JP2004527785A - Optical compensator - Google Patents

Abstract

An optical compensation element is provided.
The present invention relates to an optical compensation element (1). The optical compensating element includes a first transparent surface 4 'on which the transparent electrode 3' is fitted and a second transparent surface 4 on which a number of transparent main electrodes 33 and 36 are arranged. The above is the difference between the second transparent surface connected to the drive electrode 31 via the horizontal electrode 32 and the first and second transparent surfaces 4 and 4 ′ with the refractive index changed according to the applied voltage. And the material 2 disposed in the first position. It is an object of the present invention to propose an improved compensating element. According to the present invention, this compensation element is achieved by connecting each of the main electrodes 33 and 36 to the horizontal electrode 32 at only one point.
[Selection diagram] Fig. 1

Description

【Technical field】
[0001]
The invention relates to an optical compensating element that influences the wavefront, as claimed in the preamble of claim 1.
[Background Art]
[0002]
Such a compensating element is known from US Pat.
[0003]
[Patent Document 1]
JP-A-10-221703 [Disclosure of the Invention]
[Problems to be solved by the invention]
[0004]
An object of the present invention is to propose a compensating element that is superior to this compensating element.
[Means for Solving the Problems]
[0005]
To this end, the invention allows each main electrode, or most of the main electrode, to be connected to the horizontal electrode at only one point. The advantage of this is that up to 50% of the contact electrodes can be saved. In contrast to the main compensating element, in this case, the voltages on both sides of the main electrode do not need to exactly match each other. As a result, since the connection portion is provided only on one side of the main electrode, the degree of freedom in design is increased. Further, the number of required drive electrodes is reduced.
[0006]
The horizontal electrode and the drive electrode are made of the same material, and the cross section of the horizontal electrode is smaller than the cross section of the main electrode. This therefore has the advantage of producing a large voltage drop along the lateral electrode, so that a large potential change is achieved over a short distance. In this case, the voltage drop on the horizontal electrode is larger than the drive electrode, and the horizontal electrode does not need to have a large resistance. Another advantage is that a small voltage drop occurs on the main electrode, so that a faster potential change is achieved, even when using a smaller resistance, and that the potential as uniform as possible is all That can be achieved over the main electrode.
[0007]
Advantageously, the main electrode is arranged substantially rotationally symmetric and has a constant voltage with the same mathematical sign for a constant radius. The advantage of this is that the use of multiple switching materials, for example ferroelectric liquid crystals, gives the same switching response, in other words the same optical change, over the ring corresponding to a particular radius. If a nematic material is used as the switching material, the applied voltage may also have a different mathematical sign. The reason is that different mathematical codes do not affect the optical effects that can be achieved with these materials. However, for other switching materials, different mathematical codes will produce different responses. It is also advantageous that geometries other than rotationally symmetric geometries can be used, depending on the desired phase correction distribution.
[0008]
The present invention allows the main electrode to be in the form of a plurality of substantially closed rings having different radii and connected to the transverse electrodes on opposite sides of the ring opening. This has the advantage that as much of the available area as possible is covered with the desired main electrode shape and only a small part of the surface is occupied by the lateral electrodes, in other words, the lateral electrodes The purpose is to supply a potential when passing through the opening of the ring. Advantageously, the circular areas are covered by main electrodes which are as close as possible to one another but are arranged so as not to touch each other, with the highest possible filling factor. The transverse electrodes advantageously have a wider cross section in the open area of the ring. The advantage that this has is that in the area of connection to each ring, a greater voltage drop occurs on the lateral electrodes. This results in a lower operating voltage and a wider range of usable potentials. The potential extracted from the horizontal electrodes is held by the ring of each main electrode. This results in a rotationally symmetric rising or falling potential distribution from the outside to the inside. The shape of the potential distribution is governed by the selection of the extraction point on the horizontal electrode.
[0009]
According to one development of the invention, a correction electrode is arranged between the main electrodes. In this case, the correction electrode can be driven by another driving electrode. The advantage of providing one or more such correction electrodes is that it allows to obtain not only a continuously rising or falling potential distribution, but also any desired potential distribution.
