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Double Pulses

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Definition: two closely spaced optical pulses

German: Doppelpulse

Category: light pulses

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Optical pulses sometimes occur in the form of double (or multiple pulses). This phenomenon can arise under different circumstances, as discussed in the following.

Double Pulses in Q-switched Lasers

The normally desired mode of operation of a Q-switched laser leads to the generation of a single pulse (typically with a nanosecond duration) per Q-switching cycle. Under certain circumstances, however, multiple pulses may be emitted within a single Q-switching cycle. Typical reasons for such behavior are:

For predicting or analyzing such effects, it can be very helpful to simulate the laser dynamics numerically.

Double Pulses in Mode-locked Lasers

With mode locking of a laser, one usually obtains a single ultrashort pulse circulating in the laser resonator, or (with harmonic mode locking) multiple pulses with equidistant spacing. In some situations, however, a circulating pulse may break up into two or more closely spaced pulses, leading to multiple pulses in the output pulse train. This occurs particularly often in passively mode-locked lasers operating in the femtosecond domain, e.g. with soliton mode locking.

double pulse

Figure 1: A pair of 120-fs pulses with a 1-ps temporal spacing.

spectrum of double pulse

Figure 2: Optical spectrum of a double pulse as shown above. Both pulses are assumed to have a center frequency of 300 THz. The spectrum (solid curve) is modulated with a frequency of 1 THz, which is the inverse pulse spacing. For comparison, the dashed curve shows the spectrum of a single pulse.

Multiple pulses can sometimes form a relatively stable configuration, since there can be various physical effects which permit some kind of interaction between the pulses. These pulses can then do many resonator round trips with constant spacing and even constant phase, i.e., they remain mutually coherent. As a result of that, the optical spectrum is strongly modulated (see also a Spotlight article). In fact, such a spectrum often leads to the discovery of that phenomenon.

In other cases, however, multiple pulses can be very unstable, at least concerning their relative phase. A modulated spectrum then cannot be observed.

Detection of double pulsing of mode-locked lasers is possible, e.g. with the following methods:

See also: pulses, ultrashort pulses, Q switching, Q-switched lasers, mode locking, mode-locked lasers, photodiodes, autocorrelators, Spotlight article 2007-10-11
and other articles in the category light pulses

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