Vertical cavity surface emitting laser using photonic crystals

ABSTRACT

A vertical cavity surface emitting laser (VCSEL) using photonic crystals. Photonic crystals are formed such that the active region of the VCSEL is bounded by the photonic crystals. The photonic crystals have a periodic cavity structure that reflects light of certain wavelengths through the active region of the VCSEL such that laser light at the wavelengths is generated. Additional photonic crystals can be formed to increase the bandwidth of the VCSEL. The photonic crystals can also be combined with distributed bragg reflector layers to form the mirrors of a VCSEL.

INFORMATION

DETAILED DESCRIPTION OF THE INVENTION

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Semiconductor lasers are some of the most common light sources in optical networks. At a basic level, semiconductor lasers are essentially pn-junctions that convert electrical energy into light energy. Typically, a gain medium or active region is formed at the pn-junction between the p-type semiconductor material and the n-type semiconductor material. Often, the active region includes quantum wells that can be either compressively or tensile strained quantum wells. The active region may also include quantum dots.

In vertical cavity surface emitting lasers (VCSELs), mirrors are usually formed both above and below the active region. The mirrors reflect light or photons back and forth the through the active region. Within the VCSEL cavity that is effectively bounded by the mirrors or by this mirror system, the light resonates vertically or perpendicularly to the pn-junction. Some of the light escapes the mirror system and emerges from a surface of the VCSEL. Because the light is resonating vertically, the cavity length of a VCSEL is often very short with respect to the direction of light travel. The length of the cavity thus has an effect on the ability of a photon to stimulate the emission of additional photons, particularly at low carrier densities.

To overcome this problem, the mirrors or the mirror system of a VCSEL must be highly efficient and this high reflectivity requirement cannot be achieved through the use of metallic mirrors. VCSELs currently employ, for example, Distributed Bragg Reflector (DBR) layers. DBR layers are formed by forming or growing alternating layers of materials whose refractive index varies. These alternating layers are often formed or grown from semiconductor material or dielectric materials. Light is reflected at the junction of these materials and in order to achieve the high reflectivity required by VCSELs, many layers must be formed or grown as previously discussed.

VCSELs that operate at wavelengths on the order of 1.3 to 1.55 micrometers, as previously stated, are very difficult to manufacture or fabricate. The difficulty in fabricating VCSELs that generate light at these longer wavelengths is often related, for example, to the atomic lattice structure of the materials, the quality of the active region, the reflectivity of the mirror systems, and the type of composition of the material.

The present invention relates to a vertical cavity surface emitting laser and to methods of fabricating or manufacturing vertical cavity surface emitting lasers that generate or produce light at higher wavelengths. In the present invention, the mirror system or mirror layers of VCSELs are achieved using photonic crystals or using a combination of fewer DBR layers and photonic crystals.

A photonic crystal is a material that has a cavity structure that is related to the wavelengths emitted by the VCSEL and FIG. 1A illustrates an exemplary photonic crystal or layer. A plurality of cavities that are periodic in nature are formed or structured in the photonic crystal . Cavities and are examples of the cavities that are thus formed in the photonic crystal . Each cavity typically passes through the photonic crystal . This causes the photonic crystal to have a perforated quality in this example. It is possible for the cavity structure to be formed such that the photonic crystal is not perforated by cavities. In another example, the cavities may extend into other layers of the VCSEL. The cavities are formed or placed in the photonic crystal using an appropriate cavity structure that can vary according to the desired wavelength. The distance between cavities in the cavity structure may be related to the wavelengths of laser light that are generated by the VCSEL. In one example, the photonic crystal enables VCSELs to generate wavelengths on the order of 1.3 to 1.55 microns more easily.

