Modulators – Pulse or interrupted continuous wave modulator
Reexamination Certificate
2001-09-21
2003-10-28
Pascal, Robert (Department: 2817)
Modulators
Pulse or interrupted continuous wave modulator
C332S109000, C332S112000, C359S237000
Reexamination Certificate
active
06639482
ABSTRACT:
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 frequ
Geiger Harald
Glingener Christoph
Mohs Georg
Bell Boyd & Lloyd LLC
Glenn Kimberly E
Pascal Robert
Siemens Aktiengesellschaft
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