Abstract:
A method and drive unit for controlling a modulator ( 128 ) in which the working point of the modulator ( 128 ) is regulated using a regulating circuit ( 124 ) in such a way that the working point is stable in relation to the transmission characteristic curve of the modulator ( 128 ) for a long time and under different operating conditions.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a method for regulating the working point of a modulator. The modulator generates a modulated output radiation, for example in the visual range, from an input radiation as a function of a control signal. 
     Stable pulse sources are required to generate pulses in optical telecommunication transmission networks. A simple and cost-effective method for generating pulses from what is referred to as a continuous-wave source using high-speed optical modulators is described in German patent document no 199 24 347.6. However, the long-term stability of the pulse source is a problem with this method. In order to avoid shifting of the working point, stable modulators, in which long-term stability is achieved by means of costly structural measures, have been used at low data rates. The same problems occur with data modulators. 
     SUMMARY OF THE INVENTION 
     The present invention pertains to a simple method for regulating the working point of a modulator which ensures a stable working point of the modulator. In addition, the present invention pertains to an associated drive unit. 
     The invention is based on the fact that the working point is an essential operating parameter of the modulator. If the working point changes, the pulses generated by the modulator also change. The working point can be set very precisely when a modulator is manufactured, but it then drifts as a function of various causes. Such causes are, for example, aging of the modulator over the years or an operating temperature which changes within minutes while the modulator is operating, for example directly after switching-on. 
     The invention is also based on the fact that the working point can be easily set with respect to the transmission characteristic curve of the modulator by means of the average value of the control signal or using an auxiliary signal which ultimately influences the average value of the control signal. Furthermore, the invention is based on the idea that a deviation of the actual working point from a predefined setpoint working point results in a change in the output radiation. 
     In a method according to the invention, the average radiant power is sensed from the output radiation in at least one predefined frequency range. The average radiant power is the radiant power averaged over the frequencies. Furthermore, a periodic deflection of the working point in accordance with a working point deflection frequency is forcibly brought about. A regulating signal is generated as a function of the deflection of the working point. The average value of the control signal and/or the signal value of the auxiliary signal are changed as a function of the regulating signal in such a way that the deviation between the actual working point and the setpoint working point becomes smaller. 
     As a result of this procedure, both short-term deviations of the actual working point from the setpoint working point and long-term deviations due to a change in the transmission characteristic curve of the modulator can easily be compensated for. The average radiant power is used as a regulated variable. The voltage or the current of the control signal is used as the manipulated variable. 
     As a result of the reference to a prominent point, the regulation can also be carried out without predefining a setpoint power. For example, a minimum value, a maximum value, an inflection or another point at which a derivative has the value zero can be selected in the power curve as the reference point. 
     The methods known from regulating technology are used as regulating methods, for example, a proportional, a proportional-integral or a proportional-integral-differential regulating method. The power sensed can, if appropriate, be used directly as a regulated variable. However, very good control circuits are obtained if the regulated variable is sensed using phase-sensitive detection, which is also known as a lock-in method. Phase-sensitive detection has the advantage that the regulation can be carried out comparatively independently of interference variables, for example of signal noise. Phase-sensitive detection is explained, for example, in the book “Electronic Measurement and Instrumentation”, Klaas B. Klaassen, Cambridge University Press, 1996, pages 204 to 210. 
     In one embodiment, a derivative of the function of the working point and sensed power is used as the regulated variable. During the regulation operation, it is then possible to make reference to a point of the function at which the selected derivative has the value zero. Making reference here means that regulation is performed to the regulating point without detuning the control circuit. 
     The modulator is either a pulse modulator which is driven with a periodic control signal with a predetermined driving frequency, or a data modulator which is driven with a control signal which is dependent on the data to be transmitted, half the data rate being referred to as the driving frequency. 
     In an embodiment, the predefined frequency range contains all the frequencies of the frequencies of the output radiation which can be sensed by a transducer unit. For example a photodiode or a phototransistor is used as the transducer unit. The frequencies which can be sensed by the transducer unit are determined by its design. In addition to the transducer unit, no filters for filtering out specific frequency ranges are necessary in this embodiment. The predefined frequency range can have a very broad band, for example from 0 Hz to the gigahertz range. However, it is also possible to use transistor units which operate with a comparatively narrow band, sensing, for example, only frequencies from 0 Hz to the kilohertz range. Narrow-band transducer units can be manufactured more easily in comparison to broadband transducer units and can therefore be obtained more cost-effectively. 
