Patent Application
20070291598
The subject of the invention is related to the field of optical systems for information storage, and more specifically to the servo branch of optical drives.
The invention relates to a servo branch of an optical drive comprising detection means for detecting zero order light beams and higher order light beams. An embodiment of such a device is known from the Encyclopedia of Optical Engineering DOI: 10.1081/E-EOE 120009664, 2003.
The invention also relates to a method of measuring beam landing in such a device.
The invention also relates to a method of measuring the spherical aberration present in light beams at a detector.
Such a device usually comprises several individual detectors to detect light beams passing along the servo light path. A single light beam is split into diffracted orders between the light source and the start of the servo branch resulting in the beam being split into a zero order beam and higher order beams. An array of detectors is used to individually detect these beams. Detection is often restricted to zero order and plus and minus first order only. A problem of the known device is that, for each detector, associated drivers and electronics have to be added to the system, thus increasing cost and impacting space available in the device.
It is an object of the invention to reduce the number of detectors required in the servo branch, while still allowing for the detection of higher order beams.
According to the invention, this object is realized in that the device is characterized in that a switchable diaphragm and a device for beam deflection are placed in a path of the zero and higher order light beams. The switchable diaphragm blocks unwanted light beams, or parts of beams, while allowing selected light beams, or parts of beams, to pass. The device for beam deflection alters the path of the light, providing a further means for manipulating the selected light beams. In such a way, the required number of detectors can be reduced as the landing position of each beam order, or parts of orders, can be diverted to another position.
A further embodiment of the device is characterized in that the switchable diaphragm is arranged to selectively block at least parts of orders of light beams. Thus it is possible to selectively block orders of light, or parts of orders, when performing measurements. This feature allows information from many parts of the light beams to be accessed independently, or in combination, according to the information or signals or measurements to be determined.
A further embodiment of the device is characterized in that the device for beam deflection comprises a grating. A grating can be made small enough to fit into the limited space in the servo device. In addition, the beam angles and change in direction of the beam at the grating can be carefully defined and controlled. It is also possible to arrange a number of smaller gratings, or grating segments, with different pitches, within the device and thus tailor the grating action to a particular requirement or selected beam.
A further embodiment of the device is characterized in that the device for beam deflection is arranged to steer diffracted orders of light towards a selected position in the detection means. Higher order beams may have to be deflected through greater angles than lower order beams in order to reach the same target, for instance by using a grating pitch which is more dense. By using segments of different gratings and assigning each grating to a particular light order, specific beam deflection can be accomplished. Thus a detector may be nominated as the target for a specific light beam order. In the case of a grating, the grating can be arranged to direct the beams as required. In such a way it is possible to make selected beams incident on the same detector. This would reduce the required number of detectors in the system to one main detector, thus reducing cost of manufacture and saving space within the device. It would also be possible to reduce the number of detectors to a more limited number, from three to two, for example, depending on application requirements.
A further embodiment of the invention is characterized with the device further comprising a servo lens placed in the path of the light beams, where the switchable diaphragm is placed at a position in the light path, between the servo lens and the detector, where the diffraction orders are physically separated. This effectively means the switchable diaphragm is at a position where the lateral displacement of the beams is greater than the beam diameter. This position has been found to be the most optimum position, allowing the individual orders to be blocked for separate measurement.
A further embodiment of the invention is characterized with the device further comprising a servo lens placed in the path of the light beams, where the device for beam deflection is placed at a position in the light path, between the servo lens and the detector, where the diffraction orders are physically separated. This effectively means the device for beam deflection is at a position where the lateral displacement of the beams is greater than the beam diameter. This position may be on either side of the switchable diaphragm, depending on the angle through which the orders must be deflected and the physical arrangement of the device.
A further embodiment of the invention is characterized in that the switchable diaphragm is based on electro wetting. Such a diaphragm is extremely useful in situations where the main zero order beam must be blocked off from the detector, as the diaphragm is manipulated to block or clear different areas depending on how an electrical voltage is applied, thus removing the need for physical connection to an outer ring, as might be required for a mechanical diaphragm.
A further embodiment of the invention is characterized in that the switchable diaphragm is a liquid crystal based diaphragm. This has the advantages of the electro wetting diaphragm but, with the present state of the art, is constructed under more mature manufacturing processes, and therefore can be produced in greater numbers.
