JP4531010B2 - Optical phase modulation / demodulation circuit and optical phase modulation / demodulation method - Google Patents

Description

  The present invention relates to a modulation technique using multi-level optical phase modulation and a demodulation technique using delayed detection.

  A differential quadrature phase-shift keying (DQPSK) system has attracted attention as a narrowband modulation system that improves the frequency utilization efficiency of optical fiber transmission technology. In this modulation system, the transmission capacity can be doubled while the occupied bandwidth is the same as that of the binary differential phase-shift keying (DPSK) system (for example, non-patent) Reference 1).

On the receiving side, a delay Mach-Zehnder interferometer (Mach-Zehnder
Since demodulation is performed by delay detection using Interferometer (MZI), polarization dependency is small, and stable reception characteristics can be realized.

R. A. Griffin et al. "Optical Differential Quadrature Phase-Shift Key (oDQPSK) for High Capacity Optical Transmission", OFC2002, PaperWX6, 2002. C. R. Doerr et al. , “Simultaneous reception of both quadratures of 40-Gb / s DQPSK using simple simple demodulator”, OFC 2005, Paper PDP 12, 2005.

  However, the conventional DQPSK modulation / demodulation system has the following problems. That is, in the two delay Mach-Zehnder interferometers on the receiving side in the DQPSK system, the phase difference between the two arms is set to π / 4 for one delay Mach-Zehnder interferometer, and the other delay Mach-Zehnder interferometer is π from the former phase difference. It is necessary to shift by / 2 (that is, −π / 4).

For this reason, the transmission spectrum of the delayed Mach-Zehnder interferometer comes to a position where the frequency that becomes the transmission peak is shifted from the optical carrier frequency by FSR / 8. However, FSR is the free spectral range (Free
Spectral Range).

  In an actual DQPSK transceiver, it is necessary to adjust the center wavelength of the delayed Mach-Zehnder interferometer on the receiving side according to the wavelength of the transmission signal. At this time, the FSR / 8 is determined from the position of the optical carrier frequency at which the transmission power is maximum. Since the delay Mach-Zehnder interferometer is adjusted to a position shifted by a certain amount, an adjustment method using transmitted power cannot be used, so that complicated adjustment means such as monitoring a demodulated waveform is required.

  The present invention has been made in consideration of such a background, and has a narrow spectrum characteristic comparable to that of a conventional DQPSK signal, but the phase difference of the delayed Mach-Zehnder interferometer is zero, that is, at the optical carrier frequency. An object of the present invention is to facilitate the adjustment of the center frequency of the delayed Mach-Zehnder interferometer by setting the transmission power of the delayed Mach-Zehnder interferometer to a maximum value so as to be demodulated.

The optical phase modulation circuit of the present invention includes a CW light source that generates continuous light with constant power, and a π / 4 phase that modulates the phase to 0 or π / 4 at a frequency that is half the symbol rate of the input optical signal. Modulator and optical quadrature that divides the input optical signal into two paths , modulates the phase of each optical signal to 0 or π , shifts the phase of one optical signal by π / 2, and multiplexes again is more configured to a phase modulator, the CW light source - [pi / 4 phase modulator - optical quadrature phase modulator, or CW light source - optical quadrature phase modulator - [pi / 4 that are connected in series in the order of the phase modulator Features.

Alternatively, the optical phase modulation circuit of the present invention includes a CW light source that generates continuous light with constant power, and 3π / that modulates the phase to 0 or 3π / 4 at a frequency that is half the symbol rate of the input optical signal. 4 and the phase modulator divides the input optical signal into two paths, the phase with respect to each of the optical signal is modulated to 0 or [pi, multiplexes again shifting the phase of one of the optical signal by [pi / 2 It is more configured to the optical quadrature phase modulator, the CW light source - 3 [pi] / 4 phase modulator - optical quadrature phase modulator, or CW light source - optical quadrature phase modulator - connected in series in the order of 3 [pi] / 4 phase modulator It is characterized by that.

Alternatively, the optical phase modulation circuit of the present invention includes a CW light source that generates continuous light with constant power, and π / that modulates the phase to 0 or π / 4 at a frequency half the symbol rate with respect to the input optical signal. 4 and the phase modulator, and [pi phase modulator for modulating the phase 0 or [pi with respect to the input optical signal, modulates the phase 0 or [pi / 2 relative to the input optical signal [pi / 2 phase and modulator, the heading of the CW light source, then the [pi / 4 phase modulator and the [pi / 2 phase modulator and said [pi phase modulator is characterized by being connected in series in any order.

