BACK-PUMPED SEMICONDUCTOR MEMBRANE LASER

Abstract

A semiconductor membrane laser chip includes a planar-shaped lasing medium having an upper surface and a lower surface opposite the upper surface, the lasing medium configured to emit electromagnetic radiation at a laser wavelength λ.sub.1. A first heat spreader is bonded to one of the upper surface and the lower surface of the lasing medium. A first dielectric layer is arranged on the lower surface of the lasing medium or arranged on a lower surface of the first heat spreader when the first heat spreader is bonded to the lower surface of the lasing medium. The first dielectric layer is reflective for the laser wavelength λ.sub.1.

Claims

1-20. (canceled)

21. A semiconductor membrane laser chip comprising: a planar-shaped lasing medium having an upper surface and a lower surface opposite the upper surface, the lasing medium configured to emit an electromagnetic radiation at a laser wavelength λ.sub.1; a first heat spreader bonded to one of the upper surface and the lower surface of the lasing medium; a first dielectric layer arranged on the lower surface of the lasing medium or arranged on a lower surface of the first heat spreader when the first heat spreader is bonded to the lower surface of the lasing medium, wherein the first dielectric layer is reflective for the laser wavelength λ.sub.1.

22. The semiconductor membrane laser chip according to claim 21, wherein the planar-shaped laser medium is configured to emit the electromagnetic radiation at the laser wavelength λ.sub.1 when optically pumped by an electromagnetic radiation of a pump wavelength λ.sub.2.

23. The semiconductor membrane laser chip according to claim 22, wherein the first dielectric layer is transmissive for the electromagnetic radiation of the pump wavelength λ.sub.2.

24. The semiconductor membrane laser chip according to claim 21, further comprising a second dielectric layer arranged on the upper surface of the lasing medium or arranged on an upper surface of the first heat spreader when the first heat spreader is bonded to the upper surface of the lasing medium, the second dielectric layer having a transmissivity for the laser wavelength λ.sub.1 larger than a transmissivity of the first dielectric layer for the laser wavelength λ.sub.1.

25. The semiconductor membrane laser chip according to claim 21, further comprising a second heat spreader bonded to the other one of the upper surface and the lower surface of the lasing medium.

26. The semiconductor membrane laser chip according to claim 25, further comprising a first contact layer adjacently arranged to one of the upper surface and the lower surface of the lasing medium or adjacently arranged to a surface of one of the first heat spreader and the second heat spreader, wherein the surface of the one of the first heat spreader and the second heat spreader faces away from the lasing medium.

27. The semiconductor membrane laser chip according to claim 26, further comprising a second contact layer adjacently arranged to the other one of the upper surface and the lower surface of the lasing medium or adjacently arranged to a surface of the other one of the first heat spreader and the second heat spreader, wherein the surface of the other of the first heat spreader and the second heat spreader faces away from the lasing medium.

28. The semiconductor membrane laser chip according to claim 27, wherein at least one of the first contact layer and the second contact layer has an opening or aperture in which a corresponding one of the first dielectric layer and the second dielectric layer is arranged.

29. The semiconductor membrane laser chip according to claim 27, wherein at least one of the first contact layer and the second contact layer comprises a metal contact layer configured for soldering to a submount, wherein the submount comprises a metal body.

30. The semiconductor membrane laser chip according to claim 21, further comprising a submount comprising a metal body having a recess sized to receive the lasing medium, the first heat spreader and the first dielectric layer therein.

31. The semiconductor membrane laser chip according to claim 25, wherein at least one of the first heat spreader and the second heat spreader comprises a thermally conductive material including at least one of silicon carbide, diamond and aluminum oxide.

32. The semiconductor membrane laser chip according to claim 21, wherein the lasing medium comprises a semiconducting material including at least one of AlGaInAsP, AlInGaN, AlGaInAsSb and AlGaInNAs.

33. The semiconductor membrane laser chip according to claim 24, wherein at least one of the first dielectric layer and the second dielectric layer comprises a dielectric material including at least one of SiO.sub.2, Nb.sub.2O.sub.5, HfO.sub.2, TiO.sub.2, Al.sub.2O.sub.3 and Ta.sub.2O.sub.5.

