"CLIC" - Clean to Maintain Optical Connector Quality

There are many different optical connectors, but no matter what connector you work with, CLean and Inspect your Connectors (CLIC) as it is important to keep the end face clean and un-blemished to prevent excessive loss and return loss.

Test Fiber and Initial Preparation

Scratches, dirt, dust, and other contaminants can severely impact high-speed data transmission in optical fibers. Dirty connector end-faces are often the number one cause of poor performance, link failures and even connector damage. High insertion loss and/or high back reflection can result in transmission loss or high bit errors and poor BER.

Most measurement variations and test repeatability conditions in fiber-optic systems can be traced back to the cleanliness of optical connections. Contamination of fiber end faces not only affects optical power levels but also impacts back reflectance performance and levels which is harmful to sensitive optical components.

Dirty fiber end faces can cause back reflectance and insertion loss

Contamination

Optical connectors are susceptible to contamination from airborne particles and human body oils when exposed. Left over liquid residue from improper cleaning can also leave the fiber end face contaminated. The smaller the fiber core, the more severe the problem is likely to be, especially when considering that fiber core diameters can range from 62.5 microns all the way down to 8 microns in size.

Types of contamination viewed from a Fiber microscope

 

Cross-section-mode and multi-mode fiber zones

Inspection

Whenever possible, inspect the fiber-optic connection (connectors, bulkheads, and test interfaces) with a fiber microscope. It is recommended to wear laser safety glasses when working with fiber-optic connections. Always check that the laser source or transmitter is disconnected before cleaning the connector end faces.

How to inspect a connector end-face using a fiberscope

Cleaning Procedure

To ensure proper and effective cleaning of optical fiber connectors and interfaces, use the following fiber optics specific cleaning materials:

• Glass cleaning fluid or 99.9% Isopropyl alcohol
• Lint free soft tissues
• Ferrule cleaning sticks (1.25mm, 2.5mm or other specialty connectors like MTP/MPO or MDC versions)
• Connector reel cleaners (Cletop type or similar)
• Lint-free swab sticks

Optical fiber cleaning materials: Isopropyl alcohol, lint free soft tissues, connector reel cleaners, and a ferrule cleaner (courtesy of MicroCare™)

Clean the connector end-face by rubbing it onto a lint-free wipe dampened with isopropyl alcohol

Procedure

1. Dab the contaminated connector end-face with a wipe that has been dampened with isopropyl alcohol. The solvent will dissolve and remove contaminants that have dried and attached to the connector or fiber end-face.
2. Rub the fiber end-face perpendicularly against a dry lint free wipe several times, following the manufacturer's instructions.
3. Alternatively, use compressed air to dry the surface quickly. Do not blow or allow the connector end face to air dry as this may leave a residue behind which is often more difficult to clean and can attract even more dirt.
4. Re-inspect the fiber end-face with an optical microscope (fiber scope) to check that all the contaminants have been removed properly. If not, repeat the process.

Best Practices

• Never touch the end face of an optical fiber connector with any hands or fingers.
• Always install dust caps on unplugged fiber connectors.
• Store unused dust caps in a resealable plastic bag to prevent dust accumulating.
• Never re-use optic cleaning swabs or lint free wipes. Always discard materials that have been used.
• Ensure all alcohol or solvent residues are removed after using wet cleaning techniques
• It is recommended to re-inspect the bulkhead receptacles and connector end face using a fiber microscope following the cleaning and prior to use.

Best practices for inspection and cleaning prior to connecting an optical fiber

Fiber Connectors

Connector Types

In fiber optic networks, you will come into contact with many different connector types, the most common being described below:

SC Connector

SC officially stands for Subscriber Connector; however, some people believe that "Square Connector", is the correct name. It is a general purpose push/pull style connector. It was developed by NTT, which has an advantage in LAN/enterprise networks where duplex cabling to support TX/RX (GBIC) channels are commonly used has quickly replaced screw-type connectors.

SC connector end face

LC Connector

LC stands for Lucent Connector. It is a small form-factor fiber optic connector that uses a 1.25 mm ferrule, which is half the size of the ST / SC connectors and supports increased connector density in central offices patch panels; it is often used in LAN/enterprise networks with duplex cables to support SFP channels.

FC Connector

FC stands for Fixed Connection - it is fixed by way of a threaded barrel housing. FC connectors are typical in test and measurement environments and for single- mode applications.

FC is the fiber optic connector standard for Nippon Telephone & Telegraph (NTT) installations, developed with Nippon Electric Co. (NEC).

 

ST Connector

ST stands for Straight Tip-a quick release bayonet style connector developed by AT&T. STs were predominant in the late 80s and early 90s and are still one of the most commonly used fiber optic connectors in Utilities fiber networks. They are cylindrical with twist lock coupling, 2.5mm keyed ferrule.

ST connectors are used both short distance applications and long line systems. The ST connector has a bayonet mount and a long cylindrical ferrule to hold the fiber.

E2000 Connector

Developed by Diamond, this connector has proven its performance worldwide in CATV and telecommunication networks. The connector features a spring-loaded shutter used to protect the ferrule from dust and scratches, and to provide increased safety protection. It uses a high precision Zirconia full ceramic ferrule with an insertion loss of 0.1dB. The APC version has a return loss of better than 85dB. The connector is mostly used in Europe and defense contractors to ensure high performance with their networks.

