INTEGRATED OPTICAL DEVICE WITH COUPLING WAVEGUIDE LAYER

Abstract: An integrated optical device comprises first and second optical devices (5, 9) and an optical waveguide layer (3). Each optical device (5, 9) is arranged to be optically coupled with the waveguide layer (3). The optical devices to be coupled may be a distributed feedback semiconductor laser and an electro-absorption modulator, e.g.

Description: INTEGRATED OPTICAL DEVICE WITH COUPLING WAVEGUIDE LAYER The present invention relates to optical devices, and in particular to integrated semiconductor optical devices . BACKGROUND OF THE INVENTION Conventional approaches for the integration onto single substrates of semiconductor opto-electronic devices with different band gap energy values, for example semiconductor lasers and external modulators, fall into three main categories . In a so-called "butt-joint approach", one of the component structures (for example, a semiconductor laser) is grown epitaxially onto a semiconductor substrate. Unwanted growth areas are then removed from the surface of the substrate where another component structure (for example, a modulator) is to be formed. The second component structure is consequently grown in these areas . This approach suffers from several disadvantages; misalignment occurs between the optical wave guide layers of the different structures, and poor crystal quality and layer defamation can occur at the boundary between the two areas during the second growth step. A second approach is known as the "selected area growth (SAG)" approach. In such a technique, the substrate is patterned before the devices are grown epitaxially on the substrate. All of the component structures are grown in a single process. The optical waveguide layers are therefore self-aligning. The necessary differences in material parameters, particularly the band gap energies, are achieved by the fact that growth speed varies across a pattern substrate, which results in variations in parameters such as quantum well thickness. The main disadvantage of such a technique is that the achievable band gap difference is often limited. It is also difficult to optimise the two device structures independently because they must have the same layer structure and doping profile. In a third technique, known as quantum well intermixing (QWI) the component quantum well (Q ) structures are grown on a usual substrate in a single growth step. The band gap of a quantum well is changed by an intermixing process, which causes the QW to change material composition and geometric shape, resulting in band gap variation. The waveguide layer is again self- aligning. The main disadvantage with such a technique is that the achievable band gap differences are limited by the amount of achievable intermixing. The intermixing process is usually carried out at high temperatures and may therefore result in deterioration of the material quality. In addition, the intermixing process is sensitive to process environment and substrate surface conditions, which can result in low repeatability and control ability. It is also difficult to optimise the two device structures independently because they must have the same layer structure and doping profile. SUMMARY OF THE PRESENT INVENTION According to one aspect of the present invention, there is provided an integrated optical device comprising first and second optical devices and an optical waveguide layer, wherein each optical device is arranged to be optically coupled with the waveguide layer. According to a second aspect of the present invention, there is provided an integrated optical device comprising a substrate layer, a waveguide layer carried by the substrate layer, and a first optical device carried by the waveguide layer, wherein the first optical device is optically coupled with the waveguide layer. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates a first embodiment of the present invention; Figure 2 illustrates a second embodiment of the present inventio ; and Figure 3 illustrates a third embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As will be appreciated from Figure 1 of the accompanying drawings, embodiments of the present invention can overcome the disadvantages of the previously-considered integration techniques by providing an independent waveguide layer in the integrated device. Figure 1 illustrates a first integrated optical device embodying the present invention. The device is formed on a substrate 1 and includes layers of material which form the device structures. The substrate carries a waveguide layer 3 which is formed of a material which is able to carry optical signals, with low loss, such as a semiconductor material with wider bandgap, for example InP, GaAs or related compounds. The waveguide layer 3 is preferably enclosed by a cladding layer 4. On the cladding layer 4 , or on the waveguide layer 3 , individual optical devices 5 and 9 are provided. The first device 5 includes a device region 6 and a coupling region 7. The second device 9 includes a coupling region 10 and a device region 11. Each device is arranged such that it can couple optical signals with the waveguide layer 3. This optical coupling occurs because the devices are located in close proximity to the waveguide layer 3, and the coupling regions have propagation constants substantially equal to the waveguide layer 3. For example, a spacing of lμm enables light to be coupled between the devices and the waveguide layer 3. Each of the optical devices, however, is preferably provided with a coupling region in order to optimise the coupling between the device and the waveguide layer, as shown in Figure 1. The overall length of the device and its associated coupling region determines the mode in which optical signals are coupled between the device and the waveguide layer 3. When light is coupled into waveguide layer 3 from the devices, the lengths of the coupling regions are preferably chosen so that maximum optical power transfer occurs at the end of the devices. This serves to minimise optical loss and backscattering. When light in coupled from waveguide layer 3 into the devices, the lengths would be chosen either to facilitate maximum optical power transfer at the end of the coupling regions, or to facilitate a gradual optical power transfer along the device length. The former mode would gather most of the optical power at the entrance of the device regions, while the latter mode would give a more even optical power distribution along the device -o — length. Complete coupling can be achieved if the length of the coupling interface is equal to the beat length of the lowest order modes of the waveguide layer 3. The coupling regions 7 and 10 are arranged so that strong optical coupling occurs between those regions and the waveguide layer 3. This can be achieved, as mentioned above, by making the propagation constants of the coupling regions substantially equal to that of the waveguide layer 3. Alternatively, the lateral waveguide shape of regions 7 and 10 could be tapered. The first device is arranged such that optical signals produced or processed by the device region 6 is coupled into the waveguide layer 3 of the integrated device through the coupling region 7. The optical signals coupled into the waveguide layer 3 are then coupled with the second device 9 by way of its coupling region 10. The coupling region 10 is coupled with the device region 11 for transferring a light optical signal to the device. The integrated device is formed by growing the waveguide layer 3 (and optional protective layer 4) onto the substrate 1 and then by growing the individual devices 5 and 9 either in a single epitaxial growth operation, or in separate operations. The coupling regions of each device are formed integrally with the device regions, and so optical coupling between the two can be arranged to be strong. Since each device 5 and 9 couples with the waveguide layer of the integrated device, for transfer of signals between the two devices, the devices need not be produced as part of the same fabrication step. The waveguide layer 3 provides an independent coupling path 14 for transfer of optical signals between the devices, and so the transfer is not dependent on achieving direct coupling between the devices . Figure 2 illustrates a particular embodiment of the present invention, in which the first device is a distributed feedback semiconductor laser (device region 6) , and the second device is an electro-absorption modulator. Both devices use short lengths of coupling regions in order to optimise the coupling between the device and the waveguide layer 3. In the example shown in Figure 2, the waveguide layer 3 is passive and simply enables optical energy to travel between the two devices along optical path 14. Another embodiment of the present invention is shown in Figure 3 in which a distributed feedback laser 6 couples to the waveguide layer 3 via a coupler 7. In the Figure 3 embodiment, a second device 15 is provided which again is an electro-absorption modulator. In the Figure 3 embodiment, however, the modulator 15 is provided by a modulator layer 16 to which a bias voltage can be applied. The waveguide layer 3 then provides part of the modulator itself . It will therefore be appreciated that embodiments of the present invention are able to overcome the disadvantages associated with the prior art. For example, the Figure 3 embodiment is simple to manufacture, since only one epitaxial growth process is required, the modulator being manufactured by a simple metalling process . Examples of optical devices that can be integrated on a substrate which carries a waveguide layer in accordance with the invention, are: a Fabry-Perot cavity semiconductor laser, a distributed feedback semiconductor laser, a distributed Bragg reflector semiconductor laser, an electro-absorption modulator, a Mach-Zehnder modulator, or an electro-optic modulator. Other optical components such as detectors or amplifiers are also suitable for use in embodiments of the invention. Naturally, the above lists of devices and components are not exhaustive, and other suitable devices and components can be used in integrated devices embodying the present invention.

