JP2010078845A - Bias controller of light intensity modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a bias controller of a light intensity modulator, which causes little deterioration in signal of modulated light, accurately controls a bias point and has the suppressed cost as the whole of the controller. <P>SOLUTION: The bias controller of the light intensity modulator includes: the light intensity modulator 4 having a Mach-Zehnder type optical waveguide 5; and a direct current bias control means 28 which controls direct current bias voltage applied to the light intensity modulator, the bias controller further includes: a re-incidence means which makes a portion of a light wave radiated or emitted from the exit side of the Mach-Zehnder type optical waveguide re-incident on the exit waveguide of the Mach-Zehnder type optical waveguide; a detection means 24 which detects the light wave radiated or emitted from the entrance side of the Mach-Zehnder type optical waveguide; a clock voltage application means which applies a clock voltage together with the direct current bias voltage to the light intensity modulator; and a bias control means which controls the direct current bias voltage on the basis of the detection result of the detection means. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a bias control apparatus for an optical intensity modulator, and in particular, includes an optical intensity modulator having a Mach-Zehnder type optical waveguide and DC bias control means for controlling a DC bias voltage applied to the optical intensity modulator. The present invention relates to a bias control apparatus for a light intensity modulator.

  In the current optical transmission system, a transmission rate of 10 Gbps is mainstream. However, with the recent increase in Internet users, there is a demand for an increase in transmission capacity and an increase in transmission distance. Under such circumstances, a shift to a transmission system with a transmission rate of 40 Gbps is expected.

In the case of high-speed transmission such as a 40 Gbps transmission system, wavelength dispersion is the main factor that hinders long-distance transmission. In order to solve this problem, a carrier-suppressed RZ (Carrier-Suppressed Return-to-Zero, hereinafter referred to as CS-RZ) modulation method proposed in Patent Document 1 and the like has been proposed.
JP 2003-279912 A

  CS-RZ is superior in resistance to chromatic dispersion compared to a normal RZ modulation method, but it is essential to always control the bias point of the light intensity modulator to the bottom of the modulation curve. If such bias control is not possible, there is a problem that the signal quality is remarkably deteriorated, for example, noise is generated in the modulated output light.

As a method for controlling the bias point of the light intensity modulator, a low frequency signal is superimposed on the modulation signal which is the main signal, as shown in Patent Document 2, and the bias point is determined from the spectrum corresponding to the modulated low frequency signal. There are ways to determine and control.
JP 49-42365 A

  However, in the above method, a filter for extracting a signal corresponding to a low frequency signal is expensive, and an expensive device such as a spectrum analyzer for measuring a spectrum corresponding to the low frequency signal is required. In addition, there is a problem that the signal quality itself is deteriorated by superimposing the low frequency signal on the main signal.

On the other hand, as in Patent Documents 3 and 4, a light wave traveling in the opposite direction to the traveling direction of the modulated light propagating in the light intensity modulator is incident on the modulator and travels in the reverse direction that has passed through the modulator. It is disclosed to detect the state of the light wave and perform bias control.
Japanese Patent Laid-Open No. 4-116618 JP-A-4-263525

  However, the method disclosed in Patent Document 3 or 4 requires a detection circuit that detects a mark rate from a modulation signal that generates modulated light, and a circuit that smoothes a detection signal that detects a light wave, and the circuit is complicated. In addition, the entire apparatus becomes expensive.

  The problem to be solved by the present invention is to solve the above-described problems, reduce the signal degradation of the modulated light, accurately control the bias point, and reduce the cost of the entire apparatus. A bias control device for a light intensity modulator is provided.

  The invention according to claim 1 comprises a light intensity modulator having a Mach-Zehnder type optical waveguide and a direct current bias control means for controlling a direct current bias voltage applied to the light intensity modulator. A re-incident means for re-injecting part of the light wave emitted or emitted from the exit side of the Mach-Zehnder type optical waveguide into the exit waveguide of the Mach-Zehnder type optical waveguide, and the entrance of the Mach-Zehnder type optical waveguide Detection means for detecting a light wave emitted or emitted from the side, and a frequency is approximately half of a code speed of a transmission signal that drives the light intensity modulator, and an amplitude is approximately the half-wave voltage of the light intensity modulator. A clock voltage applying means for applying a doubled clock voltage to the light intensity modulator in combination with the DC bias voltage, and the DC bias voltage based on the detection result of the detecting means. And having Gosuru a bias adjustment means.

