All-fiber light beam modulation and collection system based on double super lenses and design method of all-fiber light beam modulation and collection system

Abstract

The invention relates to the field of integrated optical fiber microscopic endoscopy, in particular to an all-optical-fiber light beam modulation and collection system based on double super lenses and a design method of the all-optical-fiber light beam modulation and collection system. The system comprises a single-mode optical fiber, a coreless optical fiber for beam expansion and two super lenses (a super lens 1 and a super lens 2) with different functions. The super lens 1 is arranged on the end face of the fiber core of the single-mode optical fiber and focuses output exciting light to form extremely small waist spots, and the divergence angle of light beams is increased. The super lens 2 is arranged on the end face of the coreless optical fiber beam expanding area and used for refocusing the exciting light and focusing the collected fluorescence to the focus position of the super lens 1 at the same time, and the fluorescence is incident to the fiber core in parallel through the super lens 1. According to the invention, a double-super-lens system based on a high numerical aperture (NA) is fully utilized, wavefront modulation is carried out, the length of a beam expanding area is adjusted, and the loss of fluorescence in the beam expanding area is reduced, so that the collection efficiency of the fluorescence is remarkably improved, and an innovative solution is provided for the design of a super-lens-based optical fiber optical system.

Description

Technical Field

[0001] The present invention relates to the field of integrated optical fiber micro-endoscopy, and in particular to an all-optical fiber beam modulation and collection system based on a double super lens and a design method thereof. Background Art

[0002] Optical fiber is an efficient and compact photon transmission device with unique characteristics such as flexible operation, strong light confinement, and long-distance and efficient light transmission. It has important technical significance in the field of modern optics. Fiber optic technology has been widely used in fiber optic communications, light capture, nonlinear light generation, sensing, and endoscopic imaging. Among them, fiber endoscopes, as a key imaging device, have been widely used in in vivo medical imaging. In order to solve the divergence problem of the fiber end face, researchers have proposed a variety of methods for manufacturing micro-optical lenses on the fiber end face. These methods have made certain progress in realizing the beam focusing function. However, the existing technology relies on complex and expensive manufacturing processes, and it is difficult to ensure the quality consistency of optical components, which leads to fluctuations in system performance and limits the feasibility of related optical systems in practical applications.

[0003] The rapid development of metasurface technology has provided a new platform for optimizing the optical performance of traditional optical fibers. As an artificial two-dimensional metamaterial, metasurfaces have the unique ability to manipulate the amplitude, phase, and polarization of light on a subwavelength scale, thereby achieving precise control of the output light field wavefront. As an important branch of metasurfaces, superlenses have brought unprecedented opportunities for creating efficient beam focusing elements on the end face of optical fibers. Through top-down and bottom-up manufacturing methods, such as electron beam lithography, focused ion beam etching, and two-photon polymerization technology, superlenses can be made on the end face of optical fibers to achieve beam focusing. However, due to the limitation of the size of the optical fiber end face, it is difficult for superlenses to achieve sufficient wavefront manipulation and to meet the focusing requirements of ultra-high numerical aperture (NA).

[0004] In addition, the current fluorescence collection optical path design of miniature multiphoton microscopes also faces challenges. Such systems usually use a simple optical configuration in which the microscope objective is separated from the monolithic condenser, which can meet the collection requirements of ballistic fluorescence photons. However, for samples with scattering characteristics (such as living tissue), fluorescent photons are scattered in the tissue and deviate from the original trajectory. Traditional optical systems cannot effectively collect these scattered fluorescent photons, thereby reducing the collection efficiency of fluorescence signals, resulting in a decrease in the signal-to-noise ratio of fluorescence imaging and limited imaging depth. Summary of the invention

[0005] The purpose of the present invention is to provide an all-fiber beam modulation and collection system based on a double super lens and a design method thereof, so as to solve the problems of fiber end face divergence and low fluorescence collection efficiency in the prior art. The present invention realizes precise modulation of the beam and efficient collection of fluorescence signals through the design and coordinated work of the double super lens.

[0006] The present invention provides an all-fiber beam modulation and collection system based on a double superlens, comprising a single-mode optical fiber, a coreless optical fiber beam expansion region and two superlenses with different functions, the specific structure of which is as follows:

[0007] 1. Single-mode optical fiber: responsible for transmitting excitation light and collecting excited fluorescence.

[0008] 2. Superlens 1: The diameter range is 10μm-20μm, which is set at the end face of the single-mode optical fiber core. It is used to focus the light beam output by the single-mode optical fiber and collect the fluorescence focused by the superlens 2 back to the single-mode optical fiber.

