1. Introduction
Since the beginning of optical communications, fiber manufacturer have tried to give their contribution to system performance improvement by designing new fiber types with parameters tailored to reduce the impact of propagation effects. After the development of the standard single-mode fiber (SSMF), fibers with low dispersion were introduced to ease in-line optical dispersion compensation. Then, fibers with large effective area to reduce the impact of nonlinear effects were developed, and recently new fibers with low loss and even larger effective area have been introduced in the market.
In order to compare fiber types, different figure of merits (
FoM) have been proposed encompassing the effects of both nonlinearity and loss [
1A. Pilipetskii, “Nonlinearity management and compensation in transmission systems,” OFC 2009, paper OTuL5.
,
2Y. Yamamoto, M. Hirano, and T. Sasaki, “A new class of optical fiber to support large capacity transmission,” OFC 2011, paper OWA6.
]. With the introduction of coherent detection and electronic dispersion compensation [
3M. G. Taylor, “Coherent detection method using DSP for demodulation of signal and subsequent equalization of propagation impairments,” IEEE Photon. Technol. Lett. 16(2), 674–676 (2004). [CrossRef]
] for 40 and 100 Gbps systems, a new paradigm for system design has arisen: it appears that uncompensated links outperform dispersion managed solutions and that larger dispersion fibers outperform lower dispersion ones [
4V. Curri, P. Poggiolini, A. Carena, and F. Forghieri, “Dispersion compensation and mitigation of non-linear effects in 111 Gb/s WDM coherent PM-QPSK systems,” IEEE Photon. Technol. Lett. 20(17), 1473–1475 (2008). [CrossRef]
,
5D. van den Borne, V. Sleiffer, M. Alfiad, S. Jansen and T. Wuth, “POLMUX-QPSK modulation and coherent detection: the challenge of long-haul 100G transmission,” ECOC 2009, paper 3.4.1.
]. In order to deal with this scenario, in this paper we propose a
FoM definition taking into account the effects of chromatic dispersion on system performance, extending results presented in [
6A. Carena, V. Curri, G. Bosco, R. Cigliutti, E. Torrengo, P. Poggiolini, A.Nespola, D. Zeolla, and F. Forghieri “A novel figure of merit to compare fibers in coherent detection systems with uncompensated links,” ECOC 2011, paper Th.12.LeCervin.5.
] to channel spacing larger than the symbol rate. The presented closed-form of
FoM is based on a model for the impact of non-linear propagation proposed in [
7P. Poggiolini, A. Carena, V. Curri, G. Bosco, and F. Forghieri, “Analytical modeling of non-linear propagation in uncompensated optical transmission links,” IEEE Photon. Technol. Lett. 23(11), 742–744 (2011). [CrossRef]
].
First, in Sec. 2, we give the
FoM definition based on the system margin. Then, in Sec. 3, we recall the model of nonlinear interference [
6A. Carena, V. Curri, G. Bosco, R. Cigliutti, E. Torrengo, P. Poggiolini, A.Nespola, D. Zeolla, and F. Forghieri “A novel figure of merit to compare fibers in coherent detection systems with uncompensated links,” ECOC 2011, paper Th.12.LeCervin.5.
] to obtain a closed-form expression of
FoM at the Nyquist limit, i.e., for channel spacing (
Δf) equal to the symbol rate (
RS). In Sec. 4, we compare the proposed
FoM with the ones earlier presented in the literature [
1A. Pilipetskii, “Nonlinearity management and compensation in transmission systems,” OFC 2009, paper OTuL5.
,
2Y. Yamamoto, M. Hirano, and T. Sasaki, “A new class of optical fiber to support large capacity transmission,” OFC 2011, paper OWA6.
] showing how it is fundamental to include the role of chromatic dispersion in order to obtain a fair comparison between different fiber types. In order to validate the proposed
FoM we simulated a 10 channels Nyquist WDM PM-QPSK link and, in Sec. 5, displayed how simulative estimates of
FoM based on system margin are in excellent agreement with the proposed closed-form expression. In Sec. 6, we tested by simulation the applicability of the proposed
FoM to channel spacings larger than the Nyquist limit, showing that the presented
FoM works properly also in such scenarios. Finally, in Sec. 7, we validate experimentally the performance prediction obtained through the new
FoM for different fiber types and in conclusion (Sec. 8) we discuss the obtained results.
2. The figure of merit based on system margin
We consider a multi-span periodic link based on optical amplification operating at a target bit error-rate BERtarget. In such a scenario we can define the following parameters.
• Fiber loss Afiber: it is the intrinsic loss introduced by each fiber span, i.e., Afiber = αdB·Ls [dB], where αdB is the fiber loss coefficient expressed in dB/km and Ls is the span length expressed in km.