The wavelengths emitted by a VCSEL can be altered by changing characteristics or attributes of the photonic crystal. Characteristics or attributes than can be changed such that a VCSEL emits a different wavelength(s) include, but are not limited to, the cavity structure, the shape of the cavities, the angle of the cavities with respect to the surface of the photonic crystal, the depth of the cavities, the material from which the photonic crystal is formed, the thickness of the photonic crystal, and the like or any combination thereof.

As previously stated, the cavities that are formed in a photonic crystal are periodic in nature or repeating. Examples of the periodic structure of the cavities in the photonic crystal is thus illustrated in FIGS. 1A, B and C. FIG. 1A illustrates cavities that are formed using a square cavity structure as illustrated by the dashed line . FIG. 1B illustrates cavities that are formed using a honeycomb cavity structure shown by the dashed line and FIG. 1C illustrates cavities that are formed using a rhombic cavity structure shown by the dashed line . The present invention is not limited to these repeating structures but extends to other periodic cavity structures such as triangular cavity structures or other geometric cavity structures.

Cavities are not limited in shape either. The cavities and shown FIG. 1A are circular in shape and form a circular column through the photonic crystal, the cavities and are triangular in shape and form a triangular column through the photonic crystal , and the cavities and are square in shape and form a square column through the photonic crystal . The periodic cavity structure can be combined with any cavity shape and the present invention contemplates photonic crystals or layers whose cavities are of different shapes. In addition, the cavities may not pass completely through the photonic crystal, but may form a dimpled surface. Alternatively, the cavities may have a depth that extends into other layers of the VCSEL.

FIG. 2 is a block diagram that illustrates generally the structure of a VCSEL in accordance with the present invention. The VCSEL is formed on a substrate . In some cases, the light exits the VCSEL through the substrate , which is often transparent to the laser light. Usually, one side of the VCSEL is blocked to laser light such that light is only emitted from one side of the VCSEL. A lower mirror layer is formed or grown on the substrate . An active region is formed or grown on the mirror layer . On the active region , an upper mirror layer is grown or formed. As the mirror layers and repeatedly reflect light or photons through the active region , the laser light is ultimately generated and exits the VCSEL .

The active region is typically formed from a semiconductor material. The mirror layers and can be formed from or include photonic crystals or layers. The photonic crystals provide the reflectivity required by the VCSEL and are not as difficult to grow as the multiple DBR layers previously discussed. This makes VCSELs easier to fabricate and reduces cost. In addition, VCSELs that emit different wavelengths of light can be fabricated on the same wafer by controlling the cavity structures.

FIG. 3 illustrates another example of a VCSEL that incorporates photonic crystals as part of the mirror layers. In this example, the active region is bounded by a photonic crystal and a photonic crystal . The VCSEL also utilizes DBR layers and as part of the mirror layers. The upper mirror layer thus includes the DBR layers and the photonic crystal while the lower mirror layer includes the DBR layers and the photonic crystal . When photonic crystals are included as part of the mirror layers, the number of DBR layers can be reduced. In fact, the DBR layers can be omitted in one embodiment. The orientation or location of the photonic crystals with respect to the DBR layers can also be changed. In another embodiment, for example, the active region is bounded by the DBR layers which, in turn, are bounded by the photonic crystals.

FIG. 4 is a perspective view of a VCSEL that uses photonic crystals or layers as mirrors. In this example, the VCSEL includes an active region that is bounded by a photonic crystal and a photonic crystal . The photonic crystal is formed on the DBR layers . In this example, both the photonic crystal and the photonic crystal have the same periodic cavity structure. The photonic crystal and the photonic crystal have a square cavity structure and the cavities have a circular shape as shown by the cavities and .

The photonic crystals and , however, are not required to have the same periodic cavity structure. The periodic structure of the cavities selected for the photonic crystal may be affected, for example, by the DBR layers . The periodic structure of the cavities on the photonic crystals may also be influenced by the material used to form the photonic crystals. When the cavities of the photonic crystals and are first formed, they typically contain air. However, the present invention contemplates filling the cavities with another material.

