Fujitsu Corporation

Yoshihito Onoda

Terumi Chikama

Shigeki Watanabe

Takao Naito

Tetsuya Kiyonaga

Hideaki Sugiya

Hiroshi Onaka

Keiko Sasaki

Hideo Kuwahara

Takashi Shimizu

Motoyoshi Sekiya

Yasushi Sugaya

Hideyuki Miyata

Shinya Inagaki

Etsuko Hayashi

Tomoaki Takeyama

Hiroaki Tomofuji

Tadashi Okiyama

Takeshi Sakamoto

Yuji Tamura

Hiroyuki Itou

Michikazu Shima

Taiki Kobayashi

Chihiro Ohshima

Makoto Murakami

Two rare earth-doped optical fibers are connected in series and used to amplify input light. A splitter is installed between these two rare earth-doped optical fibers. The input light is monitored by having the portion of the input light that is branched off, by the splitter received by a photodiode. Excitation light output from a laser light source is guided by optical couplers and supplied to the above rare earth-doped optical fibers. A control circuit controls the output light level and, at the same time, stops the output from the laser light source when the input light level drops below a specified threshold value. The gain of the first stage rare earth-doped optical fiber while excitation light is being supplied is larger than the loss that occurs due to branching of the input light by the splitter.

Optical amplifier and an optical amplification method

A contrivance, if a WDM signal state changes, is to improve an optical transmission quality by compensating gain flatness. A first excitation control unit sets a first optical pumping light source unit to emit excitation power needed to get the same amplifying operating level of a first amplifying medium as at the time of a maximum wavelength count when allocating wavelengths after a change in wavelength count at an equal interval in a wavelength range. A wavelength allocation bias estimation unit compares a present monitor value of optical power after outputting of a gain equalizerwith wavelength equi-allocation power associated with the recognized wavelength count, and thus estimates a wavelength allocation bias occurred as a concomitant of the change in the wavelength count. A primary gradient calculation unit obtains a primary gradient quantity defines as a gain deviation from the wavelength allocation bias. A second excitation control unit sets, in a second optical pumping light source unit, the excitation power needed to cancel the primary gradient quantity. An attenuation quantity control unit controls a variable optical attenuator to fix a gain by compensating an amount of change in the sum of gains.

Optical amplifier

Optical signals (multi-wavelength light) are guided to an EDF. The EDF is supplied with pump light generated by a first and a second pump light sources. Dummy light is generated by a dummy light source and supplied to the EDF. The wavelength of the dummy light is shorter than a wavelength range in which the optical signals are allocated.

Optical amplifier for amplifying multi-wavelength light

An optical amplifying apparatus for widening the input dynamic range and lowering noises. The optical amplifying apparatus comprises a first optical amplifier, an optical attenuator, a second optical amplifier and a controller. The first optical amplifier changes the target value of output light when the variance amount of input light reaches a predetermined value. The controller changes the attenuation amount of attenuator in accordance with a difference between the target value and the changed target value of the output light level of the first optical amplifier, when the target value of the output light level of the first optical amplifier is changed.

Optical amplifying apparatus, wide-band optical amplifying apparatus, and optical communication system

Two rare earth-doped optical fibers are connected in series and used to amplify input light. A splitter is installed between these two rare earth-doped optical fibers. The input light is monitored by having the portion of the input light that is branched off by the splitter received by a photodiode. Excitation light output from a laser light source is guided by optical couplers and supplied to the above rare earth-doped optical fibers. A control circuit controls the output light level and, at the same time, stops the output from the laser light source when the input light level drops below a specified threshold value. The gain of the first stage rare earth-doped optical fiber while excitation light is being supplied is larger than the loss that occurs due to branching of the input light by the splitter.

Disclosed herein is an optical amplifier including an optical amplifying medium, a pumping source, an optical filter, and a control unit. Signal light is supplied to the optical amplifying medium. The pumping source supplies pump light to the optical amplifying medium so that the optical amplifying medium provides a gain band including the wavelength of the signal light. The optical filter is optically connected between the optical amplifying medium and the pumping source, and has wavelength selectivity depending upon temperature. The control unit controls the temperature of the optical filter according to a control signal given to the control unit. The wavelength characteristic of gain in the gain band provided by the optical amplifying medium changes according to the power of the signal light and the power and center wavelength of the pump light. When the temperature of the optical filter changes, the wavelength selectivity of the optical filter changes, so that the center wavelength of the pump light changes. Accordingly, the gain tilt of the optical amplifier can be adjusted.

