U.S. patent application number 16/170445 was filed with the patent office on 2019-09-19 for in-field laser calibration for heat-assisted magnetic recording head using temperature compensation equation.
The applicant listed for this patent is Seagate Technology LLC. Invention is credited to James E. Angelo, Alfredo Sam Chu, Steven J. Kimble, Drew Michael Mader, Franklin P. Martens.
Application Number | 20190287554 16/170445 |
Document ID | / |
Family ID | 64050672 |
Filed Date | 2019-09-19 |
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United States Patent
Application |
20190287554 |
Kind Code |
A1 |
Chu; Alfredo Sam ; et
al. |
September 19, 2019 |
IN-FIELD LASER CALIBRATION FOR HEAT-ASSISTED MAGNETIC RECORDING
HEAD USING TEMPERATURE COMPENSATION EQUATION
Abstract
A temperature compensation equation is generated during
manufacture of a heat-assisted magnetic recording (HAMR) disk drive
using initial total currents supplied to a laser diode of the disk
drive at different initial operating temperatures. The total
currents represent currents for recording data to or erasing data
from the medium. The temperature compensation equation is stored in
the disk drive, and updated, during field operation, using a
subsequent total current associated with an operating temperature
differing from the initial operating temperatures. The total
current supplied to the laser diode for a subsequent write
operation is adjusted using the updated temperature compensation
equation in response to the operating temperature at the time of
the subsequent write operation.
Inventors: |
Chu; Alfredo Sam; (Prior
Lake, MN) ; Martens; Franklin P.; (Bloomington,
MN) ; Mader; Drew Michael; (Minneapolis, MN) ;
Kimble; Steven J.; (Chanhassen, MN) ; Angelo; James
E.; (Savage, MN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Seagate Technology LLC |
Cupertino |
CA |
US |
|
|
Family ID: |
64050672 |
Appl. No.: |
16/170445 |
Filed: |
October 25, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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15919386 |
Mar 13, 2018 |
10127930 |
|
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16170445 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G11B 2005/0021 20130101;
G11B 5/012 20130101; G11B 5/5534 20130101; G11B 5/02 20130101; G11B
5/455 20130101; G11B 5/607 20130101 |
International
Class: |
G11B 5/02 20060101
G11B005/02; G11B 5/60 20060101 G11B005/60 |
Claims
1. A method, comprising: generating, for a heat-assisted magnetic
recording (HAMR) disk drive, a temperature compensation equation
that characterizes total currents supplied to a laser diode of the
disk drive across a range of operating temperatures of the disk
drive, the total currents representative of currents for recording
data to or erasing data from a magnetic recording medium; storing
the temperature compensation equation in the disk drive; updating
the temperature compensation equation using a subsequent total
current associated with an operating temperature at the time of the
updating; and adjusting, using the updated temperature compensation
equation, the total current supplied to the laser diode for a
subsequent write operation in response to an operating temperature
at the time of the subsequent write operation.
2. The method of claim 1, wherein the temperature compensation
equation is updated using the subsequent total current only when
the operating temperature is outside of the range of operating
temperatures.
3. The method of claim 1, wherein the temperature compensation
equation is updated using the subsequent total current when the
operating temperature is inside or outside of the range of
operating temperatures.
4. The method of claim 1, wherein: the temperature compensation
equation is updated in response to at least an X degree operating
temperature change relative to a minimum or a maximum of the range
of operating temperatures; and X is a number.
5. The method of claim 1, wherein: for each X degree or more change
in the operating temperature relative to a minimum or a maximum of
the range of operating temperatures, one a plurality of subsequent
total currents is measured; the temperature compensation equation
is updated in response to measuring N subsequent total currents; X
is a number; and N is an integer.
6. The method of claim 5, wherein X is a number between 2 and 8,
and N is an integer equal to or greater than 1.
7. The method of claim 1, wherein the temperature compensation
equation is at least a second order polynomial equation.
8. The method of claim 1, wherein the temperature compensation
equation is updated in response to measuring a performance metric
that exceeds a predetermined threshold at the subsequent total
current.
9. The method of claim 1, wherein updating the temperature
compensation equation comprises: writing data using a plurality of
different subsequent total currents; selecting a particular
subsequent total current of the plurality of different subsequent
total currents having an acceptable performance metric; and
updating the temperature compensation equation using the particular
subsequent total current.
10. The method of claim 1, wherein updating the temperature
compensation equation continues until the temperature compensation
equation covers a range of the operating temperatures that is at
least coextensive with a temperature range specified for the disk
drive.
11. An apparatus, comprising: a slider of a heat-assisted magnetic
recording (HAMR) disk drive movable relative to a magnetic
recording medium; a temperature sensor disposed in the disk drive
and configured to measure an operating temperature; and a
controller of the disk drive coupled to the slider and the
temperature sensor, the controller configured to: store a
temperature compensation equation in a memory of the disk drive,
the temperature compensation equation characterizing total currents
supplied to a laser diode of the disk drive across a range of
operating temperatures of the disk drive, the total currents
representative of currents for recording data to or erasing data
from a recording medium; update the temperature compensation
equation using a subsequent total current associated with an
operating temperature at the time of the updating; and adjust,
using the updated temperature compensation equation, the total
current supplied to the laser diode for a subsequent write
operation in response to an operating temperature at the time of
the subsequent write operation.
12. The apparatus of claim 11, wherein the controller is configured
to update the temperature compensation equation using the
subsequent total current only when the operating temperature is
outside of the range of operating temperatures.
13. The apparatus of claim 11, wherein the controller is configured
to update the temperature compensation equation using the
subsequent total current when the operating temperature is inside
or outside of the range of operating temperatures.
