WO2011083575A1 - 光伝送システム - Google Patents
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- WO2011083575A1 WO2011083575A1 PCT/JP2010/050096 JP2010050096W WO2011083575A1 WO 2011083575 A1 WO2011083575 A1 WO 2011083575A1 JP 2010050096 W JP2010050096 W JP 2010050096W WO 2011083575 A1 WO2011083575 A1 WO 2011083575A1
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/60—Receivers
- H04B10/66—Non-coherent receivers, e.g. using direct detection
- H04B10/69—Electrical arrangements in the receiver
- H04B10/697—Arrangements for reducing noise and distortion
- H04B10/6971—Arrangements for reducing noise and distortion using equalisation
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/50—Transmitters
- H04B10/516—Details of coding or modulation
- H04B10/548—Phase or frequency modulation
- H04B10/556—Digital modulation, e.g. differential phase shift keying [DPSK] or frequency shift keying [FSK]
- H04B10/5561—Digital phase modulation
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/60—Receivers
- H04B10/66—Non-coherent receivers, e.g. using direct detection
- H04B10/67—Optical arrangements in the receiver
- H04B10/676—Optical arrangements in the receiver for all-optical demodulation of the input optical signal
- H04B10/677—Optical arrangements in the receiver for all-optical demodulation of the input optical signal for differentially modulated signal, e.g. DPSK signals
Definitions
- the present invention relates to an optical transmission system, and more particularly to an optical transmission system in which optical waveform distortion is reduced in optical information transmission technology, and more particularly, to an optical transceiver suitable for transmitting and receiving multilevel optical information transmitted by an optical fiber
- the present invention relates to an optical transmission system.
- the amount of information that can be transmitted by a single optical fiber almost reaches the limit by using the wavelength band of the optical fiber amplifier by increasing the number of wavelengths and increasing the modulation speed of the optical signal. There is. Furthermore, in order to increase the transmission capacity of the optical fiber, it is necessary to devise a signal modulation method, pack many optical signals in a limited frequency band, and increase the utilization efficiency of the frequency band.
- Multi-level modulation technology has enabled high-efficiency transmission with a frequency utilization efficiency exceeding 10 since the 1960s.
- Multilevel modulation is considered promising in optical fiber transmission, and has been studied in the past.
- R. A. Griffin, et. Al. In “10 Gb / s Optical Differential Quadrature Phase Shift Key (DQPSK) Transmission using GaAs / AlGaAs Integration,” OFC 2002, paper PD-FD6, 2002 (Non-Patent Document 1), QPSK (Quadrature Phase Shift Keying) performing four-valued phase modulation Is reported, N. Kikuchi, K. Mandai, K. Sekine and S.
- FIG. 1 are diagrams showing a description of a complex phase plane used for light transmission, and signal point arrangements of various known modulation schemes.
- Signal points complex representation of the optical electric field at the identification time
- the complex phase plane or complex plane, phase plane, IQ plane.
- FIG. 1A is an explanatory view of signal points on the IQ plane, and each signal point is a complex orthogonal coordinate (IQ coordinate) or a polar coordinate shown by an amplitude r (n) and a phase ⁇ (n) shown in the figure. Can be displayed.
- IQ coordinate complex orthogonal coordinate
- ⁇ phase ⁇
- phase angle ⁇ (n). , 10) are shown as quaternary phase modulation (QPSK).
- (C) shows 16-ary quadrature amplitude modulation (16 QAM) widely used in radio.
- 16 QAM signal points are arranged in a grid, and four bits of information can be transmitted by one symbol.
- the value of the upper 2 bits (10xx, 11xx, 01xx, 00xx) in the Q axis coordinate and the value of the lower 2 bits (xx10, xx11, xx01, xx00) in the I axis coordinate are represented. It is known that this signal constellation can increase the distance between signal points, so that it is known that the reception sensitivity is high, and in optical communication this kind of quadrature amplitude modulation can be realized using a coherent optical receiver.
- the coherent receiver is a scheme using a local light source disposed inside the receiver for detecting the phase angle of the optical signal.
- a coherent reception method which is one of the prior art of an optical multilevel receiver, for example, M. G. Taylor, “Coherent detection method using DSP to demodulate signal and for subsequencing equalization of propagation impairments,” paper We4.
- P. No. 111, ECOC 2003, 2003 (non-patent document 4) describes a coherent optical field receiver.
- FIG. 2 is a block diagram of a conventional digital coherent optical multilevel transmission system using a polarization diversity coherent optical field receiver.
- the unmodulated laser beam output from the laser light source 106 is input to the orthogonal optical field modulator 107, and the output optical signal 109 subjected to predetermined electric field modulation is output from the output optical fiber 108 Ru.
- An information signal to be transmitted is input to the digital information input terminal 101 as a parallel (for example, m bit width) binary high-speed digital electrical signal sequence. This signal is converted into a complex multilevel information signal 103 by the complex multilevel signal generation circuit 102 in units of several bits.
- the real part i and the imaginary part q are output.
- These signals are converted into high-speed analog signals by DA converters 104-1 and 104-2, respectively, and then amplified by driver circuits 105-1 and 105-2 to modulate I and Q of orthogonal optical field modulator 107. Input to the terminal.
- the output optical signal 109 becomes an optical electric field signal having the complex multilevel signal (i, q) as the in-phase component I and the orthogonal component Q of the optical electric field.
- the optical electric field of the light amplitude / phase modulation signal is (i (n) + jq (n)) exp (j ⁇ (n)), where ⁇ (n) is an optical angular frequency of the laser light source 106.
- ⁇ (n) is an optical angular frequency of the laser light source 106.
- the configuration using the DA converter 104 in multi-level modulation is shown. However, when the number of multi-levels is small, for example, in the case of realizing 4-level phase modulation, the DA converter is not used. In some cases, two sets of binary signals are applied to the orthogonal optical field modulator.
- the output optical signal 109 is transmitted through the optical fiber transmission line 122, and after being subjected to transmission deterioration due to wavelength dispersion of the optical fiber or the like, the output optical signal 109 is input to the digital coherent optical receiver 120.
- the input optical signal 121 is divided into four sets of in-phase components of horizontal (S) polarization and orthogonal components of horizontal polarization, and in-phase components and orthogonal components of vertical (P) polarization by the polarization separation / light 90 degree hybrid circuit 113. They are separated and input to balanced optical receivers 110-1, 110-2, 110-3, and 110-4, respectively.
- the local laser light source 112 disposed in the receiver is used as a reference of the light phase of the received light, and has substantially the same wavelength as the input light signal 121.
- the output light of the local laser light source 112 is connected to the other input port of the polarization separation / light 90 degree hybrid circuit 113, and like the signal light, the balance type optical receivers 110-1, 110-2, 110-3. , 110-4.
- the input signal light interferes with local light to be converted into an electric signal, and time-sampled by the AD converters 111-1, 111-2, 111-3, and 111-4, respectively. It is converted to a digital signal.
- These digital signals are first input to the chromatic dispersion compensation circuits 114-1 and 114-2 for each polarization component, and then input to the adaptive equalization circuit 115, and waveform distortion due to modulation distortion or residual wavelength dispersion, polarization After compensation for changes in wave state, polarization dispersion, and the like, the signal is input to the phase estimation circuit 116 in the subsequent stage.
- the two sets of multi-level signals from which the phase fluctuation has been removed are input to the multi-level signal determination circuit 117, subjected to symbol determination processing, and decoded into the original bit string.
