Chromatic dispersion

1. DISPERSION
CHROMATIC
IN 10Gb/s DWDM SYSTEMS

2. The effects of chromatic dispersion are crucial for any system engineer, designer, or architect to design and
implement a 10Gb/s DWDM network. Although it’s important to understand what chromatic dispersion is, it is
imperative to understand the negative impact on optical transceiver receiver sensitivity. Although chromatic
dispersion negatively impacts receiver sensitivity, there are ways to compensate for this effect in distances over
80KM. These issues can be further impacted by the effects of reduced Optical Signal to Noise Ratio (OSNR) in optically
amplified systems. This paper will focus on chromatic dispersion, its impact on receiver sensitivity, compensation
methods, and the combined effects with reduced OSNR.
A basic example of chromatic dispersion (CD) is a rainbow. Sunlight acts as a “light source” and water droplets act as
a dispersive media where it separates the sunlight into the various colors, or wavelengths of light, to form a rainbow.
The concept of chromatic dispersion is also similar in optical fiber especially over long distances. Optical fiber is
composed of a core and cladding, each of which have differing refractive indexes. As a result, wavelengths of light
inevitably travel at slower or faster speeds as compared to others. The result is a spreading or widening of the signal
pulse at the receive end of the fiber. This causes digital signals such as “1s” and “0s” to “spread” or overlap into one
another and even become indistinguishable resulting in misinterpretations of data. See Figure 1 below for a
representation of chromatic dispersion in optical fiber.
Figure 1. Chromatic Dispersion in Optical Fiber
Figure 1 represents a digital signal input to the optical fiber. This digital input is comprised of several wavelength
components which are represented by the red, green, and blue wavelengths for purposes of representation. As the
input digital signal travels through the fiber and experiences the index of refraction of the core and cladding, it will
cause a “spreading” of the digital signal due to the various wavelength components. Shorter wavelengths (blue)
travel faster than the longer wavelengths (red) and hence the result of chromatic dispersion. The more variation in
wavelength velocity, the greater the individual pulse spreading.
The effect of chromatic dispersion is that it makes it difficult for the receiver to determine digital “1s” and “0s”. As
pulses spread into one another, the eye diagram becomes compressed and “1s” and “0s” can be mistaken which
leads to an increase in Bit Error Rate (BER). Figure 2 represents a clean eye diagram (top) as well as a compressed eye
diagram (bottom).

3. Figure 2. Eye Diagram and Decision Threshold
As you will note, in the compressed eye diagram, the sample or decision threshold window overlaps into the “1s” and
‘0s” area which leads to errors. The effect of chromatic dispersion results in a compressed eye diagram as shown in
Figure 3.
Figure 3. Eye Diagram Without and With the Effects of Chromatic Dispersion
In the eye diagram without chromatic dispersion, the decision threshold will not experience any errors as the eye
opening is wide and very clean. Whereas in the eye diagram with chromatic dispersion, the eye diagram becomes
very compressed and distorted resulting in errors.
Although chromatic dispersion can negatively impact optical communications systems, it can be measured or
approximated as well as compensated for. In some cases, optical fiber links may have documentation from initial
system commissioning or specifications may be available when the fiber manufacturer and type is known. Many good
chromatic dispersion test sets are also commercially available to measure chromatic dispersion. In standard single
mode optical fiber, the value of 18 ps/nm*km is typically used to approximate/calculate chromatic dispersion.
For market purposes, the focus will be on 10 Gb/s NRZ (Non-Return to Zero) signals since they are inexpensive and
widely deployed. Today, most 10 Gb/s optical transceivers are rated to a maximum distance of 80km. The total
chromatic dispersion tolerance is:

4. CD = (18 ps/nm*km) * (80km) = 1,440 ps/nm
10Gb/s optical transceivers will be capable of tolerating a maximum of 1,440 ps/nm of chromatic dispersion over
80km of fiber. Discussion will be forthcoming on how to compensate for chromatic dispersion using DCM
(dispersion compensation modules), but first it’s important to understand the effects of chromatic dispersion on bit
error rate (BER) and receiver sensitivity.
As previously discussed, chromatic dispersion compresses the eye diagram which results in reduced performance at
the optical layer. It becomes difficult to distinguish the difference between “1s” and “0s” which is sometimes
referred to as Intersymbol Interference (ISI) as shown in Figure 4. The result is that pulses begin to spread into one
another. This directly impacts (reduces) the receiver sensitivity of a 10 Gb/s optical transceiver’s ability to maintain
the industry minimum standard BER of 1x10-12
.
Figure 4. Example of ISI
It’s very important to understand the effects of chromatic dispersion on the receiver sensitivity. In general, the
minimum receiver sensitivity of a 10Gb/s 80KM optical transceiver is -24 dBm. What is important to note is that this
specification is typically noted to be “Back to Back” as measured with OSNR of 35 dB to achieve a BER of 1x10-12
.
“Back to Back” means that the transceiver is basically optically looped back on itself or connected to another
transceiver with minimal optical fiber connecting the two combined with optimal OSNR and no chromatic dispersion to
achieve the minimum accepted BER of 1x10-12
.
In practical use cases, this is not the representative implementation in networks. Typically, network designs range from
non-amplified links up to 80KM or amplified DWDM links extending beyond 80KM all within the 1550nm range.
Network designers and engineers need to understand the impacts of chromatic dispersion on link budgets. Chromatic
dispersion directly reduces the receiver sensitivity. Often, the term power penalty or dispersion power penalty is
specified. Generally, for a 10Gb/s 80KM optical transceiver, the dispersion power penalty is 3 dB. This means that at
80KM, on an unamplified link, where the maximum chromatic dispersion is 1,440 ps/nm, the minimum receiver
sensitivity will be reduced by 3 dB. As previously mentioned, the minimum receiver sensitivity in a ‘back to back’
configuration was -24 dBm. Hence, at 80KM, the minimum receiver sensitivity will be reduced by 3 dB to -21 dBm.
Therefore, for an unamplified 80KM link at 10Gb/s, the minimum receiver sensitivity will be -21 dBm to achieve a BER
of 1x10-12
.

