Structural optimization for two-mode fiber: Theory and experiment

Structural design of two-mode optical fiber is investigated. It is clarified theoretically that the operating
V
-value V
0
with zero group delay time difference
\Delta\tau
between the LP
01
and LP
11
modes is extended to the quasi-two-mode region above the cutoff
V
-value of the next higher order LP
21
mode. The optimum operating
V
-value
V_{0m}
is numerically determined to be 6.45 with the optimum index profile parameter
\alpha_{opt} = 2.24
for
\Delta = 0.3
percent. It is also shown theoretically that an index depression at the core center increases
\Delta\tau
due to the
V
-value deviation from V
0
, and that V
0
becomes smaller than that with no central index depression for the same α. A low-loss two-mode test fiber designed for operating in the 1.3 μm wavelength region is prepared: the core diameter is 21μm with
\alpha = 2.65
and
\Delta = 0.31
percent, and the test fiber has a precisely controlled graded-index profile with a small central index depression. Experimental results show that the two-mode propagation is maintained in the quasi-two-mode spectral region. Furthermore, small
\Delta\tau
characteristics against
V
-value deviation
\DeltaV
from V
0
are verified experimentally. It is also shown that the two-mode test fiber has about two times larger tolerance for the misalignment at a splice than a step-index single-mode fiber.