JP4828447B2 - Frequency modulator - Google Patents

Description

  The present invention relates to an optical heterodyne type frequency modulator.

  In order to obtain a broadband modulated signal having a large modulation degree, a frequency modulator using an optical heterodyne method is disclosed (for example, see Non-Patent Document 1). In a frequency modulator using this heterodyne method, direct modulation is disclosed. A modulated signal is obtained by detecting an output obtained by combining the output of the first semiconductor laser optically modulated by the output of the second semiconductor laser for local transmission with a light receiving element.

  FIG. 5 shows a conventional example of an optical heterodyne type frequency modulator. A first light source 101 that outputs light frequency-modulated by an electric input signal supplied to the electric input terminal 100, a second light source 102 that outputs local oscillation light, output light from the first light source 101, and first light An optical coupler 103 that combines the output lights of the two light sources 102, an optical detector 104 that photoelectrically converts the light combined by the optical coupler 103 and outputs a modulated signal, and an output of the optical detector 104 And an output terminal 105 for outputting a modulated signal to be output to the outside.

  FIG. 6 is a graph showing an example of the time waveform of the output light of the first light source, where (a) shows the amplitude component and (b) shows the frequency component. Since the output light from the first light source 101 is frequency-modulated, the frequency component fluctuates with time. In addition, the amplitude component of the output light from the first light source 101 also fluctuates with time due to the time change of the voltage or current of the electric input signal. FIG. 7 is an example of the frequency spectrum of the modulated signal output from the optical detector. In the frequency spectrum of the modulated signal, in addition to the frequency modulation component 110 due to the time change of the amplitude component described with reference to FIG. 6A, the amplitude modulation component 111 due to the time change of the frequency component described with reference to FIG. Appears in the low frequency region. The amplitude modulation component 111 causes deterioration of signal quality when demodulating the modulated signal.

  As a method for suppressing the amplitude modulation component, a method is disclosed in which a signal having a phase opposite to that of the electrical input signal input to the first light source is input to the second light source (see, for example, Patent Document 1). The electric signal supplied to the input terminal is divided and output by the 180 ° distributor to the first and second signals having the same amplitude and frequency and the phase being inverted by 180 °, and the first signal is the first signal. The second signal is input to the light source, and the second signal is input to the second light source. Each light source includes a light emitting element, and each light emitting element is modulated by an input signal. After the output light from the first light source and the second light source is combined by the optical coupler, it is detected by the light receiving element and the modulated signal is output. When the output light from the first light source and the second light source is combined by the optical coupler, the amplitude modulation component is canceled and the amplitude modulation component is suppressed.

In the method of suppressing the amplitude modulation component, there is a problem that the amplitude modulation component cannot be canceled if the phase of the first light source and the second light source is shifted. In particular, the characteristics of the first light source and the second light source change sensitively due to changes in temperature and injection current, and also change due to secular change, so that there is a problem that an amplitude modulation component remains.
Nobu Shibata, Koji Kikushima, Naoya Sakurai, Takashi Watanabe “Optical Video Distribution System Using FM Batch Conversion”, IEICE Transactions B, Vol. J83-B, no. 7, pp. 948-959, July 2000 Japanese Patent Laid-Open No. 11-112433

  It is an object of the present invention to provide a frequency modulator with extremely small amplitude modulation components.

  In order to achieve the above object, the present invention provides an optical heterodyne type that outputs a modulated signal by combining the output light of the first light source and the output light of the second light source with an optical coupler and performing photoelectric conversion. In the frequency modulator, a light intensity modulator for canceling the amplitude modulation component is provided at the subsequent stage of the first light source.

Specifically, the frequency modulator according to the present invention includes an input terminal to which an electrical input signal is input, and light that is directly modulated by the electrical input signal that is input to the input terminal and whose optical frequency is changed by the direct modulation. A first light source that outputs frequency-modulated light and an optical frequency-modulated light that is output from the first light source are included in the optical frequency-modulated light based on an electrical input signal that is input to the input terminal. An optical intensity modulator that outputs an intensity-modulated signal that cancels an amplitude modulation component caused by an input signal, and a local oscillation light that is combined with the optical frequency-modulated light that is output from the first light source. 2 light sources, an optical coupler that combines the optical frequency modulated light output from the light intensity modulator and the local oscillation light output from the second light source, and the combined light from the optical coupler. Photoelectrically convert the modulated signal Comprising an optical detector for force, wherein the optical path from the first light source to said optical coupler, keeping the polarization of the output optical frequency modulated light of said first light source, the from the second light source The optical path to the optical coupler is characterized in that the polarization of the local oscillation light output from the second light source is maintained .

