GB1565136A - Optical waveguides - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO OPTICAL WAVEGUIDES (71) We, STANDARD TELEPHONES AND CABLES LIMITED, a British Company of 190 Strand, London W.C.2. England., do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to the production of high mode number optical waveguides.

According to the present invention there is provided a method of making an optical waveguide in which an optically transparent crystalline material has an ionic impurity diffused into it by electric field assisted indiffusion.

Embodiments of the invention will now be described with reference to Figures 1 and 2 of the drawings which accompanied the Provisioned Specification.

A method is described herein for producing high mode number waveguides of enhanced refractive index in transparent ionic media, by means of electric field assisted indiffusion of an ionic impurity. In one example given, lithium niobate (LiNbO3) is used in conjunction with a source film of nickel oxide (NiO). Forced diffusion enables the optical depth of the waveguide to be more readily controlled than if a purely thermal process is used. It is usually necessary to employ contacting electrodes due to the high ionic conductivity of most materials at typical diffusion temperatures.

When prepared with good stoichiometry by zone refining to highest purity, typical ionic materials possess ionic leakage conductivities as low as 10- to 10.8 ohm~'cm~l at temperatures between 0.6 and 0.4 of the melting point. Several optically active materials, including lithium niobate, crystallise with unusually high defect concentrations and have high ionic leakage currents that are perhaps 104 times greater than typical ionic crystals of similar binding energy. The ionic conductivity of commercial magnesium-doped lithium niobate plates grown with congruent composition is about ten times greater even than the observed conductivity in stoichiometric lithium niobate crystals given by the following expression: log10o = 2.823 - 7150 T This is valid for T in "K between 400"C and 12008C when e is in ohm" cm~l. At 800"C, o exceeds 1.4 104 ohm"cm'. The total mobile charge in the substrate thus greatly exceeds the facial dielectric polarisation charge, notwithstanding the high DC high-temperature dielectric constant of the ferroelectric lithium niobate. Thus it is impossible to maintain an appreciable internal field in LiNbO3 when gapped electrodes are used, unless the gap is so small that the applied potential exceeds the breakdown voltage of the gap. Thus in most ionic materials, contacting electrodes are required above temperatures around 0.3 of the melting point.

It has emerged from previous electron microprobe measurements that indiffusion into Y-cut LiNbO3 plates from metallic coatings leads to low metal concentrations. This is thought to be related to the low solubility of neutral metal atoms in the mixed oxide crystal.

When films of metallic oxides were used, concentrations capable of producing refractive index changes adequate for waveguiding were readily observed. using NiO and ZnO, and it thus appears that oxide sources may be promising diffusants for fabrication of high mode number guides in LiNbO3.

The presence of the intervening oxide source layer prevents a metallic anode electrode from contacting the substrate directly when such field assisted diffusion is carried out, and to obtain an adequate potential drop across the substrate, the oxide film resistance must therefore be a negligible fraction of the substrate resistance. Fortunately this does not require an unusually low oxide resistivity provided on the oxide film is much thinner than the substrate crystal, as is usually the case.

In the production of nickel doped waveguides 20,um deep, a layer of NiO or Ni203 around lllm thick is an adequate source upon the basis of previous experiments. When the oxide resistivity is 1011 ohm cm at 4000C, or 1.2 106 ohm cm at 8000C, half of the applied voltage is lost across the oxide layer when LiNbO3 substrates 0.02 cm thick are assumed, obeying the above quoted equation. Nevertheless many refractory oxides will require traces of impurity doping to attain these conductivity levels. The films deposited by sputtering may be doped by using suitably contaminated targets. There is an alternative method of enhancing the conductivity of NiO films deposited upon LiNbO3 substrates, based on the fact that NiO acquires substantial impurity-band conduction when doped with lithium. Thus a film of NiO upon LiNbO3 could be doped with lithium by making use of the well-established outdiffusion of Li from the niobate: it would be necessary to maintain the coated substrate at a temperature near 11000C for a matter of five to ten minutes. If this process is carried on before the definition of any two-dimensional waveguides, the lateral refractive index change defining the guide wall would be reduced. Outdiffusion over the general area can perhaps be compensated during the doping of the oxide film by this technique, if the waveguide strips are defined first with any gold coatings required to form electrodes, the outdiffusion step is then carried out with a second polished slab of LiNbO3 placed in close contact with the waveguide face of the substrate. Outdiffusion from both slabs then creates an equilibrium partial pressure of Li2O vapour in the crack between the polished faces, and except for a margin around the edges of the contiguous crystals the outdiffusion from both slabs will be compensated thereby.

In making a planar guide, the surface of a Y-cut lithium niobate substrate 1 is coated with a thickness 2 of oxide adequate to ensure that the oxide source film remains intact throughout diffusion. The oxide layer and also the back face of the substrate are then covered by sputtered gold electrodes 3 and 4, about 2500 A thick. The coated substrate is heated to an appropriate temperature and a voltage applied making the oxide the anode.

Care must be taken that the actual substrate temperature is controlled by a sampling thermocouple during this process, and in order to avoid thermal runaway it is advisable to employ a current stabilised voltage supply.

