OPTICALLY RESONANT STRUCTURE

Abstract: A vertical cavity surface emitting laser comprises a resonant cavity (2) formed between a distributed Bragg reflector formed of semiconductor material (3) and an overlying distributed Bragg reflector (4) formed of dielectric material. The resonant cavity contains a layer (8) of laser active material sandwiched between spacer layers (9, 10), and a blocking layer (16, 19) which confines an energising electric current to flow through the cavity between a metallic contact layer (13) on the underside of the Bragg reflector (4) and a contact (17). In order to support a preselected resonant mode, a series of concentric rings (18) is formed in the blocking layer (16, 19), surrounding the resonant cavity (2) so as to provide a Bragg structure which reflects light from the cavity with a phase characteristic that supports the preselected transverse resonant mode.

Description: Optically Resonant Structure Field of the Invention This invention relates to an optically resonant struαure and has particular but not exclusive application to a vertical cavity surface emitting laser (VCSEL). Background In a conventional Fabry Perot diode semiconductor laser, a resonant cavity is formed extending longitudinally in the plane of the semiconductor substrate with refleαive struαures at opposite ends. In contrast, in a VCSEL, the resonant cavity is arranged vertically in the substrate with the advantage that light is transmitted perpendicularly from its surface over a larger area than with a hoπzontal struαure, which renders the VCSEL well suited to launching signals into optical telecommunications fibres. An example of a VCSEL is given in IEEE Photonics Technology Letters Vol. 7 No. 6 June 1995 pp 608-610, M. A. Fisher et al. The device consists of a semiconduαor substrate on which are formed first and second refleαive means that comprise periodic Bragg refleαive struαures, with a resonant cavity between them that includes a layer of laser aαive material with spacer layers to provide a sufficient cavity thickness to achieve resonance at a desired operating wavelength λ. The first refleαive means may comprise a plurality of interleaved layers of different semiconduαor materials with different refraαive indices of λ/4 effeαive thickness, overlying the substrate. The Bragg struαure of the second refleαive means overlies the cavity and is formed of layers of dieleαric material, with different refraαive indices, of λ/4 effeαive thickness. In order to pass current through the laser aαive material in the cavity, a metal contaα is formed on the underside of the substrate and a conduαive contaα layer is formed between the dieleαric second refleαive means and the layers in the resonant cavity. Current is confined to the cavity by a peripheral reverse-biassed junαion. The thickness of the resonant cavity is typically of the order of 1 μm, whereas in a plane perpendicular to the thickness, the cavity has a relatively large transverse dimension; the cavity is typically square or circular when viewed from above with a diameter or side length of the order of 5 ~ 20 μm. A problem that arises in conventional VCSEL struαures is that during resonance of the cavity, the transverse mode of resonance is left largely uncontrolled and the relatively large diameter of the cavity may support a number of different transverse modes, which leads to mode competition and instability. Use of a surface grating of concentric circular elements in a conventional distributed feedback ( DFB) or distributed Bragg refleαor (DBR) laser, with a horizontal, rather than vertical resonant cavity is disclosed in "Circularly Symmetric Operation of a Concentric-Circle-Grating, Surface-Emitting, AlGaAs/GaAs Quantum-Well Semiconduαor Laser", T. Erdogan et al, Appl. Phys. lett. 60 (16) 20 April 1992 pp 1921-23. With this device, a conventional transversely extending resonant cavity is provided with a surface grating which produces surface emission from the laser by second order Bragg refleαion. The circular pattern of the grating results in the emission of a circularly symmetric beam. Another device is described in US Patent No. 5 301 201 and "Zone Laser" Appl. Phys. Lett. 65 (2) 11 July 1994 pp 144-146, D. Vakhshoori et al, in which the laser cavity is divided into a plurality of concentric zones which each support an individual resonant mode. The outputs of the concentric zones sum together in a similar way to light from the zones of a Fresnel lens, thereby focusing the laser output into a single spot. In Applied Physics Letters, Vol 66, No. 21, 22 May 1995, pp 2769-2771, J. H. Ser et al, there is described a VCSEL which has an overlaid fine metal interlaced grating etched on its upper DBR, overlying its light emitting face. The purpose of the grating is to produce polarisation stabilisation, for example to seleα TM polarisation as the dominant polarisation mode. Summary of the Invention The present invention offers an alternative approach. In accordance with the invention, there is provided an optically resonant struαure comprising first and second refleαive means spaced apart so as to provide an optically resonant cavity with a relatively small thickness dimension between the refleαive means and