Polarization modulation system based on Sagnac interferometer
By using a polarization modulation system based on a Sagnac interferometer and a specific connection method between the beam splitter and the phase modulator, the problem of poor stability in existing polarization modulation systems is solved, achieving higher stability and convenience, and making it suitable for quantum key distribution systems.
Patent Information
- Application Number
- CN202423287319.5
- Authority / Receiving Office
- CN · China
- Patent Type
- Utility models(China)
- Current Assignee / Owner
- Filing Date
- 2024-12-31
- Publication Date
- 2025-11-28
- Estimated Expiration
- 2034-12-31
AI Technical Summary
Existing polarization modulation systems are not stable enough and are sensitive to environmental interference, which makes QKD engineering development difficult and inconvenient.
A polarization modulation system based on a Sagnac interferometer is adopted. By utilizing a specific connection method of the beam splitter and phase modulator, the optical pulse is ensured to be transmitted through the slow axis of the polarization-maintaining fiber, and the fast axis is blocked by a polarizer to cancel the phase noise and polarization mode dispersion introduced by the transmission medium.
It improves the stability of polarization modulation, reduces interference with the external environment, simplifies system design, eliminates the need for an additional phase compensator, and enhances the stability and convenience of the system.
Smart Images
Figure CN223613354U_ABST
Abstract
Description
TECHNICAL FIELD
[0001] The utility model relates to quantum information and optical communication equipment field, concretely relates to a polarization modulation system based on Sagnac interferometer. BACKGROUND
[0002] Quantum technology is a new track of future industry. Quantum key distribution (QKD) research adopts various modulation methods and is expanding from point-to-point application and experimental system to quantum network architecture. There are various polarization modulation systems at present, wherein in the polarization modulation system based on Mach-Zehnder interferometer, two orthogonal polarization components enter different arms of the interferometer and then a phase shift is applied between them to realize polarization encoding. Although this scheme is easy to implement, it is essentially unstable due to the sensitivity of Mach-Zehnder interferometer to external environmental interference.
[0003] And some polarization modulation systems are based on lithium niobate phase modulators, the polarization maintaining optical fiber input of which is aligned at 45°, but due to polarization mode dispersion between two orthogonal components (about 10 ps for common lithium niobate phase modulators), it is usually necessary to use high birefringent fiber or another phase modulator for additional compensation. At the same time, due to the transmission of two orthogonal components on different axes of the phase modulator, the stability of this scheme is also not ideal.
[0004] Therefore, the existing QKD engineering development is difficult, inconvenient, and the stability of the polarization modulation system is not good enough. UTILITY MODEL CONTENTS
[0005] The utility model discloses a polarization modulation system based on Sagnac interferometer to solve the problem that the stability of the existing polarization modulation system is not good enough.
[0006] To achieve the above-mentioned purposes, the technical solutions adopted by the utility model are as follows:
[0007] A polarization modulation system based on Sagnac interferometer, comprising a light source, a Sagnac interferometer and a receiving end;
[0008] The Sagnac interferometer comprises a beam splitter and a phase modulator;
[0009] The first port of the beam splitter is connected with the output end of the light source, the second port of the beam splitter is connected with one end of the phase modulator through a polarization maintaining optical fiber, the third port of the beam splitter is connected with the other end of the phase modulator through a polarization maintaining optical fiber, and the fourth port of the beam splitter is connected with the input end of the receiving end.
[0010] In the scheme, the phase noise introduced by the transmission medium can be well offset by the Saganc interferometer structure, and no additional phase compensation is needed; and the clockwise and counterclockwise light pulses in the Saganc interferometer are transmitted through the slow axis of the polarization maintaining optical fiber, so that the polarization mode dispersion and the difference in the effect of the phase modulator on different polarization state light pulses are not affected, and the stability of the polarization modulation is improved.
[0011] Preferably, the Sagnac interferometer further comprises a polarizer;
[0012] The polarizer is arranged at the second port and the third port of the beam splitter respectively, so as to realize fast axis blocking and slow axis alignment.
[0013] Preferably, the first port of the beam splitter is connected with the output end of the light source through the polarization maintaining optical fiber.
[0014] Preferably, the included angle between the Key key of the first port of the beam splitter and the slow axis of the polarization maintaining optical fiber is 45°.
[0015] Preferably, the beam splitter is a half-transmission half-reflection beam splitter.
[0016] Preferably, the lengths of the polarization maintaining optical fibers connected with the two ends of the phase modulator are different.
[0017] Preferably, the phase modulator is a lithium niobate phase modulator.
