WO2011146017A1

WO2011146017A1 - Optical coupler for hamr head

Info

Publication number: WO2011146017A1
Application number: PCT/SG2011/000187
Authority: WO
Inventors: Baoxi Xu; Guillaume Vienne; Chengwu An; Yeow Teck Toh; Cheow Wee Chia; Tow Chong Chong
Original assignee: Agency For Science, Technology And Research
Priority date: 2010-05-19
Filing date: 2011-05-19
Publication date: 2011-11-24

Abstract

An optical coupler, a heat-assisted magnetic recording (HAMR) head, and a method for coupling light from an optical fiber or a planar waveguide to a slider waveguide of a HAMR head. The optical coupler comprises a planar waveguide structure configured for receiving light from the optical fiber or the planar waveguide at a first end thereof, and for coupling light to the slider waveguide at a second end thereof; wherein the planar waveguide structure comprises a bent portion disposed between the first and second ends such that the direction of the light signal is changed between the first and second ends; and wherein cross-sections of the planar waveguide structure at the first and second ends respectively are different for facilitating coupling of the received light at the first end to the slider waveguide at the second end.

Description

OPTICAL COUPLER FOR HAMR HEAD

FIELD OF INVENTION

The present invention relates to an optical coupler, a heat-assisted magnetic recording (HAMR) head, and a method for coupling light from an optical fiber or a planar waveguide to a slider waveguide of a HAMR head.

BACKGROUND

Heat-assisted magnetic recording (HAMR) is a promising approach to overcome the so-called super-paramagnetic limit and may be able to push magnetic recording densities of hard disk drives to a few terabits per square inch (Tb/in²). By combining HAMR with bit patterned media (BPM), an areal density of about 10Tb/in² is expected to be achievable.

It has been recognised that a near-field optical head with a nano-size spot and high optical coupling efficiency is a key component for HAMR. In a HAMR head, the light's energy is typically delivered to a near-field transducer through an optical waveguide on a slider which is part of the head. One purpose of the near-field optical transducer is to concentrate the light energy to heat a recording layer of a magnetic recording media over a nano-size area. The optical waveguide is used to effectively deliver the light's energy to the near-field transducer. Typically, this waveguide has a width of a few micrometers and a thickness of about one micrometer or less. It has been recognised that a highly efficient optical delivery system is very desirable for a HAMR head because the energy delivered to the transducer is expected to locally heat the magnetic recording medium to a few hundred degrees Celsius (°C) (such as about 400 °C for iron-platinum FePt recording media).

Using current technology, low-loss propagation of light within waveguides may be achievable. However, coupling light from a source into a waveguide with high efficiency is typically a challenge. Several methods have been proposed in the prior art. One proposed method uses coupling through a grating structure. It uses parallel free-space light to irradiate the grating, and the grating couples the light into a thin waveguide on the slider, which further delivers the light to a near field optical transducer. However, the grating coupling technique suffers from the critical dependence of the coupling to the angle of incidence. The coupling efficiency is low, with less than 20% predicted by theory. One of the possible implementations of the grating coupling technique is a fiber- based method to deliver light onto the grating. As mentioned above, the small angle tolerance will limit the coupling efficiency and the applicability of this method. Also, in 'such methods, the slider has to be mounted on the suspension arm opposite to the orientation currently used in devices.

Another structure that has been proposed to couple the light into the waveguide on the slider uses a mirror to coupling light into the waveguide on the slider. Another proposed method relies on a bent waveguide to deliver light onto a grating to couple light into the waveguide on the slider. However, the output of the bent waveguide is a divergent beam. This further adversely affects the coupling efficiency to the grating since the coupling is very critically dependent on the incident angle.

Generally, the abovementioned methods are typically demanding in terms of fabrication process or have low coupling efficiency.

Therefore, there is a need for an optical coupler and a heat-assisted magnetic recording head comprising the optical coupler that seeks to address at least one of the above problems.

SUMMARY

In accordance with a first aspect of the present invention there is provided an optical coupler for coupling light from an optical fiber or a- planar waveguide to a slider waveguide of a heat assisted magnetic recording head, the optical coupler comprising a planar waveguide structure configured for receiving light from the optical fiber or the planar waveguide at a first end thereof, and for coupling light to the slider waveguide at a second end thereof; wherein the planar waveguide structure comprises a bent portion disposed between the first and second ends such that the direction of the light signal is changed between the first and second ends; and wherein cross-sections of the planar waveguide structure at the first and second ends respectively are different for facilitating coupling of the received light at the first end to the slider waveguide at the second end.

