Short Pulses for narrow linewidth

The emitted wavelength of a DFB laser is given by the spacing of its Bragg grating, which is affected by temperature. In the case of a pulsed lasers, the sudden onset of electrical dissipation will increase the temperature during the pulse, which will create chirp. We give here some information on this behaviour.

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nullAt turn-on, this effect changes the emitted wavelength. The tuning rate is approximately 14 ppm/ns at the outset of the pulse and slows down rapidly after a few ns; the exact rates varying from one laser to another. It follows that, to obtain a narrow linewidth on a slow detector, the pulse length must be kept to a minimum.

The pulsed laser from Alpes Lasers are normally tested on their datasheet using a 50 ns pulse, which results in a noticeable linewidth shown on the datasheet spectra.

A shorter pulse can be used to reduce this linewidth. Using the QCL pulser provided by Alpes Lasers, pulses as short as 20 ns can be created. Dedicated electronics may be able to achieve even shorter pulses. However the non-linear electrical behavior of QCL make the typical rise and fall-time of the pulse on the order of 8 ns, making very short pulses difficult to achieve.

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As a final note, the effective linewidth can also seem to
 depend on the amplitude of the pulse. This is because there
 is typically an overshoot at the beginning of a pulse; this is especially pronounced at low amplitude and very short pulses. It may therefore seem as though a pulse is very short, while it is in fact below threshold, with only a short overshoot being above threshold. An increase in amplitude will then show the true length of the current pulse. You can see on the figure, on the right, a typical shape for a short pulse: the actual spectral behavior will vary depending on the location of the threshold with respect to the shoulder appearing after 7.9 ns.