Abstract: A signal receiver, such as an RF-matched filter receiver, includes an optical source (e.g. a mode-locked laser) providing an optical signal, and a first optical modulator to modulate the optical signal with a received RF signal and provide a modulated optical signal. A second optical modulator modulates the modulated optical signal with a reference signal and provides a twice modulated optical signal. The modulators may be Mach-Zehnder Modulators (MZM) and/or Indium Phosphide (InP) modulators. An optical detector receives the twice modulated optical signal and provides a detected signal, and a processing unit receives the detected signal and extracts or measures cross-correlation between the received RF signal and the reference signal.

Abstract: An optical communication apparatus for controlling the operating point of an optical modulation unit appropriately. An adding unit adds a bias voltage and a pilot signal. An optical modulation unit modulates light by an electrical signal, performs further modulation by a signal from the adding unit, and outputs an optical signal. A monitor unit monitors the level of the optical signal and outputs a monitor signal. A reference signal output unit outputs a reference in phase with the pilot signal contained in the monitor signal. A synchronous pilot-signal detection unit performs synchronous detection of the monitor signal by using the reference. A synchronous noise detection unit extracts noise from the monitor signal and performs synchronous detection of the noise by using the reference. A control signal output unit subtracts the synchronous detection value of the noise from that of the monitor signal, and outputs a control signal.

Abstract: A method for transmitting digital data includes splitting a coherent optical carrier having a subcarrier into mutually coherent optical carriers, producing corresponding sequences of phase shifts in each of the mutually coherent optical carriers, and then, interfering the mutually coherent optical carriers. The interfering produces an output optical carrier whose subcarrier has modulated inphase and quadrature components with a corresponding sequence of pairs of values. The pairs of values of the modulated inphase and quadrature phase components produced by the interfering correspond to a sequence of coordinate pairs for the signal points the 4-PSK 2D, 16-QAM 2D, or 16-PSK 2D constellation.

Abstract: The present invention relates to an optical communication network and to a network element for use in such a network. The network element comprises a plurality of receivers (70-76) for receiving optical communication signals, a plurality of transmitters (54-62, 82-86) for transmitting optical communication signals, and a plurality of network connections, each network connection having an individual signal impairment characteristic. The pluralities of receivers (70-76) and transmitters (54-62, 82-86) are adapted to employ a plurality of different modulation schemes (64, 66). Furthermore, the pluralities of receivers (70-76) and transmitters (54-62, 82-86) are assigned to the network connections as a function of the individual signal impairment characteristics.

Abstract: As a capacity of an output interface of an electronic circuit is increased, there results a significant increase in the number of channels of optical signals on a semiconductor LSI, converted from electrical signals. Since a semiconductor laser used as a means for generating the optical signal is inferior in reliability to the semiconductor LSI, a problem arises as the number of optical signals is increased in that the number of the semiconductor lasers needs be reduced. To solve the problem, for an optical transmitter, use is made of a structure wherein a continuous laser beam from a semiconductor laser is subjected to modulation in external optical modulators to thereby generate optical signals instead of a structure wherein electric current of a semiconductor laser as an optical signal source is directly modulated.

Abstract: The present invention relates to an apparatus and method for stabilizing a bias voltage for a pulse generating modulator, which can automatically detect an optimal bias voltage for an external modulator, which is used in RZ or CSRZ modulation of an optical transmission signal using NRZ data to generate a reference pulse optical signal, and maintain the optimal bias voltage. In the bias voltage stabilizing method of the present invention, an output signal of the external modulator, to which the bias voltage to be stabilized is applied, is detected, a drive clock signal applied to the external modulator is detected, a mean output value of the products obtained by the multiplication of the output signal and the clock signal is periodically examined, and the bias voltage is adjusted so that the mean output value becomes “0”.

Abstract: An apparatus for accelerating assessment of an optical transmission system using Bit Error Rate (BER) tests calculates Q-factors for at least two different extinction ratios from measured test BER values, and extrapolates to determine a Q-factor for an operational extinction ratio, whereby the operational BER value for the operational extinction ratio can be calculated.

