Download Optical Reflection Measurements

Lecture-3
Optical Reflection Measurements
Optical Reflection Measurements
• Total Return Loss Technique
 A total return-loss measurement results in
a single number that represents the total
fractional power that is reflected from a
test device.
 In order to make an accurate total return
loss measurement, all unwanted powers
reflected back into the power meter should
be measured and taken into account.
 When a test device contains multiple
reflections,
a
total
return-loss
measurement can give inconsistent
results.
Optical Reflection Measurements
• Basic Concepts for Spatially Resolved
Reflectometry
 Two point spatial resolution refers to the
minimum distance between two reflectors
that can still be resolved by the
measurement system. This value is
approximately equal to the FWHM of a
single reflection on a reflectometry trace.
 The term single-point spatial resolution
refers to the accuracy for determining the
location of a single reflector.
Optical Reflection Measurements
• Basic
Concepts
for
Spatially
Resolved Reflectometry
 As spatial resolution is decreased, the effects
of fiber dispersion must eventually be
considered. This problem arises since the
accompanying increase in spectral width
causes spreading of the probe signal as it
travel along the fiber.
 The effects of dispersion depend on the
spectral content of the probe signal combined
with the length and dispersion characteristics
of the fiber.
Optical Reflection Measurements
• Basic Concepts for Spatially Resolved
Reflectometry
 The measurement of Rayleigh backscatter
(RBS) is a very powerful method for
characterizing fiber optic links. Conventional
OTDRs use RBS to determine the location of
fiber breaks, the fiber attenuation coefficient,
splice loss, and various other link
characteristics.
 The success of using RBS in long- and
midterm resolution applications has spurred a
strong
desire
that
high
resolution
reflectometry techniques be also capable of
measuring RBS.
Optical Reflection Measurements
• Basic Concepts
Reflectometry
for
Spatially
Resolved
 Two difficulties arise when attempting to
measure RBS with high spatial resolution.
 First, the strength of the RBS signal becomes
very small as the spatial resolution is
decreased.
 Second, the measured RBS amplitude
becomes very noisy because of coherent
interface effects. This noisiness is sometimes
referred to as coherent speckle.
Optical Reflection Measurements
• Basic Concepts for Spatially Resolved
Reflectometry
 Optical reflectometry can be classified into
direct-detection and coherent-detection.
 Coherent techniques more complex to
implement but it has some advantages like
larger dynamic ranges, increased signal
sensitivity, and the possibility for dispersion
cancellation.
 In case of direct detection, only the reflected
signal is incident on the detector.
Optical Reflection Measurements
• Optical Low Coherence Reflectometry
 OLCR is based on a coherent detection
scheme that is often referred to as white-light
interferometry. It uses broad spectral width
optical sources with short coherence length.
 Compared
to
other
high
resolution
reflectometry techniques, OLCR currently
offers substantial advantages in both
theoretical
performance
and
practical
implementation.
Optical Reflection Measurements
• Optical Time-domain Reflectometry
 One
advantages
of
time-domain
techniques is that measurement can be
performed over large distance range.
After a pulse is injected into the test
fiber, return signals need only be
collected as a function of time.
 Due to poor sensitivity, high-resolution
direct-detection OTDR techniques will
be limited to applications involving high
reflectivity signal.
Optical Reflection Measurements
• Optical Time-domain Reflectometry
 The poor sensitivity associated with the
m
high speed photodiodes
can be greatly
improved by using a photon counting
detection scheme.
 Photon counting can be performed using
photo-multiplier tubes or gain quenched
avalanche photodiodes (APD).
 Practical methods for photon counting at
wavelengths
longer
than
about
1
micrometer are difficult to achieve.