optimizing the simulation parameters, the fiber channel OSNR

and Q factor decreases with increasing the distance as in the Figure 2.20. As a

consequence, the BER performance is improved with the OSNR. The rate of

69

change is compensated with suitably selecting the parameters of in-line EDFA

and DCF; The OSNR of 36 dB corresponding to the Q factor of more than 7

is maintained.

(a) BER variation with Q factor

(b) BER variation with OSNR

Figure 2.20 Optimization of OSNR and Q factor for the Fiber

70

2.2.5 Disadvantages

The electrical eye is unsymmetrical for both the cases. The dynamic

decision threshold level setting in electrical domain is a complex problem as

(i) The threshold setting has to change in accordance with the number of users

(ii) Estimating the number of users in an asynchronous environment using

digital signal processing or estimating the higher order harmonic levels of the

received signal (iii) The optimum threshold set for an average number of

users beyond certain level is not proportionate and the BER performance

estimated with noise probability density function is not satisfactory

(iv) Adaptively varying the threshold tends to increase enormously the cost,

complexity in receiver preamplifier design and complexity in thresholder.

And further burst errors degrade the system severely . (v) Alternatively using

cost effective and lesser complex optical thresholding to mitigate MAI and

noise gives more than 4 dB OSNR improvements over the optimum threshold

technique. Thus the performance of OOK OCDMA with optical thresholding

is nearly the same as DPSK OCDMA without thresholding.

2.2.6 Simulation Schematic

Simulation of OOK OCDMA and DPSK OCDMA is demonstrated

using SSFBG E/D in Figure 2.21 shows the simulation setup for the

demonstration and comparative investigation of DPSK OCDMA with OOK

OCDMA. The schematic for generation of multiple access interference is

shown in Figure 2.22. The mode locked laser diode generated ~1.8-ps optical

pulse at a repetition rate of 10 GHz with a central wavelength of

1552.524 nm.

71

Figure 2.21 Shows the arrangement for OOK OCDMA and with DCU

The signal from the OOK modulator was split into two paths. The

upper path is for target OCDMA user with 10 Gb/s OOK modulation using

intensity modulator. The same schematic is used for DPSK OCDMA with

phase modulator instead of intensity modulator. The lower one is used to

generate different number and levels interferences. In the interference path,

the MAI generator can generate interferences with different 1 by tuning the

variable optical attenuators (VOAs) and different K by adjusting the optical

switches.

72

Figure 2.22 Simulation Schematic for MAI generation

Here, 1 is defined as the power contrasts ratio between single

interference and the target signal, and K is the number of interferences. The

signal and the interference are mixed and decoded by the decoder. At the

receiver, a fiber based interferometer followed by a balance detector and a

single PD was used for DPSK and OOK detection, respectively.

2.2.7 Super Structured FBG

The encoders/decoders were written with significant frequency

guard bands between frequency bins. These bins are partition of the available

bandwidth into uniform segments for coding. The original intent of the guard

bands was to reduce multiple-access interface (MAI) by assuring that when

two codes do not have a given bin in common, no energy would leak

through one bin to the other. Many codes are available, but long codes are

usually considered. The assumption of square frequency response leads to the

zeroing out of MAI after balanced detection when using constant cross

73

correlation codes. The encoders and decoders have identical codes, whereas

the complementary decoder consists of the complementary code of the

corresponding encoders.

For a given bit rate, systems with greater optical bandwidth would

offer better performance in terms of BER or, for fixed BER, could

accommodate more uses for greater capacity.

Since intensity noise is the principal noise source, and frequency

guard bands reduce the occupied effective optical bandwidth and, therefore,

the capacity. If ideal rectangular filters could be achieved, the encoders would

carve out truly orthogonal frequency bins while exploiting all available

bandwidth. Realistic FBGs that can be written in an effective manner will

have finite roll off, leading to a tradeoff between MAI and intensity noise.

This tradeoff requires the identification of further constraints on the code

family to achieve optimization. However, simulation is achieved with FBG

spectral responses with various levels of overlap between bins. These spectral

responses are then used to predict the BER floor. The optimum spectral

response is determined and a set of encoders/decoders is realized.

