GB2137746A - Apparatus for Detecting Deviations of Position from a Reference - Google Patents

Abstract

An improved position error detection system comprising two separate or partially overlapping spots of light (40, 41), being separable by controllable variation of the intrinsic properties of wavelength or polarization, with position error direction and magnitude information being determinable from recovered and detected light intensities. The use of separation by a characteristic other than longitudinal spacing enables the spots to be closely spaced or even partly overlapping in a direction transverse to the direction of motion of an object, e.g. a recorded track 44, so that both spots can be used for recovering information without phasing difficulties. <IMAGE>

Description

SPECIFICATION Apparatus for Following Recorded Tracks on Optical Discs This invention relates to apparatus for following recorded tracks on optical discs, especially to optical error sensing systems which direct a beam or beams of light at some target to form spots thereon and collect the reflected or transmitted light, thereby detecting any lateral position error between the spots of light and the target in the plane of the target. This error sensing system is useful in such areas as automated positioning or anywhere it is necessary to know the magnitude and direction of the position error between spots and their target. The present invention is useful for detecting the absolute radial position error of spots, whether focused or not, resident on spiral or concentric data pit patterns of a rotating optical data recording disc.Such a position error detection system is a necessary part of the servocontrol system required to maintain the radial alignment of read spots with rotating opticallydetectable data pit centerlines, or optical tracks, to provide for maximum signal modulation on data read-out or to radially position the write spot or spots when recording data.

In the prior art, detection of focused spot position with respect to a curvilinear pattern of optical data pits, or optical tracks, disposed on a rotating optical data recordin#g disc has been accomplished in a variety of ways. One method uses three linearly aligned and spatially separated focused spots, a focused spot being the twodimensional distribution of light intensity on a surface, in this case the disc, located at or near the focus of a convergent beam of light. Referring to Figure 1 , these three focused spots 1, 2 and 3 have a spot centerline 60 which is angularly displaced relative to a tangent to the optical track centerline 43.The focused spot 1 is positioned slightly inboard of the track centerline, the focused spot 2 is resident on the track centerline for reading data only, and the focused spot 3 is positioned slightly outboard of the optical track centerline. The light transmitted through or reflected from the disc is collected such that a real image of the disc and focused spots 1, 2 and 3 is formed. Separate photodetectors located at these images of the focused spots 1 and 3 sense variations in collected light intensity as position error is incurred. The output signals from these two photodetectors are subtracted, thus generating a signal which contains both the direction and magnitude of the tracking error.In the application of this technique, the photodetectors located at the images of the spots 1 and 3 will sense the passage of a given data pit at different times because the spots 1 and 3 are separated along the track as well as across it. The data component of the photodetectors' output signals will therefore contain a relative phase shift which, upon subtraction of the signals, can limit the ability to generate a useful error signal. To overcome this difficulty, the photodetector signals must be processed prior to subtraction, often by phase shifting, time integrating, or low-pass filtering. If low-pass filtering is used, as is commonly done, the cut-off frequency of that filter must be below the lowest data frequency.

This requirement can impose undesirable limitations on the bandwidth of both the tracking servo-control system and the data channel.

Another disadvantage of this technique is the close proximity of the inboard and outboard spot images to each other and to the center image at the photodetectors, thus requiring the photodetector elements to be very small, close together, and at a real image of the disc. Also, the focused spots on the disc surface must be spatially separated in order for their images to be separated and independently detected. This requirement prohibits the often desirable arrangement whereby the focused spots partially overlap each other at the recording surface of the disc. Also, since a diffraction grating is typically used to generate the three beams, and since retransmission of the reflected beams through the grating in a "double pass" configuration is not practical, additional constraints are imposed on the design of the optical system.

Additional prior art in this field, as described in U.S. Patent Specification No. 4,063,287, wobbles the read beam sinusoidally across the track. Variations in the resulting signal modulation contain tracking error information. However, this system can be optically and mechanically complex and requires that the read signal be intentionally degraded to yield tracking error information.

