JP5367299B2 - Light modulator - Google Patents

Description

  The present invention relates to an optical modulation device.

  Conventionally, broadband optical modulators have been studied. FIG. 6 shows an example of a conventional light modulation device. The optical modulation device 600 is an optical modulation device that uses an optical heterodyne method to obtain a phase modulation signal or a frequency modulation signal having a large modulation degree (see Non-Patent Document 1), and is a first modulated directly by an electrical signal. A light source 601, a second light source 602 for local signals, an optical coupler 603 that combines the output light of the first light source 601 and the second light source 602, and the output light of the optical coupler 603 as an optical heterodyne. An optical heterodyne detection unit 604 that detects and outputs a frequency modulation signal, and an optical transmission unit 605 that modulates the intensity of the frequency modulation signal output from the optical heterodyne detection unit 604 and outputs the modulated signal to the optical transmission line.

  The light modulation device as shown in FIG. 6 has a drawback that the quality of the demodulated signal is deteriorated because the phase noise of a light source such as a semiconductor laser is large. Then, what was shown in FIG. 7 was proposed as an optical modulation apparatus which can improve the problem of a phase noise (refer patent document 1). The light modulation device 700 includes a light source 701, an optical branching device 702 that branches output light from the light source 701 into a first optical path and a second optical path, an optical frequency shifter 703 provided in the first optical path, An optical phase modulator 704 provided in the second optical path, an optical coupler 705 that couples output light from the optical frequency shifter 703 and the optical phase modulator 704, and optical heterodyne detection of the output light from the optical coupler 705 An optical heterodyne detection unit 706 that outputs an angle modulation signal, and an optical transmission unit 707 that intensity-modulates the angle modulation signal output by the optical heterodyne detection unit 706 and outputs it to the optical transmission line. According to the optical modulator 700, the light in the first optical path is shifted by the intermediate frequency by the optical frequency shifter 703, the light in the second optical path is subjected to angle modulation by the optical phase modulator 704, and these lights are combined. As a result, a beat signal centered on the intermediate frequency is generated, and thus phase noise due to the light source is not generated in principle.

JP 2001-133824 A K. Kikushima, et al, “Optical Super Wide-Band FM Modulation Scheme and Its Application to Multi-Channel AM Video Transmission Systems,” IOOC'95 Technical Digest, vol. 5 PD2-7, pp. 33-34

  However, the conventional optical modulation device has a problem that the distortion characteristics of the angle modulation signal are not stable due to the deviation of the operating point caused by the DC drift caused by the optical phase modulator.

  An object of the present invention is made in view of such problems, and an object of the present invention is to optimize the distortion characteristics of an angle-modulated signal and stably operate it in an optical modulation device including an optical phase modulator. is there.

In order to achieve such an object, the invention described in claim 1 is directed to a light source and a first optical branch that branches unmodulated output light from the light source into a first optical path and a second optical path. An optical frequency shifter provided in the first optical path for shifting the light branched into the first optical path by an intermediate frequency, and angle-modulating the light branched into the second optical path; An optical phase modulator provided in the second optical path; an optical coupler that couples output light from the optical frequency shifter and output light from the optical phase modulator; and output light from the optical coupler. An optical heterodyne detection unit that performs heterodyne detection and outputs an angle modulation signal centered on the intermediate frequency; and an optical transmission unit that intensity-modulates the angle modulation signal output by the optical heterodyne detection unit and outputs the intensity modulation signal to an optical transmission line. An optical modulation device comprising the optical phase A DC bias control unit that generates a DC bias voltage to be input to the adjuster, a pilot signal generation unit that superimposes a pilot signal on the DC bias voltage output by the DC bias control unit, and output light from the optical phase modulator. A second optical branching device that branches, a photoelectric conversion unit that converts an optical signal branched by the second optical branching device into an electrical signal, and a signal processing unit that processes the electrical signal output by the photoelectric conversion unit The signal processing unit extracts and monitors the residual component of the pilot signal as an intensity from the electrical signal output from the photoelectric conversion unit, and controls the DC bias control unit according to the residual component It is characterized by doing.

