Complex signal field recovery system and recovery method based on native optical carrier

Abstract

The present invention discloses a complex signal field recovery system based on native optical carrier, including a transmitting end digital signal processing module, a digital-to-analog converter, an electro-optical modulator, an optical fiber link, an optical coupler, a dispersion element, a photodetector, an analog-to-digital converter and a receiving end digital signal processing module; the transmitting end digital signal processing module performs nonlinear pre-compensation and DC bias correction of the electro-optical modulator; the electro-optical modulator converts the electrical signal from the digital-to-analog converter into an optical signal carrying target information, and generates a native optical carrier at the same time; the dispersion element performs dispersion on the optical signal passing through; and the receiving end digital signal processing module performs signal recovery operations. The present invention also discloses a complex signal field recovery method, including the following steps: electro-optically modulating an optical carrier signal using an electro-optical modulator with a finite extinction ratio; receiving an optical signal using a phase recovery receiver; and performing signal field recovery based on an improved Gerchberg‑Saxton algorithm in the receiving end digital signal processing module.

Description

Technical Field

[0001] The present invention relates to the field of optical communication technology, and in particular to a complex signal field recovery system and recovery method based on native optical carrier. Background Art

[0002] With the development of technologies such as B5G and virtual reality, the telecommunications industry seeks to expand to higher throughput and lower cost per transmitted bit. Coherent detection has become a promising candidate for the next generation of short-distance optical communications because it can linearly map the full optical field to the electrical domain with the assistance of the local oscillator. Optical field impairments such as dispersion can be effectively compensated by digital signal processing. In order to eliminate the use of local oscillators, self-coherent solutions have been proposed to reduce the cost of receivers, such as Kramers-Kronig receivers, Stokes vector receivers, and carrier-assisted differential detection. In addition, a phase recovery receiver based on the improved Gerchberg-Saxton (MGS) algorithm has been verified. For high symbol rate and high spectral efficiency signal transmission, the target signal is more susceptible to damage from the non-idealities of the transceiver itself, especially for low-cost transceiver components.

[0003] However, existing phase recovery receivers require additional pilot symbols or pilot tones to eliminate phase ambiguity or initialize phase states, which results in information redundancy, rate reduction, and additional digital signal processing complexity. At the same time, existing phase recovery receivers do not consider the impact of transmitter non-idealities on target signals, such as the limited extinction ratio damage of electro-optic modulators, the bias point shift with temperature, and the nonlinear damage of electro-optical modulators.

[0004] In summary, the existing phase recovery receiver solutions and their system component solutions do not take all factors into consideration. Although they can perform signal recovery, they cannot be efficiently used in actual systems. There is still room for improvement in terms of information redundancy and rate, as well as the complexity of digital signal processing.

[0005] In addition, because the optical signal field is an in-phase orthogonal signal, information is loaded on both the amplitude and phase of the signal field, but the receiver can only collect intensity information, resulting in missing information in the phase dimension. Summary of the invention

[0006] In order to solve the above-mentioned technical problems, the present invention provides a complex signal field recovery system and recovery method based on native optical carrier.

[0007] In order to solve the above technical problems, the technical solution proposed by the present invention is:

[0008] A complex signal field recovery system based on native optical carrier includes a transmitting end digital signal processing module, a laser, a digital-to-analog converter, an electro-optical modulator, an optical fiber link, an optical coupler, a dispersion element, a photodetector, an analog-to-digital converter and a receiving end digital signal processing module;

[0009] The transmitting end digital signal processing module performs signal preprocessing; the transmitting end digital signal processing module performs electro-optic modulator nonlinear precompensation and DC bias correction;

[0010] The laser is used to supply an optical carrier to the electro-optic modulator;

[0011] The digital-to-analog converter converts the digital signal from the digital signal processing module at the transmitting end into an analog electrical signal;

[0012] The electro-optic modulator is used to convert the electrical signal from the digital-to-analog converter into an optical signal carrying target information;

[0013] The optical fiber link is used to transmit the optical signal carrying the target information to the optical coupler;

[0014] The optical coupler is used to split the optical signal carrying target information into two or more optical signals;

[0015] The dispersive element performs a dispersive effect on the optical signal passing therethrough;

[0016] The photodetector is used to convert the optical signal into a corresponding electrical signal.

