JP5391160B2 - Method for setting initial operating point of optical modulator and multi-wavelength optical modulation system - Google Patents

Description

  The present invention relates to a technique for setting an initial operating point of an optical waveguide type optical modulator having a Mach-Zehnder interferometer using an electro-optic effect, and more particularly to an optical modulator that performs optical modulation on light of a plurality of different wavelengths. The present invention relates to a technique that enables setting of an initial operating point.

  Optical communication is spreading as a means for realizing high-speed data requested in recent years. Optical communication with a communication speed of about 10 Gb / s has been put into practical use, but with the recent rapid increase in the amount of communication data, further improvement in communication capacity is eagerly desired.

  In response to this demand, phase modulation methods (ODB, DPSK, DQPSK) that modulate the phase of light with a data signal instead of the conventional intensity modulation method (NRZ, RZ communication method) that modulates the light intensity with a data signal. Etc.) are attracting attention and are being put to practical use.

  In addition to the above-described modulation schemes, wavelength multiplexing schemes (WDM, DWDM), etc. using light of a plurality of different wavelengths have been put to practical use as methods for multiplexing information transmitted over a single optical fiber. ing.

  Therefore, in the future, as an optical modulator used in the above communication method, it is necessary to be able to cope with modulation with respect to light of different wavelengths.

  As an optical modulator capable of high-speed modulation, an optical waveguide type optical modulator having a Mach-Zehnder interferometer utilizing an electro-optic effect is known.

For example, as shown in FIG. 6, the optical waveguide type optical modulator is provided on a substrate 10 (LiNbO ₃ substrate or the like) having a characteristic (electro-optic effect) in which a refractive index with respect to light changes according to the magnitude of an applied electric field. , A first waveguide 11 for light incidence, an optical branch 12 for branching the light guided to the first waveguide 11, a second waveguide 13 for propagating the branched light at equal distances, and a third guide The optical waveguide 15 for combining the light propagated through the waveguide 14 and the second waveguide 13 and the light propagated through the third waveguide 14 and the fourth waveguide 16 for emitting the combined light to the outside are formed. ing.

  Here, if the lengths of the second waveguide 13 and the third waveguide 14 are equal, the phases of the light incident on the optical waveguide 15 are equal, and are combined in phase and become light equivalent to the original incident light. The light is emitted from the fourth waveguide 16. For example, by applying an electric field of constant strength in the opposite direction to the second waveguide 13 and the third waveguide 14 (electric field application electrodes are not shown), When the refractive indexes of the waveguides 13 and 14 are changed in the opposite directions and the phases of the light incident on the optical waveguide 15 from the second waveguide 13 and the third waveguide 14 are reversed, the two mutually weaken the light. That is, the light component is hardly emitted to the fourth waveguide 16.

  That is, if the electric field applied to both the waveguides 13 and 14 is changed according to the level of the data signal, light whose intensity is modulated by the data signal can be obtained.

  The above example is a case of intensity modulation by a data signal. However, an optical modulation technique using an electro-optic effect in which the refractive index of a waveguide is changed by an electric field is also applied to phase modulation.

The phase modulation method is a differential phase modulation method (DPSK:
Differential Phase Shift Keying) and differential quaternary phase modulation (DQPSK), which is expected as a method capable of realizing a modulation speed of 40 Gbs or more.
Differential Quadrature Phase Shift Keying) is attracting attention, but these differential phase modulation systems are basically Mach-Zehnder type using the electro-optic effect, except that the processing technique for the input data is different. This is a modulation technique using an interferometer as a component.

As a well-known characteristic of an optical modulator using such a Mach-Zehnder interferometer, the intensity of the combined light with respect to the applied electric field, that is, the change in applied voltage is periodically (cos ² x).

  A voltage corresponding to the half period of this modulation characteristic is called a half-wave voltage Vπ. For example, a data signal having an amplitude Vπ with reference to the voltage Vb at point A at which the intensity is ½ of the maximum in the modulation characteristic. By superimposing D, it is possible to obtain modulated light having the maximum amplitude modulated according to the data.

