Improved coupling structures for a phase-locked laser array

Abstract: A semiconductor laser array comprises a plurality of forward-biased amplification waveguides (34) disposed in an active layer (24) of a monolithic semiconductor structure (10). At least one radiator segment (60) forms part of a selected amplification waveguide to improve coupling with adjacent amplification waveguides.

Description: The invention relates to a phased-array semiconductor laser and more particularly to a coupling structure for the laser. Such an array is known from EP-A-0 456 450, a document falling under the terms of Article 54(3) which is not relevant to the question of inventive step. Phased-array lasers include a set of directly coupled waveguide amplifiers which generate phase-locked lightwaves. Each waveguide has a terminal aperture at a laser facet that radiates the lightwaves generated in the waveguide amplifier. The use of a set of waveguide amplifiers provides more energy than a single waveguide and coupling the waveguide amplifiers forces each waveguide to generate lightwave energy only in modes that are phase-locked so that the resulting emitted light may be focused into a highly-resolved beam. It is well-known to use y-shaped waveguide amplifiers to obtain in-phase output lightwaves. Lightwaves propagating in each waveguide amplifier in the array will be deflected into adjacent waveguide amplifiers by the y-shaped branches of the amplifiers and out-of-phase modes of the light waves will radiate away from the stripes and be absorbed. A second well-known coupling method is to dispose the waveguide amplifiers close together so that their evanescent electric fields overlap. However, this method supports array modes where the modes of the individual waveguide amplifiers are not in phase. As laser applications increase improved coupling systems are needed to provide stable, in-phase operation at high power. An object of the present invention is the provision of a laser array incorporating an improved coupling structure for supporting only the in-phase mode of the array. The present invention provides a semiconductor laser array as claimed in any of the accompanying claims. By way of example only, embodiments of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 is a cross-sectional view of a heterostructure laser; Fig. 2 is a top view of part of an embodiment of the invention utilizing radiator segments to improve evanescent coupling and Fig. 3 is a top view of part of an embodiment of the invention utilizing short active stripes to enhance the in-phase array mode. Referring now to Fig. 1, there is illustrated an enlarged view of a semiconductor structure 10 comprising a plurality of epitaxially deposited layers 22-28 on substrate 20. In this example, semiconductor structure 10 is a semiconductor heterostructure. Substrate 20 may comprise n-GaAs, and epitaxial layers 22-28 may be consecutively deposited in an MO-CVD reactor as is known in the art. These epitaxially deposited layers may be, for example, as follows: Cladding layer 22 of n-Ga 1-y AL y As wherein, for example, y = 0.40; active region 24 comprising a layer of GaAs or Ga 1-x Al x As where y > x, or a single quantum well layer of GaAs or a multiple quantum well of alternating layers of GaAs and Ga 1-x Al x As or alternating layers of Ga 1-x Al x As and Ga 1-z Al z As where y > z > x; cladding layer 26 of p-Ga 1-y Al y As and a cap layer 28 of p + GaAs. In the particular example here, active region 24 comprises multiple quantum wells. This active region is comprised of four 12 nm quantum wells of Ga 1-x Al x As, wherein x = 0.05 separated by three 6 nm barriers of Ga 1-z Al z As, wherein z = 0.02. Therefore, the active region has a thickness, L z , of approximately 66 nm. An electrode arrangement 12, 14 may be provided to enable an electrical bias to be applied to the structure 10. Fig. 2 is a top view of the active layer 24. In Fig. 2 a plurality of parallel, equispaced amplication waveguides 34 are disposed in the active region 24 and forward biased to generate and amplify lightwaves under lasing conditions. A part of each alternate amplication waveguide 34 is formed by a radiator segment 60. These segments 60 are transparent, passive waveguides that are narrower than the amplication waveguide 34. The radiator segments 60 and the amplification waveguides 34 can be formed by an impurity induced layer disordering described in US-A-4 802 182. The index-guided amplication waveguides have a higher index of refraction than the neighboring areas of active layer 24 and of cladding layers 22 and 26. Thus, the light generated in the amplication waveguides is confined by the well-known phenomena of total internal reflection. The radiator segments 60 are transparent and do not need to be forward biased. Radiator segments 60 may be included in every amplification waveguide 34 for stronger coupling. Graphs 62 and 64 depict the intensity of the electric field in the lightwave modes guided by the amplification waveguide and radiator segment 34 and 60 respectively. The amplitude of the field decreases exponentially outside of an index-guided waveguide. It is well-known that the rate at which the field falls off decreases as the width of the waveguide decreases. Thus, the light spreads out for the narrow radiator segment 60 to improve evanescent coupling with adjacent amplification waveguides 34. Fig. 3 shows an alternative arrangement in which the radiator segments 60 are included in each amplification waveguide 34 and disposed along a single axis. An active region 80, comprising a small section of forward biased amplification waveguide is disposed in the active layer 24 between adjacent radiator segments 60. Graphs 82 and 84 illustrate how the active region boosts the in-phase mode and lowers its threshold. The fields from both radiating elements are positive and thus are summed at the active region. The summed field is amplified to increase the power of the in-phase mode. Graphs 86 and 88 depict how the fields cancel at the active region 84 for the out-of-phase mode. Thus, the out-of-phase mode is not enhanced by the active regions 80.

Claims: A semiconductor laser array comprising: a monolithic semiconductor structure (10), having a plurality of semiconductor heterostructure layers (22,24,26,28) disposed over a substrate (20), at least one of said layers being an active layer (24) for light amplification and propagation under lasing conditions; means, electrically coupled to said semiconductor structure, for applying an electrical forward bias to said layers in said substrate; a plurality of substantially parallel, spaced apart, two-dimensional index-guided amplification waveguides (34), disposed in the active region, oriented parallel to a first axis in said active layer, with each of said amplification waveguides having a predetermined width, for generating and amplifying light waves at a given wavelength in response to said electrical forward bias; a plurality of radiator segments (60), each forming a part of a respective amplification waveguide in said plurality, each of said radiator strips disposed in said active region, with each radiator segment of width S, for guiding said amplified lightwaves generated in said selected amplification waveguide, where each radiator segment increases the spreading of an evanescent field; and a plurality of coupling amplification regions (80), each coupling amplification region disposed in said active layer between said radiator segments of a corresponding adjacent pair of amplification waveguides, said coupling amplification region for amplifying the evanescent field radiated by said radiator segments of said corresponding pair to lower the threshold for the in-phase array mode. A laser array as claimed in claim 1, with: each radiator segment (60) of width S, with the magnitude of the width S less than the critical width required for substantially complete confinement of said generated light. A laser array as claimed in claim 1 or claim 2, wherein: each radiator segment (60) is a low-loss waveguide.

Inventors: PAOLI THOMAS L, PAOLI, THOMAS L.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1991
Main classification: H01S3
IPC codes: H01S5