Wide-spectrum visible light receiving system using achromatic lens combination

Abstract

The invention belongs to the technical field of visible light communication, and particularly relates to a wide-spectrum visible light receiving system using an achromatic lens combination. The visible light receiving system comprises an achromatic lens combination and a PD. The achromatic lens combination is composed of a large-size common optical focusing lens and a small-size super lens with a phase compensation function. The super lens takes a glass cylinder as a bottom support, and a TiO2 nano-column assembly with the same height is arranged on the glass cylinder; the optical focusing lens is located on the left side of the system, the super lens is located in the middle, and the PD is located on the rightmost side; the distance between the PD and the top of the TiO2 nanorod is equal to the focal length of the super lens on the extension line of the central line of the super lens; the layout ensures that the visible light emitted by the LED is accurately converged at the central position of the light absorption chip of the PD through the super lens, so that the light energy absorbed by the PD can be greatly enhanced, a plurality of lenses and objective lenses do not need to be used for zooming light beams to introduce extra chromatic aberration, and the signal detection and processing effects are greatly improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of visible light communication, and particularly relates to a wide-spectrum visible light receiving system using an achromatic lens combination. Background Art

[0002] With the rapid development of the sixth-generation mobile communication technology (6G), the demand for high-capacity communication is increasing day by day. Visible light communication technology is considered to be one of the most promising solutions in the 6G network because of its theoretically large communication capacity [1-3]. This technology uses light-emitting diodes (LEDs) as the transmitting end and photodetectors (PDs) as the receiving end, which not only has cost-effective advantages but also can be seamlessly integrated with existing lighting infrastructure [3]. However, in practical applications, how to improve the data transmission rate of VLC systems remains a technical problem to be solved urgently. Research shows that by optimizing the optical receiving system composed of components such as lenses and photodetectors, the communication performance of the system can be effectively improved [4,5].

[0003] In existing optical wireless communication (OWC) systems, optical lenses are generally used to achieve beam focusing. However, traditional optical lenses have a significant technical defect: their focal length is correlated with the incident wavelength, specifically manifested as the chromatic aberration phenomenon that the focal length decreases with the increase of the incident wavelength [6,7]. This characteristic is particularly prominent in OWC systems with LEDs as the light source. Since LED light sources usually have a wide spectral range, traditional optical lenses cannot focus light of all wavelengths on the same focal plane at the same time, resulting in the inability of photodetectors to fully absorb all light signals. This technical defect not only reduces the communication rate of the system but also causes significant light signal loss, severely restricting the performance improvement of VLC systems. And existing functional optical devices need to use a combination of various large-sized and complex-structured lenses and objectives. If a large-sized single achromatic metalens is designed, it requires huge computing resources and phase libraries, which pose very severe challenges to the existing computing resources and process levels.

[0004] Due to the limitations of computing resources and manufacturing processes, the current sizes of achromatic metalenses are mostly at the micron level. In imaging and communication systems, multiple lenses and objectives are needed to scale the beam, which will further introduce problems such as chromatic aberration and dispersion, restricting practical applications.

[0005] Therefore, developing a new type of optical receiving device that can overcome the chromatic aberration problem, improve the light signal reception efficiency, and does not require the assistance of many other optical devices is of great significance for improving the overall performance of visible light communication systems. Summary of the Invention

[0006] The object of the present invention is to provide a wide-spectrum visible light receiving system that can eliminate color difference, has high optical signal reception efficiency and a simple structure, so as to achieve high-efficiency high-speed free-space visible light wireless communication.

[0007] The wide-spectrum visible light receiving system provided by the present invention uses an achromatic lens combination, and its structure is shown in Figure 1 As shown, it specifically includes an achromatic lens combination and a photodetector (PD); the achromatic lens combination is composed of a large-size ordinary optical focusing lens and a small-size metalens with a phase compensation function. The two lenses are separated by a certain distance, and the centers of the lenses are on the same horizontal line; among them, the metalens with a phase compensation function includes a glass cylinder as the bottom support, and an array composed of several TiO2 nanocolumns of the same height evenly distributed on the glass cylinder substrate (the nanocolumns are all perpendicular to the substrate); the center line of the glass cylinder is the center line of the metalens. The achromatic lens combination is located on the left side of the LBVAR structure. The large-size optical lens is on the leftmost side, the metalens is in the middle, and the PD is on the rightmost side. The three are placed vertically and parallel to each other. The center of the achromatic lens group and the center of the PD are on the same horizontal line. The light absorption chip of the PD is placed parallel to the metalens on one side of the TiO2 nanocolumns of the metalens. On the extension line of the center line of the metalens, the distance from the top of the TiO2 nanocolumns is the focal length of the metalens. Such a layout design ensures that the visible light emitted by the LED is accurately converged at the center position of the light absorption chip of the PD. By changing the structural dimensions of the nanocolumns and the arrangement mode of the array, phase compensation can be achieved.

