Attenuation layer on interference mirror of gas discharge laser

The invention relates to a gas discharge laser comprising a laser tube with a longitudinal bore sealed at both ends, said tube being filled with a gas in which an active laser medium is accommodated in an optical resonator formed by at least two multilayer mirrors is formed, wherein the multi-layer mirrors have a number of alternating layers with a high and a low refractive index, which are mounted on the laser interior facing side of a substrate.

Such a gas discharge laser is known from DE-OS 2506498, in which a He-Ne gas discharge laser is described, in which the multi-layer mirrors are mounted directly on the ends of the laser tube, so that they also form the vacuum seal of the discharge space. The laser tube is connected by transverse tubes with electrode spaces outside the laser tube. In the laser tube, a discharge between the electrodes is initiated by the application of a potential difference between the electrodes, wherein a laser beam is generated by stimulated emission.

Gas discharge lasers have a wide range of applications. They are often used in measuring devices such as interferometers and the like. They are also used in devices for reading out record carriers on which information, for example image and / or sound information, is recorded in an optically readable information structure. They form therein a monochromatic radiation source, which emits a light beam, which falls on the recording medium and is reflected by this to a detection system. Such a device is described inter alia in Philips Technical Review 33, No. 7, p.198 to 202. Therein it is also mentioned that it must be ensured that not too large an amount of the light reflected and modulated on the disk surface can return to the laser. Namely, this feedback would cause undesirable fluctuations in the output power of the laser.

Unwanted light reflected off individual parts of the device, such as the outcoupling mirror, may interfere with the primary beam at the location of the detector, thereby resulting in modulation of the large modulation depth detector signal.

Such variations in output power and modulation are undesirable in many cases, even in other applications, such as in interferometers.

In DE-OS 2622560 it is described that in that a radiation-attenuating element is arranged in the radiation path of the readout beam of a readout device, the random intensity modulations in the readout beam, which are due to unwanted reflections in the radiation path, can be significantly reduced. The radiation-attenuating element is preferably mounted on the substrate of the coupling-out mirror in the form of an absorption filter. However, it turned out that this measure is not optimal, moreover, the power loss is high.

The aim of the invention is therefore to provide a gas discharge laser of the initially mentioned type, in which unwanted fluctuations in the output power and unwanted modulations are almost avoided and the outcoupling mirror on the outside has a very low reflection of unwanted light, while the reflection on the inside of the Discharge level generally increases even more and the power loss is minimal.

The gas discharge laser according to the invention of the type mentioned is characterized in that provided as Ausplplungsspiegel multi-layer mirror through which the laser beam generated by the gas discharge laser laser leaves, is provided with a metal film having an optical thickness between 0.01 and 0.03 λ, λ is the wavelength of the laser light generated, and wherein between the metal film and the substrate at least one layer and at most five layers, and the layers in the vicinity of the metal film have an optical thickness such that the optical impedance, as viewed from the substrate side, is adapted so that minimal reflection occurs from the substrate side.

The invention is based on the finding that in the multilayer mirror in this way a cavity resonator is formed, in which the unwanted reflected light is weakened by the radiation-weakening metal film.

- 3 Nr.370918

A preferred embodiment of such a gas discharge laser according to the invention is characterized in that on the substrate of the coupling-out mirror successively a layer with a high complex refractive index, a layer with a low complex refractive index, the radiation-attenuating metal film, a layer with a low complex

Refractive index and a number of layers with alternately a high and a low complex refractive index are applied.

The radiation-weakening film may consist of pure metal, but also of a cermet, ie a material of ceramic and metallic constituents.

As has been shown, optimum results can be achieved when the metal film has an optical thickness of 0.02λ. It is also particularly advantageous if the metal film is composed of one or more metals of the group formed by Ti, Ag, Cr, Al, Mg and Ni.

The gas discharge laser according to the invention can in a particularly advantageous manner in a device for reading a recording medium on which information, eg Image and / or Tonis information, in an optically readable structure are drawn, are applied, this device further includes a lens system, by means of which the readout bundle on the recording medium zuführhar a radiation-sensitive detection system; doing the described fluctuations and modulations are avoided, which would occur without the application of the invention and would affect the effect.

The invention will be explained below with reference to exemplary embodiments illustrated in the drawings. 2 shows a schematic illustration of a known 21-layer mirror, FIG. 3 shows a corresponding representation of a preferred embodiment of a coupling-out mirror according to the invention, and FIG. 4 shows an application of the gas discharge laser according to the invention in a

Device for reading a record carrier.

In the case of the gas discharge laser shown schematically in FIG. 1, multilayer mirrors -2 or 3 are fastened directly on the preferably cylindrical part of the gas discharge tube which forms the laser tube -1- by means of a two-component cement. These multilayer mirrors -2 and 3- have substrates -4 and 5-, respectively, on the insides of which multilayer packages -6 or 7--, for example, are applied by vapor deposition. With the aid of the electrodes located in side tubes -24, 25- a discharge in the laser tube -1- is generated. Stimulated emission results in a laser beam which leaves the laser via the multilayer mirror -3- provided as a coupling-out mirror. The laser described has the following laser parameters:

Length of the laser tube length of the active discharge current through the discharge inner diameter laser tube

gas filling

gas pressure

Resonator configuration Output power

<td>approximately</td><td>250</td><td>mm</td>

<td>approximately</td><td>205</td><td>mm</td>

<td></td><td>6.4</td><td>mA</td>

<td></td><td>1.8</td><td>mm</td>

15% Ne, 85% Hey

2.3 torr near hemispheric 1-2 mW at 632.8 nm

The invention is of course not limited to He-Ne lasers of the above-described side tube type, but it can also be applied to so-called coaxial lasers and known lasers, in which the multi-layer mirrors of the resonator are in some Ent45 distance from the closed with Brewster windows laser tube.

