Mode Locking Lasers can be made to operate continuously or in pulses. As an example, the bandwidth of the 632.8 nm line in the He-Ne laser is 1.5 GHz, so that the shortest pulses that a He-Ne can produce would be 0.67. ns long. This is not particularly short by modem standards. Dye lasers typically have gain bandwidths greater than 10*1000000000000 Hz and can be used to generate pulses shorter than 100 fs. This is achieved by a technique called mode-locking. To achieve shorter laser pulses, mode locking of the laser modes is done
Types of Mode Locking
In a comparison of ultrafast and normal lasers, an ultrafast laser simultaneously lases in many different modes (the more the better), but the phases of the different modes are completely uncorrected.
Active Mode Locking
A physical device is placed in the cavity, which modulates the frequencies of the cavity modes. It depends on an external source to incite oscillation in the resonant cavity, creating modulated light.
Passive Mode Locking
An intensity dependent loss-mechanism (saturable absorber) placed in the cavity causes less intense radiation to be damped out, leaving only a single, intense pulse oscillating back and forth in the cavity.
It is a special kind of passive mode locking in which the lasing medium itself has an intensity dependent index of refraction.