A further embodiment of the invention is characterized in that the switchable diaphragm is circular in shape. The requirement that the switchable diaphragm be circular relates to the measurement of spherical aberration in the system. This aberration is circularly symmetric and thus the light to be measured is preferably captured within a circular frame. The effect of this aberration is to focus light rays from the outer portions of a lens to a different focus position than light rays which pass through the central portion of a lens, thus distorting the light beam. The measurement of spherical aberration is applicable for all optical drives but is more important for certain types, depending on the wavelength of the light and the numerical aperture of the scanning objective lens, or set of lenses, that are used.
In an embodiment of the invention, a method for measurement of beam landing can be developed, the method comprising steps of:
deflection of higher order beams onto a selected detector
selection of which light orders to measure
blocking of beams to remove unwanted orders from detection
measurement of beam intensity signals and tracking error signals from the required order or orders
repetition of beam blocking and measurement steps for the required number of individual measurements
averaging to obtain an averaged signal
calculation of the average beam landing drift from the averaged signal
correction for beam landing offset
The position of the light beam detected at the detector in the servo branch should, ideally, be central but in practice is not perfectly in position. The actual location of the spot must be determined in order to correct for this discrepancy and improve the performance of the optical drive. Beam landing drift may occur due to temperature variations or due to alignment changes arising during the lifetime of the device and must be checked periodically.
Detection of the plus and minus first order diffraction beams, and the zero order beam, can provide information on the spot position within the servo branch. These beam positions are often obtained by use of three detectors, one for each beam. By means of the invention, the beams can be directed towards a main detector and thus measured at the same detector, but can be measured independently by blocking off selected beams as needed with the switchable diaphragm.
In an embodiment of the invention, a method for measurement of spherical aberration can be developed, the method comprising steps of:
restriction of beam to be measured such that whole zero order beam passes through but no higher orders pass
detection of the whole zero order beam
measurement of focus error signal
restriction of zero order beam such that an outer annulus of the beam cross-section is blocked
detection of the remaining central part of the beam
measurement of the new focus error signal
determination of difference in the focus error signals
calculation of the spherical aberration, which is characterized by the variation in focus error signals.
In the invention the beam restriction is effected by a switchable diaphragm. This allows quick and easy selection of parts of the zero order light beam. Another advantage is that only one detector is needed, provided that detector comprises at least two segments, for measurement of the focus error signal. Thus the detector need not be complex or divided into many segments, thus reducing space requirements, cost and complexity. The whole zero order light beam and the paraxial rays can be measured at the same detector but at different times.
These and other aspects of the invention and methods will be further elucidated and described with reference to the drawings, in which:
a and 2b show the standard way of beam landing detection requiring three detectors (
The embodiment of the invention displayed in
Elements, which are numbered 1 to 8 give an indication of the light path through the system in order from laser 1 to detector 8. Elements 9 and 10 are additional elements according to the invention.
These elements combine to function such that light from the laser 1 passes through the device optics to the disc 6 and is then returned from the disc 6 to the detector 8. At the start of the process, light from the laser diode 1 passes through the grating 2 and is diffracted into a zero order beam and higher order beams. Usually, only the zero order and plus and minus first orders are considered as these three beams provide a main spot for reading and writing to the disc 6 and two satellite spots which can be used for beam landing or other measurements. A beam splitter 3 is added to the device to direct the beam reflected from the disc 6 to the detector 8. The laser diode 1 emits a diverging beam, thus requiring the light path to include the collimator lens 4, which turns the diverging beam into a well collimated one. The beam thus formed is then incident on the objective lens 5 and is focused onto the disc 6. Light is reflected from the disc 6 and travels back to the beam splitter 3 where it is directed into the servo branch of the device towards the servo lens 7. The servo lens 7 directs and focuses the light onto the detector 8. A system such as that just described comprises elements of an optical drive known in the current state of the art. As an embodiment of the invention the switchable diaphragm 9 and the device for beam deflection 10 are added to the servo branch between the servo lens 7 and the detector 8. The positions of the switchable diaphragm 9 and the device for beam deflection 10 can also be interchanged, if so desired.
In an embodiment of the invention, it is possible to implement a method for beam landing measurement in which the device has a single main detector present. Beam landing measurement is an operation which has to be done only infrequently. The beam should ideally be central to the system but in practice there is always a small discrepancy. Temperature variations or alignment changes arising during lifetime can cause this discrepancy to change. Thus the beam landing position must be monitored. When the beam landing position is known, correction can be made for the deviation from central position, thus allowing optimization of the system performance.
