DE102008038961B4 - A surface-emitting semiconductor laser chip, laser arrangement with a surface-emitting semiconductor laser chip and method for producing a surface-emitting semiconductor laser chip - Google Patents

Abstract

Surface emitting semiconductor laser chip (1) with
- A first layer sequence (2), which is reflective for a primary radiation (P) having a primary wavelength λ _P , and
a layer stack (4) having at least one active region (5), wherein the layer stack (4) is applied to a main side of the first layer sequence (2) and configured to emit the primary radiation (P), wherein a thickness (D) the layer stack (4) is designed so that it is non-resonant with respect to the primary wavelength λ _P , so that an intensity of an electromagnetic field within the semiconductor laser chip (1) corresponds to that of a Fabry-Perot element with minimal transmission,
for an effective resonator length L of the semiconductor laser chip (1) with a tolerance of at most k _P / 8: L = N λ _P / 2 + k _P / 4 and N is a natural number,
- The semiconductor laser chip (1) further comprises at least one second layer (7) which is attached to the light exit surface (6) of the layer stack (4) and for the primary radiation (P) is partially reflecting, characterized in that a reflectance of the at least one second layer (7) for the primary radiation (P) is between 7% and 13%.

Description

The invention relates to a surface-emitting semiconductor laser chip, to a laser arrangement having a surface-emitting semiconductor laser chip and to a method for producing such a semiconductor laser chip.

Semiconductor-based lasers offer great advantages in terms of efficiency, maintenance and size of the component compared to gas or solid state lasers with an optically pumped YAG or YLF crystal as the amplifier medium. One way to classify semiconductor lasers is to distinguish between edge emitting and surface emitting lasers. In edge-emitting semiconductor lasers, the light emitted by the semiconductor chip often has a line-like, asymmetrical cross-section of the beam profile. In the case of surface-emitting semiconductor lasers, the quality of the beam profile is usually more favorable since the beam profile is more symmetrical. Surface emitting semiconductor lasers are also capable of producing higher intensity light than edge emitting semiconductor lasers. The optical power densities within the laser crystal can be several watts per cubic millimeter, for example in continuous wave mode.

The publication WO 02/47223 A1 relates to an optically pumped VCSEL laser comprising on a light exit side a Bragg mirror with an antireflection layer.

The pamphlets US 2002/0 071 463 A1 and US Pat. No. 6,741,629 B1 are directed to optically pumped VCSELs which have light entrance sides with a low reflectivity.

An object to be solved is to provide a surface emitting semiconductor laser chip with a high efficiency. Another object to be solved is to specify a laser arrangement with such a semiconductor laser chip. Furthermore, an object to be solved is to provide a method for producing such a surface-emitting semiconductor laser chip.

These objects are achieved by the surface emitting semiconductor laser chip according to claim 1, by the laser arrangement according to claim 5 and by the method according to claim 8.

According to the invention, the surface-emitting semiconductor laser chip comprises a first layer sequence. The first layer sequence acts for a primary radiation generated in operation of the semiconductor laser chip with a primary wavelength λ _P reflective. The layer sequence can be formed from a single or, preferably, from several layers. It is possible that the layer sequence has a reflective metallic component. If the first layer sequence is designed as a Bragg mirror and has dielectric layers or semiconductor layers with alternately low and high refractive index, the layer sequence preferably comprises 20 to 40 pairs of layers with low and high refractive index. The thickness of the respective layers is preferably one quarter of the primary wavelength λ _P , Thickness here denotes the optical thickness, that is to say the integral over the product of geometric length and refractive index for the relevant wavelength.

The first layer sequence may alternatively or additionally also have reflective metal layers, reflective layers made of a transparent conductive oxide, TCO for short, or a structure similar to a diffraction grating. Preferably, all layers of the first layer sequence are configured just. But it is equally possible that individual or all layers have a curvature, so that approximately a focusing reflector is formed.

In the event that the first layer sequence is configured as a Bragg mirror, the primary radiation has a certain effective penetration depth into the Bragg mirror and thus into the reflective, first layer sequence. The effective penetration depth can be estimated as the primary wavelength λ _P divided by four times the refractive index difference between high and low refractive index layers. If the refractive index difference between the high and low refractive index layers is about 0.5, then the effective penetration depth is λ _P , ₀ / (4x 0.5), that is, about A _P , _0/2 . In this estimation, λ _P , _{0 represents} the vacuum wavelength of the primary radiation, that is not like λ _P the wavelength divided by the refractive index of the medium. The effective penetration depth is therefore essentially a wavelength-dependent physical property of a Bragg mirror.

Surface emitting means that the semiconductor chip emits laser radiation in a planar manner on an outer surface of a semiconductor body. The surface over which the light radiation is emitted from the semiconductor laser chip has a two-dimensional character. In contrast to this, edge-emitting lasers whose light-emitting surface is linear and thus represents a substantially one-dimensional surface whose longitudinal extent is significantly larger than its transverse extent can be seen. Preferably, the surface over which the surface emitting semiconductor laser chip emits laser radiation has two major axes, each of the major axes having a greater length than an extension of a light generating region of the semiconductor laser chip in one direction perpendicular to the light emitting surface. Preferably, both major axes of the light-emitting surface are approximately the same size. "Approximately" means that the relative deviation is less than 25%, in particular less than 10%. In other words, the light-emitting surface is, for example, ellipsoidal or, preferably, circular.

Semiconductor laser chip means that the chip is based essentially on semiconductor materials. In other words, according to the invention, the semiconductor laser chip has at least one active region which is designed to generate the primary radiation which is based on a semiconductor material. It is in principle possible that the semiconductor laser chip in addition to semiconductor materials also other materials such as dielectric layers, passivation or electrically ohmic conductive layers, for example of a metal having. However, the generation of the laser radiation is based on at least one semiconductor material, such as GaAs, InGaAs, AlGaAs, GaP, InGaP, GaN, InGaN or InP. The semiconductor material may further include substances, for example in the form of dopants.

According to at least one embodiment of the surface-emitting semiconductor laser chip, the primary wavelength λ _P lies in the near-infrared spectral range, in particular between 1000 nm and 1100 nm. The primary wavelength λ _P can however also be at other wavelengths in the spectral range between 200 nm and 3500 nm.

According to the invention, the surface-emitting semiconductor laser chip has a layer stack with at least one active region. The layers of the layer stack may be grown epitaxially and expand substantially parallel to the light-emitting surface. The at least one active region is configured to generate the primary radiation having the primary wavelength λ _P during operation of the semiconductor laser chip. The primary wavelength λ _P may comprise one or more wavelength ranges. Preferably, the semiconductor laser chip has a plurality of active regions, in particular between five and 15 such regions. At least one active region is designed as a quantum dot, as a quantum wire or preferably as a quantum well.

The at least one active region may be optically or electrically pumped. The pumping of at least one active area can be direct or indirect. In direct pumping, for example, electron-hole pairs are generated in the active region itself. In the case of indirect pumping, the layer stack has, for example, barrier layers in which electron-hole pairs are generated by pumping, which then propagate into the active regions or into at least one active region and serve to generate the primary radiation there.

According to the invention, the layer stack having the at least one active region is applied to a main side of the first layer sequence. The first layer sequence and the layer stack are preferably in direct contact with one another, but it is also possible for layers which serve, for example, an electrical contacting, to be arranged between the reflective first layer sequence and the layer stack. A main side of the layer stack facing away from the first layer sequence is formed, at least in part, by a light exit surface.

The thickness of the layer stack is its optical thickness, that is the integral over the optical refractive index and a path along a running direction of the radiation generated in the stack of layers during operation of the semiconductor laser chip. The layer stack is in particular limited by the light exit surface and the reflective first layer sequence. The light exit surface can be distinguished by the fact that a material having a significantly reduced refractive index, such as air or an antireflection layer, is present on the side of the layer stack facing away from the light exit surface. "Substantially reduced" means that the refractive index difference, starting from the material forming the light exit surface, is at least 20%, preferably at least 40%.

