# Patent

## Title
Waveguide with reflective grating for localized energy intensity

## Abstract

An apparatus includes a waveguide with first and second sections, and a junction coupling the first and second waveguide sections together. The first waveguide section has a first reflective device and the second section comprising a second reflective device arranged to generate a standing wave in the waveguide with maximum energy wave intensity at a target region of the waveguide in response to an incident energy wave being provided into at least one of the waveguide sections.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This application is a divisional of U.S. patent application Ser. No. 14/321,786, filed on Jul. 1, 2014, which claims the benefit of U.S. Provisional Application Ser. No. 61/951,618 filed on Mar. 12, 2014, which is expressly incorporated by reference herein in its entirety.


BACKGROUND
In magnetic storage devices such as hard disk drives (HDD), read and write heads are used to magnetically write and read information to and from storage media, such as a magnetic storage disk. An HDD may include a rotary actuator, a suspension mounted on an arm of the rotary actuator, and a slider bonded to the suspension to form a head gimbal assembly. In a conventional HDD, the slider carries a write head and read head, and radially flies over the surface of the storage media. The magnetic media disk rotates on an axis, forming a hydrodynamic air bearing between an air bearing surface (ABS) of the slider and the surface of the magnetic media disk. The thickness of the air bearing at the location of the transducer is commonly referred to as “flying height.”
The read and write heads are mounted on a trailing edge surface of the slider, which is perpendicular to the air bearing surface (ABS). The magnetic media surface is exposed to the ABS during read and write operations. A Heat Assisted Magnetic Recording (HAMR) device or an Energy Assisted Magnetic Recording (EAMR) device is an enhanced HDD that applies heat to magnetically soften the media surface during recording, particularly useful for high capacity storage with physically smaller bit sizes. The heat may be generated by optical energy from a laser diode coupled to a waveguide, and focused by a near field transducer (NFT) formed on the slider. The NFT is arranged on or near the ABS to transit the focused optical energy to the magnetic media disk surface to produce the heating.



BRIEF DESCRIPTION OF THE DRAWINGS
Various aspects of the present invention will now be presented in the detailed description by way of example, and not by way of limitation, with reference to the accompanying drawings, wherein:
FIG. 1 shows a diagram of an exemplary hard disk drive assembly;
FIG. 2 shows an exemplary waveguide with a reflective mirror to recycle the incident energy wave;
FIG. 3 shows an exemplary waveguide having a reflective grating;
FIG. 4 shows a detail of an exemplary reflective grating;
FIGS. 5A-5C show various exemplary configurations of waveguides having reflective grating and/or mirror with single incident wave energy; and
FIGS. 5D-5F show various exemplary configurations of waveguides having reflective gratings with dual incident wave energy.



DETAILED DESCRIPTION
The detailed description set forth below in connection with the appended drawings is intended as a description of various exemplary embodiments and is not intended to represent the only embodiments that may be practiced. The detailed description includes specific details for the purpose of providing a thorough understanding of the embodiments. However, it will be apparent to those skilled in the art that the embodiments may be practiced without these specific details. In some instances, well-known structures and components are shown in block diagram form in order to avoid obscuring the concepts of the embodiments. Acronyms and other descriptive terminology may be used merely for convenience and clarity and are not intended to limit the scope of the embodiments.
The various exemplary embodiments illustrated in the drawings may not be drawn to scale. Rather, the dimensions of the various features may be expanded or reduced for clarity. In addition, some of the drawings may be simplified for clarity. Thus, the drawings may not depict all of the components of a given apparatus or method.
The word “exemplary” is used herein to mean serving as an example, instance, or illustration. Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. Likewise, the term “embodiment” of an apparatus or method does not require that all embodiments include the described components, structure, features, functionality, processes, advantages, benefits, or modes of operation.
Any reference to an element herein using a designation such as “first,” “second,” and so forth does not generally limit the quantity or order of those elements. Rather, these designations are used herein as a convenient method of distinguishing between two or more elements or instances of an element. Thus, a reference to first and second elements does not mean that only two elements can be employed, or that the first element must precede the second element.
As used herein, the terms “comprises”, “comprising,”, “includes” and/or “including”, when used herein, specify the presence of the stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.
In the following detailed description, various aspects of the present invention will be presented in the context an optical or dielectric waveguide used to assist magnetic recording on a hard disk drive (HDD). However, those skilled in the art will realize that these aspects may be extended to any suitable application where waveguides are implemented. For example, resonant electromagnetic wave energy in a waveguide antenna may be optimized according to the methods described herein. While the energy source presented in the following detailed description relates to light from a laser, those skilled in the art will also realize that the described aspects may be extended to other forms of energy or electromagnetic waves propagated in a dielectric waveguide. Accordingly, any reference to a waveguide as part of an HDD is intended only to illustrate the various aspects of the present invention, with the understanding that such aspects may have a wide range of applications.
Aspects of a waveguide include first and second sections, and a junction coupling the first and second waveguide sections together. The first waveguide section has a first reflective device and the second section has a second reflective device arranged to generate a standing wave in the waveguide with maximum energy wave intensity at a target region of the waveguide in response to an incident energy wave being provided into at least one of the waveguide sections.
Aspects of a heat assisted magnetic recording (HAMR) apparatus include a waveguide having a cladding and a core, a near field transducer, and an energy source arranged to propagate light through the waveguide to the near field transducer. The core has a plurality of protrusions extending into the cladding.
Aspects of a magnetic hard disk drive include a rotatable magnetic recording disk, a slider having a heat assisted magnetic recording (HAMR) device with a near field transducer (NFT), and a waveguide having first and second sections. A waveguide junction couples the first and second waveguide sections together. The first waveguide section has a first reflective device and the second section has a second reflective device arranged to generate a standing wave in the waveguide with maximum energy wave intensity at a target region of the waveguide in response to an incident energy wave being provided into at least one of the waveguide sections. The NFT is arranged adjacent to the waveguide at the target region. The NFT is configured to couple the energy wave to the surface of the recording disk for heat assisted magnetic recording.
Aspects of a magnetic hard disk drive include a rotatable magnetic recording disk, a slider having a heat assisted magnetic recording (HAMR) device with a near field transducer (NFT), a waveguide including a cladding and a core, the core having a plurality of protrusions extending into the cladding, and an energy source arranged to propagate light through the waveguide to the near field transducer. The NFT is arranged adjacent to the waveguide and configured to couple the light to the surface of the recording disk for heat assisted magnetic recording.