• Span budget Amax: it is the maximum tolerable loss per span in order to keep the system in-service, i.e., BER ≤ BERtarget.
Using these two parameters we can then define the system margin as the tolerable excess span loss with respect to fiber loss:
In traditional links based on dispersion management, the
system margin strongly depends on the dispersion map, modulation format and data-rate, therefore it cannot be used as universal parameter
to weight the performance of a fiber type. If we focus our analysis on transmission based on coherently-received modulation formats, we know that the optimal links are uncompensated [
4V. Curri, P. Poggiolini, A. Carena, and F. Forghieri, “Dispersion compensation and mitigation of non-linear effects in 111 Gb/s WDM coherent PM-QPSK systems,” IEEE Photon. Technol. Lett. 20(17), 1473–1475 (2008). [CrossRef]
,
5D. van den Borne, V. Sleiffer, M. Alfiad, S. Jansen and T. Wuth, “POLMUX-QPSK modulation and coherent detection: the challenge of long-haul 100G transmission,” ECOC 2009, paper 3.4.1.
]. Therefore, we can suppose to use the system margin as quality parameter for a specified fiber type. In this paper, we propose to define as fiber figure of merit (
FoM) for uncompensated links transmitting coherently-received modulation formats, the system margin, i.e.:
In general, the FoM is difficult to be interpreted as an absolute quality parameter, therefore, it can be useful to refer to a differential FoM (ΔFoM) defined with respect to a reference fiber as
3. Closed-form expression for channel spacing equal to the symbol rate
The
FoM definition proposed in
Eq. (2) has a general validity, provided that it is applied to uncompensated links transmitting coherently-received modulation formats. In general, the
FoM can be estimated by simulation or measurements. In this section, we demonstrate that a closed-form expression for the
FoM can be derived for multichannel transmission with channel spacing equal to the symbol rate
Rs.
We focus our analysis on multi-span systems based on EDFA lumped amplification completely recovering span loss. According to [
7P. Poggiolini, A. Carena, V. Curri, G. Bosco, and F. Forghieri, “Analytical modeling of non-linear propagation in uncompensated optical transmission links,” IEEE Photon. Technol. Lett. 23(11), 742–744 (2011). [CrossRef]
], the performance of modulation formats coherently-received over uncompensated links can be inferred from an effective optical signal-to-noise ratio
OSNRNL that accounts for nonlinear interference (NLI) as additional Gaussian noise with equivalent noise power
PNLI, so that:
The amount of ASE noise at the receiver (Rx) is
where
Ns is the number of spans,
A is the span loss,
F is the EDFA noise figure,
h is Plank’s constant,
ν [Hz] is the operating center frequency and
Bn [Hz] is the reference noise-bandwidth for
OSNR measurement. At the Nyquist limit, i.e., when the channel spacing
Δf [Hz] is equal to the symbol-rate
Rs [symbol/s] and channel spectra are rectangular,
PNLI [W] in a bandwidth
Bn can be accurately approximated through the following expression [
7P. Poggiolini, A. Carena, V. Curri, G. Bosco, and F. Forghieri, “Analytical modeling of non-linear propagation in uncompensated optical transmission links,” IEEE Photon. Technol. Lett. 23(11), 742–744 (2011). [CrossRef]
]:
where
γ [1/W/km] is the nonlinear coefficient,
Leff [km] is the fiber-span effective length,
PTx,ch [W] is the transmitted power per channel,
β2 [ps
2/km] is the dispersion coefficient and
Nch is the number of transmitted channels. The effective length is defined as
where
Ls [km] is the span length and
α [1/km] is the fiber loss coefficient.
Substituting
Eqs. (5) and
(6) into the expression of
OSNRNL, the span loss
A resulting for a given target
OSNRtarget can be evaluated as a function of all system parameters. Maximizing
A with respect to the transmitted power
PTx,ch, the following expression for the maximum span budget, valid for practical scenarios with
Amax >> 1, is obtained:
where
OSNRtarget is the one yielding a required target BER for a given modulation format.