[0008] The spectrum of the visible light is 400 - 700 nm.

[0009] In the present invention, the ordinary optical lens converges the parallel visible light beam; the metalens can not only converge the light beam, but also effectively solve the problem of inconsistent focal plane positions of multi-wavelength light, realize accurate focusing of the visible light spectrum, increase the absorption and conversion of light by the photodetector (PD), so as to achieve the efficient utilization of optical signals and high-speed visible light wireless communication.

[0010] In the present invention, the large-size ordinary optical focusing lens has a diameter of 4 mm, a focal length of 1 cm, and a design wavelength of 400 nm; the small-size metalens has a diameter of 0.22 mm and a focal length of 0.75 mm.

[0011] In the present invention, in the TiO₂ nanocolumn array of the metalens, rectangular nanocolumns are preferred. They have equal height, all being 800 nm, and the center distance between adjacent nanocolumns is 400 nm. The length, width, and rotation angle of the nanocolumns are variable parameters, and the control and compensation of the phase are achieved through regulation. The length and width dimensions of the nanocolumns range from 80 to 320 nm, and can cover different dispersion ranges within the optical wave range of 400 - 700 nm. Taking the focusing phase of the wavelength of 700 nm as the baseline, the theoretical dispersion range of the lens is calculated. According to the calculated theoretical dispersion range, nanocolumns with appropriate sizes that can cover this dispersion range are selected, and then the nanocolumn bodies are rotated to conform to the focusing phase of the wavelength of 700 nm.

[0012] The following further introduces the specific design of the achromatic combination lens:

[0013] (1) The designed phase of a common optical lens, that is, the focusing phase of the designed wavelength, is:

[0014]

[0015] where x and y represent the coordinates with the center of the metalens as the origin, f represents the designed focal length, and λ set is the designed wavelength.

[0016] After the parallel light beam passes through the optical lens and undergoes refraction, the electric field distribution after propagating a distance z in free space is:

[0017] E(x, y, z) = FFT -1 {FFT{E₀(x, y, 0)} · H(f x , f y , z)} (2)

[0018]

[0019] where FFT{·} is the two-dimensional Fourier transform, FFT -1 {·} is the inverse Fourier transform, E₀(x, y, 0) is the plane electric field after the light beam passes through the lens, E(x, y, z) is the plane electric field after the light beam passes through the lens and propagates a distance z, H(f x , f y , z) represents the free space transmission function, k = 2π / λ is the wave number, f x , f y is the frequency domain coordinate, and λ is the incident wavelength.

[0020] Due to the existence of chromatic aberration, the phases of lights with different wavelengths at the same propagation distance are not the same, having a phase difference Δphase. In order to eliminate chromatic aberration and simultaneously meet the focusing requirement, that is, to satisfy:

[0021]

[0022] Therefore, to compensate for the phase difference between different incident wavelengths, the focusing phase distribution equation can be rewritten with the focusing phase at a fixed wavelength as the baseline, and the phase difference Δphi between this wavelength and other wavelengths is added.

[0023] (2) Phase design of the metalens; that is, the metalens for achromatism needs to compensate for the phase of two parts, namely the phase difference (|Δphase|) of the ordinary optical lens and the phase difference (|Δphi|) of the target phases at different wavelengths, and take the absolute value of these two phase differences:

[0024] ΔP achromate = |Δphase| + |Δphi| (5)

[0025] Specifically:

[0026] First, determine the radius, focal length, design wavelength, and propagation distance of the large-sized ordinary optical lens. Then, the plane electric field of light beams with different wavelengths in the visible light spectrum passing through the lens and propagating in free space for a certain distance can be simulated using matlab software to obtain the phase distribution, and the phase difference |Δphase| between the minimum wavelength (400 nm) and the maximum wavelength (700 nm) can be calculated.

[0027] Second, determine the target phase distribution of the achromatic lens combination according to formula (4). According to the target phase distribution, the phase difference |Δphi| between the minimum wavelength (400 nm) and the maximum wavelength (700 nm) can be determined.

[0028] Finally, according to formula (4), combining these two phase differences is the phase compensation required for the metalens.

