Fiber amplifier coupler

A fiber amplifier comprising

• gain fiber means for amplifying optical signals, and
• a fiber optic coupler comprising a first single-mode optical fiber having a biconical taper section and a second single-mode optical fiber having a biconical taper section, each of said fibers having a core surrounded by a cladding of refractive index n.sub.2, said taper sections being fused together to form a coupling region, said coupling region being surrounded by a medium having a refractive index n.sub.3 that is lower than n.sub.2, an end of said first fiber being connected to said gain fiber means,
• said first fiber having first means for modifying the propagation constant of the fundamental mode in the coupling region to such an extent that the difference between the propagation constants of said first and second fibers in said coupling region would, in the absence of the following second propagation constant modifying means, differ by such an amount that less than a given percent of light power of a wavelength .lambda..sub.S would normally couple from said second fiber to said first fiber, and
• second means for modifying the propagation constant of one of said fibers by such an extent that more than said given percentage of the light power of wavelength .lambda..sub.S couples from said second fiber to said first fiber, the spectral propagation constant curves of those portions of said fibers that are in said coupling region being substantially parallel to each other in the wavelength region where maximum coupling occurs.

A fiber amplifier comprising

• gain fiber means for amplifying optical signals, and
• fiber optic coupler means having first and second coupler optical fibers, one end of said first coupler fiber being connected to said gain fiber means, the mode field diameter of said first coupler fiber being substantially matched to that of said gain fiber means and being smaller than that of said second coupler fiber, a portion of said coupler fibers being situated in a wavelength dependent light transfer relationship whereby most of the light power of a wavelength .lambda..sub.S couples between said first and second coupler fibers, and most of the light power of a wavelength .lambda..sub.P that is introduced into said first fiber remains in said first coupler fiber,
• the mode field diameter difference between said fibers causing the propagation constants of said first and second coupler fibers in said coupling region to differ by such an amount that less than a given percent of light power of a wavelength .lambda..sub.S would normally couple from said second fiber to said first fiber, and
• means for modifying the propagation constant of one of said fibers by such an extent that more than said given percentage of the light power of wavelength .lambda..sub.S couples from said second fiber to said first fiber, the spectral propagation constant curves of those portions of said fibers that are in said coupling region being substantially parallel to each other.