Optical Head with a Variable Optical Component

BRIEF DESCRIPTION OF THE DRAWINGS

The Figures are not drawn to scale and are purely schematic. The same reference numbers in different figures refer to the same elements.

FIG. 1 shows schematically a cross section of a variable optical component having bendable nano-elements in their non-bent state,

FIG. 2 shows schematically a cross section of a variable optical component having bendable nano-elements in their bent state,

FIG. 3 shows a diagram of a device for scanning a medium in which device a variable optical component according to the invention is used,

FIG. 4 shows a variable optical component with a non-uniform distribution of bendable nano-elements,

FIG. 5 shows examples of variable optical components with segmentations in the distribution of bendable nano-elements or electrodes,

FIG. 6 shows an example of an alternative construction for the variable optical component based on a stack of substrates comprising bendable nano-elements, and

FIG. 7 shows a diagram of a device for scanning a medium in which device a variable optical component is used in combination with means to control the variable optical component.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 a schematic presentation is given of a variable optical component 10 to show the principles of operation of a variable optical component with bendable nano-elements. The variable optical component comprises a substrate 4, which is in this case a glass substrate that is at the bottom side provided with a polarizer 5 and at the opposite site with electrodes 1, 2 and bendable nano-elements 3. For a transmission type variable optical component the electrodes may be made of, for example indium tin oxide (ITO). In this example a dielectric layer 6 of SiO2 covers the electrodes. The function of the layer is twofold: firstly, it planarizes the substrate 4, which simplifies the application of bendable nano-elements, and secondly, it acts as insulating barrier between the bendable nano-elements 3 and the electrodes 1, 2. In this way the bendable nano-elements will be influenced by an electric field only, and not by direct electrical contact with the electrodes 1, 2.

The bendable nano-elements 3 are in this example carbon nanotubes that have been functionalized with Si(OR)₃groups, wherein R is methyl. Functionalization of carbon nanotubes with suitable end-groups is known per se from Langmuir, vol 16 (2000), pp 3569-3573. Therein, single walled carbon nanotubes of desired length are suspended with ultra-sonification in alcohol. The carbon nanotubes have been given carboxylic end groups by oxidation. This end group is then substituted through chemical reaction with Si(OR)₃.

In order to achieve a patterned deposition, the substrate is covered with a photoresist, which is developed according to a desired pattern. Then, the photoresist material and substrate are undergoing a plasma treatment process so as to make the substrate more hydrophilic and the photoresist more hydrophobic. A suitable treatment is a sequence of an oxygen plasma treatment, a fluor plasma treatment and an oxygen plasma treatment. Bundles of carbon nanotubes will align along the surface, due to the hydrophobic interactions between the individual carbon nanotubes. As an alternative to the use of a photoresist, a mask of another material may be used to obtain the required pattern. The pattern may also be obtained by burning away carbon nanotubes according to a desired pattern by means for example of a laser bundle having sufficient intensity.

In case the applied electric field is zero, i.e. no voltage supplied to the electrodes 1 and 2, the nano-elements 3 are aligned perpendicular to the substrate 4. A radiation beam B incident on the variable optical component 10 in a direction normal to the substrate surface will pass the variable optical component substantially unhindered as the nano-elements are aligned parallel to the propagation direction of the radiation. If the electrical field is switched on, the nano-elements will bend and become curved nano-elements as shown in FIG. 2. The curved nano-elements now cover at least a substantial part of the area between the electrodes 1 and 2 and absorb that component of the beam B that has a polarization direction parallel to the tangent of the curved nano-elements. The absorption of an incident beam B will be maximal if the beam is a linearly polarized beam having its polarization direction tangent to the curved nano-elements. If the radiation beam is already sufficiently linearly polarized, such as a radiation beam originating from a semiconductor laser diode, the polarizer 5 may be omitted. The variable optical component is then preferably oriented in such a direction that the bending direction of the nano-elements matches the polarization direction of the radiation beam.

