JP2010233176A - Optical transmitter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a phase encoding optical transmitter capable of measuring wavelength. <P>SOLUTION: The optical transmitter includes: a light source; a Mach-Zehnder light modulator for output of light with an amplitude specific to modulating signals that are selectively given a first voltage potential and a second voltage potential after light from a light source is input, with the voltage potential gradually changing between the first potential and second potential; and a control means for controlling the Mach-Zehnder light modulator such that, in a communication mode, as the modulating signal turns from the first potential to the second potential, the voltage potential of the light output by the Mach-Zehnder light modulator changes, at the boundary where light output intensity becomes the minimum, from one potential of two voltage potentials that differ from each other by π to the other potential, and the light of one phase and light of the other phase output by the Mach-Zehnder LN light modulator have the same optical power, wherein the control means, in the wavelength measurement mode, performs control different from that performed during communication. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an optical transmitter used for optical transmission.

  Conventionally, a binary intensity modulation system has been widely used in optical transmission systems. The binary intensity modulation method is also called an OOK (On-Off Keying) method, and is a method in which logic “1” and logic “0” of a signal bit to be transmitted correspond to the magnitude of light intensity. This method is widely used because of the advantage that the configuration of the transmitter and the receiver is simple. As another method, an optical duobinary modulation method (ODB method) using three kinds of optical signals, a high intensity signal with an optical phase of 0 °, a high intensity signal with a phase of 180 °, and a small intensity signal is also used. Yes.

  In recent years, an optical phase modulation method that transmits information by an optical phase has been attracting attention by developing a technology for switching an optical phase that has been put to practical use in an optical duobinary modulation method. For example, the binary differential phase shift keying (DPSK) method can configure an optical transmitter / receiver in the simplest form as an optical phase modulation method, and can realize high reception sensitivity that is resistant to noise. It is expected to be put to practical use in the near future because of the advantages, and energetic research is being conducted.

  FIG. 8 is a diagram showing an outline of a conventional optical transmitter. This figure shows an example of a DPSK optical transmitter. A plurality of data is input to the multiplexer 21 from the outside via the data input line 41 and the clock line 42, and the multiplexer 21 uses the CMU (Clock Multiplication Unit) built in the multiplexer 21 based on the plurality of data as a serial data signal. Is generated. The generated serial data signal is input to the differential encoder 22. The differential encoder 22 generates a signal serving as a basis of the modulation signal from the input serial data signal, and inputs the signal to the modulator driver 23. The modulator driver 23 amplifies the input amplitude, generates a modulation signal whose amplitude (potential difference) is twice the half-wave voltage, and converts the generated modulation signal into the Mach-Zehnder niobic acid in the optical modulator module 24. Input to the lithium (LN) light modulator 101. The Mach-Zehnder LN optical modulator 101 receives light from a tunable laser 26 that is a variable wavelength light source, and the Mach-Zehnder LN optical modulator 101 outputs a modulated optical signal through the optical output terminal 12. The wavelength of the tunable laser 26 is controlled by a laser control circuit 27.

  As described above, a Mach-Zehnder LN optical modulator is often used as a phase modulation type optical modulator. One reason is that the LN optical modulator is superior in optical characteristics as compared with the semiconductor optical modulator. Another reason is that the Mach-Zehnder type optical modulator is driven by a modulation signal having an amplitude of about twice the half-wave voltage (Vπ), and even when the driving amplitude of the modulator is changed, it is 180 ° (π). The optical phase shift amount can be maintained. The Mach-Zehnder type optical modulator is an interferometer configured by connecting two phase modulators in parallel.

  However, the Mach-Zehnder type LN optical modulator has a well-known problem that the operating point drifts due to a temperature change, a secular change and the like. In order to cope with this problem, the MZ type LN optical modulator is provided with a terminal for supplying a bias voltage for controlling the operating point in addition to a terminal for inputting a modulation signal from the outside. It is set to automatically follow. The means for setting the bias voltage is called an automatic bias control circuit (hereinafter referred to as ABC circuit). The ABC circuit as a countermeasure against drift is required regardless of the modulation method when the Mach-Zehnder LN optical modulator is used. Different. For example, Patent Document 1 discloses an example of an ABC circuit method when a phase modulation method is employed. By controlling the bias voltage by the ABC circuit, the optical power of the two types of light having the phase of 0 ° and the phase of 180 ° is controlled to be the same and maximum.

  In the example of FIG. 8, the ABC circuit 25 will be described. ABC circuit 25 includes pilot signal generation circuit 105, bias voltage generation circuit 104, adder 108, pilot component detector 106, and bias calculator 107. Part of the light output from the Mach-Zehnder type LN optical modulator 101 is received by the monitoring photodiode 102 and converted into electric power, and the optical power detection circuit 103 converts the electric power into a signal, and the pilot component detector of the ABC circuit 25 106. The pilot component detector 106 detects the frequency component of the pilot signal that changes with time in the optical power from the signal from the optical power detection circuit 103. Thereby, a change in optical power due to the pilot signal can be detected. The bias calculator 107 obtains the bias voltage by feedback control. The adder 108 adds the pilot signal generated by the pilot signal generation circuit 105 and the bias voltage obtained by the bias voltage generation circuit 104. The bias voltage generation circuit 104 generates a voltage having the added value and inputs it to the Mach-Zehnder LN optical modulator 101.

