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Laser (Wikipedia), how lasers work, quantum properties of light, lasers (Photonics Encyclopedia), lasers (Colorado Physics), photonics, photonics

Chart of the Electromagnetic Spectrum

- Photonics - a comparatively recent term used to identify the scientific and technological domain fundamentally related to photons, radiation sources, lasers, interaction of laser radiation with matter, and so on. No accurate and universally accepted definition of this term is known today (and will scarcely be developed in the future). Therefore, term "photonics" is often understood as broadly as, for example, "electronics" with the corresponding substitution of photons for electrons.

- Optical Pumping - the process of energy transfer from a source to the active medium of a laser. Energy may be channeled by means of laser radiation in the case of laser pumping sources, by means of electric discharge in gases for gas-based lasers, etc. Pumping elevates the laser active medium from the state of thermal equilibrium into the so-called active, or excited, state with population inversion (generally characterised by certain higher-energy levels populated more than lower ones). Pumping can be done both in continuous and pulsed modes. The most widely used types of laser pumping at this moment involve semiconductor lasers (laser diodes), solid-state, or fibre lasers. Solid-state and fibre lasers are also, in their turn, pumped with laser diodes. Pumping of laser diodes is done, mainly, by direct electric current injection into the p-n transition of the laser element. In other cases activation of the semiconductor laser medium is done by an optical or an electron beam.

- Titanium-sapphire (Ti:Sa) laser - solid-state laser, in which a sapphire (Al2O3) single crystal containing a certain amount of titanium-ion impurities (Ti3 ) is used as its active medium. Such lasers feature very broad working spectral range (690-1100 nm). Ti:Sa laser requires powerful optical pumping in the visible green spectrum range. An Nd:YVO4 laser with frequency doubling may be used as a pumping source for titanium-sapphire lasers. See also: Ti:Sapphire laser, Titanium-Sapphire Lasers, CW single-frequency Ti:Sapphire laser

- Dye laser - liquid-based laser, in which a solution of an organic dye formed into a fast jet acts as the active medium. Working spectral range of such laser using one dye solution amounts to 50-100 nm, however the whole visible spectrum range can be covered by using different dyes. The most common spectral ranges of dye jet lasers are 550-700 nm and 275-350 nm, the latter being accessible through extra- or intra-cavity frequency doubling of the fundamental output radiation of the dye laser. Modern manufacturing technology and design of dye jet lasers provide almost the same level of ease and comfort of operation as with solid-state lasers. See also: Dye laser, Dye Lasers, dye laser, CW single-frequency Dye laser

- Single-frequency laser - laser whose output radiation spectrum contains only one frequency. Line width of radiation from such laser is determined by instabilities of the generation frequency value arising because of various external or internal perturbations of the laser resonator (mechanical, acoustic, etc.). Line width of output from a single-frequency laser can be significantly (by one, two, and more orders of magnitude) reduced by using a system of stabilisation of the laser generation frequency. Such a system must include a stable spectral reference, such as a transmission peak of an optical interferometer, atomic absorption line, etc. onto which the laser output radiation is locked. Normally, stabilisation of the laser generation frequency reduces both short-term frequency jitter and its long-term drift. See also: single-frequency lasers, single-frequency tunable laser systems

- Single-mode laser - a laser running only the lowest transverse electromagnetic mode (fundamental or TEM00) of its optical resonator. Sometimes the term "single-mode laser" is also applied to devices generating only one longitudinal electromagnetic mode of the resonant cavity, however this is not entirely correct. A single-frequency laser necessarily always operates in single mode, however a single-mode laser only running one transverse cavity mode may, at the same time, generate several longitudinal modes, correspondingly being a multi-frequency laser. Transverse intensity distribution within the beam of a single-mode lasers is normally close to Gaussian.

