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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 22, Iss. 11 — Jun. 2, 2014
  • pp: 13102–13108
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Focused ion beam post-processing of optical fiber Fabry-Perot cavities for sensing applications

Ricardo M. André, Simon Pevec, Martin Becker, Jan Dellith, Manfred Rothhardt, Manuel B. Marques, Denis Donlagic, Hartmut Bartelt, and Orlando Frazão  »View Author Affiliations


Optics Express, Vol. 22, Issue 11, pp. 13102-13108 (2014)
http://dx.doi.org/10.1364/OE.22.013102


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Abstract

Focused ion beam technology is combined with chemical etching of specifically designed fibers to create Fabry-Perot interferometers. Hydrofluoric acid is used to etch special fibers and create microwires with diameters of 15 μm. These microwires are then milled with a focused ion beam to create two different structures: an indented Fabry-Perot structure and a cantilever Fabry-Perot structure that are characterized in terms of temperature. The cantilever structure is also sensitive to vibrations and is capable of measuring frequencies in the range 1 Hz – 40 kHz.

© 2014 Optical Society of America

1. Introduction

Focused Ion Beam (FIB) is a commercially available technology developed for, and mainly employed by, the semiconductor industry. FIB systems are very similar to Scanning Electron Microscopes (SEM), but instead of an electron beam they use an ion beam, generally of gallium ions (Ga+) [1

1. S. Reyntjens and R. Puers, “A review of focused ion beam applications in microsystem technology,” J. Micromech. Microeng. 11(4), 287–300 (2001). [CrossRef]

]. They can be operated with small currents for imaging just like the SEM systems, or with higher currents for milling and sputtering. Modern systems include both an ion beam column and an electron beam column (dual-beam systems), which allow for a higher flexibility in imaging/milling the substrate [2

2. A. A. Tseng, “Recent developments in micromilling using focused ion beam technology,” J. Micromech. Microeng. 14(4), R15–R34 (2004). [CrossRef]

].

In the last few years, FIB technology has been applied to optical fibers, leading to new and interesting ways of creating very small and short optical fiber devices. FIB has been combined with optical fiber technology to create structures ranging from long-period [3

3. C. Martelli, P. Olivero, J. Canning, N. Groothoff, S. Prawer, S. Huntington, and B. Gibson, “Micromachining long period gratings in optical fibres using focused ion beam,” in Bragg Gratings, Photosensitivity, and Poling in Glass Waveguides (Optical Society of America, 2007), p. BTuC6.

] and fiber Bragg gratings for refractive index [4

4. J.-L. Kou, S.-J. Qiu, F. Xu, Y.-Q. Lu, Y. Yuan, and G. Zhao, “Miniaturized metal-dielectric-hybrid fiber tip grating for refractive index sensing,” IEEE Photon. Technol. Lett. 23(22), 1712–1714 (2011). [CrossRef]

] and temperature [5

5. J. L. Kou, S. J. Qiu, F. Xu, and Y. Q. Lu, “Demonstration of a compact temperature sensor based on first-order Bragg grating in a tapered fiber probe,” Opt. Express 19(19), 18452–18457 (2011). [CrossRef] [PubMed]

] sensing to more complex structures such as fiber-top cantilevers for very small displacement sensing [6

6. D. Iannuzzi, K. Heeck, M. Slaman, S. de Man, J. H. Rector, H. Schreuders, J. W. Berenschot, V. J. Gadgil, R. G. P. Sanders, M. C. Elwenspoek, and S. Deladi, “Fibre-top cantilevers: design, fabrication and applications,” Meas. Sci. Technol. 18(10), 3247–3252 (2007). [CrossRef]

]. Nanofiber cavities have been fabricated by milling two FBGs on a nanofiber with FIB [7

7. K. P. Nayak, F. Le Kien, Y. Kawai, K. Hakuta, K. Nakajima, H. T. Miyazaki, and Y. Sugimoto, “Cavity formation on an optical nanofiber using focused ion beam milling technique,” Opt. Express 19(15), 14040–14050 (2011). [CrossRef] [PubMed]

]. FIB has also been used for the milling of channels in microstructured optical fibers, thus allowing for selective filling of these fibers [8

