| In the 21st century,the research of life science has been upgraded to a high level,therefore fiber optical biosensor technology has received increasing attention.Among them,excessively tilted fiber grating（Ex TFG）introduces a greatly tilted grating region of approximately 10mm in the fiber core,is not only sensitive to external refractive index（RI）but also avoids sensing errors caused by environmental temperature changes,making it advantageous in the field of biological detection.Based on the principle of Ex TFG and Sagnac interferometer,the structure,principle and characteristics of a new type of optical fiber vernier RI sensor were presented and studied in detail.Ex TFG was performed as the sensing unit to modify graphene oxide（GO）film to fabricate biological immunosensor for the detection of canine distemper virus（CDV）,and the following studies have been carried out:（1）In theory,the effective refractive index of Ex TFG’s core mode and cladding mode of two polarization states were calculated based on numerical calculation tool,which explained the existence of adjacent resonant peaks in the transmission spectrum of Ex TFG.By studying the wavelength-period matching conditions,it is found that the central wavelength of TM resonant peak is smaller than that of TE resonant peak when the order of cladding mode m（30）11,which is consistent with the phenomenon observed in general in C-L band range.In addition,the waveguide dispersion factorsγatλ=1200 nm,1500nm and 1800 nm were studied.The longer the wavelength is,the closer the jump ofγis to the low order mode.At the same time,we studied the related theories of the Panda type maintaining fiber（PMF）Sagnac interferometer（PMF-SI）and analyzed its transmission spectrum characteristics.The general rule of the influence of PMF length on the free spectrum range（FSR）was summarized.Finally,the sensitivity amplification principle of vernier effect was analyzed by cascading two PMF-SIs with similar FSR.（2）Ex TFG was embedded into optical circuit to form an Ex TFG-based Sagnac interference ring（Ex TFG-SI）.The output spectrum of this structure showed a dense comb spectrum,FSR=1.36 nm,meanwhile the two polarization peak information of Ex TFG was preserved,thus the RI sensitivity of the interferometer is equal to that of ordinary Ex TFG after testing.Then the length L=2.55 m of PMF was intercepted to construct PMF-SI,and the output interference spectrum was obtained,which revealed that the FSR=1.52 nm.Then the optical fiber vernier sensor with envelope period of 13.01 nm was constructed by cascading PMF-SI and Ex TFG-SI.The experimental results showed that,in the SRI range of 1.3349-1.3423,the RI sensitivity of the proposed sensor reached-1286.40 nm/RIU,which is 8.46 times and 10.55 times higher than that of the TM and TE modes of single Ex TFG,respectively.Moreover,the temperature sensitivity of the sensor is only-114pm/℃even after vernier amplification,which is comparable to that of a typical LPFG.Finally,the spectral fluctuation of the sensor within one hour was evaluated,and the results showed that the sensor system has good stability.Due to its advantages of simple structure,high sensitivity and good stability,the sensor has certain application potential in the field of bio-detection.（3）For the proposed vernier sensor,Ex TFG was utilized as the sensing unit and GO nanofilm was modified on its surface to further immobilize CDV monoclonal antibody to complete biological functionalization.Then,immunoassay experiments were conducted by different concentrations of CDV antigen solution.The experimental results showed that the sensitivity of the immunosensor is-1.18 nm/[log（mg/ml）]in the low concentration range of1 pg/ml to 50 ng/ml,and the detection limit of CDV antigen can be as low as 1 pg/ml,with the ability to detect ultra-trace target biomolecules.In order to further validate the clinical application value of the proposed sensor,a control experiment was conducted using fetal bovine serum,toxoplasma gondii,rabies virus serum and CDV serum.The results showed that the vernier sensor has high specificity for CDV antigen and can effectively detect CDV molecules in clinical animal serum,which is of great significance in the early diagnosis of CDV. |
