WO1997013302A1 - Optically resonant structure - Google Patents

Optically resonant structure Download PDF

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Publication number
WO1997013302A1
WO1997013302A1 PCT/GB1996/002375 GB9602375W WO9713302A1 WO 1997013302 A1 WO1997013302 A1 WO 1997013302A1 GB 9602375 W GB9602375 W GB 9602375W WO 9713302 A1 WO9713302 A1 WO 9713302A1
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WIPO (PCT)
Prior art keywords
cavity
struαure
refleαive
layer
rings
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PCT/GB1996/002375
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French (fr)
Inventor
Michael John Adams
Michael Andreja Fisher
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British Telecommunications PLC
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British Telecommunications PLC
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Priority to CA002231396A priority Critical patent/CA2231396C/en
Priority to US09/043,768 priority patent/US6061381A/en
Priority to AU70908/96A priority patent/AU698782B2/en
Priority to EP96931904A priority patent/EP0852834B1/en
Priority to JP9514054A priority patent/JPH11513534A/en
Priority to DE69601948T priority patent/DE69601948T2/en
Publication of WO1997013302A1 publication Critical patent/WO1997013302A1/en
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
    • H01S5/18—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities
    • H01S5/183—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL]
    • H01S5/18308—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL] having a special structure for lateral current or light confinement
    • H01S5/18322—Position of the structure
    • H01S5/18327—Structure being part of a DBR
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02G—INSTALLATION OF ELECTRIC CABLES OR LINES, OR OF COMBINED OPTICAL AND ELECTRIC CABLES OR LINES
    • H02G15/00—Cable fittings
    • H02G15/013—Sealing means for cable inlets
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02G—INSTALLATION OF ELECTRIC CABLES OR LINES, OR OF COMBINED OPTICAL AND ELECTRIC CABLES OR LINES
    • H02G3/00—Installations of electric cables or lines or protective tubing therefor in or on buildings, equivalent structures or vehicles
    • H02G3/02—Details
    • H02G3/08—Distribution boxes; Connection or junction boxes
    • H02G3/088—Dustproof, splashproof, drip-proof, waterproof, or flameproof casings or inlets
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S2301/00—Functional characteristics
    • H01S2301/16—Semiconductor lasers with special structural design to influence the modes, e.g. specific multimode
    • H01S2301/166—Single transverse or lateral mode
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
    • H01S5/18—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities
    • H01S5/183—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL]
    • H01S5/18308—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL] having a special structure for lateral current or light confinement
    • H01S5/18319—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL] having a special structure for lateral current or light confinement comprising a periodical structure in lateral directions
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
    • H01S5/18—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities
    • H01S5/183—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL]
    • H01S5/18355—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL] having a defined polarisation
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
    • H01S5/18—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities
    • H01S5/183—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL]
    • H01S5/18361—Structure of the reflectors, e.g. hybrid mirrors
    • H01S5/18369—Structure of the reflectors, e.g. hybrid mirrors based on dielectric materials
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10S277/00—Seal for a joint or juncture
    • Y10S277/904—Viscous seal