[0029] In an ordinary receiving system, the lens only has the focusing function, and the focal length decreases as the incident wavelength increases, showing the phenomenon of chromatic aberration. In a visible light communication (VLC) system, this phenomenon makes the light beam unable to be accurately focused on the PD chip, thereby affecting the complete absorption of the visible light spot of the information carrier, reducing the photoelectric conversion efficiency, and limiting the information transmission speed. The present invention uses an achromatic lens group and can accurately align the focal planes of visible light with multiple wavelengths to the absorption layer of the PD, enhancing the light energy absorbed by the PD, and also does not require multiple lenses and objectives to scale the light beam to introduce additional chromatic aberration, which is crucial for optimizing signal detection and processing. Description of the Drawings

[0030] Figure 1 It is a schematic diagram of the structure of an existing wide-spectrum visible light receiving system.

[0031] Figure 2This is a structural diagram of the broadband visible light receiving system of the present invention.

[0032] Figure 3 For incident wavelengths (405, 450, 520, 638 nm), (a) the simulated intensity distribution of the foci of the achromatic metalens and (b) the conventional metalens in the x-z plane, and (c) the simulated results of the focal length varying with the incident wavelength. Detailed implementation manners

[0033] In the present invention, the diameter of the ordinary optical converging lens is designed according to the diameter of the light-emitting spot of the fiber collimator. The diameter of the light-emitting spot of the fiber collimator is 2 mm. In the present invention, the diameter of the ordinary optical converging lens is set to 4 mm, the focal length is 1 cm, and the designed wavelength is 400 nm to ensure that the light beam within the visible spectral range can be completely converged and incident into the LBVAR. In the TiO₂ nanocolumn array of the metalens, the rectangular nanocolumns have equal height, all 800 nm, and the center distance between adjacent nanocolumns is 400 nm; the length and width of the nanocolumns are determined according to the parameter scanning function of the simulation software for the phase compensation range of the nanocolumns within the optical wave range of 400 - 700 nm, and a numerical point is taken every 5 nm within the size range of 80 - 320 nm as the length and width of the nanocolumns for parameter scanning. Taking the focusing phase at a wavelength of 700 nm as the baseline, the theoretical dispersion range of the lens is calculated. According to the calculated theoretical dispersion range, nanocolumns with appropriate sizes that can cover this dispersion range are selected, and then the nanocolumn bodies are rotated according to the PB phase formula to conform to the focusing phase at a wavelength of 700 nm.

[0034] The present invention will be further introduced below in combination with the simulation method and simulation results.

[0035] (1) Calculation range of the dispersion of the ordinary optical converging lens

[0036] First, use the matlab software to perform free-space propagation simulation on the ordinary optical converging lens and calculate the far-field focus distribution. As Figure 3 shown, focus on analyzing the focus positions of the ordinary optical converging lens in the x-z plane within the visible spectral range (mainly 400 nm and 700 nm), and at the same time determine the placement position of the metalens. When the incident wavelengths are 400 nm and 700 nm respectively, the focal lengths are 1 cm and 0.57 cm. According to the principles of the lens diameter, focus position, and the diameter of the metalens that can be designed, the present invention determines that the metalens is placed at a position z = 0.80 cm away from the center point of the ordinary lens and determines the diameter of the metalens to be 0.22 mm. At the same time, calculate the phase distributions (phase 400 and phase 700), calculate its dispersion range (|Δphase| = |phase 700 -phase 400 |).

[0037] (2) Theoretical dispersion compensation calculation

[0038] Secondly, calculate the theoretical dispersion compensation (ΔP achromate ) of the metalens. According to the relevant parameters of the metalens (diameter of 0.22 mm, focal length of 0.75 mm and formula (4)), calculate the phase change (|Δphi| = |phi 700 -phi 400 |) in the x-y plane at the target position (0.75 mm from the top of the metalens nanocylinders) when the incident wavelengths are 400 nm and 700 nm. Add the dispersion range (|Δphase|) generated by the ordinary optical lens to the dispersion range (|Δphi|) at the target position to obtain the theoretical dispersion range that the metalens needs to compensate.

