GB2191304A - Integrated optical channel waveguide - Google Patents

Abstract

The refractive index distribution in both width and depth in a waveguide produced by indiffusion or ion exchange (eg. if titanium into lithium niobate) is controlled by an initial pattern produced on the waveguide substrate (1) by using a single stage photolithographic process. The initial pattern on the substrate is chosen so as to produce the required distribution and for each waveguide may comprise at least two strips (11), each adjacent pair of strips being separated by a respective gap (9). The refractive index variation is controlled by varying the strip width to gap ratio. Waveguide mode size can be tapered along the length of the waveguide by corresponding variations in the initial pattern. Losses at waveguide bends can be reduced by employing a pattern which increases the refractive index increase at the inside of the bend relative to the outside thereof. <IMAGE>

Description

SPECIFICATION Integrated optical channel waveguides This invention relates to integrated optical channel waveguides and in particular to the manufacture thereof.

Integrated optics is concerned with the manipulation and processing of light in channel waveguides formed near the surface in a crystal with appropriate optical properties. The crystal may be lithium niobate which has a large electro-optic effect, in which case light propagating in the waveguide may be changed in phase by the application of an electrical bias field across the waveguide. A waveguide can be formed by increasing the refractive index in a "channel" of material near the surface of the crystal. A waveguiding action similar to that in optical fibres then occurs in the channel. Such waveguides are most commonly formed by an indiffusion or ion exchange process, for example titanium indiffusion or proton exchange in lithium niobate. For optimum device design it is necessary to be able to vary the refractive index increase (An) profile in a controlled manner.Previous attempts at controlling the An profile have concentrated on varying the thickness of a titanium layer, which is deposited on the surface and then indiffused to produce the waveguide, at different parts of the waveguide. This is a process which involves multiple evaporation and/or etching steps.

According to the present invention there is provided a method of manufacturing an integrated optical channel waveguide in a substrate by an indiffusion or ion exchange process, including the step of providing an initial pattern on the surface of the substrate by a single stage photolithographic process, the initial pattern including, for the production of a single waveguide, two or more closely spaced strips, each adjacent pair of strips being separated by a respective gap, which initial pattern is such as to control the refractive index distribution in the waveguide in both width and depth.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figures la, lb and ic illustrate, respectively, and in cross-section, a deposited strip of titanium on the surface of a crystal, a waveguide channel formed by the diffusion of the titanium into the crystal (indiffusion profile) and the waveguide modes supported by the channel; Figures 2a and 2b illustrate, respectively, a single titanium strip and the corresponding diffused channel (indiffusion profile), and a pair of titanium strips presenting the same overall width as the single strip and the corresponding diffused channel (indiffused profile); Figure 3 illustrates a plan of a titanium strip which splits into two portions partway along its length and the corresponding indiffused profile at three positions along its length;; Figures 4a and 4b, respectively, indicate a plan of a curved waveguide channel and the theoretical required An profile for equalised optical path lengths around the curve, and Figures 5a and 5b, respectively, indicate a titanium strip width pattern and the corresponding An profile which approximates to the theoretical profile of Fig. 4b.

Referring firstly to Fig. 1, a waveguide can be formed in a lithium niobate crystal 1 by the deposition of a titanium strip 2 commonly 600-900A thick and 6-8,um wide and as long as required. The deposited titanium is then diffused into the crystal 1, typically by maintaining the crystal with the titanium deposited thereon at 1050"C for six hours. The titanium diffuses into the crystal and produces a waveguide channel with the cross-section (diffusion profile) illustrated in Fig. 1 b as a result of the titanium altering (increasing) the refractive index of the substrate nS by An. The refractive index of the waveguide ng is thus given by ng =n+An. Fig. 1c illustrates the waveguide modes supported.From a strip 6jum wide and 600A thick there results a waveguide supporting modes with, for example, Wx=8,um and Wy=5.5Am.

As metioned before, the An profile can be altered by correspondingly changing the thickness of the titanium strip from which it is diffused by multiple evaporation and/or etching steps. However, if instead of changing the titanium thickness, two closely spaced narrow strips 4 of titanium (Fig. 2b) which present the same total width as a single strip 7 (Fig.

2a) are indiffused, the resulting An profile 5 will be similar to that 6 produced by a single strip albeit of reduced thickness, and with correspondingly lower An. An2 > An,. If the total width of the single strip 7 is 6,um, the two strips 4 may each be 2,um wide and separated by 2Fm, the strips all being 900A thick initially. The uniformity of the indiffused An profile will be improved if an increased number of narrower strips of titanium are indiffused. A spaced plurality of strips of titanium are formed in the same basic way as a single strip by using a photolithographic process.For example, the crystal surface may be completely covered with titanium, and then a resist which is baked and exposed through a suitable mask and then developed, the mask pattern and development being such as to uncover those areas where titanium is not required and then etching the exposed titanium away. Alternatively, the bare crystal surface may be coated with resist, baked, exposed through a suitable mask, developed to remove the resist where titanium is to be deposited, and titanium evaporated over the whole surface, the remaining resist and the titanium overlying it being subsequently removed by a lift-off technique, ieaving titanium only in the required areas.The more narrow strips there are, the more demanding the photolithographic process, although only a single lithographic step is required to vary the An profile in both width Wx and depth Wy. An is thus controlled merely by varying the strip width to gap ratio.

