JP2007163941A - Four-phase phase modulation circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a parallel type four-phase phase control circuit capable of always keeping a phase difference π/2 in an orthogonal phase control part constant, thereby evading the occurrenve of receiving sensitivity penalty caused by a phase shift. <P>SOLUTION: The four-phase phase control circuit branches a part of an output optical signal of an optical coupling part, receiving the branched optical signal for a monitor and converting it into an electric signal, and detecting an AC component of power of the converted electric signal, and controls the phase of the orthogonal phase control part so that this power is minimized. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention is used in an optical transmission system using a four-phase phase shift keying code as a modulation code. In particular, the present invention relates to a quadrature control technique for stably controlling the quadrature state of the phase between the in-phase component (I component) and the quadrature component (Q component).

  In a long-distance optical transmission system, a WDM transmission technology that multiplexes and transmits a plurality of wavelengths in one optical fiber is applied, and economical and large-capacity information transmission is realized.

In a WDM transmission apparatus, conventionally, as a modulation / demodulation method, an IM-DD (Intensity Modulation Direct Detection) method in which binary intensity modulation is performed by turning on / off light intensity and detection is directly performed by a photodiode on the reception side is used. It was general. In recent years, multi-level modulation schemes have been studied in order to effectively use a limited optical transmission band and improve frequency utilization efficiency. In particular, differential four-phase phase shift keying (Differential differential) that uses a four-phase modulation method, which is a quaternary phase modulation method, and performs demodulation by a delay interferometer on the receiving side.
Quadrature Phase-Shift Keying (DQPSK) method is considered as an effective method.

  In a four-phase phase modulation circuit, light with constant intensity from a continuously oscillating CW (Continuous Wave) light source is divided into two paths (I component and Q component), and a phase difference of π / 2 is given to one signal light. In general, a parallel-type modulator configuration that generates a four-phase phase modulation signal by applying 0 / π phase modulation and then recombining is applied (see, for example, Non-Patent Document 1).

R. A. Griffin et al. , Technical Digest of OFC2002, Paper WX6

  However, the conventional four-phase phase modulation circuit has the following problems.

As the external modulator, a LiNbO ₃ optical modulator is used. In this case, the phase difference is adjusted by applying a DC voltage to the quadrature modulation unit that gives the phase difference π / 2, but the voltage necessary to give the phase difference of π / 2 drifts with time. It is known to do.

  When the phase difference applied to the quadrature phase modulation unit deviates from π / 2, there is a problem that wavelength degradation occurs when receiving with a delay interferometer on the reception side, and a reception sensitivity penalty occurs.

  The present invention has been made in view of such a background, and in a parallel type four-phase modulation circuit, the phase difference π / 2 in the quadrature phase control unit is always kept constant, and the reception sensitivity penalty due to the phase shift is reduced. The purpose is to avoid the occurrence.

  The present invention includes a CW light source that outputs continuous light with constant intensity, a light separation unit that separates continuous light into two paths, a quadrature phase control unit that shifts the phase of one optical signal by π / 2, This is a four-phase phase modulation circuit including a phase modulation unit that superimposes phase modulation on an optical signal and an optical coupling unit that couples two optical signals.

  Here, the present invention is characterized by an optical branching unit for branching a part of the output optical signal of the optical coupling unit, and an OE (Optical) for receiving the branched monitor optical signal and converting it into an electrical signal. / Electric) conversion unit, RF (Radio Frequency) power detection unit for detecting the power of the AC component of the OE converted electrical signal, and a controller for controlling the phase of the quadrature phase control unit so that the power is minimized It is in the place with.

  As a result, the phase difference π / 2 in the quadrature phase control unit can always be kept constant, and the occurrence of a reception sensitivity penalty due to a phase shift can be avoided.

