ApplicationNotesenko.com

Testing MT or MPO with
Coated End-faces GR Testing
JUNE 2021
Rev. 01

Issued by:EngineeringDepartment

Created by:Andrei Vankov
Emmanuel Kolczynski

Application Engineering Note
1.0Introduction

SENKO’s product line has fiber optic interconnects that at times feature durable and/or antireflective (AR) coatings. The purpose of the coating can vary depending on the product.
However, various coatings can be added to improve the connectors mating performance in theform of Insertion Loss (IL) and Return Loss (RL), assist with better repeatability, minimize
cleaning requirements, or even increase the durability of the connector. For example, SENKO
MT and MPO connectors can be offered with such coatings that remain compatible withconventional MT and MPO interconnects. However, these various coatings may require a
special consideration to be taken during the optical IL measurement process. A coatedconnector can either add or subtract a small amount of attenuation during the measurement
process which can result in skewed data and confusion. For example, the referencing process
can be puzzling when a coated reference end is used to test a standard non-coated compatible
connector. There are also instances where testing assemblies with various coatings should beconsidered. One would not need to take the same considerations when a coated reference isused to test connectors with the same coating.
This compensation is not atypical or a sign of degraded performance. In fact, this is common
practice and it’s good to be aware of this variance with the advancements and evolution of thefiber optic market. This document will illustrate the specifics of what needs to be consideredduring testing and measurements of end-faces with various coatings.
2.0Discussion

When light transfers from one media to another there will inevitably be a certain amount ofreflected light which needs to be considered. This is based on the value of each media’s
refractive indices. Reflection 𝑅 is defined in a formula below where 𝑛! and 𝑛" are the refractive
indices of two different materials respectively.
𝑅=(𝑛! − 𝑛" )"(𝑛! + 𝑛" )"The optical loss due to Reflection is defined as,
L = −10log(1 − 𝑅)
For a polished uncoated fiber end, the two materials are fiber glass and air with 𝑛! = 1.45 and𝑛" = 1.00 respectively. Using the formulas above, the calculated reflectivity 𝑅 is about
3.3736% or 14.7dB with optical total loss 0.149dB per each surface. These theoretical values areoften seen when test equipment measures the reflection of a clean standard polish connector.

1

Application Engineering Note
Similarly, changes to Reflection and overall Loss are contributed to by a coating. A coating canhave a different refractive index. Table 1 shows some examples of calculated insertion loss
based on the refractive index of a coating.
Table 1. Examples of optical losses per surface due to AR coatings
Coatingrefraction
index
1.33
1.46
1.50
1.60

Total IL,
dB
-0.18
-0.30
-0.35
-0.54

To demonstrate the theory, SENKO set up experiments to quantify the change in loss due to thecoating. The method that was used can be referred to as a cut-back method. This can be done
by first referencing a coated connector with the measurement equipment. Following thereference, the coated connector is replaced by either a spliced connector with no coating, or a
precisely cleaved bare fiber which can be held in a special bare fiber adapter in front of thedetector. The difference between the reference and the new measurement is the adjustmentthat will need to be considered during measurement. Some form of compensation will need tobe applied when a coated reference cable end-face is used with compatible non-coatedconnectors and vice versa. Thus, the goal of the experiment would be to quantify thecompensation factor for various scenarios.
Four samples were used to verify the theory by considering the points below:••••

The cleaving tool and splicer are verified to provide repeatable measurements prior totesting.
The IL of equipment must be stable meaning the laser should not drift after the firstreference. It may need a warmup period. Observing a stability of +/-0.01dB over 15
minutes is needed before testing.
A minimum amount of fiber is needed to be cut back. Fiber loss can influence yourresults.
The fiber management is maintained in a fixed position avoiding the influence ofpolarization and bending losses due to handling.

2Application Engineering Note

The following process can be followed to determine the difference in loss between the coated,and non-coated optical interface.
Step 1: Reference Coated MPO to set “0”.

Figure 1: Initial reference of a Coated connector

Table 2. Initial referenced loss is zero across all 12 MPO Channels
Sample #Fiber #

#1

#2

#3

#4

CoatedMPO tobespliced
withregular
MPO

1231310nm
0001550nm
0001310nm
0001550nm
0001310nm
0001550nm
0001310nm
0001550nm
0004567800000000000000000000000000000000000000009100000000000000000111200000000000000003

Application Engineering Note
Step 2a: Reference, then cut one side of Coated MPO and fusion splice the free end with aregular MPO

Figure 2: Step 2a of experiment reference using same cable with non-coated spliced on connector

Or, an alternate solution is to use a bare fiber adapter. Note, the cleave needs to be very clean
and the use of a high-quality precision cleaver is advised when using a bare fiber adapter.
Step 2b: Reference, then cut one side of the Coated MPO. Cut, cleave, load into the bare fiber
adapter, and measure.