The planar waveguide structure may comprise a tapered portion disposed between the first end and the bent portion such that at least a lateral dimension is reduced in a direction towards the bent portion. The first end may be configured for butt-coupling or for parallel coupling to the optical fiber or planar waveguide.

The second end may be configured for butt-coupling or for parallel coupling to the slider waveguide.

The planar waveguide may be formed in a flexible printed circuit board.

In accordance with a second aspect of the present invention there is provided a method for coupling light from an optical fiber or a planar waveguide to a slider waveguide of a heat assisted magnetic recording head, the method comprising using a planar waveguide structure configured for receiving light from the optical fiber or the planar waveguide at a first end thereof, and for coupling light to the slider waveguide at a second end thereof, wherein the planar waveguide structure comprises a bent portion disposed between the first and second ends such that a direction of the light signal is changed between the first and second ends; and wherein cross-sections of the planar waveguide structure at the first and second ends respectively are different for facilitating coupling of the received light at the first end to the slider waveguide at the second end.

The method may comprise reducing at least a lateral dimension of the planar waveguide structure in a direction towards the bent portion by way of a tapered portion disposed between the first end and the bent portion.

The method may comprise configuring the first end for butt-coupling or for parallel coupling to the optical fiber or planar waveguide. The method may comprise configuring the second end for butt-coupling or for parallel coupling to the slider waveguide.

In accordance with a third aspect of the present invention there is provided a heat assisted magnetic recording (HAMR) head comprising a slider structure comprising a near-field optical transducer and a slider waveguide capable of delivering light to the optical transducer; an optical coupler disposed for coupling light from an optical fiber or a planar waveguide to the slider waveguide; wherein the optical coupler comprises: a planar waveguide structure configured for receiving light from the optical fiber or the planar waveguide at a first end thereof, and for coupling light to the slider waveguide at a second end thereof; wherein the planar waveguide structure comprises a bent portion disposed between the first and second ends such that a direction of the light signal is changed between the first and second ends; and wherein cross-sections of the planar waveguide structure at the first and second ends respectively are different for facilitating coupling of the received light at the first end to the slider waveguide at the second end.

The planar waveguide structure of the optical coupler may comprise a tapered portion disposed between the first end and the bent portion such that at least a lateral dimension is reduced in a direction towards the bent portion.

The first end may be configured for butt-coupling or for parallel coupling to the optical fiber or planar waveguide. The second end may be configured for butt-coupling or for parallel coupling to the slider waveguide.

The optical fiber may comprise a glass or polymer optical fiber.

The optical fiber or planar waveguide may be configured for mounting to suspension arm coupled to the heat assisted magnetic recording head.

The optical fiber may comprise a micro-fiber. The slider structure may be directly connected to the optical coupler.

The connected slider structure and optical coupler may in turn be connected to a suspension arm for the HAMR head.

The slider structure and the optical coupler may be connected via a suspension arm for the HAMR head.

The planar waveguide structure of the optical coupler may be optically coupled to the slider waveguide of the slider structure via an aperture formed in the suspension arm.

A refractive index matching liquid or polymer material may be disposed in the aperture for facilitating the optical coupling.

The planar waveguide may be formed on a flexible printed circuit board.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be better understood and readily apparent to one of ordinary skill in the art from the following written description, by way of example only, and in conjunction with the drawings, in which:

Figure 1(a) is a schematic perspective view of an optical coupler in assembly with a slider in an example embodiment.

Figure 1(b) is a schematic side view of the optical coupler and the slider when viewed in a direction A in the example embodiment.

Figure 2(a) is a schematic illustration of coupling light from a micro-fiber via a planar optical waveguide to a slider waveguide in an example embodiment. Figure 2(b) is a schematic illustration of a fiber directly coupled to a slider waveguide in a conventional arrangement.

Figure 3 is a schematic illustration of an optical coupler coupled to a slider waveguide in an example embodiment.

^■Figure 4 shows the light distribution along the optical coupler of Figure 3.

Figure 5(a) is a schematic diagram showing production of optical couplers on a wafer in an example embodiment.

Figure 5(b) is a schematic flow chart illustrating production of optical couplers on a wafer in an example embodiment. Figure 6 is a schematic diagram illustrating an assembly of a coupling module to a slider in an example embodiment.

Figure 7 is a schematic diagram illustrating an optical coupler coupling iight to a slider waveguide using evanescent wave coupling in an example embodiment.

