WO2014056508A1

WO2014056508A1 - Mode selection laser

Info

Publication number: WO2014056508A1
Application number: PCT/DK2013/050329
Authority: WO
Inventors: Il-Sug Chung; Qijiang RAN
Original assignee: Danmarks Tekniske Universitet
Priority date: 2012-10-12
Filing date: 2013-10-14
Publication date: 2014-04-17

Abstract

The invention relates to a semiconductor mode selection laser, particularly to a VCSEL laser (200) having mode selection properties. The mode selection capability of the laser is achieved by configuring one of the reflectors (15,51) in the resonance cavity so that a reflectivity of the reflector (15) varies spatially in one dimension or two dimensions. Accordingly, the reflector (15) with spatially varying reflectivity is part both of the resonance cavity and the mode selection functionality of the laser. A plurality of the lasers configured with different mode selectors, i.e. different spatial reflector variations, may be combined to generate a laser beam containing a plurality of orthogonal modes. The laser beam may be injected into a few- mode optical fiber, e.g. for the purpose of optical communication. The VCSEL may have intra-cavity contacts (31,37) and a Tunnel junction (33) for current confinement into the active layer (34). An air-gap layer (102) may be provided between the upper reflector (15) and the SOI wafer (50) acting as a substrate. The lower reflector may be designed as a high-contrast grating (51) by etching.

Description

MODE SELECTION LASER

FIELD OF THE INVENTION

The invention relates to lasers, particularly to lasers having mode selection properties and more particularly to vertical cavity surface emitting lasers having mode selection properties.

BACKGROUND OF THE INVENTION

The bandwidth of optical communication using a single optical fiber has been greatly increased by using the multiple time channels (time-division multiplexing, TDM) and multiple wavelength channels (wavelength-division multiplexing, WDM). Recently, to further increase the bandwidth multiple space channels starts to be utilized. In this approach, there are two types of approaches that may be combined. One is the space-division multiplexing (SDM) where multiple cores are embedded in a single fiber. The other is the mode-division multiplexing (MDM) where several transverse modes are launched into a few-mode fiber to carry different information. The combined form may be that multiple few-mode cores are embedded in a fiber and each core carries several transverse modes. The mode-division multiplexing (MDM) system requires multiple light emitters and multiple detectors. In known light emitter systems for the MDM system, bulky phase filters are used to generate different transverse modes from the Gaussianlike fundamental mode and beam splitters are used to couple the generated modes into a few-mode fiber. Also in known detectors for the MDM system, the same beam splitters and phase filters are used to divide the output beam from the few-mode fiber and to detect different transverse modes, respectively.

Accordingly, there is a need for light systems for mode-division multiplexing systems which are less bulky and complex.

US2008/0279229 discloses a surface emitting semiconductor laser which includes a substrate, a lower reflective mirror formed on the substrate, an active layer formed on the lower reflective mirror, an upper reflective mirror formed on the active layer, an optical mode controlling layer formed between the lower reflective mirror and the upper reflective mirror, and a current confining layer formed between the lower reflective mirror and the upper reflective mirror. The active layer emits light. The upper reflective mirror forms a resonator between the lower reflective mirror and the upper reflective mirror. In the optical mode controlling layer, an opening is formed for selectively absorbing or reflecting off light that is emitted in the active layer. The optical mode controlling layer optically controls mode of laser light. The current confining layer confines current that is applied during driving.

US2011/0280269 discloses a vertical-cavity surface-emitting Laser (VCSEL) which incorporates a high contrast grating (HCG) to replace the top mirror of the device and which can operate at long-wavelengths, such as beyond 0.85 μηη. The HCG beneficially provides a high degree of polarization differentiation and provides optical containment in response to lensing by the HCG. The device incorporates a quantum well active layer, a tunnel junction, and control of aperture width using ion implantation. A tunable VCSEL is taught which controls output wavelength in response to controlling a micro-mechanical actuator coupled to a HCG top mirror which can be moved to, or from, the body of the VCSEL.

The inventor of the present invention has appreciated that improvements in light systems for mode-division multiplexing systems would be of benefit, and has in consequence devised the present invention.

SUMMARY OF THE INVENTION

It would be advantageous to achieve improvements in laser systems and/or detector systems for generating beams with different orthogonal modes. In general, the invention preferably seeks to mitigate, alleviate or eliminate one or more of the above mentioned disadvantages of known complex and bulky systems singly or in any combination. In particular, it may be seen as an object of the present invention to provide a method that solves the above mentioned problems, or other problems, of the prior art.

To better address one or more of these concerns, in a first aspect of the invention a vertical cavity surface emitting laser having a vertical optical axis is presented that comprises

- a central structure comprising an active region configured to generate light, - an optical resonant cavity comprising a first reflector arranged below the active region and a second reflector arranged above the active region, wherein the first and second reflectors are arranged to reflect light back to the active region, wherein the first or the second reflector is also arranged to partially transmit light away from the active region, and wherein the reflectivity of the second reflector varies from a starting high reflectivity to a middle low reflectivity being low relative to the high reflectivity and to an ending high reflectivity over at least a fraction of the width of the active region in at least one direction in a horizontal plane perpendicular to the optical axis for selection of a single transverse mode in the cavity,

- a high contrast grating having grating elements of dielectric material (or semiconductor material) separated from each other by a separation medium, where the refractive index of the grating elements is higher than the refractive index of the separation medium, where the grating elements have widths and where the sum of pairs of the grating widths and separation distances between adjacent grating elements define periods, and where the high contrast grating is configured by varying the widths and the periods so that at least one of the grating elements has a width which is different from widths of other grating elements so as to obtain the varying reflectivity of the second reflector, wherein - the functions of the second reflector for reflecting light back to the active region and for obtaining the varying reflectivity are embodied exclusively by the high contrast grating.

The VCSEL has a vertical optical axis, i.e. an axis which is vertical from a top surface from which laser emission propagates along the vertical axis.

The reflection profile defined by a starting high reflectivity, a middle low

reflectivity and an ending high reflectivity is suited for selection of first order modes such as LPllax, LPllay, LPllbx and LPllby modes. Alternatively, the laser may be configured with other reflection profiles for selection of other modes.

The high contrast grating (HCG) is known as gratings having periods lower than 1 micron or 1.5 micron and wherein the refractive- index difference between grating elements and the surrounding separation medium should be relatively high, preferably above 1.4, e.g. 1.5, or above 2. The widths of grating elements and separation distances between grating- elements may be selected according to design principles described herein. Among the plurality of grating element one or more of the grating elements may have widths that are different from the widths of other grating elements. For example, one or more grating elements may have a common width which is greater or smaller than the common width of the remaining grating elements so that the grating is configured for mode selection by two different widths of the grating elements. Advantageously, the laser may be configured with the second reflector embodied as the HCG which both provides high reflectivity for ensuring lasing in the active medium as well as the varying reflection profile for mode selection or mode discrimination. Thus, the second reflector may not contain any other reflective or absorbing structures, such as DBR, light absorbing structures or other structures, than the HCG to have both high reflectivity and a capability of mode selection.

