WO2009004581A1

WO2009004581A1 - Vertical extended cavity surface emitting laser with transverse mode control

Info

Publication number: WO2009004581A1
Application number: PCT/IB2008/052653
Authority: WO
Inventors: Johannes Baier
Original assignee: Philips Intellectual Property & Standards Gmbh; Koninklijke Philips Electronics N.V.
Priority date: 2007-07-04
Filing date: 2008-07-02
Publication date: 2009-01-08

Abstract

The present invention relates to a vertical extended cavity surface emitting laser comprising a layer sequence (2, 3, 4) on a substrate (5), said layer sequence (2, 3, 4) forming at least a first end mirror (3) and an active gain region (2) emitting fundamental laser radiation in a first wavelength region when optically or electrically pumped. A frequency converting element (7) is arranged between the first end mirror (3) and an external second end mirror (6) forming an extended cavity with the first end mirror (3). The frequency converting element (7) converts part of the fundamental laser radiation to laser radiation in a second wavelength region. At least one circular aperture (9) is arranged distant from the layer sequence (2, 3, 4) between the layer sequence (2, 3, 4) and the second end mirror (6), wherein said circular aperture (9) is dimensioned and arranged to allow formation of a fundamental transversal mode of the fundamental laser radiation and to inhibit formation of higher transversal modes. With the proposed vertical extended cavity surface emitting laser high power levels can be applied without formation of higher transversal modes. This allows an increased output power of the frequency converted laser radiation.

Description

Vertical Extended Cavity Surface Emitting Laser with Transverse Mode Control

FIELD OF THE INVENTION

The present invention relates to a vertical extended cavity surface emitting laser (VCSEL) comprising a frequency converting element arranged between a first end mirror and a second end mirror, said frequency converting element converting part of the fundamental laser radiation of a first wavelength region to laser radiation of a second wavelength region, wherein said second end mirror is highly reflective for the fundamental laser radiation and at least partly transmissive for the laser radiation of the second wavelength region. Vertical extended cavity surface emitting lasers are a promising technology for projection applications. The fundamental laser radiation of the usually IR emitting components of such a laser can be converted to blue, green and red light by second harmonic generation with non linear crystals inside the extended laser cavity.

BACKGROUND OF THE INVENTION Due to their high radiance lasers are an ideal light source for applications with high optical demands, like e.g. projection. While red and blue light can be generated directly by laser diodes at a reasonable price and effort, integrated green lasers are not available yet, hampering a broader application of lasers in display applications.

A promising approach to cost reduction in green laser devices for projection is an infrared VECSEL (Vertical Extended Cavity Surface Emitting Laser) with additional frequency doubling within the extended cavity. A single VECSEL device usually consist of three core components: the VCSEL (Vertical Cavity Surface Emitting Laser) chip, a frequency doubling element and a reflective element defining the extended cavity. In the VCSEL chip an infrared (IR) laser cavity is usually formed by two layer stacks of distributed Bragg reflectors (DBR), which enclose an active gain region made up from several quantum wells. The DBR layers are also used for feeding the operation current to the active gain region. Therefore, one is usually n-doped and the other p-doped. One DBR forming a first end mirror of the extended cavity is made highly reflective, which is typically the p-DBR with a reflectivity of > 99.8 %, while the other one, which is located towards the extended or external cavity, allows efficient coupling to the extended cavity. Typically the second DBR is an n-DBR with a reflectivity between 85% and 95%, but also lower or higher reflectivity values are possible. Furthermore, the second DBR may also be omitted in a VECSEL. The active gain region is usually electrically pumped at al level, which does not allow for the VCSEL (inner) cavity system to exceed the laser threshold, but requires the feedback of the extended (external) cavity to achieve lasing. For frequency doubling by second harmonic generation, especially for generating green laser light, Periodically Poled Lithium Niobate (PPLN) due to its high efficiency is used inside of the extended cavity. The second end mirror forming the extended cavity with the first end mirror can either be a laser mirror, a dielectric mirror coating on the frequency doubling element or a volume Bragg grating (VBG) with a narrow band reflectivity. An example for a VECSEL laser with internal frequency doubling is shown in US 6,778,582 Bl. In order to achieve lasing with a single fundamental mode, this document proposes to configure the external output cavity mirror, e.g. the second end mirror, to confine the resonant radiation within the extended cavity to a single fundamental mode. This may be achieved by an appropriate shape, reflectivity and distance of the second end mirror from the VCSEL chip.

