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EFFPI Based on Capillary or Hollow Fiber

Glass capillaries with similar dimensions to that of optical fibers, named hollow-core fibers, can be used for the formation of EFFPIs, by fusion splicing a short section of capillary between two SMFs [56], as shown in Figure 2.10. The fabrication process is simple and includes only cleaving and fusion splicing. No expensive micromachining systems are required and it is cost effective. The fringe visibility can achieve as high as 20 dB when the cavity length is around 60 |lm. The cavity length of longer than 1 mm was also demonstrated, with a relatively lower visibility of about 8 dB, which indicates the great capability of spatial frequency multiplexing. There might be a problem on the automatic alignment during the fusion splicing. As the alignment control of a splicer was usually based on the feedback of the imaging of the fiber core, the hollow- core fiber or capillary with air in the core area may require manual alignment during the fusion splicing.

Besides by the use of micromachining technologies, open-hole EFFPIs can also be formed by aligning two SMFs through an open- hole capillary with diameter at millimeter scale. The open hole can be easily ground by conventional mechanical tools. The capillary and SMFs can be bonded by CO2 laser welding to enhance the stability of the device [57]. This is a low-cost way to fabricate a sensitive EFFPI sensor, which is capable of measuring the biochemical parameters.

 
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