Application
Notesenko.com

Low Loss Connectors
and Fiber Outside Diameter
JANUARY 2024
Rev. 01

Issued by:Engineering
Department

Created by:Andrei Vankov

senko.com

Low Loss Connectors
and Fiber Outside
DiameterContents

2

Low Loss Connectors and Fiber Outside Diameter3

Introduction

4

Discussion

10

Conclusion

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Introduction
SENKO provides a wide range of connectors designed for diverse fiber optic applications. But what exactly
sets a fiber optic connector apart in terms of its merits? The primary purpose of a fiber optic connector
is to terminate the ends of fiber optic cables, ensuring they can be interconnected reliably with minimal
optical loss. After termination and interconnection, two critical parameters come into play: Insertion Loss (IL)and Reflection or Return Loss (RL). A superior connector will exhibit minimal optical loss, thanks to precisealignment of the connected fiber cores and enhanced stability.
In essence, the demand for a fiber optic connector is driven by these qualities: reduced loss, cost-effectiveness,and ease of termination. Consequently, the market has seen the introduction of numerous fiber optic
connectors, each adhering to various standards and serving specific applications. However, only a select few
dominate the market, with even fewer standing out for their exceptional quality.
One crucial factor that determines a connector’s quality is the zirconia ceramic ferrule. The degree of ferrule
concentricity and the tightness of the ferrule’s inner diameter (ID) are key factors that influence the extent oflateral misalignment during connection. Lateral misalignment, rather than longitudinal air gaps or angular
misalignment, is a primary contributor to interconnect loss.
When estimating insertion loss (IL) resulting from lateral misalignments, the following theoretical calculations
were considered. The equation used:

IL (dB) = -10 log e-U
U=

2

x
ωo

x = core misalignment;
1.619

ωo = a (0.65 + V

1.5

2.879

+ V )
6

ωo –single mode fiber (SM) spot size determined empirically with α as a fiber core radius and V as a normalized
frequency – dimensional number used to describe the characteristics of SM fibers, depending on refractive
indexes of fiber core/cladding.
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Relationship Between
Lateral Offset andOptical Loss87

Insertion Loss (dB)

6
5
4
3
2
1
0
0.1

0.6

1.1

1.6

2.1

2.6

3.1

3.6

4.1

4.6

5.1

Offset (μm)

Figure 1 Relationship between lateral offset and optical loss.

5.6

It’s reasonable to assume that a connector withtighter ferrule ID tolerances will yield superior
optical performance. However, one must also
consider the tolerances associated with thecylindrical silica glass outer diameter (OD) of thefibers. Ensuring there is no overlap in tolerances
between the ferrules and the fibers is crucial toprevent misfit issues. Connector termination
processes cannot afford production slowdowns
due to trial-and-error attempts to fit tight ferrules
onto equally tight fibers, nor can they risk scrapping
connectors due to misfit problems arising fromtolerance overlaps.

SENKO’s range of connectors offers various ferrule grades, spanning from Low Loss to Premium to Standard. These
options cater perfectly to different fiber types and applications, aligning with specific loss requirements.
Now, let’s delve into what sets SENKO’s connector offerings apart from the competition.

Discussion
There are two critical parameters in a fiber optic connection: Optical Insertion Loss (IL) and Optical Return Loss
(RL). IL measures the power loss during signal transmission, while RL measures the amount of reflected light.
Both parameters are crucial for assessing the performance and quality of optical components and connections
in fiber optic systems. Lower IL and RL values are required for efficient and reliable optical communication.
They are used to characterize the quality of optical connections and the efficiency of signal transmission infiber optic networks. Here are their definitions:

4

Optical Insertion Loss

Optical Return Loss

Optical Insertion Loss, often simply called
Insertion Loss (IL), is a measure of the amount of
optical power that is lost when light is transmitted
through an interconnect. It quantifies the
reduction in signal strength that occurs as lighttravels through a connection point. Expressed indecibels (dB), IL is typically displayed in a positive
notation. Lower insertion loss values indicate
better signal transmission efficiency.

