We introduced a laser-based noncontact shallow water depth measurement technique from a flying unmanned aerial vehicle (UAV). The water depth is measured by imaging two laser beam spots scattered from the surface and bottom of the water. The effect of water surface waves and UAV tilt angles to the depth measurement has been studied for practical applications. We have further developed this laser-based detection system consisting of a green laser, a global positioning system, a camera with a narrow field of view lens, a laser range finder, and a single-board computer. The measurement system onboard of a UAV flying over a small lake has demonstrated satisfactory water depth measurement capability. The low-cost light weight UAV-based water depth measurement should benefit water depth monitoring, mapping, and reporting in a hazardous environment offering flexibility, mobility, and remote control safe operation.
Planar concave grating wavelength demultiplexers with a flattened spectral response are realized based on SU-8 polymer waveguides. The flattened spectral response is accomplished by using an optimized multimode interference (MMI) coupler as the input aperture of the planar waveguide for all spectrally separated channels. The mode field distribution at the input of the planar waveguide is controlled by adjusting the width of the input taper connected to the MMI coupler. The devices are fabricated by cost-effective one-step UV lithography. Experimental results show that the desired flattened spectral response has been realized. The on-chip loss, crosstalk, and nonuniformity of the fabricated device are −14.8, −22, and 2.5 dB, respectively.
We report on improving lateral resolution of optical coherence tomography (OCT) for imaging of skins using multiframe superresolution technique. Through introduction of suitable slight transverse positional shifts among a series of C-scans, the superresolution processing of the lateral low resolution images at each axial depth reconstructs a high resolution image. Superresolution processing of all depth layers yields a high resolution 3D image. Using known resolution photomasks, 3 times lateral resolution improvement has been confirmed for both low and high numerical aperture OCT imaging. The superresolution processed OCT 3D skin image provides much more feature details for all subsurface depth layers within the OCT axial imaging range.
A 4-channel planar concave grating device with a flattened spectral response based on SU-8 polymer is presented.
The flattened spectral response is accomplished by using an optimized multi-mode interference coupler as the input
aperture of the device for spectrally separated channels. The mode field distribution in the input plane is controlled
by adjusting the width of input taper coupled to the multi-mode interference coupler. The effects of the input taper
width on the flattened spectral response are demonstrated in detail through simulation results. The devices are
realized by using an SU-8 polymer strip waveguide with a UV lithography technology. Experimental results show
that the flattened spectral response can be easily controlled by adjusting the taper width.
Multi-frame superresolution technique has been used to improve the lateral resolution of spectral domain optical coherence tomography (SD-OCT) for imaging of 3D microstructures. By adjusting the voltages applied to 𝑥 and 𝑦 galvanometer scanners in the measurement arm, small lateral imaging positional shifts have been introduced among different C-scans. Utilizing the extracted 𝑥-𝑦 plane en face image frames from these specially offset C-scan image sets at the same axial position, we have reconstructed the lateral high resolution image by the efficient multi-frame superresolution technique. To further improve the image quality, we applied the latest K-SVD and bilateral total variation denoising algorithms to the raw SD-OCT lateral images before and along with the superresolution processing, respectively. The performance of the SD-OCT of improved lateral resolution is demonstrated by 3D imaging a microstructure fabricated by photolithography and a double-layer microfluidic device.
Evaluation of tear film is performed by an optical reflectometer system with alignment guided by a galvanometer scanner. The reflectometer system utilizes optical fibers to deliver illumination light to the tear film and collect the film reflectance as a function of wavelength. Film thickness is determined by best fitting the reflectance-wavelength curve. The spectral reflectance acquisition time is 15 ms, fast enough for detecting film thickness changes. Fast beam alignment of 1 s is achieved by the galvanometer scanner. The reflectometer was first used to evaluate artificial tear film on a model eye with and without a contact lens. The film thickness and thinning rate have been successfully quantified with the minimum measured thickness of about 0.3 μm. Tear films in human eyes, with and without a contact lens, have also been evaluated. A high-contrast spectral reflectance signal from the precontact lens tear film is clearly observed, and the thinning dynamics have been easily recorded from 3.69 to 1.31 μm with lipid layer thickness variation in the range of 41 to 67 nm. The accuracy of the measurement is better than ±0.58% of the film thickness at an estimated tear film refractive index error of ±0.001 . The fiber-based reflectometer system is compact and easy to handle.
We report an integrated optical coherence tomography (OCT) and reflectometry system for ophthalmology imaging. The dual-functional device provides a complementary high-resolution tear film evaluation by reflectometry and anterior segment imaging by OCT, offering a more comprehensive anterior segment examination. The imaging measurement capabilities have been demonstrated on a human eye as well as on a model eye. The minimum measured tear film thickness is 0.3 μm with measurement resolution of less than ±0.58% of film thickness yet the OCT anterior segment offers a depth resolution of 7 μm with a 45-nm bandwidth superluminescent light source at 840-nm center wavelength. The integrated system has demonstrated the capability for three-dimensional imaging in the anterior segment of the eye.
A spectral domain optical coherence tomography system is integrated with an optical reflectometer to provide dualfunctional eye measurement. The system is capable of performing anterior segment imaging and tear film thickness evaluation at the same time. The axial resolution of the anterior segment imaging is 6μm while for tear film thickness measurement the resolution is about 21 nm. We use the integrated device to examine a model eye with artificial tear film. Structures such as the cornea, the ciliary muscle, and the front boundary of the crystalline lens are clearly visible. Artificial tear film thickness is determined simultaneously with anterior segment imaging. The integrated device is also flexible for separated anterior segment imaging or tear thickness evaluation.
We report the evaluation of water film on a contact lens using an improved optical reflectometry technique. A
galvanometer scanner is added to an optical reflectometry system for fast measurement beam alignment. Light from
a Tungsten Halogen light source travel down a 2×1 fiber coupler, go through the focusing lens and the galvanometer
scanner, and light up the water film on the contact lens. The air/water and water/contact lens interfaces reflect the
light back to the fiber, where the spectral dependent reflection data is acquired by the fiber coupled spectrometer for
analysis. From the reflective spectra, the water film thickness can be calculated using predictor-corrector curve
fitting method. In the scanning selection and the curve fitting calculation, a band stop filter is applied to the
reflectance spectrum to eliminate data noise.
We demonstrate cis-to-trans transition based red light holographic recording in an azobenzene-liquid crystal (LC)
polymer material following efficient crucial transition of trans-to-cis, which is prepared by selected wavelength light
pre-illumination. The presence and orientation of soft liquid crystal impregnating the polymer backbone allows the
cis intermediate states to hold for the stable red hologram recording and non-destructive readout. Using a 50 nm
bandwidth light source at the center wavelength of 575 nm, we have efficient pre-illumination on the material. The
diffraction efficiency of up to 2% can be achieved by a HeNe laser at 632.8 nm wavelength, and two-dimensional
holograms of USAF resolution target are successfully recorded.
We report an enhanced laser induced polarization switching and holography in a novel photo-reconfigurable material
system that contains polymer backbone impregnated by a soft liquid crystal material. The backbone of polymer chains is
reconfigurable and the surrounding movable liquid crystal molecules are reoriented collectively anchoring toward the
reconfigured polymer chains, resulting in remarkable enhancement of anisotropic refractive index modulation suitable
for updatable holographic recording. The dynamic polymer-liquid crystal network strengthening concept enables
significant enhancement of holographic performance with controllable updatability.
We report the use of optical reflectometry technique for evaluation of water film on contact lens. The water film can
be measured through the spectral dependent reflectance evaluation, which is carried out by illuminating the contact
lens with a white light and collecting the returning light with an optical fiber coupled to a spectrometer. Water film
thinning process has been observed on different soft contact lenses and minimum measurable thickness is about 0.85
μm. The measurement is fast and accurate. The water film measurement can be valuable for contact lens design to
improve its hydrophilic properties. The technique can be extended for the study of tear film dynamics in an eye.
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