Sealed Fiber-Optic EFPI Fabricated by Femtosecond Laser
A sealed fiber-optic EFPI was also fabricated by using fs-laser micromachining [15,16]. In the device fabrication, fs-laser pulses (A = 800 nm) of 120 fs at the repetition rate of 1 kHz were focused onto the fiber end facet by a 20x objective lens with an NA value of 0.5 and a working distance of 2.1 mm. The pulse energy used in the experiment was ~2 J A CCD camera was employed to monitor the fabrication process and record the sample morphology. A section of standard SMF-28 with the core diameter of 8.2 |!m and the nominal effective RI of 1.4682 (at 1550 nm) was mounted on a computer-controlled 3D translation stage with a 40-nm resolution. The following steps were adopted in the fabrication process, as illustrated in Figure 3.9. First, fs laser drilled a micro-hole of ~1 p,m in diameter at the center of the cleaved fiber end facet, and the fiber tip obtained was then spliced together with another cleaved SMF tip without micro-hole by a fusion splicer (ERICSSON FSU975), with a fusing current and a fusing duration of 16.3 mA and 2.0 s, respectively. The hollow sphere formed had a diameter of ~60 p,m. Such a sealed EFPI could be used as strain sensor like the open EFPI.
Based on the sealed EFPI, a refractive index sensor could be formed by drilling a microchannel vertically crossing the FP cavity, which allowed the RI liquid to readily flow in or out of the cavity,
Figure 3.9 Fiber in-line FPI cavity fabrication process. (a) The fs laser creates a micro-hole of ~1 |am in diameter at the center of cleaved fiber end facet. (b) The fiber tip with the micro-hole spliced together with another cleaved SMF tip. (c) FP cavity formed. (d) Microscope image of the fiber in-line FPI cavity. (From Liao, C. R. et al. 2012. Optics Express, 20(20), 22813-22818.) as given in Figure 3.10. In the microchannel fabrication process, the laser beam was firstly focused on the top fiber surface at the microcavity position, scanned at a speed of 2 ^m/s with a scanning distance of 40 |lm, in parallel to the fiber core axis. After one scanning cycle, the laser beam was shifted by 10 |!m, in perpendicular to the fiber axis until a 40 |lm X 40 p,m square area was drilled through, to create the top part of the microchannel. The fiber was then rotated by 180° to allow fabrication of the bottom part of the microcavity, following the
Figure 3.10 Microscope image of the fiber in-line FPI cavity with the microchannel (left). The reflection spectrum at RI = 1.315, 1.32, and 1.325, respectively. (From Liao, C. R. et al. 2012. Optics Express, 20(20), 22813-22818.)
Figure 3.11 (a) Schematic of the pressure sensor. (b) SEM image of the cut-out view of the
sensor head. (From Zhang, Y. et al. 2013. Optics Letters, 38(22), 4609-4612.) same procedure as mentioned above. The RI sensitivity obtained is ~994 nm/RIU.
A pressure sensor based on the sealed EFPI was also demonstrated. Figure 3.11 schematically illustrates the structure of the sensor. To fabricate the sensor, a micro-hole was first drilled into the cleaved end face of a SMF by fs-laser micromachining. The inner surface of the laser-drilled micro-hole had quasi-distributed structures of submicron sizes and a low optical reflectivity. The hole-drilled SMF was then spliced to another SMF to form a sealed air cavity in between two fibers. Fusion splicer settings (arc duration, arc power, etc.) were adjusted to avoid the collapsing of the hole. During fusion splice, the rough structure of the micro-hole surface was melted by the arc. As a result, the surface became smooth, and the reflectivity increased. Precision fiber cleaving was applied to cut the fiber so that a thin piece of fiber was left to perform as the diaphragm. Finally, the as-cleaved diaphragm was thinned and roughened by fs laser.
During fs-laser micromachining, the sensor was mounted on a computer-controlled high-precision 3D stage with a resolution of 0.1 |lm. Light pulses generated by a regeneratively amplified Ti:sapphire laser (Coherent RegA 9000; 200 fs pulse duration, 250 kHz repetition rate, and 800 nm central wavelength) were focused onto the fiber end face or diaphragm surface through a microscope objective (20x magnification, 0.4 NA). The laser power could be changed by adjusting the laser-beam optics, including a half-wave plate, a polarizer, and several neutral density filters. The actual laser energy used for fabrication was approximately 0.4 |lJ per pulse.
Figure 3.12 (a) Typical interference spectra of the fiber FPI sensor. (b) Pressure-induced interferogram shift of FPI sensor. (From Zhang, Y. et al. 2013. Optics Letters, 38(22), 4609-4612.)
The diameter and depth of the hole could be flexibly varied. The diaphragm thinning process was also performed layer-by-layer with a step size of 0.5 gm. The fabrication was completed when a preset depth scan was reached. Figure 3.11b shows the scanning electron microscopic (SEM) image of a sensor head. For easy visualization, half of the sensor head was cut out using fs laser to expose the sealed hole and the diaphragm. This particular sensor had a cavity with the diameter of 70 gm and the length of 100 gm.
Figure 3.12a plots the interference fringes of a typical sensor before and after fs-laser ablation. The sensor had a cavity length of about 8 gm and a hole diameter of 75 gm. The cavity length was calculated to be 7.97 gm. Figure 3.12b shows the measurement results obtained from the sensor, where the changes in cavity length are plotted as a function of the applied pressure. Within the pressure range of 6.895 X 105 Pa, the center of the diaphragm deflected linearly as the pressure changed. Based on the linear fitting curve, the sensitivity of the sensor was calculated to be 2.8 X 10-4 nm/Pa.