Abstract:
Optical impairments such as dispersion and fibre nonlinearity are compensated by generating a pre-distorted electrical signal at the transmitter. This signal is modulated onto a carrier signal, so that it is upconverted in frequency. This up converted signal is then used to modulate an optical source. Generally the optical signal will have two sidebands, one of which has the correctly pre-distorted information and the other which is unwanted. Information in the unwanted optical sideband is either filtered optically or electrically. In the preferred embodiments, the transmitter uses a tunable semiconductor laser with an integrated electroabsorption modulator to modulate the light. The preferred receiver is a coherent receiver with a tunable local oscillator laser. The receiver uses an electrical filter to remove the information in the unwanted sideband.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a method and apparatus for compensating for optical impairments electrically and a system incorporating the same.  
       BACKGROUND TO THE INVENTION  
       [0002]     Optical communications systems typically comprise of a transmitter which converts electrical signals into optical signals, an optical link over which the optical signals are transported and a receiver which converts the optical signal into an electrical signal, the electrical signal recovered at the receiver ideally being identical to the electrical signal originating at the transmitter. The optical link generally consists of optical transmission fibre to convey the signal and optical amplifiers to compensate for the loss that the fibre introduces. The optical amplifiers are generally interspersed at regular intervals along the link (e.g. every 40-200 km).  
         [0003]     One impairment the optical fibre may exhibit is chromatic dispersion. This is where different optical frequencies of the light propagate at different speeds in the fibre. This results in the optical signal being distorted. Eventually after propagating over some distance of fibre, the signal will not be recoverable unless some compensation of this effect is afforded. For example, if a data rate of 10 GBit/s is used in an intensity modulation format at 1550 nm in standard fibre, then the signal will be irrecoverable after around 200 km without some form of compensation. Commercially deployed telecommunications systems use optical dispersion compensation devices to overcome this limitation. There are a number of optical dispersion compensation devices which fulfil this role, the preferred type being dispersion compensating fibre modules. These modules consist of optical fibre of a different type to the transmission fibre, known as dispersion compensating fibre. This fibre has dispersion which is of the opposite sign to the transmission fibre, such that when the dispersion compensating fibre is coupled to the transmission fibre, the overall net dispersion is within the limit of an uncompensated system. Typically the fibre has a higher magnitude of dispersion to the transmission fibre, to reduce the length required. The fibre is generally coiled and placed in modules. A number of modules may be used, distributed at the amplifier sites and terminal sites, or even lumped at one or a few sites.  
         [0004]     However, there are a number of drawbacks to using dispersion compensating fibre modules for compensating for the optical dispersion. The most obvious disadvantage is that they tend to be very costly. However, they are also relatively large and have high optical loss. This latter feature results in more optical amplification being required which in turn increases cost and can have degrading effect on performance. A further disadvantage is that dispersion compensating fibre modules tend to be more sensitive to nonlinear distortion, thus also reducing performance. It is therefore desirable to reduce in number or eliminate completely the dispersion compensating devices to reduce the system cost and simultaneously increase the system performance.  
         [0005]     One method for removing the optical dispersion compensation devices is disclosed in U.S. patent application Ser. No. 10/262,944, filed on Oct. 3, 2002 and published as US 2004/0067064 A1 on Apr. 8 2004. In this application, instead of compensating for the optical dispersion using an optical dispersion compensating device, the dispersion is compensated by electrical means. Such a method is illustrated in  FIG. 1 . In this example the electrical signal applied to the optical transmitter is first pre-distorted by applying a digital filter. The pre-distortion function is such that after the signal has passed through the system, it can be recovered at the optical receiver. One feature of this prior art is that to adequately compensate for the dispersion of the system it is necessary to have a pre-distorted signal which modulates both the amplitude and phase of the optical signal. That is the signal applied to the modulator is a complex signal, such that it contains both amplitude and phase information in a polar coordinate system or In-phase and Quadrature components in a Cartesian coordinate system. Accordingly, it is necessary to use an optical modulator which takes a complex signal as its input, a complex modulator. Such a modulator is considerably more expensive to fabricate than a conventional modulator, as the structure is larger and there are more processing steps involved in its fabrication. Additionally a significant expense is the amplifier used to provide sufficient voltage swing to drive the amplifier. Since two such amplifiers are required, this expense is increased. A disadvantage of using a complex modulator is that its optical loss is generally significantly higher than a conventional one. A further disadvantage is that the fabrication steps involved are very different from that of a semiconductor laser, such that it has not been possible to monolithically integrate a complex modulator with a semiconductor laser on the same substrate.  