FIG. 1 shows a hard disk drive 111 including a disk drive base 114, at least one disk 113 (such as a magnetic disk, magneto-optical disk, or optical disk), a spindle motor 116 attached to the base 114 for rotating the disk 113. The spindle motor 116 typically includes a rotating hub on which disks are mounted and clamped, a magnet attached to the hub, and a stator. Rotation of the spindle motor hub results in rotation of the mounted disks 113. At least one actuator arm 115 supports at least one head gimbal assembly (HGA) 112 that includes the slider with writing and reading heads. For an EAMR/HAMR enhanced drive, a waveguide and near field transducer are included on the slider as well. During a recording operation of the disk drive 111, the actuator arm 115 rotates at the pivot 117 to position the HGA 112 to a desired information track on the disk 113.
FIG. 2 shows a waveguide 101 configured to receive an energy wave generated by an energy source 103, and may naturally guide the energy wave (e.g., light) to a Near Field Transducer (NFT) 102 arranged adjacent to the waveguide 101. The waveguide 101 and NFT 102 may be disposed on a slider of the HGA 112 shown in FIG. 1. As an example, for an optical energy wave, the energy source 103 may be a laser diode. The NFT 102 may be a passive type NFT which receives energy from the energy source 103 and couples and transfers the energy to the surface of the storage disk 113. For example, the NFT 102 may focus the energy wave from the waveguide 101 to generate a heating spot on the magnetic storage disk 113 to magnetically soften the layers of the disk 113 at a nano-sized spot to assist changing the bit state when writing to the storage disk 113. As a passive NFT, the NFT 102 may operate by a plasmon effect as optical energy interacts with a small plasmonic metal feature, such as the plasmonic disk 107. Alternatively, the small plasmonic metal feature may be configured as pin or a ridge, for example. The plasmonic metal may be one of gold (Au), silver (Ag), copper (Cu), or aluminum (Al) for example. The NFT 102 may be an antenna type NFT as shown in FIG. 2. Alternatively, the NFT 102 may be an aperture type, which utilizes a nano-sized aperture to confine an input optical energy field.
The waveguide 101 may have a slanted taper with a width approximately equal to or greater than the width of the NFT to guide the energy wave to a small area for interaction with the NFT 102. The waveguide 101 may include a core (e.g., a dielectric material core) and a cladding (e.g., a silicon dioxide material). As shown in FIG. 2, the waveguide 101 may be configured with two substantially linear sections, a first section which receives the incident energy, and a second section which includes the reflective device 103 for reflecting the incident energy. Also, the first and second sections of the waveguide 101 may be coupled as shown to form an angular junction at an angle between 0 and 180 degrees.
To recycle the energy wave which passes through the waveguide 101, the reflective device 103 may be arranged at the end of the waveguide 101, as shown in FIG. 2, so the reflected energy wave can be coupled to the NFT 102 in addition to the incident energy wave. A forward propagating wave can interact with a backward propagating wave to form a standing wave along the waveguide 101. One of several constructive interference peaks that are formed at a target region of the waveguide, such as at the junction of the first and second sections for example. The waveguide 101 may be configured with a length for the first section and the second section such that a maximum interference peak for the energy wave occurs exactly at the target region. The NFT 102 may be positioned adjacent to the target region of the waveguide 101, thus providing an optimized efficiency of energy interaction with the waveguide 101.
FIG. 3 shows a diagram of an exemplary embodiment of a waveguide 201 that is a variation of the waveguide 101 shown in FIG. 2. In addition to a reflective device 203 at the end of the waveguide 201, another reflective device 204 is arranged on the incident section of the waveguide 201. The reflective device 204 may include a grating to render it functional as a partial reflection mirror, and to further improve the recycling rate of an energy wave within the waveguide 201. The grating of the reflective device 204 may be a Partially Reflective Grating (PRG). The reflective device 203 may include a gold mirror. The waveguide 201 may be configured with a combination of grating dimensions for the reflective device 204 and length the waveguide sections to produce a standing wave pattern with antinodes 205. Reflectance of the reflective device 204, controllable by the grating composition and dimensions, may react with the incident energy wave generated by energy source 201. If the reflectance of the reflective device 204 is high enough, a trapped wave may form the standing wave inside the waveguide as shown by the anti-nodes 205. A maximum energy amplitude of the antinodes 205 may be produced at a target region of the waveguide 201, by the configuration of the reflective device 204 and the dimensions of the waveguide 201. For example, the target region of the waveguide may be the junction of the first and second waveguide sections. The NFT 202 may be positioned adjacent to the target region of the waveguide 201 at one of such anti-nodes 205.
FIG. 4 shows a diagram of an exemplary detail of the reflective device 204. Waveguide 301 includes a cladding 305 and a core 304. The reflective device 204 may include a grating 307 on one surface of the waveguide 301 and an optional reflective mirror 304 opposite to the grating 307 to collect scattered light. A plane wave may be assumed inside the waveguide if the lateral size is larger compared to the working wavelength. For a core material assumed to be non-absorptive, the peak intensity of each anti-node can be represented by Equation (1):




