Manual

 A Quick Start Guide to Fiber-to-the-Antenna (FTTA) Installation and Maintenance Testing
OTDR Testing
An OTDR typically makes four vital measurements:
y Distance — An OTDR measurement is based on time: the
round-trip time travel of each pulse sent down the fiber.
Distance is calculated by knowing the speed of light in a
vacuum and the index of refraction of the fiber glass.
y Attenuation (fiber loss) — Expressed in dB or dB/km, this
represents the loss or rate of loss between two events
along a fiber span.
y Event Loss — An event loss has a direct impact on
the total optical budget and has to be checked with
an OTDR. In general, two types of events can occur:
reflective and non-reflective.
– A reflective event loss such as a connector typically
ranges from 0.2 to 0.5 dB.
– A non-reflective event loss such as a fusion splice
typically ranges from 0.01 to 0.2 dB. This is measured
with an OTDR as the difference expressed in dB in
optical power level before and after the event.
y Reflectance — An event reflectance impacts the total
optical return loss (ORL). The ORL is the total light that is
reflected to the emitter and needs to be limited to avoid
increase of bit error rate (BER). This is measured with an
OTDR as the ratio of reflected power to incident power
of an event and is expressed as a negative dB value. The
higher the reflectance (the more light reflected back), the
worse the connection. For example, a  dB reflectance
is better than  dB value.
TEST
1550 nm
0.19 dB/km
Connection between
the OTDR and the patchcord
or launch cable.
1310 nm
0.33 dB/km
Front-end reective event
Connector
Mechanically aligns two bers
together using a self-contained
assembly.
Reectance: ~ –55 dB
Insertion loss: ~ 0.5 dB
Launch cable
Using a launch cable allows for
characterizing the connector at the
origin of the link by moving it outside
the dead zone of the OTDR connector.
The last connector can also be tested
using a receive cable.
A fusion splice uses a splicing machine
to thermally fuse two bers together.
Reectance: none
Insertion loss: < 0.1 dB
Fusion splice
Loss
An unexpected event resulting
from a strong reection causing
“echoes” on the trace.
When it appears, it is often seen
after the ber end.
Reectance: lower than echo source
Insertion loss: none
Ghost
Macro bend
Loss
Loss
1550 nm
1310 nm
A splice gain that appears
after splicing together two bers
with dierent backscatter
coecients.
Reectance: none
Insertion Loss: small gain
Gainer
Gain
Macro bending results
from physical constraints on the ber.
Bending loss is higher as
wavelength increases.
Distinguishing a bend
from a splice requires using
two dierent wavelengths.
Reectance: none (generally)
Insertion loss: varies according
to wavelength
The Attenuation Dead Zone (ADZ)
is the minimum distance after
a reective event where a
non-reective event (splice)
can be measured (usually 0.5 dB).
In this case, the events are more
closely spaced than the ADZ and
shown as one event. ADZ can be
reduced using smaller pulse widths.
Attenuation Dead Zone
The Event Dead Zone (EDZ)
is the minimum distance that
distinguishes two consecutive
unsaturated reective events.
Here the events are more closely spaced
than the EDZ and are shown as one event.
It can be reduced using smaller pulse widths.
Event Dead Zone
Distance Range species the distance that the OTDR will display on the X axis
Dynamic range determines the
observable length of the ber and
depends upon the OTDR design and
settings.
The injection level refers to the power level
in which the OTDR injects light into
the ber under test.
Poor launch conditions, resulting in
low injection levels, are the primary
reason for reductions in dynamic
range
and, therefore, measurement
accuracy.
Dynamic Range
and Injection Level
ADZ
> 0.5dB
EDZ
1.5 dB
A ber end or break refers to where the ber
terminates. The end reection depends on
the ber end cleavage and its environment.
Reectance:
• PC open to air: ~ –14 dB
• APC open to air: ~ –45 dB
Insertion loss: high (generally)
Fiber end or break
Noise
Event is hidden
Event is hidden
(L)
(L)
2
1
3
4
5
6
8
9
Noise
Loss
Loss
Fusion splice or macrobend Connector or mechanical splice
Loss
Loss
Typical Reflectance Values
Polished connector ≈ -45 dB
Ultra-polished connector ≈ -55 dB
Angled polished connector ≈ -65 dB