A Refractive index sensor based on tilted fiber Bragg grating and stimulated Brillouin scattering is proposed and demonstrated. The cladding modes of the tilted fiber Bragg grating is sensitive to the surrounding refractive index and the approach relies on refractive index dependent resonance wavelength shift of the modes. Stimulated Brillouin scattering is introduced to measure the wavelength shift due to its narrow bandwidth, which enhances the wavelength resolution to 0.25 pm and provides much higher refractive index sensitivity than traditional wavelength readout methods. This kind sensor is suitable to sense a very small variation of refractive index and the sensitivities can reach to a resolution of 1.27 × 10−4 RIU with the refractive index ranging from 1.3405 to 1.4025 and a resolution of 2.49 × 10−5 RIU ranging from 1.4025 to 1.4219, respectively.
© 2012 OSA
Surrounding refractive index (SRI) sensing is of great importance in chemical, biological and food industry since many substances can be detected by measuring the refractive index. Refractive index sensors based on fiber gratings are playing an important role in many areas due to the direct advantages such as compact size, low process cost, electrically passive operation, high sensitivity and integrated structure. Fiber gratings which can be photo-written into a silica fiber are easy to fabricate and this kind of sensors can sense SRI by monitoring the resonance wavelength shift of the fiber grating , measuring the optical power  and so on.
Compared to other kinds of fiber grating sensors, such as short period Bragg grating sensor  and long period grating sensor , tilted fiber Bragg grating (TFBG) sensors seem the most promising solution to the SRI sensing applications. TFBG is known as a kind of short period grating with a tilted index modulation pattern by an angle with respect to the fiber axis and is able to penetrate the evanescent field of the optical modes into the external medium to sense the SRI with a very high sensitivity. Many works have been done on TFBG sensors. Laffont and Ferdinand firstly proposed a TFBG sensor which shows good performance for SRI sensing . Chan and associates realized a 1 × 10−4 refractive index unit (RIU) accuracy of SRI measurement by monitoring the cladding mode resonance shift of TFBG . Miao and associates demonstrated a TFBG sensor by measuring the transmission power of TFBG with a resolution of 10−4 RIU . Guo and associates proposed a later offset structure of TFBG to sense the SRI by measuring the reflected optical power . Although TFBGs have been widely used in refractive index sensing applications, there are seldom publications about the sensor readout enhancement for improving the resolution of wavelength shift.
Because of its narrow bandwidth, optical spectrum measurement combined with Stimulated Brillouin scattering (SBS) has attracted much attention in the past two decades and been used in many applications to achieve high resolution. Domingo and associates demonstrated a high resolution optical spectrometry based on SBS with a 0.08 pm resolution and 80 dB dynamic range . Villafranca and associates achieved a complete characterization of a 10 Gb/s optical signal by using SBS . Mihelic and associates achieved a spectral resolution as low as kilohertz level with a dynamic range in excess of 90 dB .
Both TFBG and SBS are concepts of great interest in the field of fiber optic sensors and their potentialities can be usefully combined. In this paper, a refractive index sensor based on tilted fiber Bragg grating and SBS is proposed and demonstrated. The resonance wavelength shift of the TFBG’s cladding modes is corresponding to the SRI variation, and the SBS introduced in this approach is used to measure this wavelength shift. Due to the narrow bandwidth of SBS, the experimental results show that the sensitivity of the sensor is extremely high.
2. Principles and operation
The basic idea of this approach is to measure the spectrum shift of a TFBG which is immersed in different liquids with different refractive indices. SBS is used to enhance the resolution of the wavelength shift and achieve a high sensitivity refractive index sensor.
In this approach, the TFBG is a 7° tilted grating photo-written in a single-mode step-index optical fiber. The experimentally measured transmission spectrum in air is shown in Fig. 1 . The dip of rightmost is the Bragg resonance (core mode) which results from the coupling between the co- and contra- propagating core modes, and the wavelength resonances on the left side are the so called cladding modes which result from the coupling between the core mode and contra-propagating cladding modes. Because the cladding modes attenuate quickly along the fiber, the resonance peaks of those modes can only be observed in transmission spectrum, but not in the reflective spectrum. The titled angle also turns this fiber Bragg grating to an asymmetric optical waveguide and enhances the optical light coupling from the core mode to contra-propagating cladding modes. The TFBG transmission spectrum is determined not only by the structure of the grating, but also the SRI (next) and temperature. The effective indices of the cladding modes are sensitive to the SRI while the effective index of the core mode is insensitive to the SRI for single-mode optical fibers. The temperature can be kept constant in the experiment and the wavelength shift of cladding modes (△λiclad) can be written as follows :
Equation (1) shows that the wavelength shifts of cladding modes (△λiclad) are caused by the variation of the SRI (△next) and the different cladding modes may have different sensitivities to SRI changes. By monitoring the wavelength shifts of those modes at a constant temperature, the SRI can be sensed with a high resolution.