- FIG. 3 is an explanatory view of the problem of the present invention, and shows a state of modulation distortion and its equalization in a conventional digital coherent optical multilevel transmission system.
- the multi-level signal output from the complex multi-level signal generation circuit 102 is an ideal one described by digital information.
- the complex signal point arrangement is as shown in FIG.
- the position of the signal point contains no error or distortion at all.
- the multi-level signal suffers from significant waveform deterioration.
- the optical electric field of the output signal 101 causes an error in the signal point position as shown in FIG. 3B, which causes the code error rate of the received signal to be greatly deteriorated.
- FIG. 3C shows the signal point arrangement input to the adaptive equalization circuit 115, which is almost the same as the output optical signal 109 (here, for the sake of simplicity, the influence of chromatic dispersion, polarization fluctuation, and phase fluctuation) Ignore).
- the digital adaptive equalization filter such as multistage transversal filter as the adaptive equalization circuit 115 Can be As a result, as shown in FIG. 3D, it is possible to almost completely remove modulation distortion from the output signal point and prevent deterioration of transmission characteristics such as a code error rate.
- FIG. 4 is a block diagram of a phase pre-integration type optical multilevel signal transmission system using the optical direct detection proposed by us previously. This method realizes optical multilevel transmission simply by using optical delay detection without using coherent detection and local light source, and the details thereof are described in Patent Document 1: WO2009 / 060920 .
- phase pre-accumulation type optical electric field transmitter 123 (laser light source 106, orthogonal optical electric field light modulator 107, complex multilevel signal generation circuit 102, D / A converter 104, driver circuit 105, etc.) Similar to the second optical multilevel transmitter 100, a part of internal signal processing is different because optical direct detection is used.
- the multilevel signal output from the complex multilevel signal generation circuit 102 is input to the phase preintegration circuit 126, and the phase preintegration complex in which only the phase component is digitally integrated at the time interval T internally It is converted into a multilevel information signal.
- ⁇ (n) is the phase angle of the output signal
- ⁇ (n) is the past phase angle ⁇ (1). . . . It is a value obtained by cumulatively adding ⁇ (n).
- the output signal is converted to the orthogonal coordinate system again, and then input to the complex up-sampling circuit 124, which complements the sampling points so that the sampling rate is 2 samples / symbol or more. This satisfies the Nyquist theorem and enables perfect electric field equalization processing.
- the inverse function of the degradation occurring in the optical fiber transmission line 122 or the like is applied by the pre-equalization circuit 125 and converted into a complex signal i '', q ''.
- the output optical signal 109 is transmitted through the optical fiber transmission line 122, and is subjected to transmission deterioration due to wavelength dispersion of the optical fiber or the like, and then input to the noncoherent optical multilevel receiver 130 as the input optical signal 121.
- the influence of the chromatic dispersion of the optical fiber transmission line mutually cancels out with the inverse function applied in advance by the pre-equalization circuit 125, so the input optical signal 121 is equivalent to the output signal of the phase pre-integration circuit 126.
- the input optical signal 121 is branched into three optical signal paths by the optical branching device 132, and is input to the first optical delayed detector 133-1, the second optical delayed detector 133-2, and the optical intensity receiver 135. Be done.
- the first optical delay detector 133-1 is such that the delay time difference Td between the two internal optical paths is approximately equal to the symbol time T of the received optical multilevel information signal, and the optical phase difference between the two paths is zero. It is set to.
- the second optical delay line detector 133-2 is set such that the delay time difference Td between the two internal optical paths is approximately equal to T, and the optical phase difference between the two paths is ⁇ / 2.
- the output lights of the first and second optical delay detectors 133-1 and 133-2 are converted into electric signals by the balanced optical receivers 134-1 and 134-2, respectively, and then AD converters 136-1, At 136-2, they are converted into digital signals dI (n) and dQ (n). Further, the output electric signal of the light intensity receiver 135 is also converted into a digital signal P (n) by the AD converter 136-3.
- the digital signals dI (n) and dQ (n) are input to the adaptive equalization circuit 115-1 to remove a part of waveform distortion, and then input to the inverse tangent calculation circuit 137.
- a two-argument inverse tangent operation is performed using dI (n) as the X component and dQ (n) as the Y component, and the phase angle is calculated.
- ⁇ (n) is the phase difference ( ⁇ (n) ⁇ (n ⁇ 1)) from the immediately preceding symbol of the received n-th light electric field symbol. Since dI and dQ are sine and cosine components of ⁇ (n), respectively, the arctangent operation circuit 137 can calculate ⁇ (n) by performing an inverse tangent (inverse Tan) operation of four quadrants.
- One of the problems to be solved by the present invention is that, in the conventional optical multilevel transmission system using direct detection, completeness etc. of modulation distortion caused by imperfection of optical modulation on the transmission side in the optical multilevel receiver. In that it can not be That is, in the optical multilevel transmission system using the direct detection shown in FIG. 4 described above, since the optical multilevel signal restoration process inside the optical multilevel receiver is non-linear, the electric field orthogonality is made inside the transmitter. Linear distortion that occurs in modulation can not be linearly equalized.
- the adaptive equalization circuits 115-1 and 115-2 are disposed inside the noncoherent optical receiver of FIG. 4, they can be corrected by a linear transversal filter generally used as these adaptive equalization circuits. Is only a part of the distortion of the received waveform. For example, in the adaptive equalization circuits 115-1 and 115-2, equalization of imperfections in the frequency characteristics of the receiver, reflection of high frequency signals inside the receiver, deviation of the origin of signal point arrangement, amplitude error, etc. However, in principle, it is not possible to equalize modulation distortion, which is waveform distortion generated due to linear deterioration inside the optical transmitter.
- FIG. 5 shows the results of numerical calculation of modulation / demodulation of a four-level phase modulation signal using an optical transmission simulation, and shows an example of modulation distortion in a conventional optical transmission system using direct detection, adaptation on the receiving side, etc.
- Show the effects of 5A shows the signal point arrangement of the original multilevel signal generated by the complex multilevel signal generation circuit
- FIG. 5B shows the modulation distortion which is deteriorated due to the lack of the frequency characteristic inside the transmitter and the deviation of the modulation timing. It is a signal point arrangement of the output optical signal 109.
- this calculation example is a simple four-level phase modulation example, and the phase preintegration circuit 126, the preequalization circuit 125, the DA converter 104, etc. shown in FIG. 4 are not used.
- FIGS. 5C and 5D show signal point constellations of complex signals obtained by receiving and reproducing the 4-level phase modulation signal with modulation distortion by the noncoherent optical multilevel receiver 130 of FIG. Due to the influence, modulation distortion increases more than the output light signal (B).
- (C) is the case where there is no adaptive equalization circuit 115
- (D) is the case where the adaptive equalization circuit 115 is used, but the effect of the adaptive equalization is only slightly obtained, and most of the modulation distortion remains. I understand.
- the second problem to be solved by the present invention is performance in an optical multilevel modulation circuit using a plurality of optical modulators using the conventional direct detection, the performance being caused by the timing difference of the electric signal to be applied and the difference of the frequency characteristics. It is deterioration. Since these may have variations at the time of mounting, and their amounts also change due to aging deterioration or temperature characteristic deviation, it is difficult to make them completely zero on the transmission side, and causes the deterioration of transmission characteristics and become.
