Abstract:
Methods for manufacturing a polarization pinned vertical cavity surface emitting laser (VCSEL). Steps include growing a lower mirror on a substrate; growing an active region on the lower mirror; growing an upper mirror on the active region; depositing a grating layer on the upper mirror; and etching a grating into the grating layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 11/299,638, “Surface Gratings on VCSELS For Polarization Pinning,” filed Dec. 12, 2005, which is the U.S. Pat. No. 8,000,374, which claims priority to U.S. Provisional Applications 60/673,219 titled “Integrated VCSEL and Photodiode with Asymmetries for Polarization Control” filed Apr. 20, 2005 and 60/711,311 titled “Amorphous Silicon Gratings for Polarization Control” filed Aug. 25, 2005, which are incorporated herein by reference in their entireties. 
    
    
     BACKGROUND OF THE INVENTION 
     1. The Field of the Invention 
     The invention generally relates to polarization control in VCSELs. More specifically, the invention relates to using gratings for polarization control. 
     2. Description of the Related Art 
     Lasers have become useful devices with applications ranging from simple laser pointers that output a laser beam for directing attention, to high-speed modulated lasers useful for transmitting high-speed digital data over long distances. Several different types of lasers exist and find usefulness in applications at the present time. One type of laser is the edge emitter laser which is formed by cleaving a diode from a semiconductor wafer. Cleaving a diode from a semiconductor wafer forms mirrors that form a laser cavity defined by the edges of the laser diode. Edge emitter lasers may be designed to emit a laser beam more strongly from one of the edges than the other edges. However, some laser energy will be emitted at the other edges. Edge emitter lasers are commonly used when high optical power is needed. 
     A second type of commonly used laser is known as a vertical cavity surface emitting laser (VCSEL). A VCSEL is formed in part by forming a first mirror from Distributed Bragg Reflector (DBR) semiconductor layers. The DBR layers alternate high and low refractive indices so as to create the mirror effect. An active layer is then formed on the first mirror. A second mirror is formed on the active layer using more DBR semiconductor layers. Thus the VCSEL laser cavity is defined by upper and lower minors which causes a laser beam to be emitted from the surface of the laser. 
     One challenge that exists with the VCSELs mentioned above relates to polarization of optical beams. For example, in communication circuits, if polarized light is emitted from a laser device, the light can be routed using various types of beam splitters and polarization selective filters. However, often polarization in a VCSEL will change from batch to batch and depending on the operating conditions under which the VCSEL is operating. For example, a VCSEL may have one polarization at a given bias current and another polarization at a different bias current. In sensor applications, it is often important to emit a constant polarization because part of the sensing operation relates to detecting differences in polarization. Thus, it would be advantageous to effectively pin polarization in VCSELs. 
     BRIEF SUMMARY OF THE INVENTION 
     One embodiment described herein includes a method for manufacturing a polarization pinned vertical cavity surface emitting laser (VCSEL). 
     The method includes growing a lower mirror on a substrate. An active region is grown on the lower minor. An upper minor is grown on the active region. A grating layer is deposited on the upper mirror. A grating is etched into the grating layer. 
     Advantageously, the embodiments described above allow for polarization pinned VCSELs to be manufactured. This allows the VCSELs to be used in communication and sensor circuits where un-polarized VCSELs were previously not available for use. 
     These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       In order that the manner in which the above-recited and other advantages and features of the invention are 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 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. 1  illustrates a cutaway view of a VCSEL and grating; 
         FIG. 2  illustrates a detailed view of a VCSEL grating; 
         FIG. 3  illustrates another detailed view of a VCSEL grating; and 
         FIG. 4  illustrates yet another detailed view of a VCSEL grating. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Light from optical components can be described as polarized or un-polarized. Un-polarized light can more accurately be characterized as light of a random polarization. Light may be emitted randomly from a device, such as a VCSEL, in any one of an infinite number of polarizations. The polarization can be characterized by X and Y vectors or by a magnitude and angle describing the direction of the electric field, generally perpendicular to the direction of propagation, i.e. Z. In some cases, the light may be circularly polarized which may be characterized by changing X and Y values as the light propagates. If light can be facilitated in one direction, while it is inhibited in an orthogonal direction, polarized light can be produced where all, or at least for the most part, light is emitted in a single known polarization direction. On Zinc Blende substrates such as GaAs on or about 100 orientation, there are two polarization directions, namely along the (011) and (01-1) directions. These are normally along and perpendicular respectively to the major flat. Using asymmetries, light emissions can be facilitated in one direction while they are inhibited in the orthogonal direction. Exploiting asymmetries and gratings may be used to pin polarization in a VCSEL. 