Diffusion measurements for NiO on Y-cut LiNbO3 assuming Einstein's relation suggest that a process at 800"C would require an internal field in the niobate of 104 v/m applied for about 1l/2 hour. At temperatures above 400"C the leakage current is determined largely by the equation quoted above when the oxide resistance is assumed to be negligible, and exceeds ly ohm cam~2. The space charge is thus sufficiently high that the voltage V between the faces of the substrate is decided by the ratio of the film resistances. The substrate internal field is then independent of substrate dielectric constant : Ej = Q/ET = Viol1 where Q is the specific dielectric polarisation charge upon the niobate-to-oxide Snterface. If the voltage drop in the oxide layer is small, V is almost the electrode voltage. The value of Ei is required for a given waveguide penetration in a given process time drops with increasing temperature since the dopant ionic field mobility (apart from the linear factor) mirrors the thermal activation of the diffusion constant of the impurity ion. At higher temperatures the possibility of thermal runaway is somewhat more serious since the leakage current at 800"C and 104 V/m is already 15 mA, corresponding to a power density of 1.5 W / cm3 in stoichiometric LiNbO3, and may reach 150 W cm3 in congruent Mg doped crystals. The reduction in heating due to falling leakage current is somewhat offset by the need to increase potential as the temperature is reduced.

After diffusion the gold and residual oxide layers are removed by chemical cleaning and the surface finish restored by mechanical polishing.

The dimensional waveguides and networks are produced by firstly making up the sandwich structure of Figure 1 and then defining the networks in photoresist upon the anodic gold layer above the polished niobate surface. Standard selective etching techniques are employed to successively remove the gold and oxide layers from the substrate areas where waveguides are not required. During the etching the gold coating upon the reverse side of the substrate is protected by waxing and remains intact to form the cathode electrode plane during the field assisted diffusion process. This is carried out as described for the planar waveguide. Contact to the gold films is maintained by gold contact strips between which the substrate is pressed. (Figure 2).

Due to the small area of the waveguides the heating of the bulk substrate by leakage current is reduced, and as a result the local heating of the niobate within the defined guide areas may easily pass out of control of the sample thermocouple. If a current controlled voltage supply is employed, thermal equilibrium in the diffusing regions will be reached at a reproducible value of the local temperature, substantially above the actual sample-space temperature, but still controlled by the proportional oven temperature controller. An ideal method of sample temperature stabilisation involves controlling the furnace temperature from an error signal derived from the sample leakage current. Since the latter is decided by the area of the waveguide pattern being diffused, this method needs individual calibration of temperature, and is subject to variations in substrate and oxide impurity levels between batches.

After the diffusion process the substrates are cleaned and polished as before.

The edges of the strip waveguides tend to spread in the fringing fields marking the electrode edges, more markedly as the oxide film is increased in thickness. It is therefore advantageous in attempting to obtain a nearly rectangular beam area in the final waveguide to employ the thinnest oxide film that remains reliably intact during the processing. In order to allow for this lateral spreading the electrode strips used to define the waveguide area are made somewhat narrower than the desired guide width.

WHAT WE CLAIM IS: 1. A method of making an optical waveguide in which an optically-transparent crystalline material has an ionic impurity diffused into it by electric field assisted indiffusion.

2. A method as claimed in claim 1, and in which the transparent crystalline material is lithium niobate (LiNbO3) and the ionic impurity source is Uni203.

3. A method as claimed in claim 1, and in which the transparent crystalline material is lithium niobate (LiNbO3) and the ionic impurity source is NiO.

4. A method as claimed in claim 1, 2 or 3, in which the crystalline material has a gold layer applied to one face and an ionic impurity source applied to the other face, in which said source is etched to leave the desired pattern, and in which the pattern thus produced has a gold layer applied to it.

5. A method of making an optical waveguide, substantially as described with reference to the drawings accompanying the Provisional Specification.

6. An optical waveguide made by the method of any one of claims 1 to 6.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. areas may easily pass out of control of the sample thermocouple. If a current controlled voltage supply is employed, thermal equilibrium in the diffusing regions will be reached at a reproducible value of the local temperature, substantially above the actual sample-space temperature, but still controlled by the proportional oven temperature controller. An ideal method of sample temperature stabilisation involves controlling the furnace temperature from an error signal derived from the sample leakage current. Since the latter is decided by the area of the waveguide pattern being diffused, this method needs individual calibration of temperature, and is subject to variations in substrate and oxide impurity levels between batches. After the diffusion process the substrates are cleaned and polished as before. The edges of the strip waveguides tend to spread in the fringing fields marking the electrode edges, more markedly as the oxide film is increased in thickness. It is therefore advantageous in attempting to obtain a nearly rectangular beam area in the final waveguide to employ the thinnest oxide film that remains reliably intact during the processing. In order to allow for this lateral spreading the electrode strips used to define the waveguide area are made somewhat narrower than the desired guide width. WHAT WE CLAIM IS:

1. A method of making an optical waveguide in which an optically-transparent crystalline material has an ionic impurity diffused into it by electric field assisted indiffusion.

2. A method as claimed in claim 1, and in which the transparent crystalline material is lithium niobate (LiNbO3) and the ionic impurity source is Uni203.

3. A method as claimed in claim 1, and in which the transparent crystalline material is lithium niobate (LiNbO3) and the ionic impurity source is NiO.

4. A method as claimed in claim 1, 2 or 3, in which the crystalline material has a gold layer applied to one face and an ionic impurity source applied to the other face, in which said source is etched to leave the desired pattern, and in which the pattern thus produced has a gold layer applied to it.

5. A method of making an optical waveguide, substantially as described with reference to the drawings accompanying the Provisional Specification.

6. An optical waveguide made by the method of any one of claims 1 to 6.