Abstract: A method and corresponding apparatus are disclosed for controlling the phase of a second clock signal from (i) a first clock signal of different frequency and (ii) a synchronization signal. One example embodiment recovers a first clock signal from a bus carrying data and detects a synchronization signal. Both the first clock signal and the synchronization signal are provided to a clock generator, which then generates the second clock signal with a phased based on the first clock signal and the synchronization signal. The use of the example embodiment is a low cost way to easily control the phase of a second clock signal with minimal added components and/or additional signal processing.

Abstract: An optical transmission system accomplishes optical transmission over a long distance by combining a multiplexing line terminal with optical amplifiers, linear repeaters, and regenerators with optical amplifiers combined together. The system also accomplishes the optical transmission over a short distance by directly connecting the linear terminals therebetween, with an electric-to-optic converter replaced by an electric-to-optic converter having a semiconductor amplifier, with an optic-toelectric converter by an optic-to-electric converter having an avalanche photodiode as light receiver, and with no use of any optical booster amplifier and optical preamplifier in multiplexing line terminal. With these, the optical transmission system can be easily constructed depending on the transmission distance required.

Abstract: A method of transmitting bidirectional telephony communication signals on a single optical fiber. The bidirectional signals are transmitted in one direction as NRZ coded signals at a first clocking pulse rate, and in the other direction as Manchester coded signals at a second clocking pulse rate which is a multiple (preferably three times (3×)) of the first clocking pulse rate.

Abstract: A signal communication apparatus of a clock reproduction system in which clock signals are extracted from each of parallel data signals for redigitizing each of the data signals. The apparatus includes a reference clock signal generating circuit which is comprised of a clock extraction circuit for extracting a clock signal from each of a plurality of bits of received data signals, and a clock signal selection circuit for selecting one of the extracted clock signals. Alternatively, the reference clock signal generating circuit may be comprised of a data signal selection circuit for selecting one of a plurality of received data signals, and a clock extraction circuit for extracting a clock signal from the selected bit. Based on the resultant reference clock signal, clock signals are obtained that are phase-adjusted for redigitizing each bit of the received data signals.

Abstract: One embodiment of the invention relates to producing optical pulses for use on a transmission link. A light source is configured to produce an optical signal. A pulse generator is coupled to the light source. The pulse generator is configured to receive, for a first channel, the optical signal and a clock signal. The pulse generator is also configured to modify the optical signal based on the clock signal to produce an optical pulse having a predetermined pulse shape. The clock signal is associated with the predetermined pulse shape.

Abstract: The present invention is an optical clock signal extraction device, comprising first conversion means and second conversion means for enabling to extract an optical clock signal without depending on the polarization direction of an input optical signal. The first conversion means comprises a first optical converter and a continuous wave light source of which wavelength is ?2, where an input optical signal of which wavelength is ?1 and continuous wave light of which wavelength is ?2 are input to the first optical converter, and an intermediate optical signal of which wavelength is ?2 is generated and output without depending on the polarization direction of the input optical signal. The second conversion means has a second optical converter, where the intermediate optical signal is input to the second optical converter, and an optical clock signal of which wavelength is ?3 is generated and output by the passive mode locking operation of the second optical converter.

Abstract: In a receiver operable in response to a data signal, an eye aperture size is detected along a time axis by an eye aperture detection circuit and is controlled by a control circuit so that it becomes a maximum. The eye aperture detection circuit determines different decision time points of the same level arranged along a time axis and judges whether or not an error is caused to occur at each of the decision time points, so as to detect the eye aperture size and to produce detection results. The control circuit processes the detection results in accordance with a predetermined algorithm to successively vary the eye aperture size and to keep the data signal at an optimum amplitude.

