JP4026389B2 - Optical intensity modulator and optical frequency shifter - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical intensity modulator and an optical frequency shifter that shift the frequency of the optical frequency by intensity modulating an optical signal having a predetermined optical frequency incident from a light source.
[0002]
[Prior art and problems to be solved by the invention]
The optical frequency shifter changes the frequency by intensity-modulating light source light having a single optical frequency emitted from the light source and extracting a sideband component of the light intensity-modulated wave obtained by the intensity modulation. In this case, the frequency displacement amount is defined by the frequency of the modulation signal (electric signal) used for intensity modulation. In such an optical frequency shifter, a Mach-Zehnder type optical modulator is usually used as an optical intensity modulator. This Mach-Zehnder type optical modulator is excellent in high-speed response, but has a modulation operating point. However, it has a drawback that it tends to drift due to the influence of ambient temperature and the like.
[0003]
Japanese Patent Application Laid-Open No. 2000-12215 discloses a technique for feedback control of a bias voltage of a modulation signal in accordance with a frequency variation of a sideband component as a technique for overcoming the drawbacks of such a Mach-Zehnder type optical modulator. . However, this technique has a problem that a high-frequency modulation signal cannot be used due to the band limitation of the light receiver that converts the light intensity modulated wave into an electric signal, that is, the frequency displacement amount is limited. .
[0004]
The present invention has been made in view of the above-described problems, and an object thereof is to suppress the drift of the modulation operation point while eliminating the restriction on the frequency displacement amount.
[0005]
[Means for Solving the Problems]
In order to achieve the above object, in the present invention, as a first means related to a light intensity modulator, a modulation control unit that outputs a modulation signal of a predetermined frequency, and an optical signal output as a carrier wave from a light source are modulated. A light intensity modulator comprising: a modulator that modulates light intensity based on the modulation signal output from the control unit and outputs a light intensity modulated wave; wherein the modulation control unit includes the carrier wave, the light intensity modulated wave, A beat detection unit that detects a beat component of the modulator, and a bias voltage that minimizes a beat component that is a direct current component among beat components detected by the beat detection unit is superimposed on the modulation signal, and the modulator And a bias control unit that outputs to the signal.
[0006]
Further, as a second means related to the light intensity modulator, in the first means, the modulator outputs an optical output according to a bias voltage superimposed on the modulation signal output from the bias controller. It is a Mach-Zehnder type modulator having changing characteristics, and employs a configuration in which the bias voltage is set to a voltage at which the optical output of the modulator is minimized.
[0007]
As a third means related to the light intensity modulator, in the first or second means, the beat detection unit includes a carrier wave optical demultiplexer for demultiplexing a part of the carrier wave, and the light intensity modulated wave. A modulation wave optical demultiplexer that demultiplexes a part of the optical signal, a carrier wave input from the carrier wave optical demultiplexer, and an optical beam combining the light intensity modulated wave input from the modulation wave optical demultiplexer. A wave receiver, a light receiver that converts the combined light output from the optical multiplexer into a combined electric signal, and a low-pass filter that selectively outputs only a low-frequency component from the combined electric signal output from the light receiver. The composition which consists of is adopted.
[0008]
As a fourth means related to the light intensity modulator, in the third means, a configuration is adopted in which a frequency selective light receiver having a light receiving sensitivity only for a low frequency component is used instead of the light receiver and the low pass filter. To do.
[0009]
On the other hand, in the present invention, as a first means related to the optical frequency shifter, the sideband component is extracted from the light intensity modulated wave output from the light intensity modulator described in any one of the first to fourth, and the optical frequency shift is performed. A configuration is adopted in which frequency selection means for outputting as a signal is provided.
[0010]
Further, as the second means related to the optical frequency shifter, in the first means, a configuration is adopted in which the frequency selection means is an optical bandpass filter that selectively transmits the lower sideband or the upper sideband.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment of an optical intensity modulator and an optical frequency shifter according to the present invention will be described with reference to the drawings.
