Pulse laser

A pulsed laser is understood to be a laser that does not emit light continuously ( continuous wave laser, also called CW laser), but is operated in a pulsed manner. H. the light emits in time-limited portions (the pulses ). Depending on the length of the pulse, one speaks of short or ultra-short pulse lasers .

properties

While light from a continuous wave laser typically has a very narrow spectrum , this is not possible with a pulsed laser. According to the Fourier transformation , the temporal course and the spectrum of a pulse are linked: the shorter a pulse is, the greater its frequency - bandwidth .

The product of temporal and spectral width ( and the FWHM of the intensity) is called the transform limit and fulfills the inequality ${\ displaystyle \ Delta t}$ ${\ displaystyle \ Delta \ nu,}$

{\ displaystyle \ Delta t \ cdot \ Delta \ nu \ geq {\ text {const.}}}

The constant depends on the pulse shape. For a Gaussian pulse z. B.

{\ displaystyle \ Delta t \ cdot \ Delta \ nu \ geq {\ frac {2 \ ln {2}} {\ pi}} \ approx 0 {,} 44.}

Quasi-continuous wave laser (QCW)

These lasers are CW lasers whose operation is periodically interrupted (hence "quasi-continuous-wave"). During the emission, this is constant over time and has a fixed relationship to the pump rate, as in CW operation. However, since the emission is periodically interrupted, the power averaged over time is less than the peak power. This enables peak performance that would overload the beam source with uninterrupted operation. In this respect, this operation corresponds to a pulsed laser.

generation

Some laser types only emit laser pulses for physical reasons or cannot be operated efficiently as CW lasers. The first laser, the ruby laser , is such a pulse laser. The energy stored in the population inversion is "cleared away" faster by a pulse than the pump source can pump new energy into the upper laser level. Solid-state lasers pumped with flash lamps in particular emit only pulsed laser light. The energy, duration and peak power can be precisely set via a controlled power supply for the flash lamps.

Many CW lasers can also be operated in a pulsed manner by switching the pump power on and off quickly. Carbon dioxide lasers can be operated in pulsed mode up to over 1 kHz. In principle, pulses can also be generated with a combination of a CW laser and a modulator (e.g. a simple chopper ). However, such a method is not very efficient because you lose a large part of the laser power. In addition, the minimum achievable pulse duration is limited by the speed of the modulator. In practice, the aim is therefore to achieve the total population inversion , i.e. H. the entire available gain of the laser can be called up during a pulse duration.

There are different methods for generating short and ultra-short pulses. With them, top performances in the range of several GW can be achieved.

Q-switching

Under Q-switch (engl. Q-switching ) refers to the switching of the losses within the laser resonator . While the losses are kept high, a high population inversion can be built up via optical pumping. Due to the high losses, the laser cannot start to oscillate during this time. During this time, the population inversion is reduced only by spontaneous, but not by stimulated emission. If the quality of the resonator is switched to "good" and the losses are reduced, the stimulated emission starts. This consumes the population inversion that has built up within a short time, so that the energy in the optical medium is concentrated in a short pulse.

The implementation can be done by active or passive elements. In the case of an active implementation, the Q-switching is "controlled externally", e.g. B. via electro-optical or acousto-optical modulators . With a saturable absorber as a passive element, the Q-switching changes via the lighting. Due to the spontaneous emission, the absorber is gradually "saturated" until its absorption has decreased enough for the stimulated emission to begin. This leads to a further saturation of the absorber, so that the resonator quality increases further. The stimulated emission therefore continues to increase until the population inversion is consumed.

With an active Q-switch, pulse durations of a few nanoseconds could be generated. Passive Q-switches are used for shorter pulses.

Mode coupling

In the mode-locking (engl. Mode locking ) existing in the laser longitudinal be modes synchronized. Due to the in-phase superposition, the different modes interfere constructively, so that a short pulse is formed.

As with Q-switching, there are also active and passive methods here. Again, an active method is the use of an acousto-optic modulator. With mode coupling, however, this does not regulate the losses in such a way that the laser operation is completely suppressed for a certain time. Rather, the modulator is operated at a frequency that corresponds to the cycle time of a pulse in the resonator. The modulator does not have to switch between 0 and 100% transmission . A modulation of a few percent is sufficient. Passive processes can be implemented using saturable absorbers or by using the Kerr lens effect.

With mode coupling, pulse durations in the picosecond and femtosecond range are achieved. With values in the pico and nanojoule range, the pulse energies are well below the values that can be achieved with Q-switched lasers. The shortest pulses are achieved when using saturable absorbers. The pulses can be amplified afterwards, e.g. B. in a regenerative amplifier .

Laser pulses in the picosecond range were first generated in the mid-1960s by Anthony J. DeMaria and co-workers, and in the less than 1 picosecond range in 1974 by Charles Shank and Erich P. Ippen .

Measurement

In order to resolve a process in time, one needs events that are shorter than the event to be measured. Ultrashort laser pulses are the shortest events that can be artificially created. An electronic measurement with a photodiode is not possible because the speed of a photodiode is limited by the recombination time of the electron-hole pairs, which is typically in the nanosecond range.

Often the shortest available event is the pulse itself. In an autocorrelator one can measure the pulse "with itself" and thus infer the pulse duration.

Another possibility is the use of FROG ( frequency-resolved optical gating ). This can be used to record a spectrogram of the pulse and to calculate the electric field and phase from it.

Applications

Due to their high peak intensities, pulse lasers are used in a wide variety of applications. B. in material processing and ophthalmology . In the latter case, ametropia can be corrected through targeted removal of the corneal surface (e.g. LASIK surgery, Femto Lasik ).

Furthermore, because of the high intensities, effects of the non-linear optics , such as e.g. B. Frequency doubling or the Kerr effect , induce.

Due to the extremely short pulse durations, physical processes that take place on the time scale of the pulse duration can be resolved. This happens e.g. B. with the pump-probe technique .

Web links

Pulsed laser in the Encyclopedia of Laser Physics and Technology (Engl.)