FIB Milling (Tapered Fibers)
FIB has been used in a number of fiber-based applications, including fabrication of long-period gratings, micromachining of fiber tips, and milling of side access holes in structured optical fibers. As a consequence of its applications in semiconductor technology, the FIB technique is mature and much more flexible than fs-laser processing, and dual-beam instruments combining a SEM and FIB are now commercially available. This method has the advantage of allowing micromachining with more well-defined milling profiles and a better surface finishing than what can be obtained with fs-laser technology with the conventional 800 nm wavelength, without the requirements of additional etching steps.
Figure 3.20 (a) Reflective spectrum of the temperature and refractive index sensor; (b) Rl response; and (c) temperature response of the solid cavity (IFPI). (Ran, Z. L. et al. 2008. Optics Express, 16(3), 2252-2263; Ran, Z. et al. 2013. IEEE Sensors Journal, 13(5), 1988-1991.)
Figure 3.21 (a) Reflective spectrum of the FP sensor. (b) Measured phase shift of the sensor
signal driven by a 40-Hz, 1.6-g peak-to-peak acceleration signal. (From Ran, Z. et al. 2011. 21st International Conference on Optical Fiber Sensors (OFS-21), Ottawa, ON, Canada, Papers 7753109, 7753-113, and 7753-114.)
Figure 3.22 Schematic diagram of (a) sapphire fiber high-temperature sensor and (b) silica fiber sensor. (From Ran, Z. et al. 2011. 21st International Conference on Optical Fibre Sensors (OFS-21) (pp. 775317-775317). International Society for Optics and Photonics.)
Figure 3.23 (a) Photo of micro-groove on fiber end. (b) Photo of the sapphire fiber temperature sensor in splicer. (From Ran, Z. et al. 2011. 21st International Conference on Optical Fibre Sensors (OFS-21) (pp. 775317-775317). International Society for Optics and Photonics.)
Figure 3.24 Reflected spectra of (a) sapphire FP sensor and (b) silica FP sensor. (From Ran, Z. et al. 2011. 21st International Conference on Optical Fibre Sensors (OFS-21) (pp. 775317-775317). International Society for Optics and Photonics.)
Figure 3.25 Temperature response of sapphire and silica FP sensors. (From Ran, Z. et al. 2011. 21st International Conference on Optical Fibre Sensors (OFS-21) (pp. 775317-775317). International Society for Optics and Photonics.)
A Quanta 200 3D FIB with Ga+ ion source is used to mill the fiber. The use of the FIB as a milling tool allows quick and direct patterning via a sputtering process. During sputtering, a portion of the ejected atoms or molecules are redeposited on the exposed region, making it difficult to control the amount of material that is removed. In order to avoid this redeposition process, the fiber is perpendicularly aligned with the ion beam and the milling process starts from the fiber side so that a pathway will be formed for the material ejection. Furthermore, a high incident angle (close to 90°) of the ion beam can speed up the milling rate significantly, since the FIB sputtering yield roughly increases with 1/cos(0), where 0 is the angle between the surface normal and the FIB direction .
During the ion bombardment process, there are excess charges cumulated on the surface of the material. These charges will cause random deflection of the incident ion beam and thus damage the milling profile. To avoid the charging effect, the fiber is fixed on the top of conductive carbon tape that is attached to the grounded sample holder. Moreover, a charge neutralization process with the aid of an electron beam is applied to counteract the charges. A conductive metal wire is also coiled as an anchor to prevent the fiber from vibrating. The micromachining is performed as close as possible to the anchored point.
To make a compact FP fiber sensor, a conventional SMF is used. The diameter of the fiber is tapered down to 32 |!m to reduce the amount of surrounding silica that the FIB needs to penetrate before accessing the fiber core. This assists in reducing the milling time. To obtain a high milling rate, a high ion current of 20 nA is used at the beginning, which gives a relatively large beam spot size of 0.3— 0.5 p,m. As shown in Figure 3.26, the FIB milling starts from the side of the fiber to make an opening for the material ejection and reduce the material redeposition. After the speedy milling, a lower ion current (3 nA) is used to polish the two ends of the cavity with higher spatial resolution (down to 10 nm). A small additional tilt of the fiber (~3°) to the incident direction of the ion beam is applied during the polish milling, in order to get rid of the residual angle misalignment during milling on a tangent and obtain parallel end walls of the
Figure 3.26 SEM image of the milled microcavity in the 32 цт in diameter fiber taper (left). Microscope image of the light guided in a ~500-|am-long fiber taper with the FP microcavity and the shattered fiber end (right). (From Yuan, W. et al. 2011. Review of Scientific Instruments, 82(7), 076103.) microcavity. In this way, a 25-^m-long microcavity with a width of 10 ^m is fabricated in the core of the fiber. The whole milling process takes around 20 min. The finely milled end facets of the microcavity can be seen in the enlarged SEM image in Figure 3.26. It is clearly seen that the inner sidewalls of the microcavity are not perfectly parallel, since 3~7° angle misalignment occurs due to the convergence of the FIB. This will not have a noticeable negative influence on the performance of the FP fiber sensor, since the surface quality of the end facets of the microcavity will have a much stronger influence on the interference pattern.