The reflectivity of the second reflector may vary in one direction or in two orthogonal directions in a plane perpendicular to the vertical axis. In one- dimensional (ID) grating case, discrimination of polarization may be maintained while the reflectivity profile in two directions is varied.

Advantageously, the use of a reflector with a spatially varying reflectivity for creating of a resonant cavity and a mode selector provides a compact and simple laser with mode selection properties. Accordingly, by combining a plurality of such lasers with different mode selectors it is possible to generate a laser beam containing a plurality of transvers modes. Such a laser beam may advantageously be used in optical communication systems by injecting the laser beam into a few- mode fiber. Since each of the optical modes may be modulated independently, each mode may carry data information independent from other modes. In this way the communication bandwidth may be increased with a factor corresponding to the number of spatially multiplexed optical modes.

Furthermore, the reflectivity of the second reflector may be higher for a specific polarization of the incident field so as to enable selection of a single transverse mode with the specific polarization in the cavity. In an embodiment the high contrast grating is configured by varying the width tg and the period T of the grating elements so as to obtain a constant phase of the grating. Thus, the high contrast grating acting as the second reflector may be configured by varying the width (tg) and the period (T) so as to obtain a constant

reflection/transmission phase profile from the grating wherein the reflectivity profile of the grating varies along at least one planar direction. The high contrast grating may be characterized by a refractive-index difference between the grating elements and the separation medium which is above 1.4, e.g. 1.5.

The thickness of the grating elements along the vertical optical axis may be constant or substantially constant.

In an embodiment the high contrast grating is configured so that the reflectivity of the second reflector is dependent on the polarization direction of light in the cavity.

In an embodiment the vertical cavity surface emitting laser (VCSEL) comprises - a gap layer, where the gap layer is located adjacent to the high contrast grating (e.g. between the second reflector, i.e. the HCG and the central structure), where the layer has a thickness in the direction of the optical axis so that light reflected from the second reflector interferes constructively (or destructively) with light reflected from the boundary between the gap layer and the central structure and propagating towards the active region. In some VCSEL designs, the gap layer thickness can be determined so that destructive interference is resulted. In an embodiment the gap layer is of the same material as the separation medium.

In an embodiment the gap layer is of a material different than the separation medium, and where the material of the gap layer has a refractive index which is lower than the refractive index of the grating elements. In an embodiment the reflectivity of the second reflector varies in a first direction, e.g. x-direction, and the reflectivity is constant in a second direction, e.g. y- direction, perpendicular to the first direction. Alternatively, the reflectivity may vary in two directions.

In an embodiment the reflectivity of the second reflector varies from a starting low reflectivity, to a center high reflectivity being high relative to the low reflectivity and to an ending low reflectivity along the first direction over at least a fraction of the width of the active region.

In an embodiment the reflectivity of the second reflector varies from a starting high reflectivity, to a middle low reflectivity being low relative to the high reflectivity and to an ending high reflectivity along the first direction over at least a fraction of the width of the active region.

It is understood that the reflectivity profile of the second reflector to an incident field with a specific polarization may vary in such a way that the overlap integral of the reflectivity profile with the incident mode profile is larger than the overlap integral with any other modes, so that the reflectivity profile may vary from a starting low reflectivity, to a center high reflectivity being high relative to the low reflectivity and to an ending low reflectivity along the first direction over at least a fraction of the width of the active region

In an embodiment the second reflector and/or the first reflector is embodied by a high contrast grating, and the high contrast grating of the first or the second reflector is configured with a parabolic varying phase along a direction in a horizontal plane perpendicular to the optical axis for focusing the light from the laser towards a common point, or is configured with a linearly varying phase along a direction in a horizontal plane perpendicular to the optical axis for deflecting the light from the laser, or is configured with a combination of parabolic and linearly varying phases for focusing as well as deflecting the light from the laser.

In a second aspect the invention relates to an optical transmitter for injecting multiple beams into a fiber by spatial multiplexing, where the transmitter is configured for generating a plurality of optical light beams each of them having a transverse mode being orthogonal to the others, wherein the transmitter comprises:

- a plurality of vertical cavity surface emitting lasers wherein the spatial variation of the reflectivity of the second reflector is different for at least two of the lasers, and wherein at least one of the lasers is configured according to the first aspect,

- a fiber coupler for coupling the light beams from each of the lasers into an optical fiber. In an embodiment of the second aspect, the optical transmitter comprises

- one vertical cavity surface emitting laser wherein the reflectivity of the second reflector varies as defined in an embodiment along the first direction x and wherein the grating elements are parallel with the second direction y,

- one vertical cavity surface emitting laser wherein the reflectivity of the second reflector varies as defined in an embodiment along the second direction (y), and wherein the grating elements are parallel with the first direction (x)

- one vertical cavity surface emitting laser wherein the reflectance of the second reflector varies as defined in the first aspect along the second direction (y), and wherein the grating elements are parallel with the second direction (y),

- one vertical cavity surface emitting laser wherein the reflectivity of the second reflector varies as defined in the first aspect along the second direction (y), and wherein the grating elements are parallel with the first direction (x),

- one vertical cavity surface emitting laser wherein the reflectivity of the second reflector varies as defined in the first aspect along the first direction (x), and wherein the grating elements are parallel with the second direction (y), and

- one vertical cavity surface emitting laser wherein the reflectivity of the second reflector varies as defined in the first aspect along the first direction (x), and wherein the grating elements are parallel with the first direction (x). In an embodiment of the second aspect the fiber coupler is embodied by the high contrast grating by configuring the high contrast grating to direct and focus the light from each laser towards a common point.

A third aspect of the invention relates to a transverse mode receiver for measuring a single optical transverse mode in a spatially multiplexed optical signal comprising a plurality of optical transverse modes, wherein the receiver is configured for receiving light from the optical transmitter defined in the second aspect, wherein the receiver comprises

- a light sensing region,

- an optical resonant cavity comprising a first reflector arranged below the light sensing region and a second reflector arranged above the light sensing region, wherein the first and second reflectors are arranged to reflect light back to the light sensing region, wherein the first or the second reflector is also arranged to partially transmit light from the spatially multiplexed optical signal into the light sensing region, and wherein the reflectivity of the second reflector varies from a starting high reflectivity to a middle low reflectivity being low relative to the high reflectivity and to an ending high reflectivity over at least a fraction of the width of the active region in at least one direction in a horizontal plane perpendicular to the optical axis for selection of a single transverse mode in the cavity,

- a high contrast grating having grating elements of dielectric material separated from each other by a separation medium, where the refractive index of the grating elements is higher than the refractive index of the separation medium, where the grating elements have widths and where the sum of pairs of the grating widths and separation distances between adjacent grating elements define periods, and where the high contrast grating is configured by varying the widths and the periods so that at least one of the grating elements has a width which is different from widths of other grating elements so as to obtain the varying reflectivity of the second reflector, wherein

- the functions of the second reflector for reflecting light back to the light sensing region and for obtaining the varying reflectivity are embodied exclusively by the high contrast grating.

