Distributed resonant ring fiber filter

DETAILED DESCRIPTION An object of the invention is to improve the efficiency of a fiber amplifier and/or tailor the spectral output of a fiber amplifier.
Another object is to provide an improved fiber optic filter.
Yet another object is to provide a distributed fiber optic filer, the peak filter wavelength and peak attenuation of which can be readily adjusted.
Another object is to provide a temperature stable fiber optic filter.
The present invention relates to a distributed filter formed of an optical fiber having a central core, a ring core having an inner radius r.
sub.
R concentric with the central core, an inner cladding region of refractive index n.
sub.
i between the central and ring cores, and a cladding layer of refractive index n.
sub.
c surrounding the ring core.
The maximum refractive indices n.
sub.
1 and n.
sub.
2 of the central and ring cores are greater than n.
sub.
c and n.
sub.
i.
At least a portion of the optical fiber is subjected to a continuous curvature as by winding it into a coil.
The propagation constants of one core mode and one ring mode are different at wavelengths except for wavelength .
lambda.
.
sub.
0, whereby a narrow band of wavelengths including .
lambda.
.
sub.
0 is coupled between the central core and the ring core and is at least partially radiated, whereby the narrow band of wavelengths is attenuated.
This technology is especially useful for implementation of distributed loss filters in gain fibers utilized in certain fiber amplifier and laser designs where an appropriate ring core is used in addition to the conventional active ion-doped central core to obtain spectral gain shaping.
The ring structure can be designed to provide the appropriate loss for a certain fiber coil size at those wavelengths where the fiber amplifier exhibits amplified spontaneous emission.
The peak attenuation wavelength .
lambda.
.
sub.
0 of a fiber of given outside diameter can be measured, and it may be determined that a fiber having a different value of r.
sub.
R (and thus outside diameter) will result in the correct value of wavelength