Claims: CLAIMS 1. An integrated optical device comprising first and second optical devices and an optical waveguide layer, wherein each optical device is arranged to be optically coupled with the waveguide layer. 2. An integrated optical device comprising a substrate layer, a waveguide layer carried by the substrate layer, and a first optical device carried by the waveguide layer, wherein the first optical device is optically coupled with the waveguide layer. 3. A device as claimed in claim 2, comprising a second optical device carried by the waveguide layer and optically coupled with the waveguide layer. 4. A device as claimed in claim 2 or 3, wherein the first optical device is coupled with the waveguide layer by way of a coupling region of the first optical device . 5. A device as claimed in claim 2, 3 or 4, wherein the second optical device is coupled with the waveguide layer by way of a coupling region of the second optical device . 6. A device as claimed in any one of claims 2 to 5, wherein the first optical device is a distributed feedback semiconductor laser device. 7. A device as claimed in any one of claims 2 to 5, wherein the first optical device is a distributed Bragg reflector semiconductor laser device. 8. A device as claimed in any one of claims 2 to 5 , wherein the first optical device is a Fabry-Perot cavity semiconductor laser. 9. A device as claimed in claim 3 , or in any one of claims 4 to 8 when dependent upon claim 3 , wherein the second optical device is an electro-absorption modulator. 10. A device as claimed in claim 3, or in any one of claims 4 to 8 when dependent upon claim 3 , wherein the second optical device is a Mach-Zehnder modulator. 11. A device as claimed in claim 3, or in any one of claims 4 to 8 when dependent upon claim 3 , wherein the second optical device is an electro-optic modulator. 12. A device as claimed in claim 3 , or in any one of claims 4 to 8 when dependent upon claim 3 , wherein the second optical device is a optical amplifier. 13. A device as claimed in claim 3, or in any one of claims 4 to 8 when dependent upon claim 3 , wherein the second optical device is a optical detector. 14. A device as claimed in any one of claims 2 to 13, comprising further optical devices carried by and optically coupled with the waveguide layer. 15. A device as claimed in any one of the preceding claims, being a semiconductor device, wherein the or each optical device is provided by a layered structure. 16. A device as claimed in claim 15, wherein the or each optical device is formed by an epitaxial growth methos on the waveguide layer. 17. A device as claimed in any one of claims 2 to 16, wherein the waveguide layer comprises a layer of optically transmissive material, and a layer of protective material. 18. A device as claimed in claim 3, or in any one of claims 4 to 17 when dependent upon claim 3 , wherein at least part of the second optical device is provided by part of the waveguide layer. 19. An integrated optical device substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings .

Inventors: PENTY RICHARD VINCENT, WHITE IAN HUGH, YU SIYUAN, PENTY, RICHARD, VINCENT, WHITE, IAN, HUGH, YU, SIYUAN
Applicants: PENTY RICHARD VINCENT, UNIV BRISTOL, WHITE IAN HUGH, YU SIYUAN, PENTY, RICHARD, VINCENT, UNIVERSITY OF BRISTOL, WHITE, IAN, HUGH, YU, SIYUAN
Application year: 2000
Main classification: G02B6
IPC codes: G02B6, G02F1, H01S5