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Asymmetric mode coupling in arc-induced long-period fiber gratings
Optics Communications 364 (2016) 37–43
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Asymmetric mode coupling in arc-induced long-period fiber gratings A. Martinez-Rios a,n, I. Torres-Gomez a, G. Anzueto-Sanchez b, R. Selvas-Aguilar c, V.M. Duran-Ramirez d, J.A. Guerrero-Viramontes e, D. Toral-Acosta c, G. Salceda Delgado c, A. Castillo-Guzman c a
Centro de Investigaciones en Óptica, Loma del Bosque 115, Col. Lomas del Campestre, 37150 León, Guanajuato, México Centro de Investigación en Ingeniería y Ciencias Aplicadas, CIICAp, Universidad Autónoma del Estado de Morelos, UAEM, Av. Universidad No. 1001, Col. Chamilpa, C.P. 62209 Cuernavaca, Morelos, México c Universidad Autónoma de Nuevo León, Facultad de Ciencias Físico-Matemáticas, Av. Universidad S/N, Cd. Universitaria, San Nicolás de los Garza, Nuevo León C.P. 66451, México d Universidad de Guadalajara, Centro Universitario de los Lagos, Enrique Díaz de León 1144, C.P. 47460 Lagos de Moreno, Jalisco, México e Instituto Tecnologico de Aguascalientes, Av. Adolfo Lopez Mateos 1801 Ote. Fracc. Bona Gens, C.P. 20256 Aguascalientes, Ags., México b
art ic l e i nf o
a b s t r a c t
Article history: Received 26 March 2015 Received in revised form 7 October 2015 Accepted 27 October 2015 Available online 21 November 2015
An extensive experimental study of the transverse modal field characteristics of mircrobend arc-induced long-period fiber gratings is presented. A wavelength scanning of the near-field intensity pattern inside each loss band in the transmission spectrum, shows a clear asymmetry in the transverse intensity distribution resulting from the fabrication method. This asymmetry reflects as a 10.7 dB difference in the notch depths for two orthogonal polarizations. Though a one year study, it was found that that environmental conditions during fabrication strongly affects the gratings characteristics. The best performance was obtained during the autumn season, where microbend arc-induced long-period fiber gratings produce wavelength filters with short lengths (between 10 and 30 periods for depths in excess of 20 dB) and the insertion loss may be as low as 0.12 dB. & 2015 Elsevier B.V. All rights reserved.
Keywords: Long-period fiber grating Arc-induced Modal pattern Polarization state
1. Introduction Long-period fiber gratings (LPFGs) are all-fiber devices which have demonstrated to be useful in many technological applications such as high temperature sensing [1], high sensitivity refractive index sensors [2], optical switching [3], etc. In essence, LPFGs are wavelength optical band-rejection filters whose operation relies on the selective coupling of the fundamental core mode towards cladding modes when a phase matching condition is satisfied [4]. This selective coupling produces notches on the transmission spectrum, and each notch may be associated with a specific cladding mode. The symmetry of the cladding modes depends strongly on the fabrication method, which in all cases involves the periodic perturbation of the refractive index of the core and/or the cladding of the optical fiber. For the particular case of LPFGs induced by an electric arc, mechanisms such as stress relief [5], microbending [6,7], and tapering [8–10], have been used for the induction of the periodic refractive index change. Rego and coworkers have realized extensive studies on the symmetry of the cladding modes excited in LPFGs fabricated by electric arc [9,10]. n
Corresponding author. E-mail address: [email protected] (A. Martinez-Rios).
http://dx.doi.org/10.1016/j.optcom.2015.10.061 0030-4018/& 2015 Elsevier B.V. All rights reserved.
They have demonstrated that factors such as the fiber type and fabrication method may impact the symmetry of the excited cladding modes, which may be symmetric and anti-symmetric. In addition, they found that this kind of LPFGs, based on periodic micro-tapering, experiences noticeable changes in the resonant wavelengths when the arc power or pulling tension is changed. On the other hand, in the case of micro-bend based arc-induced LPFGs, the resonance wavelength only depends on the grating period and is not affected by the coupling strength [7]. In Ref. [7], the resonant cladding modes of microbend based LPFGs are assumed to be associated with LP1n anti-symmetric modes. Recently, the fabrication of LPFGs by using a CO2 laser as the heat source has been subject of considerable interest. In particular, it is found that the refractive index perturbation caused by the CO2 laser source has a character highly asymmetric, so that, in general, the excited cladding modes are highly asymmetric, i.e., there is an asymmetric mode coupling [11]. In this work, by measuring the near field intensity patterns of arc-induced LPFGs as a function of wavelength, fabricated by using a misaligned setup, we show the asymmetric mode coupling of microbending arc-induced LPFGs. The asymmetric cladding modes can be seen as anti-symmetric cladding modes, where a portion (half) of its pattern is almost lost. This asymmetry translates into a strong dependence of the response of the LPFG to the polarization
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of the input light, in particular, the notch depth can differ by 10 dB for two orthogonal polarizations. LPFGs were fabricated during a period spanning 12 months, from September 2014 to August 2015, i.e., a series of LPFGs were fabricated at each season of the year by using approximately the same fabrication parameters. In general, the position of the notch bands remained approximately around the same wavelength, except for those fabricated during the winter season, but the number of periods necessary to obtain deep bands depends strongly on the environmental conditions. As mentioned in [4], the position of the resonance wavelengths are not affected significantly by the arc power, since, as we will show, LPFGs fabricated with different arc powers in a time period of more than one month, tend to have notch bands around the same region. The changes in arc power reflects mainly as a change in the necessary periods to obtain deep notches (425 dB), and in the insertion loss. Short LPFGs were obtained during the autumn season, with notch depths as high as 35 dB, and out of band loss as low as 0.12 dB. On the other hand, when there is a season change (from autumn to winter), there is a significant red shift of the notch band positions, which can be alleviated under controlled conditions of humidity and temperature. This short LPFGs bands are highly susceptible to bending, so that they can be tuned by changing their curvature radius, which can be useful in fiber lasers and amplifiers.
