Abstract
A laser includes a network of micro-ridges of quantum cascade lasers of preset emission wavelength, the micro-ridges, which are of preset widths, forming active zones of refractive index n.sub.za that are spaced apart from each other by an inter-ridge material of refractive index n.sub.e, with n.sub.za<n.sub.e. The inter-ridge material is a group-IV material is also provided.
Claims
1. A laser comprising a network of micro-ridges of quantum cascade lasers of preset emission wavelength, the micro-ridges, which are of preset widths, forming active zones of refractive index n.sub.za that are spaced apart from each other by an inter-ridge material of refractive index n.sub.e, with n.sub.za<n.sub.e, wherein the inter-ridge material is a group-IV material.
2. The laser as claimed in claim 1, wherein the spacings between active zones are identical and/or the widths of the micro-ridges are identical.
3. The laser as claimed in claim 1, wherein the widths L of the micro-ridges are identical and the spacings D are identical and determined by where m is an uneven positive integer that is defined as the number of extrema in one oscillation between the micro-ridges, and .sub.leak is the spatial periodicity of the portion of the super mode oscillating between two ridges.
4. The laser as claimed in claim 1, wherein the group-IV material is silicon or germanium.
5. The laser as claimed in claim 1, wherein the group-IV material is amorphous.
6. The laser as claimed in claim 1, wherein with the active zones forming an effective active zone and the network of micro-ridges including two peripheral ridges, said group-IV material is also placed on the external flanks of the peripheral ridges over a width S0 determined depending on the spacings between active zones and on an overlap of the super mode with the effective active zone.
7. The laser as claimed in claim 1, wherein the active zones are heterostructures of III-IV materials.
8. The laser as claimed in claim 1, wherein the laser has an emission wavelength comprised between 3.5 m and 10 m.
9. The laser as claimed in claim 1, wherein the network comprises from 4 to 20 micro-ridges.
10. A process for fabricating a laser as claimed in claim 1, from a stack, on a substrate of refractive index n.sub.s, of a layer of an active-zone material of refractive index n.sub.za, with n.sub.s<n.sub.za, and of a top confinement layer of refractive index n.sub.cs, with n.sub.cs<n.sub.za, which comprises a step of etching said layers to the substrate in order to form the micro-ridges on the substrate, wherein it furthermore includes the following steps: depositing, in a single layer, the group-IV material on the micro-ridges and the substrate, removing the group-IV material deposited on the micro-ridges and on the substrate while leaving said material between the micro-ridges and on the external flanks of the peripheral micro-ridges over a preset width, depositing a dielectric passivating layer on the edges of the material and on the substrate, depositing a metal contact layer.
11. The process for fabricating a laser as claimed in claim 1, wherein S=0.
12. The process for fabricating a laser as claimed in claim 10, wherein step a) is carried out by vapor deposition or by atomic layer deposition.
13. The process for fabricating a laser as claimed in claim 10, wherein step b) is carried out by chemical-mechanical polishing and/or by dry or wet etching.
14. The process for fabricating a laser as claimed in claim 10, wherein that the substrate and the top confinement layer are made of InP or GaAs.
Description
[0025] Other features and advantages of the invention will become apparent on reading the following detailed description, which is given by way of nonlimiting example and with reference to the appended drawings, in which:
[0026] FIGS. 1a to 1d schematically show examples of symmetric anti-guided super-mode lasers according to the invention (FIGS. 1a and 1c) and examples of anti-symmetric anti-guided super-mode lasers (FIGS. 1b and 1d),
[0027] FIGS. 2a and 2b schematically illustrate, for a single ridge, a guided mode (FIG. 2a) and an anti-guided mode (FIG. 2b), with the refractive indices (at the top of the figures), and the amplitude of the corresponding electric field (at the bottom of the figures),
[0028] FIGS. 3a to 3f schematically illustrate various fabricating steps of a laser according to the invention,
[0029] FIG. 4 illustrates the relationship between the distance D between ridges, the thickness S of the material on the external flanks and the overlap of the super mode with the active zones.
[0030] In all the figures, elements that are the same have been referenced with the same references.
[0031] Insofar as the device may be positioned with other orientations, the directional terminology is indicated by way of illustration and is nonlimiting.
