JP2018007206A - Multi-carrier optical receiver and optical transmission system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a multi-carrier optical receiver and an optical transmission system having both tolerance to a transmission characteristic deterioration, caused by the characteristic of an optical fiber, and module miniaturization in a well-balanced manner.SOLUTION: A multi-carrier optical receiver 2 includes: an optical receiver module 21 which independently receives two optical signals with a direct detection method through an optical fiber 9; an A/D converter 22 which converts the two optical signals received by the optical receiver module 21 into respective digital signals; a pseudo signal circuit 23 which generates each digital signal of a coherent detection method in a simulated manner from the two digital signals converted by the A/D converter 22; and a DSP 24 which performs the adaptive equalization of a signal waveform to the digital signal of the coherent detection method which the pseudo signal circuit 23 generates.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a multicarrier optical receiver and a technology of an optical transmission system.

  Due to the rapid traffic increase in the data center, standardization of 100 GbE (Gigabit Ethernet) and (Ethernet is a registered trademark) and development of optical modules are progressing as a large-capacity optical transmission system. For example, the mainstream of 100 GbE optical modules is composed of 4 waves x 25 [Gbit / s] IM-DD (Intensity Modulation-Direct Detection) transceivers, and CFP4 (C form-factor pluggable) as an optical interface in the data center. Development of smaller, power-saving modules such as 4) and QSFP28 (Quad Small Form-factor Pluggable 28) is in progress.

  On the other hand, in order to connect data centers directly with a 100 GbE optical interface, a standard for long-distance transmission is being studied. For example, a small module compatible with LR4, which is a standard for 10 km transmission, has started to be marketed, and a small module compatible with ER4, which is a standard for 40 km transmission, is also planned for commercialization. In order to cover a 40km loss in an ER4-compatible optical module, improvement of the transmission / reception characteristics by improving the extinction ratio by using a Mach-Zehnder type external modulator and by improving the minimum light receiving sensitivity by APD has been studied (Non-patent Document 1). .

In addition to optical fiber loss and transmission / reception characteristics, there are optical signal waveform distortions due to optical fiber wavelength dispersion and polarization mode dispersion (PMD: Polarization-Mode Dispersion) as factors that limit the transmission distance of the optical interface.
As for chromatic dispersion, when a G.652 optical fiber having a zero dispersion wavelength in the 1.3 μm band is used, there is almost no influence of chromatic dispersion on the signal light wavelength of the existing 100 GbE in the 1.3 μm band.
On the other hand, PMD is a phenomenon in which individual components of an optical pulse incident on a fiber reach the output end at different times due to changes in refraction in the fiber. As measured values of laid optical fibers, PMD coefficients of 4.79 ps / √km (Non-patent Document 2) and 0.94 ps / √km (Non-patent Document 3) have been reported. These values were not a problem at the time of 10 Gbit / s optical signal transmission, but cannot be ignored in the transmission of a 25 Gbit / s optical signal with a narrower bit interval.

The resistance to PMD is affected by the modulation scheme (detection scheme) of the optical transmission system. The detection method is roughly classified into a “direct detection (DD)” method that directly measures the intensity of the received optical signal and a “coherent detection” method that measures the phase of the received optical signal.
First, as an example of direct detection, there is an IM-DD system in which a digital code “1” is set to light intensity “on” and a digital code “0” is modulated to light intensity “off”. Although the direct detection has a simple mechanism and can easily reduce the size of the module, it has a low resistance to PMD because it directly receives a signal component spread in time due to the influence of PMD.

On the other hand, coherent detection includes, for example, a PSK (Phase-Shift Keying) method in which the light intensity for each bit is constant, and QPSK (Quadrature Phase-Shift Keying) using 4 points in the signal space diagram is biased. Wave-multiplexed DP (Dual polarization) -QPSK is mainly used (Non-Patent Document 4). In a digital coherent system using DP-QPSK, PMD compensation by a digital signal processing technique for an electric field component of a received signal is possible, and has high resistance to PMD.
Note that EVM (Error Vector Magnitude) for received signals is known as an index for evaluating transmission characteristics including optical signal waveform distortion due to PMD and wavelength dispersion (Non-Patent Document 5).