Abstract: Disclosed is a duobinary optical transmission apparatus using a semiconductor optical amplifier (SOA). The duobinary optical transmission apparatus includes a light source for generating a carrier wave; a duobinary precoder for encoding an input (non return to zero) electric signal; a semiconductor optical amplification unit to amplify the encoded signal from the duobinary precoder, wherein the amplification unit receives an optical amplification gain difference that varies with a bias current combined with the encoded signal; and an optical band pass filter for receiving a phase-modulated optical signal from the semiconductor optical amplification unit, filtering the received optical signal to a prescribed bandwidth, and thereby generates a duobinary optical signal.

Abstract: An optical transmitter comprises an amplitude modulation unit performing amplitude modulation of only a one-side amplitude of a main signal with a low-frequency signal having a predetermined frequency. An optical modulator receives an input signal generated after the one-side amplitude modulation, and modulates an incoming light in response to the received signal in accordance with a predetermined modulation-characteristic curve to output an optical output signal. An operating point control unit applies a predetermined bias voltage to the optical modulator to control a level of the input signal substantially applied to the modulation-characteristic curve so that the one-side amplitude of the main signal is applied to a minimum portion of the modulation-characteristic curve of the optical modulator.

Abstract: A method and apparatus for transmitting optical pulses in a dispersion-managed optical transmission system comprises determining the pulse width of optical pulses in a optical transmission channel in accordance with the path-average dispersion (PAD) experienced by that channel.

Abstract: A duobinary optical transmission apparatus is provided to generate duobinary optical signals, without using an electric low-pass-filter (LPF), by restricting a bandwidth of a Mach-Zehnder-interferometer-type optical intensity modulator (MZ MOD). Alternatively, a LPF having a single pole with a single capacitor is implemented instead of an expensive LPF, thereby providing a simple transmitter with a lower production cost in comparison with a transmitter using a conventional electric LPF.

Abstract: The invention provides a method and system for selectively canceling a specified frequency band in the payload of an optical communication system. This selected frequency band is to be used for launching pilot tones for power measurement and identification. The method controls the intensity of the optical payload signal through an intensity modulator that is included in the optical communication system. The frequency band in payload to be cancelled is removed where as the other frequency components of the payload are passed without attenuation. An embodiment that uses a feed-forward unit, a feedback unit and a control unit for generating a cancellation signal for the intensity modulator is described. An advantage of the method is that it removes a continuum of payload tones in the specified frequency band to be cancelled.

Abstract: Method and apparatus for synchronizing two different types of modulators in an optical transmission system includes a first modulator generating an optical pulse train, a second modulator encoding data onto the optical pulse train, an optical filter resolving upper and lower modulation sidebands of the optical data and an analyzer measuring the optical power of modulation sidebands and converting the received optical power of the sidebands into a control signal for synchronizing the two modulators. A wedged etalon is the filter element selecting the USB and LSB from the optical data spectrum. The analyzer contains photo-detectors measuring the optical power of the filtered USB and LSB and an electronic differential amplifier producing a control signal based upon photo-detector output. The phase shifter, in response to the control signal, adapts the temporal delay of the first modulator to reduce differences between the power levels of the upper and lower sidebands.

Abstract: Laser line-width compensation is performed by detecting noise in an optical signal output by a laser of an optical communications system and controlling a complex modulator to cancel the detected noise. Line-width compensation can be performed for both amplitude and phase noise in the optical signal. Noise measurements are used to compute a complex scalar. The complex scalar is used by a compensation processor to modify an input signal that is to be transmitted by the optical communications system. The modified input signal controls the complex modulator, which modulates the output signal to cancel the noise.

Abstract: The present invention aims at providing a drive control apparatus and a drive control method with a simple configuration, which can also reliably compensate for operating point fluctuations of an optical modulator driven in accordance with a drive signal corresponding to a clock signal. To this end, the drive control apparatus of the present invention is configured to supply, to an optical modulator of which the optical output characteristics with respect to drive voltage are changed periodically, a drive signal having a waveform corresponding to a clock signal, and the amplitude of which is set to around 2V?, detect the power of an optical signal output from the optical modulator, and control an operating point of the optical modulator so that an average value of the optical signal power becomes a maximum.