  Alternatively, the four-phase modulation circuit of the present invention includes an optical branching unit that branches a part of the output optical signal of the optical coupling unit, and an OE conversion unit that receives the branched monitoring optical signal and converts it into an electrical signal. An RF signal power detection unit for detecting the AC component power of the OE-converted electrical signal, a low frequency signal generation unit for superimposing a low frequency component on the phase difference of the quadrature phase control unit, and the RF signal power detection The low-frequency signal superimposed on the phase difference of the quadrature phase control unit output from the low-frequency signal is synchronously detected with another low-frequency signal output from the low-frequency signal generation unit. A synchronous detection circuit for detecting a phase difference and a controller for controlling a phase difference of the quadrature phase control unit so that an absolute value of an output signal of the synchronous detection circuit is minimized.

  According to this, since a shift direction can be detected when a phase shift occurs, it is not necessary to estimate the shift direction, and the phase difference control procedure can be simplified.

  The optical coupling unit is composed of a 3 dB optical coupler, and an output signal is extracted from one output port of the 3 dB optical coupler, and a monitoring optical signal is extracted from the other output port instead of the optical branching unit. Can do. Alternatively, the optical coupling portion is configured by a Y branch optical waveguide, and instead of the optical branch portion, radiated light emitted from the Y branch optical waveguide to other than the output waveguide can be used as a monitoring optical signal.

  Thereby, since the optical branching unit can be omitted, attenuation of the optical power of the output signal by the optical branching unit can be avoided.

  In addition, an electric filter unit that filters an electric signal output from the OE conversion unit may be provided. At this time, the pass frequency of the electric filter unit can be set so as to remove a DC (Direct Current) component and a frequency band of 1/2 or more of the symbol rate.

  As a result, the DC component that does not change due to the phase difference of the quadrature phase control unit and the clock component of the frequency (for example, bit rate / 2 or bit rate / 4) are filtered out, and only the RF signal component that changes depending on the phase difference is removed. Electric power can be detected, and the phase difference of the quadrature phase control unit can be monitored with high accuracy.

  According to the present invention, in the parallel type four-phase modulation circuit, the phase difference between the in-phase component and the quadrature component can always be controlled to be constant at π / 2, and the reception sensitivity penalty due to the phase difference deviation can be reduced. Occurrence is suppressed, and a stable four-phase phase modulation signal can be generated.

(First Example)
A first embodiment of the present invention will be described with reference to FIGS. FIG. 1 is a diagram showing the configuration of the first embodiment. As shown in FIG. 1, the four-phase phase modulation circuit of the present invention applies intensity modulation at a frequency equal to a symbol rate (B / 2, B: bit rate), a CW light source 1 that outputs CW light with constant intensity. Clock modulation unit 2, optical separation unit 3 that separates light from CW light source 1 into two paths, quadrature phase control unit 4 that gives a phase difference of π / 2 to one of the optical signals separated into two paths, two systems of light Phase modulation sections 5-1 and 5-2 that apply phase modulation to each of the signals, an optical coupling section 6 that multiplexes two phase-modulated signals, and a part of an output signal from the optical coupling section 6 An optical branching unit 7 that branches, an OE conversion unit 8 that receives one of the optical signals branched by the optical branching unit 7 and converts it into an electrical signal, and an RF signal power that detects the AC power of the output electrical signal of the OE conversion unit 8 Depending on the output from the detection unit 9 and the RF signal power detection unit 9, the quadrature phase control unit 4 It constituted by a controller 10 for controlling the phase difference to obtain.

  Here, a feature of the present invention is that a part of the output signal is branched, the RF signal power after OE conversion is monitored, and the output power of the quadrature phase control unit 4 so that the RF signal power is minimized. This is the point to control.

  Next, the operation of the first embodiment will be described with reference to FIG. In a state where the phase difference applied by the quadrature control unit 4 is π / 2, the output optical signal has an optical waveform that periodically changes at the frequency B / 2 as shown in FIG. . Therefore, the RF signal spectrum after the OE conversion is only a symbol rate B / 2 component as shown in FIG.

