JP2011150052A - Optical transmitter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical transmitter that reduces the labor cost in a manufacturing process, by providing a simple voltage controller converging an RF amplitude adjustment voltage and a DC bias voltage applied to a Mach-Zehnder modulator by automatic control to the optimum point. <P>SOLUTION: The optical transmitter includes the Mach-Zehnder modulator disposed in a transmission path for transmitting modulation data, and a voltage controller for controlling the amplitude adjustment voltage and DC bias voltage applied to the Mach-Zehnder modulator. The optical transmitter includes a switch disposed in a power supply route of the driver power supply for driving the Mach-Zehnder modulator, and a switch controller disposed in the voltage controller and on-off-controls the switch. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an optical transmitter using a Mach-Zehnder (MZ-Zehnder) modulator (hereinafter referred to as an MZ modulator), and more particularly to initial adjustment of voltage for controlling optical phase modulation after voltage power is turned on. The present invention relates to an optical transmitter which has been improved for automation.

  Optical phase modulation such as DPSK (Differential Phase-Shift Keying) and DQPSK (Differential Quadrature Phase-Shift Keying) is widely used as a modulation method in optical communication. .

  FIG. 15 is an example of a configuration diagram of a DPSK optical modulator using an MZ modulator used in a conventional optical transmitter.

  The DPSK optical modulator 10 in FIG. 15 is composed of an MZ modulator. Further, the DPSK optical modulator 10 is formed with optical waveguides branched into two systems, and a phase modulation unit 10a and a phase modulation unit 10b for applying data for modulation to each of these optical waveguides, a phase modulation unit 10c and the phase modulation unit 10d are respectively connected in series, and the phase modulation units 10a, 10b and 10c, 10d are connected in parallel.

  A light source 11 is connected to one end of an optical waveguide of the DPSK optical modulator 10, this optical waveguide is branched into two systems, and is multiplexed again through the respective phase modulators 10a, 10b, 10c, and 10d. The end and the optical distributor 14 are connected.

  A light receiver 15 for inputting light from the light distributor 14 is connected to the light distributor 14, and a voltage controller 16 for controlling the voltage of the DPSK light modulator 10 is connected to the output terminal of the light receiver 15. Has been. A bias voltage amplifier 17 for inputting a DC bias voltage output from the voltage control unit 16 is connected to the voltage control unit 16. Further, a bias voltage amplifier 17 is connected to the phase modulators 10 c and 10 d of the DPSK optical modulator 10.

  The voltage control unit 16 includes an amplitude adjustment voltage control 16a and a DC bias voltage control means 16b. A high frequency (Rapid frequency) driver (hereinafter referred to as an RF driver 13) is connected to an amplitude adjustment voltage control means 16a of a voltage control unit 16 for inputting an RF amplitude adjustment voltage, and is connected to a driver power source and a data generator 12. Yes. Further, the RF driver 13 is connected to the phase modulators 10a and 10b of the drive DPSK optical modulator 10 for inputting the driven signal.

  Continuous light from the light source 11 is input to one end of the optical waveguide of the DPSK optical modulator 10, and the continuous light is branched into two systems, passes through each optical waveguide, is combined again, and is distributed from the other end. It is output as an optical signal Sout via the device 14.

  The data output from the data generator 12 is input to the RF driver 13, and the RF amplitude adjustment voltage is applied to the RF driver 13 by the amplitude adjustment voltage control means 16a in the voltage control unit 16 constituting the feedback path. By inputting a driver power source for driving the modulator 10 to the RF driver 13, the data input to the RF driver 13 is amplified to an appropriate amplitude voltage and is provided in each optical waveguide of the DPSK optical modulator 10. It is applied to the phase modulators 10a and 10b.

  Each phase modulation unit 10c, 10d provided in the optical waveguide of the DPSK optical modulator 10 is supplied with a predetermined voltage via a bias voltage amplifier 17 by a DC bias voltage control means 16b in the voltage control unit 16 constituting the feedback path. A DC bias voltage is applied and this DC bias voltage is adjusted appropriately. Here, by applying an RF signal, the signal is phase-modulated to zero or π and distributed to the signal Sout output to the outside and the signal input to the light receiver 15.

  If the amplitude and DC bias voltage of the RF driver are not set appropriately, the eye pattern at the time of demodulation closes, making it difficult to identify “0” or “1” in the data, resulting in transmission quality degradation and transmission. Causes deterioration. Therefore, in the optical transmitter of FIG. 15, amplitude adjustment voltage control means 16 a and DC bias voltage control 16 b are provided in the voltage control unit 16, and a predetermined voltage is fed back to the DPSK optical modulator 10 to maintain transmission quality. ing.

  Here, a control method for the DPSK modulator will be described. When controlling the RF amplitude adjustment voltage, it must be set within a certain error range with respect to the optimum point of the DC bias voltage. When controlling the DC bias voltage, it is set within a certain error range with respect to the optimum point of the RF amplitude adjustment voltage. Need to be done. Hereinafter, setting the DC bias voltage within a certain error range with respect to the optimum point of the RF amplitude adjustment voltage is referred to as a precondition.

Here, the reason why the precondition is necessary will be described below. The output optical power of the DPSK modulator 10 at a certain moment is approximated by the equation (1). Here, the output optical power of the DPSK optical modulator 10 is Pow, the output optical power reference value of the DPSK optical modulator is Pow ₀ , the bias angle of the optical phase determined by the DC bias voltage is θ _DC , and the applied voltage of the RF driver 13 The amount of change in the optical phase is θ _RF .
Pow (θ _DC , θ _RF ) = Pow ₀ · {1 + cos (θ _DC + θ _RF )} Expression (1)

Here, assuming that the modulation waveform of the RF driver 13 is a trapezoidal wave close to a square having an amplitude corresponding to a certain amount of phase change (± θ _AMP ), the average value of the output optical power is given by equation (2). Can be approximated.

  FIG. 16 shows an example of the relationship between the average optical power, the DC bias voltage, and the RF amplitude derived from the relational expression (2). (a) and (b) are graphs showing the relationship between the average optical power characteristics with respect to the DC bias voltage, and (c) and (d) are graphs showing the relationship between the average optical power characteristics with respect to the RF amplitude.

