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OTDR Studies

OTDR Studies. First look with SM OTDR failed: not enough sensitivity to see RBS signal. Nice clear signals visible with MM OTDR (on free loan until 11/05/07). Next slides show typical traces

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OTDR Studies

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  1. OTDR Studies • First look with SM OTDR failed: not enough sensitivity to see RBS signal. • Nice clear signals visible with MM OTDR (on free loan until 11/05/07). • Next slides show typical traces • RBS signal clearly visible above noise floor  can measure attenuation length of fibre in cable with laser at 856 nm • Higher resolution with 665 nm laser allows clear separation of the Fresnel peaks from the MT-12 at PPB1 and the p-i-n diode on the dogleg.

  2. OTDR Measurements APC ST MT MT MM v OTDR 3.2m or 2m 5m MT p-i-n diode ~ 2m FUT (Fibre under test)

  3. MT-12 at PPB1 Connectors at start of cable and fan-out RBS Signal above background p-i-n diode on dogleg

  4. Use delay setting to avoid peaks from start of cable & fan-out

  5. Use 665 nm laser for higher resolution to see loss at MT-12 at PPB1

  6. Use 856 nm laser to measure attenuation length of fibre in cable

  7. Measurements • Measured attenuation length of fibres in cable and loss at MT-12 at PPB1 for • All fibres in 4 ribbons associated with “problematic” channels, all channels have < 90 mA p-i-n current. • All fibres in 4 ribbons associated with “good” channels. All channels have >= 90 mA p-i-n current. • Also checked that there was no excess loss near PPB1 due to micro-bending. • Measured attenuation length of 4 ribbons in spare cable.

  8. Summary • Attenuation lengths • <good channels> = 15.1 +/- 0.45 dB/km • <bad channels> = 17.1 +/- 0.6 dB/km • <fibres spare cable> = 8.24 +/- 0.28 dB/km • Loss at MT-12 PPB1 • <good channels> = 0.13 +/- 0.02 dB • <bad channels> = 0.17 +/- 0.03 dB

  9. Discussion • There are systematic errors on the absolute values of these measurements but the relative measurements of good and problematic channels should be meaningful. • Losses at PPB1 appear to be very small and no significant difference between problematic and good channels. • The attenuation length for problematic channels is slightly longer than for good channels. Marginally statistically significant (2.7 s) • Attenuation length for fibes in spare cables is significantly longer.

  10. Averages for ribbons • Look at average attenuation lengths for each ribbon • Good • Problematic • Spare

  11. Conclusions • Some evidence for non-statistical ribbon to ribbon variations. • Difference between “good” and “problematic” ribbons small, even for worst ribbon • extra attenuation compared to good ribbons is only ~ 0.6 dB. • For this ribbon <Expected/measured pin current> = 5.9 dB • Difference between fibre in spare cable and fibres in installed cables is much larger. • Different mode coupling for fibres in installed cables compared to fibres in spare cable ??? • Still need “better” VCSELs …

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