SE522705C2 - Preparation of single mode optical waveguides - Google Patents

Abstract

A technique for the preparation of waveguides from optic material, comprises creating the total reflection by reducing the refraction index in the regions surrounding those areas which become light conducting by applying a high electric field over a certain period of time, which leads to ions being transported away through the material without them being replaced by fresh ions. Preferably the optic material is an alkali-rich glass such as soda-lime or boron silicate glass, or it is a semiconductor-doped glass, or it is based on alumina. The following waveguide preparation methods based on the use of conventional (field-assisted) ion exchange techniques to define a core and mantle, are also claimed : (i) polarisation of the substrate after the core and mantle have been defined, so that the refraction index is reduced to a value equal to or greater than that for the core by polarising the mantle ; and (ii) polarisation of the core periodically in the length direction and polarisation of the mantle continuously in the length direction, once the core and mantle have been defined, so that the average refraction index value for the core is reduced to a value less than the refraction index for the mantle.

Description

|. | »O 522 705 2. be unsuitable for planar waveguide geometry, or require expensive equipment and / or complicated chemical processes.

The most common methods of making waveguides in soft glass are ion exchange and electric-field-assisted ion exchange. The ion exchange technique has been described in [R.V. Ramaswamy and R. Srivastava, “Ion exchanged glass waveguides: a review °, J. Lightwave Technology 6, 984-1002 (l988)]. A special ion in the substrate e.g. Nai ", is replaced by another ion such as K * or Ag fl which has been brought into contact with the glass substrate usually in the form of molten salt, so that a high index region for light conduction is formed. This manufacturing method requires high temperatures for the ion exchange to take place. Another soft glass waveguide technique is electric field-assisted ion exchange as described by [GLYip, PC Noutsios, K.

Kishioka, “Characteristics of optical waveguides made by electric- fi eld assisted K * ion _ exchange °, Opt. Lett., 15, 739-791 (1990)]. An applied electric field, at a sufficiently high temperature, increases the ion exchange rate through a stimulated ion drive. A fundamental feature of the electrically field-assisted ion exchange technique for the production of optical waveguides in glass is the injection of new ions into the substrate to replace other ions expelled from the substrate.

Waveguides have been fabricated in electro-optical polymers using birefringence obtained during polarity, where the refractive index of the polymer increases significantly for light polarized parallel to the polarity direction, and decreases for light polarized perpendicular to the polarity direction. This has been reported by J.I. Tackara et al in Applied Physics Letters 52, 1031-1033, 1988. For glass, however, the induced birefringence is not sufficient to provide guidance.

Planar waveguide fabrication techniques for soft glass described above are incompatible with polishing. If the waveguide is found first, a subsequent polarity with heating and an applied field leads to ground drift and a reduction of the guidance. If the sample is first polished, the heating necessary for ion exchange will erase the high frozen field in the glass. SUMMARY OF THE INVENTION This invention describes a technique of making planar waveguides with low losses by selective polishing of soda-lime glass. In this technique, and in contrast to ion exchange and field-assisted ion exchange, areas of the sample are deliberately depleted by polarity of previously existing ions. This takes place without any form of ion exchange. As a consequence of this procedure, the refractive index in the ion-depleted areas is reduced, leaving other areas in the sample that have not been polished with a higher refractive index.

These areas can then conduct light. The production is simple because it only involves the application of a high field for a certain time on lithographically patterned electrodes on a heated sample. High-quality single-mode waveguides can be formed with this method.

DESCRIPTION OF THE DRAWINGS, Fig. 1 shows a cross section of the elements which are important for the polishing procedure.

Fig. 2 shows a cross section of a polar sample where a waveguide has been formed in accordance with this invention.

Fig. 3 shows a perspective view of the waveguide in accordance with this invention.

Fig. 4 illustrates a perspective view of the waveguide formed with a periodic structure in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION Referring to Fig. 1, it is known that when a glass sample (1) is heated in an oven, on a hot plate or in other ways, cations become mobile and the conductivity is significantly increased.

In soda-lime glass, borosilicate glass or other glass, the main charge carrier is Na fl By applying high electric fields over a glass plate (1), the electric field (2) drives the sodium ions (3) from the side of the anode (4) to the side of the cathode ( 5). In this way, a thin tube (6) is formed in the glass near the anode which is depleted of sodium atoms, hereinafter referred to as depletion tube. In the technique described here, the anode apso: 522 705 4: (7) consists of a so-called "Blocking electrode", such as a sufficiently thick aluminum strip that prevents new ions from entering the sample. This prevents ion exchange in the depletion area and the refractive index of the depletion area (6) is not raised, but is lowered instead. The depletion area (6) can not be used for guidance as its adjacent areas (8) have the initial concentration of ions and higher refractive index. A sample (1) is typically 0.2 - 5 mm thick and 0.5 - 5 cm long and wide. Commonly used temperatures and voltages for polarity are 250-500 ° C and 1-30 kV respectively.

As shown in Fig. 2 of this technique, the anode (7) is not a continuous conductor in the lateral joint but instead has at least two regions (10) and (11) separated by a gap (9). By applying high voltage between the anode (7) and the cathode (12) on a heated sample (1), an exhalation area (6) is created which is not continuous in the lateral dimension, but instead has openings (13) below the gap (9). between the two regions (10) and (11) of the anode (7). Due to the gap (9), the electric field lines (2) do not extend in a straight line from the anode (7) to the cathode (12), but instead spread under the opening (13) of the depletion tube (6). One consequence of this is that some ions migrate towards the cathode (12) also in the area below the opening (13). The result is that an urethra (14) is formed below the opening (13), which to some extent is depleted of moving ions. This area (14) then has a lower refractive index than below the opening (13). In this way, the opening in the depletion groove area (6) mainly retains its original refractive index while the glass in the area adjacent to the opening (13) has a lower refractive index, namely the area (14) below the opening (13) and the depletion areas (6) below the two parts (10) and (11) of the anode (7). The covering medium (15) may be an m lm deposited on the sample (1), air or vaccum. The end result is a structure where the aperture (13) can conduct light through total re ection and where light can be coupled into the aperture with an optical fiber or a lens as shown in Fig. 3.

Another way of making waveguides which this invention relates to is polarity of a substrate (1) with an electrode (7) having a gap (9) between the areas (10) and (11) in the lateral: 522 705 lateral the dimension in which the gap (9) is in principle periodic in the logitudinal dimension which is shown in the perspective view in Fig.4. Using such an anode in the longitudinal dimension, the mean refractive index of the opening (13) below the substantially periodic gap (9) is reduced due to the smaller polarization than the refractive index of the two parts of the depletion region (6) below the regions (10) and (11) for the anode (7). In the same way as for an anode (7) which has a continuous gap (9), one which has a substantially periodic gap (9) creates a waveguide by polarity in a one-step process without the need for ion exchange or field-assisted ion exchange.

The preferred method described above does not preclude a manufacturing method in which conventional techniques such as ionuthyte or field-assisted ion exchange are used to define a guiding core and a sheath, and a subsequent polishing of the substrate. in which the refractive index of the shell is lowered as much or more than that of the core. A possible way to achieve this is by manufacturing a waveguide with conventional ion exchange or field-assisted ion exchange and then as illustrated in Fig. 4 poles the core in principle periodically in the longitudinal dimension and poles the jacket continuously so that on average in the longitudinal dimension reduced less than for the mantle. Although the above discussion relates to sodium, it is to be understood that other cations such as potassium behave in a similar manner, and the principle of the invention is equally applicable to glasses containing such ions. It will also be appreciated that waveguides can be fabricated using the technique described above for use in various configurations for well known applications in optoelectronics and electro-optics, where additional electrodes are either needed or not needed on the substrate to control light in the waveguide. In particular, balanced and unbalanced Mach-Zehnder interferometers, Sagnac interferometers, couplers, splitters, as well as active components such as armature amplifiers and phase modulators can be fabricated using the techniques described in this invention. It is also possible and advantageous to combine the waveguides manufactured with this technology with other well-known techniques such as Bragg lattice fabrication in -ua-n 522 705 b waveguides by UV illumination, for example to make an electro-optical component which electrically controls the wavelength for maximum response.