2 shows schematically a diagram of a known 21-layer mirror. The first layer -9- on the glass substrate tS) -8- is generally a layer having a high complex refractive index (H). The next successive layers alternately have a low IL) and a high IH) complex refractive index. Because the last layer is also mostly high

It is found that such reflectors often consist of an odd number of layers. As a rule, glass or quartz substrates are used. If two parallel surfaces -10, 11- are considered and if it is required that the light beams reflected at these surfaces be in phase to amplify each other by interference, the optical thickness of the intermediate layer -12- must preferably be one fourth the wavelength in the relevant layer. When using a plurality of layers, the successive surfaces will reflect light and enhance each of the first reflections. The maximum number of layers is determined by the absorption and the scattering in the material of the layers. Such a 21-layer mirror may be briefly referred to as S (HL) H, where S represents the substrate, H the high-complex-refractive-index layers, and L the low-refractive-index layers. In this case, the layers with a high complex refractive index consist of titanium dioxide (Ti0₂) and the layers with a low complex refractive index of silica lSiO₂); the layers have an optical thickness equal to one quarter of the wavelength of the light of a He-Ne15 laser, u.zw. nd = -. 632.8 nm, where n is the refractive index of the material of the layer in question and d is the mechanical thickness.

3 shows a preferred embodiment of a coupling-out mirror for a gas discharge laser according to the invention. On a glass substrate (S) -8- is a layer having a high complex refractive index (H) of titanium dioxide (TiO₂), on which two layers separated by a titanium film -13-IM) with a low (L) complex fission number of silicon dioxide (SiO₂) (LML) and then a layer packet of the usual composition (HL)^S are attached. The titanium dioxide {TiO₂) and silica (SiO 2)₂) Layers in the vicinity of the titanium film -13- have such an optical thickness that the optical impedance, as seen from the substrate side, is adapted, so that from the substrate side, a minimal reflection occurs. The titanium film 13 has an optical thickness of 0.02λ, where λ again represents the wavelength of the laser light. The complex refractive indices of the various metals suitable for the metal film are quite different, and accordingly, for matching the optical impedance in the vicinity of the metal film, the layers lying in the vicinity of the metal film have an optical thickness which varies depending on the complex Refractive index of the metal used can be very different. The two separated by the titanium film -13- silicon dioxide (SiO₂) Layers together form an optical cavity resonator in which the light returned across the substrate is captured and attenuated. As a result, the reflectance of the outcoupling mirror on the outside decreases considerably (eg, by a factor of -10), while the transmission through the multilayer mirror only slightly (less than 20%) decreases and the Beflexion on the inside, as calculations show, even something increases.

The radiation-attenuating element according to the cited DE-OS 2622560 is an absorption filter which, for example, weakens up to 30%. A modulation depth m is thus weakened to 0.3 m. The power V is weakened proportionally to 0.3V. The weakening with the decoupling_1

Mirror according to the invention is proportional to (R)²where R is the intensity reflection coefficient. This is about 10% in the present example. The reflection is almost 100% without application of the invention and decreases by a factor of -10 from the substrate side by application of the invention. A modulation depth m is thus weakened to about 0.3 m, while the power V decreases by 20% and thus equal to 0.8 V. With a dip mirror after the

Thus, the invention is obtained a low reflection from the substrate side at a relatively low power loss.

4 shows schematically an apparatus for reading out a recording medium on which information, for example image and / or sound information, are recorded in an optically readable information structure, in which device the application of a Gasent50 charge laser of the type described above offers great advantages. In this figure, a round disc-shaped recording medium -14- iso-formed image plate) provided with information tracks -15- is shown in section. The record carrier -14- is with

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5 illuminates radiation resulting from a He-Ne laser. A readlet -17- is focused by a lens -18- onto the surface of the information tracks -15- into a small radiation spot -20- on the order of the information details in the information structure. An auxiliary lens 19- ensures that the pupil of the lens -18- is filled.

The readout beam is reflected by the information structure, passes through the lens -18- for the second time and is imaged on a detector -21-. A transverse magnetic field in the laser causes the outgoing laser beam to be polarized parallel to this magnetic field. As a result, with a - λ plate -22- and a polarizing mirror -23-, a separation between the incident and the reflected light can be obtained. However, this separation is not completely effective; Light returns to the laser. As already mentioned, it is necessary to ensure that not too large an amount of light reflected and thus modulated on the optical elements and on the surface of the information carrier 14 will return to the laser and be reflected by the outcoupling mirror again in the device can be. This can be achieved by using a gas discharge laser according to the invention. However, the gas discharge laser according to the invention is also particularly well suited for use in interferometers, because fluctuations and modulations in the laser beam are undesirable there as well.