Abstract:
An optical transmitter for transmitting a multilevel amplitude-shift-keying modulated signal includes an optical modulator for modulating an optical signal with a multilevel amplitude-shift-keying modulation, and a spectral filter adapted to increase a high-frequency component of the modulated optical signal relatively to a central frequency component. The multilevel ASK modulation is quaternary ASK and the symbol rate of the optical modulator is above 40 Gbaud. An optical link connects the optical transmitter to a quadratic direct detection optical receiver.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to the technical field of optical communication systems, in particular communications systems employing multilevel amplitude-shift-keying (ASK) modulation schemes. 
       BACKGROUND 
       [0002]    Optical fiber transmissions employing coherent detection make it possible to achieve very high data rates over long distances, e.g. typically 100 Gb/s per wavelength channel. However coherent detection solutions may be too expensive for some applications, e.g. short reach transmissions. Therefore a need exists for optical transmission techniques that achieve a high data rate and remain compatible with direct detection of the signal intensity, i.e. quadratic detection, at the destination node. 
       SUMMARY 
       [0003]    In an embodiment, the invention provides an optical transmitter for transmitting a multilevel amplitude-shift-keying modulated signal, comprising: 
         [0000]    an optical modulator for modulating an optical signal with a multilevel amplitude-shift-keying modulation, and
 
a spectral filter adapted to increase a high-frequency component of the modulated optical signal relatively to a central frequency component.
 
         [0004]    According to embodiments, such optical transmitters can comprise one or more of the features below. 
         [0005]    In embodiments, the multilevel ASK modulation is quaternary ASK. 
         [0006]    In embodiments, the symbol rate of the optical modulator is above 40 Gbaud, for example about 56 Gbaud. 
         [0007]    In embodiments, the high-frequency component of the modulated optical signal comprises a frequency higher than f 0 +0.5 R, wherein f 0  is a central frequency of the optical signal and R is the symbol rate of the optical modulator. 
         [0008]    In embodiments, the high-frequency component of the modulated optical signal comprises a frequency equal to f 0 +0.7 R. 
         [0009]    In embodiments, the spectral filter comprises a Feed Forward filter arranged to filter a baseband electrical signal that drives the optical modulator. 
         [0010]    In embodiments, the Feed Forward filter comprises a single delay-tap with a negative configurable delay-tap coefficient. 
         [0011]    In embodiments, the Feed Forward filter comprises a plurality of delay-taps with a plurality of configurable delay-tap coefficients. 
         [0012]    In embodiments, the spectral filter comprises an optical spectral equalizer for equalizing the modulated optical signal 
         [0013]    In embodiments, the optical transmitter further comprises a feedback loop including a quality measurement module for measuring a quality of the transmitted optical signal and a feedback controller for reconfiguring the spectral filter as a function of the measured quality. In embodiments, the quality measurement module is adapted to measure an eye-diagram opening of the transmitted optical signal. 
         [0014]    In embodiments, the quality measurement module is adapted to measure a power ratio between the high-frequency component of the transmitted optical signal and the central frequency component of the transmitted optical signal. 
         [0015]    In embodiments, the feedback controller is adapted to increase the absolute value of a negative delay-tap coefficient of the Feed Forward filter in response to the measured eye-diagram opening or power ratio being lower than a target value. 
         [0016]    In embodiments, the feedback controller is adapted to increase the gain of a high-frequency channel of the optical spectral equalizer in response to the measured eye-diagram opening or power ratio being lower than a target value. 
         [0017]    In embodiments, the optical transmitter further comprises a laser source for generating the optical signal. 
         [0018]    In an embodiment, the invention also provides an optical communication system comprising: 
         [0000]    the above mentioned optical transmitter,
 
an optical receiver, and
 
an optical link connecting the optical receiver to the optical transmitter in the optical domain.
 
