WO2006021298A1

WO2006021298A1 - Diode laser comprising an optical device for increasing the beam density of an output laser beam emitted by said laser

Info

Publication number: WO2006021298A1
Application number: PCT/EP2005/008393
Authority: WO
Inventors: Bernd Köhler; Jens Biesenbach; Matthias Haag
Original assignee: Dilas Diodenlaser Gmbh
Priority date: 2004-08-21
Filing date: 2005-08-03
Publication date: 2006-03-02
Also published as: JP2008511131A; DE102004040608A1; DE102004040608B4; EP1779482A1

Abstract

The invention relates to a diode laser containing an optical device (10) which is used to increase the beam density of an output laser beam (12) which is emitted by said laser to the place of an object (16), said laser being made of a plurality of partial beams (8(1), 8(2), ... 8(n)) which are produced by a plurality of diode laser elements (2(1), 2(2), ... 2(n)). The optical device (10) is arranged downstream from the diode laser elements (2(1), 2(2), ... 2(n)) and contains a first Bragg grating volume (18) which only reflects a spectral region (λB(1), ΔλB(l), … λB(2), ΔλB(2), λB(n), ΔλB(n)) of the partial beams (8(1), 8(2), ... 8(n)) emitted by the diode laser elements (2(1), 2(2), ... 2(n)), in the respective diode laser element (2(1), 2(2), ... 2(n)) and only transmits the predominant part of said spectral region (λB(1), ΔλB(l), … λB(2), ΔλB(2), … λB(n), ΔλB(n)). The average wave lengths (λB(1), λB(2), ... λB(n)) of the different partial beams (8(1), 8(2), ... 8(n)), which are respectively filtered from spectral regions (λB(1), ΔλB(1), … λB(2), ΔλB(2), λB(n), ΔλB(n)), are different from each other. The transmitted partial beams (8(1), 8(2), 8(n)) are superimposed in a collinear manner by the first Bragg grating volume (18) in the output laser beam, by means of a second grating (24).

Description

description

Diode laser with an optical device for increasing the radiance of an output laser beam emerging from it

The invention relates to a diode laser with an optical device for increasing the intensity of an output laser beam emerging from it.

As a laser-active element, a diode laser contains an opto¬ electronic semiconductor chip, the so-called laser diode, which, in particular in the case of diode lasers, can consist of a plurality of individual emitters arranged side by side. An embodiment for achieving high optical output power is a monolithic arrangement of individual emitters in a so-called laser diode bar. Such a laser diode bar is typically about 5-10 mm wide (lateral), 0.10-0.15 mm high (vertical) and has resonator lengths of between 0.3 and 2.5 mm (transversal). In the case of the so-called edge emitters, the laser radiation generated in the pn junctions of the laser diode emerges on one of the lateral sides (exit or front side, emitter face). The opposite side (back) is highly reflective mirrored and forms the rear view mirror of the resonator.

Each laser diode bar generates a narrow approximately rectangular laser beam which is composed of a plurality of partial beams which emerge from the individual emitters and whose beam properties in the lateral direction of the laser diode bar (width), the so-called slow axis, of the jet properties in the perpendicular Axis (epitaxial direction), the so-called fast axis, significantly differ.

The laser diode bars and the need in a high-performance diode lasers, several such laser diode bars complex due _^ this highly asymmetric beam properties ne¬ by side or one another to be arranged in a stack, er¬ calls in particular for the construction of a high-power diode laser usually optical devices for beam shaping, ie for beam symmetrization and Strahlüber¬ storage. This applies in particular to diode lasers in which the laser beam is coupled into an optical fiber and the line-shaped beam of the individual laser diode bars must be converted into a nearly square, or ideally round, profile. Diode lasers with such beam shaping devices are known, for example, from DE 196 45 150 C2, DE 198 46 532 C1 and DE 100 15 245 C2.

While a symmetrization of the output laser beam is possible by rearranging the sub-beams emerging from the individual emitters and thus inducing beam symmetry in principle for any number of emitters, an increase in the beam density of the output laser beam requires a collinear spatial superposition of the individual ones partial beams. However, the radiance (intensity per solid angle unit) or brightness of the radiation emitted by a beam source composed of a plurality of spatially separated individual sources and not in a coherent relationship with each other can only be increased when the individual partial beams differ in their physical properties in such a way that they can be spatially superimposed with correspondingly selectively acting beam splitters. This is possible if the sub-beams differ in their polarization or wavelength.