[0032] With reference to FIGS. 1a and 1c, an example of a laser 100 according to the invention will now be described. It includes a network of micro-ridges of quantum cascade lasers of preset emission wavelength . The micro-ridges, which are of preset widths L.sub.1, . . . , L.sub.K (K being the number of micro-ridges) that may be, but are not necessarily, identical for all the micro-ridges (L.sub.1=L.sub.K=L), form active zones 1 of refractive index n.sub.za that are spaced apart from one another by a width (or spacing) D filled with an inter-ridge material 2 of refractive index n.sub.e. These spacings may be identical but are not necessarily. So as to obtain a network that produces anti-guided super modes, n.sub.za<n.sub.e. The inter-ridge material is a group-IV material such as silicon or germanium and each spacing D between two adjacent micro-ridges (or two adjacent active zones) is determined on the basis of the wavelength , of all the widths L.sub.1, . . . , L.sub.K of the micro-ridges and of the refractive indices n.sub.za and n.sub.e in order to obtain a symmetric anti-guided super mode. The expression super mode is understood to mean the resultant mode of the whole laser 100. The permitted super modes are, to the first order, linear combinations of the modes of the active zones 1 alone. The super mode favored is the super mode that has the highest modal gain (and therefore the largest overlap) with that of the active zone.
[0033] It will be recalled that when n.sub.za>n.sub.e, each active zone 1 supports a guided mode and the electric field decreases exponentially as a function of the distance measured from the core of the active zone, as illustrated in FIG. 2a for a single ridge. This is the case for evanescent coupling between narrow ridges separated from one another by InP:Fe: the widths L.sub.1, . . . , L.sub.K, of the ridges are chosen to be sufficiently narrow that, considered separately, each of the ridges supports only the fundamental mode. It is sinusoidal in the active zone and decreases exponentially in the InP:Fe. With a plurality of ridges, the super modes are, to a first approximation, linear combinations of the modes of the ridges considered separately. The antisymmetric super mode is favored (the field in the active zones changes sign between nearest neighbors: +++, . . . ), this giving a double-lobe far field. The spacing between the micro-ridges then has an influence only on the strength of the coupling therebetween.
[0034] By filling the space between the micro-ridges with a group-IV material such as silicon, the existence of anti-guided super modes is made possible, the optical index of silicon being higher than that of the active zone (n.sub.za<n.sub.e). In this case, the mode of the individual ridges is sinusoidal inside the active zone but also outside, in the silicon, with a smaller amplitude, as may be seen in FIG. 2b for a single ridge; leaky modes or leaky waves are then spoken of. In this case, the spacing between the micro-ridges has a different impact to that played in the guided-mode micro-ridge configuration. Specifically, the resonance of the symmetric anti-guided super modes (the field in the active zones is of invariant sign) and of the antisymmetric anti-guided super modes (the field in the active zones changes sign between nearest neighbors) is theoretically obtained for an inter-ridge space D given by:
[00001]
when the spacings are identical and where m is a positive integer that is defined as the number of extrema in the oscillation between the micro-ridges, n.sub.Si (or n.sub.e) and n.sub.za are the refractive indices of the group-IV material and of the active zone, respectively, is the emission wavelength of the laser 100, .sub.leak is the spatial periodicity of the portion of the super mode oscillating between two ridges, and L is the width of the micro-ridges, which width is identical for all the ridges (L.sub.1=L.sub.K=L).
[0035] For uneven m, the symmetric super mode is favored, as illustrated in FIGS. 1a and 1c, and for even m, the antisymmetric super mode is favored, as illustrated in FIGS. 1b and 1d.
[0036] When the widths of the micro-ridges vary from one ridge to the next, each spacing D between two micro-ridges is determined by simulation of the optical mode so that the overlap of the desired mode with the adjacent active zones is maximal.