- An object of the present invention is to equalize modulation distortion in an optical transmission system using direct detection. Another object is to prevent deterioration of transmission characteristics and enhance the practicability of the optical transmission system.
- the object of the present invention is to provide an optical transmitter comprising a polar coordinate type optical phase modulator that modulates the phase of an optical signal in the phase rotation direction, a combination type two-dimensional optical differential detection receiver, and two or more AD converters
- an optical receiver provided with a differential phase calculation circuit and a digital adaptive equalizer
- the optical receiver receives the binary or more optical phase multilevel modulation signal sent from the optical transmitter, and converts the output signal of the two-dimensional optical delay detector into a high-speed digital signal by the AD converter. This is realized by inputting to the differential phase calculation circuit and performing adaptive processing of the multilevel signal after adaptively equalizing the calculated differential phase component by the digital adaptive equalizer.
- an optical amplitude modulator that does not cause phase inversion of the optical signal
- an optical intensity detector is provided inside the optical receiver
- An optical multi-level modulated signal in which both the phase and the amplitude of the optical signal are modulated is transmitted from the optical transmitter, and a differential phase component obtained from the optical receiver is an optical intensity modulation component obtained from the optical intensity modulator
- the object of the present invention can be realized by performing the determination process of the multilevel signal after combining the light amplitude modulation component which is the square root thereof.
- a Mach-Zehnder type optical modulator is used as the optical amplitude modulator, and the modulation signal applied to the modulation electrode of the Mach-Zehnder modulator is biased such that it does not extend over the extinction point which is the minimum point of the light transmission characteristic. If modulation is performed, since phase jump does not occur at the time of amplitude modulation, correction of modulation distortion according to the present invention can be effectively implemented.
- the phase modulator and the amplitude modulator are realized by a two-electrode Mach-Zehnder type modulator, the sum of the voltages applied to the two electrodes is a phase modulation component, and the difference between the voltages applied to the two electrodes is an amplitude modulation component It can be realized by modulating so that the difference between the applied voltages does not cross the extinction point of the Mach-Zehnder type optical amplitude modulator.
- the number of optical phase modulators does not have to be one, and a plurality of polar phase modulators that modulate the phase of the optical signal in the rotational direction may be cascaded.
- any one of a plurality of optical amplitude modulators that do not cause phase inversion or polar coordinate phase modulators that modulate the phase of the optical signal in the rotational direction may be cascaded.
- phase modulation signal of the optical phase modulator is a high-speed analog signal generated by a DA converter whose sampling rate is greater than 1 sample / symbol, and the phase of the generated signal becomes continuous when the phase modulation range exceeds ⁇ . This can be realized by interpolating and modulating the phase and amplitude of the signal point.
- the range of the calculated differential phase modulation exceeds ⁇ ⁇ (or 0 to 2 ⁇ ). This can be realized by performing phase unwrapping processing so as to be continuous.
- An optical transmitter comprising a polar coordinate type optical phase modulator that modulates the phase of an optical signal in the direction of phase rotation
- An optical receiver comprising a coupled two-dimensional optical delay detection receiver, two or more AD converters, a differential phase calculation circuit, and a digital adaptive equalizer; Equipped with The optical receiver receives the binary or more optical phase multilevel modulation signal sent out from the optical transmitter, The two output signals of the combined two-dimensional optical delay detection receiver are respectively converted into high-speed digital signals by the AD converter and input to the differential phase calculation circuit, and the difference calculated by the differential phase calculation circuit.
- modulation distortion can be equalized in an optical transmission system using direct detection. Further, according to the present invention, deterioration of transmission characteristics can be prevented, and the practicability of the optical transmission system can be enhanced.
- FIG. 14 is an explanatory diagram of the problem of the present embodiment, and is an explanatory diagram showing a state of equalization of modulation distortion in a conventional digital coherent optical multilevel transmission method; It is a block diagram of the phase pre-integration type
- FIG. 14 is an explanatory diagram of the problem of the present embodiment, and is an explanatory diagram showing a state of equalization of modulation distortion in a conventional digital coherent optical multilevel transmission method; It is a block diagram of the phase pre-integration type
- FIG. 1 is a configuration diagram of a direct detection light multilevel transmission system using light multilevel phase modulation in a first embodiment of the present embodiment. It is an explanatory view showing phase transition of a phase modulation signal of this embodiment in a direct detection light multilevel transmission system using multilevel phase modulation of this embodiment. It is an explanatory view showing a situation of equalization of modulation distortion in a direct detection light multilevel transmission system using multilevel phase modulation of this embodiment.
- FIG. 7 is a configuration diagram of a direct detection light multilevel transmission system using light multilevel amplitude / phase modulation according to a second embodiment of the present embodiment. It is explanatory drawing of the principle of the light amplitude modulation part in 2nd Embodiment of this Embodiment.
- FIG. 14 is a configuration diagram of a direct detection light multilevel transmission system using light multilevel amplitude / phase modulation according to a third embodiment of the present embodiment. It is explanatory drawing which shows the modulation
- FIG. 6 is a block diagram showing the configuration of a direct detection light multilevel transmission system using light multilevel phase modulation according to the first embodiment of the present invention.
- the feature of this configuration is, for example, the point that a pure multilevel phase modulation light is generated using a polar coordinate optical phase modulator on the transmission side, and differential phase components are extracted from the received signal, Modulation equalization by equalizing modulation distortion.
- the path of the optical signal is indicated by a thick line
- the path of the high frequency signal of electricity is indicated by a thin line
- the path of the parallel electric digital signal using a plurality of signal lines is indicated by an open arrow.
- the present optical transmission system includes, for example, an optical phase multilevel transmitter (optical transmitter) 200 and an optical phase multilevel receiver (optical receiver) 204.
- the optical phase multilevel transmitter 200 includes a phase multilevel signal generation circuit 202, a DA converter 104, a driver circuit 105, a laser light source 106, and a polar coordinate type optical phase modulator 201.
- the optical phase multilevel receiver 204 includes an optical splitter 132, an optical delay detection circuit (combined two-dimensional optical delay detection receiver) 133, a balanced optical receiver 134, an AD converter 136, and an arctangent operation.
- a circuit (differential phase calculation circuit) 137, a phase adaptive equalization circuit (digital adaptive equalizer) 205, an orthogonal coordinate conversion circuit 139, and a phase multilevel signal determination circuit 215 are included.
- phase multilevel signal generation circuit 202 receives m bits (m is an integer of 2 or more) parallel information signals from digital information input terminal 101. This is assigned to a signal point of an M-value (M is an integer of 2 or more) multi-level electric signal, and is output as a digital parallel signal. This signal is converted to an electric high-speed analog signal by the DA converter 104, then amplified by the driver circuit 105, and then input to the polar coordinate type optical phase modulator 201 of this embodiment, and the output light of the laser light source 106 is pure. Convert to phase modulated light.
- the polar coordinate type optical phase modulator 201 used in the present embodiment is a device having a characteristic of linearly changing the input high-speed electric signal to the phase of the optical signal.
- it can be realized by a waveguide type element utilizing the electro-optical effect of a semiconductor such as a lithium niobate substrate or the like, and is widely marketed as a phase modulator.
- FIG. 7 shows the phase modulation signal generated in this manner.
- FIG. 7A shows an example of the phase transition waveform of the phase modulation signal 203 according to the present embodiment.
- This waveform is proportional to the high-speed electrical signal applied to the polar coordinate phase modulator 201 of this embodiment, and has a feature that the waveform changes continuously when transitioning between signal points.