[0009] 3. Coreless fiber beam expansion zone: diverges the light beam focused by superlens 1 to a larger cross-sectional size to match the diameter of superlens 2.

[0010] 4. Superlens 2: The diameter range is 50μm-125μm, which is set at the end face of the coreless optical fiber expansion area, and is used to refocus the excitation light diverged after the superlens 1 is focused, and to focus the fluorescence to the focal length position of the superlens 1.

[0011] Among them, the combined design of superlens 1 and superlens 2 in the present invention draws on the principle of telescope system, wherein superlens 1 is a short focal length lens, which is used to focus the excitation light output by single-mode optical fiber in the coreless optical fiber beam expansion area to form a very small waist spot, and further diverge in the beam expansion area with a larger divergence angle. Combined with the functional design of superlens 1 and superlens 2, the light beam transmission path is optimized, and the length of the coreless optical fiber beam expansion area required to obtain the excitation light with a spot diameter range of 50μm-125μm is reduced; the excitation light wavefront modulation area is increased, and the effective collection range of fluorescence is increased, while the loss of fluorescence in the beam expansion area can be reduced, and the collection efficiency of fluorescence signals is increased by more than 5 times.

[0012] The super lens is composed of a dielectric substrate and a dielectric nanocolumn, and phase control is achieved by changing the diameter of the nanocolumn. The dielectric nanocolumn adopts a cylindrical structure to achieve polarization-independent functions and avoid interference from various polarization states in the optical fiber.

[0013] The arrangement of the dielectric nanocolumns is determined by the ideal phase distribution. In order to achieve achromatic focusing of two wavelengths, the phase delay provided by the superlens 1 needs to satisfy the two wavelengths, and its ideal phase distribution needs to satisfy the following conditions at the same time:

[0014]

[0015] Where f is the focal length, (x, y) is the coordinate of a point on the surface of the metalens to be designed, and λ i is the operating wavelength, and C is a constant term related to the wavelength, which is used to increase the degree of freedom of phase matching.

[0016] The superlens 2 needs to consider the light field regulation of the excitation light and the fluorescence at the same time. It is necessary to perform negative value processing on the phase distribution of the Gaussian beam of the excitation light and the phase distribution of the fluorescence after the superlens 1 is focused and then diverged, and superimpose the focusing phase to obtain the target phase distribution. The phase distribution of the superlens 2 needs to satisfy the following requirements at the same time:

[0017]

[0018] Where λ1 is the excitation wavelength of the single-mode fiber input, λ2 is the emission wavelength of the fluorescence, l1 is the focal position of the excitation wavelength, l2 is the transmission distance from the focused light spot of the superlens 1 to the diameter of the superlens 2 after the coreless fiber is expanded, and the focal position of the fluorescence, R is the radius of curvature of the Gaussian beam after focusing by the superlens 1, z R is the confocal parameter of the Gaussian beam after focusing by the superlens 1. C is a constant term related to the wavelength and is used to improve the degree of freedom of phase matching.

[0019] In order to obtain the unit structure closest to the ideal phase curve at each position, the particle swarm optimization algorithm is used to arrange the metalens. The value of the constant C at different wavelengths is used as the input of each population to obtain the ideal phase distribution and find the unit arrangement with the smallest difference between the actual phase and the ideal phase based on this distribution. The fitness function of each population is calculated as the evaluation standard for phase matching. Through continuous optimization and iteration of the particle swarm, it can eventually converge to the global optimal solution that minimizes the fitness function. The fitness function F can be expressed as:

[0020]

[0021] in, represents the actual phase distribution, and m is the number of radius samples.

[0022] like Figure 1 As shown, the excitation light beam of the single-mode optical fiber in the present invention is focused by the superlens 1, diverged in the beam expansion region until the excitation light spot size reaches the diameter of the superlens 2, and then refocused to the imaging plane by the superlens 2. When the fluorescence is excited, the fluorescence signal is focused to the focal length position of the superlens 1 through the superlens 2, reducing the loss of fluorescence in the beam expansion region, and finally being efficiently collected by the superlens 1 and coupled back to the single-mode optical fiber.

[0023] The beneficial effects of the present invention are:

[0024] 1. Optimization of the beam transmission path: Superlens 1 focuses the excitation light into a very small waist spot in the coreless fiber beam expansion area and increases the beam divergence angle, providing ideal beam conditions for subsequent beam expansion and refocusing, so that the excitation beam spot size matches the diameter of superlens 2, providing a sufficiently large wavefront manipulation area. At the same time, by combining the functional design of superlens 1 and superlens 2, the length of the coreless fiber beam expansion area can be adjusted, further optimizing the beam transmission path.