  On the other hand, when the phase difference in the quadrature control unit 4 deviates from π / 2, an intensity modulation component is generated in the optical waveform as shown in FIG. This intensity modulation component is an intensity modulation component that changes at random according to the data of the I component and the Q component at the symbol rate B / 2, and the RF signal spectrum has a shape as shown in FIG. . Therefore, it can be seen that the RF signal power of the electrical signal after the OE conversion is minimized at the point where the phase difference in the quadrature phase control unit 4 is π / 2.

  In order to accurately monitor the point where the phase difference π / 2 from the minimum value of the RF signal power, the DC component that does not change due to the phase difference of the quadrature phase control unit 4 and the clock component of the frequency B / 2 are filtered. It is desirable to detect the power of only the RF signal component that changes due to the phase difference.

  As shown in FIG. 3, this is realized by inserting an electric bandpass filter 11 after the OE converter 8 and removing signal components other than the intensity modulation component generated by the deviation of the phase difference from π / 2. it can. Further, in the case of CSRZ-DQPSK modulation in which RZ pulsation is performed by CS-RZ modulation in the clock modulation unit 2, as shown in FIG. 2 (e), due to the deviation of the DC bias point of the CS-RZ modulation unit, B It is known that an intensity modulation component is generated at / 4, that is, a frequency half the symbol rate.

  In such a case, the detection accuracy of the phase shift of the quadrature phase control unit 4 decreases due to the B / 4 intensity modulation component. Therefore, for CSRZ-DQPSK modulation, as shown in FIG. 2 (f), by setting the pass band of the electric bandpass filter 11 so as to remove clock components of DC and B / 4 or more, It is possible to avoid a reduction in accuracy of the quadrature phase control unit 4 due to the phase shift of the modulation unit 2.

  FIG. 4A shows an example of a measurement result of the dependence of the RF signal power after OE conversion on the voltage applied to the quadrature control unit 4. Here, in this measurement example, the transmission speed is set to B = 43 Gbit / s, and the RF signal power after the RF signal component is filtered using the electric bandpass filter 11 having a pass band of 100 k to 2.8 GHz after OE conversion is measured. It is the result. As shown in FIGS. 4B and 4C, as can be seen from the wavelength demodulated by the one time slot delay interferometer on the receiving side, the phase difference becomes π / 2 at the point where the RF signal power is minimized. It can be confirmed that the best demodulated waveform is obtained.

  The measurement result of FIG. 4 is a measurement example in the case of an RZ-DQPSK signal in which the light intensity is B / 2 and is modulated, but the present invention is an NRZ-DQPSK signal in which the B / 2 intensity modulation is not superimposed. It is also applicable to. The measurement result in the case of the NRZ-DQPSK signal is shown in FIG. In this case, even when the phase difference of the quadrature phase control unit 4 is π / 2, frequency components other than B / 2 exist in the RF signal spectrum after OE conversion. As shown, the output remains slightly even at the minimum value, but it can be confirmed that the RF signal power component is minimized when the phase difference is π / 2.

(Second embodiment)
In the configuration of the first embodiment, since a part of the signal light is branched by the optical branching unit 7 on the output side and the monitor signal light is extracted, the optical power of the output signal is attenuated in the optical branching unit 7. There is a problem that it ends up. In the present embodiment, a configuration for avoiding signal light attenuation in the optical branching unit 7 will be described.

  As shown in FIG. 6, a Y-branch optical waveguide 12 is usually used for the optical coupling unit 6 that multiplexes the signal light divided into two parts and then multiplexes them in the four-phase phase modulation circuit. In DQPSK modulation, two signal lights are combined with a phase difference of π / 2. In this case, about half of the signal light is emitted as radiated light to the cladding region other than the optical waveguide on the output side. The Therefore, it is possible to receive this radiated light at the side of the output optical waveguide and use it as an optical signal for monitoring.