extending over an area with relatively large transverse dimensions in a plane transverse to the thickness dimension, and means for refleαing a component of the optical radiation from the cavity travelling parallel to said plane, with a phase charaαeristic for supporting a preseleαed transverse resonant mode in the cavity. The refleαive means may comprise means for producing Bragg refleαion of radiation from the cavity. The Bragg refleαion may be produced by means of a pattern of material with a spatially periodic optical refraαive index charaαeristic. The struαure according to the invention may be used in a laser, for example a VCSEL, so that a layer of laser aαive material for producing optical radiation may be provided between the first and second refleαive means. The layer of laser aαive material may be spaced from the refleαive means by at least one spacer layer. The invention also includes an optically resonant struαure comprising means for providing an optically resonant cavity, optically aαive material with a relatively small thickness dimension and extending over an area with relatively large transverse dimensions in a plane transverse to the thickness dimension, wherein means are disposed around the aαive material for refleαing a component of the optical radiation therein travelling in said plane with a phase charaαeristic for supporting a preseleαed transverse resonant mode in the cavity. The means for producing Bragg refleαion may comprise a pattern of rings formed in at least one of the layers of the struαure. In the embodiment described hereinafter, the rings are formed in a blocking layer, which confines current flow through the cavity. The rings may be of different widths in order to maximise support of the preseleαed resonant mode for the cavity. The rings may comprise grooves formed in the layer or alternatively ridges which are upstanding from the layer. So-called Bragg waveguides have been proposed previously in which conventional waveguide cladding is replaced by a periodic multilayer struαure with high refleαivity that results from interference between partial refleαions at a number of the interfaces between the layers. Bragg waveguides, including antiresonant refleαing optical waveguides or ARROWs were proposed and first demonstrated in GaAs planar struαures in 1977, see P. Yeh, A. Yariv, C-S Hong, J. Opt. Soc. Am. 67, 423 (1977) and A. Y. Cho, A. Yariv, P. Yeh, Appl. Phys. Lett. 30, 471 (1977). The idea was extended to a cylindrical geometry but the benefits in respeα of a conventional cylindrical waveguide were found to be unrealisable in praαice for optical fibres because the fabrication tolerances are unrealistic and the launching of light is inefficient, see N. J. Doran K. J. Blow, IEEE J. Light. Tech. LT-1, 108 (1983). It has however been found in accordance with the invention that a periodic Bragg refleαor arrangement around the cavity of a VCSEL can support a preseleαed resonant mode in the cavity of the laser, without suffering from the previously discussed disadvantages which occur in respeα of an optical fibre. Brief Description of the Drawings In order that the invention may be more fully understood, an embodiment of a VCSEL with a cylindrical Bragg waveguide, will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic seαional view of a VCSEL with a circular Bragg struαure surrounding its cavity according to a first embodiment of the invention; Figure 2 is a top plan view of the cavity shown in Figure 1; Figure 3 is a graph illustrating the field amplitude as the funαion of radial distance from the centre of the cavity shown in Figure 2, together with the spatial periodic refraαive index variation of the Bragg struαure in the radial direαion; and Figure 4 is a seαional view corresponding to Figure 1, of a second embodiment of a device according to the invention. Detailed Description The basic VCSEL struαure shown in Figure 1 is generally similar to that described by Fisher et al supra and comprises a semiconduttor substrate 1 of InP material on which a resonant cavity 2 is formed, of thickness X between a first refleαive means 3 and a second refleαive means 4. The dimensions of the cavity are so arranged that in use, light of a predetermined wavelength λ e.g. 1.55 μm is generated by laser aαion and emitted in the direαion of arrows 5. The first refleαive means 3 is eleαrically conduαive and comprises a plurality of interleaved layers of InGaAsP 6 of bandgap equivalent wavelength 1.4 microns, and InP 7 of λ/4 effeαive thickness that are deposited on the substrate as a stack with 45 periods, so to provide a 90 layer DBR struαure. The layers 6, 7 are typically deposited by conventional gas source MBE techniques. In the resonant cavity 2, an undoped InGaAsP laser aαive material layer 8 of bandgap equivalent wavelength of 1.55 microns of a thickness corresponding to one wavelength at 1.55 μm (i.e 0.45 - 0.47 μm) is overgrown onto the first refleαive means 3 by atmospheric pressure metalorganic vapour phase epitaxy (MOVPE) between an n-doped layer of InP 9 which aαs as a underlying spacer layer, and an overlying spacer layer 10 formed of p-doped InP material. The thicknesses of the spacer layers 9,10 are seleαed so that the cavity produces resonance