  Here, the point A is called the operating point of the optical modulator, and the reference voltage Vb that gives this operating point A is called the bias voltage. In this example, the operating point is set at a position where the modulation characteristic strength is ½, but there may be a case where the modulation characteristic strength is 0 (bottom).

  Regarding the optical modulator using the Mach-Zehnder interferometer as described above, the simplest intensity modulation technique is described in Patent Document 1 below, and the technique relating to phase modulation is described in Patent Document 2.

Japanese Patent Laid-Open No. 03-251815 JP 2000-162563 A

  In the optical modulator described above, since the modulation characteristics change periodically, there are operating points that give the same modulation action at intervals of 2Vπ. However, the optical modulator is affected by the applied DC voltage, temperature change, and secular change. A change (operating point drift) occurs in the input / output characteristics. This operating point drift is one in which the modulation characteristic of FIG. 7 moves in the left-right direction (voltage axis direction), and a desired modulation operation cannot be obtained by this operating point drift.

  For this reason, conventionally, among the operating point candidates having a plurality of modulation characteristics, the power consumption is the smallest, that is, the point closest to zero volts is set as the optimal operating point, and further feedback for suppressing the operating point drift Control means were provided.

  Patent Documents 1 and 2 also disclose a technique for superimposing a low-frequency signal on a data signal, extracting the low-frequency signal component from the light emitted from the optical modulator, detecting an operating point variation, and suppressing this. ing.

  By the way, in the conventional optical modulator, the consideration about the change of the optimum operating point due to the difference in wavelength in the optical modulator that handles multi-wavelength light has not been made, and it is an important issue for realizing the multi-wavelength optical modulation system. It has become one.

  That is, the phase change of the light passing through the waveguide having a certain length of the optical modulator is wavelength-dependent, and therefore the period of the modulation characteristic (that is, the half-wave voltage Vπ) is different when the optical wavelength is different. Further, if there is a difference in the optical path length between the second waveguide 13 and the third waveguide 14 of the optical modulator 10 described above, a deviation occurs in the modulation characteristics for each wavelength according to the difference.

  For this reason, in order to support this multi-wavelength optical modulation system with the conventional technology, an optimum initial operating point is found in advance for each wavelength, and the initial operation is performed for each wavelength of light incident on the optical modulator. There was only a method of switching points, and it could not cope with simultaneous modulation of a plurality of wavelengths.

  An object of the present invention is to solve this problem and to provide an initial operating point setting method and a multi-wavelength light modulation system that can handle high-speed wavelength switching and simultaneous modulation of a plurality of wavelengths in a multi-wavelength light modulation system. .

In order to achieve the above object, an initial operating point setting method for an optical modulator according to claim 1 of the present invention is as follows.
A light modulator including a Mach-Zehnder interferometer using an electro-optic effect and having a modulation characteristic in which the intensity of emitted light periodically changes with a change in applied bias voltage . A light of a specific wavelength is incident, and the bias voltage supplied to the optical modulator is variably controlled while monitoring the intensity of light emitted from the optical modulator, and the operating point candidate of the optical modulator for the specific wavelength One of these as a reference point and storing the intensity at that time (S1 to S4),
While keeping the operating point of the optical modulator at the reference point, sequentially entering light of wavelengths other than the specific wavelength and examining the intensity for each wavelength (S5 to S8);
Inspecting a deviation from the reference intensity of the intensity obtained by the incidence of light having a wavelength other than the specific wavelength as the reference intensity, which is obtained when the specific wavelength light is incident (S9),
Determining whether all the deviations obtained for each of the other wavelength lights are within a preset allowable range (S10);
When it is determined that any of the deviations obtained for the other wavelength light is not within the allowable range, the reference point is changed to an operation point candidate next to either the + direction or the − direction. , Making each other wavelength light incident, and obtaining a deviation from the reference intensity for each other wavelength (S12 to S18);
Determining whether the deviation obtained after the change of the reference point is within the allowable range (S19);
When it is determined that the deviation obtained after the change of the reference point is not within the allowable range, comparing the deviation after the change with the deviation before the change (S20);
If the deviation after the change of the reference point is smaller than the deviation before the change, it is determined that the change direction of the reference point is correct, and the reference point is changed to the next operation point candidate in the one direction to obtain a new deviation. Is performed until the deviation falls within the allowable range. If the deviation after the change is larger than the deviation before the change, it is determined that the change direction of the reference point is incorrect and the reference point is determined. Including changing the operation point candidate next to the one direction opposite to the one direction to obtain a new deviation until the deviation falls within the allowable range (S12 to S18, S21 to S27),
The operating point of the optical modulator when the deviation obtained for the other wavelength light falls within the allowable range is determined as a common operating point for each wavelength, and a bias voltage corresponding to the common operating point is set as the bias voltage. The initial operating point of the optical modulator is set.

