# US 5,347,529 A · Method for generating a distortion-free, frequency-modulated optical signal and apparatus for the implementation of such a method > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 5,347,529 A - Document ID: US5347529A - Status: Granted - Kind: A - Application number: 08/095,401 - Filed: 1993-07-23 - Granted: 1994-09-13 - Earliest claimed: 1992-08-07 - Assignees: Siemens Aktiengesellschaft - Inventors: Reinhold Noe - Web page: https://patentgrep.com/patent/US5347529A ## CPC - H03F 1/3252: {using multiple parallel paths between input and output (H03F1/3258, H03F1/3282, H03F1/3294 take precedence)} - H04B 10/503: {Laser transmitters} - H04B 10/504: {using direct modulation} - H04B 10/58: Compensation for non-linear transmitter output - H04B 10/64: Heterodyne {, i.e. coherent receivers where, after the opto-electronic conversion, an electrical signal at an intermediate frequency [IF] is obtained} - H03F: AMPLIFIERS - H04B: TRANSMISSION ## Abstract Method and apparatus for generating a distortion-free, frequency-modulated optical signal, whereby this signal from an optical semiconductor laser driven with a modulating voltage is distorted by the modulating voltage due to a thermally conditioned frequency transfer function of the transmitter for the modulation frequency of the modulating voltage. For generating the distortion-free, frequency-modulated optical signal, the modulating voltage and/or the distorted, frequency-modulated optical signal itself and/or a superimposition signal containing this optical signal is subjected to a distortion that entirely or partially compensates the distortion of this optical signal. This method is simple and, in combination with receivers for the frequency-modulated optical signal, avoids sensitivity losses. ## Claims ### Claim 1 (Amended) A method for generating a frequency-modulated optical signal, comprising the steps of: providing a semiconductor laser having at least a pump electrode; pumping the semiconductor laser with an electrical voltage that is applied to the pump electrode of the semiconductor laser and that modulates with a modulation frequency, the semiconductor laser having a frequency transfer function with a thermal low-pass part that is dependent on the modulation frequency, said low-pass part effecting a distortion of the generated, frequency-modulated optical signal; subjecting to distortion, for generating the frequency-modulated optical signal such that the frequency-modulated optical signal is at least partially free of distortion producible by the thermal low-pass part, at least one of: a) for a semiconductor laser having in addition to the pump electrode, a further electrode for applying the modulating voltage, the modulating voltage applied to the pump electrode and the modulating voltage applied to the further electrode such that the distorted, modulating voltages together at least partially compensate the distortion of the frequency-modulated optical signal; or b) the frequency-modulated optical signal itself in order to compensate a distortion of the frequency-modulated signal with reference to a signal frequency of the frequency-modulated signal; or c) for a frequency-modulated, optical superposition signal generated by superimposing the frequency-modulated optical signal with an optical reference signal having a specific, modulatable reference frequency, the reference signal with reference to the reference frequency that at least partially compensates the distortion of the frequency-modulated optical signal contained in the superposition signal. ### Claim 2 (depends on 1) The method according to claim 1, wherein a semiconductor laser is employed whose frequency transfer function with reference to the modulating voltage applied to the pump electrode is at least approximately a linear function of the thermal low-pass part. ### Claim 3 (depends on 2) The method according to claim 2, wherein the linear function of the thermal low-pass part has a proportionality factor of the thermal low-pass part and an additive constant; wherein the additive constant and a sum of the additive constant and the proportionality factor of the thermal low-pass part are respective positive numerical values; and wherein both the modulating voltage to be supplied to the pump electrode as well as the modulating voltage to the further electrode are distorted with respective filters that the distortion of the frequency-modulated optical signal is at least partially compensated. ### Claim 4 (depends on 2) The method according to claim 2, wherein the linear function of the thermal low-pass part has a proportionality factor of the thermal low-pass part and an additive constant; wherein the additive constant and a sum of this additive constant and the proportionality factor of the thermal low-pass part are respectively negative numerical values; and wherein at least the modulating voltage to be supplied to the pump electrode is distorted with at least one filter such that the distortion of the frequency-modulated signal is at least partially compensated. ### Claim 5 (depends on 1) The method according to claim 1, wherein the laser with reference to the modulating voltage applied to the further electrode has a transfer function dependent on the modulation frequency that is equal to a low-pass transfer function multiplied by a specific proportionality constant referred to the further electrode, said low-pass transfer function having the value one for low modulation frequencies and having the value zero for extremely high modulation frequencies. ### Claim 6 (depends on 2) The method according to claim 2, wherein the modulating voltage is supplied with high-pass filtering to the pump electrode and is supplied with low-pass filtering to the further electrode. ### Claim 7 (depends on 1) The method according