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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 4 — Feb. 14, 2011
  • pp: 3124–3129
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In-fiber modal Mach-Zehnder interferometer based on the locally post-processed core of a photonic crystal fiber

Rodrigo M. Gerosa, Danilo H. Spadoti, Leonardo de S. Menezes, and Christiano J. S. de Matos  »View Author Affiliations


Optics Express, Vol. 19, Issue 4, pp. 3124-3129 (2011)
http://dx.doi.org/10.1364/OE.19.003124


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Abstract

We demonstrate a novel, compact and low-loss photonic crystal fiber modal Mach-Zehnder interferometer with potential applications to sensing and WDM telecommunications. By selectively collapsing a ~1-mm-long section of a hole next to the solid core, a pair of modes of the post-processed structure are excited and interfere at its exit. A modulation depth of up to ~13 dB and an insertion loss as low as 2.8 dB were achieved. A temperature sensitivity of −53.4 pm/°C was measured, making the device suitable for temperature sensing.

© 2011 OSA

1. Introduction

Since their first demonstration, photonic crystal fibers (PCFs) have made possible the development of new optical devices for a wide range of areas, such as sensing, metrology and optical communications [1

1. P. St. J. Russell, “Photonic-Crystal Fibers,” J. Lightwave Technol. 24(12), 4729–4749 (2006). [CrossRef]

]. These devices are a direct consequence of the unique optical properties of PCFs, which, in turn, derive from their distinct waveguide geometry. Recently, many PCF post-processing techniques have been developed [2

2. H. C. Nguyen, B. T. Kuhlmey, E. C. Magi, M. J. Steel, P. Domachuk, C. L. Smith, and B. J. Eggleton, “Tapered photonic crystal fibers: Properties, characterization and applications,” Appl. Phys. B 81(2-3), 377–387 (2005). [CrossRef]

8

8. H. Y. Choi, K. S. Park, and B. H. Lee, “Photonic crystal fiber interferometer composed of a long period fiber grating and one point collapsing of air holes,” Opt. Lett. 33(8), 812–814 (2008). [CrossRef] [PubMed]

], which has allowed for this geometry to be locally and permanently altered, thus enabling fiber structures that are optimized for specific goals. Among these structures are all-fiber Mach-Zehnder interferometers [6

6. J. Villatoro, V. P. Minkovich, and D. Monzón-Hernández, “Compact Modal Interferometer Built with Tapered Microstructured Optical Fiber,” IEEE Photon. Technol. Lett. 18(11), 1258–1260 (2006). [CrossRef]

17

17. J. Villatoro, M. P. Kreuzer, R. Jha, V. P. Minkovich, V. Finazzi, G. Badenes, and V. Pruneri, “Photonic crystal fiber interferometer for chemical vapor detection with high sensitivity,” Opt. Express 17(3), 1447–1453 (2009). [CrossRef] [PubMed]

], in which orthogonal modes play the role of the interferometer’s branches. In recent years, PCF-based modal interferometers have been actively studied and proposed for a wide range applications such as signal demodulation in optical communications [10

10. J. Du, Y. Dai, G. K. P. Lei, W. Tong, and C. Shu, “Photonic crystal fiber based Mach-Zehnder interferometer for DPSK signal demodulation,” Opt. Express 18(8), 7917–7922 (2010). [CrossRef] [PubMed]

] and strain [5

5. K. Lai, S. G. Leon-Saval, A. Witkowska, W. J. Wadsworth, and T. A. Birks, “Wavelength-independent all-fiber mode converters,” Opt. Lett. 32(4), 328–330 (2007). [CrossRef] [PubMed]

8

8. H. Y. Choi, K. S. Park, and B. H. Lee, “Photonic crystal fiber interferometer composed of a long period fiber grating and one point collapsing of air holes,” Opt. Lett. 33(8), 812–814 (2008). [CrossRef] [PubMed]

,11

11. H. Y. Choi, M. J. Kim, and B. H. Lee, “All-fiber Mach-Zehnder type interferometers formed in photonic crystal fiber,” Opt. Express 15(9), 5711–5720 (2007). [CrossRef] [PubMed]

