阅读说明：本技术 二极管激光器 (Diode laser ) 是由 哈罗·弗里切 拉尔夫·科克 巴斯蒂安·克鲁斯克 法比奥·法拉里欧 于 2015-09-16 设计创作，主要内容包括：本发明涉及一种外腔二极管激光器,其允许在期望的波长范围中的增强的总输出功率,其包括被放置在内部激光腔(10)内的有源媒介,内部激光腔(10)包括：用于输出耦合激光辐射的出射面(12),输出耦合的激光辐射(B0)具有在空间稳定的主偏振方向上较高的辐射强度及在与主偏振方向不同的次偏振方向上的相应较低辐射强度的偏振部分；用于激光辐射的波长稳定化的外部频率选择元件(14)；以及用于将输出耦合的激光辐射(B0)分成沿第一光束路径(P1)延伸的第一光束(B1)和沿第二光束路径(P2)延伸的第二光束(B2)的偏振分束器(16),第一光束路径(P1)不同于第二光束路径(P2)；其中外部频率选择元件(14)被布置在第二光束路径(P2)中。(The present invention relates to an external cavity diode laser allowing an enhanced total output power in a desired wavelength range, comprising an active medium placed within an internal laser cavity (10), the internal laser cavity (10) comprising: an exit surface (12) for outcoupled laser radiation, the outcoupled laser radiation (B0) having a polarization component with a higher radiation intensity in a spatially stable main polarization direction and a correspondingly lower radiation intensity in a secondary polarization direction different from the main polarization direction; an external frequency selective element (14) for wavelength stabilization of the laser radiation; and a polarizing beam splitter (16) for splitting the outcoupled laser radiation (B0) into a first beam (B1) extending along a first beam path (P1) and a second beam (B2) extending along a second beam path (P2), the first beam path (P1) being different from the second beam path (P2); wherein the external frequency selective element (14) is arranged in the second beam path (P2).)

1. An external cavity diode laser comprising:

an active medium placed within an internal laser cavity (10), said internal laser cavity (10) comprising an exit facet (12) adapted for outcoupling laser radiation (B0), said outcoupled laser radiation (B0) having a polarization portion of higher radiation intensity in a spatially stable main polarization direction and a corresponding lower radiation intensity in a secondary polarization direction different from said main polarization direction;

an external frequency selective element (14) placed outside the internal laser cavity (10) and adapted for wavelength stabilization of the laser radiation; and

a polarizing beam splitter (16) placed outside the internal laser cavity (10) and adapted for splitting the outcoupled laser radiation (B0) into a first beam (B1) extending along a first beam path (P1) and a second beam (B2) extending along a second beam path (P2), without a polarization rotator between the exit face (12) and the beam splitter, the first beam path (P1) being different from the second beam path (P2), the polarizing beam splitter (16) being arranged to bring the full radiation intensity of the outcoupled laser radiation (B0) from the main polarization direction into the first beam (B1) extending along the first beam path (P1);

the external frequency selective element (14) is arranged in the second beam path (P2) and the second beam (B2) with the laser intensity of the outcoupled laser radiation (B0) from the secondary polarization direction is reflected back to the internal laser cavity (10) to stabilize the frequency.

2. The external cavity diode laser according to claim 1, wherein:

the polarized portion in the secondary polarization direction results from an unpolarized portion of the outcoupled laser radiation (B0).

3. The external cavity diode laser according to claim 1 or 2, wherein:

the degree of polarization of the main polarization in the outcoupled laser radiation (B0) is 60% to 99%.

4. The external cavity diode laser according to any of the preceding claims, wherein:

the external frequency selective element (14) has a reflectivity of greater than 90% for a wavelength band of less than 2 nm.

5. The external cavity diode laser according to any of the preceding claims, wherein:

a polarization modifying means (18) is arranged in the second beam path (P2) between the beam splitter (16) and the external frequency selective element (14), wherein a polarizer (20) is located in the second beam path (P2) between the polarization modifying means (18) and the external frequency selective element (14).

6. The external cavity diode laser according to any of the preceding claims, wherein:

the external frequency selective element (14) is formed as a volume bragg grating.

7. The external cavity diode laser according to any of the preceding claims, wherein:

the external frequency selective element (14) comprises a mirror having an intensity-dependent reflection coefficient.

8. The external cavity diode laser according to any of the preceding claims, wherein:

further comprising a multi-part mirror (22), the multi-part mirror (22) comprising a plurality of parts having different reflectivities, the plurality of parts of the multi-part mirror (22) being arranged side by side, the multi-part mirror (22) being arranged in the second beam path (P2) between the beam splitter (16) and the external frequency selective element (14), wherein a deflection means (24) is arranged in the second beam path (P2) between the beam splitter (16) and the multi-part mirror (22), wherein the deflection means (24) is adapted to deflect radiation in the second beam path (P2) onto different parts of the multi-part mirror (22) depending on the intensity of the outcoupled laser radiation (B0).