Alternatively, the optical phase modulation circuit of the present invention includes a CW light source that generates continuous light with constant power and a 3π / 4 phase that modulates the phase to 3π / 4 at a frequency that is half the symbol rate of the input optical signal. a modulator, a phase modulator [pi for modulating the phase [pi to light signals inputted, and [pi / 2 phase modulator for modulating the phase [pi / 2 with respect to the input optical signal, CW a light source at the beginning, then the 3 [pi] / 4 phase modulator and the [pi / 2 phase modulator and said [pi phase modulator is characterized by being connected in series in any order.

  Furthermore, by adopting a configuration in which a light intensity modulator that modulates light intensity at the same frequency as the symbol rate is inserted at an arbitrary position, it is possible to superimpose intensity modulation at a frequency equal to the symbol rate.

In the configuration of the optical phase modulation circuit of the present invention, or prior SL [pi / 4 phase modulator in place of the 3 [pi / 4 phase modulator, at half the frequency of the symbol rate, driven by sinusoidal drive signals having phases shifted A dual electrode drive Mach-Zehnder optical modulator can also be used.

  For example, at this time, the phase difference between the arms of the sine wave drive signal is about 14 °, and the amplitude is about 0.51 times the half-wave voltage. Alternatively, the phase difference between the arms of the sine wave drive signal is about 37 °, and the amplitude is about 0.63 times the half-wave voltage.

Alternatively, or pre SL [pi / 4 phase modulator in place of the 3 [pi / 4 phase modulator, at half the frequency of the symbol rate, and are phase shifted by [pi each other are driven by different sinusoidal drive signal amplitude A double electrode drive Mach-Zehnder optical modulator can also be used.

  For example, at this time, the amplitude of the sine wave drive signal is about 0.875 times the half-wave voltage and about 0.125 times the half-wave voltage. Alternatively, the amplitude of the sine wave drive signal is about 0.625 times the half-wave voltage and about 0.375 times the half-wave voltage.

Further, when the present invention is viewed from the viewpoint of an optical phase demodulator circuit, the optical phase demodulator circuit of the present invention has an optical branch circuit that divides a received optical signal into two paths, and each optical signal divided into two paths. Te, 1 and symbol delay two delay Mach-Zehnder interferometer Ru added, and the signal output from the two delay Mach-Zehnder interferometer is composed of two balanced receivers for receiving respectively the two and wherein the phase difference given to the optical signal delay Mach-Zehnder interferometer are each 0 and [pi / 2 radians.

Further, when the present invention is viewed from the viewpoint of the optical modulation method, in the optical modulation method of the present invention and the optical modulation method using differential quadrature phase shift keying, the optical phase of even symbols is 0, π / 2, π, 3π. / 2 takes any value, the value of any of the odd symbol the light phase is π / 4,3π / 4,5π / 4,7π / 4 DOO is, the phase point that can be taken in the even symbols and the odd symbols Is shifted by π / 4 .

When the present invention is viewed from the viewpoint of the optical demodulation method, the optical demodulation method of the present invention divides the received optical signal into two paths, and adds a delay of one symbol to the optical signal divided into the two paths. one of the respectively input to the delay Mach-Zehnder interferometer, an optical respectively by signals of two balanced receiver output from the two delay Mach-Zehnder interferometer - an optical receiving method for receiving after electrical conversion, the two delay Mach-Zehnder a phase difference provided to the optical signal interferometer characterized in that each 0 and [pi / 2.

  As a result, the phase difference of the delayed Mach-Zehnder interferometer is set to zero, that is, the transmission power of the delayed Mach-Zehnder interferometer is maximized at the optical carrier frequency while having narrow spectrum characteristics comparable to those of the conventional DQPSK signal. Therefore, the center frequency of the delay Mach-Zehnder interferometer can be easily adjusted.

  According to the present invention, a quaternary phase modulation signal can be generated, and the transmission peak of the delay Mach-Zehnder interferometer used on the receiving side can be matched with the optical carrier frequency. Adjustment can be facilitated.

(First Example)
A first embodiment of the present invention will be described with reference to FIGS. FIG. 1 is a diagram showing a configuration of an optical phase modulation circuit according to a first embodiment of the present invention. As shown in FIG. 1, the optical phase modulation circuit of the present invention includes a CW light source 1 that generates continuous light with constant power, and a symbol rate (B / 2) with respect to continuous light with constant power generated by the CW light source 1. , B: bit rate), a phase modulator 2 that gives periodic phase modulation with a phase shift of 0 or π / 4 radians at half the frequency, and an input optical signal divided into two paths, It is composed of an optical quadrature phase modulator 3 that applies phase modulation with a phase shift of 0 or π to the signal, and shifts the phase of one optical signal by π / 2 radians and combines the signals again.

