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.

Figure 1 depicts a schematic diagram of the cascaded TFBG structure for SRI measurement. It is well known that in a standard FBG, the only strong coupling permitted occurs between the forward and backward-propagating core modes, for which the light is confined near the fiber axis and isolated from the surroundings by the relative thick cladding.^[20] However, for a TFBG, the tilted fringes can improve the light coupling from the forward-propagating core mode to the backward-propagating cladding modes at shorter wavelengths, and simultaneously reduce the coupling of the backward-propagating core mode. According to the phase-matching conditions, the resonant wavelengths of the backward coupled core (λ_co) and cladding (λ_cl) modes of the TFBG can be expressed as follows:

Fig. 1.

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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. 1, due to the smaller diameter, the arc power and arc time need to be adjusted for low splice loss. The photosensitivity of SMF and RD-SMF was firstly enhanced by H₂ loading for 72 h. After splicing and hydrogenation, the TFBG was UV-inscribed in series in the SMF and RD-SMF by using a frequency doubled continuous wave Ar₊ laser of 244 nm wavelength with the help of scanning phase-mask technique. The lengths of TFBG in SMF and RD-SMF are 8 mm and 10 mm, respectively. The tilt angle of the grating plane is about 6.5°, and the grating period is 543.8 nm determined by the phase mask. The fabricated cascaded TFBG was annealed at 80 °C for 48 h to release the residual hydrogen and stabilize the grating structure.

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 2(a). We can clearly observe a relative wide spectrum ranging from 1460 nm to 1560 nm held by the cladding mode resonances of the cascaded TFBG. As a comparison, the TFBGs with the same tilt angle and grating length only in the SMF and RD-SMF are shown at the bottom of Fig. 2(a). Clearly, if a TFBG with the same tilt angle of 6.5° is fabricated in the standard SMF, the cladding modes can only be excited in the spectrum range of 1500–1550 nm.

Fig. 2.

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	Fig. 2. (color online) (a) Measured transmission spectra of the cascaded TFBG (top with red line) and the TFBGs (bottom) with the same tilt angle in SMF (with magenta line) and RD-SMF (with blue line); (b) separations of adjacent cladding mode resonances at different wavelengths of the TFBGs.

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. 2(b). The separation between the adjacent resonances changes with the wavelength, and a larger separation appears at the shorter wavelength region. And also, the separation of the TFBG in SMF is smaller than that of the TFBG in RD-SMF. However, in the spectral region of 1500–1550 nm, the separation for the cascaded TFBG is mainly determined by that of the TFBG in SMF and periodically modulated by that of the TFBG in RD-SMF, and thus exhibits a periodical vibration, instead of the original exponential change with the wavelength. On the whole, the separation of the cascaded TFBG becomes smaller due to the superposition of the transmission spectra of the TFBGs in SMF and RD-SMF. Therefore, the measurement range will be expanded by the broader spectrum range of the TFBG, and also the measurement uncertainty will be reduced by the smaller separation of the comblike spectrum.

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.