US20160307590A1 - Adaptive laser output control in a hamr device - Google Patents
Adaptive laser output control in a hamr device Download PDFInfo
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- US20160307590A1 US20160307590A1 US14/688,049 US201514688049A US2016307590A1 US 20160307590 A1 US20160307590 A1 US 20160307590A1 US 201514688049 A US201514688049 A US 201514688049A US 2016307590 A1 US2016307590 A1 US 2016307590A1
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- 238000013500 data storage Methods 0.000 claims abstract description 38
- 238000000034 method Methods 0.000 claims description 43
- 238000005259 measurement Methods 0.000 claims description 7
- 238000010438 heat treatment Methods 0.000 claims description 5
- 238000010586 diagram Methods 0.000 description 12
- 230000003287 optical effect Effects 0.000 description 8
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- 238000004364 calculation method Methods 0.000 description 1
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- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B5/00—Recording by magnetisation or demagnetisation of a record carrier; Reproducing by magnetic means; Record carriers therefor
- G11B5/02—Recording, reproducing, or erasing methods; Read, write or erase circuits therefor
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
- G11B7/12—Heads, e.g. forming of the optical beam spot or modulation of the optical beam
- G11B7/125—Optical beam sources therefor, e.g. laser control circuitry specially adapted for optical storage devices; Modulators, e.g. means for controlling the size or intensity of optical spots or optical traces
- G11B7/126—Circuits, methods or arrangements for laser control or stabilisation
- G11B7/1267—Power calibration
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B5/00—Recording by magnetisation or demagnetisation of a record carrier; Reproducing by magnetic means; Record carriers therefor
- G11B5/127—Structure or manufacture of heads, e.g. inductive
- G11B5/31—Structure or manufacture of heads, e.g. inductive using thin films
- G11B5/3109—Details
- G11B5/313—Disposition of layers
- G11B5/3133—Disposition of layers including layers not usually being a part of the electromagnetic transducer structure and providing additional features, e.g. for improving heat radiation, reduction of power dissipation, adaptations for measurement or indication of gap depth or other properties of the structure
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- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
- G11B7/12—Heads, e.g. forming of the optical beam spot or modulation of the optical beam
- G11B7/125—Optical beam sources therefor, e.g. laser control circuitry specially adapted for optical storage devices; Modulators, e.g. means for controlling the size or intensity of optical spots or optical traces
- G11B7/126—Circuits, methods or arrangements for laser control or stabilisation
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
- G11B7/12—Heads, e.g. forming of the optical beam spot or modulation of the optical beam
- G11B7/125—Optical beam sources therefor, e.g. laser control circuitry specially adapted for optical storage devices; Modulators, e.g. means for controlling the size or intensity of optical spots or optical traces
- G11B7/126—Circuits, methods or arrangements for laser control or stabilisation
- G11B7/1263—Power control during transducing, e.g. by monitoring
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B5/00—Recording by magnetisation or demagnetisation of a record carrier; Reproducing by magnetic means; Record carriers therefor
- G11B2005/0002—Special dispositions or recording techniques
- G11B2005/0005—Arrangements, methods or circuits
- G11B2005/001—Controlling recording characteristics of record carriers or transducing characteristics of transducers by means not being part of their structure
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B5/00—Recording by magnetisation or demagnetisation of a record carrier; Reproducing by magnetic means; Record carriers therefor
- G11B2005/0002—Special dispositions or recording techniques
- G11B2005/0005—Arrangements, methods or circuits
- G11B2005/0021—Thermally assisted recording using an auxiliary energy source for heating the recording layer locally to assist the magnetization reversal
Definitions
- a method includes measuring a current temperature of a data storage device while in idle.
- the data storage device includes a heat assisted magnetic recording (HAMR) device.
- a threshold laser diode power output of the HAMR device is calculated at the current temperature when there is a change between a previous temperature and the current temperature. It is determined whether there is a change between the currently applied laser diode current and a new laser diode current that produced the calculated threshold laser diode power output.
- the new laser diode current is applied when there is a change in laser current from the currently applied laser diode current to the new laser diode current.
- a data storage device comprising includes a medium, a heat assisted magnetic recording (HARM) device including a laser diode for heating the medium while writing data and control circuitry.
- the control circuitry is configured to measure a current temperature of the data storage device while in idle, calculate a threshold laser diode power output of the HAMR device at the current temperature when there is a change between a previous temperature and the current temperature, determine whether there is a change between the currently applied laser diode current and a new laser diode current that produced the calculated threshold laser diode power output and write the new laser diode current to a preamp when there is a change in laser current from the currently applied laser diode current to the new laser diode current.
