JP2010217633A - Optical modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a bias control circuit capable of reducing the hard scale of an optical modulator having a plurality of places such as MZ type optical modulation sections, namely DQPSK modulators or the like, for adjusting the direct current bias. <P>SOLUTION: The optical modulator includes two Mach-Zehnder (MZ) type optical modulation sections 2 and 3, which are respectively incorporated into branch waveguides 12 and 13 of a main Mach-Zehnder type optical waveguide 1. The optical modulator includes DC bias-applying means 121 and 131 for applying the DC bias to each MZ type optical modulation section, low-frequency signal superimposing means 122, 123, 132 and 133 for superimposing a low-frequency signal on each DC bias, a light reception-detecting means 6 for detecting reception of a portion of output light or radiation light from the optical modulator, and bias control means 124 and 134 for controlling each of the DC bias-applying means based on the output of the light reception detecting means. The low-frequency signal superimposed on the DC bias applied to each MZ type optical modulation section is set as a triangular wave that has the same frequency and a phase difference of 90°. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an optical modulator, and in particular, includes two Mach-Zehnder optical modulators (hereinafter referred to as “MZ-type optical modulators”) having a Mach-Zehnder optical waveguide. The present invention relates to an optical modulator in which each MZ type optical modulator is incorporated in each branch waveguide. Further, the present invention relates to bias control of a DC bias applied to such an optical modulator.

  In a next-generation long-distance large-capacity optical communication system that requires high speed and large capacity as communication traffic increases, the introduction of multilevel modulation / demodulation coding technology is being studied. One typical example is a differential quadrature phase shift keying (DQPSK modulation) system. Compared with the conventional binary intensity modulation (OOK) method, this method has a narrower signal band, can improve the frequency utilization efficiency and increase the transmission distance, and can be expected to have higher sensitivity. It is a next-generation key technology expected to be applied to s systems and 100 Gb / s systems.

  First, in the quadrature phase shift keying (QPSK modulation) method, for each symbol “00”, “01”, “11”, and “10” composed of 2-bit data, “ “θ”, “θ + π / 2”, “θ + π”, and “θ + 3π / 2” are assigned. The principle is that “θ” and “θ + π” are first generated by the MZ modulation unit of the I (In-phase) arm, and the MZ modulation unit of the Q (Quadrature) arm performs 90 degrees after performing the same phase modulation. Phase shifts of “θ + π / 2” and “θ + 3π / 2” are generated. Here, “θ” is an arbitrary phase. Then, the receiver reproduces the transmission data by detecting the phase of the received signal.

  As a means for realizing the QPSK modulation method relatively easily, there is a DQPSK modulation method. In the DQPSK modulation, the phase change amount of the carrier wave between the value of the symbol transmitted first and the value of the symbol transmitted next (“0 ”,“ Π / 2 ”,“ π ”, and“ 3π / 2 ”) are associated with 2 bits of transmission information. Therefore, the receiver can recover the transmission data by detecting the phase difference between two adjacent symbols.

  As an example of an optical transmitter using the DQPSK modulation method, as shown in FIG. 1, two branch waveguides of one Mach-Zehnder type optical waveguide 1 (hereinafter referred to as “main Mach-Zehnder type optical waveguide”). 12 and 13 include Mach-Zehnder optical modulators (hereinafter referred to as “MZ-type optical modulators”) 2 and 3 having Mach-Zehnder optical waveguides, and at least one branching waveguide has a phase of 90 degrees. An optical phase adjustment unit 4 that performs shifting is arranged in series with the MZ type optical modulation unit. Reference numeral 10 denotes an input waveguide, reference numeral 11 denotes a branching section, reference numeral 14 denotes a multiplexing section, and reference numeral 15 denotes an output waveguide.

  For example, the same modulation signal is applied to the two MZ type optical modulation units 2 and 3 by the modulation signal driving means 5, and the MZ type optical modulation unit 2 performs the modulation corresponding to the I arm, and the MZ type optical modulation unit 3. The optical phase adjustment unit 4 performs modulation corresponding to the Q arm.

  In the optical waveguide and main Mach-Zehnder optical waveguide that constitute the MZ type optical modulator, a polarization phenomenon occurs in the substrate that constitutes the optical waveguide due to long-time electric field application or temperature change, and the modulation of the optical modulator A drift phenomenon such as a curve shift occurs.

  A predetermined DC bias (DC bias) is applied to the MZ type optical modulation units 2 and 3 and the optical phase adjustment unit 4 by the DC bias application means 21, 31, and 41. It is necessary to control the value of to the optimum state. For example, in the DQPSK modulation method, the MZ type optical modulation units 2 and 3 need to perform modulation using so-called Vπ whose phase is shifted by π as an optimum bias. In addition, the optical phase adjustment unit 4 requires optimal bias control so that the phase shift amount is 90 degrees.

  As shown in Patent Document 1 or 2, in various optical modulators including an optical modulator using the DQPSK modulation method, a method of superimposing a low frequency signal is adopted as a method for maintaining the DC bias at an optimum value. ing. Furthermore, Patent Document 3 discloses that low frequency signals having different frequencies are used when a plurality of light modulation units are provided.