14

14. S. H. Aref, R. Amezcua-Correa, J. P. Carvalho, O. Frazão, P. Caldas, J. L. Santos, F. M. Araújo, H. Latifi, F. Farahi, L. A. Ferreira, and J. C. Knight, “Modal interferometer based on hollow-core photonic crystal fiber for strain and temperature measurement,” Opt. Express 17(21), 18669–18675 (2009). [CrossRef]

], temperature [14

14. S. H. Aref, R. Amezcua-Correa, J. P. Carvalho, O. Frazão, P. Caldas, J. L. Santos, F. M. Araújo, H. Latifi, F. Farahi, L. A. Ferreira, and J. C. Knight, “Modal interferometer based on hollow-core photonic crystal fiber for strain and temperature measurement,” Opt. Express 17(21), 18669–18675 (2009). [CrossRef]

,15

15. G. Coviello, V. Finazzi, J. Villatoro, and V. Pruneri, “Thermally stabilized PCF-based sensor for temperature measurements up to 1000°C,” Opt. Express 17(24), 21551–21559 (2009). [CrossRef] [PubMed]

], refractive index [13

13. W. J. Bock, T. A. Eftimov, P. Mikulic, and J. Chen, “An inline core-cladding intermodal interferometer using a photonic crystal fiber,” J. Lightwave Technol. 27(17), 3933–3939 (2009). [CrossRef]

,16

16. R. Jha, J. Villatoro, G. Badenes, and V. Pruneri, “Refractometry based on a photonic crystal fiber interferometer,” Opt. Lett. 34(5), 617–619 (2009). [CrossRef] [PubMed]

], pressure [13

13. W. J. Bock, T. A. Eftimov, P. Mikulic, and J. Chen, “An inline core-cladding intermodal interferometer using a photonic crystal fiber,” J. Lightwave Technol. 27(17), 3933–3939 (2009). [CrossRef]

] and chemical [17

17. J. Villatoro, M. P. Kreuzer, R. Jha, V. P. Minkovich, V. Finazzi, G. Badenes, and V. Pruneri, “Photonic crystal fiber interferometer for chemical vapor detection with high sensitivity,” Opt. Express 17(3), 1447–1453 (2009). [CrossRef] [PubMed]

] sensing. This type of interferometer has long been investigated in conventional fibers [18

18. B. H. Lee and J. Nishii, “Dependence of fringe spacing on the grating separation in a long-period fiber grating pair,” Appl. Opt. 38(16), 3450–3459 (1999). [CrossRef]

], with PCFs offering the advantage of a greater control over the characteristics of the excited modes and improved sensitivity to measurands, in the case of sensor applications.

A powerful and versatile PCF post-processing technique, to date not exploited for the development of Mach-Zehnder interferometers, was demonstrated by Witkowska et al. [4

4. A. Witkowska, K. Lai, S. G. Leon-Saval, W. J. Wadsworth, and T. A. Birks, “All-fiber anamorphic core-shape transitions,” Opt. Lett. 31(18), 2672–2674 (2006). [CrossRef] [PubMed]

,5

5. K. Lai, S. G. Leon-Saval, A. Witkowska, W. J. Wadsworth, and T. A. Birks, “Wavelength-independent all-fiber mode converters,” Opt. Lett. 32(4), 328–330 (2007). [CrossRef] [PubMed]

]. It produces structural modifications to the PCF by submitting different microstructure holes to distinct air pressure levels during a tapering process. Holes that are subjected to lower pressures tend to collapse during tapering, giving rise to solid regions that can either work as additional cores or modify the pre-existent core shape. The selective expansion of holes has also been demonstrated using a similar approach [19

19. J. Ju, H. F. Xuan, W. Jin, S. Liu, and H. L. Ho, “Selective opening of airholes in photonic crystal fiber,” Opt. Lett. 35(23), 3886–3888 (2010). [CrossRef] [PubMed]

].