34. A laser arrangement comprising: a semiconductor membrane laser chip according to claim 21; and a pump laser configured to emit an electromagnetic radiation at a pump wavelength λ.sub.2; wherein the pump laser is arranged and configured to emit the electromagnetic radiation through the first dielectric layer into the lasing medium.

35. The laser arrangement according to claim 34, wherein the pump laser comprises at least one edge-emitting laser diodes or wherein the pump laser comprises at least one laser diode bars.

36. The laser arrangement according to claim 34, wherein an optical path between the pump laser and the semiconductor membrane laser chip is void of collimating or focusing optical elements.

37. The laser arrangement according to claim 34, further comprising a submount comprising a metal body and wherein the semiconductor membrane laser chip comprises at least one contact layer thermally coupled to the submount by soldering.

38. A method of manufacturing a plurality of the semiconductor membrane laser chip according to claim 21, the method comprising: providing the lasing medium on a substrate; arranging or forming the first heat spreader on the upper surface of the lasing medium facing away from the substrate; removing the substrate; arranging or forming the first dielectric layer on one of the lower surface of the lasing medium facing away the first heat spreader and an upper surface of the first heat spreader facing away the lasing medium.

39. The method according to claim 38, further comprising: arranging or forming a second heat spreader on the lower surface of the lasing medium when the first dielectric layer is arranged or formed on the upper surface of the first heat spreader.

40. The method according to claim 38, wherein the substrate comprises a wafer of a predetermined wafer size, wherein the lasing medium, the first heat spreader and the first dielectric layer extend across the wafer size and form a wafer layer stack, and wherein the method of manufacturing a plurality of the semiconductor membrane laser chips further comprises dicing the wafer layer stack into individual ones of the plurality of semiconductor membrane laser chips.

Description

BRIEF DESCRIPTION OF THE DRAWING FIGURES

[0082] FIG. 1 shows a semiconductor disk laser with a flip-chip process according to the prior art.

[0083] FIG. 2 shows a semiconductor disk laser with an intra-cavity heat spreader according to the prior art.

[0084] FIG. 3 shows an inclined pumped semiconductor membrane laser according to the prior art.

[0085] FIG. 4 shows an example of a back-pumped semiconductor membrane laser according to the present invention.

[0086] FIG. 5 shows a cross-section of the amplifier unit of the back-pumped semiconductor membrane laser.

[0087] FIG. 6 shows a partial section of a production of a large number of amplifier units on wafer scale.

[0088] FIG. 7 shows in cross-section an exemplary setup of a laser resonator with complete amplifier unit on a submount.

[0089] FIGS. 8a and 8b show in cross-section the amplifier unit for a compact component with integrated edge-emitting diode as pump source.

[0090] FIG. 9 shows a further example of the amplifier unit in which a submount is mounted on one side of the semiconductor membrane laser chip.

[0091] FIG. 10 shows a flow diagram of a manufacturing process for producing an amplifier unit.

[0092] FIG. 11 shows an example of a process of manufacturing a layer stack for producing semiconductor membrane laser chips.

[0093] FIG. 12 shows another example of a process of manufacturing a layer stack for producing semiconductor membrane laser chips.

[0094] FIG. 13 shows a further example of a process of manufacturing a layer stack for producing semiconductor membrane laser chips.

DETAILED DESCRIPTION OF THE INVENTION

[0095] The invention will now be described on the basis of the drawing figures. It will be understood that the embodiments and aspects of the invention described herein are only examples and do not limit the protective scope of the claims in any way. The invention is defined by the claims and their equivalents. It will be understood that features of one aspect or embodiment of the invention can be combined with a feature of a different aspect or aspects and/or embodiments of the invention.