9. The external cavity diode laser according to any of the preceding claims, wherein:

further comprising a deflection device (24) arranged in the second beam path (P2) between the beam splitter (16) and the external frequency selective element (14), wherein the deflection device (24) is adapted to deflect radiation in the second beam path (P2) such that only a portion of the radiation in the second beam path (P2) is directed onto the active area of the external frequency selective element (14), the amount of the radiation portion being dependent on the intensity of the outcoupled laser radiation (B0).

10. The external cavity diode laser according to any of the preceding claims, wherein:

further comprising a focusing lens (26) arranged in the second beam path (P2) between the beam splitter (16) and the external frequency selective element (14), wherein the focusing lens (26) is adapted to focus radiation in the second beam path (P2) on an active area of the external frequency selective element (14), a focusing power of the focusing lens (26) depending on an intensity of the outcoupled laser radiation (B0).

11. The external cavity diode laser according to any of the preceding claims, wherein:

comprising a plurality of internal laser cavities (10), each internal laser cavity (10) comprising an exit facet (12) adapted for outcoupling laser radiation, wherein said internal laser cavities (10) are arranged such that a plurality of stacked parallel laser beams (B01, B02, B03, B04) are directed to said beam splitter (16),

wherein the beam splitter (16) comprises a plurality of stepped portions, each of the stepped portions being adapted to split one of the plurality of stacked parallel laser beams (B01, B02, B03, B04) into a first beam (B1) extending along a first beam path (P1) and a second beam (B2) extending along a second beam path (P2), the first beam (B1) having a higher radiation intensity than the second beam (B2) and the first beam path (P1) being different from the second beam path (P2).

Technical Field

The present invention relates to a diode laser with external spectrally selective feedback.

Background

The radiation of an edge-emitting diode laser is highly divergent in a direction perpendicular to the waveguide plane (vertical direction, "fast axis") and has a relatively broad wavelength spectrum. Furthermore, the wavelength spectrum typically depends on other parameters, such as temperature. The wavelength spectrum is therefore dependent on the power provided by the laser.

According to the prior art, the wavelength can be confined and stabilized by internal or external wavelength selective elements or structures. Due to the spectrally selective feedback of the emitted radiation in the diode laser, an external limitation and stabilization of the wavelength is achieved. One example is the so-called External Cavity Diode Laser (ECDL), where the feedback is done by spectrally selective reflection on e.g. a surface grating. However, this has the disadvantage of requiring additional optical elements and making miniaturization difficult.

Another way to achieve spectrally selective feedback is to use a volume bragg grating (also referred to as VBG). The advantage of using such a VBG is that a compact, wavelength-stable laser beam source can be realized. For example, DE 102011006198 a1, US 2005/0207466a1, US 2006/0251143a1, US 7,397,837B2 and US 7,545,844B2 disclose how to place volume bragg gratings in (collimated) laser beams. However, a disadvantage of this arrangement is that the VBG is arranged within the main light path (i.e. the path along which the laser radiation is coupled out) receiving high optical energy, which may lead to a shift of the wavelength with higher radiation energy. This small shift in the peak wavelength of the locked diode laser is due to the heating of the VBG with increasing power resulting in a slight change in the locking wavelength. Furthermore, prior art laser diode systems use VBG that provides a fixed (reflectivity) percentage for the feedback signal, resulting in a high intensity feedback level for large currents of the respective (current-driven) laser diode. That is, the level of the feedback signal is higher than required (for large diode drive currents) in order to achieve wavelength stabilization, thereby reducing the overall output power of the laser diode system. On the other hand, if the feedback is optimized for large diode drive currents, it will be too small for low currents. Furthermore, the VBGs of the prior art need to be adjusted in their geometrical layout in order to achieve the necessary (low) reflectivity, thus causing undesirable adverse effects such as diffraction. Furthermore, this laser diode arrangement suffers from a reduction in the total output power, since the laser diodes used do not produce fully polarized radiation. In fact, a typical range of polarization exhibited by practical laser diodes for the main polarization direction is about 80% to 95%. That is, the unpolarized part of the emitted laser radiation disappears within the laser system, for example at an edge filter or a polarizing filter.

It is therefore an object of the present invention to provide a diode laser with wavelength stabilization which overcomes the drawbacks of the prior art and allows an increased total output power.

Disclosure of Invention

According to a first aspect of the present invention, a laser, preferably an external cavity diode laser, is disclosed, comprising an active medium (e.g. an active layer) placed within an internal laser cavity, the internal laser cavity comprising an exit facet adapted for outcoupled laser radiation having a polarization part with a higher radiation intensity in a spatially stable main polarization direction and a corresponding lower radiation intensity in a secondary polarization direction different from said main polarization direction; an external frequency selective element placed outside the internal laser cavity and adapted for wavelength stabilization of the laser radiation; and a polarizing beam splitter positioned outside the internal laser cavity and adapted for splitting outcoupled laser radiation into a first beam extending along a first beam path and a second beam extending along a second beam path, the outcoupled laser radiation having a higher radiation intensity in a spatially stable main polarization direction and a polarization portion of a corresponding lower radiation intensity in a sub-polarization direction different from the main polarization direction, the first beam path being different from the second beam path, the polarizing beam splitter being arranged to pass the entire radiation intensity of the outcoupled laser radiation from the main polarization direction into the first beam extending along the first beam path; wherein an external frequency selective element is arranged in a second beam path and the second beam with the laser intensity of the outcoupled laser radiation from the secondary polarization direction is reflected back to the internal laser cavity for frequency stabilization.