Figure 8(a) is a schematic diagram illustrating a head-gimbal-assembly in an example embodiment.

Figure 8(b) is a schematic diagram illustrating a head-gimbal-assembly in another example embodiment.

Figure 9 is a schematic diagram illustrating a head-gimbal-assembly in an example embodiment. Figure 10(a) is a schematic diagram illustrating a micro-fiber in parallel coupling with an optical waveguide in an optical coupler in an example embodiment. Figure 10(b) is a graph showing transmission efficiency based on finite differential time domain simulations of parallel coupling in an example embodiment, for two different polarizations.

Figure 11(a) is a schematic diagram illustrating an optical coupler comprising a polymer pianar waveguide in butt-coupiing configuration with a planar optical waveguide in an example embodiment.

Figure 11 (b) is a schematic diagram illustrating the optical coupler comprising the polymer planar waveguide in parallel-coupling configuration with the planar optical waveguide in another example embodiment.

Figure 12 is a schematic flowchart illustrating a method for coupling light from an optical fiber or planar waveguide to a slider waveguide of a heat assisted magnetic recording head in an example embodiment.

DETAILED DESCRIPTION

In one example embodiment, an optical coupler is configured to provide highly efficient optical coupling from a laser/light source to a near-field optical transducer of a slider. The optical coupler can comprise an optical micro-fiber with a diameter at its output facet smaller than a standard optical fiber and a planar optical waveguide coupled to the optical micro-fiber, the planar optical waveguide having a tapered section and a bent section. In the example embodiment, the diameter at the output facet refers to the diameter of the fiber including the core and cladding of the fiber. As will be appreciated by a person skilled in the art, the term micro-fiber generally relates to a fiber having a diameter from a few tenths of micrometers to a few micrometers. More specifically, in the following description, the micro-fiber is a fiber that comprises two or three sections. The first section is a fiber section having a diameter of a few tens or a few hundreds of micrometers, for example a section with a diameter of about 125 μηη. The second section is a tapered fiber section connecting the first section to the third section. The third section is a fiber section having a diameter in a range of about 0.1 ym to about 10 ym, and having a length up to a few centimeters. The third section could be omitted in different embodiments, i.e. the term micro-fiber used herein includes a tapered fiber. In the example embodiment, the micro-fiber may be collinearly butt coupled to the planar waveguide. The optical coupler may be housed in a separate module and can be assembled with a slider waveguide. The thickness of the planar optical waveguide of the optical coupler may be configured such that it is equal to or smaller than the width of the slider waveguide.

Figure 1 (a) is a schematic perspective view of an optical coupler in assembly with a slider in an example embodiment. Figure 1 (b) is a schematic side view of the optical coupler and the slider when viewed in direction A.

In the example embodiment, the optical coupler 102 is configured to provide highly efficient coupling from a laser source 104 to a near-field optical transducer 106. The near-field optical transducer 106 is disposed in the slider 1 18. The optical coupler 102 comprises an optical micro-fiber 108 with a diameter at its output facet (at numeral 1 10) smaller than a standard optical fiber and a planar optical waveguide 1 2 having a tapered section 1 14 and a bent section 1 16, such that at least a lateral dimension is reduced in a direction towards the bent region 1 16. The micro-fiber 108 is preferably, but not limited to, collinearly coupled to the planar optical waveguide 12. That is, the micro-fiber 108 may be coupled to the planar optical waveguide 1 12 through a butt- coupling method. The optical coupler 102 may be housed in a separate module and assembled separately with the slider 1 18.

As shown in Figure 1 (b), during assembly, the optical coupler 102 is assembled with a slider waveguide 120 where a size in x-direction (or thickness) of the planar optical waveguide 1 12 of the optical coupler 102 is equal to or smaller than a size in x- direction (or width) of the slider waveguide 120.

In the example embodiment, the miniature planar optical waveguide 1 12 is used to change light propagation direction from e.g. a horizontal direction (i.e. fiber 108 axis) to e.g. a vertical direction (for end-coupling into the slider waveguide 20). For example, the miniature planar optical waveguide 112 may be about 0.3μΐη to about 2.0μηι long in the x direction and about 0.2μ(η to about Ι .Ομι in the y direction. For example, for a Si₃N Si0₂ waveguide functioning as the miniature planar optical waveguide 1 12, the dimensions can be about 0.4 μιτι in the x direction and about 0.4 pm in the y direction, and having a wavelength of about 800 nm. A large refractive index contrast achievable in current planar waveguide technology allows light concentration through the tapered section 114 and the bent section 116 with high optical efficiency. For example, with refractive indices of 2.0 and 1.7 for the core and cladding materials respectively, the bent radius can be about 20 micrometers only with very low bend loss.

