Single-Frequency Terahertz Focusing Transmission Imaging System Based on QCL and Aspherical Lenses

Abstract

The present invention provides a single-frequency terahertz focusing transmission imaging system based on a QCL and an aspherical lens. A single-frequency terahertz light is emitted by a quantum cascade laser (QCL), and forms a collimated beam through a collimating lens. The aspherical lens focuses the collimated beam on the sample plane. A terahertz power detector (Golay cell) is used to detect the terahertz power transmitted through the sample closely behind the sample. Two-dimensional scanning of the sample can achieve two-dimensional terahertz transmission imaging of the sample. In the system, the aspherical lens is made of TPX material, which has smaller aberration than ordinary spherical lenses and parabolic lenses under the condition of a large numerical aperture. In this system, the waist radius of the focused beam can be reduced to below the wavelength, which helps to improve the imaging resolution.

Description

Technical Field

[0001] This system relates to the field of terahertz imaging, and particularly to a single-frequency terahertz focusing transmission imaging system based on a QCL and an aspherical lens. Background Art

[0002] The quantum cascade laser (QCL) is different from a semiconductor laser. Its lasing wavelength is determined by the energy level spacing between subbands in the conduction band. The energy level spacing between subbands can be changed by adjusting the thickness of the quantum well / barrier layer, thereby changing the lasing wavelength. Therefore, the laser wavelength output by the QCL can cover the THz band that is difficult to cover by traditional lasers. The terahertz light source used in this imaging system is a QCL with a wavelength of 3.7 THz fabricated by the Institute of Semiconductors, Chinese Academy of Sciences.

[0003] The Golay Cell is a terahertz power detector that can work at room temperature. Its principle is to use a gas chamber to absorb the energy of the incident terahertz wave. The increase in gas temperature causes the film deformation of the gas chamber wall, and then the degree of film deformation is measured to indirectly record the power of the received terahertz wave.

[0004] In existing terahertz transmission imaging technologies, imaging systems based on single-frequency terahertz all use collimated beams to irradiate the entire sample, and a terahertz camera is used to achieve two-dimensional imaging. Such a system has a very fast imaging speed. However, the beam energy is dispersed over the entire target, and the transmission power for each target point is insufficient, which results in low imaging resolution and signal-to-noise ratio. On the other hand, such an imaging system also poses high requirements on the target thickness and the power of the terahertz source, making it difficult to make the imaging system portable. Summary of the Invention

[0005] To solve the above technical problems, the present invention proposes a single-frequency terahertz focusing transmission imaging system based on a QCL and an aspherical lens, which is a single-frequency terahertz focusing transmission imaging system to achieve high-resolution imaging of the terahertz transmittance of a thin-sheet sample.

[0006] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0007] A single-frequency terahertz focusing transmission imaging system based on a QCL and an aspherical lens, comprising:

[0008] A QCL as a terahertz light source, which emits a point light source;

[0009] A spherical lens for collimating the beam, the spherical lens is placed at a position of one focal length behind the point light source to collimate the spherical light emitted by the point light source into a parallel beam;

[0010] An aspherical lens for converging the beam, which focuses the parallel beam on a point on the sample plane;

[0011] A sample placed at one focal length behind the aspherical lens is driven by a two-dimensional scanning stage to move two-dimensionally in a plane perpendicular to the optical axis so that the entire sample is scanned;

[0012] A terahertz power detector closely behind the sample to reduce errors caused by scattering and attenuation of terahertz light transmitted through the sample;

[0013] A signal generator and a current source for providing signals and energy to the light source;

[0014] A lock-in amplifier for receiving signals from the terahertz power detector;

[0015] A computer for imaging control and image generation.

[0016] Furthermore, the output wavelength of the QCL is 3.7 THz, driven by a current source, operating in a liquid nitrogen cooling environment, and a small hole with a diameter of 2 mm is provided at its output port to make it a point light source emitting spherical light.

[0017] Furthermore, the shape of the aspherical lens is based on the general formula of an even-order curve, and the parameter values of the general formula are optimized using the ray tracing algorithm and the finite element analysis method; the purpose of optimizing the aspherical lens is to minimize the beam waist radius of the focused beam.

[0018] Furthermore, the material of the aspherical lens is TPX, which is transparent in both the terahertz band and the visible band.

[0019] Furthermore, the shape of the aspherical lens satisfies the following conditions: the aspherical lens is rotationally symmetric, and its shape in the plane is enclosed by two even-order curves. The part more than 10 mm away from the axis in the region enclosed by the two even-order curves is removed; both of the two even-order curves satisfy the following general formula:

[0020]

[0021] where r is the radius from the principal optical axis and is the independent variable of the equation; a1 is the second-order coefficient, a2 is the fourth-order coefficient, a3 is the sixth-order coefficient, a4 is the eighth-order coefficient, a5 is the tenth-order coefficient, a6 is the twelfth-order coefficient, d is the intercept of the principal optical axis, c is the curvature, and k is the conic coefficient.

[0022] Furthermore, a plane mirror is provided to fold the optical path.

