Patent Application
20060078031
The present invention relates generally to blue and green-light emitting semiconductor lasers. The invention relates in particular to such semiconductor lasers grown from II-VI semiconductor materials.
Most commercially available blue-light emitting diode-lasers are made from indium gallium nitride (abbreviated InGaN), a III-V semiconductor having a general formula InxGa1-xN, where x is equal to or greater than 0.0 and less than or equal to 1.0. The lasers can be made to emit at a particular wavelength in a spectral range from about 380 nanometers (nm) in the ultraviolet region of the electromagnetic spectrum to about 460 nm in the blue region of that spectrum by selecting an appropriate value for x.
The blue region of the spectrum is defined as having a spectral range extending from about 425 nm (purplish blue) to about 490 nm (greenish blue). Accordingly, InGaN diode-lasers provide can provide light in only in the shortest 50% of the blue region of the spectrum. It would be advantageous to have a diode-laser capable of emitting light in at least the remaining 50% of the blue region of the spectrum.
Diode-lasers grown from II-VI semiconductor materials such as zinc sulfoselenide ZnSxSe1-x and ZnxCd1-xSe (where x is equal to or greater than 0.0 and less than or equal to 1.0) are capable of providing light at wavelengths in a range from about 460 nm in the blue region of the spectrum to about 530 nm in the green region of the spectrum. These lasers, unfortunately, have been found to have relatively short lifetimes, for example less than 1000 hours. It is generally believed that the short lifetime is due to the growth of color centers in the II-VI material originating from doping sites in the material. Doping of the material is necessary to provide the p and n conductive layers that provide the “diode” of the diode-laser. The color centers develop as a result of the passage of current through the diode-laser. A lifetime of less than 1000 hours is at least an order of magnitude shorter than would typically be required for a diode-laser to be commercially viable.
A possible solution to the lifetime problem of a II-VI semiconductor laser is to optically pump rather than electrically pump such a laser. This would eliminate the requirement for doping the layers of the laser and presumably greatly reduce, if not altogether eliminate, color center formation and growth. Optically pumped II-VI semiconductor lasers have been experimentally investigated. Optical pumping in these investigations has been effected using light from solid state and dye lasers. This would not be practical or cost effective for a commercial II-VI semiconductor laser.
In theory at least a II-VI semiconductor laser could be pumped using light from one or more InGaN light emitting diodes (LEDs). InGaN LEDs are commercially available from Nichia Corporation of Japan and from Cree Research Inc. of Durham, N.C. A problem that limits the possibility of InGan LED pumping in practice however, is that an LED has a relatively low brightness (radiance) as compared to diode-laser. Radiance is defined as the amount of optical power P (or flux) in Watts that can be delivered from a unit surface area and within a unit solid angle. Radiance can be analyzed with reference to an optical invariant referred to by practitioners of the art as an “etendue”. For a disc of radius R (area π R2 ), emitting (or receiving) light propagating in a medium having a refractive index n, and in a range of solid angles bounded by an angle Θ, etendue (E) is defined by an equation:
E=π sin(Θ)2n2πR2 (1)
An InGaN LED has a large etendue. The large etendue of an LED results from the emitting area thereof being large, or the solid angle containing the emission being large large, or both. Since etendue cannot be reduced through an optical system, large etendue of the LED leads to difficulty creating sufficiently high intensity of light (due to low “focusability” of the light) at the target. Additionally, in a practical InGaN LED pumped, II-VI semiconductor laser, light from a single LED would be typically insufficient to provide adequate pumping and it would be necessary to collect and deliver light from a plurality of LEDs and concentrate that light on a laser chip to be pumped. Accordingly, there is a need for a method and apparatus for collecting light from a plurality of InGaN LEDs and delivering that light to the chip to be pumped while conserving the light flux and the etendue of the LEDs, and maximizing intensity at the target by reducing its focal spot area. Conserving the light flux will provide that all of the light collected from the LEDs, except for any portion of the light lost by scatter or absorption in the apparatus, can be delivered to the chip to be pumped, while conserving etendue and reducing focal spot area ensures sufficient pumping intensity
In one aspect, an optically pumped semiconductor laser in accordance with the present invention comprises a II-VI semiconductor laser chip having a pump-light-receiving surface thereof in optical contact with a transparent refractive medium. One or more InGaN LEDs provide optical pump light for optically pumping the laser chip. The refractive medium is configured such that the pump-light is concentrated thereby on the pump-light-receiving surface of the semiconductor laser chip.