JP 2007-310288 A

  As an application destination of an optical transmitter that employs a phase modulation method such as the DPSK method, for example, there is a long-distance backbone optical transmission system using a wavelength division multiplexing (WDM) technique. This is because, when the phase modulation method is applied to the system, high reception sensitivity which is an advantage thereof can be utilized. In recent years, considering application to such optical transmission systems, so-called tunable optical transmitters that can adjust the output signal wavelength of optical transmitters to any wavelength within a certain range have been widely adopted. It has come to be. Generally, a tunable optical transmitter has a configuration in which an output of a tunable laser that is a wavelength tunable light source is connected to an optical modulator, and an output of the optical modulator is used as an output of the transmitter.

  Here, the output of the optical transmitter is not the output of the laser as a light source itself but an optical signal modulated by the optical modulator. In a tunable optical transmitter having such a configuration, for example, when measuring the wavelength for inspection of the transmitter, the modulated optical signal is transmitted to an optical wavelength measuring device, for example, a wavelength meter using a Michelson interferometer. Connect and measure. However, there is a problem that this wavelength cannot be measured by the optical wavelength measuring device.

  The effect of this problem is not limited to simply examining the output wavelength of a tunable optical transmitter. For example, an optical signal modulated by an optical modulator must be measured during calibration. This is because, in general, when a tunable optical transmitter is manufactured, it is necessary to calibrate the optical wavelength and the optical output level by incorporating a tunable laser into an optical transmitter having a laser drive circuit to emit light.

  The present invention has been made in view of the above problems, and an object thereof is to provide a phase modulation type optical transmitter capable of measuring a wavelength with high accuracy.

Of the inventions disclosed in the present application, the outline of typical ones will be briefly described as follows.

  An optical transmitter according to the present invention is a modulated signal to which a light source and light from the light source are inputted and a first potential and a second potential higher than the first potential are selectively applied. A Mach-Zehnder LN optical modulator that outputs light having an amplitude corresponding to a modulation signal in which the potential gradually changes between the first potential and the second potential; and means for acquiring a control switching signal; When the control switching signal indicates a communication mode, the light output from the Mach-Zehnder LN optical modulator has two phases different from each other by π as the modulation signal goes from the first potential to the second potential. The phase of the other phase from the phase at which the light output intensity is minimum is the boundary, and the optical power of the light of the one phase output from the Mach-Zehnder LN optical modulator and the other phase Same as optical power of phase light Control means for controlling the Mach-Zehnder type LN optical modulator so as to control the Mach-Zehnder type LN optical modulator when the control switching signal indicates a wavelength measurement mode. In contrast, the control at the time of wavelength measurement different from the control at the time of communication is performed.

  In one aspect of the present invention, the control means includes the optical power of the one-phase light output from the Mach-Zehnder LN optical modulator and the other when the control switching signal indicates a wavelength measurement mode. The wavelength measurement time control for controlling the Mach-Zehnder LN optical modulator may be performed so that the optical power of the light having the above phase is different.

  In one aspect of the present invention, the control means sets an operating point corresponding to an intermediate point between the first potential and the second potential when the control switching signal indicates the communication mode. When the communication control is performed so that the light output intensity of the optical modulator is the minimum point, and the control switching signal indicates the wavelength measurement mode, the operating point is the light output intensity of the Mach-Zehnder LN optical modulator. The wavelength measurement control may be performed so as not to make the minimum point.

  Also, in one aspect of the present invention, the control means sets the operating point to a point where the optical output intensity of the Mach-Zehnder LN optical modulator is maximum when the control switching signal indicates the wavelength measurement mode. Control at the time of wavelength measurement may be performed.

  In one aspect of the present invention, the potential difference between the first potential and the second potential when the control switching signal indicates the wavelength measurement mode is a half-wave voltage of the Mach-Zehnder LN optical modulator. It may be smaller than twice.

  Also, in one aspect of the present invention, the control means is configured such that, when the control switching signal indicates a wavelength measurement mode, the Mach-Zehnder type as the modulation signal goes from the first potential to the second potential. Control at the time of wavelength measurement for controlling the Mach-Zehnder LN optical modulator so as to pass through a potential at which the optical output intensity becomes maximum without changing the phase of the light output from the LN optical modulator may be performed.

  Further, according to one aspect of the present invention, it further includes monitor means for measuring the optical power of the light output from the Mach-Zehnder LN optical modulator, and the control means generates a modulated signal and outputs the modulated signal. Means, a pilot signal output means for outputting a first pilot signal, and when the control switching signal indicates the communication mode, the first pilot signal is output as the second pilot signal, and the control When the switching signal indicates the wavelength measurement mode, the polarity inversion means for outputting the second pilot signal obtained by inverting the polarity of the first pilot signal, the bias voltage, and the voltage of the second pilot signal And a bias voltage generating means for inputting the voltage to the Mach-Zehnder LN optical modulator, and the light measured by the monitoring means. A pilot component detection means for detecting a change in optical power due to the first pilot signal based on the power, and a first pilot signal and a second pilot signal based on the change in optical power. And bias calculating means for calculating the bias voltage so that the operating point is a point at which the optical output intensity of the Mach-Zehnder LN optical modulator is minimum when the polarities are the same.

