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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 16 — Jul. 30, 2012
  • pp: 18079–18084
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Structural microfiber long-period gratings

Li-Peng Sun, Jie Li, Long Jin, and Bai-Ou Guan  »View Author Affiliations


Optics Express, Vol. 20, Issue 16, pp. 18079-18084 (2012)
http://dx.doi.org/10.1364/OE.20.018079


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Abstract

We experimentally demonstrate a novel structural long-period grating by helically coiling one microfiber onto another with the relatively thicker diameter. Owing to the strong periodic modulation of the coiled microfiber to the evanescent field of the straight microfiber, a resonance transmission notch of ~16.2 dB can be induced for a compact device length of ~450μm only (4 helical periods). Moreover, the filtered light energy from the straight fiber can emerge again at the output of the coiled one, providing great flexibility in producing new device functions. The spectral response to external strain is investigated and wide wavelength tuning range of around 106nm is discussed.

© 2012 OSA

1. Introduction

Recently, the subwavelength-size microfibers are increasingly used for fabrication of various miniature optical devices and sensors [9

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

11

11. 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: fibrication and applications,” Adv. Opt. Photon. 1(1), 107–161 (2009). [CrossRef]

]. In this letter, we experimentally demonstrate a structural LPG by helically coiling one microfiber onto another with the relatively thicker diameter. The coiled microfiber can provide a strong modulation to the light field of microfiber, so that a resonance transmission notch of ~16.2 dB can be induced in a device length of ~450μm (4 helical periods). Moreover, the coiled microfiber can resonantly lead out the light signal from the straight one, providing a great flexibility in producing new functions of the device.

2. Fabrication and characterization

Figure 2(a)
Fig. 2 Evolution of transmission spectra in respect of the period number N, for the two LPGs with different parameters: (a) d1 = 7.05μm, d2 = 3.1μm, and Λ = 120μm; (b) d1 = 5.8μm, d2 = 3.4μm, and Λ = 112μm. The transmission notches correspond to the coupling between the LP01 modes of the two microfibers.
gives the measured evolution of transmission spectrum in the straight microfiber in respect of the period number N of a LPG, with d1 = 7.05μm, d2 = 3.1μm, and Λ = 120μm, by the use of a broad-band light source in junction to an optical spectrum analyzer. The attenuation dip corresponds to the coupling between the LP01 modes for the straight and the coiled microfibers. For the period number increasing from 0 to 9, the attenuation notch at λ = 1426nm increases gradually until it reaches the maximum value of 16.4dB at the grating length of L = 1.08mm. The resulted 3dB bandwidth is around 35nm. The strength of the attenuation notch decreases with the further increase of the grating period number, due to the back-coupling of light energy from the coiled microfiber to the straight microfiber. During the variation of period number, the resonance wavelength keeps almost unchanged. The insertion loss of the device, including the transmission loss of the microfiber, is less than 0.5dB.

Figure 2(b) shows the evolution of transmission spectrum in the microfiber with an increase in period number for a LPG with d1 = 5.8μm, d2 = 3.4μm, and Λ = 112μm. The attenuation dip corresponds to the resonant coupling between the LP01 modes for the straight and the coiled microfibers. Comparison between Figs. 2(a) and 2(b) shows that when the straight microfiber diameter decreases, the outer microfiber coil can induce a much stronger refractive-index modulation through the increased evanescent fields. An attenuation notch of 16.2dB is achieved with 4 periods only, resulting in a compact LPG with length of 450μm. The grating length is much shorter than the conventional ones in normal size fibers [1

1. A. M. Vengsarkar, P. J. Lemaire, J. B. Judkins, V. Bhatia, T. Erdogan, and J. E. Sipe, “Long–period fiber gratings as band–rejection filters,” J. Lightwave Technol. 14(1), 58–65 (1996). [CrossRef]

].