By a polarization coupling, as is known, for example, from DE 198 46 532 C1 for the coupling of two laser diode barren stacks, due to the only two independent degrees of polarization, only a maximum increase in the beam density by a factor of two can be achieved. In contrast, a wavelength coupling is fundamentally possible for a large number of partial beams, as long as they differ only sufficiently in their wavelength and can be superimposed as low as possible with the aid of spectrally selective dielectric mirror systems. In practice, it has been shown that when using dielectric mirror systems, a minimum wavelength spacing for the coupling of unpolarized partial beams in the infrared range must be about 40 nm. As a result, this method of wave coupling for the partial beams of a single laser diode bar grund¬ addition, not applicable, since the emitters of a laser diode bar usually have approximately the same wavelengths. A wavelength coupling with dielectric mirrors is therefore used only for the coupling of a plurality of laser diode bars or a plurality of stacks with different wavelengths.

The invention is based on the object of providing a diode laser with an optical device for increasing the beam density of an output laser beam emerging from it, which, with a simple structure, provides the spatial coupling of a laser beam compared to the prior art significantly increased number of partial beams allows.

The above object is achieved according to the invention, with a diode laser having the features of claim 1. According to these features, a diode laser includes an optical device for increasing the radiance of an aus¬ passing out output laser beam in an application plane, which consists of a plurality of partial beams which are generated by a plurality of diode laser elements, wherein the optical device is arranged downstream of the diode laser elements. According to the invention, the optical device contains a first volume Bragg grating, which in each case only partly reflects a spectral region of the partial beams emerging from the diode laser elements into the respective diode laser element and transmits the predominant part of only this spectral region, wherein the average wavelengths of the respective spectral components filtered from different sub-beams are different from one another. The first volume Bragg grating is followed by a second grating, preferably also a volume Bragg grating, for collinear spatial superimposition of the partial beams transmitted by the first volume Bragg grating in the output laser beam.

Due to the wavelength coupling according to the invention, the beam parameter product and thus the beam quality of a laser diode bar in the plane of the slow axis signifi¬ kant of typically 500mm * mrad at 10mm bar width by a factor formed from the ratio of bar width and single emitter width 10mm / 0.15mm to about 10mm-mrad improved. The use of a first holographic or volume Bragg grating makes it possible to narrow the sub-beams which are already emerging from the diode laser elements in a relatively narrow band with respect to their spectral bandwidth. In this way, spectrally different partial beams can be generated, which differ sufficiently with respect to their mean wavelength and are stabilized narrowband to this wavelength in order to be able to be superimposed largely loss-free with the second grid.

Although the use of a holographic grating to perform wavelength division multiplexing is already known in principle from US Pat. No. 5,691,989. However, the aim of this known application is not to increase the radiance or brightness of a laser beam at the location of an object to be changed by the laser beam in its physical properties, for example its thermal processing, but to increase the bandwidth in the optical transmission of information with an optical fiber by the simultaneous transmission of spectrally separable laser beams and their subsequent separation in a demultiplexer.

A diode laser in the sense of the present invention is a superordinate structural unit, which is composed of a plurality of diode laser elements. Such a diode laser can be either one by a group of individual emitters, which are then referred to as a diode laser element in the context of the invention, or by a group of laser diodes, for example by a group of laser diode bars, each one

Variety of single emitters included, formed arrangement be. In the latter case, the laser diode bars are then referred to as a diode laser element.

In order to enable a loss-free coupling of the sub-beams, in an advantageous embodiment of the invention, the difference between the average wavelength of spectrally adjacent transmitted sub-beams is greater than half the sum of their spectral half-widths.

A particularly compact construction is achieved if the first and / or second volume Bragg grating is constructed from a single volume Bragg grating element whose lattice properties are location-dependent, wherein preferably the respective volume Bragg grating Element has a number of regions corresponding to the number of diode laser elements, each having constant but different lattice properties.

Alternatively, the first and / or second volume Bragg gratings are constructed from a plurality of discrete volume Bragg gratings which differ in their grating characteristics, in particular the number of discrete volume Bragg gratings being equal to the number of Diode laser elements corresponds. A volume Bragg grating constructed from individual volume Bragg grating elements can be manufactured in a particularly simple manner.

In an advantageous embodiment of the first and / or second volume Bragg grating arrangement is preceded by a respective micro-optics, which is particularly integrated in each case in the volume Bragg grating arrangement. This will contribute to losses the transmission and coupling of the partial beams to or reduced in the volume Bragg gratings.