The E2000 patch cords is available in simplex and duplex versions and comply with IEC 61 754-15 and TIA/EIA 604-16 standards.

Connector Performance and Polishing

Polishing of fiber optic connectors is the process of polishing the end-face of the ceramic ferrule within the fiber optic connector. The purpose is to improve the light transfer between the mating of connectors, in order to minimize optical losses and reducing reflections. This is an important aspect of fiber optic communications as losses affect the quality of the light signals.

There are two main types of losses that are affected by the style and quality of polishing, which dictate the optical performance level of terminated fiber optic connectors:

• Insertion Loss (IL) - The most important performance indicator of a fiber optic connection. This is the loss of light signal, measured in decibels (dB), during the insertion of a fiber optic connector.
• Return Loss (RL) - Also known as back reflection, is the portion of the light signal that is reflected back to the original light source. This occurs as the light is reflected off the connector and travels back along the fiber to the light source. This indicator is measured in negative decibels (dB). When reading return loss figures, the higher the absolute value of the decibel unit means the better the performance of the interconnection.

The amount of insertion loss is affected by fiber alignment, and/or the quality of the finishing on the end of ferrule, while reflectance is affected by the style of polishing on the ceramic ferrule in a connector. There are two different styles of polishing, reflected by the shape of their finish:

• Ultra Physical Contact (UPC) - The UPC style ferrule has the shape of the PC style, except they are polished with several grades of polishing film that allows for an ultra-smooth surface. The main difference between UPC and PC is that the former have lower return loss.
• Angled Physical Contact (APC) - The APC style produces the lowest return loss when compared to other styles. The ferrule is polished to an angle of 8 degrees. The angle is calculated so that it is less than the critical angle, which ensures light is not propagated back along the fiber.

Connector Polish and Performance

 UPC and APC back reflection 

UPC and APC return loss

Typical Return loss values are:

• Polished Connector ~ -45dB
• Ultra-Polished Connector ~ -55dB
• Angled Polished Connector up to ~ -65dB or better

Fiber Cables and Fiber Patch Cords

Styles of Fiber Optic Cable vary in outer appearance, materials, application, features, and benefits. In OTDR applications, you will regularly come across the following types:

Bare Fiber

• Usually Spooled in 25km spools
• 250 to 400um acolyte buffer / polyimide
• PM Light Sensitive Bare
• Fiber Adaptor is a useful accessory and provides quick connection to check fiber length/continuity PVC or Plenum Jacketed
• Many colors that mean very little
• Used mostly in internal applications

900 μm

Patch cords

• Simplex (single fiber) or Duplex (two fibers) construction
• Singlemode or Multimode

Pig tails

• Outer Diameter is usually 1.6mm, 2.0mm, and 3.0mm
• Duplex versions - Zip cord with outer diameters of 1.6mm, 2.0mm, 3.0mm

Break Out / Fan Out

• Both Multimode and Singlemode
• Indoor / Outdoor applications
• 1.6mm, 2.0mm, and 3.0mm subunits
• 2, 4, 6, 8, 12, 24, 48, 72 or greater fiber counts Outer Jacket can be of a variety of materials
• Terminated with all styles of connectors
• Often field terminated

Distribution

• Both Multimode and Singlemode
• Indoor / Outdoor applications
• 900 μm subunits
• 2, 4, 6, 8, 12, 24, 48, 72, 144 or greater fiber counts
• Outer Jacket can be of a variety of materials
• Usually longer runs and can be terminated with almost any style of connector

Ribbon Fiber

• Commonly used in LAN and PON applications with MTP/MPO connectors

Fiber Patch Cord

In a typical GPON network, a fiber is connected directly to the customer ONT/ONU. To test the GPON network, the fiber must be temporarily disconnected from the ONT/ONU and inserted into the FX120 OLT port. An additional fiber patch cord is then used to connect the FX120 ONU port to the ONT/ONU modem.

Patch cord and optical connectors play a critical role in fiber measurements. Ensure that a high quality, clean patch cord is always used, as this can and will have profound performance on test results and traces.
 
Color Coding

The buffer or jacket on patch cords is often color-coded to indicate the type of fiber used.

 
Connector Boot

The strain relief boot that protects the fiber from bending at a connector is color-coded to indicate the type of connection.

Connectors with a plastic shell (such as SC or E2000) typically use a color-coded shell.

Standard color coding for jackets and boots (or connector shells)

Inserting the Fiber

Carefully align the optical fiber connector to the port to avoid rubbing the fiber against the external part of the port or any other surface. If the interface of the connector has an alignment key, make sure to insert it correctly into the corresponding groove.

Push the connector in and make sure the optical cables are inserted to guarantee sufficient contact. If the connector has screw bushing, screw down the connector to fix in the optical fiber. Do not over- screw the connector or it will damage the optical fiber and the port.

Preventing Inaccurate Measurement Readings

To achieve maximum power and prevent false readings, clean the optical fiber connector interfaces before inserting them into the test port.

Please ensure the correct fiber connector type is used before inserting it into the test port or connector. Mismatched connector types will damage the optical end faces and the test set.

 

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