Table of Contents
Narrow-Linewidth Lasers: Types and Features
Narrow-linewidth lasers, or lasers that have a small optical linewidth, are usually single-frequency, with low RIN and high spectral purity as a result of low phase noise.
The most common types of narrow linewidth lasers are distributed feedback laser diodes and distributed Bragg reflector lasers. Ultra-narrow linewidths can be achieved by using a single-mode fiber that contains a narrow-bandwidth fiber Bragg grating in a resonator. Special fiber Bragg grating used in distributed feedback lasers also allows for a narrow linewidth of a few kilohertz. Narrow-linewidth lasers with high output powers can be achieved in the form of longer distributed Bragg reflector lasers. Other lasers that have a narrow optical spectrum include diode-pumped solid-state bulk lasers and external-cavity diode lasers.
Achieving Ultra-Narrow Linewidth and Noise Minimization
Several difficulties emerge in the manufacturing of narrow-linewidth lasers and ultra-narrow-linewidth lasers. First of all, achieving single-frequency operation is crucial to get a narrow linewidth; the goal is a stable single-frequency operation without mode hopping over a relatively long period of time.
Secondly, minimizing noise influences from external sources is problematic. There are multiple ways of resolving the problem of external noise influences. A stable resonator setup with protection against vibration may be used. Certain lasers require specific solutions; for example, a low-noise current source should be implemented into the operation of electrically pumped lasers; an optically pumped laser should have a low-intensity noise pump source. A Faraday isolator is often used to avoid optical feedback.
Thirdly, the laser itself, its design in particular, should be manufactured with a goal of minimizing laser and phase noise. A long resonator and a high intracavity optical power can be beneficial to construct a narrow linewidth laser.
Narrow-linewidth lasers have a variety of applications: strain and temperature fiber-optic sensors, gas detection, LIDAR, and wind speed measurements. Ultra-narrow linewidth lasers are used in optical frequency metrology.