Definitions

  • This invention relates to an optically resonant stru ⁇ ure and has particular but not exclusive application to a vertical cavity surface emitting laser (VCSEL).
  • VCSEL vertical cavity surface emitting laser
  • a resonant cavity is formed extending longitudinally in the plane of the semiconductor substrate with refle ⁇ ive stru ⁇ ures at opposite ends.
  • 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 ho ⁇ zontal stru ⁇ ure, 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 semicondu ⁇ or substrate on which are formed first and second refle ⁇ ive means that comprise periodic Bragg refle ⁇ ive stru ⁇ ures, with a resonant cavity between them that includes a layer of laser a ⁇ ive material with spacer layers to provide a sufficient cavity thickness to achieve resonance at a desired operating wavelength ⁇ .
  • the first refle ⁇ ive means may comprise a plurality of interleaved layers of different semicondu ⁇ or materials with different refra ⁇ ive indices of ⁇ /4 effe ⁇ ive thickness, overlying the substrate.
  • the Bragg stru ⁇ ure of the second refle ⁇ ive means overlies the cavity and is formed of layers of diele ⁇ ric material, with different refra ⁇ ive indices, of ⁇ /4 effe ⁇ ive thickness.
  • a metal conta ⁇ is formed on the underside of the substrate and a condu ⁇ ive conta ⁇ layer is formed between the diele ⁇ ric second refle ⁇ ive means and the layers in the resonant cavity.
  • Current is confined to the cavity by a peripheral reverse-biassed jun ⁇ ion.
  • the thickness of the resonant cavity is typically of the order of 1 ⁇ m, 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 ⁇ m.
  • a problem that arises in conventional VCSEL stru ⁇ ures 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.
  • 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 sele ⁇ TM polarisation as the dominant polarisation mode.
  • an optically resonant stru ⁇ ure comprising first and second refle ⁇ ive means spaced apart so as to provide an optically resonant cavity with a relatively small thickness dimension between the refle ⁇ ive means and extending over an area with relatively large transverse dimensions in a plane transverse to the thickness dimension, and means for refle ⁇ ing a component of the optical radiation from the cavity travelling parallel to said plane, with a phase chara ⁇ eristic for supporting a presele ⁇ ed transverse resonant mode in the cavity.
  • the refle ⁇ ive means may comprise means for producing Bragg refle ⁇ ion of radiation from the cavity.
  • the Bragg refle ⁇ ion may be produced by means of a pattern of material with a spatially periodic optical refra ⁇ ive index chara ⁇ eristic.
  • the stru ⁇ ure according to the invention may be used in a laser, for example a VCSEL, so that a layer of laser a ⁇ ive material for producing optical radiation may be provided between the first and second refle ⁇ ive means.
  • the layer of laser a ⁇ ive material may be spaced from the refle ⁇ ive means by at least one spacer layer.
  • the invention also includes an optically resonant stru ⁇ ure comprising means for providing an optically resonant cavity, optically a ⁇ ive 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 a ⁇ ive material for refle ⁇ ing a component of the optical radiation therein travelling in said plane with a phase chara ⁇ eristic for supporting a presele ⁇ ed transverse resonant mode in the cavity.
  • the means for producing Bragg refle ⁇ ion may comprise a pattern of rings formed in at least one of the layers of the stru ⁇ ure.
  • 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 presele ⁇ ed resonant mode for the cavity.
  • the rings may comprise grooves formed in the layer or alternatively ridges which are upstanding from the layer.
  • Bragg waveguides have been proposed previously in which conventional waveguide cladding is replaced by a periodic multilayer stru ⁇ ure with high refle ⁇ ivity that results from interference between partial refle ⁇ ions at a number of the interfaces between the layers.
  • Bragg waveguides including antiresonant refle ⁇ ing optical waveguides or ARROWs were proposed and first demonstrated in GaAs planar stru ⁇ ures 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).
  • Figure 1 is a schematic se ⁇ ional view of a VCSEL with a circular Bragg stru ⁇ ure 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 fun ⁇ ion of radial distance from the centre of the cavity shown in Figure 2, together with the spatial periodic refra ⁇ ive index variation of the Bragg stru ⁇ ure in the radial dire ⁇ ion; and
