JP5061817B2 - Light modulator - Google Patents

Description

  The present invention relates to a light modulation device, and more particularly to a light modulation device that realizes a high extinction ratio of on / off of intensity modulation.

2. Description of the Related Art In an optical communication system in which a signal is placed on light and transmitted through an optical fiber, an optical intensity modulator that generates an optical signal by intensity-modulating laser light emitted from a light source is used. The light intensity modulator is obtained by forming a Mach-Zehnder type optical waveguide, a modulation electrode, a bias electrode, and the like on a substrate of an electro-optic crystal such as lithium niobate (LiNbO ₃ ; hereinafter abbreviated as LN).

  A Mach-Zehnder type optical waveguide has a waveguide configuration that includes a branching unit that branches input light, two arms that propagate the branched light, and a multiplexing unit that recombines the light propagated through the arms. is doing. In this multiplexing part, when the two lights to be combined are in phase, the light waves are intensified and output to each other, and in the opposite phase, the light waves cancel each other and output light is eliminated. It becomes a state. The ratio between the output light intensity in the on state and the output light intensity in the off state is called an extinction ratio and is an important index representing the performance of the light intensity modulator. The higher the extinction ratio, that is, the greater the difference in the output light intensity between the on state and the off state, the deeper the modulation depth, and the higher the quality of optical transmission.

  Here, most ideally, the output in the off state is zero, and the extinction ratio is infinite at this time, but in order to create this situation, it is necessary that the intensity of the two light beams to be merged exactly There is. However, the intensity of the combined light is asymmetric because the branching ratios of the branching portions are usually not equal due to an optical waveguide manufacturing error or the like, and the propagation loss differs between the two arms. In this case, even if the phase is reversed, the two lights are not completely canceled out, and the extinction ratio is deteriorated.

  As a method of improving the extinction ratio by making the light intensity symmetrical at the multiplexing part, for example, irradiating an excimer laser on the arm with the larger branching power, giving a defect to the waveguide, and increasing the loss. A method of balancing the intensity and intensity of light that has passed can be considered. However, this method has a problem that the loss due to the defect is wavelength-dependent and the extinction ratio also depends on the wavelength.

By the way, sub-Mach-Zehnder optical waveguides are provided on the two arms of the main Mach-Zehnder optical waveguide, respectively, and RF modulation is performed on each sub-Mach-Zehnder optical waveguide to generate sideband light (upper and lower sidebands) above and below the frequency. Optical FSK (Frequency Shift Keying) modulation, which generates and outputs frequency-modulated signal light by switching the sideband light between the upper and lower sides by selecting the phase corresponding to the data signal in the main Mach-Zehnder optical waveguide A device has been developed (see, for example, Patent Document 1). In recent years, there has been proposed a light intensity modulator that realizes a high extinction ratio by using the sub Mach-Zehnder optical waveguide as a light amount adjusting unit and operating the above-described optical FSK modulator as the above-described light intensity modulator. (For example, refer nonpatent literature 1).
Japanese Patent Laid-Open No. 2005-134897 Nichijo et al., Wavelength Characteristics of High Extinction Ratio Modulator Using Optical FSK Modulator, 2005 Proceedings of Society Conference of IEICE, September 2005, C-3-2.

However, the light intensity modulator shown in Non-Patent Document 1 simply monitors the output light intensity of the light intensity modulator when adjusting the light amount of the sub Mach-Zehnder optical waveguide. In this case, if the modulation to the main Mach-Zehnder optical waveguide is performed at the same time, the light amount adjustment based on the monitored light intensity becomes impossible. Therefore, it is necessary to adjust the light amount in a state where no modulation is performed (that is, prior to the actual operation of the light intensity modulator). For example, a high extinction ratio is realized and maintained in real time in order to respond to an environmental change or the like. There was a problem that I could not.
In addition, if the light spectrum is monitored instead of the output light intensity, it is possible to adjust the light amount in real time. In this case, it is necessary for humans to visually confirm the spectrum, which is difficult to automate. Met.
Against this background, the applicant has filed an application relating to an optical modulator capable of realizing a high extinction ratio while performing modulation as Japanese Patent Application No. 2006-116630. However, an optical communication system or the like has been required to realize a further high extinction ratio with a small number of parts in an optical modulation device.

