Inert Gas Ion Laser

1. Field of the Invention

The invention relates to an inert gas ion laser comprising a member having an elongated, gas-filled cylindrical discharge channel, two electrodes with which a gas discharge can be generated in the discharge channel and two spaced mirrors forming an optical resonator which enclose the discharge channel between them.

2. Description of the Prior Art

For years, the need for coherent light sources which should emit a continuous beam in the visible spectrum with a power between 0.5 mW and 5 mW has been covered by helium-neon lasers.

In the meantime, this type of laser has been developed into a technologically mature component with long-term stability which can also be inegrated without further ado into the most modern optical systems, for example video disc players. There unsatisfactory, however, is the fact that HeNe lasers continue to require a great deal of space. Thus, the currently smallest tube, the 0.5 mW laser offered by the Spectra-Physics Company under the designation "007", still has a length of 11.5 cm and a diameter of about 2.5 cm and further advances in this area are hardly possible for physical reasons.

Alternatives had therefore already been sought. The argo laser thereby had to appear particularly attractive: it offers amplification values per length unit that are an order of magnitude higher than in HeNe lasers. Over and above this, calculations on the basis of balance equations with which the laser intensification mechanisms and the gas discharge events can be approximately described has led to similarity laws which at least did not exclude a start-up even given very narrow, short discharge tubes (periodical Physik 215 (1968) 437 and 219 (1969) 5). Insofar as known, these scaling rules were not, however, verified for tube diameters <1 mm; efforts, instead, were exclusively concentrated on larger discharge volumes for generating medium through high output powers. Mini argon lasers may possibly not have been taken into consideration because technological problems were envisioned (cf., in this regard, U.S. Pat. No. 3,646,476, initially cited patent, particularly Col. 3, paragraph 2) or excessively high magnetic fields were considered necessary (cf., in this regard, Appl. Phys. 4 (1974) 141, particularly Eq. ( 7)).

Semiconductor lasers which are extremely compact and are already used where a light wavelength >0.7 .mu.m is acceptable and excessively high demands are not made of the reliability may possibly present serious competition. It is therefore presently entirely unresolved as to whether and, under given conditions, when laser diodes are in a position to penetrate into high-frequency areas as well.

It is therefore an object of the invention to specify a laser which can generate light with wavelengths <0.7 .mu.m, requires a low structural volume and promises a long useful life. This object is inventively achieved by a gas laser.

The invention proceeds on the basis of the finding that, given suitably selected operating parameters, a steady-state laser activity arises even given smallest capillary diameters and extremely short discharge lengths. Thus, for example, an argon laser can be realized which supplies a 1 mW strong 0.488 .mu.m line with a discharge path of 1 cm, a mirror distance of 3 cm, a discharge cross-section of 0.1 mm and a power consumption of 25 W. The construction is thereby not excessively loaded and manages, surprisingly, without magnetic field.

The range of possible operating parameters is essentially determined by the following boundary conditions:

1. A low-pressure arc discharge should be marked by electron-atom impacts and which is wall-stablized should burn in the filler gas. In this case, the population inversion becomes maximum when the products of the filling pressure p and the capillary radius R as well as of the longitudinal field strength E.sub.Z prevailing in the discharge and the capillary radius have defined values which are independent of the current density, when thus

2. The laser amplification saturates when the laser levels are emptied by electron impacts with a rate which is no longer small in comparison to the radiation decay rate. In order to avoid this saturation effect, the electron density n.sub.e dare not exceed a certain limit value (periodical Physik 264 (1973) 61, Sec. 2). This means that the discharge current density j must remain below a limit value j.sub.gr :

3. A saturation of the laser amplification also occurs when the migration rate of the laser ions to the tube wall reaches the order of magnitude of the radiation decay rate. This saturation effect does not occur when the capillary radius R remains above a limit value R.sub.gr, i.e.

applies.

4. The light suffers losses due to diffraction, respectively attenuation effects and due to reflection at the mirrors and these losses should be kept as low as possible. When the laser functions with modes ("resonant modes") at which the light propagates between the mirrors in free space, then diffraction losses dominate. They are lowest when the light oscillates in a confocal resonator configuration in the TEM.sub.oo mode and are still tolerable in this case when

applies (Bell Syst. Techn. J. 40 (1961) 453, particularly FIG. 15). When a change to smaller F-values is undertaken, then a waveguide mode is preferable wherein the light is conducted by reflections at the tube wall. Particularly deriving here are transmission losses which are dependent on the laser wavelength, the tube diameter, the mode type and the refractive index of the wall material. For the basic mode EH.sub.11 and a tube having an optical density n, the loss per length unit v derives at ##EQU1## according to Bell Syst. Techn. J. 43 (1964) 1783.

