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Nitrogen laser

Nitrogen laser

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A nitrogen laser is a gas laser
Gas laser
A gas laser is a laser in which an electric current is discharged through a gas to produce coherent light. The gas laser was the first continuous-light laser and the first laser to operate "on the principle of converting electrical energy to a laser light output...

 operating in the ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 range (typically 337.1 nm) using molecular nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

 as its gain medium, pumped
Laser pumping
Laser pumping is the act of energy transfer from an external source into the gain medium of a laser. The energy is absorbed in the medium, producing excited states in its atoms. When the number of particles in one excited state exceeds the number of particles in the ground state or a less-excited...

 by an electrical discharge.

The wall-plug efficiency
Wall-plug efficiency
In optics, wall-plug efficiency or radiant efficiency is the energy conversion efficiency with which the system converts electrical power into optical power...

 of the nitrogen laser is low, typically 0.1% or less, though nitrogen lasers with efficiency of up to 3% have been reported in the literature. The wall-plug efficiency is the product of the following three efficiencies:
  • electrical: TEA laser
    TEA laser
    A TEA laser is a transversely-excited atmospheric-pressure laser.-Invention:The CO2 TEA laser was invented in the late 1960s by Dr Jacques Beaulieu working at the Defence Research Establishment, Valcartier, in Quebec, Canada...

  • gain medium: This is the same for all nitrogen lasers and thus has to be at least 3%
    • inversion by electron impact is 10 to 1 due to Franck-Condon principle
      Franck-Condon principle
      The Franck–Condon principle is a rule in spectroscopy and quantum chemistry that explains the intensity of vibronic transitions. Vibronic transitions are the simultaneous changes in electronic and vibrational energy levels of a molecule due to the absorption or emission of a photon of the...

    • energy lost in the lower laser level: 40 %
  • optical: More induced emission than spontaneous emission

Gain medium


The gain medium is nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

 molecules in the gas phase. The nitrogen laser is a 3-level laser. In contrast to a ruby laser
Ruby laser
A ruby laser is a solid-state laser that uses a synthetic ruby crystal as its gain medium. The first working laser was a ruby laser made by Theodore H. "Ted" Maiman at Hughes Research Laboratories on May 16, 1960....

 or to other more typical 4-level lasers, the upper laser level of nitrogen is directly pumped
Laser pumping
Laser pumping is the act of energy transfer from an external source into the gain medium of a laser. The energy is absorbed in the medium, producing excited states in its atoms. When the number of particles in one excited state exceeds the number of particles in the ground state or a less-excited...

, imposing no speed limits on the pump. Pumping is normally provided by direct electron impact; the electrons must have sufficient energy, or they will fail to excite the upper laser level. Typically reported optimum values are in the range of 80 to 100 eV per Torr·cm pressure of nitrogen gas.

There is a 40 ns upper limit of laser lifetime at low pressures and the lifetime becomes shorter as the pressure increases. The lifetime is only 1 to 2 ns at 1 atmosphere. In general


The strongest lines are at 337.1 nm *wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

 in the ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

. Other lines have been reported at 357.6 nm, also ultraviolet. This information refers to the second positive system of molecular nitrogen, which is by far the most common. No vibration
Vibronic coupling
In theoretical chemistry, the vibronic coupling terms, , are proportional to the interaction between electronic and nuclear motions of molecules. The term "vibronic" originates from the concatenation of the terms "vibrational" and "electronic"...

 of the two nitrogen atoms is involved, because the atom-atom distance does not change with the electronic transition. The rotation
Rovibronic coupling
Rovibronic coupling denotes the simultaneous interactions between rotational, vibrational, and electronic degrees of freedom in a molecule. When a rovibronic transition occurs, the rotational, vibrational, and electronic states change simultaneously, unlike in rovibrational coupling...

 needs to change to deliver the angular momentum
Angular momentum
In physics, angular momentum, moment of momentum, or rotational momentum is a conserved vector quantity that can be used to describe the overall state of a physical system...

 of the photon, furthermore multiple rotational states are populated at room temperature. There are also lines in the far-red and infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

 from the first positive system, and a visible blue laser line from the molecular nitrogen positive (1+) ion.

