JP4745096B2 - Drive device for optical modulator - Google Patents

Description

  The present invention relates to an optical modulator driving device, and more particularly to an optical modulator driving device that applies an NRZ modulation signal and a clock pulse to an optical modulator having two Mach-Zehnder type optical waveguides.

  Conventionally, in the field of optical communication, a waveguide-type optical modulator in which an optical waveguide and a modulation electrode are formed on a substrate having an electro-optic effect has been widely used. In such an optical modulator, an NRZ (Non Return to Zero) modulation signal is used for an optical modulator having two Mach-Zehnder type optical waveguides in order to realize high-speed, large-capacity communication and stable long-distance transmission. And a clock pulse are applied to output an RZ (Return to Zero) optical modulation signal from the optical modulator.

FIG. 1 is a diagram showing an outline of an optical modulation method for outputting an RZ optical signal from a conventional NRZ modulation signal.
Continuous wave (CW) light is incident on an optical modulator in which two Mach-Zehnder type optical waveguides 1 and 2 are connected in series, and an NRZ optical signal is formed in the optical modulation unit including the first Mach-Zehnder type optical waveguide 1. Further, the light modulation unit including the second Mach-Zehnder type optical waveguide 2 is modulated into an RZ optical signal and emitted from the light modulator.

In the optical modulation unit including the first Mach-Zehnder type optical waveguide 1, a modulation signal of NRZ data is output from the signal source 5, and the modulation signal is amplified by the driver circuit 6 and applied to the modulation electrode 3.
In the optical modulation unit including the second Mach-Zehnder type optical waveguide 2, a clock pulse is output from the clock generator 7, and the pulse is amplified by the driver circuit 8 and applied to the modulation electrode 4.

  FIG. 2 shows an optical modulation method for outputting an RZ optical signal from an NRZ modulation signal, and in particular, a clock pulse having a frequency half that of the RZ optical signal is applied to the bottom of the modulation curve of the optical modulator. It is a figure which shows the outline of a light modulation system. In other words, the RZ optical signal spectrum is configured to be a CS (Carrier Suppressed) RZ optical signal in which the carrier wave is suppressed.

Similar to the optical modulator of FIG. 1, the optical modulation unit having two Mach-Zehnder type optical waveguides 10 and 11 and including the first Mach-Zehnder type optical waveguide 10 modulates NRZ data output from the signal source 15. The signal is amplified by the driver circuit 16 and applied to the modulation electrode 12.
On the other hand, in the optical modulation unit including the second Mach-Zehnder type optical waveguide 11, the modulation electrodes 13 and 14 are provided for the two branch waveguides constituting the Mach-Zehnder type optical waveguide. Clock pulses having a frequency half that of the RZ optical signal output from the optical modulator and having opposite phases are applied.

As the clock pulse applied to the modulation electrodes 13 and 14, the clock pulse output from the clock generator 17 is input to the 180 ° hybrid circuit 18, and two clock pulses in opposite phases are output from the 180 ° hybrid circuit 18. The signal is amplified by the driver circuits 19 and 20 and applied to the modulation electrodes 13 and 14.
An optical modulator that outputs a CS-RZ optical signal as shown in FIG. 2 is disclosed in Patent Document 1 or 2 below.
JP 2001-147411 A Japanese Patent Laid-Open No. 2002-23121

  In the optical modulation system shown in FIG. 1 or FIG. 2, two modulation signal generating means, ie, signal sources 5 and 15 and clock pulse generators 7 and 17 are required as means for generating a modulation signal, and the two are synchronized. Therefore, some synchronization means 9, 21 such as a synchronization circuit is required. For this reason, the driving device of the optical modulator is complicated, which causes high cost.

On the other hand, in Patent Document 3, as shown in FIG. 3, the NRZ modulation signal from the signal source 34 is branched into two, and one is amplified via the driver circuit 35 to obtain the first Mach-Zehnder type optical waveguide. The light is applied to the modulation electrode 32 of the light modulation unit including the waveguide 30. The other modulation signal is converted into a clock pulse through the clock extraction unit 36, controlled to a predetermined phase by the phase adjustment unit 37, then amplified by the driver circuit 38, and the second Mach-Zehnder optical waveguide 31. Is applied to the modulation electrode 33 of the light modulation section.
JP 2002-311400 A

In the optical modulation system of FIG. 3, the signal from the signal source 34 is directly input to the driver circuit 35 and amplified, so that if the signal quality from the signal source 34 deteriorates, the signal is output from the optical modulator. Degradation of the optical signal is significant. That is, the quality of the signal from the signal source has directly affected the performance of the optical modulator.
Further, as a means for generating a modulation signal as shown in FIG. 1 or FIG. 2, not only when a signal source and a clock pulse generator are used, but also when a single signal source is used as shown in FIG. The modulation signal to be applied to the modulation electrode of the optical modulation unit including each Mach-Zehnder type optical waveguide needs to be accurately adjusted in phase, and depending on the temperature change and the connection state of the signal line to the optical modulator, The quality and phase are easily changed, and a proper RZ optical signal cannot be obtained.

