Patent Application: US-201113018020-A

Abstract:
an embodiment of the invention relates to a device comprising a laser and a waveguide stripe or netlike hexagonal stripe structure , which allows propagation of multitude of the lateral modes in the waveguide stripe or stripe structure , wherein the waveguide stripe has at least one corrugated edge section along its longitudinal axis to provide preferable amplification of the fundamental lateral mode or in - phase supermode and to obtain high brightness of the emitted radiation .

Description:
the preferred embodiments of the present invention will be best understood by reference to the drawings , wherein identical or comparable parts are designated by the same reference signs throughout . it will be readily understood that the present invention , as generally described herein , could vary in a wide range . thus , the following more detailed description of the exemplary embodiments of the present invention , is not intended to limit the scope of the invention , as claimed , but is merely representative of presently preferred embodiments of the invention . fig1 shows a top view of a typical prior art single - stripe semiconductor laser 1 . the lateral and longitudinal directions are designated by reference numerals x and z , respectively . the laser 1 inter alia comprises a wave guiding stripe 3 , a rear facet 4 , and an output facet 5 . the free space outside the laser is designated by reference numeral 6 . the refractive index of the stripe 3 is larger than the refractive index of the outer areas 7 in order to provide wave guiding in the lateral direction . the output and the rear facets have high reflective and low reflective coatings , respectively , consisting of several layers ( not shown ) of low refractive index materials with definite thicknesses . for high - brightness lasers both high - power output and narrow divergence are advantageous . for the vertical direction the effective methods to achieve high brightness were disclosed [ 1 - 4 ]. for the lateral direction , tapered stripes , bend stripes ( see fig2 and 14 ) and stripe arrays are known . however the problem of avoiding poor beam quality in the lateral direction and simultaneously obtaining high power remains unresolved . in order to address these drawbacks , embodiments of the present invention are based on the suppression of higher - order lateral modes in favor of the fundamental mode in order to provide a single - lobe far field . the devices according to fig3 - 10 comprise longitudinally corrugated stripes 3 ′. the width of the stripes 3 ′ is wide enough to allow multimode propagation of radiation along the longitudinal direction . however , the corrugations of the waveguide stripes 3 ′ in the longitudinal direction cause large losses for higher - order lateral modes by increased scattering as compared to the fundamental lateral mode . thus the output radiation is single - mode ( despite the large stripe width which would allow multimode propagation ), and results in a single - lobe far field with narrow divergence in the lateral direction , which means high brightness . fig3 shows a first exemplary embodiment of a device comprising a single - stripe semiconductor laser 1 in further detail . the laser 1 may be formed by a multi - layered heterostructure consisting of iii - iv or ii - vi semiconductor materials , e . g . algaas / gaas , ingaas / gaas , ingaasp / gaas , znmgsse / znsse , ( al , ga , in ) n / gan . the structure including the layers with e . g . quantum wells provide wave guiding in the vertical direction and gain . the laser 1 according to fig3 comprises a longitudinally inhomogeneous waveguide stripe 3 ′. at both edges of the waveguide stripe 3 ′, triangular - shape corrugations are provided with “ saw - tooth ” orientation with respect to the laser output facet 5 . the refractive index of the stripe 3 ′ is larger than the refractive index of the outer areas 7 in order to provide wave guiding in the lateral direction . both , wave guiding in the vertical and the lateral directions , as well as light amplification and laser output are performed in a well understood manner . in particular , the laser output occurs as a combination of the modes guided by the heterostructure with contributions from each mode defined by the mode confinement factor and loss . the triangular - shape corrugations may be provided at both edges along the entire length of the waveguide stripe 3 ′, as shown fig3 . alternatively , only one edge may be provided with corrugations and / or only waveguide sections of the waveguide stripe 3 ′ ( i . e . not the entire stripe ) may have corrugations . fig4 shows a second exemplary embodiment of a device comprising a single - stripe semiconductor laser 1 in further detail . the laser 1 comprises a longitudinally inhomogeneous waveguide stripe 3 ′ having triangular - shape corrugations with “ saw - tooth ” orientation with respect to the laser output facet 5 . the “ sawtooth ” orientation in fig4 is opposite to the orientation shown in fig3 . fig5 shows a third exemplary embodiment of a device comprising a single - stripe semiconductor laser 1 in further detail . the laser 1 comprises a longitudinally inhomogeneous waveguide stripe 3 ′ having rectangular - shape corrugations which are symmetric with respect to the lateral axis . fig6 shows a fourth exemplary embodiment of a device comprising a single - stripe semiconductor laser 1 in further detail . the laser 1 comprises a longitudinally inhomogeneous waveguide stripe 3 ′ having rectangular - shape corrugations which are asymmetric with respect to the lateral axis . fig7 shows a fifth exemplary embodiment of a device comprising a single - stripe semiconductor laser 1 in further detail . the laser 1 comprises a longitudinally inhomogeneous waveguide stripe 3 ′ having inclined corrugations which are symmetric . at both sides of the lateral axis the corrugations are bent to the output facet direction with respect to the lateral axis . fig8 shows a sixth exemplary embodiment of a device comprising a single - stripe semiconductor laser 1 in further detail . the laser 1 comprises a longitudinally inhomogeneous waveguide stripe 3 ′ having inclined corrugations which are asymmetric . at both sides of the lateral axis , the corrugations are bent to opposite directions with respect to the lateral axis . fig9 shows a seventh exemplary embodiment of a device comprising a single - stripe semiconductor laser 1 in further detail . the laser 1 comprises a longitudinally inhomogeneous