The multi-wavelength optical modulation system according to claim 2 of the present invention is
A multi-wavelength light source (21) that emits light of a plurality of different wavelengths;
Including a Mach-Zehnder interferometer using an electro-optic effect, having a modulation characteristic in which emitted light intensity changes periodically with respect to a change in applied bias voltage, light emitted from the multi-wavelength light source, and the bias An optical modulator (22) for receiving a voltage and a modulation signal and emitting modulated light modulated by the modulation signal;
Outgoing light intensity detection means (23, 24) for detecting the outgoing light intensity of the light modulator;
A bias voltage generator (25) for generating a bias voltage to be applied to the optical modulator;
A multi-wavelength optical modulation system having a control unit (30) for controlling the baice voltage output from the bias voltage generator,
The control unit sequentially enters the light of the plurality of different wavelengths from the light source to the optical modulator, and monitors the output light intensity of the light modulator by the output of the output light intensity detection unit, The bias voltage supplied to the optical modulator is variably controlled to find a common operating point where the emitted light intensity is within a predetermined allowable range for each wavelength, and the bias voltage corresponding to the common operating point is set to the optical modulator. An initial operating point setting unit (31) for setting an initial operating point is provided ,
The initial operating point setting unit
A light having a specific wavelength out of a plurality of different wavelengths is incident on the optical modulator, and the bias voltage supplied to the optical modulator is variably controlled while monitoring the intensity of light emitted from the optical modulator. A means for obtaining one of the operating point candidates of the optical modulator as a reference point for the specific wavelength, and storing the intensity at that time;
Means for sequentially injecting light of other wavelengths other than the specific wavelength while maintaining the operating point of the optical modulator at the reference point,
Means for examining the deviation of the intensity obtained by the incidence of light of a wavelength other than the specific wavelength with respect to the reference intensity, with the intensity determined when the specific wavelength of light is incident as a reference intensity;
Means for determining whether or not all deviations obtained for each of the other wavelength lights are within a preset allowable range;
When it is determined that any of the deviations obtained for the other wavelength light is not within the allowable range, the reference point is changed to an operation point candidate next to either the + direction or the − direction. Means for making each other wavelength light incident and calculating a deviation from the reference intensity for each other wavelength;
Means for determining whether the deviation obtained after the change of the reference point is within the allowable range;
Means for comparing the deviation after the change with the deviation before the change when it is determined that the deviation obtained after the change of the reference point is not within the allowable range;
If the deviation after the change of the reference point is smaller than the deviation before the change, it is determined that the change direction of the reference point is correct, and the reference point is changed to the next operation point candidate in the one direction to obtain a new deviation. Is performed until the deviation falls within the allowable range. If the deviation after the change is larger than the deviation before the change, it is determined that the change direction of the reference point is incorrect and the reference point is determined. Means for performing a process of obtaining a new deviation by changing to an operation point candidate adjacent to the one direction opposite to the one direction until the deviation falls within the allowable range;
The operating point of the optical modulator when the deviation obtained for the other wavelength light falls within the allowable range is determined as a common operating point for each wavelength, and a bias voltage corresponding to the common operating point is set as the bias voltage. The initial operating point of the optical modulator is set .

