The polymer polydimethylsiloxane (PDMS) whose refractive index and shape vary with absorbed organic compounds
and pressure is used as the cladding layer in the tapered region of an optical fiber. The light propagation in the tapered
region is influenced by the refractive index and shape changes of the PDMS layer due to the optical evanescent field.
By monitoring the optical tapered fiber transmission power variations, an organic compound/ pressure sensor is
developed.
A pressure or touch sensor is proposed by using a tapered optical fiber and polydimethylsiloxane (PDMS)
curved films. Under pressure, the light in the tapered optical fiber is partially leaked into the films by the
evanescent field properties. The optical attenuation of the tapered optical fiber is directly related to the
pressure change and sensitive to the profile of the film. By monitoring the change of the transmitted optical
power of the tapered optical fiber, the change of pressure is measured.
The polymer polydimethylsiloxane (PDMS), which is used as a cladding layer in waveguide-based optical
components, is sensitive to the organic compounds. In this work, a compact organic compound optical
sensor with a simple fabrication is proposed by using a taped optical fiber with PDMS coating. The
sensing mechanism is based on the reaction of sensing material PDMS with chemical molecules to result in
the changes of PDMS cladding layer which cause the transmitted optical power of the taped optical fiber.
A ridge waveguide Bragg grating pressure or touch sensor is proposed. The sensor consists of an open-top ridge waveguide Bragg grating with an overlaid pressure sensing polydimethylsiloxane film. The effective index of the guided mode of the waveguide is changed by stress-induced variations in the film refractive index that are caused by increases in pressure normal to the waveguide. Pressure is then measured by monitoring the shift of the Bragg resonance resulting from the changes to the effective index. By using the smaller core size ridge waveguide, and a polymer with smaller Young's modulus, the sensitivity of the sensor is enhanced. By utilizing the polarization dependence of the sensor response, a temperature-independent pressure sensor can be realized.
The polymer polydimethylsiloxane (PDMS), which is used as a cladding layer in waveguide-based optical
components, is sensitive to some organic compounds. Absorption of organic compounds by PDMS results in
changes to the polymer's refractive index and absorption spectrum. In this work, a compact and highly sensitive
organic compound sensor based on an evanescent coupling of a Bragg grating ridge waveguide into a PDMS top
cladding layer is proposed. The sensor is an open-top Ge-doped SiO2 ridge waveguide possessing a
photoinduced Bragg grating in the waveguide core that has a cladding overlayer of PDMS. When an analyte is
applied to the top of the waveguide, changes to the refractive index and absorption of the PDMS layer result in
shifts to the Bragg resonance of the core grating. The birefringence, and temperature sensitivity of the sensor are
examined.
In this work, a ridge waveguide Bragg grating pressure or touch sensor is proposed. The sensor consists of an open top
ridge waveguide Bragg grating with a pressure sensing film: polydimethylsiloxane (PDMS) on the waveguide surface.
Under pressure, the guided mode of the waveguide accesses the film by coupling of the evanescent field. A large shift
of the Bragg wavelength occurs when the effective index of the waveguide is changed by stress-indued variations in the
film refractive index that are caused by increases in pressure. By monitoring the shifts of Bragg wavelengths in TE and
TM modes, respectively, the external pressure change and the internal strain change in the film are measured. The
sensitivities of the sensor with different waveguide structures are investigated.
In our previous work, a highly sensitive waveguide Bragg grating (WBG) sensor for measuring small changes in the refractive index of a surrounding liquid was developed [1]. We proposed a technique for creating a temperature insensitive refractometer that utilizes core and cladding modes in an open-top ridge waveguide architecture in order to discriminate between Bragg wavelength changes in temperature and refractive index [2]. In this work, a technique for creating a temperature insensitive refractometer that utilizes TE and TM modes in an open-top ridge waveguide design is presented. By using the TE mode resonance as a temperature reference, the relative shift of the TM mode can be monitored in order to measure the refractive index of liquids under test. Specifically, the device fabricated here produces a relative resonance shift of 1 pm for every 1×10-4 of measured index change, with a temperature sensitivity less than 0.2 pm/°C.
