EP1706523A1

EP1706523A1 - Treatment of crystals in order to avoid light-induced modifications of the refraction index

Info

Publication number: EP1706523A1
Application number: EP04786888A
Authority: EP
Inventors: Karsten Buse; Matthias Falk; Konrad Peithmann
Original assignee: Deutsche Telekom AG
Current assignee: Deutsche Telekom AG
Priority date: 2004-01-14
Filing date: 2004-09-30
Publication date: 2006-10-04
Also published as: WO2005068690A1; JP2007519951A; JP4638883B2; US20070155004A1; US7833345B2; DE102004002109A1

Abstract

The invention relates to a method for the treatment of a crystal, in particular a lithium niobate crystal or lithium tantalate crystal, having nonlinear optical properties. The crystal comprises foreign atoms which bring about specific absorption of radiated light. The foreign atoms are transformed into a lower valent state by means of oxidation. Electrons, which are released during oxidation, are discharged from the crystal with the aid of an external power source.

Description

P03152F

Treatment of crystals to avoid light-induced changes in the refractive index

The present invention relates to a method for treating a crystal with non-linear optical properties, in particular a lithium niobate or a lithium tantalate crystal, the crystal having foreign atoms which bring about a specific absorption of incident light, the foreign atoms being converted into a lower valence state by oxidation , so that they subsequently assume a larger, in particular the largest possible, positively charged state.

It is known that electro-optic crystals, such as the LiNb0 ₃ - and LiTaCV crystals mentioned, change their refractive index in the case of externally applied or internally constructed electric fields. In addition, these crystals are ferroelectric; They therefore have a preferred axis ("spontaneous polarization"), the direction of which can be changed ("Poland"). Such oxidic crystals are required for a large number of applications in communications technology, for light generation, as data storage and for integrated optics. For example, Bragg gratings can be stored holographically in the crystals. These are then used as narrow-band wavelength filters or as reflectors for lasers. In addition, so-called "periodically poled lithium niobate / tantalate" (PPLN / PPLT), in which the direction of spontaneous polarization changes regularly and thus the formation of periodically arranged ferroelectric domains, is suitable for the construction of frequency P03152F - 2 - doubled for laser light (SHG, "second harmonic generation") or of optical parametric oscillators (OPOs). Furthermore, LiNb0 ₃ and LiTa0 ₃ crystals are used as substrate material for waveguides. In particular, it is possible with these crystals to realize integrated optical components together with the above-mentioned applications monolithically. The electro-optical effect offers the possibility of producing fast modulators by applying fields to the wave-guiding structure. As is known, doping serves to specifically influence the optical properties of the crystals used.

Frequency conversion in periodically poled material is of particular interest for new, powerful light sources. The light is often focused in the material in order to improve the efficiency of the conversion with high light intensities. In addition, high output powers of the light sources, which can be built on the basis of frequency conversion, are desirable. However, in order to guarantee the reliable functioning of the components, they must be optimized for use with high light intensities.

It is disadvantageous here that high light intensities change the refractive index and thus the optical properties of the crystal (photorefractive effect or "optical damage"). Inhomogeneous lighting in particular makes charge carriers movable in the material, which are subject to drift, diffusion and / or Redistribute the volume photovoltaic effect, build up electrical space charge fields in the crystal and thus change the refractive index via the electro-optical effect. Impurities in the crystal are of central importance, which serve as donors or catchers for the required charge carriers, which are known to be provided by doping Doping with copper, chromium or manganese is particularly widespread doping with iron, which has proven to be very efficient for the photorefractive effect, which is useful on the one hand for recording volume-phase holograms in the crystals o Refractive index changes can be achieved in a more controlled manner and holograms, such as Bragg gratings, can be saved the more effectively the material reacts to light. P03152F - 3 -

On the other hand, the optical damage leads to a deterioration in the optical crystal properties. "Optical damage" does not go hand in hand with mechanical damage, but only causes optical inhomogeneities that influence the propagation of light. As a result, the light can no longer be guided as a result of the damage. It is defocused and losses, especially in waveguides, and one As a result, the light intensity profiles used deteriorate, making the corresponding components inefficient.

Remedial measures can be taken, among other things, by increasing the dark conductivity. This ensures that no electric fields can build up in the crystal and thus no more photorefractive effect takes place. In this

Context, reference is made to DE 10 300 080 A1, the contents of which are referred to here in full.

However, a particularly high doping of the crystals of greater than 1 x 10 ²⁵ m ^"3 , in particular with iron or comparable effective elements, increases the dark conductivity in such a way that this in turn limits the space charge fields. The crystals doped with a lot of iron have a high absorption For visible light, this leads to heating and thus to thermal expansion at high light intensities, as well as thermally induced changes in the refractive index, which in turn destroy the beam profile.

