Patent Application
20050083982
This application claims the benefit of U.S. Provisional Application No. 60/512,189, filed Oct. 20, 2003, and of U.S. Provisional Application No. 60/578,289, filed Jun. 10, 2004, the disclosures of which are hereby incorporated herein by reference.
The present invention relates, in general, to surface emitting and receiving photonic devices, and more particularly to improved surface emitting laser devices and methods for fabricating them.
Semiconductor lasers typically are fabricated by growing the appropriate layered semiconductor material on a substrate through Metalorganic Chemical Vapor Deposition (MOCVD) or Molecular Beam Epitaxy (MBE) to form an active layer parallel to the substrate surface. The material is then processed with a variety of semiconductor processing tools to produce a laser optical cavity incorporating the active layer, and metallic contacts are attached to the semiconductor material. Finally, laser mirror facets typically are formed at the ends of the laser cavity by cleaving the semiconductor material to define edges or ends of the laser optical cavity so that when a bias voltage is applied across the contacts the resulting current flow through the active layer causes photons to be emitted out of the faceted edges of the active layer in a direction perpendicular to the current flow.
The prior art also discloses processes for forming the mirror facets of semiconductor lasers through etching, allowing lasers to be monolithically integrated with other photonic devices on the same substrate. The formation of total-internal-reflection facets within an optical cavity through the creation of such facets at angles greater than the critical angle for light propagating within the cavity is also known.
The use of an etch process to form two total-internal-reflection facets at each end of a linear laser cavity, with each facet being positioned at an angle of 45° with respect to the plane of the active layer, is also described in the prior art. In such devices, light in the cavity may be directed perpendicularly upward at one end of the cavity, resulting in surface emission at one facet, while the facet at the other end of the cavity may be oppositely angled to direct the light perpendicularly downward to, for example, a high reflectivity stack below the laser structure.
The prior art also describes devices which combine etched 45° facets with cleaved facets. The resultant devices cannot be tested in full-wafer and as such suffer from the same deficiencies as cleaved facet devices. Furthermore, they are incompatible with monolithic integration in view of the need for cleaving. Chao, et al., IEEE Photonics Technology Letters, volume 7, pages 836-838, attempted to overcome these short-comings, however, by providing an interrupted waveguide structure, but the resultant device suffered from scatter at each end of the laser cavity.
Vertical Cavity Surface Emitting Lasers (VCSELs), have gained popularity over the past several years; however, VCSELs do not allow in-plane monolithic integration of multiple devices and only allow light to exit their surface mirror at perpendicular incidence. A common aspect of these prior surface-emitting devices is that the photons are always emitted from the optical cavity in a direction perpendicular to the plane of the active layers.
In accordance with the present invention, improved surface emitting semiconductor lasers are provided in which light is emitted at an emitter end of an optical cavity in a direction vertical to the plane of the laser active layer, and in which light is emitted at a reflective region at the opposite end of the cavity in the plane of the active layer. This arrangement facilitates monitoring of laser operation without adversely affecting the light output. In accordance with one form of the invention, a reflection modification layer or stack is provided on the emitter end while in another form of the invention filter elements are provided within the laser cavity, to permit operation of the laser in an essentially single longitudinal mode. Furthermore, in accordance with the invention, surface and in-plane detectors are provided on the same substrate as the laser, and multiple laser cavities are positioned to enable the emission of multiple wavelengths at a common location.
A first embodiment of the invention is directed to an improved surface emitting laser, wherein a semiconductor laser in the form of an elongated cavity on a substrate is fabricated with a succession of layers, including active layers parallel to the substrate surface, upper and lower cladding layers, and upper contact layers, with a first, 45°-angled facet at a first, or emitter end, and with a reflective region including a perpendicular facet at a second, or reflective end. The laser may include a ridge waveguide outwardly at the emitter end, while the reflective region at the second end may, in one form of the invention, include a distributed Bragg reflector (DBR) adjacent the facet. The laser device may also include a monitoring photo detector (MPD) adjacent the DBR that responds to the small amount of light that is emitted from the facet in the reflective region to monitor the intensity of the laser. The backside of the MPD preferably is designed to have a near Brewster angle so that it is essentially non-reflective to light propagating in the optical cavity and in the MPD device. The MPD may be formed as an integral part of the DBR so as to contribute to the laser reflectivity at the second end, or may be formed as a separate element. The DBR can be replaced with a reflectivity modification layer or stack.