  In (a) and (b), the average optical power periodically changes with respect to the DC bias voltage. Further, when (a) and (b) are compared, it can be confirmed that the slope is inverted at the boundary of ± (hereinafter referred to as plus or minus) 0.5 * Vπ which is the optimum point of the RF amplitude. That is, in (a), when the optimum point of the RF amplitude is from − (hereinafter referred to as minus) 0.5 * Vπ to + (hereinafter referred to as plus) 0.5 * Vπ, the point at which the average optical power becomes maximum is the optimum DC bias voltage. On the other hand, in (b), when the optimum point of the RF amplitude is (minus) 0.5 * Vπ or less and (plus) 0.5 * Vπ or more, the point where the average optical power is minimum is the DC bias. It is the optimum point of voltage.

  In (c) and (d), the average optical power periodically changes with respect to the RF amplitude. Further, when (c) and (d) are compared, it can be confirmed that the slope is inverted at the boundary of (plus or minus) 0.5 * Vπ which is the optimum point of the DC bias voltage. That is, in (c), when the optimum point of the DC bias voltage is (minus) 0.5 * Vπ to (plus) 0.5 * Vπ, the point at which the average optical power is maximum is the optimum point of the RF amplitude. In (d), when the optimum point of the DC bias voltage is (minus) 0.5 * Vπ or less and (plus) 0.5 * Vπ or more, the point where the average optical power is minimum is the optimum point of the RF amplitude. .

  Therefore, Patent Document 1 proposes an optical modulation apparatus and an optical modulator control method that can compensate for an operating point variation accompanying a variation in voltage versus optical output characteristics of the optical modulator in the optical duobinary modulation method.

  Patent Document 2 proposes a method and apparatus for controlling the bias and matching of an optical signal transmitter that applies intensity modulation and DPSK modulation to an optical signal, for example, in the RZ-DPSK modulation format.

JP 2000-162563 A

JP 2008-524655 A

  However, such an optical transmitter has the following problems. In the control method that searches for the maximum point of average optical power, the initial value setting of the DC bias voltage and RF amplitude causes the problem of convergence to a point that is not the optimal point, that is, a point that is Vπ away from the optimal point during start-up control. There is.

  As a method for avoiding the DC bias voltage and RF amplitude from converging to a point that is not the optimum point, a method of adjusting the initial value of the RF amplitude adjustment voltage is generally used in the manufacturing process. The method has a problem that labor costs are incurred when making adjustments.

  Furthermore, in order to reduce the labor cost for adjustment, it is necessary to set the initial value of the RF amplitude adjustment voltage as a fixed parameter common to the products. In order to set the initial value of the RF amplitude adjustment voltage as a common fixed parameter between products, it is necessary to select components based on the RF amplitude voltage required to change the optical phase of the MZ modulator by π and the amplitude characteristics of the RF driver. Therefore, there is a problem that labor costs are incurred.

  An object of the present invention is to reduce labor costs in the manufacturing process by providing a simple voltage control unit that can converge an RF amplitude adjustment voltage and a DC bias voltage applied to a Mach-Zehnder modulator by automatic control to an optimum point. It is to realize an optical transmitter.

In order to achieve such a problem, the invention according to claim 1 of the present invention is:
In an optical transmitter having a Mach-Zehnder modulator provided in a transmission path for transmitting modulation data, and a voltage controller for controlling an amplitude adjustment voltage and a DC bias voltage applied to the Mach-Zehnder modulator,
A switch provided in a power supply path of a driver power supply for driving the Mach-Zehnder modulator;
Switch control means provided in the voltage control unit and for controlling on / off of the switch is provided.

The invention according to claim 2 is the invention according to claim 1,
The switch control means includes
When the value of the DC bias voltage is indefinite, the switch is controlled to be turned off,
2. The optical transmitter according to claim 1, wherein the switch is turned on when the value of the DC bias voltage is at an optimum point.

The invention according to claim 3 is the invention according to claim 1 or 2,
As the Mach-Zehnder modulator, a DPSK optical modulator using a Mach-Zehnder type dielectric crystal LiNbO 3 or a DQPSK optical modulator is used.

  According to the present invention, RF-Vπ of the Mach-Zehnder modulator is provided by providing a simple voltage control unit that can converge the RF amplitude adjustment voltage and the DC bias voltage applied to the Mach-Zehnder modulator by automatic control to the optimum point. Even when there are variations in the amplitude characteristics of the RF driver, labor costs in the manufacturing process can be reduced by performing automatic control without adjusting initial values or selecting parts in the manufacturing process.

It is a block diagram which shows one Example of the DPSK optical modulator used with the optical transmitter of this invention. It is a flowchart which shows an example of the starting control sequence of a voltage control part. It is a block diagram which shows the other Example of the DPSK optical modulator used with the optical transmitter of this invention. It is a block diagram which shows the other Example of the DPSK optical modulator used with the optical transmitter of this invention. It is a block diagram which shows the other Example of the optical transmitter of this invention. It is an example of the electric field vector diagram of MZA output light when both the DC bias voltage and RF amplitude in MZA are optimal. It is an example of the figure which showed the electric field vector of the DQPSK optical modulator output light. It is the figure which showed the electric field vector when the power supply of RF driver was turned off (RF amplitude is zero) in both MZA and MZB from the optimal state shown in FIG. It is a flowchart which shows an example of the starting control sequence of the control apparatus currently used by FIG. FIG. 4 is a diagram illustrating an optimum point search principle of a phase modulation unit MZC (DC) of the MZC. It is an electric field vector diagram of light about the flowchart of FIG. It is an example of an optical electric field vector diagram when High and Low of an RF driver are asymmetrical in MZA. It is an example of a startup control flowchart of the RF amplitude and bias control device corresponding to asymmetry in the RF drive. It is an example of the electric field vector diagram of light explaining the DQPSK starting control flowchart of FIG. It is an example of the block diagram of the DPSK optical modulator using the MZ modulator used with the conventional optical transmitter. It is a graph which shows an example of the conventional light output intensity graph and a light output waveform.

  Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of a DPSK optical modulator used in the optical transmitter of the present invention. The same elements as those in FIG. 15 are denoted by the same reference numerals, and description thereof is omitted.

  The difference from the conventional example of FIG. 15 is that the voltage control unit 20 is provided with switch control means 20a having a function of controlling on / off of a driver power source for driving the DPSK optical modulator 10. Further, a switch 21 for turning on / off the driver power supply based on an output signal from the switch control means 20a is provided.

  The voltage control unit 20 includes a switch control unit 20a in addition to the amplitude adjustment voltage control unit 16a and the DC bias voltage control unit 16b that are conventionally provided. The DPSK optical modulator 10 in FIG. 1 is composed of an MZ modulator.

  Light from the light source 11 is incident on each optical waveguide of the DPSK optical modulator 10 composed of an MZ modulator. On the other hand, the data is input to the RF driver 13, driven to an appropriate amplitude voltage, and applied to each phase modulation unit 10 a, 10 b provided in each optical waveguide of the DPSK optical modulator 10.

  A predetermined voltage is applied from the RF driver 13 to the phase modulators 10a and 10b provided in the optical waveguide, and the light intensity is modulated. An optical signal whose light intensity is modulated by the DPSK optical modulator 10 is input to the optical distributor 14 and distributed to the signal Sout output to the outside and the signal input to the optical receiver 15.

  The switch control means 20a in the voltage control unit 20 controls the switch 21, that is, the connection of the driver power supply applied to the RF driver 13 is controlled. For example, when the switch 21 is on, the voltage of the driver power supply is applied to the RF driver 13, and further, based on the output current from the DPSK optical modulator 10, the amplitude adjustment voltage control means 16 a in the voltage control unit 20 has a predetermined value. The RF amplitude adjustment voltage is input to the RF driver 13. It is driven to an appropriate amplitude voltage from the RF driver 13 and applied to each phase modulator 10a, 10b provided in each optical waveguide of the DPSK optical modulator 10.

  Further, the RF driver 13, the switch 21, and the bias voltage amplifier 17 constitute feedback means. Here, the feedback means is a first feedback means, a second feedback means, and a third feedback means. The first feedback means is composed of the DC bias voltage control means 16 b of the voltage control unit 20 and the bias voltage amplifier 17. The second feedback means is composed of the amplitude adjustment voltage control means 16 a of the voltage control unit 20 and the RF driver 13. The third feedback means is composed of the switch control means 20 a of the voltage control unit 20, the switch 21 and the RF driver 13.

  Further, the feedback control of the DC bias voltage is performed by the first feedback means, so that the adjusted optical adjustment output waveform is output from the DPSK optical modulator 10.

  By feedback control, the DC bias voltage of the DPSK optical modulator 10 can be automatically controlled, and the transmission quality of the optical transmission waveform can be maintained.

  FIG. 2 is a flowchart illustrating an example of a startup control sequence of the voltage control unit.

  In step S1, the switch 21 is controlled by the switch control means 20a in the voltage control unit 20 to disconnect the driver power source connected to the RF driver 13, thereby reducing the RF amplitude to zero.

  Next, in step S2, the optimum point of the DC bias voltage is searched. By step S1, the relationship between the average optical power and the DC bias voltage becomes the state shown in FIG. That is, the optimum point of the DC bias voltage has the minimum average optical power. Therefore, the point where the average optical power, which is the optimum point of the DC bias voltage, is minimized is searched here.

  In step S3, the switch 21 is controlled by the switch control means 20a in the voltage control unit 20, and the connection of the driver power source connected to the RF driver 13 is turned on.

  In step S4, the optimum point of the RF amplitude is searched. Since the DC bias voltage is the optimum point in step S2, the relationship between the average optical power and the RF amplitude is the state shown in FIG. That is, the optimum point of the RF amplitude has the maximum average optical power. Therefore, here, the process ends when a point where the average optical power is maximized is searched.

  FIG. 3 is a block diagram showing another embodiment of the DPSK optical modulator used in the optical transmitter of the present invention. In addition, the same code | symbol is attached | subjected to the same element as FIG. 1, and description is abbreviate | omitted.

  The difference from FIG. 1 is that an RZ optical modulator 30 is provided on the output side of the DPSK optical modulator 10. Further, based on the output current from at least one of the DPSK optical modulator 10 or the RZ (return to zero) optical modulator (hereinafter referred to as the RZ optical modulator 30), that is, the current from the light receivers 15a and 15b. Thus, the amplitude adjustment voltage and DC bias voltage of the DPSK optical modulator 10 are controlled.

  FIG. 4 is a block diagram showing another embodiment of the DPSK optical modulator used in the optical transmitter of the present invention. In addition, the same code | symbol is attached | subjected to the same element as FIG. 1, and description is abbreviate | omitted.

  The difference from FIG. 1 is that a DQPSK optical modulator 40 is used instead of the DPSK optical modulator 10. Further, three Mach-Zehnder modulators are provided in the DQPSK optical modulator 40. These three Mach-Zehnder modulators are hereinafter referred to as MZA, MAB, and MZC. Further, by providing three Mach-Zehnder modulators inside the DQPSK optical modulator 40, the phase modulation units 40Aa to 40Ad and 40Ba to 40Bd of MZA and MZB are the same as the configuration of the phase modulation unit of FIG. . Further, switches 21a and 21b for MZA and MZB are provided, respectively, RF drivers 13a and 13b are connected to MZA and MZB, and bias voltage amplifiers 17a to 17c are connected to the Mach-Zehnder modulators, respectively.

  Specifically, in the DQPSK optical modulator 40, optical waveguides branched into two systems are formed, and MZA and MZB are provided. The MZA is formed with optical waveguides in which each optical waveguide is further branched into two systems, and a phase modulation unit 40Aa, a phase modulation unit 40Ab, and a phase modulation unit 40Ac for applying data for modulation to each of these optical waveguides. The phase modulators 40Ad are respectively connected in series, and the phase modulators 40Aa, 40Ab and 40Ac, 40Ad are connected in parallel. The signals are multiplexed again via the respective phase modulators 40Aa, 40Ab, 40Ac, and 40Ad, and are connected to the phase modulator 40Ca of the MZC.