5. In amplified DWDM links, where distances extend beyond 80KM, chromatic dispersion will need to be
compensated for. Dispersion Compensation Modules (DCMs) are utilized to add “negative dispersion to the
system. For example, if an amplified link has a total length of 110KM, this exceeds the rating of an 80KM 10Gb/s
transceiver by 30KM. A DCM-30 or DCM-40 (for additional margin) can be utilized to compensate for the additional
link distance. See Figure 5 below
Figure 5. Dispersion Compensation Modules (DCMs) in amplified links exceeding 80KM
In Figure 5, the total link length is 110KM. By designing in DCMs, the link length, from a chromatic dispersion standpoint appears
to be 70KM (110KM - 40KM = 70KM). As the 10Gb/s signals propagates 110KM, they experience positive chromatic dispersion.
The purpose of the DCM is to add negative chromatic dispersion which, in the example above, negates 40KM of positive chromatic
dispersion.
Figure 5 represents a simple link design, but when distances span several hundreds or even thousands of kilometers, DCMs should
be placed in line with each EDFA (Erbium Doped Fiber Amplifier) to compensate for the chromatic dispersion in each span.
Although it’s beyond the scope of this paper, one additional factor to consider in 10Gb/s systems is the impact of OSNR when
utilizing EDFAs. When systems are lossy or span long distances, EDFAs are required to extend distances. EDFAs directly introduce
a rise in the noise floor of an amplified optical signal. A rise in the noise floor negatively impacts the OSNR of a signal. OSNR is the
measure of the signal to noise floor over an RBW (Resolution Bandwidth) = 0.1nm. In a “back to back” configuration this
measurement is typically >35 dB. However, over lossy or many amplified links, OSNR can be greatly reduced. In 10Gb/s NRZ optical
systems, OSNR values of approximately 25 dB begins to negatively impact the receiver sensitivity. Typically, a reduction of 0.5 dB
to 1 dB to the minimum receiver sensitivity can be experienced at an OSNR of 25 dB. With OSNR of 23 dB, a typical reduction in
the receiver sensitivity of 1.5 dB to 2 dB is typical. Again, the reduction in receiver sensitivity is to maintain a BER of 1x10-12
. If one
combines worst case chromatic dispersion effects (80KM) of a 3 dB power penalty along with a signal OSNR of 23 dB (2 dB
reduction), the receiver sensitivity can be reduced by 5 dB. As we have previously discussed, the minimum receiver sensitivity of a
10Gb/s NRZ signal is typically -24 dBm. If we reduce that by 5 dB, the minimum receiver sensitivity is -19 dBm due to the maximum
effects of chromatic dispersion (3 dB penalty) and sub optimal OSNR of 23 dB (2 dB reduction).
110KM
DW
DM
DWDM
DWDM Equipment
Platform
Equipment
Platform
Port 1
Port N
Port 1
Port N
Port 1
Port N
Port 1
Port N
Booster
AMP
Pre AMP DCM
40 KM
DCM
40 KM
Pre AMP
Booster
AMP
DW
DM
110KM

6. In conclusion, it is very important to understand the effects of chromatic dispersion in 10Gb/s NRZ systems. When designing a link
or system, it is extremely important to understand the negative impacts of chromatic dispersion and its effect on the total link
budget while maintaining the minimum acceptable level of BER = 1x10-12
. Chromatic dispersion can be compensated for, with total
effects over long distances reduced by introducing DCMs, to better improve overall link budget. Furthermore, OSNR, especially at
reduced levels (23 dB to 24 dB range) can introduce reductions to the minimum receiver sensitivity as measured with an Optical
Spectrum Analyzer with RBW = 0.1 nm.
Although guidelines/approximations are provided above, exact measurements should be taken and modeled specifically for the
system implementation which will include specifications for optical fiber, transceivers, etc.
Coherent technologies have emerged at the 100Gb/s, 200Gb/s, 400Gb/s and now 600Gb/s rates which negate the effects of
chromatic dispersion. These optical transceivers utilized DSP (Digital Signal Processors) which electronically compensate for
chromatic dispersion thereby eliminating the effects and extending higher data rates over longer distances. We will discuss
coherent optical transceivers, in greater detail, as another topic soon.