According to the invention, the thickness of the layer stack is designed such that it is non-resonant with respect to the primary wavelength λ _P. In this case, "non-resonant" means that with respect to nodes and antinodes within the semiconductor chip, no electric light field is formed, as is the case, for example, in a resonant case in a Fabry-Perot element.

According to the invention, the surface-emitting semiconductor laser chip has a resonator. The resonator is integrated in the semiconductor laser chip and formed by the light exit surface and by the reflective first layer sequence. The light exit surface has a non-vanishing reflectivity with respect to the primary wavelength λ _P.

According to the invention, the surface-emitting semiconductor laser chip has an effective resonator length L. The effective resonator length L is formed from the sum of the thickness of the layer stack and the effective penetration depth of the primary radiation into the reflective first layer sequence. That is, the resonator length L is greater than or equal to the thickness of the layer stack. The respective thicknesses or lengths relate to the optical lengths, that is, the integral over the product of path length and refractive index.

According to the invention, the surface-emitting semiconductor laser chip comprises a first layer sequence which has a reflective effect for a primary radiation having a primary wavelength λ _P. In addition, the semiconductor laser chip comprises a layer stack having at least one active region, wherein the layer stack is applied to a main side of the first layer sequence and configured to emit the primary radiation. The thickness of the layer stack is designed to be non-resonant with respect to the primary wavelength λ _P.

In other words, the semiconductor laser chip is designed with a non-resonant amplifier structure.

In such a surface emitting semiconductor laser chip, only small optical losses occur within the semiconductor laser chip. Therefore, such a semiconductor laser chip has a high efficiency.

The efficiency of a semiconductor laser chip results from the relation of gain achieved and optical losses occurring. Both gain and optical losses are approximately directly proportional to the optical intensity in the stack of layers having the at least one active region. That is, in the case where the gain is high, the optical losses are also high. This adversely affects the overall efficiency of the semiconductor laser chip.

The optical intensity in the semiconductor laser chip is highest, for example, when the thickness of the layer stack is designed such that the layer stack is resonant with respect to the primary wavelength λ _P. That is, in the resonant case forms within the semiconductor laser chip, a standing wave, which is analogous to a resonance in a Fabry-Perot element to see. In a non-resonant case, the optical intensity in the semiconductor laser chip is correspondingly reduced.

At least three types of optical losses within the semiconductor laser chip can be reduced by non-resonant design of the thickness of the layer stack. Firstly, because of the reduced optical intensity in the semiconductor laser chip, the reabsorption of radiation within the semiconductor laser chip, for example through free carrier absorption, is also reduced. Secondly, the reduction of the absorption or reabsorption reduces the temperature and in particular the temperature gradient within the semiconductor laser chip. Third, in the resonant case, as mentioned, the light exit surface and the reflective first layer sequence act as a Fabry-Perot resonator. In the case of resonance, a Fabry-Perot element has an increased transmission, which occurs in the case of a semiconductor laser chip as an optical power loss.

One aspect of the operation of the semiconductor laser chip is therefore that can be reduced by the non-resonant structure disturbing effects by the formation of a thermal lens, for example by an optical pumping of the semiconductor laser chip. Since an effective penetration depth of the primary radiation and / or a pumping radiation into the semiconductor laser chip is reduced for a non-resonant structure of the semiconductor laser chip, in this case also the thermal lens and the losses associated therewith are weaker.

It can therefore be reduced by reducing the optical intensity in the semiconductor laser chip, the optical losses and the efficiency or the efficiency of the semiconductor laser chip can be increased. This is possible if the gain in the stack of layers is sufficiently high so that the gain need not be optimized. In such a semiconductor laser chip so not the gain, English gain, but the efficiency is optimized or improved.

According to the invention, the effective resonator length L corresponds to the formula: $$\text{L}=0.5\text{N}{\mathrm{\lambda }}_{\text{P}}+0.25{\mathrm{\lambda }}_{\text{P}}$$

N is a natural number. The tolerance for the effective resonator length L is in this case at most λ _P / 8.

In other words, the effective resonator length L corresponds to an integer multiple of half the primary wavelength λ _P , extended by λ _P / 4. That is, the effective resonator length L is thus 3λ _P / 4 or 5λ _P / 4 or 7λ _P / 4 and so on. Such an effective resonator length L reduces the Fabry-Perot effect and thus the transmission in the direction away from the layer stack through the reflective first layer sequence. The efficiency of the semiconductor laser chip is thereby increased.

Again, as in the following, the effective resonator length L is the optical and not the geometric length.

According to at least one embodiment of the surface emitting semiconductor laser chip, the tolerance with respect to the effective resonator length L is at most λ _P / 16, in particular at most λ _P / 32. A reduced tolerance of the effective resonator length L leads to a more effective Reduction of optical losses in the semiconductor laser chip and thus increases its efficiency.

In accordance with at least one embodiment of the surface emitting semiconductor laser chip, the thickness of the layer stack is designed such that the semiconductor laser chip is anti-resonant with respect to the primary wavelength λ _P. That is, the intensity of the electromagnetic field within the semiconductor laser chip corresponds to that of a Fabry-Perot element with minimal transmission. In the anti-resonant case, the optical intensity in the semiconductor laser chip is reduced. Such a choice of the thickness of the layer stack can also reduce the optical losses in the semiconductor laser chip.

In accordance with at least one embodiment of the surface-emitting semiconductor laser chip, no intensity maximum with respect to the optical intensity of the primary radiation is present at the light exit surface of the layer stack during operation of the semiconductor laser chip. Preferably, an intensity minimum is present at the light exit surface. The tolerance for this is at most λ _P / 16, in particular at most λ _P / 32. By minimizing the intensity at the light exit surface is also accompanied by a minimization of the optical intensity within the semiconductor chip. This also reduces the occurring optical losses in the half-fiber chips, whereby its efficiency is increased.

According to at least one embodiment of the surface-emitting semiconductor laser chip, the latter comprises a plurality of active regions, wherein at least one distance between two adjacent active regions corresponds to a multiple of half the primary wavelength λ _P and at least one active region is in an intensity maximum of the primary radiation during operation of the semiconductor laser chip. Towards the reflective first layer sequence, the distances between adjacent active regions may preferably increase. If the active regions are arranged in groups with at least two active regions, wherein within a group the distances between adjacent active regions are approximately the same, the distances between the groups increase in the direction of the first layer sequence. Likewise, at least half of the active regions, in particular all active regions, are arranged in an intensity maximum of the primary radiation. If active regions are placed in the intensity maxima of the primary radiation, the gain of the semiconductor laser chip increases.

According to the invention, the surface-emitting semiconductor laser chip comprises at least one second layer, which is attached to the light exit surface of the layer stack and partially reflecting the primary radiation. The second layer may be in direct physical contact with the light exit surface, this being the preferred case or separated by one or more functional layers from the layer stack. The at least one second layer is, for example, a layer which has a refractive index which lies between that of air and that of the layer stack forming material. About such a partially reflecting layer, the coupling-out efficiency can be increased from the semiconductor laser chip. In addition, the reflectivity of the light exit surface of the layer stack can be selectively adjusted via such a second layer.

According to the invention, the reflectance of the light exit surface of the layer stack with respect to the primary radiation due to the at least one second layer is between 7% and 13%. The reflectivity thus differs from that of an optimized, single-layer antireflection coating, with a reflectivity of less than 2%, and also of an interface air-light exit surface or air-semiconductor material, with a reflectivity of> 30%. Rather, a reflectivity is set, which has proven to be optimal in terms of avoiding losses in the semiconductor chip. A reflectivity of the light exit surface in said value range due to the second layer can be realized with comparatively little technical effort.

In accordance with at least one embodiment of the semiconductor laser chip, the at least one second layer is a passivation. That is, the at least one second layer protects the layer stack at least in part from chemical or physical influences that are detrimental to the layer stack. Such a second layer increases the life of the semiconductor laser chip.