  In the optical modulator shown in FIG. 1, a low frequency signal having a frequency f 1 from the low frequency signal generating means 22 is superimposed on the DC bias of the direct current bias applying means 21 and applied to the MZ type optical modulator 2. A part of the signal light outputted from the output waveguide 15 of the main Mach-Zehnder type optical waveguide 1 is taken out by using a branching means 16 such as an optical coupler, and an optical / electrical conversion means (light reception detecting means such as a light receiving element). ) 6 is converted into an electrical signal corresponding to the signal light. Then, only the component corresponding to the frequency f 1 in the electrical signal is extracted by the filter 25 and input to the DC bias control means 24.

  The direct current bias control means 24 controls the direct current bias application means 21 to change the DC bias value, and at the same time, obtains the optimum DC bias value from the change state of the electric signal input from the optical / electrical conversion means 6 through the filter 25. Control to determine.

  As for DC bias control for the MZ type optical modulation unit 3 and the optical phase adjustment unit 4, the low frequency signal generation means 32 and 42, DC bias application means 31 and 41, DC bias control means 34 and 44, filters 35 and 45, etc. And controlled in the same way. It should be noted that a low-frequency signal having a frequency f2 is used for the MZ type optical modulation unit 3, and a low-frequency signal having a frequency f3 is used for the optical phase adjustment unit 4, and the frequencies f2 and f3 are different from the frequency f1. Is done. For example, as shown in FIG. 2, 3 kHz is used as the frequency f1, 7 kHz is used as the frequency f2, and 10 kHz is used as the frequency f3.

  When there are a plurality of locations where the DC bias is adjusted, such as I-arm control, Q-arm control, and 90-degree phase shift unit of DQPSK modulation, different low-frequency oscillators are required, and three of them are required. There is a problem that the type of control procedure becomes complicated and the hardware scale increases. Further, Patent Document 2 describes that control is performed in a time-sharing manner, but has a drawback that convergence is slow, firmware is required, and a hardware scale is increased.

Japanese Patent No. 3723358 JP 2007-43638 A JP 2004-318052 A

  The problem to be solved by the present invention is to solve the above-described problems, and to an optical modulator having a plurality of locations for adjusting a DC bias, such as a plurality of MZ type optical modulators such as a DQPSK modulator. Another object is to provide a bias control circuit capable of reducing the hardware scale.

  In order to solve the above-described problems, the invention according to claim 1 includes two MZ-type optical modulation units each having a Mach-Zehnder (MZ) type optical waveguide, and each MZ is provided in each branch waveguide of the main Mach-Zehnder type optical waveguide. In an optical modulator incorporating a type optical modulation unit, a DC bias applying unit that applies a DC bias to each MZ type optical modulation unit, a low frequency signal superimposing unit that superimposes a low frequency signal on each DC bias, and the optical modulation Each of the MZ type optical modulations, comprising: a light receiving detection means for receiving and detecting a part of the output light or radiated light from the detector; and a bias control means for controlling each of the DC bias applying means based on the output of the light reception detection means. The low frequency signal superimposed on the DC bias applied to the unit is a triangular wave having the same frequency and a phase difference of 90 degrees.

  According to a second aspect of the present invention, in the optical modulator according to the first aspect, the bias control unit converts the electrical signal output from the light receiving detection unit into a filter corresponding to the even order of the specified frequency of the triangular wave. Transmission is performed, and control is performed based on the transmitted electric signal.

  According to a third aspect of the present invention, in the optical modulator according to the first or second aspect, an optical phase adjusting unit is incorporated in one of the branch waveguides, and a sine wave is superimposed on the optical phase adjusting unit. A DC bias is applied.

  According to a fourth aspect of the present invention, in the optical modulator according to the first or second aspect, an optical phase adjusting unit is incorporated in each of the branch waveguides, and each optical phase adjusting unit includes the MZ-type light. A DC bias on which a triangular wave having a frequency different from that of the triangular wave related to the modulation unit and a phase difference of 90 degrees is superimposed is applied.

  According to a fifth aspect of the present invention, in the optical modulator according to any one of the first to fourth aspects, the optical modulator is an optical transmitter that performs DQPSK modulation, and each branch waveguide includes an I arm and a Q It has the function of each arm.

  According to the first aspect of the present invention, two MZ type optical modulation units having Mach-Zehnder (MZ) type optical waveguides are provided, and each MZ type optical modulation unit is incorporated in each branch waveguide of the main Mach-Zehnder type optical waveguide. In the optical modulator, DC bias applying means for applying a DC bias to each MZ type optical modulator, low frequency signal superimposing means for superposing a low frequency signal on each DC bias, and output light or radiation from the optical modulator A DC bias applied to each MZ type optical modulation unit is provided with a light reception detection means for detecting a part of the light and a bias control means for controlling each of the DC bias application means based on the output of the light reception detection means. Since the low frequency signal superimposed on is a triangular wave having the same frequency and a phase difference of 90 degrees, the DC biases related to the two MZ type optical modulators can be controlled at a single frequency, and the bias control circuit can be controlled. Hardware scale it is possible to provide an optical modulator capable of reducing the.

