AN143

The Photonics Pioneer

Insertion Loss and Return Loss Testing of Short Cables andAssemblies (including TOSA and cassettes)
Overview
As fiber optic infrastructure and components are designed more densely and more compactly, patch cords, cassettes, and opticalsubassembly cables have followed suit by becoming shorter and shorter. These short cables, which can sometimes be just 20cmin length, present certain problems for testing return loss (RL). Most RL meters, whether pulse-based or Optical Continuous Wave
Reflectometer (OCWR) based, have difficulty accurately measuring the return loss of these cables. Often, the return loss from theback connector of the device under test interferes with the measurement of the front connector.

Figure 1:

Short MTP®/MPO - MTP®/MPO cable
Figure 2:

LC breakout cassette

Return Loss Test Method for Short DUTs
We have explored some solutions to this problem in the past (see AN-127 for more information). Here we will focus on the test
method that employs a termination cable to reduce the reflection from the back connector, as this can be used for patchcords,
cassettes, and harsh environment cables with plug and receptacle fixtures. We suggest this method because we believe it yields ahighly efficient and repeatable measurement without sacrificing much in the way of accuracy.
This test method can be performed for either
unidirectional-style tests, where the receive
cable is connected to the detectors of thetest system, or in bidirectional style tests, likein the diagram to the right, (figure 3) where
the “receive cable” is connected to a switchthat allows the light to go along the fiber
path in both directions. In fact, bidirectional
tests on short cable assemblies utilize thistest method simply because of the required
cable setup for that test.

Figure 3: Testing very short cables with a bidirectional test
setup gives total IL and RL for each connector.

1

OptoTest Corp.

SANTEC
4750
CalleCORPORATION
Quetzal5823
Ohkusa-Nenjozaka,
Komaki,
Camarillo,
CA 93012
Aichi 485-0802,
UnitedStates Japan+81-568-79-3536

Doc: AN-143 Rev. A 12/03/19

Advancing the World of Fiber Optics®
www.optotest.com
www.santec.com
The Photonics Pioneer

SANTEC USA CORPORATION
1.805.987.170 0
USA Toll Free+1-800-726-8321
sales@optotes
t.com
suppor t@optotes t.com
SANTEC
LIMITED
MTP® is EUROPE
a registered trademarkof US Connec
Tel. +44-20-3176-1550

AN143

The Photonics Pioneer

Insertion Loss and Return Loss Testing of Short Cables andAssemblies (including TOSA and cassettes)
Insertion Loss Consideration
One thing to consider with this method of testing is that the insertion loss that is measured is the sum of the IL of the twoconnectors and the fiber that connects them, and the RL is the combined reflectance of the two connectors. This means that theIL measurement is a “total IL” measurement and should have a corresponding pass/fail threshold. For RL, the measurement caneffectively be interpreted as a “worst case scenario” measurement, where a passing value means that both connectors are better
than is required.

Unterminated Short Cables

For cable assemblies where the back connector cannot be connected to a receive cable or otherwise have the reflection reduced,
the test will still measure the total RL of the assembly. When measuring RL in this way, the two reflections add together according
𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅1
𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅2to the following formula:
RL total = 10 ∙ 𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙10 �10 10 + 10 10 �

Essentially, if the two reflections are the same size, the measured reflection will increase by 3dB (e.g. 55dB to 52dB). If one reflection
is much larger than the other, the return loss will be very close to the value of the larger reflection.
Therefore, if the test is measuring the combination of a UPC connection (greater than 45dB) and an open connector (as high as
15-20dB), the result will show values much closer to 15dB than 45dB.

Figure 4:
With both ends of the short DUT connected to a sufficiently long reference-grade lead, thereflections of the two sides of the cable are measured as one combined reflection.

Figure 5:
With no receive cable connected to the short DUT, the reflections of theunterminated end overpowers the smaller reflection.

2

OptoTest Corp.

SANTEC
4750
CalleCORPORATION
Quetzal5823
Ohkusa-Nenjozaka,
Komaki,
Camarillo,
CA 93012
Aichi 485-0802,
UnitedStates Japan+81-568-79-3536

Doc: AN-143 Rev. A 12/03/19

Advancing the World of Fiber Optics®
www.optotest.com
www.santec.com
The Photonics Pioneer

SANTEC USA CORPORATION
1.805.987.170 0
USA Toll Free+1-800-726-8321
sales@optotes
t.com
suppor t@optotes t.com
SANTEC EUROPE LIMITED
Tel. +44-20-3176-1550