  The problem to be solved by the present invention is to solve the above-mentioned problems, and in an optical modulator driving device in which two Mach-Zehnder type optical waveguides are directly connected, while suppressing complexity and cost increase of the device. To provide a drive device for an optical modulator that can apply a modulated signal whose phase is accurately adjusted to the modulation electrodes of the two Mach-Zehnder type optical waveguides, and that suppresses the deterioration of the optical signal output from the optical modulator. .

In the invention according to claim 1, two Mach-Zehnder type optical waveguides are arranged in series, and a modulation electrode for modulating light passing through each Mach-Zehnder type optical waveguide is provided for each Mach-Zehnder type optical waveguide. For the optical modulator, an NRZ modulation signal is input to the first modulation electrode of one Mach-Zehnder optical waveguide, and a clock pulse is input to the second modulation electrode of the other Mach-Zehnder optical waveguide, the drive device for an optical modulator for outputting the RZ optical signal from the optical modulator, NRZ modulation signal output from a single signal source is inputted, the rewritable outputs NRZ modulated signal waveform shaping, clock extraction function and Carlo logic circuit having a, a first driver circuit for amplifying the NRZ modulation signal outputted from the logic circuit, click outputted from said logic circuit A second driver circuit for amplifying the clock pulse, and applying the output of the first driver circuit to the first modulation electrode and applying the output of the second driver circuit to the second modulation electrode The logic circuit and at least the first and second driver circuits are formed on the same connection substrate.

  The optical modulator to which the present invention is applied is not limited to one using an NRZ modulation signal in the upstream Mach-Zehnder type optical waveguide, as viewed from the traveling direction of light, but the upstream Mach-Zehnder type optical waveguide. A clock pulse may be used for the waveguide, and an NRZ modulation signal may be used for the downstream Mach-Zehnder type optical waveguide. It is also possible to use two optical modulators having one Mach-Zehnder type optical waveguide and connect them in series to form two Mach-Zehnder type optical waveguides.

  Further, not only a clock pulse having the same frequency as that of the NRZ modulation signal is applied to the second modulation electrode as shown in FIG. 1, but also a clock pulse having a frequency half that of the frequency as shown in FIG. It is also possible to apply by dividing the frequency.

The invention according to claim 2, in the driving apparatus for an optical modulator according to claim 1, between a first or second modulation electrodes from said logic circuit, and having an inverter circuit.

  According to the first aspect of the present invention, since the NRZ modulation signal and the clock pulse are generated using the logic circuit to which the NRZ modulation signal output from the single signal source is input, the modulation signal generating means One is sufficient, and no means for synchronizing the NRZ modulation signal and the clock pulse is required. As a result, the drive device for the optical modulator can be simplified, and an increase in cost can be suppressed.

In addition, since the logic circuit is a waveform shaping circuit having a clock extraction function, even if the quality of the signal from the signal source deteriorates, the logic circuit shapes the signal into an appropriate signal and the clock extracted by the logic circuit. Together with the pulse, it becomes possible to generate an optical signal with extremely high quality.
Further , since the logic circuit and the first and second driver circuits are formed on the same connection board, the signal line introduced into the connection board is only the signal line of the NRZ modulation signal, so that the connection state of the signal lines The signal line from the logic circuit to each modulation electrode to each modulation electrode can always be kept in a predetermined state on the same substrate. It is possible to apply a modulation signal whose phase is accurately adjusted. In addition, since the signal lines for determining the phase state are formed on the same substrate, it is possible to provide an optical modulator driving device that has a small temperature difference between the circuits and is extremely stable against temperature changes.

The invention according to claim 2, between the logic circuit of the first or second modulation electrodes, for an inverter circuits, a clock pulse output from the logic circuit, is output in NRZ modulation signal and antiphase despite that, more inverter circuits, and a NRZ modulated signal and the clock pulse, it is possible to apply in a proper phase state to each of the modulation electrodes.