An important property of the second harmonic generation mechanism is that the frequency conversion efficiency depends linearly on the intensity at the fundamental laser wavelength. Therefore it is highly beneficial and in most cases even essential that laser emission at the fundamental wavelength λ takes place in the transverse fundamental mode, namely the TEM₀O mode. The TEM₀O mode has the smallest transverse extension of all transverse modes and thus the highest local intensity. Its intensity distribution is a radial symmetric Gaussian profile, which can be expressed by

Io is the maximum intensity in the center of the beam, and the quantity w is often referred to as the beam radius; it changes within an optical resonator according to the longitudinal position z. The dependence of w(z) within an optical resonator with known optical components or properties can be calculated by help of the so called Ray matrix formalism, also sometimes referred to as ABCD matrix formalism.

Although the use of an extended cavity already supports laser operation in the fundamental transverse mode compared with a simple surface emitting single cavity VCSEL laser diode, at elevated power levels higher order transverse modes tend to appear even in extended cavity devices. This effect limits the power range of the laser which can be covered with good beam quality. Especially intra cavity frequency doubling becomes inefficient once the transverse mode deviates from the pure TEM₀O mode above a certain power level.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a vertical extended cavity surface emission laser (VECSEL) with a frequency converting element arranged in the extended cavity, which laser can be operated at an extended power range, in which the laser device generates only TEM₀O mode radiation.

The object is achieved with the vertical extended cavity surface emission laser according to claim 1. Advantageous embodiments of the laser are subject matter of the dependent claims or are described in the subsequent portions of the description.

The proposed VECSEL device comprises a layer sequence on a substrate, said layer sequence forming at least a first end mirror and an active gain region emitting fundamental laser radiation of a first wavelength region when excited, e.g. by optically or electrically pumping. A second end mirror forms the extended cavity with the first end mirror. A frequency converting element is arranged between the first end mirror and the second end mirror. The frequency converting element converts part of the fundamental laser radiation to laser radiation in a second wavelength region, in particular to the second harmonic of the fundamental radiation. To allow outcoupling of the laser radiation of the second wavelength region, the second end mirror is designed to be highly reflective for the fundamental laser radiation and at least partly transmissive for laser radiation of the second wavelength region. The proposed laser is characterized by at least one circular aperture which is arranged distant from the layer sequence between the layer sequence and the second end mirror. The circular aperture is designed, in particular dimensioned, and arranged to allow formation of the fundamental transversal mode TEM₀O of the fundamental laser radiation and to inhibit formation of higher transversal modes at power levels at which an identical VECSEL without said aperture would already generate higher transversal modes. The term circular aperture in the context of the proposed device means a circular region which has a higher transmission for the fundamental radiation than the surrounding regions.

The circular aperture acts as a mode aperture and due to its circular geometry favors the generation of the radial symmetric TEM₀O mode. By introducing this aperture into the external resonator, i.e. the extended cavity, higher order transverse modes which do not show radial symmetry or which take up a larger radial extension will be efficiently suppressed. To this end the diameter D of the aperture is preferably dimensioned according to the following equation: 0.8*2*w(z) < D < 1.5*2*w(z), more preferably according to 2*w(z) < D < 1.2*w(z), with 2*w(z) being the diameter of the TEM₀O mode of the fundamental laser radiation at the position z of the aperture within the extended cavity. This diameter 2*w(z) can be calculated according to the above mentioned Ray matrix formalism. In one embodiment of the proposed VECSEL device the diameter D of the aperture matches the TEM₀O mode of the fundamental laser radiation at the position at which the aperture is arranged within the extended cavity.

By efficiently inhibiting the formation of higher order transversal modes, the VECSEL device can be operated at higher powers without formation of higher transversal modes, compared to an identical VECSEL without such an aperture, resulting in a higher optical intensity in the fundamental TEM₀₀ mode. This increases the efficiency of higher harmonic generation in the frequency converting element and allows an increased output power of the laser radiation of the second wavelength region. Therefore fundamental radiation in the IR wavelength region can be efficiently converted, for example, to laser radiation in the green wavelength region.

