A coherent detection method, apparatus and system

Abstract

The application discloses a kind of coherent detection method, device and system, it is related to optical fiber communication field, the method includes: receiving the intensity modulation optical signal of transmission end transmission, wherein the intensity modulation optical signal is the intensity modulation of original signal and obtained by transmission end;Phase modulation is carried out to local light signal, and phase modulation local light signal is obtained;After the intensity modulation optical signal and the phase modulation local light signal are mixed, photoelectric detection, analog-digital conversion and digital signal receiving processing are carried out in turn, to restore original signal.By phase modulation local light, the homodyne detection of phase diversity is avoided, only a pair of balanced detector can obtain the IQ component of signal, reduce the cost of system equipment, while using intensity modulation optical signal, avoid subsequent carrier recovery algorithm, reduce system complexity.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of optical fiber communication, and in particular to a coherent detection method, device and system. BACKGROUND

[0002] With the explosive growth of intelligent terminal equipment and data services, users have higher and higher demand for bandwidth. Optical fiber access, as the mainstream access mode, has the potential to meet the high bandwidth demand of users.

[0003] Since coherent detection can obtain higher sensitivity than direct detection, coherent detection will be widely used in passive access networks in the future.

[0004] The traditional coherent detection system can be used for simultaneous demodulation of amplitude and phase, and its system structure is complex, requiring multiple pairs of balanced detectors (or balanced receivers) to achieve phase diversity, polarization diversity reception, and additional digital signal processing (DSP) to eliminate phase noise and achieve carrier recovery. The above problems result in that coherent optical communication cannot meet the requirements of short-distance communication for transmission cost.

[0005] Traditional coherent detection is mainly divided into two types: homodyne detection and heterodyne detection. Homodyne coherent detection is generally applied in core transmission networks, while heterodyne detection is more commonly used in optical and wireless hybrid networks.

[0006] The homodyne detection system requires the signal light and the local oscillator light to have the same frequency, and its advantage is that the bandwidth requirement of the device is low, but it needs to use two pairs of balanced detectors at the receiving end to recover the baseband signal, greatly increasing the cost of equipment. As shown in Figure 1 In a phase diversity homodyne receiving system, the signal light and the local oscillator light are divided into two paths after passing through a 90° hybrid, and are sent into two pairs of balanced detectors, which directly output the in-phase component and the quadrature component of the signal after completing photoelectric conversion.

[0007] For the heterodyne detection system, as shown in Figure 2a The number of balanced detectors required is only half of that of the homodyne detection system, and a 90° hybrid is not required. Although it does not need to do phase diversity, the signal directly output by the detector is still at intermediate frequency, and it still needs to be down-converted in the electrical domain to obtain the baseband signal, which increases the complexity of the system. Figure 2b The process of digital down-conversion is shown, in which the received intermediate frequency signal is multiplied by a carrier and a carrier shifted by 90°, respectively, and low-pass filtered to obtain the in-phase component I and the quadrature component Q of the baseband signal.

[0008] Figure 3The diagram illustrates the digital signal processing steps for coherent detection in single-polarization-state reception, which sequentially include IQ orthogonalization, clock recovery, dispersion compensation, frequency offset recovery, phase estimation, and decision output. Since coherent detection can obtain both amplitude and phase information of the signal, phase noise generated in the system can severely impact the received signal. Phase noise is often caused by frequency deviations and linewidths of the lasers at the transmitter and receiver. Therefore, subsequent digital signal processing requires carrier recovery algorithms such as frequency offset recovery and phase estimation to recover the original signal.

[0009] With the increasing maturity of coherent detection technology, how to reduce costs has become a major research hotspot. This requires not only a simple system structure in the optical domain, but also a reduction in signal processing complexity in the digital domain.

[0010] In summary, in order to address the cost issues of coherent detection systems in the aforementioned access network scenarios, there is an urgent need for a coherent detection solution with low equipment cost and low system complexity. Summary of the Invention

[0011] The main objective of this invention is to provide a coherent detection method, apparatus, and system to solve the problems of high equipment cost and high system complexity in traditional coherent detection systems.

[0012] This invention provides a coherent detection method, the method comprising: receiving an intensity-modulated optical signal transmitted from a transmitter, wherein the intensity-modulated optical signal is obtained by intensity modulation of an original signal by the transmitter; performing phase modulation on a local oscillator signal to obtain a phase-modulated local oscillator signal; mixing the intensity-modulated optical signal and the phase-modulated local oscillator signal and then sequentially performing photoelectric detection, analog-to-digital conversion and digital signal reception processing to recover the original signal.

[0013] In one embodiment, the step of phase-modulating the local oscillator signal to obtain a phase-modulated local oscillator signal includes: performing phase modulation on the local oscillator signal within each symbol period to obtain a phase-modulated local oscillator signal, wherein the modulation frequency of the phase-modulated local oscillator signal is consistent with the symbol rate of the intensity-modulated optical signal; or performing phase modulation twice on the local oscillator signal within each symbol period to obtain two phase-modulated local oscillator signals with different phases, wherein the modulation frequency of the phase-modulated local oscillator signal is twice the symbol rate of the intensity-modulated optical signal.

[0014] In one embodiment, receiving the intensity-modulated optical signal transmitted by the transmitting end includes: adjusting the polarization state of the intensity-modulated optical signal transmitted by the transmitting end so that the polarization state of the intensity-modulated optical signal is consistent with the polarization state of the phase-modulated local oscillator optical signal.

[0015] In one embodiment, the method further includes: using a first polarization beamsplitter to divide the phase-modulated local oscillator optical signal into two phase-modulated local oscillator optical signals with orthogonal polarization states; correspondingly, the intensity-modulated optical signal transmitted by the receiving end includes: using a second polarization beamsplitter to divide the intensity-modulated optical signal transmitted by the receiving end into two intensity-modulated optical signals with orthogonal polarization states, so as to obtain two sets of intensity-modulated optical signals and phase-modulated local oscillator optical signals with consistent polarization states.

