Abstract:
Method for controlling the operating range of a modulator, and an associated drive unit, the operating range of the modulator being controlled via a control circuit such that the operating range is stable over a long period of time and in different operating conditions, relative to the transmission characteristic of the modulator.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method for controlling the operating range of a pulse modulator. The modulator produces modulated output radiation, for example, in the optical range, from input radiation as a function of a control signal. 
     Stable pulse sources are required to produce pulses in optical message transmission networks. One simple and cost-effective method for producing pulses from a continuous-wave source via high-speed optical modulators is described in DE 199 24 347.6. However, the long term stability of the pulse source is problematic in this method. In order to avoid operating range changes, inherently stable modulators have, until now, been used for low data rates, in which the long term stability is achieved by complex design measures. The same problems also occur in data modulators. 
     One object of the invention is, therefore, to provide a simple method for controlling the operating range of a modulator, which ensures that the operating range of the modulator is fixed. Furthermore, an associated drive unit is intended to be specified. 
     SUMMARY OF THE INVENTION 
     The present invention is based on the knowledge that the operating range of the modulator close to the operating point is a major operating parameter. The operating range varies for various reasons, for example, as a function of the operating temperature, or due to aging of the modulator. Furthermore, the operating range is subject to scatter due to production tolerances between different modulators. 
     Preferably, according to the present invention, the modulator produces modulated output radiation from input radiation as a function of a control signal. The actual operating range of the modulator with respect to its transmission characteristic depends on the amplitude of the control signal. Starting from the output radiation, the mean radiation power is detected in at least one predetermined frequency range. The mean radiation power is the radiation power averaged over the frequencies. Cyclic deflection of the operating range is forced to occur at the deflection frequency. A control signal for controlling the operating range is produced as a function of the deflection of the operating range. The amplitude of the control signal is varied as a function of the control signal, so that the discrepancy between the actual operating range and the nominal operating range becomes less. 
     The operating range can be controlled very accurately and in a simple manner by detecting the radiation power and using the power as a control variable. Changes to the transmission characteristic can also be included in the control system, by indirect reference to the transmission characteristic. This procedure allows the control process to be carried out even without presetting any nominal power. 
     In one embodiment, the derivative of the function of the operating range and detected power is used as a control variable. The control process refers to a point in the function at which the derivative has the value zero. A minimum, a maximum, a point of inflection or some other point at which a derivative has the value zero in the power curve is chosen, by way of example, as the reference point. Thus, using the reference point, the control loop controls to this point without any additional tuning. 
     Known method of control engineering (e.g., analog or digital control methods based on the use of proportional, integral and differential regulators, and combinations of them) can be used for control. However, very good control loops result if the control variable is detected using phase-sensitive detection, which is also known as the lock-in method. Phase-sensitive detection has the advantage that the control process can be carried out comparatively independently of disturbance variables (e.g., signal noise). Phase-sensitive detection is further explained in the book “Electronic Measurement and Instrumentation”, Klaas B. Klaassen, Cambridge University Press, 1996, pages 204 to 210. 
     Preferably, the modulator is either a pulse modulator, which is driven at a drive frequency predetermined by a cyclic control signal, or a data modulator, which is driven by a control signal dependent on the data to be transmitted, with half the data rate being referred to as the drive frequency. 
     In one embodiment of the present invention, the frequency range, which is predetermined for controlling the operating range, includes all the output radiation frequencies which can be detected by a transducer unit. There is no need for any filters for selecting a frequency range. 
     In yet another embodiment, the frequency range, which is predetermined for controlling the operating range, includes only a portion of the output radiation frequencies detected by a transducer unit. Although this necessitates filter units being connected downstream from the transducer unit, it creates additional degrees of freedom in the choice of the control variable. 
     In one embodiment, in which only some of the frequencies detected by the transducer unit are used, the predetermined frequency range includes twice the drive frequency. The predetermined frequency range does not include other multiples of the drive frequency and the drive frequency itself. Thus, if there are any discrepancies in the actual operating range and the nominal operating range, the radiation power in the vicinity of twice the drive frequency falls considerably, so that the power in this region is highly suitable for control purposes. The frequency range used has a width of 0.3 times twice the drive frequency, in one embodiment. 
