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

3 Connectors at start of cable and fan-out RBS Signal above background MT-12 at PPB1 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  A p-i-n current. –All fibres in 4 ribbons associated with “good” channels. All channels have >= 90  A 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 – = / dB/km – = /- 0.6 dB/km – = / dB/km Loss at MT-12 PPB1 – = / dB – = / 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  ) 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 Crate/slot/Tx Attenuation dB/kmError Crate/slot/Tx Attenuation dB/kmError

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 = 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 …