Jungsoon Shin

Widely tunable chiral nematic liquid crystal optical filter with microsecond switching time

A wavelength shift of the photonic band gap of 141 nm is obtained by electric switching of a partly polymerized chiral liquid crystal. The devices feature high reflectivity in the photonic band gap without any noticeable degradation or disruption and have response times of 50 µs and 20 µs for switching on and off. The device consists of a mixture of photo-polymerizable liquid crystal, non-reactive nematic liquid crystal and a chiral dopant that has been polymerized with UV light. We investigate the influence of the amplitude of the applied voltage on the width and the depth of the reflection band.

Always-On CMOS Image Sensor for Mobile and Wearable Devices

This paper presents a low-power always-on image sensor for mobile and wearable device applications. The sensor continuously captures images for smart sensing, such as face detection, eye tracking, and gesture recognition, and it provides high-resolution images for capturing pictures with a unified sensor. The sensor employs a switchable dual mode: always-on (AO) mode with low power consumption and photo-shooting (PS) mode with high signal-to-noise ratio. For dual-mode operation with high energy efficiency, we implemented dynamic voltage scaling that provides 0.9 V analog-digital supply voltage for the AO mode and 3.3 V analog-1.8 V digital supply voltage for the PS mode. For low-voltage operation in the AO mode, a conventional four-transistor pixel operates as a charge-shared pixel. In order to suppress power consumption and frequency in conventional single-slope analog-to-digital converters (ADC), two neighboring column-parallel ADCs are reconfigured to one successive-approximation ADC for low-voltage and low-frequency operation in the AO mode with negligible area overhead. The fabricated 640 × 480 pixel prototype sensor operates at 45.5 μW (at 15 fps, 320 × 240) in the AO mode, which significantly extends battery life when performing always-on sensing.

Expansion of Smartwatch Touch Interface from Touchscreen to Around Device Interface Using Infrared Line Image Sensors

Touchscreen interaction has become a fundamental means of controlling mobile phones and smartwatches. However, the small form factor of a smartwatch limits the available interactive surface area. To overcome this limitation, we propose the expansion of the touch region of the screen to the back of the user’s hand. We developed a touch module for sensing the touched finger position on the back of the hand using infrared (IR) line image sensors, based on the calibrated IR intensity and the maximum intensity region of an IR array. For complete touch-sensing solution, a gyroscope installed in the smartwatch is used to read the wrist gestures. The gyroscope incorporates a dynamic time warping gesture recognition algorithm for eliminating unintended touch inputs during the free motion of the wrist while wearing the smartwatch. The prototype of the developed sensing module was implemented in a commercial smartwatch, and it was confirmed that the sensed positional information of the finger when it was used to touch the back of the hand could be used to control the smartwatch graphical user interface. Our system not only affords a novel experience for smartwatch users, but also provides a basis for developing other useful interfaces.

Fast polarisation-insensitive optical shutter supported by backflow in dichroic dye-doped dual-frequency liquid crystal

This paper describes a novel implementation of a dual-frequency liquid crystal optical shutter of the guest–host type. The transmissive state of the filter is obtained by applying a low-frequency electric field that brings the dichroic dye in a homeotropic orientation. The light-absorbing state is realised by a twisted planar configuration for which the absorption is quasi-independent of the polarisation. Switching between the two states occurs in about 1 ms and the devices show no scattering for wavelengths inside or outside the absorption band of the dichroic dye. Simulations and experiments reveal how a twisted state is obtained through the backflow phenomenon.

A 3-D Camera With Adaptable Background Light Suppression Using Pixel-Binning and Super-Resolution

This paper presents a CMOS time-of-flight (TOF) 3-D camera employing a column-level background light (BGL) suppression scheme for high-resolution outdoor imaging. The use of the column-level approach minimizes a pixel size for high-density pixel arrays. Pixel-binning and super-resolution can be adaptably applied for an optimal BGL suppression at given spatiotemporal resolutions. A prototype sensor has been fabricated by using 0.11 μm CMOS processes. The sensor achieved a fill factor of 24% in a pixel pitch of 5.9 μm which is the smallest among all the reported TOF cameras up to date. Measurement results showed the temporal noise of 1.47 cm-rms with a 100 ms integration time at a demodulation frequency of 12.5 MHz using a white target at 1 m distance. The non-linearity was measured as 1% over the range of 0.75 m ~ 4 m. The BGL suppression over 100 klx was achieved from indoor and outdoor experiments, while the BGL-induced offset was maintained less than 2.6 cm under 0 ~ 100 klx.

An Architecture With Pipelined Background Suppression and In-Situ Noise Cancelling for 2D/3D CMOS Image Sensor

We present a CMOS image sensor with integrated background suppression scheme for detecting small signals out of unwanted background signals. For the background suppression, differential signals with suppressed common-mode background signals are sampled within a short sub-sensing time in order to avoid the saturation from strong background signals. Analog differential signals are digitally accumulated multiple times in one integration time for high SNR. The column-parallel background suppression circuits are pipelined in order to achieve short sub-sensing time. Moreover, additional operations for the noise cancelling are merged with the background suppression and no extra timing for the noise cancelling is required during the sub-sensing time. In order to suppress stronger background signals, sensitivity can be adjusted to be decreased using in-pixel capacitors when strong background signals are present. The prototype image sensor with 1328 × 1008 pixel array has been fabricated with a 0.11 μm 1P4M CIS process. We have successfully captured images from the fabricated sensor chip with strong background signal over 10 klx scene illuminance without optical filters. The background-to-signal ratio is 32.1 dB.

Biometrics for electronic eyes: system authentication with embedded CMOS image sensor

Identically designed electronic devices such as transistors may have random variations in their physical quantities. Inspired by human biometrics, this paper presents a novel approach that enables home appliances to have unique identities based on the intrinsic features of a CMOS image sensor (CIS) embedded in their electronic eyes (i.e., inbuilt cameras). More specifically, the column component of the fixed pattern noise (FPN) that inevitably exists in most modern CISs is utilized as a unique feature of the CIS for describing a unique ID for the embedded system. To practically capture column FPNs that were originally canceled out by a correlated double sampling (CDS) process, this paper also proposes a novel readout scheme. The feasibility of the proposed CIS-based identification is assessed with real measurements using 12 CIS prototypes. In conclusion, a set of applications requiring unique device identifications are presented for demonstrating the utility of the proposed approach in the domain of consumer electronics.1

A 3D image sensor with adaptable charge subtraction scheme for background light suppression

We present a 3D ToF (Time-of-Flight) image sensor with adaptive charge subtraction scheme for background light suppression. The proposed sensor can alternately capture high resolution color image and high quality depth map in each frame. In depth-mode, the sensor requires enough integration time for accurate depth acquisition, but saturation will occur in high background light illumination. We propose to divide the integration time into N sub-integration times adaptively. In each sub-integration time, our sensor captures an image without saturation and subtracts the charge to prevent the pixel from the saturation. In addition, the subtraction results are cumulated N times obtaining a final result image without background illumination at full integration time. Experimental results with our own ToF sensor show high background suppression performance. We also propose in-pixel storage and column-level subtraction circuit for chiplevel implementation of the proposed method. We believe the proposed scheme will enable 3D sensors to be used in out-door environment.

6.3 A 45.5μW 15fps always-on CMOS image sensor for mobile and wearable devices

Most mobile devices embed a CMOS image sensor (CIS) for capturing images. In addition, a variety of sensors such as proximity, ambient light, and fingerprint sensors are integrated for device control. The integration of multiple sensors in a device requires significant power consumption, area, and cost. In contrast to multiple sensors, an always-on CIS enables advanced smart sensing, including gesture sensing, face recognition, eye tracking, and so on. Smart sensing using a CIS offers a variety of user interfaces and experiences such as touch-less control, authentication, gaming, and object recognition for the Internet of Things (IOT). A major drawback of a CIS in mobile devices is that it consumes power greater than 50mW [1], and this is not feasible for always-on sensing that is required to function with the limited energy available from the devices battery. Moreover, power reduction in a CIS causes image degradation owing to reduced SNR, which is not acceptable for capturing high-quality images. Many low-power CISs have been reported [2-3]. However, they are inadequate for use as high-resolution sensors because of the requirement of additional in-pixel circuits for device operation at low supply voltages.

Microsecond-range optical shutter for unpolarized light with chiral nematic liquid crystal

A fast electro-optic shutter is fabricated and demonstrated. The device works independently of the polarization state of the incoming light beam. Modulation between 3% transmission and 60% transmission is obtained within a wavelength range of 50 nm with a response time of 20 μs. The device consists of two partly polymerized chiral nematic liquid crystal layers separated by a half wave plate. The transmission modulation is due to a 50 nm wavelength shift of the photonic band gap of the chiral liquid crystal realized by applying an electric field over a mixture of photo-polymerized LC and non-reactive nematic LC containing a chiral dopant. The shutter features high reflectivity in the photonic band gap. We investigate the influence of the amplitude of the applied voltage on the width and the depth of the reflection band.