1. Introduction

As a representative passive optical device, long-period fiber gratings (LPFGs) couples light from the basic core mode of the fiber to the common propagation cladding mode by periodically modulating the characteristics of the fiber. The scattering of the coupled optical signal at the interface of the dielectric and cladding, the energy coupled to the cladding mode are attenuated, resulting in a discrete attenuation band in the transmission spectrum. Utilizing the band-reject characteristics, LPFGs have exhibited potential in a variety of applications, such as tunable filtering [1] and gain equalization [2]. Because the effective indices of guided modes are essentially correlated with external physical or chemical parameters, LPFGs are often used as sensors [3–10] to detect different variables, such as DNA, immunosensing, bacteria, cells, and viruses. Those applications are applicable to the development of new device fabrication methods. To date, various fabrication methods, including ultraviolet exposure [11], CO₂-laser thermal shock [12], femtosecond milling [13], and mechanical stress modulation [14], have been adopted for LPFGs in the conventional fibers [1–8] or photonic crystal fibers [15,16].

Recently, with the development of optical fiber technology, the quality requirements of optical devices have become increasingly miniaturized [17,18]. Compared with conventional single-mode fibers, microfibers are smaller in size, finer in structure, larger in evanescent field and higher in sensitivity, which makes microfibers more advantageous in the fields of communication and sensing [19]. With technology advancements, the requirements for optical devices are also increasing. Efforts have begun to explore the preparation of long-period fiber gratings on microfibers [20–27]. In terms of such manufacturing methods, mLPG can be prepared by modifying or destroying the optical fiber structure, such as through femtosecond laser writing [22], carbon dioxide laser writing [23], and arc discharge stretching [24]. However, the main challenges with these methods are that modifying the optical fiber structure results in large device insertion loss and mechanical strength reduction. Moreover, mLPG preparation by point-by-point ultraviolet laser exposure is affected by the ultraviolet photosensitivity of the optical fiber materials [25], which limits the optical sensitivity of the devices when the diameter of the optical fiber decreases, such that additional materials need to be doped in the optical fiber [26]. Compared with the above methods, the structure-modulated mLPG has attracted attention due to its unique production process and its protection of optical materials, such as was demonstrated by manufacturing periodic structures in optical fibers using point dipping with PDMS [27]. However, this method still presents the problems of instability and peeling of spot glue when measuring the refractive index [28].

Here, a structural mLPG is fabricated by helically coiling one microfiber around another, which demonstrates an mLPG with a combination of band-reject and bandpass filtering characteristics. We discussed the structure and spectral characteristics of the prepared four-port mLPG and investigate the sensing characteristics of the LPG with strain, force, temperature, and refractive index, showing that the structure exhibits typical sensitivities of −6.4 pm/µɛ, −8418.6 nm/N, 7.62 pm/°C and 2122 nm/RIU, respectively. The results are instructive for future applications of mLPG structures.

2. Fabrication

Figure 1 shows the simulation diagram and electron microscope characterization diagram of the mLPG device respectively, which comprises a helically coiled thinner microfiber, with a diameter of d₂, around a thicker microfiber, with a diameter of d₁. The surface microfiber acts as a periodic perturbation to the field of the straight microfiber so that light can be resonantly coupled between the modes with different indices. The microfibers are obtained by locally heating and adiabatically tapering a standard single-mode fiber to the subwavelength scale with the assistance of the conventional flame-brushing technique [17]. By measuring the optical transmission of standard fibers at both ends, the waveguide characteristics of microfiber during the drawing process can be monitored on-site, including propagation loss, multimode interference, and group velocity delay [29]. The fabricated fiber size is controlled by controlling the heating temperature and tapering speed, and the taper technology is optimized to ensure that the fiber still maintains suitable consistency after refinement and the fiber shape shows good symmetry in both orthogonal directions [30]. The entire drawing process is tested by connecting the light source and spectrometer, where light can pass through the fiber taper with transmission loss below 1 dB.

 

Fig. 1. (a) Simulation diagram of mLPG device; (b) Electron microscopic characterization of mLPG devices; (c) Photo of mLPG structure connected to visible light sources.