     In embodiments with predetermined frequency ranges in the radio-frequency band, and a nominal operating range of the modulator which is symmetrical about a transmission maximum (Return to Zero (RZ) operation), or which is about a transmission minimum (carrier-suppressed RZ operation), the amplitude value of the control signal is controlled via a control loop. Without being tuned, the control loop is aligned with a control point at which the mean radiation power is maximized within the frequency range predetermined for controlling the operating range. 
     In yet another embodiment, the frequency range, which is predetermined for controlling the operating range, includes only frequencies which are well below the drive frequency (i.e., the frequency is low in comparison to the drive frequency). For example, these frequencies are less than one-tenth of the drive frequency. Thus, components with low-cut-off frequencies may be used, even if the drive frequency is in the radio-frequency band. 
     If, in one embodiment, the nominal operating range of the modulator, preferably of a pulse modulator, in the low-frequency range is symmetrical about a transmission minimum (carrier-suppressed RZ operation) or is symmetrical about an operating point between a transmission point of inflection and a transmission maximum (clock RZ operation), then the amplitude value of the control signal is controlled via a control loop which is aligned, without tuning, to a control point at which the mean radiation power is a maximum within the frequency range predetermined for controlling the operating range. 
     If, in another embodiment, the nominal operating range of the modulator, preferably of a pulse modulator, in the low-frequency band is, in contrast, symmetrical about a transmission maximum (RZ operation) or is symmetrical about an operating point between a transmission minimum and a transmission point of inflection (clock RZ operation), then the amplitude value of the control signal is controlled via a control loop which is aligned, without tuning, to a control point at which the mean radiation power is a minimum within the frequency range predetermined for controlling the operating range. 
     The control loops for controlling the operating range are tuned, in other embodiments, meaning the control loop is controlled to a point close to the control point, in the nominal operating range of the modulator. The control loop can be tuned by known methods of control engineering (e.g., by deliberately applying a disturbance variable). 
     The amplitude value of the control signal can, for example, easily be varied by adjusting the gain of an amplifier at whose output the control signal is produced. A clock signal (e.g., a sinusoidal or square-wave signal) or a data signal is applied to the input of the amplifier. 
     A control variable with the correct mathematical sign can easily be obtained if a small discrepancy is forced to occur between the operating range and the nominal operating range, for control purposes. The power is detected for at least two operating ranges. 
     The control variable with the correct mathematical sign is then derived from the detected power. One method which operates with forced discrepancies in the operating range is phase-sensitive detection. Phase-sensitive detection is also referred to as the lock-in method, see Klaas B. Klaassen, “Electronic Measurement and Instrumentation” Cambridge University Press, 1996, pages 204 to 210. 
     In one embodiment, the discrepancy in the operating range is forced via a periodic deflection signal at a predetermined deflection frequency. The deflection signal is additively or subtractively superimposed on the control signal. A signal which is dependent on the detected power is multiplied by a cyclic reference signal, whose frequency matches the deflection frequency. A signal resulting from the multiplication is used, after low-pass filtering and preferably after subsequent integration, for varying the amplitude of the control signal. The cut-off frequency of the low-pass filter governs the response time of the control loop which is, for example, between 10 and 100 milliseconds. The refinement is based on the knowledge that the signal value of the DC component that passes through the low-pass filter is a measure of the first derivative of the power function. 
     The deflection signal has a cosine or sinusoidal waveform. However, other deflection signals (e.g., a square waveform) are also used. If the reference signal frequency corresponds to a multiple of the deflection frequency, then it is possible to detect points at which higher derivatives are zero, for instance, a point of inflection at twice the deflection frequency. 
     The operating point can be controlled in a similar way at the same time. The deflection frequency for controlling the operating point and the deflection frequency for controlling the operating range are chosen appropriately. Deflection frequencies that differ from one another are thus used, for example a deflection frequency of 3 kHz and a deflection frequency of 5 kHz. 
     In a pulse modulator or a data modulator, the input radiation is produced via a continuous wave light source or via a pulsed light source. 
     In further embodiments, the drive frequency and the data rate are more than 1 Gigahertz or 1 GBit/s, respectively, preferably 5 Gigahertz and 5 GBit/s, respectively. In yet another embodiment, the modulator operates in the optical band. A modulator which contains a Mach-Zehnder interferometer is suitable. The modulator&#39;s transmission characteristic is, for example, in cosine form, or sinusoidal. However, modulators with other transmission characteristics are also used. 