In embodiments of the transverse mode receiver the reflectivity of the second reflector may alternatively vary from a starting low reflectivity, to a center high reflectivity being high relative to the low reflectivity and to an ending low reflectivity along the first direction over at least a fraction of the width of the active region.

Accordingly, the transverse mode receiver may be configured for measuring first or second order modes. In general the reflectors of the transverse mode receiver may be configured similarly to the reflectors of the vertical cavity surface emitting laser.

A fourth aspect of the invention relates to an optical transmitter-receiver which comprises

- the optical transmitter according to the second aspect,

- a plurality of transverse mode receivers according to the third aspect, wherein the spatial variation of the reflectivity of the second reflector is different for at least two of the transverse mode receivers,

- a fiber out-coupler for coupling the optical modes from the fiber into each of the transverse mode receivers.

A fifth aspect of the invention relates to a method for selecting a mode in a vertical cavity surface emitting laser having a vertical optical axis by providing : - a central structure comprising an active region configured to generate light,

- an optical resonant cavity comprising a first reflector arranged below the active region and a second reflector arranged above the active region, wherein the first and second reflectors are arranged to reflect light back to the active region, wherein the first or the second reflector is also arranged to partially transmit light away from the active region, and wherein the reflectivity of the second reflector varies from a starting high reflectivity to a middle low reflectivity being low relative to the high reflectivity and to an ending high reflectivity over at least a fraction of the width of the active region in at least one direction in a horizontal plane perpendicular to the optical axis for selection of a single transverse mode in the cavity, and

- a high contrast grating having grating elements of dielectric material separated from each other by a separation medium, where the refractive index of the grating elements is higher than the refractive index of the separation medium, where the grating elements have widths and where the sum of pairs of the grating widths and separation distances between adjacent grating elements define periods, and where the high contrast grating is configured by varying the widths and the periods so that at least one of the grating elements has a width which is different from widths of other grating elements so as to obtain the varying reflectivity of the second reflector, wherein - the functions of the second reflector for reflecting light back to the active region and for obtaining the varying reflectivity are embodied exclusively by the high contrast grating. In general the various aspects of the invention may be combined and coupled in any way possible within the scope of the invention. These and other aspects, features and/or advantages of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In summary the invention relates to a semiconductor mode selection laser, particularly to a VCSEL having mode selection properties. The mode selection capability of the laser is achieved by configuring one of the reflectors in the resonance cavity so that a reflectivity of the reflector varies spatially in one dimension or two dimensions. Accordingly, the reflector with a spatially varying reflectivity is part both of the resonance cavity and part of the mode selection functionality of the laser. A plurality of the lasers configured with different mode selectors, i.e. different spatial variations in reflectivity profiles, may be combined to generate a laser beam containing a plurality of orthogonal modes. The laser beam may be injected into a few-mode optical fiber, e.g. for the purpose of optical communication.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which

Fig. 1 schematically illustrates the principle of a mode-selective VCSEL,

Fig. 2 shows further details of an embodiment of a mode-selective VCSEL, Fig. 3 shows details of an alternative embodiment of a mode-selective VCSEL, Fig. 4 illustrates a reflectivity profile and phase profile of the HCG reflector in comparison with mode shapes of the mode-selective VCSEL,

Figs. 5-10 show six different mode shapes, six grating designs, and six reflectivity profiles,

Fig. 11 provides an overview of different mode shapes and grating designs for selection of the mode shapes, Fig. 12 illustrates a transmitter with six VCSELs for injecting 6 different modes into a multimode fiber,

Fig. 13 illustrates a transmitter-receiver system,

Fig. 14 illustrates a HCG based detector,

Fig. 15 illustrates design principles for designing a HCG to obtain mode selection, Fig. 16 illustrates different designs of a HCG, and

Fig. 17 shows details of an alternative embodiment of a mode-selective VCSEL.

DETAILED DESCRIPTION OF AN EMBODIMENT

Fig. 1 illustrates an embodiment of a vertical cavity surface emitting laser 100, equally referred to as a VCSEL or just a laser in the description. The laser 100 is configured to have mode-selection properties. The laser 100 has a vertical optical axis 110. The VCSEL structure includes a central structure 111 comprising an active region 104 configured to generate light. Contact layers 103 and 105 may be located above and/or below the active region 104, respectively, for guiding current from metal contacts (not shown) to the active region 104. For this, either layer 103 or layer 104, or both of them may have a function to confine current such as an oxide aperture or tunnel junction. The active region 104 may comprise one or more quantum wells, quantum wire layers, or quantum dot layers, configured for light generation around a wavelength of interest as is well known for the skilled person. The number of quantum wells, quantum wire layers, or quantum dot layers is selected so as to achieve sufficient light amplification in the active region. The active region is located in a resonant cavity, i.e. an optical resonant cavity comprising a first reflector 106 arranged below the active region and a second reflector 101 arranged above the active region. Since the reflectors constitute the resonant cavity, it is understood that the first and second reflectors are arranged to reflect light back to the active region 104. In order to emit light out, the first or the second reflector is arranged to partially transmit light, i.e. to transmit light away from the cavity. Layer 107 constitutes a support for the other layers, i.e., is a substrate. In Fig. 1, the second reflector is a high-index-contrast grating (HCG) mirror. The HCG mirror requires to be surrounded by a low refractive index material. Thus, a gap layer 102 is arranged between the second reflector 101 and the central structure 111. In the configuration shown in Fig. 1, the gap layer 102 is air but it can be any other non-absorbing low refractive index material such as oxide. The medium above the second reflector 101 as well should be a low refractive index material.

The gap layer 102 may be arranged so that a bottom boundary surface of the gap layer 102 is in contact with a boundary surface of the central structure and so that an upper boundary surface of the gap layer 102 is in contact with the second reflector 101.

The reflectivity of the second reflector 101 varies in at least one direction in a horizontal plane perpendicular to the optical axis 110 so as to enable selection of a single transverse mode in the cavity. This function of the second reflector is described in detail below.

The second reflector 101 is embodied by a high contrast grating (HCG) configured so as to obtain the spatially-varying reflectivity profile of the second reflector. The design of the HCG is described in detail below.

The reflectivity profile of the first reflector 106 is constant or substantially constant of an area of the reflector. The first reflector 106 may be configured as a distributed Bragg reflector (DBR), a HCG or other reflector with a high reflectivity value preferably higher than 99%, 99.5%, or 99.95 %.

In order to implement the mode-selective properties, at least one of two reflectors should be a HCG mirror of which the reflectivity profile is spatially modulated. In the specific configuration shown in Fig. 1, this HCG with a modulated reflectivity profile is the second reflector. But it can be located as the first reflector as well. This can be decided, depending on the nature of applications and their design.