  According to the invention of claim 2, the bias control means transmits the electrical signal output from the light receiving detection means through the filter corresponding to the even order of the specified frequency of the triangular wave, and controls based on the transmitted electrical signal. The shape of the output signal of the light receiving detection means can be more accurately identified. For example, it is possible to determine which MZ type optical modulation unit has a deteriorated DC bias and perform DC bias control. The band of the filter to be transmitted here is a set value that takes into consideration the even-order frequency of the regular frequency of the triangular wave.

  According to the invention of claim 3, the optical phase adjustment unit is incorporated in one of the branch waveguides of the main Mach-Zehnder type optical waveguide, and a DC bias superimposed with a sine wave is applied to the optical phase adjustment unit. Therefore, since it is possible to detect the bias fluctuation of the two MZ type optical modulation units using a triangular wave and to detect the bias fluctuation of the optical phase adjustment unit using a sine wave, each DC bias is detected. Can be optimally controlled.

  According to the invention of claim 4, the optical phase adjustment unit is incorporated in each of the branch waveguides of the main Mach-Zehnder type optical waveguide, and each optical phase adjustment unit is different from the triangular wave related to the MZ type optical modulation unit. Since a DC bias on which a triangular wave having a frequency and a phase difference of 90 degrees is superimposed is applied, the bias fluctuation of the two MZ type optical modulation units is detected using the triangular wave related to the MZ type optical modulation unit, and 2 Since it is possible to detect the bias fluctuation of the two optical phase adjustment units using triangular waves having different frequencies related to the optical phase adjustment unit, each DC bias can be optimally controlled.

  According to the invention of claim 5, the optical modulator is an optical transmitter that performs DQPSK modulation, and each branch waveguide of the main Mach-Zehnder optical waveguide has the functions of an I arm and a Q arm, Also in the optical modulator used for the DQPSK modulation method, it is possible to provide an optical modulator that can reduce the hardware scale of the bias control circuit.

It is a figure explaining the structure of the conventional bias control circuit in an optical modulator. It is a figure which shows an example of the low frequency signal utilized for the bias control circuit of the optical modulator of FIG. It is a figure which shows an example of the bias control circuit in the optical modulator of this invention. It is FIG. (1) explaining the principle of the bias control which concerns on the optical modulator of this invention. It is FIG. (2) explaining the principle of the bias control which concerns on the optical modulator of this invention. It is FIG. (3) explaining the principle of the bias control which concerns on the optical modulator of this invention. It is a figure which shows the example of the low frequency signal utilized for the bias control circuit of the optical modulator of FIG. It is a figure which shows the calculation model of the bias control circuit which concerns on the optical modulator of this invention. It is a graph which shows the calculation result using the calculation model shown in FIG. 10 is a graph obtained by normalizing the graph of FIG. 9 with a value of t / T = 0.625. It is a figure which shows the relationship between a trapezoid waveform and a triangular wave. It is a figure explaining the variation factor in the bias control circuit which concerns on the optical modulator of this invention. It is a figure which shows the mode of the sampling with respect to the fluctuation | variation of a sine wave. It is the figure which showed the convergence state in DC bias control with the locus | trajectory in an I / Q plane. It is a figure which shows the relationship between the bias characteristic of an optical modulator, and each area | region (D, E) of FIG. It is a figure which shows an example of the control flow used for the bias control circuit which concerns on the optical modulator of this invention. It is a figure which shows the locus | trajectory of a bias point at the time of controlling with the control flow of FIG. It is a figure which shows the example of the waveform utilized when Fourier-transforming the graph shown in FIG. It is a figure which shows the other example of the bias control circuit in the optical modulator of this invention. FIG. 6 is a diagram (No. 1) for explaining the principle of bias control when an optical phase adjustment unit is provided in two branch waveguides of a main Mach-Zehnder type optical waveguide in the optical modulator of the present invention. FIG. 6 is a diagram (No. 2) for explaining the principle of bias control when an optical phase adjustment unit is provided in two branch waveguides of a main Mach-Zehnder type optical waveguide in the optical modulator of the present invention. It is a figure which shows the example of the low frequency signal utilized for the bias control circuit of the optical modulator of FIG.

Hereinafter, the optical modulator according to the present invention will be described in detail using preferred examples.
As shown in FIG. 3, the present invention includes two MZ type optical modulators (2, 3) each having a Mach-Zehnder (MZ) type optical waveguide, and each branch guide of the main Mach-Zehnder type optical waveguide (1). In an optical modulator in which each MZ type optical modulator is incorporated in the waveguide (12, 13), DC bias applying means (121, 131) for applying a DC bias to each MZ type optical modulator, and a low frequency for each DC bias A low-frequency signal superimposing means (122, 123, 132, 133) for superimposing a signal, a light receiving detection means (6) for receiving and detecting a part of output light or radiated light from the light modulator, and the light receiving detection means Bias control means (124, 134) for controlling each of the DC bias applying means on the basis of the output of the low frequency signal superimposed on the DC bias applied to each MZ type optical modulation section. Phase difference Characterized in that it is a 0 degrees and becomes a triangular wave (f1, f1 + π / 2).

