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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 10 — May. 20, 2013
  • pp: 12111–12121
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Characteristics of microfiber Fabry-Perot resonators fabricated by UV exposure

Jie Li, Xiang Shen, Li-Peng Sun, and Bai-Ou Guan  »View Author Affiliations


Optics Express, Vol. 21, Issue 10, pp. 12111-12121 (2013)
http://dx.doi.org/10.1364/OE.21.012111


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Abstract

We inscribe a Fabry-Perot (FP) resonator in the microfiber utilizing the 193-nm UV exposure and the phase mask technique. Some new characteristics including strong polarization dependence and large spectral dispersion in contrast to the conventional counterparts are measured, which are attributed to the two-fold symmetry of index change in the grating and the dispersion of the effective grating length, respectively. The thinner microfiber can generally generate stronger polarization dependence. The FP spectral dependencies on external strain, temperature, and refractive index are also investigated. Our fabricated structures can have potential of acting as photonic sensors or polarization related filters.

© 2013 OSA

1. Introduction

In recent years, optical microfibers with subwavelength diameters have been attracting great research interests because of their compactness, large surface optical intensity, tight light confinement, excellent mechanical strength, and controllable large waveguide dispersions [1

1. L. Tong, R. R. Gattass, J. B. Ashcom, S. He, J. Lou, M. Shen, I. Maxwell, and E. Mazur, “Subwavelength diameter silica wires for low-loss optical wave guiding,” Nature 426(6968), 816–819 (2003). [CrossRef] [PubMed]

]. Microfibers can be achieved by heating and tapering commercial optical fibers. Due to the subwavelength size of the waveguide in the transverse direction, large evanescent field effect becomes one of the most interesting properties in contrast to the conventional counterparts, which permits a strong interaction between the microfiber and the surroundings and makes the structure very sensitive to the change of the ambient medium when acting as refractive index sensors. Nowadays a great number of microfiber based devices such as microfiber knots/loops, coils, and gratings have been implemented [2

2. G. Brambilla, F. Xu, P. Horak, Y. Jung, F. Koizumi, N. P. Sessions, E. Koukharenko, X. Feng, G. S. Murugan, J. S. Wilkinson, and D. J. Richardson, “Optical fiber nanowires and microwires: Fabrication and applications,” Adv. Opt. Photon. 1(1), 107–161 (2009). [CrossRef]

], which exhibit great potential of applications ranging from sensing to quantum dynamic experiments. On the other hand, the Fabry-Perot (FP) resonator has become a key photonic element of both optical communication and sensing systems. Formation of FP resonators in optical fibers has received considerable research attention in the past [3

3. C. E. Lee and H. F. Taylor, “Sensors for smart structures based on the Fabry-Perot interferometer,” in Fiber optic smart structures, (Eric Udd, New York, 1995), 249–269.

9

9. S. C. Kaddu, D. J. Booth, D. D. Garchev, and S. F. Collins, “Intrinsic fibre Fabry-Perot sensors based on co-located Bragg gratings,” Opt. Commun. 142(4-6), 189–192 (1997). [CrossRef]

], due to their advantages such as easy design, high fineness, possibility of multiplexing, and compatibility with optical fiber systems. Various in-fiber FP configurations have been designed utilizing either two [5

5. S. Legoubin, M. Douay, P. Bernage, P. Niay, S. Boj, and E. Delevaque, “Free spectral range variations of grating-based Fabry-Perot filters photowritten in optical fibers,” J. Opt. Soc. Am. A 12(8), 1687–1694 (1995). [CrossRef]

8

8. Y. O. Barmenkov, D. Zalvidea, S. Torres-Peiró, J. L. Cruz, and M. V. Andrés, “Effective length of short Fabry-Perot cavity formed by uniform fiber Bragg gratings,” Opt. Express 14(14), 6394–6399 (2006). [CrossRef] [PubMed]

] or more [9

9. S. C. Kaddu, D. J. Booth, D. D. Garchev, and S. F. Collins, “Intrinsic fibre Fabry-Perot sensors based on co-located Bragg gratings,” Opt. Commun. 142(4-6), 189–192 (1997). [CrossRef]

] gratings, which show great potential for use in detecting of measurands (temperature, strain, vibration, acoustics, and humidity) [3

3. C. E. Lee and H. F. Taylor, “Sensors for smart structures based on the Fabry-Perot interferometer,” in Fiber optic smart structures, (Eric Udd, New York, 1995), 249–269.

,4

4. T. Zhu, D. Wu, M. Liu, and D. W. Duan, “In-line fiber optic interferometric sensors in single-mode fibers,” Sensors (Basel) 12(12), 10430–10449 (2012). [CrossRef] [PubMed]

] or short-cavity fiber lasing [7

7. B. Jacobsson, V. Pasiskevicius, and F. Laurell, “Single-longitudinal-mode Nd-laser with a Bragg-grating Fabry-Perot cavity,” Opt. Express 14(20), 9284–9292 (2006). [CrossRef] [PubMed]

,8

8. Y. O. Barmenkov, D. Zalvidea, S. Torres-Peiró, J. L. Cruz, and M. V. Andrés, “Effective length of short Fabry-Perot cavity formed by uniform fiber Bragg gratings,” Opt. Express 14(14), 6394–6399 (2006). [CrossRef] [PubMed]

].

