Light guiding for vertical external cavity surface emitting laser

Abstract

The present invention relates to an active gain layer stack (21) for a vertical emitting laser device, the active gain layer stack (21) comprising a semiconductor material, wherein the semiconductor material is structured such that it forms at least one mesa (24) extending in a vertical direction. A transversally neighboring region (25) that at least partly surrounds said mesa (24) has a second refractive index (n.sub.2)At least part of said mesa (24) has a first refractive index (n.sub.1) and a part of the neighboring region (25) transversally adjacent to said part of the mesa (24) has second refractive index (n 2)Said first refractive index (n.sub.1) is higher than said second refractive index (n.sub.2) and a diameter in transversal direction of said mesa (24) is chosen such that a transversal confinement factor in the active gain layer stack (21) is increased. The present invention also relates to a laser device including such a stack, further to a method of operation of such a stack, and also to a method of manufacturing of such a stack. The VECSEL comprises a IV-VI semiconductor gain material grown on the lower mirror and an external cavity mirror. A plurality of mesa (22) may be grown on a single substrate (23). Anti-guiding is prevented by the lower refractive index of the surrounding material (25) improving the single transversal mode operation.

Claims

1. An active gain layer stack for a vertical emitting laser device, the active gain layer stack comprising a semiconductor material, wherein the active gain layer stack is structured such that it forms at least one mesa extending in a vertical direction and a transversally neighboring region at least partly surrounding said mesa, wherein at least part of said mesa has a first refractive index (n.sub.1) and wherein a part of the neighboring region transversally adjacent to said part of the mesa has a second refractive index (n.sub.2), wherein said first refractive index (n.sub.1) is higher than said second refractive index (n.sub.2), wherein a diameter in a transversal direction of said mesa is chosen such that transversal confinement of laser light in the active gain layer stack is increased, wherein said semiconductor material has the refractive index (n.sub.1) that decreases with increasing temperature, and wherein said neighboring region comprises at least one of a vacuum region, a gas-filled region, and a liquid-filled region.

2. The active gain layer stack according to claim 1, wherein the semiconductor material is a IV-VI semiconductor material.

3. The active gain layer stack according to claim 1, wherein the neighboring region surrounds said mesa over substantially the entire vertically extending height of said mesa, and preferably encircles said mesa transversally completely.

4. The active gain layer stack according to claim 1, wherein the vertically extending height of the mesa is substantially equal to or smaller than a height of the active gain layer stack.

5. The active gain layer stack according to claim 1, wherein the active gain layer stack includes a substrate, wherein the vertically extending height of the mesa extends substantially down onto or into said substrate, wherein preferably, at least part of said substrate acts as a mirror.

6. The active gain layer stack according to claim 1, wherein the diameter of the mesa in transversal direction is in a range extending from 5 to 500 micrometers.

7. The active gain layer stack according to claim 1, wherein the active gain layer stack has a plurality of individual said mesas, wherein these individual mesas have the same or different emission characteristics.

8. A method of manufacturing of the active gain layer stack according to claim 1, wherein the active gain layer stack is manufactured by applying etching techniques or by locally changing the material properties after epitaxial growth.

9. A vertical emitting laser device including a first mirror with at least one first mirror layer, a second mirror with at least one second mirror layer forming an optical cavity with said first mirror, and an active region between the first and the second mirror, wherein said active region includes at least one active gain layer stack comprising a semiconductor material, wherein the active gain layer stack is structured such that it forms at least one mesa extending in a vertical direction and a transversally neighboring region at least partly surrounding said mesa, wherein at least part of said mesa has a first refractive index (n.sub.1) and wherein a part of the neighboring region transversally adjacent to said part of the mesa has a second refractive index (n.sub.2), wherein said first refractive index (n.sub.1) is higher than said second refractive index (n.sub.2), wherein a diameter in a transversal direction of said mesa is chosen such that transversal confinement of laser light in the active gain layer stack is increased, wherein said semiconductor material has the refractive index (n.sub.1) that decreases with increasing temperature, and wherein said neighboring region comprises at least one of a vacuum region, a gas-filled region, and a liquid-filled region.

10. The vertical emitting laser device according to claim 9, wherein said active gain layer stack is provided on the first mirror, wherein a height of the mesa in vertical direction is at least 10% to 100% percent of a height of said active gain layer stack, wherein preferably said height of the mesa in vertical direction is larger than said height of the active gain layer stack, such that the mesa extends over the height of the active gain layer stack into the first mirror, such that the mesa comprises a mirror layer, wherein more preferably, the mesa extends in vertical direction over the entire height of the active gain layer stack and the first mirror onto or into a substrate whereupon the first mirror is provided on, such that the mesa comprises the gain layer stack and a mirror layer or it comprises the gain layer stack, a mirror layer, and a substrate layer.