waveguide stripe 3 ′ having triangular - shape corrugations which are symmetric in respect to the lateral axis . fig1 shows an eighth exemplary embodiment of a device comprising a single - stripe semiconductor laser 1 in further detail . the laser 1 comprises a longitudinally inhomogeneous waveguide stripe 3 ′ having triangular - shape corrugations which are asymmetric in respect to the lateral axis . the effective discrimination of higher - order lateral modes in corrugated waveguides can be proven with the help of the two - dimensional wave equation [ 15 ] ( d 2 / dx 2 + d 2 / dz 2 ) e ( x , z )= k 2 n 2 ( x , z ) e ( x , z ) ( 1 ) solved numerically as a eigenvalue - eigenstate problem with respect to the wavenumber k and the lateral - longitudinal mode field e ( x , z ) for the refractive index and gain n ( x , z ) versus the lateral and longitudinal coordinates x and z , respectively . boundary conditions corresponding to non - reflecting boundaries at the top , bottom and right side ( perfectly matched layers ) and perfectly conducting boundary at the left side of the computation domain can be used . the complex eigenvalue k obtained defines the frequency and the loss ( lifetime ) of the modes e ( x , z ) which take into account field diffraction and radiation into the lateral direction for the refractive index profile including the contribution of the stripe corrugation . the rear and output facets typically consisting of several layers with definite thicknesses and refractive indices , providing high reflection and anti - reflection , respectively , can also be taken into account in the refractive index profile n ( x , z ) for the solution of eq . ( 1 ). in the one - dimensional case with n ( z )= n our numerical solution of eq . ( 1 ) gives the well - known longitudinal modes of a cavity formed by a finite - length slab . these modes consist of two counter - propagating plane waves in the cavity and an out - going plane wave in free space . their wavenumbers satisfy the well - known laser generation condition [ 15 ] where l is the cavity length , r 1 = 1 and r 2 are the refractive coefficients at the left rear and right output facets of the cavity , respectively . the solution of eq . ( 1 ) allows to find the two - dimensional counterpart of the laser condition ( 2 ), i . e . to find the complex wavenumbers k and the lateral - longitudinal profiles e ( x , z ) of the waveguide modes for a given distribution of the refractive index and gain n ( x , z ). by this way it allows to prove that higher - order lateral modes with multi - lobe far fields have larger loss than the fundamental mode for the corrugated stripes . fig1 - 13 show further embodiments of devices having edge - emitting semiconductor lasers . the embodiments provide high power and low divergence in the lateral direction based on multi - stripe arrays with field - coupled multiple stripes . far - order field coupling of the stripes provided by the waveguide structure , where each stripe couples with all the stripes in the same extent ( not dominantly with the nearest neighbors ) causes preferential guiding and amplification of the in - phase lateral supermode of the stripe array . the out - of - phase supermode of the stripe array has larger loss due to scattering at the stripe structure and contributes less to the output of the laser array . radiation from multiple stripes is coherent and phase - locked , resulting in a single - lobe far field with small divergence in the lateral direction proportional to the number of the stripes . the exemplary embodiment shown in fig1 refers to a multiple - stripe semiconductor laser 1 with a netlike stripe structure 10 . according to the embodiment of fig1 , the netlike stripe structure 10 is a hexagonal structure for field coupling of two arrays 11 each of which consists of three straight stripes 12 . the number of the straight stripes 12 in the vicinity of the rear and output laser facet is not limited by three as shown and could be arbitrary . the hexagonal openings 13 of the netlike stripe structure 10 have a lower refractive index than the wave guiding stripes 14 of the netlike stripe structure 10 . the exemplary embodiment shown in fig1 also comprises a hexagonal structure 10 , but no arrays of straight stripes . the hexagonal structure 10 consists of , but is not limited to , three periods in the lateral direction as shown and could be arbitrary . the low - index openings 13 of the stripe structure 10 have a hexagonal shape . the exemplary embodiment shown in fig1 comprises a netlike stripe structure 20 with round openings 13 . the embodiment may or may not comprise straight stripes 12 as shown in fig1 . the netlike structure 20 consists of , but is not limited to , three periods in the lateral direction as shown and could be arbitrary . the round openings 13 have a lower refractive index than the waveguide stripes 14 of the netlike stripe structure 20 . the embodiments shown in fig3 - 13 may consist of or comprise epitaxial layers of iii - iv or ii - vi semiconductor materials grown by metal organic vapor phase epitaxy ( movpe ) or molecular beam epitaxy ( mbe ). the single - or multi - stripe edge - emitting lasers may be manufactured by photolithography and ion - beam - assisted etching . this standard technology allows forming corrugated boundaries of the stripes with characteristic sizes of the corrugations of the order of several wavelengths which are sufficient to provide substantial discrimination of the higher - order lateral modes . the netlike stripe structures 10 , and 20 as shown in fig1 - 13 may also be manufactured by photolithography and ion - beam - assisted etching . in summary , embodiments of the invention relate to single - stripe edge - emitting semiconductor lasers with a stripe with corrugated boundary between areas with different refractive indices across the stripe to achieve lower lateral divergence of the output emission . the corrugations cause the discrimination of the lateral modes in wide stripes due to higher losses of the higher - order modes scattered into the lateral directions as compared to the fundamental lateral mode . the output emission of the stripe has high power and low divergence of the far field both due to wider stripe width as compared to a straight stripe . similarly , a high - brightness diode laser may be made of a multiple - stripe array with coupling of the stripes through a stripe structure having a hexagonal , a round or another shape . multiple - 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