Data storage device calibrating laser write power for heat assisted magnetic recording

Description

BACKGROUND

Data storage devices such as disk drives may comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and embedded servo sectors. The embedded servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo controller to control the actuator arm as it seeks from track to track.

Data is typically written to the disk by modulating a write current in an inductive coil to record magnetic transitions onto the disk surface in a process referred to as saturation recording. During readback, the magnetic transitions are sensed by a read element (e.g., a magnetoresistive element) and the resulting read signal demodulated by a suitable read channel. Heat assisted magnetic recording (HAMR) is a recent development that improves the quality of written data by heating the disk surface with a laser during write operations in order to decrease the coercivity of the magnetic medium, thereby enabling the magnetic field generated by the write coil to more readily magnetize the disk surface.

FIG. 1 shows a prior art disk format 2 as comprising a number of servo tracks 4 defined by servo sectors 6₀-6_N recorded around the circumference of each servo track. Each servo sector 6_i comprises a preamble 8 for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 10 for storing a special pattern used to symbol synchronize to a servo data field 12. The servo data field 12 stores coarse head positioning information, such as a servo track address, used to position the head over a target data track during a seek operation. Each servo sector 6_i further comprises groups of servo bursts 14 (e.g., N and Q servo bursts), which are recorded with a predetermined phase relative to one another and relative to the servo track centerlines. The phase based servo bursts 14 provide fine head position information used for centerline tracking while accessing a data track during write/read operations. A position error signal (PES) is generated by reading the servo bursts 14, wherein the PES represents a measured position of the head relative to a centerline of a target servo track. A servo controller processes the PES to generate a control signal applied to a head actuator (e.g., a voice coil motor) in order to actuate the head radially over the disk in a direction that reduces the PES.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a prior art disk format comprising a plurality of servo tracks defined by servo sectors.

FIG. 2A shows a data storage device in the form of a disk drive comprising a head actuated over a disk according to an embodiment.

FIG. 2B shows a head according to an embodiment comprising a laser configured to heat the disk during write operations.

FIG. 2C is a flow diagram according to an embodiment wherein an operating write power applied to the laser is calibrated based on an estimated coefficient C of an error function.

FIG. 3A shows an embodiment wherein a track width metric is generated by maintaining the head at a substantially constant radial location while reading a slanted test pattern according to an embodiment.

FIG. 3B shows an embodiment wherein a track width metric is generated by moving the head at a substantially constant velocity radially over the disk while reading a concentric test pattern according to an embodiment.

FIG. 3C shows an embodiment wherein a read signal generated by reading the test pattern comprises a pulse having a peak amplitude and a full width half maximum (FWHM).

FIG. 4A illustrates the peak amplitude of the read signal versus a ratio of a track width metric (TWM) to the coefficient C of an error function according to an embodiment.

FIG. 4B illustrates the laser power versus the track width metric according to an embodiment.

FIG. 5 is a flow diagram for calibrating the operating write power for the laser by iteratively updating the coefficient C of the error function.

DETAILED DESCRIPTION

FIG. 2A shows a data storage device in the form of a disk drive according to an embodiment comprising a disk 16, and a head 18 actuated over the disk 16, wherein the head 18 (FIG. 2B) comprises a laser 20 configured to heat the disk 16 while writing data to the disk 16. The disk drive further comprises control circuitry 22 configured to execute the flow diagram of FIG. 2C, wherein a first write power is applied to the laser and a first test pattern is written to the disk (block 24). The first test pattern is read while the head crosses over a width of the first test pattern to generate a first read signal (block 26), and a first track width metric and a first peak amplitude are generated based on the first read signal (block 28). A second write power is applied to the laser and a second test pattern is written to the disk (block 30). The second test pattern is read while the head crosses over a width of the second test pattern to generate a second read signal (block 32), and a second track width metric and a second peak amplitude are generated based on the second read signal (block 34). The first track width metric, the first peak amplitude, the second track width metric, and the second peak amplitude are processed to estimate a coefficient C of an error function:
PeakAmplitude=Amax*erf(TrackWidthMetric/C).
An operating write power for the laser is calibrated based on the estimated coefficient C of the error function.

In the embodiment of FIG. 2B, the head 18 comprises a suitable write element 40 (e.g., an inductive write coil), a suitable read element 42 (e.g., a magnetoresistive element), and a suitable fly height actuator (FHA) 44 (e.g., a thermal element). Any suitable laser 20 may also be employed in the head 18, such as a suitable laser diode, together with any other suitable optical components for implementing HAMR, such as a suitable waveguide and a suitable near field transducer for focusing the laser beam onto the surface of the disk 16. In one embodiment, an operating write power applied to the laser 20 during write operations is calibrated in order to achieve sufficient reliability of the recorded data. Applying too low an operating write power may undersaturate the media, whereas too high an operating write power may cause inter-track interference (ITI) due to an excessive track width.

