Laser amplifier system and laser assembly

Abstract

The invention relates to a laser amplifier system and to a laser assembly, in particular a laser amplifier system with a controllable beam and a laser assembly which comprises the laser amplifier system according to the invention. A laser amplifier system according to the invention comprises a waveguide region (A) with a waveguide (10) for guiding laser radiation; an amplifier region (B) with an amplifier (20), said amplifier region (B) being designed 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), said intermediate region (C) being designed to provide a variable thermal lens (30) for a controllable adaptation of the laser radiation overcoupled into the amplifier (20). The laser assembly according to the invention comprises a laser amplifier system according to the invention and a suitable laser radiation source.

Description

[0001] title

[0002] Laser amplifier system and laser arrangement

[0003] Description

[0004] The present invention relates to a laser amplifier system and a laser arrangement, in particular a beam-controllable laser amplifier system and a laser arrangement comprising the laser amplifier system according to the invention.

[0005] State of the art

[0006] Laser amplifiers are used to amplify laser radiation coupled into them. The laser amplifiers can be designed as a single element or as an integrated component of a laser. In particular, tapered amplifiers integrated using semiconductor technology, which can be combined with semiconductor lasers as a single element or as part of them in a common semiconductor system, are a frequently encountered design of laser amplifiers. The amplifiers are designed as active regions within which the energy of an incident laser radiation can be increased many times over. Fan-shaped amplifiers are characterized by their width widening in the lateral direction from a narrow input side to a wider output side, whereby the widening can be freely structured using 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 fan-out amplifiers with a widening that deviates from a geometric trapezoidal structure (or, more generally, amplifiers with a variable lateral cross-section) can also be included. Although the present disclosure relates in particular to fan-out amplifiers or trapezoidal amplifiers, it is expressly not limited to such amplifier types.

[0007] The aforementioned tapered amplifiers are used, for example, as amplifiers in tapered lasers. However, the use of 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, for example, 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 differently set operating points, the lenses used to condition the radiation must then be repositioned, which can represent a considerable amount of work 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 glued in place.This disadvantage of known tapered lasers has so far been circumvented by a precisely defined operating point. However, this also largely fixes the emitted output power during operation and cannot be meaningfully adjusted.

[0008] Disclosure of the invention

[0009] 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.

[0010] These objects are achieved according to the invention by the features of independent claims 1 and 10. 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 expedient manner and can be supplemented by explanatory facts from the description and / or details from the figures, whereby further advantageous embodiments of the invention are demonstrated.

[0011] 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 designed such that the laser radiation is coupled from a first end of the waveguide 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 designed to provide a variable thermal lens for controllably adapting the laser radiation coupled into the amplifier.

[0012] In the context of this disclosure, an amplifier is understood to mean any form of active region for laser amplification. This can be, in particular, a general fan-out 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. Waveguides and associated waveguide regions are well known in the art and are used in a variety of ways for waveguiding (radiation guidance). The waveguides can be designed as structures with (active) and without (passive) an amplification effect for the guided waves. Amplifiers connected to them for amplifying the waves guided in the waveguide are also well known to those skilled in the art (e.g., from the tapered laser).For details, please refer to the relevant specialist literature.

[0013] 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 designed to provide a variable thermal lens for controllably adjusting the laser radiation coupled into the amplifier. This means that after guided radiation is coupled out of the waveguide and before the laser radiation is coupled into the amplifier region, it must be transmitted through the intermediate region, and the propagation can be influenced by a thermal lens that can be formed in the intermediate region. The design and effect of thermal lenses are also well known in the prior art and are fully understood from a physical perspective.

[0014] The present invention is directed to the application of such a thermal lens for coupling laser radiation from a waveguide into an amplifier, and in particular into a fan-out amplifier or a tapered amplifier. By designing a thermal lens in the intermediate region that is adapted to the operating point of the amplifier, the spatial-optical properties of the laser radiation can be controllably influenced. 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.