[0023] From the above technical solutions, it can be seen that the single-frequency terahertz focusing transmission imaging system based on QCL and aspherical lens of the present invention has the following beneficial effects:

[0024] (1) The terahertz optical path structure is simple, without the need to calibrate a large number of optical elements, and the system has high robustness;

[0025] (2) The Golay cell is used as the terahertz power detector, and the single-point sampling speed is fast. It only takes 100 ms to complete the acquisition of the terahertz transmission power of a sample point.

[0026] (3) A specially designed aspherical lens is used as the focusing lens, which can make the waist radius of the focused beam less than the wavelength, enabling the imaging system to have a high resolution approximate to the wavelength. Description of the Drawings

[0027] Figure 1 It is a schematic structural diagram of the single-frequency terahertz focusing transmission imaging system based on QCL and aspherical lens of the present invention;

[0028] Figure 2 It is a shape sectional view of the aspherical lens of the present invention;

[0029] Figure 3 It is an axial distribution diagram of the electric field intensity simulation of the aspherical lens of the present invention;

[0030] Figure 4 It is a transverse distribution diagram of the electric field intensity simulation of the aspherical lens of the present invention;

[0031] Figure 5 It is a measured transverse light intensity distribution diagram at the focus of the aspherical lens of the present invention. The dotted line represents the light intensity value of the maximum value 1 / e 2 of the light intensity, and the length of the curve intercepting the dotted line is twice the waist radius;

[0032] Figure 6 It is an imaging result diagram of a Siemens star-shaped sample with a gold-plated silicon substrate in a specific embodiment of the present invention. The left image size is 10 mm, and the right image is an enlarged schematic diagram at the center of the left image, with an image size of 1.2 mm;

[0033] Figure 7 It is an imaging diagram (left) and a physical diagram (right) of a tissue section with a thickness of 100 microns of a dry rabbit liver in a specific embodiment of the present invention. Detailed Embodiments

[0034] In order to make the purpose, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0035] The single-frequency terahertz emitter QCL and the aspherical lens for focusing used in the single-frequency terahertz focusing transmission imaging system of the present invention are both designed to obtain high resolution at the wavelength level while ensuring that the complexity of the imaging system is not high.

[0036] As Figure 1 shown, the single-frequency terahertz focusing transmission imaging system based on QCL and aspherical lens of the present invention includes:

[0037] A quantum cascade laser (QCL) as a terahertz light source, with an output wavelength of 3.7 THz, which is driven by a current source and needs to work in a liquid nitrogen cooling environment. A small hole with a diameter of 2 mm is placed at its output port to make the light source a point source that can emit spherical light.

[0038] A spherical lens for collimating the beam. The spherical lens should be placed at a position of one focal length after the light source point, and its function is to collimate the spherical light emitted by the point source into a parallel beam.

[0039] An aspherical lens for converging the beam. Since the beam has been collimated, theoretically the aspherical lens can be placed at any position after the collimating lens. However, in practice, considering the attenuation of terahertz light in the air medium, the distance between the two lenses should not be too far. Its function is to focus the parallel beam on a point on the sample plane. The shape of the aspherical lens is based on the general formula of even curves, and the parameter values are optimized using the ray tracing algorithm and the finite element analysis method according to the requirements of this embodiment. The purpose of optimizing the aspherical lens is to minimize the waist radius of the focused beam. For different specific embodiments, the optimized parameters do not need to be exactly equal to the parameter values in this specific embodiment.

[0040] The shape of the aspherical lens satisfies the following conditions: The aspherical lens is rotationally symmetric, and its shape in the plane is surrounded by two even curves above and below. The part more than 10 mm away from the axis in the area surrounded by the two even curves is removed. Both of the two even curves satisfy the following general formula:

[0041]

[0042] where r is the radius from the principal optical axis and is the independent variable of the equation. The meanings of the parameters are shown in Table 1:

[0043] Table 1

[0044]

[0045] For the upper and lower surfaces of the aspherical lens, the values of the parameters are shown in Table 2 (where R is the reciprocal of the curvature c, that is, the radius of curvature):

[0046] Table 2

[0047]

[0048] The cross-sectional shape of the aspherical lens is as shown in Figure 2 . In the figure, the Z-axis is the rotation symmetry axis of the lens and needs to be collinear with the principal optical axis in the imaging system; point O is the vertex of the aspherical lens closer to the light source side, and the X-axis is the direction of the lens diameter in the lens cross-sectional view. The diameter of this lens is 20 mm, and the edge thickness is 4.677 mm.

[0049] The beam focused by this aspherical lens is a Gaussian beam, and its waist radius reflects the radius of the beam at the focal point (with the 1 / e 2 as the boundary). It can be considered that the waist radius is the radius of the light spot where the focal point irradiates on the sample plane. The waist radius of the beam focused by the aspherical lens directly determines the lateral resolution of the imaging system, as shown in Figure 3 . The design goal of this aspherical lens is to obtain the smallest possible waist radius. In fact, the simulated value of the waist radius of this aspherical lens is 69.9 μm (which can be obtained from Figure 4 ), and the measured value is 73 μm (marked in Figure 5 ). The simulated value is very close to the measured value and is lower than the wavelength (80 μm) of the terahertz wave used.