  FIG. 3 exemplifies an optical modulator used for an optical transmitter of the DQPSK modulation system, but the present invention is not limited to this. For example, MZ type optical modulation is provided in each branch waveguide (12, 13). When only the units (2, 3) are provided, the application can be applied to various optical modulators such as an SSB (Single Side Band) modulator. Moreover, what uses the code | symbol similar to the prior art example of FIG. 1 means the same functional component. In the following, the description will focus on the DQPSK modulation type optical modulator. For example, the same modulation signal is applied to the two MZ type optical modulation units 2 and 3 by the modulation signal driving means 5, and the MZ type optical modulation unit 2 performs the modulation corresponding to the I arm, and the MZ type optical modulation unit 3. The optical phase adjustment unit 4 that performs 90-degree phase shift performs modulation corresponding to the Q arm.

  In the optical modulator of FIG. 3, a DC wave supplied from the DC bias applying unit 121 (131) is a triangular wave having a frequency f1 (a triangular wave having a phase difference of 90 degrees at the frequency f1) generated from the low frequency signal generating unit 122 (132). It is superimposed on the bias by the superimposing means 123 (133) and applied to the MZ light modulator 2 (3). A part of the signal light outputted from the output waveguide 15 of the main Mach-Zehnder type optical waveguide 1 is taken out using a branching means 16 such as an optical coupler, and a light receiving detection means (light / electric conversion means such as a light receiving element). ) 6 is converted into an electrical signal corresponding to the signal light. Then, only the component corresponding to the frequency f1 in the electrical signal is extracted by the filter 125 and input to the DC bias control means 124 (134). The light receiving detection means 6 can detect not only a part of the signal light but also the radiated light emitted from the multiplexing unit 14 as necessary.

  In the DC bias control means 124 (134), based on the electric signal input from the light receiving detection means 6 through the filter 125 (the filter that transmits the signal of the frequency f1 and its even order), the optimum DC bias is obtained. The DC bias applying means 121 (131) is controlled. In FIG. 3, the DC bias control means is provided corresponding to each MZ type optical modulation section, but it can also be integrated into one control means.

  For the optical phase adjustment unit 4 that performs 90-degree phase shift with the Q arm, the low frequency signal generator 142 generates a sine wave low frequency (frequency f2 is different from the triangular wave frequency, as in the conventional bias control circuit. Is superimposed on the DC bias from the DC bias applying means 141 by the superimposing means 143 and applied to the optical phase adjusting unit 4. Then, a part of the signal light is detected by the light receiving detection means 6 and an electric signal is output. The electric signal is input to the DC bias control means 144 via the transmission filter 145 having the frequency f2. The DC bias control means 144 controls the DC bias application means 141 so as to obtain an optimum DC bias based on the input electric signal.

  Hereinafter, a method of identifying a DC bias state using a triangular wave as a low frequency signal, which is performed by the DC bias control unit 124 (134), will be described.

  The feature of the optical modulator of the present invention is that, as shown in FIG. 3, for each MZ type optical modulation unit constituting the I arm and Q arm, a triangular wave having a phase difference of 90 degrees is superimposed on a DC bias in synchronization with each other. Applied. As a result, as shown in FIGS. 4 to 6, the change in light intensity due to the low frequency signal applied to the I arm is plotted on the horizontal axis, and the change in light intensity due to the low frequency signal applied to the Q arm is plotted on the vertical axis. The locus of the superimposed signal is a diamond type A. The light intensity is proportional to the square of the length of the vector B.

  When a normal bias is set for the I arm and the Q arm as shown in FIG. 4A, the vector B indicating the light intensity draws a diamond-shaped locus indicated by A. The relationship between the light intensity change at this time and the triangular wave superimposed on the I arm is shown in FIG. The light intensity change becomes a time response waveform (time change of the light intensity E output from the optical modulator) as shown in the graph (1) of FIG. 4B, and the trapezoid whose upper side is substantially parallel to the time axis. A waveform (note that the rising of both shoulders of the trapezoidal waveform is emphasized in the graph (1) of FIG. 4B. For reference, a theoretical waveform is illustrated in FIG. 10 described later). .

  A graph (2) in FIG. 4B shows a waveform corresponding to a temporal change of the DC bias intensity F applied to the I arm shown in FIG. 3, particularly a low frequency signal (triangular wave) portion to be superimposed. .

  As shown in FIG. 5A and FIG. 6A, when the DC bias applied to the I arm and the Q arm is unbalanced, the diamond-shaped locus A by the triangular wave superimposed on the I arm and the Q arm is It will deviate from the normal position shown in FIG. Then, when the state of the light intensity change is seen, the trapezoidal waveform is broken, for example, an asymmetric trapezoid with a lower right shoulder (upward to the left shoulder) as shown in FIG. 5 (b) and FIG. 6 (b), the trapezoidal waveform is represented by a straight line. However, as shown in FIG. The shoulder becomes a slightly raised waveform.) In addition, it is possible to identify which bias deviation of the I arm or the Q arm is based on which side of the shoulder is raised. This is seen on the time axis, but can also be identified on the frequency axis (phase) as described later.

  However, FIG. 5A and FIG. 6A exemplify a case where the DC bias is larger than a specified value in order to easily understand the state in which the DC bias is unbalanced. For this reason, when the DC bias of the I arm is increased, the shoulder is lowered to the right (up the left shoulder). However, as will be described later, when the DC bias is controlled so as not to exceed a specified value, The state in which the DC bias is unbalanced means that the DC bias is smaller than a specified value, and the change in light intensity at that time rises to the right, opposite to FIG. 5B (FIG. 6B). And the same shape). The same applies to the Q arm.

  FIG. 7 shows an example of a low-frequency signal used in the optical modulator of FIG. 3, and the frequency f1 of the triangular wave applied to the MZ type optical modulator is 5 kHz, and the optical phase adjuster The frequency f2 of the sine wave applied to is exemplified as 3 kHz. In order to identify on the frequency axis in the superposition of the triangular wave, it is convenient because the maximum signal amplitude can be obtained in the even order by monitoring the frequency of the even order, particularly the second harmonic (2 * f1). .

Next, the response waveform when drawing the diamond trajectory is theoretically verified using the calculation model shown in FIG. When the diamond trajectory surrounding the coordinate point (Ex, Ey) is expressed by the functions f (x) and f (y) of the XY coordinate system, the following mathematical expression 1 is obtained. Here, A is the amplitude value of the triangular wave, T is the period of the triangular wave, and t is the elapsed time.

  The amplitude of the coordinate P = (f (x), f (y)) (corresponding to the light intensity of the frequency f1 detected by the light receiving detection means) is obtained by the following mathematical formula [Formula 2].

  When the value of the equation [Equation 2] is obtained in the interval from time t to 0 to 1.25T, the following equation [Equation 3] is obtained.

  Here, when the DC bias of (A, Ex, Ey) = (0.01, 1.0, 1.0) is normal, (A, Ex, Ey) = (0.01, 0.99, 1.0), (0.01, 0.97, 1.0), FIG. 9 shows the result of calculating the formula [Equation 3] when only the DC bias on the X-axis side (corresponding to the I arm) is degraded as in (0.01, 0.95, 1.0).

  When FIG. 9 is seen, since the coordinate point (Ex, Ey) changes, the level of the amplitude value is different for each case, so-called offset occurs, but the response waveform is almost a trapezoidal waveform. Is easily understood. Here, in order to clarify the difference between the waveforms, the offset was subtracted and overwritten as shown in FIG. That is, adjustment was made so that the amplitude values of all the graphs coincided with each other at the normalized time t / T = 0.625. Referring to FIG. 10, it can be easily understood that the slope of the graph increases as the DC bias on the X-axis side deteriorates. In addition, this inclination occurs when the normalized time t / T is in the range of 0.5 to 0.75 (the range of the condition (3) in [Expression 3] above).

  From the result of FIG. 10, when the DC bias of the MZ type optical modulation unit on the I arm side deteriorates (meaning a case corresponding to a decrease in Ex, this is the level of the output light from the MZ type optical modulation unit of the I arm. Is lower than the specified value), it is easily understood that the asymmetric trapezoidal waveform rises to the right. On the other hand, when the DC bias of the MZ type optical modulation unit on the Q arm side deteriorates (meaning a case corresponding to a decrease in Ey, this means that the level of the output light from the MZ type optical modulation unit of the Q arm is less than the specified value. It means that it becomes lower.), It becomes an asymmetric trapezoidal waveform with a lower right shoulder (up left shoulder). However, it is preferable to control so that the values of Ex and Ey do not both become 1 or more (so that the DC bias falls within a specified value).

  As shown in FIG. 10, the relationship between the triangular wave and the manner in which a trapezoidal response waveform is formed by the triangular wave superimposed on the I and Q arms is shown in FIG. The upper side of the trapezoidal waveform is formed between the apex of the triangular wave superimposed on the I arm (pulse width standardization time 0.5) and the apex of the triangular wave superimposed on the Q arm (0.75 at the same time). The When the DC bias of the I arm deteriorates and is lower than the specified value, the slope of the upper side rises to the right as shown by reference C, and it is determined that the I arm side should be corrected.

  On the other hand, when the DC bias of the Q arm deteriorates and is lower than the specified value, the slope of the upper side falls to the right as shown by symbol C, and it is determined that the Q arm side should be corrected.

  Therefore, as shown in FIG. 11, the trapezoidal response waveform is sampled by monitoring the amplitudes (light intensity values) at the time points 0.5 and 0.75 in the pulse width standardization time. And Q-arm degradation can be identified. Moreover, since this sampling time is the time at which the two triangular waves to be superimposed just reach the top, means for monitoring in synchronization with the triangular wave is also effective. The pulse width normalization time of 0 to 0.25 (1.0 to 1.25) is a period corresponding to the bottom side of the trapezoidal waveform. (See FIG. 5B or 6B). However, the direction of the inclination is opposite to the upper side, and the sampling time is performed at the bottom points of the two triangular waves to be superimposed.