2. Fabrication of LPFGs The LPFGs were fabricated by using procedure similar to that used in Refs. [6–10], which consisted in the use of a fusion splicer Fitel S-175 in manual mode, with both fiber holders removed from the fusion splicer, so that the fiber (SMF28) is supported only by the splicer V-grooves. Each V-groove was transversely displaced on opposite directions by 60 μm, so that the applied arc was highly asymmetric, i.e. a misaligned fabrication setup was used. The fiber was supported externally from one side by a fiber holder attached to a motorized stage (Thorlabs 150 mm Linear Translation Stage, Stepper Motor), with a displacement resolution of 0.1 μm, and controlled by a Thorlabs Benchtop Stepper Motor Controller. The other side of the fiber pass through a channeled plastic disk and has attached a 1.35 gr weight which keeps the fiber under tension. After applying the first arc (period 1), the motorized stage pulls the fiber to the next position with high accuracy. During fabrication, the transmission characteristics of the LPFGs were monitored by using a 1550 nm broadband LED source and a YOKOGAWA AQ-6370 optical spectrum analyzer. Up to 65 LPFGs were fabricated by this method, by using a discharge time of 200 ms, while arc powers of 15 and 20 W were used. Fig. 1 shows a graph of the center resonance wavelength λR as a function of grating period Λ. Only notches with depths in excess of 15 dB were included in this graph. Despite the LPFGs were fabricated on different days (different environmental conditions), and in some cases with different arc power, it is observed that resonant wavelengths tend to fall along the same curves (dashed lines in Fig. 1), indicating that each line defines coupling to a specific cladding mode. The dashed lines in Fig. 1 can be fitted directly to the function that define the position of the resonance wavelengths λR ¼ ΛΔneff, so that the slope of each line gives the value of the effective refractive index difference Δneff for coupling to a specific cladding mode. In Fig. 1, we have labeled each resonance line from the lowest to the highest order as modes 1, 2, and 3, with effective refractive index differences of 0.001441819, 0.001839374, and 0.002712637, respectively. In Table 1, the parameters of 19 LPFGs and the fabrication date are shown. Before 10/24/2014 all gratings were fabricated by using a 20 W arc power, while at later dates 15 W of arc power was used. Most of the experimental data were
Fig. 1. Center notch wavelengths as a function of LPFG period. The points joined by the dotted curves were taken at different environmental conditions (winter).
taken during the autumn (dashed lines in Fig. 3), while a few data were taken during the winter (January 2015). For LPFGs fabricated on spring (April 2015) and summer (July 2015) we found that the resonances appeared at the same position as those observed in Autumn 2014, however, in the first case as much as 100 periods were required to observe depths in excess of 10 dB, while in the second case as much as 60 periods were required. As can be observed there is a major impact of the environmental conditions, mainly ambient humidity and temperature, and for our particular geographical location autumn seems to be the best season for the LPFGs fabrication. It is worth to mention that the same behavior is observed with other modal interference devices such as those based on tapered fibers.
3. Measurement of the near field intensity patterns We have avoided to designate the cladding modes through which the core mode is coupled as LPlm or HElm since the asymmetric writing setup has important consequences in the cladding mode distribution. To clarify this point the near field transverse pattern distribution (NFIP) was measured as a function of wavelength. For this purpose, after the LPFG was fabricated and its transmission spectrum measured, the fiber section containing the LPFG was cleaved a few mm after the grating end. The other end of the fiber was coupled to a tunable fiber laser source, and the NFIP measured at wavelengths steps of 1 nm in the 1450–1590 nm range, by collimating the light coming out the LPFG with a 40 microscope objective and a ccd camera XEVA XC-130. Figs. 2 and 3 show the transmission spectra of LPFGs with periods of 572 μm and 532 μm, respectively. The photos on each figure show the NFIPs at the center wavelength of the deep notches. In the case of LPFG with 572 μm period, it is observed that the shorter wavelength resonance (1461 nm) is the lowest order mode (mode 1), and the longer wavelength resonance (1535 nm) is a higher order mode (mode 2). The mode 1 shows an intense lobe very close to the core mode followed by two less intense lobes with a crescent shape on one direction, and another wider crescent shape, lobe on the other direction, i.e., three lobes in one direction and one in the other direction. The mode 2, in contrast, has three crescent shape lobes in one direction, and two less intense lobes in the other direction. On the other hand, mode 3 can be observed in Fig. 3, for a grating period of 532 μm, and in this case four more intense lobes can be observed in one direction. All these modes may be
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Table 1 Parameters of 14 of the fabricated LPFGs. Period (μm)
Notch wavelength (nm)
Notch depth (dB)
10 dB Bandwidth (nm)
Out of band loss (dB)
Number of periods
Fabrication date
500 510 520 525 530 545 572 585 590 615 620 625 638 670
1492.4 1528.2 1558 1559 1452 1577 1480.4 1616.6 1461 1535 1482.4 1558.4 1484.6 1565.6 1535.3 1620.2 1526.2091 1590.596 1539.3991 1628.8959 1568.6 1606.8
29.294 25.799 27.463 33.002 16.608 31.48 29.98 23.33 25.01 29.10 35.95 28.53 35.38 24.30 23.41 20.28 35.49 15.34 27.50 15.87 26.20 27.19
5 6.4 8.6 4.6 4.6 5.2 12 13.2 6 7.2 5.8 7.4 12.2 8.4 16.2 3.51 20.51 12.11 7.8 8
0.926 0.12 0.624 0.674 0.574 1.005 0.216 0.182 0.672 0.2 1.429 1.565 0.5 1.186
33 30 20 20 27 25 21 23 14 26 11 11 22 17
10/27/2014 11/5/2014 10/28/2014 10/28/2014 10/27/2014 10/27/2014 10/31/2014 10/29/2014 10/28/2014 11/7/2014 09/30/2014 09/30/2014 10/31/2014 10/24/2014