The upper contact layer, which may be a low bandgap semiconductor material to allow ohmic contacts to be formed, preferably incorporates an aperture in the region of the 45°-angled facet to remove light-absorbing layers and to improve the efficiency of the device.
In another embodiment of the invention, the portion of the top surface of the optical cavity at the emitter end, which extends over the 45° angled facet and includes the aperture described above, which surface is parallel to the active layers, may be coated with a dielectric layer or stack to modify the reflectivity at the laser output.
The laser device may also incorporate multiple filters at the reflective region to produce still another embodiment in which the device may operate with an essentially single longitudinal mode. Such filters may be formed by etching through the semiconductor layers to form spaced filter elements in series between the end facet of the laser and the distributed Bragg reflector provided in the first embodiment. The longitudinal mode may alternatively be produced by a reflection modification layer or stack on the end facet of the laser in the reflective region.
In accordance with another embodiment of the invention, an optical detector is located on the substrate beside the laser and is integrally formed with the laser in that it uses the same epitaxial structure on the substrate as is used for the laser. In this case, the epitaxial layers are etched during the etching of the laser cavity to fabricate a detector region that occupies a surface of the substrate adjacent to the laser. Suitable electrodes are deposited on the detector region so that light that impinges on it will be detected. This allows both a light emitter and a light detector to be integrally formed on a single substrate side-by-side.
In still another embodiment of the invention the light detector is an integral in-plane detector with a 45° angled facet which is located beside the laser and is fabricated from the same epitaxial structure as is used for the laser. The detector is elongated, and may be generally parallel to the laser axis to conserve space on the substrate. The light to be detected impinges on the detector surface above the angled facet, and is directed into the active region of the detector through the total internal reflection of the angled facet. This surface-receiving detector can be made to be extremely fast by controlling its length and width. The surface above the 45°-angled facet that is parallel to the active layers may be coated with a dielectric layer or stack to make it antireflective for even better operation of the detector.
In order to provide a selectable wavelength output, in accordance with another embodiment of the invention, multiple surface emission laser cavities may be positioned so that their emitting ends are clustered adjacent to each other, with the cavities extending outwardly; for example, as spokes surrounding a central hub. The epitaxial structure in each laser device may be slightly different to cause a different wavelength to be emitted from each one. The proximity of the emitting ends then allows the outputs from all of the lasers to be easily combined into one receiving medium, such as a fiber, and by selectively activating the lasers, a selected wavelength, or wavelengths, can be transferred into the fiber.
In still another embodiment, lasers in accordance with the present invention may be positioned at any desired angle on a substrate to maximize their packing density, since chemically assisted ion beam etching (CAIBE) is used to form the devices, and this process etches uniformly without dependence on the crystallographic planes of the semiconductor material. Thus, for example, they can be placed diagonally on a rectangular substrate. Conventional cleaving does not permit such an orientation.
In another embodiment of the invention, instead of forming the reflective end of the laser with a vertical facet, it may be desirable to etch it at a 45° angle to produce vertical emission at both ends. This surface above this second angled facet may also have an aperture in the contact layer to prevent absorption, and may incorporate a reflection modification layer or stack.
The foregoing, and additional objects, features and advantages of the present invention will be apparent to those of skill in the art from the following detailed description of preferred embodiments thereof, taken with the accompanying drawings, in which:
Turning now to a more detailed description of the invention, a surface emitting semiconductor laser 10 fabricated on a substrate 12 is illustrated diagrammatically in
As is conventional in the fabrication of solid state ridge lasers, the substrate 12 may be formed, for example, of a type III-V compound, or an alloy thereof, which may be suitably doped. The substrate includes a top surface 14 on which is deposited, as by an epitaxial deposition such as Metalorganic Chemical Vapor Deposition (MOCVD) or Molecular Beam Epitaxy (MBE), a succession of layers generally indicated at 16 which form an optical cavity 18 that includes an active region 20. A horizontal cavity semiconductor laser structure such as the optical cavity 18 typically contains upper and lower cladding regions 19 and 19′, formed from lower index semiconductor material than the active region 20, such as InP, adjacent the active region 20, which may be formed with InAlGaAs-based quantum wells and barriers. A transition layer 21 of InGaAsP is formed on the top surface of cladding region 19.