Apart from acting as the index modulation in the composite LPG, the coiled microfiber itself can guide light with large evanescent fields. When light is resonantly coupled from the fundamental mode for the straight microfiber to that of the coiled microfiber, light can be readily dropped down at the coiled microfiber output. Figure 3
Fig. 3 Transmission characteristics of the LPG with d1 = 5.8μm, d2 = 3.4μm, Λ = 112μm, and N = 4.
shows the transmission characteristics of the LPG with d1 = 5.8μm, d2 = 3.4μm, Λ = 112μm, and N = 4, where the green solid line is the transmission spectrum from A to D as shown in Fig. 1(a), the black dash-and-dot line denotes the transmission spectrum from A to C. It is clear that, the outputs from port C and port D are complementary to each other. One shows band-rejection characteristics and the other shows band-pass characteristics. The insertion loss from A to D is smaller than 0.3 dB, which may be attributed to the non-coupled mode remaining in the central microfiber.

As a broadband light is launched in the structure from port B, the light that satisfies the resonant conditions can be coupled into the central microfiber through the evanescent field interaction. The transmission spectrum from B to C, shown as the red dot line in Fig. 3, is similar to that from A to D, which is consistent to the reversibility of light since the coupling system is left-right symmetrical, as shown in Fig. 1(a). The insertion loss of transmission from port B to C is 0.8dB only. The small difference between the spectrum from A to D and that from B to C may be induced by the dissimilarity of leakage loss between the transition regions of microfibers near the respective input/output ports. Experiment also indicates that the transmission spectrum from B to D is a little bit similar to that from A to C. But the transmission loss from B to D is extremely large (>15dB, close to the background loss of the spectrum analyzer detection). The loss can be attributed to the radiation of light from the coiled microfiber to the unguided modes of the straight microfiber. In a transmission system, if a signal power is attenuated by 15 dB, the signal is no longer deemed useful.

3. Spectral response to external strain

By stretching the straight microfiber but keeping the coiled microfiber in a free state, we measure the spectral responses of the LPG to the axial strain. Figure 4
Fig. 4 (a) Resonance wavelength as a function of the axial strain. (b) The transmission spectra corresponding to the strains: 0 με (solid curve), 5150 με (broken curve), and 10300με (dashed curve).
gives the resonance wavelength as a function of the strain in the uniform microfiber, for the structure with d1 = 8.6μm, d2 = 2.8μm, Λ = 140μm, and L = 1.62mm. The dots in Fig. 4 are the experimentally measured results and the solid line is the linear fitting result. For the strain varying from 0 to 10300με, the resonance wavelength shifts from 1580 to 1474nm, producing an average tuning efficient of around −10.6pm/με. As the grating is stretched, the resonance wavelength, as determined by the phase-matching condition in Eq. (1), can be blueshifted (Fig. 4). The change of the grating pitch may affect the dissimilar propagation constants of the two microfibers via the inclination angle θ. On the other hand, it has been argued the waveguide dispersion can have a strong influence on the strain coefficient [13

13. X. W. Shu, L. Zhang, and I. Bennion, “Sensitivity characteristics of long-period fiber gratings,” J. Lightwave Technol. 20(2), 255–266 (2002). [CrossRef]

]. From [14

14. H. F. Xuan, W. Jin, and M. Zhang, “CO2 laser induced long period gratings in optical microfibers,” Opt. Express 17(24), 21882–21890 (2009), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-24-21882. [CrossRef] [PubMed]

, 15

15. H. F. Xuan, W. Jin, and S. J. Liu, “Long-period gratings in wavelength-scale microfibers,” Opt. Lett. 35(1), 85–87 (2010). [CrossRef] [PubMed]

], the waveguide dispersion gives dλ/dΛ<0 for the LPGs in microfibers, which may be extended to explain that the strain coefficient is negative for the present case.

The strain coefficient is similar to that of the resonance at the coupling of the higher order modes in the conventional long-period fiber gratings [13

13. X. W. Shu, L. Zhang, and I. Bennion, “Sensitivity characteristics of long-period fiber gratings,” J. Lightwave Technol. 20(2), 255–266 (2002). [CrossRef]

]. Compared with the conventional ones, the microfiber LPGs can have the larger wavelength tuning range thanks to the good stretchability of the micro-size fibers. The whole experiment has been carried out in over 30 minutes. At the beginning of the fiber stretching, the force associated with strain is weaker than that associated with surface forces. The dip wavelength can shift almost linearly with the change of the axial strain. After the strain is unloaded the spectrum can be recovered very well, as shown in Fig. 4, suggesting that the two microfibers are kept in good contact with each other in this process. But as we further increase the axial strain sufficiently, the force associated with strain become stronger than that associated with surface forces. As a result of this, both the grating pitch and the dip wavelength are unchanged as the fiber is stretched. Investigation shows that the maximum wavelength tuning range of the LPG is ~106nm. In order to widen the wavelength tuning range, the LPG should be packaged so that the relative position between coiled and the straight microfiber keep unchanged in the external strain loading.