[0010]
The invention makes it possible for both sides of the compensating element to have structured electrodes. This has the advantage that different optical effects are obtained or compensated for by the structuring of the different electrodes. In this case, not only a rotationally symmetric arrangement but also other surface distributions are provided. Thus, a cylindrical lens, an optical wedge, a spherical lens, and / or an aspheric lens are combined into a single compensating element to provide a potential distribution to provide tilt, focus, defocus. ), And / or astigmatism.
[0011]
Advantageously, different voltages are applied to the lateral electrodes. The present invention allows the lateral electrodes to have a variable cross section. The advantage this has is that a potential distribution with any desired voltage drop and thus a non-linear rise is obtained. Thus, by modulating the lateral electrodes in such a manner, any desired phase configuration for the compensating element can be obtained.
[0012]
The supply electrode is arranged between the main electrode and the horizontal electrode, and the contact points of the main electrode with the horizontal electrode are advantageously arranged such that the contact points are not equally spaced. This represents another advantageous variant for optimizing the potential distribution.
[0013]
According to the invention, different voltages are applied to the drive electrodes. This has the advantage that, depending on the desired phase distribution, different voltages are applied to the main electrode, and by varying the potential distribution in such a way in relation to the phase / voltage characteristics of the material used, That is, a phase distribution is obtained. Therefore, different potential distributions are obtained for the same voltage difference on the drive electrodes.
[0014]
Similarly, it is advantageous to apply different voltages to the electrode structures arranged on both sides of the material having a variable refractive index. In the simplest case, i.e., even if the first structure of these electrode structures is flat, the zero point of the potential applied to the other electrode structure of the lateral electrode is applied to the first electrode structure. By changing the voltage, it is possible to shift along the horizontal electrodes. This makes it possible to obtain different phase distributions without having to change the voltage applied to other electrode structures.
[0015]
An apparatus according to the invention for reading and / or writing to an optical recording medium has a compensation element according to the invention. This has the advantage, in particular in the case of optical recording media with high recording density, that it is possible to compensate for wavefront disturbances caused, for example, by tilting or different layer thicknesses. Such wavefront disturbances have a particularly significant effect on the accuracy of reading and writing on such optical recording media. These wavefront disturbances are optimally compensated for in the device of the present invention.
[0016]
Other advantages of the present invention are also included in the description of the exemplary embodiments below. It is self-evident that the invention is not limited to the examples described, but includes modifications that the skilled person will be familiar with.
BEST MODE FOR CARRYING OUT THE INVENTION
[0017]
The compensating element according to the present invention uses, for example, a liquid crystal. The liquid crystal is a material having a variable refractive index and modulates the phase of an incident light beam according to a local electric field. In particular, a liquid crystal device having an electrode structure with very good performance is described in the following text. In this case, an internal voltage drop occurs, and the electrode that modulates the wavefront can use this internal voltage drop. This step allows a large number of electrodes to operate with a low level of drive complexity. The large number of electrodes allows a high-resolution representation of the phase distribution and thus a good correction of the wavefront. In particular, the elements are constructed to also compensate for time-varying wavefront impairments. The method of operation will be described first with reference to two elements that correct for coma and spherical aberration.
[0018]
The liquid crystal element described in this specification is used to correct wavefront disturbances such as coma and spherical aberration in an optical system. In this case, the characteristics of the liquid crystal element are as simple as possible, but nevertheless, many contrivances have been made, and the electrode structure has a minimum required driving unit. The drive is supplied with only one drive voltage, preferably an AC voltage of 1 kHz and of adjustable amplitude of about 2-10 V. The internal voltage drop that occurs in the electrode structure allows for a continuous wavefront deformation. In an exemplary embodiment, the electrode structure consists of only one transparent conductive layer of indium tin oxide, also called ITO, which has a uniform surface resistance and can therefore be produced very easily. However, other transparent conductive materials, such as polymers and the like, can also be used for this purpose.
