Reflective Refractometer Research Articles

Rare Earth-Doped Microfiber Bragg Grating Refractive Index Sensor With Self-Photothermal Manipulation

We demonstrate a microfiber Bragg grating as the reflective refractometer with the function of self-photoheating. The rare earth-doped fiber provides not only the photosensitivity to the UV modulation but also the effective pump laser absorption and photoheating conversion. The rare earth-doped microfiber Bragg grating, which is cladding-removed by the hydrofluoric acid, can detect the tiny refractive index change with a resolution of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> RIU scale. The self-photoheating could be realized by 980 nm pump absorption and non-radiative transition process of the ytterbium and erbium ions, which are well-preserved by the core structure after etching. Monitored by the reflective grating signal, the photoheating can manipulate 30 °C increase of the device in the air, 8 °C in the ethanol, and 4 °C in the water. The second time-scale heating/cooling response allows the sensor to act as ad optical switch for engineering temperature. The work may open a new route for accelerative biosensing and pollutant determination and treatment.

Dual-point reflective refractometer based on parallel no-core fiber/FBG structure

Abstract A novel dual-point reflective fiber-optic refractometer based on multimode interference (MMI) effect and fiber Bragg grating (FBG) reflection is proposed and experimentally demonstrated, which adopts parallel structure. Each point of the refractometer consists of a single mode-no core-single mode fiber (SNS) structure cascaded with a FBG. Assisted by the reflection of FBG, refractive index (RI) measurement can be achieved by monitoring the peak power variation of the reflected FBG spectrum. By selecting different length of the no core fiber and center wavelength of the FBG, independent dual-point refractometer is easily realized. Experiment results show that the refractometer has a nonlinear relationship between the surrounding refractive index (SRI) and the peak power of the reflected FBG spectrum in the RI range of 1.3330–1.4086. Linear relationship can be approximately obtained by dividing the measuring range into 1.3330–1.3611 and 1.3764–1.4086. In the RI range of 1.3764–1.4086, the two sensing points have higher RI sensitivities of 319.34 dB/RIU and 211.84 dB/RIU, respectively.

Reflective Optical Fiber Refractometer Based on Long-Period Grating Tailored Active Bragg Grating

A reflective optical fiber refractometer based on a long-period grating (LPG) followed by an active fiber Bragg grating (AFBG) is proposed. The AFBG is inscribed in a short section of cobalt-doped optical fiber, which can be heated by pump laser and acts as a sensing signal indicator. The LPG functions as a bridge between the surrounding refractive index (RI) and the AFBG reflection, tailoring the power of pump laser for heating. Experimental results demonstrate that the wavelength response of this fiber grating-based reflective refractometer can be easily adjusted, and a significantly improved sensitivity within the RI range from 1.3403 to 1.4654 has been achieved.

Reflective fiber refractometer based on fiber Bragg grating inscribed in thin-core-fiber

A novel reflective refractometer based on a fiber Bragg grating (FBG) inscribed in a thin-core-fiber (TCF) stub was demonstrated. The cladding modes are excited by the core diameter mismatch between the single-mode-fiber (SMF) and TCF. The core mode and specific cladding modes in the TCF are reflected by the FBG and partly recoupled into the back-propagating core mode in the SMF. Experimental results demonstrate that the power of the cladding mode reflection changes with the surrounding refractive index (SRI). The temperature-immune SRI measurement with linear sensitivity of 54.55 dB per refractive index unit has been achieved by the cost-effective power detection within the refractive index range from 1.33 to 1.42. To eliminate the measurement deviation caused by the fluctuation of light source, the simple self-referencing detecting has been carried out by measuring the ratio of the cladding mode reflection and the core mode reflection.

Simple and compact reflective refractometer based on tilted fiber Bragg grating inscribed in thin-core fiber

A simple and compact reflective refractometer based on a tilted fiber Bragg grating (TFBG) inscribed in an ultra-high photon-sensitive thin-core fiber is proposed and experimentally demonstrated. The reflective refractometer utilizes a short piece of thin-core fiber containing one TFBG to ensure the recoupling of cladding modes. The reflection spectra occur in two well-defined wavelength bands that correspond to the Bragg core mode and cladding modes, respectively. It is found that the power of the collected cladding modes changes with the external refractive index (RI), while that of the Bragg core mode remains unaffected and can be used as the temperature reference. High RI sensitivity and temperature immunity of the proposed reflective refractometer are experimentally achieved.

Reflective Refractometer Based on a Thin-Core Fiber Tailored Multimode Fiber Bragg Grating

A temperature-independent reflective refractometer configuration based on a core-diameter-mismatch is proposed and experimentally demonstrated. The configuration consists of a short section of thin-core fiber (TCF) with a multimode fiber (MMF) tip inscribed with a fiber Bragg grating (FBG). The TCF excites cladding modes into the downstream MMF via the mismatch-core splicing interface, and the parts of the core and cladding modes are reflected back to the lead-in fiber core by the MFBG. The recoupling efficiency of the core and cladding modes is highly dependent on the surrounding refractive index (SRI) of the TCF and MMF. Experimental results show that some recoupled modes show a high sensitivity to SRI. Power-referenced and temperature-independent SRI measurement with improved sensitivity for resolutions with SRI values near 1.33 is achieved through the wavelengths of selective mode monitoring.

Reflective fiber-optic refractometer based on a thin-core fiber tailored Bragg grating reflection

A novel reflective refractometer based on a thin-core fiber (TCF) sandwiched between a leading single-mode fiber (SMF) and a fiber Bragg grating (FBG) imprinted SMF stub was demonstrated. The reflection from the fiber stub occurs in two well-defined wavelength bands, corresponding to the Bragg core mode and cladding modes. The TCF section functions as a tailorable bridge between the FBG core mode reflection and the surrounding refractive index (SRI). Linear response with enhanced sensitivity of 133.26 dB/refractive index unit for temperature-immune SRI measurement within the biologically desirable sensing range of 1.33-1.41 has been achieved via cost-effective power detection.