Abstract:
A method and system for optimization of an optical transmissions signal which is modulated with a binary data signal, wherein a control device ensures that the operating point and the modulation signal of a Mach-Zehnder modulator are set optimally, with the fundamental frequency and/or the first harmonic frequency of the transmission signal being selected for this purpose, and an optimum setting is reached when the amplitude of the fundamental frequency has reached a maximum value and the amplitude of the harmonic frequency has reached a minimum value.

Description:
BACKGROUND OF THE INVENTION 
     Mach-Zehnder modulators (MZI Mach-Zehnder interferometers) are used in optical transmission systems whose data rates are 10 Gbit/s or more since, at these high data rates and with the present-day state of the art, neither direct modulation of a laser nor modulation using electroabsorption modulators is expedient. Generally, in addition to the modulation signal, MZI modulators require a bias voltage for setting the operating point in order to achieve a balanced output signal and, hence, a balanced eye shape for the received signal. Any deviation of the operating point from this value leads to distortion in the optical transmission signal and, hence, to greater error rates and/or to a reduced range. Like all interferometer arrangements, which react extremely sensitively to a very small optical path length change, the operating point also varies with the environmental conditions, in most available modulators. 
     An MZI modulator is described, for example, in the “Designer&#39;s Guide to External Modulation”, UTP, 1289 Blue Hills Avenue, Bloomfield, Conn., pages 4-6. When the modulator is being fully driven, any change in the operating point leads to overdriving, as a result of which, after initially rising, the optical power falls once again, despite the control signal increasing, during the transition from blocking after switching on from “0” to “1”. In the event of overdriving, a transmitted “1 bit” peaks in the region of the rising 1 flank, resulting in a relatively high frequency structure in which some of the spectral power is contained in higher frequencies in particular; for example, at twice the fundamental frequency of the data signal, or at the data rate. 
     Patent specification U.S. Pat. No. 5,710,653 discloses a transmission system having a module for external modulation of a signal, in which harmonic frequencies of the modulated signal are suppressed. A first method uses two Mach-Zender interferometers, arranged in parallel, as modulators, to whose inputs the signal to be modulated is supplied with different amplitudes (80% and 20%), and whose two modulated output signals are combined such that a harmonic frequency from the second output signal is isolated from the fundamental frequency, is inverted and then amplified such that the harmonic frequencies in the resultant modulated signal compensate for one another by addition between the first modulated output signal and the processed isolated harmonic frequency of the second modulated output signal. In the second method, only one modulator is used in order to suppress second-order and third-order harmonic frequencies in the modulated signal. In this case, a distortion network is required to suppress the third-order harmonic frequency in addition to controlling the operating point of the modulator. These two methods suppress third-order harmonic frequencies. An additional control loop is provided for setting the operating point of the modulator or modulators in phase quadrature in order to eliminate the second-order harmonic frequency. 
     Patent Specification U.S. Pat. No. 5,629,792 discloses a further arrangement and method for modulation of a signal, with closed-loop control of the operating point of the modulator. The closed-loop control controls the operating point of the modulator by measuring the power level of the fundamental frequency of the modulated signal emitted by the modulator. This closed-loop process does not overcome, nor does it minimize, the influences of interference harmonic frequencies in the modulated signal. 
     An object of the present invention is, therefore, to specify a method which provides as simple a solution approach as possible for suppression of harmonic frequencies. Another aim is to specify a suitable system for doing the same. 
     Additional features and advantages of the present invention are described in, and will be apparent from, the following Detailed Description of the Invention and the Figures. 
     SUMMARY OF THE INVENTION 
     This object is achieved by using the signal at the harmonic frequency which, in each case, has been filtered out to derive a control signal which controls the operating point of the modulator, such that the at least one harmonic frequency reaches a minimum amplitude or power. 
     The advantage of this solution is that it requires considerably less complexity than the prior art for suppressing harmonic frequencies. 
     The method makes use of an effect, which occurs when the modulator is overdriven, for regulating the operating point. It is particularly advantageous in this case to combine operating point control with control of the modulation signal. All major parameters are kept constant by the control process. 
     It is also advantageous for the control criteria to be obtained at the receiving end, and to be transmitted via a service channel. This also makes it possible to partially compensate for distortion caused by the transmission path. 
     Additional features and advantages of the present invention are described in, and will be apparent from, the following Detailed Description of the Invention and the Figures. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 shows an outline circuit diagram of a system according to the present invention. 
     FIG. 2 shows a waveform of the associated control voltage. 
