Round-trip continuous variable quantum key distribution system source noise monitoring method and device

Abstract

The application discloses a kind of round-trip continuous variable quantum key distribution system source noise monitoring method and device, wherein method includes the following steps: signal receiving end provides laser source, and the laser signal generated by laser source is split, one laser signal is retained in signal receiving end, another laser signal is sent to signal modulation end;Signal modulation end receives laser signal, and laser signal is divided into reference light and signal light using asymmetric beam splitter;Signal light is reflected, then signal light is modulated by Gauss or discrete, and signal light is attenuated by variable optical attenuator;Real-time monitoring of laser source noise is completed by passive beam splitter combined with coherent detector, and signal light is sent back to signal receiving end;Signal receiving end carries out coherent detection to signal light, and security quantum key extraction is completed after data post-processing.The application can effectively improve the security and performance of round-trip continuous variable quantum key distribution system.

Description

Technical Field

[0001] This invention relates to the field of quantum key distribution technology, and in particular to a method and apparatus for monitoring source noise in a round-trip continuous variable quantum key distribution system. Background Technology

[0002] Quantum Key Distribution (QKD) technology, based on the uncertainty principle and no-cloning theorem in quantum physics, possesses provable unconditional security and detectability against eavesdropping. It has broad application prospects in fields such as medicine, finance, government affairs, and defense, and is currently a hot topic of international technological competition. QKD is divided into two main categories based on different technical approaches: Discrete Variable (DV) and Continuous Variable (CV). DV-QKD technology, based on single-photon coding and single-photon detection, has been relatively mature. CV-QKD technology, as a novel and competitive technology, offers advantages such as higher detection efficiency, higher security bit rate over short to medium distances, and better compatibility with existing network infrastructure.

[0003] In recent years, CV-QKD has made significant progress in both theoretical research and experimental techniques. However, practical applications such as integrated photonics and quantum access networks place higher demands on CV-QKD schemes, necessitating the development of more compact and noise-tolerant CV-QKD solutions. The round-trip CV-QKD scheme possesses self-compensation capabilities for polarization jitter and phase drift. This scheme requires only one laser to achieve key exchange between the communicating parties and generates local oscillator light at the receiving end for coherent detection of quantum signals. This not only eliminates many practical security issues caused by the simultaneous transmission of signal light and local oscillator light in the quantum channel but also avoids the requirement to control the relative frequency drift of two freely operating lasers in a local oscillator CV-QKD scheme. Therefore, the round-trip CV-QKD scheme is an effective approach to advancing the practical application of CV-QKD technology in the future.

[0004] However, the round-trip transmission structure of lasers requires the signal to be transmitted through an untrusted quantum channel before being loaded with quantum key information, leaving a security vulnerability for eavesdroppers to launch quantum hacking attacks. In 2016, a research team at Shanghai Jiao Tong University first proposed a round-trip CV-QKD scheme based on dual-phase modulation and conducted preliminary experimental verification. In 2020, Spanish researchers proposed a bidirectional scheme to suppress Rayleigh backscattering noise. In this scheme, the distribution of the pre-modulated laser signal and the transmission of the post-modulated quantum signal are carried out through two independent optical fibers, resulting in the signal light lacking polarization self-compensation capability. To resist quantum hacking attacks, in both of the above schemes, laser source noise is treated as untrusted noise, which significantly reduces the system performance. In 2022, Russian researchers proposed a trusted noise model, which assumes that the distribution of the pre-modulated laser signal and the coherent detection of the post-modulated quantum signal are not manipulated by an eavesdropper (Eve). This assumption obviously overestimates the security code rate of the actual system.

[0005] In summary, the existing round-trip CV-QKD scheme limits the system's security and performance, thus restricting its practical application and promotion. Summary of the Invention

[0006] This invention addresses the problem that laser source noise has a significant impact on the performance and security of round-trip continuous variable quantum key distribution systems. It proposes a source noise monitoring method and device for round-trip continuous variable quantum key distribution systems, which constructs a round-trip continuous variable quantum key distribution method with better security and higher noise tolerance from the perspectives of laser source noise monitoring and noise suppression.

[0007] The technical solution adopted in this invention is as follows:

[0008] A method for monitoring source noise in a round-trip continuous-variable quantum key distribution system includes the following steps:

[0009] S1. The signal receiving end provides a laser source and splits the laser signal generated by the laser source into beams, one of which is retained inside the signal receiving end and the other is sent to the signal modulation end.

