A stokes parameter encoder and a method of parameter encoding

Abstract

The application discloses a Stokes parameter encoder, which comprises a fiber optical circulator and an optical path round trip path; a polarization maintaining fiber Faraday rotator is arranged on the optical path round trip path to rotate the polarization of light by 45 degrees; the fiber optical circulator is a polarization maintaining fiber optical circulator; the fiber optical polarization beam splitter is a polarization maintaining fiber optical polarization beam splitter; the delay fiber is a polarization maintaining delay fiber; and the fiber optical phase modulator is a polarization maintaining fiber optical phase modulator. The application further discloses a parameter encoding method of the encoder. The above technical scheme can perfectly offset the rotation angle deviation problem caused by the device, can guarantee the stability of the rotation angle regardless of the angle deviation of the device, is easy to realize the driving circuit, is easy to integrate and small in size due to the pure fiber structure, and can realize the continuous variable quantum key distribution of the complete GG02 protocol based on the Stokes encoder.

Description

Technical Field

[0001] This invention belongs to the technical field of quantum cryptography in quantum communication, and specifically relates to a continuous-variable quantum key distribution system employing Stokes parametric encoding. This invention also relates to the encoder and its parametric encoding method. Background Technology

[0002] Continuous variable quantum key distribution technology lags behind discrete variable quantum key distribution, but its advantage in code generation over short distances has attracted great attention in this field.

[0003] Since the advent of the GG02 protocol, continuous variable quantum key distribution has been gradually developed and its security has been proven. The longest distance of fiber-optic continuous variable quantum key distribution system has exceeded 200 kilometers.

[0004] However, the development of free-space continuous-variable quantum key distribution systems has been relatively slow. Until 2009, D. Elser et al. verified the feasibility of Stokes parameters being transmitted in free space and measured the noise levels during atmospheric transmission. Although the technology has advanced, to date, a complete GG02 protocol continuous-variable quantum key distribution system has not been implemented using Stokes coding. The main reasons are as follows:

[0005] First, the polarization rotation angle in optical fiber is not as stable as in free space. Existing literature all chooses free space modulation, which will lead to the rotation angle deviation caused by the device.

[0006] Secondly, building a pure free space optical encoder requires the use of an EOM modulator. The half-wave voltage of the EOM modulator is very high, reaching two to three hundred volts. With such a high half-wave voltage, if the frequency reaches above MHz, it is difficult for the drive circuit to achieve this. Summary of the Invention

[0007] This invention provides a Stokes parametric encoder, the purpose of which is to overcome angular rotation error and ensure the stability of the rotation angle.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0009] The Stokes parametric encoder of the present invention includes an optical fiber circulator and an optical path round trip. A polarization-maintaining fiber Faraday rotator is provided on the optical path round trip to rotate the polarization of light by 45°.

[0010] The fiber optic circulator mentioned above is a polarization-maintaining fiber optic circulator.

[0011] The encoder is equipped with a fiber polarization beam splitter, which is a polarization-maintaining fiber polarization beam splitter that combines and splits H and V light beams.

[0012] The encoder is equipped with a delay fiber, which is a polarization-maintaining delay fiber to delay the optical signal.

[0013] The encoder is equipped with an optical fiber phase modulator, which is a polarization-maintaining optical fiber phase modulator that loads phase information onto the optical signal.

[0014] Specifically:

[0015] The Stokes parametric encoder of the present invention includes an optical fiber circulator CIR1, an optical fiber circulator CIR2, an optical fiber polarization beamsplitter PBS1, an optical fiber polarization beamsplitter PBS2, a delay fiber DL1, a delay fiber DL2, an optical fiber phase modulator PM1, an optical fiber phase modulator PM2, and an optical path for round trip. A polarization-maintaining fiber Faraday rotator FR is provided on the optical path for round trip to rotate the polarization of the light by 45°. Both the optical fiber circulators CIR1 and CIR2 are polarization-maintaining fiber circulators. Both the optical fiber polarization beams PBS1 and PBS2 are polarization-maintaining fiber polarization beamsplitters, combining and splitting H and V light beams. Both the delay fibers DL1 and DL2 are polarization-maintaining delay fibers, delaying the optical signal. Both the optical fiber phase modulators PM1 and PM2 are polarization-maintaining fiber phase modulators, loading phase information onto the optical signal.

[0016] The internal connection relationship of the parametric encoder is as follows:

[0017] The fiber optic circulator CIR1 includes port 1, port 2 and port 3; wherein, port 1 is the input signal port of the encoder; port 2 is connected to the polarization-maintaining fiber Faraday rotator FR, and port 3 is connected to the fiber optic circulator CIR2.

[0018] The polarization-maintaining fiber Faraday rotator FR includes port 4 and port 5; wherein, port 4 is the input and output port of the polarization-maintaining fiber Faraday rotator FR, and port 4 is connected to port 2; port 5 is the output and input port of the polarization-maintaining fiber Faraday rotator FR, and port 5 is connected to the fiber polarization beam splitter PBS1.

[0019] The fiber polarization beam splitter PBS1 includes ports 6, 7, and 8. Port 6 is the beam combining port of the fiber polarization beam splitter PBS1 and is connected to port 5. Ports 7 and 8 are the two beam splitting ports of the PBS. Port 7 is connected to the fiber phase modulator PM1. Port 8 is connected to one end of the delay fiber DL1.

[0020] The fiber phase modulator PM1 includes port 9 and port 10. Port 9 is the input / output port of the fiber phase modulator PM1 and is connected to port 7. Port 10 is the input / output port of the fiber phase modulator PM1 and is connected to the other end of the delay fiber DL1.

[0021] The fiber optic circulator CIR2 includes ports 11, 12, and 13; wherein, port 11 is connected to port 3; port 12 is connected to the fiber polarization beam splitter PBS2; and port 13 is the output signal port of the encoder.

[0022] The fiber polarization beam splitter PBS2 includes ports 14, 15, and 16; wherein, port 14 is the beam combining port of the fiber polarization beam splitter PBS2, and port 14 is connected to port 12; ports 15 and 16 are the two beam splitting ports of the PBS, port 15 is connected to the fiber phase modulator PM2; and port 16 is connected to one end of the time delay fiber DL2.

[0023] The fiber phase modulator PM2 includes port 17 and port 18; wherein, port 17 is the input / output port of fiber phase modulator PM1, and port 17 is connected to port 15; port 18 is the input / output port of fiber phase modulator PM1, and port 18 is connected to the other end of fiber DL1.

[0024] To achieve the same inventive objective as the aforementioned technical solution, this invention also provides a parametric encoding method for the Stokes encoder described above, the technical solution of which is:

[0025] Linearly polarized light H is input to port 1 of fiber optic circulator CIR1, and reaches fiber optic Faraday rotator FR through port 2; after rotation, the linearly polarized light becomes H+V.

[0026] Then, through the fiber polarization beam splitter PBS1, H and V are propagated in clockwise and counterclockwise directions, respectively; the phase difference loaded by the fiber phase modulator PM1 is the phase modulation of one of the components, and the modulation follows a Rayleigh distribution.

[0027] The modulated signal then passes through the fiber Faraday rotator FR, and the output light consists of Stokes parameters S1 and S3; S1 is the local oscillator light required by the continuous variable quantum key distribution system. The rotated beam reaches the second ring for modulation, the main purpose of which is to perform phase traversal, and the modulation follows a uniform distribution.

[0028] The modulated beam contains three components: S1, S2, and S3. S1 is the Stokes parametric encoded local oscillator light required by the system, while S2 and S3 are the Stokes parametric encoded signal lights, which follow two-dimensional Gaussian modulation.

[0029] This completes the encoding of the entire Stokes parameter, and finally, it can be accurately transmitted to free space via port 13 of the fiber optic circulator CIR2.

[0030] The present invention adopts the above-mentioned technical solution, which can perfectly offset the rotation angle deviation problem caused by the device. No matter how much the device angle deviation is, the stability of the rotation angle can be guaranteed. The driving circuit is easy to implement. The pure fiber structure is easier to integrate and miniaturize. The complete GG02 protocol continuous variable quantum key distribution can be realized based on the Stokes encoder. Attached Figure Description

[0031] The following is a brief explanation of the contents shown in the attached diagram and the markings therein:

[0032] Figure 1 This is a connection diagram of the Stokes parametric encoder of the present invention.

[0033] The symbols 1 to 18 in the diagram represent ports 1 to 18, respectively. Detailed Implementation

[0034] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings, so as to help those skilled in the art to have a more complete, accurate and in-depth understanding of the inventive concept and technical solution of the present invention.

[0035] like Figure 1 The structure of the present invention shown is a Stokes parametric encoder, including an optical fiber circulator CIR1, an optical fiber circulator CIR2, and an optical path round trip.

[0036] To address the problems and shortcomings of existing technologies and achieve the invention objective of overcoming angular rotation errors and ensuring the stability of rotation angles, the technical solution adopted by this invention is as follows:

[0037] like Figure 1 As shown, the Stokes parametric encoder of the present invention includes an optical fiber circulator CIR1, an optical fiber circulator CIR2, an optical fiber polarization beam splitter PBS1, an optical fiber polarization beam splitter PBS2, a delay fiber DL1, a delay fiber DL2, an optical fiber phase modulator PM1, an optical fiber phase modulator PM2, and an optical path round trip; on the optical path round trip, a polarization-maintaining fiber Faraday rotator FR is provided to rotate the polarization of the light by 45°.

[0038] Both the fiber optic circulator CIR1 and fiber optic circulator CIR2 mentioned above are polarization-maintaining fiber optic circulators.

[0039] Both the fiber polarization beam splitter PBS1 and fiber polarization beam splitter PBS2 are polarization-maintaining fiber polarization beam splitters, which combine and split H and V light beams.

[0040] Both the delay fiber DL1 and the delay fiber DL2 mentioned above are polarization-maintaining delay fibers, which delay the optical signal.

[0041] Both the fiber phase modulator PM1 and the fiber phase modulator PM2 are polarization-maintaining fiber phase modulators, which load phase information onto the optical signal.

[0042] The beneficial effects of the above-mentioned technical solution in this invention are:

[0043] First, by using a fiber Faraday rotator (FR) to rotate the polarization state of light and placing it on the optical path, the rotation angle deviation caused by the device can be perfectly offset. No matter how much the device angle deviation is, the stability of the rotation angle can be guaranteed.

[0044] Second, the encoder adopts an optical fiber structure, and the modulator in the encoder is a phase modulator used in optical fiber communication. Its half-wave voltage is generally less than 4V, and the modulation rate can be up to GHz. The drive circuit is easy to implement.

[0045] Third, the pure fiber optic structure makes it easier to integrate and miniaturize.

[0046] Fourth, continuous variable quantum key distribution of the complete GG02 protocol can be implemented based on Stokes coding.

[0047] The specific structure is as follows: Figure 1 As shown, the internal connection relationship of the parametric encoder is as follows:

[0048] The fiber optic circulator CIR1 includes port 1, port 2, and port 3; wherein, port 1 is the input signal port of the encoder; port 2 is connected to the polarization-maintaining fiber Faraday rotator FR, and port 3 is connected to the fiber optic circulator CIR2; light is transmitted in the direction from port 1 to port 2 and from port 2 to port 3.

[0049] The polarization-maintaining fiber Faraday rotator FR includes port 4 and port 5; wherein, port 4 is the input (output) port of the polarization-maintaining fiber Faraday rotator FR, and port 4 is connected to port 2; port 5 is the output (input) port of the polarization-maintaining fiber Faraday rotator FR, and port 5 is connected to the fiber polarization beam splitter PBS1.

[0050] The fiber polarization beam splitter PBS1 includes ports 6, 7, and 8. Port 6 is the beam combining port of the fiber polarization beam splitter PBS1 and is connected to port 5. Ports 7 and 8 are the two beam splitting ports of the PBS. Port 7 is connected to the fiber phase modulator PM1. Port 8 is connected to one end of the delay fiber DL1.

[0051] The fiber phase modulator PM1 includes port 9 and port 10. Port 9 is the input (output) port of the fiber phase modulator PM1 and is connected to port 7. Port 10 is the output (input) port of the fiber phase modulator PM1 and is connected to the other end of the delay fiber DL1.

[0052] The fiber optic circulator CIR2 includes ports 11, 12, and 13; wherein, port 11 is connected to port 3; port 12 is connected to the fiber polarization beam splitter PBS2; and port 13 is the output signal port of the encoder.

[0053] The fiber polarization beam splitter PBS2 includes ports 14, 15, and 16; wherein, port 14 is the beam combining port of the fiber polarization beam splitter PBS2, and port 14 is connected to port 12; ports 15 and 16 are the two beam splitting ports of the PBS, port 15 is connected to the fiber phase modulator PM2; and port 16 is connected to one end of the time delay fiber DL2.

[0054] The fiber optic phase modulator PM2 includes port 17 and port 18; wherein, port 17 is the input (output) port of the fiber optic phase modulator PM1, and port 17 is connected to port 15; port 18 is the output (input) port of the fiber optic phase modulator PM1, and port 18 is connected to the other end of the fiber optic DL1.

[0055] To achieve the same inventive objective as the aforementioned technical solution, this invention also provides a parametric encoding method for the Stokes encoder described above, the technical solution of which is:

[0056] Linearly polarized light H is input to port 1 of fiber optic circulator CIR1, and reaches fiber optic Faraday rotator FR through port 2; after rotation, the linearly polarized light becomes H+V.

[0057] Then, through the fiber polarization beam splitter PBS1, H and V are propagated in clockwise and counterclockwise directions, respectively; the phase difference loaded by the fiber phase modulator PM1 is the phase modulation of one of the components, and the modulation follows a Rayleigh distribution.

[0058] The modulated signal then passes through the fiber Faraday rotator FR, and the output light consists of Stokes parameters S1 and S3; S1 is the local oscillator light required by the continuous variable quantum key distribution system. The rotated beam reaches the second ring for modulation, the main purpose of which is to perform phase traversal, and the modulation follows a uniform distribution.

[0059] The modulated beam contains three components: S1, S2, and S3. S1 is the Stokes parametric encoded local oscillator light required by the system, while S2 and S3 are the Stokes parametric encoded signal lights, which follow two-dimensional Gaussian modulation.

[0060] This completes the encoding of the entire Stokes parameter, and finally, it can be accurately transmitted to free space via port 13 of the fiber optic circulator CIR2.

[0061] The specific analysis is as follows:

[0062] like Figure 1 As shown, the linearly polarized light H emitted by the light source reaches port 1 of the fiber circulator CIR1, and then reaches the fiber Faraday rotator FR through port 2. The actual angle of the fiber Faraday rotator FR cannot reach a stable 45 degrees, especially under different temperature conditions. Therefore, the actual linearly polarized light can be considered to have rotated to the right by an angle 'a' after passing through the fiber Faraday rotator FR.

[0063] After rotation, the beam is split by the fiber polarization beam splitter PBS1. The H component is transmitted clockwise without modulation, while the V component is transmitted counterclockwise with modulation phase ψ1. The modulation of ψ1 follows a Rayleigh distribution.

[0064] Finally, the beams are combined using the fiber polarization beam splitter PBS1.

[0065] After the bundle is combined, H becomes V, and V becomes H. This results in the rotation angle becoming 90°-a after passing through the loop modulation information. Then, after being rotated right by an angle a by the fiber Faraday rotator FR, the final angle is 90°-a+a. That is, the two rotations can always maintain a 90° rotation, which can perfectly offset the rotation angle deviation caused by the device. No matter how much the device angle deviation is, the stability of the rotation angle can be guaranteed.

[0066] At this point, the outgoing light in the optical fiber contains Stokes parameters S1 and S3. S1 is the Stokes-coded local oscillator light for continuous variable quantum key distribution. The rotated beam reaches the second ring (optical fiber circulator CIR2) for modulation.

[0067] Similarly, after the fiber polarization beam splitter PBS2 splits the beam, the H component is transmitted clockwise without modulation, and the V component is transmitted counterclockwise with phase modulation ψ2. The modulation of ψ2 follows a uniform distribution, and the phase is traversed.

[0068] The modulated beam contains three components: S1, S2, and S3. S1 is the Stokes parametric encoded local oscillator light required for continuous variable quantum key distribution, while S2 and S3 are the Stokes parametric encoded signal lights, which follow two-dimensional Gaussian modulation.

[0069] The present invention has been described above by way of example with reference to the accompanying drawings. Obviously, the specific implementation of the present invention is not limited to the above-described manner. Any non-substantial improvements made using the inventive concept and technical solution of the present invention, or the direct application of the inventive concept and technical solution of the present invention to other occasions without modification, are all within the protection scope of the present invention.

Claims

1. A Stokes parametric encoder, comprising an optical fiber circulator CIR1, an optical fiber circulator CIR2, an optical fiber polarization beam splitter PBS1, an optical fiber polarization beam splitter PBS2, a time-delay fiber DL1, a time-delay fiber DL2, an optical fiber phase modulator PM1, an optical fiber phase modulator PM2, and an optical path for round trip. Its features are: Along the optical path, a polarization-maintaining fiber Faraday rotator FR is installed to rotate the polarization of the light by 45°; both fiber circulators CIR1 and CIR2 are polarization-maintaining fiber circulators; both fiber polarization beamsplitters PBS1 and PBS2 are polarization-maintaining fiber polarization beamsplitters to combine and split H and V beams; both delay fibers DL1 and DL2 are polarization-maintaining delay fibers to delay the optical signal; both fiber phase modulators PM1 and PM2 are polarization-maintaining fiber phase modulators to load phase information onto the optical signal. The internal connection relationship of the parametric encoder is as follows: The fiber optic circulator CIR1 includes port 1, port 2 and port 3; wherein, port 1 is the input signal port of the encoder; port 2 is connected to the polarization-maintaining fiber Faraday rotator FR, and port 3 is connected to the fiber optic circulator CIR2. The polarization-maintaining fiber Faraday rotator FR includes port 4 and port 5; wherein, port 4 is the input and output port of the polarization-maintaining fiber Faraday rotator FR, and port 4 is connected to port 2; port 5 is the output and input port of the polarization-maintaining fiber Faraday rotator FR, and port 5 is connected to the fiber polarization beam splitter PBS1. The fiber polarization beam splitter PBS1 includes ports 6, 7, and 8. Port 6 is the beam combining port of the fiber polarization beam splitter PBS1 and is connected to port 5. Ports 7 and 8 are the two beam splitting ports of PBS1. Port 7 is connected to the fiber phase modulator PM1. Port 8 is connected to one end of the delay fiber DL1. The fiber phase modulator PM1 includes port 9 and port 10. Port 9 is the input / output port of the fiber phase modulator PM1 and is connected to port 7. Port 10 is the input / output port of the fiber phase modulator PM1 and is connected to the other end of the delay fiber DL1. The fiber optic circulator CIR2 includes ports 11, 12, and 13; wherein, port 11 is connected to port 3; port 12 is connected to the fiber polarization beam splitter PBS2; and port 13 is the output signal port of the encoder. The fiber polarization beam splitter PBS2 includes ports 14, 15, and 16; wherein, port 14 is the beam combining port of the fiber polarization beam splitter PBS2, and port 14 is connected to port 12; ports 15 and 16 are the two beam splitting ports of PBS2, and port 15 is connected to the fiber phase modulator PM2; port 16 is connected to one end of the delay fiber DL2. The fiber optic phase modulator PM2 includes port 17 and port 18; wherein, port 17 is the input / output port of the fiber optic phase modulator PM2, and port 17 is connected to port 15; port 18 is the input / output port of the fiber optic phase modulator PM2, and port 18 is connected to the other end of the fiber optic DL2.

2. The parametric encoding method of the Stokes parametric encoder according to claim 1, characterized in that: The parametric encoding method of the Stokes parametric encoder is as follows: Linearly polarized light H is input to port 1 of fiber optic circulator CIR1, and reaches fiber optic Faraday rotator FR through port 2; after rotation, the linearly polarized light becomes H+V. Then, through the fiber polarization beam splitter PBS1, H and V are propagated in clockwise and counterclockwise directions, respectively; the phase difference loaded by the fiber phase modulator PM1 is the phase modulation of one of the components, and the modulation follows a Rayleigh distribution. The modulated signal then passes through the fiber Faraday rotator FR, and the output light consists of Stokes parameters S1 and S3; S1 is the local oscillator light required by the continuous variable quantum key distribution system. The rotated beam reaches the second ring for modulation, the main purpose of which is to perform phase traversal, and the modulation follows a uniform distribution. The modulated beam contains three components: S1, S2, and S3. S1 is the Stokes parametric encoded local oscillator light required by the system, while S2 and S3 are the Stokes parametric encoded signal lights, which follow two-dimensional Gaussian modulation. This completes the encoding of the entire Stokes parameter, and finally, it can be accurately transmitted to free space via port 13 of the fiber optic circulator CIR2.

Images