- HARM heat assisted magnetic recording
- a method comprising includes measuring a current temperature of a data storage device while in idle.
- the data storage device includes a heat assisted magnetic recording (HAMR) device.
- a threshold laser diode power output of the HAMR device is calculated at the current temperature when there is a change between a previous temperature and the current temperature that is greater than a threshold value. It is determined whether there is a change between the currently applied laser diode current and a new laser diode current that produced the calculated threshold laser diode power output.
- the new laser diode current is written to a preamp when there is a change in laser current from the currently applied laser diode current to the new laser diode current.
- FIG. 1 is a schematic diagram of exemplary components of a data storage device including a head stack assembly and a medium.
- FIG. 2 is a schematic diagram of a side view of a head gimbal assembly (HGA).
- HGA head gimbal assembly
- FIG. 3 illustrates an enlarged diagram of a trailing end of a slider of the HGA illustrated in FIG. 2 .
- FIG. 4 illustrates a graphical representation illustrating the relationship of applied laser current to sensed laser output power at different temperatures.
- FIGS. 5 and 6 illustrate graphical representations illustrating threshold laser power output relative to temperature using a photo diode.
- FIG. 7 is a block diagram illustrating a method of calibrating laser diode current in a HAMR device during preheating of the laser according to one embodiment.
- FIG. 8A is a block diagram illustrating a method of calculating a threshold laser power output according to one embodiment.
- FIG. 8B is a block diagram illustrating a method of calculating a threshold laser power output according to another embodiment.
- FIG. 9 a graphical representation of the method of FIG. 8B .
- FIG. 10 is a graphical representation of bit error rate as a function of temperature to verify the effectiveness of the method illustrated in FIG. 7 .
- the drive performance of a Heat Assisted Magnetic Recording (HAMR) device varies with respect to the drive temperature if the applied laser current is not changed as the temperature changes. That performance variation may be based on the effects of temperature on the laser diode of HAMR device.
- the threshold of laser diode power output increases as data storage device or drive temperature increases. This disclosure describes a method of determining a threshold laser power output in a HAMR device against temperature when the data storage device is in idle and as a background task and compensating for the magnetic performance change caused by the change in threshold laser power output by adjusting the laser current.
- FIG. 1 is a simplified block diagram of an exemplary data storage device 100 that can be used in embodiments described herein.
- Data storage device 100 includes control circuitry 102 , which is used for controlling operations of data storage device 100 with the use of programming stored in memory 104 .
- Control circuitry 102 may be coupled to a buffer 106 through a read/write channel 110 .
- Buffer 106 can temporarily store user data during read and write operations and may include the capability of temporarily storing access operations pending execution by control circuitry 102 .
- Data storage device 100 includes storage medium or magnetic recording medium (i.e., disc) 108 and a suspension 116 supporting a transducer 118 (in this case a HAMR transducer or device) that can read and write data to medium 108 .
- the storage medium 108 is illustrated as being a rotatable disc.
- Data storage device 100 also includes a preamplifier (preamp) 107 for generating a write signal applied to transducer 118 during a write operation, and for amplifying a read signal emanating from transducer 118 during a read operation.
- preamp 107 also includes compensation circuitry 109 .
- Control circuitry 102 executes read and write operations on data storage medium 108 . These read/write operations executed by control circuitry 102 may be performed directly on data storage medium 108 or through read/write channel 110 .
- Read/write channel 110 receives data from control circuitry 102 during a write operation, and provides encoded write data to data storage medium 108 via preamp 107 . During a read operation, read/write channel 110 processes a read signal via preamp 107 in order to detect and decode data recorded on data storage medium 108 . The decoded data is provided to control circuitry 102 and ultimately through an interface 112 to an external host 114 .
- External host 114 contains logic (e.g., a processor) capable of issuing commands to data storage device 100 .
- FIG. 1 illustrates external host 114 as being a single host, data storage device 100 can be connected through interface 112 to multiple hosts. Via interface 112 , data storage device 100 receives data and commands from external host 114 and can provide data to external host 114 based on commands executed by control circuitry 102 .
- FIG. 2 illustrates an enlarged side view of a head gimbal assembly (HGA) 120 illustrating a suspension 116 supporting a slider 122 by a gimbal 124 .
- Slider 122 includes transducer 118 , which is rotatable relative to suspension 116 via gimbal 124 .
- Transducer 118 is located at a trailing edge of slider 122 and is held proximate to surface 109 of medium 108 for reading and writing data.
