1 Optical switching fabrics for terabit packet switches
1.2 Modeling optical devices
None of the proposed optical fabrics includes any signal regeneration besides pure linear optical amplification. Using the common terminology introduced in [1], we have at most 1R regeneration of the signals inside the optical fabric, while we exclude 2R and 3R regeneration. As a result, physical layer impairments may accumulate when increasing the port count N or the number of planes S, so that the characterization of the used optical devices becomes crucial to effec-tively assess each architecture’s ultimate scalability. In performing the analy-sis described in this chapter, we observed that a first-order scalability assess-ment based on theoretical insertion loss values gives unrealistic results. As a clear example, the AWG in the WR architecture has an insertion loss that in a first approximation does not depend on the number of input/output ports, thus leading to a theoretical “infinite scalability.” Clearly, we needed a more accurate second-order assessment capable of capturing other important effects that characterize commercial devices, such as polarization dependence, excess losses, channel uniformity, and crosstalk. Despite their different nature, all these effects can be expressed as an input/output equivalent power penalty which accounts for both actual physical power loss and the equivalent power penalty introduced by other second-order transmission impairments, as described below.
We only focused our study on optical components, as fiber-related effects (e.g., dispersion, attenuation, non-linearities, cross-phase modulation, etc.) are likely to be negligible in the proposed architectures, mainly due to the short distances involved.
Optical switching fabrics for terabit packet switches 11
1.2.1 Physical model
The following physical-layer effects are taken into account in our analysis. See [4] for details.
Insertion Loss (IL):We indicate as insertion loss the total worst-case power loss, which includes all effects related to internal scattering due to the splitting process and also non-ideal splitting conditions, such as material defects, or manu-facturing inaccuracies. In the case ofn-port splitters, the splitting process gives a minimum theoretical loss increasing with 10 logndB, but extra loss contributions due to non-ideal effects, often referred to as Excess Losses (EL), must also be considered.
Uniformity (U):Due to the large wavelength range typically covered by multi-port devices, different transmission coefficients exist for different wavelengths.
Over the full WDM comb, the propagation conditions vary slightly from center channels to border ones. Similar uneven behaviors appear in different spatial sections of some components. These differences are taken into account by the U penalty component, which is often referred to as the maximum IL variation over the full wavelength range in all paths among inputs and outputs.
Polarization Dependent Loss (PDL):The attenuation of the light crossing a device depends on its polarization state due to construction geometries, or to material irregularities. Losses due to polarization effects are counted as a penalty in the worst propagation case.
Crosstalk (X):Asignal out of a WDM demultiplexing port always contains an amount of power, other than the useful one, belonging to other channels passing through the device. This effect is generally referred to as crosstalk. For a given useful signal at wavelengthλ, the crosstalk is usually classified [1] as either out-of-band, when the spurious interfering channels appear at wavelengths spectrally separated fromλ, or as in-band crosstalk, when they are equal toλ. For the same amount of crosstalk power, this latter situation is much more critical in terms of overall performance [16]. Both types of crosstalk translate into a power penalty at receivers dependent on the amount of interfering power.
For out-of-band crosstalk, also called incoherent crosstalk, the contribution from adjacent wavelength channelsXA is usually higher than the contribution from non-adjacent channelsXNA. Following the formalism presented in [17], the overall crosstalk relative power level, expressed in dimensionless linear units, can be approximated as follows
X(w) = 2XA+ (w−3)XNA, (1.1) where X(w) is the total amount of crosstalk power present on a given port, normalized to the useful signal power out of that port;wis the number of wave-length channels, which is typically equal to the numbernof ports of the device.
Out-of-band crosstalk is present on any WDM filtering device, such as WDM
12 D. Cuda, R. Gaudino, G. A. Gavilanes Castillo, and F. Neri
demultiplexers, 1 :nAWGs, and optical filters, due to the fact that their ability to transmit/reject out-of-band signals does not behave as an ideal step transfer function. As such, incoherent crosstalk is present in all our proposed architec-tures. Typical values for multiplexers [7] are−25 dB and −50 dB for adjacent XA and non-adjacent XNA channels, respectively. The equation above can be transformed into an equivalent power penalty (in dB), labeled OX. Following the approximations presented in [17], OX is equal to:
OX(w)|dB= 10 log10(1 +X(w)). (1.2) In-band crosstalk, or coherent crosstalk, is caused by interference from other channels working on the same wavelength as the channel under consideration.