Fig. 2 shows further details of an embodiment of a VCSEL 200 having mode- selection properties and operating at long wavelengths e.g. 1310 nm or 1550 nm. The mode-selective VCSEL 200 comprises two HCG mirrors 15 and 51 and an active region 30 between them. The grating mirror 15 is an example of the second reflector 101, and the grating mirror 51 is an example of the first reflector 106. The top grating mirror 15 is made in a III-V semiconductor layer 10, while the bottom grating mirror 51 is made in the silicon layer 52 of a silicon-on-insulator (SOI) wafer 50. The layer 20 is a sacrificial etching layer which can be selectively removed against layers 10 and 30 so as to form an air gap 102 underneath the grating 15. Layers 32 and 34 are moderately n-doped and p-doped layers, respectively while a tunnel junction 33 consists of two thin highly n-doped and p- doped layers. When the voltage applied to a metal contact 31 is higher than that applied to the other metal contact 37, a reverse bias is formed between the layers 32 and 34 and no current passes from the layer 32 to the layer 34 except through the tunneling junction 33 where the electrons in the conduction band in the highly n-doped layer can tunnel into the valence band in the highly p-doped layer. In this way, the current flow can be confined within the area of the tunnel junction 33. Layer 35 is an example of the active region 104 and is configured for generation of light and typically consists of several quantum wells or quantum dot layers. Layer 36 is a moderately n-doped layer. In Fig. 2 either HCG 15 being the second reflector or HCG 51 being the first reflector may be configured with a spatially varying reflectivity in order to implement the mode-selective properties.

In general, in a configuration wherein both the first and second reflectors are embodied by HCGs, the HCG which is not used for mode selection may be configured for other purposes, e.g. for providing beam focusing or beam deflection. However, in a configuration with one or two HCGs, the HCG which is used for mode selection may additionally be configured for other purposes, e.g. for providing beam focusing or beam deflection.

Fig. 2 also illustrates the design of the high contrast grating (HCG) 15 having grating elements 11 separated from each other by a separation medium 14, where the refractive index of the grating elements is higher than the refractive index of the separation medium. The design of the HCG 15 is characterized by a width 12 of the grating elements and a period 13, i.e. the width 12 plus the separation distance between adjacent grating elements 11. The HCG is further characterized by the thickness 20 of the gap layer 102. The thickness 20 in the direction of the optical axis 110 is selected so that light reflected from the second reflector 101 (at the boundary between the reflector 101 and the gap layer 102) interferes constructively or destructively with light reflected from the boundary between the gap layer 102 and the central structure. Thereby, the reflectivity from can be made higher or lower than the reflectivity of the HCG alone in the case of the constructive interference or destructive interference, respectively. Thereby, the reflected light from boundaries and propagating towards the active region achieves a high reflection due to the constructive interference. The specific configuration of the HCG results in a specific variation in reflectivity of the second reflector.

The grating elements 11 are typically made of a high-refractive-index material such as a dielectric material or a semiconductor. The separation medium 14 which surrounds the grating elements 11 are made from a low- refractive- index material such as air or oxide. The refractive- index difference between the grating elements 11 and the separation medium should be relatively high, preferably above 1.4, e.g. 1.5, or above 2. The period, duty cycle (width 12 of the grating elements divided by period 13) and thickness of the grating elements are chosen so that the grating provides high reflectivity, e.g. greater than 99.5% or 99.9% over broad wavelength range, e.g. 140 nm. For example, a HCG designed for operating at 1550-nm wavelength has a period of 700 nm, a duty cycle of 50 %, and a thickness of 500 nm.

The gap layer 102 may be of the same material as the separation medium 14, e.g. air. Alternatively, the gap layer 102 may be made from a material which is different from the material of separation medium 14. The material of the gap layer 102 should have a refractive index which is lower than the refractive index of the grating elements 11. Another gap layer 40 should have a low refractive index. For example, it can be air, an oxide, or a polymer.

Fig. 17 shows details of an alternative embodiment of a VCSEL 1700 having mode-selection properties and operating at long wavelengths, e.g., 1310 nm or 1550 nm. The laser structure is the same as the VCSEL 200 except for the top mirror being replaced by a distributed Bragg reflector (DBR) 1710.

Fig. 3 shows details of an alternative embodiment of a VCSEL 300 having mode- selection properties and operating at short wavelengths, e.g., 850 nm, 980 nm, or 1060 nm. The mode-selective VCSEL 300 comprises a HCG reflector 15 and a distributed Bragg reflector (DBR) 250 and a n active region 230 between them . The DBR 250 is a n exam ple of the first reflector 106. The layer 220 is a solid layer having a low refractive index, so as to form a low index layer underneath the grating 15. Layer 232 is an electrically conducting layer and layer 233 is an electrica lly insulating layer for providing current confinement. Layer 234 for light generation is equivalent to layer 35 of Fig . 2. Layer 235 is equivalent to layer 36 of Fig . 2.

Fig . 4 illustrates a reflectivity profile 402 a nd a n associated phase profile 401 of the second reflector 101, 15 in com parison with mode shapes 403-404 of the optical cavity of the VCSEL. The reflectivity profile 402 shows that the reflectivity varies in the x d irection in a horizonta l plane perpendicular to the optica l axis 110.

The mode intensity sha pes 403-404 shown in Fig . 4 a re commonly known as the linearly polarized modes : LPoix and LPnax. The index x denotes the electric field of the mode is polarized a long x d irection . The LPoix mode shape has a single central intensity maxim um, i .e. the light intensity of this mode is maximal in the center of the beam em itted by the laser 100. The LPnax mode shape has two pea ks of high intensity maxim um displaced symmetrica lly relative to the spatial center of the mode, i .e. the light intensity profile of this mode shows two peaks displaced sym metrica lly relative to the center of the bea m em itted by the laser 100.

The reflectivity profile 402 has a m inim um reflectivity Ri in the center of the output aperture of the laser and a maxima l reflectivity Rh elsewhere. The phase profile 401 is constant, i .e. the phase of transm itted a nd reflected light from the second reflector is not altered by the second reflector 101, 15.

Due to the smaller reflectivity value in the center of the reflectivity profile, the mode LPoix experiences a la rger m irror loss, whereas the mode LPnax experiences a smaller m irror loss, which means the threshold gain for LPnax will be sma ller than that for LPoix. Accord ingly, by proper design of the second reflector 101 and, thereby, the reflectivity profile 402, it is possible to achieve that the threshold moda l ga in in the cavity of the laser 100 config ured so that only a single transverse mode in the cavity is am plified sufficiently to generate a laser light output from the laser 100. With reference to the two modes in Fig . 4, the visualization shows that only the LPnax mode will receive sufficient amplification in the cavity.

