Abstract:
This invention relates to the field of radiant energy information storage devices and provides for a low reading level, an intermediate level and a high writing level of power from the radiation source so as to lower the overall power requirements of the radiation source and increase availability and accuracy of the verification read directly after or during writing, without causing unwanted damage to the information recording media. A circuit is described to accomplish this. The circuit basically employs two power sources, one for reading power, one for writing power and an intermediate (&#34;Pedestal&#34;) circuit to employ a portion of the writing power source&#39;s power to provide the step-up in power to the intermediate level.

Description:
This invention relates to the field of laser power level control, and more particularly to where such control is employed in optical disk storage systems. 
     BACKGROUND OF THE INVENTION 
     Generally, in optical recording, the same beam which is used for reading information from tracks on optical media is also used for recording information on the same media. Where such is the case, two distinctly different power levels are used for reading and writing operations. Usually a very low power reading beam is employed to attenuate concerns over the potential problems involved with reading and re-reading an area in a short period of time, which, if the read beam is strong enough, could result in damage to the disk media surface and loss of recorded information. Depending on the type of media employed such damage could result from any number of potential causes including, unwanted ablation due to successive heating, or other types of changes in the physical structure of the recording substrate or even changes in the protective surface or carrier layers of the recording media. Therefore it is important, especially to applications which may require reading and re-reading of the same track areas, that the read power be very low. A higher power is used to &#34;write&#34; on the media, that is, to cause detectable modulations in its information bearing layer(s). 
     In the implementation of many optical disk storage systems to which this invention is primarily directed, a direct read during write operation is used to ensure that the data has been written correctly. That is, the data written is read immediately to verify it. 
     In an implementation employing media such as that employed in the preferred embodiment reflective surface is removed during writing causing ablation in the media surface and resulting in a nonreflective pit in the information bearing layer. 
     SUMMARY OF THE INVENTION 
     The writing power required to form equal sized holes turns out to be significantly less when the laser is already operating at a &#34;pedestal&#34; level substantially above that of the lowest reading power level. In other words, a step-up in power before stepping up to full writing power with the laser will require less overall power expenditure to form a hole, or write, on the media. That is, a lower &#34;full&#34; power is required to &#34;write&#34; than without using the pedestal level. Unfortunately, this substantial step-up in power before a write, if continued during reading, is likely to damage the disk media surface as mentioned. The inventors herein have designed a circuit to take advantage of the power savings and laser duty cycle savings (thus increasing useful life of the laser) inherent in using a step-up in power (the power pedestal) before stepping up to a full write power. Again, this full write power, when used with the media employed in the preferred embodiment, allows for lower pulsed power to accomplish the same ablation on the disk surface media than would otherwise be required. 
     Maintaining a power pedestal for a short period after the write pulse also has the effect of enhancing the readability of the just-written pit (it is called a &#34;pit&#34; with the media described herein but may be called a modulation in the more general case). The problem of reduced readability may be due to temporary loss of efficiency of the reading photodiode and associated electronics or the fact that the beam is not centered on the pit or modulation immediately after the write pulse or some combination of these or other factors, but whatever the cause, this invention helps to overcome it and enhances verification of what has been written. 
     This invention provides for pulsing a laser with smaller transient power changes than ordinarily required for writing on a data disk. In the implementation described, the injection of a small additional current to the read current on the order of 3 to 10 milliamps raises the read power of the laser by an additional 50% to a little over 1 milliwatt, this being a sufficient &#34;pedestal&#34; level to achieve the results described above in the media used with the preferred embodiment, but different values for the power levels may be required with different media. 
     A circuit illustrative of the preferred embodiment is also described. A further understanding of the features, qualities and advantages of the instant invention will be become readily apparent when the detailed description of the preferred embodiment is read in conjunction with the accompanying drawings taken as limited only by the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a recording media structure for use with the preferred embodiment, illustrating the layered construction of the media, the impingement of a radiation beam upon same, and the formation of data holes in that media structure. 
     FIG. 2 is circuit diagram illustrating one practical implementation of the invention. 