In providing the change in the light propagation direction in the planar optical waveguide 112, the example embodiment can advantageously configure the fiber axis 108 to be substantially horizontal (x-y plane). As will be described in more detail below with reference to Figures 8 and 9, this can advantageously facilitate disposition of the fiber substantially along a suspension arm of a head-gimbal-assembly, for mounting to the suspension arm, while avoiding sharp fiber bends otherwise required to guide the optical fiber from the suspension arm to the slider waveguide 120 for direct, vertical coupling. Sharp fiber bends, may result in fiber breakage and can add to the overall geometrical footprint of the head-gimbal-assembly, and may incur substantial bend losses.

In the example embodiment, the light from the micro-fiber 108 is not only bent through the planar optical waveguide 112, but is also reshaped so as to suit the geometry of the slider waveguide 20 (having the near-field optical transducer 106 at its output). This reshaping can provide a high coupling efficiency into the slider waveguide 120. In other words, cross-sections of the planar optical waveguide 112 at a first end 122 and at a second end 124 respectively are different for facilitating coupling of the received light from the micro-fiber 108 at the first end 122 to the slider waveguide 120 at the second end 124.

Figure 2(a) is a schematic illustration of coupling light from the micro-fiber 108 via the planar optical waveguide 112 to the slider waveguide 120 in the example embodiment. Figure 2(b) is a schematic illustration of a fiber 204 directly coupled to a slider waveguide 206 in a conventional arrangement.

The capture and reshaping of the light beam through the bent section 116 of the planar optical waveguide 1 2 is schematically illustrated in Figure 2(a). The micro-fiber 108 couples to the planar optical waveguide 112 and the light is reshaped through the bent section 16 and coupled , into the slider waveguide 120. The inventors have recognized that a larger refractive index difference between core material and cladding material of a fiber/waveguide can allow a small bent radius with very small light leakage (or known as bend loss). For example, with refractive indices of 2.0 and 1.7 for the core and cladding materials respectively, the bent radius could be only about 20 micrometers with very low bend loss. Microstructured (also called 'photonics crystal') waveguides can offer a very high refractive index contrast due to the presence of air in the cladding. If the structures immediately surrounding the core are substantially smaller than the wavelength, the cladding has an effective refractive index close to 1.0.

In contrast, Figure 2(b) shows a fiber direct coupling configuration for comparison, illustrating a smaller coupling/overlap region 202 between a fiber 204 being directly coupled to a slider waveguide 206.

In a situation when a hard disk drive is in operation, a slider is typically flying over a magnetic recording medium. The space between the slider and the magnetic recording medium is typically a few nanometers. Therefore, the flying stability of the slider is typically critical to the performance of a hard disk drive system. in the above example embodiment, the small diameter (at numeral 110 of Figure 1) of the micro-fiber 108 (Figure 1) at the planar optical waveguide 112 (Figure 1) input facet can have two significant advantages. The small size of the micro-fiber 108 (Figure 1) advantageously reduces the weight of the micro-fiber 108 as compared to a standard fiber. Furthermore, the flexibility of the micro-fiber 108 is also increased. As a result, the slider's 118 (Figure 1) ability to fly over magnetic recording medium is not affected by the presence of the optical fiber or fiber plus coupler system. This is in contrast to an expected significant disturbance when using a standard fiber on a slider. Figure 3 is a schematic illustration of an optical coupler 300 coupled to a slider waveguide 306 in an example embodiment. Simulations of light distribution of the optical coupler 300 were carried out using the so-called finite differential time domain (FDTD) method. The FDTD method is described in K. Yee, "Numerical solution of initial boundary value problems involving Maxwell's equations in isotropic media," Antenaa and Propagation, IEEE Transactions on 14, 3002-307 (1966) and A. Taflove "Application of the finite-difference time-domain method to sinusoidal steady state electromagnetic penetration problems", IEEE Transactions on Electromagnetic Compatibility 22: 191— 202(1980), both documents being incorporated herein by reference. The optical coupler 300 comprises a micro-fiber 302 coupled to a planar optical waveguide 304. The planar optical waveguide 304 is in turn coupled to the slider waveguide 306. In the simulations, light was input as a mode of the micro-fiber 302 which propagated into the planar optical waveguide 304 and to the slider waveguide 306. The slider Waveguide 306 acted as the output (see numeral 308) as the near-field transducer was not considered.