14. The apparatus of claim 11, wherein: the controller is
configured to update the temperature compensation equation in
response to at least an X degree operating temperature change
relative to a minimum or a maximum of the range of operating
temperatures; and X is a number.
15. The apparatus of claim 11, wherein: the controller is
configured to measure one of a plurality of subsequent total
currents in response to each X degree or more change in the
operating temperature relative to a minimum or a maximum of the
range of operating temperatures; the controller is configured to
update the temperature compensation equation in response to
measuring N subsequent total currents; X is a number; and N is an
integer.
16. The apparatus of claim 15, wherein X is a number between 2 and
8, and N is an integer equal to or greater than 1.
17. The apparatus of claim 11, wherein the temperature compensation
equation is at least a second order polynomial equation.
18. The apparatus of claim 11, wherein the controller is configured
to: measure a performance metric at the subsequent total current;
and update the temperature compensation equation using the
subsequent total current in response to the performance metric
exceeding a predetermined threshold.
19. The apparatus of claim 11, wherein the controller is configured
to: write data using a plurality of different subsequent total
currents; select a particular subsequent total current of the
plurality of different subsequent total currents having an
acceptable performance metric; and update the temperature
compensation equation using the particular subsequent total
current.
20. The apparatus of claim 11, wherein the controller is configured
to update the temperature compensation equation until the
temperature compensation equation covers a range of the operating
temperatures that is at least coextensive with a temperature range
specified for the disk drive.
Description
RELATED PATENT DOCUMENTS
[0001] This application is a continuation of U.S. patent
application Ser. No. 15/919,386, filed Mar. 13, 2018, which is
incorporated herein by reference in its entirety.
SUMMARY
[0002] Various embodiments are directed to a method comprising
generating, during manufacture of a heat-assisted magnetic
recording (HAMR) disk drive, a temperature compensation equation
using initial total currents supplied to a laser diode of the disk
drive at different initial operating temperatures. The total
currents represent currents for recording data to or erasing data
from the medium. The method comprises storing the temperature
compensation equation in the disk drive, and updating, during field
operation, the temperature compensation equation using a subsequent
total current associated with an operating temperature differing
from the initial operating temperatures. The method also comprises
adjusting, using the updated temperature compensation equation, the
total current supplied to the laser diode for a subsequent write
operation in response to the operating temperature at the time of
the subsequent write operation.
[0003] Other embodiments are directed to an apparatus comprising a
slider of a HAMR disk drive movable relative to a magnetic
recording medium. The slider comprises a writer, a reader, a
near-field transducer, and an optical waveguide for communicating
light from a laser diode to the near-field transducer. A
temperature sensor is disposed in the disk drive and configured to
measure an operating temperature. A controller of the disk drive is
coupled to the slider and the temperature sensor. The controller is
configured to store a temperature compensation equation in a memory
of the disk drive. The temperature compensation equation
characterizes total currents supplied to the laser diode of the
disk drive at different initial operating temperatures. The total
currents represent currents for recording data to or erasing data
from the medium. The controller is also configured to update,
during field operation, the temperature compensation equation using
a subsequent total current associated with an operating temperature
differing from the initial operating temperatures. The controller
is further configured to adjust, using the updated temperature
compensation equation, the total current supplied to the laser
diode for a subsequent write operation in response to the operating
temperature at the time of the subsequent write operation.
[0004] The above summary is not intended to describe each disclosed
embodiment or every implementation of the present disclosure. The
figures and the detailed description below more particularly
exemplify illustrative embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] Throughout the specification reference is made to the
appended drawings, where like reference numerals designate like
elements, and wherein:
[0006] FIG. 1 is a perspective view of a hard drive slider
configured for heat-assisted magnetic recording (HAMR) in
accordance with embodiments described herein;
[0007] FIG. 2 is a cross-sectional view showing details of a HAMR
slider according to various implementations;
[0008] FIG. 3 illustrates a HAMR slider in accordance with some
aspects described herein;
[0009] FIG. 4 is a generalized graph characterizing output optical
power of a laser diode of a HAMR slider in response to current
supplied to the laser diode;
[0010] FIG. 5 is a graph showing how total current, I.sub.TOTAL,
supplied to a laser diode of a HAMR slider changes non-linearly
across temperature;
[0011] FIG. 6 illustrates a method of generating and updating a
temperature compensation equation that characterizes the total
current, I.sub.TOTAL, in accordance with various embodiments;
[0012] FIG. 7 illustrates a method of updating a temperature
compensation equation that characterizes the total current,
I.sub.TOTAL, in accordance with various embodiments;
[0013] FIG. 8 illustrates a process of updating a temperature
compensation equation that characterizes the total current,
I.sub.TOTAL, during field operation of a particular HAMR disk drive
in accordance with various embodiments;
[0014] FIG. 9 is a graph showing plots of total current,
I.sub.TOTAL, versus temperature for a conventional laser diode
calibration approach and one that uses a temperature compensation
equation in accordance with embodiments of the present
disclosure;
[0015] FIG. 10 is a graph showing plots of bit error rate (BER)
versus temperature for a conventional laser diode calibration
approach and one that uses a temperature compensation equation in
accordance with embodiments of the present disclosure; and
[0016] FIG. 11 is a block diagram of a system for calibrating a
laser of a HAMR head using a temperature compensation equation
approach in accordance with various embodiments.
[0017] The figures are not necessarily to scale. Like numbers used
in the figures refer to like components. However, it will be
understood that the use of a number to refer to a component in a
given figure is not intended to limit the component in another
figure labeled with the same number.