[0039] (3) Theoretical simulation of the lens combination

[0040] Finally, take the phase (phase 700 ) in the x-y plane of an ordinary optical converging lens with an incident wavelength of 700 nm at z = 0.8 cm as the baseline. Assuming that the nanocylinders constituting the metalens can provide linear phase compensation in the visible spectrum (400 - 700 nm), the phase distribution of the large-aperture achromatic lens combination at a specific incident wavelength is as follows:

[0041]

[0042] where λ inis the incident wavelength. The diameter of the large lens in the achromatic lens combination is 4 mm, the diameter of the metalens is 0.22 mm, and the focal length of the lens combination is 8.75 mm. The present invention uses the angular spectrum method to simulate the transmission of visible light beams in free space, and focuses on analyzing the intensity distribution and focal position of the achromatic combination lens and the conventional lens in the x-z plane when the incident wavelengths are 400, 450, 500, 550, 600, 650, and 700 nm. According to theoretical calculations, when the incident wavelengths are 400, 450, 500, 550, 600, 650, and 700 nm, the focal positions are 8.673, 8.658, 8.658, 8.673, 8.696, 8.720, and 8.742 mm respectively, and the average focal length is 8.689 mm, with a small deviation compared to the designed focal length (8.750 mm), and the maximum focal length deviation is 0.092 mm. At the same time, a common lens with the same parameters is designed, and the focal length range in the visible spectral range is 4.981 - 8.733 mm. The focal length of the lens decreases with the increase of the incident wavelength, and the maximum focal length deviation reaches 3.769 mm. Compared with the conventional lens, the large-aperture achromatic lens combination shows a smaller focal length deviation, indicating that the correlation between the focal length of the achromatic lens combination and the incident wavelength is reduced, thus verifying the lens design strategy of the present invention and achieving a 97.6% performance improvement, as Figure 3 shown.

[0043] (4) Calculation of PD received optical intensity

[0044] The LBVAR system designed by the present invention improves the communication rate by enhancing the optical energy absorbed by the PD. As mentioned in the above simulation results, the average focal length of the achromatic lens combination is 8.689 mm. The present invention uses GaAs-PIN (Newport, 818-BB-45A) as the PD, and its optical absorption area is 60×60 μm 2 . Therefore, it is necessary to calculate the optical intensity energy on the x-y plane at z = 8.689 mm to calculate the percentage of the total energy in the 60×60 μm 2 area. Based on the assumption that visible light can completely pass through the metalens without being absorbed by the glass substrate and TiO2 nanocolumns, first use matlab to extract and analyze the optical energy on the x-y plane of 4×4 mm 2 at z = 8.689 mm, and consider it as the total energy after the incident light is diffracted by the lens. Then calculate the spot energy in the central area of 60×60 μm 2 area and calculate the percentage. The results are compiled in Table 1. As expected, the achromatic lens combination maintains almost the same spot intensity at different incident wavelengths, in contrast to the conventional lens, whose spot intensity shows significant differences, especially in the case of the green and red light spectra, where the light intensity is reduced by about 90%.

[0045] Table 1. Percentage of total energy in the achromatic and conventional metalens with 60×60μm at z = 8.689mm 2 within the area

[0046]

[0047]

[0048] The advantages of the present invention are as follows: By using an achromatic lens combination in optical wireless communication, visible light of multiple wavelengths can be accurately focused on the absorption chip of the PD without the assistance of many other optical devices. Compared with conventional lenses, in the case of the green and red light spectra, the light intensity of the large-aperture achromatic lens combination increases by about 90%. This shows that the visible light receiving system composed of the large-aperture achromatic lens combination and the PD exhibits superior communication capabilities in optical wireless communication, providing an option for realizing faster 6G wireless communication.

[0049] References:

[0050] 1. N. Chi, Y. Zhou, Y. Wei, and F. J. I. V. T. M. Hu, "Visible light communication in 6G: Advances, challenges, and prospects," vol. 15, no. 4, pp. 93 - 102, 2020.

[0051] 2. J. Shi et al., "AI-enabled intelligent visible light communications: Challenges, progress, and future," vol. 9, no. 8, p. 529, 2022.

[0052] 3. S. U. Rehman et al., "Visible light communication: A system perspective overview and challenges," vol. 19, no. 5, p. 1153, 2019.

[0053] 4. J. Li, P. Zou, X. Ji, X. Guo, and N. Chi, "High-speed visible light communication utilizing monolithic integrated PIN array receiver," vol. 494, p. 127027, 2021.

[0054] 5. N. He et al., "High-speed duplex free space optical communicationsystem assisted by a wide-field-of-view metalens," vol.10, no.9, pp.3052-3059, 2023.

[0055] 6. M. Khorasaninejad et al., "Metalenses at visible wavelengths:Diffraction-limited focusing and subwavelength resolution imaging," vol 352, no.6290, pp.1190-1194, 2016.