Figure 3: Alternate method using Bare Fiber Adapter

4Application Engineering Note
Table 3. Optical losses when coated connector is replaced with a non-coated standard connector
Sample#Fiber #Coated
MPO tobe
splicedwith
regularMPO

1234
567
89101112Max
MinAVG#11310nm
0.17
0.18
0.14
0.18
0.16
0.14
0.15
0.14
0.14
0.15
0.14
0.14
0.18
0.14
0.15

#21550nm
0.18
0.18
0.16
0.2
0.19
0.17
0.17
0.16
0.15
0.17
0.17
0.16
0.2
0.15
0.17

1310nm
0.18
0.17
0.14
0.13
0.13
0.14
0.17
0.13
0.12
0.14
0.12
0.13
0.18
0.12
0.14

#3
1550nm
0.19
0.19
0.17
0.16
0.16
0.16
0.19
0.16
0.15
0.15
0.15
0.15
0.19
0.15
0.17

1310nm
0.12
0.12
0.13
0.13
0.1
0.12
0.12
0.13
0.11
0.12
0.11
0.12
0.13
0.1
0.12

#4
1550nm
0.14
0.15
0.16
0.14
0.13
0.15
0.16
0.15
0.13
0.15
0.16
0.15
0.16
0.13
0.15

1310nm
0.18
0.18
0.19
0.20.23
0.19
0.18
0.17
0.24
0.2
0.18
0.16
0.24
0.16
0.19

1550nm
0.17
0.17
0.19
0.19
0.22
0.2
0.18
0.17
0.22
0.2
0.19
0.17
0.22
0.17
0.19

The above study indicates that there is an average difference of 0.15 dB between the coatedand non-coated MPO. As a result, the theoretical and real measurements align with each other.
For this reason, a compensation should be applied in certain test cases. The following arerepresentations on how to apply the loss compensations and when.
Note, LC’s and MPO’s are used for visual representation. The same principle applies for othertypes of coated and non-coated connectors.

5Application Engineering NoteSituation 1 - Compensation needed
The reference is performed with a coated MPO in the detector. The measurement is taken witha non-coated LC in the detector. This means a -0.15dB compensation should be applied to themeasured IL value.

Figure 4: Coated to Non-coated

The reference is performed with a non-coated LC in the detector. The measurement is takenwith a coated MPO in the detector. This means a +0.15dB compensation should be applied tothe measured IL value.

Figure 5: Non-coated to coated6

Application Engineering NoteSituation 2: No compensation needed
The reference is performed with a non-coated LC in the detector. The measurement is takenwith a non-coated LC in the detector. This means no compensation should be applied to themeasured IL value.

Figure 6: Non-coated to Non-coated

The reference is performed with a coated MPO in the detector. The measurement is taken witha coated MPO in the detector. This means no compensation should be applied to the measuredIL value.

Figure 7: Coated to Coated7

Application Engineering Note
3.0

SummaryIn conclusion, important to remember a few considerations when testing connectors withcoated end faces. The difference in the refractive index between coated and uncoatedconnectors would require you to add or subtract a compensation factor to the data collected.
This is because a reflection and loss measurement will then be referenced prior to testing thestandard jumpers in order to take more accurate measurements of the loss of the standard
connector alone. Referencing a master can be compared to zeroing a load cell before weighing
something. Therefore, if the master test jumper has a coating on it which causes attenuation tochange, and that jumper is referenced in the detector of the test equipment, the testequipment has now taken a measurement of the jumper with adjusted attenuation. Then, if
you plug in a non-coated or differently coated connector in the detector to test, the datameasured will have discrepancies because you would not be comparing two end faces withmatching conditions. This is only the case when the master jumper and the device under test do
not have corresponding coatings on their respective end faces. It is possible to apply thetheoretical value, then verify it by using the cut-back method.
Thus, anytime one is using a coated connector as a master jumper, a similar quantification to
the example shown above, may be necessary.

8Application Engineering Note1