  MZB is also configured in the same manner as MZA, that is, phase modulator 40Ba and phase modulator 40Bb, phase modulator 40Bc and phase modulator 40Bd are connected in series, and phase modulators 40Ba, 40Bb and 40Bc, 40Bd are connected in parallel. ing. The signals are recombined via the respective phase modulators 40Ba, 40Bb, 40Bc, and 40Bd, and connected to the MZC phase modulator 40Cb. The signals are recombined via the MZC phase modulators 40Ca and 40Cb and connected to the optical distributor 14.

  That is, the light source 11 is connected to one end of the optical waveguide of the DQPSK optical modulator 40, and this optical waveguide is branched into two systems, through the respective phase modulators 40Aa to 40Ad, 40Ba to 40Bd, 40Ca, and 40Cd, The signal is multiplexed again and the other end is connected to the optical distributor 14.

  A light receiver 15 for inputting light from the light distributor 14 is connected to the light distributor 14, and a voltage control unit 23 for controlling the voltage of the DQPSK light modulator 40 is connected to the output terminal of the light receiver 15. Has been. Bias voltage amplifiers 17a to 17c for inputting the DC bias voltage output from the voltage control unit 23 are connected to the voltage control unit 23, respectively. Further, bias voltage amplifiers 17a to 17c are connected to the phase modulation units 40Ac, 40Ad, 40Bc, 40Bd, 40Ca, and 40Cb of the DQPSK optical modulator 40, respectively.

  The voltage control unit 16 includes an amplitude adjustment voltage control 16a and a switch control means 23a in addition to the DC bias voltage control means 16b. The RF drivers 13a and 13b are connected to the amplitude adjustment voltage control means 16a of the voltage control unit 23 for inputting the RF amplitude adjustment voltage, and are connected to the switches 21a and 21b. The RF driver 13a is connected to the phase modulators 40Aa and 40Ab of the DQPSK optical modulator 40 that inputs the driven signal, and the phase of the DQPSK optical modulator 40 that inputs the driven signal is connected to the RF driver 13b. Modulators 40Ba and 40Bb are connected.

  By feedback control, the DC bias voltage of the DQPSK optical modulator 40 can be automatically controlled, and the transmission quality of the optical transmission waveform can be maintained.

  FIG. 5 is a block diagram showing another embodiment of the optical transmitter of the present invention. In addition, the same code | symbol is attached | subjected to the same element as FIG. 1, and description is abbreviate | omitted.

  The difference from FIG. 1 is that a DQPSK optical modulator 40 is used instead of the DPSK optical modulator 10, and an RZ optical modulator 50 is provided on the output side of the DQPSK optical modulator 40. The amplitude of the DQPSK optical modulator 40 is based on the current corresponding to the output optical power of at least one of the DQPSK optical modulator 40 and the RZ optical modulator 50, that is, based on the output current from the light receiver 15. The adjustment voltage and DC bias voltage are controlled. Here, the phase data control of the DQPSK optical modulator 40 is to control the phase data of at least one Mach-Zehnder modulator among the MZA, MZB, and MZC in the DQPSK optical modulator 40.

  FIG. 6 shows an example of an electric field vector diagram of MZA output light when both the DC bias voltage and RF amplitude in MZA are optimal.

  (a) is an electric field vector diagram when the input data is zero, (b) is an electric field vector diagram when the input data is 1, and (c) is a power supply of the RF driver with the DC bias voltage in an optimum state. It is an electric field vector when it is turned off (RF amplitude is zero), and (d) is a display in which three states (a) to (c) are superimposed.

  The phase modulation units of the MZA inside the DQPSK optical modulator 40 are 40Aa (RF), 40Ab (RF), 40Aa (DC), and 40Ab (DC). Here, 40Aa (RF) and 40Ab (RF) are phase modulation units 40Aa and 40Ab when the power supply of the RF driver is turned on in a state where the DC bias voltage is optimal, and 40Ac (DC) and 40Ad (DC) are DC. These are phase modulation units 40Ac and 40Ad when the power supply of the RF driver is turned off (RF amplitude is zero) with the bias voltage in an optimum state.

  In (a), when the data input to the RF driver is set to zero and the power supply of the RF driver is turned on, the electric field vector of MZA inside the DQPSK optical modulator 40 in the phase modulation unit 40Aa and the phase modulation unit 40Ab is Can be confirmed. On the other hand, in (b), when the data input to the RF driver is set to 1 and the power of the RF driver is turned on, the electric field of the MZA inside the DQPSK optical modulator 40 in the phase modulation unit 40Aa and the phase modulation unit 40Ab. It can be confirmed that the vector faces downward. In (c), when the power of the RF driver is turned off, since the power is turned off, the electric field vectors of the phase modulation unit 40Ac and the phase modulation unit 40Ad are horizontal lines. Moreover, it can confirm that it has faced the opposite direction. In (d), the electric field vectors in various states can be confirmed by displaying the three states (a) to (c) in an overlapping manner.

  Moreover, since MZB is the same as MZA, description is abbreviate | omitted.

  FIG. 7 shows the case where both the MZA and MZB have the optimum DC bias voltage and RF amplitude as shown in FIG. 6, and the electric field vector of the DQPSK optical modulator output light when the MZC DC bias voltage is optimum. FIG.

  Here, MZA (0) is the MZA vector when the data input to MZA is zero, MZA (1) is the MZA vector when the data input to MZA is 1, and MZB (0) is MZB vector when data input to MZB is zero, and MZB (1) is an MZB vector when data input to MZB is 1.

  Also, it is confirmed that MZA (0) and MZB (0) are orthogonal, and MZA (1) and MZB (1) are orthogonal, so that the electric field vectors of MZA output light and MZB output light are orthogonal. it can.