According to definition reported in
Eq. (2), using
Eq. (8) we can express the
FoM in the following closed-form:
Note that various system parameters (
F, Ns, OSNRtarget and the span length
Ls) giving the first factor of
Eq. (8) do not appear in
Eq. (9) because they can be neglected as they exactly cancel out when calculating
. Analyzing in detail
Eq. (9), we observe that for a large occupied bandwidth
BWDM =
Nch·Rs (e.g., C-band and
Rs = 30 Gbaud, means
Nch≈160), log
(πNch2)>>log
(|β2|LeffRs2). Hence, the term log
(|β2|LeffRs2) becomes negligible, and the term
Rs2/log
(πNch2) is fiber-independent and cancels out when calculating
. As a result, for practical scenarios using several channels, we can re-write the
FoM expression in more compact form:
which is dependent only on fiber parameters and independent of
all other system parameters. This property has in fact a quite fundamental meaning:
ΔFoM, i.e., the gain or loss of system margin when replacing one fiber with a different one,
is independent of the system on which fibers are compared and therefore qualifies as a very powerful and general indicator of fiber performance. In addition,
Eq. (10) clearly highlights how different fiber parameters impact the
FoM. Obviously non-linearity makes it worse whereas dispersion improves it, although with different weights: a doubling of non-linearity causes a 3 dB decrease of
FoM whereas a doubling of dispersion causes a 1.5 dB improvement. Note that dispersion, whose inclusion is the principal novelty in our
FoM definition, improves it because of its impact on the generation of nonlinearity given by the ratio
Leff/LD (see
Eq. (6)) where
LD is the dispersion length defined as
LD =
1/ (
|β2|Rs2). As a final comment it is important to remark that the use of the simplified expression of
Eq. (10), instead of complete expression given by
Eq. (9), implies always a limited inaccuracy within fractions of dBs, even for a small number of channels (e.g.,
Nch = 10). Moreover, such an inaccuracy decreases with the increasing of dispersion.
4. Earlier FoM definitions and fiber comparison
After introducing the original closed-form expression for the
FoM (
Eqs. (8) and
(9)), we compared it with two earlier
FoMs proposed in the literature [
1A. Pilipetskii, “Nonlinearity management and compensation in transmission systems,” OFC 2009, paper OTuL5.
,
2Y. Yamamoto, M. Hirano, and T. Sasaki, “A new class of optical fiber to support large capacity transmission,” OFC 2011, paper OWA6.
]:
Note that their original expressions used the effective area (
Aeff) rather than the nonlinear parameter
γ = 2π·n2/(
λ·Aeff). We have replaced
Aeff with
γ to ease the comparison with the one we propose.
FoM1 takes into account only the scaling in the nonlinear coefficient together with the difference in span loss.
FoM2 is an upgrade of
FoM1 considering also the interplay between nonlinearity and loss through the effective length. The
FoM we propose in
Eq. (10) is therefore a further upgrade that takes into account the simultaneous effect of nonlinearity and fiber dispersion, together with fiber loss.
We compared the three
FoMs considering different fiber types: a standard single mode fiber (SSMF), a pure silica core fiber (PSCF) and a non-zero dispersion-shifted fiber (NZDSF). Fiber parameters are displayed in
Table 1
together with
ΔFoM values, where SSMF has been selected as the reference fiber. Chromatic dispersion of the NZDSF is very low, therefore it can be considered as a worst-case.
Table 1 Fiber parameters and ΔFoM comparison with Ls = 100 km.
| | | | | | | ΔFoM [dB]
|
|---|
| Α [dB/km] | Γ [1/W/km] | Β [ps2/km] | D [ps/nm/km] | ΔFoM1
[dB] | ΔFoM2
[dB] | Eq (10) | Nch = 10
Eq (9) |
|---|
SSMF
| 0.21
| 1.26
| −21.3
| 16.7
|
| 0.0
| 0.0
| 0.0
|
PSCF
| 0.18
| 1.00
| −26.2
| 20.6
| 4.0
| 3.4
| 4.1
| 4.0
|
| NZDSF | 0.22 | 2.00 | −3.3 | 2.6 | −3.0 | −2.8 | −7.0 | −6.2 |
Note that for the
FoM we propose, results are shown either using the simplified expression given by
Eq. (10), or using the complete expression (
Eq. (9)) with only 10 channels. It can be observed that for large dispersion (PSCF) there are no differences, while reducing dispersion a limited difference appears: −7 dB against −6.2 for NZDSF.
Analyzing
Table 1, for the PSCF it can be observed that the three
ΔFoMs present small differences limited within 0.7 dB. While, for the NZDSF, whose dispersion value is much smaller than the reference fiber (SSMF) one,
ΔFoM1 and
ΔFoM2 display a mismatch larger than 4 dB with respect to the
ΔFoM we propose. Such a large difference confirms the need for the inclusion of the dispersion parameters in a
FoM definition that allows fair comparison between fibers presenting large differences in transmission parameters.