In recent years, integrating FP resonators to microfibers has been of great interest and many new possibilities may be opened up. Among the configurations as demonstrated in the literatures [10

10. S. S. Wang, Z. F. Hu, Y. H. Li, and L. M. Tong, “All-fiber Fabry-Perot resonators based on microfiber Sagnac loop mirrors,” Opt. Lett. 34(3), 253–255 (2009). [CrossRef] [PubMed]

15

15. M. Ding, P. Wang, T. Lee, and G. Brambilla, “A microfiber cavity with minimal-volume confinement,” Appl. Phys. Lett. 99(5), 051105 (2011). [CrossRef]

], the grating-assisted FP structures can have a much strong longitudinal confinement in a small size so that light can be effectively captured in the central dielectric fiber with a high Q factor. In contrast to the interferometers of Mach-Zehnder or Sagnac loop [16

16. Y. H. Li and L. M. Tong, “Mach-Zehnder interferometers assembled with optical microfibers or nanofibers,” Opt. Lett. 33(4), 303–305 (2008). [CrossRef] [PubMed]

,17

17. J. Li, L. P. Sun, S. Gao, Z. Quan, Y. L. Chang, Y. Ran, L. Jin, and B. O. Guan, “Ultrasensitive refractive index sensors based on rectangular silica microfibers,” Opt. Lett. 36(18), 3593–3595 (2011). [CrossRef] [PubMed]

], a FP resonator can have interference fringes which have good finesse of wavelength. Previously, a microfiber FP resonator is realized by etch-eroding [12

12. W. Liang, Y. Huang, Y. Xu, R. K. Lee, and A. Yariv, “Highly sensitive fiber Bragg grating refractive index sensors,” Appl. Phys. Lett. 86(15), 151122 (2005). [CrossRef]

] or adiabatically tapering [13

13. J. J. Zhang, Q. Z. Sun, R. B. Liang, J. H. Wo, D. M. Liu, and P. Shum, “Microfiber Fabry-Perot interferometer fabricated by taper-drawing technique and its application as a radio frequency interrogated refractive index sensor,” Opt. Lett. 37(14), 2925–2927 (2012). [CrossRef] [PubMed]

] a segment of commercial single-mode fiber between two standard fiber Bragg gratings, which exhibits high sensitivity to surrounding refractive index. However, an additional loss or overlong cavity is readily produced since the two transition regions of fiber taper are included in the cavity. To decrease the device size, an alternate method is to inscribe gratings directly in the uniform microfiber region. A resonator has been formed by carving notches in a micro-size waveguide using the focused ion beam milling technique [14

14. 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]

,15

15. M. Ding, P. Wang, T. Lee, and G. Brambilla, “A microfiber cavity with minimal-volume confinement,” Appl. Phys. Lett. 99(5), 051105 (2011). [CrossRef]

]. Besides the small size, the fabricated structure exhibits a high index contrast in the periodic structure and has potential in the application of quantum dynamic experiments. Even though, the milling procedure is relatively complex and time consuming because of the low efficiency of nanostructuring. To improve the mechanical properties of microfiber devices, high manufacture efficiency with no distinct physical damage is required.

2. Theory

Figure 1
Fig. 1 Schematic diagram of the microfiber FP resonator fabricated by UV laser exposure. The incident side has a larger index change than the shadow side of the microfiber.
depicts the schematic diagram of the microfiber FP resonator that contains a segment of microfiber sandwiched by two mFBGs, where the parameters d, Λ, Lg, and L0 are the diameter of microfiber, the grating period, the physical grating length, and the separation between the two gratings, respectively. The microfiber is fabricated by heating and stretching a general Ge-doped 62.5/125μm multimode fiber (manufactured by Corning, NA = 0.275) down to the subwavelength scale using the oxyhydrogen-flame brushing technique [1

1. L. Tong, R. R. Gattass, J. B. Ashcom, S. He, J. Lou, M. Shen, I. Maxwell, and E. Mazur, “Subwavelength diameter silica wires for low-loss optical wave guiding,” Nature 426(6968), 816–819 (2003). [CrossRef] [PubMed]

,2

2. G. Brambilla, F. Xu, P. Horak, Y. Jung, F. Koizumi, N. P. Sessions, E. Koukharenko, X. Feng, G. S. Murugan, J. S. Wilkinson, and D. J. Richardson, “Optical fiber nanowires and microwires: Fabrication and applications,” Adv. Opt. Photon. 1(1), 107–161 (2009). [CrossRef]

]. The microfiber size at the taper waist is controlled by tuning the heating temperature and stretching speed on both sides. Compared to the version from a single-mode fiber, a microfiber drawn from the multimode fiber can provide a relatively large photosensitive Ge-doped region in the microfiber and thus a high efficiency of grating writing [18

18. Y. Ran, L. Jin, Y. N. Tan, L. P. Sun, J. Li, and B. O. Guan, “High-efficiency ultraviolet-inscription of Bragg gratings in microfibers,” IEEE Photon. J. 4(1), 181–186 (2012). [CrossRef]

]. In the subwavelength-scale microfiber region, the guided-mode energy spreads out of the Ge-doped core and the whole microfiber acts as the “core” and the surrounding air acts as the “cladding” of the structure. The intermodal interference emerging in the original multimode fiber has been suppressed in the tapered fiber since most of the high-order modes have been filtered out at the waist of fiber taper. The two untapered ends of the microfiber are fusion spliced to conventional single-mode fibers. The total device insertion loss is typically 1.5dB, which is mainly induced by the mode mismatch between the single-mode and multi-mode fibers. The transmission loss of mode in the fiber taper can be smaller than 0.5dB with optimization of fiber tapering technique.

