Abstract:
An optical position detector which has adjacent detector cells connected to each other in a zig-zag pattern offers a wider dynamic range than a detector with a vertical separation line during transition between a read laser power and a write laser power. This allows use of a fast response bi-cell in place of a slower position sensing device (PSD). The optical position detector has detector cells coupled to each other in a zig-zag pattern. Each detector cell produces a signal when a light beam is focused onto the cell to indicate a location of the light beam relative to the cell.

Description:
BACKGROUND 
     The present disclosure relates to determination of a position of an optical spot relative to an optical system. 
     Optical systems often use position sensing detectors (PSDs) to determine the position of an optical spot that is incident upon the active surface of a device. The optical spot may be a reflected laser beam from the surface of an optical recording medium. The PSD device is constructed using photodetectors, such as photo-diodes or PIN-diodes, in a bi-cell, quadrant or lateral configuration. Signals required for the X and Y displacement of the spot is found by suitably subtracting currents from adjacent cells, followed by normalization to the total intensity of the optical spot. However, with rapid increase in the optical processor speed, the need for faster and more accurate position detectors is desirable. 
     The PSD device has relatively low output impedance, which often causes offsets when fed to current sensing pre-amplifiers. This may lead to power adjustment problems in a laser beam source during a transition between read and write operations of the optical system. A read operation of an optical system often involves a reflected laser beam modulated with both tracking information and the data stored on the optical recording medium. A write operation involves modulating the laser beam via an optical modulation including beam intensity, phase, and polarization at a laser beam source. Therefore, the transition between these two operations may cause considerable delay in the efficient operation of the optical system. 
     SUMMARY 
     An enhanced optical position detector having detector cells is disclosed. The detector cells are connected to each other in a jagged or curved line where segments of the jagged or curved line are interleaved in a saw-tooth shape. The line segments are symmetric about a line drawn through middle of the segments. Each detector cell produces a signal proportional to an amount of light incident on each cell. The signal indicates a location of the optical spot relative to the detector. The saw-tooth shape of the jagged or curved separation line allows the detector to be relatively insensitive to a displacement of the optical spot along a direction of the jagged or curved line. 
     In another aspect, a differencing element which operates to difference the signals from adjacent detector cells to produce a differencing signal is disclosed. The differencing signal indicates the position of the optical spot. 
     Other features and advantages will become apparent from the following description and drawings, and from the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a conventional optical detector; 
     FIG. 1B is a conventional optical detector with a diagonal separation line; 
     FIG. 2A shows a plot of the subtracting signal of the cells in the conventional optical detector; 
     FIG. 2B shows a plot of the subtracting signal normalized to the summing signal; 
     FIG. 3 is a zig-zag-patterned optical detector system; 
     FIGS. 4A and 4B show plots of the subtracting and the normalized signals for the zig-zag-patterned optical detector system; 
     FIG. 4C shows a plot of the gain of the zig-zag-patterned system; 
     FIGS. 5A through 5C are several different embodiments of the zig-zag-patterned optical detector; 
     FIG. 5D is a three-cell detector system; 
     FIGS. 6A and 6B are further embodiments of the zig-zag-patterned optical detector; and 
     FIG. 7 is a block diagram of a magneto-optic storage system. 
    
    
     DETAILED DESCRIPTION 
     A conventional optical detector  100 , shown in FIG. 1A, includes two photodetector cells A and B that are manufactured from a single piece of doped semiconductor material, and are placed adjacent to each other. A grounded vertical line  102  separates the two cells. Two electrical leads  104 ,  106 , one from each cell, carry photocurrent from the cells A and B, respectively. 
     When a beam of light  108  hits the detector  100  and scans the detector  100  horizontally, each cell produces a signal that depends on the location of the beam  108  within the detector  100 . The amplitude of the signal coming from each cell is determined by the coverage of the beam  108  incident upon each cell, and hence the amplitude from a cell is higher when more of the beam  108  is illuminating that cell. Therefore, the position of the beam  120  relative to the vertical line  102  is often determined by differencing the signals coming from the two adjacent cells. The result is a subtracting signal, A−B or B−A. 
