| Superconducting nanowire single photon detector(SNSPD),as a new type of photodetector,is now widely used in laser ranging,quantum computing,biological fluorescence imaging,deep space communication,space detection and other frontier fields.For single-pixel SNSPD,it operates in a strong non-linear mode,if multiple photons are absorbed at the same time,the detector will only produce one output signal.However,many applications,e.g.,quantum communication,biofluorescence imaging and non-classical light source characterization,the required detector not only needs to respond to incident photons,but also needs to resolve the number of incident photons and the spatial information.Therefore,a device that uses multiple SNSPDs to form an array to realize photon number resolving and photon space position resolving,has a wide application prospect.Series-structured,parallel-structured and multi-element area array SNSPD have the function of photon number resolving.A series-structured SNSPD is composed of multiple nanowires connected in series,and each nanowire is connected in parallel with a resistor,all pixels share the same bias current and share the same readout,which reduces the number of coaxial lines and equivalently increased the load capacity of the refrigerator.However,series-structured SNSPD requires an additional high input impedance low temperature amplifier circuit to improve the output signal-to-noise ratio.As for multi-element area array SNSPD,since each pixel uses a separate bias current,it not only has the capability of spatial resolving,but also effectively reduces the mutual interference between each pixel.After the electrical pulses output by each pixel are combined into one output by power,the number of photons can also be resolved according to the amplitude of the output pulse.As for the readout requirement of series-structure SNSPD,we tested and analyzed the characteristic parameters of the high input impedance amplify integrated circuit in the temperature range of liquid helium.By adjusting the operating point,a stable gain of 20 d B is obtained at 2 K,and the bandwidth range is 100 k Hz ? 1.5 GHz,but power consumption is only 1 m W.Moreover,we designed the interconnection scheme of the cryogenic amplify integrated circuit and 6-pixel series-structure SNSPD.After solving the problem of SNSPD superconducting critical current compression,caused by noise and uneven heat dissipation,the signal-to-noise ratio of the output signal was significantly improved.Finally,we successfully achieved the 6 photons resolving.In addition,as for the application of area array structure SNSPD in the field of deep space communication,according to its photon number resolving function,we proposed a photon enhanced counting mode.Based on this working mode,the communication simulation experiment was carried out.Compared with the traditional optical communication,due to the long transmission distance,strong light attenuation,and the influence of various factors such as atmospheric turbulence,the optical signal reaching the detector is usually a few photons or even one single photon in the deep space optical communication.In this case,the photon enhanced counting mode can effectively filter out the influence of background noise and extract signal photon information.In the experiment,considering the uncertainty of the communication system,we introduced Bayesian estimation to select the threshold in the photon enhanced counting mode.Compared with SNSPD receiving system which do not have photon enhanced counting mode,the bit error rate(BER)limit of the communication system could be reduced by at least two orders of magnitude,which shows that SNSPD array combined with the photon enhanced counting mode communication receiving system has a huge application potential in the deep space optical communication. |
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