11. The vertical emitting laser device according to claim 9, wherein a diameter of the mesa in transversal direction is substantially equal to a cavity mode spot size of a TEM00 mode.

12. The method of manufacturing according to claim 8, wherein the diameter and form of the mesa is chosen such that only selected transversal modes are lasing.

13. A method of operation of the vertical emitting laser device according to claim 9, wherein said laser device is operated by pumping one or at least one mesa, wherein each distal surface of a pumped mesa forms an active lasing spot.

14. The method of operation according to claim 13, wherein a plurality of mesas is pumped simultaneously or consecutively, and/or wherein the vertical emitting laser device is operated by applying a switching between different lasing regimes during operation.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Preferred embodiments of the invention are described in the following with reference to the drawings, which are for the purpose of illustrating the present preferred embodiments of the invention and not for the purpose of limiting the same. In the drawings,

(2) FIG. 1 shows a schematic sketch of a typical embodiment of the laser;

(3) FIG. 2 shows a typical arrangement of lasing spots for a lasing area;

(4) FIG. 3 shows the refractive index gradient for III-V and IV-VI semiconductor materials during a laser pulse;

(5) FIG. 4 shows a schematic sketch of a mesa provided on the active gain layer stack according to FIG. 2; and

(6) FIG. 5 shows a diagram containing an output spectrum of a VECSEL in accordance with the setup of the previous figures.

DESCRIPTION OF PREFERRED EMBODIMENTS

(7) For the purposes of the present invention, a VECSEL-type laser device may be broadly defined as follows: A VECSEL-type laser is a surface-emitting semiconductor laser having a first (bottom) mirror and a second (top) mirror which is disposed at a distance from the first mirror, forming an optical cavity with the first mirror. An active region (gain medium) comprising at least one active semiconductor layer is disposed in the cavity, the semiconductor layer being oriented such that the resulting laser beam in the cavity will be substantially perpendicular to the layer plane. The active region may optionally comprise quantum wells or quantum dots. The active region contains regions (lasing area) fabricated in such a way, that the refractive index within this region remains higher than its surroundings during operation of the laser. The area where lasing is possible may be limited by macroscopic parts, such as a sample mount. The first and/or the second mirror serve as an output coupler for an output beam. The laser may be optically or electrically pumped.

(8) The lasing spots are typically round, but other shapes may be chosen for different reasons including better heat flux, selection of one or more specific transversal modes, polarization selection of the emitted light, or passivation. Their size is chosen to be approximately at the minimum of threshold power. The lasing threshold decreases with smaller size and thus stronger confinement, but increases for to small diameters as additional losses are induced. The optimal size depends on multiple parameters of the cavity such as its optical length and the curvatures of the mirrors. With decreasing cavity length and increasing curvature the size becomes smaller, down to a few micrometers.

(9) The manufacturing may be done on wafer level or individually for smaller chips. In both cases one or more lasing spots with uniform or individual properties are placed at the area where lasing will occur. Typically a symmetric repetitive arrangement of multiple lasing spots is chosen for each such lasing area. During epitaxial deposition of the light generating IV-VI semiconductors a shadow mask is used to structure the active region. The mask or a slightly larger one can also be used on any number of beforehand deposited layers, including partial layers and layers forming a mirror. Alternatively, lasing spots are etched after growth using standard lithography and etching techniques. The etch depth can be through any number of layers, including partial layers and layers forming a mirror, down to the substrate. The size of the etched structure can be slightly larger in the mirror layers. The low refractive material can be either using vacuum or any gas mixture including air. The space around the lasing spot can also be filled with other materials with a refractive index lower than the IV-VI semiconductors used in the active region, for example to improve the heat removal from the active region. In both cases, a passivation of the vertical walls of the lasing spot may be applied. The refractive index gradient structure may also be produced by changing the composition of the IV-VI semiconductor layers using ion implementation at selected areas.

(10) Either flat or curved mirrors can be used. In case of flat mirrors, aligned substantially parallel, all lasing spots of one lasing area are lasing with comparable threshold, output powers, and wavelengths. In case of deviation from perfect parallelism, or when a curved mirror is used, emission wavelengths are different for individual lasing spots. In case of a curved mirror, lasing characteristics for the individual lasing spots differ, too. The lowest threshold is achieved for the one lasing spot where the mirror curvature aligns optimal with the laser cavity.