- Selection of laser radiation wavelength - isolation of a narrow radiation line within a laser out of a broad generation (or gain) spectrum. With the help of radiation wavelength selection it may be possible to reduce the width of the output radiation line and also to shift, or detune, the output line within the gain spectral band of the laser active medium. In widely tuneable lasers the radiation wavelength is done by one or several (2-3) spectrally selective optical elements with different degree of selectivity. For preliminary spectral selection of laser radiation the coarsest element is used (for instance, a birefringent filter in CW lasers or a diffraction grating/prism in pulsed lasers). For more refined spectral isolation mostly Fabry-Perot etalons are used. For smooth continuous (or quasi-continuous) detuning of the laser radiation line, synchronous continuous alignment of the used spectrally selective elements is necessary.

- Tuneable laser - a laser whose output radiation wavelength can be changed within a certain spectral range exceeding considerably the width of the laser generation line. It is far from every type of lasers that possess this possibility. However tuneable lasers may be based on solid-state, liquid, fibre, semiconductor, hybrid, and other media. The largest tuneable ranges are available from solid-state lasers: Ti:Sapphire laser (690-1100 nm), Cr:ZnSe laser (1970-2760 nm), Fe:ZnSe laser (3950-5050 nm), and liquid-based dye-jet lases (400-700 nm). Considerable progress made recently in the development of highly efficient non-linear crystals has brought optical parametric oscillators pumped by fixed-wavelength lasers as an alternative to tuneable lasers in a number of applications. The output wavelength of such oscillator is adjusted by changing the orientation of the non-linear crystal or its temperature.

- Active and passive stabilisation of the laser generation frequency - methods of stabilisation aimed at narrowing of the laser radiation line width and at reduction of output line spectral drift over a long time. In passive stabilisation techniques this is achieved by choosing laser cavity materials with low thermal expansion coefficient (such as Invar, pyroceramics, quartz, etc.), isolation of the laser resonator from mechanical vibration by using materials with good mechanical and acoustic damping properties (rubber, foam plastic, and others). Line width of the output from passively stabilised single-frequency lasers usually amounts to several MHz. In systems with active stabilisation of the generation frequency, an external frequency reference is used (e.g. interferometer transmission peak, atomic or molecular absorption line, etc.), onto which the laser output frequency is locked with the help of an electronic control system. Line width of an actively-stabilised single-frequency laser normally does not exceed 1 MHz, and in high-end advanced systems may be brought down to several kHz or even Hz.

- Computer-controlled tuneable laser system - laser apparatus, in which the wavelength of output radiation can be changed by a computer in a broad range according to a user-specified program. The output wavelength is controlled through spectrally selective optical elements with electro-mechanical drive installed in the laser cavity. Control signals from the computer are fed into these drives through a special hardware interface. For laser wavelength measurement in such systems usually a high-precision fast wavelength meter is included. See also: tunable laser, tunable lasers (book), tunable lasers, autoscanned ultra-wide-tunable laser system

- Resonant frequency doubler for continuous-wave (CW) single-frequency lasers - laser accessory for doubling of optical radiation frequency by use of a non-linear crystal installed in an independent (external) high-Q optical resonator. Second-harmonic generation efficiency of such doublers is fairly high for CW radiation and reaches 25-40% at the input radiation power of 1 W. Resonant frequency doubler is a very useful addition to a CW single-frequency laser, allowing efficient conversion of its radiation into a shorter-wavelength range. See also: second harmonic generation, frequency doubling, resonant frequency doubler

- Non-linear crystal - an artificial (as a rule) optical crystal in which non-linear conversion of radiation wavelength is possible with acceptable efficiency-second- and third-harmonic generation or generation of sum and difference frequencies. Such conversion is possible because of a non-linear response of atomic oscillators to powerful light field-anharmonicity of the atomic oscillator leads to polarisation of the optical medium varying at frequencies (or wavelengths) different from those of the incident light. Energy transfer from the incident light wave (or waves) to optical oscillations at other frequencies (harmonics or waves with sum/difference frequency) is optimal when phase velocities of interacting waves are equal. This particular case is usually called the condition of phase matching.