The metastable lower level lifetime is 40 μs, thus, the laser self-terminates, typically in less than 20 ns. This type of self-termination is sometimes referred to as “bottlenecking in the lower level”.
This is only a rule of thumb as is seen in many other lasers:
The helium-neon laser
Helium-neon laser
A helium–neon laser or HeNe laser, is a type of gas laser whose gain medium consists of a mixture of helium and neon inside of a small bore capillary tube, usually excited by a DC electrical discharge.- History of HeNe laser development:...

 also has a bottleneck as one decay step needs the walls of the cavity and this laser typically runs in continuous mode.
Several organic dyes with upper level lifetimes of less than 10 ns have been used in continuous mode.
The Nd:YAG laser
Nd:YAG laser
Nd:YAG is a crystal that is used as a lasing medium for solid-state lasers. The dopant, triply ionized neodymium, typically replaces yttrium in the crystal structure of the yttrium aluminium garnet , since they are of similar size...

 has an upper level lifetime of 230 µs, yet it also supports 100 ps pulses.

Repetition rates can range as high as a few kHz, provided adequate gas flow and cooling of the structure are provided. Cold nitrogen is a better medium than hot nitrogen, and this appears to be part of the reason that the pulse energy and power drop as the repetition rate increases to more than a few pulses per second. There are also, apparently, issues involving ions remaining in the laser channel.

Air, which is 78% nitrogen, can be used, but more than 0.5% oxygen poisons the laser.

Optics


Nitrogen lasers can operate within a resonator cavity,
but due to the typical gain of 2 every 20 mm they more often operate
on superluminescence alone;
though it is common to put a mirror at one end such that the output is emitted from the opposite end.

For a 10 mm wide gain volume diffraction
Diffraction
Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...

 comes into play after 30 m along the gain medium,
a length which is unheard of.
Thus this laser does not need a concave lens or refocusing lenses and beam quality improves along the gain medium.
The height of the pumped volume may be as small as 1 mm, needing a refocusing lens already after 0.3 m.
A simple solution is to use rounded electrodes with a large radius, so that a quadratic pump profile is obtained.

Electrical


The gain medium is usually pumped by a transverse electrical discharge. When the pressure is at (or above) 1013 mbar (atmospheric pressure
Atmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...

), the configuration is called a TEA laser Transverse Electrical discharge in gas at Atmospheric pressure
TEA laser
A TEA laser is a transversely-excited atmospheric-pressure laser.-Invention:The CO2 TEA laser was invented in the late 1960s by Dr Jacques Beaulieu working at the Defence Research Establishment, Valcartier, in Quebec, Canada...

, this is also used for pressures down to 30 mbar.

Microscopic description of a fast discharge


In a strong external electric field this electron creates an electron avalanche
Electron avalanche
An electron avalanche is a process in which a number of free electrons in a medium are subjected to strong acceleration by an electric field, ionizing the medium's atoms by collision , thereby forming "new" electrons to undergo the same process in successive cycles...

 in the direction of the electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...

 lines.
Diffusion
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...

 of electrons and elastic scattering
Elastic scattering
In scattering theory and in particular in particle physics, elastic scattering is one of the specific forms of scattering. In this process, the kinetic energy of the incident particles is conserved, only their direction of propagation is modified .-Electron elastic scattering:When an alpha particle...

 at a buffer gas
Buffer gas
A buffer gas is an inert or nonflammable gas. In the Earth's atmosphere, nitrogen acts as a buffer gas. A buffer gas adds pressure to a system and controls the speed of combustion with any oxygen present...