[0010] S2. After receiving the laser signal, the signal modulation end uses an asymmetric beam splitter to split the laser signal into a reference beam and a signal beam, wherein the power of the reference beam is greater than the power of the signal beam.

[0011] S3. The signal modulation end reflects the signal light, then performs Gaussian modulation or discrete modulation on the signal light, and attenuates the signal light through a variable optical attenuator;

[0012] S4. The signal modulation end uses a passive beam splitter combined with a coherent detector to perform real-time monitoring of the laser source noise and sends the signal light back to the signal receiving end.

[0013] S5. The signal receiver performs coherent detection on the signal light and completes secure quantum key extraction after data post-processing.

[0014] Further, step S1 includes: splitting the laser signal generated by the laser source, wherein one laser signal is retained locally at the signal receiving end as the local oscillator light, and used to coherently detect the quantum signal transmitted back from the signal modulation end; the other laser signal enters the quantum channel through the circulator at the signal receiving end and is transmitted to the signal modulation end.

[0015] Furthermore, in step S1, the laser source is generated by means of a narrow-linewidth laser.

[0016] Further, step S2 includes: after the laser signal enters the signal modulation end, it first passes through a circulator, and then is split into a high-power beam and a low-power beam by an asymmetric beam splitter. The low-power beam is used to be modulated to generate signal light, and the high-power beam is used as a reference beam to perform coherent detection with the modulated part of the signal light in order to monitor the magnitude of the laser source noise, i.e., the quantum signal noise.

[0017] Furthermore, in step S3, the method for reflecting the signal light includes: reflecting it through a 90° Faraday rotating mirror to convert any polarization state into an orthogonal state, so as to suppress all birefringence effects and polarization loss that occur during the transmission of the optical signal.

[0018] Furthermore, the 90° Faraday rotator includes a 45° Faraday rotator and a plane mirror.

[0019] Further, step S4 includes: the signal light is attenuated by a variable optical attenuator and then split into two beams by a beam splitter. One beam of signal light enters the quantum channel after passing through a circulator and is transmitted back to the signal receiver. The other beam of signal light interferes with the reference light generated in step S2, and the laser source noise is monitored by a balanced homodyne detector or a heterodyne detector. At the same time, based on the monitoring results, the signal variance is modulated to the optimal value using a variable optical attenuator to suppress the laser source noise. The reference light is rotated 90° by a 90° Faraday rotator before interference so that the polarization of the reference light is consistent with the polarization of the signal light.

[0020] Further, in step S5, the signal light transmitted back to the signal receiving end passes through a circulator and is coherently detected with the local oscillator light through a balanced zero-difference detector or a heterodyne detector; the local oscillator light is first rotated 90° by a 90° Faraday rotating mirror so that the polarization of the local oscillator light is consistent with the polarization of the signal light.

[0021] Furthermore, in step S5, when calculating the secure code rate, it is necessary to establish an entanglement equivalent theoretical model under laser source noise monitoring conditions; the data post-processing includes basis comparison, parameter estimation, data negotiation, error correction, and private key amplification.

[0022] A source noise monitoring device for a round-trip continuous variable quantum key distribution system includes a signal modulation end and a signal receiving end;

[0023] The signal receiving end includes a laser source, a beam splitter, and a signal light detection module. The laser signal generated by the laser source is split by the beam splitter, with one laser signal retained inside the signal receiving end and the other laser signal sent to the signal modulation end. The light detection module performs coherent detection on the signal light sent by the signal modulation end, and completes secure quantum key extraction after data post-processing.

[0024] The signal modulation end includes an asymmetric beam splitter, an optical signal reflection module, a signal modulation module, a variable optical attenuator, a passive beam splitter, and a coherent detector. The asymmetric beam splitter splits the laser signal transmitted from the signal receiver into a reference beam and a signal beam, wherein the power of the reference beam is greater than the power of the signal beam. The optical signal reflection module reflects the signal beam, the signal modulation module performs Gaussian modulation or discrete modulation on the signal beam, and the variable optical attenuator attenuates the signal beam. Finally, the passive beam splitter, combined with the coherent detector, performs real-time monitoring of the laser source noise and sends the signal beam back to the signal receiver.

