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Journal of Physics Conference Series

High extinction ratio integrated optical modulator for quantum telecommunication systems

Aleksandr V. Tronev, Mikhail V. Parfenov, Peter M. Agruzov, Igor V. Il'ichev, Alexander V. Shamray

Published Jan 1, 2018• 9 citations
Publisher

Abstract

A method for increasing the extinction ratio of integrated optical Mach-Zehnder modulators based on LiNbO 3 via the photorefractive effect is proposed. The influence of the photorefractive effect on the X- and Y-splitters of intensity modulators is experimentally studied. An increase in the modulator extinction ratio by 17 dB (from 30 to 47 dB) is obtained. It is shown that fabricated modulators with a high extinction ratio are important for quantum key distribution systems.

DOI
10.1088/1742-6596/951/1/012002

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A method for increasing the extinction ratio of integrated optical Mach-Zehnder modulators based on LiNbO 3 via the photorefractive effect is proposed. The influence of the photorefractive effect on the X- and Y-splitters of intensity modulators is experimentally studied. An increase in the modulator extinction ratio by 17 dB (from 30 to 47 dB) is obtained. It is shown that fabricated modulators with a high extinction ratio are important for quantum key distribution systems.
Imperfections in integrated photonics manufacturing have a detrimental effect on the maximal achievable visibility in interferometric architectures. These limits have profound implications for further technological developments in photonics and in particular for quantum photonic technologies. Active optimization approaches, together with reconfigurable photonics, have been proposed as a solution to overcome this. In this Letter, we demonstrate an ultrahigh (>60 dB) extinction ratio in a silicon photonic device consisting of cascaded Mach-Zehnder interferometers, in which additional interferometers function as variable beamsplitters. The imperfections of fabricated beamsplitters are compensated using an automated progressive optimization algorithm with no requirement for pre-calibration. This work shows the possibility of integrating and accurately controlling linear-optical components for large-scale quantum information processing and other applications.
Modulators based on the Mach-Zehnder interferometer (MZI) structure are widely used in continuous-variable quantum key distribution (CVQKD) systems. MZI-based variable optical attenuator (VOA) and amplitude modulator can reshape the waveform and control the intensity of the coherent state signal to realize secret key information modulation in the CVQKD system. However, these devices are not ideal; internal and external effects like non-linear effects and temperature may degrade their performance. In this paper, we analyzed the security loophole of CVQKD under the photorefractive effect (PE), which originates from the crystal characteristics of lithium niobate (LN). It is found that the refractive index change of modulators because of PE may lead to an overestimate or underestimate of the final secret key rate. This allows Eve to perform further attacks like intercept-resend to get more secret key information. To solve this problem, several countermeasures are proposed, which can effectively eliminate potential risks.
We propose and demonstrate an approach to generate stable and high extinction ratio light pulses with an extinction ratio 80 dB. To this end, a high stable bias locking technique is proposed, and a small size, easily integrated, cost-effective pulse generator based on delay line chips is designed and constructed. The pulse generator can generate voltage >12.5 V at a load impedance of 50 Ω, which is enough to drive the lithium niobate-based Mach-Zehnder (LNMZ) intensity modulator with a half wave voltage ~6-8 V. The pulse width and delay time can be programmed directly via digital I/O ports. The bias locking technique utilizes a successive scanning and fitting method which overcomes the sensitivity limit of the photodetector and can achieve the bias voltage accurately and quickly. By this method, the bias points of two or more cascaded LNMZ intensity modulators can be locked stably. The presented system can be easily integrated into the continuous variable quantum key distribution system.