In the experimental setup, encoding/decoding process is achieved

by FBG that is working in transmission. The apodization profile A (z) of an

FBG is the modulation index envelope that will be written in the fiber. The

spectral response of a highly chirped FBG is simply an inverse translation of

the gating apodization profile along the z-axis of the fiber. Basically, no

modulation index (A=0) leads to a transmission bin, whereas a modulation

index (A=1) leads to a non transmission bins, i.e., a reflective bin. An

apodization profile based on super-Gaussian lobes is used in order to

minimize the ripples in spectral response that cause MAI.

74

The encoder is an SSFBG working in transmission that takes a

broadband source and filters out all spectral content, expect those frequencies

included in the users unique spectral code. All the users in the system share

the same optical bandwidth and contain frequency elements for the same

band; they access the channel asynchronous and without coordination. An

N1 coupler is used to combine all signals onto one fiber.

The duration of the encoded signal and the decoded signal are about

800 ps and 1.6 ns, respectively. Therefore, in the interference path, the data

rate is intentionally converted to 622 Mb/s to avoid the inter symbol

interference. However as in the signal path the data are transmitted at 10 Gb/s

data rate, the interference results in the performance degradation. But on the

performance comparison, interference is considered as a fixed level

interference to the received signal and neglected by taking all the

measurement against the relative interference level 1, which is proportional

to the absolute interference level

Figures 2.23 to 2.35 show the simulation results for the E/D for the

desired user for various conditions indicated in the relevant figures.

Photodetector 1 (PD 1) is in the upper arm and photodetector 2 (PD 2) is in

the lower arm of the differential detection scheme.

75

Figure 2.23 Encoded Spectrum after the 1st SSFBG encoder

Figure 2.24 Encoded spectrum at the input of Star coupler for the

Desired user

76

Figure 2.25 Encoded signal at the input of star coupler

Figure 2.26 Encoded signal at the input of Star coupler for the Desired

user

77

Figure 2.27 Decoded signal after 1st SSFBG

Figure 2.28 Decoded Spectrum at the input of PD 1

78

Figure 2.29 Decoded Spectrum at the input of PD 2

Figure 2.30 Waveform at the input of PD2

79

Figure 2.31 Waveform at the input of PD1

Figure 2.32 Spectrum after Star coupler

80

Figure 2.33 Waveform after Star coupler

Figure 2.34 Reflected spectrum from the 1st SSFBG in the PD2

81

Figure 2.35 Reflected spectrum from the 2nd

SSFBG in the PD2

The schematic in Figure 2.34 shows the complete arrangement

carried out for OOK OCDMA and programmable decoder for multiple users

with DCU. This scheme simulates the OOK OCDMA system with intensity

modulator and direct detection using PIN photodetector. At different stages of

the proposed schematic, the signal and its spectrum are taken but their

inferences are not given. Also, why the difference in the power level between

signal and spectrum

2.3 DPSK OCDMA SYSTEM

The use of PSK format requires that the phase of the optical carrier

remain stable so that phase information can be extracted at the receiver

without ambiguity. This requirement puts a stringent condition on the

tolerable line widths of the transmitter laser and local oscillator. The line

width requirement can be somewhat relaxed by using a variant of the PSK

82

format, known as differential phase-shift keying (DPSK). In the case of

DPSK, information is coded by using the phase difference between two

neighboring bits. For instance, if k represents the phase of the kth bit, the

phase difference = k-k-1 is changed by or 0 depending on whether kth

bit is a 1 or 0 bit. The advantage of DPSK is that the transmittal signal can be

demodulated successfully as long as the carrier phase remains relatively stable

over the duration of two bits.

Bartlomiej Kozicki et al (2008) demonstrated a flexible optical

performance monitoring (OPM) method for phase modulated signals. The

OSNR was measured within the range of 20 to 35dB with accumulated CD

between -600 and +600ps/nm and PMD precisely for high capacity optical

networks. OPM method supported NRZ, RZ-DPPSK and RZ-DQSK formats.

However, the complications in the measurement method were not addressed.