8. F. Wang, W. Yuan, O. Hansen, and O. Bang, “Selective filling of photonic crystal fibers using focused ion beam milled microchannels,” Opt. Express 19(18), 17585–17590 (2011). [CrossRef] [PubMed]

]. Fabry-Perot (FP) cavities have also been fabricated using FIB. Most are created on tapered fiber tips in order to reduce the required FIB milling [9

9. J. L. Kou, J. Feng, Q. J. Wang, F. Xu, and Y. Q. Lu, “Microfiber-probe-based ultrasmall interferometric sensor,” Opt. Lett. 35(13), 2308–2310 (2010). [CrossRef] [PubMed]

11

11. W. Yuan, F. Wang, A. Savenko, D. H. Petersen, and O. Bang, “Note: Optical fiber milled by focused ion beam and its application for Fabry-Pérot refractive index sensor,” Rev. Sci. Instrum. 82(7), 076103 (2011). [CrossRef] [PubMed]

] to a minimum. One downside of FIB processing is the time it takes to mill large quantities of material. This is why, in the vast majority of the works involving FIB processing of optical fiber, another, faster technique is used to somehow reduce the amount of material to be milled. The most common method is tapering, either by creating nanofibers or tapered fiber tips.

In our effort, another technique is used instead of tapering: hydrofluoric acid etching of specifically designed fibers [12

12. S. Pevec, E. Cibula, B. Lenardic, and D. Donlagic, “Micromachining of optical fibers using selective etching based on phosphorus pentoxide doping,” IEEE Photon. J. 3(4), 627–632 (2011). [CrossRef]

]. This allows for the creation of microwires and thus reduces the FIB milling time necessary to create a structure. This paper reports about two different sensing FP structures milled with FIB. The FP structures were milled on 15 μm-diameter microwires. One of the structures is a simple FP cavity created by milling an indentation in the microwire. This structure was characterized as a high temperature sensor. The other structure is a microwire cantilever FP structure and was characterized as a temperature and vibration sensor.

2. Fabrication

The fabrication of the Fabry-Perot 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 focused ion beam technology. This two-step process allows for the much faster fabrication of microstructures than using solely 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.

2.1 Microwire fabrication with chemical etching

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 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 [13

13. S. Pevec and D. Donlagic, “All-fiber, long-active-length Fabry-Perot strain sensor,” Opt. Express 19(16), 15641–15651 (2011). [CrossRef] [PubMed]

,14

14. S. Pevec and D. Donlagic, “Miniature micro-wire based optical fiber-field access device,” Opt. Express 20(25), 27874–27887 (2012). [CrossRef] [PubMed]

]. This technique is used to create microwires which are then further post-processed using FIB technology. After splicing the structure forming fibers (SFF) to a single-mode fiber (SMF), the SFF is cleaved to the desired length (see Figs. 1(a)
Fig. 1 Microwire fabrication process: (a) SMF-SFF fusion splicing; (b) cleaving to desired length; (c) SFF-cMMF fusion splicing; (d) cMMF cleaving (30-40 μm); (e) etching; (f) final structure; (g) SEM micrograph of etched microwire; (h) structure forming fiber cross-section.
1(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 structure forming fiber was cleaved using an ultrasonic YORK FK 11 cleaver set at a tensile strength of 2 N. The splicing was performed by a filament fusion splicer (Vytran FFS 2000) that led to splices with losses below 0.2 dB [14

14. S. Pevec and D. Donlagic, “Miniature micro-wire based optical fiber-field access device,” Opt. Express 20(25), 27874–27887 (2012). [CrossRef] [PubMed]

].

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 μm/min for pure silica and 31 μm/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 minutes. The structure that remains after chemical etching consists of a microwire with a diameter of 15 μm, 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 Fig. 1(f)). 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 etching, 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 [14

14. S. Pevec and D. Donlagic, “Miniature micro-wire based optical fiber-field access device,” Opt. Express 20(25), 27874–27887 (2012). [CrossRef] [PubMed]

]. The guiding losses for the microwires are below 0.4 dB for diameters of 15 μm.

