The linewidth of the laser does not change…in theory at least. In high performance lasers, such as OEwaves’ OE4030, the linewidth is stabilized by self-injection locking to a microresonator, so to change the laser noise and linewidth parameters of microresonator must change. Since the temperature of the microresonator and the current and temperature of the laser are controlled at fixed values, under normal operating conditions the laser linewidth does not change.
However, in many other lasers, the linewidth can fluctuate for a variety of reasons - from day-to-day or even from second-to-second. For example, in fiber optic sensor and communications networks where various network elements may impact the noise of the laser beam, and its linewidth. In other cases, changes in the environmental conditions such as temperature, or in voltage or current can cause the laser linewidth to change or fluctuate. In these instances, monitoring of the laser linewidth can be used to account for the system performance degradation or other operational considerations.
OEwaves has designed additional optional functionality for the OE4000 Multi-function Noise Analyzer to help in monitoring the laser linewidth. The Linewidth Monitoring option provides a real-time capability for measurement of the Lorentzian linewidth at rates up to 0.1 Hz, with sub-Hertz sensitivity and a 60 dB dynamic range. Lasers operating anywhere in the visible to near IR can be monitored remotely with a GUI interface and/or command script capability. So now, every system operator can watch the winking of the linewidth and record it for diagnostics and tests.