This paper provides a detailed description of an optical spectrum analyser constructed from a single birefringent optical fibre. The operation of the analyser system for the measurement of optical chirp is reported and limits on this measurement technique are established. Also for the first time, measurement of optical linewidths down to frequency resolutions of a few MHz is reported. The analyser system is simple in construction, very inexpensive, and can allow time resolved measurements to be made.
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