Optical reading apparatus capable of correcting aberration

An optical reading apparatus with aberration-correcting functionality includes a light source, a lens, a liquid crystal layer and an electrode set. The light source of the optical reading apparatus emits light, which is converged onto a storing medium by the lens for accessing data stored in the storage medium. The liquid crystal layer is disposed in an optical path between the light source and the storage medium. The electrode set provides a variable electrical field across the liquid crystal layer for changing the refractive index of the liquid crystal layer. Aberration of the optical reading apparatus can thus be corrected.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical reading apparatus, and more particularly, to an optical reading apparatus capable of correcting coma aberration.

2. Description of the Prior Art

Lenses are used for converging, refracting or transmitting light in many optical systems. Ideally, aberration of an image can be reduced by proper lens design. However, the unavoidable aberration, caused by material itself or manufacturing factors in manufacturing processes, causes image deviations. Certain unpredictable factors during operations, i.e. the data surface of the disk having a tilt angle caused by disk misplacement, can also aggravate aberrations in the optical system and thus affect display quality or data access.

Certain major aberrations in an optical system are spherical aberration, coma aberration, astigmatism, curvature of field, etc. Coma aberration in an optical system refers to monochromatic aberration inherent to certain optical designs, or due to imperfection in the lens or a tilted storage medium. Coma aberration results in off-axis images resembling coma tails. Coma aberration represents a spherical aberration of a lens when the incident light is off-axis, and often occurs when the data surface of the storage medium has a large tilt angle with respect to the incident light. Coma aberration is most significant at image rims and can be represented by the following formula:

where d is the thickness of the disk substrate, NA is the numerical aperture (NA) of the optical pick-up, λ is the wavelength of incident laser, θ is the tilt angle between the incident laser and the disk substrate, and n is the refractive index of the light in the disk substrate.

With the development of technology, storage media with higher storage capacity is required.FIG. 1shows a table listing certain specifications of similar storage media available in the market. Compared to the conventional compact disc (CD) and the digital versatile disc (DVD), Blu-ray disk (BD) and the high density digital versatile disc (HD-DVD) have the potential to become the future storage media for their higher storage capacity. BD and HD-DVD drives access data using a laser of a wavelength of 405 nm. Besides, a BD drive further adopts a high NA OPU (optical pick up) for increasing the data capacity and the 0.1 mm thick cover layer.

Coma aberration is inversely proportional to wavelength and proportional to the thickness of the disk substrate and to the third power of NA. Therefore, the tolerances of tilted angle θ in a BD system are largely reduced by using a 0.1 mm thick cover layer. Coma aberration reduces the optical quality more seriously in an HD-DVD system.

A schematic diagram of a prior art optical system20disclosed in U.S. Publication 2004/0042356 is shown inFIG. 2. The optical system20includes a storage medium21, an optical pick-up22, a spindle motor24, a tilt corrector26, a land pre-pits (LPP) signal generator27, a radio frequency (RF) signal generator28, and a tilt correction calculator29. An optical sensor disposed on the optical pick-up22can transmit signals measured from the storage medium21to the LPP signal generator27and the RF signal generator28. The tilt correction calculator29generates a tilt correction value based on LPP signals and RF signals respectively received from the LPP signal generator27and the RF signal generator28, and sends the tilt correction value to the tilt corrector26. Therefore, the optical system20can adjust the angle of the optical pick-up22when accessing the storage medium21for reducing coma aberration. In the optical system20, an extra tilt corrector is required for correcting aberration, thereby increasing the manufacturing costs and complicating system operations.

A schematic diagram of a prior art optical pick-up30disclosed in U.S. Publication 2002/0067672 is shown inFIG. 3. The optical pick-up30includes light sources10and12, an optical sensor14, a conflux prism31, a collimator lens32, a grating33, a polarized beam splitter (PBS)34, a condenser lens35, a liquid crystal layer36, an object lens37, and an actuator38. The optical pick-up30reduces the aberration when accessing the storage medium21using the actuator38that includes a focus region and a tracking region. Based on signals measured by the optical sensor14, the focus region of the actuator38adjusts the distance between the object lens37and the storage medium21, and the tracking region of the actuator38adjusts the angle of the object lens37. In the optical pick-up30, an extra actuator is required for correcting aberration, thereby increasing the manufacturing costs and complicating system operations.