DETAILED DESCRIPTION
[0018] The present disclosure relates to heat-assisted magnetic
recording, which can be used to increase areal data density of
magnetic media. HAMR may also be referred to as energy-assisted
magnetic recording (EAMR), thermally-assisted magnetic recording
(TAMR), and thermally-assisted recording (TAR). In a HAMR device,
information bits are recorded in a storage layer at elevated
temperatures in a specially configured magnetic media. The use of
heat can overcome superparamagnetic effects that might otherwise
limit the areal data density of the media. As such, HAMR devices
may include magnetic write heads for delivering electromagnetic
energy to heat a small confined media area (spot size) at the same
time the magnetic write head applies a magnetic field to the media
for recording.
[0019] A HAMR read/write element, sometimes referred to as a
slider, recording head, read head, write head, read/write head,
etc., includes magnetic read and write transducers similar to those
on current hard drives. For example, data may be read by a
magnetoresistive sensor that detects magnetic fluctuations of a
magnetic media as it moves underneath the sensor. Data is written
to the magnetic media by a write coil that is magnetically coupled
to a write pole. The write pole changes magnetic orientation in
regions of the media as it moves underneath the write pole in
response to an energizing current applied to the write coil. A HAMR
slider also includes a source of energy, such as a laser diode, to
heat the media while it is being written to by the write pole. An
optical delivery path is integrated into the HAMR slider to deliver
the energy to the surface of the media.
[0020] The optical delivery path of a HAMR slider may include a
plasmonic transducer proximate a media-facing surface (e.g.,
air-bearing surface, contact surface). The plasmonic transducer
shapes and transmits the energy to a small region on the medium.
The plasmonic transducer is sometimes referred to as a near-field
transducer (NFT), optical antenna, surface plasmon resonator, etc.,
and may include a plasmonic metal such as gold, silver, copper,
aluminum, etc., and alloys thereof. The plasmonic transducer for a
HAMR device is very small (e.g., on the order of 0.1 to a few light
wavelengths, or any value therebetween) and creates a localized
region of high power density in the media through an
electromagnetic interaction. This results in a high temperature
rise in a small region on the media, with the region reaching or
exceeding the Curie temperature having dimensions less than 100 nm
(e.g., .about.50 nm).
[0021] With reference to FIG. 1, a perspective view shows a HAMR
slider assembly 100 according to a representative embodiment. The
slider assembly 100 includes a laser diode 102 located on input
surface 103 of a slider body 101. In this example, the input
surface 103 is a top surface, which is located opposite to a
media-facing surface 108 that is positioned over a surface of a
recording media (not shown) during device operation. The
media-facing surface 108 faces and is held proximate to the moving
media surface while reading and writing to the media. The
media-facing surface 108 may be configured as an air-bearing
surface (ABS) that maintains separation from the media surface via
a thin layer of air.
[0022] The laser diode 102 delivers light to a region proximate a
HAMR read/write head 106, which is located near the media-facing
surface 108. The energy is used to heat the recording media as it
passes by the read/write head 106. Optical coupling components are
formed integrally within the slider body 101 (near a trailing edge
surface 104 in this example) and function as an optical path that
delivers energy from the laser diode 102 to the recording media via
a near-field transducer 112. The near-field transducer 112 is near
the read/write head 106 and causes heating of the media during
recording operations.
[0023] The laser diode 102 in this example may be configured as
either an edge-emitting laser or surface-emitting laser. Generally,
the edge-emitting laser emits light from near a corner edge of the
laser and a surface emitting laser emits light in a direction
perpendicular to a surface of the laser body, e.g., from a point
near a center of the surface. An edge-emitting laser may be mounted
on the top surface 103 of the slider body 101 (e.g., in a pocket or
cavity) such that the light is emitted in a direction parallel to
(or at least non-perpendicular to) the media-facing surface. A
surface-emitting or edge-emitting laser in any of these examples
may be directly coupled to the slider body 101, or via an
intermediary component such as a submount (not shown). A submount
can be used to orient an edge-emitting laser so that its output is
directly downwards (negative y-direction in the figure).
[0024] While the example in FIG. 1 shows a laser diode 102 directly
mounted to the slider body 101, the waveguide system 110 discussed
herein may be applicable to any type of light delivery
configuration. For example, a laser may be mounted on the trailing
edge surface 104 instead of the top surface 103. In another
configuration known as free-space light delivery, a laser may be
mounted external to the slider 100, and coupled to the slider by
way of optic fiber and/or waveguide. An input surface of the slider
body 101 may include a grating or other coupling feature to receive
light from the laser via the optic fiber and/or waveguide.
[0025] With reference now to FIG. 2, a cross-sectional view shows
details of a HAMR apparatus 200 according to an example embodiment.
Near-field transducer 112 is located proximate a media-facing
surface 202 (e.g., ABS), which is held near a magnetic recording
media 204 during device operation. In the orientation of FIG. 2,
the media-facing surface 202 is arranged parallel to the x-z plane.
A waveguide core 206 may be disposed proximate the NFT 112, which
is located at or near the media writing surface 214.
[0026] The waveguide core 206 is surrounded by cladding layers 208,
210. The waveguide core 206 and cladding layers 208, 210 may be
made from dielectric materials. Generally, the dielectric materials
are selected so that the refractive index of the waveguide core
layer 206 is higher than refractive indices of the cladding layers
208, 210. This arrangement of materials facilitates efficient
propagation of light through the waveguide. Light is delivered from
the waveguide core 206 along the negative y-direction where it is
coupled to the NFT 112. The NFT 112 delivers surface plasmon
enhanced, near-field electromagnetic energy along the y-axis where
it exits at the media writing surface 214. This may result in a
highly localized hot spot (not shown) on the media surface 214 when
the media 204 placed in close proximity to surface 202 of the
apparatus 200. Further illustrated in FIG. 2 is a write pole 212 of
the read/write head that is located alongside the NFT 112. The
write pole 212 generates a magnetic field (e.g., perpendicular
field) used in changing the magnetic orientation of the hotspot
during writing.