Abstract: A method is directed to recovering a digital data content in a communication system, wherein the digital data content has been converted into an analog signal based on a primary clock, at a transmitter for transmitting to a receiver. The method comprises receiving the analog signal by the receiver. The analog signal is converted into a digital signal, based on a clock of the receiver. The digital signal is interpolated at the desired interpolation point, if digital signal in time has been shifted by an amount equal to or larger than a predetermined time length. The interpolated digital signal is recovered back to the digital data content, with the assist of an estimated channel impulse response. The channel impulse response is retrained every time when the interpolation point is significantly changed.

Abstract: Method and apparatus for reducing intra-channel distortions of received data resulting from non-linear signal propagation includes parallel detection subcircuits for determining output values of sequentially provided optical data bits such that each of the sequentially provided optical data bits is processed by only one of the parallel detection subcircuits. Two parallel detection subcircuits process the input optical data bits according to even valued and odd valued clock signals. Each subcircuit has first and second signal analyzers to detect the value of the input optical data bit and a first memory unit and a second memory unit connected to the first and second signal analyzers. The input value of clock information provided to the first and second memory units of the first parallel subcircuit are 180° out of phase with input values indicative of a clock speed provided to the second parallel subcircuit.

Abstract: The invention relates to the distribution of a synchronization signal in an optical communication system which is inherently asynchronous. In order to accomplish a cost-efficient mechanism for transmitting a synchronization signal in such a system, the amplitude of a payload signal is modulated with the synchronization signal, whereby an amplitude-modulated payload signal is obtained. This amplitude-modulated payload signal is transmitted as an optical signal to the opposite end of an optical link, where the synchronization signal is separated from the payload signal.

Abstract: Processing a received optical signal in an optical communication network includes equalizing a received optical signal to provide an equalized signal, demodulating the equalized signal according to an m-ary modulation format to provide a demodulated signal, decoding the demodulated signal according to an inner code to provide an inner-decoded signal, and decoding the inner-decoded signal according to an outer code. Other aspects include other features such as equalizing an optical channel including storing channel characteristics for the optical channel associated with a client, loading the stored channel characteristics during a waiting period between bursts on the channel, and equalizing a received burst from the client using the loaded channel characteristics.

Abstract: A method for remodulation of a modulated optical signal is disclosed which uses a disturbed line signal and an optical clock signal derived from the undisturbed original line signal modulated with the bitrate frequency feeding both signals in a Raman amplifying fiber connected to at least one Raman pump running the clock signal as Raman pump wavelength for the line signal.

Abstract: An apparatus for adaptively receiving, compensating, and transmitting data in optical fiber communication networks are provided. A receiver according to this invention includes at least one optical device for compensating distortion in a channel of an optical signal, at least one photodetector circuit for converting the optical signal into an electrical signal, at least one electronic device for further compensating the distortion in the electronic signal, a clock and data recovery circuit for generating a recovered data signal and a clock signal from the electronic signal, and a post-processing circuit.

Abstract: Disclosed herein is a method of optically producing a clock, in which clock components are extracted from the spectrum of a transmitted optical signal and the influence of noise and jitter on the extracted clock components is reduced, thus improving the stability of an optical communication system. In the clock producing method of the present invention, a plurality of clock components are extracted by filtering out frequency bands between neighboring intensity peaks, and then a clock having reduced noise and jitter is produced by logically ANDing two or more of the plurality of clock components extracted at the step of extracting the plurality of clock components.

Abstract: The invention provides a transceiver circuit for communication that enhances the operation frequency of a synchronous digital circuit up to the maximum frequency of a flip-flop and inhibits the occurrence of jitters. A clock signal synchronized with data at f1/n Hz is converted by a multiplier so that the signal has a frequency of “n” times so as to use the clock signal for triggering a flip-flop the operation frequency of which is f1 b/s in the synchronous digital circuit. The multiplier is arranged in the vicinity of the flip-flop triggered by the clock signal of f1 Hz so as to avoid the effect of the deterioration of the operation frequency by interconnect capacitance. The maximum operation frequency of the transceiver circuit determined based upon the operating frequency of the synchronous digital circuit can be enhanced up to the maximum operation frequency of the flip-flop.