Choi et al (2008) described a method for in-band OSNR monitoring

of DPSK and differential quadrature phase shift keying (DQPSK) signals by

analyzing the spectrum obtained from self heterodyne detection. The

technique was insensitive to the effects of CD and PMD. Complexity

involved was not addressed. DPSK receiver design applied to strong optical

filtering for NRZ and RZ modulation formats were analyzed by Christian

Malouin et al (2007). For NRZ-DPSK, the measured OSNR penalty was

obtained for a perfect one bit DI.

Asynchronous demodulation cannot be used in the case of PSK

format because the phase of the transmitter laser and the local oscillator are

not locked and can drift with time. A variant of PSK, known as DPSK, can be

demodulated by using an asynchronous DPSK receiver. However the use of

the DPSK format permits asynchronous demodulation by using delay scheme.

The idea is to multiply the received bit stream by a replica of it that has been

83

delayed by one bit period. The resulting signal has a component of the form

cos(k-k-1), where k is the phase of the kth bit, which can be used to recover

the bit pattern since information is encoded in the phase difference k-k-1.

Such a scheme requires phase stability only over a few bits and can be

implemented by using DFB semiconductor lasers. The filtered current is

divided into two parts, and one part is delayed by exactly one bit period. The

product of two currents contains information about the phase difference

between the two neighboring bits and is used by the decision current to

determine the bit pattern. The BER calculation is more complicated for the

DPSK case because the signal is formed by the product of two currents.

Hideaki Furukawa et al (2010) demonstrated optical packet

switching and buffering operation of dense wavelength division multiplexing

(DWDM)/NRZ-DPSK with optical payload data rate of 640 Gb/s (64

wavelength x 10 Gb/s). Interference level of DPSK and detailed comparison

to OOK was not carried out.

All optical data format conversion to and from DPSK were

proposed and numerically demonstrated by Jian Wang et al (2008).

Multicasting, multi channel and ultrahigh speed (160 GB/s) format

conversions were also demonstrated by simulation. However, the deviation to

the actual application was not discussed. Chromatic dispersion limited by the

hybrid ASK-DPSK modulation format were studied by Jian Zhao et el (2007)

for enhancing the transmission reach, also shown that electronic equalization

of ASK and DPSK separately did not improve the CD tolerance of ASK-

DPSK signal. Other modulation formats were not discussed.

Mohammad Alfiad et al (2008) demonstrated the DPSK receiver

employing MZI with less than one bit delay. The enhanced CD tolerance of

84

4000 ps/nm at 2 dB penalty was achieved for 10.7 Gb/s NRZ-DPSK. Other

than one bit delay was difficult to achieve.

The block schematic of DPSK OCDMA system is shown in

Figure 2.36. The optical laser diode is driven by the clock waveform to

generate 1.8 ps pulses at a repetition rate of 10 GHz with a central wavelength

of 1552.24 nm. This laser source waveform is modulated using a phase

modulator (PM) in accordance with the DPSK encoded data. DPSK data is

obtained from pseudorandom bit sequence generator in NRZ format. This

NRZ data is encoded using a precoder and phase modulated with the laser

output. The phase modulated DPSK signal is further encoded using OCDMA

encoder corresponding to the desired user by the OCDMA encoder. All the

other OCDMA encoded signals are MAI corresponding to 31 users which are

coupled to the fiber through a star coupler. DPSK OCDMA decoder and one

bit delay Mach Zehnder delay Interferometer (MZI) with balanced detection

is used at the receiving end. The decoder performs the reverse operation of

OCDMA encoder.

Simulation of DPSK OCDMA is carried out using the schematic

shown in Figure 2.21 through the following modifications (i) replacing the

Intensity Modulator (IM) by a Phase modulator (PM) and (ii) replacing the

single photodetector by the Balanced Modulator. However the simulation

schematic remains the same as given in Figure 2.22.

85

Figure 2.36 DPSK-CDMA systems

Commonly used external modulator makes use of LiNbO3

waveguides in a Mach-Zehnder (MZ) configuration. The performance of

external modulators is quantified through the on-off ratio, called extinction

ratio and the modulation bandwidth. LiNbO3 modulators provide an on-off

ratio in excess of 20 and can be modulated at speeds up to 75 GHz. The

driving voltage is typically 5 V but can be reduced to near 3 V with a suitable

design. Other materials can also be used to make external modulators. A

polymeric electro-optic MZ modulator requires only 1.8 V for shifting the

phase of a 1550 nm signal by in one of the arms of the MZ interferometer.