A schematic diagram of a prior art optical pick-up40disclosed in U.S. Publication 2004/0125711 is shown inFIG. 4. The optical pick-up40includes a light source42, a spindle motor43, a PBS44, an optical sensor45, a liquid crystal layer46, a control circuit47, lenses L1-L3, and indium tin oxide (ITO) electrodes ITO1and ITO2. In the optical pick-up40, the control circuit47sends correction voltages to the electrodes ITO1and ITO2based on signals measured by the optical sensor45in an open-loop manner. The aberration, which occurs when the optical pick-up40is accessing the storage medium21, can thus be corrected. However, applying voltages to the electrodes ITO1and ITO2cannot improve aberration effectively, and different degrees of aberrations cannot be corrected flexibly.

To reduce coma aberration in the prior art optical systems, an additional control circuit (such as the tilt corrector26or the actuator38) or two ITO electrodes are required for adjusting the angle of the lens. This makes the prior art optical systems more expensive and complicated. However, coma aberration cannot be corrected efficiently and flexibly.

SUMMARY OF THE INVENTION

The claimed invention provides a light-converging device capable of correcting aberration comprising a lens for converging light onto a storage medium for accessing data stored in the storage medium; a liquid crystal layer integrated with the lens and disposed in an optical path along which the light-converging device converges light onto the storage medium; and an electrode set electrode set for providing a variable electrical field across the liquid crystal layer.

The claimed invention also provides an optical reading apparatus capable of correcting aberration comprising: a light-emitting device for providing light; a lens for converging light provided by the light-emitting device onto a storage medium for accessing data stored in the storage medium; a liquid crystal layer disposed in an optical path between the light-emitting device and the storage medium; and an electrode set disposed in the optical path between the light-emitting device and the storage medium for changing an electrical field across the liquid crystal layer. The electrode set provides a variable electrical field across the liquid crystal layer.

DETAILED DESCRIPTION

The present invention corrects coma aberration by using a liquid crystal layer and generating a variable electrical field across the liquid crystal layer.

A diagram of an optical system50according to a first embodiment of the present invention is shown inFIG. 5. The optical system50includes a light source52, a liquid crystal layer54, a lens56, a power source58, electrodes A1-A4, and switches SW1-SW4. The light source52can include a laser diode capable of providing light for accessing the storage medium21. The lens56is disposed in an optical path between the light source52and the storage medium21for converging light emitted by the light source52onto the surface of the storage medium21, thereby accessing data stored in the storage medium21.

In the first embodiment of the present invention, the lens56can be made of plastic or glass and fabricated by using injection molding technology or by a founding process. The lens56includes an upper lens55and a bottom lens57, between which the electrodes A1-A4can be fabricated by using physical vapor deposition (PVD) or chemical vapor deposition (CVD) techniques. Subsequently, the liquid crystal layer54is formed, and the upper lens55and the bottom lens57of the lens56are sealed. As shown inFIG. 1, the electrodes A1-A4are disposed at two sides of the liquid crystal layer54. The electrodes A1and A4are disposed at a first side of the liquid crystal layer54, wherein the electrode A1is parallel to the first side of the liquid crystal layer54and the electrode A4is not parallel to the first side of the liquid crystal layer54. The electrodes A2and A3are disposed at a second side of the liquid crystal layer54, wherein the electrode A2is parallel to the second side of the liquid crystal layer54and the electrode A3is not parallel to the second side of the liquid crystal layer54. Therefore, the electrodes A1and A2are disposed at different sides of the liquid crystal layer54and parallel to each other, and the electrodes A3and A4are disposed at different sides of the liquid crystal layer54and parallel to each other. The power source58provides voltages to the electrodes disposed at the two sides of the liquid crystal layer54for changing the electrical field across the liquid crystal layer54in a direction along which light travels. In the first embodiment of the present invention, the power source58is electrically connected to the electrodes A1-A4via the switches SW1-SW4, respectively.