Abstract: An all-optical clock recovery system for recovering the clock from a received optical signal with a short response time and without patterning effects includes a first optical clock recovery device adapted to supply a first optical clock signal in response to the received optical signal and a second optical clock recovery device adapted to supply a second optical clock signal in response to the first optical clock signal. Applications include regenerating optical packets in asynchronous optical packet-switched telecommunication networks.

Abstract: A clock recovery circuit has a phase comparator circuit, a phase adjusting circuit, and a duty cycle correction circuit. The phase comparator circuit carries out phase comparison between an input signal and an output signal, and outputs a phase control signal proportional to a phase difference between the input signal and the output signal. The phase adjusting circuit receives the phase control signal from the phase comparator circuit, adjusts the phase of the input signal, and produces the output signal, and the duty cycle correction circuit receives the output signal from the phase adjusting circuit, and corrects the duty cycle of the output signal.

Abstract: An optical communication system configured to operate with optical signals at lower signal to noise ratios than previously contemplated. The communication system includes a receiver having an optical pre-processor coupled between a demultiplexer and a detector. The optical pre-processor includes either an optical polarization section having a polarization rotator and an optical polarizer, a phase modulation section that includes a phase modulator and a dispersion element and a clock recovery circuit, or an amplitude modulation section that includes an amplitude modulator clock recovery circuit and a spectral shaping filter.

Abstract: A method and system for communicating a clock signal over an optical link includes receiving a multimodulated optical information signal including non-intensity modulation for a data signal and intensity modulation for a clock signal. The clock signal is recovered based on the intensity modulation of the multimodulated optical information signal. The non-intensity modulation for the data signal is converted to intensity modulation for the data signal. The data signal is recovered from the intensity modulation for the data signal using the clock signal.

Abstract: An apparatus for simultaneous OTDM demultiplexing, electrical clock recovery and optical clock generation, and optical clock recovery using a traveling-wave electroabsorption modulator. The apparatus includes a TW-EAM and a PLL coupled thereto. The TW-EAM includes a first, a second, a third, and a fourth. The first port is used for an optical input and the third port is used for optical output. The second port is coupled to an input, and the fourth port is coupled to an output, of the PLL. When the first port receives optical input, the second port produces a photocurrent to be applied to the PLL, and the fourth port receives a recovered clock produced by the PLL, and the third port produces demultiplexed data and an optical clock. Using the same configuration, the apparatus produces a recovered optical clock signal.

Abstract: Method and apparatus for recovering a clock and data from a data signal. One method of the invention includes receiving the data signal having a first data rate and receiving a clock signal having a first clock frequency, and alternating between a first level and a second level. The data signal is stored when the clock signal alternates from the first level to the second level, and the stored data signal is provided as a first signal a first amount of time later. The first signal is stored when the clock signal alternates from the first level to the second level, and the stored first signal is provided as a second signal a second amount of time later. A third signal is provided by delaying the first signal for a third amount of time. The third signal is stored when the clock signal alternates from the second level to the first level, and the stored third signal is provided as a fourth signal a fourth amount of time later. A fifth signal is provided by delaying the data signal a fifth amount of time.

Abstract: An apparatus for simultaneous OTDM demultiplexing, electrical clock recovery and optical clock generation, and optical clock recovery using a traveling-wave electroabsorption modulator. The apparatus includes a TW-EAM and a PLL coupled thereto. The TW-EAM includes a first, a second, a third, and a fourth. The first port is used for an optical input and the third port is used for optical output. The second port is coupled to an input, and the fourth port is coupled to an output, of the PLL. When the first port receives optical input, the second port produces a photocurrent to be applied to the PLL, and the fourth port receives a recovered clock produced by the PLL, and the third port produces demultiplexed data and an optical clock. Using the same configuration, the apparatus produces a recovered optical clock signal.

