Guolu Yin

Simultaneous measurement of strain and temperature by employing fiber Mach-Zehnder interferometer

We demonstrated a novel fiber in-line Mach-Zehnder interferometer (MZI) with a large fringe visibility of up to 17 dB, which was fabricated by misaligned splicing a short section of thin core fiber between two sections of standard single-mode fiber. Such a MZI could be used to realize simultaneous measurement of tensile strain and temperature. Tensile strain was measured with an ultrahigh sensitivity of -0.023 dB/μɛ via the intensity modulation of interference fringes, and temperature was measured with a high sensitivity of 51 pm/°C via the wavelength modulation of interference fringe. That is, the MZI-based sensor overcomes the cross-sensitivity problem between tensile strain and temperature by means of different demodulation methods. Moreover, this proposed sensor exhibits the advantages of low-cost, extremely simple structure, compact size (only about 10 mm), and good repeatability.

High-sensitivity strain sensor based on in-fiber rectangular air bubble

We demonstrated a unique rectangular air bubble by means of splicing two sections of standard single mode fibers together and tapering the splicing joint. Such an air bubble can be used to develop a promising high-sensitivity strain sensor based on Fabry-Perot interference. The sensitivity of the strain sensor with a cavity length of about 61 μm and a wall thickness of about 1 μm was measured to be up to 43.0 pm/με and is the highest strain sensitivity among the in-fiber FPI-based strain sensors with air cavities reported so far. Moreover, our strain sensor has a very low temperature sensitivity of about 2.0 pm/°C. Thus, the temperature-induced strain measurement error is less than 0.046 με/°C.

Simultaneous measurement of pressure and temperature by employing Fabry-Perot interferometer based on pendant polymer droplet.

We investigated a novel and ultracompact polymer-capped Fabry-Perot interferometer, which is based on a polymer capped on the endface of a single mode fiber (SMF). The proposed Fabry-Perot interferometer has advantages of easy fabrication, low cost, and high sensitivity. The variation of the Fabry-Perot cavity length can be easily controlled by using the motors of a normal arc fusion splicer. Moreover, the enhanced mechanical strength of the Fabry-Perot interferometer makes it suitable for high sensitivity pressure and temperature sensing in harsh environments. The proposed interferometer exhibits a wavelength shift of the interference fringes that corresponds to a temperature sensitivity of 249 pm/°C and a pressure sensitivity of 1130 pm/MPa, respectively, around the wavelength of 1560 nm.

Asymmetrical in-fiber Mach-Zehnder interferometer for curvature measurement.

We demonstrated a compact and highly-sensitive curvature sensor based on a Mach-Zehnder interferometer created in a photonic crystal fiber. Such a Mach-Zehnder interferometer consisted of a peanut-like section and an abrupt taper achieved by use of an optimized electrical arc discharge technique, where only one dominating cladding mode was excited and interfered with the fundamental mode. The unique structure exhibited a high curvature sensitivity of 50.5 nm/m-1 within a range from 0 to 2.8 m-1, which made it suitable for high-sensitivity curvature sensing in harsh environments. Moreover, it also exhibited a temperature sensitivity of 11.7 pm/°C.

Long Period Fiber Gratings Inscribed by Periodically Tapering a Fiber

A promising technique for inscribing long period fiber gratings (LPFGs) was demonstrated by only using a commercial splicer. The commercial splicer was developed secondarily to build up a new program for periodically tapering a single mode fiber. High-quality LPFGs with a low insertion loss of ~1 dB and a large resonant attenuation of more than -30 dB were achieved. The achieved periodic tapers exhibited an excellent reproducibility with a small error of less than ±0.3 μm. To the best of our knowledge, it is the minimum reproducibility error of tapers achieved by arc discharge technique so far. Near mode fields of three LPFG samples with different pitches were observed to investigate the mode coupling in the taper-inscribed LPFGs. In addition, the resonant wavelengths of our taper-inscribed LPFGs exhibited a blue shift first and then red shift with an increased number of grating periods, resulting from residual stress relaxation together with physical deformation.