To enhance the sensitivity of the refractive index sensor, SBS is introduced in this approach. Due to the bandwidth of the Brillouin gain spectrum is within the order of a few tens of MHz, this interaction takes place in a very narrow-bandwidth spectral resonance, which is useful to achieve a very high resolution for the wavelength measurement. SBS is a well-known nonlinear effect in optical fiber, which is the result of the interactions among an acoustic wave, a Stokes wave and a pump wave. The Stokes wave and the pump wave counter-propagate in the fiber and create a moving interference pattern which induces an acoustic wave via electrostriction. The acoustic wave induces a periodic modulation of the refractive index and makes the Stokes wave amplify by the pump wave.
Figure 2 shows the scheme of the experimental setup. The TFBG was immersed in the different SRI solutions at a constant temperature. Different SRI solutions ranging from 1.3405 to 1.4219 were made by the calibrated sugar solutions with different concentrations. The light emitted from a tunable laser source (TLS) is split into two paths by an optical coupler. In the upper path, a microwave signal fm + fB (fB is the Brillouin frequency shift, which equals to 10.52GHZ in this approach) is applied to the electro-optic modulator (EOM) to realize double sideband suppressed carrier (DSB-SC) modulation . The sidebands are amplified by an erbium doped fiber amplifier (EDFA) before it transmits through the TFBG. The upper sideband of this modulated light will act as a pump wave. In the lower path, the split light is phase modulated by a microwave signal fm using an electro-optic phase modulator (EOPM) and the upper sideband is considered as a seed of Stokes wave. The phase modulated light is launched into a 3.1 km dispersion shifted fiber (DSF) through an isolator (ISO). When the power of the pump wave is larger than the threshold of SBS after it transmits through the TFBG, the seed of Stokes wave will interacts with the pump wave counter-propagating from a circulator and the SBS will be excited. So if the upper sideband of the phase modulated light is located in the Brillouin gain spectrum, it will be a seed of Stokes wave and be amplified by the pump wave. The π-phase-difference of the phase modulated sidebands will cancel each other while beating with the optical carrier at a PD, and the phase modulated microwave signal cannot be detected. When the upper sideband of phase modulated light is amplified by SBS in this approach, the amplitude-equality of the two sidebands is broken and the phase modulation to intensity modulation conversion is achieved. Then the modulated microwave signal can be detected by the following PD. In our experiments, a radio frequency cymoscope is placed after the PD to pick up the amplitude of the microwave signal fm and makes the system easier to monitor the refractive index change using a digital oscilloscope.
In common configurations of SBS, both sidebands of DSB-SC modulation are used to excite SBS [13–15]. In this approach, only the upper sideband is utilized and the lower sideband will affect the accuracy and should be eliminated. The fundamental of the operation with SBS excitation is shown in Fig. 3 . A microwave signal fm + fB is modulated on the incident light with a DSB-SC modulation (Fig. 3(a)). TFBG can be used as a narrow band filter. By adjusting the frequency of DSB-SC modulated microwave signal fm + fB, the wavelengths of the sidebands can be changed. The lower sideband can be tuned to the resonance wavelength of a cladding mode to gain large intensity attenuation while the upper sideband can be tuned to the peak of the cladding mode to gain large transmittance (Fig. 3(b)). Thus, only the upper sideband of the DSB-SC modulation can exceed the threshold to excite SBS as a pump wave (Fig. 3(c)) and interacts with the upper sideband of the phase modulation (Fig. 3(d)). The amplified signal can be obtained (Fig. 3(e)) and detected at the end of link (Fig. 3(f)).
Through the TFBG, the pump wave will experience different attenuation with different SRI solution because of the wavelength shift of cladding mode. The attenuation reduces the power of pump wave and the strength of SBS, which determines the amplitude of detected signal fm. Thus, by sweeping the microwave signal fm, the maximum amplitude of detected signal fm will be obtained at a different frequency of fm. In that case, the relationship of frequency fm detected at the PD with SRI can be obtained.