seen as LP1x modes (anti-symmetric modes) where a lobe is missing in one direction, and the coupling is stronger towards the opposite direction, and are similar to those observed in LPFGs fabricated with a CO2 laser source [11]. For the LPFG with Λ ¼ 532 μm the deepest peak around 1578 nm may be seen as an asymmetric LP14 mode (Mode 3), while for the LPFG with Λ ¼ 572 μm, the deepest peak around 1535 nm may be seen as an asymmetric LP13 mode (Mode 2). A closer view to the evolution of the NFIP may be obtained by viewing the animations Media1 (Λ ¼572 μm) and Media2 (Λ ¼ 532 μm), associated with Figs. 2 and 3, which were made from the experimental data by using Mathematica. In these animations we show the NFIP and the corresponding transmission as a function of wavelength in steps of 1 nm. In order to have a clearer picture of the transverse intensity pattern distribution, the images shown in these animations were filtered so that the lowest intensity zones are not observed. Nevertheless, as can be observed in the above animations, the transverse pattern evolves even inside the notch band, so that, sometimes it may be difficult to identify or label each pattern as an specific mode. What is clear, is the asymmetric mode coupling occurring as a result of the fabrication setup. This asymmetric mode coupling is a common characteristic of LPFGs written by a CO2 laser [6], and impacts the response of the gratings to the state
of polarization, and to the mechanical perturbations which is dependent on the fiber orientation. Supplementary material related to this article can be found online at http://dx.doi.org/10.1016/j.optcom.2015.10.061. For comparison, Fig. 4 shows the transmission spectrum of a LPFG fabricated without the transverse offset on the fusion splicer electrodes, and by improving the alignment of the fiber with respect to the fiber holder on the fusion splicer and the motorized translation stage. In this case one of the most important mechanisms for the grating formation is the periodic microtapering. As can be observed, even after 65 periods it is not possible to reach a notch depth comparable with that obtained with the misaligned setup, i.e., the notch depth in the micro-bend LPFG is 16.2 dB higher and its length is 3 times smaller. Thus, the refractive index change associated with the microbend arc-induced LPFGs is considerable higher, taking into account that the same tension and arc power were used in the aligned and misaligned setups. In between, by reducing the transverse offset or misalignment a reduction in the strength of the refractive index change and asymmetry can be expected.
Fig. 2. Transmission spectrum for a LPFG with a period of 572 μm. The photographs on the right show the measured near-field transverse intensity patterns corresponding to each resonant mode. Media1 shows the NFIP pattern evolution as a function of wavelength.
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Fig. 3. Transmission spectrum for a LPFG with a period of 532 μm. Media2 show the NFIP pattern evolution as a function of wavelength.
4. Sensitivity of the LPFGs to bending The asymmetric character of the excited cladding modes means that the transmission characteristics should be sensitive to parameters such polarization state of the input light and it should show significant sensitivity to stress or deformation caused by bending. In order to evaluate the bending sensitivity, a LPFG with Λ ¼ 510 μm was mounted on a metallic flexible blade [upper left inset in Fig. 5(a)], coated with a layer of low index Teflon, and glued on the metallic blade with red silicone. Mounted in this wave we ensure that the bending deformation is always applied on the same direction. As can be observed in Fig. 5(a), after the LPFG is mounted on the metallic blade, the notch depth decreases and there is a slight shift towards shorter wavelengths. Fig. 5 (b) shows the transmission spectrum of a LPFG with Λ ¼ 640 μm mounted on the metallic blade. The mounted LPFG was attached
to a single pivoting mechanism, so that, the bending state was changed by pushing the blade from the free end. The following animation shows the evolution in the transmission spectrum as the bending state is changed (Media3), for a LPFG with Λ ¼640 μm, which corresponds to a NFIP of the lowest order mode (Mode 1), as can be inferred from Fig. 1. This is the lowest order mode (asymmetric LP12 mode), so that it was possible to glue the LPFG directly on the metallic blade with red silicone without excessive leaking loss, even in the case of the higher order mode (asymmetric LP14) shown in Fig. 5(a). It is observed that after gluing the notch band shifts towards shorter wavelength. During the glue curing, the LPFG is compressed so that the bending state and period are changed. The lateral stress increase the refractive index so overcoupling occurs, so the notch depth reaches saturation and decreases. Moreover, the longitudinal compression of the LPFG slightly reduce the period and the attenuation bands slight
Fig. 4. Transmission spectrum for a LPFG with a period of 532 μm and using a perfectly aligned setup.
A. Martinez-Rios et al. / Optics Communications 364 (2016) 37–43
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Fig. 5. (a) Transmission spectrum of a LPFG with Λ ¼ 510 μm (Mode 3), just after fabrication (solid light line), and when the LPFG is mounted and glued on a flexible metallic blade (solid thick line). The upper left inset shows a photography of the LPFG mounted on the metallic blade, while the lower right inset is a photography of the NFIP corresponding to this notch. (b) Transmission spectrum of a LPFG with 640 μm mounted on a metallic blade at the zero bending state. Media3 shows an animation of the evolution of the transmission spectrum with bending applied on this LPFG.
shift towards shorter wavelength. Supplementary material related to this article can be found online at http://dx.doi.org/10.1016/j.optcom.2015.10.061.