- Transducer 118 includes a magnetic writer coil (not illustrated), a reader (not illustrated) and an optical near field transducer (NFT) 129 , which is described below.
- NFT optical near field transducer
- a HAMR transducer such as a transducer 118 , uses an energy source to locally heat a small portion of a recording medium to overcome superparamagnetic effects that limit the areal data density of a magnetic medium, such as medium 108 .
- the heating of the medium raises a region of the medium's temperature above a set temperature, allowing for it to be magnetized by a magnetic writer.
- the medium quickly cools as it rotates away from the energy source and therefore magnetically freezes the written pattern for stable, long-term storage of data.
- FIG. 3 illustrates an enlarged diagram of a trailing end of slider 122 .
- HAMR transducer 118 may include optical components, such as an optical wave guide 119 , that direct, concentrate and transform light energy from a laser assembly 126 to heat medium 108 .
- Laser assembly 126 includes a laser diode that receives a current input and applies laser energy onto medium 108 through optical wave guide 119 .
- the HAMR medium hot spot may need to be smaller than the diffraction limit of light.
- NFT optical near field transducer
- Control of the applied laser energy in a HAMR device is essential to performance. If the heat energy imparted to the medium 108 is too low then medium 108 is not sufficiently heated, and the recorded signal is of a poor quality. If the energy is too high, the recorded signal of adjacent tracks may be partially erased. Moreover, the energy can change even if the current of the heat energy is constant. For example, the laser energy for a given laser current varies with temperature and also varies with other effects, such as with laser diode aging or other component aging. For example, as components age, the amount of applied laser current needed to achieve the same degree of media heating may vary.
- laser diode input current may be controlled by a register in preamplifier 107 ( FIG. 1 ).
- Preamplifier 107 contains a digital-to-analog converter (DAC) to convert the programmed register value into an applied current.
- DAC digital-to-analog converter
- the laser energy output from transducer 118 ( FIGS. 2 and 3 ) onto medium 108 can vary. Even if the current to the laser diode is accurate and constant, the power output from the laser diode may not. For example, a forward voltage drop of the laser diode can cause this relationship to vary.
- the preamplifier's applied current may not always be accurate and may also vary. Temperature has a strong effect on all of these variations.
- bit error rate the bit error rate of the written track on the media
- ATI adjacent track interference
- Temperature can be sensed periodically using a thermistor 128 , for example.
- Laser output power can also be sensed in real-time, for example, with a sensor such as a photodiode 127 or, in another embodiment, with a bolometer 131 , which is a detector that changes light into temperature.
- photodiode 127 can be, in one embodiment, part of laser assembly 126 , which can be manufactured on each transducer and can be used to measure the laser power or energy within the recording head.
- the arrow within transducer 118 in FIG. 3 illustrates the path of laser energy through optical wave guide 119 from laser assembly 126 to NFT 129 .
- laser energy emanates from laser assembly 126 and energy from NFT 129 heats a portion of medium 108 .
- bolometer 131 can be coupled to optical wave guide 119 and may also measure laser output power in recording head 118 .
- the laser diode In a HAMR drive, there are three general modes of operation for the laser diode in a HAMR drive. When idle, the diode is fully off or inactive (no applied current). When writing data, the diode is fully on or active with an applied current sufficient to record or erase data to medium 108 . In preparation for writing, the laser diode is partially on or biased with a current insufficient to record or erase data to medium 108 .
- FIG. 4 illustrates a graphical representation 240 illustrating the relationship of applied laser current or laser diode (LD) current (on the x-axis) to sensed laser output power (on the y-axis) at different temperatures as measured during the engineering phase.
- the sensed laser output power is measured by, for example, photodetector 127 or bolometer 131 , and is typically measured in terms of sensor voltage or current.
- Photodetector 127 converts photons to electrons, which in turn lead to a voltage that can be measured by preamp 107 .
- Bolometer 131 measures the power of incident electromagnetic radiation via the heating of the material of optical wave guide 119 with a temperature-dependent electrical resistance.
- the relationship can be, but not limited to, linear, and therefore can be described by equation(s) or tables that model such a relationship. In other embodiments, where the relationship is more complex, curve fitting can be used.
- the threshold laser diode power output changes as a function of temperature. If temperature is increased, threshold laser power output is increased. Therefore, as temperature increases, applied laser diode current should be increased to maintain performance.
- FIGS. 5 and 6 illustrate graphical representations 340 and 440 illustrating threshold laser diode power output relative to temperature using a photodetector.
