Patent Application
20070053397
The present invention relates to semiconductor lasers and methods for making semiconductor lasers and, more particularly, to angled facet laser cavities and methods for their manufacture.
Conventional semiconductor lasers can be divided into two classes: edge emitters and surface emitters. Edge emitters have a layered structure of optical materials grown by an epitaxial process on a suitable substrate in a manner such that a waveguide region is formed near the surface of the wafer containing an optical gain medium. The gain region in conventional edge emitters is typically very thin, being about one-half of the emission wavelength thick to ensure single-mode operation. The waveguide is formed into a rectangular laser cavity or resonator by cleaving the substrate and waveguide structure such that the cleaved facets are substantially normal to the waveguide axis. The optical field grows in the laser cavity and light exits through one or both of the cleaved facets, hence the term edge emitter. The vast majority of conventional edge emitters confine the light to the waveguide using total internal reflection. In this approach, the waveguide is formed with a material with a high index surrounded by a cladding of low index material. Light incident on the interface between the waveguide and the cladding at an angle exceeding the critical angle is totally internally reflected back into the waveguide.
While confinement to the waveguide in the lateral direction can be accomplished by index guided structures such as ridge waveguides or buried channel waveguides, one class of edge emitting lasers are grating confined waveguides such as the α-DFB. Grating confined waveguides employ a surface-relief distributed Bragg reflector (DBR) for lateral confinement. The nature of the Bragg reflector is such that confinement to the waveguide stripe occurs only over a narrow range of incident angles for the operation wavelength. The α-DFB is a grating confined waveguide with the waveguide stripe angled with respect to the exit facet. This places additional self-consistency constraints on the allowable field configurations in the waveguide. The result is that the laser can be made broader in the lateral direction, while still maintaining a single lateral mode profile.
Surface emitter lasers include vertical cavity surface emitting lasers (VCSELs) using distributed Bragg reflectors (DBR). In VCSELs, the laser cavity is oriented in the vertical, or growth direction, perpendicular to the surface of the wafer. The laser cavity is formed by placing an active layer, or gain medium, in an optical cavity between a pair of DBRs. The DBR is a structure composed of alternating high and low index materials with controlled thickness such that light at the laser wavelength attempting to exit the cavity is reflected back into the cavity at normal incidence. DBRs can be constructed with arbitrarily high reflectivities by adding more high/low index pairs. A conventional VCSEL is shown in
DBR structures have also been used to create an edge emitter laser. In this structure, the thickness of the DBR pairs are changed so that instead of reflecting laser light at normal incidence, the high reflection occurs at an angle with respect to the optical axis. The structure looks similar to the VCSEL including a rectangular laser cavity, but conducts light horizontally rather than vertically. As with other edge emitters, cleaved facets serve as end mirrors forming a resonator. This is the concept of an epitaxial transverse Bragg waveguide. As shown in
It is desirable to create even more powerful semiconductor lasers having improved beam quality. Brightness, or peak on-axis beam power density in the far field diffraction pattern, is one measure of beam quality which is of particular interest. Increasing the output power of a semiconductor laser can be accomplished in two ways: 1) pumping the structure harder; or 2) increasing the volume of the active material. These two methods increase beam power at the expense of beam quality in all conventional types of semiconductor lasers to the point where beam quality degrades beyond acceptable levels prior to achieving the desired output brightness. Pumping a structure harder causes nonlinearities associated with non-uniform current injection, thermal lensing in the medium, filamentation, and spatial hole burning, all of which reduce beam quality and decrease brightness. Increasing the volume of the active material in the laser structure is successful up to the point where the laser cavity can support multiple transverse modes. At this point, further increases in cavity volume do not correspond to single mode emission, a requirement for achieving high power in the central lobe of the far-field diffraction pattern.
Currently, all index guided edge emitter lasers employ a cleaved facet to provide feedback into the gain region via Fresnel reflection. The devices are predominately grown on (100) surface wafers, using (110) cleavage planes for feedback. The cleaved facets may be coated so as to change their reflectivity to a higher or lower value from that of the uncoated materials. The (110) plane intersects the (100) surface at normal incidence. With facets oriented substantially normal to the waveguide axis, an edge emitter geometry laser employing index guiding for confinement to the plane containing the active medium will become multi-mode with increasing thickness as soon as the waveguide thickness is such that multiple bounce angles inside the waveguide core are supported (for thicknesses greater than about λ/2 n).