- This example is four-level phase modulation, and the phase modulation signal is a central time t, t + T, t + 2T. . . .
- the discrete value of one of the phases 3 ⁇ / 4, ⁇ / 4, - ⁇ / 4 and -3 ⁇ / 4 is taken.
- the true signal point positions are indicated by white circles a to f.
- FIG. 7B shows signal point changes on the complex plane, and in the polar coordinate phase modulator used in the present embodiment, the electric field of the optical signal when transiting between the signal points a to f. Has the feature of moving along the rotational direction (phase rotational direction). For example, in the case of transition from the signal point d to e, the transition does not proceed linearly, but along the phase rotation direction as shown in FIG. 7 (B). That is, modulation distortion generated by phase modulation of an optical multilevel signal is linearly converted to modulation distortion of a phase component by using a polar coordinate type phase modulator having such features.
- a binary phase modulator can be realized by a Mach-Zehnder (MZ) type optical phase modulator
- a simple Mach-Zehnder (MZ) type optical phase modulator is a polar coordinate type phase modulator intended in the present embodiment. is not. That is, in the MZ type optical modulator, when an input electrical signal is applied, a phase change of 0 or ⁇ can be produced before and after the extinction point, but the amplitude passes through the origin (amplitude zero) along the way This is because the phase component instantaneously and discontinuously inverts as the voltage greatly changes, and the above-mentioned condition “the phase modulation is approximately proportional to the applied voltage” is not satisfied.
- the phase multilevel signal generated in this manner is transmitted through the optical fiber transmission line 122 and then received by the optical phase multilevel receiver 204 of the present embodiment.
- the present receiver is an optical multilevel receiver using optical direct detection as in FIG. In this example, only the reception of the phase multilevel signal is premised, so the light intensity receiver 135 is not used.
- the largest difference between this configuration and the conventional optical multilevel receiver using the conventional direct detection of FIG. 4 is that the phase adaptive equalization circuit 205 is disposed immediately after the inverse tangent arithmetic circuit 137 to adaptively correct the differential phase ⁇ . The point is that
- FIG. 8 shows the correction effect of modulation distortion in the present embodiment.
- the signal point (A) of the original multi-level signal generated by the complex multi-level signal generation circuit 202 is greatly degraded by the imperfection of the frequency characteristic of the high frequency signal in the transmitter 200, and the signal point of the optical phase modulation signal 203
- the arrangement is greatly disturbed as shown in FIG. 8 (B).
- 8C and 8D show complex values obtained by receiving this four-level phase modulation signal with modulation distortion by the optical phase multilevel receiver 204 of FIG. 6 and reproducing the detected differential phase component by the orthogonal transformation circuit 139. It is a signal point arrangement of a signal.
- the amplitude of each multilevel signal is set to a constant value (1).
- FIG. 8C shows the case where the phase adaptive equalization circuit 205 is not provided
- FIG. 8D shows the case where the phase adaptive equalization circuit 205 is used. It can be seen that the modulation distortion is almost completely equalized by this configuration. .
- FIG. 9 is a block diagram of a direct detection light multilevel transmission system using light multilevel amplitude / phase modulation according to the second embodiment of the present invention.
- the feature of this configuration is that, for example, in addition to the phase modulation of the configuration of FIG. 6, an optical amplitude modulator 211 is introduced so that the amplitude of light can also be modulated, and light intensity reception is enabled so that detection of amplitude component becomes possible It is a point in which the unit 135 is introduced and expanded so that both amplitude and phase can be used for information transmission.
- multi-value amplitude / phase modulation that can be used in such a configuration, for example, 16-value amplitude / phase modulation shown in FIG.
- This modulation can be generated by independently modulating the amplitude of the optical signal with two values and the phase with eight values on the transmission side, and receiving the amplitude component and the phase component (differential phase component) independently on the reception side.
- the polar coordinate multilevel signal generation circuit 212 outputs a complex multilevel signal in which two-dimensional polar coordinates of the amplitude information r and the phase information ⁇ are expressed. Both are converted to high frequency electric signals by the DA converters 104-1 and 104-2, respectively, amplified by the driver circuit 105, and then input to the optical amplitude modulator 211 and the polar coordinate optical phase modulator 201, respectively.
- These two light modulators are cascade-connected to the laser light source 106, and when the laser light passes through the inside, they apply optical multilevel amplitude modulation and optical multilevel phase modulation, respectively, It is configured to be converted into a phase modulation signal 213.
- the optical amplitude modulation component and the optical phase modulation component are applied independently, so that no waveform interference occurs between them, and modulation of the phase component is performed.
- distortion and modulation distortion of the amplitude component are independently transferred to the light amplitude and phase modulation signal 213.
- FIG. 10 is an explanatory example of the principle of light amplitude modulation used in this configuration, and shows an example of using a non-chirp MZ type light modulator as the light amplitude modulator 211.
- FIG. 10A shows a signal point arrangement of an optical signal generated by binary intensity modulation.
- the amplitude value is a binary value of a and b (0 ⁇ a ⁇ b), and the amplitude modulation is performed because it is not chirped. It is assumed that no phase change occurs with it.
- Such modulation applies, for example, a small amplitude binary electric digital signal modulated with an information signal to an X-cut MZ type optical modulator as shown in FIG. 10B.
- the electric signal level L0 of symbol 0 and the electric signal level L1 of symbol 1 are applied to the shoulder portion of the sinusoidal light transmission characteristic of the MZ modulator as shown in the drawing (there is no extinction point). do it. If L0 and L1 are sufficiently small and sufficiently close to the extinction point (point of zero transmittance) of the light transmission characteristics, the conversion characteristics of the electric signal and the optical electric field become almost linear. It is possible to convert linearly.
- FIG. 10B shows the light phase of the MZ modulator, as described above, the light phase rapidly changes from 0 to ⁇ at the extinction point.
- the MZ modulator for amplitude modulation, it is possible to maintain the linearity of the optical phase modulation so that the optical phase inversion does not occur. desirable.
- the differential phase component ⁇ (n) and the amplitude are the same as in the noncoherent optical multilevel receiver 130 using the direct detection of FIG.
- the modulation component r (n) is detected.
- These are respectively input to the phase adaptive equalization circuit 205 of this embodiment and the amplitude adaptive equalization circuit 214 of this embodiment, are adaptively equalized, and modulation distortion is removed for each component. Since this example is multilevel transmission that independently modulates the amplitude and phase of the multilevel signal, the adaptively equalized differential phase component and amplitude component are independently the phase multilevel signal determination circuit 215, and the amplitude multilevel signal.
- the signal is input to the determination circuit 216 to demodulate the multilevel signal.
- the amplitude adaptive equalization circuit 214 is provided immediately after the square root circuit 138, this is an example in which light modulation is performed so that the light amplitude becomes linear on the transmission side (for example, the configuration of FIG. 9). It is effective in removing modulation distortion. If the amplitude of the high frequency signal is proportional to the intensity of the output light, as in the case of intensity modulation of a semiconductor laser, it is more effective to insert an adaptive equalization circuit immediately before the square root circuit 138. Also, the number of adaptive equalization circuits need not be one, and even in the configuration of FIG. 9, the frequency characteristics etc. of the light intensity receiver 135 can be corrected by separately arranging the adaptive equalization circuit immediately before the square root circuit 138. There is an advantage that a correction effect can be obtained.