  As factors affecting the light intensity change of the signal light, not only the bias fluctuation of the MZ type optical modulation section but also the bias fluctuation of the optical phase adjustment section, as shown in FIG. 12, the MZ of the I arm or the Q arm is shown. The bias variation of the optical modulation unit changes along the coordinate point (black dot) along either the X axis or the Y axis, but the bias variation of the optical phase adjustment unit determines the distance between the origin, the coordinate point, and the distance. It changes along the circumference of the radius. When a triangular wave is used as a signal to be superimposed on the bias of the optical phase adjustment unit, for example, as shown in FIG. 22, an I arm near the frequency f1 (30 kHz) of the triangular wave superimposed on the bias of the MZ type optical modulation unit. And a band that does not interfere with the Q arm. In addition, it is also effective to use a low-frequency sine wave for the DC bias control of the optical phase adjustment unit, which is used by being superimposed on the DC bias. In such a sampling method using a sine wave, as shown in FIG. 13, the maximum value and the minimum value are obtained by sequential sampling, and the amplitude of the sine wave is obtained from the difference between them.

  As in the present invention, by using a triangular wave, the light modulation unit to be controlled, such as the I arm and the Q arm, can be specified by the inclination of the light intensity change. Accurate DC bias control is possible.

  When performing control in consideration of the light intensity parameter, if there is an initial bias point α in the D region in the I / Q plane shown in FIG. Go to the optimal bias point, as in a. However, if the control is performed without taking the absolute value of the light intensity into consideration, it is not the optimum bias point at which the amplitudes of the I arm and Q arm are maximized, but simply the size of the I arm and Q arm as shown by the locus b in the figure. The control is directed to any point where is equal.

  Further, when there is an initial bias point β in the E region, the control follows a process such as a trajectory c or a trajectory d in the figure and goes to a point where the sizes of the I arm and the Q arm are equal. This is due to the bias characteristic of the optical modulator shown in FIG. 15, and the Vπ / 2 voltage is a voltage that divides the D region and the E region in FIG.

  As an example, FIG. 16 shows a flowchart of DC bias control using this method. First, in step (1) of FIG. 16, the slope (V_balance) of the upper side of the trapezoidal waveform is the same as the amplitude (V (t = 0.50)) at the above-described pulse width normalization time of 0.5 at 0.75. (V (t = 0.75)) is extracted from the sampling data, and the two are compared and determined. As shown in FIG. 11, if the trapezoidal waveform rises to the right, it is determined that the I arm is degraded, and if it is a trapezoidal waveform that descends to the right, it is determined that the Q arm is degraded.

  Next, if it is determined that the I arm has deteriorated, the bias voltage of the I arm is increased by ΔV (a predetermined voltage value set in advance) in step (2). In step (3), assuming that the slope of the sampling data before changing the bias voltage by the predetermined voltage ΔV is (V_balance_n), the sampling data after changing the bias voltage by the predetermined voltage ΔV (V_balance_n + 1) is compared. . As a result, when the light intensity value increases due to the change of ΔV, the process returns to step (2), and increases again by the predetermined voltage ΔV, and the change state of the sampling data (V_balance_n) and (V_balance_n + 1) is changed to step (3). )

  When this is repeated and the bias voltage of the I arm is changed so that the light intensity value does not increase any more, that is, when the maximum value of the light intensity value due to the change of the bias voltage of the I arm is reached, the step ( The determination in 3) is “No”, and the process moves to step (4).

  In step (4), the bias voltage of the Q arm is increased by a predetermined voltage ΔV. Then, in step (5), as in step (3) of the I arm, sampling data (V_balance_n) before changing the bias voltage by the predetermined voltage ΔV and sampling after changing the bias voltage by the predetermined voltage ΔV The data (V_balance_n + 1) is compared. As a result, if V_balance is decreased due to a change in ΔV, the bias voltage is further increased, so that the process returns to step (4) and is increased again by a predetermined voltage ΔV. Similarly, the change state of the sampling data of V_balance Is determined in step (5).

  By repeating this, the bias voltage of the Q arm is changed, and the sampling data value of V_balance (t = 0.75) does not increase any more, that is, (V_balance_n + 1) = (V_balance_n) due to the change of the bias voltage of the Q arm When the value is reached, the determination in step (5) is “No”.

  When the state of this DC bias control is shown in an I / Q plan view, the initial bias point δ in FIG. 17 is moved in the direction parallel to the I arm axis in steps (2) and (3) described above, and then In steps (4) and (5), the axis moves in the direction parallel to the Q arm axis, and finally reaches the optimum bias point ε.