The FP spectral characteristics are co-governed by the resonance condition in the cavity and the reflectivity of the two Bragg gratings. As shown in Fig. 1, the microfiber gratings on both sides provides strong reflection so that light can be confined in the center with the so-called “cavity modes”. In our study, we fabricate the two microfiber gratings using the same inscription condition. Generally, when the fundamental mode is incident into an microfiber Bragg grating, either the reversely-propagating fundamental mode or higher-order mode can be excited, with the phase-matching condition: (n1 + n2)Λ = λB, where n1 and n2 represent the effective indices of the two modes that are coupled to each other and λB represents the reflected Bragg wavelength. Utilizing the coupled-mode theory for two counter-propagating waves [19

19. T. Erdogan, “Fiber grating spectra,” J. Lightwave Technol. 15(8), 1277–1294 (1997). [CrossRef]

], the reflectivity of an mFBG can be expressed as R = sinh2[(κ2−σ2)0.5Lg]/{cosh2[(κ2−σ2)0.5Lg]−(σ/κ)2}, where κ is a coupling coefficient and σ = 2πn¯(λ−1λB−1) is the detuning from the Bragg wavelength λB, where n¯ = (n1 + n2)/2 represents the mean effective index of two coupled modes. In particular, when σ = 0, the maximum reflectivity is given as Rmax = tanh2(κLg). Here the coupling coefficient κ is
κ=πλGeΔnE1E2*dxdy,
(1)
where Δn represents the photoinduced index modulation within the microfiber and E1 and E2 represent the transverse electric fields of the two coupled modes, respectively. From Eq. (2), the magnitude of coupling coefficient is determined by the overlap between the mode fields and the photo-induced index profile in the cross section of microfiber. The effective grating length, defined as the distance for the light wave propagating into a microfiber Bragg grating before it is reflected back, is needed for a proper estimate of the cavity length [9

9. S. C. Kaddu, D. J. Booth, D. D. Garchev, and S. F. Collins, “Intrinsic fibre Fabry-Perot sensors based on co-located Bragg gratings,” Opt. Commun. 142(4-6), 189–192 (1997). [CrossRef]

]. Then the phase change acquired by the wave from an individual grating is 2k0n¯Leff, where k0 = 2π/λ represents the wavenumber in vacuum. In the FP cavity, constructive interference occurs when the total phase change of the light wave traveling over a round trip in the cavity equals a number of 2π, known as the resonance condition:
2k0n¯(L0+2Leff)=2mπ,
(2)
where m is an integer. From Eq. (2), the resonance wavelength is determined by the mean effective index n¯, the physical length L0, and the effective grating length Leff. Any change in those three parameters will shift the interferometric fringes. As demonstrated in [9

9. S. C. Kaddu, D. J. Booth, D. D. Garchev, and S. F. Collins, “Intrinsic fibre Fabry-Perot sensors based on co-located Bragg gratings,” Opt. Commun. 142(4-6), 189–192 (1997). [CrossRef]

], the higher reflectivity of the gratings can in general produce the shorter Leff.

3. Experimental results and analysis

In mFBG’s writing process, we place a microfiber in parallel to the phase mask with a distance of around 100μm. The 193nm ArF excimer laser with a single-pulse output energy of ~3 mJ and a repetition rate of 200 Hz is adopted. The laser spot is stringently aligned to the microfiber axis, which produces an energy density of 120mJ/cm2 on the microfiber surface. After the laser beam passing through the phase mask, a periodic interference laser field is formed, which generates a photoinduced index change in the microfiber based on the two-photon effect. The grating writing typically takes around ~1min. The fabricated grating has a pitch of ~536nm and a total length of ~2.5mm, determined by the phase mask period of 1072nm and the output aperture of laser beam, respectively. To achieve a FP structure, after one grating is fabricated, the laser beam moves along the microfiber with a distance to write another one with the same parameters. The transmission spectrum can be monitored through a broadband source (BBS) in junction to optical spectrum analyzer (OSA), as shown in Fig. 1.

