Laser amplifier system and laser arrangement

Abstract

Laser amplifier system, including: a waveguide area (A) with a waveguide (10) for guiding laser radiation; an amplifier section (B) with an amplifier (20), wherein the amplifier section (B) is configured such that the laser radiation from a first end (A1) of the waveguide (10) is coupled into an input side (B1) of the amplifier (20); and an intermediate region (C) between the first end (A1) of the waveguide (10) and the input side (B1) of the amplifier (20), wherein the intermediate region (C) is configured to provide a variable thermal lens (30) for controllable adaptation of the laser radiation coupled into the amplifier (20); wherein the provision of the variable thermal lens (30) is effected by optical heating of the intermediate area (C) or the intermediate area (C) for the provision of the variable thermal lens (30) by electrical heating of the intermediate area (C) comprises an ohmic resistance element (32) applied to the intermediate area (C) as a means for local heating of the intermediate area (C).

Description

[0001] The present invention relates to a laser amplifier system and a laser arrangement, in particular a beam-controlled laser amplifier system and a laser arrangement comprising the laser amplifier system according to the invention. State of the art

[0002] Laser amplifiers are used to amplify laser radiation coupled into them. These amplifiers can be designed as individual elements or as integrated components of a laser. Semiconductor-integrated tapered amplifiers, in particular, are a common type of laser amplifier. These amplifiers can be combined with semiconductor lasers as individual elements or integrated into a common semiconductor system. The amplifiers are designed as active regions within which the energy of incident laser radiation can be increased many times over. Tapered amplifiers are characterized by a lateral widening from a narrow input side to a wider output side, with this widening being freely structured via appropriate boundary functions.A common embodiment in the prior art is a trapezoidal amplifier with a trapezoidal edge region. However, the term "trapezoid" is sometimes interpreted broadly, so that it can also include fan-shaped amplifiers with a widening that deviates from a geometric trapezoidal structure (or, more generally, amplifiers with a variable lateral cross-section). Although the present disclosure relates specifically to fan-shaped or trapezoidal amplifiers, it is expressly not limited to such types of amplifiers.

[0003] These trapezoidal amplifiers are used, for example, as amplifiers in trapezoidal lasers. However, using such amplifiers is challenging because their spatial-optical properties (especially the position of the lateral beam waist) depend directly on the set operating point (see, e.g., Fiebig, C. et al., 12W high-brightness single-frequency DBR tapered diode laser, Electron. Lett. 44, 1253-1255 (2008)). The operating point is defined by the set electrical current flow through the amplifier. For different operating points, the lenses used to process the radiation must be repositioned, which can be quite labor-intensive depending on the application. When these lasers are used in micromodules, this is generally not possible at all due to the lenses typically being permanently bonded in place.This disadvantage of known trapezoidal lasers has so far been circumvented by a precisely defined operating point. However, this also largely determines the emitted output power during operation, preventing any meaningful modification.

[0004] US 5,539,571 A relates to a differentially pumped optical semiconductor amplifier arrangement with a wide-strip emitter and a main oscillator power amplifier (MOPA) that uses such an amplifier arrangement. US 2018 / 0205200 A1 relates to a laser device and, in particular, to a tunable laser diode with a distributed Bragg reflector in which a wavelength of laser light is electrically and thermally tunable. Disclosure of the invention

[0005] It is therefore an object of the present invention to provide a laser amplifier system and a laser arrangement with which the disadvantages of the prior art can be overcome or at least significantly reduced, and in which the spatial-optical properties no longer depend on the operating point. In particular, a beam-optimized laser amplifier system and a laser arrangement comprising the laser amplifier system according to the invention are to be provided for this purpose.

[0006] These problems are solved according to the invention by the features of independent claims 1 and 7. Advantageous embodiments of the invention are contained in the dependent claims. The features listed individually in the claims can be combined with one another in a technologically meaningful way and can be supplemented by explanatory details from the description and / or details from the figures, thereby showing further advantageous embodiments of the invention.

[0007] A first aspect of the present invention relates to a laser amplifier system comprising a waveguide region with a waveguide for guiding laser radiation; an amplifier region with an amplifier, wherein the amplifier region is configured such that the laser radiation from a first end of the waveguide is coupled into an input side of the amplifier;and an intermediate region between the first end of the waveguide and the input side of the amplifier, wherein the intermediate region is configured to provide a variable thermal lens for controllable adaptation of the laser radiation coupled into the amplifier, wherein the provision of the variable thermal lens is effected by optical heating of the intermediate region or the intermediate region for providing the variable thermal lens by electrical heating of the intermediate region comprises an ohmic resistance element applied to the intermediate region as a means for local heating of the intermediate region.