Kang et al (2008) demonstrated all optical byte pattern recognition

embedded in PSK data streaming at 40 Gb/s and used matched filtering to

generate autocorrelation pulse. Reconfigurable Silica Planar Lightwave

Circuit (PLC) delay line filter based correlation filter complicated the system

hardware. Performance of 40 Gb/s DPSK demodulator in silicon-on-insulator

(SOI) was presented by Karsten Voigt et al (2008). The delay interferometer

manufactured in 4 m rib waveguide was presented for operation at 1550 nm.

Although advanced temperature control for real applications was not carried

out. Teh et al (2002) reported the fabrication and application of 255 chip

86

320 Gchip/s quaternary phase SSFBG for optical code generation and

recognition in a four channel WDM/OCDM. However, the SSFBG was used

with fixed codes.

In the simulation work, 31 active users are included to characterize

the interference. The simulation is carried out with and without optical

thresholder (OT) as given in section 2.2.3 for DPSK OCDMA. The

combination of optical thresholder with MZI and pin photo detectors used for

balanced detection suppresses the MAI. DPSK OCDMA is superior to OOK

OCDMA with advantages of improved receiver sensitivity, better tolerance to

beat noise and multiple access interference without optical and dynamic

threshold setting. Differentially phase coded OCDMA systems are susceptible

to coherent beat noise and MAI. Differentially Phase coding scheme require

additional components and the system complexity is also increased due to

laser diode phase noise and coherence problems. Sensitivity at high data rates

is affected by the noise of the source and interference level of the system.

2.3.1 Generation of DPSK Signal

The schematic to generate the DPSK signal in shown in the

Figure 2.37.

87

Figure 2.37 Generation of DPSK Signal

Laser diode operating at 1552.5244 nm corresponding to 193.1 THz

with output power of 0dBm, line width of 0.1 MHz, without polarization

reference and with zero phases generates the optical signal. The data is

generated using a PRBS generator and precoded using one bit delay precoder

to get the DPSK data and applied to the LiNb Mach-Zehnder modulator

(MZM) to obtain the modulated spectrum.

The modulated signal is then encoded with super structured fiber

Bragg grating (SSFBG) in the encoder. This encoded output corresponds to

the desired user. Other users operating from 1550 nm with channel spacing of

0.645 nm (0.08 THz spacing).All the encoded outputs are combined through a

star coupler and then to the fiber. The fiber lengths are varied from 50 kms

to 100 kms to study the OSNR obtained at the input of the photodetector. The

88

general parameters of the system are i) sample rate = 1.3648 THz, ii) number

of samples = 4096, iii) samples per bit = 32, with a sequence length = 128

bits. Encoded signal from the star coupler has the following characteristics

and are shown in the Table 2.2.The electrical eye diagram characteristics are

given in Table 2.3. Table shows the maximum and minimum values of the

encoded signal and noise for any of the possible number of users (encoders)

Table 2.2 Encoded signal characteristics

Signal power

(dBm)

Noise power

(dBm) OSNR(dB)

Minimum Value -6.5226704 -12.03328 2.0042

Maximum value -6.2082046 -8.7438653 5.5679

Ratio of Max/min 0.53143461 3.2893425 3.2893

Table 2.3 Eye Diagram Comparison

Parameters

Eye Characteristics

for DPSK OCDMA

(a.u)

Eye Characteristics

for OOK OCDMA

(a.u)

Maximum Q factor 3.838 2.871

Minimum BER 5.943e-5 170.1e-5

Eye height 0.1982e-6 - 1.824e-6

Threshold level - 9.103e-6 14.60e-6

Decision instant 0.458 0.703

89

2.3.2 E/D with Walsh Code

The encoders and the decoder are composed of fiber pigtailed

bulk-optics-based femtosecond pulse shapers. Briefly, the encoded data

streams are collimated onto diffraction gratings, spatially spreading out the

spectral components of the incident pulses. The spread spectrum is incident

upon a spatial light phase modulator (SLPM), which applies a phase shift to

different portions of the spectrum, as designated by the OCDMA codes.