     In another embodiment, the predefined frequency range contains only a portion of the frequencies of the output radiation which can be sensed by a transducer unit. This portion is determined by the design of a filter unit connected downstream of the transducer unit. The filter unit is, for example, a low-pass filter, a bandpass filter or a high-pass filter. In this development, changes which occur in the spectrum as a function of the working point are used. As a result of the selection of one or more suitable frequency ranges it is possible to obtain very large signal differences between the power in the setpoint working point and the power when there are deviations from the setpoint working point. 
     In one refinement of the method with a filter unit, the predefined frequency range includes a frequency which corresponds to the driving frequency. Twice the driving frequency and multiples of twice the driving frequency are not contained in the frequency range. The refinement is based on the fact that when there are deviations from the setpoint working point a considerable power increase occurs in the vicinity of the driving frequency in the power density spectrum. The power which can be sensed in the vicinity of the driving frequency is dependent on the magnitude of the deviation between the setpoint working point and actual working point. 
     If, in a further refinement of the method with a filter unit, in particular with a pulse modulator, the setpoint working point lies at a transmission maximum value—what is referred to as RZ (return to zero) mode—or at a transmission minimum value—what is referred to as carrier suppressed RZ mode—the average value of the control signal and/or the signal value of the auxiliary signal is regulated using a control circuit, which is adjusted, without detuning, to a regulating point at which the average power within the predefined frequency range is at a minimum. 
     In another embodiment, the predefined frequency range contains only frequencies which lie far below the driving frequency, i.e. are low frequency in comparison to the driving frequency. For example, the frequencies are smaller than a tenth of the driving frequency. The signals to be processed thus have lower frequencies. Components are used which are configured for limiting frequencies which lie far below the driving frequency. If the driving frequency lies, for example, in the gigahertz range, components for the kilohertz range are suitable for processing because these components still sense the average power required for the regulation. Circuits required for the method can therefore be constructed cost-effectively without high-frequency components. 
     If the setpoint working point is at a transmission minimum value in a refinement with a low frequency range—in particular in the case of a pulse modulator—the average value of the control signal and/or the signal value of the auxiliary signal is regulated using a control circuit which is adjusted to a regulating point at which the average power within the predefined frequency range is at a maximum. 
     If, on the other hand, the setpoint working point is at a transmission maximum value in an alternative refinement with a low frequency range—in particular in the case of a pulse modulator—the average value of the control signal and/or the signal value of the auxiliary signal is regulated using a control circuit which is adjusted to a regulating point at which the average power within the predefined frequency range is at a minimum. 
     If the setpoint working point lies between a transmission maximum value and a transmission minimum value of the transmission characteristic curve (what is referred to as clock RZ mode) in a further alternative refinement with a low frequency range—in particular in the case of a pulse modulator—a regulating point at which the average power is at a minimum value or at a maximum value is selected. 
     If the setpoint working point lies between a transmission maximum value and a transmission minimum value, preferably at an inflection, in a further alternative refinement with a low frequency range in the case of a data modulator, the average value of the control signal and/or the signal value of the auxiliary signal is regulated using a control circuit which is adjusted to a regulating point at which the function of the average power and of the working point has an inflection. 
     In another embodiment, the control circuit for regulating the working point is not detuned, so that the control circuit is regulated to the regulating point at the setpoint working point of the modulator. In what is referred to as the clock RZ mode, the control circuit is detuned. 
     A regulated variable with a correct sign can be easily acquired in an embodiment if a small deviation of the working point is forcibly brought about for regulating purposes. The power is then sensed at least two different working points. Phase-sensitive detection, for example, is based on forcibly bringing about such small deviations of the working point in such a way, said detection also being referred to as a lock-in method, see, for example, Klaas B. Klaassen, “Electronic Measurement and Instrumentation”, Cambridge University Press, 1996, pages 204 to 210. 
