Coherent light generators – Having an applied magnetic field
Reexamination Certificate
2002-04-12
2004-09-21
Ip, Paul (Department: 2828)
Coherent light generators
Having an applied magnetic field
C372S055000
Reexamination Certificate
active
06795462
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a gas laser comprising a container intended to receive a gas or a gas mixture as laser-active medium, the container having an axis along which the gas laser emits its laser radiation, and comprising further means for generating a plasma in the container and means for generating a magnetic field in the container. A gas laser of this kind is known from “Laser” by F. K. Kneubühl, M. W. Sigirst, published by B. G. Teubner Stuttgart, 1988, pp. 232 to 271.
More specifically, the invention concerns rare-gas lasers, for example helium-neon lasers, ion lasers, especially argon ion lasers, and molecular lasers, especially CO
2
lasers.
STATE OF THE ART
Excitation of the active medium in a gas laser usually is initiated by an electric discharge. Free electrons and ions are produced in an electric gas discharge. These charge carriers gain kinetic energy due to the acceleration in the electric field of the gas discharge. The kinetic energy of the electrons so gained can be transferred, by inelastic collision, to other gas particles and can excite the latter to higher levels. The movement of the ions is, generally, of no importance as only the free electrons contribute to the excitation of the gas atoms, gas ions or molecules. A stimulating emission of radiation can then take place from the higher levels.
A helium neon laser is a neutral atom gas laser in which a direct-current or high-frequency discharge is excited in order to obtain a laser-active plasma. The efficiency of the laser, defined as quotient of optic performance and electric power input, is, typically, as low as 0.1%. This is due, among other things, to the rather inefficient excitation mechanism (Kneubühl/Sigrist, loc. cit., p. 240).
A typical argon ion laser comprises, in a vessel configured as a tube, a cascade-like electrode arrangement in which a high-intensity arc discharge is excited between the electrodes in the gas filling, which typically has a gas pressure of 0.01-1 mbar, in order to obtain a laser-active plasma with a degree of ionisation in an order of magnitude of 10
−4
to 10
−2
. The efficiency of an argon ion laser is likewise very low, being less than 0.1%. The beam quality, for example the beam jitter and its maximum intensity, depend on the age of the tube. Due to interaction between the plasma and the tube, the life of the tube is heavily restricted, being typically only 2000 hours for a laser power of 20 W. At the end of that time, the tube, which costs some ten thousand D-marks, must be replaced. In addition, the tube requires intensive cooling, the cooling power being typically in the range of up to 40 kW. It has been known in connection with an argon ion laser to apply a longitudinal magnetic field, i.e. a magnetic field that extends in parallel to the longitudinal axis of the laser tube, in order to concentrate the discharge on the axis and to reduce the damaging effects the plasma has on the tube wall (Kneubühl/Sigrist, loc. cit. p. 246). However, this measure is only little effective and has not succeeded in increasing the efficiency of the laser to over 0.1%.
CO
2
lasers are excited by direct-current discharges or electromagnetic fields in the radio-frequency range, depending on the particular design. Their efficiency is, typically, between 10% and 15%, depending on the particular design and operating mode.
The present invention has for its object to improve the efficiency of gas lasers and extend the life of the container that encloses the laser-active medium.
This object is achieved by a gas laser having the features defined in claim 1. Advantageous further improvements of the invention are the subject-matter of the sub-claims.
BRIEF DESCRIPTION OF THE INVENTION
Gas lasers comprise a container intended to receive a gas or a gas mixture as laser-active medium, the container having an axis along which the gas laser emits its laser radiation. For exciting the laser-active medium, a source, associated to the container, of an electromagnetic alternating field is used to produce a plasma in which the electromagnetic alternating field is injected into the laser-active medium in the container. For producing a magnetic field in the container, a group of at least two pairs of magnet poles is provided, with the poles extending at a distance from, and along the axis of the container and being arranged around the axis with alternating polarity so that the magnetic field emanating from them exhibits an intensity sink in the region of the axis. As a result, a plasma column is produced which represents the essential portion of the laser-active volume, the geometric dimensions of which are determined by the confinement of the plasma in a static or slowly variable magnetic multipole field. Using energy irradiated and/or injected from the electromagnetic alternating field, electric fields with azimuthal or axially parallel components are produced that cause the plasma to become denser in the magnetic multipole field.
This provides the following essential advantages:
Due to the interaction between the charge carriers present in the plasma and the magnetic field and the electromagnetic alternating field, the charge carriers, especially the electrons are driven away from the container wall and toward the container axis, where the magnetic field exhibits an intensity sink and, preferably, disappears. This greatly reduces interaction between the plasma and the container wall.
The reduced interaction between the plasma and the container wall has the result to extend the life of the container.
As a result of the reduced interaction between the plasma and the container wall, the degree of heating-up of the latter, and thus the cooling power required, are reduced.
The reduced interaction between the plasma and the container wall reduces the level of absorption of gas components by the container wall and, as a result thereof, maintains the optimum composition and the optimum pressure of the gas over a longer period of time.
Due to the reduced interaction between the electrons of the plasma and the container wall, more electrons, and electrons of higher energy are available for gas-exciting collisions, whereby the efficiency of the laser is increased.
Because the electrons of the plasma are driven into the intensity sink of the magnetic field, the electron density and the collision probability of the electrons increases in that region so that the light yield and, with in, the efficiency of the laser, increase. First test have shown that compared with the prior art the efficiency of, for example, an argon ion laser can be increased by a factor of ten to twenty.
As a result of the concentration of the excitation-triggering electrons in the near environment of the axis of the container, the zone emitting the laser radiation becomes narrower so that the laser beam becomes thinner and more intensive.
As a result of the reduced interaction between the plasma and the container wall and the increase of the electron density in the near environment of the container axis, a rise in electron temperature (kinetic energy of the electrons) occurs in this region, which temperature may be further increased by an increase in amplitude of the electromagnetic alternating field. The increase in amplitude of the electromagnetic alternating field in turn is facilitated by the reduced interaction between the plasma and the container wall.
The invention renders possible a higher electron temperature, which in turn permits the excitation of higher energy levels and, thus, laser radiation with shorter wave lengths, down to the roentgen range.
The high electron temperature, that can be reached with a relatively low HF power, leads to a considerably lower build-up threshold of the laser, from an energetic point of view.
The invention permits plasmas to be generated in the laser container, in which the electron temperature is much higher than the ion temperature, which is desirable as such.
The reduced interaction between the plasma and the container wall permits the use of containers made fro
Christiansen Jens
Christiansen Kai
Christiansen Silke
Jacoby Joachim
Christiansen Jens
Ip Paul
Levisohn, Berger & Langsam LLP
Nguyen Phillip
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