DIODE LASER HAVING REDUCED BEAM DIVERGENCE

Abstract

The present disclosure relates to a diode laser having reduced beam divergence. Some implementations reduce a beam divergence in the far field by means of a deliberate modulation of the real refractive index of the diode laser. An area of the diode laser (e.g., the injection zone), may be structured with different materials having different refractive indices. In some implementations, the modulation of the refractive index makes it possible to excite a supermode, the field of which has the same phase (in-phase mode) under the contacts. Light, which propagates under the areas of a lower refractive index, obtains a phase shift of π after passing through the index-guiding trenches. Consequently, the in-phase mode is supported and the formation of the out-of-phase mode is prevented. Consequently, the laser field can, in this way, be stabilized even at high powers such that only a central beam lobe remains in the far field.

Claims

1. A diode laser, comprising: first n-conducting functional layers; an active layer which is suitable for generating electromagnetic radiation and which is arranged on the first functional layers; second p-conducting functional layers which are arranged on the active layer; a p-type contact area, wherein, in order to form a Talbot filter section, the p-type contact area comprises a longitudinal-lateral structuring in the area of an injection zone, wherein individual p-type contacts are formed by the structuring; and at least one facet for coupling out electromagnetic radiation along a second axis, wherein the first functional layers, the active layer and the second functional layers are stacked along a third axis, wherein the longitudinal-lateral structuring comprises index-guiding trenches having a lower refractive index compared with the refractive index of the second p-conducting functional layers between the individual p-type contacts of the p-type contact area formed by the structuring.

2. The diode laser of claim 1, wherein the first n-conducting functional layers comprise an n-type cladding layer and an n-type waveguide layer.

3. The diode laser of claim 1, wherein the second p-conducting functional layers comprise a p-type waveguide layer and a p-type cladding layer.

4. The diode laser of claim 1, wherein the longitudinal-lateral structuring along a first axis is structured alternately with refractive indices which differ from one another.

5. The diode laser of claim 1, wherein the longitudinal-lateral structuring along the second axis is structured alternately with refractive indices which differ from one another.

6. The diode laser of claim 1, wherein the index-guiding trenches along the third axis project into a p-type cladding layer or penetrate the latter.

7. The diode laser of claim 6, wherein the index-guiding trenches along the third axis project into the p-type waveguide layer.

8. The diode laser of claim 1, wherein the longitudinal-lateral structuring along the first axis comprises at least six of the index-guiding trenches.

9. The diode laser of claim 1, wherein the longitudinal-lateral structuring along the second axis comprises at least two of the index-guiding trenches.

10. The diode laser of claim 1, wherein the index-guiding trenches along the first axis comprise first index-guiding trenches and second index-guiding trenches, wherein the second index-guiding trenches along the first axis constitute the respective outermost index-guiding trenches.

11. A diode laser, comprising: first n-conducting functional layers; an active layer which is suitable for generating electromagnetic radiation and which is arranged on the first functional layers; second p-conducting functional layers which are arranged on the active layer; a p-type contact area, wherein, in order to form a Talbot filter section, the p-type contact area comprises a longitudinal-lateral structuring in the area of an injection zone, wherein individual p-type contacts are formed by the structuring; and at least one facet for coupling out electromagnetic radiation along a second axis, wherein the first functional layers, the active layer and the second functional layers are stacked along a third axis, wherein the longitudinal-lateral structuring comprises index-guiding trenches having a lower refractive index compared with the refractive index of the second p-conducting functional layers between the individual p-type contacts of the p-type contact area formed by the structuring, wherein the first n-conducting functional layers comprise an n-type cladding layer and an n-type waveguide layer, wherein the second p-conducting functional layers comprise a p-type waveguide layer and a p-type cladding layer.

12. The diode laser of claim 11, wherein the longitudinal-lateral structuring along a first axis is structured alternately with refractive indices which differ from one another.

13. The diode laser of claim 11, wherein the longitudinal-lateral structuring along the second axis is structured alternately with refractive indices which differ from one another.

14. The diode laser of claim 11, wherein the index-guiding trenches along the third axis project into a p-type cladding layer or penetrate the latter.

15. The diode laser of claim 14, wherein the index-guiding trenches along the third axis project into the p-type waveguide layer.

16. The diode laser of claim 11, wherein the longitudinal-lateral structuring along the first axis comprises at least six of the index-guiding trenches.

17. The diode laser of claim 11, wherein the longitudinal-lateral structuring along the second axis comprises at least two of the index-guiding trenches.

18. The diode laser of claim 11, wherein the index-guiding trenches along the first axis comprise first index-guiding trenches and second index-guiding trenches, wherein the second index-guiding trenches along the first axis constitute the respective outermost index-guiding trenches.

