Highly wavelength-dependent evanescent tunneling loss in dispersion-engineered tapered double-cladding fibers

The techniques of modifying the propagation constant beta(omega) of optical fibers by using different materials or structures can be called dispersion engineering. Since the variations of beta(omega) can also substantially affect the propagation losses through the mode cutoff effects, the dispersion engineering on beta(omega) has been employed to achieve fundamental-mode cutoff (LP_01-C) filters. The LP_01-C fiber filters with a high cutoff slope are essential for building practical silica-based S-band erbium-doped fiber amplifiers and lasers so as to expand the communication bandwidth to 1450 nm [1-3]. A sharp LP_01-C filter providing high wavelength-dependent losses [3] is also useful for wavelength tuning in ultra-high gain efficiency fiber lasers. The cutoff slope can be increased by modifying waveguide dispersion or by modifying material dispersion. However, the achieved cutoff slopes by simply modifying either one of the dispersions are not good enough to clearly separate the guiding and antiguiding wavelengths.