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§5 Multiplexed and distributed sensors

1. Basic sensor array topologies

2. Time division multiplexing (TDM)

3. Wavelength division multiplexing (WDM)

4. Optical time domain reflectometry (OTDR)

5. Optical low coherence reflectometry (OLCR)

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Basic sensor array topologies

Advantages of multiplexing:(1)Many sensors share the same source/detector and processing

electronics and this helps to reduce the cost of the system. (2)Lower fiber count in telemetry cables and ease of E/O interfacing

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Time Division Multiplexing (TDM) TDM ladder network

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TDM star network

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Wavelength division multiplexing

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Optical time domain reflectometry

(OTDR)

OTDR Basics

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Spatial resolutions

Single point resolution:

The accuracy for determining the location of a single reflector

Two-point resolution:

The minimum distance between the two reflectors that can be resolved by the measurement system

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Spatial resolution of OTDR

sg

r tv

Z 2

22rpst

p is the optical pulse width r is the response time of the receiver.

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A single pulse OTDR

The response time of the detector usually limit the minimum spatial resolution e.g., a response time of100ps corresponds to about 10mm spatial resolution

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OTDR fiber signature

From the OTDR trace, fiber attenuation, the losses and positions of splices, connectors, bend and crack, can be determined.

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Backscatter signal analysis

)exp()( 0 zPzP

34.4dB

sa

tPtP ggw νexpνk2

10rs

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Backscatter signal analysis

sSk

4

1

s

mn

NAS

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is proportional to

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Optical low coherence reflectormetry (OLCR)

tXPPPPI dutrefdutrefd ,cos2

2cL

Coherence length:

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A practical implementation

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BPF parameters

m

dv

f2

c

md L

vf

2

2cL

Center frequency:

Bandwidth:

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OLCR measurement of a pigtailed-EELED p

ackage

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Spatial resolution of OLCR

2

2

1

g

cr nLz

Measurement range: Limited by the scanning distance of the mirror

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Backscatter measurement

Question: for a fixed fiber, how does the measured backscatter power level relate to the spectral width or the coherence length of the source?

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Comparison between OTDR and OLCR p

erformance Spatial Resolution

Reflection Sensitivity

Dynamic Range

Measurement Range 

DD-OTDR 5 mm -50 dB 50 dB > 100 m

OLCR < 2 m -162 dB >120dB >lm

OLCR has a better spatial resolution, a higher refection sensitivity and a largerdynamic range, but smaller measurement range. It is often used to characterize optical waveguide components.

OTDR has a large measurement range and is often used to measure fiber loss and to characterize long fiber optic cables.

Question: the measurement range of an OLCR system is typical limited by the scanning range of the reference mirror? Find a way to improve the measurement range.