Further, the invention according to claim 2 is characterized in that, in claim 1, the signal processing unit extracts the residual distortion component of the pilot signal as an intensity from the electric signal output from the photoelectric conversion unit, and monitors the residual signal. The DC bias controller is controlled so that the distortion component is minimized.

According to a third aspect of the present invention, in the first aspect, the signal processing unit extracts and monitors the residual signal component of the pilot signal as an intensity from the electric signal output from the photoelectric conversion unit, and the residual signal The direct current bias control unit is controlled so that the signal component becomes maximum.

According to a fourth aspect of the present invention, there is provided a light source, a first optical branching device that branches unmodulated output light from the light source into a first optical path and a second optical path, and the first optical path. An optical frequency shifter provided in the first optical path that shifts the light branched into the first optical path and an angle modulation of the light branched into the second optical path are provided in the second optical path. An optical phase modulator, an optical coupler for combining the output light from the optical frequency shifter and the output light from the optical phase modulator, and optical heterodyne detection of the output light of the optical coupler to obtain the intermediate frequency An optical modulation device comprising: an optical heterodyne detection unit that outputs an angle modulation signal with a center; and an optical transmission unit that intensity-modulates the angle modulation signal output by the optical heterodyne detection unit and outputs the intensity modulation signal to an optical transmission line, DC via input to the optical phase modulator A DC bias control unit for generating a voltage, and a pilot signal generator for superimposing a pilot signal on a DC bias voltage, wherein the DC bias control unit outputs a second optical splitter for splitting the output light from the optical coupler A photoelectric conversion unit that converts an optical signal branched by the second optical branching device into an electrical signal, a signal demodulation unit that demodulates an angle modulation signal output from the photoelectric conversion unit, and the signal demodulation unit A signal processing unit that processes the demodulated electrical signal to be output, wherein the signal processing unit monitors the demodulated electrical signal output by the signal demodulation unit, and according to the demodulated electrical signal The DC bias control unit is controlled.

According to a fifth aspect of the present invention, in the fourth aspect, the signal processing unit monitors the demodulated electric signal output from the signal demodulating unit, and the distortion component of the demodulated pilot signal is minimized. The direct-current bias control unit is controlled as described above.

According to a sixth aspect of the present invention, in the fourth aspect, the signal processing unit monitors the demodulated electric signal output from the signal demodulating unit, and the signal component of the demodulated pilot signal is maximum. The direct-current bias control unit is controlled as described above.

  According to the present invention, a pilot signal different from an electric signal and a DC bias voltage are input to the optical phase modulator, and the DC bias voltage is controlled while monitoring the residual component of the pilot signal, thereby distorting the angle modulated signal. The characteristics can be optimized and stabilized.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

(First embodiment)
FIG. 1 shows a modulation device according to the first embodiment. The light modulation device 100 is provided in a first light path, a light source 101, a first light splitter 102 that branches unmodulated output light from the light source 101 into a first light path and a second light path. An optical frequency shifter 103, an optical phase modulator 104 provided in the second optical path, an optical coupler 105 for coupling output light from the optical frequency shifter 103 and the optical phase modulator 104, and an output of the optical coupler 105 An optical heterodyne detection unit 106 that detects optical heterodyne and outputs an angle modulation signal, and an optical transmission unit 107 that modulates the intensity of the angle modulation signal output from the optical heterodyne detection unit 106 and outputs the result to an optical transmission line. The optical frequency shifter 103 shifts the light branched into the first optical path by the intermediate frequency. An electric signal is input to the optical phase modulator 104 as a modulation input, and the light branched into the second optical path is subjected to angle modulation (frequency modulation or phase modulation). The angle modulation signal is a heterodyne signal centered on an intermediate frequency equal to the frequency difference between the output light of the optical frequency shifter 103 and the output light of the optical phase modulator 104.