  In this case, when the phase difference of the output signal light is π / 2, the phase difference of the radiated light is −π / 2 in the optical coupling unit 6 as shown in FIG. Since the RF signal power after OE conversion is minimized even at a phase difference of −π / 2 (Vb = 15 V), the phase difference of the quadrature phase control unit 4 is adjusted so that the phase difference of the emitted light is minimized. By doing so, the orthogonal state of the output signal light can be kept optimal.

  Further, as shown in FIG. 7, it is possible to use a 3 dB optical coupler 13 in place of the Y branch optical waveguide 12 in the optical coupling unit 6. In this case, one output port of the 3 dB optical coupler 13 may be used as an optical output, and the other output port may be used as a monitoring optical signal. When the phase difference of the output signal light is π / 2, the phase difference of the monitor optical signal is −π / 2, and the RF signal after the OE conversion of the monitor optical signal is performed as in the case of the Y branch optical waveguide 12. By adjusting the phase difference of the quadrature phase control unit 4 so as to minimize the power, the quadrature state of the output signal light can be kept optimal. When the 3 dB optical coupler 13 is used in the optical coupling unit 6, it is possible to obtain monitor light having almost the same intensity as the output signal light as compared with the Y branch optical waveguide 12. It can be said that it is excellent in terms of efficiency.

(Third embodiment)
In the first embodiment, the configuration is shown in which the RF signal power after OE conversion of the output signal is monitored and the phase difference of the quadrature phase control unit 4 is controlled so that the RF signal power is minimized. This method is characterized in that the configuration is simple, but there is a problem that the control method becomes complicated because the direction of the shift cannot be detected when a phase shift occurs. Therefore, the third embodiment shows a four-phase phase modulation circuit configuration capable of detecting the direction of phase shift.

  FIG. 8 shows a configuration example of the third embodiment. In this embodiment, the low-frequency signal output from the low-frequency signal generation unit 14 is superimposed on the control signal supplied to the quadrature phase control unit 4. Then, the low frequency signal superimposed on the phase difference of the quadrature phase control unit 4 output from the RF signal power detection unit 9 is phase-detected with another low frequency signal output from the low frequency signal generation unit 14. Thus, the phase shift direction in the quadrature phase control unit 4 is detected.

Next, the operation of this embodiment will be described with reference to FIG. As shown in FIG. 9, the dependency of the RF signal power on the phase shift is a cosine type output characteristic, and the RF signal power is minimized when the phase difference is π / 2. Here, when the component of the frequency f ₀ that is sufficiently lower than the transmission speed B is superimposed on the control signal applied to the quadrature phase control unit 4, the time change of the RF signal power is shown in FIG. It becomes like this.

That is, when the phase difference is π / 2, the RF signal power changes at a frequency of 2f ₀ as shown in A of FIG. Therefore, the output signal is zero when detected at the frequency f ₀ . Next, when the phase difference decreases from π / 2, a component of f ₀ appears as shown in B of FIG. Further, when the phase difference exceeds π / 2, the component of f ₀ appears as shown in FIG. 9C, but it can be seen that the phase of the RF signal is inverted compared to the case of B. Therefore, it can be seen that the output after the phase detection is inverted between B and C, and the direction of deviation can be detected.

  As described above, it is possible to always control the phase difference to be constant at π / 2 by superimposing the DC component on the signal supplied to the quadrature phase control unit 4 so that the output after phase detection becomes zero.

  According to the present invention, in the parallel type four-phase modulation circuit, the phase difference between the in-phase component and the quadrature component can always be controlled to be constant at π / 2, and the reception sensitivity penalty due to the phase difference deviation can be reduced. Since generation can be suppressed and a stable four-phase phase modulation signal can be generated, a high-quality four-phase phase modulation signal can be generated.