The multi-wavelength optical modulation system according to claim 3 of the present invention is the multi-wavelength optical modulation system according to claim 2 ,
The control unit is provided with a drift suppression unit (32) that detects and suppresses the drift of the operating point initially set in the optical modulator by the initial operating point setting unit.

  As described above, in the present invention, a plurality of light beams having different wavelengths are sequentially incident on the optical modulator, and the bias voltage is variably controlled while monitoring the emitted light intensity of the optical modulator, and the light is output for each wavelength. A common operating point where the light intensity falls within a predetermined allowable range is found, and a bias voltage corresponding to the common operating point is set as the initial operating point of the optical modulator.

  Thus, since the operating point common to each wavelength is found, it is not necessary to switch the operating point at the time of wavelength switching, high-speed wavelength switching is possible, and multi-wavelength simultaneous modulation can be supported.

Configuration diagram of an embodiment of the present invention The flowchart which shows the process sequence of the principal part of embodiment of this invention. The flowchart which shows the process sequence of the principal part of embodiment of this invention. The flowchart which shows the process sequence of the principal part of embodiment of this invention. The figure for demonstrating the process in the case of 3 wavelengths The figure which shows the schematic structure of the conventional optical modulator The figure which shows the relationship between the modulation characteristic and the operating point of the optical modulator

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 shows a configuration of an optical modulation system 20 to which the present invention is applied.

  In FIG. 1, a multi-wavelength light source 21 emits light having a plurality of n different wavelengths λ1 to λn, a structure for selectively or combining light of a plurality of fixed wavelength light sources, and a variable wavelength. Although the structure may be such that the emission wavelength of the light source is switched, it has a function of selectively emitting at least light of each wavelength λ1 to λn in order to set an operating point of the optical modulator 22 described later. To do.

The optical modulator 22 is a Mach-Zehnder interferometer having the waveguide structure shown in FIG. 6 on a substrate (LiNbO ₃ substrate or the like) having an electro-optic effect whose refractive index changes according to the strength of an applied electric field. Is a waveguide type optical modulator that receives light emitted from the multi-wavelength light source 21 and a signal in which a modulation signal is superimposed on a bias voltage, and emits modulated light modulated by the modulation signal. . The optical modulator 22 including the Mach-Zehnder interferometer using the electro-optic effect has a modulation characteristic that the emitted light intensity changes periodically with respect to the change of the applied bias voltage.

  The light emitted from the light modulator 22 is branched by the light branching device 23 and enters the light receiver 24. The light receiver 24 outputs a signal corresponding to the intensity of the emitted light from the light modulator 22 to the control unit 30.

  On the other hand, the bias voltage generator 25 generates a bias voltage to be applied to the optical modulator 22, and the output bias voltage is variably controlled by the control unit 30.

  The bias voltage and the modulation signal (data signal) of the multi-wavelength modulation system 20 are superimposed by the superimposing unit 26 and supplied to the optical modulator 22. The modulation signal is generated by modulation signal generation means (not shown), but the phase and amplitude of the signal depend on the modulation method. Although the bias voltage and the modulation signal are superimposed by the superimposer 26 here, a so-called AC / DC separation input configuration in which the bias voltage and the modulation signal can be separately input to the optical modulator may be employed.

  For example, in the case of simple intensity modulation as described above, the modulation signal may be a data signal having an amplitude Vπ (simple 1 or 0 repeated data for setting an initial operating point) may be used, and binary or quaternary phase modulation may be performed. In this case, multiple series of signals that have undergone the encoding process and inversion process in the previous stage are input, but as described above, the basic operation of the optical modulator is the same, and periodic modulation is performed. Characteristic intensity intermediate values and inflection points (maximum and minimum points) are used as operation point candidates.

