TEA laser

The CO2 TEA laser was invented in the late 1960s by Dr Jacques Beaulieu working at the Defence Research Establishment, Valcartier, in Quebec. 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 100W 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.

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 thyratron 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.

The first true 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.



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.

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.

The double-discharge method required to initiate stable high-pressure gas discharges can be used both below and above atmospheric pressure, and these devices too can be referred to as TEA lasers. Commercial Excimer lasers operating in the ultra-violet use a double-discharge regime very similar to the CO2 TEA laser. Using Krypton, Argon or Xenon Chloride or Flouride gas buffered with helium to 2 – 3 atmospheres of pressure, Excimer lasers can produce megawatt pulses of ultra-violet laser light.

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 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.

Applications

 * nitrogen laser
 * carbon dioxide laser
 * excimer laser