Since the refractive index of the liquid crystal layer54is related to the applied electrical field, which can be adjusted by applying voltages to the electrodes A1-A4using the power source58, coma aberration in the optical system50can be corrected.FIGS. 6 and 7are diagrams of equivalent optical systems60and70of the optical system50. InFIG. 6, the power source58is electrically connected to the electrodes A1and A3via the switches SW1and SW3, respectively. Since the distance between the electrode A3and the liquid crystal layer54varies, the electrodes A1and A3provide the liquid crystal layer54with a different electrical field in the direction of light propagation. Therefore, the light passes through the liquid crystal layer54at different locations of different refractive indexes. In the above-mentioned case, the lens of the optical system50functions as an equivalent lens66shown inFIG. 6. Similarly, inFIG. 7, the power source58is electrically connected to the electrodes A2and A4via the switches SW2and SW4, respectively. Since the distance between the electrode A4and the liquid crystal layer54varies, the electrodes A2and A4provide the liquid crystal layer54with a different electrical field in the direction of light propagation. Therefore, the light passes through the liquid crystal layer54at different locations of different refractive indexes. In this case, the lens of the optical system50functions as an equivalent lens76shown inFIG. 7. As shown inFIGS. 6 and 7, when the power source58supplies voltages to the electrodes, the lens of the optical system50functions as the equivalent lens66or76, with which coma aberration due to the tilted storage medium21can be corrected by changing the direction of light propagation. Therefore, when the storage medium21is not properly displaced and has a large tilt angle, the optical system50of the present invention can determine the output voltage of the power source58and which electrodes should be electrically connected to the power source58based on the measured coma aberration. The refractive index of the liquid crystal layer54can thus be adjusted for correcting the coma aberration, thereby improving the accessing performance of the optical system50.

A diagram of an optical system80according to a second embodiment of the present invention is shown inFIG. 8. The optical system80includes the light source52, a liquid crystal layer84, a lens86, the power source58, electrodes B1-B4, and the switches SW1-SW4. The optical system80differs from the optical system50in that the electrodes B1-B4are disposed outside the lens86. The electrodes B1-B4are disposed at two sides of the liquid crystal layer84. The electrodes B1and B4are disposed at a first side of the liquid crystal layer84, wherein the electrode B1is parallel to the first side of the liquid crystal layer84and the electrode B4is not parallel to the first side of the liquid crystal layer84. The electrodes B2and B3are disposed at a second side of the liquid crystal layer84, wherein the electrode B1is parallel to the second side of the liquid crystal layer84and the electrode B3is not parallel to the second side of the liquid crystal layer84. The electrodes B3and B4are parallel to each other. In other words, the parallel electrodes B1and B2are disposed at different sides of the liquid crystal layer84outside the lens86and, and the parallel electrodes B3and B4are disposed at different sides of the liquid crystal layer84outside the lens86. Therefore, when the storage medium21is not properly displaced and has a large tilt angle, the optical system80of the present invention can also determine the output voltage of the power source58and which electrodes should be electrically connected to the power source58based on the measured coma aberration. The refractive index of the liquid crystal layer84can thus be adjusted for compensating the coma aberration, thereby improving the accessing performance of the optical system80.

A diagram of an optical system90according to a third embodiment of the present invention is shown inFIG. 9. The optical system90includes the light source52, a liquid crystal layer94, a lens96, the power source58, electrodes C1-C4, and the switches SW1-SW4. The optical system90differs from the optical system50in that the electrodes C2and C3are disposed outside the lens96, while the electrodes C1and C4are disposed inside the lens96. The electrodes C1-C4are disposed at two sides of the liquid crystal layer94. The electrodes C1and C4are disposed at a first side of the liquid crystal layer94inside the lens96, wherein the electrode C1is parallel to the first side of the liquid crystal layer94and the electrode C4is not parallel to the first side of the liquid crystal layer94. The electrodes C2and C3are disposed at a second side of the liquid crystal layer94outside the lens96, wherein the electrode C2is parallel to the second side of the liquid crystal layer94and the electrode C3is not parallel to the second side of the liquid crystal layer94. The electrodes C3and C4are parallel to each other. Therefore, when the storage medium21is not properly displaced and has a large tilt angle, the optical system90of the present invention can also determine the output voltage of the power source58and which electrodes should be electrically connected to the power source58based on the measured coma aberration. The refractive index of the liquid crystal layer94can thus be adjusted for compensating the coma aberration, thereby improving the accessing performance of the optical system90.

A diagram of an optical system100according to a fourth embodiment of the present invention is shown inFIG. 10. The optical system100includes the light source52, a liquid crystal layer104, a lens106, a protecting layer103, the power source58, electrodes D1-D4, and the switches SW1-SW4. The optical system100differs from the optical system50in that the liquid crystal layer104and two electrodes are disposed outside the lens106, and the optical system100further includes the protecting layer103. In the optical system100, the electrodes D1and D4are disposed at a first side of the liquid crystal layer104inside the lens106, wherein the electrode D1is parallel to the first side of the liquid crystal layer104and the electrode D4is not parallel to the first side of the liquid crystal layer104. The electrodes D2and D3are disposed at a second side of the liquid crystal layer104outside the lens106, wherein the electrode D2is parallel to the second side of the liquid crystal layer104and the electrode D3is not parallel to the second side of the liquid crystal layer104. The electrodes D3and D4are parallel to each other. Therefore, when the storage medium21is not properly displaced and has a large tilt angle, the optical system100of the present invention can also determine the output voltage of the power source58and which electrodes should be electrically connected to the power source58based on the measured coma aberration. The refractive index of the liquid crystal layer104can thus be adjusted for compensating the coma aberration, thereby improving the accessing performance of the optical system100.

