Single direction loss test procedure using a source & meter

For standards compliant test procedures & equipment, please refer to Standards Center. For more background theory, please read on...


This is the simplest form of attenuation measurement. Patch leads should be of similar type to the DUT. F or multimode measurements, a mandrel wrap forms part of the source reference condition. Communication is required between the two operators.


1. Clean the connectors. Connect the source and meter using two patch leads and a through connector (or one patch lead and no through connector for the building cabling method), set the source and meter wavelengths. Allow the source to warm up and stabilise. Measure and record the reference power P1 dBm [For meters which have a relative dB measurement facility, put the meter in relative mode and set the reference]. For dual wavelength work, also reference the other wavelength at this point, after a further warm up period.

Note: it is good practice to occasionally check your patch leads by reversing them (eg the other ends go into the through connector), and re-measuring the reference value, which should only deviate within the expected tolerance of your connectors.

Light Source and Optical Power Meter with 2 Cords and Through Connector

2. Leave the patch leads connected to the instruments (or the one patch lead on the source for the building cabling method), since removing them adds to measurement uncertainty.

3. Insert the DUT into the measurement set up, using an using extra through connector if needed, and record measurement P2 in dBm [if the same reference value was used, the attenuation is displayed directly in dB. Step 5 is not required]

Light Source and Optical Power Meter with 2 Cords, DUT and Two Through Connectors

4. Remeasure the source power as in set-up 1, to verify the source drift remain within acceptable limits.

5. Calculate the attenuation (dB) of the link as: Link attenuation in dB = P1 - P2

6. Repeat at the same wavelength for the other fibers (the order of 5 & 6 may depend on source warm up characteristics)

7. For dual wavelength measuring, change the source wavelength, wait for the source to warm up, then go back to step 3.


  • May require operators in two places.
  • May require operators to swap instruments if measuring bi-directional losses.
  • The time to swap instruments to other end of link can create problems with source stability and battery life.
  • It may be impossible to use a single integrated LTS, if that is what you have.

In this situation 'Autotest' type features can result in better measurement confidence, lower skill requirements and twice the productivity for dual wavelength measurements, since both wavelengths are measured in one operation, and the attenuation is displayed directly.


One Optical Patch Cord Patch Cord
Fiber Optic Connectorised Link Connectorised link
Fiber Optic Through Adaptor Through adaptor
Fiber Optic Source incl. mandrel wrap if multimode Source, incl. mandrel wrap if multimode
Optical Power Meter Meter
DUT Device Under Test DUT Device Under Test (system being tested)

[instructions on use of the relative mode are in italics]



This application note assists in the following application areas...

About Kingfisher

Kingfisher International are specialist designers and manufacturers of handheld fiber optic test equipment.

Our equipment is used in all phases of fiber optic manufacture, installation and maintenance. Our comprehensive documentation and resources help you get started easily.

Kingfisher Products are Australian made, with a global distribution & support network spanning over 70 countries.

You may print, photocopy and redistribute this document for educational use only, provided that this page is printed as is, without modification. This content may not be used commercially.