  As the light source 101, for example, a distributed feedback semiconductor laser can be used. As the optical splitter 102, for example, a fusion stretch type optical directional coupler can be used. The optical frequency shifter 103 can be composed of, for example, an optical SSB modulator in which a plurality of Mach-Zehnder interferometers are combined. The optical coupler 105 may be, for example, a fusion-stretching type optical directional coupler as with the optical splitter 102. In this case, the input / output of the optical splitter 102 may be used in the reverse direction.

  In the optical modulation device 100 according to the first embodiment, a pilot signal and a DC bias voltage different from the electrical signal are superimposed on the optical phase modulator 104. If the optical phase modulator 104 is an ideal device, the optical signal output from the optical phase modulator 104 is only a phase modulation signal and cannot be extracted as an intensity signal. Residual signal components and residual distortion components occur due to imperfections in the device such as reflection.

  These residual components are monitored, and the DC bias voltage input to the optical phase modulator 104 is controlled so that the residual signal component is maximized or the residual distortion component is minimized. Specifically, in addition to the above configuration, a DC bias control unit 111 that generates a DC bias voltage to be input to the optical phase modulator 104, and a pilot that superimposes a pilot signal on the DC bias voltage output by the DC bias control unit 111 A signal generation unit 112; a second optical branching unit 113 that branches the output light of the optical phase modulator 104; an optical / electrical conversion unit 114 that converts the optical signal branched by the second optical branching unit 113 into an electrical signal; A signal processing unit 115 for processing an electrical signal output from the photoelectric conversion unit 114 is further provided. The signal processing unit 115 extracts the residual distortion component of the pilot signal as an intensity from the electric signal output from the photoelectric conversion unit 114, and uses the DC bias voltage generated by the DC bias control unit 111 so as to minimize the intensity. Control. The signal processing unit 115 can convert a temporal electrical signal output from the photoelectric conversion unit 114 into a frequency spectrum, and can control a DC bias voltage according to the frequency spectrum. In this way, it is possible to improve the distortion characteristics of the angle modulation signal output from the optical heterodyne detection unit 106. For example, when the pilot signal is 1 kHz, a 2 kHz residual distortion component (a distortion component remaining due to the input of the pilot signal) may be extracted. In the signal processing unit 115, the bias voltage may be controlled so that the residual signal component of the pilot signal (the signal component remaining after the pilot signal is input) is maximized.

  The frequency of the pilot signal to be used may be any frequency, and a plurality of frequencies may be used. However, a frequency different from the electric signal input to the optical phase modulator 104 is desirable.

(Second Embodiment)
FIG. 2 shows a modulation apparatus according to the second embodiment of the present invention. The light modulation device 200 is provided in the first optical path, the light source 101, the first optical splitter 102 that branches the unmodulated output light from the light source 101 into the first optical path and the second optical path. An optical frequency shifter 103, an optical phase modulator 104 provided in the second optical path, an optical coupler 105 for coupling output light from the optical frequency shifter 103 and the optical phase modulator 104, and an output of the optical coupler 105 An optical heterodyne detection unit 106 that detects optical heterodyne and outputs an angle modulation signal, and an optical transmission unit 107 that modulates the intensity of the angle modulation signal output from the optical heterodyne detection unit 106 and outputs the result to an optical transmission line. The optical frequency shifter 103 shifts the light branched into the first optical path by the intermediate frequency. An electric signal is input to the optical phase modulator 104 as a modulation input, and the light branched into the second optical path is subjected to angle modulation (frequency modulation or phase modulation). The angle modulation signal is a heterodyne signal centered on an intermediate frequency equal to the frequency difference between the output light of the optical frequency shifter 103 and the output light of the optical phase modulator 104. These are the same as those of the modulation device 100 according to the first embodiment.