A technique for creating a temperature insensitive refractometer that utilizes TE and TM modes in an open-top ridge
waveguide design is presented. By using the TE mode resonance as a temperature reference, the relative shift of the
TM mode can be monitored in order to measure the refractive index of liquids under test. Specifically, the device
fabricated here produces a relative resonance shift of 1 pm for every 1×10-4 of measured index change, with a
temperature sensitivity less than 0.2 pm/°C. To increase the sensitivity of these devices, a theoretical model is
developed to investigate the performance of some potential waveguide structures. Relationships between the waveguide
core size, refractive index distribution, as well as the relative evanescent sensitivity of TE and TM modes are
examined.
Experimentally, in the open-top ridge waveguides, the sensitivities of core and cladding resonances to the surrounding medium's refractive index are different while the temperature sensitivities are similar. Based on these characteristics, a temperature insensitive refractometer has been proposed. To increase the sensitivity of these devices, a theoretical model is developed to investigate the performance of some potential waveguide and Bragg grating structures. Relationships between the waveguide core size, refractive index distribution, tilt angle of the Bragg gratings as well as the relative evanescent sensitivity of the core and cladding modes are examined. As a result, we find that sensitivity can be enhanced by decreasing the waveguide core size, making the effective index of the waveguide close to the expected refractive index of the analyte, and incorporating tilt in the Bragg grating structures. Furthermore, the inclusion of tilt also appears to reduce the grating's birefringence for the waveguide structure examined.
Bragg grating refractometers have been previously disclosed by other authors but sometimes suffer from temperature instability due to well know grating characteristics. In order to overcome this limitation, a temperature insensitive refractometer is developed by using a cladding mode resonance as a temperature reference, with the relative shift of the core mode resonance used to measure the refractive index of substances under test. Specifically, the device fabricated here produces a relative resonance shift of 1 pm for every 5×10-4 of measured index change, with a temperature sensitivity less than 0.5 pm/°C.
A simple numerical method is developed to analyze how the intrinsic birefringence of silica-based ridge waveguides changes with waveguide dimensions and UV irradiation. Identical Bragg gratings were induced on waveguides with different widths which varied from 5 µm×6 µm to 9 µm×6 µm with ArF excimer irradiation and a phase mask. The variation of the waveguide effective index and birefringence as a function of the waveguide dimensions and UV processing are observed and quantified by monitoring the shifts in Bragg wavelength with UV irradiation. With UV irradiation, zero birefringence was easily realized in the waveguides, having an initial birefringence of <2.5×10–4. The mechanism for controlling the waveguide birefringence with UV irradiation is verified both in the theoretical analysis and experimentally. Additionally, the ±0.1-nm variation in Bragg wavelength for waveguides with the same nominal width corresponds to a ±0.1-µm dimensional change in the actual waveguides geometry. This result is used as a way for improving quality control over the waveguide dimensions obtained from the photolithographic and RIE processes.
Optical Bragg grating sensors based on side polished or etched waveguides have been demonstrated for the measurement of refractive index [1, 2, 3, 4]. However, these devices typically exhibit polarization dependent behavior for index values around 1.3. In this report, a ridge waveguide Bragg grating (WBG) sensor with high sensitivity, for refractive index measurement in liquids is presented. The device is based on a small core size silica open top cladding ridge waveguide and polarization independent Bragg gratings (PIBG) written and optimized using UV light [5,6,7]. The WBG is surrounded by a liquid analyte and is accessed via evanescent field interaction of the guided waveguide mode with the liquid layer. In the theoretical analysis, enhancement of sensitivity by optimizing waveguide structures is proposed. In the experiment, Bragg grating is induced in the open top cladding ridge waveguide using a phase mask and excimer laser radiation at 193 nm. A series of refractive index matching liquids are used to test the device. Results indicate the sensitivity is as high as 50 pm of wavelength shift for a change of the index 3×10-4. This technology can offer many advantages over previously proposed waveguide sensors, including enhanced sensitivity, and dynamic measurement range, better polarization stability, and a simpler fabrication processes.