     In one refinement, for regulating purposes the deviation of the working point is forcibly brought about using a periodic deflection signal with a predefined deflection frequency. The deflection signal is preferably added to the control signal. A signal which is dependent on the sensed power is multiplied by a periodic reference signal whose frequency corresponds to the deflection frequency. A signal which results from the multiplication is used, after low-pass filtering and preferably after subsequent integration, to change the average value of the control signal and/or to change the signal value of the auxiliary signal. The limiting frequency of the low-pass filter determines the response time of the control circuit, which is, for example, between 10 milliseconds and 100 milliseconds. As a result of this method, the derivative of the power curve is ultimately used as a regulated variable. Depending on the phase of the deflection signal (π/2 or 3 π/2), the power can be regulated to a maximum value or a minimum value. The deflection signal has a cosine-shaped or sine-shaped profile. However, other deflection signals are also used, for example, signals with a square-wave profile. If the reference signal has a frequency which corresponds to a multiple of the deflection frequency, points can be detected at which higher derivatives are zero, for example an inflection at twice the deflection frequency. 
     At the same time as the working point, it is also possible to regulate the working range in a similar way. The deflection frequency for regulating the working point and the deflection frequency for regulating the working range are selected in such a way that the control circuits operate independently of one another. Thus, deflection frequencies which are different from one another are used, for example a deflection frequency of 3 kHz and a deflection frequency of 5 kHz. 
     The input radiation is generated in a pulse modulator or a data modulator using a continuous-wave light source or a pulsed radiation source. A pulse modulator forms a pulse light source, for example. 
     The driving frequency of the modulator is more than 1 gigahertz, preferably 5 gigahertz or 20 gigahertz, in some embodiments. In another embodiment, the modulator operates in the visual range. For example, the modulator contains a Mach-Zehnder interferometer. The transmission characteristic carve of the modulator is, for example, cosine-shaped or sine-shaped. However, modulators with other transmission characteristic curves are also used. 
     The invention also relates to a drive unit for carrying out the above mentioned methods. The technical effects which have been mentioned for the methods also apply to the drive unit and its embodiments. 
     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 a transmission characteristic curve of a pulse modulator with input radiation which is constant over time, and the profile of a control signal. 
     FIG. 2 shows the power density spectrum of the output radiation of the pulse modulator given optimum working parameters. 
     FIG. 3 shows the power density spectrum of the output radiation of the pulse modulator given a deviation of the actual working point by 10 percent and the setpoint working range. 
     FIG. 4 shows the average HF radiant power of the pulse modulator in a predefined frequency range as a function of the working point deviation between the actual working point and the setpoint working point. 
     FIG. 5 shows a block circuit diagram for a drive unit of the pulse modulator containing HF components. 
     FIG. 6 shows the average LF radiant power of the pulse modulator as a function of the working point deviation. 
     FIG. 7 shows a block circuit diagram for a drive unit, operating at low frequency, of a pulse modulator according to a second exemplary embodiment, and 
     FIG. 8 shows a block circuit diagram of a drive unit, operating at low frequency, of a data modulator. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows in its upper part a transmission characteristic curve  10  of a modulator, for example, a pulse modulator, with input radiation which is constant over time. The driving current is plotted in volts on an abscissa axis  12 . An ordinate axis  14  shows transmission values T. The transmission characteristic curve  10  has a cosine-like profile starting at the voltage 0 volts. The transmission drops from a maximum value 1 to virtually 0 at a voltage U 1 . A working point AP 2  lying at a transmission minimum value is associated with the voltage U 1 . As the voltage increases, the transmission T increases until at a voltage U 2  a working point AP 1  is reached which lies at a maximum value of the transmission characteristic curve  10 . If the voltage is increased further, the transmission drops, and a minimum value is reached again at a voltage U 3 . 
     The working point AP 1  at the transmission maximum value is also referred to as the RZ (return to zero) working point. In the RZ mode, the working point AP 1  should always lie at the transmission maximum. If the transmission characteristic curve  10  of the pulse modulator changes, it is necessary to readjust the working point AP 1  by changing the voltage U 2 . As aging of the pulse modulator occurs, the transmission characteristic curve  10  is compressed, stretched or displaced in the direction of the abscissa axis  12  and/or in the direction of the ordinate axis  14 . If the pulse modulator is operated at the working point AP 1 , an optimum working range AB 1  lies precisely between the voltages U 1  and U 3 . 
