CN105093375A - Outdoor visible light communication receiving end natural background light filtering method - Google Patents

Abstract

The invention discloses a method for filtering natural background light at an outdoor visible light communication receiving end, comprising the following steps: Step 1, analyzing the spectral characteristics of white light LEDs and the spectral characteristics of sunlight, and according to the spectral characteristics of the white light LED and the sunlight The comparison result of spectral feature selects the induced transmission filter; Step 2, the structure of the filter in the design step 1; The filter is made up of several layers of thin films; Step 3, according to the angle of incidence and the The relationship between the passbands of the optical filter is analyzed to obtain that the optimal incident angle of the incident light is 0°≤θ≤10°, and the concept of an optical angle selector is proposed; step 4, designing an optical filter for noise filtering; the optical filter The device is composed of an incident light angle selector, a filter, a focusing lens and a photodetector. The filtering method proposed by the present invention can effectively filter out the background light, and proposes that the optimal incident angle θ of the incident light is 0°≤θ≤10°, which reduces the deformation of the passband shape of the optical filter with the change of the θ angle To a certain extent, the quality of communication is guaranteed.

Description

A natural background light filtering method at the receiving end of outdoor visible light communication

technical field

The invention belongs to the field of visible light wireless communication technology, optical filtering technology, and visible light communication technology of LED lamps between mobile vehicles, and in particular relates to a natural background light filtering method of an outdoor visible light communication receiving end.

Background technique

Visible Light Communication technology (VisibleLightCommunication, VLC) refers to the use of light in the visible light band as an information carrier, without using optical fiber and other wired channel transmission media, and a communication method that directly transmits optical signals in the air. LED car lights have the characteristics of high efficiency, low energy consumption, long life, environmental protection, high brightness and high temperature resistance, small size, and good stability. The flickering pulse light intensity modulation that cannot be sensed realizes the purpose of communication between LED lights and traditional lighting functions at the same time. Visible light wireless communication is an emerging wireless optical communication method, which has great research value and broad application prospects.

The document "ADual-ReceivingVisible-LightCommunicationSystemforIntelligentTransportationSystem" introduces the communication mode between traffic lights and vehicles in the intelligent transportation system (IntelligentTransportationSystem, ITS) and the communication mode between vehicles, and proposes a diversity receiving device. In this paper, LED traffic lights are used for visible light communication. LED traffic lights are composed of a single LED red light, yellow light, and green light. Therefore, only one color of light is received at the same time, so the filter at the receiving end only needs to pass one color of light; this document is for traffic lights, but the present invention is for white light LEDs, which do not belong to the same field.

The document "OutdoorVisibleLightCommunicationForInter-VehicleCommunicationUsingControllerAreaNetwork" applies visible light communication to the vehicle's Controller Area Network (ControllerAreaNetwork, CAN), which mainly studies the communication between vehicles. The receiving system in this paper adds a layer of light shield on the outer layer of the receiver to reduce the interference of background light on the receiver, and does not analyze and filter the background spectrum.

In the patent "Tunable Thin Film Filter" (application number 200620040819.1), the tuning range of the filter length is from 1.4um to 1.6um, which is not applicable to the visible light band.

In the patent "optical filter" (application number 200880020360.0), it is suitable for an image sensor with a plurality of sensor pixels arranged. The filter mainly suppresses the light in the direction of optical irradiation, and is convenient for stripping the illumination of multiple array sensors after filtering; it is different from the technical subject of the present invention.

The patent "optical low-pass filter" (application number 200920136004.7) is used for color distortion during daytime shooting and suppresses the generation of moiré false signals; it is completely different from the technical subject of the present invention.

The invention is suitable for visible light communication between LED headlamps of automobiles. The spectral range emitted by automotive LED headlights is 412nm to 775nm, and there is a lot of overlap between signal light and sunlight spectral ranges in this band. Therefore, a new type of optical filter is designed to allow the LED carrier light signal to pass through as much as possible, while filtering out more solar background light and improving the signal-to-noise ratio at the receiving end.