     Referring now to  FIG. 1 , a cutaway view of a VCSEL structure is illustrated. The VCSEL  100  is formed from an epitaxial structure. The epitaxial structure may include, for example, various layers formed through a process such as molecular beam epitaxy (MBE) or other appropriate method. The example shown in  FIG. 1  illustrates a GaAs substrate  102 . The VCSEL  100  includes a lasing cavity formed by a lower mirror  104  and an upper mirror  106 . A lower mirror  104  is grown on the substrate  102 . The lower minor  104 , in this example includes alternating layers of higher and lower index of refraction materials. Each interface between higher and lower index of refraction materials causes reflection. Thus by using an appropriate number of alternating layers, a given reflectivity can be achieved. 
     An upper minor  106  is grown on an active region  108 . The upper mirror  106  is similar to the lower mirror  104  in that it generally comprises a number of layers that alternate between a high index of refraction and a lower index of refraction. Generally, the upper mirror  106  has fewer minor periods of alternating high index and low index of refraction layers, to enhance light emission from the top of the VCSEL  100 . 
     Between the mirrors  104 ,  106  is an active region  108  that includes quantum wells. The active region  108  forms a PN junction sandwiched between the lower mirror  104  and an upper mirror  106 , which are doped such that they are of opposite conductivity types (i.e. a p-type mirror and an n-type mirror). In alternative embodiments, the minors may be un-doped or the mirrors may be dielectric minors. Free carriers in the form of holes and electrons are injected into the quantum wells when the PN junction is forward biased by an electrical current. At a sufficiently high bias current the injected minority carriers form a population inversion (i.e. a higher concentration of free carriers in the conduction band than electrons in the valance band) in the quantum wells that produces optical gain. Optical gain occurs when photons in the active region cause electrons to move from the conduction band to the valance band which produces additional photons. When the optical gain is equal to the loss in the two mirrors, laser oscillation occurs. The free carrier electrons in the conduction band quantum well are stimulated by photons to recombine with free carrier holes in the valence band quantum well. This process results in the stimulated emission of photons, i.e. coherent light. 
     The active region  108  may also include or be formed near an oxide aperture  110  formed using one or more oxide layers  112 . The oxide aperture  110  serves both to form an optical cavity and to direct the bias current through the central region of the cavity that is formed. To form the oxide aperture  110 , a trench  114  is etched down to the oxide layer  112  to expose the oxide layer  112 . An oxide  116  is then grown by subjecting the exposed oxide layer  112  to various chemicals and conditions so as to cause portions of the oxide layer  112  to oxidize. The oxidized portion  116  becomes optically lower in index of refraction and electrically insulating so as to form the boundaries of the oxide aperture  110 . 
     As illustrated in  FIG. 1 , a grating  122  may be included in a grating layer  124  on the VCSEL  100  to control polarization. The grating  122  may be formed over the oxide aperture  110  so as to provide the polarizing effect to light emitted from the oxide aperture  110 . In the example shown, a low index of refraction layer  126  is formed through a deposition process on a high index of refraction layer of the top minor  106 . A high index of refraction layer  128  is formed through a deposition process on the low index of refraction layer  126 . The grating  122  is then formed into the high index of refraction layer  128  or alternatively into both the high index of refraction layer  128  and a portion of the low index of refraction layer  126 . In one embodiment where the grating  122  is formed only into the high index of refraction layer  128 , the low index of refraction layer  126  may be used as an etch stop layer for etching the grating  122  into the high index of refraction layer  128 . 
     In one embodiment, the high index of refraction layer  128  and low index of refraction layer  126  are thin layers as thin layers seem to exhibit better polarization and optical transmission characteristics. The high index of refraction layer  128  may be for example Si or Si 3 N 4 . The low index of refraction layer  126  may be for example SiO 2 . The index of refraction for the low index of refraction layer  126  may be for example between 1.2 and 2.5. The index of refraction for the high index of refraction layer  128  may be for example between 1.8 and 5. In any case, the high index of refraction layer  128  has, in one embodiment, an index of refraction that is 0.3 or greater than the index of refraction for the low index of refraction layer  126 . 