  The present invention has been made in view of the above points, and an object of the present invention is to stably realize an extremely high extinction ratio even while performing modulation with a modulation signal, and to reduce the number of components. The object is to provide a configurable light modulation device.

The present invention has been made to solve the above-described problem, and the first and second sub Mach-Zehnder optical waveguides are provided on the two arms of the main Mach-Zehnder optical waveguide to which light having the frequency f ₀ is input, respectively. The first phase difference adjusting means adjusts the output light intensity of the sub Mach-Zehnder optical waveguide by giving a phase difference to the light passing through both arms of at least one of the sub-Mach-Zehnder optical waveguides. by modulating the light at a modulation frequency f _m by with modulation means for imparting a phase difference between the bias in the light passing through the two arms of the main Mach-Zehnder optical waveguides by phase difference adjusting means, the frequency from the main Mach-Zehnder optical waveguides f ₊₁ = an optical modulator that outputs light having a component of f ₀ + _{f m} and _f -1 _{= f} 0 -f _m, the output of the optical modulator And branch means for branching into two, separating means for outputting light consisting of frequency f ₊₁ and f _-1 components with taking out the frequency f ₀ component from One of the branched light to the transmission path, said extracted frequency a first light detecting means for measuring the power of the light composed of the f ₀ component, and a second light measuring means for measuring the power of the other branched light having the components of the frequencies f ₀ , f ₊₁ , and f ₋₁ . of the light detection means, wherein the light modulator, the light receiving power of the first light detecting means is a minimum when subjected to modulation at a modulation frequency f _m by the modulating means and the second light detecting means The light modulation device is characterized in that the first and second phase difference adjusting means are controlled so that the received light power becomes maximum.

According to the present invention, output light having frequency components f ₀ , f ₊₁ , and f ₋₁ is output from the optical modulator by modulation, and only power and all components of the frequency f ₀ component are included from the output light. Since the power is measured and each phase difference adjusting means of the optical modulator is controlled based on these two received light powers, it is possible to stably optimize the extinction ratio while performing the modulation. Since each phase difference adjusting means is controlled so that the frequency f ₀ component is minimized, the output light intensities of the two sub Mach-Zehnder optical waveguides are equal with high accuracy. That is, the output light from the optical modulator has a high extinction ratio. In addition, since the output light having a high extinction ratio is further input to the separating means, the frequency f _{+ 1} and f- ₁ components are separated and output to the transmission line, so that the extinction ratio is extremely high. The signal light can be output to the transmission line. At the same time, the separation means is also used as means for obtaining light of the frequency component to be measured by the first light detection means, so that the number of parts does not increase and the cost of the light modulation device is increased. It is possible to suppress the rise. Further, since the power of the signal components (frequency f ₊₁ and f ₋₁ ) is controlled to be maximized by the phase difference adjusting means, the output efficiency of the optical modulator is maximized.

  Further, the present invention provides the above optical modulation device, wherein the first phase difference adjusting means shifts the phase difference of the light passing through both arms with respect to the sub Mach-Zehnder optical waveguide having a higher light intensity. The intensity is attenuated, and the output light intensity of the two sub Mach-Zehnder optical waveguides is controlled to be equal.

  According to the present invention, since the output light intensities of the two sub Mach-Zehnder optical waveguides are set to be equal, a high extinction ratio can be realized. Further, since the light intensity having the higher light intensity is attenuated, the output of the optical modulator can be maximized while minimizing the loss of light.