- the threshold laser power output has a linear relationship with respect to the change in environmental temperature in the data storage device or drive.
- threshold laser power output is greater and therefore so is laser diode current.
- threshold laser power output is lower and therefore laser diode current can be lower.
- FIG. 7 is a block diagram 550 illustrating a method of calibrating (or setting) laser diode current in a HAMR device, such as device 118 , while the laser diode in laser assembly 126 is preheating according to one embodiment.
- the method illustrated in block diagram 550 is performed when data storage device 100 is idle (i.e., when the transducer is not processing commands) and by control circuitry 102 .
- a current temperature is measured.
- the change in temperature between the previous temperature and the current temperature is determined.
- a threshold value is 5 degrees. If the change in temperature is greater than the threshold value, then the method passes to block 558 and threshold laser diode power output is calculated at the current temperature. If the change in temperature is less than the threshold value, then the method passes to the end.
- FIGS. 8A and 8B illustrate two embodiments of this calculation.
- FIG. 8A is a block diagram 670 illustrating a method of calculating threshold laser diode power output at the current temperature according to one embodiment.
- HGA 120 seeks to a reserved track on medium 108 .
- a laser diode current is applied at block 673 and at block 674 laser power output (M COUNT or M 1 ) is measured at the applied laser diode current using, for example, photodetector 127 .
- a new laser diode current that is different from the previously applied laser diode current is applied at block 675 and at block 676 laser power output (M COUNT+1 or M 2 ) is measured at the new laser diode current using, for example, photodetector 127 .
- the method passes to block 678 and M 1 is calculated to be the threshold laser power output. If not, the method passes back to block 674 , the count is increased by one and a new laser diode current is applied that is different from the previous laser diode current.
- M COUNT or M 2 to M COUNT+1 or M 3 This method continues to loop until the slope between two laser diode power outputs with respect to laser diode current is greater than a criteria. When the slope is greater than the criteria, the method passes to 678 and M COUNT is calculated to be the threshold laser power output at the current temperature.
- FIG. 8B is a block diagram 770 illustrating another method of determining a threshold laser diode power output at the current temperature according to another embodiment.
- the method illustrated in FIG. 8B is a four-point fitting method. While the method illustrated in FIG. 8A is more accurate, the method illustrated in FIG. 8B reduces the amount of time it takes to determine a threshold laser power output. Based on a generic profile of laser power output versus laser current with respect to temperature (such as the profile shown in the graphical representation illustrated in FIG. 6 ), a threshold laser diode power output can be estimated.
- base level laser diode currents can be identified (i.e., laser diode currents where laser diode power output is below the estimated threshold laser diode power output) and upper level laser diode currents can be identified (i.e., laser diode currents where laser diode power output is above the estimated threshold laser diode power output).
- HGA 120 seeks to a reserved track on medium 108 and the count is set to one.
- a first base level laser diode current is applied at block 773 and at block 774 laser diode output power is measured (M COUNT or M 1 ) using, for example, photodetector 127 .
- the count is increased by one so that the count is now equal to two.
- the count is increased by one again so that the count is now equal to three. This time at block 776 it is determined that the count is not less than three, so the method passes to block 778 where a first upper level laser diode current is applied.
- laser diode power output is measured (M COUNT or M 3 ) using, for example, photodetector 127 .
- the count is increased by one so that the count is equal to four and at block 781 it is determined whether the count is less than five. Since the count is equal to four, the method passes to block 782 and a second upper level laser diode current is applied that is different from the first upper level laser diode current. The method returns to block 779 where laser output power is measured (M COUNT or M 4 ) using, for example, photodetector 127 .
- the count is increased by one again so that the count is now equal to five. This time at block 776 it is determined that the count is not less than five, so the method passes to block 783 .
- a block 783 and as illustrated in FIG. 9 a linear slope is fitted to M 1 and M 2 .
- a linear slope is fitted to M 3 and M 4 .
- threshold laser diode power output is calculated. The calculated threshold laser diode power output is the point where the linear slopes intersect as is illustrated in FIG. 9 . The point where the linear slopes intersect will also give the laser diode current where threshold laser power output occurs. This is labeled as LDI threshold in FIG. 9 and is the new laser diode current.
- the method in FIG. 7 passes to block 562 .
- the laser diode current that preamp 107 is currently applying to the laser diode is compared to the new laser diode current that produces a threshold laser diode power output calculated in block 558 . If the two values are different, then the method passes to block 564 and the new laser diode current is written to preamp 107 . If there is no change between the two values, then the method passes to the end and no new laser diode current is written to preamp 107 . In this way, the magnetic performance is compensated when a change in threshold laser diode power output is caused by a drive temperature change. This compensation is performed by adjusting the laser diode current set in preamp 107 .