According to at least one embodiment of the surface emitting semiconductor laser chip, this is indirectly optically pumpable. Such pumping of the semiconductor laser chip is also referred to as barrier pumps. That is, light having a pump wavelength λ _R is irradiated into the semiconductor laser chip. This pump light is absorbed in barrier layers in which electron-hole pairs are generated. These electron-hole pairs then propagate into the at least one active region and subsequently serve via recombination to generate the primary radiation. The barrier layers are preferably transparent to the primary radiation and designed to be absorbent for the pump radiation. Such a semiconductor laser chip has a high efficiency and a high beam quality.

In accordance with at least one embodiment of the semiconductor laser chip, the at least one second layer designed as an antireflection layer with respect to the pump wavelength λ _R.

According to at least one embodiment of the surface emitting semiconductor laser chip, this is designed for a continuous wave operation. That is, the semiconductor laser chip is not pulsed. In particular, continuous wave operation means that a repetition rate is at most 10 Hz and a pulse duration is at least 0.1 s.

In addition, a laser arrangement with at least one surface-emitting semiconductor laser chip is specified. The laser arrangement comprises a surface emitting semiconductor laser chip as described above.

According to the invention, the laser arrangement comprises at least one surface emitting semiconductor laser chip and at least one pumping light source which is designed for pumping the semiconductor laser chip.

According to the invention, the laser arrangement comprises at least one external mirror which has a reflective effect on the primary radiation generated by the semiconductor laser chip. The external mirror is preferably a highly reflecting dielectric mirror or a metal mirror. The external mirror may have a curvature such that the external mirror, for example, has a focusing effect.

According to the invention, an external resonator of the laser arrangement is formed by the first layer sequence of the at least one semiconductor laser chip and by the at least one external mirror.

According to the invention, the laser arrangement comprises at least one surface-emitting semiconductor laser chip, as described above. Furthermore, the laser arrangement comprises at least one pumping light source for pumping the semiconductor laser chip. In addition, the laser arrangement has at least one external mirror which has a reflective effect on the primary radiation, so that an external resonator of the laser arrangement is formed by the first layer sequence of the semiconductor laser chip and by the at least one external mirror.

Such a laser arrangement can have a compact design and permits a conversion of the radiation generated by the semiconductor laser chip into a secondary radiation having a secondary wavelength λ _S different from the primary radiation.

In accordance with at least one embodiment of the laser arrangement, the resonator of the laser arrangement comprises a wavelength-selective element. Optionally, the polarization of the primary and / or secondary radiation can also be defined via the wavelength-selective element. About such a wavelength-selective element, the spectral properties of the laser array can be adjusted specifically.

According to at least one embodiment of the laser arrangement, this comprises a wavelength-selective element in conjunction with a semiconductor chip, in which the light exit surface has a reflectivity in the range of 4% to 20%, in particular between 7% and 13%.

wobei N eine natürliche Zahl ist. Das heißt, die Wellenlängen λ_Res erfüllen eine Resonanzbedingung im Sinne eines Fabry-Perot-Effekts bezüglich der effektiven Resonatorlänge L. Für solche Wellenlängen λ_Res, bezüglich denen der Resonator des Halbleiterlaserchips resonant ist, wirkt also das wellenlängenselektive Element sperrend. „Sperrend“ kann hierbei bedeuten, dass die Transmission für resonante Wellenlängen λ_Res um einige Prozent unterhalb der Transmission für anti-resonante Wellenlängen λ_P liegt. Der Wellenlängenbereich, in dem das Element sperrend wirkt, ist bevorzugt so gewählt, dass im gesamten Verstärkungsbereich des Schichtenstapels solche Wellenlängen λ_Res unterdrückt sind. Ein derartiges wellenlängenselektives Element verhindert, dass der Halbleiterlaserchip in einen resonanten Modus gerät, in dem die optische Intensität im Halbleiterlaserchip hoch ist und somit auch die optischen Verluste groß sind. Über ein solches wellenlängenselektives Element kann eine bestimmte, insbesondere bezüglich der effektiven Resonatorlänge des Halbleiterlaserchips anti-resonante Primärwellenlänge λ_P gezielt eingestellt werden.According to at least one embodiment of the laser arrangement, the wavelength-selective element in a wavelength range of 0.8 λ _P to 1.2 λ _P blocking for wavelengths λ _Res , with a wavelength tolerance of at most λ _P / 16 with respect to the effective resonator length L of the semiconductor laser chip, the condition fulfill: $${\mathrm{\lambda }}_{\text{Res}}=\left(2\text{L}\right)/\text{N}\text{.}$$

where N is a natural number. That is, the wavelengths λ _Res fulfill a resonance condition in the sense of a Fabry-Perot effect with respect to the effective resonator length L. For wavelengths λ _Res , with respect to which the resonator of the semiconductor laser chip is resonant, then the wavelength-selective element acts as a blocking element. In this context, "blocking" can mean that the transmission for resonant wavelengths λ _res is a few percent below the transmission for anti-resonant wavelengths λ _P. The wavelength range in which the element has a blocking effect is preferably chosen so that such wavelengths λ _{res are} suppressed in the entire amplification range of the layer _stack . Such a wavelength-selective element prevents the semiconductor laser chip from entering a resonant mode in which the optical intensity in the semiconductor laser chip is high and hence the optical losses are also large. By means of such a wavelength-selective element, a specific, in particular with respect to the effective resonator length of the semiconductor laser chip anti-resonant primary wavelength λ _P can be set specifically.

In accordance with at least one embodiment of the laser arrangement, the wavelength-selective element is an etalon. Etalons can be very compact and, in addition to the wavelength selectivity, also have a polarization selectivity. Via an etalon, it is thus possible to determine both the spectral properties and the polarization properties of the laser arrangement. A laser arrangement with an etalon is for example in the document WO 2008/028454 A1 whose disclosure content is hereby incorporated by reference.

In accordance with at least one embodiment of the laser arrangement, the resonator of the laser arrangement comprises at least one crystal for frequency doubling of the primary radiation. By way of frequency doubling, in particular near-infrared light with wavelengths in the range of 1 μm, in particular between 0.95 μm and 1.15 μm, can be converted into green light with wavelengths, in particular in the range from 510 nm to 570 nm.

In accordance with at least one embodiment of the laser arrangement, during operation of the semiconductor laser chip, a thermal lens is formed in the latter, via which the primary radiation is focused into the crystal for frequency doubling. For a given pump power, a temperature gradient is formed in the semiconductor laser chip. This temperature gradient leads to a variation of the optical refractive index in a direction parallel to the light exit surface. This refractive index gradient has the effect of a lenticular element, in particular a condenser lens. Via this thermal lens, the primary radiation can be focused in the crystal. Such a thermal lens can eliminate an external lens outside the semiconductor laser chip and increase the doubling efficiency in the crystal.

There is also provided a method of manufacturing a surface emitting semiconductor laser chip. By means of the method, a surface-emitting semiconductor laser chip is produced, as described above.

• - Bereitstellen eines Halbleiterlaserchips mit der ersten Schichtenfolge und dem Schichtenstapel, wobei die effektive Resonatorlänge L des Halbleiterlaserchips mit einer Toleranz von höchstens λ_P/16 einem ganzzahligen Vielfachen der Hälfte der Primärwellenlänge λ_P entspricht, und
• - Reduzierung der effektiven Resonatorlänge L des Halbleiterlaserchips mit einer Toleranz von höchstens λ_P/16 um einen Wert von ((2 N - 1) λ_P)/4, wobei N eine natürliche Zahl ist.

The method according to the invention comprises the following steps:

• Providing a semiconductor laser chip having the first layer sequence and the layer stack, wherein the effective resonator length L of the semiconductor laser chip with a tolerance of at most λ _P / 16 corresponds to an integer multiple of half the primary wavelength λ _P , and
• - Reduction of the effective resonator length L of the semiconductor laser chip with a tolerance of at most λ _P / 16 by a value of ((2 N - 1) λ _P ) / 4, where N is a natural number.

The provision of the semiconductor laser chip may include epitaxial growth of the first layer sequence and / or the layer stack on a growth substrate. The epitaxially grown layers can then be transferred from the growth substrate to a support or remain on the growth substrate. The fact that the effective resonator length L corresponds to a multiple of half the primary wavelength λ _P means that the semiconductor laser chip is resonant with respect to the primary wavelength λ _P.