2. Post-processing method

Finally, the setup shown in Fig. 2
Fig. 2 Experimental setup used for collapsing the selected hole in the PCF microstructure.
was used to pressurize and locally heat the PCF. The PCF end with a single sealed hole was fixed to the tip of a syringe, which in turn was connected to an air compressor. This assembly was attached to a computer-controlled motorized uniaxial translation stage with a repeatable incremental movement of 0.8 μm. A section of the fiber, located a few centimeters from its end, was positioned at the fiber fusion splicer and the air compressor was turned on, producing a pressure increase of ~5 bar in all holes except the sealed one. The fiber splicer then produced an arc discharge that locally heated the PCF. The sealed hole, with a lower internal pressure then collapsed generating a solid region. The arc current, arc duration and air pressure were chosen so as to collapse only the sealed hole, while all other holes kept approximately their original dimensions.

This procedure allowed a channel length of ~200 μm to be collapsed. The motorized translation stage was then used to axially displace the PCF by 150-180 μm before the procedure was repeated. Individually post-processed regions were, therefore, stitched together, which made possible to obtain control over the device length. We observe that this method can be regarded as a simplification of the technique described in [4

4. A. Witkowska, K. Lai, S. G. Leon-Saval, W. J. Wadsworth, and T. A. Birks, “All-fiber anamorphic core-shape transitions,” Opt. Lett. 31(18), 2672–2674 (2006). [CrossRef] [PubMed]

,5

5. K. Lai, S. G. Leon-Saval, A. Witkowska, W. J. Wadsworth, and T. A. Birks, “Wavelength-independent all-fiber mode converters,” Opt. Lett. 32(4), 328–330 (2007). [CrossRef] [PubMed]

], in which fiber tapering is not necessary (because the required fiber modification is sufficiently small) and a conventional fiber fusion splicer replaces the flame as a heat source. After cleaving both fiber tips, the post-processed PCF was optically characterized.

3. Interferometer structure and analysis

When the mode of the original PCF reaches the modified region it ceases to be a waveguide mode, becoming a superposition of modes of the new structure. As the cross section of the original PCF core overlaps with only one half of that of the modified core, excitation of a few low order modes is achieved. These modes then evolve independently with different propagation constants. At the end of the post-processed region, if the accumulated phase difference is a multiple of 2π, the original transverse intensity distribution is recovered and light can couple back into the pristine PCF core. Otherwise, part or all the light is lost to the cladding. The two limits of the modified core then naturally act as efficient beam splitters and combiners, resulting in low interferometer insertion losses.

4. Device characterization

The post-processed PCF was characterized with the setup shown in Fig. 3
Fig. 3 Experimental setup for optically characterizing the post-processed PCF.
, in which the radiation from a commercial infrared (800-1700 nm) supercontinuum light source (Toptica FFS-Cont) was coupled to one end of the PCF via an objective lens, being collected at the opposite end with another objective lens and a singlemode fiber (SMF) connected to an optical spectrum analyzer (OSA).

The transmission spectrum obtained with a post-processed PCF with a 1.5-mm-long collapsed hole is shown in Fig. 4(a)
Fig. 4 Transmission spectra of PCFs with 1.5-mm (a) and 0.75-mm (b) long collapsed holes. (c) Light at the output of a PCF cleaved at the modified region and showing light propagation along the original core as well as the collapsed region.
. As expected, a modulated spectrum was obtained, presenting a 17-nm periodicity and a modulation depth of up to 9.5 dB. A single periodicity was observed to dominate over the whole measured spectrum (from at least 900 to 1600 nm), which indicates that two modes are preferentially excited within the interferometer. With the use of Eq. (2) a modal index difference of ~4 × 10−2 is calculated, which is of the same order of magnitude of those found in some other PCF interferometers that use core modes [15

15. G. Coviello, V. Finazzi, J. Villatoro, and V. Pruneri, “Thermally stabilized PCF-based sensor for temperature measurements up to 1000°C,” Opt. Express 17(24), 21551–21559 (2009). [CrossRef] [PubMed]

]. As in [15

15. G. Coviello, V. Finazzi, J. Villatoro, and V. Pruneri, “Thermally stabilized PCF-based sensor for temperature measurements up to 1000°C,” Opt. Express 17(24), 21551–21559 (2009). [CrossRef] [PubMed]

], the relatively high value of ∆n may indicate that one of the coupled modes is beyond its cutoff, but presents a sufficiently low loss along the short device length. Indeed, the modulation depth was observed to reduce for longer wavelengths (in some cases 0.5-dB depth observed at 1550 nm), for which the mode would be further away from cutoff.

