Basic Principles and Applications of Single-Frequency, Narrow-Linewidth Lasers

Principles of Narrow-Linewidth Lasers with Fiber Bragg Grating Feedback：

TeraXion's PureSpectrum NLL series utilizes a technology that combines a phase-shifted fiber Bragg grating filter with a highly stable driver circuit. Its core principle is to monitor the laser frequency in real time using an optical discriminator, leveraging the narrowband filtering properties of the fiber Bragg grating to suppress multi-longitudinal mode oscillations, while maintaining wavelength stability through a temperature control module. This design integrates the low-noise characteristics of a semiconductor laser with narrow linewidth performance in a compact package, achieving linewidths on the order of 5 kHz and making it suitable for high-precision applications such as LiDAR and distributed fiber sensing.

Basic Principles of Narrow Linewidth Implementation in Semiconductor Lasers：

Single-frequency, narrow-linewidth semiconductor lasers achieve single-longitudinal mode output through a DFB (distributed feedback) or DBR (distributed Bragg reflector) structure. The key features are:

1) carrier injection to induce population inversion;

2) cavity feedback to enhance stimulated emission;

3) suppression of transverse mode competition. By optimizing the epitaxial layer structure and cavity surface coating process, spontaneous emission noise can be reduced, wavelength stability can be improved, and narrow linewidth output can be achieved.

Applications: Optical communications and sensing, LiDAR, and ranging. Coherent optical communication systems, long-distance LiDAR, and inter-satellite laser communications, among others. For example, using a single-frequency narrow-linewidth laser (with linewidths as low as kHz) as a light source can improve receiver sensitivity by 7-14dB, extend ultra-long-haul fiber transmission distances, and support advanced modulation methods such as phase and frequency modulation..‌