  The control unit 30 includes an initial operating point setting unit 31 and a drift suppression unit 32. The initial operating point setting unit 31 sequentially enters light having a plurality of different wavelengths λ1 to λn from the multi-wavelength light source 21 to the optical modulator 22 and monitors the intensity of light emitted from the optical modulator 22 based on the output of the light receiver 24. However, the bias voltage supplied to the optical modulator 22 is variably controlled to find a common operating point where the emitted light intensity is within a predetermined allowable range for each wavelength, and the bias voltage corresponding to the common operating point is optically modulated. Set to the initial operating point of the device 22.

  The drift suppression unit 32 detects and suppresses the drift of the operating point set in the optical modulator 22 by the initial operating point setting unit 31, and the configuration and processing method of the drift suppression unit 32 are described above. As described in Patent Documents 1 and 2, etc., a low-frequency signal is superimposed on the modulation signal, the low-frequency signal component is extracted from the emitted light, and the superimposed low-frequency signal and the detected low-frequency signal , The operating point drift due to temperature or the like is detected, and the bias voltage is changed by the drift, thereby suppressing the operating point drift.

  As described above, the processing of the initial operating point setting unit 31 sequentially enters the light having a plurality of different wavelengths λ1 to λn to the optical modulator 22 and monitors the emitted light intensity of the optical modulator 22, By variably controlling the bias voltage, finding a common operating point where the emitted light intensity is within a predetermined allowable range for each wavelength, and setting the bias voltage corresponding to the common operating point as the initial operating point of the optical modulator Various methods can be considered for the specific processing.

  One of them is to obtain all the operating point candidates (specified by the distinction between intermediate value, maximum value and minimum value and the slope of the characteristic) determined by the modulation method with the modulation characteristics for all wavelengths, and the emitted light intensity is predetermined. This is a method for finding a common operating point that is within an allowable range, each operating point candidate for each wavelength is within a certain error range, and further the absolute value of the bias voltage is minimized to reduce power consumption. However, this method requires processing time because it is necessary to find all operating point candidates for each wavelength.

  Therefore, a method of shortening the processing time will be described based on the flowcharts of FIGS. As shown in FIG. 2, first, light having a specific wavelength (λ1) out of a plurality of different wavelengths λ1 to λn is incident on the optical modulator 22, and the intensity of the emitted light is monitored while the bias voltage is set. By variably controlling, one of the operating point candidates of the optical modulator 22 for the specific wavelength λ1, that is, the one having the minimum absolute value for the purpose of reducing power consumption is obtained as a reference point, and the intensity at that time is stored (S1). ~ S4).

  Here, for the sake of simplicity of explanation, it is assumed that the operating point candidate is an intermediate point with a positive slope of intensity, and the emitted light intensity detected at that time is detected in an unmodulated state where no data signal is given. The intensity of the emitted light when modulation is performed by giving reference data of the amplitude of Vπ corresponding to the shortest wavelength among ˜λn may be used.

  FIG. 5 shows the modulation characteristics of each wavelength when n = 3, and the operating point candidates (A1, A2,..., B1, B2 determined by the modulation method of the modulation characteristic M1 of the specific wavelength λ1 indicated by the solid line. ,..., Is an intermediate point with positive slope, the first reference point is the A1 point with + polarity and close to 0 volts, and the emitted light intensity P1 at that time is obtained as a reference value.

  Next, with the operating point of the optical modulator 22 held at the reference point (A1), other wavelength light λ2 to λn other than the specific wavelength are sequentially incident, and the emitted light intensity for each wavelength including the inclination is examined (S5). ~ S8). The inclination is detected by slightly increasing / decreasing the bias voltage and determining the change in the light intensity. In the following explanation, it is assumed that the information on the intensity of the emitted light includes the positive / negative symbols. . That is, if the slope is positive and the absolute value of the intensity is P, it is + P, and if the slope is negative and the intensity is P, it is -P.

