A real-time gas vacuum sensor interrogated based on a microwave photonic method at high speed and high resolution is proposed and demonstrated. The sensor is a microfiber Mach-Zehnder interferometer (mMZI) with its wavelength sensitive to the gas concentration. Instead of detecting the wavelength shift of the mMZI spectrum in the optical domain, we convert the mMZI spectrum to the time domain by spectral shaping and wavelength-to-time (SS-WTT) mapping and apply a digital signal processor (DSP) to realize the cross-correlation to estimate the wavelength shift of the mMZI. The sensitivity and resolution of the proposed gas vacuum sensor are -0.586 ps/ppm and 34.13 ppm with a concentration range from 0 to 1.4×104 ppm, respectively.
We demonstrate an all-solid photonic bandgap fiber modal interferometer by concatenating two tapers separated with a middle section of the fiber. Unlike the conventional devices, our structure has a lower effective index in the core and a higher effective index in the cladding, which produce novel sensing characteristics. The measured sensing sensitivities are ~63pm/°C for temperarure and ~1.74nm/N for the axial stress, respectively.
We investigate the spectral characteristics of Brillouin scattering in micro-scaled silica fibers with diameter of ~2μm. A multiple-peak structure in contrast to the conventional counterparts is observed. The measured temperature and strain sensitivities are ~0.8MHz/°C and ~0.05MHz/με, respectively, corresponding to a fiber diameter of 2.01μm. A comparison with the conventional single-mode fiber is made in our manuscript.
In this paper, an abnormal grating evolution was recorded during microfiber Bragg grating (mFBG) inscription under 193nm excimer laser. Within 20 minutes exposing, a Type IIa FBG could be obtained with above 20dB strength in 8.5 μm microfiber. This regenerated mFBG had good survival ability against high temperature up to 800 °C. Moreover, the strain response of the regenerated grating was enlarged by the microfiber structure. Thus, highly sensitive strain sensor with considerable temperature resistance could be obtained, which had potential applications in gas/oil and aerospace territory.
We realize a microdroplet-etched fiber Fabry-Perot resonator. Strong polarization discrimination is achieved due to the asymmetric fiber cross section in the cavity, which should be useful for improving the measurement precision in the refractive index (RI) sensing application. The measured RI sensitivities are ~133.8 nm/RI-unit for the x polarization and 117.1 nm/RI-unit for the y polarizations, respectively. Simultaneously, the temperature effect can be eliminated by monitoring the peak difference of the two polarizations, which have the similar temperature coefficient but different RI responses.
We demonstrated a highly sensitive evanescent-wave-based water salinity sensor using a rectangular optical microfiber Sagnac interferometer. The microfiber has a rectangular cross-section with widths of 4.0 μm by 2.5 μm and total length of 36 mm. For water salinity from 0‰ to 40‰, a high sensitivity of 1.95 nm/(1‰) was achieved at the wavelength of 1550 nm, indicating a detection limit of 0.01‰. The proposed sensor has advantages of high sensitivity, compact size, ease for fabrication, and potentially low-cost. It is very useful for undersea applications and manufacture process controlling where monitoring small change in water salinity is required.
A magnetic field sensor is demonstrated by placing a bent-fiber taper modal interferometer inside a magnetic fluid sealed with an organic glass base. Owing to the strong refractive index dependency of the interferometer and magneto-optics property of the fluid, our sensor exhibits high sensitivity to the external magnetic field change. A linear wavelength dependency of ~58pm/Oe is experimentally obtained within a magnetic field range from 30 to 80 Oe. Our structure is featured of high sensitivity, fiber-compatibility, compactness, and robustness.
A highly-birefringent elliptic microfiber is fabricated by use of the CO2-laser machining and fusion tapering methods. The fiber ellipse can be well controlled with modification of the CO2 laser output power. Both positive and negative sensitivities are observed for the structure to be used in the refractive index sensing application, in contrast to the previously-reported microfiber devices. Moreover, the maximum obtained sensitivity is as high as 195348nm/RIU (refractive index unit) around refractive index of 1.35887, which is one order of magnitude higher than other microfiber counterparts. The temperature-cross sensitivity of 0.007nm/°C is quite low.
We demonstrate a highly-sensitive current sensor by packaging a single taper-based modal interferometer into a copper
tube that is filled with alcohol and surrounded with chrome-nickel wire. As the flowing current in the chrome-nickel wire
is changed, the interference spectrum varies accordingly with sensitivity as high as 1014.5 nm/A2 . Our results are
promising for the current sensing and the electric-tunable filtering.
We demonstrate a temperature-independent displacement sensor by inscribing a periodic grating in a microfiber taper with assistance of the 193-nm ultraviolet exposure technique. The obtained bandwidth is as large as 29.64nm for the grating with diameter of 3.8~6.38μm and length of 6.2mm, respectively. When the displacement is increased from 0 to 1.08mm, the reflecting bandwidth reduces to 3.38nm gradually, producing an average sensitivity of around −22.8nm/mm. The minimum displacement of measurement is ~4.39×10−4mm considering the wavelength resolution of 10pm in the optical spectrum analyzer. Moreover, the temperature-cross sensitivity is suppressed.