The amplitude modulation component included in the optical frequency modulation light is an amplitude component due to an electric input signal input to the input terminal. For this reason, the light intensity modulator modulates the intensity with an intensity modulation signal corresponding to the electrical input signal input to the input terminal, so that the amplitude is not affected by the change in the characteristics of the first light source and the second light source. The modulation component can be canceled out. Since the amplitude modulation component can be stably suppressed, it is possible to provide a frequency modulator having a very small amplitude modulation component.
Further, by maintaining the polarization of the frequency-modulated light and the local oscillation light, the frequency-modulated light and the local oscillation light can be incident on the optical coupler with the same polarization. Thereby, a stable output signal can be obtained.

  The frequency modulator according to the present invention includes a frequency difference control unit that controls a frequency difference between the optical frequency modulated light output from the first light source and the local oscillation light output from the second light source to be constant. preferable. By keeping the frequency difference between the first light source and the second light source constant, the carrier frequency of the modulated signal can be stabilized. As a result, it is possible to more effectively suppress the amplitude modulation component included in the modulated signal by compensating for the characteristic change due to the temperature and aging of each light source.

  In the frequency modulator according to the present invention, an amplitude modulation component extractor that extracts an amplitude modulation component by the electric input signal included in the optical frequency modulation light from a signal obtained by photoelectrically converting the combined light combined by the optical coupler. And a light intensity modulator control circuit that increases or decreases the signal intensity of the intensity modulation signal so that the signal output extracted by the amplitude modulation component extractor approaches zero. By controlling the level of the intensity modulation signal for intensity modulation of the light intensity modulator, the amplitude modulation component from the first light source can be efficiently suppressed by the feedback effect.

  In the frequency modulator according to the present invention, it is preferable that light intensity adjusting means for adjusting the intensity of the locally modulated light output from the second light source is further provided between the second light source and the optical coupler. By further including the light intensity adjusting means, the intensity of the local oscillation light input to the optical coupler can be adjusted without controlling the injection current to the second light source. Thereby, a modulated signal can be obtained with high efficiency.

  According to the present invention, it is possible to cancel the amplitude modulation component without being affected by the change in the characteristics of the first light source and the second light source. For this reason, since the amplitude modulation component can be stably suppressed, it is possible to provide a frequency modulator having an extremely small amplitude modulation component.

Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiment described below is an example of the configuration of the present invention, and the present invention is not limited to the following embodiment.
(Embodiment 1)
FIG. 1 is a configuration diagram of a frequency modulator according to the first embodiment. The frequency modulator 91 includes an input terminal 10, a distributor 16, a first light source 11 for modulation, a light intensity modulator 17, a second light source 12 for local oscillation, an optical coupler 13, An optical detector 14 and an output terminal 15 are provided. The input terminal 10 receives an electrical input signal from the outside and outputs the electrical input signal to the distributor 16. The distributor 16 receives an electrical input signal and outputs the electrical input signal to the first light source 11 and the light intensity modulator 17. The first light source 11 receives an electrical input signal and outputs optical frequency modulated light that has been optical frequency modulated to the light intensity modulator 17. The optical intensity modulator 17 receives the optical frequency modulated light and the electrical input signal, and outputs the optical frequency modulated light subjected to the intensity modulation to the optical coupler 13. The second light source 12 outputs the local oscillation light to the optical coupler 13. The optical coupler 13 receives the optical frequency modulated light and the local oscillation light, and outputs the combined light to the optical detector 14. The optical detector 14 receives the combined light and outputs the modulated signal to the output terminal 15. The output terminal 15 receives the modulated signal and outputs the modulated signal to the outside.

  The electric input signal input to the input terminal 10 is distributed by the distributor 16 into two electric input signals. One electrical input signal is input to the first light source 11 and optical frequency modulated. The other electric input signal is input to the light intensity modulator 17 and becomes an intensity modulation signal for modulating the light intensity of the optical frequency modulated light frequency-modulated by the first light source 11. The two electric input signals are signals having the same waveform as that of the electric input signal input to the input terminal 10 or signals having the same waveform and inverted phases as those of the electric input signal input to the input terminal 10. Preferably there is.

