WO2006000729A1

WO2006000729A1 - Device for sampling part of a light beam originating from a light-emitting optoelectronic component

Info

Publication number: WO2006000729A1
Application number: PCT/FR2005/050436
Authority: WO
Inventors: François Marion
Original assignee: Commissariat A L'energie Atomique
Priority date: 2004-06-14
Filing date: 2005-06-13
Publication date: 2006-01-05
Also published as: FR2871583A1

Abstract

The invention relates to a method of device for sampling part of a light beam originating from a light-emitting electronic component. According to the invention, the aforementioned component consists of a substrate comprising first and second faces and an active zone which is disposed on the first face and which is designed to emit the light beam in the direction of the second face. The inventive device comprises an annular mirror (20) which is disposed on the second face and which can sample the outer annular part (22) of the light beam (16), said annular mirror and light beam (16) being coaxial. The device is particularly suitable for regulating the luminous intensity of the beam.

Description

DEVICE FOR COLLECTING A PART OF A LUMINOUS BEAM FROM AN OPTOELECTRONIC LIGHT - EMITTING COMPONENT

DESCRIPTION

TECHNICAL FIELD

The present invention relates to a device for sampling a portion of a light beam from a light emitting electronic component (in English "light emitting electronic component"). It applies in particular to the control (in English "control") of the power of the luminous emission of the component. The invention is more particularly usable with VCSEL type components, ie vertical cavity surface emitting lasers, or with cavity electroluminescent diodes. resonant (in English "resonant cavity light emitting diodes"). It finds applications in particular in all systems that use optical links and, more generally, in all systems that include light emitters and require control of the light output of these emitters, the latter being, for example, VCSELs or RCLED that is to say light-emitting diodes resonant cavity (in English "cavity cavity light emitting diodes"). Recall that a RCLED is a structure that is almost identical to a VCSEL but whose mirrors have a lower reflectivity. The invention applies, for example, to high-speed optical links, to intra-chip optical connections, to intra-card optical links and to free-space optical links. All these links need a perfectly stabilized luminous power, automatically controlled by an APC type system (for "Automatic Power Control").

STATE OF THE PRIOR ART

The level of compactness of the opto ^¬ electronic components must respond to the growing miniaturization of the transceiver modules. In addition, these modules are asked to be more and more efficient while being less and less expensive. As a result, techniques are constantly being sought to increase the integration density of the components on chips (in English "chips"). Refer to the following documents:

[1] Vertical-Cavity Lasers with an Intracavity Resonant Detector, Sui F. Lim et al., IEEE Journal of Selected Topics in quantum electronics, vol. 3, No. 2, 1997, pages 416 to 421 [2] Power Control of VCSEL Arrays Using Monolithically Integrated Focal Plane Detectors, Mohammad Azadeh et al., Journal of Lightwave Technology, vol. 20, No. 8, 2002, pages 1478 to 1484 [3] International Application WO 03/000019, published January 3, 2003, Integrated photodetector for VCSEL feedback control.

In a conventional light emission system, two distinct components are generally implanted: the emission component, for example a laser stripe laser or a VCSEL, and a detection component making it possible to control the light power emitted, from the removal of a fraction of it, as is done in APC type systems. In the case of a Fabry-Perot type laser, it is easy to use a photodiode for sampling. Indeed, the two faces of the laser being accessible and light being emitted even by the face opposite to the actual emission face, it is easy to have a photodiode facing this opposite face. On the other hand, in the case of a VCSEL, the means associated with the latter for taking a part of the light power is generally external: a photodetector component is placed near the VCSEL (see for example document [3] ). Indeed, the two mirrors of very high reflectivity of VCSEL are not necessarily accessible and lateral leakage of light is extremely low. However, it has already been proposed to manufacture a monolithic structure comprising a VCSEL and means for detecting evanescent lateral light waves originating from this VCSEL, and / or an additional resonant cavity for the detection, this cavity being integrated in the stack of layers of VCSEL. It should be noted, however, that the integration of such a cavity increases the production time and lowers the manufacturing efficiency.

