Photonics Connector Care

About this Article
Written and presented at the 1995 NCSL Workshop & Symposium by Peter Linowitz.

Qualitative Assessment on the Effects of Damaged Connectors and Optical Interfaces in Fiber Optic Measurements

In a world in which fiber optics becomes more a part of our lives, more people have become involved in its activities ranging from metrology through to production and installation. In much the same way that BNC and N-Type connectors are used on a daily basis without further thought, the dangers of fiber optic connectors being used in a similar fashion become more real. To quantify or predict the effects that a damaged fiber optic connector or optical interface can have on measurements can be difficult, but sometimes a qualitative assessment of the damage is all that is required in order to ensure that these connectors are kept in a condition of optimum performance.

Background

In order to visualize the effects that damaged fiber optic connectors can have, it is necessary to have a basic understanding of the optical fiber and connector. The following will give a brief overview.

Figure 1

Figure 1 shows the basic construction of two typical optical fibers. In common, they both consist of a glass fiber 125 microns (um) in diameter. The central portion, referred to as the Core, consists of glass with a refractive index slightly higher than that of the glass surrounding it, (Cladding). The diameter of the Core determines whether the light within the fiber propagates in a singlemode or multimode fashion. Typical values for the core diameter are 9um for singlemode and 50um for multimode as shown in figure 1. Light only propagates, or travels, within the core.

Figure 2

Typical construction of a fiber optic connector is shown in figure 2. The main component of the connector is the ferrule. This may be constructed from a metallic or ceramic material and for the more commonly used connectors such as FC/PC, ST, DIN and Diamond HMS-10 will have a diameter of 2.5 millimeters (mm). The optical fiber runs along the length of the ferrule and exits centrally at its end face. It is here that the fiber and ferrule end face are cut/polished to the connectors specification. Typical values of return loss for a connector can range from 14-60dB depending on connector specification.

Also shown in figure 2 is the key. This is a protrusion that ensures that the connector always aligns in the correct orientation when connected to its female counter part.

Figure 3

In its simplest form an interface may consist of two fiber optic connectors butted end to end using an alignment collar or tube such that the fibers within each ferrule are aligned in the same plane. The type of connector used will determine whether the fibers are in physical or nonphysical contact.

In the case where the light from an optical fiber is required to pass into or out of an optical component or system, it may be necessary to modify the divergent light that emits from the end of the fiber to match the characteristics of that component or system. Figure 3 shows such an interface in its simplest form. Here a lens is used as the interface. Its purpose is to collect the divergent beam of light that emits from the end of the optical fiber and convert it into a collimated or parallel beam. The converse is also true for this type of interface in that the lens can take a collimated beam of light and focus it into an optical fiber.

Conditions Affecting Performance

When light exits a glass fiber and enters air, a back reflection in the order of 3.5% of the incident light is produced. This equates to a return loss of 14.5dB and is known as a Fresnel Reflection. For some applications this is acceptable and an interface of the nonphysical contact type is used. For other applications where maximum transmission and improved return loss are required, especially in systems using singlemode fibers, other techniques need to be employed. The simplest of these ensure that the two glass fiber ends are in physical contact with one another, thereby excluding the air that causes the Fresnel Reflection. This can be achieved by providing the glass fiber at the end of the ferrule with a highly polished flat or convex end face. This ensures a good physical contact between two mating fibers of the same type, provided they are clean and undamaged. Typical return losses that can be achieved using this technique are 30-40dB. In order to achieve return losses up to 60dB additional techniques need to be employed. This can include both flat and curved angled cuts and polishes to the ferrule/glass fiber end face in physical and nonphysical contact forms.

In singlemode applications where core diameters of 9um need to be aligned to ensure maximum performance, quite clearly any degradation of ferrule or glass fiber end face can have a significant effect on connector performance. This could be caused by dust or grit between two connectors in contact with one another. This can lead to physical damage of the glass fiber end face itself. Where the damage is severe the damage can in turn be transferred to other good connectors that come into contact with it.

Presentation

The presentation is designed to compliment the information within this paper and hence has a high pictorial content. The intent is to familiarize and raise the level of awareness to the following :

• The fiber optic connector
• The variety of connector types available
• The dimensions involved
• Damage sustained to connectors and the resultant effect on measurements
• Damage sustained to a lens type optical interface and the resultant effect on measurements

Conclusion

Although robust, fiber optic connectors are susceptible to microscopic damage that is not immediately obvious to the naked eye. This damage can have significant effects on measurements being made, whether in a laboratory, production or field environment. Although microscopic examination of connectors is not always practical, an awareness of these effects in conjunction with good practices can ensure that their performance is always optimized.

Bibliography

1. Senior, John M. : Optical Fiber Communications - Principles and Practice,1985, Prentice-Hall International, London. ISBN 0-13-638248-7
2. Hentschel, Christian : Fiber Optics Handbook, 1988, Hewlett Packard. p/n 5952-9654 ISBN 3-9801677-0-4
3. Radermacher, Wilhelm : A High-Precision Optical Connector for Optical Test and Instrumentation, Hewlett Packard Journal, Vol.38, No. 2, February 1987, p.28-30
4. Application Note 366-2 : How to Measure Return Loss of Optical Components, 1988, Hewlett Packard, p/n 5952-9661
5. Handbook : Lightwave Connection Techniques for Better Measurements, 1991, Hewlett Packard, p/n 08703-90028

Selected Photographs

Here are selected photographs that were used to illustrate the points made in the presentation.

Picture 1 -- Optical fiber as seen at the end of connector ferrule. Note the dimensions; 125 micro-metre fiber diameter and 9 micron for the core. The core is where the lightwave actually travels.

Picture 2 -- An example of how NOT to clean an optical interface ! Cleaning with a pin can destroy the ferrule/fiber end-face. Any connector mated to this is likely to be damaged itself.

Picture 3 -- A ploughed field from the air ? Actually, it's a close-up of the fibre-end shown in picture 2. Note the damage around the edge of the fiber but, more importantly, the scratches across the core surface causing light scattering and reflection.

Picture 4 -- Mating in the presence of foreign objects isn't recommended either ! General damage at another fibre-end, perhaps caused by poor cleaning practice.

Picture 5 -- Reprocessing picture 4 emphasizes the problem.

Picture 6 -- Deeply pitted fiber end-face, probably caused by dirt particles being ground into the fiber during connector-mating.

Picture 7 -- The amazing view of Uranus from the Hubble Space Telescope ? No, just the previous photo reprocessed to emphasize the mountainous nature of the damaged fiber

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Photonics Connector Care (World_of_Micrometers_basic2007)