Abstract: This invention provides a technique for realizing low-cost optical signal waveform monitoring with improved realtimeness to be applied to signal quality monitoring in an actual optical transmission system, and a technique for stably controlling an optical transmitter/receiver and various compensators by means of this waveform monitoring. Opening/closing of a optical gate is controlled by means of a clock signal synchronized with an optical signal input from a photocoupler and having a period equal to the bit interval of data or N (N: a positive integer) times longer than the bit interval to allow each pulse of the optical signal for one bit of data to pass through the optical gate for only part of the time width of the gate. A photoelectric conversion element to which the optical signal transmitted through the optical gate for only part of the time width obtains an average light intensity of the input optical signal. Information on this average light intensity is output to a monitoring output section.

Abstract: A method and apparatus for optical clock recovery using optical filters with fixed reflective wavelengths is provided. In the apparatus, a first filter receives an optical signal and extracts one out of two spectral lines corresponding to two side peaks that are second in magnitude to a peak and exist at both sides of a peak on the spectrum of the received optical signal. A second filter extracts a spectral line corresponding the largest peak on the spectrum of the received optical signal. An attenuator equalizes the power levels of the two spectral lines extracted by the first and second filters. A beating generation unit receives signals corresponding to the two equalized spectral lines and generates beating between the equalized spectral lines, thereby recovering a clock component.

Abstract: The present invention provides a robust solution to the task of re-aligning data at the transmit end of a fiber optic or other high performance serial link, and also offers flexibility in the circuit board design approach. A high performance analog phase locked-loop circuit is used to simultaneously provide clock recovery for multiple bit streams. The power dissipation required to perform clock recovery is thereby reduced to a fraction of that required in conventional transmit systems. This analog phase locked loop produces plural phase output signals. An output multiplexer selects one phase for use in electrical to optical conversion.

Abstract: This optical clock phase-locked loop circuit includes an oscillator 12, an optical coupler 2, an optical cross-correlation detection device 3 that outputs light containing the correlating components of two lights from the coupled light of optical coupler 2, an optical band pass filter 4 that extracts light of a wavelength that contains cross-correlating components, an optical receiver 5, a phase comparator 6 that compares the phases of the output signal of the oscillator 12 and the cross-correlation signal from the optical receiver 5, a voltage controlled oscillator 7 that changes the oscillation frequency and phase corresponding to the output of the phase comparator 6, an optical pulse generator 8 that generates a optical clock pulse containing an nth harmonic component, an optical coupler 10 that divides optical clock, and an optical modulator 11 driven by low-frequency oscillator 12 that modulates optical clock from optical coupler 10.

Abstract: A system and method for improving the transmission quality of a WDM optical communications system begins by determining the bit-error rate for an optical channel before forward error correction is performed at a receiver. The pre-corrective bit-error rate is fed back through a feed back circuit that includes a parameter adjustment module which adjusts an optical signal parameter based on the bit-error rate. As examples, the signal parameter may be a channel power, dispersion, signal wavelength, the chirp or eye shape of an optical signal. The feedback circuit may also adjust various parameters within the WDM system, including amplifier gain, attenuation, and power for one or more channels in the system. By adjusting these parameters based on a pre-corrective bit-error rate, transmission quality is improved and costs are lowered through a reduction in hardware.

Abstract: An optical transmission system for compensating for transmission loss includes a transmitting apparatus for serializing a plurality of n (n is a natural number)-bit channel data received from the outside in response to a predetermined clock signal, converting the serialized channel data and the predetermined clock signal into a current signal whose magnitude changes corresponding to an error detection signal, and outputting optical signals having optical output power corresponding to the magnitude of the current signal, a first optical fiber for transmitting the optical signals, a receiving apparatus for recovering the n-bit channel data and the predetermined clock signal from the optical signals received through the first optical fiber, detecting transmission loss generated when the optical signals are transmitted and received, optically converting the transmission loss, and outputting the optically converted transmission loss as the error detection signal, and a second optical fiber for transmitting the opt

Abstract: This invention discloses a method to easily extract a header from an optical packet. An optical data transmission method to transmit an optical packet composed of a header and data containing steps of generating a second clock which has a frequency equal to one integer of that of a first clock carrying the data and synchronizes with the first clock, and carrying the header information on the second clock.