[0016] In one embodiment, the step of mixing the intensity-modulated optical signal and the phase-modulated local oscillator optical signal and then sequentially performing photoelectric detection, analog-to-digital conversion, and digital signal reception processing to recover the original signal includes: mixing the intensity-modulated optical signal and the phase-modulated local oscillator optical signal with consistent polarization states; performing photoelectric detection on the mixed optical signal to obtain an electrical signal containing the in-phase component and quadrature component of each symbol; performing analog-to-digital conversion on the electrical signal containing the in-phase component and quadrature component of each symbol; and performing decision demodulation on the digital signal containing the in-phase component and quadrature component of each symbol after analog-to-digital conversion to recover the original signal.

[0017] In one embodiment, the step of performing decision demodulation on the digital signal containing the in-phase and quadrature components of each symbol after analog-to-digital conversion to recover the original signal includes: extracting the in-phase and quadrature components of each symbol; and performing modulo decision on the in-phase and quadrature components of each symbol to recover the original signal.

[0018] In one embodiment, the method further includes: before performing a modulo decision on the in-phase and quadrature components of each symbol, performing orthogonalization processing on the extracted in-phase and quadrature components of each symbol, so as to perform a modulo decision on the in-phase and quadrature components of each symbol after orthogonalization processing.

[0019] This invention also provides a coherent detection method, the method comprising: an induction modulator modulating an original signal at a transmitting end to obtain an intensity modulated optical signal; a phase modulator modulating a local oscillator signal at a receiving end to obtain a phase modulated local oscillator signal; and mixing the phase modulated local oscillator signal with the intensity modulated optical signal transmitted at the transmitting end, followed by photoelectric detection, analog-to-digital conversion, and digital signal reception processing to recover the original signal.

[0020] This invention also provides a coherent detection device, comprising: a signal receiving module for receiving an intensity-modulated optical signal transmitted from a transmitter, wherein the intensity-modulated optical signal is obtained by intensity modulation of an original signal by the transmitter; a phase modulation module for phase modulation of a local oscillator optical signal to obtain a phase-modulated local oscillator optical signal; and a signal recovery module for mixing the intensity-modulated optical signal and the phase-modulated local oscillator optical signal and then sequentially performing photoelectric detection, analog-to-digital conversion, and digital signal reception processing to recover the original signal.

[0021] In one embodiment, the phase modulation module includes: an adjustable electrical delay line for controlling the phase modulation of the local oscillator optical signal; and a phase modulator for performing phase modulation on the local oscillator optical signal in each symbol period under the control of the adjustable electrical delay line to obtain a phase-modulated local oscillator optical signal, wherein the modulation frequency of the phase-modulated local oscillator optical signal is consistent with the symbol rate of the intensity-modulated optical signal; or performing phase modulation twice on the local oscillator optical signal in each symbol period to obtain two phase-modulated local oscillator optical signals with different phases, wherein the modulation frequency of the phase-modulated local oscillator optical signal is twice the symbol rate of the intensity-modulated optical signal.

[0022] In one embodiment, the signal receiving module includes a polarization controller for adjusting the polarization state of the intensity-modulated optical signal transmitted by the transmitting end, so that the polarization state of the intensity-modulated optical signal is consistent with the polarization state of the phase-modulated local oscillator optical signal.

[0023] In one embodiment, the device further includes: a first polarization beam splitter for splitting the phase-modulated local oscillator light signal into two phase-modulated local oscillator light signals with orthogonal polarization states; correspondingly, the signal receiving module includes: a second polarization beam splitter for splitting the intensity-modulated light signal transmitted by the transmitting end into two intensity-modulated light signals with orthogonal polarization states, so as to obtain two sets of intensity-modulated light signals and phase-modulated local oscillator light signals with consistent polarization states.

[0024] In one embodiment, the signal recovery module includes: an optical coupler for mixing the intensity-modulated optical signal and the phase-modulated local oscillator optical signal with consistent polarization states; a balanced detector for photoelectric detection of the mixed optical signal to obtain an electrical signal containing an in-phase component and a quadrature component for each symbol; an analog-to-digital converter for performing analog-to-digital conversion on the electrical signal containing the in-phase component and the quadrature component for each symbol; and a digital signal receiver for decision demodulating the digital signal containing the in-phase component and the quadrature component for each symbol after analog-to-digital conversion to recover the original signal.

[0025] In one embodiment, the digital signal receiver is specifically used to extract the in-phase and quadrature components of each symbol, and to perform modulo decision on the in-phase and quadrature components of each symbol to recover the original signal.

[0026] In one embodiment, the digital signal receiver is further configured to orthogonalize the extracted in-phase and quadrature components of each symbol before performing a modulo decision on the in-phase and quadrature components of each symbol, so as to perform a modulo decision on the in-phase and quadrature components of each symbol after orthogonalization.

[0027] This invention also provides a coherent detection system, the system comprising a transmitter and a receiver, wherein: the transmitter is used to intensity modulate the original signal to obtain an intensity-modulated optical signal; and the receiver includes the aforementioned coherent detection device.