at the desired operating wavelength of λ - 1.55 μm (i.e. with the overall cavity dimension X being around Iμm). The second refleαive means 4 comprises an interleaved stack of dieleαric material consisting of layers 11 of Al 2 0 3 material interleaved with layers 12 of Si material, the layers being deposited by conventional evaporation techniques. The layers 11, 12 have an effeαive wavelength thickness of λ/4 and are of different refraαive indices so as to aα as a DBR in a similar manner to the first refleαive means 3, although the layers 11, 12 are eleαrically non- conduαing. In order to energise the laser, an eleαric current is passed through the resonant cavity 2, from an eleαrically conduαive p-doped GalnAsP contaα layer 13 of bandgap equivalent wavelength 1.4 microns on the underside of the dieleαric refleαor 4, the layer 13 being conneαed to a metal contaα layer 14 formed by vapour deposition on an intermediate contaα layer 15 formed of p + GalnAs material, deposited on the layer 13. An external eleαrical connettion, not shown, is provided to the layers 13, 14. A current blocking layer 16 formed of n-doped InP material is deposited on the upper spacer layer 10 and, by conventional photolithography and etching, as for the struαure described by Fisher et al supra, a central aperture D of diameter d, typically of the order of 5-20 μm, is formed, which defines the aαive region for the cavity. It can thus be seen that the cavity of the VCSEL has a relatively small vertical thickness dimension X between the first and second refleαive means 3, 4 and a relatively large transverse dimension of in a horizontal plane, transverse to the thickness dimension. Likewise, the thickness dimension w of the aαive layer 8 is small in comparison with the transverse dimension The dimension w is typically of the range of 0.45-.47μm and the dimension X is in the region of lμm. This is in contrast to a conventional edge emitting laser which extends longitudinally in the plane of the substrate, for which a typical length is in the range of 300 to 500 μm, or greater. It will be seen that the eleαrical contaα layer 13 forms direα eleαrical contaα with the spacer layer 10 in the central aperture D so that current passes through the materials in the cavity, in this region. The current can pass through the semiconduαor materials that form the first refleαive means 3, and the substrate 1 so as to reach a Au metal contaα 17 on the underside of the substrate. However, the blocking layer 16, produces a reverse p-n junαion with the p-doped spacer layer 10 so as to confine current flow through the central aperture D. Thus, the current passes through the layer 8 in this aαive region to generate optical radiation which resonates in the cavity 2. It will be understood that due to the distributed nature of the multi-layer first and second refleαive means 3, 4, the optical path length over which light resonates between them has an average length which is greater than the physical dimension X shown in Figure 1. In accordance with the invention, a series of circular rings 18 is formed in the uppermost surface of the blocking layer 16. As shown in Figure 2, six rings lδj-lδj or more may be preferred although, in Figure 1, only four rings are shown for ease of illustration. As shown in the drawings, the central region D has a circular periphery and rings 18 are concentric with the circular central region. In the embodiment shown in Figure 1, the rings are formed as upstanding ridges from the general planar extent of the blocking layer 16. This is achieved during manufacture, prior to the deposition of the layers 11 to 15, by eleαron beam lithography and seleαive etching of the blocking layer 16. Initially, the layer 16 is laid down with a continuous thickness y and is subsequently etched seleαively by an amount z so as to define the ridges 18. In an alternative embodiment (not shown), grooves can be etched into the blocking layer. The eleαrical contaα layers 13, 14 and 15 fill the spaces between the rings 18 when subsequently deposited on the layer 16. The n-doped InP material of the layer 16 has a different optical refraαive index from the materials that form the eleαrode layers 13, 14 and 15, so that the rings 18 form a refleαive Bragg struαure for light from the cavity 2 which, whilst travelling in the direαion of arrows 5 has a component in a plane parallel to the plane of the optically aαive layer 8. The rings 18 and the intervening material of layers 13, 14, 15 produce a spatially periodic refraαive index pattern which confines the component travelling parallel to the aforesaid plane to the centre of the circular region D. Figure 3 shows the spatially periodic pattern is seleαed so that the phase of the refleαed light construαively supports a preseleαed resonant mode for the resonant cavity 2. Typically, the mode comprises the HE π mode which has a field amplitude as a funαion of radial distance outwardly from the centre O of the central region D, as shown in Figure 3. The field amplitude has a substantially Gaussian peak at the centre O and decays towards the outer edge of the central region D. The spatially periodic refraαive index pattern formed by the ridges 18 1 -18 4 is shown in Figure 3 as a