Abstract:
A heat-assisted magnetic recording system may include, but is not limited to: at least one magnetic recording read/write head; at least one laser diode configured to illuminate at least a portion of at least one magnetic recording medium; at least one laser power level sensor configured to detect a power level of the at least one laser diode; and a controller configured to modify one or more power level settings associated with the at least one laser diode in response to one or more output signals of the at least one laser power level sensor.

Description:
BACKGROUND 
     Heat-assisted magnetic recording (HAMR) is a technology that magnetically records data on high-stability media using laser thermal assistance to first heat the material. HAMR takes advantage of high-stability magnetic compounds such as iron platinum alloy. These materials can store single bits in a much smaller area without being limited by the same superparamagnetic effect that limits the current technology used in hard disk storage. 
     High-density digital recording requires small grain size hence increased susceptibility to thermally-induced paramagnetism and decay of written information. Increasing the ferromagnetic anisotropy of the medium may reduce the paramagnetic phenomena but may require excessive write fields. 
     Heat-Assisted Magnetic Recording (HAMR) may resolve the writeability versus longevity dilemma by using a beam from a laser diode to raise the temperature of the medium in the vicinity of a written transition to near the Curie point, allowing the writer to switch the medium. 
     SUMMARY 
     The present invention proposes means to implement a sample-data approach to laser power monitoring in HAMR. A HAMR system may include, but is not limited to: at least one magnetic recording read/write head; at least one laser diode configured to illuminate at least a portion of at least one magnetic recording medium; at least one laser power level sensor configured to detect a power level of the at least one laser diode; and a controller configured to modify one or more power level settings associated with the at least one laser diode in response to one or more output signals of the at least one laser power level sensor. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The numerous advantages of the disclosure may be better understood by those skilled in the art by referencing the accompanying figures in which: 
         FIG. 1  shows a head/slider assembly and its relation to a recording medium in a HAMR system; 
         FIG. 2  shows a pre-amplifier chip containing elements employed in a HAMR system; 
         FIG. 3  shows an embodiment of a HAMR system; 
         FIG. 4  shows waveforms representative of the operations of HAMR system; 
         FIG. 5  shows a transfer function of a laser diode employed in a HAMR system; and 
         FIG. 6  shows an embodiment of a HAMR system. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIGS. 1 and 2 , an exemplary embodiment of a HAMR system  100  implementation is shown. A carrier/slider unit  101  may contain at least one magnetoresistive read head  102  and at least one inductive write head  103 . The slider unit  101  may further include a laser diode  104 , an optical waveguide  105  and a plasmonic resonator  106 . The waveguide  105  and the plasmonic resonator  106  may collimate the laser illumination  107  onto a confined region of medium  108  below the write head  103 , thus locally heating the medium  108  to near the Curie temperature and enabling its switching by the write head  103 . As a result of collocation, high laser power and poor light transduction-to-medium efficiency (e.g. ˜100 mW and &lt;1%, respectively) may lead to heating of the slider unit  101 , which in turn may affect laser power emitted at a given current for the laser diode  104 . 
     During a write process, the laser diode  104  may either continuously illuminate a spot on the medium  108  beneath the write head  103  or may be pulsed (e.g. one pulse per bit-write time). 
     The slider unit  101  may further include a photodiode  109  configured to monitor the operation of the laser diode  104  in order to allow for optimization of the laser diode  104 . A fraction of the laser light generated by the laser diode  104  may be directed onto the photodiode  109 , for example, by use of a semi-transmissive back-facet on the resonant cavity of the laser diode  104 . 
     The HAMR system  100  may sample the photodiode  109  signal during intervals when no toggling of write head  103  driver signals or laser diode  104  driver signals are present. Although the present description assumes that the laser diode  104  is pulsed during a write operation in sympathy with data bit cells (e.g. a “pulse write”), the HAMR system  100  is also applicable to “continuous write” systems in which the laser diode  104  is remains on during course of data writing. 
     The photodiode  109  may be periodically activated only during the (read-only) servo bursts or elsewhere in read data. During activation, the laser diode  104  may be illuminated to a calibration power sufficiently low as not to cause long-term loss of servo data. Alternatively, laser calibration could occur over read data. Although described in reference to the photodiode  109 , it is possible that other power/temperature monitoring means could be employed (e.g. the ability to sense temperature-induced change in the resistance of the read head  102  or a use of a dedicated bolometer). 
     Referring to  FIG. 2  a schematic representation of a pre-amplifier  110  configured for control of the various components of the slider unit  101  is shown. The pre-amplifier  110  may include a write driver  111  configured to regulate the operation of the write head  103 , a read amplifier  112  configured to receive read signals from the read head  102 , a laser driver  113  configured to regulate operation of the laser diode  104  and a photodiode channel  114  configured to monitor operation of the photodiode  109 . 