The main idea of the invention is to arrange an external frequency selective element outside the main beam path comprising at least a major part, i.e. more than 50%, of the outcoupled laser radiation power. Thus, the external frequency selective element may be placed in an additional feedback (i.e. second) beam path, different from the main (i.e. first) beam path, wherein the intensity of the radiation incident on the frequency selective element is controlled by a suitable intensity control means, so that the additional feedback beam path has a lower radiation intensity, resulting in less thermal stress of the frequency selective element.

Preferably, the polarized portion in the secondary polarization direction results from an unpolarized portion of the outcoupled laser radiation.

Preferably, the degree of polarization of the main polarization in the outcoupled laser radiation is between 60% and 99%.

Preferably, the active medium is constituted by a laser diode formed as an edge-emitting laser diode. Preferably, the internal laser cavity is formed by at least two opposing mirrors. Alternatively, the active medium can consist of a solid state laser with a wide gain bandwidth, such as Yb doped material or a wire selection in a solid state laser material with several discrete wires close to each other. Preferably, the active medium is adapted to emit radiation having a wavelength with a maximum intensity in the range of 400nm to 2900nm, preferably in the range of 750nm to 1100nm, more preferably in the range of 1400nm to 1600nm or 1700nm to 2000 nm. Preferably, the active medium comprises a semiconductor material. More preferably, the active medium is composed of a semiconductor material.

Preferably, the intensity incident to the frequency selective element is controlled by the intensity control means such that the total intensity of the ECDL according to the invention is reduced or increased when it is increased or decreased, respectively. More preferably, the intensity in the feedback beam path is kept constant or substantially constant by the intensity control means. Even more preferably, the intensity in the feedback beam path is controlled such that a constant intensity is fed back into the internal laser cavity, or more particularly into the active medium within the internal laser cavity.

Preferably, the intensity control means may be a single element arranged in the second beam path. More preferably, the intensity control means consists of a beam splitter and/or a frequency selective element arranged in the second beam path in an integrated manner. Even more preferably, the intensity control means consists of a combination of a single element and an integrated element arranged in the second beam path. The intensity control means may be formed as passive intensity control means or as active intensity control means.

Preferably, the radiation intensity of the first beam is greater than 60% of the radiation intensity of the outcoupled laser radiation, whereas the radiation intensity of the second beam is less than 40% of the radiation intensity of the outcoupled laser radiation, more preferably, the radiation intensity of the first beam is greater than 70% of the radiation intensity of the outcoupled laser radiation, whereas the radiation intensity of the second beam is less than 30% of the radiation intensity of the outcoupled laser radiation, more preferably, the radiation intensity of the first beam is greater than 80% of the radiation intensity of the outcoupled laser radiation, whereas the radiation intensity of the second beam is less than 20% of the radiation intensity of the outcoupled laser radiation, even more preferably, the radiation intensity of the first beam is greater than 90% of the radiation intensity of the outcoupled laser radiation, whereas the radiation intensity of the second beam is less than 10% of the radiation intensity of the outcoupled laser radiation.

Preferably, the external frequency selective element is formed as a reflector. More preferably, the reflector is formed as a volume bragg grating, also referred to as VBG. Preferably, the (maximum) reflectance (reflectance) of the reflector at the specific wavelength for which said external frequency selective element is designed is greater than 60%, preferably greater than 70%, more preferably greater than 80%, more preferably greater than 90%, more preferably greater than 95%, and still more preferably greater than 99%. Preferably, the reflectance of the reflector at a second wavelength different from the specific wavelength at which reflectance is maximum and greater than 5nm (more preferably greater than 4nm, more preferably greater than 3nm, more preferably greater than 2nm, still more preferably greater than 1nm) is less than 60% of the maximum reflectance, more preferably less than 40% of the maximum reflectance, more preferably less than 20% of the maximum reflectance, more preferably less than 10% of the maximum reflectance, still more preferably less than 5% of the maximum reflectance. In other words, the reflector exhibits a high maximum reflectance over a small wavelength range, thereby generating a wavelength-specific feedback signal that is coupled back into the internal laser cavity, thereby stabilizing the lasing wavelength. The reflection bandwidth for a reflector having a reflectance of 50% or more of the maximum reflectance is preferably less than 2nm, more preferably less than 1nm, more preferably less than 0.5nm, and still more preferably less than 0.2 nm.

Preferably, in order to obtain a compact design, the distance between the exit facet of the internal laser cavity and the beam splitter is relatively small. In other words, the distance between the exit facet of the internal laser cavity and the beam splitter is preferably less than 100cm, more preferably less than 50cm, more preferably less than 25cm, more preferably less than 10cm, still more preferably less than 5 cm. Preferably, also in order to obtain a compact design, the distance between the beam splitter and the external frequency selective element is relatively small. In other words, the distance between the beam splitter and the external frequency selective element is preferably less than 100cm, more preferably less than 50cm, more preferably less than 25cm, more preferably less than 10cm, still more preferably less than 5 cm.