     The present invention also relates to a drive unit for carrying out the methods mentioned above. The technical effects cited for the method also apply to the drive unit and to 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 of a pulse modulator and the waveform of a control signal. 
     FIG. 2 shows the spectral power density spectrum of the output radiation from the pulse modulator with optimum operating parameters. 
     FIG. 3 shows the spectral power density spectrum of the output radiation when there is a 10 percent discrepancy between the actual operating point and the nominal operating range. 
     FIG. 4 shows the mean radio-frequency radiation power in a predetermined frequency range as a function of the operating range discrepancy between the actual operating range and the nominal operating range. 
     FIG. 5 shows a block diagram of an exemplary embodiment of a drive unit containing radio-frequency components in accordance with the principles of the present invention. 
     FIG. 6 shows the mean low-frequency radiation power as a function of the operating range discrepancy. 
     FIG. 7 shows a block diagram of an exemplary embodiment of a drive unit operating at low frequency in accordance with the principles of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The upper part of FIG. 1 shows a transmission characteristic  10  of a pulse modulator. The drive voltage is plotted in volts on an abscissa axis  12 . Transmission values T are shown on an ordinate axis  14 . The transmission characteristic  10  has a cosine waveform starting at a voltage of 0 volts. The transmission falls from a maximum value of 1 to virtually 0 at a voltage U 1 . The voltage U 1  has an associated operating point AP located at a transmission minimum. As the voltage increases, the transmission T rises once again until, at a voltage U 2 , an operating point AP 1  is reached which is located at a maximum on the transmission characteristic  10 . If the voltage is increased further, then the transmission falls, and reaches a minimum once again at a voltage U 3 . 
     The operating point AP 1  at the transmission maximum is also referred to as an RZ operating point (Return to Zero). In RZ operation, the operating point AP 1  should always be located at the transmission maximum. If the transmission characteristic  10  of the pulse modulator varies, then the operating point AP 1  must be readjusted by varying the voltage U 2 . As the age of the pulse modulator increases, the transmission characteristic  10  will be compressed or stretched in the direction of the abscissa axis  12 , and/or in the direction of the ordinate axis  14 . Furthermore, the transmission characteristic may be shifted with respect to the abscissa axis. If the pulse modulator is operated at the operating point AP 1 , then there is an optimum operating range AB 1  precisely between the voltages U 1  and U 3 . 
     However, the modulator may also be operated at the operating point AP 2 , at which the drive voltage fluctuates about the voltage U 1 . This method of operation is referred to as suppressed-carrier operation. The optimum operating range at the operating point AP 2  is between the voltage 0 volts and the voltage U 2 . 
     However, the pulse modulator may also be operated at an operating point AP 3  which is located between the two operating points AP 1  and AP 2 . In an exemplary embodiment, the operating point AP 3  is located below the point of inflection of the transmission characteristic  10  in the vicinity of the operating point AP 2 . The optimum operating range for the operating point AP 3  is symmetrical about this operating point AP 3  between the voltage U 1  and a voltage that is less than the voltage U 2 . 
     The lower part of FIG. 1 shows the voltage waveform of a control signal  20  as a function of time t, plotted on an abscissa axis  22 . An ordinate axis  24  is used to show the voltage values U in volts. 
     The control signal  20  is used to drive the pulse modulator at the operating point AP 1 . At a time t 0 , the control signal  20  is at the voltage U 1 , so that the modulator passes only minimal output radiation. At a later time t 1 , the control signal voltage is U 2 . Thus, the modulator passes the input radiation virtually unimpeded. A light pulse appears at the output of the modulator. At a time t 2 , the control signal voltage is U 3 , so that the modulator is once again operating at a transmission minimum, and is essentially not passing any light. At a time t 3 , the control signal voltage is once again U 2 , so that a second light pulse is produced. At a later time t 4 , the control signal  20  once again has the voltage value U 1 , so that no radiation reaches the output of the modulator. Two light pulses are thus emitted during one cycle of the control signal  20 . 
     The mean value of the sinusoidal control signal  20  governs the operating point, see the voltage U 2 . The amplitude of the control signal  20  governs the operating 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 from the pulse modulator at the optimum operating point AP 1  and in the optimum operating range AB 1 , see the upper part of FIG.  1 . The frequency is plotted with respect to the data rate on the abscissa axis  52 . In an exemplary embodiment, the data rate is 10 Gigabits per second, and the control signal drive frequency is 5 Gigabits per second. The signal power, as detected via a photodiode and via a spectrum analyzer, is plotted on a logarithmic scale on the ordinate axis  54 . The signal power has been normalized. 
