CN113497705A - Polarization modulator, driving method and quantum key distribution system - Google Patents
Polarization modulator, driving method and quantum key distribution system Download PDFInfo
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Abstract
The invention provides a polarization modulator, a driving method and a quantum key distribution system, which comprise a first-stage interference structure and a second-stage interference structure which are cascaded, wherein a bias phase shifter is arranged on one arm of the first-stage interference structure, and two arms of the second-stage interference structure are respectively provided with a phase shifter; an offset phase shifter configured to be driven by a direct current voltage, the phase shifter configured to be driven by a pulse voltage in a single-ended push-pull manner; the system greatly improves the system integration level and reduces the cost of an optical system; the problem of unstable polarization state preparation under dynamic high-speed modulation is solved, and the number and the driving voltage of dynamic driving circuits are reduced.
Description
Technical Field
The disclosure belongs to the technical field of quantum key distribution, and particularly relates to a polarization modulator, a driving method and a quantum key distribution system.
Background
The statements in this section merely provide background information related to the present disclosure and may not necessarily constitute prior art.
Polarization encoded Quantum Key Distribution (QKD) systems achieve key distribution with photon polarization states as the information carrier. Photon polarization state fabrication is critical in QKD systems. The accuracy and stability of photon polarization state preparation directly determine the long-term bit error rate and safety of the commercial QKD system.
According to the inventor, two photon polarization state preparation schemes are mainly included at present, wherein one scheme is that classical discrete devices (a polarization beam splitter, a phase shifter, an optical fiber, a flange and the like) are combined and connected by using the existing commercial quantum key distribution scheme to build a polarization modulator/module with the required function. The polarization modulator/module manufactured by the scheme is large in size and not beneficial to integration. The influence of the environmental temperature and the vibration is obvious, and the stability is poor.
The other scheme is to adopt the silicon photonic chip integration technology to realize the function of the quantum key transmitting end optical system. As shown in fig. 1: a two-stage mach-zehnder interferometer (MZ interferometer) structure is employed. The polarization modulator is composed of two stages of MZ interferometers connected in series and is used for preparing a horizontal polarization state H, a vertical polarization state V, a +45 DEG polarization state P and a-45 DEG polarization state N. Existing QKD products all employ pulsed light sources, which require phase shifters in two-stage interferometric structures to be driven with pulsed voltages. And when the phase difference of the two arms of the signal light in the first-stage interference structure is 0 and pi, the horizontal polarization state and the vertical polarization state are respectively and correspondingly prepared. Under the premise that the phase difference of the first two arms is pi/2, the phase difference of the second two arms is 0 and pi, and a + 45-degree polarization state and a-45-degree polarization state are correspondingly prepared respectively. In QKD systems, signals of several hundred MHz or even GHz are typically employed. With such high frequency modulation, it is difficult to drive to achieve perfect phase difference. And the pi/2 phase shift is in the middle position of the phase shift-power interference curve, the slope is the largest, the influence of phase shift jitter on the interference power output is larger, and the dependence of the polarization state modulation of the second-stage interference structure on the light intensity ratio of the upper arm and the lower arm of the second-stage interference structure is stronger. Therefore, the slight deviation of the phase difference of the first-stage modulation has a very significant influence on the preparation of the polarization state of the second-stage interference structure. Secondly, the scheme adopts two independent dynamic driving circuits, so that the cost is high.
Disclosure of Invention
In order to solve the above problems, the present disclosure provides a polarization modulator, a driving method, and a quantum key distribution system, which greatly improves the system integration level and reduces the cost of an optical system; the problem of unstable polarization state preparation under dynamic high-speed modulation is solved, and the number and the driving voltage of dynamic driving circuits are reduced.
According to some embodiments, the following technical scheme is adopted in the disclosure:
a polarization modulator is integrated on a silicon substrate and comprises a first-stage interference structure and a second-stage interference structure which are cascaded, wherein a bias phase shifter is arranged on one arm of the first-stage interference structure, and two arms of the second-stage interference structure are respectively provided with a phase shifter;
the offset phase shifter is configured to be driven by a direct current voltage, and the phase shifter is configured to be driven by a pulse voltage in a single-ended push-pull manner.