  FIG. 8 is a diagram showing an electric field vector when the power supply of the RF driver is turned off (RF amplitude is zero) for both MZA and MZB from the optimum state shown in FIG.

  Also, the phase modulation unit for applying the MZA DC bias voltage inside the DQPSK optical modulator is 40Ac (DC) and 40Ad (DC), and the phase modulation unit for applying the MZB DC bias voltage is 40Bb (DC) and 40Bd ( DC). Here, 40Ac (direct current), 40Ad (direct current), 40Bb (direct current), 40Bd (direct current) are phase modulation units when the power of the RF driver is turned off (RF amplitude is zero) with the optimum DC bias voltage. is there.

  Since the electric field vectors of 40Ac (direct current) and 40Ad (direct current) are directed to cancel each other, the light passing through the phase modulation unit 40Ac (direct current) and the phase modulation unit 40Ad (direct current) may cancel each other. I can confirm. Further, the combined light VMZA of the electric field vectors of 40Ac (DC) and 40Ad (DC) becomes a zero vector.

  Further, since the electric field vectors of 40Bb (direct current) and 40Bd (direct current) are directed to cancel each other, light passing through the phase modulation unit 40Bb (direct current) and the phase modulation unit 40Bd (direct current) cancel each other. I can confirm that. Further, the combined light VMZB of the electric field vectors of 40Bb (direct current) and 40Bd (direct current) becomes a zero vector.

  Therefore, the output light from MZA and MZB becomes a zero vector. Further, it can be confirmed that the electric field vector of the light passing through the phase modulation unit 40Ac and the phase modulation unit 40Ad is orthogonal to the electric field vector of the light passing through the phase modulation unit 40Bb (MZb1) and the phase modulation unit 40Ad.

  FIG. 9 is a flowchart showing an example of a startup control sequence of the control device used in FIGS.

  In step S11, the RF driver is turned off. In other words, the switches 21a and 21b for controlling the MZA and MZB RF drivers are controlled by the switch control means in the control unit, and the RF amplitude is made zero by turning off the power of the RF drivers.

  Next, in step S12, the maximum point of average optical power is searched for MZC.

  FIG. 10 is an optimum point search principle diagram of the phase modulation unit MZC (DC) of the MZC. That is, it shows the relationship between the MZC DC bias voltage and the modulator output optical power when the RF amplitude of MZA and MZB in the DQPSK optical modulator is zero.

  Here, it can be confirmed from FIG. 10 that the electric field vector of the output light from the MZA and the output light from the MZB has the same phase at the maximum average optical power point VC_MAX. That is, the average optical power maximum point VC_MAX corresponds to a point where the phase difference between the electric field vectors of the output light from the MZA and the output light from the MZB is zero.

  Returning to step S12 of FIG. 9, the maximum value of the output optical power is stored as PowC_MAX, and the DC bias voltage when searching for the maximum average optical power point is stored as VC_MAX.

  Next, in step S13, the minimum point of average optical power is searched for MZC.

  Here, it can be confirmed from FIG. 10 that the electric field vectors of the output light from the MZA and the output light from the MZB are in opposite phases at the average optical power minimum point VC_MIN. That is, the average optical power minimum point VC_MIN corresponds to a point where the phase difference between the electric field vectors of the output light from the MZA and the output light from the MZB is π.

  Returning to step S13 in FIG. 9, the minimum value of the output optical power is stored as PowC_MIN, and the DC bias voltage at the time of searching for the average optical power minimum point is stored as VC_MIN.

  In step S14, | PowC_MAX−PowC_MIN |> threshold is determined. That is, it is determined whether or not the value obtained by subtracting the average optical power minimum point PowC_MIN stored in step S13 from the average optical power maximum point PowC_MAX stored in step S12 is larger than the threshold value. Proceed to S21.

  Here, is it sufficient to determine whether the value obtained by subtracting the average optical power minimum point PowC_MIN stored in step S13 from the average optical power maximum point PowC_MAX stored in step S12 is larger than the threshold value? It is for judging. That is, when the output light from MZA and MZB becomes zero due to the DC bias point of MZA and the DC bias point of MZB, the average optical power from the DQPSK optical modulator is constant regardless of the DC bias voltage of MZC, The search accuracy of the average optical power maximum and minimum points VC_MAX and VC_MIN is lowered. In order to avoid such a state, a value obtained by subtracting the average optical power minimum point PowC_MIN stored in step S13 from the average optical power maximum point PowC_MAX stored in step S12 is calculated.

  It is determined whether or not the value obtained by subtracting the average optical power minimum point PowC_MIN stored in step S13 from the average optical power maximum point PowC_MAX stored in step S12 is larger than the threshold value. If it is larger, that is, if sufficient detection sensitivity is obtained, step S15 is performed. If it is small, that is, if sufficient detection sensitivity cannot be obtained, the process proceeds to step S21.

  In step S15, the voltage of MZC is set to (VC_MAX + VC_MIN) / 2 for MZC.

  Here, from FIG. 10, (VC_MAX + VC_MIN) / 2, that is, VC_MAX and VC_MIN, which are intermediate points between VC_MAX and VC_MIN, are added and divided by 2, the values of the output light from MZA and the output light from MZB It can be confirmed that the phase difference is π / 2. The point where the phase difference between the output light from MZA and the output light from MZB is π / 2 is the optimum point of DC bias in MZC. That is, the optimum point of MZC where MZA and MZB are orthogonal to each other is set.

  Returning to FIG. 9, in steps S12 and S13 as the DC bias of MZC, the maximum and minimum points of the average optical power, which is the optimal point of DC bias, in MZC, that is, the respective optimal points VC_MAX and VC_MIN are added. Set to the value divided by 2, to obtain the optimum state.

  In step S16, the minimum point of average optical power is searched for MZA (direct current). That is, the optimum point of DC bias is searched in MZA.