The nano-elements can be bent by means of an electric field having a strength in the range from 0.1 to 5 V/μm. The voltage for generating the electrical field may be a DC voltage. However, an alternating current with a frequency between a few Hz and some kHz, preferably about 50 Hz (e.g. the mains frequency), can be used.

In general a variable optical component, for example such as described above with reference to FIG. 1, may be of a transmission type or a reflection type. The transmission type variable optical component as described above can be converted into a reflective type by using, for example, a reflective substrate or by arranging a reflective layer between the substrate and the electrode configuration. This reflective layer may have optical characteristics as used in optical beam splitters or fixed apodisation filters having wavelength depend characteristics, e.g. for one wavelength range the layer is transmittive and for another wavelength range it is reflective.

In FIG. 3 an optical head 100 is schematically shown in combination with a storage medium 13. It illustrates an optical system according to a first embodiment of the invention. The optical head 100, which can be used in an optical storage apparatus, comprises a radiation source 11 for generating a radiation beam, a variable optical component 10, a beam splitter 15, an objective lens 12 used as a lens for focusing the radiation beam onto a medium and a radiation detector 14. In this example the medium is an optical storage medium. A radiation beam emitted from the radiation source 11 is incident on the variable optical component 10, and the radiation beam transmitted through the variable optical component 10 is incident on the beam splitter 15. The radiation beam transmitted through the beam splitter enters an objective lens 12 that focuses the radiation beam onto a data layer of the optical storage medium (optical disc) 13. This optical disc can be a read-only disc, such as CD-ROM or BD-ROM, or a disc on which data recordings can be made such as DVD+R or DVD+RW. The radiation reflected from the data layer of the optical disc 13 enters the objective lens 12 and is split by the beam splitter 15 from the radiation beam of the radiation source 11 and directed towards the radiation detector 14. This radiation detector may be used for data signal generation and/or servo signals generation such as a focusing and tracking signal.

The variable optical component 10 appropriately varies the intensity distribution of the radiation beam emitted by the radiation source 11 when it is transmitted through the variable optical component. In a read mode the variable optical component can be set in an ‘on-state’ with the nano-elements in a bent state, in order to be able to attenuate the radiation beam incident on the variable optical component. The attenuation of the variable optical component is set at such a level that the radiation source is operating at a sufficiently low laser noise level, while maintaining a usable radiation intensity level on the radiation detector for the signals generation. In a write mode the variable optical component can be set in an ‘off-state’ with the nano-elements in a substantially non-bent state, such that the radiation beam transmitted through the variable optical component is substantially unaffected.

When the variable optical component 10 is positioned in the optical path between the beam splitter 15 and the objective lens 12, the radiation emitted from the radiation source and reflected by the optical disc 13 will reach the radiation detector 14 while passing the variable optical component 10 twice. In a read mode with the variable optical component in an on-state the radiation intensity distribution will have been varied twice. This may result in a radiation intensity level on the radiation detector that is too low for good quality servo and or data signals. Therefore it is preferable to position the variable optical component 10 in the optical path between the radiation source 11 and the beam splitter 15 as in that case the radiation emitted from the radiation source and reflected by the optical disc 13 will reach the radiation detector 14 while passing the variable optical component 10 only once and thus a higher radiation intensity level is achieved compared to the variable optical component position between radiation source and beam splitter.

