1. Introduction

Fiber optical sensors based on long period gratings (LPGs) have attracted considerable attention in the past few decades [1–4]. Refractive index (RI) is an important parameter that is useful in environment monitoring, food industry and bio/chemical application, and many LPG based RI sensors have been proposed and demonstrated owing to the intrinsic high RI sensitivity of LPGs [3]. However, standard LPGs usually exhibit their highest sensitivity for RI larger than 1.4, i.e. those approaching that of optical fiber, while a much smaller sensitivity for low RI region (1.33–1.35), where many aqueous solutions fall into. To enhance the sensitivity of LPGs in low RI region, we can design the LPG to work close to or at the turning point of the dispersion curve [4], but a careful control of the fabrication conditions is required [5]. Moreover, the cross sensitivity to ambient perturbations is also increased, and the resonant bandwidth of turning point LPGs (TP-LPGs) is usually too broad to be well identified [6]. Modification of the fiber structure, including thin film coating [7–11], and reducing the cladding thickness by chemical etching or tapering [12–17], is also an effective method. But additional processing of the LPGs is required, and the gratings become fragile when the cladding thickness is reduced. Recently, several schemes integrating the aforementioned methods are also proposed [18–21]. However, despite the high sensitivity realized, a complex post-processing process is involved [21].

Fundamentally, the sensitivity of an evanescent wave based RI sensor is usually governed by the fraction of light spreading into the surrounding solution [22, 23]. In terms of LPGs, the resonant cladding modes become less confined when the difference between their effective index and the surrounding RI decreases. This is why previously reported LPGs show an increased RI sensitivity when the surrounding RI increases close to that of fiber [3]. To enhance the RI sensitivity of LPGs in low refractive index range, one simple way is sensing with cladding modes that have smaller effective indexes. For the aqueous solutions considered here, the coupled cladding modes should have an effective index around that of water, i.e. 1.33. Those modes will be “weakly guided” in aqueous solutions and consequently, a high sensitivity can be achieved.

To realize such a high order cladding modes coupling in normal single mode fiber (SMF), a much smaller grating period is required according to the phase matching condition [23]. From the fabrication point of view, however, the grating period is too small to be introduced with the conventional CO₂ laser or electronic arc [25, 26]. UV laser exposure method works well in terms of the grating period, but considering the intrinsically low coupling coefficients for high order cladding modes [24], it is not easy to achieve a strong enough coupling unless a large tilt is introduced [27], and the requirement of photosensitivity may limit its application.

To overcome the aforementioned problems, in this work, an eccentric LPG with a period of 15 $\text{μm}$ is fabricated with a femtosecond laser. No photosensitivity is required for the inscription, and coupling coefficients to cladding mode are significantly enhanced, as has been demonstrated effective in highly localized FBG [28–31], by the eccentric localized modification. Compared with the standard LPG, the fabricated small period LPG is compact in size, and the RI sensitivity is successfully improved in low RI region.

2. Fabrication and analysis of the grating

In our experiments, the femtosecond laser centered at 520 nm with a pulse duration of 350 fs and a repetition rate of 200 kHz was used to introduce the periodic refractive index change. Figure 1(a) shows the schematic of the fabrication configuration. The laser pulses were focused by an oil immersion objective lens (Olympus, 63×). A section of stripped SMF (YOFC, G. 652), which has no further pre-processing such as cladding diameter reduction or hydrogen loading, was fixed on the high resolution three dimensional stage. The fabrication process was monitored by a CCD camera. The laser was focused into the fiber core with a 1.5 $\text{μm}$ offset from the core center, and was periodically switched on and off when the fiber was translated along the fiber axis with a speed of 10$\text{μm/s}$. Here the grating period was designed to be 15_$\text{μm}$ with a 50% duty cycle, and 270 periods were introduced for the LPG discussed in this paper, which corresponds to a length of 4.05 mm. Figure 1(b) is the microscope image of the fabricated LPG viewed along the laser direction, where the eccentric modification can be seen.

 

Fig. 1 (a) Schematic of grating fabrication configuration. (b) Microscope view of the fabricated LPG.