Abstract: An optical receiver is provided with inputs for receiving two fiber optic lines. The two fiber optic lines carry redundant data. An optical interface passes the respective signals therein to a comparison unit. The comparison unit compares the signals and determines which has the better quality and passes the better quality signal on to a processing interface. Clock data for operating the comparison unit is extracted from the optical signals.

Abstract: A bit rate-independent optical receiver ensures transparency with respect to changes of the transmission format and its associated bit rate in relation to an optical communications system. The optical receiver includes an opto-electric converter for converting an input optical signal into an electric signal, an amplifier circuit for amplifying the electrical signal, a bit rate-sensing circuit for generating a sensing signal with a voltage level determined on the basis of the bit rate of the electrical signal, a bit rate-recognition circuit for generating a recognition signal further amplified from the sensing signal, and a clock/data recovery circuit for reproducing a clock signal and data from the amplified electrical signal in accordance with a control signal.

Abstract: An electro-optic clock recovery circuit comprising an electro-optic modulator circuit operatively coupled to; a photodetector circuit; a buffer circuit operatively coupled to said photodetector circuit; a voltage controlled oscillator operatively coupled to said buffer circuit and to said electro-optic modulator circuit; wherein the electro-optic modulator circuit receives and modulates a multi-channel input data stream, the photodetector circuit responsive to the modulated input data stream drives the voltage controlled oscillator in a phase locked loop through the buffer circuit, and the modulator circuit is responsive to the voltage controlled oscillator and is driven at a harmonic of the voltage controlled oscillator.

Abstract: The present invention regenerates optical signals in an intact optical state according to a low-speed optical clock. In one embodiment, an optical splitter applies a portion of optical pulse signals to a clock recovery circuit and the rest to an optical splitter. The optical splitter applies half of the input signal to an optical regenerator and the rest to another optical regenerator. The clock recovery circuit regenerates an electric clock signal with half the frequency of the input optical pulse and applies the clock to an optical clock pulse generator. The optical clock pulse generator generates an optical clock signal with half the frequency of the input optical pulse whose phase matches with that of the optical signal at the optical regenerator. An optical regenerator regenerates the optical signal from the optical splitter in an intact optical state. A polarization combiner combine the optical signals from the optical regenerators in mutually orthogonal polarization.

Abstract: A scanning optical monitoring system and method are appropriate for high speed scanning of a WDM signal band. The system and method are able to identify dropped channels or, more generally, discrepancies between the determined or detected channel inventory and a perpetual inventory for the WDM signal, which perpetual inventory specifies the channels that should be present in the WDM signal assuming proper operation of the network. The system includes a tunable optical filter that scans a pass band across a signal band of a WDM signal to generate a filtered signal. A photodetector then generates an electrical signal in response to this filtered signal. A decision circuit compares the electrical signal to a threshold and a controller, which is responsive to the decision circuit, inventories the channels in the WDM signal.

Abstract: All optical clock recovery includes a transmitter for generating an optical timing signal. The transmitter includes a semiconductor laser for the production of a dynamically synchronizable timing signal, the laser having an external resonator for feedback of the timing signal to the laser, the feedback having a delay time greater than a relaxation oscillation time for the laser, and the laser outputting an optical timing signal having a characteristic dynamic. The transmitter supplies the optical timing signal to a receiver configured to receive the timing signal and to synchronize to the laser on receipt of the timing signal, such that the receiver outputs a recovered timing signal having the characteristic dynamic.