[0027] FIG. 3 shows a side view of a read/write transducer 302
configured for heat-assisted magnetic recording according to a
representative embodiment. The read/write transducer 302 may be
used in a magnetic data storage device, e.g., a hard disk drive.
The read/write transducer 302 may also be referred to herein as a
slider, read/write head, recording head, etc. The read/write
transducer 302 is coupled to an arm 304 by way of a suspension 306
that allows some relative motion between the read/write transducer
302 and arm 304. The read/write transducer 302 includes read/write
transducers 308 at a trailing edge that are held proximate to a
surface 310 of a magnetic recording medium 311, e.g., magnetic
disk. The read/write transducer 302 further includes a laser 320
and a waveguide 322. The waveguide 322 delivers light from the
laser 320 to components (e.g., a near-field transducer) near the
read/write transducers 308.
[0028] When the read/write transducer 302 is located over surface
310 of recording medium 311, a flying height 312 is maintained
between the read/write transducer 302 and the surface 310 by a
downward force of arm 304. This downward force is counterbalanced
by an air cushion that exists between the surface 310 and an air
bearing surface 303 (also referred to herein as a "media-facing
surface") of the read/write transducer 302 when the recording
medium 311 is rotating. It is desirable to maintain a predetermined
slider flying height 312 over a range of disk rotational speeds
during both reading and writing operations to ensure consistent
performance. Region 314 is a "close point" of the read/write
transducer 302, which is generally understood to be the closest
spacing between the read/write transducers 308 and the magnetic
recording medium 311, and generally defines the head-to-medium
spacing 313.
[0029] To account for both static and dynamic variations that may
affect slider flying height 312, the read/write transducer 302 may
be configured such that a region 314 of the read/write transducer
302 can be configurably adjusted during operation in order to
finely adjust the head-to-medium spacing 313. This is shown in FIG.
3 by a dotted line that represents a change in geometry of the
region 314. In this example, the geometry change may be induced, in
whole or in part, by an increase or decrease in temperature of the
region 314 via one or more heaters 316. A thermal sensor 315 is
shown situated at or near the close point 314 (e.g., adjacent the
read/write transducers 308, such as near the near-field transducer)
or can be positioned at other location of the ABS 303.
[0030] Turning now to FIG. 4, there is illustrated a generalized
graph characterizing output optical power of a laser diode of a
HAMR slider (e.g., as measured by a photodiode) in response to
current supplied to the laser diode. The laser diode is in a
non-conducting state until a threshold current, I.sub.TH, is
reached. At I.sub.TH, the laser diode begins to conduct or lase.
When recording data on a magnetic recording medium, the current
supplied to the laser diode is increased to I.sub.EFF (referred to
as effective current). At I.sub.EFF, the current supplied to the
laser diode is sufficient to facilitate an increase in media
temperature to the Curie Temperature for recording data on the
medium. A current I.sub.BIAS represents a current supplied to the
laser diode that is lower than I.sub.EFF. At I.sub.BIAS, the
current supplied to the laser diode (bias current) is not
sufficient to facilitate recording of data. I.sub.BIAS is typically
set between I.sub.EFF and I.sub.TH, but can also be set lower than
I.sub.TH. (not shown). As with I.sub.TH, I.sub.BIAS changes as the
laser diode temperature changes. The laser diode current is set at
I.sub.BIAS prior to performing a write or when performing various
non-recording tasks, such as when performing a seek or when the
slider is over servo wedges between writes.
[0031] The sum of I.sub.BIAS and I.sub.EFF is referred to as
I.sub.TOTAL (total current), which is the total current applied to
the laser diode during write operations. In some embodiments,
I.sub.TOTAL is achieved by adjusting I.sub.EFF while I.sub.BIAS is
fixed. In other embodiments, I.sub.TOTAL is achieved by adjusting
I.sub.EFF and adjusting I.sub.BIAS. I.sub.BIAS can be adjusted
based on the output of a photodiode, the output of a thermal sensor
(e.g., a dual-ended temperature coefficient of resistance sensor),
or by some other means or algorithm.
[0032] It is been found that the laser diode current I.sub.TOTAL is
non-linear across temperature. The amount of non-linearity of
I.sub.TOTAL is head and media dependent. As such, a global
compensation approach cannot be applied. FIG. 5 is a graph showing
how I.sub.TOTAL 502 changes non-linearly across temperature. More
particularly, FIG. 5 shows that I.sub.TOTAL 502 varies non-linearly
between about 18 and 18.7 mA across a temperature range of
20.degree. to 75.degree. C. The magnitude of I.sub.TOTAL
non-linearity increases at the extremes (low and high) of the
temperature range. FIG. 5 also shows a straight-line average
I.sub.TOTAL 504 derived from performing a linear regression on the
I.sub.TOTAL data points. It can be appreciated that using the
straight-line average I.sub.TOTAL 504 to set I.sub.TOTAL of the
laser diode at a given operating temperature would result in a
suboptimal current supplied to the laser diode for recording.