[0025] 2. Improved fluorescence collection efficiency: Through the phase modulation of fluorescence by superlens 2, the fluorescence is collimated and refocused, minimizing the loss of fluorescence signals. The coordinated design of superlens 1 and superlens 2 enables the excited fluorescence to be smoothly collected into the single-mode optical fiber. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a simplified optical path diagram of the all-fiber light beam modulation and collection system of the present invention. The solid line represents the propagation path of the excitation light, and the dotted line represents the propagation path of the fluorescence, showing the optical path behavior of the light beam after passing through the coreless fiber beam expansion area and the double super lens;

[0027] Figure 2 The schematic diagram of the structure of the super lens of the present invention is shown in FIG. The super lens is composed of a dielectric substrate and periodically distributed dielectric nano cylinders;

[0028] Figure 3 1 is a light intensity distribution diagram of the superlens 1 in the embodiment of the present invention at the excitation light wavelength (λ1=1060nm) and the fluorescence wavelength (λ2=548nm);

[0029] Figure 4 1 and 1 are light intensity distribution diagrams of the superlens 2 in the embodiment of the present invention at the excitation light wavelength (λ1=1060nm) and the fluorescence wavelength (λ2=548nm). DETAILED DESCRIPTION

[0030] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0031] like Figure 1 As shown, the present invention provides an all-fiber modulation and collection system based on a double superlens, which includes a single-mode optical fiber, a coreless optical fiber expansion region, a superlens 1 and a superlens 2.

[0032] 1. Single-mode optical fiber: core diameter d = 9 μm, used to transmit an excitation light beam with a wavelength of λ1 = 1060 nm.

[0033] 2. Coreless fiber beam expansion zone: used to expand the excitation light beam focused by the superlens 1 to a larger cross-sectional size.

[0034] 3. Superlens 1: diameter D1 = 16 μm, focal length f = 5 μm, used to focus the excitation light and collect fluorescence into the optical fiber core.

[0035] 4. Superlens 2: diameter D2 = 100 μm, used to refocus the excitation light after expansion and focus the fluorescence to the focal position of superlens 1.

[0036] The excitation light with wavelength λ1=1060nm is transmitted to superlens 1 through single-mode optical fiber, and is focused to the imaging plane by superlens 2 after being diverged in the coreless optical fiber beam expansion area, thereby exciting fluorescence. The excited fluorescence (wavelength λ2=548nm) is focused to the focal length position of superlens 1 by superlens 2, and finally collected by superlens 1 and coupled back to the core of the single-mode optical fiber.

[0037] Among them, the period of the dielectric cylinder on the superlens is U, and according to the Nyquist sampling theorem, its value is U=320nm. The height H of the dielectric cylinder is H=600nm in order to achieve a phase coverage of at least 0 to 2π at two wavelengths λ1=1060nm and λ2=548nm.

[0038] Based on this, the design goal of the metalens 1 in this embodiment is to achieve achromatic focusing at wavelengths of λ1=1060nm and λ2=548nm. The particle swarm is used to optimize the value of the constant C at different wavelengths to obtain the ideal phase distribution and find the unit arrangement with the smallest difference between the actual phase and the ideal phase according to this distribution. Through multiple iterations, the structural unit with the smallest matching error can be found to construct the metalens 1. The optimized metalens 1 is imported into the physical simulation software (such as Lumerical) for simulation. The simulation results are as follows: Figure 3 As shown, the focal positions at the two wavelengths are consistent with the design goals, verifying the dual-wavelength achromatic focusing function of the superlens 1.

[0039] Furthermore, the design of the superlens 2 in this embodiment needs to consider the light field regulation of the excitation light and the fluorescence at the same time. The phase distribution of the Gaussian beam after being focused by the superlens 1 and diverging for a distance in the beam expansion region is processed negatively, and the focusing phase is superimposed to obtain the target phase distribution, and the focusing distance l1 = 25 μm.

[0040] Among them, the electric field of the Gaussian beam can be expressed as:

[0041]

[0042] in is the phase part, w(z) is the spot radius of the Gaussian beam at a fixed position z perpendicular to the light transmission direction. When z = 0, the beam radius is w0, called the beam waist radius; R is the curvature radius of the Gaussian beam; z R is the Rayleigh length, which represents the time from light transmission to the increase of the spot radius to the beam waist. Times the length of the position.

[0043] The excitation light with a wavelength of λ1=1060nm is focused by superlens 1 with a spot radius of 0.5μm. The spot radius w(z) diverges a distance z=l2=110μm in the beam expansion zone and matches the size of superlens 2 to ensure that the entire superlens can fully manipulate the wavefront of the light beam.