Mengyuan Huang, et al., “25Gb / s Normal Incident Ge / Si Avalanche Photodiode,” ECOC2014, We.2.4.4, (2014). D. Breuer, et al., “Measurements of PMD in the installed fiber plant of Deutsche Telekom,” LEOS Summer Topical Meetings, MB2.1, pp.5-6, 2003. Toshiya Matsuda, et al., “PMD Design for High-Speed WDM Backbone Network Systems Based on Field PMD Measurements”, IEICE Trans. Commun., Vol. E94-B, no. 5, pp. 1303-1313, 2011. Kazuro Kikuchi, “Phase-Diversity Homodyne Detection of Multilevel Optical Modulation With Digital Carrier Phase Estimation,” IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 12, NO. 4, JULY / AUGUST 2006. W. Freude, et al., “Quality Metrics for Optical Signals: Eye Diagram, Q-factor, OSNR, EVM and BER,” Proc. ICTON 2012, Mo. B1.5, 2012.

In order to reduce the cost and power consumption of optical transmission systems, the optical module connected to the optical fiber is reduced in size and power consumption, and the transmission characteristic deterioration due to the characteristics of the optical fiber is suppressed to extend the transmission distance. It is necessary to achieve both.
However, the IM-DD system is excellent in miniaturization, but is vulnerable to transmission characteristic deterioration due to the characteristics of the optical fiber. On the other hand, the existing 100G digital coherent system using the DP-QPSK signal is excellent in resistance to transmission characteristic deterioration due to the characteristics of the optical fiber, but is difficult to apply to a small optical module because of its complicated configuration. In this way, both types are pros and cons.

  SUMMARY OF THE INVENTION Accordingly, it is a main object of the present invention to provide a multicarrier optical receiver and an optical transmission system that balance both resistance to transmission characteristic deterioration due to characteristics of optical fibers and downsizing of a module in a balanced manner.

In order to solve the above problems, the multicarrier optical receiver of the present invention has the following features.
In other words, the multicarrier optical receiver includes an optical receiver module that independently receives two optical signals via an optical fiber by a direct detection method;
An A / D converter for converting each of the two optical signals received by the optical receiver module into a digital signal;
A pseudo signal circuit that pseudo-generates a coherent detection type digital signal from the two digital signals converted by the A / D converter;
A digital signal processor that performs adaptive equalization of the signal waveform with respect to the digital signal of the pseudo coherent detection method generated by the pseudo signal circuit;
The pseudo signal circuit is
A delay circuit that adjusts the delay of the two digital signals output from the A / D converter so that the centers of the pulses coincide with each other;
For each of the two digital signals output from the delay circuit, the amplitude is converted by normalizing each signal, and the two digital signals after normalization are combined on a two-dimensional complex plane. And an amplitude conversion circuit that uses the pseudo coherent detection digital signal.

  As a result, the direct detection system module, which is easy to miniaturize, and the digital signal processor that corrects the transmission characteristic deterioration due to the characteristics of the optical fiber are connected via the pseudo signal circuit, thereby improving the characteristics of the optical fiber. It is possible to achieve a balance between the resistance to the transmission characteristic deterioration and the downsizing of the module.

In the present invention, the digital signal processor comprises:
An adaptive equalization circuit that performs signal waveform shaping by adaptive equalization processing on a digital signal of the coherent detection method that is pseudo-generated by the pseudo signal circuit;
A phase correction circuit for correcting a phase rotation component for the output signal of the adaptive equalization circuit;
And an identification circuit for demodulating a data signal sequence with respect to the output signal of the phase correction circuit.

  As a result, it is possible to enhance the resistance against transmission characteristic degradation caused by the characteristics of the optical fiber, even for a pseudo-coherent detection digital signal.

In the present invention, the optical receiver module receives an optical signal using two sets of IM-DD systems as a direct detection system,
The amplitude conversion circuit synthesizes a QPSK signal as the pseudo coherent detection digital signal.

  This makes it possible to balance the resistance against deterioration of transmission characteristics due to the characteristics of the optical fiber and the miniaturization of the module in a well-balanced manner by using an inexpensive and widely used intensity-modulating optical receiver module.

In the present invention, the optical receiver module receives an optical signal using two sets of M-value amplitude modulation direct detection methods as a direct detection method,
The amplitude conversion circuit synthesizes an M ² -QAM (Quadrature Amplitude Modulation) signal as the pseudo coherent detection digital signal.

  This makes it possible to balance the resistance against deterioration of transmission characteristics due to the characteristics of the optical fiber and the miniaturization of the module in a well-balanced manner by using an amplitude-modulating optical receiver module with a simple mechanism.