Figure 4(b) shows the transmission spectrum of a post-processed PCF with half the collapsed length (0.75 mm). As expected, a longer modulation period is observed (~31 nm), which is reasonably close to the 34-nm period expected from Eq. (2) and the calculated Δn. The improved modulation depth of 12.7-dB with the shorter interferometer is consistent with the assumption that the higher order mode is beyond cutoff but may also be related to the limited axial homogeneity of the collapsed hole obtained with the stitching technique.

At the spectral range shown in Figs. 4(a) and 4(b), the measured modulation depth is suitable for both sensing and filtering applications, comparing favorably with those observed in a number of reported PCF interferometers [7

7. J. Villatoro, V. P. Minkovich, V. Pruneri, and G. Badenes, “Simple all-microstructured-optical-fiber interferometer built via fusion splicing,” Opt. Express 15(4), 1491–1496 (2007). [CrossRef] [PubMed]

,9

9. J. H. Lim, H. S. Jang, K. S. Lee, J. C. Kim, and B. H. Lee, “Mach-Zehnder interferometer formed in a photonic crystal fiber based on a pair of long-period fiber gratings,” Opt. Lett. 29(4), 346–348 (2004). [CrossRef] [PubMed]

,11

11. H. Y. Choi, M. J. Kim, and B. H. Lee, “All-fiber Mach-Zehnder type interferometers formed in photonic crystal fiber,” Opt. Express 15(9), 5711–5720 (2007). [CrossRef] [PubMed]

,13

13. W. J. Bock, T. A. Eftimov, P. Mikulic, and J. Chen, “An inline core-cladding intermodal interferometer using a photonic crystal fiber,” J. Lightwave Technol. 27(17), 3933–3939 (2009). [CrossRef]

,14

14. S. H. Aref, R. Amezcua-Correa, J. P. Carvalho, O. Frazão, P. Caldas, J. L. Santos, F. M. Araújo, H. Latifi, F. Farahi, L. A. Ferreira, and J. C. Knight, “Modal interferometer based on hollow-core photonic crystal fiber for strain and temperature measurement,” Opt. Express 17(21), 18669–18675 (2009). [CrossRef]

] (the highest reported depths are of the order of 18 dB [10

10. J. Du, Y. Dai, G. K. P. Lei, W. Tong, and C. Shu, “Photonic crystal fiber based Mach-Zehnder interferometer for DPSK signal demodulation,” Opt. Express 18(8), 7917–7922 (2010). [CrossRef] [PubMed]

]). Figure 4(c) shows an optical image of light as it couples out from another PCF, which was cleaved at the modified region. It is seen that light is guided both via the original core and via the collapsed hole.

The optical transmission spectrum of the 10-mm-long sample (not shown) exhibits a lower modulation depth (~2 dB) and a less regular periodicity. Although the lower depth is in line with the assumption that one of the coupled modes is beyond cutoff, and agrees with the result reported in [14

14. S. H. Aref, R. Amezcua-Correa, J. P. Carvalho, O. Frazão, P. Caldas, J. L. Santos, F. M. Araújo, H. Latifi, F. Farahi, L. A. Ferreira, and J. C. Knight, “Modal interferometer based on hollow-core photonic crystal fiber for strain and temperature measurement,” Opt. Express 17(21), 18669–18675 (2009). [CrossRef]

], both these spectral features are also believed to be a consequence of the lack of axial homogeneity. Work is under way to improve the post-processing system, but for high homogeneity the flame-brushing technique may be necessary. In any case, the interferometer is observed to present a better performance for short post-processed lengths, for which the use of the splicer is demonstrated to be suitable.