5. The discharge must take place in a body that is not excessively loaded by the dissipated heat.

The dissipated power, which amounts to more than 99% of the supplied power P, is to be limited such that the temperature at the inside tube wall does not exceed a critical value which lies far below the melting temperature and such that the thermally conditioned stresses do not lead to breakage; the differences in longitudinal expansion between inside and outside of the tube are especially critical. For cylindrical-symmetrical discharge tubes having the length L.sub.e and the supplied power P=U.multidot.i ##EQU2## is valid for the temperature at the inside tube wall (T.sub.i) in the steady-state mode, wherein

T.sub.a =temperature at the outside tube wall

A=coefficient of thermal conductivity [Ws/cm.degree.K.]

R.sub.a =outside tube radius [cm].

T.sub.a is kept to a defined value by means of cooling and T.sub.i should not lie more than 200.degree. K. above this. So that the tube does not break, its length should remain below a limit length L.sub.gr to which ##EQU3## applies, wherein .eta.=coefficient of thermal expansion [.degree.K..sup.-1 ]

.sigma..sub.B =breaking stress [kp/cm.sup.2]

E=modulus of elasticity [kp/cm.sup.2 ].

When T is eliminated from equation (7) and (8) and L.sub.gr =L.sub.e is inserted, then ##EQU4## results, with ##EQU5## For a typical use case, the ArI laser with the 0.488 .mu.m line and a cylindrical-symmetrical beryllium oxide tube, the enumerated dimensioning rules produce the following numerical values:

The required type of gas discharge is established when

j=discharge current density

applies. In this range, the differential gain g.sub.o and the output power .PHI. have maximum values when

and

apply. The length-normalized power P/L can be calculated from (10) and (12):

i=discharge current.

Valid under these preconditions are ##EQU6## and, because of (10),

The electron impact emptying of the laser level plays no part for the discharge current densities and discharge currents up to the limit values

Following from this current limitation as limit values for the values g.sub.o, P and .PHI./L are

When the output power is summed over all lines, then

The migration losses of the laser ions to the tube wall are negligible when the tube radius is greater than

When the laser operates with a confocal mirror arrangement and a TEM.sub.oo mode, then Eq. (5) supplies

The mirror spacing can be even greater in practice. Given very thin cross sections (R<2.times.10.sup.-2 cm), the mirrors can even lie apart by 2 to 3 times the L-value from the Fresnel number F. Resulting from Eq. (6) in waveguide mode is that the losses can be neglected in comparison to the gain values when

applies.

For the tube, EQS. (7), (8) and (9) supply, with the material constants for BeO and the assumption .DELTA.T=100.degree. K.

and

One can see that the physically prescribed geometry limits can be realized without further ado with beryllium oxide.

With its short structural length, the proposed miniature laser allows a single-frequency operation. It is therefore particularly suitable for generating stable-frequency signals or for emitting amplitude and/or frequency modulated light.

The proposed solution shall now be explained in greater detail with reference to two preferred exemplary embodiments in conjunction with the attached drawing. In the FIGS., mutually corresponding parts are provided with the same reference characters. Shown therein are:

FIG. 1, an argo laser in a longitudinal section.

FIG. 2, the laser of FIG. 1 in the section II-II.

FIG. 3, a longitudinal section of another argon laser.

FIG. 4, the quantity P/L as a function of R.

FIG. 5, the quantity .PHI. as a function of R.

FIG. 6, the quantity i as a function of R.

FIG. 7, the quantities p and E.sub.z dependent on R.

FIG. 8, the limit length of a BeO discharge tube, as a function of R.

FIG. 9, .PHI. entered relative to R, for a TEM.sub.oo wave in a confocal resonator.

In detail, the gas laser of FIG. 1 contains a solid body 1, an anode 2, a cathode pot 3 with a cathode 4, as well as two resonator mirrors 5, 6.

The body 1 has the form of a plate which has an area of 30.times.20 mm.sup.2, is 5 mm thick and is composed of beryllium oxide. This plate is provided with a 0.1 mm large center bore ("discharge channel") 7 which penetrates the plate 1 concentrically relative to its center axis parallel to the longitudinal edge. The discharge channel is somewhat expanded at its two ends and is terminated there by respectively one of the two mirrors 5, 6. Lenses having a mirrored convex outside and a coated planar inside function as mirrors in the present case.

The anode 2 is composed of a copper sheet having a diameter of 40 mm and a thickness of 4 mm. This sheet covers one of the two base surfaces of the plate 1. The other base surface of the plate is covered by the cathode pot 3. As may be derived from the FIGS., the pot has the shape of a hollow cylinder 8 open at one end which is provided with an outwardly directed flange 9 at its open end. The flange, which contacts the plate 1, has roughly the same outside dimensions as the anode 2. The cathode 4 is situated in the inside of the hollow cylinder, being a commercially available electric discharge cathode in the present case.