  In the optical modulator according to the present invention, the second phase difference adjusting unit is controlled so that a phase difference of light passing through both arms of the main Mach-Zehnder optical waveguide is π. And

According to the present invention, it is possible to control the output light of two sub Mach-Zehnder optical waveguides whose light intensity is asymmetric so that the light intensity is made uniform and the frequency f ₀ component is minimized. Thereby, a high extinction ratio can be realized.

  According to the present invention, in the above-described optical modulation device, in the optical modulator, a first step in which the light receiving powers of the first and second light detection units are both maximized, A second step in which the second phase difference adjusting means is controlled so that the light reception power of the first photodetector is minimized; and the first step so that the light reception power of the first photodetector is further reduced. And a third step in which the phase difference adjusting means is controlled.

  According to the present invention, in the control for each phase difference adjusting means, it is possible to perform control for obtaining a high extinction ratio without causing the control result to diverge. It is also possible to perform automation control according to the above steps.

  Further, the present invention provides the above optical modulation apparatus, wherein each Mach-Zehnder optical waveguide is constituted by an optical waveguide formed on a substrate having an electro-optic effect, and each of the phase difference adjusting means and the modulating means is included in the optical waveguide. It is characterized by comprising an electrode for applying an electric field.

  According to the present invention, the optical modulator can be configured as an optical waveguide element manufactured by a semiconductor process, and miniaturization and mass productivity of the optical modulation device can be achieved.

Further, according to the present invention, first and second sub Mach-Zehnder optical waveguides are respectively provided on two arms of a main Mach-Zehnder optical waveguide to which light having a frequency f ₀ is input, and at least the first phase difference adjusting unit performs at least A phase difference is given to light passing through both arms of one sub Mach-Zehnder optical waveguide to adjust the output light intensity of the sub-Mach-Zehnder optical waveguide, and both the main Mach-Zehnder optical waveguides are adjusted by the second phase difference adjusting means. by with modulation means for imparting a phase difference between the bias in the light passing through the arm by modulating the light at a modulation frequency f _m, the frequency _{f +1 =} f 0 ₊ f _m and f _{-1 =} f ₀ from the main Mach-Zehnder waveguide An optical modulator that outputs light having a component of −f _m , and output light of the optical modulator is converted to light having a frequency f ₀ component and frequency f Separating means for separating the light comprising the ₊₁ and f ₋₁ components into two, third light detecting means for measuring the power of the light comprising the separated frequency f ₀ component, and the separated frequency f ₊₁ and includes a fourth light detecting means for measuring the power of light consisting of f _-1 component, and a branching means for outputting to the transmission line light consisting of the separated frequency f ₊₁ and f _-1 component, wherein optical modulator, as the light receiving power of the received light power is smallest and the fourth light detecting means of said third light detecting means when performing the modulation at a modulation frequency f _m by the modulation means is maximum, The light modulation device is characterized in that the first and second phase difference adjusting means are controlled.

According to the present invention, output light having frequency components f ₀ , f ₊₁ , and f ₋₁ is output from the optical modulator by modulation, and light of frequency f ₀ component and frequencies f ₊₁ and f _{− are} output from the output light. _Since light of _one component is separated and each power is measured, and each phase difference adjusting means of the optical modulator is controlled based on these two light receiving powers, the optimum extinction ratio is stably maintained while performing modulation. Can be performed. Since each phase difference adjusting means is controlled so that the frequency f ₀ component is minimized, the output light intensities of the two sub Mach-Zehnder optical waveguides are equal with high accuracy. That is, the output light from the optical modulator has a high extinction ratio. In addition, since the output light having a high extinction ratio is input to the separating means for separating each component and the separated frequencies f ₊₁ and f ₋₁ are output to the transmission line, an extremely high extinction ratio. Can be output to the transmission line. At the same time, the separating means is also used as means for obtaining light of the frequency component to be measured by the third light detecting means, so that the number of parts does not increase and the cost of the light modulation device is increased. It is possible to suppress the rise. Further, since the power of the signal components (frequency f ₊₁ and f ₋₁ ) is controlled to be maximized by the phase difference adjusting means, the output efficiency of the optical modulator is maximized.

