CN114157370A

CN114157370A - Collimating light path design method for increasing working distance of underwater wireless optical communication system

Info

Publication number: CN114157370A
Application number: CN202111462893.8A
Authority: CN
Inventors: 徐敬; 张皓; 杨兴启; 张雨凡
Original assignee: Zhejiang University ZJU
Current assignee: Zhejiang University ZJU
Priority date: 2021-12-02
Filing date: 2021-12-02
Publication date: 2022-03-08

Abstract

The invention belongs to the technical field of underwater wireless optical communication, and discloses a method for designing a collimation light path for increasing the working distance of an underwater wireless optical communication system, which is suitable for a system with an array form adopted by a light emitting end and a light detecting end. The invention firstly carries out theoretical analysis on a straight optical path, then carries out optical path simulation in software, and finally carries out verification in an optical communication system. The result shows that the designed optical lens group has better collimation and collection effects on optical signals transmitted in the system, the problem of uneven light spot illumination on a detection surface caused by mutual conjugation of the two ends of light emission and light detection in the traditional light path collimation method is solved, the utilization rate of optical power is effectively improved, and the working distance of the system is further prolonged.

Description

Collimating light path design method for increasing working distance of underwater wireless optical communication system

Technical Field

The invention relates to the technical field of underwater wireless optical communication, and designs a coaxial lens group for light path collimation so as to prolong the working distance of an array-form light emitting and light receiving underwater wireless optical communication system.

Background

The exploration and development of marine resources have profound influence on the future development of human beings. In recent years, the development of marine resources is increasing in all countries, and various marine observation devices are developed and put into use in the ocean. The demand for underwater wireless communication is also greatly increased at present because communication links need to be established between the equipment and the base station and between the equipment and the equipment for transmitting instructions and data, and the traditional cable communication mode brings many limitations to the use of underwater equipment. Nowadays, underwater communication modes mainly include radio frequency communication, underwater acoustic communication and optical communication. Because the attenuation of high-frequency electromagnetic waves in water is large, the transmission distance of a high-speed radio frequency communication technology which is widely applied in air in water is extremely short; the underwater acoustic communication is the most widely applied underwater communication mode at present, and although the underwater acoustic communication has the advantage of long transmission distance, the application range of the underwater acoustic communication is limited by the defects of low speed, narrow bandwidth, time prolongation, susceptibility to water environment and the like. In comparison, optical communication has the advantages of good security, abundant bandwidth resources, low time delay and the like, and is very suitable for high-speed communication at a working distance of several tens of meters to one hundred meters, so that the optical communication has attracted extensive attention and research in recent years.

In current underwater wireless optical communication systems, common light source choices include Laser Diodes (LDs) and Light Emitting Diodes (LEDs). The former has the advantages of high bandwidth, high power density and good directivity, so that the device is suitable for underwater long-distance communication, but has strict requirements on the alignment of light beams; the light emitted by the LED can provide illumination, and underwater image acquisition is facilitated. Due to the rapid improvement of the semiconductor processing technology, the smaller the volume of the LED is, the higher the optical power density is, so that more research works select the LED as the light source. In order to prolong the transmission distance of an optical communication system, many researches use an array formed by a plurality of LEDs to replace a single LED as a light source, so that the optical power reaching a detection surface after passing through an underwater channel is effectively improved, and the transmission distance of the system is further prolonged. The LED is used as a semiconductor device, is similar to a photodiode in structure, and can also be used as a detector in a photovoltaic mode. Similar to the LED array as a light source, the array formed by a plurality of LEDs is selected as a detector, so that the detection area of light can be increased, and the transmission distance of the system can be prolonged. Because the LED has a large divergence angle, it is often necessary to add appropriate optical elements at both ends of light emission and light detection in an optical communication system to improve the directivity of an optical signal, i.e., collimate an optical path, and further improve the utilization rate of optical power. In the traditional light path collimation method, a lens is added at the light emitting end to collimate, and a lens is added at the light detecting end to converge, so that the two ends of light emission and light detection are in a conjugate relation. Under the form of array at the transceiving end, the optical power at the detection end is not uniformly distributed any more, but carries the information of the light source image, so that the signal light cannot be concentratedly covered on the effective detection surface, and the optical power is wasted.

Disclosure of Invention

The invention designs a novel collimation light path, which is mainly applied to an underwater wireless optical communication system adopting array light emission and optical detection, solves the problem of mutual conjugation of a light emission array and an optical detection array caused by the traditional collimation light path, and effectively prolongs the transmission distance of the system.