In one embodiment, the write power applied to the laser and the resulting track width written on the disk may be represented by the above error function an example of which is illustrated in FIG. 4A. The error function correlates a track width metric with a peak amplitude of the read signal while reading a test pattern on the disk as the head moves across a width of the test pattern. Both the peak amplitude and track width metric are dependent on the amount of write power applied to the laser. In one embodiment, the above error function may be represented as a Gauss error function:

erf ⁡ ( x ) = 2 π ⁢ ∫ 0 x ⁢ ⅇ - t 2 ⁢ ⁢ ⅆ t
In one embodiment, the operating write power for the laser may be selected based on a target peak amplitude as defined by the error function. For example, in one embodiment the error function such as shown in FIG. 4A may be estimated, and a write power for the laser that corresponds to a target point in the error function may be selected as the operating write power for the laser.

FIG. 3A shows an embodiment wherein a test pattern 46 may be written to the disk 16 while moving the head 18 at a substantially constant velocity radially over the disk 16 resulting in a slanted test pattern. After writing the test pattern 46, the control circuitry 22 may maintain the head 18 at a substantially constant radial location while using the read element 42 to read the test pattern 46. FIG. 3C shows an example pulse that is generated as the read element 42 passes over the test pattern 46, wherein the pulse comprises a peak amplitude and a full width half maximum (FWHM) both of which depend on the physical width of the test pattern 46 as well as the physical width of the read element 42. As the physical width of the test pattern increases (by increasing the write power applied to the laser 20), the peak amplitude of the resulting read signal pulse will eventually saturate as illustrated in FIG. 4A. In one embodiment, the operating write power for the laser may be selected relative to the point in FIG. 4A where the peak amplitude begins to saturate. FIG. 3B illustrates an alternative embodiment for generating the read signal pulse shown in FIG. 3C by moving the read element 42 at a substantially constant velocity across the width of a concentric test pattern 48 (e.g., a test pattern 48 written while maintaining the head 18 at a substantially constant radial location).

In one embodiment, the error function such as shown in FIG. 4A including the coefficient C may be estimated by writing the test pattern to the disk using at least two different write powers applied to the laser, and by generating a corresponding peak amplitude and track width metric of the pulse generated by reading the test pattern. Any suitable track width metric may be generated, wherein in one embodiment the track width metric may be generated by integrating the read signal representing the pulse such as shown in FIG. 3C, and in another embodiment the track width metric may be generated based on a product of the peak amplitude and the FWHM of the pulse. In the example embodiment shown in FIG. 4A, the peak amplitude and track width metric are generated for a test pattern written at three different laser powers. The three initial data points 49 are then used to estimate the above described error function including the coefficient C, for example, using a suitable curve fitting technique.

Once the error function has been estimated, a target peak amplitude may be selected based on the error function, such as the peak amplitude 50 shown in the example of FIG. 4A. From the target peak amplitude there is a corresponding target value 47 in FIG. 4A for the ratio:
TrackWidthMetric/C
and from the estimated coefficient C there is a corresponding target track width metric:
TargetTrackWidthMetric=TargetRatio·C
In one embodiment, the track width metrics generated by writing the test pattern at the different laser write powers are processed to generate a power function representing a relationship between the laser power to the track width metric, an example of which is shown in FIG. 4B. That is, the initial three data points 49 used to estimate the error function shown in FIG. 4A may correspond to three data points 51 in FIG. 4B used to estimate the power function 52 (e.g., based on a liner curve fit). Based on the above target track width metric a corresponding write power Δ1′ for the laser is generated based on the estimated power function 52.

In one embodiment, the initial write power selected for the laser based on the initial estimate of the error function and the power function (FIGS. 4A and 4B) may be used as the operating write power for the laser. In another embodiment, the initial estimate of the error function shown in FIG. 4A and/or the initial estimate of the power function 52 shown in FIG. 4B may not be sufficiently accurate. Accordingly in one embodiment the process of estimating the error function and the power function is iterated until both functions converge to a congruent result.

