We have seen in a previous article that External Cavity Diode Lasers (ECDL) have a narrow-linewidth laser spectrum. But that under some external influence, a specific current or temperature, another mode suddenly takes over all the optical power and, for a short while, there may be mode-hopping or even power in both modes. And this is the kind of laser spectrum that one should clearly avoid for spectroscopic applications.
First approach would be to use an optical spectrum analyzer (OSA) for measuring the laser spectrum. Nonetheless mode hopping is a fast behaviour. And OSA offers a slow measurement rate of maximum 10 Hz which is not fast enough for a correct observation of this behavior. And moreover their absolute accuracy is too weak to make accurate measurements on the real mode-hop-free range of your ECDL. Another approach would be to use a wavelengthmeter. Wavelengthmeters do offer a better accuracy and higher measurement rate. Nonetheless most wavelengthmeters are made for measuring only single-mode lasers and when facing a mode hopping they give an erratic value. The erratic value will clearly be a sign that your ECDL is out of the mode-hop-free range but it will leave you blind. The most suitable approach is to use a laser spectrum analyzer with fast measurement rate and high absolute accuracy, such as ZOOM Spectra. When measuring an ECDL, one can have both the information on the laser spectrum of the ECDL to check it is well single-mode and the accurate limits of its mode-hop-free range.