Fiber optic switching system with link monitoring

Claim 1

A fiber optic switching apparatus with link monitoring capability comprising: fiber optic signal generating means operable to output light of a first wavelength and a second wavelength; a fiber optic switch having an input and an output, said input being connected to receive the output of the signal generating means, the switch providing an optical path between the switch input and output and having an optical filter transparent to light of the first wavelength and opaque to light of the second wavelength, said filter being moveable into and out of the optical path, wherein the switch has a first state in which the filter is withdrawn from the optical path and a second state in which the filter is inserted into the optical path; and a receiver coupled to said switch output which includes: photo detection means coupled to said switch output and operable to produce electrical signals in response to light at the first and second wavelengths; switch state detection means responsive to the electrical signals to indicate the switch state based on the presence of light of the second wavelength; and link fault detection means responsive to said electrical signals to indicate a link fault in the absence of light of the first wavelength; wherein, in a no-fault condition of the link, light of the first wavelength is incident upon the photodetection means regardless of the state of the switch.