Ferrules and V-Groove Blocks Measurements

Contents

• Introduction
• Ferrules
• C Technologies Ferrules
• FiberGuide Ferrules
• V-groove Blocks
• Overall Dimensions
• Fiber Spacing
• Fiber Alignment
• Conclusion

Introduction

The ferrules and the V-groove blocks of two prototype fiber optic harnesses, each of which was fabricated by C Technologies and FiberGuide Industries for multi-object spectroscopy, were inspected and measured for mechanical tolerances. This report presents the measurements and observations made on those parts.

Each harness has twenty fiber optic lines with both ends terminated. At one end, the free fibers are terminated with stainless steel ferrules for insertion into an aluminum plate at the telescope focal plane. At the other end, the fibers are stripped and placed, with close spacing, in a small V-groove block, which is to be mounted at the slit of the spectrograph.

Ferrules

The diameter of each ferrule was measured for both C Technologies and FiberGuide fiber harnesses. Three measurements were taken at three locations along the ferrule, from shoulder to tip (Figure 1). Approximately, the same locations were measured on all the ferrules. The diameters along those locations are tabulated (Table 1 and Table 2). A Mitutoyo micrometer, model # 293-765-10, was used to measure these diameters. The micrometer zero value was checked after 3 or 4 ferrules had been measured. The diameters were only taken in the axis parallel to the anchor block. The design specification for the ferrule's diameter was 2.154 ± 0.003 mm. The three measurements were averaged and incorporated into a histogram plot for each set of ferrules (Figure 2 and Figure 3).

Fig. 1: Three locations measured along the ferrule.

            Ferrule Diameters (+/- .00127 mm)  
fiber #   location 1   location 2   location 3   Average
   1       2.14884      2.14884      2.14884     2.14884
   2       2.15138      2.15138      2.15138     2.15138
   3       2.15138      2.15138      2.15138     2.15138
   4       2.15265      2.15265      2.15265     2.15265
   5       2.15138      2.15138      2.15138     2.15138
   6       2.15138      2.15138      2.15138     2.15138
   7       2.14884      2.14884      2.14884     2.14884
   8       2.15138      2.15138      2.15138     2.15138
   9       2.15138      2.15138      2.15138     2.15138
  10       2.15138      2.15138      2.15138     2.15138
  11       2.15265      2.15265      2.15265     2.15265
  12       2.15138      2.15138      2.15138     2.15138
  13       2.15265      2.15138      2.15138     2.15180
  14       2.15011      2.15011      2.15011     2.15011
  15       2.15138      2.15138      2.15138     2.15138
  16       2.15265      2.15265      2.15265     2.15265
  17       2.15138      2.15138      2.15138     2.15138
  18       2.15392      2.15392      2.15392     2.15392
  19       2.15392      2.15392      2.15392     2.15392
  20       2.15265      2.15265      2.15265     2.15265
Stadev                                           0.00132
Minimum                                          2.14884
Maximum                                          2.15392

Table 1: Diameter measurements of C Technologies ferrules. (Design specification of the diameter is 2.154 +/- 0.003 mm)

            Ferrule Diameters (+/- .00127 mm)      
fiber #   location 1   location 2   location 3   Average
   1       2.15392      2.15392      2.15392     2.15392
   2       2.15519      2.15519      2.15519     2.15519
   3       2.15265      2.15265      2.15265     2.15265
   4       2.15138      2.15138      2.15138     2.15138
   5       2.15138      2.15265      2.15265     2.15223
   6       2.15138      2.15265      2.15265     2.15223
   7       2.15392      2.15392      2.15392     2.15392
   8       2.15138      2.15138      2.15138     2.15138
   9       2.15265      2.15265      2.15265     2.15265
  10       2.15138      2.15138      2.15138     2.15138
  11       2.15392      2.15265      2.15265     2.15307
  12       2.15265      2.15138      2.15138     2.15180
  13       2.15773      2.15773      2.15646     2.15731
  14       2.15646      2.15646      2.15519     2.15604
  15       2.15646      2.15646      2.15519     2.15604
  16       2.15392      2.15392      2.15392     2.15392
  17       2.15519      2.15392      2.15392     2.15434
  18       2.15265      2.15265      2.15265     2.15265
  19       2.15392      2.15392      2.15265     2.15350
  20       2.15265      2.15265      2.15138     2.15223
Stdev                                            0.00170
Minimum                                          2.15138
Maximum                                          2.15731

Table 2: Diameter measurements of FiberGuide ferrules. (Design specification of the diameter is 2.154 +/- 0.003 mm)

Fig. 2: Histogram plot of ferrule diameter of C Technologies harness.

Fig. 3: Histogram plot of ferrule diameter of FiberGuide harness.

The following paragraphs describe the measurements and observations made on the ferrules.

C Technologies Ferrules

The only aberration that was found in this group of ferrules was that the diameters on the three locations (Figure 1) along one ferrule were not identical (Table 1) and differed by as much as 1.27 micron (or 50 micro inches which is at the resolution limit of the micrometer).

FiberGuide Ferrules

Problems which were found on the FiberGuide ferrules were:

1. The measuring surface of the micrometer was contaminated by fine dark particles found on some of the ferrules. Under a microscope, the surfaces of these ferrules were not machined smoothly. Fine dark particles which probably resulted from machining were found imbedded in those surfaces which contaminated the micrometer's measuring surface.

