A surface plasmon resonance sensor based on a dual-groove photonic crystal fiber (PCF) for refractive index (RI) sensing with ultra-wide measurement range and high sensitivity is designed and theoretically investigated. The upper and right grooves of the PCF as different sensing channels are coated with Au or Au-TiO 2 compound film. The influence of the parameters on the sensing performance of the designed sensor are analyzed. Numerical results show that the maximum wavelength sensitivity (WS) of 6800 nm/RIU with a wavelength resolution of 1.47× 10 −5 RIU and maximum amplitude sensitivity (AS) of 5440 RIU −1 for the x polarization in the ultra-wide measurement range from 1.25 to 1.43 have been achieved. For y polarization, the proposed sensor has a maximum WS of 13200nm/RIU and AS of 3465 RIU −1 in the detection range from 1.39 to 1.43. The corresponding wavelength resolution is obtained about 7.58× 10 −6 RIU. Moreover, the ultra-wide range and high sensitivity of the sensor can be flexibly adjusted by a polarization controller to meet different practical requirements. Therefore, the proposed sensor would be a suitable candidate for medical testing, bio-sensing and environmental monitoring.
In this paper, a continuously tunable microwave photonic filter (MPF) with optical frequency comb (OFC) generated by recirculating frequency shifter (RFS) based on an inphase/quadrature (I/Q) modulator is proposed and experimentally demonstrated. A chirped fiber Bragg grating (CFBG) is inserted into the RFS loop to act as an optical bandpass filter. The central frequencies of two passbands of the MPF in 20 GHz range have been tuned linearly from 7.67 GHz to 9.04 GHz, and 15.34 GHz to 18.03 GHz, respectively by changing the frequency of the driving microwave signal onto the I/Q modulator from 20 GHz to 17 GHz. When the frequency of the driving microwave signal is decreased, OFC with smaller tone spacing is generated, and then the MPF's passbands move to higher frequency. So that, the proposed MPF's passbands with high frequencies can be achieved by applying driving microwave signal with low frequency to generate the OFC. By varying the bandwidth of the CFBG, the number of comb lines can be changed, which results in different bandwidth of the MPF's passband. Furthermore, in our experiment by carefully adjusting bias voltage of the Mach-Zenhder modulator, when the frequencies of driving microwave signal onto the I/Q modulator is 20 GHz and 18 GHz, one passband in the 20 GHz range can be suppressed at the frequency of 15.34 GHz, and 8.52 GHz, respectively.
In this paper, we have proposed and experimentally demonstrated a multiplexed sensing interrogation technique based on a flexibly switchable multi-passband RF filter with a polarization maintaining fiber (PMF) Solc-Sagnac loop. A high-order Solc-Sagnac loop can be used as a spectrum slicer as well as sensing heads, and a multi-passband microwave photonic filter (MPF) can be achieved together with a dispersive medium. Environmental parameter variations will cause a frequency shift of the corresponding passband of the MPF, so by employing only one Sagnac loop, it is possible to monitor several environmental parameters simultaneously. In this article, we have demonstrated and analyzed the performance of the flexibly switchable multi-passband MPF by using a second-order Solc-Sagnac loop. To demonstrate the temperature sensing capabilities of our interrogation system, we have applied temperature changes individually to Sensor Head 1 ( L P M F 1 ≈ 0.97 m ) only, Sensor Head 2 ( L P M F 2 ≈ 2.97 m ) only, and both Sensor Head 1 and 2 in the experiment. By monitoring frequency shift of the MPF’s passbands, the sensitivities for Sensor Head 1 and Sensor Head 2 have been estimated to be -0.275 ± 0.011 MHz/℃ and -0.811 ± 0.013 MHz/℃ respectively, which show a good sensing linearity and stability. By utilizing the longer length of the sensing PMF, higher sensitivity can be achieved. By using Solc-Sagnac loop with higher order, more sensors can be multiplexed.
In this paper, a continuously tunable microwave photonic filter (MPF) with optical frequency comb generated by recirculating frequency shifter (RFS) based on an IQ modular is proposed and experimentally demonstrated. The optical comb generated by the proposed scheme possess good flatness and high optical signal to noise ratio. Optical frequency comb with 63 comb lines can be obtained by RFS based on the single-sideband recirculating frequency shifter (SSB-RFS) scheme, by employing the optical filter with bandwidth of 10 nm and the modulation microwave signal with the frequency of 20 GHz. In addition, the modulation depth of the stable optical frequency comb exceeds 15dB, and the flatness fluctuation of optical frequency is less than 3dB. MPF with narrow bandwidth passbands has been implemented by taking the generated optical frequency comb as the optical source together with a dispersive medium. By changing the frequency of the modulation signal, the number and wavelength interval of optical frequency comb can be changed. As a result, the free spectrum range (FSR) and bandwidth of the MPF can be tuned and in our experiment the central frequencies of two passbans of the MPF has been tuned linearly from 7.38 GHz to11.30 GHz, and 14.65 GHz to 19.53 GHz, respectively.
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