EP1719220A4

EP1719220A4 - Methods for polarization control for vcsels

Info

Publication number: EP1719220A4
Application number: EP05723667A
Authority: EP
Inventors: Jin Kim
Original assignee: Finisar Corp
Current assignee: Finisar Corp
Priority date: 2004-02-25
Filing date: 2005-02-25
Publication date: 2008-08-06
Also published as: WO2005082010A2; CA2556709A1; WO2005082010A3; EP1719220A2

Abstract

A vertical cavity surface emitting laser (VCSEL) (40) having strain or stress elements (41) added for stable polarization control. The elements may disturb the symmetrical stress of the VCSEL structure to induce stable polarization control of its output. Such elements may be attached to the top surface or the sides of the VCSEL (40), or may be inserted in some of the layers of the VCSEL (40) structure. Some elements may involve the removal of material from the sides of the VCSEL (40).

Description

METHODS FOR POLARIZATION CONTROL FOR VCSELS BACKGROUND OF THE INVENTION Field of the Invention The present invention pertains to lasers and particularly to vertical cavity surface emitting lasers (VCSELS). More particularly, the invention pertains to polarization control of VCSELS. Polarization stability over the operating range is desirable for semiconductor lasers in communications and other applications. However, vertical-cavity surface- emitting lasers, due to their symmetry, usually do not meet this condition of a stable polarized output. SUMMARY The present invention describes devices and techniques that may he implemented to introduce polarization stability in VCSELS. VCSEL output polarization may be achieved by breaking the optical-loss and/or optical-gain symmetry of a VCSEL in the two orthogonal in-plane directions. Stress-inducing features may be formed on the top or bottom surface of a VCSEL, on the side of a VCSEL, or within a VCSEL to produce a break in its symmetry of stresses to effect a polarizing influence upon an output of the VCSEL. Compactness and compatibility of the invention with existing production methods may add to the usefulness of the techniques of this invention. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a perspective of an illustrative VCSEL; Figure 2 is a sectional view of the VCSEL of Figure 1; Figures 3 a, 3b and 3 c illustrate induced polarization with stress plates on top of the VCSEL; Figures 4a, 4b and 4c illustrate induced polarization with stress plates on the sides of the VCSEL; Figures 5a, 5b and 5c illustrate induced polarization with stress elements in the layer portions of the VCSEL; Figures 6a, 6b and 6c illustrate induced polarization with portions of the stack etched away and stress elements in the layer portions of the VCSEL; and Figures 7a, 7b and 7c illustrate induced polarization with just portions of the stack etched away. DESCRIPTION Figures 1 and 2 illustrate an example of a planar, current-guided, vertical cavity surface emitting laser (VCSEL) 10 having periodic layer pairs for top and bottom distributed Bragg reflector (DBR) mirror stacks, that may use the polarization control of the present invention. Formed on the bottom of a substrate 14 may be a bottom contact 12. Substrate 14 may be doped with impurities of a first type. A first-type doped mirror stack 16 may be formed on substrate 14. Formed on stack 16 may be a first-type doped spacer layer 20. The first-type doped bottom spacer layer 20 and a second-type doped top spacer layer 24 may sandwich an active region 22. A second-type doped mirror stack 26 may be formed on a top spacer layer 24. A metal layer 28 may be formed as a contact on a portion of stack 26 at an interface 32. The emission region may have a passivation layer 30. An isolation region 29 may restrict the area of the current flow 27 through the active region. Region 29 may be formed by an ion implantation and/or oxidation. A diameter 13 may be set to provide the desired active area, and thus the gain aperture of the VCSEL 10. Further, a diameter 11 of an aperture 34 for light 31 may be set by the resistance of the second-type doped mirror stack 26, particularly through the non- conductive region 29. Thus, non-conductive region 29 may perform the gain guiding function. Diameter 13 may be limited by fabrication limitations, such as lateral straggle during the implantation or oxidation step. The present apparatus and method for providing stable polarization control of a