     In one embodiment, the dimensions of the detector  100  is 3 mm in width and 1 mm in length. The vertical separation line  102  indicates the reference point of the x-axis. 
     The optical detector  100  is insensitive to vertical displacement of the beam  108  because the separation line  102  between the two cells A and B are vertical, and hence, has no effect on the subtracting signal. However, a slightly varied optical detector, shown in FIG. 1B, with a diagonal separation line between the cells, is sensitive to any vertical displacement. 
     A plot of the subtracting signal is shown in FIG.  2 A. The figure shows the difference of the amplitudes of signals A and B versus position relative to the separation line  102  as the beam of light  108  moves across the optical detector  100  in the X direction  110 . The amplitudes of the subtracting signal range between −0.2 mA and +0.2 mA. 
     The dynamic range of the optical detector  100 , which is the range of positions of the beam  108  within which accurate measurements can be made, falls within the linear range of the subtracting signal. The plot of the subtracting signal shows the dynamic range  200  to be approximately 0.5 mm for beam width of 0.8 mm. 
     FIG. 2B shows a plot of the output signal normalized to the summing signal which represents the total electrical intensity produced by the beam  108  falling on the cells A and B. An advantage of monitoring the normalized signal versus the subtracting signal is the fact that the normalized signal is less sensitive to the strength of the beam  108  and the sensitivity of the detector  100 . 
     The normalized plot shows the dynamic range  200  of the optical detector  100  between approximately −0.25 mm and +0.25 mm. The amplitude of the normalized signal in the dynamic range  200  is about 2.4 volts, which results in a slope of 4.8 volts/mm. 
     The gain of the optical detector  100  which is represented by the slope of the normalized signal, shown in FIG. 2B, is plotted in FIG.  2 C. The nominal gain  202  is shown to be about 4.8 volts/mm and the range  204  over which the gain is within 10% of the nominal gain  202  is only about 0.5 mm. 
     A zig-zag-patterned optical detector system  300  (FIG.3) includes a position detector  302 , such as a PIN-diode position sensor. The detector  302 , formed as an integrated circuit on a single piece of semiconductor material, has two cells A and B connected in a zig-zag pattern  304 . Two electrical leads  306 ,  308 , one from each cell, carry photocurrent from the cells A and B, respectively. As beam of light  322  hits the detector  302  and scans the detector  302  from left to right  324 , each cell produces a signal that depends on the location of the beam  322 . Each of the detector cells A and B has a curved or jagged boundary line that is conforming to a boundary line of an adjacent cell. The boundary line curved or jagged is in such a pattern that a straight line drawn in the middle of the boundary line along the direction of the boundary line touches each cell in at least two points. Each detector cell produces a signal that is proportional to an amount of light incident on said each cell and indicates a location of the optical spot of the beam  322  relative to the detector. 
     The op-amps  310 ,  312  amplify the photocurrents generated by the photodetector cells A and B and supply the amplified signal to the subtracting circuit  316  and the summing circuit  314  for generating the normalized output signal  320 , (A−B)/(A+B). This output signal  320  is linearly related to the position of the beam  322  on the photodetector cells A and B. 
     FIGS. 4A and 4B show plots of the subtracting and normalized signals for the zig-zag-patterned optical detector system  300 . The plots show the response of the detector  302  as the beam of light  322 , having beam width of 0.8 mm, horizontally scans the detector  302 . The plots are similar in shape to the respective signals for the conventional detector  100 . However, the enhanced system  300  with zig-zag-patterned optical detector  302  shows output signals having lower amplitudes and significantly longer linear regions, which results in wider dynamic range and higher responsitivity. The plots show the dynamic range of the zig-zag-patterned optical detector  302  to be approximately 2.0 mm. 