2.2 Cavity fabrication with focused ion beam

The second structure is similar save that a whole section of the microwire is removed instead of just a half cylinder section (see Fig. 2 bottom). 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 a FP cavity, being that the reflecting interfaces are the fiber top and the silica-to-air interface at the air gap signaled in Fig. 2 bottom. In this case, the cavity has a much greater length of approx. 1025 μm.

3. Results

3.1 Optical spectra

The Fabry-Perot structures were analyzed in a simple reflection setup consisting of an optically amplified spontaneous emission (ASE) broadband source @ 1550 nm, an optical circulator, and an optical spectrum analyzer. The resulting optical spectra of both structures after focused ion beam milling show a channeled spectrum typical of a low finesse FP structure, but they have completely different fringe spacings, as the cavity lengths are different (see Fig. 2). Estimating the cavity length using the fringe spacing can provide a better understanding of where the reflections are taking place. The following expression can easily be derived from the resonance wavelength expression for a FP cavity, taking into account two consecutive resonant wavelengths λ1 and λ2:

L=λ1λ22neffΔλ
(1)

Using values obtained from the reflection spectra one obtains lengths of approximately 172 μm and 1026 μm for indented FP and cantilever FP structures, respectively. Comparing these lengths with the SEM micrographs one readily identifies that the fringes come from reflections at the fiber top and at the first interface of the milled section (both signaled in Fig. 2). The lengths measured from the SEM micrographs for these two structures are 167 μm and 1025 μm, which are in good accordance, given the uncertainty of measuring the SEM micrographs.

3.2 Temperature

For temperature characterization, the Fabry-Perot structures were placed inside a tubular oven, and the temperature was varied from 100 to 550 °C. An optical spectrum analyzer with a resolution of 1 pm was used to acquire the spectra. The wavelengths were tracked using MATLAB® to determine the spectral shift. For each temperature measurement, the system (oven + sensor) is stabilized at each temperature point for approximately 2 minutes to ensure a stable and uniform temperature in the oven. Both structures present a similar, slightly quadratic behavior, but two linear regimes can be defined (see Fig. 3
Fig. 3 Temperature response of both Fabry-Perot structures: indented FP cavity (full dots, blue line) and FP cantilever structure (hollow dots, red line).
). For low temperatures (100-300 °C), sensitivities of 11.5 pm/K and 12.3 pm/K were obtained for indented FP and cantilever FP, respectively. For higher temperatures (300-550 °C), 14.2 pm/K and 15.5 pm/K were obtained for indented FP cavity and cantilever, respectively. These results are in accordance with the only other paper that FP cavities in optical fiber milled with FIB [10

10. J. L. Kou, J. Feng, L. Ye, F. Xu, and Y. Q. Lu, “Miniaturized fiber taper reflective interferometer for high temperature measurement,” Opt. Express 18(13), 14245–14250 (2010). [CrossRef] [PubMed]

]. Kou et al. milled a FP cavity on a tapered fiber tip and obtained a temperature response given by a third order polynomial with an average sensitivity of 17 pm/K, similar to the sensitivities obtained in this work [10

10. J. L. Kou, J. Feng, L. Ye, F. Xu, and Y. Q. Lu, “Miniaturized fiber taper reflective interferometer for high temperature measurement,” Opt. Express 18(13), 14245–14250 (2010). [CrossRef] [PubMed]

]. When considering structures milled with femtosecond laser micromachining, sensitivities in a wide range have been obtained from nearly insensitive structures (0.074 pm/K [15

15. T. Wei, Y. Han, H.-L. Tsai, and H. Xiao, “Miniaturized fiber inline Fabry-Perot interferometer fabricated with a femtosecond laser,” Opt. Lett. 33(6), 536–538 (2008). [CrossRef] [PubMed]

]) to highly sensitive ones (51.5 pm/K [16

16. L. Zhao, L. Jiang, S. Wang, H. Xiao, Y. Lu, and H.-L. Tsai, “A high-quality Mach-Zehnder interferometer fiber sensor by femtosecond laser one-step processing,” Sensors 11(1), 54–61 (2011). [CrossRef] [PubMed]

]). Structures with a similar sensitivity (14.7 pm/K [17

17. L. Yuan, T. Wei, Q. Han, H. Wang, J. Huang, L. Jiang, and H. Xiao, “Fiber inline Michelson interferometer fabricated by a femtosecond laser,” Opt. Lett. 37(21), 4489–4491 (2012). [CrossRef] [PubMed]

]) have been reported.