Download Full Size | PDF

During fabrication, relatively large and relatively small diameters microfiber were prepared using the conical method mentioned above. The coarser microfibers are first fixed on two rotatable fiber scaffolds and maintain good contact with the thinner microfibers. Then, by rotating the two optical fibers simultaneously, the fine and coarse fibers move relative to each other such that the fine fibers wrap around the coarse fibers, the device insertion loss caused by the structure formed throughout the entire process is below 0.1 dB in the case of well matching. During winding, we monitor the transmission spectrum of a grating using a broadband light source (BBS) and an optical spectral analyzer (OSA). When the diameters of the two fibers are uniform, the helical structure around the straight fibers is considered to be equidistant. When light propagates in the grating, the guiding mode propagating in the same direction is coupled, the optical fiber wrapped around the device periodically modulates the ambient refractive index of the straight optical fiber, the observed dip is most possibly caused by the resonant coupling of HE₁₁ mode and HE₂₁ mode [23].When one end of a straight microfiber is input to detect the output signal from the other end of the straight microfiber, the transmission spectrum shown by the red line in Fig. 2(a) is obtained. When the detection end is changed to a relatively coiled microfiber port, the obtained spectral signal is shown by the blue line in Fig. 2(a). The two spectral signals are effectively complementary, which demonstrates the four-port characteristics of the prepared fiber grating. Therefore, this part of the light signal is detected directly by changing the access light path of the device. This phenomenon proves that compared with the traditional long-period grating, the long period grating developed here can simultaneously demonstrate favorable bandpass and band-reject characteristics, which enables broad applications in optical communication. Furthermore, the coupling between the two fibers can be increased by increasing the number of microfibers wrapped in the mLPG manufacturing process or by adjusting the length of the tail fibers at the natural descending end of the wrapped microfibers. The grating period length can be directly changed from the geometric structure, thus making the device parameters highly tunable. As shown in Fig. 2(b), the resonance peak of the grating blueshifts with increasing grating pitch, which is related to the inherent dispersion characteristics of microfiber long period gratings [23,24].

 

Fig. 2. (a) Transmission spectral characteristics and four port characteristics of the mLPG with d₁= 7.5 µm and d₂= 4.5 µm; (b) Transmission spectrum of mLPG samples with different periods with d₁= 6.93 µm and d₂= 2.75 µm.

Download Full Size | PDF

3. Sensing characteristics

To avoid long-term fatigue and even fracture of the elastically bent wire (due to bending stress), the formed structure is subsequently annealed through flame heating at a high temperature of ∼1300 °C. The heating requires approximately 0.1 s until the plastic deformation of the coiled microfiber is maintained. Figure 3 shows a comparison of the transmission spectra of a mLPG before and after annealing. The variation in the transmission spectrum is mainly attributed to the release of stress during heating of the microfibers. Another alternative approach is encapsulation immobilization of the fiber-optic device using a sol solution prepared from high-substituted hydroxypropyl cellulose, allowing the encapsulated device to withstand tensile forces as high as 1 N, which is suitable for detection in work of liquid environments at low temperatures [31].

 

Fig. 3. Transmission spectral characteristics of the mLPG with d₁= 8.5 µm, d₂= 4.65 µm, Λ=116 µm, and L = 0.58 mm. Solid curve: before annealing; dashed curve: after annealing.

Download Full Size | PDF

The strain response is measured using a displacement platform to pull the stretched microfiber and keep the crimped microfiber in a free state. Figure 4(a) shows the transmission spectra of the structural mLPG corresponding to several tensile strains. Within the strain range from 0 to 7050 µɛ, the dip wavelength blueshifts from 1593 nm to 1549 nm. The variation in the spectral attenuation is attributed to the slight modification of the coupling strength during fiber stretching, and the strain response of the LPFG mainly depends on the change in grating period and the difference in the refractive index of the fiber core and cladding. Due to the existence of the photoelastic effect, the change in the external applied stress causes a change in the resonance wavelength. Figure 4(b) details the measured dip wavelength as a function of axial strain. The wavelength decreases almost linearly with increasing strain, with a tuning efficiency of approximately −6.4 pm/µɛ. The strain sensitivity of mLPG is directly affected by the dispersion factor, and the dispersion factor is negative due to the negative growth relationship between the resonance wavelength and the grating period, resulting in the negative strain sensitivity of the grating [25]. The measured dip wavelengths during strain loading matches those measured during strain unloading very well. Distinct from conventional fiber devices, the structure can be readily stretched due to the low stiffness of the thinned diameter, enabling the structure to be used in sensitive force sensing. For this purpose, we built a precise force-sensing system to improve the measurement accuracy when characterizing the device.

