EXPERIMENT NO. 8
To excite and identify the low-order LP modes in weakly guided few-mode
C. L. Chen,
Irwin, (1996), Chapter 8.
2. Yeh, C., "Guided waves modes in cylindrical optical fibers,"
IEEE Trans. on
, Vol. E-30, pp. 43-51, (1987).
Gloge, D., "Weakly guided fibers,"
., Vol. 10, pp. 2252-2258, (1971).
, Academic Press, Inc., New York, (1982).
Assume the fiber used in the experiment has V= 4.00.
Use Ref.  to determine
the expected LP modes supported by the fiber, and the radiant intensity distribution of
the LP modes. See, for example, p. 452 of Ref. . It is usually very difficult to excite one
and only one mode. The pattern you observe may in fact be a combination of several LP
modes. The only exception is the LP
Arrange your setup as shown in Figure 1.
Use a linearly polarized HeNe laser
and a 20X (or 40X) objective to couple light into the few-mode fiber.
Verify that the
beam entering the fiber coupler is linearly polarized.
Measure the power of the
polarized beam before coupling it into the fiber.
Figure 1 Setup to observe mode patterns
Try to align the fiber axis exactly along the light beam to excite only the lowest
order mode (or low-order modes).
The output radiant intensity pattern should appear