[0019]
The elements described in the following text are used to compensate for coma or spherical aberration, especially in devices for reading or writing to optical recording media, for example DVD pickup heads. In particular, for future generations of such devices where shorter wavelength light will be used, active compensation will be required. The trend for DVDs is to use an objective lens with a protective layer thickness of 0.1 mm and a numerical aperture of NA = 0.85. Spherical aberrations that occur when switching between different layers of a multilayer optical recording medium, for example from layer I to layer II, must be compensated. If the substrate is relatively thick, the tolerance for disk tilt can be significantly increased by active tilt compensation.
[0020]
A compensation element, shown in cross-sectional form in FIG. 1 and in the form of a liquid crystal compensation element 1, modulates the wavefront using local changes in the refractive index of the thin liquid crystal layer 2.
[0021]
The local refractive index distribution is generated by a suitable electrode structure 3 and can be changed by applying a voltage. Using the mesh technique, in other words the voltage drop occurring on the electrode structure 3, the number of electrodes that need to be driven can be greatly reduced. FIG. 1 shows a cross section through such an element. The electrode structures 3, 3 'are made of a transparent conductive material, in this case indium tin oxide ITO, and are applied to the glass substrates 4, 4' by photolithography. The polyimide layer 5 is used for standard alignment of the liquid crystal 2 and is polished in a preferred direction after being rotated by spin coating. The cell is filled with liquid crystal 2 in a vacuum and finally sealed.
[0022]
The spacer 6 prevents contact between the two electrode surfaces and controls the thickness of the cell. The electrode structure 3 'is shown here as a ground electrode without any special structure.
[0023]
The electrode structure for spherical aberration is shown in FIG. The electrode design shown in FIG. 2 makes it possible to compensate for a wavefront similar to a conical or spherical form. In this case, only two drive electrodes 31 are required. Thus, two or more π (Pi) phase modulations can be achieved depending on the cell thickness of the liquid crystal layer and the materials used.
[0024]
The thin lateral electrodes 32 are provided with the aid of two wide drive electrodes 31. Because the lateral electrodes are narrow, they have a greater resistance for the same surface resistance. Therefore, the voltage that drops on the drive electrode 31 drops almost entirely across the horizontal electrode 32. Each desired potential is picked up by the main electrode 33 at a different point along the lateral electrode 32. Thus, the main electrodes 33 are at their respective associated potentials. The number of phase stages and the phase distribution, such as linear distribution, logarithmic distribution, etc., can be achieved by the number and location of the extraction points.
[0025]
In this exemplary embodiment, the main electrode 33 is in the form of a circular ring, the main electrode being connected on the left hand part to the horizontal electrode 32, but on the right of the center the horizontal electrode 34 is connected to the right hand drive electrode 31 To each other.
[0026]
The electrode structure for correcting coma is shown in FIG. The electrode design shown makes it possible to compensate for a wavefront similar to a coma. In this case, similarly, only two drive electrodes 31 are required.
[0027]
The difference from the spherical correction element shown in FIG. 2 lies in the nature of voltage extraction. Here, the horizontal electrode 32 causing a voltage drop is located outside the modulation area. The supply electrode 35 extracts a desired potential from the horizontal electrode 32, and the potential is passed to the main electrode 36.
[0028]
The shape of the main electrode 36 in this case depends on the desired phase modulation and the potential distribution required for this purpose. The number of main electrodes 36 depends on the desired phase quantization. The potential of each of the individual main electrodes 36 is determined by the extraction point 37 of the lateral electrode 32 over a specific operating range of the liquid crystal curve (see also FIG. 4 in the present context) so as to allow as effective a correction as possible. Optimized by appropriate potential extraction at
[0029]
Selection of the operating voltage range and selection of the extraction point 37 provides the two degrees of freedom required for continuous modulation.