[0017] The analog-to-digital converter converts the electrical domain analog signal from the photodetector into a digital domain signal and sends it to the receiving end digital signal processing module;

[0018] The receiving end digital signal processing module performs a signal recovery operation.

[0019] As a further improvement of the above technical solution:

[0020] Preferably, the transmitting end digital signal processing module is additionally provided with a DC bias correction algorithm, which cooperates with an existing nonlinear pre-compensation algorithm for the electro-optic modulator to pre-compensate the device nonlinearity of the in-phase orthogonal electro-optic modulator.

[0021] Preferably, the DC bias correction algorithm is expressed as:

[0022] E target (t) = E DPD (t)-α·sign( <E DPD (t)>) <E DPD (t)>

[0023] Among them, E targetis the DC bias correction output signal, α is the DC bias correction factor; E DPD is the signal after digital predistortion; sign(·) is the sign function; <·> is the averaging operator.

[0024] Preferably, the extinction ratio of the electro-optic modulator is limited, or a bias point of the electro-optic modulator is caused to have a temperature drift, and the output modulated optical signal of the electro-optic modulator has a native optical carrier.

[0025] Preferably, the receiving end digital signal processing module performs signal recovery operations, and the internal components are phase recovery, dispersion compensation, subcarrier demultiplexing, signal equalization, judgment and signal-to-noise ratio calculation based on the MGS algorithm. Based on the phase recovery capability of the native optical carrier system with the improved Gerchberg-Saxton algorithm, the dependence on additionally inserted pilot symbols or pilot tones is eliminated.

[0026] The present invention also provides a complex signal field recovery method based on native optical carrier, using the above recovery system, the recovery method comprises the following steps:

[0027] Step S1: performing nonlinear pre-compensation and DC bias correction of the electro-optic modulator in the digital signal processing module at the transmitting end;

[0028] Step S2: At the transmitter end of the system, an electro-optical modulator with a finite extinction ratio is used to electro-optically modulate the optical carrier signal;

[0029] Step S3: At the system receiver end, a phase recovery receiver is used to receive the optical signal;

[0030] Step S4: Perform signal field recovery based on the improved Gerchberg-Saxton algorithm in the digital signal processing module at the receiving end.

[0031] The complex signal field recovery system and method based on native optical carrier provided by the present invention have the following advantages compared with the prior art:

[0032] (1) The complex signal field recovery system and method based on native optical carrier of the present invention utilizes the limited extinction ratio characteristic of low-cost "co-phase orthogonal optoelectronic modulator" to introduce native residual optical carrier into the modulated optical signal to enhance the phase recovery capability of the system. The existing solutions do not consider the beneficial role of the native residual optical carrier of the low-cost "co-phase orthogonal optoelectronic modulator" in promoting the communication system, nor do they propose a DC offset correction algorithm.

[0033] (2) The complex signal field recovery system and recovery method based on native optical carrier of the present invention designs a DC bias correction algorithm to cooperate with the existing nonlinear pre-compensation algorithm for electro-optical modulators, reducing the extinction ratio index of the "co-phase orthogonal optoelectronic modulator" to 10dB or below, while the system performance remains basically unchanged, enhancing the manufacturing tolerance of the modulator. This scheme is the first to perform flexible correction of modulator nonlinear pre-compensation and DC bias point for the system of low-cost "co-phase orthogonal optoelectronic modulator" with phase recovery receiver, greatly reducing the modulator extinction ratio index to 10dB.

[0034] (3) The complex signal field recovery system and recovery method based on the native optical carrier of the present invention, the finite extinction ratio "in-phase orthogonal optoelectronic modulator" is matched with a phase recovery receiver, which eliminates the reliance of the existing published solutions on additionally inserted pilot symbols (pilot-symbols) or pilot tones (pilot-tones), and effectively alleviates the complexity of the digital signal processing module. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 The schematic diagram of the recovery system of the present invention is shown in FIG.

