Description :: Our patents

The guessed invention falls into field of laser technique and can be used at making technological complexes of laser welding and cutting.

With the use of high-power lasers for treating materials the major value has an opportunity of operation in a pulse - periodic mode. When welding the depth of a weld rises, and when cutting the bur and unevenness of walls diminishes.

There are some techniques of a pulse - periodic mode embodying:

Modulation of pumping;
Q-switching of a cavity, including, for example, overlapping of a cavity caustic with opaque screen;
Usage of intracavity saturated absorbents.

With reference to high-power lasers the number of possible techniques is sharply narrowed down. So, for example, the technique 3 already does not transit because of considerable intracavity aberrations, thermal nature, resulting in sharp lowering quality of the laser output radiation.

The modulation of pumping is less effective, than the Q-switching in the cavity, and is more expensive technique of a problem solution. The Q-switching of the cavity can be implemented, changing in time curvature of one mirror.

The pulse - periodic laser is known [1], including stable resonator, formed by a transparent output mirror, mirror with controllable curvature, restricting diaphragm and excitation chamber. As a mirror with controllable curvature the bimorph deformable mirror is [2,3], which allows to drive its curvature with frequencies up to several kHz.

The given device operates as follows. With delivery of a regulating voltage the bimorph deformable mirror starts periodically to be curved with frequency of a controlling signal. With variation of a controllable mirror curvature radius the size of the basic mode caustic of the stable resonator varies. A dimensional change of the basic mode in the field of diaphragm finding [4] leads to modulation of losses and Q-factor of the cavity. The Q-switching of the cavity carries on to modulation of the laser output power

Thus, driving curvature of one mirror surface it is possible to drive an output power of the laser.

The important virtue of the unstable resonator with a controllable back or intermediate rotary mirror is that the spatial parameters of an output beam, such as a curvature radius of a wave field and, appreciably, size of a beam, remain constant during modulation of cavity losses. They are defined with a curvature radius of an output mirror and size of a diaphragm.

A deficiency of the known device is the necessity of transparent mirrors application for an outlet of radiation, that restricts application of stable cavity for lasers with a major level of power because of a low threshold of damage of a transparent mirror and major thermal deformations.

In the other known device – analog - pulse - periodic laser with the unstable resonator, containing a mirror with controllable curvature, this deficiency is solved [5]. The unstable resonator contains only total reflecting mirrors. In the unstable resonator useful losses are diffraction, thus the radiation is output around of one mirror as a ring. The internal size of a ring is peer to the size of a lead-out mirror d, and external size-

D = d M,

where M - coefficient of cavity magnification.

With action on a controllable mirror, there is a curving of its surface and thus cavity magnification coefficient - M varies, and as the unstable resonator losses for pass are featured by the formula:

t = 1 - 1/M²,

therefore, losses of the cavity vary also.

A deficiency of this device is that simultaneously with variation of cavity losses change parameters of output radiation too: an exterior diameter of radiation and radius of wave front curvature. It brings that in time varies not only laser power, but also spatial performances of its radiation. After a focusing of radiation the variation of wave front curvature radius will give scanning of focusing spot along axial line, that essentially restricts an area of such laser application.

The device - pulse-periodic laser with stable-unstable resonator- selected by us as the prototype is also known [7, 8].

In it the pulse - periodic mode is implemented due to modulation of the power supply, and as the optical resonator is selected stable-unstable. The resonator is in one plane unstable and in perpendicular plane - stable. Virtues of stable-unstable resonator are:

Possibility to use only reflective optics, which can be derived to lasers with a high output power;
In such laser it is possible to fill with radiation much more volume of activ medium, than in the laser with stable resonator.

Deficiency of the prototype is the fact, that for receiving pulse-periodic mode is used pulse pumping technique or active medium pumping modulation technique, which is in comparison with the technique of cavity losses modulation rather less effective and which requires application of high-voltage key devices. This substantially complicates the device and diminishes the reliability. Using of high-voltage, high-current key devices for switching (modulation) of pumping current gives also in oscillation of different pulsing radio disturbances. A problem of the invention is the simplification of high-power pulse-periodic laser construction, pinch of efficiency, pinch of reliability, elimination of radio disturbances.

The problem is solved thanks to, that in known device, including the stable-unstable resonator, which is fulfilled stable only in one plane, and in another (orthogonal first) plane it is fulfilled under the plan of the unstable resonator, there is cylindrical, with variable in time radius, mirror with controllable curvature. And cylinder generator is in the plane of cavity instability. In this device the pulse - periodic operational mode of the laser is implemented owing to modulation of cavity losses. With action on a controllable mirror by controlling signal it starts periodically to be curved. The curvature alternation of this mirror brings to an alternation of the basic mode size in a stable plane of the resonator and, thereof, to variation of diffraction losses on a lead-out mirror.

Using of the guessed invention, as compared to prototype, ensures in the laser a pulse - periodic mode by modulation of cavity losses, without using of pumping modulation, that is without using of pumping current switching. Because of this laser efficiency and reliability is much higher, than in the device - prototype, the radio disturbances are nonexistent, the construction of all device is simplified. Let's mark, that the pulse - periodic mode is implemented at using only total reflecting optics, that enables to use given device at major levels of beam power.

