GB2221791A - Surface emitting laser - Google Patents

Description

222 179 19 SURFACE EMITTING LASERS RD 19, 523P The invention described

herein was made in the performance of work under NASA Contract No. NAS 1-17441 and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958 (72 Stat. 435; 42 U. S.C. 2457).of the united States of America.

Background of the Invention

The present invention relates to surface emitting lasers, Surface emitting lasers have an advantage over edge emitting lasers in that since their light emitting surface (the area of a grating) is larger, the power density is lower, and therefore, more power can be generated without destructive heating effects. -Further, the active section of a surface emitting laser can be made longer than that of a Fabry-Perot XFP) cavity laser for more gain without spurious frequency generation due to the use of the grating. For still higher power outputs surface emitting lasers can have their outputs combined using an optical waveguide and a grating or distributed Bragg reflector (DBR) such as shown in FIGURE 1 of the article "Dynamically stable 00 Phase Mode Operation Of A Grating-surface-emitting Diode-laser Array," by N. W. Carlson et al., Optics Letters, Volume 13, No. 4, April 1988, pp. 312-314. However, in such devices, dae to losses in the waveguide, the phase locking of the numerous lasers may not be sufficient to prevent spurious frequency generation as well as an incoherent light beam with a broad main beamwidth and high amplitude and broad beamwidth sidelobes. It is known from P. Zory tt al., "Grating-Coupled Double-Reterostructure A1GaAs Diode Lasers," ME Journal Of Quantum Electronics, Volume QE-11, No. 7, July 1975, pp. 451-457, to longitudinally align two RD 19, 523 lasers. However, the power is limited to that of a single pair of lasers.

It is, therefore, desirable to have a high power output from surface emitting lasers with good phase locking 5 and a coherent output light beam.

Summary of the Invention

A surface emitting semiconductor laser device in accordance with the invention for emitting an output light signal perpendicular to a major surface thereof, comprises a substrate having first and second opposing major surfaces; first contact means over said first major surface of said substrate; first and second longitudinally spaced apart and laterally aligned laser regions defining a central region therebetween and said laser regions being disposed on said second major surface of said substrate; an optical medium extending over the central region and the first and second laser regions thr6ugh which light generated by the first and second laser regions propagates; a capping_layer and second contact means overlying the optical medium; and a single optical grating surface etched into the second contact means and the capping layer to extend over said central region in optical communication with the optical medium to define said major surface of said laser device, wherein the first and second laser regions are disposed on opposing sides longitudinally of the grating surface, said grating surface having grating periods for phase locking and combining the light propagating in the optical medium and generated by said first and second laser regions and for allowing said output light signal to be emitted therethrough perpendicular to said major surface of said laser device.

Brief Description of the Drawings

FIGURE 1 is a side cross-sectional view of a double heterostructure-large optical Cavity (DE-LOC) laser used in a first embodiment of the invention; FIGURE 2 is a top view of FIGURE 1; RD 19, 523 FIGURE 3 is a side cross-sectional view of a quantum well (QW) laser used in a second embodiment of the invention; and FIGURE 4 is a top view of an embodiment of the invention showing a plurality of laterally adjacent lasers.

Detailed Description of the Preferred Embodiments

FIGURE 1 shows a device5 generally designated by numeral 10, comprising an N-contact 11, such as sintered Ni/Ge/Au, underneath a substrate 12, such as GaAs, of N-conductivity type with a doping level of about 10 is cm-3 and a thickness of about 100pm (micrometers). The central top of the substrate 12 has a 1pm deep channel 13 (described in detail below) so that the lasers (described below) are of the channeled substrate planar (M) type.

