JP2011166249A - Optical transmitter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical transmitter which can carry out high bit rate optical transmission by reducing the load of an electric device, and can prevent deterioration of transmission quality of the optical transmission waveform by changes over time, environmental fluctuations, or the like. <P>SOLUTION: The optical transmitter equipped with at least two DQPSK optical modulators is further provided with: a 1/2 wavelength plate which receives the output from one DQPSK optical modulator; and a polarized beam combiner which performs cross polarization multiplexing by receiving the output from the 1/2 wavelength plate and the output from the other DQPSK optical modulator; and a voltage control unit which controls a bias voltage to be applied to the phase modulation unit of the DQPSK optical modulator by the output from the polarized beam combiner. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an optical transmitter used for optical transmission, and more particularly to an optical communication system that transmits differential quadrature phase-shift keying (hereinafter referred to as DQPSK) signal light using a polarization multiplexing method. The present invention relates to an optical transmitter.

  In recent years, instead of the conventional 10 Gbps transmission system, construction of an optical transmission system of 40 Gbps or more is required. Therefore, as a method for transmitting high-speed optical signals exceeding 40 Gbit / s over long distances, it has excellent noise immunity, is resistant to signal waveform distortion caused by chromatic dispersion and polarization dispersion, and aims to secure a transmission capacity equivalent to or higher than conventional ones. The DQPSK light modulation method is known. In recent years, the increase in transmission capacity has accelerated, and in the future, an optical transceiver exceeding 100 Gbit / s will be required. In the DQPSK optical modulation system, a DQPSK modulator including two sets of differential phase modulators is used. Currently, optical transmitters that do not place a load on electrical devices by polarization multiplexing are being studied.

  FIG. 6 is a block diagram showing a configuration example of a conventional optical transmitter.

  In FIG. 6, the DQPSK optical modulator 1 has optical waveguides branched into two systems, and each of these optical waveguides has a Mach-Zehnder (MZ) modulator (hereinafter referred to as an MZ modulator). Each is connected in parallel. Here, the MZ modulators connected in parallel are MZ11I and MZ11Q.

  A light source 2 is connected to one end of the optical waveguide of the DQPSK optical modulator 1. This optical waveguide is branched into two systems, and the output side of the MZ11Q and the phase shift unit 12 are connected to each other via the MZ11I and MZ11Q. A DQPSK precoder 3 is differentially connected to optical waveguides branched into two systems of the DQPSK optical modulator via amplifiers 4I and 4Q.

  The DQPSK precoder 3 precodes the data input to the transmitter so that the data received by the DQPSK receiver becomes desired data. This precoded signal is input to the DQPSK optical modulator 1 via the amplifiers 4I and 4Q. The DQPSK modulator 1 performs optical phase modulation on the continuous light output from the light source 2 with the precoded data signal output from the DQPSK precoder 3. Each data output from MZ11I and MZ11Q is assigned a state in which the optical phase difference from the previous bit is 0 ° and 180 °. Further, in the MZ 11, the optical phase is shifted by 90 ° by the phase shift unit 12. That is, the output from the phase shift unit 12 is assigned two phase states of 90 ° and 270 °. Data phase-modulated by the MZ modulators of MZ11I and MZ11Q are combined and interfered by the DQPSK modulator 1, and generated as a DQPSK optical signal. Here, there are four phase states, that is, 45 °, 135 °, 225 °, and 315 °.

  Patent Document 1 proposes an optical receiver that can be miniaturized with a simple configuration and that has low power consumption and low cost for the DQPSK wave multiplex system.

JP 2009-182888

  However, such an optical transmitter has the following problems. In a 40 Gbit / s DQPSK modulation system that is currently commercialized, a pair of differential phase modulators handles 20 Gbit / s data. Therefore, in order to realize a 100 Gbit / s DQPSK modulation method, it is necessary to handle 50 Gbit / s data with a pair of differential phase modulators, and there is a problem that a large load is applied to the electric device to be used.