In order to highlight the dependence of
ΔFoM on fiber dispersion, in
Fig. 1
we plotted it with respect to
β2 for three different pairs of (
α,
γ) values, again taking as reference the SSMF of
Table 1. It can be clearly observed that on the dispersion range of practical transmission fibers the
FoM varies over a range of 6 dB independently of the value of nonlinear and loss coefficients. This plot gives a further confirmation of the need to properly include dispersion in a quality parameter allowing comparison between fiber types.
Fig. 1 ΔFoM as a function of dispersion, for span length L
s = 100 km, with fiber loss and non-linearity as parameters. Labeled dots represent the fibers listed in
Table 1 5. Simulative validations
In order to validate the proposed
FoM definition, we carried out an analysis aimed at calculating by simulation the
FoM for different fiber types and comparing such results with the closed-form expressions of
Eqs. (8) and
(9). For simulations we used the same set-up described in [
8E. Torrengo, R. Cigliutti, G. Bosco, A. Carena, V. Curri, P. Poggiolini1, A. Nespola, D. Zeolla, and F. Forghieri. “Experimental validation of an analytical model for nonlinear propagation in uncompensated optical links,” ECOC 2011, paper We.7.B.2.
] with
Δf = Rs in order to have a reference system scenario used also for the experimental validation that is presented in Sec. 7. The channel comb was made of 10 optically-shaped Nyquist-WDM PM-QPSK channels at
RS = 30 Gbaud (120 Gbps) spaced
Δf = RS. The link was a 8 spans uncompensated link with each span made of
LS = 100 km of fiber, a variable optical attenuator (VOA) and an EDFA completely recovering the span loss.
We considered the three fiber types (SSMF as reference, PSCF and NZDSF) whose parameters are reported in
Table 1. The target
BER was established to be
BERtarget = 3·10
−3 corresponding to
OSNRtarget = 16.3 dB that includes realistic Tx impairments and crosstalk by fitting to the back-to-back performance of the experimental set-up.
For each considered fiber type we swept the transmitted power per channel
Ptx,ch from −5 up to + 7 dBm and varied the attenuation of the VOA in order to evaluate the maximum span loss, the span budget
Amax, ensuring the transmission operating below
BERtarget after 8 spans. Results are plotted in
Fig. 2
for the three fiber types. The plotted curves display the expected qualitative parabolic behavior characterized by an optimal power representing the best trade-off between advantages of power enlargement ad detrimental effects of nonlinearities. Also the hierarchy between fibers is the expected one showing performance advantages with the increasing of fiber dispersion due to beneficial effects of chromatic dispersion in mitigating nonlinearities.
Fig. 2 Simulative results of maximum span budget vs. the power per channel for the fibers whose parameters are listed in
Table 1 used in the considered 8 spans, PM-QPSK, R
S = 30 Gbaud,10 channel system with Δf = R
s. Evaluations of FoM according to
Eq. (2) are reported on the graph.
From a quantitative evaluation of maxima for each curve, and with the knowledge of the fiber loss parameter and span length, the
FoM can be easily evaluated using
Eq. (2). Calculations are reported on the graph showing
FoM = 12.1 dB for the PSCF,
FoM = 8.0 dB for the SSMF and
FoM = 1.7 dB for the NZDSF. Considering the SSMF as a reference, we can evaluate
ΔFoM, obtaining
ΔFoM = 4.1 dB for the PSCF and
ΔFoM = −6.3 dB for the NZDSF: results in excellent agreement with the ones based on the closed-form expressions presented in
Table 1. Such an agreement validates the proposed
FoM for
Δf = Rs and confirms the need of inclusion of dispersion in giving a proper hierarchy to fiber types used to transmit coherently-received modulation formats on uncompensated links.
6. Extension of FoM definition to Δf > Rs
The proposed closed-form expression of
FoM is so far proposed and validated only for links operated at the Nyquist limit, i.e., multi-channel systems based on
Δf = RS. The reason is that for such scenarios the NLI presents an easy-to-be-handled closed-form expression [
7P. Poggiolini, A. Carena, V. Curri, G. Bosco, and F. Forghieri, “Analytical modeling of non-linear propagation in uncompensated optical transmission links,” IEEE Photon. Technol. Lett. 23(11), 742–744 (2011). [CrossRef]
].
For larger channel spacings there indeed exists an expression for the NLI that still is a noise-like Gaussian process [
9P. Poggiolini, A. Carena, V. Curri, G. Bosco, and F. Forghieri, “A simple and accurate model for non-linear propagation effects in uncompensated coherent transmission links,” ICTON 2011, paper We.B1.3.
], but it does not have a closed-form, therefore it is difficult to infer the general trends with respect to the system parameters.