  In the example of FIG. 5, the modulation characteristic M2 of wavelength λ2 is indicated by a dotted line, and the modulation characteristic M3 of wavelength λ3 is indicated by a broken line. The bias voltage is Vb1 corresponding to the reference point A1, and each characteristic at this bias voltage Vb1 The emission intensities + P2 and -P3 of M2 and M3 are greatly separated from the characteristic M1 due to a wavelength difference or an optical path difference, and the difference from the intensity + P1 at the specific wavelength λ1 is large. In addition, the slope of the characteristic M3 is negative, which is different from the case of the specific wavelength λ1.

  And the intensity | strength P1 calculated | required at the time of incidence | injection of the light of specific wavelength (lambda) 1 is made into reference | standard intensity | strength, and deviation (P1-P2) with respect to the reference | standard intensity | strength P1 of intensity | strength P2-Pn obtained by incidence | injection of light λ2-λn other than a specific wavelength. , (P1-P3),..., (P1-Pn) are examined (S9), and whether or not all the deviations obtained for the other wavelength lights P2 to Pn are within a preset allowable range. Determine (S10). This allowable range depends on the accuracy required for the system, but is set to ± 5% of the reference strength P1, for example.

  Since each intensity detected as described above includes characteristic inclination information as a sign, the absolute value of the deviation between the inconsistent inclinations becomes extremely large.

  That is, as shown in FIG. 5, when the slope of the characteristic M3 is negative, the intensity is (−P3), so the deviation is (+ P1) − (− P3) = P1 + P3, which is very high. As a matter of course, it becomes a large value and does not fall within the allowable range described above.

  Even if the slopes match, the deviation does not fall within the allowable range if it is far away from the reference intensity (+ P1), such as the intensity (+ P2) of the characteristic M2.

  Thus, when it is determined that the deviation is not within the allowable range, the reference of the operating point of the optical modulator 22 is changed to the adjacent operating point candidate A2 in the + direction, and each other wavelength light is changed in the same manner as described above. Incident light is obtained and an intensity deviation with respect to the reference intensity P1 for each other wavelength is obtained (S12 to S18).

  In the example of FIG. 5, the bias voltage Vb2 in the operating point candidate A2 is closer to the intensity intermediate point of the modulation characteristics M2 and M3 and the characteristic inclination matches the reference, but the intensity deviation is still large. It is determined that the deviation is outside the allowable deviation (S19).

  Then, since the slopes coincide with each other as in this time, the maximum of the current deviation becomes smaller than the maximum of the previous deviation (S20), so that the change direction is assumed to be correct, and the reference of the operating point of the optical modulator 22 is again obtained. Is changed to the operating point candidate A3 adjacent to the + direction, each other wavelength light is incident in the same manner as described above, and an intensity deviation with respect to the reference intensity P1 for each other wavelength is obtained (S12 to S18).

  In the example of FIG. 5, the bias voltage Vb3 at the operating point candidate A3 substantially matches the intensity intermediate point between the modulation characteristics M2 and M3, and the emitted light intensities + P2 and + P3 at the wavelengths λ2 and λ3 are closer to the reference intensity P1. The deviation falls within an allowable range ± W / 2.

  In this way, when the deviation obtained for the other wavelength light falls within the allowable range (S19), the operating point of the optical modulator 22 at that time is determined as the common operating point for each wavelength, and the common operating point is determined. Is set as the initial operating point of the optical modulator 22 (S11). Further, even when the deviation obtained for the other wavelength light within the first operation point candidate A1 falls within the allowable range (S10), the operation point of the optical modulator 22 at that time is set as the common operation for each wavelength. The bias voltage corresponding to the common operating point is set as the initial operating point of the optical modulator 22.

  On the other hand, when the deviation does not fall within the allowable range in the determination process of S19, as described above, the maximum of the previous deviation is compared with the maximum of the current deviation, and the current deviation is smaller. Performs the same process by changing the operating point candidate to the same direction (in this case, the + side) assuming that the operating point change direction is correct (S20).