Contents of the invention

In order to solve the problem that outdoor visible light communication signal light and sunlight spectrum overlap more, resulting in the decrease of the signal-to-noise ratio at the receiving end, and the difficulty of separating the signal and noise, the present invention carefully analyzes the solar spectrum and the automobile LED headlight spectrum. Based on the similar and non-similar areas between them and the bandwidth relationship between the two, a natural background light filtering method at the receiving end of outdoor visible light communication is proposed at the front end of the receiving end. The filtering method designed in the present invention The optical filter can filter out the solar background light in non-similar areas, filter out the signal light in similar areas, and retain as much signal light bandwidth as possible, improving the signal-to-noise ratio. The technical solutions adopted are:

A method for filtering natural background light at an outdoor visible light communication receiving end, comprising the following steps:

Step 1, analyzing the spectral characteristics of the white light LED and the spectral characteristics of sunlight, and selecting a filter according to the comparison result of the spectral characteristics of the white light LED and the spectral characteristics of sunlight;

Step 2, designing the structure of the optical filter described in step 1; the optical filter includes several layers of thin films;

Step 3, according to the relationship between the incident angle of light entering the filter and the passband of the filter, an incident light angle selector is designed;

Step 4, design an optical filter for noise filtering; the optical filter includes the incident light angle selector described in step 3, the optical filter, focusing lens and photodetector described in step 2; the incident The light angle selector is arranged on the incident surface of the optical filter, the focusing lens is arranged on the outgoing surface of the optical filter, and the photodetector is arranged at the rear focal point of the focusing lens.

Preferably, the filter described in step 1 is an induced transmission filter.

Preferably, the several layers of films described in step 2 include: incident layer, base layer, metal layer, anti-reflection film stack, matching film layer; the metal layer, the anti-reflection film stack, the matching film layers are located between the incident layer and the base layer, and the metal layer is located in the center between the incident layer and the base layer, and both sides of the metal layer are sequentially arranged from the inside to the outside. There are the matching film layer and the anti-reflection film stack, the matching film layers on both sides of the metal layer are symmetrically distributed with the metal layer as the center of symmetry, and the anti-reflection film stacks on both sides of the metal layer are arranged around the metal layer. The metal layer is symmetrically distributed around the center of symmetry.

Preferably, the anti-reflection film stack includes: a high refractive index λ/4 film layer H and a low refractive index λ/4 film layer L; the high refractive index λ/4 film layer H and a low refractive index λ/4 film The layers L are alternately arranged to form a 2P+1 layer film, and both ends of the 2P+1 layer film are high refractive index λ/4 film layers H;

Among them, P is an integer coefficient, and λ is the center wavelength.

As preferably, the design method of described optical filter is:

Step 2.1, superimposing a layer of matching film layer with refractive index n _F and phase thickness δ on the surface of the metal layer, the optical thickness of the matching film layer is between λ/4 and λ/2; where λ is central wavelength;

Step 2.2, stacking an anti-reflection film stack on the matching film layer;

Step 2.3, setting an incident layer and a base layer on the antireflection film stack.

Preferably, both the incident layer and the base layer are glass, the metal layer is silver, the high refractive index λ/4 film layer H is zinc sulfide, the matching film layer and the low refractive index λ The /4 film layer L is all cryolite.

Preferably, the thickness of the metal layer is 26.80 nm, and the thickness of the matching film layer is m times the thickness of the low refractive index λ/4 film layer.

Preferably, the λ=491.9nm, the P=2, and the m=1.67.

Preferably, the light receiving angle of the incident light angle selector in step 3 is 0°≤θ≤10°.

Beneficial effects of the present invention:

(1) The present invention designs an optical filter with a new structure for the receiving end of the outdoor LED optical communication system, which can effectively filter out the background light. In addition, it also proposes that the optimal incident angle θ of the incident light is 0°≤ θ≤10°, which reduces the degree of deformation of the passband shape of the filter with the change of θ angle, and ensures the quality of communication.

(2) The designed optical filter filters out a lot of sunlight background noise, which can greatly improve the communication quality of the whole system. After formula calculation and simulation experiment analysis, the background light power is reduced by 26% compared with the original Around 42.5% improvement in signal-to-noise ratio (SNR).

Description of drawings

Fig. 1 is a system block diagram of a visible light communication receiving end;

Fig. 2 is a solar spectrum and a white light LED spectrum diagram;

Figure 3 is a schematic diagram of the basic film structure of the induced projection filter;

Fig. 4 is the optical filter passband shape figure when the silver (Ag) layer thickness is 26.8nm;

Figure 5 is a comparison diagram of white light LED spectrum, sunlight spectrum and filter film passband;

Fig. 6 is the change diagram of the passband shape of the optical filter with the θ angle;

Fig. 7 is a top view of the overall structure of the optical filter.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.