  Moreover, in order to implement | achieve an apparatus by giving a great load to the electric device to be used, there exists a problem that the expense corresponding to a great load starts.

  Furthermore, there is a problem that it is more difficult to maintain the transmission quality of the optical transmission waveform as the data transmission speed increases.

  An object of the present invention is to provide a plurality of DQPSK optical modulators and to control a bias voltage applied to a half-wave plate, a polarization beam combiner, and a phase modulation unit of the optical modulator on the output side of the DQPSK optical modulator By providing a bias voltage control unit, DQPSK signals can be orthogonally polarized and multiplexed to reduce the load on electrical devices and perform high-speed bit-rate optical transmission. In addition, optical transmission waveforms due to aging and environmental variations It is to realize an optical transmitter capable of preventing degradation of transmission quality.

In order to solve such a problem, the invention according to claim 1 of the present invention,
In an optical transmitter equipped with a DQPSK optical modulator that performs phase modulation,
Provide at least two DQPSK optical modulators,
A half-wave plate for inputting the output from one of the DQPSK optical modulators,
The output from the half-wave plate and the output from the other DQPSK optical modulator are input, and a polarization beam combiner that performs orthogonal polarization multiplexing,
And a voltage control unit for controlling a bias voltage applied to the phase modulation unit of the DQPSK optical modulator based on an output from the polarization beam combiner.

The invention according to claim 2
Provide at least two phase modulation transmitters that perform phase modulation with pre-coded signals for continuous light,
A half-wave plate for inputting the output from one of the phase modulation transmitters;
The output from this half-wave plate and the output from the other phase modulation transmitter are input, a polarization beam combiner that performs orthogonal polarization multiplexing,
And a voltage control unit for controlling a bias voltage applied to the phase modulation unit of the phase modulation transmitter according to an output from the polarization beam combiner.

The invention according to claim 3
Provide at least two intensity modulation transmitters that perform intensity modulation with signals encoded for continuous light,
A half-wave plate for inputting the output from one of the intensity modulation transmitters;
The output from this half-wave plate and the output from the other intensity modulation transmitter are input, a polarization beam combiner that performs orthogonal polarization multiplexing,
A voltage control unit that controls a bias voltage applied to the intensity modulation unit of the intensity modulation transmitter is provided based on an output from the polarization beam combiner, and intensity modulation is performed.

  According to the present invention, by providing a half-wave plate, a polarization beam combiner, and a bias voltage control unit that controls a bias voltage applied to the phase modulation unit of the optical modulator on the output side of the DQPSK optical modulator. Because it can process at half the transmission bit rate, the load on the electrical device can be greatly reduced, the timing of skew adjustment etc. can be relaxed, and the transmission quality of the optical transmission waveform can be prevented from deteriorating due to aging and environmental fluctuations. Can do.

It is the block diagram which showed one Example of this invention. It is a block diagram which shows an example of the simulation block at the time of optical transmitter operation | movement confirmation. It is a block diagram of the other Example of this invention. It is a block diagram of the other Example of this invention. It is a block diagram which shows an example of a structure of a polarization multiplexing receiver. It is the block diagram which showed the structural example of the conventional optical transmitter.

  Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of an optical transmitter according to the present invention. The same elements as those in FIG. 6 are denoted by the same reference numerals, and description thereof is omitted.

  Conventionally, one DQPSK optical modulator 1 is provided, whereas in the present invention, two DQPSK optical modulators 1a and 1b are provided in parallel. Further, in the present invention, the half-wave plate 20 for inputting the output from one DQPSK optical modulator 1a, and the output from the other DQPSK optical modulator 1b together with the output from the half-wave plate 20 And a polarization beam combiner 21 (hereinafter referred to as PBC21) that performs orthogonal polarization multiplexing, and a bias voltage applied to each phase modulation unit of the DQPSK optical modulators 1a and 1b by the output from the PBC21 A voltage control unit 22 is provided for controlling the above.

