Transmission of intense femtosecond 825 nm pulses progressively produces a waveguide at the entrance of a heavily Ge-doped silicate fiber. The waveguide behaves as a multimillimeter long-fiber bandpass filter that scatters away light with wavelengths shorter or longer than 850 nm. This phenomenon has been correlated with the ~800 nm photosensitivity producing type I-IR fiber Bragg gratings in side-written lightly Ge-doped silicate fibers and low-loss waveguides in pure silica bulk glass. A model incorporating color center formation is proposed to understand the underlying mechanism.
Continuum generation from normally dispersive ultrahigh-numerical-aperture fibers deteriorates in relatively short times, limiting its application as a practical optical source for high-resolution optical coherence tomography. We find that reversible light-induced structural modification of fiber optic materials, rather than permanent optical damage, is responsible for this deterioration. By examining how the optical properties of corresponding light-induced waveguides depend on pumping wavelength, we isolate a waveguide that is beneficial for stable continuum generation. The performance deterioration due to the formation of other waveguides can be reversed by overwriting them with this particular waveguide.
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