TEA laser
Encyclopedia
A TEA laser is a transversely-excited atmospheric-pressure laser
.
working at the Defence Research Establishment, Valcartier, in Quebec
, Canada
. The development was kept secret until 1970 when brief details were published.
C K N Patel
, working at the Bell Telephone Laboratories in 1963, first demonstrated laser output at 10.6 µm from a low pressure RF excited CO2 gas discharge. With the addition of nitrogen and helium and using a DC electrical discharge, CW powers of around 100 W were achieved. By pulsing the discharge using higher voltages or Q-switching
using a rotating mirror, pulse powers of a few kilowatts could be obtained, but this was the practical limit.
Higher peak powers could only be achieved by increasing the density of excited CO2 molecules. The capacity for stored energy per unit volume of gas increases linearly with density and thus gas pressure, but the voltage needed to achieve gas breakdown and couple energy into the upper laser levels, increases at the same rate. The practical solution avoiding very high voltages was to pulse the voltage transversely to the optical axis (rather than longitudinally as was the case for low pressure lasers), limiting the breakdown distance to a few centimetres. This allowed the use of manageable voltages of a few tens of kV. The problem was how to initiate and stabilize a glow discharge at these much higher gas pressures, without the discharge degenerating into a bright high-current arc, and how to achieve this over a useful volume of gas.
provided the high voltage pulses.
These first “Pin-Bar” TEA lasers, operating at around one pulse per second, were easy and cheap to construct. By operating at atmospheric pressure, complex vacuum and gas-handling systems could be avoided. They could produce MW peak powers of a few 100 ns
duration capable of breaking down air if brought to a focus with a short focal-length lens. Disadvantages were poor gain symmetry, dissipation in the resistors and size.
Pearson and Lamberton used a streak-camera to verify the sequence of events. As the voltage erected across the electrodes, field emission from the thin wire resulted in a sheet discharge between itself and the anode. Since the subsequent main discharge started from the cathode, it was suggested that photoemission was the initiating mechanism. Subsequently, other workers demonstrated alternative methods for achieving preionisation. These included dielectrically isolated wires and electrodes, sliding spark arrays, electron beams and pins impedance-loaded with capacitors.
The original Pearson-Lamberton TEA laser could be operated at around one pulse per second when switched with a spark gap discharging a capacitor resistively charged from a DC power supply. By circulating the gas between the electrodes, using lossless capacitor charging and replacing the spark-gap with a thyratron, repetition rates in excess of a thousand pulses
per second were subsequently achieved with various designs of TEA laser.
, and these devices too can be referred to as TEA lasers. Commercial excimer laser
s operating in the ultraviolet use a double-discharge regime very similar to the CO2 TEA laser. Using krypton
, argon
or xenon
chloride or fluoride gas buffered with helium
to 2–3 atmospheres of pressure, Excimer lasers can produce megawatt pulses of ultraviolet laser light.
Beside this first TEA CO2 Lasers are furthermore used for surface preparation in industrial environments since the mid 90’s. Applications are:
• the selective or complete paint stripping, known as selective laser coating removal (SLCR) in the field of Aircraft maintenance or repair; this selective stripping process was approved in 2001 as the first laser stripping process by OEM’s and Aircraft maintenance centres;
• the activation or cleaning of surfaces for painting and gluing;
• the removal of contamination or coating layers, as preparation for bonding or welding;
• wear free cleaning of moulds and tools, e.g. tire moulds or moulds to produce skins for automotive interior parts;
The advantage of this specific laser is the combination of the CO2 specific wavelength, mainly 10.6µm, with the high energy level of the short pulses (~2µsec).
References:
1. C K N Patel, Interpretation of CO2 Optical Maser Experiments, Phys. Rev. Lett., 12, No 21, pp 588 – 590, May 1964
2. A J Beaulieu, Transversely Excited Atmospheric Pressure CO2 Lasers, Appl. Phys Lett., 16, No 12, pp 504 – 505, June 1970
3. P R Pearson and H M Lamberton, Atmospheric Pressure CO2 Lasers Giving High Output Energy per Unit Volume, IEEE J Quant Elec., 8, No2, pp 145 – 149, February 1972.