- FIG. 10 is a graphical representation of bit error rate as a function of temperature to verify the effectiveness of the method illustrated in FIG. 7 .
- both the one track and triple track bit error rate (BER) plot drops with a fixed laser diode current as temperature increases.
- the one track and the triple track BER plots remain steady and relatively flat with optimized laser diode current being applied as temperature increases. Steady BER provides better magnetic performance.
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Abstract
Description
- A method includes measuring a current temperature of a data storage device while in idle. The data storage device includes a heat assisted magnetic recording (HAMR) device. A threshold laser diode power output of the HAMR device is calculated at the current temperature when there is a change between a previous temperature and the current temperature. It is determined whether there is a change between the currently applied laser diode current and a new laser diode current that produced the calculated threshold laser diode power output. The new laser diode current is applied when there is a change in laser current from the currently applied laser diode current to the new laser diode current.
- A data storage device comprising includes a medium, a heat assisted magnetic recording (HARM) device including a laser diode for heating the medium while writing data and control circuitry. The control circuitry is configured to measure a current temperature of the data storage device while in idle, calculate a threshold laser diode power output of the HAMR device at the current temperature when there is a change between a previous temperature and the current temperature, determine whether there is a change between the currently applied laser diode current and a new laser diode current that produced the calculated threshold laser diode power output and write the new laser diode current to a preamp when there is a change in laser current from the currently applied laser diode current to the new laser diode current.
- A method comprising includes measuring a current temperature of a data storage device while in idle. The data storage device includes a heat assisted magnetic recording (HAMR) device. A threshold laser diode power output of the HAMR device is calculated at the current temperature when there is a change between a previous temperature and the current temperature that is greater than a threshold value. It is determined whether there is a change between the currently applied laser diode current and a new laser diode current that produced the calculated threshold laser diode power output. The new laser diode current is written to a preamp when there is a change in laser current from the currently applied laser diode current to the new laser diode current.
- This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. The claimed subject matter is not limited to implementations that solve any or all disadvantages noted in the background.
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FIG. 1 is a schematic diagram of exemplary components of a data storage device including a head stack assembly and a medium. -
FIG. 2 is a schematic diagram of a side view of a head gimbal assembly (HGA). -
FIG. 3 illustrates an enlarged diagram of a trailing end of a slider of the HGA illustrated inFIG. 2 . -
FIG. 4 illustrates a graphical representation illustrating the relationship of applied laser current to sensed laser output power at different temperatures. -
FIGS. 5 and 6 illustrate graphical representations illustrating threshold laser power output relative to temperature using a photo diode. -
FIG. 7 is a block diagram illustrating a method of calibrating laser diode current in a HAMR device during preheating of the laser according to one embodiment. -
FIG. 8A is a block diagram illustrating a method of calculating a threshold laser power output according to one embodiment. -
FIG. 8B is a block diagram illustrating a method of calculating a threshold laser power output according to another embodiment. -
FIG. 9 a graphical representation of the method ofFIG. 8B . -
FIG. 10 is a graphical representation of bit error rate as a function of temperature to verify the effectiveness of the method illustrated inFIG. 7 . - The drive performance of a Heat Assisted Magnetic Recording (HAMR) device varies with respect to the drive temperature if the applied laser current is not changed as the temperature changes. That performance variation may be based on the effects of temperature on the laser diode of HAMR device. In general, the threshold of laser diode power output increases as data storage device or drive temperature increases. This disclosure describes a method of determining a threshold laser power output in a HAMR device against temperature when the data storage device is in idle and as a background task and compensating for the magnetic performance change caused by the change in threshold laser power output by adjusting the laser current.