Hereinafter, the present invention will be described in detail using preferred examples.
FIG. 4 shows an outline of an optical modulator driving apparatus according to the present invention.
The optical modulator used in the present invention has two Mach-Zehnder type optical waveguides 40 and 41 arranged in series. However, two optical modulators having one Mach-Zehnder type optical waveguide may be connected in series.
Modulation electrodes 42 and 43 are formed in the light modulation section including each Mach-Zehnder type optical waveguide. The modulation electrode includes a signal electrode that applies a modulation signal and a ground electrode that is grounded.

  Also, the modulation electrode can be provided independently for the two branch optical waveguides of the Mach-Zehnder type optical waveguide as shown in FIG. In this case, a clock pulse output from a logic circuit 45, which will be described later, is converted to a half frequency by a frequency divider and input to the 180 ° hybrid circuit shown in FIG. Two clock pulses in opposite phases can be output, and each signal can be amplified by a driver circuit and applied to each modulation electrode. A differential output type driver circuit incorporating a 180 ° hybrid circuit and a driver circuit can also be used.

The modulation signal applied to the modulation electrodes 42 and 43 is generated by a single signal source 44. An NRZ modulation signal is output from the signal source 44 and input to the logic circuit 45.
As the logic circuit 45 used in the present invention, a waveform shaping circuit having a clock extraction function is used. This is not only to extract the clock pulse from the NRZ modulation signal, but even if the NRZ modulation signal is degraded, it is shaped into an appropriate signal by the logic circuit and combined with the clock pulse extracted by the logic circuit. It is possible to supply a modulated signal with high quality. Thereby, an extremely high quality optical signal can be generated.
The logic circuit 45 outputs an NRZ modulation signal corresponding to the input signal and a clock pulse having the same frequency as the frequency of the input signal. However, the phase of the output NRZ modulation signal and the clock pulse is in an opposite phase state.

The NRZ modulation signal output from the logic circuit 45 is amplified to a predetermined value by the driver circuit 46 and applied to the first modulation electrode 42 of the first Mach-Zehnder optical waveguide 40.
On the other hand, the clock pulse output from the logic circuit 45 is introduced into the driver circuit 49 through the inverter circuit 47 and the phase adjuster 48, amplified to a predetermined signal intensity, and then the second pulse of the second Mach-Zehnder type optical waveguide. Applied to the modulation electrode 43.

The inverter circuit 47 and the phase adjuster 48 are not indispensable in the present invention. The length of the signal line from the logic circuit 45 to the first modulation electrode 42 and the signal from the logic circuit 45 to the second modulation electrode 43 are not necessary. The length of the line can be omitted if the phase of the modulation signal applied to each modulation electrode can be set in a predetermined relationship.
Further, the inverter circuit and the phase adjuster can be provided on the first modulation electrode side using the NRZ modulation signal.

With the above configuration, CW light is incident on the first Mach-Zehnder optical waveguide to form an NRZ optical signal, and the NRZ optical signal is incident on the second Mach-Zehnder optical waveguide to convert it to an RZ optical signal. Output from the modulator.
The optical modulator driving device of the present invention is not limited to applying an NRZ modulation signal to the first modulation electrode 42 as shown in FIG. 4, but applies a clock pulse to the first modulation electrode 42, and the second It is also possible to apply an NRZ modulation signal to the modulation electrode.

Next, the case where the connection substrate 110 is used in the drive device of the present invention will be described with reference to FIG.
Reference numeral 100 denotes a substrate having an electro-optic effect, and two Mach-Zehnder type optical waveguides 101 and 102 are formed on the substrate. Further, a modulation electrode for modulating light passing through each Mach-Zehnder type optical waveguide is formed. Here, in order to simplify the drawing, only the signal electrodes 103 and 104 are shown. The material and structure constituting the optical modulator are not particularly limited. For example, as a substrate having an electrooptic effect, lithium niobate, lithium tantalate, PLZT (lead lanthanum zirconate titanate), And quartz based materials are available. The optical waveguide on the substrate can be formed by diffusing Ti or the like on the substrate surface by a thermal diffusion method or a proton exchange method. Further, the signal electrode, the ground electrode, and the like constituting the modulation electrode can be formed by forming a Ti / Au electrode pattern, a gold plating method, or the like. Furthermore, if necessary, a buffer layer such as dielectric SiO ₂ can be provided on the surface of the substrate after the optical waveguide is formed.