     The gratings  122  are designed to maximize the reflection of the desired polarization and to minimize the reflectivity of the perpendicular polarization so the VCSEL  100  will lase in the polarization of highest reflectivity and be inhibited from lasing in the perpendicular direction. VCSELs prefer to lase with polarizations in the (110) planes, so it may be advantageous to select one of the desired directions, although this is not required. 
       FIG. 2  illustrates a more detailed view of the gratings  122  used for polarization control.  FIG. 2  illustrates a grating  122  that includes a number of protrusions  130 . The protrusions  130  may be etched into the high index of refraction layer  128  which may be, for example, comprised of Si or Si 3 N 4 . 
     To form the grating  122 , the low index of refraction layer  126  and high index of refraction layer  128  are deposited, as opposed to grown as is typically done with the minor layers in the top mirror  106  and bottom mirror  104 , on a high index of refraction layer of the top mirror  106  of the VCSEL  100  ( FIG. 1 ). This can be done, for example, (PECVD (plasma enhanced vapor deposition), by sputtering or evaporation at a reasonably low substrate temperature. The low index of refraction layer  126  and high index of refraction layer  128  may be thin layers. The following is described with thicknesses described in terms of optical thicknesses such that one wave thickness is the thickness of a full wave in that material. For example silicon nitride has an index of refraction of about 2 at 850 nm. A wave of silicon nitride is therefore 850 nm/2.0 or 425 nm. In some embodiments, the low index of refraction layer  126  may have a wave optical thickness of between 0.04 and 0.18. An optical thickness of about 0.1 wavelengths for the low index of refraction layer  126  functions well in one embodiment. The high index of refraction layer  128  may be, in some embodiments, between 0.15 and 0.5 wavelengths thick. A thickness of about 0.25 wavelengths seems to function well in one exemplary embodiment. The combined thickness of the low index of refraction layer  126  and high index of refraction layer may be, in one embodiment, 0.25 waves thick. This may be done to create a low reflectivity region outside the grating where it is not etched to provide mode control. Examples of this are illustrated in U.S. Pat. No. 5,940,422, titled “Laser With Improved Mode Control” which is incorporated herein by reference in its entirety. 
     The high index of refraction layer  128  is then etched using in one embodiment direct write electron beam lithography. Direct write electron beam lithography involves depositing an electron beam resist layer on a surface to be etched. An electron beam is then used to expose the resist. The electron beam can be controlled in much the same way as electrons in a conventional cathode ray tube television are controlled. The electron beam is used to pattern a region that will be etched leaving the protrusions  130 . A reactive ion etch (RIE) or other appropriate etch is then used to etch portions of the high index of refraction layer  128  to produce the grating  122 . In one embodiment, a stop etch may be used that selectively etches the high index of refraction layer, but stops on the low index of refraction layer  126 . In this embodiment, the high index layer  128  may be Si 3 N 4 , and low index layer  126  may be SiO 2 . 
     The protrusions  130  are formed periodically. For example, the protrusions may be formed according to a period  132 . The period may be for example in the rage of about 1 to 2 wavelengths of the laser as measured in air. This period allows for low cost photolithography while maintaining adequate discrimination between the two competing orthogonal polarizations. 
     The protrusions  130  may also be etched according to a particular duty cycle. The duty cycle is the width  134  of the protrusions with respect to a period  132 . The duty cycle should be less than 50% although above 50% still functions. In particular, the duty cycle may be anywhere from 10% to 60%. One embodiment functions particularly well with a 30% duty cycle. 
     Various alternatives embodiments will now be described in conjunction with  FIG. 2 . It may be preferable to use thin layers for the high index of refraction layer  128  and low index of refraction layer  126 . For example, in one embodiment illustrating an 850 nm VCSEL, the low index of refraction layer  126  may comprise 70 nm of SiO 2 . The high index of refraction layer  128  may comprise a 100 nm layer of Si 3 N 4 . The period  132  of the protrusions  130  may be for example between 1 and 2 microns. It may be advantageous to maximize the period for ease of fabrication. While the period  132  is illustrated at 1 to 2 microns, other values may also be used. However, at about 750 nm, due to resonance, the loss factor, a measure of loss of one polarization with respect to the orthogonal polarization, exhibited suggests that periods about this value should be avoided. Even in the 1 to 2 micron range, another resonance is exhibited at about 1690 nm. However, the loss factor still remains significant. At a 1690 nm period  132 , there may be some loss of reflectivity of the higher reflected polarization. This can be compensated for by adding more DBR periods in the top minor  106 . 
