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,
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