Tunable Er-doped fiber ring laser with single longitudinal mode operation based on Rayleigh backscattering in single mode fiber

A tunable and single longitudinal mode Er-doped fiber ring laser (SLM-EDFRL) is proposed and demonstrated based on Rayleigh backscattering (RBS) in single mode fiber-28e (SMF-28e). Theory and experimental study on formation of SLM from normal multi-mode ring laser is demonstrated. The RBS feedback in 660 m SMF-28e is the key to ensure SLM laser oscillation. This tunable SLM laser can be tuned over 1549.7-1550.18 nm with a linewidth of 2.5-3.0 kHz and a side mode suppression ratio (SMSR) of ~72 dB for electrical signal power. The tuning range is determined by the bandpass filter and gain medium used in the experiment. The laser is able to operate at S+C+L band.

Intensity-Modulated Strain Sensor Based on Fiber In-Line Mach–Zehnder Interferometer

We demonstrated a novel intensity-modulated strain sensor based on a fiber in-line Mach-Zehnder interferometer with a large fringe visibility of up to 17 dB, which was fabricated by splicing a section of thin core fiber between two sections of single mode fibers with one misalignment-spliced joint. Such a strain sensor exhibited an ultrahigh sensitivity of -0.023 dBm/με within a measurement range of 500 με, which is about one order of magnitude higher than that reported in references. Displacement and stress distributions at the misalignment spliced joint were simulated by use of finite element method. In addition, the proposed strain sensor has an advantage of compact size of ~10 mm.

Compact tunable multibandpass filters based on liquid-filled photonic crystal fibers

We demonstrated a compact tunable multibandpass filter with a short size of about 9 mm and a high wavelength-tuning sensitivity of up to -2.194  nm/°C by means of filling a liquid with a high refractive index of 1.700 into the air holes of a photonic crystal fiber (PCF). Such a PCF-based filter maintains an almost constant bandwidth and a large extinction ratio of more than 40 dB within the whole wavelength tuning range of more than 100 nm. Moreover, the transmission spectrum of the PCF-based filter is insensitive to the stretch force and the curvature of the fiber.

Simultaneous Refractive Index and Temperature Measurement With LPFG and Liquid-Filled PCF

We proposed and demonstrated a novel scheme for simultaneous measurement of temperature and refractive index (RI) by cascading a liquid-filled photonics crystal fiber (PCF) to a long period fiber grating (LPFG) written in a single-mode fiber. The liquid-filled PCF showed a high-temperature sensitivity of 1.695 nm/°C, which is two orders of magnitude higher than that of the LPFG (0.0642 nm/°C). The LPFG exhibited a blue shift with the increased RI. In contrast, liquid-filled PCF showed a total immunity to the surrounding RI. So, the temperature was solely measured by the liquid-filled PCF, and the RI information was extracted from the total response of the LPFG with the temperature effect having been compensated by the liquid-filled PCF.

Highly birefringent phase-shifted fiber Bragg gratings inscribed with femtosecond laser

We demonstrate a highly birefringent phase-shifted fiber Bragg grating (PS-FBG) inscribed in H2-free fiber with a near-infrared femtosecond Gaussian laser beam and uniform phase mask. The PS-FBG was fabricated from an ordinary fiber Bragg grating (FBG) in a case in which overexposure was applied. The spectral evolution from FBG to FS-FBG was observed experimentally with a decrease in transmission loss at dip wavelength, blueshift of the dip wavelength, decrease in the cladding mode loss, and an increase in the insertion loss. A high birefringence was demonstrated experimentally with the existence of PS-FBG only in TM polarization. The formation of the PS-FBG may be due to a negative index change induced by the higher intensity in the center of the Gaussian laser beam.