[0044] Furthermore, for the excited fluorescence, a point light source is selected for simulation, and the spherical wave phase distribution emitted from the point light source to the superlens 2 is processed negatively, and the focusing phase is superimposed to obtain the target phase distribution, and the focusing distance is l2=110μm.

[0045] Based on this, the particle swarm optimization is used to optimize the value of the constant C of the phase distribution of the metalens 2 at different wavelengths to obtain the ideal phase distribution and find the unit arrangement with the smallest difference between the actual phase and the ideal phase according to this distribution. Through multiple iterations, the structural unit with the smallest matching error can be found to construct the metalens. The optimized metalens 2 is imported into the physical simulation software (such as Lumerical) for simulation. The simulation results of the two wavelengths are shown as follows: Figure 4 As shown, the focus position obtained by simulation calculation is consistent with the focus position required by the super lens 2.

[0046] In this embodiment, the metalens 1 and metalens 2 provided by the present invention work together, and the fluorescence collection efficiency achieved by the present invention is 6.5%, which is significantly higher than the 0.99% of the traditional spatial multiplexing single metalens, and is increased by more than 6.5 times. Through particle swarm optimization and dual-wavelength phase control, the system can accurately control the light field and achieve efficient focusing and collection of excitation light and fluorescence.

[0047] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. An all-fiber beam modulation and collection system based on a dual superlens, comprising a single-mode optical fiber, a coreless optical fiber beam expansion region, and two superlenses with different functions, namely, superlens 1 and superlens 2; characterized in that: The two superlenses have different diameters. The superlens 1 is placed on the end face of the single-mode optical fiber core, with a diameter range of 10μm-20μm, and is used to focus the excitation light beam output by the single-mode optical fiber and collect the fluorescence from the superlens 2 back into the single-mode optical fiber; the superlens 2 is placed on the end face of the coreless optical fiber expansion zone, with a diameter range of 50μm-125μm, and is used to refocus the light beam after passing through the coreless optical fiber expansion zone, and at the same time focus the excited fluorescence to the focal length position of the superlens 1.

2. The all-fiber beam modulation and collection system based on a double superlens according to claim 1, characterized in that: The combined design of the superlens 1 and the superlens 2 draws on the principle of the telescope system, wherein the superlens 1 is a short focal length lens, which is used to first focus the excitation light output by the single-mode optical fiber in the coreless optical fiber expansion region to form a very small waist spot, and further diverge in the expansion region with a larger divergence angle.

3. The all-fiber beam modulation and collection system based on a double superlens according to claim 1, characterized in that: By combining the functional designs of superlens 1 and superlens 2, the light beam transmission path is optimized and the length of the coreless optical fiber beam expansion zone required to obtain excitation light with a spot diameter range of 50μm-125μm is reduced; the excitation light wavefront modulation area is increased, and the effective collection range of fluorescence is increased. At the same time, the loss of fluorescence in the beam expansion zone is reduced, and the collection efficiency of fluorescence signals is increased by more than 5 times.

4. The all-fiber beam modulation and collection system based on a double superlens according to claim 1, characterized in that: The superlens 2 modulates the excitation light that is focused and then diverged by the superlens 1, by performing negative value processing on the Gaussian beam phase distribution of the excitation light and superimposing the focusing phase to achieve the effect of collimation first and then focusing. Phase distribution of the excitation light modulated by the superlens 2: Among them, λ1 is the excitation wavelength of the single-mode fiber input, l1 is the focusing position of the excitation wavelength, l2 is the distance from the focused light spot of superlens 1 to the spot size reaching the diameter of superlens 2 through the coreless fiber beam expansion zone, R is the radius of curvature of the Gaussian beam after focusing through superlens 1, and zR is the confocal parameter of the Gaussian beam after focusing through superlens 1.

5. The all-fiber beam modulation and collection system based on a double superlens according to claim 1, characterized in that: The superlens 2 performs phase modulation on the excited fluorescence. The excited fluorescence has a spherical wave phase distribution. The superlens 2 performs negative value processing on the spherical wave phase distribution of the fluorescence and superimposes the focusing phase to achieve the effect of collimating the fluorescence first and then focusing it, thereby obtaining the target phase distribution. The specific phase distribution is: Among them, λ2 is the emission wavelength of fluorescence, l2 is the transmission distance after the focused light spot of superlens 1 is expanded and the spot size reaches the diameter of superlens 2, which is also the focusing position of superlens 2 on fluorescence, and l1 is the distance that fluorescence is transmitted to superlens 2.

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