  We investigated the cause of optical transmittance degradation in tapered
fibers. Degradation commences immediately after fabrication and it eventually
reduces the transmittance to almost zero. It is a major problem that limits
applications of tapered fibers. We systematically investigated the effect of
the dust-particle density and the humidity on the degradation dynamics. The
results clearly show that the degradation is mostly due to dust particles and
that it is not related to the humidity. In a dust free environment it is
possible to preserve the transmittance with a degradation of less than the
noise (+/- ?0.02) over 1 week.
