Yu-Jen Wang

Extended depth-of-focus 3D micro integral imaging display using a bifocal liquid crystal lens.

We present a three dimensional (3D) micro integral imaging display system with extended depth of focus by using a polarized bifocal liquid crystal lens. This lens and other optical components are combined as the relay optical element. The focal length of the relay optical element can be controlled to project an elemental image array in multiple positions with various lenslet image planes, by applying different voltages to the liquid crystal lens. The depth of focus of the proposed system can therefore be extended. The feasibility of our proposed system is experimentally demonstrated. In our experiments, the depth of focus of the display system is extended from 3.82 to 109.43 mm.

Electrically adjustable location of a projected image in augmented reality via a liquid-crystal lens

An augmented reality (AR) system involving the electrically tunable location of a projected image is implemented using a liquid-crystal (LC) lens. The projected image is either real or virtual. By effectively doubling the LC lens power following light reflection, the position of a projected virtual image can be made to vary from 42 to 360 cm, while the tunable range for a projected real image is from 27 to 52 cm on the opposite side. The optical principle of the AR system is introduced and could be further developed for other tunable focusing lenses, even those with a lower lens power. The benefits of this study could be extended to head-mounted display systems for vision correction or vision compensation. We believe that tunable focusing LC optical elements are promising developments in the thriving field of AR applications.

Eliminate coherence spike in reflection-type pump-probe measurements.

The coherence spike of femtosecond laser pulses in the reflection-type pump-probe measurements has been systematically studied in the semiconductor (100) InP. By varying the setup of the pump-probe measuring system, i.e. the polarizations of pump and probe pulses, the incident angles of pump and probe beams, and the interval of delay time between pump and probe pulses, the dramatic changes in the strength of coherence spike could be clear observed. Furthermore, the proposed methods to remove the coherence spike from the transient reflectivity curves have been demonstrated in the time-domain measurements.

Single-shot detection of mid-infrared spectra by chirped-pulse upconversion with four-wave difference frequency generation in gases

Summary form only given. Single-shot detection of an entire MIR spectrum (500-5000cm-1) has been required for advanced molecular spec-troscopy such as pump-probe spectroscopy to trace ultrafast structural dynamics of molecules, real-time molecular imaging of biological tissues, etc. However, due to low pixel numbers, low sensitivity, and high cost of multi-channnel MIR detectors, the bandwidth has been limited to ~500 cm-1 at direct measurement of MIR spectra by using dispersive infrared spectrometers.Chirped-pulse upconversion is an alternative approach to detect an MIR spectrum with single-shot. By converting the wavelength of coherent MIR pulse to visible range, it becomes possible to detect MIR spectra with a visible spectrometer, which has much higher performance than MIR spectrometers. However, the bandwidth of the chirped-pulse upconversion has still been limited to ~1000 cm-1 because of the limited transmission range of the nonlinear crystals [1]. In this contribution, we have demonstrated ultrabroadband detection of MIR spectra on a single-shot basis using chirped-pulse upconversion with four-wave difference frequency generation (FWDFG) in gases. The schematic of the method is shown in Fig. 1(a). By using a gas as a nonlinear medium, the detection bandwidth becomes dramatically broad due to wide transmission range of gas media. Experimental demonstration of the scheme was realized with the system described as follows. We generated sub-single-cycle MIR pulses by using four-wave mixing of the fundamental and the second harmonic of Ti:sapphire amplifier (Femtopower compactPro, FEMTOLASERS) output through filamentation in air, which is basically the same generation scheme as that reported in Ref. 2 and 3. A small portion of the fundamental pulse (0.1 mJ) before the compressor of the Ti:sapphire amplifier was used as a chirped pulse, whose pulse duration was 10.3 ps. The chirped pulse Eref(t - τ) and the MIR pulse (EIR(t), 0.5 μJ) were focused into xenon with a parabolic mirror (f=50 mm) and generated a FWDFG signal, E2 ref(t - τ)EIR(t), which spread from 400 nm to 550 nm. The spectrum of the FWDFG signal was measured with a conventional spectrometer with a camera EMCCD (ProEM+1600, Princeton Instruments). The camera was synchronized with the repetition rate (1 kHz) of the laser and the spectrum was measured with a single shot, namely within 1 ms. A typical spectrum is shown as the upper curve in Fig. 1(b). By using retrieval algorithm from the upconverted spectrum to the original MIR spectrum [4] including the nonlinear chirp of the reference pulse, it was possible to retrieve the MIR spectrum shown as the lower curve in Fig. 1(b). Fine structure due to absorption of carbon dioxide (~2300 cm-1) and water vapor (~1600 cm-1 and ~3700 cm-1) in air was clearly observed. At the conference, we plan to show the application of the system to MIR absorption spectroscopy.

Extended depth-of-field 3D endoscopy with synthetic aperture integral imaging using an electrically tunable focal-length liquid-crystal lens.

Conventional synthetic-aperture integral imaging uses a lens array to sense the three-dimensional (3D) object or scene that can then be reconstructed digitally or optically. However, integral imaging generally suffers from a fixed and limited range of depth of field (DOF). In this Letter, we experimentally demonstrate a 3D integral-imaging endoscopy with tunable DOF by using a single large-aperture focal-length-tunable liquid crystal (LC) lens. The proposed system can provide high spatial resolution and an extended DOF in synthetic-aperture integral imaging 3D endoscope. In our experiments, the image plane in the integral imaging pickup process can be tuned from 18 to 38 mm continuously using a large-aperture LC lens, and the total DOF is extended from 12 to 51 mm. To the best of our knowledge, this is the first report on synthetic aperture integral imaging 3D endoscopy with a large-aperture LC lens that can provide high spatial resolution 3D imaging with an extend DOF.

A large bistable negative lens by integrating a polarization switch with a passively anisotropic focusing element

A bistable negative lens with a large aperture size (~10mm) by integrating a polarization switch of ferroelectric liquid crystals (FLCs) with a passively anisotropic focusing element is demonstrated. The proposed lens not only exhibits electrically tunable bistability but also fast response time of sub-milliseconds. The tunable lens power is from 0 to -1.74 Diopters. The electro-optical properties and imaging performances are demonstrated. The impact of this study is to provide a solution of electrically bistable liquid crystal lenses for the applications of portable devices, wearable devices and colored ophthalmic lenses.

A Polarizer-Free Liquid Crystal Lens Exploiting an Embedded-Multilayered Structure

Development of liquid crystal (LC) lenses is limited by the power law. The aperture size of LC lenses is traded for the lens power. In this letter, we prove theoretically and experimentally that the aperture size is not limited by the power law based on a polarizer-free LC lens exploiting an embedded-multilayered structure. By adding numbers of LC layers, the aperture size of the LC lens can be enlarged without lowering the tunable lens power. The optical theory of the polarization independence of the LC lens is derived. The wavefronts are measured after light propagates through the LC lens to discuss the polarization independence and image adjustment. The impact of this study is to show the possibility of LC lenses with large aperture size for wearable devices and ophthalmic applications.

A polarized bifocal switch based on liquid crystals operated electrically and optically

A polarized bifocal switch based on liquid crystals (LC) operated electrically and optically is demonstrated. The bifocal switch mainly consists of two parts: a LC layer as a polarization switch and two polymeric layers for modulation of polarization dependent spatial phase difference which results in a positive or a negative lensing effect. The orientations of the LC molecules in the polarization switch are manipulated either in electrically switching (ES) mode or optically rewritten (ORW) mode. The bifocal switch with an aperture size of 10 mm exhibits two discrete lens powers (−1.39 Diopter and +0.7 Diopter) with different polarization states no matter in ES mode or in ORW mode. ORW mode is also a bistable mode. The related mechanism and electro-optical performance are discussed and demonstrated. Such a versatile optical switch is capable of not only switching between a positive lens power and a negative lens power, but also switching between two linear polarization states which can be useful in optica...

Augmented reality with image registration, vision correction and sunlight readability via liquid crystal devices

Augmented reality (AR), which use computer-aided projected information to augment our sense, has important impact on human life, especially for the elder people. However, there are three major challenges regarding the optical system in the AR system, which are registration, vision correction, and readability under strong ambient light. Here, we solve three challenges simultaneously for the first time using two liquid crystal (LC) lenses and polarizer-free attenuator integrated in optical-see-through AR system. One of the LC lens is used to electrically adjust the position of the projected virtual image which is so-called registration. The other LC lens with larger aperture and polarization independent characteristic is in charge of vision correction, such as myopia and presbyopia. The linearity of lens powers of two LC lenses is also discussed. The readability of virtual images under strong ambient light is solved by electrically switchable transmittance of the LC attenuator originating from light scattering and light absorption. The concept demonstrated in this paper could be further extended to other electro-optical devices as long as the devices exhibit the capability of phase modulations and amplitude modulations.

A liquid crystal and polymer composite film for liquid crystal lenses

Liquid crystal (LC) lenses offer novel opportunities for applications of ophthalmic lenses, camera modules, pico projectors, endoscopes, and optical zoom systems owing to electrically tunable lens power. Nevertheless, the tunable lens power and the aperture size of LC lenses are limited by the optical phase resulting from limit birefringence of LC materials. Recently, we developed a liquid crystal and polymer composite film (LCPCF) as a separation layer and an alignment layer for a multi-layered structure of LC lenses in order to enlarge the polarization-independent optical phase modulation. However, the physical properties and mechanical properties of the LCPCF are not clearly investigated. In this paper, we show the mechanical and physical properties of the LCPCF. The anchoring energy of the LCPCF is comparable with the standard rubbing-induced alignment layer. The transmission efficiency is around 97% neglecting the Fresnel reflection. The surface roughness is under 2 nm by using AFM scanning. The bending strength test indicates that the LCPCF can hold the LC material with reasonable deformation. We believe this study provides a deeper insight to the LC lens structure embedded with LCPCF.