[0030]
A characteristic of the liquid crystal driving technique, also referred to as the liquid crystal calibration curve, shown in FIG. 4, is that the local phase shift in degrees (°) for a compensating element whose local potential plotted in volts (V) is predetermined by the electrode structure. Indicated by This phase shift / voltage characteristic shows the phase shift that occurs when a potential difference is applied to the electrode structures 3, 3 '. The phase distribution that is as suitable as possible is generated by appropriately selecting the driving range together with the selection of the extraction point 37 on the horizontal electrode 32.
[0031]
For example, to form a distribution whose curvature is as spherical as possible for spherical correction, the main electrode 33 has a curvature between 3.5V on the innermost ring and 5.2V on the outermost ring. Potential is recommended. The distribution can be optimized to an ideal spherical shape by appropriate selection of the extraction points 37 along the lateral electrodes 32. If a different curve distribution is desired, a range of about 2 volts and 2.7 volts may be used, for example, between 2.7 volts and 3.3 volts, and, for example, above 6 volts. , Resulting in a fairly linear range.
[0032]
The following should be noted with respect to switching response and switching time. That is, the switching time of the nematic liquid crystal is substantially determined by the thickness of the cell and the material used. In this case, the maximum achievable phase shift is directly proportional to the cell thickness. Switching times of less than 10 ms can be achieved with a wavefront correcting nematic material having a value between the peak and the valley of less than half λ, in other words, less than half the wavelength of the light used. Above 2λ, it is only possible to achieve switching times of a few hundred milliseconds.
[0033]
There are various liquid crystal mixtures, as well as other materials, such as quartz, polymers, polymer liquid crystal composites, that can be used as phase shift materials in such devices. Nematic liquid crystal is a very suitable material for the switching process, and nematic liquid crystal is not too fast, due to high birefringence, in the range from tens of milliseconds to several seconds, and at the same time has good transfer properties, It has low driving voltage, good polarization characteristics, and low price. Generally, different materials must be used to reduce switching time. However, different materials have other disadvantageous effects. In the case of quartz, these disadvantageous effects are, for example, small changes in the refractive index and large driving voltages, or, in the case of ferroelectric liquid crystals, low birefringence, which has the property of changing polarization. If the idea according to the invention is used for these faster materials, the disadvantages mentioned above are secondary factors compared to the advantages achieved by the invention.
[0034]
As described above, the internal voltage drop utilized by the present invention also allows for the production of more complex electrodes and phase distributions associated with the complex electrodes, for example, top spherical combination elements or 2D top correction elements. Until now, liquid crystal compensating elements for aberration correction have been produced only by directly connecting supply lines to individual surface electrodes. In this case, the individual main electrodes are driven directly. Such elements always have large sudden phase changes, the magnitude of which is governed by the number of control electrodes. In the case of 1 * 1 wavefront correction, assuming five drive electrodes, the wavefront is corrected in only one-fifth step. The device according to the invention requires only a single drive voltage and also allows very precise phase quantization. For example, this is 1/50 for 50 main electrodes.
[0035]
The concept according to the invention of the voltage tapping for the compensating element 1 as well as the concept of the supply electrode 35 for the main electrode of any desired shape is particularly advantageous, since the electrode design and the electrode design to compensate for spherical and coma aberrations The same applies to the element. Devices having various structures of the counter electrode are also provided by the present invention. In this case, according to the invention, the electrode structures 3, 3 'are arranged on opposing glass substrates 4, 4'. Therefore, coma and spherical aberration are compensated by, for example, one element. With the element according to the invention, having a relatively complex main electrode, any desired aberrations, switching arrangements such as checker gratings, wedge arrangements, etc., asymmetric spherical corrections, any desired radial symmetry corrections, or local to the element A special function that can be integrated into one is made possible. The scope of the present invention also includes devices having internal electrodes that use more than one potential drop. In this case, not only a solution using two or more horizontal electrodes 32 and one voltage supply, but also a solution using two or more horizontal electrodes 32 and two or more voltage supplies is provided. A multi-layer electrode structure according to the invention can be created, for example, using an insulating layer and a voltage conduit.