[0015] A laser amplifier system according to the invention preferably further comprises a substrate; and a semiconductor layer structure arranged on the substrate, wherein the waveguide in the waveguide region is formed as a ridge waveguide in the semiconductor layer structure, the amplifier region is formed 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, for example, of trapezoidal amplifiers in the prior art. The substrate can also be removed after the semiconductor layer structure has been produced and is therefore not essential to the invention.To form the thermal lens in the intermediate region, a means for locally heating the intermediate region is provided. Local heating of the semiconductor layer structure causes a change in the refractive index in this region via the thermo-optical effect, whereby the gradient of the resulting heat flow directly determines the spatial refractive index profile of the resulting thermal lens. Through targeted design of the means for local heating and appropriate control (see thermal engineering), the precise shape of the thermal lens can be defined or varied.

[0016] The variable thermal lens is preferably provided by optical heating of the intermediate region. In particular, the heating can be achieved by local exposure to laser radiation with a defined beam profile at a fixed power. The beam profile and / or power used can be variable 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 separate from each other and essentially non-overlapping.

[0017] The variable thermal lens is preferably 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 inside the semiconductor layer structure.

[0018] Preferably, an ohmic resistance element is applied to the intermediate region for electrical heating. This constitutes a resistance heater as a means of locally heating the intermediate region. Resistance heaters are common in the field of laser technology and are used, for example, for phase adjustment of waveguide sections through thermal length changes or for stabilizing spectral filter elements. By specifically designing the ohmic resistance element, the precise shape of the thermal lens can be defined or varied with appropriate control via an applied voltage. The ohmic resistance element can, for example, be designed as an extended surface heater by means of a conductor structure applied to the intermediate region.

[0019] The ohmic resistance element is preferably designed as a continuous conductor strip with contacts at its ends. It is thus a linear resistance heater (also referred to as a linear heating element or heating strip) with which a strip-shaped heat flow pattern can be provided in the intermediate region. The continuous conductor strip can be straight or curved.

[0020] Preferably, the continuous conductor strip has a rectilinear extension section designed to match the propagation direction of the laser radiation in the intermediate region (straight line heating element). It is also preferred that the continuous conductor strip is designed to be meandering at least in a section along the propagation direction of the laser radiation in the intermediate region, or that the continuous conductor strip has a freeform shape. With a meandering design of the continuous conductor strip, this can be used to heat a relatively large surface of the intermediate region. Simply widening a linear, continuous conductor strip to increase area coverage would reduce the ohmic resistance of the conductor strip and thus lead to reduced heat input or higher required operating voltages to achieve the necessary heating current.The use of a general freeform profile enables not only a 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.

[0021] Preferably, the waveguide is designed as a passive waveguide or, at least in one section, as an active waveguide. In particular, the waveguide can also be designed as a waveguide laser for the direct generation and provision of the laser radiation to be coupled into the amplifier. A waveguide designed as a passive waveguide, on the other hand, serves solely for waveguiding, so that the corresponding laser radiation must be provided externally and coupled into the waveguide accordingly.

[0022] A laser amplifier system according to the invention preferably further comprises a means for controlling the variable thermal lens. The control means can, in particular, be a device for controlling a current flow, an irradiated 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 control means can also comprise 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.An amplifier system according to the invention can preferably be used in amplifier types with a structure that is laterally variable in width along the optical axis, in particular one that widens or widens in the direction of the facet.

[0023] 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 tapered amplifier) ​​by means of said waveguide.

[0024] 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 tapered amplifier region with a tapered amplifier). The amplifier can be part of a complete laser in which additional 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.

[0025] Preferably, a metallic heating strip can be formed in this region, which can be electrically contacted externally via metallic conductor tracks 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. In addition, the position of the heating strip on the intermediate region should be precisely defined, depending on the shape of the amplifier. However, different strip layouts can also be addressed using integrated switching elements. The heating strip can be electrically isolated from the amplifier and operated independently via the control means.