Still other prior art embodies pre-grooved embossed or molded discs having grooves into which or between which data pits are written.

Either a single read beam undergoes phase modulation as tracking error is incurred, or the previously described three-spot method is used to follow the pre-grooves. The disadvantages of this method are the undesirable degradation of the read signal and the complex media fabrication techniques required.

Finally, the U.S. Patent Specification No.

4,067,044 employs sinusoidally undulating tracks to modulate the read beam's photodetector signal. Again, a disadvantage of this system is the necessary degradation of the read signal caused by the deliberate introduction of tracking error.

The present invention provides apparatus for detection of position error with respect to a reference comprising means for emitting at least two beams of electromagnetic radiation selectively separable by intrinsic properties thereof, means for directing the beams into at least two spots, means for acting on the spots, means for collecting radiation from the spots, means for separating the collected radiation into at least two beams, each having radiation properties corresponding to the emitted beams, and means for independently detecting the separate beam intensities of the separated beams and for producing a signal indicating position error associated therewith.

The intrinsic radiation property of the electromagnetic radiation may be polarization radiation or wavelength.

The spots may be formed in partially overlapping relationship or so that they are separate.

There now follows a detailed description which is to be read with reference to Figures 2 to 7B of the accompanying drawings of several apparatuses according to the invention; it is to be clearly understood that these apparatuses have been selected for description to illustrate the invention by way of example and not by way of limitation.

In Figures 2 to 7B of the accompanying drawings: Figures 2 is a cross-section of the optical disc and optics housing system constructed according to the principles of the present invention; Figure 3A is a schematic illustration of an optical track showing two partially overlapping focused spots positioned for zero tracking error in the system of Figure 2; Figure 3B is a schematic illustration of an optical track showing two separate focused spots positioned for zero tracking error in the system of Figure 2; Figures 4A, 4D, and 4G are schematic illustrations showing the partially overlapping focused spots relative to data pits for three tracking error conditions;; Figures 4B, 4E and 4H illustrate the photodetector signal modulation (vertical axis) associated with each focused spot, as a function of tracking error distance (horizontal axis); Figures 4C, 4F and 41 illustrate the tracking error signal (vertical axis) plotted as a function of tracking error distance (horizontal axis) for the conditions of Figures 4A, 4D and 4G, respectively; Figure 5 is a schematic illustration of one preferred embodiment using two beams with mutually perpendicular electric field polarizations; Figures 5A and 5B are diagrammatic representations of the polarization at Section A-A of Figure 5 expressed as a single resultant vector and as component vectors, respectively;; Figure 5C is a diagrammatic illustration of the angularly separate perpendicularly polarized beams of Section B-B of Figure 5; Figure 5D is a diagrammatic illustration of the polarized beams focused to form spots 40 and 41 at Section C-C of Figure 5; Figure 6 is a schematic illustration of a second preferred embodiment using two light beams having different wavelengths; Figure 7A is a schematic illustration of a third preferred embodiment also using two beams with mutually perpendicular electric field polarizations; and Figure 7B is a schematic illustration of a variation of the embodiment of Figure 7A which offers improved optical power transmission.

Referring to Figure 2, rigid optical data recording disc 1 is rotatably mounted about an axis 31, which is driven by a motor (not shown) to cause disc rotation. An optical housing 10 is movably mounted to maintain a constant spacing with respect to the surface of the disc 1 , while moving radially relative to the rotational axis 31 of the disc. Thus, the housing 10 can be positioned above any optical track on the disc 1.

The optical track on the disc can comprise either a series of optically detectable data pits disposed along a centerline, or a pregroove, or other curvilinear feature which can be used for reference. An optically detectable data pit consists of any local alteration of the recording medium which can be detected by a read-out beam. Such data pits may be created during the manufacture of the disc, or they may be optically recorded with a modulated focused laser beam.

