SUPPORT MEMBER

Abstract: A support member (11) for optical devices has a device support face (12) which tapers between two sides (22, 25). Such a support member (11) can be used to support any one of a set of optical devices, the optical devices in the set being of different lengths, while still providing access to both ends of the optical device. The support member (11) has particular application in optical communications.

Description: This invention relates to support members for use in supporting optoelectronic semiconductor devices in the field of optical communications. Optoelectronic semiconductor devices are used in combination with other elements in optical communication systems. It is generally necessary to provide connections to the devices for the input or output of optical power. In order to facilitate the making of these connections it is advantageous to mount the optoelectronic semiconductor device on a support member. The nature of many optoelectronic semiconductor devices, for instance semiconductor stripe lasers, is that, in use, the optical activity with which the device is concerned is located in a longitudinal volume extending along the length of the device. Input or output of optical power to or from the device must be made at an end of the longitudinal volume. In a number of applications of such devices a port for input or output of optical power is necessary at both ends of the longitudinal volume. Any support member on which the device is mounted for use in these applications must therefore be designed to allow access to both ends. We have observed that the quality of optical signal achieved can be impaired by its transmission into or out of the device concerned and that this can present serious difficulties. In particular, if the device is mounted on a face of a support member which extends beyond either end of the device's longitudinal volume, partial reflection of the signal can occur at the support face of the support member. Subsequent interference between reflected and unreflected components of the signal can then cause fluctuations of amplitude and phase distrubance. It can therefore be important that the support member face does not extend beyond the ends of the longitudinal volume of an optoelectronic semiconductor device. Two further factors which can influence the design of a support member are firstly the fragility of optoelectronic semiconductor devices and secondly the potential function of the support member as a heat sink. Taking the first factor; because an optoelectronic semiconductor device is fragile, to be efficient the support member should support it throughout its length, that is from end to end. Taking the second factor; active devices may tend to generate unwelcome heat during use. For instance, in semiconductor stripe lasers, which generate heat in use, an increase in temperature of the laser material leads to an increase in the input current threshold at which it will start to lase. The higher input current required in turn leads to a further rise in temperature. Hence, the support member can play an important role by conducting heat away from the device. To do this efficiently, it is important that the support member is in contact with the device concerned throughout the latter's length. Optoelectronic semiconductor devices, however, can vary considerably in length. For instance, a semiconductor laser having an integral cavity for frequency control may measure about 1mm in length, in contrast to lasers without the cavity and for use for instance as amplifiers which may be only 100µm in length. In Patents Abstracts of Japan, Vol. 5, No. 137 (E-72) (809), 29 August 1981, at page 127E72 there is described a semiconductor diode, the major layers of which form an elongate tapered mesa structure. The purpose of the tapering, which is from a width of 1 to 2 µm at one end of the mesa to a width of between 30 to 100µm at the other end, is to increase coupling efficiency with a multimode optical fibre aligned with the wide end of the mesa. As an initial step in the production of the device, a suitably tapered mesa structure is etched in an n-type Ga 0.7 Al 0.3 As cladding layer which has been grown on an n-type GaAs substrate. An active layer of p-type Ga 0.9 Al 0.1 As is then grown on the mesa, with additional p-type layers being grown thereover to produce the desired tapered-mesa device. The n-type cladding layer is of course part of the device rather than being a device support. Moreover, the thermal conductivities of GaAlAs and GaAs are about 0.5 W.cm⁻¹( o C)⁻¹. There is no teaching in this document suggesting the use of a tapered mount for a device. In British Patent Application 2065918, there is described a device package for a semiconductor laser to which an optical fibre is coupled. The invention therein concerns a solder seal for locking the optical fibre into alignment with the laser, the solder seal also providing an hermetic seal between the fibre and the device package. The laser diode is shown mounted on the upper surface of a rectangular section column of oxygen-free, high-conductivity copper. The column's upper surface is shown as being deeper than the length of the laser diode, the rear facet of the laser, which illuminates a remotely mounted back-facet monitor diode, being shown to be positioned well short of the column's edge. Again there is no teaching relating to the use of a tapered mounting surface. It is an object of the present invention to provide a support member which is suitable for use with any one of a set of optoelectronic semiconductor devices, the devices of the set being of different lengths. In a first aspect the present invention provides a member for supporting an optoelectronic semiconductor device and for removing the heat generated by said device, comprising a block of material having a planar support face to receive an optoelectronic semiconductor device such as a semiconductor laser chip, said block of material having a thermal conductivity at 20 o C, in a direction away from the device support face, of at least 2 W.cm⁻¹( o C)⁻¹, said support surface being provided with two opposing straight sides, wherein the distance between the midpoints of said two opposing sides is less than or equal to 1.5mm and wherein said two opposing straight sides are non-parallel and lie at an angle in the range 1 o