  According to the present invention, it is possible to stably realize a high extinction ratio even while performing modulation with a modulation signal. Thereby, a high quality optical communication system can be constructed. Further, the configuration for realizing a high extinction ratio is simple.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
<< First Embodiment >>
FIG. 1 is a functional block diagram of a light modulation device 1 according to the first embodiment of the present invention.

In the figure, laser light having a frequency f ₀ emitted from a light source (not shown) is input to the optical modulator 10. The optical modulator 10 is a predetermined structure of the optical waveguide and electrodes having waveguide element formed using the LN substrate (details will be described (described later) in FIG. 2), the frequency f _m of the input optical Modulated with the modulation signal, output light having frequency components f ₀ , f ₊₁ , and f ₋₁ is output. here,
_{f +1} = f 0 + f _{m
f -1} = f 0 -f _m
And Incidentally, when performing the modulation at the frequency _{f m,} it is also generated high-order component _f 0 + 2f _m and _f 0 + 3f _m, where these in order to simplify the explanation is neglected.

  The output light passes through the branching means 11 and the separating means 12 and is output from the optical modulation device 1 as a main output and sent to the transmission path (optical fiber).

  The light input to the branching unit 11 is branched into two and output to the separating unit 12 and the second light detecting unit 14b. The branching unit 11 is a fiber-type optical coupler and has a branching ratio of −10 dB (output light to the separation unit: output light to the second light detection unit = 10: 1), for example.

The separation means 12 outputs only the f ₀ component of the frequency components (f ₀ , f ₊₁ , f ₋₁ ) of the input light to the first light detection means 14 a, and only the f ₊₁ and f ₋₁ components. Is output to the transmission line as the main output.
FIG. 3 is a diagram showing a specific configuration example of the separating unit 12. The separating means 12 has a configuration in which an optical circulator 121 and a fiber bragg grating (FBG) 122 are connected.

The light input to the separating means 12 is sent to the fiber grating 122 side through the optical circulator 121 (the optical circulator 121 is an optical element that outputs light only in the direction indicated by the arc-shaped arrow in the figure). . The fiber grating 122 is an optical element in which a diffraction grating having a predetermined pitch is formed, reflects only light having a wavelength (frequency) corresponding to this pitch, and transmits light having other wavelengths. Here, it is assumed that a fiber grating 122 that selectively reflects only the frequency f ₀ component is used. The light having the frequency f ₀ reflected by the fiber grating 122 is again output to the first light detection means 14 a through the optical circulator 121, while the lights having the frequencies f ₊₁ and f ₋₁ that are transmitted through the fiber grating 122 are Output to the transmission line side.

Although the separating means 12 is configured to use one optical circulator 121 and one fiber grating 122 one by one, the optical circulator 121 and the fiber grating 122 may be combined in two stages or in multiple stages in series. By adopting such a multi-stage configuration, the frequency selectivity of light to be filtered can be increased, and the accuracy of control to be described later can be improved. In the case of a multistage configuration, it is desirable that the rear portion of the fiber grating 122 that does not serve as an output end to the transmission line is subjected to a non-reflection termination process. This is to prevent the components of the frequencies f ₊₁ and f ₋₁ transmitted through the fiber grating 122 from being reflected back in the path ahead and mixed into the output light to the first light detection means 14a. is there.
In general, since the wavelength selection characteristic of the fiber grating 122 is temperature-dependent, it is desirable to control the temperature by putting the separating means 12 in a thermostatic chamber.

Returning to FIG. 1, the light of the frequency f ₀ component output from the separating means 12 is input to the first light detecting means 14a, and its power P1 is measured. The other light branched from the branching means 11 is input to the second light detecting means 14b, and the power P2 including all components of the frequencies f ₀ , f ₊₁ and f ₋₁ is measured. Note that the spectrum of light input to each photodetection means is in the state shown in FIG. 4, and the received light powers P1 and P2 correspond to the power of the spectrum components indicated by the vertical solid lines in the figure.