Preferably, the exit face (also referred to as front face) comprises a reflectivity for the emitted laser radiation in the range from 0.1% to 12%, more preferably from 0.1% to 6%, even more preferably from 0.1% to 3%. Preferably, the latter comprises a reflectivity ranging from 80% to 99.999%, more preferably from 95% to 99.99%, still more preferably from 99.0% to 99.9% for the emitted laser radiation.

Preferably, the beam splitter is one selected from a polarizing beam splitter, a thin film Polarizer, and a Glan laser Polarizer (also known as a "Glan-Taylor Polarizer"). That is, the laser diode according to an aspect of the present invention aims to increase the total output power by employing these portions of unpolarized emitted light of the laser diode to achieve frequency stabilization. In conventional laser diode systems, the unpolarized part of the emitted laser radiation disappears in the laser system, for example at an edge filter or a polarizing filter, whereas, according to the present invention, the unpolarized part of the emitted laser radiation is outcoupled into the second beam path to be (frequency selectively) reflected back into the internal laser cavity for frequency (or wavelength) stabilization. In order to obtain a sufficient feedback signal for frequency stabilization, it is preferred to use laser diodes which do not produce fully polarized radiation. Preferably, the laser diode is formed such that the degree of polarization (for the main polarization direction) ranges from 60% to 99%, more preferably from 70% to 97%, more preferably from 80% to 95%, still more preferably from 85% to 92%.

Preferably, the polarization modifying means is arranged in a second beam path between the beam splitter and the external frequency selective element, wherein the polarizer is located in the second beam path between the polarization modifying means and the external frequency selective element. An advantage of this preferred embodiment is that the intensity of the wavelength specific feedback signal, which is coupled back into the internal laser cavity, can be controlled. That is, the overall intensity of the laser diode can be controlled by the pumping energy, for example by controlling the drive current of the laser diode. The intensity of the radiation in the second beam path (and accordingly the intensity of the feedback signal) is related to the intensity of the radiation outcoupled from the laser diode. Therefore, when the overall laser diode emission intensity increases, which may mean thermal stress of an external frequency selective element formed as a VGB, for example, the intensity of the feedback signal increases. However, the strength of the feedback signal only needs to be above a certain (absolute) threshold. It is therefore advantageous to adapt the polarization modifying means such that the intensity of the radiation at the VBG is independent of the total intensity of the laser diode, more particularly to control the polarization in the second beam path such that the intensity of the radiation passing through the polarizer ranges between 100% and 200%, more preferably between 100% and 150%, even more preferably between 100% and 120%, of a (minimum) threshold value necessary for stable laser operation of the laser diode. The polarization modifying means is preferably formed as an electro-optical polarizer. More preferably, the polarization modifying means is formed as a Pockels cell. The polarizer located in the second beam path between the polarization modifying means and the frequency selective element is preferably formed as a thin film polarizer arranged at a Brewster angle with respect to the main beam propagation direction in the second beam path.

Preferably, the polarization modifying means is connected to (the control unit of) the (current-driven) laser diode. Preferably, the polarization modifying means is connected to means for detecting (or determining) the total radiation intensity of the laser diode or its equivalent.

According to an alternative preferred embodiment, the polarization modifying means is arranged in a first beam path between the beam splitter and the exit facet of the internal laser cavity, wherein the polarizer is located in a second beam path between the beam splitter and the external frequency selective element. According to this preferred embodiment it is also possible to obtain a controlled feedback signal for a laser diode emitting (substantially) fully polarized radiation.

According to an alternative preferred embodiment, the external frequency selective element comprises a mirror having an intensity dependent reflection coefficient in order to adapt the intensity of the feedback signal independently of the total intensity of the laser diode. Such mirrors with intensity dependent reflection coefficients are known in the art, for example from "Thermo-optically driven mirrors for laser applications" by Michel et al (appl. phys. b, pp.721-724 (2004)). Preferably, the mirror with an intensity dependent reflection coefficient is adapted such that the intensity of the feedback signal varies by less than 30%, more preferably by less than 20%, more preferably by less than 10%, even more preferably by less than 5%, irrespective of the overall intensity of the laser diode.

Preferably, the mirror with an intensity-dependent reflection coefficient is connected to (the control unit of the) laser diode. Preferably, the mirror with an intensity-dependent reflection coefficient is connected to means for detecting the total radiation intensity of the laser diode or its equivalent.

According to an alternative preferred embodiment, the beam splitter is a non-polarizing beam splitter. According to this preferred embodiment it is also possible to obtain a controlled feedback signal for the laser diode emitting fully polarized radiation.