According to the survey, currently the car light LED is generally made of blue LED chips plus yellow phosphorous phosphor; the white light LED in the present invention refers to the car light LED. As shown in Figure 2, the dotted line is the standard white LED spectrum, and the solid line is the standard sunlight spectrum. It can be found that the white LED spectrum overlaps with the solar spectrum in the 472nm to 775nm band, and the LED spectral region in the 412nm to 472nm band with a relative intensity greater than 34% and less than 94.67% does not overlap with the solar spectrum. Therefore, an optical thin-film filter is designed at the receiving end to keep the LED carrier light as much as possible, and at the same time filter out as much background sunlight as possible to ensure a high-quality signal-to-noise ratio.

As shown in Figure 1, it is a system block diagram of the visible light communication receiving end. The optical communication receiving system includes an optical filtering module, a photodetector, a digital signal amplification circuit, a digital signal/analog signal converter, an analog amplifier, and a signal receiving module; The filter module, photodetector, digital signal amplification circuit, digital signal/analog signal converter, analog amplifier and signal receiving module are connected in sequence; the optical filter module receives visible light in the atmosphere and filters out noise, and the photodetector receives the optical filter module The output visible light signal is converted into an electrical signal 1 and then output to a digital signal amplifier circuit. The digital signal amplifier circuit amplifies the electrical signal 1 and outputs it to a digital signal/analog signal converter for D/A conversion. The analog amplifier converts D/A The final analog electrical signal is amplified and then sent to the signal receiving module for processing. The present invention is mainly designed specifically for the optical filter module.

The specific design process is as follows:

(1) Analyze the spectral characteristics of white light LED and sunlight, and select the filter according to the comparison results of white light LED spectral characteristics and sunlight spectral characteristics.

As shown in Figure 2, the peak of the white LED spectrum is located at 459nm, and the relative intensity reaches 94.67%. The LED spectral region from 412nm to 472 band and the relative intensity is greater than 34.00% and less than 94.67% does not overlap with the solar spectrum, so this region The LED spectrum must be filtered out. At the same time, the overall shape of the LED spectrum is analyzed, which is a split double peak shape, the main peak is located at 459nm, the peak transmittance is 94.67%, and the bandwidth is 20nm, the secondary peak is located at 568nm, the peak transmittance is 66.40%, and the bandwidth is 83nm.

Combined with the above analysis, select the induced transmission filter, which has a high peak transmittance and a wide cut-off area, so it is not only suitable for a passband that does not require a very narrow pass band, but also suitable for a very high peak transmittance rate and wide cutoff for various applications. The absorption of the metal film is not only determined by the optical constants of the metal film itself (refractive index n, extinction coefficient k and thickness) but also closely related to the admittance of the adjacent medium. As long as the admittance of the matching film stack on the substrate side is correctly selected, the potential transmittance of the entire film system can be maximized. If an appropriate anti-reflection film stack is designed on the incident side at the same time, the reflection of the entire film system will be reduced to close to zero. At this point the maximum possible transmittance of the metal film can be exploited. In addition, when the thickness of the central metal layer increases, the transmittance curve will change significantly, and split double peaks will appear, which can better fit the LED spectral shape, which is also one of the reasons for choosing this filter.

(2) Design the structure and material selection of the induced transmission filter.

The induced transmission filter is composed of several layers of thin films, and the basic film structure is shown in Figure 3. Where G is a glass medium. H is a λ/4 film layer with a higher refractive index, and L is a λ/4 film layer with a lower refractive index. The two layers of H and L alternately form a 2P+1 anti-reflection film stack (P is an integer coefficient , λ is the central wavelength, and the thicknesses of H and L are both λ/4). mL is the matching film layer (m is the film thickness coefficient, that is, the thickness of the matching film layer is m times the thickness of the λ/4 film layer with a lower refractive index).

The specific design process is as follows: firstly, a layer of matching film layer with refractive index n _F and phase thickness δ is superimposed on the metal surface. Its function is to make the equivalent admittance Y of the metal film combined with it a real number. Let Y be equal to the real number μ, At the same time, the calculated phase thickness δ=arctan (-n _F /k) (wherein δ takes a value in the second quadrant, at this time, the thinnest matching film layer can be obtained, and its optical thickness is between λ/4 and λ/2 ). The value of the parameter m is calculated by the formula m=δ/(π/4).