  Next, when it is determined in step (1) of FIG. 16 that the Q arm is deteriorated, steps (6) and (7) are repeated, and the bias voltage of the Q arm is changed to the maximum slope (negative value) of the maximum V_balance. ). When (V_balance_n + 1) = (V_balance_n) is reached in step (7), the process proceeds to steps (8) and (9) with a determination of “No” to adjust the bias voltage of the I arm. When (V_balance_n + 1) = (V_balance_n) is reached due to the change in the bias voltage of the I arm, the determination in step (9) becomes “No”, and the DC bias control ends. As a result, as shown in the I / Q plan view of FIG. 17, the initial bias point γ moves along the illustrated arrow, and finally reaches the optimum bias point ε.

  The optimum bias point in the DC bias control flow illustrated in FIG. 16 is the result of comparing the bias change in each arm of the I arm or the Q arm with the sampling data of V_balance from STEP2 to STEP5 (control from STEP6 to STEP9). In the same manner, sequential control is performed so that the slope of the upper side of the trapezoidal waveform is always kept parallel.

  As described above, when the output signal from the light receiving detection means is viewed on the time axis, the change in the DC bias can be identified from the change in the trapezoidal waveform, but further, the response waveform (light intensity change) due to the superposition of the triangular wave However, since it is known in advance that the trapezoidal waveform has a tilted top, analysis in the frequency domain (frequency axis) is also possible.

  In order to perform an analysis on the frequency axis, a Fourier transform of a trapezoidal waveform shown in FIG. 18A and a Fourier transform of a square wave shown in FIG.

  The Fourier transform of the trapezoidal waveform is shown in the following [Equation 4]. T represents the period of the trapezoidal waveform, and Tr represents the time width of rising or falling of the trapezoidal waveform.

  From the mathematical expression shown in [Expression 4], the trapezoidal waveform is a harmonic component of 1, 3, 5... With the sinc function as the magnitude of the frequency component, but the rising part of the trapezoid is also expressed as the sinc function. It can be seen that the magnitude of higher order components is reduced. The parameter obtained after the Fourier transform is the spectral intensity of the frequency. During normal (normal) control, the first-order and third-order odd-order frequencies appear at high levels. On the contrary, when even order such as second order and fourth order appear, it appears as a phenomenon of waveform deterioration. Therefore, it is possible to determine the waveform deterioration by monitoring the frequency spectrum intensity.

  Next, the case of using a Fourier transform of a square wave whose top is inclined will be described. In the case of this waveform, since it cannot be said to be an odd function or an even function, the Fourier series expansion is considerably complicated. Therefore, as shown in FIG. 18B, a Fourier transform of a waveform in which the top is inclined with a square wave is used. A mathematical formula obtained by performing the Fourier transform is shown in the following [Equation 5]. Here, T represents the period of the square wave, and τ represents the pulse width of the square wave.

  By using the value of T × a appearing in the third term of f (t), the equation a shown in [Equation 5] can be used to detect a which is an inclination component without being buried in the frequency component of the square wave. Moreover, since the T × a component is inverted by the sign of a, the sign can be identified. It can also be seen that identification and control on the frequency axis is possible by monitoring the spectral intensity of odd-order or even-order frequencies.

  Next, as shown in FIG. 19, the 90-degree phase shift unit in the optical modulator of the DQPSK modulation system is provided with light provided in each of the two branch waveguides (12, 13) of the main Mach-Zehnder type optical waveguide. The case where it comprises with a phase adjustment part (7, 4) is demonstrated.

  The bias control circuit according to the MZ type optical modulator of FIG. 19 is the same as the optical modulator of FIG. The optical modulator of FIG. 19 is characterized in that the 90-degree phase shift unit is configured by two optical phase adjustment units (4, 7), and for controlling the DC bias applied to the optical phase adjustment unit. Similar to the MZ type light modulator, a low frequency signal of a triangular wave is used.

  Each optical phase adjuster (4, 7) performs a 45 degree phase shift, and the two are combined to realize a 90 degree phase shift. The method of DC bias control of the optical phase adjusting unit is that the triangular wave of the frequency f2 (triangular wave having a phase difference of 90 degrees at the frequency f2) generated from the low frequency signal generating unit 142 (172) is the DC bias applying unit 141 (171). Is superimposed by the superimposing means 143 (173) and applied to the optical phase adjusting unit 4 (7). However, the frequency f2 is a frequency different from the frequency f1 used in the MZ type optical modulation unit.

  A part of the signal light output from the output waveguide 15 of the main Mach-Zehnder type optical waveguide 1 is taken out using a branching means 16 such as an optical coupler, and the light receiving detection means 6 such as a light receiving element handles the signal light. Convert to electrical signal. Then, only the component corresponding to the frequency f2 in the electrical signal is extracted by the filter 145 and input to the DC bias control means 144 (174).

  In the DC bias control means 144 (174), based on the electric signal input from the light receiving detection means 6 through the filter 145 (the filter that transmits the signal of the frequency f2 and its even order), an optimum DC bias is obtained. The DC bias applying means 141 (171) is controlled. In FIG. 19, the DC bias control means is provided corresponding to each MZ type optical modulation section and each optical phase adjustment section, but it can also be integrated into one control means.