[0028] The present invention provides a coherent detection method, apparatus and system that avoids the use of phase diversity by phase modulating the local oscillator optical signal. Only a pair of balanced detectors are needed to obtain the I and Q components of the signal, reducing the system equipment cost. At the same time, the use of intensity modulated optical signal avoids the need for subsequent carrier recovery algorithms, reducing system complexity. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of a phase diversity zero-difference receiving system;

[0030] Figure 2a This is a schematic diagram of a phase diversity heterodyne receiving system;

[0031] Figure 2b This is a schematic diagram of the digital down-conversion process;

[0032] Figure 3 This is a schematic diagram of the digital signal processing for coherent detection in the case of single polarization state reception;

[0033] Figure 4 This is a schematic flowchart of a coherent detection method provided in an embodiment of the present invention;

[0034] Figure 5a This is a first schematic block diagram of a coherent detection system provided in an embodiment of the present invention;

[0035] Figure 5b This is a second schematic block diagram of a coherent detection system provided in an embodiment of the present invention;

[0036] Figure 6 This is a signal mapping constellation diagram of a four-level pulse amplitude modulation signal (PAM4) provided in an embodiment of the present invention;

[0037] Figure 7 This is a schematic diagram of the local oscillator phase modulation method and the signal IQ component reception method provided in the embodiments of the present invention;

[0038] Figure 8 This is a schematic diagram of the suboptimal local oscillator phase modulation method provided in an embodiment of the present invention;

[0039] Figure 9 This is a schematic diagram of the signal processing process provided in an embodiment of the present invention;

[0040] Figure 10a and Figure 10b These are, respectively, the signal constellation diagram and the signal magnitude diagram obtained by simulation when the received optical power is -20dBm and there is no frequency offset between the local oscillator light and the signal light, as provided in the embodiments of the present invention;

[0041] Figure 11a , Figure 11b , Figure 11c These are, respectively, the signal constellation diagram before orthogonalization, the signal constellation diagram after orthogonalization, and the signal magnitude diagram obtained by simulation when the received optical power is -20dBm and there is a 5GHz frequency offset between the local oscillator light and the signal light, as provided in the embodiments of the present invention.

[0042] Figure 12a and Figure 12b These are, respectively, the signal constellation diagram and the signal magnitude diagram obtained by simulation when the received optical power is -25dBm and there is no frequency offset between the local oscillator light and the signal light, as provided in the embodiments of the present invention;

[0043] Figure 13a , Figure 13b , Figure 13c These are, respectively, the signal constellation diagram before orthogonalization, the signal constellation diagram after orthogonalization, and the signal magnitude diagram obtained by simulation when the received optical power is -25dBm and there is a 5GHz frequency offset between the local oscillator light and the signal light, as provided in the embodiments of the present invention. Detailed Implementation

[0044] It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0045] Figure 4 This is a schematic flowchart of a coherent detection method provided in an embodiment of the present invention, as shown below. Figure 4 As shown, the method may include:

[0046] Step S101: Receive the intensity-modulated optical signal transmitted by the transmitter, wherein the intensity-modulated optical signal is obtained by the transmitter performing intensity modulation on the original signal.

[0047] Step S102: Phase modulate the local oscillator signal to obtain a phase-modulated local oscillator signal.

[0048] Specifically, step S102 may include: performing phase modulation on the local oscillator signal within each symbol period to obtain a phase-modulated local oscillator signal, wherein the modulation frequency of the phase-modulated local oscillator signal is consistent with the symbol rate of the intensity-modulated optical signal. Alternatively, step S102 may include: performing phase modulation twice on the local oscillator signal within each symbol period to sequentially obtain two phase-modulated local oscillator signals with different phases, wherein the modulation frequency of the phase-modulated local oscillator signal is twice the symbol rate of the intensity-modulated optical signal.

[0049] Step S103: The intensity-modulated optical signal and the phase-modulated local oscillator optical signal are mixed and then subjected to photoelectric detection, analog-to-digital conversion and digital signal reception processing in sequence to recover the original signal.

[0050] In one embodiment, step S101 may include: using a polarization controller to adjust the polarization state of the intensity-modulated optical signal transmitted from the transmitting end, so that the polarization state of the intensity-modulated optical signal is consistent with the polarization state of the phase-modulated local oscillator optical signal. In another embodiment, the method further includes: using a first polarization beamsplitter to divide the phase-modulated local oscillator optical signal into two phase-modulated local oscillator optical signals with orthogonal polarization states. Correspondingly, step S101 may include: using a second polarization beamsplitter to divide the intensity-modulated optical signal transmitted from the transmitting end into two intensity-modulated optical signals with orthogonal polarization states, so as to obtain two sets of intensity-modulated optical signals and phase-modulated local oscillator optical signals with consistent polarization states. That is, in addition to using a polarization controller to control the polarization of the received intensity-modulated optical signal, two polarization beamsplitters can also be used to perform polarization diversity on the intensity-modulated optical signal and the phase-modulated local oscillator optical signal respectively, so as to perform coherent detection on intensity-modulated optical signals and phase-modulated local oscillator optical signals with the same polarization state. That is, the polarization controller and the first and second polarization beam splitters are both designed to keep the polarization state of the intensity-adjusted optical signal consistent with that of the phase-modulated local oscillator optical signal.

[0051] Step S101 may further include: adjusting the received optical power of the intensity-modulated optical signal transmitted by the transmitting end before adjusting the polarization state.

[0052] Whether it is a set of intensity-modulated optical signals with consistent polarization obtained by using a polarization controller and the phase-modulated local oscillator optical signal, or any set of two sets of intensity-modulated optical signals with consistent polarization obtained by using a polarization beam splitter and the phase-modulated local oscillator optical signal, signal recovery can be performed using step S103.

[0053] Step S103 may include: mixing the intensity-modulated optical signal and the phase-modulated local oscillator optical signal with consistent polarization states; performing photoelectric detection on the mixed optical signal to obtain an electrical signal containing the in-phase component and quadrature component of each symbol; performing analog-to-digital conversion on the electrical signal containing the in-phase component and quadrature component of each symbol; and performing decision demodulation on the digital signal containing the in-phase component and quadrature component of each symbol after analog-to-digital conversion to recover the original signal.