overlying the substrate 12 is an N-cladding layer 14 of N-conductivity type. The layer 14 also provides a barrier to holes. overlying the layer 14 is an active layer 16 with a thickness between about 500 to 2000A (Angstroms), 0 preferably about 800A. The active layer 16 is not intentionally doped, and typically comprises Alz Gal-ZAs wherein 0 z 9 0. 13. A barrier layer 18, which provides a barrier for electrons, overlies the active layer 16 and has 0 a thickness of about 200-1000A and is not intentionally doped. It is to be understood that the layers 16 and 18 normally acquire some doping from their respective adjacent layers during fabrication. A large optical cavity (LOC) or optical medium waveguide layer 20 overlies the layer 18 and typically comprises Al y Ga 1-Y As ' wherein 0.15 9 y 5 0.4, with a thickness between about 0.25 to lpm and 17 3 N-conductivity doping of about 5 x 10 cm- In the middle-of the upper surface of the waveguide layer 20 is a granting surface 22 comprising surface corrugations with a peak-to-valley amplitude of about 1000 A and with a spacing of about A/nel where A is the wavelength of the generated light and ne is the effective index of refraction of the guided mode. The profile of the corrugations is chosen such that the A/ne periodic structure comprises significant- RD 19, 523 components at A/nel for example, by using V grooves where the width of the top of the groves is about half of the intergroove spacing. A P- cladding layer 24 has segments 24a and 24b that overlie the ends of the layer 20, respectively, and is P-conductivity type doped. The layers 14, 18, and 24 typically are of AlxGal-X As, wherein 0.3 5 x 5 O.S. The cladding layers 14 and 24 typically have a thickness of about 1pm and.a doping level of about 17 -3 X 10 cm A capping layer 26 has segments 26a and 26b that overlie the layer segments 24a and 24b, respectively, and typically is of GaAs P-conductivity doped with a doping level between about 1018 to 1019CM-3 and thickness of about 0.5pm. A P-contact layer 28 has segments 28a and 28b that overlie the layer segments 26a and 26b, respectively, and typically comprises successive layers of Ti/Pt/Au, with the Ti layer next to the layer 26. At the sides of the structure are reflective facet layers 30a and 30b such as appropriately cleaved ends with a dielectric stack of alternate layers of Si02 and Al 2 0 3- Typically about three such pairs (six layers) are used, each layer having a thickness of about one quarter wavelength, such as shown in U.S. Patent No. 4,092,659.

It will be appreciated that longitudinally aligned DH-LOC lasers 32a and 32b are formed by the structure described above, each having a length L1 preferably of about 200pm. The grating length L2 is preferably about 300pm. Thus, the lasers or laser regions 32a and 32b are spaced apart on opposing sides of the grating surface 22 with the grating 22 extending over a central region of the laser device between the laser regions 32a and 32b. As shown in FIGURE 2 the device 11 has a width W of about 300pm." The top of the channel 13 at the top of the substrate 12, indicated by the dotted lines 34a and 34b, is preferably about 4 to 8pm, wide, while the bottom of the channel 13, indicated by the dotted lines 36a and 36b, is narrower. The sidewalls therebetween, designated 38a and 38b, make an angle of about 57 degrees with the top of the substrate 12.

RD 19, 523 This embodiment can be made by the liquid phase epitaxial process with appropriate reagents and dopants all as known in the art. The channel 13 can be formed by etching the substrate 12 along its 111A plane using Caros solution at 200C, which in this case is a mixture of H2SO4/"202/H20 in a 5/1/1 ratio by volume. similarly,. the respective segments of the layers 24, 26, and 28 can be formed by etching the central portions of the layers 24, 26, and 28, while masking their end portions, which are part of the lasers 32a and 32b. A preferrential etchant, such as 1/1/8 Caros solution can be used.

In operation, a positive voltage is applied to the P-contact 28 and a negative voltage to the N-contact 11. Holes are injected from the P-contact 28 into the active layer 16 with the cladding layer 14 providing a barrier against further downward movement by holes.

Similarly, electrons are injected from the N-contact 11 into the active layer 16 with the barrier layer 18 providing a barrier against further upward movement by electrons. At a threshold current, population inversion occurs and, therefore, stimulated emission of photons.

Photons generated by both laser regions 32a and 32b are present in the waveguide 20 and a first portion of the photons incident on the grating surface, through the interaction with the X/ne, component of the grating, is emitted perpendicular to the waveguide 20 as indicated by the arrows 34. A second portion of the photons incident on the grating surface 22 is reflected back into the laser regions 32a and 32b through the action of the A/n e component of the grating surface 22, thereby increasing the optical feed back and enhancing the lasing action. Because the amount of reflection is dependent on the wavelength of the light generated by each laser device, a significant amount of feedback is present only at one specific period.

The light generated by the laser regions 32a and 32b is thereby frequency locked. The remaining portion of the photons incident on the grating surface 22 generated by each of the laser regions 32a and 32b is transmitted RD 19, 523 through the optical medium 20 to the other laser region, thereby phase locking the two laser regions together. Since both laser regions 32a and 32b share the same device, the light generated by both laser regions is locked in both frequency and phase, and is emitted through the grating surface perpendicular to the grating surface.

It will be appreciated that since only two longitudinally aligned laser regibns 32a and 32b are used, the waveguide 20 can be relatively short and, hence, have a low loss, therefore, and phase and frequency locking of the two lasers 32a and 32b is greater than if more such lasers are used. In turn, this results in stabilizing the longitudinal mode of the lasers 32 resulting in a single emission frequency compared to the plurality of modes-in an FabryPerot cavity laser. Further, more light power output is available compared to using just a single laser.