Optical Return Loss, also known as Return Lossor Optical Reflectance, measures the amountof light that is reflected back towards the source
when it encounters a discontinuity or reflection
point in the optical interconnect. In other words,
it quantifies the amount of optical power that
does not get transmitted through the system andis instead reflected. Expressed in decibels (dB).

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To minimize both IL and RL in fiber optic systems, precise alignment of fiber cores without any air gap
between them is crucial. This process begins with the use of high-quality ceramic ferrules that exhibit
near-zero eccentricity and concentricity values and have tightly controlled fiber hole inner diameters (IDs).
Additionally, RL is managed during the manufacturing process by automated polishing of ferrules to meet
specified geometrical standards and ensuring a defect-free optical endface.
In the past, ferrule and fiber tolerances were less stringent compared to today’s requirements, driven by theneed to reduce optical losses. To achieve this, a technique known as “Tuning” was commonly employed.
Tuning involved orienting the fiber core in a specific position, typically aligning it with the key direction of
the connector. By aligning the fiber core offset (also referred to as eccentricity and concentricity error) of
each ferrule in the same direction, the total lateral offset between the joining fiber cores could be reduced
compared to random orientation, thereby minimizing optical loss.
However, it’s important to note that this tuning technique is effective only when a tuned connector is matedwith another tuned connector, ensuring that the fiber cores are rotated and placed in the same orientation.
Mating a tuned connector with a standard one negates the advantages of low loss achieved through tuning,which is a common occurrence in the field. Therefore, tuning is not a viable or practical option.
So, what are the critical factors that connector manufacturers need to consider in achieving low insertion loss
measurements? Assuming that connectors are clean, free from foreign particles and defects, and adhere to
GR-326 requirements in terms of geometry, the most critical aspects are:

Critical factors for Low Insertion Loss
1.)

Ferrule Hole Diameter

GR 326

The ferrule is the most important component in a fiber optic connector
STANDARDS
that ultimately bears responsibility for optical loss values. For a minimum
Insertion Loss, you should use the tightest-tolerance ferrule hole diameter
available. For example, if the typical values for a single-mode fiber glass Outer Diameter (OD) are
124.5μm to 124.9μm the desirable ferrule Inner Diameter (ID) should be 125μm to 125.5μm.

2.)Ferrule Hole Eccentricity
Ferrule hole eccentricity measures the deviation or displacement of the center of one shapefrom the center of another shape. In the context of a ferrule, eccentricity refers to the linear
distance between the true center of the ferrule hole and its ideal position. It’s crucial for the holein the ferrule to be perfectly centered; otherwise, proper fiber alignment cannot be achieved.

3.)

Ferrule Hole Concentricity
Ferrule hole concentricity assesses the alignment of the centers of two or more shapes. In
the context of a ferrule, it refers to the roundness and positioning of the bore hole within theferrule. If the bore hole deviates from a perfectly round shape, you introduce another variableknown as ferrule hole concentricity, which measures the alignment of the bore hole’s inside
diameter (ID) with its outside diameter (OD). An oval-shaped bore hole, rather than a perfectlyround one, will not secure an optical fiber in perfect alignment.

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Ferrule manufacturers typically specify ferrule hole concentricity to simplify terminology, assuming all three
aspects mentioned above are in context. In this case, for SENKO Low Loss ferrules concentricity value isconsidered to be ≤0.3 μm, SENKO Premium Loss concentricity value is considered to be ≤0.6 μm, and theStandard Loss concentricity value is considered to be ≤1.0 μm.
Therefore, it’s important to use ferrules considering the above tolerances for optimum performance.
1

Ferrule hole diameter

Insidediameter

125 μm fiber Ceramic
inside ferrule ferrule
hole is too large

2

Eccentricity

9 μm
fiber core

Eccentricity
inside ferrule hole is
not in true center

3

Concentricity or Ovality

Fiber

Concentricity
inside ceramic diameternot round shape

Figure 2 Examples of defects in ceramic ferrules that result in lateral misalignments.
Note, the images are exaggerated for demonstrative purposes

