GB2223323A - Optical devices using curved, concentric coupled optical waveguides - Google Patents

Abstract

The invention discloses optical waveguide devices based on curved directional couplers in which the two coupled waveguides 10, 12 form adjacent concentric arcs. The built-in asynchronism of such a device can be accurately defined, is insensitive to fabrication variations and requires only one stage of photolithography (in the case of monolithic versions of the device). Applications to optical switching and wavelength duplexing are described. <IMAGE>

Description

OPTICAL DEVICES USING CURVED, CONCENTRIC COUPLED OPTICAL WAVEGUIDES Technical Background Directional Couplers Optical directional couplers are well-known and much utilised devices in both monolithic (integrated) optical and fibre-optic formats. When monomode dielectric waveguides run parallel and close together for a distance L before diverging, light input to one (the primary) waveguide couples across to the other (the secondary) waveguide (see Figure 1). Because this phenomenon is essentially due to the interference (or "beating") of the two modes of the coupled system, the light amplitudes are cosine and sine functions respectively, in the primary and secondary guides, of distance down the guides.

Transfer Length. Loins an important parameter for describing a coupler. If L=Lo then all light is transferred to and remains in the secondary guide. L0 has a reciprocal relationship to the Coupling Coefficient K which depends on the guide dimensions, spacing and material constants.

Asvnchronous Directional Couplers.

Complete transfer, as described above, requires not only that L=Lo (or L=MxLo where M is an odd integer), but also that the two guides have the same propagation constant (13). ) . The propagation constant is inversely related to the velocity of light in the guide.

Under these circumstances. the essential phase relationship between the light in the two guides is preserved. This is automatically the case when the guides are identical, but can also be arranged between non-identical guides. If the propagation constants differ - i.e. the coupler is asynchronous - transfer is incomplete and spatially more rapid. Asynchronism increases the effective coupling coefficient. but reduces the maximum light transfer. For very short couplers (L < < Lo) these two effects nearly cancel.

Asvmmetric Coupler Wavelength Duplexers All directional couplers have some wavelength sensitivity which is of limited usefulness of itself. Two wavelengths which are required to be separated (one seeing a "through state and the other, a "cross" state) cannot in general be uniquely and independently setup with any ease.

A different, known approach utilises wavelength dependent asynchronism. When guides have different widths, the propagation constant (8) changes with wavelength at different rates (see Figure 2a). At some unique wavelength (see Figure 2b) waveguides with different cross sections can be arranged to be synchronous (for a good "cross" state) provided that the wider guide is also the shallower or is made of lower refractive-index material. Both options are difficult to carry out in practice and to control accurately.

Optical Switches Most optical switches operate by changing a coupler, electrooptically, so that a synchronous coupler becomes asynchronous. If L=L0 initially, the default state is the so-called "cross" switch state; two independent inputs transfer across to opposite ports without mixing. If the coupler has been fabricated in an electrooptic material, applying an electric field across one of the two guides causes asynchronism. Sufficient field can double the effective coupling coefficient so that all transferred light (now much less than 100%) returns to the primary guide in the given length.

Zero net transfer gives the "through" switch state (see Figures 3 and 4a).

The Reverse Delta-Beta (Ass ) Switch A problem with the simple switch described above is the requirement for an exact L=L0 default condition. Since L0 is very sensitive to fabrication variations this is difficult to achieve with sufficient accuracy.

If the electrically induced asynchronism is reversed in sense half-way down the coupler (reverse ss ) both switch states can be accessed for a wide range of L/Lo ratio (1 sL/LO < 3). Figures 3 and 4b both illustrate this latter type of switch whilst Figure 5 illustrates the computed responses to applied voltage for both uniform and reversed Ass switches.

It is seen that the reverse AB switch would benefit from some built-in asynchronism whose sense reverses half-way down. This would pre-bias the switch mid-way between states. Thus a simple polarity reversal of the switching voltage could be used to access either switch state. However, reliance on electrical pre-bias of the required degree would risk electrical breakdown and thus severely restrict design scope.

The present invention relates to a technique for the accurate, predictable and simply-implemented built-in asynchronism which simulates that which would result from a depth or index difference, and is thus useful for building-in the required pre-bias geometrically.

Accordingly to the present invention, there is provided an optical coupler comprising a pair of dielectric waveguides extending in closely spaced parallel relationship for a predetermined distance, the waveguides, over the predetermined distance being curved in a common plane so as to form concentric arcs.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which: Figures 1 to 5 illustrate aspects of the technical background to the present invention; Figure 6 diagrammatically illustrates a basic form of an optical coupler in accordance with the present invention; Figure 7 diagrammatically illustrates a reverse AB switch in accordance with the present invention; Figure 8 diagrammatically illustrates a wavelength duplexer in accordance with the present invention; and Figures 9 and 10 illustrate graphically the response of an ideal duplexer and the effect of curved concentive guides on the propagation constant versus wavelength relationship (cf Figures 2a and 2b).

Referring to Figure 6 and 1 ova, an optical coupler constructed in accordance with the present invention may be described very simply: a coupled guide-pair 10, 12 comprising monomode dielectric waveguides is made to be curved in the lateral plane in such a way that the two guides form concentric circular arcs. If the guides 10, 12 are otherwise identical, their slightly different radii R and R+R nevertheless introduce a substantial asynchronism. Light in the outer guide 12 must propagate with a greater linear velocity ( and guidewavelength) if it is to stay in phase with light in the inner guide 10 ie synchronism now requires a consistent angular phase velocity.

The degree of asynchronism due to curvature is fixed by the two radii of curvature. It is thus easily controllable at the device design stage and cannot be inadvertently changed by fabrication uncertainties and error. Typically, the designer knows the difference between the radii (aR = the guide separation + guide width) that he wants to use and the minimum radius R for low loss. The required coupler length L will also be known. Simple geometrical concepts then determine the precise radii and arc angle.

Referring to Figure 7, there is shown a reverse Ass switch using this inventive concept. The coupler resembles an S-bend. The guides 14, 16 have cross-connected electrodes 18, 20 as shown.

Figure 8 illustrates an asynchronous duplexer comprising guides 22, 24 which are of different widths. The disparity due to the different radii can be used to obtain 100% coupling at a particular wavelength. Conversely, if the wavelengths to be coupled are known, the differences in radii can be calculated to achieve the required coupling.

It will be appreciated that the guides 10,12, 14, 16 and 22, 24 are co-planar. The radius of curvature must, of course, be finite to achieve the advantages of the present invention.

The various couplers of the present invention may be realised in integrated, monalithic, planar format or by the use of optical fibres.

As is well known, the guides must not be spaced at a greater distance than that permissible for coupling to take place.

Generally, where the guides are of different transverse dimensions, the inner (smaller radius) guide will have the greater dimension. It is possible, with the device of Figure 7, to provide a uniform Ass switch instead of a reverse Ass switch.

Other variations are possible within the scope of the present invention.

Claims (8)

1. An optical coupler comprising a pair of dielectric waveguides extending in closely spaced parallel relationship for a predetermined distance, the waveguides, over the predetermined distance, being curved in a common plane so as to form concentric arcs.

2. A coupler as claimed in claim 1 wherein the waveguides are of different transverse dimension in the common plane.

3. A coupler as claimed in claim 2 wherein that waveguide on the inside of the curve has the greater transverse dimension.

4. A coupler as claimed in any preceding claim further including electrodes for applying an electric field thereacross.

5. A coupler as claimed in any preceding claim further including a second oppositely curved coupler of which the waveguides are continuous with the respective waveguides of the first coupler.

6. A coupler as claimed in claims 4 and 5 wherein the electrodes of the second coupler are cross-connected with the electrodes of the first coupler so that the field applied across each coupler extends in the same direction relative to the curvature of the coupler.

7. A coupler as claimed in any preceding claim fabricated in integrated, monolithic planar format.

8. An optical coupler substantially as hereinbefore described with reference to and as illustrated in Figure 6 or Figure 7 or Figure 8 of the accompanying drawings.