The present invention provides the multi-carrier optical receiver,
A multi-carrier optical transmitter configured to transmit an optical signal to the multi-carrier optical receiver via the optical fiber,
The optical transmission module of the multicarrier optical transmitter transmits an optical signal by a direct detection method corresponding to the optical reception module.

  As a result, the optical transmitter and the optical receiver can be installed as a set, and can be introduced as an optical transmission system that balances resistance to deterioration of transmission characteristics due to the characteristics of the optical fiber and miniaturization of the module in a balanced manner.

The present invention transmits n sets of optical signals from the multicarrier optical transmitter to the multicarrier optical receiver as a set of two optical signals,
The pseudo signal circuit performs pseudo processing of n digital signals from 2n digital signals by performing n sets of processes for synthesizing two digital signals into one digital signal of the pseudo coherent detection method. It is characterized by producing | generating.

  As a result, even when the number of signal channels is large, it is possible to introduce an optical transmission system that balances resistance to deterioration of transmission characteristics due to the characteristics of the optical fiber and miniaturization of the module.

  ADVANTAGE OF THE INVENTION According to this invention, the multicarrier optical receiver and optical transmission system which can balance the tolerance with respect to the transmission characteristic deterioration resulting from the characteristic of an optical fiber, and size reduction of a module in good balance can be provided.

FIG. 1A is an overall configuration diagram of an optical transmission system. FIG. 1B is a detailed configuration diagram of a part of the optical transmission system. It is explanatory drawing of the processing content of the delay circuit concerning this embodiment. It is explanatory drawing of the processing content of the amplitude converter circuit concerning this embodiment. FIG. 4A shows an experimental environment for evaluation. FIG. 4B shows the evaluation result in FIG. FIG. 5A shows an experimental environment for evaluation. FIG. 5B shows the evaluation result in FIG.

  Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a configuration diagram of an optical transmission system. FIG. 1A is an overall configuration diagram, and FIG. 1B is a partial detailed configuration diagram.
The optical transmission system of FIG. 1A transmits a signal from the optical transmission module 11 of the multicarrier optical transmitter 1 to the optical reception module 21 of the multicarrier optical receiver 2 via the optical fiber 9.
The multicarrier optical transmitter 1 and the multicarrier optical receiver 2 may support one-way communication or two-way communication. When the bidirectional communication is supported, the multicarrier optical transmitter 1 has the optical reception module 21, and the multicarrier optical receiver 2 has the optical transmission module 11.

Here, in the optical transmission system of FIG. 1 (a), as a detection method between the optical transmission module 11 and the optical reception module 21, a direct detection method (multi-carrier) that receives two wavelength signals independently is used. This makes it possible to reduce the size and power consumption of the module.
The employed direct detection method may be, for example, two sets of intensity modulation direct detection methods (IM-DD method) or two sets of M-value amplitude modulation direct detection methods.

  The multi-carrier optical receiver 2 includes an A / D (Analog-to-Digital) converter 22, a pseudo signal circuit 23, a DSP (Digital Signal Processor) 24, Have The A / D converter 22 converts two received signals that have been subjected to O / E (Optical / Electronic) conversion in the optical reception module 21 into digital signals.

The DSP 24 is a circuit that performs adaptive equalization such as PMD compensation and signal identification for signals of a coherent detection method such as a QPSK signal. However, the optical transmission module 11 and the optical reception module 21 transmit signals of the direct detection method, and the phase information of the optical signal is lost. Therefore, if a digital signal in which the phase information of the optical signal is lost is input to the DSP 24 as it is, the DSP 24 cannot perform adaptive equalization.
Therefore, the pseudo signal circuit 23 generates a pseudo QPSK signal to be input to the subsequent DSP 24 based on the digital signal output from the preceding A / D converter 22. Thereby, unlike the normal digital coherent method, the phase information of the optical signal in the received signal is lost, so that it is not possible to completely compensate for the transmission characteristic deterioration due to the characteristics of the optical fiber such as chromatic dispersion and PMD. However, in a region where the transmission distance is relatively short, such as a metro network, it is possible to sufficiently enhance the resistance against transmission characteristic degradation caused by the characteristics of the optical fiber.