The optical data recording disc 1 can be of the reflectance or transmittance type, incident light being either reflected from or transmitted through the disc during data read-out. In either case, a read beam focused on the surface of the disc is collected after interaction with the optical recording medium and the intensity of the beam is sensed with a photodetector. The presence of optically detectable data pits 44, Figure 1, causes light reflected from or transmitted through the disc to be amplitude or phase modulated. This optical modulation of a read beam is covered by a photodetector into amplitude modulation of an electrical signal, thereby allowing data to be extracted. The signal modulation, herein defined as peak-to-peak signal amplitude, is a maximum if the spot of light focused on the disc is centered on the optical track, otherwise the signal modulation will be reduced.This reduction of signal modulation which results from tracking errors also provides the means for detecting such errors. Thus the optical tracks become the reference means for detecting tracking errors.

The present invention makes use of two readout beams which form two focused spots 40 and 41 on the surface of the optical data recording disc 1; these spots can be either partially overlapping or separate as shown in Figures 3A and 3B, respectively. These pots have their centers aligned on a radial axis 45, this axis being perpendicular to the optical track centerline 43.

Each spot 40 and 41 is associated with a separate photodetector which sense light collected only from that spot, and which gives rise to a separate signal. The modulation of each such photodetector signal is determined in part by the area of the corresponding focused spot, either 40 or 41 are equidistant from optical track centerline 43. Data pits 44 thus act equally on the focused spots 40 and 41 causing the two resulting photodetector signals to be equally modulated as the disc rotates. This zero tracking error condition will exist whether the focused spots are partially overlapping or separate as shown in Figures 3A and 3B, respectively.

Figures 4A, 4D and 4G illustrate the conditions of zero tracking error, outboard tracking error, and inboard tracking error, repectively. Referring to Figure 4A, it is seen that for zero tracking error the data pits 44, thus the optical track centerline 43, are aligned with a focused spot pattern centerline 47. Due to the absence of tracking error, the photodetector output signals are equally modulated as shown at 142 in Figure 48. In this Figure, curves 140 and 141 plot photodetector signal modulation as a function of tracking error distance for the spots 40 and 41.The curves 140 and 141 are symmetrically located with respect to the focused spot pattern centerline 47. Only the point 142 in Figure 4B corresponds to the zero tracking error condition of Figure 4A; all other points on the curves 140 and 141 correspond to conditions with non-zero tracking error.

For any given tracking error condition, the tracking error signal is obtained by subtracting the modulation of the photodetector signal associated with the spot 41 from that of the spot 40. For the zero tracking error situation of Figure 4A, the modulation of the two signals is equal (140, Figure 4B) and the resulting tracking error signal is zero as shown at 46 in Figure 4C. Curve 1 50 is the locus of many possible tracking error signals for many possible tracking error conditions.

Note that if the optical track consists of a pregroove or other reference feature which contains no data pits, the tracking error signal is obtained by subtracting the amplitudes of the two photodetector signals which will contain no modulation due to data.

Figure 4D illustrates the condition in which the optical track centerline 43 is outboard of the focused spot pattern centerline 47. The track centerline 43 has shifted away from the focused spot 40 causing a reduction in the modulation of the associated photodetector signal as shown at 143, Figure 4E. The track centerline 43 has shifted toward the focused spot 41 causing an increase in the modulation of the associated photodetector signal as shown at 144, Figure 4E.

Subtracting 144 from 143 yields a negative tracking error signal as shown at 48, Figure 4F.

Figure 4G illustrates the condition where the optical track centerline 43 is inboard of the focused spot pattern centerline 47. The track centerline 43 has shifted toward the focused spot 40, resulting in an increase in modulation of the photodetector signal associated with that spot.

The track centerline 43 has shifted away from the focused spot 41 causing a reduction in the modulation of its associated photodetector signal.