  If it is determined in step S20 that the current deviation is larger, it is determined that the operating point change direction is incorrect, and the operating point candidate is displayed in the reverse direction (in this case, the-side) as shown in FIG. (B1, B2,...) And the same processes as S13 to S19 are performed (S21 to S28).

  By performing the above processing until the deviation falls within the allowable range, an operating point common to each wavelength can be found efficiently.

  In the above-described embodiment, the inclination of the characteristic at the operating point is conscious and a common operating point is obtained such that they match for each wavelength. The common operating point may be obtained by ignoring the tilt information.

  In the above embodiment, the slope information is assigned to the intensity code so that the deviation does not match in the deviation calculation greatly exceeds the allowable range. However, the intensity deviation calculation and the slope mismatch do not match. These determinations may be performed separately.

  Further, in the embodiment, when it is determined that the intensity deviation in the first operation point candidate is not within the allowable range, the operation point candidate is changed to the next operation point candidate on the + side (which is higher than the current state). However, it may be changed to the next operating point candidate on the negative side (lower than the current level).

  In this way, after the setting of the initial operating point is completed, the multi-wavelength optical modulation system 20 modulates the light of each wavelength with the data signal according to a predetermined schedule or the like and emits it. In the meantime, the control unit 30 performs feedback control for suppressing drift of the operating point of the optical modulator 22. Here, the operating point drift acts almost equally on the modulation characteristics of each wavelength, that is, the modulation characteristics M1 to M3 of each wavelength as shown in FIG. It works equally for all wavelengths.

  In addition, when light of a plurality of wavelengths is simultaneously incident on the optical modulator 22 and is modulated with the same data signal, light of one wavelength is selected and a low frequency signal component is selected from the selected light. And the drift suppression control may be performed.

  In the above embodiment, the optical branching device 23 and the light receiving device 24 for detecting the intensity of the light emitted from the optical modulator 22 are shown separately from the optical modulator 22. An optical modulator to which a monitor function) is added is realized, and when such an optical modulator is used, the monitor output may be input to the control unit 30.

  DESCRIPTION OF SYMBOLS 20 ... Multi-wavelength type optical modulation system, 21 ... Multi-wavelength light source, 22 ... Optical modulator, 23 ... Optical splitter, 24 ... Light receiver, 25 ... Bias voltage generator, 26 ... Superimposer , 30 ... Control unit, 31 ... Initial operating point setting unit, 32 ... Drift suppression unit

Claims (3)