  In one aspect of the present invention, the first pilot signal is a low-frequency signal having a frequency lower than that of the modulation signal, and the pilot component detection means is the low-frequency signal included in the optical power measured by the monitoring means. The frequency component of the frequency signal may be detected as the change in the optical power.

  According to the present invention, it is possible to perform wavelength measurement with sufficient accuracy by the light output from the optical transmitter.

It is the block diagram which showed an example of the structure of the optical transmitter-receiver which concerns on embodiment of this invention. It is a figure which shows the structure of the optical transmission part which concerns on embodiment of this invention. It is a figure which shows the relationship between the optical output characteristic of an MZ type | mold LN modulator, and an input-output signal in communication mode. It is a schematic diagram which shows the structure of a Michelson interferometer. It is a schematic diagram which shows the interference fringe produced | generated by the optical signal of a communication mode. It is a figure which shows an example of the relationship between the optical output characteristic of an MZ type | mold LN modulator, and an input-output signal in wavelength measurement mode. It is a figure which shows the other example of the relationship between the optical output characteristic of a MZ type | mold LN modulator, and an input-output signal in wavelength measurement mode. It is a figure which shows the structure of the conventional optical transmitter.

  Hereinafter, embodiments of the present invention will be described step by step with reference to the drawings. Hereinafter, an optical transceiver using a binary DPSK method, which is one of phase modulation methods, will be described.

  FIG. 1 is a block diagram showing an example of the configuration of an optical transceiver 1 according to an embodiment of the present invention. The optical transmitter 2 is on the upper side of FIG. 1 and corresponds to the optical transmitter. An external data input line 41 and a clock line 42 are connected to the multiplexer 21 (MUX) of the optical transmission unit 2 via the connector 11. The multiplexer 21 multiplexes the data input in parallel at a relatively low speed than the data input line 41 and the clock input from the clock line 42, and further uses a CMU (Clock Multiplication Unit) in the multiplexer 21 to generate a high bit. Converted to rate serial data signal. This serial data signal is encoded by the differential encoder 22. As the differential encoder 22, for example, the same encoder as an ODB precoder mounted in an ODB transmitter may be used. Note that the multiplexer 21 may incorporate a differential encoder 22.

  The output of the differential encoder 22 is input to the modulator driver 23 and amplified to an amplitude sufficient to drive a Mach-Zehnder LN optical modulator (hereinafter referred to as MZ type LN modulator) 101. When a Mach-Zehnder type modulator is used, the drive amplitude of the modulator 101 is preferably about twice the half-wave voltage (Vπ) of the Mach-Zehnder type LN modulator. Theoretically, if the amplitude is twice that of the half-wave voltage (Vπ), the optical loss associated with the modulation is minimized. However, even if the drive amplitude is a value far from double, for example, 1 or 3 times, optical phase modulation of 0, π (180 °) can be performed.

  The signal amplified by the modulator driver 23 is input to the MZ type LN modulator 101 as a modulation signal. The MZ type LN modulator 101 receives light from the light source, and outputs light (optical signal) having an amplitude corresponding to the modulation signal and the bias voltage from the ABC circuit 25 via the optical output terminal 12. Here, the amplitude of the light output from the MZ type LN modulator 101 can be both positive and negative. Negative amplitude light is observed as light having a positive amplitude light that is shifted by π.

  Here, the MZ type LN modulator is selected as the optical modulator among the Mach-Zehnder type optical modulators because it has superior optical characteristics. On the other hand, since the MZ type LN modulator has the property of causing drift with temperature change and secular change, the MZ type LN modulator is generally provided with a terminal for obtaining a bias voltage from the outside, and drifts from the outside. It is possible to perform control corresponding to. A circuit for automatically controlling the bias voltage is an automatic bias control circuit 25. The automatic bias control circuit is also called an ABC (Automatic Bias Control) circuit. The ABC circuit 25 is a feedback control circuit that sets the bias voltage of the MZ type LN modulator 101 by monitoring the optical power of the light output from the MZ type LN modulator 101 by the monitoring photodiode (PD) 102. Here, the optical power is the time average of the light output intensity over a time sufficiently longer than the time during which a signal corresponding to 1 bit is transmitted.

  A tunable laser 26 is used as a variable wavelength light source input to the MZ type LN modulator 101. A laser control circuit 27 is connected to the tunable laser 26 in order to control the output of the tunable laser 26. The MZ type LN modulator 101 and the monitoring photodiode 102 constitute an optical modulator module 24.

  An optical receiver 3 is shown on the lower side of FIG. The optical signal input to the optical receiver 3 via the optical input terminal 13 is delayed and detected by the delay interferometer 31. The delay-detected light is input to a balanced PD-TIA (Photodiode-Transfer Impedance Amplifier) 32 and subjected to optical / electrical conversion (O / E conversion). FIG. 1 shows a configuration in which two optical outputs of the delay interferometer are connected to each photodiode (PD) of the balanced PD-TIA 32. Note that an optical signal can be received even in a configuration in which only one output of the delay interferometer is connected to a one-input PD-TIA. In that case, however, in principle, the reception sensitivity is degraded by 3 dB compared to the configuration of FIG. To do. The output of the balanced PD-TIA 32 is input to a clock data recovery (CDR) circuit built in the demultiplexer 34 (DeMux), and clock extraction and data identification / reproduction are performed. The identified and reproduced data is converted into low-speed parallel data and a clock in the demultiplexer 34 and output from the data output line 43 and the clock line 44 of the optical receiver 3 via the connector 11, respectively.

