There are usually two cases of beat notes suitable for linewidth measurement, as shown in Figure 1A. The linewidth of the beat signal is the sum of the widths of the two lasers participating in the beat. Two lasers with the same or similar frequencies interfere and produce a beat signal with a lower frequency. Where Δ v = v t + v r, v t is the linewidth of the tested laser, v r is the linewidth of the reference laser, and v b is the difference between the output frequencies of the two lasers mentioned above (also the center frequency of the beat notes). The mixed signal of two incoherent lasers, each with a Lorentzian line shape, still has a Lorentzian line shape, and the PSD of the beat notes can be expressed as. Optical beat notes are necessary to obtain the PSD. The power spectrum contains more-intuitive linewidth information, and it is relatively easy to obtain therefore, a large proportion of linewidth measurement experiments focus on the former. Two methods are mainly used for linewidth measurement: directly calculating the laser linewidth using the power spectrum density (PSD) of the laser and deducing the linewidth indirectly based on the relationship between the phase noise and linewidth. Narrow-Linewidth Laser Measurement Method Finally, a summary and an outlook for the future development of narrow-linewidth measurements are provided. In this article, typical methods for measuring narrow-linewidth lasers are reviewed, and the characteristics of each method, as well as the status of its development, are summarized. In the past few decades, laser frequency stabilization and mode selection have matured, and many narrow-linewidth measurement schemes are constantly being updated. Specific devices must be built for lasers with a narrower linewidth (sub-megahertz) to measure the linewidth. The rapid development and application of narrow-linewidth lasers have resulted in higher requirements for laser linewidth measurement technology. At present, the resolution of a commercial optical spectrum analyzer based on diffraction gratings is approximately 0.05 nm (gigahertz level) 1, and the resolution of Fabry–Pérot interferometers can reach a few megahertz 2. Therefore, precise measurement of the linewidth value is a prerequisite for the application of narrow-linewidth lasers.ĭifferent types of laser produce a broad coverage of linewidths, as large as tens of gigahertz and as small as subhertz. The linewidth value, an essential parameter of the laser, directly affects the accuracy of the narrow-linewidth laser in detection, sensitivity in sensing, and bit error rate in communication. They deduced that the spectral profile of the laser is Lorentzian and calculated its width (full width at half height). Scully and Lamb proposed the laser quantum theory. Therefore, the laser linewidth reflects the physical and frequency stability of the laser. The main reason for the linewidth generation is the phase fluctuation caused by spontaneous radiation and the noise induced by mechanical and temperature factors. Narrow-linewidth lasers with extremely low phase noise and a large coherence length have been widely used as a high-spectral-purity light source in gravitational wave detection, optical atomic clocks, lidar, high-speed coherent optical communication, and distributed optical fiber sensing. In this article, narrow-linewidth measurement methods and their research progress are reviewed to provide a reference for researchers engaged in the development, measurement, and applications of narrow-linewidth lasers. With the continuous improvement of laser coherence, the requirements for laser linewidth measurement technology are increasing, which also promotes the rapid development of narrow-linewidth lasers and their applications. The spectral resolution of a high-precision Fabry–Pérot interferometer can only reach the megahertz level. Traditional grating spectrum measurement technology has a wide wavelength scanning range and an extended dynamic range, but the spectral resolution can only reach the gigahertz level. An ultranarrow-linewidth output with a linewidth as narrow as subhertz has been generated with a theoretical coherence length over millions of kilometers. 4Science and Technology on Electro-Optical Information Security Control Laboratory, Tianjin, ChinaĪ narrow-linewidth laser with excellent temporal coherence is an important light source for microphysics, space detection, and high-precision measurement.3Department of Physics and Astronomy, MQ Photonics Research Centre, Macquarie University, Sydney, NSW, Australia.2Hebei Key Laboratory of Advanced Laser Technology and Equipment, Tianjin, China.1Center for Advanced Laser Technology, Hebei University of Technology, Tianjin, China.Zhenxu Bai 1,2,3* Zhongan Zhao 1,2 Yaoyao Qi 1,2 Jie Ding 1,2 Sensen Li 4 Xiusheng Yan 4 Yulei Wang 1,2 Zhiwei Lu 1,2
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