Mode hops are often provoked by external influences as laser case temperature, length drifts of the laser resonator, injection current and optical feedback. And as a matter of fact, mode hops often result from attempts to tune the wavelength of a laser. Let’s see here the two main causes behind laser mode hopping: temperature and injection current.
The laser cavity can support many different wavelengths or longitudinal modes. In laser diodes, these modes are separated by typically 10 to 300 pm (5 to 100 GHz) depending on the laser resonator. Mode hopping occurs for specific laser temperatures. However, the laser is a very small piece of semiconductor material mounted in the centre of a laser package. Therefore the temperature of the laser itself is not the same as the case temperature.
Since there is electrical current flowing through the laser, there is Joule heating that causes the laser to be hotter than the case. Heat flows from the laser to the case. And if the case temperature increases, the current must decrease to keep the same laser temperature. Mode hopping occurs for specific values of laser case temperature and injection current. This means that mode hopping can be eliminated by carefully controlling these two parameters.
Mode hopping can be caused by other external influences. For example, a random noise as mirror vibrations will create length drifts. And these length drifts will shift the resonator mode frequencies without shifting the gain maximum. Another identified cause to mode hopping is the external optical feedback. As a general rule with most lasers, reflections of the laser beam back into the laser cavity should be avoided.
Figure 1. External Cavity Diode Laser (ECDL) wavelength scan measured with a ZOOM Spectra laser spectrum analyzer. It shows a free spectral range (FSR) of about 20 pm / 9 GHz with laser mode hops of about 30-40 pm / 13-18 GHz.
The stability map is distinct for each laser. It can be measured thanks to a laser spectrum analyser, preferably high resolution and high rate. It is a reliable means of determining which values of temperature and current will result in wavelength stability.
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