  Hereinafter, the optical transmitter 2 will be described in more detail. FIG. 2 is a diagram showing a configuration of the optical transmission unit 2 according to the embodiment of the present invention. In addition, although demonstrated as the optical transmitter / receiver 1 here, the structure is not changed even if it removes the optical receiver 3 and it is a single optical transmitter. FIG. 2 is a diagram specifically illustrating the configuration of the ABC circuit 25 in detail.

  The data input line 41, clock line 42, multiplexer 21, differential encoder 22, modulator driver 23, optical modulator module 24, tunable laser 26, and laser control circuit 27 shown in FIG. 2 are those described in FIG. Is the same. The ABC circuit 25 includes a pilot signal generation circuit 105, a bias voltage generation circuit 104, an adder 108, a pilot component detector 106, a bias calculator 107, and a polarity inversion circuit 109.

  The pilot signal generation circuit 105 outputs a signal having a frequency sufficiently lower than that of the modulation signal, for example, a low frequency signal of 1 kHz, as a pilot signal for bias control. The pilot signal may be a sine wave or another waveform such as a rectangular wave.

  The polarity inversion circuit 109 is a characteristic part of the present invention, and is means for switching a method of controlling the bias voltage of the MZ type LN modulator 101 performed by the ABC circuit 25. Specifically, the control method is switched between a communication mode for performing data communication and a wavelength measurement mode for performing wavelength measurement. The polarity inversion circuit 109 receives the pilot signal from the pilot signal generation circuit 105 and outputs a signal to the adder 108. The polarity inversion circuit 109 has a function of switching whether the polarity of the output signal is the same as or opposite to that of the pilot signal. A signal for switching the polarity of the output signal is input from the outside of the optical transceiver 1. For example, since a method of operating the optical transceiver 1 or transmitting / receiving data from the outside via an I2C interface is standardized in a 300 pin optical transceiver, a command for switching between the communication mode and the wavelength measurement mode is provided. In addition, it may be sent from the I2C interface to the optical transmitter.

  The polarity inversion circuit 109 outputs a signal having the same polarity as the pilot signal in the communication mode, and outputs a signal obtained by inverting the polarity of the pilot signal in the wavelength measurement mode. The signal output from the polarity inversion circuit 109 is added to the bias voltage to be applied to the MZ type LN modulator 101 calculated by the bias calculator 107 in the adder 108. The added voltage is applied to the MZ type LN modulator 101 via the bias voltage generation circuit 104.

  The MZ type LN modulator 101 outputs light having an amplitude corresponding to the modulation signal input from the modulator driver 23 and the voltage input from the bias voltage generation circuit 104. The monitoring photodiode 102 detects light branched from the optical output of the MZ type LN modulator 101 or leaked light generated when the MZ type LN modulator 101 is extinguished, and converts it into a current. The current converted by the monitoring photodiode 102 is converted into a voltage signal by the optical power detection circuit 103. The signal converted into the voltage signal is input to the pilot component detector 106 of the ABC circuit 25. The pilot component detector 106 detects a change in optical power due to the pilot signal from the voltage signal and the pilot signal input from the pilot signal generation circuit 105. The output from the pilot component detector 106 is input to the bias calculator 107. The bias calculator 107 obtains the bias voltage by feedback control.

FIG. 3 is a diagram showing the relationship between the optical output characteristics of the MZ type LN modulator 101 and the input / output signals in the communication mode. In the upper left of the figure, there is a graph of a characteristic curve showing the relationship between the input signal of the MZ type LN modulator 101 and the optical output intensity. Input signals of the MZ type LN modulator 101 are a bias voltage and a modulation signal. In this figure, the pilot signal is not shown. In the characteristic curve graph, since the vertical axis represents the light output intensity, the phase differs by π at the point where the light output intensity is minimum. The light output at the peak of the potential V ₋₁ is π different in phase from the light output at the peak of the potential V ₁ .

There is a graph of the input signal in the lower left of the figure. The vertical axis is time and the horizontal axis is potential. The graph progresses as you go down. The horizontal axis is shared with the characteristic curve graph. The graph shown represents the modulated signal to the electric potential change corresponding to the amplitude centered on the potential V _0. The modulation signal is selectively given V ₋₁ and V ₁ as its potential. For example, if the bit of the serial data signal is logic “0”, the same potential as the previously applied potential is applied, and if the logic is “1”, a potential different from the previously applied potential is applied. If granted the modulation signal voltage is different from the immediately preceding, i.e. when switched between the potential V _-1 and V _1, (in continuous) the potential is gradually changed by changing. Note that an intermediate potential between the potentials V ₋₁ and V ₁ is V ₀ . Here, where the point corresponding to the potential V ₀ at the center of the amplitude is located on the characteristic curve of the MZ type LN modulator 101 is determined by the bias voltage. This point is called the operating point. In this figure, the bias voltage is controlled so that the operating point is the point where the light output intensity is minimum.