to claim 1, wherein the semiconductor laser is a TTG laser. ### Claim 8 (depends on 2) The method according to claim 2, wherein with which frequency ranges of the modulation frequency, wherein a magnitude of a modulation sensitivity referred to the pump electrode that is established by a relationship dependent on the modulation frequency between a change of the signal frequency of the frequency-modulated optical signal and a change of the modulating voltage to be applied to the pump electrode and/or a magnitude of a modulation sensitivity referred to the further electrode which is established by a relationship dependent on the modulation frequency between a change of the signal frequency of the frequency-modulated optical signal and a change of the modulating voltage to be applied to the further electrode that causes this change, has at least an approximately flat curve and covers an entire range between a lowest-first frequency and a second frequency that is higher in comparison to this first frequency. ### Claim 9 (depends on 8) The method according to claim 8, wherein a semiconductor laser is employed, whereby the magnitude of the modulation sensitivity referred to the pump electrode has at least an approximately flat curve at least in a frequency range between a defined, third frequency of the modulation frequency that is higher in comparison to the first frequency and lower in comparison to the second frequency, as well as for modulation frequencies that are higher than a defined, fourth frequency that is higher in comparison to the third frequency, and whereby the magnitude of the modulation sensitivity referred to the further electrode has at least an approximately flat curve at least in a frequency range between the first frequency and a fifth frequency of the modulation frequency that is higher in comparison to the fourth frequency. ### Claim 10 (depends on 1) The method according to claim 1, wherein the method further comprises the steps of: generating an electrical error signal from the frequency-modulated optical signal output by the semiconductor laser, said electrical error signal being at least approximately proportional with a specific proportionality constant to the thermal low-pass part of the frequency transfer function of the semiconductor laser and to the modulating voltage; controlling a controllable frequency converter with this error signal, said controllable frequency converter effecting a frequency shift of the signal frequency of the frequency-modulated optical signal; and selecting the proportionality constant of the error signal such that the optical frequency of an optical output signal at an output of the frequency converter is essentially proportional to the modulated voltage and is independent of the thermal low-pass part of the frequency transfer function of the semiconductor laser. ### Claim 11 (depends on 1) The method according to claim 1, wherein the method further comprises the steps of: generating an amplitude signal that is dependent on an intermediate frequency of an intermediate frequency signal acquired from the superimposed signal such that said amplitude signal with reference to a specific amplitude level has at least two level crossings with slopes inclined in a same direction at two respective specific characteristic frequency values of the intermediate frequency that differ from one another; and, when the frequency-modulated optical signal output by the semiconductor laser is undistorted, intermediate frequency values occur that are equal to the characteristic frequency values; and at least approximately adjusting back, given a frequency-related deviation of the intermediate frequency values from the characteristic frequency values, the intermediate frequency values in turn to the characteristic frequency values by a change of the reference frequency of the reference signal that is effected by an output signal generated from a constantly identified amplitude level of the amplitude signal. ### Claim 12 (depends on 1) The method according to claim 1, wherein the method further comprises the steps of: deriving a signal that corresponds to the modulating voltage from an intermediate frequency signal acquired from the optical superimposed signal; and distorting the derived signal with a low-pass filter part of the frequency transfer function that is dependent on the modulation frequency that is at least approximately equal to a transfer function of the semiconductor laser that is dependent on the modulation frequency; and generating, by means of the distorted derived signal, a frequency modulation impressed on the reference signal for at least partial compensation of the distortion of the frequency-modulated optical signal that is contained in the intermediate frequency signal. ### Claim 13 (depends on 12) The method according to claim 12, wherein the method further comprises the steps of: continuously checking the signal dependent on the intermediate frequency to determine whether, at the characteristic frequency values, a deviation of an actual amplitude level of this signal from a specific amplitude level and, thus, a deviation of the intermediate frequency values from the characteristic frequency values is present; generating an optical signal that is dependent on such a deviation; and producing, by means of said output signal and the distorted signal superimposed on one another, a change of the reference frequency of the reference signal for at least partial compensation of the distortion of the frequency-modulated optical signal that is contained in the superimposition signal. ### Claim 14 (depends on 1) The method according to claim 1, wherein the method further comprises the steps of: deriving a signal that corresponds to the modulating voltage from an intermediate frequency signal acquired from