Here, a feature of the present invention is that in addition to the optical quadrature phase modulator 3 that performs normal DQPSK modulation, a phase modulator that performs periodic modulation of 0 or π / 4 at a period of half the symbol rate. 2 is added. Even if the order of the phase modulator 2 and the optical quadrature phase modulator 3 is changed, a similar output signal can be obtained.

Further, in place of the optical quadrature modulator 3 as shown in FIG. 1, as shown in FIG. 2, 0 or [pi phase modulation and 0 or [pi / 2 phase modulation and connected in series to perform a two phase It can also be realized by the modulators 4 and 5. In this case, three phase modulators 2, 4, and 5 are connected in series with the phase modulator 2 for π / 4 shift, and these phase modulators 2, 4, and 5 are connected to each other. It can be configured in any order.

The drive signal for driving the phase modulator 2 that performs phase modulation of 0 or π / 4 is maximum (π / 4) or at the center of the time slot with respect to the phase change applied by the optical quadrature phase modulator 3. Good modulation characteristics can be obtained by adjusting the phase of the drive signal so as to be the minimum (0).

  FIG. 3 shows the constellation of the output signal of the optical phase modulation circuit of this embodiment. FIG. 3 also shows a constellation of a conventional DQPSK modulation method for comparison. In the conventional DQPSK modulation, the phase takes four points of π / 4, 3π / 4, 5π / 4, and 7π / 4, whereas in the modulation system of this embodiment, the odd symbols are π / 4, 3π / 4, There are 4 points of 5π / 4 and 7π / 4, and even symbols are 4 points of 0, π / 2, π, 3π / 2, and the phase points that can be taken by even and odd symbols are shifted by π / 4. It can be seen that the points are different.

  In this embodiment, RZ conversion is not performed, but it is also possible to superimpose intensity modulation at a frequency equal to the symbol rate by using a pulse light source as a transmission light source or adding an intensity modulator for RZ conversion. It is.

  In the present embodiment, the case where the phase shift amount is π / 4 has been described, but it is needless to say that the constellation of FIG. 3 can be obtained similarly even if the phase shift amount is 3π / 4. .

  The configuration of the receiving circuit of this embodiment is shown in FIG. In the receiving circuit of this embodiment, the input signal is branched into two by the optical branching circuit 10 and input to the delay Mach-Zehnder interferometers MZI # 1 and # 2 which respectively give a delay of one symbol. Here, in the two delay Mach-Zehnder interferometers MZI # 1 and # 2, the optical phase difference between the arms is set to be 0 and π / 2. These two delayed Mach-Zehnder interferometers MZI # 1 and # 2 convert the phase modulation signal into two intensity modulation signals and receive them by balanced receivers 9-1 and 9-2.

The transmission characteristics of each port of the delay Mach-Zehnder interferometers MZI # 1 and # 2 in the receiving circuit of this embodiment are shown in FIG. In the case of the receiving circuit of the present embodiment, the phase difference between the arms is zero in the delay Mach-Zehnder interferometer MZI # 1.
The output of the port is maximized at the optical carrier frequency, and the output of the destructive port is minimized at the carrier frequency.

Since the optical spectrum of the modulated signal of this embodiment has a peak at the carrier frequency, the delay Mach-Zehnder interferometer MZI # 1 is constitutive.
What is necessary is just to adjust the phase difference of the delay Mach-Zehnder interferometer MZI # 1 so that the transmission power of the port is maximized. As for the delayed Mach-Zehnder interferometer MZI # 2, by using the configuration shown in Non-Patent Document 2, the two delayed Mach-Zehnder interferometers MZI # 1 and # 2 are moved while maintaining the frequency difference on the frequency axis. Therefore, the delay Mach-Zehnder interferometer MZI # 2 can be automatically adjusted by adjusting the center frequency of the delay Mach-Zehnder interferometer MZI # 1.

  On the other hand, in the conventional DQPSK signal, as shown in FIG. 5, the transmission peaks of the delayed Mach-Zehnder interferometers MZI # 1 and # 2 are shifted from the optical carrier frequency, so that the delayed Mach-Zehnder interferometer MZI using the average optical power is used. Adjustment of # 1 and # 2 is difficult.

  As described above, by using the optical phase modulation circuit of the present embodiment, the phase difference between the delay Mach-Zehnder interferometers MZI # 1 and # 2 in the receiving system can be set to 0 and π / 2. Therefore, the delay Mach-Zehnder interferometer MZI The transmission peaks of # 1 and # 2 can be matched to the optical carrier frequency, and there is an advantage that the delay Mach-Zehnder interferometers MZI # 1 and # 2 can be easily adjusted.

