† Corresponding author. E-mail:
Project supported by the National Natural Science Foundation of China (Grant Nos. 61775182 and 61505165) and Marie Skłodowska-Curie Individual Fellowships in the European Union’s Horizon 2020 Research and Innovation Programme (Grant No. 660648).
We proposed and experimentally demonstrated a cascaded tilted fiber Bragg grating (TFBG) for enhanced refractive index sensing. The TFBG is UV-inscribed in series in ordinary single-mode fiber (SMF) and reduced-diameter SMF with the same tilt angle, and then excites two sets of superposed spectral combs of cladding modes. The cascaded TFBG with total length of 18 mm has a much wider wavelength range over 100 nm and narrower wavelength separation than that of a TFBG only in the SMF, enabling an enlarged range and a higher accuracy of refractive index measurement. The fabricated TFBG with the merits of enhanced sensing capability and temperature self-calibration presents great potentials in the biochemical sensing applications.
Surrounding refractive index (SRI) and temperature measurements are very important in various fields such as biochemical industry, environmental monitoring, and life science. Recently, optical fiber-based SRI sensors have attracted more interests because of their advantages of compact structure, remote interrogation, high sensitivity, etc. Tilted fiber Bragg gratings (TFBGs) belong to the short-period grating family, but the grating planes are slanted with respect to the fiber axis. The slanted grating planes can make the light propagating in the fiber core penetrate to the cladding even to much deeper surrounding medium, without any requirement of special alteration in the fiber cladding, such as etching, side-polishing, etc. TFBGs can therefore be used as SRI sensors with the combinations of advantages in mature fiber Bragg grating technology and resonant excitations of cladding modes, providing a promising structure for SRI sensing.[1] Based on the above merits, the TFBGs have been applied in the sensing of many parameters that can be converted to the measurement of SRI.[2–7] In the meantime, the core mode resonance provides an absolute power and wavelength reference, which can therefore be used to eliminate uncertainties such as the power fluctuation of the light source and the variation of the ambient temperature.[1, 8]
The tilt angle of a TFBG is an important parameter, which can determine the spectrum range of the cladding modes and the SRI measurement range. For the TFBG with a small tilt angle, it usually is highly sensitive in the SRI region above 1.4.[9–12] For covering the lower SRI region such as the refractive index of water where most biochemical applications require, it is a common method to increase the tilt angle of the TFBG.[1, 13] Another way is to coat some materials onto the TFBG surface. For instance, by coating the TFBG with a gold film to excite surface plamon resonance and smartly using its polarized spectral information, the SRI measurement range can be effectively extended even to air.[14–17] By integrating carbon nanomaterials with complex refractive index such as graphene or carbon nanotube onto the TFBG, the sensing capacity in the low refractive index region can also be enhanced based on the interaction between the nanomaterials and the cladding modes.[9,18,19]
In this work, we report a cascaded TFBG with the tilt angle of 6.5Å in ordinary single-mode fiber (SMF) and reduced-diameter SMF (RD-SMF) for the enhancement of the SRI sensing performance, without any coating or larger tilt angle. Excitation and superposition of two sets of cladding mode resonances will broaden the spectral range over 100 nm and reduce the separation between the cladding modes, achieving the enlarged range and enhanced accuracy of the SRI measurement. In addition, we investigate the SRI response in the range of 1.30–1.45, and obtain the sensitivities of 510.48 nm/RIU and 494.12 nm/RIU in different SRI regions. The fabricated TFBG with a small tilt angle can achieve a wide range of SRI measurement, and has the potentials in the applications such as biochemical and environmental sensing.
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![]() | Fig. 1. (color online) Schematic model of the cascaded TFBG. The inset is the photograph of the splicing joint between the SMF and RD-SMF. |
For a specific cladding mode, if its effective index is lower than the SRI, the cladding modes will no longer be confined in the fiber cladding, leaking into the surrounding medium. However, if the effective index is higher than the SRI, the cladding mode is totally confined by the relative thick cladding. When the effective index is equal to the SRI, regarding as a cutoff state, the wavelength of the cladding mode is marked as a cutoff wavelength, which is analogous to the critical angle of an Abbe refractometer.[21]
In the device fabrication, a segment of 15 mm long RD-SMF was spliced between standard SMFs by a fiber arc fusion splicer. The core/cladding diameter of the employed RD-SMF is approximately 3.8/80 μm. As displayed in the inset of Fig.
We characterized the transmission spectrum of the cascaded TFBG by an experimental system, including a super-continuum (superK) light source and an optical spectrum analyzer (OSA). Launched from the superK source, the broadband light covering the spectrum range of 1400–1600 nm was transmitted into the cascaded TFBG, and the transmission spectrum was monitored and recorded by the OSA with a minimum wavelength resolution of 0.02 nm. The measured spectrum of the cascaded TFBG around the air is depicted at the top of Fig
As the discreteness of the cladding modes will reduce the measurement accuracy of the SRI, we tracked the separations of the adjacent mode resonances of the TFBGs in SMF and RD-SMF and cascaded TFBG, and the results are shown in Fig.
In the experiment, we use the experimental system as described in the report[22] to examine the SRI response of the cascaded TFBG. The broadband light from the superK source is coupled in the TFBG, and the transmission signal is monitored and recorded by the OSA. And then, a series of refractive index matching liquids with the range of 1.300–1.450 and the interval of 0.005 are injected into the sample cell in turn to change the SRI around the TFBG.
Figure
![]() | Fig. 3. (color online) (a) Spectral evolution of the cascaded TFBG with the SRI change, and (b) the extracted wavelengths of the core mode and cut-off mode resonances versus the SRI. |
From the spectral evolution of the cascaded TFBG shown in Fig.
The change of the temperature around the cascaded TFBG will affect the grating period and the effective indices of the core mode and cladding modes via thermal expansion and thermal-optic effects, as a result, the transmission spectrum of the TFBG will globally shift with temperature. Then, we experimentally investigated the temperature response by placing the TFBG in a temperature-controllable heating chamber with a calibrated temperature probe. We heated the TFBG from 0 °C to 80 °C, and recorded the spectrum in the interval of 10 °C. Different from the SRI response results, as the temperature increases, the spectral evolution demonstrates a constant coupling intensity but shifted resonant wavelength. Figure
We proposed, fabricated, and demonstrated a cascaded TFBG with two sets of cladding mode resonances for enhanced SRI sensing performance. Relying on the broad spectrum range of more than 100 nm of the cascaded TFBG, the SRI sensing range can be effective extended to 1.30–1.45. Meanwhile, by tracking the cutoff mode and core mode resonances, the SRI and temperature can be simultaneously obtained according to the strictly linear responses to temperature and SRI. The response coefficients of the temperature and SRI are 11.83 pm/°C and 510.48 nm/RIU (or 494.12 nm/RIU), respectively. Therefore, the cascaded TFBG in SMF and RD-SMF with rich spectral properties and enhanced sensing capacity could be desirable in-fiber devices for biochemical sensing, environmental monitoring, and other signal processing applications.
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