[0054] In one implementation, the step of performing decision demodulation on the digital signal containing the in-phase and quadrature components of each symbol after analog-to-digital conversion to recover the original signal includes: extracting the in-phase and quadrature components of each symbol; and performing modulo decision on the in-phase and quadrature components of each symbol to recover the original signal.

[0055] In another embodiment, the step of performing decision demodulation on the digital signal containing the in-phase and quadrature components of each symbol after analog-to-digital conversion to recover the original signal includes: extracting the in-phase and quadrature components of each symbol; performing orthogonalization processing on the extracted in-phase and quadrature components of each symbol; and performing modulo decision on the in-phase and quadrature components of each symbol after orthogonalization processing to recover the original signal.

[0056] Accordingly, the present invention provides a method for implementing... Figure 1 The coherent detection apparatus of the embodiment may include: a signal receiving module, a phase modulation module, and a signal recovery module, wherein:

[0057] The signal receiving module is used to receive the intensity-modulated optical signal transmitted by the transmitter, wherein the intensity-modulated optical signal is obtained by the transmitter performing intensity modulation on the original signal.

[0058] The phase modulation module is used to phase modulate the local oscillator optical signal to obtain a phase-modulated local oscillator optical signal. Specifically, the phase modulation module includes: an adjustable electrical delay line for controlling the phase modulation of the local oscillator optical signal; and a phase modulator for phase modulating the local oscillator optical signal under the control of the adjustable electrical delay line to obtain a phase-modulated local oscillator optical signal. In one embodiment, the phase modulator, under the control of the adjustable electrical delay line, performs phase modulation on the local oscillator optical signal in each symbol period to obtain a phase-modulated local oscillator optical signal, wherein the modulation frequency of the phase-modulated local oscillator optical signal is consistent with the symbol rate of the intensity-modulated optical signal. In another embodiment, the phase modulator, under the control of the adjustable electrical delay line, performs phase modulation twice on the local oscillator optical signal in each symbol period to obtain two phase-modulated local oscillator optical signals with different phases, wherein the modulation frequency of the phase-modulated local oscillator optical signal is twice the symbol rate of the intensity-modulated optical signal.

[0059] The signal recovery module is used to mix the intensity-modulated optical signal and the phase-modulated local oscillator optical signal and then perform photoelectric detection, analog-to-digital conversion and digital signal reception processing in sequence to recover the original signal.

[0060] In one embodiment, the signal receiving module may include a polarization controller for adjusting the polarization state of the intensity-modulated optical signal transmitted from the transmitting end, so that the polarization state of the intensity-modulated optical signal is consistent with the polarization state of the phase-modulated local oscillator optical signal. In another embodiment, the device further includes a first polarization beamsplitter for splitting the phase-modulated local oscillator optical signal into two phase-modulated local oscillator optical signals with orthogonal polarization states; correspondingly, the signal receiving module includes a second polarization beamsplitter for splitting the intensity-modulated optical signal transmitted from the transmitting end into two intensity-modulated optical signals with orthogonal polarization states, to obtain two sets of intensity-modulated optical signals and phase-modulated local oscillator optical signals with consistent polarization states. That is, in addition to using a polarization controller to control the polarization of the received intensity-modulated optical signal, two polarization beamsplitters can also be used to perform polarization diversity on the intensity-modulated optical signal and the phase-modulated local oscillator optical signal respectively, so as to perform coherent detection on intensity-modulated optical signals and phase-modulated local oscillator optical signals with the same polarization state. That is, the polarization controller and the first and second polarization beamsplitters are all for ensuring that the polarization state of the intensity-modulated optical signal is consistent with that of the phase-modulated local oscillator optical signal.

[0061] In addition, the signal receiving module may also include: an adjustable optical attenuator, used to adjust the received optical power of the intensity-modulated optical signal transmitted by the transmitting end before polarization state adjustment.

[0062] When a polarization controller is used to obtain a set of intensity-modulated optical signals with consistent polarization states and the phase-modulated local oscillator optical signal, the signal recovery module includes a set of devices, namely an optical coupler, a balanced detector, an analog-to-digital converter, and a digital signal receiver. When a polarization beam splitter is used to obtain two sets of intensity-modulated optical signals with consistent polarization states and the phase-modulated local oscillator optical signal, the signal recovery module includes two sets of devices, each including an optical coupler, a balanced detector, an analog-to-digital converter, and a digital signal receiver, to perform signal recovery using the polarization states of one set of intensity-modulated optical signals with consistent polarization states and the phase-modulated local oscillator optical signal, respectively.

[0063] The system includes: an optical coupler for mixing the intensity-modulated optical signal and the phase-modulated local oscillator optical signal with consistent polarization; a balanced detector for photoelectric detection of the mixed optical signal to obtain an electrical signal containing the in-phase and quadrature components of each symbol; an analog-to-digital converter for performing analog-to-digital conversion on the electrical signal containing the in-phase and quadrature components of each symbol; and a digital signal receiver for performing decision demodulation on the digital signal containing the in-phase and quadrature components of each symbol after analog-to-digital conversion to recover the original signal. In one embodiment, the digital signal receiver is specifically used to extract the in-phase and quadrature components of each symbol and perform modulus-based decision on the in-phase and quadrature components of each symbol to recover the original signal. In another embodiment, the digital signal receiver is specifically used to extract the in-phase and quadrature components of each symbol; to perform orthogonalization processing on the extracted in-phase and quadrature components of each symbol; and to perform modulo decision on the orthogonalized in-phase and quadrature components of each symbol to recover the original signal. Compared with the previous embodiment, this embodiment is applicable to the case of IQ component imbalance, that is, firstly, the extracted in-phase and quadrature components of each symbol are orthogonalized to eliminate the IQ imbalance problem, and then modulo decision is performed on the orthogonalized in-phase and quadrature components of each symbol.

