In augmented reality (AR), focal accommodation for real objects and augmented imagery is essential for comfortable viewing, and this requires a depth of field (DoF) in AR displays. Wide DoFs have been achieved in various display systems, but these systems are often bulky, highly power-consuming, and computationally complex. Hence, we propose a simple and effective optical method to extend the DoF in AR displays. The proposed method harnesses a focus tuning block consisting of a multifocal lens (MFL) and a focus tunable lens. Through the multi-variable focusing unit, virtual imagery is displayed at spatially separated focal depths using the MFL; then, the focus tunable lens dynamically tunes the foci. This enables the individual virtual objects to be sequentially focused on the same image plane, helping to register them with the real objects in the user’s eye accommodation. The proposed approach was numerically simulated, and its performance was experimentally evaluated with an AR display equipped with the focusing unit, which extended the DoF from 0.3 m (3.3 diopters) to 2 m (0.5 diopters).
We analyzed regular polygonal ring resonators based on multi-mode waveguide using finite-difference time-domain simulation. It consists of the regular polygonal ring waveguide, total internal reflection mirror, and MMI coupler. In general, multi-mode waveguide-based resonator is difficult to use as sensors because of poor output characteristics. By using the low reflectance of the higher-order mode compared to the fundamental mode in the TIR mirror, we designed a regular polygonal ring resonator that can be used as sensors even when a multi-mode waveguide is used instead of a single-mode waveguide. In fabrication, the multi-mode waveguide has a wider line width than the single-mode waveguide, which reduces the process cost and enables mass production. The width and height of the multi-mode waveguide are designed to be 2.5 μm and 2 μm, respectively using SU-8 polymer. The regular hexagon ring resonator shows the highest Q-factor of 1.03×104 among the various regular polygonal ring resonators.
In this paper, we designed hexagonal ring resonator using localized surface plasmon resonance (LSPR) phenomenon to enhance the sensitivity which is a significant factor in bio-chemical sensors. We used a hexagonal ring resonator structure to eliminate the bending loss which is one of the prime factors affects sensitivity. The sensing area of the hexagonal ring resonator with LSPR is deposited metal nanoparticle on cladding which makes difference with general sensing region of the hexagonal ring resonator. In this sensing region, the wavelength of light should be longer than the size of the nanoparticle because the metal nanoparticle reacts the light in specific condition. The sensitivity of the resonator can be improved with using this phenomenon. We used finite difference time domain (FDTD) methods for theoretical analysis. Also, we optimized the structure to reduce LSPR loss and enhance the sensitivity by adjusting type, size, thickness of the metal nanoparticle. As a simulation result, we verified that sensitivity of hexagonal ring resonator with LSPR can be 2.5 times higher than without LSPR.
We propose a rectangular resonator sensor structure with butterfly MMI coupler using SOI. It consists of the rectangular resonator, total internal reflection (TIR) mirror, and the butterfly MMI coupler. The rectangular resonator is expected to be used as bio and chemical sensors because of the advantages of using MMI coupler and the absence of bending loss unlike ring resonators. The butterfly MMI coupler can miniaturize the device compared to conventional MMI by using a linear butterfly shape instead of a square in the MMI part. The width, height, and slab height of the rib type waveguide are designed to be 1.5 μm, 1.5 μm, and 0.9 μm, respectively. This structure is designed as a single mode. When designing a TIR mirror, we considered the Goos-Hänchen shift and critical angle. We designed 3:1 MMI coupler because rectangular resonator has no bending loss. The width of MMI is designed to be 4.5 μm and we optimize the length of the butterfly MMI coupler using finite-difference time-domain (FDTD) method for higher Q-factor. It has the equal performance with conventional MMI even though the length is reduced by 1/3. As a result of the simulation, Qfactor of rectangular resonator can be obtained as 7381.
In this paper, we propose temperature sensing method by using optical beating. When temperature changes, a peak wavelength of the sensing laser varies slightly. However, with limitation of the optical spectrum analyzer’s (OSA) spectral resolution (sub-nm), it is hard to measure the exact quantity of the wavelength variation. Therefore, we used electrical spectrum analyzer (ESA) and two lasers to obtain the wavelength shift. We used DFB-LD (distributed feedback laser diode) and TLS (tunable laser source) to get beating signal. Each of laser has 1550 nm of wavelength, -20 dBm of intensity and 108 of Q factor. We varied temperature by 0.1 °C from 17.4 °C to 18.4 °C using TEC (temperature controller). We observed 0.01 nm/°C of wavelength change through OSA and 9.5 GHz/°C of beating frequency change through ESA. With this result, we verified that we can measure relative temperature change with having ultra-fine resolution of 9.5×10-7 °C theoretically for the ESA resolution bandwidth of 1 kHz. This detecting ability can be applied to highly sensitive temperature sensor.
In this paper, total internal reflection (TIR) mirror is carefully simulated for silicon nitride polygonal ring resonator sensor structure. Polygonal resonator has recently attracted much attention for applications in bio and chemical sensors because it does not have a bending loss, and it has an advantage of using MMI coupler. In polygonal resonator sensor design, high Q-factor and low TIR mirror loss are extremely significant factors. Therefore, critical angle and Goos-Hanchen shift should be considered in the design of TIR mirror. When cladding material is SiO2, the critical angle of SiNx waveguide is about 44.99 degrees and the Goos-Hanchen shift is about 400 nm at 1.55 μm wavelength. For the rib type waveguide, we designed it to have 3 μm width, 1 μm height, and 0.5 μm etching depth for decreasing TIR mirror loss. As simulation results of FDTD, reflectivitities of polygonal TIR mirrors are 79% for pentagon, 95% for hexagon and 98% for octagon, respectively. According to the simulations, Q-factors for hexagonal and octagonal resonators can be obtained as high as 1.55 x 104 and 1.72 x 104, respectively.
KEYWORDS: Signal to noise ratio, Signal detection, Optical amplifiers, Interference (communication), Telecommunications, Modulation, Sensors, Linear filtering, Capacitors, Cognitive informatics, Optical sensors, Signal processing, Optical design
Lock-in amplifier (LIA) has been widely used in optical signal detection systems because it can measure small signal under high noise level. Generally, The LIA used in optical signal detection system is composed of transimpedance amplifier (TIA), phase sensitive detector (PSD) and low pass filter (LPF). But commercial LIA using LPF is affected by flicker noise. To avoid flicker noise, there is 2ω detection LIA using BPF. To improve the dynamic reserve (DR) of the 2ω LIA, the signal to noise ratio (SNR) of the TIA should be improved. According to the analysis of frequency response of the TIA, the noise gain can be minimized by proper choices of input capacitor (Ci) and feed-back network in the TIA in a specific frequency range. In this work, we have studied how the SNR of the TIA can be improved by a proper choice of frequency range. We have analyzed the way to control this frequency range through the change of passive component in the TIA. The result shows that the variance of the passive component in the TIA can change the specific frequency range where the noise gain is minimized in the uniform gain region of the TIA.
Position-sensitive photomultiplier tubes (PSPMTs) in array are used as gamma ray position detector. Each PMT converts the light of wide spectrum range (100 nm ~ 2500 nm) to electrical signal with amplification. Because detection system size is determined by the number of output channels in the PSPMTs, resistive network has been used for reducing the number of output channels. The photo-generated current is distributed to the four output current pulses according to a ratio by resistance values of resistive network. The detected positions are estimated by the peak value of the distributed current pulses. However, due to parasitic capacitance of PSPMTs in parallel with resistor in the resistive network, the time constants should be considered. When the duration of current pulse is not long enough, peak value of distributed pulses is reduced and detected position error is increased. In this paper, we analyzed the detected position error in the resistive network and variation of time constant according to the input position of the PSPMTs.
Lock-in amplifier (LIA) has been proposed as a detection technique for optical sensors because it can measure low signal in high noise level. LIA uses synchronous method, so the input signal frequency is locked to a reference frequency that is used to carry out the measurements. Generally, input signal frequency of LIA used in optical sensors is determined by modulation frequency of optical signal. It is important to understand the noise characteristics of the trans-impedance amplifier (TIA) to determine the modulation frequency. The TIA has a frequency range in which noise is minimized by the capacitance of photo diode (PD) and the passive component of TIA feedback network. When the modulation frequency is determined in this range, it is possible to design a robust system to noise. In this paper, we propose a method for the determination of optical signal modulation frequency selection by using the noise characteristics of TIA. Frequency response of noise in TIA is measured by spectrum analyzer and minimum noise region is confirmed. The LIA and TIA circuit have been designed as a hybrid circuit. The optical sensor is modeled by the laser diode (LD) and photo diode (PD) and the modulation frequency was used as the input to the signal generator. The experiments were performed to compare the signal to noise ratio (SNR) of the minimum noise region and the others. The results clearly show that the SNR is enhanced in the minimum noise region of TIA.
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