MZA and MZB, when the RF amplitude is zero, when the electric field vectors of light output from MZA and MZB are VMZA and VMZB, the output optical power from the DQPSK optical modulator is as shown in Equation (3) Become.
Pow = | VMZA + VMZB | ²
= | VMZA | ² + | VMZB | ² +2 (VMZA / VMZB) (3)

Since VMZA and VMZB are orthogonal at step S15 in FIG. 9, equation (3) becomes equation (4). That is, it is a simple addition of the output optical power from the MZA and the output optical power from the MZB.
Pow = | VMZA | ² + | VMZB | ² Formula (4)

  When the DC bias voltage of MZB is fixed and the DC bias voltage of MZA is changed according to equation (4), when the output optical power from MZA is maximum, the output optical power from the DQPSK optical modulator is also maximum. When the output optical power from the MZA is minimum, the output optical power from the DQPSK optical modulator is also minimum.

  Since the MZA has the same configuration as the DPSK optical modulator, the relationship between the output optical power with respect to the DC bias and the RF amplitude in the MZA has the same characteristics as the DPSK optical modulator.

  Here, since the RF amplitude is set to zero in step S11 of FIG. 9, the point where the average optical power of MZA is the minimum in FIG. 16B is the optimum point of the DC bias voltage of MZA. Also, since the output optical vectors of MZA and MZB are orthogonal, the point where the average optical power from the DQPSK optical modulator is minimum is the point where the average optical power of MZA is minimum, and the DC bias voltage of MZA It becomes the optimal point.

  Therefore, in the MZA, the DC bias voltage of the MZA is optimized by searching for the DC bias voltage point at which the average optical power is minimum.

  Next, in step S16, MZA and MZB are exchanged. In step S17, the minimum point of average optical power is searched for MZB (direct current). That is, the optimum point of the DC bias voltage is searched for in MZB. Since the RF amplitude is set to zero in step S11, the point where the average optical power of the MZA is minimum in FIG. 16B is the optimum point of the DC bias voltage of the MZA. Also, since the output optical vectors of MZA and MZB are orthogonal, the point where the average optical power from the DQPSK optical modulator is minimum is the point where the average optical power of MZB is minimum, and the DC bias voltage of MZB It becomes the optimal point.

  Therefore, the DC bias voltage of MZB is optimized by searching for the DC bias voltage point at which the average optical power is minimum in MZB.

  In step S18, the RF driver is turned on. That is, the switches 21a and 21b for controlling the power supply of the RF drivers of MZA and MZB are controlled by the switch control means in the control unit, and the power of each RF driver is turned on.

  In step S19, the optimum point of the RF driver amplitude voltage is searched for in MZA. By step S11, the relationship between the average optical power and the DC bias voltage becomes the state shown in FIG. That is, the optimum point of the RF driver amplitude has the maximum average optical power. Therefore, the point where the average optical power, which is the optimum point of the DC bias voltage, is maximized is searched here. That is, the optimum point of the RF driver amplitude in MZA is searched.

  When the data input to the MZA RF driver is data A and the data input to the MZB RF driver is data B, the combinations of (data A, data B) are (0,0), (0, It is assumed that there are four patterns 1), (1,0), and (1,1), and the appearance probability of each data is 1/4.

  The combination of the electric field vectors of the output light from MZA and MZB when each data is input is (VMZA (0), VMZB (0)), (VMZA (0), VMZB (1)), (VMZA ( 1), VMZB (0)), (VMZA (1), VMZB (1)), and when the appearance probability of each data is 1/4, the average optical power of the output light from the DQPSK optical modulator is Approximated by equation (5).

  As shown in FIG. 5, in the case of a combination of an RF driver and a DQPSK optical modulator in which voltages having opposite signs and the same absolute value are applied to MZA 40Aa (RF) and 40Ad (RF), the steps of FIG. If VMZA and VMZB are orthogonalized in DC state by S15, VMZA (i) and VMZB (j) are orthogonal to any i and j with i = 0 or 1, j = 0 or 1 To do. Thereby, Formula (5) becomes like Formula (6). That is, it is a simple addition of the output optical power from the MZA and the output optical power from the MZB.

  In other words, when the DC bias voltage of MZB is fixed and the DC bias voltage of MZA is changed, when the average value of the output optical power from MZA is the maximum, the average value of the output optical power from the DQPSK optical modulator is also the maximum Thus, when the average value of the output light power from the MZA is minimized, the average value of the output light power from the DQPSK optical modulator is also minimized.

  Since the MZA has the same configuration as the DPSK optical modulator, the relationship of the output optical power with respect to the DC bias voltage and the RF amplitude in the MZA has the same characteristics as the DPSK optical modulator.

  Here, since the DC bias voltage of the MZA is the optimum point by step S16 in FIG. 9, the point where the average optical power of the MZA is maximum in FIG. 16C is the optimum point of the RF amplitude of the MZA. .

  Since VMZA (i) and VMZB (j) are orthogonal, the point where the output optical power output from the DQPSK optical modulator is maximum is the point where the output optical power from MZA is maximum, It becomes the optimum point of RF amplitude.

  Therefore, in the MZA, the RF amplitude of the MZA is optimized by searching for the RF amplitude that maximizes the average optical power.

  In step S20, the object is MZA in step S19, but the object is replaced with MZB, and the same process as in step S19 is performed. That is, the maximum point of average optical power is searched for MZB (RF). That is, the optimum point of the DC bias voltage is searched for in MZB.

  Therefore, in the MZB, the RF amplitude of the MZB is optimized by searching for the RF amplitude of the MZB that maximizes the average optical power.

  In step S21, the DC bias voltage is changed for MZA (DC) and MZB (DC). If it is determined in step S15 that the detection sensitivity cannot be obtained, the DC bias voltages of MZA and MZB are changed so that the bias point at which the detection sensitivity is obtained is obtained. That is, the DC bias voltage is changed in MZA and MZB, and the process proceeds to step S12. That is, if it is determined in step S14 that the detection sensitivity cannot be obtained, the process proceeds from step S21 to step S12, and then proceeds to step S13 and step S14. Therefore, the process proceeds from step 12 to step S21 until the detection sensitivity is obtained in step S14, and steps S12, S13, and S14 are repeated. Further, as a method of changing the bias, there are a method of adding or subtracting a fixed value, or a method of changing the value randomly within a certain range.

