H01S 3/08

The invention falls into field of laser technique and can be used at making high-power lasers with high-quality radiation.

The flowing gas laser with the stable-unstable resonator is known [1]. The laser represents the excitation chamber, drawn along a stream of active medium and the optical cavity, which axis is perpendicular to a stream. The cavity, formed by reflecting mirrors, is stable in a plane, perpendicular stream, and unstable in a plane, parallel stream. The output radiation looks like two spots, opposite in unstable plane, and the axis of the cavity transits between them.

At a misalignment of the cavity in unstable plane, its axis comes nearer to one spot edge, it diminishes in size and intensity and as a result there is one output beam.

The circumscribed above laser with the stable-unstable resonator has the following properties:

1. All laser mirrors are total reflecting, and, therefore, output power can be enough major.
2. The output radiation has compact section.
3. If in a stable plane the resonator is single-mode with the help of a diaphragm or because of the restricted sizes of mirrors, the radiation has very much excellence.
4. The radiation almost completely fills the excitation chamber and consequently such laser has high efficiency.

However, the circumscribed above laser has essential disadvantage. For actual lasers the size of a caustic in stable plane is rather small. So for a wave length 10,6 μm, distance between mirrors L =1 m, size of basic mode d =10 mm, and therefore it is necessary to use the excitation chamber with small section and so to diminish a laser power, or to apply multimode in stable plane the stable-unstable resonator. Quality of radiation in this case is sharply aggravated. Thus, there is a technical conflict between radiation excellence and laser high output power.

With the purpose of the indicated conflict elimination in paper [2] is offered the laser, including stable-unstable resonator, in which are used toroidal mirrors, which allow to implement in stable plane the so-called convex-concave stable resonator, and in unstable plane, parallel to direction of stream motion, unstable resonator. This laser is selected as prototype. The example of embodying of such laser is given in paper [3] for 5-kW CO₂ –laser. There was used a project, in which one mirror is concave in both planes, but with different curvature radiuses - R_x3 and R_y3, and other mirror is convex in both planes, but with different curvature radiuses – R^x₄ and R_y4 too. If the radiuses R_y3 and R_y4 are close on absolute value, but are opposite on a sign and fulfill requirement in stability, i.e. if

(1 – L/R_y3) (1- L/R_y4) < 1,

the size of basic mode caustic amplifies.

Therefore, for a single-mode operation increases overall dimension of the excitation chamber or, otherwise, in a multimode case diminishes the number of modes.

However, prototype has essential disadvantages:

1. There are used very dear and requiring man-hours per job mirrors with toroidal surface.
2. Simultaneously with basic mode size increase in toroidal cavity is sharply incremented the radiation axis sensitivity to small mirrors misalignments. So, in an above shown example sensitivity K, i.e. the ratio of axis displacement angle of radiation to an angular mirror misadjustment is equal 64, while the size of the basic mode is only 18 mm. The further magnification of the basic mode at the expense of curvature radiuses approach R_y3 and R_y4 gives the even greater magnification of sensitivity, and consequently lasers with such cavities are practically not very suitable for industrial usage.

Problems, solved by the offered invention, are:

1. Security of compact single-mode radiation in widely aperture excitation chamber.
2. Lowering in price of a construction at the expense of a failure from using toroidal mirrors.
3. Pinch of cavity alignment stability.

The above-stated problems are solved because of the flowing gas laser contains the excitation chamber, drawn along a stream of mixed gas, optical cavity, which axis transits across mixed gas streamline, being stable and unstable in two inter perpendicular planes. Thus a plane, in which the resonator is unstable, is canted in relation to mixed gas streamline, while the cavity caustic crosses almost all stream section.

The flowing gas laser, according to the invention, is arranged as follows (fig.1). It contains gas stream 1, discharge chamber 2, the cavity, formed by mirrors 3 and 4. The resonator unstable plane is canted angular q > arcsin h/b, where

b - cavity caustic size in unstable plane,

h - breadth of excitation chamber channel.

As all excited gas transits through cavity caustic at any caustic size in stable plane, it ensures a high efficiency of the cavity at anyone, somehow major, gap in the excitation chamber channel. At the same time on beam size inside the cavity in stable plane now there are no restrictions, therefore it is possible to use, for example, flat-convex stable resonator, and consequently, mirror 3 - cylindrical convex, and mirror 4 - spherical.

In the above described laser the output radiation looks like two spots, while in stable plane looks like Gaussian, and in unstable plane has rectangular, or close to triangular, distribution.

It is obvious, that, in case of extended along stream of gas mixture excitation chamber, it is possible the application of folded multipass cavity (fig.2). In a case of induced misaligned cavity, according to deviation of one mirror in unstable plane, the axis of the cavity displaces, the size and intensity of one spot grow, and second - impinge. In this case we have stable-unstable cavity with unilateral outlet of radiation.

For practice is important, that laser output radiation has a flat wave front.

It is implemented in a case, when laser resonator in unstable plane is confocal, i.e. focal points standings of end mirrors coincide. Thus two cases are possible. The first case is implemented, when joint focus is outside of the cavity (fig.4) - so-called " a positive branch " of unstable resonator. In the second case, " a negative branch " of unstable resonator – joint focus is between end mirrors inside the cavity (fig.5,6).

The outlet of radiation in such cavity can be realized with two techniques. First, with the help of a mirror 3 it is as a strip, drawn in stable plane, while the basic mode in stable plane is oozed with the help of a diaphragm 6 (fig. 3a). Secondly, with the help of a lead-out mirror 5, having a rectangular bore (fig. 3b), while in stable plane breadth of a bore is defined from a ratio b = 1,5 d, where d - basic mode diameter of stable resonator on a level 0,86 intensities, and in a plane of instability

d_H = D/M,

where D - size of beam caustic in unstable plane.

