A polarization modulation method and system based on a Sagnac interferometer

Abstract

The application provides a polarization modulation method and system based on a Sagnac interferometer, belongs to the technical field of quantum communication, and has the following steps: presetting a polarization state of a polarization measurement module, adjusting a system frequency and a frequency of a modulation voltage signal, so that light intensity detected by the polarization measurement module is stable; the modulation voltage signal adopts a rectangular wave, and a duty cycle of the rectangular wave is adjusted to 50%; a delay is adjusted, so that a forward light pulse is at a high level; a high-level modulation voltage is adjusted until the light intensity detected by the polarization measurement module is maximum, and the modulation voltage at this time is recorded; different polarization states of the polarization measurement module are set, the system frequency, the duty cycle of the modulation voltage and the delay are kept unchanged, the high-level modulation voltage is adjusted until the light intensity detected by the polarization measurement module is maximum, and the modulation voltage corresponding to different theta is recorded respectively, the polarization modulation rate limitation of the Sagnac ring system is broken, and the polarization instability problem caused by the transmission delay of the electric signal is solved.

Description

Technical Field

[0001] This invention relates to the field of quantum communication technology, specifically to a polarization modulation method and system based on a Sagnac interferometer. Background Technology

[0002] Polarization modulation is a crucial step in existing quantum key distribution protocols. A schematic diagram of a current polarization modulator based on a Sagnac interferometer is shown below. Figure 1 As shown: After the light pulse emitted by the light source passes through the polarization controller PC, the polarization state is modulated to... It enters the Sagnac interferometer, and is then separated into polarization states by the PBS. and Two beams with an intensity ratio of 1:1. Entering the interferometer clockwise, the light... The light enters the interferometer in a counter-clockwise direction. The phase modulator (PM) is located on the Sagnac interferometer loop and performs phase modulation. The counter-clockwise and clockwise light are combined by the PBS and output.

[0003] During modulation, a specific optical pulse generation frequency and phase modulation voltage signal can be set to make the counterclockwise rotation... When light passes through PM, PM is at a high level; the phase of the light changes counterclockwise. And clockwise When light passes through PM, PM is at a low level, and the clockwise light phase remains unchanged. Considering half-wave loss, the phase difference between the clockwise and counterclockwise light is... When the light is combined and output by the PBS, the polarization state of the light changes to... This achieves the modulation of the polarization state.

[0004] As can be seen from the above principles, the key to polarization modulation lies in stabilizing the phase difference between clockwise and counterclockwise light. To achieve this, Figure 1 The polarization modulator based on the Sagnac interferometer requires that PM be at a high level during the complete passage of each counterclockwise optical pulse, and at a low level during the complete passage of each clockwise optical pulse. Therefore, the time interval between two adjacent pulses must be at least [value missing]. ,in To modulate the width of the voltage square wave, Let L be the refractive index of the PM waveguide and L be the length of the PM waveguide. The speed of light in a vacuum means that the modulation frequency is limited by the time interval between two adjacent pulses. In one example, PM is used, and L is 4 cm. It is approximately 0.6 ns, and it also needs to be... The redundancy setting limits the polarization modulation frequency of the optical pulses, preventing the achievement of a 1.25 GHz modulation frequency. In QKD, to achieve higher data rates, the optical pulse transmission frequency needs to be increased, thus requiring a higher polarization modulation frequency.

[0005] Existing technology proposes to utilize the difference between the forward and reverse directions of a photon phase (PM) using a sinusoidal modulation voltage for adjustment. In this scheme, clockwise and counterclockwise pulses are modulated simultaneously, but the counterclockwise pulse is located at the peak of the sinusoidal waveform, while the clockwise pulse undergoes an integer number of sinusoidal cycles. This results in a phase change for the counterclockwise pulse, while statistically, the phase remains unchanged after clockwise modulation. However, the sinusoidal modulation voltage requires an extremely narrow pulse width; otherwise, the pulse width will cause some light to fall on non-peak areas, leading to different phase changes for photons within the same pulse in the PM, introducing additional phase errors into the phase modulation. Summary of the Invention