  That is, the switch control means controls the switches 21a and 21b to be turned off when the value of the DC bias voltage is indefinite (step S1), and switches 21a and 21b when the value of the DC bias voltage is at the optimum point (step S17). 21b is turned on. Here, turning off the RF driver means turning off the switch 21a and the switch 21b. Further, turning on the RF driver means turning on the switch 21a and the switch 21b.

  FIG. 11 is an electric field vector diagram of the light described with reference to the flowchart of FIG. (a) is an electric field vector diagram of light corresponding to the end point of step S15 in FIG. 9, (b) is an electric field vector diagram of light corresponding to the end point of step S17 in FIG. 9, and (c) is FIG. It is an electric field vector diagram of light corresponding to the end time of step S20.

  Also, the phase modulation unit for applying the MZA DC bias voltage inside the DQPSK optical modulator is 40Ac (DC) and 40Ad (DC), and the phase modulation unit for applying the MZB DC bias voltage is 40Bb (DC) and 40Bd ( DC). Here, in (b), 40Ac (direct current), 40Ad (direct current), 40Bb (direct current), and 40Bd (direct current) are when the DC bias voltage is optimal and the RF driver power is turned off (RF amplitude is zero) It is a phase modulation part.

  In (b), since the electric field vectors of 40Ac (direct current) and 40Ad (direct current) are directed to cancel each other, light passing through the phase modulation unit 40Ac (direct current) and the phase modulation unit 40Ad (direct current) is mutually transmitted. It can be confirmed that they cancel each other. Further, the combined light VMZA of the electric field vectors of 40Ac (DC) and 40Ad (DC) becomes a zero vector.

  Further, since the electric field vectors of 40Bc (direct current) and 40Bd (direct current) are directed to cancel each other, light passing through the phase modulation unit 40Bc (direct current) and the phase modulation unit 40Bd (direct current) cancel each other. I can confirm that. Further, the combined light VMZB of the electric field vectors of 40Bc (direct current) and 40Bd (direct current) becomes a zero vector.

  Therefore, the output light from MZA and MZB becomes a zero vector. Further, it can be confirmed that the electric field vector of the light passing through the phase modulation unit 40Ac and the phase modulation unit 40Ad is orthogonal to the electric field vector of the light passing through the phase modulation unit 40Bb (MZb1) and the phase modulation unit 40Ad.

  In (c), MZA (0) is the MZA vector when the data input to MZA is zero, MZA (1) is the MZA vector when the data input to MZA is 1, and MZB (0 ) Is an MZB vector when data input to MZB is zero, and MZB (1) is an MZB vector when data input to MZB is 1.

  Also, it is confirmed that MZA (0) and MZB (0) are orthogonal, and MZA (1) and MZB (1) are orthogonal, so that the electric field vectors of MZA output light and MZB output light are orthogonal. it can.

  For example, the following method is effective as a search method for the minimum or maximum point of the average optical power in steps S2 and S4 in FIG. 2 and steps S12, S13, S16, S17, S19, and S20 in FIG. .

  For example, as a first search method, the dither signal of frequency f in the target control unit is added and input, and the detected current from the photoreceiver is synchronously detected with the reference signal of frequency f, and this synchronous detection is performed. The output voltage is controlled by negative feedback or positive feedback with respect to the target value of zero.

  By controlling in this way, a zero cross point where the synchronous detection value is switched from negative to positive or a zero cross point where the synchronous detection value is switched from positive to negative is searched. The zero-cross point that switches from negative to positive or the zero-cross point that switches from positive to negative corresponds to the point where the average optical output power is maximized on the one hand and the point where the average optical output power is minimized on the other hand. This correspondence is determined by the configuration of a LiNbO 3 modulator (hereinafter referred to as an LN modulator) using a Mach-Zehnder type dielectric crystal, that is, a DPSK optical modulator, a DQPSK optical modulator, and an RZ optical modulator.

  As a second search method, a control voltage that maximizes or minimizes the current from the light receiver is searched based on the DC component of the measured value of the current from the light receiver. Furthermore, as a first specific example, the voltage applied to the target control unit is swept within a certain range, and simultaneously with the sweep, the value of the current from the light receiver is acquired. Among the acquired values of current from the light receiver, the control voltage when the maximum or minimum value is obtained is determined as the point at which the average optical power is maximized or the point at which it is minimized. As a second specific example, the control voltage at the past N points and the current from the light receiver with respect to the control voltage at the N point are acquired and held. Based on the N-point control voltage and the current data sequence from the receiver, a linear approximation is performed using a method such as the least square method, and the current characteristics from the receiver with respect to the control voltage at the current value of the control voltage. Estimate the slope of. The estimated slope is negatively or positively fed back with respect to the target value of zero to search for the maximum point (maximum point) or the minimum point (minimum point) of the current characteristic from the light receiver with respect to the control voltage.

  In this way, even when there are component variations in the RF-Vπ of the LN modulator and the amplitude characteristics of the RF driver, by performing automatic control without adjusting initial values and selecting components in the manufacturing process, A simple controller that can converge to the correct optimum point can be realized, and labor costs in the manufacturing process can be reduced.

  In the embodiment shown in FIGS. 1 to 11, the characteristics of the RF driver are based on the premise that the high and low are symmetrical with respect to the center voltage. The voltage amplitude may be different depending on whether the value is zero or one. Therefore, FIG. 12 shows an embodiment where the RF driver High and Low are asymmetric.

  FIG. 12 shows an example of an optical electric field vector diagram when the RF driver High and Low of the MZA are asymmetric.

  When the RF driver High and Low are asymmetrical, the light passing through the phase modulators 40Aa and 40Ac and the phase modulators 40Ab and 40Ad has zero RF amplitude, and the passing light when only the DC bias voltage is applied (40Ac ( DC), 40Ad (DC)) as a reference, the phase difference is not π / 2. In this case, in the activation control flowchart shown in FIG. 9, after the RF driver power supply in step S18 is turned on, the orthogonality is not maintained between the output light from the MZA and the output light from the MZB, and the optimum point of the RF amplitude And the maximum point of the average optical power does not match, and the optimum point search cannot be performed accurately. This asymmetry depends on the design or manufacturing process of the RF driver. If all manufactured by the same design and manufacturing process show the same characteristics, MZB is the same as MZA There are characteristics. FIG. 13 shows an improvement in the DQPSK activation control flow shown in FIG. 9 by utilizing the same characteristics as MZA for MZB.