     However, the modulator can also be operated at the working point AP 2  at which the driving voltage fluctuates about the voltage U 1 . This mode of operation is referred to as operation with suppressed carrier because no spectral line occurs at the carrier frequency, i.e. at the frequency of the input radiation, in the optical frequency spectrum of the output radiation. The optimum working range at the working point AP 2  lies between the voltage 0 volts and the voltage U 2 . 
     However, the pulse modulator can also be operated at a working point AP 3  which lies between the two working points AP 1  and AP 2 . In the exemplary embodiment, the working point AP 3  lies below the inflection of the transmission characteristic curve  10  in the vicinity of the working point AP 2 . The optimum working range for the working point AP 3  lies symmetrically around this working point AP 3  between the voltage U 1  and a lower voltage than the voltage U 2 . 
     In the lower part of FIG. 1, the voltage profile of a control signal  20  is illustrated as a function of the time t plotted on the abscissa axis  22 . An ordinate axis  24  is used to represent the voltage values U in volts. 
     The control signal  20  is used to drive the pulse modulator at the working point AP 1 . At a time t 0 , the control signal  20  has the voltage U 1  so that the modulator only transmits a minimum output radiation. At a later time t 1 , the control signal has the voltage U 2 . This means that the modulator transmits the input radiation almost unimpeded. A light pulse appears at the output of the modulator. At a time t 2 , the control signal has the voltage U 3  so that the modulator operates at a transmission minimum value again and essentially transmits no light. At a time t 3 , the control signal has the voltage value U 2  again, with the result that a second light pulse is generated. At a later time t 4 , the control signal  20  has the voltage value U 1  again, so that no radiation passes to the output of the modulator. During a period of the control signal  20 , two light pulses are therefore emitted. 
     The average value of the sine-shaped control signal  20  determines the working point, see voltage U 2 . The amplitude of the control signal  20  determines the working range AB 1 , see the difference between the voltages U 3  and U 1 . 
     FIG. 2 shows the power density spectrum  50  of the output radiation of the pulse modulator at the optimum working point AP 1  and optimum working range AB 1 , see the upper part of FIG.  1 . The frequency related to the data rate is plotted on an abscissa axis  52 . In the exemplary embodiment, the data rate  10  is in gigahertz and the control signal has a driving frequency of 5 gigahertz. The signal power sensed using a photodiode and a spectrum analyzer is plotted on a logarithmic scale on an ordinate axis  54 . The signal power has been standardized. 
     The power density spectrum  50  shows four power peaks  56  to  62  at the frequency/data rate values 1, 2, 3 and 4. The peak values of the power peaks  56 ,  58 ,  60  and  62  lie at approximately 0.6; 0.08; 0.0007 and at 8×10 −6  in this sequence. 
     FIG. 3 shows the power density spectrum  70  of the output radiation of the pulse modulator given a deviation of the actual working point from the setpoint working point AP 1  of 10 percent. The working range AB is correspondingly displaced, but has an unchanged width. An abscissa axis  72  shows the ratio of the frequency to the data rate, in the same way as the abscissa axis  52  of FIG.  2 . An ordinate axis  74  shows the standardized radiant power in a logarithmic representation. Power peaks  76  to  82  lie at the same frequencies as the power peaks  56  to  62  in this sequence, see FIG.  2 . The peak values of the power peaks  76 ,  78  and  80  lie considerably below the peak values of the power peaks  56 ,  78  and  80 . Given deviations from the optimum working point, further power peaks  84 ,  86 ,  88  and  90  occur at the driving frequency and between the power peaks  76 ,  78 ,  80  and  82 . The power peak  84  has a peak value of approximately 0.03 which lies below the peak value 0.25 of the power peak  76 . The peak values of the power peaks  86 ,  88  and  90  each lie approximately between the peak values of the respective adjacent power peaks  76 ,  78 ,  80  and  82 . 