  • Figure 4 is a se ⁇ ional view corresponding to Figure 1, of a second embodiment of a device according to the invention.
  • the basic VCSEL stru ⁇ ure 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 refle ⁇ ive means 3 and a second refle ⁇ ive means 4.
  • the dimensions of the cavity are so arranged that in use, light of a predetermined wavelength ⁇ e.g. 1.55 ⁇ m is generated by laser a ⁇ ion and emitted in the dire ⁇ ion of arrows 5.
  • the first refle ⁇ ive means 3 is ele ⁇ rically condu ⁇ ive and comprises a plurality of interleaved layers of InGaAsP 6 of bandgap equivalent wavelength 1.4 microns, and InP 7 of ⁇ /4 effe ⁇ ive thickness that are deposited on the substrate as a stack with 45 periods, so to provide a 90 layer DBR stru ⁇ ure.
  • the layers 6, 7 are typically deposited by conventional gas source MBE techniques.
  • an undoped InGaAsP laser a ⁇ ive material layer 8 of bandgap equivalent wavelength of 1.55 microns of a thickness corresponding to one wavelength at 1.55 ⁇ m (i.e 0.45 - 0.47 ⁇ m) is overgrown onto the first refle ⁇ ive means 3 by atmospheric pressure metalorganic vapour phase epitaxy (MOVPE) between an n-doped layer of InP 9 which a ⁇ s as a underlying spacer layer, and an overlying spacer layer 10 formed of p-doped InP material.
  • MOVPE atmospheric pressure metalorganic vapour phase epitaxy
  • the thicknesses of the spacer layers 9,10 are sele ⁇ ed so that the cavity produces resonance at the desired operating wavelength of ⁇ - 1.55 ⁇ m (i.e. with the overall cavity dimension X being around I ⁇ m).
  • the second refle ⁇ ive means 4 comprises an interleaved stack of diele ⁇ ric 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 effe ⁇ ive wavelength thickness of ⁇ /4 and are of different refra ⁇ ive indices so as to a ⁇ as a DBR in a similar manner to the first refle ⁇ ive means 3, although the layers 11, 12 are ele ⁇ rically non- condu ⁇ ing.
  • an ele ⁇ ric current is passed through the resonant cavity 2, from an ele ⁇ rically condu ⁇ ive p-doped GalnAsP conta ⁇ layer 13 of bandgap equivalent wavelength 1.4 microns on the underside of the diele ⁇ ric refle ⁇ or 4, the layer 13 being conne ⁇ ed to a metal conta ⁇ layer 14 formed by vapour deposition on an intermediate conta ⁇ layer 15 formed of p + GalnAs material, deposited on the layer 13.
  • An external ele ⁇ rical connettion 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 stru ⁇ ure described by Fisher et al supra, a central aperture D of diameter d, typically of the order of 5-20 ⁇ m, is formed, which defines the a ⁇ ive 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 refle ⁇ ive means 3, 4 and a relatively large transverse dimension of in a horizontal plane, transverse to the thickness dimension.
  • the thickness dimension w of the a ⁇ ive layer 8 is small in comparison with the transverse dimension
  • the dimension w is typically of the range of 0.45-.47 ⁇ m and the dimension X is in the region of l ⁇ m.
  • 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 ⁇ m, or greater.
  • the ele ⁇ rical conta ⁇ layer 13 forms dire ⁇ ele ⁇ rical conta ⁇ 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 semicondu ⁇ or materials that form the first refle ⁇ ive means 3, and the substrate 1 so as to reach a Au metal conta ⁇ 17 on the underside of the substrate.
  • the blocking layer 16 produces a reverse p-n jun ⁇ ion with the p-doped spacer layer 10 so as to confine current flow through the central aperture D.
  • the current passes through the layer 8 in this a ⁇ ive 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 refle ⁇ ive 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.
  • a series of circular rings 18 is formed in the uppermost surface of the blocking layer 16.
  • six rings l ⁇ j-l ⁇ j or more may be preferred although, in Figure 1, only four rings are shown for ease of illustration.
  • the central region D has a circular periphery and rings 18 are concentric with the circular central region.
  • 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 ele ⁇ ron beam lithography and sele ⁇ ive etching of the blocking layer 16.
  • the layer 16 is laid down with a continuous thickness y and is subsequently etched sele ⁇ ively by an amount z so as to define the ridges 18.
  • grooves can be etched into the blocking layer.
  • the ele ⁇ rical conta ⁇ 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 refra ⁇ ive index from the materials that form the ele ⁇ rode layers 13, 14 and 15, so that the rings 18 form a refle ⁇ ive Bragg stru ⁇ ure for light from the cavity 2 which, whilst travelling in the dire ⁇ ion of arrows 5 has a component in a plane parallel to the plane of the optically a ⁇ ive layer 8.
