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Optical Test Leads MUST be of higher quality than the network they are testing. Protrusion and Undercut can wreak havoc
on installed network connectors during the testing process. FOT UPC, High Performance Test Leads are 100% tested and
visually inspected prior to shipment. All FOT Test Leads come complete with OLTS and Interferometer test reports.
on installed network connectors during the testing process. FOT UPC, High Performance Test Leads are 100% tested and
visually inspected prior to shipment. All FOT Test Leads come complete with OLTS and Interferometer test reports.
In this FOT NOTE we will explain and discuss the key criteria that affect the
subsequent performance of polished fiber optic connectors. These are:
1. Back Reflection, Reflectance, or Return Loss
Backreflection is light reflected back through the fiber toward the source, which transmits
the lightwave. The light reflection occurs at the contact point of two connectors when
they are mated. A high level of Backreflection will cause transmission problems for systems
that depend on the speed and clarity of a fiber system, since the desired high data rates
can encounter bit errors if the signal is distorted.
The current industry Back-reflection standard is <-55dB.
subsequent performance of polished fiber optic connectors. These are:
- Back-reflection (measurement)
- Insertion Loss (measurement)
- Apex Offset (end-face geometry)
- Radius of Curvature (end-face geometry)
- Fiber Undercut/Protrusion (end-face geometry)
- Acceptable and Unacceptable End-faces on fiber optic connectors
1. Back Reflection, Reflectance, or Return Loss
Backreflection is light reflected back through the fiber toward the source, which transmits
the lightwave. The light reflection occurs at the contact point of two connectors when
they are mated. A high level of Backreflection will cause transmission problems for systems
that depend on the speed and clarity of a fiber system, since the desired high data rates
can encounter bit errors if the signal is distorted.
The current industry Back-reflection standard is <-55dB.
| Doctor Fiber |
"Let's start out by learning a bit about fiber optics connectors and
why it is important for test leads to be of the highest quality
available."
why it is important for test leads to be of the highest quality
available."
Connectors are commonly referred to as PC, SPC, UPC, and APC. These are terms,
which describe connector end-faces and also relate to the Backreflection designation:
which describe connector end-faces and also relate to the Backreflection designation:
| Doctor Fiber |
"We can learn more about these various types of
polishes for fiber optics connectors. Read on!"
polishes for fiber optics connectors. Read on!"
- PC (Physical Contact)
- SPC (Super Physical Contact)
- UPC (Ultra Physical Contact)
- APC (Angled Physical Contact)
The angle of choice is 8 degrees. This angle deflects Backreflection to <-65db.
| Doctor Fiber |
| "Do you know how to use these tools? If not, better get a dose of fiber at BDI DataLynk. They teach procedures . . . NOT OPINIONS!" |
| Fiber To The Desk Takes The Stand |
| The US Courts Project in Texas Will Prove to be one of the Largest 10Gig, FTTD Projects in Recent History. Project was Designed by BDI DataLynk and all Technicians Installing and Certifying the Project were Trained by BDI DataLynk. Read About it Right HERE! |
2. Insertion Loss
Insertion loss is the amount of optical power lost at the interface of two connectors. Poor
insertion loss readings are generally a result of fiber misalignment, separation between
connections (also referred to as ‘air-gap’) and/or the quality of the finish on the end of
the connector.
Insertion loss is a function of the polishing equipment and the technique used to perform
the polishing. A machine that produces poor end-face geometry will almost always
generate unacceptable levels of loss. The currently stated standard for insertion loss is
<0.5db, but the commonly expected level, has become <0.3db. In addition to the above
performance characteristics, there is a specified product geometry-specified to assure
reliability and ongoing proper connector performance under adverse conditions such as
vibration and temperature cycling. These characteristics depend on the high level of
control that a mechanical polisher provides.
3. Apex Offset
The term Apex defines the highest point on the spherical surface at the end-face of the
connector. Apex Offset is the measured distance between the center of the fiber and the
actual high point of a polished connector.
Although Apex Offset describes a physical condition of the polished fiber, rather than a
performance parameter, it is considered an acceptance criterion in itself. An excessive
Apex Offset contributes to high Insertion Loss and high Backreflection readings.
4. Radius of Curvature
Radius of curvature is the measurement of a connector's end-face spherical condition.
The radius generated on a connector end-face affects connector performance, and so is
specified-the radius must be such that when mated with another connector most of the
compression that occurs is applied to the material that surrounds the fiber (also referred
to as ferrule absorption). In general, the ferrules used are pre-radiused. The radius is
maintained during polishing by applying pressure between the connector and a resilient
polishing surface, by application of a weight or by setting compression dimensionally (it
is increasingly rare, but the forming of a flat ended ferrule into a PC end is still done
through the same basic technique of applying pressure against a resilient surface). The
harder the resilient polishing surface the larger will be the resultant connector radius
(more flat). Conversely, the softer the polishing surface the smaller the connector
radius. A proper radius, in conjunction with fiber under-cut, allows for correct fiber-to-
connector compression. The industry specification for radius of curvature is 8-25mm.
This range allows for maximum connector performance.