Preferably, the (non-polarizing) beam splitter comprises a plurality of sections with different partial reflectivities, which are arranged side by side (but not necessarily directly adjacent). Preferably, the laser diode device further comprises means for moving the beam splitter (preferably laterally) preferably with respect to the propagation direction of the outcoupled laser radiation. Preferably, the laser diode device further comprises means for controlling the (lateral) movement of the beam splitter. Preferably, the means for controlling the (lateral) movement of the beam splitter is adapted for controlling the movement of the beam splitter in dependence on the intensity of the outcoupled laser radiation. Preferably, the means for controlling the lateral movement of the beam splitter is adapted to control the movement of the beam splitter such that the intensity of the feedback signal varies by less than 30%, more preferably by less than 20%, more preferably by less than 10%, even more preferably by less than 5%, irrespective of the overall intensity of the laser diode. Preferably, the plurality of portions having different partial reflectivities directly adjoin each other or directly adjoin their equivalents.

Preferably, the means for controlling the lateral movement of the beam splitter are connected to (the control unit of the) laser diode(s). Preferably, the means for controlling the movement of the beam splitter is connected to means for detecting the total radiation intensity of the laser diode or its equivalent.

According to a preferred embodiment (instead of the means for moving the beam splitter and the means for controlling the movement of the beam splitter), the laser diode further comprises a multi-part mirror comprising a plurality of parts having different reflectivities, which are arranged side by side (but not necessarily directly adjoining, e.g. directly adjoining on a circle), which multi-part mirror is arranged in the second beam path between the (preferably non-polarizing) beam splitter and the external frequency selective element, wherein the deflection means is arranged in the second beam path between the beam splitter and the multi-part mirror, wherein the deflection means is adapted to deflect radiation in the second beam path onto different parts of the multi-part mirror depending on the intensity of outcoupled laser radiation. Preferably, the plurality of portions having different partial reflectances directly adjoin each other.

According to a part of the present invention, a laser diode device comprises a plurality of diode lasers with internal laser cavities, each internal laser cavity comprising an exit facet adapted for outcoupling laser radiation, wherein the internal laser cavities are arranged such that a plurality of stacked laser beams is directed onto a common beam splitter. Preferably, the plurality of stacked laser beams are arranged parallel to each other.

Preferably, the common beam splitter comprises a plurality of stepped portions, each of the stepped portions being adapted to split one of the plurality of stacked parallel laser beams into a first beam extending along a first beam path and a second beam extending along a second beam path, each first beam having a higher radiation intensity than each second beam and each first beam path being different from each second beam path.

Preferably, all the stepped portions have the same inclination angle with respect to the propagation direction of the plurality of stacked laser beams. Preferably, the stepped portions are arranged equidistant from each other.

According to a preferred embodiment, the laser diode device further comprises a deflection device arranged in the second beam path between the beam splitter and the external frequency selective element, wherein the deflection device is adapted to deflect the radiation in the second beam path such that only a portion of the radiation in the second beam path is directed onto the active area of the external frequency selective element, the amount of the radiation portion being dependent on the intensity of the outcoupled laser radiation. Preferably, no further elements are arranged in the second beam path between the deflection means and the external frequency selective element. Preferably, the deflection means is formed as an acousto-optic modulator (AOM) or a Spatial Light Modulator (SLM).

According to a preferred embodiment, the laser diode device further comprises an intensity adapted focusing lens arranged in the second beam path between the beam splitter and the external frequency selective element, wherein the focusing lens is adapted to focus the radiation in the second beam path on the active area of the external frequency selective element, the focusing power of the focusing lens being dependent on the intensity of the outcoupled laser radiation. Such intensity-dependent focusing lenses are known in the art, for example from R.Koch, "Self-adaptive optical elements for compensation of thermal lengthening effective silicon diode-pumped solid state lasers-

General and preliminary experiments (adaptive Optics for compensating thermal lensing in diode-pumped solid state lasers-recommended and preliminary experiments) "(Optics Communications 140(1997), 158-164). Preferably, no further elements are arranged in the second beam path between the focusing lens and the external frequency selective element.

According to another aspect of the present invention, a laser (preferably a diode laser) is disclosed, comprising an active medium (e.g. an active layer) positioned within a laser cavity, the laser cavity comprising an exit facet adapted for outcoupling laser radiation; an external frequency-selective element placed outside the laser cavity and adapted for wavelength stabilization of the laser radiation, and a beam splitter adapted for splitting the outcoupled laser radiation into a first beam extending along a first beam path and a second beam extending along a second beam path, the first beam having a higher radiation intensity than the second beam and the first beam path being different from the second beam path, wherein the external frequency-selective element is arranged in the second beam path.

The main idea of this aspect of the invention is to arrange the external frequency selective element outside the main beam path comprising at least a major part, i.e. more than 50%, of the outcoupled laser radiation power. Thus, the external frequency selective element may be placed in an additional feedback (i.e. second) beam path different from the main (i.e. first) beam path, the additional feedback beam path having a lower radiation intensity, resulting in less thermal stress of the frequency selective element.

Drawings

Hereinafter, the present invention will be described in more detail. The examples given are suitable for describing the invention, but do not limit it in any way. In particular, the invention is not limited to diode laser cavities comprising an active medium and an internal laser cavity. The active medium and/or the internal laser cavity may be included in any suitable laser system.