[0008] For the purposes of this disclosure, an amplifier is understood to be any form of active region for laser amplification. This can be, in particular, a general fanning amplifier and, more specifically, a ("true") trapezoidal amplifier with a trapezoidal edge region. However, the present invention is not limited to trapezoidal amplifiers and can be used with a variety of different amplifier types.

[0009] Waveguides and associated waveguide sections are well known in the art and are used in a variety of ways for wave guidance (radiation guidance). The waveguides can be designed as structures with (active) or without (passive) amplification of the guided waves. Amplifiers downstream of the waveguides for amplifying the waves are also well known to those skilled in the art (e.g., from trapezoidal lasers). For further details, please refer to the relevant technical literature.

[0010] A laser amplifier system according to the invention further comprises an intermediate region between the first end of the waveguide and the input side of the amplifier, wherein the intermediate region is configured to provide a variable thermal lens for controllable adaptation of the laser radiation coupled into the amplifier. This means that after the extraction of guided radiation from the waveguide and before the coupling of the laser radiation into the amplifier region, it must be transmitted through the intermediate region, and its propagation can be influenced by a thermal lens that can be formed in the intermediate region. The design and function of thermal lenses are also well known in the prior art and are physically fully understood.

[0011] The present invention relates to the application of such a thermal lens for coupling laser radiation from a waveguide into an amplifier, and in particular into a fanning amplifier or a trapezoidal amplifier. By means of a thermal lens configuration in the intermediate region adapted to the operating point of the amplifier, the spatial-optical properties of the laser radiation can be controlled. The object of the invention is thus achieved precisely by providing an intermediate region with a controllable thermal lens, via which the spatial-optical properties of the laser radiation can be adapted to the respective operating point of the amplifier.

[0012] Preferably, a laser amplifier system according to the invention further comprises a substrate and a semiconductor layer structure arranged on the substrate, wherein the waveguide in the waveguide region is designed as a finned waveguide in the semiconductor layer structure, the amplifier region is designed as an active region with an active layer within the semiconductor layer structure and an overlying metallic contact layer, and the intermediate region comprises a means for locally heating the intermediate region. The active layer can extend over the entire semiconductor layer structure or be limited to the active regions. The described structure of the amplifier region corresponds to the usual structure of, for example, trapezoidal amplifiers in the prior art. The substrate can also be removed after the semiconductor layer structure has been created and is therefore not essential to the invention.

[0013] To form the thermal lens in the intermediate region, a means for locally heating this region is provided. Local heating of the semiconductor layer structure causes a change in the refractive index in this region via the thermo-optic effect. The spatial refractive index profile of the forming thermal lens is then directly determined by the gradient of the resulting heat flow. By precisely designing the means for local heating and controlling it accordingly (see thermal engineering), the exact shape of the thermal lens can be defined or varied.

[0014] Preferably, the variable thermal lens is provided by optical heating of the intermediate region. In particular, the heating can be achieved by local application of laser radiation with a defined beam profile at a specified power. The beam profile and / or the power can be varied to adapt the variable thermal lens. Preferably, the spectra of the laser radiation used to heat the intermediate region and the laser light to be amplified in the amplifier are separated and essentially do not overlap.

[0015] Preferably, the variable thermal lens is provided by electrically heating the intermediate region. In such an embodiment, a defined input of thermal energy occurs via a current flow in the intermediate region. To adapt the variable thermal lens, the current path and / or the current intensity can be changed. The current path can be freely defined within the intermediate region; in particular, it can also be a current path within the semiconductor layer structure.

[0016] For electrical heating of the intermediate region, a resistive element is applied to it. This is therefore a resistance heater used for the local heating of the intermediate region. Resistive heaters are common in laser technology and are used, for example, for phase matching of waveguide sections by thermal length changes or for stabilizing spectral filter elements. By specifically designing the resistive element, the exact shape of the thermal lens can be defined or varied when controlled by an applied voltage. The resistive element can, for example, be designed as an extended surface heating element using a conductor structure applied to the intermediate region.

[0017] Preferably, the resistive element is designed as a continuous conductor strip with contacts at its ends. It is therefore a linear resistance heater (also known as a line heating element or heating strip) that provides a strip-shaped heat flow in the intermediate area. The continuous conductor strip can be straight or curved.