2. The front surfaces of ferrules #14 and 20 were not machined flat. There was a noticeable 'facet' to the surface when light is reflected of the fronts of these ferrules. The non-flat section, however, does not affect the fiber optics on either fiber.

3. On 10 of the ferrules, the diameters on three locations along the ferrules are not identical (Table 2).

V-groove Blocks

The following dimensions were taken on these V-groove blocks.

1. The front and back widths and length of the block.

2. The spacing between fibers.

3. The angular alignment of the fibers on the V-groove blocks.

Overall dimensions

The V-groove blocks are tapered from front to back. A Mitutoyo's Digimatic Caliper, model # CD-6"P, was used to measure the widths on the front and back of the block and also the length. The following table shows these dimensions.

                Front width (mm)    Back width (mm)   Length (mm) 
C Technologies  7.1374 ± 0.0127     7.0993 ± 0.0127     4.85 
FiberGuide      7.2003 ± 0.0127     7.0993 ± 0.0127    10.4* 
Design spec.    7.1323 ± 0.0254     7.0917 ± 0.0254     5.0#
*The length of the V-groove part is only 5.4 mm where the width of the block measured 7.1755 ± 0.0127.
#This dimension was flexible.

Fiber Spacing

A Nikon microscope, model # MMH23010, was used to measure the fiber spacing. The zoom control of the microscope was adjusted so that 10 divisions on the microscope's scale corresponded to 1 mm. To determine fiber spacing, the distance between the centers of fibers #1 and 20 was measured. Then that distance was divided by the fiber spacing which was 19. Average fiber spacings for the two blocks are shown below.

                 Fiber spacing (mm)
C Technologies   0.3589 ± 0.0005
FiberGuide       0.3600 ± 0.0005
Design specs.    0.360 ± 0.030

Under the microscope, the V-groove block assembly produced by FiberGuide appeared to be less well controlled. The two plates which make up a block, the V-groove and the cover plates, were not aligned. Part of an end fiber was exposed due to the misalignment. Also, there was excess adhesive attached to the sides of the block. The C Technologies V-groove block appeared to be well fabricated.

Furthermore, under the microscope, the grooves on the FiberGuide's block were wide and U-shaped rather than V-shaped. It was seen that some of the fibers contacted only on one side of the grooves. The grooves on the C Technologies' block were V-shaped.

Fiber Alignment

An effort was made to determine the alignment of the fibers in the V-groove blocks. The set up went as follows. The V-groove block was set at 254 mm from a piece of paper which was taped to the wall. A small plug plate machined by the UW Physics Machine Shop (originally for hole concentricity and tolerance tests) was mounted on the front of a laser, manufactured by Aerotech, model # LSR05P. Each fiber was plugged in the hole which was lined up with the laser output. The output from the V-groove end appeared on the paper. The light spot on the paper was traced with a pencil. On the C Technologies harness, fibers # 2 and 20 illuminated the paper and were traced. On the FiberGuide harness, fibers # 1 and 20 illuminated the paper and were also traced. It should be noted that the fibers on the V-groove blocks are arranged randomly, i. e., fiber # 2 may or may not appear in the second groove of the block. The positions of the fibers on the V-groove blocks are as follows.

          C Technologies                   FiberGuide
Fiber #   Position on block*      Fiber #  Position on block*
   2              4                  1             7
  20             17                 20            19
*Arranged from right to left (from 1 to 20) as one faces the output end of the block.

The determination of the fiber alignment in the V-groove blocks was done by comparing the distance between two fiber spots which appeared on the paper to that of the design specification. From the design specifications, calculated at 254 mm away, the distances between the centers of the light cones should be 5.545 mm between fibers # 1 and 20, and 6.158 mm between fibers # 2 and 20. Using the Mitutoyo Caliper to measure the distances of the light spots appeared on the paper for each harness, the distance between fibers # 2 and 20 of C Technologies harness was about 5.23 mm while the distance between fibers # 1 and 20 of FiberGuide harness was about 9.00 mm. The uncertainty in tracing the light spots was estimated to be ± 1.0 mm while uncertainty in measuring the locations of centers of two light spots on the paper was about ± 0.10 mm.

Other observations made on the test were:

1. The output light spots from the FiberGuide fibers were about two times larger than the ones made by C Technologies'. The average diameter of the light spots produced by C Technologies fiber harness was about 37 mm.

2. No light came out of fiber #11 of FiberGuide harness. Light was seen through the fiber's plastic tubing at the anchor block. It seemed to indicate that the fiber was broken at the anchor block.

Conclusion

Two prototype fiber optic harnesses, manufactured by C Technologies and FiberGuide Industries, were obtained. The ferrules and V-groove blocks of both harnesses were measured for fabrication tolerance assessment. Most of the diameters of the ferrules on both harnesses meet the design specifications. On the FiberGuide harness, as many as 10 ferrules possessed varying diameters along the section which would be inserted into the plug-plate. Most of the ferrules on C Technologies harness had undersized diameters, but systematic, i.e., the distributions of measured parameters were Gaussian, which was in contrast with those from FiberGuide.

The fiber spacing on the V-groove blocks manufactured by both companies met the specifications. The overall dimensions of the V-groove block fabricated by C Technologies were within design specifications, while one dimension on the FiberGuide V-groove block did not meet the design specification. Finally, the angular alignment of the fibers in the V-groove blocks of both harnesses did not seem to be well-constrained by the groove in the V-groove blocks.