The input signal shown in this figure is a superposition of all possible changes in the input signal by any serial data signal. From the graph, the period in which the potential is not constant time change from V _-1 or _{V 1,} the potential to _{V 1} from V _-1, from _{V 1} to V _-1, remains _{V 1,} and the remains of V _-1 It can be seen that there is a period during which either transition occurs. A period in which the above two periods are set corresponds to a time for transmitting information for one bit transmitted from the optical transmitter. In this embodiment, the potential difference between V ₁ and V ₋₁ is slightly smaller than 2Vπ, which is twice the half-wave voltage. The reason for this will be described later in the description of the wavelength measurement mode.

On the right side of the figure, there is a graph showing the optical output intensity of the optical signal output from the MZ type LN modulator 101. The horizontal axis is time, and the vertical axis is the light output intensity. The vertical axis is shared with the characteristic curve graph. This graph is created by plotting the light output intensity on the characteristic curve at the potential of the input signal (modulation signal) for each time using the two graphs on the left side. As can be seen from the characteristic curve at the upper left of the figure, when the potential of the modulation signal is V ₋₁ or V ₁ , the light output intensity is the maximum (P1), which is a peak of the light output intensity graph of the optical signal. When the potential of the modulation signal is switched between V ₋₁ and V _1, it passes through the potential V ₀ at which the light output intensity is minimized on the characteristic curve, and therefore becomes a valley of the light output intensity graph of the optical signal. . When the potential of the modulation signal does not change between V ₋₁ and V ₁ , the light output intensity remains P1. At the point where the light output intensity is minimum, the components of the light phase 0 and π cancel each other out inside the MZ type LN modulator 101, and if limited to these phases, the amplitude and the light output intensity become 0. Yes.

  Hereinafter, feedback control performed by the ABC circuit 25 in the communication mode will be described in detail. In the communication mode, the bias voltage applied to the MZ type LN modulator 101 is feedback-controlled so that the operating point is a point on the characteristic curve where the light output intensity is minimum. The pilot component detector 106 detects a change in the optical power due to the pilot signal based on the optical power measured by the monitoring photodiode 102 or the like. As a specific example of the pilot component detector 106, there is a synchronous detector that integrates the signal and the pilot signal with a multiplier and passes through a low-pass filter. Then, the frequency component of the pilot signal of optical power is detected as a change in optical power. The bias calculator 107 controls the bias voltage so as to maximize the average value of the optical power output for a fixed time from the change of the pilot signal detected by the pilot component detector 106 and the optical power. More specifically, when the optical power increases when the pilot signal is positive, the bias voltage is increased according to the amount, and even when the optical power is decreased, the bias voltage is decreased according to the amount. The time average value can be controlled to be maximum. A method as described in Patent Document 1 may be used.

Here, when the optical power output becomes maximum, the points on the characteristic curve corresponding to the potentials V ₋₁ and V ₁ of the modulation signal have the same optical output intensity as shown in FIG. 3 in the binary phase modulation method. It will be the maximum point. Since the characteristic curve is symmetric with respect to the potential at which the light output intensity is minimum, the potential V ₀ is the potential at which the light output intensity is minimum, and the light output intensity minimum point is the operating point. The above relationship applies to the binary phase modulation method (theoretically, the amplitude is larger than Vπ and smaller than 3Vπ).

In the communication mode, the optical output intensities of the optical signal (phase 0) corresponding to the potential V _{−1 of the} modulation signal and the optical signal (phase π) corresponding to the potential V ₁ are equally maximized as described above. Further, since the optical phase is 0 and π are generated at the same frequency statistically, the light power is the same for the light of phase 0 and the light of phase π except for the period when the potential is switched. . Further, during the period when the potential of the modulation signal is switched, the change in the light output intensity is symmetric with respect to the potential at the operating point, and the phases are opposite with respect to the potential at the operating point. The power will be the same. That is, the optical power of the phase 0 light output from the MZ type LN modulator 101 is the same as the optical power of the light of phase π.

  Details of the problem that the wavelength cannot be accurately measured when such an optical signal is measured with a wavelength meter will be described below. FIG. 4 is a diagram showing an outline of the configuration of the Michelson interferometer. Michelson interferometers are often used for measuring the wavelength of laser light. In the Michelson interferometer, the light output from the same light source is divided by the half mirror 51 into two routes, the upper direction and the right direction in the figure, reflected by the upper fixed mirror 52 and passed through the half mirror 51 to receive the light receiver 54. , And interference caused by the light reflected by the right movable mirror and reflected downward by the half mirror 51 and reaching the light receiver 54 is measured. The optical path difference can be changed by moving the movable mirror 53, and if the intensity of the interference light for each optical path difference is measured by the light receiver 54, the change in the intensity of the interference light due to the optical path difference is measured in the same manner as the interference fringes. can do. In the Michelson interferometer, the wavelength is calculated by counting the number of peaks and valleys of interference fringes for a length (optical path difference) corresponding to a sufficient number of wavelengths. Since the moving speed of the movable mirror 53 is sufficiently slower than the change of the optical signal, the intensity of the interference light measured by the light receiver 54 is an average of the intensity of the interference light for a sufficiently long time.