     In the example shown in  FIG. 2  where the VCSEL is an 850 nm VCSEL, the duty cycle of the protrusions  130  may be about 30%. In other words the width  134  of the protrusion  130  is 30% of the period  132 . Other duty cycles may be used successfully, such as between 10% and 60%. However below 50%, and at about 30% appears to be at or near the optimal duty cycle value. 
     For a 1280 nm VCSEL, the low index of refraction layer  126  may be 65 nm of SiO 2 . The high index of refraction layer  128  may be 220 nm of Si 3 N 4 . The period  132  may be about 1.3 to 2.4 microns. In the 1280 nm case, resonance may occur at a 1280 nm period  132  and as such, periods of about this value should be avoided. The period  132  may be for example between 1600 nm and 2300 nm. While other periods may be used, periods below 850 nm become difficult to print using the etching process. Above 2400 nm, the reflectance difference between the two orthogonal polarizations weakens, making polarization more difficult to achieve. 
     In an alternative embodiment of an 850 nm VCSEL, the low index of refraction layer  126  may be 40 nm of SiO 2 . The high index of refraction layer  128  may be 75 nm of Si. In this example the period  132  is still about 1 to 2 microns and the duty cycle is about 30%. 
     In the example shown in  FIG. 2  the low index of refraction layer  126  may also be used as passivation for the VCSEL  100  ( FIG. 1 ). Typically, III-V semiconductors such as GaAs and AlGaAs can degrade when exposed to open air. By passivating the semiconductor materials with a layer such as SiO 2 , the degradation can be slowed or prevented altogether. Thus, when the low index of refraction layer  126  is also used as an etch stop layer in the grating formation etch process, the low index of refraction layer  126  may be used as passivation for the VCSEL  100  ( FIG. 1 ). 
     Referring now to  FIG. 3 , an alternative embodiment of an 850 nm VCSEL grating  122  is illustrated. In the example shown in  FIG. 3 , the low index of refraction layer  126  is about 70 nm of SiO 2  and the high index of refraction layer  128  is about 100 nm of Si 3 N 4 . In this example etching of the protrusions  130  is performed using a timed etch system where the etchant etches into both the high index of refraction layer  128  and low index of refraction layer  126 . In the embodiment shown in  FIG. 3 , etching is done such that about 50 nm of the low index of refraction layer  126  remains on the VCSEL top minor  106 . This 50 nm layer may function as passivation protection for the VCSEL top minor  106 . As with the examples previously illustrated herein, the period is about 1 to 2 microns. Also, the duty cycle, or the width of the protrusion  134  as compared to the period  132 , is about 30%. 
     Referring now to  FIG. 4 , an embodiment is illustrated where the grating  122  uses silicon, such as amorphous or polycrystalline silicon. In this example, silicon (Si) is used as the high index of refraction layer  128  on a lower index of refraction layer  126 , which may be for example, SiO 2  or of Si 3 N 4 . Si has a much higher index of refraction than Si 3 N 4 . The index of refraction for Si may be 3.2 to 4.3 depending on deposition and wavelength. As such, Si gratings can be thinner than other types of gratings. Further, an increased difference in index of refraction by using silicon increases the reflective difference between the two competing polarizations.  FIG. 4  further illustrates a using a SiO 2  overlay  136 . The overlay  136  may be for example a ¼λ. In the example shown, the Si high index of refraction layer  128  is 50 nm. The SiO 2  low index of refraction layer is 40 nm. The period  132  is 1 to 2 microns. And the duty cycle is 30%. 
     Embodiments described herein can be further optimized to promote single modedness of the VCSEL  100 . When the grating  122  is former essentially circularly over the aperture  110 , and when the grating layer  124  is at a thickness to act like an anti-reflective (AR) coating outside of the circular region of the grating  122 , the fundamental mode is selected. 
     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 that come within the meaning and range of equivalency of the claims are to be embraced within their scope.