         [0019]    According to embodiments, such optical communication systems can comprise one or more of the features below. 
         [0020]    In embodiments, the optical receiver is a quadratic direct detection receiver. 
         [0021]    In embodiments, a range of the optical communication system is shorter than 100 km, preferably shorter than 10 km. 
         [0022]    Aspects of the invention are based on the idea of generating a high-rate multilevel ASK signal that can be successfully demodulated with a quadratic direct detection receiver. Aspects of the invention stem for the observation that power equalization makes it possible to reduce inter-symbol interferences in a high-rate multilevel ASK signal. Aspects of the invention are based on the idea of employing a feedback loop to update the coefficients of a spectral filter to maintain an optimal configuration of the spectral filter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter, by way of example, with reference to the drawings. 
           [0024]      FIG. 1  is a functional representation of an optical transmitter suitable for generating a quaternary ASK-modulated optical signal. 
           [0025]      FIG. 2  is a graph showing the power spectrum of a quaternary ASK-modulated optical signal. 
           [0026]      FIG. 3  is a functional representation of an optical transmitter in accordance with an embodiment. 
           [0027]      FIG. 4  is a functional representation of a Feed Forward filter employed in the optical transmitter of  FIG. 3 . 
           [0028]      FIGS. 5 and 6  are two graphs showing the transfer function of the filter of  FIG. 4  for two different values of the single tap coefficient. 
           [0029]      FIG. 7  is a graph showing the power spectrum of a quaternary ASK-modulated optical signal obtainable with the transmitter of  FIG. 3 . 
           [0030]      FIG. 8  is a graph showing the power spectrum of a quaternary ASK-modulated optical signal and two spectral bands monitored by a quality assessment module. 
           [0031]      FIG. 9  is a flow chart showing the operations of a feedback loop in an embodiment. 
           [0032]      FIG. 10  is a functional representation of an optical transmitter in accordance with another embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0033]      FIG. 1  shows an optical transmitter  1  for generating a modulated optical signal at 100 Gb/s based on 4-level amplitude shift keying (4-ASK). Such a 4-ASK modulated signal can be used for short reach transmissions using direct detection. 
         [0034]    The transmitter  1  comprises a binary signal generator  2  that generates four NRZ-coded binary streams  3  at a clock rate of 28 GHz. Two multiplexers  4  are arranged to receive two binary streams  3  each. The multiplexer  4  is driven by a clock signal  5  at 56 GHz to interleave both binary streams  3  into a 56 Gb/s NRZ-coded binary stream  6 . Both of the resulting 56 Gb/s binary streams  6  are fed to a 2-bits digital-to-analog converter  7  that generates a 4-level driving signal I and the complementary 4-level signal Ī to drive a Mach-Zehnder modulator  8  (MZM) in a push-pull mode. 
         [0035]    The push-pull mode means that binary driving signals having mutually reversed phases are input to a data input terminal and an inverted data input terminal of the MZM  8  and the peak voltages of those binary driving signals are set to a half wavelength voltage of the Mach-Zehnder interferometer. 
         [0036]    The MZM  8  receive an optical carrier wave from a laser source  9  and outputs a modulated optical signal  10  carrying a 4-ASK modulation at a symbol rate R=56 Gbaud, i.e. equivalent to 112 Gb/s. 
         [0037]    The power spectrum of the resulting modulated signal  10  can be seen in  FIG. 2 . The ability to generate the multi-level modulated signal at such a high baud rate is limited by the combined bandwidth of the electrical drivers  7  and the optical modulator  8 . Commercial optical modulators with low Voltage drive usually have 3 dB-bandwidth below 35 GHz. 
         [0038]    The bandwidth limitation of the transmitter induces large filtering of high frequency components, yielding a quite closed 4-level optical eye diagram. The bandwidth limitation for the 56 Gbaud 4-ASK signal  10  is rendered visible by the 12 dB attenuation of the spectral components located at 0.7×R from the carrier frequency f 0  in  FIG. 2 . These limitations induce inter-symbol interferences (ISI) that severely degrade the performance of the transmitter. 
         [0039]    Equalization can be employed to mitigate such bandwidth limitations by enhancing spectral content at high frequencies. For instance, feed forward equalizers (FFE) can be used and implemented either in the electrical or the optical domain. 
         [0040]    With reference to  FIGS. 3 to 10 , there will now be described embodiments of a similar optical transmitter in which optical or electrical spectral equalization is employed to mitigate the inter-symbol interferences (ISI) induced penalties. Elements identical or similar to those of  FIG. 1  are designated by the same numeral as in  FIG. 1 . 
         [0041]    In the embodiment of  FIG. 4 , spectral equalization is achieved in the electrical domain by a Feed Forward Equalizer  11  arranged between the digital-to-analog converter  7  and the optical modulator  8 . An amplifier may also be arranged between DAC  7  and Feed Forward Equalizer  11 . 
         [0042]    In an embodiment depicted in  FIG. 4 , the FFE  11  is a single delay-tap equalizer having a single tap coefficient a. In  FIG. 4 , τ designates the delay of one symbol. The transfer function H(f) of this single delay-tap equalizer is: 
         [0000]        H ( f )=(1 +a.e   −i2πfτ )/2  (Eq. 1)
 
         [0043]      FIGS. 5 and 6  illustrate the filter gain as a function of frequency for two different values of tap coefficient a. Particularly, negative values of the tap coefficient a result in amplification of the high frequency spectral components with respect to the central frequency component at f 0 . By increasing the absolute value of the delay-tap coefficient a, the pre-emphasis of high frequency components is increased. Indeed the equalizer response in  FIG. 6  has a more pronounced pre-emphasis of high frequency components than equalizer response in  FIG. 5 . 
         [0044]    By adjusting the delay-tap coefficient a, the transfer function H(f) of the equalizer can be modified to provide an optimal amplification of the high frequency components that have been attenuated by the electrical driver, e.g. the high-frequency component at 
         [0000]        f−f   0 =0.7 R   (Eq. 2)
 