The block diagram of the 4-phase phase modulation circuit of a 1st Example. The figure which shows the output optical waveform and RF signal spectrum of a four-phase phase modulation circuit. The block diagram of the four-phase phase modulation circuit of a 1st Example (example using an electric band pass filter). The figure which shows the phase shift dependence of RF signal power, and a demodulation optical wavelength (example of RZ-DQPSK). The figure which shows the phase shift dependence of RF signal power, and a demodulation optical wavelength (example of NRZ-DQPSK). The block diagram of the 4-phase phase modulation circuit of a 2nd Example (example which used Y branch optical waveguide for the optical coupling part). The block diagram of the 4 phase phase modulation circuit of a 2nd Example (example which used 3 dB optical coupler for the optical coupling part). The block diagram of the 4-phase phase modulation circuit of a 3rd Example. The figure which shows the dependence with respect to the orthogonal phase control part phase difference of RF signal power.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 CW light source 2 Clock modulation part 3 Optical separation part 4 Quadrature phase control part 5-1, 5-2 Phase modulation part 6 Optical coupling part 7 Optical branching part 8 OE conversion part 9 RF signal electric power detection part 10 Controller 11 Electric band pass Filter 12 Y branch optical waveguide 13 3 dB optical coupler 14 Low frequency signal generator 15 Synchronous detection circuit

Claims (6)

A CW (Continuous Wave) light source that outputs continuous light with a constant intensity, a light separation unit that separates continuous light into two paths, a quadrature phase control unit that shifts the phase of one optical signal by π / 2, In a four-phase phase modulation circuit configured by a phase modulation unit that superimposes phase modulation on an optical signal and an optical coupling unit that combines two optical signals
An optical branching unit for branching a part of the output optical signal of the optical coupling unit;
An OE (Optical / Electric) converter that receives the branched monitor optical signal and converts it into an electrical signal;
An RF (Radio Frequency) power detection unit for detecting the power of the alternating current component of the OE converted electrical signal;
And a controller for controlling the phase of the quadrature phase control unit so that the power is minimized. A CW light source that outputs continuous light with a constant intensity, a light separation unit that separates continuous light into two paths, a quadrature phase control unit that shifts the phase of one optical signal by π / 2, and a phase for each optical signal In a four-phase modulation circuit configured by a phase modulation unit that superimposes modulation and an optical coupling unit that couples two optical signals,
An optical branching unit for branching a part of the output optical signal of the optical coupling unit;
An OE converter that receives the branched optical signal for monitoring and converts it into an electrical signal;
An RF power detector that detects the power of the AC component of the electrical signal that has undergone OE conversion;
A low-frequency signal generation unit that superimposes a low-frequency component on the phase difference of the quadrature phase control unit;
The quadrature phase is obtained by synchronously detecting a low frequency signal superimposed on the phase difference of the quadrature phase control unit output from the RF power detection unit with another low frequency signal output from the low frequency signal generation unit. A synchronous detection circuit for detecting the phase difference of the control unit;
And a controller for controlling a phase difference of the quadrature phase control unit so that an absolute value of an output signal of the synchronous detection circuit is minimized.   2. The optical coupling unit is composed of a 3 dB optical coupler, an output signal is extracted from one output port of the 3 dB optical coupler, and a monitoring optical signal is extracted from the other output port instead of the optical branching unit. Or a 4-phase phase modulation circuit according to 2;   3. The optical coupling unit is constituted by a Y branch optical waveguide, and instead of the optical branch unit, radiated light emitted from the Y branch optical waveguide to a part other than the output waveguide is used as a monitor optical signal. 4 phase modulation circuit.   5. The four-phase modulation circuit according to claim 1, further comprising an electric filter unit that filters an electric signal output from the OE conversion unit.   6. The four-phase phase modulation circuit according to claim 5, wherein the pass frequency of the electric filter section is set so as to remove a frequency band of 1/2 or more of a DC (Direct Current) component and a symbol rate.

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