[0012]
FIG. 1 is a functional configuration diagram of a light intensity modulator and an optical frequency shifter in the present embodiment. In this figure, reference numeral 1 is a light source, 2, 4 and 5 are optical splitters, 3 is an external modulator, 6 is an optical multiplexer, 7 is a BPF (band pass filter), 8 is a light receiver, and 9 is an LPF (low pass). Filter) 10 is an A / D (A / D converter), 11 is a control unit, 12 is a D / A (D / A converter), 13 is a bias circuit, and 14 is a signal source.
[0013]
Among these components, the optical branching units 4 and 5, the optical multiplexer 6, the optical receiver 8, the LPF 9, the A / D 10, the control unit 11, the D / A 12, the bias circuit 13, and the signal source 14 surrounded by a two-point difference line are The modulation control unit MC is configured. Among the components constituting the modulation control unit MC, the optical branching units 4 and 5, the optical multiplexer 6, the optical receiver 8 and the LPF 9 surrounded by a dotted line constitute a beat detection unit BS and the remaining A / D 10. The control unit 11, the D / A 12, the bias circuit 13, and the signal source 14 constitute a bias control unit BC.
[0014]
The light source 1 generates light source light (carrier wave a) having a frequency ν and outputs it to the optical splitter 2. The optical branching device 2 branches and outputs the carrier wave a to the external modulator 3 and the optical branching device 4. The external modulator 3 is a Mach-Zehnder type optical modulator, which modulates the intensity of the carrier wave a with the modulation signal b (electric signal) input from the bias circuit 13 and outputs it to the optical branching unit 5 as a light intensity modulated wave c. The optical branching device 4 branches the carrier wave a, outputs one to the outside as main line output light, and outputs the other to the optical multiplexer 6. The optical branching device 5 branches the light intensity modulated wave c, outputs one to the BPF 7, and outputs the other to the optical multiplexer 6.
[0015]
The optical combiner 6 combines the carrier wave a and the light intensity modulated wave c, and outputs the combined light d to the light receiver 8. The BPF 7 separates only the upper sideband component or the lower sideband component from the light intensity modulated wave c and outputs it as the main line output light to the outside. The light receiver 8 converts the combined light d into an electrical signal (light reception signal) and outputs it to the LPF 9. The LPF 9 extracts only the direct current vicinity component (beat signal) of the received light signal and outputs it to the A / D 10. The A / D 10 converts the beat signal into a digital signal (beat data) and outputs it to the control unit 11.
[0016]
The control unit 11 outputs bias data corresponding to the beat data to the D / A 12. Here, the bias data is data set by the control unit 11 so that the beat data sequentially decreases in time series. The D / A 12 converts such bias data into an analog signal (bias voltage) and outputs it to the bias circuit 13. The bias circuit 13 superimposes a bias voltage, which is a DC component, on the modulation signal (only the AC component) input from the signal source 14 and outputs the superimposed signal to the external modulator 3. The signal source 14 oscillates a modulation signal (only an AC component) having a single frequency f and outputs it to the bias circuit 13.
[0017]
Next, operations of the thus configured optical intensity modulator and optical frequency shifter will be described in detail with reference to FIGS.
[0018]
First, FIG. 2 is a diagram showing operating characteristics of the external modulator 3. As shown in this figure, the operating characteristics of the external modulator 3 are such that the optical output changes in a sine wave shape with respect to the bias voltage. For such an operation characteristic, the control unit 11 minimizes the fundamental wave component a0 (the component having the same frequency as that of the carrier wave a) included in the light intensity modulated wave c so that the beat signal takes a minimum value. The bias voltage is sequentially feedback-controlled so that That is, the control unit 11 operates so as to maintain the point A at which the light output takes the minimum value as the modulation operation point. The operating point of the light intensity modulator is generally set at a point B whose operating characteristics are close to a straight line, for example, but in the external modulator 3 of this embodiment, the point A at which the light output takes the minimum value is the modulating operation. Set to a point.