Abstract: The optical transmitter has a feature that the transmitter comprises a light-emitting device, an optical splitter, a dispersion controller including a first dispersion generator and a waveform monitor, and a processing unit. The optical splitter splits light emitted from the light-emitting device, and guides one of split light into the first dispersion generator. The first dispersion generator adds dispersion, an amount of which is predefined by the processing unit so as to reflect dispersion of the optical path to the receiving station, to the split light and outputs dispersed light to the waveform monitor. The processing unit maintains the dispersed light output from the dispersion generator to have a predetermined quality.

Abstract: A method and system for modulating an optical signal to encode data therein. The method comprises the steps of directing the optical signal through a filter mechanism having a passband function including a center wavelength, and modulating the center wavelength of the optical signal to establish a difference between the center wavelengths of the filter mechanism and the optical signal to represent a data value. With the preferred implementation of the invention, a transmit device is used to encode the data in the optical signal, and a receive device is provided to decode the signal. The transmit device modulates the center wavelength of the optical signal to establish a difference between the center wavelength and a predefined wavelength to encode data in the optical signal, and the transmit device then transmits the optical signal. The receive device receives the optical signal from the transmit device and processes that signal to identify the encoded data.

Abstract: The invention describes methods and systems for monitoring the performance of an optical network by marking a group of optical signals with a set of identification tags which are unique to network characteristics. In the preferred embodiments, fiber identification (FID) and bundle identification (BID) tags are encoded into optical signals by marking an optical signal with low frequency dither tones whose frequencies are unique to the fiber section and to a bundle of fibers respectively. Detecting of the FID and BID tones provides more effective and accurate monitoring of performance of the optical network and allows determining of the network topology, e.g. paths of optical channels and traffic load through different fiber sections in the network. Other sets of hierarchically arranged identifiers encoded into optical signals have also been proposed, including band, conduit, city, region, country, etc. identifiers, as well as identifiers related to network security and service characteristics.

Abstract: An optical transmission controller for controlling an optical transmission system is presented. The optical transmission controller according to the present invention includes a laser controller, a modulator block controller, and a modulation driver controller. In some embodiments, the optical transmission controller is formed on a single integrated circuit chip. In some embodiments, the optical transmission controller includes a digital signal processor or microprocessor executing instructions for providing signals to a laser module, a modulator block and modulation drivers.

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: An optical transmission controller includes a semiconductor-laser, a semiconductor laser driver, a monitor photoreceptor, a waveform detector and a phase relation adjuster. The semiconductor laser driver allows the semiconductor laser to output an optical signal. The monitor photoreceptor monitors the optical signal output. The waveform detector detects a fall state of the optical signal output. The phase relation adjuster adjusts a phase relation between a fall timing of an input current and a variation timing of a relaxation oscillation of the optical signal output in accordance with a detection result of the waveform detector.

Abstract: An electro-optical device includes an amplifier having an input receiving a data signal, a crossing adjustment input, and an output. The device includes a light source and a Mach-Zehnder modulator having a first input coupled to the output of the amplifier, an optical input coupled to the laser, a bias input, and an optical output. A bias circuit is coupled to the bias input of the Mach-Zehnder modulator and preferably includes a pilot signal generator connected to the crossing adjustment input of the amplifier, and a feedback circuit coupled between the optical output and bias input of the Mach-Zehnder modulator and coupled to the pilot signal generator. The bias circuit applies the pilot signal to the crossing adjust input of the amplifier which forces the modulator to the correct bias point for duobinary transmission. Modulation on +1 and ?1 levels is eliminated, and only the 0 level is modulated. The feedback loop forces modulated portion of the waveform to null bias.

Abstract: First and second carrier modulators each modulate a carrier having a different frequency from each other with a baseband input signal. First and second variable wavelength optical modulators each convert the modulated signal into an optical signal having a first or second wavelength. An optical multiplexer multiplexes the optical signals, and sends a multiplexed signal to an optical transmission line. A wavelength separator individually outputs wavelength components of the multiplexed signal. First and second optical receivers each convert these wavelength components into an electrical signal. First and second filters each pass only the signal components of each different frequency. First and second burst demodulators each demodulate the modulated signal. With such a structure, a large-capacity optical communication apparatus which is capable of simultaneously using the same wavelength without requiring wavelength management in optical transmitting circuits can be achieved at a low cost.

