OPTICAL BACKSCATTER
REFLECTOMETRY (OBR) ˗
OVERVIEW AND APPLICATIONS
INTRODUCTION
REFLECTANCE AND RETURN LOSS
MEASURING RETURN LOSS
OPTICAL TIME-DOMAIN REFLECTOMETERS (OTDR)
OPTICAL LOW-COHERENCE REFLECTOMETRY (OLCR)
OPTICAL FREQUENCY-DOMAIN REFLECTOMETRY (OFDR)
OPTICAL BACKSCATTER REFLECTOMETRY (OBR)
ULTRA-HIGH SPATIAL RESOLUTION AND NO DEAD ZONES
ADDITIONAL MEASUREMENT CAPABILITIES OF OBR
APPLICATIONS OF OBR
CHARACTERIZATION OF SHORT FIBER NETWORKS
COMPONENT AND WAVEGUIDE CHARACTERIZATION
LATENCY AND LENGTH MEASUREMENT
LUNA OBR REFLECTOMETERS
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Optical communications technology is rapidly
evolving to meet the ever-growing demand for
ubiquitous connectivity and higher d ata rates. As
signaling rates increase and modulation schemes
become more complex, guaranteeing a hi gh-fidelity
optical transmission medium is becoming even more
critical. Additionally, modern networks are relying
more on photonic integrated circuits (PICs) based on
silicon photonics or other developing technologies,
introducing additional variables into the design and
deployment of robust high bandwidth optical
systems. Measurement and full characterization o f
loss along the light path is a fundamental tool in the
design and optimization of these components and
fiber optic networks.
Different types of reflectometers are available to
measure return loss, insertion loss, and event
location for different types of optical systems. While
optical time domain reflectometers (OTDRs) are a
standard tool for medium to long span fiber optic
networks, optical backscatter reflectometry (OBR)
offers a unique c ombination of ultra-high spatial
resolution and sensitivity that make it a very
important tool for shorter fiber spans, modern
photonic integrated circuits (PICs) and silicon
photonics.
Whether characterizing a miniaturized PIC or
troubleshooting a long-haul fiber optic span,
understanding and quantifying the loss along the
optical path is a very important step when optimizing
performance or resolving transmission problems.
Return loss (RL) is defined as the ratio of light
reflected back from a device under test (P
R
) to the
light launched into that device (P
in
). RL is typically
expressed as a negative number in decibels (dB).
High levels of optical return loss can lower signal-to-
noise ratios, contribute to higher bit-error rates
(BER), interfere with the operation of the light
source, and generally compromise the performance
of the optical component or system.
The two primary phenomena that cause return loss
are Fresnel back reflection and Rayleigh backscatter
(Figure 1). Fresnel back reflection occurs when light
transitions through different media with different
refractive indices (n
i
). In an optical fiber, for example,
Fresnel reflections are caused by air gaps, cracks in
the core, misalignment of fiber cores in splices,
macro bends, etc. Rayleigh backscattering, on the
other hand, is an intrinsic property of optical media
and is caused by the natural impurities and
imperfections in the optical fiber core or media.
Rayleigh backscattering occurs along the entire
length of the optical fiber or light path.