       OBJECT TO THE INVENTION  
       [0006]     The invention seeks to provide an improved method and apparatus for compensating optical impairments electrically using an optical modulator which does not require a complex input. The invention also seeks to provide an improved method and apparatus for compensating optical impairments electrically using an optical modulator of a design which may be monolithically integrated with a semiconductor laser.  
       SUMMARY OF THE INVENTION  
       [0007]     According to a first aspect of the invention, there is provided a pre-distortion unit for compensating optical transmission impairments of a communications signal conveyed through an optical communications system comprising: at least one electrical filter for modifying an electrical input signal using a pre-distortion function to generate a pre-distorted electrical signal which mitigates the impairments in an optical system; and at least one upconverter for modulating the pre-distorted electrical signal onto an electrical carrier signal for modulation of an optical signal to generate a corresponding pre-distorted optical signal. This unit may be used to compensate for optical transmission impairments electrically, with the advantage that the pre-distorted signal has a single electrical output which may be applied to an optical modulator; the optical modulator not needing to be a complex optical modulator. It has the further advantage that it may be used with a low cost optical modulator such as an electrosborption modulator or a monolithically integrated laser electroabsorption modulator. It has the further advantage that it may be used with a directly modulated laser.  
         [0008]     According to a second aspect of the invention, there is provided a system for compensating for optical transmission impairments on an optical signal conveyed through an optical communications system, the system comprising: at least one electrical filter for modifying an electrical input signal using a pre-distortion function to generate a pre-distorted electrical signal which mitigates the impairments in an optical system; and at least one upconverter for modulating the pre-distorted electrical signal onto an electrical carrier signal; and an optical transmitter for generating an optical signal and modulating said optical signal, using the upconverted pre-distorted electrical signal; and an optical receiver for converting the optical signal into an electrical signal. This unit is particularly advantageous as it may be used to compensate for optical transmission impairments electrically and does not need a complex optical modulator. It has the further advantage that it enables a low cost optical modulator such as an electrosborption modulator or an monolithically integrated laser electroabsorption modulator to be used. It has the further advantage that it enables a directly modulated laser to be used.  
         [0009]     According to a third aspect of the invention, there is provided a method of compensating optical impairments of an optical signal conveyed through an optical communications system, the method comprising the steps of: filtering an electrical input signal using a pre-distortion function to generate a pre-distorted electrical signal which mitigates the impairments in an optical system; and modulating an RF carrier with a pre-disorted electrical signal to generate an upconverted pre-distorted electrical signal modulating an optical source using the upconverted pre-distorted electrical signal to generate a corresponding pre-distorted optical signal for transmission through the optical communications system. This method has the advantage that it may be used to compensate for optical transmission impairments electrically and does not need a complex optical modulator. It has the further advantage that it enables a low cost optical modulator such as an electrosborption modulator or a monolithically integrated laser electroabsorption modulator to be used. It has the further advantage that it enables a directly modulated laser to be used.  
         [0010]     According to a fourth aspect of the invention, there is provided a pre-distorted optical signal suitable for being conveyed through a communications system, which has been filtered by a pre-distortion function which mitigates impairments in an optical system; and upconverted to an RF frequency; and upconverted to an optical frequency. This signal has the advantage that its generation does not need a complex optical modulator. It has the further advantage that it enables a low cost optical modulator such as an electrosborption modulator or an monolithically integrated laser electroabsorption modulator to be used. It has the further advantage that it enables a directly modulated laser to be used.  
         [0011]     According to a fifth aspect of the invention, there is provided a pre-distortion unit in which the electrical filter is a digital filter, with the advantages that a more complicated pre-distortion function may be used and higher signal integrity may be maintained.  
