Abstract: An optical fiber amplification. system includes: a first optical fiber amplifier including a first optical amplifying fiber including a core portion doped with a first rare-earth. element, a first input unit configured to receive first signal light, an excitation-light source configured to output pump light, a pump light combiner configured to input the pump light to the first optical amplifying fiber, and a residual pump light recovery device configured to recover residual pump light; and a second optical fiber amplifier including a second optical amplifying fiber including a core portion doped with a second rare-earth. element, a second input unit configured to receive second signal light, and a residual pump light combiner configured to input, to the second optical amplifying fiber, the residual pump light recovered by the residual pump light recovery device.

Abstract: The present invention relates to a coherent optical communication apparatus and method. According to the invention, the optical communication apparatus receives a modulated optical signal, which is generated by modulating an optical signal with a first electrical signal obtained by adding a second electrical signal carrying information to be transmitted and a reference electrical signal, and converts the modulated optical signal to a third electrical signal by coherent detection. Then the apparatus detects an amount of fluctuation of the reference electrical signal included in the third electrical signal, and compensates the second electrical signal included in the third electrical signal using the amount of fluctuation.

Abstract: A present invention provides an optical communication apparatus and method which can use a data modulator and an OFDM modulator more narrow band than a transmission band. The optical communication apparatus comprise an intensity modulation means for modulating intensity of continuous light with a sine wave, a means for separating optical signal outputted by said intensity modulation means into a bottom sideband and an upper sideband and outputting, a first modulation means for modulating said upper sideband with a first electrical signal, a second modulation means for modulating said bottom sideband with a second electrical signal, and a multiplex means for multiplexing an output signal from said first modulation means and an output signal from said second modulation means and outputting.

Abstract: The present invention relates to a coherent optical communication apparatus and method. According to the invention, the optical communication apparatus receives a modulated optical signal, which is generated by modulating an optical signal with a first electrical signal obtained by adding a second electrical signal carrying information to be transmitted and a reference electrical signal, and converts the modulated optical signal to a third electrical signal by coherent detection. Then the apparatus detects an amount of fluctuation of the reference electrical signal included in the third electrical signal, and compensates the second electrical signal included in the third electrical signal using the amount of fluctuation.

Abstract: To improve transmission characteristics of a CS-RZ signal, a laser diode (LD) (12) outputs a continuous light (CW light) having a wavelength &lgr;s (frequency f0). A CS-RZ pulse train shaper (14) generates a CS-RZ optical pulse train at a bit rate (B) from the CW output light from the LD (12). A data modulator (16) intensity-modulates an output optical pulse train from the shaper (14) according to a data (D) to be transmitted. An optical bandpass filter (18) passes through all of a component of carrier frequency (f0) and a component of frequency f1 (=f0+B/2) and transmits a small amount of component of frequency f2 (=f0−B/2).

Abstract: The soliton pulse compression optical fiber comprises a core and a clad that surrounds the core for compressing the injected light pulse in width. The maximum relative index difference between the core and the clad is between about 1.2% and 2.5%. The core diameter of the soliton pulse compression optical fiber varies along the length of the soliton pulse compression optical fiber and the core has a step-like refractive index distribution. The optical fiber perform of the soliton pulse compression optical fiber comprises a core preform, which is to be a core of the soliton pulse compression optical fiber, and a clad preform that surrounds the core preform. The maximum relative index difference between the core perform and the clad preform is also between about 1.2% and 2.5%.

Abstract: An object of this invention is to realize 20 Gbit/s or more high speed WDM transmission over 6000 km. A dispersion compensating optical transmission line consists of a first optical fiber composed of an optical fiber whose chromatic dispersion becomes zero at a 1.3 &mgr;m band, a second optical fiber composed of an optical fiber whose chromatic dispersion becomes zero at a 1.3 &mgr;m band, and a dispersion/dispersion slope compensator which is substantially a linear optical device disposed between the first and second optical fibers to compensate for chromatic dispersion and dispersion slope of the first and second optical fibers.