 molecule spreads the avalanche perpendicular to the field.
Inelastic scattering
Inelastic scattering
In particle physics and chemistry, inelastic scattering is a fundamental scattering process in which the kinetic energy of an incident particle is not conserved . In an inelastic scattering process, some of the energy of the incident particle is lost or gained...

 creates photons, which create new avalanches centimeters away.
After some time the electric charge in the avalanche becomes so large that following Coulomb's law
Coulomb's law
Coulomb's law or Coulomb's inverse-square law, is a law of physics describing the electrostatic interaction between electrically charged particles. It was first published in 1785 by French physicist Charles Augustin de Coulomb and was essential to the development of the theory of electromagnetism...

 it generates an electric field as large
as the external electric field. At regions of increased field strength the avalanche effect is enhanced.
This leads to electric arc
Electric arc
An electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on...

 like discharges called streamers.
A mix of a noble gas
Noble gas
The noble gases are a group of chemical elements with very similar properties: under standard conditions, they are all odorless, colorless, monatomic gases, with very low chemical reactivity...

 (up to 0.9) and nitrogen enhance elastic scattering of electrons over electron multiplying and
thus widens avalanches and streamers.

Spark gap
Spark gap
A spark gap consists of an arrangement of two conducting electrodes separated by a gap usually filled with a gas such as air, designed to allow an electric spark to pass between the conductors. When the voltage difference between the conductors exceeds the gap's breakdown voltage, a spark forms,...

s use a high density of gas molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

s and a low density of initial electrons to favor streamers.
Electrons are removed by a slowly rising voltage.
A high density gas increases the breakdown field, thus shorter arcs can be used with lower inductance and the capacity between the electrodes is increased. A wide streamer has a lower inductance.

Gas laser
Gas laser
A gas laser is a laser in which an electric current is discharged through a gas to produce coherent light. The gas laser was the first continuous-light laser and the first laser to operate "on the principle of converting electrical energy to a laser light output...

s use low density of gas molecules and a high density of initial electrons to prevent streamers.
Electrons are added by preionisation not removed by oxygen, because nitrogen from bottles is used. Wide avalanches can excite more nitrogen molecules.

Inelastic scattering heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...

s up a molecule, so that in a second scattering the probability of electron emission is increased.
This leads to an arc. Typically arcing occurs after lasing in nitrogen.
The streamer in the spark gap discharges the electrodes only by means of image charge, thus when the streamer touches
both electrodes most of the charge is still available to feed the arc, additional charge is stored on the distribution plates. Thus arcing in the spark gap starts before lasing.

Conditions for pulsed avalanche discharges are described by Levatter and Lin.

Electrodynamics




The electronics is a circuit composed of a spark gap
Spark gap
A spark gap consists of an arrangement of two conducting electrodes separated by a gap usually filled with a gas such as air, designed to allow an electric spark to pass between the conductors. When the voltage difference between the conductors exceeds the gap's breakdown voltage, a spark forms,...

, a capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...

, and the discharge through the nitrogen.
First the spark gap and the capacitor are charged.
The spark gap then discharges itself and voltage is applied to the nitrogen.

An alternative construction uses two capacitors connected as a Blumlein
Alan Blumlein
Alan Dower Blumlein was a British electronics engineer, notable for his many inventions in telecommunications, sound recording, stereo, television and radar...

 generator. Two capacitors are connected so that one plate is a common earth, the others are each connected to the spark gap electrodes. These capacitors are often constructed from a single layer of printed circuit board, or similar stack of copper foil and thin dielectric. The capacitors are linked through an inductor, a simple air-spaced coil. One capacitor also has a small spark gap across it. When HT is applied, the two capacitors are charged slowly, effectively linked by the inductor. When the spark gap reaches its triggering voltage, it discharges and quickly reduces that capacitor's voltage to zero. As the discharge is rapid, the inductor acts as an open circuit and so the voltage difference across the transverse spark gap (between the two capacitors) rises rapidly until the main spark gap discharges, firing the laser.