An angled facet 22 is formed at a first, or emitter end 24 of the cavity 18 by a masking and etching process in which the facet is etched downwardly and inwardly at or near a 45° angle with respect to the surface 14. This facet is angled to cause light generated in the optical cavity to be emitted in a direction that is essentially perpendicular, or close to perpendicular, to the plane of the active region 20 and to the surface 14. The emitter end facet 22 is substantially totally internally reflective so that light propagating along the longitudinal axis of the optical cavity 18 is reflected in a direction perpendicular to this axis, and this travels vertically upwardly in the direction of arrow 26, as viewed in the FIGS.
At a second, or reflective end of the optical cavity, generally indicated at 28, an end facet 30 is formed at an angle of 90° with respect to the longitudinal axis of the cavity, and thus substantially perpendicular to the active region 20 of the laser. In addition, a distributed Bragg reflector (DBR) element 32 and a monitoring photo detector (MPD) 34 are formed at end 28, facet 30 and elements 32 and 34 being formed through masking and etching in known manner. A ridge 36 extending between emitter end 24 and reflective end 28 is formed by masking and etching the optical cavity 18 above the active region 20 to form the ridge-type laser 10. At the emitter end 24, the ridge 36 is widened, or tapered outwardly, as at edges 38 and 40, to provide an open area 41 above the facet 22 to allow the beam 26 to emerge through the top surface 42 of the optical cavity 18 without distortion.
The back of the MPD portion 34 which is the left-hand end as viewed in
A top electrical contact layer 48 on the top surface 42 of the ridge 36 is typically a low bandgap semiconductor, such as InGaAs, that allows ohmic contacts to be formed with a metal layer applied to it. The transition layer 21 typically is a semiconductor having a bandgap that is between that of the upper cladding layer 19 and that of the contact layer 48, and in some cases may have a variable bandgap. The contact layer and the transition layer may absorb the light generated in the laser. For example, if an optical cavity 18 having the materials described above generates laser light with 1310 nm in wavelength, the InGaAs contact layer 48 will absorb this light after it is reflected upward from the 45° total internal reflection facet illustrated at 22. Additionally, if the bandgap of the InGaAsP transition layer 21 is smaller than about 0.95 eV, corresponding to a wavelength of 1310 nm, then the transitional layer will also lead to absorption. Removal of any absorbing layers is, therefore, important to the efficient and reliable operation of the laser. This is accomplished, as illustrated in
The top electrode is deposited on contact layer 48 on the laser and MPD, and a second electrode 54 is deposited on a bottom surface 56 of the substrate, so that a bias voltage can be applied across the ridge 36 between the electrodes to produce lasing. A zero or negative bias can also be applied across the MPD to allow it to generate an electrical current based on the light that impinges upon it. Laser light propagating in the optical cavity 18 will be reflected by facet 22 to exit vertically at first end 24, as indicated by arrow 26, and some light will exit horizontally, in the plane of the active region 20, through the facet 30 at second end 28. Some of the light exiting through facet 30 will be reflected back into the cavity by the DBR reflector 32 and some will pass through reflector 32 to impinge on the front surface 58 of the MPD 34, where it will be detected. Light which passes through the MPD will be dissipated by facet 44, as indicated by arrow 60 (
A laser cavity can be optimized by using reflectivity modification coatings. In conventional cleaved-facet lasers, one facet may have a high reflectivity coating while the other facet may be coated to lower reflectivity, for example 90% and 10% reflectivity, respectively, so that most of the laser light emerges from the lower reflectivity facet. In short cavities both facets may have high reflectivity to reduce the cavity round-trip loss, but typically one facet will have a lower reflectivity than the other, for example 99.9% and 99.0% nominal reflectivity, respectively, to allow most of the laser light to emerge from the lower reflectivity facet. In a second embodiment of the invention, illustrated in
Instead of having the back end facet 30 of the laser cavity 18 be a vertical facet, that facet can also be etched at a 45-degree angle as illustrated in
Single longitudinal mode lasers are more desirable than multi-longitudinal mode lasers in many applications. One such application is in data communications where longer reaches of communications are obtained with a single longitudinal mode lasers compared to a multi-longitudinal laser.