Then choose to add a group of λ/4 film layers with alternating high and low refractive index to eliminate the reflection of this real number admittance. Assuming that the refractive index of the film layer adjacent to the incident medium is n _H , and the film layer alternating with it is n _L , if the multilayer film has an odd number of 2P+1 layers, then Where n ₀ is herein the refractive index of the glass substrate (n ₀ =1.52).

H is zinc sulfide (ZnS), n _ZnS = 2.47, L is cryolite, n _F = 1.3, the material of the metal film is silver (Ag), n = 0.139, k = 2.8, calculated m = 1.67, P = 2 . When designing the anti-reflection film system, the reflectivity value of the actually infinitely thick silver film is adopted. When the silver layer is thinner, the reflectivity decreases slightly, so the anti-reflection film can have fewer layers. The film structure of the filter is G|HLH1.67LAg1.67LHLH|G. Then determine the thickness of the silver (Ag) layer and the specific value of the central wavelength λ, because the passband shape is a split double peak, so its central wavelength is difficult to determine; first we constantly adjust the thickness of the silver (Ag) layer, As the thickness of the Ag layer decreases, the passband presents split double peaks and gradually reaches our expected peak transmittance, which is closer to the shape of the passband we require. After many experiments and comparisons, the silver layer is finally selected. The thickness is 26.80nm; the thickness of the incident layer glass and the base layer glass is specified according to the process level of the coating.

Then we take the value of the central wavelength λ, because the basic shape of the passband is a split doublet. The wavelength changes, so the central wavelength λ is difficult to determine. After continuous debugging and comparison with the LED spectrum, the central wavelength λ=491.9nm is finally determined.

As shown in Figure 4, it is the passband shape of the entire optical filter, the incident angle θ=0°, the abscissa is the wavelength (in nm), and the ordinate is the transmittance. The main peak is at 459nm, the half width is 13nm, and the peak transmittance is 94.67%. The secondary peak is at 518nm, the half width is 23nm, and the peak transmittance is 50.1%.

As shown in Figure 5, the solid line is the passband shape of the filter we designed (incident angle θ = 0°), and the other two are the sunlight spectrum and white LED spectrum.

(3) According to the relationship between the incident angle of light entering the filter and the passband of the filter, an incident light angle selector is designed.

As shown in Figure 6, assuming that the θ angle is the incident angle of light entering the filter, as the θ angle increases, the passband of the filter moves to a direction with a smaller wavelength and the transmittance decreases continuously, and the shape of the passband changes. It's getting worse and worse. The change of the shape of the passband will directly lead to the filtering effect of the filter, so the incident light angle must be controlled.

It can be seen that when the angle of incidence θ = 0°, the shape of the passband of the filter is the most ideal. Table 1 is the different eigenvalue parameters of the passband shape corresponding to different incident angles.

Table 1 Passband shape eigenvalue parameters corresponding to incident angle

It can be concluded from Table 1 that when the incident angle is 0°≤θ≤10°, the shape of the passband changes slightly with the angle, the overall passband shifts 7nm to the short wavelength direction, and the change degree of the main peak transmittance is less than 1.24%. , the corresponding bandwidth has almost no change, the change degree of the sub-peak transmittance is less than 1.86%, and the corresponding bandwidth change degree is less than 0.89%. When θ=15°, the degree of variation of the primary and secondary peaks is 5.39% and 4.97%, respectively, and the corresponding bandwidth changes of the primary and secondary peaks are 2.4% and 2.2%. When θ=20°, the variation degrees of the primary and secondary peaks are 15.78% and 10.91% respectively, and the corresponding bandwidth variation degrees of the primary and secondary peaks are 26.61% and 8.33%. When θ=25°, the variation degrees of the primary and secondary peaks are 39.35% and 19.52%, respectively, and the corresponding bandwidth variation degrees of the primary and secondary peaks are 76.61% and 30.26%.

To sum up, the optimum receiving angle of the incident light angle selector is 0°≤θ≤10°.

(4) Design an optical filter for noise filtering.