Optical amplifier and system including the same

Disclosed is a bidirectional optical amplifier circuit having a function that achieves reliable detection of reflections from an exposed end at low cost. Received optical power is detected; if the detected power is less than a prescribed discrimination level, it is then decided that the detected light is not received signal light but transmitted signal light sent in the direction opposite from it and reflected at an exposed fiber end, and the bias current for a laser diode in the opposite path is reduced to lower the optical output level of an EDF.

Bidirectional optical amplifier circuit

A control apparatus for an optical amplifier such as an erbium-doped optical-fiber (EDF) amplifier suppresses overshooting of ALC control at power on and at the time of input light restoration from an off condition, while ensuring quick starting. At power on and at the time of input light restoration from an off condition, an idling current (IDC) reference value generating circuit generates an IDC reference voltage that increases to a value sufficiently greater than a value in a normal operating condition with a time constant corresponding to a rise time of the EDF, and thereby controls a laser diode for pumping the EDF. The voltage may be set to a value larger than the value in the normal operating condition for a predetermined period of time by using a timer.

Control apparatus for optical amplifier

A receiver for coherent optical communication in a phase diversity system. An optical hybrid circuit receives a signal beam and receives a local-oscillator beam which has a predetermined phase shift. The optical hybrid circuit branches each beam into pairs, each pair having one signal beam and one local-oscillator beam. These beam pairs are output to first and second photodetectors which each apply a photoelectric conversion, thus producing first and second output signals. The output signals are output to an electrical 90.degree. hybrid circuit which shifts the first signal by 90.degree. and adds it to the second signal to produce a first output signal, and shifts the second signal by 90.degree. and adds it to the first signal to produce a second output signal. The first and second output signals are applied to first and second demodulators, respectively, which delay the signals by a delay corresponding to their frequency. The demodulators output the delayed signals to an adder which adds the two signals to produce an output signal. As a result, the receiver is capable of compensating the wavelength dispension is an optical fiber to secure an enhanced receiving sensitivity.

Receiver for coherent optical communication

After mixing the signal light and the local oscillation light with a mixing circuit, two intersecting polarization components are divided by a polarization splitter and respective components are subjected to heterodyne detection by optical receivers. Thereafter, in an electric signal stage, the intermediate frequency signal outputted from the optical receivers are applied to a 90-degree hybrid coupler and the output signal from one of the output ports is demodulated by a demodulator. On the other hand, an intermediate frequency signal outputted from the optical receivers are subjected to a subtraction to provide a principal axis angle monitor signal for the signal light. The signal outputted from the output port from the 90-degree hybrid coupler is used to monitor the elliptic ratio of the signal light and these monitor signals are applied to the polarization control circuit. The polarization control circuit drives the polarization operation apparatus based on said monitor signal to control the polarization state (i.e. a principal axis angle and the elliptic ratio) of the signal light to achieve the optimum state.

Heterodyne receiver for coherent optical communication

A direct modulation PSK system and method, in a coherent optical fiber transmission system, in which an injection current supplied to a laser diode is adapted to be directly varied is disclosed. In this system or method, a modulating current pulse is superposed on a bias current for the laser diode so that a specified phase condition may be satisfied. Thereby, a differential coding circuit and an external modulator being indispensable for a DPSK system become unnecessary and, in addition, the system becomes less susceptible to the influence of wavelength dispersion than the DPSK system and able to reproduce the carrier.

Direct modulation phase-shift-keying system and method

A polarized wave diversity optical receiver for coherent optical communication including: an optical local oscillating circuit (7) for oscillating local oscillating light; a mixing circuit (8) for mixing signal light and the local oscillating light to obtain two polarized components; a detecting circuit (9) for detecting the polarized component to output intermediate frequency signals (e.sub.S and e.sub.P); and a frequency control circuit (11) for controlling, in accordance with the intermediate frequency signals (e.sub.S and e.sub.P), the oscillating frequency of the optical local oscillating circuit (7). To ensure that the intermediate frequency does not disappear, the frequency control circuit (11) outputs a combined signal of a sum and a difference of the intermediate frequency signals (e.sub.S and e.sub.P).

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Kawasaki, Japan

Yoshihito Onoda