[0026] The heating strip should be designed and controlled in such a way that a temperature gradient is generated lateral to the preferred longitudinal propagation direction of the amplified laser radiation. This lateral temperature gradient can advantageously produce a thermo-optical lens effect on the laser radiation propagating longitudinally in the intermediate region. The longitudinal extension of the heating element can be used to pre-control or adjust the achievable refractive power of the thermal lens. The direction of the electrical flow (longitudinal or lateral) is not essential for the formation of the thermal lens. However, the longitudinal direction is preferred due to a potentially larger interaction distance. The effect of the heating element can be controlled by the applied electrical voltage.The effect of the heating element is preferably adjusted such that the spatial characteristics of the output radiation behind the amplifier remain as unchanged as possible, even with a changed operating point. This applies in particular to the longitudinal position of the lateral beam waist. With a fixed collimation optics in the beam path after the amplifier, rectification or collimation of the radiation can 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 layer system can first be constructed on an n-doped GaAs substrate, which, on the one hand, enables vertical waveguiding and, at the same time, can amplify laser radiation guided therein. The active zone (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; wavelengths from approximately 626 nm to approximately 1180 nm can be generated using different material systems (e.g., GaInP). For an emission wavelength of approximately 1120 nm, InGaAs can be used as the optically active material. For the specified wavelength, optical powers of over 10 W have already been demonstrated from an amplifier system according to the invention.However, the achievable optical performance depends on the material system used and can vary accordingly in the specified wavelength range.

[0028] The present invention utilizes a local change in the refractive index to adjust the spatial-optical properties. The location of the targeted change in the refractive index influences the efficiency of the amplifier system. By positioning and designing the appropriate means for local heating, a high optical effect can be achieved with low energy consumption. Positioning a means for local heating in the intermediate region directly in front of an amplifier region with a variable cross-section enables a particularly high efficiency of the invention. Thus, with the lowest possible heat input, the laser radiation can be particularly strongly influenced at the specified position.

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

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

[0031] The various embodiments of the invention mentioned in this application can be advantageously combined with one another, unless otherwise stated in the individual case.

[0032] Brief description of the drawings

[0033] The invention and the technical environment 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 cited. In particular, unless explicitly stated otherwise, it is also possible to extract partial aspects of the facts explained in the figures and combine them with other components and findings from the present description. They show:

[0034] Fig. 1 is a schematic isometric view of a first exemplary embodiment of a laser amplifier system according to the invention;

[0035] Fig. 2 is a schematic isometric view of a second exemplary embodiment of a laser amplifier system according to the invention;

[0036] Fig. 3 is a schematic cross-sectional view of the formation of the thermal lens in a semiconductor layer structure below a continuous conductor strip applied to the intermediate region of an ohmic resistance element according to the invention; and

[0037] Fig. 4 is a schematic representation of the temperature distribution in the semiconductor material of the semiconductor layer structure shown in Fig. 3.

[0038] Detailed description of the drawings

[0039] 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 designed 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 designed to provide a variable thermal lens 30 for controllably adjusting the laser radiation coupled into the amplifier 20. The waveguide 10 can be designed as a passive waveguide or, at least in one section, as an active waveguide.Laser radiation can be coupled into the waveguide 10 via the second end A2 of the waveguide 10. After amplification, this radiation can then be coupled out, preferably at the output side B2 of the amplifier 20.

[0040] The waveguide 10 is designed as a ridge waveguide in the waveguide region A. The amplifier region B is intended to be an active region with a metallic contact layer 60 lying thereon. The structure in the intermediate region C is a means for locally heating the intermediate region C. The variable thermal lens 30 can be provided in particular by electrically heating the intermediate region C. For this purpose, an ohmic resistance element 32 is applied to the intermediate region C as a continuous conductor strip 34 with end contacts 36. In the embodiment shown, the continuous conductor strip 34 has a rectilinear extension section designed to match the propagation direction of the laser radiation in the intermediate region C.A variable adaptation of the thermal lens 30 to the respective operating point of the amplifier 20 according to the invention can be carried out by applying a variable operating voltage UHS via a corresponding means for controlling the variable thermal lens 30.

