高斯白噪声相位调制的激光光谱展宽

激光器是现代光学中一种常用的光源，分析其谱宽特性对于激光的研究具有重要意义。由于高性能窄线宽激光的半高全宽通常在几十MHz以内，难以直接满足宽光谱领域的应用需求，因而无法发挥其在成本和性能方面的优势，这在一定程度上限制了激光的发展。如果在激光内部直接进行调制实现线宽展宽，又会导致频率的严重漂移，破坏稳频特性。为了在激光器波长稳定性不变的前提下得到宽光谱，提出了一种以高斯白噪声为驱动电信号的外部相位调制方法。以激光相位噪声和光谱特征之间的关系为基础，通过理论计算阐明了相位噪声对于光谱的展宽效应，并基于OptiSystem光仿真软件和数值仿真分析了在不同白噪声参数下的光谱演变过程，明确了要得到好的展宽和载波抑制效果需要高功率、大带宽的噪声信号源。搭建的外部相位调制系统采用10 GHz带宽的信号源，通过两个前置预放大器以及一个高饱和输出功率放大器将噪声信号由-17 dBm最高提升至28 dBm，以此驱动半波电压3.7 V的铌酸锂电光相位调制器，并在放大链路中加入低通滤波器和可调衰减器调节信号参数。研究对象为初始线宽20 kHz的分布式反馈半导体激光器，通过光谱分析仪观测和对比调制前后的激光光谱，得到了与理论及仿真相吻合的结果，并拟合计算出展宽分量最高可达65 GHz，且载波也得以明显地抑制；此外，基于扫描式法布里-珀罗共焦腔评估了激光频率的漂移情况，证明外部调制并未对激光的波长稳定性造成影响，综合论证了高斯白噪声的相位调制方案能够实现优越的光谱展宽效果。由于调制后的光谱无边带和次峰，且能实现数倍于调制带宽的展宽，因此相较于正弦函数和伪随机编码等其他信号而言，高斯白噪声具有明显的优势。该调制方法不依赖于激光器的本征线宽，具有广泛的应用前景，为相位调制和光谱展宽的相关研究提供了参考依据。

Laser Spectrum Broadening Method Based on Phase Modulation of Gaussian White Noise

Laser is a kind of common light source in modern optics. It is of great significance to analyze its spectrum width characteristics for research. Because the full width at half maximum (FWHM) of high-performance narrow-linewidth lasers is usually within tens of MHz, it is very difficult for them to directly meet the application requirements in the fields that need broadband source and give full play to their advantages in terms of cost and performance, which also limits the development of laser to a certain extent. If the laser is directly modulated to achieve linewidth broadening, it will cause serious frequency drift and destroy its important frequency stability. In order to obtain the wide spectrum without degrading wavelength stability, an external phase modulation method using Gaussian white noise (GWN) as a driving signal is reported. Based on the relationship between laser phase noise and spectral characteristics, the broadening effect of phase noise on the spectrum is clarified through theoretical calculation. The spectrum evolution process with different parameters of white noise is analyzed based on OptiSystem optical simulation tool and numerical simulation. The noise source with high power and wide bandwidth is needed for broadening and carrier suppression of high efficiency. The external phase modulation system uses a 10 GHz noise source. The signal is increased from -17 to 28 dBm by two preamplifiers and a high saturation output power amplifier to drive a lithium niobate (LiNbO₃) electro-optic phase modulator with the half-wave voltage of 3.7 V. The low-pass filter and variable attenuator are added to the amplification link to adjust the signal parameters. The research object is a distributed feedback (DFB) semiconductor laser with an initial linewidth of 20 kHz. It can be seen by observing and comparing the laser spectra with and without modulation using an optical spectrum analyzer (OSA) that the experimental results are consistent with the theory and simulation. The maximum broadened component is up to 65 GHz and the carrier is also obviously suppressed. In addition, the laser frequency drift is evaluated based on the scanning Fabry-Pérot confocal cavity. It is proved that the external modulation does not affect the wavelength stability of the laser, so the phase modulation scheme of GWN can achieve superior spectrum broadening. Because the modulated spectrum has no sideband or minor peak and the broadened component is several times the modulation bandwidth, GWN has obvious advantages compared with others, such as sinusoidal signal and pseudo-random binary sequence (PRBS) signal. This promising method does not depend on the intrinsic linewidth of laser, and the work can be used as a reference for the research of phase modulation and spectrum broadening.