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In this study we demonstrate a monolithic liquid crystal (LC) diffraction grating with multiple diffraction modes using the techniques of polymer stabilization and novel electrode design. It can be applied to a single-path rewritable optical pick-up (OPU) with multiple wavelengths for the tracking compatibility of CD, DVD, and blue-light storage systems.
©2007 Optical Society of America
1. Introduction
Backward compatibility is one of the most important issues in the progress of optical storage systems. It has been very clear in past DVD development and will be apparent in future blue-light systems also. Many studies regarding this topic have been presented. They can be attributed to many categories, such as improvements in optical system design, component design, and servo methodologies. One important improvement is the birth of the monolithic two-wavelength integrated-laser diode (TWIN-LD) [1–4]. It integrates two laser cavities, one for a 650 nm-wavelength diode and the other for a 780 nm-wavelength diode, in a single package. That integration greatly simplifies the optical system design from two complicated paths to one simple path and therefore reduces the manufacturing cost as well as the assembly complexity. Currently, the TWIN-LDs are being widely applied to optical pick-ups (OPUs) of DVD read-only drives and are being actively promoted for rewritable OPUs [5–7].
Fig. 1. Optical designs for (a) the multipath blue-light OPU that uses three gratings and three lasers and (b) the single-path blue-light OPU that uses a monolithic multimode grating and a monolithic three-wavelength laser.
As we know, most rewritable OPUs adopt the tracking servo of differential push-pull (DPP) for both CD and DVD recordable disks [8–11]. There are three tracking spots that are generated by a three-beam (3-B) grating projected on the land and adjacent grooves. Since the track pitches of CDs and DVDs are different, the grating for generating tracking beams of each wavelength must be designed to have proper period and depth for separating the laser beams on adjacent tracks in optimized diffraction efficiencies. As a result, two 3-B gratings corresponding to 650 nm and 780 nm lights are necessary for a conventional rewritable DVD OPU using two discrete lasers. Also, the orientation corresponding to each grating needs to be adjusted for each laser, respectively, while assembling it to the head. These requirements imply the challenge of fulfilling a single-path rewritable DVD OPU using a TWIN-LD to perform DPP tracking on both CD and DVD recordable disks. It must be accompanied by a monolithic 3-B grating for achieving dual diffraction modes. Each mode provides a different diffraction effect corresponding to each laser wavelength, respectively. A similar situation will apply to the next-generation rewritable blue-light system, once the monolithic three-wavelength laser device that integrates the 780, 650, and 405 nm laser chips in one package is invented and applied to the single-path blue-light OPU for being backwardly compatible for all recordable disks as shown in Fig. 1. Therefore, a monolithic 3-B grating that can provide triplex diffraction modes (multimode) is eagerly anticipated for the upcoming blue-light system. Figure 2 shows the comparison of beams on tracks among three disk types. There are cross angles θ 1 and θ 2 between the orientations of tracking spots. They depend on both the track pitches and the separations between the zero-order and the first-order spots.
Although a commercialized two-surface 3-B grating design having two different surface-relief patterns on both sides of the glass or plastic plate exists, it appears to have some shortcomings. One surface is optimized for diffracting the 650 nm light into diffraction orders in an adequate ratio for the DVD tracking. The optical path difference (OPD) of the groove on the surface must be in multiples of 780 nm so that it is almost fully transparent to the other wavelength. The same constraints have to be obeyed for the other surface so as to diffract the 780 nm light and to penetrate the 650 nm light for the CD tracking. Two grating patterns are fixedly etched or injection-molded on the surfaces. Therefore, in order to avoid the power loss, the producing accuracy and the manufacturing tolerance are very tight. Besides, the crosstalk between diffractions from both patterns is unavoidable because of the dependence of the wavelength shift on the temperature. Moreover, this two-surface design cannot be extended to that of a monolithic triplex-mode grating because it needs three different patterns. Under the circumstances, constructing a grating that has electrically switchable diffraction properties is a better way to resolve the difficulties stated above. It is reasonable to select the liquid crystal (LC) as the grating material for its modulating characteristics [12–19]. Recently we demonstrated a dual-mode grating using a novel electrode design [20]. In this study we further combine the techniques of a polymer-stabilized liquid crystal (PSLC) with complementary electrodes to present a multimode grating in a monolithic structure. It can achieve the features of switching among three distinct diffraction modes. The diffraction efficiency is tunable for obtaining the required power ratio projected on any type of disk. This device has the potential of being applied to a single-path blue-light OPU with backward compatibility.
2. Design configuration
The key design concept of the multimode grating is based on the novel electrodes, which can provide three operation modes. They can be switched by the external electrical connections for changing different diffractions. Figure 3(a) shows the schematic diagram. There are top and bottom electrodes patterned on two transparent conducting plates. The top electrode has spatially periodic varying stripes with the period calculated for the tracking of the first disk type and the first laser wavelength, whereas the bottom electrode has stripes with the different period for the tracking of the second disk type and the second laser wavelength. There are two complementary electrodes designed for compensating the top and bottom grating electrodes in order to form the common electrodes. For electrical isolation, a gap exists between the complementary and grating electrodes. Its width should be assigned to be as thin as possible for decreasing the discontinuity of the common electrode.
Fig. 3. (a). Schematic diagram of the multimode grating configuration and (b) cross angles between the grating electrode patterns and the PSLC patterns.
The LC mixed with ultraviolet- (UV) curable monomers is filled within two transparent conducting plates. The mixture needs to be exposed to UV light through the photomask of the third grating pattern that is calculated for the third disk type and the third laser wavelength. After the UV curing, the grating pattern will be stabilized inside the LC by the polymer networks. Two electrode grating patterns and one PSLC grating pattern are mutually crossed by angles as shown in Fig. 3(b). These angles relate to those described in Fig. 2. For the diffraction of the first or second wavelength, as one grating electrode is turned on for diffracting the laser beam, the complementary electrode of the other grating pattern will be connected to its corresponding grating electrode. Figure 4(a) shows the operation called the electrode-mode (E-mode) in which the bottom grating electrode dominates the diffraction effect, whereas the top electrode is accompanied by the complementary electrode as the common electrode. The contrary operation is shown in Fig. 4(b) for the top grating diffraction. The grating effects of above two modes are generated by the electrode patterns, so the applied voltage must overcome the higher threshold voltage to direct the LC molecule under the electrode region, whether it has been cured or not. Figure 4(c) shows the operation called the PSLC-mode (P-mode) where both the top and bottom grating electrodes are connected to their corresponding complementary electrodes for forming two common electrodes, respectively. The grating operated at this mode is generated by the uniform electrical field that comes from the common electrodes and drives the preformed pattern inside the LC/polymer mixture. This electrical field should only be able to direct the LC molecules in the uncured region and not to direct those in the UV-cured region.
3. Fabrication method
The proposed grating was fabricated and assembled according to the processes of common LC devices. Two photomasks that address the grating electrodes, the complementary electrodes, and the orientation, were designed for pattern transfer. The grating periods depend on the OPU system design and are not absolutely given. For functional verification and easy fabrication, they were assigned to be 340 µm and 154 µm for the top and bottom electrodes, respectively. The isolation gap width was set to be 5 µm. Figure 5 is an example that shows the electrodes on the photomask. The top and bottom electrode patterns were etched on two transparent glass plates that had been coated with indium-tin-oxide (ITO) conducting film. Two plates were then coated with polyimide and treated by horizontal rubbing. They were separated by 5 µm ball spacers and carefully aligned to assure the orientation as shown in Fig. 3(b) in the LC cell assembly. The cross angle was also functionally set to be 12 degrees, corresponding to the top electrode. The nematic LC that has the birefringence of Δn=0.12 was mixed with 5 wt% UV-curable monomer (Merck RM257). The LC/polymer mixture was injected into the cell. After being annealed, the cell was in proximity contact with the third photomask having the third grating period of 200 µm and was aligned with the cross angle of -12 degrees, corresponding to the top electrode during the UV exposure. Figure 6 shows the schematic construction and the UV exposure process for the grating. The UV light source with peak wavelength of ~365 nm and intensity of ~17 mW/cm2 was used to cure the device for forming the polymer networks inside the cell.
4. Experimental results
We tested the fabricated grating for verifying its mode-switching and diffraction effects. The He-Ne collimated laser beam was first used as the light source to illuminate the device. Though not the exact wavelength for the DVD, CD, or blue-light system, it doesn’t lose the validity of the functions. Figure 7 shows the diffraction patterns that were captured from the screen behind the device. Figure 7(a) shows the pattern as all electrodes were floated. It reveals that the device is almost transparent to the incident light. Owing to the OPD contribution of the conducting film thickness, weak diffraction occurs but its intensity is rather low and does not degrade the device’s functions. Figure 7(b) shows the diffraction pattern as the E1-mode described in Fig. 4(a) was operated. For the purpose of good contrast, we tuned the applied voltage to get higher first-order efficiency. As stated above, the alignments of both the electrodes and the P-mode photomask were followed by setting the top electrode as the reference. Therefore, three diffracted spots were spread counterclockwise to the horizontal line by an angle θ 1 that was assigned to be around 12 degrees in the fabrication process. Similar statements are suitable for Fig. 7(c) where the E2-mode described in Fig. 4(b) was operated. Their diffracted spots were located almost on the horizontal direction. Figure 7(d) shows the diffraction obtained from the operation of P-mode that was described in Fig. 4(c). In this figure, three diffracted beams were spread clockwise to the horizontal line by the other angle θ 2 that was assigned to be around -12 degrees. These experimental results apparently present different diffraction properties that are generated from different electrodes and operation modes. Besides, their spread orientations also verify the cross angles, which were assigned on the alignment of the device fabrication.
For assuring the modulating ability of the device, we measured its diffraction distribution in primary orders corresponding to the applied voltage. Figure 8 gives the efficiencies where the solid line denotes the curve for the zero-order as well as the dashed line for the positive first-order. These data were calculated by using the transmitted zero-order power in the off mode as the denominator. The tunable diffraction efficiencies on the zero and first orders are practically suitable for any type of rewritable OPU applications. Figure 8(a) is the data measured from the sample that was cured for 5 minutes, whereas Fig. 8(b) is that from the sample with a curing time of 6 minutes. They show the differences on the P-mode first-order efficiencies and the threshold voltages. The longer the curing time is, the higher the P-mode efficiency will be. However, the longer curing time will simultaneously increase the operation voltage.
Fig. 7. Pictures that show the diffraction effects as the multimode grating operated under (a) the turn-off mode, (b) the E1-mode, (c) the E2-mode, and (d) the P-mode. The cross angles of the spot orientations are the same as those in the electrode design.
Fig. 8. Diffraction efficiencies measured from (a) the sample cured for 5 minutes and (b) the sample cured for 6 minutes corresponding to three operation modes in the zero and first orders. These data were calculated by using the transmitted zero-order power in the off mode as the denominator.
For further verification of the application to the three-wavelength OPU, we tested the device’s diffractions using three different laser wavelengths. Figure 9 is the result that shows the switching ability corresponding to 780, 650, and 405 nm, respectively. Figure 10 shows the diffraction efficiencies measured from the same sample for Fig. 8(a) that correspond to three different wavelengths. Owing to the lasting exposure by the blue laser light whose wavelength is near the UV curing light, the measuring data appear to be a slight variation between Figs. 8(a) and 10. Nevertheless, they give evidence that this device really possesses the capability of providing three diffraction modes and can be applied to the use of single-path, three-wavelength, and blue-light OPUs.
5. Discussion and conclusion
We have demonstrated a monolithic multimode grating using the techniques of the PSLC and novel electrode design. The experimental results are satisfactory and meet the requirement of developing a single-path blue-light OPU that has all backward compatibility. Since the diffraction mode is switchable and the diffraction efficiency is tunable, this device will not cause power loss and diffraction crosstalk. Moreover, the fabrication processes are relatively simple and similar to common LC devices, so it possesses high potential for being applied to real applications.
Fig. 9. Pictures that show the diffraction effects obtained from three incident beams of different wavelengths.
Fig. 10. Diffraction efficiencies measured from the same sample for Fig. 8(a) that correspond to three different wavelengths.
There are still some challenges that must be overcome. First, the UV intensity and the curing duration must be well-optimized to get the best device performance for all modes. It includes improving the zero- and first-order efficiencies, avoiding the crosstalk among modes, and suppressing high-order diffractions. Second, since the wavelength of 405 nm for a blue-light OPU is near the peak wavelength of ~365 nm for the UV curing light, the PSLC grating mode will be gradually destroyed under a long period of operating a blue-light OPU. A new deep-UV curable prepolymer material is eagerly anticipated. Third, the isolation gap between the grating electrode and the complementary electrode results in some unwanted disturbance of the electric field distribution required for forming a common electrode. These above problems should be investigated further in the future.
Acknowledgments
The authors thank the National Science Council and the Ministry of Economic Affairs of Taiwan. This work was supported in part by the NSC under grant NSC 95-2221-E-005-148- and by MOEA under grant 95-EC-17-A-07-S1-011.
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