Ofidium Pty. Ltd.

The University of Melbourne

Other

Arthur James Lowery

Liang Bangyuan Du

Jean Armstrong

Mark Geoffrey Sceats

Rodney S Tucker

Richard David Lauder

Brendon James Schmidt

Phillip Clyde Roberts Gurney

Hongbing Gan

Noriaki Onodera

An apparatus () mitigates cross-channel nonlinear distortion of an optical signal () carried on one of a plurality of wavelength channels () in a wavelength division multiplexed (WDM) transmission system (). The apparatus includes a first optical receiver () which is arranged to detect a measure () of aggregate optical power of the plurality of wavelength channels. A nonlinear dispersion compensator includes means () for applying a phase modulation to the optical signal in proportion to the measure of aggregate optical power.

Multichannel nonlinearity compensation in an optical communications link

A method for transmitting digital information having a predetermined bit-rate (B) via a nonlinear optical channel includes determining a combination of transmission parameters comprising a number (N) of subcarriers, an information symbol rate (S) for each subcarrier, and a number (b) of bits per symbol on each subcarrier, such that B=NSb. An optical signal is generated comprising the plurality (N) of orthogonal optical subcarriers. Each subcarrier is modulated to carry a proportion (Sb/B) of the total bit-rate (B) of the digital information. The optical signal is then transmitted to a corresponding optical receiver via the nonlinear optical channel. The received signal has a measurable signal quality (Q) which is a function of the transmission parameters (N, S, b). Determining the combination of transmission parameters comprises selecting the parameters such that the associated signal quality is greater than a corresponding quality for a single-carrier transmission, and greater than a corresponding quality employing a larger number (M&#x3e;N) of subcarriers.

Optical OFDM transmission

An optical signal () is modulated with a transmitted electrical signal comprising an information-bearing component () and a pilot tone () having a predetermined frequency. Transmission of the signal results in distortion, including nonlinear optical transmission impairments. A method of receiving the signal includes detecting the optical signal to produce a received electrical signal (X) comprising a distorted variant of the transmitted electrical signal. The pilot tone is extracted from the received electrical signal using a filtering operation () having a predetermined characteristic, and a compensation signal determined based upon the extracted pilot tone. The compensation signal is applied to the received electrical signal (X) to produce a compensated signal (Y) having reduced distortion. The predetermined characteristic of the filtering operation () is represented by one or more parameters having values selected so as to substantially maximize a measure of quality of the compensated signal.

Electronic compensation of cross-phase modulation

An equalizer () for equalization of a signal transmitted via an optical fiber link from a transmitter to a corresponding receiver employs a backpropagation model () which comprises one or more sequential segments collectively representing an inverse fiber link. Each sequential segment comprises a linear backpropagation element (), and a non-linear backpropagation element () having an associated compensation bandwidth (). The equalizer () generates a distortion-mitigated signal by computing, for each sequential segment in turn, a first linear compensated signal from a signal input to the segment in accordance with the linear backpropagation element (), and a non-linear compensated signal from the first linear compensated signal in accordance with the non-linear backpropagation element (). Computation of the non-linear compensation signal comprises limiting a bandwidth of a compensation signal derived from the first linear compensated signal in accordance with the associated compensation bandwidth ().

Method and system for non-linearity compensation in optical transmission systems

A method of generating an information-bearing optical signal () from input digital information () includes generating () an information-bearing electrical signal comprising an in-phase (I) component and a quadrature (Q) component. Each of the components has a predetermined baseband bandwidth requirement (B). The I and Q signal components are combined () with corresponding I and Q components of a radio frequency (RF) carrier (). The carrier has a central frequency greater than the signal bandwidth requirement (B). An optical source is modulated () with the combined I and Q signal and carrier components, in order to produce a modulated optical signal () which comprises an optical carrier corresponding with the RF carrier, and substantially only a single information-bearing optical sideband in an optical frequency domain, corresponding with the information-bearing electrical signals. Transmitters implementing the method are also disclosed.