  The input terminal 10 receives an electrical input signal. The input terminal 10 is a terminal for inputting an electric input signal into the frequency modulator 91 from the outside of the frequency modulator 91. If the first light source 11 is a distributed feedback (DFB) laser, the electric input signal is, for example, a signal obtained by changing the injection current to the first light source 11 according to the optical frequency to be modulated.

  The distributor 16 distributes the electric input signal into two. Here, the phases of the two electrical input signals may be the same or inverted, but a phase that can cancel the amplitude modulation component by the light intensity modulator 17 is preferable. Since the distributor 16 generates an intensity modulation signal, the configuration of the light intensity modulator 17 and the frequency modulator 91 can be simplified. For example, when the light intensity modulator 17 increases or decreases the light output intensity with the same sign with respect to the increase or decrease of the input signal, the distributor 16 has an intensity that is opposite in phase to the electrical input signal that directly modulates the first light source 11. It is preferable to output the modulation signal to the light intensity modulator 17. On the other hand, when the light intensity modulator 17 increases or decreases the intensity of the optical output with the opposite sign with respect to the increase or decrease of the input signal, the distributor 16 preferably generates signals having the same phase.

  The first light source 11 is directly modulated by an electrical input signal input to the input terminal 10 and outputs optical frequency modulated light whose optical frequency is changed by the direct modulation. The first light source 11 is, for example, a semiconductor laser. Here, in the high frequency region, the optical frequency output from the semiconductor laser increases and decreases with the same sign as the input signal increases and decreases. Also, the intensity of the output light of the semiconductor laser increases and decreases with the same sign with respect to the increase and decrease of the input signal.

  The light intensity modulator 17 converts the optical frequency modulated light output from the first light source 11 into an amplitude modulation component based on the electrical input signal included in the optical frequency modulated light based on the electrical input signal input to the input terminal 10. The intensity is modulated with an intensity modulation signal to be canceled and output. For example, an intensity modulation signal is generated based on the electrical input signal output from the distributor 16, and the intensity of the optical frequency modulation light is modulated with the intensity modulation signal. The amplitude modulation component by the electric input signal included in the optical frequency modulation light is generated by a change in the intensity of the electric input signal input to the input terminal 10. For this reason, the optical intensity modulator 17 acquires the electrical input signal input to the input terminal 10 and modulates the intensity of the optical frequency modulated light with the intensity modulation signal corresponding to the intensity change of the electrical input signal. It is possible to cancel the amplitude modulation component due to the electric input signal included in the modulated light.

  The intensity modulation signal is, for example, a signal having the same waveform as the electric input signal. Alternatively, the intensity modulation signal is a signal obtained by inverting the phase of a signal having the same waveform as the electric input signal. By intensity modulation with the same waveform corresponding to the intensity change of the electrical input signal or with a waveform that is inverted only in phase, the amplitude modulation component due to the electrical input signal contained in the optical frequency modulated light can be canceled exactly. . The phase of the intensity modulation signal is determined according to the input / output characteristics of the light intensity modulator 17. For example, when the light intensity modulator 17 increases or decreases the intensity of the output light with the same sign as the input signal increases or decreases, the intensity modulated signal is a signal obtained by inverting the phase of a signal having the same waveform as the electrical input signal. It is. Further, when the light intensity modulator 17 increases or decreases the intensity of the output light with the opposite sign with respect to the increase or decrease of the input signal, the intensity modulation signal is a signal having the same waveform as the electrical input signal.

