基于混沌激光的无后处理多位物理随机数高速产生技术研究

Chaotic laser-based ultrafast multi-bit physical random number generation without post-process

提出一种基于混沌激光的无后处理多位物理随机数高速提取方法.该方法在光域中利用锁模激光器作为光时钟,通过太赫兹光非对称解复用器完成对混沌激光的超低抖动光采样,无需射频时钟及后续逻辑处理过程的参与,经多位比较量化可直接产生优质物理随机数.并以光反馈半导体激光器这一典型的混沌激光产生装置作为熵源对所提方法进行了原理性实验论证.结果显示,光反馈半导体激光器产生的6 GHz混沌激光经5 GSa/s实时、低抖动光采样后,利用并行输出型多位比较器对所获混沌脉冲序列进行量化处理,选取最低有效位4位,可直接产生速率达20 Gb/s的随机数.该随机数速率由选取的量化结果最低有效位数和光采样率联合决定,而当前光采样率受限于所用混沌激光熵源的带宽.本文工作可为硬件上实现更高速物理随机数的实时、在线产生提供有力的技术和理论支撑.

Random numbers have great application value in the fields of secure communications, which are commonly used as secret keys to encrypt the information. To guarantee that the information is absolutely secure in the current highspeed communication, the applied random keys should possess a generation speed not less than the encrypted data rate,according to ＂one-time pad＂ theory found by Shannon（Shannon C E 1949 Bell. Syst. Tech. J. 28 656）Pseudo-random numbers generated by algorithm may easily reach a fast speed, but a certain periodicity makes them difficult to meet the aforementioned demand of information security. Utilizing physical stochastic phenomena can provide reliable random numbers, called physical random number generators（RNGs）. However, limited by the bandwidth of the conventional physical sources such as electronic noise, frequency jitter of oscillator and quantum randomness, the traditional physical RNG has a generation speed at a level of Mb/s typically. Therefore, real-time and ultrafast physical random number generation is urgently required from the view of absolute security for high-speed communication today.With the advent of wideband photonic entropy sources, in recent years lots of schemes for high-speed random number generation are proposed. Among them, chaotic laser has received great attention due to its ultra-wide bandwidth and large random fluctuation of intensity. The real-time speed of physical RNG based on chaotic laser is now limited under 5 Gb/s, although the reported RNG claims that an ultrafast speed of Tb/s is possible in theory.The main issues that restrict the real-time speed of RNG based on chaotic laser are from two aspects. The first aspect is ＂electrical jitter bottleneck＂ confronted by the electrical analog-to-digital converter（ADC）. Specifically, most of the methods of extracting random numbers are first to convert the chaotic laser into an electrical signal by a photodetector, then use an electrical ADC driven by radio frequency（RF） clock to sample and quantify the chaotic signal in electronic domain. Unfortunately, the response rate of ADC is below Gb/s restricted by the aperture jitter（several picoseconds） of RF clock in the sample and hold circuit. The second aspect comes from the complex post-processes,which are fundamental in current RNG techniques to realize a good randomness. The strict synchronization among postprocessing components（e.g., XOR gates, memory buffers, high-order difference） is controlled by an RF clock. Similarly,it is also an insurmountable obstacle to achieve an accurate synchronization due to the electronic jitter of the RF clock.In this paper, we propose a method of ultrafast multi-bit physical RNG based on chaotic laser without any postprocess. In this method, a train of optical pulses generated by a GHz mode-locked laser with low temporal jitter at a level of fs is used as an optical sampling clock. The chaotic laser is sampled in the optical domain through a low switching energy and high-linearity terahertz optical asymmetric demultiplexer（TOAD） sampler, which is a fiber loop with an asymmetrical nonlinear semiconductor optical amplifier. Then, the peak amplitude of each sampled chaotic pulse is digitized by a multi-bit comparator（i.e., a multi-bit ADC without sample and hold circuit） and converted into random numbers directly.Specifically, a proof-of-principle experiment is executed to demonstrate the aforementioned proposed method. In this experiment, an optical feedback chaotic laser is used, which has a bandwidth of 6 GHz. Through setting a sampling rate to be 5 GSa/s and selecting 4 LSBs outputs of the 8-bit comparator, 20 Gb/s（= 5 GSa/s × 4 LSBs） physical random number sequences are obtained. Considering the ultrafast response rate of TOAD sampler, the speed of random numbers generated by this method has the potential to reach several hundreds of Gb/s as long as the used chaotic laser has a sufficient bandwidth.