Optical transmitter

A receiver for recovering transmitted information carried by a received optical signal that has been affected by dispersion includes an optical splitter having an input port arranged to receive the received optical signal, and a plurality of output ports. A proportion of optical power at the input port is transmitted to each of the output ports. A plurality of optical detectors is operably connected to respective output ports of the optical splitter, for generating a corresponding plurality of electrical signals. Optical phase shifters are disposed between the output ports of the optical splitter and respective optical detectors. As a result, each optical phase shifter applies a frequency dependent phase shift to an optical signal passing therethrough. An electronic processor includes analog and/or digital electronic components configured to combine two or more of the plurality of electrical signals, or information recovered separately therefrom, in order to provide improved accuracy or reliability of information recovery as compared with detecting and processing only the received optical signal. The receiver is advantageously able to mitigate the effects of frequency-dependent fading which may occur in intensity modulation/direct detection optical transmission systems due to dispersion in optical transmission paths.

Reception of signals transmitted over a dispersive optical channel

A method and apparatus for receiving digital information transmitted via an optical signal over an optical channel wherein the optical signal includes an optical carrier () and at least one information-bearing optical sideband () in the optical frequency domain. The transmitted optical power is distributed between the optical carrier () and the optical side band (). The received optical signal is processed in order to increase the received power in the optical carrier () relative to the power in the optical side-band (). The processed optical signal is detected in order to produce a corresponding electrical signal. The electrical signal is then processed in order to recover the digital information. Advantageously, increasing the optical carrier power relative to the power in the information-bearing sideband results in improved quality of the detected electrical signal, enabling an improved power budget to be achieved, which in turn allows for longer transmission distances, increased transmitted information rates, and/or improved transmission error performance as compared with conventional direct detection optical transmission systems.

Method and apparatus for improving reception of optical signals

A system () for transmitting digital information includes a transmitting apparatus () for generating an optical signal bearing digital information, a dispersive optical channel (), and a receiving apparatus () for receiving the optical signal. The dispersive optical channel () is disposed to convey the optical signal from the transmitting apparatus () to the receiving apparatus (). The transmitting apparatus includes an encoder () for encoding digital information into a series of blocks, each including a plurality of data symbols corresponding with one or more bits of digital information. A signal generator () generates a time-varying signal corresponding with each of said blocks. An optical transmitter () is arranged to apply the time-varying signal to an optical source () to produce an optical signal which includes an optical carrier and substantially only a single information bearing optical sideband in an optical frequency domain, the sideband corresponding with the time-varying signal. The receiving apparatus () includes an optical detector () for detecting the optical signal to produce a corresponding received time-varying electrical signal. The receiver further includes means () for generating a series of received data blocks from the time-varying electrical signal. An equalizer () performs an equalization of received data symbols included in each data block to mitigate the effect of dispersion of the optical channel, thereby enabling the transmitted data symbols to be recovered.

Methods and apparatus for optical transmission of digital signals

A method of transmitting information over a non-linear optical channel includes the step () of generating an information-bearing signal, preferably an OFDM signal, which includes a plurality of closely-spaced sub-carriers in the frequency domain. A time-varying phase modulation is determined (), which is a first function, and preferably a linear function, of the transmitted optical power corresponding with the information-bearing signal. The information-bearing signal and the time-varying phase modulation are applied () to an optical source in order to generate a corresponding transmitted optical signal having substantially the stated transmitted optical power characteristic. The first function of transmitted optical power is selected so as to mitigate the effect of the non-linearity of the optical channel upon the transmitted optical signal. In alternative arrangements, a time-varying phase modulation, being a second function of optical power, is computed () and applied () to a signal received following transmission through a non-linear optical channel. The two alternative arrangements provide, respectively, for pre-compensation and post-compensation of non-linear propagation effects that may be carried out entirely within the electrical domain, for example using digital signal processing techniques.