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FIG. 1 is a simplified block diagram of an exemplarydata storage device 100 that can be used in embodiments described herein.Data storage device 100 includescontrol circuitry 102, which is used for controlling operations ofdata storage device 100 with the use of programming stored inmemory 104.Control circuitry 102 may be coupled to abuffer 106 through a read/writechannel 110.Buffer 106 can temporarily store user data during read and write operations and may include the capability of temporarily storing access operations pending execution bycontrol circuitry 102. -
Data storage device 100 includes storage medium or magnetic recording medium (i.e., disc) 108 and asuspension 116 supporting a transducer 118 (in this case a HAMR transducer or device) that can read and write data tomedium 108. In the embodiment illustrated inFIG. 1 , thestorage medium 108 is illustrated as being a rotatable disc.Data storage device 100 also includes a preamplifier (preamp) 107 for generating a write signal applied totransducer 118 during a write operation, and for amplifying a read signal emanating fromtransducer 118 during a read operation. In some embodiments,preamp 107 also includescompensation circuitry 109. -
Control circuitry 102 executes read and write operations ondata storage medium 108. These read/write operations executed bycontrol circuitry 102 may be performed directly ondata storage medium 108 or through read/writechannel 110. Read/writechannel 110 receives data fromcontrol circuitry 102 during a write operation, and provides encoded write data todata storage medium 108 viapreamp 107. During a read operation, read/writechannel 110 processes a read signal viapreamp 107 in order to detect and decode data recorded ondata storage medium 108. The decoded data is provided to controlcircuitry 102 and ultimately through aninterface 112 to anexternal host 114. -
External host 114 contains logic (e.g., a processor) capable of issuing commands todata storage device 100. AlthoughFIG. 1 illustratesexternal host 114 as being a single host,data storage device 100 can be connected throughinterface 112 to multiple hosts. Viainterface 112,data storage device 100 receives data and commands fromexternal host 114 and can provide data toexternal host 114 based on commands executed bycontrol circuitry 102. -
FIG. 2 illustrates an enlarged side view of a head gimbal assembly (HGA) 120 illustrating asuspension 116 supporting aslider 122 by agimbal 124.Slider 122 includestransducer 118, which is rotatable relative tosuspension 116 viagimbal 124.Transducer 118 is located at a trailing edge ofslider 122 and is held proximate tosurface 109 ofmedium 108 for reading and writing data.Transducer 118 includes a magnetic writer coil (not illustrated), a reader (not illustrated) and an optical near field transducer (NFT) 129, which is described below. - A HAMR transducer, such as a
transducer 118, uses an energy source to locally heat a small portion of a recording medium to overcome superparamagnetic effects that limit the areal data density of a magnetic medium, such asmedium 108. The heating of the medium raises a region of the medium's temperature above a set temperature, allowing for it to be magnetized by a magnetic writer. The medium quickly cools as it rotates away from the energy source and therefore magnetically freezes the written pattern for stable, long-term storage of data. -
FIG. 3 illustrates an enlarged diagram of a trailing end ofslider 122.HAMR transducer 118 may include optical components, such as anoptical wave guide 119, that direct, concentrate and transform light energy from alaser assembly 126 toheat medium 108.Laser assembly 126 includes a laser diode that receives a current input and applies laser energy ontomedium 108 throughoptical wave guide 119. The HAMR medium hot spot may need to be smaller than the diffraction limit of light. One way to achieve such small hot spots is to use an optical near field transducer (NFT) 129. - Control of the applied laser energy in a HAMR device is essential to performance. If the heat energy imparted to the
medium 108 is too low thenmedium 108 is not sufficiently heated, and the recorded signal is of a poor quality. If the energy is too high, the recorded signal of adjacent tracks may be partially erased. Moreover, the energy can change even if the current of the heat energy is constant. For example, the laser energy for a given laser current varies with temperature and also varies with other effects, such as with laser diode aging or other component aging. For example, as components age, the amount of applied laser current needed to achieve the same degree of media heating may vary. - In one embodiment, laser diode input current may be controlled by a register in preamplifier 107 (
FIG. 1 ).Preamplifier 107 contains a digital-to-analog converter (DAC) to convert the programmed register value into an applied current. The laser energy output from transducer 118 (FIGS. 2 and 3 ) ontomedium 108 can vary. Even if the current to the laser diode is accurate and constant, the power output from the laser diode may not. For example, a forward voltage drop of the laser diode can cause this relationship to vary. In addition, the preamplifier's applied current may not always be accurate and may also vary. Temperature has a strong effect on all of these variations. - There are two parameters that are critical to drive quality—the bit error rate (BER) of the written track on the media and the degradation imparted to adjacent tracks (adjacent track interference or ATI) by the write operation. Changes in laser power impact both of these parameters. Unfortunately, to perform BER and ATI measurements well, many revolutions of writing are required. In addition, experimentally performing these measurements may cause degradation to the data on adjacent tracks. Therefore, performing BER and ATI measurements are not practical to perform on a frequent basis while the drive is in normal operation.
