4. OTDR Testing
GN Netttest
CMA4000
OTDR
OTDR (Optical Time Domain Reflectometer) technology is designed
to provide a single ended test of any cable. Utilizing sophisticated
algorithms, the equipment is able to calculate exact length and
approximate loss of “events” along the cable span.
5. OTDR Testing
1. Generates a baseline trace:
A “visual” of the link.
2. Can identify and evaluate
specific events in the link.
3. Cable acceptance tool.
4. Fault location tool.
5. Excellent documentation
capabilities.
6. Limited use in short length
networks. <50ft
Noyes M600 OTDR
6. Fault-Locate (Using an OTDR)
Work Area
PC Telecom Horizontal
Outlet Cross-connect
Telecom Room
MM
Main Network
Cross-connect Equipment
Launch Equipment Room
Cable
OTDR
8. OTDR Types
Wavetek MTS5100
Fluke
OptiFiber
Most common OTDRs use a “console” design allowing the
user to upgrade or swap between MM and SM modules.
These offer similar analytical features to the lab quality
OTDRs, but are more rugged and field portable. Files can be
saved to various media and later downloaded to a PC.
9. OTDR Types
Anritsu CMA5000
Exfo
FTB-150
More common console OTDRs. Files can be saved to
various media and later downloaded to a PC.
10. OTDR Types
Noyes M200 Noyes OFL200
OTDR Noyes M100 OTDR
OTDR
Micro-OTDRs are the next generation of fast, economical
test sets for field use. These models offer fewer features than
the larger console design and are currently not upgradeable.
Many manufacturers are focusing on development of these
types of OTDRs for size and weight reasons.
11. OTDR Functionality
Basic OTDR function LCD Display
Control
Unit
Splitter
Laser Transmitter Fiber under Test
Detector
OTDR
Connector
12. OTDR Trace Analysis
Physical cable plant
as displayed on
Loss
OTDR screen
Network under test
13. Launch Cables
Launch cables vary from simple reel (or “ring style”) through larger
“lunch box” style suitcases. Most modern OTDRs don’t require a
launch suppression longer than 250-500’, but many older models
needed delay lines of 1000’ or more.
14. Using an OTDR w/ Launch Cable
Use of a launch cable assures the user that the front end connector of
the network will be accurately measured. If the launch cable is too
short, the front end connector will be consumed in the deadzone.
Likewise, a receive cable assures the technician that the far end
connector is not broken and the span has continuity.
Cable under test End
Launch Cable Event
OTDR Launch Noise
Port event Receive Floor
Cable
15. OTDR Trace Analysis
OTDR Panel Splice Closure Panel
Launch Cable Receive Cable
Connect the OTDR to a launch (suppression/reference)
cable. The secondary end of the launch cable will be
connected to an access panel at one end of the fiber
optic span under test. Optionally, a receive cable can be
attached at the far end.
16. OTDR Trace Analysis
Power Loss
Network Under Test
Splice
Distance Scale
OTDR
Launch Cable Receive Cable
17. OTDR Trace Analysis
Most commonly, users manipulate two cursors, “A” and “B”, to
illustrate what is referred to as “two point loss” on an OTDR result.
This can be used to show loss in a single event or in a group of
events. These cursors can be individually moved left and right to
specific points on the result.
A B
18. OTDR Trace Analysis
Power Loss
A B
Distance Scale
Use cursor/markers to isolate individual events, such as
the repair splice location (above)…
19. OTDR Trace Analysis
Power Loss
A B
Distance Scale
…or the two point loss (attenuation) of an entire
network span (above)…
21. OTDR Setup - Range
OTDRs have four basic setup requirements regardless of brand:
Range/Resolution, Pulse Width, Index of Refraction and Time (number of
averages). If any of these settings contradicts another, the results will be
poor. The first one to consider is “Range” or distance of fiber to test.
Many OTDRs have automatic length detection functions, but if the length
is known, the user can set the range manually. The range setting should
be adjusted to no less than 1.5 to 2x the fiber span under test.
2975’ span
under test
Set to
>6000’
22. OTDR Setup - Range: Summary
Too short: less than Good: about 1.5x to Too long: much larger
link length 2x link length than link length
Link Link Link
Can’t see entire link – Good trace – can Trace is “squashed”
unpredictable results see end of fiber. into left side of display.
23. OTDR Setup - Pulse Width
Longer pulse widths are used for longer range tests. As distance
increases, pulse width must go up, otherwise traces will appear
“noisy” and rough. Similarly, short traces will be inconclusive if long
pulse widths are used (events may be missed or clipped). Long cable
span=longer pulse width, Short cable span= short pulse width
“Short”
Fiber run
under test
>6500’
“Long”
Fiber run
under test
>10,000’
24. OTDR Setup - Pulse Width: Summary
Too narrow: About right: Too wide:
Link Link Link
Where is this
this event?
Trace “disappears” Events can be seen Can’t resolve events
into noise floor. and trace is smooth.
25. Index of Refraction (IOR)
In review, the Index of Refraction is a way of measuring the speed of light
in a material. Light travels fastest in a vacuum, such as outer space. The
actual speed of light in a vacuum is 300,000 kilometers per second, or
186,000 miles per second. Index of Refraction is calculated by dividing the
speed of light in a vacuum by the speed of light in some other medium
(such as glass in the case of fiber optics!).
Medium Typical Index of Refraction Speed
Vacuum 1.0000 Faster
Air 1.0003
Water 1.33
Cladding 1.46
Core 1.48 Slower
Speed of Light in a Vacuum
Index of Refraction = Speed of Light in a Medium
26. OTDR Setup – Index Of Refraction
Each different optical glass fiber has a different refractive index
profile consistent with it’s type and manufacture process. Typical
G.652.B singlemode fiber from Draka has an index number of 1.467
@ 1310nm and 1.468 @ 1550nm. Note that the longer the
wavelength, the faster the light travels through the core.
The user must set the OTDR to the proper GIR (Group Index of
Refraction). If the GIR is not set to the proper number, the OTDR
may overestimate or underestimate linear cable footage. Since the
index is a measure of the speed of light, if the GIR is not set
properly, the OTDR cannot calculate the proper footage.
If the actual index is not known, use the machine’s default or the
following guidelines:
MM 850nm – 1.496
MM 1300nm – 1.491
(Corning SMF28e) SM 1310nm - 1.4677
(Corning SMF28e) SM 1550nm – 1.4682
27. Index Of Refraction: Summary
As discussed earlier, Index of Refraction is a measure of the
speed of light in a medium. If the Group Index of Refraction (GIR)
setting in the OTDR does not match that of the fiber under test,
the results will show incorrect distances as a result.
GIR set at Launch OTDR thinks
1.462 Cord Footage is 9,800’
10,000’ of fiber
GIR 1.4677 @ 1310nm
28. OTDR Averaging Time
Averaging time refers to how long the user allows the device to take
samples (a.k.a. how long the test “runs”). The longer the
testing/averaging time allowed, the better the result. Eventually, enough
data is averaged for a good test and continuing to test won’t yield any
more of an accurate result.
Corning
OV1000
MUTOA
Launch
Cord
29. OTDR Setup - Averages: Summary
Too few: About right: Too many
Link Link Link
Trace is noisy – noise Trace is smooth. Trace is smooth but
floor is too high. waste of time.
30. OTDR Trace Analysis
LSA lines are an effective method of getting more accurate test
results. Most OTDRs have loss estimation based on the simple 2-point
method, but use of LSAs obtain better accuracy through events by
calculating lead-in slope and tail-out slope. See below for an example:
Lead in Area
Tail out Area
A B