In the WR case, the same wavelength can be generated simultaneously at the input of many AWG ports. Due to the AWG actual transfer functions, some amount of in-band power leaks to other device ports; this behavior is described in data sheets as adjacent/non-adjacent port crosstalk (XA and XNA, respec-tively, defined for physical ports instead of wavelength channels as for incoherent crosstalk). For the other architectures, space switching is not ideal so that a small portion of the useful linecard input power leaks into other switching planes, as will be explained in Section1.2.3. The impact of this crosstalk is typically high given its in-band characteristics, and the equivalent IX power penalty (in dB) for optimized decision-threshold in the receiver can be estimated [16] by
IX(n)|dB=−10 log10(1−X(n)Q2), (1.3) whereQis the target eye-opening quality factor in linear units, determining the target Bit Error Rate (BER) (typicallyQlies in the range from 6 to 7 in linear scale for BER between 10−9 and 10−12). X(n) represents here the normalized crosstalk power from the othernin-band sources (for example multi-plane non-ideal switches or crosstalk between spatially adjacent ports in AWG devices) relative to the useful signal in consideration (see Section1.2.3).
1.2.2 Device characterization
The previously described power penalties enable the characterization of pas-sive optical devices of a given number n of ports in terms of the overall power penalty, which takes into account all actual passive losses introduced by the components plus equivalent losses introduced by crosstalk impairments. We will denote withLDem(n),LSpl(n), andLAW G(n) the “equivalent” losses introduced by muxes/demuxes, couplers/splitters, and AWGs of n ports, respectively. To estimate these power penalties, a detailed study has been carried out in order to find reasonable values for realistic commercial devices, by analyzing a large num-ber of commercial device datasheets [18,19,20]. As a result, we collected typical realistic values of each parameter for the different devices. Linear and logarith-mic regression methods have been used to derive analytical formulas that fit well on datasheet values and can estimate unknown ones. For the same device type,
Optical switching fabrics for terabit packet switches 13
0 10 20 30 40
0 5 10 15 20
Number of ports (n)
L Splitter [dB]
Total penalty Ideal splitter EL U PDL
Figure 1.5 Power penalties for 1 :ncouplers/splitters.
the values reported in datasheets from different vendors were usually very simi-lar, and are often dictated by the the specification of some relevant international standard. For instance, most commercial 1 :nsplitters have values that are set by current PON standards. Thus, the values that we considered can be assumed as fairly general and consistent among different optical component vendors.
As an example, the estimated losses for coupler/splitter devices are shown in Figure1.5. These plots report the contribution of each of the individual effects described in Section1.2.1, and the resulting total equivalent power penalty. In both cases, the ideal logn-like loss dominates over the contributions of other parameters like U, PDL, and EL. However, as the number of ports increases, so does the relative contribution of these second-order parameters. For instance, 20 ports contribute 3–4 dB of additional penalty with respect to the ideal case.
The characterization of AWGs is shown in Figure1.6. While the power penalty of these devices should ideally be independent of the number of ports due to the wavelength routing property, we observe that the IL values inferred from datasheets show a dependency on the number of ports that contributes logarith-mically to the power penalty. More notably, we found that out-of-band crosstalk effects are negligible, while in-band crosstalk has a significant impact in the case of AWGs of sizen:n. In this case, crosstalk increases exponentially the power penalty, limiting the realistically useful size of the AWG device to about 10–
15 ports (which cause 13–18 dB of equivalent losses). This rather strong limit is confirmed by several experimental works, such as [7, 16], and is in contrast with many studies on switching architectures in which AWGs with large port counts are indicated as very promising components for the implementation of large optical switches.
Tunable transmitters are a key component in these architectures. They are modeled as sources characterized by a given Optical Signal-to-Noise Ratio
14 D. Cuda, R. Gaudino, G. A. Gavilanes Castillo, and F. Neri
10 20 30 40
0 5 10 15 20 25 30
Number of ports (n)
L AWG [dB]
Total penalty IL U PDL IX+OX penalty
Figure 1.6 Power penalties forn:nAWGs.
