Revista de Engenharia Elétrica e Tecnologia Eletrônica

Laser excitation ultrasound in light absorbing liquids imputing laser radiation through optical fiber with colloidal coating of distal tip by a single layer of transparent spheres

Bredikhin Vladimir

Efficient converters of optical laser radiation into high frequency acoustic for medicine and technologies are an important applied problem. A possible solution is to inject radiation into a liquid through the fiber, the distal end of which is covered by a layer of transparent microspheres.  Microspheres act as lens here creating highly concentrated areas of light radiation in the liquid. In the presence of light absorption in the liquid, there arises a system of local heated volumes which leads to an optoacoustic (OA) response due to the thermoelastic effect. From this point of view, the layer of transparent microspheres at the distal tip of the fiber in a light absorbing medium can be considered to be a fiber laser–acoustic converter (LAC). Two opposite schemes of LAC are investigated experimentally in this report.

At first (a) we investigate ultrasound [1] excited by laser radiation through a quartz optical fiber Ø 1 mm with LAC — a coating at the distal tip of the fiber with Ø 0.96 μm polysterene (PS) spheres. The laser is YAG: Nd laser with λ = 1.064 μm, and distilled water is used as medium (light absorbtion coefficient α ≈ 0.1 cm^-1). The laser generated in the zero transverse mode regime (beam diameter ≈2 mm) a train of pulses with a total duration of ≈300 ns with a spike frequency of ≈ 2 × 10⁵ Hz using an optical passive modulator. This configuration of the experiment allows studying the basic parameters of the system in “primeval” form, avoiding the influence of more complex effects, such as thermal self-defocusing and superheated liquid states.

The second (b), opposite case is a use the coating of Ø 200 μm glass spheres on a glass substrate as the LAC in the laser beam ( approx. Ø 1 mm) of second harmonic (λ = 0.532 μm) with impulse time 15 ns. The media is water - ink solution (α ≈ 100 cm^-1) in this case [2].

Os revestimentos constituídos por esferas de 1 e 200 μm de diâmetro (ver Fig) são aplicados na face da ponta da fibra através de uma tecnologia de 2 fases. Em primeiro lugar, forma-se uma única camada de esferas sobre uma placa de vidro plana. De seguida, a camada única obtida é colada na extremidade da fibra com uma fina camada pré-aplicada de adesivo ótico de cianoacrilato. Uma única camada de esferas de pequena dimensão (até 10 μm) é depositada na placa a partir de uma solução coloidal. As esferas grandes são espalhadas numa camada sobre uma placa plana (dentro de uma área de enchimento limitada). As microfotografias das esferas na ponta de uma fibra de 1 mm no adesivo são