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Microtapered long period gratings: Non-destructive fabrication, highly sensitive torsion sensing, and tunable broadband filtering
Infrared Physics and Technology 102 (2019) 103000
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Microtapered long period gratings: Non-destructive fabrication, highly sensitive torsion sensing, and tunable broadband filtering
T
Jihong Liua, Minhui Chenga, Xudong Kongb, Dongdong Hana, Jun Donga, Wenfeng Luoa, ⁎ Kaili Rena, a b
School of Electronic Engineering, Xi’an University of Posts and Telecommunications, Xi’an 710121, China State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Xi’an 710119, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Microtapered long period fiber grating Optical fiber sensors Broadband filters
A promising technology for non-destructive fabricating microtapered long period fiber gratings (MT-LPFGs) is demonstrated by periodically slight tapering single-mode fiber. Owing to the optimized microtapering technique, high-quality LPFGs with dip attenuations of about 40 dB, insertion loss < 0.5 dB are easily fabricated by precisely controlling the slightly tapering process. In addition, based on the characteristics of the MT-LPFG, not only highly strain, bending and torsion sensing are investigated experimentally, but also all-fiber broadband and extremely deep (−70 dB) band-rejection filters are proposed and demonstrated. The performances of the sensors and filters are significantly improved several times compared with conventional technology.
1. Introduction As a kind of all-fiber versatile device, long period fiber gratings (LPFGs) have drawn considerable attention in a variety of applications in the fields of optical communications, chemistry, industry, civil engineering, etc. [1–8]. In the past few decades, many methods have been developed to fabricate LPFGs. To further improve the performance of the LPFGs, efficient and steady techniques for fabricating the LPFGs are proposed. Recently, a flexible non-photosensitive based technique, microtapered method, is proposed and demonstrated for fabrication of the LPFGs [9–17]. In this method, the microtapered long-period fiber gratings (MT-LPFGs) can be formed by periodically tapering a single mode fiber (SMF) with various heating sources. Unlike the conventional UV writing method [18] in which fiber materials with high photosensitivity are generally required, the proposed method not only removes the need for the photosensitivity of the fiber, but also ensures the excellent spectral characteristics of the fabricated gratings. Compared with conventional LPFGs, the MT-LPFGs are more stable, simpler to fabricate and more sensitive to surrounding environments. Therefore, various applications based on the MT-LPFGs, such as ambient sensors [17], stress sensors [11], and wavelength-selective couplers [15], have been developed owing to their distinctive optical characteristics. Until now, several heating methods for fabricating MT-LPFGs have been proposed and demonstrated, including arc discharge, filament heating and CO2 laser heating, etc. [9–17]. Among them, arc discharge
⁎
techniques are widely used for softening fibers due to its simplicity and practicability [9–14]. However, the degeneration of the electrodes, which takes responsibility for the unstable arc duration and electric current, will greatly affect the quality of the fabricated MT-LPFGs [10–14]. In the previous work, we proposed a method of heating with a double CO2 laser beam, the quality of MT-LPFG can be significantly improved. Meanwhile, for the important application of filters, LPFGs has attracted much attention in recently years due to its simplicity, temperature resistance, low cost, easy fabrication, compatible with the fiber communication system and good stability. However, LPFGs typically has a narrow bandwidth of few nanometers, which limits its application, such as the gain equalization in broadband optical amplifiers and broadband filtering. To solve this problem, several methods have been developed. A tunable band-rejection filter with a bandwidth of tens of nanometer, which is constructed by a pair of helical LPFGs with opposite helicities, was proposed [19]. After that, a flat-top band-rejection filter was proposed by using two cascaded helical LPFGs with opposite helicities [20]. Unfortunately, the aforementioned filters suffer from a limited rejection depth. Recently, an improved method for enhancing the rejection depth of the two successively cascaded helical LPFGs with opposite helicities was proposed [21], which is realized by employing an retroreflector on one side of the fiber, as a result, the rejection depth has been greatly improved. However, auxiliary optical devices undoubtedly increase the cost and complexity of the system.
Corresponding author. E-mail address: [email protected] (K. Ren).
https://doi.org/10.1016/j.infrared.2019.103000 Received 23 April 2019; Received in revised form 30 July 2019; Accepted 31 July 2019 Available online 01 August 2019 1350-4495/ © 2019 Elsevier B.V. All rights reserved.
Infrared Physics and Technology 102 (2019) 103000
J. Liu, et al.
In this paper, an efficient and flexible method for non-destructive fabrication of the MT-LPFGs is proposed and experimentally demonstrated. Differing from the conventional heating methods, a double CO2 laser is used as the heating source in this scheme. The laser is split into two beams to illuminate the SMF from two counter directions, ensuring the fiber is heated uniformly. Moreover, the MT-LPFG is made by more times of slightly microtapered, so the spectral quality of the grating can be more precisely controlled during fabrication. As a result, highquality MT-LPFGs with uniform periods, low insertion loss, and high extinction ratio have been achieved readily. In addition, the sensing and filter characteristics have been investigated experimentally. Compared with the conventional LPFG, the proposed MT-LPFG has higher strain, bending and torsion sensitivities. Finally, an all-fiber broadband filter constructed with two-cascaded MT-LPFGs is demonstrated, which has a considerable bandwidth of ∼15 nm with an extremely high rejection efficiency of up to 99.9%. 2. Fabrication technology and sensing characteristics The experimental setup for fabrication of the MT-LPFG in a standard SMF (Corning SMF-28e(R)) is shown in Fig. 1(a). A double CO2 laser beams with output power of 4 W is focused to soften the SMF. Different from the conventional softening method in which the fiber is heated based on the one-sided laser irradiation [16]. In this scheme, the CO2 laser is split into two beams to illuminate the fiber from two contrary sides with an angle of ∼170° and focused with a diameter of ∼0.3 mm. As a result, the fiber will be fully softened by the uniform and continuous heating source. A couple of translation motors affixed with fiber holders are used for fixing and stretching fibers. Transmission spectrum of the MT-LPFG is monitored by a broadband light source (NKT Phonics, Superk Extreme) and an optical spectrum analyzer (OSA, YOKOGAWA, AQ6370C) in fabrication process. It is worth to mention that the whole fabrication procedures are completed in a commercial fusion splicer (Fujikura, LZM-100) and precisely controlled by a computer. In the experiments, the standard SMF is firmly clamped by two fiber holders, and a double CO2 laser beam is employed as a heating source. It is worth noting that, unlike the previous taper fabrication method, we propose a new method for non-destructive fabrication of the MT-LPFGs with lower loss and more accurate extinction ratio. As we mentioned above, the original beam is split into two beams to illuminate the fiber from different directions, which can be uniformly softened after 800 ms of the illumination. Once the softening process is finished, the laser stops working immediately, while the left and right translation motors start to advance toward the left direction. The left translation motor has a step distance of one period, e.g., 400 μm, and the right translation motor is slightly shorter of 390 μm. When one taper is finished, the fiber will be moved by one period along the axial direction to form the next adjacent taper. Hereafter, a desired MT-LPFG can be readily obtained by adjusting the number of repetitions of the above cycle step or the distance between two adjacent tapers. The microscopic image of the fabricated MT-LPFG is shown in Fig. 1(b), as one can observe in this figure, the fabricated MT-LPFG shows a smooth surface without obvious flaws. Note that, during the fabrication process, the translation motors are precisely controlled with a high stepping resolution of 0.02 μm to slightly taper the fiber. Due to the slightly tapering technique, the MT-LPFG will be fabricated with an extremely small deformation (see Fig. 1(b)). Benefit from the small deformation, the insertion loss can be significantly reduced, and the fabricated grating has a stronger ability to prevent breakage caused by external forces in practical applications. It is worth to mention that, while the shape profile of the periodically tapered section is slight, which is a little difficult to discriminate in Fig. 1(b), as seen in Fig. 1(c), the fabrication process may induce a periodic up-tapering in the core, which results in a periodic index modulation along the fiber [22]. As well known that, for most of the SMF, the core is doped with the Germanium, the heating process should account for the core material
Fig. 1. (a) Experimental setup for fabricating the MT-LPFGs. (b) Microscopic image of the MT-LPFG. (c) Sketch of the real shape of the slightly tapered zone at each period showing extremely small deformation of the cladding and core expansion along the slight taper.