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The transmission spectrum of the fabricated LPG is shown in Fig. 2(a), where super-continuum light source (YSL Super-continuum Source CS-5) and an optical spectrum analyzer (OSA, YOKOGAWA AQ6370C) were used. We can see that though the LPG is short in length, a strength of around 8 dB is realized. The insert loss is relatively high (~4 dB), which results from the Mie scattering [32]. Since the Mie scattering is wavelength dependent, the loss at short wavelength side is higher. The insert loss can be reduced with decreased pulse energy [32], but the coupling efficiency will also be reduced, so for real application, a balance between the compactness and the insert loss should be considered. Splitting of the resonance bands observed in Fig. 2(a) is attributed to the offset inscription which gives rise to the coupling to asymmetrical cladding modes [33]. Figure 2(b) shows the high order Bragg reflection spectrum of the grating [34], where index matching gel was used to eliminate the unwanted background reflection. The Bragg resonance results from the much smaller grating period than normal LPG, but is hard to be distinguished in the transmission spectrum because of the limited grating periods (270) and the eccentric inscription, which enhances the coupling to cladding modes but weakens the self-coupling of core mode, and the high Bragg grating order may also contribute to the low reflectivity.

 

Fig. 2 (a) Transmission and (b) reflection spectrum of the eccentric LPG. The reflection spectrum was measured with index matching gel to eliminate the background reflection.

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The effective index of the resonant cladding modes can be well estimated at wavelength 1 and wavelength 2 (indicated by the vertical line in Fig. 2) with the phase matching condition:

where ${\lambda }_{LPG}$ is the resonance wavelength of the LPG, $nef{f}_{co}$ and $nef{f}_{cl,i}$ are the effective index of the fundamental core mode and $i^{th}$cladding mode, and $\Lambda$ is the grating period. The reason for choosing wavelength 1 and wavelength 2 is that at these wavelengths, the Bragg reflection and the LPG resonances almost overlap, so the effective index of the core mode can be calculated first from the phase-matching condition for fiber Bragg grating (FBG):

Here N denotes that the reflection results from the $N^{th}$order Bragg resonances, which can be un-ambiguously determined together with $nef{f}_{co}$, considering that the effective index of core mode should be around the RI value of the fiber core (germanium-doped silica) and fiber cladding (pure silica). The calculated results are listed in Table 1, from which we can see that coupling to cladding modes with effective index close to water is achieved with the 15 $\text{μm}$ period LPG.

Table 1. Estimated effective index at the indicated wavelengths

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The polarization dependence of the LPG is also characterized. We used a polarizer followed by a polarization controller to control the polarization of the light entering the LPG. Figure 3 shows the measured results, where the two polarizations are set to be along (P1) and vertical to (P2) the direction of the inscription laser [31]. We can see from Fig. 3 that the resonance at ~1565 nm shows an opposite polarization dependence with a smaller wavelength splitting (3.9 nm), indicating that the resonance arises from coupling to different set of cladding modes, e. g., the asymmetrical modes, as has been previously reported in LPG fabricated with electric arc technology [33]. The strong polarization dependence mainly results from two reasons: one is the birefringence introduced by the eccentric inscription; the other is the intrinsic properties of high order cladding modes, which are far away from the weakly guided regime, and vectorial effects should be considered [28, 35].

 

Fig. 3 Polarization dependence of the LPG transmission spectrum.

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3. Response to surrounding refractive index and temperature

In order to assess the RI sensitivity of the LPG in low refractive index range, the proposed grating was immersed into NaCl solutions of different concentration. The grating was kept straight during the measurement to alleviate unwanted perturbation. Figure 4 shows the spectral evolution of the small period LPG when the surrounding RI is increased from 1.3328 to 1.3544 (top to bottom). We can see that resonant wavelengths have a red shift with increasing surrounding RI, and when the surrounding RI reaches 1.3544, the two peaks at the longest wavelength side spread and the coupling becomes weak, indicating that the corresponding cladding modes are no longer guided. It can be seen from Fig. 4 that the resonant peak does not spread immediately after the surrounding RI exceeds its effective index, which can be explained by the fact that the surrounding RI is still lower than the RI of fiber cladding, so the cladding modes, though become leaky, can still propagate a short distance. As the RI sensitivity of an LPG increases with surrounding RI for guided cladding modes, the point where the resonant peaks almost disappear, here around 1.3544, corresponds to the highest RI sensitivity that certain cladding mode can reach.