4.1-Insertion loss

The insertion loss of the modified PCF structure was determined around a wavelength of 1550 nm by tuning an external cavity semiconductor laser to one of the interferometer transmission maxima and by measuring its power in the PCF after and, subsequently, before the post-processed region. Measured losses mounted to ~2.8 dB, which are significantly lower than those of most PCF interferometers, as insertion losses of up to ~9-10 dB have been reported [7

7. J. Villatoro, V. P. Minkovich, V. Pruneri, and G. Badenes, “Simple all-microstructured-optical-fiber interferometer built via fusion splicing,” Opt. Express 15(4), 1491–1496 (2007). [CrossRef] [PubMed]

,10

10. J. Du, Y. Dai, G. K. P. Lei, W. Tong, and C. Shu, “Photonic crystal fiber based Mach-Zehnder interferometer for DPSK signal demodulation,” Opt. Express 18(8), 7917–7922 (2010). [CrossRef] [PubMed]

,15

15. G. Coviello, V. Finazzi, J. Villatoro, and V. Pruneri, “Thermally stabilized PCF-based sensor for temperature measurements up to 1000°C,” Opt. Express 17(24), 21551–21559 (2009). [CrossRef] [PubMed]

,17

17. J. Villatoro, M. P. Kreuzer, R. Jha, V. P. Minkovich, V. Finazzi, G. Badenes, and V. Pruneri, “Photonic crystal fiber interferometer for chemical vapor detection with high sensitivity,” Opt. Express 17(3), 1447–1453 (2009). [CrossRef] [PubMed]

]. The low insertion loss, together with compactness, are the main achievements of our device. Further loss reduction may be possible if its axial homogeneity is improved.

4.2-Temperature dependence

5. Conclusions

Acknowledgments

This work is supported by CNPq (including support via Rede de Nanofotônica and INCT FOTONICOM), CAPES/PROCAD, FAPESP, and Fundo Mackenzie de Pesquisa. The scanning electron micrograph of Fig. 1(b) was kindly provided by C. R. Biazoli.

References and links

1.

P. St. J. Russell, “Photonic-Crystal Fibers,” J. Lightwave Technol. 24(12), 4729–4749 (2006). [CrossRef]

2.

H. C. Nguyen, B. T. Kuhlmey, E. C. Magi, M. J. Steel, P. Domachuk, C. L. Smith, and B. J. Eggleton, “Tapered photonic crystal fibers: Properties, characterization and applications,” Appl. Phys. B 81(2-3), 377–387 (2005). [CrossRef]

3.

H. Bartelt, H. Lehmann, R. Willsch, R. Spittel, J. Mörbitz, and M. Balakrishnan, “Enhanced functionality by selective filling of microstructured optical fibers,” Proc. SPIE 7839, 783909, 783909-4 (2010). [CrossRef]

4.

A. Witkowska, K. Lai, S. G. Leon-Saval, W. J. Wadsworth, and T. A. Birks, “All-fiber anamorphic core-shape transitions,” Opt. Lett. 31(18), 2672–2674 (2006). [CrossRef] [PubMed]

5.

K. Lai, S. G. Leon-Saval, A. Witkowska, W. J. Wadsworth, and T. A. Birks, “Wavelength-independent all-fiber mode converters,” Opt. Lett. 32(4), 328–330 (2007). [CrossRef] [PubMed]

6.

J. Villatoro, V. P. Minkovich, and D. Monzón-Hernández, “Compact Modal Interferometer Built with Tapered Microstructured Optical Fiber,” IEEE Photon. Technol. Lett. 18(11), 1258–1260 (2006). [CrossRef]

7.

J. Villatoro, V. P. Minkovich, V. Pruneri, and G. Badenes, “Simple all-microstructured-optical-fiber interferometer built via fusion splicing,” Opt. Express 15(4), 1491–1496 (2007). [CrossRef] [PubMed]

8.

H. Y. Choi, K. S. Park, and B. H. Lee, “Photonic crystal fiber interferometer composed of a long period fiber grating and one point collapsing of air holes,” Opt. Lett. 33(8), 812–814 (2008). [CrossRef] [PubMed]

9.

J. H. Lim, H. S. Jang, K. S. Lee, J. C. Kim, and B. H. Lee, “Mach-Zehnder interferometer formed in a photonic crystal fiber based on a pair of long-period fiber gratings,” Opt. Lett. 29(4), 346–348 (2004). [CrossRef] [PubMed]

10.