The photonic crystals can be formed, for example, from GaAs, AlGaAs, InGaAs, InP, GaInAsP, AlGaInAs, InGaAsN, InGaAsSb, and the like. The photonic crystals can also be formed from dielectric materials that can be deposited in a thin film. The material used to fill the cavities also extends to similar materials. The resonance frequency of the photonic crystal can be altered or changed if the refractive index of the material used to form the photonic crystal and/or fill the cavities is tunable.

In another example of the present invention, only one photonic crystal is provided as one of the other mirror layers. The other mirror layer is formed, for example, using DBR layers. In another example of the present invention, more than one photonic crystal is used. The addition of more photonic crystals extends the bandwidth of the VCSEL. More specifically, the upper and/or the lower mirror layer may include more than one photonic crystal. Each photonic crystal may be formed from a different material and each photonic crystal may have a different cavity structure. Other attributes of the photonic crystals, described above, may be independent of other photonic crystals in the VCSEL.

The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to describe the manner in which the above-recited and other advantages and features of the invention can be obtained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:

FIG. 1A is a perspective view of a photonic crystal or layer with a square periodic cavity structure;

FIG. 1B is a top view of a photonic crystal or layer that has a honeycomb periodic cavity structure;

FIG. 1C is a top view of a photonic crystal or layer that has a rhombic periodic cavity structure;

FIG. 2 illustrates a vertical cavity surface emitting laser where the mirror layers are formed from photonic crystals;

FIG. 3 illustrate a vertical cavity surface emitting laser where the mirror layers are formed from a combination of photonic crystals and distributed Bragg Reflector layers; and

FIG. 4 illustrates an active region of a vertical cavity surface emitting laser, where the active region is bounded by one mirror layer that includes a photonic crystal on one side and by a mirror layer that includes both a photonic crystal and Distributed Bragg Reflector layers on the other side.

CLAIMS

1. A vertical cavity surface emitting laser comprising: a lower mirror layer formed on a substrate, wherein the lower mirror layer comprises a lower photonic crystal having a cavity structure that includes one or more cavities and at least one distributed bragg reflector layer; an upper mirror layer; and an active region formed between the lower mirror layer and the upper mirror layer wherein photons are reflected between the lower mirror layer and the upper mirror layer through the active region.

2. A vertical cavity surface emitting laser as defined in claim 1, wherein the upper mirror layer further comprises an upper photonic crystal that has a cavity structure.

3. A vertical cavity surface emitting laser as defined in claim 2, wherein the cavity structure of the lower photonic crystal and the cavity structure of the upper photonic crystal are periodic.

4. A vertical cavity surface emitting laser as defined in claim 3, wherein the cavity structure of the lower photonic crystal is substantially the same as the cavity structure of the upper photonic crystal.

5. A vertical cavity surface emitting laser as defined in claim 1, wherein the one or more cavities the lower photonic crystal are filled with air, and wherein the one or more cavities of the upper photonic crystal are filled with air.

6. A vertical cavity surface emitting laser as defined in claim 1, wherein the lower mirror layer comprises a plurality of distributed bragg reflector layers.

7. A vertical cavity surface emitting laser as defined in claim 6, wherein the distributed bragg reflector layers are formed between the active region and the lower photonic crystal.

8. A vertical cavity surface emitting laser as defined in claim 6, wherein the distributed bragg reflector layers are formed between the substrate and the lower photonic crystal.

9. A vertical cavity surface emitting laser as defined in claim 1, wherein the upper mirror layer further comprises one or more distributed bragg reflector layers.

10. A vertical cavity surface emitting laser as defined in claim 2, wherein the upper mirror layer further comprises an extra photonic crystal.

11. A vertical cavity surface emitting laser as defined in claim 1, wherein light with a wavelength of at least 1.3 microns is emitted by the vertical cavity surface emitting laser.

