Performance Assessment of High Density Wavelength Division by zti51661


									    Performance Assessment of High Density Wavelength Division Multiplexing
          Systems with Dispersion Supported Transmission at 10 Gbit/s

                                            Mário M. Freire
               Department of Mathematics and Computer Science, University of Beira Interior
                        Rua Marquês d'Ávila e Bolama, P-6200 Covilhã, Portugal

                                           Henrique J. A. da Silva
                    Department of Electrical Engineering, Pole II of University of Coimbra
                              Pinhal de Marrocos, P-3030 Coimbra, Portugal

                        Abstract                              solution for high-capacity DST on long spans of SMF is
    This paper presents a simulation methodology for          the optical transmission of wavelength division
performance assessment of wavelength division                 multiplexed (WDM) 10 Gbit/s channels. We have
multiplexing (WDM) systems using directly modulated           discussed the impact of single- and double-cavity Fabry-
quantum-well lasers and optically preamplified direct         Perot (FP) demultiplexers on the performance of
detection receivers. The methodology is based on a pure       dispersion supported transmission of two [5] and three [6]
semi-analytical method which combines noiseless WDM           10 Gbit/s WDM channels separated 1 nm. In a further
transmission simulation with noise analysis in those          study, we have assessed the system performance for three
systems. Using this approach, we discuss performance          10 Gbit/s WDM channels separated 0.5 nm, assuming that
implications of single-cavity, double-cavity and three-       a three-mirror FP filter is used as demultiplexer [7].
mirror Fabry-Perot (FP) demultiplexers for dispersion             In this paper, a modeling and simulation methodology
supported transmission (DST) of three WDM channels            is presented for performance assessment of multichannel
separated 1 nm. Assuming that a three-mirror filter is        optical communication systems. Using this methodology,
used as demultiplexer, the performance of a 40 Gbit/s         we compare the performance of three-channel WDM-DST
capacity four-channel WDM-DST system is assessed for          with 1 nm channel spacing for single-cavity, double-cavity
0.5 nm channel spacing. The robustness of the                 and three-mirror FP demultiplexers. The performance of a
multichannel DST method against pattern dependencies is       40-Gbit/s capacity four-channel WDM-DST system is
also investigated for this channel spacing.                   assessed for 0.5 nm channel spacing, assuming that a
                                                              three-mirror Fabry-Perot filter is used as demultiplexer.

