Abstract: Communications having high security against information leakage can be established in a current optical fiber network in the following manner. (1) A sender and a receiver share a seed key in advance, and then transmit and receive random numbers superimposed on carrier light accompanied with fluctuations, where transmission basis is determined by a random number. The sender and the receiver check a shared basis determined by the seed key with a random basis and employ only a random number signal superimposed on a slot for the shared basis that coincides with the random basis, and share the random numbers between the sender and the receiver. Here, since the carrier light has fluctuations, a bit error exists in the received signals. However, because of the seed key, a legitimate receiver can receive a signal with a bit error rate smaller than an eavesdropper.

Abstract: Regardless of a transmission basis, a phase of a signal light with reference to a phase of local light is measured, and an output light from an antisqueezed light source in a transponder device is modulated in accordance with the measured phase. Since information obtainable in the transponder device is only the relative phase of the signal light with reference to the phase of the local light, and includes fluctuations corresponding to an antisqueezed component of the signal light, even if the information in the transponder device is eavesdropped on, the information cannot be decoded easily.

Abstract: An antisqueezed light generator system is built with only the components for optical communications with long-term reliability. A cw-LD light is made pulses by an intensity modulator and amplified by an optical amplifier. The amplified optical pulses are made short by high-order soliton pulse compression effect at a first optical fiber and peak power is increased. A fluctuation is expanded in a phase direction through propagation in a second optical fiber. Because an initial fluctuation is amplified by the optical amplifier, the fluctuation expanded in the phase direction is increased to the extent of the amplification and sufficient antisqueezing strength can be obtained.

Abstract: For conventional cipher communications based on the principles of quantum mechanics, the photon number per signal needs to be controlled below 1 or to a mesoscopic level in order to make quantum-mechanical properties remarkable. This invention is intended to provide a quantum-mechanical communications' method that is practical even with a macroscopic number of photons. Antisqueezed light is transmitted using a random sender basis. The legitimate recipient can use a secret key to know the random sender basis, and thus to receive the information accurately without being affected by antisqueezing. However, because eavesdroppers unknowing of the sender basis must use haphazard bases in an attempt to receive the signals, the eavesdroppers are significantly affected by antisqueezing and the respective signal bit-error-rates are increased. This makes eavesdropping impossible, thus ensuring secure communications between the legitimate senders and recipients.

Abstract: In a cryptographic key distribution system by the phase modulation using a single photon state or a faint LD light, there is required an interferometer independent on polarization and stabilized against thermal fluctuations in order to make a transmission distance longer. Cryptographic key distribution systems are generally low in cryptographic-key-generating efficiency, and an improvement in the efficiency is demanded. In the present invention, two interferometers are disposed within the receiver so as to require no phase modulator within the receiver, thereby achieving a polarization-independent receiver. The pulses are paired, and the signal is transmitted with the relative phase, and the interval of the paired pulses is sufficiently reduced to set the optical path within the interferometer in the receiver to be smaller, thereby achieving the interferometer stabilized against thermal fluctuations.

Abstract: In an optical detection method that requires a reference light such as homodyne detection, a signal light and the reference light must be equal to each other in the wavelength, and the phase relation between them must be maintained constant. In order to satisfy this requirement, the signal light and the reference light are extracted from the same light source and made equal to each other in the wavelength. The signal light and the reference light are transmitted so as to be temporally superimposed on each other with orthogonal polarizations to the same optical path, thereby making the external environments equal to each other to maintain the constant phase relation.

Abstract: Signals can be superimposed on optical phase even when low-coherency light is used, and a bit rate and a signal coding format similar to those used in ordinary optical communications can be used. A transmitter includes an asymmetric interferometer or an antisqueezed light generator to convert a train of single pulses into a train of dual pulses. A receiver also includes an asymmetric interferometer that provides the same delay time as that between the dual pulses. The receiver allows pulses originating in the same light source to interfere, so that signals can be superimposed on the phase even when a low-coherency light source is used. The delay time (optical path length difference) provided in the asymmetric interferometer is set to be longer than half the period of the pulses outputted from the optical pulse source.

