1. Field of the Invention
The present invention relates to waveguides that propagate light at multiple discreet speeds—equivalently, multiple discreet transverse modes—and that transport telecommunications signals, generate or amplify light, transport electromagnetic power, or are used for decorative or display purposes.
2. Description of Related Art
Optical fiber waveguides that transport telecommunications signals are typically designed and manufactured to allow light to propagate at just one speed, to ensure that a signal arrives at its destination in a single, brief instant. Waveguides that generate or amplify light, such as those doped with rare-earth ions, are also typically designed and manufactured to allow light to propagate at just one speed, in this case to ensure that the pattern of radiation emitted by the waveguides may be focused to the tightest possible spot. Such a radiation source is said to be “diffraction limited.”
Waveguides that transport telecommunications signals or that generate or amplify light may also be designed and manufactured to allow light to propagate at multiple discreet speeds (in multiple discreet transverse radiation patterns, or “modes”). Such waveguides are sometimes more economical to manufacture or to interconnect, and the benefits of the single-speed fibers may be retained by preferentially attenuating light that has propagated at undesired speeds or by selectively exciting light that propagates at one preselected speed.
An advantage of the selective-excitation approach is that light that propagates in a high-order mode—a mode that forms many well-defined rings or spots in a plane transverse to the propagation direction of the light—travels at an effective index that differs more significantly, when compared to the differences that naturally arise in conventional waveguides, from the effective indices of its neighboring modes. This inherent advantage simplifies the task of selectively exciting and de-exciting a desired mode, but unfortunately a large fraction of the power guided by the high order circularly-symmetric modes of conventional waveguides tends to be located near the central axis of the waveguide, and this hot-spot may reduce the threshold for undesired nonlinear propagation artifacts and waveguide damage.
Waveguides that allow light to propagate at only one speed most often distribute their guided power in the shape that is Gaussian, or nearly Gaussian, in the plane transverse to the propagation direction of light. Waveguides may also be designed so that their guided power is flat, or nearly flat, in the transverse plane. Since the peak power density of a flattened-mode waveguide is lower than that of a Gaussian-mode waveguide, the flattened-mode waveguide has a higher (and thus more desirable) threshold for nonlinear propagation artifacts and waveguide damage.