  FIG. 2 shows an example of a frequency spectrum, where (a) shows the frequency spectrum of the modulated signal when no intensity modulation signal is input to the light intensity modulator, and (b) shows the intensity modulation by the light intensity modulator. The frequency spectrum of the modulated signal when a signal is input is shown. In the frequency spectrum of the modulated signal shown in FIG. 2A, not only the frequency modulation component 110 obtained by optical frequency modulation of the first light source but also the amplitude modulation component 111 generated by the electric input signal obtained by modulating the first light source appears. . The light intensity modulator suppresses the amplitude modulation component 111 by intensity-modulating the optical frequency modulated light output from the first light source with an intensity modulation signal. For example, when the phase or signal intensity of the intensity modulation signal necessary for the light intensity modulator to cancel out the amplitude modulation component or the bias thereof is obtained in advance, it is converted into the obtained phase or signal intensity, A function for shifting the signal intensity is provided in at least one of the distributor and the light intensity modulator, or a configuration having the function is provided between the distributor and the light intensity modulator. Furthermore, the intensity modulation signal is preferably a signal intensity that can make the amplitude modulation component 111 zero. For example, a light intensity modulation control unit described later is provided. Further, at least one of the distributor and the light intensity modulator is provided with a configuration for that purpose. FIG. 2B shows the frequency modulation component 110 and the amplitude modulation component 111 of the modulated signal when the intensity modulation signal is input to the light intensity modulator. Since the amplitude modulation component included in the modulated signal is canceled by the intensity modulation signal, the amplitude modulation component 111 due to the electric input signal can be suppressed.

  The second light source 12 outputs local oscillation light to be combined with the optical frequency modulated light output from the first light source 11. The optical coupler 13 combines the optical frequency modulated light output from the light intensity modulator 17 and the local oscillation light output from the second light source 12. The optical detector 14 photoelectrically converts the combined light combined by the optical coupler 13 and outputs a modulated signal.

  Here, the optical waveguide and components between the first light source 11 and the optical coupler 13 and the optical waveguide and components between the second light source 12 and the optical coupler 13 are of the polarization maintaining type. Is preferably used. The polarization of the propagating light is stabilized so that the frequency-modulated light output from the first light source 11 and the local oscillation light output from the second light source 12 are incident on the optical coupler 13 with the same polarization. Keep it. When guided by a normal optical fiber, it was difficult to obtain a stable output signal because the polarizations did not match. However, by using a polarization maintaining component such as a polarization maintaining fiber, A stable output signal can be obtained. The components for maintaining the polarization are described in, for example, the polarization maintaining component in Section 5.6 of the “Next Generation Ultra High-Speed Optical Communication Technology” of the Technical Information Association.

  Furthermore, the frequency modulator 91 preferably includes a frequency difference control unit 81. The frequency difference control unit 81 controls the frequency difference between the optical frequency modulated light output from the first light source 11 and the local oscillation light output from the second light source 12 to be constant. When the first light source 11 and the second light source 12 are in a free-run state, the output intensity and the modulation degree of the output light of the first light source 11 and the second light source 12 cannot be maintained at the same values as the initial values. . However, since the frequency difference control unit 81 keeps the frequency difference between the optical frequency modulated light output from the first light source 11 and the local oscillation light output from the second light source 12, the first light source 11 and the second light source 11. Thus, it is possible to compensate for fluctuations in characteristics associated with changes over time and temperature of the light source 12. Since the frequency difference between the first light source 11 and the second light source 12 can be made constant, the phase of the frequency-modulated light and the local oscillation light can be matched, and the carrier frequency of the modulated signal can be stabilized. Since the modulated signal is generated stably, the signal intensity of the amplitude modulation component can be more effectively suppressed.

  For example, the frequency difference control unit 81 detects a beat signal obtained by inputting the optical frequency modulated light output from the first light source 11 and the local oscillation light output from the second light source 12 to the optical coupler 13. An optical detector 26, a prescaler 21 that divides the beat signal detected by the optical detector 26, a reference frequency oscillator 23 that oscillates a predetermined reference frequency, and a waveform of the reference frequency that the reference frequency oscillator 23 oscillates. A phase comparator 22 that detects a phase difference from the divided waveform of the beat signal output from the prescaler 21, and a control circuit 24 that changes the frequency of the second light source 12 in accordance with the phase difference detected by the phase comparator 22. And comprising.