Fig. 5 shows a mode shape LPoix of the cavity of the VCSEL 100, 200, 300 which has a single central intensity peak. Fig. 5 shows that the mode shape LPoix has an intensity distribution similar to a Gaussian distribution. The electric field of the mode LPoix is polarized in the x-direction as indicated by polarization direction 501. Fig. 5 shows the design 511 of the HCG of the second reflector 10, 101 wherein grating elements 11 are oriented in the y direction - perpendicular to the polarization direction 501. This type of one-dimensional grating is called

transverse-magnetic (TM) grating which provides higher reflectivity to an incident field of which polarization is perpendicular to the grating element orientation. Even though all examples in this description use only TM gratings for explaining the invention, one may equivalently use a transverse electric (TE) grating that provides higher reflectivity to an incident field with a polarization parallel to the grating orientation. The grating elements has a width tg and the sum of the grating width tg and the separation distance ts between adjacent grating elements defines a period T. Accordingly, the duty cycle is given by width tg divided by period T.

This particular design results in a reflectivity profile 521 for x-polarized incident field and a reflection profile 531 for y-polarized incident field . Accordingly, the HCG is configured so that the reflectivity of the second reflector 10, 101 is dependent on the polarization direction (501-506) of field in the laser cavity. As a general rule (for TM gratings), elongated grating elements 11 which extend in a direction perpendicular to the polarization direction generate a higher reflectivity as compared to elongated grating elements 11 which extend in a direction parallel to the polarization direction.

Due to the particular design 511 of the grating, the reflectivity of the second reflector varies in both the x and y directions. The reflectivity profile 521, R_x{x,y) for x polarized incident field has a high reflectivity value, e.g. above 98%, at the location where the intensity profile of x-polarized mode LPoix, | ELPOIX( , ) | has its peak values, while the reflectivity profile 531, R_y(x,y) for y polarized incident field has lower values over the location where the intensity profile of y-polarized mode LPoiy, I E_poi_y(x,y) I has its peak values. Thus, the modal reflectivity for LPoix, RLPOIX is higher than that for LPoiy, R_poi_y, J|E_Lpoi_y(¾ y)| .₅n(¾ y)dA ( i)

POl y

j] ^ELP01 y ( ^X> Υ) dA

That is, the mirror loss for the LPoix mode is smaller than that for the LPoiy mode. It means that LPoix will be the lasing mode with the smallest threshold gain to compensate all types of losses. Even though the reflectivity profile 521 varies symmetrically in two dimensions (x and y directions), a symmetric reflectivity profile which varies only in either x or y direction and is constant in the other direction would also be suitable for selection of the LPoix mode. Accordingly, the reflectivity profile 521 enables selection of a single transverse mode, i.e. the LPoix mode in the cavity of the laser 100, 200, 300.

Fig. 6 shows the intensity profile of LPoiy mode of the cavity of the VCSEL 100, 200, 300 which has a single central intensity peak. Fig . 6 shows that the mode shape LPoiy has an intensity distribution similar to the LPoix mode. The electric field of the LPoiy mode is polarized in the y-direction as indicated by polarization direction 502.

Fig. 6 shows the design 512 of the HCG of the second reflector 10, 101 wherein grating elements 11 are oriented in the x direction - perpendicular to the polarization direction 502. This particular design 512 results in a reflectivity profile 522 for y-polarized incident field and a reflectivity profile 532 for x-polarized field. Due to the particular design 512 of the grating the reflectivities of the second reflector vary in both the x and y directions. The reflectivity profile 522 for y polarized incident field has a high reflectivity value, e.g. above 98%, at the location where the intensity profile of y-polarized mode LPoiy has its peak values, while the reflectivity profile 531 for x polarized incident field has lower values over the location where the x-polarized mode profile LPOlx has peak values. Thus, the modal reflectivity for LPoiy is higher than that for LPoix.

That is, the mirror loss for the LPoiy mode is smaller than that for the LPoix mode. It means that LPoiy will be the lasing mode with the smallest threshold gain to compensate all types of losses. Even though the reflection profile 522 varies in two dimensions, a similar reflectivity profile which only varies in the x or y direction and has a constant high reflectivity value in the other direction would also be suitable for selection of the LPoiy mode. Accordingly, the reflection profile 522 enables selection of a single transverse mode, i.e. the LPoiy mode in the cavity of the laser 100, 200, 300.

Fig. 7 shows the intensity profile of LPnax mode of the cavity of the VCSEL 100, 200, 300 which has two intensity peaks located along the x-direction and symmetrically relative to a low intensity area located in the center of the mode. The electric field of the LPnax mode is polarized in the x-direction as indicated by polarization direction 503.

Fig. 7 shows the design 513 of the HCG of the second reflector 10, 101 wherein grating elements 11 are oriented in the y direction - perpendicular to the polarization direction 503.

This particular design 513 results in a reflectivity profile 523 for x-polarized incident field and a reflectivity profile 533 for y-polarized incident field. Due to the particular design 513 of the grating the reflectivity of the second reflector varies only in the x direction. The reflectivity profile 523 has a low reflectivity value, e.g. below 95%, at the location where the mode shape LPnax has its low intensity dip and a high reflectivity value, e.g. above 98%, where the mode LPnax has intensity peaks. Thereby, the LPnax mode has a relatively higher modal reflectivity than other modes, becoming the lasing mode. As defined in Eq. (1), the modal reflectivity is an overlap integral of the reflectivity profile of a reflector and the intensity profile of an incident mode.

Accordingly, the reflectivity profile 523 enables selection of a single transverse mode, i.e. the LPnax mode in the cavity of the laser 100, 200, 300.

Fig. 8 shows a mode shape LPnay of the cavity of the VCSEL 100, 200, 300 which has two intensity peaks located along the y-direction similarly to the LPnax mode. The electric field of the mode shape LPnay is polarized in the y-direction as indicated by polarization direction 504. The design 514 of the HCG of the second reflector 10, 101 shows that the grating elements 11 are oriented in the x direction - perpendicular to the polarization direction 504 of the LPnay mode.

This particular design 514 results in a reflection profile 524 for y-polarized incident field and a reflectivity profile 534 for x-polarized incident field. The reflection profile 524 has a low reflectivity value, e.g. below 95%, at the location where the mode shape LPnay has its low intensity dip and a high reflectivity value, e.g.

above 98%, where the mode LPnay has intensity peaks. Thereby, the LPnay mode has a relatively higher modal reflectivity than other modes, becoming the lasing mode.

Accordingly, the reflection profile 524 enables selection of a single transverse mode, i.e. the LPnay mode in the cavity of the laser 100, 200, 300. Fig. 9 shows a mode shape LPiibx of the cavity of the VCSEL 100, 200, 300 which has two intensity peaks located along the x-direction and symmetrically relative to a low intensity area located in the center of the mode. The electric field of the mode shape LPiibx is polarized in the x-direction as indicated by polarization direction 505. The design 515 of the HCG of the second reflector 10, 101 shows that the grating elements 11 are oriented in the y direction - perpendicular to the polarization direction 505 of the LPiibx mode.