     The gain of the enhanced system  300  is plotted in FIG.  4 C. The nominal gain  400  is shown to be approximately 1.17 volts/mm and the dynamic range  402  over which the gain is within 10% of the nominal gain  400  is increased four-fold over the conventional detector  100  to about 2.0 mm. 
     Several different embodiments of the zig-zag-patterned optical detector  302  are shown in FIGS. 5A through 5C. Each detector  302  has two cells, A and B, which are coupled in a saw-tooth shape with a varying number of teeth. The number of teeth or the length of the zig-zag pattern, along with the beam width, determines the dynamic range of the optical detectors. Therefore, the dynamic range of the zig-zag-patterned detector  302  can be further increased over that of the conventional detector  100  by increasing the number of teeth in the detector pattern. For example, in FIGS. 4A and 4B, the curves will get flatter and the regions where the curves are linear will get wider as the length of zig-zag pattern increases. This will also be reflected in the gain curve of FIG. 4C with wider dynamic range  402 . 
     FIG. 5D shows a three-cell configuration, with cells A, B and C, in which the cells are connected in zig-zag patterns. This configuration is used to detect the position of a light beam over a wider range. 
     Further embodiments of the zig-zag-patterned detector  302  are shown in FIGS. 6A and 6B. FIG. 6A shows a saw-tooth pattern with rounded tips, while FIG. 6B shows another pattern in the shape of inter-leaved fingers. Both patterns will increase the size of the linear region as long as the patterns are symmetric about the center vertical line. 
     FIG. 7 is a block diagram of an optical storage system  700  which includes a zig-zag-patterned position sensor  718 . The system  700  provides an user interface  704  of data input  706  through main electronic control  702  which is preferably implemented to monitor and control all components and subsystems. The user interface  704  includes, but is not limited to, a computer keyboard, a display, electrical and mechanical switches and control buttons. The system  700  also includes an optical storage medium  708  in the form of a disk or other format. In some embodiments, the disk is a magneto-optic disk, a write-once disk, a phase-change disk, or a read-only disk. 
     In one embodiment, a flying read/write head  714  and the optical medium  708  are positioned relative to each other so that the optical spacing therebetween is less than one wavelength of the light produced by the light source  710 . This is known as the near-field configuration. An air-bearing surface is preferably implemented at the base of the flying head  714  to maintain a desired focus without conventional servo optics for focusing. Alternatively, a non-near-field configuration can also be used with the flying head  714 , in which case the separation between the flying head and the recording layer does not allow efficient coupling of evanescent waves and thus a conventional servo focusing system is needed to directly focus the beam onto the recording surface. 
     In a readout operation, a reflected laser beam usually is modulated with both tracking information and the data stored on the optical medium  708 . In a recording operation, the reflected laser beam from the optical medium  708  is encoded with beam-tracking information. Recording data onto the optical medium  708  can be done by either modulating a writing beam via an optical modulation including beam intensity, phase, and polarization either at the light source  710  or at the beam relay system  712 , or directly modulating the state of the optical medium  708  through thermal or electromagnetic methods. Thus, a transition from the readout operation to the recording operation can cause delays in the system. 
     One of the advantages of the zig-zag-patterned position sensor  718  is that it provides a more linear transfer function and wider dynamic range than the conventional PSDs. As a result, a laser beam source  710  in the optical storage system  700  recovers more quickly when switching between the recording and the readout operation. 
     Although only a few embodiments have been described in detail above, those of ordinary skill in the art certainly understand that modifications are possible. For example, the two photodetector cells A and B can be connected with patterns other than the zig-zag pattern, such as a figure-eight pattern. Further, the photodetector cells with zig-zag pattern can be extended to have more than two or three cells, such as a quadrant configuration. All such modifications are intended to be encompassed within the following claims, in which: