Laser having a brief discharge between two elongated electrodes

The invention concerns a travelling wave laser and has application to the reduction of the divergence of laser beams. The electrodes between which the travelling electric discharge for the excitation of the amplifier gas takes place have a cross-section in the shape of two polished rounded-off points facing each other, so as to prevent spurious reflections of light towards the discharge zone and to reduce the divergence of the output beam which is converged by means of a simple optical system.

BACKGROUND OF THE INVENTION 
1. Field Of The Invention 
The present invention concerns a laser having a brief discharge between two 
elongated electrodes. 
2. Description Of The Prior Art 
In such a laser, the direction of light emission is generally defined 
mainly by the shape of the space in which the light amplification takes 
place and not by a resonant optical cavity. The main direction of emission 
is then that along which the light is amplified for the longest time, i.e. 
along the channel situated between the two elongated electrodes. The 
problem is to obtain light emission having as little divergence as 
possible. 
This problem arises more particularly in the case of a laser having a brief 
discharge of the travelling wave type. Such a laser comprises, 
conventionally, a flat-plate transmission line with conductive surfaces 
having a parabolic edge; a spark gap placed at the focus of the parabola 
short-circuits the two conductive surfaces after the line has been charged 
and thus generates a cylindrical travelling electric wave front in the 
line which is converted to a plane wave front by reflection at the edges; 
a laser channel is formed by interrupting one of the conductive surfaces 
with a rectilinear slot whose edges constitute the two elongated 
electrodes and by keeping a gas, such as nitrogen, in that slot. This 
channel is disposed at an angle to the axis of the parabola; the two 
electrodes are electrically connected by an induction coil for the 
charging of the transmission line. A localized electric discharge is thus 
obtained between the electrodes which moves along the channel at the speed 
of light and which effects a very rapid excitation of the nitrogen. 
This excitation enables the selective amplifying of a travelling light wave 
accompanying the discharge zone. Such a laser is described in the article 
by M.B. GODARD - "A simple high-power large efficiency N.sub.2 ultraviolet 
laser", I.E.E.E. Journal of Quantum Electronics, vol. QE-10 N.degree. 2, 
February, 1974, pp. 147 to 153. 
The electrodes between which the travelling electric discharge appears are 
polished and have the form of plane parallel surfaces with rounded edges. 
Such a form is, indeed, simple and conventional for setting up an electric 
discharge between two electrodes when the firing voltage is required to be 
well determined and when damaging of the electrodes is to be prevented. 
The pulse laser beam thus obtained is not parallel and has a divergence, 
which can be partly eliminated by placing a lens in the path of the beam 
having a focus which coincides with a first end of the laser channel, i.e. 
with the zone in which the electric discharge begins and from which the 
light wave is subsequently amplified all along the channel. The residual 
is, however, still a hindrance and cannot be sufficiently corrected by 
optical means placed in the beam. 
Preferred embodiments of the present invention provide a laser having a 
brief discharge between two elongated electrodes, with a beam of reduced 
divergence. 
The present invention provides a brief discharge laser comprising an 
enclosure for containing a gas capable of amplifying light when excited by 
an electric discharge, two parallel elongated electrodes disposed in the 
enclosure, and having adjacent faces which form two sides of an elongated 
laser channel running from a first end to a second end, electric means for 
so generating a brief discharge across the faces of the electrodes that, 
in operation, a light wave appearing at the first end of the channel and 
propagating towards the second end is amplified as it passes through 
excited gas and leaves the enclosure through a suitably placed window in 
the enclosure, and a convergent optical system placed in the path of light 
leaving the second end of the channel and having a focus substantially 
coinciding with the first end of the channel; the shape of said adjacent 
faces of each of the electrodes as seen in a cross-sectional view taken in 
a plane perpendicular to the channel being that of a rounded-off point 
directed towards the other electrode whereby any light propagating 
obliquely along the channel and being reflected from either electrode is 
reflected outside the channel and therefore ceases to be amplified during 
operation of the laser. 
A brief discharge travelling wave laser is known, whose electrodes have the 
form of a rounded point, when seen in a cross-section view taken through a 
plane perpendicular to the channel containing the active gas. See the 
British periodical "Opto Electronics" vol. 4 N.degree. 1, February, 1972, 
pages 43-49, publishing an article by D. Basting et al, "A simple high 
power nitrogen laser". The laser described therein supplies a beam having 
great divergence of 66 milliradians in one direction and of 26 
milliradians in the other. Its teaching has therefore never been taken 
into consideration when it was sought to reduce the divergence of the 
output beam of a laser. 
The efficiency of the present invention resides in the fact that the 
irreducible part of the divergence of known brief discharge lasers was 
essentially due to the fact that a part of the excitation energy of the 
gas was used for amplifying light waves which were propagated obliquely in 
the laser channel by successive reflections on the plane parts of the 
adjacent faces of the electrodes.

In FIGS. 1 and 2, the laser comprises an elongated enclosure 2, made of 
methyl polymethacrylate containing nitrogen at a pressure lying between 30 
and 60 millibars, for example 50 millibars. Two elongated stainless steel 
electrodes 4 and 6 are arranged parallel to each other in the enclosure 
and form the edges of a laser channel 8 which is 90 cm long. 
A flat-plate transmission line is formed by two thin copper sheets 10 and 
12, one of whose edges forms a parabola 14, separated by an insulating 
sheet 16 made of polyester or polyimide, having a thickness of 75 microns. 
Another edge of the sheet 10 is in contact with the electrode 4. Another 
sheet 18, made of copper, is placed in the extension of the sheet 10 and 
has one edge in contact with the electrode 6. It is connected to the sheet 
10 by an inductance coil 20. 
An electric generator 22 charges the flat-plate transmission line thus 
formed to a voltage of 1 to 10 kV, for example 5 kV. 
A spark gap 24, placed at the focus of the parabola 14, then sets up a 
discharge of the transmission line by suddenly connecting the sheet 10 to 
the sheet 12. This results in the production of an electric wave in the 
transmission line. This wave is initially circular, then, by reflection on 
the parabola 14, it becomes a rectilinear wave which is perpendicular to 
the axis of that parabola and which is propagated parallel to the axis of 
the parabola towards the channel 8. When it reaches the channel 8, it sets 
up a brief electric discharge therein between the electrodes 4 and 6. 
Because of the inclination of the channel in relation to the electric wave, 
the discharge is propagated from a first end 26 of the channel up to a 
second end 28 at the speed of light. A light wave appearing at the first 
end 26 and going towards the second end 28 therefore accompanies the 
electric discharge. Provided the light wave has a wavelength of 3371 
angstroms, it is amplified along the whole channel 8 by the nitrogen which 
is excited by the electric discharge, whereas a light wave having the same 
wavelength, but with a very different direction of propagation will be 
amplified only over a very short path. 
Since the amplified wave begins at the first end 26, a convergent lens 30 
whose focus coincides with the first end 26 is used to obtain a parallel 
output beam. Nevertheless, there remains a divergence of the beam, for 
certain waves are reflected by the electrodes 4 and 6, forming a slight 
angle with the axis of the channel 8, this imparting to them a propagation 
speed in the direction of the channel which is very close to that of the 
electric discharge. These waves, having the suitable wavelength of 3371 
angstroms are also therefore amplified along the whole channel. 
The shape of the electrodes 4 and 6 shown in FIG. 3 makes it possible to 
avoid these reflections and the spurious amplification. Each of these 
electrodes is polished and has, in its cross-section, a rounded off tip or 
point directed towards the other electrode. 
The radius of curvature of the rounded off tip, for example 1 mm, is 
preferably less than a quarter of the width of the channel 8, situated 
between the points, for example 6 or 7 mm. This radius should not, 
however, be less than 2% of the channel width. Each tip forms an arc of a 
circle comprised, preferably, between 120.degree. and 170.degree., so that 
the thickness of the electrodes may become greater on moving away from the 
channel 8. Beyond their tips, the electrodes have the general form of 
blades with a thickness of 5 mm and have two faces parallel to the plane 
of the sheets 10, 12 and 18. The face furthest from the sheets (such as 
40) remains parallel to that general direction up to the rounded off 
point. The other face (such as 42) is in contact with the sheet 10 (or the 
sheet 18) up to the edge of that sheet, which constituted by an edge line 
such as 44. Beyond the edge, the said other face extends away from the 
plane of the sheets up towards the rounded-off point. 
This makes it possible to move the discharge zone away from the insulating 
sheet 16. The discharge zone is shown by a dotted line between the 
rounded-off points. 
The sheets 10, 12, 16 and 18, which are resilient, are fixed on a support 
46 which ensures the mechanical rigidity of the assembly. 
The support has two grooves cut in it under parts of the electrodes 4 and 6 
which are in contact with the sheets 10 and 18 and in the vicinity of the 
edges such as 44. 
Means for pressing flexibly the assembly formed by the sheets 10, 12, 16 
and 18 against the electrodes 4 and 6 and for ensuring proper contact 
between the electrodes 4 and 6 and the sheets 10 and 18 are arranged in 
the said grooves. These means are constituted by resilient tubes 48 and 
50. 
The stand 46 forms, on the other hand, a depression 52 adjacent to the 
inside space of the enclosure 2, so as to bring the sheets 16 and 12 away 
from the channel 8. This is ensured by making the bottom of the depression 
52 communicate with a vacuum pump, not shown. 
The laser which has just been described makes it possible to obtain with 
the lens 30, a beam having a divergence of 0.3 milliradians, whereas if 
the usual form of electrodes had been used, the divergence would have been 
approximately 10 milliradians, using an analogous lens. The duration of 
the light pulse is generally comprised between 2 and 12 ns. 
It can sometimes be useful to arrange, in the vicinity of the spark gap 24, 
a reflector to prevent the circular wave generated at the spark gap from 
reaching directly the laser channel 8. Nevertheless, this precaution is 
not indispensable, for the direct wave reaches the channel almost at the 
same time as the wave reflected by the parabola. 
Inasmuch as concerns the choice of voltages to be applied between the 
metallic sheets 10 and 12, it should be observed that the lower these 
voltages are, the more the distance between the electrodes 4 and 6 can be 
reduced, for a constant pressure. It is desirable to bring it to less than 
9 mm, this making it possible to reduce further the residual divergence of 
the beam. The pressure is, to great advantage, comprised between 30 and 60 
millibars. Since the energy of the electric discharge cannot be much 
reduced, it is an advantage to choose a small distance between the sheets 
10 and 12, i.e., a small thickness for the dielectric sheet 16, of less 
than 130 microns. In practice, for a thickness of 75 microns, a voltage of 
5 kV is suitable with a distance between electrodes of 6.7 mm. It is, 
however, practically possible to go down to a voltage of 1 kV with a 
distance between electrodes of 2 mm and a thickness of the sheet 16 of 25 
microns. 
Although one embodiment of the invention which appeared to afford an 
advantage has been described, that embodiment using, to set up the 
electric discharge in the laser channel, a flat-plate transmission line 
having a parabolic edge, it must be understood that other forms of 
transmission lines can also be used. It is, for example, possible to store 
the electric energy in a set of discrete capacitors.