In an embodiment of the invention the variable optical component has a substantially uniform distribution of bendable nano-elements. When the variable optical component is in an on-state during a read mode, the absorption, due to the bent nano-elements, is substantially homogeneous over the cross section of the radiation beam with the variable optical component. In this case the radiation intensity distribution of the transmitted radiation beam is attenuated. The electric field (driver-field) determining the bending angle of the bendable nano-elements depends on the applied voltage over the electrodes and the electrode configuration. The applied voltage therefore determines the attenuation of the radiation beam. When the variable optical component is applied in an optical head, the variable optical component can be in an off-state during a write mode in which the bendable nano-elements are in the substantially non-bent state. To obtain a high radiation power level onto the medium during the write mode the coupling efficiency, which is the ratio of the amount of radiation power coupled into the optics over the total emitted radiation power output of the radiation source, can be set at a high level by using a high numerical aperture for the coupling of the radiation into the optics towards the medium. In a read mode the high coupling efficiency can result into such a low radiation power emitting level that the radiation source is not operating in a stable output power range resulting in output power noise. Increasing the radiation output of the radiation source to such a value that the noise is at an acceptable level, can result in too much radiation power focused on the medium. To reduce the noise in the emitted beam out of the radiation source and still keep the level of the radiation power onto the medium at a low enough level, the transmission of the radiation path from radiation source to the medium needs to be reduced. The transmission of the variable optical component can be set such that the output power level of the emitted radiation beam during the read mode results in a sufficient low noise level in the emitted beam out of the radiation source without having a too high radiation power level onto the medium.

As an example, some data based on the results from a study of Li et al. (Li, Z. M. et al., Phys. Rev. Lett. 87(2001), 1277401-1/4) are used to design an attenuator using a substantially uniform distribution of bendable nano-elements. At a wavelength of 405 nm (corresponding to a photon energy of about 3.1 eV) the absorption for a radiation beam with a polarization perpendicular to the bendable nano-element axis is about 40% (absorption OD˜0.35), while the transmission for the polarization direction parallel to the bendable nano-element axis is about 0% (absorption OD˜5.5). These figures are based on a specific density of bendable nano-elements as well as dimensions of the bendable nano-elements.

The transmission T of the system can be written in a dependency of the absorption coefficient α and L as a measure for the dimensions of the bendable nano-elements according to

T˜e^−α,L (1)

The optical density OD, which is the logarithm of the transmission T, is therefore linearly dependent on αL. From the data of the publication of Li et al. the ratio of the OD for the radiation polarization direction parallel to the axis of bendable nano-elements and the OD for the radiation polarization direction perpendicular to the axis of the bendable nano-elements is found to be about 15. This ratio can be used when considering other configurations with other densities and dimensions of bendable nano-elements.

A configuration with a transmission of about 0.1% (OD=3) for a radiation beam with a polarization direction parallel to the axis of bendable nano-elements will, using said ratio, result in a transmission of about 63% (OD˜0.2) for a polarization perpendicular to the axis of the bendable nano-elements. A configuration with a transmission of about 10% (OD=1) for a radiation beam with a polarization direction parallel to the axis of bendable nano-elements will result in a transmission of about 86% (OD˜0.067) for a polarization perpendicular to the axis of the bendable nano-elements.

As intermediate bending angles for the bendable nano-elements are possible, so, between non-bent and fully bent, not the fall range of the ratio may be used in an application. For each application the driver-field or driver-fields can be tuned to such a level that the bending angles result in the required absorption level.

It may be possible, that to achieve a sufficiently homogenous absorption over the cross section with the radiation beam, multiple electrodes are required to generate substantially the same bending angles for all the nano-elements in that cross section.

In another embodiment of the invention the variable optical component has a non-uniform distribution of bendable nano-elements. Such a non-uniform distribution of bendable nano-elements does not have to be rotational symmetric. It can have any distribution suitable for the application, for example, a density distribution variation in one direction and in a direction perpendicular to said one direction a uniform density distribution.

FIG. 4 shows a schematic example of a cross section of such a variable optical component 10 having a non-uniform distribution of bendable nano-elements 3. The density of bendable nano-elements in the center of the variable optical component is higher than in the outer region. The variable optical component in this example has a rotational symmetry along the optical axis 16. When the variable optical component is placed in a radiation beam of for example an optical head, and no driver-field is applied (off-state) such that the bendable nano-elements are in a substantially non-bent state with respect to the direction of the radiation beam passing the nano-elements, there is no absorption and thus no effect on the radiation intensity distribution of the passing radiation beam. This can be the state of the variable optical component during a write mode of the optical head, in which a high efficiency from radiation source to medium is required. When a driver-field is applied (on-state) between the electrodes 1, 2 the bendable nano-elements can be bent. When the bending direction is in the direction of the polarization of the radiation beam (so parallel to the polarization direction of the radiation beam), the absorption in the center part of the variable optical component will be larger than in the outer part. A first advantage is that the required radiation emitting power of the radiation source in a read-mode is increased to reduce the noise generated by the radiation source as discussed in previous embodiments. A second advantage is that the intensity distribution in the radiation beam is modified in such way that the rim-intensity is increased and thereby reducing the focus spot size on the medium, so, increasing the readout resolution (see also Tawa L. et al., SPIE Vol. 3864 (1999), p. 37-39).

For optical systems such as CD, DVD and BD the rim-intensity is important with respect to the read-out quality of the data from the disc. In applications such as optical recording the radiation source is an edge emitting semiconductor laser. The far field aspect ratio of these lasers is usually such that the beam divergence of the emitted beam in the direction parallel to the active layer of the laser is smaller than the beam divergence in the direction perpendicular to the active layer of the laser. Using a coupling lens to couple the emitted radiation beam into the optical system, the numerical aperture of this lens will be limited by the minimum requirements on the rim-intensity of the resulting radiation beam. This will be determined by the beam divergence angle in the parallel direction of the active layer of the laser. As a result the rim-intensity in the direction corresponding to the perpendicular direction will be high. This means that quite a substantial amount of emitted radiation will not be used (falling outside the numerical aperture of the coupling lens) especially in the direction corresponding to the perpendicular direction. The coupling lens can be integrated in the objective lens of the optical head. The objective lens is then of the finite conjugate type.

In conventional beam shaping optics usually anamorphic prisms are used to reshape the substantially elliptical radiation intensity distribution of the emitted radiation beam into a radiation beam with a more circular radiation intensity distribution. With the same requirement on the minimum rim-intensity more radiation can be coupled into the optical system. Such anamorphic prisms require a perfectly parallel beam at its entrance, therefore making a collimator lens between the radiation source and the beam shaper necessary. This means that additional optical components are needed in the readout and servo optical path of the optical head to focus the radiation reflected by the disc onto a detector. Furthermore there are positional stability requirements between the radiation source and the collimator lens; defocus of the laser with respect to the collimator lens will result in astigmatism in the beam exiting the anamorphic prism and thus in a reduction of the optical quality of the radiation beam.

It is also possible to increase the rim-intensity by reducing the intensity in the center part of the beam while substantially not affecting the intensity at the outer part (rim) of the beam by using for example a variable optical component such as shown in and discussed in relation to FIG. 4. In this situation the coupling efficiency can be increased by applying a relatively large numerical aperture for the coupling lens thereby reducing the rim-intensity, and accordingly reduce the intensity at the center portion of the radiation beam in order to end up with corrected (relative) rim intensities are obtained. For optical systems requiring high rim-intensities such as BD the loss of radiation due to the absorption in the variable optical component can be less than the gain in coupling efficiency due to the larger numerical aperture of the coupling lens. A gain in coupling efficiency of about 50% or more can be obtained in designs for such optical systems. In the above description the variable optical component will be in an on-state in the writing mode to obtain the required absorption in the central portion of the radiation beam passing the variable optical component.

The application of a variable optical component with beam shaper functionality does not require the tight tolerances as needed for anamorphic prisms or lens type beam shapers.

In both situations where the variable optical component has a uniform or a non-uniform distribution of bendable nano-elements, the electrode configuration for applying a driver-field or multiple driver-fields can be segmented. FIG. 5 shows some examples of possible segmentations for a variable optical component. The bendable nano-elements can be located within in a circular area, such as for example in circular cross section of the variable optical component with the radiation beam. In FIG. 5A annular segments 51, 52, 53, 54 of the electrode configuration in combination with a homogeneous density distribution of bendable nano-elements is schematically shown. Such a configuration gives, for example, the possibility to tune the absorption per segment per laser, because semiconductor lasers as applied in optical storage applications are not all identical. The radiation emission angles (FWHM) in both the parallel and perpendicular directions spread between production batches of the lasers. Another advantage is that when such segmented devices are in an optical head applying multiple lasers (for example, a red laser for DVD and a blue-violet laser for BD), the variable optical component segment absorptions can be differently tuned for each laser. The driver-field between the segments can then be adapted to fit to the requirements for each laser type. The number of segments is not limited to the number shown in the figure. In FIG. 5B an example is shown with segmentation in only one direction. Segments 55a and 55b, but also 56a and 56b, 57a and 57b, 58a and 58b, can be all addressed individually or as pairs. In the application this direction will most likely be in the direction of the parallel farfield of the laser, such that the rim-intensity in that direction can be increased. Although the previous description is related to electrode configurations it can also be related to segments of distributions of bendable nano-elements.

This segmentation can also be such that each segment can be individually addressed in order to apply a specific driver-field per segment. The driver-field can be different per segment per write and read mode. The segment can have any shape, such as square pixels, annular rings, or other shape. It is also possible that the distribution of bendable nano-elements is segmented, for example such as schematically shown in FIG. 5C. In the example of FIG. 5C the central segment 51a has a highest density of bendable nano-elements and in the outer segment 54a a lowest density of bendable nano-elements.

A similar effect as obtained with a non-uniform distribution of bendable nano-elements on the variable optical component can also be obtained when using a combination of a uniform intensity distribution and a non-uniform driver-field applied by a suitable electrode configuration. Due to the non-uniform driver-field applied the bending angle of the bendable nano-elements is not homogeneous over the cross section and thus is the absorption also not homogeneous over the cross section of the radiation beam.

Variable optical components can be designed and made having combinations of the features described in this application. Furthermore it is also possible that the variable optical component comprises a stack of substrates with bendable nano-elements. In FIG. 6 an example of such a stacked device is shown. The individual substrates 4, 4′and 4″ comprise bendable nano-elements 3, 3′and 3″ as well as electrodes 1,2, 1′, 2′ and 1″, 2″ for applying a driver-field to the bendable nano-elements. The number of substrates is not limited to the amount shown in FIG. 6. The substrates with bendable nano-elements are separated by means of separators 30. In FIG. 6 six of such separators are shown. The separators can be of glass, plastic or any other suitable material. The top substrate 4″ with bendable nano-elements 3″ can be covered with a cover 31. The cover can be made of glass, plastic or other suitable transparent material. The stack as shown in FIG. 6 can be considered as a 3-dimensional segmentation of the variable optical component in the direction of its optical axis 16. The individual substrates (4, 4′, 4″) can comprise a uniform or non-uniform distribution of bendable nano-elements. It is also possible that each substrate with bendable nano-elements has segments with distributions of bendable nano-elements or comprises a segmented (multiple) electrode configuration. The electrodes on the substrates in the stack can be addressed simultaneously, i.e. as if it were a single stack, or individually.

An advantage of such a stack is that the bendable nano-elements can be distributed on the substrate in a uniform density, which is easier to manufacture than for example a non-uniform distribution of a bendable nano-elements in which the density of bendable nano-elements in the center of the variable optical component is higher than in the outer region. Also the functionality of a variable optical component such as example shown in FIG. 5c can be made with a device as shown in FIG. 6 using a stack with multiple substrates while each substrate has a uniform density distribution of bendable nano-elements.

In FIG. 7 a schematic representation is given for a variable optical component 10 used in an optical head 100 including means for controlling the variable optical component as well as the amount of radiation out of the radiation source. The radiation source 11 emits a radiation beam passing the variable optical component 10. The transmitted radiation beam passes the beam splitter 15 and is coupled into the lens 12 that focuses the radiation beam onto a medium 13. The radiation reflected by the medium is passing the lens and directed by the beam splitter 15 towards a radiation detector 14. Making use of the astigmatism generated in the beam by means of a cylindrical lens 18 an astigmatic focusing method is possible. It is also possible to have a reflection type variable optical component in the optical head.

A forward sense detector 19 is positioned in a part of the emitted radiation beam to measure and control the radiation output of the radiation source. In this case part of the emitted radiation is directed towards the forward sense detector 19 via a beam splitter 23. The output signal of the forward sense detector is an input signal for the radiation source controller 20, which controls the amount of radiation out of the radiation source 11 to for example a read level and a write level.

The amount of radiation that has passed the variable optical component is also monitored using a monitor detector 21. This is, for example, to monitor the effect of the settings of the variable optical component in the read or write mode radiation power towards the medium. The output signal of the monitor detector 21 can be an input signal for a variable optical component controller 22. This variable optical component controller 22 controls the driver-field or fields of the variable optical component. These driver-field or fields are required to bend the bendable nano-elements of the variable optical component.

When the system is set in a read mode the radiation source controller 20 sets the radiation output of the radiation source at a predetermined value. Optionally this can be done making use of the signal from the forward sense detector 19. The variable optical component controller 22 sets the variable optical component 10 in a read mode. This can be a predetermined setting with respect to the driver-field or fields necessary to obtain the required absorption of the radiation beam passing the variable optical component. The monitor detector 21 can monitor the radiation beam power transmitted through the variable optical component. Fine-tuning of the settings of the variable optical component controller can be done using the output of the monitor detector 21. The radiation beam passes thought the lens 12 and is focused in the medium 13 to readout the data. The radiation reflected by the medium is transmitted through the lens 12 and directed towards the signal detector 14. Focusing of the radiation beam can be controlled by a commonly used focusing method, such as the astigmatic focusing method.

When the system is set in a write mode, the controller 22 sets the variable optical component 10 in a write mode. This can be such that there is substantially no absorption due to the bendable nano-elements, or such that there is absorption in a way that beam shaping is applied. The output power of radiation source 10 is set at a write power level by the radiation source controller 20. Radiation source controller 20 can make use of the output signal of the forward sense detector 19 during the settings and/or the read and/or write mode of the system. Also the variable optical component controller 22 can make use of the output signal of the monitor detector 21 during the settings and/or read and/or write mode of the system.

The monitor detector 21 can be a single detector or a segmented detector suitable for measuring the transmission of a single segment or multi-segment variable optical component. In case of a multi-segmented monitor detector it is possible to fine-tune each segment of the variable optical component 10 via its controller 21.

The radiation source controller 20 and variable optical component controller 22 as well as radiation source controller 20 and the monitor detector 21 may have electrical interactions in order to set the output power of the radiation source 11 and the setting of the variable optical component 10. The dashed lines in FIG. 7 represent such possible interactions. These electrical interactions have the advantage that timing of the settings of the radiation source output power and the settings of the variable optical component can be controlled in a better way. It is also possible that, with the radiation source controller linked to the controller of the variable optical component, the settings can be controlled and/or tuned for a multiple of radiation sources in the optical head.

This radiation source controller 20 and/or the variable optical component controller 22 can be located on the optical head or on in another part of the optical system comprising the optical head.

The variable optical component can be positioned in radiation beam having a vergence or in a parallel beam parallel beam.

The variable optical components as described can also be used in other optical applications were a radiation beam intensity is to be modified. Such applications can for example be laser printers, microscopy, etc.

Optical Head with a Variable Optical Component

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