Abstract: A squeezed light generator comprises an arbitrary optical fiber, a means for temporally separating two linearly polarized components, two Faraday rotators and a high-reflection mirror. Pulse lights that are temporally separated into two orthogonally polarized components at an intensity ratio of 50:50 are reciprocatively propagated in the optical fiber, and the polarized light is rotated by 90° in an outward transmission. Since those two polarized components pass through the optical paths which are accurately equal to each other in the outward and homeward transmissions, those two polarized components interfere with each other accurately at 50:50 after reciprocation through the fiber. The interfered beam is separated by a polarizing beam splitter that is high in an extinction ratio. When the polarized lights before inputting the fiber and after reciprocating coincide with each other, it is unnecessary to maintain the polarization in the fiber propagation, and an arbitrary fiber can be used.

Abstract: Regardless of a transmission basis, a phase of a signal light with reference to a phase of local light is measured, and an output light from an antisqueezed light source in a transponder device is modulated in accordance with the measured phase. Since information obtainable in the transponder device is only the relative phase of the signal light with reference to the phase of the local light, and includes fluctuations corresponding to an antisqueezed component of the signal light, even if the information in the transponder device is eavesdropped on, the information cannot be decoded easily.

Abstract: For conventional cipher communications based on the principles of quantum mechanics, the photon number per signal has needed to be controlled below 1 or to a mesoscopic level in order to make quantum-mechanical properties remarkable. This invention is intended to provide a quantum-mechanical communications' method that is practical even in the condition of a macroscopic number of photons. Antisqueezed light is transmitted using a random sender basis. The legitimate recipient can use a secret key to know the random sender basis and thus to receive the information accurately without being affected by antisqueezing. However, because eavesdroppers unknowing of the sender basis must use haphazard bases in an attempt to receive the signals, the eavesdroppers are significantly affected by antisqueezing and the respective signal bit-error-rates are increased. This makes eavesdropping impossible, thus ensuring secure communications between the legitimate sender and recipient.

Abstract: An antisqueezed light generator system is built with only the components for optical communications with long-term reliability. A cw-LD light is made pulses by an intensity modulator and amplified by an optical amplifier. The amplified optical pulses are made short by high-order soliton pulse compression effect at a first optical fiber and peak power is increased. A fluctuation is expanded in a phase direction through propagation in a second optical fiber. Because an initial fluctuation is amplified by the optical amplifier, the fluctuation expanded in the phase direction is increased to the extent of the amplification and sufficient antisqueezing strength can be obtained.

Abstract: In a cryptographic key distribution system by the phase modulation using a single photon state or a faint LD light, there is required an interferometer independent on polarization and stabilized against thermal fluctuations in order to make a transmission distance longer. Cryptographic key distribution systems are generally low in cryptographic-key-generating efficiency, and an improvement in the efficiency is demanded. In the present invention, two interferometers are disposed within the receiver so as to require no phase modulator within the receiver, thereby achieving a polarization-independent receiver. The pulses are paired, and the signal is transmitted with the relative phase, and the interval of the paired pulses is sufficiently reduced to set the optical path within the interferometer in the receiver to be smaller, thereby achieving the interferometer stabilized against thermal fluctuations.

Abstract: A squeezed light generating method using the Kerr effect of an optical fiber is relatively easy, but the requirements in manufacture such that a beam must be divided accurately at 50:50 are large. In addition, it is desirable that the squeezed light can be generated with arbitrary wavelength and pulse width from the viewpoint of the communication application. For that reason, the squeezed light generator of this invetion is made up of an arbitrary optical fiber, a means for temporally separating two linearly polarized components, two Faraday rotators and a high-reflection mirror. Pulse lights that are temporally separated into two orthogonally polarized components at an intensity ratio of 50:50 are reciprocatively propagated in the optical fiber, and the polarized light is rotated by 90° in an outward transmission.