[0036] Figure 2 The schematic diagram of the digital signal processing module at the transmitting end of the present invention is shown in FIG.

[0037] Figure 3 The different extinction ratios (E inner ,E outer ) under the conditions of (a) Global-SNR and (b) carrier signal power ratio and correction factor.

[0038] Figure 4 This is a diagram of the modulated optical signal output by the electro-optical modulator of the present invention.

[0039] Figure 5 Schematic diagram of the internal structure of the electro-optic modulator of the present invention.

[0040] Figure 6 This is a flow chart of the digital signal processing module at the receiving end of the present invention. DETAILED DESCRIPTION

[0041] The specific embodiments of the present invention are described in detail below. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0042] Terminology explanation:

[0043] ER: extinction ratio, extinction ratio (an inherent parameter in the manufacture of electro-optic modulators).

[0044] DPD: digital pre-distortion, digital pre-distortion.

[0045] CSPR: carrier-to-signal power ratio, carrier signal power ratio.

[0046] Tx: Transmitter.

[0047] Rx: Receiver, receiver.

[0048] DSP: digital signal processor, digital signal processor (module).

[0049] ADC: analog-to-digital converter.

[0050] DAC: digital-to-analog converter.

[0051] SSMF: standard single-mode fiber, standard single-mode fiber.

[0052] PR: Phase retrieval, phase recovery.

[0053] AWGN: Additive White Gaussion Noise, additive white Gaussian noise.

[0054] IQ Mod.: in-phase and quadrature modulator, in-phase and quadrature modulator.

[0055] like Figure 1 As shown, the complex signal field recovery system based on native optical carrier of the present invention comprises a transmitting end digital signal processing module (M1), a laser (M2), a digital-to-analog converter (M3), an electro-optic modulator (M4), an optical fiber link (M5), an optical coupler (M6), a dispersion element (M7), a photodetector (M8), an analog-to-digital converter (M9) and a receiving end digital signal processing module (M10). The native optical carrier mentioned in the present invention is generated together with the optical signal during the electro-optical modulation process. The reasons for the generation of the native optical carrier are: (1) the extinction ratio of the electro-optical modulator is not infinite; (2) due to temperature drift, the electro-optical modulator does not work perfectly at its extinction point.

[0056] In this embodiment, Figure 2As shown, the digital signal processing module (M1) at the transmitting end performs signal preprocessing to make the target signal more suitable for transmission in the optical fiber channel. The internal components are random bit sequence generation, bit to quadrature amplitude modulation (QAM) symbol mapping, pulse shaping filtering, subcarrier multiplexing, nonlinear compensation of electro-optic modulator and DC bias point correction.

[0057] Furthermore, the transmitting end digital signal processing module (M1) performs nonlinear pre-compensation and DC bias correction of the electro-optic modulator by adding a DC bias correction algorithm to the transmitting end digital signal processing module (M1) and coordinating with the existing nonlinear pre-compensation algorithm of the electro-optic modulator (M4) to compensate for the device nonlinearity of the in-phase orthogonal electro-optic modulator, thereby reducing the extinction ratio index of the in-phase orthogonal electro-optic modulator to 10dB or less. Figure 3 At the same time, the manufacturing tolerance of the modulator can be enhanced while restoring the system performance unchanged, and the manufacturing difficulty of the in-phase orthogonal optoelectronic modulator can be alleviated.

[0058] In this embodiment, the DC offset correction algorithm is expressed as follows:

[0059] E target (t) = E DPD (t)-α·sign( <E DPD (t)>) <E DPD (t)>

[0060] Among them, E target is the DC bias correction output signal, α is the DC bias correction factor; E DPD is the signal after digital predistortion; sign(·) is the sign function; <·> is the averaging operator.