     FIG. 3 shows a variant of the system according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The system illustrated in FIG. 1 contains a Mach-Zehnder modulator (MZI)  4 , to which an optical signal OS is supplied from a laser  3 . The modulator  4  is modulated with a data signal DS, which is supplied from a data source, as a modulation signal U DAT , via a controllable amplifier  2 . The modulated optical transmission signal OSM is transmitted. A small portion of the signal is supplied via an optical splitter  5  to an optoelectrical transducer  6  with a downstream amplifier  7 , and is demodulated. The electrical data signal DS 1  recovered in this way essentially contains the modulation signal U DAT  or data signal DS. The data signal DS 1  is supplied via an amplifier  7  to two filters, the bandpass filters  8  and  10 . The first bandpass filter  8  filters the fundamental frequency GW out of the data signal DS 1 ; that is to say, its pass frequency is at half the bit rate. A low-pass-filtered 01 bit sequence essentially results in a sinusoidal voltage at a frequency corresponding to half the data rate (a derived data signal also may be used instead of the NRZ data signal). The output voltage from the first bandpass filter  8  is supplied directly or via a measurement transducer  9 , such as a rectifier or a power measurement device, as a control signal U R1  to a control device  12 . In the exemplary embodiment, a second bandpass filter  10  is provided and tuned to a harmonic frequency OW, preferably the first. Its output voltage is also supplied directly or via a second measurement transducer  11  as a further control signal U R2  to the control device  12 . Additional bandpass filters also may be provided for filtering out further harmonic frequencies, and their output voltages can be combined. The control device produces, via a regulator  17 , a control signal U BIAS , which governs the operating point of the modulator  4 . 
     As already mentioned, the first bandpass filter BP 1  filters out the fundamental frequency GW. Deviations from the operating point or overdriving caused by an excessively large modulation signal lead to a reduction in the amplitude of the fundamental frequency (sinusoidal signal), since the harmonics which then occur result in the fundamental frequency spectral component decreasing. A corresponding situation applies to the control signal U R1  obtained from the sinusoidal signal. An opposite situation applies to the wave form for the harmonic frequencies. Their amplitudes and, thus, the amplitudes of the control signals U R2 , . . . increase when overdriving occurs. 
     The diagram illustrated in FIG. 2 shows the relationship between the power levels P of the fundamental frequency GW and of the first harmonic frequency OW, and the operating point of the modulator. The power (amplitude) of the fundamental frequency has a clear optimum in the region of the ideal operating point. Here, the amplitude of the harmonic frequency (of the harmonic frequencies) is at a minimum which is more strongly dependent on the operating point and which is particularly highly suitable for optimization of the operating point. It is particularly advantageous to combine the two control signals U R1  and U R2  for operating point control, such as by addition, with one of these signals being inverted, since this leads to the control characteristic having a steeper profile. FIG. 2 shows the main values for the operating point control signal (bias voltage). The optimum occurs at about 4.8 V. 
     A control device can at the same time be used to convert the data signal DS emitted from the data source to an optimum modulation signal U DAT  by using a further control signal U MOD  to control the amplifier  2 , this optimum modulation signal U DAT  being that which leads to a transmission signal with the maximum amplitude and the minimum harmonic content (maximum modulation level). It is sufficient to use the fundamental frequency for control purposes in order to maximize the modulation signal U DAT . 
     During a control process, the control device can be used to produce changes in the control signals U BIAS  and U MOD  in both directions on a trial basis in order to reach the respective optimum setting. The operating point U BIAS  (control signal/bias voltage) and the modulation signal U DAT  may, for example, be adjusted alternately. The frequency of the operating point and/or the amplitude of the modulation signal likewise may be swept (periodic variation by a small amount via frequency-sweep voltages U W1 , U W2 ) in order to determine the mathematical sign of any control error, with the control process being carried out based on the lock-in principle. 
     It is, of course, also possible to control the amplitude of the laser signal OS and, hence, the amplitude of the transmission signal OSM. 
     FIG. 3 shows a variant in which the control signals U R1 , U R2  are derived from the received signal OEM at the end of a transmission path (at the receiving end). The control process also can, in this way, take account of the line characteristics. The demodulation of the received signal OEM is carried out in a receiving device  20 . The demodulated data signal DS 1  is once again evaluated via filters  8 ,  10  and is converted by the measurement transducers  9 ,  11  to control signals U R1 , U R2  which, after inversion of one control signal via an inverting amplifier  13 , are combined in an adder  16  to form a resultant control signal U R . 
     The control device  12  may be arranged at the receiving end or at the transmitting end. In this exemplary embodiment, the combined control signal U R  is transmitted via a service channel  22  to the control device  12  arranged at the transmitting end, in order to optimize the operating point and/or the amplitude of the modulation signal. 
     Although the present invention has been described with reference to specific embodiments, those of skill in the art will recognize that changes may be made thereto without departing from the spirit and scope of the present invention as set forth in the hereafter appended claims.