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The basis of the technique is to induce a fixed phase relationship between the modes of the laser's resonant cavity. The laser is then said to be phase-locked or mode-locked. Interference between these modes causes the laser light to be produced as a train of pulses. Depending on the properties of the laser, these pulses may be of extremely brief duration, as short as a few femtoseconds.
Although lasers are popularly thought to emit light of a single, pure frequency or wavelength, this is not actually true. All lasers produce light over some natural bandwidth or range of frequencies. A laser's bandwidth of operation is determined primarily by the gain medium that the laser is constructed from, and the range of frequencies that a laser may operate over is known as the gain bandwidth. For example, a typical helium- neonThis page is about the chemical element. For the automobile, see Dodge Neon. For a new semiconductor technology see ARM Neon is the chemical element in the periodic table that has the symbol Ne and atomic number 10. A colorless nearly inert noble gas, neo ( HeNeA helium-neon laser usually called a HeNe laser is a small gas laser of a type often used in laboratory demonstrations of optics. Its usual operation wavelength is 632. 8 nm, in the red portion of the visible spectrum. Schematic diagram of a helium-neon l) gas laser has a gain bandwidth of approximately 1.5 G HzHz or hz may mean: Herero language ( ISO 639 alpha-2, hz Hertz, unit of frequency Lists of two-letter combinations. (around 0.002 nmA nanometre ( American spelling: nanometer is 10−9 metres—or one billionth of a metre. Its symbol is nm . It is an SI measure of length, commonly used in measuring the wavelengths of visible light, ultraviolet radiation and gamma rays; amongst other wavelength range), whereas a titanium-doped sapphire ( Sapphire is the single crystal form of aluminium oxide (AlO). It can be found naturally as gemstones or manufactured in large crystal boules for a variety of applications. Sapphire Gems Sapphire is any gemstone-quality corundum that is not red in color.) solid-state laser has a bandwidth of about 128 T HzHz or hz may mean: Herero language ( ISO 639 alpha-2, hz Hertz, unit of frequency Lists of two-letter combinations. (a 300 nm wavelength range).
The second factor which determines a laser's emission frequencies is the optical cavity or resonant cavity of the laser. In the simplest case, this consists of two plane (flat) mirrorThis article is about the reflective surfaces. A mirror is a reflective surface that is smooth enough to be able to form an image. The best known example is the plane mirror that most people have at home. In it, a parallel beam of light changes its directs facing each other, surrounding the gain medium of the laser (this arrangement is known as a Fabry-PerotIn optics, a Fabry-Perot interferometer or etalon is typically made of a transparent plate with two reflecting surfaces, or two parallel highly-reflecting mirrors. Its transmission spectrum as a function of wavelength exhibits peaks of large transmission cavity). Since light is a wave, when bouncing between the mirrors of the cavity the light will constructively and destructively interfere with itself, leading to the formation of standing waves between the mirrors.
These standing waves form a discrete set of frequencies, known as the longitudinal modes of the cavity. These modes are the only frequencies of light which are self-regenerating and allowed to oscillate by the resonant cavity; all other frequencies of light are suppressed by destructive interference. For a simple plane-mirror cavity, the allowed modes are those for which the separation distance of the mirrors L is an exact multiple of half the wavelength of the light λ, such that L = q λ/2, when q is an integer known as the mode order.
In practice, the separation distance of the mirrors L is usually much greater than the wavelength of light λ, so the relevant values of q are large (around 105 to 106). Of more interest is the frequency separation between any two adjacent modes q and q+1; this is given (for an empty linear resonator of length L) by Δν:
where c is the speed of light.
Using the above equation, a small laser with a mirror separation of 30 cm has a frequency separation between longitudinal modes of 0.5 GHz. Thus for the two lasers referenced above, with a 30 cm cavity the 1.5 GHz bandwidth of the HeNe laser would support up to 3 longitudinal modes, whereas the 128 THz bandwidth of the Ti:sapphire laser could support approximately 250000 modes.
Each individual longitudinal mode has itself some bandwidth or narrow range of frequencies over which it operates, but typically this bandwidth is much smaller than the inter-mode frequency separation.
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