The flow diagram of FIG. 5 illustrates an example of this embodiment wherein the initial data points for the peak amplitude and track width metric are generated (block 54) for a number of different write powers applied to the laser. An initial estimate of the error function shown in FIG. 4A is generated, including an initial estimate of the coefficient C (block 56), and an initial estimate of the power function shown in FIG. 4B is generated (block 58). A target track width metric is generated based on a target ratio (TWM/C) that corresponds to a target point in the error function shown in FIG. 4A (block 60). A Nth write power corresponding to the target track width metric as determined using the power function of FIG. 4B is applied to the laser (block 62) and an Nth test pattern is written to the disk at the Nth write power (block 64). The Nth test pattern is read to generate an Nth read signal (block 66), and an Nth track width metric and an Nth peak amplitude for the resulting read signal pulse is generated (block 68). The error function shown in FIG. 4A is updated including updating the estimate of the coefficient C (block 70) and the power function shown in FIG. 4B is updated (block 72) by curve fitting all of the data points including the new data point (Nth track width metric and Nth peak amplitude). For example, the estimated power function shown in FIG. 4B may be updated from the initial linear function 52 to an updated linear function 78 based on the initial data points 51 and the new data point Δ1. If the delta update to the error function and/or the power function is less than a threshold, the write power in FIG. 4B that corresponds to the final target track width metric is selected as the operating write power for the laser (block 74). Otherwise the target point in the updated error function of FIG. 4A may be determined, which may correspond to an updated target ratio (TWM/C), and an updated target track width metric (block 76) based on the updated coefficient C:
TargetTrackWidthMetric=TargetRatio·C
The flow diagram of FIG. 5 may then be repeated from block 62 until the error function and the power function converge to a congruent result. In the example shown in FIGS. 4A and 4B, the error function and power function may converge to a congruent result after executing three iterations of the flow diagram of FIG. 5.

In one embodiment, when initially estimating the error function such as shown in FIG. 4A the initial write power levels applied to the laser may be selected to ensure the laser 20 is not damaged as well as ensure the initial data points (e.g., the initial three data points 49) fall on the non-linear segment (the curve) of the error function. This embodiment may help ensure the initial data points provide a sufficiently accurate initial estimate of the error function.

In the embodiment of FIG. 4B, the data points may be curve fitted based on a linear function when estimating the power function, as well as to updated the power function at block 72 in the flow diagram of FIG. 5. However, in other embodiments the data points may be curve fitted based on a suitable non-linear function, such as a suitable polynomial function. In one embodiment, employing a linear function to estimate the power function in FIG. 4B may help ensure the laser 20 is not damaged due to an excessive write power, whereas employing a more sophisticated non-linear function may decrease the convergence time by reducing the number of iterations in FIG. 5.

Any suitable control circuitry may be employed to implement the flow diagrams in the above embodiments, such as any suitable integrated circuit or circuits. For example, the control circuitry may be implemented within a read channel integrated circuit, or in a component separate from the read channel, such as a disk controller, or certain operations described above may be performed by a read channel and others by a disk controller. In one embodiment, the read channel and disk controller are implemented as separate integrated circuits, and in an alternative embodiment they are fabricated into a single integrated circuit or system on a chip (SOC). In addition, the control circuitry may include a suitable preamp circuit implemented as a separate integrated circuit, integrated into the read channel or disk controller circuit, or integrated into a SOC.

In one embodiment, the control circuitry comprises a microprocessor executing instructions, the instructions being operable to cause the microprocessor to perform the flow diagrams described herein. The instructions may be stored in any computer-readable medium. In one embodiment, they may be stored on a non-volatile semiconductor memory external to the microprocessor, or integrated with the microprocessor in a SOC. In another embodiment, the instructions are stored on the disk and read into a volatile semiconductor memory when the disk drive is powered on. In yet another embodiment, the control circuitry comprises suitable logic circuitry, such as state machine circuitry.

In various embodiments, a disk drive may include a magnetic disk drive, an optical disk drive, etc. In addition, while the above examples concern a disk drive, the various embodiments are not limited to a disk drive and can be applied to other data storage devices and systems, such as magnetic tape drives, solid state drives, hybrid drives, etc. In addition, some embodiments may include electronic devices such as computing devices, data server devices, media content storage devices, etc. that comprise the storage media and/or control circuitry as described above.

The various features and processes described above may be used independently of one another, or may be combined in various ways. All possible combinations and subcombinations are intended to fall within the scope of this disclosure. In addition, certain method, event or process blocks may be omitted in some implementations. The methods and processes described herein are also not limited to any particular sequence, and the blocks or states relating thereto can be performed in other sequences that are appropriate. For example, described tasks or events may be performed in an order other than that specifically disclosed, or multiple may be combined in a single block or state. The example tasks or events may be performed in serial, in parallel, or in some other manner. Tasks or events may be added to or removed from the disclosed example embodiments. The example systems and components described herein may be configured differently than described. For example, elements may be added to, removed from, or rearranged compared to the disclosed example embodiments.

While certain example embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions disclosed herein. Thus, nothing in the foregoing description is intended to imply that any particular feature, characteristic, step, module, or block is necessary or indispensable. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the embodiments disclosed herein.