[0033] Embodiments of the disclosure are directed to generating a
temperature compensation equation that characterizes the total
current, I.sub.TOTAL, supplied to a laser diode of a HAMR slider
across a range of operating temperatures of the disk drive. The
temperature compensation equation is preferably at least a second
order polynomial equation (e.g., a quadratic equation). In some
embodiments, the temperature compensation equation can be a third
or fourth order polynomial equation (continuous or piecewise). An
initial temperature compensation equation characterizing the total
current, I.sub.TOTAL, is typically established at the time of disk
drive manufacture for a minimal number (e.g., 2) of operating
temperatures. The temperature compensation equation for selecting
the total current, I.sub.TOTAL, supplied to the laser diode is
updated during field operation of the HAMR disk drive. For example,
the temperature compensation equation can be updated during field
operation in response to a difference between the current operating
temperature and previous operating temperatures within a
temperature range covered by the temperature compensation
equation.
[0034] FIG. 6 illustrates a method of generating a temperature
compensation equation that characterizes the total current,
I.sub.TOTAL, in accordance with various embodiments. The method
according to FIG. 6 involves generating 602, during HAMR disk drive
manufacture, a temperature compensation equation using initial
total currents supplied to a laser diode of the disk drive at a
minimal number of different initial operating temperatures. For
example, the temperature compensation equation generated during
disk drive manufacture can be based on two initial operating
temperatures (e.g., 20.degree. C. and 50.degree. C.). The method
involves storing 604 the temperature compensation equation in the
disk drive. The method also involves updating 606, during field
operation, the temperature compensation equation using a subsequent
total current associated with an operating temperature differing
from the initial operating temperatures or initial operating
temperature range. The method further involves adjusting 608, using
the updated temperature compensation equation, the total current
supplied to the laser diode for a subsequent write operation in
response to the operating temperature at the time of the subsequent
write operation. It is understood that the method shown in FIG. 6
is performed for each recording head of a HAMR disk drive.
[0035] Limiting the number of operating temperatures when
generating the initial temperature compensation equation in the
factory is important from a manufacturing cost/time standpoint.
Developing a temperature compensation equation during manufacturing
using multi-temperature testing across a wide range of operating
temperatures is very expensive and time consuming. Also,
conventional production equipment has a limited temperature range
which is significantly smaller than the operating temperature range
specified for disk drive product. As such, it is not presently
possible to develop, at the time of disk drive manufacture, a
temperature compensation equation for characterizing the total
current, I.sub.TOTAL, across the full range of operating
temperatures specified for HAMR disk drive product. Because the
temperature compensation equation is updated in the field according
to embodiments of the disclosure, the total current, I.sub.TOTAL,
can be characterized across the full range of operating
temperatures experienced by the disk drive.
[0036] FIG. 7 illustrates a method of updating a temperature
compensation equation that characterizes the total current,
I.sub.TOTAL, during field operation of a disk drive in accordance
with various embodiments. The method according to FIG. 7 involves
providing 702 a temperature compensation equation. The temperature
compensation equation can be an initial temperature compensation
equation generated at the time of manufacture or a temperature
compensation equation that has been updated in the field. In either
case, the temperature compensation equation characterizes total
currents for two or more operating temperatures of a HAMR disk
drive that define an operating temperature range.
[0037] The method of FIG. 7 involves measuring 704 a change in the
disk drive operating temperature (Op Temp) relative to an operating
temperature range associated with the temperature compensation
equation. For example, the temperature compensation equation
provided in block 702 may characterize two or more total currents
within an operating temperature range of 20.degree. C. and
50.degree. C. The change in disk drive operating temperature
measured in block 704 is made relative to the limits (e.g.,
20.degree. C. and 50.degree. C.) of the operating temperature range
associated with the temperature compensation equation.
[0038] During operation of the disk drive in the field, the
operating temperature of the disk drive is measured. The disk drive
temperature can be measured continuously or periodically (e.g.,
every 5, 10, 30 minutes, hourly). A check 706 is made to determine
if the operating temperature has changed by X degrees or more
relative to the closest limit of the operating temperature range
associated with the temperature compensation equation. The variable
X is typically an integer between 2 and 8, such as 5, but can also
be a real number. If the operating temperature has not changed by X
degrees or more, processing returns to block 704. If the operating
temperature has changed by X degrees or more, an in-field laser
performance test (ILPT) is performed 708 at the current operating
temperature.
[0039] According to some embodiments, an ILPT involves a test of
the laser diode's performance. For example, the ILPT can involve
writing data to a track of a magnetic recording medium, reading the
data, and measuring a metric of writeability, such as BER. As
another example, the output optical power of the laser diode can be
measured, such as by use of a photodetector or a bolometer. It is
understood that performance metrics other than BER and output
optical power can be measured in block 708. If, as tested at block
710, the performance metric is better than a predetermined
threshold (e.g., a BER threshold or an output optical power
threshold), no changes are made to laser diode operation or to the
temperature compensation equation, and processing returns to block
704. If the performance metric is poorer than the predetermined
threshold, an ILPT calibration is performed 712. In some
embodiments, blocks 708 and 710 are not included (as indicated by
the dashed line), and an ILPT calibration is performed 712 in
response to determining (at block 706) that the operating
temperature has changed by X degrees or more relative to the
closest limit of the operating temperature range associated with
the temperature compensation equation.
[0040] In some embodiments, an ILPT calibration 712 involves
writing tracks of data while varying the total current,
I.sub.TOTAL, between a minimum and maximum value, and measuring a
performance metric (e.g., BER) for each of the total current
values. For example, the total current, I.sub.TOTAL, can be swept
at increments between 100 and 120 DAC values. A DAC value
represents a value of a digital-to-analog converter output. For
example, a particular I.sub.TOTAL DAC value can correspond to a
particular amount of current (in mA) supplied to the laser diode.
DAC values are generally incremented and decremented in fixed step
sizes. The ILPT calibration returns a total current, I.sub.TOTAL,
that has an acceptable (e.g., maximum, optimum or otherwise useful)
performance metric for the current operating temperature (e.g., a
BER better than a predetermined threshold). The total current,
I.sub.TOTAL, returned by the ILPT calibration is stored in a memory
of the disk drive.
