The simple and highly sensitive measurement of the refractive index (RI) of liquids is critical for designing the optical instruments and important in biochemical sensing applications. Intensity modulation-based polymer optical fiber (POF) RI sensors have a lot of advantages including low cost, easy fabrication and operation, good flexibility, and working in the visible wavelength. In this review, recent developments of the intensity modulation POF-based RI sensors are summarized. The materials of the POF and the working principle of intensity modulation are introduced briefly. Moreover, the RI sensing performance of POF sensors with different structures including tapered, bent, and side-polished structures, among others, are presented in detail. Finally, the sensing performance for different structures of POF-based RI sensors are compared and discussed.

A plastic optical fiber (POF) probe with a macro-bending tapered structure was proposed and optimized for the refractive index (RI) sensing. The principle of operation is based on the macro-bending loss and evanescent wave, which are introduced by the macro-bending tapered structure on the fiber, and modulated by the environmental RI. A heating and drawing method and thermal setting process were used to fabricate a tapered POF and form a stable macro-bending structure for the probe. The probes with cladding and without cladding were fabricated. By changing the taper waist and curvature radius, the sensing performance of the probe was optimized. The highest sensitivity of the probe with cladding reached 937%/RIU in the RI range of 1.33-1.41. By removing the cladding, the RI sensing range was expanded to 1.33-1.45, and the sensitivity was about 800%/RIU, lower than the ones with cladding. In addition, the temperature dependence of the sensor was investigated. The POF probe is a low cost solution for RI sensing applications, which has the features of simple structure, compact, and intensity modulation at visible wavelength.

We proposed a peanut-shape structure in-line fiber Mach-Zehnder interferometer (MZI) for simultaneous measurement of refractive index (RI) and temperature. Two kinds of demodulation methods were investigated and compared. The wavelength-related and phase-related character matrices of the in-line fiber MZI were determined for simultaneous measurement of RI and temperature, respectively. For the wavelength-related measurement, the highest temperature and RI sensitivities were 0.0709 nm/°C and -47.3620nm/RIU, respectively. The phase-related measurement was with a lower measuring errors compared to the wavelength-related measurement, and the highest temperature and RI sensitivities were -0.0632rad/°C (0.0764 nm/°C) and 77.0995rad/RIU (-93.2429nm/RIU), respectively. The minimum measuring errors of temperature and RI were 0.3800°C and 0.0004 RIU, respectively.

We proposed a bent core-offset in-line fiber Mach–Zehnder interferometer for refractive index (RI) sensing. It was fabricated by the splicing method and consists of a standard single-mode fiber sandwiched between two core-offset joints, with a core-offset distance of about 6 μm. The RI sensing performance of this so-obtained interferometer was evaluated and compared with that of straight ones. Straight interferometers exhibited a relatively low sensitivity (15.326 nm/RIU), inadequate for RI measurements. However, this sensitivity was greatly improved by simply bending them into U-shaped probes. The RI sensitivity was enhanced as the curvature radius was reduced; the highest sensitivity (358.039 nm/RIU) was achieved with an interferometer having a length of 30 mm and a bend radius of 16.42 mm. Hence, the experimentally confirmed high sensitivity of our proposed sensor, combined with its simple and compact structure, easy fabrication, and low cost, makes it a promising candidate for RI sensing applications.

In this paper, we introduce a refractive index (RI) sensor based on the surface plasmon resonance (SPR) effect. A commercial plastic optical fiber (POF) is fabricated with a D-shaped macrobend structure by heat setting and side polishing techniques to expose the core and enhance RI sensitivity. Gold is sputtered onto the polished surface of the POF, generating a film with a thickness of approximately 10 nm. When the RI of the environment changes, the propagation attenuation in the macrobend section and the SPR spectrum change simultaneously. Therefore, the RI can be determined by the dual mechanism of bending loss and spectral shift. The experimental results show that as the RI of the immersion liquid varies from 1.33 to 1.40, the sensing probe exhibits a bending loss change and a spectral shift. This result indicates that the sensitivities of the probe in regard to bending loss and SPR are 43 dB/RIU and 1570 nm/RIU, respectively.

An enhanced plastic optical fiber (POF)-based surface plasmon resonance (SPR) sensor is proposed by employing a double-sided polished structure. The sensor is fabricated by polishing two sides of the POF symmetrically along with the fiber axis, and a layer of Au film is deposited on each side of the polished region. The SPR can be excited on both polished surfaces with Au film coating, and the number of light reflections will be increased by using this structure. The simulation and experimental results show that the proposed sensor has an enhanced SPR effect. The visibility and full width at half maximum (FWHM) of spectrum can be improved for the high measured refractive index (RI). A sensitivity of 4284.8 nm/RIU is obtained for the double-sided POF-based SPR sensor when the measured liquid RI is 1.42. The proposed SPR sensor is easy fabrication and low cost, which can provide a larger measurement range and action area to the measured samples, and it has potential application prospects in the oil industry and biochemical sensing fields.