diffusion in fabricating, thus leading to a core expansion at the waist of tapering section due to the thermal diffusion of the core dopant. As such, the refractive index of the core will be reduced due to the core expansion at the taper waist. The refractive index modulation of the MT-LPFG is mainly considered as a result of the periodic tapering of the fiber and the residual stress relaxation [23]. The coupling properties are similar to that of the conventional LPFG, the MT-LPFG couples the fundamental core mode to the matched discrete cladding modes when the phase-matching condition is satisfied: i λ i = (neff , co − neff , cl )Λ,
(1)
i where neff , co and neff , cl are the effective indices of the fundamental core mode and the i-th order cladding mode, respectively. Λ is the period of MT-LPFG, and λ i is the resonance wavelength. Fig. 2(a) shows the transmission spectrum of the fabricated MTLPFG with a grating period of 400 μm and a grating length of 26 mm (corresponding to 65 periods), which exhibits four distinct resonance dips within a relatively wide wavelength range. In Fig. 2(a), the yellow curve represents the spectrum in the absence of MT-LPFG, and the purple curve represents the original transmission spectrum of the MTLPFG mixed with irrelevant noise. The green1 curve indicates the real transmission spectrum, which is obtained by subtracting the spectrum in the absence of MT-LPFG from the original transmission spectrum. Obviously, the shape profile of the transmission spectrum is regular and smooth. A strong extinction ratio of about 38.7 dB can be observed at the resonance wavelength of 1578.8 nm, which is identified to be the LP07 mode [24]. Besides, the other three resonance dips corresponding to LP04, LP05 and LP06 modes can also be easily distinguished but with a weaker coupling efficiency than that of the LP07 mode. In our experiments, only the deepest dip corresponding to LP07 mode is of concern to us. As shown in Fig. 2(a), the insertion loss of the fabricated MT-LPFG is lower than 0.5 dB. The excellent transmission spectral performance has
1 For interpretation of color in Fig. 2, the reader is referred to the web version of this article.
2
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Fig. 2. (a) Transmission spectrum of the fabricated MT-LPFG. (b) Computed extinction ratio of transmission spectrum versus the length of MT-LPFG.
The result shows a good linear relationship of the wavelength shift on axial strain with a sensitivity of 0.85 pm/με, which is twice larger than that of the previous LPFGs [26,27]. The high strain sensitivity can be explained in part by the fact that due to its tapering structure, the grating period will be increased under the action of stretching, resulting in phase mismatching expressed by Eq. (1), further changing the resonance wavelength. For the whole MT-LPFG, the tapered and untapered regions will be stretched uniformly under the same longitudinal forces as a result of the condition of mechanical equilibrium [28]. The effect of the increased periods induced by the tensile force is dominated in our situation where the amount of tapers is relatively large (65 tapers). Meanwhile, different with conventional LPFGs [29], the small change in transmission loss gives a potential to MT-LPFGs to be used as strain sensors as well as wavelength-tunable filters. The setup for bending measurements is illustrated in Fig. 4. The fiber containing the MT-LPFG is mounted onto a flexible metal sheet of L = 90 mm in length that left side is fixed. A pair of fiber holders are used to prevent the fiber from moving on the metal sheet during bending. The MT-LPFG stand in the center between two holders. As the stepping length changed from 0 to 0.35 mm with an interval of 0.01 mm, the MT-LPFG will be bent under the movement of the right translation motor. The curvature C of the bent fiber is given by the following expression [30]:
confirmed the superiority of this fabrication technique. The extinction ratio of transmission spectrum, T, has a regular change under different grating length, which can be described as [25]:
T = 10log10 (cos2 (κL)),
(2)
where κ is the coupling constant and L is the length of MT-LPFG. According to Eq. (2), the corresponding simulation result is shown in Fig. 2(b). Note that only two modes are considered: the LP01 fundamental core mode and LP07 cladding mode. It is clearly seen that the extinction ratio T have a periodic increase and decrease as the grating length increases, and T can periodically reaches the maximum value at a series of certain grating length. It is worth noting that, small changes in grating length can dramatically affect the extinction ratio, especially near the wavelengths corresponding to the maximum values of extinction ratio. Therefore, precise control of the grating length is the key to fabricate a high-quality MT-LPFG. In our experiment, based on a slight taper to produce high-precision refractive index modulation as well as the extremely small deformation of the fiber in fabricating, the MT-LPFG can be fabricated in a smaller grating period but a relatively larger number of tapers, which makes it easier for the grating length to reach the position corresponding to the maximum extinction ratio. As such, the extinction ratio of the MT-LPFG is able to be more precisely controlled. In addition, the strain characteristics of the MT-LPFG are measured by applying continuous tension along the axial direction. All the following experiments are conducted at a room temperature (22 °C). The MT-LPFG is clamped by two fiber holders which is mounted on the left and right translation motors, respectively. In the case where the distance between the left and right fiber holders is 90 mm, the tensile strain length varies from 0 to 0.3 mm with a step of 0.02 mm and the strain can be worked out. Fig. 3(a) shows the changes of the transmission spectrum with the increasing axial strain. The dependence of the resonance wavelength shift on the axial strain is plotted in Fig. 3(b).