STATEMENT OF THE INVENTION

The present invention aims to overcome the above disadvantages. It proposes to integrate the light pickup photodiode as close as possible to the chip of the VCSEL or RCLED, more generally of the light emitting electronic component, and to exploit a part of the light beam. emitted by this component, in particular the external part, or peripheral part, of this beam. Especially in the case of a VCSEL, this part is the least useful of the beam. More generally, the present invention solves the problem of taking part of the light beam emitted by a light-emitting electronic component, in particular a VCSEL or a RCLED, in order to use the light taken off, by example to control the power of the light beam. Specifically, the present invention relates to a device for sampling a portion of a light beam from a light-emitting electronic component, this component comprising a substrate having first and second faces, and an active zone arranged. on the first face and intended to emit the light beam in the direction of the second face, this device being characterized in that it comprises an annular mirror, or annular reflector, which is arranged on the second face and able to take the external part annular light beam, this annular mirror and the light beam being coaxial. According to a particular embodiment of the device according to the invention, the light emitting electronic component is a vertical cavity surface emitting laser or a resonant cavity light emitting diode. The device according to the invention may further comprise an annular photodetector which is arranged on the first face and provided for detecting the part taken by the mirror, after reflection of this part by this mirror, this annular photodetector and the annular mirror being coaxial . In the case where the device which is the subject of the invention is provided with the annular photodetector, this photodetector is preferably monolithically integrated with the light-emitting electronic component. According to a particular embodiment of the invention, the device further comprises means for regulating the light power of the beam emitted by the light-emitting electronic component, using the portion of this beam, which is picked up by the mirror. Preferably, in the case where the device which is the subject of the invention is provided with the annular photodetector, these regulation means comprise this annular photodetector. The present invention also relates to a matrix of light-emitting electronic components, each component being provided with a device according to the invention.

The invention has various advantages: it makes it possible to use strips or matrices of elementary light emitting electronic components such as VCSELs or RCLEDs, provided with photodetectors and sampling devices according to the invention, and hybridize these VCSELs or these RCLEDs on a control circuit, the latter being for example of the CMOS type, it makes it possible to reduce substantially the manufacturing costs of components of the VCSEL or RCLED type whose luminous power is controlled automatically, it allows the manufacturing a matrix of light-emitting elementary components, for example VCSELs or RCLEDs, whose pitch is not very high, this matrix comprising means for controlling the light power emitted by each elementary component, and - it makes it possible to diaphragm the light beam emitted by an electronic light-emitting component, thanks to the presence of a reflector on the rear face of this component.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood on reading the description of exemplary embodiments given below, purely by way of indication and in no way limiting, with reference to the appended drawings in which: FIG. 1 is a diagrammatic sectional view of FIG. an example of known VCSEL, - Figure 2A is a schematic and partial longitudinal sectional view of a VCSEL provided with a device according to the invention, - Figure 2B is a schematic and partial cross-sectional view of the VCSEL of FIG. 2A, FIG. 2C schematically and partially illustrates a variant of the VCSEL of FIG. 2A,

FIGS. 3A to 3D schematically illustrate steps of a method of manufacturing a VCSEL equipped with a device according to the invention, and FIGS. 4A to 4E schematically illustrate steps of another method of manufacturing a VCSEL equipped with a device according to one invention.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