A DBR can be designed to operate at a specific wavelength over a very narrow range of incident angles by using a periodic structure with a small index contrast between the two materials making up the DBR. The result is that a waveguide formed with distributed Bragg reflectors for confinement can be made large in transverse dimensions without allowing additional higher order modes. Two problems arise, however, concerning high power operation: 1) practical concerns in the epitaxial growth of the DBR stack limits minimum realizable index contrast, and hence the minimum breadth of the angular reflectance spectrum; and 2) linear Fabry-Perot cavities formed with normally cleaved planes are subject to filamentation and spatial hole burning destroying high power, single mode operation.
Thus, there is a need to overcome these and other problems of the prior art to provide a laser cavity design which allows growth of an optically thick epitaxial gain region, effectively increasing the active volume of the structure, while maintaining single mode behavior and suppressing the onset of many of the deleterious effects associated with high pump levels.
According to various embodiments, a semiconductor laser or angled faceted emitter is provided. The semiconductor laser can include a first distributed Bragg reflector (DBR) and a second DBR. An active layer can be disposed between the first DBR and the second DBR. The semiconductor laser can further include a highly reflective facet at an end of the active layer and a partially reflective facet at an end of the active layer opposite the highly reflective facet. The highly reflective facet, the partially reflective facet, the first DBR, and the second DBR can bound a laser cavity having a cross sectional shape that is not rectangular and the laser cavity can propagate a guided mode normal to the highly reflective facet.
According to various embodiments, a method of operating a semiconductor laser is provided. The method can include propagating light in a zig-zag path within a laser cavity comprising a gain medium, wherein the laser cavity has a cross sectional shape that is not rectangular. The gain medium can be optically pumped and light can be emitted from the laser cavity at an angle greater than 0° and less than 90° with respect to a surface of a distributed Bragg reflector forming a bottom of the laser cavity.
According to various embodiments, a method of making a semiconductor laser is provided. The method of making the semiconductor laser can include providing a substrate, forming a first distributed Bragg reflector (DBR) on the substrate, forming an active layer over the first DBR, and forming a second DBR over the active layer. The method can further include forming a first facet at a first end of the active layer and a second facet at a second end of the active layer, wherein the first facet and the second facet are disposed at an angle that is not normal relative to a surface of the first DBR.
According to various embodiments, another semiconductor laser or angled faceted emitter is provided. The semiconductor laser can include a distributed Bragg reflector (DBR) and an active layer disposed over the DBR and further comprise a highly reflective facet and a partially reflective facet. The semiconductor laser can further include a material disposed at a top surface of the active layer, wherein the highly reflective facet, the partially reflective facet, the first DBR, and the top surface of the active layer bound a laser cavity having a cross sectional shape that is not rectangular.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description, serve to explain the principles of the invention.
In the following description, reference is made to the accompanying drawings that form a part thereof, and in which is shown by way of illustration specific exemplary embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention and it is to be understood that other embodiments may be utilized and that changes may be made without departing from the scope of the invention. The following description is, therefore, not to be taken in a limited sense.
Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements. Moreover, all ranges disclosed herein are to be understood to encompass any and all sub-ranges subsumed therein. For example, a range of “less than 10” can include any and all sub-ranges between (and including) the minimum value of zero and the maximum value of 10, that is, any and all sub-ranges having a minimum value of equal to or greater than zero and a maximum value of equal to or less than 10, e.g., 1 to 5.
As used herein, the terms “shape” and “cross sectional shape” are used interchangeably with reference to the shape of a laser cavity and refer to a cross sectional view of the laser cavity that includes both facets of the laser cavity.
As depicted in
Referring to
In various embodiments, first DBR 210 and/or second DBR 220 can be, for example, alternating layers of GaAs and AlGaAs. Active layer 230 can itself be formed of one or more layers of quantum wells, quantum dots, quantum wires or other gain medium known to one of ordinary skill in the art, such as but not restricted to, for example, InGaAs. In various other embodiments, semiconductor laser 200 can include a plurality of active layers 230.
In operation, first DBR 210 and/or second DBR 220 can be designed so that a wavelength is resonant with the DBRs at an angle of incidence such that the guided mode propagates normal to highly reflective facet 235 and/or partially reflective facet 236. As represented by the arrows, a zig-zag mode pattern can be established in the laser cavity. This can reduce spatial hole burning, filamentation, and thermal lensing. With sufficiently thick waveguides, the waveguide formed by the top and bottom surfaces of the laser cavity supports multiple field configurations, limited by the angular reflectivity of the DBR. However, only field configurations with nearly normal incidence to the angled facet, highly reflective facet 235 and/or partially reflective facet 236, are reflected back into self-consistent laser modes, further restricting the transverse mode profile such that a large volume single mode, and hence high brightness, laser is obtained. The result is a single transverse (vertical) mode laser with substantially thicker active region and emission normal to the angled facets 235 and 236. The beam can exit the laser cavity through partially reflective facet 236 at an angle greater than zero and less than 90 degrees with respect to a surface of first DBR 210.
