Laser diode spectrum stability can be very important for applications where the single-mode characteristics, wavelength stability and narrow linewidth are decisive, as for example in absorption spectroscopy, in Raman spectroscopy, for atom/ion trapping and cooling, Bose-Einstein condensates, interferometry and microscopy.
In this case, the laser diode must include a system to perform an operating point search which finally provides a stabilized Laser diode. This initial search and the continuous stabilization are possible thanks to a loop which can use different technics (wavelength selective feedback, wavelength measurement, etc.).
In the present practical case, the search of the operating point after the start up of the laser is obtained thanks to a loop based on the measurement of current, temperature and laser power and the adjustment of the laser driver current and the Peltier thermal regulation parameters. Typically, it is known that these three measurements settled at the targeted values guarantee a stable laser mode. The loop is monitored by an embedded firmware in the electronics of the laser.
As soon as this operating point is reached, the stability of the laser spectrum is based on temperature stability and low noise current.
We can take the opportunity of this practical case to clarify the dependencies between laser wavelength measurement, linewidth and drift. It is important to understand that the linewidth is related to a measurement duration. All phenomenons which induce a jitter of the laser wavelength within the duration of a measurement broaden the linewidth. Slower phenomenons (with typical time constant above the measurement duration) induce drift from one measurement to another. Therefore, to be strictly rigorous, the linewidth and drift have to be related to the timescale corresponding to an application. A range between nanosecond and seconds must be considered. Fast phenomenon such as laser current noise, vibrations and acoustic noise add to the linewidth when measurement duration is typically above µs, while temperature and air pressure changes generally cause drifts. However, it has to be considered that some slight temperature changes may occur very quickly: it can be the case of an opened door or the switch off of the light of a lab for instance.
For the most demanding applications regarding the laser spectrum, more sophisticated laser locking systems can eliminate laser drifts and achieve kHz range linewidths.