## 1 Introduction  
Random numbers are of great significance in many fields, including statistical analysis, numerical simulation, fundamental science [1] and cryptography [2]. The randomness of a random number generator (RNG) could seriously affect the applications of high security demands, one of the outstanding example is the quantum key distribution (QKD) system in quantum cryptography [3; 4; 5; 6]. Based on the determined algorithm, a classical random number generator, also called a pseudo-RNG, could generate pseudo random sequences in an efficient method, expanding the randomness from short random seeds with extremely fast speed, and compatible with portable devices. However, due to the deterministic and predictable intrinsic nature of computational algorithms, pseudo random number generators face severe security issues in applications such as secure communication systems. In contrast, quantum random number generators (QRNG) [7; 8; 9], based on the intrinsic random nature of quantum processes, stand out as a promising alternative for its non-deterministic and unpredictable characteristics. QRNG schemes are classified into three categories, based on various requirements of physical devices, namely full-device-independent [10; 11; 12], semi-device-independent [13; 14; 15; 16; 17; 18], and full-trusted-device, i.e., practical QRNG schemes. Among the three categories, practical QRNG has been developed rapidly due to its convenience and huge demand. By adopting a certain post-processing method, practical QRNG scheme is information theoretically proved secure under the trusted-device scenario, which is adequent for the security requirements of most applications. During the last two decades, plenty of practical QRNG schemes have been realized with high generation rate and relatively low cost, including primarily developed discrete variable schemes, which measure photon path (spatial mode) [19; 20; 21], photon arrival time (temporal mode) [22; 23; 24; 25; 26; 27], photon number distribution [28; 29; 30]. A couple of years later, another category with relatively better performance was proposed, namely the continuous variable schemes. Due to the utilization of conventional high bandwidth photo-detector (PD) and analog-to-digital converter (ADC), continuous variable schemes could reach a generation rate several orders of magnitude higher than its discrete counterpart, while the volume of practical devices is also much more compact, and thus far more popular in recent research. Continuous variable QRNG schemes measure phase fluctuation of laser [31; 32; 33; 34; 35; 36; 37; 38; 39], vacuum fluctuation of quantum state [40; 41; 42; 43; 44; 45], amplified spontaneous noise (ASE) [46; 47; 48; 49; 50] and so on. Among the schemes mentioned above, phase noise, or phase fluctuation, is one of the major schemes adopted as random source (the other is vacuum fluctuation), for its high generation rate and relatively simple implementation setup. Traditional phase noise schemes utilize a self-delayed interference structure, thus inevitably introducing a delay-line in an unbalanced Mach-Zender interferometer (MZI) in the experimental setup [31; 32; 38; 39]. However, due to the bandwidth of phase noise in the laser, it usually takes several meters for the delay-line to reduce the auto-correlation coefficient of raw data effectively. In practical devices, space for the delay-line is one of the most difficult parts to be compressed, unless this part can be totally removed by adopting a novel scheme. In this paper, we demonstrate a QRNG scheme based on phase noise with a delay-free structure. We theoretically analyze our scheme, and point out three main frequency components in the electric signal to be measured. After adopting appropriate implementation settings, namely center wavelength of laser diodes and passband of a band-pass filter, we distill phase noise from the original signal for further randomness extraction.