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Aihan Yin, Li Li, Aiyun Zhan, Xinliang Zhang УДК 004.73
ANALYSIS OF MODULATION FORMAT IN WDM-PON OPTICAL COMMUNICATION SYSTEM
I. Introduction
With the rapid development of the next generation of optical fiber transmission information system, 40Gbps optical fiber transmission system and its wavelength division multiplexing (WDM) system have been the focus of research. In order to enhance the capacity of the system and diminish the degradation of performance which would be caused by the loss of transmission, systems engineering and optimization would be important. Thereinto, the optical code-pattern would be the important factor which decides the spectrum efficiency, transmission quality and the dispersive tolerance of the system. Thus, the chosen of code-pattern is the first factor in the high speed optical transmission system [1-2].
The application of optical fiber transmission system and dense-wavelength-division-multiplexing (DWDM) system make the dispersive tolerance markedly decrease, and the non- linearity effect has an impact on the system performance. Traditional NRZ code-pattern would not have met the demand, while need other new modulation format. People propose many code-patterns in terms of 40Gb/s optical transmission system [3-11], such as RZ [3], and also propose the RZ of the carrier suppressive (CS-RZ), single sideband-RZ (SSB-RZ), duojdecimal-RZ (d-RZ), mend duojdecimal-RZ (MD-RZ) [4], RZ-DPSK [5-6], full spectrum return-to-zero (FSRZ) and chirp return-to-zero (CRZ). DPSK and RZ are better than NRZ in the term of anti-noise. RZ-DPSK is better than NRZ at the influence of anti-dispersive. CS-RZ, CRZ and the DPSK are better than NRZ in the term of nonlinear effect. SSB is better than NRZ in term of spectrum effectiveness. Recently, some code-patterns are appeared which include differential quadrature phase keying (DQPSK) [7], intensity modulation-DPSK (IM-DPSK) [8], and the Prefiltered CS-RZ [9]. NRZ-DPSK, RZ-DPSK and CS-RZ-DPSK use the phase change of the close codes showing the signal "0" and "1". Their spectrums are smoothness and do not have any linear spectrum. IM-DPSK is to diminish the optical intensity change of close codes, in order to de-
crease the chatter and non-linearity. Differential quadrature phase keying (DQPSK) makes the phase change in the code of two bytes. Prefiltered CS-RZ is used to filter the CS-RZ in order to decrease the width of the spectrum and enhance the spectrum efficiency.
In the paper, take the 40Gbps for example, the way of modelization of NRZ, RZ and CS-RZ with computer analyzes the optical spectrum. The 40 Gbps signal transmitted in 200 km G.652 fiber by way of single channel with erbium-dope-fiber-amplifier (ED-FA) is simulated for these three formats. Thus, we can get the better code-pattern with high spectrum efficiency and high tolerance for the optical noise and the non-linearity effect.
II. The principle and characteristic of the code-PATTERN
A. The principle of chosen code pattern
There are three principles for the modulation
format that we should follow: firstly, the compact modulation signal spectrum is good at enhancing the operating factor of the spectrum and the dispersive tolerance of group velocity; secondly, a high non-linearity tolerance; thirdly, the structure of the transmitter and receiver are simple as soon as possible.
B. The principle of NRZ
We usually use the Mach-Zehnder modulator (MZM) and the consecutive wave (CW) laser in the modulation system. Except for the NRZ, their appearance would be performed by the two concatenation of MZM. These two concatenations of MZM play a different role. The first MZM is used to bring various pulses by the drive of the clock signal. The second one is used to load the data.
Figure1 is the frame of the NRZ signal of the optics. When transmitted the "1"in the NRZ, optical signal impulse occupies a whole bit-time; when there is no optical pulse, the signal is "0". The realization of the coding is simple, only needing a high speed exterior modulator that can work effectively at the speed of 10Gbps. The advantage of NRZ is the simplicity of application, low cost and high spectrum efficiency, which can be used widely into the synchronous digital
СИСТЕМНЫМ АНАЛИЗ И МЕЖДИСЦИПЛИНАРНЫМ ПОДХОД В ИССЛЕДОВАНИЯХ
hierarchy (SDH) and wavelength division multiplexing (WDM) system. Under the 10Gbps system and more less, we use the NRZ modulation model. The disadvantage of NRZ is that the transition doesn't return zero between two codes, the sensitivity for transmission loss. So it is not suitable for high speed and the extra long-distance transmission.