The step of reducing the effective resonator length L follows the provision of the semiconductor laser chip. In this case, the effective resonator length L is reduced by an odd-numbered multiple of the quarter-wavelength primary wavelength λ _P , that is, for example, λ _P / 4, λ 3 _P / 4, λ _P / 4 and so forth. Such a reduction of the effective resonator length L means that the semiconductor laser chip previously resonant with respect to the primary wavelength λ _{P is} now non-resonant, in particular anti-resonant.

The manufacturing tolerance in the individual method steps with respect to the respective optical thicknesses of the layers is, according to at least one embodiment, highest λ _P / 32.

By such a method, semiconductor laser chips having a non-resonant or anti-resonant resonator effective length L having high efficiency and high efficiency can be effectively and high-yielded.

In other words, first a resonant semiconductor laser chip is produced in which the gain is optimized and thus the optical intensity in the layer stack is maximized. In this case, the primary wavelength λ _P lies in particular at the wavelength at which the optical amplification predetermined by the material of the at least one active region has a maximum during laser action. This maximum is in particular at approximately 1055 nm. Subsequently, the thickness of the layer stack is reduced so that the semiconductor laser chip is, for example, anti-resonant at the primary wavelength λ _P. In particular, the primary wavelength λ _P does not change by reducing the thickness. However, the optical losses in the semiconductor laser chip decrease because the intensity in the semiconductor laser chip is reduced and the transmission through the reflective first layer sequence is reduced due to the Fabry-Perot effect.

A difference between a resonant and an anti-resonant semiconductor laser chip thus lies in the effective resonator length L.

According to at least one embodiment of the method, before the step of reducing the effective resonator length L, the function of the semiconductor laser chip is tested at the primary wavelength λ _P in the resonant case. A resonantly constructed semiconductor laser chip can, regardless of the reflectivity of the light exit surface, show a laser action, in particular in the case of a light exit surface with a reflectivity> 30% in the case of an air-semiconductor material interface. In the case of a non-resonant or anti-resonant semiconductor laser chip, a light exit surface with a reflectivity of between 2% and 20%, in particular between 7% and 13%, may be required for this purpose. So it's an im Frame of manufacture allows early testing of semiconductor laser chips.

In accordance with at least one embodiment of the method, the step of reducing the effective resonator length L is carried out with the aid of at least one etching stop layer. That is, preferably within the layer stack, there is an etch stop layer at such a position that the etching is automatically terminated at an effective resonator length L corresponding to an odd-numbered multiple of the quarter-wavelength primary wavelength λ _P. All active regions of the layer stack are preferably located between the etch stop layer and the reflective first layer sequence. By using an etch stop layer, the effective resonator length L can be reduced particularly easily and accurately.

In accordance with at least one embodiment of the method, the step of reducing the effective resonator length L is carried out by wet-chemical etching.

In accordance with at least one alternative embodiment of the method, the semiconductor laser chip can also be produced directly, for example by epitaxial growth, as a non-resonant chip. In other words, the layer stack is thus grown such that its thickness is so large that an effective resonator length L results, so that the semiconductor laser chip is non-resonant or anti-resonant with respect to the primary wavelength λ _P. This eliminates the operation of reducing the effective resonator length L. However, testing of the component in production is difficult.

Some fields of application in which surface-emitting semiconductor laser chips or laser arrangements described here can be used are, for example, display devices, illumination devices for projection purposes, headlights, light emitters or else devices for general illumination.

In the following, a surface-emitting semiconductor laser chip described here, a laser arrangement described here and a method described here will be explained in more detail with reference to the drawing.

The same reference numerals indicate the same elements in the individual figures. However, there are no scale relationships shown, but individual elements can be shown exaggerated for better understanding.

• 1 und 2 schematische Darstellungen von resonanten Halbleiterlaserchips (A) sowie schematische Darstellungen des Verlaufs der optischen Intensität (B, C),
• 3 eine schematische Darstellung einer nicht erfindungsgemäßen Abwandlung eines Halbleiterlaserchips (A) sowie eine schematische Darstellung des Intensitätsverlaufs (B),
• 4 eine schematische Darstellung des Aufbaus eines erfindungsgemäßen, hier beschriebenen Halbleiterlaserchips,
• 5 und 6 schematische Darstellungen des optischen Intensitätsverlaufs sowie des Brechungsindexverlaufs für resonante und hier beschriebene nicht-resonante Halbleiterlaserchips,
• 7 eine schematische Illustration von Verlustmechanismen bezüglich der optischen Leistung eines Halbleiterlaserchips,
• 8 eine schematische Darstellung eines Fabry-Perot-Effekts sowie der Verstärkung im resonanten (A, B) sowie im anti-resonanten Fall (C, D),
• 9 eine schematische Darstellung einer durch eine thermische Linse hervorgerufenen Phasenstörung für drei verschiedene Wellenlängen (A) sowie Lage der Wellenlängen bezüglich der spektralen Position einer Resonanz eines zugrunde liegenden Halbleiterlaserchips (B),
• 10 eine schematische Darstellung der Ausprägung einer thermischen Linse von Halbleiterlaserchips in Abhängigkeit von einer maximalen Temperatur in einer Pumpregion für vier verschiedene Reflektivitäten einer Lichtaustrittsfläche,
• 11 eine schematische Darstellung eines Ausführungsbeispiels einer hier beschriebenen Laseranordnung,
• 12 schematische Darstellungen der Ausgangsleistung (A), der Emissionswellenlänge (B) und der optischen Verluste (C) eines Halbleiterlaserchips in Abhängigkeit von der Reflektivität der Lichtaustrittsfläche,
• 13 schematische Darstellungen der Leistungsdaten eines Halbleiterlaserchips mit einem Etalon und mit einem externen Resonator,
• 14 und 15 schematische Darstellungen eines Vergleichs zwischen einem resonanten und einem anti-resonanten Halbleiterlaserchip, und
• 16 eine schematische Veranschaulichung eines Herstellungsverfahren für einen Halbleiterlaserchip.

Show it:

• 1 and 2 schematic representations of resonant semiconductor laser chips (A) and schematic representations of the course of the optical intensity (B, C),
• 3 1 is a schematic representation of a non-inventive modification of a semiconductor laser chip (A) and a schematic representation of the intensity profile (B),
• 4 a schematic representation of the structure of a semiconductor laser chip according to the invention described here,
• 5 and 6 schematic representations of the optical intensity profile and the refractive index profile for resonant and here described non-resonant semiconductor laser chips,
• 7 a schematic illustration of loss mechanisms with respect to the optical power of a semiconductor laser chip,
• 8th a schematic representation of a Fabry-Perot effect and the amplification in the resonant (A, B) and in the anti-resonant case (C, D),
• 9 a schematic representation of a caused by a thermal lens phase disturbance for three different wavelengths (A) and position of the wavelengths with respect to the spectral position of a resonance of an underlying semiconductor laser chip (B),
• 10 1 is a schematic representation of the characteristic of a thermal lens of semiconductor laser chips as a function of a maximum temperature in a pumping region for four different reflectivities of a light exit surface;
• 11 a schematic representation of an embodiment of a laser arrangement described herein,
• 12 schematic representations of the output power (A), the emission wavelength (B) and the optical losses (C) of a semiconductor laser chip as a function of the reflectivity of the light exit surface,
• 13 schematic representations of the performance data of a semiconductor laser chip with an etalon and with an external resonator,
• 14 and 15 schematic representations of a comparison between a resonant and an anti-resonant semiconductor laser chip, and
• 16 a schematic illustration of a manufacturing method for a semiconductor laser chip.

In 1A is a surface emitting semiconductor chip 1 represented by a layer stack 4 comprising a thickness D. The thickness D is designed so that the semiconductor laser chip 1 resonant with respect to a primary wavelength λ _P one in the layer stack 4 generated primary radiation P is. The primary radiation P is symbolized by a thick, arrowed line.

