Vibration Sensor Fabricated by Combination of Chemical Etching and FIB
FIB technology is combined with chemical etching of specifically designed fibers to create FPIs. Hydrofluoric acid is used to
Figure 3.36 SEM photo of a typical sensor with a sensitivity of -550 nm/bar (x2000, left). Deflection of sensor diaphragm versus pressure at high pressures showing nonlinear behavior (sensor sensitivity 550 nm/bar, right). (From Cibula, E. et al. 2009. Optics Express, 17(7), 5098-5106.)
etch special fibers and create microwires with diameters of 15 |lm. These microwires are then milled with an FIB to create two different structures: an indented FP structure and a cantilever FP structure that are characterized in terms of temperature .
The fabrication of the FP structures can be divided into two steps: the fabrication of the microwire by chemical etching micromachining, and the milling of a gap in the microwire with FIB technology. This two-step process allows for the much faster fabrication of microstructures than solely using FIB on standard fiber. Accessing the light guiding region with FIB would take too long on a standard fiber, and the structures would be very poorly defined, due to the high aspect ratio necessary.
This micromachining technique is based on the much higher etching rate of phosphorus pentoxide-doped silica when compared to pure silica. This way, structure-forming fibers (SFFs) can be engineered with pure silica regions and P2O5-doped regions so that, after etching, only the pure silica regions remain, leaving just the desired microstructure [34,35]. This technique is used to create microwires, which are then further post-processed using FIB technology. After splicing SFF to SMF, the SFF is cleaved to the desired length (see Figure 3.37a-f). To prevent etching from the top of the fiber, an additional short section of a coreless all-silica multimode fiber (cMMF) is spliced to the top of the SFF. The SFF was cleaved using an ultrasonic YORK FK 11 cleaver set at a tensile strength of 2 N. The splicing was performed
Figure 3.37 Microwire fabrication process: (a) SMF—SFF fusion splicing; (b) cleaving to desired length; (c) SFF-cMMF fusion splicing; (d) cMMF cleaving (30-40 цт); (e) etching; (f) final structure; (g) SEM micrograph of etched microwire. (From Andre, R. M. et al. 2014. Optics Express, 22(11), 13102-13108.)
by a filament fusion splicer (Vytran FFS 2000) that led to splices with losses below 0.2 dB .
The whole structure is then placed inside a HF solution with 40% concentration. Initially, only pure silica is in contact with the solution and, consequently, the whole structure is etched uniformly, but when the outer silica shell is etched away and the acid comes into contact with the doped region, preferential etching of the P2O5-doped silica occurs. The P2O5 concentration of the SFF is about 8.5 mol%, which means that the etching rate of the P2O5-doped region is about 30 times higher than the etching rate of pure silica. Etching in 40% HF at room temperature (~25°C) with no stirring leads to etching rates of 1 p,m/min for pure silica and 31 |lm/min for the P2O5-doped region. The process was concluded by rinsing the structures in distilled water. The total etching times depend on the desired microwire diameter and the external temperature and can range from 15 to 20 min. The structure that remains after chemical etching consists of a microwire with a diameter of 15 p,m, aligned with the single-mode lead-in fiber core and two side support beams that, due to the complete misalignment with the SMF core, do not guide light (see Figure 3.37f). These side support beams give the microwire protection and help the whole structure retain its form. Even though the microwire, when in air, supports several modes after being etched, practically only one mode is launched by the SMF in the current configuration. This required special care in structure design as described in detail in Reference 35. The guiding losses for the microwires are below 0.4 dB for diameters of 15 p,m.
After the microwire is created, an FIB is used to mill the microwire and create two different FP structures. Before FIB milling, the microwires were sputter-coated with a thin tantalum film (ca. 50 nm). This is necessary to avoid charging during electron beam and ion beam operation of the fiber, as silica is nonconductive. The charging will affect the milling because it will cause the ion beam to drift from its intended spot position, effectively reducing the resolution and quality of the milled structures. In the milling of these structures, an ion current of approximately 1 nA is used for a primary coarse milling of the cavities. After this, a polishing is performed using a much smaller current of 100-300 pA. The currents were adjusted so that the primary milling times did not exceed 1 h and the secondary polishing times did not exceed 20 min to avoid charging and consequent drift effects. The surface quality is rough after the primary milling due to the high current employed and also due to the redeposition of some of the milled material but, after the polishing, the surface roughness greatly decreases. Flat, parallel walls can be obtained because the aspect ratio of the milled structure is not high . The first structure milled consists of an indentation in the microwire (see Figure 3.38a). The reflections at both silica-to-air interfaces (signaled in Figure 3.38a) result in a low-finesse FP cavity. The cavity has a length of approximately 167 p,m.
The second structure is similar save that a whole section of the microwire is removed instead of just a half-cylinder section (see Figure 3.38b). This results in a completely cleaved microwire that is suspended from the fiber top side. The microwire stays in place due to the side support beams that still remain after the milling process. This structure also behaves as an FP cavity, being that the reflecting interfaces are the fiber top and the silica-to-air interface at the air gap signaled in Figure 3.38b. In this case, the cavity has a much greater length of approx. 1025 p,m.
In the cantilever FP structure, the microwire is solely suspended by one of its ends as opposed to the indented FP structure where both ends of the microwire are fixed. This suspended microwire has freedom to move relative to the bulk input fiber, allowing for aligned and misaligned positions. Using this property, it is possible to apply this structure as a vibration sensor. The structure was attached to an
Figure 3.38 (a) Indented FP cavity SEM micrographs and related optical reflection spectrum, (b) Cantilever FP structure micrograph and related optical reflection spectrum. (From Andre, R. M. et al. 2014. Optics Express, 22(11), 13102-13108.)
Figure 3.39 FP cantilever structure: time responses (left) and related fast Fourier transforms (right) when an external frequency is applied. (From Andre, R. M. et al. 2014.
Optics Express: 22(11), 13102-13108.) acoustic vibrating system that produced a vibration frequency in the range from 1 Hz to 40 kHz, as shown in Figure 3.39.