As shown in Figure 7, it is a top view of the overall structure of the optical filter, which is composed of an incident light angle selector, an optical filter, a focusing lens and a photodetector; the incident light angle selector is arranged at the incident light angle of the optical filter The focusing lens is arranged on the exit surface of the filter, and the photodetector is arranged at the rear focal point of the focusing lens. First, the light is directed to the light angle selector from different angles, and only the light within the range of the optimal incident angle (0°≤θ≤10°) can pass through, and then the incident light is incident on the surface of the filter at a specified angle , and then the visible light passing through the filter is converged and incident on the surface of the photodetector through the focusing lens. Through the optical filter, the deformation problem caused by the shape of the passband of the filter film due to the angle change is solved, and at the same time, a large amount of sunlight background noise is filtered, which further ensures the communication quality of the entire communication system.

The above descriptions are only used to describe the technical solutions and specific embodiments of the present invention, and are not intended to limit the protection scope of the present invention. It should be understood that any improvements or equivalent replacements made without departing from the essence of the present invention will be Fall into the protection scope of the present invention.

Claims (9)

1. A method for filtering natural background light at an outdoor visible light communication receiving end, comprising the following steps: Step 1, analyzing the spectral characteristics of the white light LED and the spectral characteristics of sunlight, and selecting a filter according to the comparison result of the spectral characteristics of the white light LED and the spectral characteristics of sunlight; Step 2, designing the structure of the optical filter described in step 1; the optical filter includes several layers of thin films; Step 3, according to the relationship between the incident angle of light entering the filter and the passband of the filter, an incident light angle selector is designed; Step 4, design an optical filter for noise filtering; the optical filter includes the incident light angle selector described in step 3, the optical filter, focusing lens and photodetector described in step 2; the incident The light angle selector is arranged on the incident surface of the optical filter, the focusing lens is arranged on the outgoing surface of the optical filter, and the photodetector is arranged at the rear focal point of the focusing lens. 2 . The method for filtering natural background light at the receiving end of outdoor visible light communication according to claim 1 , wherein the filter described in step 1 is an induced transmission filter. 3 . 3. A method for filtering natural background light at the receiving end of outdoor visible light communication according to claim 1 or 2, characterized in that the several layers of thin films described in step 2 include: an incident layer, a base layer, and a metal layer , anti-reflection film stack, matching film layer; the metal layer, the anti-reflection film stack, and the matching film layer are all located between the incident layer and the base layer, and the metal layer is located between the incident layer layer and the base layer, both sides of the metal layer are sequentially provided with the matching film layer and the anti-reflection film stack, and the matching film layers on both sides of the metal layer The distribution is symmetrical with the metal layer as the center of symmetry, and the anti-reflection film stacks on both sides of the metal layer are symmetrically distributed with the metal layer as the center of symmetry. 4. A method for filtering natural background light at the receiving end of outdoor visible light communication according to claim 3, wherein the anti-reflection film stack comprises: a film layer H with a high refractive index λ/4 and a film layer H with a low refractive index λ/ 4 film layers L; the high refractive index λ/4 film layer H and the low refractive index λ/4 film layer L are alternately arranged to form a 2P+1 film, and both ends of the 2P+1 film are high refractive Ratio λ/4 layer H; Among them, P is an integer coefficient, and λ is the center wavelength. 5. A method for filtering natural background light at an outdoor visible light communication receiving end according to claim 3 or 4, wherein the design method of the optical filter is: Step 2.1, superimposing a layer of matching film layer with refractive index n _F and phase thickness δ on the surface of the metal layer, the optical thickness of the matching film layer is between λ/4 and λ/2; where λ is central wavelength; Step 2.2, stacking an anti-reflection film stack on the matching film layer; Step 2.3, setting an incident layer and a base layer on the antireflection film stack. 6. A method for filtering natural background light at an outdoor visible light communication receiving end according to claim 3 or 4, wherein the incident layer and the base layer are both glass, the metal layer is silver, and the The high refractive index λ/4 film layer H is zinc sulfide, and the matching film layer and the low refractive index λ/4 film layer L are both cryolite. 7. A method for filtering natural background light at an outdoor visible light communication receiving end according to claim 3 or 4, wherein the thickness of the metal layer is 26.80 nm, and the thickness of the matching film layer is low refractive index m times the thickness of the λ/4 film. 8. A method for filtering natural background light at the receiving end of outdoor visible light communication according to any one of claims 4-7, wherein the λ=491.9nm, the P=2, and the m=1.67 . 9. The method for filtering natural background light at the receiving end of outdoor visible light communication according to claim 1, wherein the light receiving angle of the incident light angle selector described in step 3 is 0°≤θ≤ 10°.