the superimposition signal; supplying the modulating voltage to the semiconductor laser pre-distorted by a high-pass transfer function for the modulation frequency; distorting the derived signal by a low-pass transfer function of the modulation frequency that is at least approximately complementary to the high-pass transfer function; and generating, by means of said distorted signal, a frequency modulation impressed on the reference signal for at least partial compensation of a distortion of the frequency-modulated optical signal that is contained in the superimposition signal. ### Claim 15 An apparatus for generating a frequency modulated optical signal, comprising: a semiconductor laser having a pump electrode and a further electrode; an electrical voltage that is applied to the pump electrode of the semiconductor laser and that modulates with a modulation frequency, the semiconductor laser having a frequency transfer function with a thermal low-pass part that is dependent on the modulation frequency, said low-pass part effecting a distortion of the generated, frequency-modulated optical signal; a high-pass filter means for high-pass filtering of the modulating voltage to be supplied to the pump electrode of the semiconductor laser; a low-pass filter means for low-pass filtering of the modulating voltage to be supplied to the further electrode of the semiconductor laser; the low-pass filter means having a predetermined low-pass transfer function for the modulation frequency that has a value one for modulation frequencies that are extremely low in comparison to a predetermined frequency and that has a value zero for modulation frequencies that are extremely high in comparison to the predetermined frequency; and the high-pass means having a high-pass transfer function is equal to a value of one minus the low-pass transfer function of the low-pass filter means; wherein the frequency-modulated signal is generated at least partially free of the distortion caused by the thermal low-pass part. ### Claim 16 (depends on 15) The apparatus according to claim 15, wherein a magnitude of a modulation sensitivity referred to the pump electrode that is established by a relationship dependent on the modulation frequency between a change of the signal frequency of the frequency-modulated optical signal and a change of the modulating voltage to be applied to the pump electrode and a magnitude of a modulation sensitivity referred to the further electrode which is established by a relationship dependent on the modulation frequency between a change of the signal frequency of the frequency-modulated optical signal and a change of the modulating voltage to be applied to the further electrode that causes this change, has at least an approximately flat curve and covers an entire range between a lowest-first frequency and a second frequency that is higher in comparison to this first frequency, and wherein the magnitude of the modulation sensitivity referred to the pump electrode has at least an approximately flat curve at least in a frequency range between a defined, third frequency of the modulation frequency that is higher in comparison to the first frequency and lower in comparison to the second frequency, as well as for modulation frequencies that are higher than a defined, fourth frequency that is higher in comparison to the third frequency, and the magnitude of the modulation sensitivity referred to the further electrode has at least an approximately flat curve at least in a frequency range between the first frequency and a fifth frequency of the modulation frequency that is higher in comparison to the fourth frequency, and wherein the predetermined frequency lies between the fourth frequency and the fifth frequency. ### Claim 17 (depends on 15) The apparatus according to claim 15, wherein the apparatus further comprises a multiplication means preceding or following the low-pass filter means for multiplying the unfiltered, modulating voltage or the modulating voltage filtered by this low-pass filter means by a factor that is equal to an additive constant of the frequency transfer function in the form of a linear function of the thermal low-pass part of this frequency transfer function divided by a specific proportionality constant referred to the further electrode. ### Claim 18 (depends on 15) The apparatus according to claim 15, wherein the apparatus further comprises a multiplication means preceding or following the high-pass filter means for multiplying the unfiltered, modulating voltage or the modulating voltage filtered by this high-pass filter means with a factor that is equal to a specific proportionality constant referred to the further electrode divided by an additive constant of the frequency transfer function in the form of a linear function of the thermal low-pass part of this frequency transfer function. ### Claim 19 (depends on 15) The apparatus according to claim 15, wherein the high-pass filter means has a capacitor connected into a signal path for the modulating voltage that leads to the pump electrode of the semiconductor laser and has a resistor connected between this signal path and a reference potential. ### Claim 20 (depends on 15) The apparatus according to claim 15, wherein the low-pass filter means has a resistor connected into a signal path for the modulating voltage that leads to the further electrode of the semiconductor laser and has a capacitor connected between the second signal path and a reference potential. ### Claim 21 (depends on 15) The apparatus according to claim 15, wherein the low-pass filter means has an inductance connected in a signal path for the modulating voltage that leads to the further electrode of the semiconductor laser and has a resistor connected between this signal path and the reference potential. ### Claim 22 (depends on 15) The apparatus according to claim 15, wherein the high-pass filter means has a capacitor connected into a signal path for the modulating voltage that leads to the pump electrode of the semiconductor laser and has a resistor connected between this signal path and a reference potential, wherein the low-pass filter means has a resistor connected into a signal path for the modulating voltage that leads to the further electrode of the semiconductor laser and has a capacitor connected between the second signal path and a reference potential, and wherein a capacitance of the capacitor and a value of resistance of the resistor of the high-pass filter means as well as a value of resistance of the resistor and a capacitance of the capacitor of the low-pass filter means are selected such that a product of the high-pass filter means formed of the capacitance and the value of resistance is equal to a product formed from the value of resistance and from the capacitance of the low-pass filter means. ### Claim 23 (depends on 15) The apparatus according to claim 15, wherein the high-pass filter means has a capacitor connected into a signal path for the modulating voltage that leads to the pump electrode of the semiconductor laser and has a resistor connected between this signal path and a reference potential, wherein the low-pass filter means has an inductance connected in a signal path for the modulating voltage that leads the further electrode of the semiconductor laser and has a resistor connected between this signal path and the reference potential, and wherein a capacitance of the capacitor and a value of resistance of the resistor of the high-pass filter means as well as an inductance of the inductor and a value of resistance of the resistor of the low-pass filter means are selected such that a product formed of the capacitance and the value of resistance of the high-pass filter means is equal to a quotient of the inductance of the low-pass filter means divided by the value of resistance thereof. ### Claim 24 An apparatus for generating a frequency modulated optical signal, comprising: a semiconductor laser having at least a pump electrode; an electrical voltage that is applied to the pump electrode of the semiconductor laser and that modulates with a modulation frequency, the semiconductor laser having a frequency transfer function with a thermal low-pass part that is dependent on the modulation frequency, said low-pass part effecting a distortion of the generated, frequency-modulated optical signal; an amplitude signal that is dependent on an intermediate frequency of an intermediate frequency signal acquired from the superimposed signal being generated such that said amplitude signal with reference to a specific amplitude level has at least two level crossings with slopes inclined in a same direction at two respective specific characteristic frequency values of the intermediate frequency that differ from one another and, when the frequency-modulated optical signal output by the semiconductor laser is undistorted, intermediate frequency values occur that are equal to the characteristic frequency values, and given a frequency-related deviation of the intermediate frequency values from the characteristic frequency values, the intermediate frequency values being in turn at least approximately adjusted back to the characteristic frequency values by a change of the reference frequency of the reference signal that is effected by an output signal generated from a constantly identified amplitude level of the amplitude signal; a frequency discriminator for generating the amplitude signal that is dependent on the intermediate frequency from the intermediate frequency signal; and a control means, whereby a continuously identified amplitude level of the amplitude signal is used as an actual value and a specific amplitude level is used as a rated value, said control means generating an output signal that is dependent on a deviation of this actual value from this rated value that is supplied as a control signal to a frequency-modulatable, optical reference signal transmitter generating the reference signal for the purpose of a change of the reference frequency of the reference signal such that the deviation of the actual value from the rated value is at least approximately eliminated; wherein a frequency-modulated signal is generated that is at least partially free of the distortion caused by the thermal low-pass part. ### Claim 25 (depends on 24) The apparatus according to claim 24, wherein the apparatus further comprises a frequency-modulatable, optical reference signal transmitter that generates the reference signal and has a frequency modulation characteristic that is at least approximately constant over a modulation frequency range provided for the frequency-modulated optical signal. ### Claim 26 (depends on 25) The apparatus according to claim 25, wherein the frequency-modulatable reference signal transmitter that generates the reference signal is a tunable TTG laser. ### Claim 27 An apparatus for generating a frequency modulated optical signal, comprising: a semiconductor laser having at least a pump electrode; an electrical voltage that is applied to the pump electrode of the semiconductor laser and that modulates with a modulation frequency, the semiconductor laser having a frequency transfer function with a thermal low-pass part that is dependent on the modulation frequency, said low-pass part effecting a distortion of the generated, frequency-modulated optical signal; a signal that corresponds to the modulating voltage being derived from an intermediate frequency signal acquired from the optical superimposed signal, the derived signal being distorted with a low-pass filter part of the frequency transfer function that is dependent on the modulation