Figure 1 is a schematic sectional view of a conventional VCSEL in a plane containing the X axis of this VCSEL. The latter is formed on a substrate 2 and comprises a resonant cavity 4 delimited by a lower mirror 6 and an upper mirror 8. The lower mirror 6 rests on the substrate 2. The cavity 4 comprises an active zone 10 surrounded by a lateral layer 12. When the VCSEL is suitably polarized (in English "biased") (by means not shown), the active zone 10 emits a light beam 14 which is substantially conical and whose axis is the X axis The half-angle at the apex OC of this beam depends on the design of the VCSEL but this angle is generally between 10 ° and 20 °. FIG. 2A is a schematic partial sectional view of a VCSEL 15 which is provided with a device according to the invention. More precisely, this FIG. 2A is a half section of the VCSEL 15 by a plane containing the Z axis of this VCSEL. We see half of the VCSEL. Figure 2B is section II of Figure 2A. It is therefore a half-section of the VCSEL along a plane that is perpendicular to the Z axis of this VCSEL. This axis Z also constitutes the axis of the substantially conical light beam 16 emitted by the VCSEL when this last is suitably polarized (by means not shown). The half-angle at the top of this beam is still noted OC in Figure 2A. In accordance with the invention, the VCSEL 15 of FIG. 2A is provided with a mirror 20 which is disposed near the VCSEL and is provided for taking an outer portion 22, or peripheral portion, of the light beam 16. This portion 22 is delimited in FIG. 2A, by the half-angle cone at the vertex OC and another cone of the half-angle at the vertex β, where β is smaller than OC. The VCSEL is formed on a substrate 24 and comprises a resonant cavity 26 delimited by a lower mirror 28 and an upper mirror 30, the lower mirror 28 resting on the substrate 24. The resonant cavity 26 comprises an active zone 32 which emits the beam bright 16 when the VCSEL is properly polarized. This active zone 32 is surrounded by a lateral layer of an oxide 34. In the example of FIG. 2A, the light beam 16 is emitted in the direction of the lower face of the substrate 24, that is to say the face opposite to that carrying the mirror 28. The mirror 20 according to the invention is formed on this underside of the substrate. This mirror 20 is totally reflective vis-à-vis the beam 16. It is also seen that the underside of the substrate 24 has an anti-reflection layer 36 and the mirror 20 is formed on the anti-reflection layer 36. This mirror 20 is an annular mirror whose axis is the Z axis of the beam 16. The dimensions of this annular mirror are chosen to intercept the desired peripheral portion of the beam 16. The inner radius and the outer radius of the annular mirror 20 are therefore chosen for this purpose. In the example of FIG. 2A, the VCSEL is also provided with a photodiode 38 intended to detect the external part 22 of the beam 16, which is picked up by the mirror 20, after reflection of this part by this mirror 20. In For example, the photodiode 38 is an annular photodiode whose axis is also the Z axis. It can be seen that this photodiode 38 actually receives the light 40 resulting from the reflection of the peripheral portion 22 by the mirror 20. This is an internal reflection since the light intercepted by the mirror 20 and the light reflected by the mirror do not leave the substrate. Note further that the elementary device comprising the VCSEL 15, the annular mirror 20 and the photodiode 38 is an integrated device for regulating the light power of the beam 16 emitted by the VCSEL, by exploiting the light that is reflected by the rear face of this device (lower face of the substrate 24). It is specified that the annular photodiode 38 is a cavity photodiode (in English "cavity photodiode") because it is delimited by the mirrors 28 and 30 which also delimit the resonant cavity 26. In addition, this photodiode can be constructed around the oxide layer 34 (annular layer). In this photodiode, the material 41 between the mirrors 28 and 30 is a quantum wells stack which is preferably the same as the stack of the active zone 32. This configuration has the advantage that, in this way, the response peaks of the VCSEL and the photodiode coincide since the resonant wavelengths are the same. The anode of this photodiode is constituted for example by the lower Bragg mirror N (reference 28) and its cathode by the upper Bragg mirror doped P for example (reference 30). There is thus a cavity photodiode, which is monolithically integrated directly around the VCSEL, the transmission and reception devices being advantageously able to be constructed from the same stacks of quantum wells and mirror layers. It should also be noted that the annular mirror 20, or annular reflector, constitutes a diaphragm which reflects the light intercepted towards the front face of the elementary device, that is to say towards the face where the two mirrors 28 and 30 are located, whereas the non-intercepted portion of the beam 16 passes through the mirror 20 for use outside the VCSEL. FIG. 2A also shows the two electrodes 42 and 44 making it possible to collect the current generated by the photodiode 38 when it receives the light reflected by the mirror 20. The electrodes 46 and 48 for the control of the VCSEL are also seen. Advantageously, the photodiode and the VCSEL could have a common electrode. FIG. 2A also shows an annular structure 50 whose axis is the Z axis and which rests on the substrate 24, isolating the photodiode 38 and the resonant cavity 26. There is also another insulating annular structure 52 on which is the electrode 44. The material of these structures 50 and 52 may be for example a polymer. Vias may be provided in this polymer to ensure the resumption of contact at the buried face of the VCSEL and / or the photodiode. In the example of FIG. 2A, provision is made for a resumption of contact 54 relative to the VCSEL and a contact recovery 55 relating to the photodiode. FIG. 2C schematically illustrates the fact that the material 41 can be laterally oxidized and thus framed by two annular oxide layers 41a. The injection current of the VCSEL 15 can be slaved to the current which is generated by the photodiode 38 when the latter receives the light reflected by the mirror 20. It is therefore possible to regulate the excitation current of the VCSEL and therefore the power of the light beam 16 generated by this VCSEL. The electronic means for regulating this power comprise the photodiode 38. The remainder of these means is not shown. The invention thus allows the planar integration of a VCSEL matrix, each VCSEL comprising a surveillance photodiode (in English). "monitoring") which allows the regulation of the transmission power of the VCSEL. Such a way does not require to modify the vertical structure of the VCSEL and does not affect the performance of these. In addition, such a technique is compatible with the technique of flip-chip (in English "flip-chip"). The resonant cavity 26 of the VCSEL 15 as well as the lower mirror 28 and the upper mirror 30, which delimit this cavity, are