This particular design 515 results in a reflection profile 525 for x-polarized incident field and a reflectivity profile 535 for y-polarized incident field. The reflection profile 525 has a low reflectivity value, e.g. below 95%, at the location where the mode shape LPiibx has its low intensity dip, and a high reflectivity value, e.g. above 98%, where the mode LPiibx has intensity peaks. Thereby, as explained for the other modes, the reflectivity profile 525 enables selection of a single transverse mode, i.e. the LPiibx mode in the cavity of the laser 100, 200, 300.

Fig. 10 shows a mode shape LPnby of the cavity of the VCSEL which is similar to the LPiibx mode except that the electric field of the mode shape LPnby is polarized in the y-direction as indicated by polarization direction 506. The design 516 of the HCG of the second reflector 10, 101 shows that the grating elements 11 are oriented in the x direction - perpendicular to the element direction for the design 515.

The design 516 results in a reflectivity profile 526 for y-polarized incident field and a reflectivity profile 536 for x-polarized incident field. As explained for the other modes, the reflectivity profile 526 enables selection of a single transverse mode, i.e. the LPnby mode.

Fig. 11 provides an overview of the content of Figs. 5-10. Thus, the first row in Fig. 11 shows the mode names, the second row shows the mode shapes and polarization directions, the third row shows the principal design of the high contrast gratings and the fourth row shows the reflectivity profiles for x and y polarized light for selection of the mode shape given in the column. Accordingly, each column shows the mode shape, polarization direction, grating design and reflection profiles for each mode.

Since Fig. 11 only shows the principal design of the high contrast gratings, reference is made to Figs. 5-10 and 16 and the corresponding descriptions for explanations and examples of grating designs. Accordingly, as from the examples given above it is clear that the local reflectivity in 521-526 of the second reflector may vary in a first direction and that and the reflectivity may be constant in a second direction perpendicular to the first direction. Alternatively, the local reflectivity of the second reflector may vary both in a first direction and a perpendicular a second direction so that the local reflectivity varies two-dimensionally.

In order to achieve mode selection of modes having a central peak intensity, the local reflectivities 521-526 of the second reflector 10, 101 may vary from a starting low reflectivity value, to a center high reflectivity value being high relative to the low reflectivity and to an ending low reflectivity at least along a first direction over a width of the optical modes in the cavity, i.e. over at least a fraction of the width of a the active region. Furthermore, the grating element orientation may be chosen to be perpendicular to the polarization of the mode selected for lasing, since the grating has higher reflectivity for the incident field with a polarization perpendicular to the grating element orientation. In order to achieve mode selection of modes having a central low intensity, i.e. a central intensity dip, the reflectivity profiles 521-526 of the second reflector 10, 101 may vary from a starting high reflectivity value, to a middle low reflectivity value being low relative to the high reflectivity value and to an ending high reflectivity value along the first direction over at least a fraction of the width of the active region at least along a first direction over a width of the optical modes in the cavity, i.e. over at least a fraction of the width of the active region.

Furthermore, the grating element orientation may be chosen to be perpendicular to the polarization of the mode selected for lasing, since the grating has higher reflectivity for the incident field with a polarization perpendicular to the grating element orientation.

Other variations of the reflectivity profiles 521-526 of the second reflector may be envisaged for selection of similar or differently shaped modes. The six different modes LPoix, LPoiy, LPnax, LPnay, LPiibx, and LPiiby are suited for injection into optical fibers having radially symmetric transverse refractive index profiles, i.e. index profiles where the refractive index depends only on the radial coordinate and not on the azimuthal coordinate. Such fibers are capable of guiding linearly polarized (LP) modes of light.

Fig. 15 shows graphs 1501, 1503 showing contours 1502 of constant reflectivity and contours 1504 of constant reflection (or transmission) phase as a function of grating period T and grating width tg.

Graph 1501 shows how the reflectivity of a grating (e.g. the grating of the second reflector 101) can be varied between two or more values along path AB. For example, the reflectivity may be varied between the values at point A and B by using gratings configured with two different widths tg and two different periods T as can be read from graph 1501.

As shown in graph 1503, by choosing the points A and B so that they are located on a contour of constant phase 1504, it is possible to obtain a constant phase of the grating along at least one direction wherein the reflectivity of the second reflector varies between values at points A and B. By a constant phase of the grating it is understood that the grating affects light which is transmitted or reflected from the grating with a phase change which is constant or substantially constant along at least one direction wherein the reflectivity of the second reflector varies, or is constant in a plane perpendicular to the vertical optical axis 110.

In mode selection gratings, if the phase profile of transmittivity of the second grating is made constant throughout the second grating 101, the output beam will propagate along the vertical optical axis 110, experiencing diverging in x-y plane as is normally occurring.

It is possible to configure the HCG grating, i.e. the second reflector 101, so that the output beam from the laser 100 is deflected, i.e. directed at an angle relative to the vertical optical axis 110 and/or focused. To implement the beam deflection/focusing as well as mode selection, it may be advantageous to use double HCG mirrors as in VCSEL 200 shown in Fig. 2. Then, the beam selection capability can be implemented in the HCG 51 being the first reflector and the beam deflection/focusing can be implemented in the HCG 15 being the second reflector which in this case would also be the a beam emitting reflector.

Deflection of the output beam may be achieved by configuring a grating 15, 51 so that its transmission phase profile of light from the laser 100 varies linearly in one direction across the grating the while the reflectivity profile is constant throughout the grating. Light output from the grating with a linearly varying phase profile along a direction in a horizontal plane perpendicular to the optical axis 110, will have a deflection from the optical axis 110. For example, the linearly varying transmission phase profile as well as the constant reflectivity profile may be obtained by varying the grating period T and grating width tg of the second grating along line AC as shown in graphs 1501, 1503. Focusing of the output beam may be achieved by configuring the second grating so that it modifies the phase of light from the laser 100, so that light outputted from the grating has a parabolic varying phase along a direction in a horizontal plane perpendicular to the optical axis 110. The parabolic varying phase of the light beam from the laser causes focusing of the beam.

For example, the parabolic varying phase may be obtained by varying the grating period T and grating width tg along line BC as shown in graphs 1501, 1503, similarly as in the implementation of the linearly varying phase case. To get the linear phase profile, the change of T and tg along line BC should be uniform as shown in 1505 (i.e. with equal step lengths along path BC traversed in one direction), since the phase change along line BC is almost linear. Analogously, one may adjust the change of T and tg along line BC non-uniformly (i.e. with equal step lengths along path BC traversed back and forth so that the small steps near point C are located at the center of the grating) so as to get parabolic phase profile, as shown in 1506. In an embodiment the second reflector and/or the first reflector is embodied by a high contrast grating wherein the parabolic varying phase and the linearly varying phase are combined by superposing step length changes from path 1505 with step length changes from path 1506.

Fig. 16 show specific HCG designs for mode selections can be made on basis of the design rules described in connection with Fig. 15.

