Patent Publication Number: US-2018053525-A1

Title: Optical storage system divider based draw verification

Description:
TECHNICAL FIELD 
     This disclosure relates to techniques for real-time verification of written data in an optical storage system. 
     BACKGROUND 
     Optical recording devices such as optical disk and optical tape drives commonly use an Optical Pickup Unit (OPU) or read/write head to write and retrieve data from associated optical media. Conventional OPUs may utilize different wavelength semiconductor laser diodes with complex beam path optics and electromechanical elements to focus and track the optical beam within one or more preformatted tracks on the medium to write or store the data and subsequently read the data. Data written to the medium with a laser at higher power may be verified in a separate operation or process after writing using a lower laser power, or may be verified during the write operation by another laser or laser beam. The ability to read and verify the data during the write operation may be referred to as Direct Read After Write (DRAW). 
     Current OPUs may use a diffraction grating or similar optics in the laser path to generate three beams from a single laser element including a higher power beam used for reading/writing data and for focusing, and two lower power satellite beams used for tracking. The three beams are focused to three corresponding spots on the surface of the optical storage medium used by the various optical and electromechanical elements of the OPU. In general, the higher power spot is positioned in the center or middle between the two satellite spots. In addition to reading/writing data and focusing, the center spot may also be used for one particular type of tracking operation in some applications. The lower power satellite spots generated from the lower power side beams are typically used for another type of tracking operation for specific types of media. 
     SUMMARY 
     Optical storage systems and methods of performing direct read after write for the same utilize circuitry and/or controllers configured to process data read directly after writing to remove noise introduced by the writing. Because the writing process involves high-frequency writing strategy pulses in the laser&#39;s optical power for creating the crystal phase change on the optical recording layer of the media, the direct read laser power signal from the laser light sensor during the write contains modulation of the written data and the high-frequency writing pulses. Division of the read signal by the writing strategy signal, for example, can cancel out the noise to recover and verify the written data. The delay or bias associated with the signals may be tuned to improve the signal quality associated with the recovered data. 
     In one embodiment, an optical storage system includes an optical head that splits a light beam into a higher power main beam and at least one lower power side beam, and a controller. The controller alters an optical medium via the higher power main beam to write data to the medium while processing a first signal resulting from the at least one lower power side beam being reflected from the medium and a second signal resulting from scatter of the higher power main beam to remove noise from the first signal caused by the higher power main beam to generate output indicative of the data directly after writing. 
     In another embodiment, an optical storage system includes an optical head and controller arrangement that writes data to an optical medium via a higher power main beam, reads, directly after writing, feedback from the medium containing the written data and noise resulting from the higher power main beam, removes the noise from the feedback by dividing the feedback with data indicative of the higher power main beam, and generates output indicative of the written data. 
     In yet another embodiment, a method for performing direct read after write on an optical medium includes splitting a light beam into a higher power main beam and at least one lower power side beam, and writing data to the medium by altering the medium via the higher power main beam. The method also includes, while performing the writing, processing a first signal resulting from at least one of the lower power side beams being reflected from the medium and a second signal resulting from scatter of the higher power main beam to remove noise from the first signal caused by the higher power main beam, and generating output resulting from the processing indicative of the data. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are block diagrams illustrating operation of an example optical data storage system or method with direct read after write (DRAW) capability. 
         FIG. 2  is a block diagram illustrating operation of the optical pickup unit (OPU) of  FIGS. 1A and 1B  having a coherent light beam split or divided into a center beam and two satellite or side beams to provide DRAW capability. 
         FIGS. 3A through 3C  are diagrams illustrating components associated with RF and FM signal wave forms, and the result of their division. 
         FIG. 4  is another block diagram illustrating operation of the example optical data storage system of  FIGS. 1A and 1B . 
         FIG. 5  is a block diagram illustrating an example DRAW demodulation circuit. 
         FIG. 6  is a plot comparing, for the same data, a read signal generated during a read operation (top waveform) and a DRAW division output signal generated by the DRAW demodulation circuit of  FIG. 5  during a write operation (bottom waveform). 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments of the present disclosure are described herein. However, the disclosed embodiments are merely exemplary and other embodiments may take various and alternative forms that are not explicitly illustrated or described. The Figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one of ordinary skill in the art to variously employ the present invention. As those of ordinary skill in the art will understand, various features illustrated and described with reference to any one of the Figures may be combined with features illustrated in one or more other Figures to produce embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. However, various combinations and modifications of the features consistent with the teachings of this disclosure may be desired for particular applications or implementations. 