EXAMPLES Commercially available soda-lime glass having a thickness of 3 mm was used for waveguide fabrication in accordance with the technique described in this invention. On square pieces with a side length of 5 cm were patterned on one side of the samples over a 4 x 4 cm large surface, an approx. 0.3 μm thick aluminum electrode which was used as an anode. The pattern in the aluminum film as they were photographed consisted of a large number of gaps (9) with widths ranging from 2 microns to 12 microns as well as a number of wide gaps (about 200 microns). The waveguides were formed below the gaps (9) in the anode (7). The soda-lime samples were polished at 280 ° C, and the polishing pressure was increased in steps of * approx. 700 V to a maximum voltage of 3 kV. The integrated current flowing through the polarity circuit was typically 1_0 ° -10 ° C and the polarity time was about 12 hours. The cathode (12) was made of stainless steel and was kept in direct contact with the glass plate (1). The patterned anode was electrically contacted at the edges with a U-shaped piece that was placed upside down on top of the sample.Once the samples were polished, they were sawn into pieces, the edges polished and the waveguides were characterized.

A conventional near field measurement setup with a HeNelaser was used. It was found that for a polarity voltage of 3 kV, waveguides with widths of 2, 3, and 6 μm became single mode. Even wider gaps provided waveguides. The fact that a waveguide is formed means that the refractive index is lowered not only under the electrode but also under the channel itself. This can to some extent be explained by curved field lines, but it is also probable that the field distribution develops during the poling process so that the amount of cations is depleted during the channel as well.

An Nd: YAG laser at 1.06 μm was used to determine the refractive index difference between the unpolar core and the polar shell. The laser beam was focused on the sample and the reflected and transmitted signal was detected. Spatial filtering was used for the transmitted signal to identify the contribution of wavy and non-wavy light. In the first round of measurements, the light was connected to the sample and it was then translated vertically and horizontally. The reflected signal gave the position of the edge of the sample. For a sample (12 μm wide waveguide), it is confirmed that the transmitted signal was constant for a vertical translation of as much as 19 μm (FWHM). The sample was then repositioned horizontally so that light was switched on under the anode (7), ie. in the depleted area (6) with lower refractive index. The sample was then translated vertically again in the same way. As expected, the transmitted signal was very small when the light was focused near the surface and transmission took place only 19 μm below the surface consistent with the waveguide network. This result shows that the mechanism of waveguide fabrication is really the creation of an area with a lower refractive index by polarity. 19 μm below the surface, the reflected signal shows a discontinuity which corresponds to the edge of the refractive index change. From this discontinuity one can calculate a change in refractive index by polarity of -0.0_23, which corresponds to a percentage change of -1.5%. _

Claims (9)

10 15 “èo 25. 522 705. . . : | now PATENT REQUIREMENTS A method of manufacturing optical waveguides in an optical material comprising the steps of: - moving ions in the optical material from at least a first region in the vicinity of a surface in said optical material so that said region becomes ion-depleted, a second region is maintained substantially depleted in the vicinity of said surface in said optical material, and that no external ions are applied to said second region in the optical material. A method according to claim 1, characterized in that the ion depletion takes place by means of polishing with an anode comprising at least one gap in the lateral dimension during which said second area in the vicinity of said surface in the optical material is maintained depleted. 3. A method according to claim 2, characterized in that the gap is continuous in the longitudinal dimension. A method according to claim 2, characterized in that the gap is periodic in the longitudinal dimension. 5. A method according to claim 3 or 4, characterized in that the optical material is an alkali-rich glass, such as soda-lime or borosilicate glass. 6. A method according to claim 3 or 4, characterized in that the optical material is a semiconductor doped glass. 7. A method according to claim 3 or 4, characterized in that the optical material is based on alumina (alumina (Al¿h)). u:; »Ø | -O Method according to claim 5, 6 or 7, characterized in that the optical material is heated for a part of the time that a voltage is applied. 9. A method according to claim 8, characterized in that the voltage exceeds 100 V during at least part of the manufacture.