Nondestructive measurement of core radius, numerical aperture, and cutoff wavelength for single-mode fibers.

A method is proposed for determining the core diameter, numerical aperture, and cutoff wavelength of single-mode fibers using the end separation loss measured at two light wavelengths. A spacer with adequate end separation is used to reduce the fluctuation in connection losses caused by multiple Fresnel reflections between the fiber end faces and by small lateral offsets. The cutoff wavelengths of four 2-m long fibers measured by this method agreed with those in which abrupt changes in the near-field patterns were observed.

Measuring fiber connection loss using steady-state power distribution: a method.

This paper describes conditions for the reproducible measurement of optical fiber connection losses. The steady-state power distribution is characterized by the width of the far-field pattern (FFP). A method is proposed to determine the power distribution in the fiber from the measured FFP. Using this method, connection losses of graded-index fibers are calculated for various widths of the FFP. Calculated results are verified experimentally. The width of the FFP must be controlled to within an accuracy of 3% for a 0.05-dB reproducibility of the connection loss. Connection loss dependence on mismatch of fiber parameters is also calculated. Calculation shows that variations in core radius, numerical aperture, and index gradient have to be controlled to within 3%, 6%, and 0.3, respectively, for the same reproducibility.

Determination of modal power distribution in graded-index optical waveguides from near-field patterns and its application to differential mode attenuation measurement.

A technique is introduced that determines power distribution in fibers from the measured near-field pattern, assuming that: (1) the optical power distributes uniformly among degenerated modes with the same propagation constant, (2) enough modes are excited to ensure the validity of calculation by geometrical optics, and (3) the phase of each propagation mode has no correlation. Experiments verifed that the fibers have the function of flattening power distribution among modes with the same propagation constant. This fact shows that assumption (1) does not severely limit the applicability of the technique. Wave optical calculation is done to determine the numbers of modes that must be excited to satisfy assumption (2). As an example of application of the technique, differential mode attenuation of graded-index fibers is determined from longitudinal variation of the measured near-field pattern.