[0064] The coherent detection method and apparatus provided in the above embodiments of the present invention are applied to the receiving end. By phase-modulating the local oscillator optical signal, it avoids the zero-difference detection of phase diversity. Only a pair of balanced detectors are needed to obtain the IQ components of the signal, which reduces the system equipment cost. At the same time, by using intensity-modulated optical signal, it avoids the subsequent carrier recovery algorithm, which reduces the system complexity.

[0065] This invention also provides a coherent detection method, which includes: a transmitter performing intensity modulation on an original signal to obtain an intensity-modulated optical signal; a receiver performing phase modulation on a local oscillator optical signal to obtain a phase-modulated local oscillator optical signal; and mixing the phase-modulated local oscillator optical signal with the intensity-modulated optical signal transmitted by the transmitter, followed by sequential photoelectric detection, analog-to-digital conversion, and digital signal reception processing to recover the original signal. Specifically, the transmitter obtains an intensity-modulated optical signal that allows the receiver to determine the signal based on its signal modulus by applying a bias voltage and amplitude adjustment to the original information. The processing steps at the receiver are similar to those at the transmitter. Figure 1 The implementation methods are the same and will not be repeated here.

[0066] Accordingly, this embodiment also provides a coherent detection system for the above-described coherent detection method. The system includes a transmitter and a receiver, wherein: the transmitter is used to intensity modulate the original signal to obtain an intensity-modulated optical signal; the receiver includes the aforementioned coherent detection device, which will not be described again here.

[0067] The coherent detection method and system provided in the above embodiments of the present invention avoid the use of phase diversity by phase modulating the local oscillator optical signal. Only a pair of balanced detectors are needed to obtain the I and Q components of the signal, which reduces the system equipment cost. At the same time, the intensity modulated optical signal avoids the need for subsequent carrier recovery algorithms, which reduces the system complexity.

[0068] The following is combined with Figure 1 , Figures 5a-13c The present invention will be described in detail below.

[0069] Figure 5a This is a first schematic block diagram of a coherent detection system provided in an embodiment of the present invention, such as... Figure 5aAs shown, the coherent detection system based on phase-modulated local oscillator light provided in this embodiment includes: an electroabsorption modulated laser (or electroabsorption modulator, intensity modulator, Electroabsorption Modulated Distributed Feedback Laser, EML), a distributed feedback laser (DFB), a pulse signal generator, a balanced detector (or balanced photodiode, BPD), a polarization controller (PC), a phase modulator (PM), an adjustable electrical delay line (τ), an analog-to-digital converter (ADC), a digital signal receiving module (equivalent to the aforementioned digital signal receiver), a single-mode optical fiber (SSMF), an adjustable optical attenuator (or variable optical attenuator, VOA), and an optical coupler (OC).

[0070] At the transmitting end, the pulse signal generator produces a four-level pulse amplitude modulation (PAM4) pulse signal, which drives the electroabsorption optical modulator to perform intensity modulation. The intensity-modulated optical signal is then transmitted via single-mode fiber or back-to-back (B2B) transmission.

[0071] At the receiving end, the optical signal in the single-mode fiber is first fed to an optical attenuator to adjust the received optical power. After the polarization state is adjusted by a polarization controller, it is mixed with the local oscillator signal. The local oscillator signal is generated by a distributed feedback laser and then passed through a phase modulator. An adjustable electrical delay line is used to control the phase modulation of the local oscillator signal. The mixed optical signal is input to a balanced receiver for photoelectric detection. The resulting electrical signal is then converted from analog to digital for digital reception and subsequent signal processing.

[0072] The following uses optical communication simulation software to verify the zero-difference detection system of phase-modulated local oscillator light using PAM4 modulation format in this embodiment. The electro-absorption modulated laser used in the simulation has a center wavelength of 1550 nm and a linewidth of 1 MHz, and the distributed feedback laser also has a center wavelength of 1550 nm and a linewidth of 1 MHz. The center wavelength of the electro-absorption modulator is tunable to simulate the frequency shift between the signal light and the local oscillator light.

[0073] At the transmitting end, a pseudo-random sequence generator produces a binary sequence, which, after bit mapping, is used by a pulse signal generator to produce a PAM4 signal with a rate of 50 Gbit / s and a baud rate of 25 Gbaud. This PAM4 signal drives an electro-absorption optical modulator to generate an intensity-modulated optical signal (i.e., an intensity-modulated optical signal), which is then transmitted back-to-back after electro-optic modulation. The bit mapping constellation diagram of the PAM4 signal is shown below. Figure 6 As shown, by adjusting the bias voltage of the intensity modulator and the amplitude of the pulse signal, the following can be obtained: Figure 6 The PAM4 signal in the image. In this case, even if there is significant phase noise causing the received signal constellation diagram to rotate, resulting in a constellation diagram with four rings, the signal can still be judged based on its magnitude, thus avoiding the need to use a carrier recovery algorithm.

[0074] At the receiving end, the signal first passes through an adjustable optical attenuator to adjust the received optical power and then a polarization controller to adjust its polarization state to match that of the local oscillator light. Afterward, it is input to an optical coupler with a coupling ratio of 50% to mix with the local oscillator light. A balanced detector with a bandwidth of 50 GHz and a sensitivity of 1 A / W is then used to perform photoelectric conversion on the mixed optical signal.

[0075] The local oscillator light is generated by a distributed feedback laser and then phase-modulated. The phase modulator is driven by a 25 GHz binary pulse signal, such that the two levels of the pulse signal correspond to 0° phase modulation and 90° phase modulation, respectively. During the 90° modulation phase of the local oscillator light, the balanced detector corresponds to the in-phase component of the output signal, while when the modulation phase is 0°, the balanced detector corresponds to the quadrature component of the output signal.