Therefore, we followed a heuristic approach and tested by simulation the applicability of the proposed
FoM definition to channel spacings larger than the symbol rate. To pursue such an objective, we considered the same system scenario used for the simulative validation presented in Sec. 5 and redid simulations enlarging the channel spacing from 30 GHz (
Δf = RS) up to 50 GHz (
Δf = 5/3·RS). For each considered scenario, we swept the transmitted power per channel and varied the VOA level in order to estimate by simulation the maximum span budget at the link receiver for
BERtarget = 3·10
−3. Then, from the collected results we were able to evaluate the
FoM according to
Eq. (2) for each scenario, and considering the SSMF as a reference we derived
ΔFoM vs.
Δf for PSCF and NZDSF.
Results of this analysis are pictorially presented in
Fig. 3
as
ΔFoM vs.
Δf for the considered fibers. It can be clearly observed that the behavior of the curves is practically flat with respect to
Δf and the constant level is in excellent agreement with the values predicted by
Eqs. (8) and
(9) reported in
Table 1, last column. This result confirms the applicability of the proposed
FoM definition also to system scenarios based on channel spacing larger than the Nyquist limit. And from a physical interpretation point of view, it says that the scaling of performance hierarchy between fibers with respect to loss, dispersion and nonlinearity is independent of the channel spacing.
Fig. 3 Simulative evaluations of
ΔFoM for the fibers whose parameters are listed in
Table 1 used in the considered 8 spans, PM-QPSK, R
S = 30 Gbaud,10 channels system with Δf = 1·R
s up to Δf = 5/3·R
s 7. Experimental validation
As a further validation, we carried out experiments aiming at measuring the maximum achievable span budget and consequently evaluate the
FoM. The experimental set-up was the same as described in [
8E. Torrengo, R. Cigliutti, G. Bosco, A. Carena, V. Curri, P. Poggiolini1, A. Nespola, D. Zeolla, and F. Forghieri. “Experimental validation of an analytical model for nonlinear propagation in uncompensated optical links,” ECOC 2011, paper We.7.B.2.
] with ten 30 Gbaud (120 Gbps) PM-QPSK optically-shaped channels.
We tested experimentally SSMF and NZDSF with parameters specified in
Table 1. Span lengths were
Ls = 102 km (SSMF) and
Ls = 100 km (NZDSF). In order to reduce the linear crosstalk between channels, we did not consider the Nyquist limit case and we used a channel spacing
Δf = 1.1·Rs, i.e., 33 GHz.
Results in terms of maximum span budget vs.
PTx for BER = 3·10
−3, after 8 spans propagation, are shown in
Fig. 4b
together with the analytical prediction based on the NLI model [
9P. Poggiolini, A. Carena, V. Curri, G. Bosco, and F. Forghieri, “A simple and accurate model for non-linear propagation effects in uncompensated coherent transmission links,” ICTON 2011, paper We.B1.3.
]. The measured maximum span budgets were 31.2 dB and 25.7 dB, for SSMF and NZDSF, respectively. In
Fig. 4a simulative results are presented for the same scenario, i.e., for
Δf = 33 GHz, for three fiber types. The excellent agreement between simulations, experiments and theory can be clearly observed giving a further cross-validation of both theoretical predictions and simulative algorithms. Calculating from the experimental results the system margin difference according to
Eq. (1), we obtained a
of −6.1 dB for NZDSF vs. SSMF. Comparing this measurement with the analytical result of
Table 1 (
ΔFoM = −6.2 dB) and the simulative calculation (
ΔFoM = −6.3 dB), we can observe that differences are of the order of fractions of dBs, i.e., they are within the inaccuracy of both experiments and simulations. Hence, we can conclude that also the experiments confirm the validity of the proposed
FoM within a scenario of channel spacing larger than the Nyquist limit. Note that, instead, the
ΔFoMs calculated according to
FoM1 and
FoM2 definitions are −3.0 dB and −2.8 dB, respectively, as reported in
Table 1. Clearly, they greatly underestimate propagation penalty confirming the need of inclusion of dispersion effect in
FoM definition in order to fairly compare fiber types over a wide range of chromatic dispersion values.
Fig. 4 Simulative (a) and experimental (b) evaluations of maximum span budget for the fibers whose parameters are listed in
Table 1 used in the considered 8 spans link, PM-QPSK, R
S = 30 Gbaud,10 channels system with Δf = 1.1·R
s (33 GHz). Together with experimental results plotted as points (b), NLI model [
9P. Poggiolini, A. Carena, V. Curri, G. Bosco, and F. Forghieri, “A simple and accurate model for non-linear propagation effects in uncompensated coherent transmission links,” ICTON 2011, paper We.B1.3.
] results are plotted as continuous lines.