VCSEL output are simpler, more effective and easier to make and put into effect than other devices and approaches in the related art. The present approach and apparatus may involve using polarization inducing stress components applied in or on VCSEL 10 in various ways, as long as the induced stress results in different stresses in two orthogonal in-plane directions. Various configurations of polarization control are implemented with an example of a VCSEL for illustrative purposes. Certain VCSEL components, such as contact metals are omitted in some of the illustrative figures for simplicity. In Figures 3a through 7c, the material type, size, shape, number, orientation and placement of the components may be of various kinds since those factors do not detract from of the invention. Figures 3a, 3b and 3c show the application of strain or stress elements (stressors) 41 on top of a VCSEL 40. Even though VCSELS in the figures are circular, they may be square, rectangular or another shape. Elements 41 may be applied on the top or bottom of VCSEL 40. Elements 41 may be patterned dielectric material such as silicon nitride or silicon dioxide. Elements 41 may also be a metal or other appropriate stress causing material. The material of elements 41 may or may not be lattice-mismatched relative to the VCSEL. Elements 41 may be deposited or grown with a pattern of a mask on the VCSEL surface 42. On the other hand, the material for elements 41 may be deposited with an etch stop under it and the patterns may be etched with a mask applied on the deposited material. The strain or stress-inducing elements 41 may be placed on and attached to the top of surface 42 or be recessed slightly into top surface 42. Only one element 41 may be used for inducing the asymmetric stress in VCSEL 40. The induced stress may be an expansive stress 43 or a compressive stress 44 applied to the VCSEL, as long as the stresses within the VCSEL are different in two orthogonal directions in a plane parallel to surface 42. That is, the stresses, whether positive or negative, are different from stress 45 which may be natural and have a direction orthogonal to the direction of stress 43 or 44. This stress differential may induce a stable polarization of an output 31 from VCSEL 40. In the configuration of VCSEL 50, strain or stress inducing elements (stressors)

51 may be applied to or deposited on opposing sides of the VCSEL, as illustrated in Figures 4a, 4b and 4c. Elements 51 may be patterned dielectrics, oxidized materials, metals or other items. Stress inducing elements 51 may be attached to the external side surface of VCSEL 50 or be recessed slightly into the side surface of the VCSEL. Stress 53 or 54 may be on the side of the top mirror and extend down to the bottom of a trench, mesa or some place of structural interference. . Stress 53 or 54 may occur upon the attachment or deposit of elements 51. Instead, only one element 51 may be attached or deposited. Resultant stresses 53 or 54 and 55 caused by elements 51 in the VCSEL 50 structure may be different in the two orthogonal directions in a plane parallel to surface 52. Such differences of the orthogonal stresses may induce a stable polarization in an output 31 of VCSEL 50. In the configuration of VCSEL 60, strain or stress inducing elements (stressors) 61 may be situated in the VCSEL side and applied to various layers of a DBR mirror of the VCSEL 60, as illustrated in Figures 5a, 5b and 5c. These elements 61 may be oxidized portions 67 of layers in the upper DBR mirror stack of VCSEL 60. Portions 67 of layers in the lower DBR mirror stack may also be oxidized to effect a stress on VCSEL 60. Oxidation of layers may be discussed in U.S. Patent No. 5.903,588, issued May 11, 1999, to Guenter et al., and entitled "Laser with a Selectively Changed Current Confining Layer", which is hereby incorporated in this description by reference. For an illustrative example, low index layers of an AlGaAs DBR or other layers may be selectively oxidized in the opposite sides of the VCSEL 60. However, there may be just one or more selectively oxidized portions 67 of the layers in just one side of VCSEL 60. Instead, this portion or these portions 67 may be etched out and filled in with other materials such as a dielectric or other material resulting in elements 61 to cause a stress 63 or 64 in VCSEL 60. The resultant stress 63 or 64 and stress 65 in the VCSEL 60 structure may be different in two orthogonal directions in a plane parallel to the surface 62, as shown in Figure 5b. This stress differential relative to the two orthogonal directions may be sufficiently significant to induce a stable polarization in an output 31 of VCSEL 60. In the configuration of VCSEL 70, strain or stress inducing elements (stressors)

