OTDR Studies

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
FUT (Fibre under test)
2m
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 lt 90 mA
  p-i-n current.
• All fibres in 4 ribbons associated with good
  channels. All channels have gt 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
• ltgood channelsgt 15.1 /- 0.45 dB/km
• ltbad channelsgt 17.1 /- 0.6 dB/km
• ltfibres spare cablegt 8.24 /- 0.28 dB/km
• Loss at MT-12 PPB1
• ltgood channelsgt 0.13 /- 0.02 dB
• ltbad channelsgt 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

Crate/slot/Tx Attenuation dB/km Error
0 8 0 10.55 0.35
2 14 1 15.06 0.54
1 7 1 17.58 0.32
3 15 0 17.14 0.27
Crate/slot/Tx Attenuation dB/km Error
0 6 0 15.38 0.60
0 7 0 12.71 1.34
2 12 0 18.83 0.15
2 12 1 21.63 0.50
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 ltExpected/measured pin currentgt
  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