External Cavity Diode Lasers (ECDL) are laser diode systems with an external grating. They are mainly appreciated for three of their characteristics. They offer a narrow-linewidth laser spectrum with most often linewidth of only tens of kHz (< 0.001 pm) which is very much appreciated for spectroscopic applications. They also offer very interesting tunability range from 370 nm to 1770 nm. And they can be quite easily stabilized thanks to the use of wavelengthmeters and/or absorption cells or other kind of stabilization set-ups (this will be discussed in another article). But when one get interest in an ECDL, he will quickly take a close look at the laser spectrum of its laser and check the mode-hop-free range.
Indeed ECDL offer tens of nanometers of tunability. But this tunablity relies on the use of piezo parts. And the dark side is that it implies a limited free spectral range (FSR). The free spectral range often limits the optical frequency range in which the laser can be used. The FSR most often indicates the achievable mode-hop-free tuning range. Under some external influence, another mode suddenly takes over all the optical power and, for a short while, there may even be power in both modes. And this is the kind of laser spectrum that one should clearly avoid for spectroscopic applications.