A diagram of an optical system110according to a fifth embodiment of the present invention is shown inFIG. 11. The optical system110includes the light source52, a liquid crystal layer114, a lens116, a protecting layer113, the power source58, electrodes E1-E4, and the switches SW1-SW4. The optical system110differs from the optical system50in that the liquid crystal layer114and the electrodes E1-E4are disposed outside the lens116, and the optical system110further includes the protecting layer113. In the optical system110, the electrodes E1and E4are disposed at a first side of the liquid crystal layer114outside the lens116, wherein the electrode E1is parallel to the first side of the liquid crystal layer114and the electrode E4is not parallel to the first side of the liquid crystal layer114. The electrodes E2and E3are disposed at a second side of the liquid crystal layer114outside the lens116, wherein the electrode E2is parallel to the second side of the liquid crystal layer114and the electrode E3is not parallel to the second side of the liquid crystal layer114. The electrodes E3and E4are parallel to each other. Therefore, when the storage medium21is not properly displaced and has a large tilt angle, the optical system110of the present invention can also determine the output voltage of the power source58and which electrodes should be electrically connected to the power source58based on the measured coma aberration. The refractive index of the liquid crystal layer114can thus be adjusted for compensating the coma aberration, thereby improving the accessing performance of the optical system110.

The electrodes used in the optical systems50,80-110can include indium tin oxide (ITO). ITO is transparent material and does not greatly influence light propagation. The optical systems50,80-110can include conventional CD drives, DVD drives, BD drives or HD-DVD drives. The power source58can be electrically connected to corresponding electrodes via the switches SW1-SW4or by using other methods. Based on different optical systems, the electrodes can be disposed at different angles with respect to the liquid crystal layer for providing flexible aberration adjustments.

A diagram of an optical system120according to a sixth embodiment of the present invention is shown inFIG. 12. The optical system120includes a light source52, a liquid crystal layer124, a lens126, electrodes A1and A2, and power supply units128and129. The light source52can include a laser diode capable of providing light for accessing the storage medium21. The lens126is disposed in an optical path between the light source52and the storage medium21for converging light emitted by the light source52onto the surface of the storage medium21, thereby accessing data stored in the storage medium21. The liquid crystal layer124is formed inside the lens126for providing incident light with different refractive indexes according to different applied electrical fields. The electrodes A1and A2have different sheet resistance and are disposed at two sides of the liquid crystal layer124, respectively. The power supply unit128is coupled to first ends of the electrodes A1and A2and includes a power source PS1and switches SW1-SW2. The power supply unit129is coupled to second ends of the electrodes A1and A2and includes a power source PS2and switches SW3-SW4. The power supply units128and129provide voltages to the electrodes A1and A2for changing an electrical field established across the liquid crystal layer124in a direction along which light travels.

In the sixth embodiment of the present invention, the lens126can be made of plastic or glass and can be fabricated by using injection molding technology or by a founding process. The lens126includes an upper lens125and a bottom lens127, between which the electrodes A1and A2can be fabricated by using physical vapor deposition or chemical vapor deposition techniques. Subsequently, the liquid crystal layer124is formed, and the upper lens125and the bottom lens127of the lens126are sealed.

Since the refractive index of the liquid crystal layer124is related to the applied electrical field, which can be adjusted by applying voltages to the electrodes A1and A2using the power supply units128and129, coma aberration in the optical system120can be compensated. An electrical field E established across the liquid crystal layer124can be represented by the following formula:
E=V/d

where V represents the voltage difference across the two sides of the liquid crystal layer124, and d represents the distance between the electrodes A1and A2.

When the optical system120is operating in a first mode, a diagram of an equivalent optical system130is shown inFIG. 13. InFIG. 13, the switches SW1and SW4are turned off (open-circuited), and the switches SW2and SW3are turned on (short-circuited). Therefore, the power source PS1is electrically connected to the first ends of the electrodes A1and A2, and the power source PS2is electrically isolated from the second ends of the electrodes A1and A2. When the optical system120is operating in the first mode, the current supplied by the power source PS1flows from the first end of the electrode A1to the first end of the electrode A2via the second ends of the electrodes A1and A2, in a sequence a-b-c-d as designated inFIG. 13. The current direction during the first mode is indicated by solid arrows inFIG. 13.

When the optical system120is operating in the first mode, a diagram showing the voltage level of the electrodes A1and A2is shown inFIG. 14. InFIG. 14, the electrode voltages obtained at the designated points a-d are represented by Va-Vd, respectively. The slopes representing the voltage variations of the electrodes A1and A2are designated as m1and m2, respectively. Due to different travel distance of the current, the voltages obtained at the points a-d of the electrodes A1and A2have the following relationship: Va>Vb>Vc>Vd. Also, since the electrodes A1and A2are parallel to each other and the distance between the electrodes is constant, the electrical field established across the liquid crystal layer124is related to the voltage difference between the electrodes A1and A2. At this time, the lens of the optical system120functions as an equivalent lens136shown inFIG. 13. The equivalent lens136, similar to a lens with a chamfer angle φ1, can change the refracting angle of the incident light for correcting coma aberration due to large tilt angle of the storage medium21.

When the optical system120is operating in a second mode, a diagram of an equivalent optical system150is shown inFIG. 15. InFIG. 15, the switches SW1and SW4are turned on, and the switches SW2and SW3are turned off. Therefore, the power source PS1is electrically isolated from the first ends of the electrodes A1and A2, and the power source PS2is electrically connected to the second ends of the electrodes A1and A2. When the optical system120is operating in the second mode, the current supplied by the power source PS2flows from the second end of the electrode A1to the second end of the electrode A2via the first ends of the electrodes A1and A2, in a sequence b-a-d-c as designated inFIG. 15. The current direction during the second mode is indicated by solid arrows inFIG. 15.

When the optical system120is operating in the second mode, a diagram showing the voltage level of the electrodes A1and A2is shown inFIG. 16. InFIG. 16, the electrode voltages obtained at the designated points a-d are represented by Va-Vd, respectively. The slopes representing the voltage variations of the electrodes A1and A2are designated as m1and m2, respectively. Due to different travel distance of the current, the voltages obtained at the points a-d of the electrodes A1and A2have the following relationship: Vb>Va>Vd>Vc. Also, since the electrodes A1and A2are parallel to each other and the distance between the electrodes is constant, the electrical field established across the liquid crystal layer124is related to the voltage difference between the electrodes A1and A2. At this time, the lens of the optical system120functions as an equivalent lens156shown inFIG. 15. The equivalent lens156, similar to a lens with a chamfer angle φ2, can change the refracting angle of the incident light for correcting coma aberration due to large tilt angle of the storage medium21.

The electrodes A1and A2of the optical system120have different sheet resistance, resulting in different slopes m1and m2. The chamfer angles φ1and φ2can be adjusted by changing the sheet resistance of the electrodes A1and A2. Therefore, when the storage medium21is not properly displaced and has a large tilt angle, the voltages of the power source PS1and PS2can be determined and the switches SW1-SW4can be turned on or turned off based on the measured coma aberration. The refractive index of the liquid crystal layer124can thus be adjusted for compensating different degrees of coma aberration, thereby improving the accessing performance of the optical system50.

A diagram of an optical system170according to a seventh embodiment of the present invention is shown inFIG. 17. The optical system170includes the light source52, a liquid crystal layer174, a lens176, electrodes B1and B2, and the power supply units128and129. The optical system100differs from the optical system120in that the electrodes B1and B2are disposed outside the lens176. The electrodes B1and B2also have different sheet resistance. In the seventh embodiment of the present invention, the lens176can be made of plastic or glass and can be fabricated by using injection molding technology or by a founding process. The electrodes B1and B2can be fabricated outside the lens176using physical vapor deposition or chemical vapor deposition techniques. Therefore, when the storage medium21is not properly displaced and has a large tilt angle, the voltages of the power source PS1and PS2can be determined and the switches SW1-SW4can be turned on or turned off based on the measured coma aberration. The refractive index of the liquid crystal layer174can thus be adjusted for compesating different degrees of coma aberration, thereby improving the accessing performance of the optical system170.

A diagram of an optical system180according to an eighth embodiment of the present invention is shown inFIG. 18. The optical system180includes the light source52, a liquid crystal layer184, a lens186, electrodes C1and C2, and the power supply units128and129. The optical system180differs from the optical system120in that the electrode C1is disposed inside the lens186, while the electrode C2is disposed outside the lens186. The electrodes C1and C2also have different sheet resistance. In the eighth embodiment of the present invention, the lens186can be made of plastic or glass and can be fabricated by using injection molding technology or by a founding process. The lens186includes an upper lens185and a bottom lens187, between which the electrode C1can be fabricated by using physical vapor deposition or chemical vapor deposition techniques. Subsequently, the liquid crystal layer184is formed, and the upper lens185and the bottom lens187of the lens186are sealed. The electrode C2can be fabricated outside the lens186by using physical vapor deposition or chemical vapor deposition techniques. Therefore, when the storage medium21is not properly displaced and has a large tilt angle, the voltages of the power source PS1and PS2can be determined and the switches SW1-SW4can be turned on or turned off based on the measured coma aberration. The refractive index of the liquid crystal layer184can thus be adjusted for compensating different degrees of coma aberration, thereby improving the accessing performance of the optical system180.

A diagram of an optical system190according to a nineth embodiment of the present invention is shown inFIG. 19. The optical system190includes the light source52, a liquid crystal layer194, a lens196, electrodes D1and D2, and the power supply units128and129. The optical system190differs from the optical system120in that the liquid crystal layer194and the electrodes D2are disposed outside the lens196, while the electrode D1is disposed inside the lens196. The electrodes D1and D2also have different sheet resistance. In the nineth embodiment of the present invention, the lens196can be made of plastic or glass and can be fabricated by using injection molding technology or by a founding process. The lens196includes an upper lens195and a bottom lens197, between which the electrode D1can be fabricated by using physical vapor deposition or chemical vapor deposition techniques. Subsequently, the liquid crystal layer194and the electrode D2are formed, and the upper lens195and the bottom lens197of the lens196are sealed. Therefore, when the storage medium21is not properly displaced and has a large tilt angle, the voltages of the power source PS1and PS2can be determined and the switches SW1-SW4can be turned on or turned off based on the measured coma aberration. The refractive index of the liquid crystal layer194can thus be adjusted for compensating different degrees of coma aberration, thereby improving the accessing performance of the optical system190.

A diagram of an optical system200according to a tenth embodiment of the present invention is shown inFIG. 20. The optical system200includes the light source52, a liquid crystal layer204, a lens206, electrodes E1and E2, and the power supply units128and129. The optical system020differs from the optical system120in that the liquid crystal layer204and the electrodes E1, E2are disposed outside the lens206. The electrodes E1and E2also have different sheet resistance. In the tenth embodiment of the present invention, the lens206can be made of plastic or glass and can be fabricated by using injection molding technology or by a founding process. The liquid crystal layer204and the electrodes E1, E2can be fabricated outside the lens206by using physical vapor deposition or chemical vapor deposition techniques. Therefore, when the storage medium21is not properly displaced and has a large tilt angle, the voltages of the power source PS1and PS2can be determined and the switches SW1-SW4can be turned on or turned off based on the measured coma aberration. The refractive index of the liquid crystal layer204can thus be adjusted for compensating different degrees of coma aberration, thereby improving the accessing performance of the optical system200.

The electrodes used in the optical systems120,170-200can include ITO. ITO is a transparent material and does not greatly influence light propagation. Also, the sheet resistance of the electrodes can be adjusted by changing the doping concentration of oxide/tin/indium. In the sixth through tenth embodiments of the present invention illustrated above, the electrodes A1-E1coupled to the positive terminals of the power sources PS1and PS2have higher sheet resistance, while the electrodes A2-E2coupled to the negative terminals of the power sources PS1and PS2have lower sheet resistance. Other electrodes having different sheet resistance relationships can also be used in the present invention for correcting coma aberration flexibly. The optical systems120,170-200can include conventional CD drives, DVD drives, BD drives, HD-DVD drives, or other optical reading apparatuses.

In the present invention, the refractive index of the liquid crystal layer is adjusted by applying different voltages to the electrodes (the first through the fifth embodiments) or by applying voltages to the electrodes with different sheet resistance (the sixth through the tenth embodiments) in order to reduce coma aberration. Since no extra tilt corrector or actuator is required for adjusting the angle of the lens, the optical system of the present invention is relatively inexpensive, simple and can correct coma aberration more flexibly.