  FIG. 5 is a schematic diagram illustrating interference fringes generated by an optical signal in the communication mode. If the signal of the optical transmitter is 40 Gbps, the optical path length corresponding to one bit is about 7.5 mm (hereinafter referred to as α). When the optical path difference is larger than this, it always interferes only with another bit of light. In that case, the case where the phases of the light of the two interfering bits are the same and the case where they are shifted by π occur at the same rate as viewed statistically. Further, as described above, the optical power is the same in both phases. Therefore, the time-averaged interference light intensity distribution is a distribution in which interference fringes of light having the same phase and interference fringes of light having a phase shifted by π are superimposed and averaged. In the interference fringes of light having the same phase and the interference fringes of light having a phase shifted by π, the positions of peaks and valleys of the intensity of the interference light are opposite to each other. Two lines indicated by broken lines in the range larger than the optical path difference α in FIG. 5 indicate these interference fringes. As a result of superimposing two interference lights having the same light intensity amplitude and opposite valleys and valleys, as indicated by a straight line in FIG. 5, when the optical path difference is larger than α, the light intensity is constant regardless of the optical path difference. .

  In a range where the light intensity is constant, the number of peaks and valleys of the interference fringes cannot be counted from the measured light intensity, resulting in poor accuracy of the measured wavelength.

  FIG. 6 is a diagram illustrating an example of the relationship between the optical output characteristics of the MZ type LN modulator and the input / output signals in the wavelength measurement mode. The upper left of the figure is a graph of a characteristic curve showing the relationship between the bias voltage and modulation signal of the MZ type LN modulator 101 and the optical output intensity, which is the same as that described in FIG.

The lower left of the figure is a graph of an input signal, which represents a modulation signal whose potential changes with a certain amplitude around a potential V ₀ that is the center of the amplitude of the modulation signal. The axis of the graph is the same as in FIG. The method of giving the modulation signal is the same as in the communication mode. In this figure, the bias voltage is controlled so that the operating point is the point where the light output intensity is maximum. The potential difference between V ₋₁ and V ₁ is slightly smaller than 2Vπ, which is twice the half-wave voltage as in FIG.

The right side of the figure is a graph showing the optical output intensity of the optical signal output from the MZ type LN modulator 101. The axis of the graph is the same as in FIG. As can be seen from the graph of the characteristic curve, when the potential of the modulation signal is V ₋₁ or V ₁ , the light output intensity is smaller (P0) than when other potentials are switched, and thus the optical output of the optical signal. It becomes the trough of the graph of intensity. When the potential of the modulation signal is switched between V ₋₁ and V _1, it passes through the potential V ₀ at which the light output intensity is maximum on the characteristic curve, and thus becomes a peak of the light output intensity graph of the light signal. . Here, the optical signal corresponding to the peak of the light output intensity is in the form of a pulse train. When the potential of the modulation signal does not change between V ₋₁ and V ₁ , the light output intensity remains P0. As can be seen from FIG. 6, if the amplitude of the modulation signal is smaller than 2Vπ, the phase of the optical signal output from the MZ type LN modulator is the same (π in this figure).

  In the wavelength measurement mode, the bias voltage automatically applied to the modulator by the ABC circuit 25 changes due to the polarity inversion of the pilot signal. Since the optical output intensity characteristic with respect to the input voltage of the MZ type LN modulator is a sine curve, the operating point becomes the point where the optical output intensity is maximum as shown in FIG. The bias voltage is controlled so that

  Then, since only the optical signal having the same phase is output from the MZ type LN modulator 101, the situation where the peaks and valleys of the interference fringes overlap and disappear as in the communication mode does not occur. Therefore, the wavelength can be measured. In the present embodiment, the wavelength can be measured with a simple configuration in which a polarity inversion circuit is provided.

Strictly speaking, when the amplitude of the modulation signal is strictly 2Vπ, light of phase 0 or π is not output when the potential is V ₋₁ or V ₁ , so that there is a possibility that interference does not occur due to the optical path difference. . In order to cope with this, the amplitude of the modulation signal is preferably shifted from 2Vπ. This is because light is output even when the potential is V ₋₁ or V ₁ , and interference occurs although it is small. More specifically, the amplitude of the modulation signal should be smaller than 2Vπ. This is because the light output as shown in FIG. 6 always has the same phase and can be measured without the interference fringes being weakened.

  As shown in FIG. 6, when the bias is set so that the maximum point of the optical output intensity becomes the operating point, the output optical waveform from the MZ type LN modulator 101 is RZ-OOK (Return to Zero-On Off Keying). Similar to the modulation method, it is expressed by the intensity of the light output intensity. For example, in the communication mode of the DPSK transmitter, when an output logic is selected in which the logic “1” is an optical phase change and the logic “0” is an optical phase unchanged, the Hi / Low of the electrical input is an logic “1”. A positive logic RZ-OOK optical signal is obtained in which light is ON because of transition, and Hi / Low of the electrical input does not transition when logic is “0”, and light is OFF. On the other hand, when an output logic is selected in which logic “1” is no change in optical phase and logic “0” is an optical phase change, negative logic in which light is OFF with logic “1” and light ON with logic “0”. RZ-OOK optical signal can be obtained. In the latter case, if the output logic of the differential encoder is inverted in conjunction with the polarity inversion of the pilot signal so that Hi / Low of the electrical input transitions at logic “1” and does not transition at logic “0”, A positive logic RZ-OOK optical signal is obtained.