In the optical modulation device 200 according to the second embodiment, in addition to the above configuration, a DC bias control unit 211 that generates a DC bias voltage to be input to the optical phase modulator 104, and a DC bias output by the DC bias control unit 211. A pilot signal generator 212 for superimposing the pilot signal on the voltage, a second optical splitter 213 for splitting the output light of the optical coupler 105 , and an optical signal branched by the second optical splitter 213 are converted into electrical signals. And a signal demodulator 215 that demodulates the angle modulation signal output from the photoelectric converter 214, and a signal processor 216 that processes the demodulated electrical signal output from the signal demodulator 215. Provide. The signal processing unit 216 can convert the demodulated temporal electrical signal output from the signal demodulation unit 215 into a frequency spectrum, and can control the DC bias voltage according to the frequency spectrum. For example, when the pilot signal is 1 kHz, the demodulated electric signal includes distortion components such as 2 kHz and 3 kHz in addition to 1 kHz. The DC bias voltage may be controlled so that the 2 kHz or 3 kHz distortion component is minimized. Further, the DC bias voltage may be controlled so that the demodulated 1 kHz pilot signal is maximized.

  Note that the distortion component of the pilot signal refers to the distortion component of the demodulated pilot signal. For example, when the pilot signal is 1 kHz, a distortion component such as 2 kHz or 3 kHz is output from the signal demodulator. It refers to the distortion component.

  As in the first embodiment, the frequency of the pilot signal to be used may be any frequency, and a plurality of frequencies may be used, but the frequency of the pilot signal is different from that of the electric signal input to the optical phase modulator 104. Is preferable.

(Third embodiment)
FIG. 3 shows a modulation device according to the third embodiment. The optical modulation device 300 is the modulation device 100 according to the first embodiment, wherein two optical phase modulators are provided in series, the first electric signal 1 is supplied to the first optical phase modulator 304A, and the second electric signal is supplied. The signal is phase-divided by dividing the band as in the optical phase modulator 304B. At this time, the optical phase modulators 304A and 304B are respectively provided with DC bias controllers 311A and 311B and pilot signal generators 312A and 312B, and the residual components are monitored in the manner described in the first embodiment. In this case, it is desirable that the first pilot signal and the second pilot signal have different frequencies. With this configuration, the modulation band of each optical phase modulator can be narrowed, and the input voltage of the optical phase modulator can be lowered, so that high-quality angular modulation can be performed.

  As shown in FIG. 4, the electric signal is distributed by the distributor 316, and the delay is adjusted by the delay unit 317 so that the branched signals are out of phase with each other, so that the first optical phase modulator 304A and the second optical signal are adjusted. You may input into the phase modulator 304B. With this configuration, the phase shift amount can theoretically be doubled.

(Fourth embodiment)
FIG. 5 shows a modulation device according to the fourth embodiment. The optical modulation device 500 includes the second optical phase modulator 504B in series with the optical frequency shifter 103 in the modulation device 100 according to the first embodiment. The electric signal is distributed by the distributor 516, and the delay is adjusted by the delay unit 517 so that the branched signals are opposite in phase, and then input to the first optical phase modulator 504A and the second optical phase modulator 504B. To do. With this configuration, the phase shift amount can theoretically be doubled.

It is a figure which shows the modulation apparatus which concerns on 1st Embodiment. It is a figure which shows the modulation apparatus which concerns on the 2nd Embodiment of this invention. It is a figure which shows the modulation apparatus which concerns on 3rd Embodiment. It is a figure which shows the modification of the modulation apparatus which concerns on 3rd Embodiment. It is a figure which shows the modulation apparatus which concerns on 4th Embodiment. It is a figure which shows an example of the conventional light modulation apparatus. It is a figure which shows the conventional optical modulation apparatus which can improve the problem of a phase noise. It is a figure which shows the relationship between the residual component of a pilot signal, and a bias voltage.

Explanation of symbols

DESCRIPTION OF SYMBOLS 101 Light source 102 1st optical splitter 103 Optical frequency shifter 104 Optical phase modulator 105 Optical coupler 106 Optical heterodyne detection part 107 Optical transmission means 111 DC bias control part 112 Pilot signal generation part 113 2nd optical splitter 114 Light Electrical converter 115 Signal processor 211 DC bias controller 212 Pilot signal generator 213 Second optical splitter 214 Photoelectric converter 215 Signal demodulator 216 Signal processor 304A First optical phase modulator 304B Second light Phase modulator 311A First DC bias controller 311B Second DC bias controller 312A First pilot signal generator 312B Second pilot signal generator 315 Signal processor 316 Divider 317 Delay unit 504A First light Phase modulator 504B Second optical phase modulator 51 DESCRIPTION OF SYMBOLS 1A 1st DC bias control part 511B 2nd DC bias control part 512A 1st pilot signal generation part 512B 2nd pilot signal generation part 513 2nd optical branching device 514 Photoelectric conversion part 515 Signal processing part 516 Distribution 517 Delay Unit