C=
1 2h = L−l R 2 h + 2
( )
2
, (3)
where R is the radius of curvature, h is the arch height of the bent fiber, and l is the stepping length. In the experiment, the stepping length range of 0 to 0.35 mm corresponds to a curvature range of 0 to 3.35 m−1. Fig. 5(a) shows the spectra of the fabricated MT-LPFG with different curvatures, a red shift with increase of the curvature and a significant reduction of the resonance dip can be observed. The
Fig. 3. (a) Transmission spectra of the MT-LPFG with different strains. (b) The dependence of wavelength shift on the applied axial strain. 3
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Fig. 4. Experimental setup for bending measurements of the MT-LPFG.
band-rejection filters with a large tunable range of transmission loss are always desired in fiber communications, which can tune the transmission loss dynamically. To test the performance of MT-LPFG as a losstunable filter, the transmission loss is measured at the wavelength of 1578.6 nm as the stepping length increases from 0 to 0.35 mm, as shown in Fig. 5(c). Note that the transmission does not have a significant change until the stepping length is larger than 0.03 mm. It can be attributed to the initial slight axial tension ensuring the fiber straight at the beginning. As a result, the push rendered by translation motor will be counterbalanced to some degree by the initial axial tension. From Fig. 5(c), it is seen that a wide loss tuning range from −1 dB to −38 dB has been achieved, meaning that this kind of MT-LPFGs can be used as a loss-tunable band-rejection filter with a large tunable range. For the first time, the torsion properties of the MT-LPFG are also studied in our experiments. Both ends of the MT-LPFG are tightly clamped by two fiber holders mounted on two rotation stages. Two
response of the wavelength shift to different curvatures of the grating is shown in Fig. 5(b). It is seen that the relationship of the curvature versus resonance wavelength shift possess a good linearity with a range of 1.15–2.57 m−1. The bending sensitivity is measured to be 13.7 nm/ m−1, which is almost twice larger than that of the conventional LPFGs [27]. Therefore, the MT-LPFGs may find great potential applications as bending sensors. In the bending sensing experiment, we found that the MT-LPFG has an obvious and regular variation of loss at a certain wavelength, which makes it possible for the MT-LPFG to be used as a loss-tunable filter. A possible explanation for the large loss variation is that the applied bending may generates an asymmetric strain distribution on the crosssection of fiber. Correspondingly, the photoelastic effect induced by the asymmetric strain distribution causes an asymmetric index distribution. The index perturbation will increase as the curvature increases, which may seriously affect the coupling efficiency. Generally, loss-tunable
Fig. 5. (a) Transmission spectra of the MTLPFG with different curvatures. (b) The dependence of wavelength shift on the curvatures, which shows linear correlation when the curvature is larger than 1.15 m−1. (c) Measured variation in transmission loss at the wavelength of 1578.6 nm (corresponding to the peak wavelength of LP07 mode) as a function of stepping length (in step of 0.01 nm).
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Fig. 6. (a) Transmission spectra of the MTLPFG with different twist angles. The twist angle changes from −720° to 720° with a step of 90°, and the angle −720°, 720° and 0° (original) are marked by blue arrow, red arrow and black arrow, respectively. (b) The dependence of wavelength shifts on the twist rate. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
fiber holders are 90 mm apart and the length of the MT-LPFG is 26 mm. The twist rate τ can be expressed as [30]:
τ =
πβ , 180L
(4)
where the β is the twist angle, and L (=90 mm) is the distance between left and right fiber holders. In the experiments, the positive and negative angles refer to clockwise direction and counterclockwise direction, respectively. The transmission spectra of the MT-LPFG with different twist angles are shown in Fig. 6(a). The resonance dip has a red shift when the MT-LPFG is twisted in clockwise direction. Conversely, when twisting the MT-LPFG in counterclockwise direction, the resonance dip shifts to shorter wavelengths. Due to the presence of asymmetric strain modulation caused by the double CO2 laser beams illumination on the cross-section of the fiber, the grating is a directionality torsion sensor. When the grating is twisted counterclockwise or clockwise, the transmission spectrum will shift toward different directions as the twisting direction changes [31]. At the same time, the twist induced shearing force within the MT-LPFG will result in a torsional photoelastic effect, thus generating index perturbation in this grating, as such, it will seriously affect the coupling efficiency as the torsional force increases. Fig. 6(b) depicts the relationship of wavelength shift against the twist rate. The result shows a roughly linear relationship between wavelength shift and twist rate with the relative value of about 0.973 and the torsion sensitivity of 0.169 nm m rad−1. This sensitivity is about 2.5 times higher than that of the conventional LPFG [27]. Based on the directionality of the wavelength shift in different torsion directions, the MT-LPFG has a great prospect for applications as a direction-discernible torsion sensor.
Fig. 7. Transmission spectra of the fabricated MT-LPFGs, where the blue solid line represent the first fabricated MT-LPFG1 with a period of 397 μm and the red solid line represent the second fabricated MT-LPFG2 with a period of 400 μm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3. Filtering characteristics of the cascaded MT-LPFGS In this section, a couple of MT-LPFGs, which are slightly different in period, are utilized in constructing the two-cascaded MT-LPFGs. Initially, the first MT-LPFG (MT-LPFG1) is fabricated with a grating period of 397 μm and a length of 25.8 mm (65 periods). After that, the second MT-LPFG (MT-LPFG2) is also fabricated but with a slightly longer period of 400 μm and a length of 26 mm (65 periods). In order to prohibit the cladding mode produced in MT-LPFG1 from coupling back with the MT-LPFG2, there is a 15 cm spatial separation between the two gratings without removing the fiber coating. Therefore, it is expected that the cladding mode produced in MT-LPFG1 will be depleted before reaching the MT-LPFG2 due to the absorption of the fiber coating. Transmission spectra of the two MT-LPFGs are plotted in Fig. 7 with blue and red solid lines, respectively. Note that, due to the little difference in period (3 nm), there is a 10.4 nm separation of the resonance peaks between the MT-LPFG1 and the MT-LPFG2. The transmission spectrum of the two-cascaded MT-LPFGs is shown in Fig. 8. For comparison, the transmission spectrum of the MT-LPFG2 is also labeled by
Fig. 8. Transmission spectra of the MT-LPFG2 (with a period of 400 μm) and the two-cascaded MT-LPFGs.
black solid line in Fig. 8. As one can observe, the rejection band has a central wavelength of ∼1573 nm. The bandwidth of both cascaded MTLPFGs and MT-LPFG2 at a rejection level about 30 dB is given in Fig. 8. A wide bandwidth of ∼15 nm is achieved, which is at least 7.5 times larger than that of the single MT-LPFG. To fully access the optimal broadband rejection performance, precise design and fabrication of the two MT-LPFGs with specific periods is prerequisite. Although there is a slight fluctuation at the bottom of the rejection band, it is generally reaching an extremely high rejection efficiency of up to 99.9% at the 5
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Fig. 9. Transmission spectra of the two-cascaded MT-LPFGs with different twist angles in (a) counterclockwise direction and (b) clockwise direction. (Only MTLPFG2 section is twisted).