The iron occurs in at least the two charge states 2+ and 3+. While iron 3+ as an electron scavenger does not cause absorption in the visible spectral range, iron 2+ as an electron donor leads to a broad absorption band for green to blue light. In order to change the valence state of the built-in doping centers, it is known to subject the oxidic crystals to thermal treatment at temperatures around 1000 ° C. (“tempering”). For example, tempering in an oxygen atmosphere leads to an oxidation of the crystals which transfers the iron from the valence state 2+ to the state 3+. However, this process cannot take place to any great extent, so that with the known treatment method part of the iron always remains in the state 2+ P03152F. 4th

special highly doped crystals, for a not inconsiderable residual absorption. For example, absorption cannot be reduced to below 2 mm ^"1 by conventional methods.

For other oxidic materials, such as potassium niobate, it is known that thermal treatment in an electric field instead of oxidation leads to a reduction. The reloading of the built-in centers results in a negative valence state.

The object of the invention is to provide a method which can be implemented inexpensively with simple means and with which the crystals are almost completely oxidized and the residual absorption in the crystals in question here, in particular in lithium niobate or lithium tantalate Can be reduced to a minimum.

This object is achieved by a method having the characterizing features of claim 1. Advantageous embodiments of the method are mentioned in the subclaims.

An essential aspect of the invention is to enable the Fermi level in the crystals to be lowered significantly by "assisted" oxidation. The invention ultimately lies in the annealing of the highly doped crystals, which is carried out in connection with an electric field during the temp, an additional high voltage of up to 1200 V is applied to the crystal, which sometimes generates currents of more than 10 mA, with which the electrons released during the oxidation are actively removed from the crystal Residual absorption and enriches valence states with little, advantageously none, absorption in the visible spectral range. The particular advantage of the invention is thus that the absorption is suppressed while at the same time having a high dark conductivity. The advantages of the invention occur particularly with lithium niobate and lithium tantalate crystals appearance. P03152F

If the parameters are chosen accordingly, the method according to the invention can be used to achieve, for example, Fe ²⁺ concentrations of less than 1 × 10 ²³ m ^"3. As stated, it is known that thermal treatment in an oxygen atmosphere at normal pressure leads to oxidation However, the concentration of Fe ²⁺ , in particular in the case of high total iron doping, cannot be reduced below values of 4 x 10 ²⁴ m ^"3 .

As already mentioned, it is known to carry out thermal treatments on potassium niobate crystals under the simultaneous application of electrical voltage. However, in all cases known to date, such a treatment led to a reduction in which the charge carrier donors and charge traps present are increasingly occupied by electrons and the Fermi level is increased. This effect, known as "electroreduction", therefore goes in the wrong direction.

Even though it is conceivable to remove the electrons released during the oxidation from the crystal by other means, it is particularly simple and therefore advantageous to use the method already described, the oxidation by applying thermal energy to the crystal and support with an electric field.

In addition, the method according to the invention can be used particularly advantageously in connection with the method described in DE 10 300080 A1. Then the foreign atoms are first introduced into the crystal by doping before the supported oxidation process is carried out. The foreign atoms are thus doping elements. It is also advantageous if the doping elements are not converted into any valence state by the supported oxidation according to the invention, but rather into the lowest possible valence state that can be achieved with the means used. Even if this is only possible in a few cases, it is important that the oxidation state achieved has no absorption.

As also described in DE 10 300 080 A1, it is advantageous if the doping elements used to increase the dark conductivity are extrinsic P03152F

Ions, especially iron ions in a concentration of more than 1 x 10 ²⁵ m ^"3 , are.

In order to be able to carry out the method as effectively and simply as possible, it is advantageous if the crystal is arranged between two electrodes, in particular between two metal electrodes, which are connected to a voltage source. One of the electrodes can be designed as a corona electrode which is not in contact with the crystal, the corona electrode in particular being connected to the negative pole of the voltage source. In this way, particularly high fields can be generated without contact within the crystal. In another easy to implement form, the crystal can simply be clamped between two electrodes.

In order to ensure that the free electrons are “washed out” as completely as possible, a sufficient voltage should be applied between the electrodes. In the case of contacting electrodes, this is in the range of a few volts, in particular approximately 10 V, with corona electrodes

Voltages of several 100 V, in particular between 800 V and 1200 V, in particular approximately 1000 V, are operated. The choice of the voltage is to be made in such a way that the application of the electric field within the crystal leads to currents between 0.01 mA and 15 mA, in particular about 10 mA. It is also advantageous if the application of thermal energy according to the invention within the crystal leads to temperatures between 300 ° C. and 1200 ° C., in particular between 800 ° C. and 900 ° C. In addition, the time of treatment should be chosen so that the desired effect is achieved.

The method according to the invention makes it possible to largely completely transfer the iron contained in the crystals to the 3+ charge state. As stated, this valence state does not cause absorption in the visible spectral range. This effectively suppresses absorption while maintaining high dark conductivity. The achieved residual absorption is below 0.4 mm ^"1 in the described choice of parameters. It is an advantage of the invention that P03152F - 7 -

that the low optical absorption minimizes thermal effects and limits losses.

