WO1990000752A1

WO1990000752A1 - An optical system

Info

Publication number: WO1990000752A1
Application number: PCT/GB1989/000763
Authority: WO
Inventors: Eric Ian Drummond
Original assignee: Plessey Overseas Limited
Priority date: 1988-07-06
Filing date: 1989-07-06
Publication date: 1990-01-25
Also published as: GB2220501A; GB8816064D0

Abstract

An optical system coupling a waveguide such as a laser diode (4) to a waveguide such as an optical fibre (6) must transform the beam ellipticity for efficient coupling. The optical system comprises a combination of lenses (8a, 8b, 10) providing independent coupling in two orthogonal directions. The combination of lenses includes first and second lenses each having a respective index which may be homogeneous or inhomogeneous. This enables the optical coupler to provide efficient, low-loss coupling and has particular advantages when coupling an array of waveguides on a semiconductor to optical fibre ''pigtails''.

Description

AN OPTICAL SYSTEM

The present invention relates to an optical system and in particular, but not exclusively to, an optical system comprising an optical coupler for coupling waveguides such as laser diodes.

Certain waveguides, e.g. conventional lasers, or optical fibres provide a circularly symmetrical beam output. Optical couplers for such waveguides are relatively simple, matching the numerical apertures. However, some waveguides provide an elliptically symetrical beam output.

Gaussian Optics describe the shape of a beam in three

dimensions in terms of the diameter of the focussed waist of the beam .

A general gaussian beam has an elliptical profile at the waist. Laser diodes are examples of waveguides providing a neargaussian elliptical beam output due to the geometry of the output region. Typically the dimensions of the output region of a laser diode are in the order of 3μm wide and 0.5μm deep. Consequently in the far field the beam divergence in the horizontal plane is relatively narrow and in the vertical plane relatively wide.

Laser diodes are usually coupled to optical fibres, which have acircular cross-sectional beam input, but laser diodes may be coupled to any other waveguide having the same or a dissimilar beam eliipticity. An optical coupler in order to match efficiently the numerical apertures of the waveguides must transform the beam ellipticity. This can be achieved by an optical coupler which permits independent magnification of the beam waist in two perpendicular directions (see Figure 1 ).

Hitherto bulk optical lenses with pan-cylindrical surfaces have been used in an anamorphic system e.g. wide screen cinemas to give dissimilar magnification in perpendicular directions (see Figure 2). In photographing or projection, the subject or screen respectively are both distant and also reflect light diffusely over an extremely wide angle. This large numerical aperture is impossible to match from a distance, so photographic lenses are just made with as large an aperture as possible.

Anamorphic systems have a relatively small numerical

aperture and so matching the numerical apertures is not especially important. Whereas the optical system coupling waveguides subject of the present invention, must provide very accurate numerical aperture matching due to the dimensions of the waveguides in combination with having large numerical apertures.

Consequently, applying an anamorphic system on a reduced scale to waveguides such as laser diodes would encounter prohibitive difficulties not only in the accuracy of matching the numerical apertures but also of providing highly polished lenses with minimum aberrations.

Furthermore an anamorphic system would not easily permit extension to closely-packed arrays of waveguides. If a single pair of crossed cylindrical lenses were used to cover an array of waveguides. then it would employ off-axis imaging and eccentric apertures which would require either compound lenses or aspherics to give high numerical aperture matching and wide field of view with low aberrations. Since semiconductor waveguides require very high numerical aperture matching such an optical coupler would be impractical. Also this type of optical coupler would introduce an angular misalignment as shown in Figure 3.

A known optical coupler suitable for waveguides such as laser diodes includes a transverse homogeneous cylindrical lens. The transverse lens is transverse since its axis is orthogonal to the path of the beam. The coupler matches the ellipticity of the laser diode to the optical fibre through focussing in the vertical plane. However increasing focussing in the vertical plane means the beam in the horizontal plane becomes defocussed thereby introducing an astigmatism as shown in Figure 4.

An object of the present invention is to provide an efficient, low-loss, optical system suitable for coupling waveguides such as laser diodes.

Another object of the present invention is to provide an optical system suitable for coupling an array of waveguides.

According to the present invention there is provided an optical system comprising an optical coupler and at least two waveguides, the optical coupler enabling at least one of the waveguides to be coupled to at least one other of the waveguides and the optical coupler including a combination of transverse cylindrical lenses providing orthogonally independent coupling.

Advantageously the combination of transverse lenses

comprises a number of first lenses, the or each first lens having a first axis and a number of second lenses the or each second lens having a second axis whereby each first axis is substantially

orthogonal to each second axis.

The present invention will be described with reference to the accompanying drawings, of which:

Figure 1 is a schematic diagram of elliptical beam conversion in the vertical plane (Figure 1 a) and the horizontal plane (Figure 1b):

Figure 2 is a schematic diagram of an anamorphic system according to the prior art;

Figure 3 is a schematic diagram of a single lens illustrating angular misalignment in array imaging;

Figure 4 is a schematic diagram of the cross-section of a cylindrical lens illustrating an astigmatism;

Figure 5 is a schematic diagram of a first embodiment of the present invention (Figure 5a) and a schematic diagram illustrating a beam propagated by the first embodiment (Figure 5b):

Figure 6 is a schematic diagram of a second embodiment of the present invention;

Figure 7 is a schematic diagram of cross-sections of a

homogeneous circular lens 7a, a generalised Luneburg lens 7b. and a Luneburg lens 7c each propagating a beam:

Figure 8 is a schematic diagram of a cross-section of a diffused cylindrical lens array;

Figure 9 is a schematic diagram of five cross-sections of five Dshaped lenses: and

Figure 10 is a schematic diagram of two forms of optical switch.

A first embodiment of the present invention is shown in Figure 5. An optical coupler 2 enables a beam to propagate from one waveguide such as a laser diode 4 to another waveguide such as an optical fibre 6. For the purposes of this description a waveguide is any optical conductor, for propagating electromagnetic radiation, having dimensions no greater than approximately 2cm wide and 10 μ m deep. The optical coupler 2 comprises first and second transverse homogeneous cylindrical lenses 8a and 8b respectively and a diffused cylindrical graded index (GRIN) lens array 10. The beam consists of two orthogonal components for convenience termed vertical and horizontal. The horizontal component passes undeviated through both cylindrical lenses 8a, 8b and is focussed by the GRIN lens array 10. The vertical component is focussed by the cylindrical lenses 8a, 8b and passes through the GRIN lens array 10 with little deviation. The horizontal and vertical components of the bearr. are thus focussed independently at the output aperture. The optical coupler 2 has transformed the beam from an elliptical beam to a circular beam suitable for propagation by the optical fibre 6.

The optical coupler 2 provides more degrees of design freedom than available hitherto, since the cylindrical lenses 8 each have an index and a radius and there are possibly three different media surrounding the lenses 8, providing a maximum of seven degrees of freedom. This increase in degrees of design freedom will allow a reduction of spherical aberration without the complexity of graded index fabrication. Further, a higher focussing power is available which meets the requirement with laser diodes of high numerical aperture without needing high-index lenses.

A second embodiment of the present invention is shown in Figure 6, which consists of the optical coupler 2 positioned on an optical microbench 12, e.g. v-grooves etched in silicon. The

cylindrical lens 8b is mounted in a groove 14 in the optical micro bench 12. The other cylindrical lens 8a is permitted to rotate about the axis of the cylindrical lens 8b, thereby allowing "translation" of the lens 8a in an arc about the same axis. This permits a degree of beam-steering freedom in the vertical plane.

Having a degree of beam-steering freedom in the vertical plane is particularly advantageous in coupling semiconductor devices.

Semiconductor devices have vertical mode depths in the order of 1 μm, as opposed to other waveguides such as Ti: LiNbO₃ which have vertical mode depths of approximately 7 to 10μm. The optical coupler 2 can steer the beam more accurately in the vertical plane thus increasing the efficiency of the coupler 2.

Furthermore, having a degree of beam-steering freedom will also enable a particularly advantageous assembly technique to be used. This assembly technique is the "flip-chip" technique which employs surface tension in molten solder droplets to pull into alignment similar metal patterns on adjacent surfaces between which the solder is sandwiched. Thus a metal pattern on the surface of an inverted waveguide chip may be self -aligned to a pattern on a silicon v-groove chip holding the fibres and lenses. Variations in groovedepth and solder thickness occur among fabricated chips, limiting the height accuracy to about 1μm , which is sufficient for LIN60₃. but not for the sub-micron mode-heights in semiconductor waveguides.

The beam-steering capability of the optical coupler 2 enables many microns of compensation for height errors. In Figure 7a there is shown a cross-section of an homogeneous circular lens propagating a beam. The lens creates a relatively large spherical aberration of the beam as can be shown by the indistinct focal point A. Using an inhomogenous lens enables such spherical aberration to be minimised. In Figure 7b a cross-section of a

generalised Luneberg lens illustrates a diverging beam being

propagated and focussed with minimal loss. A Luneberg lens shown in Figure 7c enables a parallel beam to be converged with minimum aberration. Such lenses can be used as the lenses 8 in the optical coupler 2 to minimise spherical aberrations in the vertical

components of the beam, thereby optimising the coupling efficiency.