- the configuration of the optical amplitude modulator of the present embodiment is not limited to this example.
- phase modulation associated with the intensity modulation be zero, but when using an optical amplitude modulator that produces linear phase rotation with respect to the modulation voltage of the amplitude component (or intensity component),
- the accompanying phase modulation component can also be equalized by providing an adaptive equalization filter that corrects the phase component from the light amplitude component r (n) (or the light amplitude component P (n)) in the unit.
- An equalization effect of high modulation distortion is obtained in the form of
- the modulation distortion of the phase modulation component can be used without any problem since the equalization effect can be obtained independently.
- FIG. 11 is a block diagram of a direct detection light multilevel transmission system using light multilevel amplitude / phase modulation in the third embodiment of the present embodiment.
- the point using the interlock modulation of the amplitude and the phase and the point using the multistage dependent modulation of the phase are major features.
- alteration by the combination of a high-speed binary signal is employ
- the feature is that the modulation of the amplitude and the phase is not completely independent, but a part thereof is in an interlocking relationship.
- a modulation signal can be generated, for example, by first generating binary amplitude / phase modulation in which both the amplitude and phase change as shown in FIG. 12A, and further superposing four-level phase modulation on this. It is.
- the polar coordinate multilevel signal generation circuit 212 In the optical amplitude / phase multilevel transmitter 210 of FIG. 11, the polar coordinate multilevel signal generation circuit 212 generates 1-bit amplitude information r and 2-bit phase information ⁇ 1 and ⁇ 2. These binary high frequency electric signals are amplified to desired amplitudes by the driver circuits 105-1, 105-2, 105-3, and then the optical amplitude phase modulator 226 and polar coordinate type optical phase modulator 201-1, respectively. , And is applied to the polar coordinate type optical phase modulator 201-2.
- the optical amplitude phase modulator 226 subjects the input light to the binary amplitude / phase modulation shown in FIG.
- the polar coordinate type optical phase modulator 201-1 performs binary phase modulation of phase amplitude ⁇
- the polar coordinate type optical phase modulator 201-2 performs binary phase modulation of phase amplitude ⁇ / 2.
- four-value phase modulation in which two different amplitude phase modulations are added is superimposed on the two-value amplitude / phase modulation of FIG. 12A, and the eight-value amplitude / phase modulation of FIG. Can be generated.
- phase modulators or amplitude-phase modulators
- all of the phase modulation components and phase modulation distortion applied by each modulator are linearly added in the phase domain. Therefore, it is possible to adaptively equalize the modulation distortion of the phase component by the phase adaptive equalization circuit 205 in the optical amplitude / phase multilevel receiver 219 of this embodiment. Note that since it is necessary to receive an 8-level amplitude and phase modulation signal inside the receiver 219, the differential phase component ⁇ (n) and the amplitude component r (n) separately received and adaptively equalized are orthogonal coordinates.
- This signal is different from the original optical multilevel signal r (n) exp (j ⁇ (n)), and is affected by the phase ⁇ (n-1) of the previous symbol, so that the multilevel signal judgment is simply performed.
- symbol determination can be performed by a method such as using MLSE (maximum likelihood sequence estimation) method in the phase multilevel signal determination circuit 117, for example.
- FIG. 12C is a first configuration example of the optical amplitude phase modulator 226 used in this configuration.
- the MZ type optical modulator 223 and the polar coordinate type optical phase modulator 201 are connected in cascade.
- the input binary electric signal is divided into two, amplified by the driver circuits 105-4 and 105-5, and applied to the MZ optical modulator 223 and the polar coordinate optical phase modulator 201, respectively.
- the locus of the signal point becomes as shown in the right figure of FIG. 12C, and desired binary amplitude / phase modulation is realizable.
- FIG. 12D shows a second configuration example of the light amplitude phase modulator 226 used in this configuration.
- This is an example using an integrated light modulator including the MZ light modulator 223, and the principle of such a waveform generation method is disclosed in detail in Patent Document 2: WO2008 / 026326.
- the input light 221 is branched into two, one of which is input to the MZ optical modulator 223 and the other to the waveguide unit 228.
- the binary electric signal 220 is amplified by the driver circuit 105-4 and then input to the MZ type optical modulator 223.
- the binary electric signal is applied so that the extinction point of the MZ modulator is at the amplitude center, and the output optical signal is binary phase modulation.
- the binary phase-modulated light has its phase angle ⁇ rotated by the light phase adjustment area 224 and its amplitude is attenuated by the light attenuator 225.
- the output light 227 of this MZ section is binary phase modulation light rotated by ⁇ as in the signal point arrangement of FIG.
- an electrical signal applied to the MZ modulator is applied across the extinction point, and the MZ modulator is used as a nonpolar coordinate phase modulator.
- the phase transition between the two signal points AB is linear, and can be regarded as a substantially linear phase rotation which does not cause a phase discontinuity.
- the MZ type phase modulator that causes phase discontinuities when using as a modulator that does not generate phase discontinuities as a whole by using optical interference as in this example, It can be regarded as a polar coordinate phase modulator according to the embodiment.
- this example shows an example of generation of an optical multilevel signal not using an optical DA converter, even if a DA converter is used to drive a part or all of the optical modulators, it is possible to apply this embodiment. There is no problem at all.
- a multi-level signal may be generated by a DA converter, and a part of the light modulators may be driven by the multi-level signal.
- a method may be considered in which a digital signal processing circuit is also used on the transmission side to pre-equalize part of modulation distortion.
- application of the present embodiment is effective because signal constellations with higher accuracy can be obtained by applying and equalizing modulation distortion remaining on the receiving side.
- the DA converter is not used, the equivalent modulation distortion can be reduced by the pre-enhancement of the drive signal and the correction circuit of the transmitter band. Even in this case, the present embodiment can be further applied. It is.
- FIG. 13 is a block diagram of a direct detection light multilevel transmission system using light QAM modulation in the fourth embodiment of the present invention.
- the arrangement of the phase pre-integration circuit 126 enables arbitrary multi-level modulation to be performed, the phase up sampling circuit 218 is added to improve the continuity of phase rotation, dispersion pre
- the feature of the configuration is that the signal processing circuit 230 is added and that the phase unwrap / speed conversion circuit 231 is disposed on the receiving side. It is not necessary to use all of these functions at the same time, and some functions may be arbitrarily selected and implemented as needed.
- the polar coordinate multi-level signal generation circuit 212 assigns a complex QAM signal (for example, the 16 QAM signal of FIG. 1C) to the input information signal, and its phase component ⁇ and amplitude component r Output Among them, the phase component is input to the phase preaccumulation circuit 126, and the phase component is integrated for each symbol. This phase pre-integration cancels the detection effect of the differential phase on the receiving side as described in the prior art of FIG. 4, and enables transmission of an arbitrary QAM signal.
- a complex QAM signal for example, the 16 QAM signal of FIG. 1C
- the amplitude information and the integrated phase information are respectively input to the amplitude up sampling circuit 217 of the present embodiment and the phase up sampling circuit 218 of the present embodiment, are upsampled to about twice the sampling rate, and further polar coordinates.
- Signal point interpolation is performed above.
- the polar coordinate interpolation has an effect of preventing a detection error of phase rotation on the reception side.
- FIG. 14A is an explanatory view of the time waveform. Assuming that signal points a to f are central phase angles of multi-level symbol times t to t + 5T, the upsampled waveform is added with signal points A to F obtained by interpolating phases on polar coordinates at symbol boundary times. Although not shown in this example, interpolation is performed on the polar component plane as in the case of the amplitude component.