     The change in the power density spectrum  50  to the power density spectrum  70 , apparent from FIGS. 2 and 3, given deviations from the working point, can be used to regulate the working point of the pulse modulator. In particular the occurrence of the power peak  84  at the driving frequency which is 5 gigahertz in the exemplary embodiment is used to regulate the working point. The objective is to minimize the peak value of this power peak. A frequency range FB 1  surrounding the power peak  84  has a width of approximately 0.3×f, f being the driving frequency. The frequency range FB 1  is centered around the driving frequency f. During the regulation of the working point using HF components (high frequency), only the power peak  84  is used within the frequency range FB 1 . 
     The power density spectrum of the output radiation is also used to regulate the working range AB. FIG. 3 illustrates a frequency range FB 2  which surrounds the power peak  76  and which contains the frequencies used to regulate the working range. The frequency range FB 2  has a width of approximately 0.3×2f, f being the driving frequency. When deviations of the working range from the setpoint working range occur, the peak value of the power peak  76  lying in the frequency range FB 3  drops. 
     FIG. 4 shows the average HF radiant power in the frequency range FB 1  as a function of the working point deviation between the setpoint working point AP 1  and the actual working point, see FIG.  1 . The relationship is illustrated by a power function  100  with parabola-like profile in accordance with a function f(APD), APD being the working point deviation. The working point deviation is plotted as a percentage on an abscissa axis  102 . An ordinate axis  104  discloses values for the radiant power. The radiant power has been standardized to a power which occurs at approximately −4 percent or +4 percent deviation of the actual working point from the setpoint working point. If the actual working point and the setpoint working point correspond, the signal power has the minimum value, see measuring point MP 1 . As the deviation increases, the signal power increases continuously in both deviation directions. The objective of the regulation of the working point is therefore to minimize the radiant power within the frequency range FB 1 , i.e. to set it to the minimum value. The derivation of the power function  100  is suitable as the regulated variable. A regulating circuit which utilizes the relationships explained with reference to FIG. 4 is explained in more detail below with reference to FIG.  5 . 
     FIG. 5 shows a block circuit diagram for a drive unit  120  containing HF components, a photodiode  122 , a working point regulating circuit  124  and a working range regulating circuit  126 . The drive unit  120  is used to drive a pulse modulator  128  which contains what is referred to as a Mach-Zehnder interferometer (MZI) and has the transmission characteristic curve  10 , see FIG.  1 . The modulator  128  modulates the radiation generated by a laser diode  134 , as a function of a working point regulating voltage  130 , generated by the working point regulating circuit  124 , and as a function of a working range regulating voltage  132  generated by the working range regulating circuit  126 . The laser diode  134  operates in the continuous-wave mode (cw—continuous wave), so that the input radiation of the modulator  128  has a constant radiant power. 
     The modulated output radiation is divided with a ratio of 1:10 at a radiation splitter  136 . The greater part of the modulated radiation is fed to a data modulator (not illustrated) which modulates the output radiation in accordance with data to be transmitted, see arrow  138 . The smaller part of the output radiation is transmitted to the photodiode  122  by the radiation splitter  136  using an optical waveguide  140 . 
     The photodiode  120  has a limiting frequency lying in the gigahertz range and is thus a high-frequency diode. The current flowing across the photodiode  122  depends on the radiation striking the photodiode  122 . The current signal which is dependent on the radiation or a voltage signal acquired therefrom is used as the input signal for the working point regulating circuit  124  and as the input signal for the working range regulating circuit  126 , see arrows  142  and  144 . 
     The working point regulating circuit  124  contains a high-frequency bandpass filter  146  at whose input there is the signal coming from the photodiode  122 . The bandpass filter  146  transmits essentially only signals with frequencies which lie within the frequency range FB 1 . Signals with frequencies which lie outside the frequency range FB 1  are heavily damped. A high-frequency power meter  148  is connected downstream of the bandpass filter  146 . The power meter  148  contains a rectifier diode with a limiting frequency lying in the high-frequency range. A signal whose value depends on the radiant power within the frequency range FB 1  is output at the output of the power meter  148 . This signal is multiplied, in a multiplication unit  150 , by a reference signal which is generated by a signal generator  152 . The following applies to the voltage u r (t) of the reference signal: 
     
       
           u   r ( t )=   u     r ·cos(ω t+φ 1)  (1), 
       
     
     u r (t) being the instantaneous value of the voltage of the reference signal as a function of the time t,  u   r  being the maximum value of the voltage of the reference signal, ω being a reference angular frequency and φ1 being an adjustable phase. The reference angular frequency ω is 2π times the deflection frequency for the working point. 