[0041] 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 that shown in Fig. 1, therefore the reference numerals and their respective assignment to individual features also apply accordingly. In contrast to the first embodiment according to Fig. 1, however, the continuous conductor strip 34 is meander-shaped at least in one section along the propagation direction of the laser radiation in the intermediate region C. As a result, a larger area can be heated with the continuous conductor strip 34. As shown in the illustration, the meander-shaped conductor strip can be formed with a variable lateral cross-section, for example with a widening or broadening structure.However, when developing a corresponding layout, the heat input dissipated into the underlying semiconductor material and the shape of the thermal lens 30 formed by the heating must be taken into account.

[0042] 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 resistance 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 attached to a suitable submount 70 as shown.

[0043] When a current flows perpendicular to the illustrated plane through the continuous conductor strip 34, the underlying regions of the semiconductor layer structure 50 heat up, which can result, 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 additionally shown only schematically, approximately at the level of the active layer 52, i.e., in the region in which the laser radiation also propagates. The refractive index gradient thus generated in the semiconductor layer structure 50 can then be used for the controlled adaptation of the propagation properties of the laser radiation within a laser amplifier system according to the invention.

[0044] Figure 4 shows a schematic representation of the temperature distribution in the semiconductor material of the semiconductor layer structure 50, as shown in Fig. 3. Depending on the strip width of the continuous conductor strip 34, the resulting heat flow 38, for example, results in 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 region via the thermo-optical effect, whereby the gradient of the resulting heat flow 38 also directly determines the spatial refractive index profile of the developing thermal lens 30.

[0045] List of reference symbols

[0046] 10 waveguides

[0047] 20 amplifiers (e.g. trapezoidal amplifiers)

[0048] 30 thermal lens

[0049] 32 ohmic resistance element

[0050] 34 continuous conductor strips

[0051] 36 contacts

[0052] 38 Heat flow

[0053] 40 Substrat

[0054] 50 semiconductor layer structure

[0055] 52 active layer

[0056] 60 metallic contact layer

[0057] 70 submount

[0058] A waveguide region

[0059] A1 first end (of the waveguide)

[0060] A2 second end (of the waveguide)

[0061] B amplifier range

[0062] B1 Input side (of the amplifier)

[0063] B2 Output side (of the amplifier)

[0064] C Intermediate area

Claims

Patent claims 1. A laser amplifier system, comprising: 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 designed 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 designed to provide a variable thermal lens (30) for controllably adapting the laser radiation coupled into the amplifier (20).

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 formed as a ridge waveguide in the semiconductor layer structure (50), the amplifier region (B) is formed as an active region with an active layer (52) within the semiconductor layer structure (50) and an overlying metallic contact layer (60), and the intermediate region (C) comprises a means for locally heating the intermediate region (C).

3. Laser amplifier system according to claim 1 or 2, wherein the provision of the variable thermal lens (30) is effected via optical heating of the intermediate region (30).

4. Laser amplifier system according to claim 1 or 2, wherein the provision of the variable thermal lens (30) is effected by electrical heating of the intermediate region (C).

5. Laser amplifier system according to claim 4, wherein an ohmic resistance element (32) is applied to the intermediate region (C) for electrically heating the intermediate region (C).

6. Laser amplifier system according to claim 5, wherein the ohmic resistance element (32) is designed as a continuous conductor strip (34) with end contacts (36).

7. Laser amplifier system according to claim 6, wherein the continuous conductor strip has a straight extension section designed to match the propagation direction of the laser radiation in the intermediate region, the continuous conductor strip is designed to be meander-shaped at least in a section along the propagation direction of the laser radiation in the intermediate region (C), or the continuous conductor strip has a free-form course.

8. 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.

9. A laser amplifier system according to any one of the preceding claims, further comprising means for controlling the variable thermal lens (30).

10. A 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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