The Colliding Pulse Mode-Locked Laser

For this situation the time separation At = T/2 between the passage of successive pulses through the amplifier achieves the maximum value of one half of the round-trip time T. This means that the amplifying medium has a maximum time to recover its inversion after it had been depleted by the foregoing pulse. 

The total pulse intensity in the absorber where the two pulses collide is twice that of a single pulse. This means larger saturation and less absorption. Both effects lead to a maximum net gain if the two pulses collide in the absorber jet. 

Technique Mode locker Laser Typical pulse duration Typical pulse energy 

CPM Passive mode locking and eventual synchroneous pumping Ring dye laser 100 fs 1 nJ 

At a proper choice of the amplifying gain and the absorption losses this situation will automatically be realized in the passively mode-locked ring dye laser. It leads to an energetically favorable stable operation which is called Colliding-Pulse Mode (CPM) locking and the whole system is termed a CPM laser. This mode of operation results in particularly short pulses down to 50 fs. There are several reasons for this pulse shortening  

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