5. Fiber Undercut/Protrusion
When a fiber is recessed inside a connector ferrule the term used is “Fiber Undercut.”
When a fiber protrudes above the ferrule it is called “Fiber Protrusion.” Measurement of
this characteristic is accomplished by using an interferometer. An Interferometer displays
the offset of the interference lines that pass over the fiber.
Most polishing sequences begin with aggressive materials, silicon carbide to remove epoxy
and diamond lapping films for the beginning and intermediate polishing, that remove both
the ferrule and fiber at the same rate. During the last polishing step, however, less
aggressive material, usually silicon dioxide, is used because it attacks only the fiber. If an
aggressive film is used or a less aggressive film is used too much, undercut can occur.
Keep in mind that even if a 0.3 micron film is used, there are still airborne dust particles
floating around that land on the film and become a part of the polishing medium.
Excessive Fiber Undercut is usually specified as more than 50nm. Fiber Undercut is a
condition that affects both Backreflection and Insertion Loss. When connectors are
mated, the ferrule material surrounding the fiber compresses, which optimally allows
fibers with an acceptable undercut/protrusion to make contact. Fibers that do not make
intimate contact have an air-gap. An air-gap will produce unacceptable Backreflection &
Insertion Loss measurements.
Both Undercut and Protrusion are a result of the polishing process. If excessive
protrusion is present, fiber chipping and/or cracking may take place during the connector
mating process and can actually damage the connector on the test fiber segment.
6. CHART OF ACCEPTABLE AND UNACCETABLE END-FACE FINISHES ON FIBER OPTIC
Insertion loss is the amount of optical power lost at the interface of two connectors. Poor
insertion loss readings are generally a result of fiber misalignment, separation between
connections (also referred to as ‘air-gap’) and/or the quality of the finish on the end of
the connector.
Insertion loss is a function of the polishing equipment and the technique used to perform
the polishing. A machine that produces poor end-face geometry will almost always
generate unacceptable levels of loss. The currently stated standard for insertion loss is
<0.5db, but the commonly expected level, has become <0.3db. In addition to the above
performance characteristics, there is a specified product geometry-specified to assure
reliability and ongoing proper connector performance under adverse conditions such as
vibration and temperature cycling. These characteristics depend on the high level of
control that a mechanical polisher provides.
3. Apex Offset
The term Apex defines the highest point on the spherical surface at the end-face of the
connector. Apex Offset is the measured distance between the center of the fiber and the
actual high point of a polished connector.
Although Apex Offset describes a physical condition of the polished fiber, rather than a
performance parameter, it is considered an acceptance criterion in itself. An excessive
Apex Offset contributes to high Insertion Loss and high Backreflection readings.
4. Radius of Curvature
Radius of curvature is the measurement of a connector's end-face spherical condition.
The radius generated on a connector end-face affects connector performance, and so is
specified-the radius must be such that when mated with another connector most of the
compression that occurs is applied to the material that surrounds the fiber (also referred
to as ferrule absorption). In general, the ferrules used are pre-radiused. The radius is
maintained during polishing by applying pressure between the connector and a resilient
polishing surface, by application of a weight or by setting compression dimensionally (it
is increasingly rare, but the forming of a flat ended ferrule into a PC end is still done
through the same basic technique of applying pressure against a resilient surface). The
harder the resilient polishing surface the larger will be the resultant connector radius
(more flat). Conversely, the softer the polishing surface the smaller the connector
radius. A proper radius, in conjunction with fiber under-cut, allows for correct fiber-to-
connector compression. The industry specification for radius of curvature is 8-25mm.
This range allows for maximum connector performance.
5. Fiber Undercut/Protrusion
When a fiber is recessed inside a connector ferrule the term used is “Fiber Undercut.”
When a fiber protrudes above the ferrule it is called “Fiber Protrusion.” Measurement of
this characteristic is accomplished by using an interferometer. An Interferometer displays
the offset of the interference lines that pass over the fiber.
Most polishing sequences begin with aggressive materials, silicon carbide to remove epoxy
and diamond lapping films for the beginning and intermediate polishing, that remove both
the ferrule and fiber at the same rate. During the last polishing step, however, less
aggressive material, usually silicon dioxide, is used because it attacks only the fiber. If an
aggressive film is used or a less aggressive film is used too much, undercut can occur.
Keep in mind that even if a 0.3 micron film is used, there are still airborne dust particles
floating around that land on the film and become a part of the polishing medium.
Excessive Fiber Undercut is usually specified as more than 50nm. Fiber Undercut is a
condition that affects both Backreflection and Insertion Loss. When connectors are
mated, the ferrule material surrounding the fiber compresses, which optimally allows
fibers with an acceptable undercut/protrusion to make contact. Fibers that do not make
intimate contact have an air-gap. An air-gap will produce unacceptable Backreflection &
Insertion Loss measurements.
Both Undercut and Protrusion are a result of the polishing process. If excessive
protrusion is present, fiber chipping and/or cracking may take place during the connector
mating process and can actually damage the connector on the test fiber segment.
6. CHART OF ACCEPTABLE AND UNACCETABLE END-FACE FINISHES ON FIBER OPTIC
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