Fig. 1 shows a schematic cross-sectional view of a laser diode arrangement according to a first preferred embodiment of the present invention;

fig. 2 shows a schematic cross-sectional view of a laser diode arrangement according to a second preferred embodiment of the present invention;

fig. 3 shows a schematic cross-sectional view of a laser diode arrangement according to a third preferred embodiment of the present invention;

fig. 4a to 4d show schematic cross-sectional views of a laser diode arrangement according to a fourth preferred embodiment of the present invention;

fig. 5 shows a schematic cross-sectional view of a laser diode arrangement according to a fifth preferred embodiment of the present invention;

fig. 6 shows a schematic cross-sectional view of a laser diode arrangement according to a sixth preferred embodiment of the present invention;

fig. 7 shows a schematic cross-sectional view of a laser diode arrangement according to a seventh preferred embodiment of the present invention.

Wherein:

10. an internal laser cavity; 12. an exit surface; 14. a frequency selective element; 16. a beam splitter; 18. a polarization modifying device; 20. a polarizer; 22. a multi-part mirror; 24. a deflection device; 26. a focusing lens; 28. a curved mirror;

b0, outcoupled laser radiation; b1, a first light beam; b2, a second light beam; b3, partial beam of the second light beam; b4, partial beam of the second light beam; b01, laser beam; b02, laser beam; b03, laser beam; b04, laser beam; p1, first beam path; p2, second beam path.

Detailed Description

Fig. 1 shows a schematic cross-sectional view of a laser diode arrangement according to a first preferred embodiment of the present invention.

Diode laser cavities are used to pump solid state lasers and serve applications of plastic welding, soldering, cladding and heat conduction welding, all of which are major markets. Due to their limited power and beam quality, deep fusion welding and, more importantly, cutting with conventional diode laser systems is not cost effective. The deep fusion welding requires a beam with a mass in the range of 10mm mrad to 20mm mrad with a power in the range of 1kW to 6kW and greater. The cutting requires a beam with a mass of less than 10mm mrad and a power level in the range of 2kW to 4kW, in particular for cutting thin gauges of material to a few millimetres of about 3mm mrad and 2kW to 3kW, and for thicker gauges of material about 7mm mrad.

Optical stacks are known for power scaling, and many different configurations are available for laser bars and single emitters. The spectral stacking allows for scaling of brightness and power. For subsequent spectral combinations of multiple diodes with different wavelengths, a narrow and stable spectrum of a single diode is required. Multiple single emitters, each rated at, for example, 12W, may be stacked in the fast axis, for example, with a monolithic array of Slow Axis Collimators (SAC). Although the most preferred embodiments of the present invention relate to a single emitter diode, it should be understood that optical and/or spectral stacking may be advantageously deployed in all embodiments of the present invention.

The power and brightness of these systems enable cutting and welding using diode lasers. This technique can be transferred to other wavelengths including 793nm and 1980 nm. The optimized spectral combination enables further improvements in spectral brightness and power. Fast control electronics and miniaturized switching power supplies make it possible to have pulse rise times of less than 10 mus with continuously adjustable pulse widths from 20 mus to cw.

In a preferred embodiment, an externally disposed volume bragg grating stabilizes the wavelength and narrows the line width to less than 1 nm. Wavelength stabilization using an external frequency selective element such as VBG reflects part of the emitted light with the desired wavelength back into the diode. The design of the external resonator, i.e. the diode front facet reflectivity (i.e. the outcoupling facet reflectivity) and the reflectivity and dimensions of the external frequency selective element determine the resulting line width and locking range. Typically, the linewidth is narrowed from 5nm (fwhm) to 0.3nm (fwhm) spectrum, corresponding to 95% of the power in less than 1 nm. With proper resonator design, the drive current is changed from a threshold (e.g., 0.5A) to full power (e.g., 12A), with the peak wavelength nearly constant. The resonator design determines the locking range and higher locking ranges are achieved with higher reflectivity, but the power loss increases.

According to a first preferred embodiment of the present invention, as shown in fig. 1, a laser diode comprises an active layer and an internal laser cavity 10, comprising an exit facet 12 through which laser radiation B0 of the laser diode is coupled out.

Laser diodes are typically based on a double heterostructure for the 9xx nm region, built in sandwich fashion, with aluminum arsenide (Al)_xGa₍₁X) As) in a broadband diode laser with an output power of 12W at 10A drive current and an emission wavelength of 970nm in this embodiment, the emission surface has an area of about 96 × 4 μm with a 3% topcoat, a small height of the emission area and a high difference between the refractive indices of the laser material and air result in a high divergence of up to 23 °, the so-called fast axis, the divergence in the slow axis being only 4 °.

The degree of polarization is determined by the gain within the waveguide, which is different for pi (te) and sigma (tm) polarized light and slightly greater for sigma polarized light. The degree of polarization depends on the strain of the embedded quantum well. The polarization can be switched from TE to TM by compressive or tensile strain in the quantum well structure. This change in tension affects the heavy hole gain and the light hole gain differently. But this also has an effect on the performance of the laser diode, which means that there is a fixed polarization at optimum performance.

