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What are short pulse lasers?


Frequently Asked Questions

As more understanding is gained about the way laser light interacts with materials, more laser materials processing is being undertaken by 'short pulse length' (or 'ultra-fast') pulsed lasers. For a given laser pulse energy, the shorter the pulse, the higher the peak laser power available. Materials processing applications have already been performed using pulses ~50 femtoseconds (50 x 10-15 seconds) in duration and producing peak powers in the terra watt range, using commercially available lasers. Off particular interest at the moment is whether nanosecond pulses, rather than femtosecond pulses, will be adequate for commercial applications, nanosecond pulses being significantly easier to obtain with the high average powers required for commercial viability. Attosecond (10-18s) pulses have also been achieved from experimental ti:sapphire lasers in recent years.

Nanosecond Lasers

Commercially the most important way to shorten laser pulse length is to employ the technique of Q-switching, a technique which lends itself conveniently to the solid state laser. Like any oscillator, a laser cavity has a quality factor Q, that measures the loss or gain in the cavity. The factor is defined as the ratio of the energy stored per pass to the energy dissipated per pass. Normally, the Q factor of a laser cavity is constant. However, if the Q factor is kept artificially low, say by putting a lossy optical element in the cavity, energy will gradually accumulate in the laser medium because the Q factor is too low for laser oscillation to occur and dissipate the energy. If the loss is suddenly removed, the result is a large population inversion in a high Q cavity, producing a high power pulse of light, typically a few nanoseconds to a few hundred nanoseonds long, in which the energy is emitted. The rapid change in cavity Q is called Q-switching. The technique only works for a laser medium capable of storing energy for a time much longer than the Q-switched pulse duration. The Nd:YAG laser can be supplied with relatively inexpensive electro-optic 'Q-switch' modulators to produce the short nanosecond pulses. The electro-optic modulator relies on the effects of electric fields on the refractive index of certain non-linear materials. With both frequency multiplying and Q-switching capabilities available on solid state lasers, the opportunities for further commercialisation of the solid state laser in materials processing applications are significant.

Femtosecond Lasers

The generation of ultra short light pulses - first picosecond (10-12 ) and now femtosecond (10-15 ) - has been made possible by the development of a technique known as "mode locking". Under the condition that the cavity length in a laser must equal an integral number of wavelengths, many modes can exist. Each mode has a slightly different frequency and wavelength. In a mode locked laser, the electric field associated with the different modes must add constructively at one point and destructively elsewhere to create a high intensity spike.

This leads to a dramatic increase in the peak intensity of the laser output for a very short time, while at other times little or no output is produced. This mode locking corresponds to a short pulse of light travelling back and forth between the laser mirrors. Today's femtosecond lasers are based on self mode locking in titanium doped sapphire solid state lasers, and in principle are relatively simple in concept, as can be seen from Fig.1. The system consists of several mirrors, the Ti:sapphire crystal, a pair of prisms and a pumping laser source. Light reflected from the output window is focused through the Ti:sapphire crystal, and re-collimated by another mirror before passing through the prisms to the cavity and mirror. Four mirrors are used in this system to obtain a very tight focus through the Ti-sapphire. When a pulse of light is introduced from the pump laser (which can be a frequency doubled Nd:YAG or Nd:YLF laser) the light intensity at the focus can be very high, increasing the refractive index at the centre of the crystal. Thus high intensity light sees the crystal as a lens, while low intensity light is unaffected. This results in a self focusing effect for a short high intensity pulse, which is used to preferentially select the pulsed mode locked set of modes that provide short pulses. The intensity of any pulse propagating within the laser increases faster than that of the continuous light. Eventually the pulse consumes all the energy pumped into the laser and the continuous wave output is suppressed. The prisms perform a critical function in this type of laser. When such short pulses travel along the laser cavity, the prism pair equalises the time it takes for the slightly different wavelengths in the laser pulse to travel back and forth across the laser, thus ensuring that the short pulse retains its original shape.

Fig. 1 Schematic representation of a mode locked Ti:sapphire laser
Fig. 1 Schematic representation of a mode locked Ti:sapphire laser

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