A quantum key distribution ring network

Abstract

The present invention belongs to the technical field of optical secure communication, and discloses a quantum key distribution ring network, which comprises a central node and N intermediate nodes to form a ring network; when quantum key distribution is carried out between the central node and any one of the intermediate nodes, both of them perform random phase modulation respectively to generate a symmetric key; when quantum key distribution is carried out between any two intermediate nodes, both of them perform random phase modulation respectively, and the central node does not perform phase modulation. After signal transmission and detection are completed, the central node announces the measurement result to generate a symmetric key. Compared with the prior art, the present invention can realize quantum key distribution between any two nodes in the ring without a trusted relay. The structure of the intermediate node is simple, only phase modulation and optical intensity monitoring are required, there is no need to set both a sending end and a receiving end at the same time, and there is no need for polarization compensation and phase compensation, which reduces resource consumption, increases stability, and greatly reduces the deployment and operation costs.

Description

Technical Field

[0001] The present invention relates to the technical field of optical secure communication, and particularly relates to a quantum key distribution ring network. Background Art

[0002] With the development of technology and society, data and communication security have become increasingly important. Quantum key distribution can provide information-theoretic security for both communication parties and can be used to ensure the security of data transmission and communication. However, the key generation rate of current quantum key distribution systems cannot meet the growing demand for encryption rates.

[0003] In existing quantum key distribution networking solutions, quantum key distribution processes need to be carried out separately between each pair of nodes. Conventional quantum key distribution devices distinguish between the sending end and the receiving end. Therefore, multiple devices need to be deployed at each node, resulting in a relatively high networking cost and a large occupied volume. In addition, in a conventional ring network structure, direct quantum key distribution between non-adjacent nodes is not possible, and the nodes between them need to be used as trusted relays for secure key transfer. However, the prerequisite for a trusted relay is that the nodes are trusted, which greatly limits the actual security. Patent CN 115883089 B proposes a ring network structure in which the same quantum key distribution device is deployed at each network node, which can serve as both the sending end and the receiving end at the same time. However, the device structure in this solution is complex, and real-time polarization compensation and phase compensation of the channel are required, increasing the complexity of network devices and resource consumption, and still requiring a trusted relay. Summary of the Invention

[0004] In view of the above-mentioned defects in the prior art, the present invention proposes a quantum key distribution ring network.

[0005] The technical solution of the present invention is realized as follows:

[0006] A quantum key distribution ring network includes N intermediate nodes and a central node, where N is an integer not less than 2;

[0007] Each node includes an input port and an output port;

[0008] The input ports and output ports of these N + 1 nodes are connected by optical fiber channels between adjacent nodes to form a ring network. Specifically: each pair of nodes is connected by an optical fiber channel; among them, the output port and the input port of the central node are respectively connected to the input port of intermediate node 1 and the output port of intermediate node N; the input port and the output port of intermediate node j are respectively connected to the output port of intermediate node j - 1 and the input port of intermediate node j + 1, where 2 ≤ j ≤ N - 1;

[0009] The central node is used to generate a dual-pulse signal through a laser and an unbalanced interferometer, and modulate the phase difference between the two sub-pulses in the dual-pulse signal to 0, π / 2, π, or 3π / 2, and then output it to the optical fiber channel; and is used to make the input dual-pulse signal enter the unbalanced interferometer in reverse for interference, and use a single-photon detector to detect the two interference results generated.

[0010] The intermediate node is used to modulate the phase difference between the two sub-pulses in the input dual-pulse signal to 0, π / 2, π, or 3π / 2, and attenuate it to the single-photon level and then output it.

[0011] When quantum key distribution is carried out between the central node and any one of the intermediate nodes, both of them perform random phase modulation respectively. After completing signal transmission and detection, operations such as basis vector comparison and post-processing are carried out to generate a symmetric key.

[0012] When quantum key distribution is carried out between any two intermediate nodes, both of them perform random phase modulation respectively, and the central node does not perform phase modulation. After completing signal transmission and detection, the central node announces the measurement results. Subsequently, the two intermediate nodes carry out operations such as basis vector comparison and post-processing to generate a symmetric key.

[0013] Preferably, the central node includes a polarization processing module, which is used to perform polarization processing such as polarization randomization and polarization excitation on the input dual-pulse, and make its horizontally polarized component enter the unbalanced interferometer in reverse.