In the technical scheme, firstly, all the components are integrated on the silicon substrate, so that the problems of large volume, poor stability and the like in a typical discrete device mode in the prior art are effectively solved, the integration level is greatly improved, and the cost of an optical system is also reduced.
Secondly, the phase shifter of the second-level interference structure is driven by pulse driving voltage in a single-ended push-pull mode, and the two-arm phase shifter can be driven by one path of driving signal at the same time, so that half-wave voltage is reduced, the chirp phenomenon of the modulator is reduced, and the bandwidth of the modulator is improved. The problems that two paths of independent dynamic driving circuits are needed, the cost is high and the mutual influence between two stages is large in the second mode in the prior art are solved.
As an alternative embodiment, the first-stage interference structure includes a 1 × 2 beam splitter, an offset phase shifter, and a 2 × 2 beam splitter, where the 1 × 2 beam splitter is used to divide the signal light into two beams in equal proportion, an output end of the 1 × 2 beam splitter is connected to different input ends of the 2 × 2 beam splitter through waveguides, respectively, and the offset phase shifter is additionally disposed on one path.
The 2 x 2 beam splitter is used for converging two beams of optical signals and forming interference.
As an alternative embodiment, the offset phase shifter may be a thermally tuned phase shifter or a silicon based phase shifter based on carrier dispersion effects.
Of course, in other alternative embodiments, other types of offset phase shifters may be substituted as appropriate or desired.
As an alternative embodiment, the splitting ratio of the 1 × 2 beam splitter and the 2 × 2 beam splitter is 1: 1.
As an alternative embodiment, the second-stage interference structure includes two phase shifters and a polarization combiner, two output ends of the 2 × 2 beam splitter are respectively connected to one phase shifter, and two output ends of the two phase shifters are respectively connected to two input ends of the polarization combiner.
As an alternative embodiment, a BIAS electrode is added in the electrode structure of the modulator, and is used for inputting a direct current BIAS signal, so that the modulator operates at a target BIAS point.
As an alternative embodiment, the phase shifter is a PN type phase shifter.
Further, the PN type phase shifter is a silicon-based phase shifter based on a carrier dispersion effect, and the optional doping structure is a PIN type or a PN type.
Of course, in other alternative embodiments, other types of phase shifters may be substituted as appropriate or desired.
Based on the driving method of the polarization modulator, the bias phase shifter in the first-stage interference structure is driven by direct-current voltage, and the phase shifter of the second-stage interference structure is driven by pulse driving voltage in a single-ended push-pull mode.
Specifically, the dynamic pulse voltage drives the two-arm phase shifter in a push-pull manner, so that the phase shifter modulates four phase differences to two paths of signal light, and the four phase differences are converged and synthesized on the polarization beam combiner to form a required polarization state: when the phase difference of the second-stage interference two-arm signals is 0, the synthetic polarization state is +45 degrees; when the phase difference is pi/2, the synthesized polarization state is right-handed circular polarization; when the phase difference is pi, the synthetic polarization state is-45 degrees; when the phase difference is 3 pi/2, the resultant polarization state is left-handed circular polarization.
A quantum key distribution system comprises the polarization modulator.
Compared with the prior art, the beneficial effect of this disclosure is:
the integral structure of the polarization modulator is monolithically integrated on a silicon substrate, so that the integration level is improved, the size is reduced, and the investment cost is also reduced.
One path of the first-stage interference structure is additionally provided with an offset phase shifter; by driving the offset phase shifter with a direct current voltage; the refractive index of the waveguide can be changed, so that the effect of modulating the phase difference of the two-arm optical signals is achieved; the light intensity distribution of the second-order interference structure can be strictly and stably controlled; and when preparing each polarization state, the stability of direct current drive is far higher than dynamic drive.
According to the modulator, the BIAS electrode is added on the basis of the electrode structure of the traditional modulator and is used for inputting a direct current BIAS signal, the modulator is driven in a single-ended push-pull mode, two phase shifters can be driven simultaneously by one path of pulse drive, compared with other modulation methods, the amplitude of the drive voltage can be obviously reduced when the same modulation depth is achieved, and a drive circuit is not added when the drive voltage is reduced.
The single-ended push-pull mode driving modulator can obviously reduce the influence of chirp on the modulator, can improve the power stability of each prepared polarization state, reduces half-wave voltage and improves the bandwidth of the modulator.
The polarization modulator is used for a Quantum Key Distribution (QKD) system, the volume of the QKD optical system can be reduced, and the integration level of the QKD system is greatly improved; the power stability of optical signals is improved in a QKD system with high-speed modulation, and the long-term bit error rate and the safety of a commercial quantum key distribution system are facilitated.
Drawings
The accompanying drawings, which are included to provide a further understanding of the disclosure, illustrate embodiments of the disclosure and together with the description serve to explain the disclosure and are not to limit the disclosure.
FIG. 1 is a schematic diagram of a prior art configuration employing a two-stage Mach-Zehnder interferometer of the present disclosure;
FIG. 2 is a schematic diagram of a polarization modulator of the present embodiment;
fig. 3 is a cross-sectional structural view of the PN type phase shifter of the present embodiment;
FIG. 4 is a schematic diagram of a second-order interference structure according to the present embodiment;
FIG. 5 is an equivalent circuit diagram of the second-order interferometric structure modulation operation of the present embodiment.
Wherein: 1. 1 x 2 beam splitter, 2, offset phase shifter, 3, 2 x 2 beam splitter, 4, PN type phase shifter, 5, PN type phase shifter, 6, polarization synthesizer, 7, S electrode, 8, BIAS electrode, 9, GND electrode.
The specific implementation mode is as follows:
the present disclosure is further described with reference to the following drawings and examples.
It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the 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 example embodiments according to the present disclosure. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, and it should be understood that when the terms "comprises" and/or "comprising" are used in this specification, they specify the presence of stated features, steps, operations, devices, components, and/or combinations thereof, unless the context clearly indicates otherwise.
In the present disclosure, terms such as "upper", "lower", "left", "right", "front", "rear", "vertical", "horizontal", "side", "bottom", and the like indicate orientations or positional relationships based on those shown in the drawings, and are only relational terms determined for convenience in describing structural relationships of the parts or elements of the present disclosure, and do not refer to any parts 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 connected", "connected", and the like are to be understood in a broad sense, and mean either a fixed connection or an integrally connected or detachable connection; may be directly connected or indirectly connected through an intermediate. The specific meanings of the above terms in the present disclosure can be determined on a case-by-case basis by persons skilled in the relevant art or technicians, and are not to be construed as limitations of the present disclosure.
As described in the background art, in the first conventional scheme, the polarization modulation is implemented by using discrete devices in the conventional commercial quantum key distribution scheme, so that the device has a large volume, high power consumption and high cost, and is difficult to implement system integration and poor in system stability. In the second conventional scheme, as shown in fig. 1, it is difficult for the driver to achieve a perfect phase difference under the high frequency modulation of the key distribution system. The first-stage modulation phase difference has slight deviation, so that the preparation of the polarization state of the second-stage interference structure is obviously influenced, at least two independent dynamic driving circuits are needed, and the cost is higher.
In order to solve the above problem, the present disclosure proposes a high-stability polarization modulator integrated on a silicon substrate. Discrete devices such as a phase shifter, a beam splitter and the like in the traditional QKD system are integrated on a silicon substrate, so that the system integration level is greatly improved, and the cost of an optical system is reduced.
The disclosure also provides an electrode design structure and a driving method of the modulator, which can solve the problem of unstable polarization state preparation under dynamic high-speed modulation, and simultaneously reduce the number of dynamic driving circuits and driving voltage.
In order to make the technical solution for realizing the above object more clear to those skilled in the art, the following detailed description is made by way of example.
Example one
A high-stability polarization modulator integrated on a silicon substrate comprises a 1 x 2 beam splitter 1, an offset phase shifter 2, a 2 x 2 beam splitter 3, a PN type phase shifter 4, a PN type phase shifter 5 and a polarization synthesis device 6.
As shown in fig. 2, the 1 × 2 beam splitter 1, the offset phase shifter 2 and the 2 × 2 beam splitter 3 together form a first-order interference structure of the polarization modulator. The 1 × 2 splitter splits the signal light into two beams in equal proportion, and the two outlets are respectively connected with the two input ends of the 2 × 2 splitter 3 through optical waveguides.
In some embodiments, the splitting ratio of the 1 × 2 beam splitter and the 2 × 2 beam splitter is 1: 1.
It should be noted that, since the 2 × 2 beam splitter 3 is a common part of the first-stage interference structure and the second-stage interference structure, in the present embodiment, the first-stage interference structure and the second-stage interference structure both include the 2 × 2 beam splitter 3, but in the description of other embodiments, the 2 × 2 beam splitter 3 may be separately divided into a part of the first-stage interference structure or the second-stage interference structure, which are merely different in expression manner and do not represent a difference in technical solutions or a difference in protection ranges.
The TE mode signal light is split into two beams after passing through the 1 × 2 beam splitter 1, and the two beams are respectively input to the upper arm and the lower arm of the first-stage interference structure. One of the paths additionally passes through an offset phase shifter 2.
The bias phase shifter 2 is driven by direct current voltage, the refractive index of the waveguide can be changed, and the effect of modulating the phase difference of the two-arm optical signals is achieved. The two-arm optical signals with phase difference are converged and form interference at the 2 x 2 beam splitter.
As shown in fig. 3, the cross-sectional structure of the PN-type phase shifter includes P +, P, N, N + arranged in sequence, wherein P, N portion is a ridge waveguide portion.
Wherein, P + and N + are P type heavy doping and N type heavy doping respectively, and P, N is P type doping and N type doping respectively.
In this embodiment, the first-stage offset phase shifter is driven by a constant dc signal, and the light intensity distribution of the second-stage interference structure can be strictly and stably controlled. Compared with the scheme of preparing H, V, P, N, the embodiment reduces the requirement of one dynamic driving circuit. And when preparing each polarization state, the stability of direct current drive is far higher than dynamic drive.
In some embodiments, the offset phase shifter in the first-stage interference structure may be a thermally tuned phase shifter or a silicon-based phase shifter based on carrier dispersion.
Of course, this does not mean that the offset phase shifter can be used with only the two types described above, and in other embodiments, other phase shifters can be substituted.
As shown in fig. 4, the second-order interference structure specifically includes: a 2 × 2 beam splitter 3, a PN type phase shifter 4, a PN type phase shifter 5, a polarization synthesizer 6, an S electrode 7, a BIAS electrode 8, and a GND electrode 9.
After interfering with each other, the signal light is output from the two light outlets respectively by the 2 × 2 beam splitter 3, and is connected to the PN phase shifter 4 and the PN phase shifter 5 through optical waveguides respectively. The two PN type phase shifters are driven by dynamic pulse voltage and modulate the phase difference of the upper and lower arms signal light. The signals are then combined and combined into the polarization state by the polarization combining device 6. The polarization synthesis device 6 receives two paths of input optical signals, rotates the polarization state of one path of input signal by 90 degrees, keeps the other path unchanged, and then performs polarization synthesis on the two paths of input optical signals and outputs the signals.
Pulse driving voltage signal VSInput into the modulator through S electrode, BIAS electrode is used to make the modulator work at target BIAS point VBIAS. The GND electrode 9 is used for receiving a GND signal.
The phase shift amount of the PN type phase shifter 4 and the PN type phase shifter 5 is controlled by a dynamic driving circuit, and the two-arm phase shifter is driven by one driving circuit in a single-ended push-pull mode.
As shown in fig. 5, in an ideal equivalent circuit diagram, the diodes i and ii are equivalent PN or PIN diodes formed by phase shifter doping regions on the upper arm and the lower arm of the second stage of the MZ interferometric structure, and the resistor iii is used for impedance matching of the input signal terminal.
In this embodiment, a BIAS electrode is added to the electrode structure of the conventional modulator for inputting a dc BIAS signal.
The signal electrode (S electrode) is arranged on one side of the two waveguides, and the driving electric signal is applied to the PN junction or the PIN junction through the signal electrode.
Pulse driving voltage signal VSMay be positive or negative. Can be formed with VBIASFor biasing, the drive voltage is at-VSV and 2SAnd/2, a push-pull drive alternating between the two.
Dynamic driving voltage is input into the modulator through an S electrode, four kinds of pulse voltage are respectively sent down, and the two-arm phase shifter is driven in a single-ended push-pull mode, so that the phase shifter modulates four kinds of phase differences (0, pi/2, pi and 3 pi/2) to two paths of signal light. Then, the needed polarization states are merged and synthesized on a polarization beam combiner:
when the phase difference of the second-stage interference two-arm signals is 0, the synthetic polarization state is +45 degrees; when the phase difference is pi/2, the synthesized polarization state is right-handed circular polarization; when the phase difference is pi, the synthetic polarization state is-45 degrees; when the phase difference is 3 pi/2, the resultant polarization state is left-handed circular polarization.
The phase shifters are driven in a single-ended push-pull mode, and two phase shifters can be driven simultaneously by one path of pulse driving.
The single-ended push-pull mode driving modulator can remarkably reduce the influence of chirp on the modulator. The two-phase shifter series structure is adopted, the capacitance value is reduced, the transmission loss of the driving signal on the traveling wave electrode can be reduced, and the modulation bandwidth is improved.
In some embodiments, the PN phase shifter is a silicon-based phase shifter based on carrier dispersion effect, and the optional doping structure is PIN type or PN type.
In the first embodiment, the bias phase shifter in the first-stage interference structure is driven by the direct-current voltage, the polarization state preparation is more stable, the requirement of a path of dynamic driving circuit is reduced, the modulator of the second-stage interference structure is driven by the pulse driving voltage in a single-ended push-pull mode, the two-arm phase shifter can be driven by a path of driving signal at the same time, the half-wave voltage is reduced, the chirp phenomenon of the modulator is reduced, and the bandwidth of the modulator is improved. Can be prepared into four polarization states, namely a polarization state of +45 degrees, a right-handed circular polarization state, a polarization state of-45 degrees and a left-handed circular polarization state, and has wide application range.
Example two
A Quantum Key Distribution (QKD) system including a silicon-based integrated polarization modulator provided by embodiment one.
The quantum key distribution system provided by the embodiment integrates the polarization modulator on the silicon substrate, so that the volume of the QKD optical system is reduced, and the integration level of the QKD system is greatly improved.
While reducing the cost of the optical system.
The bias phase shifter in the first-stage interference structure is driven by the direct-current voltage, the light intensity ratio of two-arm signals in the second-stage interference structure can be strictly controlled, and the stability of the polarization angle of each prepared polarization state is improved. And the requirement of one path of dynamic driving circuit is reduced, and the cost of the peripheral driving circuit is reduced.
Four polarization states can be prepared by adopting a single-ended push-pull mode to drive the second-stage interference structure, only one driving circuit is needed, and the driving voltage is reduced without adding a driving circuit. And the influence of the chirp phenomenon on the modulator is reduced, and the power stability of the optical signal is further improved in the QKD system with high-speed modulation.
The above description is only a preferred embodiment of the present disclosure and is not intended to limit the present disclosure, and various modifications and changes may be made to the present disclosure by those skilled in the art. Any modification, equivalent replacement, improvement and the like made within the spirit and principle of the present disclosure should be included in the protection scope of the present disclosure.
Although the present disclosure has been described with reference to specific embodiments, it should be understood that the scope of the present disclosure is not limited thereto, and those skilled in the art will appreciate that various modifications and changes can be made without departing from the spirit and scope of the present disclosure.
Claims (10)
1. A polarization modulator integrated on a silicon substrate, comprising a first level interference structure and a second level interference structure in cascade, characterized in that: an offset phase shifter is arranged on one arm of the first-stage interference structure, and two arms of the second-stage interference structure are respectively provided with a phase shifter;
the offset phase shifter is configured to be driven by a direct current voltage, and the phase shifter is configured to be driven by a pulse voltage in a single-ended push-pull manner.
2. A polarization modulator according to claim 1, wherein: the first-stage interference structure comprises a 1 x 2 beam splitter, a bias phase shifter and a 2 x 2 beam splitter, wherein the 1 x 2 beam splitter is used for dividing signal light into two beams in equal proportion, the output end of the 1 x 2 beam splitter is respectively connected with different input ends of the 2 x 2 beam splitter through waveguides, and the bias phase shifter is additionally arranged on one path.
3. A polarization modulator according to claim 1, wherein: the bias phase shifter is a thermally tuned phase shifter or a silicon-based phase shifter based on a carrier dispersion effect.
4. A polarization modulator according to claim 2, wherein: the splitting ratio of the 1X 2 beam splitter to the 2X 2 beam splitter is 1: 1.
5. A polarization modulator according to claim 1 or 2, wherein: the second-stage interference structure comprises two phase shifters and a polarization synthesizer, two output ends of the 2 x 2 beam splitter are respectively connected with one phase shifter, and output ends of the two phase shifters are respectively connected with two input ends of the polarization synthesizer.
6. A polarization modulator according to any one of claims 1 to 5, wherein: a BIAS electrode is additionally arranged in an electrode structure of the modulator and used for inputting a direct current BIAS signal so that the modulator works at a target BIAS point.
7. A polarization modulator according to any one of claims 1 to 5, wherein: the phase shifter is a PN type phase shifter;
or, the PN type phase shifter is a silicon-based phase shifter based on a carrier dispersion effect, and the doping structure is PIN type or PN type.
8. The method of driving a polarization modulator according to any one of claims 1 to 7, wherein: and the offset phase shifter in the first-stage interference structure is driven by direct-current voltage, and the phase shifter of the second-stage interference structure is driven by pulse driving voltage in a single-ended push-pull mode.
9. The driving method according to claim 8, wherein: the dynamic pulse voltage drives the two-arm phase shifter in a single-end push-pull mode, so that the phase shifter modulates four phase differences to two paths of signal light, and the four phase differences are converged and combined into a required polarization state on the polarization beam combiner:
when the phase difference of the second-stage interference two-arm signals is 0, the synthetic polarization state is +45 degrees; when the phase difference is pi/2, the synthesized polarization state is right-handed circular polarization; when the phase difference is pi, the synthetic polarization state is-45 degrees; when the phase difference is 3 pi/2, the resultant polarization state is left-handed circular polarization.
10. A quantum key distribution system is characterized in that: comprising a polarization modulator according to any one of claims 1 to 7.
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| CN1550825A (en) * | 2003-01-22 | 2004-12-01 | ��Ѹ�Ƽ���˾ | Direct optical n-state phase shift keying |
| US6798557B1 (en) * | 2003-05-22 | 2004-09-28 | Lucent Technologies Inc. | Direct optical N-state phase shift keying |
| US20110170161A1 (en) * | 2010-01-12 | 2011-07-14 | Alcatel-Lucent Usa Inc. | Nested mach-zehnder modulator |
| CN104301041A (en) * | 2014-10-13 | 2015-01-21 | 北京大学 | Silicon substrate integrated coherent light transmitter chip and transmitter |
| CN104330905A (en) * | 2014-11-12 | 2015-02-04 | 天津工业大学 | Two-dimensional grating coupling-based silicon-based QPSK (Quadrature Phase Shift Keying) photomodulator |
| CN109962770A (en) * | 2017-12-14 | 2019-07-02 | 科大国盾量子技术股份有限公司 | Silicon-based monolithic integrates quantum key distribution sender chip structure and its encapsulating structure |
| CN207977967U (en) * | 2017-12-29 | 2018-10-16 | 浙江九州量子信息技术股份有限公司 | A kind of polarization encoder QKD system based on silicon substrate integrated chip |
| CN109039463A (en) * | 2018-08-15 | 2018-12-18 | 南京航空航天大学 | Microwave photon broadband phase-moving method and microwave photon broadband phase shift chip |
| CN110601752A (en) * | 2019-08-16 | 2019-12-20 | 武汉光迅科技股份有限公司 | Chirp measuring device and method, computer readable storage medium |
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| CN114465668A (en) * | 2022-01-28 | 2022-05-10 | 中国科学技术大学 | Phase modulation unit applied to quantum key distribution system on chip |
| CN114465668B (en) * | 2022-01-28 | 2023-10-20 | 中国科学技术大学 | Phase modulation unit applied to quantum key distribution system on chip |
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