Laser mode hopping detection for heat-assisted magnetic recording device
Methods and apparatuses for detecting mode hopping in a laser diode or other optical energy source in heat-assisted magnetic recording. An output power of the laser diode or other optical energy source is measured and the output power is differentiated over time to determine a rate of change. If it is determined that the rate of change exceeds a threshold value, a fault signal is asserted indicating a potential mode hopping event.
1. A method for detecting mode hopping in a laser diode of a storage device, the method comprising steps of:
receiving a time-variant signal from a photodetector optically connected to the laser diode while the laser diode is engaged for writing data to a recording media of the storage device, the time-variant signal indicating a light energy output of the laser diode;
differentiating the signal over time to produce a differentiated signal representing a rate of change in the light energy output of the laser diode;
determining whether the differentiated signal exceeds a threshold value indicating a rapid change in an oscillation wavelength of the laser diode; and
upon determining that the differentiated signal exceeds the threshold value, asserting a fault signal to a controller of the storage device indicating a potential write-fault due to a mode hopping event in the laser diode.
2. The method of claim 1 , wherein the photodetector comprises a bolometer.
3. The method of claim 1 , wherein differentiating the signal over time is performed by a high-pass filter.
4. The method of claim 1 , wherein differentiating the signal over time is performed by an AC-coupled differentiator circuit.
5. The method of claim 1 , wherein determining whether the amplified differentiated signal exceeds a threshold value is performed by a comparator.
6. The method of claim 1 , wherein the steps are performed by a fault logic circuit in a preamplifier of a hard disk drive (“HDD”) device implementing heat-assisted magnetic recording (“HAMR”) technology.
7. A storage device configured to:
measure a voltage output of a photodetector optically connected to a laser diode in an optical heating element of a read/write head configured for heat-assisted magnetic recording (“HAMR”) during a write operation of data to a recording media of the storage device, the voltage output representing a light energy output of the laser diode;
determine a rate of change in the voltage output of the photodetector over time;
determine whether the rate of change in the voltage output of the photodetector exceeds a threshold value indicating a rapid change in an oscillation wavelength of the laser diode; and
if the rate of change of the voltage output of the photodetector exceeds the threshold value, assert a fault signal to a controller of the storage device indicating a potential write fault.
8. The storage device of claim 7 , wherein determining the rate of change of the voltage output of the photodetector over time is performed by a differentiation circuit.
9. The storage device of claim 8 , wherein the differentiation circuit comprises a high-pass filter.
10. The storage device of claim 8 , wherein the photodetector comprises a bolometer.
11. The storage device of claim 8 , wherein an output of the differentiation circuit is compared to a threshold voltage level corresponding to the threshold value in a comparator to determine if the rate of change of the voltage output of the photodetector exceeds the threshold value.
12. The storage device of claim 8 , wherein the controller is configured to, upon assertion of the fault signal, perform error recovery to avoid loss of the data.
13. The storage device of claim 8 , wherein the measuring, determining, and asserting are performed by a preamplifier component of the storage device.
14. A preamplifier for a hard disk drive (“HDD”) implementing heat-assisted magnetic recording (“HAMR”), the preamplifier comprising:
a differentiator circuit configured to receive a signal from a photodetector and produce a differentiated signal representing a rate of change in light energy output of a laser diode at any given time, the photodetector optically coupled to the laser diode in an optical heating element of a read/write head in the HDD; and
a threshold detector configured to determine if the differentiated signal exceeds a threshold voltage level indicating a rapid change in an oscillation wavelength of the laser diode, and upon determining that the differentiated signal exceeds the threshold voltage level, assert a fault signal to a controller of the HDD.
15. The preamplifier of claim 14 , further comprising an amplifier configured to amplify the differentiated signal.
16. The preamplifier of claim 14 , further comprising a low pass filter to filter out high-frequency noise in the signal from the photodetector.
17. The preamplifier of claim 14 , further comprising a rectifier for rectifying the differentiated signal.
18. The preamplifier of claim 14 , wherein differentiator circuit comprises a high-pass filter configured in a programmable band-pass filter.