After we have established the differences in concentricity values among the ferrule manufacturers, we will
explore how these variations may impact the manufacturing process on the production floor, which is
responsible for terminating connectors with optical fibers.
The outer diameter (OD) tolerances for optical single-mode fibers can vary depending on the specific fiber
specifications, manufacturer, and industry standards. However, there are common industry standards thatprovide guidance on these tolerances. The International Telecommunication Union (ITU) has establishedstandards for optical fibers, including single-mode fibers. According to ITU-T G.652, which defines thecharacteristics of standard single-mode optical fibers (also known as G.652 fibers), the typical outer diameter(OD) tolerance for these fibers is within the range of ± 0.5μm. It’s important to note that different types ofsingle-mode fibers, such as those optimized for specific applications or wavelength ranges (e.g., G.657 fibers
for bend-insensitive applications or G.655 fibers for long-haul applications), may have slightly different OD
tolerances. Therefore, it’s advisable to understand the specific fiber specification provided by the manufacturer
or perform your own study for precise information on OD tolerances for the particular type of single-mode
fiber you are using or considering.

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However, if you look at the specification sheets for Corning® SMF-28e+® LL optical fiber the Glass Geometry
will look like as follows:
Dimension Specifications
Glass Geometry

Coating Geometry

Fiber Curl

≥ 4.0 m radius of curvature

Coating Diameter

242 ± 5 μm

Cladding Diameter

125 ± 0.7 μm

Coating-Cladding Concentricity< 12 μm

Core-Clad Concentricity≤ 0.5 μm

Cladding Non-Circularity≤ 0.7 %

Figure 3 Fiber glass specifications for Corning® SMF-28e+® LL opticalfiber as published.

In practice, leading fiber manufacturers often maintain significantly tighter tolerances for the outer diameter(OD) than ±0.5 μm. The actual statistical data from a fiber manufacturer is typically proprietary and not publicly
shared. However, SENKO gained extensive experience in estimating actual fiber sizes from various suppliers.
Typically, the ODs of leading fibers range from 124.7 μm to 125.3 μm, with a normal Gaussian distribution
centered around the 124.9 μm value.
In the following example, we will demonstrate a straightforward method to estimate the implications ofusing super-tight ferrules with an assumed inner diameter (ID) range of 125.0 to 125.7 μm. To calculate thepercentage of ferrule fallout due to tolerance overlap in the above scenario, we will assume that the ferrule
sizes follow a Gaussian probability distribution (normal distribution). Using statistical calculation for a number
N (mean, standard deviation) we have the following:

Problem: Calculate the Percentage
of Ferrule Fallout
Ferrule ID (A) ~ N (125.35, 0.117)
Fiber OD (B) ~ N (125, 0.1)
The probability that the fiber is lager than the hole, P(B > A) = P(B - A)
The probabilistic variable C = B - A also follows a normal distribution:
C ~ N (125 - 125.35, 0.1 + 0.117) = N (-0.35, 0.217)
Our task is to find P(C > 0) - the probability of fiber being oversized.
In order to do this, we need to calculate the Z-factor and consult
standard normal distribution tables N(0,1) or a calculator that isavailable on-line. Z = -0.35 / 0.217 = -1.613
We look up the table or use an on-line calculator (in the on-line calculator
example you can simply set Mean=-0.35, SD=0.217 and compare it to zero).

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Low Loss Connectors and Fiber Outside Diameter

Figure 4 Calculator for standard
normal distribution.

Answer: 5.3%
or 5-6 ferrules out of 100
could not go over the fiber

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SENKO is aware of tolerance overlaps, drawing from its extensive experience with fibers from leading
manufacturers. To mitigate misfit scenarios, SENKO shifts its ferrule ID distribution to the right, thus adjusting
the normal curve accordingly. This approach minimizes the likelihood of overlap and misfit between theferrules and the fiber distribution, all while ensuring the tightest and most manufacturable fit of the fiber into
the ferrules. This, in turn, results in perfect core alignment and the lowest possible optical loss.
How optical loss is defined and what is the lowest possible loss? The losses are categorized by Grades that are
defined in IEC 61753-1 document.
Insertion Loss criteria based on Random mating in the document IEC 61753-1 titled Attenuation of Random
Mated Connectors which defines a certain minimum IL value of randomly mated connectors in a field setting
and categorizes them into four different grades. It is important to note that the IEC 61753-1 defines connection
performance grades for single-mode and multimode jumpers in a controlled environment. The standard
gives four grades for IL from A (best) to D (worst) and RL, with grades from 1 (best) to 4 (worst). Grade “A” isnot officially ratified by the IEC document, but most likely it will have similar optical performance reflected inTable 1 below.