to 30 o with respect to each other, the dimensions of the support face being such that the member can be utilized for a range of sizes of optoelectronic semiconductor devices. In a second aspect, the present invention provides an optical assembly comprising: an optoelectronic semiconductor device having two ports located on opposing facets; and a heat sink comprising a block of material having a planar device support face for supporting said optical semiconductor device, said support face having two opposing straight sides and the width of said optoelectronic semiconductor device at least in the regions of said ports being substantially smaller that the lengths of said two opposing straight sides, wherein said straight sides are non-parallel and are inclined at an angle in the range of 1 o to 30 o with respect to each other, and wherein the optoelectronic semiconductor device is positioned so that each port overlies at least a point in a respective one of said non-parallel straight sides with at least one of said straight sides being angularly offset with respect to the overlying port. It should be noted that no word used herein should be taken to imply a necessary orientation of a support member in space. Such words, including "overlies" and "uppermost", are used for convenience of description only. An advantage of heat sinks, or support members, constructed in accordance with the present invention is that they are susceptible to mass production. One support member may be suitable for use with varying types of optoelectronic semiconducting device and that support member can therefore be usefully reproduced in quantity. An advantage of optical assemblies according to the present invention is that each optoelectronic semiconductor device and its respective support member do not have to be matched in size to a fine tolerance. Production of both the support member and the optoelectronic semiconductor device may thus be made simpler. A further advantage of the present invention is that the length of the support member is reduced whilst retaining its usefulness for a range of optical device lengths. Examples of applications in which support members according to embodiments of the invention are particularly useful include the following: i) a semiconductor stripe laser being used to regenerate a signal transmitted by an optical fibre, and ii) an external cavity being used in conjunction with a semiconductor stripe laser in order to lock the output signal of the laser to a chosen frequency mode. In example i) the laser is positioned between the ends of two optical fibres and the signal is transmitted from one fibre to the other via the longitudinal volume of the laser. In example ii) the external cavity picks up and returns optical power at one end of the longitudinal volume of the laser and an optical signal, modified by the presence of the cavity, is emitted at the other end of the longitudinal volume. A support member and an optical assembly according to the invention will now be described with reference to the accompanying figures in which: Figure 1 shows a perspective view of an optoelectronic semiconductor mounted on the support member; Figure 2 shows a top plan view of the arrangement of Figure 1; Figure 3 shows a cross section taken along the line AA in Figure 2; Figure 4 shows a perspective view of an optical assembly incorporating the support member and optoelectronic semiconductor device of Figure 1. Referring to Figures 1 and 2, the support member 11 comprises a block of material which has a planar face 12 on which an optical device can be supported. The planar face is hereinafter referred to as the support face 12. The optical device is a semiconductor stripe laser 13 and can be represented as a shallow cuboid. Each major face of the cuboid is coated with a metal layer 14a, 14b, the metal layer 14b on the lower face having a narrow strip 15 of greater depth which extends the length of the cuboid. The metal layers 14a, 14b provide contacts to the laser 13 through which a driving current can be supplied. In use the driving current leads to lasing activity in a buried longitudinal volume 16 which lies parallel to and between the upper metal layer 14a and the strip 15, also extending the length of the laser 13. Each end of the longitudinal volume 16 constitutes an input/output site or port 17, 18 for optical power to be fed into or out of the laser 13. The laser 13 in the direction of the longitudinal volume 16 is 200µm long while the strip 15 is 2µm wide. (It should be noted that the laser 13 is not shown to an accurate scale.) The width of the longitudinal volume 16 is less than or equal to the width of the stripe 15. The support member 11 is a right prism on a base 19 which is a trapezium, two adjacent internal angles of which trapezium are 90°. The uppermost face, that is the support face 12, of the support member 11 is therefore also a trapezium, congruent with the base 19. The remaining two internal angles of the support face 12 are 100° and 80° respectively. The distance between its parallel sides 20, 23 is 2mm and the distance between the midpoints of its non-parallel sides 22, 25 is 200µm, that is, the same length as the laser 13 and its longitudinal volume 16. The distance between the base 19 and the support face 12 of the support member 11 is 1.5mm. The laser 13 is mounted on the support face 12 such that the metal layer 14b which is provided with a stripe 15 is parallel and adjacent to the support face 12. Further, the laser 13 is positioned such that the stripe 15 lies parallel to, and midway between, the parallel sides 20, 23 of the support face 12, while the ports 17, 18 of the laser 13 substantially overlie the non-parallel sides 22, 25. It will be clear that although a first port 17 may be aligned with one of the non-parallel sides 22, the second port 18 will be out of alignment with the other of the non-parallel sides 25 by 10°. Since, as a result of the above arrangement, the stripe 15 lies symmetrically above the line joining the midpoints of the two non-parallel sides 22, 25, the laser 13 will show an overhang and an equal underhang on either side of said line with respect to the non-parallel side 25 concerned. Referring to Figures 2 and 3, it will also be clear that the maximum overhang 31 that will exist between the stripe 15 and the support member 11 occurs