An electrodynamics model of a fiber-optic hydrogen sensor with a taper in a special photonic-crystal fiber coated by a nano-scale palladium film is proposed. The transmission spectrum of the sensor is explained by the interference and mutual transformation of local modes in the taper. As a result, the model contains 3m+1 free parameters, where m is the number of local modes taken into account. The parameters are determined by the least square method from the experimental transmission spectrum. This approach allows us to determine the dispersion characteristics of a nanoscale palladium film in a hydrogen atmosphere and optimize the taper from the standpoint of maximizing the sensitivity of the sensor.
In this report, we inform in detail on fabrication of a special (3 rings of air-holes) index-giuding air-silica microstructured optical fiber (IG MOF) with different air-hole diameters in the cladding (irregular cladding) and its application for a broadband supercontinuum (SC) generation by femtosecond laser pulses. For comparison, supercontinuum generation in a special nonlinear air-silica IG MOF with regular cladding is also investigated. Dispersion properties of the investigated fibers were numerically predicted and experimentally verified. Broadband SC generation from visual wavelengths up to 1600 nm in such fibers, both with the length of 1 m, was observed.
An Ytterbium-Holmium co-doped all-fiber CW laser is reported. The fiber used in the laser setup has been fabricated through the conventional MCVD process in conjunction with the SD technique and finally drawn using a standard fiberdraw tower. The laser was built in a linear Fabry-Perot configuration in which two fiber Bragg gratings reflecting at 2.05 μm were used as the cavity couplers. Under 1.06-μm in-core pumping of the fiber, CW lasing at 2.05 μm was provided due to energy transfer Yb3+→Ho3+. The laser demonstrated low threshold (~0.8 W), moderate slope efficiency of lasing (~8.4% when measured vs. pump power launched into the active fiber), and high stability: during 6 hours its output power fluctuated within a 3% range. The laser spectrum width at a 3-dB level using an optical spectrum analyzer with a 37-pm resolution was measured to be ~70 pm.
An all-fiber mode interferometer consisting of a short segment of photonic crystal fiber (PCF) fusion spliced to standard single mode optical fiber and pressed on localized regions is proposed for multi-parameter sensing. In our configuration, the physical parameter being sensed changes the fringe contrast (or visibility) of the interference pattern and also causes a shift to the same. To achieve this dual effect the device is pressed on localized regions over a few millimeters. In this manner we introduce losses and effective refractive index changes to the interference modes, hence visibility and shift to the interference pattern. Our interferometer is suitable for monitoring diverse physical parameters such as weight, force, pressure, load, etc. The advantage is that no temperature or power fluctuations compensation is required.
The influence of microbends on a phase shift and attenuation in a classical Corning SMF-28 fiber and in a special microstructured single-mode fiber with a limited number of air-hole rings in the cladding has been investigated. The largest measured phase shift of 750 deg/mm was obtained for the classical fiber at displacement magnitude of 117 µm. This corresponds to 1.3×10−3 change in the effective refractive index of the fundamental LP01 mode in a microbent optical fiber section with length of 24 mm. The phase shift for the investigated microstructured fibers is in 3.9 times less. The cutoff of radiation intensity takes place at displacement magnitudes of 55 and 270 µm for the microstructured fiber and for the SMF-28 fiber, respectively. It is shown that the sensitivity of the microstructured fiber to the displacement magnitude is in about five times higher than that of the Corning SMF-28 fiber.
We report an in-reflection photonic crystal fiber (PCF) interferometer which exhibits high sensitivity to different volatile
organic compounds (VOCs), without the need of any permeable material. The interferometer is compact, robust, and
consists of a stub of PCF spliced to standard optical fiber. In the splice the voids of the PCF are fully collapsed, thus
allowing the excitation and recombination of two core modes. The device reflection spectrum exhibits very regular
interference pattern which shifts differently when the voids of the PCF are infiltrated with VOC molecules. The volume
of voids responsible for the shift is around 500 picoliters whereas the detectable levels are in the nanomole range.
By using two optical fibers and a capillary it is possible to measure the refractive index of liquids. Light
leaving a fiber is sent transversally to a capillary that behaves as a cylindrical lens when liquids are
inserted in it. Focused light is collected by a second fiber and sent to a detector.
A defocused transfer function of experimental microscopic bright-field images, closely connected with the evaluation of the relative refractive index profile of axially symmetric optical fibers, is presented. It is shown that with this function it is possible to obtain, with high precision, the defocused bright-field images of investigated fibers from their focus images and then to easily evaluate the relative refractive index profiles of the fibers.
A simple experimental technique for the measurement of refractive index profiles of axially symmetric optical fibers is presented. This method is based on image phase gradients introduced into a transmitted optical field by a fiber sample. An image of the phase gradients is obtained using a technique based on bright field microscopy. Then a relative refractive index profile of optical fibers is reconstructed by means of the inverse Abel transform.
Using a simple non-destructive and non-interferometric technique, we obtained an approximation of the average index
profile for a partially symmetrical air-silica microstructured optical fiber. The method proposed in [1] for conventional
fibers, where an image of the phase gradients is introduced into a transmitted optical field by a fiber sample, was used.
An image of the phase gradients was obtained using a technique based on bright field microscopy. Then, an average of
the refractive index profile for optical fibers was reconstructed using the inverse Abel transform.
The fabrication of a compact all-fibre modal interferometer that can be used to sense different parameters is reported.