In order to achieve the purpose, the invention adopts the following specific scheme:

a method for designing a collimation light path for increasing the working distance of an underwater wireless optical communication system comprises a green light LED array and an emitting end condenser lens L₁An objective lens L₂Receiving end condenser lens L₃And a red LED detection array;

the green LED array converts the modulated electric signal into an optical signal; the transmitting end condenser lens collects green light emitted by the LED array into a light path as much as possible and images a light source; the objective lens is used for collimating and homogenizing the light path and receiving the light source image; the receiving end condenser converges the optical signal on an effective detection surface of the detector array; the red LED detection array converts the optical signal into an electric signal;

further, the LED array is composed of 9 0402 packaged green LEDs which are connected in series, and the light-emitting wavelength is 530 nm; the LED detection array consists of 4 0402 packaged red LEDs, and the light-emitting wavelength is 630 nm.

Furthermore, the invention designs a collimation light path consisting of a transmitting end condenser lens, an objective lens and a receiving end condenser lens, collects light emitted by the green light LED array into the system as much as possible to reduce the waste of light power, and collimates and homogenizes the light simultaneously, so that a light signal forms a uniform light spot with a certain size after passing through a water channel, and the uniform light spot is converged again near the receiving array and then irradiates an effective detection surface of the detector array. Compare traditional collimation light path, in light emission end collimation promptly, receiving end convergence, this design mainly has two big improvements: the working distance of the system is increased, the light path focuses on collection and utilization of optical signals, and waste of optical power is reduced, so that the optical signals still have enough optical power density when reaching a receiving end through a water channel, and the requirement of a detector array on light intensity is met; the conjugate relation is changed, the conjugate relation between a light emitting end and a light receiving end in the traditional collimation mode is broken through, so that the light power reaching the detection array is approximately uniformly distributed, the light source image information is not carried, and the effective area of the detection array can be effectively covered.

Furthermore, the invention uses the green light LED array as the light source of the system, and a condenser lens is arranged at the position of a distance l behind the light source, so that the light source is amplified to form a real image on the image surface while optical signals are collected as much as possible. The objective lens is placed on the real image surface of the light source, at a distance l' from the emission end condenser. Then, a condenser lens at the receiving end is placed at the rear distance d of the objective lens₁And thereafter by a distance d₂Where the detector is placed. In the collimation light path designed in this way, the light source array, the transmitting end condenser lens and the objective lens form an illumination system, and the objective lens and the receiving end condenser lens form a projection system. Because the light source image (the exit window of the illumination system) is superposed with the objective lens (the aperture diaphragm of the projection system), the transmitting end condenser is the aperture diaphragm of the illumination system and the incident window of the projection system, so that each point of the transmitting end condenser is imaged by the light beams with the same aperture angle, and the illumination on the image surface can be uniform.

The illumination distribution on the collecting lens at the receiving end is

Wherein k is₁Is the condenser transmittance, k₂Is the transmittance of the objective lens, L is the brightness of the light source, and D is that of the objective lensExit pupil diameter, W 'is the image viewing angle, f'₂Is the objective focal length. l' ≈ beta₂f'₂，l'₂Is the projection distance. Beta is a₂Is the magnification of the image. The illumination on the receiving end condenser is proportional to the square of the relative aperture of the objective lens and inversely proportional to the square of the image magnification.

In order to enhance the illumination on the image plane, the objective lens needs to have as large a relative aperture as possible, and the spot of the light transmitted through the objective lens after passing through the underwater channel is as small as possible. In order to collect as much light as possible at the receiving end, the aperture of the receiving end condenser lens placed in front of the detection plane needs to be as large as possible, and finally the LED array for detection is placed behind the receiving end condenser lens. The lens group homogenizes a light source consisting of 9 LED lamp bead arrays into approximately uniform light spots, the light spots penetrate through an underwater channel after being collimated, and finally, light is converged into a high-illumination light spot matched with an effective detection surface on a receiving end LED detector plane, so that the problem that an emitting end plane and a receiving end plane form a conjugate relation is effectively solved, and the working distance of the system is prolonged.