         [0012]     According to a sixth aspect of the invention there is provided a pre-distortion unit in which there are a plurality of electrical filters and a plurality of upconverters, each with different carrier frequencies and combined to form a composite signal. This arrangement has the advantage that multiple channels may be transmitted using a single modulator and thus the number of components and hence cost may be reduced.  
         [0013]     According to a seventh aspect of the invention, there is provided a method in which the impairment is dispersion, with the advantage that dispersion compensating modules are not required and that the cost is reduced.  
         [0014]     According to an eight aspect of the invention, there is provided a method in which the impairment is fibre-nonlinearity, with the advantage that the performance will be improved and as a result longer transmission distances will be possible.  
         [0015]     According to a ninth aspect of the invention, there is provided a method in which the impairment is modulator chirp in addition to dispersion or fibre nonlinearity, with the advantage that a less ideal modulator may be used. If an electroabsorption modulator is used, a greater tolerance to chirp will mean that the wavelength window of operation will be increased.  
         [0016]     According to a tenth aspect of the invention, there is provided a system in which the receiver comprises a local oscillator optical source for coherently interfering with the received signal. This has the advantage that the filtering of the unwanted signal components may be performed electrically, rather than optically.  
         [0017]     According to an eleventh aspect of the invention, there us provided a system wherein the receiver has an interferometer for decoding a differential phase modulation format. This has the advantage that the performance will be improved and as a result longer transmission distances will be possible.  
         [0018]     The invention is also directed to a method by which the described apparatus operates and including method steps for carrying out every function of the apparatus.  
         [0019]     The invention also provides for a system for the purposes of communications which comprises one or more instances of apparatus embodying the present invention, together with other additional apparatus.  
         [0020]     The invention also provides for computer software in a machine-readable form and arranged, in operation, to carry out every function of the apparatus and/or methods.  
         [0021]     The preferred features may be combined as appropriate, as would be apparent to a skilled person, and may be combined with any of the aspects of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]     In order to show how the invention may be carried into effect, embodiments of the invention are now described below by way of example only and with reference to the accompanying figures in which:  
         [0023]      FIG. 1  shows a prior art system for compensating for optical impairments using an electrical pre-distortion of the original signal.  
         [0024]      FIG. 2  shows a block diagram schematically illustrating the principle components and operations of a device for compensating for optical impairments electrically in accordance with a first embodiment of the present invention.  
         [0025]      FIG. 3  shows a block diagram schematically illustrating the principle components and operations of a system for compensating for optical impairments electrically in accordance with a first embodiment of the present invention.  
         [0026]      FIG. 4  shows example spectra at various points in the system of  FIG. 3  in accordance with a first embodiment of the present invention.  
         [0027]      FIG. 5  shows a block diagram schematically illustrating the principle components and operations of a modified device for compensating for optical impairments electrically in accordance with a second embodiment of the present invention.  
         [0028]      FIG. 6  shows a block diagram schematically illustrating the principle components and operations of a system for compensating for optical impairments electrically in accordance with a third embodiment of the present invention.  
         [0029]      FIG. 7  shows example spectra at various points in the system of  FIG. 6  in accordance with a third embodiment of the present invention.  
         [0030]      FIG. 8  shows a block diagram schematically illustrating the principle components and operations of a system for compensating for optical impairments electrically in accordance with a fourth embodiment of the present invention.  
         [0031]      FIG. 9  shows example spectra at various points in the system of  FIG. 8  in accordance with a fourth embodiment of the present invention.  
         [0032]      FIG. 10  shows a block diagram schematically illustrating the principle components and operations of a system for compensating for optical impairments electrically in accordance with a fifth embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF INVENTION  
       [0033]     Any range or device value given herein may be extended or altered without losing the effect sought, as will be apparent to the skilled person for an understanding of the teachings herein.  