1. Introduction                                               2. Modeling and simulation methodology
    Single-channel lightwave systems operated at 10               The most appropriate performance measure for digital
Gbit/s are being introduced commercially, but long term       optical communication systems is the average error
trends indicate that signaling rates double every two years   probability or bit error rate (BER). In the methodology
and it is predicted that there will be a need for 40 Gbit/s   outlined here, the average error probability is estimated
by the end of the decade [1]. As a consequence, the next      using a pure semi-analytical method, which combines
generation of commercial lightwave transmission systems       noiseless transmission simulation of the signal and
will have transmission capacities over 10 Gbit/s using        interfering channels with noise analysis in wavelength-
wavelength division multiplexing (WDM) and optical            selective optically preamplified direct-detection receivers.
amplifiers [2]. The method of dispersion supported            In the following, a description of the model used for
transmission (DST) has shown to be very powerful for          simulation of the WDM-DST system is presented. This is
optical transmission at 10 Gbit/s over long spans of          followed by the BER estimation and the noise analysis.
standard singlemode fiber (SMF) [3]. However, in a                Fig. 1(a) shows the block diagram of a WDM-DST
recent DST experiment at 20 Gbit/s, the link length was       system with N channels. A brief description of the model
reduced to 53 km SMF [4], as expected from the principle      used for each system component follows. In this paper, we
of dispersion supported transmission [3]. Thus, one           assume the signal channel was represented by its lowpass
equivalent, and each lowpass equivalent of the other              dNb    I   N    N    N
channels (interfering channels) was frequency juxtaposed              =    − b − b + b ,                           (1)
                                                                   dt   qVw τ cap τ n τ esc
relatively to the lowpass equivalent of the signal channel,
as shown in Fig. 1(b).                                            dN w   N    N     N  N − N0
                                                                       = b − w − w − g0 w      S , (2)
               DRIVER MQW
                                                                   dt   τ cap τ esc τn  1 + εS
       PPG                                                        dS      N − N0     S         N
                                                                     = Γg0 w      S−    + Γβ sp w            ,     (3)
                       λ1                                         dt       1 + εS    τp        τn
               DRIVER MQW
                                        WDM                       dφ α
                                                                      = Γg0 ( N w − N wr ) + (1 − Γ ) gb w ( Nb − Nbr ) ,
                                        MUX        EDFA
                       λ2                                          dt  2                                Vs
               DRIVER MQW                                         Nb = N s         ,                               (5)
       PPG                                                                   Vw
                       λN                                     where Nb is a fictitious density, Ns is the carrier density in
                                                              the SCH, Nw is the carrier density in the quantum wells, S
                     EDFA     SMF           EDFA     SMF
                                                              is the photon density in the laser cavity, φ is the phase of
                                                              the optical field, I is the injection current, q is the
                                                              electronic charge, Nwr is the carrier density in the
             FPF       EDFA        PIN PD     AMP LPF         quantum wells for the reference bias level, Nbr is the
                                                              fictitious density corresponding to the carrier density in
                                                              the SCH for the reference bias level, and the other
                                  (a)                         symbols are defined in table I, which was obtained from
                                                              [8]-[9]. Some lasers exhibit a strongly non-linear light
        Interfering          Signal         Interfering       versus current characteristic above threshold due to
        Channels            Channel          Channels         thermal effects. As that was the case of the laser used by
                                                              Wedding in DST experiments [3], such effects have been
     ch. 1     ch. nsc-1 ch. nsc ch. nsc+1 ch. N              taken into account by expressing the bimolecular
             ...                         ...                  recombination lifetime, tn, as [9]:

             ... −∆ω          0          ∆ω ... Ω=ω−ω             τ n = τ n0e − KT I 0 ,                           (6)
                                  (b)                         where where tn0 and, KT are given in table I, and I0 is the
Fig. 1. WDM-DST system with N channels.                       laser mean input current.
     (a): Block diagram;                                          At the WDM optical multiplexer output, the total
     (b): Lowpass equivalent spectrum.                        electric field is the sum of the input electric fields. Erbium
                                                              doped fiber optical amplifiers (EDFAs) are assumed to be
    The pseudopattern generator (PPG) provides a              used in the configurations of booster, in-line, and
maximal-length pseudorandom binary sequence (PRBS)            preamplifier, as in reported DST experiments [3]. It is
with 27-1 bits at 10 Gbit/s. Each one of the N optical        assumed that these optical amplifiers act as wideband
transmitters consists of a laser driver and a MQW-DFB         linear repeaters with the same optical gain of the booster,
laser. Assuming the laser driver behaves as a non ideal       in-line and preamplifier EDFAs used in the DST
current source, the NRZ drive current applied to the laser    experiments. An equivalent noise bandwidth of 1.25 THz
is generated with exponential rising and falling edges. For   and a spontaneous emission factor of 0.3 dB have been
modeling and simulation of the dynamic response of            considered for the optical preamplifier, as in [7]. The
quantum-well lasers, a new rate equation model [8]-[9]        standard singlemode fiber (SMF) was modeled using the
has been used, which takes into account carrier transport     lowpass transfer function with first order dispersion of
effects. This model describes the carrier dynamics in the     16.2 ps/( at 1532 nm. The transfer functions of
quantum wells and in the separate confinement                 single- and double-cavity FPF have been modeled as in
heterostructure (SCH) layers, and the photon dynamics in      [5]. The finesse of the single-cavity FPF was considered
the laser cavity, yielding the following set of equations     to be 150, and for the double-cavity filter with equal
written in terms of volumetric densities:                     cavities, the finesse of each one was also considered to be
150. The three-mirror Fabry-Perot filter (TMF) has been            noise voltage, σ2sh is the variance of the shot noise
modeled as in [7]. The reflectivities of the outer mirrors of      voltage, and σ2th is the variance of the thermal noise
the TMF were considered to be 0.8 and the reflectivity of          voltage. The optical amplifier noise model we use here is
the center mirror was chosen so that the frequency                 based on the model originally derived for single-channel
response of the filter is of the second-order Butterworth          systems using semiconductor optical amplifiers [11], and
type. A PIN photodiode, with a 3-dB cutoff frequency of            further extended to fiber amplifiers [12]. The model is
9.35 GHz, is assumed to be used. The receiver main                 used for multichannel systems, and signal dependent noise
amplifier (AMP) and the lowpass filter (LPF) have been             terms are evaluated for each bit of the PRBS. The signal
jointly modeled as a lowpass RC filter with the 3-dB               photocurrent, Ik, is obtained by simulation, which, for
bandwidth required by the DST method.                              multichannel systems, is contaminated by crosstalk due to
                                                                   the imperfect response of the optical filter used as
           TABLE I. MQW-DFB laser parameters                       demultiplexer. Thus, for multichannel systems, the
           Description              Parameter         Value        variance of the signal-ASE beat noise voltage includes the
 Volume of the quantum wells           Vw            18 µm3        contribution of the signal-ASE (signal channel) plus the
      Volume of the SCH                Vs            72 µm3        crosstalk-ASE beat noises. Being Isp the spontaneous
   Optical confinement factor           Γ             0.093        emission noise photocurrent given by [12]:
                                       βsp            10-4                        ηq
                                                                                     n sp ( G − 1)hνBo La ,
  Spontaneous emission factor
                                                                       I sp = 2                                       (9)
  Differential gain in the wells       g0         4⋅10-12 m3/s                    hν
     Parameter of the SCH              gb        4.17⋅10-13 m3/s
                                                                   the variance of the noise voltage terms are given by:
 Carrier density at transparency       N0          1⋅1024 m-3
  Bimolecular recombination            τn0          0.718 ns                                      Be
                                                                       σ 2− sp = 2 Z R I k I sp
                                                                         s                           ,              (10)
 Transport time across the SCH            τcap      56.8 ps
 Thermionic emission time out             τesc      225 ps                          2 2 B         B 
                                                                       σ 2 − sp = Z R I sp e  1 − e  ,            (11)
      of the quantum wells                                                                Bo     2 Bo 
         Photon lifetime                   τp       3.95 ps
      Differential quantum
       efficiency per facet
                                           η      0.0442 W/A                         2
                                                                       σ 2 = 2 Be qZ R I k + I sp ,
                                                                         sh              [           ]              (12)
    Gain compression factor                 ε    2.33⋅10-23 m3                 2 2
                                                                       σ 2 = Z R I th Be ,                          (13)
 Linewidth enhancement factor              α          3.22               th
        Thermal Constant                   KT         15.9
                                                                   where Be is the electrical bandwidth, Bo is the optical
     Emission wavelength                  λnsc      1532 nm
         (signal channel)                                          bandwidth, η is the quantum efficiency of the PIN
                                                                   photodiode, q is the electronic charge, h is Planck's
    For performance evaluation, a pure semi-analytical             constant, ν is the optical frequency, G is the optical
method has been used, which combines noiseless                     preamplifier gain, La is the loss between the optical
transmission simulation of the signal and interfering              preamplifier output and the photodetector input, nsp is the
channels with receiver noise analysis. Using the Gaussian          spontaneous emission factor of the EDFA, ZR is the
approximation, the average error probability may be                receiver transimpedance, and Ith is the spectral current
estimated by [10]:                                                 density of the thermal noise, which is assumed to be 25
                                                                   pA/ Hz .
                L    v( τ k ) − Vth 
               ∑ Q
    Pe =                             ,                  (7)       3. Simulation results and discussion
           L             σ( τ k ) 
               i =1                 
                                                                       In this section, we assess the performance of WDM
where L is the length of the used PRBS, Q is the well              systems with dispersion supported transmission at 10
known Q-function, v(τk) is the value of the signal voltage         Gbit/s. The performance assessment was focused on
at the sampling instants tk, Vth is the decision threshold         channel 2 (signal channel). Synchronous data patterns are
level, and σk is the standard deviation of the noise voltage       assumed to be transmitted in all channels, as in [5]-[7],
for the k-th bit of the PRBS, which is given by:                   since this is the worst case for crosstalk. For each fiber
                                                                   length, the system parameters, namely the bias current, the
    σ 2 = σ 2− sp + σ 2 − sp + σ 2 + σ 2 ,
      k     s         sp         sh    th                (8)       modulation current, the bandwidth of the FPF, and the
                                                                   receiver cutoff frequency, have been adjusted in order to
where σ2s-sp is the variance of the signal-ASE beat noise          minimize the EDFA preamplifier input mean optical
voltage, σ2sp-sp is the variance of the ASE-ASE beat               power for an average error probability of 10-9.
3.1. WDM-DST with 1 nm channel spacing                         1 dB for distances ranging from 24.5 to 315 km. If a
                                                               double or a single-cavity FP demultiplexer is used,
    In the following, the transmission of three 10 Gbit/s      crosstalk penalties are less than 0.8 and 2 dB,
WDM channels is studied for 1 nm channel spacing, being        respectively, in the region of small linear increase of
the emission wavelengths of the lasers of λ1=1533 nm           dispersion penalty of the DST method (80-270 km).
(channel 1), λ2=1532 nm (channel 2), and λ3=1531 nm
(channel 3).                                                                           -10

