Data storage device modifying write operation when a laser mode hop is detected

[0001] This application is a divisional of U.S. patent application Ser. No. 14/497,285, filed on Sep. 25, 2014, which is hereby incorporated by reference in its entirety.

[0002] 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.

[0003] 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.

[0019] FIG. 1A shows a data storage device in the form of a disk drive according to an embodiment comprising a first head 2 actuated over a first disk surface 4, wherein the first head 2 (FIG. 1B) comprises a laser 6 configured to heat the first disk surface 4 while writing data to the first disk surface 4. The disk drive further comprises control circuitry 8 configured to execute the flow diagram of FIG. 1C, wherein a write power is applied to the laser when executing a first write operation (block 10). When a transient increase in an output power of the laser is detected during the first write operation (block 12), the first write operation is paused (block 14).

[0020] Any suitable head 2 may be employed, and in the embodiment of FIG. 1B, the head 2 comprises a suitable write element 16 (e.g., an inductive write coil) and a suitable read element 18 (e.g., a suitable magnetoresistive element). In addition, any suitable laser 6 may be employed, such as a suitable laser diode. In one embodiment, the write power applied to the laser 6 during write operations is calibrated in order to achieve an acceptable reliability of the recorded data. For example, a write power may be calibrated so as to provide a sufficient saturation of the media as well as a target magnetic write width (track width). Any suitable technique may be employed to calibrate the laser power, such as by writing a test pattern to the disk at different write powers and evaluating a quality metric of the resulting read signal when reading the test pattern.

[0021] In one embodiment, when the write power is applied to the laser 6 during a write operation, the laser 6 may exhibit a transient in output power (mode hop), for example, at a particular write power the laser may split into multiple frequencies, thereby reducing the intensity at the fundamental frequency used to heat the disk surface. If a mode hop down occurs during a write operation, the reliability of the recorded data may decrease since the media may be under saturated, and if a mode hop up occurs, the increased intensity of the laser beam may erase data in adjacent data tracks (intertrack interference). The propensity of the laser 6 to exhibit a mode hop during a write operation may depend on general degradation of the laser 6 over time, as well as environmental conditions such as temperature. For example, an increase in the temperature during normal operation of the disk drive may shift the laser 6 into an operating region where the calibrated write power results in a mode hop. Accordingly, in one embodiment the control circuitry 8 of FIG. 1A detects when a mode hop occurs during a write operation and takes suitable action, such as pausing a write operation when a mode hop up is detected, or triggering a write-verify when a mode hop down is detected (FIG. 2B). In addition, in one embodiment the control circuitry 8 may recalibrate the write power applied to the laser 6 when the mode hop persists.

[0022] Any suitable technique may be employed to detect when the laser 6 exhibits a mode hop during a write operation. In one embodiment illustrated in FIG. 2A, the head 2 may comprise a suitable photodiode 20 configured to detect the intensity of the laser beam emitted by the laser 6. In the example embodiment of FIG. 2B, the photodiode 20 may be biased with a voltage such that the current flowing through the photodiode 20 represents the output power of the laser 6, including the output power when a mode hop occurs. In one embodiment, the graph shown in FIG. 2B may shift left or right as the laser degrades over time, and/or as environmental conditions (e.g., ambient temperature) changes, and therefore the laser power where a mode hop occurs may shift such that it overlaps with the write power applied to the laser. In addition, the graph shown in FIG. 2B may shift during a write operation due to the continuous heating of the laser 6 and the resulting shift of the mode hop region may overlap with the write power applied to the laser.

[0023] FIG. 3A illustrates an embodiment wherein when a mode hop up is detected during a first write operation (during a first revolution of the disk), the first write operation is paused. The power applied to the laser is decreased for a cool-down interval, and after the cool-down interval, the write power is applied to the laser to execute a second write operation. In the example of FIG. 3A, the second write operation completes the first write operation during a second revolution of the disk. In this example, the cool-down interval may allow the laser 6 to cool sufficiently so that the remainder of the first write operation may be completed without the laser 6 exhibiting another mode hop. In another embodiment, a mode hop may be detected multiple times such that the first write operation may be executed in multiple segments over multiple revolutions of the disk until the last segment is successfully written before a mode hop occurs. In one embodiment, the control circuitry may execute at least one read operation during the cool-down interval as shown in FIG. 3A before completing the first write operation. For example, in one embodiment the second part of the first write operation may be processed together with other read operations as part of a rotation position optimization (RPO) algorithm that selects the next access command to execute in order to minimize a seek latency of the head and a rotational latency of the disk.

[0024] FIG. 3B illustrates an embodiment wherein after the cool-down interval the control circuitry may be configured to execute a second write operation that is separate from the first write operation. That is, the RPO algorithm may select a different write operation to execute before selecting the second part of the first write operation. In the example shown in FIG. 3B, the second write operation may be executed during the same revolution as the first part of the first write operation, but in other embodiments the second write operation may be executed during a different revolution of the disk depending on the sequence of access commands selected by the RPO algorithm.

[0025] FIG. 4 shows an embodiment wherein when a transient decrease in the output power of the laser is detected during a first write operation, a write-verify is triggered. The write-verify is then executed during a second revolution of the disk by reading the written data to verify the recoverability. In this manner, when the recording quality decreases due to a transient decrease in the output power of the laser, the write operation continues but the recoverability of the written data is verified. In an embodiment described in greater detail below, when the write-verify fails, at least part of the first write operation is retried, and if the transient decrease in the output power of the laser persists, the write power for the laser is recalibrated.

[0026] FIG. 5 is a flow diagram according to an embodiment that expands on the flow diagram of FIG. 1C, wherein when a transient increase in the output power of the laser is detected (block 12) during a first write operation and the first write operation is paused (block 14), the control circuitry waits for a cool-down interval (block 24) and then executes a second write operation (block 26) which may be a different write operation or a continuation of the first write operation. If the transient increase in the output power of the laser is again detected during the second write operation (block 28), the write power for the laser is recalibrated (block 30). In one embodiment, the write power for the laser may be recalibrated so as to shift operation of the laser away from the mode hop region. For example, the write power may be decreased and a write-verify mode enabled to compensate for the decrease in recording quality. In another embodiment, the write power may be increased and the data of subsequent write operations written to an area of the disk surface having a lower track density (lower tracks per inch (TPI)) to compensate for the increase in the magnetic write width (track width). In another embodiment described below, the fly height of the head may be increased (by adjusting a fly height actuator (FHA) 22 of the head shown in FIG. 2A) so that the write power of the laser may be increased while maintaining the same magnetic write width. In one embodiment, recalibration of the laser write power may be delayed until a certain number transient increases in the output power has been detected, that is, when the mode hop persists due, for example, to a change in the ambient temperature of the disk drive.

[0027] In one embodiment, the control circuitry may recalibrate the laser write power such as described above when mode hop events are being detected and a change in an environmental condition has been detected (e.g., a change in ambient temperature) that is likely causing the mode hop events to occur. When the environmental condition reverts back to a normal level, the control circuitry may again recalibrate the laser write power to achieve better reliability and/or better performance by disabling the write-verify mode.

[0028] FIG. 6A is a flow diagram according to an embodiment wherein when a transient decrease (mode hop down) in the output power of the laser is detected (block 36) while executing a first write operation (block 34), a write-verify is triggered (block 38). When the write-verify is selected for execution (block 40), the data written during at least part of the first write operation is read from the disk surface to verify the recoverability (block 42). If the data is unrecoverable (block 44), then at least part of the first write operation is retried (block 46). When a mode hop down is again detected during the retry write operation (block 48), the laser write power is recalibrated (block 50) such as described above, and at least part of the first write operation is retried (block 52). FIG. 6B is a flow diagram illustrating an alternative embodiment wherein when the write-verify operation fails (block 44), the laser write power is recalibrated (block 50), and at least part of the first write operation is retried (block 52). Other embodiments may recalibrate the laser write power based on other criteria, such as after a predetermined number of mode hop down events are detected, or based on a degree to which a write-verify fails (e.g., a number of unrecoverable data sectors).

[0029] FIG. 7A is a flow diagram according to an embodiment wherein when a mode hop event is detected (block 56) while executing a first write operation (block 54), a pre-lase parameter of the laser is adjusted (block 58). In this embodiment, a pre-lase power is applied to the laser for a pre-lase interval that precedes a write operation in order to “warm up” the laser prior to applying the write power to the laser, thereby improving the recording quality of the data at the beginning of the write operation. However, warming up the laser during a pre-lase interval may also result in a mode hop during the write operation due to the laser temperature exceeding a threshold. Accordingly in one embodiment when a mode hop event is detected (or a series of mode hop events are detected), a pre-lase parameter is adjusted, for example, to reduce the amount of heating during write operations. For example, in the flow diagram of FIG. 7B at least one of a pre-lase power applied to the laser and/or a pre-lase interval may be decreased (block 60) in order to decrease the overall heating of the laser during write operations. Also in the embodiment of FIG. 7B, after adjusting the pre-lase parameter(s), the control circuitry may enable a write-verify mode (block 62) to compensate for the lower write quality, particularly at the beginning of write operations. In one embodiment, the degree to which the pre-lase parameter(s) are adjusted may be proportional to a length of the ensuring write operation (e.g., the longer the write operation the lower the pre-lase power and/or the shorter the pre-lase interval).

[0030] FIG. 8 is a flow diagram according to an embodiment wherein when a mode hop event is detected (block 66) while executing a first write operation (block 64), a fly height of the head is adjusted (block 68) such as by decreasing a control signal applied to the FHA 22 of FIG. 2A in order to increase the fly height. A corresponding adjustment may then be made to the laser write power (e.g., by increasing the write power) during subsequent write operations (block 70). This embodiment may shift the laser into an operating region away from the mode hop region while maintaining a target magnetic write width (track width) and signal-to-noise ratio (SNR).

[0031] FIG. 9 is a flow diagram according to an embodiment wherein when a mode hop event is detected (block 74) while executing a first write operation (block 72), at least part of the write operation is retried by writing data to a second disk surface (block 76). That is, in one embodiment the disk drive comprises a second head actuated over a second disk surface (not shown) that may be used to perform write operations when the first head is exhibiting mode hop events (e.g., due to an increase in the ambient temperature of the disk drive). In one embodiment, the second head may comprise a laser used to heat the second disk surface during write operations, wherein the laser of the second head may have different operating characteristics and therefore not exhibit mode hop events under the same operating conditions as the first head. In yet another embodiment, the second head and second disk surface may be of a non-HAMR type such that the second head does not require a laser and therefore the second head cannot suffer from mode hop events. In one embodiment, the control circuitry may monitor environmental conditions and/or perform test writes to the first disk surface during an idle mode in order to determine when it is safe to again write to the first disk surface.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.