  The optical detector 26 is, for example, a photodetector that photoelectrically converts combined light. The reference frequency oscillated by the reference frequency oscillator 23 is an initial value between the optical frequency modulated light output from the first light source 11 and the local oscillation light output from the second light source 12 when the optical detector 26 detects a beat signal. It is the frequency difference in the state. By providing the prescaler 21 and the phase comparator 22, the frequency difference between the optical frequency modulated light output from the first light source 11 and the local oscillation light output from the second light source 12 is detected. The relative characteristic deviation from the initial state of the second light source 12 can be detected. By detecting the relative characteristic shift, even when the characteristics of the first light source 11 and the second light source 12 change, the optical detector 14 can stably output the modulated signal. Further, in order to detect the frequency difference between the optical frequency modulated light output from the first light source 11 and the local oscillation light output from the second light source 12, a reference frequency oscillator 23, a prescaler 21 and a phase comparator 22 are used. Thus, the response speed of the frequency difference control unit 81 can be increased as compared with the configuration for measuring the frequency. For example, the control circuit 24 changes the frequency of the second light source 12 so that the difference between the frequency of the beat signal detected by the optical detector 26 and the reference frequency becomes zero. The control circuit 24 changes the temperature and driving current of the second light source 12 in order to change the frequency of the second light source 12. The frequency difference control unit 81 may be configured to acquire the optical frequency modulated light output from the first light source 11 and the local oscillation light output from the second light source 12 and detect these frequency differences. .

  Furthermore, in order to more effectively suppress the amplitude modulation component in the light intensity modulator 17, the frequency modulator 91 preferably includes a light intensity modulation control unit 82. The intensity modulation control unit 82 includes, for example, a photodetector 18, an amplitude modulation component extractor 19, and a light intensity modulator control circuit 20. The photodetector 18 photoelectrically converts the combined light combined by the optical coupler 13. For example, a photo detector. The amplitude modulation component extractor 19 extracts an amplitude modulation component based on an electrical input signal included in the optical frequency modulated light output from the first light source 11 from the photoelectrically converted signal of the photodetector 18. For example, the amplitude modulation component 111 is extracted from the modulated signal as shown in FIG. 2B by a low-pass filter that transmits a frequency band lower than the frequency modulation component 110. The light intensity modulator control circuit 20 increases or decreases the signal intensity of the intensity modulation signal of the light intensity modulator 17 so that the signal output extracted by the amplitude modulation component extractor 19 approaches zero. The light intensity modulator control circuit 20 is, for example, an automatic gain control circuit.

  The light intensity modulation control unit 82 converts a part of the combined light combined by the optical coupler 13 into an electric signal by the photodetector 18, and then extracts and extracts the amplitude modulation component by the amplitude modulation component extractor 19. By controlling the gain of the light intensity modulator control circuit 20 with the amplitude modulation component, the level of the intensity modulation signal for intensity modulation of the light intensity modulator 17 is controlled, and the amplitude modulation component from the first light source 11 is obtained by the feedback effect. Can be reduced efficiently. FIG. 3 is a graph showing an example of the modulated signal output from the optical detector, where (a) shows the time waveform of the amplitude component, (b) shows the time waveform of the frequency component, and (c) shows the frequency spectrum. Since the amplitude modulation component is suppressed by the feedback effect, the time component of the amplitude component is eliminated as shown in FIG. 3A, and only the time component of the frequency component is left as shown in FIG. As shown in (c), it is possible to generate a modulated signal in which only the frequency modulation component appears in the frequency spectrum.

  If the frequency modulator 91 includes the frequency difference control unit 81 and the light intensity modulation control unit 82, the frequency difference control unit 81 stabilizes the frequencies of the first light source 11 and the second light source 12, so that It is possible to effectively suppress amplitude modulation components that compensate for temperature and aging. Further, a polarization maintaining type optical waveguide and components between the first light source 11 and the optical coupler 13 and an optical waveguide and components between the second light source 12 and the optical coupler 13 are used. If used, a stable modulated signal output with good quality can be obtained.

(Embodiment 2)
FIG. 4 is a configuration diagram of a frequency modulator according to the second embodiment. The frequency modulator 92 shown in FIG. 4 includes a light intensity adjusting unit 25 that adjusts the intensity of the locally modulated light output from the second light source 12 described with reference to FIG. 1 and between the second light source 12 and the optical coupler 13. Further prepare for. The light intensity adjusting means 25 is, for example, an optical attenuator that attenuates light. By further including the light intensity adjusting means 25, the intensity of the local oscillation light output from the second light source 12 with respect to the intensity of the optical frequency modulated light output from the first light source 11 can be optimized. The output intensity of the local oscillation light can also be controlled by controlling the injection current to the second light source 12, but if the injection current is changed, the modulation characteristics and the average optical frequency also fluctuate simultaneously, so that it does not depend on the injection current. Control is preferred. By optimizing the intensity of the local oscillation light by the light intensity adjusting means 25, it is possible to efficiently obtain a high-quality modulated signal in a stable state without changing the characteristics of the second light source 12. it can. In the above embodiment, an optical attenuator is used as the light intensity adjusting means 25. However, an optical amplifier or a combination of an optical amplifier and an optical attenuator may be used.

