the concept of mode-locked laser

Could someone give me a clear explaination about the concept of mode-locked laser?Or recommend some reading material to me?

Thanks!

I assume you are looking for a basic level discussion. So I?ll try to make it simple:
Imagine a simplified laser structure; a long active medium and two mirrors at the end
If you can generate a very narrow pulse of light, that will bounce back and forth between the mirrors (at each reflection loses some power outside, that power is recovered during the travel in the active medium). The output will be periodic pulses, separated by the internal roundtrip time.
Normally if the active medium is powered continuously then the light in the laser will also be continuous, not pulse.
So a mode locked laser is one that uses some means of generating (and maintaining) a pulse traveling inside. This is achieved usually by the presence of some sort of switch (electro-optic, electro-absorptive, acousto-optic etc) inside the laser cavity.
Mathematically speaking what happens is that a laser oscillates on multiples of the fundamental resonant frequency of the cavity (actually it is more complicated but for now it is something like that). Typically the central frequency of interest corresponds to the mode order 5000 or so. But other modes will also be present like order 5001, 5002, 4999, 4998 etc. If you combine these modes with the right phase (and amplitude) you can obtain the desired pulse. That is were the mode locking or coupling name comes from, the modes are locked in phase. In practice, as I said, a switch is used, blocking everything except the desired pulse interval ? this way the pulse is obtained from an otherwise uniform random combination of modes.