3.1 Spectrum of a single mFBG

A photoinduced mFBG in general has weak polarization dependence, as reported in [18

18. Y. Ran, L. Jin, Y. N. Tan, L. P. Sun, J. Li, and B. O. Guan, “High-efficiency ultraviolet-inscription of Bragg gratings in microfibers,” IEEE Photon. J. 4(1), 181–186 (2012). [CrossRef]

,20

20. Y. Zhang, B. Lin, S. C. Tjin, H. Zhang, G. H. Wang, P. Shum, and X. L. Zhang, “Refractive index sensing based on higher-order mode reflection of a microfiber Bragg grating,” Opt. Express 18(25), 26345–26350 (2010). [CrossRef] [PubMed]

]. However, in this paper, we show that the polarization dependence can be very strong when the microfiber diameter becomes small sufficiently. Figure 2(a)
Fig. 2 (a) Typical transmission spectra of the x- and y-polarized states for an mFBG with d = 1.85. Insets are the mode field profiles. Distinct polarization dependence especially at band b is observed. (b) Primary linearly-polarized field sketches for the fundamental (LP01) and the second-order (LP11) modes.
records typical transmission spectra of an mFBG with diameter of d = 1.85um. A silica microfiber can support multiple modes since V = πd(nmicrofiber2−1)1/2/λ>2.405. Strong polarization dependence is observed. As shown in the figure, reflection notches occur around wavelengths of 1454.7 nm, 1396.0nm, 1381.2nm, 1320.5nm, and 1299.0nm corresponding to the coupling from the fundamental HE11x,y mode to the modes HE11x,y (band a), TE01 (band b), HE21(1),(2) and TM01 (band c), EH11(1),(2) (band d), and HE31(1),(2) and HE12x,y (band e), respectively. The superscripts (1) and (2) represent the differential polarization states for the waveguide mode. Those states are degenerate only in an ideal circularly-symmetric optical fiber. The corresponding mode electric field profiles are plotted as insets in Fig. 2(a). Among them, we define the modes HE11x,y and HE12x,y as the symmetric modes since the mode fields are radially symmetric, and the rest modes as the asymmetric modes since the mode fields are non-radially symmetric along the cross section of microfiber. Assuming the photoinduced index modulation Δn is radially symmetric, based on Eq. (1), we can achieve κ = 0 for the overlap between the symmetric and asymmetric modes and the photosensitive microfiber area. Thereby the coupling only occurs between the symmetric modes similar to the conventional fiber Bragg gratings [19

19. T. Erdogan, “Fiber grating spectra,” J. Lightwave Technol. 15(8), 1277–1294 (1997). [CrossRef]

,21

21. A. Kersey, M. Davis, H. Patrick, M. LeBlanc, K. Koo, C. Askins, M. Putnam, and E. Friebele, “Fiber grating sensors,” J. Lightwave Technol. 15(8), 1442–1463 (1997). [CrossRef]

]. For the present case, the index modulation Δn is two-fold symmetric as previously described. We achieve κ≠0 for the overlap between the symmetric and asymmetric modes, suggesting that the coupling could occur between them.

The transmission spectrum exhibits distinct polarization dependence. Especially, for band b, the maximum extinction ratio of around 10 dB occurs, which can be demonstrated using the linearly polarized modes of Gloge [22

22. R. C. Youngquist, J. L. Brooks, and H. J. Shaw, “Two-mode fiber modal coupler,” Opt. Lett. 9(5), 177–179 (1984). [CrossRef] [PubMed]

]. Figure 2(b) shows schematically the two cross-polarized states of the fundamental LP01 mode and the four forms of the second-order LP11 mode. The coupling occurs only between the sets of the same linear polarization state [22

22. R. C. Youngquist, J. L. Brooks, and H. J. Shaw, “Two-mode fiber modal coupler,” Opt. Lett. 9(5), 177–179 (1984). [CrossRef] [PubMed]

]. Considering a UV laser beam irradiating from top down into the microfiber, the index change is symmetric with regard to the y axis but asymmetric with regard to the x axis. Band b corresponds to the coupling between HE11 and TE01 modes. The HE11 mode corresponds to the sets of LP01 while the TE01 mode can be decomposed into the two degenerate linearly-polarized sets of LP11-1 and LP11-3, with each having two-fold lobes and the arrows indicating the field vector direction. For the coupling between HE11 and TE01, based on Eq. (1), we achieve κ ≠0 for the x-polarized modes and κ = 0 for the y-polarized modes, respectively, which can demonstrate the high extinction ratio of polarizations in band b shown in Fig. 2(a). Other mode reflections can also be explained using the same method.

To express the asymmetry degree of index change in the microfiber, we assume that the mFBG has an average index change of Δn1 in the upper region and Δn2 in the below region, respectively, with regard to the x axis shown in Fig. 2(b). Based on Eq. (1), from band b, the calculated coupling coefficient is 7.79cm−1, which corresponds to an index difference of Δn1−Δn2 = 7.07 × 10−4. Accordingly, from band a, the reflectivity 98.5% corresponds to a coupling coefficient of 11.2 cm−1, producing a sum of index change with Δn1 + Δn2 = 8.62 × 10−4. Hence the difference of index change is around (Δn1−Δn2)/(Δn1 + Δn2) = 82.0%. Such large asymmetry is useful for the polarization manipulation of the device. The reflectivity of mFBG can be improved by increasing the writing laser intensity or the grating length. The large asymmetry of index change can be achieved by decreasing the microfiber diameter and using the optimized UV exposure condition.