 

Fig. 4. (a) Transmission spectra corresponding to several different strains, with d₁= 7.1 µm, d₂= 3.7 µm, Λ=114 µm, and L = 0.57 mm, respectively. (b) Resonance wavelength shift function of the mLPG force sensor under different axial strains.

Download Full Size | PDF

In the force sensing experiment, an experimental system is established by using a method similar to the fiber Bragg grating force measurement system to measure the applied axial force [32]. One side of the straight microfiber is fixed on a fiber holder, while an axial force is applied at the other end by adding weights. In this experiment, the applied force varied from 0 N to 6.86 × 10⁻³ N. From the data, we calculated the sensitivity for the mLPG as −8418.6 nm/N using linear fitting, which is a higher value than the previously reported force sensors in Refs. [32–34]. The force measurement repeatability is also evaluated by recording spectra with increasing and decreasing forces. As shown in Fig. 5, data from the two curves coincide, showing that the device exhibits good repeatability. Indeed, there are many prospective applications for the force sensor at this scale.

 

Fig. 5. Resonant wavelength offset function of the mLPG force sensor under different forces.

Download Full Size | PDF

The temperature response is investigated by placing the mLPG into a resistance air furnace. Figure 6(a) plots the dip wavelength as a function of temperature. With the temperature increases from 29 °C to 800 °C, the dip wavelength redshifts from 1361.53 nm to 1366.98 nm, giving a sensitivity of approximately 7.62 pm/°C. The temperature response of the mLPG is affected by the thermal expansion coefficient, thermal optical coefficient, and dispersion factor of the optical fiber, and the positive and negative effects of these factors lead to the positive sensitivity of temperature [35]. Low temperature sensitivity greatly reduces the effect of cross-sensitivity in the RI measurement.

 

Fig. 6. (a) Measured (dots) and fitted (solid curves) wavelengths at transmission dip versus external temperature in air, with d₁= 6.8 µm, d₂= 3.2 µm, Λ=154 µm, and L = 0.92 mm, respectively. (b) Measured (dots) and fitted (solid curves) wavelength as a function of external refractive index, with d₁= 6.6 µm, d₂= 4.5 µm, Λ=200 µm, and L = 1.0 mm.

Download Full Size | PDF

The sensitivity of the structural mLPG to the external refractive index is measured by immersing the structure into an aqueous solution of sucrose, and the index of the solution is modified by changing the sucrose concentration at room temperature. For the increase in RI from 1.3334 to 1.3551, the spectral dip redshifts from 1473.40 to 1521.14 nm. Figure 6(b) shows the detailed dip wavelength as a function of RI. The wavelength increases almost linearly with increasing RI, the refractive index sensitivity of mLPG is determined by the dispersion factor and the dependence on external refractive index changes, here both of which are negative, therefore the refractive index sensitivity is positive [35]. The measured sensitivity is approximately 2122 nm/RIU, which is comparable to that of the CO₂-laser-fabricated mLPG [23].

4. Conclusion

In conclusion, we fabricate a novel structured microfiber long-period grating and demonstrate its unique characteristics. We measured sensitivities of −6.4 pm/µɛ, −8418.6 nm/N, 7.62 pm/°C and 2122 nm/RIU for the strain, force, temperature, and refractive index, respectively. Due to the limitations of microfiber in different sensing applications, we cannot detect multiple parameters on one device, but the structure still offers many distinctive advantages over previously reported gratings, including compactness, flexibility, robustness, and a large evanescent field effect. It therefore demonstrates utility in possible applications, such as its simultaneous bandpass and band-reject characteristics are similar to a long-period waveguide grating coupler [36], with the difference that our device only needs to be implemented on one grating instead of requiring multiple gratings for matching. And its broadband filtering characteristics also provide possibilities for the construction of fiber lasers [37,38].