[0056] 7. N. Meinzer et al., "Plasmonic meta-atoms and metasurfaces," vol.8, no.12, pp.889–898 2014。

Claims

1. A wide-spectrum visible light receiving system using an achromatic lens combination, characterized in that: Specifically, it includes an achromatic lens combination and a photodetector (PD); the achromatic lens combination is composed of a large-sized ordinary optical focusing lens and a small-sized super lens with a phase compensation function, the two lenses are separated by a certain distance, and the centers of the lenses are on the same horizontal line; wherein the super lens includes a glass cylinder as a bottom support, and an array composed of a plurality of TiO2 nano columns of equal height uniformly arranged on the glass cylinder substrate, and the nano columns are all perpendicular to the substrate; the center line of the glass cylinder is the center line of the super lens; the achromatic lens combination is located on the left side of the LBVAR structure, the leftmost is a large-sized optical lens, the super lens is in the middle, and the rightmost is the PD, the three are placed vertically and parallel to each other, and the center of the achromatic lens group is on the same horizontal line as the center of the PD; the light absorption chip of the PD is placed parallel to the super lens on one side of the TiO2 nano column of the super lens; on the extension line of the center line of the super lens, the distance from the top of the TiO2 nano column is the focal length of the super lens; such a layout ensures that the visible light emitted by the LED is accurately converged at the center position of the light absorption chip of the PD through the super lens; wherein, phase compensation is achieved by changing the structural size of the nano column and the arrangement of the array; The spectrum of the visible light is 400-700nm.

2. The wide-spectrum visible light receiving system according to claim 1, characterized in that: The large-sized ordinary optical focusing lens has a diameter of 4 mm, a focal length of 1 cm, and a design wavelength of 400 nm; the small-sized superlens has a diameter of 0.22 mm and a focal length of 0.75 mm.

3. The wide-spectrum visible light receiving system according to claim 2, characterized in that: In the TiO2 nanocolumn array of the superlens, the nanocolumns are rectangular nanocolumns with equal heights of 800nm, and the center distance between adjacent nanocolumns is 400nm; the length and width of the nanocolumns and the rotation angle of the column are variable parameters, and phase control and compensation are achieved through regulation; the length and width of the nanocolumns range from 80 to 320nm, which can cover different dispersion ranges within the light wave range of 400 to 700nm; the focusing phase of a wavelength of 700nm is used as a baseline to calculate the theoretical dispersion range of the lens, and based on the calculated theoretical dispersion range, a nanocolumn of a suitable size that can cover this dispersion range is selected, and then the nanocolumn body is rotated to meet the focusing phase of a wavelength of 700nm.

4. The wide-spectrum visible light receiving system according to claim 3, characterized in that: The specific design of the achromatic combination lens is as follows: (1) The design phase of an ordinary optical focusing lens, that is, the focusing phase of the design wavelength is: Where x and y represent the coordinates with the center of the lens as the origin, f represents the designed focal length, and λ set is the design wavelength; After a parallel light beam is refracted through a lens, the electric field distribution after propagating a distance z in free space is: E(x,y,z)=FFT -1 {FFT{E0(x,y,0)}·H(f x ,f y ,z)} (2) Among them, FFT{·} is the two-dimensional Fourier transform, FFT -1 {·} is the inverse Fourier transform, E0(x,y,0) is the plane electric field after the light beam passes through the lens, E(x,y,z) is the plane electric field after the light beam passes through the lens and propagates a distance z, H(f x ,f y ,z) represents the free space transmission function, k = 2π / λ is the wave number, f x ,f y is the frequency domain coordinate, λ is the incident wavelength; Due to the existence of chromatic aberration, light of different wavelengths has different phases at the same propagation distance, and has a phase difference Δphase. In order to eliminate chromatic aberration and meet the focusing requirements at the same time, that is, to meet: In order to compensate for the phase difference between different incident wavelengths, the focusing phase distribution equation is rewritten to take the focusing phase of a fixed wavelength as the baseline and supplement the phase difference Δphi between this wavelength and other wavelengths; Therefore, the achromatic metalens compensates for the phase difference of the ordinary optical lens and the phase difference of the target phase of different wavelengths, that is: ΔP achromate =|Δphase|+|Δphi| (5) Specifically: Firstly, the radius, focal length, design wavelength and propagation distance of a large-size ordinary optical lens are determined. Matlab software is used to simulate the plane electric field of light beams of different wavelengths in the visible light spectrum after they pass through the lens and propagate a certain distance in free space, and the phase distribution is obtained. The phase difference |Δphase| between the minimum wavelength of 400nm and the maximum wavelength of 700nm is calculated. Secondly, the target phase distribution of the achromatic lens combination is determined according to formula (4), and the phase difference |Δphi| between the minimum wavelength 400nm and the maximum wavelength 700nm is determined according to the target phase distribution; Finally, according to formula (5), the phase compensation required by the metalens is obtained.

Images