[0096] FIG. 4 shows an example of the semiconductor membrane laser according to the present invention implemented as a back-pumped semiconductor membrane laser. Here, the lasing medium 510 is pumped by radiation 150 of a pump laser 160 (see FIG. 5) from the back of the semiconductor membrane laser. A second heat spreader 520b is coated with a coating 410 which is transparent to light from the pump laser 160 but reflects light of the wavelength of the light generated in the active region, i.e. in a lasing or amplifier medium 510. The lasing medium 510 and hence the layer stack of the lasing or amplifier medium 510 is sandwiched by a first heat spreader 520a and the second heat spreader 520b. A lower surface of the second heat spreader 520b, i.e. facing away from the lasing medium 510, is provided with the coating 410. Typically, the coating 410 is provided or formed by a dielectric layer 535b.

[0097] FIG. 5 shows an example of the semiconductor membrane laser 500 according to one aspect of the invention. The steps of the manufacturing process for the semiconductor membrane laser 500 are illustrated in FIG. 10. It will be appreciated that the steps set out in FIG. 10 and explained below are merely exemplary. In particular, the order of some of the steps could be changed.

[0098] The semiconductor membrane laser 500 comprises a semiconductor amplifier or lasing medium 510 (which is also called semiconductor membrane) which is located between an upper or first heat spreader 520a and a lower or second heat spreader 520b and selectively applied dielectric layers 535a,b and metal contact layers 530a,b. The semiconductor amplifier medium 510 is created by depositing a layer stack of semiconductor material on a substrate in step 1000 of FIG. 10 using an epitaxial process. It will be appreciated that the terms “upper” and “lower” used in this description are merely used to distinguish different elements shown in the drawing figures.

[0099] With the presently illustrated example the planar-shaped lasing medium 510 is sandwiched between the first and second heat spreaders 520a, 520b. Here, an upper surface 511a of the lasing medium 510 is in contact with a lower surface 522a of the first heat spreader 520a. A lower surface 511b of the lasing medium 510 is in contact with an upper surface 521b of the second heat spreader 520b. An upper surface 521a of the first heat spreader 520a facing away the lasing medium 510 is provided with the second dielectric layer 535a and with the first contact layer 530a. A lower surface 522b of the second heat spreader 520b is provided with or is in contact with the first dielectric layer 535b and with the second contact layer 530b.

[0100] The first and the second contact layers 530a, 530b may each comprise an opening or recess 532a, 532b in the layer structure extending all through the thickness of the respective first and second contact layers 530a, 530b. In the opening or recesses 532a, 532b there is provided the respective dielectric layer 535a, 535b. Since the contact layers 530a, 530b typically comprise a metal or are made of a metallic material the recesses or through openings extending through the contact layers 530a, 530b provide unobstructed optical beam propagation.

[0101] Examples of the semiconductor amplifier medium 510 include but are not limited to the following material systems: [0102] AlGaInAsP (on GaAs substrate)—e.g. GaInAs quantum wells embedded in GaAs(P) barriers for laser emission in the near infrared spectral range (approx. 850-1200 nm). [0103] AlGaInP (on GaAs substrate)—e.g. GaInP quantum wells embedded in AlGaInP barriers for laser emission in the red spectral range (approx. 630-700 nm). [0104] AlInGaN (on GaN/Al.sub.2O.sub.3/SiC substrate)—e.g. InGaN quantum wells for laser emission in the blue/green spectral range (approx. 400-550 nm). [0105] AlGaInNAs (on GaAs substrate)—e.g. GaInNAs quantum wells embedded in GaAs barriers for laser emission in the near infrared spectral range (>1200 nm). [0106] GaAsSb (on GaSb substrate)—e.g. GaInAsSb quantum wells embedded in GaAs barriers for laser emission in the short wavelength infrared spectral range (around 2 μm). [0107] AlGaInAsP (on InP substrate)—e.g. GaInAs quantum wells embedded in AlGaInAs barriers for laser emission in the short wavelength infrared spectral range (around 1.6 μm).

[0108] The upper surface of semiconductor amplifier medium 510 is cleaned in step 1010 of FIG. 10 and then the upper heat spreader 520a is applied to the cleansed upper surface 511a of the semiconductor amplifier medium 510 by means of a plasma-activated bonding process to form a direct contact. The substrate is removed from the lower surface 511b of the semiconductor amplifier medium 510 in step 1020 of FIG. 10, for example by wet chemical etching, and, if required, in step 1030 of FIG. 10 the lower heat spreader 520b is attached to the lower surface 511b of the semiconductor amplifier medium 510 using the same bonding process.