[0076] In one implementation, the specific phase modulation method of the local oscillator light at the receiving end can be as follows: Figure 7 As shown, ideally, the modulation frequency of the local oscillator's phase modulation is the same as the symbol rate of the signal, and it is interleaved with the received symbols. Specifically, the modulation phase of the local oscillator is changed at the midpoint of each symbol period. In this way, the balanced detector outputs the in-phase and quadrature components of each symbol in the order of IQQI in the diagram. Receiving the signal in this order allows for the complete recovery of each symbol. The use of an adjustable electrical delay line ensures that the phase modulation of the local oscillator and the received symbol are exactly offset by half a symbol period.

[0077] A digital storage oscilloscope is used to receive the signal output from the balanced detector, i.e., analog-to-digital conversion (ADC). The oscilloscope's sampling rate is set to twice the signal symbol rate, which is 50 Gsample / s. Since the received signal contains the in-phase component I and the quadrature component Q of each symbol, these two components need to be extracted from the serial signal data in a specific order. The IQ component reception order is as follows: Figure 7 As shown, taking three received symbols as an example, the 90° phase modulation period of the local oscillator corresponds to the first half-cycle of the 1st and 3rd symbols and the second half-cycle of the 2nd symbol, while the 0° phase modulation period corresponds to the second half-cycle of the 1st and 3rd symbols and the first half-cycle of the 2nd symbol. Therefore, the in-phase and quadrature components of each of these three symbols should be extracted in the order of IQQIIQ. Similarly, the final output signal is in the form of repeated IQQI, and the two components of each symbol should be extracted in this order.

[0078] In another embodiment, the specific phase modulation method of the local oscillator light at the receiving end can be as follows: Figure 8 As shown, the same effect as the aforementioned implementation method can be achieved. Specifically, this method requires the modulation phase of the local oscillator light to be changed twice per symbol period, that is, the frequency of the local oscillator light phase modulation is twice the signal symbol rate (in the aforementioned implementation method, the two rates are the same), which is 50 GHz. At this time, for each symbol period, in the first half of the period, the balanced detector will output the in-phase component of the symbol, and in the second half of the period, the balanced detector will output the quadrature component. In subsequent digital reception, the in-phase and quadrature components must be extracted in the order of IQIQ.

[0079] Figure 5b This is a second schematic block diagram of a coherent detection system provided in an embodiment of the present invention, as shown below. Figure 5b As shown, polarization diversity reception is used instead of... Figure 5a The polarization controller usage illustrated here specifically involves a first polarization beamsplitter splitting the phase-modulated local oscillator (PBX) signal into two orthogonally polarized PBX signals. A second polarization beamsplitter splits the intensity-modulated (IMM) signal transmitted from the transmitter into two orthogonally polarized IIM signals, resulting in two sets of IIM signals and PBX signals with consistent polarization states. In this embodiment, with polarization diversity, it is no longer necessary to adjust the signal polarization state using a polarization controller. Instead, a polarization beamsplitter (PBS) splits the signal into two orthogonal polarization states, which are then optically mixed with the phase-modulated PBX signals of the two polarization states before detection. Subsequent digital signal processing requires additional polarization demultiplexing to recover the single-polarization signal. The remaining algorithms are consistent with single-polarization reception, achieving the same level of performance as... Figure 5a The same effect is achieved through the implementation method.

[0080] This invention provides a low-cost coherent detection technology for passive access network scenarios. It is based on phase-modulated local oscillator light for coherent detection. The frequency of the local oscillator laser used is the same as that of the signal laser at the transmitting end, so it still falls within the scope of zero-difference detection. Compared with the traditional local oscillator light method, this invention replaces phase diversity with phase modulation of the local oscillator light. Compared with the traditional phase diversity zero-difference detection system, this invention only requires a pair of balanced detectors to demodulate the signal and does not require a 90° optical mixer, which greatly reduces the cost of the equipment.

[0081] Furthermore, since the in-phase and quadrature components of each symbol are obtained from two adjacent samples, the frequency deviation of the transceiver lasers will cause an imbalance in the IQ components of the signal.

[0082] The imbalance of the sign IQ components caused by the laser frequency shift can be expressed by the following formula:

[0083] I n =Acos(φ+2π2nΔfT)

[0084] Q n =Asin(φ+2π(2n+1)ΔfT)

[0085] This formula represents the IQ components of the nth received symbol, where A is the signal strength, T represents the sampling time, φ represents the phase noise, and the last term is the cumulative phase offset due to the frequency offset Δf. The phase offset between the I and Q components of the same symbol is 2πΔfT.

[0086] The IQ imbalance was eliminated using a Schmidt orthogonalization algorithm. Measurements showed that a 3dB power compensation was achieved for a frequency offset of -5 to 5.5 GHz. Therefore, the frequency offset tolerance in this embodiment is approximately ±Rs / 5, where Rs represents the symbol rate.

[0087] Figure 9 This is a schematic diagram of the signal processing procedure provided in an embodiment of the present invention, such as... Figure 9 As shown, the subsequent digital signal processing can include: after extracting the in-phase and quadrature components of each symbol, performing orthogonalization processing on the signal to eliminate IQ imbalance, directly taking the modulus to recover the original signal, and finally calculating the bit error rate.

[0088] It should be noted that traditional phase diversity reception zero-difference systems, such as Figure 1 As shown, its final output of two signals can be represented as:

[0089]

[0090]

[0091] The in-phase and quadrature components in the above two equations together determine the final signal:

[0092]

[0093] In the formula, R represents the responsivity of the photodiode, and P... s P represents signal power. LO θ represents the local oscillator power. s θ represents the modulation phase of the signal itself. n This represents phase noise.

