OPTICAL COMPONENT AND WAVEGUIDE ASSEMBLY

Abstract: A method of positioning an optical component such as a laser diode (36) in alignment with an optical waveguide (30) comprises forming an elongate V-shaped groove (29) and a depression (31) in a substrate (28). A laser diode (3) is then mounted in the depression (31) and is accurately located therein. An optical fibre (30) is mounted in the groove (29). The relative positions of the depression (31) and the wave­ guide (30) are such that in use an optical beam may be coupled between the optical component and the waveguide.

Description: This invention relates to assemblies comprising at least one elongate optical waveguide and a locating formation formed in a common substrate and an optical component mounted in the locating formation in alignment with the waveguide See, for example, EP-A-0 006042. Recent developments in the field of optical communication have lead to the more widespread use of monomode waveguides such as monomode optical fibres. It is particularly important in this case to devise a method for accurately positioning an optical component such as a laser chip or photosensor in alignment with an optical waveguide. By using a locating formation it is possible to align accurately a component with an optical waveguide since the component is located by the formation which is the same substrate as the waveguide. This is particularly suitable where the substrate comprises a single crystal since the locating formation can be accurately formed in the substrate using ion etching techniques. For example, a single crystal of silicon is a particularly suitable substrate for a number of reasons. These include the fact that silicon is readily available in large sizes to the most exacting standards of purity and perfection; photolithographic and etching techniques for defining surface and sub-surface geometries are highly developed; and most importantly large anisotropic etching rates exist between different crystallographic axes in the crystal. Furthermore, silicon has particularly useful electrical, mechanical and optical properties which enable the crystal to be used for opto-electronic sub-assemblies such as electro-optical modulators. Such structures also enable an optical component to be mounted in the same substrate as other optical devices. Another substrate which is particularly useful is lithium niobate which is commonly used as a basis for modulators and other electro-optic components. In the case of lithium niobate, ion etching techniques are not generally suitable but the locating formation can be formed using an ion beam milling method. The present invention provides an assembly comprising at least one elongate optical waveguide and a locating formation formed in a common substrate and an optical component mounted in the locating formation such that, in use, an optical beam may be directly coupled between the optical component and the waveguide, characterised in that a) the locating formation is a V-groove, b) the axis of the locating formation is transversely offset relative to the longitudinal axis of the optical waveguide. The invention is particularly suitable for use with laser chips which have a line emission from an edge of the chip. It has been very difficult up to now accurately to align the edge with an optical waveguide. The optical component may be positioned by causing optical radiation to be coupled between the optical component and the at least one waveguide, adjusting the position of the at least one or each waveguide, and monitoring the optical power coupled between the component and the at least one waveguide to determine the positions corresponding to maximum power coupling. The optical component may typically be bonded in the locating formation by for example soldering. The at least one optical waveguide may be formed by diffusing a suitable material into the substrate to change the refractive index of the substrate or by forming a substantially V-shaped groove in the substrate and mounting an optical fibre in the groove. Other methods are also possible. With this method, two optical waveguides are produced which are accurately aligned and, in the case where the optical component comprises a laser chip, may end adjacent opposite facets of the laser chip. This is useful for a number of reasons. Firstly, access to both facets helps to improve the spectral response of the laser chip. Secondly, a single laser chip can be used for both transmitting and receiving in an optical communication system; and thirdly monitoring of the laser chip can be carried out. The V-shaped locating formation may have an included angle substantially equal to the angle between supporting surfaces of the optical component. This is particularly useful where the optical component comprises a laser chip or an optical sensing chip since these chips typically have a pair of angled supporting surfaces enabling the chip to the accurately positioned within the locating formation. An example of an assembly in accordance with the invention will now be described with reference to the accompanying drawings, in which:- Figures 1 to 4 are an exploded, perspective view and side views of two examples of an optical device located relative to a waveguide by a locating formation of known type; Figure 5 is a perspective view of an embodiment of the invention; and Figure 6 is a view taken in the direction A in Figure 5. The assembly shown in Figure 1 comprises a single crystal silicon chip or substrate 28 having an elongate V-shaped groove 29 for receiving an optical fibre 30. A depression 31 having a triangular prismatic form is etched into the substrate through an intermediate portion of the groove 29 using an anistropic etching technique. The depth of the depression 31 is chosen in a conventional manner by defining the surface area of the substrate 28 which is etched. The depth is chosen such that the included angle between opposite faces 32, 33 of the depression 31 is accurately known. The detector chip 36 would need to be fabricated to produce a compatible included angle between its side walls 34, 35 and this could be achieved by using a diamond saw set at the required angle to divide up a silicon slice into separate chips. The depth is also chosen such that simply by inserting the chip 36 into the depression 31, the optical fibre 30 will be aligned with the centre of the 30 to 50 micron light sensitive area of the chip 36. As previously mentioned, the alignment between the chip 36 and the optical fibre 30 is set by the relative depths of the V-shaped groove 29 and the depression 31 which are defined by the surface window size used for the self-limiting edge. Furthermore accurate alignment between the groove 29 and depression 31 is achieved by etching them together using different window sizes. The chip 36 and the fibre 30 could be attached to the silicon substrate 28 by suitable metallisation and bonding. The chip 36 is typically 500µm square and 200µm thick. Figure 2 illustrates the use of the angled end surface 37 of the depression 31 as the combined bonding and lower contacting surface. Any optical penalty caused by the angled detector surface will be minimised by the use of an anti-reflection coating on the chip 36 and the lens ended fibre 30. If required, a mirror image substrate 38 defining a clamp could be mounted on the substrate 28 as shown in Figure 3. The grooves 29 in each substrate 28, 38 could be used to provide a route for the second electrical connection to an outside circuit but other configurations including metal tracks on the surface of one of both of the substrates 28, 38 are possible. An extension of the principle shown in Figure 3 would allow a balanced photodiode configuration to be considered with the fibres in-line as shown in Figure 4, using a parallel arrangement of grooves and fibres. In Figure 4 a second detector 36' is bonded to the face of the depression 31 opposite to the face 37 and each detector 36, 36' is received in complementary depressions 39, 40 in the upper substrate 38. In addition a second optical fibre 30' extends between the substrate 28, 38 to the detector 36'. For optical receivers where the first stage amplifier is a bipolar silicon transistor or FET the associated circuits could be diffused into the silicon chip 28 adjacent to the photodiode 36. In contrast to the light sensitive area of a photodiode, the light emission from a laser diode is from a region situated on two opposite sides of the chip and is situated about 1.5 microns from one of the faces. One of the main requirements in the mounting of a laser diode is that the centre line of the light emitting region is aligned with the axis of the adjacent optical fibre. Figures 5 and 6 illustrate an embodiment of the invention for achieving this mounting configuration. In this example, two V-shaped grooves 41, 42 are etched in a silicon substrate 43, the two grooves being parallel but offset from one another. A laser diode chip 46 is bonded to a sloping side 44 of the groove 42 while an optical fibre 45 is positioned in the groove 41. As before, the depths of the grooves 41, 42 are defined by a horizontal masking process and additionally the fibre diameter and concentricity is accurately defined. Thus, the alignment accuracy is dictated by the width of the chip 46. The lens ended optical fibre 45 is tapered to allow an efficient fibre to laser distance to be obtained.

Claims: An assembly comprising at least one elongate optical waveguide (41, 45) and a locating formation (42) formed in a common substrate (43) and an optical component (46) mounted in the locating formation (42) such that, in use, an optical beam may be directly coupled between the optical component and the waveguide, characterised in that a) the locating formation is a V-groove, b) the axis of the locating formation (42) is transversely offset relative to the longitudinal axis of the optical waveguide (41, 45). An assembly as claimed in claim 1 in which the optical component comprises a laser chip (46). An assembly as claimed in claim 1 in which the optical component comprises an optical sensing chip. An assembly as claimed in any preceding claim in which the substrate comprises lithium niobate. An assembly as claimed in any one of claims 1 to 3 in which the substrate (43) comprises silicon. An assembly as claimed in claim 5 in which the optical waveguide comprises an optical fibre (45) mounted in a V-groove (41). An assembly as claimed in any one of claims 1 to 5 in which the optical waveguide comprises a portion of the substrate into which a suitable material has been diffused. An assembly as claimed in either of claims 5 and 6 in which each V-groove (41, 42) is an anisotropically etched V-groove (41, 42). An assembly as claimed in any preceding claim in which the included angle of the V-groove is substantially equal to the angle between the supporting surfaces of the optical component. An assembly as claimed in any preceding claim in which the optical component is bonded to the surfaces of the locating formation. An assembly as claimed in any preceding claim in which the substrate comprises a single crystal.

Inventors: STANLEY, IAN WILLIAM, DR.
Applicants: BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY
Application year: 1986
Main classification: G02B6
IPC codes: G02B6, G01J3, H01S5, G01D5, G02B7, H01S3, G02B26