3.3 Vibration

4. Conclusions

In this work, the combination of chemical etching and micromachining of specially designed and doped fibers with focused ion beam milling was proposed and demonstrated. To the authors’ knowledge, this is the first time that this has been attempted and achieved. Two different Fabry-Perot structures were generated by FIB milling of a microwire created by chemical etching. One consists of a milled indentation on the microwire while in the other, the microwire is totally cleaved and lies suspended on just one end, leaving it susceptible to vibration. The initial fiber design allows for this because of the existence of side support beams that hold the microwire in place. This free-standing microwire is what allows this structure to work as a vibration sensor. Most fiber vibrations sensors are based on using two distinct fibers for input and output and an external system to hold them in place, using one fiber as a cantilever [18

18. J. Kalenik and R. Pająk, “A cantilever optical-fiber accelerometer,” Sens. Actuators A Phys. 68(1-3), 350–355 (1998). [CrossRef]

]. The sensor presented here has the advantage of being completely integrated in the fiber, with no need for external support structures. Temperature sensitivities from 11.5 to 15.5 pm/K were obtained for both the FIB milled FP structures when considering a temperature range from 100 to 550 °C. As for vibration, the cantilever system detects frequencies from 1 Hz to 40 kHz, and potentially as high as the first resonance frequency of 360 kHz.

This work showed that a FIB can be used for processes such as selective cleaving and creating simple structures that could not be achieved otherwise with such high optical quality. FIB can also be combined with other post-processing techniques to create new and novel optical sensors and devices opening doors for miniaturized and fiber-integrated elements.

Acknowledgments

This work was supported by the COST Action TD1001. The work of Ricardo André was supported in part by Fundação para a Ciência e Tecnologia under the grant SFRH/BD/84048/2012. This work was also partially funded by the Slovenian Research Agency (ARSS) program no. P2-0368. The P2O5-doped fibers used in this research were kindly produced and supplied by Optacore d.o.o., Slovenia.

References and links

1.

S. Reyntjens and R. Puers, “A review of focused ion beam applications in microsystem technology,” J. Micromech. Microeng. 11(4), 287–300 (2001). [CrossRef]

2.

A. A. Tseng, “Recent developments in micromilling using focused ion beam technology,” J. Micromech. Microeng. 14(4), R15–R34 (2004). [CrossRef]

3.

C. Martelli, P. Olivero, J. Canning, N. Groothoff, S. Prawer, S. Huntington, and B. Gibson, “Micromachining long period gratings in optical fibres using focused ion beam,” in Bragg Gratings, Photosensitivity, and Poling in Glass Waveguides (Optical Society of America, 2007), p. BTuC6.

4.

J.-L. Kou, S.-J. Qiu, F. Xu, Y.-Q. Lu, Y. Yuan, and G. Zhao, “Miniaturized metal-dielectric-hybrid fiber tip grating for refractive index sensing,” IEEE Photon. Technol. Lett. 23(22), 1712–1714 (2011). [CrossRef]

5.

J. L. Kou, S. J. Qiu, F. Xu, and Y. Q. Lu, “Demonstration of a compact temperature sensor based on first-order Bragg grating in a tapered fiber probe,” Opt. Express 19(19), 18452–18457 (2011). [CrossRef] [PubMed]

6.

D. Iannuzzi, K. Heeck, M. Slaman, S. de Man, J. H. Rector, H. Schreuders, J. W. Berenschot, V. J. Gadgil, R. G. P. Sanders, M. C. Elwenspoek, and S. Deladi, “Fibre-top cantilevers: design, fabrication and applications,” Meas. Sci. Technol. 18(10), 3247–3252 (2007). [CrossRef]

7.

K. P. Nayak, F. Le Kien, Y. Kawai, K. Hakuta, K. Nakajima, H. T. Miyazaki, and Y. Sugimoto, “Cavity formation on an optical nanofiber using focused ion beam milling technique,” Opt. Express 19(15), 14040–14050 (2011). [CrossRef] [PubMed]

8.

F. Wang, W. Yuan, O. Hansen, and O. Bang, “Selective filling of photonic crystal fibers using focused ion beam milled microchannels,” Opt. Express 19(18), 17585–17590 (2011). [CrossRef] [PubMed]

9.

J. L. Kou, J. Feng, Q. J. Wang, F. Xu, and Y. Q. Lu, “Microfiber-probe-based ultrasmall interferometric sensor,” Opt. Lett. 35(13), 2308–2310 (2010). [CrossRef] [PubMed]

10.

J. L. Kou, J. Feng, L. Ye, F. Xu, and Y. Q. Lu, “Miniaturized fiber taper reflective interferometer for high temperature measurement,” Opt. Express 18(13), 14245–14250 (2010). [CrossRef] [PubMed]

11.

W. Yuan, F. Wang, A. Savenko, D. H. Petersen, and O. Bang, “Note: Optical fiber milled by focused ion beam and its application for Fabry-Pérot refractive index sensor,” Rev. Sci. Instrum. 82(7), 076103 (2011). [CrossRef] [PubMed]

12.

S. Pevec, E. Cibula, B. Lenardic, and D. Donlagic, “Micromachining of optical fibers using selective etching based on phosphorus pentoxide doping,” IEEE Photon. J. 3(4), 627–632 (2011). [CrossRef]

13.

S. Pevec and D. Donlagic, “All-fiber, long-active-length Fabry-Perot strain sensor,” Opt. Express 19(16), 15641–15651 (2011). [CrossRef] [PubMed]

14.

S. Pevec and D. Donlagic, “Miniature micro-wire based optical fiber-field access device,” Opt. Express 20(25), 27874–27887 (2012). [CrossRef] [PubMed]

15.

T. Wei, Y. Han, H.-L. Tsai, and H. Xiao, “Miniaturized fiber inline Fabry-Perot interferometer fabricated with a femtosecond laser,” Opt. Lett. 33(6), 536–538 (2008). [CrossRef] [PubMed]

16.

L. Zhao, L. Jiang, S. Wang, H. Xiao, Y. Lu, and H.-L. Tsai, “A high-quality Mach-Zehnder interferometer fiber sensor by femtosecond laser one-step processing,” Sensors 11(1), 54–61 (2011). [CrossRef] [PubMed]

17.

L. Yuan, T. Wei, Q. Han, H. Wang, J. Huang, L. Jiang, and H. Xiao, “Fiber inline Michelson interferometer fabricated by a femtosecond laser,” Opt. Lett. 37(21), 4489–4491 (2012). [CrossRef] [PubMed]

18.

J. Kalenik and R. Pająk, “A cantilever optical-fiber accelerometer,” Sens. Actuators A Phys. 68(1-3), 350–355 (1998). [CrossRef]

OCIS Codes
(050.2230) Diffraction and gratings : Fabry-Perot
(060.2310) Fiber optics and optical communications : Fiber optics
(060.2370) Fiber optics and optical communications : Fiber optics sensors
(160.2290) Materials : Fiber materials
(230.4000) Optical devices : Microstructure fabrication

ToC Category:
Sensors

History
Original Manuscript: March 18, 2014
Revised Manuscript: May 15, 2014
Manuscript Accepted: May 17, 2014
Published: May 22, 2014

Citation
Ricardo M. André, Simon Pevec, Martin Becker, Jan Dellith, Manfred Rothhardt, Manuel B. Marques, Denis Donlagic, Hartmut Bartelt, and Orlando Frazão, "Focused ion beam post-processing of optical fiber Fabry-Perot cavities for sensing applications," Opt. Express 22, 13102-13108 (2014)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-11-13102