(Second embodiment)
In the first embodiment, phase modulation of 0 or π / 4 is added by the phase modulator 2, but in the second embodiment, this phase modulation and intensity modulation for RZ conversion are performed by one optical modulator. The structure which enables is shown. The configuration of the second embodiment is shown in FIG. Here, a double-electrode drive Mach-Zehnder optical modulator 6 is used instead of the phase modulator 2 shown in FIG. 1, and a clock signal having a frequency half the symbol rate is used, and each of the double-electrode drive Mach-Zehnder optical modulator 6 is used. The phase of the clock signal applied to the arm is shifted by the phase shifter 7.

  The modulation component of the output signal from the two-electrode drive Mach-Zehnder optical modulator 6 driven by a sine wave signal of frequency fm with a phase difference of δ is:

と表される。ここで、Ｖπは光位相シフトがπとなる印加電圧であり、Ｖは印加する正弦波信号の振幅である。式１より、δ≒１４°、Ｖ≒０．５１Ｖπで駆動することにより位相変化がπ／４、光強度の繰り返し周波数が２ｆmのパルス列が得られる。

It is expressed. Here, Vπ is an applied voltage at which the optical phase shift is π, and V is the amplitude of the sine wave signal to be applied. From Equation 1, a pulse train having a phase change of π / 4 and a light intensity repetition frequency of 2 fm can be obtained by driving at δ≈14 ° and V≈0.51Vπ.

FIG. 7 shows how the light intensity and phase change when the phase change calculated from Equation 1 is π / 4. As can be seen from this result, the double electrode drive Mach-Zehnder optical modulator 6 is used, and it is driven by a sine wave signal whose frequency is half the symbol rate and whose phase is shifted by about 14 °, so that an alternating of 0 or π / 4 is obtained. A phase pulse train can be generated, and by inputting this signal to the optical quadrature phase modulator 3, a π / 4 shift DQPSK signal can be generated as in the first embodiment. In order to generate an alternating pulse train having a phase change of 0 or 3π / 4, driving may be performed at δ≈37 ° and V≈0.63Vπ.

  Up to this point, the case where driving is performed centering on the point where the transmittance of the both-electrode driving Mach-Zehnder optical modulator 6 is maximized has been described. However, focusing on the point where the transmittance of the both-electrode driving Mach-Zehnder optical modulator 6 is minimized. It is also possible to drive. In this case, as shown in FIG. 8, in addition to the phase shifter 7, an attenuator 11 for adjusting the amplitude is used.

  When driving at a point where the transmittance of the both-electrode drive Mach-Zehnder optical modulator 6 is minimized, a signal given to each of the both-electrode drive Mach-Zehnder optical modulator 6 is:

とした場合に、両電極駆動マッハツェンダ光変調器６からの出力信号の変調成分は

In this case, the modulation component of the output signal from the both-electrode drive Mach-Zehnder optical modulator 6 is

と表される。従って、０またはπ／４の位相変調を行う位相変調の場合（Δ＝０．２５）は、振幅は半波長電圧の０．６２５倍及び０．３７５倍、０または３π／４の位相変調を行う位相変調の場合（Δ＝０．７５）は、振幅は半波長電圧の０．８７５倍及び０．１２５倍とすればよいことがわかる。

It is expressed. Therefore, in the case of phase modulation that performs phase modulation of 0 or π / 4 (Δ = 0.25), the amplitude is 0.625 times and 0.375 times the half-wave voltage, and phase modulation of 0 or 3π / 4 is performed. In the case of phase modulation to be performed (Δ = 0.75), it can be seen that the amplitude may be 0.875 times and 0.125 times the half-wave voltage.

  According to the present invention, a quaternary phase modulation signal can be generated, and the transmission peak of the delay Mach-Zehnder interferometer used on the receiving side can be matched with the optical carrier frequency. Adjustment can be facilitated. Thereby, the DQPSK system can be easily realized.