The semiconductor laser chip 1 includes a reflective first layer sequence 2 , This consists according to 1A essentially of a reflective, metallic layer. At the layer sequence 2 opposite side of the layer stack 4 there is a light exit surface 6 of the layer stack 4 , About the refractive index jump between the layer stack 4 surrounding air and that the layer stack 4 forming semiconductor material acts the light exit surface 6 , as well as the first layer sequence 2 , at least partially reflecting for the primary radiation P. As a result of the light exit surface 6 and from the first layer sequence 2 a resonator in the semiconductor laser chip 1 educated. Because the reflective first layer sequence 2 is designed metallic and thus the primary radiation P no significant depth of penetration into the first layer sequence 2 has, the thickness D of the layer stack 4 practically equal to an effective resonator length L of the semiconductor laser chip 1 ,

In 1B schematically is the course of the optical intensity I in a direction x perpendicular to the light exit surface 6 shown. Since the intensity I is proportional to the square of the electric field, the period length with respect to the intensity I is only half the period length of the electromagnetic field. That is, two maxima with respect to the intensity I are not separated from each other by a whole primary wavelength λ _P , but only half a primary wavelength λ _P. All details of lengths or wavelengths refer in the following to the wavelength occurring in the corresponding medium, unless otherwise described. That is, the refractive index of the medium along the direction of extension of the light must be taken into account. The height of the intensity maxima is within the entire layer stack 4 in about the same size.

At the semiconductor laser chip 1 according to 1A it is a resonant chip. That is, similar to a Fabry-Perot element are in the resonance case at the ends of the light exit surface 6 and first layer sequence 2 formed resonator with the thickness D maxima with respect to the optical intensity I. Since outside the layer stack 4 the refractive index is significantly lower than inside, the wavelength is on the layer stack 4 opposite side of the light exit surface 6 increased accordingly. The effective resonator length L in this case is equal to the thickness D of the layer stack 4 and is N k _P / 2, where N is a natural number.

The semiconductor laser chip 1 according to 2A is also a resonant chip. The reflective first layer sequence 2 Here is a sequence of low refractive index layers 2a alternating with layers with high refractive index 2 B built up. The first layer sequence 2 So corresponds to a Bragg mirror, which is highly reflective for the primary radiation P. The layer stack 4 is in turn located in a resonator, the light exit surface 6 and first layer sequence 2 is formed. Because the first layer sequence 2 is constructed of a plurality of dielectric or semiconducting layers, the primary radiation P has a certain effective penetration depth in the first layer sequence 2 on. This penetration depth can be estimated as the wavelength A _P , _{0 of} the primary radiation in vacuum divided by four times the refractive index difference between low refractive index layers 2a and high refractive index layers 2 B , Is the refractive index of the layers 2a in about 3 and the refractive index of the layers 2 B in about 3.5, the refractive index difference is about 0.5. The effective penetration depth is thus approximately 0.5 λ _{P, 0} . Is the refractive index of the layer stack 4 at about 3.5, the effective penetration depth is about 1.75 λ _P. The effective resonator length L corresponds to the sum of the thickness D of the layer stack 4 and the effective penetration depth into the first layer sequence 2 , The resonator length L is thus in the case of a Bragg mirror as a reflective first layer sequence 2 greater than the thickness D of the layer stack 4 ,

In 2 B the schematic course of the optical intensity I is shown. Along the x-axis, the intensity increases toward the stack of layers 4 in the first layer sequence 2 approximately exponentially, within the layer stack 4 the intensity I is evenly distributed on average.

2C shows an alternative representation of the intensity profile. Here, the vertical dashed line indicates the effective penetration depth into the first layer sequence 2 , the vertical dotted line is the thickness D of the layer stack 4 , Since it is a resonant semiconductor laser chip 1 is located at the light exit surface 6 as well as at the fictitious point of the effective penetration into the first layer sequence 2 , which have the distance L from each other, each a maximum of intensity I. For ease of illustration, the effective penetration depth in the reflective first layer sequence 2 shortened drawn.

The layer stack 4 has three active areas 5 on. The active areas 5 are positioned at a distance T to each other, which is half of the Primary wavelength λ _P corresponds. All active areas 5 are in regions of maximum intensity I.

In 3A is a non-inventive modification of a non-resonant semiconductor laser chip 1 shown. The layer stack 4 has a thickness D or the semiconductor laser chip 1 an effective resonator length L on, so that the semiconductor laser chip 1 is anti-resonant.

In 3B is again shown schematically the course of the intensity I along the x-axis. The effective resonator length L is again determined by the thickness D of the layer stack 4 and the effective penetration depth into the first layer sequence 2 , The three active areas 5 in the layer stack 4 are in maxima of intensity I and λ _P / 2 are off to each other. The effective resonator length L is chosen such that at the light exit surface 6 a minimum of the optical intensity I is present, an intensity maximum at the fictitious point of the effective penetration depth. The effective resonator length L thus corresponds to an odd-numbered, integer multiple of the quarter-wavelength primary wavelength λ _P. The point of the effective penetration into the first layer sequence 2 is in 3B indicated by a vertical dashed line, the thickness D of the layer stack 4 through a vertical dotted line.

An embodiment according to the invention of the surface-emitting, non-resonant semiconductor laser chip 1 is in 4 illustrated. 26 pairs of low refractive index layers 2a and high refractive index layers 2 B form the first reflective layer sequence 2 , which is designed as a Bragg mirror. On a main page of the first layer sequence 2 is the layer stack 4 applied. The layer stack 4 has barrier layers 16 and four active areas 5 that follow each other alternately. The barrier stories 16 are designed to absorb light of pump radiation R having a wavelength λ _R. About the pump radiation R are in the barrier layers 16 free electron-hole pairs formed. Through energy relaxation, these electron-hole pairs become active in the quantum wells 5 captured. By recombination of the electron-hole pairs, the primary radiation P is finally generated. The layer stack 4 is at the first layer sequence 2 opposite side of the light exit surface 6 limited.

Optionally, the active areas 5 of tension compilation layers 15 be enclosed. About the tension compensation layers 15 can the crystal lattice between barrier layers 16 and active areas 5 be adapted to each other. Likewise, at the the layer stack 4 opposite side of the light exit surface 6 at least one partially reflective second layer 7 be upset.

The semiconductor laser chip 1 may be based on the gallium arsenide material system. As layers 2 B with high refractive index of the first layer sequence 2 then serve layers of GaAs, as layers 2a low refractive index AlAs or AlGaAs-95. AlGaAs-95 means that about 95% of the lattice sites of gallium atoms, with respect to pure GaAs, are replaced by aluminum atoms. The layer thickness of the layers 2a . 2 B each quarter of the primary wavelength λ _P. At an emission wavelength of about 1000 nm and a refractive index of about 4, the respective layer thicknesses of the layers are 2a . 2 B in the order of 60 nm. The total thickness of the Bragg mirror or the first layer sequence 2 then lies at about 5 microns. The first layer sequence 2 may be grown on a non-subscribed GaAs substrate.

The barrier stories 16 of the layer stack 4 are made of GaAs and designed to absorb pump radiation R with a wavelength λ _R of approximately 808 nm. The active areas 5 forming quantum wells are made of InGaAs having a thickness in the range of 6 nm to 12 nm. The strain compensation layers 15 have a thickness in the range of 15 nm to 30 nm and are made of GaAsP, for example. The thickness of the individual barrier layers 16 is in the embodiment according to 4 each about the same size, wherein the layer thicknesses of the first layer sequence 2 seen from first and last barrier layer 16 may have different thicknesses, in particular those at the light exit surface 6 lying barrier layer 16 has a thickness such that the semiconductor laser chip 1 , as in accordance with 3 , is anti-resonant and the active area 5 each lie in maxima of the optical intensity I. The manufacturing tolerances with respect to the individual layers are preferably below 10 nm, in particular at approximately 5 nm. This corresponds approximately to λ _P / 60.