     The voltage u i  of the input signal for the multiplication unit  150  which is output by the power meter  148  can be described by the following formula: 
     
       
           u   i ( t )= f ( x   0   + u0 ·cos (ω· t ))  (2), 
       
     
     u i  designating the instantaneous value of the input signal as a function of the time, x 0  designating the working point,  u 0    designating the maximum value of the forced deflection around the actual working point, ω designating the reference frequency and f( . . . ) designating the function illustrated in FIG.  4 . 
     The multiplication unit  150  generates an output signal which, in addition to portions with multiples of the reference frequency ω, also contains a DC element. The DC element is a measure of the derivative of the function illustrated in FIG.  4  and is filtered out using a low-pass filter  154  and transmitted to an integrator unit  156 . Signal elements with the reference frequency ω and signal elements with a frequency which corresponds to a multiple of the reference frequency ω are heavily damped by the low-pass filter  154  and thus do not arrive at the integrator unit  156 . The integrator unit  156  integrates the signal present at its input over time and thus supplies the integrator element for regulation. At the output end, the integrator unit  156  is connected to one of the inputs of a summing element  158 . The other input of the summing element  158  is connected to an output of the signal generator  152  at which a deflection signal is present, the value of which deflection signal changes in accordance with a cosine function with the reference frequency ω. The output of the summing element  158  also forms the output of the working point regulating circuit  124 . 
     By setting the phase φ1 it is possible to cause the working point regulating circuit  124  to drive the pulse modulator  128  in such a way that the radiant power within the filter range FB 1  is minimized, and the actual working point is thus regulated to the setpoint working point at the transmission maximum value. The reference frequency ω is suitably selected and lies, for example, in the kilohertz range. 
     The working range regulating circuit  126  is essentially of the same design, and thus also has the same function, as the working point regulating circuit  124 . The working range regulating circuit  126  thus contains, in the sequence from the input to the output, a bandpass filter  160 , a power meter  162 , a multiplication unit  164 , a low-pass filter  166 , an integrator unit  168  and a summing element  170 . The bandpass filter  160  transmits only signals with a frequency lying within the frequency range FB 2 . Furthermore, the working range regulating circuit  126  contains a signal generator  172  which generates a reference signal which changes in accordance with the function cos(ηt+φ2), η being a reference angular frequency which differs from the reference frequency ω. The reference angular frequency η is 2π times the deflection frequency for the working range. φ2 is an adjustable phase of the signal. The signal generator also generates a further deflection signal which changes in accordance with the function cos νt. This deflection signal is applied to the other input of the summing element  170 . 
     The output of the summing element  170  is simultaneously the output of the working range regulating circuit  126  which generates the working range regulating voltage  132 . This regulating voltage  132  is used to adjust the gain of an amplifier  174 . An input signal  176  with a sine-shaped profile and the driving frequency is present at the input of the amplifier  174 . An output signal  178  of the amplifier  174  corresponds, even as far as the average value, with the control signal  20  and is used to drive the pulse modulator  128 . 
     Furthermore, the amplifier  174  has an output (not illustrated) at which a voltage which is proportional to the output power of the amplifier is output. This voltage is used to detune the working range regulating circuit  126  by subtracting the voltage from the signal within the control loop using a subtractor element (not illustrated) between the low-pass filter  166  and integrator element  168 , see also FIG.  7 . 
     By suitably selecting the phase φ2, it is possible to ensure that the working range regulating circuit regulates the radiant power within the frequency range FB 2  to a maximum value. At the same time, the amplitude of the control signal  20  is thus regulated in such a way that the actual working range corresponds to the setpoint working range AB 1 . 
     By suitably selecting the reference frequencies ω and ν it is possible to ensure that the regulation of the working point operates independently of the regulation of the working range, and on the other hand that the regulation of the working range also operates independently of the regulation of the working point. Suitable values are, for example, 3 kHz and 5 kHz for the reference frequency ω and the reference frequency ν, respectively. 
     FIG. 6 shows the average LF radiant power as a function of the working point position which is illustrated on an abscissa axis  190 . The variable Vπ is used as a unit, i.e. the voltage which is required to displace the working point in the radiant direction on the transmission characteristic curve  10  by 180° or π. An ordinate axis  192  shows the standardized radiant power in the low frequency range which is emitted by the pulse modulator. 
     A continuous RZ characteristic curve  194  applies to a working point lying at the transmission maximum value of the characteristic curve  10 , for example to the working point AP 1 . When the actual working point and the setpoint working point correspond, the characteristic curve  194  has a minimum value which is suitable for regulating the working point as a regulating point. 
     A dashed CSRZ (Carrier Suppressed Return to Zero) characteristic curve  196  applies to a setpoint working point at the transmission minimum value of the characteristic curve  10 , for example to the working point AP 2 , see FIG.  1 . When the actual working point and the setpoint working point correspond, the characteristic curve  196  has a transmission maximum value which is suitable for regulating the working point. 
     A characteristic curve  198  which is represented by a dotted line applies to the operation of the pulse modulator at a working point which lies between a transmission maximum value and a transmission minimum value, for example to the working point AP 3 , see FIG.  1 . This operation is also referred to as clock RZ mode. The characteristic curve  198  has a transmission minimum value which is suitable for regulating the working point in the clock RZ mode. However, the associated control circuit is to be detuned in such a way that the setpoint working point is regulated. At the setpoint working point, the average radiant power is then near to the minimum value, see measuring point MP 4 . 
     Three circuits which are suitable for regulating the working point, in each case, one of the three operating modes of the modulator, are described below with reference to FIG.  7 . 
     FIG. 7 shows a block circuit diagram for a low-frequency drive unit  220  of a pulse modulator  128   b , which also has the transmission characteristic curve  10 , see FIG.  1 . The essential difference between the drive unit  220  and the drive unit  120 , see FIG. 6, is that the drive unit  220  does not contain any high-frequency components. Otherwise, the design of the drive units  120  and  220  is the same so that circuit modules with the same design and same function are designated by the same reference symbols, but a “b” is placed after them in order to distinguish them. This applies in particular to reference symbols  124   b  to  140   b . Instead of the HF photodiode  122 , a photodiode  222  which has a limiting frequency lying in the LF range, for example a limiting frequency of 10 kHz, is used in the drive unit  220 . The deflection frequency lies within the bandwidth of the photodiode  222 . The current flowing through the LF photodiode  222  changes as a function of the output radiation impinging on the photodiode  222 . In this case, only the low-frequency elements of the output radiation result in a change in the diode current. The photodiode  222  also averages over frequencies which lie above the frequency range sensed by it. The diode current or a voltage derived therefrom is used as input variable for the working range regulating circuit  124   b  and as input variable for the working range regulating circuit  126   b , see arrows  224  and  226 . 
     The working point regulating circuit  124   b  contains, from its input to its output, a multiplication unit  150   b , a low-pass filter  154   b , an integrator unit  156   b  and a summing element  158   b . In addition, the regulating circuit  124  contains a signal generator  152   b  which in turn generates a reference signal and a deflection signal. The reference signal changes in accordance with the function cos(ωt+φ3). The deflection signal changes in accordance with the function cos ωt. 
     The working range regulating circuit  126   b  contains, from the input to the output, a multiplication unit  164   b , a low-pass filter  166   b , a subtractor element  228 , an integrator  168   b  and an adder element  170   b . In addition, the regulating circuit  126   b  contains a signal generator  172   b  which generates a reference signal and a deflection signal. The reference signal changes in accordance with the function cos(νt+φ4). The deflection signal changes in accordance with the function cos νt. The output of the working range regulating circuit  126   b  is connected to the control input of an amplifier  174   b . An input signal  176   b  which has a sine-shaped profile with the driving frequency is present at the amplifier  174   b . An output signal  178   b  is used to drive the modulator  128   b  and corresponds to the control signal  20  as far as the average value, see FIG.  1 . In addition, the amplifier  174   b  generates a detuning signal  230  whose signal value changes as a function of the average output power of the amplifier  174   b . The detuning signal  230  is present at a further input of the subtractor element  228  and is used to detune the regulating circuit  126   b  for regulating the working range. 