  In the DQPSK optical modulators 1a and 1b, optical waveguides branched into two systems are formed, and an MZ modulator is connected to each of these optical waveguides. Here, it is assumed that the connected MZ modulators are MZ11Ia, MZ11Qa and MZ11Ib, MZ11Qb.

  A light source 2 is connected to one end of the optical waveguide of each of the DQPSK optical modulator 1a and the DQPSK optical modulator 1b. Each optical waveguide is branched into two systems. In the DQPSK optical modulator 1a, the optical waveguide is branched into two systems, MZ11Ia and MZ11Qa. Further, the output side of the MZ11Qa and the phase shift unit 12a are connected. The DQPSK optical modulator 1b is the same as the DQPSK optical modulator 1a. In the DQPSK optical modulator 1b, the optical waveguide is branched into two systems, MZ11Ib and MZ11Qb. Further, the output side of the MZ11Qb and the phase shift unit 12b are connected.

  The DQPSK precoder 3a is connected to optical waveguides branched into two systems of the DQPSK optical modulator 1a via amplifiers 4Ia and 4Qa. The DQPSK precoder 3b is connected to optical waveguides branched into two systems of the DQPSK optical modulator 1b via amplifiers 4Ib and 4Qb.

  The DQPSK precoder 3a precodes the data input to the transmitter so that the data received by the DQPSK receiver becomes predetermined data. The precoded signals are amplified by the amplifiers 4Ia and 4Qa and input to the DQPSK optical modulator 1a. The DQPSK modulator 1a performs optical phase modulation on the continuous light output from the light source 2 with the precoded signal output from the DQPSK precoder 3a. Each data output from MZ11Ia and MZ11Qa is assigned a state in which the optical phase difference from the previous bit is 0 ° and 180 °. In MZ11Qa, the optical phase is shifted by 90 ° by the phase shift unit 12a. That is, two phase states of 90 ° and 270 ° are assigned to the output from the phase shift unit 12a. Data that has undergone phase modulation by the MZ modulators of MZ11Ia and MZ11Qa are combined and interfered by the DQPSK modulator 1 to generate a DQPSK optical signal. Here, there are four phase states, that is, 45 °, 135 °, 225 °, and 315 °. Since the DQPSK optical modulator 1b performs the same operation as the DQPSK optical modulator 1a, the description of the DQPSK optical modulator 1b is omitted.

  Here, the polarization state of light indicates the oscillation direction of the optical electric field. When the electric field component is transverse to the incident surface, it is referred to as TE (Transverse Electric) polarization, hereinafter referred to as TE polarized light, and the magnetic field component is transverse to the incident surface. In this case, TM (Transverse Magnetic) polarization, hereinafter referred to as TM polarization.

  In FIG. 1, the optical outputs from the DQPSK modulators 1a and 1b are output in a TE polarization state due to structural factors of the DQPSK modulators 1a and 1b. The half-wave plate 20 sets the polarization plane to 45 ° with respect to the TE polarized light output from the DQPSK modulator 1a, and enters the TE polarized light from the DQPSK modulator 1a. The polarization plane can be rotated by 90 ° while being held.

  The PBC21 is an element that combines birefringent crystals. In addition, the PBC 21 includes an output from the half-wave plate 20 provided for the output from one DQPSK optical modulator 1a, that is, TM polarized light whose polarization plane is rotated by 90 °, and the other DQPSK light. An output from the modulator 1b, that is, TE polarized light is input, and orthogonal polarization multiplexing is performed. That is, the polarization direction of the light input to the PBC 21 is matched, and the TM polarization and the TE polarization are incident on the PBC 21 so that the polarization multiplexing of the light can be performed, and the polarization multiplexed light can be output.

