Sensing photoacoustic waves brings a lot of loss since the detector distance is in the order of millimeters which also leads to noise in the measured signal. To solve this problem, we used an optically trapped silica particle as a transducer in this study. We used two laser sources, one for optical tweezers (976 nm, CW) and a fiber laser for photoacoustic imaging (SHG output: 532 nm, pulsed). The fiber laser was produced in our laboratory whose pulse duration is 8 ns, pulse energy is 10 µJ, and pulse repetition frequency is 65 kHz. The separation between them in the sample plane is 8 µm. The green laser excited several absorbing mediums such as trypan blue, horse hair, black ink and gold thin film. We tracked the position of trapped silica particle (5µm diameter) when the green laser is on and off. We observed dramatic difference between two states. We have validated that this effect is fully photoacoustic by changing the frequency of the green laser with a chopper which led to the exact same frequency when we calculated the Fourier transform of the position distribution of the trapped silica particle. Also, when we change the power of the green laser, the amplitude of the Fourier transformation of the position distribution of the trapped silica particle changes in the same way.
This work presents an analytical approach for the solution of the tissue diffusion equation based on the bound- ary measurements. We consider a bioluminescent point source in both homogeneous and heterogeneous circular turbid media. The point source is described by the Dirac delta function. Analytical expressions for the strength and position of the point source are obtained introducing boundary measurements and then applying appropriate boundary conditions. In addition, numerical simulations are performed for the position of the source. Calculations show that that the analytical results are in a good accordance with the numerical results.
The representation theorems of the convolution type and the correlation type are used to obtain the superposition of the Green's function and its time reversal counterpart for the photoacoustic wave equation. Based on the representation theorems, an interferometry relation providing the Green's function between sources and receivers is obtained. The reciprocity theorems for a spherical geometrical system consisting of sources located on the boundary of the inner spherical region and transducers located on the outer boundary are utilized. Therefore, the measurement would be observed at one of the detectors if there were a photoacoustic point source at the other one.
Photoacoustic microscopy, as an imaging modality, has shown promising results in imaging angiogenesis and
cutaneous malignancies like melanoma, revealing systemic diseases including diabetes, hypertension, tracing drug
efficiency and assessment of therapy, monitoring healing processes such as wound cicatrization, brain imaging and
mapping. Clinically, photoacoustic microscopy is emerging as a capable diagnostic tool. Parameters of lasers used
in photoacoustic microscopy, particularly, pulse duration, energy, pulse repetition frequency, and pulse-to-pulse
stability affect signal amplitude and quality, data acquisition speed and indirectly, spatial resolution. Lasers used
in photoacoustic microscopy are typically Q-switched lasers, low-power laser diodes, and recently, fiber lasers.
Significantly, the key parameters cannot be adjusted independently of each other, whereas microvasculature and
cellular imaging, e.g., have different requirements. Here, we report an integrated fiber laser system producing
nanosecond pulses, covering the spectrum from 600 nm to 1100 nm, developed specifically for photoacoustic
excitation. The system comprises of Yb-doped fiber oscillator and amplifier, an acousto-optic modulator and a
photonic-crystal fiber to generate supercontinuum. Complete control over the pulse train, including generation
of non-uniform pulse trains, is achieved via the AOM through custom-developed field-programmable gate-array
electronics. The system is unique in that all the important parameters are adjustable: pulse duration in the range
of 1-3 ns, pulse energy up to 10 μJ, repetition rate from 50 kHz to 3 MHz. Different photocoustic imaging probes
can be excited with the ultrabroad spectrum. The entire system is fiber-integrated; guided-beam-propagation
rendersit misalignment free and largely immune to mechanical perturbations. The laser is robust, low-cost and
built using readily available components.
In this work, Fourier transform based analytical solution to photoacoustic wave equation is obtained for an optically absorbing spherical object warmed up by a pulsed laser for rectangular and Gaussian radial profiles by treating the temporal profile of the laser as Gaussian. The photoacoustic signal is investigated as a function of time for different locations outside the spherical object. An expression including the dependency of the laser parameters on the photoacoustic signal is presented.