  • the rings 18 and the intervening material of layers 13, 14, 15 produce a spatially periodic refra ⁇ ive 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 sele ⁇ ed so that the phase of the refle ⁇ ed light constru ⁇ ively supports a presele ⁇ ed resonant mode for the resonant cavity 2.
  • the mode comprises the HE ⁇ mode which has a field amplitude as a fun ⁇ ion 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 refra ⁇ ive index pattern formed by the ridges 18 1 -18 4 is shown in Figure 3 as a fun ⁇ ion of radial distance r, for a situation where the central region D has a diameter d of 9.2 ⁇ m (r - 4.6 ⁇ m).
  • 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 presele ⁇ ed resonant mode for the resonant cavity 2. In one example, the innermost ridge 18 j was wider than the other ridges.
  • the a ⁇ ual 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 refle ⁇ ing the optical field that occurs outside the central region D.
  • the HE n mode has to compete with other modes and thus tends to be unstable, but the provision of the Bragg stru ⁇ ure provided by the rings 18 permits stabilisation of the presele ⁇ ed transverse mode.
  • the blocking layer comprises a layer 19 of semi-insulating InP which is sele ⁇ ively re-grown around the cavity.
  • the a ⁇ ive and spacer layers 8, 9, 10 are formed on the first refle ⁇ ive 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.
  • the confining Bragg stru ⁇ ure 18 is then etched in the top of the layer 19 in the manner described previously with reference to Figure 1.
  • the conta ⁇ ing layers 13, 15 are then formed over the entire top surface, whereas the metallic conta ⁇ layer 14 is formed only on the surface that lies outside of the area D where light is emitted.
  • the second refle ⁇ ive means 4 is then formed over the top surface, as previously described, and a suitable ele ⁇ rical conne ⁇ ion is made to the layer 14, not shown, in order to provide an external conne ⁇ ion, e.g. by localised etching and metallisation or other conventional techniques.
  • the spaces between the ridges 18 need not necessarily be filled with material and could be left empty.
  • the spaces between the ridges could be filled with a different material, for example polyimide material.
  • the ridges 18 could be made in other layers of the stru ⁇ ure, for example, in either of the underlying spacer layers 9, 10.
  • the invention is not necessarily restri ⁇ ed to lasers and it will be appreciated that the cavity 2 could be used as a passive cavity, without the need for ele ⁇ rical conta ⁇ s, 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 stru ⁇ ures as described in this reference, in terms of ruggedness.
  • 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 refle ⁇ ive layers 11, 12 are replaced by an anti-refle ⁇ ion stack, and the second refle ⁇ ive means comprises a mirror external to the semicondu ⁇ or substrate, and reference is dire ⁇ ed to "A CW. external cavity surface emitting laser" by J. V. Sandusky A. Mukherjee and S.R. Brueck paper TuEll at the Conference on Semicondu ⁇ or lasers: Advanced Devices and Applications (Keystone, Colorado, August 21-23, 1995) - Optical Society of America 1995 Technical Digest Series, Volume 20, pp 153-155.

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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 Al203 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 HEπ 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 181-184 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 18j 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 A1203 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 HEn 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 PC13 has been described in a paper by M. J. Harlow, P. C. Spurdens and R.H. Moss: "The influence of PC13 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 cooperating with second refleαive means extemal to the substrate.
PCT/GB1996/002375 1995-09-29 1996-09-26 Optically resonant structure Ceased WO1997013302A1 (en)

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AU7090896A (en) 1997-04-28
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CN1198265A (en) 1998-11-04
US6061381A (en) 2000-05-09
CA2231396C (en) 2001-02-27
EP0852834A1 (en) 1998-07-15
CA2231396A1 (en) 1997-04-10
JPH11513534A (en) 1999-11-16
AU698782B2 (en) 1998-11-05
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JP2008022024A (en) 2008-01-31
EP0852834B1 (en) 1999-03-31

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