Photoacoustic imaging (PAI) detects ultrasonic signals from tissue optical absorption. Lipids, with strong absorption at 1.7 μm, are valuable for arteriosclerosis and myocardial infarction imaging. We introduce a high-speed wideband wavelength tunable Raman fiber laser for photoacoustic spectroscopy of lipids. Pump lasers adjust from 1560 to 1600 nm, with a 1 ns pulse width and 100 kHz repetition rate. The pump light combines with seed light and passes the Raman fiber. The seed light amplifies by matching the Raman gain spectrum generated through stimulated Raman scattering. We obtained photoacoustic images in the 1670 - 1720 nm range, demonstrating our lasers for lipid imaging.
Photoacoustic imaging (PAI) is a label-free biomedical imaging technique based on photoacoustic effects. PAI can differentiate various tissues by the different absorption coefficients of each molecule. Therefore, the wavelength band around 1700 nm is advantageous for finding lipid tissue because lipids have a higher absorption coefficient than water at around 1700 nm. In this paper, we demonstrate a stimulated Raman scattering (SRS) pulsed fiber laser with a variable repetition rate for PA generation in 1700 nm. The pulse repetition rate and the pulse energy are inversely proportional under the given optical gain. Therefore, by utilizing a pulse laser with variable repetition rate, we can determine the optimal pulse energy and measurement speed required by the system.
Photoacoustic imaging (PAI) is a technique for detecting ultrasonic signals occurred from the optical absorption properties of biological tissues. The contrast of PAI is determined by the degree of optical absorption properties of biological tissues. PAI uses absorption contrast to distinguish endogenous factors to image biological tissues. Among various endogenous contrast agents, a lipid with high absorption at 1.7 μm is one of the main agents for identifying diseases such as arteriosclerosis and myocardial infarction.
We present a high-speed wavelength-switchable active mode-locked Raman fiber laser using active mode-locking (AML) and stimulated Raman scattering (SRS) in a 1.7 um band for photoacoustic generation of lipids. the wavelength of 1.7 um band is oscillated to generate high pulse energy on the order of hundreds of nano-Joules. PA signals of lipids can be acquired to prove the feasibility of label-free PAI of lipids.
We present a dual-wavelength laser using stimulated Raman scattering (SRS) effect which have high pulse repetition rate for fast functional photoacoustic (PA) imaging. This laser has a high pulse repetition rate of 300 kHz and high pulse energy more than 200 nJ. We periodically modulated the electro-optic modulator voltage from 0 to 168 V to switch the polarization of the output light. Two different pulse lights separated by polarization switching were used to generate different SRS peaks using Raman fibers with lengths of 5 and 20 m. The operating wavelength of this laser was switched to 545 nm and 603 nm using SRS effect and polarization switching. Wavelength dependent fast functional PA images of blood vessels and gold nanorods were obtained using a dual-wavelength switchable SRS pulsed-laser.
Optical-resolution photoacoustic microscopy (OR-PAM), has been widely used and studied as noninvasive and in-vivo imaging technique, can achieve a high resolution and high contrast image. OR-PAM is combined with optical absorption contrast and detection of acoustic wave generated by thermal expansion. Recently, nanoparticles and dyes have been used as contrast agents of OR-PAM. To obtain functional OR-PAM image such as a distribution image of blood vessels and nanoparticles, a tunable dye laser or optical parametric oscillator (OPO) should be needed at more two wavelength. However, because these lasers have a low pulse repetition rate (10 Hz ~ 10 kHz), a functional OR-PAM image with real-time display has been limited.
In our previous study, we demonstrated high-speed OR-PAM using an Ytterbium fiber laser and a graphics processing unit (GPU) technique at 300 kHz-pulse repetition rates. Although this Ytterbium fiber laser has a high pulse repetition rate, it is not comfortable for functional imaging owing to lasing at only single wavelength. Therefore, in this study, we used a high-speed interlaced illumination method at 532 nm and 1064 nm for real-time display functional OR-PAM. For high-speed interlaced illumination of two wavelength, we applied second harmonic generation effect and a high-speed optical switching using an electro-optic modulator. Therefore, we could obtain maximum amplitude projection (MAP) images about distributions of blood vessels and nanoparticles, simultaneously, with 500 x 500 pixels and a real-time display of approximately 0.5 fps.
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