[0018] Preferably, the phase loaded by the phase modulator is 0, π / 2, π or 3π / 2.
[0019] Preferably, the Key key of the second port of the beam splitter is aligned with the slow axis of the polarization maintaining optical fiber.
[0020] Preferably, the Key key of the third port of the beam splitter is aligned with the fast axis of the polarization maintaining optical fiber.
[0021] The beneficial technical effects of the present application are as follows:
[0022] The polarization modulation system based on the Sagnac interferometer can well offset the phase noise introduced by the transmission medium, and no additional phase compensation is needed; and the clockwise and counterclockwise light pulses in the Saganc interferometer are transmitted through the slow axis of the polarization maintaining optical fiber, so that the polarization mode dispersion and the difference in the effect of the phase modulator on different polarization state light pulses are not affected, and the stability of the polarization modulation is improved. BRIEF DESCRIPTION OF DRAWINGS
[0023] Figure 1 The present application is a module connection schematic diagram;
[0024] Figure 2 Figure 1 is a structural schematic diagram of a beam splitter in the utility model;
[0025] Wherein: 1, light source; 2, Sagnac interferometer; 21, beam splitter; 22, phase modulator; 23, polarizer; 3, receiving end. DETAILED DESCRIPTION
[0026] In order to make the purpose, technical scheme and advantages of the utility model more clearly, the following will be further detailed with examples, but the scope of protection of the utility model is not limited to the following specific examples.
[0027] Example 1
[0028] As shown in Figure 1, a polarization modulation system based on Sagnac interferometer 2 includes light source 1, Sagnac interferometer 2 and receiving end 3. Figure 1
[0029] The Sagnac interferometer 2 includes beam splitter 21 and phase modulator 22.
[0030] The first port of the beam splitter 21 is connected with the output end of the light source 1, the second port of the beam splitter 21 is connected with one end of the phase modulator 22 through polarization maintaining optical fiber, the third port of the beam splitter 21 is connected with the other end of the phase modulator 22 through polarization maintaining optical fiber, and the fourth port of the beam splitter 21 is connected with the input end of the receiving end 3.
[0031] In the specific implementation process, the Sagnac interferometer structure can better offset the phase noise introduced by the transmission medium, without additional phase compensation; and the clockwise and counterclockwise light pulses in the Sagnac interferometer are transmitted through the slow axis of the polarization maintaining optical fiber, so they will not be affected by the polarization mode dispersion and the difference in the effect of the phase modulator 22 on different polarization state light pulses, thereby improving the stability of polarization modulation.
[0032] Example 2
[0033] A polarization modulation system based on Sagnac interferometer 2 includes light source 1, Sagnac interferometer 2 and receiving end 3.
[0034] The Sagnac interferometer 2 includes beam splitter 21 and phase modulator 22.
[0035] The first port of the beam splitter 21 is connected with the output end of the light source 1, the second port of the beam splitter 21 is connected with one end of the phase modulator 22 through polarization maintaining optical fiber, the third port of the beam splitter 21 is connected with the other end of the phase modulator 22 through polarization maintaining optical fiber, and the fourth port of the beam splitter 21 is connected with the input end of the receiving end 3.
[0036] More specifically, the first port of the beam splitter 21 is connected with the output end of the light source 1 through a polarization maintaining optical fiber.
[0037] More specifically, the angle between the Key of the first port of the beam splitter 21 and the slow axis of the polarization maintaining optical fiber is 45°.
[0038] In the specific implementation process, the short light pulse emitted by the light source 1 enters the beam splitter 21 along the polarization maintaining optical fiber, and the incident polarization state can be represented as:
[0039]
[0040] More specifically, as shown in Figure 2 the Sagnac interferometer 2 further comprises a polarizer 23;
[0041] The polarizer 23 is arranged at the second port and the third port of the beam splitter 21 respectively, so as to realize fast axis blocking and slow axis alignment.
[0042] In the specific implementation process, the fast axis blocking and slow axis alignment realized by the polarizer 23 ensure that only the polarization component along the slow axis enters the Sagnac loop.
[0043] More specifically, the beam splitter 21 is a half-transmission half-reflection beam splitter.
[0044] In the specific implementation process, the 45° linearly polarized light pulse is divided into two beams through the beam splitter 21 and enters the Saganc loop, and the reflected light (transmitted light) is transmitted along the slow axis of the polarization maintaining optical fiber in the Saganc loop through the second (third) port of the beam splitter 21 clockwise (counterclockwise). The polarization state of the light pulse finally coupled to the polarization maintaining optical fiber in the Saganc loop can be represented as:
[0045]
[0046] wherein c and a represent clockwise and counterclockwise, and -π / 2 is the phase difference introduced by the reflection of the beam splitter 21.
[0047] More specifically, the lengths of the polarization maintaining optical fibers connected between the two ends of the phase modulator 22 and the beam splitter 21 are different.
[0048] In the specific implementation process, since the lengths of the polarization maintaining optical fibers connected between the two ends of the phase modulator 22 and the beam splitter 21 in the Saganc loop are different, the times of the two light pulses in the clockwise and counterclockwise channels reaching the phase modulator 22 are different, and there is a fixed delay. Therefore, by adjusting the delay of the control electric signal of the phase modulator 22, the phase can be accurately loaded on one of the two light pulses, so that the two light pulses have a certain phase difference. The polarization state after the modulation of the phase modulator 22 can be represented as:
[0049]
[0050] More specifically, the Key of the second port of the beam splitter 21 is aligned with the slow axis of the polarization maintaining fiber.
[0051] More specifically, the Key of the third port of the beam splitter 21 is aligned with the fast axis of the polarization maintaining fiber.
[0052] In the specific implementation, the Key of the second port and the third port of the beam splitter 21 are respectively aligned with the slow axis and the fast axis, so the third port also plays a role of 90° polarization rotation. The polarization states of the clockwise and counterclockwise light pulses after interference after returning to the beam splitter 21 are:
[0053]
[0054] Therefore, the output light pulses can be controlled by the phase modulator 22 to add phase .
[0055] More specifically, the phase modulator 22 is a lithium niobate phase modulator.
[0056] More specifically, the phase loaded by the phase modulator 22 is 0, π / 2, π or 3π / 2.
[0057] In the specific implementation, when 0, π / 2, π, 3π / 2 are taken, the output polarization states are respectively:
[0058]
[0059]
[0060] The four mutually unbiased bases meet the requirements of the BB84 protocol and can be used for quantum key distribution. If necessary, the four polarization states can be converted into the common |H>, |V>, |L>, |R> through unitary transformation.
[0061] According to the disclosure and teaching of the above description, the person skilled in the art of the present application can also change and modify the above embodiments. Therefore, the present application is not limited to the specific embodiments disclosed and described above, and some modifications and changes of the present application should fall within the protection scope of the claims of the present application. In addition, although some specific terms are used in the present specification, these terms are only for convenience and do not constitute any limitation on the present application.
Claims
1. A polarization modulation system based on a Sagnac interferometer, characterized in that, The light source, the Sagnac interferometer and the receiving end are connected in series. The Sagnac interferometer comprises a beam splitter and a phase modulator. The first port of the beam splitter is connected with the output end of the light source, the second port of the beam splitter is connected with one end of the phase modulator through a polarization maintaining optical fiber, the third port of the beam splitter is connected with the other end of the phase modulator through a polarization maintaining optical fiber, and the fourth port of the beam splitter is connected with the input end of the receiving end.
2. A polarization modulation system based on Sagnac interferometer according to claim 1, characterized in that, The Sagnac interferometer further comprises a polarizer. The polarizers are arranged at the second port and the third port of the beam splitter respectively, so as to realize fast axis blocking and slow axis alignment.
3. A polarization modulation system based on Sagnac interferometer according to claim 1, characterized in that, The first port of the beam splitter is connected with the output end of the light source through a polarization maintaining optical fiber.
4. A polarization modulation system based on Sagnac interferometer according to claim 3, characterized in that, The included angle between the Key key of the first port of the beam splitter and the slow axis of the polarization maintaining optical fiber is 45°.
5. A polarization modulation system based on Sagnac interferometer according to claim 1, characterized in that, The beam splitter is a half-transmission half-reflection beam splitter.
6. A polarization modulation system based on Sagnac interferometer according to claim 1, characterized in that, The lengths of the polarization maintaining optical fibers connected with the two ends of the phase modulator are different.
7. A polarization modulation system based on Sagnac interferometer according to claim 1, characterized in that, The phase modulator is a lithium niobate phase modulator.
8. A polarization modulation system based on Sagnac interferometer according to claim 1, characterized in that, The phase loaded by the phase modulator is 0, π / 2, π or 3π / 2.
9. A polarization modulation system based on Sagnac interferometer according to claim 1, characterized in that, The Key key of the second port of the beam splitter is aligned with the slow axis of the polarization maintaining optical fiber.
10. A polarization modulation system based on Sagnac interferometer according to claim 1, characterized in that, The Key key of the third port of the beam splitter is aligned with the fast axis of the polarization maintaining optical fiber.