Nippon Telegraph and Telephone Corporation

Matsushita Electric Industrial Co., Ltd.

Koji Kikushima

Satoshi Ikeda

Masaru Fuse

Kuniaki Utsumi

Katsuyuki Fujito

Kiyomi Kumozaki

Naoya Sakurai

Hiroshi Nakamoto

Seiichiro Kawashima

Nori Shibata

Katsushi Iwashita

Kouichi Suto

Chisei Kishimoto

In an optical transmission system by frequency-division multiplexing, the interference due to distortions of a first signal against a second signal can be reduced. In the optical transmission system where a modulated optical signal by the first signal is further modulated with the second signal for transmission, distortions of the first signal corresponding to a frequency band of the second signal is extracted. The extracted signal is phase-inverted and then adjusted in phase and amplitude with respect to the distortions of the first signal to obtain a correction signal. By intensity-modulating the first signal containing the distortions by the correction signal, the distortions of the first signal against the second signal can be cancelled out. Alternatively, by intensity-modulating the first signal containing the distortions by a combined signal of the correction signal and the second signal, the distortions of the first signal against the second signal can be cancelled out.

Optical transmitting device, optical transmission system, optical transmitting method and optical transmission method

The optical signal receiver of this invention is an optical signal receiver that receives and frequency-demodulates an optical signal, comprising: an optical branch device for splitting an input optical signal into two optical signals (); an optical delay line for delaying one of the two branched optical signals (); a first photoelectric conversion circuit () for converting the optical signal from the optical delay line into the first electrical signal (); a second photoelectric conversion circuit () for converting the other optical signal of the two branched optical signals into a second electrical signal; the rectangular-wave forming circuit () for outputting a single rectangular-wave signal using the first electrical signal from the first photoelectric conversion circuit and the second electrical signal from the second photoelectric conversion circuit as inputs; and a smoothing circuit () for smoothing the rectangular-wave signal from the rectangular-wave forming circuit.

Optical signal receiver, optical signal receiving equipment, and optical signal transmitting system

The invention provides an optical signal transmitter low in noise and in distortion and provides an optical signal transmission system using this optical signal transmitter. The optical signal transmitter includes a plurality of frequency modulation means for distributing an electric signal into a plurality of electric signals and applying frequency modulation to the distributed electric signals to output and a multiplexing mean for multiplexing a plurality of signals output from the plurality of frequency modulation means and outputting a multiplexed signal. The plurality of frequency modulation means are set to be substantially equal to each other in frequency deviation and in intermediate frequency and to be substantially identical to each other in the phase of each output.

Optical signal transmitter and optical signal transmission system

A wavelength multiplex transmission system is provided to reduce crosstalk among wavelengths and reduce deterioration of signals. At a transmission apparatus, one input signal is differentially divided into two; each of the two is converted to an optical signal; and then they are wavelength-multiplexed and transmitted. When crosstalk is arisen in the wavelength multiplex transmission system, the crosstalk is superposed on the two optical signals. The crosstalk is equally superposed on each of the signals with inverted polarities. Accordingly, converting the optical signals to electrical signals and then differentially combining them at a receiving apparatus, their signal components are accumulated, while their crosstalk components are cancelled out. Thereby, in the wavelength multiplex transmission system, deterioration of optical signals due to crosstalk can be reduced.

Wavelength multiplex transmission systems

A branch portion 107 branches a modulating signal into two signals in opposite phases. One of them is inputted to an FM laser element 102. The other one is adjusted in propagation delay and in amplitude and then is inputted to an IM suppressing laser element 110. The FM laser element 102 outputs an optical-frequency-modulated signal around a wavelength .lambda.1, whose optical intensity is also modulated. A local light source 104 outputs light at a wavelength .lambda.0, which is different from the oscillation wavelength .lambda.1 of the FM laser element 102 by .DELTA..lambda.. The IM suppressing laser element 110 outputs an optical-intensity-modulated signal. The three lights are combined and inputted to a photodetection portion 106. The photodetection portion 106 applies a heterodyne detection to inputted lights to output an FM modulated signal corresponding to a beat signal of the outputted optical signal from the FM laser element 102 and the outputted light from the local light source 104 at frequency corresponding to the difference .DELTA..lambda. between the original two wavelengths, and also cancels the average-value variation component in the FM modulated signal with an electrical signal produced by square-law detecting the optical-intensity-modulated signal from the IM suppressing laser element 110, thereby producing an ideal FM modulated signal.

FM modulation device

An electrical signal to be frequency-modulated is branched into first and second signals. An FM laser 102 oscillates light having a wavelength .lambda.1 in a steady state condition, and outputs an optical signal obtained by converting an amplitude change in the inputted first signal into an optical intensity change and an optical frequency change. An optical detecting portion 106, to which the optical signal from the FM laser 102 and light having a wavelength .lambda.0 outputted from a local light source 104 are inputted, converts an optical intensity modulation component of the inputted optical signal as a current amplitude change by operating square-law detection characteristics, and generates a beat signal at a frequency corresponding to a difference in wavelength .DELTA..lambda. between the two inputted optical signals. A discrimination portion 110 discriminates using a threshold value generated on the basis of the second signal and converts the beat signal into the pulse signal. A high frequency component of the pulsed beat signal is removed by an LPF 311. Consequently, it is possible to generate an ideal wide band FM modulation signal by removing an amplitude variation and an average value variation from the beat signal generated by using an optical frequency modulating operation of a laser diode and heterodyne detection.

Frequency modulator

The AM/FM converter of the present invention comprises an optical frequency modulation section which receives multi-channel AM video signals and outputs an optical-frequency-modulated optical signal beam; and an optical heterodyne detection section comprising an optical frequency oscillator composed of a semiconductor local laser which outputs a local oscillator optical beam having an optical frequency separated by a designated intermediate frequency, an optical coupler, and a photoelectric converter composed of a photodiode, which receives the optical signal beam and outputs an electrical batched FM modulation signal which is frequency-modulated and frequency-converted to the intermediate frequency. By means of this configuration, the invention offers an AM/FM converter wherein the frequency band is made considerably wider.

Growing community of inventors

Ichikawa, Japan

Koji Kikushima