Dr. Bob Q&A on Ferrules
The following question didn’t fall into Dr. Bob’s area of expertise, but Bishop & Associates’ Karl Jalbert, offered this response.
Dear Dr. Bob,
Why hasn’t the industry considered moving away from the ceramic ferrule to the borosilicate glass ferrules?
Working as a DOD contractor, we have almost exclusively used the glass ferrules when deploying ST, SC, and FC-style connector terminations. The main reason is that we achieved a nearly 100-percent yield rate for delivering a low-loss connection with a PC finish. These ferrules are field repairable at several times, while test results repeat a low-loss measurement. We can use epoxy, UV, and anaerobic adhesives. No special or expensive termination kit is required. They offer good quality polishing pucks, rubber mat, and a four-step quick polishing process that is extremely forgiving.
We purchase these from a small OEM that seems great at engineering and product/process development, but absent when it comes to marketing this product. I have been involved with fiber terminations since the late ‘70s and have never come across such an easy and fast way of terminating FO connectors while delivering a super smooth PC polish. For the single-mode terminations, where back reflections/return-loss are an issue, we use a field polisher to resolve this budget issue. Again, you have several attempts to get it right, if needed. The end-user likes them because they are truly designed for repairability, rather than replaceability, should the end-face become damaged during use.
Now that many of the fiber cabling infrastructures are carrying more bandwidth, we feel there needs to be more attention placed on the weakest link—the connector and its termination methods.
I find that very few contractors, much less the end customer, know of this product and/or the polishing process. Only a few of the bandwidth hogs at Navy/Spawar, Goddard, NOAA, NRO, and other Pentagon sectors are aware of this design.
We have thrown rocks at this product and the polishing process, and it just seems to be what it is. What are your findings for this type of ferrule?
Richard C. Schaum, Project Coordinator
Navy Combat Systems Integration
The reason is that borosilicate glass is relatively soft. What makes the glass ferrules easy to work with is the same thing that makes them vulnerable to vibration and mating or coupling damage. Borosilicate glass is less dense than ordinary glass (having a refractive index of 0.92) and is typically used in the manufacture of step-index (non-data type) optical fiber. Basically, it is easy to use as a ferrule material because it is the same type of glass that is used in the fiber itself. It has a very low thermal expansion coefficient, about one-third that of ordinary glass. This reduces material stresses caused by temperature gradients, thus making it more resistant to breaking. This is why it is a popular material for ferrules. Also, its thermal expansion coefficient is close to that of the fiber. Nonetheless, borosilicate glass begins to soften around 821°C (1510°F). While more resistant to thermal shock than other types of glass, borosilicate glass can still crack or shatter when subject to rapid or uneven temperature variations. When broken, it tends to crack into large pieces, rather than shattering. In other words, it will snap rather than splinter.
Zirconia ceramic and alumina ceramic used for ferrules is extremely hard. For example: there is a scale used to rate the hardness of minerals called the Mohs scale (named after Friedrich Mohs, 1812). It lists the mineral hardness standards, from one to ten, with talc being the softest and diamond being the hardest.
- Talc (Mg3Si4O10(OH)2
- Diamond (pure carbon)
Alumina and zirconic ceramic is rated nine on the Mohs scale.
Like borosilicate glass, aluminum oxide also has a favorable thermal expansion coefficient when compared to the glass fiber. It exhibits extraordinary performance in thermal cycling and weathering conditions. Aluminum oxide is also known as alumina, and is the main component of bauxite, the principal ore of aluminum.
Zirconium dioxide is one of the most studied ceramic materials. Pure ZrO2 has a monoclinic crystalline structure at room temperature, and transitions to tetragonal and cubic. The volume expansion caused by the cubic-to-tetragonal-to-monoclinic transformation induces very large stresses and will cause pure ZrO2 to crack upon cooling from high temperatures. To prevent this, several different oxides are added to zirconia to stabilize the tetragonal and cubic phases. They include magnesium, yttrium oxide, calcium oxide, cerium oxide, and others. All this jargon really means is that ceramic ferrules are engineered to be the most dependable under harsh conditions, especially under thermal cycling.
Mostly, because ceramic ferrules are extruded, not machined, the concentricity of the fiber-hole relative to the outside diameter of the ferrule can be held to about one micron concentricity, and about the same for diameter tolerance. This is especially necessary when aligning single-mode fibers.
Now, you ask, how can the ferrule’s manufacturer guarantee such tolerances? Simple. They measure every one and only keep the tight ones. The not-so-tight ones and those with looser concentricity are either used for multimode applications, or simply discarded altogether.
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