                                                                 for channel 2 [dBm]
    The average error probability against mean optical                                                DST (ch. 2)
                                                                                       -15            TM
power at the input of the EDFA preamplifier is shown in

Fig. 2, for single-channel DST, and for three-channel DST                                             DC
over 204 km SMF. As can be seen, the mean optical                                                     SC
power required to achieve a BER of 10-9 is -26.17 dBm                                  -25
for single-channel DST, and -26.25, -25.88 and -25.42
dBm for three-channel DST with a three-mirror (TM), a                                  -30
double-cavity (DC), and a single-cavity (SC)
demultiplexer, respectively. Therefore, the crosstalk                                  -35
penalty, at BER=10-9, is less than 0.3 and 0.8 dB for                                        0   50 100 150 200 250 300 350
double-cavity and single-cavity demultiplexers, whereas
                                                                                                    Fibre length [km]
for the three-mirror demultiplexer no crosstalk penalties
are estimated. The rejection of interfering channels is        Fig. 3. Receiver sensitivity for channel 2 after three-
about 16 dB (FWHM=40 GHz) and 20 dB (FWHM=60                   channel DST via different fiber lengths, considering a
GHz), for single- and double-cavity FP demultiplexers,         single-cavity (SC), a double-cavity (DC), or a three-
respectively, whereas for the three-mirror demultiplexer,      mirror (TM) FPF for selection of channel 2. For
crosstalk levels are about 35 dB (FWHM=30 GHz)                 comparison, the receiver sensitivity for single-
bellow the signal level.                                       channel DST is also displayed.

               -1                                              3.2. WDM-DST with 0.5 nm channel spacing
                                                                   The low crosstalk penalties obtained with a three-
  log (BER)

               -5                                              mirror demultiplexer allow a reduction of the channel
               -7                                              spacing. In this section, we consider a 10 Gbit/s four-