The pseudo signal circuit 23 includes a delay circuit 23a and an amplitude conversion circuit 23b.
The delay circuit 23 a controls the delay of the digital signal output from the A / D converter 22.
FIG. 2 is an explanatory diagram of processing contents of the delay circuit 23a.
First, the digital signal before delay adjustment will be described with reference to reference numerals 101 and 102.
At reference numeral 101, the circuits in the delay circuit 23a (the tau ₁ shown in FIG. 1 (b) and an upper, tau ₂ to the lower side), the optical signal detected directly, respectively, are input. The horizontal axis of the optical signal is the time axis, and the vertical axis is the light intensity.
At 102, the light intensities of the two waveforms at the reference time indicated by the dotted vertical line of 101 are shown as the horizontal axis (I axis) of the graph. In the upper graph, since the heights of the two waves are located at the upper limit and the lower limit at the reference time, the size of the I-axis of the reference numeral 102 is also in the range from the upper limit (I = + 1) to the lower limit (I = 0) Two dots).

Next, the digital signal after delay adjustment will be described with reference to reference numerals 111 and 112.
In reference numeral 111, the upper graph from the state of reference numeral 101 remains unchanged, but the upper and lower graphs both reach the upper and lower limit peaks at the reference time by adjusting the time of the lower graph slightly (shifted to the left). Adjusted to
Reference numeral 112 is a graph corresponding to the reference numeral 111, and the upper and lower graphs are both shown in the range from the upper limit (I = + 1) to the lower limit (I = 0) at the reference time.

As described above with reference to FIG. 2, the delay circuit 23 a adjusts the delay with respect to the output signal of the A / D converter 22 so that the bit center positions coincide. Since the delay adjustment process of the delay circuit 23a is a signal process for two uncorrelated signals, it is only necessary that the centers of the pulses coincide with each other, and the maximum delay amount is half of 1 bit. Therefore, the circuit of the delay circuit 23a is simple and contributes to miniaturization.
On the other hand, in the prior art such as the optical transmission system described in Japanese Patent Application Laid-Open No. 2015-65516, since signal processing is performed on two correlated signals, it is necessary to adjust the delay in bit units so that the bit patterns match. The delay amount is a delay difference between the transmission lines. Therefore, the delay adjustment processing in the prior art becomes complicated and it is difficult to reduce the size.

FIG. 3 is an explanatory diagram of the processing contents of the amplitude conversion circuit 23b.
Reference numeral 121 is the same graph as reference numeral 112 in FIG. That is, the output signal of the delay circuit 23a becomes the input signal of the amplitude conversion circuit 23b.
Reference numeral 122 is a signal obtained by normalizing the signal indicated by the reference numeral 121 by the amplitude conversion circuit 23b. The lower limit value (the leftmost point) of the horizontal axis (I axis) extends from I = 0 to I = -1. Hereinafter, regarding the two signals denoted by reference numeral 122, the signal indicated by the upper graph is referred to as signal I ₁ (t), and the signal indicated by the lower graph is referred to as signal I ₂ (t).

A signal space diagram denoted by reference numeral 123 (a signal point arrangement diagram on a two-dimensional complex plane) shows the result of the amplitude conversion circuit 23b combining the two signals denoted by reference numeral 122. Reference numeral 123 denotes a signal space diagram composed of a horizontal axis (I axis) and a vertical axis (Q axis), and indicates a polarization multiplexed QPSK pseudo signal using four points on the circumference. For example, a point on the first quadrant (both the I axis and the Q axis are positive) is associated with the digital value “11”.
The amplitude conversion circuit 23b generates the pseudo signal Ex (t) based on the signal of reference numeral 122 by the calculation formula “Ex (t) = I ₁ (t) + jI ₂ (t)”.

  Returning to FIG. 1B, the DSP 24 includes an adaptive equalization circuit 24a, a phase correction circuit 24b, and an identification circuit 24c. As described above, the DSP 24 is a circuit that performs adaptive equalization on the QPSK signal. The pseudo signal Ex (t) output from the amplitude conversion circuit 23b also performs adaptive equalization in the same manner as the QPSK signal. be able to. For example, a circuit described in JP-A-2015-65516 may be used as a circuit that performs adaptive equalization for a QPSK signal.

The adaptive equalization circuit 24a corrects the waveform of the pseudo signal Ex (t) output from the amplitude conversion circuit 23b (performs signal waveform shaping). Specifically, the adaptive equalization circuit 24a uses the complex signal EX (t) obtained by performing adaptive signal processing of an FIR (Finite Impulse Response) filter by maximum likelihood estimation from the complex signal input Ex (t), and the phase correction circuit 24b. Output to. Note that the algorithm for realizing the maximum likelihood estimation may be an arbitrary algorithm such as CMA (Constant Modulus Algorithm) or DD-LMS (Decision Directed Least Mean Square) described in JP-A-2015-65516. .
Incidentally, Non-Patent Document 4, it is described that updates the tap coefficients h ₁₁ of the FIR filter by using the output of the adaptive equalization circuit 24a (Formula 1) (Formula 2). That is, the "the Tap update" of the adaptive equalization circuit 24a of FIG. 1 (b), a mechanism for updating the tap coefficients h ₁₁ of the FIR filter.