Abstract: A method and system for self-synchronizing an optical packet network by selecting a seed pulse from among the data pulses within a packet having no synchronization marker, transforming the seed pulse to be optically distinguishable from the remaining data pulses after the packet has been transmitted, and extracting that seed pulse for use in synchronizing the operation of the network. In one embodiment, the process is practiced using a intensity modifier (such as a fast-saturation, slow-recovery amplifier) to modify the seed pulse intensity, and an intensity discriminator (such as an unbalanced NOLM, or a dispersion-shifted fiber and bandpass filter) to extract the differentiated seed pulse.

Abstract: A method and apparatus for optical performance monitoring that provides for multi-rate and multi-protocol monitoring includes, identifying a protocol associated with each of a plurality of communication signals using respective data rates extracted therefrom, determining, for each of the plurality of communication signals, a respective bit-error rate (BER), and generating an alarm indicative of BER excursions beyond a protocol appropriate BER threshold level.

Abstract: An optical transmitter transmits a data signal developing a specific spectrum at a predetermined frequency, while an optical receiver detects amplitude information on the spectrum included in the data signal to control the received data signal to a constant amplitude on the basis of the detected amplitude information. This can provide an optical communication system and an optical receiver capable of functioning well even if the optical S/N is in a poor condition.

Abstract: A system that transmits amplitude modulated data in a wavelength-encoded format and then uses a wavelength-sensitive receiver to convert the received optical signal back to the original amplitude modulated data. This system enables transmission of optical signals that are less sensitive to attenuation and attenuation changes. This system is applicable to data in digital, multilevel, or analog formats.

Abstract: A transmitter comprises an oscillator, a phase lock loop, a serializer, and an electrical-to-optical converter. The oscillator is enclosed in a metal shield. The metal shield is soldered to a ground ring of the printed circuit board. In one embodiment, the oscillator is voltage-controlled oscillator.

Abstract: Disclosed are an apparatus for regenerating a distorted optical signal through reamplification, regenerating, and retiming processes in an optical communication network and a method thereof. The apparatus includes an optical clock generation section for generating an optical clock signal of a specified frequency, an optical signal sampling section for sampling a distorted non-return-to-zero (NRZ) optical signal in synchronization with the clock signal from the optical clock generation section, an optical signal regenerating section for regenerating an output signal of the sampling section, and a return-to-zero (RZ)/NRZ conversion section for converting the optical signal reshaped by the optical signal regenerating section into an NRZ optical signal. The apparatus overcomes the limitations of the signal processing speed produced in the electric 3R type regenerator, and enable the signal regenerating to be performed without the necessity of an optical clock extraction apparatus.

Abstract: A bit-rate independent optical receiver and a method thereof. In the bit-rate independent optical receiver, an optoelectric converter converts an input optical signal to an original electrical signal, a bit rate identifying unit forms a resultant signal by performing an exclusive-OR (XOR) logic operation on the original electrical signal received from the optoelectric converter and a second signal corresponding to the original electrical signal delayed by a predetermined quantity of time, and detects a bit rate from the resultant signal, a reference clock generator generates a reference clock signal according to the detected bit rate, and a clock and data recovery circuit recovers a clock signal and data from the input signal according to the reference clock signal.

Abstract: A clock recovery circuit for synchronizing a clock signal having frequency of approximately f0 with an optical data signal having a frequency of N×f0, where N is an arbitrary rational number, includes a local oscillator for generating the clock signal, a sampler for producing an output signal indicative of a phase difference between the clock signal and the optical data signal, an optical detector coupled to detect the output signal as an electrical signal, and a mixer for isolating at least one harmonic of the electrical signal and for downconverting the at least one harmonic to a baseband error signal. The local oscillator is tuned in response to the baseband error signal to synchronize the clock signal with the optical data signal.

Abstract: Disclosed is a self-healing apparatus and method of an optical receiver that is capable of avoiding error due to temperature variation in the optical receiver when determining the bit rate.

