Technical data

Testing PROFIBUS
A.2 Testing fiber-optic cable
PROFIBUS Network Manual
System Manual, Edition 04/2009, C79000-G8976-C124-03
247
Measuring methods
In the main, two test methods are used:
1. The optical power source and meter
2. Optical time domain reflectometer (OTDR)
As of version 3, the PROFIBUS OLM has an integrated diagnostic option with which the
quality of the received optical signal can be checked.
A.2.2 Optical power source and meter
All the components of a fiber-optic link such as the fiber, connectors, couplers and splices
contribute to losses. The total loss must be below the available optical power budget
between the optical transmitter and receiver.
The optical power source and meter method allows the link loss to be measured. Light from
a defined light source is sent through the link and the resulting losses are measured with an
optical power meter.
This method can be used for plastic, PCF, multimode and singlemode fiberoptic cables.
Since attenuation depends on wavelength, the measurement must be performed at the same
wavelength that will later be used during operation (650 nm, 850 nm or 1300 nm)
Arrangement for measuring loss
The arrangement for measuring loss consists of a light source and an optical power meter.
The light source is first connected to the receiver via a reference fiber. The optical power
measured at the receiver is the reference value for a link without attenuation. Following this,
the reference fiber is opened and the link to be measured is inserted. The meter compares
the optical power received now with the previously measured reference value and calculates
the attenuation of the inserted link on this basis.
Step 1: Reference measurement
Light source
Optical power meter
Step 2: Line measurement
Light source
Optical power meter
Figure A-2 Measuring the total loss of a fiber-optic link
Evaluating the results of the loss measurements
An optical power budget is available between an optical transmitter and optical receiver. This
identifies the difference between the minimum power launched by the transmitter and the
minimum optical power required at the receiver. The optical power budget is normally