funαion of radial distance r, for a situation where the central region D has a diameter d of 9.2 μm (r - 4.6 μm). The radial width of the ridges 18 may change from ridge to ridge. Furthermore, the spacing between the ridges may differ. The specific ridge configuration is chosen to support the preseleαed resonant mode for the resonant cavity 2. In one example, the innermost ridge 18 j was wider than the other ridges. The aαual width may be computed according to the principles set out by Doran et al, supra. Alternating layers lla of A1 2 0 3 and 12a of Si are formed over the rings 18. The alternating layers lla, 12a form a DBR for refleαing the optical field that occurs outside the central region D. Without the provision of the ridges 18, the HE n mode has to compete with other modes and thus tends to be unstable, but the provision of the Bragg struαure provided by the rings 18 permits stabilisation of the preseleαed transverse mode. Another configuration of the struαure is shown in Figure 4, and can be considered as a modification of the device struαure shown in Figure 1. Like parts are marked with the same reference numbers. In the embodiment of Figure 4, the blocking layer comprises a layer 19 of semi-insulating InP which is seleαively re-grown around the cavity. In order to form the struαure, the aαive and spacer layers 8, 9, 10 are formed on the first refleαive stack 3 as described with reference to Figure 1, and then a mesa of diameter d, corresponding to the previously described area D for light emission, is formed from layers 8, 9, 10, by conventional lithography and etching. A planar, semi-insulating layer 19 of InP is then re-grown all around the mesa. A suitable technique for the re-growth that uses PC1 3 has been described in a paper by M. J. Harlow, P. C. Spurdens and R.H. Moss: "The influence of PC1 3 on the planarisation and seleαivity of InP re-growth by atmospheric pressure MOVPE", Proceedings of the 7th International Convention on Indium Phosphide and Related Materials, 9-13 May 1995, Sapporo, Japan. The semi-insulating InP layer 19 was iron doped to provide eleαron traps in the layer, but it will be appreciated that other suitable dopants could be used. The regrown layer 19 has the advantage of providing a flat surface that is contiguous with the upper surface of the spacer layer 10, that can receive the various layers of the refleαor struαure 4, so that the step in the refleαor shown in Figure 1 around the edge of the region D, is avoided in the struαure of Figure 2. Reference is also direαed to "Very Rapid and Seleαive Epitaxy of InP around Mesas of height up to 14μm by Hydride Vapour Phase Epitaxy" S. Lourdudoss et al Conference Paper IPRM '94. The confining Bragg struαure 18 is then etched in the top of the layer 19 in the manner described previously with reference to Figure 1. The contaαing layers 13, 15 are then formed over the entire top surface, whereas the metallic contaα layer 14 is formed only on the surface that lies outside of the area D where light is emitted. The second refleαive means 4 is then formed over the top surface, as previously described, and a suitable eleαrical conneαion is made to the layer 14, not shown, in order to provide an external conneαion, e.g. by localised etching and metallisation or other conventional techniques. Modifications and variations of the described struαures are possible. For example, the spaces between the ridges 18 need not necessarily be filled with material and could be left empty. Alternatively, the spaces between the ridges could be filled with a different material, for example polyimide material. Also, the ridges 18 could be made in other layers of the struαure, for example, in either of the underlying spacer layers 9, 10. Furthermore, the invention is not necessarily restriαed to lasers and it will be appreciated that the cavity 2 could be used as a passive cavity, without the need for eleαrical contaαs, for example, to be used as an all optical switch or bistable element. Passive cavities are well known per se in the literature of microresonators, and have application in all-optical switching, logic, bistability, etc, see for example the paper "GaAs- AlAs Monolithic Microresonator Arrays" J.L. Jewel, A Scherer, S.L. McCall, A.C. Gossard & J.H. English, Appl Phys Lett, Vol 51, No 2, pp 94-96 (1987). The transverse mode confinement produced by the present invention represents an advance over the use of pillar struαures as described in this reference, in terms of ruggedness. In another modification, the rings 18 and the central region D are made elliptical in order to impart a predetermined polarisation to the emitted light for example with an ellipticity of 10-30%. The invention is also applicable to a VCSEL in which the refleαive layers 11, 12 are replaced by an anti-refleαion stack, and the second refleαive means comprises a mirror external to the semiconduαor substrate, and reference is direαed to "A CW. external cavity surface emitting laser" by J. V. Sandusky A. Mukherjee and S.R. Brueck paper TuEll at the Conference on Semiconduαor lasers: Advanced Devices and Applications (Keystone, Colorado, August 21-23, 1995) - Optical Society of America 1995 Technical Digest Series, Volume 20, pp 153-155.