The tracking error signal is again obtained by subtracting the modulation of the photodetector signal due to the spot 41 from that due to the spot 40, which yields a positive value shown at 50, Figure 41. By examining many possible tracking error conditions, the tracking error signal 1 50 can be plotted as a function of tracking error distance as shown in Figures 4C, 4F and 41. This curve has the very desirable characteristics of having the maximum slope and a sign change as the track passes through the zero tracking error condition. Thus the direction of the tracking error is given the sign of the tracking error signal, and the magnitude of the tracking error is proportional to the magnitude of the tracking error signal within the linear region of the curve 150.

The present invention relies on either of two different intrinsic properties of the light which forms the focused spots 40 and 41 to effect separation and convenient detection of the reflected or transmitted light. One embodiment of the present invention uses light beams having mutually perpendicular polarizations as the intrinsic separation property. Although the following descriptions refer to operation using a reflectance type optical disc, a transmittance type disc could be used with only minor optical layout modifications.

Referring to Figure 5, a laser light source 2 emits a monochromatic, linearly polarized light beam 100. The laser 2 is oriented such.that the plane of polarization of the beam 100 forms an angle of approximately 45 degrees with the horizontal as shown in Figure 5A. Such a beam is identical to and can be regarded as two superimposed component beams of equal intensity polarized in the horizontal and vertical planes as shown in Figure 58.;The beam 100 is projected through a birefringent prism 3, such as a Rochon or Wollaston type, which causes the horizontally polarized component to be refracted by a different angular amount than the vertically polarized component. Figure 5C illustrates the angularly separated beams exiting the prism 3, beam 101 providing the focused spot 40, and beam 102 the focused spot 41.These perpendicularly polarized beams then pass without deviation through a beamsplitter 4, and through an objective lens 5, to form the separate or partially overlapping focused spots 40 and 41 resident on the surface of the optical data recording disc 1. Due to rotation of the disc 1, pits 44 are caused to pass through the focused spots 40 and 41. Upon reflection from the disc 1, the beams 101 and 102 are modulated and return back through the lens 5, and are diverted from the lens axis by the beamsplitter 4. The reflected beams 101 and 102 are then directed to a polarization beamsplitter 6, where the beam 101 is transmitted therethrough, while the beam 102 is reflected therefrom. Lenses 7 collect the light onto photodetectors 8 and 9.

Figure 6 represents a second preferred embodiment of the present invention using light beams having different wavelengths as the intrinsic separation property. Laser light sources 20 and 21 project monochromatic beams, 110 and 111, respectively of different wavelengths undeviated through beamsplitters 23 and 22, respectively, these beams being then combined by a dichroic beamsplitter 24. The tilt of the beamsplitter 24 is adjusted to angularly separate the two beams entering an objective lens 25. The beam 110 forms the focused spot 40 (not shown), and the beam 111 forms the focused spot 41 (not shown). The objective lens 25 focuses the two beams into the separate or partially overlapping focused spots 40 and 41, resident on the surface of the optical data recording disc 1 in the same manner as the objective lens 5 focuses the two beams in Figure 5.

Depending on the type of optical disc used, amplitude or phase modulated light collected from the focused spots 40 and 41 is reflected back through the objective lens 25. Light returning from the focused spots 40 and 41 is received at photodetectors 27 and 28, respectively, from lenses 26 via beamsplitters 24, 22 and 23. Note that the separation of the focused spots 40 and 41 can be adjusted by changing the tilt of the beamsplitter 24.

In a third preferred embodiment illustrated in Figure 7A, a polarization beamsplitter is used instead of a birefringent prims to separate the input beam into two linearly polarized, angularly displaced output beams. A beam 120 emitted by a linearly polarized laser source 48 is directed through an amplitude beamsplitter 49 and into a polarization beamsplitter 50. The polarization beamsplitter divides the beam 120 into two perpendicularly polarized component beams, 121 and 122, each of which is directed to a separate plane mirror 51 and 52, respectively. Upon reflection from these mirrors, the two beams are returned into the polarization beamsplitter 50 and are directed back toward the amplitude beamsplitter 49.A portion of the light in each beam is reflected from the amplitude beamsplitter 49, through a second amplitude beamsplitter 29, and is focused by an objective lens 53 onto the surface of the optical data recording disc 1, forming two separate or partially overlapping focused spots 40 and 41. If the beam 120 from the laser source 48 is linearly polarized in a plane oriented at 45 degrees to the principal sections of the polarization beamsplitter 50, the component beams 121 and 122 will have similar intensities.

