Ultrashort Laser Pulse Phenomena, 2nd Edition

Hybrid Mode-Locking 345

Mode locked laser

Dye jet

Feedback

Glass

plate

Figure 6.2 Typical synchronously pumped dye laser. The length of the dye laser cavity has to be

matched to the repetition rate of the pump pulses. The noise in a synchronously pumped laser can

be reduced by reinjection of a portion of the output ahead of the main intracavity pulse. A thin glass

plate on the output mirror intercepts and reflects part of the beam into the cavity, with the desired

advance. The fraction of energy reflected (of the order of 10

−6

) is determined by the overlap of the

aperture and the glass plate. (Adapted from Peter et al. [13].)

fields E(t) (which essentially implies ηE(t) ˜)? Both calculation and experi-

ment have demonstrated a dramatic noise reduction by seeding the cavity with

a small fraction of the pulse in advance of the main pulse [13]. The emphasis

here is on small; only a fraction of the order of 10

−7

(not exceeding 10

−5

)of

the output power should be reinjected. A possible implementation would consist

of reflecting back a fraction of the output pulse delayed by slightly less than a

cavity round-trip. This amounts to a weakly coupled external cavity. A much

simpler implementation demonstrated by Peter et al. [13] consists in inserting a

thin glass plate (microscope cover glass for instance) in front of the output mirror

(Figure 6.2). The amount of light reinjected is adjusted by translating the glass

plate in front of the beam. The timing of the reinjected signal is determined by

the thickness of the plate.

6.2. HYBRID MODE-LOCKING

Synchronous pumping alone can be considered as a good source of ps rather

than fs pulses. The disadvantages of this technique, as compared to passive

mode-locking, are:

• a longer pulse duration,

• larger amplitude and phase noise,

• the duration of the pulses of the train are often randomly distributed, [3] and