12. A vertical cavity surface emitting laser comprising: a substrate; a lower mirror layer formed on the substrate, wherein the lower mirror layer includes a plurality of distributed bragg reflector layers; an active region formed on the lower mirror layer, wherein the active region is lattice matched to the lower mirror layer; and an upper photonic crystal that is formed and lattice matched to the active region, wherein the upper photonic crystal includes a plurality of cavities, wherein light is reflected through the active region by the lower mirror layer and the upper photonic crystal such that laser light is produced at a wavelength that is related to a cavity structure of the upper photonic crystal.

13. A vertical cavity surface emitting laser as defined in claim 12, wherein the lower mirror layer further comprises a lower photonic crystal that has a periodic cavity structure, wherein the lower photonic crystal is formed from an n-type semiconductor material and wherein the upper photonic crystal is formed from a p-type semiconductor material.

14. A vertical cavity surface emitting laser as defined in claim 12, wherein the active region comprises a plurality of quantum wells or quantum spots.

15. A vertical cavity surface emitting laser as defined in claim 12, wherein the cavity structure of the lower photonic crystal is a periodic structure and wherein each cavity has a shape that affects the wavelength emitted by the vertical cavity surface emitting laser.

16. A vertical cavity surface emitting laser as defined in claim 15, wherein the cavity structure of the upper photonic crystal is a periodic structure and wherein each cavity has a shape that affects the wavelength emitted by the vertical cavity surface emitting laser, and wherein the periodic structure of the upper photonic crystal is the same as the periodic structure of the lower photonic crystal.

17. A vertical cavity surface emitting laser as defined in claim 13, wherein the plurality of cavities of the lower photonic crystal and the plurality of cavities of the upper photonic crystal are filled with air.

18. A vertical cavity surface emitting laser as defined in claim 13, wherein the cavities of the lower photonic crystal and the cavities of the upper photonic crystal are formed by either dry etching or lithography.

19. A vertical cavity surface emitting laser as defined in claim 13, further comprising one or more distributed bragg reflector layers formed between the lower photonic crystal and the active region.

20. A vertical cavity surface emitting laser as defined in claim 13, further comprising a third photonic crystal formed on the upper photonic crystal.

21. A vertical cavity surface emitting laser as defined in claim 12 wherein light with a wavelength of at least 1.3 microns is emitted by the vertical cavity surface emitting laser.

22. A method for fabricating vertical cavity surface emitting laser, the method comprising: forming a lower mirror layer on a substrate comprising one or more distributed bragg layers; forming an active region on the first mirror layer; and forming an upper mirror layer that includes an upper photonic crystal on the active region such that light is reflected through the active region by the lower mirror layer and the upper mirror layer, wherein a periodic cavity structure is formed in the upper photonic crystal.

23. A method as defined in claim 22, wherein the active region is formed on the one or more distributed bragg layers.

24. A method as defined in claim 22, wherein forming a lower mirror layer on a substrate further comprises: forming a lower photonic crystal on the one or more distributed bragg layers; and forming a cavity structure in the lower photonic crystal, wherein the cavity structure of the lower photonic crystal is substantially the same as the cavity structure of the upper photonic crystal.

25. A method as defined in claim 22, further comprising: forming an additional photonic crystal on either the upper photonic crystal or the lower photonic crystal; and forming a cavity structure in the additional photonic crystal.

26. A method as defined in claim 24, further comprising changing an index of either the lower photonic crystal or the upper photonic crystal by filling cavities with a material whose refractive index is different than a refractive index of either the lower photonic crystal or the upper photonic crystal.

27. A method as defined in claim 22, wherein light with a wavelength of at least 1.3 microns is emitted by the vertical cavity surface emitting laser.

COPYRIGHT

User acknowledges that Fairview Research and its third party providers retain all right, title and interest in and to this xml under applicable copyright laws. User acquires no ownership rights to this xml including but not limited to its format. User hereby accepts the terms and conditions of the License Agreement.