  When both the I arm and the Q arm are provided with an optical phase adjustment unit that is a 45-degree shift unit, and a DC bias in which a triangular wave is superimposed is applied to each optical phase adjustment unit, as shown in FIGS. As in the case of the mold light modulator, a diamond locus A is obtained. The response waveform also becomes a trapezoidal waveform. 20 (a) and 20 (c), the horizontal axis represents the light intensity change corresponding to the low frequency signal applied to the I arm, and the vertical axis represents the light intensity change corresponding to the low frequency signal applied to the Q arm. Then, a state in which the locus of the superimposed signal becomes a diamond type A is shown.

  When a normal bias is set for the I arm and the Q arm as shown in FIG. 20 (a), as shown in FIG. 20 (b), the time response waveform (the vertical axis is from the optical modulator). The output light intensity changes with time, and the horizontal axis indicates the time axis. The upper side is a trapezoidal waveform whose upper side is substantially parallel to the time axis (actually, the both shoulders of the trapezoid are raised slightly). However, when the phase shift difference changes from 90 degrees as shown in FIG. 21A, the response waveform becomes an asymmetric trapezoidal waveform as shown in FIG. 21B, which is the same as the DC bias control of the MZ type optical modulator. In addition, identification / control on the time axis and identification / control on the frequency axis are possible. In particular, it is preferable that the frequency of the low-frequency signal applied to the phase modulation unit is set to about 10 times that of the low-frequency signal applied to the MZ type optical modulation unit so that two different low-frequency signals do not interfere with each other. Specifically, the monitor frequency of the MZ type optical modulation unit is set to a frequency of 10 KHz or less, the phase modulation unit is set to a frequency of 100 KHz or less, and is set in an area that is not affected when monitoring a high-order frequency.

  FIG. 22 shows an example of a low-frequency signal used in the optical modulator of FIG. 19, and the triangular wave frequency f1 applied to the MZ type optical modulation unit uses 30 kHz, and the optical phase adjustment unit The frequency f2 of the triangular wave applied to is exemplified as 3 kHz. In order to discriminate on the frequency axis in the superposition of the triangular wave, a 30 KHz triangular wave is used for bias control of the MZ type optical modulation unit of the I arm or the Q arm, and the double frequency of 60 KHz is monitored and controlled. Further, for the bias control of the optical phase adjustment unit (90 degree shift), a 3 KHz triangular wave is used and the double frequency of 6 KHz is monitored and controlled.

  As described above, according to the present invention, a bias capable of reducing the hardware scale for an optical modulator having a plurality of locations for adjusting a DC bias, such as a plurality of MZ type optical modulators, such as a DQPSK modulator. A control circuit can be provided.

DESCRIPTION OF SYMBOLS 1 Main Mach-Zehnder type | mold optical waveguide 2, 3 MZ type | mold optical modulation part 4, 7 Optical phase adjustment part 5 Modulation signal drive means 6 Light reception detection means (optical / electrical conversion means)
DESCRIPTION OF SYMBOLS 10 Input waveguide 11 Branch part 12, 13 Branch waveguide 14 Combined part 15 Output waveguide 16 Branch means 21, 31, 41, 121, 131, 141, 171 DC bias application means 22, 32, 42, 142 Sine wave low frequency signal generating means 23, 33, 43, 123, 133, 143, 173 Superimposing means 24, 34, 44, 124, 134, 144, 174 DC bias control means 25, 35, 45, 125, 145 Filters 122, 132, 142, 172 for passing electrical signals of frequencies Triangular wave low frequency signal generating means

Claims (5)

In an optical modulator comprising two MZ type optical modulators having Mach-Zehnder (MZ) type optical waveguides, and incorporating each MZ type optical modulator unit in each branch waveguide of the main Mach-Zehnder type optical waveguide,
DC bias applying means for applying a DC bias to each MZ type light modulator;
Low frequency signal superimposing means for superimposing a low frequency signal on each DC bias;
A light receiving detection means for receiving and detecting a part of output light or radiated light from the light modulator;
Bias control means for controlling each of the DC bias application means based on the output of the light receiving detection means,
An optical modulator characterized in that the low frequency signal superimposed on the DC bias applied to each MZ type optical modulator is a triangular wave having the same frequency and a phase difference of 90 degrees.   2. The optical modulator according to claim 1, wherein the bias control unit transmits the electrical signal output from the light receiving detection unit through a filter corresponding to an even order of a specified frequency of the triangular wave, and transmits the transmitted electrical signal. The optical modulator is controlled based on the above.   3. The optical modulator according to claim 1, wherein an optical phase adjustment unit is incorporated in one of the branch waveguides, and a DC bias superimposed with a sine wave is applied to the optical phase adjustment unit. Characteristic light modulator.   3. The optical modulator according to claim 1, wherein an optical phase adjusting unit is incorporated in each of the branching waveguides, and each optical phase adjusting unit has a frequency different from that of the triangular wave related to the MZ type optical modulating unit. An optical modulator, wherein a DC bias on which a triangular wave having a phase difference of 90 degrees is superimposed is applied.   5. The optical modulator according to claim 1, wherein the optical modulator is an optical transmitter that performs DQPSK modulation, and each branch waveguide has a function of each of an I arm and a Q arm. An optical modulator characterized by.

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