Laser light that circulates in two counter-propagating beams helps form robust and short pulses at very high pulse repetition rates. The method, known as colliding pulse mode-locking, has recently been demonstrated with a vertical-external-cavity semiconductor laser (VECSEL). VECSELs are lasers that can emit pulses at repetition rates even beyond 100 GHz. Here, Laurain and colleagues set a new milestone for pulse duration and power from this type of a laser and present a comprehensive analysis of what happens when the two beams circulate in different directions inside the laser cavity. In particular, they show that the interference of the beams leads to a periodic intensity pattern on cavity mirrors. This helps to initiate pulses with an intensity-dependent reflector. Moreover, the beams tend to balance one another and synchronize in time. As a result, short and stable pulses appear more easily than in a regular laser. The demonstrated combination of high pulse repetition rate and high peak power can boost the speed and quality of many nonlinear imaging and optical sampling techniques.
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