SUMMARYIn order to realize beam forming with an identical radiation characteristic over a wide bandwidth, this paper describes a method of design for two-dimensional FIR fan filters with wedge-shaped transition regions. In order to provide excellent amplitude approximations in a wide range from low to high frequencies with different transition range widths, a method based on the two-dimensional Fourier series approximation is used. The approximate amplitude characteristic of the target fan filter for which the widths of the passband and the transition band are specified in proportion to the frequency from the origin is greatly degraded at low frequencies near the origin. To reduce this degradation, the target amplitude characteristic at low frequencies of the fan filter is replaced with a rectangular cross-sectional shape. The width of the rectangular amplitude characteristic is set such that the amplitude of the designed result takes the specified value on the center line. Design examples are given for an axially symmetric fan filter and a diagonally symmetric fan filter. Further, the characteristics of the beam formers using these filters are derived and the effectiveness of the method is demonstrated.

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A highly sensitive and compact butt-coupled-type fiber-optic acoustic sensor has been developed based on the Fabry-Perot interferometer (FPI) for use in remote sensing or sensing in narrow spaces. The system consists of a sensor head, a controller, and an optical fiber transmission line. The sensor head is based on the FPI, and is formed from the highreflectance-coated confronting surfaces of an optical fiber and a diaphragm. The sensitivity of the sensor head is related to the finesse of the FPI. As the finesse increases, so does the sensitivity. To ensure system stability and increase the dynamic range, the controller is composed of a DFB laser diode with a wavelength of 1300 nm, double optical isolators, an automatic temperature controller, and an automatic power controller. 4:22 GG12. A fiber-optic, interferometric, acceleration canceling, flexural disk hydrophone.

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