5. Sensitivity of the LPFGs to the polarization state In order to evaluate the response to the polarization state of the input light a new set of LPFGs was fabricated in August 2015. The transmission spectra of three of the gratings is shown in Fig. 6. It is worth to note that the required number of periods increased with respect to that observed in September 2014 and January 2015, but it is less than that observed in the spring of 2015. The LPFG with 66 period of 600 μm was used to measure the sensitivity to the polarization state. A stabilized tunable diode
Fig. 6. Transmission spectra of three LPFGs fabricated on August 2015.
laser source is coupled to the input end of the LPFG (see Fig. 7), on which a polarization controller has been added in order to find the intensities of the low and fast polarization components in a wavelength range from 1520 to 1570 nm. This range corresponds to a peak at 1542 nm of the transmission spectrum of this long period grating. On the output end the transmitted spectrum is monitored by an optical spectrum analyzer (OSA). The procedure used to find the low and fast intensities at every wavelength was as follows: with the laser set at the initial wavelength value of the range, it was adjusted the positions of the 3 fiber polarization controller paddles in order to get the minimal intensity transmission measured by the OSA; then again the 3 paddles of the fiber polarization controller were adjusted but this time to get the maximal transmission measured by the OSA. Both values were registered and captured. Then, the laser was set to the next wavelength value of the range and the 3 paddles where adjusted to get the maximum and minimum transmission value, again both values where registered and captured. At every wavelength the maximum and minimum values were captured, and the spectra in the range for both maximal and minimal were obtained in this way. These two values (maximum and minimum), are the two orthogonal polarization components which are transmitted through the long period grating. The way to fit the paddles of the manual polarization controller was as follows, first one of λ/4 paddle were adjusted until get the minimum transmission value, then both, the same λ/4 and the one of λ/2, were moved at the
Fig. 7. Setup used to find the polarization response of a long period grating.
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Fig. 8. Left graph shows the transmission spectrum for two orthogonal polarizations labeled as Maximum (solid line) and Minimum (dashed line); Right graph shows the transmission spectrum obtained with an unpolarized broadband source.
same time until get a new minimal transmission value, finally, adjusting the other λ/4 paddle, the minimal transmission value was obtained and captured. This paddle sequence adjustment was the same used to find the maximum transmission value. From the measurement of the minimum and maximum transmission at each wavelength, i.e., for two distinct orthogonal polarizations, we obtained the transmission spectra shown in the left graph of Fig. 8. As a reference the right graph on Fig. 8 shows the transmission spectra obtained by the use of an un-polarized broadband source. There is a significant difference of more than 10 dB between the notch depths obtained for the two orthogonal polarizations, this fact and the asymmetry observed in NFIP proves that the polarizing effect of the device is favored by the fabrication method. The asymmetry of the fabrication method creates a microbend due to a lateral stress induced by the controlled misaligning of the fiber holders during fabrication [4], and hence the LPFG is in fact a periodic microbend structure.
6. Discussion The mechanism for the LPFG formation is based on the permanent geometrical deformation caused by the induced microbend. The microbending induces an asymmetry in the transversal effective refractive index profile, which results in a change in the effective index. This asymmetry causes the polarization dependent transmission shown on the last section (see Fig. 8), which also can be observed in the measured NFIP's. One of the major objectives of the present study was to register the changes in the LPFG's performance characteristics with the environmental conditions during fabrication, a factor that is relevant for uncontrolled environments despite that in our geographical location there are not extreme changes in the weather conditions. As we have observed, shorter LPFGs are obtained during the autumn season, meaning that the strength of the refractive index change is higher. Through the years we have learned that there are some seasons that are particularly convenient for fabrication of modal coupling devices (LPFGs and tapered fiber based interferometers),
and other seasons are convenient for the fabrication of low loss devices, and this study helped us to clearly identify them.
7. Conclusions In conclusion, we have identified the mode patterns associated with the resonance bands at a given grating period of arc-induced LPFGs fabricated by using a misaligned setup. In this way microbend based LPFGs were obtained. An animation of the nearfield transverse mode pattern evolution as a function of wavelength using a tunable laser shows how the coupling to asymmetric cladding modes is achieved versus the resonant bands of the grating. The asymmetry of the modes is related with the fabrication setup, which enables the formation of rejection bands as deep as 35 dB with less than 30 periods, while insertion loss may be as low as 0.12 dB. In addition, it was demonstrated that this asymmetry reflects as a polarization dependent transmission of the LPFGs, where for a particular case it was found that the notch depth for two distinct orthogonal polarizations differs by 10.7 dB. On the other hand, comparison of arc-induced LPFGs with Λ ¼532 μm fabricated by using aligned and misaligned fabrication setups, and by using the same arc power and tension, showed that the refractive index changes obtained in the misaligned setup (microbend based) are considerably higher. In particular, the microbend LPFG was 16.2 dB deeper and 3 times shorter that the LPFG fabricated by the aligned setup. On the other hand, the extensive experimental data allows a clear identification of the mode pattern and effective refractive index difference associated with each resonance band at a given grating period. In addition, it was found that the major variation in the position of the loss notches comes from the variations in the environmental conditions during the LPFGs fabrication. References [1] G. Rego, O. Okhotnikov, E. Dianov, V. Sulimov, High-temperature stability of long-period fiber gratings produced using an electric arc, J. Lightwave Technol.
