optimum fiber strength and splice reflectivity if desired the reflectivity of the splice can be monitored during the fabrication process using an optical time domain reflectometer otdr as illustrated in fig2 in such a setup short pulses at 0 83 u03bcm from a diode laser 12 are injected into an optical fiber 2 which has been provided with a series of dielectric mirrors 7 the optical pulses reflected from a beam splitter 15 are then monitored with a silicon avalanche photodiode 14 it should be understood that other laser wavelengths such as 1 3 u03bcm and other photodetector types such as germanium avalanche photodiodes may be used it should be further appreciated that a fiber optic directional coupler could be used as a beam splitter fig3 a 3c illustrate two distinct methods for forming variably reflective taps in a continuous length of optical fiber as noted some vanadium oxides undergo a thermally induced dielectric to metal transition thereby enabling the construction of variably reflective mirrors fig3 a illustrates a variable vo 2 or v 2 o 5 reflector 20 formed in a length of optical fiber 2 placed across this reflector 20 is a resistive film or wire 22 which is formed in contact with the fiber 2 when an electrical current is placed through this resistive film or wire 22 the fiber 2 is heated sufficiently to drive the reflector 20 through its phase transition and thus alter its overall reflectivity when the current is turned off or reduced the reflector 20 cools sufficiently to.