In addition to the bore 7, the plate 1 contains three recesses which connect the discharge channel either to the anode (recess 10) or to the cathode space (recesses 11 and 12). The recesses 10 and 11, they define the length of the discharge path, are at a distance of about 1 cm from one another, and the recess 12, it is relatively narrow and serves for pressure equalization, is situated in the region of the anode recess 10. In order to prevent discharge paths through the recess 12, this opening could be displaced even closer to the mirror 6 and could discharge into the cathode space via a connecting channel in the flange 9. It can be demonstrated that axial pressure gradients in a mini argon laser (R<4.times.10.sup.-2 cm) having m equalization lines can be prevented when ##EQU7## applies (R.sub.1, L.sub.1 =radius and length respectively of every equalization line in cm). Given this dimensioning rule, the lines are also punch through proof at the same time.

The interior of the laser contains argon under a pressure of 5 Torr. During operation, a low pressure arc discharge is maintained on the discharge path of the bore 7 with a charge current of 200 mA and a discharge voltage of 130 V. The gain factor g.sub.o lies at about 0.15 cm.sup.-1 and the output power .PHI. amounts to roughly 1 mW.

The exemplary embodiment of FIG. 3 is constructed as follows: the beryllium oxide plate is composed of two halves 13, 14 which have arisen by means of a cut in the center plane of the plate parallel to the base area. The FIG. shows that the discharge channel 7 is introduced into the bottom surface of the upper half 13. This channel likewise has expanded ends into which a respective mirror carrier 15, 16 is inserted. Every carrier is composed of a small metal tube with an applied stop web which accepts one of the mirrors 5, 6 and simultaneously serves as discharge anode.

The cathode pot 3 is designed as a hollow cylinder completely closed at the floor side and closed with a perforated disk 17 at the plate side. The cathode space communicates with the discharge channel via the opening of the perforated disk 17 and a central plate recess 11. In order to enable a gas return, the upper plate half 13 also contains two further channels, a cross-channel 18 and an annular groove 19. The cross-channel proceeds perpendicular to the discharge channel 7 and cuts it in the region of the central recess 11; the annular groove connects two expanded ends of the discharge channel to the ends of the cross-channel. For the purpose of a better heat elimination, the upper plate half also carries a cooling member, schematically indicated in the FIG. as block 20. For the rest, the geometrical dimensions and operating parameters are similar to those in the first exemplary embodiment.

The laser modification of FIG. 3 has specific advantages: all plate recesses can be realized in a relatively simple way; in particular, the capillaries need not be bored. All parts can be soldered vacuum-tight to one another in a traditional metal-ceramic-(glass) technique. The tube also allows high heating temperatures.

The following FIGS., for the 0.488 .mu.m line of the ArII, again show the optimum, respectively critical operating and dimensioning values dependent on the radius of the discharge column. Suitable parameters and the appertaining output power for a prescribed discharge cross section can be effortlessly identified with the assistance of this curve apparatus.

FIGS. 4, 5 and 6 show the maximum values for the quantity P/L, .PHI./L and, respectively, i (curves 21, 22 and, respectively 22). All curves are defined for R-values <2.times.10.sup.-2 cm by the limit current density j.sub.gr and are defined for larger radii by the demand jR.ltoreq.50A/cm.

FIG. 7 presents Eqs. (11) and (12) in curves 24 and 25, respectively.

The critical length of a BeO member is entered in FIG. 8, namely for a TEM.sub.oo mode in a confocal mirror arrangement (curve 26). The curve shows that the maximum member length in the inventively prescribed range of inside cross section is defined only by diffraction losses. Only given an inside. diameter of about 1 mm would the rise of the straight line 26 be cut off by the mechanically defined limit length. At this point, let it again be pointed out that a mirror spacing between 2L and 3L is possible given very small radii (R<2.times.10.sup.-2 cm) for the Fresnel number F employed (F=R.sup.2 /.lambda.L).

The output power .PHI. shown in FIG. 9 (curve 27) is limited in the first segment by saturation effects (Eqs. (15) and (20)) and is limited in the second segment beginning at R=2.times.10.sup.-2 cm by diffraction losses (Eqs. (16) and (24)).

The invention is not limited merely to the illustrated exemplary embodiments. Thus, other laser transitions of the ionized argon or other ionized inert gases can be employed, for instance the lines of ArIII lying at 0.3638 .mu.m or 0.3511 .mu.m, or the 0.3507 .mu.m line of KrII. Insofar as the conditions must thereby be redetermined, this is at least possible without extraordinary outlay for the most important laser transitions. Independently thereof, the laser could also be operated in pulsed mode instead of continuous wave mode and, in individual cases, could also be employed as amplifier. Further left to the discretion of a person skilled in the art is to also structurally modify the laser. For example, the discharge channel can be lent an angular cross section and/or be operated in a mixed mode in which the discharge tube functions as a wave guide and maintains a defined distance from the mirrors; also conceivable is the employment of different channel plate materials such as aluminum oxide, boron nitride, eloxiated aluminum or certain crystals.

As is apparent from the foregoing specification, the invention is susceptible of being embodied with various alterations and modifications which may differ particularly from those that have been described in the preceeding specification and description. It should be understood that I wish to embody within the scope of the patent warranted hereon all such modifications as reasonably and properly come within the scope of my contribution to the art.