  Here, the first and second light detection means 14a and 14b are configured by a photodiode (PD) for detecting the power (intensity) of the received light. The respective light receiving powers P1 and P2 are transmitted from the respective light detection means to the control means 17.

The control means 17 controls the modulation operation of the optical modulator 10 based on the received light powers P1 and P2. As will be described later, this control is performed individually for the electrodes provided in each of the three Mach-Zehnder optical waveguides (MZ-A, MZ-B, and MZ-C). The control means 17 is constituted by a personal computer or other general control device.
Further, the MZ-C, addition of a control signal from the control unit 17, a modulation signal of frequency f _m are also inputted. This modulation signal is a signal generated by the modulation signal generation unit 15 and converted into a voltage having a predetermined amplitude by the RF driver 16.

Next, the optical modulator 10 will be described with reference to FIG. FIG. 2 is a configuration diagram of the optical modulator 10.
In the figure, an optical modulator 10 includes a main Mach-Zehnder optical waveguide (MZ-C) 101, a first sub Mach-Zehnder optical waveguide (MZ-A) 102 provided in each arm of the MZ-C 101, a second sub It has an optical waveguide composed of a Mach-Zehnder optical waveguide (MZ-B) 103. Further, MZ-A102 and MZ-B103 are provided with DC electrodes (first phase difference adjusting means) 106a and 106b for adjusting the phase difference of light passing through each arm of the Mach-Zehnder unit, the -C101, the Mach-Zehnder portion DC electrode on the light passing through the two arms provide a phase difference between the bias of the (second phase difference adjusting means) 104, and a modulation electrode for modulating the light at a frequency f _m (Modulation means) 105 is provided.

  Although not shown, the optical modulator 10 is formed on an LN substrate in which each element is a crystal having an electro-optic effect, and the refractive index of the optical waveguide is generated by an electric field applied from each electrode. As a result of the change, phase change is given to the light passing through the optical waveguide.

  Here, when the input voltage is changed in the DC electrodes 106a and 106b of the MZ-A102 and MZ-B103 sections, the phase difference of light passing through each arm of the sub Mach-Zehnder optical waveguide can be adjusted. As a result, the intensity of light output from the sub Mach-Zehnder optical waveguide can be changed.

Further, when the input voltage is changed at the DC electrode 104 of the MZ-C101 unit, the phase difference of light (light output from the MZ-A102 and MZ-B103) that passes through both arms of the main Mach-Zehnder optical waveguide is adjusted. be able to. As a result, the modulation operating point of modulation (modulation by the modulation electrode 105) in the main Mach-Zehnder optical waveguide can be changed.
For example, when the phase difference π is given by the DC electrode 104, the output of the optical modulator 10 at the time of non-modulation becomes zero because the output of each sub Mach-Zehnder optical waveguide interferes with the opposite phase. In this state, when the modulation frequency f _m by the modulation electrode 105, the side band generated in the frequency f ₊₁ and f _-1, is outputted as a modulated signal light. However, when the output light intensity of each sub Mach-Zehnder optical waveguide is asymmetric, the output during non-modulation does not become zero, and the frequency f ₀ component remains and is output during modulation. (State of FIG. 4A).

Therefore, in the present light modulation device 1, each of the DC electrodes 106a of the MZ-A102, MZ-B103, and MZ-C101, using the light powers P1 and P2 measured by the first and second light modulation means 14a and 14b, The phase difference applied by 106b and 104 is controlled. In this control, the DC electrodes 106a, 106b, and 104 are adjusted so that the light reception power P1 of the first light modulation means 14a is minimized and the light reception power P2 of the second light modulation means 14b is maximized.
The specific procedure is as follows.