19. A diode laser, comprising: first n-conducting functional layers; an active layer which is suitable for generating electromagnetic radiation and which is arranged on the first functional layers; second p-conducting functional layers which are arranged on the active layer; a p-type contact area, wherein, in order to form a Talbot filter section, the p-type contact area comprises a longitudinal-lateral structuring in the area of an injection zone, wherein individual p-type contacts are formed by the structuring; and at least one facet for coupling out electromagnetic radiation along a second axis, wherein the first functional layers, the active layer and the second functional layers are stacked along a third axis, wherein the longitudinal-lateral structuring comprises index-guiding trenches having a lower refractive index compared with the refractive index of the second p-conducting functional layers between the individual p-type contacts of the p-type contact area formed by the structuring, wherein the index-guiding trenches along the third axis project into a p-type cladding layer or penetrate the latter, wherein the index-guiding trenches along the third axis project into the p-type waveguide layer.

20. The diode laser of claim 19, wherein the index-guiding trenches along the first axis comprise first index-guiding trenches and second index-guiding trenches, wherein the second index-guiding trenches along the first axis constitute the respective outermost index-guiding trenches.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The invention is explained below in exemplary embodiments with reference to the associated drawing, wherein:

[0035] FIG. 1 shows an exemplary embodiment of the diode laser according to the invention;

[0036] FIG. 2a shows a representation of a longitudinal-lateral structuring of a further exemplary embodiment of the diode laser according to the invention;

[0037] FIG. 2b shows a charge carrier density distribution and phase of the forward-propagating optical field at the positions marked in FIG. 2a;

[0038] FIG. 2c shows a lateral-vertical (x,y) cross-section of the diode laser according to the invention;

[0039] FIG. 2d shows a lateral-vertical (x, y) cross-section of a further exemplary embodiment of the diode laser according to the invention;

[0040] FIG. 3 shows an example of a modulation of the refractive index in conventional stripe-geometry lasers under (a); and regarding this [0041] (b): shows a field amplitude (in arbitrary units) of the in-phase supermode during an index guiding in accordance with the refractive index variation in (a); [0042] (c): shows a field amplitude (in arbitrary units) of the out-of-phase supermode during an index guiding in accordance with the refractive index variation in (a); [0043] (d): shows a field amplitude (in arbitrary units) of the in-phase supermode during an anti-index guiding in accordance with the refractive index variation in (a); [0044] (e): shows a field amplitude (in arbitrary units) of the out-of-phase supermode during an anti-index guiding in accordance with the refractive index variation in (a); [0045] (f): shows a far-field intensity of an in-phase supermode of a conventional diode laser; [0046] (g): shows a far-field intensity of an out-of-phase supermode of a conventional diode laser;

[0047] FIG. 4 shows a representation of a lateral far field of an exemplary embodiment of the diode laser according to the invention;

[0048] FIG. 5 shows a representation of the longitudinal-lateral structuring of a laser having a conventional Talbot filter section;

[0049] FIG. 6a shows a representation of a longitudinal-lateral gain/loss modulation of a conventional diode laser;

[0050] FIG. 6b shows a representation of the lateral-vertical cross-section of a gain/loss modulation of a conventional diode laser.

DETAILED DESCRIPTION OF THE DRAWINGS

[0051] FIG. 1 shows an exemplary embodiment of the diode laser according to the invention of the Talbot type having longitudinal-lateral refractive index and gain modulation. The vertical layer structure matches that of a conventional high-power laser diode. This consists of at least one active layer 1, which is surrounded by the p-type waveguide 5, p-type cladding layer 6, n-type waveguide 2 and n-type cladding layer 3, which are in turn mounted on the substrate 4. The laser is restricted by a front and rear facet. The rear facet can optionally be supplemented by a wavelength-stabilizing DBR section. The lateral injection zone 16 can optionally be restricted by second index-guiding trenches 15 which are etched through the p-type cladding layer 6 or, additionally, through parts of the p-type waveguide layer 5, and stabilize the laser field.

[0052] The p-type contact area 9 is characterized in that the layers in the vertical direction beneath this area, that is to say the p-type contact layer 7, the p-type cladding layer 6 and the p-type waveguide layer 5, are conductive. The first index-guiding trenches 8, on the other hand, do not conduct current so that a structuring of the p-type contact area results in a gain/loss modulation in the active layer 1. In order to make the structuring of the p-type contact area clear, the p-type metallization has been omitted in the representation in FIG. 1. A lateral-vertical cross section of FIG. 1, which also includes the p-type metallization, is depicted in FIG. 2c.

[0053] The p-type contact area 9 of the exemplary embodiment is structured in accordance with formula 1.

