In this presentation, we revisit MMF CD measurement, and point to the principal mode group dependence of MMF CD.

1. SM
Panduit Confidential and Proprietary
Zero Dispersion Modes and its Effects on
Characterization of MMF Chromatic Dispersion
Bulent Kose, Rick Pimpinella, Jose M.
Castro, Yu Huang, Asher Novick
Panduit Laboratories, Panduit Corp.
OFC March 2015

2. SM
Introduction
- 2 -
As a cost effective solution, VCSELs coupled to Multimode fiber (MMF) make up
the majority of short optical reaches in data center and enterprise networks
10G-SR, 40G-SR4, 100G-SR10, 100G-SR4*, 16GFC, 32GFC etc.
Modal and chromatic dispersion are main limiting MMF channel impairments for
short optical reaches (850nm)
With increasing application speeds, VCSEL spectral width got wider, therefore
increasing channel chromatic dispersion (CD) impairment
10G-SR: 0.45nm
40/100G SR4/SR10: 0.65nm
100G-SR4: 0.6nm
In this paper, we revisit MMF CD measurement, and point to the principal mode
group dependence of MMF CD

3. SM
Impact of CD on Ethernet Channel Link Reach
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90
100
110
120
130
140
150
160
170
180
190
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70
MaximumReach(m)
Spectral Width (nm)
100GBASE-SR4
OM4
Calculated reach for 100GBASE-SR4 using the IEEE Link Model
Reach is chromatic dispersion limited
Similar results for 10Gb/s Ethernet (10GBASE-SR)

4. SM
MMF Chromatic Dispersion Measurements
- 4 -
Current CD measurement standards (IEC 60793-1-42)
Method B, spectral group delay in time domain:
Send temporally narrow pulse over the MMF under test at different wavelengths and measure group
delay/dispersion
Use Sellmeir coefficient curve fits to compute lo and So
Group delays at each measurement wavelength calculated as the centroid of the temporal
pulse
Around the critical zero dispersion wavelength of 1300nm, centroid of measured pulse
becomes noisy, due to high modal dispersion
Due to graded index profile, individual MMF mode groups undergo different chromatic
dispersion
3000
3200
3400
3600
3800
4000
4200
4400
4600
4800
5000
1220 1240 1260 1280 1300 1320 1340
GroupDelay(ps/km)
Wavelength (nm)
Temporal Delays Around lo
2500
7500
12500
17500
22500
850 950 1050 1150 1250 1350
GroupDelay(ps/km)
Wavelength (nm)
Temporal Delays as a Function of Wavelength

5. SM
MMF DMD and Mode Group Chromatic Dispersion
Measurement Set-up
- 5 -
Super continuum white light
Source (6ps, 20 MHz)
Std SMF
Monochromator DUT - MMF
Mechanical stage for
controlled offset launch
Sampling
Oscilloscope
Std SMF
Control
computer
Light Source: Supercontinuum white light source with a narrow temporal pulse output (6 ps FWHM)
Standards (i.e. IEC 60793-1-42) require overfilled launch for CD measurements
Mono-chromator
Computer controlled offset launch mechanism
Radial modes are used instead of mode groups for modal dependence of CD
High speed sampling scope with an Agilent 86105C photodetector plugin module
The centroid of temporal pulse from radial offset excitation are computed
Group delay, g
Zero dispersion wavelength (lo) and dispersion slope (So) are computed from the fitted group delay
3-term Sellmeier equation

6. SM
MMF CD: Impact of Launch Condition
- 6 -
Group delays for 5 fibers using single mode vs overfilled launch (fitted from measurements, 3
terms used)
The shift in measured zero dispersion wavelength clearly visible for all 5 fibers
Around 10nm
Actual dispersion values do not diverge much around 850nm
0
50
100
150
200
250
300
1250 1270 1290 1310 1330 1350
GroupDelay(ps/km)
Wavelength (nm)
F1, SM
F2, SM
F3, SM
F4, SM
F5, SM
F1, OFL
F2, OFL
F3, OFL
F4, OFL
F5, OFL
10000
11000
12000
13000
14000
15000
16000
17000
18000
850 870 890 910 930 950
GroupDelay(ps/km)
Wavelength (nm)
F1, SM
F2, SM
F3, SM
F4, SM
F5, SM
F1, OFL
F2, OFL
F3, OFL
F4, OFL
F5, OFL

7. SM
CD vs Offset Launch for MMF
- 7 -
Launch narrow temporal pulse at different wavelengths, at radial offsets
Similar to DMD measurements - (i.e. TIA-FOTP-220)
Zero dispersion wavelength (lo) and dispersion slope (So) measured at radial offsets for 5 fibers
Radial offsets relate to the mode groups through p-matrix
Lower offsets correspond to lower order mode groups
Significant zero dispersion wavelength difference of about 15nm observed between LOM and
HOM for all measured fibers
lo tends to shorter wavelengths for HOM
Dispersion slope also varies as a function of radial offset

8. SM
MMF Mode Group CD – from Offset Data
- 8 -
From the offset data, zero dispersion wavelength and dispersion slope can be calculated for
each mode group
As an illustrative example one radial offset, 10 mm, is selected in which at least five MGs can be
observed
Group delays as a function of the wavelength for all the MG present in this radial offset is shown on the
right
For a relatively narrow range of wavelengths around zero dispersion wavelength, individual
mode CD’s can be evaluated using this measurement
Change in the number of propagating modes with wavelength makes it impossible to scan wider
wavelength range with this method

9. SM
MMF Mode Group CD - Theory
- 9 -
Group delay per mode group:
Where optimum alpha profile is:
Derivative of group delay with respect to wavelength can be computed
numerically to find lo for each mode group from profile parameters
αopt, ∆ and N1 can be obtained from the Sellmeier coefficients of the dopants used in
the fiber
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l
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
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






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





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
l
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l
d
d
N
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opt



1
12
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
l
l
l
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d
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Nd
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td R
M
gg

10. SM
MMF CD – Arbitrary Launch Condition
- 10 -
Chromatic dispersion (lo, So) measured at individual radial offsets can be scaled
by coupled power, P(r) to compute total MMF CD
i.e. for SM launch P(r) = 1 only when r = 0
Utilizing P(r) and offset launch method, we can compute CD for arbitrary launch
conditions
Standardized launch condition such as 10 VCSEL used in worst case modal
bandwidth (TIA-FOTP 220) can allow more realistic CD measurement
𝜆 𝑜 = 0
𝑎
𝑃 𝑟 𝜆 𝑜 𝑟 𝑑𝑟
0
𝑎
𝑃 𝑟 𝑑𝑟
𝑆 𝑜 = 0
𝑎
𝑃 𝑟 𝑆 𝑜 𝑟 𝑑𝑟
0
𝑎
𝑃 𝑟 𝑑𝑟
𝐷 = 0
𝑎
𝑃 𝑟 𝐷 𝑟 𝑑𝑟
0
𝑎
𝑃 𝑟 𝑑𝑟

11. SM
Panduit Confidential and Proprietary
- 11 -
Summary/Conclusion
In this work, chromatic dispersion is focused on as a primary impairment
for MMF links with wider spectral width of higher speed VCSELs
Input launch dependence of measured zero dispersion wavelength and
dispersion slope is highlighted
lo for single mode launch is about 10nm higher than overfilled
Furthermore, variation of chromatic dispersion for MMF mode groups is
studied theoretically and experimentally
Offset launch method similar to DMD measurement utilized
Difference in lo of about 15nm is observed between LOM and HOM
Encircled flux compliant launch condition for CD measurements is
recommended, to reduce measurement variability in MMF CD
measurements