Alternatively, the barrier layers 16 towards the first layer sequence 2 have increasing thicknesses such that the distances T between individual, adjacent active areas 5 towards the first layer sequence 2 are also enlarged. The active areas 5 can also be arranged in groups. Within a group, the active areas 5 have the same distances from each other. Toward the first layer sequence 2 In addition, the distances between individual, neighboring groups then increase preferentially. By increasing the distances between adjacent active areas 5 or groups in the direction of the first layer sequence 2 can be compensated that the pump radiation R reinforced in the barrier layers 16 near the light exit surface 6 is absorbed, and the intensity of the pump radiation R in the direction of the first layer sequence 2 towards decreasing exponentially. To compensate for this effect and, therefore, in each of the barrier layers 16 In each case an approximately equal number of electron-hole pairs is generated, the thickness of the barrier layers increases 16 towards the first layer sequence 2 exponential. In order to ensure complete absorption of the pump radiation R, the entire stack of layers has 4 a thickness D corresponding to a geometric thickness of the order of 2.5 μm.

As second layer 7 Si ₃ N _{4 can} serve. The thickness of the second layer 7 as an antireflective coating 17 The refractive index of Si ₃ N _{4 is} in the range of 2.0, compared to a refractive index of approximately 3.5 of the layer stack 4 , is a high extraction efficiency from the semiconductor laser chip 1 guaranteed.

In the 5A . 5B schematically are two resonant semiconductor laser chips 1 illustrated. It is the refractive index n, see left ordinate axis, and the intensity I plotted, see right axis of ordinate, each along a direction x perpendicular to the light exit surface 6 , The horizontal arrows indicate which curve refers to which ordinate axis.

At the light exit surface 6 Both have semiconductor laser chips 1 in each case a maximum of the intensity I of the optical power during operation of the semiconductor laser chip 1 on. Due to the resonant structure, the intensity I is within the semiconductor laser chip 1 , especially within the layer stack 4 , comparatively high. Toward the layer stack 4 away the intensity I falls within the configured as Bragg mirror first layer sequence 2 exponentially.

According to 5A are the active areas 5 , plotted as regions of increased refractive index, in maxima of the standing wave with respect to intensity I. The active areas 5 thus represent a resonant periodic gain structure, also referred to as Resonant Periodic Gain or RPG.

According to 5B is the distance between individual adjacent active areas 5 near the light exit surface 6 first constant and then take in the direction of the first layer sequence 2 towards approximately exponential. Furthermore, a second layer 7 in the form of an antireflective coating 17 in the embodiment according to 5B available. The anti-reflective coating 17 has a refractive index of about 1.6.

In the 6A . 6B . 6C is the dependence of the intensity I within the layer stack 4 illustrated by the resonance condition, the type of representation follows the 5A . 5B , In 6A is a resonant semiconductor laser chip 1 shown. According to 6B the wavelength λ _{P of} the primary radiation P is shifted by 6 nm with respect to a resonance wavelength λ _Res . The intensity I of the light field within the semiconductor laser chip 1 This reduces by about a factor 2 , In a further shift of the wavelength by 5 nm away from the resonance, ie a wavelength shift of 11 nm compared to the resonance case, shown in 6C , almost halves the optical intensity I compared to 6B , Compared to a resonant semiconductor laser chip 1 is the optical intensity I within the semiconductor laser chip 1 So in a non-resonant semiconductor chip 1 significantly reduced.

The in the 6A . 6B . 6C illustrated semiconductor layer stack 4 can optionally according to 5B be designed.

Occurring optical losses and loss mechanisms are in 7 illustrated. This is, outside the operation of the semiconductor laser chip 1 whose reflectance is measured. A radiation P _In with the wavelength λ _{P of} the primary radiation P is perpendicular to a semiconductor laser chip 1 blasted. Much of the light P _In is from the semiconductor laser chip 1 reflected. This reflected light is designated P _Refl . Part of the radiation P _In , which is in the semiconductor laser chip 1 penetrates, is in the layer stack 4 absorbed, designated as P _Abs . The absorption occurs, for example, at active areas 5 , impurities in the layer stack 4 or disturbances of the crystal lattice. Part of the radiation is also transmitted through the Bragg mirror, which is the reflective first layer sequence 2 forms, transmits. This radiation component is marked P _Trans . In particular, the radiation components P _Trans and, at least partially, P _Abs contribute to the optical power loss. If P _{In is} known and P _{Refl is} measured, the sum of P _Abs and P _Trans can be determined. The quotient P _Refl / P _In is a reflectivity R of the semiconductor laser _chip 1 ,

The losses due to the absorption in the layer stack 4 are approximately proportional to the optical intensity I in the layer stack 4 , Therefore, these losses are reduced, in which also the optical intensity I in the layer stack 4 is reduced, as by an anti-resonant structure of the semiconductor laser chip 1 happens.

Another loss mechanism based on the Fabry-Perot effect and concerning the transmitted light power P _Trans is in 8th illustrated. In 8A is the reflectivity p of the semiconductor laser chip 1 , with unpumped semiconductor laser chip 1 . as a function of the wavelength λ. The reflectivity p has values of more than 98% in a spectral range from approximately 1010 nm to approximately 1090 nm. This spectral range corresponds to the spectral reflection width of the first layer sequence 2 , In the center of this range at about 1050 nm, corresponding to the primary wavelength λ _P , there is a minimum at which the reflectivity p drops well below 99%. This minimum is due to the Fabry-Perot effect in the resonant case where the effective resonator length L is an integer multiple of λ _P / 2.

A corresponding representation in the case of an anti-resonant semiconductor laser chip 1 is in 8C illustrated. To ensure comparability are in 8A and 8C the optical losses due to absorption in the layer stack 4 each same size. In the range around 1050 nm there is a spectrally broad maximum of the reflectivity p. The reflectivity p here has values of well over 99% and is thus significantly higher at this wavelength than in the case of a resonant semiconductor laser chip 1 according to 8A , That is, the optical losses due to the Fabry-Perot effect are reduced from about 1.2% in the resonant case to about 0.6% in the anti-resonant case. These optical losses are thus approximately halved.

In 8B is the reflectivity p with respect to the wavelength λ in the operation of the semiconductor laser chip 1 drawn. In the area of the resonant wavelength λ _Res of 1050 nm, a clear maximum is shown. Due to the reinforcement in the layer stack 4 the reflectivity p appears higher than 1 and is about 1.03 or 103%.

In the anti-resonant case, as in 8D can be seen, the maximum of the reflectivity p in the case of a gain is lower, at about 1.016, and spectrally much wider.

Thus, both the gain and the optical losses are reduced in the anti-resonant case. In the anti-resonant case, the reduced gain suffices for the operation of the semiconductor laser chip 1 Thus, the anti-resonant configuration is more efficient in terms of efficiency due to the reduced optical losses. The parameter reflectivity p and thus the optical losses are in particular in laser arrangements or semiconductor laser chips 1 particularly relevant, which have an external resonator, in which only a small light output is coupled out per revolution. In other words, in such an external resonator, the light often passes through the external resonator, often on the semiconductor laser chip 1 reflects and in particular the losses due to transmission through the first layer sequence 2 through come to fruition. Such an external resonator with a low decoupling rate is used, for example, to frequency-double near-infrared radiation to ensure a high intensity within the external resonator and thus to increase the power of doubled light, since the doubling is an optically non-linear process.

Due to absorption of the primary radiation P in the layer stack 4 and due to absorption of the shorter wavelength pump radiation R, the semiconductor laser chip heats up 1 in the regions in which the pump radiation R is absorbed and the primary radiation P is generated. As a result, a temperature distribution is formed in the semiconductor laser chip 1 out. The temperature inside the pumped semiconductor region is highest here, in the edge regions the temperature decreases. Due to this temperature gradient, a thermal lens is formed, since the refractive index of the semiconductor material depends on the temperature. This means that in the center of the pumped area the refractive index is greater than at the edge areas. This corresponds to a condenser lens. Since the absorption of primary radiation P is approximately proportional to the optical intensity I of the primary radiation P in the semiconductor laser chip 1 is the thermal lens is more pronounced, the higher the optical intensity I in the semiconductor laser chip 1 is.