  In other words, the optical transmitter according to the present embodiment can easily operate as an RZ-OOK transmitter by switching the polarity of the pilot signal. Since RZ-OOK signals can be received by RZ-OOK optical receivers and widely used NRZ-OOK optical receivers, this function can be used for DPSK existing optical transmission devices and optical measuring instruments. A DPSK optical transmitter can be easily connected.

Hereinafter, a method for calibrating a tunable laser in the tunable DPSK optical transmitter according to the present embodiment will be described. First, the reason why calibration is necessary will be described.
For example, in a DFB (Distributed Feed-Back) array type tunable laser that is currently popular, the wavelength and optical power are controlled by the temperature of the laser and the current injected into the laser. The wavelength is measured with a wavelength locker such as an etalon filter mounted inside the tunable laser module, and the optical power is measured with a monitoring photodiode mounted on the tunable laser module or a monitoring photodiode mounted on the modulator module. measure. During the operation of the optical transmitter, the wavelength and optical power are controlled using these measured values. In order to perform this control, it is necessary to know in advance the relationship between the value measured by the wavelength locker, which is a wavelength monitor, and the monitoring photodiode, which is an optical power monitor, and the actual wavelength or optical power. Specifically, the value measured by the wavelength locker and the monitoring photodiode is measured to determine whether the optical wavelength and optical power at the output of the optical transmitter correspond to the specified values. A control target value with the monitor value of the PD that monitors the optical power is determined. Making this determination is the calibration of the tunable laser.

  Therefore, when calibrating a tunable laser, both the optical wavelength and optical power of the transmitter output must be measured. In the conventional OOK system, the optical wavelength and optical power of the transmitter output can be measured simultaneously or sequentially using a wavelength meter and an optical power meter. In the DPSK system, the optical wavelength of the transmitter output cannot be measured accurately during normal operation. First, the optical power is measured in normal operation, and then the pilot signal polarity is inverted as described above to operate the optical modulator ( The wavelength may be measured by moving the bias point. When the optical power is measured again, the polarity inversion of the pilot signal is stopped and the operation as a normal DPSK transmitter is resumed.

  At this time, the automatic bias control is reset and the control is restarted from the initial value of the operating point, or the operating point immediately before the pilot signal polarity inversion is held in the transmitter, and the held value is initialized. It is preferable to resume automatic bias control to the value. The reason for this is that if the pilot polarity is switched repeatedly without resetting the bias voltage control, the modulator bias voltage searches for the maximum and minimum points of the optical output intensity each time the switching is performed, This is because the movement may be repeated and the upper limit or the lower limit of the voltage that can be applied to the modulator may be reached.

  In the description so far, the operating point of the MZ type LN optical modulator is completely inverted in order to measure the wavelength, but the operating point does not necessarily have to be inverted. If the operating point determined by the bias voltage is somewhat deviated from the point where the light output intensity is minimum, the wavelength can be measured.

  FIG. 7 is a diagram showing another example of the relationship between the optical output characteristics of the MZ type LN modulator and the input / output signals in the wavelength measurement mode.

  The upper left of the figure is a graph of a characteristic curve showing the relationship between the bias voltage and modulation signal of the MZ type LN modulator 101 and the optical output intensity, which is the same as that described in FIG.

The lower left of the figure is a graph of the input signal, and represents a modulation signal whose potential changes with an amplitude centered on the potential V ₀ which is the center of the amplitude of the modulation signal. The axis of the graph is the same as in FIG. The method of giving the modulation signal is the same as in the communication mode. In this figure, the bias voltage is controlled so that the operating point deviates from the point where the light output intensity is minimum. The potential difference between V ₋₁ and V ₁ is slightly smaller than 2Vπ, which is twice the half-wave voltage as in FIG. The phases of light output at the potential V ₋₁ and the potential V ₁ are shifted from each other by π. When the potential of the modulation signal switches between V ₋₁ and V ₁ , the light output intensity becomes maximum at the top of the peak of the phase π of the characteristic curve. Also, since the operating point is at the peak of phase π, when switching, the period of outputting phase π light is longer than the period of outputting phase 0 light.

The right side of the figure is a graph showing the optical output intensity of the optical signal output from the MZ type LN modulator 101. The axis of the graph is the same as in FIG. As can be seen from the correspondence with the characteristic curve, the light output intensity stabilizes at Pb after a small peak where the light output intensity starts from the point where the light output intensity is minimum and reaches the maximum (Pa), and then the light output intensity again. The peak that ends at the point where the light output intensity is minimum after passing through a small peak at which the maximum is (Pa) is the peak of the optical signal of phase π. Then, the peak that ends at the point where the light output intensity is minimum is the peak of the optical signal of phase 0. The optical signal of phase π has an optical power greater than that of the optical signal of phase 0 by the amount of time and optical output intensity corresponding to at least two small peak portions in the peak of phase π. As a result, even if the interference fringes having the same phase and the interference fringes having the phase shifted by π are overlapped in the Michelson interferometer, the intensity of the light intensity of the interference fringes having one phase (π in this figure) remains. Therefore, the wavelength can be measured. In addition, when the amplitude is smaller than 2Vπ, the light output intensity Pb at the potential V ₁ becomes larger than the light output intensity Pc at the potential V _−1. In this case, the wavelength remains stronger than in the case of, and the wavelength can be measured more reliably.