The HCG design 1601 comprises central gratings 1622 and outer grating 1621. The central gratings 1622 has have a smaller period T and smaller grating widths tg than the outer gratings 1621. The design provides a reflectivity profile 1611 for selection of LPoi modes, i.e. zero order modes. The central grating 1622 having a period T and a width tg corresponding to point A in Fig. 15 provides a high reflectivity value given by the reflectivity value of point A in 1501. The outer gratings 1621 having a period T and a width tg corresponding to point B in Fig. 15 provides a low reflection given by the reflectivity value of point B in 1501.

An alternative design for selection of LPoi modes is the design 1602, wherein the reflectivity profile 1612 has a high central reflection and reflectivity-dips at the spatial location where LPn, i.e. first order modes, has a high intensity. The reflectivity-dips are obtained by configuring the grating with one or more gratings 1623 at the spatial location where the reflectivity-dip should be located. The width tg and period T for gratings 1623 and 1624 may be selected correspondingly with gratings 1621 and 1622 in the grating design 1601.

The HCG design 1603 comprises center gratings 1626 having large grating widths tg and outer gratings 1625 having smaller grating widths. The design provides a reflectivity profile 1613 for selection of LPn modes, i.e. first order modes. The central gratings 1626 having a period T and a width tg corresponding to point B in Fig. 15 provides a low reflection given by the reflectivity value of point B. The outer gratings 1625 having a period and a width tg corresponding to point A in Fig. 15 provides a high reflection given by the reflectivity value of point B.

The alternative HCG design 1603a for creating the reflectivity profile 1613 shows how transition from the central grating 1626 to the outermost grating elements 1629 can be made smoother by having one or more intermediate gratings 1625a having widths tg being smaller than the widths of the center elements 1626 and greater than the widths of the outermost grating elements 1629. Having smooth transition may reduce un-wanted scattering loss at the transition interface from elements 1626 to elements 1629.

An alternative design for selection of LPn modes is the design 1604, wherein the reflectivity profile 1614 has a low central reflectivity and higher side reflectivity- values. The design 1604 differs from design 1603 by having a narrower

reflectivity-dip obtained by configuring the grating with one or more gratings

1626, 1628 at the spatial location where the reflectivity-dip should be located. The width tg and period T for grating-elements 1627 and 1628 may be selected correspondingly with grating-elements 1625 and 1626 in the grating design 1603. For the designs 1603 and 1604, typical values of the period width tg and period T are dependent on operating wavelengths. For example, at 1550 nm wavelength, the values can be 200-1200 nm for the narrow widths (tg), 200-1200 nm for the broader widths (tg), 400-1400 nm for the small periods (T) and 400-1400 nm for the broader periods (T).

Fig. 12 shows an optical transmitter 1200 for generating a plurality of optical light beams each of them having a transverse mode being orthogonal to the others. The transmitter 1200 is further configured for injecting multiple beams into a fiber 1220. The optical transmitter 1200 comprises six lasers 1201-1206 each configured according to one of the embodiments of the invention to generate one out of the six different modes LPoix, LPoiy, LPnax, LPnay, LPiibx, and LPiiby. That is, the optical transmitter 1200 comprises a plurality of lasers 100, 200, and 300 wherein the spatial variation of the local reflectivity of the second reflector 101 is different for at least two of the lasers 100. The light beams (illustrated by solid lines 1231 or dashed lines 1232) having different mode shapes can be coupled into a single multi-mode fiber 1220 by a fiber coupler 1210. The fiber coupler 1210 may be embodied by a focusing lens configured to focus and direct the beams 1231 into the fiber. Alternatively, the fiber coupler 1210 may be embodied by the HCG, i.e. the second reflector 101, by configuring the HCG to direct and/or focus the beams from each laser (illustrated by beams 1232) towards a common point, i.e. towards the entrance aperture of the fiber 1220.

Fig. 13 illustrates a transmitter-receiver system 1300 comprising the optical transmitter 1200 and an optical receiver 1301. The output beams from the plurality of lasers 1201-1206 are coupled into the multi-mode fiber 1220 by spatial multiplexing which is possible since each of the lasers 1201-1206 generates a transverse output mode which is orthogonal to output modes of the other lasers 1201-1206. In the other end of the fiber 1220, the plurality of output modes are demultiplexed by the optical receiver 1301. Accordingly, the optical receiver 1301 comprises a plurality of transverse mode receivers 1401-1406 (see. Fig. 14) - one for each of the optical modes coupled into the fiber 1220 - configured to detect the light intensity for a single mode in the spatially

multiplexed output signal. I.e. the spatial variation of the reflectivity of the second reflector is different for at least two of the transverse mode receivers, e.g. the spatial reflectivity variation of the reflector may be different for each of the six transverse mode receivers in order to enable detection of six orthogonal optical modes. The optical receiver 1301 further comprises a fiber out-coupler (e.g. array of beam splitters) for coupling the optical modes from the fiber 1220 into each of the transverse mode receivers.

Fig. 14 shows one of the plurality of transverse mode receivers 1401 comprised by the optical receiver 1301 which is configured for measuring a single optical transverse mode in a spatially multiplexed optical signal comprising a plurality of optical transverse modes. The optical receiver 1401 comprises a resonant cavity configured with a first reflector 106 arranged below a light sensing region 1404 and a second reflector 101 arranged above the light sensing region 1404. The first and second reflectors 106, 101 of the transverse mode receiver 1401 are configured similarly to the first and second reflectors of the laser 100, 200, 300 in order to select one out of a plurality of modes contained in the spatially

multiplexed optical signal. Accordingly, the plurality of transverse mode receivers 1401-1406 may consist of a first receiver 1401 configured with a second reflector 101 configured with the HCG 513, a second receiver 1402 configured with a second reflector 101 configured with the HCG 514, a third receiver 1403 configured with a second reflector 101 configured with the HCG 515, and so forth in order to obtain receivers capable of detecting different optical transverse modes.

The light sensing region 1404 may comprise an absorption layer for receiving incident photons and generating charged carriers, i.e. electrons, in dependence of the amount of absorbed photons.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A vertical cavity surface emitting laser (100, 200, 300, 1700) having a vertical optical axis (110), comprising

- a central structure (111) comprising an active region (104) configured to generate light,

- an optical resonant cavity comprising a first reflector (106, 51, 240, 1710) arranged below the active region and a second reflector (101, 15, 1751) arranged above the active region, wherein the first and second reflectors are arranged to reflect light back to the active region, wherein the first or the second reflector is also arranged to partially transmit light away from the active region, and wherein the reflectivity of the second reflector varies from a starting high reflectivity to a middle low reflectivity being low relative to the high reflectivity and to an ending high reflectivity over at least a fraction of the width of the active region in at least one direction in a horizontal plane perpendicular to the optical axis for selection of a single transverse mode in the cavity,

- a high contrast grating having grating elements (11) of dielectric material separated from each other by a separation medium (14), where the refractive index of the grating elements is higher than the refractive index of the separation medium, where the grating elements have widths (tg) and where the sum of pairs of the grating widths (tg) and separation distances (ts) between adjacent grating elements define periods (T), and where the high contrast grating is configured by varying the widths (tg) and the periods (T) so that at least one of the grating elements has a width which is different from widths of other grating elements so as to obtain the varying reflectivity of the second reflector, wherein

- the functions of the second reflector for reflecting light back to the active region and for obtaining the varying reflectivity are embodied exclusively by the high contrast grating.

