RU222926U1 - Optoacoustic probe based on an axicon lens for optical resolution optoacoustic microscopy - Google Patents

Abstract

The utility model relates to the field of optical resolution optoacoustic microscopy. The developed device is a probe consisting of a conical axicon lens for focusing laser radiation onto the sample and a piezoelectric detector for recording the ultrasonic response. The use of an axicon lens makes it possible to lengthen the focal area of laser radiation. The technological result of the proposed utility model is to combine the optical radiation focusing system and the ultrasonic sensor into a single element. This configuration allows us to avoid the problem of adjusting the relative position of the axicon and the detector to combine the optical and acoustic focuses when scanning an object. 4 ill.

Description

The utility model relates to the field of optoacoustic microscopy (OAM), a method for non-invasive visualization of biological tissue. The utility model is an optoacoustic (OA) probe, a key part of an optical resolution (OP) optoacoustic microscope. OAM-OR allows you to obtain images of the smallest tissue structures with high contrast. Spatial resolution in OAM-OR systems is determined by the wavelength of laser radiation and can reach a value of less than 1 μm with an imaging depth of the order of 1 mm. The principle of OAM is the generation of ultrasonic waves when the medium absorbs probing laser radiation. OAM-OR involves the use of focused optical radiation. A three-dimensional image of tissue in OAM-OR occurs when the OA sample is scanned spatially with a probe.

In the simplest model of an OAM-OR microscope, a focusing lens or objective is used to focus optical radiation with a Gaussian profile. In this configuration of the OAM-OR microscope, there is a problem of low depth of field of the image due to the short depth focus (tens of micrometers). Focus blur limits the quality of the resulting 3D image and makes it impossible to visualize thick tissue. One option for increasing the depth of focus is to use Bessel beams instead of Gaussian beams.

The patent CN 101918811 “Confocal photoacoustic microscopy with optical lateral resolution)) (IPC A61B 5/1455, A61B 8/00, G01N 21/00, G01N 29/00, G02B 27/30, published 07/31/2013) is described optical resolution confocal optoacoustic microscopy system. Optical radiation is focused onto the sample using a lens while separating the optical and acoustic paths using a system of acoustic prisms. The disadvantages in this case are the small depth of focus (DOF depth of field) and the difficulty of mutual adjustment of the positions of the optical and acoustic elements.

The patent PM RU 215019 “Optical transparent sensor for optoacoustic microscopy of optical resolution” (IPC G01H 9/00, G01N 29/06, published November 24, 2022) describes an optically transparent ultrasonic sensor for optoacoustic microscopy of optical resolution. A lens is used to focus the laser radiation. The disadvantage when using a lens is the low DOF value.

The above systems use laser radiation with a Gaussian beam profile to probe the sample. Application CN 114324183 “Large-focal-depth ultraviolet acoustic microscopic imaging system and imaging method” (IPC G01N 21/01, G01N 21/17, published 02/14/2022) presents an ultraviolet optoacoustic microscopy system with an extended depth focus ( DOF) by converting a Gaussian beam into a Bessel-like beam. The disadvantage of this configuration is the supply of probing optical radiation and detection of optoacoustic signals from opposite sides of the sample, which imposes restrictions on the sample. In addition, the multi-element nature of the optical system complicates its setup and alignment.

The closest analogue of the claimed device is described in the article “Reflection-mode switchable subwavelength Bessel-beam and Gaussian-beam photoacoustic microscopy in vivo” (Byullee Park, Hoyong Lee, Seungwan Jeon, Joongho Aim, Hyung H. Kim, Chulhong Kim, Journal of Biophotonics, 12(2) 2018). It implements a system of optoacoustic microscopy of optical resolution with the ability to change the operating mode with a Bessel (BB) or Gaussian (GB) beam. To convert a laser beam with a Gaussian profile into a beam with a Bessel profile, an axicon lens is used in the work. An ultrasonic detector in the form of a narrow strip was located under the optical lens. The disadvantage of the prototype is the multi-element optical system, which leads to the problem of adjusting the relative position of the optical elements.

The problem to be solved by this useful model is the development of an optoacoustic probe design based on an axicon lens, which allows combining optical and acoustic foci when scanning an object without additional adjustment.

The technical result is achieved due to the fact that the developed optoacoustic probe, like the prototype device, contains an axicon lens, an ultrasonic detector with a receiving element made of piezo film. What is new is that along the axis of the axicon there is a cylindrical hole in which a cylindrical ultrasonic detector is located.

The device is illustrated by the following figures.

Figure 1 shows an optoacoustic probe: a) diagram of the probe head, b) photograph of the probe head, c) photograph of the optoacoustic probe.

Figure 2 shows a diagram of the path of optical rays in an optoacoustic probe.

Figure 3 shows the profile of the dependence of radiation intensity on depth (Z coordinate).

Figure 4 shows the profile of the dependence of the radiation intensity on the radial coordinate (X coordinate).

The head 1 of the optoacoustic probe shown in Fig. 1a consists of a conical axicon lens 2 with a through cylindrical hole along the axis in the center, into which a cylindrical ultrasonic detector 3 is inserted. The ultrasonic detector 3 is made on the basis of a piezoelectric film, in this case a PVDF-TrFe piezopolymer was used.

Figure 1c shows a photograph of an optoacoustic probe in a single device on rods together with a single-mode fiber radiation focusing system.

Figure 2 shows a diagram of the path of optical rays in an optoacoustic probe, where 4 is an adapter with a fixed output end of a single-mode optical fiber, 5 is a collimating lens, 1 is the head of an optoacoustic probe.

Using the axicon lens 2, laser radiation is focused on the sample and the Gaussian radiation profile is converted into a Bessel profile. The output beam profile and focal depth are determined by the angle α of axicon 2 and the refractive index n of the material from which the axicon 2 lens is made. The intensity of the Bessel beam, depending on spatial coordinates, is determined by the following formula

where k is the wave number;

n is the relative refractive index of the axicon material;

α - axicon angle;

w ₀ - radius of the laser beam;

I ₀ - intensity of input radiation;

(r, z) - radial and longitudinal coordinates, respectively.

Based on the calculation results, a sample of an axicon lens 2 with an angle α=49° was made from K8 glass with a refractive index n=1.5191 at a wavelength of 532 nm. With these parameters, the calculated depth of focus (DOF) is 26.8 mm at FWHM (full width at half maximum) in water, the diameter of the focal spot at FWHM is 1.1 µm in water. An ultrasonic detector 3 with an aperture of 3.5 mm is glued into the lens hole.

Figure 3 shows the longitudinal distribution of radiation intensity in the region of space behind the axicon. The arrow indicates the FWHM value.

Figure 4 shows the transverse distribution of radiation intensity in the region of space behind the axicon. The arrow indicates the FWHM value.

Thus, in the developed optoacoustic probe, a cylindrical ultrasonic detector is located in the axicon hole, i.e. optical radiation focusing systems and ultrasonic sensor are combined into a single element. This configuration avoids additional adjustment of the relative position of the ultrasonic detector and axicon.

Claims (1)

An optoacoustic probe for optoacoustic microscopy of optical resolution, containing an axicon lens, an ultrasonic detector with a receiving element made of piezo film, characterized in that a cylindrical hole is made along the axis of the axicon, in which a cylindrical ultrasonic detector is located.