C. The principle of RZ and CS-RZ
Figure2 is the frame about the principle of the generation of RZ and CS-RZ, which is all composed by the two concatenation of MZM. The technology of RZ code prevails recently, which is used in the high speed of 40Gps optical transmission system. In the pulse sequence of RZ code, the transition area which connects "1" amplitude of electric field has the independent time envelope. Because modulation format of RZ has the different transition all the time in the code bits, thus it can bring more "neatness" optical signal in order to unscramble the receiver. The advantage of RZ is the low average of optical power; higher ability on anti-non-linearity effect and anti-polarization mode dispersion (PMD) [12]. RZ code is also more conducive to clock recovery. Because the consecutive "1" of NRZ is a whole, the eye pattern of RZ code stretches bigger, the better ability of anti-error-code performance, and provides the improvement on 3dB of the optical signal noise ratio (OSNR).
The CS-RZ code is based on the traditional RZ code, and join the phase separation of n in each adjacent sign bit (no matter the sign bit is "0" or "1"). The phase separation of the carrier can be regarded as the signal with a minus but the carrier is invariability. The typical value of this signal with positive and negative ambipolar is '0', so there is no pinnacle in the zero frequency because of without A function (impulse function), after multiplying the according carrier, and there is also no pinnacle in the carrier. In the CS-RZ, because the sign about consecutive code of amplitude of electric field is reversed, we can get the low width of spectrum. With the high power, it not only increases the dispersive capacity, but also enhances the resist of the non-linearity of self-phase modulation (SPM) and four-wave-mixing (FWM), and so on.
Bias Conttol
^ MZM
- diirih/c Aldcii-it'eil
40Gb/s optical NRZ signal
40Gb/s electrical NRZ signal
Fig. 1. Blocks diagram of NRZ
CW-Laser MZM MZM ^^
ienal I
40Gb/s electrical RZ signal
Fig. 2. Blocks diagram of RZ/CS-RZ
III. Code type wave shape and spectral analysis
A. The wave analysis of code type
At present, we often use MZM to produce modulation code; MZM is the coupler about two-interconnected-3dB. While a AL optical path difference is existed between two wave-guides, there can be phase modulation which produced by the two signals at the output. Such as the figure 1, with the Mach-Zehnder interferometer (MZI) modulator modulating for CW laser lamp-house, the electric 40Gbps NRZ signal (27-1 pseudo-random binary sequence) would become 40Gbps NRZ optical signal. For the figure2, the first MZI has the same working theory as MZI of the figure 1, only changing the electric 40Gbps of NRZ into RZ (dutyfactor is 0.5); the second MZI should be controlled by the sine-wave clock and voltage bias on two spur track of MZI, getting through to modulate the amplitude, frequency, phase and the voltage bias, then we can get the optical signal of RZ and CS-RZ.
Thus, we might as well input light field:
Em (t ) = E0I ^ . (1)
Passing the MZM, after the phase modulation, the light field would be:
Emt (t)= ^ [> W + j (t)] , (2) <PX (t) = y ^[Vim sin(,t)+V ] , (3) ^2 (t) = n V~ [V2 m sm(®2t+^2 ) ] , (4)
where the Vim=©1(=2f1n), ^ (i=1,2 are the amplitude, angular frequency, phase of the clock signal on two spur track of MZI separately. V1, V2 can provide the voltage bias on two spur track of MZI separately. When the single arm works causing the appearance of
phase separation, Vn is the switch voltage from the max to the min. In order to make the MZM working at the station without chirp, we can add the voltage of two single arms to a fixed voltage bias. That is V1 (t) +V2 (t) =Vbias, thus the output of the light field is:
Eout (t) = En (t)
cos
П
2Vn
(V - V )
V in b ias J
(5)
The output of the light intensity is:
CW-Laser
ИРКУТСКИЙ ГОСУДАРСТВЕННЫЙ УНИВЕРСИТЕТ ПУТЕЙ СООБЩЕНИЯ
Pout (t ) = Pn (t )C0S2
П
(V - V )
V in bias /
2Vn
(6)
RZ code with the dutyfactor 50% can set the
input of the electrical clock signal:
V V
Vin = sin m0t, the bias voltage is . (7)
Then the light field and light intensity of RZ is by equation (3), (4):
/ n
Eou, (t ) = Ein (t )C0S (^nnsin®0t ^ ,
Pout (t) = Pp [1 + cos (nsin©0t)] .