3.2 Spectral characteristics of FP resonator

A FP resonator is formed by writing two equal mFBGs into the microfiber under the same exposure condition. After writing the first 2.5-mm grating, the UV laser is moved along the microfiber to write the second one. The displacement of the UV beam is controlled using a high precision translation stage. During formation of the second grating, some resonance peaks occur in the grating bands, which can be monitored by OSA. The polarization dependence is in general unobvious unless the microfiber diameter become small sufficiently. In our experiments, we study two types of FP resonators in thicker and thinner microfibers, corresponding to weaker and stronger polarization dependences, respectively. Figure 3(a)
Fig. 3 (a) Transmission spectrum of FP cavity with microfiber diameter of 3.8μm. (b)Details of interferometric spectra (solid blue curve) and effective grating lengths (dashed red curve) in the grating bands.
plots a transmission spectrum of FP with d = 3.8μm and L0 = 0.25mm. Since the transmission characteristics of the two polarizations are similar to each other, an unpolarized light is used to monitor the transmission spectrum. Due to the existence of FP cavity between the two mFBGs, multiple interferometric peaks occur at the separated grating bands centered at 1527.0nm (HE11x,y), 1501.6nm (TE01), 1498.6nm (HE21(1),(2) and TM01), 1469.6nm (EH11(1),(2)), and 1459.3nm (HE31(1),(2) and HE12x,y), respectively, as marked by symbols a~e in Fig. 3(a). Figure 3(b) shows the details of FP spectrum. Compared to a microfiber Mach-Zehnder interferometer or Sagnac loop [16

16. Y. H. Li and L. M. Tong, “Mach-Zehnder interferometers assembled with optical microfibers or nanofibers,” Opt. Lett. 33(4), 303–305 (2008). [CrossRef] [PubMed]

,17

17. J. Li, L. P. Sun, S. Gao, Z. Quan, Y. L. Chang, Y. Ran, L. Jin, and B. O. Guan, “Ultrasensitive refractive index sensors based on rectangular silica microfibers,” Opt. Lett. 36(18), 3593–3595 (2011). [CrossRef] [PubMed]

], a FP resonator has much narrowed bandwidth. For example, the bandwidth is smaller than 0.043nm around 1527.0nm at band a. The peak interval and bandwidth are co-determined by the cavity length and the reflectivity of the mFBGs, respectively. Generally, the smaller cavity length can generate a larger peak interval and the higher grating reflectivity generates the more narrowed bandwidth in the interferometric fringes.

For example, in band a, the interval of spectral peaks varies from 0.21nm to 0.43nm with a rate of ~0.073 within the wavelength range from 1525.8nm to 1528.3nm, as shown in Fig. 3(b). Based on Eq. (3), the calculated contribution from the variation of λ to the Δλ change is in an order of 10−2, while the contribution from the variation of Leff, n¯, or n¯ is in an order of 10−4 only. Thereby the dispersion of the FP spectrum shown in Fig. 3 is mainly attributed to the variation of Leff, which corresponds to around −424.61 at 1525.8nm and 236.90 at 1528.3nm, respectively. The minimum effective length of mFBG occurs with 1113.0 μm at the center of the grating band.

4. Responses to elongation strain, temperature, and refractive index

Any external perturbation such as strain, temperature, or refractive index may alter the spectral properties of the microfiber FP resonator, of which the study is necessary for future applications of the device. Figure 5(a)
Fig. 5 Peak wavelengths as functions of (a) applied strain and (b) temperature, respectively, at differential grating bands for a microfiber FP structure with d = 3.8μm and L0 = 0.25mm.
shows variations of FP peaks with applied strain, as marked by symbols a~e, at differential grating bands, with diameter d = 3.8μm and L0 = 0.25mm. As an elongation strain is applied, both the interferometric fringes and the grating bands reshift almost as a whole, due to the increasing of the cavity length as well as the grating pitch. As shown in Fig. 5(a), for the strain increasing from 0με to 3000με (micro strain), the measured sensitivities are 1.09pm/με (micro strain) at 1523.9nm (band a), 1.10 pm/με at 1498.2nm (band b), 1.12 pm/με at 1495.8nm (band c), 1.12 pm/με at 1466.7nm (band d), and 1.02 pm/με at 1456.5nm (band e), respectively. The experimental results are comparable to that of the conventional fiber Bragg grating (~1.21pm/ at 1550nm) [21

21. A. Kersey, M. Davis, H. Patrick, M. LeBlanc, K. Koo, C. Askins, M. Putnam, and E. Friebele, “Fiber grating sensors,” J. Lightwave Technol. 15(8), 1442–1463 (1997). [CrossRef]

] for the similar waveguiding structures. On the other side, the peak intervals decrease slightly due to the increase of FP cavity, consistent to Eq. (2). For example, at 1523.91nm in band a, the measured intervals are 0.317nm at 0 με and 0.310nm at 3000με, respectively, producing a decreasing rate of ~2.37 × 10−3pm/με. Considering a 10pm wavelength resolution of OSA, the minimum strain change that can be detected is ~8.9με.