The device consists of a tapered large-mode-area microstructured optical fibre with collapsed air holes over a localized
region. The tapering is carried out by slowly elongating the fibre while it is heated with a high-temperature oscillating
flame torch. This non adiabatic method allows the collapsing of the air holes and transforms a section of the
microstructured fibre into a solid one. As a consequence the fundamental HE11 mode is coupled to the HE1m cladding
modes which can beat or interfere. This makes the transmission of the device versus the wavelength to exhibit an
oscillatory pattern. Such a pattern shifts with strain, high temperature, or refractive index. The device is compact, can
operate in a broad range of wavelengths, and can be fabricated in a few minutes which makes it attractive for optical
sensing.
We report experimental evidence of light-induced changes in the transmission spectrum of human venous blood under the action of low-intensity radiation from He-Ne laser. We have applied a sensitive registration technique and the transmission spectra of diluted and nondiluted heparinized human venous blood have been analyzed before, after and in the course of irradiation. When irradiating nondiluted blood, reproducible variations of the blood transmission spectrum in the range of 640 - 805 nm have been observed for the first time. We found these changes correlate with oxygen saturation. The changes in transmission spectra and oxygen saturation of venous blood after 1-3 procedures of irradiation can be used as a method to detect an individual sensitivity to intravenous laser irradiation.
Large-mode-area photonic crystal fibers with a limited number of air channels in the cladding are investigated theoretically and experimentally. An impact of the relative hole diameter on single-mode operation, the fiber transmission, and bending loss is addressed in detail.
New linear effects for the coherent ultrashort laser pulse trains are considered. The processes of envelope compression for two pulses and regular and irregular multiplying a number of pulses due to temporal interference are predicted.
Proc. SPIE. 4750, ICONO 2001: Quantum and Atomic Optics, High-Precision Measurements in Optics, and Optical Information Processing, Transmission, and Storage
KEYWORDS: Optical fibers, Ultrafast phenomena, Mirrors, Dispersion, Solitons, Single mode fibers, Fiber lasers, Nonlinear optics, Analog electronics, Laser optics
We report on collective effects for the limited train of periodical laser pulses when multiplying the number of pulses in the train occurs at some distance in dispersive medium. This phenomena can be treated as one of the cases of the temporal interference of coherent laser pules.
At present, foolproof methods are needed for control of low-power light biomedical effects. Transmission spectra of whole blood from 108 patients were recorded before and after action of low-intensity He-Ne laser and extracorporal UV irradiation. We have observed changes in the blood transmission spectrum caused by oxygen saturation changes in venous blood. It has shown that He-Ne laser irradiation more selectively acts to venous blood than extracorporal UV irradiation.
We describe a fiber optic voltage sensor with optically controlled sensitivity.The sensor operates at two widely separated wavelength (633nm and 976nm) one of whichis a control signal (976nm).We show that at a properly chosen wavelength of the control signal and of the phase-retarding element, variation of the control signal allow increase or decreases in the sensitivity of the sensor.A theoretical analysis of sensitivity as a function of the optical power of a control signal is presented. WE have demonstrated experimentally variation of the sensor ,s sensitivity from 0.01% to 0 per 1Vms of control power change in the range of 0÷7 ?W.
We report on experimental proves of light-induced changes in the transmission spectrum of human venous blood under the action of low- intensity radiation from He-Ne laser. The transmission spectra of diluted and nondiluted heparinized human blood have been analyzed before, after and in the course of irradiation. The reproducible variations of the blood transmission spectrum in the range 730-780 nm have been observed. These changes have been shown to correlate with oxygen saturation. The decreased oxygen saturation in venous blood is stabilized at the level of about 70-80% from the initial one after 5-8 procedures of laser irradiation.
The influence of photoconductivity upon operation of a Bi12TiO20-based optical voltage sensor is reported. We have demonstrated, that the presence of photoconductivity defines non-uniform sensitivity of the sensor in the cross section of an infrared light beam. The sensitivity of the sensor is shown to depend on the intensity of light as a consequence of photoconductivity of a crystal. An agreement between our theoretical model and experimental results has been demonstrated.
Some nontrivial features of nonlinear transformation of frequency in the ring fiber resonator of free running YAG:Nd laser have been experimentally studied. Fiber of resonator has been matched with the laser active medium using microobjectives. Fibers of different lengths with the core diameters from 340 to 10 µm have been used. Nonlinear frequency transformation has been observed for fibers with core diameter of 60 pm and investigated lenths of resonator 43, 100 and 226 m. For all these fiber resonators in the antiStokes region spectral lines of 505, 532 and 558 nm have been presented. In the region of the Stokes component generation of the stimulated Raman scattering the row of narrow (with width less then 0.2 nm) spectral lines in the spectral region of 1127. . .1131 nm has been observed. The amount of the lines has been increased from 1 to 12 with increasing of the laser pumping power. In the anti-Stokes region for the long fibers (100 m and 226 m) the third harmonic of the main laser frequency additionally has been generated. Here it is also observed the two groups of lines in the spectral region of 1129 nm and 1137 nm, the amount of which depends from the pumping power of laser.In the same fibers under the excitation by the external YAG:Nd laser with the same power of radiation the spectral lines in the anti-Stokes region have not been observed and in the Stokes region the cascade generation of the stimulated Raman scattering has been taken place. The connection of the observed features with the spectral selective properties of the fiber ring resonator is discussed.
The polarization sensors on the singlemode optical fibers are investigated. The segment of the optical fiber clamped between two cylindrical or plane surfaces rotates the plane of polarization of the linear polarized light on the output of the fiber. The angle of rotation is proportional to the strength of clamping of the surfaces. This phenomenon can be used for the creation fiber-optical sensors of pressure or temperature.
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