Conditions of hydrogen loading were studied for shortening of the exposure time for fabrication of long-period fiber grating sensors using a low-pressure mercury lamp. By an increased hydrogen pressure of 135 atm, shortening of the exposure time to 2/3 that of 120 atm was obtained. By increasing the loading time from 4 to 12 weeks, the exposure time was shortened to 3/4 that of the same pressure, or half that of 120 atm for 3 weeks. The temperature rise of the fiber during exposure was measured to be 16 °C or less. Distribution of hydrogen molecules in the fiber was studied by a numerical analysis solving a diffusion equation. The result of the analysis agreed with the pressure dependence of the measured hydrogen concentration and the exposure time for fabrication. For the longer loading time, neither the calculated nor the measured hydrogen concentrations increased: the shortening of the exposure time by the longer loading time was not explained. The temperature and strain sensitivities were lower than those of hydrogen loading at 120 atm except that a temperature sensitive and strain insensitive long-period grating was obtained with a period of 460 μm.
Wavelength tuning with a current of a long-wavelength vertical-cavity surface-emitting laser (VCSEL) with a spectral
line width of 30 MHz was studied. To stabilize the power, saturated amplification of the VCSEL output by an erbiumdoped
fiber amplifier (EDFA) was performed. For wavelength tuning by 2.5 nm, there was a 3-fold change in the
VCSEL output power; however, the variation in the amplified output power was within ±13%. For wavelength tuning of
4 nm, the variation was ±20%. Application to fiber Bragg grating sensor interrogation was discussed.
Second-harmonic generation (SHG) and electro-optic (EO) modulation were studied on thermally poled twin-hole fiber.
Metal electrode wires were inserted into the side holes. The typical poling condition was 2.5 kV, 300 °C, and 40 min.
SHG was measured using a Q-switched Nd:YAG laser. The SH power did not depend on the applied forward or reverse
voltages. SHG without poling was also measured, then the maximum power was about 1/18 that of the poled SHG. EO
modulation was performed using a twin-hole fiber inserted to a fiber-optic Mach-Zehnder interferometer. An AC
modulation voltage was applied to the electrodes together with a DC bias voltage. Without poling, the modulation output
was obtained only when a DC bias voltage was applied simultaneously. After poling, a modulation output was obtained
without any bias voltage, and for the forward DC bias the modulation output increased with the bias voltage. For the
reverse DC bias the modulation output showed the minimum for a bias voltage. The origin of the second-order
nonlinearities and the other effects in the above SHG and EO modulation are discussed considering charge layers.
A 1.3-μm vertical-cavity surface-emitting laser (VCSEL) with a fiber Bragg grating (FBG) as an external cavity was developed. VCSELs emit light vertically from the semiconductor substrate. They have a circular output beam and therefore have a merit of easy coupling with optical fibers. Long-wavelength VCSELs in the 1.3 or 1.55-μm bands have recently been developed, which are suitable for long-distance telecommunication using single mode fibers. In this study, we coupled a 1310-nm VCSEL with an FBG as a wavelength-selective external cavity. The FBG was inscribed into a single mode fiber and the reflectivity was 47%. A rod lens was used for coupling of the VCSEL and a fiber. Grating temperature was varied for tuning of the Bragg wavelength. The spectral width without using an FBG was 85 pm FWHM, then the spectral width decreased to 64 pm FWHM by using an FBG at a temperature of 50°C. Further narrowing to 52 pm FWHM was obtained by an optimized diode current. It is expected that the wavelength dispersion in optical fiber transmission can be reduced by using an FBG.
The second-order nonlinearity in poled optical fiber is promising for application to electro-optic switching and modulation, second-harmonic generation (SHG), and frequency conversion. In this paper, we poled a twin-hole fiber, which is similar to PANDA fiber but the strain applying part is vacant. Electrode wires were inserted into the side holes, and the fiber was poled with a voltage of 2.5 kV at 300°C for 40 min. We measured the SHG using a linearly polarized Q-switched Nd3+:YAG laser. The SH power was highest for polarization parallel with the direction of two holes. The SH power had a maximum for the fiber length of 5 cm and decreased for longer fiber lengths. We analyzed this phase matching considering cladding modes. We calculated numerically the propagation constants of the cladding modes. We showed that the ~40th-order cladding mode of the SH wave and the fundamental core mode of the fundamental wave are in phase matching. We also performed an SHG of poled twin-hole fiber using a 260-fs passively mode-locked Er3+-doped fiber laser as a fundamental-wave source. The SH signal from the poled fiber was proportional to the 1.82-th power of the fundamental power, and the polarization dependence agreed with that measured with an Nd3+:YAG laser. We discussed an application of the poled twin-hole fiber.
Low-temperature sensing by a fiber Bragg grating fixed on Teflon substrates was studied. The temperature sensitivity at 77 K was 33 pm/K, which is 1.5 times higher than that of the sensor using a poly(methyl methacrylate) substrate.
Bending sensitivity of Bragg gratings in multimode graded-index fiber is investigated experimentally. Bragg gratings in dispersion shifted fiber at a 3-mode propagating wavelength are also used and compared. Polarization and temperature dependence is also measured.
The authors systematically investigated the second-order susceptibility of the cathode-side face of poled glass. This report consists of three parts. The first is a technique to pole a set of multi-pieces of glasses for distinguishing the second harmonic (SH) signals from the anode-side piece and from the cathode-side piece. The SH signal from the cathode- side piece was only one hundredth of that from the anode- side piece, and those from the middle pieces are further less. The SH signal increased with the number of glasses in a set. The second is a technique to give a further identification of the SH signals from each face. For this purpose, we looked for the materials, which are opaque to the second-harmonics. We found two kinds of new materials: lead silica and Pyrex glasses. They don't transmit the UV light, but transmit the visible light. We detected the second harmonics at 266 nm from the face of the glass facing to the detector. The SH signal in poled lead silica increased exponentially with the increase in the lead percentage. The third is a new technique of doping F- into the cathode-side face, which increases the SH signal from the cathode-side face by two orders. It reached two times of that from the anode-side face.
The investigations on the multi-cascade processes of four-wave mixing (FWM) in liquid cored optical fiber is reported. The core material is ethylbenzene and the wavelength of its first order. Stokes is 562 nm by the pumping light of 532 nm. The dye laser is tunable and so are its FWMs. The frequencies of FWMs and their cascade FWM among the frequency difference of the first order Stokes line and the dye laser, (omega) 1s - (omega) d, and the dye laser; or it and the second order Stokes line; or it and the third order Stokes line; or it and the pumping light are (omega) d -((omega) 1s - (omega) d), (omega) d -2((omega) 1s - (omega) d), and (omega) d -3((omega) 1s - (omega) d); or (omega) 2s - ((omega) 1s - (omega) d) and (omega) 2s - 2((omega) 1s - (omega) d); or (omega) 3s - ((omega) 1s - (omega) d) and (omega) 3s - 2((omega) 1s - (omega) d); or (omega) p -((omega) 1s - (omega) d), (omega) p -2((omega) 1s - (omega) d), and (omega) p -3((omega) 1s - (omega) d), respectively. In the output of liquid cored optical fiber there are Stokes lines, the broadening of each line, the dye laser, the pumping light, the FWMs and their cascade FWMs. We point out that the FWM process is not only to initiate those obtained FWMs and their cascades, but also to contribute to the stimulated Raman scattering and the stimulated Rayleigh wing scattering for generating higher orders of Stokes and anti-Stokes line and for expanding the spectrum widely in the red-shifted broadening and the blue-shifted broadening in liquid cored optical fiber.
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