[0018] Preferably, the continuous conductor strip has a straight section (straight line heating element) that is shaped to match the direction of propagation of the laser radiation in the intermediate region. It is also preferred that the continuous conductor strip has a meandering shape at least in one section along the direction of propagation of the laser radiation in the intermediate region, or that the continuous conductor strip has a freeform shape. With a meandering configuration, the continuous conductor strip can be used to heat a relatively large surface area of ​​the intermediate region. Simply widening a linear continuous conductor strip to increase the area coverage would lead to a reduction in the ohmic resistance of the conductor strip and thus to reduced heat input or higher operating voltages required to achieve the necessary heating current.The use of a general freeform gradient enables not only high area coverage but also a particularly specific design of the heat input into the semiconductor layer structure and thus of the resulting thermal lens.

[0019] Preferably, the waveguide is configured as a passive waveguide or at least in one section as an active waveguide. In particular, the waveguide can also be configured as a waveguide laser for the direct generation and provision of the laser radiation to be coupled into the amplifier. A waveguide configured as a passive waveguide, on the other hand, serves solely for wave guidance, so that the corresponding laser radiation must be provided externally and coupled into the waveguide accordingly.

[0020] Preferably, a laser amplifier system according to the invention further comprises a means for controlling the variable thermal lens. The means for controlling can, in particular, be a device for controlling a current flow, an incident optical radiation, or another device for influencing the heat input into the intermediate region with regard to, for example, its shape, distribution, intensity, and direction of variation. The means for controlling can also include a logic circuit for selecting a heat input optimally matched to the selected operating point of the amplifier and thus an optimal thermal lens for correcting corresponding deviations in the spatial-optical properties of external optics or systems.

[0021] An amplifier system according to the invention can preferably be used with amplifier types having a structure that varies laterally in width along the optical axis, in particular widening or broadening in the direction of the facet.

[0022] A second aspect of the present invention relates to a laser arrangement comprising a laser amplifier system according to the invention; and a laser radiation source, wherein the laser radiation emitted by the laser radiation source is coupled into a second end of the waveguide for amplification in the laser amplifier. A laser arrangement according to the invention thus consists of a laser radiation source coupled to the amplifier (e.g., a trapezoidal amplifier) ​​by means of said waveguide.

[0023] A laser amplifier system according to the invention thus comprises at least three regions: a waveguide region, an intermediate region, and an amplifier region with a variable lateral cross-section (e.g., a trapezoidal amplifier region with a trapezoidal amplifier). The amplifier can be part of a complete laser in which further technical elements (e.g., Bragg reflectors or facet coatings) are integrated. The novel intermediate region according to the invention is used as a refraction element with controllable refractive power. This region is preferably integrated monolithically into the laser amplifier system according to the invention.

[0024] Preferably, a metallic heating strip can be formed in this area, which can be externally electrically contacted via metallic conductors and bond pads. In a preferred embodiment, the heating strip can be designed as a simple straight metal strip. The heating strip itself can have a precisely defined width and length. Additionally, the position of the heating strip on the intermediate area should be precisely defined, depending on the shape of the amplifier. However, different strip layouts can also be addressed by means of integrated switching elements. The heating strip can be electrically isolated from the amplifier and operated independently via the control means.

[0025] The heating strip should be designed and controlled such that a temperature gradient lateral to the preferred longitudinal propagation direction of the amplified laser radiation is generated. This lateral temperature gradient advantageously creates a thermo-optic lensing effect on the laser radiation propagating longitudinally in the intermediate region. The refractive power of the thermal lens can be pre-controlled or adjusted by the longitudinal extension of the heating element. The direction of the electrical current (longitudinal or lateral) is not essential for the formation of the thermal lens itself. However, the longitudinal direction is preferred due to a potentially larger interaction distance.

[0026] The effect of the heating element can be controlled by the applied electrical voltage. The heating element's effect is preferably adjusted such that the spatial characteristics of the output radiation downstream of the amplifier remain as unchanged as possible, even with a different operating point. This applies particularly to the longitudinal position of the lateral beam waist. With a constant collimation optic in the beam path downstream of the amplifier, rectification or collimation of the radiation can thus be achieved for different operating points.

[0027] In an exemplary embodiment, the amplifier system according to the invention can be implemented in a III-V semiconductor system (GaAs). For this purpose, a layered system can first be built up on an n-doped GaAs substrate, which enables vertical waveguiding and simultaneously amplifies laser radiation guided within it. The active zone (the zone in which charge carriers are spatially and energetically bound) can be designed as a single or multiple quantum well. The material composition of the quantum well determines the emission wavelength, whereby wavelengths from approximately 626 nm to approximately 1180 nm can be generated with different material systems (e.g., GaInP). For an emission wavelength of approximately 1120 nm, InGaAs can be used as the optically active material. Optical powers of over 10 W have already been demonstrated from an amplifier system according to the invention for the specified wavelength.However, the achievable optical performance depends on the material system used and can vary accordingly within the specified wavelength range.

