JP2011075913A - Method of controlling bias of optical modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To control, with a simple configuration, a bias of an optical modulator having a nested waveguide to be used for DQPSK (Differential Quadrature Phase Shift Keying) modulation. <P>SOLUTION: The optical modulator has a nested waveguide configured by mounting two sub-Mach-Zehnder waveguides SMZ1 and SMZ2 on two branched waveguides, respectively, in one main Mach-Zehnder waveguide MMZ. The optical modulator further includes modulation parts ME1 to ME3. The method includes: while applying low-frequency signals having the same frequency (f) and 90 degree different phase to the modulation part provided on each of the sub-Mach-Zehnder waveguides, detecting a portion of lightwave output from the corresponding sub-Mach-Zehnder waveguide in each of the sub-Mach-Zehnder waveguides, and performing bias control BC1 and BC2 of the modulation parts in the sub-Mach-Zehnder waveguide based on detection values; and detecting a portion of the lightwave output from the main Mach-Zehnder waveguide, and performing bias control BC3 of the modulation parts in the main Mach-Zehnder waveguide based on the detection values. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a bias control method for an optical modulator, and more particularly to a nested waveguide in which two sub Mach-Zehnder waveguides are incorporated in two branch waveguides of one main Mach-Zehnder waveguide. The present invention relates to a bias control method for an optical modulator having a modulation section in all Mach-Zehnder type waveguides.

  DQPSK modulation (Differential Quadrature Phase Shift keying) in the optical communication field, ROF (Radio Over Fiber) that modulates the intensity of an optical signal with a radio signal (modulated carrier wave) and transmits it over an optical fiber An optical modulator including a nested optical waveguide (nested waveguide) is used for an optical SSB modulation device used in the system and an optical frequency shifter used in the optical measurement field.

  The nested waveguide means a nested optical waveguide in which two sub Mach-Zehnder waveguides are incorporated one by one in two branch waveguides of one main Mach-Zehnder waveguide. In an optical modulator having a nested waveguide, a modulation unit having a modulation electrode for modulating a light wave propagating in the optical waveguide is provided in each of the main Mach-Zehnder waveguide and the two sub-Mach-Zehnder waveguides. Provided.

  The optical modulator is formed on a substrate having an electro-optic effect, such as lithium niobate, and the bias point of the modulation signal that drives the optical modulator is affected by the temperature change of the optical modulator and the pyroelectric effect accompanying the driving. A temperature drift phenomenon that shifts due to an influence and a DC drift phenomenon that shifts with time according to the applied voltage occur. In order to compensate for these, it is necessary to perform bias control in each modulation section.

  As a bias control method of an optical modulator having a nested waveguide, as in Patent Documents 1 to 3, a method of performing bias control by inputting a plurality of frequency signals to individual modulation units, or Patent Document 4 In addition, there is a method of performing bias control by detecting a spectrum of an RF (Radio Frequency) component from output light from the main Mach-Zehnder type waveguide.

  However, in the bias control method using a plurality of frequency signals, control signals corresponding to a plurality of frequencies are required for the control itself, and the configuration of the control system is complicated. Moreover, since signals of a plurality of frequencies are mixed in some cases, it is difficult to accurately separate each frequency component from the detection signal for control. Furthermore, when the control is performed with a sufficient interval between the frequencies, the frequency band required for the control signal becomes too wide.

  In addition, in the bias control method performed by detecting the RF component, the components used for detecting the spectrum of the RF component are expensive, and compared with the case of using a general low-frequency signal, printing for the RF signal is performed. This causes problems such as difficulty in board design.

JP 2007-133176 A JP 2007-43638 A Japanese Patent No. 4083657 JP 2007-163941 A

  The problem to be solved by the present invention is to solve the above-mentioned problems and realize bias control of an optical modulator having a nested waveguide, particularly an optical modulator that performs DQPSK modulation, with a simple configuration. It is an object to provide a bias control method for an optical modulator capable of satisfying the requirements.

  In order to solve the above-mentioned problem, the invention according to claim 1 includes a nested waveguide in which two sub Mach-Zehnder waveguides are incorporated in two branch waveguides of one main Mach-Zehnder waveguide. In a bias control method for an optical modulator having a modulation unit in all Mach-Zehnder type waveguides, each of the modulation units of the sub-Mach-Zehnder type waveguide has a low phase difference of 90 degrees at the same frequency f. First, in each sub-Mach-Zehnder type waveguide, a part of the light wave output from the sub-Mach-Zehnder type waveguide is detected and the sub-Mach-Zehnder type waveguide is detected based on the detected value. Bias control of the modulation unit of the waveguide is performed, then a part of the light wave output from the main Mach-Zehnder type waveguide is detected, and the main Mach-Zehnder type waveguide is detected based on the detected value And performing bias control of the modulation unit.

  According to a second aspect of the present invention, in the bias control method for an optical modulator according to the first aspect, the optical modulator is an optical modulator that performs DQPSK modulation, and the modulation unit of the sub-Mach-Zehnder waveguide The bias control performs bias control so that the frequency f component of the detected value is the minimum value or the frequency 2f component is the maximum value, and the bias control of the modulation unit of the main Mach-Zehnder type waveguide is the frequency of the detected value. The bias control is performed so that the 2f component becomes the maximum value.

  According to the first aspect of the present invention, there is provided a nested waveguide in which two sub-Mach-Zehnder waveguides are incorporated one by one in two branching waveguides of one main Mach-Zehnder-type waveguide. In the bias control method for an optical modulator having a modulation unit in a type waveguide, while applying low frequency signals having a phase difference of 90 degrees at the same frequency f to each of the modulation units of the sub Mach-Zehnder type waveguide, First, in each sub-Mach-Zehnder type waveguide, a part of the light wave output from the sub-Mach-Zehnder type waveguide is detected, and bias control of the modulation unit of the sub-Mach-Zehnder type waveguide is performed based on the detected value Next, a part of the light wave output from the main Mach-Zehnder type waveguide is detected, and the bias control of the modulation unit of the main Mach-Zehnder type waveguide is performed based on the detected value. For performing, since it is possible to bias control in the low-frequency signal having a single frequency, it is possible to simplify the configuration required in the generation of low frequency signal, no complicated configuration of the control system. Furthermore, since the low-frequency signal is applied only to the modulation unit of the sub-Mach-Zehnder type waveguide, it is possible to omit applying the low-frequency signal to the modulation unit of the main Mach-Zehnder type waveguide, and further simplify the configuration. It becomes possible.

  According to the second aspect of the present invention, the optical modulator is an optical modulator that performs DQPSK modulation, and the bias control of the modulation unit of the sub Mach-Zehnder waveguide has a minimum frequency or a frequency of 2f. The bias control is performed so that the component becomes the maximum value, and the bias control of the modulation unit of the main Mach-Zehnder type waveguide performs the bias control so that the frequency 2f component of the detected value becomes the maximum value. However, the bias of each modulation section can be optimally controlled with a simple configuration.

It is the schematic explaining the bias control method of the optical modulator of this invention. It is a figure which shows the optical spectrum distribution by which the low frequency signal modulation in code | symbol AC of FIG. It is a figure explaining the synthetic | combination state of an optical spectrum component. It is a graph which shows the relationship between the bias point and the monitor level of the frequency 2f component regarding the output light of the main Mach-Zehnder type waveguide.

The optical modulator bias control method of the present invention will be described in detail below.
As shown in FIG. 1, the present invention is a nest in which two sub Mach-Zehnder waveguides (SMZ1, SMZ2) are incorporated one by one in two branch waveguides of one main Mach-Zehnder waveguide (MMZ). In the bias control method of an optical modulator having a type waveguide, and including all the Mach-Zehnder type waveguides with modulation units (ME1 to ME3), each modulation unit of the sub-Mach-Zehnder type waveguide has the same frequency. First, in each sub Mach-Zehnder type waveguide, light waves (A, B) output from the sub-Mach-Zehnder type waveguide while applying low-frequency signals (AC1, AC2) having a phase difference of 90 degrees at f. And a bias control (BC1, BC2) of the modulation section of the sub-Mach-Zehnder type waveguide is performed based on the detected value, and then the main Mach-Zehnder-type waveguide is controlled. Detecting a portion of the outputted optical wave (C), and performs bias control of the modulation of the main Mach-Zehnder type waveguide on the basis of the detection value (BC3).

  In the optical modulator bias control method of the present invention, since a single frequency is used for the low frequency signal applied to the modulation section, the configuration necessary for generating the low frequency signal can be simplified and controlled. The system configuration is not complicated. In addition, since the low-frequency signal is applied only to the modulation unit of the sub-Mach-Zehnder type waveguide, it is possible to omit application of the low-frequency signal to the modulation unit of the main Mach-Zehnder type waveguide, and further simplify the configuration. It becomes possible.

Next, the bias control method of the present invention will be described by taking an optical modulator that performs DQPSK modulation as an optical modulator.
As shown in FIG. 1, the modulation unit (ME1) of the sub Mach-Zehnder type waveguide (SMZ1) has a low frequency signal (frequency f, phase 0 °) (AC1) to the DC bias output from the bias control circuit BC1. ) Is superimposed and applied to the modulation electrode constituting the modulation section. When superimposing, a bias T circuit (BT1) or the like can be used.

  The modulation unit is configured by a modulation electrode arranged along the branch waveguide of the sub Mach-Zehnder type waveguide. The modulation electrode includes a signal electrode and a ground electrode, and may use a single signal electrode or two signal electrodes corresponding to two branch waveguides.

  In addition, in the modulation unit, it is possible to separately provide a modulation electrode for inputting a modulation signal corresponding to an optical signal to be transmitted and a bias electrode for applying a DC bias. It is not necessary to apply a DC bias to the bias electrode and superimpose the low frequency signal and the DC bias as shown in FIG.

  A part of the light wave output from the sub Mach-Zehnder type waveguide (SMZ1) is detected, and a signal related to the detected value is input to the bias control circuit (BC1). As a light wave to be detected, a light wave having an in-phase component with the output light can be obtained by a method of directly detecting from the output waveguide of the sub-Mach-Zehnder type waveguide (SMZ1) as shown in FIG. Can be detected. Further, it is possible to detect a light wave having a component opposite in phase to the output light by a method of detecting the radiated light of the multiplexing unit or a detection method of configuring the multiplexing unit with a coupler.

  The bias control circuit (BC1) controls the DC bias applied to the modulation unit (ME1) based on the detected value detected. Specifically, only the frequency f component is extracted from a detection signal output by a detector such as a light receiving element using a bandpass filter or the like, and the DC bias is controlled so that the extracted detection value is minimized.

  Alternatively, only the frequency 2f component can be extracted from the detection signal, and the DC bias can be controlled so that the extracted detection value becomes maximum. Since the bias control method in the modulation section of the sub Mach-Zehnder type waveguide is the same as the bias control method in the optical modulator having one Mach-Zehnder type waveguide, various conventional bias control methods can be applied.

  On the other hand, the bias control method of the modulation unit (ME2) of the other sub Mach-Zehnder type waveguide (SMZ2) is such that the low frequency signal (frequency f, phase 90 °) applied to the modulation unit (ME2) is Exactly the same bias control method can be used except that the phase differs from the Mach-Zehnder type waveguide (SMZ1) by 90 ° (in FIG. 1, it is + 90 ° but may be −90 °).

  Next, a bias control method for the modulation unit (ME3) of the main Mach-Zehnder waveguide (MMZ) will be described. Until the bias control of the modulation unit of the main Mach-Zehnder type waveguide is performed, the DC bias of the modulation unit (ME1, ME2) of the two sub Mach-Zehnder type waveguides (SMZ1, SMZ2) is determined by the bias control method described above. Optimized.

  In bias control of the modulation unit (ME3) of the main Mach-Zehnder type waveguide (MMZ), an output obtained by combining the light waves output from the two sub-Mach-Zehnder type waveguides without applying a low frequency signal to the modulation unit Based on the light, the DC bias applied to the modulation unit (ME3) is controlled.

  As a method for detecting a part of the light wave output from the main Mach-Zehnder type waveguide (MMZ), it is possible to detect either the in-phase component or the anti-phase component of the output light as in the case of the sub Mach-Zehnder type waveguide. .

  The bias control circuit (BC3) controls the DC bias applied to the modulation unit (ME3) based on the detected value detected. Specifically, only the frequency 2f component is extracted from the detection signal output from a detector such as a light receiving element using a band-pass filter or the like, and the DC is adjusted so that the extracted detection value is maximized. Control the bias.

  A bias control method related to the modulation unit (ME3) of the main Mach-Zehnder type waveguide (MMZ) will be described in more detail.

  The light waves output from the two sub-Mach-Zehnder type waveguides are modulated by a low-frequency signal f (f is also referred to as a dither frequency). The output light (modulation component) of the waveguide is expressed by Equation (1) for SMZ1 and Equation (2) for SMZ2. Here, m represents the degree of modulation by dither, and w represents the dither modulation angular frequency. Further, it is assumed that the bias of the sub-Mach-Zehnder type waveguide is appropriately set to the phase difference π and there is no even-order modulation component.

  Expressions (1) and (2) mean that the light wave modulated by the modulation unit of the sub-Mach-Zehnder type waveguide has an optical spectrum of frequency ± f component and ± 3f component. Although higher-order components exist than the components, the dither modulation is shallow (the intensity of the low-frequency signal to be applied is weak), so the intensity of the higher-order components becomes small enough to be ignored.