  First, the three DC electrodes 106a, 106b, and 104 are adjusted to set the light receiving powers P1 and P2 to a maximum value (first step). At this time, in MZ-A102 and MZ-B103, the phase difference in each arm is zero, and the output light intensity of each sub Mach-Zehnder optical waveguide is maximum (but asymmetric). Also in MZ-C101, the phase difference between the two arms (the phase difference of the output light of each sub Mach-Zehnder optical waveguide) is zero.

Next, the DC electrode 104 of the MZ-C 101 is adjusted in the above state to set the light receiving power P1 to a minimum state (second step). At this time, in MZ-C101, the phase difference of the output light of each sub-Mach-Zehnder optical waveguide is π, and each light interferes with an opposite phase, whereby the intensity of the frequency f ₀ component in the output light of the optical modulator 10 is increased. It is the minimum. However, since the output light intensity of each sub Mach-Zehnder optical waveguide remains asymmetric, the frequency f ₀ component remains and does not become zero (true minimum value).

Then, the DC electrode 106a of the MZ-A 102 and the DC electrode 106b of the MZ-B 103 are adjusted little by little to select the DC electrode (referred to as DC electrode 106a) that changes in the direction in which the received light power P1 decreases. Further, by adjusting the selected DC electrode 106a, the received light power P1 is set to a state where the true minimum value is obtained (third step). At this time, the intensity of the output light from the MZ-A 102 having a high output light intensity is attenuated by the phase difference adjustment by the DC electrode 106a and becomes equal to the output light intensity of the MZ-B 103. As a result, the frequency f ₀ component becomes zero, and only the components of the frequencies f ₊₁ and f ₋₁ are output from the optical modulator 10, thereby realizing a high on / off extinction ratio in the modulation of the modulation signal f _m. .

  In addition, after performing control by said 1st-3rd step, the phase state of the output light of each Mach-Zehnder optical waveguide may change with time, for example by the change of environmental temperature etc., for example. In order to correct this variation, the control of the second and third steps is executed constantly or repeatedly at regular intervals, so that more accurate light modulation can be realized.

Thus, according to this embodiment monitors the light having the component of the modulation signal f frequency f ₀ output from the optical modulator 10 when performing the modulation with _m, f _+1, and f _-1 the power P1 of the frequency f ₀ component extracted by separation means 12 in the first light detecting means 14a with measuring the power P2 of all components in the second light detecting means 14b is measured to the light-receiving power P1 and P2 Based on this, the phase difference applied by each DC electrode of each Mach-Zehnder optical waveguide MZ-A, MZ-B, MZ-C of the optical modulator 10 is controlled. The control is performed so that the received light power P1 is minimized and the received light power P2 is maximized. As a result, phase difference control for optimizing the extinction ratio while performing modulation is possible, and even when the optical modulation device 1 is actually operating in an optical communication system, it is stably high in real time. An extinction ratio can be obtained.
Further, the output light from the optical modulator 10 having a high extinction ratio is input to the separating means 12 to separate the frequency f _{+ 1} and f- ₁ components, and the separated frequency f _{+ 1} and f- ₁ components to the transmission line. Since the signal is output, signal light having an extremely high extinction ratio can be output to the transmission line. Further, since the separating means 12 is also used as a means for obtaining light of a frequency component to be measured by the first light detecting means 14a, the number of parts does not increase and the cost of the light modulation device increases. Can be suppressed.

<< Second Embodiment >>
In the above embodiment, although to carry out the control by using the the power P1 of the frequency f ₀ component and the power P2 of all components, in place of the power P2 of the entire component, the frequency f ₊₁ and f _-1 ingredient It is also possible to perform control using the light power P3 consisting of
Therefore, the light modulation device 2 according to the second embodiment of the present invention has the configuration shown in FIG.

In FIG. 5, the output light from the optical modulator 10 is output as a main output from the optical modulation device 2 through the separating means 12 and the branching means 11 in this order, and sent to the transmission line. The light from the optical modulator 10 input to the separating means 12 is separated into two light, that is, light having a frequency f ₀ component and light having frequency f ₊₁ and f ₋₁ components. The former light has its power P1 'measured by the third light detection means 14c, and the latter light is input to the fourth light detection means 14d via the branching means 11, and its power P3 is measured. In the present embodiment, the spectrum of the light input to each light detection means is in the state shown in FIG.