In a particularly advantageous embodiment of the invention, the first and / or second volume Bragg grating is constructed from one or more PTR elements. These enable a particularly narrow-band stabilization of the wavelength of the respective component beams emerging from the diode laser elements, and thus a particularly effective and loss-free wavelength coupling of the partial beams stabilized in this way.

For further explanation of the invention reference is made to the Ausfüh¬ approximately examples of the drawing. Show it:

1 and 2 different embodiments of a diode laser according to the invention in a respective schematic Prinzipdar¬ position.

Referring to FIG. 1, a diode laser including a plurality n of DIO denlaserelementen 2 ^(1), 2 ⁽²⁾ ... 2 ^(n>, which are in the exemplary embodiment of FIG play arranged in a row next to each other and form a diode laser assembly 4 The diode laser elements 2 ⁽¹⁾ , 2 ^<2) ,... 2 ⁽ⁿ⁾ shown in the figure can be both the individual emitter of a laser diode bar and laser diode bars which are arranged one above the other in a stack are. Each diode laser element 2 ⁽¹⁾ , 2 ⁽²⁾ , ... 2 ⁽ⁿ⁾ emits a partial beam 8 ^{1) , 8 ⁽²⁾ ,... 8 ^(n> .

On the output side, the diode laser assembly 4 is followed by a micro-optics 6, which form a collimation of the partial beams 8 ^U) , 8 ^{(2. 3} ⁾ respectively emerging from the diode array elements 2 ⁽¹⁾ , 2 ⁽²⁾ , ⁾ , ... 8 ^<n) in the direction of the fast axis. This micro-optics 6 is a single one in the case where the diode laser assembly 4 is a single laser diode bar and the diode laser elements 2 ⁽¹⁾ , 2 ⁽²⁾ , ... 2 ⁽ⁿ⁾ are single emitters cylindrical lens. When the diode laser assembly shown in the figure is composed of a plurality of laser diode bars serving as diode laser elements 2 ⁽¹⁾ , 2 ⁽²⁾ , ... 2 ⁽ⁿ⁾ , an array of n-single microlenses is used as the micro-optics 6. In an alternative embodiment, a collimation in the direction of the slow axis is additionally performed with the micro-optics 6.

The partial beams 8 ⁽¹⁾ , 8 ^<2) ,... 8 ^<π) emerging from the diode laser assembly 4 or the collimator 6 are coupled into an optical device 10, the output side being a

Output laser beam 12 is generated, in which all sub-beams 8 ⁽¹⁾ , 8 ^{! 2>} , ... 8 ^{(n) are} collinear superimposed and directly or with an only indicated in the figure beam guiding and beam shaping device, for example with a fiber optic, be performed together and superimposed to an object 16 in order to cause there with high intensity each targeted according to the intended physical effect. This object can be, for example, a workpiece to be machined or the laser-active medium of a solid-state laser, which is to be pumped optically with the diode laser.

The optical device 10 comprises on the input side a first holographic grating or volume Bragg grating 18, which of each partial beam 8 ⁽¹⁾ , 8 ⁽²⁾ ,... 8 ^{(n) has} only a narrow spectral range with a central wavelength λ _B ⁽¹⁾ , λ _B ⁽²⁾ , ... λ _B ^<n> and the half width Δλ _B ⁽¹⁾ , Δλ _B ^<2) , ... Δλ _B ⁽ⁿ⁾ which is smaller than the half-width of the emitted respective partial beam 8 ⁽¹⁾ , 8 ^<2> , ... 8 ⁽ⁿ⁾ is partially reflected back into the respective diode laser element 2 ⁽¹⁾ , 2 ^<2) , ... 2 ⁽ⁿ⁾ and transmits the predominant part only of this spectral range. The feedback caused by the reflected-back component leads to a so-called self-seeding of the respective diode laser element 2 ⁽¹⁾ , 2 ⁽²⁾ ... 2 ^<n> and thus to a spectral narrowing of the respective output radiation, so that the diode denlaserelemente 2 ⁽¹⁾ , 2 ^(2> ... 2 ^(n> in the stationary state only partial beams 8 ⁽¹⁾ , 8 ⁽²⁾ , ... 8 ^(n> emit their Mitten¬ wavelengths on the wavelengths λ _B ⁽¹⁾ , λ _B ^<2) , ... λ _B ^(n> stabilized and Siert in their Halbwertsbreite Δλ _B ⁽¹⁾ , Δλ _B ⁽²⁾ , ... .DELTA.λ _B ^{ln) are} correspondingly concentrated, ie Without the first volume Bragg grating 18, the sub-beams 8 ⁽¹ ⁾ would have emerged from the diode laser elements 2 ⁽¹⁾ , 2 ⁽²⁾ , ... 2 ⁽ⁿ⁾ ⁾ , 8 ⁽²⁾ ,... 8 ^{<n) have} approximately the same wavelength λ _A and the same half-width Δλ _A. A suitable volume Bragg grating is, for example, a photo-thermal h-refractive component, as for example from the