The best results are achieved when the circular aperture is positioned at the beam waist of the fundamental laser radiation or at least close to this beam waist. At this position maximum mode selectivity is achieved within the extended cavity. The beam waist location can be calculated by help of the above mentioned Ray matrix formalism. In one embodiment more than one circular aperture is arranged within the extended cavity. With more than one circular aperture a further improvement of the mode selection is achieved allowing the operation of the VECSEL at even higher power levels without formation of higher order transversal modes. In one of the preferred embodiments, the circular aperture(s) is/are formed by a coating on one or both surfaces of the frequency converting element. Since the frequency converting element usually is positioned in or near the beam waist of the fundamental laser radiation within the cavity, the circular apertures automatically are close to this beam waist. In an alternative embodiment or additionally, the circular aperture may be formed as a coating on the internal reflective surface of the second end mirror. The coating may either be absorbing, leaving an opening at the center for higher transmission of the fundamental radiation, or may modify the transmission or reflection of the optical surface in another way such that the formation of the fundamental mode is supported but sufficient losses are added to unwanted higher order transverse modes to prevent them from being generated. In a very advantageous embodiment the coating is an antireflective coating for the fundamental radiation which is only applied in a circular region centered at the longitudinal axis of the fundamental radiation in the extended cavity. This circular region forms the mode selective aperture. Outside of this circular region no antireflective coating is applied on the surface, thus causing higher losses due to higher reflection for higher transverse modes. This embodiment combines the antireflective coating, which in many cases is necessary for intracavity surfaces, with the proposed circular aperture and therefore avoids the need for any further coating or element to form the circular aperture.

The frequency converting element is preferably a frequency doubling element, in particular a nonlinear optical crystal designed for second harmonic generation. Since one of the preferred applications is the generation of green laser light, preferably a PPLN crystal is used as the frequency doubling element generating laser radiation in the green wavelength region from fundamental IR radiation emitted, for example, by Ga containing VCSEL chips. Nevertheless, also other frequency converting elements may be used as well as other VCSEL chips generating fundamental radiation in other wavelength regions. It is obvious for the skilled person that the present invention is not limited to special wavelength regions or frequency converting elements. The layer sequence on the substrate is preferably a DBR layer stack forming a first end mirror and a quantum well structure forming the active gain region as known in the art. The present invention allows the use of a common VCSEL chip as the layer sequence on the substrate, in which a second (inner) DBR is formed from a layer stack with the active gain region sandwiched between the two DBRs. As already explained in the introductory portion, this second DBR is designed to have a sufficient transmission for the fundamental laser radiation, allowing the operation of the VECSEL at high powers without reaching the lasing threshold without the feedback of the extended cavity. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described herein after.

BRIEF DESCRIPTION OF THE DRAWINGS

The proposed VECSEL is described in the following by way of examples in connection with the accompanying in figures without limiting the scope of protection as defined by the claims. The figures shown:

Fig. 1 a schematic view of a first embodiment of a VECSEL according to the proposed invention; Fig. 2 a top view of the SHG element of Fig. 1 indicating the circular aperture; Fig. 3 a schematic view of a second embodiment of the proposed

VECSEL;

Fig. 4 a schematic view of a third embodiment of the proposed VECSEL; Fig. 5 a schematic view of a fourth embodiment of the proposed

VECSEL.

DETAILED DESCRIPTION OF EMBODIMENTS

Figure 1 shows a first embodiment of the proposed VECSEL device in a schematic view. The VECSEL device consists of a surface emitting semiconductor laser (VCSEL) chip 1 formed by an electrically pumped active gain medium 2 embedded between two distributed Bragg reflectors 3, 4 which act as mirrors for the inner laser cavity. The active gain medium may consist for example of a layer stack of InGaAs quantum wells embedded in GaAs. The lower DBR 3 is highly reflective (reflectivity > 99.8%) for the fundamental IR laser radiation, while the reflectivity of the upper DBR 4 is smaller in order to allow feedback from the external cavity. One of the DBRs is p- doped and the other n-doped to enable efficient current feeding into the gain region 2. The electrodes for applying the necessary voltage are not shown in the figures. Nevertheless the construction of such a VCSEL chip with a transparent substrate 5 is commonly known in the art, for example from US 6,778,582 Bl . In most of the known VCSEL chips n- and p-type doping are formed out as indicated in Figure 1 in which the DBR 3 with higher reflectivity is p-doped. Nevertheless, doping in the reversed order is also possible.

The external cavity is formed by a mirror 6 placed and adjusted above the DBR 4 with lower reflectivity. This mirror 6 has a high reflectivity of preferentially > 99% at the generated IR wavelength of the fundamental radiation and preferentially an anti-reflection coating for the second harmonic wavelength in order to achieve efficient out-coupling of laser radiation of this wavelength. The mirror 6 may be a conventional laser mirror, for example with a dielectric coating, or any other mirror like a volume Bragg grating (VBG) with suited properties for IR reflection and transmission of the second harmonic wavelength. Preferably this external mirror 6 has a curved shape in order to support formation of a beam waist within the extended cavity. Nevertheless it is also possible and known in the art to use a planar external mirror or a VBG, which may also include a holographic lens.