In one preferred embodiment of the present invention, a plurality of InGaN LEDs provides optical pump light, and the refractive medium is configured as an immersion lens having positive optical power. In another preferred embodiment of the present invention, a plurality of InGaN LEDs provides optical pump light, and the refractive medium is configured as a tapered light pipe, tapering from a widest dimension at an entrance end thereof to a narrowest dimension at an exit end thereof, with the pump-light-receiving surface of the semiconductor laser chip being at the exit end of the light pipe.
The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain the principles of the present invention.
Referring now to the drawings, wherein like features are designated by like reference numerals,
Chip 22 includes a substrate 24 on which a basic edge-emitting heterostructure 25 of semiconductor layers of the chip are epitaxially grown. In this example, these layers include a lower cladding layer 26, a lower waveguide layer 27, an active (quantum-well) layer 28, an upper waveguide layer 29 and an upper cladding layer 30. The function of such layers in an edge-emitting heterostructure is well known to those skilled in the art. Heterostructure 25 is but one example of a heterostructure suitable for use in embodiments of the present invention and should not be considered as limiting the invention. Those skilled in the art will be aware that, in more complex heterostructures, the function of certain individual layers of heterostructure 25 may be achieved using two or more layers. Further, the terminology “upper” and “lower” as applied to layers of heterostructure 25 is used merely for convenience of description and does not necessarily relate to a gravitationally determined “up” or “down”.
In a II-VI semiconductor laser chip, a preferred material system has a general composition AxB1-xCyD1-y, where x is equal to or greater than zero and less than or equal to one; y is equal to or greater than zero and less than or equal to one; where A and B are selected from a group of group II elements consisting of (Zn, Cd, Mg, Be, Sr, and Ba); and where C and D are selected from a group of group VI elements consisting of (S, Se, and Te). The selection of materials for A, B, C, and D and the values of x and y for active layer 27, inter alia, determines the emitting (lasing) wavelength of laser chip 22. Materials and x and y values for other layers are selected, inter alia, according to the selection of materials and values for the active layer.
Continuing with reference to
Pump light receiving surface 34 of chip 22 is immersed in, i.e., in optical contact with, a fluid refractive medium 42. The terminology “refractive medium” here means that the medium has a refractive index greater than about 1.0, i.e., greater than the refractive index or air or vacuum. The terminology “immersed in” or “in optical contact with” means that there is no optically effective air or vacuum space between the pump-light-receiving surface of the chip and the refractive medium with which the pump-light-receiving surface is in optical contact (or is “immersed”).
In laser 20, fluid refractive medium 42 is confined in a vessel 44, sealed to heat sink base 40, and made from a solid refractive medium 46 such as glass, fused silica, or sapphire. Vessel 44 has a dome-shaped, here hemispherical, outer surface portion 48, preferably arranged symmetrically over laser chip 22. This hemispherical (positive) optical surface is effectively the last optical surface encountered by pump-light rays before the rays are concentrated on semiconductor laser chip 22. The terminology “effectively the last optical surface”, here, means that any refraction provided by the interface between the fluid and the vessel is negligible.
Preferably, fluid refractive medium 42 and solid refractive medium 46 have about the same refractive index. That. being the case, the solid and fluid refractive media function as a continuous refractive medium with negligible, if any, Fresnel reflection loss between the solid and fluid media. The vessel and the fluid therein can be considered as an immersion lens 49 in which laser chip 22 is immersed. Immersion lens 49 has positive optical (dioptric) power. One fluid medium suitable for use as fluid medium 44 is a laser grade fluid, catalog number 20310, available from Cargille Laboratories of Cedar Grove, N.J.
Continuing with reference to
The collected, etendue-minimized, pump light (rays 52B) is directed by a positive lens 56 as a converging beam, indicated by converging rays 54C, to hemispherical surface 48 of vessel 44, i.e., to the effectively continuous refractive medium provided by the solid medium 46 of the vessel and the fluid medium 42 therein. Surface 48 of vessel 44 provides a positive refractive lens surface that further converges rays 54C, as indicated by rays 54D, such that all of the light from the LEDs, except for that lost by scatter, absorption, and reflection, is concentrated on pump-light-receiving surface 34 of semiconductor laser chip 22. Reflection losses, of course, can be minimized by provision of suitable antireflection coatings on any optical surfaces in the path of pump light from the LEDs to the laser chip. As chip 22 is an edge-emitting laser chip, laser output is a diverging beam propagating in a general direction perpendicular to the general direction of propagation of pump light, and passes through fluid medium 42 and a cylindrical wall portion 49 of vessel 44.