         [0034]     Referring firstly to  FIG. 1  there is shown a prior art transmitter for compensating for the chromatic dispersion of a telecommunications system by using an electrical pre-distortion of the original signal. There is an optical transmitter  105  comprising an optical source  106  coupled to a complex optical modulator  107 , which is capable of modulating the amplitude and phase of the optical signal in a Cartesian format by way of applying in-phase (I)  104   a  and quadrature (Q)  104   b  signal components. The information to be transmitted is coupled to an electrical pre-distortion unit  100 , comprising two paths to generate the I component  104   a  and the Q component  104   b  of the pre-distorted signal. Each respective path comprises an electrical filter  101   a  and  101   b , a serial to parallel converter  102   a  and  102   b , and a digital-to analogue converter  103   a  and  103   b . The electrical filters  101   a  and  101   b  respectively take as inputs the I and Q components of the required pre-distortion transfer function, generated by a processor  108  adapted to determine the correct coefficients for mitigation of the impairments  
         [0035]      FIG. 2  shows a schematic block diagram in accordance with the first embodiment of the present invention. Here there is an optical transmitter  215  comprising an optical source  216  (which may be a semiconductor laser and may be wavelength tunable) and an optical modulator  206 . The optical modulator  206  is different from the complex optical modulator  217  in that it only takes a single input  206  and can therefore not modulate the amplitude and phase of an optical signal independently. This modulator  206  may be an electroabsorption modulator with a single electrical input. The modulator  206  may be monolithically integrated on the same substrate as the optical source. The information to be transmitted is coupled to a different electrical pre-distortion unit  210  comprising two paths to generate the I  214   a  and Q  214   b  signal components of the pre-distorted signal. The two paths each comprise an electrical filter  211   a  and  211   b , which may be digital filters, a serial to parallel converter  212   a  and  212   b  and a digital to analogue converter  213   a  and  213   b . The electrical filters  211   a  and  211   b  respectively take as inputs the I and Q components of the required pre-distortion transfer function. The I and Q signals  214   a  and  214   b , are coupled to an upconverter unit  200 . The upconverter  200  takes the I signal  214   a  and an RF signal  204  to produce a signal  205   a  which is the composite modulated signal of  214   a  on the RF carrier  204 . This signal  205   a  is similar to  214   a , but shifted in frequency by RF carrier  204 . Such an element can be achieved by a multiplying function or by some other nonlinear element such as a mixer. Similarly, the Q signal  214   b  is upconverted to signal  205   b  using upconverter  201   b . However, the RF signal input to  201   b  is the RF signal  204  modified in phase by  202 , such that the phase of the RF signal applied to  201   b  is approximately 90 degrees out of phase compared to the RF signal applied to  201   a . The two signals  205   a  and  205   b  are combined using a conventional combiner  203  to produce a single output  206 .  
         [0036]      FIG. 3  shows the first embodiment of the present invention in an optical system with an optical receiver. There is an optical pre-distortion unit  210  coupled to an optical transmitter  215  including optical modulator  205 . The optical system is coupled to the optical link  300  which may comprise of optical fibre, optical amplifiers and other optical components. The optical link in coupled to a coherent receiver  301 . The coherent receiver  301  comprises an optical source  302  which may be wavelength tunable and may be shared with another unit such as an optical transmitter for the opposite direction. The signal from the optical source  302  is combined with the signal from the link  300  with an optical coupler  303 . The optical coupler  303  combines its input signals in a polarisation diverse form such that it generates two output signals with a different polarisation relationship between the input signals. The difference in this polarisation relationship between the output ports is approximately orthogonal, such that there is always a combined beat signal at one or other of the ports. Such a coupler can be implemented with a conventional coupler followed by a polarisation beam splitter, with the polarisation angle of the optical source  302  aligned such that there is approximately equal source power from  302  delivered to each arm of the beam splitter. The two outputs from the optical coupler  303  are detected independently with an optical detector  304   a  and  304   b , an electrical filter  305   a  and  305   b  an electrical envelope detector  306   a  and  306   b  and subsequently combined with  305  and threshold detected into an digital signal with  308 . The coherent optical receiver is similar to a conventional coherent optical receiver except that the filtering means  305   a  and  305   b  has a different characteristic as will be later apparent. The envelope detector may be implemented as a squaring function, which itself may be realised as a mixer with the two inputs matched in phase and amplitude response and commoned together.  
         [0037]      FIG. 4  shows example spectra at various points in the system for the first embodiment.  FIG. 4   a  shows the spectra of the electrical signal  206  from the pre-distortion unit  210 . This assumes a bit rate of 10 Gbit/s, an RF signal  204  of 10 GHz and an electrical bandwidth for the optical modulator  205  of 10 GHz. Here it can be seen that the modulated data has been upconverted onto a 10 GHz carrier. The electrical spectrum is shown in the conventional way with both positive and negative frequencies, though negative frequencies are not observable.  