The laser diode exhibits a degree of polarization of about 93% for the main polarization direction, which in this embodiment is parallel polarization. The outcoupled laser radiation B0 is directed to a polarizing beam splitter 16 which transmits all radiation with parallel polarization into a first beam path P1, forming a first beam B1. The intensity of the first beam B1 is about 95% of the intensity of the outcoupled laser beam B0. Furthermore, polarizing beam splitter 16 deflects a portion of radiation B0 that is not parallel polarized into a second beam path P2, thereby forming a second beam B2. An advantage of the first embodiment of the invention is that portions of the radiation B0 which differ from the main polarization direction (parallel) can be used for wavelength stabilization in the second beam path which has only an intensity of about 5% of the intensity of the outcoupled laser beam B0, thereby minimizing thermal stress of the frequency selective element 14 formed as a VBG. The VBG14 reflects only a narrow portion of 0.3nm (fwhm) of the beam B2 back to the laser diode, thereby stabilizing the wavelength of the outcoupled laser beam B0.

Fig. 2 shows a schematic cross-sectional view of a laser diode arrangement according to a second preferred embodiment of the present invention. The embodiment of fig. 2 is similar to the embodiment of fig. 1, but further comprises a polarization modifying device 18 and a polarizer 20 located in the second path P2. Another advantage of this embodiment is that the intensity of the second beam B2 can be adjusted (before reaching the VBG 14) so that the intensity of the second beam B2 is below a predetermined value set so that the intensity of the second beam B2 reflected back into the laser diode is sufficient to enable stable laser operation at the desired wavelength at which the VBG14 is designed. At the same time, the predetermined value is set such that the intensity of the second light beam B2 directed onto the VBG14 is low enough to avoid thermal stress at the VBG 14.

Fig. 3 shows a schematic cross-sectional view of a laser diode arrangement according to a third preferred embodiment of the present invention. The embodiment of fig. 3 is similar to the embodiment of fig. 1, but also comprises a beam splitter 16 having four different sections 16-1, 16-2, 16-3 and 16-4, each of the sections 16-1, 16-2, 16-3 and 16-4 having a different reflectance for the outcoupled laser radiation B0. Preferably, beam splitter 16 is a non-polarizing beam splitter. The laser diode device according to the third preferred embodiment further comprises means 16-5 for moving the beam splitter 16 along the longitudinal axis indicated by the arrow. An advantage of this embodiment is that the intensity of the second beam B2 reflected back into the laser diode can be controlled by moving the beam splitter 16 so that only one of the portions 16-1, 16-2, 16-3 and 16-4 is selected to split the beam B0 into the first beam B1 and the second beam B2 so that the intensity of the second beam B2 is high enough to achieve stable laser operation and low enough to avoid thermal stress at the VBG 14. In order to adapt the intensity of the second light beam B2 independently of the intensity of the outcoupled laser beam B0, the means 16-5 for moving the beam splitter 16 are preferably connected to the means for detecting the intensity of the outcoupled laser beam B0.

Fig. 4a to 4d show schematic cross-sectional views of a laser diode arrangement according to a fourth preferred embodiment of the present invention. The embodiments of fig. 4a to 4d are similar to the embodiment of fig. 1, but each embodiment has a non-polarizing beam splitter 16 instead of a polarizing beam splitter, and further comprises a deflection device 24 and mirrors 22 and 28, which may be multi-part mirrors 22 or curved mirrors 28. An advantage of this embodiment is that the deflection means 24 and the (multi-part) mirror 22, 28 are fixed in this embodiment, whereas the beam splitter 16 of the third embodiment, which is also a partial multi-part mirror, is arranged to be movable along its longitudinal axis. The deflection means 24 is formed, for example, as an AOM. AOM 24 may be controlled to deflect beam B2 according to the intensity of second beam B2.

As shown in the embodiment in FIG. 4a, the curved multi-section mirror 28 enables beams deflected at different angles by the AOM 24 and impinging on the curved mirror 28 at different positions 28-1, 28-2 and 28-3 to be collimated and deflected onto the VBG 14. Multi-portion mirror 28 may include different portions 28-1, 28-2, and 28-3 each having a different reflectance for each outcoupled laser radiation of second beam path P2 (beam B2). The VBG14 then reflects the beam back into the laser diode for wavelength stabilization. The AOM 24 can be adjusted to deflect the beam B2 such that the intensity of the second beam B2 at the VBG14 is high enough to achieve stable laser operation and low enough to avoid thermal stress at the VBG 14. In order to adapt the intensity of the second light beam B2 independently of the intensity of the outcoupled laser beam B0, the AOM 24 is preferably connected to a device for detecting the intensity of the outcoupled laser beam B0.

Instead of using a curved mirror 28, it may alternatively be advantageous to use a stepped mirror 22 having a plurality of planar portions 22-1, 22-2 and 22-3 as shown in FIG. 4 b. The planar segments 22-1, 22-2 and 22-3 have different angles relative to the VBG14 to collimate the divergent radiation from the AOM 24 that impinges on the stepped mirror 22. Different portions 22-1, 22-2, and 22-3 may each have a different reflectance for each outcoupled laser radiation of second beam path P2 (beam B2).

