Ken Koizumi

Optical characteristics of a light-focusing fiber guide and its applications

A lenslike glass fiber guide with a parabolic variation of refractive index has been developed. This optical guide named SELFOC® has the following characteristics: simultaneous transmission of laser beams modulated by wide-band signals through narrow space; optical image transmission; realization of a lens with tiny aperture or with extremely short focal length; and the possibility of being bent with a small radius of curvature without spoiling transmission characteristics. In the case of a typical fiber guide with length 1 meter and diameter 0.3 mm, transmission loss is about 0.2 dB and depolarization is about 20 dB at wavelength 0.63μ. The mode pattern of a laser beam after passing through the fiber guide is scarcely deformed. The fiber guide can be used as a transmission line or lens, in optical communication, optical data processing, and optical instruments.

A frequency-tunable mode-locked CW Nd:Glass laser

Mode locking and frequency tuning of a CW light-focusing Nd : glass laser called the SELFOC Nd : glass laser were achieved. Using an acoustooptic modulator having a modest degree of modulation (about 3 percent), almost perfectly mode-locked pulses as short as 49 ps were obtained. Pulsewidth was measured with an intensity-correlation method using the second-harmonic generation in KDP. Most of the observed shapes of the intensity-correlation curves were in agreement with theories previously presented by several authors. When the laser was mode locked, the long-term averaged optical spectrum was observed to narrow considerably. Some experimental evidence that the CW SELFOC Nd:glass laser could be considered to have a homogeneously broadened line was also found. Frequency tuning was achieved by using a tilted etalon in a cavity. When a 50-μm-thick etalon was used, the tuning range was more than 40 A. Pulses obtained from a frequency-tuned mode-locked laser were as short as about 100 ps.

Actively initiated quasi-passive mode-locking in a CW SELFOC Nd:glass laser

Actively initiated quasi-passive mode-locking has been achieved in a small-power CW SELFOC Nd:glass laser. Three means have been taken to enhance the pulse intensity incident on a saturable dye to a magnitude sufficient for nonlinear absorption initiation: 1) tight focusing, 2) controlled spiking oscillation, and 3) auxiliary active mode-locking. Mode-locked pulses having an 11-12-ps pulsewidth were obtained. Interpretation concerning the observed mode-locked-pulse intensity-correlation curves is presented. To obtain satellite-pulse-free mode-locked pulses, a modified mode-locking process with sufficiently long buildup time is considered.

Gradient-Index Micro-Optic Glasses Based On Ion-Exchange Techniques

Due to unique characteristics of gradient-index glass rod as a micro-lens, a variety of micro-optic devices for communication use and new optical components for photo-copiers and facsmile machines have been developed. Ion-exchange techniques which were used to the fabrication of gradient-index micro-lens have also been applied to the fabrication of such new micro-optic glasses as 2-dimensional matrix lenses and optical waveguide circuits. This paper reviews the fabrication processes and applications of gradient-index micro-optic glasses based on ion-exchange techniques.