     The power density spectrum  50  has four power peaks  56 ,  58 ,  60 ,  62  at the frequency/data rate values  1 ,  2 ,  3  and  4 . The peak values of the power peaks  56 ,  58 ,  60  and  62  are, in this sequence, at approximately 0.6; 0.08; 0.0007 and 8×10 −6 . 
     FIG. 3 shows the power density spectrum  70  of the output radiation from the pulse modulator when there is a 10 percent discrepancy between the actual operating point and the nominal operating point AP 1 . The operating range AB is shifted in a corresponding manner, but its width is not changed. As with the abscissa axis  52  in FIG. 2, an abscissa axis  72  shows the ratio of the frequency to the data rate. An ordinate axis  74  shows the normalized radiation power on a logarithmic scale. Power peaks  76 ,  78 ,  80 ,  82  occur in this sequence at the same frequencies as the power peaks  56 ,  58 ,  60 ,  62 , see FIG.  2 . The peak values of the power peaks  76 ,  78  and  80  are in this case considerably lower than the peak values of the power peaks  56 ,  58  and  60 , respectively. In the event of discrepancies from the optimum operating point, further power peaks  84 ,  86 ,  88  and  90  occur at the drive frequency and between the power peaks  76 ,  78 ,  80  and  82 . The power peak  84  has a peak value of approximately 0.03, which is below the peak value of 0.25 at the power peak  76 . The peak values of the power peaks  86 ,  88  and  90  are in each case approximately between the peak values of the respectively adjacent power peaks  76 ,  78 ,  80  and  82 . 
     The change between the power density spectrum  50  and the power density spectrum  70 , which can be seen in FIGS. 2 and 3 when there are any discrepancies from the operating point can be utilized to control the operating point of the pulse modulator. The occurrence of the power peak  84  at the drive frequency (i.e., at 5 Gigahertz) is used, in particular, to control the operating point. The aim is to minimize the peak value of this power peak. A frequency range FB 1  surrounding the power peak  84  has a width of about 0.3·f, where f is the drive frequency. The frequency range FB 1  is centered about the drive frequency f. If the operating point is controlled using radio-frequency components, only the power peak  84  is used within the frequency range FB 1 . 
     The power density spectrum of the output radiation is likewise used to control the operating range AB. FIG. 3 shows a frequency range FB 2  surrounding the power peak  76 , and containing the frequencies used to control the operating range. The frequency range FB 2  has a width of about 0.3·2f, where f is the drive frequency. If there are any discrepancies between the operating range and the nominal operating range, this leads to a reduction in the peak value of the power peak  76  in the frequency range FB 2 . 
     FIG. 4 shows a power function  110  of the mean radio-frequency radiation power in the frequency range FB 2  as a function of the operating range discrepancy between the nominal operating range AB 1  and the actual operating range, see FIG.  1 . An abscissa axis  112  shows the discrepancy between the nominal operating range AB 1  and the actual operating range AB as a percentage. This discrepancy is proportional to the discrepancy between the amplitude of the control signal and the nominal value. An ordinate axis  114  shows the normalized radiation power. The power function  110  has a maximum when there is a discrepancy of approximately 3 percent between the nominal operating range AB 1  and the actual operating range. The power function  110  has a profile which is similar to an inverted parabola, so that the radiation power falls continuously to the left and right of a discrepancy of −3 percent. A measurement point MP 3  to the right alongside the measurement point MP 2  is associated with an amplitude discrepancy of 0 percent. The aim of the operating range control must therefore be to achieve a radiation power associated with measurement point MP 3 . This may be done using a control loop which maximizes the radiation power which is, however, tuned such that it is controlled to the measurement point MP 3 . A circuit for such a control loop is explained below, with reference to FIG.  5 . 
     FIG. 5 shows a block diagram for a drive unit  120  which contains radio-frequency components, and contains a photodiode  122 , an operating point control circuit  124  and an operating range control circuit  126 . The drive unit  120  is used for driving a pulse modulator  128 , which contains what is referred to as a Mach-Zehnder interferometer (MZI) and has a transmission characteristic  10 , see FIG.  1 . The modulator  128  modulates the radiation produced by a laser diode  134 , as a function of an operating point control voltage  130  produced by the operating point control circuit  124 , and as a function of an operating range control voltage  132  produced by the operating range control circuit  126 . The laser diode  134  operates in the continuous wave mode, so that the input radiation to the modulator  128  has a constant radiation power. 