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19 (2010) 1574–1579. [2] A. Martinez-Rios, D. Monzon-Hernandez, I. Torres-Gomez, Highly sensitive cladding-etched arc-induced long-period fiber gratings for refractive index sensing, Opt. Commun. 283 (2010) 958–962. [3] Zhi-Gang Zang, Wen-xuan Yang, Theoretical and experimental investigation of all-optical switching based on cascaded LPFGs separated by an erbium-doped fiber, J. Appl. Phys. 109 (2011) 103106. [4] A.M. Vengsarkar, Paul J. Lemaire, Justin B. Judkins, Vikram Bhatia, Turan Erdogan, John E. Sipe, Long-period fiber gratings as band-rejection filters, J. Lightwave Technol. 14 (1996) 58–65. [5] M. Akiyama, K. Nishide, K. Shima, A. Wada, R. Yamauchi, A novel long-period fiber grating using periodically released residual stress of pure-silica core fiber, in: Proceedings of Optical Fiber Communication Conference, 1998 OSA Technical Digest Series, Optical Society of America, 1998, Vol. 2, paper ThG1. [6] S.G. Kosinski, A.M. Vengsarkar, Splicer-based long-period fiber gratings, in:
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[10] [11]
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Proceedings of Optical Fiber Communication Conference, 1998 OSA Technical Digest Series, Optical Society of America, 1998, Vol. 2, paper ThG3. I.K. Hwang, S.H. Yun, B.Y. Kim, Long-period fiber gratings based on periodic microbends, Opt. Lett. 24 (1999) 1263–1265. M. Smietana, W.J. Bock, P. Mikulic, J. Chen, Increasing sensitivity of arc-induced long-period gratings—pushing the fabrication technique toward its limits, Meas. Sci. Technol. 22 (2011) 015201 1–6. Gaspar Rego, Oleg V. Ivanov, Paulo V.S. Marques, Demonstration of coupling to symmetric and antisymmetric cladding modes in arc-induced long-period fiber gratings, Opt. Express 14 (2006) 9594–9599. O.V. Ivanov, G. Rego, Origin of coupling to antisymmetric modes in arc-induced long-period fiber gratings, Opt. Express 15 (2007) 13936–13941. Yiping Wang, Review of long period fiber gratings written by CO2 laser, J. Appl. Phys. 108 (2010) 08110 1–18.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Asymmetric mode coupling in arc-induced long-period fiber gratings A. Martinez-Rios a,n, I. Torres-Gomez a, G. Anzueto-Sanchez b, R. Selvas-Aguilar c, V.M. Duran-Ramirez d, J.A. Guerrero-Viramontes e, D. Toral-Acosta c, G. Salceda Delgado c, A. Castillo-Guzman c a
Centro de Investigaciones en Óptica, Loma del Bosque 115, Col. Lomas del Campestre, 37150 León, Guanajuato, México Centro de Investigación en Ingeniería y Ciencias Aplicadas, CIICAp, Universidad Autónoma del Estado de Morelos, UAEM, Av. Universidad No. 1001, Col. Chamilpa, C.P. 62209 Cuernavaca, Morelos, México c Universidad Autónoma de Nuevo León, Facultad de Ciencias Físico-Matemáticas, Av. Universidad S/N, Cd. Universitaria, San Nicolás de los Garza, Nuevo León C.P. 66451, México d Universidad de Guadalajara, Centro Universitario de los Lagos, Enrique Díaz de León 1144, C.P. 47460 Lagos de Moreno, Jalisco, México e Instituto Tecnologico de Aguascalientes, Av. Adolfo Lopez Mateos 1801 Ote. Fracc. Bona Gens, C.P. 20256 Aguascalientes, Ags., México b
art ic l e i nf o
a b s t r a c t
Article history: Received 26 March 2015 Received in revised form 7 October 2015 Accepted 27 October 2015 Available online 21 November 2015
An extensive experimental study of the transverse modal field characteristics of mircrobend arc-induced long-period fiber gratings is presented. A wavelength scanning of the near-field intensity pattern inside each loss band in the transmission spectrum, shows a clear asymmetry in the transverse intensity distribution resulting from the fabrication method. This asymmetry reflects as a 10.7 dB difference in the notch depths for two orthogonal polarizations. Though a one year study, it was found that that environmental conditions during fabrication strongly affects the gratings characteristics. The best performance was obtained during the autumn season, where microbend arc-induced long-period fiber gratings produce wavelength filters with short lengths (between 10 and 30 periods for depths in excess of 20 dB) and the insertion loss may be as low as 0.12 dB. & 2015 Elsevier B.V. All rights reserved.
Keywords: Long-period fiber grating Arc-induced Modal pattern Polarization state
1. Introduction Long-period fiber gratings (LPFGs) are all-fiber devices which have demonstrated to be useful in many technological applications such as high temperature sensing [1], high sensitivity refractive index sensors [2], optical switching [3], etc. In essence, LPFGs are wavelength optical band-rejection filters whose operation relies on the selective coupling of the fundamental core mode towards cladding modes when a phase matching condition is satisfied [4]. This selective coupling produces notches on the transmission spectrum, and each notch may be associated with a specific cladding mode. The symmetry of the cladding modes depends strongly on the fabrication method, which in all cases involves the periodic perturbation of the refractive index of the core and/or the cladding of the optical fiber. For the particular case of LPFGs induced by an electric arc, mechanisms such as stress relief [5], microbending [6,7], and tapering [8–10], have been used for the induction of the periodic refractive index change. Rego and coworkers have realized extensive studies on the symmetry of the cladding modes excited in LPFGs fabricated by electric arc [9,10]. n
Corresponding author. E-mail address: [email protected] (A. Martinez-Rios).
http://dx.doi.org/10.1016/j.optcom.2015.10.061 0030-4018/& 2015 Elsevier B.V. All rights reserved.
