In a waveguide-type display for augmented reality, the image is injected in the waveguide and extracted in front of the eye appearing superimposed on the real world scene. An elegant and compact way of coupling these images in and out is by using blazed gratings, which can achieve high diffraction efficiencies, thereby reducing stray light and decreasing the required power levels. This study investigates the fabrication of blazed gratings with grayscale electron beam lithography and the subsequent replication of the realized 3D grating structures in a polymer material with ultraviolet nanoimprint lithography. As such, diffractive elements are realized on a waveguide sheet, with very good control over the dimensions and the profile of the printed features. Blazed gratings are designed for green light (λ= 543 nm) and a diffraction angle of 43°. Making use of a PMMA resist and by carefully optimizing the electron-beam parameters, electron dose distributions and development step, blazed gratings with a pitch of 508 nm and a fill factor of 0.66 are achieved. Finally, a master is realized with two blazed gratings, 3 cm apart, which are replicated using ultraviolet nanoimprint lithography onto a waveguide sheet. The in- and outcoupling of an image through these two blazed gratings is shown, appearing sharp and non-distorted in the environment, and a throughput efficiency of 17.4% is confirmed.
In this work, the reverse replication of circular micro grating structures on glass substrates is implemented using an ultra-violet curable resin and a polydimethylsiloxane (PDMS) mold which has the same structure as the original circular grating master. Two different techniques (“double PDMS replication” and “polymer- PDMS replication”) are employed to fabricate those reversed circular micro grating structures. Surface profiling measurements show that in case of the polymer-PDMS replication the dimensions of the resulting circular grating structures closely approximate those of the master, while the grating height is slightly decreased in case of the double PDMS replication technique, mainly due to the use of the releasing agent. For both methods, the grating slopes of the circular gratings are almost unchanged, leading to the desired optical performance. The two techniques are quite useful for more accurate reverse replications of micro optical and photonic structures.
In this article we report the fabrication of large arrays of micro-optical gratings using soft embossing with elastic Polydimethylsiloxane (PDMS) molds and ultra-violet (UV) curable resins. Three different kinds of resins are used to replicate the master gratings in a process akin to a roll to roll process. The optical surface profiling measurements show that the dimensions of the replicated gratings closely approximate those of the master gratings. Optical diffractions of these gratings are also measured and analyzed.
LED-based projectors have numerous advantages compared to traditional projectors. They are more compact, they exhibit a larger color gamut and a longer lifetime, the supply voltage is lower and they can even operate on batteries. LEDs can switch rapidly (possibility to pulse) and they have a high dimming ratio (contrast considerations). However, they have low optical power per étendue, although this is also improving consistently. With an efficient illumination engine design we can build an LED projector with a moderate light output and with superior properties. We present a relatively compact LED projector with two liquid crystal on silicon (LCOS) light valves (LVs). One of these LVs alternately modulates red and blue information, while the other permanently modulates green information to achieve a good color balance. Additionally, we apply some methods to increase the brightness on the screen. Our two-LCOS approach results in a compact, efficient LED projector that produces 171 lm projected D65 flux.
LED based projectors have numerous advantages compared to traditional projectors: they are more compact, exhibit a larger color gamut and a longer lifetime, the supply voltage is lower, the absence of ultra violet, infrared radiation and mercury vapour, etc. Furthermore LED's can switch on and off very rapidly (possibility to pulse them) and they have a high dimming ratio that can be used to improve the contrast. However, there is also an important disadvantage: the optical power per unit of etendue (luminance) of an LED is significantly lower than that of e.g. an UHP-lamp. Because of this and the etendue limitation of the projector (small light valve, f-number projection lens), the projected flux on the screen will not be high. Despite this shortcoming, LED's are still very interesting for low power applications because of their superior properties. However we have to collect the available light flux optimally and combine multiple LED's with high luminance within the available system etendue. In this paper we have studied collection optics that collect the LED flux with high optical efficiency and collimation and reshape the spot in a uniform illuminated rectangle with the sizes of the micro display. We have designed 'Gradually Tapered Light Pipes', 'Elliptical Reflectors' and 'Parabolic Reflectors'. Furthermore we have combined many of these LED/collector combinations to get a high luminance illumination engine for LED based projectors.
LED-based projectors have numerous advantages compared to traditional projectors, such as compactness, larger color gamut, longer lifetime, and lower supply voltage. As LEDs can switch rapidly, there is the possibility to pulse. However, there is also an important disadvantage. The optical power per unit of étendue of an LED is significantly lower than, e.g., an ultra-high-performance (UHP) lamp. This problem can be remedied partly by pulsing the LEDs. If one drives an LED with a pulsed current source, the peak luminance can be higher, albeit the average luminance will not increase. By pulsing two LEDs alternately (50% duty cycle), their increased flux can be added up in time and will generate a higher average flux within the same étendue. We combine the LEDs with a polarizing beam splitter (PBS) and change the polarization of one LED with a switchable retarder. The achieved substantial net gain after all losses is 36%.
Led based projectors have numerous advantages compared to traditional projectors, such as: compact, larger color gamut, longer lifetime, lower supply voltage, etc. As LED's can switch rapidly, there is the possibility to pulse. However, there is also an important disadvantage. The optical power per unit of etendue of a LED is significantly lower than e.g. an UHP-lamp (approximately 50 times). This problem can be remedied partly by pulsing of the LED’s. If one drives a LED with a pulsed current source, the peak luminance can be higher, albeit that the average luminance will not increase. By pulsing X LED's alternately, their increased flux can be added up in time and will generate a higher average flux within the same etendue. This can be carried out in a number of different configurations. The first configuration uses moving components where a number of LED's (e.g. 8) are mounted on a carrousel and consecutively the pulsed LED is brought in the light path of the projector to fill up the time with its peak flux. An alternative without moving components can be reached with 2 LED's which are combined with a PBS. By alternately pulsing the LED's with 50% duty cycle and changing the polarisation of one LED with a switchable retarder, one can combine the flux of both LED's in the same etendue. Because of its fast switching time ferro-electric retarders are used here. This can be extended further to 4,8,16... LED's, at the price of a larger and more complicated optical architecture.
We developed a measurement method for the characteristics of microdisplays specifically aimed at vertically aligned nematic reflective cells. It allows determination of contrast ratio and cell gap, and gives good estimates for the pretilt angle and the elastic surface-coupling constant. The set-up consists of a laser source, high quality polarisers, a beamsplitter mirror, a quarter-wave plate and a sensitive photodiode. A model for the polarization changes in the light caused by each component allows the extraction of the initial phase retardation induced by the cell and gives a first estimate of the thickness. Simulation of the director configuration in liquid crystals is then used to enhance the accuracy by taking into account the properties of a real LC cell. Matching of the simulation and the measurements yields the required values together with a calibrated simulation model.
Liquid crystal on silicon (LCOS) is becoming an established technology for personal viewers and projection applications. However no unique technology is used at present. One has different silicon backplane architectures as well as liquid crystal technologies. Analog addressed DRAM backplanes are often preferred in projection applications, however these architectures often require higher voltage addressing schemes than the standard used 5 V CMOS. Using oblique evaporation of SiO2, a reflective vertically aligned nematic liquid crystal technology was developed enabling the manufacturing of microdisplays for projection applications in a standard 5 V CMOS backplane technology. The developed technology showed acceptable optical performance and might be an interesting alternative to the present used microdisplay technologies.
Recently LCOS microdisplays are becoming available for personal IT applications, despite some problems which are less critical in poly silicon or amorphous silicon based displays. The most common problems which must be encountered are the polarization of the pixels and the light shielding of the silicon substrate. In this paper a methods proposed which solves the light shielding and pixel flatness problem. A non-critical back-end processing which can be applied outside the silicon foundry has been developed. The effectiveness of the light shielding on a working demonstrator display is shown. To avoid light losses caused by a polarization filter, a polymer dispersed LC has been chosen. By decreasing the cell gap we made the PDLC voltage compatible with a standard 3 micrometers CMOS process and its response fast enough to be used for video applications. It is shown that this choice is very suited in direct view and portable applications. The realized prototype has 3 bit grey levels and is video compatible and can be used in a number of applications, such as personal viewers, PDAs and data displays.