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261 results on '"Choi, Young Soo"'

61. Measurements of the farfield characteristics of an automotive ultrasonic sensor with a vertical temperature gradient

Author

Choi Young Soo, Sang-Hyun Kim, and Won-Suk Ohm

Subjects
Acoustics and Ultrasonics, Acoustics, Temperature measurement, law.invention, Lidar, Arts and Humanities (miscellaneous), Thermocouple, law, Speed of sound, ComputerSystemsOrganization_SPECIAL-PURPOSEANDAPPLICATION-BASEDSYSTEMS, Ultrasonic sensor, Radar, Image sensor, Sound pressure, Geology
Abstract

With the coming of autonomous vehicles on the horizon, a variety of sensor platforms such as ultrasonic, radar, lidar, and image sensors are currently under development. Among these, ultrasonic sensors play a key role in the autonomous parking system. An ultrasonic sensor emits and receives a sound pulse (usually centered around a few dozen kHz), and computes the distance to a nearby object assuming a constant sound speed. However, the assumption of constant sound speed is often invalid because of the meteorological conditions, and could cause errors in distance estimation. In this talk, we discuss the farfield characteristics of a typical automotive ultrasonic sensor in the presence of a vertical stratification of temperature hence sound speed. The sound speed profile on a hot summer day is derived from temperature measurements with a vertical thermocouple array, and the farfield acoustic pressure is measured using a 2-D array of microphones. The measurements indicate that refraction due to the stratification of sound speed can be significant enough to alter the farfield performance of an ultrasonic sensor, and thus should be taken into account in the design of the autonomous parking system. With the coming of autonomous vehicles on the horizon, a variety of sensor platforms such as ultrasonic, radar, lidar, and image sensors are currently under development. Among these, ultrasonic sensors play a key role in the autonomous parking system. An ultrasonic sensor emits and receives a sound pulse (usually centered around a few dozen kHz), and computes the distance to a nearby object assuming a constant sound speed. However, the assumption of constant sound speed is often invalid because of the meteorological conditions, and could cause errors in distance estimation. In this talk, we discuss the farfield characteristics of a typical automotive ultrasonic sensor in the presence of a vertical stratification of temperature hence sound speed. The sound speed profile on a hot summer day is derived from temperature measurements with a vertical thermocouple array, and the farfield acoustic pressure is measured using a 2-D array of microphones. The measurements indicate that refraction due to the stratific...

Published
2018

69. Saturation characteristics of [Cr.sub.4+]:YAG crystals and dye films for passive Q switches

Author

Choi, Young-Soo

Subjects
Saturation (Electronics) -- Measurement, Crystals -- Electric properties, Astronomy, Physics
Abstract

Measurements of the saturated transmission and the undepleted ground-state population density of both [Cr.sup.4+]:YAG crystals and dye films as saturable absorbers have been experimentally attempted to understand the additional unsaturated losses in a high Q state. The saturated transmission of a saturable absorber is less than its maximum at a high saturation fluence because of the undepleted ground-state population density. Higher saturated transmission and lower undepleted ground-state population density have been observed for [Cr.sup.4+]:YAG crystals than have been observed for dye films at high optical densities. OCIS codes: 140.0140, 140.3540, 140.3580.

Published
2001

70. Asymmetry of the forward and backward Raman gain coefficient at 1.54 [micro]m in methane

Author

Choi, Young Soo

Subjects
Raman effect -- Measurement, Pumping machinery -- Performance, Electromagnetic pumps -- Evaluation, Astronomy, Physics
Abstract

The threshold pump energies at 1.06 [micro]m and Raman gain coefficients of stimulated Raman scattering (SRS) at 1.54 [micro]m have been measured in both single-pass and half-resonator configurations at various methane pressures. In the single-pass configuration, the Raman gain coefficients of the backward and forward SRS are 0.32 and 0.23 cm/GW at 95 atm, respectively. The backward Raman gain coefficient is higher than the forward Raman gain coefficient. This gain reduction is caused by the depletion of local pump intensity, the phase-matching conditions, and the transient effect of the high amplification rate in the forward SRS process. In the half-resonator configuration, the Raman gain coefficient was 0.46 cm/GW at 75 atm of methane. [C] 2001 Optical Society of America OCIS codes: 140.0140, 140.3550.

Published
2001

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