  The control means 17 of the light modulation device 2 controls the modulation operation of the light modulator 10 based on the received light powers P1 'and P3. In this control, the DC electrodes 106a, 106b, and 104 are adjusted so that the light reception power P1 'of the third light modulation means 14c is minimized and the light reception power P3 of the fourth light modulation means 14d is maximized. The only difference from the first embodiment is that the received light power used for the control is changed from P1 and P2 to P1 'and P3, and the specific procedure of the control is as described above.

Thus, according to this embodiment monitors the light having the component of the modulation signal f frequency f ₀ output from the optical modulator 10 when performing the modulation with _m, f _+1, and f _-1 , the frequency _{f +1} in the fourth optical detecting unit 14d, and thereby measure the power P3 of _{f -1} 'to measure, those receiving power P1' power P1 of the frequency _{f 0} component in the third light detecting means 14c and P3 Based on the above, it is possible to perform phase difference control for optimizing the extinction ratio while performing modulation.
Further, since the output light having a high extinction ratio from the optical modulator 10 is input to the separating means 12 and the components of the frequencies f _{+ 1} and f- ₁ are separated to become output light to the transmission line, it has an extremely high extinction ratio. The signal light can be output to the transmission line. Further, since the separating unit 12 is configured to be used also as a unit for obtaining the light of the frequency component to be measured by the third light detecting unit 14c, the number of parts does not increase and the cost of the light modulation device increases. Can be suppressed.

As described above, the embodiment of the present invention has been described in detail with reference to the drawings. However, the specific configuration is not limited to the above, and various design changes and the like can be made without departing from the scope of the present invention. It is possible to
For example, the optical modulator 10 had been modulated with a single modulation frequency f _m which is generated by the modulation signal generating unit 15 generates a modulated signal comprising a data signal to be transmitted by the modulation signal generator 15, Even when modulation is performed using this data signal, a high extinction ratio can be realized by the same control.

Further, the specific configuration of the separating unit 12 is not limited as long as it is a component having an optical filter function based on a known technique. For example, a dielectric multilayer filter using interference by a multilayer film can be applied. In addition, by using a wavelength selective optical element that is usually used, the frequency f ₀ component is reflected (or transmitted), and the frequency f ₊₁ and f ₋₁ components are transmitted (or reflected) to be reflected. You may make it take out with an optical circulator.
Further, when automatic control is not performed, the operation of the control means 17 may be manually performed by a human.
Further, as a light source of laser light input to the optical modulator 10, for example, a DFB laser whose stabilization is controlled with a wavelength accuracy <± 1 GHz and an optical output power accuracy <± 0.1 dB is used. Can be obtained.

1 is a functional block diagram of a light modulation device according to a first embodiment of the present invention. It is a block diagram of an optical modulator. It is an internal block diagram of a isolation | separation means. It is the figure which showed the spectrum of the light input into the photon detection means of FIG. It is a functional block diagram of the optical modulation apparatus by the 2nd Embodiment of this invention. It is the figure which showed the spectrum of the light input into the photon detection means of FIG.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1, 2 ... Light modulation apparatus 10 ... Optical modulator 11 ... Branch means 12 ... Separation means 14a ... 1st light detection means 14b ... 2nd light detection means 14c ... 3rd light detection means 14d ... 4th light detection means 15 ... Modulation signal generator 16... RF driver 17... Control means 101... Main Mach-Zehnder optical waveguide 102. Electrode 121 ... Optical circulator 122 ... Fiber grating

Claims (6)