(8)
(9)
With the above formula: the optical impulse cycle is 2n/œo, the angle frequency is œo, the full width at half maximum (FWHM) is n/œ o.
CS-RZ code would be set the input of electrical clock signal, Vin = Vn sin a>0t, the voltage bias is Vn
Kut (t) = Em (t)
cos
П
v 2V;
V П
V Sin®0t - Vn)
Put (t) = Pn (t)sin2
-Sin®0t
, (10)
(11)
P (f ) = f0 ]Z J2n+I I П
( 2n +1) /0
x cos
n( f - fc )
2 f
2 J 2n(f - fc )[(f - fc )2 -(2n +1)2 f2 ]
да
f - fc ±(2n - 1)f0]. (13)
From the above analysis, the CS-RZ optical spectrum has a distance of stair linear spectrum that is 40GHz. There is no linear spectrum existed, which is caused by the change of n phase between the two pulses. The power spectrum of the RZ code is:
g(t) = ^7 U (jl + 2t J2n (n) cos(2nffl„t) - 2% J2n+1 i-4)sin [(2n +1) ®0t]
+S( f - fc ± 2nf0).
(14)
From the expression of the RZ optical spectrum, we can know the linear spectrum existed in the RZ optical spectrum.
From the formula, the FWHM of the NRZ signal is 25ps; the FWHM of RZ is 12.5ps and CS-RZ is 12.5ps, too. The figure below is the spectrum of modulation format; figure3 is the wave and the spectrum of NRZ; the spectrum width of NRZ is about 80GHz (the distance between secondary line spectrum); fig-ure4 is the wave and the spectrum of RZ code, and spectrum width is 160GHz; figure5 is the wave and the spectrum of CS-RZ code, and spectrum width is 120GHz with no carrier wave. This section through comparing of NRZ, RZ and CS-RZ between the single pulse wave and the spectrum, getting the concrete transmission characteristic of various modulations formats [5] [10].
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From the above formula: the optical impulse cycle is n/&>0 ; angle frequency is 2©o; the FWHM is
2n/3© o.
B. Optical spectrum analysis of the code pattern
According to the above academic expression of various code-models, using the method of optical spectrum towards random signal in the communication theory, the power spectrum density of the random pulse sequence concludes two parts: consecutive spectrum [Pu(©)] and linear spectrum [Pv(©)].Through computing the power spectrum of NRZ code is:
1 2 1
Pe (f) = - Tssa (n( f - f)Ts) + ^(l - f), (12)
1.5518 □ 1.5521 □ 1.5524 □ 1.5527 □ 1.553 □ Wavelength (m)
.5518 □ 1.5521 □ 1.5524 □ 1.5527 □ 1.553 □ Wavelength (m)
(a) Spectrum of NRZ (b) Spectrum of NRZ-DPSK
Fig. 3. Wave and spectrum of NRZ
=l Optical Spectrum Analyzer_2 3 Optical Spectrum Analyzer
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1.551S □ 1.5521 □ 1.5524 □ 1.5527 □ 1.553 □ Wavelength (m>
1.551S □ 1.5521 □ 1.5524 □ 1.5527 □ 1.553 □ Wavelength (m)
(a) Spectrum of RZ (b) Spectrum of RZ-DPSK
Fig. 4. Wave and spectrum of RZ
si Optical Spectrum Analyzer_1 -
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1.5513 □ 1.5521 □ 1.5524 □ 1.5527 □ 1.553 □ Wavelength (m)
1.551S □ 1.5521 □ 1.5524 □ 1.5527 □ 1.553 □
Wavelength (m)
(a) Spectrum of CS-RZ (b) Spectrum of CS-RZ-DPSK
Fig. 5. Wave and spectrum of CS-RZ
C. The analysis of spectrum by three kinds of modulation formats
N = 0
The impulse width of NRZ is the biggest in these six kinds of modulation formats, so it is easy to occur intersymbol interference, sensitive to the transmission loss, infective to the non-linear effect, but the spectrum of NRZ is narrow which is good for decreasing the interference of inter-channel WDM system.