Here, μ is a step size parameter. Unlike normal coherent reception, signal rotation associated with the frequency difference or phase difference between signal light and local light does not occur. However, since the phase rotation component accompanying the adaptive equalization circuit 24a is generated, the phase correction circuit 24b corrects the phase rotation component.
Then, the identification circuit 24c demodulates the data signal sequence based on the complex signal output from the phase correction circuit 24b and outputs it to the subsequent stage (another device).

  As described above, in the optical transmission system described with reference to FIGS. 1 to 3, the example in which two wavelength signals are independently received as the multicarrier direct detection method has been described. Here, since each wavelength signal is independent, by repeating the signal processing similar to (Equation 1) and (Equation 2) of the adaptive equalization circuit 24a, not only two wavelength signals but also 2n (where n is It can also be extended to multi-carrier signals with an integer of 2 or more. That is, the pseudo signal circuit 23 is expanded to generate n sets of pseudo QPSK signals from 2n digital signals output from the A / D converter 22.

Further, the pseudo signal generated by the pseudo signal circuit 23 is not limited to the QPSK signal, and may be an M ² QAM (Quadrature Amplitude Modulation) signal in which points are arranged in a grid pattern in the signal space diagram. For example, a 4-QAM signal with M = 2 has one point in each quadrant and a total of four points, resulting in the same signal space diagram as the QPSK signal.

FIG. 4 shows the result of evaluating the EVM (Error Vector Magnitude) of the optical transmission system.
FIG. 4A shows an experimental environment for evaluation. As the optical transmission module 11, a total of 4 NRZ signals of 4 × 25.8 Gbit / s arranged at a wavelength of 50 GHz in the range of 15551.72 nm to 1552.92 nm are transmitted. A multiplexer is configured to input a transmission signal from one transmitter (Tx) to one optical fiber 9 with a signal light power of 0 dBm / channel. As this transmission line condition, a G.652 optical fiber having a zero dispersion wavelength of 1310 nm is used, and the optical loss value in the optical signal band is 0.3 dB / km. In the optical receiver module 21, the duplexer outputs the signal transmitted through the optical fiber 9 to the four receivers (25G Rx).

In the graph of FIG. 4B, PMD compensation is not performed on the received signal output from the A / D converter 22 (that is, the pseudo signal circuit 23 and the DSP 24 are not provided), “conventional IM-DD”, and PMD. It is a graph which compares EVM with "this invention" which compensates (that is, the pseudo signal circuit 23 and DSP24 are provided like FIG. 4A).
The “conventional IM-DD” and the “present invention” are common in that the IM-DD is used as the modulation / demodulation method of the optical transmission module 11 → the optical reception module 21, so that the module can be easily downsized.
Further, as shown in the graph of FIG. 4B, the “present invention” has good transmission characteristics because the EVM value is low not only for a short distance but also for a middle distance (˜20 km). In other words, the transmission distance satisfying 25% or less of EVM corresponding to the bit error rate 1E-4 that can be corrected by the Reed-Solomon [255,239], which is an existing FEC, is less than 15 km in the “conventional IM-DD”. Is extended to 20km.

FIG. 5A shows a configuration in which a PC 31 and a DGD (Differential Group Delay) 32 are added to the transmission path of the optical fiber 9 with respect to the experimental environment of FIG.
The PC 31 is a polarization controller that adjusts the received signal to a polarization state that maximizes the waveform degradation caused by the DGD 32. The DGD 32 is a DGD emulator that is provided at the end of the transmission line and simulates PMD by giving DGD = 3 · PMD [ps] corresponding to the risk factor 1E-4.
In FIG. 5 (a), a transmission signal is transmitted using a 4 x 25.8 Gbit / s NRZ signal having a wavelength arrangement in the range of 1295.56 nm to 1309.14 nm, which is a 100 GbE specification, as a transmission signal, with a signal light power of 0 dBm / channel. To enter. As a transmission line condition, a G.652 optical fiber having a zero dispersion wavelength of 1310 nm is used, the PMD coefficient is 1 ps / √km, and the optical loss value in the optical signal band is 0.4 dB / km.

