Hikaru Takahara

Visibility of speckle patterns: effect of the optical guide length in coherent light.

Speckle patterns produced by coherent light have been studied, but few studies of speckle patterns passing through optical waveguides (fiber optics) with coherent light are found. We have investigated the visibility of speckle patterns produced through an optical guide with coherent light. The experimental arrangement is shown in Fig. 1. A He-Ne laser (6328-Å wavelength) was used as the coherent light source. Laser light was matched to the optical guide with lens L1 and collimated with lens L2. Diffuser D was illuminated with collimated laser light. The diffused laser beam was Fourier-transformed with lens L 3 and passed through aperture A; the speckle patterns were produced on the film P with lens L4. Before examining the visibility of speckle patterns we studied the relationship of the size of the speckle pattern to the aperture diameter. Glass fiber was used as the optical guide. The aperture diameters were 1.0 mm, 1.5 mm, 2.0 mm, 5.0 mm, and 10.0 mm. The glass fibers were 5 cm, 10 cm, and 50 cm in length. The result is shown in Fig. 2. The size of the speckle pattern decreases as the diameter of the aperture increases; it does not depend on the length of the guide. Next, the effect of the length of the optical guide on the visibility of the speckle patterns was studied. The following formula was used as a measurement of visibility:

Visibility of interference fringes: effect of the optical guide length in coherent light

The visibility of the interference fringes produced by a wavefront of coherent light passing through an optical guide, divided, and recombined is studied. A cladding fiber and a light-focusing fiber are used as the optical guide. The relationship of the visibility of the interference fringes to the length of these fibers is determined. The visibility of the interference fringes produced through the light-focusing fiber and those produced through the cladding fiber are almost independent of the length of the fiber despite random internal reflection.

Coherence of a Laser Beam Passing through an Optical Fibre

The coherence of a laser beam passing through multimode fibres shows considerable variation. Experiments have been carried out in order to investigate this variation. The visibility of interference fringes is taken to be a measure of coherence. A He-Ne laser (632·8 nm wavelength) was used, with a multimode step-index fibre, a multimode graded fibre (Selfoc fibre) and a single-mode step-index fibre. The azimuthal angle of polarization and the degree of polarization of a beam passing through a single mode fibre show little variation compared to a multimode fibre. The visibility of interference fringes produced by two beams passing through either one or two single-mode fibres have an extremely small variation compared to the multimode fibre. The variation of the coherence of a beam passing through a multimode fibre is due to the superposition of different modes produced by the beams being transmitted. The value of visibility obtained by inserting a polarizer to get equal azimuthal angles of polarization is o...

Fundamental studies on laser radiation therapy: dispersion of a laser beam passing through optical fibres

Abstract For laser internal therapy in medical applications, optical fibres are required to propagate the laser beam. The far-field pattern of the laser beam at the exit face of the optical fibre is studied using a multimode step-index fibre and a graded index Selfoc fibre. The degree of dispersion, or the output angular distribution, of the beam passing through the Selfoc fibre is much smaller than that passing through the multimode step-index fibre. Consequently, the energy density in the far-field pattern of the beam passing through the Selfoc fibre is very much larger than that for the beam passing through the multimode step-index fibre. It is concluded that the Selfoc fibre is more effective in laser internal therapy.

Fiber gyroscope using a polarization-maintaining optical fiber and a fiber coupler

A fiber gyroscope usin a polarization-maintaining optical fiber(PMF) has been studied. A phase shift of the laser beam passing through the PMF by an ultrasonic wave has been observed In this paper, the fiber gyroscope using the PMF and a polarization-maintaining fiber coupler(PMC) is presented. Moreover, the Sagnac phase modulation by the ultrasonic wave is studied.