We demonstrated a simple method for temperature-independent refractive index measurement by use of two cascaded fiber Bragg gratings fabricated in single-mode fiber and microfiber, respectively. The reflective peaks of the two FBGs exhibit almost identical temperature sensitivity of 10.1 pm/°C and different responses to ambient refractive index. Based on the differential measurement method, of the issue of temperature cross-sensitivity for FBG sensors is solved. The refractive-index sensitivity of the sensor is 17.22 nm/RIU when the diameter of microfiber is 6.5 μm.
A Mach-Zehnder interferometer (MZI) based on a pair of long period gratings (LPGs) fabricated by silica microfiber for sensing applications is demonstrated. Each LPG with only 6 deformations was fabricated by using a pulsed CO2 laser to periodically modify the surface of the microfiber through only one scanning cycle. Owing to the relatively large effective refractive index (RI) difference between the fundamental and higher order modes of the microfiber LPG, the size of the microfiber MZI can reach as short as 8.84mm when the diameter of the microfiber is 9.5μm. The microfiber MZI can exhibit a high sensitivity of around 2225nm per refractive index unit and temperature sensitivity of only 11.7 pm/°C. Featured with the easy fabrication, excellent compactness, high sensitivity and stability, the microfiber MZI has potential in the microfiber-based devices and sensors.
High current sensitivity is obtained based on a microfiber that is wrapping around a chrome-nickel (CrNi) wire. Due to the strong heating effect of the CrNi wire with the flowing electric current, the mode index and the loop length of microfiber are changed, resulting in the shift of resonant wavelength. The measured current responsivity is as high as 220.65nm/A2, which is in two or three magnitude orders than the previously-obtained ones. We study the influence of component size to the structure performance, which is useful for future applications of current sensing or tuning devices.
We demonstrate an ultrasensitive temperature sensor by sealing a highly-birefringent microfiber into an alcoholinfiltrated copper capillary. With a Sagnac loop configuration, the interferometric spectrum is strongly dependent on the external refractive index (RI) with sensitivity of 36800nm/RIU around RI=1.356. As mainly derived from the ultrahigh RI sensitivity, the temperature response can reach as high as −14.72 nm/°C in the range of 30.9-36.9 °C. The measured response time is ~8s, as determined by the heat-conducting characteristic of the device and the diameter of the copper capillary. Our sensor is featured with low cost, easy fabrication and robustness.
We demonstrate four-port long-period gratings formed by winding an optical microfiber with another thinner
microfiber. The surrounding thinner microfiber not only induces a strong refractive-index perturbation in the center
microfiber, but also collects and leads out the light resonantly coupled from the fundamental mode to high-order
modes, providing flexibility for applications as optical filters and sensors. The devices exhibit temperature
sensitivity of 7.6 pm/°C, strain sensitivity of -10.6 pm/μ(epsilon) and refractive-index sensitivity of 2012.6 nm/RIU.
A compact microfiber sensor is implemented with the twist of a continuous rectangular microfiber. The structure can exhibit extremely-high sensitivity of around 24,373nm per refractive-index unit and temperature stability of better than 0.005nm/oC, implying a great suppression of cross-sensitivity. Thia sensor is featured with compact size, high sensitivity, easy fabrication, robustness, and low connection loss with all-fiber system.
We demonstrated a novel method for temperature-independent refractive index measurement by use of a Bragg grating
fabricated in a highly birefringent rectangular microfiber. The two reflective peaks corresponding to two polarization
axes exhibit almost identical temperature sensitivity of 12.01 pm/°C and different responses to ambient refractive index
of 38.9 and 46nm/RIU at RI of 1.36, respectively. By monitoring the wavelength separation between the two peaks,
temperature-independent refractive index measurement can be achieved.
Orientation-recognized two-dimensional vibration sensor based on a polarization-controlled cladding-to-core recoupling is demonstrated experimentally. A compact structure in which a short section of multi-mode fiber stub containing a weakly tilted fiber Bragg grating (TFBG) is spliced to another single-mode fiber without any lateral offset. Several well defined lower-order cladding resonances in reflection show different polarization dependence due to the tilted grating vector excitation. Both orientation and amplitude of the vibration can be determined unambiguously via dual-path power detection of the orthogonal-polarimetric odd-cladding-modes. Meanwhile, the unwanted power fluctuations and temperature perturbations can be definitely removed via core mode monitoring.
In this paper, temperature compensated microfiber Bragg grating (mFBG) is realized by use of a liquid with a negative
thermo-optic coefficient. The effects of grating elongation and the index change of silica glass are compensated by the
liquid through evanescent-field interaction. As a result, the reflective wavelength shifts by only 30 pm when the
temperature varies from 15 to 60°C. The proposed method is promising due to the compactness and high flexibility of
the device.
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