frequency that is at least approximately equal to a transfer function of the semiconductor laser that is dependent on the modulation frequency, and this distorted signal being employed for generating a frequency modulation impressed on the reference signal for at least partial compensation of the distortion of the frequency-modulated optical signal that is contained in the intermediate frequency signal; a means for generating an optical superimposition signal by superimposition of the frequency-modulated optical signal output by the semiconductor laser and the reference signal that is generated by an optical reference semiconductor laser and has the specific reference frequency; a means for generating an electrical intermediate frequency signal from the optical superimposition; a means for generating the derived signal that at least approximately corresponds to the modulating voltage from the intermediate frequency signal; and a means for quantized feedback of the derived signal to a reference signal transmitter that generates the reference signal and for distorting the derived signal with the transfer function that is at least approximately equal to the low-pass part of the frequency transfer function the semiconductor laser, the reference signal transmitter being an optical transmitter that can be frequency-modulated by the distorted derived signal from the means for quantized feedback; wherein the frequency-modulated signal is generated at least partially free of the distortion caused by the thermal low-pass part. ### Claim 28 (depends on 27) The apparatus according to claim 27, wherein the apparatus further comprises an adder or subtractor means for superimposing the output signal that is dependent on a deviation of the actual value from the rated value and the distorted derived signal and supplying these signals superimposed on one another to the frequency-modulatable reference signal transmitter. ### Claim 29 An apparatus for generating a frequency modulated optical signal, comprising: a semiconductor laser having at least a pump electrode; an electrical voltage that is applied to the pump electrode of the semiconductor laser and that modulates with a modulation frequency, the semiconductor laser having a frequency transfer function with a thermal low-pass part that is dependent on the modulation frequency, said low-pass part effecting a distortion of the generated, frequency-modulated optical signal; a signal that corresponds to the modulating voltage being derived from an intermediate frequency signal acquired from the superimposition signal, the modulating voltage being supplied to the semiconductor laser pre-distorted by a high-pass transfer function for the modulation frequency, the derived signal being distorted by a low-pass transfer function of the modulation frequency that is at least approximately complementary to the high-pass transfer function, and this distorted signal being employed for generating a frequency modulation impressed on the reference signal for at least partial compensation of a distortion of the frequency-modulated optical signal that is contained in the superimposition signal; a high-pass filter means having a specific high-pass transfer function for high-pass filtering of the modulating voltage to be applied to the pump electrode of the semiconductor laser; a means for generating an optical superimposition signal by superimposing the frequency-modulated optical signal output by the semiconductor laser and the optical reference signal that is generated by an optical reference signal transmitter and has the specific reference frequency; a means for generating the derived signal that at least approximately corresponds to the modulation signal from the optical superimposition signal; and a means for quantized feedback of the derived signal to a reference semiconductor laser that generates the reference signal and for distorting the derived signal with the low-pass transfer function that is at least approximately complementary to the high-pass transfer function, the reference semiconductor laser being an optical transmitter than can be frequency-modulated by the distorted derived signal; wherein the frequency-modulated signal is generated at least partially free of the distortion caused by the thermal low-pass part. ### Claim 30 An apparatus for generating a frequency modulated optical signal, comprising: a semiconductor laser having at least a pump electrode; an electrical voltage that is applied to the pump electrode of the semiconductor laser and that modulates with a modulation frequency, the semiconductor laser having a frequency transfer function with a thermal low-pass part that is dependent on the modulation frequency, said low-pass part effecting a distortion of the generated, frequency-modulated optical signal; an electrical error signal generated from the frequency-modulated optical signal output by the semiconductor laser, said electrical error signal being at least approximately proportional with a specific proportionality constant to the thermal low-pass part of the frequency transfer function of the semiconductor laser and to the modulating voltage; a controllable frequency converter controlled with said error signal, said controllable frequency converter effecting a frequency shift of the signal frequency of the frequency-modulated optical signal, the proportionality constant of the error signal being selected such that the optical frequency of an optical signal at an output of the frequency converter is essentially proportional to the modulated voltage and is independent of the thermal low-pass part of the frequency transfer function of the semiconductor laser; an optical splitter for dividing the frequency-modulated optical signal into two signal parts; the controllable frequency converter, to which one