CN103969841A - All optical fiber high-speed polarization controlling system and method - Google Patents

Abstract

The present invention relates to an all-fiber high-speed polarization control system and method. The system includes: a pulse laser, used to generate a laser pulse with a constant polarization state; a polarization controller, used to convert the laser pulse generated by the pulse laser to 45° The polarization direction; the circulator is used to input the laser pulse with 45° polarization direction and output the synthesized polarized light; the polarization beam splitter is used to divide the 45° polarized light passing through the circulator into two beams with equal intensity and orthogonal polarization direction Laser pulses Λ1 and Λ2; and a bidirectional Sagnac ring structure is built by using a polarization beam splitter and a phase modulator, so that the laser pulses Λ1 and Λ2 are superimposed at the polarization beam splitter after passing through optical paths with opposite propagation directions but equal distances Outputting light of a specific polarization state; and an electro-optic phase modulator, used to phase modulate one of the laser pulses Λ1 and Λ2, thereby controlling the phase difference between the laser pulses Λ1 and Λ2 and finally controlling the polarization state of the light.

Description

An all-fiber high-speed polarization control system and method

technical field

The present invention relates to the technical field of polarization control, in particular to an all-fiber high-speed polarization control scheme based on the bidirectional Sagnac ring working mode, which can realize the control of the polarization direction of the light field with high precision, high stability and high speed, and specifically relates to a All-fiber high-speed polarization control system and method.

Background technique

As an emerging field, quantum secure communication is one of the current international research hotspots. The two parties in the communication load a sequence of true random number bits on a string of single photons, and finally establish the same key through the quantum channel, thereby realizing quantum key distribution. . The usual experimental systems are divided into phase encoding and polarization encoding, and no matter which encoding scheme is adopted, the precise control of the polarization state is directly related to the stability and bit error rate of the system. In free-space quantum secure communication systems, very high-speed electro-optical phase modulators can be used to achieve polarization control. In 1992, Bennett et al. used a Pockels cell to realize the random output of four polarization states of light in a free-space quantum secure communication demonstration system. However, in more mature all-fiber quantum key distribution systems, fiber optic polarization controllers are usually used to adjust and correct the polarization state. The traditional optical fiber polarization control device is a passive device that controls the polarization direction of the output light by squeezing the optical fiber. This mechanical control method can only be used for the adjustment of the general static polarization state, and it can usually be used in the phase-encoded all-fiber quantum key distribution system, but for the polarization-encoded all-fiber quantum key distribution system, photon The polarization state is modulated at high speed, and it is difficult to achieve precise and efficient control with this mechanical passive fiber polarization controller. In fact, the research on polarization control started almost at the same time as the appearance of single-mode optical fiber; with the rapid development of optical fiber communication, polarization control technology has been constantly updated. In 1979, Johnson first proposed a fiber-optic polarization controller based on electromagnetic extrusion. Since then, there have been various polarization controllers such as electro-optic crystal type, Faraday rotation type, and delay coupling type. In 1989, Aarts and Khoe developed a new type of endless polarization controller, which solved the reset problem of polarization control and laid the foundation for its practical application. In 2002, Hirabayashi and Amano successfully developed a low-voltage liquid crystal polarization controller. In 2003, Yoshino et al proposed a high-speed all-fiber polarization controller. In 2011, Fan Fei et al proposed a multifunctional magnetophotonic crystal terahertz tunable polarization control device. Nevertheless, for a polarization-encoded quantum key distribution system, a high-speed and precise polarization controller is still required to achieve efficient information loading. Therefore, high-speed, accurate, and low-cost polarization controllers are still the main research direction of polarization control. In the quantum key distribution experimental system, in order to obtain high-speed information loading, multiple lasers are usually used to generate various polarized light and then use a polarization beam splitter to couple these differently polarized light into the same channel to replace the polarization controller. Although this method solves the problem of precise and efficient polarization control. But its biggest disadvantage is that the implementation cost is very high, and the control system is very complicated, which requires very high accuracy and synchronization of the control circuit.

Therefore, there is currently no solution with simple structure, low cost, and high stability that can solve the high-speed and precise control of polarization in optical fibers.

Contents of the invention

The purpose of the present invention is to provide an all-fiber high-speed polarization control system and method to overcome the above problems.