Abstract: An optical transmitter for transmitting a first output data signal and a second output data signal, the optical transmitter comprising: a phase-locked-loop, the phase-locked-loop operable to receive a reference clock signal; a clock-recovery circuit, the clock-recovery circuit coupled to the phase-locked-loop, the clock-recovery circuit operable to receive a first input data signal; a latch-decision circuit, the latch-decision circuit coupled to the clock-recovery circuit; a latch, the latch coupled to the latch-decision circuit, the latch operable to receive the first input data signal and the second input data signal; a first electro-optical converter, the first electro-optical converter coupled to the latch, the first electro-optical converter operable to transmit the first output data signal; and a second electro-optical converter, the second electro-optical converter coupled to the latch, the second electro-optical converter operable to transmit the second output data signal.

Abstract: A system and method of selecting and viewing communication traffic transmitted over an optical channel selected from among a plurality of possible channels without disrupting the communication traffic occurring over the selected channel or other channels is presented. The system and method includes an optical channel analyzing switch which taps each of the possible plurality of channels and selects a specific channel for routing to a network analyzer. The signal on the selected channel, prior to being analyzed by the network analyzer, undergoes clock and data recovery and retiming/recombination to mitigate contamination from routing and switching the selected original signal between the signal source and the network analyzer. The retimed and recombined channel signal results in a signal, as presented to the network analyzer, which is representative of the original channel signal.

Abstract: The invention has for its object to provide a monitoring unit, particularly for optical burst mode signals in asynchronous optical networks based on, e.g., Internet or ATM (ATM=asynchronous transfer mode). The monitoring unit for burst mode signals comprises a detector for performing a Q-factor measurement in dependence on at least one variable and at least one fixed amplitude threshold value and on at lest one variable and at least one fixed phase value. This detector contains a burst mode amplitude detector for generating a fixed amplitude threshold value for each burst, a burst mode clock recovery circuit for generating a fixed phase value for each burst, a burst detector for detecting the beginning of each burst, and a control logic for generating a variable amplitude threshold value and a variable phase value for each burst.

Abstract: According to a PLL circuit of the present invention, an output of a phase comparator is adjusted according to a space-to-mark transition-probability of an input signal so that an output of a voltage controlled oscillator has a predetermined frequency and phase. Therefore, even when a phase of a timing clock is set other than at 0, an output of the PLL circuit can be kept at the set phase, irrespective of the space-to-mark transition-probability. By using the PLL circuit as such in an optical communication apparatus and an optical communication system, a discrimination point can be kept almost fixed, and therefore, it is possible to lower an error rate.

Abstract: In an ultrahigh-speed clock extraction circuit wherein a local pulse generating light source 22 for generating a local optical pulse stream synchronized in bit phase with an input optical signal pulse stream is placed in a phase-locked loop, when repetition frequencies of the input optical signal pulse stream and the local optical pulse stream bear a particular relationship, a frequency demultiplier 32 and multipliers 43 and 52 are set so that the frequency of a modulation signal for an optical modulator 41 and a frequency which is a natural-number multiple of the modulation signal frequency, and the frequency of a down-converted version of an optical pulse stream output from a photodetector 42 differ from each other.

Abstract: A wavelength converter and a wavelength division-multiplexing optical communication apparatus automatically generate clock signals of a specified frequency that match send/receive digital signals and regenerates timing for the send/receive digital signals. The wavelength converter includes: optical/electrical signal converters (2A, 2B) that convert input optical signals into electric digital signals; clock generator circuits (4A, 4B) that automatically identify the transmission mode for digital signals and generate phase-synchronized clocks of a specified frequency that match the signal transmission mode; timing regeneration circuits (6A, 6B) that regenerate clock timing for the digital signals based upon the phase-synchronized clocks from the clock generator circuits; and electric/optical converters (3A, 3B) that convert the digital signals from the timing regeneration circuits into optical signals of a specified wavelength.