2. A vertical cavity surface emitting laser according to claim 1, wherein the high contrast grating is configured by varying the width (tg) and the period (T) so as to obtain a constant phase of the grating along the at least one direction wherein the reflectivity of the second reflector varies.

3. A vertical cavity surface emitting laser according to claim 1, wherein the high contrast grating is characterized by a refractive- index difference between the grating elements and the separation medium which is above 1.4.

4. A vertical cavity surface emitting laser according to claim 1, wherein the thickness of the grating elements along the vertical optical axis is constant or substantially constant.

5. A vertical cavity surface emitting laser according to claim 1, wherein the high contrast grating is configured so that the reflectivity of the second reflector (10, 101) is dependent on the polarization direction (501-506) of light in the cavity.

6. A vertical cavity surface emitting laser according to claim 1, comprising

- a gap layer (102), where the gap layer is located between the second reflector (10, 101) and the central structure (111) where the layer has a thickness (20) in the direction of the optical axis so that light reflected from the second reflector interferes constructively or destructively with light reflected from the boundary between the layer and the central structure and propagating towards the active region.

7. A vertical cavity surface emitting laser according to claim 6, wherein the gap layer (102) is of the same material as the separation medium (14).

8. A vertical cavity surface emitting laser according to claim 6, wherein the gap layer (102) is of a material different than the separation medium (14), and where the material of the gap layer has a refractive index which is lower than the refractive index of the grating elements (11).

9. A vertical cavity surface emitting laser according to claim 1, wherein the reflectivity of the second reflector (10, 101) varies in a first direction and wherein the reflectivity is constant in a second direction perpendicular to the first direction.

10. A vertical cavity surface emitting laser according to claim 9, wherein the reflectivity of the second reflector (10, 101) varies from a starting low reflectivity, to a center high reflectivity being high relative to the low reflectivity and to an ending low reflectivity along the first direction over at least a fraction of the width of the active region.

11. A vertical cavity surface emitting laser according to any of the preceding claims, wherein the second reflector and/or the first reflector is embodied by a high contrast grating, and wherein the high contrast grating of the first or the second reflector is configured with a parabolic varying phase along a direction in a horizontal plane perpendicular to the optical axis for focusing the light from the laser towards a common point, or is configured with a linearly varying phase along a direction in a horizontal plane perpendicular to the optical axis for deflecting the light from the laser, or is configured with a combination of parabolic and linearly varying phases for focusing as well as deflecting the light from the laser.

12. An optical transmitter (1200) for injecting multiple beams into a fiber (1220) by spatial multiplexing, where the transmitter is configured for generating a plurality of optical light beams each of them having a transverse mode (LPOlx, LPOly, LPllax, LPllay, LPllbx, LPllby) being orthogonal to the others, wherein the transmitter comprises:

- a plurality of vertical cavity surface emitting lasers wherein the spatial variation of the reflectivity of the second reflector is different for at least two of the lasers, and wherein at least one of the lasers is configured according to claim 1,

- a fiber coupler (1210) for coupling the light beams from each of the lasers into an optical fiber.

13. An optical transmitter according to claim 12, comprising

- one vertical cavity surface emitting laser wherein the reflectivity of the second reflector varies as defined in claim 10 along the first direction (x) and wherein the grating elements are parallel with the second direction (y),

- one vertical cavity surface emitting laser wherein the reflectivity of the second reflector varies as defined in claim 10 along the second direction (y), and wherein the grating elements are parallel with the first direction (x)

- one vertical cavity surface emitting laser wherein the reflectivity of the second reflector varies as defined in claim 1 along the second direction (y), and wherein the grating elements are parallel with the second direction (y), - one vertical cavity surface emitting laser wherein the reflectivity of the second reflector varies as defined in claim 1 along the second direction (y), and wherein the grating elements are parallel with the first direction (x),

- one vertical cavity surface emitting laser wherein the reflectivity of the second reflector varies as defined in claim 1 along the first direction (x), and wherein the grating elements are parallel with the second direction (y), and

- one vertical cavity surface emitting laser wherein the reflectivity of the second reflector varies as defined in claim 1 along the first direction (x), and wherein the grating elements are parallel with the first direction (x).

14. An optical transmitter according to claim 12, wherein the fiber coupler (1210) is embodied by the high contrast grating by configuring the high contrast grating with a parabolic varying phase along a direction in a horizontal plane

perpendicular to the optical axis for focusing the light from each laser towards a common point.

15. A transverse mode receiver (1401) for measuring a single optical transverse mode in a spatially multiplexed optical signal comprising a plurality of optical transverse modes, wherein the receiver is configured for receiving light from the optical transmitter defined in claim 12, wherein the receiver comprises

- a light sensing region (1404),

- an optical resonant cavity comprising a first reflector (106) arranged below the light sensing region and a second reflector (101) arranged above the light sensing region, wherein the first and second reflectors are arranged to reflect light back to the light sensing region, wherein the first or the second reflector is also arranged to partially transmit light from the spatially multiplexed optical signal into the light sensing region, and wherein the reflectivity of the second reflector varies from a starting high reflectivity to a middle low reflectivity being low relative to the high reflectivity and to an ending high reflectivity over at least a fraction of the width of the active region in at least one direction in a horizontal plane perpendicular to the optical axis for selection of a single transverse mode in the cavity,

16. An optical transmitter-receiver (1300) comprising :

- the optical transmitter (1200) according to claim 12,

- a plurality of transverse mode receivers (1401) according to claim 15, wherein the spatial variation of the reflectivity of the second reflector is different for at least two of the transverse mode receivers,

- a fiber out-coupler for coupling the optical modes from the fiber (1220) into each of the transverse mode receivers.

17. A method for selecting a mode in a vertical cavity surface emitting laser having a vertical optical axis by providing :

- an optical resonant cavity comprising a first reflector (106, 51, 250) arranged below the active region and a second reflector (101, 15) arranged above the active region, wherein the first and second reflectors are arranged to reflect light back to the active region, wherein the first or the second reflector is also arranged to partially transmit light away from the active region, and wherein the reflectivity of the second reflector varies from a starting high reflectivity to a middle low reflectivity being low relative to the high reflectivity and to an ending high reflectivity over at least a fraction of the width of the active region in at least one direction in a horizontal plane perpendicular to the optical axis for selection of a single transverse mode in the cavity, and