INDEX AND GAIN COUPLED DISTRIBUTED FEEDBACK LASER
A method of forming a laser involves forming, on a substrate, a first epitaxial part of the laser that includes at least an active region layer surrounded by first and second waveguide layers. A dielectric layer is formed over the first epitaxial part. Two or more mask openings are patterned within the dielectric layer. The mask openings extend normal to a light-propagation direction of the laser and are spaced apart in the light-propagation direction of the laser. A second epitaxial part of the laser is formed in the mask openings using selective area epitaxy. The second epitaxial part includes a refractive grating with three-dimensional grating features.
This application is a divisional of U.S. application Ser. No. 16/448,415 filed on Jun. 21, 2019, which is incorporated herein by reference in its entirety.
SUMMARYThe present disclosure is directed to an index and gain coupled distributed feedback laser. In one embodiment, a method of forming a laser, involves forming, on a substrate, a first epitaxial part of the laser that includes at least an active region layer surrounded by first and second waveguide layers. A dielectric layer is formed over the first epitaxial part. Two or more mask openings are patterned within the dielectric layer. The mask openings extend normal to a light-propagation direction of the laser and are spaced apart in the light-propagation direction of the laser. A second epitaxial part of the laser is formed in the mask openings using selective area epitaxy. The second epitaxial part includes a refractive grating with three-dimensional grating features. The active region, first and second waveguide layers, and three-dimensional refractive grating may be formed of III-V semiconductors. The III-V semiconductors may include combinations of two or more of AlN, GaN, InN, AlGaN, GaInN, and AlGaInN.
These and other features and aspects of various embodiments may be understood in view of the following detailed discussion and accompanying drawings.
The discussion below makes reference to the following figures, wherein the same reference number may be used to identify the similar/same component in multiple figures.
The present disclosure is generally related to semiconductor lasers. In particular, the disclosure describes the structure and fabrication of a specialized distributed feedback (DFB) laser that is both index-coupled and gain-coupled. The structure of the laser can result in laser light being emitted with a particularly narrow linewidth and good side mode suppression. It is also robust against environmental fluctuations. Applications for such a DFB laser can include, among other things, spectroscopy, communication, and atom/ion interaction (atomic clocks, quantum computing, etc.).
Generally, DFB lasers are either index-coupled (modulation of real part of refractive index) or complex-coupled (modulation of imaginary part of the refractive index). Modulation of the real part of the refractive index is typically achieved by etching a diffraction grating into the semiconductor material. For example, the pattern can be etched into the top cladding of the heterostructure resulting in a semiconductor/air grating. Or the grating can be placed closer to the active zone (e.g., etching of lower cladding) and the rest of the laser heterostructure is epitaxially overgrown. In such a case, the grating is embedded in the heterostructure and not necessarily observable at the surface.
Modulation of the imaginary part of the refractive index can be realized by modulation of either the gain (e.g., through patterning of active zone) or the loss (e.g., with a metal grid on the surface that adds modulated absorption losses). In any case, lasing of one (or two w/o phase shifter) specific longitudinal mode(s) is supported in DFB-type lasers. For various applications, pure single-mode operation with a narrow emission linewidth is useful for accessing the full potential of the application (e.g., spectroscopy, communication, atom/ion interaction, etc.). A laser described below can induce modulation in both the imaginary and real part of the refractive index, thereby providing improvements in device performance in terms of linewidth, mode stability, threshold, etc.
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The mask openings extend normal to the light-propagation direction (along the X-direction in this example) and are spaced apart in the light-propagation direction of the laser. The mask openings 108 modulate a current flow into the active region. A three-dimensional refractive grating 112 is formed out of the dielectric mask openings via selective area epitaxy with proper adjustments of the growth conditions. The refractive grating 112 include grating features (e.g., trapezoidal stripes in this example) spaced apart in the light-propagation direction of the laser 100. Other shapes for the grating include stripes/channels with triangular or rectangular cross-sections. The refractive grating 112 provides modulation of the real part of the effective refractive index of the laser heterostructure. Arrow 114 represents a current that flows through the layers of the laser diode 100. The laser diode 100 would generally be packaged together with electrodes (anode, cathode), electrical contact layers, outer casing/coating, etc. These additional features are known in the art and so are not illustrated here.
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The laser grating 112 may be formed using selective area epitaxy, which involves growing an epitaxial layer in targeted regions through voids in a patterned amorphous dielectric mask. In reference again to
In a subsequent second epitaxial run the laser heterostructure, which includes part 210 is completed. Growth conditions are chosen for development of 3-D structures (e.g., nano-stripes with trapezoidal cross section) via selective area epitaxy. For example, this technique is described in “Metal organic vapour phase epitaxy of GaN and lateral overgrowth” by Pierre Gibart 2004 Rep. Prog. Phys. 67 667 for the goal of improving the GaN material property through reduction of threading dislocations. Here, selective area epitaxy is used as fabrication method that allows forming the shape of the refractive grating 112 without etching or milling. The patterned dielectric 110 not only acts as mask material for the selective area growth, but also fulfills the function of current confinement due to its electrically insulating property.