at a corner of the stripe 15 and will be equal to 1/2 W tan 10° where W is the width of the stripe 15. In the arrangement described above the maximum overhang of the stripe 15, and therefore the maximum underhang, is 0.088µm. Typically, in use, the longitudinal volume 16 may be 3µm wide and the maximum overhang of the longitudinal volume 16 would then be 0.132µm. At present, it is only generally possible to bond a laser to a support member to within a tolerance in position of about 2µm. Therefore a maximum overhang of 0.132µm can be considered to be insignificant. Referring to Figure 4, an optical signal amplifying assembly incorporating the support member 11 and laser 13 described above comprises two optical fibre ends 41, 42, the support member 11, the laser 13, two fibre mounts 43, 44 and a base 45. (Arrangements suitable for supplying a driving current to the laser 13 will be clear to a person skilled in the art of optical communications and are not therefore described.) The support member 11 is placed between the fibre mounts 43, 44 on the base 45, the fibre mounts 43, 44 being adjacent to the non-parallel sides 22, 25 of the support member 11. The laser 13 is positioned on the support member 11 as described above and the two optical fibre ends 41, 42 are aligned with the longitudinal volume 16 of the laser 13. The fibre ends 41, 42 are held in place on the fibre mounts 43, 44 by the use of a suitable solder material 46 and extend to within 50µm of each respective end 17, 18 of the longitudinal volume 16. In use, the signal emitted by a first optical fibre end 41 is transmitted onto one port 18 of the longitudinal volume 16 of the laser 13. It is then amplified before being emitted from the other port 17 of the longitudinal volume 16 into a second optical fibre end 42. Typically a semiconductor stripe laser 13 may be operated at a temperature in the range 10-70°C. The material out of which the support member 11 is constructed may be diamond, which typically has a thermal conductivity at room temperature of 20 W.cm⁻¹(°C)⁻¹. The support member 11 will then act as a heat sink conducting heat away from the longitudinal volume 16 of the laser 13. It may be necessary to provide a further heat sink on which the diamond support member 11 is mounted. In the optical signal amplifying assembly described the base 45 will act as a further heat sink. In general, however, the thermal requirements of the optical device concerned will determine at least in part what material will be selected for the support member 11 and any combination with a sub-mount that may be required. Other examples of suitable materials for the support member 11 are copper whose thermal conductivity at room temperature is typically 4 W.cm⁻¹(°C)⁻¹, or aluminium whose thermal conductivity at room temperature is typically just above 2 W.cm⁻¹(°C)⁻¹. The laser 13 described can be mounted by soldering on the support member 11. To do this the device support face 12 of the support member 11 is first metallised over its whole surface with a titanium coating. A eutectic alloy of gold and tin is then deposited onto the titanium coating under low pressure and used as a soldering material with which to attach the laser 13. By coating the whole surface of the device support face 12 of the support member 11 the position of the laser 13 on the face 12 can be freely adjusted after the laser 13 is placed on the face 12 and before the alloy is hardened. The laser 13 can therefore as a first step be positioned on the face 12 such that its longitudinal volume 16 extends parallel to the parallel sides 20, 23 of the support member 11 and then as a second step be slid in a direction perpendicular to said sides 20, 23 until the ports 17, 18 of the longitudinal volume 16 are aligned as required with the edges of the support face 12. It will be clear that where the support member 11 is to act as a heat sink it may be particularly important that the support face 12 should be in contact with as great a proportion of the surface closest to the longitudinal volume of the associated device 13 as possible. It may then be preferable to mount the device such that the surface closest to the longitudinal volume is entirely supported on the support face 12 rather than, as described above and shown in Figure 2, having a slight wedge-shaped overhang at one port 18. The dimensions which are selected for the support member 11 will be at least partially determined by the dimensions and characteristics of the optoelectronic semiconductor devices with which it is intended to use the support member. If one port could be misaligned substantially without affecting an optical device's usefulness then the angle between the non-parallel sides 21, 25 of the support face could be increased, for example, to 20°. This has the advantage of reducing the length of the support member 11 while retaining its usefulness for the same range of optical device lengths. For many applications, however, the angle has been found required to be in the range 7°-15°. Alternatively, if facets at both ends of a device require to be substantially aligned with an edge of the support member 11, the support face 12, instead of being in the form of a quadrilateral with two adjacent internal angles of 90°, could be in the shape of a symmetrical truncated triangle. Again, alternatively, the longitudinal volume 16 of the laser 13 need not be arranged parallel to the parallel sides 20, 23 of the support face 12 but may be for instance at an angle such that each port 17, 18 is misaligned with its respective edge by only half the angle between the non-parallel sides 22, 25 of the support face 12. Although it is not necessary that the support face 12 should have any two sides parallel, it is generally convenient in manufacturing of the support member 11 that this should be the case. The dimensions given in the example described above for the support face 12 may clearly be varied to suit any range of sizes of optoelectronic semiconductor device with which the support member 11 may be intended for use. For instance, the distance between the midpoints of the non-parallel sides 22, 25 might be up to 1.5 mm for use with, say, a semiconductor stripe laser having an integral external cavity. Alternatively it may be as little as 100µm for use with a semiconductor laser intended for frequency locking applications.

