KI 7340/7740 UM-12
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ACCURACY CONSIDERATIONS
DWDM Loss and Power Measurements
The DWDM bands are typically within 1525 ~ 1610 nm band. For this
application, an InGaAs detector calibrated at 1550 nm gives good
absolute accuracy at all DWDM wavelengths.
Testing optical loss at 1610 or 1625nm is common, since this
represents the worst case attenuation in most systems.
High power InGaAs detectors may be required to measure the system
power level.
CWDM Loss and Power Measurements
The CWDM band is from 1270 ~ 1610 nm at 20 nm spacing, however,
1490 ~ 1610 nm is more common since it avoids the water absorption
peak at 1383 nm.
The most common operating wavelengths for FTTH PON systems are
currently 1310 / 1490 / 1550 nm.
As in DWDM application, an InGaAs detector gives good absolute
accuracy.
Because of the broad operating wavelengths encountered, the power
meter calibration wavelength needs to be matched to the measured
wavelength, or absolute errors may be excessive.
Many systems are measured at 1390 nm to test for the water peak,
and at 1625 nm to measure the high end attenuation limit.
At 1625 nm, the fiber attenuation is actually very sensitive to
wavelength, since the silica infra-red absorption zone is reached. So
the wavelength tolerance of this particular laser is a major issue,
especially since many older source instruments have a tolerance of 30
nm.
ORL Measurements
ORL measurements are typically divided into two classes:
1. Acceptance of installed cable systems, usually 20 ~ 35 dB. This is
generally easily accomplished with an ORL meter. In these
measurements, the provided ORL terminator may be usefully placed
at the far end of the system.
2. Acceptance of components and patch leads, which may involve
levels well below this. These measurements require an understanding
of sources of reflection, and require additional steps.
In Two Way mode, the port isolation can be an important contributor
to the noise floor. A special instrument version has higher isolation for
component acceptance testing. Using this equipment, very high
throughput can be achieved in a factory environment.
When setting up ORL testing, a strong understanding of both PC and
APC connector reflection effects may be required.