  The optical spectrum of the light wave output from each sub-Mach-Zehnder type waveguide is as shown in FIG. 2 (a) (or FIG. 2 (b)). Become. This is to control the DC bias applied to the modulation section of the sub-Mach-Zehnder type waveguide so that the detection value becomes the minimum value in the above-described frequency f component (or extract only the frequency 2f component and extract the detected value Is controlled to be the maximum value).

  Next, the two light waves emitted from the sub Mach-Zehnder type waveguide are combined in a relationship of angle θ by a bias voltage applied to the modulation unit ME3.

The state of the combined light wave is shown in FIG. FIG. 3A shows a state of multiplexing in the frequency −3f component or + f component, where the vector of the output light from the sub-Mach-Zehnder waveguide (SMZ1) is s ₁ and the other sub-Mach-Zehnder type is shown. the vector of the output light from the waveguide (SMZ2) and s _2, and the combined resultant vector displayed in s _3.

  The amplitude value and phase of the combined vector in FIG. 3A are expressed by Expression (3) and Expression (4).

FIG. 3B shows a state of multiplexing in the frequency −f component or +3 component, where t ₁ is the vector of the output light from the sub Mach-Zehnder type waveguide (SMZ1), and the other sub Mach. the vector of the output light from Zehnder type waveguide (SMZ2) and t _2, and the combined resultant vector displayed in t _3.

  The amplitude value and phase of the combined vector in FIG. 3B are expressed by Expression (5) and Expression (6).

  A state in which light waves output from the sub-Mach-Zehnder waveguide are synthesized is expressed by Expression (7) using Expressions (1) to (6) above.

The state of the optical spectrum of the combined light wave indicated by symbol C in FIG. 1 is displayed as shown in FIG. The output (detection value) I of the combined light from the light receiving element is I = E · E ^* . When this is calculated and arranged, the coefficient of the double frequency term (2f component) of the dither modulation frequency is expressed by the equation ( It is proportional to the expression expressed in 8).

  In Expression (8), the detection output of the optical spectrum in FIG. 2C is a 2f component generated when square detection is performed, which is the product of the −3f component and the −f component, the −f component and the f component. And a composite value of the product of the f component and the 3f component.

  From the result of the equation (8), the phase depends on the DC bias applied to the modulation unit (ME3) of the main Mach-Zehnder type waveguide (MMZ) regardless of the depth of modulation of the dither (low frequency signal intensity). As θ changes, the light intensity of the double frequency component changes in proportion to the absolute value of sin θ. For this reason, it is understood that the DC bias of the modulation unit (ME3) can be controlled to an optimum value by detecting a part of the output light of the main Mach-Zehnder type waveguide.

  The change of the monitor level (detection value) of the 2f component expressed by the equation (8) with respect to the bias point (phase θ) is shown in FIG. In DQPSK modulation, it is only necessary to set θ = π / 2. Therefore, as shown by the set bias point in FIG. 4, it is necessary to perform bias control so that the double frequency monitor signal is maximized. .

  As described above, according to the present invention, an optical modulator that can realize bias control of an optical modulator having a nested waveguide, particularly an optical modulator that performs DQPSK modulation, with a simple configuration. A bias control method can be provided.

AC1, AC2 Low frequency signal BC1 to BC3 Bias control circuit BT1, BT2 Bias T circuit ME1 to ME3 Modulator MMZ Main Mach-Zehnder type waveguide SMZ1, SMZ2 Sub-Mach-Zehnder type waveguide

Claims (2)

The two branch waveguides of one main Mach-Zehnder type waveguide have a nested waveguide in which two sub-Mach-Zehnder type waveguides are incorporated one by one, and all the Mach-Zehnder type waveguides have a modulation section. In an optical modulator bias control method,
While applying low frequency signals having the same frequency f and different phases by 90 degrees to each of the modulation sections of the sub-Mach-Zehnder type waveguide,
First, in each sub-Mach-Zehnder type waveguide, a part of the light wave output from the sub-Mach-Zehnder type waveguide is detected, and bias control of the modulation unit of the sub-Mach-Zehnder type waveguide is performed based on the detected value And
Next, a part of the light wave output from the main Mach-Zehnder type waveguide is detected, and bias control of the modulation unit of the main Mach-Zehnder type waveguide is performed based on the detected value. Bias control method. The bias control method for an optical modulator according to claim 1,
The optical modulator is an optical modulator that performs DQPSK modulation,
The bias control of the modulation unit of the sub-Mach-Zehnder waveguide performs bias control so that the frequency f component of the detected value is the minimum value or the frequency 2f component is the maximum value,
A bias control method for an optical modulator, wherein the bias control of the modulation section of the main Mach-Zehnder type waveguide is performed so that the frequency 2f component of the detected value becomes a maximum value.

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