Hong-Shik Lee

Color filter based on a subwavelength patterned metal grating

A color filter incorporating a subwavelength patterned grating in a metal film perforated with a square array of circular apertures on a quartz substrate was accomplished. Its performance was enhanced by applying a dielectric overlay to the grating layer so as to match the refractive indices of the media on either side of it. The device was designed by utilizing the finite-difference time-domain method and implemented by adopting the electron-beam direct-writing technique. Two different devices were fabricated with the structural parameters: the grating height of 50 nm and the pitch of 340 nm for the red color and 260 nm for the green color. For the red color filter the center wavelength was 680 nm and the peak transmission 57%, while for the green color one the center wavelength was 550 nm and the peak transmission 50%. It was confirmed the introduction of the index matching overlay led to an increase of ~15% in the transmission efficiency and helped combine double bands into a single dominant band as well, thereby improving the color selectivity of the filter.

Color filter incorporating a subwavelength patterned grating in poly silicon

A color filter based on a subwavelength patterned grating in poly silicon was proposed and realized on a quartz substrate. It was produced by utilizing the laser interference lithography technique to feature wide effective area compared to the costly e-beam lithography. An oxide layer was introduced on top of the silicon grating layer as a mask to facilitate the silicon-etching and to enhance the filtering selectivity as well. The structural parameters for the device include the grating pitch and height of 450 nm and 100 nm respectively, the silicon stripe width of 150 nm, and the oxide thickness of 200 nm. The fabricated device offered a spectral response suitable for a blue color filter, exhibiting the center wavelength of approximately 460 nm, the bandwidth approximately 90 nm and the peak transmission 40%. The positional dependence of its performance was examined to find the effective area of 3 x 3 mm(2), where the variation in the relative transmission efficiency and in the center wavelength was less than 10% and 2 nm respectively. Finally, the influence of the angle of the incident beam upon the transfer characteristics of the device was investigated to reveal that the rate of change in the relative transmission was equivalent to about 1.5%/degree.

Compact laser transmitter delivering a long-range infrared beam aligned with a monitoring visible beam

A compact laser transmitter, which takes advantage of an optical subassembly module, was proposed and demonstrated, providing precisely aligned collinear IR and visible beams. The collimated IR beam acts as a long-range projectile for simulated combat, carrying an optical pulsed signal, whereas the visible beam plays the role of tracking the IR beam. The proposed laser transmitter utilizes IR (λ(1)=905 nm) and visible (λ(2)=660 nm) light sources, a fiber-optic collimator, and a beam combiner, which includes a wavelength division multiplexing (WDM) filter in conjunction with optical fiber. The device was built via the laser welding technique and then evaluated by investigating the characteristics of the generated light beams. The IR collimated beam produced had a Gaussian profile and a divergence angle of ~1.3 mrad, and the visible monitoring beam was appropriately collimated to be readily discernible in the vicinity of the transmitter. The two beams were highly aligned within an angle of 0.004 deg as anticipated. Finally, we performed a practical outdoor field test to assess the IR beam with the help of a receiver. An effective trajectory was observed ranging up to 660 m with an overall detectable beam width of ~60 cm.

Highly angle tolerant filter incorporating serially cascaded a-Si based etalons and its application to a compact receiver

A highly angle tolerant spectral filter has been implemented taking advantage of three-stage serially concatenated resonators in dielectric films, each of which involves a high-index cavity in a-Si, sandwiched with a pair of SiO2 films. For the constituent etalons, the free spectral range is individually adjusted by differentiating the thickness of the cavity, so that a primary infrared pass-band could be attained to present enhanced roll-off characteristics in conjunction with an appropriate bandwidth. The a-Si cavities relating to the three etalons are selected to be 117, 234, and 468-nm thick, while the SiO2 layer is uniformly 150-nm thick. The filter is actually created on a silica glass substrate, by alternately depositing SiO2 and a-Si films. The observed center wavelength, bandwidth, and peak transmission efficiency are about 900 nm, 120 nm, and over 90%, respectively, for normal incidence. In response to an angle change amounting to 60°, the relative wavelength shift and the variation in peak transmission become barely 0.03 and 8%, respectively. Finally, a detecting cell is constructed by integrating the prepared filter with a photodiode, thus rendering a 3-dB angular bandwidth of 90°. By adequately arranging three detecting cells in a fixture, a compact, portable optical receiver could then be constructed. For incoming collimated light at λ = 905 nm, the infrared receiver exhibits an extended 3-dB angular acceptance as large as 160°.

Precision Alignment of Infrared–Visible Beams Exploiting a Dual-Laser Transmitter Based on an Integrated Optic Beam Aligner

Precision-aligned co-collimated infrared (IR) and visible light beams were efficiently generated and simultaneously delivered via a transmitter, which was constructed by exploiting an integrated optic beam aligner in conjunction with a collimating lens. The beam aligner comprises two optimally separated silica waveguides preventing mode coupling, and delivers parallel beams at λ=905 and 650 nm. The aligner was fiber-pigtailed to the corresponding laser diodes and combined with an aspheric lens to construct a miniature dual-laser transmitter. Gaussian-like IR and visible beams, with divergence angles of 0.085 and 0.09°, respectively, were highly aligned with an angle of 0.075°.

Dual Beam Generator Incorporating a Guided-Wave Beam Aligner Hybrid Integrated with Infrared and Visible Laser Diodes

A compact dual beam generator was realized by exploiting an integrated optic beam aligner accompanied by an aspheric lens, to deliver collinear infrared (IR) and visible collimated beams. The aligner entails straight and bent silica waveguides for IR and visible channels, respectively, which are hybrid integrated with laser diodes at λ=905 and 605 nm. The guided-wave aligner is fixed to IR and visible laser diodes via active power monitoring, then combined with a lens to construct a 35-mm-long beam generator. The produced IR and visible collimated beams feature profiles with a divergence of ∼0.1° and an angular alignment of 0.13°.

Compact Infrared/Visible Laser Transmitter Featuring an Extended Detectable Trajectory

A miniaturized laser beam transmitter, in which a visible laser module at λ=650 nm is precisely stacked upon an infrared (IR) module at λ=905 nm, has been proposed and constructed to provide an IR collimated beam in conjunction with a collinear monitoring visible beam. In particular, the IR beam is selectively dispersed through a perforated sheet diffuser, so as to create a rapidly diverging close-range beam in addition to a highly defined long-range beam simultaneously. The complementary close-range beam plays a role in mitigating the blind region in the vicinity of the transmitter, which is inevitably missed by the main long-range beam, thereby uniformly extending the transmitters effective trajectory that is sensed by a receiver. The proposed transmitter was designed through numerical simulations and then fabricated by incorporating a diffuser sheet, perforated with an aperture of 2 mm. For the manufactured transmitter, the IR long-range beam was observed to have divergences of ~2.3 and 1.6 mrad in the fast and slow axes, respectively, while the short-range beam yielded a divergence of ~24 mrad. The angular alignment between the long-range IR and visible beams was as accurate as ~0.5 mrad. According to an outdoor feasibility test involving a receiver, the combination of the IR long- and short-range beams was proven to achieve a nearly uniform trajectory over a distance ranging up to ~600 m, with an average detectable cross-section of ~60×80cm2.

Color Filter Based on a Sub-wavelength Patterned Metal Grating

A color filter incorporating a subwavelength patterned grating in a metal film perforated with a square array of circular apertures on a quartz substrate was accomplished. Its performance was enhanced by applying a dielectric overlay to the grating layer so as to match the refractive indices of the media on either side of it. The device was designed by utilizing the finite-difference time-domain method and implemented by adopting the electron-beam direct-writing technique. Two different devices were fabricated with the structural parameters: the grating height of 50 nm and the pitch of 340 nm for the red color and 260 nm for the green color. For the red color filter the center wavelength was 680 nm and the peak transmission 57%, while for the green color one the center wavelength was 550 nm and the peak transmission 50%. It was confirmed the introduction of the index matching overlay led to an increase of ~15% in the transmission efficiency and helped combine double bands into a single dominant band as well, thereby improving the color selectivity of the filter.

Color Filter Incorporating a Fabry-Perot Etalon

Three color filters based on a simple Fabry-Perot etalon, which consists of an oxide thin film sandwiched in between two silver films, were demonstrated with no additional infrared cutoff filter included.

Color Filter Based on a Sub-Wavelength Patterned Poly-Silicon Grating Fabricated using Laser Interference Lithography

A color filter was proposed and demonstrated by incorporating a subwavelength patterned 1-dimensional grating in poly silicon. It was produced by employing the laser interference lithography method, providing much wider effective area compared to the conventional e-beam lithography. A layer was introduced on top of the silicon grating layer as a mask for the etching of the silicon, facilitating the etching of the silicon layer. It was theoretically found that the selectivity of the filter was also improved thanks to the oxide layer. The parameters for the designed device include the grating pitch of 450 nm, the grating height of 100 nm and the oxide-layer height of 200 nm. As for the fabricated filter, the spectral pass band corresponded to the blue color centered at 470 nm and the peak transmission was about 40%. Within the effective area of , the variation in the relative transmission efficiency and in the center wavelength was less than 10% and 2 nm respectively. Finally, the influence of the angle of the incident beam upon the transfer characteristics of the device was investigated in terms of the rate of the relative transmission efficiency, which was found to be equivalent to 1.5%/degree.