II. SENSOR DESIGN AND FABRICATION
A cleaved lead-in optical fiber [single-mode fiber (SMF-28)]
was spliced with a 3.3-mm-long hollow fiber with inside diameter
(I.D.) of 75 m and outside diameter (O.D.) about 125 m
by using a optical fiber fusion splicer (Sumitomo, Type-36).
Another cleaved optical fiber with O.D. of 60–65 m was inserted
into the hollow fiber from the other end as a reflecting
fiber to form an FP cavity between these cleaved two fiber end
faces. The thinner fiber can be obtained by etching or drawing
a standard optical fiber (125- m O.D.). The lead-in fiber was
connected to a white light interferometer system [8]. The air
gap was preadjusted by the splicer stage holding the reflecting
fiber tail, and monitored on-line by the white light system. The
air gap can be set from zero to millimeters depending on the
coherence length of the light source and application requirements.
Because the stage of the fiber fusion splicer has precisely
controlled alignment and movement, the sensor fabrication is
simple, convenient, and fast. Once the air gap was adjusted to
near the desired value, the electric arc was applied to bond the
reflection fiber with the hollow fiber end. In addition, the sensor
air gap can be adjusted with a resolution as high as nanometers
by properly controlling the power, duration, and number of
electric arcs as shown in Fig. 2. To our knowledge, this is the
first time that the length of an optical fiber FP cavity was precisely
controlled by electric arcs, which is a useful technique for
sensor fabrication and investigation of FP interferometers.