A light modulator including a Mach-Zehnder interferometer using an electro-optic effect and having a modulation characteristic in which the intensity of emitted light periodically changes with a change in applied bias voltage . A light of a specific wavelength is incident, and the bias voltage supplied to the optical modulator is variably controlled while monitoring the intensity of light emitted from the optical modulator, and the operating point candidate of the optical modulator for the specific wavelength One of these as a reference point and storing the intensity at that time (S1 to S4),
While keeping the operating point of the optical modulator at the reference point, sequentially entering light of wavelengths other than the specific wavelength and examining the intensity for each wavelength (S5 to S8);
Inspecting a deviation from the reference intensity of the intensity obtained by the incidence of light having a wavelength other than the specific wavelength as the reference intensity, which is obtained when the specific wavelength light is incident (S9),
Determining whether all the deviations obtained for each of the other wavelength lights are within a preset allowable range (S10);
When it is determined that any of the deviations obtained for the other wavelength light is not within the allowable range, the reference point is changed to an operation point candidate next to either the + direction or the − direction. , Making each other wavelength light incident, and obtaining a deviation from the reference intensity for each other wavelength (S12 to S18);
Determining whether the deviation obtained after the change of the reference point is within the allowable range (S19);
When it is determined that the deviation obtained after the change of the reference point is not within the allowable range, comparing the deviation after the change with the deviation before the change (S20);
If the deviation after the change of the reference point is smaller than the deviation before the change, it is determined that the change direction of the reference point is correct, and the reference point is changed to the next operation point candidate in the one direction to obtain a new deviation. Is performed until the deviation falls within the allowable range. If the deviation after the change is larger than the deviation before the change, it is determined that the change direction of the reference point is incorrect and the reference point is determined. Including changing the operation point candidate next to the one direction opposite to the one direction to obtain a new deviation until the deviation falls within the allowable range (S12 to S18, S21 to S27),
The operating point of the optical modulator when the deviation obtained for the other wavelength light falls within the allowable range is determined as a common operating point for each wavelength, and a bias voltage corresponding to the common operating point is set as the bias voltage. An initial operating point setting method for an optical modulator, wherein the initial operating point is set for the optical modulator. A multi-wavelength light source (21) that emits light of a plurality of different wavelengths;
Including a Mach-Zehnder interferometer using an electro-optic effect, having a modulation characteristic in which emitted light intensity changes periodically with respect to a change in applied bias voltage, light emitted from the multi-wavelength light source, and the bias An optical modulator (22) for receiving a voltage and a modulation signal and emitting modulated light modulated by the modulation signal;
Outgoing light intensity detection means (23, 24) for detecting the outgoing light intensity of the light modulator;
A bias voltage generator (25) for generating a bias voltage to be applied to the optical modulator;
A multi-wavelength optical modulation system having a control unit (30) for controlling the baice voltage output from the bias voltage generator,
The control unit sequentially enters the light of the plurality of different wavelengths from the light source to the optical modulator, and monitors the output light intensity of the light modulator by the output of the output light intensity detection unit, The bias voltage supplied to the optical modulator is variably controlled to find a common operating point where the emitted light intensity is within a predetermined allowable range for each wavelength, and the bias voltage corresponding to the common operating point is set to the optical modulator. An initial operating point setting unit (31) for setting an initial operating point is provided,
The initial operating point setting unit
A light having a specific wavelength out of a plurality of different wavelengths is incident on the optical modulator, and the bias voltage supplied to the optical modulator is variably controlled while monitoring the intensity of light emitted from the optical modulator. A means for obtaining one of the operating point candidates of the optical modulator as a reference point for the specific wavelength, and storing the intensity at that time;
Means for sequentially injecting light of wavelengths other than the specific wavelength while maintaining the operating point of the optical modulator at the reference point, and checking the intensity for each wavelength;
Means for examining the deviation of the intensity obtained by the incidence of light of a wavelength other than the specific wavelength with respect to the reference intensity, with the intensity determined when the specific wavelength of light is incident as a reference intensity;
Means for determining whether or not all deviations obtained for each of the other wavelength lights are within a preset allowable range;
When it is determined that any of the deviations obtained for the other wavelength light is not within the allowable range, the reference point is changed to an operation point candidate next to either the + direction or the − direction. Means for making each other wavelength light incident and calculating a deviation from the reference intensity for each other wavelength;
Means for determining whether the deviation obtained after the change of the reference point is within the allowable range;
Means for comparing the deviation after the change with the deviation before the change when it is determined that the deviation obtained after the change of the reference point is not within the allowable range;
If the deviation after the change of the reference point is smaller than the deviation before the change, it is determined that the change direction of the reference point is correct, and the reference point is changed to the next operation point candidate in the one direction to obtain a new deviation. Is performed until the deviation falls within the allowable range. If the deviation after the change is larger than the deviation before the change, it is determined that the change direction of the reference point is incorrect and the reference point is determined. Means for performing a process of obtaining a new deviation by changing to an operation point candidate adjacent to the one direction opposite to the one direction until the deviation falls within the allowable range;
The operating point of the optical modulator when the deviation obtained for the other wavelength light falls within the allowable range is determined as a common operating point for each wavelength, and a bias voltage corresponding to the common operating point is set as the bias voltage. A multi-wavelength optical modulation system, characterized by being set to an initial operating point of an optical modulator. According to the control unit, which detects drift of the initial set operating point on the optical modulator by the initial operating point setting unit, wherein the suppressing drift suppressing portion (32) is provided Item 3. The multi-wavelength optical modulation system according to Item 2 .

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