precisely controlled, and a high extinction ratio of ∼40 dB has been achieved. The sensing features of strain, bending and torsion of the fabricated MT-LPFGs are experimentally investigated. Experimental results indicate that the strain and bending sensitivities can reach up to 0.85 pm/με and 13.7 nm/m−1 with good linear relationship, respectively, which is found to be several times higher than conventional LPFGs, and a high torsion sensitivity of 13.7 nm/m−1 is achieved with the abilities of rotational direction identification and twist rate measurement. Furthermore, by simply cascading two MT-LPFGs with small period gap, a broadband band-rejection filter has been obtained successfully based on the combination of two dips. Finally, an extremely high extinction ratio of ∼70 dB are readily achieved by merely twisting one of the MT-LPFGs contained in the two-cascaded MT-LPFGs. Considering the superior filtering properties that we demonstrated above, it is envisaged that this kind of two-cascaded LPFGs may find promising applications for the aspect of filtering.
top position of the fluctuation. As such, this slight fluctuation has negligible effects on the filtering performance in practical applications. At the same time, a wide bandwidth of 30 nm is also obtained at a rejection level of 15 dB. Obviously, two spectra are overlapped directly without modes interference. When the MT-LPFG1 and MT-LPFG2 are cascaded, and the properties of the newly obtained spectrum are largely determined by the property of each grating and the separation of their resonance peaks. Therefore, we should find a balance between the depth and bandwidth of the rejection band. To investigate the tunability of the two-cascaded MT-LPFGs, a measurement setup similar to that used in the torsional sensing experiment is utilized. Note that only the section containing the MTLPFG2 of the two-cascaded MT-LPFGs is fixed between two fiber holders while the section of MT-LPFG1 is well kept to isolate its interference on the spectrum of MT-LPFG2. The left and right fiber holders are separated by 90 mm. The negative angles are defined when the fiber is twisted in a counterclockwise direction. Fig. 9(a) shows the evolution of the transmission spectrum when the section of MT-LPFG2 of the twocascaded MT-LPFGs was twisted from 0 to −720° with a step of 90°. At beginning, the bandwidth decreases as the twist angle increases. When the twist angle is larger than −180°, the bandwidth does not change anymore, while the depth of the dip varies as the twist angle continues to change. Unlike the helical LPFG, which has little change in transmission loss as a torsion applied [6], the MT-LPFG has a significant change in transmission loss as the torsion increases. This is due to the coupling efficiency of the grating decreases as the torsion angle increases as we mentioned above. An amazing growth in extinction ratio of more than 35 dB are achieve by simply twisting the two-cascaded MT-LPFGs. It is worth noting that, an extremely high extinction ratio of ∼70 dB has been obtained at the twist angle of 450°. As shown in Fig. 9(a), it is apparent that there is no modes interference throughout the torsion process. Therefore, the two-cascaded MT-LPFGs could make great significance in the field of fiber communication when used as a single-band-rejection filter. Twisting the two-cascaded MT-LPFGs in a clockwise direction, similarly, only MT-LPFG2 section is twisted, the measurement results are shown in Fig. 9(b). It is clearly seen that only the right half of the spectrum have a red shift, which can be attributed to the tuning method we adopt of partial twisting to the two-cascaded MT-LPFGs. The variation of the right half spectrum is consistent with that of the single MTLPFG investigated before.
Funding information National Natural Science Foundation of China (NSFC) (61535015, 61805198). Declaration of Competing Interest There is no conflict of interest. References [1] H.J. Patrick, A.D. Kersey, F. Bucholtz, Analysis of the response of long period fiber gratings to external index of refraction, J. Lightw. Technol. 16 (1998) 1606–1612, https://doi.org/10.1109/50.712243. [2] W. Zhang, L. Huang, K. Wei, P. Li, B. Jiang, D. Mao, F. Gao, T. Mei, G. Zhang, J. Zhao, High-order optical vortex generation in a few-mode fiber via cascaded acoustically driven vector mode conversion, Opt. Lett. 40 (2016) 5082–5085, https://doi.org/10.1364/OL.41.005082. [3] Y.G. Han, S.B. Lee, C.S. Kim, J.U. Kang, U.C. Paek, Y. Chung, Simultaneous measurement of temperature and strain using dual long-period fiber gratings with controlled temperature and strain sensitivities, Opt. Express 11 (2003) 476–481, https://doi.org/10.1364/oe.11.000476. [4] K. Wei, W. Zhang, L. Huang, D. Mao, F. Gao, T. Mei, J. Zhao, Generation of cylindrical vector beams and optical vortex by two acoustically induced fiber gratings with orthogonal vibration directions, Opt. Express 25 (2017) 2733–2741, https:// doi.org/10.1364/OE.25.002733. [5] T. Zhu, Y.J. Rao, Y. Song, K.S. Chiang, M. Liu, Highly sensitive temperature-independent strain sensor based on a long–period fiber grating with a CO2-laser engraved rotary structure, IEEE Photon. Technol. Lett. 21 (2009) 543–545, https:// doi.org/10.1109/lpt.2009.2014566. [6] X. Kong, K. Ren, L. Ren, J. Liang, H. Ju, Chiral long-period gratings: fabrication, highly sensitive torsion sensing, and tunable single-band filtering, Appl. Opt. 56 (2017) 4702–4707, https://doi.org/10.1364/ao.56.004702. [7] R. Subramanian, C. Zhu, H. Zhao, H. Li, Torsion, strain, and temperature sensor based on helical long-period fiber gratings, IEEE Photon. Technol. Lett. 30 (2018) 327–330, https://doi.org/10.1109/lpt. 2017.2787157.