Table 1 IEC 61753-1 Proposed Attenuation Grade Jumpers

Attenuation Grade

Attenuation Random Mated IEC 61300-3-34*

Grade A≤ 0.07 dB mean

≤ 0.15 dB max for >97% of samples

Grade B

≤ 0.12 dB mean

≤ 0.25 dB max for >97% of samples

Grade C≤ 0.25 dB mean

≤ 0.50 dB max for >97% of samples

Grade D≤ 0.50 dB mean

≤ 1.00 dB max for >97% of samples

Return Loss Grade

Return Loss Random Mated IEC 61300-3-6

Grade 1

≥ 60 dB (mated) and ≥ 55 dB (unmated)

Grade 2≥ 45 dB

Grade 3≥ 35 dB

Grade 4

≥ 26 dB

Specification
0.1 dB Connector

Each-to-each Values
approx 0.2 dB
possibly higher if different manufacturers are
combined or unadjusted connectors are usedapprox. 2 dB
unclear range of tolerance

Grade C≤ 0.25 dB mean, ≤ 0.50 dB max≤ 2.5 dB

Grade B

≤ 0.12 dB mean, ≤ 0.25 dB max≤ 1.2 dB

Grade A*≤ 0.07 dB mean, ≤ 0.12 dB max≤ 0.70 dB* Note, Grade A is not specified at time of writing, but assumed to be as shown

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Budget for 10 Connections

Low Loss Connectors and Fiber Outside Diametersenko.com
SENKO offers ferrules that comply with any of the discussed grades from IEC 61753-1. The available ferrule
offerings fall into three categories: Low Loss, Premium, and Standard. Today, SENKO customers have theoption to order Low Loss ferrules with a maximum concentricity specification of 0.3μm, Premium ferrules witha maximum concentricity specification of 0.5μm, and Standard ferrules. The typical concentricity of Standard
ferrules is also 0.5μm, with a slightly larger standard deviation compared to Premium ones.
SENKO’s intermetability data using Low Loss ferrules demonstrate the ability to achieve Grade A, as proposed
by IEC. However, during production, there may be occasional misfits due to the tight tolerances between
the fiber OD and ferrule ID. Nevertheless, when working with leading fiber manufacturers, the misfit should
not exceed 2-3%. Even in situations where the ferrule does not fit on a stripped fiber initially, it is likely to fit
on the same fiber lot on a different area of the fiber, as the fiber OD varies slightly over the fiber’s length. This
trade-off results in premium optical loss, regarded as the best in the industry. If the stripped fiber does not fit
a specific ferrule, it is common practice to put the ferrule off to the side and try another ferrule on the same
stripped fiber. Afterwards, the ferrule that was put off to the side can be used for the next stripped fiber.
Low Loss connectors are used to manufacture reference cables, and in hyperscale data centers, where thelink loss budget for 400GB+ transceivers can be as low as 1-2 dB, achieving minimal connection loss is critical.
In emerging quantum computing, specifically Quantum Key Distribution (QKD) systems, there is a need for
repeatable Low Loss connections to meet the requirements of Quantum Networking applications.
Premium ferrules are the most common SENKO offerings that meet the demands of today’s most interconnect
applications and are often recommended and used. With Premium ferrules, achieving IEC Grade B is possible
without any production line slowdowns due to fallout or misfit. Furthermore, by employing SENKO Premium
connectors and the ferrule tuning technique, it is possible to exceed Grade B Loss requirements.
Standard ferrules are often utilized in SENKO multimode connectors, or situation where Low Loss is not critical.
The ferrules are still designed to have maximum tolerances on concentricity that align with single-mode
requirements.
For a comprehensive summary of the anticipated optical losses under random mating conditions for all
available ferrule types by SENKO, refer to the table provided below. This data originates from a study performed
using SENKO single-mode LC Unibody connectors in a 10x10 random mating testing scenario.
Connector TypeIL Against Master