[0109] The two heat spreaders, i.e. the upper heat spreader 520a and the lower heat spreader 520b, are brought as complete wafers by means of plasma-activated bonding processes into direct, monolithic contact in steps 1010 and 1030 of FIG. 10 with the semiconductor amplifier medium 510, which is also of wafer size. As a result of this direct contact, heat dissipation (i.e. dissipation of thermal energy) in operation from the amplifier medium 510 at the interfaces 515a and 515b between the amplifier medium 510 and the upper heat spreader 520a or the lower heat spreader 520b is substantially uninhibited.

[0110] The two heat spreaders 520a and 520b are made, for example of diamond or silicon carbide with good optical qualities to allow passage of the laser radiation. Silicon carbide (SiC) is monocrystalline and has a very high optical quality at wafer-size scale with good surface finish available. Its thermal conductivity can be up to 400 W/mK. Diamond is also monocrystalline, but currently does not yet have a high optical quality with good surface finish available at wafer scale but has a very good thermal conductivity of up to 2000 W/mK.

[0111] Aluminum oxide (monocrystalline) can also be used and has very high optical quality with good surface finish available at wafer scale, but with a low thermal conductivity of only ˜25 W/mK.

[0112] The combination of the semiconductor amplifier medium 510 and the heat spreaders 520a and 520b are termed a “wafer layer stack” 110 (as shown in FIGS. 11-13).

[0113] Subsequently in step 1040 of FIG. 10, the top 525a and the bottom 525b of the wafer layer stack are selectively provided with dielectric layers 535a and 535b by deposition or metal contact layers 530a and 530b by metallization using lithography or shadow masks. As can be seen in the wafer 600 shown in FIG. 6, individual—here shown in a circle—surfaces (light-opening window or aperture) are provided with the dielectric layers 535 and the adjacent surrounding area—here shown in a square—is provided with the metal contact layers 530. Between the metal contact layers 530, so-called sawing lines 610 remain uncoated along the lines in which the sawing or splitting process in step 1050 of FIG. 10 for separation or dicing of the semiconductor membrane laser chips takes place later. In a non-limiting example, the wafer 600 is a 4-inch wafer and the edge length of the amplifier unit of 1.5 mm and a width of the saw lines of 0.1 mm results in approx. 3,000 semiconductor membrane laser chips as amplifier units.

[0114] It will be seen that the deposition of the dielectric layers 535a and 535b as well as the metal contact layers 530a and 530b takes place symmetrically on both the top 525a and the bottom 525b of the wafer layer stack. However, the dielectric layers 535a and 535b have different functions on the two sides as will now be explained. The bottom of the wafer layer stack is assumed to be the direction from which the pump light 150 is received (as shown in FIGS. 4 and 5). The function of the upper or second dielectric layer 535a deposited on the upper heat spreader 520a may be to enable a high transmission at the wavelength λ.sub.1 of the generated laser mode in the amplifier medium 510. On the other hand, the function of the lower or first dielectric layer 535b applied to the lower heat spreader 520b is to enable a high reflection at the wavelength λ.sub.1 of the generated laser mode in the amplifier medium 510 and a high transmission at the wavelength λ.sub.2 of the pump laser 160 used to pump the amplifier medium 510. Alternatively, the lower or first dielectric layer 535b is arranged to reflect light at the wavelength λ.sub.2 of the pump laser 160 used to create a resonator for the pump wavelength, and thus an increased absorption efficiency. The material used in the dielectric layers 535a,b can be SiO.sub.2, Nb.sub.2O.sub.5, HfO.sub.2 TiO.sub.2, Al.sub.2O.sub.3 and Ta.sub.2O.sub.5, but this is not limiting of the invention.

