Research Of Up-conversion Single-photon Detectors Based On Periodically Poled Lithium Niobate Waveguides

Over the past decade,optical quantum information applications,including quantum communication,quantum computation,and quantum precise measurement,have attracted intensive attention from scientists.Quantum key distribution(QKD)is the closest practical branch in the field of quantum information and has been considered as the most secure form of communication yet devised.Single-photon detectors(SPDs)operating in the 1.55-?m telecom band are a core part of quantum communication over optical fiber,and the SPD performance directly limits the transmission distance and key rate of fiber-based QKD.Whereas,the performance of up-conversion SPDs satisfies the need of long-distance QKD,where the detection spectral range of well-developed silicon avalanche photodiodes(Si-APDs)is extended into the 1.55-?m telecom band utilizing sum-frequency generation(SFG)in periodically poled lithium niobate(PPLN)waveguides.However,practical applications of PPLN devices based on congruent lithium niobate(CLN)involving visible or ultraviolet wavelengths are limited to low-power and high-temperature operation due to the effects of photorefractive damage and green-induced infrared absorption.In contrast,5 mol.%Mg:LN is of superior properties and has been identified as the most promising material for the fabrication of PPLN devices.Whereas,unexpected leakage current complicates the high-quality periodic poling of large-size wafers of 5 mol.%Mg:LN,which makes it difficult to fabricate PPLN waveguide chips of sufficient length,resulting in low detection efficiency and high dark count rate of the corresponding up-conversion SPDs.Based on the widespread applications of PPLN waveguides in the field of quantum secure communication,this paper carried out the following work:1.Polarization reversal characteristics of CLN and 5 mol.%Mg:LN during electric-field periodic poling were investigated,and the weak negative feedback mechanism and the easy formation of leakage current were confirmed to be the two key factors affecting the poling quality of 5 mol.%Mg:LN.Aiming at the existing problems in the periodic poling process of 5 mol.%Mg:LN,the spatial configuration of the patterned photoresist and the poling voltage waveform were improved to suppress the undesired side-wide extension of inverted domains.Based on the investigation of formation mechanism of the leakage current,the approach of using SiO2 dielectric layer to suppress the leakage current was proposed,and the influence of SiO2 layer thickness on the periodic poling of 5 mol.%Mg:LN was investigated.By selecting optimum thickness of the SiO2 layer and combining with the multipulse application method,high-quality periodic poling of 76.2 mm-diameter wafers of 5 mol.%Mg:LN was realized by the electric-field poling method using liquid electrodes.2.The high poling quality of PPMgLN was demonstrated by the characterizations of domain-inverted structures and second harmonic generation(SHG)performance.Well-defined periodic domain-inverted structures with a period of 20 ?m and a duty cycle of?50%were fabricated on the +z surface of 5 mol.%Mg:LN wafers with a diameter of 76.2 mm.The domain inversion had almost penetrated the-z surface and exhibited a duty cycle of45? 5%from the+z surface to a depth of?300 ?m,which is optimal for the first-order QPM.Besides,the SHG conversion efficiency of a single pass through the 25 mm bulk PPMgLN is?2.35%/W,corresponding to an effective nonlinear optical coefficient of 14.2 pm/V,slightly lower than the theoretical value of 17.2 pm/V,and the discrepancy between the measured and theoretical conversion efficiency is mainly due to the Fresnel loss.3.Differences in hydrogen ion diffusion characteristics of CLN and 5 mol.%Mg:LN were investigated,and the main factors affecting the fabrication process and properties of reverse-proton-exchanged(RPE)PPMgLN waveguides were analyzed.By studying the effects of mode filter widths,waveguide widths,and hydrogen ion diffusion depths on waveguide transmittances and beam modes of 1310-nm signal and 1950-nm pump,the reasonable parameters of waveguide design and fabrication were determined.Based on the obtained 76.2 mm-diameter wafers of PPMgLN,PPMgLN waveguides with the length of 52 mm were fabricated via the RPE technique.The waveguide transmittances,QPM tuning curve,and SFG(Sum-frequency Generation)conversion efficiency of the RPE PPMgLN waveguide were measured respectively,indicating that the waveguide transmittances of the 13 10-nm signal and the 1950-nm pump are 24.3%and 27.1%,the phase-matching wavelength is 1301.61 nm at 25?,and the SFG conversion efficiency is?10.96%.By analyzing the reasons for the low SFG conversion efficiency of the RPE PPMgLN waveguide,an improved scheme was proposed for the subsequent waveguide performance optimization experiments.4.Based on an integrated dual-channel RPE PPLN waveguide and the polarization diversity configuration,we demonstrated a compact all-fiber polarization-independent up-conversion single-photon detector.The dual-channel PPLN waveguide is composed of two adjacent independent waveguides with the same design parameters.The QPM tuning curves and SFG conversion efficiencies of these two up-conversion waveguides were measured respectively,indicating that the phase-matching wavelengths of the two waveguides are 1546.58 nm and 1546.62 nm at 25 0C with the maximum signal photon conversion efficiencies of 73.8%and 74.3%respectively.By selecting the appropriate components and adjusting the input pump power to compensate for the differences of the maximum signal photon conversion efficiencies between the two waveguides,the detection efficiencies for H polarization and V polarization were adjusted to the same value.As a result,a polarization-independent up-conversion single-photon detector was realized.Based on this detector,the polarization-independent single-photon counting at 1550 nm was demonstrated,achieving a high detection efficiency of 29.3%,a low dark count rate of 1600 cps,and a polarization dependent loss of 0.1 dB.