Patent Application
20130016749
1. Field of the Invention
The present invention relates to a surface emitting laser diode.
2. Background Art
A surface emitting laser diode emits light perpendicular to its major surface. In a typical surface emitting diode, a linear diffraction grating is formed perpendicular to a laser stripe. Also, a surface emitting laser diode wherein a plurality of stripe-shaped active regions are radially arranged on a circular diffraction grating has been proposed (for example, refer to Japanese Patent Laid-Open No. 01-105590).
In a conventional surface emitting laser diode, a horizontal far field pattern (FFP) differed from a vertical FFP. Also, high output light could not be outputted.
In view of the above-described problems, an object of the present invention is to provide a surface emitting laser diode which can emit high output light whose horizontal far field pattern and vertical far field pattern are the same.
According to the present invention, a surface emitting laser diode includes: a semiconductor substrate; a first semiconductor layer of a first conductivity type on the semiconductor substrate; an active layer on the first semiconductor layer; a second semiconductor layer of a second conductivity type on the active layer; and a second order diffraction grating in the first semiconductor layer or the second semiconductor layer. The second order diffraction grating has a pattern which includes concentric circles, a spiral, or a polygon. An active region including the first semiconductor layer, the active layer, and the second semiconductor layer is circular or polygonal.
The present invention makes it possible to emit high output light having the same horizontal far field pattern and vertical far field pattern.
Other and further objects, features and advantages of the invention will appear more fully from the following description.
A surface emitting laser diode according to the embodiments of the present invention will be described with reference to the drawings. The same components will be denoted by the same symbols, and repeated description thereof is omitted.
On an n-type semiconductor substrate 1, an n-type clad layer 2, an active layer 3, a p-type clad layer 4, and a p-type contact layer 5 are sequentially laminated. The n-type semiconductor substrate 1 is GaAs or GaN. An n-type clad layer 2 and a p-type clad layer 4 are AlGaInP, AlGaAs, GaN, or the like. An active layer 3 is GaAs, AlGaAs, InGaP, AlGaInP, InGaN, GaN, or the like. A p-type contact layer 5 is of GaAs, GaN, or the like.
A second order diffraction grating 6 is present in the p-type clad layer 4. Here, if λ0 denotes a Bragg wavelength, Neff denotes the equivalent refractive index in a medium, and Λ denotes the period of the diffraction grating, the oscillation wavelength λ of a semiconductor laser is represented by λ=λ0/Neff=2Λ/m (m=1, 2, . . . ). The case wherein the integer m is 1 is a first order diffraction grating, and the case wherein the integer m is 2 is a second order diffraction grating. The period of the second order diffraction grating is twice the period of the first order diffraction grating.
In the present embodiment, the pattern of the second order diffraction grating 6 includes concentric circles having an outer diameter of 100 μm. The active region 7, including the n-type clad layer 2, the active layer 3, the p-type clad layer 4, and the p-type contact layer 5, is also circular in a top view. A high-reflectivity film 8 is located on the entire side surfaces of the active region 7. The high-reflectivity film 8 is gold, platinum, titanium, molybdenum, tantalum, nickel, or the like; or a multilayer film of these metals. The phase of the central portion of the second order diffraction grating 6 is shifted by ¼λ against the oscillation wavelength λ.
Above the second order diffraction grating 6, an annular p-side electrode 9 having a circular opening is located on the p-type contact layer 5. An n-side electrode 10 is located on the back face of the n-type semiconductor substrate 1. The p-side electrode 9 and the n-side electrode 10 are gold, platinum, titanium, molybdenum, tantalum, nickel, or the like; or may be a multilayer film of these metals.
The substrate used for LDs is selected in conformity with the material for the grown crystals. In the case of the AlGaInP system, the use of an off-axis substrate makes the crystallinity of the grown crystals better, and the light emitting efficiency of LDs higher. Although wet etching can be used for forming the circular patterns, dry etching makes the crystalline orientation dependency smaller, and facilitates etching circularly and vertically. Furthermore, wet etching and dry etching can be combined.
Next, the operation of the above-described surface emitting laser diode will be described. When a voltage is supplied between the p-side electrode 9 and the n-side electrode 10, light is generated in the active layer 3, and radiated to various directions. The radiated light resonates and oscillates at the side faces of the second order diffraction grating 6 and the active region 7. The oscillating light is output vertically in
Next, the effect of the present embodiment will be described. Since the second order diffraction grating 6 is circular and the active region 7 is also circular, the horizontal far field pattern and the vertical far field pattern of the emitted light has the same pattern. Furthermore, since light can be guided from all directions in the circular active region 7, high power output light can be emitted.
Furthermore, with regard to the oscillation wavelength λ, the phase of the center portion of the second order diffraction grating 6 is shifted by ¼λ. Thereby, single wavelength output light can be obtained.
In addition, the high-reflectivity film 8 covers the entire side face of the active region 7. The light not reflected by the second order diffraction grating 6 inside the active region 7 is reflected by the high-reflectivity film 8 in the peripheral portion of the active region 7. Thereby, the light emitting efficiency can be elevated. When the reflectance of the high-reflectivity film 8 is 100%, light is not emitted from the side surface of the active region 7, but is emitted only from the upper surface.
Furthermore, the semiconductor substrate 1 has a larger band gap than the band gap of the active layer 3. Thereby, since the absorption of light by the semiconductor substrate 1 is decreased, light can be efficiently reflected from the back face of the semiconductor substrate 1.
The surface emitting laser diodes according to the first to fifth embodiments have a DBR (Distributed Bragg Reflector) structure wherein the second order diffraction grating 6 is located in part of the active region 7. However, the invention is not limited to this, but may be of a DFB (Distributed Feedback) structure wherein the second order diffraction grating 6 is located on the entire region of the active region 7. Furthermore, although the second order diffraction grating 6 is located in the p-type clad layer 4, the invention is not limited to this, and the second order diffraction grating 6 may be located in the n-type clad layer 2.
Although detailed epi-structures, such as the structure of the active layers have not been described in the first to sixth embodiments, the structure of the active layers may be an SQW (Single Quantum Well) or an MQW (Multi Quantum Well) structure. Furthermore, a structure having a band discontinuity relaxing layer, for relaxing the different band gaps between the clad layer and the contact layer, is also feasible.
Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.
The entire disclosure of Japanese Patent Application No. 2011-154481, filed on Jul. 13, 2011, including specification, claims, drawings, and summary, on which the Convention priority of the present application is based, is incorporated herein by reference in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2011-154481 | Jul 2011 | JP | national |