They have demonstrated that factors such as the fiber type and fabrication method may impact the symmetry of the excited cladding modes, which may be symmetric and anti-symmetric. In addition, they found that this kind of LPFGs, based on periodic micro-tapering, experiences noticeable changes in the resonant wavelengths when the arc power or pulling tension is changed. On the other hand, in the case of micro-bend based arc-induced LPFGs, the resonance wavelength only depends on the grating period and is not affected by the coupling strength [7]. In Ref. [7], the resonant cladding modes of microbend based LPFGs are assumed to be associated with LP1n anti-symmetric modes. Recently, the fabrication of LPFGs by using a CO2 laser as the heat source has been subject of considerable interest. In particular, it is found that the refractive index perturbation caused by the CO2 laser source has a character highly asymmetric, so that, in general, the excited cladding modes are highly asymmetric, i.e., there is an asymmetric mode coupling [11]. In this work, by measuring the near field intensity patterns of arc-induced LPFGs as a function of wavelength, fabricated by using a misaligned setup, we show the asymmetric mode coupling of microbending arc-induced LPFGs. The asymmetric cladding modes can be seen as anti-symmetric cladding modes, where a portion (half) of its pattern is almost lost. This asymmetry translates into a strong dependence of the response of the LPFG to the polarization
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of the input light, in particular, the notch depth can differ by 10 dB for two orthogonal polarizations. LPFGs were fabricated during a period spanning 12 months, from September 2014 to August 2015, i.e., a series of LPFGs were fabricated at each season of the year by using approximately the same fabrication parameters. In general, the position of the notch bands remained approximately around the same wavelength, except for those fabricated during the winter season, but the number of periods necessary to obtain deep bands depends strongly on the environmental conditions. As mentioned in [4], the position of the resonance wavelengths are not affected significantly by the arc power, since, as we will show, LPFGs fabricated with different arc powers in a time period of more than one month, tend to have notch bands around the same region. The changes in arc power reflects mainly as a change in the necessary periods to obtain deep notches (425 dB), and in the insertion loss. Short LPFGs were obtained during the autumn season, with notch depths as high as 35 dB, and out of band loss as low as 0.12 dB. On the other hand, when there is a season change (from autumn to winter), there is a significant red shift of the notch band positions, which can be alleviated under controlled conditions of humidity and temperature. This short LPFGs bands are highly susceptible to bending, so that they can be tuned by changing their curvature radius, which can be useful in fiber lasers and amplifiers.
2. Fabrication of LPFGs The LPFGs were fabricated by using procedure similar to that used in Refs. [6–10], which consisted in the use of a fusion splicer Fitel S-175 in manual mode, with both fiber holders removed from the fusion splicer, so that the fiber (SMF28) is supported only by the splicer V-grooves. Each V-groove was transversely displaced on opposite directions by 60 μm, so that the applied arc was highly asymmetric, i.e. a misaligned fabrication setup was used. The fiber was supported externally from one side by a fiber holder attached to a motorized stage (Thorlabs 150 mm Linear Translation Stage, Stepper Motor), with a displacement resolution of 0.1 μm, and controlled by a Thorlabs Benchtop Stepper Motor Controller. The other side of the fiber pass through a channeled plastic disk and has attached a 1.35 gr weight which keeps the fiber under tension. After applying the first arc (period 1), the motorized stage pulls the fiber to the next position with high accuracy. During fabrication, the transmission characteristics of the LPFGs were monitored by using a 1550 nm broadband LED source and a YOKOGAWA AQ-6370 optical spectrum analyzer. Up to 65 LPFGs were fabricated by this method, by using a discharge time of 200 ms, while arc powers of 15 and 20 W were used. Fig. 1 shows a graph of the center resonance wavelength λR as a function of grating period Λ. Only notches with depths in excess of 15 dB were included in this graph. Despite the LPFGs were fabricated on different days (different environmental conditions), and in some cases with different arc power, it is observed that resonant wavelengths tend to fall along the same curves (dashed lines in Fig. 1), indicating that each line defines coupling to a specific cladding mode. The dashed lines in Fig. 1 can be fitted directly to the function that define the position of the resonance wavelengths λR ¼ ΛΔneff, so that the slope of each line gives the value of the effective refractive index difference Δneff for coupling to a specific cladding mode. In Fig. 1, we have labeled each resonance line from the lowest to the highest order as modes 1, 2, and 3, with effective refractive index differences of 0.001441819, 0.001839374, and 0.002712637, respectively. In Table 1, the parameters of 19 LPFGs and the fabrication date are shown. Before 10/24/2014 all gratings were fabricated by using a 20 W arc power, while at later dates 15 W of arc power was used. Most of the experimental data were
Fig. 1. Center notch wavelengths as a function of LPFG period. The points joined by the dotted curves were taken at different environmental conditions (winter).
taken during the autumn (dashed lines in Fig. 3), while a few data were taken during the winter (January 2015). For LPFGs fabricated on spring (April 2015) and summer (July 2015) we found that the resonances appeared at the same position as those observed in Autumn 2014, however, in the first case as much as 100 periods were required to observe depths in excess of 10 dB, while in the second case as much as 60 periods were required. As can be observed there is a major impact of the environmental conditions, mainly ambient humidity and temperature, and for our particular geographical location autumn seems to be the best season for the LPFGs fabrication. It is worth to mention that the same behavior is observed with other modal interference devices such as those based on tapered fibers.
