THORLABS vytran PTR308 User Manual page 15

PTR308 Automatic Recoater with Linear Proof Tester Chapter 6: Recoat and Proof Test Process
Chapter 6 Recoat and Proof Test Process
The PTR308 has right and left-hand fiber holding blocks that are used to position the fiber for recoating and grip
the fiber for proof testing. Once the fiber is loaded into the fiber holding blocks, the recoat and proof test processes
are performed automatically, either independently or in sequence.
The purpose of the recoat is to maintain the strength and flexibility of a stripped fiber or fusion splice by protecting
the glass surface from damage. The recoat material is nominally the same as the original coating on the fiber, and
it will therefore have similar performance characteristics. It should be noted that recoating a splice does not make
the splice stronger, it simply protects what strength is already there. The handling and processing of the fiber prior
to recoating will therefore determine the strength and reliability of the recoated section. By performing a proof test
after the recoat is completed, the reliability of the recoated section can be assured. The required proof test level for
long term reliability will depend upon the stresses that the recoated section of fiber will see during its lifetime. A
general rule of thumb is to proof test 3 to 5 times higher than the maximum expected load on the fiber.
To perform a recoat, the original fiber must enter into the recoat mold from both sides so that recoat material can
be injected up to, and ideally over, the strip interface regions. This means that the original diameter of the fiber must
be smaller than the diameter of the recoat mold and that the length of the recoat must be shorter than the length of
the mold (50 mm). The standard recoat mold size for nominal 250 micron coated fiber is 280 microns. This provides
room for an "overlap" region where the recoat can flow over the original coating. An overlap length of 2 to 5 mm is
recommended from a reliability standpoint.
When the recoat process is initiated, a slight tension is applied to the fiber through the movement of the right-hand
fiber holding block. The mold is then closed around the fiber, which is centered within the mold cavity by alignment
guides (early models) or alignment pins (newer models) built into the mold gripper assembly. The fact that the
original coating enters into the mold at both sides helps to ensure that the exposed section of fiber is centered within
the mold and that the bare glass does not touch the sides of the mold cavity. Once the mold is closed, the
programmed Quantity of recoat material is injected into the mold at the programmed Recoat Rate. The curing LEDs
are then turned on for the programmed Cure Time to cure the recoat material. In order to account for the fact that
the recoat material will shrink slightly during cure, an additional small percentage of the Recoat Quantity (the Shrink
Quantity) is injected into the mold during an initial percentage of the Lamp Time (the Shrink Time). Once the Cure
Time is complete the mold will open and the recoated section of fiber should be free from the mold.
After the recoat is completed a proof test should be performed to ensure that the recoated fiber meets the minimum
strength requirement for its intended application. When the proof test process is initiated, the right-hand fiber holding
block will first move to the left to release any load that is on the fiber. The load reading is then zeroed and the right-
hand fiber holding block begins moving to the right to apply tension to the fiber. Rubber inserts within the fiber
holding blocks tightly grip the fiber without damaging the original coating. The programmed Ramp Time determines
how long the fiber holding block will take to reach the programmed Peak Tension. Once the Peak Tension is
reached, the load will be held for the programmed Hold Time, after which the load is quickly released. If the fiber
breaks during the proof test process, the maximum tension achieved (the breaking strength) will be recorded.
Note: The proof tester measures the load that is applied to the fiber, which can be displayed in units of grams,
Newtons or pounds. By dividing the applied load by the cross-sectional area of the fiber, the system can also display
the tensile stress applied to the fiber, in units of kpsi (kilo-pounds-per-square inch). The cross-sectional area is
calculated from the Fiber Diameter parameter, which must be entered correctly for the fiber under test. Because
the coating has a very low modulus compared to the glass fiber, any load applied to the fiber is carried almost
entirely by the glass. It is therefore very important to make sure that the diameter of the fiber is entered (e.g. 125
microns) and not the diameter of the coating.
Rev C, October 19, 2020 Page 13