[0036]
Other exemplary embodiments are described in the following documents. These show variants with electrode distributions that are particularly good at correcting wavefronts. Using all the extensions to the electrode structure shown in the following figures, a rotationally symmetric phase distribution with almost any desired morphology is now generated. The examples described above mainly described spherical distributions, but also include aspherical distributions, which are described in more detail herein. The graph shows a schematic of the voltage distribution falling across the horizontal electrodes. The phase shift obtained in the liquid crystal layer is achieved with the aid of the phase shift / voltage characteristics shown in FIG. Thus, wavefronts that are corrected in various ways can be generated depending on the selected voltage range.
[0037]
FIG. 5 shows an electrode structure according to the invention having a correction electrode, a drive electrode 31, a lateral electrode 32 and a main electrode 33 corresponding to the electrodes described in connection with FIG. The opening 34 is kept somewhat wide and allows the passage of correction electrodes 38, 39 connected to at least one of the main electrodes 33.
[0038]
This configuration is shown in detail in FIG. FIG. 6 shows the lateral electrode 32 and the main electrode 33 and the correction electrodes 38, 39, which are only partially, ie, at best, imperfect. Like the horizontal electrodes 32, these correction electrodes pass through the opening 34 and are connected to one of the main electrodes 33 at the contact point 30. This main electrode 33 is also connected to the horizontal electrode 32 in the left-hand area of the electrode structure 3 (not shown here), which changes the potential distribution on the horizontal electrode 32. This expanded spherical electrode distribution allows for improved phase matching or generation of higher order rotationally symmetric distributions. Using the correction electrodes 38, 39, the spherical distribution is changed.
[0039]
FIG. 7 shows the voltage drop U plotted along the vertical axis versus the radius R plotted on the horizontal axis. The mirror image axis 7 for the voltage distribution is shown by the radius R = 0, in other words at the center of the annular main electrode 22. K1 and K2 indicate the points at which the correction electrodes act. As can be seen, the maximum radius Rmax and the potential distribution between the points K1 and K2 at which the correction electrode acts are almost linear, but a kink exists at the points K1 and K2.
[0040]
FIG. 8 shows a different distribution of voltage drops corresponding to FIG. 5, wherein the voltage on the correction electrodes 38, 39 is such that the slope between points K1 and K2 is opposite to the slope of the rest of the distribution. Has been selected to be.
[0041]
FIG. 9 shows an electrode structure having the modified horizontal electrode 32. As described in connection with FIG. 1, the horizontal electrode 32 in the left-hand area is connected to the main electrode 33, while the opening 34 is provided in the right-hand area. The horizontal electrode 32 has a thickened area 8 in the left hand area. Therefore, the resistance of the region changes locally. In other words, the extracted voltages change with respect to each other, even if the distance between the extraction points is the same. Once again, this allows the wavefront to ideally match the desired characteristics.
[0042]
FIG. 10 shows a schematic, enlarged, modified lateral electrode 32. In this case, the thickened area 8 is present in a constant wide cross-sectional area that is uniform and extends over a predetermined range, but is also created in an area of irregular width, as further shown on the right. .
[0043]
FIG. 11 shows the electrode structure from FIG. 2 with a variable tap on the lateral electrode 32. The extraction points 37 at which the main electrodes 33 are connected to the lateral electrodes 32 are no longer equidistantly arranged, but are at different distances from each other, in contrast to FIG. A supply electrode 35 is provided to connect between the horizontal electrode 32 and the main electrode 33.
[0044]
This is shown in detail in FIG.
[0045]
FIG. 13 shows the voltage drop for a horizontal electrode having a constant cross section. The graph in this case corresponds to the graphs of FIGS. The graph shows a linear distribution with a tilt angle α over the radius. In this case, α cannot exceed the maximum value αmax = 45 °.
[0046]
FIG. 14 shows the voltage drop corresponding to FIG. 13, but for a lateral electrode with a variable cross section. The voltage rise α changes so as to respond to the change in the cross section, but also in this case, the voltage rise cannot exceed the upper limit value αmax.
