Narrow linewidth lasers are lasers that emit light with a very narrow spectral linewidth. This means that the light waves emitted by the laser are very close together in frequency. Narrow linewidth lasers are generally suitable for single frequency lasers, i.e. lasers oscillating on a single cavity mode with low phase noise and high spectral purity.

Types of Narrow Linewidth Lasers
Distributed feedback (DFB) lasers: DFB lasers use a grating structure within the laser cavity to produce a narrow linewidth output. DFB lasers are relatively inexpensive and easy to manufacture, making them a popular choice for many applications. However, DFB lasers can have relatively poor frequency stability, which can limit their use in some applications.

External cavity lasers (ECLs): ECLs use an external cavity in conjunction with the laser diode to produce a narrow linewidth output. ECLs offer superior frequency stability to DFB lasers, making them a good choice for applications where high accuracy is required. However, ECLs are more expensive and complex to manufacture than DFB lasers.

Quantum cascade lasers (QCLs): QCLs are a type of semiconductor laser that can emit light at very high frequencies, making them ideal for applications such as telecommunications and spectroscopy. QCLs are relatively new technology, and they are still under development. However, they offer the potential for very narrow linewidths and high output powers, making them a promising technology for a variety of applications.

Microchip lasers: Microchip lasers are a type of semiconductor laser that are very small and compact. Microchip lasers are ideal for applications where size and weight are important factors, such as in fiber optic communications and medical diagnostics. Microchip lasers offer good performance and are relatively inexpensive, making them a good choice for a variety of applications.

Features of Narrow Linewidth Lasers
Narrow spectral linewidth: Narrow linewidth lasers emit light with a very narrow spectral linewidth. This means that the light waves emitted by the laser are very close together in frequency.

High frequency stability: Narrow linewidth lasers are typically very stable in frequency. This means that the frequency of the laser does not change very much over time.

High output power: Narrow linewidth lasers can typically produce high output powers. This makes them suitable for a variety of applications, such as optical communications and spectroscopy.

Compact size: Narrow linewidth lasers can be made in a very compact size. This makes them ideal for applications where size and weight are important factors.

Low cost: Narrow linewidth lasers can be made at a relatively low cost. This makes them a cost-effective option for a variety of applications.

Applications of Narrow Linewidth Lasers
Optical frequency metrology： Narrow linewidth lasers are used to measure the frequency of other light sources. This is important for a variety of applications, such as developing new optical components and standards.

Optical spectroscopy: Narrow linewidth lasers are used to study the absorption and emission of light by atoms and molecules. This information can be used to identify and characterize different materials.

Optical communications: Narrow linewidth lasers are used to transmit data over optical fiber networks. The narrow linewidth of the lasers helps to ensure that the data is transmitted accurately and without errors.

There are a number of different technologies that can be used to create narrow linewidth lasers. The most common technology is called a Fabry-Perot laser. Fabry-Perot lasers use a resonant cavity to create a narrow linewidth. Other technologies that can be used to create narrow linewidth lasers include ring lasers, fiber lasers, and semiconductor lasers.

Narrow linewidth lasers are a valuable tool for a variety of applications. The narrow linewidth of the lasers allows them to be used for precise measurements and for transmitting data over long distances. As the technology for creating narrow linewidth lasers continues to improve, these lasers will become even more useful in a wider range of applications.