An investigation of an optical fibre amplifier loop for intra-cavityand ring-down cavity loss measurements

Abstract

We present the design and initial investigation of a fibreoptical system which may be used both for intra-cavity and for ring-downmeasurements of absorption losses. The system consists of a fibre loopcontaining a length of erbium-doped fibre pumped at 980 nm, with gainadjustment below or above threshold for the two types of operation. Thefibre loop is constructed from standard fibre optical components andincludes a micro-optical gas cell. The intended application is formeasurement of levels of trace gases which possess near-IR absorption lines withinthe gain bandwidth of the erbium fibre amplifier. We discuss the key issuesinvolved in operation of the system and the level of sensitivityrequired. Our initial experimental investigations have demonstrated thatring-down times of several microseconds can be obtained, which can bealtered through adjustment of the attenuation or gain factor of the loop.Gain control is one of the most important issues and we explain how thismay be achieved.