- FIG. 14B is an example illustrating transition to signal points d to e on a two-dimensional complex plane, and clearly showing the effect of interpolation.
- phase angle of the signal points d to e exceeds ⁇ , it is difficult to determine whether the signal transition path left the option on the transition plane or leftward on the complex plane if interpolation is not performed. Become. In particular, when interpolation of signal points is performed on the same orthogonal coordinates as that for generating a multi-level QAM signal, the interpolation point is located between points d and e as shown by D 'in FIG. 14 (B). An incorrect signal point transition 234 results, and the adaptive equalization on the receiving side will not operate normally. Therefore, by designating the point D interpolated on the polar coordinates as an intermediate sample, the correct signal point transition 233 is ensured, whereby the adaptive equalization on the receiving side can be correctly performed.
- the up-sampling rate is exactly doubled, but this is a figure in consideration of the performance of the subsequent chromatic dispersion pre-equalization.
- the value does not necessarily have to be doubled, and may be any value in principle if it is a value exceeding one time.
- the upsampled phase / amplitude information is input to the dispersion pre-equalization circuit 230, and dispersion pre-equalization processing is performed to cancel the influence of the chromatic dispersion of the optical fiber transmission line 122 in advance.
- This can use the technique described in the prior art of FIG.
- the digital signal after chromatic dispersion pre-equalization is output in polar coordinates of amplitude and phase, is converted to a high frequency electric signal by DA converters 104-1 and 104-2, and is desired by driver circuits 105-1 and 105-2.
- the present invention can be realized by utilizing a polar coordinate type arbitrary optical field modulator in which an optical amplitude modulator and a polar coordinate type optical phase modulator are connected in cascade The embodiment can be applied.
- the optical amplitude modulator 211 does not cause phase inversion. That is, when an MZ modulator is used for the optical amplitude modulator 211, it should be used so that the high frequency electric signal to be applied does not extend over the extinction point of the extinction characteristic causing phase inversion as described in FIG. This point is a major difference from the conventional MZ type optical amplitude modulator and the configuration of an arbitrary electric field modulator using the same.
- one Mach-Zehnder modulator may be used as a polar coordinate phase modulator. Such an arrangement is disclosed in detail in US Pat.
- FIG. 15 shows the configuration of the two-electrode MZ modulator 300.
- This modulator is widely used as an intensity modulator of light, and the input light 221 is branched into two optical waveguides 228-1 and 228-2 by the optical coupler 301-1 and then re-connected by the optical coupler 301-2. The light is synthesized and output as an output light 222.
- the modulation electrodes 302-1 and 302-2 are disposed in each of the optical waveguides, and the ends thereof are connected to the termination resistors 302-1 and 303-2. At this time, voltages applied to the modulation electrodes 302-1 and 302-2 are denoted by VL and VR, respectively.
- the average value of the voltages VL (t) and VR (t) applied to the two electrodes is the phase modulation amount, and the difference is the amplitude modulation amount.
- the present modulator also satisfies the above amplitude condition and is applied to the present embodiment. It becomes possible.
- modulation distortion due to interference may occur between the two voltages because the phase modulation component and the amplitude modulation component are added and subtracted after being added and subtracted. . Even in such a case, in this embodiment, modulation distortion can be reduced and received by using a butterfly-type adaptive equalization filter that cancels out intersymbol interference between amplitude components and phase components inside the receiver. It becomes.
- the optical QAM signal receiver 236 of the present embodiment shown in FIG. 13 detects an optical signal as in the above-described embodiment, but the difference from the above-described embodiment is the AD converter 136- 1, 136-2 after setting the sampling rate to approximately twice the symbol rate and detecting the differential phase component .phi. (N) by the arctangent operation circuit 137, the phase unwrapping process by the phase unwrapping / velocity conversion circuit 231. And the sampling rate downsampled to 1 sample / symbol.
- phase unwrapping it is determined whether the phase shift amount has exceeded the output range (normally + ⁇ to - ⁇ ) of the inverse tangent calculation circuit 137, and if it exceeds this range, the output signal range is expanded to output the output signal.
- a to F are signal points at symbol boundaries observed by sampling at a double speed with a DA converter.
- the phase at point D is + 0.5 ⁇ and the phase at point E is + 0.45 ⁇
- the amount of phase rotation decreases to ⁇ or less during the transition from point d to point D and a phase jump to + ⁇ occurs.
- a path passing through three points D, e and E and a phase jump from + ⁇ to ⁇ occurs again becomes a correct phase transition. If such phase discontinuities occur and adaptive equalization processing is performed, the equalization processing becomes incomplete and it becomes impossible to completely remove modulation distortion.
- phase unwrapping process the phases of the signal points are shifted by 2 ⁇ and connected so that the phase change between the signal points is, for example, less than or equal to ⁇ , thereby preventing occurrence of phase discontinuity.
- signal points D, e, E are shifted to 2 ⁇ on the negative side and signal points D ′, e ′, E ′ are connected to D ′, e ′, E ′, f in order from point d. If a dotted path (path after unwrapping) is taken, the amount of phase change between signal points can be made less than or equal to ⁇ .
- This unwrapping process is performed by the phase unwrapping / speed conversion circuit 231.
- the output signal P of the light intensity receiver 135 is also sampled by the AD converter 136-3 at approximately twice the symbol rate, and then input to the square root circuit 138 and the speed converter circuit 232. It is converted to one sample / symbol again. It is not necessary to use this configuration for the amplitude component because it does not require unwrapping processing as in the case of phase, but this configuration matches the phase component with the processing timing, and improves the accuracy of the amplitude calculation. It has the effect of
- the polar coordinate phase modulation on the transmission side and the detection of the differential phase on the reception side have a linear relationship even in an optical multilevel transmission system using direct detection (or noncoherent detection), and the phase region
- modulation distortion can be removed.
- phase jump does not occur at the time of amplitude modulation. Distortion correction can be implemented effectively.
- an optical multilevel signal can be generated even by connecting an optical amplitude modulator and a phase modulator in cascade, and an effect that a complex multilevel signal can be generated without using a DA converter is obtained.
- the sampling rate of the AD converter greater than 1 sampling / symbol and performing phase unwrapping, it becomes possible to correct the phase rotation discontinuity, so that the scope and effect of the present invention can be further enhanced. Can be significantly enhanced.
- the present embodiment can be applied to non-coherent optical fiber transmission of, for example, optical multilevel signals in the field of optical communication, in particular, optical phases, or optical multilevel signals with good transmission efficiency in which amplitude and phase are modulated.
- the present invention is also applicable to an optical multilevel transmitter, an optical multilevel receiver, and an optical multilevel transmission system used for such optical fiber transmission.
