EFFPI Based on Microfabrication Technologies
Nowadays, microfabrication technologies have been developed greatly, including chemical etching, excimer laser micromachining, femtosecond laser micromachining, and FIB milling, just to name a few. These technologies were widely used for the fabrication of various microelectromechanical systems (MEMS) and optical waveguides. The development of fiber-optic sensors also benefits a lot from these technologies, which make available EFFPIs with small size and high stability, as compared with ferrule aligned EFFPIs. The detailed description of the fabrication technologies can be found in Chapter 3. Here, we are going to describe the characteristics of the fabricated EFFPI structures.
Chemical etching methods have been used for shaping the microstructure of optical fibers for a long time. The fiber-optic probes with sharp tapers at nanometer scale were chemically etched for near-field optical microscopy [44,45], while the etching occurred at the interface between the etching solution and air due to the surface tension effect. Different from the fabrication of nanofiber tapers, the EFFPI structure can be fabricated by another chemical etching mechanism, using the etching rate difference between the fiber core and cladding [46,47], as well as the fusion splicing process.
Usually, HF acid, sometimes buffered HF acid (HF mixed with NH4F), was used for the etching. In order to obtain a higher sensing performance, EFFPI can also be fabricated by chemically etching Er-doped fibers with mixed hydrochloric (HCl) and HF acid and fusion splicing, as shown in Figure 2.9. The sensor performance is greatly improved by the chemical reaction between HCl acid and doped Er2O3. A maximum fringe contrast of ~24 dB is obtained , comparable to that of MEFPI sensors fabricated by excimer lasers. The MEFPI sensor has high mechanical strength as the etching rate difference between fiber core and cladding is enlarged. This kind of sensor is insensitive to temperature while highly sensitive to strain, with sensitivities of ~0.65 pm/°C and —3.15 pm/^?, respectively.
It is worth noting that the insensitivity to temperature is a universal advantage of EFFPI sensors, partially due to the low thermo-expansion coefficient of air and the self-compensation of the structure. The self-compensation means that, as the temperature increases, the cavity length remains almost the same, because the cladding part of the EFFPI tends to stretch the structure and increase the cavity length, while the fiber core part tends to be expanded into the cavity and make the cavity length shorter.
Figure 2.9 (a-c) Microscopic images of the etched optical fiber and (d) EFFPI sensor by chemical etching.
The main advantage of chemically etched EFFPIs is the simple and low-cost fabrication process. However, the final sensing performance is strongly dependent on the fusion splicing process, as the electric charge intensity and period need to be optimized through the fusion splicing program in order to melt the surface for two-beam interference. The fusion splicing process lacks repeatability and may also be influenced by the residual etching solution. Another disadvantage is that the chemically etched surface is cone-shaped, which is not preferred for high-quality EFFPIs.
The laser micromachining technologies were employed for fabricating EFFPI structures, by using either the 157-nm excimer laser or femtosecond laser micromachining systems [48-50]. High-quality surfaces can be fabricated and the interference fringes were quite good and the fringe contrast can be as high as 26 dB. The smoothness of the fabricated surface is very good by using a mask-based fabrication technique and using a laser spot with uniform intensity distribution. By combining the fabricated micro-hole with a diaphragm, high- performance pressure sensors can be formed. This kind of EFFPI sensors is especially useful for strain and pressure sensing at high- temperature environments . In addition, refractive index sensing for biochemical applications was also performed by fusion splicing the laser-etched fiber with hollow-core photonic crystal fiber (PCF) or introducing a micro-hole at the end as an inlet for injecting aqueous samples .
Femtosecond (fs) laser micromachining technology has been widely used in recent years due to its capability of cold ablation. Very low heat effect during fabrication is a big advantage of this technology. Now microstructures, and even nanostructures, have been fabricated by fs laser micromachining methods. In 2007, Rao et al.  fabricated EFFPI structures in both SMF and PCF by the fs laser micromachining. Wei et al.  further enhanced the performance of EFFPI by using a Coherent fs laser and also monitoring the interference spectra of the fabricated device in real time. The fringe visibility of the fabricated EFFPI microstructure was enhanced up to 14 dB and was used for high-temperature sensing of up to 1100°C. Good linearity was observed for the high-temperature sensing.
EFFPI structures are especially useful for physical parameter sensing in harsh environments. However, after ablation by the fs laser, the mechanical strength of EFFPI was not as good as that of the fiber before the ablation. The only exception is the self-enclosed allfiber in-line EFFPI structure ; only the fiber core part is ablated and the fiber cladding can withstand relatively large strain and also dynamic strain.
The most promising aspect of the micromachined EFFPI is for biochemical applications, as the micronotch is not only a fine micro interferometer, but also an open sampling channel for biochemical reagents. When the refractive index of the medium within the cavity was changed, the interference fringes of the reflective spectra would shift .
Besides the chemical etching and laser micromachining methods, FIB milling  was also used for the fabrication of EFFPI. By using the FIB milling technology, a compact open-hole EFFPI refractive index sensor was fabricated and integrated with a microfluidic channel. The thermal effect makes the reflective surface smooth; however, it may sometimes introduce deformations of the fabricated device and degrade the fringe visibility and sensing performance. Because of the strong thermal effect, it is very difficult to use the CO2 laser pulses to fabricated precise EFFPI microstructures in optical fibers.
Figure 2.10 EFFPI structure formed by splicing a capillary in between two SMFs.