Abstract: Detection of an optical pulse position uses an optical pulse string with a determined repetitive ratio and an electric clock signal with a same frequency as the repetitive ratio of the optical pulse string. A phase of the electric clock signal oscillator is shifted and supplied to an optical modulator. The optical modulator modulates the optical pulse string based on the electric clock signal and outputs a modulated optical signal. A photo detector converts the modulated optical signal output from the optical modulator to an electric signal. The phase shift amount of the electric clock signal is controlled to maximize an output from the photo detector. Additionally, a dither signal may be used in the control of the phase shift, more than the optical modulator may be employed, and/or more than color light source may be employed. The use of at least one of feed forward and feedback control provided by maximizing an output of the photo detector allows an optical pulse having a short width to be realized.

Abstract: An object of this invention is to generate a high-speed optical signal pulse with a narrow spectrum width of a constant polarization. An optical pulse generator (10) generates an optical pulse of S polarization. An optical splitter (12) splits the optical pulse output from the optical pulse generator (10) into two portions maintaining its polarization and applies one portion to a data modulator (14) and the other to a data modulator (16). The data modulators (14 and 16) digitally modulate the optical output pulses from the optical splitter (12) according to data #1 and #2 respectively. A Faraday rotator (18) converts the optical output from the data modulator (14) from S polarization to P polarization. An optical delay (20) delays the optical pulse from the data modulator (16) in order to multiplex with the optical pulse from the Faraday rotator (18) in the time domain.

Abstract: Detection of an optical pulse position uses an optical pulse string with a determined repetitive ratio and an electric clock signal with a same frequency as the repetitive ratio of the optical pulse string. A phase of the electric clock signal oscillator is shifted and supplied to an optical modulator. The optical modulator modulates the optical pulse string based on the electric clock signal and outputs a modulated optical signal. A photo detector converts the modulated optical signal output from the optical modulator to an electric signal. The phase shift amount of the electric clock signal is controlled to maximize an output from the photo detector. Additionally, a dither signal may be used in the control of the phase shift, more than the optical modulator may be employed, and/or more than color light source may be employed. The use of at least one of feed forward and feedback control provided by maximizing an output of the photo detector allows an optical pulse having a short width to be realized.

Abstract: Detection of an optical pulse position uses an optical pulse string with a determined repetitive ratio and an electric clock signal with a same frequency as the repetitive ratio of the optical pulse string. A phase of the electric clock signal oscillator is shifted and supplied to an optical modulator. The optical modulator modulates the optical pulse string based on the electric clock signal and outputs a modulated optical signal. A photo detector converts the modulated optical signal output from the optical modulator to an electric signal. The phase shift amount of the electric clock signal is controlled to maximize an output from the photo detector. Additionally, a dither signal may be used in the control of the phase shift, more than the optical modulator may be employed, and/or more than color light source may be employed. The use of at least one of feed forward and feedback control provided by maximizing an output of the photo detector allows an optical pulse having a short width to be realized.

Abstract: An optical transmission device using return-to-zero optical pulses as transmission optical pulses, which is capable of enhancing a transmission distance as well as a wavelength range with satisfactory transmission characteristics and enabling an easy realization of the wavelength division multiplexing optical transmission. The optical transmission device is formed by an optical transmitter for transmitting transmission optical pulses by superposing digital data signals onto return-to-zero optical pulses, and a modulator for applying one of a phase modulation and a frequency modulation in synchronization with a transmission rate of the transmission optical pulses, to the transmission optical pulses transmitted by the optical transmitter. This optical transmission device can be utilized in forming a wavelength division multiplexing optical transmission apparatus and an optical transmission system.

Abstract: An optical pulse generator, capable of generating ultrashort optical pulses suitable for optical soliton transmission, includes a DFB laser 10 for continuous laser oscillation, an electroabsorbtion modulator 12 for creating a sequence of optical pulses of the pulse width 14.6 ps from optical output of the laser 10. Output from an optical modulator 12 enters into a dispersion decreasing fiber 16 via an optical isolator 14. The dispersion decreasing fiber 16 has chromatic dispersion that decreases from 13.7 ps/nm/km to 2.3 ps/nm/km with distance, and its fiber length is 15 km. Pump laser beams from pump lasers 20, 24 are introduced to the dispersion decreasing fiber 16 by optical couplers 18, 22, and the fiber 16 functions as a Raman amplifier. When the Raman gain is 2.4 dB, the pulse width is compressed from 14.6 ps to 5.8 ps, approximately, even when the power of input pulses to the dispersion decreasing fiber 16 complies with the soliton condition.