Claims: Claims 1. An optically resonant struαure comprising first and second refleαive means (3, 4) spaced apart so as to provide an optically resonant cavity (2) with a relatively small thickness dimension (X) between the refleαive means and extending over an area with relatively large transverse dimensions (d) in a plane transverse to the thickness dimension, characterised by refleαive means (18) for refleαing a component of optical radiation from the cavity travelling parallel to said plane, with a phase charaαeristic for supporting a preseleαed transverse resonant mode in the cavity. 2. A struαure according to claim 1 wherein the refleαive means (18) comprises means disposed around the cavity for producing Bragg refleαion of radiation from the cavity. 3. A struαure according to claim 2 wherein the means for producing Bragg refleαion comprises a pattern of material (13, 14, 15, 16,19) with a spatially periodic optical refraαive index charaαeristic. 4. A struαure according to claim 2 or 3 including a layer (8) of laser aαive material for producing optical radiation that is refleαed in said preseleαed resonant mode between the first and second refleαive means, for laser operation. 5. A struαure according to claim 4 wherein the layer (8) of laser aαive material is disposed between the first and second refleαive means. 6. A strutture according to claim 5 including a spacer layer (9, 10) between the layer of laser aαive material (8) and the first or the second refleαive means. 7. A struαure according to claim 4, 5 or 6 wherein at least one of the refleαive means comprises a multiple layer configuration (6, 7; 11, 12). 8. A struαure according to anyone of claims 2 to 7 wherein the means for producing Bragg refleαion comprises a pattern of rings (18,-18^). 5 9. A struαure according to claim 8 wherein the rings comprise grooves. 10. A struαure according to claim 8 wherein the rings comprise upstanding ridges (18). 10 11. A struαure according to any one of claims 8 to 10 wherein the rings are circular and concentric. 12. A structure according to any one of claims 8 to 10 wherein the rings 15 are generally parallel and disposed elliptically around the cavity. 13. A struαure according to any one of claims 8 to 12 wherein the rings are of different widths. 20 14. A structure according to any one of claims 8 to 13 wherein spaces between the rings are filled with material (13, 14, 15) of a refraαive index different from that of the material of the layer (16,19) in which the rings are formed. 25 15. A structure according to any one of claims 8 to 14 wherein the rings have been formed by lithography and etching. 16. A structure according to any one of claims 8 to 15 including a current blocking layer (16,19) for confining current to the cavity, the pattern of rings 30 being formed in the blocking layer (16,19). 17. A struαure according to claim 16 wherein the blocking layer (16) has been seleαively etched to bound the perimeter of the cavity. 18. A struαure according to claim 16 wherein the blocking layer (19) has been seleαively grown to bound the perimeter of the cavity. 19. A struαure according to claim 18 wherein the blocking layer (19) is formed of InP material. 20. A struαure according to any one of claims 4 to 19 wherein the first refleαive means comprises a plurality of interleaved layers (6, 7) of semiconduαor materials having different refraαive indices, on a semiconduαor substrate, the second refleαive means comprises a plurality of interleaved layers (11, 12) of dieleαric materials having different refraαive indices, and the cavity includes an undoped semiconduαor laser aαive material (8) sandwiched between spacer layers (9, 10) of semiconduαor material of opposite conduαivity type. 21. A VCSEL characterised by a Bragg struαure (18) around its cavity (2) for supporting a predetermined transmission mode (HE ! ,). 22. An optically resonant struαure comprising means for (3, 4) providing an optically resonant cavity (2), optically aαive material (8) with a relatively small thickness dimension (w) and extending over an area with relatively large transverse dimensions (d) in a plane transverse to the thickness dimension, characterised by means (18) around the aαive material (8) for refleαing a component of the optical radiation therein travelling in said plane with a phase charaαeristic for supporting a preseleαed transverse resonant mode in the cavity. 23. A struαure according to claim 22 including a substrate (1) with said optically aαive material formed as a layer (8) thereon, and said means for defining the cavity includes first refleαive means (3) formed on the substrate, for coopera t ing with second refleαive means extemal to the substrate.

Inventors: ADAMS MICHAEL JOHN, FISHER MICHAEL ANDREJA, ADAMS, MICHAEL, JOHN, FISHER, MICHAEL, ANDREJA
Applicants: ADAMS MICHAEL JOHN, BRITISH TELECOMM, FISHER MICHAEL ANDREJA, ADAMS, MICHAEL, JOHN, BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY, FISHER, MICHAEL, ANDREJA
Application year: 1996
Main classification: H01S3
IPC codes: H01S5