A connection substrate 110 made of a low dielectric loss material such as alumina or aluminum nitride is disposed beside the optical modulator. On the connection substrate 110, a logic circuit 111 and driver circuits 112 and 114 are arranged, and a phase adjusting means 113 such as an inverter circuit or a phase adjuster is provided as necessary in detail as described in the description of FIG. Has been placed.
Terminators 130 and 131 are further arranged near the optical modulator to prevent reflection of the modulated signal propagating through the signal electrodes 103 and 104.

The optical modulator, the connection substrate 110, and the terminators 130 and 131 are hermetically sealed in a housing 140 made of metal or the like to constitute an optical modulator module.
An input optical fiber 105 and an output optical fiber 106 are connected to the optical modulator from the outside of the housing 140. Further, the signal line of the signal source 120 is introduced into the connection substrate 110 from the outside of the housing 140 and is connected to a line connected to the logic circuit 111.

Thus, the signal line introduced into the optical modulator module is only one of the signal lines for introducing the NRZ modulation signal from the signal source 120, and the connection work is easy and the two Mach-Zehnder type optical waveguides are connected. There is no need to consider the phase shift of the modulation signal applied to each modulation electrode.
In addition, a logic circuit, a driver circuit, and the like are arranged on the connection substrate 110, and the state of the signal line from the logic circuit to each modulation electrode through each driver circuit is always in a predetermined state on the same substrate. It is possible to hold it. Thereby, it is possible to apply a modulation signal whose phase is accurately adjusted to each modulation electrode. Furthermore, since the signal lines and circuit elements that determine the phase state are formed on the same substrate, it is possible to provide an optical modulator driving device that is extremely stable against temperature changes.

  As described above, according to the present invention, in an optical modulator driving device in which two Mach-Zehnder type optical waveguides are directly connected, the complexity and cost of the device are suppressed, and the two Mach-Zehnder type optical waveguides are suppressed. Therefore, it is possible to provide an optical modulator driving apparatus that can apply a modulated signal whose phase is accurately adjusted to the modulation electrode according to the present invention, and that suppresses deterioration of the optical signal output from the optical modulator.

It is the schematic which shows the conventional RZ light modulation system. It is the schematic which shows the conventional RZ light modulation system, and shows what uses a 1/2 frequency for a clock pulse. It is the schematic which shows the conventional RZ light modulation system, and shows what produces | generates a clock pulse using an NRZ modulation signal. It is the schematic of the drive device of the optical modulator which concerns on this invention. It is a figure which shows what uses a connection board | substrate for the drive device of the optical modulator which concerns on this invention.

Explanation of symbols

1, 2, 10, 11, 30, 31, 40, 41, 101, 102 Mach-Zehnder type optical waveguides 3, 4, 12, 13, 14, 32, 33, 42, 43 Modulating electrodes 5, 15, 34, 44, 120 NRZ modulation signal source 7, 17 clock pulse generator 6, 8, 16, 19, 20, 35, 38, 46, 49, 112, 114 driver circuit 9, 21 synchronization means 18 180 ° hybrid circuit 36 clock extraction Unit 37 phase adjusting unit 45, 111 logic circuit 47 inverter circuit 48 phase adjuster 100 optical modulator substrate 103, 104 signal electrode 105, 106 optical fiber 110 connection substrate 113 phase adjusting means 130, 131 terminator 140 optical modulator module Enclosure

Claims (2)

Two Mach-Zehnder type optical waveguides are arranged in series, and one modulation modulator for modulating light passing through each Mach-Zehnder type optical waveguide is provided for each Mach-Zehnder type optical waveguide. An NRZ modulation signal is input to the first modulation electrode related to the Mach-Zehnder optical waveguide, a clock pulse is input to the second modulation electrode related to the other Mach-Zehnder optical waveguide, and an RZ optical signal is received from the optical modulator. In the drive device of the optical modulator to be output,
NRZ modulated signal outputted from a single signal source is inputted, and Carlo logic circuit Yusuke The rewritable outputs NRZ modulated signal waveform shaping, clock extraction function,
A first driver circuit for amplifying an NRZ modulation signal output from the logic circuit;
A second driver circuit for amplifying a clock pulse output from the logic circuit;
The output of the first driver circuit is applied to the first modulation electrode, and the output of the second driver circuit is applied to the second modulation electrode.
At least the logic circuit and the first and second driver circuits are formed on the same connection substrate. In the drive apparatus for an optical modulator according to claim 1, between a first or second modulation electrodes from said logic circuit, an optical modulator driving apparatus characterized by having an inverter circuit.

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