Figure 4 shows the light distribution along the entire optical coupler 300 of Figure 3. It can be observed that when the light propagates within the tapered section of the planar optical waveguide 304 (Figure 3), the light distribution is reduced to a smaller size (see numeral 402) to match the mode distribution within the planar optical waveguide 304 (Figure 3). It can be observed that the propagation loss within the bent section of the planar optical waveguide 304 (Figure 3) is very small, as can be seen from there being substantially no difference in brightness of light before (at numeral 404) and after (at numeral 406) the bent section. After the light is coupled into the slider waveguide 306 (Figure 3), the light intensity (at numeral 408) is reduced. This is due to the larger size of the slider waveguide 306 (Figure 3). However, the light is refocused (at numeral 410) at the output 308 (Figure 3). In this simulation of Figure 4, the size of the bent section of the planar optical waveguide 304 (Figure 3) in the x-direction (compare Figure 1 for axis directions) is about 0.5 μηη while the size of the slider waveguide 306 (Figure 3) in the x- direction is about 2 μητ

Figure 5(a) is a schematic diagram showing production of optical couplers e.g. 502, 504 on a wafer 506 in an example embodiment. A plurality of optical couplers e.g. 502, 504 can be fabricated in the form of separate coupling modules e.g. 508, 510 in parallel on the wafer 506 using conventional planar technology. This allows for mass production at low cost. After cutting (e.g. at 512, 514) and lapping, the coupling modules e.g. 508, 510 can be assembled with sliders.

Figure 5(b) is a schematic flow chart illustrating production of optical couplers on a wafer in an example embodiment. At step 516, a silicon wafer substrate 518 is provided. At step 520, a layer 522 of lower cladding material is deposited. At step 524, a layer 526 of resist is spin-coated on the layer 522. At step 528, a mask 530 is used to conduct lithography exposure on the layer 526 using a UV light source. At step 532, resist development is carried out to obtain patterns 534. At step 536, a layer 538 of core material is deposited. At step 540,; resist liftoff is conducted to remove the remainder of the resist layer 526. At step 542, a layer 544 of upper cladding material is deposited over the remainder of the core material layer 538. At step 546, chemical mechanical polishing (CMP) is carried out to obtain the optical couplers 548 on the substrate 518. Figure 6 is a schematic diagram illustrating an assembly of a coupling module to a slider in an example embodiment. The coupling module 600 comprises a planar optical waveguide 602 (compare coupling module 508). The coupling module 600 comprises a trench 604. During assembly, a microfiber 606 can be aligned and fixed into the trench 604 to couple to the waveguide 602. The slider 608 comprises a slider waveguide 610. The coupling module 600 and the slider 608 are assembled into one unit. During assembly, the coupling module 600 is aligned with the slider 608 and mounted onto the slider 608 using e.g. adhesive glue. It can be seen that the thickness of the planar optical waveguide 602 corresponds to the width of the slider waveguide 610. In another example embodiment, an optical coupler couples laser/light energy into a slider waveguide by means of evanescent wave, instead of by using butt-coupling as is used in Figure 1 (a).

Figure 7 is a schematic diagram illustrating an optical coupler coupling light to a slider waveguide using evanescent wave coupling in an example embodiment. As shown at coupling area 708, the bending waveguide 702 of the optical coupler 704 is not directly coupled to the slider waveguide 706. In this approach, the mode field distributions of the two waveguides 702, 706 within the coupling area 708 are preferably designed to be almost the same and the coupling length, /, and distance, d, are preferably adjusted so as to obtain a desired coupling efficiency. In this embodiment, the optical coupler comprises an extension portion 7l0 to be received in a corresponding ledge portion 712 formed in the cladding 714 for the waveguide 706. The extension portion 710 and corresponding ledge portion 712 advantageously facilitate alignment of the optical coupler 704 and the slider 716 during assembly. Again, cross-sections of the planar optical waveguide 702 at a first end 718 and at a second end 720 respectively are different for facilitating coupling of the received light from the micro-fiber 722 at the first end 718 to the slider waveguide 706 in the coupling area 708 at the second end 720.

