JP2010002850A - Bias control method of optical modulator, and optical modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To ensure accuracy in control of each DC bias in at least a pair of optical modulation parts by preventing an adverse effect of the DC bias applied to one modulation part while controlling the bias point of the other optical modulator. <P>SOLUTION: In an optical modulator comprising a substrate having an electrooptical effect, an optical waveguide 4 formed on the substrate, at least a pair of optical modulation parts 2A and 1B disposed in parallel to modulate optical waves by applying a modulation signal to the optical waveguide, and a multiplexing part multiplexing the optical waves modulated by the pair of optical modulation parts 2A and 2B, each DC bias in the pair of optical modulation parts 2A and 2B is controlled. The DC bias applied to one optical modulation part 2A is changed, and a low frequency signal is superimposed to the other optical modulation part 2B, thereby detecting a change in light quantity of the multiplexed optical wave. Based on the detected change, the DC bias of the one optical modulation part 2A is controlled. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an optical modulator bias control method and an optical modulator.

  In the optical communication field, an optical modulator is used as an electrical / optical conversion element. There are many types of optical modulators, including those utilizing the electro-optic effect. In an optical modulator using a substrate having an electro-optic effect such as lithium niobate, the output optical characteristics change depending on the elapsed time due to changes in the amount of direct current applied for driving control and the operating temperature. This is called drift.

As a method for controlling the drift phenomenon, Patent Documents 1 and 2 superimpose a low-frequency signal on the drive signal of the optical modulator, and monitor a change in the amount of light related to the low-frequency signal included in the output light from the optical modulator. Thus, the bias point with respect to the actual applied voltage is detected. The bias point is automatically corrected so as to optimize the optical response characteristics by feeding back to the bias compensation circuit that controls the DC bias applied to the optical modulator.
JP 49-42365 A JP 3-251815 A

In Patent Document 3, a plurality of light modulation units are provided in an optical modulator, a low frequency signal is superimposed for each light modulation unit, and a light amount change related to the low frequency signal is monitored for each light modulation unit. Each bias point is feedback controlled.
Japanese Patent Laid-Open No. 2004-318052

  For an optical modulator having a plurality of optical modulators, there are the following problems in controlling the bias point as described in Patent Document 3. First, a point at which the light amount of the low frequency signal input to the light modulation unit is minimized is set as a bias point (FIG. 1). A bias point control method will be described with reference to FIG. An optical waveguide 4 is formed on the optical modulator 1, Mach-Zehnder type optical modulation units 2A and 2B are arranged in parallel, the optical modulation units 2A and 2B are further combined, phase-adjusted and output To do. Light from the light source 6 is made incident on the optical waveguide 4 through the polarization controller 7, intensity-modulated by the light modulators 2 </ b> A and 2 </ b> B, combined and output, and the light quantity is measured by the power meter 8.

  Here, when the drift phenomenon control method described in Patent Document 3 is used, the three DC biases 5A (DC-I), 5B (DC-Q), and 5C (DC-P) are controlled separately. It will be necessary. The following two methods are conceivable as methods for such compensation control.

(1) As shown in FIG. 3, the light receivers 11A, 11B, and 11C are installed corresponding to the three light modulators 2A, 2B, and 2C, respectively. And the output light quantity which concerns on a low frequency signal is measured, respectively, and the feedback control of the direct current bias input into each optical modulation part is performed via control circuit 10A, 10B, 10C, respectively. However, in this control method, it is necessary to provide a light receiver for each light modulation unit, and the apparatus becomes complicated. If the number of light modulators increases, it is not realistic.

(2) When a light receiver is not provided for each light modulation unit, for example, as shown in FIG. 4, a light receiver 11 is installed downstream of the combined light light modulation unit 2C. However, since the outputs of the optical modulators 2A and 2B are combined, if the applied low frequency signal is the same signal, it cannot be determined from which optical modulation unit the output signal is.

  Therefore, as shown in FIG. 4, the frequencies of the low-frequency signals 12A, 12B, and 12C oscillated from the low-frequency signal generation sources 9A, 9B, and 9C are made different from each other. As a result, signals related to low frequency signals having different frequencies can be separately analyzed and fed back from the output light received by the same light receiving element 11. However, this method requires different signal sources 9A, 9B, 9C. As another method, the frequency of the low-frequency signals 12A, 12B, and 12C may be the same, but the oscillation timing may be shifted.

  However, in these methods, when the bias point of one optical modulation unit 2A is controlled, a DC drift is generated by a DC bias applied to another optical modulation unit 2B, and the waveform of the output light wave is May change. For this reason, accurate control is very difficult.

  An object of the present invention is to control the DC bias applied to the other optical modulation unit while controlling the bias point of one optical modulation unit when controlling each DC bias in at least one pair of optical modulation units. It is to enable accurate control by preventing interference.

The present invention provides a substrate having an electro-optic effect, an optical waveguide formed on the substrate, and at least a pair of parallel arrangements for modulating a light wave by applying a modulation signal to the optical waveguide. This is a bias control method for controlling each DC bias voltage in a pair of optical modulators with respect to an optical modulator provided with an optical modulator and a multiplexer that multiplexes the light waves modulated by the pair of optical modulators. And
By changing the DC bias voltage applied to one light modulation unit and superimposing a low-frequency signal on the other light modulation unit, the light amount change of the combined light wave is detected, and this detected light amount change Based on this, the DC bias voltage of one of the light modulation units is controlled.

The operation principle according to the present invention will be described with reference to FIGS.
As shown in FIG. 9, the outputs of the pair of light modulation units 2 </ b> A and 2 </ b> B are combined by a multiplexing unit 20 and output. One light modulation unit is, for example, I-arm, and the other light modulation unit is, for example, Q-arm. It is assumed that each of the optical modulation units 2A and 2B is provided with predetermined control ports (electrodes) 3A and 3B, respectively.

  Here, one optical modulator 2A is set as a control target. In this case, the DC bias 5A applied to one of the light modulation units 2A is changed, and the low frequency signal 12B is superimposed on the other light modulation unit 2B. Then, the light amount change of the light wave after the combination is detected, and the direct current bias of one of the light modulation units 2A is controlled based on the detected light amount change. In this method, while controlling the bias point of one light modulation unit 2A, the other light modulation unit 2B can be controlled without applying a DC bias. Therefore, it is possible to prevent interference of the DC bias applied to the other optical modulation unit and perform accurate control.

  Furthermore, since the same signal frequency can be used for controlling the bias point, the control circuit can be simplified in that case.

  Furthermore, since the light can be turned off, the drive voltage Vπ of the light modulation unit to be controlled can be measured.

The operation principle of the present invention will be described below.
When high-frequency modulation is applied to the light wave by the light modulation unit, the output light Eout from the modulation unit is expressed by Equation (1) with respect to the input light E0.
Eout = E0 (exp (jφ1) + exp (jφ2)) (1)
(φ1−φ2 = phase difference)

It is assumed that the low frequency signal is continuously applied to the modulation unit 2B. In this state, when the output light from the modulation unit 2A becomes the maximum, the output light intensity E1 can be expressed as in Expression (2).
E1 = E0 (exp (jφ1) + exp (jφ2)) / 4 (2)

Therefore, the combined output light Eon can be expressed by Equation (3).
Eon = E1 + E0 / 2
Eon = E0 (exp (jφ1) + exp (jφ2)) / 4 + E0 / 2 (3)
The waveform of the combined output light Eon is, for example, as shown in FIG.

Further, when the output light intensity of the light modulation unit 2A is minimum, that is, in the off state, the output light E2 can be expressed as shown in Expression (4).
E2 = E0 (exp (jφ1) + exp (jφ2)) / 4 (4)
Therefore, the combined output light wave Eoff can be expressed as shown in Equation (5).
Eoff = E2 + 0
Eoff = E0 (exp (jφ1) + exp (jφ2)) / 4 (5)

Therefore, the waveform of the combined output light wave Eoff is as shown in FIG. That is, in the off state, the intensity waveform of the output light wave is a cosine square curve (cos θ) ² , and the intensities of adjacent peak tops are substantially equal.

Further, the state shown in FIG. 6 is obtained between the ON state (FIG. 5) and the OFF state (FIG. 7) (while changing). In this case, the waveform of the output light wave is distorted from (cos θ) ^2, and the intensities of adjacent peak tops greatly differ.

A specific control example will be described. As shown in FIG. 9, the bias point of the light modulation unit 2A is controlled.
(1) First, a DC bias voltage is applied to the three modulation units 2A, 2B, and 2C so that the amount of light output from the optical modulator is maximized.
(2) As shown in FIG. 9, a low frequency voltage (with a frequency of, for example, 1 kHz) is applied to the port 3B that has been applied to the light modulation unit 2B that is not a control target, at a value equal to or higher than the drive voltage Vπ. At this time, the output signal has a waveform as shown in FIG. 12, for example.

(3) The DC bias voltage applied to the optical modulation unit 2A to be controlled is gradually increased from 0 V, and the change in the waveform output from the modulator is observed with an oscilloscope.
(4) When a certain DC bias voltage is reached, the waveform of the output light appearing on the oscilloscope changes to a cosine square ((cos θ) ² ) curve as shown in FIG. When the waveform is adjusted, the output from the modulation unit 2A side is determined to be in the off-light state by the applied DC bias voltage. This is the point at which adjacent peak top values are equal.

  For example, in the control method described in Patent Document 3, for example, as shown in FIG. 10, a DC bias voltage and a low-frequency signal voltage are simultaneously applied to the modulation unit 2A to be controlled. Moreover, the low frequency signal needs to have a very small amplitude. This method is fundamentally different from the method of the present invention as shown in FIG.

  On the other hand, the same method as above may be used to control the bias point of the light modulator 2B. This example will be described with reference to FIG. In this case, the light modulator 2B is considered as one light modulator, and the light modulator 2A is considered as the other light modulator.

(1) First, a DC bias voltage is applied to the three modulation units 2A, 2B, and 2C so that the amount of light output from the optical modulator is maximized.
(2) As shown in FIG. 11, a low frequency voltage is applied to the port 3A that has been applied to the light modulation unit 2A that is not a control target, at a value equal to or higher than the drive voltage Vπ. At this time, the output signal has a waveform as shown in FIG. 12, for example.

(3) The direct current bias voltage applied to the optical modulation unit 2B to be controlled is gradually increased from 0 V, and the change in the waveform output from the modulator is observed with an oscilloscope.
(4) When a certain DC bias voltage is reached, the waveform of the output light appearing on the oscilloscope changes to a cosine square ((cos θ) ² ) curve as shown in FIG. When the waveform is adjusted, the output from the modulation unit 2B side is determined to be in the off-light state by the applied DC bias voltage. This is the point at which adjacent peak top values are equal.

  The optical modulator of the present invention is not limited as long as it modulates light characteristics, and may be a light intensity modulator or an optical phase modulator. The light intensity modulator may be an optical amplitude modulator using a Mach-Zehnder type optical waveguide. The optical phase modulator means one that applies phase modulation to incident light and extracts a phase modulation signal from the emitted light. The type is not particularly limited, and various phase modulation schemes such as DQPSK and SSB can be used.

The phase modulation method using a plurality of phase modulation units is not particularly limited, and DQPSK (Differential Quadrature Phase Shift
Keying), SSB (Single Side Band amplitude modulation), DPSK `` Differential
Various phase modulation methods such as “Phase Shift Keying: Differential Phase Shift Keying” can be adopted.

  In a preferred embodiment, the amplitude of the low frequency signal applied to the other optical modulation unit is set to be equal to or higher than the drive voltage Vπ of the optical modulation unit.

  According to the present invention, it is possible to determine the off-light state even when the amplitude of the low-frequency signal applied to the other light modulation unit is the drive voltage Vπ of the light modulation unit. That is, FIG. 8 shows an extinction curve obtained by applying an amplitude of 5 V when Vpi = 5 [V].

  Here, when the modulation unit is in the ON state, the slope is not 0 at the amplitude voltage of 5 V, and the absolute value is considerably large. When the modulation unit is in the ON state, the slope is 0 when the amplitude voltage is 0 V, and the slope is 0 when the amplitude voltage is 10 V.

  On the other hand, when the modulation unit is in the off state, the slope is 0 when the amplitude voltage is 0V and when the amplitude voltage is 5V. Therefore, when the amplitude voltage is 5 V, it can be determined that the light is extinguished, that is, it is possible to determine the off state.

  In a preferred embodiment, the amplitude of the low-frequency signal applied to the other optical modulation unit is set to be not less than twice the drive voltage Vπ (2Vπ) of the optical modulation unit.

  In the method of the present invention, in the extinction curve of the combined light wave, control is performed so that the light amounts of adjacent peak tops of the extinction curve approach each other. In a preferred embodiment, the ratio of the light amounts of adjacent peak tops in the extinction curve is 1: 0.98 to 1.02, particularly preferably both are equal.

  The waveform of the low frequency signal is not particularly limited and includes a sine wave, a rectangular wave, and a triangular wave, and a sawtooth wave is particularly preferable.

  The frequency of the low frequency signal is preferably 100 Hz to 1 GHz, and particularly preferably 100 Hz to 300 MHz.

  In the present invention, it is also possible to apply low frequency signals having different frequencies corresponding to the respective modulation units. For example, in the example of FIG. 4, low-frequency signal sources 9A, 9B, and 9C are provided corresponding to the respective modulation units.

  However, in a preferred embodiment, a low frequency signal having the same frequency is applied to each modulator. In this case, it is not necessary to provide a different low-frequency signal source corresponding to each modulation section, so that the apparatus becomes very simple and advantageous. For example, in the examples of FIGS. 14 and 15, the low frequency signal 12 having the same frequency is applied from the common low frequency signal source 9 to each of the modulation units 2A, 2B, and 2C.

  In the example of FIG. 14, a part of the output from the modulation unit 3 </ b> C is received by the light receiver 11 through the branch optical waveguide. In the example of FIG. 15, a part of the output from the modulation unit 3 </ b> C is received by the light receiver 11 via the directional coupler 13, for example.

  In the present invention, it is necessary to apply a high frequency modulation signal (RF), a direct current bias voltage (DC), and a low frequency signal (AC) to each optical modulation unit. This application method is not particularly limited.

  For example, as schematically shown in FIG. 16A, a high frequency modulation signal (RF), a DC bias voltage (DC), and a low frequency signal (AC) can be applied to the same port 30. .

  Alternatively, as schematically shown in FIG. 16B, a high frequency modulation signal (RF) is applied to the port 31, and a direct current bias voltage (DC) and a low frequency signal ( AC) can be applied.

  Alternatively, as schematically shown in FIG. 16 (c), a high frequency modulation signal (RF) and a low frequency signal (AC) are applied to the port 33, and a DC bias voltage ( DC) can be applied.

It is a graph which shows the relationship between an extinction curve and a bias point. It is a figure which shows typically the whole structure of an optical modulator. It is a block diagram which shows typically the control apparatus of a comparative example. It is a block diagram which shows typically the control apparatus which can be used by this invention. It is a graph which shows an extinction curve when the light modulation part to be controlled is in an on state. It is a graph which shows an extinction curve when the light modulation part to be controlled is in a state between ON and OFF. It is a graph which shows an extinction curve when the light modulation part to be controlled is in an off state. It is a graph which shows the relationship between an amplitude voltage and a light quantity when the amplitude of a low frequency voltage is equal to the drive voltage of a modulation | alteration part. It is a schematic diagram for demonstrating the control method by this invention. It is a schematic diagram for demonstrating the control method of a comparative example. It is a schematic diagram for demonstrating the control method by this invention. It is an extinction curve when the light modulator to be controlled is in an on state (Example). FIG. 10 is a graph showing an extinction curve when the light modulation unit to be controlled is in an off state [Example]. FIG. It is a block diagram which shows typically the control apparatus which can be used by this invention. It is a block diagram which shows typically the control apparatus which can be used by this invention. (A), (b), (c) is a schematic diagram which shows the application port of a high frequency modulation signal (RF), a direct-current bias voltage (DC), and a low frequency signal (AC), respectively.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Optical modulator 2A, 2B, 2C Optical modulation part 3A, 3B, 3C Application port 4 Channel type optical waveguide 5A, 5B, 5C DC bias source 9A, 9B, 9C Low frequency signal source 10A, 10B, 10C Control circuit 11, 11A, 11B, 11C

Claims (6)

A substrate having an electro-optic effect, an optical waveguide formed on the substrate, and at least a pair of optical modulation units arranged in parallel to modulate a light wave by applying a modulation signal to the optical waveguide; A bias control method for controlling each DC bias voltage in the pair of optical modulators with respect to an optical modulator comprising a multiplexer that multiplexes the light waves modulated by the pair of optical modulators,
By changing a DC bias voltage applied to one of the light modulation units and superimposing a low frequency signal on the other light modulation unit, a change in the amount of light wave of the combined light wave is detected, and this detected A bias control method for an optical modulator, comprising: controlling a DC bias voltage of the one optical modulator based on a change in light amount.   The method according to claim 1, wherein an amplitude of the low frequency signal applied to the other light modulation unit is equal to or higher than a drive voltage Vπ of the other light modulation unit.   2. The method according to claim 1, wherein an amplitude of the low frequency signal applied to the other light modulation unit is set to be not less than twice a drive voltage Vπ of the other light modulation unit.   The method according to any one of claims 1 to 3, wherein, in the extinction curve of the combined light wave, control is performed so that the light amounts of adjacent peak tops of the extinction curve are equal. .   The method according to claim 1, wherein the waveform of the low-frequency signal is a sawtooth wave. A substrate having an electro-optic effect, an optical waveguide formed on the substrate, and at least a pair of optical modulation units arranged in parallel to modulate a light wave by applying a modulation signal to the optical waveguide; An optical modulator comprising a multiplexing unit that multiplexes the light waves modulated by the pair of optical modulation units,
By changing a DC bias voltage applied to one of the light modulation units and superimposing a low frequency signal on the other light modulation unit, a change in the amount of light wave of the combined light wave is detected, and this detected An optical modulator that controls a DC bias voltage of the one optical modulator based on a change in the amount of light.

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