OPHIR vega User Manual page 67

value, and the second will activate a new value. If you
suspect that the Vega has a permanent zero offset, then
disconnect the sensor while it is in the power
measurement mode.
5.4.6. Measuring loss using the dB offset function
Since dBm is a logarithmic measurement, the ratio
between two measurements will be the difference
between the dBm measurements. For instance, if you
want to measure the loss in a fiber optic cable where the
measurement before the cable is 1mW = 0dBm and the
measurement after the cable is 0.1mW = -10dBm. The
ratio is then 1:10 = 0.1 and the dB loss is 0 – (-10) = 10dB.
The dB offset function allows you to easily measure this.
To do so do as follows:
1. When measuring the reference value press
O". The value changes to 0 dB (note that now the
units are dB, a relative value instead of dBm, an
absolute value).
2. Now make your second measurement and the value
of the difference in dB = ratio in numerical units will
be shown.
Note:
If there is a zero offset in the reference value, you
cannot subtract this using the dB offset function.
Instead, before the start of the measurement, press
"Offset" and subtract the zero offset. Then follow
steps 1 and 2 above. The zero offset subtracted when
"Offset" was pressed will be saved in the dBm scale
and you can now use the dB-O setting to measure
true ratio without zero offset problems.
5.4.7. Average and Measuring Very Low Power
Measurement
If the laser power is fluctuating, the Vega can display the
average power readings with averaging periods varying
from 1s to 1 hour. When measuring very low powers,
such as picowatt measurements using the PD300-IRG or
PD300-UV, there will be a rather large zero offset coming
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