The speed of either circuit is increased in two steps.
Firstly, the inductance
Inductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...

 of all components is reduced by shortening and widening conductors and by squeezing the circuit into a flat rectangle. The total inductance is the sum of the components:
object length thickness width width inductance inductance inductance capacity oscillation
as coil as wire measured coil theory wire theory plate theory period
unit m m m m nH nH nH nF ns
spark gap 2E-02 1E-02 2E-02 1E-05 10 12.57 13.70 0.0004
metal tape 2E-02 2E-02 4E-02 5E-03 12.57 5.32 0.0004
cap. 1 2E-01 4E-04 3E-01 0.34 2.6563
metal tape 2E-02 2E-02 3E-01 1.68 0.0027
laser channel 1E-02 2E-02 3E-01 0.84 0.0013
metal tape 2E-02 2E-02 3E-01 1.68 0.0027
cap. 2 3E-01 4E-04 3E-01 0.50 3.9844
spark osc. 22.90 2.6563 49
disch. osc. 5.03 1.5938 18


The intense discharge is reported to distort oscilloscopes nearby.
This can be reduced by building the laser symmetrically into a grounded cylinder with the spark gap at the bottom, the laser at the top,
capacitor 1 left and right, and capacitor 2 left and right stacked onto capacitor 1.
This has the further advantage of reducing the inductance.
And this has the disadvantage that the laser channel cannot be inspected for sparks anymore.

Secondly, transmission line
Transmission line
In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...

 theory and waveguide
Waveguide
A waveguide is a structure which guides waves, such as electromagnetic waves or sound waves. There are different types of waveguides for each type of wave...

 theory is applied to achieve a traveling wave excitation.
Measured nitrogen laser pulses are so long that the second step is unimportant.
From this analysis it follows that:
  • the end mirror and the spark gap are on the same side
  • a long narrow laser at atmospheric pressures is ineffective

Spark gap


Paschen's law
Paschen's law
Paschen's Law, named after Friedrich Paschen, was first stated in 1889. He studied the breakdown voltage of gas between parallel plates as a function of pressure and gap distance. The voltage necessary to arc across the gap decreased up to a point as the pressure was reduced. It then increased,...

 states that the length of the spark gap is inverse-proportional to the pressure.
For a fixed length to diameter ratio of the spark, the inductance
Inductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...

 is proportional to the length
(source http://www.consultrsr.com/resources/eis/induct5.htm, compare with: dipole antenna
Dipole antenna
A dipole antenna is a radio antenna that can be made of a simple wire, with a center-fed driven element. It consists of two metal conductors of rod or wire, oriented parallel and collinear with each other , with a small space between them. The radio frequency voltage is applied to the antenna at...

 ).
Thus the electrodes of the spark gap are glued or welded on a dielectric spacer-ring.
To reduce the danger due to the pressure, the volume is minimized.
To prevent sparks outside space ring in the low pressure the spacer usually gets thicker outwards in an s-shaped manner.

Connection between spark gap and laser channel based on traveling wave theory:
  • The low inductance spark gap
    Spark gap
    A spark gap consists of an arrangement of two conducting electrodes separated by a gap usually filled with a gas such as air, designed to allow an electric spark to pass between the conductors. When the voltage difference between the conductors exceeds the gap's breakdown voltage, a spark forms,...

     may be inserted into a strip transmission line
  • biconical spark gap
  • biconical spark gap
  • biconical spark gap


The breakdown voltage is low for helium, medium for nitrogen and high for SF6 (http://www.freepatentsonline.com/4237404.html),
though nothing is said about the spark thickness variations.

8E10A/s are possible with a spark gap (http://www.springerlink.com/content/r456v73254ul7758/fulltext.pdf)
this nicely matches the typical rise times of 1E-8s and typical currents of 1E3A occurring in nitrogen lasers.