Abstract: Incident light 1 is inputted to an electro-absorption-type optical modulator 4 via an optical circulator 2 and a lens 3, and subjected to intensity modulation that corresponds to a modulation signal 8. The optical signal outputted by the electro-absorption-type optical modulator 4 is inputted to a Faraday rotator 6 via a lens 5, and the plane of polarization is rotated 45.degree.. The optical signal transmitted by the Faraday rotator 6 is totally reflected by a totally reflecting mirror 7, inputted for the second time to the Faraday rotator 6, then passed through the lens 5 after the plane of polarization has been rotated 45.degree. by this Faraday rotator 6, and readmitted by the electro-absorption-type optical modulator 4. The output of the electro-absorption-type optical modulator 4 is emitted via the lens 3 and the optical circulator 2. The polarization dependence of insertion loss can be eliminated because the plane of polarization is rotated 90.degree.

Abstract: An optical frequency shifter which has no insertion loss polarization dependency and no polarization mode dispersion is formed by: at least one ultrasonic wave generator for generating ultrasonic waves; at least one electric acousto-optic element forming at least one diffraction grating therein upon being applied with the ultrasonic waves generated by the ultrasonic wave generator; at least one polarization rotator for rotating a polarization plane of a first primary diffracted light produced by the diffraction grating, by 90.degree.

Abstract: An optically amplifying transmission system available for wider signal bands includes an optical transmission line 42 divided into two sections 42a and 42b, optically amplifying repeaters 48a having 7 m-long erbium-doped optical fibers to repeat transmission optical fibers 46a in the section 42a, and optically amplifying repeaters 48b having 10.8 m-long erbium-doped optical fibers to repeat transmission optical fibers 46b in the section 42b. The transmission optical fibers 46a an 46b are optical fibers absolutely identical in construction and length.

Abstract: An optical transmission system which permits long-distance, ultrafast, high-capacity optical soliton transmission by suppressing timing jitter with simple means. In an optical transmission system which uses a transmission line composed of an optical fiber for transmitting therethrough a lightwave signal having digital information added to a return-to-zero lightwave pulse and a plurality of optical amplifying repeaters for compensating for losses by the optical fiber and in which the pulse compression effect by the nonlinear optical effect on the optical pulse and the pulse expansion effect by the dispersion effect are compensated, at least one optical band limited element which has a flat group delay characteristic near the center frequency is disposed in the transmission line at intervals of a period Z equal to the soliton period Z.sub.sol and the product, aB.sub.sol.spsb.

Abstract: An optical amplifying-repeating transmission system is disclosed which is composed of an optical fiber for transmitting a lightwave signal with digital information added to return-to-zero lightwave pulses and a plurality of optical amplifying repeaters inserted in the optical fiber for transmission use. The mean value of wavelengths at which the wavelength dispersion of the optical fiber is zero is smaller than the wavelength of the lightwave signal which is transmitted over the system. The accumulated wavelength dispersion value of the optical fiber tends to increase with the distance of transmission, from a macroscopic viewpoint. The optical fiber for transmission is divided into a plurality of sections.

Abstract: An optical amplifying transmission system is disclosed in which a large quantity of optical fiber whose zero dispersion wavelength is longer than the wavelength of the lightwave signal is introduced to provide an RZ lightwave signal transmission system. In this system, the average zero dispersion wavelength by the optical fiber is shorter than the wavelength of the lightwave signal, the zero dispersion wavelength by the optical fiber whose length is in the range of one-thirds to two-thirds of the entire length of the optical fiber is longer than the wavelength of the lightwave signal, and the absolute value of the wavelength dispersion is larger than one-half the absolute value of the average wavelength dispersion value over the entire length of the optical fiber.