[0025] The beneficial effects of this invention are as follows:

[0026] This invention employs a passive beam splitting combined with coherent detection monitoring method to monitor the source noise of a round-trip continuous-variable quantum key distribution (RTG) system in real time. At the signal modulation end, a portion of the signal light is split, and source noise is monitored in real time through homodyne detection. Combined with a variable optical attenuator, effective suppression of source noise is achieved. By combining source noise monitoring and suppression, the security and performance of the RTG system are effectively improved. The source noise monitoring method proposed in this invention does not require the introduction of an additional laser, thus not increasing the complexity or cost of the RTG system. Furthermore, the structure used does not require the signal light to be pulsed, simplifying system configuration. This invention can effectively promote the practical application of RTG in scenarios such as quantum access networks. Attached Figure Description

[0027] Figure 1 The diagram shown is a schematic flowchart of the source noise monitoring method according to Embodiment 1 of the present invention.

[0028] Figure 2 The diagram shown is a schematic block diagram of the source noise monitoring device according to Embodiment 1 of the present invention.

[0029] Figure 3 The diagram shown is a schematic diagram of the source noise monitoring implementation case of Embodiment 2 of the present invention. Detailed Implementation

[0030] To provide a clearer understanding of the technical features, objectives, and effects of the present invention, specific embodiments are now described. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention; that is, the described embodiments are only a part of the embodiments of the invention, not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0031] Example 1

[0032] This embodiment provides a method for monitoring source noise in a round-trip continuous-variable quantum key distribution system, such as... Figure 1 As shown, it includes the following steps:

[0033] S1. The signal receiving end provides a laser source and splits the laser signal generated by the laser source into two beams. One beam of laser signal is retained inside the signal receiving end, and the other beam is sent to the signal modulation end. Preferably, the laser source can be generated by a narrow linewidth laser and split into two beams by a beam splitter. One beam of laser signal enters the quantum channel through the circulator of the signal receiving end and is transmitted to the signal modulation end. The other beam of laser signal is retained locally at the signal receiving end as the local oscillator light, used for coherent detection of the quantum signal returned from the signal modulation end. The signal light and the local oscillator light can be pulsed light or continuous light, mainly determined by the detection effect and the complexity of actual implementation.

[0034] S2. After receiving the laser signal, the signal modulation end uses an asymmetric beam splitter to divide the laser signal into a reference beam and a signal beam, wherein the power of the reference beam is greater than the power of the signal beam. Preferably, after the laser signal enters the signal modulation end, it first passes through a circulator, and then is split into a high-power beam and a low-power beam by an asymmetric beam splitter. The low-power beam is used to be modulated to generate the signal beam, and the high-power beam serves as the reference beam and is coherently probed with the modulated portion of the signal beam to monitor the magnitude of the source noise (quantum signal noise).

[0035] S3. The signal modulation end reflects the signal light, then performs Gaussian or discrete modulation on the signal light, and attenuates the signal light through a variable optical attenuator. Preferably, low-power signal light can be reflected by a 90° Faraday rotator mirror; subsequently, the signal modulation module modulates the signal light, and a variable optical attenuator attenuates the signal light. This 90° Faraday rotator mirror can consist of a 45° Faraday rotator and a common plane mirror. The function of the Faraday rotator mirror is to convert any polarization state into its orthogonal state to suppress all birefringence effects and polarization losses that occur during optical signal transmission.

[0036] S4. The signal modulation end uses a passive beam splitter combined with a coherent detector to perform real-time monitoring of the laser source noise and sends the signal light back to the signal receiving end. Preferably, the signal light is attenuated by a variable optical attenuator and then split into two beams by a beam splitter. One beam interferes with the reference light generated in step S2, and then passes through a balanced homodyne (or heterodyne) detector to monitor the source noise level. Simultaneously, based on the monitoring results, the quantum signal variance is modulated using a variable optical attenuator to optimize it, thereby suppressing the source noise. The other beam passes through a circulator and enters the quantum channel, then is transmitted back to the receiving end. Before interference, the reference light is rotated 90° by a Faraday rotator mirror to ensure its polarization matches that of the signal light, guaranteeing optimal interference performance.