The desired frequency doubled radiation is generated in a frequency converting element 7 of a suited nonlinear material for second harmonic generation, preferentially Periodically Poled Lithium Niobate (PPLN), which is placed in the extended cavity. One of the optical surfaces of this frequency converting element 7 is coated with an absorbing or reflecting material with a circular opening 9 in the coating 8, which is shown in the top view of the frequency converting element 7 in Figure 2. The diameter D of this opening 9 is adapted to the beam diameter 2w of the TEM₀O mode at the location of the coated optical surface. The frequency doubling element 7 is aligned in such a way that the center of the circular opening 9 in the coating 8 coincides with the longitudinal axis of the extended cavity formed between the external mirror 6 and the VCSEL chip 1.

Due to this circular aperture, i.e. the absorption coating 8 with the opening 9, higher order transversal modes of the generated IR laser radiation are suppressed in favor of the fundamental TEM₀O mode. This allows electrically pumping of the active gain medium 2 with higher power levels without the generation of such higher transversal modes. The higher intensity in the TEM₀O mode then results in a higher efficiency of second harmonic generation within the frequency converting element 7 and therefore in a higher output power of this frequency doubled radiation.

It may be advantageous to provide the optical surfaces of the frequency converting element 7 completely with an additional anti-reflection coating at least for the wavelength of the fundamental radiation in order to minimize reflection losses for the desired TEM₀₀ mode in the external cavity.

Another embodiment of the proposed VECSEL is shown in Figure 3. In this figure the external mirror 6 is formed by an additional coating 10 on the frequency converting element 7 or, as depicted in the figure, on the absorption coating 8 forming the circular aperture 8 on this frequency converting element 7. The additional coating 10 is highly reflective for the fundamental IR laser wavelength λ and at least partly, preferentially fully, transmissive for the frequency doubled laser radiation at λ/2. The further components of the VECSEL are the same as in Figure 1. For an explanation of these additional components it is referred to the description of Figure 1.

A third embodiment of the proposed VECSEL is shown in Figure 4. This embodiment distinguishes from the embodiment of Figure 1 in that the absorption coating 8 with the circular aperture 9 is applied to the intracavity surface of the external mirror 6 and not to the surface of the frequency converting element 7. The further components are the same as already explained with reference to Figure 1.

Finally Figure 5 shows a further example of a VECSEL according to the present invention. In this example, both surfaces of the frequency converting element 7 are coated with an antireflective coating 11 for the fundamental radiation only in a circular region which has a center coinciding with the longitudinal axis of the extended cavity formed between the external mirror 6 and the VCSEL chip 1. This circular region forms the circular aperture 9, since it results in lower losses for the fundamental radiation passing through the circular region, i.e. for the fundamental transverse mode, than for fundamental radiation passing outside of said circular region, i.e. for higher order transversal modes. Therefore in this embodiment the two end facets of the frequency converting element 7 simultaneously act as mode apertures. The diameters Dl and D2 of the two apertures are chosen in a suited way according to the TEM₀O mode radii at both facets. With two circular apertures for mode control the VECSEL can be pumped at even higher power levels without occurring of higher transversal modes.

In order to achieve an efficient second harmonic generation the laser may be frequency controlled to allow for efficient frequency doubling. This is sometimes necessary since the frequency bandwidth of Periodically Poled Lithium Niobate (PPLN) is typically < 2nm/(crystal length in mm). As the second harmonic generation efficiency depends quadratically on the crystal length, crystal dimensions in excess of 3mm are normally used leading to a required wavelength stabilization of better than lnm, which is a considerably high demand. With a frequency control, this demand can be fulfilled.

Furthermore, second harmonic generation can only be obtained for a well defined polarization of the laser light. Therefore additional measures have to be taken to make sure that only light with the correct polarization is generated. Fundamental laser light having other polarization would be wasted. Polarization control can for example be achieved by introducing a two-dimensional line grating structure into the external cavity. This grating structure can be placed in an anti-node of the longitudinal laser field, which should preferentially coincide with the top surface of the substrate.

The proposed VECSEL device is applicable in fields in which visible lasers, especially green lasers are required. A preferred application is the field of projection applications in which the proposed VECSEL may be used as one or several of the laser light sources in a laser projection device. 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. The different embodiments described above and in the claims can also be combined. 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. For example, the VECSEL device may also be formed of an array of light emitting VCSEL chips in a side by side arrangement to provide a VECSEL array. In this case, the array preferably shares a single transparent substrate. Furthermore, also VCSEL chips with the substrate on the opposed side may be used.