[0041] After the ILPT calibration 712, a performance test is
performed 714 using the total current, I.sub.TOTAL, obtained from
the ILPT calibration. The ILPT 714 can measure a performance metric
such as BER or output optical power of the laser diode. If the
performance metric is better than a predetermined threshold, as is
tested at block 716, processing proceeds to block 720. If the
performance metric is poorer than the predetermined threshold, the
total current, I.sub.TOTAL, returned by the ILPT calibration 712 is
discarded and processing returns to block 704.
[0042] The total current, I.sub.TOTAL, returned by the ILPT
calibration 712 can also serve as a performance metric that is
evaluated at block 716. The ILPT 714 can determine if the total
current, I.sub.TOTAL, returned by the ILPT calibration 712 is an
acceptable current. For example, if the total current, I.sub.TOTAL,
returned by the ILPT calibration 712 changes by less than a
predetermined amount (e.g., <10% or 20%) relative to the
presently-used total current, I.sub.TOTAL, then the total current,
I.sub.TOTAL, returned by the ILPT calibration can be considered
acceptable, in which case processing proceeds to block 720. If the
total current, I.sub.TOTAL, returned by the ILPT calibration 712
changes by more than the predetermined amount relative to the
presently-used total current, I.sub.TOTAL, then the total current,
I.sub.TOTAL, returned by the ILPT calibration can be considered
unacceptable, in which case the total current, I.sub.TOTAL,
returned by the ILPT calibration 712 is discarded and processing
returns to block 704.
[0043] In some embodiments, a check 720 is made to determine how
many times (N) ILPT calibration has been performed since the last
update to the temperature compensation equation. N is an integer
equal to or greater than 1 (e.g., N=1, 2 or 3). If ILPT calibration
has not been performed more than N times, processing returns to
block 704. If ILPT calibration has been performed more than N
times, the temperature compensation equation is updated 724 using
the stored values of total current, I.sub.TOTAL, and associated
temperatures from the previous ILPT calibrations. In other
embodiments, the temperature compensation equation is updated after
each ILPT calibration (e.g., block 720 is excluded). During
operation of the disk drive in the field, the updated temperature
compensation equation can be used to adjust 726 the total current,
I.sub.TOTAL, applied to the laser diode for a subsequent write
operation in response to the operating temperature at the time of
the subsequent write operation. It is understood that the method
shown in FIG. 7 is performed for each recording head of a HAMR disk
drive.
[0044] FIG. 8 illustrates a process of updating a temperature
compensation equation that characterizes the total current,
I.sub.TOTAL, during field operation of a particular HAMR disk drive
in accordance with various embodiments. In the illustrative example
shown in FIG. 8, it is assumed that the specified temperature range
for disk drive operation is 5-65.degree. C. As such, is it desired
that the temperature compensation equation be updated over time in
the field to characterize the total current, I.sub.TOTAL, over the
entire specified temperature range. It is also assumed that an ILPT
is self-invoked by the disk drive in the field for every X degree
Celsius of operating temperature change, such that X equals
5.degree. C. It is further assumed that once the disk drive has
performed N (e.g., N=2) self-invoked ILPT calibrations, a
controller or processor of the disk drive updates the temperature
compensation equation, which is at least a second order polynomial
equation. As previously discussed, the temperature compensation
equation can be a third or a fourth order polynomial equation.
[0045] At a time t=t.sub.0, an initial temperature compensation
equation is generated at the time of disk drive manufacture. In
this illustrative example, a total current, I.sub.TOTAL, is
determined at two different temperatures, 20.degree. C. and
50.degree. C. The total current at each of the two different
temperatures results in an acceptable (e.g., maximum, optimal or
otherwise useful) performance metric (e.g., BER better than a
predetermined threshold). The temperature compensation equation is
generated using the total currents at these two different
temperatures. It is understood that the total currents associated
with more than two temperatures can be used to generate the initial
temperature compensation equation during manufacture. The initial
temperature compensation equation is stored in the disk drive and
subject to updating in the field. It can be seen that the
temperatures associated with the initial temperatures compensation
equation range from 20.degree. C. to 50.degree. C. Typically, the
temperature compensation equation is not updated (but can be
updated in some embodiments) for operating temperatures that fall
within the temperature range associated with the initial
temperatures compensation equation.
[0046] At time t=t.sub.1, the operating temperature of the disk
drive is measured at 18.degree. C. The lower limit of the
temperature range of 20-50.degree. C. is compared to the operating
temperature at time t=t.sub.1. Because the difference between
20.degree. C. and 18.degree. C. is less than 5.degree. C. (X=5), an
ILPT is not performed. At time t=t.sub.2, the operating temperature
of the disk drive is measured at 15.degree. C. The lower limit of
the temperature range of 20-50.degree. C. is compared to the
operating temperature at time t=t.sub.2. Because the difference
between 20.degree. C. and 15.degree. C. is equal to 5.degree. C.,
an ILPT is performed. Assuming an ILPT calibration is performed
(e.g., BER better than a predetermined threshold at 15.degree. C.),
a total current, I.sub.TOTAL, for 15.degree. C. is returned. In
some embodiments, the temperature compensation equation is updated
after each ILPT calibration is performed. In other embodiments, as
in the case of FIG. 8, the temperature compensation equation is
updated after N (e.g., N=2) ILPT calibrations have been performed.
It can be seen that at time t=t.sub.2, the temperature range has
increased from 20-50.degree. C. to 15-50.degree. C.