[0037] S5. The signal receiver performs coherent detection on the signal light and completes secure quantum key extraction after data post-processing. Preferably, the signal light transmitted back to the signal receiver is coherently detected with the local oscillator light through a balanced homodyne (or heterodyne) detector after passing through a circulator. The local oscillator light is first rotated 90° by a Faraday mirror to ensure its polarization matches that of the signal light, guaranteeing optimal interference. During secure code rate calculation, an entanglement equivalent theoretical model under laser source noise monitoring conditions needs to be established. Preferably, data post-processing includes basis comparison, parameter estimation, data negotiation, error correction, and private key amplification.

[0038] Accordingly, this embodiment also provides a source noise monitoring device for a round-trip continuous variable quantum key distribution system, including a signal modulation end and a signal receiving end. The signal receiving end includes a laser source, a beam splitter, and a signal light detection module. The laser signal generated by the laser source is split by the beam splitter, with one beam retained inside the signal receiving end and the other beam sent to the signal modulation end. The light detection module performs coherent detection on the signal light sent by the signal modulation end, and after data post-processing, secure quantum key extraction is completed.

[0039] The signal modulation end includes an asymmetric beam splitter, an optical signal reflection module, a signal modulation module, a variable optical attenuator, a passive beam splitter, and a coherent detector. The asymmetric beam splitter splits the laser signal sent by the signal receiver into a reference beam and a signal beam, where the power of the reference beam is greater than that of the signal beam. The optical signal reflection module reflects the signal beam, and the signal modulation module performs Gaussian modulation or discrete modulation on the signal beam, and the variable optical attenuator attenuates the signal beam. Finally, the passive beam splitter, combined with the coherent detector, performs real-time monitoring of the laser source noise and sends the signal beam back to the signal receiver.

[0040] like Figure 2 As shown, the laser signal is generated by a laser source at the signal receiver and then transmitted to the signal modulation end via a communication channel, which is an optical fiber channel. Upon reaching the signal modulation end, the optical signal is split into two beams: a high-power beam serves as a reference beam for monitoring the signal beam, and a low-power beam serves as the signal beam. After being processed by a polarization rotation module, the signal beam rotates 90 degrees from the polarization direction of the incident signal beam, meaning the incident and outgoing beams are orthogonal. This can be achieved through reflection using a 90-degree Faraday rotator mirror or other common optical devices. The optical signal is input to the signal modulation module to obtain the modulated optical signal required by the CV-QKD protocol. The modulation process results in a relatively large modulation variance, and signal modulation includes, but is not limited to, Gaussian modulation and discrete modulation. After passing through a tunable attenuator, the optical signal is split again. One beam is transmitted back to the signal receiver via the transmission channel, while the other beam interferes with the high-power reference beam for real-time monitoring of the signal beam. Based on the monitoring results, the attenuation coefficient of the tunable attenuator is adjusted to apply strong attenuation to the signal beam, suppressing laser source noise and maintaining optimal system performance. The quantum signal light transmitted back to the signal receiver interferes with the local oscillator light emitted by the receiver, completing the measurement and decoding of the quantum signal light. Then, after data post-processing, steps such as parameter estimation, data error correction, and private key amplification are completed. Finally, the two communicating parties achieve a completely consistent random key sharing.

[0041] The transmission of the pre-modulated laser signal from the signal receiver to the signal modulator, and the transmission of the post-modulated quantum signal from the signal modulator to the signal receiver, are both carried out in the same optical fiber. Since the polarization directions of the round-trip laser signals are orthogonal, it is not required that the laser signal be a pulsed light.

[0042] This invention can achieve laser source noise suppression, and the principle is as follows:

[0043] For round-trip continuous-variable quantum key distribution systems, laser source noise can be divided into two main categories: noise affecting performance (such as modulation noise introduced by the imperfections of the signal modulation module devices, which is not controlled by the eavesdropper) and noise affecting security (noise introduced when the laser signal first passes through the transmission channel, which is controlled by the eavesdropper). In the scheme proposed in this invention, the magnitude of the laser source noise is determined by source noise monitoring, and a tunable attenuator is used to suppress the laser source noise. First, a larger intensity modulation signal is applied to increase the modulation variance. Then, based on the measurement results of the source noise monitoring, the signal intensity is attenuated to an appropriate level using the tunable attenuator. Since the laser source noise controlled by the eavesdropper is introduced during the first transmission of the optical signal, the intensity modulation process only increases the modulation noise and does not amplify the noise introduced by the eavesdropper. During the subsequent strong attenuation process, the signal light is attenuated to a smaller value. At this point, both the modulation noise and the source noise introduced by the eavesdropper are attenuated together. By selecting an appropriate tunable attenuation coefficient, the source noise controlled by the eavesdropper can be attenuated to a smaller value. Therefore, the above method can suppress the source noise introduced by the eavesdropper. In summary, the laser source noise monitoring device proposed in this invention can effectively improve the security and performance of round-trip continuous variable quantum key distribution devices.