designed so that the detection quantum efficiency of the associated photodiode 38 is maximum. By way of example, the cavity 38 and the mirrors 28 and 30 are designed so as to maximize the reflected photonic current with respect to a photonic current which enters the photodiode at an angle of incidence of the order of 5 ° to 10 °. Indeed, one can, for example, quite conceive a mirror allowing the light it receives at an angle of incidence of 5 ° to 10 ° (compared to a normal to this mirror) and reflecting the light that it receives under a zero incidence compared to this normal, this design using the techniques of quantum engineering (quantum engineering). Similarly, it is possible to make a resonant cavity 26 which is for example optimized with respect to any wavelength shifts created by temperature gradients between the VCSEL and the cavity photodiode 38 associated therewith. FIGS. 3A to 3D diagrammatically illustrate various steps of a method of manufacturing a VCSEL provided with a mirror and an annular photodiode. A substrate 56 (FIG. 3A) is used on which a resonant cavity layer 58 is formed delimited by two mirror layers 60 and 62, the mirror layer 60 being formed on the substrate 56. In a conventional manner, the mirrors which delimit the resonant cavity are preferably DBR mirrors, that is to say distributed Bragg reflector mirrors (in English "distributed Bragg reflectors"). Then, as in a conventional method for manufacturing VCSEL (FIG. 3B), the mirror layers 60, 62 and the layer 58 are etched to the substrate 56, for example by reactive plasma etching. delimit the VCSEL and the associated annular photodiode. The annular trenches 72 and 74 are thus formed between which the photodiode is comprised. These trenches have the same axis Z (axis of the light beam, intended to be emitted by the VCSEL). The inner radius of the annular trench 74 is greater than the outer radius of the annular trench 72. Later, by lateral oxidation or implantation of protons, the "restricted" active area 70 of the VCSEL is formed, this zone forming a Z-axis disk. . During this step, the photodiode may or may not be protected, for example by means of a nitride layer, to avoid the presence of oxide or not. In FIG. 3B, the oxide 75 which surrounds the "restricted" active zone 70 is seen. In addition, this FIG. 3B illustrates the case where the photodiode has not been protected by the nitride, so that layers annular oxide 75 frame the annular resonant cavity 76 of the photodiode. On the contrary, FIGS. 3C and 3D illustrate the case where the photodiode has been protected by the nitride, so that the resonant cavity 76 is not framed by the oxide. In fact, in practice, several identical VCSELs are simultaneously formed on the substrate 56 and annular trenches such as the trench 74, making it possible to delimit the different VCSELs. The various electrodes (not shown), which are necessary for the operation of each VCSEL and each annular photodiode, are then formed according to the techniques known to those skilled in the art. There are two connections by VCSEL and two connections per photodiode, an electrode that can be common to the VCSEL and the photodiode. Subsequently, the substrate 56 is thinned until it has a thickness D (FIG. 3C) allowing the light reflected by the mirror according to the invention, which is subsequently formed, to flow back through this substrate while being hardly absorbed by this last, to reach the annular photodiode (D <500 μm). An antireflection layer (not shown) is then deposited on the rear face 77 of the substrate, from which this substrate has been thinned, and an annular mirror 78 is then formed on the rear face of the thinned substrate (FIG. 3D ). The axis of this mirror 78 is also the Z axis. FIGS. 4A to 4E schematically illustrate steps of another method for manufacturing a VCSEL that can be used for a mirror and an annular photodiode. In this other method, the mirror that comprises this device is formed on the VCSEL after hybridizing this VCSEL on a CMOS processing circuit. The steps schematically illustrated by FIGS. 4A and 4B and the structures obtained by these steps are identical to those described with reference to FIGS. 3A and 3B and the same references correspond to the same elements (with the exception that, in the case 4B, the photodiode is protected so that the resonant cavity is not framed by the oxide, and the various electrodes (not shown) which are necessary for the operation of each VCSEL and each annular photodiode are then formed. Then, the structure 80 thus obtained is hybridized, by the flip-chip technique, on a wafer of a control circuit 82, for example CMOS type (Figure 4C), by means of solder balls (English "solder balls") 84 that is formed beforehand on the structure 80. Pads (English "pads") connection (not shown), are also provided on the structure 80 and the circuit 82 for this hybridization. In practice, as already mentioned, several VCSELs are simultaneously manufactured on the substrate 56, and these VCSELs are collectively hybridized on the control circuit 82. Then (FIG. 4D), the substrate is thinned Hybridized VCSELs until this substrate has a thickness D allowing the light reflected by the mirror according to the invention, subsequently formed on each VCSEL, to flow back through the substrate while being practically not absorbed by the latter, to reach the associated annular photodiode. Then (FIG. 4E), for each VCSEL, an annular mirror 86 is formed on the rear face of the thinned substrate so that this mirror 86 and the active area 70 of the VCSEL have the same Z axis. A VCSEL matrix is thus obtained. each VCSEL being provided with a mirror and a photodiode which allow the regulation of the light power emitted by this VCSEL. The following is a purely indicative and in no way limiting reference, with reference to FIG. 2A, numerical values relating to the structure represented in FIG. 2A. For example, it is desired to form light-emitting components whose light output can be controlled on a reduced surface: a 10 × 10 VCSEL matrix is formed, for example, with a pitch of 250 μm, which is the standard pitch of the connections. optics. The substrate used is given a residual thickness D of 100 μm. Each VCSEL is capable of emitting a cone light beam with a half-angle at the apex of 10 °. The part of the beam, which is not picked up by the annular mirror, is a conical beam whose half-angle at the apex is 5 °. The annular mirror has an inner radius of 17 μm and an outer radius of 35 μm. Active zone 32 (FIG. 2A) has a radius of 15 μm. The annular structure 45 has an inner radius of 30 μm and an outer radius of 50 μm. One can even form a VCSEL matrix provided with annular mirrors and photodiodes so that the integration step is equal to 125 microns or less. As a purely indicative and in no way limiting, examples of materials that can be used in the structures described with reference to FIGS. 3A to 3D and 4A to 4E are given below. substrate 56: GaAs - mirror DBR 60: AlGaAs / GaAs - mirror DBR 62: AlGaAs / GaAs - layer (cavity) 58: GaInNAs - active zone 70: GaInNAs - mirrors 78 and 86: Ti

Examples of the invention, which have been given, relate to VCSELs. However, the invention is not limited to such components: the person skilled in the art can adapt the examples given to the case of RCLEDs. In addition, the examples given relate to the removal of an external portion of the light beam from an electronic light emitting component. However, the invention is not limited to this: for certain applications, it may be advantageous to take a portion (not necessarily annular) of the beam, more towards the center of the latter, the part thus taken away being likely to be more representative of the emitted beam (in particular because there is no edge effect). Those skilled in the art can adapt these examples given to the sampling of such a part.

Claims

1. Device for removing a portion (22) of a light beam (16) from a light emitting electronic component (15), this component comprising a substrate having first and second faces, and an active area arranged on the first face and designed to emit the light beam in the direction of the second face, this device being characterized in that it comprises an annular mirror (20) which is arranged on the second face and able to take the annular outer portion (22) of the light beam (16), this annular mirror and the light beam (16) being coaxial.

The device of claim 1, wherein the light emitting electronic component is a vertical cavity surface emitting laser (15) or a resonant cavity light emitting diode.

3. Device according to any one of claims 1 and 2, this device further comprising an annular photodetector (38) which is arranged on the first face and provided for detecting the part (22) taken by the mirror, after reflection of this partly by this mirror (20), this annular photodetector and the annular mirror being coaxial.

4. Device according to claim 3, wherein the photodetector (38) is integrated so monolithic to the light emitting electronic component (15).

5. Device according to any one of claims 1 to 4, this device further comprising means (38) for regulating the light power of the beam (16) emitted by the component (15), using the part (22) of this beam, which is taken by the mirror (20).

6. Device according to any one of claims 3 and 4, this device further comprising means for regulating the light power of the beam (16) emitted by the component (15), using the part (22) of this beam, which is picked up by the mirror (20), these regulation means comprising the photodetector (38).

7. matrix of light-emitting electronic components, each component (15) being provided with the device according to any one of claims 1 to 6.