[00002]$\begin{array}{cc}\frac{2{\left({\Lambda }_x\right)}^2{n}_{\mathrm{eff}}}{{\lambda }_0}={\Lambda }_z& \left(1\right)\end{array}$

[0054] The half-period in the lateral direction W.sub.2=Λ.sub.x/2 lies at a 1 μm vacuum wavelength at 3 to 10 μm. The half-period in the propagation direction W.sub.1=Λ.sub.z/2 is also produced in accordance with formula 1 and lies at a 1 μm vacuum wavelength and n.sub.eff≈3.3 at 100 μm to 2000 μm.

[0055] In addition to the longitudinal-lateral structuring of the p-type contact area 9 in the injection area 16, a longitudinal-lateral modulation of the refractive index is introduced. This modulation is created in the exemplary embodiment by first index-guiding trenches 8 having a material of a lower refractive index compared with the refractive index of the p-type waveguide layer 5 and of the p-type cladding layer 6. The number of the lateral first index-guiding trenches 8 having a width of W.sub.2, which are periodically separated by a distance W.sub.2 through the p-type contact area, is eight in the exemplary embodiment in FIG. 1. The number of the longitudinal first index-guiding trenches having a length of W.sub.1, which are periodically separated by a distance W.sub.1 through the p-type contact area, is two in the exemplary embodiment in FIG. 1.

[0056] These first index-guiding trenches 8 having a material of a lower refractive index can be generated by etching off the p-type cladding layer 6 or, additionally, by etching off parts of the p-type waveguide layer 5. Either an insulating layer 11, e.g., 100 nm SiN, is added to the etched areas, with the p-type metallization 10 thereabove, as depicted in FIG. 2c, or the etched areas are overgrown with a semiconductor material 13 which has a lower refractive index than the p-type waveguide layer 5 and p-type cladding layer 6, as represented in FIG. 2d. Since these areas are conductive, these areas of another refractive index still have to additionally be implanted so that the structuring of the p-type contact area 9 also results in a gain/loss modulation in the active zone 1. Current, which is injected into the p-type contact area 9 via the p-type metallization 10, spreads in the lateral x-direction so that the implantation should at least penetrate the p-type contact layer 7 but, better still, should additionally penetrate the p-type cladding layer.

[0057] FIG. 2a shows a representation of a longitudinal-lateral structuring of the diode laser according to the invention which is depicted in FIG. 1. The longitudinal-lateral structuring having alternating areas of p-type contact zone 9 and first index-guiding trenches 8, which are utilized to adapt the phase, are structured in accordance with formula 1. The modulation of the refractive index by the first index-guiding trenches 8 must be selected such that the phase condition,

Δφ(x,z)=k.sub.0Λ.sub.z/2.Math.(n.sub.0(x,z)−n.sub.1(x,z))=π,

is met for the operating point, wherein n.sub.0(x, z) and n.sub.1(x, z) are, in each case, the real refractive index changes under the areas of the p-type contact zone 9 and of the first index-guiding trenches 8. An exemplary lateral profile of the charge carrier density (LTD) is depicted at the z-position 3.82 mm at the top of FIG. 3b. At the bottom of FIG. 3b, the phase of the forward-propagating optical field is shown at the positions marked in FIG. 2a. The regions of a low charge carrier density correspond to lateral regions of the index-guiding trenches. It can be seen that the phase of the optical field which propagates in these regions during the propagation from the z-position 3.63 mm to the z-position 4 mm obtains a phase shift of π with respect to the field which propagates in regions of high charge carrier density.

[0058] FIGS. 2c and 2d show exemplary embodiments of the first index-guiding trenches 8. In both cases, semiconductor material of the p-type contact layer 7, the p-type cladding layer 6 and/or the p-type waveguide layer 5 is etched off. In the case of FIG. 3c, an insulating layer 11, e.g., 100 nm SiN, is introduced into the etched areas, thereabove lies the p-type metallization 10. In the case of FIG. 3d, the etched areas are overgrown with a semiconductor material 13 which has a lower refractive index than the p-type waveguide layer 5 and p-type cladding layer 6. The disadvantage of this process is that an elaborate two-step epitaxy is necessary, which is why the former method is preferred. In the latter case, the newly grown semiconductor material is conductive and, for this reason, it has to be additionally implanted 12 in these areas so that a gain/loss modulation up to the active zone is guaranteed.

[0059] An example of a modulation of the real refractive index in conventional stripe-geometry lasers is depicted in FIG. 3a. At the same time, this profile also corresponds to the modulation of the real refractive index in the laser according to the invention at a longitudinal z-position, wherein n.sub.0(x, z) and n.sub.1(x, z) are the real refractive index changes in each case under the areas of the p-type contact zone 9 and of the first index-guiding trenches 8. In FIGS. 3b to 3e, field amplitudes of noteworthy supermodes of this conventional stripe-geometry laser are depicted.

[0060] Index-guided supermodes, in which the intensity maxima, that is to say the square of the absolute value of the depicted field amplitudes, match the maxima of the real refractive index modulation, are depicted in FIGS. 3b and 3c. Anti-index-guided supermodes, in which the intensity maxima match the minima of the real refractive index modulation, are depicted in FIGS. 3d and 3e. The in-phase supermodes, in which the phase matches neighboring intensity peaks, are depicted in FIGS. 3b and 3d. The out-of-phase supermodes, in which the phase of neighboring intensity peaks shows a phase difference of π in each case, are depicted in 3c and 3e. The far field of the in-phase supermodes from FIGS. 3b and 3d is depicted in FIG. 3f. It has the desired central beam lobe, as well as side peaks at far-field angles of +φ.sub.0=λ.sub.0/Λ.sub.x, wherein λ.sub.0 is the vacuum wavelength and Λ.sub.x is the full period of the lateral index modulation. The far field of the out-of-phase supermodes from FIGS. 3c and 3e is depicted in FIG. 3g. It shows a high divergence and has two distinct intensity peaks at ±φ≈λ.sub.0/2Λ.sub.x.

[0061] FIG. 4 shows the far-field intensity of a specific embodiment of the laser according to the invention in arbitrary units. The active layer 1 of this embodiment consists of a 12 nm-thick InGaAs quantum trench for laser light emission at a wavelength of 905 nm. The active layer is situated between 1.1 μm-thick n-type waveguide layers and 0.2 μm-thick p-type waveguide layers made of Al(0.40)Ga(0.60)As having a doping of 1 E17 cm.sup.−3, restricted in each case by 2 μm-thick n-type and p-type cladding layers made of Al(0.45)Ga(0.55)As having a doping of 1 E18 cm.sup.−3.

[0062] A central beam lobe can be clearly seen, wherein the side peaks, which can be seen in FIG. 3f, have been suppressed. The central beam lobe has a power content of 40% at a far-field angle of Θ.sub.40%=0.4. In this specific exemplary embodiment, the full lateral period Λ.sub.x=6 μm and the full longitudinal period Λ.sub.z=260 μm. The difference in refractive index brought about by index trenches is Δn.sub.0=−5.5.Math.10.sup.−3. The injection zone 16 has a width of 270 μm and a length of 3 mm. It is restricted laterally by second index-guiding trenches 15 which additionally stabilize the laser field. A wavelength-stabilizing DBR element having a length of 1 mm is situated on the rear facet, so that the entire laser has a length of 4 mm.

[0063] FIG. 5 shows the longitudinal-lateral structuring of a laser having a conventional Talbot filter section. It comprises structured p-type contact areas 9 which result in a gain/loss modulation. These regions can theoretically have both a higher and a lower real refractive index than the adjacent regions so that the supermode can be both index-guided and anti-index-guided. However, the publications indicated above deal with anti-index guided lasers. There are no design-specific restrictions for the length of these sections in the longitudinal z-direction. The laser additionally has conventional Talbot filter sections having a length which corresponds to half the Talbot length Λ.sub.z/2. The sections are longitudinal-laterally unstructured and the optical field can run freely here.

[0064] A longitudinal-lateral gain/loss modulation in accordance with the prior art, without additional modulation of the real refractive index by index-guiding trenches, is shown in FIG. 6a, and the lateral-vertical cross-section of such a modulation is shown in FIG. 6b. The longitudinal-lateral zone consists of periodic sequences of p-type contact zone 9 and non-conducting implanted zones 12, wherein the full period of the lateral modulation Λ.sub.x and the full period of the longitudinal modulation Λ.sub.z comply with formula 1. To ensure that current, which is injected into the p-type contact area 9 via the p-type metallization 10, does not spread in the lateral x-direction, the implantation has to penetrate at least the p-type contact layer 7 but, better still, has to additionally penetrate the p-type cladding layer. Thus, a gain/loss modulation up to the active zone 1 can be guaranteed.

LIST OF REFERENCE NUMERALS

[0065] 1 Active layer [0066] 2 n-type waveguide layer [0067] 3 n-type cladding layer [0068] 4 Carrier substrate [0069] 5 p-type waveguide layer [0070] 6 p-type cladding layer [0071] 7 p-type contact layer [0072] 8 First index-guiding trenches [0073] 9 p-type contact area [0074] 10 p-type metallization [0075] 11 Insulating layer [0076] 12 Implantation [0077] 13 Semiconductor material of a lower refractive index than the material of the p-type layers [0078] 14 Free-running Talbot filter section [0079] 15 (Optional) second index-guiding trenches [0080] 16 Injection zone [0081] 17 Residual layer thickness [0082] W1 Half-period in the propagation direction [0083] W2 Half-period in the lateral direction [0084] W3 Width of the (optional) second index-guiding trenches [0085] X First axis [0086] Y Third axis [0087] Z Second axis