This is in 9A illustrated. In 9A is the radial position r plotted in microns against the phase noise q in rad. The in the semiconductor laser chip 1 Deposited thermal power is according to 9A Assuming 1 W, the diameter of the pumping beam R is approximately 100 microns. The curve marked M corresponds to a spherical mirror with a radius of curvature of 10 mm. This spherical mirror serves as a reference. In the case of a resonant wavelength at 1054 nm corresponds to the expression of the thermal lens at radial positions r up to about 50 microns in a good approximation of such a mirror. Spectrally distant from a resonant wavelength, for example, at 1060 nm and 1065 nm, the effect of the thermal lens, recognizable by the flatter course of the corresponding curves, decreases sharply.

In 9B the reflectivity R is plotted against the wavelength λ. In the 9A shown wavelengths are in 9B symbolized by vertical arrows. The shape of the reflectivity curve is similar to that in 8B shown.

The expression of the thermal lens is also dependent on how strong the light exit surface is reflective for the primary radiation P, as in 10 explained in more detail. In the 10A to 10D in each case the wavelength λ is plotted against a maximum temperature t occurring in a pumping region. Grayscale coded and with Profile lines are marked, the respective focal lengths of the thermal lens registered. The smaller the value for the focal length, the stronger the thermal lens is pronounced. For all four 10A to 10D It can be seen that the thermal lens is more pronounced at a certain wavelength λ with increasing temperature t.

According to 10A has the light exit surface 6 no reflectivity, so the reflectivity is exactly 0%. Due to this, the expression of the thermal lens is almost independent of the wavelength λ, since the semiconductor laser chip 1 in the case of a vanishing reflectivity at the light exit surface 6 has no internal resonator, and thus no resonance or anti-resonance condition can be set. For temperatures t in the range of 360 K, the focal length of the thermal lens is approximately 45 mm.

According to 10B has the light exit surface 6 a reflectivity of 2%. This corresponds to a reflectivity, which is a semiconductor laser chip 1 with an optimized, single-layer antireflective coating 17 having. Because of the finite reflectivity of the light exit surface 6 together with the first layer sequence 2 a resonator-like structure is formed, depending on the wavelength λ, also resonance and anti-resonance conditions are met. The resonance condition in which the effective resonator length L of the light exit surface 6 and first layer sequence 2 formed internal resonator of the semiconductor laser chip 1 a multiple of half the wavelength of the primary wavelength λ _P , the thickness of the thermal lens has a maximum. That is, at already a relatively low temperature t of about 340 K, a focal length of 50 mm is achieved. In the anti-resonant case, at about 1030 nm and 1080 nm, this thickness of the thermal lens is reached only at about 375 K.

The wave-shaped course of the thickness of the thermal lens essentially follows the intensity profile as a function of the wavelength λ in the semiconductor laser chip 1 , This illustrates once again that the intensity I in the semiconductor laser chip 1 highest in the resonant case and lowest in the anti-resonant case.

In 10C is the reflectivity of the light exit surface 6 about 5%. The with respect to the wavelength λ oscillating expression of the thermal lens is therefore enhanced. Due to the higher reflectivity of the light exit surface 6 In the anti-resonant case, the intensity I in the semiconductor chip is reduced and increased in the resonant case. Therefore, the effect of the thermal lens compared to 10B reduced at the anti-resonant wavelengths.

According to 10D corresponds to the reflectivity of the light exit surface 30%. This corresponds to a semiconductor laser chip 1 who does not have second layer 7 having. Here the oscillating course of the effect of the thermal lens is most pronounced. In the anti-resonant case, the optical intensity I is in the semiconductor laser chip 1 towards lower reflectivities of the light exit surface 6 further reduced. The shift of the sinusoidal wave-like structure towards a sawtooth-like structure is also due to a temperature effect. This shift with increasing temperature is related to the refractive index, which likewise increases with increasing temperature, as a result of which the resonances and anti-resonances shift to longer wavelengths λ.

In 11 is a laser arrangement 100 with a surface emitting semiconductor laser chip 1 shown. The semiconductor laser chip 1 is on a substrate 10 applied. About the substrate 10 For example, a heat sink and a mechanically stable mounting surface can be realized. The semiconductor laser chip 1 is optically from a pump light source 13 pumped out, which emits the pump radiation R, symbolized by a thin arrow line. The wavelength λ _{R of} the pump radiation is approximately 808 nm. The primary radiation P, symbolized by a thick arrow line, is from the semiconductor laser chip 1 emitted. The emission of the primary radiation P takes place approximately perpendicular to the light exit surface 6 of the semiconductor laser chip 1 , The coupling of the pump radiation R takes place at an angle of approximately 45 °. With respect to the primary radiation P, the semiconductor laser chip 1 as well as two mirrors 8a . 8b an external resonator of the laser array 100 dar. Both mirrors 8a . 8b are highly reflective of the primary radiation P. From the first mirror 8a the primary radiation P is in the direction of a doubling crystal 12 forwarded, is in the doubling crystal 12 partly into a secondary radiation S with a secondary wavelength λ _S ≈ λ _P / 2, symbolized by an arrow-dashed line, converted. Subsequently, both converted S and primary radiation P strike the second mirror 8b and become back towards the first mirror 8a reflected. On the way back, another part of the primary radiation P is converted into the secondary radiation S. The first mirror 8a is transmissive to the secondary radiation S, so that it is coupled out of the external resonator.

Optionally, the external resonator of the laser arrangement 100 a wavelength-selective element 11 , which is designed for example as etalon. About the wavelength-selective element 11 it is also possible that the polarization of the primary radiation P is set.

In 12 are the optical power of the primary radiation P, the emission wavelength λ _Em and the optical losses O _L in the semiconductor laser chip 1 as a function of the power of the pump radiation R for different reflectivities of the light exit surface 6 drawn. The respective curves, designated with the respective reflectivity at the light exit surface 6 in%, are simulations for the case of a laser array 100 , in addition to the surface-emitting semiconductor laser chip 1 an external mirror 8th having a reflectivity of 99% and corresponding to 1% of the primary radiation P from the laser array 100 couples out. The simulated laser arrangement 100 has no doubling crystal and no wavelength-selective element. The semiconductor laser chip 1 is an anti-resonant chip. The individual curves are each with the percentage of the reflectivity of the light exit surface 6 characterized.

According to 12A is compared to the power of the pump radiation R from the laser arrangement 100 decoupled power of the primary radiation P applied. With a high reflectivity of the light exit surface 6 the laser threshold is exceeded only at higher powers of the pump radiation R, so that the individual curves with increasing reflectivity of the light exit surface 6 only start at higher pump powers. The slope of the curves, up to a critical maximum value of the power of the pump radiation R, a maximum value at a reflectivity of the light exit surface 6 from about 10% up. Thus, at least for pump powers in the range greater than about 0.8 W, a reflectivity of about 10% is a preferred value.

In 12B is compared to the power of the pump radiation R, the emission wavelength λ _Em plotted. The respective curves vary by a few nanometers. At a reflectivity of the light exit surface 6 of 30%, the emission wavelength λ _Em deviates significantly.

In 12C are, in percent, the optical losses O _L in the semiconductor laser chip 1 compared with the power of the pump radiation R applied. At a reflectivity of the light exit surface 6 of about 10%, the optical losses are lowest. A reflectivity of the light exit surface of about 10% is also a preferred value with respect to this aspect.

With a reflectivity according to the invention in the range of approximately 10%, therefore, there is an efficient coordination between the optical losses and the optical intensity in the semiconductor laser chip 1 , and thus the reinforcement, given.

Is the reflectivity of the light exit surface 6 relatively high, for example, 30%, so there is a possibility that the semiconductor laser chip 6 , without additional measures, automatically switches from an anti-resonant wavelength to a resonant wavelength, see 12B , This possibility is given in particular, since the amplification bandwidth of the amplification medium or of the active regions 5 and the spectral width of the reflectivity of the first layer sequence 2 can be up to 80 nm. In this approximately 80 nm wide spectral range, in principle, a laser action is possible.