  The meaning of “substantially ½” or “substantially double” in the present invention is not limited to a value that is multiplied by an exact ½ or 2, but is within a range where the object of the present invention can be achieved. This means that a certain amount of deviation can be allowed from the numerical values of 2 and 2.

  According to a second aspect of the present invention, in the bias control apparatus for a light intensity modulator according to the first aspect, the re-incident means is provided on the optical path of the outgoing light emitted from the exit waveguide of the Mach-Zehnder optical waveguide. It is characterized by the said reflection means.

  According to a third aspect of the present invention, in the bias control apparatus for a light intensity modulator according to the first aspect, the re-injection means branches a part of the outgoing light emitted from the exit waveguide of the Mach-Zehnder type optical waveguide. And branching light introducing means for introducing the branched branched light in the direction opposite to the propagation direction of the outgoing light on the optical path of the outgoing light.

  According to a fourth aspect of the present invention, in the bias control apparatus for a light intensity modulator according to the first aspect, the re-incident means converts the emitted light emitted from the multiplexing point of the Mach-Zehnder type optical waveguide into the Mach-Zehnder optical waveguide. It is characterized by having a radiated light introducing means for introducing light in the direction opposite to the propagation direction of the emitted light on the optical path of the emitted light emitted from the exit waveguide of the Zehnder type optical waveguide.

  According to a fifth aspect of the present invention, there is provided the bias control apparatus for an optical intensity modulator according to any one of the first to fourth aspects, wherein the light wave emitted in the reverse direction from the entrance waveguide of the Mach-Zehnder optical waveguide is Separating means for separating the light wave incident from the entrance waveguide into the zender-type optical waveguide from the optical path, and the separated separated light are detected by the detecting means.

  According to a sixth aspect of the present invention, in the bias control apparatus for an optical intensity modulator according to any one of the first to fourth aspects, the detection means is emitted from a Y branch point on the entrance side of the Mach-Zehnder type optical waveguide. It is characterized by detecting radiated light.

  According to a seventh aspect of the present invention, in the bias control apparatus for an optical intensity modulator according to any one of the first to sixth aspects, the optical intensity modulator is an optical intensity modulator used for a carrier suppression RZ modulation method. It is characterized by that.

  According to the first aspect of the present invention, there is provided a light intensity modulator bias control apparatus comprising: a light intensity modulator having a Mach-Zehnder type optical waveguide; and a DC bias control means for controlling a DC bias voltage applied to the light intensity modulator. A re-incident means for re-injecting part of the light wave emitted or emitted from the exit side of the Mach-Zehnder type optical waveguide into the exit waveguide of the Mach-Zehnder type optical waveguide, and the entrance of the Mach-Zehnder type optical waveguide Detection means for detecting a light wave emitted or emitted from the side, and a frequency is approximately half of a code speed of a transmission signal that drives the light intensity modulator, and an amplitude is approximately the half-wave voltage of the light intensity modulator. Clock voltage application means for applying a doubled clock voltage to the light intensity modulator in combination with the DC bias voltage, and the DC bias voltage based on the detection result of the detection means. Because having a bias adjustment means for controlling the, without using expensive parts such as a spectrum analyzer, and the state of the bias point of the optical intensity modulator can be easily determined.

  According to the second aspect of the present invention, the re-incident means is a reflecting means provided on the optical path of the outgoing light emitted from the exit waveguide of the Mach-Zehnder type optical waveguide. Is possible. Further, by adjusting the reflectance and transmittance of the reflecting means, it is possible to make adjustment so as to ensure the amount of detection light necessary for bias control while suppressing a decrease in the amount of signal light.

  According to the invention of claim 3, the re-incident means includes a branching means for branching a part of the outgoing light emitted from the exit waveguide of the Mach-Zehnder optical waveguide, and the branched branched light on the optical path of the outgoing light. Since it has branching light introduction means that introduces light in the direction opposite to the direction of propagation of the emitted light, it is possible to effectively extract only a part of the emitted light that is signal light, thereby suppressing degradation of the signal light. It becomes possible to do.