[0115] It was noted above that the order of the manufacturing steps set out in FIG. 10 is not limiting of the invention. For example, the deposition of the dielectric layers 535a and 535b as well as the metal contact layers 530a and 530b can be changed and will depend on the design of the semiconductor membrane chip. Similarly, the bonding of the substrate and the subsequent removal of the substrate may be carried out in a different order. It would also be possible to apply the dielectric layers 535a and 535b after dicing of the semiconductor membrane chips.

[0116] Finally, the individual ones of the semiconductor membrane laser chips are fixed or soldered in step 1060 of FIG. 10 to a submount 700, as shown in FIG. 7 using a soldering process—e.g. using a pre-formed soldering foil 710 or any other metallic fasteners, e.g. in form of a metallic plate. Alternatively, the solder can be previously deposited on the submount 700 or on the semiconductor membrane laser chip. This submount 700 comprises a metal body, such as but not limited to, copper or brass, which may or may not be coated with gold. The metal body has a high thermal conductivity and has a recess 720. The recess 720 is adapted to the thickness of the semiconductor membrane laser chips and the thickness of the soldering foil 710 in such a way that the semiconductor membrane laser chip is flush with the surface of the submount 700 on the other side and therefore the metal contact layer 530b can be connected to the submount 700 via another soldering foil 710 or metallic fastener.

[0117] The submount 700 has an upper window 730a and a lower window 730b which align respectively with the upper dielectric layer 535a and the lower dielectric layer 535b such that the dielectric layers 535a and 535b remain optically freely accessible through the recess 720 and enable light to pass through the submount 700. The heat or thermal energy from both sides of the upper heat spreader 520a and the lower heat spreader 520b is dissipated to the submount 700, since the remaining area of the upper and lower sides of the semiconductor membrane laser chip is available for the heat transfer between the upper heat spreader 520a and the lower heat spreader 520b and the submount 700.

[0118] The example shown in FIG. 7 is a linear resonator geometry with a single external mirror 180 coupling the laser beam 175 out of the semiconductor membrane laser. The design of the amplifier unit on the submount 700 allows good access to the amplifier medium 510 from both sides of the semiconductor membrane laser. The semiconductor membrane laser is pumped by a pump laser 160 which is able to focus a beam through the lower dielectric layer 535b to the amplifier layer 510 at an angle of 180°. This means that the lateral size of a focusing lens as part of the pump optics is not limited by the geometry of the submount 700. Preferably, an optical path between the pump laser 160 and the lasing medium 510 can be void of any optical components, such as a focusing of collimating optical arrangement. The optical path may be void of any refractive or diffractive optical elements. In an alternative arrangement, the individual ones of the semiconductor laser chips and the submount 700 can also be arranged so that the more accessible side of the submount 700 is on the side with the upper dielectric layer 535a, thus pointing in the direction of an output coupler mirror 180 and the outcoupled laser beam 170, in order to take advantage of the good accessibility for particularly compact resonator geometries.

[0119] A similar concept is shown in FIG. 8A which only has a single upper heat spreader 520a and no lower heat spreader 520b compared to the designs shown in FIGS. 5 and 7. The lower and hence the first dielectric layer 535b and the lower metal contact layer 530b are applied directly to the amplifier medium 510. This design enables the placement of an edge-emitting laser diode 162 (see FIG. 5) as a pump source at a very small distance from the amplifier medium 510. The distance is selected depending on the emission profile of the edge-emitting laser diode 162 and the thickness of the lower dielectric layer 535b so that the pump beam 150 has a circular shape in the plane of the amplifier medium 510. At this defined distance (which has a typical optical path length in the range of 10 to 100 μm) the plane of the amplifier medium 510 is between the near field and the far field of the pump beam 150 and the beam diameters in the two beam dimensions of the pump beam “fast axis” and “slow axis” are the same. In this arrangement, no optics are required to focus the pump beam 150, which makes it possible to produce particularly compact and cost-effective components consisting of an amplifier unit with integrated pump source.

[0120] The semiconductor membrane laser shown in FIG. 8B also has no lower heat spreader 520b and further has no lower metal contact layer 530b. There is therefore also no need for a lower window through which the light from the pump laser 160 needs to pass.

