We demonstrate the first application of the periodic damage tracks in optical fibers formed by the fiber fuse. A refractive index sensor with a sensitivity of 350 nm/refractive index unit (RIU) is experimentally demonstrated. This technique can allow mass production by incorporating small sections of the damaged fiber into each device. It can also be applied widely in different types of sensing systems by coating sensing material on the damaged fiber’s cladding surface.
In this paper we presented the simultaneous measurement of liquid level and specific gravity with two fiber Bragg
grating (FBG) sensors and utilized the relationship between liquid level and pressure sensors. One FBG was fixed on the
surface of a cantilever beam as a liquid level sensor and another FBG was embedded in a polymer-half-filled metal
cylinder as a pressure sensor. The two sensors exhibited an approximately linear response and can be applied to measure
the unknown liquid level and specific gravity of liquid. This system can be applied for the oil tanks and batteries
monitoring.
We experimentally demonstrate that a 3×3 port fused fiber grating coupler can be implemented by combining the technique of fabricating fused-fiber couplers as well as that of writing fiber Bragg gratings for application in all-fiber optical filters. A Bragg grating with a reflectivity of 65% is written into the coupling region of a 3×3 fiber coupler with the coupling ratios of 60, 24, and 16%, respectively, for three different output ports. This device can be applied in optical multi/demultiplexing and add/drop filters or in special optical filters.
In this paper, we proposed a chemical-gas fiber sensor based on the poly-aniline film coated on the surface of an etched fiber grating and experimentally demonstrated for detecting ammonia gas. This sensing mechanism is based on the testing gas to interact with the poly-aniline coating film on the surface of the fiber grating to cause the thin-film index change and then to create the Bragg wavelength shift or grating reflectivity variation. The sensitivity and response time of this sensor for measuring ammonia gas are around 0.73 nm per percent concentration and tens of mini-seconds respectively, which depend on the optical characteristics of fiber grating, the diameter of fiber cladding, and the constituents of chemical sensing film. This sensor may provide a simple, reliable, repeatable and non-destructive fiber sensing technique.
We demonstrate that a multichannel fiber grating can be fabricated using a heating method based on differences in the thermal expansion of the phase mask and the fiber. The separation between two neighboring channels in a multichannel grating can be precisely tuned by means of temperature control. A three-wavelength-channel equal-spacing grating is implemented under three different temperatures of 25, 160, and 300°C, respectively. In addition, a chirp fiber grating is manufactured, whereby the temperature gradient is adjusted to obtain an increased, adjustable bandwidth, which can be used in optical filter applications.
KEYWORDS: Digital signal processing, Wavelets, Wavelet transforms, Signal processing, Linear filtering, Software development, Interferometry, Discrete wavelet transforms, 3D image processing, Detection and tracking algorithms
This paper presents a DSP-base defect recognition system by using wavelet transform and the gray level co-occurrence matrix (GLCM). It can be used to detect the texture image of wafer surface which is captured from a laser interferometer. Wavelet analysis associated with the entropy criterion appears to be a good method for recognizing automatically the defects of the interferometric patterns. Three-dimensional plots of the GLCM for various captured
images have been compared and discussed. The parameter of entropy has been calculated from the GLCM and can be used as an indicator for surface flatness.
This study demonstrates that the cladding modes of a tilted superstructure fiber grating can be coupled back to the core mode to create induced channels by acousto-optic interactions when the acoustic wave traveling along the fiber axis vibrates the fiber. This phenomenon is based on the acoustic wave vector matching the difference between the core mode and the cladding mode wave vector. Acoustic power levels can be used to control the reflectivity and number of the induced wavelength channels. Moreover, the wavelength location of the induced channel can be tuned by varying the acoustic wave frequency. Thus, the proposed device may provide a switchable multiwavelength comb filter for applications in wavelength-division multiplexing systems, fiber lasers, or fiber sensors.