Figure 8(a) is a schematic diagram illustrating a head-gimbal-assembly in an example embodiment. Figure 8(b) is a schematic diagram illustrating a head-gimbal- assembly in another example embodiment. In these example embodiments, an assembled slider head structure (i.e. in the assembled form of a head-gimbal-assembly HGA 800) is provided. The assembled optical coupler 801 and slider 806 including the magnetic and optical heads function as a heat-assisted magnetic recording (HAMR) head. The micro-fiber 802 can be fixed either beneath or above a suspension arm 804 of the HGA 800, the slider 806 in turn being mounted at an acute angle to the suspension arm (see Figures 8(a) and (b) respectively). Due to the small size and resulting small weight and large flexibility of the micro-fiber 802, there is advantageously a negligible effect on the flying stability of the slider 806. In the example embodiment, the suspension arm 804 can be connected to the optical coupler 801 using attachment means including but not iimited to adhesive, brackets etc. Figure 9 is a schematic diagram illustrating a head-gimbal-assembly in another example embodiment, in which an optical coupler 902 and a slider 906 are disposed on opposite sides of a suspension arm 904, which is in turn mounted at an acute angle to the slider 906. The optical coupler 902 and slider 906 are respectively mounted/fixed on the suspension arm 904 of the HGA 900, using attachment means including but not Iimited to adhesive, brackets etc.. The suspension arm 904 is configured for enabling optical coupling between the planar waveguide 910 of the optical coupler 902 and the slider waveguide 912 on the siider 906, for example by way of an aperture (not shown) formed in the suspension arm 904. A refractive index matching iiquid or a polymer material can be disposeed in the aperture at the connecting point 908 between the optical planar waveguide 910 of the optical coupler 902 and the siider waveguide 912 of the slider 906.

It is noted again that, in providing the change in the light propagation direction in the planar optical waveguide 803, 910, the example embodiments can advantageously configure the fiber axis to be substantially horizontal (x-y plane). As shown in Figures 8 and 9, this can advantageously facilitate disposition of the fiber substantially along the suspension arm 804, 904 of the head-gimbal-assembly 800, 900, for mounting to the suspension arm 804,: 904, while avoiding sharp fiber bends otherwise required to guide the optical fiber from the suspension arm to the slider waveguide for direct, vertical coupling. Sharp fiber bends, , may result in fiber breakage and can add to the overall geometrical footprint of the head-gimbal-assembly, and may incur substantial bend losses. Figure 10(a) is a schematic diagram illustrating a micro-fiber in parallel coupling with a optical waveguide in an optical coupler in an example embodiment. The optical coupler 1006 comprises the micro-fiber 1002 and the planar optical waveguide 1004. The micro-fiber 1002 runs parallel to, and is in contact or close proximity to, a portion of the planar optical waveguide 1004 of the optical coupler 1006 in an evanescent coupling configuration. In this embodiment, the mode field distributions of the micro-fiber 1002 and the waveguide 1004, within the coupling area 1008, are preferably designed to be almost the same and the coupling length is preferably adjusted so as to obtain a desired coupling efficiency. For alignment purposes, it is advantageous to pattern a groove 1009 over the coupling region 1008, as shown in Fig.10(a).

.

Figure 10(b) is a graph showing transmission efficiency based on finite differential time domain simulations of parallel coupling in an example embodiment. The graph shows parallel coupling efficiency between a microfiber and a planar waveguide core for two polarizations. The microfiber and the planar waveguide core are assumed to have a refractive index of about 2.0 while the cladding of the waveguide has a refractive index of about 1.45. The microfiber is circular in cross-section with a diameter of 0.4 μιη and the waveguide core is rectangular in cross-section with dimensions 0.4 μηι and 0.3 μηη in the y and z-directions respectively. Propagation is along the x-direction. L is the overlap length for the microfiber and the planar waveguide. Plot 1010 shows the transmission efficiency for input light polarized along the z-direction and plot 1012 shows the transmission efficiency for input iight polarized along the y-direction. As can be observed, the first optimum value of overlap length 'L', giving the highest coupling efficiency, lies between L=2 and L=3. The transmission curve is expected to be periodic and efficient coupling can also be obtained at other values of 'L'. In alternative example embodiments, the micro-fiber may be replaced with a polymer: planar waveguide. In these embodiments, the polymer waveguide can extend from the light source to the optical coupler. The polymer waveguide can be fabricated together with a flexible printed circuit board (PCB). In these embodiments, the polymer waveguide comprises a rectangular core structure.