  The voltage control unit 22 is a bias voltage feedback unit that automatically controls the bias voltage of the signal that has undergone orthogonal polarization multiplexing in the PBC 21. The polarization multiplexed light from the PBC 21 is input to the voltage controller 22, and feedback control of the bias voltage is performed, so that an adjusted optical adjustment output waveform is output from the DPSK optical modulator 1a. By feedback control, the bias voltage applied to the DPSK optical modulator 1a can be automatically controlled, and deterioration of the transmission quality of the optical transmission waveform due to secular variation or environmental variation can be prevented.

  An optical transmitter can be configured by the DQPSK optical modulators 1a and 1b, the half-wave plate 20, the basic optical element such as the PBC 21, and the voltage controller 22.

  In addition, DQPSK precoders 3a and 3b, amplifiers 4Ia, 4Qa, 4Ib, and 4Qb, and DQPSK optical modulators 1a and 1b are provided, and a half-wave plate 20 is provided for the output of one DQPSK optical modulator 1a. By rotating the output deflection state by 90 °, orthogonal polarization multiplexing can be performed by providing PBC21 on the output side of the half-wave plate 20 and the output side of the DQPSK optical modulator 1b. it can. Since orthogonal polarization multiplexing can be performed, processing can be performed at a bit rate that is half the transmission bit rate, and the load on the electrical device can be greatly reduced. In addition, polarization multiplexed light can be output by performing orthogonal polarization multiplexing in the PBC21.

  Furthermore, since the bit rate to be processed can be made lower than the transmission bit rate, a timing adjustment system such as skew adjustment can be relaxed.

  FIG. 2 is a block diagram showing an example of a simulation block when the operation of the optical transmitter according to the present invention is confirmed. The same elements as those in FIG. 1 are denoted by the same reference numerals and description thereof is omitted.

  The PBC 21a separates the orthogonal polarization multiplexed signal into two linear components whose polarizations are orthogonal by the PBS 21b. Each of the separated signals is converted from a phase modulation signal to an intensity modulation signal by passing through a time delay due to 1-bit optical fibers 23a to 23d and an optical phase delay due to phase shift units 24a to 24d. .

  The intensity-modulated signal is input to photodetectors 25a to 25d having differential inputs, and converted from light to electric signals. The outputs of the photodetectors 25a to 25d are set to an appropriate amplitude when inputted to the amplifiers 26a to 26d, and converted into a digital signal when the appropriately amplified electric signal is inputted to the data buffers 27a to 27d. . When this digital signal is input to MUX28A, 28B and DEMUX29A, 29B, the bit rate is set to an arbitrary speed. When this digital signal is input to error detector 30, the bit error rate is set. Is measured.

  FIG. 3 is a configuration diagram of another embodiment of the present invention.

  The difference from FIG. 1 is that a phase modulation transmitter 31a is provided instead of providing the DQPSK optical modulator 1a. Similarly, instead of providing the DQPSK optical modulator 1b in FIG. 1, a phase modulation transmitter 31b is provided. Since FIG. 3 operates in the same manner as FIG. 1, description thereof is omitted.

  In FIG. 1, the phase modulation method is only supported for DQPSK, whereas in FIG. 3, instead of providing DQPSK optical modulators 1a and 1b, phase modulation transmitters 31a and 31b are provided, so that other than DQPSK. A phase modulation method can also be supported. For example, differential phase shift keying (hereinafter referred to as DPSK) and phase shift keying (hereinafter referred to as PSK) can also be supported.

  Here, the phase modulation transmitters 31a and 31b perform optical phase modulation on the continuous light output from the light source 2 with the precoded signals output from the precoders 3a-1 and 3b-1.

  Here, DPSK is a method for discriminating the phase on the basis of 0 ° every time the phase of the sine wave transmitted previously is transmitted. PSK is one of modulation methods for converting a digital value into an analog signal, and is a method for expressing information by combining a plurality of waves whose phases are shifted.

