Biomedical Engineering Reference
In-Depth Information
FIGURE 3.3
Scanning electron micrograph of a tapered fiber-optic probe fabricated via a heated pulling process.
greater variability in fiber tips from batch to batch, it does not require any special equip-
ment and can provide tips with much greater taper angles than are generally produced via
heated pulling processes. Two different types of chemical etching have been reported in
the literature: (1) Turner etching
35,36
and (2) tube etching.
37
In the first method, Turner etch-
ing, a silica fiber is placed in the meniscus between hydrofluoric acid (HF) and an organic
overlayer, causing the HF to etch the silica while the organic overlayer protects the region
of fiber suspended in it. By changing the position of the fiber in this meniscus during the
etching process, as well as various other parameters, it is possible to accurately control the
taper angle and degree of taper of these fiber-optic tips. However, due to the dual chemi-
cal nature of the etchant solution, environmental parameters such as vibrations can cause
significant batch-to-batch variations in the characteristics of the fiber tips.
To overcome the issues associated with this dual chemical etchant, a process known
as tube etching was developed. In this process, a silica core fiber having a polymer
cladding is placed into a solution of HF and the silica core is allowed to dissolve while
the polymer cladding remains unaffected. As the silica core is etched it rises above the
surface of the HF solution. At this point, capillary action draws HF up the walls of the
polymer cladding where it reaches the silica core of the fiber again and then drains back
into the solution below, forming a smooth tip with a large taper angle. The entire etch-
ing process takes approximately 2 hours. To remove the surrounding polymer cladding
and expose the silica tip for use, the fibers can either be placed in a suitable organic
solvent to dissolve the cladding or burned off using a high-temperature flame. While
this process can produce more reproducible tapered fiber-optic probes than Turner
etching, difficulties associated with submerging multiple fibers to the same depth in the
HF solution tend to make it less reproducible than fibers produced via the heated
pulling process.