4. Conclusion

In conclusion, we demonstrate a structural LPG by helically coiling a thinner microfiber on a thicker microfiber. Transmission notch of ~16.2dB is achieved in a device length of around 450μm only. The insertion loss of the device can be smaller than 0.5dB. Moreover, the coiled microfiber can resonantly lead out the light signal being transmitted in the microfiber. This novel LPG device has distinctive advantages of compactness and flexibility for future applications as optical filters and sensors.

Acknowledgments

This work was supported by National Natural Science Foundation of China (Grant No. 11004085, and 1110411), the Research Fund for the Doctoral Program of Higher Education (Grant No. 20114401110006), and the Fundamental Research Funds for the Central Universities (Grant No. 21609102).

References and links

1.

A. M. Vengsarkar, P. J. Lemaire, J. B. Judkins, V. Bhatia, T. Erdogan, and J. E. Sipe, “Long–period fiber gratings as band–rejection filters,” J. Lightwave Technol. 14(1), 58–65 (1996). [CrossRef]

2.

S. W. James and R. P. Tatam, “Optical fiber long–period gratings sensors:characteristics and application,” Meas. Sci. Technol. 14(5), 49–61 (2003). [CrossRef]

3.

Y. J. Rao, Y. P. Wang, Z. L. Ran, and T. Zhu, “Novel fiber-optic sensors based on long-period fiber gratings written by high-frequency CO2 laser pulses,” J. Lightwave Technol. 21(5), 1320–1327 (2003). [CrossRef]

4.

I. K. Hwang, S. H. Yun, and B. Y. Kim, “Long-period fiber gratings based on periodic microbends,” Opt. Lett. 24(18), 1263–1265 (1999). [CrossRef] [PubMed]

5.

S. Savin, M. J. F. Digonnet, G. S. Kino, and H. J. Shaw, “Tunable mechanically induced long-period fiber gratings,” Opt. Lett. 25(10), 710–712 (2000). [CrossRef] [PubMed]

6.

S. Oh, K. R. Lee, U. C. Paek, and Y. Chung, “Fabrication of helical long-period fiber gratings by use of a CO2 laser,” Opt. Lett. 29(13), 1464–1466 (2004). [CrossRef] [PubMed]

7.

C. Jáuregui and J. M. López-Higuera, “Virtual long-period gratings,” Opt. Lett. 30(1), 14–16 (2005). [CrossRef] [PubMed]

8.

V. I. Karpov, M. V. Grekov, E. M. Dianov, K. M. Golant, S. A. Vasiliev, O. I. Medvekov, and R. R. Khrapko, “Mode-field converters and long-period gratings fabricated by thermo-diffusion in nitrogen-doped silica-core fibers,” in Digest of European Conference on Optical Communication (Institute of Electrical Engineers, London, 1997), paper 2–56.

9.

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

10.

G. Brambilla, “Optical fibre nanowires and microwires: a review,” J. Opt. 12(4), 1–19 (2010). [CrossRef]

11.

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: fibrication and applications,” Adv. Opt. Photon. 1(1), 107–161 (2009). [CrossRef]

12.

J. D. Love, W. M. Henry, W. J. Stewart, R. J. Black, S. Lacroix, and F. Gonthier, “Tapered single-mode fibres and devices Part 1: Adiabaticity criteria,” IEEE Proc. J.: Optoelectron. 138, 343–354 (1991).

13.

X. W. Shu, L. Zhang, and I. Bennion, “Sensitivity characteristics of long-period fiber gratings,” J. Lightwave Technol. 20(2), 255–266 (2002). [CrossRef]

14.

H. F. Xuan, W. Jin, and M. Zhang, “CO2 laser induced long period gratings in optical microfibers,” Opt. Express 17(24), 21882–21890 (2009), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-24-21882. [CrossRef] [PubMed]

15.