The dutyfactor of RZ code will reduce; impulse width is smaller than NRZ, which can restrain the non-linear effect of optical fiber, suitable to work in the high-power and long-distance transmission condition, but the spectrum of RZ code is wide, dispersive capacity decreases markedly, which is not good for management.
The impulse width of CS-RZ is smaller than NRZ; spectrum width is between the NRZ and RZ. The part of carrier wave is prohibited, thus it is not only becoming smaller and increasing the dispersive capacity, but also becoming stronger on the ability of resisting the non-linear effect.
The spectrum of NRZ is the most compactive in the one-lever sideband and baseband, which result in the cost of eye pattern stretch becoming smaller comparatively caused by the wave variation and the interference. Because NRZ spectrum contains the residual carrier element, it would be affected by the FWM interference in the low dispersive optical fiber.
The spectrum of RZ has the linear spectrum obviously in the one-lever sideband and baseband; one-lever sideband will overlap in the adjacent channel of WDM system which has minor interval. So it can arouse badly interference between the adjacent channels. Then its optical spectrum width is bigger than NRZ and CS-RZ so as not to assure the optical efficiency in the receiver.
In parallel to NRZ, the optical spectrum shape of CS-RZ is prohibited in the frequency element of carrier wave. But the two interval linear-spectrums with a period of a byte frequency are appeared, which is 40GHz in this article. When the frequency interval of WDM channel is bigger than 100GHz, this characteristic can diminish the loss of FWM, but it can result in serious loss of FWM when the frequency interval is 75GHz. The spectrum width of CS-RZ is between NRZ and RZ.
In the baseband and first-order sideband which don't conclude the linear-spectrum-element, three kinds of DPSK signals have no pinnacle because of itself characteristic, which can prohibite the non-linear effect in the optical fiber such as FWM and SRS. In terms of the spectrum shape, there are some successive performance between the NRZ-DPSK and NRZ, RZ-DPSK and RZ, CS-RZ-DPSK and CS-RZ. The spectrum width of NRZ-DPSK is the smallest, CS-
RZ-DPSK is the following and RZ-DPSK is the biggest.
IV. The analysis of transmission performance
A. The performance of anti-dispersive code format
Different codes have different spectrums; the widths of various spectrums determine the dispersion
which has different effects. Dispersion is the width of the pulse wave caused by diverse waves transmitted in the optical fiber differently. When the pulse width comes to some extent, it appears the inter-codes interference badly. Towards the transmission system of 40Gbps, we should consider the anti-dispersion characteristic of different codes [13]. Such as figure5, it is compared the anti-dispersion characteristic of DPSK and RZ, NRZ. With the same input power, the peak value power of RZ is two times of NRZ, thus after the transmission of optical fiber, the RZ power can converge intensely, but the power of NRZ is dispersive, using the RZ code is good for enhancing the acceptable sensitivity so as to decrease the requirement of OSNR in the transmission system. We can conclude the optimal optical anti-dispersive performance of RZ-DPSK.
B. The anti-PMD ability of code format
In the standard single-mode optical fiber, the base mode is composed by a couple of vertical polarisation modes. Only when it is located in the round symmetry with refractive index, the half of the whole optical power of each-mode carrier transmits in the same speed and arrives at the optical port simultaneity and becomes the single mode. Because the practical fiber with different refractive indexes is not isotropy, the speed of mode will be influenced. It can be transmitted with different speeds to cause the dispersion of the signals. This phenomenon is called polarization mode dispersion. PMD has a little effect on the speed under 10Gbps, which it isn't basically considered. Considering the 10Gbps and the above speed, the less-extrusive polarization mode dispersion can become the key factor of limiting the performance of system [14]. When the speed of the single channel comes to 40Gbps, polarization mode dispersion would emerge the high-level effect obviously, which has an effect on the transmission performance of the system severely. Assumed the power of PMD to be 1dB, 10Gbps nonelectric relay system has the maximum transmission distance 200km, thus correspondingly the system of 40Gbps is only 25km, so the linear modulation code should be considered the ability of anti-PMD dispersion effect. The anti-PMD performance of each code compares below, such as figure7. Through the com-
ИРКУТСКИЙ ГОСУДАРСТВЕННЫЙ УНИВЕРСИТЕТ ПУТЕЙ СООБЩЕНИЯ
parison we can get the conclusion that RZ code has the best ability of anti-PMD.