     The processes, methods, logic, or strategies disclosed may be deliverable to and/or implemented by a processing device, controller, or computer, which may include any existing programmable electronic control unit or dedicated electronic control unit. Similarly, the processes, methods, logic, or strategies may be stored as data and instructions executable by a controller or computer in many forms including, but not limited to, information permanently stored on various types of articles of manufacture that may include persistent non-writable storage media such as ROM devices, as well as information alterably stored on writeable storage media such as floppy disks, magnetic tapes, CDs, RAM devices, and other magnetic and optical media. The processes, methods, logic, or strategies may also be implemented in a software executable object. Alternatively, they may be embodied in whole or in part using suitable hardware components, such as Application Specific Integrated Circuits (ASICs), Field-Programmable Gate Arrays (FPGAs), state machines, controllers or other hardware components or devices, or a combination of hardware, software and firmware components. 
     Referring now to  FIGS. 1A and 1B , block diagrams illustrating operation of an example optical data storage system or method with direct read after write (DRAW) capability are shown.  FIG. 1A  is a side view diagram and  FIG. 1B  is a top or plan view diagram. In this embodiment, optical data storage system  10  is an optical tape drive  12  that receives an optical data storage medium  14 , which is an optical tape  16 . While illustrated and described with reference to an optical tape drive, those of ordinary skill in the art will recognize that the teachings of the present disclosure may also be applied to various other types of optical data storage devices that may use various types of write-once or re-writable optical media, such as optical discs. The optical tape  16  is a ½ inch (12.7 mm) wide tape having a plurality of tracks  36  generally extending across the width of the tape and may vary in length depending on the desired storage capacity and performance characteristics as illustrated and described in greater detail herein. Other tape configurations and dimensions, however, are also possible. The optical tape  16  may be wound on an associated spool  17  contained within a protective case or cartridge  18  that is manually or automatically loaded or mounted in the optical tape drive  12 . Transport mechanism  24  moves the optical tape  16  through a carriage and past at least one optical pickup unit (OPU) or optical head  20  to a take-up spool  22  that typically remains within the tape drive  12 . The OPU  20  writes data to, and reads data from, the optical tape  16  as the transport mechanism  24  moves the optical tape  16  between the cartridge  18  and take-up spool  22  in response to at least one controller and associated electronics  26 . As explained in greater detail below, data may be read/written to the optical tape  16  in one or more of the tracks  36  in a serpentine fashion as the tape travels in either direction past the OPU  20 , i.e., either from the cartridge  18  to the take-up spool  22 , or from the take-up spool  22  to the cartridge  18 . 
     The optical head  20  may include associated optics and related electromechanical servo controlled devices, represented generally by reference numeral  30 , that split or divide a light beam, such as a laser beam, into two or more beams that are focused to corresponding spots on the storage medium  16  for reading/writing data as illustrated and described in greater detail with reference to  FIG. 2 . Various servo mechanisms (not specifically illustrated) may be used to position/align the beams with a selected one of the tracks  36  on the optical tape  16 . 
       FIG. 2  is a block diagram illustrating operation of the optical pickup unit (OPU)  20  having a coherent light beam split or divided into a center beam  40  and two satellite or side beams  44 ,  48  to provide DRAW capability. The beams  40 ,  44 ,  48  may be generated by a single or common coherent light source, such as a laser diode, for example. The source beam travels through associated optics, that may include a diffraction grating, for example, to divide or split the source beam into the center beam  40 , first side beam  44 , and second side beam  48  and to focus the beams to corresponding spots  50 ,  54 , and  58 , respectively, on the surface of the optical tape  16  within a selected one of the tracks  36 . The three optical spots  50 ,  54 , and  58  are manipulated by various optical and electrometrical elements of the OPU  20  to write and retrieve data from the optical tape  16 . 
     The optical elements used to split the source beam and focus the resulting beams to the spots  50 ,  54 , and  58  may be designed to provide higher power to the center beam  40  and center spot  50  with lower power to the side beams  44 ,  48  and associated spots  54 ,  58 . For example, the center spot  40  may contain about  90 % of the source beam power with the side beams  44 ,  48  dividing the remaining  10 % of the source beam power. The center beam  40  is modulated by the OPU  20  to generate write marks  60  during writing of data to the optical tape  16 , which may require about ten times more average power than to read previously stored data (such as about 7 mW to write data and about 0.3 mW to read data, for example). As such, if the source beam is modulated and produces sufficient power for writing data using the center beam/spot  40 / 50 , the side beams  44 ,  48  will be modulated in a like manner but will contain insufficient power to alter the tape  16 . 
     In this embodiment, the spots  50 ,  54 , and  58  are mechanically aligned in the OPU manufacturing process to correspond to the axes of the data tracks  36 . In addition, the satellite spots  54 ,  58  are generally symmetrically positioned relative to the center spot  50  so that transit distance (d) of the tape  16  between the center spot  50  and either of the satellite/side spots  54 ,  58  is substantially the same. Other embodiments may include a distance (d) of between about 10-20 μm—although other distances are also contemplated. 
     Certain conventional optical storage devices use the center spot  50  from the higher power emitting beam  40  for reading, writing, and focusing in addition to one type of tracking operation. The satellite spots  54 ,  58  formed by the lower power side beams  44 ,  48  are used for another type of tracking for specific types of media. In these applications, the side spots  54 ,  58  may not be aligned with one another, or with the center spot  50  along a single one of the tracks  36 . 
     As previously described, the source laser beam is operated at a higher power (relative to operation during a data read/retrieval) and modulated to write the data marks  60  on a selected one of the tracks  36  on the optical tape medium  16 . However, only the center beam  40  emits enough power to the optical tape  16  to actually alter the structure of the optically active layer. The satellite beams  44 ,  48 , having much lower power as determined by the diffraction grating power distribution, do not alter the tape  16 . They, however, have enough power after being reflected from the optical tape  16  to detect the data marks  60 . Therefore depending on the direction of travel of the optical tape  16 , the reflection from one or both of the associated satellite spots  54 ,  58  can be detected by the OPU  20  and used to verify the data marks  60  directly after being written by the main beam/spot  40 / 50  to provide DRAW operation. While the reflected beam associated with one of the satellite beams  44 ,  48  (depending on the direction of travel of the tape  16 ) contains information associated with the data marks  60  on the tape medium  16 , the reflected beam is heavily contaminated by the modulation of the center beam  40  and other noise sources and generally exhibits a very low signal to noise ratio (SNR). 
     Here, some of the DRAW systems and algorithms contemplated use a demodulation/division method to verify written data during the write operation in real-time. For example during the write operation, the written data is decoded (read) from a reflected laser light signal by a high frequency demodulation circuit (divider circuit). Then, the signal quality of the decoded written data can be calculated by a Bit-Error-Rate (BER) detector in order to verify the written data. As a result, the time between data writing and data decoding in this example is less than 1 msec. 
     As mentioned above, data written by the main spot  50  could be read back by one of the satellite spots  54 ,  58  after a few micro seconds. The satellite spots  54 ,  58 , however, only have a fraction of the light intensity of the main spot  50 . Thus, the light intensity signal detected by either one of the satellite spots  54 ,  58  is modulated (distorted) with high-frequency laser pulses used for writing. In order to better decode the written data, the satellite spot reflected laser light intensity signal (referred to as the RF signal) can be demodulated from the main spot writing laser pulse signal (referred to as the FM signal as it can be measured by a laser light front monitor detector) using, for example, a DRAW demodulation circuit in order to reverse the modulation caused by the writing pulsation of the laser diode. Also, frequency responses of the RF signal and FM signal can be matched by applying a matched filter before the demodulation. A filter and high-frequency demodulator, therefore, can be designed for decoding and verifying written data during the write operation. Thus, the demodulation and verification of written data can be in real-time. 
     Certain DRAW circuits contemplated herein require much less calculation cost by using a high frequency demodulator (e.g., analog high speed divider). This enables the operation of DRAW for multiple channels (e.g., 24 channels) simultaneously. Other advantages may include small size, low cost, and high speed for multi-channel designs. 
       FIG. 3A  shows that the RF signal detected by a corresponding RF chip (e.g., a photodetector chip, PDIC, placed at the end of the reflected optical light path of the OPU  20 ) not only contains data associated with the written mark being read but also the writing strategy waveform embodied by the main spot  50  at the time the written mark was being read. That is, the RF signal is subject to noise introduced by the writing strategy waveform.  FIG. 3B  shows that the FM signal detected by a corresponding FM chip (e.g., a front monitor chip, FMIC, placed at the laser light output path of the OPU  20 ) from the scatter associated with the center beam  40  is essentially the writing strategy waveform.  FIG. 3C  shows that the division of the RF signal by the FM signal via a DRAW circuit yields the written mark. 
