The amplification of spontaneous emission is used to initiate laser action. As the phase of spontaneous emission is inherently random, the phase of the coherent laser emission (the carrier phase) will also be random. This prevents phase-resolved detection of the laser field, inhibiting their application to studies where the control of the amplitude and of the phase can be particularly interesting (metrology, mode-locking etc). The “ultra-fast THz spectroscopy” group at LPA-ENS has recently shown how the carrier phase of a semiconductor laser can be fixed by forcing the device to demonstrate laser action on an injected coherent pulse that has a well-defined phase (i.e. phase seeding).
The semiconductor laser that was phase seeded in this work was a terahertz (THz) quantum cascade laser (QCL). This laser source is very promising for the THz region but the coherent detection of its emission remains problematic. The LPA team was able to inject into the device an ultra-fast THz pulse that initiated the laser action of the QCL. This permitted the direct coherent detection of the QCL emission in the time domain, giving insights into the phase-resolved build-up of the laser field. An additional outcome of this work is that QCLs can now be used as powerful sources for time-domain spectroscopy.
D. Oustinov, et al. “Phase seeding of a terahertz quantum cascade laser”, Nature Communications 1:69 doi : 10.1038 / ncomms1068 (2010).
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