  The graph of FIG. 5B shows the EVM with respect to the transmission distance, as in FIG. In the “conventional IM-DD”, the transmission distance satisfying EVM of 25% or less is limited to about 27 km due to the influence of DGD, whereas DGD is compensated even at 40 km by using the “present invention”. I understand that.

As described above, in the optical transmission system according to the present embodiment, a direct detection method such as IM-DD, which is widely used as an inexpensive transmission / reception method, is used for the optical transmission module 11 and the optical reception module 21 at the front stage. In addition to being applied to the optical module, signal waveform distortion is compensated for by the maximum likelihood approximation of the adaptive equalization circuit 24a for the DSP 24 at the rear stage. In order to input the direct detection signal to the PMD compensation (adaptive equalization of signal waveform) mechanism of coherent detection, a pseudo signal circuit 23 is newly prepared. As a result, it is possible to provide an optical transmission system that balances the extension of the transmission distance by improving the resistance to the deterioration of the transmission characteristics due to the characteristics of the optical fiber and the reduction in size and power consumption of the module in a well-balanced manner.
In addition to enabling the use of an inexpensive optical module, it is expected that the development cost of the digital signal processing portion can be reduced by diverting existing ones to the mounting contents and design contents of the DSP 24.

DESCRIPTION OF SYMBOLS 1 Multicarrier optical transmitter 2 Multicarrier optical receiver 9 Optical fiber 11 Optical transmission module 21 Optical reception module 22 A / D converter 23 Pseudo signal circuit 23a Delay circuit 23b Amplitude conversion circuit 24 DSP (digital signal processor)
24a Adaptive equalization circuit 24b Phase correction circuit 24c Identification circuit

Claims (6)

An optical receiver module that independently receives two optical signals via an optical fiber by direct detection; and
An A / D converter for converting each of the two optical signals received by the optical receiver module into a digital signal;
A pseudo signal circuit that pseudo-generates a coherent detection type digital signal from the two digital signals converted by the A / D converter;
A digital signal processor that performs adaptive equalization of the signal waveform with respect to the digital signal of the pseudo coherent detection method generated by the pseudo signal circuit;
The pseudo signal circuit is:
A delay circuit that adjusts the delay of the two digital signals output from the A / D converter so that the centers of the pulses coincide with each other;
For each of the two digital signals output from the delay circuit, the amplitude is converted by normalizing each signal, and the two digital signals after normalization are combined on a two-dimensional complex plane. A multi-carrier optical receiver, comprising: an amplitude conversion circuit that converts the pseudo coherent detection digital signal. The digital signal processor is
An adaptive equalization circuit that performs signal waveform shaping by adaptive equalization processing on a digital signal of the coherent detection method that is pseudo-generated by the pseudo signal circuit;
A phase correction circuit for correcting a phase rotation component for the output signal of the adaptive equalization circuit;
The multicarrier optical receiver according to claim 1, further comprising: an identification circuit that demodulates a data signal sequence with respect to an output signal of the phase correction circuit. The optical receiver module receives an optical signal using two sets of IM-DD (Intensity Modulation-Direct Detection) systems as a direct detection system,
3. The multicarrier optical reception according to claim 1, wherein the amplitude conversion circuit synthesizes a QPSK (Quadrature Phase-Shift Keying) signal as the pseudo coherent detection digital signal. 4. Machine. The optical receiver module receives an optical signal using two sets of M-value amplitude modulation direct detection methods as a direct detection method,
The multi-carrier light according to claim 1, wherein the amplitude conversion circuit synthesizes an M ² -QAM (Quadrature Amplitude Modulation) signal as the pseudo coherent detection digital signal. Receiving machine. The multi-carrier optical receiver according to claim 1 or 2,
A multi-carrier optical transmitter configured to transmit an optical signal to the multi-carrier optical receiver via the optical fiber,
The optical transmission system of the multicarrier optical transmitter transmits an optical signal by a direct detection method corresponding to the optical reception module. Transmitting two sets of optical signals from the multicarrier optical transmitter to the multicarrier optical receiver as one set, and n sets of optical signals,
The pseudo signal circuit performs pseudo processing of n digital signals from 2n digital signals by performing n sets of processes for synthesizing two digital signals into one pseudo coherent detection digital signal. The optical transmission system according to claim 5, wherein the optical transmission system is generated as follows.

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