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 transmitter and methods of generating an optical signal having SBS suppression are described. An optical transmitter having SBS suppression according to the present invention includes a signal generator that generates a SBS suppression signal. A laser generates a line width broadened optical signal having AM noise. A signal processor generates a modified SBS suppression signal from the SBS suppression signal. A modulator modulates the line width broadened optical signal having the AM noise with a payload modulation signal and with the modified SBS suppression signal to generate a payload modulated optical signal having SBS suppression and reduced AM noise.

Abstract: A system for matching a optical filter characteristic of a first filter tunable in wavelength with an optical first signal comprises a modulator for modulating at least a part of the first signal with a modulation signal before being applied to the first filter. An analyzing unit derives a control signal for tuning the first filter by analyzing the modulated first signal after passing the first filter in conjunction with the modulation signal.

Abstract: The present invention relates to an optical sender applicable to WDM (wavelength division multiplexing), and a primary object of the present invention is to prevent interchannel crosstalk in WDM. The optical sender includes a light source for outputting a light beam, an optical modulator for modulating the light beam in accordance with a main signal to output an optical signal, and a unit for shutting down the optical signal when receiving at least one of a power alarm relating to on/off of power supply and a wavelength alarm relating to the wavelength of the light beam.

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: Adjusting power for a WDM optical transmission system comprising emitter means, an optical transmission line, and receiver means, the method being characterized in that for each wavelength channel, the power emitted for said channel by the emitter means is adjusted as a function of the optical powers received for said channel at a plurality of points distributed along the transmission line.

Abstract: Methods and apparatus for maintaining a pre-determined ratio of a pilot tone power and a total mean optical power of an optical signal are provided. One apparatus includes an optical source coupled to provide an optical signal in response to a DC bias and an AC bias corresponding to a pilot tone signal. A processor is coupled to compute each of an AC component and a DC component of the optical signal from the same digitally sampled signal. A feedback control is coupled to vary at least one of the DC bias and the AC bias in accordance with a difference between a pre-determined value and a ratio of the AC and DC components.

Abstract: The influences on transmission quality caused by chirp and self-phase modulation are at least largely corrected by way of an optimally set operating point of the modulator (2). Suitable criteria are obtained in control loops in order to maintain the optimal setting.

Abstract: A high-speed optical transmitter having an optical modulator and a driver circuit while maintaining optical modulator performance. The high-speed optical transmitter comprises the optical modulator which modulates and outputs an input light in accordance with an applied voltage, and a driver circuit which outputs the applied voltage into the optical modulator and has an emitter follower circuit at an output stage thereof.

Abstract: The optical transmission apparatus has a Mach-Zehnder optical modulator, a light source for applying an optical signal of a continuous light to the optical modulator, a driving circuit for inputting a driving signal into the optical modulator, a branching filter for taking out a part of an output optical signal, a photodiode for converting the taken-out output optical signal into an electric signal, a band pass filter for extracting a frequency component of a driving signal included in this electric signal, an error signal generator circuit for generating an error signal of a bias voltage that minimizes a value of the extracted frequency component, and a bias voltage control circuit for applying a bias voltage added with this error signal to the optical modulator.

Abstract: An improved analog optical system which provides improved dynamic range as well as sensitivity relative to known analog optical systems. The analog optical system includes a Mach-Zehnder modulator (MZM) operated with a low bias to improve sensitivity. In accordance with an important aspect of the invention, the optical system utilizes two optical wavelengths with two effective bias points to cancel even ordered distortion associated with low biasing. Two lasers having different wavelengths are applied to the Mach-Zehnder modulator by way of a wavelength division multiplexer (WDM). Alternately, a single laser producing two optical carriers having different wavelengths could be used in place of the two single wavelength lasers and the WDM. The modulator bias control circuit forces two optical carriers to two bias points on opposite sides of the minimum bias point thus, providing equal modulation depth with opposite sign on each of the two optical carriers.

Abstract: A device and technique for aligning an optical carrier signal (e.g., a soliton pulse train) with a data signal in a transmitter. According to the invention, the device is configured to analyze the radio frequency (RF) spectrum of the transmitter's output. In one implementation, the device evaluates the amount of energy in a certain frequency band located near a selected null of the RF spectrum. In another implementation, the device examines the shape of the RF spectrum within that frequency band. In either case, based on the analysis, the device adjusts the phase of the clock signal driving an electro-optic (E/O) modulator in the transmitter. Such adjustment reduces misalignment between the optical carrier signal and data resulting, e.g., from thermal effects in the E/O modulator. The device may be used, e.g., in long-haul optical transmission systems operating at 10 GBit/s.