It is possible to pick up rectangle sides b and d_H so, that the radiation, going out the laser with unilateral output would have the peer sizes in both planes:

d_H (M - 1) = 1,5 d_0,86 1/L

For the flat-curved stable cavity beam size on a flat mirror is peer:

d _0,86 = 2 ( l / p L (R - L))^1/2

Coefficient of magnification for the confocal cavity:

M - 1 = 2L / R₃

Therefore: d_H = 1.5R₃ / L ( l / p L (R₄-L))^1/2

There is example of invention embodying. In the flowing gas 5 kW laser was implemented stable-unstable 5-pass resonator, circumscribed above. While the angle θ = 90⁰, L = 6,5 m, D = 50 mm, R_H = 26 m, R (cylinder) = -13 m, and cylinder generator was oriented in parallel to gas stream. The size of restricting diaphragm in stable plane was18 mm, and mirror size in unstable plane was 20 mm. Thus magnification coefficient M in unstable plane is peer M = R₄ / R₃ = 2. In case of bilateral output of radiation are obtained two beams of Gaussian view in stream plane and rectangular shape across a stream (fig.1, plan A-A), and at cavity misalignment in unstable plane was implemented radiation unilateral outlet with the aperture 20x18 mm. While inside the cavity its caustic was 40 mm and overlapped active gas stream almost completely. A measuring of power distribution in long-range zone, i.e. in lens focus, have shown, that quality coefficient of such beam, i.e. ratio of beam divergence to diffraction divergence, was close to 1.

Laser sensitivity to a misalignment, determined under the formula from chapter 5 paper [3], for our example is peer 4, i.e. it is less in 15 times, than in the prototype.

Following invention development is the execution of stable-unstable resonator in circumscribed above laser as self-filterable in unstable plane. The plan of self-filterable stable-unstable resonator is given (fig. 3d). Here a mirror 3 is concave in unstable plane and flat in stable plane, a mirror 4 is spherical concave.

Confocal requirement: R₃ / 2 + R₄ / 2=L

Slot breadth - d and focal distance F₃ = R₃ / 2 are fixed by a ratio d = (2l F)^1/2 so, that at reflection from a mirror 3 through a slot of a mirror 5 transits only first maximum of beam diffraction pattern. In this case slot in a mirror 5 will filter beam aberrations and to a mirror 4, and then on an output, the beam of radiation will be guided with close to Gaussian intensity distribution, except its central part.

We shall mark remarkable property such self-filterable stable-unstable resonator: the complete losses on pass in it are much less, than in usual self-filterable cavity and are defined by the formula: t = 1 – 1 / M

There is example of invention embodying on the last formula claim:

Cavity length:

L = 6 m

F₃ = 1 m, R₃ = 2× F₃ = 2 m

F₄ = 5 m, R ₄= 2 F₄ = 10 m

Slot breadth:

d = (2 λ F)^1/2 = (2 10^-5 10)^1/2 m = 1,4 10^-2m = 14 mm

M = D / d = 55 mm / 14 mm = 4

The complete losses: t = 1 – 1/4 = 75 %

Thus the output radiation has ellipse shape with axes ratio 55 mm / 18 mm = 3 and cut strip with breadth 14 mm. With the help of cylindrical mirrors, forming a telescope with coefficient of diminution 3 in unstable plane, the beam with elliptic section is easy for conversing to a beam with circular section. Thus slot breadth will also decrease in 3 times and become less than 5 mm.

What is claimed is:

1. A flowing gas laser with a stable – unstable resonator, comprising an excitation chamber, drawn along a stream of a mixed gas, and an optical resonator, being stable and unstable accordingly in two mutually perpendicular planes, passing through an optical axis transverse a stream, characterized in that a resonator unstable plane is inclined relative to a streamline, while a resonator caustic crosses almost all stream section.
2. The flowing gas laser according to claim 1, wherein the laser resonator is single-mode in a stable plane.
3. The flowing gas laser according to claims 1,2, wherein a corner between a streamline and an unstable resonator plane is peer 90⁰.
4. The flowing gas laser according to claims 1-3, wherein its resonator in an unstable plane is confocal.
5. The flowing gas laser according to claims 1-4, wherein its resonator is fulfilled multipass.
6. The flowing gas laser according to claims 1-5, wherein its resonator is aligned in unstable plane in such a manner, that its axis transits near a mixed gas stream edge, and the outlet of radiation is unilateral.
7. The flowing gas laser according to claims 2-4, wherein its resonator contains at any rate one cylindrical and one spherical mirror, oriented in such a manner that will derivate in a stable plane flat- concave resonator and in an unstable plane a telescopic unstable resonator.
8. The flowing gas laser according to claims 1-5 and 7, wherein its resonator is fulfilled self-filterable in an unstable plane.

 

 

1. O.L.Bourn et al. A Novel Stable – unstable resonator for beam control of rare – gas Halide lasers. Optics communications, v. 31, No 2, 1979, p.p. 193 – 196.
2. A. Borghese et al. Unstable – stable resonators with toroidal mirrors. Applied Optics, v.20, No 20, 1981, p.p. 3547 – 3552.
3. V. Fantini et al. A 5kW cw CO₂ – laser for industrial applications. Inst. Phys. Conf. Ser. No 72, 1984, p.p. 17 – 20.