Test Setup
For the experiment the FO 8400 was fit with a thinner base plate, resulting in a test fiber position slightly below the height required for proper testing, as shown in Figure 1. By the insertion of plastic shim stock between the fiber cartridge and base plate, the test fiber could be raised in small steps until it was in the plane of the rotating sensor fiber and the effect on MFD could be measured.

Figure 1: Experiment Setup

The hole that the fiber cartridge is inserted into is slightly larger than the cartridge. While the cartridge is meant to be pushed to the front, right side corner in order to align it before testing, the hole allows 0.01 to 0.02 in. of play. The cartridge was tested in two different positions: with the cartridge pushed to the front and right (denoted as FR) and to the front and left (FL). For accurate measurement, the cartridge must always be pushed to the full forward position.

Test Procedure
The test fiber was cleaved and mounted in the fiber cartridge. The fiber is properly aligned in the cartridge when it lies in the groove and its tip lines up with the front of the cartridge as shown in Figure 2. The cartridge was then inserted into the FO 8400, and MFD was measured in the FR and FL positions. After completing data collection for the cartridge resting on the thin base plate, plastic shim stock was inserted under the cartridge to raise it up in 0.010 in. steps.

Figure 2: Proper Fiber Positioning

Results
The results of testing are presented in Table 1. Left or right justification of the fiber cartridge caused little change in measured MFD - on the order of 0.005 µm. As the fiber was vertically translated, the MFD (and the peak intensity) achieved a peak level between 0.010 in. and 0.020 in., and then decreased, as shown in Figure 4. Within 0.020 in.of the peak, MFD changed by approximately 0.02 µm.

Table 1: The Effect of Test Fiber Alignment on Mode Field Diameter