Non-linearity compensation in an optical transmission

An apparatus and method are provided for receiving an optical signal having an optical carrier component and at least one information-bearing optical sideband. In general, an optical filter arrangement () is used to separate the optical carrier component from the information-bearing optical sideband. First and second optical splitters () divide the optical power in the optical carrier and the optical sideband, respectively, into corresponding sub-components. The sub-components of the optical carrier have substantially orthogonal polarisation states, which is an optional characteristic of the sideband sub-components. First and second optical coupling devices () respectively each combine one of the optical carrier sub-components with a corresponding one of the optical sideband sub-components. Optical detectors () detect the outputs of the combiners (). The arrangement is able to achieve a number of the benefits of coherent optical heterodyne receivers, particularly when used in combination with advanced coding and modulation methods, while avoiding the need, and associated complexity, of providing an optical oscillator at the receiver.

Method and apparatus for reception of optical signals

A method of communicating digital information over a dispersive optical channel includes encoding the digital information into a plurality of data blocks, each of which includes a number of bits of the information. A time-varying electrical signal is generated which corresponds with each of said data blocks. The time-varying electrical signal is applied to an optical transmitter () to generate an optical signal which includes an asymmetrically amplitude limited transmitted signal modulated onto an optical carrier. The optical signal is then transmitted over the dispersive optical channel (). At a receiving apparatus () the optical signal is detected to produce an electrical signal which corresponds with the asymmetrically amplitude limited transmitted signal. A frequency domain equalization of the electrical signal mitigates the effect of dispersion of the optical channel () on the transmitted optical signal, and the equalized signal is decoded to recover the encoded data blocks and the corresponding transmitted digital information. The method enables bipolar signals to be transmitted over a dispersive unipolar optical channel, and reduces or eliminates the need to apply a high optical bias level at the transmitter, thereby improving optical power efficiency and enabling output power levels to be maintained below applicable safe levels, while simultaneously enabling the effects of channel dispersion to be substantially mitigated.

A method of transmitting digital information over a dispersive optical channel includes encoding the digital information into a series of data blocks, wherein each block comprises a plurality of substantially orthogonal frequency domain subcarriers. Each data block is then divided into at least two frequency bands, each band comprising a plurality of contiguous subcarriers. A frequency/time transformation is then performed, in order to form a corresponding plurality of transformed bands, each transformed band comprising a sequence of time domain data samples. A time delay is applied to at least one of the transformed bands relative to at least one other of the transformed bands. The bands are then combined to produce an electrical signal waveform embodying the digital information. Finally, an optical source is modulated using the electrical signal waveform, to produce a corresponding optical signal for transmission over the dispersive optical channel. The invention enables a reduction in the duration of guard intervals that may need to be inserted into the transmitted data blocks, in order to avoid received signal degradation due to inter-symbol interference caused by dispersion in the optical channel. Transmission overheads may thereby be reduced, resulting in an improvement in transmission efficiency.

Optical OFDM transmission with improved efficiency

An optical device () having a number of waveguides (), an input port () coupled to the waveguides () such that light incident on the input port () is split for propagation on the waveguides, and an output port () receiving light propagated on the waveguides () at a point on an output plane of incidence. The waveguides () induce respective delays in the propagated light to induce a wavelength dependent group delay. Delays between adjacent waveguides are chosen to be non-constant, to introduce quadratic, cubic or higher-order phase differences between different waveguides. Applications include dispersion compensation, pulse compression, and passband broadening in arrayed waveguide grating multiplexers and demultiplexers.

Optical device for dispersion compensation

An optical communications system comprises, among other things, a laser source; an optical waveguide interconnected to the laser source to carry an optical signal from the source to an optical receiver; an optical receiver interconnected to the optical waveguide for decoding the signal; and a mechanical modulator adapted to substantially continuously mechanically perturb a portion of the optical waveguide so as to reduce Rayleigh backscattering from the optical waveguide.

Growing community of inventors

Kew, Australia

Arthur James Lowery