- Optical multi-value transmitter 101 Digital information input terminal (m bits) 102: complex multilevel signal generation circuit 103: complex multilevel information signal 104: DA converter 105: driver circuit 106: laser light source 107: orthogonal optical electric field modulator 108: output optical fiber 109: output optical signal 110: balance type optical Detector 111: AD converter 112: local laser light source 113: polarization separation / light 90 degree hybrid circuit 114: wavelength dispersion compensation circuit 115: adaptive equalization circuit 116: phase estimation circuit 117: multilevel signal determination circuit 120: Digital coherent optical receiver 121: Input optical signal 122: Optical fiber transmission line 123: Phase pre-accumulation type optical multi-level transmitter 124: Complex up-sampling circuit 125: Pre-equalization circuit 126: Phase pre-accumulation circuit 130: Non-coherent light Receiver 132: Optical splitter 133: Optical delay detection circuit 134: Balanced optical receiver 135: Optical intensity receiver 13 : AD converter 137:
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Abstract
Description
光多値送信器100では、レーザ光源106から出力される無変調のレーザ光が直交光電界変調器107に入力され、所定の電界変調を施した出力光信号109が出力光ファイバ108から出力される。伝送すべき情報信号は、並列(例えばmビット幅)の2値高速デジタル電気信号列としてデジタル情報入力端子101に入力される。本信号は複素多値信号生成回路102で、数ビットごとにまとめて複素多値情報信号103に変換される。本信号は、2次元のIQ平面上で(i(n)、q(n))(nはサンプル番号)と表現されるデジタル電気多値信号であり、時間間隔T(=シンボル時間)毎にその実部iと虚部qが出力される。これらの信号はそれぞれDA変換器104-1、104-2で高速アナログ信号に変換された後、ドライバ回路105-1、105-2によって増幅され、直交光電界変調器107のI、Q2つの変調端子に入力される。これによって、出力光信号109は複素多値信号(i、q)を光電界の同相成分Iと直交成分Qに持つ光電界信号となる。なお、光振幅・位相変調信号の光電界は(i(n)+jq(n))exp(jω(n))であり、ω(n)はレーザ光源106の光角周波数である。なお、本例では多値変調の際に、DA変換器104を用いる構成を示したが、多値数の少ない場合、例えば4値位相変調などを実現する場合には、DA変換器を用いず直交光電界変調器に2組の2値信号を印加する場合もある。
前記光送信器から送出された2値以上の光位相多値変調信号を前記の光受信器で受信し、前記二次元光遅延検波器の出力信号を該AD変換器で高速デジタル信号に変換して差動位相算出回路に入力し、算出された差動位相成分を前記デジタル適応等化器で適応等化したのちに多値信号の判定処理を行うことによって実現できる。
前記光送信器から光信号の位相と振幅がともに変調された光多値変調信号を送信し、前記の光受信器から得られる差動位相成分に、光強度変調器から得られる光強度変調成分ないしはその平方根である光振幅変調成分を合成したのちに多値信号の判定処理を行うことで、本発明の目的が実現できる。
この際、前記の光振幅変調成分の一部が光位相変調成分の一部と連動し、同一の情報信号で変調するようにすれば、QAM変調などのより複雑な多値変調を生成し、かつ本発明が適用できるようになる。
光信号の位相を位相回転方向に変調する極座標型光位相変調器を備えた光送信器と、
結合型二次元光遅延検波受信器と2個以上のAD変換器と差動位相算出回路とデジタル適応等化器を備えた光受信器と、
を備え、
前記光送信器から送出された2値以上の光位相多値変調信号を前記光受信器で受信し、
前記結合型二次元光遅延検波受信器の2つの出力信号を前記AD変換器でそれぞれ高速デジタル信号に変換して前記差動位相算出回路に入力し、該差動位相算出回路で算出された差動位相成分を前記デジタル適応等化器で適応等化したのちに多値信号の判定処理を行うことを特徴とした光伝送システムが提供される。
(第1の実施の形態)
図6は、本発明の第1の実施の形態における、光多値位相変調を用いた直接検波光多値伝送方式の構成を示す構成図を示している。
すなわち、このような特徴を持つ極座標型位相変調器を利用することで光多値信号の位相変調によって発生する変調歪は、位相成分の変調歪に線形に変換される。
また、図9は本発明の第2の実施の形態における、光多値振幅・位相変調を用いた直接検波光多値伝送方式の構成図である。
本構成の特徴は、例えば、図6の構成の位相変調に加えて、光の振幅も変調できるように光振幅変調器211を導入し、また振幅成分の検出が可能となるように光強度受信器135を導入し、振幅と位相の両方を情報伝送に用いることができるように拡張した点である。このような構成で利用できる多値振幅・位相変調の例としては、例えば図1(D)の16値振幅位相変調が挙げられる。本変調は送信側では光信号の振幅を2値、位相を8値で独立に変調することで生成でき、また受信側では振幅成分と位相成分(差動位相成分)をそれぞれ独立に受信することで復調できる。
図10(A)は2値強度変調によって生じる光信号の信号点配置であり、本例では振幅値はaとb(0<a<b)の2値であり、無チャープであるため振幅変調に伴う位相の変化は生じないものとする。このような変調は例えば、XカットのMZ型光変調器に図10(B)のように情報信号で変調された小振幅の2値の電気デジタル信号を加える。この際、シンボル0の電気信号レベルL0とシンボル1の電気信号レベルL1が、図示のようにMZ変調器の正弦波状の光透過特性の肩の部分にかかる(消光点をまたがない)ようにすればよい。L0、L1が十分小で光透過特性の消光点(透過率ゼロの点)に十分近ければ、電気信号と光電界の変換特性はほぼ線形となるため、駆動信号の変調歪を光振幅変化に線形に変換することが可能となる。
図11は本実施の形態の第3の実施の形態における、光多値振幅・位相変調を用いた直接検波光多値伝送方式の構成図である。
例えば、振幅と位相の連動変調を用いた点、および位相の多段従属変調を用いた点が大きな特徴である。またDA変換器を利用せずに、高速2値信号の組み合わせによって多値変調を生成する構成を採用している。本構成で利用できる多値振幅・位相変調の例としては、例えば図12(B)の8値振幅位相変調(ないしは8値QAM変調)が挙げられる。その特徴は振幅と位相の変調が完全には独立になっておらず、その一部が連動関係にある点である。このような変調信号は、例えば最初に図12(A)のような振幅と位相がともに変化する2値振幅・位相変調を生成し、これにさらに4値の位相変調を重畳することで生成可能である。
MZ型光変調器223と極座標型光位相変調器201を縦続接続した構成である。本例では、入力された2値電気信号を2つに分割し、それぞれドライバ回路105-4、105-5で増幅して、MZ型光変調器223と極座標型光位相変調器201に印加する。この際、ドライバ回路の出力信号の振幅を、初段のMZ光変調器223で無チャープの振幅r=b-a(振幅値aからb)の2値振幅変調を生成するように、また同時に後段の極座標型光位相変調器201で振幅φ0の位相変調が生じるように設定しておけば、信号点の軌跡は図12(C)右図のようになり、所望の2値振幅・位相変調が実現できる。