[0019]
On the other hand, FIG. 3 is a diagram showing the spectrum of the optical output of the external modulator 3, that is, the light intensity modulated wave c, with the operating point set to the point A in this way. Since the light intensity modulation wave c is obtained by intensity-modulating the carrier wave a having the frequency ν with the modulation signal having the modulation operating point as the point A and the modulation signal having the frequency f, the fundamental wave component c0 having the same frequency as the fundamental wave component a0 is obtained. The upper sideband component c1 of the frequency (ν + f) shifted upward by the frequency f from c0, the lower sideband component c2 of the frequency (ν−f) shifted downward by the frequency f, and the point A are the operations of the external modulator 3. Since it is set in the non-linear region in the characteristics, it is composed of higher-order upper and lower sideband components (not shown because the level is small).
[0020]
FIG. 3A shows a spectrum component a0 having a frequency ν corresponding to the carrier wave a. FIG. 3B shows the spectrum of the light intensity modulated wave c, that is, the fundamental wave component c0, the upper sideband component c1, and the lower sideband component c2. FIG. 3C shows the beat components db, d1, and d2 of the combined light d. Since the multiplexed light d is a sum of the carrier wave a and the light intensity modulated wave c, the beat component db of the fundamental wave component a0 of the carrier wave a having the same frequency ν and the fundamental wave component c0 of the light intensity modulated wave c, and the carrier wave Beat components d1 and d2 of the fundamental wave component a0 of a and the upper sideband component c1 and lower sideband component c2 of the light intensity modulated wave c are generated.
[0021]
FIG. 4 is a diagram showing the level relationship between the fundamental wave component c0 and the upper and lower sideband components c1 and c2 in the optical output of the external modulator 3. As shown in FIG. 4, when the bias voltage is changed, the levels change opposite to each other. That is, when the fundamental wave component c0 has the maximum value, the upper and lower side band components c1 and c2 have the minimum value, and when the fundamental wave component c0 has the minimum value, the upper and lower side band components c1 and c2 have the maximum value. Moreover, the fundamental wave component c0 and the upper and lower sideband components c1 and c2 have a steep change rate before and after taking the minimum value, while the change rate before and after taking the maximum value is very gentle. The modulation operation point A of the external modulator 3 described above corresponds to the point A at which the fundamental wave component c0 takes the minimum value as shown in FIG.
[0022]
Here, among the beat components db, d1, and d2, only the beat component db that is a component near the direct current passes through the LPF 9 and is input to the A / D converter 10. Since the beat component db is generated by adding the fundamental wave component a0 of the carrier wave a and the fundamental wave component c0 of the light intensity modulated wave c by the optical multiplexer 6, the level thereof is the same as the level of the fundamental wave component a0. It depends on the level of the wave component c0. The level of the fundamental wave component a0 is constant because it is a component of the carrier wave a. On the other hand, since the fundamental wave component c0 is a component of the light intensity modulated wave c, it depends on the bias voltage set by the control unit 11. The control unit 11 performs feedback control of the bias voltage so that the beat component db, that is, the fundamental wave component c0 that changes depending on the bias voltage, takes a minimum value. As a result, the modulation operation point of the light intensity modulated wave c is controlled. Is maintained at point A.
[0023]
According to the present embodiment, the modulation operation point of the light intensity modulated wave c is determined based on the beat signal obtained by receiving the beat component db, that is, the DC component, of the combined light d output from the optical combiner 6 by the light receiver 8. Since feedback control is performed, that is, the light intensity modulated wave is not directly input to the light receiver and converted into an electric signal as in the prior art, the beat component db of the combined light d output from the optical multiplexer 6 is received by the light receiver. Since the light is received at 8, it is possible to suppress the drift of the modulation operating point without being subjected to the band limitation by the light receiver 8.
[0024]
In addition, the modulation operating point A is set to a point where the rate of change of the fundamental wave component c0 of the light intensity modulated wave c changes steeply as shown in FIG. 4, so the stability of the modulation operating point is extremely high. Further, since the upper and lower sideband components c1 and c2 of the light intensity modulated wave c have the maximum value at the modulation operating point A, the high level upper and lower sideband components c1 and c2, that is, the sideband component c1 having a good S / N ratio. , C2 can be obtained.