[0047]
FIG. 15 shows a voltage drop corresponding to FIG. 14, but when the supply electrode 35 is used. By increasing and decreasing the distance between the extraction points 37, which is possible in this case, it is possible to achieve an inclination α greater than αmax in the graph of FIG. This tilt is also due to the small cross section of the supply electrode 35, which allows, among other things, a tighter interleaving.
[Brief description of the drawings]
[0048]
FIG. 1 is a diagram showing a cross section penetrating a compensation element.
FIG. 2 is a diagram showing an electrode structure for spherical aberration.
FIG. 3 is a diagram showing an electrode structure for coma aberration.
FIG. 4 is a diagram showing phase shift / voltage characteristics.
FIG. 5 is a diagram showing an electrode structure having a correction electrode.
FIG. 6 is a detailed view from FIG. 5;
7 shows a voltage drop across a lateral electrode according to a first variant with respect to FIG. 5;
FIG. 8 shows a voltage drop across a lateral electrode according to a second variant with respect to FIG. 5;
FIG. 9 is a diagram showing an electrode structure having a modified horizontal electrode.
FIG. 10 is a detailed view from FIG. 9;
FIG. 11 is a diagram showing an electrode structure having a variable extraction.
FIG. 12 is a detailed view from FIG. 11;
FIG. 13 is a diagram illustrating a voltage drop with respect to a horizontal electrode having a constant cross section.
FIG. 14 is a diagram showing a voltage drop with respect to a lateral electrode having a variable cross section.
FIG. 15 is a diagram showing a voltage drop with respect to a horizontal electrode having a supply electrode.
[Explanation of symbols]
[0049]
DESCRIPTION OF SYMBOLS 1 Optical compensation element 2 Material 3 Structured electrode 3 'Structured electrode, transparent electrode 4 Second transparent surface 4' First transparent surface 5 Polyimide layer 6 Spacer 8 Variable cross section 31 Drive electrode 32 Horizontal electrode 33 Transparent main electrode 34 Horizontal electrode 35 supply electrode 36 transparent main electrode 37 contact 38 correction electrode 39 correction electrode

Claims (10)

A first transparent surface (4 ') on which the transparent electrode (3') is fitted;
A second transparent surface (4) on which a number of transparent main electrodes (33, 36) are arranged, wherein two or more of the main electrodes are connected to a drive electrode (31) via a horizontal electrode (32); A second transparent surface respectively connected,
An optical compensating element (1) having a material (2) disposed between the first and second transparent surfaces (4, 4 ′), the refractive index being changed according to an applied voltage; ,
An optical compensation element, wherein each of the main electrodes (33, 36) is connected to the horizontal electrode (32) at only one point. The optical compensation element according to claim 1, wherein the horizontal electrode (32) and the drive electrode (31) are made of the same material, and a cross section of the horizontal electrode is smaller than a cross section of the main electrode. A device according to any one of the preceding claims, characterized in that the main electrode (33) is arranged substantially rotationally symmetric and has a constant voltage with the same mathematical sign for a constant radius (R). The optical compensation element according to any one of the preceding claims. The method according to any of the preceding claims, characterized in that the main electrode (33) is in the form of a plurality of substantially closed rings having different radii and connected to the lateral electrode (32) on opposite sides of the opening. The optical compensation element according to claim 1. Optical compensation element according to any one of the preceding claims, characterized in that at least one correction electrode (38) is arranged between the main electrodes (33, 36) and can be driven by another drive electrode. . Optical compensation element according to any of the preceding claims, characterized in that both surfaces (4, 4 ') have structured electrodes (3, 3'). Optical compensation element according to any one of the preceding claims, characterized in that different voltages can be applied to the drive electrode (31). Optical compensation element according to any of the preceding claims, characterized in that the lateral electrode (32) has a variable cross section (8). A supply electrode (35) is arranged between the main electrodes (33, 36) and the horizontal electrodes (32), and the contact points (37) of the supply electrodes with the horizontal electrodes (32) are not arranged at equal intervals. The optical compensation element according to claim 1, wherein the optical compensation element is arranged. Apparatus for reading and / or writing to optical recording media, comprising a compensating element (1) according to any one of the preceding claims.