As the optical fiber and cable industry unfolded, several terms were coined to describe specific properties that were new and different from conventional wire processing. One of those that stayed around was the term “Loose Tight Buffer.”
Over the past fifteen to 20 years the term was used to define both a specific property as well as a product problem. This resulted in many different definitions and a broad set of requirements for a type of optical cable. That has meant many different products to many different users. As we move forward the time is past due to create a definition of what exactly is a loose tight buffer and how is it measured. This article proposes that the various tight buffer requirements be defined based on end-uses such as termination with an epoxy polish connector, a fusion splice termination, and mechanical field splice connectors. The various environments that such cables and terminations are expected to function in are also in need of clearer definition.
Why Loose Tight Buffer?
As the methods of termination and interconnection continued to evolve, two generic methods of cable design evolved. The most common design was a gel filled loose tube which initially contained only one optical waveguide per tube but could contain many tubes (for multi-fiber cables), and a very robust simplex cable design commonly known as tight buffer (a.k.a. tight bound). The loose tube design needed a termination enclosure such as a splice case or termination rack. Initially these were fusion spliced, separated or furcated into individual tubes for termination. For low count optical cables the alternative was an insulation or “buffer” to make the 125/250 um fiber more resistant to handling and termination. A 900 um standard emerged shortly after the SMA optical connector was standardized. This allowed for a solid epoxy bond to an engineering plastic and the glass optical waveguide, making a robust termination that could be handled many times with little chance of breakage.
Other methods of termination included fusion splicing as well as mechanical splices. Many of these methods evolved to enable estimation of the splice loss prior to permanently sealing the splice. One such technique is the use of local injection and detection (LID). Due to the need to access optical power thru the optical waveguide, coating removal of the buffer for some distance beyond the splice was required. Typically this occurred in a connector at one end and a fusion splice at the other end. Tight buffer cables now needed to have a removable buffer layer in order to be compatible with such termination systems. These splices were also placed in housings where the amount of space for slack storage was minimal and a 900 um coated fiber takes up 13 times the amount of space compared to a 250 um coated fiber. For one fiber this is not a significant issue but place 24 or 72 or 144 fibers in a splice case or rack and the difference is significant.
A second reason to create a loose close fitting buffer is specialty fibers, which are far more sensitive to mechanical stresses. These came on the scene in uses that required mechanical protection and flexibility, making a rigid loose tube design unacceptable. These fibers may be as small as 60 um cladding with a 150 um coating, or as large as 1 mm cladding and 1.4 mm coating. In each case, the reasons for being able to strip off a coating related to the specific application.
Items such as splicing and splice slack storage were common needs and in many cases, large scale field installers using existing equipment for fusion splicing and mechanical field connector termination needed to have a standard medium (size coating) to terminate and train to.
Enter the Loose Tight Buffer
The logical evolution to a removable (loose) tight buffer followed. Due to varying reasons and lengths of tight buffer removal required, many different specifications propagated. In some cases the buffer was nothing more than a very small loose buffer using a hard engineering material such as nylon that was easily removed using existing loose tube tools. In other cases the lack of excess length control and mechanical robustness made this design limited in usefulness. One area of concern was that in optical waveguide connector termination, any gap between the buffer and coating would act as a wicking agent for epoxy to migrate from the connector up through the interstitial space and into the flexible cable. This would almost always cause a fiber break just outside of the cable connector interface. As a result, many cable specifications called out no gap between the acrylate coating and the buffer material, while also requiring a strip ability of from 2 to 10 cm.