Figure 11(a) is a schematic diagram illustrating an optical coupler 1100 comprising a polymer planar waveguide 1102 in butt-coupling configuration with a planar optical waveguide 1104 in an example embodiment/ In this example embodiment, the polymer planar waveguide 1102 is directly butt-coupled to the planar optical waveguide 1104.

Figure 11 (b) is a schematic diagram illustrating the optical coupler 1100 comprising the polymer planar waveguide 1 02 in parallel-coupling configuration with the planar optical waveguide 1104 in another example embodiment. The polymer planar waveguide 1102 comprises a polymer core and polymer cladding. The refractive index of the waveguide 1102 can be 1.5 to 2.5, wherein the refractive index of the core is larger than the refractive index of cladding, in this example embodiment, the polymer planar waveguide 1102 is parallel coupled to the planar optical waveguide 1104 by means of evanescent wave (compare Figure 10(a)). The mode field distributions of the two ^'. waveguides 1102, 1104 within the coupling area 1106 are preferably designed to be almost the same and the coupling length is preferably adjusted so as to obtain a desired coupling efficiency.

Figure 12 is a schematic flowchart 1200 illustrating a method for coupling light from an optical fiber or planar waveguide to a slider waveguide of a heat assisted magnetic recording head in an example embodiment. At step 1202, a planar waveguide structure configured for receiving light from the optical fiber or planar waveguide at a first end thereof, and for coupling light to the slider waveguide at a second end thereof is used. Indicated at step 1204, the planar waveguide structure comprises a bent portion disposed between the first and second ends such that a direction of the light signal is changed between the first and second ends, and cross-sections of the planar waveguide structure at the first and second ends respectively are different for facilitating coupling of the received light at the first end to the slider waveguide at the second end.

In the above example embodiments, the optical fiber may be a glass or polymer optical fiber. One material used in glass fibers is silica, with a refractive index n of 1.45 around a wavelength of about 800nm. To obtain a refractive index contrast, the microfiber can be attached to the planar coupler unit with a fluoropolymer (e.g. of refractive index down to 1.29 for Telon AF2400) or with an aerogel, whose refractive index can be very close to 1.0 [L Xiao, . D.W. Grogan, W.J. Wadsworth, E. England, and T. A. Birks, "Stable low-loss optical nanofibres embedded in hydrophobic aerogel, Opt. Express 19(2), 764-769 (2011) incorporated herein by reference]. Using a fiber material of larger refractive index than silica can offer more flexibility for the choice of the cladding material. Examples of high refractive index materials that can be used to make microfibers include flints such as Schott SF6 (n_d=1.81) 7SF57 (n_d=1.85), N-SF66 (n_d=1.92), P-SF68 (n_d=2.00), bismuth silicates (n_d=2.02) [G. Brambilla et at., Optical fiber nanowires and microwires: fabrication and applications, Adv. Opt. Photon. 1 , 107- 161 (2009) incorporated herein by reference]; terurrites (n_d=2.05) [Z. Ma, S.-S. Wang, Q. Yang, L.-M. Tong, "Near-field characterization of optical micro/nanofibers," Chin. Phys. Lett. 24 (10), 3006-3008 (2007) incorporated herein by reference], and chalcogenides (n_d=2.7) [D-l Yeom, E. C. Magi, M. R. E. Lamont, M. A. F. Roelens, L. B. Fu, and B. J. Eggleton, "Low-threshold supercontinuum generation in highly nonlinear chalcogenide nanowires, Opt. Lett. 33 (7), 660-662 (2008) incorporated herein by reference]. For polymers, poly-(methyl methacrylate) (PMMA) can be used as fiber material. Microfibers can also be constructed using polystyrene (PS), poiyacrylamide (PAM), and/or polyaniline/polystyrene (PANI/PS) [F. Gu, L. Zhang, X.F. Yin, and L.-M. Tong, " Polymer single-nanowire optical sensors, Nano Lett. 8 (9), 2757-2761 (2008) incorporated herein by reference].

Further, in the above example embodiments, although the planar waveguide structure on the coupler is described as having a tapering section in one plane, it will be appreciated that the optical waveguide is not limited as such and. can include a tapering section with tapering in other planes or forms depending on different manufacturing techniques. It will be appreciated by a person skilled in the art that numerous variations and/or modifications may be made to the present invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.