- Two parameters that can be sensed regularly without performance degradation include temperature and laser output power. Temperature can be sensed periodically using a
thermistor 128, for example. Laser output power can also be sensed in real-time, for example, with a sensor such as aphotodiode 127 or, in another embodiment, with abolometer 131, which is a detector that changes light into temperature. In the embodiment illustrated inFIG. 3 ,photodiode 127 can be, in one embodiment, part oflaser assembly 126, which can be manufactured on each transducer and can be used to measure the laser power or energy within the recording head. The arrow withintransducer 118 inFIG. 3 illustrates the path of laser energy throughoptical wave guide 119 fromlaser assembly 126 toNFT 129. As shown, laser energy emanates fromlaser assembly 126 and energy fromNFT 129 heats a portion ofmedium 108. In an alternative embodiment,bolometer 131 can be coupled tooptical wave guide 119 and may also measure laser output power inrecording head 118. - There are three general modes of operation for the laser diode in a HAMR drive. When idle, the diode is fully off or inactive (no applied current). When writing data, the diode is fully on or active with an applied current sufficient to record or erase data to
medium 108. In preparation for writing, the laser diode is partially on or biased with a current insufficient to record or erase data tomedium 108. -
FIG. 4 illustrates agraphical representation 240 illustrating the relationship of applied laser current or laser diode (LD) current (on the x-axis) to sensed laser output power (on the y-axis) at different temperatures as measured during the engineering phase. The sensed laser output power is measured by, for example,photodetector 127 orbolometer 131, and is typically measured in terms of sensor voltage or current.Photodetector 127 converts photons to electrons, which in turn lead to a voltage that can be measured bypreamp 107.Bolometer 131 measures the power of incident electromagnetic radiation via the heating of the material ofoptical wave guide 119 with a temperature-dependent electrical resistance. As illustrated bygraphical representation 240, the relationship can be, but not limited to, linear, and therefore can be described by equation(s) or tables that model such a relationship. In other embodiments, where the relationship is more complex, curve fitting can be used. As also illustrated inFIG. 4 , the threshold laser diode power output changes as a function of temperature. If temperature is increased, threshold laser power output is increased. Therefore, as temperature increases, applied laser diode current should be increased to maintain performance. -
FIGS. 5 and 6 illustrategraphical representations FIGS. 5 and 6 , the threshold laser power output has a linear relationship with respect to the change in environmental temperature in the data storage device or drive. Ingraph 340, as temperature increases so does the threshold laser power output. Ingraph 440, at higher temperatures, threshold laser power output is greater and therefore so is laser diode current. At lower temperatures, threshold laser power output is lower and therefore laser diode current can be lower. -
FIG. 7 is a block diagram 550 illustrating a method of calibrating (or setting) laser diode current in a HAMR device, such asdevice 118, while the laser diode inlaser assembly 126 is preheating according to one embodiment. In other words, the method illustrated in block diagram 550 is performed whendata storage device 100 is idle (i.e., when the transducer is not processing commands) and bycontrol circuitry 102. - At
block 552, a current temperature is measured. Atblock 554, the change in temperature between the previous temperature and the current temperature is determined. Atblock 556, it is determined whether there is a change in temperature between the previous and the current temperature. More specifically, it is determined whether this change in temperature is greater than a threshold value. One exemplary threshold value is 5 degrees. If the change in temperature is greater than the threshold value, then the method passes to block 558 and threshold laser diode power output is calculated at the current temperature. If the change in temperature is less than the threshold value, then the method passes to the end.FIGS. 8A and 8B illustrate two embodiments of this calculation. -
FIG. 8A is a block diagram 670 illustrating a method of calculating threshold laser diode power output at the current temperature according to one embodiment. Atblock 672,HGA 120 seeks to a reserved track onmedium 108. A laser diode current is applied atblock 673 and atblock 674 laser power output (MCOUNT or M1) is measured at the applied laser diode current using, for example,photodetector 127. A new laser diode current that is different from the previously applied laser diode current is applied atblock 675 and atblock 676 laser power output (MCOUNT+1 or M2) is measured at the new laser diode current using, for example,photodetector 127. - At
block 677, it is determined whether the slope between M1 and M2 is greater than a criteria. If it is, the method passes to block 678 and M1 is calculated to be the threshold laser power output. If not, the method passes back to block 674, the count is increased by one and a new laser diode current is applied that is different from the previous laser diode current. At block MCOUNT or M2 to MCOUNT+1 or M3. This method continues to loop until the slope between two laser diode power outputs with respect to laser diode current is greater than a criteria. When the slope is greater than the criteria, the method passes to 678 and MCOUNT is calculated to be the threshold laser power output at the current temperature. -