  
    
      
        
          I
          m
        
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              (
              
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                -
                
                  R
                  1
                
              
              )
            
            ⁢
            
              
                (
                
                  1
                  +
                  
                    
                      R
                      2
                    
                  
                
                )
              
              2
            
          
          
            
              (
              
                1
                -
                
                  
                    
                      R
                      1
                    
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                      R
                      2
                    
                  
                
              
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            2
          
        
      
    
    
      
        (
        1
        )
      
    
  




where:

R1 is reflectance of the reflective device 204, and
R2 is the reflectance of the reflective device 303.
Assuming a practically achievable reflectance R2 that varies from 0 to 90%, reflectance of the R1 for reflective device 204 may be tunable to maximize the peak intensity. As an example, reflective device 204 may be configured with a reflectance R1=60%, where reflective device 303 has a reflectance R2=90%.
In another embodiment, the reflective device 204 may be configured as a retro-reflecting grating 307 and the mirror 306 configured to collect scattered light. The reflectance R1 of reflective device 204 may be tuned by the grating parameters such as grating pitch, grating depth, grating duty cycle and overall size of the grating area. The reflective device 303 may be configured as a mirror with a thin gold layer and with a reflectance greater than 90%. The grating 307 may have a 250 nm pitch, grating depth of 90 nm, and duty cycle of 50% for an assumed working wavelength of 836 nm, and a core material that is Ta2O5 with refractive index of 2.1. For the cladding material 305 in this example, Al2O3 with refractive index of 1.65 may be used. The core 304 dimensions may be 150 nm thickness and 300 nm width for example. The reflectance and transmission of this grating 307 can be tuned by the overall size of grating area or the total number of grating pairs, with a grating pair defined as a single “grate” unit (i.e., one peak and one valley).
The length of core 304 may be configured to render the resonance of the energy wave inside the waveguide. As an example, a waveguide may generate a resonance having peak intensity at an antinode 205 using a core length of approximately 2300 nm for a core width 300 nm and core thickness 150 nm and grating configured with pitch of 250 nm, grating depth of 90 nm and 40 grating pairs.
The waveguide may be configured with a grating size (i.e., number of grating pairs) based on estimating a loss for each trip of incident and reflective energy wave interaction with the grating, thus determining the optimal number of grating pairs to maximize the intra-cavity intensity.
FIGS. 5A-5F show additional exemplary embodiments of waveguides having variations for the reflective devices 203, 204 as shown in FIG. 3.
The waveguides shown in FIGS. 5A-5C are configured in an angular configuration similar to the waveguide 201 shown in FIG. 3, with a first section of the waveguide that receives the injected incident energy wave and a second section of the waveguide for reflecting the energy wave to form a standing wave.
FIG. 5A shows an exemplary embodiment of a waveguide configured with a grating 501 and a grating 502, both of which may be configured as an etched grating with the core partially etched away and filled with a cladding material. Grating 501 may be a partially reflective grating and grating 502 may be a highly reflective grating.
FIG. 5B shows an exemplary embodiment of a waveguide configured with a mirror 503 in the first waveguide section and a grating 504 in the second waveguide section The mirror 503 may be formed by a thin Au film with a reflectance that may be tuned by altering the thickness of the Au film. The mirror 503 may function as a partially reflective mirror. The grating 504 may be configured as a highly reflective grating.
FIG. 5C shows an exemplary embodiment of a waveguide configured with two mirrors, the mirror 507 being partially reflective, and the mirror 508 being highly reflective.
The exemplary waveguide embodiments shown in FIGS. 5D-5F are suitable for dual incident energy wave injection, as shown by incident energy wave input into each of the two waveguide sections. With such a configuration, a mirror with Au film, such as mirror 503 as shown in FIG. 5B and mirrors 504, 505 as shown in FIG. 5C, is eliminated.
FIG. 5D shows an exemplary embodiment of a waveguide configured with etched gratings 507 and 508, which may be formed by etching the waveguide core material and lining etched surfaces with a reflective film.
FIGS. 5E and 5F show exemplary embodiments of waveguides that are formed with Width Varying Grating (WVG). As such, the grating 509 and the grating 510 shown in FIG. 5E, and the gratings 511, 512 shown in FIG. 5F maintain the full waveguide interior core dimensions to reduce energy loss for the incident wave and the reflected wave in each section of the waveguide. Along each of gratings 509-512, the cross-sectional area of the waveguide interior is not diminished compared to the cross-sectional area of the waveguide interior elsewhere. The gratings 509-512 for the configurations shown in FIGS. 5E and 5F are constructed by extending the waveguide exterior to form the grating pairs, contrasted with the etched gratings 507 and 508 described above with respect to FIG. 5D. As shown in FIG. 5E, the gratings 509, 510 may be configured as square-shaped extensions of the waveguide width. As shown in FIG. 5F, the gratings 511, 512 may be configured as curve-shaped extensions of the waveguide width. The external gratings 509-512 may be formed by extensions of the core material as protrusions into the cladding material of the waveguide.
The various aspects of this disclosure are provided to enable one of ordinary skill in the art to practice the present invention. Various modifications to exemplary embodiments presented throughout this disclosure will be readily apparent to those skilled in the art, and the concepts disclosed herein may be extended to other devices. Thus, the claims are not intended to be limited to the various aspects of this disclosure, but are to be accorded the full scope consistent with the language of the claims. All structural and functional equivalents to the various components of the exemplary embodiments described throughout this disclosure that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the claims. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims. No claim element is to be construed under the provisions of 35 U.S.C. §112(f) unless the element is expressly recited using the phrase “means for” or, in the case of a method claim, the element is recited using the phrase “step for.”

## Claims

1. A heat assisted magnetic recording (HAMR) apparatus, comprising:
a waveguide comprising a cladding and a core, the core having a plurality of protrusions extending into the cladding;
a near field transducer;
an energy source arranged to propagate light through the waveguide to the near field transducer, and
wherein the waveguide comprises first and second sections coupled together at a junction and wherein the waveguide is configured to optimize a normalized peak intensity of the light at a target region of the waveguide.

2. The apparatus of claim 1, wherein the plurality of protrusions are square-shaped.

3. The apparatus of claim 1, wherein the plurality of protrusions are curve-shaped.

4. The apparatus of claim 1, wherein the plurality of protrusions are partially reflective.

5. The apparatus of claim 1, wherein at least one of the first and second sections comprises the plurality of protrusions, wherein the waveguide is configured to have a number of protrusions that optimizes a normalized peak intensity of the light at a target region of the waveguide.

6. The apparatus of claim 5, wherein each of the first and second sections of the waveguide have a respective portion of the plurality of protrusions, wherein each of the first and second sections of the waveguide receive an incident light wave from the energy source and a reflected light wave reflected by the plurality of protrusions of the opposite waveguide section to produce a standing wave,
wherein the standing wave includes at least one antinode formed at the target region.

7. The apparatus of claim 6, wherein the near field transducer is arranged adjacent to the target region of the waveguide.

8. A magnetic hard disk drive, comprising:
a rotatable magnetic recording disk;
a slider comprising a heat assisted magnetic recording (HAMR) device having a

near field transducer (NFT);
a waveguide comprising a cladding and a core, the core having a plurality of protrusions extending into the cladding; and
an energy source arranged to propagate light through the waveguide to the near field transducer;
wherein the NFT is arranged adjacent to the waveguide, the NFT configured to couple the light to the surface of the recording disk for heat assisted magnetic recording and wherein the waveguide comprises first and second sections coupled together at a junction and wherein the waveguide is configured to optimize a normalized peak intensity of the light at a target region of the waveguide.