[0050] The placement direction of the aspherical lens in the imaging system is: the terahertz beam is incident from the lower surface (more prominent) and converges to the upper surface (smoother).

[0051] The sample placed at one focal length behind the aspherical lens is driven by a two-dimensional scanning stage and can move two-dimensionally in a plane perpendicular to the optical axis to scan the entire sample;

[0052] The terahertz power detector closely behind the sample is placed closely behind the sample to reduce the errors caused by the scattering and attenuation of the terahertz light transmitted through the sample. The terahertz power detector is a Golay Cell.

[0053] A signal generator and a current source for providing signals and energy to the light source, a lock-in amplifier for receiving the signals from the terahertz power detector, and a computer for imaging control and image generation. Among them, the current source outputs a current of 1.8 A to drive the terahertz light source according to the voltage signal provided by the signal generator. The lock-in amplifier receives the signals from the terahertz power detector, amplifies them, and then transmits them to the computer.

[0054] For the above optical devices, except for the sample, the principal optical axes of the light source, the small hole, the spherical lens, the aspherical lens, and the terahertz power detector are all adjusted to be collinear. In the optical path, a plane mirror can be placed for convenient placement of the devices, which has no impact on the imaging system.

[0055] Using a specific embodiment of the imaging system, the imaging effects on different samples are as follows Figure 6 , Figure 7 shown. Figure 6 It is a diagram showing the imaging result of a Siemens star-shaped sample with gold plating on a silicon substrate. The size of the left image is 10 mm, and the right side is magnified at the center with an image size of 1.2 mm. Figure 7 It is a comparison between the imaging diagram of a dried rabbit liver tissue section with a thickness of 100 microns and the actual object.

[0056] In summary, the present invention designs a single-frequency terahertz focusing transmission imaging system based on QCL and an aspherical lens, achieving high resolution close to the wavelength. And compared with THz-TDS, the complexity of the system of the present invention is greatly reduced, and the sampling speed is greatly accelerated, having high application value in the field of imaging identification of biological tissue slices.

[0057] The above specific embodiments have further detailed the purpose, technical solution and beneficial effects of the present invention. It should be understood that the above is only the specific embodiments of the present invention and is not used to limit the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A single-frequency terahertz focusing transmission imaging system based on QCL and an aspherical lens, characterized in that, Comprising: A QCL serving as a terahertz light source, which emits a point light source; A spherical lens for collimating the light beam. The spherical lens is placed at a position of one focal length behind the point light source to collimate the spherical light emitted by the point light source into a parallel light beam; An aspherical lens for converging the light beam, which focuses the parallel light beam onto a point on the sample plane; A sample placed at a position of one focal length behind the aspherical lens, which is driven by a two-dimensional scanning stage and moves two-dimensionally in a plane perpendicular to the optical axis so that the entire sample is scanned; A terahertz power detector closely behind the sample to reduce the errors caused by the scattering and attenuation of the terahertz light transmitted through the sample; A signal generator and a current source for providing signals and energy to the light source; A lock-in amplifier for receiving the signals from the terahertz power detector; A computer for imaging control and image generation; The shape of the aspherical lens is based on the general formula of an even-order curve, and the parameter values of the general formula are optimized using the ray tracing algorithm and the finite element analysis method. The purpose of optimizing the aspherical lens is to minimize the waist radius of the focused light beam; The shape of the aspherical lens satisfies the following conditions: The aspherical lens is rotationally symmetric and collinear with the principal optical axis in the imaging system. Its shape in the plane is enclosed by two even-order curves. The part more than 10 mm away from the axis in the region enclosed by the two even-order curves is removed. Both of the two even-order curves satisfy the following general formula: (1) Where, r is the radius from the principal optical axis and is the independent variable of the equation; a1 is the second-order coefficient, a2 is the fourth-order coefficient, a3 is the sixth-order coefficient, a4 is the eighth-order coefficient, a5 is the tenth-order coefficient, a6 is the twelfth-order coefficient, d is the intercept of the principal optical axis, c is the curvature, and k is the conic coefficient; The lens diameter is 20 mm and the edge thickness is 4.677 mm; The parameter values of the upper and lower surfaces of the aspherical lens are shown in the following table: Where, R is the reciprocal of the curvature c, that is, the radius of curvature; The placement direction of the aspherical lens in the imaging system is: The terahertz light beam is incident from the lower surface and converges to the upper surface.

2. The single-frequency terahertz focusing transmission imaging system based on QCL and aspherical lens according to claim 1, characterized in that, The output wavelength of the QCL is 3.7 THz, which is driven by a current source and operates in a liquid nitrogen cooling environment. A small hole with a diameter of 2 mm is provided at its output port to make it a point light source emitting spherical light.

3. A single-frequency terahertz focusing transmission imaging system based on QCL and an aspherical lens according to claim 1, wherein The material of the aspherical lens is TPX, which is transparent in both the terahertz band and the visible band.

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