J. Du, Y. Dai, G. K. P. Lei, W. Tong, and C. Shu, “Photonic crystal fiber based Mach-Zehnder interferometer for DPSK signal demodulation,” Opt. Express 18(8), 7917–7922 (2010). [CrossRef] [PubMed]

11.

H. Y. Choi, M. J. Kim, and B. H. Lee, “All-fiber Mach-Zehnder type interferometers formed in photonic crystal fiber,” Opt. Express 15(9), 5711–5720 (2007). [CrossRef] [PubMed]

12.

J. Villatoro, V. Finazzi, V. P. Minkovich, V. Pruneri, and G. Badenes, “Temperature-insensitive photonic crystal fiber interferometer for absolute strain sensing,” Appl. Phys. Lett. 91(9), 091109 (2007). [CrossRef]

13.

W. J. Bock, T. A. Eftimov, P. Mikulic, and J. Chen, “An inline core-cladding intermodal interferometer using a photonic crystal fiber,” J. Lightwave Technol. 27(17), 3933–3939 (2009). [CrossRef]

14.

S. H. Aref, R. Amezcua-Correa, J. P. Carvalho, O. Frazão, P. Caldas, J. L. Santos, F. M. Araújo, H. Latifi, F. Farahi, L. A. Ferreira, and J. C. Knight, “Modal interferometer based on hollow-core photonic crystal fiber for strain and temperature measurement,” Opt. Express 17(21), 18669–18675 (2009). [CrossRef]

15.

G. Coviello, V. Finazzi, J. Villatoro, and V. Pruneri, “Thermally stabilized PCF-based sensor for temperature measurements up to 1000°C,” Opt. Express 17(24), 21551–21559 (2009). [CrossRef] [PubMed]

16.

R. Jha, J. Villatoro, G. Badenes, and V. Pruneri, “Refractometry based on a photonic crystal fiber interferometer,” Opt. Lett. 34(5), 617–619 (2009). [CrossRef] [PubMed]

17.

J. Villatoro, M. P. Kreuzer, R. Jha, V. P. Minkovich, V. Finazzi, G. Badenes, and V. Pruneri, “Photonic crystal fiber interferometer for chemical vapor detection with high sensitivity,” Opt. Express 17(3), 1447–1453 (2009). [CrossRef] [PubMed]

18.

B. H. Lee and J. Nishii, “Dependence of fringe spacing on the grating separation in a long-period fiber grating pair,” Appl. Opt. 38(16), 3450–3459 (1999). [CrossRef]

19.

J. Ju, H. F. Xuan, W. Jin, S. Liu, and H. L. Ho, “Selective opening of airholes in photonic crystal fiber,” Opt. Lett. 35(23), 3886–3888 (2010). [CrossRef] [PubMed]

OCIS Codes
(120.3180) Instrumentation, measurement, and metrology : Interferometry
(060.5295) Fiber optics and optical communications : Photonic crystal fibers

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: January 3, 2011
Revised Manuscript: January 29, 2011
Manuscript Accepted: January 29, 2011
Published: February 2, 2011

Citation
Rodrigo M. Gerosa, Danilo H. Spadoti, Leonardo de S. Menezes, and Christiano J. S. de Matos, "In-fiber modal Mach-Zehnder interferometer based on the locally post-processed core of a photonic crystal fiber," Opt. Express 19, 3124-3129 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-4-3124