                                                               channel WDM-DST system with 0.5 nm channel spacing.
               -9            TM
                                                                   Fig. 4 shows the receiver sensitivity for channel 2 after
              -11                                              single-channel and four-channel DST via different fiber
              -13            DST(ch.2)                         lengths. In order to investigate the robustness against
                                                               pattern dependencies for 0.5 nm channel spacing, the
              -15                                              same and the complementary PRBS, with respect to signal
                    -35   -33   -30   -28   -25   -23   -20    channel, was considered for optical transmission in the
                          Mean optical power [dBm]             interfering channels. As can be seen in Fig. 4, the
                                                               differences in crosstalk penalties, using the same and the
Fig. 2. Average error probability for channel 2 versus         complementary PRBS, are less or equal than 0.37 dB for
mean optical power at the input of the optical                 distances ranging from 50 up to 318 km, and less or equal
preamplifier, after DST via 204 km SMF of three 10             than 1.56 dB in the whole range from 0 up to 318 km. For
Gbit/s WDM channels, considering a single-cavity               both cases, crosstalk penalties are less or equal than 1.1
(SC), a double-cavity (DC), or a three-mirror (TM)             dB in the region of small linear increase of dispersion
FPF for selection of channel 2.
                                                               penalty of the DST method (80-270 km). For distances
    The receiver sensitivity for channel 2, versus fiber       ranging from 100 up to 318 km, these crosstalk penalties
length, is shown in Fig. 3 for single-channel DST and for      are less or equal than 0.44 and 0.67 dB, considering the
three-channel DST with a single-cavity, a double-cavity,       same and the complementary PRBS, respectively. The
or a three-mirror FP demultiplexer. For performance            reduction of crosstalk penalties for these link lengths
comparison, the receiver sensitivity for single-channel        follows the narrowing of the laser spectra for both cases:
DST is also shown. As can be seen, if a three-mirror           the laser frequency deviation of 15 GHz at 80 km rapidly
demultiplexer is used, crosstalk penalties are less than 0.1   decreases with fiber length, being of 4.8 GHz at 270 km.
dB in the region of small linear increase of dispersion            Fig. 5 shows the receiver sensitivity for channel 2 after
penalty of the DST method (80-270 km), and are less than       single-channel DST (ch.2) and multichannel DST (ch.1 +
ch.2, ch.1 + ch.2 + ch.3, and ch.1 + ch.2 + ch.3 + ch.4) via         small linear increase of dispersion penalty of the DST
different fiber lengths. The same PRBS was considered to             method (80-270 km), if a three-mirror FP demultiplexer is
be transmitted in all channels. Comparing the system                 used. The robustness of the multichannel DST method
performance for two-channel and four-channel DST, the                against pattern dependencies is verified for long-distance
differences in crosstalk penalties are less or equal than            WDM-DST. Compared with performance studies for
0.76 dB in the region of small linear increase of                    WDM-DST systems with 1 nm channel spacing, the use of
dispersion penalty of the DST method (80-270 km). For                three-mirror demultiplexers allows an increase by a factor
three-channel and four-channel DST, the differences in               of two in the frequency utilization efficiency with low
crosstalk penalties are less or equal than 0.13 dB in the            crosstalk penalties. However, the reduction of channel
region of small linear increase of dispersion penalty of the         spacing to 0.5 nm makes WDM-DST systems more
DST method (80-270 km), and less or equal than 0.46 dB               susceptible to laser/demultiplexer misalignments.
in the whole range from 0 up to 318 km.
                                         DST (ch. 2)
   for channel 2 [dBm]

                          -15                                        [1] M. A. Newhouse, L. J. Button, D. Q. Chowdhury, Y. Liu,
                                         Same PRBS
                                                                     and V. L. da Silva, “Optical amplifiers and fibers for