As the number of electrons increases Coulomb's law
states that also the field strength increases.
The strong field accelerates the avalanche.
A slow rise time of the voltage lets the electrons drift towards the anode before they can generate an avalanche.
Electrophilic molecules capture electrons before they can generate an avalanche.
Thermal effects destabilize a homogeneous discharge electron and ion diffusion stabilizes it.
Reference:
4. J I Levatter and S C Lin, Necessary conditions for the homogeneous formation of pulsed avalanche discharges at high gas pressures, J.Appl.Phys. 51, No2, pp 210 – 222, January 1980.
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
.
Invention
The CO2 TEA laser was invented in the late 1960s by Dr Jacques BeaulieuJacques Beaulieu
A. Jacques Beaulieu, is a Canadian physicist who invented the first transversely excited atmospheric carbon dioxide laser or TEA laser in 1968.In 1978, he was awarded the Royal Society of Canada's Thomas W. Eadie Medal...
working at the Defence Research Establishment, Valcartier, in Quebec
Quebec
Quebec or is a province in east-central Canada. It is the only Canadian province with a predominantly French-speaking population and the only one whose sole official language is French at the provincial level....
, Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
. The development was kept secret until 1970 when brief details were published.
C K N Patel
C. Kumar N. Patel
C. Kumar N. Patel developed the carbon dioxide laser in 1963; it is now widely used in industry for cutting and welding, as a laser scalpel in surgery, and in laser skin resurfacing...
, working at the Bell Telephone Laboratories in 1963, first demonstrated laser output at 10.6 µm from a low pressure RF excited CO2 gas discharge. With the addition of nitrogen and helium and using a DC electrical discharge, CW powers of around 100 W were achieved. By pulsing the discharge using higher voltages or Q-switching
Q-switching
Q-switching, sometimes known as giant pulse formation, is a technique by which a laser can be made to produce a pulsed output beam. The technique allows the production of light pulses with extremely high peak power, much higher than would be produced by the same laser if it were operating in a...
using a rotating mirror, pulse powers of a few kilowatts could be obtained, but this was the practical limit.
Higher peak powers could only be achieved by increasing the density of excited CO2 molecules. The capacity for stored energy per unit volume of gas increases linearly with density and thus gas pressure, but the voltage needed to achieve gas breakdown and couple energy into the upper laser levels, increases at the same rate. The practical solution avoiding very high voltages was to pulse the voltage transversely to the optical axis (rather than longitudinally as was the case for low pressure lasers), limiting the breakdown distance to a few centimetres. This allowed the use of manageable voltages of a few tens of kV. The problem was how to initiate and stabilize a glow discharge at these much higher gas pressures, without the discharge degenerating into a bright high-current arc, and how to achieve this over a useful volume of gas.
CO2 TEA laser
Beaulieu (in 1970) reported a transversely-excited atmospheric-pressure CO2 laser. His solution to the problem of arc formation was to have a conducting bar facing a linear array of pins with a separation of a few centimetres. The pins were individually loaded with resistors forcing the discharge from each pin into a low current brush or glow discharge which fanned out towards the bar. The laser cavity probed 100-200 of these discharges in series providing the laser gain. A fast discharge capacitor rapidly switched across the laser electrodes using a spark gap or thyratronThyratron
A thyratron is a type of gas filled tube used as a high energy electrical switch and controlled rectifier. Triode, tetrode and pentode variations of the thyratron have been manufactured in the past, though most are of the triode design...
provided the high voltage pulses.
These first “Pin-Bar” TEA lasers, operating at around one pulse per second, were easy and cheap to construct. By operating at atmospheric pressure, complex vacuum and gas-handling systems could be avoided. They could produce MW peak powers of a few 100 ns
Nanosecond
A nanosecond is one billionth of a second . One nanosecond is to one second as one second is to 31.7 years.The word nanosecond is formed by the prefix nano and the unit second. Its symbol is ns....
duration capable of breaking down air if brought to a focus with a short focal-length lens. Disadvantages were poor gain symmetry, dissipation in the resistors and size.
