A polarization encoding device and method that combines optical fiber and free space

Abstract

This invention relates to a polarization encoding device and method combining optical fiber and free space, comprising a polarization-maintaining laser, four intensity modulators, a random signal source, two polarization-maintaining polarization couplers, two fiber collimators, a half-wave plate, and a beam splitter. By controlling the intensity modulators, a light pulse is randomly selected from one of the four light sources. This pulse is then combined with a signal from a polarization-maintaining polarization coupler that is orthogonal to the pulse. The two combined beams then pass through fiber collimators into free space, where they are non-orthogonally combined via a free space optical path, resulting in four non-orthogonal polarization states. The intensity modulators are randomly selected based on the random signal, and one of the four polarization states is randomly output at the combining port. This invention, through its encoding method combining optical fiber and free space, not only solves the problem of inconsistent wavelengths among multiple lasers but also reduces interference from optical fiber on polarization changes. This improves polarization accuracy and ensures the security of quantum state preparation.

Description

Technical Field

[0001] This invention belongs to the field of quantum communication technology and relates to a polarization coding device and method that combines optical fiber and free space. Background Technology

[0002] Quantum secure communication is a secure communication method different from classical communication. It can generate a completely consistent, unconditionally secure key between the communicating parties. This key can encrypt classical information using a "one-time pad" method, thus ensuring high security of information transmission and attracting widespread attention. In the polarization coding scheme based on the BB84 protocol, two basis vectors of orthogonal polarization states are loaded onto the photon state, with the included angle between the two pairs of basis vectors maintained at 45°.

[0003] Specifically, this encoding scheme typically includes a free-space encoding scheme using multiple lasers and an all-fiber encoding scheme using fiber polarization rotation technology. Free-space polarization encoding uses a multi-laser scheme, where light pulses emitted by multiple lasers (LDs) travel through different paths to obtain different specific polarization states. By controlling whether the lasers emit light, the desired polarization state is prepared. The light pulses are then combined and output via a beam combiner (e.g., a beam splitter prism). In the all-fiber encoding scheme, the light emitted from a single laser can be split into four paths by a fiber beam splitter. Passive encoding (selecting the optical path based on time division or optical switching) or active encoding (phase modulation, etc.) are used to obtain different specific polarization states after traveling through different paths, thus preparing the desired polarization state. The different polarization states are then combined by a fiber beam combiner and output into a single-mode fiber.

[0004] Among the existing technologies for polarization coding, free-space polarization coding, which relies on multiple lasers emitting light to generate four polarization states, requires four lasers, each driven individually. Due to differences in laser manufacturing processes and the difficulty in ensuring complete consistency of their driving signals, the emission characteristics of multiple lasers inevitably differ (e.g., spectral shape, center wavelength, and pulse time-domain characteristics). The four polarization states may also exhibit some discriminability, making it difficult to meet the practical security requirements of quantum communication. Achieving high-speed, stable modulation of the optical pulse polarization state using a single laser is a crucial issue for polarization-coded quantum key distribution applications. While the all-fiber polarization coding scheme reduces the number of light sources by using fiber beam splitters and can perform orthogonal polarization state modulation with high precision by utilizing the polarization-maintaining performance of polarization-maintaining fibers, it still faces challenges when the polarization direction of polarized light is precisely modulated to 45°. If active coding is used, the electronic modulation signal must have a stable and accurate time relationship with the arrival time of the optical pulse signal at the phase modulator, which makes the optical path stability requirement high and the electronic modulation difficult. On the other hand, when using fiber devices to modulate 45°, the polarization state is extremely unstable in the fiber channel, especially single-mode fiber, which is difficult to maintain stably, making it impossible to accurately maintain the 45° polarization direction of the input light. Summary of the Invention

[0005] The technical problem solved by this invention is to overcome the shortcomings of the prior art and propose a polarization encoding device and method that combines optical fiber and free space to solve the problem of mode mismatch that may be caused by multi-laser schemes and the inability of all-fiber encoding schemes to stably maintain a 45° polarization state.

[0006] The solution of the present invention is: a polarization encoding device that combines optical fiber and free space, comprising a polarization-maintaining laser, a polarization-maintaining fiber beam splitter, a first intensity modulator, a second intensity modulator, a third intensity modulator, a fourth intensity modulator, a random signal source, a first polarization-maintaining polarization coupler, a second polarization-maintaining polarization coupler, a first fiber collimator, a second fiber collimator, a half-wave plate, and a beam splitter prism.

[0007] The slow-axis laser emitted by the polarization-maintaining laser is split into four optical paths by the polarization-maintaining fiber beam splitter. Each optical path is controlled by an intensity modulator to control the on / off state of the optical path. The on / off state of the four intensity modulators is controlled by a random signal source, so that only one optical path is emitted at any given time.

[0008] The outputs of the first and second intensity modulators are connected to the input of the first polarization-maintaining polarization coupler, and the third and fourth intensity modulators are connected to the input of the second polarization-maintaining polarization coupler. The first, second, third, and fourth intensity modulators modulate continuous light into pulsed light. The output of the first polarization-maintaining polarization coupler is connected to the first fiber collimator, and the output of the second polarization-maintaining polarization coupler is connected to the second fiber collimator. Through the two fiber collimators, the polarized light enters the free-space coded optical path from the fiber optic coded optical path.

[0009] In the free-space coded optical path, the output light of the second fiber collimator is rotated by a half-wave plate and then combined with the output light of the first fiber collimator through a beam splitter.

[0010] Furthermore, the polarization-maintaining laser outputs continuous light.

[0011] Furthermore, the random signal source uses a random number generator to generate random signals, and the first intensity modulator, the second intensity modulator, the third intensity modulator, and the fourth intensity modulator are controlled by the same random signal, so that only one intensity controller emits light at any given time.

[0012] Furthermore, the connecting optical fibers used in the polarization encoding device are all polarization-maintaining optical fibers.

[0013] Furthermore, the first polarization-maintaining polarization coupler converts the two slow-axis lasers output by the first intensity modulator and the second intensity modulator at different times into slow-axis lasers and fast-axis lasers in the polarization-maintaining fiber, and outputs them from the same channel;

[0014] The second polarization-maintaining polarization coupler converts the two slow-axis lasers output by the third intensity modulator and the fourth intensity modulator at different times into slow-axis lasers and fast-axis lasers in the polarization-maintaining fiber, and outputs them from the same channel; wherein the slow-axis lasers and fast-axis lasers are orthogonal to each other.

[0015] Furthermore, the angle between the slow axis of the half-wave plate and the slow axis of the polarization-maintaining fiber is 22.5°.

[0016] Furthermore, the beam splitter enables the two beams to be combined at a non-orthogonal angle of 45°.

[0017] Furthermore, a polarization coding method that combines optical fiber and free space is provided, comprising the following steps:

[0018] A polarization-maintaining laser generates horizontally polarized continuous light, which is uniformly split into four beams by a polarization-maintaining fiber beam splitter. These beams then enter four intensity modulators, whose operating states are controlled by random signals. Random extinction is performed on any three beams, while the exit time of the beams not randomly extinct by the intensity modulators is controlled, thus modulating the continuous light into pulsed light. The polarization state of the outputs from the four intensity modulators is slow-axis laser.

[0019] The two slow-axis lasers output from the first intensity modulator and the second intensity modulator are converted into mutually orthogonal slow-axis lasers and fast-axis lasers in the polarization-maintaining fiber by the first polarization-maintaining polarization coupler. They are then output to the first fiber collimator through the same output channel of the polarization-maintaining polarization coupler. The polarized light enters the free space coding optical path through the fiber coding optical path.

[0020] The two slow-axis lasers output from the third and fourth intensity modulators are converted into mutually orthogonal slow-axis and fast-axis lasers in the polarization-maintaining fiber by the second polarization-maintaining polarization coupler. They are then output to the second fiber collimator through the same output channel of the second polarization-maintaining polarization coupler. The polarized light enters the free space coding optical path through the fiber coding optical path.

[0021] The two beams output from the second fiber collimator are rotated by 45° using a half-wave plate to modulate the desired polarization direction.

[0022] The output light from the second fiber collimator is rotated by a half-wave plate and then combined with the output light from the first fiber collimator through a beam splitter.

[0023] Furthermore, when the random signal arbitrarily selects an intensity modulator that does not perform extinction, one of four polarization states—0°, 45°, 90°, and 135°—is randomly output at the beam combining port of the beam splitter.

[0024] The beneficial effects of this invention compared to the prior art are:

[0025] (1) This invention uses a polarization-maintaining fiber beam splitter to split a single laser into multiple paths, and combines four intensity modulators to control the output of the optical path. The intensity modulators achieve high-speed pulse modulation of the continuous light emitted by the laser, which solves the problem of achieving high-speed and stable modulation of the polarization state of optical pulses using a single laser in the polarization-coded quantum key distribution system.

[0026] (2) The two pairs of orthogonal polarization states output by the two polarization-maintaining polarization couplers of the present invention enter free space through the fiber collimator. The combined beam output by the second fiber collimator is rotated using a 22.5° half-wave plate, which realizes 45° non-orthogonal coupling of the two pairs of basis vectors in free space, avoids the instability of polarization state transmission in fiber optic channels (especially single-mode fiber optic channels), and improves modulation accuracy. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the principle of a polarization encoding device that combines optical fiber and free space according to the present invention. Detailed Implementation

[0028] The invention will now be further described with reference to the accompanying drawings.

[0029] like Figure 1 As shown, a polarization encoding device combining optical fiber and free space according to the present invention includes a polarization-maintaining laser 1, a polarization-maintaining fiber beam splitter 2, a first intensity modulator 3, a second intensity modulator 4, a third intensity modulator 5, a fourth intensity modulator 6, a random signal source 7, a first polarization-maintaining polarization coupler 8, a second polarization-maintaining polarization coupler 9, a first fiber collimator 10, a second fiber collimator 11, a half-wave plate 12, and a beam splitter 13; wherein, the polarization-maintaining fiber beam splitter 2 is a 1×4 polarization-maintaining fiber beam splitter; and all connecting optical fibers used in the polarization encoding device are polarization-maintaining fibers.

[0030] The slow-axis laser emitted by polarization-maintaining laser 1 is split into four optical paths by polarization-maintaining fiber beam splitter 2. Each optical path uses an intensity modulator to control the on / off state of the optical path. The on / off state of the four intensity modulators is controlled by the same random signal, so that only one path emits light at any given time. In this embodiment, the random signal source 7 uses a random number generator to generate the random signal.

[0031] The outputs of the first intensity modulator 3 and the second intensity modulator 4 are connected to the input of the first polarization-maintaining polarization coupler 8, and the third intensity modulator 5 and the fourth intensity modulator 6 are connected to the input of the second polarization-maintaining polarization coupler 9. The first polarization-maintaining polarization coupler 8 converts the two slow-axis lasers output from the first intensity modulator 3 and the second intensity modulator 4 at different times into slow-axis and fast-axis lasers in the polarization-maintaining fiber, and outputs them from the same channel. The second polarization-maintaining polarization coupler 9 converts the two slow-axis lasers output from the third intensity modulator 5 and the fourth intensity modulator 6 at different times into slow-axis and fast-axis lasers in the polarization-maintaining fiber, and outputs them from the same channel. The slow-axis and fast-axis lasers are orthogonal to each other.

[0032] The output of the first polarization-maintaining polarization coupler 8 is connected to the first fiber collimator 10, and the output of the second polarization-maintaining polarization coupler 9 is connected to the second fiber collimator 11. Through the two fiber collimators, polarized light enters the free space coded optical path from the fiber coded optical path.

[0033] In the free-space coded optical path, the output light from the second fiber collimator 11 is rotated by the half-wave plate 12 and then combined with the output light from the first fiber collimator 10 through the beam splitter 13. The slow axis of the half-wave plate 12 and the slow axis of the polarization-maintaining fiber form an angle of 22.5°; the beam splitter 13 achieves a 45° non-orthogonal beam combination of the two beams.

[0034] Specifically, the polarization-maintaining laser 1 outputs continuous light; the first intensity modulator 3, the second intensity modulator 4, the third intensity modulator 5, and the fourth intensity modulator 6 modulate the continuous light into pulsed light.

[0035] The specific encoding process using the polarization encoding device described in this invention is as follows:

[0036] A polarization-maintaining laser 1 generates horizontally polarized continuous light, which is uniformly split into four beams by a polarization-maintaining fiber beam splitter 2. These beams then enter first intensity modulator 3, second intensity modulator 4, third intensity modulator 5, and fourth intensity modulator 6, respectively. A random signal controls the operating states of these four modulators, randomly extincting any three beams while simultaneously controlling the exit time of the beams not randomly extincted by the intensity modulators, thus modulating the continuous light into pulsed light. The polarization state output from all four intensity modulators is slow-axis laser. To ultimately modulate the output to four polarization directions—0°, 90°, 45°, and 135°—the beams are first divided into two groups: first intensity modulator 3 and second intensity modulator 4 form one group, and third intensity modulator 5 and fourth intensity modulator 6 form the other.

[0037] The two slow-axis lasers output from the first intensity modulator 3 and the second intensity modulator 4 are converted into mutually orthogonal slow-axis and fast-axis lasers in the polarization-maintaining fiber by the first polarization-maintaining polarization coupler 8. If the slow-axis laser output from the first polarization-maintaining polarization coupler 8 is defined as the 0° direction, then the fast-axis laser orthogonal to it is defined as the 90° direction. The first polarization-maintaining polarization coupler 8 outputs the polarized light in both the 0° and 90° directions to the first fiber collimator 10 through the same output channel. The polarized light then enters the free-space coding optical path through the fiber coding optical path.

[0038] The two slow-axis lasers output from the third intensity modulator 5 and the fourth intensity modulator 6 are converted into mutually orthogonal slow-axis and fast-axis lasers in the polarization-maintaining fiber by the second polarization-maintaining polarization coupler 9. If the slow-axis laser output from the second polarization-maintaining polarization coupler 9 is defined as the 0° direction, then the fast-axis laser orthogonal to it is defined as the 90° direction. The second polarization-maintaining polarization coupler 9 outputs the polarized light in both the 0° and 90° directions to the second fiber collimator 11 through the same output channel. The polarized light then enters the free-space coded optical path through the fiber optic coded optical path.

[0039] Since there is a 45° angle between the first group of 0° and 90° polarized light and the second group of 0° and 90° polarized light in the four polarization states required by the BB84 protocol, the two beams of light output from the second fiber collimator 11 are rotated by 45° using a 22.5° half-wave plate 12 to complete the modulation of the required polarization direction.

[0040] The output light from the second fiber collimator 11 is rotated by the half-wave plate 12, and the resulting two sets of laser pulses are input to the beam splitter 13. Each set contains a slow-axis laser and a fast-axis laser with mutually orthogonal polarization directions.

[0041] In this invention, when a random signal is randomly selected to select an intensity modulator without extinction, one of the four polarization states (0°, 45°, 90°, 135°) can be obtained, thus achieving stable polarization encoding.

[0042] Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make possible changes and modifications to the technical solutions of the present invention by utilizing the methods and techniques disclosed above without departing from the spirit and scope of the present invention. Therefore, any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solutions of the present invention shall fall within the protection scope of the technical solutions of the present invention.

Claims

1. A polarization encoding device that combines optical fiber and free space, characterized in that, Includes a polarization-maintaining laser (1), a polarization-maintaining fiber beam splitter (2), a first intensity modulator (3), a second intensity modulator (4), a third intensity modulator (5), a fourth intensity modulator (6), a random signal source (7), a first polarization-maintaining polarization coupler (8), a second polarization-maintaining polarization coupler (9), a first fiber collimator (10), a second fiber collimator (11), a half-wave plate (12), and a beam splitter (13); The slow-axis laser emitted by the polarization-maintaining laser (1) is split into four optical paths by the polarization-maintaining fiber beam splitter (2). Each optical path is controlled by an intensity modulator. The on / off state of the four intensity modulators is controlled by a random signal source (7), so that only one path of light is emitted at any given time. The outputs of the first intensity modulator (3) and the second intensity modulator (4) are connected to the input of the first polarization-maintaining polarization coupler (8), and the outputs of the third intensity modulator (5) and the fourth intensity modulator (6) are connected to the input of the second polarization-maintaining polarization coupler (9). The first intensity modulator (3), the second intensity modulator (4), the third intensity modulator (5), and the fourth intensity modulator (6) modulate continuous light into pulsed light. The output of the first polarization-maintaining polarization coupler (8) is connected to the first fiber collimator (10), and the output of the second polarization-maintaining polarization coupler (9) is connected to the second fiber collimator (11). Through the two fiber collimators, the polarized light enters the free space coded optical path from the fiber coded optical path. In the free space coded optical path, the output light of the second fiber collimator (11) is rotated by the half-wave plate (12) and then combined with the output light of the first fiber collimator (10) through the beam splitter (13). The connecting optical fibers used in the polarization encoding device are all polarization-maintaining optical fibers. The angle between the slow axis of the half-wave plate (12) and the slow axis of the polarization-maintaining fiber is 22.5°; The beam splitter (13) enables the two beams to be combined at a non-orthogonal angle of 45°.

2. The polarization encoding device that combines optical fiber and free space according to claim 1, characterized in that, The polarization-maintaining laser (1) outputs continuous light.

3. The polarization encoding device that combines optical fiber and free space according to claim 1, characterized in that, The random signal source (7) generates random signals using a random number generator. The first intensity modulator (3), the second intensity modulator (4), the third intensity modulator (5), and the fourth intensity modulator (6) are controlled by the same random signal, so that only one intensity modulator emits light at any given time.

4. The polarization encoding device that combines optical fiber and free space according to claim 1, characterized in that, The first polarization-maintaining polarization coupler (8) converts the two slow-axis lasers output by the first intensity modulator (3) and the second intensity modulator (4) at different times into slow-axis lasers and fast-axis lasers in the polarization-maintaining fiber, and outputs them from the same channel; The second polarization-maintaining polarization coupler (9) converts the two slow-axis lasers output by the third intensity modulator (5) and the fourth intensity modulator (6) at different times into slow-axis lasers and fast-axis lasers in the polarization-maintaining fiber, and outputs them from the same channel; wherein the slow-axis lasers and fast-axis lasers are orthogonal to each other.

5. A polarization coding method based on the device described in claim 4, characterized in that, Includes the following processes: The polarization-maintaining laser (1) generates horizontally polarized continuous light, which enters the polarization-maintaining fiber beam splitter (2) and is uniformly divided into four beams, which enter four intensity modulators respectively. The working state of the four intensity modulators is controlled by a random signal, and any three beams are randomly extincted. At the same time, the output time of the beams that are not randomly extincted by the intensity modulators is controlled, and the continuous light is modulated into pulsed light. The polarization state of the output of the four intensity modulators is slow-axis laser. The two slow-axis lasers output from the first intensity modulator (3) and the second intensity modulator (4) are converted into mutually orthogonal slow-axis lasers and fast-axis lasers in the polarization-maintaining fiber by the first polarization-maintaining polarization coupler (8). They are output to the first fiber collimator (10) through the same output channel of the first polarization-maintaining polarization coupler (8), and the polarized light enters the free space coding optical path through the fiber coding optical path. The two slow-axis lasers output by the third intensity modulator (5) and the fourth intensity modulator (6) are converted into mutually orthogonal slow-axis lasers and fast-axis lasers in the polarization-maintaining fiber by the second polarization-maintaining polarization coupler (9). They are output to the second fiber collimator (11) through the same output channel of the second polarization-maintaining polarization coupler (9), and the polarized light enters the free space coding optical path through the fiber coding optical path. The light output from the second fiber collimator (11) is rotated by 45° using a half-wave plate (12) to complete the modulation of the required polarization direction; The output light from the second fiber collimator (11) is rotated by the half-wave plate (12) and then combined with the output light from the first fiber collimator (10) through the beam splitter (13).

6. The polarization coding method combining optical fiber and free space according to claim 5, characterized in that, When a random signal is randomly selected to select an intensity modulator without extinction, one of four polarization states of 0°, 45°, 90°, and 135° is randomly output at the beam combining port of the beam splitter (13).

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