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References

  1. S. Reyntjens, R. Puers, “A review of focused ion beam applications in microsystem technology,” J. Micromech. Microeng. 11(4), 287–300 (2001). [CrossRef]
  2. A. A. Tseng, “Recent developments in micromilling using focused ion beam technology,” J. Micromech. Microeng. 14(4), R15–R34 (2004). [CrossRef]
  3. C. Martelli, P. Olivero, J. Canning, N. Groothoff, S. Prawer, S. Huntington, and B. Gibson, “Micromachining long period gratings in optical fibres using focused ion beam,” in Bragg Gratings, Photosensitivity, and Poling in Glass Waveguides (Optical Society of America, 2007), p. BTuC6.
  4. J.-L. Kou, S.-J. Qiu, F. Xu, Y.-Q. Lu, Y. Yuan, G. Zhao, “Miniaturized metal-dielectric-hybrid fiber tip grating for refractive index sensing,” IEEE Photon. Technol. Lett. 23(22), 1712–1714 (2011). [CrossRef]
  5. J. L. Kou, S. J. Qiu, F. Xu, Y. Q. Lu, “Demonstration of a compact temperature sensor based on first-order Bragg grating in a tapered fiber probe,” Opt. Express 19(19), 18452–18457 (2011). [CrossRef] [PubMed]
  6. D. Iannuzzi, K. Heeck, M. Slaman, S. de Man, J. H. Rector, H. Schreuders, J. W. Berenschot, V. J. Gadgil, R. G. P. Sanders, M. C. Elwenspoek, S. Deladi, “Fibre-top cantilevers: design, fabrication and applications,” Meas. Sci. Technol. 18(10), 3247–3252 (2007). [CrossRef]
  7. K. P. Nayak, F. Le Kien, Y. Kawai, K. Hakuta, K. Nakajima, H. T. Miyazaki, Y. Sugimoto, “Cavity formation on an optical nanofiber using focused ion beam milling technique,” Opt. Express 19(15), 14040–14050 (2011). [CrossRef] [PubMed]
  8. F. Wang, W. Yuan, O. Hansen, O. Bang, “Selective filling of photonic crystal fibers using focused ion beam milled microchannels,” Opt. Express 19(18), 17585–17590 (2011). [CrossRef] [PubMed]
  9. J. L. Kou, J. Feng, Q. J. Wang, F. Xu, Y. Q. Lu, “Microfiber-probe-based ultrasmall interferometric sensor,” Opt. Lett. 35(13), 2308–2310 (2010). [CrossRef] [PubMed]
  10. J. L. Kou, J. Feng, L. Ye, F. Xu, Y. Q. Lu, “Miniaturized fiber taper reflective interferometer for high temperature measurement,” Opt. Express 18(13), 14245–14250 (2010). [CrossRef] [PubMed]
  11. W. Yuan, F. Wang, A. Savenko, D. H. Petersen, O. Bang, “Note: Optical fiber milled by focused ion beam and its application for Fabry-Pérot refractive index sensor,” Rev. Sci. Instrum. 82(7), 076103 (2011). [CrossRef] [PubMed]
  12. S. Pevec, E. Cibula, B. Lenardic, D. Donlagic, “Micromachining of optical fibers using selective etching based on phosphorus pentoxide doping,” IEEE Photon. J. 3(4), 627–632 (2011). [CrossRef]
  13. S. Pevec, D. Donlagic, “All-fiber, long-active-length Fabry-Perot strain sensor,” Opt. Express 19(16), 15641–15651 (2011). [CrossRef] [PubMed]
  14. S. Pevec, D. Donlagic, “Miniature micro-wire based optical fiber-field access device,” Opt. Express 20(25), 27874–27887 (2012). [CrossRef] [PubMed]
  15. T. Wei, Y. Han, H.-L. Tsai, H. Xiao, “Miniaturized fiber inline Fabry-Perot interferometer fabricated with a femtosecond laser,” Opt. Lett. 33(6), 536–538 (2008). [CrossRef] [PubMed]
  16. L. Zhao, L. Jiang, S. Wang, H. Xiao, Y. Lu, H.-L. Tsai, “A high-quality Mach-Zehnder interferometer fiber sensor by femtosecond laser one-step processing,” Sensors 11(1), 54–61 (2011). [CrossRef] [PubMed]
  17. L. Yuan, T. Wei, Q. Han, H. Wang, J. Huang, L. Jiang, H. Xiao, “Fiber inline Michelson interferometer fabricated by a femtosecond laser,” Opt. Lett. 37(21), 4489–4491 (2012). [CrossRef] [PubMed]
  18. J. Kalenik, R. Pająk, “A cantilever optical-fiber accelerometer,” Sens. Actuators A Phys. 68(1-3), 350–355 (1998). [CrossRef]

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