                                         Complementary PRBS          multiwavelength systems”, in Proc. LEOS'95, San Francisco,
                                                                     Vol. 2, pp. OC 5.1, 1995.
                          -25                                        [2] D. A. Fishman, and J. A. Nagel, “Next generation WDM
                                                                     lightwave Systems”, in Proc. LEOS'95, San Francisco, Vol. 2,
                          -30                                        pp. WDM 1.1, 1995.
                                                                     [3] B. Wedding, B. Franz, and B. Junginger, "10-Gb/s optical
                          -35                                        transmission up to 253 km via standard single-mode fiber using
                                0   50   100 150 200 250 300 350     the method of dispersion-supported transmission", IEEE J.
                                                                     Lightwave Tech., Vol. 12, No. 10, pp. 1720-1727, 1994.
                                         Fibre length [km]           [4] B. Wedding, K. Köffers, B. Franz, D. Mathoorasing, C.
                                                                     Kazmierski, P. Monteiro, J. Nuno Matos, "Dispersion-supported
Fig. 4. Receiver sensitivity for channel 2 versus fiber              transmission at 20 Gbit/s over 53 km standard singlemode
length after single-channel, and four-channel DST                    fibre", Electron. Lett., Vol. 31, No. 7, pp. 566-568, 1995.
with 0.5 nm channel spacing.                                         [5] M. M. Freire and H. J. A. da Silva, "Performance assessment
                          -10                                        of two-channel dispersion-supported transmission systems using
                                         ch.2 (DST)                  single- and double-cavity Fabry-Perot filters as demultiplexers",
    for channel 2 [dBm]

                          -15            ch.1 + ch.2                 IEEE Photon. Technol. Lett., Vol. 7, No. 11, pp. 1360-1362,

                                         ch.1 + ch.2 + ch.3
                          -20                                        [6] M. M. Freire and H. J. A. da Silva, "Performance assessment
                                         ch.1 + ch.2 + ch.3 + ch.4
                                                                     of WDM dispersion supported transmission systems using single
                          -25                                        and double-cavity Fabry-Perot demultiplexers", in Proc.
                                                                     LEOS'95, San Francisco, Vol. 2, pp. OC 6.4, 1995.
                          -30                                        [7] M. M. Freire, A. M. F. de Carvalho, and H. J. A. da Silva,
                                                                     “performance implications of three-mirror Fabry-Perot
                          -35                                        demultiplexers for 10-Gb/s WDM dispersion-supported
                                0   50    100 150 200 250 300 350    transmission with 0.5 nm channel spacing”, IEEE Photon.
                                                                     Technol. Lett, Vol. 9, No. 9, pp. 1261-1263, 1996.
                                           Fibre length [km]
                                                                     [8] R. F. S. Ribeiro, J. R. F. da Rocha, A. V. T. Cartaxo, H. J. A.
Fig. 5. Receiver sensitivity for channel 2 versus fiber              da Silva, B. Franz, and B. Wedding, "FM response of quantum-
length, after DST via SMF of one, two, three and                     well lasers taking into account carrier transport effects", IEEE
four 10 Gbit/s WDM channels separated 0.5 nm.                        Photon. Technology Lett., Vol. 7, No. 8, pp. 857-859, 1995.
                                                                     [9] R. F. S. Ribeiro, "Simulation of DST dispersion range",
4. Conclusions                                                       contribution to deliverable 25 of TRAVEL-RACE 2011,
                                                                     University of Aveiro, Portugal, 1994.
   Using a simulation methodology for performance                    [10] M. C. Jeruchim, “Techniques for estimating the bit error
assessment of WDM-DST systems, we have discussed                     rate in the simulation of digital communication systems”, J.
                                                                     Select. Areas Commun. Vol. SAC-2, No. 1, pp.153-170, 1984.
performance implications of single-cavity, double-cavity
                                                                     [11] N. A. Olsson, “Lightwave systems with optical amplifiers”,
and three-mirror FP demultiplexers for dispersion                    IEEE J. Lightwave Technol., Vol. 7, No. 7, pp. 1071-1082,
supported transmission of three 10 Gbit/s WDM channels               1989.
separated 1 nm. It was shown that crosstalk penalties for            [12] Y. K. Park, and S. W. Granlund, “Optical preamplifier
10 Gbit/s four-channel WDM-DST, with 0.5 nm channel                  receivers: application to long-haul digital transmission”, Optical
spacing, are less or equal than 1.1 dB in the region of              Fiber Technol., Vol. 1, pp. 59-71, 1994.

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