IL Random Mating

IL Random Tuned

Unibody Low Loss (dB)

≤0.05 mean / ≤0.15 max≤0.125 mean / ≤0.25 max≤0.02 mean / ≤0.14 max

Unibody Premium≤0.085 mean / ≤0.20 max≤0.15 mean / ≤0.305 max≤0.04 mean / ≤0.17 max

Standard UPC≤0.12 mean / ≤0.30 max≤0.25 mean / ≤0.50 max

N/A

Table 2 Random mated optical losses for SENKO Low Loss, Premium and Standard ferrules.

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Conclusion
SENKO provides a diverse range of connectors featuring Low Loss,
Premium, and Standard ferrules, each tailored to meet various loss
specifications. Selecting the most appropriate connector for a given
application hinges on a thorough understanding of the optimal network
optical loss budget and specific requirements.
It’s important to recognize that tighter ferrule inner diameters andconcentricity result in reduced lateral misalignment between mated cores,
leading to the lowest optical loss, as defined by the IEC 61753-1 standard
titled Attenuation of Random Mated Connectors. However, it’s not always
necessary to opt for Low Loss connectors, as this choice can potentially
impact mass production efficiency when dealing with tightly fit fibers
into ferrules, especially if the fibers are sourced from manufacturers withless stringent cladding OD tolerances.

SENKO’s quality
ferrule types:
Low Loss
PremiumStandard

In many applications, Premium connectors offer reliable and adequate
performance. At SENKO, we encourage our customers to invest in quality
connectors that not only adhere to the strictest tolerances for ferrule
hole diameter, concentricity, and eccentricity but also align with their
optical budget requirements.
For optimal results, it’s important to consider that the fiber itself shouldhave extremely low cladding OD tolerances, virtually zero core/cladding
concentricity, and the production line must adhere to factory termination
processes that comply with GR-326-CORE standards. By incorporating
connectors that meet these rigorous tolerance criteria through worldclass manufacturing processes, you set the stage for creating toptier fiber optic cable assemblies that consistently deliver outstanding
performance and long-term reliability.

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Low Loss Connectors and Fiber Outside Diameter

GR 326
STANDARDSsenko.com

References
1. IEC 61753-1:2018 © IEC 2018 Fibre optic interconnecting devices and passive components - Performance standard – Part 1:
General and guidance
2. IEC 61300-3-4. Edition 3.0 2009-01 Fibre optic interconnecting devices and passive components – Basic test and measurement
procedures – Part 3-4: Examinations and measurements – Attenuation of random mated connectors
3. Telcordia Technologies Generic Requirements for Singlemode Optical Connectors and Jumper Assemblies GR-326-CORE Issue
4, February 2010
4. The Relationship between Insertion Loss and Premium Ferrules, SENKO Application Note, Released Date: February 26th, 2021
5. Random Mating IL versus IL by Master Jumper, SENKO Application Note, Released Date: November 1, 2021
6. Online Statistics Education: An Interactive Multimedia Course of Study,
onlinestatbook.com/2/normal_distribution/normal_distribution.html. Accessed 1/18/2024.

https://

Biography
Andrei Vankov, is an Application Engineer at SENKO Advanced Components. He received his BS from Thomas Edison
State College and his MSEE from Pennsylvania State University. He began his career in 1993 at Sumitomo ElectricLightwave Corp as a Fiber Optic Manufacturing Engineer where he worked on active and passive components using
Kaizen methods in Yokohama, Japan. As a Senior Optical Design Engineer in Franklin, MA (founded as Advanced
Interconnect) Andrei Vankov developed various passive optical components and packaging integration to meet
Telcordia industry standards. He designed optical interconnects, including optical backplanes (MTP, HBMT, PHD,
OGI), and a fiber optic SMPTE compatible Broadcast Connector for HD applications. In 2013-2020 Andrei worked at
Nokia division Radio Frequency Systems (RFS) where he provided leadership for an LTE RAN launch project team.
Andrei holds several US and European Patents in fiber optics interconnect technology.

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Low Loss Connectors and Fiber Outside Diameter