  The output side of the phase modulation transmitter 31a is connected to the input side of the half-wave plate 32. The output side of the half-wave plate 32 is connected to the PBC 33. The output side of the phase modulation transmitter 31b is connected to the PBC33. With this connection, a circuit for performing orthogonal polarization multiplexing can be configured. Since orthogonal polarization multiplexing can be performed, processing can be performed at a bit rate that is half the transmission bit rate, and the load on the electrical device can be greatly reduced. Furthermore, the timing adjustment system such as skew adjustment can be relaxed.

  Further, the output side of the PBC 33 is connected to the voltage controller 34. The voltage control unit 34 controls the bias voltage applied to each of the phase modulation transmitters 31a and 31b based on the output from the PBC 33. Similar to the voltage control unit 22, the voltage control unit 34 is a bias voltage feedback unit that automatically controls the bias voltage of the signal that has undergone orthogonal polarization multiplexing in the PBC 33. The polarization multiplexed light from the PBC 33 is input to the voltage controller 34, and feedback control of the bias voltage is performed, so that adjusted optical adjustment output waveforms are output from the phase modulation transmitters 31a and 31b, respectively. The bias voltage applied to each of the phase modulation transmitters 31a and 31b can be automatically controlled by feedback control, and deterioration of the transmission quality of the optical transmission waveform due to secular variation or environmental variation can be prevented.

  FIG. 4 is a configuration diagram of another embodiment of the present invention.

  The difference from FIG. 1 is that FIG. 1 is a phase modulation transmitter, whereas FIG. 4 is an intensity modulation transmitter.

  The difference from FIG. 1 is that an intensity modulation transmitter 41a is provided instead of the light source 2, the amplifiers 4Ia, 4Qa, and the DQPSK optical modulator 1a. Similarly, instead of providing the light source 2, the amplifiers 4Ib, 4Qb, and the DQPSK optical modulator 1b in FIG. 1, an intensity modulation transmitter 41b is provided. Since FIG. 4 operates in the same manner as FIG. 1, description thereof is omitted.

  In FIG. 1, phase modulation was performed, whereas in FIG. 4, instead of providing the light source 2, the amplifier 4Ia, 4Qa, and the DQPSK optical modulator 1a, the intensity modulation transmitter 41a was provided, so that the intensity modulation method was adopted. Can also respond. For example, the intensity modulation transmitter includes ON OFF keying (hereinafter referred to as OOK).

  Here, the intensity modulation transmitters 41a and 41b perform intensity modulation on the continuous light output from the light source 2 with the encoded signals output from the encoders 5a and 5b.

  Here, on / off keying is a kind of modulation system that represents digital data by turning on / off light, and is the simplest formation of intensity modulation.

  The output side of the intensity modulation transmitter 41a is connected to the input side of the half-wave plate 32. The output side of the half-wave plate 32 is connected to the PBC 33. The output side of the intensity modulation transmitter 41b is connected to the PBC 33. With this connection, a circuit for performing orthogonal polarization multiplexing can be configured. Since orthogonal polarization multiplexing can be performed, processing can be performed at a bit rate that is half the transmission bit rate, and the load on the electrical device can be greatly reduced. Furthermore, the timing adjustment system such as skew adjustment can be relaxed.

  Further, the output side of the PBC 33 is connected to the voltage controller 34. The voltage control unit 34 controls the bias voltage applied to each of the intensity modulation transmitters 41a and 41b based on the output from the PBC 33. Similar to the voltage control unit 22, the voltage control unit 34 is a bias voltage feedback unit that automatically controls the bias voltage of the signal that has undergone orthogonal polarization multiplexing in the PBC 33. The polarization multiplexed light from the PBC 33 is input to the voltage controller 34, and feedback control of the bias voltage is performed, so that adjusted optical adjustment output waveforms are output from the intensity modulation transmitters 41a and 41b, respectively. The bias voltage applied to each of the intensity modulation transmitters 41a and 41b can be automatically controlled by feedback control, and deterioration of the transmission quality of the optical transmission waveform due to secular variation or environmental variation can be prevented.