3. Result and discussion
Different cladding modes have different sensitivities of SRI and two different cladding modes are measured in our experiment. The resonance wavelengths are 1534.94 nm for cladding mode A and 1539.75 nm for cladding mode B, respectively. The wavelength of the TLS is set close to the peak wavelength of the chosen cladding mode, which makes the SBS is sensitive to the wavelength shift and the power attenuation of the pump wave.
The amplitudes of fm detected at the end of the link as a function of microwave frequency for different refractive indices of cladding mode A are shown in Fig. 4 . The peaks show that the excited maximal strengths of SBS are at 3.78 GHz, 4.26 GHz, 4.52 GHz and 5.30 GHz corresponding to the SRIs of 1.3496, 1.3523, 1.3538 and 1.3580, respectively. The peak frequency increases with increasing SRI. Figure 5 shows the SRI as a function of the detected microwave frequency fm for different cladding modes A and B, respectively. The relationships between microwave frequency and the refractive index are not quite linear and the refractive index sensitivity is different with different cladding modes, which are determined by the characters of TFBG. For cladding mode A, the refractive index changes from 1.3405 to 1.4025 corresponds to the microwave frequency from 1.98 GHz to 16.16 GHz, and the refractive index changes from 1.4025 to 1.4219 corresponds to the microwave frequency from 16.61 GHz to 39.97 GHz. Cladding mode B obtains a less sensitivity of SRI than cladding mode A, and the refractive index changes from 1.3405 to 1.4078 corresponds to the microwave frequency from 1.98 GHz to 11.21 GHz while the refractive index changes from 1.4078 to 1.4219 corresponds to the microwave frequency from 11.21 GHz to 18.02 GHz.
Besides the characteristics of the TFBG, the sensitivity of this approach is mainly determined by the full width at half maximum (FWHM) bandwidth of SBS, which measured in this experiment is about 30 MHz and enhances the resolution of wavelength shift to 0.25 pm. Take cladding mode A for example, by monitoring the wavelength shift with the resolution of 0.25 pm, the sensitivities of the TFBG immersed in different SRI solutions can reach to a resolution of 1.27 × 10−4 RIU with the refractive index ranging from 1.3405 to 1.4025 and a resolution of 2.49 × 10−5 RIU ranging from 1.4025 to 1.4219, respectively. The SBS is extremely sensitive to the environment because the Brillouin frequency shift varies with temperature (~1 MHz/þC). In the experiment, the temperature is kept at 24þC and the amplitude of variation is less than 1 degree centigrade, which means the Brillouin frequency shifts less than 1 MHz during the experiment. Compared to the FWHM of 30 MHz, the frequency shift is quite small.
The wavelength scanning technology is usually used to measure the cladding mode wavelength shift of TFBG with different SRI, and the minimum scanning interval determines the wavelength resolution. Optical spectrum analyzer is commonly utilized to measure the wavelength and the resolution is much lower than narrow-linewidth lasers. Compared to these measuring methods, a microwave modulated light can provide much higher wavelength resolution by using one sideband of the light. The wavelength of sideband can be adjusted by changing the microwave frequency and the resolution is higher than the methods above. However, a narrow tunable bandwidth filter will be required to prevent the carrier and the other sideband and make sure only one sideband is used to monitor the wavelength shift. SBS introduced in this approach replaces the traditional optical filter with a narrow bandwidth (10MHz-30MHz, depending on the power of pump wave) and wide wavelength tunability, which overcomes the disadvantages of the one sideband scanning method and provides a very high wavelength resolution.
In this approach, a refractive index sensor based on tilted fiber Bragg grating and SBS with high SRI sensitivity is experimentally demonstrated. The cladding modes of tilted fiber Bragg grating is sensitive to SRI and the resonance wavelength shifts with the variation of SRI. To measure the wavelength shift with a high resolution, SBS is introduced in the approach to enhance the wavelength resolution to 0.25 pm due to its filter character and narrow bandwidth. By choosing the appropriate cladding mode, the sensitivities of the sensor in the experiment reach to a resolution of 1.27 × 10−4 RIU with the refractive index ranging from 1.3405 to 1.4025 and a resolution of 2.49 × 10−5 RIU ranging from 1.4025 to 1.4219, respectively. As a readout method of wavelength shift, SBS shows good performance and highly enhances the sensitivity of refractive index sensor.
This work was supported in part by the National Natural Science Foundation of China (grant Nos. 61171036 and 61177003), the Natural Science Foundation of Zhejiang Province of China (No. R1090354), and the Fundamental Research Funds for the Central Universities (No. 2010XZZX002-9).
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