Additional phase shifts are applied to compensate for dispersion in

transmission fiber components and bulk optic components, collimators are

employed. Ultimately, the eight encoder channels and single decoder require

only a pair of pulse shapers. After reflecting back into the fiber, circulators

route the encoded signals through the remainder of the system. All OCDMA

encoded data sets then combine to single decoder that applies the conjugate

phase code of the desired signal. Since both the circulator and the diffraction

grating of the pulse shapers are polarization, the decoder doubles as a

polarization. The equivalent functions can be performed using compact and

fiber-based arrayed waveguide gratings, which can spread and recombine the

spectrum.

FBG based E/D for the 2-D time-spreading/wavelength hopping

optical coding was proposed by Ye Zhang et al (2008). FBG based codes

were given and the spectral analysis were also carried out. However, the

OCDMA system was tested at the data rate of 5Gb/s only. Petropoulos et al

(2001) investigated the benefits of using time gating in the detection process

for OCDMA system that comprises of bipolar 64-chip long SSFBG encoders

and decoders. It was shown that correlation combined with time gating

detection provided resilience to the distorting effects of dispersion and MAI.

However, the number users and the data rate were minimal for the OCDMA

system analyzed.

90

Huiszoon et al (2007) presented the integrated device that

performed the cost-effective parallel encoding and decoding (E/D) in SAC-

OCDMA. However, the performance analysis were not presented or

compared with other types of E/D. Julien Penon et al (2007) developed the

methodology for numerical optimization of frequency response of FBG, and

maximized the capacity of spectral amplitude coded OCDMA (SOC-

OCDMA). Optical encoders were realized and demonstrated for the

incoherent SAC-OCDMA system with seven simultaneous users. However,

error free operation (BER

91

transimpedance amplifier (TIA). These parameter values are chosen for the

application at 1.5 m instead of 2 m.

A Balanced receiver was demonstrated to Suppress nonlinearities

by Alexander et al (2008). However their works were in the frequency range

of MHz only for IM/DD.

Uni-traveling carrier photodiode (UTC) and traveling wave photo

detector were described by Andreas Beling et al (2008) for more than

100GHz applications. These are not essential for OCDMA system with BD.

However, for detection at high date rates of more than 80 Gb/s the traveling

wave photo detector is suitable.

PMD insensitive and dispersion tolerant in - band OSNR monitor

based on beat noise measurement was demonstrated by Bakaul (2008).

Balanced receivers limited the suitability for high speed applications.

Yannick Keith Lize et al (2007) demonstrated the technique for a

simultaneous CD and PMD monitoring method using a partial bit delay in

MZI. The technique increases CD monitoring sensitivity by a factor of two

for NRZ intensity modulation format and a factor of five for a DPSK

modulation format. However, realizing partial delay in MZI was critical.

Xin Chen et al (2007) analyzed lumped dispersion compensation

for 40 Gb/s RZ-DPSK transmission. The scheme was flexible simple and low

cost solution for DPSK link design. However, OCDMA network analysis was

not made. Ilya Lyubomirsky et al (2008) proposed and analyzed differential

quadrature phase shift keying (DQPSK) receiver architecture based on optical

frequency discriminator filtering and direct detection for enhanced CD

tolerance. However, OSNR sensitivity was poor.

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The balanced detector for DPSK OCDMA is illustrated in

Figure 2.38. Balanced detection eliminates MAI for codes with fixed cross

correlation, leaving only intensity noise. The upper arm contains an FBG with

the decoder identical to the encoder for the data to be received. In the lower

arm, the complementary decoder is orthogonal to the encoder spectral

response. It contains only the frequency bins that are not present in the

encoder. An optical attenuator is inserted to achieve balanced detection and is

adjusted according to the used codes.

Figure 2.38 Balanced Detection Scheme

Optimum source and fiber characteristics are analyzed for the OCDMA

system. Performance of OOK OCDMA system is analyzed with optical

thresholding and direct detection. DPSK OCDMA system with balanced

detection is analyzed.