71 may be situated in the VCSEL side and applied to various layers of a DBR mirror of the VCSEL, as illustrated in Figures 6a, 6b and 6c. These elements 71 may be oxidized portions of layers in the upper and/or lower DBR mirror stacks of VCSEL 70. For an illustrative example, low index layers of an AlGaAs DBR or other layers may have portions 77 selectively oxidized or etched out of two sides diametrically opposed to each other in VCSEL 70. Instead, selectively etched portions 77 of the layers in the sides of the VCSEL 70 may be filled in with another material such as a dielectric or metal resulting in elements 71 to induce a stress 73 or 74 in VCSEL 70. However, portions 77 layers on just one side may be oxidized or etched out and filled in with a stress-causing material. Also, portions on the other sides of VCSEL 70 creating voids 76 may be removed from the original profile 78 of the VCSEL to result in more stress-inducing elements 71 on the other sides of VCSEL 70. The induced stress of elements 71 may cause the resultant stress 73 or 74 and stress 75 in the VCSEL 70 structure to be different in two orthogonal directions in a plane parallel to the surface 72, as shown in Figure 6b. This stress differential relative to the two orthogonal directions may be sufficiently significant to induce a stable polarization in an output 31 of VCSEL 70. In the configuration of VCSEL 80, strain or stress inducing elements (stressors) 81 may involve just the removal of portions of layers from the DBR mirror stack resulting in stress-causing voids 86 situated at the VCSEL sides, as illustrated in Figures 7a, 7b and 7c. The resultant VCSEL 80 may have an asymmetrical shape such as an oblong one in the case of an originally round VCSEL as shown by profile 88. These elements 81 may be on one side or on both sides diametrically across from each other in the VCSEL. The removal or etching away portions of layers may apply to the upper DBR mirror stack of VCSEL 80. Some removal of material from the VCSEL may also occur in the lower mirror as voids 86 to form elements 81, at least where it is possible before reaching a structure interruption such as the bottom of a trench or a mesa. There may be just one void 86 on one side for a stress-inducing element 81. Element or elements 81 may induce a stress 83 or 84 in VCSEL 80. The resultant stress 83 or 84 and stress 85 in the VCSEL 80 structure may be different in two orthogonal directions in a plane parallel to the surface 82, as shown in Figure 7b. This stress differential relative to the two orthogonal directions may be sufficiently significant to induce a stable polarization in an output 31 of VCSEL 80. One or more strain or stress inducing elements may be used to effect an asymmetry of stress distribution in the laser system or VCSEL. A stress inducing element may be one of those disclosed in this description or may of another kind that may induce a stress in the laser system or VCSEL. Various combinations of different kinds of stress-inducing elements may be used for causing an asymmetry in the stress distribution in a laser or VCSEL, such as in one of its mirror stacks. Such asymmetry of a distribution of stresses may cause an effective polarization of light emitted by the laser system or device. Although the invention has been described with respect to at least one illustrative embodiment, many variations and modifications will become apparent to those skilled in the art upon reading the present specification. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.

Claims

CLAIMS 1. A polarization stable laser system comprising: a substrate; a first mirror stack situated on the substrate; an active region situated on a first mirror stack; a second mirror stack situated on the active region; and a stress-inducing element proximate to the second mirror stack. 2. The system of claim 1, wherein the laser system has an asymmetry of stress distribution in its structure at least partially caused by the stress-inducing element. 3. The system of claim 2, wherein the asymmetry of stress distribution effects at least some polarization of an output of the laser system. 4. The system of claim 3, wherein the stress-inducing element is attached on a top surface of the second mirror stack. 5. The system of claim 4, wherein the stress-inducing element is deposited on the top surface of the second mirror stack. 6. The system of claim 5, further comprising a second stress-inducing element deposited on the top surface of the second mirror stack. 7. The system of claim 6, wherein the laser system is a vertical cavity surface emitting laser (VCSEL). 8. The system of claim 3, wherein the stress-inducing element is attached to a side of the second mirror stack. 9. The system of claim 8, wherein the laser system is a vertical cavity surface emitting laser. 10. The system of claim 9, further comprising a second stress-inducing element proximate to a side of the second mirror stack. 11. A polarization stable laser system comprising: a mirror stack; an active region situated in the mirror stack; and a stress-inducing element proximate to the mirror stack. 12. The system of claim 13, wherein the stress-inducing element causes at least partially an asymmetry of stress distribution in the laser system. 13. The system of claim 12, wherein the asymmetry of the stress distribution effects a polarization of an output of the laser system

14. The system of claim 13, wherein the stress-inducing element is situated proximate to a perimeter of at least one layer of the mirror stack. 15. The system of claim 14, wherein the laser system is a vertical cavity surface emitting laser. 16. A laser system comprising: means for reflecting; means for generating laser light situated in the means for reflecting; means for effecting an asymmetric stress distribution in the means for reflecting so as to at least partially polarize a laser light that may be generated by the means for generating laser light. 17. The system of claim 16, wherein the means for effecting an asymmetric stress distribution is a stress-inducing element attached to the means for reflecting. 18. The system of claim 16, wherein the means for effecting an asymmetric stress distribution is a removal of material from at least one side of the means for reflecting. 19. The system of claim 17, wherein the stress-inducing element is attached to a top of the means for reflecting. 20. The system of claim 17, wherein the stress-inducing element is attached to a side of the means for reflecting.