[0061] In this embodiment, the laser (M2) is used to supply an optical carrier to the electro-optic modulator, and may be an external cavity laser (ECL), a distributed feedback laser (DFB), or the like.

[0062] In this embodiment, the digital-to-analog converter (M3) converts the digital signal from the transmitting end digital signal processing module (M1) into an analog electrical signal.

[0063] In this embodiment, the electro-optic modulator (M4) performs electro-optic modulation, so that the analog electrical signal of the digital-to-analog converter (M3), that is, the electrical domain signal, continues to electro-optically modulate the optical carrier from the laser (M2) to generate an optical signal carrying target information. In this embodiment, the internal structure of the electro-optic modulator (M4) is as follows: Figure 5As shown, the structure is the same as that of the traditional electro-optic modulator. In actual use, the extinction ratio of the electro-optic modulator is limited (the actual device is usually below 30dB), and the bias point of the electro-optic modulator has temperature drift. Therefore, the output modulated optical signal of the electro-optic modulator has a native optical carrier, such as Figure 4 shown.

[0064] The physical principle of electro-optical modulation can be expressed as:

[0065]

[0066] Among them, g I / g Q and g P Represents the internal Mach-Zehnder interferometer MZI I With MZI Q and an external Mach-Zehnder interferometer MZI P The power separation ratio is g. Since the optical waveguide itself has optical power loss, g I / g Q and g P It also includes the power loss difference in the upper and lower arms of each Mach-Zehnder interferometer. πI 、V πQ and V πP Represents MZI I With MZI Q and MZI P Half-wave voltage; V I (t),V Q (t) and V P (t) respectively represent the effect on MZI I With MZI Q and MZI P The extinction ratio (ER) in the electro-optic modulator is related to the power difference factor g in the upper and lower arms of the Mach-Zehnder interferometer MZI. I / g Q and g P The relationship can be expressed as:

[0067] ER m =((1+g m ) / (1-g m )) 2 ,m∈{I,Q,R}

[0068] In this embodiment, the optical fiber link (M5) is used to transmit the optical signal carrying the target information to the optical coupler (M6), and the optical fiber link (M5) can be compatible with various optical fiber types.

[0069] In this embodiment, the optical coupler (M6) is used to split the optical signal carrying the target information into two or more optical signals. In this embodiment, only the case of splitting into two paths is described, and in other embodiments, it can be split into any number of signals. The two split optical signals enter different photodetectors respectively, and one optical signal first passes through the dispersion element (M7) and then enters the photodetector.

[0070] In this embodiment, the dispersive element (M7) performs a dispersive effect on the passing optical signal. The dispersive element may be a component with a dispersive effect, such as an ordinary optical fiber or an optical filter.

[0071] In this embodiment, the photodetector (M8) is used to convert the optical signal into a corresponding electrical signal, and can be various types of photodetectors.

[0072] In this embodiment, the analog-to-digital converter (M9) converts the electrical domain analog signal from the photodetector (M8) into a digital domain signal.

[0073] In this embodiment, the receiving end digital signal processing module (M10) performs a signal recovery operation, such as Figure 6 As shown, the internal components are phase recovery based on the MGS algorithm, dispersion compensation, subcarrier demultiplexing, signal equalization, decision and signal-to-noise ratio calculation.

[0074] Although the phase recovery of the system based on the native optical carrier and the MGS algorithm is the same as the prior art, the traditional solution requires the insertion of additional pilot symbols (pilot-symbols) or pilot tone (pilot-tone) signals in the target signal and has a dependency on the additional signal. The phase recovery based on the improved Gerchberg-Saxton algorithm in this embodiment eliminates the dependency on the above-mentioned additionally inserted pilot symbols (pilot-symbols) or pilot tone (pilot-tone), does not require insertion operations and overhead in the application, and effectively alleviates the complexity of the digital signal processing module.