Pearson and Lamberton
The first true (non pin-bar) TEA laser was realised by Pearson and Lamberton working at the UK MOD Services Electronic Research Laboratory at Baldock. They used a pair of Rogowski-profiled electrodes separated by one or two centimetres. Their double-discharge design coupled part of the discharge energy to a thin wire running parallel to and offset from one side of the electrodes. This served to “preionise” the gas resulting in a uniform volumetric glow-discharge. Of equal importance to preionisation, was the need for the discharge to be very fast. By dumping energy into the gas rapidly, high-current arcs had no time to form.The original Pearson-Lamberton TEA laser could be operated at around one pulse per second when switched with a spark gap discharging a capacitor resistively charged from a DC power supply. By circulating the gas between the electrodes, using lossless capacitor charging and replacing the spark-gap with a thyratron, repetition rates in excess of a thousand pulses
per second were subsequently achieved with various designs of TEA laser.
'Double-discharge' method
The double-discharge method required to initiate stable high-pressure gas discharges can be used both below and above atmospheric pressureAtmospheric 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...
, and these devices too can be referred to as TEA lasers. Commercial excimer laser
Excimer laser
An excimer laser is a form of ultraviolet laser which is commonly used in the production of microelectronic devices , eye surgery, and micromachining....
s operating in the ultraviolet use a double-discharge regime very similar to the CO2 TEA laser. Using krypton
Krypton
Krypton is a chemical element with the symbol Kr and atomic number 36. It is a member of Group 18 and Period 4 elements. A colorless, odorless, tasteless noble gas, krypton occurs in trace amounts in the atmosphere, is isolated by fractionally distilling liquified air, and is often used with other...
, argon
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...
or xenon
Xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. The element name is pronounced or . A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...
chloride or fluoride gas buffered with 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...
to 2–3 atmospheres of pressure, Excimer lasers can produce megawatt pulses of ultraviolet laser light.
Uses
Currently, TEA CO2 lasers are used extensively for product marking. A logo, serial number or "best-before" date is marked on to a variety of packaging materials by passing the laser light through a mask containing the information, and focusing it down to an intensity which ablates the material to be marked.Beside this first TEA CO2 Lasers are furthermore used for surface preparation in industrial environments since the mid 90’s. Applications are:
• the selective or complete paint stripping, known as selective laser coating removal (SLCR) in the field of Aircraft maintenance or repair; this selective stripping process was approved in 2001 as the first laser stripping process by OEM’s and Aircraft maintenance centres;
• the activation or cleaning of surfaces for painting and gluing;
• the removal of contamination or coating layers, as preparation for bonding or welding;
• wear free cleaning of moulds and tools, e.g. tire moulds or moulds to produce skins for automotive interior parts;
The advantage of this specific laser is the combination of the CO2 specific wavelength, mainly 10.6µm, with the high energy level of the short pulses (~2µsec).
References:
1. C K N Patel, Interpretation of CO2 Optical Maser Experiments, Phys. Rev. Lett., 12, No 21, pp 588 – 590, May 1964
2. A J Beaulieu, Transversely Excited Atmospheric Pressure CO2 Lasers, Appl. Phys Lett., 16, No 12, pp 504 – 505, June 1970
3. P R Pearson and H M Lamberton, Atmospheric Pressure CO2 Lasers Giving High Output Energy per Unit Volume, IEEE J Quant Elec., 8, No2, pp 145 – 149, February 1972.
Microscopic description of the discharge
In most over-voltage spark gaps avalanches of electrons move towards the anode.As the number of electrons increases 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...
states that also the field strength increases.
The strong field accelerates the avalanche.
A slow rise time of the voltage lets the electrons drift towards the anode before they can generate an avalanche.
Electrophilic molecules capture electrons before they can generate an avalanche.
Thermal effects destabilize a homogeneous discharge electron and ion diffusion stabilizes it.
Reference:
4. J I Levatter and S C Lin, Necessary conditions for the homogeneous formation of pulsed avalanche discharges at high gas pressures, J.Appl.Phys. 51, No2, pp 210 – 222, January 1980.