Claims

1 An optical coupler; for coupling light from an optical fiber or a planar waveguide to a slider waveguide of a heat assisted magnetic recording head, the optical coupler comprising:

a planar waveguide structure configured for receiving light from the optical fiber or the planar waveguide at a first end thereof, and for coupling light to the slider waveguide at a second end thereof;

wherein the planar waveguide structure comprises a bent portion disposed between the first and second ends such that the direction of the light signal is changed between the first and second ends; and

wherein cross-sections of the planar waveguide structure at the first and second ends respectively are different for facilitating coupling of the received light at the first end to the slider waveguide at the second end.

2. The coupler as claimed in claim 1 , wherein the planar waveguide structure comprises a tapered portion disposed between the first end and the bent portion such that at least a lateral dimension is reduced in a direction towards the bent portion.

3. The coupler as claimed in claims 1 or 2, wherein the first end is configured for butt-coupling or for parallel coupling to the optical fiber or planar waveguide.

4. The coupler as claimed in any one of the preceding claims, wherein the second end is configured for butt-coupling or for parallel coupling to the slider waveguide.

5. The coupler as claimed in any one of the preceding claims, wherein the planar waveguide is formed in a flexible printed circuit board.

6. A method for coupling light from an optical fiber or a planar waveguide to a slider waveguide of a heat assisted magnetic recording head, the method comprising: using a planar waveguide structure configured for receiving light from the optical fiber or the planar waveguide at a first end thereof, and for coupling light to the slider waveguide at a second end thereof ,

wherein the planar waveguide structure comprises a bent portion disposed between the first and second ends such that a direction of the light signal is changed between the first and second ends; and

wherein cross^sections of the planar waveguide structure at the first and second ends respectively are different for facilitating coupling of the received light at the first end to the slider waveguide at the second end.

7. The method as claimed in claim 6, comprising reducing at least a lateral dimension of the planar waveguide structure in a direction towards the bent portion by way of a tapered portion disposed between the first end and the bent portion.

8. The method as claimed in claims 6 or 7, comprising configuring the first end for butt-coupling or for parallel coupling to the optical fiber or planar waveguide.

9. The method as claimed in any one of claims 6 to 8, comprising configuring the second end for butt-coupling or for parallel coupling to the slider waveguide.

10. A heat assisted magnetic recording (HAMR) head comprising:

a slider structure comprising a near-field optical transducer and a slider waveguide capable of delivering light to the optical transducer;

an optical coupler disposed for coupling light from an optical fiber or a planar waveguide to the slider waveguide;

wherein the optical coupler comprises:

wherein the planar waveguide structure comprises a bent portion disposed between the first and second ends such that a direction of the fight signal is changed between the first and second ends; and wherein cross-sections of the planar waveguide structure at the first and second ends respectively are different for facilitating coupling of the received light at the first end to the slider waveguide at the second end.

11. The HAMR head as claimed in claim 10, wherein the planar aveguide structure of the optical coupler comprises a tapered portion disposed between the first end and the bent portion such that at least a lateral dimension is reduced in a direction towards the bent portion.

12. The HAMR head as claimed in claims 10 or 11 , wherein the first end is configured for butt-coupiing or for parallel coupling to the optical fiber or planar waveguide.

13. The HAMR head as claimed in any one of claims 10 to 12, wherein the second end is configured for butt-coupiing or for parallel coupling to the slider waveguide.

14. The HAMR head as claimed in any one of claims 10 to 13, wherein the optical fiber comprises a glass or polymer optical fiber.

15. The HAMR head as claimed in any one of claims 10 to 14, wherein the optical fiber or planar waveguide is configured for mounting to a suspension arm coupled to the heat assisted magnetic recording head.

16. The HAMR head as claimed in any one of claims 10 to 15, wherein the optical fiber comprises a micro-fiber.

17. The HAMR head as claimed in any one of claims 10 to 15, wherein the slider structure is directly connected to the optical coupler.

18. The HAMR head as claimed in claim 17, wherein the connected slider structure and optical coupler are in turn connected to a suspension arm for the HAMR head;

19. The HAMR head as claimed in any one of claims 10 to 16, wherein the slider structure and the optical coupler are connected via a suspension arm for the HAMR head.

20. The HAMR head as claimed in claim 19, wherein the planar waveguide structure of the optical coupler is optically coupled to the slider waveguide of the slider structure via an aperture formed in the suspension arm.

21. The HAMR head as claimed in claim 20, wherein a refractive index matching liquid or polymer material is disposed in the aperture for facilitating the optical coupling.

22. The HAMR head as claimed in any one of claims 10 to 21 , wherein the planar waveguide is formed on a flexible printed circuit board.