[0028] The present invention utilizes a local change in refractive index to adjust the spatial-optical properties. The location of the targeted refractive index change influences the efficiency of the amplifier system. With favorable positioning and design of the corresponding local heating means, a high optical effect can be achieved with low energy expenditure. Positioning a local heating means in the intermediate region directly in front of an amplifier section with a variable cross-section enables a particularly high efficiency of the invention. Thus, with minimal heat input, the laser radiation can be strongly influenced at this location.

[0029] The amplifier's operating point can thus be dynamically adjusted during operation without altering the spatial beam characteristics of the emitted radiation. This allows the amplifier's optical output power to be varied without requiring adjustments to downstream optical components. This represents a significant improvement over the current state of the art for both macroscopic and microscopic setups (especially those where optics are no longer positioned manually but via micro-adjusters, e.g., a hexapod).

[0030] Further preferred embodiments of the invention result from the features mentioned in the respective dependent claims.

[0031] Unless otherwise stated in individual cases, the various embodiments of the invention mentioned in this application can be advantageously combined with one another. Brief description of the drawings

[0032] The invention and its technical context are explained in more detail below with reference to the accompanying figures. It should be noted that the invention is not intended to be limited by the exemplary embodiments shown. In particular, unless explicitly stated otherwise, it is also possible to extract partial aspects of the situations described in the figures and combine them with other elements and findings from the present description. The figures show: Fig. 1 a schematic isometric representation of a first exemplary embodiment of a laser amplifier system according to the invention; Fig. 2 a schematic isometric representation of a second exemplary embodiment of a laser amplifier system according to the invention; Fig. 3 a schematic cross-sectional representation of the formation of the thermal lens in a semiconductor layer structure below a continuous conductor strip of an ohmic resistance element according to the invention applied to the intermediate area; and Fig. 4 a schematic representation of the in Fig. 3 adjusting the temperature distribution in the semiconductor material of the semiconductor layer structure. Detailed description of the drawings

[0033] Fig. Figure 1 shows a schematic isometric representation of a first exemplary embodiment of a laser amplifier system according to the invention. The laser amplifier system shown comprises a waveguide region A with a waveguide 10 for guiding laser radiation; an amplifier region B with an amplifier 20, wherein the amplifier region B is configured such that the laser radiation is coupled from a first end A1 of the waveguide 10 into an input side B1 of the amplifier 20; and an intermediate region C between the first end A1 of the waveguide 10 and the input side B1 of the amplifier 20, wherein the intermediate region C is configured to provide a variable thermal lens 30 for controllable adaptation of the laser radiation coupled into the amplifier 20. The waveguide 10 can be configured as a passive waveguide or, at least in one section, as an active waveguide.Laser radiation can be coupled into the waveguide 10 via its second end A2. After amplification, this radiation can then preferably be coupled out at the output side B2 of the amplifier 20.

[0034] The waveguide 10 is designed as a finned waveguide in waveguide region A. Amplifier region B is an active area with a metallic contact layer 60. The structure in the intermediate region C serves as a means for locally heating this region. The variable thermal lens 30 can be provided, in particular, by electrically heating the intermediate region C. For this purpose, an ohmic resistor 32, in the form of a continuous conductor strip 34 with end contacts 36, is applied to the intermediate region C. In the illustrated embodiment, the continuous conductor strip 34 has a straight section designed to match the direction of propagation of the laser radiation in the intermediate region C. The thermal lens 30 can be variably adapted to the respective operating point of the amplifier 20 according to the invention by applying a variable operating voltage U.HS via a suitable means for controlling the variable thermal lens 30.

[0035] Fig. Figure 2 shows a schematic isometric representation of a second exemplary embodiment of a laser amplifier system according to the invention. The laser amplifier system shown largely corresponds to the one described in Figure 2. Fig. As shown in Figure 1, the reference numerals and their respective assignment to individual features also apply accordingly. In contrast to the first embodiment according to... Fig. In this case, the continuous conductor strip 34 is meander-shaped at least in one section along the direction of laser radiation propagation in the intermediate region C. This allows a larger area to be heated by the continuous conductor strip 34. As shown in the illustration, the meander-shaped conductor strip can have a variable lateral cross-section, for example, with a widening or broadening structure. However, when developing a corresponding layout, the heat input into the underlying semiconductor material and the shape of the thermal lens 30 formed by the heating must be taken into account.