MZ型光変調器223を含む集積型光変調器を利用した例であり、このような波形生成法の原理は特許文献2:WO2008/026326号公報に詳細に開示されている。
図13は本発明の第4の実施の形態における、光QAM変調を用いた直接検波光多値伝送方式の構成図である。
本例では、例えば、位相予積算回路126を配置することにより任意の多値変調を行えるようにした点、位相アップサンプル回路218を追加して位相回転の連続性を高めた点、分散予等化回路230を追加した点、また受信側に位相アンラップ/速度変換回路231を配置した点が構成の特徴である。なお、これらの各機能はすべて同時に利用する必要はなく、必要に応じていくつかの機能を任意に選択して実装すればよい。
なお特別な例として、一個のマッハツェンダ型変調器を極座標位相変調器として利用するケースが挙げられる。このような構成は特許文献3:United States Patent 7023601に詳細に開示されている。
位相のアンラップ処理とは位相の変移量が逆正接演算回路137の出力範囲(通常は+π~-π)を越えたかどうかを判定し、越えた場合には出力信号の範囲を拡張して出力信号の位相の連続性を保つ手法である。例えば、図14(C)の信号点dで観測された逆正接演算回路137の出力信号、すなわち差動位相Δφ=-0.9π、また信号点eでは差動位相Δφ=-0.6π、信号点fでは差動位相Δφ=-πの点であったとする。もしサンプリング速度が1サンプル/シンボルであれば、破線のようにa~fを滑らかに接続するような位相変化が生じたと解釈するしかない。これに対し、A~FはDA変換器で2倍速でサンプリングを行って観測したシンボル境界での信号点である。このときD点の位相が+0.5π、E点の位相が+0.45πであれば、d点からD点への遷移の途中で位相回転量が-π以下になり+πへの位相ジャンプが発生し、図の実線のようにD、e、Eの3点を通過して再び、+πから-πへの位相ジャンプが発生したとする経路が、正しい位相遷移となる。このような位相不連続が生じたまた適応等化処理を行うと、等化処理が不完全となり、変調歪を完全に除去することができなくなる。
上述の各実施の形態によると、直接検波(ないしは非コヒーレント検波)を用いた光多値伝送方式においても送信側の極座標位相変調と受信側の差動位相の検出が線形関係になり、位相領域の適応等化フィルタを用いることで変調歪が除去できるようになるという効果がある。
また本発明における、光多値信号は光振幅変調器や位相変調器を縦続接続しても生成でき、DA変換器を用いなくても複雑な多値信号が生成できるという効果が得られる。
101:デジタル情報入力端子(mビット)
102:複素多値信号生成回路
103:複素多値情報信号
104:DA変換器
105:ドライバ回路
106:レーザ光源
107:直交光電界変調器
108:出力光ファイバ
109:出力光信号
110:バランス型光検出器
111:AD変換器
112:局発レーザ光源
113:偏波分離・光90度ハイブリッド回路
114:波長分散補償回路
115:適応等化回路
116:位相推定回路
117:多値信号判定回路
120:デジタルコヒーレント光受信器
121:入力光信号
122:光ファイバ伝送路
123:位相予積算型光多値送信器
124:複素アップサンプル回路
125:予等化回路
126:位相予積算回路
130:非コヒーレント光受信器
132:光分岐器
133:光遅延検波回路
134:バランス型光受信器
135:光強度受信器
136:AD変換器
137:逆正接演算回路
138:平方根回路
139:直交座標変換回路
200:本実施の形態の光位相多値送信器
201:極座標型光位相変調器
202:位相多値信号生成回路
203:本実施の形態の光位相変調信号
204:本実施の形態の光位相多値受信器
205:本実施の形態の位相適応等化回路
210:本実施の形態の光振幅・位相多値送信器
211:光振幅変調器
212:極座標多値信号生成回路
213:本実施の形態の光振幅・位相変調信号
214:本実施の形態の振幅適応等化回路
215:位相多値信号判定回路
216:振幅多値信号判定回路
217:本実施の形態の振幅アップサンプル回路
218:本実施の形態の位相アップサンプル回路
219:本実施の形態の光振幅・位相多値受信器
220:2値電気信号
221:入力光
222:出力光
223:MZ型光変調器
224:光位相調整領域
225:光アッテネータ部
226:光振幅位相変調器
227:MZ部の出力光
228:導波路部
229:導波路部の出力光
230:分散予等化回路
231:位相アンラップ/速度変換回路
232:速度変換回路
233:正しい信号点遷移
234:間違った信号点遷移
235:本実施の形態の光QAM信号送信器
236:本実施の形態の光QAM信号受信器
300:2電極型MZ変調器
301:光カプラ
302:変調電極
303:終端抵抗
Claims (10)
- 光信号の位相を位相回転方向に変調する極座標型光位相変調器を備えた光送信器と、
結合型二次元光遅延検波受信器と2個以上のAD変換器と差動位相算出回路とデジタル適応等化器を備えた光受信器と、
を備え、
前記光送信器から送出された2値以上の光位相多値変調信号を前記光受信器で受信し、
前記結合型二次元光遅延検波受信器の2つの出力信号を前記AD変換器でそれぞれ高速デジタル信号に変換して前記差動位相算出回路に入力し、該差動位相算出回路で算出された差動位相成分を前記デジタル適応等化器で適応等化したのちに多値信号の判定処理を行うことを特徴とした光伝送システム。 - 請求項1に記載の光伝送システムにおいて、
前記光送信器は光信号の位相反転を引き起こさない光振幅変調器を備え、
前記光受信器は光強度検出器を備え、
前記光送信器から光信号の位相と振幅がともに変調された光多値変調信号を送信し、
前記光受信器で得られる差動位相成分と、前記光強度検出器から得られる光強度変調成分又はその平方根である光振幅変調成分とについて多値信号の判定処理を行うことを特徴とする光伝送システム。 - 請求項2に記載の光伝送システムにおいて、前記光送信器における光振幅変調が光位相変調と連動し、同一の情報信号で変調されることを特徴とした光伝送システム。
- 請求項2に記載の光伝送システムにおいて、
前記光振幅変調器がマッハツェンダ型の光変調器であり、
前記マッハツェンダ型の光変調器の変調電極に印加する変調信号が光透過特性の最小点である消光点をまたがないように該変調信号をバイアスした状態で変調することを特徴とした光伝送システム。 - 請求項2に記載の光伝送システムにおいて、
前記光位相変調器および前記光振幅変調器を、2電極マッハツェンダ型の変調器で実現し、
2つの電極に印加する電圧の和が位相変調成分、2つの電極に印加する電圧の差が振幅変調成分となるようにし、前記印加する電圧の差が前記マッハツェンダ型光振幅変調器の消光点をまたがないようにして変調することを特徴とした光伝送システム。 - 請求項1に記載の光伝送システムにおいて、
前記光位相変調器が光信号の位相を位相回転方向に変調する複数の極座標型光位相変調器を縦続接続した構成であることを特徴とする光伝送システム。 - 請求項2に記載の光伝送システムにおいて、
前記光振幅変調器は、位相反転を引き起こさない光振幅変調器と、光信号の位相を位相回転方向に変調する第2の極座標型光位相変調器とを縦続接続した光振幅位相変調器であることを特徴とする光伝送システム。 - 請求項2に記載の光伝送システムにおいて、
前記光位相変調器と前記光振幅変調器は、位相反転を引き起こさない前記光振幅変調器と、光信号の位相を位相回転方向に変調する前記極座標型光位相変調器を複数個とを縦続接続した構成であることを特徴とする光伝送システム。 - 請求項1に記載の光伝送システムにおいて、
前記光位相変調器の位相変調信号はサンプリング速度が1サンプル又はシンボルより大なるDA変換器で生成された高速アナログ信号であり、位相変調範囲がπを超える場合でも生成信号の位相が連続となるように信号点の位相と振幅を補間して変調することを特徴とする光伝送システム。 - 請求項1に記載の光伝送システムにおいて、
前記光受信器内に配置される前記AD変換器のサンプリング速度が1サンプル又はシンボルより大となるようにし、算出された差動位相成分の範囲が±π又は0から2πを越えても連続となるように位相のアンラップ処理を行うことを特徴とする光伝送システム。
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