Claims (6)

A light source;
A first optical branching device for branching unmodulated output light from the light source into a first optical path and a second optical path;
An optical frequency shifter provided in the first optical path for shifting the light branched into the first optical path by an intermediate frequency;
An optical phase modulator provided in the second optical path for angle-modulating the light branched into the second optical path;
An optical coupler for combining the output light from the optical frequency shifter and the output light from the optical phase modulator;
An optical heterodyne detection unit that outputs an angle modulated signal centered on the intermediate frequency by optical heterodyne detection of the output light of the optical coupler;
An optical modulation device comprising: an optical transmission means for intensity-modulating an angle modulation signal output by the optical heterodyne detection unit and outputting the intensity modulation signal to an optical transmission line;
A DC bias controller for generating a DC bias voltage to be input to the optical phase modulator;
A pilot signal generator for superimposing a pilot signal on a DC bias voltage output by the DC bias controller;
A second optical branching device for branching the output light from the optical phase modulator;
A photoelectric conversion unit that converts an optical signal branched by the second optical splitter into an electrical signal;
A signal processing unit that processes an electrical signal output by the photoelectric conversion unit;
The signal processing unit extracts and monitors the residual component of the pilot signal as an intensity from the electrical signal output from the photoelectric conversion unit, and controls the DC bias control unit according to the residual component. Light modulation device. The signal processing unit extracts and monitors the residual distortion component of the pilot signal as an intensity from the electrical signal output from the photoelectric conversion unit, and controls the DC bias control unit so that the residual distortion component is minimized. The light modulation device according to claim 1. The signal processing unit extracts and monitors the residual signal component of the pilot signal as an intensity from the electrical signal output from the photoelectric conversion unit, and controls the DC bias control unit so that the residual signal component is maximized The light modulation device according to claim 1. A light source;
A first optical branching device for branching unmodulated output light from the light source into a first optical path and a second optical path;
An optical frequency shifter provided in the first optical path for shifting the light branched into the first optical path by an intermediate frequency;
An optical phase modulator provided in the second optical path for angle-modulating the light branched into the second optical path;
An optical coupler for combining the output light from the optical frequency shifter and the output light from the optical phase modulator;
An optical heterodyne detection unit that outputs an angle modulated signal centered on the intermediate frequency by optical heterodyne detection of the output light of the optical coupler;
An optical modulation device comprising: an optical transmission means for intensity-modulating an angle modulation signal output by the optical heterodyne detection unit and outputting the intensity modulation signal to an optical transmission line;
A DC bias controller for generating a DC bias voltage to be input to the optical phase modulator;
A pilot signal generator for superimposing a pilot signal on a DC bias voltage output by the DC bias controller;
A second optical splitter for splitting the output light from the optical coupler,
A photoelectric conversion unit that converts an optical signal branched by the second optical splitter into an electrical signal;
A signal demodulating unit that demodulates an angle modulation signal output by the photoelectric conversion unit;
A signal processing unit for processing the demodulated electrical signal output by the signal demodulating unit,
The signal processing unit monitors the demodulated electric signal output from the signal demodulating unit and controls the DC bias control unit in accordance with the demodulated electric signal. The signal processing unit monitors the demodulated electrical signal output from the signal demodulating unit, and controls the DC bias control unit so that a distortion component of the demodulated pilot signal is minimized. The light modulation device according to claim 4. The signal processing unit monitors the demodulated electrical signal output from the signal demodulating unit, and controls the DC bias control unit so that a signal component of the demodulated pilot signal is maximized. The light modulation device according to claim 4.

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