A simple numerical method is developed to analyze changes in intrinsic birefringence of ridge waveguides with waveguide dimensions and UV irradiation. Experimentally, Bragg gratings were written on different core size ridge waveguides using the phase mask technique and ArF laser irradiation. By monitoring the shifts in Bragg wavelength with UV irradiation, the variation of the waveguide birefringence with waveguide dimension and UV processing is observed and quantified. The mechanism of the waveguide birefringence controlled with UV irradiation is verified both in the theoretical analysis and experiment.
A method of Bragg gratings written in silica-on-silicon planar waveguides to be used to monitor the overall uniformity of the waveguides and grating processing is presented. By measuring the shift of Bragg wavelength with UV exposure time, the initial effective index nOeff and birefringence B0 of the planar waveguides are measured accurately. With one phase mask, Bragg gratings induced on different waveguides with widths that vary from 4.6 to 8.8 μm, result in variations of NOeff and βO of 1.5 x 10-3/μm and 1 x 10-4/μm, respectively. The result is used as a way of improving control over the waveguide dimensions obtained from the photolithographic and RIE processes, and optimizing the design of ridge waveguide structures to compensate the waveguide birefringence. This will improve the quality of the PLCs that include symmetric Bragg grating structures: MZI-OADM etc. By writing Bragg gratings on the linear taper planar waveguide, a chirped grating response is realized.
The dispersion curves of tapered fibers at various geometries are characterized by using white-light interferometry. In this presentation, the white-light interference patterns at frequencies near the taper’s second zero-dispersion frequency are measured and discussed in detail. A more convenient formula is proposed to fit the experimental data at these frequencies. As a result, small dispersion values can be calculated more efficiently.
Traditionally in the process of writing fiber Bragg gratings with a phase mask the fiber is placed near or in close contact with the mask. With low coherence excimer sources this is necessary because the fringe visibility is greatly reduced beyond 500 μm. As a result of these limitations there has been increased interest in understanding the interference phenomena associated with a phase mask. During the past year we studied the beam interference phenomena associated with ultrafast gratings. We observed that with these coherent sources it was possible to write gratings remotely (Phase Mask-Fiber distance of ≈1 cm). In addition to this we observed evidence of walk-off between mask orders that significantly affected the interference patterns. In this paper we demonstrate that a frequency doubled Argon-ion laser, being a coherent source, can be used to inscribe fiber Bragg gratings at large distances from the phase mask (> 15 mm). We will demonstrate that walk-off between mask orders will change the interference profile along the grating length. We show that the spectral profile correlates with the calculated interference pattern. Beam walk-off effects play a role in the inscription of any photonic device with a phase mask. This remote writing technique can be used to tailor the index modulation pattern in the fiber and could potentially be used to produce two beam interference gratings even in the presence of a significant zero order amplitude.
For real-time 3D imaging using chirped optical pulses and a femtosecond optical Kerr shutter to measure the shape accurately, we need to develop a spectral imaging method which has a high-resolution in both space and spectrum. The spectral property of the light which is generated from the chirped pulse by the femtosecond optical Kerr shutter, the wavelength is measured as the line spectrum. Therefore, the wavelength can be determined by two detectors with different spectral properties. To obtain the line spectral image, we proposed a pair of filters, the transmissivities of which vary monotonously with the wavelength, one increases with the wavelength, the other decreases. Since the ratio between the transmissivities of the two filters changes monotonously for wavelength range of 550 - 750 nm, the wavelength of line spectrum is determined uniquely for the range with a simple function of the ratio. These filters were set in front of two monochromatic CCDs which are aligned to take an image. The resolution to determine the wavelength is tested with a monochrometer and the standard deviation for each pixel is estimated to be about 10 nm. Compared the methods of color CCD and spectrometer, this method of pair-CCDs has high spatial resolution, uniform spectral resolution, wide spectral measurement range, and simple setup.
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