- Femtosecond laser - laser generating ultra-short light pulses. As a rule, such pulses are generated in mode-locked regime at a high repetition rate in the range of 1 MHz - 3 GHz. Femtosecond pulses with durations of dozens and hundreds of femtoseconds can be available both from solid-state and all-fibre lasers, as well as from hybrid fibre/bulk laser systems. Owing to relatively high peak output power of femtosecond pulses they can be quite easily spectrally transformed with different methods of non-linear optics (parametric generation, second-harmonic generation, etc.). See also: femtosecond lasers, ultrashort pulse, femtosecond lasers, femtosecond Ti:Sapphire laser,

- Super-continuum - coherent optical radiation with ultra-broad spectrum. The spectrum of super-continuum may cover an optical spectrum octave or even more (a spectrum octave is a wavelength range extending from a given value to its double, for instance, from 400 to 800 nm, etc.). Super-continuum is generated in different optical media under impact of powerful laser pulses or continuous radiation. Special optical fibres (PCF / microstructured, tapered) are the most widely used type of medium for super-continuum generation.

- Fibre laser - a laser whose resonant cavity is formed by an optical fibre. Radiation of such laser is entirely or partially generated within this fibre. In implementations where the radiation path is entirely contained in the optical fibre are usually called all-fibre designs. In cases where a part of the radiation path goes through free space or optical elements other than fibres such lasers are called hybrid or fibre/bulk. Fibre lasers are suitable for both generation of continuous radiation and ultra-short (femtosecond, picosecond) light pulses. See also: fiber laser, how fibre lasers work, fiber lasers, fiber femtosecond laser,

- Optical telecommunications - systems for high-speed data transfer based on transmission of short light pulses inside optical fibres. Optical fibre lines are, to date, the most advanced means of massive data transfer. Optical fibre cables are laid underground, alond the ocean floor, are drawn together with the electric power lines, etc. Information transmitted along an optical fibre is not sensitive to external interference and difficult to tap. Lasers (semiconductor - and fibre-based) are used as light sources for optical fibre lines. In order to maintain signal strength along large distances (50-100 km) laser-pumped optical amplifiers are installed.

- Laser cutting - a technology of material destruction by focused powerful laser beam. Such beam locally brings the material to its melting or even boiling temperature at which the material is rapidly evaporated, oxidised, or blown out of the cutting zone by forced stream of gas. Laser cutting technology allows precise cutting of sheet metal or other materials, pipes, and other volumetric parts. Laser cutting is currently one of the major technologies for cut-out of automotive body parts. By changing parameters of the laser radiation it is possible to adjust the heating impact of the radiation on the material and to achieve other ways of material processing such as laser welding, marking, engraving, laser drilling or laser tempering.

- Laser TV - an optical projection system in which colour images are formed with the help of light from three laser sources; red, green, and blue. Because of recent advances in laser technologies and miniaturisation of laser devices it will become possible in the foreseeable future to build ultra-compact projection TV units even into hand-held accessories, such as cellular phones. See also: ultra-compact laser projectors

- Application Fields of Lasers Developed in Laser Systems Laboratory:
atom cooling, trapping, and quantum manipulation, high resolution spectroscopy, Bose-Einstein condensation, atom trap trace analysis, atomic clocks, formation of cold molecules, laser isotope separation, nano-structure fabrication, wide-range spectral studies of quantum semiconductor objects and nano-strustured materials, nano-scale atomic lithography, material processing, new technologies of high-density information recording, ultrafast spectroscopy, nano-optics, THz applications, seeding an amplifier system. See also: laser applications, lasers for nanotechnologies

- Reviews, popular papers:
A.Galvanauskas. High Power Fiber Lasers.

- Supplementary info:
laser adventure, optical parametric generation and amplification, quantum optics and atom optics links, nonlinear optics, notes for modern and nonlinear optics, photonics crystals, optical negative-index metamaterials

- Popular Laser Topics:
laser pointer, laser cutting,

- Laser & Photonics e-books:
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D.Meschede. Optics, Light and Lasers. The Practical Approach to Modern Aspects of Photonics and Laser Physics ... >>>
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M.Young. Optics and Lasers including Fibers and Optical Waveguides ... >>>
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C.Ye. Tunable external cavity diode lasers ... >>>

H.Metcalf, P.Straten. Laser Cooling and Trapping ... >>>
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J.Crisp, B.Elliott. Introduction To Fiber Optics ... >>>
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