4. Conclusions In summary, a simple method for non-destructive fabrication of high-quality MT-LPFGs is demonstrated. Attributing to the slightly tapering technique, the extinction ratio of the grating is able to be more 6
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cascaded helical fiber gratings, Opt. Express 24 (2016) 5442–5447, https://doi.org/ 10.1364/oe.24.005442. C. Zhu, H. Zhao, P. Wang, R. Subramanian, H. Li, Enhanced flat-top band-rejection filter based on reflective helical long-period fiber gratings, IEEE Photon. Technol. Lett. 29 (2017) 964–966, https://doi.org/10.1109/lpt.2017.2700524. T.E.P. Culebro, A.M. Rios, D.T. Acosta, K.M. Rosa, L.F.E. Gomez, J.M.S. Hernandez, G.S. Delgado, Characteristics of LPFGs written by a CO2-laser glass processing system, J. Lightw. Technol. 37 (2019) 1301–1309, https://doi.org/10.1109/JLT. 2019.2892326. L.Y. Shao, J. Zhao, X. Dong, H.Y. Tam, C. Lu, S. He, Long-period prating fabricated by periodically tapering standard single-mode fiber, Appl. Opt. 47 (2008) 1549–1552, https://doi.org/10.1364/ao.47. 001549. F. Esposito, R. Ranjan, S. Campopiano, A. Iadicicco, Experimental study of the refractive index sensitivity in arc-induced long period gratings, IEEE Photon. J. 9 (2017) 1–10, https://doi.org/10.1109/JPHOT.2016.2634784. K. Ren, L. Ren, J. Liang, X. Kong, H. Ju, Y. Xu, Z. Wu, Online fabrication scheme of helical long-period fiber grating for liquid-level sensing, Appl. Opt. 55 (2016) 9675–9679, https://doi.org/10.1364/AO.55.009675. P. Caldas, G. Rego, O.V. Ivanov, J.L. Santos, Characterization of the response of a dual resonance of an arc-induced long-period grating to various physical parameters, Appl. Opt. 49 (2010) 2994–2999, https://doi.org/10.1364/ao.49.002994. Y.J. Rao, Y.P. Wang, Z.L. Ran, T. Zhu, Novel fiber-optic sensors based on longperiod fiber gratings written by high-frequency CO2 laser pulses, J. Lightw. Technol. 21 (2003) 1320–1327, https://doi.org/10.1109/jlt.2003.810561. C.Y. Lin, L.A. Wang, A wavelength- and loss-tunable band-rejection filter based on corrugated long-period fiber grating, IEEE Photon. Technol. Lett. 12 (2001) 332–334, https://doi.org/10.1109/68.917842. Y.P. Wang, D.N. Wang, W. Jin, Y.J. Rao, G.D. Peng, Asymmetric long period fiber gratings fabricated by use of CO2 laser to carve periodic grooves on the optical fiber, Appl. Phys. Lett. 89 (2006) 151105, , https://doi.org/10.1063/1.2360253. X. Cao, D. Tian, Y. Liu, L. Zhang, T. Wang, Sensing Characteristics of helical longperiod gratings written in the double-clad fiber by CO2 laser, IEEE Sens. J. 18 (2018) 7481–7485, https://doi.org/10.1109/jsen.2018.2855038. V. Budinski, D. Donlagic, Fiber-optic sensors for measurements of torsion, twist and rotation: a review, Sensors 17 (2017) 443, https://doi.org/10.3390/s17030443.
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Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
Microtapered long period gratings: Non-destructive fabrication, highly sensitive torsion sensing, and tunable broadband filtering
T
Jihong Liua, Minhui Chenga, Xudong Kongb, Dongdong Hana, Jun Donga, Wenfeng Luoa, ⁎ Kaili Rena, a b
School of Electronic Engineering, Xi’an University of Posts and Telecommunications, Xi’an 710121, China State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Xi’an 710119, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Microtapered long period fiber grating Optical fiber sensors Broadband filters
A promising technology for non-destructive fabricating microtapered long period fiber gratings (MT-LPFGs) is demonstrated by periodically slight tapering single-mode fiber. Owing to the optimized microtapering technique, high-quality LPFGs with dip attenuations of about 40 dB, insertion loss < 0.5 dB are easily fabricated by precisely controlling the slightly tapering process. In addition, based on the characteristics of the MT-LPFG, not only highly strain, bending and torsion sensing are investigated experimentally, but also all-fiber broadband and extremely deep (−70 dB) band-rejection filters are proposed and demonstrated. The performances of the sensors and filters are significantly improved several times compared with conventional technology.
1. Introduction As a kind of all-fiber versatile device, long period fiber gratings (LPFGs) have drawn considerable attention in a variety of applications in the fields of optical communications, chemistry, industry, civil engineering, etc. [1–8]. In the past few decades, many methods have been developed to fabricate LPFGs. To further improve the performance of the LPFGs, efficient and steady techniques for fabricating the LPFGs are proposed. Recently, a flexible non-photosensitive based technique, microtapered method, is proposed and demonstrated for fabrication of the LPFGs [9–17]. In this method, the microtapered long-period fiber gratings (MT-LPFGs) can be formed by periodically tapering a single mode fiber (SMF) with various heating sources. Unlike the conventional UV writing method [18] in which fiber materials with high photosensitivity are generally required, the proposed method not only removes the need for the photosensitivity of the fiber, but also ensures the excellent spectral characteristics of the fabricated gratings. Compared with conventional LPFGs, the MT-LPFGs are more stable, simpler to fabricate and more sensitive to surrounding environments. Therefore, various applications based on the MT-LPFGs, such as ambient sensors [17], stress sensors [11], and wavelength-selective couplers [15], have been developed owing to their distinctive optical characteristics. Until now, several heating methods for fabricating MT-LPFGs have been proposed and demonstrated, including arc discharge, filament heating and CO2 laser heating, etc. [9–17]. Among them, arc discharge
⁎
techniques are widely used for softening fibers due to its simplicity and practicability [9–14]. However, the degeneration of the electrodes, which takes responsibility for the unstable arc duration and electric current, will greatly affect the quality of the fabricated MT-LPFGs [10–14]. In the previous work, we proposed a method of heating with a double CO2 laser beam, the quality of MT-LPFG can be significantly improved. Meanwhile, for the important application of filters, LPFGs has attracted much attention in recently years due to its simplicity, temperature resistance, low cost, easy fabrication, compatible with the fiber communication system and good stability. However, LPFGs typically has a narrow bandwidth of few nanometers, which limits its application, such as the gain equalization in broadband optical amplifiers and broadband filtering. To solve this problem, several methods have been developed. A tunable band-rejection filter with a bandwidth of tens of nanometer, which is constructed by a pair of helical LPFGs with opposite helicities, was proposed [19]. After that, a flat-top band-rejection filter was proposed by using two cascaded helical LPFGs with opposite helicities [20]. Unfortunately, the aforementioned filters suffer from a limited rejection depth. Recently, an improved method for enhancing the rejection depth of the two successively cascaded helical LPFGs with opposite helicities was proposed [21], which is realized by employing an retroreflector on one side of the fiber, as a result, the rejection depth has been greatly improved. However, auxiliary optical devices undoubtedly increase the cost and complexity of the system.
Corresponding author. E-mail address: [email protected] (K. Ren).
https://doi.org/10.1016/j.infrared.2019.103000 Received 23 April 2019; Received in revised form 30 July 2019; Accepted 31 July 2019 Available online 01 August 2019 1350-4495/ © 2019 Elsevier B.V. All rights reserved.