In
In the current state of the art, the three-spots push pull method could be used to measure beam landing. This situation is shown in
The zero order beam 12 is focused onto a main detector 15 while the first order beams 13 and 14 are incident on auxiliary detectors 16 and 17, respectively. Usually, the main detector 15 is subdivided into quadrant segments and the auxiliary detectors 16 and 17 into two halves. It is possible to use more segmented detectors, but this would also add to the complexity and cost of the device. The position of the different beams on their respective detectors is determined and a calculation is performed to obtain a value for the beam coordinates. This correction is used to optimize the system performance.
An embodiment of the invention is shown in
In the method described above, the main detector was described as the central detector normally used to detect the zero order. Depending on the arrangement of switchable diaphragm and device for beam deflection chosen, it would also be possible to choose a different detector position and to redirect the zero order towards that detector in addition to deflecting the higher order beams as described. Alternatively, the number of detectors in the device could be reduced, but not necessarily to a single detector, and the beams directed as chosen.
A method for beam landing measurement according to the invention is illustrated as a flowchart in
Zero and higher order beams in the servo branch pass through the servo lens on the way to the detector means. According to the invention, the detector means comprises a single detector or a reduced number of detectors. A detector can be selected, and by means of the beam deflection device the light beams can be directed towards this detector 26. The beam deflection device may be a grating, grating segment, lens or mirror, for example, but the purpose is to control the direction of each beam order, independently from the rest of the orders.
For the measurement of beam landing, it is usual in the art to choose three beams—the zero order and plus and minus first orders. In principle, it is possible to use still higher orders to gain information, and, with the implementation of the invention, the possibility of doing this is increased, as each order no longer requires a dedicated detector. Thus the next step in the method must be to select which light orders should be measured 27.
It is possible to measure all orders together at the detector but a better practice is to select orders, measure each order separately and then average the results. To achieve separation of the orders, the invention provides for a blocking mechanism, a switchable diaphragm, which can be set-up to allow only one order, or a group of selected beam orders, to pass through, while blocking unwanted orders. Thus steps are included in the method such that the unwanted orders are removed from detection 28, the beam intensity signal and tracking error signals from the required order, or orders, are measured 29.
A check is then made to see if other orders must still be measured 30. If so, steps 28, 29 and 30 are repeated until no further measurements are needed. The averaged signal is then obtained from all the data gathered 31. This averaged signal is then used to calculate the average beam landing drift 32. This calculation provides input for correction for beam landing offset 33, necessary to optimize the performance of the optical drive.
A method for spherical aberration measurement according to the invention is illustrated as a flowchart in
Spherical aberration is an important issue in many types of optical drive as the aberration causes a blurring of the main reading and writing spot and impacts device performance. The measurement of the level of aberration present in a device is therefore important. Spherical aberration causes the focus position of different rays to vary, depending on the point the ray is incident on the lens, according to the distance from the lens center. Spherical aberration is a property of the whole beam and is defined as a focus difference between paraxial and marginal rays.
In a device according to the invention, it is possible to arrange the diaphragm to block off parts of a beam of light such that the amount of spherical aberration seen by the different beams will be different and thus the focus position of the different beams will not be identical. This difference can be measured and related to the amount of spherical aberration present in the device.
It is possible to measure spherical aberration according to the invention using several orders of light beams in the device, but more customary to concentrate measurements on the zero order beam.
The first step in the measurement process is to restrict the higher order beams to prevent them passing down the path to the detector, and thus also to set the aperture of the switchable diaphragm so that the zero order is fully filling the aperture 34. The light from the zero order beam is then detected 35. The focus error signal is measured to determine position of focus for this section of the aberrated beam 36. The aperture position is then changed to further restrict the zero order beam so that the outer annulus of the beam is blocked 37. The focus position is now changed to the detected focus position of the remaining central part of the beam 38. Thus the new focus error signal is measured 39. The difference between the focus error signals measured in steps 36 and 39 is determined. This characterizes the amount of spherical aberration in the system, which can then be calculated 41.
This method relies on the presence of a switchable diaphragm in the device according to the invention but also takes advantage of the effect of the device for beam deflection in that only a single detector, for instance a detector split into several segments, is needed to carry out both measurements.
| Number | Date | Country | Kind |
|---|---|---|---|
| 04104678.0 | Sep 2004 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB05/53092 | 9/20/2005 | WO | 3/20/2007 |
