The laser is a light source that exhibits unique properties. Since the first demonstration of a ruby laser by Maiman in 1960, there has been a phenomenal development in the field of lasers. Lasers dominate our modern world in a variety of forms ranging from tiny diode lasers in all CD and DVD players to large industrial lasers, used extensively for cutting and welding, e.g. in automobile manufacturing. In fact, lasers are used in thousands of applications in every section of modern society, including consumer electronics, information and communications technology, entertainment, science and industry, the medical field and defence. The laser triggered the photonics revolution and is the foundation of modern photonics. Modern laser research involves fundamental laser physics, creative development of novel laser concepts and advanced experimental work and diagnosis. All this can lead to the development of new lasers, which will fulfil the requirements of current and future demands in science and industry. Unique properties of lasers
Lasers range in size from tiny diode lasers to large systems the size of a football field. All of these have three basic properties in common, which separate lasers from ordinary light sources: Monochromaticity: conventional light sources emit light consisting of a broad range of wavelengths (i.e. colours); a laser, on the other hand, emits only a very narrow range of wavelengths. Directionality: conventional light sources, like a light bulb, emit light in all directions, while lasers can emit light that spreads (‘diverges’) only very little with distance. However, all laser beams eventually diverge as they move through space. Coherence: Some consider coherence to be the most fundamental property of laser light, i.e. where all parts of the electromagnetic waves are in phase.
Principles of laser operation
The word ‘laser‘ is an acronym for light amplification by stimulated emission of radiation. It is worth noting that the term ‘light’ refers to a wide spectrum of wavelengths from soft x-ray, through ultraviolet (UV) and visible to far infrared (IR). The two most important basic principles for laser science are the quantum nature of light and a process called stimulated emission (both accredited to Einstein). The quantum nature of light refers to light quantised into discrete energy portions, nowadays called photons. Basic components of a laser
A laser basically consists of three parts: a resonant optical cavity called the optical resonator, a laser gain medium (also called active laser medium) and a pump source to excite the particles in the gain medium. The optical resonator consists of at least two mirrors between which the light bounces up and down resonantly. In most cases, one or more mirrors are curved, so that a resonant optical mode forms. This mode defines the laser beam. Modern dielectric mirrors used in lasers typically have a reflectivity of up to 99,9%. However, one of the end mirrors is usually only partially reflective, so that a portion of the light is transmitted. This mirror is called the output
coupler. The transmitted part forms the laser output. In order to operate, the laser requires a gain medium in the resonator, which amplifies light and thus compensates for the loss through the output coupler. Lasers are typically classified by the type of gain medium they employ (gas laser, solid-state laser, dye laser, semiconductor laser, etc.). The stimulated emission process takes place in the gain medium. The gain medium amplifies light of any direction. However, only the light that bounces up and down between the resonator mirrors is amplified many times and therefore reaches a high intensity. In a continuous wave (CW) laser, the gain in the laser gain medium and the loss from the output coupler plus other losses are in equilibrium. The fact that the photon energy has to match a given energy transition makes the laser monochromatic. Since the amplification process maintains the phase and direction of the light, the laser output is directional and coherent. The active particles in the laser gain medium need to be in a state of inversion for the laser to operate. To reach this state requires some pumping process, which lifts them into the required energy state. Typical pumping processes are electrical current in a gas or semiconductor laser or optical pumping in a solid-state or dye laser. Optical pumping is
typically achieved either by flash lamps or by another laser.
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