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In this paper, an efficient and flexible method for non-destructive fabrication of the MT-LPFGs is proposed and experimentally demonstrated. Differing from the conventional heating methods, a double CO2 laser is used as the heating source in this scheme. The laser is split into two beams to illuminate the SMF from two counter directions, ensuring the fiber is heated uniformly. Moreover, the MT-LPFG is made by more times of slightly microtapered, so the spectral quality of the grating can be more precisely controlled during fabrication. As a result, highquality MT-LPFGs with uniform periods, low insertion loss, and high extinction ratio have been achieved readily. In addition, the sensing and filter characteristics have been investigated experimentally. Compared with the conventional LPFG, the proposed MT-LPFG has higher strain, bending and torsion sensitivities. Finally, an all-fiber broadband filter constructed with two-cascaded MT-LPFGs is demonstrated, which has a considerable bandwidth of ∼15 nm with an extremely high rejection efficiency of up to 99.9%. 2. Fabrication technology and sensing characteristics The experimental setup for fabrication of the MT-LPFG in a standard SMF (Corning SMF-28e(R)) is shown in Fig. 1(a). A double CO2 laser beams with output power of 4 W is focused to soften the SMF. Different from the conventional softening method in which the fiber is heated based on the one-sided laser irradiation [16]. In this scheme, the CO2 laser is split into two beams to illuminate the fiber from two contrary sides with an angle of ∼170° and focused with a diameter of ∼0.3 mm. As a result, the fiber will be fully softened by the uniform and continuous heating source. A couple of translation motors affixed with fiber holders are used for fixing and stretching fibers. Transmission spectrum of the MT-LPFG is monitored by a broadband light source (NKT Phonics, Superk Extreme) and an optical spectrum analyzer (OSA, YOKOGAWA, AQ6370C) in fabrication process. It is worth to mention that the whole fabrication procedures are completed in a commercial fusion splicer (Fujikura, LZM-100) and precisely controlled by a computer. In the experiments, the standard SMF is firmly clamped by two fiber holders, and a double CO2 laser beam is employed as a heating source. It is worth noting that, unlike the previous taper fabrication method, we propose a new method for non-destructive fabrication of the MT-LPFGs with lower loss and more accurate extinction ratio. As we mentioned above, the original beam is split into two beams to illuminate the fiber from different directions, which can be uniformly softened after 800 ms of the illumination. Once the softening process is finished, the laser stops working immediately, while the left and right translation motors start to advance toward the left direction. The left translation motor has a step distance of one period, e.g., 400 μm, and the right translation motor is slightly shorter of 390 μm. When one taper is finished, the fiber will be moved by one period along the axial direction to form the next adjacent taper. Hereafter, a desired MT-LPFG can be readily obtained by adjusting the number of repetitions of the above cycle step or the distance between two adjacent tapers. The microscopic image of the fabricated MT-LPFG is shown in Fig. 1(b), as one can observe in this figure, the fabricated MT-LPFG shows a smooth surface without obvious flaws. Note that, during the fabrication process, the translation motors are precisely controlled with a high stepping resolution of 0.02 μm to slightly taper the fiber. Due to the slightly tapering technique, the MT-LPFG will be fabricated with an extremely small deformation (see Fig. 1(b)). Benefit from the small deformation, the insertion loss can be significantly reduced, and the fabricated grating has a stronger ability to prevent breakage caused by external forces in practical applications. It is worth to mention that, while the shape profile of the periodically tapered section is slight, which is a little difficult to discriminate in Fig. 1(b), as seen in Fig. 1(c), the fabrication process may induce a periodic up-tapering in the core, which results in a periodic index modulation along the fiber [22]. As well known that, for most of the SMF, the core is doped with the Germanium, the heating process should account for the core material
Fig. 1. (a) Experimental setup for fabricating the MT-LPFGs. (b) Microscopic image of the MT-LPFG. (c) Sketch of the real shape of the slightly tapered zone at each period showing extremely small deformation of the cladding and core expansion along the slight taper.
diffusion in fabricating, thus leading to a core expansion at the waist of tapering section due to the thermal diffusion of the core dopant. As such, the refractive index of the core will be reduced due to the core expansion at the taper waist. The refractive index modulation of the MT-LPFG is mainly considered as a result of the periodic tapering of the fiber and the residual stress relaxation [23]. The coupling properties are similar to that of the conventional LPFG, the MT-LPFG couples the fundamental core mode to the matched discrete cladding modes when the phase-matching condition is satisfied: i λ i = (neff , co − neff , cl )Λ,
(1)
i where neff , co and neff , cl are the effective indices of the fundamental core mode and the i-th order cladding mode, respectively. Λ is the period of MT-LPFG, and λ i is the resonance wavelength. Fig. 2(a) shows the transmission spectrum of the fabricated MTLPFG with a grating period of 400 μm and a grating length of 26 mm (corresponding to 65 periods), which exhibits four distinct resonance dips within a relatively wide wavelength range. In Fig. 2(a), the yellow curve represents the spectrum in the absence of MT-LPFG, and the purple curve represents the original transmission spectrum of the MTLPFG mixed with irrelevant noise. The green1 curve indicates the real transmission spectrum, which is obtained by subtracting the spectrum in the absence of MT-LPFG from the original transmission spectrum. Obviously, the shape profile of the transmission spectrum is regular and smooth. A strong extinction ratio of about 38.7 dB can be observed at the resonance wavelength of 1578.8 nm, which is identified to be the LP07 mode [24]. Besides, the other three resonance dips corresponding to LP04, LP05 and LP06 modes can also be easily distinguished but with a weaker coupling efficiency than that of the LP07 mode. In our experiments, only the deepest dip corresponding to LP07 mode is of concern to us. As shown in Fig. 2(a), the insertion loss of the fabricated MT-LPFG is lower than 0.5 dB. The excellent transmission spectral performance has
1 For interpretation of color in Fig. 2, the reader is referred to the web version of this article.
2
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Fig. 2. (a) Transmission spectrum of the fabricated MT-LPFG. (b) Computed extinction ratio of transmission spectrum versus the length of MT-LPFG.
The result shows a good linear relationship of the wavelength shift on axial strain with a sensitivity of 0.85 pm/με, which is twice larger than that of the previous LPFGs [26,27]. The high strain sensitivity can be explained in part by the fact that due to its tapering structure, the grating period will be increased under the action of stretching, resulting in phase mismatching expressed by Eq. (1), further changing the resonance wavelength. For the whole MT-LPFG, the tapered and untapered regions will be stretched uniformly under the same longitudinal forces as a result of the condition of mechanical equilibrium [28]. The effect of the increased periods induced by the tensile force is dominated in our situation where the amount of tapers is relatively large (65 tapers). Meanwhile, different with conventional LPFGs [29], the small change in transmission loss gives a potential to MT-LPFGs to be used as strain sensors as well as wavelength-tunable filters. The setup for bending measurements is illustrated in Fig. 4. The fiber containing the MT-LPFG is mounted onto a flexible metal sheet of L = 90 mm in length that left side is fixed. A pair of fiber holders are used to prevent the fiber from moving on the metal sheet during bending. The MT-LPFG stand in the center between two holders. As the stepping length changed from 0 to 0.35 mm with an interval of 0.01 mm, the MT-LPFG will be bent under the movement of the right translation motor. The curvature C of the bent fiber is given by the following expression [30]:
confirmed the superiority of this fabrication technique. The extinction ratio of transmission spectrum, T, has a regular change under different grating length, which can be described as [25]:
T = 10log10 (cos2 (κL)),
(2)
where κ is the coupling constant and L is the length of MT-LPFG. According to Eq. (2), the corresponding simulation result is shown in Fig. 2(b). Note that only two modes are considered: the LP01 fundamental core mode and LP07 cladding mode. It is clearly seen that the extinction ratio T have a periodic increase and decrease as the grating length increases, and T can periodically reaches the maximum value at a series of certain grating length. It is worth noting that, small changes in grating length can dramatically affect the extinction ratio, especially near the wavelengths corresponding to the maximum values of extinction ratio. Therefore, precise control of the grating length is the key to fabricate a high-quality MT-LPFG. In our experiment, based on a slight taper to produce high-precision refractive index modulation as well as the extremely small deformation of the fiber in fabricating, the MT-LPFG can be fabricated in a smaller grating period but a relatively larger number of tapers, which makes it easier for the grating length to reach the position corresponding to the maximum extinction ratio. As such, the extinction ratio of the MT-LPFG is able to be more precisely controlled. In addition, the strain characteristics of the MT-LPFG are measured by applying continuous tension along the axial direction. All the following experiments are conducted at a room temperature (22 °C). The MT-LPFG is clamped by two fiber holders which is mounted on the left and right translation motors, respectively. In the case where the distance between the left and right fiber holders is 90 mm, the tensile strain length varies from 0 to 0.3 mm with a step of 0.02 mm and the strain can be worked out. Fig. 3(a) shows the changes of the transmission spectrum with the increasing axial strain. The dependence of the resonance wavelength shift on the axial strain is plotted in Fig. 3(b).