Drawings

Fig. 1 is a schematic diagram of an underwater wireless optical communication system according to the present invention;

FIG. 2 is a simulation diagram of the optical path of the collimating lens set

FIG. 3 is a printed circuit board model of the transmitting and receiving terminals of the present invention;

Detailed Description

In order to make the technical problems, technical solutions and advantages to be solved by the present invention clearer, the following detailed description is made with reference to specific embodiments and accompanying drawings;

firstly, various electronic components and lenses used in the invention are all available products; the invention can be completely reproduced by the technical knowledge of the electronic circuit and the geometric optical knowledge mastered by the technical knowledge of the electronic circuit on the basis of reading and understanding the application documents by the technicians in the field;

the invention designs a method for designing a collimation light path for improving the working distance of an underwater wireless optical communication system, and solves the problem of light power waste caused by mutual conjugation of a light emitting end and a light detecting end in the traditional collimation light path. The flow of the underwater wireless optical communication system applied by the method is shown in figure 1, and comprises a green light LED array, an emitting end condenser lens, an objective lens, a receiving end condenser lens and a red light LED detection array;

fig. 2 is a simulation diagram of a collimated light path design, wherein a condenser lens at the transmitting end is a convex lens with the diameter of 25mm and the focal length of 45mm, and is placed at a distance l behind a light emitting array. A large-caliber square Fresnel lens with the side length of 250mm is selected as an objective lens and is placed on a real image surface of a light source, wherein the image distance is determined by a Newton formula:

where f 'is the focal length of the condenser, l is the object distance, and l' is the image distance. The collecting lens at the receiving end selects a large-diameter Fresnel lens with the side length of 250mm and the focal length of 200mm, and is away from the objective lens by a distance d₁Finally, the LED detection array is arranged behind the condenser lens d₂To (3). In this design l is 55mm, l' is 247.5mm, d₁Is 7m, d₂Is 200 mm. Taking point (i) on the LED array in FIG. 2 as an example, light emitted from (i) is incident on L from different angles₁The three beams of light are indicated by the numbers 1, 2, 3, respectively. Point (i) is at L₂A real image (i ') is formed, and three rays from the point (i') are L₂Uniformly distributed in L after deflection₃The above. With the above relationship, each point on the LED array is uniformly projected to L₃The above. Thus, at L₃There is a large spot of light with nearly uniform illumination. Finally pass through₃Focusing to obtain a high-power-density light spot with uniform illumination on the plane of the detector.

FIG. 3 is a printed circuit board rendering of green and red LED arrays as proposed in the present invention; the transmitting end adopts a single-layer aluminum substrate processing technology, so that the overheating problem of the high-power LED is avoided;

finally, it should be noted that the above-mentioned list is only a specific embodiment of the present invention. It is obvious that the present invention is not limited to the above embodiments, but many variations are possible. All modifications which can be derived or suggested by a person skilled in the art from the disclosure of the present invention are to be considered within the scope of the invention.

Claims

1. A design method of a collimation light path for prolonging the transmission distance of an underwater wireless optical communication system is characterized in that the lens group solves the problem of light power waste caused by mutual conjugation of two ends of light emission and light detection in the traditional light path collimation method; the system comprises a green light LED array, an emission end condenser lens L₁An objective lens L₂Receiving end condenser lens L₃A red LED detection array;

the green LED array converts the modulated electric signal into an optical signal; the transmitting end condenser lens collects green light emitted by the LED array into a light path as much as possible and images a light source; the objective lens is used for collimating the light path and receiving the light source image; the receiving end condenser converges the optical signal on an effective detection surface of the detector array; the red LED detection array converts the optical signal into an electrical signal.

2. The method for designing the collimation optical path for increasing the working distance of the underwater wireless optical communication system according to claim 1, wherein the collimation optical path consisting of the transmitting end condenser lens, the objective lens and the receiving end condenser lens collects light emitted by the green light LED array into the system as much as possible to reduce the waste of optical power, and collimates the light simultaneously, so that a diffuse spot with a certain size is formed after an optical signal passes through a water channel, and the light is converged again near the receiving array and then irradiates an effective detection surface of the detector array.

3. The method according to claim 1, wherein the light source array is placed between a focal length of one time and a focal length of two times of the transmitting end condenser lens, so that the condenser lens collects as many optical signals as possible, and the light source is amplified to form a real image. And placing an objective lens on the real image surface of the light source, placing a receiving end condenser lens behind the objective lens, separating the actual working distance of the system, and placing an optical detection array behind the objective lens. The collimation light path designed in the way can achieve uniform illumination on the image surface.

4. The method as claimed in claim 1, wherein the relative aperture between the objective lens and the receiver condenser lens is as large as possible to improve the illumination on the detection surface. The lens group homogenizes the array light source into approximately uniform light spots, passes through an underwater channel after being collimated, and finally converges light into a high-illumination light spot matched with an effective detection surface on a detector plane.