We demonstrated that a high-sensitivity fiber sensor based on a superstructure fiber grating (SFG) can simultaneously measure the pressure and temperature by encapsulating the grating in a polymer-half-filled metal cylinder, in which there are two openings on opposite sides of the wall filled with the polymer to sense the pressure. The mechanism of sensing pressure is to transfer the pressure into the axial extended-strain. According to the optical characteristics of an SFG composed of a fiber Bragg grating (FBG) and long period grating (LPG), the various pressure and temperature will cause the variation of the center-wavelength and reflection simultaneously. Thus, the sensor can be used for the measurement both of the pressure and temperature. The pressure sensitivity of 2.28×10-2MPa-1 and the temperature sensitivity both of 0.015nm/°C and -0.143dB/°C are obtained.
The induced cladding-mode performance in a tilted superstructure fiber grating (SFG) is first experimentally demonstrated. As a slanted SFG is heated to shift wide-band loss dips for obtaining the multi-narrow-band reflection moved at various positions of the loss dip, the strength and central wavelength of the induced cladding-mode depend on the loss depth and the positive/negative sides of the dip, respectively. The characteristics may provide the third physical-parameter measurements in a simultaneous multi-parameters fiber sensor.
An all-fiber pressure sensor based on a fiber Bragg grating with the pressure sensitivity of 2.2x10-2 MPa-1 has been demonstrated. The physical configuration includes a FBG encapsulated in a polymer-half-filled metal cylinder with its end bonded to the central of a round plate attached to the surface of polymer, and the Young’s modulus of the polymer is four orders lower than FBG. This cylinder has two opening on opposite side of the wall at the polymer part. Under the pressure environment, the polymer can be pressurized along one radial direction only, and responds an axial force acting on the round plate, producing an axial strain on FBG. With a nice linearity, this sensor should be applied potentially for the measurement of mediums pressure, liquid level and depth underwater.
The application of the acoustic-induced vibration on a fiber Bragg grating has been proposed as the function of controlling reflectivity levels and switching reflection wavelengths. Moreover, a switchable multi-wavelength optical filter is expected to develop for various applications in optics. Thus an acousto-optic interaction in a superstructure fiber grating (SFG) can provide a multi-wavelength reflective filter with the function of switching the operation wavelength. In this paper, we experimentally demonstrated that the channels of a blazed SFG could be increased or switched as the acoustic waves were launched into the fiber. When the acoustic wave is applied in the fiber and travels along the fiber axis, the cladding modes of a blazed SFG can couple back to the core mode by acousto-optic interaction in the fiber. The grating reflectivity and the number of the induced wavelength channels can be controlled by acoustic flexural amplitude. Thus, this device acts as a switchable multi-wavelength comb filter for the applications in a WDM system, or in fiber lasers or in fiber sensors.
Flexural waves on fiber and fiber Bragg gratings generated with applied ultrasounds and loading were used for switching the reflection wavelength of fiber Bragg grating and for controlling the transmission spectra of fiber. By producing lateral vibration of a fiber Bragg grating, which has been etched in cladding, it was found that the reflection wavelength could be switched from the Bragg wavelength to other wavelengths, which corresponded to the conditions of strong coupling between core mode and cladding modes. By adjusting the applied voltage for acoustic waves, the relative strength of different reflection wavelengths could be well adjusted. Theoretical analysis of phase-matching condition indicated that the diameter of etched cladding played a key role in controlling the switched reflection wavelength. Flexural waves were also generated on fiber by applying loading with one or two periodically corrugated plates. By adjusting the orientation of the periodical corrugation, loading level, and the relative position of the two corrugated plates, depressions of transmission spectrum at various wavelengths could be well controlled. The spectral depressions were caused by the coupling between core mode and various cladding modes. Such depressions were used for gain equalization of erbium-doped fiber amplifiers. The equalization operation was more efficient with curved fiber under loading. Control of fiber curvature for various forms of spectral depression will be discussed.
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