At the emitter end 120 of the laser, the ridge 140 is enlarged outwardly, as illustrated by side walls 142 and 144, to form an open area 145 to allow the beam 126 to be emitted through the surface of the first end without distortion, as described above with respect to
After the etching steps described above, a top electrical contact layer (not shown) such as that described with respect to
A second electrical contact layer (not shown) is deposited on the bottom surface of the substrate, so that a bias voltage can be applied across the ridge to produce lasing and a zero or negative bias can be applied across the MPD to allow it to generate an electrical current based on the light that impinges upon it. The laser light so produced in the optical cavity will exit vertically at first end 120, as indicated by arrow 126, and longitudinally at second end 128, where some light will be transmitted through the facet 130, through filters 132, and through the DBR element 134, and will impinge on the front end 150 of the MPD 136 to be detected by the MPD and then dissipated at the back facet 146 of the MPD.
As is the case with the device of
Although single DBR elements 32 and 134 are illustrated in the embodiments of
In modern systems, it is highly desirable to have a transmitter of light and a detector of light side-by-side on a single substrate, or chip. Having such a combination is even more desirable if the devices are made out of the same material. Accordingly, in the embodiment of the invention illustrated in
Area detector 160 is located adjacent to surface emitting laser 158, as illustrated, and is fabricated from the same layers 16 as were deposited on the substrate to form the optical cavity. The detector is masked and etched in these layers during the masking and etching steps used for forming the second end 28 of the laser, which steps include formation of the vertical end facet 30 (which is perpendicular to the active layer of the laser), the distributed Bragg reflector (DBR) element 32, and the monitoring photo detector (MPD) 34.
The area detector 160, in the illustrated configuration, may be generally rectangular with a top surface 162 that receives an impinging beam 164 within a detection area 166, and uses the same active layer 20 as the one used in the laser 10. A top electrical contact 168 is applied on the top surface 162 of the detector, while leaving the area of detection 166 free of this contact. A bottom contact 170 is also applied to the back of the substrate 12 and a negative or zero bias is applied between the top and the bottom contacts 168 and 170 to allow an incoming beam 164 to be detected by the detector.
In another embodiment of the invention, illustrated in
The in-plane detector 180 is located adjacent and generally parallel to the surface-emitting laser 176. Detector 180 incorporates an elongated body portion 182 having a longitudinal axis that is illustrated as being parallel to the axis of the optical cavity 18 of laser 10; however, it will be understood that these axes need not be parallel. The detector body is fabricated in the deposited layers 16 from which the laser optical cavity is formed, using the same masking and etching steps. A reflective input facet 184 is formed at a first, input end 186 of the detector, with facet 184 being etched at or near a 45° angle with respect to the surface of substrate 178 during the formation of facet 22 on laser 10. The body portion 182 and a back facet 188 are formed during the masking and etching steps used to form the second, or reflector, end 28, the vertical end facet 30, the distributed Bragg reflector (DBR) element 32, and the monitoring photo detector (MPD) 34 of laser 176. Although the detector back facet 188 is shown as being perpendicular to the plane of the active layer 20 of the deposited material, it will be understood that this facet can be etched at an angle other than the perpendicular.
The in-plane detector 180 includes a top surface region 200 for receiving an impinging light beam 202 to be detected (
The reflectivity of areas 166 (
It will be understood that multiple lasers and/or detectors such as those described above can be fabricated on a single substrate in the form of an array, to thereby enable applications such as parallel optical interconnects, wavelength selectivity, and the like. For example, multiple lasers of different wavelengths such as the array 218 illustrated in
The output ends 230, 232, 234 and 236 of the four lasers each include an angled facet, and these are formed in the same masking step, but with four separate etching steps. A slight deviation from a 45° angle etch in each of the etching steps can be used to guide the four beams slightly away from the perpendicular so that they impinge on the centrally located object, such as the fiber 242. The back facets, filtering elements, and the MPDs for the four lasers are formed through a common masking and etching step. Finally, the ridge structure is formed through masking and etching, and the devices are metallized on the top and the bottom surfaces to provide electrical contacts, as described above.
The radial array 218 of the lasers is possible because the CAIBE process that is used in fabricating the lasers provides a uniform etch that does not depend on the crystallographic planes of the semiconductor crystal. This allows surface-emitting lasers to be positioned in any desired configuration on the substrate, as illustrated in
Although the present invention has been illustrated in terms of preferred embodiments, it will be understood that variations and modifications may be made without departing from the true spirit and scope thereof as set out in the following claims.
| Number | Date | Country | |
|---|---|---|---|
| 60512189 | Oct 2003 | US | |
| 60578289 | Jun 2004 | US |