 

Fig. 4 LPG transmission spectra in different surround refractive index.

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The evolution of the peak wavelength at the shortest wavelength side (labeled as Peak 1 in Fig. 4) is depicted in Fig. 5(a). The peak wavelength shifts by ~4.2 nm when the surrounding RI is increased from 1.3328 to 1.3544, exhibiting an average RI sensitivity of ~194 nm/RIU; while peaks at long wavelength side (labeled as Peak 2 and Peak 3 in Fig. 4) are more sensitive with average sensitivities of ~505 nm/RIU and ~494 nm/RIU, which are comparable with the sensitivity of fiber grating based plasmonic optic sensor in this RI range [36, 37]. To have a comparison, standard LPG with a period of 370 $\mu m$ has a sensitivity of only 29 nm/RIU in the RI range from 1.333 to 1.393 [38], suggesting an improvement of at least one order of magnitude for the small period LPG. The wavelength evolution is fitted with a quadratic curve, and the RI sensitivity at 1.3544 is estimated to be 246 nm/RIU, 846 nm/RIU, and 842 nm/RIU for Peak 1, Peak 2 and Peak 3, respectively. Note that while Peak 2 holds the highest sensitivity for this RI region, the peaks at shorter wavelength side have a large measuring range, and will achieve their highest sensitivity at a higher surrounding RI. Therefore, a high sensitivity can also be realized in other RI region with a proper choice of the exited cladding modes based on the proposed principle, e.g. sensing with the cladding modes that have an effective index close to surrounding RI.

 

Fig. 5 Wavelength response of (a) the peak around 1415 nm; (b) the peaks around 1660 nm and 1670 nm to different surrounding refractive index.

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To enhance the measuring accuracy of an LPG based sensor, the crosstalk with surrounding perturbation should be considered, especially the temperature cross sensitivity in RI sensing application. The temperature sensitivity of the small period LPG is characterized to evaluate the influence of temperature fluctuation. Figure 6(a) shows the measured wavelength shift against temperature for Peak 1, presenting a sensitivity of 9.8${\text{pm/}}^{\text{o}}\text{c}$. Figure 6(b) depicts that for Peak 2 (red) and Peak 3 (blue), with a sensitivity of 7.9${\text{pm/}}^{\text{o}}\text{c}$ and 6.5${\text{pm/}}^{\text{o}}\text{c}$. These sensitivities are comparable with that of FBGs but much smaller than that of normal LPGs, which can alleviate the demand for accurate temperature compensation required in TP-LPGs [6]. Further control and compensation of the wavelength shift due to temperature changes is to measure the Bragg reflection shift simultaneously because the Bragg reflection is independent of the surrounding RI.

 

Fig. 6 Temperature response of (a) the peak around 1415 nm; (b) the peaks around 1660 nm.

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4. Summary

In summary, we have demonstrated that the sensitivity of LPGs in low RI region can be enhanced by simply reducing the grating period, while the temperature cross sensitivity is alleviated. With a proper choice of the resonant cladding modes, the proposed method can also be used to improve the sensitivity in other RI region. The intrinsically low coupling coefficients for high cladding modes are significantly increased with the eccentric localized inscription supported by the femtosecond laser, as a result, LPG with acceptable strength can be realized with a length of only 4.05 mm. The reduced grating length will alleviate the volume of the sample consumed for each detection. Moreover, the grating can be fabricated in one step without any pre- or post-processing, making the proposed method much more universal. The combined advantages of easy fabrication, compact size, low temperature cross sensitivity, and high RI sensitivity in low RI region make the proposed grating promising for future bio/chemical application in water and water based solution.