[0047] At time t=t.sub.3, the operating temperature of the disk
drive is measured at 13.degree. C. The lower limit of the
temperature range of 15-50.degree. C. is compared to the operating
temperature at time t=t.sub.3. Because the difference between
15.degree. C. and 13.degree. C. is less than 5.degree. C., an ILPT
is not performed. At time t=t.sub.4, the operating temperature of
the disk drive is measured at 9.degree. C. The lower limit of the
temperature range of 15-50.degree. C. is compared to the operating
temperature at time t=t.sub.4. Because the difference between
15.degree. C. and 9.degree. C. is greater than 5.degree. C., an
ILPT is performed. Assuming an ILPT calibration is performed (e.g.,
BER is better than a predetermined threshold at 9.degree. C.), a
total current, I.sub.TOTAL, for 9.degree. C. is returned. It can be
seen that ILPT calibrations were performed at times t=t.sub.2 and
t=t.sub.4. Assuming N is set to 2, the temperature compensation
equation is updated at time t=t.sub.4 using the total currents
obtained at 9.degree. C., 15.degree. C., 20.degree. C., and
50.degree. C. The temperature range associated with the updated
temperature compensation equation is increased from 15-50.degree.
C. to 9-50.degree. C. at time t=t.sub.4. At time t=t.sub.5, the
operating temperature of the disk drive is measured at 5.degree. C.
The lower limit of the temperature range of 9-50.degree. C. is
compared to the operating temperature at time t=t.sub.5. Because
the difference between 9.degree. C. and 5.degree. C. is less than
5.degree. C., an ILPT is not performed.
[0048] At time t=t.sub.6, the operating temperature of the disk
drive is measured at 54.degree. C. The upper limit of the
temperature range of 9-50.degree. C. is compared to the operating
temperature at time t=t.sub.6. Because the difference between
54.degree. C. and 50.degree. C. is less than 5.degree. C., an ILPT
is not performed. At time t=t.sub.7, the operating temperature of
the disk drive is measured at 58.degree. C. The upper limit of the
temperature range of 9-50.degree. C. is compared to the operating
temperature at time t=t.sub.7. Because the difference between
58.degree. C. and 50.degree. C. is greater than 5.degree. C., an
ILPT is performed. Assuming an ILPT calibration is performed (e.g.,
BER is better than a predetermined threshold at 58.degree. C.), a
total current, I.sub.TOTAL, for 58.degree. C. is returned. At time
t=t.sub.8, the operating temperature of the disk drive is measured
at 62.degree. C. The upper limit of the temperature range of
9-58.degree. C. is compared to the operating temperature at time
t=t.sub.8. Because the difference between 62.degree. C. and
58.degree. C. is less than 5.degree. C., an ILPT is not
performed.
[0049] At time t=t.sub.9, the operating temperature of the disk
drive is measured at 65.degree. C. The upper limit of the
temperature range of 9-58.degree. C. is compared to the operating
temperature at time t=t.sub.9. Because the difference between
65.degree. C. and 58.degree. C. is greater than 5.degree. C., an
ILPT is performed. Assuming an ILPT calibration is performed (e.g.,
BER is better than a predetermined threshold at 65.degree. C.), a
total current, I.sub.TOTAL, for 65.degree. C. is returned. It can
be seen that ILPT calibrations were performed at times t=t.sub.7
and t=t.sub.9. Assuming N is set to 2, the temperature compensation
equation is updated at time t=t.sub.9 using the total currents
obtained at 9.degree. C., 15.degree. C., 20.degree. C., 50.degree.
C., 58.degree. C., and 65.degree. C. The temperature range
associated with the updated temperature compensation equation is
increased from 9-58.degree. C. to 9-65.degree. C. at time
t=t.sub.9.
[0050] At time t=t.sub.10, the operating temperature of the disk
drive is measured at 68.degree. C. The upper limit of the
temperature range of 9-65.degree. C. is compared to the operating
temperature at time t=t.sub.10. Because the difference between
68.degree. C. and 65.degree. C. is less than 5.degree. C., an ILPT
is not performed. In some embodiments, an ILPT process is not
performed for an operating temperature that exceeds the specified
temperature range of the disk drive. In the present example, the
specified temperature range is 5-65.degree. C., and the operating
temperature at time t=t.sub.10 exceeds 65.degree. C., in which case
ILPT is not performed for the excessively high operating
temperature. In other embodiments, an ILPT process can be performed
for an operating temperature (e.g., 70.degree. C.) that exceeds the
specified temperature range of the disk drive.
[0051] For example, and with reference to time t=t.sub.11, the
operating temperature of the disk drive is measured at 3.degree. C.
The lower limit of the temperature range of 9-65.degree. C. is
compared to the operating temperature at time t=t.sub.11. Because
the difference between 3.degree. C. and 9.degree. C. is greater
than 5.degree. C., an ILPT is normally performed. However, the
measured operating temperature of 3.degree. C. is outside of the
specified temperature range of the disk drive, and an ILPT would
not normally be performed (but can be performed in some
embodiments). In this illustrative example, an ILPT is performed at
time t=t.sub.ii. Assuming an ILPT calibration is performed (e.g.,
BER is better than a predetermined threshold at 3.degree. C.), a
total current, I.sub.TOTAL, for 3.degree. C. is returned. Because
the operating temperature is outside the specified temperature
range of the disk drive, the temperature compensation equation is
updated at time t=t.sub.11 using the total currents obtained at
3.degree. C., 9.degree. C., 15.degree. C., 20.degree. C.,
50.degree. C., 58.degree. C. and 65.degree. C., irrespective of the
value of N. The temperature range associated with the updated
temperature compensation equation is increased from 9-65.degree. C.
to 3-65.degree. C. at time t=t.sub.11. It can be seen that the
updated temperature compensation equation at time t-t.sub.ii spans
the specified temperature range of the disk drive (e.g.,
5-65.degree. C.). As such, the ILPT and temperature compensation
equation updating processes in the field can be terminated. In some
embodiments, the ILPT and temperature compensation equation
updating processes can be performed repeatedly in the field during
the life of the HAMR drive, and need not be terminated in the
manner described above.