     The modulated output voltage is split in a radiation splitter  136  in a ratio of about 1:10. The majority of the modulated radiation is supplied to a data modulator, which is not illustrated but which modulates the output radiation in accordance with the data to be transmitted, see the arrow  138 . The smaller portion of the output radiation is transmitted from the radiation splitter  136  via an optical waveguide  140  to the photodiode  122 . 
     The photodiode  120  has a cut-off frequency in the Gigahertz range, and is thus a radio-frequency diode. The current flowing through the photodiode  122  depends on the radiation striking the photodiode  122 . The current signal, which is dependent on the radiation, or a voltage signal obtained from it, is used as the input signal to the operating point control circuit  124  and as the input signal to the operating range control circuit  126 , see the arrows  142  and  144 . 
     The operating point control circuit  124  contains a radio-frequency bandpass filter  146 , to whose input a signal coming from the photodiode  122  is applied. The bandpass filter  146  essentially transmits only signals at frequencies which are within the frequency range FB 1 . Signals at frequencies which are outside the frequency range FB 1  are highly attenuated. The bandpass filter  146  is followed by a radio-frequency power meter  148 . The power meter  148  contains a rectifier diode with a cut-off frequency in the radio-frequency range. A signal whose value depends on the radiation power within the frequency range FB 1  is emitted at the output of the power meter  148 . This signal is not applied in a multiplication unit  150  by a reference signal which is produced by a signal generator  152 . The voltage u r (t) of the reference signal is given by: 
     
       
           u   r ( t ) = u     r · cos ( ωt+Φ 1)  (1) 
       
     
     where u r (t) is the instantaneous value of the voltage of the reference signal as a function of time t,  u   r  is the maximum value of the voltage of the reference signal, ω is a reference circular frequency, and Φ1 is an adjustable phase. The reference circular frequency ω is 2π times the deflection frequency of the operating point. 
     The voltage u i  of the input signal emitted from the power meter  148  to the multiplication unit  150  can be described by the following formula: 
     
       
           u   i ( t )= f ( x   0   + u   0· cos ( ω·t )  (2), 
       
     
     where u i  is the instantaneous value of the input signal,  u 0 is the maximum value of the forced deflection about the actual operating point x 0 , ω is the reference frequency and f( . . . ) is a function to describe the relationship between the operating point and the mean detected radiation power. 
     The multiplication unit  150  produces an output signal which, in addition to components with multiples of the reference frequency ω, also contains a DC component. The DC component is a measure of the derivative of the function illustrated in FIG. 4, is filtered out via a low-pass filter  154 , and is transmitted to an integration unit  156 . Signal components at the reference frequency ω and signal components at a frequency which corresponds to a multiple of the reference frequency ω are highly attenuated by the low-pass filter  154  and are thus not passed to the integration unit  156 . The integration unit  156  integrates the signal applied to its input over time, and thus supplies the integration component of the control system. On the output side, the integration unit  156  is connected to a first input of an addition element  158 . The other input of the addition element  158  is connected to an output of the signal generator  152 , at which a deflection signal is produced whose value varies in accordance with a cosine function with the reference frequency ω. The output of the addition element  158  also forms the output of the operating range control circuit  124 . 
     Adjustment of the phase Φ 1  allows the operating point control circuit  124  to drive the pulse modulator  128  such that the radiation power within the filter range FB 1  is minimized, and the actual operating point is thus controlled to the transmission maximum of the nominal operating point. The reference frequency ω is selected as appropriate and is, for example, in the kilohertz range. 
     The operating range control circuit  126  is essentially constructed in the same way, and hence also operates in the same way, as the operating point control circuit  124 . The operating range control 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 integration unit  168  and an addition element  170 . The bandpass filter  160  transmits only signals at a frequency which is within the frequency range FB 2 . The operating range control circuit  126  also contains a signal generator  172 , which produces a reference signal which varies in accordance with the function cos (η+φ2), where η is a reference circular frequency that is not the same as the reference circular frequency ω. The reference circular frequency η is 2ω times the deflection frequency of the operating range. φ2 is an adjustable signal phase. Furthermore, the signal generator produces a further deflection signal, which varies in accordance with the function cos υt. This deflection signal is applied to the other input of the addition element  170 . 
     The output of the addition element  170  is also output of the operating range control circuit  126 , which produces the operating range control voltage  132 . This control voltage  132  is used to adjust the gain of an amplifier  174 . An input signal  176  with a sinusoidal waveform and at half the drive frequency is applied to the input of the amplifier  174 . An output signal  178  from the amplifier  174  matches the control signal  20 , except for the mean value, and is used to drive the pulse modulator  128 . 
     The amplifier  174  also has an output, which is not illustrated, at which a voltage that is proportional to the output power of the amplifier is emitted. This voltage is used for tuning the operating range control circuit  128  by using a subtraction element, which is not illustrated, between the low-pass filter  166  and the integration element  168  to subtract the voltage from the signal within the control loop, see also FIG.  9 . 
     By suitable choice of the phase φ2, it is possible for the operating range control circuit  126  to control the radiation power within the frequency range FB 2  to the value represented by the measurement point MP 3  in FIG.  5 . At the same time, the amplitude of the control signal  20  is thus controlled such that the actual operating range matches the nominal operating range AB 1 . 
     By suitable choice of the reference frequencies ω and υ, it is possible for the operating point to be controlled independently of the operating range control, and for the operating range control to operate independently of the operating point control. Suitable values are, for example, 3 kHz and 5 kHz, respectively, for the reference frequency ω and for the reference frequency υ. 
     FIG. 6 shows the mean low-frequency radiation power as a function of the operating range discrepancy for three modulator operating modes. The mean low-frequency radiation power covers, for example, frequencies from 0 Hz to 10 Hz. 
     An abscissa axis  210  shows the amplitude of the control signal. The unit Vπ is once again used as the unit, with an operating range of 2 Vπ being the nominal operating range for RZ operation and for CSRZ operation, and including a range of 360° or 2 π radians on the transmission characteristic  10 . The power transmitted by the modulator in the low-frequency range is plotted, in normalized form, on an ordinate axis  212 . 
     An RZ characteristic  214  applies to an operating point at the transmission maximum on the characteristic  10 , for example, to the operating point AP 1 . The characteristic  214  has a transmission minimum at an operating range of about 1.22•2 Vπ, and this is suitable for controlling the operating range and the amplitude. The control loop can be tuned so that the nominal operating range is located to the left alongside the transmission maximum, at the amplitude 2 Vπ. 
     A CSRZ characteristic  216  applies to an operating mode in which the nominal operating point is at a transmission minimum on the transmission characteristic  10 , for example, at the operating point AP 2 . The characteristic  216  has a transmission maximum at about 1.22•2 Vπ, which is suitable for controlling the operating range. The control loop can be tuned so that the operating range once again has a width of 2 Vπ. 
     A characteristic  218  relates to clock RZ operation, in which the nominal operating point is between the transmission maximum and the transmission minimum on the transmission characteristic  10 , for example, at the operating point AP 3 , see FIG.  1 . The characteristic  218  has a transmission maximum for an operating range of about 1.22•2 Vπ, and has a transmission minimum for about 0 Vπ, both of which are suitable for controlling the operating range via a control loop. The control loop can be tuned such that the nominal operating range is, for example, considerably less than Vπ, see the measurement point MP 5  at 0.4 Vπ. 
     Circuits with control loops for controlling the operating range in each of one of three operating modes of the modulator will be explained below with reference to FIG.  7 . 
     FIG. 7 shows a block diagram for a drive unit  220 , which operates at low frequency, in a pulse modulator  128   b , which likewise has a transmission characteristic  10 , see FIG.  1 . The major difference between the drive unit  220  and the drive unit  120 , see FIG. 6, is that the drive unit  120  contains radio-frequency components. Otherwise, the construction of the drive units  120  and  220  is identical, so that circuit modules with the same construction and the same function have the same reference symbols. However, the reference symbols have the suffix  b  in order to distinguish between them. This applies in particular to the reference symbols  124   b  to  140   b . Instead of the radio-frequency photodiode  122 , the drive unit  220  uses a photodiode  220  which has a cut-off frequency in the low-frequency range, for example a cut-off frequency of 10 kHz. The deflection frequency is within the bandwidth of the photodiode  220 . The current flowing through the low-frequency photodiode  222  varies as a function of the output radiation striking the photodiode  220 . Only the low-frequency components of the output radiation result in a variation in the diode current. The photodiode  222  also averages over the frequency range above its cut-off frequency. The diode current, or a voltage derived from it, is used as the input variable to the operating point control circuit  124   b , and as the input variable to the operating range control circuit  126   b , see the arrows  224  and  226 . 