Upon reflection from the disc 1, the beams 121 and 122 are modulated by pits 44 and are returned through the objective lens 53 and reflected from the beamsplitter 29. Light returning from the focused spots 40 and 41 is received at photodetectors 59 and 57, respectively, from lens 58 and 56, respectively, via the beamsplitter 55.

A useful variation of this embodiment is illustrated in Figure 7B. The amplitude beamsplitter 49 of Figure 7A has been replaced by an offset plane mirror 54 located such that a beam 130 from the laser source 48 to a beamsplitter 50 is not obstructed. In addition, mirrors 51 and 52 have been tilted such that output beams 131 and 132 from the beamsplitter 50 are entirely reflected by a mirror 54. A primary advantage of this embodiment is the very high optical power transmittance of the optical subsystem comprised of elements 50,51,52 and 54 which can approach 100%. This compares to a maximum transmittance of 25% for the corresponding subsystem of the embodiment shown in Figure 7A (elements 49, 50, 51 and 52).

The embodiments shown in Figures 7A and 7B have several advantages including independent control of the propagation direction of each output beam, hence the position and separation of the two focused spots on the disc can be continuously varied by adjusting the tilt of the mirrors 51 and 52; the orientation of the planes of polarization is independent of the orientation of the focused spots on the disc: the mirrors 51 and 52 may be dynamically positioned (by galvonometer for example) to actively control the location and/or separation of the focused spots on the disc; each beam is separately accessible in the space between the polarization beamsplitter 50 and each mirror 51 and 52, thus allowing insertion of additional optical components to selectively modify either beam. The use of crystalline optical materials is not required.

As can be readily appreciated from the above description, the present invention provides an improved light beam position error detection system comprising two separate or partially overlapping focused spots resident on the optical data recording disc surface, these spots being separable at various locations by the intrinsic properties of wavelength or polarization. Position error direction and magnitude information is determinable from the collected reflected or transmitted light. Advantages of the present invention include direct optical track position sensing without reference to external features, no time delay between the two photodetector signals used to form the tracking error signal, substantially reduced sensitivity of the tracking error signal to rotational errors (yaw) between the track centerline 43 and the spot centerline 45 in Figure 3A, and each of separation and detection of the reflected or transmitted beams. Another advantage of this invention is that the primary data read-out signal can be generated directly from the two focused spots by summing their associated photodetector output signals. The data read-out signal thus generated is less sensitive to tracking errors than the corresponding signal derived from a single read spot.

Claims (6)

1. Apparatus for detection of position error with respect to a reference comprising: means for emitting at least two beams of electromagnetic radiation selectively separable by intrinsic properties thereof; means for directing the beams into at least two spots; means for acting on the spots: means for collecting radiation from the spots; means for separating the collected radiation into at least two beams, each having radiation properties corresponding to the emitted beams; means for independently detecting the separate beam intensities of the separated beams and for producing a signal indicating position error associated therewith.

2. Apparatus according to claim 1, wherein the intrinsic radiation property of electromagnetic radiation is polarization radiation.

3. Apparatus according to claim 1, wherein the intrinsic property of the electromagnetic radiation is wavelength.

4. Apparatus according to any one of the preceding claims wherein the means for directing the beams into two spots is arranged to form the spots in partially overlapping relationship.

5. Apparatus according to any one of the claims 1 to 3 wherein the means for directing the beams into two spots is arranged to form separate spots which do not overlap.

6. Apparatus for following recorded tracks on optical discs substantially as hereinbefore described with reference to the accompanying drawings.