FIG. 8B is a block diagram 770 illustrating another method of determining a threshold laser diode power output at the current temperature according to another embodiment. The method illustrated inFIG. 8B is a four-point fitting method. While the method illustrated inFIG. 8A is more accurate, the method illustrated inFIG. 8B reduces the amount of time it takes to determine a threshold laser power output. Based on a generic profile of laser power output versus laser current with respect to temperature (such as the profile shown in the graphical representation illustrated inFIG. 6 ), a threshold laser diode power output can be estimated. Based on this estimation base level laser diode currents can be identified (i.e., laser diode currents where laser diode power output is below the estimated threshold laser diode power output) and upper level laser diode currents can be identified (i.e., laser diode currents where laser diode power output is above the estimated threshold laser diode power output). - At
block 772,HGA 120 seeks to a reserved track onmedium 108 and the count is set to one. A first base level laser diode current is applied atblock 773 and atblock 774 laser diode output power is measured (MCOUNT or M1) using, for example,photodetector 127. Atblock 775, the count is increased by one so that the count is now equal to two. Atblock 776, it is determined whether the count is less than three. Since the count is equal to two, the method passes to block 777 and a second base level laser diode current is applied that is different from the first base level laser diode current. The method returns to block 774 where laser diode power output is measured (MCOUNT or M2) using, for example,photodetector 127. - At
block 775, the count is increased by one again so that the count is now equal to three. This time atblock 776 it is determined that the count is not less than three, so the method passes to block 778 where a first upper level laser diode current is applied. Atblock 779, laser diode power output is measured (MCOUNT or M3) using, for example,photodetector 127. Atblock 780, the count is increased by one so that the count is equal to four and atblock 781 it is determined whether the count is less than five. Since the count is equal to four, the method passes to block 782 and a second upper level laser diode current is applied that is different from the first upper level laser diode current. The method returns to block 779 where laser output power is measured (MCOUNT or M4) using, for example,photodetector 127. - At
block 780, the count is increased by one again so that the count is now equal to five. This time atblock 776 it is determined that the count is not less than five, so the method passes to block 783. Ablock 783 and as illustrated inFIG. 9 , a linear slope is fitted to M1 and M2. Atblock 784 and as illustrated inFIG. 9 , a linear slope is fitted to M3 and M4. Atblock 785 and based on these two linear slopes, threshold laser diode power output is calculated. The calculated threshold laser diode power output is the point where the linear slopes intersect as is illustrated inFIG. 9 . The point where the linear slopes intersect will also give the laser diode current where threshold laser power output occurs. This is labeled as LDI threshold inFIG. 9 and is the new laser diode current. - With reference back to
FIG. 7 , after threshold laser diode power output has been identified atblock 558, which was calculated by the method illustrated inFIG. 8A or by the method illustrated inFIG. 8B , the method inFIG. 7 passes to block 562. Atblock 562, the laser diode current that preamp 107 is currently applying to the laser diode is compared to the new laser diode current that produces a threshold laser diode power output calculated inblock 558. If the two values are different, then the method passes to block 564 and the new laser diode current is written topreamp 107. If there is no change between the two values, then the method passes to the end and no new laser diode current is written topreamp 107. In this way, the magnetic performance is compensated when a change in threshold laser diode power output is caused by a drive temperature change. This compensation is performed by adjusting the laser diode current set inpreamp 107. -
FIG. 10 is a graphical representation of bit error rate as a function of temperature to verify the effectiveness of the method illustrated inFIG. 7 . InFIG. 10 , both the one track and triple track bit error rate (BER) plot drops with a fixed laser diode current as temperature increases. The one track and the triple track BER plots, however, remain steady and relatively flat with optimized laser diode current being applied as temperature increases. Steady BER provides better magnetic performance. - Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.
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US9916849B1 (en) * | 2016-11-09 | 2018-03-13 | Seagate Technology Llc | Method and apparatus for detecting and remediating in response to insufficient or excessive HAMR optical power |
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JP2005317178A (en) * | 2004-03-29 | 2005-11-10 | Sharp Corp | Recording/reproducing device, storage medium, driving method of recording/reproducing device, semiconductor laser life estimation method, program, program storage medium and semiconductor laser |
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US8451696B2 (en) | 2011-10-31 | 2013-05-28 | HGST Netherlands B.V. | Temperature sensor in a thermally assisted magnetic recording head |
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US9916849B1 (en) * | 2016-11-09 | 2018-03-13 | Seagate Technology Llc | Method and apparatus for detecting and remediating in response to insufficient or excessive HAMR optical power |
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