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

A data storage device is disclosed comprising a first head actuated over a first disk surface, wherein the first head comprises a laser configured to heat the first disk surface while writing data to the first disk surface. A write power is applied to the laser when executing a first write operation, and the first write operation is paused when a transient increase in the output power of the laser is detected. In another embodiment, a write-verify of the written data is executed when a transient decrease in the output power of the laser is detected.

BACKGROUND

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.

DETAILED DESCRIPTION

FIG. 1Ashows a data storage device in the form of a disk drive according to an embodiment comprising a first head2actuated over a first disk surface4, wherein the first head2(FIG. 1B) comprises a laser6configured to heat the first disk surface4while writing data to the first disk surface4. The disk drive further comprises control circuitry8configured to execute the flow diagram ofFIG. 1C, wherein a write power is applied to the laser when executing a first write operation (block10). When a transient increase in an output power of the laser is detected during the first write operation (block12), the first write operation is paused (block14).

Any suitable head2may be employed, and in the embodiment ofFIG. 1B, the head2comprises a suitable write element16(e.g., an inductive write coil) and a suitable read element18(e.g., a suitable magnetoresistive element). In addition, any suitable laser6may be employed, such as a suitable laser diode. In one embodiment, the write power applied to the laser6during 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.

In one embodiment, when the write power is applied to the laser6during a write operation, the laser6may 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 laser6to exhibit a mode hop during a write operation may depend on general degradation of the laser6over 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 laser6into an operating region where the calibrated write power results in a mode hop. Accordingly, in one embodiment the control circuitry8ofFIG. 1Adetects 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 circuitry8may recalibrate the write power applied to the laser6when the mode hop persists.

Any suitable technique may be employed to detect when the laser6exhibits a mode hop during a write operation. In one embodiment illustrated inFIG. 2A, the head2may comprise a suitable photodiode20configured to detect the intensity of the laser beam emitted by the laser6. In the example embodiment ofFIG. 2B, the photodiode20may be biased with a voltage such that the current flowing through the photodiode20represents the output power of the laser6, including the output power when a mode hop occurs. In one embodiment, the graph shown inFIG. 2Bmay 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 inFIG. 2Bmay shift during a write operation due to the continuous heating of the laser6and the resulting shift of the mode hop region may overlap with the write power applied to the laser.

FIG. 3Aillustrates 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 ofFIG. 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 laser6to cool sufficiently so that the remainder of the first write operation may be completed without the laser6exhibiting 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 inFIG. 3Abefore 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.

FIG. 3Billustrates 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 inFIG. 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.

FIG. 4shows 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.

FIG. 5is a flow diagram according to an embodiment that expands on the flow diagram ofFIG. 1C, wherein when a transient increase in the output power of the laser is detected (block12) during a first write operation and the first write operation is paused (block14), the control circuitry waits for a cool-down interval (block24) and then executes a second write operation (block26) 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 (block28), the write power for the laser is recalibrated (block30). 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)22of the head shown inFIG. 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.

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.

FIG. 6Ais a flow diagram according to an embodiment wherein when a transient decrease (mode hop down) in the output power of the laser is detected (block36) while executing a first write operation (block34), a write-verify is triggered (block38). When the write-verify is selected for execution (block40), the data written during at least part of the first write operation is read from the disk surface to verify the recoverability (block42). If the data is unrecoverable (block44), then at least part of the first write operation is retried (block46). When a mode hop down is again detected during the retry write operation (block48), the laser write power is recalibrated (block50) such as described above, and at least part of the first write operation is retried (block52).FIG. 6Bis a flow diagram illustrating an alternative embodiment wherein when the write-verify operation fails (block44), the laser write power is recalibrated (block50), and at least part of the first write operation is retried (block52). 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).

FIG. 7Ais a flow diagram according to an embodiment wherein when a mode hop event is detected (block56) while executing a first write operation (block54), a pre-lase parameter of the laser is adjusted (block58). 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 ofFIG. 7Bat least one of a pre-lase power applied to the laser and/or a pre-lase interval may be decreased (block60) in order to decrease the overall heating of the laser during write operations. Also in the embodiment ofFIG. 7B, after adjusting the pre-lase parameter(s), the control circuitry may enable a write-verify mode (block62) 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).

FIG. 8is a flow diagram according to an embodiment wherein when a mode hop event is detected (block66) while executing a first write operation (block64), a fly height of the head is adjusted (block68) such as by decreasing a control signal applied to the FHA22ofFIG. 2Ain 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 (block70). 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).

FIG. 9is a flow diagram according to an embodiment wherein when a mode hop event is detected (block74) while executing a first write operation (block72), at least part of the write operation is retried by writing data to a second disk surface (block76). 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.

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.