(11) A first preferred embodiment of a VECSEL according to the present invention is schematically illustrated in FIGS. 1 and 2. It is to be noted that the dimensions are not to scale. In particular, the thickness of the individual parts of the laser along the vertical direction is strongly exaggerated in relation to the dimensions in the horizontal plane, in order to illustrate the sequence of layers in the laser device. A VECSEL 10 consisting of a first mirror 11, a second mirror 12 and a gain layer region 13, and an external cavity 14.

(12) The active gain layer region 13 is provided on the second mirror 12. It may alternatively be provided on the first mirror 11. The mirrors 11, 12 are highly reflective for the laser emission wavelength and are realized using well known technologies, e.g. metallic coatings or distributed Bragg reflectors on suitable substrates. The active region 13 consists of IV-VI semiconductor materials as commonly used in laser devices. It is structured with lasing spots 22 standing out of a surface 23, and grouped within the lasing area 31. The schematic refractive index profile over a lasing spot 22 and its close surroundings changes from a normal IV-VI gradient 32 to the induced profile 33, as shown in FIG. 3. Also shown in FIG. 3 is the refractive index gradient 31 for III-V semiconductor materials for comparison.

(13) The layouts in FIGS. 1 and 2 are just examples. Either of the two mirrors may be flat (planar) or curved. As the spot diameter of the cavity mode on the active region is typically below about 0.1 mm, the lasing spot layout may be made very compact. In addition, a single lasing spot is sufficient for lasing. A total device size of below about 111 mm3 is possible.

(14) An example of a concrete set-up realized in the laboratory uses 55 mm.sup.2 lasing area. The active region was grown epitaxially directly on the first mirror. The curved second mirror was positioned in approximately 50 micrometer distance. The lasing spots and lasing spot regions were created using wet etching. Lasing spot sizes or mesa diameters in transversal direction are the range of 5 to 500 micrometers, preferably sizes of about 40 micrometers were chosen, in relation to the mirror curvature of 100 mm and the cavity length. The device was optically pumped using a commercial 1.55 m laser diode, operated in pulsed mode at 20 kHz repetition rate and with 10 ns pulse width, and recorded with a Fourier transform infrared spectrometer. Compared to devices without defined lasing spots, the threshold power is reduced at least by a factor of five. Peak output power is approximately 10 mWp, limited by the power of the pumping diode. The emission wavelength depends on operating temperature and cavity length. FIG. 5 shows an example of such laser emission spectrum 41.

(15) FIG. 4 shows a schematic sketch of a cross section of an active gain layer stack 21 with a mesa 24 extending in vertical direction (upwards direction in FIG. 4) and having a first refractive index n.sub.1. A multi-layer structure is indicated by the broken lines. The distal surface 22 in vertical direction of the mesa 24 forms the lasing spot 22 during lasing action, i.e. if the mesa 24 is pumped. The mesa 24 is surrounded transversally completely by an adjacent neighboring region 25 with a lower second refractive index n.sub.2, here a gas or vacuum. The difference of the refractive indices n.sub.1 and n.sub.2 at working temperature, i.e. while the mesa 24 is lasing, is such that a transversal confinement factor in the active gain layer stack 21 is enhanced in comparison to the situation, where the active gain layer stack 21 does not have a neighboring region with a lower refractive index n.sub.2 as described above. The spatial variation of the refractive index follows the curve 33 in FIG. 3, where the jump continuities in curve 33 correspond to the transversal edge of the mesa 24. By having such a spatially varying refractive index, the mesa-structuring of gain layer stack 21 overcomes the disadvantage of the anti-guiding effect known from the state of the art. The active gain layer stack 21 is provided on a substrate 23 and well suited for use in a VECSEL or VCSEL.

(16) Preferably, a plurality of mesas 24 are provided on the gain layer stack or package 21, wherein the plurality of mesas 24 is preferably arranged in a regular pattern with typical transversal distances (i.e. edge-to-edge clearance) between next neighboring mesas 24 in the order of a 1 micrometer up to 200 micrometers.

(17) FIG. 5 shows an output spectrum of a VECSEL in accordance with the setup of the previous figures.

(18) TABLE-US-00001 LIST OF REFERENCE SIGNS 11 first mirror 12 second mirror 13 active region 14 external cavity 15 pump laser light in 16 generated laser light out 21 active gain layer stack, possibly with mirror below 22 lasing spot 23 substrate 24 lasing spot region 25 neighbor region to 24 31 Refractive index in III-V semiconductor materials during operation 32 Refractive index in IV-VI semiconductor materials during operation 33 Refractive index in IV-VI semiconductor materials during operation achieved by this invention 41 emission spectrum n.sub.1 first refractive index n.sub.2 second refractive index