1. 1 §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)

2. 2 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

3. 3 Time Division Multiplexing (TDM) TDM ladder network

4. 4 TDM star network

5. 5 Wavelength division multiplexing

6. 6 Optical time domain reflectometry (OTDR) OTDR Basics

7. 7 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

8. 8 Spatial resolution of OTDR is the optical pulse width is the response time of the receiver.

9. 9 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

10. 10 OTDR fiber signature From the OTDR trace, fiber attenuation, the losses and positions of splices, connectors, bend and crack, can be determined.

11. 11 Backscatter signal analysis

12. 12 Backscatter signal analysis is proportional to

13. 13 Optical low coherence reflectormetry (OLCR) Coherence length:

14. 14 A practical implementation

15. 15 BPF parameters Center frequency: Bandwidth:

16. 16 OLCR measurement of a pigtailed-EELED package

17. 17 Spatial resolution of OLCR Measurement range: Limited by the scanning distance of the mirror

18. 18 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?

19. 19 Comparison between OTDR and OLCR performance Spatial Resolution Reflection Sensitivity Dynamic Range Measurement Range DD-OTDR5 mm-50 dB50 dB> 100 m OLCR < 2  m -162 dB>120dB>lm OLCR has a better spatial resolution, a higher refection sensitivity and a larger dynamic 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.