AN-127
Measuring RL on Short Cables: A Detailed Approach
Over view
In the 1990s, a new method of testing return loss revolutionized the fiber optics industry. Requiring no mandrels
or matching gel to measure the return loss of a cable, the optical time-domain reflectometry (OTDR) method
streamlined the production process by cutting down on the amount of work required to reference and takereturn loss measurements. However, this technology brought with it a small caveat: the laser’s pulse widthalong with a few other factors limit the resolution of the measurement. This is easily seen in the following traces
recorded on a singlemode OP930.
NE W

RL

1550nm

54.20

Setup

RLdBm

dBHPR

dB

OP930

Figure 1: Return Loss scan of 3.0 meter PC-PC cable

RL

1550nm

54.20

dB

Setup

RL
dBm
dB

HPR

OP930

Figure 2: Return Loss scan of 1.5 meter PC-PC cableCompare the two scans; while one clearly shows both front- and back-end reflections, the other shows the twopeaks beginning to merge into one large peak rather than two small peaks. At 1.5 meters, the peaks are closeenough to each other that they begin to interfere with one another and resulting in the cable’s “worst casescenario” overall. This allows the user to provide an “order-of-magnitude” RL test; if the cable passes, it definitelypasses, but if it fails, further tests would be necessary to determine if the cable is truly failing. With the industrywide move towards 100G Ethernet and beyond, more cables are needed in less space which in turn requires theincreased use of shorter cables.
The principal aim of this Application Note is to provide testing methods for cables which are shorter than therecommended length rather than to instruct how to perform a reference cycle or measure standard-length cables,
although these topics will be briefly discussed. For a more detailed walkthrough of how to perform reference
cycles and test cables with OptoTest equipment, please consult the manual for the device or software applicationor other pertinent Application Notes.

1

SANTEC CORPORATION
OptoTestCorp.

5823 Calle
Ohkusa-Nenjozaka,
Komaki,
4750
Quetzal
Aichi 485-0802,
JapanCamarillo,
CA 93012
Doc:AN-127 Rev. A 12 / 04 /13+81-568-79-3536

www.santec.com

SANTEC USA CORPORATION
8 05.987.170 0USA Toll Free+1-800-726-8321
sales@ optotest.com
engineering @ optotest.com
SANTEC EUROPE LIMITED
Tel. +44-20-3176-1550

AN-127

Measuring RL on Short Cables: A Detailed Approach

Referencing Return Loss
Referencing return loss is an important step and an improper reference can lead to inaccuracy in measurements
during tests. To reference return loss, connect the launch lead(s) to the front panel and leave the endface of thefinal launch lead unmated. If the final endface of the launch assembly is an angle-polished connector (APC), aN E Wpoint
three to six inch APC-to-PC reference reflector cable can be mated to that endface to provide a reference
for the unit.
NE W

APCsidePCside
Figure 3: Reference Reflector

Measuring Return Loss on Cables 3 to 2,400 Meters Long
Cables that measure three meters and longer
can be tested with relatively few restraints. In
general, any far-end reflection will not influence
the measurements of the connector-under-test.
Using the mandrel-free method allows the user
to test without implementing a mandrel wrap,a vital factor for high output production lines
and when testing multimode fibers, especiallybend-insensitive multimode fiber (BIMMF). Thismethod also eliminates the needs for messy
index matching gels and index matching blocks.
Please note that creating the 14dB Fresnel
reflection is important to a proper reference andtherefore creating highly accurate test results.

RL

1300nm

-0.023dB

SourceRefDetectordBm

1310 nm
1550 nm

OP930

IL/RL Test Set

HPR*Figure 4: Standard cable setup to measure ILRL on a connector
for any cable 3m to 2400mMeasuring Return Loss on Cables Shorter than 3 Meters
In order to yield the most accurate results for a device-under-test (D.U.T), OptoTest units use the OTDR method of
measuring return loss with methods that have been developed specifically for testing return loss on cables lessthan two meters apart. This section will focus on effective and efficient test methods which will provide the mostaccurate return loss results.
It is important to note that although mandrels can be used to attenuate the far end reflection of a cable, mandrels
alone cannot always be counted on to sufficiently reduce a large reflection enough to ensure that only the front
end reflection is being measured. This is especially true for multimode cable because it will only increase theattenuation for the high order modes and not the more tightly coupled modes.

*HPR High Performance Reference Cable

2SANTEC CORPORATION
OptoTest
Corp.

5823 Calle
Ohkusa-Nenjozaka,
Komaki,
4750
Quetzal
Aichi 485-0802,
Japan
Camarillo,
CA 93012Doc:
AN-127 Rev. A 12 / 04 /13+81-568-79-3536

www.santec.com

SANTEC USA CORPORATION
8 05.987.170 0
USA Toll Free+1-800-726-8321
sales@ optotest.com
engineering @ optotest.com
SANTEC EUROPE LIMITED
Tel. +44-20-3176-1550

AN-127

Measuring RL on Short Cables: A Detailed Approach

Measuring Return Loss on Cables between 1.5 and 3 Meters
Cables in this range can usually be tested using standard procedures if the second end of the cable is anunmated APC connector or has a similarly small reflection.
NE W
RL

1300nm

-0.023Detector

SourceRef

APC

dBm

dB

1310 nm
1550 nm

OP930

IL/RL Test Set

HPRFigure 5: Test setup for cables with an open APC connector (1.5 m to 3.0 m)

If the reflection associated with the second connector is larger than approximately 45dB, other test methods
will need to be employed to ensure the accuracy of the return loss measurement.
The recommended method for testing these cables is to utilize a golden cable with a high quality polishbecause it will yield more consistent results. Since current PC-PC connections typically exhibit return loss
values lower than -40dB for multimode or -53dB for singlemode, it does a significantly better job of reducingthe reflection on the second connector than a matching block which will typically yield about a -30dB reflection.