OSNRT X, corresponding to the ratio between the useful laser power and the noise floor due to spontaneous emission inside the laser. Though in standard WDM transmissionOSNRT X gives negligible effects, we will show later that it can be relevant for very large optical fabrics, due to the presence of N lasers, each one contributing a low (but non-negligible) noise floor.
For the receivers, in order to address scalability at different bit rates, we fol-lowed the analysis presented in [21] which, inferring from many different com-mercial datasheets, proposes a sensitivity slope vs. bit-rate Rb of 13.5 dB for a ten-fold bit-rate increase. Following this model, and assuming a given sensitivity at, say, 10 Gb/s, the receiver sensitivity at other bitrates in dBm is estimated as follows:
PS(Rb)|dBm=PS(10 Gb/s)|dBm+ 13.5 log10 Rb
10 Gb/s. (1.4) All architectures exhibit a common amplification and demultiplexing output stage. Regarding EDFAamplifiers, we have assumed that the EDFAs operate in the saturated regime; that is, EDFAs show a constant output power, which is split over the N/S channels that cross it simultaneously. The nominal EDFA output power is assumed to be Ptot,outEDF A, which can be set by means of gain locking techniques. Let AEDF A be the EDFApower gain. The AEDF A used in noise calculations is obtained considering the ratio between the total EDFA output power,Ptot,outEDF A, and the total EDFAinput power,Ptot,inEDF A. Furthermore, we characterized EDFAs by a noise figureFEDF A.
Finally, regarding the SOA-based space switches, we based our analysis on the characteristics of one of the few commercially available specific SOA-based switches [22]. In the “on” state, the SOAis assumed to have a noisy behavior
Optical switching fabrics for terabit packet switches 15
characterized by a noise figure FSOA. In the “off” state, a realistic switching Extinction Ratio (ER) is considered. This ratio turns out to be relevant for large multi-plane solutions, since it generates in-band and out-of-band crosstalk.
Besides, we assumed a gain transparency condition for the full switch, where the SOAgain compensates the passive losses of the 1 :S splitter required to implement space switching inside the linecard.
1.2.3 Multi-plane-specific issues
Some considerations must be introduced when considering multi-plane architec-tures.
Switching extinction ratio:Due to the finite extinction ratio of SOA-based switching devices, crosstalk arises across planes just before the EDFA. In the worst case, the number of crosstalk contributions is one in-band component for each plane, and the resulting crosstalk impact is given by the coherent expression IX as shown before in Eq. (1.3), that here depends on S and on the nominal switch extinction ratioER(in linear units). As a result, the crosstalk penalty in multi-plane configurations due to this effect can be estimated as:
IX(S)|dB=−10 log10(1−(S−1)ER Q2). (1.5) Cross noise floor accumulation: In general, lasers and SOAs as optical sources generate Amplified Spontaneous Emission (ASE) noise when operating in the “on” state, which in turn is sent to the selected planes. Although indi-vidually the resulting noise floor levels are quite low, all their spectra add up, and for a high number of planes the noise accumulates, resulting in an intrinsic limitation to scalability. We took this effect into account in our model: we con-sidered the maximum number of noise floor sources per plane (corresponding to all linecards transmitting), and evaluated the noise accumulation accordingly on each switching plane.
Optimum number of switching planes:The choice of the optimal number of switching planesS is a critical design choice and depends on the following considerations. First, a largeS brings the advantage of reducing TTx tunability, but it increases the amount of coherent crosstalk. Second, smaller values of S reduce the number of optical components, hence the overall complexity. Third, both very large and very small values ofSintroduce larger excess losses, thereby reducing the power budget.
Anumerical analysis suggests that there is an optimum value ofS, which also depends on the linecard bit-rateRb, and is roughly close toS≈√
N; this value ofS shows the maximum OSNR at receivers, and thus allows better scalability in terms of aggregate bandwidth.
16 D. Cuda, R. Gaudino, G. A. Gavilanes Castillo, and F. Neri