[0006] To address the aforementioned problems, this invention proposes a polarization modulation method based on a Sagnac interferometer, which overcomes the polarization modulation rate limitation imposed by the Sagnac interferometer and uses a rectangular wave as the modulation voltage waveform, reducing the limitation on pulse width. The method includes the following steps: S1: The input light is split into two beams by a polarization beam splitter, and they enter the Sagnac interference ring in clockwise and counterclockwise directions, respectively. The two beams are modulated by phase modulators in the ring and then combined by interference from the polarization beam splitter. S2: Apply a periodically varying modulation voltage signal to the phase modulator; S3: Set the emission frequency of the optical pulse and the operating frequency of the modulation voltage signal to both be f; S4: By adjusting the delay parameter through feedback, when the light pulse propagating in the counterclockwise direction is incident on the phase modulator, the modulation voltage signal is exactly in the high-level range, and the light pulse remains in the high-level range throughout its entire journey through the phase modulator, thus obtaining the phase change. ; S5: Within the time T that the light pulse propagating in a clockwise direction passes through the phase modulator, the modulation voltage signal completes n cycles of change, thus obtaining the phase change amount. ; S6: After the two light pulses are combined by interference, the output polarization state is... Modulated light.

[0007] Furthermore, the period of the modulated voltage signal is 2T / n, where T is the propagation time of the optical signal through the phase modulator. , Let L be the refractive index of the phase modulator waveguide, L be the physical length of the phase modulator, and c be the speed of light in vacuum.

[0008] Furthermore, the operating frequency f is f = n / (2T), where n is a positive integer.

[0009] Furthermore, the modulated voltage signal is a rectangular wave, with equal durations of high and low levels within a single cycle, a duty cycle of 50%, and the light pulse propagating counterclockwise obtains a phase change after passing through the phase modulator. .

[0010] Furthermore, the feedback control in step S4 includes: S41, the duration T of the pre-calibration optical pulse passing through the phase modulator; S42. Set the system operating frequency and the frequency of the modulation voltage signal to f=n / (2T); S43. Implement feedback compensation calibration for the system delay parameters to ensure that the light pulse propagating in the counterclockwise direction is always in the high-level range of the modulation voltage throughout the entire process of passing through the phase modulator; S44. Adjust the proportion of high and low levels within a single cycle of the modulation voltage signal so that the duration of the high level is 50% of the system cycle; S45. Fine-tune the delay parameter. When the detection light intensity of the polarization measurement module remains stable after fine-tuning forward or backward, lock the current delay parameter so that the light pulse propagating in the counterclockwise direction is at the peak position of the high level of the modulation voltage square wave. S46. Adjust the amplitude of the high level of the modulation voltage signal to make the detection light intensity of the polarization measurement module reach its maximum value.

[0011] Furthermore, the input light is split into two beams by a polarization beam splitter, with a beam splitting ratio of 99:1.

[0012] This invention also proposes a polarization modulation system based on a Sagnac interferometer, comprising: A light source, used to generate and output periodic light pulses according to the system's operating frequency; The polarization modulation module is used to receive the optical pulse and split it into two beams that propagate clockwise and counterclockwise along the interference ring, while applying differentiated phase modulation to the two beams. A beam splitter is used to split the output light modulated by the polarization modulation module into a main channel light and a feedback channel light. The polarization measurement module is used to receive the feedback channel light, measure its polarization state, and output the corresponding light intensity detection signal. The feedback control unit, connected to the polarization measurement module, the light source and the polarization modulation module, is used to execute the polarization modulation method based on the Sagnac interferometer as described in any one of claims 1 to 6 according to the light intensity detection signal, so that the polarization state of the output light is stabilized in a preset state.

[0013] Furthermore, the feedback control unit performs dynamic feedback during the quantum key distribution communication process, controls the light source to emit polarization reference light before emitting the signal light pulse, and dynamically adjusts the system delay parameters and modulation voltage signal parameters based on the measurement results of the polarization reference light by the polarization measurement module to maintain the stability of polarization modulation.

[0014] Compared with the prior art, the present invention has the following beneficial effects: This invention introduces a Sagnac interferometer structure, a polarization measurement system, and a feedback mechanism, by setting the system frequency to... This effectively breaks through the limitation of the polarization modulation rate of the Sagnac ring system, realizing high-speed periodic modulation of optical pulses; at the same time, setting the modulation voltage to a square wave solves the problem of phase instability caused by the transmission delay of electrical signals.

[0015] This invention uses a beam splitter to divide the light into a 99% main channel and a 1% feedback channel. The vast majority of the energy is used for quantum communication transmission, and only a very small amount is used for real-time monitoring, which greatly improves the overall efficiency of the system. Regardless of the position of the light pulse in the PM (edge ​​or center), the optimized square wave modulation keeps the phase change constant, eliminating the error caused by time jitter. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of a polarization modulator based on a Sagnac interferometer. Figure 2 This is a schematic diagram showing the directions of optical and voltage signals; Figure 3 A schematic diagram of the timing sequence of photoelectric signals and voltage signals in the same direction; Figure 4 This is a schematic diagram of the timing of photoelectric signals and voltage signals in opposite directions. Figure 5 This is a schematic diagram of the polarization modulation system. Detailed Implementation

[0017] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make several changes and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0018] The technical solution of this invention is based on the co-directional optical signal and voltage signal of PM ( Figure 1 The middle Sagnac ring (counterclockwise direction) and the opposite direction ( Figure 1 The noncommutativity of the Sagnac ring (clockwise direction). Figure 1 In the process, the polarization beam splitter (PBS) splits the input light into two beams, which enter the Sagnac ring in clockwise (CW) and counterclockwise (CCW) directions, respectively. These two beams are modulated by the PM within the ring and then combined by interference from the PBS.

[0019] like Figure 2 The diagram showing the directions of the optical and voltage signals illustrates that for counterclockwise light, its propagation direction is the same as the transmission direction of the modulation voltage signal applied to the PM (in the same direction); for clockwise light, its propagation direction is opposite to the transmission direction of the modulation voltage signal applied to the PM (in opposite directions).

[0020] When a constant voltage is applied, light propagating in the same direction and light propagating in opposite directions undergoes the same phase change. The key to this invention lies in using a periodically varying modulation voltage. For counterclockwise light propagating in the same direction, by precisely controlling the delay, the modulation voltage can be kept at a high level throughout the entire time period it passes through the PM, thus obtaining a fixed phase change. For clockwise light propagating in opposite directions, due to its relative motion with the voltage signal, it will experience multiple cycles of the modulation voltage waveform within the time T it takes to pass through the PM.

[0021] according to Figure 2 The diagram clearly distinguishes the physical relationship in these two cases: when the optical signal and the modulation voltage signal propagate in the same direction, the optical pulse remains synchronized with the modulation voltage signal, and remains in the high-level range of the modulation voltage throughout the entire path; however, when they propagate in opposite directions, if the modulation voltage adopts a narrow square wave form (such as...), the optical pulse will not travel in the same direction. Figure 4 As shown, during the propagation period of a light pulse in the phase modulator (PM), the corresponding modulation voltage undergoes high-low level switching, with the light pulse partially in the high-level range and partially in the low-level range. This invention utilizes this physical characteristic by selecting specific light pulse emission periods and modulation voltage signal periods. This ensures that when a light pulse propagates in opposite directions in the phase modulator (PM), the corresponding modulation voltage signal completes exactly an integer number of cycles, thus achieving a stable difference in the phase change of the light signal during opposite-direction propagation compared to when it propagates in the same direction. Specifically, during same-direction propagation, the light pulse remains in the high-level range of the modulation voltage throughout. While the light pulse propagates in opposite directions, although it experiences high-low level switching of the modulation voltage, the overall phase change remains stable. If the light pulse propagates in opposite directions without experiencing an integer number of modulation voltage signal cycles, the time integral value of the modulation voltage will fluctuate, ultimately leading to irregular changes in the phase change of the light signal during opposite-direction propagation.

[0022] Existing technology applies a periodically varying modulation voltage to a phase modulator (PM). When a light pulse propagating counter-clockwise (CCW) along a Sagnac interference ring enters the PM, the modulation voltage is in a high-level range. Conversely, when a light pulse propagating clockwise (CW) passes through the PM, the modulation voltage remains in a low-level range throughout its passage. The core objective is to generate differentiated yet constant phase changes in the clockwise and counter-clockwise propagating light pulses, thereby creating a stable phase difference. However, this technology suffers from a limited light pulse emission period. A sufficiently long low-level range must be reserved between the high-level ranges of two adjacent counter-clockwise light pulses, and this low-level range must meet the timing requirements of the clockwise light pulse's passage through the PM. Ultimately, this severely restricts the light pulse emission frequency. This invention overcomes the aforementioned technical problems. When a clockwise propagating light pulse enters the PM, there is no rigid limitation on the high or low level of the modulation voltage. By adjusting the modulation voltage, the clockwise light pulse in the PM travels through exactly an integer number of modulation voltage signal cycles. This ensures that the clockwise light pulse obtains a constant phase change and also achieves a stable difference in phase change between the clockwise and counterclockwise light pulses, thus fulfilling the core requirement of polarization modulation.

[0023] The propagation time of the optical signal through the phase modulator (PM) is ,in, Let be the refractive index of the PM waveguide, L be the physical length of the PM, and c be the speed of light in vacuum. Synchronous transmission is achieved when the optical signal and the modulated voltage signal propagate in the same direction because the transmission speed of the electrical signal in the PM is custom-designed to perfectly match the propagation speed of the optical signal in the PM medium. However, when the optical signal and the modulated voltage signal propagate in opposite directions, the relative change in time between them over the time interval T during the optical signal's passage through the PM is 2T. The relevant timing characteristics are as follows: Figure 3 Photoelectric signal propagation timing, Figure 4 The timing sequence of the photoelectric signals propagating in opposite directions is shown.

[0024] exist Figure 3 and Figure 4In the timing design, the period of the modulation voltage signal is set to 2T. Within the time T that the optical pulse takes to pass through PM, it completes phase modulation for exactly one modulation voltage signal cycle. The relationship between the relative positions of the signals is detailed by the arrows in the figure. Based on this, as long as the 2T time range includes an integer number of modulation voltage signal cycles, and the phase change of the optical signal caused by each modulation voltage signal cycle remains constant, and preferably the high-level duty cycle of the modulation voltage signal is configured to be close to 50%, it can be ensured that the phase change of the optical pulse remains constant when the optical signal and the modulation voltage signal propagate in opposite directions. It should be noted that the positions of the optical signal in the same-direction and opposite-direction propagation modes shown in the figure are only illustrative examples. In actual engineering applications, the position of the optical signal is not limited to the edge region.

[0025] In addition, to achieve stable polarization modulation, it is necessary to ensure the phase modulation stability when the optical signal and the modulation voltage signal propagate in the same direction. To achieve this technical requirement, the system operating frequency and the output frequency of the modulation voltage periodic waveform must be kept strictly consistent. At the same time, a narrow optical pulse signal design scheme is adopted to ensure that the optical pulse is in the high-level range of the modulation voltage throughout the entire process of entering the PM for phase modulation.

[0026] To ensure that the optical pulse produces a constant phase change in the phase modulator (PM), it is necessary to solve the problem of accurately matching the transmission speed of the electrical signal and the optical signal inside the PM.

[0027] The forward optical signal propagates in the same direction as the optical pulse and the modulation voltage signal. Customized design ensures that the transmission speed of the electrical signal within the PM perfectly matches the propagation speed of the optical signal in its medium, achieving strict synchronization on the time axis and guaranteeing the real-time performance of phase modulation. The backward optical signal propagates in the opposite direction to the modulation voltage signal. Within the duration T of the optical pulse passing through the PM, the modulation voltage signal completes n cycles of dynamic change (n is a positive integer). Setting the period of the modulation voltage square wave to 2T / n and configuring its high-level duty cycle to approximately 50% ensures a constant phase change throughout the entire propagation process of the optical pulse within the PM, regardless of the incident timing.

[0028] Specifically, the present invention achieves stable polarization modulation through the following steps: Frequency and delay presets: The modulation voltage signal uses a periodic rectangular wave composed of high and low levels. The emission frequency of the optical pulse and the operating frequency of the modulation voltage signal are strictly consistent and both are set to [specified values]. n is a selected positive integer.

[0029] Square wave parameter optimization: Within a single cycle of the modulated voltage rectangular wave, the duration ratio of high and low levels remains constant. It is preferable to configure the duty cycle to 50%, that is, the duration of high and low levels is equal within a single cycle.

[0030] Feedback-based delay compensation: This method precisely compensates for delay parameters through feedback control, ensuring that the delay along the path is correct. Figure 1 When a light pulse propagating counterclockwise through the Sagnac interference ring is incident on PM, the modulation voltage signal is exactly in the high-level range.

[0031] Modulation voltage amplitude control: When the modulation voltage signal is in the low-level range, no modulation voltage is applied to the PM; the modulation voltage amplitude in the high-level range is precisely controlled so that during forward modulation, after the optical signal completes modulation in the high-level range, the phase change is stabilized. .

[0032] Phase-locked and polarization-modulated output: Based on the above design, along Figure 1 In the Sagnac interference ring, the light pulse propagating counterclockwise remains in the high-level modulation voltage range throughout its passage through the PM, with a stable phase change of φ. In the clockwise direction, the relative positions of the light signal and the modulation voltage signal change over n cycles on the time axis during the passage through the PM, yielding the phase change. Furthermore, the duration of the high and low levels is equal within a single cycle, with a duty cycle of 50%. At this time, the light pulse propagating in the counterclockwise direction obtains a phase change after passing through the phase modulator. After the two optical pulses are combined by interference from a polarization beam splitter (PBS), the polarization state of the output light is modulated to... This achieves precise modulation of the target polarization state. In this invention, the system operating frequency can be set to... Multiples of integers, breaking through The frequency limit is set to achieve high-speed polarization modulation of optical pulses.

[0033] In a specific embodiment, to achieve the above-mentioned polarization stabilization modulation process, the system needs to perform the following control steps in sequence: 1. Precisely adjust the system operating frequency and the frequency of the modulation voltage signal so that both are at their optimal levels. Integer multiples of; 2. Implement feedback compensation calibration for system delay parameters to ensure that the forward-propagating light pulse remains in the high-level range of the modulation voltage throughout its entire journey through the PM; 3. Adjust the relevant parameters of the square wave modulation voltage within a single cycle to ensure that the polarization modulation bias is precisely matched with the system's preset polarization state.

[0034] This invention achieves the above-mentioned modulation and control effect through a polarization modulation system, such as... Figure 5 As shown, the polarization modulation system includes: a light source, a polarization modulation module, a beam splitter, a polarization measurement module, and a feedback control unit. (1) Light source: Generates and outputs periodic light pulses based on the system's operating frequency; (2) Polarization modulation module: adopts Figure 1 The polarization modulation device based on the Sagnac interferometer shown is used to receive the optical pulse and split it into two beams that propagate clockwise and counterclockwise along the interference ring. The phase modulation module (PM) applies differentiated phase modulation to the two beams. (3) Beam splitter: The light output from the light source is split into non-uniform light intensity beams. Most of the light is used as the main channel output light for quantum communication transmission, and a small part of the light is used as the feedback channel monitoring light input to the polarization measurement module. The preferred beam intensity ratio is 99:1. (4) Polarization measurement module: accurately detects the polarization state of the monitoring light in the feedback channel and outputs a light intensity detection signal; (5) Feedback control unit, connected to the polarization measurement module, the light source and the polarization modulation module, is used to execute the polarization modulation method based on the Sagnac interferometer according to the light intensity detection signal output by the polarization measurement module, and adjust the system operating frequency, delay parameters and period parameters of the modulation voltage signal so that the polarization state of the modulated light pulse continuously approaches the preset polarization state.

[0035] The optical pulses output from the main channel will enter the subsequent phase and intensity modulation stage. After signal modulation is completed, they will be connected to the quantum communication link to realize quantum signal transmission.

[0036] In a preferred embodiment, the polarization measurement module adopts a light intensity detection architecture based on polarization state matching degree. Under this architecture, the higher the matching degree between the polarization state of the light pulse to be measured and the preset polarization state of the system, the greater the ratio of the light intensity detected by the module to the input light intensity.

[0037] During the communication process of quantum key distribution (QKD), the system dynamically adjusts the system operating frequency, delay parameters, and modulation voltage signal through the following steps to continuously ensure the stability of polarization modulation.

[0038] (a) Polarization premodulation before transmitting signal pulse (1) The target polarization state of the polarization measurement module is preset, and the operating frequency of the system and the frequency of the modulation voltage signal are precisely adjusted to keep the light intensity detected by the polarization measurement module stable. It should be noted that if the light pulse emission frequency and the modulation voltage signal frequency do not match, the polarization state of the light pulse will fluctuate periodically, which will cause the light intensity detected by the polarization measurement module to change. The light intensity stability referred to in this invention is essentially the stability of the polarization state of the light pulse, and its quantization threshold can be set to the amount of change of the detected light intensity over time being less than 1%.

[0039] (2) Adjust the proportion of high and low levels in a single cycle of the modulation voltage signal so that the duration of the high level is 50% of the system cycle.

[0040] (3) Finely adjust the system delay parameters. When the detection light intensity of the polarization measurement module remains stable after finely adjusting the delay parameters forward or backward, lock the current delay parameters. At this time, the forward-propagating light pulse is exactly at the peak position of the high level of the modulation voltage square wave.

[0041] (4) Adjust the amplitude of the high level of the modulation voltage signal until the detection light intensity of the polarization measurement module reaches the maximum value, and record the amplitude of the high level modulation voltage at this time.

[0042] (5) Reset the multiple target polarization states of the polarization measurement module, targeting Different polarization states are configured with different phase angles θ; while keeping the system operating frequency, the proportion of high / low level modulation voltage signal duration and the system delay parameters unchanged, the amplitude of the high-level modulation voltage is adjusted to the maximum detection light intensity of the polarization measurement module, and the amplitude of the high-level modulation voltage corresponding to different phase angles θ is recorded one by one.

[0043] In a preferred embodiment, the frequency adjustment in step (2) can be achieved through pre-calibration: the propagation time T of the light pulse through the phase modulator (PM) is measured in advance, and the system operating frequency and the frequency of the modulation voltage signal are directly set to... , where n is a selected positive integer.

[0044] (II) Dynamic feedback of polarization modulation in quantum key distribution (QKD) communication process In the communication process of quantum key distribution (QKD), a polarization reference light can be sent before the signal light is emitted. Based on the polarization state detection results of the polarization reference light by the polarization measurement module, the system implements feedback control on the system delay parameters and related parameters of the modulation voltage signal, thereby continuously maintaining the stability of polarization modulation.

[0045] Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. Unless otherwise specified, the embodiments and features of the present invention can be arbitrarily combined with each other.

Claims

1. A polarization modulation method based on a Sagnac interferometer, characterized in that, Includes the following steps: S1: The input light is split into two beams by a polarization beam splitter, and they enter the Sagnac interference ring in clockwise and counterclockwise directions, respectively. The two beams are modulated by phase modulators in the ring and then combined by interference from the polarization beam splitter. S2: Apply a periodically varying modulation voltage signal to the phase modulator; S3: Set the emission frequency of the optical pulse and the operating frequency of the modulation voltage signal to both be f; S4: By adjusting the delay parameter through feedback, when the light pulse propagating in the counterclockwise direction is incident on the phase modulator, the modulation voltage signal is exactly in the high-level range, and the light pulse remains in the high-level range throughout its entire journey through the phase modulator, thus obtaining the phase change. ; S5: Within the time T that the light pulse propagating in a clockwise direction passes through the phase modulator, the modulation voltage signal completes n cycles of change, thus obtaining the phase change amount. ; S6: After the two light pulses are combined by interference, the output polarization state is... Modulated light.

2. The polarization modulation method based on a Sagnac interferometer according to claim 1, characterized in that, The period of the modulated voltage signal is 2T / n, where T is the propagation time of the optical signal through the phase modulator. , Let L be the refractive index of the phase modulator waveguide, L be the physical length of the phase modulator, and c be the speed of light in vacuum.

3. The polarization modulation method based on a Sagnac interferometer according to claim 2, characterized in that, The operating frequency f is f = n / (2T), where n is a positive integer.

4. The polarization modulation method based on a Sagnac interferometer according to claim 1, characterized in that, The modulated voltage signal is a rectangular wave, with the duration of the high and low levels being equal within a single cycle, a duty cycle of 50%, and the light pulse propagating counterclockwise obtains a phase change after passing through the phase modulator. .

5. The polarization modulation method based on a Sagnac interferometer according to claim 1, characterized in that, The feedback control in step S4 includes: S41, the duration T of the pre-calibration optical pulse passing through the phase modulator; S42. Set the system operating frequency and the frequency of the modulation voltage signal to f=n / (2T); S43. Implement feedback compensation calibration for the system delay parameters to ensure that the light pulse propagating in the counterclockwise direction is always in the high-level range of the modulation voltage throughout the entire process of passing through the phase modulator; S44. Adjust the proportion of high and low levels within a single cycle of the modulation voltage signal so that the duration of the high level is 50% of the system cycle; S45. Fine-tune the delay parameter. When the detection light intensity of the polarization measurement module remains stable after fine-tuning forward or backward, lock the current delay parameter so that the light pulse propagating in the counterclockwise direction is at the peak position of the high level of the modulation voltage square wave. S46. Adjust the amplitude of the high level of the modulation voltage signal to make the detection light intensity of the polarization measurement module reach its maximum value.

6. The polarization modulation method based on a Sagnac interferometer according to claim 1, characterized in that, The input light is split into two beams by a polarization beam splitter, with a beam splitting ratio of 99:

1.

7. A polarization modulation system based on a Sagnac interferometer, characterized in that, A polarization modulation method based on a Sagnac interferometer as described in any one of claims 1-6, comprising: A light source, used to generate and output periodic light pulses according to the system's operating frequency; The polarization modulation module is used to receive the optical pulse and split it into two beams that propagate clockwise and counterclockwise along the interference ring, while applying differentiated phase modulation to the two beams. A beam splitter is used to split the output light modulated by the polarization modulation module into a main channel light and a feedback channel light. The polarization measurement module is used to receive the feedback channel light, measure its polarization state, and output the corresponding light intensity detection signal. The feedback control unit, connected to the polarization measurement module, the light source and the polarization modulation module, is used to execute the polarization modulation method based on the Sagnac interferometer as described in any one of claims 1 to 6 according to the light intensity detection signal, so that the polarization state of the output light is stabilized in a preset state.

8. The polarization modulation system based on a Sagnac interferometer according to claim 7, characterized in that, The feedback control unit performs dynamic feedback during quantum key distribution communication, controls the light source to emit polarization reference light before emitting signal light pulse, and dynamically adjusts the system delay parameters and modulation voltage signal parameters based on the measurement results of the polarization reference light by the polarization measurement module to maintain the stability of polarization modulation.

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