[0094] Phase noise, primarily caused by frequency deviations and linewidths of the laser sources at the transmitter and receiver, is unavoidable and causes rotation of the constellation diagram of the received signal. When intensity-modulated signals are coherently received, both their intensity and phase are detected; this signal can be expressed as follows:

[0095]

[0096] Since the intensity signal does not have its own modulation phase, the phase of the received signal only exists due to the phase offset caused by frequency offset and phase noise. Ideally, with a phase offset of 0°, the signal has a value only in the in-phase component. However, when a phase offset exists, the signal phase will rotate, meaning the signal has values ​​in both the in-phase component I and the quadrature component Q, ultimately resulting in a rotation of the signal constellation diagram.

[0097] In this embodiment of the invention, after the signal is transmitted using an intensity modulation format, the receiver can perform decision demodulation of the signal by calculating the modulus value after obtaining the in-phase component and quadrature component of the signal, without the need for an additional carrier recovery algorithm, thus reducing the complexity in the digital domain.

[0098] Taking a received optical power of -20dBm and no frequency offset between the local oscillator light and the signal light as an example, Figure 10a The constellation diagram for receiving signals has been rotated into four rings due to phase noise. Figure 10b The magnitude diagram of the signal, from Figure 10b The four levels can be clearly distinguished, and at this point, direct PAM4 signal decision can be obtained to achieve a lower bit error rate.

[0099] Taking a received optical power of -20dBm and a 5GHz frequency offset between the local oscillator light and the signal light as an example, Figure 11a The diagram shows the signal constellation when IQ component imbalance exists. Because the signal is received in the IQQI order, the phase mismatch angles between the in-phase and quadrature components corresponding to the IQ and QI symbols are opposite, thus generating...Figure 11a The constellation diagram shows the unbalanced superposition of two IQ components. After using an orthogonalization algorithm to eliminate the IQ component imbalance caused by frequency offset, the constellation diagram is restored to the form shown below. Figure 11b The four rings shown lead to the following result: Figure 11c The PAM4 signal modulus diagram shown can still be clearly distinguished by the naked eye, and the bit error rate is still relatively low.

[0100] Taking a received optical power of -25dBm and no frequency offset between the local oscillator light and the signal light as an example, such as Figure 12a and Figure 12b As shown, with Figure 11a and Figure 11b Similarly, at this time, the optical signal-to-noise ratio is low, and the modulus diagram of the received signal still shows four levels, but a small amount of overlap has appeared. At this time, the bit error rate for making a decision is relatively high.

[0101] Taking a received optical power of -25dBm and a 5GHz frequency offset between the local oscillator light and the signal light as an example, if Figure 13a , Figure 13b , Figure 13c As shown, with Figure 11a and Figure 11b Similarly, the magnitude of the PAM4 signal obtained in this case is... Figure Four The overlap between the two levels is significantly greater than in the case without frequency offset. To address the IQ component imbalance caused by frequency offset, the use of orthogonalization algorithms will, to some extent, increase the bit error rate.

[0102] The embodiments of the present invention have the following technical effects:

[0103] 1. The receiver of this invention uses phase modulation of the local oscillator light instead of phase diversity, which saves half of the receiver hardware overhead and reduces costs compared to the traditional phase diversity zero-difference detection scheme.

[0104] 2. The subsequent digital signal processing in this embodiment of the invention does not require an additional carrier recovery algorithm, and the orthogonalization algorithm is used to solve the additional IQ component imbalance problem, which has low complexity and is easy to implement.

[0105] The preferred embodiments of the present invention have been described above with reference to the accompanying drawings, but this does not limit the scope of the invention. Any modifications, equivalent substitutions, and improvements made by those skilled in the art without departing from the scope and spirit of the present invention should be considered within the scope of the present invention.

Claims

1. A coherent detection method, characterized in that, The method includes: The receiver receives an intensity-modulated optical signal transmitted from the transmitter, wherein the intensity-modulated optical signal is obtained by the transmitter performing intensity modulation on the original signal; Phase modulation of the local oscillator signal yields a phase-modulated local oscillator signal; The intensity-modulated optical signal and the phase-modulated local oscillator optical signal are mixed and then sequentially subjected to photoelectric detection, analog-to-digital conversion, and digital signal reception processing to recover the original signal; The step of phase-modulating the local oscillator signal to obtain a phase-modulated local oscillator signal includes: The local oscillator optical signal is phase-modulated within each symbol period to obtain a phase-modulated local oscillator optical signal, wherein the modulation frequency of the phase-modulated local oscillator optical signal is consistent with the symbol rate of the intensity-modulated optical signal; or The local oscillator optical signal is phase-modulated twice within each symbol period to obtain two phase-modulated local oscillator optical signals with different phases. The modulation frequency of the phase-modulated local oscillator optical signal is twice the symbol rate of the intensity-modulated optical signal.

2. The method according to claim 1, characterized in that, The intensity-modulated optical signal transmitted by the receiving and transmitting end includes: A polarization controller is used to adjust the polarization state of the intensity-modulated optical signal transmitted from the transmitter so that the polarization state of the intensity-modulated optical signal is consistent with the polarization state of the phase-modulated local oscillator optical signal.

3. The method according to claim 1, characterized in that, The method further includes: The phase-modulated local oscillator optical signal is split into two phase-modulated local oscillator optical signals with orthogonal polarization states using a first polarization beam splitter. Accordingly, the intensity-modulated optical signal transmitted by the receiving and transmitting end includes: Using a second polarization beam splitter, the intensity-modulated optical signal transmitted from the transmitter is divided into two intensity-modulated optical signals with orthogonal polarization states, so as to obtain two sets of intensity-modulated optical signals and phase-modulated local oscillator optical signals with consistent polarization states.