C=
1 2h = L−l R 2 h + 2
( )
2
, (3)
where R is the radius of curvature, h is the arch height of the bent fiber, and l is the stepping length. In the experiment, the stepping length range of 0 to 0.35 mm corresponds to a curvature range of 0 to 3.35 m−1. Fig. 5(a) shows the spectra of the fabricated MT-LPFG with different curvatures, a red shift with increase of the curvature and a significant reduction of the resonance dip can be observed. The
Fig. 3. (a) Transmission spectra of the MT-LPFG with different strains. (b) The dependence of wavelength shift on the applied axial strain. 3
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Fig. 4. Experimental setup for bending measurements of the MT-LPFG.
band-rejection filters with a large tunable range of transmission loss are always desired in fiber communications, which can tune the transmission loss dynamically. To test the performance of MT-LPFG as a losstunable filter, the transmission loss is measured at the wavelength of 1578.6 nm as the stepping length increases from 0 to 0.35 mm, as shown in Fig. 5(c). Note that the transmission does not have a significant change until the stepping length is larger than 0.03 mm. It can be attributed to the initial slight axial tension ensuring the fiber straight at the beginning. As a result, the push rendered by translation motor will be counterbalanced to some degree by the initial axial tension. From Fig. 5(c), it is seen that a wide loss tuning range from −1 dB to −38 dB has been achieved, meaning that this kind of MT-LPFGs can be used as a loss-tunable band-rejection filter with a large tunable range. For the first time, the torsion properties of the MT-LPFG are also studied in our experiments. Both ends of the MT-LPFG are tightly clamped by two fiber holders mounted on two rotation stages. Two
response of the wavelength shift to different curvatures of the grating is shown in Fig. 5(b). It is seen that the relationship of the curvature versus resonance wavelength shift possess a good linearity with a range of 1.15–2.57 m−1. The bending sensitivity is measured to be 13.7 nm/ m−1, which is almost twice larger than that of the conventional LPFGs [27]. Therefore, the MT-LPFGs may find great potential applications as bending sensors. In the bending sensing experiment, we found that the MT-LPFG has an obvious and regular variation of loss at a certain wavelength, which makes it possible for the MT-LPFG to be used as a loss-tunable filter. A possible explanation for the large loss variation is that the applied bending may generates an asymmetric strain distribution on the crosssection of fiber. Correspondingly, the photoelastic effect induced by the asymmetric strain distribution causes an asymmetric index distribution. The index perturbation will increase as the curvature increases, which may seriously affect the coupling efficiency. Generally, loss-tunable
Fig. 5. (a) Transmission spectra of the MTLPFG with different curvatures. (b) The dependence of wavelength shift on the curvatures, which shows linear correlation when the curvature is larger than 1.15 m−1. (c) Measured variation in transmission loss at the wavelength of 1578.6 nm (corresponding to the peak wavelength of LP07 mode) as a function of stepping length (in step of 0.01 nm).
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Fig. 6. (a) Transmission spectra of the MTLPFG with different twist angles. The twist angle changes from −720° to 720° with a step of 90°, and the angle −720°, 720° and 0° (original) are marked by blue arrow, red arrow and black arrow, respectively. (b) The dependence of wavelength shifts on the twist rate. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
fiber holders are 90 mm apart and the length of the MT-LPFG is 26 mm. The twist rate τ can be expressed as [30]:
τ =
πβ , 180L
(4)
where the β is the twist angle, and L (=90 mm) is the distance between left and right fiber holders. In the experiments, the positive and negative angles refer to clockwise direction and counterclockwise direction, respectively. The transmission spectra of the MT-LPFG with different twist angles are shown in Fig. 6(a). The resonance dip has a red shift when the MT-LPFG is twisted in clockwise direction. Conversely, when twisting the MT-LPFG in counterclockwise direction, the resonance dip shifts to shorter wavelengths. Due to the presence of asymmetric strain modulation caused by the double CO2 laser beams illumination on the cross-section of the fiber, the grating is a directionality torsion sensor. When the grating is twisted counterclockwise or clockwise, the transmission spectrum will shift toward different directions as the twisting direction changes [31]. At the same time, the twist induced shearing force within the MT-LPFG will result in a torsional photoelastic effect, thus generating index perturbation in this grating, as such, it will seriously affect the coupling efficiency as the torsional force increases. Fig. 6(b) depicts the relationship of wavelength shift against the twist rate. The result shows a roughly linear relationship between wavelength shift and twist rate with the relative value of about 0.973 and the torsion sensitivity of 0.169 nm m rad−1. This sensitivity is about 2.5 times higher than that of the conventional LPFG [27]. Based on the directionality of the wavelength shift in different torsion directions, the MT-LPFG has a great prospect for applications as a direction-discernible torsion sensor.
Fig. 7. Transmission spectra of the fabricated MT-LPFGs, where the blue solid line represent the first fabricated MT-LPFG1 with a period of 397 μm and the red solid line represent the second fabricated MT-LPFG2 with a period of 400 μm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3. Filtering characteristics of the cascaded MT-LPFGS In this section, a couple of MT-LPFGs, which are slightly different in period, are utilized in constructing the two-cascaded MT-LPFGs. Initially, the first MT-LPFG (MT-LPFG1) is fabricated with a grating period of 397 μm and a length of 25.8 mm (65 periods). After that, the second MT-LPFG (MT-LPFG2) is also fabricated but with a slightly longer period of 400 μm and a length of 26 mm (65 periods). In order to prohibit the cladding mode produced in MT-LPFG1 from coupling back with the MT-LPFG2, there is a 15 cm spatial separation between the two gratings without removing the fiber coating. Therefore, it is expected that the cladding mode produced in MT-LPFG1 will be depleted before reaching the MT-LPFG2 due to the absorption of the fiber coating. Transmission spectra of the two MT-LPFGs are plotted in Fig. 7 with blue and red solid lines, respectively. Note that, due to the little difference in period (3 nm), there is a 10.4 nm separation of the resonance peaks between the MT-LPFG1 and the MT-LPFG2. The transmission spectrum of the two-cascaded MT-LPFGs is shown in Fig. 8. For comparison, the transmission spectrum of the MT-LPFG2 is also labeled by
Fig. 8. Transmission spectra of the MT-LPFG2 (with a period of 400 μm) and the two-cascaded MT-LPFGs.
