Data storage device updating laser power during non-write mode 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 servo tracks defined by servo sectors.

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

FIG. 2B shows a head according to an embodiment comprising a laser configured the heat the disk while writing data to the disk.

FIG. 2C is a flow diagram according to an embodiment wherein while writing data to the disk, an updated power setting for the laser is stored in a staging register, and during a non-write mode, the updated power setting is transferred from the staging register to an operating register in order to update an operating power for the laser.

FIG. 2D shows control circuitry according to an embodiment comprising a staging register and an operating register for storing the power setting for a laser driver.

FIG. 3 shows an embodiment wherein the control circuitry comprises a controller and a preamp comprising the staging register and operating register.

FIG. 4A shows an embodiment wherein the non-write mode for transferring the power setting from the staging register to the operating register comprises an interval when the head is passing over an inter-sector gap during a write operation.

FIG. 4B shows an embodiment wherein the non-write mode for transferring the power setting from the staging register to the operating register comprises an interval when the head is reading a servo sector on the disk.

DETAILED DESCRIPTION

FIG. 2A shows a data storage device in the form of a disk drive according to an embodiment comprising a head 16 actuated over a disk 18, wherein the head 16 (FIG. 2B) comprises a laser 20 configured to heat the disk 18 while writing data to the disk. The disk drive further comprises control circuitry 22 configured to execute the flow diagram of FIG. 2C, wherein while writing data to the disk, an updated power setting for the laser is stored in a staging register (block 24), and during a non-write mode, the updated power setting is transferred from the staging register to an operating register in order to update an operating power for the laser (block 26).

In the embodiment of FIG. 2A, servo sectors 28₀-28_N define a plurality of servo tracks 30, wherein data tracks are defined relative to the servo tracks at the same or different radial density. In an embodiment where the servo sectors 28₀-28_N are recorded at the same data rate, the servo sectors 28₀-28_N form servo wedges that extend radially across the disk 18 as shown in FIG. 2A. Other embodiments may employ zoned servo sectors wherein the data rate may vary across the radius of the disk, thereby forming servo wedges within each servo zone. The control circuitry 22 processes a read signal 32 emanating from the head 16 to demodulate the servo sectors 28₀-28_N and generate a position error signal (PES) representing an error between the actual position of the head and a target position relative to a target track. The control circuitry 22 filters the PES using a suitable compensation filter to generate a control signal 34 applied to a voice coil motor (VCM) 36 which rotates an actuator arm 38 about a pivot in order to actuate the head 16 radially over the disk 18 in a direction that reduces the PES. The servo sectors 28₀-28_N may comprise any suitable head position information, such as a track address for coarse positioning and servo bursts for fine positioning. The servo bursts may comprise any suitable pattern, such as an amplitude based servo pattern or a phase based servo pattern (e.g., as shown in FIG. 1).

In the embodiment of FIG. 2B, the head 16 comprises a suitable write element 40 (e.g., an inductive coil), a suitable read element 42 (e.g., a magnetoresistive element), and a suitable fly height actuator (FHA) 44 configured to actuate the head 16 vertically over the disk 18. Any suitable FHA 44 may be employed, such as a heater that actuates through thermal expansion, or a piezoelectric actuator that actuates through mechanical deflection. The head 16 may also comprise other suitable optical components associated with the laser 20, such as a near field transducer configured to focus the laser onto the surface of the disk.

During a write operation, the output power of the laser may vary (e.g., decrease) due to the self heating effect of the laser. Accordingly, in one embodiment it may be desirable to adjust the laser power during a write operation in order to compensate for the variations in output power and thereby maintain a target recording quality. However, if the laser power is adjusted while writing to a data sector on the disk, the resulting transient in the recorded data (e.g., amplitude and/or phase shift) may render the data sector unrecoverable. FIG. 2D illustrates an embodiment that avoids this transient in the recorded data while enabling the laser power to be adjusted during a write operation. An updated power setting is first stored in a staging register 46 while writing data to the disk (e.g., while writing to a data sector). When the disk drive enters a non-write mode during the write operation, the updated power setting is transferred from the staging register 46 to an operating register 48, wherein the operating register 48 configures a laser driver 50 to apply a power (e.g., via a current) to the laser 20. In this manner, the laser power is updated during a non-write mode of a write operation so as to avoid the transient that would otherwise occur in the written data.

In one embodiment shown in FIG. 3, the control circuitry 22 of FIG. 2A comprises a suitable controller IC 52 (e.g., comprising a microprocessor) and a preamp IC 54 comprising the staging register 46, operating register 48, and laser driver 50. The controller 52 may communicate with the preamp 54 (e.g., over a serial interface) in order to program the registers of the preamp 54, including to program the staging register 46 with an updated power setting. In one embodiment, the controller 52 may program the staging register 46 during a write operation while the preamp 54 is concurrently writing data to a data sector using the current power setting stored in the operating register 48. In one embodiment, the controller 52 may apply a control signal to the preamp 54 to indicate that the disk drive has entered a non-write mode during the write operation, wherein the preamp 54 may react to this control signal by transferring the updated power setting from the staging register 46 to the operating register 48.

The disk drive may enter a non-write mode during a write operation for any suitable reason, thereby enabling the transfer of the updated power setting from the staging register 46 to the operating register 48. FIG. 4A shows an embodiment wherein the disk drive may enter a non-write mode during a write operation while the head is over an inter-sector gap between data sectors. FIG. 4B shows an embodiment wherein the disk drive may enter a non-write mode during a write operation while the head is reading a servo sector. In one embodiment, the updated power setting stored in the staging register 46 may be transferred to the operating register 48 near the beginning of the non-write mode so that the transient of the laser driver 50 and laser 20 settles out before continuing to write data to the disk (e.g., in the embodiment of FIG. 4A where the laser 20 may remain powered during the inter-sector gap even though user data is not being written to the disk). In one embodiment, the non-write mode may include the interval when writing the preamble of a data sector since a transient in the preamble may not render the data sector unrecoverable. In general, in one embodiment a non-write mode may refer to an interval when user data is not being written to the disk.

The control circuitry 22 may determine in any suitable manner that the power setting for the laser 20 needs updating during a write operation. In one embodiment, the control circuitry 22 may update the power setting based on an open loop power setting profile that may vary depending on the length of the write operation. That is, the control circuitry 22 may calibrate a power setting profile for the laser by evaluating the laser's output power during a calibration operation. The laser's output power may be measured in any suitable manner, such as by writing a periodic pattern (e.g., 2T pattern) to a test track and measuring the amplitude of the resulting read signal while reading the track. In another embodiment, the head 16 may comprise a suitable photodiode configured to measure the output power of the laser during a write operation. In the embodiment wherein the head 16 comprises a photodiode, the control circuitry 22 may update the power setting for the laser using a feedback loop during a write operation. That is, the control circuitry 22 may compare the laser's output power as measured by the photodiode to a target power, and generate an updated power setting based on the difference (error) during a write operation.

In one embodiment, the power setting stored in the staging register 46 and the operating register 48 may comprise multiple components that may be updated individually. That is, the staging register 46 and the operating register 48 may each comprise two or more registers that may be updated individually. For example, in one embodiment the power setting for the laser may comprise a lasing setting that corresponds to a lasing threshold for the laser, and an offset setting that is added to the lasing setting. In one embodiment, the lasing setting may be updated to account for a change in the lasing threshold due, for example, to a change in the ambient temperature or due to the laser degrading over time. In one embodiment, the control circuitry 22 may update only the laser power offset setting during a write operation since the lasing threshold typically remains unchanged during the relatively short interval of a write operation.

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.