[0037] If only the group-IV material is left on the edges of the peripheral micro-ridges 10 shown in FIGS. 1a to 1d, the super mode will also oscillate in all this region, this decreasing the overlap with the active zone and therefore the modal gain. To avoid this, excess group-IV material is etched in order to leave only a narrow zone on the external flanks of the peripheral ridges, as shown in FIGS. 1a to 1d and in FIG. 3. By adjusting the thickness S of this zone, it is possible to change the overlap and the nature of the dominant super mode. FIG. 4 shows the overlap of the super mode having the maximum overlap as a function of S (x-axis) and of the distance D between the micro-ridges (y-axis). For this simulation, L=2 m, =4.5 m, there are 8 micro-ridges, D varies from 2.8 m to 4.5 m and S varies from 0 m to 5 m. It may be seen that even if the resonance is always about a given distance D between the micro-ridges of 3.1 m in our simulation, the value of the overlap of the dominant super mode may fluctuate greatly with S. It is maximal for a nonzero S value (S=0.5 in our simulation) and tends to decrease on the whole when S increases because the super mode is allowed to make more oscillations outside of the active zone. The periodicity of the maxima is related to that of the oscillations of the super mode in this region. This corroborates the fact that anti-guided coupling is more sensitive to lateral losses than evanescent coupling, the field being stronger at the edge of the component, and that these losses may be used to filter undesirable super modes.
[0038] D and S may therefore be chosen by calculation and simulation to obtain the desired emission super mode.
[0039] To fabricate an anti-guided super-mode laser with a single-lobe emission, it is possible to proceed in the following way described with reference to FIGS. 1b, 1d and 3a-3f, starting with an InP or GaAs substrate 3 of refractive index n.sub.s<n.sub.za in lieu of bottom confinement layer of the mode of the active zones 1. The substrate typically has a thickness comprised between 100 m and 1 mm. On the substrate, a stack of the following layers is deposited: a layer of a material of the active zone 1, of refractive index n.sub.za and of thickness comprised between 300 nm and 10 m; and a top confinement layer 6 of thickness comprised between 2 and 10 m, which may consist of the same material as the substrate but does not necessarily, as shown in FIG. 3a. The refractive index n.sub.cs of this top confinement layer is such that n.sub.cs<n.sub.za. The layer of material of the active zones is a quantum-well heterostructure of III-V materials, such as AlInAs, GaInAs, AlAs, AlGaAs, GaAs.
[0040] These two last layers are etched to the substrate in order to form micro-ridges, each micro-ridge being intended to form an active zone 1 covered with a top confinement layer 6, as shown in FIG. 3b.
[0041] A group-IV material 2 of refractive index n.sub.e, with n.sub.za<n.sub.e, is deposited in a single layer on the micro-ridges and on the substrate in order to bury the micro-ridges in said material, as shown in FIG. 3c. Practically, an amorphous group-IV material 2 is used, which is easier to synthesize in particular on a III-V substrate. This deposition is advantageously carried out by vapor deposition in order to obtain a thick and conformal deposit (of about 1 to 5 m) or by atomic layer deposition.
[0042] The amorphous material 2 deposited on the micro-ridges is removed and that deposited on the substrate (except between the micro-ridges) is also removed, and preferably partially removed so as to leave the amorphous material on the external flanks of the peripheral micro-ridges over a preset width S, as shown in FIG. 3d. This material 2 is therefore present between the micro-ridges and optionally on these external flanks. This removal is carried out by chemical-mechanical polishing and/or by wet or dry etching; mention may be made, by way of example of dry etching, of reactive-ion etching using an inductively coupled plasma (or RIE-ICP). However, it is possible for S=0 and, in this case, the amorphous group-IV material is entirely removed from the substrate except of course between the micro-ridges.
[0043] Next, as shown in FIG. 3e, a dielectric passivating layer 4 of thickness comprised between 100 nm and 1.5 m, such as of silica (SiO.sub.2) or of silicon nitride (Si.sub.3N.sub.4), is deposited on the edges of the material 2 (but not on the peripheral micro-ridges) and on the substrate 3, before a metal contact 5, such as an alloy based on gold (titanium/platinum/gold (Ti/Pt/Au) or gold/germanium/nickel/gold (AuGe/Ni/Au) for example) of total thickness comprised between 100 nm and 600 nm is deposited, as shown in FIG. 3f.
[0044] This dielectric passivating layer 4 may also be deposited on the material 2.
[0045] The main fields of exploitation for power lasers in the mid infrared are for example optical countermeasures and spectroscopy.
