Abstract:
In a magneto-optical storage device, a digitally adjustable preamplifier stage balances polarization signal levels through use of a digital compensation value. A separate digital compensation value is stored for the signals corresponding to each detector head. Each time a different detector head is activated, the digital compensation value for the polarization signals corresponding to that head is accessed. The accessed digital compensation value is input to a digital to analog converter (DAC), which produces a voltage for setting the gain of the adjustable amplifier.

Description:
This application hereby claims the benefit of commonly assigned provisional application with serial No. 60/092,863, titled “Magneto-Optical Preamplifier”, which was filed on Jul. 17, 1998. 
    
    
     FIELD OF INVENTION 
     This invention pertains to the field of magneto-optical storage technology. More specifically, this invention pertains to a preamplifier which compensates for imbalances in the optical and electrical paths of separate data signals. 
     BACKGROUND OF THE INVENTION 
     Information stored at a point on a magneto-optical storage surface is generally detected by analyzing the polarization of a beam of light that has been reflected off the point on the surface. A detector head receives the reflected light beam for analysis. The angle of polarization of the light beam rotates upon reflection in a manner that is dependent upon the magnetic field present at the point of reflection on the storage surface. Information is stored on the surface in the form of magnetic fields oriented in different directions. Differences in the resulting angle of polarization following reflection indicate the state of information stored on the surface at the point of reflection, since the direction of angular rotation of the polarity is determined by the direction of the magnetic field. The magnitude of the angular differences corresponding to different magnetic fields is typically small, on the order of plus or minus half a degree. Variations in the amplitude of the reflected light beam, however, are often as large as plus or minus ten percent of the total magnitude. Given the relatively large amount of amplitude fluctuation, accurate detection of polarization angles is generally difficult. 
     One way to determine variations in polarization angle is to split the reflected light beam into two orthogonally polarized component light beams, A and B. This can be done with a polarizing beam splitter. The beam splitter is oriented at a 45° angle from the polarization angle of the incoming beam. In the absence of any rotation of polarization, the A and B component light beams are of equal magnitude. With the beam splitter oriented this way, any rotation of the polarization angle results in one component having a larger magnitude than the other. The direction of the polarization angle rotation determines which of the components has the larger magnitude. 
     The A and B components can be detected by separate photodetectors, each generating electrical signals based on the magnitude of either the A or B polarization component of the reflected light beam. The direction of angular rotation, and consequently the direction of the magnetic field at a reflection point on the surface, can be determined by subtracting the A and B components, with the desired information appearing in the sign of the difference signal. Fluctuations in amplitude, which should appear equally in both the A and B components, should not affect the sign of the result of the subtraction. This is known as common mode rejection. 
     In reality, because the A and B components follow different paths from the beam splitter to the module which implements the differencing, and these paths have slightly different optical and electrical properties, the A and B components each undergo a slightly different variation. These unequal variations can introduce a bias into the difference signal, so that the result of differencing the signals is inaccurate. Because the polarization angle rotations to be detected are very small, even slight inaccuracies introduced from the paths of the signals can result in the polarization angle information being masked by the errors, making it difficult or impossible to extract the desired information. 
     In order to minimize the inaccuracies in the difference signal, compensation of either the A or B signal is necessary to account for the optical and electrical path differences. This is conventionally done by varying a potentiometer setting which adjusts a gain for one or both signals. The potentiometer is set so that, when no polarization angle rotation is present in the reflected light beam, the result of the differencing operation is as close to zero as possible. 
     Such a solution to the problem of individual path variation is generally not sufficient where a magneto-optical drive uses multiple detector heads with the differencing of signals being performed in a single module. A magneto-optical storage device can use more than one detector head to accommodate multiple storage surfaces. Because the paths for the A and B components corresponding to each detector head are different, a separate gain adjustment is necessary for each detector head. It would be impractical, however, to adjust a potentiometer on the differencing module each time a different detector head is selected. What is needed for such a magneto-optical storage device is a mechanism for automatically compensating for the individual path variations. 
     SUMMARY OF THE INVENTION 
     The present invention solves this problem through the use of a digitally adjustable preamplifier stage. Balancing of the A and B signals is performed by at least one amplifier with a gain that is adjustable through a digital compensation value. In one embodiment, a digital compensation value is stored for the A and B signals corresponding to each detector head. Each time a different detector head is activated, the digital compensation value for the A and B signals corresponding to that head is accessed. The accessed digital compensation value is input to a digital to analog converter (DAC), which produces a voltage for setting the gain of the adjustable amplifier. The compensation required for the signals from each detector head is applied when that detector head is activated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an illustration of a magneto optical storage device. 
     FIG. 2 is an illustration an optics assembly for a magneto optical storage device. 
     FIG. 3 is an illustration of a magneto optical detector head. 
     FIG. 4 is an illustration of the A and B polarization components resulting from two polarized light beams. 
     FIG. 5 is an illustration of a polarization beam splitter and photodetectors used to convert a reflected light beam into A and B component signals. 
     FIG. 6 is an illustration of one embodiment of a preamplifier according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, a magneto optical (MO) data storage system  500  employing multiple Winchester-type flying detector heads  506  is shown (only one detector head  506  is shown). MO storage system  500  includes a set of double-sided MO disks  507  (only one disk is shown). Each detector head  506  is coupled to a rotary actuator magnet and coil assembly  520  by an actuator arm  505 , which positions detector head  506  over the surface of MO disk  507 . In operation, MO disks  507  are rotated by spindle motor  595 , which rotation generates aerodynamic lift forces between detector head  506  and MO disk  507 . The lift forces are opposed by equal and opposite spring forces applied by actuator arm  505 . Detector heads  506  are each maintained between a minimum and maximum flying height over the surface of MO disk  507  over a full radial stroke of actuator arm  505 , thus preventing contact between detector head  506  and MO disk  507 . At rest, detector heads  506  are maintained statically in a storage condition away from the surfaces of MO disk  507 . 
     MO storage system  500  further includes laser optics assembly  501  optically coupled to optical switch  504 . As shown in FIGS. 1 and 2, laser optics assembly  501  generates linearly-polarized laser beam  591  from Fabry-Perot (FP) diode laser source  631 , through lens  633 . Assembly  501  also receives reflected laser beam signals  592  and  593  with rotated tracking and phase information from return optical fibers  510  and  512 . There is one set of fibers  510  and  512  for each detector head  506 . A set of single-mode polarization maintaining optical fibers  502  optically couple optical switch  504  to each detector head  506 . 
     Laser optics assembly  501  also includes coupling lenses  638  and  639 , and photodiodes  635  and  636 . Signal-intensity reflected laser beams  592  and  593  from each detector head  506  are coupled through respective return optical fibers  510  and  512  to the photodiodes  635  and  636 , which operate as intensity-sensitive detectors. The present invention is not limited to the aforementioned arrangement of optical elements, as other techniques for directing incident laser beam  591  and for detecting the intensity of reflected laser beams  592  and  593  are well known. 
     In FIG. 1, a representative optical path between detector head  506  and laser optics assembly  501  is illustrated. The optical path includes optical switch  504  and single-mode optical fiber  502 . Optical switch  504  selectively couples incident laser beam  591  to a selected one of the single-mode optical fibers  502 . Incident laser beam  591  is directed by single-mode optical fiber  502  to detector head  506 , where it is reflected onto surface recording layer  749  of MO disk  507 , as illustrated in FIG.  3 . During recording, incident laser beam  591  is selectively routed to one side of one of the MO disks  507  and focused to one of many optical spots  740  on MO recording layer  749 . Incident laser beam  591  heats spot  740  to approximately the Curie point, thus lowering its coercivity. The optical intensity of incident laser beam  591  is preferably held constant, while a time varying vertical-bias magnetic field is used to define a pattern of “up” and “down” magnetic domains perpendicular to the surface of layer  749 . This technique is known as magnetic field modulation (MFM). Subsequently, as spot  740  cools, the information is encoded on layer  749 . 
     During readout, incident laser beam  591 , which is at a lower intensity than during recording, is selectively routed to one of the MO disks  507 , reflecting off one of the spots  740 . The Kerr effect causes reflected laser beam  594  from layer  749  to have rotated polarization  763  of either clockwise or counter-clockwise sense, depending on the magnetic domain polarity of spot  740 . 
     Referring now to FIG. 4, a polarization vector  10   a  is illustrated. Vector  10   a  corresponds to the polarization angle of a light beam that has been reflected off magneto-optical storage surface  749  and which has reentered the detector portion of detector head  506 . The A and B axes are orthogonal, so vector  10   a  can be defined by the addition of A and B vector components (A a  and B a ). The orientation of the orthogonal axes are set so that, in the absence of a magnetic field at surface  749 , and in the absence of any polarization angle rotation, the magnitudes of the A and B components are equal. The magnitudes of the A and B components of vector  10   a  are indicated in FIG.  4 . Because the magnitude of both the A and B components are equal, vector  10   a  corresponds to no magnetic field at reflection point  740  on surface  749 . Vector  10   b  corresponds to a slight counter-clockwise rotation of the polarization angle, and the magnitude of vector  10   b  is larger than the magnitude of vector  10   a . This illustrates the effect of common mode amplitude fluctuation. When the A and B components of vector  10   b  are subtracted, the direction of rotation can be determined. While the magnitude of the difference between the A and B components will change with a common mode amplitude fluctuation, the sign of the difference between the two components will not be affected by amplitude fluctuations. 
     Referring now to FIG. 5, in detector head  506  reflected polarized light beam  594  is routed to polarizing beam splitter  600 , which produces two resulting light beams  592  and  593 . One resulting light beam contains the A polarization component of the reflected light beam, and the other resulting light beam contains the B polarization component. As illustrated in FIG. 1, light beams  592  and  593  are routed, by fibers  510  and  512 , to optics assembly  501 , where they are converted into signals  100  and  200 . Signals  100  and  200  are routed to a preamplifier, such as preamplifier  60 . In a magneto-optical storage device which includes more than one detector head  506 , each corresponding to a unique storage surface  749 , one set of light beams  592  and  593  are produced for each detector head  506 . Each light beam  592  is carried by a corresponding fiber  510 , and each light beam  593  is carried by a corresponding fiber  512 . In one embodiment, all fibers  510  are bundled together at lens  638 , and all fibers  512  are bundled together at lens  639 . When information from a different storage surface  749  is read, the corresponding light beams  592  and  593  produce signals  100  and  200 , which are passed to preamplifier  60 . It will be appreciated that each individual signal path from beam splitters  600 , through photodetectors  635  and  636 , and into preamplifier  60 , will generally have unique optical and electrical properties. The path variations cause repeatable variations in the amplitudes of signals  100  and  200  at preamplifier  60 . The result is that, upon arriving at preamplifier  60 , either signal  100  or signal  200  should be modified to compensate for the different paths. Without compensating for the different optical and electrical paths, a difference signal based on signal  100  and signal  200  will generally be inaccurate. 
     Referring now to FIG. 6, in preamplifier  60  current signal  100  is initially passed to transimpedance amplifier  300 , and current signal  200  is initially passed to transimpedance amplifier  302 . Transimpedance amplifier  300  produces voltage signals  102  and  104  from current signal  100 . Signal  102  is proportional to signal  100 , and signal  104  is the negative of signal  102 . In this embodiment, both signals  102  and  104  are positive with respect to ground. With no input signal applied, output signals  102  and  104  are both equal to a positive DC common mode voltage. As current into input  100  increases, signal  102  rises above the common mode output level, and signal  104  falls below it. The difference between signals  102  and  104  is proportional to the input  100 . This proportionality is the gain of transimpedance amplifier  300 . Voltage signals  202  and  204  produced by transimpedance amplifier  302  are similarly proportional to signal  200 , with signal  204  being the negative of signal  202 . 
     Signals  102  and  104  are passed to multiplier cell  304 . In this embodiment, multiplier cell  304  is a standard Gilbert multiplier cell. Signals  102  and  104  constitute one differential input to multiplier  304 . The other differential input to multiplier  304  comes from constant voltage source  160 . Multiplier  304  is also connected to two current sources  150 , and lines  316  and  318 . The product of the differential signals from transimpedance amplifier  300  and voltage source  160  determines the level of current flowing from lines  316  and  318  through current sources  150  to ground. The sum of output currents  316  and  318  is always equal to the sum of the two source currents  150 . The difference between currents  316  and  318  is proportional to the difference between input voltages  102  and  104 . The gain from signals  102  and  104  to currents  316  and  318  is defined by voltage source  160 . Because signals  102  and  104  are the signals of interest, the signal from voltage source  160  is referred to herein as the gain for multiplier  304 . A multiplier such as multiplier  304  is considered to be a specialized form of amplifier. Current signal  318  changes in a negative sense with regard to current signal  316 , although current flows in the same direction for both signals. As with voltage signals  102  and  104 , there is a positive DC offset current for current signals  316  and  318 . 
     Multiplier cell  306  operates in the same manner as multiplier  304 . Multiplier  306  accepts voltage signals  202  and  204  and produces current signals  320  and  322 , where signal  320  is proportional to an amplified version of  202 , and signal  322  is proportional to an amplified version of  204 . Unlike multiplier  304 , however, the value by which multiplier  306  multiplies differential signals  202  and  204  to create signals  320  and  322  is not fixed. Rather, the value is set through digital to analog converter (DAC)  308 . By changing a digital compensation value input to DAC  308 , the gain of multiplier  306  can be varied above and below that of multiplier  304 . In one embodiment, DAC  308  is able to set the gain of multiplier  306  over a range of 35% below the gain of multiplier  304  to 35% above the gain of multiplier  304 . In other embodiments this range can be larger or smaller. 
     DAC  308  changes the gain of amplifier  306  to compensate for the differences between the optical and electrical paths of signal  100  and signal  200 . If there were no differences in these paths, then no compensation would be necessary, and in that case DAC  308  should be set to cause multiplier  306  to have the same gain as multiplier  304 . When DAC  308  is set to properly compensate for path differences, the resulting current signals  316 ,  318 ,  320 , and  322  are directly comparable. 
     Current signals  316  and  322  are added directly to get current signal  332 , and current signals  318  and  320  are added to get current signal  334 . Because signal  316  is proportional to signal  100 , and signal  322  is proportional to the negative of signal  200 , signal  332  is proportional to signal  100  minus signal  200 . Because signal  318  is proportional to the negative of signal  100 , and signal  320  is proportional to signal  200 , signal  334  is proportional to signal  200  minus signal  100 . Current signals  332  and  334  are converted to voltage signals  336  and  338 , respectively, through resistors  350  and  352  which are connected to constant supply voltage  360 . Voltage signals  336  and  338  are input to the non-inverting and inverting inputs, respectively, of difference amplifier  312 . Signal  400  is output from amplifier  312  and is proportional to the difference of signal  100  minus compensated signal  200 . Signal  402  is also output from amplifier  312 , and is proportional to the difference of compensated signal  200  minus signal  100 . Signals  400  and  402  are output from preamplifier  60 , and indicate the direction of angular rotation  763  of the polarization present in light beam  594  reflected from magneto-optical storage surface  749 . The output of a comparator operating on these two signals can indicate the state of the data stored on magneto-optical storage surface  749 . 
     In the embodiment described, signals  332  and  334  are produced by directly adding together currents. By adding currents, rather than voltages, a more accurate result is obtained over a wider bandwidth, with better rejection of the common-mode signal. 
     When DAC  308  properly compensates for the path differences of signals  100  and  200 , both output data signals  400  and  402  carry difference information from signals  100  and  200 . This difference information corresponds directly to the direction of magnetic fields on magneto-optical storage surfaces  749  which represent the data stored on the surfaces. Signals  400  and  402  are used by magneto-optical device  500  as data signals. 
     In order for DAC  308  to properly compensate for path differences, it must have the correct digital compensation value fed to it. In most cases, the effects of path differences are due to properties of the optical and electrical mechanisms, and do not change significantly over time. For this reason, only a single digital compensation value is necessary for each detector head  506  in magneto-optical device  500 . In one embodiment, this value is determined for each head  506  in the system, and the values are stored in long-term memory, possibly on magneto-optical surface  749  itself. Alternately, the values can be determined at each power-up of magneto-optical device  500 , with the values stored in volatile memory. When a new detector head  506  is activated, the compensation value associated with that head  506  is retrieved from storage, and is loaded into DAC  308 . In the embodiment described, the compensation value is retrieved the same drive firmware which coordinates the switching of active heads  506 . 
     Whether it is performed every time magneto-optical storage device  500  is powered up, or just once in the initial calibration of the system, it is necessary to determine the correct digital compensation value for each detector head  506 . In one embodiment, the compensation values are calculated by monitoring data signals  400  and  402  at a time when signals  100  and  200  should be the same. When signals  100  and  200  are the same, the difference of data signal  400  minus data signal  402  should be zero. For each detector head  506 , a known modulation signal is applied to the laser power level while laser beam  591  is reflected off spot  740  on a portion of surface  749  where no magneto-optical data is present. Signals  100  and  200  should be identical, since they carry no data. DAC  308  is stepped over a range of values, to determine which value minimizes the feedthrough of the known modulation signal to signals  400  and  402 . This digital value is then stored for later use in conjunction with that detector head  506 . The same process is repeated for all of the detector heads  506 , generating a lookup table of compensation values to be used in correcting for the optical and electrical differences in each path. 
     The above description is included to illustrate the operation of exemplary embodiments and is not meant to limit the scope of the invention. The scope of the invention is to be limited only by the following claims. From the above description, many variations will be apparent to one skilled in the art that would be encompassed by the spirit and scope of the present invention. For example, it is contemplated that in some embodiments of the invention the gain of both multipliers  304  and  306  are variable. Also, it is understood that a variety of different types of amplifiers can be used in place of multipliers  304  and  306 .