     FIG. 3 is a generalized schematic illustration of an information storage device in which this invention may be used. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The adoption of a read pedestal power before implementation of a write pulse in a laser beam, or the more general implementation of a slightly stepped up power before and after a full power use of the laser beam, may result in uses and advantages not currently understood. The preferred embodiment herein described is expected to be read as illustrative of this teaching only and not limited to the particular media and implementation described by the preferred embodiment. 
     The media described by FIG. 1 is merely for illustration and it should be noted that intensity levels of laser pulses, their duration, size and so forth may vary considerably from those given herein without deviating from the invention. 
     Referring now to FIG. 1 in which the workpiece, or recording media, is generally referred to with the numeral 10, the laser beam at the pictured instant in time is referred to generally with the numeral 20 having a leading edge 21 and trailing edge 22. Arrow 40 indicates the direction of movement of the workpiece media 10. Where the media described herein is employed, this travel is at a rate of approximately 0.3 microns per 60 nanosecond time interval, said 60 nanosecond time interval being equivalent to the ordinary duration of a write pulse such as that required to produce hole 36 in a tellurium or rhodium surface such as layer 32, which is the information-bearing layer in the preferred embodiment. This time period must be larger without the &#34;pedestal&#34; power (or step-up in laser intensity) immediately preceeding the write. As is understood by those in the art, pulse duration, pulse intensity, media or laser movement speed, and media material modulability or deformability are proportionally related in obvious ways. For example, a stronger pulse requires less duration to write, other factors being equal. 
     The clear or translucent substrate layer 38 is usually a glass, the next clear layer 37 may be a photopolymer lacquer and the information bearing layer 32 is a tellurium or rhodium surface, bounded on the side opposite the substrate 38 by air or another substrate layer(not shown). Again, this structure and composition of this media is provided for illustrative purposes only; the invention may be modified to work with other media in ways obvious to those skilled in the art. 
     The longitudinal diameter of hole 36 (from leading edge 36a to trailing edge 36b) produced by the incidence of a write pulsed laser beam of a duration of 60 nanoseconds, with this media, in the preferred embodiment is substantially 1 micron, which is the same longitudinal diameter as laser beam 20 from leading edge 21 to trailing edge 22. 
     Note that it is not fully clear how the process of ablation of the information bearing tellurium or rhodium layer 32 occurs, but empiricly it is clear that a direct read after write does not return the same amount of modulation that a later read would over the same hole (that is, where both the read and the direct read after write laser power levels are the same low level). It is also clear that by adopting the read pedestal taught by this invention and continuing it after a write pulse, a more perceptible modulation in verification signal is produced, rather than the attenuated verification signal which would occur without using such a pedestal directly after the write pulse. This read pedestal power level concept may readily be applied to other media by varying the values for pulse duration and intensity as well as by varying the ratio of the three intensity levels. 
     The use of a read pedestal power level as described herein results in more accurate detection or confirmation of written information on the same pass as the write pass, and does not require more than one radiating beam of radiation. 
     FIG. 3 is provided to illustrate a typical system which could employ the invention. A laser 2 which is controlled by the invention&#39;s circuitry 1, casts a beam 20 through optical path 3 to media 10. Media 10 reflects beam 20 through quarterwave plate 4 in optical path 3 and by beam splitter/mirror 5 onto detector 6 which yields an electrical signal for focusing, tracking, reading and feedback information. 
     To facilitate the description of this invention a circuit diagram of a preferred embodiment is described. 
     Referring now to FIG. 2 in which a circuit diagram (generally referred to with the numeral 11) depicting the configuration employed by the preferred embodiment is shown, and comprises mainly: write current source line (WCS) 35 and read current source line (RCS) 25 with associated current level maintenance circuits (write current source circuit 30 and read current source circuit 20 respectively); write pedestal circuit 40; write pulse control circuit 60; and laser diode 50. Basically, write window line (WW) 45 in pedestal circuit 40 turns the pedestal circuit 40 &#34;off&#34; by supplying current through line 41 during read-only operations, thereby allowing read current circuit 20 to maintain the voltage and current at conductor junction 51. During a write window, a &#34;hi&#34; signal on line 45 disables current flow through line 41, causing a demand for current through line 42 thus increasing current flow from laser diode 50 through junction 51 into line 52, thus turning &#34;on&#34; the pedestal circuit 40. During an actual write pulse, this current flow from laser diode 50 is further increased by switching inputs &#34;a&#34; and &#34;b&#34; to circuit 60, causing current to flow through transistor Qll and not through transistor Q10 as would otherwise be the case, thus providing an additional demand for current from laser diode 50, directly from write current circuit 30 (via line 36 and junction 51). 
     A fuller explanation of the operation of circuit 11 begins with the assumption that the read current source line (RCS) 25 applies a constant level of current drain although in practice the read current source may be varied in response to feedback from the laser&#39;s performance so as to keep the output constant despite temperature or other idiosyncratic variations in a particular unit. This feedback would be responsive to the amount of light produced by the laser diode as measured from the point of its application to the record media and the feedback loop would maintain that intensity level constant by increasing or decreasing the current of line (RCS) 25. Use of a feedback loop to control laser output is well known in the art and is the subject of U.S. Pat. No. 4,122,409, for example and there may be many and various implementations of feedback loop means for the purpose of maintaining a constant laser output level known or obvious to those in the art. Therefore, no particular feedback loop is described. 
     Read current source circuit 20 is thus a current sink or drain which can be adjusted to draw more current when laser diode 50 is not producing the appropriate light intensity for an adequate read beam level, or less when the level produced is too high. 
     Read current circuit 20, employing transistors Q3, and Q4 resistors R4 and R5 and capacitor C2 connected as shown, is known in the art as a current mirror circuit, because the current flowing though one side is always proportional to that flowing through the other side. That is, the voltage at point 23 and point 24 are equivalent and the amount of current flowing through point 21 and 22 are proportional. Adjustment of the values of Resistors R4 and R5 will yield different. proportions of currents. For example, if R4 is 100 ohms, the current on line 24 is 10 milliamps and R5 is 10 ohms, current at line 23 will be 100 milliamps. If the value of R5 were then changed to 11 ohms current on line 23 would then be 90.9 milliamps. 
     The function of the current mirror circuits may be emulated in any number of ways. For example, the substitution of a reliable potentiometer, which is adjustable responsive to feedback determined by laser radiation level would be fine so long as laser output may be maintained at a constant level. 
     Read current source feedback circuitry is not described herein, not being necessary to the understanding of the invention. However, its use is of benefit because output characteristics of the laser may vary over its life, or operating temperature range. 
     Thus, during a read-only operation, the only current drain associated with conductor junction 51 is line 52 from circuit 20, and transistors Q 7  and Q 11  are open circuits. Current flows to write current source circuit 30 through line 36 from the positive voltage tied to the collector of transistor Q10 in write control circuit 60, because of a &#34;hi&#34; at line &#34;b&#34; to the base of transistor Q10 and a &#34;lo&#34; at line &#34;a&#34; to the base of Q11. Also, line 43 in write pedestal circuitry 40 does not provide a current drain because all the current required by line 47 is supplied through line 41 not line 42 when line WW 45 is low. 
     The current level of write current source WCS 35 should be subject to the same type of feedback as RCS 25. This obviates any need to adjust the current level flowing through write pedestal circuitry 40 because its current drain at line 43, is proportional to the current at line 32. Write current circuit 30 is constructed as a current mirror in the preferred embodiment, similarly to read current circuit 20; with one of the transistor pair, Q 2 , having its collector tied to the current input (here line WCS 35, said input also tied to the bases of both transistors (Q 1  and Q 2 ) and via a capacitor (C 3 ) to a current drain (line 39). Here also the emmitters of both transistors (Q 1  and Q 2 ) are linked to the current drain line through a resistor each (R3 to Q2; R 1 , for Q 1 ). Write current circuit 30 also provides its current drain to laser diode 50 from the collector of one of its transistors, here Q 1 . This current drain, however, must be selected by the control circuit 60 to cause a write power level pulse to issue from laser diode 50. 
     The write pedestal circuit 40 uses a portion of the power drain from the write current circuit 30 to provide a pedestal level, or stepped up amount of current drain of the laser diode 50. 
     In the preferred embodiment when it is determined that a write operation is to be performed in an approaching sector, the write window line (WW) 45 is enabled for the expected duration of that sector or for a &#34;write window,&#34; that is, for the period of time during which the radiation beam will be traveling over a track length determined to be that sector. In the preferred embodiment, a determination that a sector will be written in will cause line WW 45 to be &#34;hi&#34; for the given amount of time determined to be a sector length passing time, for example, 4 milliseconds. The actual time during which the laser is at full write power is determined by the amount of time that the write pulse inputs &#34;a&#34; and &#34;b&#34; to control circuit 60 are switched, usually about 60 nanoseconds. The &#34;hi&#34; impulse on line WW 45 to the base of transistor Q12 (a PNP transistor, all other transistors in the preferred embodiment are NPN type) stops the emitter of transistor Q12 from conducting and prevents current flow across line 41. Transistor Q5, therefore, must draw current from line 42 across the emitter-tied-to-base transistor Q7 (acting as a signal diode) from line 43. This additional current drain must be supplied by line 52, thus increasing the current flow across laser diode 50 and therefore, increasing the light output to the read pedestal level. At the end of a sector during which a write operation is to be performed, WW 45 goes to low, thus disabling the power pedestal circuit 40 and again making current flow from laser diode 50 depend solely on read current source circuit 20. 
     To implement an actual write pulse, higher than the pedestal level, input &#34;a&#34; of circuit 60 switches to 37 hi&#34; and input &#34;b&#34; of circuit 60 switches to &#34;lo&#34; causing current to be drawn through transistor Q11 to supply the requirements of line 36. 
     To complete the understanding of this circuit 11, the demand for current of lines 47 and 46 must be explained. This can best be done by example, in which the negative voltage indicated at current drain line 39 is minus 5 volts, and the voltage drop across resistors R1 and R3 is one volt, requiring for instance, an assumed current strength of 10 milliamps at point 33 with a resistor value for R3 of 100 ohms and a resistor value for R1 of 10 ohms to result in a current at point 34 of 100 milliamps (a gain of 10). Since the voltage drop across resistor R1 is one volt, voltage potential on line 46 would be minus 4 volts in this example. The voltage on line 44, there being approximately one diode drop across transistor Q6 (connected as shown), would be about minus 3.3 volts. Therefore, the voltage on line 47 would also have to be a negative 4 volts. If the value of resistor R2 is 40 times the value of resistor R1, i.e. 400 ohms, (and the voltage drop across resistor R2 is one volt) then the current on line 47 would be 4 milliamps. Therefore, 4 milliamps is drawn down conductor line 42 across transistor Q7 operating as sort of a signal diode. (Line 41 is open because the line WW 45 is &#34;hi&#34;  for a write window.) This current on line 42 is drawn across line 43 and line 52 through conductor junction 51 from laser diode 50 thereby boosting the light output of laser diode 50 to the pedestal level approximately 30 to 40 percent above the nominal (read) level. Therefore, in short, the write current source circuit 30 controls the current drain through the write pedestal circuitry 40 to increase the current drain on laser diode 50 to the required pedestal level as a supplemental drain to that of read current source circuit 20 during a write window. 
     Note that the actual power level of the laser diode required for reading, pedestal level, or writing will vary significantly from media type to media type and the values set forth herein may be easily adapted to various media by those skilled in the art. 
     Transistors Q8 and Q9 are shown tied to a base, but the &#34;base&#34; (or ground) must merely be a more negative voltage than the anode of the laser diode to operate. These transistors Q9 and Q8 are included in the conducting lines 36 and 52 simply because lines 36 and 52 may be long cables, and this is a common way to remove parasitic capacitance noise from long cables found to be effective with this invention.