  FIG. 13 is an example of a startup control flowchart of the RF amplitude and bias control device corresponding to the asymmetry in the RF drive.

  The difference from FIG. 9 is that when searching for the RF amplitude optimum point of the target MZA in step S39 in FIG. 13, only the RF driver of the MZA is turned on and the power of the MZB RF driver is turned off. Also, in step S41 of FIG. 13, when searching for the RF amplitude optimum point of the target MZB, only the MZB RF driver is turned on, and the MZA RF driver is turned off.

  FIG. 14 is an example of an electric field vector diagram for explaining the DQPSK activation control flowchart of FIG.

  (a) is an electric field vector diagram of light at the time of step S37 in FIG. 13, (b) is an electric field vector diagram of light at the time of step S39 in FIG. 13, and (c) is a graph of step S41 in FIG. FIG. 14 is an electric field vector diagram of light at a time point, and FIG. 14D is an electric field vector diagram of light at a time point of step S42 in FIG.

  (a) shows a case where the RF driver power is turned off for both MZA and MZB (RF amplitude is zero). Also, the phase modulation unit for applying the MZA DC bias voltage inside the DQPSK optical modulator is 40Ac (DC) and 40Ad (DC), and the phase modulation unit for applying the MZB DC bias voltage is 40Bb (DC) and 40Bd ( DC). Here, 40Ac (direct current), 40Ad (direct current), 40Bb (direct current), 40Bd (direct current) are phase modulation units when the power of the RF driver is turned off (RF amplitude is zero) with the optimum DC bias voltage. is there.

  Since the electric field vectors of 40Ac (direct current) and 40Ad (direct current) are directed to cancel each other, the light passing through the phase modulation unit 40Ac (direct current) and the phase modulation unit 40Ad (direct current) may cancel each other. I can confirm. Further, the combined light VMZA of the electric field vectors of 40Ac (DC) and 40Ad (DC) becomes a zero vector.

  Further, since the electric field vectors of 40Bb (direct current) and 40Bd (direct current) are directed to cancel each other, light passing through the phase modulation unit 40Bb (direct current) and the phase modulation unit 40Bd (direct current) cancel each other. I can confirm that. Further, the combined light VMZB of the electric field vectors of 40Bb (direct current) and 40Bd (direct current) becomes a zero vector.

  Therefore, the output light from MZA and MZB becomes a zero vector. Further, it can be confirmed that the electric field vector of the light passing through the phase modulation unit 40Ac and the phase modulation unit 40Ad is orthogonal to the electric field vector of the light passing through the phase modulation unit 40Bb and the phase modulation unit 40Ad.

  (b) shows the case where the RF driver power supply of the MZA is turned on and the power supply of the MZB RF driver is turned off. When the data input to the RF driver is set to zero and the power of the RF driver is turned on, the electric field vector of MZA inside the DQPSK optical modulator 40 is upward in the phase modulation unit 40Aa and the phase modulation unit 40Ab. Can be confirmed. On the other hand, when the data input to the RF driver is set to 1 and the power supply of the RF driver is turned on, the electric field vector of MZA in the DQPSK optical modulator 40 in the phase modulation unit 40Aa and the phase modulation unit 40Ab It can be confirmed that it is suitable. When the power of the RF driver is turned off, since the power is turned off, the electric field vectors of the phase modulation unit 40Aa and the phase modulation unit 40Ab are horizontal lines. Moreover, it can confirm that it has faced the opposite direction. In addition, electric field vectors in various states can be confirmed.

  (c) is a case where the RF driver power supply of MZA is turned off and the power supply of MZB RF driver is turned on. Compared with (b), it can be confirmed that (c) is an electric field vector diagram retreated by π / 2.

  (d) shows the case where the power supply of the RF driver is turned on for both MZA and MZB. The MZA vector when the data input to the MZA is zero and the MZB vector when the data input to the MZB is zero are orthogonal and the MZA vector when the data input to the MZA is 1, and the MZB Since the MZB vector when the input data is 1 is orthogonal, it can be confirmed that the electric field vectors of the MZA output light and the MZB output light are orthogonal.

  As described above, according to the present invention, by providing a simple voltage control unit that can converge the RF amplitude adjustment voltage and the DC bias voltage applied to the Mach-Zehnder modulator by automatic control to the optimum point, LN modulation is performed. Labor costs in the manufacturing process can be reduced by performing automatic control without adjusting initial values or selecting parts in the manufacturing process even when there are component variations in the RF-Vπ and RF driver amplitude characteristics. Can be reduced.

DESCRIPTION OF SYMBOLS 10 DPSK optical modulator 20, 22, 23 Voltage control part 16a Amplitude adjustment voltage means 16b DC bias voltage control means 20a, 22a, 23a Switch control means 21 Switch 21a, 21b Switch 40 DQPSK optical modulator

Claims (3)

In an optical transmitter having a Mach-Zehnder modulator provided in a transmission path for transmitting modulation data, and a voltage controller for controlling an amplitude adjustment voltage and a DC bias voltage applied to the Mach-Zehnder modulator,
A switch provided in a power supply path of a driver power supply for driving the Mach-Zehnder modulator;
An optical transmitter, comprising: a switch control unit that is provided in the voltage control unit and controls on / off of the switch. The switch control means includes
When the value of the DC bias voltage is indefinite, the switch is controlled to be turned off,
2. The optical transmitter according to claim 1, wherein the switch is turned on when the value of the DC bias voltage is at an optimum point.   3. The optical transmitter according to claim 1, wherein a DPSK optical modulator or a DQPSK optical modulator using a Mach-Zehnder type dielectric crystal LiNbO3 is used as the Mach-Zehnder modulator.

Images