RL

1300nm

-0.023dB

RefSourceDetectordBm

1310 nm
1550 nm

OP930

IL/RL Test Set

HPRHPRPCPC

Figure 6: Test setup for PC-PC cables. The second PC connection diminishes the rear reflection on the DUT

Matching gel will also typically suffice for negating the effects of the second reflection on cables of this length,
but is not quite as repeatable or consistent of an option as the termination cable method and it takes more timeto clean up matching gel residue left on the ferrule than to disconnect a golden cable.

3SANTEC CORPORATION
OptoTest
Corp.

5823 Calle
Ohkusa-Nenjozaka,
Komaki,
4750
Quetzal
Aichi 485-0802,
Japan
Camarillo,
CA 93012Doc:
AN-127 Rev. A 12 / 04 /13+81-568-79-3536

www.santec.com

SANTEC USA CORPORATION
8 05.987.170 0
USA Toll Free
+1-800-726-8321
sales@ optotest.com
engineering @ optotest.com
SANTEC EUROPE LIMITED
Tel. +44-20-3176-1550

AN-127

Measuring RL on Short Cables: A Detailed Approach
Measuring Return Loss on Cables Shorter than 1.5 Meters
For cables shorter than 1.5 meters, it becomes increasingly necessary to have their 2nd reflection negated inorder to provide accurate readings on the front reflection.
NE W

RL

Setup

1550nm

54.20

dB

RL
dBm

HPR

dB

OP930

Figure 7: Return Loss scan of 1.0 meter PC-PC cable showing that the pulses are so close that they cannot be distinguishedfrom each other and look more like one wide pulse

For cables in this range, it is best
to utilize a combination of matching
gel and a termination stub to getaccurate RL measurements. Anothereffective combination for cables inthis category would be a termination
cable and a mandrel to isolate thefront connector’s reflection from thecombination of the two connectors’reflections. The combination of the two
methods (mandrel or matching gel andtermination cable) produces a back-end
connection with negligible return loss
so that the contribution from the front
connector can be measured without
noticeable interference.

RL

1550nm

54.20

dBRL

1300nm

-0.023dBSource

RefDetectordBm

1310 nm
1550 nm

OP930

IL/RL Test Set

HPRTermination
Cable
MANDREL

Figure 8: Test setup with DUT mandrel wrapped and terminated to get rid of back reflection

MANDREL

Setup

RLdBmdBHPR

OP930

Figure 9: Return Loss scan of 1.0 meter PC-PC cable with mandrel showing backend of DUT attenuated enough to not affectthe front reflection measurement

Please note that if a mandrel is used, it should have a small diameter and the cable should be wrapped around it
several turns. Remember that this mandrel wrap is not being used to condition the launch, but to attenuate thereflection of the back connector. Since it is harder to attenuate the more tightly coupled modes in a multimodefiber, using a mandrel in this situation is not as effective as it would be with singlemode fiber.

4SANTEC CORPORATION
OptoTest
Corp.

5823 Calle
Ohkusa-Nenjozaka,
Komaki,
4750
Quetzal
Aichi 485-0802,
Japan
Camarillo,
CA 93012
Doc:AN-127 Rev. A 12 / 04 /13+81-568-79-3536

www.santec.com

SANTEC USA CORPORATION
8 05.987.170 0
USA Toll Free
+1-800-726-8321
sales@ optotest.com
engineering @ optotest.com
SANTEC EUROPE LIMITEDTel. +44-20-3176-1550

AN-127

Measuring RL on Short Cables: A Detailed Approach

Summary
In summation, the OTDR-style return loss meter is by far the most accurate way to test return loss regardlessof cable lengths, fiber core size, and connector types. The back-end connector on cables shorter than threemeters can interfere with the front end return loss values—yielding a worst case scenario overall. However,byNE Wutilizing a golden cable or another method of attenuating the reflection of the second connector, the true returnloss of the first connector can be tested accurately.

Table 1In short,the following table can be used to determine what method to use when testing short cables:NE W

5Cable Length

Far-End Termination

Singlemode Cable

Multimode CableA

Longer than 3 meters

Angle-polished or UPC-polishedNo restrictions based on cable length

No restrictions based on cable length

B

Between 1.5 and 3 meters

Angle-polishedNo restrictions based on cable length

3 meter termination cable

UPC-polished3 meter termination cable

3 meter termination cableAngle-polished

Termination cable with mandrel before
the back-end connection

Termination cable with matching gel inthe back-end connection

UPC-polished

Termination cable with mandrel before
the back-end connection

Termination cable with matching gel inthe back-end connection

C

1.5 meters and shorter

SANTEC CORPORATION
OptoTest
Corp.

5823 Calle
Ohkusa-Nenjozaka,
Komaki,
4750
Quetzal
Aichi 485-0802,
Japan
Camarillo,
CA 93012Doc:
AN-127 Rev. A 12 / 04 /13+81-568-79-3536

www.santec.com

SANTEC USA CORPORATION
8 05.987.170 0
USA Toll Free
+1-800-726-8321
sales@ optotest.com
engineering @ optotest.com
SANTEC EUROPE LIMITED
Tel. +44-20-3176-1550