Figure 8 illustrates a cross-section of the diffused cylindrical lens array 10 propagating two beams. The graded index profile provides efficient focussing of the horizontal component of the beam. An homogenous cylindrical lens can be used to focus the horizontal component but using the GRIN lens array 10 improves coupling efficiency and enables consistent alignment when coupling an array of waveguides. Comparing Figure 8 with Figure 3, the GRIN lens array 10 clearly illustrates the advantages of all on-axis imaging and apertures, symmetrical diffraction effects and minimum aberrations.

Also the GRIN lens array 10 can be cheaply and easily fabricated using existing technologies such as ion-exchange in glass or diffusion in polymers.

Sufficient beam-steering may be available in either lateral or vertical directions or both, not only to optimise coupling of each guide of one array to one guide of a second array, but also to switch coupling to different guides of the second array, or even to a third array of guides displaced slightly from the second array. Lateral displacement may be obtained by translating the lens array 10

(Figure 10a), or vertical displacement by rotating lens 8a about lens 8b (Figure 10b).

The optical coupler 2 could alternatively use cylindrical lenses having a D-shaped cross-section or hollow rod lenses instead of the cylindrical lenses 8 and/or the GRIN lens array 10. Cross-sections of five commonly used D-shaped lenses are shown in Figure 9. The focussing capabilities and potential aberration for each lens is known in the art. Consequently, such lenses can be used in the optical coupler giving another degree of design freedom to optimise the coupling efficiency for each application of the optical coupler 2.

The principle application of the optical coupler 2 is for coupling an array of waveguides on a semiconductor to optical fibre " pigtails" . Present mode ellipticities from such waveguides are in the order of 5 to 10 and previously proposed couplers have given only typically 20% coupling efficiency. The optical coupler 2 of the present invention significantly increases this coupling efficiency to greater than 40% . and ideally close to 100%. Other applications of the optical coupler 2 include laser and laser array coupling and coupling of hi-bi fibres.

The optical coupler 2 may be made in two separable parts which mate to form the aligned coupler, but separate between the orthogonal lens-groups to enable rapid interchange of waveguide devices or fibre-harnesses. The relatively expanded beam between the parts permits increased tolerance of displacement between the mating parts of the optical edge-connector. The present invention has been described with reference to particular embodiments and it would be appreciated by a person skilled in the art that modifications may be made without departing from the scope of the present invention.

Claims

CLAIMS :-

1 . An optical system comprising an optical coupler and at least two waveguides, the optical coupler enabling at least one of the waveguides to be coupled to at least one other of the waveguides and the optical coupler comprising a combination of transverse

cylindrical lenses providing orthogonally independent coupling.

2. An optical system as claimed in claim 1 , wherein the

combination of transverse lenses comprises a number of first lenses, the or each first lens having a first axis and a number of second lenses, the or each second lens having a second axis whereby each first axis is substantially orthogonal to each second axis..

3. An optical system as claimed in claim 2, wherein the firs; axis of at least one first lens is able to rotate about the first axis of another first lens to provide beam steering.

A. An optical system as claimed in claim 2 or claim 3, wherein the second axis of at least one second lens is able to rotate about the second axis of another second lens to provide beam steering.

5. An optical system as claimed in any one of the preceding claims . wherein at least one transverse lens has an inhomogeneous index.

6. An optical system as claimed in any one of the preceding claims, wherein each respective waveguide is coupled to another respective waveguide.

7. An optical system as claimed in any one of the preceding claims, wherein the optical system further comprises means for providing optical switching.

8. An optical system as claimed in claim 7 when dependent on claim 5, wherein the or each transverse lens is mounted for lateral displacement whereby lateral displacement of the transverse lens or lenses provides optical switching between the guides of different fibre optic arrays.

9. An optical coupler substantially as hereinbefore described with reference to Figures 5 to 10.

10. An optical coupler comprising a combination of transverse cylindrical lenses for providing orthogonally independent optical coupling within an optical system, the optical coupler being formed of at least two separable parts which mate to form the aligned optical coupler.

1 1 . An optical coupler as claimed in claim 10 wherein the optical coupler is separable into parts each of which comprises a respective orthogonal lens-group whereby there is provided a means for enabling rapid interchange of waveguide devices or fibre-harnesses.

12. An optical system comprising an optical coupler as claimed in any one of claims 9, 10 or 11.