The microcavity is milled near the tip of the fiber taper and the taper end is shattered in order to reduce the Fresnel reflection, as shown in Figure 3.26. An experiment is carried out to test this microcavity as an FP fiber sensor. As shown in Figure 3.27, an FP interference pattern with an extinction ratio of about 15 dB and a pitch of about 37.4 nm is generated. Estimated with the wavelength of Xv1 and Xv2, that is, Xv1 = 1583.45 nm and Xv2 = 1546.05 nm, and using the factory-defined refractive index value of the liquid at 25°C, that is, n = 1.3, a 25.274-^m FP cavity and a pitch of 37.28 nm are obtained from the relation 4 nL = mkv, where m is an odd integer, which is 83 for Xv1 and 85 for Xv2. The whole spectrum shifts linearly toward shorter wavelength with increased temperature. An increased extinction ratio is also observed, which is due to the decrease in the refractive index of the liquid and the corresponding increase of the Fresnel reflection at the silica/liquid interface. By tracking Xv1 as shown in Figure 3.3, a total shift of 22.85 nm was introduced by a temperature increase of 38.8°C, which corresponds to an estimated decrease in the refractive index by 1.32 X 10-2. This leads to a sensitivity of —1731 nm/RIU. Taking into account the OSA resolution of 0.01 nm, the FP fiber sensor has a detection limit of —5.78 X 10-6 RIU.
A fiber-integrated slot-type microresonator with strongly improved mirror reflectivity for precise refractive index sensing is shown in Figure 3.28. It is straightforward to implement and allows very simple and quick access to the actual sensing area. This resonator is formed by a vertical, micrometer-sized slot deeply milled into the core of a step-index fiber (SMF-28 type) using FIB. The resonator has its sides coated with a high-refractive index (HI) layer, which strongly
Figure 3.27 FP reflection spectrum for different temperatures of the refractive index liquid (left). Shift of Хл versus temperature; labeled numbers indicate the estimated refractive indices (right). (From Yuan, W. et al. 2011. Review of Scientific Instalments, 82(7), 076103.)
Figure 3.28 Reflectivity-enhanced in-fiber microresonator for precise refractive index sensing. (a) Schematic of the in-fiber microresonator with the deeply cut slot and the tapered section. (b) SEM of the fabricated fiber resonator (base fiber: SMF-28). (c) 1D transfer-matrix model of the resonator (green: slot representing the actual sensing area; pink: high-refractive index layers; light blue: silica). The parameters dC and dL indicate the extensions of cavity and layers; nF, nL, and nC the refractive indices of fiber, layer, and slot. (d) Extended FP model with cavity extension Ad and additional phase shift Аф (color code is identical to that in (c)). (From Wieduwilt, T. et al. 2014. Optics Express, 22(21), 25333-25346.)
Figure 3.29 SEM of the focused-ion-beam milled and refined fiber-based microcavity (side view). The dashed yellow line represents the section of the guiding core. (From Wieduwilt, T. et al. 2014. Optics Express, 22(21), 25333-25346.)
Figure 3.30 Measured spectral distribution of the normalized reflectivity (left). Distribution of the measurement reflection resonances as a function of mode order of the fiber-integrated FPR (right). (From Wieduwilt, T. et al. 2014. Optics Express, 22(21), 25333-25346.)
increases the amplitude of the reflected light and leads to a significantly enhanced fringe reflectivity difference (A RF = Rmax — Rmin). The sensor allows measuring refractive indices within the range of silica glass, which is impossible using an uncoated resonator due to diminishing ARF.
The FP resonator was implemented by FIB milling (dual-beam system Lyra XMU [TESCAN]), a silica step-index fiber (SMF-28, Figure 3.29). Before milling, the fiber was polished at an angle of ~3° with respect to the fiber axis. The fiber was presputtered with platinum and electrically ground to avoid electrostatic charge accumulation on the fiber surface during the FIB processing. A two-step process was used: (i) high-current milling (10 nA) for cavity excavation and (ii) low-current processing (1 nA) to improve the parallelism and flatness of the cavity walls. The final cavity had a width and height of ~24.85 and ~18 p,m, respectively.
The reflection spectra of the in-fiber FPR are measured, as shown in Figure 3.30. The spectral positions of the experimentally determined resonance wavelengths decrease almost purely linearly with increasing mode order (Figure 3.30), again resembling the typical behavior of an FPR.