The invention is explained below with reference to FIGS. 1 to 4. The figures show: FIG. 1 a schematic representation of a device for the thermal and electrical oxidation of a crystal by means of a corona electrode,

2 shows a schematic representation of the device for the thermal and electrical oxidation of a crystal by means of metallic electrodes sitting on the crystal surface, FIG. 3 shows an absorption spectrum of a largely oxidized, highly iron-doped crystal and

Figure 4: an absorption spectrum of a highly iron-doped crystal oxidized by conventional methods.

In the context of the invention, the suitability of LiNb0 ₃ crystals when examined with focused continuous wave laser light was examined in particular. Crystals doped with relatively much iron (1 wt.% Fe ₂ 0 ₃ in the melt) were systematically optimized. The thermal treatment method according to the invention was used at temperatures between 300 ° C. and 1200 ° C. to which the crystals were subjected.

According to the invention, a thermal treatment with simultaneously applied voltage in iron-doped LiNb0 ₃ or LiTa0 ₃ crystals is used for an almost complete oxidation. A voltage is applied to the crystal 1 to be treated. According to FIG. 1, the application of the voltage can be accomplished either by a single electrode 2 which is applied to the crystal 1 and by a further corona electrode 3 which is not in contact with the crystal 1. In this case, a voltage of approximately 1000 V is applied. In the example according to FIG. 2, electrodes 7, for example made of metal, are applied directly to crystal 1. In this example, a voltage of approximately 10 V is sufficient. P03152F - 8 -

The voltage is made available by a voltage supply 4 which can be switched or regulated. A current measuring device 5 enables the currents that occur to be monitored. The crystal 1 is then heated to temperatures between 300 and 1250 ° C. while the voltage is applied and is kept at this temperature for a few hours, here 900 minutes. This treatment leads to strong oxidation of the crystal.

An example of a corresponding treatment is shown in FIG. 3. The absorption coefficient is plotted against the wavelength in the diagram. The absorption lies in the wavelength range from 500 nm to 1100 nm below 0.2 mm ^"1. An exception is the absorption band at 482 nm, which results from a forbidden crystal field transition. However, this is much narrower than that usually caused by Fe ²⁺ Absorption shoulder, as clearly shown in Figure 4. Figure 4 shows the absorption spectrum of a comparable crystal with an identical doping amount, which has been subjected to a conventional oxidation treatment. the values for the absorption are below 4 mm ^"1 .

In comparison with the conventional treatment, the method according to the invention achieves absorption values which are at least one order of magnitude below the values to be provided to date. This results in a significantly reduced optical absorption in the visible spectral range with a very high dark conductivity.

Claims

P03152F - 9 claims

1. A process for the treatment of a crystal with nonlinear optical properties, in particular a lithium niobate or lithium tantalate crystal, the crystal having foreign atoms which cause specific absorption of incident light, the foreign atoms being converted to a lower valence state by oxidation, characterized in that that electrons released during the oxidation are removed from the crystal with the help of an external power source.

2. The method according to claim 1, characterized in that the oxidation is supported by the application of thermal energy to the crystal and with an electric field.

3. The method according to claim 1 or 2, characterized in that the foreign atoms are doping elements which have been supplied to the crystal before the supported oxidation by doping.

4. The method according to claim 3, characterized in that the doping elements are converted into the lowest possible valence state by the supported oxidation.

5. The method according to any one of the preceding claims, characterized in that the doping elements are extrinsic ions, in particular iron, copper and / or manganese ions of a concentration of more than 1 x 10 ²⁵ m ^"3 , which increase the dark conductivity of the crystal. P03152F - 10 -

6. The method according to any one of the preceding claims, characterized in that the crystal is arranged between two electrodes, in particular two metal electrodes, which are connected to a voltage source.

7. The method according to claim 6, characterized in that one of the electrodes is a corona electrode which is not in contact with the crystal, in particular the corona electrode being connected to the negative pole of the voltage source.

8. The method according to any one of claims 6 and 7, characterized in that a voltage between 1 V and 1200 V is applied between the electrodes, wherein in the case of a corona electrode in particular about 1000 V and in the case of two contacting electrodes in particular about 10 V are applied ,

9. The method according to any one of the preceding claims, characterized in that currents between 0.01 mA and 15 mA, in particular between 1 mA and about 10 mA, are generated in the crystal by the application of the electric field.

10. The method according to any one of the preceding claims, characterized in that the application of thermal energy leads to temperatures between 300 ° C and 1200 ° C, in particular between 800 ° C and 900 ° C.

11. Optical, in particular nonlinear optical component having a crystal with nonlinear optical properties, the crystal having foreign atoms which bring about a specific absorption of incident light, the foreign atoms being converted into a lower valence state by the assisted oxidation according to one of the preceding claims, thereby characterized in that the crystal has a residual absorption of less than 0.4 mm ^"1 .