In order to achieve the above object, the present invention provides an all-fiber high-speed polarization control system, characterized in that the system includes:

a pulsed laser for generating laser pulses of a constant polarization state;

A polarization controller is used to convert the laser pulse generated by the pulsed laser into a polarization direction of 45°;

A polarization beam splitter is used to divide the laser pulse with a polarization direction of 45° into two laser pulses Λ1 and Λ2 with equal intensity and orthogonal polarization direction; and a bidirectional Sagnac ring structure is built using a polarization beam splitter and a phase modulator, After the laser pulses Λ1 and Λ2 are transmitted along the two-way Sagnac ring, that is, the propagation directions are opposite and the optical paths are equal, light of a specific polarization state is superimposed and output at the polarization beam splitter;

Wherein, the polarization controller and the polarization beam splitter are connected through a circulator.

The above polarization controller adopts PC device.

The above-mentioned electro-optic phase modulator works in a single-line polarization state, and both the input and output ports of the device use polarization-maintaining optical fibers.

Based on the above system, the present invention also provides an all-fiber high-speed polarization control method, the method comprising:

Step 101) Generate a laser pulse with a constant polarization state by a pulse laser, and the constant polarization state is 45° linearly polarized light;

Step 102) Using a polarization beam splitter to split the laser pulse into two laser pulses Λ1 and Λ2 with equal intensities and orthogonal polarization directions;

Step 103) Transmitting the laser pulses Λ1 and Λ2 to the polarization beam splitter through the Sagnac ring, and performing phase modulation on the laser pulse Λ1 or Λ2 by the phase modulator during transmission;

Step 104) The laser pulses Λ1 and Λ2 transmitted to the polarization beam splitter will be superimposed to generate and output light of a certain polarization state;

Wherein, the Sagnac ring is constructed by using a polarization beam splitter and a phase modulator.

After the above step 104), it also includes:

Step 105) Changing the phase difference between the two laser pulses Λ1 and Λ2 through the modulation of the electro-optic phase modulator, and then changing the polarization state of the superimposed output light.

An all-fiber high-speed polarization control scheme based on a bidirectional Sagnac ring working mode proposed by the present invention can directly output light of various polarization states at one port without subsequent coupling operations. It has the characteristics of simple structure, low cost, high control precision, fast control speed and high stability. Since the electric control is adopted in this scheme, various disadvantages caused by mechanical control are avoided.

The all-fiber high-speed polarization control scheme of the present invention is characterized in that a laser pulse with a constant polarization state (45° linearly polarized light) is generated by a pulse laser, and a polarization beam splitter is used to split into two laser pulses with equal intensity and orthogonal polarization directions Λ1 And Λ2. And utilize polarized beam splitter and phase modulator to build a bidirectional Sagnac ring structure, described laser pulse Λ1 and Λ2 along described bidirectional Sagnac ring, promptly propagating direction is opposite and after optical path equal transmission, at polarizing beam splitter The superposition occurs to output light of a certain polarization state. In this process, the phase modulator will modulate the phase of the laser pulse Λ1 or Λ2. The polarization state of the light generated by the superposition of the two beams at the polarization beam splitter depends on the phase difference between the two beams Λ1 and Λ2, that is, the modulation phase of the phase modulator. If the phase modulator is loaded with voltages corresponding to the four phases of 0, π/2, π and 3π/2, it can generate 45° linearly polarized light, right-handed circularly polarized light, 135° linearly polarized light and left-handed circularly polarized light. These four polarization states are exactly the polarization states that need to be generated at high speed in the BB84 protocol of quantum key distribution, and they are polarized light belonging to two groups of conjugated groups respectively.

The advantage of the present invention is that, an all-fiber high-speed polarization control scheme based on the bidirectional Sagnac ring working mode of the present invention can accurately realize the control of the polarization state of light at high speed, and has good stability due to the adoption of the bidirectional Sagnac structure , strong anti-interference ability, simple device, easy integration, and low manufacturing cost. In a word, the solution provided by the present invention realizes a single port to output various polarization states without subsequent coupling operation, the phase control accuracy is 10 ^-3 rad, and the working rate can reach 2 GHz. The extinction ratio can reach 30dB. Due to the high precision, modulation speed and stability of this scheme, and the use of an all-fiber optical circuit with simple devices and low cost, it is easy to integrate, and it is expected to have a good application prospect in the field of optical communication such as quantum secret communication.

Description of drawings

Fig. 1-a is the optical path diagram that the polarized light of the present invention generates;

Figure 1-b is a total optical path diagram including polarized light generation and detection provided by the present invention;

Fig. 2 is a diagram of detection results of polarized light generated by the present invention.

Drawing logo:

LD:: pulsed laser; PC:: polarization controller;

Cir:: circulator; PBS-A, PBS-B: polarization beam splitter;

PM: phase modulator; λ/2:: half-wave plate;

D1, D2: optical power meter

Detailed ways

The present invention will be described in detail below in conjunction with the accompanying drawings and embodiments.

The invention provides an all-fiber high-speed polarization control scheme. Specifically, the pulse laser generates a laser pulse with a constant polarization state (45° linearly polarized light), and uses a polarization beam splitter to split it into two beams with equal intensity and positive polarization direction. Intersecting laser pulses Λ1 and Λ2. And use polarization beam splitter and phase modulator to build a bidirectional Sagnac ring structure, after described laser pulse Λ1 and Λ2 are transmitted along the described bidirectional Sagnac ring, that is, the direction of propagation is opposite and the optical paths are equal, Superposition takes place at the polarization beam splitter to output light of a certain polarization state. In this process, the phase modulator will modulate the phase of the laser pulse Λ1 or Λ2. The polarization state of the light generated by the superposition of the two beams at the polarization beam splitter depends on the phase difference between the two beams Λ1 and Λ2, that is, the modulation phase of the phase modulator.

The above solution uses a polarization beam splitter to split a 45° linearly polarized light into two laser pulses Λ1 and Λ2 with equal intensity and orthogonal polarization directions.

A bidirectional Sagnac ring structure is built by using a polarization beam splitter and a phase modulator as described above, and the laser pulses Λ1 and Λ2 are transmitted along the two-way Sagnac ring, and the propagation direction is opposite but the optical path is equal, thereby ensuring the stability and anti-interference of the system ability.

The polarization state of the superimposed output light is changed by modulating the phase between the two beams of linearly polarized light with equal light intensity and orthogonal polarization directions. The phase modulator only needs to phase modulate either one of the laser pulses Λ1 or Λ2.

Figure 1-a shows the optical path diagram of polarized light generation in the present invention. Figure 1-b contains two parts: polarized light generation and detection.

Polarized light generation part:

A laser pulse with a constant polarization state was generated by a pulsed laser (Advanced Laser Diode Systems, PIL131DFB-SM) with a wavelength of 1310 nm, a pulse width of 20 ps, and a repetition rate of 1 MHz. The laser pulse is converted to a polarization direction of 45° by using a traditional optical fiber coil structure polarization controller PC. After the light passes through the circulator Cir, it is split into two laser pulses Λ1 and Λ2 with equal intensity and orthogonal polarization directions by the first polarization beam splitter PBS-A. For convenience, we define the former as vertical polarization direction and the latter as parallel polarization direction. After the two beams of light travel in opposite directions but equal in distance, they are superimposed at PBS-A to form a bidirectional Sagnac ring. In this process, the phase modulator PM will phase modulate the laser pulse Λ1 in the vertical polarization direction. The electro-optic phase modulator used is produced by the 44th Research Institute of China Electronics Technology Group Corporation in Chongqing, and works in a single-line polarization state. Both the input and output ports of the device use polarization-maintaining optical fibers. Its half-wave voltage is about 4.2V. Selecting voltages of 0, 2.1, 4.2, and 6.3V can realize phase modulation of 0, π/2, π, and 3π/2, respectively, and can generate 45° linearly polarized light, right-handed circular Polarized light, 135° linearly polarized light and left-handed circularly polarized light. These four polarization states are exactly the polarization states that need to be generated at high speed in the BB84 protocol of quantum key distribution, and they are polarized light belonging to two groups of conjugated groups respectively. The modulation frequency is 1MHz, the highest modulation rate can reach 2GHz, and the precision is 10 ^-3 rad. The result of the superimposition of the two beams of light at PBS-A depends on the phase difference between the two beams of light, that is, the modulation phase of the PM. The light pulse generated by the superposition is output through the circulator Cir.

The detection part that generates polarized light:

The generated polarized light is measured with a half-wave plate, a second polarizing beam splitter PBS-B and optical power meters D1 and D2. The role of the half-wave plate is to rotate the polarization of the light by 45°. As shown in Table 1, polarized light in different directions produced by different phase differences passes through the half-wave plate, and the change in polarization direction leads to different results measured by the optical power meter after passing through the polarization beam splitter PBS-B. The two lines of the optical power meter D1 and D2 indicate the percentage of the measured light intensity.

Table 1 Measurement results of different polarized light

It can be seen from Table 1 that the light of different linear polarization states generated by the system can be reflected by the different counts of D1 and D2.

Fig. 2 is a graph showing detection results of generated polarized light. It can be seen from the figure that different modulation voltages correspond to different polarized light outputs. When the phase modulator is loaded with a voltage of 4.2V, there will be 135° linearly polarized light output from the Sagnac ring. After arriving at the half-wave plate of the detection system, the polarization state will be rotated by 45° and become horizontal linearly polarized light, which will finally pass through the polarization beam splitter and be detected by D2, and D1 will not detect any light output. It can be seen from the figure that when the optical power output by D1 is 0 and the optical power output by D2 is maximum, the corresponding modulation voltage value of the phase modulator is exactly 4.2V, and the theory and experiment agree very well. However, due to the different detection efficiencies of the two optical power meters D1 and D2 in the analyzer optical path and the difference in the coupling efficiency between the two output ports of the polarization beam splitter PBS-B and the optical power meter, the maximum value of the optical power of D1 in the figure is It is different from the maximum value of D2 optical power.

When this polarization control scheme is applied to the polarization-encoded quantum key distribution system based on the BB84 protocol, only the phase difference needs to be modulated to 0, π/2, π and 3π/2, respectively, and 45° linear polarization can be generated respectively. Light, right-handed circularly polarized light, 135° linearly polarized light and left-handed circularly polarized light. The polarization control rate of this scheme is completely dependent on the operating frequency of the phase modulator. At present, the operating frequency of commercial phase modulators on the market can reach 40GHz, so the fastest polarization control rate of this scheme can reach 40GHz. The experiment in this scheme is only a proof of principle; due to the limitation of the operating frequency of the phase modulator used in the laboratory, the demonstration of this scheme can only reach 2GHz, but this can fully meet the current quantum density based on polarization encoding. required polarization control rate in key distribution systems.

Finally, it should be noted that the above embodiments and accompanying drawings are only used to illustrate the all-fiber high-speed polarization control solution of the present invention, and are not limiting. Although the present invention has been described in detail with reference to the embodiments, those skilled in the art should understand that various combinations, modifications, or equivalent replacements of the technical solutions of the present invention will not depart from the spirit and scope of the technical solutions of the present invention, and all of them should be fall within the scope of the claims of the present invention.

Claims (5)

1. An all-fiber high-speed polarization control system, characterized in that the system comprises: a pulsed laser for generating laser pulses of a constant polarization state; A polarization controller is used to convert the laser pulse generated by the pulsed laser into a polarization direction of 45°; A polarization beam splitter is used to divide the laser pulse with a polarization direction of 45° into two laser pulses Λ1 and Λ2 with the same intensity and orthogonal polarization directions; and a bidirectional Sagnac ring structure is built using the polarization beam splitter and phase modulator, After the laser pulses Λ1 and Λ2 are transmitted along the two-way Sagnac ring, that is, the propagation directions are opposite and the optical paths are equal, light of a specific polarization state is superimposed and output at the polarization beam splitter; Wherein, the polarization controller and the polarization beam splitter are connected through a circulator. 2. The all-fiber high-speed polarization control system according to claim 1, wherein the polarization controller adopts a PC device. 3. The all-fiber high-speed polarization control system according to claim 1, wherein the electro-optical phase modulator works in a single polarization state, and both input and output ports of the device use polarization-maintaining optical fibers. 4. An all-fiber high-speed polarization control method, the method is based on the system of claim 1, and the method comprises: Step 101) Generate a laser pulse with a constant polarization state by a pulse laser, and the constant polarization state is 45° linearly polarized light; Step 102) Using a polarization beam splitter to split the laser pulse into two laser pulses Λ1 and Λ2 with equal intensities and orthogonal polarization directions; Step 103) Transmitting the laser pulses Λ1 and Λ2 to the polarization beam splitter through the Sagnac ring, and performing phase modulation on the laser pulse Λ1 or Λ2 by the phase modulator during transmission; Step 104) The laser pulses Λ1 and Λ2 transmitted to the polarization beam splitter will be superimposed to generate and output light of a certain polarization state; Wherein, the Sagnac ring is constructed by using a polarization beam splitter and a phase modulator. 5. The all-fiber high-speed polarization control method according to claim 4, characterized in that, after the step 104), it also includes: Step 105) Changing the phase difference between the two laser pulses Λ1 and Λ2 through the modulation of the electro-optic phase modulator, and then changing the polarization state of the superimposed output light.