3. Measurement of the near field intensity patterns We have avoided to designate the cladding modes through which the core mode is coupled as LPlm or HElm since the asymmetric writing setup has important consequences in the cladding mode distribution. To clarify this point the near field transverse pattern distribution (NFIP) was measured as a function of wavelength. For this purpose, after the LPFG was fabricated and its transmission spectrum measured, the fiber section containing the LPFG was cleaved a few mm after the grating end. The other end of the fiber was coupled to a tunable fiber laser source, and the NFIP measured at wavelengths steps of 1 nm in the 1450–1590 nm range, by collimating the light coming out the LPFG with a 40 microscope objective and a ccd camera XEVA XC-130. Figs. 2 and 3 show the transmission spectra of LPFGs with periods of 572 μm and 532 μm, respectively. The photos on each figure show the NFIPs at the center wavelength of the deep notches. In the case of LPFG with 572 μm period, it is observed that the shorter wavelength resonance (1461 nm) is the lowest order mode (mode 1), and the longer wavelength resonance (1535 nm) is a higher order mode (mode 2). The mode 1 shows an intense lobe very close to the core mode followed by two less intense lobes with a crescent shape on one direction, and another wider crescent shape, lobe on the other direction, i.e., three lobes in one direction and one in the other direction. The mode 2, in contrast, has three crescent shape lobes in one direction, and two less intense lobes in the other direction. On the other hand, mode 3 can be observed in Fig. 3, for a grating period of 532 μm, and in this case four more intense lobes can be observed in one direction. All these modes may be
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Table 1 Parameters of 14 of the fabricated LPFGs. Period (μm)
Notch wavelength (nm)
Notch depth (dB)
10 dB Bandwidth (nm)
Out of band loss (dB)
Number of periods
Fabrication date
500 510 520 525 530 545 572 585 590 615 620 625 638 670
1492.4 1528.2 1558 1559 1452 1577 1480.4 1616.6 1461 1535 1482.4 1558.4 1484.6 1565.6 1535.3 1620.2 1526.2091 1590.596 1539.3991 1628.8959 1568.6 1606.8
29.294 25.799 27.463 33.002 16.608 31.48 29.98 23.33 25.01 29.10 35.95 28.53 35.38 24.30 23.41 20.28 35.49 15.34 27.50 15.87 26.20 27.19
5 6.4 8.6 4.6 4.6 5.2 12 13.2 6 7.2 5.8 7.4 12.2 8.4 16.2 3.51 20.51 12.11 7.8 8
0.926 0.12 0.624 0.674 0.574 1.005 0.216 0.182 0.672 0.2 1.429 1.565 0.5 1.186
33 30 20 20 27 25 21 23 14 26 11 11 22 17
10/27/2014 11/5/2014 10/28/2014 10/28/2014 10/27/2014 10/27/2014 10/31/2014 10/29/2014 10/28/2014 11/7/2014 09/30/2014 09/30/2014 10/31/2014 10/24/2014
seen as LP1x modes (anti-symmetric modes) where a lobe is missing in one direction, and the coupling is stronger towards the opposite direction, and are similar to those observed in LPFGs fabricated with a CO2 laser source [11]. For the LPFG with Λ ¼ 532 μm the deepest peak around 1578 nm may be seen as an asymmetric LP14 mode (Mode 3), while for the LPFG with Λ ¼ 572 μm, the deepest peak around 1535 nm may be seen as an asymmetric LP13 mode (Mode 2). A closer view to the evolution of the NFIP may be obtained by viewing the animations Media1 (Λ ¼572 μm) and Media2 (Λ ¼ 532 μm), associated with Figs. 2 and 3, which were made from the experimental data by using Mathematica. In these animations we show the NFIP and the corresponding transmission as a function of wavelength in steps of 1 nm. In order to have a clearer picture of the transverse intensity pattern distribution, the images shown in these animations were filtered so that the lowest intensity zones are not observed. Nevertheless, as can be observed in the above animations, the transverse pattern evolves even inside the notch band, so that, sometimes it may be difficult to identify or label each pattern as an specific mode. What is clear, is the asymmetric mode coupling occurring as a result of the fabrication setup. This asymmetric mode coupling is a common characteristic of LPFGs written by a CO2 laser [6], and impacts the response of the gratings to the state
of polarization, and to the mechanical perturbations which is dependent on the fiber orientation. Supplementary material related to this article can be found online at http://dx.doi.org/10.1016/j.optcom.2015.10.061. For comparison, Fig. 4 shows the transmission spectrum of a LPFG fabricated without the transverse offset on the fusion splicer electrodes, and by improving the alignment of the fiber with respect to the fiber holder on the fusion splicer and the motorized translation stage. In this case one of the most important mechanisms for the grating formation is the periodic microtapering. As can be observed, even after 65 periods it is not possible to reach a notch depth comparable with that obtained with the misaligned setup, i.e., the notch depth in the micro-bend LPFG is 16.2 dB higher and its length is 3 times smaller. Thus, the refractive index change associated with the microbend arc-induced LPFGs is considerable higher, taking into account that the same tension and arc power were used in the aligned and misaligned setups. In between, by reducing the transverse offset or misalignment a reduction in the strength of the refractive index change and asymmetry can be expected.
Fig. 2. Transmission spectrum for a LPFG with a period of 572 μm. The photographs on the right show the measured near-field transverse intensity patterns corresponding to each resonant mode. Media1 shows the NFIP pattern evolution as a function of wavelength.
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Fig. 3. Transmission spectrum for a LPFG with a period of 532 μm. Media2 show the NFIP pattern evolution as a function of wavelength.
4. Sensitivity of the LPFGs to bending The asymmetric character of the excited cladding modes means that the transmission characteristics should be sensitive to parameters such polarization state of the input light and it should show significant sensitivity to stress or deformation caused by bending. In order to evaluate the bending sensitivity, a LPFG with Λ ¼ 510 μm was mounted on a metallic flexible blade [upper left inset in Fig. 5(a)], coated with a layer of low index Teflon, and glued on the metallic blade with red silicone. Mounted in this wave we ensure that the bending deformation is always applied on the same direction. As can be observed in Fig. 5(a), after the LPFG is mounted on the metallic blade, the notch depth decreases and there is a slight shift towards shorter wavelengths. Fig. 5 (b) shows the transmission spectrum of a LPFG with Λ ¼ 640 μm mounted on the metallic blade. The mounted LPFG was attached
to a single pivoting mechanism, so that, the bending state was changed by pushing the blade from the free end. The following animation shows the evolution in the transmission spectrum as the bending state is changed (Media3), for a LPFG with Λ ¼640 μm, which corresponds to a NFIP of the lowest order mode (Mode 1), as can be inferred from Fig. 1. This is the lowest order mode (asymmetric LP12 mode), so that it was possible to glue the LPFG directly on the metallic blade with red silicone without excessive leaking loss, even in the case of the higher order mode (asymmetric LP14) shown in Fig. 5(a). It is observed that after gluing the notch band shifts towards shorter wavelength. During the glue curing, the LPFG is compressed so that the bending state and period are changed. The lateral stress increase the refractive index so overcoupling occurs, so the notch depth reaches saturation and decreases. Moreover, the longitudinal compression of the LPFG slightly reduce the period and the attenuation bands slight
Fig. 4. Transmission spectrum for a LPFG with a period of 532 μm and using a perfectly aligned setup.