     The operating point control circuit  124   b  contains, from its input to its output, a multiplication unit  150   b , a low-pass filter  154   b , an integration unit  156   b , and an addition element  158   b . Furthermore, the control circuit  124  contains a signal generator  152   b , which produces a reference signal and a deflection signal. The reference signal varies in accordance with the function cos (ωt+φ3). The deflection signal varies in accordance with the function cos (ωt). 
     The operating range control circuit  126   b  contains, from the input to the output, a multiplication unit  164   b , a low-pass filter  166   b , a subtraction element  228 , an integration element  168   b  and an addition element  170   b . In addition, the control circuit  126   b  contains a signal generator  172   b , which produces a reference signal and a deflection signal. The reference signal varies in accordance with the function cos (υt+φ4). The deflection signal varies in accordance with the function cos (υt). The output of the operating range control circuit  126   b  is connected to the control input of an amplifier  174   b . An input signal  176   b  which has a sinusoidal waveform at the drive frequency is applied to the amplifier  174   b . An output signal  178   b  is used to drive the modulator  128   b  and, except for the mean value, matches the control signal  20 , see FIG.  1 . Furthermore, the amplifier  174   b  produces a tuning signal  230 , whose signal value varies as a function of the mean output power from the amplifier  174   b . The tuning signal  230  is applied to a further input of the subtraction element  228 , and is used to tune the control circuit  126   b  in order to control the operating range. 
     If the modulator  128   b  is operated at the operating point AP 1 , see FIG. 1, then the phase φ 3  is set such that the operating point control circuit  124   b  controls the emitted power to a minimum. This results in the operating point being controlled to the transmission maximum. The phase φ 4  is thus chosen such that the operating range control circuit  126   b  controls the power of the output radiation in the low-frequency cut-off region on the basis of tuning to a control point, which is shown in addition to that in FIG. 6, with minimum transmission. Thus, the amplitude of the control signal is regulated to the value 2 Vπ. 
     The deflection frequencies ω and υ differ from one another, and are, for example, 3 kHz and 5 kHz, respectively. 
     In yet another embodiment, the modulator  128   b  is operated at the operating point AP 2 , that is to say at the transmission minimum. The phase φ 3  of the operating point control circuit  124   b  is set such that the output radiation transmitted by the modulator  128   b  in the low-frequency range assumes a maximum value. The phase φ 4  is chosen such that, although the output radiation from the modulator  128  is maximized, the tuning when the control loop is in the steady state is somewhat away from the maximum, see FIG. 6, characteristic  216 . 
     If, in another embodiment, the modulator  128   b  is operated at the operating point AP 3 , see FIG. 1, then the phase φ 3  is set such that the operating point control circuit  124   b  minimizes the power in the low-frequency range of the output radiation from the modulator  128   b  and, owing to the tuning, controls it at a value which is somewhat offset from the minimum. The operating point is thus also controlled at the nominal operating point AP 3 . The phase φ 4  in the operating point AP 3  operating mode is chosen such that the operating range control circuit  126   b  likewise minimizes the mean power of the output radiation from the modulator  128   b  in the low-frequency range. However, via the tuning of the control circuit  126   b , the operating range is controlled at a considerably lower value than 2 Vπ. 
     In another embodiment, the modulator  128   b  is a data modulator, which is driven by an input signal  176   b  or output signal  178   b  dependent on data. 
     The operating point is somewhat offset from the point of inflection of the transmission characteristic. The operating range is somewhat less than 1 Vπ. The input radiation is produced by the laser diode  134   b , which is operated in the continuous wave mode. The operating point control circuit controls at a minimum, and is tuned. The operating range control circuit likewise controls at a minimum, and is also tuned. 
     In a further embodiment for a data modulator, a pulse modulator is used instead of the laser diode, for example the pulse modulator, including the control units, shown in FIG.  7 . 
     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 invention as set forth in the hereafter appended claims.