Super-High Speed Fiber Optical Communication System Design and Simulation Kaikai Xu
David Cheng
Xingfa Huang
Department of Electrical Engineering Department of Electrical Engineering California State University California State University Fullerton,CA Fullerton,CA +1-714-278-3734 +1-714-278-3734
[email protected] du
[email protected]
ABSTRACT WDM is the most classic techniques in optical networks. WDM technology allows the capacity of a fiber by the multiwavelengths transmission increases to several dozens times, and even several hundred times by comparing with the singlewavelength transmission capacity. In view of the broad prospects of WDM communication systems, this essay has done an in-depth study and research on WDM. Then, design an 80 Gbps denseWDM communication system and analyze the performance of this communication system by BER standard. And the simulation results show that: the design is feasible.
Categories and Subject Descriptors
SISC Institute, CETC Chongqing, China
[email protected]
DWDM uses cooling laser, it needs cooling technology to stabilize the wavelength, the difficulty of realization to be very big, and cost is very high. CWDM avoids this difficulty, the DFB laser which used by CWDM system does not need cooling, thus largely reduces the cost, entire cost of CWDM system only 30% of DWDM. Based on ITU-T G.692 recommendations, the absolute reference frequency of DWDM system should be 193.1 THz (corresponding to wavelength 1552.52 nm), the frequency spacing of each two wavelengths should be several times more than the entire 100 GHz (correspond to approximately 0.8 nm of wavelengths spacing).
I.6.6 [Simulation and Modeling]: Optical communication system Design and Simulation – 80GbpsWDM, LinkSim
Thus, use cooling technology to the laser output for wavelength stability, is very important. Wavelength shifting should be maintained to be much less than 1 nm (about 100 GHz).
General Terms
2. SIMULATION ENVIRONMENT
Design
Simulator used in this WDM system is LinkSim. This simulator is supplied by Rsoft Design Group, which is a famous manufacturer for optical simulation. From simulation, the performance and cost of a system can be predicted and decided.
Keywords Optical communication system, DWDM, LinkSim Simulation
1. INTRODUCTION The wavelength division multiplexing (WDM) is one kind of transmission technology in the optical fiber communications. It uses the characteristic that an optical fiber is possible to transmit many different wavelength light carriers simultaneously to divide the possible applied wavelength range by optical fiber into certain wave bands, each wave band made an independent channel to transmit one predetermined wave length light signal. The essence of WDM is Optical Frequency Division Multiplexing (OFDM), only because the light wave is usually used the wave length, not the frequency, to describe, the monitor and control. In contrast, WDM with low density is called the coarse wavelength division multiplexing (CWDM).
3. SYSTEM DESIGN A complete optical communication WDM system usually has three main components: optical transmitter, optical fiber, optical receiver and multiplexer (including optical gating). To reach 80Gbps, 8 channels with 10Gbps each are multiplexing in to one fiber. The digital signal used is NRZ signal. Because
f 3− dB , NRZ = (1 / 2) f 3− dB , RZ
at the same rate.
3.1 Optical Transmitter InGaAsP(1550nm) Fabry-Parot multi-mode laser:
L=
mλ m c = × 2 2 nf
⇒ 2
λ c [(m + 1) − m ] = c ; ∆λ = ∆v ∆v = f 1 − f 2 = c 2nL 2nL While, L : length of resonant cavity; m : mode constant; c : velocity of light(vacuum); n : refractive index of active region;
Figure.1 shows characteristics of this laser. L = 345 × 10 −4 cm , −8
Lactive = 500 × 10 cm and I 0 = 30mA . By direct modulation, rise time of laser modulation bandwidth to be: slightly increases with
I0 ,
t r will limit on
f 3− dB = 0.35 / t r
.Data rate
but larger current will induce high
power to drift wavelength λ . And ∆λ of this laser is commonly 0.8nm. In DWDM, this is unacceptable.
2 π Vsignal +Vbias −Voffset , I 0 ≥ 1 I 0 sin 2 V I i Ron / off = 1 I 0 1 PI < , I i Ron / off Ii Ron / off
,
I 0 is the modulated optical power intensity, I i is electric power intensity.
Ron / off
is the maximum ratio of optical power (dBm).
Figure.2 shows a 10Gbps NRZ signal modulated by M-Z and DFB. Power provided by DFB is 1mW. Both bandwidth and linewidth provided by this method is acceptable by DWDM.
Current-Power relation of F-B laser
Figure.2 PSD of external modulated 10Gbps NRZ
3.2 Optical Fiber For long distance and high rate communication, single-mode fiber is used. And this fiber can efficiently couple with DFB singlemode laser. Two important factors that will effect on communication quality Cut-off Frequency of F-B laser Figure.1 Testing of F-B laser InGaAsP(1550nm)DFB single-mode laser: Linewidth of this spectrum is about 0.2nm. By optical gating, this is single-mode laser. Based on q
(λ B / 2n ) = Λ , optimal length
of optical gating Λ for wavelength at 1550nm can be defined. For such a high transmission rate, External Modulator (MachZehner) is used. Using this method, rise time of laser will not be constraint of bandwidth. Model of M-Z modulator: Modulator Frequency response:
Vsignal ( f ) Vsignal (0)
=
Coef 1
, f (GHz ) is frequency of
1 + (Coef 2 • f )
optical carrier. Transmitting power intensity response:
are: attenuation
α and dispersion D(λ ) . As:
f 3− dB = 1 / 2∆τ ; ∆τ = D(λ )L∆λ , L is length of fiber. Two single mode fibers are introduced in this paper. One is corningsmf-28-1550, which belongs to G.652, as single-mode fiber with zero-dispersion at 1312nm and α = 0.25dB / km . Another is corningleaf-1550, which belongs to G.655, as singlemode fiber with dispersion-shifting and α = 0.25dB / km . Figure.3 shows characteristics of these two fibers.
3.4 Optical Filter and Electronic Filter Optical filter will be used at de-multiplexer to separate 8 signals on different wavelength carriers. Figure.6 shows Gaussian Filter that used in this WDM system. 3-dB bandwidth of this filter is 0.1nm. Order is one. Electronic filter will be used after receiver to smooth signal. This filter can be expressed as:
Figure.3 corningsmf-28-1550 and corningleaf-1550 G.652:
D(1312 ) = 0; D(1550 ) = 17.7 ps /(nm × km)
G.655:
D(1565) = 0; D(1550 ) = 4.3 ps /(nm × km)
N 1 H Bessel ( f ) = ∑ k =1 s k f
Among them,
sk
,
k
sk =
2
(2 N − k )! k!( N − k )!
N −k
is derived from the Bessel pole. And can derive
response function of low-pass filter:
3.3 Optical Receiver Suppose the emitting optical power is 1mW, after transmitted 100km by 0.25dB/km fiber. The optimal power reached to receiver is -25dBm. If reached power is more than 1µW , PIN diode is workable. Figure.4 shows construction of this receiver.
k f ( f ) = ∑ s k f norm • j B k =1 N
H
Bess LP
−1
Among them, the normalized frequency
f norm = ln(2) • (2 N − 1) ; N ≤ 10 ; id
Figure.4 scheme of optical receiver To avoid noise increase without constrain bandwidth of receiver, capacitor with much smaller value than PIN, and resistor with a large value are included in this circuit. Attribution of this circuit is shown in Figure.5
Gaussian Optical Filter
Bessel Digital Filter Figure.6 Filters Figure.5 attribution of Figure.4
3.5 BER Consideration Algorithms are as follows:
4. SIMULATION RESULTS
(2πCT ) f 2 4kT + 4kTΓ Rf gm 2
Si,circuit( f ) =
This equation, express noise intensity of preamplifier;
Γ :channel
feedback resistance; capacitance;
noise factor;
CT
Rf
:
:total input
g m :transconductance of amplifier input.
i2shot = 2qIBeff ; i2shot,dark = 2qIdarkBeff
Equation.2
This equation, express noise intensity of shot noise; current;
4.1 Simulation 80Gbps WDM Layout
Equation.1
I dark
Beff
:dark current;
I
:signal
: 3-dB bandwidth of
transimpedance receiver.
i
2
s − sp
Figure.7 WDM model
f end
∑P P (f )
2
= 4 R Beff
s
Equation.3
n
f = f start
This equation, express noise intensity of signal-spontaneous ASE(amplified spontaneous emission); Ps :power of signal; power of noise;
f start
and
f end
Pn :
represent the begin and end of
4.2 Optimization We are interested in obtain information about the best emitting power of laser and the optimal transmitting distance and the maximum number of channel. We considered practical operating conditions for all system parameters. Figure.8 shows performance of 1 channel is on, the others are off.
ASE power spectrum, respectively.
i 2 sp − sp = 4 R 2 Beff
f end
f end
∑ ∑ P ( f )P ( f n
1
n
2
)
Equation.4
f1 = f start f 2 = f start
This equation, express spontaneous-spontaneous ASE noise intensity;
i 2 RIN = I 2 N RIN Beff
Equation.5
bit error rate can be expressed as:
BER =
1 Q ; erfc Q= 2 2
I1 − I 0 i
2
n1
+
i 2 n0
Figure.8 Figure.9a shows performance of this system with corningsmf-281550. Figure.9b shows performance of this system with corningleaf-1550.
Figure.9a
Choose Channel#1 as an example, Figure.10a is eye diagram of output of common SM fiber system,
BER ≈ 10 −12 . Figure.10b is eye diagram of output of −24 Dispersion-Shifted Fiber, BER ≈ 10 By using Dispersion-Shifted fiber, Figure.11a shows 8 channel signals are commonly multiplexed. If Gaussian optical filter with 3-dB bandwidth of 0.1nm is used in multiplexer, this system can work much better by reducing interference, which shows in Figure.11b. And the second choice is chosen for all the experiment testing above. The same Gaussian filter must be used at de-multiplexer. Figure.9b In WDM, each two channels will have interactions to decrease BER. If dispersion- shifted fiber replaces common single-mode fiber to do dispersion compensation, BER decreases dramatically while the emitting power is kept at the same value. Based on this standard: to keep received power at PIN is more than 1µW , maximal transmitting distance is 160km for 10mW and 120km for 1mW, while attenuation coefficient of fiber is 0.25dB/km. If some unavoidable and unpredictable factor are considered, 100km will be a conservative decision.
4.3 System Performance Based on scanning results above, suppose laser output is 5mW, Figure.10 shows the system performance by BER and Eye Diagram.
Figure.11a
Figure.11b Figure.10a
5. CONCLUSION This paper gives a simulated scheme of DWDM. 8 channels are multiplexed in 8nm region to reach 80Gbps transmission. Through this design, no optical amplifier is used. As Cband(1530~1565nm) can provide total linewidth of 35nm to make 35 channels multiplexing possible, the transmitted rate can reach to 350Gbps. From design and discussion above, we can conclude that. the more channels are multiplexed, the larger BER value will be generated. If emitting power is lower, such as 5mW is used to transmit a distance that is much longer than 100km, EDFA may be used. The problem is that EDFA may interfere in signal by its noise without effectively amplifying signal. Figure.10b
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