8 76 5
■s
£
<S 3 0» 3
* ч p-
10 -1
2 4 6 8
V2*L*D[l04(GHz)!]
10
Fig. 6. The contrast of some different modulate methods
0.4
EateT
showed in table 1. The various simulations of codes are showed in figure8.
The best remaining dispersion of NRZ is 10ps/nm; dispersion capacity of 1dB Q value is 60ps/nm. Due to the bad performance of the system, when the Q value is 16.9dB (BER=10-12), the remaining dispersion scope is 45ps/nm. When using the CS-RZ code, the best remaining dispersion is -15ps/nm, and the dispersion capacity of 1dB Q value is 60ps/nm, which is the same as NRZ code. But when the Q is 16.9dB with the good performance of system, the scope of the remaining dispersion is 100ps/nm. No matter we use the NRZ or CS-RZ, the dispersion capacity of the system is also extraordinary small. Thus, accurate dispersion management is absolutely necessarily in the 40Gbps transmission system. We can get the eye pattern through the computer simulations which are showed in the figure 8. When the Q is 6 from the eye pattern of the receiver, we can get the same pattern between RZ and CS-RZ, because they are also the RZ, and the difference is only the dutyfac-tor between the two. The dutyfactor of CS-RZ is bigger than the RZ, and the upper jitter of NRZ is comparative severity.
Eye Diagram Analyzer 1.0
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- 1 Шк
:
Fig. 7. The contrast of anti-disperse of PMD
V. The format of transmission system and the results of simulations
Make the system configuration of simulations. Transmitter uses the CW laser, which is with the wavelength of 1550nm. With the input MZM of NRZ electrical signal generates the optical NRZ and can be sent to circuitry; The CS-RZ is composed by two MZM concatenation; the first one produces the NRZ and be sent to the second level; From the output CS-RZ of the second level and the second MZM modulator, RF port transmits the sin clock signals when the transmission speed is the half of the whole. The transmission speed is 40Gbps and the signal sequence is 27-1. The transmission optical fiber is composed by the five different single- mode fibers (SMFs) with the length of 50km, and the whole length is 250km. The dispersion compensation uses the dispersion compensation fiber (DCF). The parameter of SMF and DCF is
Time (bit period) Time (bit period)
(a) The figure of receiver (NRZ) (b) The figure of receiver (RZ) 5 Eye Diagram Analyzer 1.0
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Time (bit period)
(c) The figure of receiver (CS-RZ) Fig. 8. The figure of receiver (Q=6)
VI. Conclusions
In this article, it produces 40Gbps optical signals of three modulation formats through computer simulation and compares its wave shape and optical spectrum. Then the performance of dispersion compensation is simulated through the no-zero dispersion shifted fiber by single-channel transmission mode. We can get the results: The ability of anti-nonlinear and anti-PDM of RZ and CS-RZ is stronger than NRZ under the complete dispersion compensation condi-
tion. We can enhance the quality of transmission through appropriate dispersion compensation. In the future we can expand more workings on the research from the following aspects to go along the research on the new code: carry on the research on the non-linear phenomena in the high-speed optical fiber transmission and get the way to restrain them. Do our more research on the dispersion management for the better transmission performance.
Item name SMF DCF
Attenuation(dB/m) 0.275 0.625
Disperse (ps/(nm*km)) 17 -85
Disperse slope (ps/(nm2*km)) 0.058 -0.029
Available area (^m2) 80 14.4
Non- linearity index (10-20D/W) 2.5 3.2
Table 1
The parameter of SMF and DCF
VII. Acknowledgement
This work was supported by the Jiangxi Provincial Department of Education GJJ08242, the fund of Jiangxi Provincial Postgraduate Innovation Project YC08B027 and fund of East China Jiaotong University 06ZKXX03.
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