[0014] Preferably, the intermediate node uses a polarization-independent phase modulation module to implement the phase modulation function.

[0015] Preferably, the intermediate node further includes a second variable optical attenuator VOA2 and an optical intensity monitoring module. The former is used to adjust the output optical intensity, and the latter is used to monitor the output optical intensity in real time.

[0016] Preferably, the optical intensity monitoring module of the intermediate node includes a third beam splitter BS3 and a photodetector PD. The input port of BS3 is connected to the output port of VOA2; one output port of BS3 is connected to PD, and the other output port is directly output.

[0017] Preferably, the central node includes a laser LD, an intensity modulator IM, a first circulator CIR1, a first single-photon detector SPD1, a second single-photon detector SPD2, a first beam splitter BS1, a second beam splitter BS2, a first phase modulator PM1, a first variable optical attenuator VOA1, an optical isolator ISO, and a polarization processing module.

[0018] LD is used to generate an optical pulse signal.

[0019] IM is used to modulate the optical intensity of the optical pulse signal to generate a signal state or a decoy state.

[0020] Two output ports of BS1 and two input ports of BS2 are connected by optical fibers with unequal lengths to form an unbalanced interferometer, which is used to generate two sub-pulses emerging from one output port of BS2 from an intensity-modulated optical pulse signal; the two sub-pulses emerging from the other output port of BS2 are isolated by ISO;

[0021] PM1 is used to randomly modulate the phase difference between the two sub-pulses to be one of 0, π / 2, π, and 3π / 2;

[0022] VOA1 is used to adjust the output optical intensity of the central node;

[0023] CIR1 is used to transmit the optical pulse modulated by optical intensity to one input port of BS1, and to transmit the interference result emerging from one input port of BS1 to SPD1;

[0024] SPD1 and SPD2 are respectively used to detect the interference results emerging from the two input ports of BS1.

[0025] Preferably, the polarization processing module includes a depolarizer and a polarizer. The depolarizer is used to passively randomize the polarization state of the input optical signal; the polarizer is used to pass the optical signal of the horizontal polarization component.

[0026] Preferably, the polarization-independent phase modulation module includes a second circulator CIR2, a first polarization beam splitter PBS1, a Faraday rotator FR, and a second phase modulator PM2. The second port of CIR2 is connected to the input port of PBS1; the two ports of PM2 are respectively connected to the two output ports of PBS1 through two polarization-maintaining optical fibers F1 and F2 of equal length to form a first Sagnac loop; wherein, a FR with a polarization rotation angle of 90° is provided on F1.

[0027] Preferably, the polarization-independent phase modulation module includes a third circulator CIR3, a second polarization beam splitter PBS2, a Faraday mirror FM, and a third phase modulator PM3. The second port of CIR3 is connected to one input port of PBS2; one output port of PBS2 is connected to FM through a polarization-maintaining optical fiber F3; the two ports of PM3 are respectively connected to the other output port and the other input port of PBS2 through a polarization-maintaining optical fiber F4 and a polarization-maintaining optical fiber F5 to form a second Sagnac loop; wherein, F4 = 2F3 + F5.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] The present invention provides a quantum key distribution ring network. Without the need for a trusted relay, quantum key distribution between any two nodes within the ring can be achieved. The intermediate nodes have a simple structure and only need to perform phase modulation and optical intensity monitoring. There is no need to set both a transmitting end and a receiving end simultaneously, nor is there a need for polarization compensation and phase compensation, reducing resource consumption, increasing stability, and significantly reducing deployment and operation costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a schematic diagram of the quantum key distribution ring network of the present invention;

[0031] Figure 2 It is a schematic diagram of the central node of the present invention;

[0032] Figure 3 It is a schematic diagram of the intermediate node of the present invention;

[0033] Figure 4 It is a schematic diagram of the first embodiment of the polarization-independent phase modulation module of the present invention;

[0034] Figure 5 It is a schematic diagram of the second embodiment of the polarization-independent phase modulation module of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0035] The present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.

[0036] As Figure 1 shown, a quantum key distribution ring network includes N intermediate nodes and a central node, where N is an integer not less than 2;

[0037] Each node includes an input port and an output port;

[0038] The input ports and output ports of these N + 1 adjacent nodes are connected by optical fiber channels to form a ring network. Specifically: Each pair of nodes is connected by an optical fiber channel; among them, the output port and input port of the central node are respectively connected to the input port of intermediate node 1 and the output port of intermediate node N; the input port and output port of intermediate node j are respectively connected to the output port of intermediate node j - 1 and the input port of intermediate node j + 1, where 2 ≤ j ≤ N - 1;

[0039] The central node is used to generate a double-pulse signal through a laser and an unbalanced interferometer, and modulate the phase difference between the two sub-pulses in the double-pulse signal to 0, π / 2, π, or 3π / 2 and then output it to the optical fiber channel; and is used to make the input double-pulse signal enter the unbalanced interferometer in the reverse direction for interference, and use a single-photon detector to detect the two interference results generated.

[0040] The intermediate node is used to modulate the phase difference between two sub-pulses in the input double-pulse signal to 0, π / 2, π, or 3π / 2, and attenuate it to the single-photon level before outputting;

[0041] When quantum key distribution is carried out between the central node and any one of the intermediate nodes, both of them perform random phase modulation respectively. After signal transmission and detection, operations such as basis vector comparison and post-processing are carried out to generate a symmetric key;

[0042] When quantum key distribution is carried out between any two intermediate nodes, both of them perform random phase modulation respectively, and the central node does not perform phase modulation. After signal transmission and detection, the central node announces the measurement results, and then the two intermediate nodes perform operations such as basis vector comparison and post-processing to generate a symmetric key.

[0043] The central node includes a polarization processing module, which is used to perform polarization processing such as polarization randomization and polarization excitation on the input double-pulse, so that the horizontal polarization component enters the unbalanced interferometer in the reverse direction.

[0044] The intermediate node uses a polarization-independent phase modulation module to implement the phase modulation function.

[0045] The intermediate node also includes a second variable optical attenuator VOA2 and an optical intensity monitoring module. The former is used to adjust the output optical intensity, and the latter is used to monitor the output optical intensity in real time.

[0046] The optical intensity monitoring module of the intermediate node includes a third beam splitter BS3 and a photodetector PD. The input port of BS3 is connected to the output port of VOA2; one output port of BS3 is connected to PD, and the other output port is directly output.

[0047] The central node includes a laser LD, an intensity modulator IM, a first circulator CIR1, a first single-photon detector SPD1, a second single-photon detector SPD2, a first beam splitter BS1, a second beam splitter BS2, a first phase modulator PM1, a first variable optical attenuator VOA1, an optical isolator ISO, and a polarization processing module.

[0048] LD is used to generate an optical pulse signal;

[0049] IM is used to modulate the optical intensity of the optical pulse signal to generate a signal state or a decoy state;

[0050] The two output ports of BS1 and the two input ports of BS2 are connected by optical fibers with different lengths to form an unbalanced interferometer, which is used to make an intensity-modulated optical pulse signal generate two sub-pulses emerging from one output port of BS2; the two sub-pulses emerging from the other output port of BS2 are isolated by ISO;

[0051] PM1 is used to randomly modulate the phase difference between the two sub-pulses to be one of 0, π / 2, π, and 3π / 2;

[0052] VOA1 is used to adjust the output optical intensity of the central node;

[0053] CIR1 is used to transmit the optically intensity-modulated optical pulse to an input port of BS1, and to transmit the interference result emerging from an input port of BS1 to SPD1;

[0054] SPD1 and SPD2 are respectively used to detect the interference results emerging from the two input ports of BS1.

[0055] The specific working process is as follows:

[0056] As Figure 2 shown, the laser LD of the central node generates an optical pulse signal, which is modulated by IM into a signal state or a decoy state and then enters an unbalanced interferometer composed of BS1 and BS2 through CIR1, generating two paths of double-pulse signals. One path of double-pulse signal emerges from an output port of BS2, undergoes random phase modulation by PM1 and the optical intensity is attenuated to the single-photon level by VOA1, or directly passes through PM1 and VOA1 without phase modulation and attenuation, and finally emerges from the output port of the central node; the other path of double-pulse emerges from the other output port of BS2, is isolated by ISO, and emerges from the input port of the central node without passing through the polarization output module, so it cannot enter the optical fiber channel and will not affect the quantum key distribution process.

[0057] Quantum key distribution can be carried out between the central node and any one of the intermediate nodes, and also between any two intermediate nodes. The following is a separate description:

[0058] (1) Quantum key distribution is carried out between the central node and any one intermediate node j,

[0059] The double-pulse signal generated by the central node undergoes random phase modulation by PM1 and the optical intensity is attenuated to the single-photon level by VOA1, obtaining 4 kinds of phase-encoded quantum states

[0060] ,

[0061] Among them, are the 4 kinds of phase differences between the two sub-pulse time states and randomly modulated by PM1.

[0062] The quantum state enters the input port of intermediate node j after passing through the optical fiber channel and other intermediate nodes between the output port of the central node and the input port of intermediate node j.

[0063] AsFigure 3 As shown, the quantum state first passes through a polarization-independent phase modulation module for the time state to perform random phase modulation . Subsequently, it passes through VOA2 without attenuation, and after passing through the optical fiber channel and other intermediate nodes between the output port of the intermediate node j and the input port of the central node, it reaches the input port of the central node, and the quantum state becomes

[0064] ,

[0065] After passing through the optical fiber channel, the polarization of the quantum state will change due to factors such as the environment. After entering the input port of the central node, the polarization of the quantum state is first randomized by the polarization processing module, and after polarization, a quantum state with horizontal polarization is generated, and it enters the unbalanced interferometer in the reverse direction through ISO. The time state and components respectively reach the two output ports of BS1 through the long arm and the short arm of the unbalanced interferometer at the same time, and the two interfere on BS1 to generate two interference results respectively emerging from the two input ports of BS1

[0066] ,

[0067] This process is the standard process of the phase-encoded BB84 protocol. After both parties complete signal transmission and detection, operations such as basis vector comparison and post-processing are performed to generate a symmetric key. At the intermediate node, after the quantum state passes through BS3, a part is split off and enters the photodetector PD for detection to monitor the optical intensity passing through the intermediate node in real time, and it can monitor whether there is a Trojan horse light injected by an eavesdropper.

[0068] (2) Quantum key distribution is performed between intermediate node i and intermediate node j.

[0069] As Figure 2 shown, the double-pulse signal generated by the central node passes through PM1 for random phase modulation, VOA1 without phase modulation and without attenuation, and exits from the output port of the central node and enters the optical fiber channel.

[0070] The double-pulse signal reaches intermediate node i after passing through several optical fiber channels and intermediate nodes. As Figure 3 shown, the quantum state first passes through a polarization-independent phase modulation module to modulate a random phase for its time state , and then passes through VOA2 to attenuate the optical intensity to the single-photon level to obtain 4 kinds of phase-encoded quantum states

[0071] ,

[0072] Among them, are the two sub-pulse time states randomly modulated by PM1 and Four kinds of phase differences among them.

[0073] After the quantum state passes through the optical fiber channel and several intermediate nodes, it enters the input port of intermediate node j. First, the polarization-independent phase modulation module is used to perform random phase modulation on the time state for random phase modulation , and then passes through VOA2 without attenuation. After passing through the optical fiber channel and other intermediate nodes, it reaches the input port of the central node, and the quantum state becomes

[0074] ,

[0075] After passing through the optical fiber channel, the polarization of the quantum state will change with factors such as the environment. After entering the input port of the central node, the polarization of the quantum state is first randomized by the polarization processing module, and after polarization, a horizontally polarized quantum state is generated and enters the unbalanced interferometer in the reverse direction through ISO. The time state and components respectively pass through the long arm and the short arm of the unbalanced interferometer and reach the two output ports of BS1 at the same time. The two interfere on BS1 to generate two interference results respectively emerging from the two input ports of BS1

[0076] ,

[0077] This process is the standard process of the phase-encoded BB84 protocol. After both parties complete signal transmission and detection, they perform operations such as basis vector comparison and post-processing to generate a symmetric key. At the intermediate node, after the quantum state passes through BS3, a part is split and enters the photodetector PD for detection to monitor the optical intensity passing through the intermediate node in real time, and whether there is a Trojan horse light injected by an eavesdropper can be monitored.

[0078] As Figure 4 shown, Embodiment 1 of the polarization-independent phase modulation module:

[0079] The polarization-independent phase modulation module includes a second circulator CIR2, a first polarization beam splitter PBS1, a Faraday rotator FR, and a second phase modulator PM2. The second port of CIR2 is connected to the input port of PBS1; the two ports of PM2 are respectively connected to the two output ports of PBS1 through two equal-length polarization-maintaining fibers F1 and polarization-maintaining fiber F2 to form a first Sagnac loop; among them, a FR with a polarization rotation angle of 90° is provided on F1.

[0080] The specific working process is as follows:

[0081] A quantum state with arbitrary polarization enters the input port of PBS1 through CIR2 and is divided into a horizontally polarized component and a vertically polarized component. The horizontally polarized component enters the first Sagnac loop and propagates counterclockwise, passing through PM2 and FR in sequence, and is phase-modulated by φ. The vertically polarized component enters the first Sagnac loop and propagates clockwise, passing through FR and PM2 in sequence. Since the lengths of polarization-maintaining fiber F1 and polarization-maintaining fiber F2 are equal, the vertically polarized component is also phase-modulated by φ. The two polarized components return to PBS1 simultaneously for polarization beam combination. Since both have passed through polarization-maintaining fiber F1 and polarization-maintaining fiber F2 and have the same phase modulated by PM2, the phases between the two polarized components are consistent. And due to the optical rotation effect of FR, the polarization rotates by 90° compared to the incident polarization after polarization beam combination, and finally is output through CIR2 with the phase modulated to φ. Therefore, polarization-independent phase modulation can be achieved.

[0082] As Figure 5 shown, Embodiment 2 of the polarization-independent phase modulation module:

[0083] The polarization-independent phase modulation module includes a third circulator CIR3, a second polarization beam splitter PBS2, a Faraday mirror FM, and a third phase modulator PM3. The second port of CIR3 is connected to an input port of PBS2; an output port of PBS2 is connected to FM through a polarization-maintaining fiber F3; the two ports of PM3 are respectively connected to the other output port and the other input port of PBS2 through a polarization-maintaining fiber F4 and a polarization-maintaining fiber F5 to form a second Sagnac loop; where F4 = 2F3 + F5.

[0084] The specific working process is as follows:

[0085] A quantum state with arbitrary polarization enters an input port of PBS2 through CIR2 and is divided into a horizontally polarized component and a vertically polarized component. The horizontally polarized component reaches FM after being transmitted by PBS2, is reflected and the polarization rotates by 90°, reaches PBS2 again and is reflected, enters the second Sagnac loop and propagates counterclockwise, and is phase-modulated by φ through PM3. The vertically polarized component is reflected by PBS2 and enters the second Sagnac loop and propagates clockwise, is phase-modulated by PM3 and then reaches PBS2 and is reflected, reaches FM and is reflected again, and finally is transmitted from PBS2. Since the length relationship of polarization-maintaining fiber F3, polarization-maintaining fiber F4, and polarization-maintaining fiber F5 satisfies F4 = 2F3 + F5, the vertically polarized component is also phase-modulated by φ. The two polarized components return to PBS2 simultaneously for polarization beam combination. Since the polarization-maintaining fiber paths passed through are the same and the phases modulated by PM3 are the same, the phases between the two polarized components are consistent. The polarization rotates by 90° compared to the incident polarization after polarization beam combination, and finally is output through CIR3 with the phase modulated to φ. Therefore, polarization-independent phase modulation can be achieved.

[0086] As can be seen from various embodiments of the present invention, the present invention proposes a quantum key distribution ring network. Without the need for a trusted relay, quantum key distribution between any two nodes within the ring can be achieved. The structure of the intermediate nodes is simple, only phase modulation and optical intensity monitoring are required, there is no need to set up both a transmitting end and a receiving end at the same time, and there is no need for polarization compensation and phase compensation, reducing resource consumption, increasing stability, and greatly reducing the deployment and operation costs.

Claims

1. A quantum key distribution ring network, characterized in that: including a central node and N intermediate nodes each having an input port and an output port, where N is an integer not less than 2; The input ports and output ports of these N+1 adjacent nodes are connected via optical fiber channels to form a ring network; The central node is used to generate a double pulse signal through a laser and an unequal-arm interferometer, and modulate the phase difference between the two sub-pulses in the double pulse signal to 0, π / 2, π or 3π / 2 and then output it to the optical fiber channel; and to make the input double pulse signal enter the unequal-arm interferometer in reverse to interfere, and use a single-photon detector to detect the generated two-way interference results; The intermediate node is used to modulate the phase difference between two sub-pulses in the input double pulse signal to 0, π / 2, π or 3π / 2, and output it after attenuating it to the single photon level; When quantum key distribution is performed between the central node and any intermediate node, the two nodes perform random phase modulation respectively, and after completing signal transmission and detection, basis vector comparison and post-processing operations are performed to generate a symmetric key. When quantum key distribution is performed between any two intermediate nodes, the two perform random phase modulation respectively, and the central node does not perform phase modulation. After completing signal transmission and detection, the central node announces the measurement results, and then the two intermediate nodes perform basis vector comparison and post-processing operations to generate a symmetric key.

2. The quantum key distribution ring network according to claim 1, characterized in that: The central node includes a polarization processing module for performing polarization randomization and polarization processing on the input double pulses so that the horizontal polarization components thereof enter the unequal-arm interferometer in the reverse direction.

3. The quantum key distribution ring network according to claim 1, characterized in that: The intermediate node uses a polarization-independent phase modulation module to achieve phase modulation function.

4. The quantum key distribution ring network according to claim 3, characterized in that: The intermediate node also includes a second adjustable attenuator VOA2 and a light intensity monitoring module, the former is used to adjust the output light intensity, and the latter is used to monitor the output light intensity in real time.

5. The quantum key distribution ring network according to claim 4, characterized in that: The light intensity monitoring module of the intermediate node includes a third beam splitter BS3 and a photodetector PD. The input port of BS3 is connected to the output port of VOA2; one output port of BS3 is connected to PD, and the other output port is directly output.

6. The quantum key distribution ring network according to claim 1, 2, 3 or 4, characterized in that: The central node includes a laser LD, an optical intensity modulator IM, a first circulator CIR1, a first single photon detector SPD1, a second single photon detector SPD2, a first beam splitter BS1, a second beam splitter BS2, a first phase modulator PM1, a first adjustable attenuator VOA1, an optical isolator ISO and a polarization processing module. LD is used to generate optical pulse signals; IM is used to modulate the light intensity of optical pulse signals to generate signal states or decoy states; The two output ports of BS1 and the two input ports of BS2 are connected through optical fibers of unequal lengths to form an unequal-arm interferometer, which is used to generate two sub-pulses emitted from one output port of BS2 from an intensity-modulated optical pulse signal; the two sub-pulses emitted from the other output port of BS2 are isolated by ISO; PM1 is used to randomly modulate the phase difference between the two sub-pulses to be one of 0, π / 2, π, and 3π / 2; VOA1 is used to adjust the output light intensity of the central node; CIR1 is used to transmit the light pulse after light intensity modulation to an input port of BS1, and to transmit the interference result emitted from an input port of BS1 to SPD1; SPD1 and SPD2 are respectively used to detect the interference results emitted from the two input ports of BS1.

7. The quantum key distribution ring network according to claim 6, characterized in that: The polarization processing module includes a depolarizer and a polarizer. The depolarizer is used to passively randomize the polarization state of the input optical signal; the polarizer is used to pass the optical signal of the horizontal polarization component.

8. The quantum key distribution ring network according to claim 3, 4 or 5, characterized in that: The polarization-independent phase modulation module includes a second circulator CIR2, a first polarization beam splitter PBS1, a Faraday rotator FR and a second phase modulator PM2. The second port of CIR2 is connected to the input port of PBS1; the two ports of PM2 are respectively connected to the two output ports of PBS1 through two equal-length polarization-maintaining fibers F1 and F2, forming a first Sagnac ring; wherein F1 is provided with a FR with a polarization rotation angle of 90°.

9. The quantum key distribution ring network according to claim 3, 4 or 5, characterized in that: The polarization-independent phase modulation module includes a third circulator CIR3, a second polarization beam splitter PBS2, a Faraday mirror FM and a third phase modulator PM3. The second port of CIR3 is connected to an input port of PBS2; an output port of PBS2 is connected to FM through a polarization-maintaining fiber F3; two ports of PM3 are respectively connected to another output port and another input port of PBS2 through a polarization-maintaining fiber F4 and a polarization-maintaining fiber F5 to form a second Sagnac ring; wherein the length relationship among the polarization-maintaining fiber F3, the polarization-maintaining fiber F4 and the polarization-maintaining fiber F5 satisfies F4=2F3+F5.

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