Whereas only titanium diffusion into lithium niobate has been described for the waveguide formation, the principle of varying the An profile by controlling the initial pattern laid down in the waveguide formation is applicable to other indiffusion processes and also to ion exchange processes. Ion exchange processes involve forming a protective pattern by a photolithographic masking process, which pattern exposes those areas of the crystal surface where the waveguide is to be formed and then disposing the crystal in the appropriate material, at an appropriate temperature and for the appropriate time to achieve ion exchange.

One example of an ion exchange process is proton exchange in lithium niobate.

Using the above mentioned control of An profile it is possible to taper the optical mode size in channel waveguides. A key parameter in the design of optical waveguides is the waveguide mode size. This refers to the width Wx of the power distribution measured in a plane perpendicular to the direction of propagation (Fig. 1 c). The waveguide mode size is strongly dependent on the An distribution and the magnitude of the peak An. The method of controlling the An distribution described above by varying the strip width to gap ratio can thus be used to taper the waveguide mode size along the waveguide.

In Fig. 3 a gap 9 is introduced into a titanium strip 8 which is, overall, say 6,um wide.

At one end of the strip 8 there is thus one thick finger 10 whereas at the other end there are two thin fingers 11 which are each, say, 2,um wide and separated by a 2,um gap. The indiffused profile for the thick finger has a high An, that for the thin fingers has a low An whereas where the gap begins and gradually increases there is a medium An. Thus the An tapers along the waveguide and the waveguide mode size along the waveguide is con trolley simply by a single photolithographic step to define the fingers and their widths and the gaps accordingly. Preferably there is a larger mode size near the edge of a crystal and an associated end of the waveguide in order to facilitate coupling of light into the waveguide, such as from an optical fibre.

Another application for the method of control of the refractive index distribution of the present invention is in order to reduce loss in integrated optical channel waveguide bends and curves. In a conventional curved optical channel waveguide 12 (Fig. 4a) 6Am wide, the optical path lengths at the inner 13 and outer 14 eges of the waveguide (of radii R, and R2, R2=R,+6,um) are different, leading to loss of optical power. If the refractive index can be increased at the inner edge, that is the refractive index increase An can be increased, to equalise the optical path lengths, then losses can be reduced, or smaller radii curves can be utilised without introducing significant extra (excess) loss. Fig. 4b shows the theoretical An profile required across the width of the waveguide for equalised optical path lengths; Fig. 5a illustrates a possible strip-gap titanium pattern for producing such a An profile, and the profile this pattern produces is illustrated in Fig. 5b. The pattern comprises from left-toright (outside edge-to-inside edge) a 1'ism strip, a 1,tm gap, a 1Am strip, an 0.5m gap and a 2.5,am strip. This example shows particularly well the control of refractive index distribution in width achieved by use of multiple strips of titanium of different widths and with different gaps.

Claims (6)

1. A method of manufacturing an integrated optical channel waveguide in a substrate by an indiffusion or ion exchange process, including the step of providing an initial pattern on the surface of the substrate by a single stage photolithographic process, the initial pattern including, for the production of a single waveguide, two or more closely spaced strips, each adjacent pair of strips being separated by a respective gap, which initial pattern is such as to control the refractive index distribution in the waveguide in both width and depth.

2. A method as claimed in claim 1 and involving the indiffusion of titanium into a lithium niobate substrate wherein for the strips of the initial pattern there are produced corresponding strips of titanium from a layer of titanium deposited on the lithium niobate substrate, which strips of titanium are subsequently indiffused into the lithium niobate substrate to form the waveguide.

3. A method as claimed in claim 1 or claim 2 wherein optical mode size along the length of a waveguide is controlled by correspondingly varying the width-to-gap ratio of the two or more strips along the length of the waveguide.

4. A method as claimed in claim 1 or claim 2 wherein the waveguide is curved and the strips of the pattern are correspondingly curved, the width-to-gap ratio thereof being chosen to increase the refractive index increase at the smaller radius edge of the waveguide relative to the larger radius edge thereof whereby to reduce optical power loss at the curve.

5. A method of manufacturing an integrated optical channel waveguide with the refractive index distribution thereof controlled in both width and depth substantially as herein described with reference to the accompanying drawing.

6. An integrated optical channel waveguide manufactured by a method according to any one of the preceding claims.