     All elements on the slider unit  101  may be served by a common flex-on-suspension (FOS)  115 . Because write and read operations may be mutually exclusive, crosstalk from high-level writer signals into the pre-amplifier  110  front end may not pose problems. However, as conductors on the FOS  115  may be in close proximity, and generally cannot be provided with isolating shielding, the low-level photodiode  109  signal may be subject to high crosstalk levels from the high-level laser diode  104  drive and write head  103  drive signals. 
     The laser power in the HAMR system  100  may be regulated using sample-data closed-loop feedback from the photodiode  109  coupled to the laser diode  104 . Use of a sample-data approach may assure that the FOS  115  transmission-line environment is quiet when the photodiode  109  is active. During photodiode  109  activation, the magnetic head and (in pulsed-laser mode) laser diode drivers may be dormant and their associated conductors of the FOS  115  may be static. 
     Referring to  FIG. 3 , a block diagram of an exemplary embodiment of the HAMR system  100  is shown. In this embodiment, the power-regulation loop-closure may occur in microcode or in dedicated hardware in a system controller  116 . As described above, the pre-amplifier  110  may include the laser driver  113 . The pre-amplifier  110  may implement associated laser-current control values: laser threshold current (I THR ), normal pulse current (I pulse ); and a calibration current (I cal ). I THR  specifies the threshold current used for both normal write and calibration operations; I pulse  establishes the incremental pulse-over-I THR  current used in normal write functions. I cal  establishes the incremental laser current to be added to I THR  during measurement of laser diode  104  intensity during loop updates. 
     The laser-current control values may be programmed into associated registers for the laser threshold current (I THR Reg); normal pulse current (I pulse Reg); and calibration current (I cal Reg), respectively. To relieve stringent microcode timing requirements, these registers may be preferably realized in double-buffered form. 
     The photodiode  109  and photodiode channel  114  may be periodically activated during intervals when no writing is occurring, (e.g. during the servo position error signal generation bursts which may be interleaved with data). During such servo bursts, or other suitable calibration regions, the laser diode  104  may be driven with a current equal to I THR +I cal  sufficient to illuminate the laser and cause a measurable photodiode output, yet insufficient to cause long-term deterioration of prewritten servo data. 
     A multi-channel trans-impedance amplifier (TIA) may convert the photodiode  109  current to voltage, which is then held stable for digitization in a track/hold (T/H) circuit which may feed an analog-to-digital convertor (ADC). As disc files commonly employ a multiplicity of heads, the TIA may be configured to serve multiple heads and offer rapid recovery after write operations. The TIA and TH may have sufficiently fast acquisition time to acquire completely the photodiode  109  output level during a short servoburst interval. The held voltage may be subsequently digitized by the ADC, whose conversion process may overlap into the post-servoburst region. A comparatively slow/low cost successive-approximation ADC may thus be employed. 
     Because of wide slider-to-slider transmissivity tolerance differences of the laser diode  104 -to-medium  108  and laser diode  104 -to-photodiode  109  optical paths, photodiode  109  sensitivity, and spread in magnetic properties of the medium  108 , power calibration of the laser diode  104  must be performed in relative fashion. Initially, optimal laser power for each slider unit  101  may determined based on recording performance (e.g. by iterative write/reads with various laser powers). 
     Once an optimal laser current value I pulse  is determined for a specific slider unit  101 , the following laser-power-control procedure may be followed (assumes influence of temperature on laser optical power is dominated by I THR  variation):
         1. Back-off laser current used for calibration by factor (e.g., α˜0.4: I cal =α*I pulse ) and store the resulting I cal  to I cal Reg. Backoff prevents gradual data destruction which would may occur were the full I pulse  current used during calibration cycles.   2. Measure resulting photodiode  109  output with laser current at I THR +I cal . Output is PD setpoint .   3. Throughout a multi-record write, and possibly for some time prior in order to allow for loop settling, periodically illuminate the laser with current I THR +I cal ; sample (e.g. during servoburst) photodiode  109  output, and regulate it to PDsetpoint value by varying I THR . Note that the same I THR  applies to both writing and calibration operations. Microcode may implements the control algorithm. When a photodiode  109  output is ready, microcode operation may be interrupted to perform the control update, which is then written back into the double-buffered I THR Reg register.       

     The above procedure may be refined to include temperature influence on current-to-optical power conversion slope or more elaborate algorithms (e.g. a discrete-time proportional-plus-integral control update where the integral term is be initialized with I THR ). Other sample data loop-closing algorithms may be employed, and are supported by the hardware structure of the invention. 
     Waveforms associated with above described operations of the HAMR system  100  of  FIGS. 1-3  appear in  FIG. 4 . 
       FIG. 5  shows a laser diode electro-optical static transfer function on which is indicated the quantities referred to above. Assertion of an EnableLaser signal activates the laser diode  104  at I THR , supporting rapid light buildup when a LaserWriteGate signal is asserted. During assertion of LaserWriteGate, in pulse mode operation, laser current is pulsed between I THR  and I peak  under influence of LaserWriteData. LaserWriteData pulses once per data-bitcell time, and originates in the controller circuits which may also generate run-lengthencoded write data which may drive the magnetic recording head. In continuous mode, laser current rises to and remains at I THR +I peak  for the duration of assertion of LaserWriteGate. 