[0036] Fig. Figure 3 shows a schematic cross-sectional view of the formation of the thermal lens 30 in a semiconductor layer structure 50 below a continuous conductor strip 36 of an ohmic resistor element 32 according to the invention, which is applied to the intermediate region C. The illustration shows a substrate 40 and a semiconductor layer structure 50 arranged on the substrate 40, wherein the continuous conductor strip 34 of the said means for locally heating the intermediate region C is applied in the intermediate region C. An included active layer 52 is also shown in the semiconductor layer structure 50, which, however, does not necessarily have a direct function in the intermediate region C. The laser amplifier system according to the invention can be mounted on a suitable submout 70 as shown.

[0037] When current flows perpendicular to the plane of representation shown through the continuous conductor strip 34, the underlying areas of the semiconductor layer structure 50 heat up, resulting, for example, in a heat flow 38, indicated by the arrows. The thermal lens 30 formed by the local heating of the semiconductor layer structure 50 is shown schematically approximately at the level of the active layer 52, i.e., in the area where the laser radiation also propagates. The refractive index gradient generated in the semiconductor layer structure 50 can then be used to control the propagation characteristics of the laser radiation within a laser amplifier system according to the invention.

[0038] Fig. Figure 4 shows a schematic representation of how to move into Fig.3. The temperature distribution in the semiconductor material of the semiconductor layer structure 50 is determined. Depending on the width of the continuous conductor strip 34, the resulting heat flow 38 leads, for example, to the approximately bell-shaped temperature distribution shown. The local heating of the semiconductor layer structure 50 induces a change in the refractive index in this area via the thermo-optic effect, whereby the spatial refractive index profile of the forming thermal lens 30 is directly determined by the gradient of the resulting heat flow 38. Reference symbol list 10 waveguides 20 amplifiers (e.g., trapezoidal amplifiers) 30 thermal lenses 32 ohmic resistance element 34 continuous conductor strips 36 contacts 38 Heat flow 40 substrate 50 Semiconductor layer structure 52 active layer 60 metallic contact layer 70 Submount A waveguide area A1 first end (of the waveguide) A2 second end (of the waveguide) B Amplifier section B1 Input side (of the amplifier) B2 Output side (of the amplifier) C Intermediate range

Claims

[1] Laser amplifier system comprising: a waveguide area (A) with a waveguide (10) for guiding laser radiation; an amplifier section (B) with an amplifier (20), wherein the amplifier section (B) is configured such that the laser radiation from a first end (A1) of the waveguide (10) is coupled into an input side (B1) of the amplifier (20); and an intermediate region (C) between the first end (A1) of the waveguide (10) and the input side (B1) of the amplifier (20), wherein the intermediate region (C) is configured to provide a variable thermal lens (30) for controllable adaptation of the laser radiation coupled into the amplifier (20); wherein the provision of the variable thermal lens (30) is effected by optical heating of the intermediate area (C) or the intermediate area (C) for the provision of the variable thermal lens (30) by electrical heating of the intermediate area (C) comprises an ohmic resistance element (32) applied to the intermediate area (C) as a means for local heating of the intermediate area (C). [2] Laser amplifier system according to claim 1, further comprising: a substrate (40); and a semiconductor layer structure (50) arranged on the substrate (40), wherein the waveguide (10) in the waveguide region (A) is designed as a finned waveguide in the semiconductor layer structure (50), and the amplifier region (B) is designed as an active area with an active layer (52) within the semiconductor layer structure (50) and an overlying metallic contact layer (60). [3] Laser amplifier system according to claim 1 or 2, wherein the ohmic resistance element (32) is designed as a continuous conductor strip (34) with end contacts (36). [4] Laser amplifier system according to claim 3, wherein the continuous conductor strip has a straight extension section designed to suit the propagation direction of the laser radiation in the intermediate region, the continuous conductor strip is meander-shaped at least in one section along the propagation direction of the laser radiation in the intermediate region (C), or the continuous conductor strip has a free-form profile. [5] Laser amplifier system according to one of the preceding claims, wherein the waveguide (10) is designed as a passive waveguide or at least in one section as an active waveguide. [6] Laser amplifier system according to one of the preceding claims, further comprising a means for controlling the variable thermal lens (30). [7] Laser arrangement comprising: a laser amplifier system according to any one of the preceding claims; and a laser radiation source, wherein the laser radiation emitted by the laser radiation source is coupled into a second end of the waveguide (A2) for amplification in the laser amplifier.

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