  According to the invention of claim 4, the re-incident means is configured to cause the light emitted from the combining point of the Mach-Zehnder type optical waveguide to be emitted on the optical path of the outgoing light emitted from the exit waveguide of the Mach-Zehnder type optical waveguide. Since there is a radiated light introducing means that introduces the emitted light in a direction opposite to the propagation direction of the emitted light, the emitted light from the light intensity modulator that is the signal light is not used, and therefore loss of signal light does not deteriorate. In addition, an optical system for extracting a part of the emitted light such as an optical coupler is not necessary, and the structure of the entire apparatus can be further simplified.

  According to the fifth aspect of the present invention, the light wave emitted in the reverse direction from the entrance waveguide of the Mach-Zehnder type optical waveguide is separated from the optical path of the light wave incident from the entrance waveguide to the Mach-Zehnder type optical waveguide. Since the separation means and the separated separated light are detected by the detection means, the light wave incident from the entrance waveguide and the light wave emitted in the reverse direction can be effectively separated, and the reverse light wave necessary for bias control is obtained. It becomes possible to detect only more accurately. In addition, reverse light waves run back on the optical path including the light intensity modulator and re-enter the semiconductor laser light source to destabilize the laser oscillation. It can also be prevented.

  According to the invention of claim 6, the detecting means detects a part of the light wave emitted in the reverse direction from the light intensity modulator in order to detect the emitted light emitted from the Y branch point on the entrance side of the Mach-Zehnder type optical waveguide. It can be easily extracted. In addition, an optical system for extracting a part of the light wave such as a circulator becomes unnecessary, and the structure of the entire apparatus can be further simplified.

  According to the invention of claim 7, since the light intensity modulator is an optical intensity modulator used in the carrier suppression RZ modulation method, there is little signal deterioration of the modulated light, and the bias point can be controlled more accurately. Thus, it is possible to realize bias control of the light intensity modulator suitable for the carrier suppression RZ modulation method.

Hereinafter, the present invention will be described in detail using preferred examples.
As shown in FIGS. 1 to 4, the present invention has a light intensity modulator 4 having a Mach-Zehnder type optical waveguide 5 and a DC bias control means 23 for controlling a DC bias voltage applied to the light intensity modulator. In the bias control device of the light intensity modulator, re-incident means for re-entering the exit waveguide of the Mach-Zehnder optical waveguide part of the light wave emitted or emitted from the exit side of the Mach-Zehnder optical waveguide; Detection means 24 for detecting a light wave emitted or emitted from the entrance side of the Mach-Zehnder type optical waveguide, and a frequency is approximately ½ of a code speed of a transmission signal for driving the light intensity modulator, and an amplitude is the light. Based on the detection result of the clock voltage applying means for applying a clock voltage that is approximately twice the half-wave voltage of the intensity modulator to the light intensity modulator together with the DC bias voltage. , And having a bias adjustment means for controlling the DC bias voltage.

  In the first embodiment shown in FIG. 1, the re-incident means is the reflecting means 8 provided on the optical path (7, 9) of the outgoing light b emitted from the exit waveguide of the Mach-Zehnder optical waveguide. It becomes possible to construct the re-incident means simply. In addition, if necessary, it is possible to adjust the reflectance and transmittance of the reflecting means 8 so as to secure a sufficient amount of detection light necessary for bias control while suppressing a decrease in the amount of signal light. It becomes.

  In FIG. 1, reference numerals 1 and 3 are optical waveguides such as optical fibers for guiding incident light to a light intensity modulator, reference numeral 6 is an electrode used when applying a DC bias to a Mach-Zehnder optical waveguide, and reference numerals 7 and 9 are An optical waveguide such as an optical fiber for guiding a light wave emitted from the light intensity modulator. Reference numeral 2 denotes a circulator which is a separating means for separating the light wave traveling in the reverse direction through the light intensity modulator. Arrows a to d indicate the traveling direction of the light wave (or part of the light wave).

  Further, reference numeral 20 denotes a clock signal generator that oscillates at approximately half of the code rate of the transmission signal applied to the light intensity modulator, and reference numeral 21 denotes a half-wave voltage of the light intensity modulator 4 for the voltage amplitude of the clock signal. In particular, an amplifier 22 for making the voltage approximately half the half-wave voltage in the Mach-Zehnder type optical waveguide 5 to be controlled, the reference numeral 22, is a synthesis for adding the clock signal voltage to the DC bias voltage output from the bias controller 23. Means.

