Yasuo Mizutani

Thermal tuning of mechanically induced long-period fiber grating

We have developed a wideband tunable optical filter that uses a long-period fiber grating (LPFG) in which both resonance wavelength and its signal attenuation can be adjusted. We create the grating mechanically by pressing a spring coil to an optical fiber. We achieve continuous fine tuning of wavelength and attenuation by varying the temperature of the LPFG. The adjustable ranges of the LPFG are more than 200 nm in resonance wavelength and more than 10 dB in signal attenuation.

Wavelength evolution of fiber Bragg gratings fabricated from hydrogen-loaded optical fiber during annealing

The shift in the central wavelength of fiber Bragg gratings (FBGs) during annealing of hydrogen-loaded optical fiber exposed to ultraviolet (UV) laser irradiation through a phase mask is studied and shown to be caused by thermal diffusion of hydrogen out of the fiber for both short-period and long-period FBGs. Reloading FBGs with hydrogen followed by a second annealing cycle without exposing the fiber to the UV laser reproduces the details of the wavelength evolution observed during first annealing following UV irradiation. This shows that the wavelength shift of the grating during annealing is determined by diffusion of hydrogen gas out of the optical fiber for both short-period and long-period FBGs.

Evolution of Optical Fiber Temperature during Fiber Bragg Grating Fabrication Using KrF Excimer Laser

The temperature distribution in an optical fiber during the fabrication of fiber Bragg gratings (FBGs) using a KrF excimer laser with a phase mask has been analyzed experimentally for typical fabrication conditions. The fluence of UV laser light at the fiber surface has been varied up to 420 mJ/cm2. These experiments show that (1) scanning electron microscope (SEM) images of the fiber surface facing the excimer laser beam reveal partial physical damage apparently owing to partial melting, so that the surface temperature has increased beyond the softening point of silica glass, which is approximately 1200°C, (2) the optical spectrum transmitted through the FBG during laser irradiation contains spikes coincident in time with the laser pulses that correspond to a near instantaneous shift of the FBG spectrum to a higher temperature spectrum representing fiber core heating of approximately 8°C, and (3) analysis of the energy absorbed by the fiber indicates a bulk temperature rise of approximately 3°C in the fiber. The resulting large variation in the temperature rise over the fiber cross section from a few °C to 1200°C, along with partial physical damage on the surface, will certainly induce large internal stresses in the fiber material and reduce the mechanical strength of the FBG.

Dynamic evolution of the spectrum of long-period fiber Bragg gratings fabricated from hydrogen-loaded optical fiber by ultraviolet laser irradiation

Long-period fiber Bragg gratings fabricated by exposure of hydrogen-loaded fiber to UV laser light exhibit large-scale dynamic evolution for approximately two weeks at room temperature. During this time two distinct features show up in their spectrum: a large upswing in wavelength and a substantial deepening of the transmission minimum. The dynamic evolution of the transmission spectrum is explained quantitatively by use of Malos theory of UV-induced quenching [Electron. Lett. 30, 442 (1994)] followed by refilling of hydrogen in the fiber core and the theory of hydrogen diffusion in the fiber material. The amount of hydrogen quenched by the UV irradiation is 6% of the loaded hydrogen.

Dependence of fiber Bragg grating characteristics on its length

A fiber Bragg grating (FBG) is normally made with a length of 10 to 30 mm. Depending on the application, however, a short length may be desired, especially in sensor applications for local detection. Under the most common fabrication conditions using the phase mask method, we have fabricated FBGs with lengths ranging from 0.35 to 15 mm using different excimer laser irradiation times. Although 0.35-mm-long FBGs can be made with laser exposure times exceeding 1 h, the practical minimum FBG length appears to be around 3 mm with exposure times in the range of 3 to 5 min.

Spectral Response of Fiber Bragg Gratings Connected in Series with Different Wavelengths

When fiber Bragg gratings (FBG) with different reflection wavelengths are connected in series, the reflection spectrum may be affected by cladding modes depending on the order in which the FBGs are connected and on which side the signal light is incident. It is analyzed and experimentally demonstrated that, in order to obtain proper reflection signals from multiple FBGs, the FBGs must be connected in order from those with shorter wavelengths to those with longer wavelengths and signal light must be incident on the shortest wavelength FBG.

Fabrication of Fiber Gratings with Different Bragg Wavelengths Using a Single Phase Mask

We present a method of fabricating multichannel fiber Bragg gratings using a single mask under control of the applied tension to the optical fiber. The Bragg wavelength of fiber Bragg gratings is investigated as a function of the amount of tension applied to an optical fiber during the photoimprinting process. The Bragg wavelength changes linearly with the amount of tension applied to the optical fiber. Using the linear relation, an optical wavelength filter consisting of five fiber Bragg gratings with five different wavelengths is fabricated successfully using one phase mask.

Effect of Spherical Aberration on Fabrication of Fiber Bragg Gratings

Cylindrical convex lenses are commonly used in fabricating fiber Bragg gratings (FBGs) by the phase mask method to increase the energy fluence of UV laser light and thereby enhance FBG productivity. We have found that lens aberrations can affect the efficiency of FBG fabrication by this method. Comparisons of the experimental measurements of FBG transmission minimum as a function of fiber position during irradiation by focussed excimer laser light with ray tracing calculations demonstrate the effect of spherical aberration, particularly for lenses with shorter focal lengths.