         [0038]      FIG. 4   b  shows the optical signal emerging from the optical transmitter  215 . Here it may be seen that there is a central carrier and the frequency scale is now relative to this carrier (so both positive and negative frequencies on this scale are observable). In addition to the carrier, there are two optical sidebands  401  and  402  each containing the data spectrum. In general only one of these sidebands will have the correct pre-distortion signal for the optical system and the other will be the complex conjugate. Which is which depends on whether there is phase lead or phase lag introduced by  202 .  
         [0039]      FIG. 4   c  shows the electrical spectrum emerging from the optical detectors  304   a  and  304   b  assuming they have a 20 GHz bandwidth (again only positive frequencies are observable). Now both sidebands  401  and  402  lie at positive frequencies and so are observable. However, due to the frequency roll-off of the optical detectors  304   a  and  304   b , the higher frequency sideband  402  is attenuated relative to the lower frequency sideband  401 . Superimposed onto  FIG. 4   c  is an example filter characteristic for filtering means  305   a  and  305   b . In a conventional coherent receiver the filtering means  305   a  and  305   b  would only have the function of high pass filtering the signal to remove the unwanted components below 10 GHz which are direct detected (i.e. present even in the absence of any signal from  302 ). In the present invention, the filtering means  305   a  and  305   b  has the additional function of suppressing the unwanted sideband. In this example, the phase of  202  has been selected such that the lower sideband  401  contains the correct pre-distorted signal. The filtering means  305   a  and  305   b  therefore has a characteristic such as to substantially remove the upper sideband  402 . In this example the filter function has been designed such that it only selects the upper portion of sideband  401 . This has the advantage that the RF frequency from  204  can be of lower frequency as only one half of sideband  402  is required to recover the original signal.  
         [0040]      FIG. 4   d  shows the signal after it has been envelope detected with  306   a  and  306   b . This signal is now ideally nominally the same as the original signal, although any nonideality in the performance of the components may cause this to deviate.  
         [0041]      FIG. 5  shows a second embodiment of the present invention in which there is an optical transmitter  215  and an optical pre-distortion unit  210 . Here there are two paths and two electrical filters  211   a  and  211   b , taking additional inputs from the I and Q components of the pre-distortion transfer function respectively. In this embodiment, these digital signals are directly fed into the upconverter unit  200 , which is now implemented in a digital form. The output from the upconverter unit is then a single digital signal which is coupled to a serial to parallel converter  212  and a digital to analogue converter  213 . This configuration has the advantage that more of the processing is performed digitally for greater fidelity and only a single digital to analogue converter is required. However this digital to analogue converter must have a wider bandwidth.  
         [0042]      FIG. 6  shows a third embodiment in which a direct detection receiver is used instead of a coherent receiver. Here there is also an optical transmitter  215 , a pre-distortion unit  210  an optical link  300  and a receiver  301 . However in this instance instead of removing the unwanted sideband  401  by an electrical filtering means, an optical filtering means is used  500 . This filter may positioned anywhere within the optical part of the system, e.g. before, after or within the optical link  300 . The optical filtering means may be positioned within the optical transmitter  215  and may also be combined within an optical element used for establishing the wavelength of the optical source. Typically a laser package would incorporate an optical filter on the rear facet for monitoring its wavelength. By moving this element to the front facet, the element would perform the dual functions of wavelength monitoring and unwanted sideband removal. The optical filter  500  may instead be incorporated within the receiver. The advantage of placing this element at the receiver is that it has the desirable effect of reducing optical noise occurring from the optical amplifiers. The receiver itself comprises an optical detector  304 , optionally a squaring element  501  and a threshold detector  308 .  