black solid line in Fig. 8. As one can observe, the rejection band has a central wavelength of ∼1573 nm. The bandwidth of both cascaded MTLPFGs and MT-LPFG2 at a rejection level about 30 dB is given in Fig. 8. A wide bandwidth of ∼15 nm is achieved, which is at least 7.5 times larger than that of the single MT-LPFG. To fully access the optimal broadband rejection performance, precise design and fabrication of the two MT-LPFGs with specific periods is prerequisite. Although there is a slight fluctuation at the bottom of the rejection band, it is generally reaching an extremely high rejection efficiency of up to 99.9% at the 5
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Fig. 9. Transmission spectra of the two-cascaded MT-LPFGs with different twist angles in (a) counterclockwise direction and (b) clockwise direction. (Only MTLPFG2 section is twisted).
precisely controlled, and a high extinction ratio of ∼40 dB has been achieved. The sensing features of strain, bending and torsion of the fabricated MT-LPFGs are experimentally investigated. Experimental results indicate that the strain and bending sensitivities can reach up to 0.85 pm/με and 13.7 nm/m−1 with good linear relationship, respectively, which is found to be several times higher than conventional LPFGs, and a high torsion sensitivity of 13.7 nm/m−1 is achieved with the abilities of rotational direction identification and twist rate measurement. Furthermore, by simply cascading two MT-LPFGs with small period gap, a broadband band-rejection filter has been obtained successfully based on the combination of two dips. Finally, an extremely high extinction ratio of ∼70 dB are readily achieved by merely twisting one of the MT-LPFGs contained in the two-cascaded MT-LPFGs. Considering the superior filtering properties that we demonstrated above, it is envisaged that this kind of two-cascaded LPFGs may find promising applications for the aspect of filtering.
top position of the fluctuation. As such, this slight fluctuation has negligible effects on the filtering performance in practical applications. At the same time, a wide bandwidth of 30 nm is also obtained at a rejection level of 15 dB. Obviously, two spectra are overlapped directly without modes interference. When the MT-LPFG1 and MT-LPFG2 are cascaded, and the properties of the newly obtained spectrum are largely determined by the property of each grating and the separation of their resonance peaks. Therefore, we should find a balance between the depth and bandwidth of the rejection band. To investigate the tunability of the two-cascaded MT-LPFGs, a measurement setup similar to that used in the torsional sensing experiment is utilized. Note that only the section containing the MTLPFG2 of the two-cascaded MT-LPFGs is fixed between two fiber holders while the section of MT-LPFG1 is well kept to isolate its interference on the spectrum of MT-LPFG2. The left and right fiber holders are separated by 90 mm. The negative angles are defined when the fiber is twisted in a counterclockwise direction. Fig. 9(a) shows the evolution of the transmission spectrum when the section of MT-LPFG2 of the twocascaded MT-LPFGs was twisted from 0 to −720° with a step of 90°. At beginning, the bandwidth decreases as the twist angle increases. When the twist angle is larger than −180°, the bandwidth does not change anymore, while the depth of the dip varies as the twist angle continues to change. Unlike the helical LPFG, which has little change in transmission loss as a torsion applied [6], the MT-LPFG has a significant change in transmission loss as the torsion increases. This is due to the coupling efficiency of the grating decreases as the torsion angle increases as we mentioned above. An amazing growth in extinction ratio of more than 35 dB are achieve by simply twisting the two-cascaded MT-LPFGs. It is worth noting that, an extremely high extinction ratio of ∼70 dB has been obtained at the twist angle of 450°. As shown in Fig. 9(a), it is apparent that there is no modes interference throughout the torsion process. Therefore, the two-cascaded MT-LPFGs could make great significance in the field of fiber communication when used as a single-band-rejection filter. Twisting the two-cascaded MT-LPFGs in a clockwise direction, similarly, only MT-LPFG2 section is twisted, the measurement results are shown in Fig. 9(b). It is clearly seen that only the right half of the spectrum have a red shift, which can be attributed to the tuning method we adopt of partial twisting to the two-cascaded MT-LPFGs. The variation of the right half spectrum is consistent with that of the single MTLPFG investigated before.
Funding information National Natural Science Foundation of China (NSFC) (61535015, 61805198). Declaration of Competing Interest There is no conflict of interest. References [1] H.J. Patrick, A.D. Kersey, F. Bucholtz, Analysis of the response of long period fiber gratings to external index of refraction, J. Lightw. Technol. 16 (1998) 1606–1612, https://doi.org/10.1109/50.712243. [2] W. Zhang, L. Huang, K. Wei, P. Li, B. Jiang, D. Mao, F. Gao, T. Mei, G. Zhang, J. Zhao, High-order optical vortex generation in a few-mode fiber via cascaded acoustically driven vector mode conversion, Opt. Lett. 40 (2016) 5082–5085, https://doi.org/10.1364/OL.41.005082. [3] Y.G. Han, S.B. Lee, C.S. Kim, J.U. Kang, U.C. Paek, Y. Chung, Simultaneous measurement of temperature and strain using dual long-period fiber gratings with controlled temperature and strain sensitivities, Opt. Express 11 (2003) 476–481, https://doi.org/10.1364/oe.11.000476. [4] K. Wei, W. Zhang, L. Huang, D. Mao, F. Gao, T. Mei, J. Zhao, Generation of cylindrical vector beams and optical vortex by two acoustically induced fiber gratings with orthogonal vibration directions, Opt. Express 25 (2017) 2733–2741, https:// doi.org/10.1364/OE.25.002733. [5] T. Zhu, Y.J. Rao, Y. Song, K.S. Chiang, M. Liu, Highly sensitive temperature-independent strain sensor based on a long–period fiber grating with a CO2-laser engraved rotary structure, IEEE Photon. Technol. Lett. 21 (2009) 543–545, https:// doi.org/10.1109/lpt.2009.2014566. [6] X. Kong, K. Ren, L. Ren, J. Liang, H. Ju, Chiral long-period gratings: fabrication, highly sensitive torsion sensing, and tunable single-band filtering, Appl. Opt. 56 (2017) 4702–4707, https://doi.org/10.1364/ao.56.004702. [7] R. Subramanian, C. Zhu, H. Zhao, H. Li, Torsion, strain, and temperature sensor based on helical long-period fiber gratings, IEEE Photon. Technol. Lett. 30 (2018) 327–330, https://doi.org/10.1109/lpt. 2017.2787157.
4. Conclusions In summary, a simple method for non-destructive fabrication of high-quality MT-LPFGs is demonstrated. Attributing to the slightly tapering technique, the extinction ratio of the grating is able to be more 6
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