Mode hopping is characterized by a stochastic exchange of power between two longitudinal modes of a laser, inducing a high-level intensity noise in the laser’s output. You can read more about why is laser mode hopping so bad for many applications in a previous article. We will review here why laser mode hopping occurs in semiconductor lasers.
The mode wavelengths and the gain peak wavelength depend on the laser’s temperature: the mode wavelengths shift with temperature at about typically 0.06 nm/°C, while the gain peak wavelength shifts at about 0.25 nm/°C. The mode shift is due to changes in the index of refraction of the semiconductor as well as the thermal expansion of the material. The latter causes the mode wavelengths to increase as the laser cavity expand. The gain peak shift is due to the change in the bandgap with temperature. A drift of the temperature of the gain medium will shift the wavelength of maximum gain while not shifting the frequencies of the resonator modes to the same extent. So, the previously lasing mode may then no longer be the mode with highest gain, and the power of a competing mode with higher gain can quickly rise. For a single-mode laser, the laser will therefore alternates between single-mode operation and mode hopping. This is true for index-guided lasers well above threshold.
For a gain-guided semiconductor laser, there is no mode hopping but mode oozing: the modes “ooze” as the modes and gain peak shift.
You can guess from this explanation which external influences can cause laser mode hopping. We will detail those influences that can lead a semiconductor laser to mode hop in a next article.
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