of the two signal parts is supplied, effecting a frequency conversion at the supplied signal part dependent on a supplied control signal; and an error signal generator to which the other signal part is supplied and which, dependent on this signal part, generates an error signal that is supplied to the frequency converter as a control signal; wherein the frequency-modulated signal is generated at least partially free of the distortion caused by the thermal low-pass part. ### Claim 31 (depends on 5) The method according to claim 5, wherein the modulating voltage is supplied with high-pass filtering to the pump electrode and is supplied with low-pass filtering to the further electrode. ### Claim 32 (depends on 5) The method according to claim 5, wherein with which frequency ranges of the modulation frequency, wherein a magnitude of a modulation sensitivity referred to the pump electrode that is established by a relationship dependent on the modulation frequency between a change of the signal frequency of the frequency-modulated optical signal and a change of the modulating voltage to be applied to the pump electrode and/or a magnitude of a modulation sensitivity referred to the further electrode which is established by a relationship dependent on the modulation frequency between a change of the signal frequency of the frequency-modulated optical signal and a change of the modulating voltage to be applied to the further electrode that causes this change, has at least an approximately flat curve and covers an entire range between a lowest-first frequency and a second frequency that is higher in comparison to this first frequency. ### Claim 33 (depends on 32) The method according to claim 32, wherein a semiconductor laser is employed, whereby the magnitude of the modulation sensitivity referred to the pump electrode has at least an approximately flat curve at least in a frequency range between a defined, third frequency of the modulation frequency that is higher in comparison to the first frequency and lower in comparison to the second frequency, as well as for modulation frequencies that are higher than a defined, fourth frequency that is higher in comparison to the third frequency, and whereby the magnitude of the modulation sensitivity referred to the further electrode has at least an approximately flat curve at least in a frequency range between the first frequency and a fifth frequency of the modulation frequency that is higher in comparison to the fourth frequency. ### Claim 34 (depends on 7) The method according to claim 7, wherein with which frequency ranges of the modulation frequency, wherein a magnitude of a modulation sensitivity referred to the pump electrode that is established by a relationship dependent on the modulation frequency between a change of the signal frequency of the frequency-modulated optical signal and a change of the modulating voltage to be applied to the pump electrode and/or a magnitude of a modulation sensitivity referred to the further electrode which is established by a relationship dependent on the modulation frequency between a change of the signal frequency of the frequency-modulated optical signal and a change of the modulating voltage to be applied to the further electrode that causes this change, has at least an approximately flat curve and covers an entire range between a lowest-first frequency and a second frequency that is higher in comparison to this first frequency. ### Claim 35 (depends on 34) The method according to claim 34, wherein a semiconductor laser is employed, whereby the magnitude of the modulation sensitivity referred to the pump electrode has at least an approximately flat curve at least in a frequency range between a defined, third frequency of the modulation frequency that is higher in comparison to the first frequency and lower in comparison to the second frequency, as well as for modulation frequencies that are higher than a defined, fourth frequency that is higher in comparison to the third frequency, and whereby the magnitude of the modulation sensitivity referred to the further electrode has at least an approximately flat curve at least in a frequency range between the first frequency and a fifth frequency of the modulation frequency that is higher in comparison to the fourth frequency. ### Claim 36 (depends on 11) The method according to claim 11, wherein the method further comprises the steps of: continuously checking the signal dependent on the intermediate frequency to determine whether, at the characteristic frequency values, a deviation of an actual amplitude level of this signal from the specific amplitude level and, thus, a deviation of the intermediate frequency values from the characteristic frequency values is present; generating an optical signal that is dependent on such a deviation; and producing, by means of said output signal and the distorted signal superimposed on one another, a change of the reference frequency of the reference signal for at least partial compensation of the distortion of the frequency-modulated optical signal that is contained in the superimposition signal. ### Claim 37 (depends on 19) The apparatus according to claim 19, wherein the low-pass filter means has an inductance connected in a signal path for the modulating voltage that leads to the further electrode of the semiconductor laser and has a resistor connected between this signal path and the reference potential. ### Claim 38 (depends on 24) The apparatus according to claim 24, wherein the apparatus further comprises an adder or subtractor means for superimposing the output signal that is dependent on a deviation of the actual value from the rated value and the distorted derived signal and supplying these signals superimposed on one another to the frequency-modulatable reference signal transmitter.