It should be noted that while positive lens 56 is described in this and other below-described embodiments of the inventive laser as a single optical element, the converging function of the lens may by achieved by a plurality of optical elements without departing from the spirit and scope of the present invention. Using a plurality of elements may be found advantageous for avoiding optical aberrations, as is known in the art.
The etendue E of LEDs 50, the numerical aperture (NA) of immersion lens 49 formed by vessel 44 and fluid refractive medium 42 , its refractive index n, and radius R of the focal spot on the target are related to each other by equation similar to (1), which can be re-written in the form:
E=πNA2πR2 (2)
where NA=n sin(ΘLens) and ΘLens is the bounding angle of light converging in the lens, i.e., in the refractive medium. For a hemispherical lens, as depicted in
I=PπNA2/E (3)
where, again E, is the etendue, and P is the power, of LEDs 50.
It is emphasized, here, that a primary problem addressed by the present invention related to taking light from LED light sources of relatively low power and large etendue and concentrating that light on an OPS chip with sufficient intensity to exceed a lasing threshold of the OPS chip, which threshold is an intensity threshold. The etendue of an LED, E in equation (3), cannot be reduced by an optical system and can at best be conserved. This conservation is achieved in laser 20 by microlenses 50 and collecting lens 56.
Increasing the power of an LED, E in equation (3), is beyond the scope of the present invention. For purposes of this invention P must be considered as fixed at whatever is the instant state of the art for InGaN LEDs.
Accordingly, in the present invention NA of equation (3) is maximized. This can be achieved by maximizing the bounding angle of the solid angle of light incident on a pump-light-receiving surface of OPS chip 22 or by maximizing the refractive index of the refractive medium in which the surface of the chip is immersed. This is achieved in laser 20 by immersion lens 49.
One factor that limits CW power of InGaN LEDs is the generating of heat during operation. As an object of the present invention is to increase the intensity of pump light on semiconductor laser chip 22, this intensity may be increased by operating the InGaN LEDs in a pulsed manner. This will allow a higher power output than the CW limit (and higher intensity on the chip) during delivery of a pulse while maintaining time-averaged power in a sequence of pulses below the CW limit. Clearly, if pump light is delivered in a sequence of pulses, laser output will be in a corresponding series of pulses. This can be useful, however, in laser machining operations, for example.
A cylinder 68 made from solid refractive medium 46 is sealed to immersion lens 49 and base 40 of heat sink 36. Cylinder 66 retains a fluid refractive medium 42 that is index matched to the refractive index of solid refractive medium 46. Fluid refractive medium 42 fills gap 67 between pump-light-receiving surface 34 of the laser chip and piano surface 66 of the immersion lens, and thereby ensures optical contact between the surfaces, i.e., immersion of the pump-light-receiving surface in the lens. It should be noted that the thickness of gap 67 is exaggerated in
Laser 60 of
The sealing problem can be avoided altogether if surface 34 of laser chip 22 and plano surface 66 of lens 62 are brought into direct optical contact. The term “direct optical contact” as used herein means that the two surfaces are bonded together by Van der Waals attraction between molecules of each surface with the surfaces being in actual physical contact, or with any space therebetween being very much less than a wavelength of the pump light, for example, less than a few nanometers. In this direct optical contact condition, no intervening fluid is necessary to establish immersion of pump-light-receiving surface 34 and the refractive medium 46 of the lens.
Those skilled in the art will recognize that directly optically contacting the surfaces of the chip and the immersion lens can not be counted on as being a 100% successful operation. It may occasionally be found impossible to establish direct contact between surface of the lens and the surface of the laser chip. This may be due to point defects in the surface accumulated during manufacture, or to failure to achieve sufficient flatness of the surfaces during manufacture. It certain applications, however, benefits accruing from a reliable, fluid-free laser may be found to outweigh a less than 100% yield in optical contacting.