The block diagram of the optical phase modulation circuit of a 1st Example. The figure which shows the structural example by a serial type phase modulator. The figure which shows the output optical signal constellation of the optical phase modulation circuit of a 1st Example. The figure which shows the transmission characteristic of the structure of the receiving circuit of a 1st Example, and a delay Mach-Zehnder interferometer. The figure which shows the transmission characteristic of the delay Mach-Zehnder interferometer for the conventional DQPSK. The block diagram of the optical phase modulation circuit of a 2nd Example. The figure which shows the intensity | strength waveform and phase waveform of the output signal of the dual drive type Mach-Zehnder modulator in a 2nd Example. The block diagram using the attenuator of the optical phase modulation circuit of 2nd Example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 CW light source 2, 4, 5 Phase modulator 3 Optical quadrature phase modulator 6 Double electrode drive Mach-Zehnder optical modulator 7 Phase shifter 8 Receiving circuit 9-1, 9-2 Balanced receiver 10 Optical branch circuit 11 Attenuator MZI # 1, MZI # 2 delay Mach-Zehnder interferometer

Claims (12)

A CW light source that generates continuous light of constant power;
A π / 4 phase modulator that modulates the phase to 0 or π / 4 at a frequency half the symbol rate with respect to the input optical signal;
An optical quadrature modulator that divides an input optical signal into two paths, modulates the phase of each optical signal to 0 or π, shifts the phase of one optical signal by π / 2, and combines the signals again; Consisting of
An optical phase modulation circuit comprising: a CW light source—π / 4 phase modulator—optical quadrature phase modulator or a CW light source—optical quadrature phase modulator—π / 4 phase modulator connected in series. A CW light source that generates continuous light of constant power;
A 3π / 4 phase modulator that modulates the phase to 0 or 3π / 4 at a frequency half the symbol rate with respect to the input optical signal;
An optical quadrature modulator that divides an input optical signal into two paths, modulates the phase of each optical signal to 0 or π, shifts the phase of one optical signal by π / 2, and combines the signals again; Consisting of
An optical phase modulation circuit comprising: a CW light source-3π / 4 phase modulator-optical quadrature phase modulator or a CW light source-optical quadrature phase modulator-3π / 4 phase modulator connected in series in this order. A CW light source that generates continuous light of constant power;
A π / 4 phase modulator that modulates the phase to 0 or π / 4 at a frequency half the symbol rate with respect to the input optical signal;
A π phase modulator that modulates the phase of the input optical signal to 0 or π;
A π / 2 phase modulator that modulates the phase of the input optical signal to 0 or π / 2,
An optical phase modulation circuit characterized in that a π / 4 phase modulator, the π / 2 phase modulator, and the π phase modulator are connected in series in an arbitrary order, starting with a CW light source. A CW light source that generates continuous light of constant power;
A 3π / 4 phase modulator that modulates the phase to 0 or 3π / 4 at a frequency half the symbol rate with respect to the input optical signal;
A π phase modulator that modulates the phase of the input optical signal to 0 or π;
A π / 2 phase modulator that modulates the phase of the input optical signal to 0 or π / 2,
An optical phase modulation circuit characterized in that a 3W / 4 phase modulator, the π / 2 phase modulator, and the π phase modulator are connected in series in an arbitrary order, starting with a CW light source.   5. The optical phase modulation circuit according to claim 1, wherein a light intensity modulator that modulates light intensity at the same frequency as the symbol rate is inserted at an arbitrary position.   2. A dual electrode drive Mach-Zehnder optical modulator driven by a sine wave drive signal whose phase is shifted at a frequency half the symbol rate is used in place of the π / 4 phase modulator or the 3π / 4 phase modulator. 5. The optical phase modulation circuit according to any one of 4 to 4.   7. The optical phase modulation circuit according to claim 6, wherein the phase difference between the arms of the sine wave drive signal is about 14 °, and the amplitude is about 0.51 times the half-wave voltage.   The optical phase modulation circuit according to claim 6, wherein the phase difference between the arms of the sine wave drive signal is about 37 °, and the amplitude is about 0.63 times the half-wave voltage.   Instead of the π / 4 phase modulator or the 3π / 4 phase modulator, a double-electrode drive Mach-Zehnder driven by a sinusoidal drive signal having a phase shifted by π and having a different amplitude at a frequency half the symbol rate. The optical phase modulation circuit according to claim 1, wherein an optical modulator is used.   10. The optical phase modulation circuit according to claim 9, wherein the amplitude of the sine wave drive signal is about 0.875 times the half-wave voltage and about 0.125 times the half-wave voltage.   The optical phase modulation circuit according to claim 9, wherein the amplitude of the sine wave drive signal is about 0.625 times the half-wave voltage and about 0.375 times the half-wave voltage. In an optical modulation method using differential quadrature phase shift keying,
The optical phase of the even symbol takes one of the values 0, π / 2, π, 3π / 2, and the optical phase of the odd symbol has one of π / 4, 3π / 4, 5π / 4, 7π / 4. Take the value,
An optical modulation method characterized in that the possible phase points are shifted by π / 4 between even symbols and odd symbols.

Images