[0044] Example 2

[0045] This embodiment is based on embodiment 1:

[0046] like Figure 3 As shown, this embodiment provides a specific implementation case for real-time round-trip continuous variable quantum key distribution system laser source noise monitoring. The signal receiver mainly includes a laser module and a balanced homodyne detection module. The continuous light generated by the laser is split into two beams by a symmetrical beam splitter 1. One beam is retained locally at the receiver as the local oscillator; the other beam enters the quantum channel through a circulator 1.

[0047] After transmission through a quantum channel, the laser signal enters the signal modulation end, which mainly includes a signal modulation module and a laser source noise monitoring module. The laser signal entering the modulation end first passes through a circulator 2, and then through a filter used to prevent potential quantum hacking attacks such as Trojan horse attacks. Subsequently, the laser signal is split into two beams by an asymmetric beam splitter 3: a high-power beam serves as the reference beam, and a low-power beam serves as the signal beam. The signal beam passes through a beam splitter 5 and is reflected by a Faraday rotator mirror, with the polarization direction of the reflected light rotated 90 degrees relative to the incident light. The light reflected by the Faraday rotator mirror is reflected again by beam splitter 5, and then undergoes quantum key modulation of the signal beam through a Gaussian modulation module. Next, a variable optical attenuator adjusts the signal beam to the quantum signal level. Finally, a balanced beam splitter 6 splits the signal beam into two beams; one beam interferes with the high-power reference beam, and the source noise is monitored by a null detector 2. The reference light, after passing through beam splitter 4, is reflected by Faraday rotator mirror 3 and rotates 90 degrees. It is then reflected again by beam splitter 4, and its polarization direction is consistent with that of the quantum signal light. The other signal light enters the quantum channel after passing through circulator 2.

[0048] The quantum signal returning to the signal receiver interferes with the local oscillator light after passing through circulator 1, and coherent detection of the quantum signal is achieved through null detector 1. The local oscillator light is rotated 90 degrees by Faraday rotator 1, and the measurement basis is randomly selected through phase modulator. Based on this, the two communicating parties can share an initial key with channel noise. Finally, after data post-processing to complete parameter estimation, data error correction, and private key amplification, the two communicating parties achieve a completely consistent secure key sharing.

[0049] In the proposed solution of this invention, in order to suppress source noise, during the signal modulation process, the signal light is first subjected to signal modulation with a large modulation variance. Then, combined with the real-time source noise monitoring results, a strong attenuation is introduced into the signal light by adjusting the variable optical attenuator, thereby effectively suppressing the source noise introduced by the eavesdropper.

[0050] It should be noted that, for the sake of simplicity, the foregoing method embodiments are described as a series of actions. However, those skilled in the art should understand that this application is not limited to the described order of actions, as some steps may be performed in other orders or simultaneously according to this application. Furthermore, those skilled in the art should also understand that the embodiments described in the specification are preferred embodiments, and the actions and modules involved are not necessarily essential to this application.

Claims

1. A method for monitoring source noise in a round-trip continuous-variable quantum key distribution system, characterized in that, Includes the following steps: S1. The signal receiving end provides a laser source and splits the laser signal generated by the laser source into beams, one beam of laser signal is retained inside the signal receiving end, and the other beam of laser signal is sent to the signal modulation end; S2. After receiving the laser signal, the signal modulation end uses an asymmetric beam splitter to split the laser signal into a reference beam and a signal beam, wherein the power of the reference beam is greater than the power of the signal beam; S3. The signal modulation end reflects the signal light, then performs Gaussian modulation or discrete modulation on the signal light, and attenuates the signal light through a variable optical attenuator; S4. The signal modulation end uses a passive beam splitter combined with a coherent detector to perform real-time monitoring of the laser source noise and sends the signal light back to the signal receiving end; S5. The signal receiver performs coherent detection on the signal light and extracts the secure quantum key after data post-processing; In step S3, the method for reflecting the signal light includes: reflecting the light through a 90° Faraday rotating mirror to convert any polarization state into an orthogonal state, thereby suppressing all birefringence effects and polarization loss that occur during the transmission of the optical signal. Step S4 includes: the signal light is attenuated by a variable optical attenuator and then split into two beams by a beam splitter. One beam of signal light enters the quantum channel after passing through a circulator and is transmitted back to the signal receiver. The other beam of signal light interferes with the reference light generated in step S2, and the laser source noise is monitored by a balanced homodyne detector or a heterodyne detector. At the same time, based on the monitoring results, the signal variance is modulated to the optimal value using a variable optical attenuator to suppress the laser source noise. The reference light is rotated 90° by a 90° Faraday rotator before interference to make the polarization of the reference light consistent with that of the signal light.

2. The source noise monitoring method for a round-trip continuous variable quantum key distribution system according to claim 1, characterized in that, Step S1 includes: splitting the laser signal generated by the laser source into beams, wherein one beam of laser signal is retained locally at the signal receiving end as the local oscillator light, and used to coherently detect the quantum signal transmitted back from the signal modulation end; the other beam of laser signal enters the quantum channel through the circulator at the signal receiving end and is transmitted to the signal modulation end.

3. The source noise monitoring method for a round-trip continuous variable quantum key distribution system according to claim 1, characterized in that, In step S1, the laser source is generated by means of a narrow linewidth laser.

4. The source noise monitoring method for a round-trip continuous variable quantum key distribution system according to claim 1, characterized in that, Step S2 includes: after the laser signal enters the signal modulation end, it first passes through a circulator, and then is split into a high-power beam and a low-power beam by an asymmetric beam splitter. The low-power beam is used to be modulated to generate signal light, and the high-power beam is used as a reference beam to perform coherent detection with the modulated part of the signal light in order to monitor the magnitude of the laser source noise, i.e., the quantum signal noise.

5. The source noise monitoring method for a round-trip continuous variable quantum key distribution system according to claim 1, characterized in that, The 90° Faraday rotator includes a 45° Faraday rotator and a plane mirror.

6. The source noise monitoring method for a round-trip continuous variable quantum key distribution system according to claim 1, characterized in that, In step S5, the signal light transmitted back to the signal receiving end passes through a circulator and is coherently detected with the local oscillator light by a balanced zero-difference detector or a heterodyne detector; the local oscillator light is first rotated 90° by a 90° Faraday rotator mirror so that the polarization of the local oscillator light is consistent with the polarization of the signal light.

7. The source noise monitoring method for a round-trip continuous variable quantum key distribution system according to claim 1, characterized in that, In step S5, when calculating the secure code rate, it is necessary to establish an entanglement equivalent theoretical model under laser source noise monitoring conditions; the data post-processing includes basis comparison, parameter estimation, data negotiation, error correction, and private key amplification.

8. A source noise monitoring device for a round-trip continuous-variable quantum key distribution system, employing the source noise monitoring method for a round-trip continuous-variable quantum key distribution system as described in claim 1, characterized in that, The device includes a signal modulation end and a signal receiving end; The signal receiving end includes a laser source, a beam splitter, and a signal light detection module. The laser signal generated by the laser source is split by the beam splitter, with one laser signal retained inside the signal receiving end and the other laser signal sent to the signal modulation end. The light detection module performs coherent detection on the signal light sent by the signal modulation end, and completes secure quantum key extraction after data post-processing. The signal modulation end includes an asymmetric beam splitter, an optical signal reflection module, a signal modulation module, a variable optical attenuator, a passive beam splitter, and a coherent detector. The asymmetric beam splitter splits the laser signal transmitted by the signal receiver into a reference beam and a signal beam, wherein the power of the reference beam is greater than the power of the signal beam. The optical signal reflection module reflects the signal beam, the signal modulation module performs Gaussian modulation or discrete modulation on the signal beam, and the variable optical attenuator attenuates the signal beam. Then, a passive beam splitter combined with a coherent detector is used to complete the real-time monitoring of the laser source noise and send the signal light back to the signal receiving end.

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