  FIG. 5 is a configuration diagram illustrating an example of a configuration of a polarization multiplexing receiver.

  FIG. 5 is a polarization multiplexing receiver that receives a signal from the polarization multiplexing transmitter of FIG. A signal output from the PBC 33 of the polarization multiplexing transmitter of FIG. 3 is input to the PBC 35. In FIG. 4, by providing the PBS 35, the input signal is separated into two types of linearly polarized waves and input to the respective intensity and phase modulation receivers 36a and 36b.

  For example, the intensity modulation receiver is OOK, and the phase modulation transmitter is PSK.

  Note that the present invention assumes a bit rate of 28 Gbit / s for one MZ modulator, that is, the entire system of FIG. 2 assumes a transmission of 112 times Gbit / s. As a result of the simulation, it was confirmed that there was no error.

  As described above, according to the present invention, a DQPSK precoder, an amplifier, and two DQPSK optical modulators are provided, and an output deflection is provided by providing a half-wave plate with respect to the output of one DQPSK optical modulator. By rotating the state by 90 ° and providing a PBC on the output side of the half-wave plate and the output side of the other DQPSK optical modulator, orthogonal polarization multiplexing can be performed.

  Since orthogonal polarization multiplexing can be performed, processing can be performed at a bit rate that is half the transmission bit rate, and the load on the electrical device can be greatly reduced. Furthermore, the timing of skew adjustment and the like can be relaxed.

  The voltage control unit is a bias voltage feedback means for automatically controlling the bias voltage of the signal subjected to orthogonal polarization multiplexing in the PBC. By this feedback control, a DPSK optical modulator, a phase modulation transmitter, or Since the bias voltage applied to each of the intensity modulation transmitters can be automatically controlled, it is possible to prevent the transmission quality of the optical transmission waveform from being deteriorated due to aging fluctuations or environmental fluctuations.

  For this reason, optical DQPSK signals are orthogonally polarization multiplexed to reduce the addition of electrical devices, enabling high-speed bit rates, for example, optical transmission with a bit rate exceeding 100 Gbit / s, as well as aging and environmental fluctuations. Therefore, it is possible to realize an optical transmitter capable of preventing the deterioration of the transmission quality of the optical transmission waveform due to the above.

1a, 1b DQPSK optical modulator
20, 32 1/2 wave plate
21, 21a, 21b, 33 Polarizing beam combiner
22, 34 Voltage controller
31a, 31b intensity and phase modulation transmitter
41a, 41b intensity modulation transmitter

Claims (3)

In an optical transmitter equipped with a DQPSK optical modulator that performs phase modulation,
Provide at least two DQPSK optical modulators,
A half-wave plate for inputting the output from one of the DQPSK optical modulators,
The output from the half-wave plate and the output from the other DQPSK optical modulator are input, and a polarization beam combiner that performs orthogonal polarization multiplexing,
An optical transmitter comprising: a voltage control unit that controls a bias voltage applied to the phase modulation unit of the DQPSK optical modulator based on an output from the polarization beam combiner. Provide at least two phase modulation transmitters that perform phase modulation with pre-coded signals for continuous light,
A half-wave plate for inputting the output from one of the phase modulation transmitters;
The output from this half-wave plate and the output from the other phase modulation transmitter are input, a polarization beam combiner that performs orthogonal polarization multiplexing,
An optical transmitter comprising: a voltage control unit that controls a bias voltage applied to the phase modulation unit of the phase modulation transmitter based on an output from the polarization beam combiner. Provide at least two intensity modulation transmitters that perform intensity modulation with signals encoded for continuous light,
A half-wave plate for inputting the output from one of the intensity modulation transmitters;
The output from this half-wave plate and the output from the other intensity modulation transmitter are input, a polarization beam combiner that performs orthogonal polarization multiplexing,
An optical transmitter comprising: a voltage control unit that controls a bias voltage applied to the intensity modulation unit of the intensity modulation transmitter based on an output from the polarization beam combiner;

Images