A cascade of spark gaps allows to use a weak trigger pulse to initiate
a streamer in the smaller gap, wait for its transition into in arc, and then for this arc to extend into the larger gap
(http://home.earthlink.net/~jimlux/hv/hvtrigsg.htm).
Still the first spark gap in the cascade needs a free electron to start with, so jitter is rather high.

Preionisation


Avalanches homogenize a discharge fast mostly along the field lines. With a short duration (<10 ms) since the last laser pulse
enough ions are left over so that all avalanches overlap also laterally. With low pressure (<100 kPa) the max charge carrier density is low
and the electromagnetic driven transition from avalanche to spark is inhibited.

In other cases
UV radiation homogenizes a discharge slowly perpendicular to a discharge.
These are brought into balance by placing two linear discharges next to each other 1 cm apart.
The first discharge is across a smaller gap and starts early.
Due to the low number of initial electrons streamers typically 1 mm apart are seen.
The electrodes for the first discharge are covered by a dielectric, which limits this discharge.
Therefore the voltage is able to rise further until avalanches can start in the second gap.
These are so many that they overlap and excite every molecule.

With about 11 ns the UV generation, ionisation, and electron capture are in a similar speed regime as the nitrogen laser pulse duration
and thus as fast electric must be applied.

Excitation by electron impact


The upper laser level is excited efficiently by electrons with more than 11 eV, best energy is 15 eV.
The electron temperature in the streamers only reaches 0.7 eV. Helium due to its higher ionisation energy and
lack of vibrational excitations increases the temperature to 2.2 eV.
Higher voltages increase the temperature. Higher voltages mean shorter pulses.

[9] Characteristics of a wire preionized Nitrogen Laser with Helium as Buffer gas. Appl. Phys. B 35, 131-133

Typical devices


The gas pressure in a nitrogen laser ranges from a few mbar to as much as several bar. Furthermore, air provides significantly less output energy than pure nitrogen or a mixture of nitrogen and helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

. The pulse energy ranges from µJ
Joule
The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...

 to mJ (a commercial version using a spark gap delivers 300 microJ) and a peak power in the range of kW to more than 3 MW can be achieved. The pulse temporal width is between a few hundred picoseconds (typically at 1 atmosphere partial pressure
Partial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....

 of nitrogen) and a maximum of approximately 30 nanoseconds at reduced pressure (typically some dozens of Torr), though fwhm pulsewidths of 6 to 8 ns are typical.

Amateur construction


The transverse discharge nitrogen laser has long been a popular choice for amateur home construction, owing to its simple construction and simple gas handling. It was described by Scientific American
Scientific American
Scientific American is a popular science magazine. It is notable for its long history of presenting science monthly to an educated but not necessarily scientific public, through its careful attention to the clarity of its text as well as the quality of its specially commissioned color graphics...

in 1974, as one of the first laser home-construction articles.

Applications

  • Transverse optical pumping of dye laser
    Dye laser
    A dye laser is a laser which uses an organic dye as the lasing medium, usually as a liquid solution. Compared to gases and most solid state lasing media, a dye can usually be used for a much wider range of wavelengths. The wide bandwidth makes them particularly suitable for tunable lasers and...

    s
  • measurement of air pollution
    Air pollution
    Air pollution is the introduction of chemicals, particulate matter, or biological materials that cause harm or discomfort to humans or other living organisms, or cause damage to the natural environment or built environment, into the atmosphere....

     (Lidar
    LIDAR
    LIDAR is an optical remote sensing technology that can measure the distance to, or other properties of a target by illuminating the target with light, often using pulses from a laser...

    )
  • Matrix-assisted laser desorption/ionization
    Matrix-assisted laser desorption/ionization
    Matrix-assisted laser desorption/ionization is a soft ionization technique used in mass spectrometry, allowing the analysis of biomolecules and large organic molecules , which tend to be fragile and fragment when ionized by more conventional ionization methods...


External links