  In the first and second embodiments, the light detectors 14 and 26 and the light detector 18 are described as independent light detectors, but the present invention is not limited to this. For example, any two of the light detectors 14 and 26 and the light detector 18 may be combined with one light detector. Furthermore, all of the photodetectors 14, 26 and the photodetector 18 may be combined with one optical detector.

  Since the present invention outputs a modulated signal with high quality in which the amplitude modulation component is suppressed, it is possible to transmit signal light whose optical intensity is modulated with a high quality FM signal in an optical transmission system.

1 is a configuration diagram of a frequency modulator according to Embodiment 1. FIG. It is an example of a frequency spectrum, (a) shows the frequency spectrum of the modulated signal when no intensity modulation signal is inputted to the light intensity modulator, and (b) shows the intensity modulation signal inputted to the light intensity modulator. Shows the frequency spectrum of the modulated signal. It is a graph which shows an example of the modulated signal output from an optical detector, (a) is a time waveform of an amplitude component, (b) is a time waveform of a frequency component, (c) shows a frequency spectrum. 6 is a configuration diagram of a frequency modulator according to Embodiment 2. FIG. It is a conventional example of an optical heterodyne type frequency modulator. It is a graph which shows an example of the time waveform of the output light of a 1st light source, (a) shows an amplitude component and (b) shows a frequency component. It is an example of the frequency spectrum of the modulated signal output from an optical detector.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10,100 Input terminal 11,101 1st light source 12,102 2nd light source 13,103 Optical coupler 14,104 Photodetector 15,105 Output terminal 16 Divider 17 Optical intensity modulator 18 Optical detector 19 Amplitude Modulation component extractor 20 Light intensity modulator control circuit 21 Prescaler 22 Phase comparator 23 Reference frequency oscillator 24 Control circuit 25 Light intensity adjusting means 26 Optical detector 81 Frequency difference control unit 82 Light intensity modulation control unit 91, 92 Frequency modulator 110 Frequency modulation component in frequency spectrum of modulated signal 111 Amplitude modulation component in frequency spectrum of modulated signal

Claims (4)

An input terminal to which an electrical input signal is input;
A first light source that is directly modulated by an electrical input signal input to the input terminal and outputs optical frequency modulated light having an optical frequency changed by the direct modulation;
An intensity-modulated signal for canceling the amplitude-modulated component due to the electrical input signal contained in the optical frequency-modulated light, based on the electrical input signal input to the input terminal, from the optical frequency-modulated light output from the first light source A light intensity modulator that outputs an intensity modulated signal at
A second light source for outputting local oscillation light for multiplexing with the optical frequency modulated light output from the first light source;
An optical coupler for combining the optical frequency modulated light output from the light intensity modulator and the local oscillation light output from the second light source;
An optical detector that photoelectrically converts the combined light combined by the optical coupler and outputs a modulated signal;
Equipped with a,
The optical path from the first light source to the optical coupler maintains the polarization of the optical frequency modulated light output from the first light source,
The frequency modulator , wherein an optical path from the second light source to the optical coupler maintains a polarization of the local oscillation light output from the second light source .   The frequency difference control part which controls further the frequency difference of the optical frequency modulation light which the said 1st light source outputs, and the local oscillation light which the said 2nd light source outputs, It is characterized by the above-mentioned. Frequency modulator. An amplitude modulation component extractor for extracting an amplitude modulation component by the electric input signal included in the optical frequency modulation light from a signal obtained by photoelectrically converting the combined light combined by the optical coupler;
A light intensity modulator control circuit that increases or decreases the signal intensity of the intensity modulation signal so that the signal output extracted by the amplitude modulation component extractor approaches zero;
Frequency modulator according to claim 1 or 2, further comprising a. A light intensity adjusting means for adjusting the intensity of the local modulated light output from the second light source, any one of claims 1 to 3, further comprising between said optical coupler and the second light source A frequency modulator according to 1.

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