The operation of the bias control apparatus for the light intensity modulator of the present invention will be described.
As shown in FIG. 1, the light wave incident from the optical waveguide 1 travels in the direction of arrow a, passes through the optical waveguide 3, and enters the Mach-Zehnder optical waveguide 5 of the light intensity modulator 4. The incident light wave undergoes intensity modulation according to a modulation signal (transmission signal) applied to the light intensity modulator. Note that a bias voltage obtained by adding a clock signal voltage, which will be described later, is applied to the light intensity modulator, but the light wave traveling in the direction of the arrow c is not affected by the clock signal voltage.

  The light wave emitted from the light intensity modulator 4 passes through the optical waveguide 7 and reaches the reflecting means 8 such as a half mirror. In the reflecting means 8, most of the light wave is transmitted and enters the optical waveguide 9 as indicated by an arrow b. A part of the light wave reflected by the reflecting means 8 travels in the direction opposite to the arrow b as indicated by the arrow c and reenters the light intensity modulator 4.

  The light wave traveling in the reverse direction through the light intensity modulator 4 is not affected by the clock signal voltage obtained by the clock signal generator 20 and the amplifier 21 but affected by the DC bias voltage output from the bias controller 23. The light is emitted from the entrance side of the light intensity modulator.

  The light wave emitted from the light intensity modulator 4 is separated in the direction of the arrow d by the separating means 2 such as a circulator, and deflected to the optical waveguide 10. The light wave emitted from the optical waveguide 1 enters the light detection means 24 such as a light receiving element, and the light intensity is detected.

  The detection signal from the light detection means 24 is incident on the bias control means 23, and the bias control means controls the bias voltage applied to the light intensity modulator.

  As a bias control method, for example, the bias voltage output from the bias control means 23 is changed continuously or stepwise, and the bias voltage is set so that the value detected by the light detection means 24 takes the minimum value or the maximum value. To do. In particular, in the case of the CS-RZ modulation method, the detection value of the light detection unit 24 is set to be the lowest value. In the bias control means, the detection value from the light detection means can be compared with a preset value, and the bias voltage can be controlled so that the detection value matches the set value.

In the second embodiment shown in FIG. 2, another embodiment is disclosed as the re-incident means.
Specifically, branching means 32 such as an optical coupler is used to branch a part of the outgoing light emitted from the optical waveguide 7 on the exit side of the light intensity modulator.

  Most of the light waves that pass through the branching means 32 travel in the direction of the arrow g and enter the optical waveguide 9 as signal light. Also, some of the light waves branch in the direction of the arrow h and pass through the optical waveguide 33 to the branched light introducing means 30 such as a circulator. The branched light travels in the direction of arrow i and reenters the light intensity modulator 4. The handling of the light wave emitted from the entrance side of the light intensity modulator and the bias control method are the same as those in the first embodiment, and thus the description thereof is omitted.

  In the third embodiment shown in FIG. 3, radiated light k emitted from the multiplexing point of the Mach-Zehnder type optical waveguide 5 is used instead of the signal light emitted from the light intensity modulator.

  The light wave emitted from the Mach-Zehnder type optical waveguide 5 enters the optical waveguide 40 and reaches the radiated light introducing means 30 such as a circulator provided in the optical waveguide 7 on the emission side. The radiated light from the radiated light introducing means travels in the direction of arrow l, travels in the reverse direction of the optical waveguide 7, and reenters the light intensity modulator 4. Thus, in Example 3, since the emitted light from the light intensity modulator 4 which is signal light is not used, loss degradation of the signal light does not occur. In addition, an optical system for extracting a part of the emitted light such as an optical coupler is not necessary, and the structure of the entire apparatus can be further simplified.

  The emitted light emitted from the light intensity modulator 4 may be incident on optical means such as an optical fiber located away from the modulator 4 as shown in FIG. It is also possible to directly connect an optical fiber to the substrate constituting the vessel 4.

  The fourth embodiment shown in FIG. 4 uses the radiated light e emitted from the Y branch point on the entrance side of the Mach-Zehnder type optical waveguide 5.

  In FIG. 1, the reflecting means 8 shown in the first embodiment is used as the re-incident means. However, the present invention is not limited to this, and the re-incident means shown in the second or third embodiment can be used.