     A voltage of the RF signal, V RF , can be represented as 
         V   RF   =k   RF   ×φ×R    (1)
 
     where k RF  is a constant associated with the RF chip, φ is the writing strategy modulated light intensity, and R is the changed reflectivity of the medium indicative of a written mark. And, a voltage of the FM signal, V FM , can be represented as 
         V   FM   =k   FM ×φ  (2)
 
     where k FM  is a constant associated with the FM chip. Dividing (1) by (2) yields k×R, where k is k RF /k FM . Because k RF  and k FM  are known, R can be obtained free of influence from V FM . 
       FIG. 4  shows the OPU  20  and an FMIC chip  23  arranged to receive laser light from a laser diode  21 . That is, the FM signal from the FMIC chip  23  represents the direct light output of the laser diode  21  without any modification by the OPU  20  or media  16 . And, a PDIC chip  25  is arranged to receive light reflected from the media  16  and through the OPU  20 . 
       FIG. 5  shows the at least one controller and associated electronics  26  implementing an analog DRAW demodulation circuit  62  to perform the signal division described above. In this example, the circuit  62  includes an RF signal input stage  64 , a direct current (DC) bias  66 , an all-pass delay filter  68 , and a low pass filter  70 . The circuit  62  also includes an FM signal input stage  72 , a low pass filter  74 , a multiplier  76 , an op-amp  78 , an inverter  80 , and a DRAW division output  82 . The signal process flow associated with the RF signal is the input stage  64  to the DC bias  66 , the DC bias  66  to the all-pass delay filter  68 , the all-pass delay filter  68  to the low pass filter  70 , and the low pass filter  70  to the op-amp  78 . The sequence of these elements, however, may be rearranged as necessary. The low pass filter  70 , for example, may come before the DC bias  66 , etc. The signal process flow associated with the FM signal is the input stage  72  to the low pass filter  74 , the low pass filter  74  to the multiplier  76 , and the multiplier  76  to the op-amp  78 . The final leg of the signal process flow is the op-amp  78  to the multiplier  76  and to the inverter  80 , and the inverter  80  to the DRAW division output  82 .  FIG. 5  shows but one example of a demodulator or divider arrangement that includes a multiplier, op-amp, and inverter. Any suitable such arrangement, however, may be used. And although the elements of the DRAW demodulation circuit  62  are shown to be implemented in analog form, they of course may be implemented in digital form. In embodiments that implement at least the RF path in digital form, the all-pass delay filter  68  may take the form of a Farrow structure phase delay interpolator, which may allow for finer delay adjustment relative to other delay operations. 
     To better align the RF and FM signals for division, the DC bias  66  applies a DC bias to the RF signal. In the example of  FIG. 5 , the bias is +1.3 volts. This value, however, may change depending on design considerations, medium configuration, etc. Also due to the differing frequency responses of the RF and FM chips associated with the OPU  20 , the all-pass delay filter  68  applies a delay to the DC biased RF signal for synchronization purposes. In other embodiments, the all-pass delay filter  68  may be in the FM signal path. The frequency associated with the writing strategy can be on the order of  165  megahertz. This value, however, may change with tape speed, writing speed, etc. As such, the low pass filters  70 ,  74  filter out frequency content associated with the RF and FM signals respectively, in this example, greater than  50  megahertz for better performance in the demodulation stage. This value may also change with tape speed, writing speed, writing strategy pattern, etc. A calibration procedure may be performed to select the appropriate bias and delay values prior to operating at run time. 
       FIG. 6  shows the similarity between a read signal generated during a read operation (top waveform) and, for the same data, a DRAW division output signal generated by DRAW division output  82  during a write operation (bottom waveform). 
     While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms encompassed by the claims. The words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the disclosure and claims. As previously described, the features of various embodiments may be combined to form further embodiments that may not be explicitly described or illustrated. While various embodiments may have been described as providing advantages or being preferred over other embodiments or prior art implementations with respect to one or more desired characteristics, those of ordinary skill in the art recognize that one or more features or characteristics may be compromised to achieve desired overall system attributes, which depend on the specific application and implementation. These attributes include, but are not limited to: cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. As such, embodiments described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics are not outside the scope of the disclosure and may be desirable for particular applications.