Graded-index Multimode Optical Fiber Research Articles

Mode division multiplexing in optical fibers

Two modes have been launched into a conventional 10-m long multimode graded-index optical fiber using spatial filtering techniques. The measured cross talk between the two modes in the baseband is -20 dB at the output end of the fiber. Modal multiplexing thus appears to be possible over short fiber lengths.

Multimode graded-index optical fibers: comparison of two Wentzel–Kramers–Brillouin formulations

Light propagation in graded-index optical fibers is described by two WKB methods, a classical formulation and the Langer–McKelvey approach. Explicit errors in these methods are evaluated for a fiber profile of near-parabolic shape and for one far from parabolic shape by comparing the methods with the results of evanescent wave theory that involve asymptotic error estimates. The most accurate formulation in general is the classical method, but in special cases the Langer–McKelvey approach is slightly more accurate. One example shows an inaccuracy of −672.5 to −672.1 psec/km in modal group delay for the latter method when the maximum intermodal group delay difference is 2.84 nsec/km.

Efficient conversion of light over a wide spectral range by four-photon mixing in a multimode graded-index fiber

The efficient conversion of 1.06-mum radiation in a graded-index multimode optical fiber oscillator into a small number of spectral components propagating in specific modes of the fiber and falling in the spectral range spanning from the blue to 1.53 mum is reported. The effect is shown to result primarily from phase-matched four-photon mixing. The converted light is found to propagate as individual fiber modes which are identified. The wavelengths of the spectral components of the output of the oscillator are governed primarily by fiber geometry. Analysis of the results also indicates the presence and gives a measure of the fiber-core birefringence sensed by low-order modes of propagation of the graded-index multimode fibers we have studied. The efficiency of the oscillator source and the specific characteristic wavelengths available with common graded-index fibers suggest several possible applications, which are discussed.

Intensity self-pulsations in (GaAl)As injection lasers operating in an external cavity

We present an experimental and theoretical study of a self-pulsing (GaAl)As injection laser operating in an external cavity. We have observed suppression of the self-pulsations when the external cavity is in the range 6<L<10 cm. Suppression of self-pulsations can also be obtained by using a multimode graded index optical fiber as the external resonator. These results can be explained by a model which includes the effects of an external cavity and electron trapping. For long cavity lengths, the self-pulsation frequency locks to an external cavity harmonic with no significant quenching, as observed in some earlier experiments.

Side launch excitation of selected modes in graded-index optical fibers

Excitation of single guided modes in a multimode graded-index optical fiber has been demonstrated. A side launch technique is employed which permits external selection of input conditions to match the axial and tangential wavenumber components of individual modes. Good agreement is obtained between observed mode order numbers and order numbers calculated from input parameters.

Pulse separating in transmission characteristics of multimode graded index optical fibers

The transmission characteristics of multimode graded index optical fibers, whose refractive index profiles were nearly parabolas, were measured. No change of the output pulse waveform could be detected at a 1-km length with a lowest-order mode excitation. In the case of mismatching excitation, output pulse waveforms separated into two pulse peaks. This was caused by the refractive index profile that was not a single function but a combination of two functions.