The example of concrete execution of the invention is figured in a fig. 1. Here: 1 - cylindrical mirror with variable curvature, and generator of the cylinder coincides with an axis X; 2 - cavity cylindrical convex mirror, and generator of the cylinder coincides with an axis Y; 3 – cavity spherical concave mirror; 4 - flat mirror; 5 - lead-out mirror; 6 - output radiation; 7 - excitation chamber for active medium; 8 - active medium stream in excitation chamber and cavity.

The device operates as follows. With absence of driving voltage the shape of surface of the mirror 1 is flat, thus the cavity properties define curvature radiuses of mirrors 2 and 3 in mutually orthogonal planes.

Test, whether the resonator is stable or unstable, is the quantity Q [6],

Q = (1 - L/R2) (1 - L/R3),

where L - mirror range; R2, R3 - curvature radiuses of mirrors 2 and 3; if Q < 1, the resonator is stable, if Q > 1, the resonator is unstable.

In our example a mirror 2 - cylindrical, therefore R^x₂ = - R, R^y₂ =∞;

Mirror 3 - spherical with R^x3=R^y3=R+2L, thus

M = 2L/R+1 - coefficient of magnification.

In this case

Q^x=(1-L/R^x₂)(1-L/R^x₃)=(L²+2LR+R²)/(2LR+R²)> 1

therefore, the resonator is unstable in a plane XOZ

Q^y=(1-L/R^y₂)(1-L/R^y₃)=(L+R)/(2L+R)

therefore, the resonator is stable in a plane YOZ.

That is, in our case we really have in a plane XOZ the unstable resonator, and in orthogonal plane - stable. The mirror 5 is a restricting diaphragm in stable plane and lead-out mirror in unstable plane of the cavity.

With driving voltage delivery on a mirror 1 it will turn from flat in cylindrical, oriented so, that cylinder generator will coincide with a plane XOY, i.e. with unstable plane of the cavity. Thus the time-varying curvature of this mirror will influence on cavity parameters only in stable plane YOZ. Thus the size of cavity caustic in stable plane will vary slightly as contrasted to diaphragm mirror size, thus diffraction losses will also vary in time.

We shall mark, that the curvature of radiation front, impinging on a mirror 5 in stable plane YOZ, will be defined by curvature of a mirror 2 in this plane and is peer 0 at any variations of curvature of a mirror 1. In unstable plane radiation parameters are defined by parameters of mirrors in this plane and do not depend on curvature variation of a mirror 1. Therefore, the delivered problem is fulfilled.

The output radiation in adjusting cavity, circumscribed above, consists of two spots. To implement compact output radiation it is necessary to disadjust or to bias an output mirror so, that the optical axis of the cavity was biased on quantity a little bit smaller, than half of mirror size. In this case the radiation will have two, strongly distinguished on power (in 10 and more time), spots. Then the spot of smaller power is guided to an absorbent and the high-power spot is used only. Such radiation has the best focusing properties, than radiation, consisting from two spots.

If as activ medium the stream of gas, transiting through excitation chamber, drawn in one direction, is used and the axis of the cavity is traversal to gas stream, there is a problem of the coordination of discharge chamber height and beam widths of the cavity. The fact is, that with the purpose of laser output power pinch tend to increase a gap of excitation chamber and frequently it becomes much more, than basic mode size in stable plane of the cavity. It gives laser efficiency lowering.

To increase an output power of the pulse - periodic laser, we offer to unfold unstable plane of the resonator across to a stream, as it is shown at fig. 2, and to bias downstream slightly the cavity concerning the excitation chamber. In this case a generator of controllable cylindrical mirror 1 oriented perpendicularly to stream, and a generator of cylindrical mirror 2 oriented in parallel to active medium stream. In such configuration it is possible to fill with radiation practically any gap of excitation chamber and thus to provide high-power, with stable spatial parameters, radiation of the pulse - periodic laser.

What is claimed is:

A pulse-periodic laser, comprising a stable-unstable resonator, which has a stable plane and perpendicular to it an unstable plane, and being a single-mode laser in a stable plane, characterized in that the resonator includes a mirror with controllable curvature, which when is affected by a driving periodic signal receives surface cylindrical shape with curvature, variable in time, and a generator of this cylindrical surface coincides with an unstable resonator plane.
A pulse-periodic laser according to claim1, wherein a stable - unstable resonator is multipass.
A pulse - periodic laser according to claim 2, wherein the mirror with controllable curvature is an intermediate rotary mirror.
A pulse - periodic laser according to claims 1, 2, 3, wherein the lead-out mirror biased concerning an optical axis of the resonator in an unstable plane on distance a little bit smaller halves of hole in it, at the expense of that the unilateral outlet of radiation is implemented.
A pulse - periodic laser according to claims 1, 2, 3, 4, wherein active medium is the stream of gas, excited by glow discharge, which is flowing transversely across the resonator , and the plane of instability of the resonator oriented across a stream, and plane of stability along a stream.