[0075] The complex signal field recovery method based on native optical carrier of the present invention comprises the following steps:

[0076] Step S1: performing nonlinear pre-compensation and DC bias correction of the electro-optic modulator in the digital signal processing module at the transmitting end;

[0077] Step S2: At the transmitter end of the system, a low-cost electro-optical modulator with a limited extinction ratio is used to electro-optically modulate the optical carrier signal;

[0078] Step S3: At the system receiver end, a phase recovery receiver is used to receive the optical signal;

[0079] Step S4: Perform signal field recovery based on the improved Gerchberg-Saxton algorithm in the digital signal processing module at the receiving end.

[0080] Furthermore, in step S1, the digital signal from the transmitting end is mainly processed, specifically:

[0081] (1) Convert the target information into a binary bit sequence;

[0082] (2) Based on Gray coding, the binary bit sequence is converted into a symbol sequence of quadrature amplitude modulation (QAM);

[0083] (3) Based on the channel bandwidth information, the symbol sequence is “upsampled” and “pulse shaped” to obtain a sample point sequence that matches the channel bandwidth;

[0084] (4) Repeat (1)-(3) multiple times to obtain multiple sample point sequences s carrying different information i (i=1,2,…,N);

[0085] (5) Multiple sample sequences are aggregated into a signal s in a frequency division multiplexing manner, and its in-phase and orthogonal components are: v i , v q ;

[0086] (6) Based on the extinction ratio of the IQ modulator, i.e., R I , R Q and R P , and get the predistortion coefficient matrix:

[0087]

[0088] (7) Based on the predistortion coefficient matrix, the predistorted sample sequence is obtained:

[0089]

[0090] in,

[0091]

[0092] (8) Perform DC offset correction on the pre-distorted sample sequence to obtain the target output sequence:

[0093]

[0094] Where α represents the DC bias correction factor;

[0095] (9) The target output sequence is loaded into the digital-to-analog converter and the actual physical link transmission is performed, i.e., step S2.

[0096] Furthermore, in step S2, specifically:

[0097] (1) In the transmitter, the optical carrier generated by the laser and the analog electrical signal generated by the digital-to-analog converter are input into an IQ modulator with a relatively small extinction ratio to generate an optical signal carrying the target information (called the target optical signal). The generated optical signal contains a residual optical carrier. Since the residual optical carrier is generated together with the target optical signal, it is called a native optical carrier to distinguish it from the additional insertion method.

[0098] (2) The original optical carrier accompanies the target optical signal and passes through the optical fiber, the dispersion function of which is h CD (t), arrives at the receiver, i.e. step S3.

[0099] Furthermore, in step S3, specifically:

[0100] (1) The target optical signal and the original optical carrier reaching the receiver are split into multiple branch signals by an optical coupler. Preferably, two branches are taken as an example for explanation.

[0101] (2) The above two branch signals pass through the dispersion elements with known dispersion values ​​respectively. Assume that the transfer functions of the dispersion elements in the two branches are 1 and h respectively. D (t) and received by the photodetector.

[0102] (3) The two analog electrical signals output by the photodetector are respectively collected by analog-to-digital converters and subjected to digital signal processing at the receiving end, i.e., step S4.

[0103] Furthermore, in step S4, the digital signal processing at the receiving end is mainly performed, specifically:

[0104] (1) Performing a square root operation on the two digital signals collected by the analog-to-digital converter to obtain signals a(t) and b(t);

[0105] (2) Use a low-pass filter to extract the phase component of the original optical carrier signal and obtain

[0106] (3) With the above a(t), b(t), h D (t), Signal bandwidth h BW (t), the number of cycles N is input, and the following algorithm is executed to obtain the output signal s(t) after phase recovery, where represents the convolution operator, · represents the product operator, and ∠· represents the phase operator:

[0107]

[0108] (4) Perform dispersion compensation operation on the algorithm output signal s(t) to obtain the dispersion compensated signal

[0109]

[0110] (5) Perform frequency demultiplexing on the dispersion compensated signal to obtain a multi-channel signal

[0111] (6) Based on a time feedforward equalizer (FFE), a time domain equalization operation is performed on the obtained multi-path signal to obtain an equalized signal;

[0112] (7) performing signal symbol determination on each equalized signal to obtain a symbol determination signal;

[0113] (8) Based on the symbol decision signal, calculate the signal-to-noise ratio (SNR) of each signal i (i=1,2,…,N).

