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Sixth Symposium on Novel Optoelectronic Detection Technology and Applications

Physical-layer key generation and distribution using gain-switched laser diode

Yibo Liu, Huatao Zhu, Xin Yi Zhang, 田八林

Published May 11, 2026• 0 citations
Publisher

Abstract

The phase fluctuations that occurs during the pulse generation process in a gain-switched laser diode (GSLD) can be utilized to generate true random numbers. However, it is difficult to ensure consistency when using GSLD for key distribution. For the first time, a key distribution scheme that employs GSLD is proposed and demonstrated by a proof-of-concept experiment. By controlling the GSLD’s modulation current and bias current, optical pulses with random phase fluctuations can be generated. Using a Mach-Zehnder interferometer (MZI), the phase fluctuations of light pulses are converted into amplitude fluctuations. In order to improve the consistency of the key distribution system, the difference in the time delay between Alice’s and Bob’s MZIs is controlled to be 1 ps. A key distribution with a waveform consistency of 0.94 and a key consistency of 0.95 is achieved, providing a reference scheme for real-time secure communication.

DOI
10.1117/12.3108099

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The phase fluctuations that occurs during the pulse generation process in a gain-switched laser diode (GSLD) can be utilized to generate true random numbers. However, it is difficult to ensure consistency when using GSLD for key distribution. For the first time, a key distribution scheme that employs GSLD is proposed and demonstrated by a proof-of-concept experiment. By controlling the GSLD’s modulation current and bias current, optical pulses with random phase fluctuations can be generated. Using a Mach-Zehnder interferometer (MZI), the phase fluctuations of light pulses are converted into amplitude fluctuations. In order to improve the consistency of the key distribution system, the difference in the time delay between Alice’s and Bob’s MZIs is controlled to be 1 ps. A key distribution with a waveform consistency of 0.94 and a key consistency of 0.95 is achieved, providing a reference scheme for real-time secure communication.
This article proposes and experimentally demonstrates a covert key distribution scheme based on the phase fluctuation of a gain-switched laser diode (GSLD). The gain-switched pulse is used to generate the covert signal through spectral broadening and temporal spreading. The amplified spontaneous emission (ASE) light is used to generate ASE noise in the public channel. Eavesdroppers cannot perceive signals hidden in the public noise. The experimental results show that the correlation coefficient of the key waveforms reaches 0.95, and the scheme’s key generation rate (KGR) reaches 1.88 Gbit/s through a 25 km single-mode fiber (SMF). Moreover, the binary bitstreams have passed the NIST statistical test suite. The covert key distribution scheme proposed in this article provides an efficient way for the method of “one-time-pad” key distribution in high-speed optical communication systems.
We demonstrate robust, high-speed random number generation using interference of the steady-state emission of guaranteed random phases, obtained through gain-switching a semiconductor laser diode. Steady-state emission tolerates large temporal pulse misalignments and therefore significantly improves the interference quality. Using an 8-bit digitizer followed by a finite-impulse-response unbiasing algorithm, we achieve random number generation rates of 8 and 20 Gb/s, for laser repetition rates of 1 and 2.5 GHz, respectively, with a ±20% tolerance in the interferometer differential delay. We also report a generation rate of 80 Gb/s using partially phase-correlated short pulses. In relation to the field of quantum key distribution, our results confirm the gain-switched laser diode as a suitable light source, capable of providing phase-randomized coherent pulses at a clock rate of up to 2.5 GHz.
Abstract In the implementation of quantum key distribution, Security certification is a prerequisite for social deployment. Transmitters in decoy-BB84 systems typically employ gain-switched semiconductor lasers (GSSLs) to generate optical pulses for encoding quantum information. However, the working state of the laser may violate the assumption of pulse independence. Here, we explored the dependence of intensity fluctuation and high-order correlation distribution of optical pulses on driving currents at 2.5 GHz. We found the intensity correlation distribution had a significant dependence on the driving currents, which would affect the final key rate. By utilizing rate equations in our simulation, we confirmed the fluctuation and correlation originated from the instability of gain-switched laser driven at a GHz-repetitive frequency. Finally, we evaluated the impact of intensity fluctuation on the secure key rate. This work will provide valuable insights for assessing whether the transmitter is operating at optimal state in practice.
We demonstrate a high bit-rate quantum random number generator by interferometric detection of phase diffusion in a gain-switched DFB laser diode. Gain switching at few-GHz frequencies produces a train of bright pulses with nearly equal amplitudes and random phases. An unbalanced Mach-Zehnder interferometer is used to interfere subsequent pulses and thereby generate strong random-amplitude pulses, which are detected and digitized to produce a high-rate random bit string. Using established models of semiconductor laser field dynamics, we predict a regime of high visibility interference and nearly complete vacuum-fluctuation-induced phase diffusion between pulses. These are confirmed by measurement of pulse power statistics at the output of the interferometer. Using a 5.825 GHz excitation rate and 14-bit digitization, we observe 43 Gbps quantum randomness generation.
Gain-switched lasers are in demand in numerous quantum applications, particularly in systems of quantum key distribution and in various optical quantum random number generators. The reason for this popularity is natural phase randomization between gain-switched laser pulses. The idea of such randomization has become so familiar that most authors use it without regard to the features of the laser operation mode they use. However, at high repetition rates of laser pulses or when pulses are generated at a bias current close to the threshold, the phase randomization condition may be violated. This paper describes theoretical and experimental methods for estimating the degree of phase randomization in a gain-switched laser. We consider in detail different situations of laser pulse interference and show that the interference signal remains quantum in nature even in the presence of classical phase drift in the interferometer provided that the phase diffusion in a laser is efficient enough. Moreover, we formulate the relationship between the previously introduced quantum reduction factor and the leftover hash lemma. Using this relationship, we develop a method to estimate the quantum noise contribution to the interference signal in the presence of phase correlations. Finally, we introduce a simple experimental method based on the analysis of statistical interference fringes, providing more detailed information about the probabilistic properties of laser pulse interference.
Gain-switching of laser diodes is a well-established technique for generating optical pulses with random phases, where the quantum randomness arises naturally from spontaneous emission. However, the maximum switching rate is limited by the time required for phase diffusion: at high repetition rates, residual photons in the cavity seed subsequent pulses, leading to phase correlations, which degrade randomness. We present a method to overcome this limitation by employing an external source of spontaneous emission in conjunction with the laser. Our results show that this approach substantially suppresses interpulse phase correlations, restoring phase randomization at repetition rates as high as 10 GHz. This technique opens new opportunities for high-rate quantum key distribution and quantum random number generation.