Lasers

A laser oscillator usually comprises an optical resonator (laser resonator, laser cavity) in which light can circulate (e.g. between two mirrors), and within this resonator a gain medium (e.g. a laser crystal), which serves to amplify the light. Without the gain medium, the circulating light would become weaker and weaker in each resonator round trip, because it experiences some losses, e.g. upon reflection at mirrors. However, the gain medium can amplify the circulating light, thus compensating the losses if the gain is high enough. The gain medium requires some external supply of energy – it needs to be “pumped”, e.g. by injecting light (optical pumping) or an electric current (electrical pumping → semiconductor lasers). The principle of laser amplification is stimulated emission.

A laser can not operate if the gain is smaller than the resonator losses; the device is then below the so-called laser threshold and only emits some luminescence light. Significant power output is achieved only for pump powers above the laser threshold, where the gain can reach the level of the resonator losses.

If the gain is larger than the losses, the power of the light in the laser resonator rises very rapidly, starting e.g. with low levels of light from fluorescence. Note that the resonator round-trip time is usually very small (e.g. a few nanoseconds, sometimes even much less), so that even a small net round-trip gain implies rapid exponential growth of the intracavity power. As high laser powers saturate the gain, the laser power will in the steady state reach a level so that the saturated gain just equals the resonator losses (→ gain clamping). Before reaching this steady state, a laser usually undergoes some relaxation oscillations. The threshold pump power is the pump power where the small-signal gain is just sufficient for lasing.

Some fraction of the light power circulating in the resonator is usually transmitted by a partially transparent mirror, the so-called output coupler mirror. The resulting beam constitutes the useful output of the laser. The transmission of the output coupler mirror can be optimized for maximum output power (see also: slope efficiency).

A high degree of spatial coherence of the laser radiation can be achieved, essentially because the light emission is triggered (stimulated) by the intracavity radiation (i.e., the light is circulating in the laser resonator), rather than occurring spontaneously. In the stimulated emission process, the laser-active ions are made to emit light in the direction of already existing light, and also with the same optical phase. The resulting amplitude and phase profile of the laser beam is largely determined by the properties of the laser resonator, not by the laser gain medium.

Temporal coherence is a different issue, and it has completely different origins. Some laser gain media can emit light only in a narrow spectral range. Even if that is not the case, a laser often emits light only at a precisely defined wavelength or frequency, because the conditions are such that a net zero round-trip gain is possible only for that wavelength. A laser may be tuned to the exact wanted wavelength e.g. by using a tunable intracavity bandpass filter, such as a Lyot filter. Again, the mechanism of stimulated emission is of crucial importance: the laser-active ions can be made to emit at exactly the optical frequency of already existing light. The smaller the emission linewidth (i.e., the narrower the optical spectrum of emitted light), the higher the degree of temporal coherence.

Interestingly, even ultrashort pulses can exhibit a very high degree of temporal coherence, in that case involving coherence between subsequent pulses in a regular pulse train. This is related to the formation of a frequency comb as the optical spectrum.

Some lasers are operated in a continuous fashion, whereas others generate pulses, which can be particularly intense. There are various methods for pulse generation with lasers, allowing the generation of pulses with durations of microseconds, nanoseconds, picoseconds, or even down a few femtoseconds (→ ultrashort pulses from mode-locked lasers). Often, a laser medium can accumulate some amount of energy over some “pumping” time in order to then release it within a much shorter time.

The optical bandwidth (or linewidth) of a continuously operating laser may be very small when only a single resonator mode can oscillate (→ single-frequency operation). In other cases, particularly for mode-locked lasers, the bandwidth can be very large – in extreme cases, it can span about a full octave. The center frequency of the laser radiation is typically near the frequency of maximum gain, but if the resonator losses are made frequency-dependent, the laser wavelength can be tuned within the range where sufficient gain is available. Some broadband gain media such as Ti:sapphire and Cr:ZnSe allow wavelength tuning over hundreds of nanometers.

Due to various influences, the output of lasers always contains some noise in properties such as the output power or phase. For pulsed lasers, additional quantities can come into play, for example the timing jitter. For more details, see the article on laser noise.

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