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Fig. 5. (a) Transmission spectrum of a LPFG with Λ ¼ 510 μm (Mode 3), just after fabrication (solid light line), and when the LPFG is mounted and glued on a flexible metallic blade (solid thick line). The upper left inset shows a photography of the LPFG mounted on the metallic blade, while the lower right inset is a photography of the NFIP corresponding to this notch. (b) Transmission spectrum of a LPFG with 640 μm mounted on a metallic blade at the zero bending state. Media3 shows an animation of the evolution of the transmission spectrum with bending applied on this LPFG.
shift towards shorter wavelength. Supplementary material related to this article can be found online at http://dx.doi.org/10.1016/j.optcom.2015.10.061.
5. Sensitivity of the LPFGs to the polarization state In order to evaluate the response to the polarization state of the input light a new set of LPFGs was fabricated in August 2015. The transmission spectra of three of the gratings is shown in Fig. 6. It is worth to note that the required number of periods increased with respect to that observed in September 2014 and January 2015, but it is less than that observed in the spring of 2015. The LPFG with 66 period of 600 μm was used to measure the sensitivity to the polarization state. A stabilized tunable diode
Fig. 6. Transmission spectra of three LPFGs fabricated on August 2015.
laser source is coupled to the input end of the LPFG (see Fig. 7), on which a polarization controller has been added in order to find the intensities of the low and fast polarization components in a wavelength range from 1520 to 1570 nm. This range corresponds to a peak at 1542 nm of the transmission spectrum of this long period grating. On the output end the transmitted spectrum is monitored by an optical spectrum analyzer (OSA). The procedure used to find the low and fast intensities at every wavelength was as follows: with the laser set at the initial wavelength value of the range, it was adjusted the positions of the 3 fiber polarization controller paddles in order to get the minimal intensity transmission measured by the OSA; then again the 3 paddles of the fiber polarization controller were adjusted but this time to get the maximal transmission measured by the OSA. Both values were registered and captured. Then, the laser was set to the next wavelength value of the range and the 3 paddles where adjusted to get the maximum and minimum transmission value, again both values where registered and captured. At every wavelength the maximum and minimum values were captured, and the spectra in the range for both maximal and minimal were obtained in this way. These two values (maximum and minimum), are the two orthogonal polarization components which are transmitted through the long period grating. The way to fit the paddles of the manual polarization controller was as follows, first one of λ/4 paddle were adjusted until get the minimum transmission value, then both, the same λ/4 and the one of λ/2, were moved at the
Fig. 7. Setup used to find the polarization response of a long period grating.
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Fig. 8. Left graph shows the transmission spectrum for two orthogonal polarizations labeled as Maximum (solid line) and Minimum (dashed line); Right graph shows the transmission spectrum obtained with an unpolarized broadband source.
same time until get a new minimal transmission value, finally, adjusting the other λ/4 paddle, the minimal transmission value was obtained and captured. This paddle sequence adjustment was the same used to find the maximum transmission value. From the measurement of the minimum and maximum transmission at each wavelength, i.e., for two distinct orthogonal polarizations, we obtained the transmission spectra shown in the left graph of Fig. 8. As a reference the right graph on Fig. 8 shows the transmission spectra obtained by the use of an un-polarized broadband source. There is a significant difference of more than 10 dB between the notch depths obtained for the two orthogonal polarizations, this fact and the asymmetry observed in NFIP proves that the polarizing effect of the device is favored by the fabrication method. The asymmetry of the fabrication method creates a microbend due to a lateral stress induced by the controlled misaligning of the fiber holders during fabrication [4], and hence the LPFG is in fact a periodic microbend structure.
6. Discussion The mechanism for the LPFG formation is based on the permanent geometrical deformation caused by the induced microbend. The microbending induces an asymmetry in the transversal effective refractive index profile, which results in a change in the effective index. This asymmetry causes the polarization dependent transmission shown on the last section (see Fig. 8), which also can be observed in the measured NFIP's. One of the major objectives of the present study was to register the changes in the LPFG's performance characteristics with the environmental conditions during fabrication, a factor that is relevant for uncontrolled environments despite that in our geographical location there are not extreme changes in the weather conditions. As we have observed, shorter LPFGs are obtained during the autumn season, meaning that the strength of the refractive index change is higher. Through the years we have learned that there are some seasons that are particularly convenient for fabrication of modal coupling devices (LPFGs and tapered fiber based interferometers),
and other seasons are convenient for the fabrication of low loss devices, and this study helped us to clearly identify them.
7. Conclusions In conclusion, we have identified the mode patterns associated with the resonance bands at a given grating period of arc-induced LPFGs fabricated by using a misaligned setup. In this way microbend based LPFGs were obtained. An animation of the nearfield transverse mode pattern evolution as a function of wavelength using a tunable laser shows how the coupling to asymmetric cladding modes is achieved versus the resonant bands of the grating. The asymmetry of the modes is related with the fabrication setup, which enables the formation of rejection bands as deep as 35 dB with less than 30 periods, while insertion loss may be as low as 0.12 dB. In addition, it was demonstrated that this asymmetry reflects as a polarization dependent transmission of the LPFGs, where for a particular case it was found that the notch depth for two distinct orthogonal polarizations differs by 10.7 dB. On the other hand, comparison of arc-induced LPFGs with Λ ¼532 μm fabricated by using aligned and misaligned fabrication setups, and by using the same arc power and tension, showed that the refractive index changes obtained in the misaligned setup (microbend based) are considerably higher. In particular, the microbend LPFG was 16.2 dB deeper and 3 times shorter that the LPFG fabricated by the aligned setup. On the other hand, the extensive experimental data allows a clear identification of the mode pattern and effective refractive index difference associated with each resonance band at a given grating period. In addition, it was found that the major variation in the position of the loss notches comes from the variations in the environmental conditions during the LPFGs fabrication. References [1] G. Rego, O. Okhotnikov, E. Dianov, V. Sulimov, High-temperature stability of long-period fiber gratings produced using an electric arc, J. Lightwave Technol.
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