4. The method according to claim 2 or 3, characterized in that, The process of mixing the intensity-modulated optical signal and the phase-modulated local oscillator optical signal, and then sequentially performing photoelectric detection, analog-to-digital conversion, and digital signal reception processing to recover the original signal includes: The intensity-modulated optical signal and the phase-modulated local oscillator optical signal with the same polarization state are mixed; The mixed optical signal is photoelectrically detected to obtain an electrical signal containing the in-phase and quadrature components of each symbol; The electrical signal containing the in-phase and quadrature components of each symbol is subjected to analog-to-digital conversion; The digital signal, which contains the in-phase and quadrature components of each symbol after analog-to-digital conversion, is demodulated to recover the original signal.

5. The method according to claim 4, characterized in that, The decision demodulation of the digital signal containing the in-phase and quadrature components of each symbol after analog-to-digital conversion to recover the original signal includes: Extract the in-phase and quadrature components of each symbol; For each symbol, modulo decision is performed on the in-phase and quadrature components to recover the original signal.

6. The method according to claim 5, characterized in that, The method further includes: Before performing a modulo decision on the in-phase and quadrature components of each symbol, the extracted in-phase and quadrature components of each symbol are orthogonalized so that a modulo decision can be performed on the in-phase and quadrature components of each symbol after orthogonalization.

7. A coherent detection method, characterized in that, The method includes: The transmitter modulates the original signal to obtain an intensity-modulated optical signal; The receiving end performs phase modulation on the local oscillator optical signal to obtain a phase-modulated local oscillator optical signal. This phase-modulated local oscillator optical signal is then mixed with the intensity-modulated optical signal transmitted by the transmitting end and sequentially subjected to photoelectric detection, analog-to-digital conversion, and digital signal reception processing to recover the original signal. The step of phase-modulating the local oscillator signal to obtain a phase-modulated local oscillator signal includes: The local oscillator optical signal is phase-modulated within each symbol period to obtain a phase-modulated local oscillator optical signal, wherein the modulation frequency of the phase-modulated local oscillator optical signal is consistent with the symbol rate of the intensity-modulated optical signal; or The local oscillator optical signal is phase-modulated twice within each symbol period to obtain two phase-modulated local oscillator optical signals with different phases. The modulation frequency of the phase-modulated local oscillator optical signal is twice the symbol rate of the intensity-modulated optical signal.

8. A coherent detection device, characterized in that, The device includes: A signal receiving module is used to receive an intensity-modulated optical signal transmitted from a transmitter, wherein the intensity-modulated optical signal is obtained by the transmitter performing intensity modulation on the original signal; The phase modulation module is used to perform phase modulation on the local oscillator optical signal to obtain a phase-modulated local oscillator optical signal. The signal recovery module is used to mix the intensity-modulated optical signal and the phase-modulated local oscillator optical signal, and then sequentially perform photoelectric detection, analog-to-digital conversion, and digital signal reception processing to recover the original signal; The phase modulation module includes: An adjustable electrical delay line is used to control the phase modulation of the local oscillator optical signal; A phase modulator is used to perform phase modulation on the local oscillator optical signal in each symbol period under the control of an adjustable electrical delay line to obtain a phase-modulated local oscillator optical signal, wherein the modulation frequency of the phase-modulated local oscillator optical signal is consistent with the symbol rate of the intensity-modulated optical signal; or to perform phase modulation on the local oscillator optical signal twice in each symbol period to obtain two phase-modulated local oscillator optical signals with different phases, wherein the modulation frequency of the phase-modulated local oscillator optical signal is twice the symbol rate of the intensity-modulated optical signal.

9. The apparatus according to claim 8, characterized in that, The signal receiving module includes: A polarization controller is used to adjust the polarization state of the intensity-modulated optical signal transmitted by the transmitter so that the polarization state of the intensity-modulated optical signal is consistent with the polarization state of the phase-modulated local oscillator optical signal.

10. The apparatus according to claim 8, characterized in that, The device further includes: The first polarization beam splitter is used to split the phase-modulated local oscillator optical signal into two phase-modulated local oscillator optical signals with orthogonal polarization states. Accordingly, the signal receiving module includes: The second polarization beam splitter splits the intensity-modulated optical signal transmitted from the transmitter into two intensity-modulated optical signals with orthogonal polarization states, so as to obtain two sets of intensity-modulated optical signals and phase-modulated local oscillator optical signals with consistent polarization states.

11. The apparatus according to claim 9 or 10, characterized in that, The signal recovery module includes: An optical coupler is used to mix the intensity-modulated optical signal and the phase-modulated local oscillator optical signal, which are of the same polarization state. A balanced detector is used to perform photoelectric detection on the mixed optical signal to obtain an electrical signal containing the in-phase and quadrature components of each symbol. An analog-to-digital converter is used to perform analog-to-digital conversion on the electrical signal containing the in-phase and quadrature components of each symbol; A digital signal receiver is used to perform decision demodulation on the digital signal after analog-to-digital conversion, which contains the in-phase and quadrature components of each symbol, in order to recover the original signal.

12. The apparatus according to claim 11, characterized in that, The digital signal receiver is specifically used to extract the in-phase and quadrature components of each symbol, and to perform modulo decision on the in-phase and quadrature components of each symbol to recover the original signal.

13. The apparatus according to claim 12, characterized in that, The digital signal receiver is further configured to orthogonalize the extracted in-phase and quadrature components of each symbol before performing a modulo decision on the in-phase and quadrature components of each symbol, so as to perform a modulo decision on the in-phase and quadrature components of each symbol after orthogonalization.

14. A coherent detection system, characterized in that, The system includes a transmitter and a receiver, wherein: The transmitting end is used to intensity modulate the original signal to obtain an intensity modulated optical signal; The receiving end includes the coherent detection device according to any one of claims 8-13.

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