         [0045]      FIG. 7  shows the spectrum of the same 4-ASK modulated signal  10  as discussed in  FIG. 2 , where the driving signals are now spectrally equalized with the transfer function shown in  FIG. 6 . 
         [0046]    The filtering reduces the power discrepancy between low and high frequency components of the modulated signal. As can be seen in  FIG. 7 , the equalized spectrum is more flat so that the corresponding eye diagram is as more open. 
         [0047]    In a preferred embodiment, the optimal configuration of the equalizer  11  bringing the best performance is determined automatically as a function of the characteristics of both the electrical driver and the optical modulator. This is especially useful if the optical receiver  15  that is employed at the destination relies on quadratic direct detection. 
         [0048]    In order to adjust the equalizer response as a function of the characteristics of the 4-ASK transmitter  1 , the transmitter of  FIG. 3  further comprises a feedback loop  20 . Using a 1% optical splitter  12  followed by a wideband photodiode  13 , a quality assessment module  21  assesses the quality of the equalized signal and provides a quality measurement signal  23  to a control module  22  that reconfigures the FFE  11  as a function of the measurement. For example, the quality assessment module  21  measures the flatness of the spectrum through spectral analysis as shown by arrow  24  or the opening of the eye diagram through time domain analysis as shown by arrow  25 . 
         [0049]    The control module  22  selects an optimal equalizer configuration that yields the best quality of the 4-ASK equalized signal either by tuning the filter coefficients or by selecting predefined filter profiles stored in a look-up table. 
         [0050]    In an embodiment depicted in  FIGS. 8 and 9 , the quality assessment module  21  comprises a pair of band pass filters  31  and  32  to measure the power spectral density of the signal at different frequencies, typically in first a frequency band  33  close to the carrier frequency f 0  and a second frequency band  34  located at 0.7×R from the carrier frequency f 0 . To assess the balance between low and high frequency spectral components, the quality assessment module  21  computes the power ratio in dB obtained from these two measurements and provides that data as the measurement signal  23  to the control module  22 . The control module  22  compares the computed ratio to a preset target value. The target value can be defined using a calibration process. In the example shown in  FIG. 7 , the power ratio of 5 dB between low and high frequency components is considered as a suitable target value. 
         [0051]    If the computed ratio is above the preset target, the control module  22  adjusts the filter  11 , e.g. the absolute value of the delay-tap coefficient a of the filter  11  is increased. 
         [0052]    In an embodiment, the quality assessment module  21  measures the opening of the eye diagram by computing a Time domain quality factor defined as: 
         [0000]        Q=Σ   1   3 ( I   k+1   −I   k )/(σ k+1 −σ k ).  (Eq. 3)
 
         [0053]    Where I k  stands for the mean value of the k-th level of the signal and σ k  stands for its standard deviation. 
         [0054]    In the above embodiment, equalization is performed in the electrical domain using the FFE  11 . However, the same equalization could be performed in the optical domain using an optical spectral equalizer, e.g. a device known as WaveShaper  0  available from Finisar Corp. USA. A corresponding embodiment is illustrated in  FIG. 10 , where the optical spectral equalizer  30  is arranged between the MZM  8  and the optical splitter  12 . A similar feedback loop  20  can be applied whatever the type of equalization chosen, either electrical as in  FIG. 3  or optical as in  FIG. 10 . In an embodiment, an amplifier is arranged between DAC  7  and optical modulator  8 . 
         [0055]    The above transmitters have been successfully tested in a short reach optical communication system with an optical link  17  of 1 km. However, the achievable range of such systems depends on a number of parameters as will be appreciated by those skilled in the art. Optical amplifiers and electrical amplifiers can be arranged at different points in the system to adjust the range. 
         [0056]    Elements such as the quality assessment and control modules could be e.g. hardware means like e.g. an ASIC, or a combination of hardware and software means, e.g. an ASIC and an FPGA, or at least one microprocessor and at least one memory with software modules located therein. 
         [0057]    The invention is not limited to the described embodiments. The appended claims are to be construed as embodying all modification and alternative constructions that may be occurred to one skilled in the art, which fairly fall within the basic teaching here, set forth. 
         [0058]    The use of the verb “to comprise” or “to include” and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. Furthermore, the use of the article “a” or “an” preceding an element or step does not exclude the presence of a plurality of such elements or steps. 
         [0059]    In the claims, any reference signs placed between parentheses shall not be construed as limiting the scope of the claims.