Improved coupling structures for a phase-locked laser array

Abstract: Coupling structures (40) for an index-guided laser array utilize passive transparent waveguides to improve coupling and phase-lock the array. The passive waveguides may be in the form of at least one y-coupled waveguide array (36, 38). The passive waveguides are oriented co-axially to, and are coupled to, amplification waveguides (34).

Description: The invention relates to a phased-array semiconductor laser and more particularly to a coupling structure for the laser. 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 (see US-A- 4 719 634). 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 one of the accompanying claims. The present invention is an improved structure for phase-locking a laser array in the in-phase mode. According to the invention as defined in claim 1, a coupling structure formed of transparent, passive waveguides and y-couplers is disposed between arrays of parallel amplification waveguides. Because the coupling structure is transparent it is not forward-biased and the charge carrier density is very low. Accordingly, phase distortions due to carriers are reduced to achieve stable output at high power. 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 a passive coupling region using y-shaped couplers. 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=0.20. 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 of a preferred embodiment of the invention. The semiconductor structure 10 is divided into first, second, and third parts I, II, and III separated by first and second boundaries 30 and 32. A set of index-guided amplification waveguides 34 are oriented along a horizontal axis and disposed in the active region of first and third parts I and III. The first and third parts are electrically forward-biased so that lightwaves are generated and amplified in the amplification waveguides 34 under lasing conditions. A set of low-loss, index-guided passive waveguides 36 and y-couplers 38 are disposed in the active layer 24 of the second section II to form a coupling structure 40. A first set of passive waveguides 36 are oriented co-axially to the amplification waveguides 34 in the first part I and coupled thereto at the first boundary 30 and a second set are oriented co-axially to the amplification waveguides 34 in the third region III and coupled thereto at the second boundary 32. Each y-coupler 38 couples an adjacent pair of passive waveguides 36 in the second set to one of the passive waveguides 36 in the first set. Thus, due to the geometry of the array, one more passive waveguide exists on one side of the coupler than on the other side. A set of graphs 50, 52 is superimposed on the structure depicted in Fig. 2 to indicate the magnitude of the electric fields of the lightwaves guided in the amplification waveguides 34. These graphs are highly oversimplified but serve to describe the function of the coupling structure 40. Each graph 50, 52 depicts the magnitude of the electric field on a horizontal y-axis as a function of the lateral position (x-axis) of the amplification waveguide 34. The graphs depict the various possible array modes and do not illustrate the actual operation of the system. In region III the out-of-phase array mode 52 is depicted. The electric fields in adjacent waveguides point in different directions. In region I the in-phase array mode 50 is depicted where all the electric fields point in the same direction. Generally, there are at least as many array modes as waveguides in the array and all modes except the in-phase mode should be suppressed to form a highly resolved output beam. The y-couplers cause out-of-phase modes to radiate away from the y-couplers and cause the lightwaves generated in the amplification waveguides 34 to be phase-locked in the in-phase array mode. A method for forming the index-guided amplification waveguides 34 and low-loss index-guided passive waveguides and y-couplers 38 and 36 is fully disclosed in U.S. Patent No. 4,802,182 to Thornton et al.. The index-guided amplification waveguides, passive waveguides, and y-couplers 34, 36, and 38 in active layer 24 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 amplification waveguides and low-loss waveguides 30 and 32 is confined by the well- known phenomena of total internal reflection. As described in the above-referenced Thornton et al. patent, the passive waveguides 36 and y-couplers 38 are integrally formed in the active medium and have an energy bandgap greater than the energy of the amplified lightwaves so that the coupling structure is nearly transparent to the amplified waveguides and does not need to be forward-biased to transmit the lightwaves. These transparent waveguides 36 can be narrower than the amplification waveguides 34. If the transparent waveguides 36 are sufficiently narrow they provide the additional function of stripping higher order modes and thus increase the stability of the laser. The coupling structure 40 is not forward-biased and thus functions as a passive element. Accordingly, the charge carrier density in the y-shaped couplers 38 is very low so that phase distortions induced by charged carriers are eliminated and phase-locked operation at high power is stabilized. Furthermore, the regions of the coupling structure 40 where the narrow passive waveguides 36 are coaxial with an amplification waveguide 34 are more effective in radiating the out-of-phase array mode.

Claims: A semiconductor laser array comprising: a monolithic semiconductor structure (10), divided into first, second and third parts (I,II,III) with the first and second parts separated by a first boundary (30), and the second and third parts separated by a second boundary (32), and each of said parts having at least an active layer (24) for amplification and propagation of light waves under lasing conditions; means for electrically forward biasing only said first and third parts; a first plurality of amplification waveguides (34) disposed in said first part for propagating only an in- phase array mode and terminating at said first boundary; a second plurality of amplification waveguides (34) disposed in said third part for propagating only an in- phase array mode and terminating at said second boundary; and a plurality of parallel, low-loss passive waveguides (36) disposed only in said second part and oriented co-axially and co-extensively coupled to the amplification waveguides at said first and second boundaries in the form of a y-coupled waveguide array (36, 38). A laser array as claimed in claim 1, wherein at least one of said first, second, or third parts is formed utilizing a layer disordering technique. A laser array as claimed in claim 1 or claim 2, wherein at least one of said first, second, or third parts is formed utilizing impurity induced layer disordering.

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