Abstract: A high efficiency optical feedback modulator and method of operation are provided. The high efficiency optical feedback modulator (30) includes an optical modulator (52) and an optical feedback system (54). The optical modulator (52) includes first and second optical inputs (66a, 68a) and first and second optical outputs (66b, 68b). The optical feedback system (54) is coupled between the second optical output (68b) and the second optical input (68a). The second optical input (68a) receives an optical feedback signal (70) from the optical feedback system (54). The first optical input (66a) receives an input light beam (56). The input light beam (56) and the optical feedback signal (70) are modulated with an electronic input signal (26a) to produce a high modulation depth optical signal (36) output from the first optical output (66b).

Abstract: An optical transmission system includes a current source for outputting a drive current, a semiconductor laser for converting the drive current into a stimulated emission light and producing the same, a drive circuit for converting a transmission data signal into a modulation control signal and outputting the same, an optical modulator for receiving the stimulated emission light and a shading control signal for interrupting light emission and producing and outputting transmission signal light by changing an amount of transmission of the stimulated emission light according to the shading control signal, and a shading element for receiving the transmission signal light and a shading control signal for interrupting light emission and interrupting the transmission of the transmission signal light according to the shading control signal whereby an optical signal having wrong optical wavelength is rapidly prevented.

Abstract: The present invention relates to an optical communication apparatus for detecting the intensity of light in an optical modulator or the intensity of a modulation signal and for controlling the operating point of the optical modulator based on the result of detection. In the optical communication apparatus having such a configuration, the operating point of the optical modulator can be kept stable even when the input light or the modulation signal is temporarily non-existent in the optical communication apparatus.

Abstract: A method and system for generating both return-to-zero (RZ) and carrier suppressed return-to-zero (CSRZ) shaped signals using only a single optical modulator. The system includes: a switch for receiving a data signal and a clock signal as inputs, and outputting a voltage signal; a unit for controllably adjusting the phase of said clock signal before input to the switch; an optical modulator for receiving a continuous wave light (CW) signal and the voltage signal as inputs, and outputting one of an RZ and a CSRZ signal. To generate a CSRZ signal, the optical modulator is biased at a transmission minimum level signal. To generate an RZ signal, the optical modulator is biased at a transmission maximum level and the clock signal is phase shifted. Also disclosed is an optical communication transceiver including a plurality of optical modulator circuits generating both RZ and CSRZ signals.

Abstract: A wavelength stabilizing apparatus utilized in an optical communication system for controlling a light wave output from a tunable optical component is disclosed. The wavelength stabilizing apparatus includes a coarse-tuning element, a fine-tuning element, and a servo element. When the wavelength stabilizing apparatus is used, the light wave output from the tunable optical component is directed into the coarse-tuning element and the fine-tuning element, respectively, and then transformed into electric signals to be received by the servo element. Particularly, the electric signals from the coarse-tuning element are served as basis for coarse-tuning and channel recognition of the light wave output from the tunable optical component while the electric signals from the fine-tuning element are served for fine-tuning and servo control of the light wave output from the tunable optical component.

Abstract: A wavelength Division Multiple Access (WDMA) free space broadcast technique for optical backplanes and interplanar communications for providing free space optical interconnects between multiple circuit cards in a computer system or networking device which is compatible with existing electrical backplanes. Current equipment can easily be upgraded in the field to take advantage of this new approach by simply replacing existing printed circuit boards, without requiring a complete redesign of the copper backplane.

Abstract: Provided are a multiplexing circuit for multiplexing a memory output signal output by a memory for temporarily accumulating a low-order group signal with an overhead bit necessary for optical digital transmission and a pattern generation circuit for generating an unfixed pattern having no fixed value. An unfixed pattern is applied to the multiplexing circuit while the memory outputs a fixed pattern. A selection circuit provided between the multiplexing circuit and the memory selects an unfixed pattern between a fixed pattern output by the memory and an unfixed pattern output by the pattern generation circuit.

Abstract: Methods and apparatus related to maintaining a pre-determined ratio of a pilot tone power and a total mean optical power of an optical signal are provided. One method includes the steps of detecting the AC and DC components of an optical signal having a pilot tone signal. An amplitude of the pilot tone signal is varied in accordance with a difference between a pre-determined value and a ratio of the AC and DC components. Another method includes the step of generating an optical signal in accordance with a DC bias and an AC bias corresponding to a pilot tone signal. The AC and DC components of the optical signal are detected. At least one of the DC bias and the AC bias is varied in accordance with a difference between a pre-determined value and a ratio of the AC and DC components. An apparatus includes an optical source providing an optical signal in response to a DC bias and an AC bias corresponding to a pilot tone signal.

Abstract: In the application of the optical transmitter to any system for the compensation of passing degradation, the invention provides the optical transmitter which maintains constant modulator output and the highly reliable optical transmission by tuning, the alpha parameter for given systems. As the optical element back-face output power detected by PD6 depends on the optical element driving current amount, the electronic absorption amount of the EA optical modulator monitored by the electronic absorption monitor depends on the optical output of the optical element 5 and the EA optical modulator driving point of the driving part 8. It is possible to detect a change in the driving point in the EA optical modulator by taking a difference between the amount monitored by the PD 6 and the amount monitored by the electronic absorption monitor.

Abstract: A semiconductor optical amplifier (SOA) is placed in the optical path between the tunable laser and the external modulator on a DWDM optical transmitter. The modulator transfer function is measured using low-level amplified spontaneous emission light output from the SOA in order to find the modulator bias corresponding to minimum transmission. The external modulator is biased to the point of minimum transmission to reduce the transmitter output power during laser turn-on. The SOA bias is also turned off to provide further attenuation as the laser is turned on. Similarly, to avoid emitting off-wavelength light during wavelength changes, the transmitter output is attenuated with a combination of low SOA bias current and biasing the modulator to its minimum transmission point. In both cases, the laser wavelength is allowed to stabilize without interfering with adjacent DWDM channels.

Abstract: The optical transmitter comprises a light source which inputs continuous optical signal and a driving circuit which inputs driving signal to an optical modulator of Mach-Zehnder type. A photo coupler separates a part of the output of the optical modulator. A photo diode converts this part into electric signal. A band-pass filter and a pre-amplifier extract a frequency component of the driving signal contained in the electric signal. A mixer conducts synchronous detection between the driving signal and the extracted frequency component. Finally, a bias voltage control circuit controls a bias voltage based on a result of the phase comparison.

Abstract: An optical transmitter including a variable optical attenuator and a controller. The variable optical attenuator attenuates a light to be transmitted from the optical transmitter in accordance with a drive current of the attenuator. The attenuator has an attenuation versus drive current characteristic curve with a peak so that attenuation increases with increasing drive current on a side of the peak ascending to the peak and attenuation decreases with increasing drive current at an opposite side of the peak descending from the peak. The controller monitors the attenuated light and controls the drive current to maintain an attenuation amount near the peak. The attenuator is, for example, a Faraday rotator.

Abstract: A communication system and more particularly to a variable rate differential phase shift keying (DPSK) communication system with minimal hardware that does not have power or performance penalties associated with known DPSK modulation systems is disclosed. The DPSK modulation system in accordance with the present invention includes a transmitter, which includes a carrier signal source, a phase modulator and a DPSK encoder for modulating a carrier signal. The modulated carrier signals may be amplified, for example, in optical communication systems by a rare earth element doped fiber amplifier. The signals are continuously transmitted to a multi-rate receiver through a communication channel, for example, free space. The multi-rate receiver includes a single demodulator, for example, a single optical interferometer, used for multiple integer sub-harmonic data rates which demodulates the modulated signal.

Abstract: A method and apparatus for modifying the extinction ratio of a modulated optical signal by adapting a modulator driver signal in response to differences in spectral regions of the modulated optical signal. In another embodiment for modifying the extinction ratio of a modulated optical signal, a modulator signal is adapted in response to differences between a profile of the modulated optical signal and a desired profile.

Abstract: An optical link is operative for transmitting a microwave signal in modulated optical form over an optical fiber. The optical link has a transmitter and a receiver. The transmitter splits and converts the microwave signal into two separate optical signals, each of which represents either the positive varying part of the microwave signal or the negative varying part of the microwave signal. The receiver combines the two separate optical signals and converts them back into the microwave signal.