As shown in the embodiment of fig. 4c, the VBG14 may be utilized and positioned such that divergent radiation from the AOM 24 impinges on the VBG 14. The VBG14 is formed as a multi-part VBG including a plurality of parts having different reflectance. For example, as shown in FIG. 4c, the VBG14 may have a gradually increasing reflectance along an axis x located in its cross-section. In this embodiment, a curved mirror 28 adapted for reflecting impinging radiation back into the laser diode 10 is utilized, i.e. the curved mirror 28 is formed such that all rays from the VBG14 are reflected back to its own end. In this case, the curved mirror 28 may have a uniform reflectance along its inner surface.

Instead of using a curved (uniform reflecting) mirror 28, it may alternatively be advantageous to use a stepped mirror 22 having a plurality of planar portions 22-1, 22-2 and 22-3 as shown in fig. 4 d. The planar segments 22-1, 22-2 and 22-3 have different angles relative to the VBG14 such that all rays from the VBG14 are reflected back to their ends. Different portions 22-1, 22-2, and 22-3 may have the same reflectance for each outcoupled laser radiation of second beam path P2 (beam B2). The AOM 24 can be adjusted to deflect the beam B2 such that the intensity of the second beam B2 at the VBG14 is high enough to achieve stable laser operation and low enough to avoid thermal stress at the VBG 14. In order to adapt the intensity of the second light beam B2 independently of the intensity of the outcoupled laser beam B0, the AOM 24 is preferably connected to a device for detecting the intensity of the outcoupled laser beam B0.

Fig. 5 shows a schematic cross-sectional view of a laser diode arrangement according to a fifth preferred embodiment of the present invention. The embodiment of fig. 5 is similar to the embodiment of fig. 4, but does not include a multi-part mirror 22 located between the deflection means 24 and the frequency selective element 14. This embodiment has the advantage that a simplified arrangement can be utilized which can control the intensity of the second light beam B2 at the VBG14 to be high enough to enable stable laser operation and low enough to avoid thermal stress at the VBG 14. In detail, the AOM 24 is arranged relative to the position of the VBG14 such that the second beam B2 is directed fully towards the VBG14 only for relatively low intensities of the second beam B2 that are low enough to avoid thermal stress at the VBG 14. When an increase in the intensity of the outcoupled laser radiation B0 results in an increase in the intensity of the second beam B2, the AOM 24 is controlled such that the beam B2 is deflected such that only a portion B4 of the second beam B2 reaches the VBG14, while the other portion B3 of the second beam B2 passes through the VBG14 without hitting the active (reflective) region of the VBG 14. Thus, the intensity of beam B4 at VBG 24 can be controlled by AOM 24 to be high enough to achieve stable laser operation and low enough to avoid thermal stress at VBG 14. In order to adapt the intensity of the second light beam B4 independently of the intensity of the outcoupled laser beam B0, the AOM 24 is preferably connected to a device for detecting the intensity of the outcoupled laser beam B0 or to a device for detecting the intensity of the second light beam B2.

Fig. 6 shows a schematic cross-sectional view of a laser diode arrangement according to a sixth preferred embodiment of the present invention. According to this embodiment, an intensity adapted focusing lens 26 is arranged in the second beam path P2 having a focusing characteristic depending on the intensity of the second light beam B2. That is, the lens 26 is arranged relative to the position of the VBG14 such that the second light beam B2 is only directed completely at the VBG14 at a relatively low intensity that is low enough to avoid thermal stress at the VBG 14. When an increase in the intensity of the outcoupled laser radiation B0 results in an increase in the intensity of the second beam B2, the lens 26 is controlled such that the beam B2 has a high divergence after passing through the lens 26, for example by adapting the focal length of the lens 26 to be much smaller than the distance between the lens 26 and the VBG 14. In this case, only a portion of the second light beam B2 reaches the VBG14, while another portion of the second light beam B2 passes through the VBG14 without striking the active (reflective) region of the VBG14 due to high divergence. Thus, the intensity of the beam at the VBG14 can be controlled by the lens 26 to be high enough to achieve stable laser operation and low enough to avoid thermal stress at the VBG 14. In order to adapt the intensity of the second light beam B2 independently of the intensity of the outcoupled laser beam B0, the lens is preferably connected to means for detecting the intensity of the outcoupled laser beam B0 or to means for detecting the intensity of the second light beam B2.

Fig. 7 shows a schematic cross-sectional view of a laser diode arrangement according to a seventh preferred embodiment of the present invention. In this embodiment, the optical stack is implemented using multiple single emitter laser diodes stacked together to produce parallel laser beams B01, B02, B03, and B04. The beam splitter 16 includes a plurality of stepped portions, each portion having a partially reflective surface that splits each of the laser beams B01, B02, B03, and B04 into a first beam extending along a first beam path P1 and a second beam extending along a second beam path P2. The stepped portion of the beam splitter 16 is formed such that the optical path from the exit face 12 of each laser diode to the VBG14 is the same for all the laser beams B01, B02, B03, and B04. These laser beams B01, B02, B03, and B04 can advantageously use a common VBG14, thereby reducing the overall cost of the stacked laser diode device.