US 6,586,141 Bl is known. On the basis of such so-called PTR elements, volume Bragg gratings with very good spectral selectivity and high diffraction efficiency are known. These PTR elements are also characterized by high mechanical, optical and thermal stability.

FIG. 1 shows an exemplary embodiment in which the lattice constant of the first volume Bragg grating continuously varies as a function of the spatial coordinate, as illustrated by the arrow 20 in the figure. Instead of such a continuous variation of the lattice constant is also a discontinuous variation of the lattice constant within the first volume Bragg grating, so that each diode laser element 2 ⁽¹⁾ , 2 ^(2> ... 2 ^{<n>) is assigned} a region with a constant lattice property in each case.

The partial beams emerging from the first volume Bragg grating 18 meet after collimation in the slow axis with a micro-optics 22 on a second grating 24 arranged within the optical device 10, which is preferably likewise a volume Bragg grating and in which an incoherent collinear superposition of the narrow-band spectral narrow beams 8 ⁽¹⁾ , 8 ^<2) ,... 8 ^{(n) is} performed. Also, the second grid is preferably a PTR element and analogous to the first volume Bragg grating 18 has a location-dependent inner grating structure. The lattice structure to be designed in such a manner at the position i has that the partial beam with the Wellenlän¬ ge λ _B ^(l> maximum reflection, while the already überla¬ siege partial beams having the wavelengths λ _B ^{(l + 1),} λ _B ^{<1 +2)} , ... λ _B ^{(1 + n>) must} be transmitted with as little loss as possible.

In order to enable an efficient wavelength coupling and thus an efficient increase of the radiance or brightness, the difference of the mean wavelength λ _B ⁽¹⁾ , λ _B ^{2) ,... Λ _B ^(n> spectrally adjacent transmitted partial beam ¬ len 8 ⁽¹⁾ , 8 ^(2> , ... 8 ^(n> greater than half the sum of their respective spectral half-width Δλ _B ⁽¹⁾ , Δλ _B ⁽²⁾ , ... Δλ _B ⁽ⁿ⁾ so that the following condition is met:

^λ A (ΔΛ _B + ΔΛ _B } S Λ _B - Λ _B

With the known PTR elements, such as those offered by PD-LD Inc., Pennington, New Jersey, USA or Ondax Inc., Monrovia, California, USA, it is possible to narrowing the laser beams emerging from a single diode laser element having a half width of about 3 to 6 nm onto a narrowband range with a half width <0.2 nm so that, for example, up to 30 individual emitters of a laser diode bar can be sufficiently separated in their wavelength a wavelength coupling to er¬ possible. Since it is also possible to set the laser diode bars arranged in a stack during manufacture to different central wavelengths, which differ from one another by a few nm, it is fundamentally possible to collinearly superpose all partial beams of such a stack by wavelength coupling. Alternatively, it is also possible to set the partial beams emerging from a laser diode bar of a stack to a single wavelength with the aid of the first volume Bragg grating, and collinear the narrowband laser beams emerging from the individual laser diode bars by wavelength coupling to overlay.

In the exemplary embodiment according to FIG. 2, the first volume Bragg grating 18 is constructed from a plurality of discrete volume Bragg grating elements 18 ^U) , 18 ⁽²⁾ ,... 18 ⁽ⁿ⁾ , which in the exemplary embodiment according to FIG the number n of the diode laser elements 2 ⁽¹⁾ , 2 ⁽²⁾ , ... 2 ^<n> . In principle, the second grating 24 can also be constructed from a plurality of discrete volume Bragg grating elements. Furthermore, it is possible to integrate micro-optics 6, first volume Bragg grating 18 and second grating 24 into a monolithic component, so that the micro-optics 22 shown in FIGS. 1 and 2 are no longer required due to their compact construction. The arrangements according to the invention enable a very compact and stable construction, since the first volume Bragg grating 18 can be mounted directly with the collimator 6 or, as explained above, can be produced together with the collimator 6 as a monolithic component. The distance d from the output facets of the diode laser element 2 ^(1> , 2 ⁽²⁾ ... 2 ^<n) to the first volume Bragg grating 18 is only a few millimeters and is typically less than 3 mm, so that the sensitivity is largely reduced compared to mechanical loads.