         [0043]      FIG. 7  shoes example spectra at various points in the system for the third embodiment.  FIG. 7   a  shows the electrical spectra applied to the optical transmitter  215 .  FIG. 7   b  shows the optical spectra emerging from the optical transmitter  215 . Also shown in  FIG. 7   b  is the characteristic of the filtering means  500 , used to select the wanted sideband  401  and attenuate the unwanted sideband  402 . In this example, a low-cost filter with a relatively broad cut-off has been used, such that a significant proportion of the carrier  403  will also pass through.  FIG. 7   c  shows the electrical spectrum after the optical detector  304 . Because a reasonable proportion of the carrier signal  403  is present, there is a beating of this carrier with the wanted sideband  401 , which results in the baseband signal being upconverted to the frequency offset of the carrier (10 GHz in this example). As a result an envelope detection element  501  is used to downconvert this signal to the baseband. The final demodulated signal is shown in  FIG. 7   d.    
         [0044]      FIG. 8  shows a fourth embodiment in which a sharper roll-off filtering means  500  is used. Here there is an optical transmitter  215 , a pre-disortion unit  210 , an optical link  300  and a receiver  301 . Here the roll-off of the filtering means is sharp enough so that both the optical carrier  403  and the unwanted sideband are substantially eliminated, such that the squaring element may be omitted.  
         [0045]      FIG. 9  shows example spectra at various points in the system for the fourth embodiment.  FIG. 9   a  is the electrical spectra from the pre-distortion unit  210 .  FIG. 9   b  is the optical sprectra emerging from the optical transmitter  215 . Superimposed on  FIG. 9   a  is the characteristic of filtering means  500 . Here the roll-off is sharp to substantially reduce the carrier  403  in addition to the unwanted sideband  402 .  FIG. 9   c  shows the electrical signal after the optical detector  304 . This signal has been demodulated to baseband.  
         [0046]      FIG. 10  shows a fifth embodiment in which a differential phase shift keyed (DPSK) modulation format is used. There is an encoding unit  1007  for differentially encoding the data to be transmitted, a pre-distortion unit  210 , an optical transmitter  215  a filtering means  500  an optical link  300  and a DPSK receiver  301 . The DPSK receiver  301  comprises a DPSK demodulator  1000 , an optical detector  1004 , a threshold detector  1005  and a decoding unit  1006 . The DPSK demodulator  1000  comprises a Mach-Zehnder interferometer (MZI) formed between two optical  1002  and  1003 . The MZI is constructed such that there is a path difference between the arms  1001  of approximate equivalent length to the bit perioid in the data. The optical detector  1004  may comprise of a balanced receiver, or a pair of receivers or a single receiver. The DPSK receiver has the advantage of increased sensitivity and therefore increased performance compared to a direct detection receiver. At least part of the filtering means may be performed in the MZI. In particular, by appropriate phase adjustment between the arms it is possible to substantially null the carrier component, thereby reducing the amount of filtering required by other components. It is also possible to integrate the filter and the MZI together, which may be in the form of an integrated planar circuit. The DPSK receiver may also be implemented using a local oscillator laser, an electrical MZI and an electrical filtering means.  
         [0047]     There are many variations of modular known to those skilled in the art that may be used within the optical transmitter. These include, but are not limited to Mach-Zehnder modulators, directional coupler modulators and electroabsorption modulators. Additionally, it is possible to modulate the optical source itself directly. In the case of a semiconductor laser this achieved by modulating the current drive to the laser, and is known as a directly modulated laser.  
         [0048]     There are many variations of receiver known to those skilled in the art that may also include a filtering means for removing the unwanted sideband  402 . These include but are not limited to direct detection receivers, heterodyne receivers, homodyne receivers, intradyne receivers and may use a variety of modulation formats including but not limited to amplitude modulation, phase modulation, multisymbol coding schemes such as QPSK and QAM.  
         [0049]     In addition to dispersion, there are many other impairments which may be compensated, including but not limited to fibre nonlinearity (e.g. self phase modulation) and modulator chirp. Chirp is where the modulator may have the characteristic of modulating both amplitude and phase simultaneously but not independently. In such circumstances chirp can have the effect of causing a distortion to the transmitted waveform.  
         [0050]     As well as compensating for a single channel, it is possible to transmit multiple channels by having a number of digital filters and upconverters within the pre-distortion unit, each having different carrier frequencies. These signals can then all be combined together to form a single electrical signal comprising many carriers. This electrical signal can then be applied to an optical modulator to allow multiple channels to be transmitted using a single modulator.