CN113541932B - Polarization modulator, method and quantum key distribution system for improving polarization contrast - Google Patents
Polarization modulator, method and quantum key distribution system for improving polarization contrast Download PDFInfo
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- 230000010287 polarization Effects 0.000 title claims abstract description 111
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L9/00—Cryptographic mechanisms or cryptographic arrangements for secret or secure communications; Network security protocols
- H04L9/08—Key distribution or management, e.g. generation, sharing or updating, of cryptographic keys or passwords
- H04L9/0816—Key establishment, i.e. cryptographic processes or cryptographic protocols whereby a shared secret becomes available to two or more parties, for subsequent use
- H04L9/0852—Quantum cryptography
- H04L9/0858—Details about key distillation or coding, e.g. reconciliation, error correction, privacy amplification, polarisation coding or phase coding
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/50—Transmitters
- H04B10/516—Details of coding or modulation
- H04B10/5161—Combination of different modulation schemes
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- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/50—Transmitters
- H04B10/516—Details of coding or modulation
- H04B10/532—Polarisation modulation
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- H—ELECTRICITY
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- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/50—Transmitters
- H04B10/516—Details of coding or modulation
- H04B10/54—Intensity modulation
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- H04B10/516—Details of coding or modulation
- H04B10/548—Phase or frequency modulation
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- H—ELECTRICITY
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- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/50—Transmitters
- H04B10/564—Power control
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- H—ELECTRICITY
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- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/70—Photonic quantum communication
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Abstract
The present disclosure provides a polarization modulator, a method and a quantum key distribution system for improving polarization contrast, wherein the polarization modulator comprises at least two stages of interference structures, two arms of a first stage of interference structure are provided with phase modulators, and an arm of a second stage of interference structure is provided with a power controller for compensating power loss of the phase modulators; the present disclosure can solve the problem of poor polarization contrast due to phase shifter absorption.
Description
Technical Field
The disclosure belongs to the technical field of polarization modulation, and particularly relates to a polarization modulator, a method and a quantum key distribution system for improving polarization contrast.
Background
The statements in this section merely provide background information related to the present disclosure and may not necessarily constitute prior art.
Polarization-encoded QKD systems are key distribution implemented with photon polarization states as information carriers, and photon polarization state preparation is critical in QKD systems. The accuracy and the polarization extinction ratio of photon polarization state preparation directly determine the long-term error rate and the safety of a commercial QKD system.
To the knowledge of the inventors, the preparation of photon polarization state at present mainly comprises the following two schemes:
scheme one: taking Chinese patent 201721907018.5 as an example for explanation, the technical scheme adopts a silicon photon on-chip integration technology to realize the function of an optical system of a quantum key transmitting end. As shown in fig. 1, a two-stage MZ interference structure is employed. The polarization modulator is composed of two stages of MZ interferometers connected in series and is used for preparing horizontal (|H >), vertical (|V >), 45 degrees (|P >) and-45 degrees (|N >) polarization states. The phase shifter in the two-stage interference structure generally adopts a carrier dispersion type phase shifter with higher bandwidth, and needs to be driven by dynamic pulse voltage. When the phase difference of two arms in the first-stage interference structure is modulated to 0 and pi, the second-stage interference phase difference is arbitrary, and the polarization states of |H > |V > can be prepared correspondingly. On the premise that the phase difference of the two arms of the first stage is pi/2, when the phase difference of the two arms of the second stage is modulated to 0 and pi, the polarization states of |P > | and |N > are correspondingly prepared.
This approach actually controls the magnitude of the phase shift by controlling the carrier concentration to change the waveguide refractive index. But changing the phase shift also changes the absorption efficiency of the waveguide for the optical signal. The higher the carrier concentration in the region where the waveguide overlaps the phase shifter, the higher the absorption of the optical signal by the waveguide. When preparing the horizontal polarization state (|H >), the phase difference of two arms of the first-stage interference structure is required to be 0, so that the interference constructive of the optical signals occurs from the upper arm of the second-stage interference structure, and the interference destructive of the lower arm occurs. The preparation of the vertical polarization state (|V >) requires that the phase difference of two arms of the first-stage interference structure is pi, and the optical signals are constructive in the upper arm and the lower arm of the second-stage interference structure. However, considering the absorption of the optical signal by the phase shifter, the absorption of the |h > state and the |v > state by the phase shifter is inconsistent, resulting in a perfect interference constructive or destructive state not being achieved. The polarization included angle between the |H > state and the |V > state prepared finally is poor. The second-order interference structure also faces the same problem in preparing + -45 ° (|p >, |n >) polarization states. Since the interference arm splitting ratio is fixed, this solution cannot compensate for this problem.
Scheme II: taking patent 201721742376.5 as an example, as shown in fig. 2, the first intensity modulator is used for modulating the optical signal intensity, and is used for preparing a signal state and a decoy state. The second-stage intensity modulator is driven by direct-current voltage and is used for stabilizing the light intensity proportion of two arms of the polarization modulator. The pulse driving voltage drives a polarization modulator phase shifter for preparing + -45 DEG (|P >, |N >) polarized light, left-handed circularly polarized light (|L >) and right-handed circularly polarized light (|R >).
The second-stage interference structure of the scheme also adopts a carrier dispersion type phase shifter, and has the same problem as the first scheme in the preparation of four polarization states. In the second scheme, the phase shift difference of the two arms of the second-stage intensity modulator is adjusted, so as to redistribute the light intensity ratio of the two arms in the polarization modulator. One of the pairs of polarization states can be perfectly adjusted during the adjustment process, but the other pair of angles is very poor, so in order to take care of the contrast between all the polarization states, a compromise is usually adopted, and the phase shift difference between the two arms of the second-stage intensity modulator is adjusted so that the two pairs of angles are consistent, but the two pairs of angles also have a certain deviation. The fundamental reason is that the phase shifter of the carrier dispersion effect generates the wanted different phase shifts of the optical signal under different voltages, but also generates the power attenuation of the optical signal, so that the two arms of the polarization modulator can be compensated to a certain extent by the redistribution of the optical power of the two arms of the polarization modulator by the second-stage intensity modulator. However, the compensation result cannot be considered with all polarization states, and a compromise effect is often finally needed, and the polarization contrast ratio is still low.
In summary, the existing polarization modulation schemes have the problem of poor polarization contrast caused by the absorption of the phase shifter.
Disclosure of Invention
In order to solve the above problems, the present disclosure provides a polarization modulator, a method and a quantum key distribution system for improving polarization contrast, which can solve the problem of poor polarization contrast caused by the absorption of a phase shifter.
According to some embodiments, the present disclosure employs the following technical solutions:
a polarization modulator for improving polarization contrast comprises at least two stages of interference structures, wherein phase modulators are arranged on two arms of a first stage of interference structure, and power control devices for compensating power loss of the phase modulators are arranged on arms of a second stage of interference structure.
As an alternative implementation manner, the first-stage interference structure is a power distribution module, and specifically includes a first beam splitter, a phase modulator disposed on two arms, and a second beam splitter that are sequentially connected.
Alternatively, the phase modulator is driven by an external dynamic modulation signal to produce a set amount of phase shift in the passing optical signal.
As an alternative embodiment, the second-stage interference structure includes a first polarization modulation module, a second polarization modulation module, and a beam combiner, wherein:
the first polarization modulation module and the second polarization modulation module both comprise a primary interference structure, the interference structure comprises a beam splitter, two arms and a polarization synthesizer, the two arms comprise a phase modulator, at least one arm is provided with a power control device, the power control device is configured to balance the light intensity proportion of the two arms and compensate for inconsistent power caused by waveguide loss difference and phase-related loss of the phase modulator, and the two arms of optical signals synthesize a required polarization state in the polarization synthesizer;
one arm of the primary interference structure of the second polarization modulation module is additionally provided with a bias phase shifter, and the bias phase shifter is configured to add an initial phase difference to the two arms;
the combiner is configured to combine the polarization states output by the first and second polarization modulation modules.
As an alternative embodiment, both arms of the primary interference structure of the first polarization modulation module include a phase modulator and a power control device.
Alternatively, a bias phase shifter is disposed in the upper arm of the primary interference structure of the second polarization modulation module.
Alternatively, the bias phase shifter is driven with a DC bias voltage, and the drive voltage is unchanged. As an alternative embodiment, the components of the polarization modulator are integrated on a silicon substrate.
The working method of the polarization modulator comprises the following steps:
the power distribution module divides an input optical signal into two beams, freely controls the optical power proportion of the two beams of light through power modulation, generates corresponding phase shift quantity, and enters the corresponding polarization modulation module, a power control device of the polarization modulation module balances the light intensity proportion of two arms, compensates for inconsistent power caused by waveguide loss difference and phase-related loss of a phase modulator, synthesizes required polarization states, and combines the polarization states output by the two polarization modulation modules into one beam.
As an alternative embodiment, the optical power ratio of the optical signal at the two light outlets is freely controlled by controlling the phase shift of the two-arm phase modulator of the power distribution module.
When the phase difference of the two arms is 0, all optical signals go on the way and enter the first polarization modulation module; when the phase difference of the two arms is pi, the optical signals all go down and enter the second polarization modulation module.
A quantum key distribution system comprising the above-described polarization modulator.
Compared with the prior art, the beneficial effects of the present disclosure are:
according to the method, after the power control device is used for balancing the light intensity proportion of the two arms of the interference structure, the power distribution module is matched with the two polarization modulation modules to jointly prepare the polarization states, the power control device is used for compensating the phase-related loss and the waveguide loss difference, and the polarization contrast ratio of the four polarization states can be greatly improved.
The phase shifter is formed by combining two-stage interference structures, each stage is respectively modulated by at least one modulator at high-speed phase, and a power control device is used for compensating power loss caused by the phase modulator, so that the problem of poor polarization contrast caused by the absorption of the phase shifter is solved.
Drawings
The accompanying drawings, which are included to provide a further understanding of the disclosure, illustrate and explain the exemplary embodiments of the disclosure and together with the description serve to explain the disclosure, and do not constitute an undue limitation on the disclosure.
FIG. 1 is a schematic diagram of a first prior art approach;
FIG. 2 is a schematic diagram of a second prior art approach;
fig. 3 is a schematic structural view of the present embodiment.
The specific embodiment is as follows:
the disclosure is further described below with reference to the drawings and examples.
It should be noted that the following detailed description is illustrative and is intended to provide further explanation of the present disclosure. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs.
It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of exemplary embodiments in accordance with the present disclosure. As used herein, the singular is also intended to include the plural unless the context clearly indicates otherwise, and furthermore, it is to be understood that the terms "comprises" and/or "comprising" when used in this specification are taken to specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof.
In the present disclosure, terms such as "upper", "lower", "left", "right", "front", "rear", "vertical", "horizontal", "side", "bottom", and the like indicate an azimuth or a positional relationship based on the azimuth or the positional relationship shown in the drawings, are merely relational terms determined for convenience in describing structural relationships of the various components or elements of the present disclosure, and do not denote any one of the components or elements of the present disclosure, and are not to be construed as limiting the present disclosure.
In the present disclosure, terms such as "fixedly coupled," "connected," and the like are to be construed broadly and refer to either a fixed connection or an integral or removable connection; can be directly connected or indirectly connected through an intermediate medium. The specific meaning of the terms in the disclosure may be determined according to circumstances, and should not be interpreted as limiting the disclosure, for relevant scientific research or a person skilled in the art.
As described in the background art, both existing two solutions have the problem that the polarization contrast ratio generated by the absorption of the phase shifter is poor, and cannot compensate or compensate the result, and cannot consider all the polarization states, so as to affect the long-term error rate and the safety of the commercial QKD system.
The embodiment provides a high-polarization contrast polarization modulator integrated on a silicon substrate, which can solve the problem of poor polarization contrast caused by the absorption of a phase shifter.
The two-stage interference structure is adopted to combine, and each stage is respectively modulated by at least one modulator at high-speed phase motion. As shown in fig. 3, after the optical signal is accessed, the optical signal passes through the first-stage MZ interference structure (i.e., the power distribution module). The light output ports of the module are provided with two paths, the optical signals are subjected to power modulation in the chip through the power distribution module, and the optical power proportion of the optical signals at the two light output ports can be freely controlled through the power modulation. And controlling the phase shift of the two-arm phase modulator, and when the phase difference of the two arms is 0, all optical signals go on the way and enter the first polarization modulation module, and no optical signals enter the second polarization modulation module. Of course, when the phase difference of the two arms is pi, the optical signals all go down and enter the second polarization modulation module, and no optical signal enters the first polarization modulation module.
The first polarization modulation module is composed of a first-order MZ interference structure. The two arms of the interference structure are respectively provided with a power control device and a phase modulator, wherein the power control device is used for balancing the light intensity proportion of the two arms and compensating the problem of inconsistent power caused by waveguide loss difference and phase-related loss of the phase modulator. It should be noted that, in the figure, two power control devices are provided on each of the two arms, so the design is only for convenience of later debugging, and in principle, the same effect can be achieved by single-arm power control, and the number of the power control devices is not limited. The two-arm optical signals are combined into the desired polarization state in a polarization combiner, and in this example the first polarization modulation module is used to output the |p > state and the |n > state.
It should be noted that in various embodiments, power loss from the phase shifter is compensated for using a power control device, including but not limited to, a tunable optical attenuator, an intensity modulator, and the like, that is capable of modulating the optical power output.
Meanwhile, in each stage of interference structure, the number of the power control devices is not limited. The principle is that the power compensation function can be realized.
In this embodiment the power control device is driven with a dc bias voltage, the driving voltage being kept constant.
The second polarization modulation module is substantially identical in structure to the first polarization modulation module. The two arms of the interference structure are respectively provided with a power control device and a phase modulator, the difference is that a bias phase shifter is added in the upper arm for adding an initial phase difference pi/2 to the two arms, the two arms of optical signals are synthesized into a required polarization state in a polarization synthesizer, and a second polarization modulation module is used for outputting an |R > state and an |L > state in the example.
The polarization states output by the two polarization modulation modules are combined into one beam through an off-chip 2X 1 beam combiner and transmitted into a transmission optical fiber.
After the signal is accessed to the chip, the signal firstly enters the power distribution module on the left side. In the module, the signal is equally divided into two paths, and the two paths are respectively divided into an upper arm and a lower arm, and the optical signals of the two arms are all transmitted through a phase modulator. Is driven by an external dynamic modulation signal, so that the optical signal generates phase shift after passing through a phase modulator. The phase shift produced by the two arm modulators is different and the optical signal phase difference occurs.
If the |P > state and the |N > state are prepared, the phase difference of two arms of the power distribution module is 2N pi, N is a natural number, and at the moment, the optical signals completely exit from the upper port after being interfered at the 2 x 2 beam combiner and enter the polarization modulation module 1. The polarization modulation module 1 is a first-stage MZ interference structure, wherein two arms are respectively provided with a VOA (Variable Optical Attenuator, adjustable optical attenuator) and a phase modulator. It is assumed that the phase-dependent loss of the phase modulator and the difference in loss of the two arms are not considered. After the signal enters the polarization modulation module 1, the signal is equally divided into two paths by 1:1, and the two paths are respectively moved to the upper arm and the lower arm of the modulation module. The phase modulator is driven by an external dynamic electrical signal to produce the desired phase shift. When preparing the |P > state, the phase difference of the upper and lower arms is 2n pi, n is a natural number, and the two-arm signals are synthesized into the |P > state at the polarization synthesizer. When preparing the |N > state, let the phase difference of the upper and lower arms be (2n+1) pi, N be natural number, at this time, the two arm signals are synthesized into the |N > state at the polarization synthesizer. However, the phase modulator often adopts a carrier dispersion type phase shifter, and in actual operation, there is a phase correlation loss, that is, an additional loss is generated to the optical signal when different phase shifts are generated. This loss results in a variation in the angle between the |p > and |n > states due to the different power of the two-arm optical signals, and a large preparation error between the two polarization states. Two VOAs are introduced in the polarization modulation module 1 for power compensation. The included angle between the |P > state and the |N > state can be optimized by driving the VOA attenuation through direct current voltage. Simulation results show that the method can lead the included angle on the sphere of the poincare with the state of |P > and the state of |N > to reach the ideal 180 degrees.
If the |R > state and the |L > state are prepared, the phase difference of two arms of the power distribution module is made to be (2n+1) pi, n is a natural number, and at the moment, the optical signals completely exit from the lower port and enter the polarization modulation module 2 after being interfered at the 2 x 2 beam combiner. The polarization modulation module 2 is substantially similar in structure to the polarization modulation module 1, except that a bias phase shifter is added in the upper arm. The bias phase shifter is driven with a DC bias voltage for generating pi/2 phase difference between the two arms and is constant. When preparing the |R > state, the external dynamic drive signal acts on the phase modulator to create a 0 phase difference. At this time, the optical signals sum to pi/2 in phase difference of both arms, and the two-arm optical signals are combined at the polarization combiner and combined into an |R > state. When preparing the L state, an external dynamic drive signal acts on the phase modulator to create a pi phase difference. The optical signals add up to 3 pi/2 at the two-arm phase difference, and the two-arm optical signals are combined at the polarization combiner and combined into the |L > state. Similarly, the two-arm VOA is used to compensate for the two-arm waveguide inconsistencies and the phase-dependent losses of the phase modulator, so that the angle between the |R > state and the |L > state can reach 180 degrees.
The angle between the |P > state and the |R > state and the angle between the |P > state and the |L > state are ideally 90 degrees on the poincare sphere. After the included angles between the |P > state and the |N > state and between the |R > state and the |L > state are ensured, the included angles between the |P > state and the |R > state or between the |P > state and the |L > state are approximately 90 degrees by finely adjusting the phase shift of the bias phase shifter.
Simulation results show that the error between the included angle on the poincare sphere and the ideal included angle of the polarization states prepared by the scheme is within 1 DEG, and the polarization-dependent loss PDL does not deteriorate. Has great benefit to the QKD system code rate and the safety.
It should be noted that fig. 3 is only a schematic diagram, where the beam splitter is a device with polarization independent optical power splitting function, including but not limited to a power splitter device such as a 1×2mmi.
The beam combiner shown in fig. 3 refers to a device having a beam combining function, which is used to interfere an optical signal or combine a target polarization state, and includes, but is not limited to, 2×1mmi, 2×2mmi, etc. power combining, beam splitter devices, polarization rotating beam splitters, 2D gratings, etc. polarization combining devices.
The foregoing description of the preferred embodiments of the present disclosure is provided only and not intended to limit the disclosure so that various modifications and changes may be made to the present disclosure by those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present disclosure should be included in the protection scope of the present disclosure.
While the specific embodiments of the present disclosure have been described above with reference to the drawings, it should be understood that the present disclosure is not limited to the embodiments, and that various modifications and changes can be made by one skilled in the art without inventive effort on the basis of the technical solutions of the present disclosure while remaining within the scope of the present disclosure.
Claims (9)
1. A polarization modulator for improving polarization contrast, characterized by: the device comprises at least two stages of interference structures, wherein two arms of a first stage of interference structure are provided with phase modulators, and arms of a second stage of interference structure are provided with power control devices for compensating power loss of the phase modulators;
the second-stage interference structure comprises a first polarization modulation module, a second polarization modulation module and a beam combiner, wherein:
the first polarization modulation module and the second polarization modulation module both comprise a primary interference structure, the interference structure comprises a beam splitter, two arms and a polarization synthesizer, the two arms comprise a phase modulator, at least one arm is provided with a power control device, the power control device is configured to balance the light intensity proportion of the two arms and compensate for inconsistent power caused by waveguide loss difference and phase-related loss of the phase modulator, and the two arms of optical signals synthesize a required polarization state in the polarization synthesizer;
one arm of the primary interference structure of the second polarization modulation module is additionally provided with a bias phase shifter, and the bias phase shifter is configured to add an initial phase difference to the two arms;
the combiner is configured to combine the polarization states output by the first and second polarization modulation modules.
2. A polarization modulator for improving polarization contrast as recited in claim 1, wherein: the first-stage interference structure is a power distribution module and specifically comprises a first beam splitter, a phase modulator and a second beam splitter which are sequentially connected, wherein the phase modulator is arranged on two arms.
3. A polarization modulator for improving polarization contrast as recited in claim 2, wherein: the phase modulator is driven by an external dynamic modulation signal, so that the passing optical signal generates a set phase shift amount.
4. A polarization modulator for improving polarization contrast as recited in claim 1, wherein: the two arms of the primary interference structure of the first polarization modulation module comprise a phase modulator and a power control device.
5. A polarization modulator for improving polarization contrast as recited in claim 1, wherein: and a bias phase shifter is arranged in the upper arm of the primary interference structure of the second polarization modulation module.
6. A polarization modulator for improving polarization contrast as recited in claim 1, wherein: the bias phase shifter is driven with a DC bias voltage, and the drive voltage is unchanged.
7. A method of operating a polarization modulator according to any one of claims 1 to 6, characterized by: the method comprises the following steps:
the power distribution module divides an input optical signal into two beams, freely controls the optical power proportion of the two beams of light through power modulation, generates corresponding phase shift quantity, and enters the corresponding polarization modulation module, a power control device of the polarization modulation module balances the light intensity proportion of two arms, compensates for inconsistent power caused by waveguide loss difference and phase-related loss of a phase modulator, synthesizes required polarization states, and combines the polarization states output by the two polarization modulation modules into one beam.
8. The method of operation of claim 7, wherein: the phase shift of the two-arm phase modulators of the power distribution module is controlled, so that the optical power proportion of the optical signals at the two optical outlets is freely controlled.
9. A quantum key distribution system, characterized by: comprising a polarization modulator as claimed in any one of claims 1 to 6.
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