First and second sub Mach-Zehnder optical waveguides are respectively provided on two arms of the main Mach-Zehnder optical waveguide to which light having a frequency f ₀ is input, and at least one sub-Mach-Zehnder optical waveguide is provided by the first phase difference adjusting means. A phase difference is given to the light passing through both arms of the waveguide to adjust the output light intensity of the sub Mach-Zehnder optical waveguide, and further, the light passing through both arms of the main Mach-Zehnder optical waveguide is adjusted by the second phase difference adjusting means. by modulating the light at a modulation frequency _{f m} by with modulation means for imparting a phase difference between the bias, the component of the frequency _f +1 _{= f} 0 + _{f m} and _f -1 _{= f} 0 -f _m from the main Mach-Zehnder waveguide An optical modulator that outputs the light it has,
Branching means for branching the output light of the optical modulator into two;
Separating means for extracting the frequency f ₀ component from one of the branched lights and outputting the light comprising the frequency f ₊₁ and f ₋₁ components to the transmission line;
First light detection means for measuring the power of the light composed of the extracted frequency f ₀ component;
Second light detection means for measuring the power of the other branched light having components of frequencies f ₀ , f ₊₁ , and f ₋₁ ;
With
The optical modulator, as the light receiving power of the received light power is smallest and the second light detecting means of said first light detecting means when performing the modulation at a modulation frequency f _m by the modulation means is maximum The light modulation device is characterized in that the first and second phase difference adjusting means are controlled. The first phase difference adjustment means attenuates the light intensity by shifting the phase difference of the light passing through both arms with respect to the sub Mach-Zehnder optical waveguide having the higher light intensity, thereby reducing the light intensity of the two sub-Mach-Zehnder optical waveguides. The light modulation device according to claim 1, wherein the output light intensities are controlled to be equal. 3. The light according to claim 1, wherein the second phase difference adjusting unit is controlled so that a phase difference of light passing through both arms of the main Mach-Zehnder optical waveguide is π. Modulation device. In the optical modulator,
A first step in which the light receiving power of the first and second light detecting means is set to a maximum;
A second step in which the second phase difference adjusting means is controlled so that the light receiving power of the first photodetector is minimized;
A third step in which the first phase difference adjusting means is controlled so that the light receiving power of the first photodetector is further reduced;
The light modulation device according to claim 1, wherein the optical modulation device is sequentially executed. Each Mach-Zehnder optical waveguide is constituted by an optical waveguide formed on a substrate having an electro-optic effect,
The optical modulation device according to any one of claims 1 to 4, wherein each of the phase difference adjustment means and the modulation means is configured by an electrode for applying an electric field to the optical waveguide. First and second sub Mach-Zehnder optical waveguides are respectively provided on two arms of the main Mach-Zehnder optical waveguide to which light having a frequency f ₀ is input, and at least one sub-Mach-Zehnder optical waveguide is provided by the first phase difference adjusting means. A phase difference is given to the light passing through both arms of the waveguide to adjust the output light intensity of the sub Mach-Zehnder optical waveguide, and further, the light passing through both arms of the main Mach-Zehnder optical waveguide is adjusted by the second phase difference adjusting means. by modulating the light at a modulation frequency _{f m} by with modulation means for imparting a phase difference between the bias, the component of the frequency _f +1 _{= f} 0 + _{f m} and _f -1 _{= f} 0 -f _m from the main Mach-Zehnder waveguide An optical modulator that outputs the light it has,
Separation means for separating the output light of the optical modulator into two light components, _{ie, a} light component having a frequency f ₀ component and a light component having frequencies f ₊₁ and f ₋₁ ;
Third light detection means for measuring the power of light comprising the separated frequency f ₀ component;
A fourth photodetecting means for measuring the power of the light composed of the separated frequency f _{+ 1} and f- ₁ components;
Branching means for outputting the light having the separated frequencies f _{+ 1} and f- ₁ components to a transmission line;
With
The optical modulator, as the light receiving power of the received light power is smallest and the fourth light detecting means of said third light detecting means when performing the modulation at a modulation frequency f _m by the modulation means is maximum The light modulation device is characterized in that the first and second phase difference adjusting means are controlled.

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