If, for example, the effective resonator length L is 10.25 λ _P in the anti-resonant case, this could correspond, for example, to 10 λ _{Res of} the resonant wavelength λ _Res . The wavelength difference between λ _P and λ _Res , would then, at a wavelength λ _P of approximately 1050 nm, only about 30 nm. In other words, both the anti-resonant wavelength λ _P and the resonant wavelength λ _Res in the gain range with respect to laser activity. The jumping from an anti-resonant to a resonant wavelength is favored in particular by the fact that the amplification is higher in the resonant case than in the anti-resonant case.

In order to prevent such jumping of the wavelength at which the laser action takes place, as in 11 shown in the external cavity of the laser array 100 a wavelength-selective element 11 be introduced. This wavelength-selective element 11 may be transmissive for an anti-resonant wavelength λ _P and, at the same time, for resonant wavelengths λ _Res that are in the gain region of the layer stack 4 lie, have a blocking effect. In this case, "blocking" means that the transmission for resonant wavelengths λ _Res is a few percent below the transmission for anti-resonant wavelengths λ _P. For example, the transmission with respect to the wavelength λ _{P is} more than 99.9%, and the transmission with respect to the wavelength λ _{Res is} less than 96%. By such an arrangement, jumping from an anti-resonant wavelength λ _P to a resonant wavelength λ _{Res is} effectively suppressed.

The effect of a wavelength-selective element 11 is in 13 illustrated. In 13A is the pump power of the pump radiation R compared to the power of the primary radiation P of a laser arrangement 100 , for example as in 12 shown shown. In the case of a free-running laser of an anti-resonant arrangement, the semiconductor laser chip falls 1 in operation automatically to a resonant wavelength λ _Res of about 1032 nm, see 13B , As a result, the efficiency is significantly reduced, see 13A , In case of an external resonator of the laser arrangement 100 located wavelength-selective element 11 , shown in the curve marked E, this effect does not occur and the power of the primary radiation P is significantly higher at powers of the pump radiation R above about 0.5 W.

In 13B is the wavelength stabilizing property of the wavelength-selective element 11 shown. The primary wavelength λ _P remains the same over the entire range of the pump power of the pump radiation R.

This will be shown in 13C , also the percent optical losses O _L compared to a free-running operation without a wavelength-selective element 11 significantly reduced.

In 14 are, in percent, the optical losses O _{L of} a resonant semiconductor laser chip and an anti-resonant semiconductor laser chip 1 compared to each other. The curve with respect to the resonant semiconductor laser chip is marked "Res". Both semiconductor laser chips 1 have a resonant periodic amplifier structure, RPG for short, with six quantum wells. At a pump power of, for example, 0.5 W, the optical losses in the resonant chip at about 0.3%, the anti-resonant semiconductor laser chip 1 only at 0.16%. The optical losses O _L are thus almost halved due to the anti-resonant design of the semiconductor laser chip 1 whose efficiency is significantly increased.

In 15 are simulated performance data shown in 15B , with measured data, shown in 15A , compared. Simulation and calculation are in good agreement. Opposite a resonant semiconductor laser chip has the anti-resonant semiconductor chip 1 an improved by about 25 to 30% performance of the primary radiation P at a pump power of about 1.4 W. The measured values were taken with an external resonator with a length of 25 mm and an external mirror 8th determined with a reflectivity of 99.7%.

Due to the reduced intensity in the semiconductor laser chip 1 and because of the avoidance of Fabry-Perot losses in the anti-resonant case, so is the efficiency of an anti-resonant semiconductor laser chip 1 significantly increased compared to a resonant semiconductor laser chip.

In 16 FIG. 3 is a schematic of a fabrication process for a non-resonant surface emitting semiconductor laser chip 1 shown. According to 16A includes the layer stack 4 three active areas 5 and an etch stop layer 18 , The effective resonator length L corresponds to an integer multiple of half the primary wavelength λ _P.

In 16B is shown that the irradiation of the pump radiation R of the semiconductor laser chip 1 Is tested. This test allows early in the manufacturing process to determine if the semiconductor laser chip 1 is a functional component. This reduces rejects in later process steps and reduces manufacturing costs.

In the process step according to 16C is etched through an etch process to an etch stop layer 18 reduces the effective resonator length L until it is an anti-resonant semiconductor laser chip. The thickness around which the layer stack 4 is reduced, is an odd multiple of λ _P / 4.

According to 16D is then according to the invention a second layer 7 in the form of an antireflection coating 17 applied, over the targeted the reflectivity of the light exit surface 6 of the layer stack 4 is set.

Claims (11)

A surface-emitting semiconductor laser chip (1) having - a first layer sequence (2) which is reflective for a primary radiation (P) having a primary wavelength λ _P , and - a layer stack (4) having at least one active region (5), the layer stack (4 ) is applied to a main side of the first layer sequence (2) and configured to emit the primary radiation (P), wherein a thickness (D) of the layer stack (4) is made non-resonant with respect to the primary wavelength λ _P such that an intensity of an electromagnetic field within the semiconductor laser chip (1) corresponds to that of a Fabry-Perot element with minimal transmission, - for an effective resonator length L of the semiconductor laser chip (1) with a tolerance of at most k _P / 8: L = N λ _P / 2 + k _P / 4 and N is a natural number, - the semiconductor laser chip (1) further comprises at least one second layer (7), which at the light exit surface (6) of the layer stack (4) and for the primary radiation (P) is partially reflecting, characterized in that a reflectance of the at least one second layer (7) for the primary radiation (P) is between 7% and 13%. Surface emitting semiconductor laser chip (1) according to Claim 1 in which the tolerance for the effective resonator length L is at most λ _P / 16. Surface-emitting semiconductor laser chip (1) according to one of the preceding claims, in which there is no intensity maximum of the primary radiation (P) at a light exit surface (6) of the layer stack (4) facing away from the first layer sequence (2) during operation of the semiconductor laser chip (1) , A surface-emitting semiconductor laser chip (1) according to one of the preceding claims, wherein at least one distance (T) between two adjacent active regions (5) is a multiple of half the primary wavelength λ _P and at least one active region (5) during operation of the Semiconductor laser chips (1) in an intensity maximum of the primary radiation (P) is located, and increase in at least one case, the distances (T) between adjacent active areas (5) in the direction of the first layer sequence (2). Laser arrangement (100) with at least one surface-emitting semiconductor laser chip (1) according to one of the preceding claims, - At least one pump light source (13) for pumping the semiconductor laser chip (1), and - With at least one external, for the primary radiation (P) reflecting mirror (8), so that of the first layer sequence (2) and of the at least one reflective mirror (8), a resonator (9) is formed. Laser arrangement (100) according to Claim 5 in which the resonator (9) comprises a wavelength-selective element (11). Laser arrangement (100) according to Claim 6 in which the wavelength-selective element (11) in a wavelength range of 0.8 λ _P to 1.2 λ _P acts blocking for wavelengths λ _Res, for those with a wavelength tolerance of at most λ _P / 16 with respect to the effective resonator length L of the semiconductor laser chip ( 1) applies: $${\mathrm{\lambda }}_{\text{Res}}=\left(2\text{L}\right)/\text{N}\text{.}$$

where N is a natural number. Method by which a surface-emitting semiconductor laser chip (1) according to one of the Claims 1 to 4 providing, comprising the steps: - providing a semiconductor laser chip (1) with the first layer sequence (2) and the layer stack (4), wherein the effective resonator length L of the semiconductor laser chip (1) with an allowance of at most λ _P / 16 an integral multiple corresponds to half of the primary wavelength λ _P , and - reducing the effective resonator length (L) of the semiconductor laser chip (1) with a tolerance of at most λ _P / 16 by a value of ((2 N - 1) λ _P ) / 4, where N a natural number is. Method according to Claim 8 wherein before the step of reducing the effective resonator length L, the function of the semiconductor laser chip (1) at the primary wavelength λ _{P is} tested. Method according to Claim 8 or 9 wherein the step of reducing the effective resonator length L is performed using at least one etch stop layer. Method according to one of Claims 8 to 10 wherein the step of reducing the effective resonator length L is via wet chemical etching.

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