     If the modulator  128   b  is operated at the working point AP 1 , see FIG. 1, the phase φ3 is set in such a way that the working point regulating circuit  124   b  regulates the emitted power to a minimum value, see FIG. 6, characteristic curve  194 . This results in the working point being regulated to the transmission maximum value. The phase φ4 is selected in such a way that, due to the detuning of the control circuit, the working range regulating circuit  126   b  regulates the power of the output radiation in the low-frequency limiting range to a point lying near to a regulating point with a minimum transmission. As a result of this, the amplitude of the control signal is regulated to the value 2Vπ. 
     The deflection frequencies ω and ν are different from one another, for example 3 kHz and 5 kHz. 
     In a further exemplary embodiment, the modulator  128   b  is operated at the working point AP 2 , i.e. at the transmission minimum. The phase φ3 of the working point regulating circuit  124   b  is set in such a way that the output radiation transmitted in the low-frequency range by the modulator  128   b  assumes a maximum value, see FIG. 6, characteristic curve  196 . The phase φ4 is selected in such a way that the output radiation of the modulator  128  is maximized but lies somewhat outside the maximum value owing to the detuning when the control circuit is adjusted. 
     If, in a further exemplary embodiment, the modulator  128   b  is operated at the working point AP 3 , see FIG. 1, the phase φ3 is set in such a way that the working point regulating circuit  124   b  minimizes the power in the low-frequency range of the output radiation of the modulator  128   b  and is regulated to a value lying quite close to the minimum value, due to the detuning, see FIG. 6, characteristic curve  198 . As a result, the working point is also regulated to the setpoint working point AP 3 . The phase φ4 is selected in the operating mode of the working point AP 3  in such a way that the working range regulating circuit  126   b  also minimizes the average power of the output radiation of the modulator  128   b  in the low-frequency range. However, the detuning of the regulating circuit  126   b  causes the working range to be regulated to a significantly lower value than 2Vπ. 
     FIG. 8 shows a block circuit diagram of a low-frequency drive unit  250  of a data modulator  128   c , which also has the transmission characteristic curve  10 , see FIG.  1 . The setpoint working point of the data modulator  128   c  lies at an inflection of the transmission characteristic curve, see working point AP 4  in FIG.  1 . The working range is Vπ and lies symmetrically around the working point AP 4 . A further difference between the drive unit  250  and the drive unit  220 , see FIG. 7, is that the drive unit  250  only regulates the working point. The working range is not regulated because it remains virtually unchanged over the years, or because deviations of the working range are not so serious for the data modulator  128   c . However, a voltage value U 5  can be changed in order to set the working range. Furthermore, instead of the input signal  176   b , an input signal  252  which is dependent on data is used. The maximum data rate is 10 GHz. Otherwise, the drive units  250  and  220  are of identical design so that circuit modules with the same design and same function are designated by the same reference symbols, but, instead of the “b” a “c” is placed after them in order to distinguish them. This applies in particular to reference symbols  124   c ,  128   c ,  130   c ,  134   c  to  140   c ,  150   c ,  154   c  to  158   c ,  174   c  and  178   c . A photodiode  222   c  is of the same design as the photodiode  222 , and an arrow  224   c  corresponds to the arrow  224 . 
     Instead of the signal generator  152   b , a signal generator  254  is used which generates a cosine-shaped deflection signal with a deflection frequency f 1 . For the multiplication in the multiplication unit  150   c , the signal generator  254  generates a reference signal with twice the deflection frequency f 1 . This measure results at the output of the low-pass filter  154   c  in a DC element which corresponds to twice the derivative of the power function. The DC element is used to regulate with respect to the inflection. The phase of the signal generated by the signal generator  254  is to be set in such a way that it is regulated to the inflection. 
     It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present invention and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims.