CN108398192B - System for accurately measuring coherence time of chaotic light field by utilizing high-speed time-to-time conversion - Google Patents
System for accurately measuring coherence time of chaotic light field by utilizing high-speed time-to-time conversion Download PDFInfo
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Abstract
The invention belongs to the field of optical measurement, provides a system for accurately measuring the coherence time of a chaotic light field by utilizing high-speed time-to-digital conversion, and solves the problems that the measurement accuracy of the second-order coherence time of the light field is low, the measurement process is easily influenced by noise and the like in the prior art. The system comprises a coupler, an HBT system, a first constant ratio phase detector, a first time-to-digital converter, a first photomultiplier, a second constant ratio phase detector, a second time-to-digital converter, a memory and a data processor, wherein the HBT system comprises a filter and a spectroscope. The chaotic light field coherent time value obtained by the invention is stable, is less influenced by noise, and has simple mathematical process; the method can be widely applied to the field of measuring the second-order coherent time of chaotic light fields and thermal light fields and improving experimental precision of the coherent time of the obtained light fields, such as quantum optics.
Description
Technical Field
The invention belongs to the field of optical measurement, and particularly relates to a system for accurately measuring the coherent time of a chaotic light field by utilizing high-speed time-to-digital conversion.
Background
The chaotic laser field meets basic requirements of information safety and cryptography due to the characteristics of wide frequency spectrum, noise-like, long-term unpredictability, high complexity and the like, has attracted extensive attention and intensive research of people under the trend of the popularization of online trading of electronic commerce and financial securities at present, and is applied to various fields of high-speed remote chaotic secret communication, rapid physical random number generation, chaotic key distribution based on public channels and the like. In 1994, Colet et al first discussed the possibility of achieving chaotic synchronization of two solid state lasers (see document p]Opt, L ett, 1994, 19(24): 2056-2058), from now on, chaotic optical communication taking optical signals as chaotic carriers began to be developed, then, over a decade, the generation and application aspects of relevant chaotic optical fields are rapidly developed, and in 2005, eight research organizations in seven countries in Europe finish 120km transmission distance, 1Gb/s rate and 10 bit error rate based on semiconductor lasers in commercial optical communication networks-7Chaotic secure communication. In 2007, "Taiwan QinghuaUniversity "Baker in forest et al studied the application of chaotic optical communication in Radio Over Fiber communication technology" (see document F.Y. L in, M.C.Tsai. electronic communication in Radio-Over-Fiber communication devices [ J.]Opt.express, 2007, 15(2): 302-311) is promoted by the high-speed development of optical communication, researches on chaotic laser secure communication are active all the time, but researches on related chaotic light fields are mainly focused on macroscopic dynamics characteristics such as time domains and frequency domains, and a method for judging and analyzing the chaotic light fields mainly comprises the steps of determining information in multiple aspects such as a bifurcation point, a universal fernbaum constant (universal ferben constant) and periodicity by observing time domain intensity fluctuation, spectrum width and a Lyapunov index (L yapunnovex) and a bifurcation diagram of an analysis system, finally determining a chaotic state and distinguishing the chaotic state from noise, but cannot obtain more information in aspects such as high-order coherence of the light fields and photon statistical distribution.
The coherent time of the light field is related to the second-order coherence of the light field, and the theoretical formula for measuring the second-order coherence is
Wherein n is1(t)，n2(t + τ) is the number of photons recorded by the two detectors at t, t + τ, respectively, and the delay time is τ. If an actual second-order coherence function curve is to be obtained, it is very troublesome to manually draw points by τ.
Disclosure of Invention
The invention provides a system for accurately measuring the coherence time of a chaotic light field by utilizing high-speed time-to-digital conversion, aiming at solving the problem that the difference between the measured values of the second-order coherence and the coherence time of the chaotic light field and a theoretical value is large in the prior art, and the coherence time of the chaotic light field can be accurately measured by utilizing the high-speed time-to-digital conversion and the data processing of convolution.
In order to solve the technical problems, the invention adopts the technical scheme that: a system for accurately measuring the coherence time of a chaotic light field by utilizing high-speed time number conversion comprises a coupler, an HBT system, a first constant ratio phase discriminator, a first time number converter, a first photomultiplier, a second constant ratio phase discriminator, a second time number converter, a memory and a data processor, wherein the HBT system comprises a filter and a spectroscope, light output by the chaotic light field enters the HBT system through the coupler, is filtered by the filter and then is divided into two beams of light output by the spectroscope to be respectively received by the first photomultiplier and the second photomultiplier, the signal output end of the first photomultiplier is connected with the input end of the first constant ratio phase discriminator, the output end of the first constant ratio phase discriminator is connected with the input end of the first time number converter, the output end of the first time number converter is connected with the memory, the signal output end of the second photomultiplier is connected with the input end of the second constant ratio phase discriminator, the output of second constant ratio phase discriminator with the input of second time-to-digital converter is connected, the output of second time-to-digital converter with the memory is connected, first time-to-digital converter and second time-to-digital converter are used for the record respectively first constant ratio phase discriminator and the time of second constant ratio phase discriminator input pulse to it sends the digital signal of record time to give the memory, the memory is used for storing the digital signal time of the sending of first time-to-digital converter and second converter, and send for data processor, data processor is used for the basis the output signal of memory calculates the coherent time that obtains the chaotic light field.
The first photomultiplier and the second photomultiplier are two-channel single-photon detectors of Aurea Technology L YNXEA.
The data processor calculates the chaos light field coherent time by the following steps:
normalizing the collected data to obtain data D1(τ)；
The normalized data D1(tau) performing convolution operation to obtain a convolution ninth-order value D9(τ) convolution formula of
Wherein τ represents time, n is 2, 3, 4, … … 9;
removing the ninth order value D9(τ) falling part of the data, then for the ninth order value D9(tau) carrying out normalization processing to obtain the ninth-order second-order coherence of the light field
According to the formula g(2)(τ)＝1+b*exp(2τ/τc) The ninth order second order coherence of the light field
Fitting is carried out by substituting to obtain the coherent time of the light field.
Compared with the prior art, the invention has the following beneficial effects:
(1) compared with the traditional method, the method can measure the coherence time of the chaotic light field generated by the semiconductor laser, does not need to manually change the delay time on the single-photon detector, but completely records the delay time which is transited from 0 to a set gate width value in one measurement and carries out numerical value statistics (the time interval between two photons recorded by the time-to-digital converter is equivalent to the delay time when the next photon lags the previous photon), effectively avoids the problem that all information of second-order coherence change cannot be effectively obtained due to the inaccuracy of artificially set delay time, and provides convenience for obtaining the accurate coherence time of the chaotic light field;
(2) obtaining photon time interval distribution by carrying out self-convolution on the acquired data, and obtaining the photon time interval distribution through 9-order second-order coherence g(2) 9(tau) carrying out coherence time fitting, and improving the measurement precision of coherence time;
(3) when the resolution time of the detector is higher, the distribution of the measured photon pairs is closer to the time interval distribution of the theoretical photon pairs, and the experimental value which is close to the theoretical value can be theoretically obtained by infinite convolution through a mathematical processing method by utilizing the distribution which is close to the theoretical value. The method provides an idea for obtaining the second-order coherence with higher measurement precision, namely the resolution time of the detector is improved by a technical innovation means, so that the time interval distribution of photon pairs with higher accuracy can be obtained, the more accurate the obtained second-order coherence of the light field is, the more accurate the coherence time of the light field obtained by fitting is. The method provided by the patent can be used for obtaining the second-order coherence with higher precision of different light fields such as thermal light, chaotic light and bunching type light, so that more accurate light field coherence time can be obtained, and the microscopic property of the chaotic light on the quantum level can be known.
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FIG. 1 is a schematic diagram of a system for accurately measuring the coherence time of a chaotic light field by using high-speed time-to-digital conversion according to the present invention;
FIG. 2 is a timing diagram of the best chaotic state obtained;
FIG. 3 is a graph of the resulting spectrum of the best chaotic state;
FIG. 4 is raw data measured by the apparatus of FIG. 1;
FIG. 5 is a diagram of second-order coherence of each order value obtained by data processing, in which the first-order second-order coherence is arranged from bottom to top
Second order degree of coherence to ninth order
FIG. 6 is a pair
Performing fitting results;
in fig. 1: 1-a distributed feedback semiconductor laser; 2-temperature control voltage source; 3-a temperature controlled current source; 4-a polarization controller; 5-a circulator; 6-80:20 fiber coupler; 7-a first variable optical attenuator; an 8-50:50 non-polarizing beam splitter; 9-a photodetector; 10-bandwidth oscilloscopes or spectrographs; 11-a second variable optical attenuator; 12-a spectroscope; 13-an HBT system; 14-a filter; 15-a first photomultiplier tube; 16-a second photomultiplier tube; 17-a first constant ratio phase detector; 18-a second constant ratio phase detector; 19-a first time-to-digital converter; 20-a second time to digital converter; 21-accumulation buffer.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below, and it is obvious that the described embodiments are some embodiments of the present invention, but not all embodiments; all other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
As shown in fig. 1, a schematic structural diagram of a system for accurately measuring a coherence time of a chaotic light field by using high-speed time conversion according to an embodiment of the present invention includes a coupler, an HBT system 13, a first photomultiplier 15, a first constant ratio phase detector 7, a first time-to-digital converter 19, a second photomultiplier 16, a second constant ratio phase detector 18, a second time-to-digital converter 20, a memory 21, and a data processor. The HBT system comprises a filter 14 and a beam splitter 12, the filter 14 being arranged before the light passes through the beam splitter 12 for filtering out stray light in space.
In this embodiment, the chaotic light source is mainly composed of a distributed feedback semiconductor laser 1 and an external cavity optical feedback system. The central wavelength of the distributed feedback semiconductor laser 1 is stabilized at 1554nm under the control of the temperature control voltage source 2 and the temperature control current source 3, and the threshold current is Ith which is 10.6 mA. Light emitted by the distributed feedback semiconductor laser 1 enters an 80:20 optical fiber coupler 6 after passing through a polarization controller 4 and a circulator 5, and 80% of the light enters the circulator 5 and is fed back to the laser 1 after passing through a first variable optical attenuator 7, so that an optical feedback loop with the external cavity delay time of 125ns is formed. A 20% end is coupled into a 50:50 non-polarizing beam splitter 8 to split the two beams of equal intensity: one of the beams is used as a monitoring signal and is sent into a photoelectric detector 9, and then the monitoring signal is accessed into a 36GHz bandwidth oscilloscope or a 26.5G frequency spectrograph 10 to ensure whether the light formed by a feedback loop is in a chaotic state or not so as to ensure that the measured light is a chaotic light field, the other beam enters a second variable optical attenuator 11, and the second variable optical attenuator 11 can be used for adjusting the intensity of the incident chaotic light field.
As shown in fig. 1, the light emitted from the second variable optical attenuator 11 is chaotic light whose intensity can be adjusted. The chaotic light enters the HBT system 13 through the coupler, and is filtered by the filter 14, and is received by the first photomultiplier 115 and the second photomultiplier 16 respectively after being divided into two light output by the spectroscope 12, the signal output end of the first photomultiplier 15 is connected with the input end of the first constant ratio phase detector 17, the output end of the first constant ratio phase detector 17 is connected with the input end of the first time converter 19, the output end of the first time converter 19 is connected with the memory 21, the signal output end of the second photomultiplier 16 is connected with the input end of the second constant ratio phase detector 18, the output end of the second constant ratio phase detector 18 is connected with the input end of the second time converter 20, the output end of the second time converter 20 is connected with the memory 21, the first time converter 19 and the second time converter 20 are respectively used for recording the pulse input time of the first constant ratio phase detector 17 and the second constant ratio phase detector 19, and sending the digital signal of the recording time to the memory 21, where the memory 21 is configured to store the digital signals sent by the first time-to-digital converter 19 and the second time-to-digital converter 20, and send the digital signals to the data processor, and the data processor is configured to calculate the coherence time of the chaotic light field according to an output signal of the memory.
Wherein the first photomultiplier and the second photomultiplier may be two-channel single photon detectors of the Aurea Technology L ynxea. ntr. m2 type.
When the output of the distributed feedback semiconductor laser 1 is adjusted, the laser is enabled to be in a state that the bias current is 15.9mA which is a threshold value of 1.5 times, when the temperature is controlled to be 9.5k omega A, the light output generated by the semiconductor laser is enabled to pass through the polarizer, the circulator and the attenuator, the output of the light passes through a part of the output of the polarizer, the circulator and the attenuator, the output of the light enters the feedback loop, the other part of the output of the light is used as the output end, the intensity of the feedback light is enabled to be changed from small to large through the adjustable knob in the previous path of attenuation. The best chaotic state can be determined by using a 36GHz bandwidth oscilloscope or a 26.5G spectrometer, as shown in fig. 2 and 3, which are a timing diagram and a frequency spectrum diagram of the best chaotic state. Then, the programmable optical attenuator VA2 is adjusted to enable the light intensity not to exceed the maximum light intensity received by the single-photon detector SPD, the quantum efficiency of the single-photon detector SPD is set to be 25%, the dead time is 4us, the delay time is 0ns, the gate width is 100ns, the trigger frequency is 312.5k, the coincidence measurement interface of the single-photon detector SPD is opened, statistics is started to be carried out, the distribution probability of the chaotic light field photon pairs is obtained, the memory 21 records and stores data after enough time, and how 4 the obtained data are recorded is shown.
The data processor calculates the chaotic light field coherent time by the following steps:
(1) normalizing the collected data (as shown in FIG. 4) to obtain data D1(τ), the normalization process is to divide each data by the sum of the statistical number of each delay time.
(2) The normalized data D1(tau) performing convolution operation to obtain a ninth convolution D9(τ), the convolution equation is:
where τ represents time, and n is 2, 3, 4, … … 9.
(3) Removing the ninth convolution D9(τ) descending part of the data, and for the ninth convolution D9(tau) carrying out normalization processing to obtain the ninth-order second-order coherence of the light field
The normalization process may be: calculating the average value A of the flat part in the data of the ninth order convolution value, and calculating the ninth order value D of the convolution9Dividing (tau) by the average value A to obtain the ninth-order second-order coherence g of the optical field(2) 9(τ)。
The theoretical formula of the second order coherence is:
wherein
Is the average light intensity of the light field detected by the detector, as shown in FIG. 5, for each order of convolution Dn(τ) (n ═ 2, …, 9) and D1(tau) normalizing to obtain second-order coherence of each order value
As can be seen from fig. 5, as the convolution order increases, the result of data processing performed on experimental data is closer to the theoretical value, and therefore, the data fitting of the coherence time is performed through the ninth-order second-order coherence degree, so that a more accurate coherence time can be obtained. Because the high-order part above the ninth order is omitted in the convolution process, and the error caused by the part becomes larger along with the larger delay time, the invention removes the descending part in the ninth order data in the calculation process, and takes the flat part in the ninth order convolution, namely the data in the interval of 40-60 ns of the uppermost curve in fig. 5 to calculate the average value A, thereby carrying out the normalization processing on the data.
(4) According to the formula g(2)(τ)＝1+b*exp(2τ/τc) The ninth order second order coherence of the light field
Fitting is carried out by substituting to obtain the coherent time of the light field.
As shown in FIG. 6, the method utilizes the ninth-order second-order coherence
Fitting to obtain a graph with a coherence time of 0.65ns and a correction decision coefficient of 5.14E-4。
Finally, it should be noted that: the above embodiments are only used to illustrate the technical solution of the present invention, and not to limit the same; while the invention has been described in detail and with reference to the foregoing embodiments, it will be understood by those skilled in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some or all of the technical features may be equivalently replaced; and the modifications or the substitutions do not make the essence of the corresponding technical solutions depart from the scope of the technical solutions of the embodiments of the present invention.
Claims (3)
1. A system for accurately measuring the coherence time of a chaotic light field by utilizing high-speed time-to-digital conversion is characterized by comprising a coupler, an HBT system, a first constant ratio phase discriminator, a first time-to-digital converter, a first photomultiplier, a second constant ratio phase discriminator, a second time-to-digital converter, a memory and a data processor, wherein the HBT system comprises a filter and a spectroscope, light output by the chaotic light field enters the HBT system through the coupler, is filtered by the filter and then is respectively received by the first photomultiplier and the second photomultiplier after being divided into two light outputs by the spectroscope, the signal output end of the first photomultiplier is connected with the input end of the first constant ratio phase discriminator, the output end of the first constant ratio phase discriminator is connected with the input end of the first time-to-digital converter, and the output end of the first time-to-digital converter is connected with the memory, the signal output part of the second photomultiplier is connected with the input end of the second constant ratio phase discriminator, the output end of the second constant ratio phase discriminator is connected with the input end of the second time-to-digital converter, the output end of the second time-to-digital converter is connected with the memory, the first time-to-digital converter and the second time-to-digital converter are respectively used for recording the time of the input pulse of the first constant ratio phase discriminator and the second constant ratio phase discriminator, and sending the digital signal of the recording time to the memory, the memory is used for storing the digital signal of the sending of the first time-to-digital converter and the second time-to-digital converter and sending to the data processor, and the data processor is used for calculating the coherent time of the chaotic light field according to the output signal of the memory.
2. The system for accurately measuring the coherence time of a chaotic light field by high-speed time-to-digital conversion according to claim 1, wherein the first photomultiplier tube and the second photomultiplier tube are two-channel single photon detectors of Aurea Technology L YNXEA. NTR. M2 type.
3. The system for accurately measuring the coherence time of the chaotic light field by using the high-speed time-to-digital conversion as claimed in claim 1, wherein the step of the data processor calculating the coherence time of the chaotic light field comprises:
normalizing the collected data to obtain data D1(τ)；
The normalized data D1(tau) performing convolution operation to obtain a convolution ninth-order value D9(τ) convolution formula of
Where τ denotes time, n is 2, 3, 4, … … 9, and t denotes an integral variable of convolution operation;
removing the ninth order value D9(τ) falling part of the data, then for the ninth order value D9(tau) carrying out normalization processing to obtain the ninth-order second-order coherence of the light field
According to the formula g(2)(τ)＝1+b*exp(2τ/τc) The ninth order second order coherence of the light field
Fitting by substituting to obtain the coherent time of the light field, wherein b represents a bunching factor, taucRepresenting the coherence time of the light field.
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