[0052] In the illustrative example of FIG. 8, the variable X is a
fixed integer of 5.degree. C. In some embodiments, the variable X
can vary depending on how far away a measured operating temperature
is from the temperature range associated with the temperature
compensation equation (e.g., from a mid-point of the temperature
range). For example, X can be reduced from 5.degree. C. to
3.degree. C. for operating temperatures that are Y degrees away
from a mid-point of the temperature range.
[0053] FIG. 9 is a graph showing plots of total current,
I.sub.TOTAL, versus temperature for a conventional laser diode
calibration approach and one that uses a temperature compensation
equation in accordance with embodiments of the present disclosure.
Plot 902 is a linear extrapolation of I.sub.TOTAL values based on
two I.sub.TOTAL values at 20.degree. C. and 50.degree. C. Plot 904
is based on a temperature compensation equation generated and
updated in accordance embodiments of the disclosure. Plot 904
accurately characterizes the non-linearity of I.sub.TOTAL for a
particular HAMR head and medium, resulting in optimal current being
supplied to the laser diode for recording. In contrast, plot 902
poorly characterizes the non-linearity of I.sub.TOTAL, particularly
at low and high operating temperatures, resulting in suboptimal
current being supplied to the laser diode for recording.
[0054] FIG. 10 is a graph showing plots of bit error rate versus
temperature for a conventional laser diode calibration approach and
one that uses a temperature compensation equation in accordance
with embodiments of the present disclosure. Plot 1002 characterizes
the BER resulting from writing data using I.sub.TOTAL values
produced using the conventional laser diode calibration approach
shown FIG. 9 (plot 902). Plot 1004 characterizes the BER resulting
from writing data using I.sub.TOTAL values produced using the
temperature compensation equation approach shown FIG. 9 (plot 904).
FIG. 10 demonstrates that writing data using I.sub.TOTAL values
produced using the temperature compensation equation approach of
the present disclosure results in a significantly better BER when
compared to a conventional laser diode calibration approach.
[0055] FIG. 11 is a block diagram of a system for calibrating a
laser diode of a HAMR head using a temperature compensation
equation methodology in accordance with various embodiments. FIG.
11 shows a portion of a HAMR drive 1100 which includes a slider
1102 upon which a laser diode 1104 is mounted. A photodetector 1106
can be mounted on or in close proximity to the laser diode 1104. A
power supply 1108 is coupled to the laser diode 1104 and provides a
supply current (e.g., I.sub.TOTAL, I.sub.BIAS) to the laser diode
1104. An output of the photodetector 1106 is coupled to the power
supply 1108. The power supply 1108 can adjust the current supplied
to the laser diode 1104 in response to the photodetector
output.
[0056] The slider 1102 includes a number of components including an
optical waveguide 1112 which is optically coupled to the laser
diode 1104 via an optical coupler 1110. The optical waveguide 1112
extends from the optical coupler 1110 to an NFT 1114 situated at
the air bearing surface 1103 proximate the optical waveguide 1112.
In some embodiments, a bolometer 1122 is situated proximate the
optical waveguide 1112. A writer 1116 is situated proximate the NFT
1114. A writer heater 1117 is situated proximate the writer 1116
and configured to thermally actuate the writer 1116 (e.g., writer
pole/NFT). A contact sensor 1118 may be situated proximate the
writer 1116 and NFT 1114 (e.g., at a close point of the writer). A
reader 1120 is shown positioned away from the writer 1116 and
contact sensor 1118. A reader heater 1121 is positioned proximate
the reader 1120 and configured to thermally actuate the reader
1120. A temperature sensor 1140 (e.g., a thermistor or
thermocouple) is provided in the HAMR drive 1100 to measure an
operating temperature of the drive 1100. In some embodiments, the
temperature sensor 1140 is positioned close to the laser diode 1104
or the slider 1102. In other embodiments, the temperature sensor
1140 is positioned away from the slider 1102 but within the
enclosure of the drive 1100.
[0057] The HAMR drive 1100 further includes a controller 1130
(e.g., microprocessor or microcontroller) coupled to a non-volatile
memory 1132. A temperature compensation equation can be stored in
the memory 1132, as well as data associated with ILPT processes.
The controller 1130 is configured to implement the ILPT processes
and update the temperature compensation equation in a manner
discussed hereinabove. For example, the controller 1130 is
configured to implement executable instructions corresponding to
the flow charts and other figures discussed hereinabove.
[0058] The power supply 1108 is also coupled to a preamplifier
1132. The preamplifier 1132 can provide the power supply 1108 with
signals corresponding to I.sub.TOTAL values for setting the total
current supplied to the laser diode 1104 during write operations in
accordance with the temperature compensation equation.
[0059] Systems, devices or methods disclosed herein may include one
or more of the features structures, methods, or combination thereof
described herein. For example, a device or method may be
implemented to include one or more of the features and/or processes
above. It is intended that such device or method need not include
all of the features and/or processes described herein, but may be
implemented to include selected features and/or processes that
provide useful structures and/or functionality. Various
modifications and additions can be made to the disclosed
embodiments discussed above. Accordingly, the scope of the present
disclosure should not be limited by the particular embodiments
described above, but should be defined only by the claims set forth
below and equivalents thereof.
* * * * *