A further positive effect is the reduction of the temperature dependence of the wavelength of the emitted laser radiation by the first volume Bragg grating 18, since this stabilizes the wavelength to a narrowband range.

Claims

claims

1. diode laser with an optical device (10) for increasing the radiance of an output laser beam (12) emerging from it at the location of an object (16) which consists of a plurality of partial beams (8 ^(1> , 8 ^<2) , ... 8 ⁽ⁿ⁾ ), which are generated by a plurality of diode laser elements (2 ⁽¹⁾ , 2 ⁽²⁾ , ... 2 ⁽ⁿ⁾ ), in which the optical device (10) the diode laser elements ( 2 ⁽¹⁾ , 2 ^<2) , ... 2 ^<n) ) and contains a first volume Bragg grating (18) each having only one spectral range (λ _B ⁽¹⁾ , Δλ _B ⁽¹⁾ , ..., λ _B ^<2> , Δλ _B ⁽²⁾ , ... λ _B ^(n> , Δλ _B ⁽ⁿ⁾ ) which consists of the diode laser elements (2 ⁽¹⁾ , 2 ⁽²⁾ , ... 2 ⁽ⁿ⁾ ) partial rays (8 ⁽¹⁾ , 8 ^<2> , ... 8 ^(n> ) partially into the respective diode laser element (2 ⁽¹⁾ , 2 ⁽²⁾ , ... 2 ⁽ⁿ⁾ ) zu¬ backreflectiert and the vast majority of only this Spekt¬ ralbereichs (λ _B ⁽¹⁾ , Δλ _B ⁽¹⁾ , ..., λ _B ⁽²⁾ , Δλ _B ^<2) , ... λ _B ⁽ⁿ⁾ , Δλ _B ⁽ⁿ⁾ ) trans mitted, the mitt leren wavelengths (λ _B ⁽¹⁾ , λ _B ^(2> ,... λ _B ⁽ⁿ⁾ ) of the spectral regions respectively filtered from different partial beams (8 ⁽¹⁾ , 8 ⁽²⁾ , ... 8 ⁽ⁿ⁾ ) (λ _B ^(1> , Δλ _B ^{! 1)} , ..., λ _B ^<2) , Δλ _B ⁽²⁾ , ... λ _B ⁽ⁿ⁾ , Δλ _B ⁽ⁿ⁾ ) are different from each other, as well as with a second grating (24) for collinear spatial superimposition of the partial beams (8 ⁽¹⁾ , 8 ^<2> , ... 8 ⁽ⁿ⁾ ) transmitted by the first volume Bragg grating (18) in the output laser beam (12).

2. diode laser according to claim 1, wherein the second grating is a volume Bragg grating is provided.

3. Diode laser according to claim 1 or 2, in which the difference of the mean wavelength of spectrally adjacent transmissive Sub-beams is greater than half the sum of their spectral half-widths.

4. Diode laser according to claim 2 or 3, wherein the first and / or second volume Bragg grating is constructed from a single volume Bragg grating element whose lattice properties are location-dependent.

5. Diode laser according to claim 4, wherein the volume Bragg grating element has a number of diode laser elements ent speaking number of areas each having constant but mutually different lattice properties.

6. diode laser according to claim 2 or 3, wherein the first and / or second volume Bragg grating is constructed of a plurality of dis¬ creter volume Bragg grating elements that differ in their lattice properties.

7. Diode laser according to claim 6, in which the number of discrete volume Bragg grating elements corresponds to the number of diode laser elements.

8. Diode laser according to one of the preceding claims, wherein the first volume Bragg grating and / or second grating each precedes a micro-optics.

9. diode laser according to one of claims 2 to 8, wherein the micro-optics is integrated in each case in the volume Bragg grating.

10. Diode laser according to one of claims 2 to 9, wherein the first volume Bragg grating and / or second grating is constructed of one or more PTR elements.