[0121] FIG. 9 shows a further example of the semiconductor membrane laser in which the submount 700 is not located around the semiconductor membrane laser 500 but on one edge 910 of the semiconductor membrane laser 500. Solder 930 is placed on the edge 910 and a thermal connection between the submount 700 and the semiconductor membrane laser 500 established.

[0122] In a further aspect, a GRIN (graded refractive index) lens can be manufactured in such a way that the GRIN lens through which the pump beam 820 from a side emitting diode 810 passes is in direct contact with the upper dielectric layer 535a or the lower dielectric layer 535b. This reduces energy losses by enabling the pump laser light from the pump laser 160 to be focused in the plane of the active region with the amplifier medium 510.

[0123] It will be appreciated that the semiconductor membrane laser described in this disclosure may include further mirrors, such as those for a V-shaped or Z-shaped cavity. Furthermore, the generated laser beam 170 in the resonator may include further intra-cavity elements, such as non-linear crystals (e.g. SHG (second harmonic generation) crystals, birefringent filters (BRF), etalons, and absorbers).

[0124] One method of producing a laser chip is for instance illustrated in FIG. 11. Here, in step a) there is provided a substrate 100 with a layer of a lasing medium 510. In a subsequent step b) the layer of a first heat spreader 520a is arranged or formed on an upper surface 511a of the lasing medium 510. Thereafter and as illustrated in step c) the substrate 100 is removed and in a further step d) the first dielectric layer 535b is deposited or arranged on the lower surface 511b of the lasing medium 510 thus forming a multi-layer stack 110 of wafer 600. Subsequently the multi-layer-stack is cut into individual laser chips 500 of appropriate transverse size. In general, the order of steps to be performed for manufacturing a multi-layer stack 110 may vary. A removal of the substrate 100 may only take place after the lasing medium 510 is mechanically stabilized, e.g. through application of a heat spreader 520a.

[0125] In FIG. 12 another way of manufacturing such laser chips 500 as described before is illustrated. Here, in step a) there is provided a substrate 100 with a layer of a lasing medium 510. In a subsequent step b) a first heat spreader 520a is arranged or formed on an upper surface 511a of the lasing medium 510. Thereafter and as illustrated in step c) the substrate 100 is removed and in a further step d) the first dielectric layer 535b is deposited or arranged on the upper surface of the first heat spreader 520a facing away the lasing medium 510 thus forming a multi-layer stack 110 of wafer 600. Subsequently, the multi-layer-stack 110 is cut into individual laser chips 500 of appropriate transverse size. Removal of the substrate 100 may also take place after deposition of the first dielectric layer 535b on the first heat spreader 520a. With some examples and contrary to the illustrated sequence of steps of FIG. 12 the first dielectric layer 535b may be deposited or coated on the first heat spreader 520a before the first heat spreader 520a is bonded or connected to the lasing medium 510. Further alternatively, the isolated first heat spreader 520a may be provided with the first dielectric layer 535b. The substrate 100 with the layer of the lasing medium 510 as illustrated in step a) of FIG. 12 may be separately prepared and may be then bonded with the first heat spreader 520a, which is prefabricated with the first dielectric layer 535b.

[0126] In FIG. 13 a further example of manufacturing a semiconductor membrane laser chip 500 comprising a multi-layer stack 110 of wafer 600 is schematically illustrated. Here, in step a) a substrate 100 is provided with a layer or with multiple internal layers of a lasing medium 510. Thereafter, as illustrated in step b) and on top of the lasing medium 510 there is provided the first heat spreader 520a. Thereafter and since the first heat spreader 520a provides mechanical stability to the lasing medium 510, the substrate 100 may be removed in step c). After removal of the substrate 100 a second heat spreader 520b is provided on that surface of the lasing medium 510 that faces away the first heat spreader 520a as shown in step d). Here, the second heat spreader 520b may be bonded to the lasing medium 510. Thereafter and as illustrated in step e) there is provided at least a first dielectric layer 535b on top of one of the first heat spreader 520a and the second heat spreader 520b.