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In summary, DFB laser is described with modulation of both the real and imaginary part of the refractive index. The laser achieves index- and complex-coupling—modulation of both real and imaginary part of the refractive index using structural modulation via a 3-D periodic pattern and gain modulation through periodic, lateral current confinement. Strong light-matter coupling enables single-mode laser operation with narrow emission linewidth.
A fabrication technique is also disclosed for manufacturing the DFB laser. In this procedure, selective area epitaxy 3-D growth has advantages compared to etching that is conventionally used to form 3-D grating features. The proposed technique can achieve atomically smooth surfaces with no etch damage. It is also anticipated that imperfections in the fabrication of the sub-micron sized growth mask can be partially neutralized in the epitaxial growth step resulting in a more homogeneous diffraction grating. Strain management through selective area epitaxy avoids severe crack formation that otherwise results from tensile-strained AlGaN films (on GaN).
Unless otherwise indicated, all numbers expressing feature sizes, amounts, and physical properties used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the foregoing specification and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by those skilled in the art utilizing the teachings disclosed herein. The use of numerical ranges by endpoints includes all numbers within that range (e.g. 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5) and any range within that range.
The foregoing description of the example embodiments has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the embodiments to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. Any or all features of the disclosed embodiments can be applied individually or in any combination are not meant to be limiting, but purely illustrative. It is intended that the scope of the invention be limited not with this detailed description, but rather determined by the claims appended hereto.
Claims
1. A method of forming a laser, comprising:
- forming, on a substrate, a first epitaxial part of the laser comprising at least an active region layer surrounded by first and second waveguide layers;
- forming a dielectric layer over the first epitaxial part;
- patterning two or more mask openings within the dielectric layer, the mask openings extending normal to a light-propagation direction of the laser and are spaced apart in the light-propagation direction of the laser; and
- forming, in the mask openings using selective area epitaxy, a second epitaxial part of the laser comprising a refractive grating comprising three-dimensional nano-stripes.
2. The method of claim 1, wherein the three-dimensional nano-stripes comprise trapezoidal cross-sections.
3. The method of claim 1, wherein the three-dimensional nano-stripes comprise rectangular cross-sections.
4. The method of claim 1, wherein the three-dimensional nano-stripes comprise triangular cross-sections.
5. The method of claim 1, wherein the mask openings align with a center of the nano-stripes.
6. The method of claim 1, wherein the active layer region, first and second waveguide layers, and three-dimensional refractive grating are formed of III-V semiconductors.
7. The method of claim 6, wherein the III-V semiconductors are selected from a group consisting of AlGaInN, AlGaInAs, AlGaInP, and AlGaInSb.
8. The method of claim 1, wherein the dielectric layer is formed of SiO2.
9. The method of claim 1, further comprising forming an electron blocking layer between one of the first and second waveguide layers and the dielectric layer.
10. The method of claim 1, further comprising forming a conductive cover layer over the nano-stripes, the cover having a different refractive index than the refractive grating and filling in gaps between the nano-stripes.
11. The method of claim 10, wherein the conductive cover layer has a lower refractive index than that of the refractive grating.
12. The method of claim 10, wherein the conductive cover layer is in contact with the dielectric layer.
13. The method of claim 10, wherein the conductive cover layer is formed of ITO.
14. The method of claim 1, wherein the nano-stripes extend beyond the edges of the mask openings in the light propagation direction.
15. The method of claim 1, wherein the nano-stripes comprise surfaces facing away from the active region that are atomically smooth.
16. The method of claim 1, wherein using selective area epitaxy comprises using metalorganic vapor-phase epitaxy.
17. The method of claim 1, wherein using selective area epitaxy comprises using molecular beam epitaxy.
18. The method of claim 1, wherein the use of selective area epitaxy forms the nano-stripes without etching or milling, resulting in atomically smooth surfaces on the nano-stripes.
19. A method of forming a laser heterostructure, comprising:
- performing a first epitaxial run in which a first part of the laser heterostructure is grown in an unpatterned manner, the first part comprising an active region layer surrounded by first and second waveguide layers;
- interrupting the first epitaxial run to deposit a dielectric mask over an upper layer of the first part, the mask comprising openings extending normal to a light-propagation direction of the laser heterostructure and are spaced apart in the light-propagation direction of the laser heterostructure; and
- performing a second epitaxial run in which selective area epitaxy is used to form a second part of the laser heterostructure, the second part comprising three-dimensional nano-stripes formed in the openings.
20. The method of claim 19, wherein the first and second parts of the laser heterostructure are formed of III-V semiconductors.
Type: Application
Filed: Aug 30, 2021
Publication Date: Dec 16, 2021
Inventor: Thomas Wunderer (Santa Cruz, CA)
Application Number: 17/460,499