[0114] The complex signal field recovery system and recovery method based on native optical carrier of the present invention, the finite extinction ratio "in-phase orthogonal optoelectronic modulator" is matched with a phase recovery receiver, which eliminates the reliance of the existing published solutions on the additional insertion of pilot symbols (pilot-symbols) or pilot tones (pilot-tone), and effectively alleviates the complexity of the digital signal processing module.

[0115] The above implementation cases are only preferred embodiments of the present invention and are not intended to limit the present invention in any form. Although the present invention has been disclosed as above with preferred embodiments, they are not intended to limit the present invention. Therefore, any simple modification, equivalent changes and modifications made to the above embodiments according to the technical essence of the present invention without departing from the content of the technical solution of the present invention shall fall within the scope of protection of the technical solution of the present invention.

Claims

1. A complex signal field recovery system based on native optical carrier, characterized in that: It includes a transmitting end digital signal processing module, a laser, a digital-to-analog converter, an electro-optical modulator, an optical fiber link, an optical coupler, a dispersion element, a photodetector, an analog-to-digital converter and a receiving end digital signal processing module; The transmitting end digital signal processing module performs signal preprocessing; the transmitting end digital signal processing module performs electro-optic modulator nonlinear precompensation and DC bias correction; The laser is used to supply an optical carrier to the electro-optic modulator; The digital-to-analog converter converts the digital signal from the digital signal processing module at the transmitting end into an analog electrical signal; The electro-optic modulator is used to convert the electrical signal from the digital-to-analog converter into an optical signal carrying target information; The optical fiber link is used to transmit the optical signal carrying the target information to the optical coupler; The optical coupler is used to split the optical signal carrying target information into two or more optical signals; The dispersive element performs a dispersive effect on the optical signal passing therethrough; The photodetector is used to convert the optical signal into a corresponding electrical signal. The analog-to-digital converter converts the electrical domain analog signal from the photodetector into a digital domain signal and sends it to the receiving end digital signal processing module; The receiving end digital signal processing module performs a signal recovery operation; The transmitting end digital signal processing module is provided with a DC bias correction algorithm, which cooperates with the existing nonlinear pre-compensation algorithm for the electro-optic modulator to pre-compensate the device nonlinearity of the in-phase orthogonal electro-optic modulator; The DC offset correction algorithm is expressed as: E target (t)=E DPD (t)-α·sign(<E DPD (t)>)·<E DPD (t)> Among them, E target is the DC bias correction output signal, α is the DC bias correction factor; E DPD is the signal after digital predistortion; sign(·) is the sign function; <·> is the averaging operator.

2. The complex signal field recovery system based on native optical carrier according to claim 1, characterized in that: The extinction ratio of the electro-optic modulator is limited, or the bias point of the electro-optic modulator has a temperature drift, and the output modulated optical signal of the electro-optic modulator has a native optical carrier.

3. The complex signal field recovery system based on native optical carrier according to claim 1, characterized in that: The receiving end digital signal processing module performs signal recovery operations, and the internal components are phase recovery based on the MGS algorithm, dispersion compensation, subcarrier demultiplexing, signal equalization, judgment and signal-to-noise ratio calculation.

4. A complex signal field recovery method based on native optical carrier, characterized in that: Using the recovery system of any one of claims 1 to 3, the recovery method comprises the following steps: Step S1: performing nonlinear pre-compensation and DC bias correction of the electro-optic modulator in the digital signal processing module at the transmitting end; Step S2: At the system transmitter end, an electro-optical modulator with a finite extinction ratio is used to electro-optically modulate the optical carrier signal; Step S3: At the system receiver end, a phase recovery receiver is used to receive the optical signal; Step S4: Perform signal field recovery based on the improved Gerchberg-Saxton (MGS) algorithm in the digital signal processing module at the receiving end.

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