FIGURE 3 shows a second embodiment of the laser regions 32a and 32b which are of the QW type. Only the laser 32a is shown as it will be understood thaqt the laser 32b is identical. Elements of FIGURE 3 that correspond to elements in FIGURE 1 are given corresponding reference numerals. The cladding layers 14 and 24 are between about 0.5 to 2.5pm thick and comprise AlxGal_xAs, wherein 0.4;S x 9 1, with a doping level between about 10 17 to x 1018 cm-3 of the appropriate conductivity type dopant.

The central portion of the cladding 24 comprises the DBR 22 0 and is about 1000A thick at the valleys of the DBR 22.

Undoped confining layers 36 and 40 are between about 500 to 4000X thick and comprise AlxGal_xAs, wherein 0.15 9 x 9 0.60,_and can be either graded or ungraded. The undoped quantum well layer 38 is between about 10 to 400A thick and comprises AlXiGal_xAs, wherein 0 9 x:5 1.

In general, the QW embodiment of FIGURE 3 has a lower threshold current, reduced variation of the threshold current with temperature, and increased differential quantum efficiency compared to DR-LOC lasers.

FIGURE 4 shows a device in accordance with an embodiment of the invention to obtain more light output 11 RD 19, 523 power compared to using just a single pair of lasers, while maintaining phase and frequency coherency. In this embodiment, the substrate 12 is laterally extended compared to FIGURE 2, as are the reflective facet layers 30a and 30b and the grating surface 22. The grating surface 22 thereby comprises only a single integral means for phase locking and combining the outputs of all of the lasers to achieve a high coherency. For the sake of clarity, only the channels 13,are shown of the CSP-LOC laser regions 32a and 32b.

Each of the five channels 13 extends under only a pair of corresponding longitudinally aligned lasers as in FIGURE 1. Thus, there are a total of ten lasers. Also, the channels 13a, 13b, 13c, 13d, and 13e are mutually laterally aligned with a typical center-to-center spacing 11d11 between about 4 to 10pm. Thus, the lasers have their lateral optical modes (parallel to the junction plane) coupled together resulting in phase and frequency coupling and coherency for the entire array. The entire array will, therefore, provide single wavelength light output from the grating surface 22 normal to the substrate 12. Depending upon L1 and the efficiency of grating surface 22 it is possible to increase the light output by about 10 to 50 times that of a single laser. QW lasers as shown in FIGURE 3 could also be used in FIGURE 4 instead of DH-LOC lasers.

-a- RD 19, 523

Claims (5)

-CLAIMS

1. A surface emitting semiconductor laser device for emitting an output light signal perpendicular to a major surface thereof, comprising:

a substrate having first and second opposing major surfaces; first contact means over said first major surface of said substrate; first and second longitudinally spaced apart and laterally aligned laser regions defining a central region therebetween and said laser regions being disposed on said second major surface of said substratei; an optical medium extending over the central region and the first and second laser regions through which light generated by the first and second laser regions propagates; a capping layer and second contact means overlying the optical medium; a single optical grating surface etched into the second contact means and the capping layer to extend over said central region in optical communication with the optical medium to define said major surface of said laser device, wherein the first and second laser regions are disposed on opposing sides longitudinally of the grating surface, said grating surface having-grating periods for phase locking and combining the light propagating in the optical medium and generated by said first and second laser regions and for allowing said output light signal to be emitted therethrough perpendicular to said major surface of said laser device.

2. The device of claim 1 wherein the first and second laser regions each comprise a plurality of laterally aligned phase-locked lasers disposed on said second major 1; k -g- RD 19, 523 surface of said substrate with corresponding spaced-apart lasers of each of the first and.second longitudinal laser regions being longitudinally aligned.

3. The device of claim 2 wherein each of the lasers comprises a double heterojunction large optical cavity laser.

4. The device of claim 2 wherein each of said lasers comprise a quantum well laser.

5. A semiconductor laser device substantially as hereinbefore described with reference to Figures 1 and 2, or to Figure 3, or to Figure 4.

published 1990 atThe Patent Office, State House. 6671 High Holborn. London WC1R4TP. Further copies maybe obtained from The PatentOMce Sales Branch. St Mary Cray. Orpingtor.. Kei t BR5 3RD. Pr,rted by IAjlliplex techniques Itc. St Mary Cray. Kent. Con- l.87