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References

  1. P. St. J. Russell, “Photonic-Crystal Fibers,” J. Lightwave Technol. 24(12), 4729–4749 (2006). [CrossRef]
  2. H. C. Nguyen, B. T. Kuhlmey, E. C. Magi, M. J. Steel, P. Domachuk, C. L. Smith, and B. J. Eggleton, “Tapered photonic crystal fibers: Properties, characterization and applications,” Appl. Phys. B 81(2-3), 377–387 (2005). [CrossRef]
  3. H. Bartelt, H. Lehmann, R. Willsch, R. Spittel, J. Mörbitz, and M. Balakrishnan, “Enhanced functionality by selective filling of microstructured optical fibers,” Proc. SPIE 7839, 783909, 783909-4 (2010). [CrossRef]
  4. A. Witkowska, K. Lai, S. G. Leon-Saval, W. J. Wadsworth, and T. A. Birks, “All-fiber anamorphic core-shape transitions,” Opt. Lett. 31(18), 2672–2674 (2006). [CrossRef] [PubMed]
  5. K. Lai, S. G. Leon-Saval, A. Witkowska, W. J. Wadsworth, and T. A. Birks, “Wavelength-independent all-fiber mode converters,” Opt. Lett. 32(4), 328–330 (2007). [CrossRef] [PubMed]
  6. J. Villatoro, V. P. Minkovich, and D. Monzón-Hernández, “Compact Modal Interferometer Built with Tapered Microstructured Optical Fiber,” IEEE Photon. Technol. Lett. 18(11), 1258–1260 (2006). [CrossRef]
  7. J. Villatoro, V. P. Minkovich, V. Pruneri, and G. Badenes, “Simple all-microstructured-optical-fiber interferometer built via fusion splicing,” Opt. Express 15(4), 1491–1496 (2007). [CrossRef] [PubMed]
  8. H. Y. Choi, K. S. Park, and B. H. Lee, “Photonic crystal fiber interferometer composed of a long period fiber grating and one point collapsing of air holes,” Opt. Lett. 33(8), 812–814 (2008). [CrossRef] [PubMed]
  9. J. H. Lim, H. S. Jang, K. S. Lee, J. C. Kim, and B. H. Lee, “Mach-Zehnder interferometer formed in a photonic crystal fiber based on a pair of long-period fiber gratings,” Opt. Lett. 29(4), 346–348 (2004). [CrossRef] [PubMed]
  10. J. Du, Y. Dai, G. K. P. Lei, W. Tong, and C. Shu, “Photonic crystal fiber based Mach-Zehnder interferometer for DPSK signal demodulation,” Opt. Express 18(8), 7917–7922 (2010). [CrossRef] [PubMed]
  11. H. Y. Choi, M. J. Kim, and B. H. Lee, “All-fiber Mach-Zehnder type interferometers formed in photonic crystal fiber,” Opt. Express 15(9), 5711–5720 (2007). [CrossRef] [PubMed]
  12. J. Villatoro, V. Finazzi, V. P. Minkovich, V. Pruneri, and G. Badenes, “Temperature-insensitive photonic crystal fiber interferometer for absolute strain sensing,” Appl. Phys. Lett. 91(9), 091109 (2007). [CrossRef]
  13. W. J. Bock, T. A. Eftimov, P. Mikulic, and J. Chen, “An inline core-cladding intermodal interferometer using a photonic crystal fiber,” J. Lightwave Technol. 27(17), 3933–3939 (2009). [CrossRef]
  14. S. H. Aref, R. Amezcua-Correa, J. P. Carvalho, O. Frazão, P. Caldas, J. L. Santos, F. M. Araújo, H. Latifi, F. Farahi, L. A. Ferreira, and J. C. Knight, “Modal interferometer based on hollow-core photonic crystal fiber for strain and temperature measurement,” Opt. Express 17(21), 18669–18675 (2009). [CrossRef]
  15. G. Coviello, V. Finazzi, J. Villatoro, and V. Pruneri, “Thermally stabilized PCF-based sensor for temperature measurements up to 1000°C,” Opt. Express 17(24), 21551–21559 (2009). [CrossRef] [PubMed]
  16. R. Jha, J. Villatoro, G. Badenes, and V. Pruneri, “Refractometry based on a photonic crystal fiber interferometer,” Opt. Lett. 34(5), 617–619 (2009). [CrossRef] [PubMed]
  17. J. Villatoro, M. P. Kreuzer, R. Jha, V. P. Minkovich, V. Finazzi, G. Badenes, and V. Pruneri, “Photonic crystal fiber interferometer for chemical vapor detection with high sensitivity,” Opt. Express 17(3), 1447–1453 (2009). [CrossRef] [PubMed]
  18. B. H. Lee and J. Nishii, “Dependence of fringe spacing on the grating separation in a long-period fiber grating pair,” Appl. Opt. 38(16), 3450–3459 (1999). [CrossRef]
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