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. The mere fact that measures are recited in mutually different dependent claims does not indicate that a combination of these measures can not be used to advantage. Any reference signs in the claims should not be construed as limiting the scope of these claims.

LIST OF REFERENCE SIGNS:

1 VCSEL chip

2 Active gain region 3 Lower DBR

4 Upper DBR

5 Substrate (transparent)

6 External Mirror

7 Frequency converting element 8 Absorbing coating

9 Circular aperture

10 Mirror coating

11 Antireflective coating

Claims

CLAIMS:

1. Vertical extended cavity surface emission laser comprising

- a layer sequence (2, 3, 4) on a substrate (5), said layer sequence (2, 3, 4) forming at least a first end mirror (3) and an active gain region (2) emitting fundamental laser radiation of a first wavelength region when excited, - a second end mirror (6) forming an extended cavity with the first end mirror (3), and

- a frequency converting element (7) arranged between the first end mirror (3) and the second end mirror (6), said frequency converting element (7) converting part of the fundamental laser radiation to laser radiation of a second wavelength region, wherein said second end mirror (6) mainly reflects the fundamental laser radiation and is at least partly transmissive for the laser radiation of the second wavelength region, and wherein at least one circular aperture (9) is arranged distant from the layer sequence (2, 3, 4) between the layer sequence (2, 3, 4) and the second end mirror (6), said circular aperture (9) being designed and arranged to allow formation of a fundamental transversal mode of the fundamental laser radiation and to inhibit formation of higher transversal modes at power levels at which an identical VECSEL without said aperture (9) would already generate higher transversal modes.

2. Vertical extended cavity surface emission laser according to claim 1, wherein said circular aperture (9) is dimensioned to have a diameter which is between 80% and 150% of the diameter of the TEM₀O mode of the fundamental laser radiation at the position of the aperture (9) within the extended cavity.

3. Vertical extended cavity surface emission laser according to claim 1, wherein said circular aperture (9) is dimensioned to have a diameter matching the diameter of the TEM₀O mode of the fundamental laser radiation at the position of the aperture (9) within the extended cavity.

4. Vertical extended cavity surface emission laser according to claim 1, wherein said fundamental laser radiation forms a beam waist in said extended cavity and said at least one circular aperture (9) is arranged at or at least close to said beam waist.

5. Vertical extended cavity surface emission laser according to claim 1, wherein said circular aperture (9) is formed by a coating (8, 11) on a surface of the frequency converting element (7).

6. Vertical extended cavity surface emission laser according to claim 1, wherein two of said circular apertures (9) are arranged within the extended cavity and formed by coatings (8, 11) on two opposing surfaces of the frequency converting element (7).

7. Vertical extended cavity surface emission laser according to claim 1, wherein said circular aperture (9) is formed by a coating (8, 11) on an intracavity surface of the second end mirror (6).

8. Vertical extended cavity surface emission laser according to claim 1, wherein two of said circular apertures (9) are arranged within the extended cavity and formed by coatings (8, 11) on an intracavity surface of the second end mirror (6) and on a surface of the frequency converting element (7).

9. Vertical extended cavity surface emission laser according to any one of claims 5, 6, 7 and 8, wherein said coating (11) is an antireflection coating for the fundamental laser radiation, said coating (11) being only applied in a circular region which represents said circular aperture (9).

10. Vertical extended cavity surface emission laser according to any one of claims 5, 6, 7 and 8, wherein said coating (8, 11) is formed of a material modifying transmission or absorption properties of the surface for the fundamental laser radiation such that the formation of higher transversal modes of the radiation is inhibited.

11. Vertical extended cavity surface emission laser according to claim 1, wherein said layer sequence (2, 3, 4) is designed to form a first distributed Bragg reflector as the first end mirror (3), a quantum well structure as the active gain region (2) and a second distributed Bragg reflector (4), said quantum well structure being arranged between the first and second distributed Bragg reflector.

12. Vertical extended cavity surface emission laser according to claim 1, wherein the frequency converting element (7) is a nonlinear optical crystal designed for second harmonic generation of the fundamental laser radiation.

13. Vertical extended cavity surface emission laser according to claim 1, wherein said active gain region (2) is designed to emit fundamental laser radiation in the infrared wavelength region and said frequency converting element (7) is designed to convert the fundamental laser radiation to laser radiation in the green wavelength region.

14. Laser projection device comprising one or several laser light sources, wherein at least one of said laser light sources is a vertical extended cavity device according to any one of the preceding claims.