Abstract
A narrow-linewidth laser with excellent temporal coherence is an important light source for microphysics, space detection, and high-precision measurement. An ultranarrow-linewidth output with a linewidth as narrow as subhertz has been generated with a theoretical coherence length over millions of kilometers. 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. The spectral resolution of a high-precision Fabry–Pérot interferometer can only reach the megahertz level. 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. 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.
Highlights
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 [1, 2], optical atomic clocks [3, 4], lidar [5, 6], high-speed coherent optical communication [7, 8], and distributed optical fiber sensing [9, 10]
Linewidth measurement technology is an essential part of the research and development process of narrow-linewidth lasers
Measurement technology based on Delayed self-heterodyne interferometry (DSHI) methods is developing rapidly, and its application is the most extensive among the methods
Summary
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 [1, 2], optical atomic clocks [3, 4], lidar [5, 6], high-speed coherent optical communication [7, 8], and distributed optical fiber sensing [9, 10]. In 2018, Pavlov et al [32] achieved a single-frequency output by using self-injection locking They used a narrow-linewidth tunable fiber laser to perform heterodyne measurement with the tested laser and obtained a 340-Hz Lorentzian width and 1.7-kHz Gaussian width. When the output power of the tested laser is large, the long-delay fiber generates stimulated Brillouin scattering, which is opposite to the direction of laser transmission; in this case, the incident pump energy is converted into Stokes wave and sound wave energy, increasing the transmission loss and even making the PD unable to detect the signal. In 1986, Richter [50] derived the power spectral density function of the beat notes under the DSHI structure and reported that the measurement result would be more accurate with a delay time much longer than the coherence time of the tested laser. In 2018, Bai [58] successfully measured a laser with a linewidth of 98 Hz using
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