H. F. Xuan, W. Jin, and S. J. Liu, “Long-period gratings in wavelength-scale microfibers,” Opt. Lett. 35(1), 85–87 (2010). [CrossRef] [PubMed]

OCIS Codes
(050.2770) Diffraction and gratings : Gratings
(060.2340) Fiber optics and optical communications : Fiber optics components
(230.3990) Optical devices : Micro-optical devices

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: May 14, 2012
Revised Manuscript: July 3, 2012
Manuscript Accepted: July 6, 2012
Published: July 23, 2012

Citation
Li-Peng Sun, Jie Li, Long Jin, and Bai-Ou Guan, "Structural microfiber long-period gratings," Opt. Express 20, 18079-18084 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-16-18079


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References

  1. A. M. Vengsarkar, P. J. Lemaire, J. B. Judkins, V. Bhatia, T. Erdogan, and J. E. Sipe, “Long–period fiber gratings as band–rejection filters,” J. Lightwave Technol.14(1), 58–65 (1996). [CrossRef]
  2. S. W. James and R. P. Tatam, “Optical fiber long–period gratings sensors:characteristics and application,” Meas. Sci. Technol.14(5), 49–61 (2003). [CrossRef]
  3. Y. J. Rao, Y. P. Wang, Z. L. Ran, and T. Zhu, “Novel fiber-optic sensors based on long-period fiber gratings written by high-frequency CO2 laser pulses,” J. Lightwave Technol.21(5), 1320–1327 (2003). [CrossRef]
  4. I. K. Hwang, S. H. Yun, and B. Y. Kim, “Long-period fiber gratings based on periodic microbends,” Opt. Lett.24(18), 1263–1265 (1999). [CrossRef] [PubMed]
  5. S. Savin, M. J. F. Digonnet, G. S. Kino, and H. J. Shaw, “Tunable mechanically induced long-period fiber gratings,” Opt. Lett.25(10), 710–712 (2000). [CrossRef] [PubMed]
  6. S. Oh, K. R. Lee, U. C. Paek, and Y. Chung, “Fabrication of helical long-period fiber gratings by use of a CO2 laser,” Opt. Lett.29(13), 1464–1466 (2004). [CrossRef] [PubMed]
  7. C. Jáuregui and J. M. López-Higuera, “Virtual long-period gratings,” Opt. Lett.30(1), 14–16 (2005). [CrossRef] [PubMed]
  8. V. I. Karpov, M. V. Grekov, E. M. Dianov, K. M. Golant, S. A. Vasiliev, O. I. Medvekov, and R. R. Khrapko, “Mode-field converters and long-period gratings fabricated by thermo-diffusion in nitrogen-doped silica-core fibers,” in Digest of European Conference on Optical Communication (Institute of Electrical Engineers, London, 1997), paper 2–56.
  9. L. M. Tong, R. R. Gattass, J. B. Ashcom, S. L. He, J. Y. Lou, M. Y. Shen, I. Maxwell, and E. Mazur, “Subwavelength-diameter silica wires for low-loss optical wave guiding,” Nature426(6968), 816–819 (2003). [CrossRef] [PubMed]
  10. G. Brambilla, “Optical fibre nanowires and microwires: a review,” J. Opt.12(4), 1–19 (2010). [CrossRef]
  11. 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: fibrication and applications,” Adv. Opt. Photon.1(1), 107–161 (2009). [CrossRef]
  12. J. D. Love, W. M. Henry, W. J. Stewart, R. J. Black, S. Lacroix, and F. Gonthier, “Tapered single-mode fibres and devices Part 1: Adiabaticity criteria,” IEEE Proc. J.: Optoelectron.138, 343–354 (1991).
  13. X. W. Shu, L. Zhang, and I. Bennion, “Sensitivity characteristics of long-period fiber gratings,” J. Lightwave Technol.20(2), 255–266 (2002). [CrossRef]
  14. H. F. Xuan, W. Jin, and M. Zhang, “CO2 laser induced long period gratings in optical microfibers,” Opt. Express17(24), 21882–21890 (2009), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-24-21882 . [CrossRef] [PubMed]
  15. H. F. Xuan, W. Jin, and S. J. Liu, “Long-period gratings in wavelength-scale microfibers,” Opt. Lett.35(1), 85–87 (2010). [CrossRef] [PubMed]

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