High-power picosecond lasers have a wide range of applications in high-precision drilling, solar cell cutting, and ultrafast spectroscopy. The main oscillator power amplifier (MOPA) technology is an effective scheme to generate high-power picosecond pulses. The thermal effect of the laser gain medium significantly impacts both the laser power output and beam quality. This paper prioritizes constructing a thermal effect model of the gain medium and simulates the temperature field distribution inside the crystal under different pumping modes. Then the pulse string picosecond laser output was successfully realized using the self-developed picosecond fiber seed laser and a two-stage end-face pumped traveling wave amplifier. The maximum average output power of 26.1 W, 28 W, and 29.6 W was obtained when the pulse string contained 2, 5, and 10 sub-pulses, respectively, corresponding to pulse energies of 130.5 µJ, 140 µJ, and 148 µJ at a repetition rate of 200 kHz.
Marine instruments deployed in seawater inevitably experience biofouling, which severely reduces their service life and hinders ocean monitoring. Marine biofouling greatly affects the service life of marine optical instruments and thus has a detrimental impact on ocean monitoring. The fouling community exhibits an attachment succession phenomenon. Macroscopic fouling organisms have adherent and stubborn attachments, whereas microorganisms during early fouling stages are easy to remove, but excessive cleaning also greatly increases energy consumption. Therefore, monitoring biofouling and selecting appropriate removal timing is critical. Due to the complex and dynamic nature of the marine environment, in-situ detection of microbial fouling on optical window of marine optical instrument is challenging because of many factors such as target characteristics, seawater turbidity, light refraction and scattering. Currently, there are no mature technologies available for in-situ fouling detection so as to remove timely micro fouling. To solve this problem, this study deployed thin poly methyl methacrylate (PMMA) coupons within the coastal seawaters of Qingdao, followed by in-situ mapping of photoacoustic signals using a self-built excitation and detection platform, along with along with of transmittance spectrum analysis on fouled PMMA thin films using PerkinElmer LAMBDA750. By combining results from both techniques with microscopic morphology analysis, we explored the relationship between microbial fouling and photoacoustic signal. The research results will provide a novel approach and technical basis for in-situ detection and timely clearance of microbial fouling on optical windows of marine optical instruments.
Filtered back-projection (FBP) is a classic imaging method in photoacoustic tomography in both frequency and time domain. However, the FBP algorithm itself will produce large artifacts, especially under extremely low signal-to-noise ratio which makes the acquisition, analysis and processing of photoacoustic signals (PASs) more difficult. Traditional filtering methods may cause modal aliasing of the filtered signal, loss of effective information, and the introduction of new interference in the process of using de-noising algorithms, which increases the difficulty of signal extraction, so select the appropriate filtering method has become an important part of this imaging mode. In addition, many algorithms tend to ignore the phase delay caused by filtering, which has a great impact on the accuracy of cross-correlation processing and distance inversion. According to the physical and mathematical principles of each step of the algorithm, this paper introduces the Empirical Mode Decomposition (EMD) algorithm to decompose the collected PASs, and puts forward the non-phase delay sifting proposal without introducing additional phase delay. A new strategy aimed at linear frequency modulation light sources under the framework of FBP algorithm. By keeping the center frequency of the ultrasonic transducer and the modulation signal unchanged, the validity of the algorithm is verified through experiments, which improved the reconstruction quality of the image. The new EMD strategy can effectively extract low signal-to-noise ratio of PASs and complete the photoacoustic tomographic reconstruction in FBP mode with higher quality.
In recent years, bladder cancer has been a serious health concern around the world. As a rapidly growing imaging technique, photoacoustic imaging (PAI) was now being explored as an alternative for bladder imaging due to its non-invasive and non-ionizing nature. It was essential to know absorbed light distribution in bladder tissue which would influence the imaging depth and range of PAI. In the paper, optical model of human bladder was established, in which diffused light source was delivered through the urethra into the bladder cavity for endoscopic illumination. And Monte Carlo simulation method was adopted to calculate the light absorption distribution (LAD) in the bladder tissue. The shape and wavelength of light source were investigated in the simulation. The relevant conclusions would be significant for optimizing the light illumination in a PAI system for bladder cancer detection.
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