Claims: A member for supporting an optoelectronic semiconductor device and for removing the heat generated by said device, comprising a block of material (11) having a planar support face (12) to receive an optoelectronic semiconductor device such as a semiconductor laser chip, said block of material having a thermal conductivity at 20 o C, in a direction away from the device support face, of at least 2 W.cm⁻¹( o C)⁻¹, said support surface being provided with two opposing straight sides (22,25), characterised in that the distance between the midpoints of said two opposing sides (22,25) is less than or equal to 1.5mm and in that said two opposing straight sides are non-parallel and lie at an angle in the range 1 o to 30 o with respect to each other, the dimensions of the support face being such that the member can be utilized for a range of sizes of optoelectronic semiconductor devices. An optical assembly comprising: an optoelectronic semiconductor device (13) having two ports (17, 18) located on opposing facets; and a heat sink (11) comprising a block of material having a planar device support face (12) for supporting said optoelectronic semiconductor device (13), said support face (12) having two opposing straight sides (22, 25) and the width of said optoelectronic semiconductor device (13) at least in the regions of said ports (17, 18) being substantially smaller than the lengths of said two opposing straight sides (22, 25); characterised in that said straight sides (22, 25) are non-parallel and are inclined at an angle in the range of 1 o to 30 o with respect to each other, and in that the optoelectronic semiconductor device (13) is positioned so that each port (17, 18) substantially overlies at least a point in a respective one of said non-parallel straight sides (27, 25) with at least one of said straight sides (25) being angularly offset with respect to the overlying port (18). A support member or optical assembly as claimed in claim 1 or claim 2, wherein the two non-parallel straight sides are the two longest sides with which the face is provided. An optical assembly as claimed in claim 2 or claim 3, wherein the distance between the two ports is less than or equal to 1.5mm. An assembly or member as claimed in any one of the preceding claims, wherein the device support face is in the shape of a quadrilateral. An assembly or member according to claim 5, wherein the quadrilateral has its two other opposing sides which are parallel. An assembly or member according to claim 6, wherein the quadrilateral has two adjacent internal angles of 90 o . An optical assembly as claimed in claim 7, wherein one of said ports (17) is substantially coplanar with the edge (22) of the support face (12) that lies between the 90 o angles. An assembly or member as claimed in claim 6 or claim 7, wherein the distance between said two parallel sides is about 2mm, and the distance between said midpoints of said two opposing non-parallel sides is about 200µm. An assembly or member according to any one of claims 1 to 8, wherein said distance between the mid points of said non-parallel sides is less than or equal to 150µm. An assembly or member as claimed in any one of the preceding claims, wherein the block of material has a thickness of 1.5mm. An assembly or member according to any one of the preceding claims, wherein the angle between the two opposing, non-parallel straight sides lies in the range 1 to 20 o . An assembly or member according to claim 12, wherein said angle lies in the range 7 to 15 o . An optical assembly as claimed in any one of claims 2 to 13, wherein the support member has a thermal conductivity at 20 o C, in a direction away from the device support face, of at least 2 W.cm⁻¹( o C)⁻¹. An assembly or member as claimed in any one of the preceding claims, wherein said thermal conductivity is at least 15 W.cm⁻¹( o C)⁻¹. An assembly or member as claimed in any one of the preceding claims, wherein said support face is metallised with titanium. An assembly or member as claimed in claim 16, wherein said support face is further metallised over the titanium with a gold-tin eutectic. An assembly or member according to any one of claims 1 to 17, wherein the material is copper. An assembly or member according to any one of claims 1 to 17, wherein the material is diamond. An optical assembly according to any one of claims 2 to 19, wherein in use the optical device consumes electrical power. An optical assembly according to claim 20, wherein the optical device is a semiconductor laser. An optical assembly according to claim 21, wherein the laser is provided with an external cavity.

Inventors: COLLINS, JOHN VINCENT
Applicants: BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY
Application year: 1985
Main classification: G02B6
IPC codes: H01L33, G02B6, H01S5, H01L21, G01M11, H01L31, G02B7