Optical demodulator

Abstract: An optical demodulator comprises a nonlinear element which responds to both light and a microwave ∼ submillimeter wave (far infrared radiation). An optical signal modulated by a modulating wave in which a subcarrier consisting of the microwave ∼ submillimeter wave is modulated by a signal wave, and a local oscillated signal consisting of a microwave ∼ submillimeter wave the same in frequency as the subcarrier are added to the nonlinear element, to fetch the signal wave by a heterodyne system.

Description: The present invention relates to an optical demodulator used in a multi-channel optical communication system. Conventionally, an optical demodulator for optical communication generally uses an avalanche photodiode which is capable of being high-speed response. The maximum operation frequency of the avalanche photodiode in the response is several tens of GHz. US-A-4 856 095 discloses a demodulator-downconverter for recovering information from a microwave modulated optical carrier signal, which demodulates a microwave optical signal by converting it from optical to electrical form and shifts the resultant demodulated electrical signal to a lower frequency. To this, a light sensitive single-gate three-electrode field-effect transistor (OPFET) is used, which simultaneously performs the demodulation function and the down-conversion function. Consequently, this demodulator exhibits great economy and efficiency. However, the use of three terminal elements gives rise to a large parasitic capacity, and so operating frequencies of several tens of THz are not accessible for this method. Since, however, as potential of the optical communication, demodulation up to several tens of THz is possible, it cannot be said that, if multi-channel ability is not utilized up to the demodulation of several tens of THz, advantages of optical communication in which light is used as a carrier are not sufficiently exhibited. In this connection, a prior art technique has not sufficiently utilized the advantages of the optical communication, and development of the optical demodulator which operates at frequency exceeding several tens of GHz has been desired. It is an object of the invention to provide an optical demodulator capable of demodulating an optical signal modulated by a microwave ∼ submillimeter wave which is an electromagnetic wave whose frequency exceeds several tens of GHz. This object is achieved by an optical demodulator according to claim 1. Further advantageous embodiments of the invention are defined in the dependent claims. Thus, there is provided an optical demodulator comprising a semiconductor diode such as a GaAs-Schottky diode as a nonlinear element which responds to both light and a microwave ∼ submillimeter wave (far infrared radiation), wherein an optical signal modulated by a modulating wave in which a subcarrier consisting of the microwave ∼ submillimeter wave is modulated by a signal wave, and a local oscillated signal consisting of a microwave ∼ submillimeter wave the same in frequency as the subcarrier are added to the semiconductor diode, to fetch the signal wave by a heterodyne system. The optical signal is directly irradiated to the semiconductor diode by the use of an optical fiber or the like. The local oscillated signal is applied to the semiconductor diode through a line or the like which uses a dielectric such as a waveguide, a microstrip line or the like. When the optical signal is irradiated to the semiconductor diode, electric current whose frequency is the same as the modulated wave flows through the semiconductor diode, by optical conduction effects. Simultaneously, when the local oscillated signal is applied to the semiconductor diode, electric current the same in frequency as the subcarrier flows through the semiconductor diode by voltage of the local oscillated signal. By nonlinearity of a current-voltage characteristic of the semiconductor diode, electric current, that is, a signal wave, having differential frequency between the modulated wave and the subcarrier flows through the semiconductor diode. This is demodulation or detection of the optical signal due to a heterodyne system. With the above arrangement of the invention, it is possible to demodulate the optical signal modulated by an electromagnetic wave whose frequency exceeds the microwave ∼ submillimeter wave, that is, several tens of GHz. As described above, according to the optical demodulator of the invention, it is possible to demodulate the optical signal modulated by the microwave ∼ submillimeter wave which is the electromagnetic wave whose frequency exceeds several tens of GHz. As a result, it is possible to sufficiently exhibit advantages of the optical communication which uses optic or light as a carrier. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic view showing a first embodiment of an optical demodulator according to the invention; Fig. 2 is a graphical representation showing a current-voltage characteristic of a GaAs schottky diode that is one of semiconductor diodes; and Fig. 3 is a perspective view showing a second embodiment of the invention. DESCRIPTION OF THE EMBODIMENTS Various embodiments of the invention will hereunder be described in detail with reference to the accompanying drawings. Fig. 1 is a schematic view showing a first embodiment of an optical demodulator according to the invention. The reference numeral 1 denotes a waveguide circuit on which a back short, i.e., a variable short-circuit plate 2 is mounted. The reference numeral 3 denotes a semiconductor diode which is loaded on the waveguide circuit 1 and which has such an energy level that, when an optical signal is irradiated, optical conductive current is generated. Further, a current-voltage characteristic of the semiconductor diode 3 has, in combination, high speediness which can respond to a microwave ∼ submillimeter wave. Specifically, the semiconductor diode 3 responds both to the microwave ∼ submillimeter wave (far infrared radiation) which are light and electromagnetic waves of frequency exceeding several tens of GHz, although their principles are different from each other. The reference numeral 4 denotes a coaxial line. The embodiment of the invention has been constructed as described above. Accordingly, when the optical signal modulated by a modulating wave (frequency: f m + Δf) which is formed such that the subcarrier consisting of the microwave (frequency: f m ) is modulated by a signal wave (frequency: Δf) is irradiated to the semiconductor diode 3 through an optical fiber or the like. current iS whose frequency is f m + Δf flows through the semiconductor diode 3 by optical conduction effects. On the other hand, when a local oscillated signal consisting of the microwave whose frequency is f m is simultaneously applied to the semiconductor diode 3 through the waveguide circuit 1, current iL whose frequency is f m flows through the semiconductor diode 3 by the voltage of the local oscillated signal. Due to nonlinearity of the current-voltage characteristic of the semiconductor diode 3, current IF (intermediate frequency) having a differential frequency Δf between the modulated wave and the local oscillated signal, that is, a signal wave flows through the semiconductor diode 3. This is demodulation (detection) of the optical signal due to the heterodyne system. The signal wave can be drawn out or fetched to the outside through the coaxial line 4 and the like by a diode loading part of the waveguide circuit 1. Fig. 2 is a graph showing a current-voltage characteristic of a GaAs schottky diode that is one of the semiconductor diodes 3. In Fig. 2, "presence" and "absence" indicate presence and absence of irradiation of a He-Ne laser light, respectively. According to the graph illustrated in Fig. 2, the graph indicates that the characteristic curve varies depending upon presence and absence of the irradiation of the laser light. Thus, it will be clear that there is produced an IF output due to nonlinearity of the current-voltage characteristic of the GaAs schottky diode. Fig. 3 is a perspective view showing a second embodiment of the invention. The arrangement is such that a microstrip line 6 and a semiconductor diode 3 as well as a microstrip line 7 are formed on a semiconductor substrate 5, an optical signal is irradiated to the semiconductor diode 3, a local oscillated signal that is a microwave is simultaneously applied to the semiconductor diode 3 through the microstrip line 6, and an IF is fetched out of the microstrip line 7.

Claims: An optical demodulator, comprising a nonlinear element which responds to both light waves and microwave-submillimeter waves, wherein an optical signal modulated by a modulating wave in which a subcarrier consisting of a microwave-submillimeter wave is modulated by a signal wave and a local oscillated signal consisting of the microwave-submillimeter wave the same in frequency as said subcarrier are added to the nonlinear element, and means for fetching the optical conductive current produced by said nonlinear element by a heterodyne system, characterised in that a semiconductor diode (3) loaded on a waveguide circuit (1) is used as said nonlinear element for obtaining the signal wave from said modulated optical signal. An optical demodulator according to claim 1, wherein said waveguide circuit (1) is a microstrip line (6, 7). An optical demodulator according to claim 1 or 2, wherein said semiconductor diode (3) is a GaAs schottky diode.

Inventors: MIZUNO KOJI, MIZUNO, KOJI
Applicants: JAPAN RES DEV CORP, RESEARCH DEVELOPMENT CORPORATION OF JAPAN
Application year: 1992
Main classification: H04B10
IPC codes: H01L31, H04B10