Minimization of the laser linewidth
The linewidth of a laser is directly related to the type of laser. The laser linewidth can be minimized by optimizing the laser design and minimizing the influence of external noise as much as possible. The first step is to determine which is dominant, quantum noise or classical noise, as this will affect the subsequent measurements.
When the intracavity power is high, the resonator loss is low and the resonator round-trip time is long, the quantum noise of the laser (mainly spontaneous emission noise) has a very small impact. Classical noise may be caused by mechanical creep. The fluctuations can be mitigated by using a compact short laser resonator, but sometimes in shorter resonators, the length fluctuations can produce a stronger effect. Reasonable mechanical design can reduce the coupling between the laser resonator and the external radiation, and also minimize the thermal drift effect. There are also thermal fluctuations in the gain medium, which are caused by the fluctuations in pump power.
For better noise performance, other active stabilizing devices need to be adopted, but it is best to initially use practical passive methods. The linewidth of single-frequency solid-state laser and fiber laser is several kilohertz, and sometimes even less than 1KHz. By using the active stabilization method, a line width of less than 1KHz can be obtained. The linewidth of laser diodes is usually in the MHZ range and can also be reduced to KHz, such as in external cavity diode lasers, especially for optical feedback diodes with a high-precision reference cavity.
It is worth noting that in some cases, a laser light source is not needed to produce a very narrow light width:
1. When the coherence length is long, the coherence effect (due to weak parasitic reflection) will disrupt the shape of the light beam. In laser projection displays, the speckle effect can interfere with the quality of the return surface.
2. When light is transmitted in active or passive optical fibers, narrow line widths cause some problems due to stimulated Brillouin scattering. At this point, the line width needs to be increased, for example, by modulating the laser diode with current or using an optical modulator to rapidly jitter the instantaneous frequency. Line width is used in other situations. Line width is also used to describe the width of optical decay
Post time: Dec-09-2025