  As a specific method of bias control, for example, in the wavelength measurement mode, the bias voltage is shifted by an appropriate amount from the bias voltage applied to the MZ type LN modulator 101 in the communication mode under the control of the ABC circuit 25. There is a method of fixing to the value. Further, it is preferable to provide means for holding the bias voltage value applied immediately before shifting the bias voltage. This is because it is possible to smoothly return to the previous automatic bias control by resetting the bias value held when returning from the release as the initial value of control. In this method, although it is difficult to use the DPSK optical transmitter as a simple RZ-OOK transmitter, the purpose of accurately measuring the optical transmitter output wavelength during laser calibration can be achieved.

1 optical transceiver, 2 optical transmitter, 3 optical receiver, 11 connector, 12 optical output terminal, 13 optical input terminal, 21 multiplexer, 22 differential encoder (differential encoding unit), 23 modulator driver, 24 light Modulator module, 25 Automatic bias control circuit, 26 Tunable laser, 27 Laser control circuit, 31 Delay interferometer, 32 Balanced PD-TIA, 33 Control circuit, 34 Demultiplexer, 41 Data input line, 42, 44 Clock line , 43 Data output line, 51 Half mirror, 52 Fixed mirror, 53 Movable mirror, 54 Light receiver, 101 Mach-Zehnder LN optical modulator, 102 Monitor photodiode, 103 Optical power detection circuit, 104 Bias voltage generation circuit, 105 Pilot Signal generation circuit, 106 pilot component detector, 107 Ass calculator, 108 adder, 109 polarity inversion circuit.

Claims (8)

A light source;
A modulation signal to which light from a light source is input and which is selectively given a first potential and a second potential that is higher than the first potential, the first potential and the second potential A Mach-Zehnder LN optical modulator that outputs light having an amplitude corresponding to a modulation signal whose potential gradually changes between
Means for obtaining a control switching signal;
When the control switching signal indicates a communication mode, the light output from the Mach-Zehnder LN optical modulator has two phases different from each other by π as the modulation signal goes from the first potential to the second potential. The phase of the other phase from the phase at which the light output intensity is minimum is the boundary, and the optical power of the light of the one phase output from the Mach-Zehnder LN optical modulator and the other phase Control means for performing communication control for controlling the Mach-Zehnder LN optical modulator so that the optical power of the phase light is the same, and
The control means, when the control switching signal indicates a wavelength measurement mode, performs a wavelength measurement control different from the communication control for the Mach-Zehnder LN optical modulator.
An optical transmitter characterized by that. When the control switching signal indicates a wavelength measurement mode, the control means determines whether the optical power of the one phase light and the optical power of the other phase light output from the Mach-Zehnder LN optical modulator are The wavelength measurement time control for controlling the Mach-Zehnder LN optical modulator to be different is performed.
The optical transmitter according to claim 1. When the control switching signal indicates the communication mode, the control means sets the operating point corresponding to the middle of the first potential and the second potential to the minimum light output intensity of the Mach-Zehnder LN optical modulator. When the communication control is performed so that the point is a point, and the control switching signal indicates the wavelength measurement mode, the wavelength measurement is performed so that the operating point is not the point where the light output intensity of the Mach-Zehnder LN optical modulator is minimum. Time control,
The optical transmitter according to claim 2. The control means performs the wavelength measurement control so that, when the control switching signal indicates the wavelength measurement mode, the operating point becomes a point of maximum light output intensity of the Mach-Zehnder LN optical modulator,
The optical transmitter according to claim 3. The potential difference between the first potential and the second potential when the control switching signal indicates the wavelength measurement mode is smaller than twice the half-wave voltage of the Mach-Zehnder LN optical modulator,
The optical transmitter according to claim 3, wherein the optical transmitter is provided. When the control switching signal indicates a wavelength measurement mode, the control means is configured to control the light output from the Mach-Zehnder LN optical modulator as the modulation signal goes from the first potential to the second potential. Performing wavelength measurement control for controlling the Mach-Zehnder LN optical modulator so as to pass through a potential at which the light output intensity becomes maximum without changing the phase;
The optical transmitter according to claim 2. Monitoring means for measuring the optical power of the light output from the Mach-Zehnder LN optical modulator,
The control means includes
Modulation signal output means for generating and outputting the modulation signal;
Pilot signal output means for outputting a first pilot signal;
When the control switching signal indicates the communication mode, the first pilot signal is output as the second pilot signal, and when the control switching signal indicates the wavelength measurement mode, the first pilot signal is output. Polarity inversion means for outputting the second pilot signal obtained by inverting the polarity of the pilot signal;
A bias voltage generating means for generating a voltage obtained by adding a bias voltage and the voltage of the second pilot signal, and inputting the voltage to the Mach-Zehnder LN optical modulator;
Pilot component detection means for detecting a change in optical power due to the first pilot signal based on the optical power measured by the monitoring means;
If the first pilot signal and the second pilot signal have the same polarity based on the change in the optical power, the operating point is the minimum optical output intensity of the Mach-Zehnder LN optical modulator. Bias calculating means for calculating the bias voltage so as to be
The optical transmitter according to claim 4. The first pilot signal is a low frequency signal having a lower frequency than the modulation signal;
The pilot component detection means detects a frequency component of the low frequency signal included in the optical power measured by the monitoring means as a change in the optical power,
The optical transmitter according to claim 7.

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