Narrow Linewidth Laser Technology and Development(Invited)

Narrow linewidth lasers have extremely high spectral purity,high peak spectral density,ultralong coherence length and extremely low phase noise,so they are used as core light sources in fields of gravitational wave detection,optical clocks,cold atom physics,coherent optical communication,optical precision measurement and microwave photonic signal processing. With the development of laser and its application research, higher and higher requirements are placed on the comprehensive parameter performance of lasers. Narrow linewidth lasers are developing in the direction of ultra-narrow linewidth, ultra-stable time-frequency parameter and wavelength arbitrary tunability. Among them, the laser linewidth is the key factor that determines laser coherence,and it has always been the focus of scientists' research on laser technology. The essence of the existence of linewidth is that the laser will be affected by the phase and intensity disturbances caused by spontaneous radiation in the gain during operation,which makes the laser output signal inherently broadened by Gaussian white noise. In addition,the laser is also easily affected by the classical noise caused by the temperature change and vibration in the external environment,which further widens the laser linewidth. The above- mentioned factors have greatly reduced the coherence of the laser,thus limiting the development of the laser in promoting scientific research and industrial application to a great extent. Therefore,laser linewidth compression technology has become a key scientific issue for obtaining high- coherence light sources. At present,lasers with fiber- doped rare earth ions and semiconductors as gain have the advantages of long life, small size, low cost, high reliability,and easy industrialization,and have become the most studied and widely used solid-state lasers. However,due to the lack of control technology,the linewidth of conventional short- cavity fiber and semiconductor lasers is usually maintained at the order of tens of kHz or even MHz,and it is difficult to meet the requirements of various technological developments for the performance of laser linewidth parameters. Based on the interaction principle of the spontaneous emission and stimulated emission in the laser cavity,the structure innovation of the laser cavity is the main research routine to achieve the extreme control of narrow linewidth laser parameters. Since the invention of the laser,it has gradually experienced three stages of cavity structures to suppress the spontaneous emission:the main cavity laser,the fixed external-cavity feedback laser, and the adaptive distributed feedback laser. Here, the laser frequency stabilization technology based on the external servo electrical feedback and the external cavity feedback technology based on the photon lifetime extension are the common means to realize the narrow linewidth laser output. However, the extra-cavity servo electrical feedback technology not only requires highprecision and high-sensitivity external detection and control devices,but also requires precise control of the operating environment of the reference cavity. Hence,this laser frequency stabilization technology has a complex structure and high cost, which is not conducive to the large- scale integrated development of lasers. The latest laser structure with adaptive distributed feedback is mainly based on the fixed spatiotemporal perturbation provided by the distributed feedback,which deeply suppresses the random spatiotemporal perturbation by the spontaneous emission, so as to achieve the laser linewidth deep compression of the laser cavity with the feature of wavelength self- adaptation. This review first introduces the application requirements and structure evolution of narrow linewidth lasers,and then introduces the research progress of the main cavity laser and the fixed external-cavity feedback laser. Then,the adaptive distributed feedback laser recently developed is introduced. The physical ideas,core devices and system performance of this new type of laser are discussed. Eventually,the application characteristics of narrow linewidth lasers are introduced in typical fields of the distributed optical fiber sensing, laser coherent communication and on- chip optical information processing. The development trends of narrow linewidth lasers are also prospected.