We built the photoacoustic imaging (PAI) systems to image oral soft tissue. To reduce the cost and size significantly, the PAI system used an intensity-modulated continuous-wave (CW) diode laser with a wavelength of 532nm and output peak power of 200mW as an excitation source, combined with a condenser microphone as photoacoustic signals detector. The Pulse Width Modulation technique was applied to form a square wave fluctuation of laser radiation by using certain duty cycle on singlefrequency of 17.8 kHz. Sample of this study was Sprague Dawley rats tongue on plasticine media, irradiated by modulated CW diode laser. The result of this study showed that modulated laser exposure on one certain spot of the sample produced PA-signals in polynomial correlation with a duty cycle of laser modulation. Based on it, the photoacoustic imaging then was done by using gradual duty cycles, i.e., 20%, 30%, and 40%. This study also showed that using CW diode laser which is modulated with low duty cycle can produce most accurate PA image, as well as keeping the sample from the high energy of laser exposure that may cause biological changes. Furthermore, the maximum duty cycle to modulate laser for oral soft tissue imaging in this system was 30%.

A CO2 laser has the advantages of being high in power and having many laser lines in the 9–11 µm infrared region. Thus, a CO2 laser photoacoustic spectrometer (PAS) can have a multi-component measurement capability for many gas compounds that have non-zero absorption coefficients at the laser lines, and therefore can be applied for measuring several volatile organic compounds (VOCs) in the human breath. We have developed a CO2 laser PAS system for detecting acetone in the human breath. Although acetone has small absorption coefficients at the CO2 laser lines, our PAS system was able to obtain strong photoacoustic (PA) signals at several CO2 laser lines, with the strongest one being at the 10P20 line. Since at the 10P20 line, ethylene and ammonia also have significant absorption coefficients, these two gases have to be included in a multi-component measurement with acetone. We obtained the lowest detection limit of our system for the ethylene, acetone, and ammonia are 6 ppbv, 11 ppbv, and 31 ppbv, respectively. We applied our PAS system to measure these three VOCs in the breath of three groups of subjects, i.e., patients with lung cancer disease, patients with other lung diseases, and healthy volunteers.

The aim of this research is to improve the sensitivity of ammonia gas sensor (hereafter referred to as sensor) based on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) by employing the doping dye of bromocresol green (BCG). The doping process was carried out by mixing the BCG and the PEDOT:PSS in a solution with an optimum ratio of 1 : 1 in volume. The sensor was fabricated by using spin-coating technique followed by annealing process. For comparison, the BCG thin film and the PEDOT:PSS thin film were also deposited with the same method on glass substrates. For optical characterization, a red-light laser diode with a 650 nm wavelength was used as light source. Under illumination with the laser diode, the bare glass substrate and BCG film showed no absorption. The sensor exhibited linear response to ammonia gas for the range of 200 ppm to 800 ppm. It increased the sensitivity of sensor based on PEDOT:PSS with BCG doping being about twofold higher compared to that of without BCG doping. Furthermore, the response time and the recovery time of the sensor were found very fast. It suggests that the optical sensor based on BCG-doped PEDOT:PSS is promising for application as ammonia gas sensor.

The feasibility of a diode laser and condenser microphone-based photoacoustic imaging system for dental anatomy characterization has been investigated. The sample of this study was human teeth illuminated by a diode laser with a wavelength of 532 nm. The laser and detector were fixed in a static position while the sample was moved in the X-Y direction. A laser diode illuminated the sample at 17-20 kHz frequencies combined with 30%, 35%, 40%, 45%, 50%, and 55% of the duty cycles to investigate optimal laser irradiation for dental anatomy imaging. The acoustic intensity was measured ten times to investigate the characterization of dental anatomical structure, i.e., enamel, dentin, and pulp. The sample was then scanned using the system to determine the characterization of the dental structure in the photoacoustic image. The results of this study reveal that the optimal frequency and duty cycle of laser exposure to produce the photoacoustic image of the sample are 19 kHz and 50%, respectively. The maximum acoustic intensities of enamel, dentin and pulp are -71,8 dB, -70,8 dB, -70,5 dB, respectively. Whereas the minimum acoustic intensities of enamel, dentin and pulp are -72,0 dB, -70,9 dB, -70,6 dB respectively. In this study, a photoacoustic imaging system based on a diode laser and a condenser microphone can generate photoacoustic images of dental anatomical structures. The optical absorption of pulp is stronger than the dentin and enamel layer. Hence the pulp area emits the highest acoustic intensity and emerges as a red area in the photoacoustic image.

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