The temperature dependency of FP spectrum is measured by placing the FP cavity into an oven in air. The temperature of oven can be continuously controlled from 40 to 153 °C. Figure 5(b) plots several peak wavelengths as functions of temperature at the different grating bands. While the peaks redshift almost linearly with the increasing of environmental temperature, the peak intervals are almost unchanged. The measured sensitivity is 0.010~0.012 nm/°C for the peak wavelength ranging from 1456nm to 1523nm. Such sensitivity is very analogous to the conventional fiber Bragg grating (~0.01nm /°C at 1550nm) [21

21. A. Kersey, M. Davis, H. Patrick, M. LeBlanc, K. Koo, C. Askins, M. Putnam, and E. Friebele, “Fiber grating sensors,” J. Lightwave Technol. 15(8), 1442–1463 (1997). [CrossRef]

] due to the similar thermo-optics coefficient in silica fibers. For a 10pm wavelength resolution, the achieved temperature precision of measurement is ~0.8°C.

The spectral characteristics of microfiber FP resonator are sensitive to external refractive index due to its large evanescent field effect. In our experiment, we place the fabricated FP resonator into an aqueous solution of sucrose and tune the solution’s index by changing the sucrose concentration at room temperature. Figure 6(a)
Fig. 6 (a) Peak wavelengths as functions of external refractive index and (b) transmission spectra at differential grating bands for the microfiber FP resonator with d = 6.4μm and L0 = 0.25mm.
shows the variations of peak wavelength with the surrounding refractive index for the FP resonator with d = 6.4μm and L0 = 0.25mm. Figure 6(b) shows the FP transmission spectra. Within the index range from 1.33 to 1.41, the measured sensitivities are 1.729nm/RIU (refractive-index unit) around 1555.7nm (band a) and 18.46nm/RIU around 1542.0nm (band b), respectively. As expected, the sensitivity in the higher-order mode band is much larger than the lower-order mode band [20

20. Y. Zhang, B. Lin, S. C. Tjin, H. Zhang, G. H. Wang, P. Shum, and X. L. Zhang, “Refractive index sensing based on higher-order mode reflection of a microfiber Bragg grating,” Opt. Express 18(25), 26345–26350 (2010). [CrossRef] [PubMed]

], because the higher-order mode can have the larger evanescent field than the lower-order one. The FP spectrum shows good fineness compared to the single mFBG, as previously discussed. Besides, with an increase in the external refractive index, the peak intervals become small slightly due to the increasing of the effective cavity length as shown in Eq. (2). From Fig. 6, for the index increasing from 1.33 to 1.41, the measured peak interval varies from 0.361nm to 0.359nm with a changing rate of −0.033nm/RIU at band a and from 0.378nm to 0.351nm with a changing rate of −0.38nm/RIU at band b, respectively. Considering the 10pm wavelength resolution of OSA, the best detection precision for refractive index change is around 5.42 × 10−4 RIU at the work wavelength of 1542.0nm.

5. Conclusion

Acknowledgments

This work is supported by the National Science Found for Distinguished Young Scholars of China (61225023), the National Natural Science Foundation of China (60736039, 11004085), the Research Fund for the Doctoral Program of Higher Education (20114401110006), the Project of Science and Technology New Star of Zhujiang in Guangzhou city (2012J2200062), and the Fundamental Research Funds for the Central Universities.

References and links

1.

L. Tong, R. R. Gattass, J. B. Ashcom, S. He, J. Lou, M. Shen, I. Maxwell, and E. Mazur, “Subwavelength diameter silica wires for low-loss optical wave guiding,” Nature 426(6968), 816–819 (2003). [CrossRef] [PubMed]

2.

G. Brambilla, F. Xu, P. Horak, Y. Jung, F. Koizumi, N. P. Sessions, E. Koukharenko, X. Feng, G. S. Murugan, J. S. Wilkinson, and D. J. Richardson, “Optical fiber nanowires and microwires: Fabrication and applications,” Adv. Opt. Photon. 1(1), 107–161 (2009). [CrossRef]

3.

C. E. Lee and H. F. Taylor, “Sensors for smart structures based on the Fabry-Perot interferometer,” in Fiber optic smart structures, (Eric Udd, New York, 1995), 249–269.

4.

T. Zhu, D. Wu, M. Liu, and D. W. Duan, “In-line fiber optic interferometric sensors in single-mode fibers,” Sensors (Basel) 12(12), 10430–10449 (2012). [CrossRef] [PubMed]

5.

S. Legoubin, M. Douay, P. Bernage, P. Niay, S. Boj, and E. Delevaque, “Free spectral range variations of grating-based Fabry-Perot filters photowritten in optical fibers,” J. Opt. Soc. Am. A 12(8), 1687–1694 (1995). [CrossRef]

6.

G. E. Town, K. Sugden, J. A. R. Williams, I. Bennion, and S. B. Poole, “Wide-band Fabry-Perot-like filters in optical fiber,” IEEE Photon. Technol. Lett. 7(1), 78–80 (1995). [CrossRef]

7.

B. Jacobsson, V. Pasiskevicius, and F. Laurell, “Single-longitudinal-mode Nd-laser with a Bragg-grating Fabry-Perot cavity,” Opt. Express 14(20), 9284–9292 (2006). [CrossRef] [PubMed]

8.

Y. O. Barmenkov, D. Zalvidea, S. Torres-Peiró, J. L. Cruz, and M. V. Andrés, “Effective length of short Fabry-Perot cavity formed by uniform fiber Bragg gratings,” Opt. Express 14(14), 6394–6399 (2006). [CrossRef] [PubMed]

9.

S. C. Kaddu, D. J. Booth, D. D. Garchev, and S. F. Collins, “Intrinsic fibre Fabry-Perot sensors based on co-located Bragg gratings,” Opt. Commun. 142(4-6), 189–192 (1997). [CrossRef]

10.

S. S. Wang, Z. F. Hu, Y. H. Li, and L. M. Tong, “All-fiber Fabry-Perot resonators based on microfiber Sagnac loop mirrors,” Opt. Lett. 34(3), 253–255 (2009). [CrossRef] [PubMed]

11.

X. B. Zhang, J. L. Li, Y. Li, W. Y. Wang, F. F. Pang, Y. Q. Liu, and T. Y. Wang, “Sensing properties of intrinsic Fabry-Perot interferometers in fiber tapers,” Proc. SPIE 8421, 842189, 842189-4 (2012). [CrossRef]

12.

W. Liang, Y. Huang, Y. Xu, R. K. Lee, and A. Yariv, “Highly sensitive fiber Bragg grating refractive index sensors,” Appl. Phys. Lett. 86(15), 151122 (2005). [CrossRef]

13.

J. J. Zhang, Q. Z. Sun, R. B. Liang, J. H. Wo, D. M. Liu, and P. Shum, “Microfiber Fabry-Perot interferometer fabricated by taper-drawing technique and its application as a radio frequency interrogated refractive index sensor,” Opt. Lett. 37(14), 2925–2927 (2012). [CrossRef] [PubMed]

14.

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]

15.

M. Ding, P. Wang, T. Lee, and G. Brambilla, “A microfiber cavity with minimal-volume confinement,” Appl. Phys. Lett. 99(5), 051105 (2011). [CrossRef]

16.

Y. H. Li and L. M. Tong, “Mach-Zehnder interferometers assembled with optical microfibers or nanofibers,” Opt. Lett. 33(4), 303–305 (2008). [CrossRef] [PubMed]

17.

J. Li, L. P. Sun, S. Gao, Z. Quan, Y. L. Chang, Y. Ran, L. Jin, and B. O. Guan, “Ultrasensitive refractive index sensors based on rectangular silica microfibers,” Opt. Lett. 36(18), 3593–3595 (2011). [CrossRef] [PubMed]

18.

Y. Ran, L. Jin, Y. N. Tan, L. P. Sun, J. Li, and B. O. Guan, “High-efficiency ultraviolet-inscription of Bragg gratings in microfibers,” IEEE Photon. J. 4(1), 181–186 (2012). [CrossRef]

19.

T. Erdogan, “Fiber grating spectra,” J. Lightwave Technol. 15(8), 1277–1294 (1997). [CrossRef]

20.

Y. Zhang, B. Lin, S. C. Tjin, H. Zhang, G. H. Wang, P. Shum, and X. L. Zhang, “Refractive index sensing based on higher-order mode reflection of a microfiber Bragg grating,” Opt. Express 18(25), 26345–26350 (2010). [CrossRef] [PubMed]

21.

A. Kersey, M. Davis, H. Patrick, M. LeBlanc, K. Koo, C. Askins, M. Putnam, and E. Friebele, “Fiber grating sensors,” J. Lightwave Technol. 15(8), 1442–1463 (1997). [CrossRef]

22.

R. C. Youngquist, J. L. Brooks, and H. J. Shaw, “Two-mode fiber modal coupler,” Opt. Lett. 9(5), 177–179 (1984). [CrossRef] [PubMed]

OCIS Codes
(060.3735) Fiber optics and optical communications : Fiber Bragg gratings
(140.3945) Lasers and laser optics : Microcavities
(050.6624) Diffraction and gratings : Subwavelength structures

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: March 25, 2013
Revised Manuscript: May 3, 2013
Manuscript Accepted: May 7, 2013
Published: May 10, 2013

Citation
Jie Li, Xiang Shen, Li-Peng Sun, and Bai-Ou Guan, "Characteristics of microfiber Fabry-Perot resonators fabricated by UV exposure," Opt. Express 21, 12111-12121 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-10-12111


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References

  1. L. Tong, R. R. Gattass, J. B. Ashcom, S. He, J. Lou, M. Shen, I. Maxwell, and E. Mazur, “Subwavelength diameter silica wires for low-loss optical wave guiding,” Nature426(6968), 816–819 (2003). [CrossRef] [PubMed]
  2. G. Brambilla, F. Xu, P. Horak, Y. Jung, F. Koizumi, N. P. Sessions, E. Koukharenko, X. Feng, G. S. Murugan, J. S. Wilkinson, and D. J. Richardson, “Optical fiber nanowires and microwires: Fabrication and applications,” Adv. Opt. Photon.1(1), 107–161 (2009). [CrossRef]
  3. C. E. Lee and H. F. Taylor, “Sensors for smart structures based on the Fabry-Perot interferometer,” in Fiber optic smart structures, (Eric Udd, New York, 1995), 249–269.
  4. T. Zhu, D. Wu, M. Liu, and D. W. Duan, “In-line fiber optic interferometric sensors in single-mode fibers,” Sensors (Basel)12(12), 10430–10449 (2012). [CrossRef] [PubMed]
  5. S. Legoubin, M. Douay, P. Bernage, P. Niay, S. Boj, and E. Delevaque, “Free spectral range variations of grating-based Fabry-Perot filters photowritten in optical fibers,” J. Opt. Soc. Am. A12(8), 1687–1694 (1995). [CrossRef]
  6. G. E. Town, K. Sugden, J. A. R. Williams, I. Bennion, and S. B. Poole, “Wide-band Fabry-Perot-like filters in optical fiber,” IEEE Photon. Technol. Lett.7(1), 78–80 (1995). [CrossRef]
  7. B. Jacobsson, V. Pasiskevicius, and F. Laurell, “Single-longitudinal-mode Nd-laser with a Bragg-grating Fabry-Perot cavity,” Opt. Express14(20), 9284–9292 (2006). [CrossRef] [PubMed]
  8. Y. O. Barmenkov, D. Zalvidea, S. Torres-Peiró, J. L. Cruz, and M. V. Andrés, “Effective length of short Fabry-Perot cavity formed by uniform fiber Bragg gratings,” Opt. Express14(14), 6394–6399 (2006). [CrossRef] [PubMed]
  9. S. C. Kaddu, D. J. Booth, D. D. Garchev, and S. F. Collins, “Intrinsic fibre Fabry-Perot sensors based on co-located Bragg gratings,” Opt. Commun.142(4-6), 189–192 (1997). [CrossRef]
  10. S. S. Wang, Z. F. Hu, Y. H. Li, and L. M. Tong, “All-fiber Fabry-Perot resonators based on microfiber Sagnac loop mirrors,” Opt. Lett.34(3), 253–255 (2009). [CrossRef] [PubMed]
  11. X. B. Zhang, J. L. Li, Y. Li, W. Y. Wang, F. F. Pang, Y. Q. Liu, and T. Y. Wang, “Sensing properties of intrinsic Fabry-Perot interferometers in fiber tapers,” Proc. SPIE8421, 842189, 842189-4 (2012). [CrossRef]
  12. W. Liang, Y. Huang, Y. Xu, R. K. Lee, and A. Yariv, “Highly sensitive fiber Bragg grating refractive index sensors,” Appl. Phys. Lett.86(15), 151122 (2005). [CrossRef]
  13. J. J. Zhang, Q. Z. Sun, R. B. Liang, J. H. Wo, D. M. Liu, and P. Shum, “Microfiber Fabry-Perot interferometer fabricated by taper-drawing technique and its application as a radio frequency interrogated refractive index sensor,” Opt. Lett.37(14), 2925–2927 (2012). [CrossRef] [PubMed]
  14. 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. Express19(15), 14040–14050 (2011). [CrossRef] [PubMed]
  15. M. Ding, P. Wang, T. Lee, and G. Brambilla, “A microfiber cavity with minimal-volume confinement,” Appl. Phys. Lett.99(5), 051105 (2011). [CrossRef]
  16. Y. H. Li and L. M. Tong, “Mach-Zehnder interferometers assembled with optical microfibers or nanofibers,” Opt. Lett.33(4), 303–305 (2008). [CrossRef] [PubMed]
  17. J. Li, L. P. Sun, S. Gao, Z. Quan, Y. L. Chang, Y. Ran, L. Jin, and B. O. Guan, “Ultrasensitive refractive index sensors based on rectangular silica microfibers,” Opt. Lett.36(18), 3593–3595 (2011). [CrossRef] [PubMed]
  18. Y. Ran, L. Jin, Y. N. Tan, L. P. Sun, J. Li, and B. O. Guan, “High-efficiency ultraviolet-inscription of Bragg gratings in microfibers,” IEEE Photon. J.4(1), 181–186 (2012). [CrossRef]
  19. T. Erdogan, “Fiber grating spectra,” J. Lightwave Technol.15(8), 1277–1294 (1997). [CrossRef]
  20. Y. Zhang, B. Lin, S. C. Tjin, H. Zhang, G. H. Wang, P. Shum, and X. L. Zhang, “Refractive index sensing based on higher-order mode reflection of a microfiber Bragg grating,” Opt. Express18(25), 26345–26350 (2010). [CrossRef] [PubMed]
  21. A. Kersey, M. Davis, H. Patrick, M. LeBlanc, K. Koo, C. Askins, M. Putnam, and E. Friebele, “Fiber grating sensors,” J. Lightwave Technol.15(8), 1442–1463 (1997). [CrossRef]
  22. R. C. Youngquist, J. L. Brooks, and H. J. Shaw, “Two-mode fiber modal coupler,” Opt. Lett.9(5), 177–179 (1984). [CrossRef] [PubMed]

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