[0025]
In the above embodiment, a Mach-Zehnder type optical modulator is used as the external modulator 3, but the present invention is not limited to this, and can be applied to other types of optical modulators. .
[0026]
【The invention's effect】
As described above, according to the present invention, a modulation control unit that outputs a modulation signal of a predetermined frequency and an optical signal output as a carrier wave from a light source are based on the modulation signal output from the modulation control unit. In a light intensity modulator comprising a modulator that modulates light intensity and outputs a light intensity modulated wave, the modulation control unit includes a beat detection unit that detects a beat component of the carrier wave and the light intensity modulated wave, Since it comprises a bias control unit that superimposes a bias voltage on the modulation signal and outputs it to the modulator such that the beat component that is a direct current component among beat components detected by the beat detection unit is minimized, It is possible to suppress the drift of the modulation operating point while eliminating the limitation of the frequency displacement amount by the light receiver in the beat detection unit.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a functional configuration of an embodiment of the present invention.
FIG. 2 is a characteristic diagram showing the operation of one embodiment of the present invention.
FIG. 3 is a characteristic diagram showing the spectrum of various signals in one embodiment of the present invention.
FIG. 4 is a characteristic diagram showing a level relationship between a carrier wave and a sideband in one embodiment of the present invention.
[Explanation of symbols]
MC …… Modulation control unit BS …… Beat detection unit BC …… Bias control unit 1 …… Light source 2 …… Optical splitter 3 …… External modulator ( modulator )
4 …… Optical demultiplexer for carrier wave 5 …… Optical demultiplexer for modulated wave 6 …… Optical multiplexer 7 …… BPF (optical bandpass filter; frequency selection means)
8 …… Receiver 9 …… LPF (low-pass filter)
10 …… A / D
11 …… Control unit 12 …… D / A
13 …… Bias circuit 14 …… Signal source

Claims (6)

Based on the modulation signal output from the modulation control unit, a modulation control unit (MC) that outputs a modulation signal (b) having a predetermined frequency, and an optical signal output as a carrier wave (a) from the light source (1). A light intensity modulator comprising: a modulator (3) that modulates light intensity and outputs a light intensity modulated wave (c);
The modulation control unit (MC)
A beat detector (BS) for detecting beat components of the carrier wave and the light intensity modulated wave;
A bias voltage that minimizes a beat component (d b ) that is a direct current component among beat components (d b , d 1 , d 2 ) detected by the beat detector is superimposed on the modulation signal. A light intensity modulator comprising: a bias control unit (BC) that outputs to the modulator. The modulator is a Mach-Zehnder type modulator having a characteristic that an optical output changes according to a bias voltage superimposed on the modulation signal output from the bias control unit,
The light intensity modulator according to claim 1, wherein the bias voltage is set to a voltage at which an optical output of the modulator is minimized. The beat detector (BS)
An optical demultiplexer (4) for demultiplexing a part of the carrier;
A modulated wave demultiplexer (5) for demultiplexing a part of the light intensity modulated wave;
An optical multiplexer (6) for combining the carrier wave input from the carrier wave optical demultiplexer (4) and the light intensity modulated wave input from the modulated wave optical demultiplexer (5);
A light receiver (8) for converting the combined light output from the optical combiner (6) into a combined electrical signal;
The light intensity modulator according to claim 1 or 2, comprising a low-pass filter (9) that selectively outputs only a low-frequency component from the combined electrical signal output from the light receiver (8). .   4. The light intensity modulator according to claim 3, wherein a frequency selective light receiver having a light receiving sensitivity only for a low frequency component is used instead of the light receiver and the low pass filter.   5. A light comprising frequency selection means (7) for extracting a sideband component from a light intensity modulated wave output from the light intensity modulator according to claim 1 and outputting it as an optical frequency shift signal. Frequency shifter.   6. The optical frequency shifter according to claim 5, wherein the frequency selection means (7) is an optical bandpass filter that selectively transmits the lower sideband or the upper sideband.

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