     LaserWriteGate is equivalent to the conventional WriteGate, which enables writing in the write head  103 . 
     Referring to  FIG. 6 , a block diagram of an exemplary embodiment of the HAMR system  100  is shown. As shown in  FIG. 6 , a loop closure may occur in the pre-amplifier  110 , and at each sample/calibration instant, full update of laser power may occur. A gated operational transconductance amplifier (OTA) in conjunction with output capacitor C, performs an integral (k/s) operation when the OTA is enabled. When disabled, the capacitor C retains its charge/voltage until the next sample instant, when these variables may be again updated. Loop action trims the threshold current to establish equality between PD setpoint  and measured output of the photodiode  109 . 
     Although the exemplary embodiments of  FIGS. 4 and 6  allocate the loop-closure law to the controller  116  and pre-amplifier  110 , respectively, this partition is not required. 
     In a further implementation of a multi-photodiode (multi-slider) trans-impedance amplifier (TIA), the amplifier may be divided into front-end bias control OTA-C sections. 
     In operation, the (reverse-biased) photodiode  109  may be connected differentially between the sources of two complementary common-gate input stages which feed current mirrors; the mirror outputs may be connected to programmable-value load resistors. The differential voltage developed across the load resistors is buffered and transmitted to a T/H or a gated integrator. 
     It is believed that the present invention and many of its attendant advantages will be understood by the foregoing description. It is also believed that it will be apparent that various changes may be made in the form, construction and arrangement of the components thereof without departing from the scope and spirit of the invention or without sacrificing all of its material advantages. The form herein before described being merely an explanatory embodiment thereof. It may be the intention of the following claims to encompass and include such changes. 
     The foregoing detailed description may include set forth various embodiments of the devices and/or processes via the use of block diagrams, flowcharts, and/or examples. Insofar as such block diagrams, flowcharts, and/or examples contain one or more functions and/or operations, it will be understood by those within the art that each function and/or operation within such block diagrams, flowcharts, or examples may be implemented, individually and/or collectively, by a wide range of hardware, software, firmware, or virtually any combination thereof. In one embodiment, several portions of the subject matter described herein may be implemented via Application Specific Integrated Circuits (ASICs), Field Programmable Gate Arrays (FPGAs), digital signal processors (DSPs), or other integrated formats. However, those skilled in the art will recognize that some aspects of the embodiments disclosed herein, in whole or in part, may be equivalently implemented in integrated circuits, as one or more computer programs running on one or more computers (e.g., as one or more programs running on one or more computer systems), as one or more programs running on one or more processors (e.g., as one or more programs running on one or more microprocessors), as firmware, or as virtually any combination thereof, and that designing the circuitry and/or writing the code for the software and or firmware would be well within the skill of one of skill in the art in light of this disclosure. 
     In addition, those skilled in the art will appreciate that the mechanisms of the subject matter described herein may be capable of being distributed as a program product in a variety of forms, and that an illustrative embodiment of the subject matter described herein applies regardless of the particular type of signal bearing medium used to actually carry out the distribution. Examples of a signal bearing medium include, but may be not limited to, the following: a recordable type medium such as a floppy disk, a hard disk drive, a Compact Disc (CD), a Digital Video Disk (DVD), a digital tape, a computer memory, etc.; and a transmission type medium such as a digital and/or an analog communication medium (e.g., a fiber optic cable, a waveguide, a wired communications link, a wireless communication link (e.g., transmitter, receiver, transmission logic, reception logic, etc.), etc.). 
     Those having skill in the art will recognize that the state of the art has progressed to the point where there may be little distinction left between hardware, software, and/or firmware implementations of aspects of systems; the use of hardware, software, and/or firmware may be generally (but not always, in that in certain contexts the choice between hardware and software may become significant) a design choice representing cost vs. efficiency tradeoffs. Those having skill in the art will appreciate that there may be various vehicles by which processes and/or systems and/or other technologies described herein may be effected (e.g., hardware, software, and/or firmware), and that the preferred vehicle will vary with the context in which the processes and/or systems and/or other technologies may be deployed. For example, if an implementer determines that speed and accuracy may be paramount, the implementer may opt for a mainly hardware and/or firmware vehicle; alternatively, if flexibility may be paramount, the implementer may opt for a mainly software implementation; or, yet again alternatively, the implementer may opt for some combination of hardware, software, and/or firmware. Hence, there may be several possible vehicles by which the processes and/or devices and/or other technologies described herein may be effected, none of which may be inherently superior to the other in that any vehicle to be utilized may be a choice dependent upon the context in which the vehicle will be deployed and the specific concerns (e.g., speed, flexibility, or predictability) of the implementer, any of which may vary. Those skilled in the art will recognize that optical aspects of implementations will typically employ optically oriented hardware, software, and or firmware.