As shown by the arrows, a zig-zag mode pattern can also be established in the parallelogram shaped laser cavity, so that spatial hole burning, filamentation, and thermal lensing are reduced. Aside from the larger active volume, the exit aperture of angled faceted emitter 300 is scaled by the reciprocal of the sine of the facet tilt angle, and thus is larger than the exit aperture of a similar edge emitter with normally cleaved facet. The larger aperture can provide reduced beam divergence and lower power density at the facet mitigating optical facet damage.
Another exemplary embodiment of a semiconductor laser or angled faceted emitter in accordance with the present teachings is shown in
In various embodiments, the angled faceted emitter can allow thicker epitaxial gain/waveguide regions operating in a single transverse mode. Moreover, the zig-zag path can allow high-power. In contrast with a VCSEL, the exit aperture of an angled faceted emitter can be separate from the top of the device. Thus, the entire top of the angled faceted emitter can be used to make electrical contact to the gain region.
In accordance various other embodiments, a generalized transverse Bragg waveguide can be combined with an angled faceted emitter to form a single device geometry. Generalized transverse Bragg waveguides are disclosed in U.S. patent application Ser. No. 11/231,812, filed on Sep. 22, 2005, the disclosure of which is incorporated by reference herein in its entirety. A Generalized Transverse Bragg Waveguide (GTBW) recognizes that a DBR with periodicity oriented at an angle to the waveguide axis can serve to provide lateral confinement.
Referring to FIGS. 5A-C, a semiconductor laser 500 can include a first DBR 510, an active layer 530 disposed on the first DBR 510, and a second DBR 520 disposed on active layer 530. The periodicity of first DBR 510 and second DBR 520 can be oriented at an angle to a waveguide axis 505, as depicted by the arrow in
An exemplary embodiment of a semiconductor laser in accordance with the present teachings is shown in the cross sectional schematic of
Another exemplary embodiment of a semiconductor laser in accordance with the present teachings is shown in the cross sectional schematic of
Fabrication of exemplary angled faceted emitters can be accomplished by one or more of: 1) selective wet etch; 2) angled reactive ion etch (RIE) or chemically assisted ion beam etch (CAIBE); 3) growth on mis-oriented substrate; 4) mechanical lapping and/or polishing and 5) faceted crystal growth. Selective wet etching and faceted growth can be used to form trapezoidal shaped laser cavities cross sections, while growth on mis-oriented substrates can form a parallelogram shaped laser cavity cross sections. Ion beam etching as well as lapping and/or polishing can generate either trapezoidal or parallelogram laser cavity shapes.
For selective wet etching, advantage can be taken of the crystal plane-dependent etch rates of GaAs based semiconductors, exposing the (111) plane facet. The (111) plane makes approximately a 55 degree angle with the (100) surface. In order for light to be normally incident on the (111) plane it must reflect off the top and bottom of the cavity at this same angle. Use of a properly designed DBR can provide such a reflection for the bottom of the laser cavity.
While exemplary embodiments have been described as being disposed on a substrate, one of ordinary skill in the art will understand that the substrate is not a requirement for the devices to operate. Appropriate growth, selective etching and lift-off processes can be used to separate the device from the growth substrate for free standing operation or subsequent deposition on another substrate (e.g. wafer bonding).
While the invention has been illustrated with respect to one or more implementations, alterations and/or modifications can be made to the illustrated examples without departing from the spirit and scope of the appended claims. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular function. Furthermore, to the extent that the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in either the detailed description and the claims, such terms are intended to be inclusive in a manner similar to the term “comprising.”
Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.
This application claims priority from U.S. Provisional Patent Application Ser. No. 60/641,783, filed Jan. 7, 2005, which is hereby incorporated by reference in its entirety.
This invention was developed under Contract F49620-03-1-0013 between the University of New Mexico and the AFOSR. The U.S. Government may have certain rights to this invention.
| Number | Date | Country | |
|---|---|---|---|
| 60641783 | Jan 2005 | US |