  The radiated light e enters the light detection means 24, and the signal output from the detection means is input to the bias control means 23 and used for bias control as described in the first embodiment. However, since the light wave propagating through the optical waveguide 1 in the reverse direction and the emitted light e have a light intensity opposite to each other, when the bias control of the CS-RZ modulation method is performed, the light intensity of the emitted light e is The bias voltage is controlled so as to be the maximum value.

  As described above, according to the present invention, it is possible to accurately control the bias point with little signal degradation of the modulated light, and to control the bias of the light intensity modulator that suppresses the cost of the entire apparatus. An apparatus can be provided.

It is the schematic which shows Example 1 of the bias control apparatus of the light intensity modulator which concerns on this invention. It is the schematic which shows Example 2 of the bias control apparatus of the light intensity modulator which concerns on this invention. It is the schematic which shows Example 3 of the bias control apparatus of the optical intensity modulator which concerns on this invention. It is the schematic which shows Example 4 of the bias control apparatus of the optical intensity modulator which concerns on this invention.

Explanation of symbols

1, 3, 7, 9, 10, 31, 33, 40 Optical waveguide (optical fiber)
2,30 Circulator 4 Light intensity modulator 5 Mach-Zehnder type optical waveguide 6 Control electrode 8 Reflecting means (half mirror)
20 clock signal generator 21 amplifier 22 voltage addition means 23 bias control means 24 light detection means 32 light separation means (optical coupler)

Claims (7)

In a light intensity modulator bias control device comprising: a light intensity modulator having a Mach-Zehnder type optical waveguide; and a DC bias control means for controlling a DC bias voltage applied to the light intensity modulator.
Re-incident means for re-entering a part of the light wave emitted or emitted from the exit side of the Mach-Zehnder type optical waveguide into the exit waveguide of the Mach-Zehnder type optical waveguide;
Detecting means for detecting a light wave emitted or emitted from the entrance side of the Mach-Zehnder type optical waveguide;
A clock voltage whose frequency is approximately ½ of the code rate of the transmission signal driving the light intensity modulator and whose amplitude is approximately twice the half wavelength voltage of the light intensity modulator is combined with the DC bias voltage. Clock voltage applying means for applying to the light intensity modulator;
A bias control apparatus for a light intensity modulator, comprising: a bias adjustment unit that controls the DC bias voltage based on a detection result of the detection unit.   The bias control apparatus for a light intensity modulator according to claim 1, wherein the re-incident means is a reflecting means provided on an optical path of outgoing light emitted from an exit waveguide of the Mach-Zehnder optical waveguide. A bias control device for a light intensity modulator.   2. The bias control apparatus for a light intensity modulator according to claim 1, wherein the re-incident means includes a branching means for branching a part of outgoing light emitted from an exit waveguide of the Mach-Zehnder optical waveguide, and the branched light. A bias control device for a light intensity modulator, comprising: branched light introducing means for introducing the branched light in a direction opposite to the propagation direction of the emitted light on the optical path of the emitted light.   2. The bias control apparatus for a light intensity modulator according to claim 1, wherein the re-incident means converts the emitted light emitted from the multiplexing point of the Mach-Zehnder type optical waveguide into an exit waveguide of the Mach-Zehnder type optical waveguide. A bias control device for a light intensity modulator, comprising: a radiated light introducing means for introducing light in a direction opposite to a propagation direction of the emitted light on an optical path of the emitted light emitted from the light intensity modulator.   5. The bias control device for an optical intensity modulator according to claim 1, wherein a light wave emitted in a reverse direction from an entrance waveguide of the Mach-Zehnder optical waveguide is guided to the Mach-Zehnder optical waveguide. A bias control apparatus for a light intensity modulator, characterized by separating means for separating light waves incident from a waveguide from an optical path, and detecting the separated separated light by the detecting means.   5. The bias control apparatus for a light intensity modulator according to claim 1, wherein the detecting means detects radiated light emitted from a Y branch point on an entrance side of the Mach-Zehnder optical waveguide. A bias control device for a light intensity modulator.   7. The light intensity modulator according to claim 1, wherein the light intensity modulator is a light intensity modulator used in a carrier suppression RZ modulation method. Device bias control device.

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