Lens 82 has a domed (hemispherical) portion 84 that provides the desired positive refractive power of the lens, a base portion 85 that provides an additional thermal conduction path for the heat sink function of the lens, and a submount portion 86 protruding from the base portion. Pump-light-receiving surface 32, the upper surface of heterostructure 25 (see
An advantage of the arrangement of laser 80 is that there is a pump-light receiving surface of laser chip 22, here, surface 34 thereof, that is that is not bonded to an opaque heat sink. This provides that additional pump light can be directed to the laser chip via this surface. By way of example,
Each of the five embodiments of the inventive optically pumped, edge-emitting, II-VI semiconductor lasers described above is based on concentrating pump light from a plurality of InGaN LEDs on an edge-emitting II-VI semiconductor laser chip. The inventive pump light delivery method, however, is also applicable to delivering pump light from a plurality of InGaN LEDs to a surface-emitting II-VI semiconductor laser chip. By way of example,
In laser 92 an optical arrangement for concentrating optical pump light is similar to the arrangement of laser 80 of
Laser chip 96 is directly optically contacted to base 85 of immersion lens 82A via a surface 102 of the mirror structure, which surface, in this arrangement, serves as the pump-light-receiving surface. Chip 96, which can also be referred to as an OPS structure, in this example, is initially grown on a semiconductor substrate 24 (see
Those skilled in the art will recognize that a surface-emitting laser chip will usually require a higher pump power to exceed a lasing threshold than would an edge emitting laser chip of the same materials. Because of this, more heat will be generated in the surface-emitting chip. Accordingly, it may be found advantageous to bond an auxiliary heat sink to base 85 of immersion lens 82A, although this should not be considered as limiting the present invention. In laser 90 a heat sink 104 (depicted in cross-section), including microchannels 106 through which a cooling fluid can be circulated, is bonded to base 85 of immersion lens 82A.
Continuing with reference to
Laser chip 96 is directly optically contacted to plano surface 66 of immersion lens 62 via an exposed surface 103 gain structure 98. This surface serves as the pump-light-receiving surface. Surface 102 of mirror structure 100 of the laser chip is bonded to a heat sink 37. Similar to the approach described with reference to laser 92, chip 96 is grown on a substrate 24 beginning with the gain structure. After the mirror structure is grown, the chip, still on the substrate, is bonded to heat sink 37. Once the chip is bonded to the heat sink, substrate 24 is etched away to expose surface 103 of gain structure 98. This surface is then preferably directly optically contacted to surface 66 of immersion lens 62. Contact between surfaces 103 and 66 may also be made via a fluid refractive medium 42 retained in a cylindrical sleeve 68 as described above with respect to laser 60 of
Continuing with reference to
In all embodiments of the inventive laser discussed above, pump light collected with minimized etendue for an array of InGaN LEDs is concentrated on a II-VI semiconductor laser chip, either an edge-emitting chip or a surface-emitting chip, by what is actually or effectively an immersion lens having positive dioptric power. Set forth below is a description of further embodiments of the invention wherein the immersion lens is replaced by a light-pipe arrangement arranged to concentrate the collected pump light on an edge-emitting II-VI semiconductor laser chip.
As light pipe 134 is arranged such that it is narrower in the general direction of propagation of the pump light, the angle of incidence (and reflection) of a light ray entering the vessel, at a wall of the vessel, becomes greater with each successive reflection in the vessel. This can be seen in
Light pipe 134 can be defined as having a “cut-off” angle θ. This cut-off angle is the maximum angle, in free space, from a direction (dashed line 136) parallel to the general propagation direction of pump, at which pump light entering the light pipe can reach pump-light-receiving surface 34 of edge-emitting II-VI semiconductor laser chip 22. At any greater angle, light could be reflected back out of the light pipe without reaching the chip. In a light pipe having a circular cross-section the cut-off angle is dependent on the ratio of the output diameter and the input diameter of the light pipe and the refractive index of the medium.
It should be noted that in certain embodiments of the inventive laser wherein a fluid refractive medium is disposed between the pump-light-receiving surface of a semiconductor laser chip and a solid refractive medium to establish optical contact therewith, the fluid refractive medium, in theory at least, could be substituted by a transparent optical cement. In practice however, such a cemented contact may be subject to failure due to local heat generated in the chip or due to absorption of optical pump light in the cement itself.
From the detailed description of the present invention presented above, those skilled in the art will recognize, without further illustration or detailed description that the arrangement of laser 150 leaves pump light receiving surface 32 of semiconductor laser-chip free to receive pump-light from a second LED array via any of the above described examples of immersion lenses or light pipes in accordance with the present invention. Those skilled in the art will likewise recognize that the light-pipe arrangements of lasers 130, 140, and 150 of
It should be noted that collecting pump light with minimized etendue from a plurality of InGaN LEDs is not limited to the method and apparatus described above with reference to lasers 20, 60, and 70 respectively. By way of example, a description of another preferred prior-art apparatus for collecting the InGaN LED pump light, which can be used with any above-described embodiment of the present invention, is set forth below with reference to
The fibers are bundled together as illustrated to create the effect of a single continuous light source. This provides minimized divergence in the emitting area, and accordingly minimizes etendue. The fibers additionally provide a convenient means of transporting light from the LED array. Only three LEDs, three microlenses, and three optical fibers are depicted in
In summary, the present invention is described above as a preferred, and other embodiments. The invention is not limited, however, to the embodiments described and depicted. Rather, the invention is limited only by the claims appended hereto.