Abstract: In order to provide a compact device easy to handle and adjust for use in bloodless measurement of the glucose concentration, in which the angle of polarization varies in synchronism with the magnetic field modulation, the direction of applying the magnetic field is so arranged as to cross the optical axis.

Abstract: In an optical detection method that requires a reference light such as homodyne detection, a signal light and the reference light must be equal to each other in the wavelength, and the phase relation between them must be maintained constant. It is generally difficult that the a local light source located on a detecting unit satisfies the above conditions, and the difficulty becomes particularly remarkable in the case where a faint signal light is transmitted, as ordinarily used in quantum communications. In order to remove the difficulty, the signal light and the reference light are extracted from the same light source and made equal to each other in the wavelength. When both of the signal light and the reference light are transmitted for a long distance, a relative phase relation between the signal light and the reference light cannot be maintained constant due to a difference in the external environments of a transmission path between the signal light and the reference light.

Abstract: The invention provides an optical waveguide for enabling the reduction of a coupling loss caused by difference in the size of a mode field diameter between optical waveguide modes different in the mode field diameter and a coupling loss caused by reflection on a boundary caused by the difference of media and an optical device using it. Structure provided with a mode field diameter converter provided with an antireflection part required to reduce a coupling loss between a microoptical circuit and a single mode optical fiber is integrally produced using effective refractive index control based upon photonic crystal structure.

Abstract: The present invention provides a technique of reducing an incident/outgoing loss of a photonic crystal. On each of incident/outgoing sides of a photonic crystal, an antireflection layer made of a photonic crystal is disposed. At the incident side of a photonic crystal 200 having an effective refractive index n2, a photonic crystal 100 having an effective refractive index n1 satisfying the relation of n1<n2 is disposed, and the thickness of the photonic crystal 100 is controlled so that reflection components from interfaces 210 and 110 cancel each other out by interference, thereby reducing a total reflection loss on the incident side. The outgoing side is similarly constructed.

Abstract: In a solid-state laser in which a gain crystal is polished to have the Brewster angle or a solid-state laser comprising a dichroic concave mirror to which light enters at an incidence angle which is not zero, astigmatism generally occurs in pumping light. By tilting a focusing lens for pumping light with respect to the optical axis of the pumping light, the astigmatism is compensated. The tilting angle is determined in such a manner that synthetic focusing points in the sagittal and tangential planes, of a series optical system of a focusing lens, a dichroic concave lens, and a gain crystal are calculated and the focusing points almost coincide with focusing points in a cavity mode.

Abstract: A solid-state laser having an excitation light source, a lens system for focusing an excitation light produced from the light source, and a laser resonator which is laser-oscillated in response to the focused excitation light. The resonator includes only a gain medium and a solid-state medium having dispersion of a wavelength dependence opposite to that of the gain medium, and surfaces of the gain medium and the solid-state medium, which are respectively at opposite sides of opposing surfaces of the gain medium and the solid-state medium, are reflectively coated and act as mirror surfaces. At least one of the other surfaces of the gain medium and the solid-state medium, which ate positioned inside the resonator and are opposite to each other, is polished to a curved surface. The gain medium is excited by the focused excitation light so that the laser oscillation is made in the resonator.

Abstract: A solid-state laser having an excitation light source, a lens system for focusing an excitation light produced from the light source, and a laser resonator which is laser-oscillated in response to the focused excitation light. The resonator includes only a gain medium and a solid-state medium having dispersion of a wavelength dependence opposite to that of the gain medium, and surfaces of the gain medium and the solid-state medium, which are respectively at opposite sides of opposing surfaces of the gain medium and the solid-state medium, are reflectively coated and act as mirror surfaces. At least one of the other surfaces of the gain medium and the solid-state medium, which are positioned inside the resonator and are opposite to each other, is polished to a curved surface. The gain medium is excited by the focused excitation light so that the laser oscillation is made in the resonator.