Abstract: A perfluorinated polyimide comprising a repeating unit represented by general formula (1): ##STR1## and a perfluorinated poly(amic acid) comprising a repeating unit represented by general formula (6): ##STR2## wherein R.sub.1 is a tetravalent organic group; and R.sub.2 is a divalent organic group, provided that chemical bonds between carbon atoms and monovalent elements contained in R.sub.1 and R.sub.2 are exclusively carbon-to-fluorine bonds; methods for preparing them; and optical material including the perfluorinated polyimide. 1,4-Bis(3,4-dicarboxytrifluorophenoxy)tetrafluorobenzene dianhydride, 1,4-difluoropyromellitic anhydride, 1,4-bis(3,4-dicarboxytrifluorophenoxy)tetrafluorobenzene, 1,4-difluoropyromellitic acid, and 1,4-bis(3,4-dicyanotrifluorophenoxy)tetrafluorobenzene as well as methods preparing them. The perfluorinated polyimide has a thermal stability and has a low optical loss in an optical communication wavelength region (0.8 to 1.7 .mu.m).

Abstract: An optical waveguide which has properties including low optical losses, facile fabrication, controllable core-cladding refractive index ratio, and a high heat resistance is provided. The optical waveguide for light transmission therethrough has a core made of a polyimide obtained from at least one tetracarboxylic acid dianhydride and at least one diamine. The polyimide is (a) a polyimide homopolymer derived from a diamine represented by formula [I] as follows: ##STR1## (b) a polyimide copolymer derived from at least one diamine including the diamine represented by the formula [I]; or (c) a mixture of the polyimide homopolymers.

Abstract: In a cross-polarization interference cancellation system operable in response to desired and undesired or interference source signals which are obtained from polarized waves having planes of polarization orthogonal to each other and which are produced in accordance with clock signals having a predetermined clock frequency, an interference signal is included in the desired signal and is derived from the interference source signal to be removed from the desired signal by the use of an interference canceller and to be produced as an interference free signal. The interference free signal is demodulated into demodulated signals, error signals, and quadrant detection signals by a demodulator unit. In this event, the quadrant detection signals alone are stably demodulated by the use of specific clock signals which are sent through a frequency doubler operable to double the predetermined clock frequency of the clock signals.

Abstract: In order to rapidly resynchronize a QAM demodulator by effectively operating a transversal equalizer, when the demodulator remains in synchronism, a transversal filter forming part of the equalizer is allowed to operate normally. On the other hand, in the event that the demodulator falls into asynchronism, (a) the transversal filter intermittently operates and receives a first constant voltage during the discontinuous operations if a deviated message point is detected within a predetermined region and (b) the transversal filter is alternately supplied with the first constant voltage and a second constant voltage if a deviated message point is outside the predetermined of the region.

Abstract: In a demodulation system comprising a demodulator (22) which demodulates a quadrature amplitude modulated signal into first and second sets of output digital signals by the use of a reference carrier signal (CA), a specific phase control signal is produced in a transient state by discriminating a plurality of areas and a plurality of zones defined on a phase plane of the modulated signal. The areas are defined by dividing the phase plane by both a pair of quadrant reference axes and a pair of auxiliary reference axes phase shifted by .pi./4 (radians) relative to the quadrant decision axes. The zones are defined along and adjacent to the auxiliary reference axes. First and second detection signals (S.sub.1 and S.sub.2) are produced by a read-only memory (35) as results of discrimination of the areas and the zones, respectively. The specific phase control signal is switched to a normal phase control signal produced in a normal state in the known manner.