Dense wavelength division multiplexing underwater wireless optical communication system

Abstract

The invention discloses a dense wavelength division multiplexing underwater wireless optical communication system, and the system comprises a transmitter which comprises a plurality of direct modulation semiconductor lasers with the wavelengths of lambda1, lambda2, lambda3,... lambdan, and a beam combining element, and is used for achieving dense wavelength division multiplexing in blue and green light wave bands; the receiver comprises a collimator and a diffraction grating, light is scattered into an irregular light spot after being propagated underwater for 20-200 meters, the light spot is collected by a receiving lens and compressed in the collimator, and the diffraction grating is used for demultiplexing and separating light with different wavelengths; and the digital signal processing module is used for capturing the optical signal by utilizing photomultiplication and converting the optical signal into an electric signal, the electric signal is captured by an oscilloscope, and signal synchronization, decoding and channel equalization are executed on a computer. By adopting the technical scheme provided by the invention, the signal transmission stability of communication under water with different turbidity is improved, and the transmission distance and the data rate are remarkably improved.

Description

Technical Field

[0001] The invention belongs to the technical field of underwater wireless optical communication, and in particular relates to a dense wavelength division multiplexing underwater wireless optical communication system. Background Art

[0002] Because the blue-green wavelengths from 450nm to 550nm have a relatively low attenuation window, underwater wireless optical communication (UWOC) has shown high speed and low latency in clean water and ocean water. However, light of different wavelengths has different attenuation coefficients in the same water quality, that is, the communication effect of light of different wavelengths for different water quality is different. Therefore, traditional UWOC using a single wavelength or sparse blue-green wavelength division multiplexing technology is difficult to tolerate changes in water turbidity. Summary of the invention

[0003] The technical problem to be solved by the present invention is to provide a dense wavelength division multiplexing underwater wireless optical communication system to improve the signal transmission stability in communication under water qualities of different turbidity and significantly increase the transmission distance and data rate.

[0004] To achieve the above object, the present invention adopts the following technical solution:

[0005] A dense wavelength division multiplexing underwater wireless optical communication system, comprising: a transmitter, a receiver and a digital signal processing module; wherein:

[0006] The transmitter includes wavelengths λ1, λ2, λ3, ... λ n Multiple directly modulated semiconductor lasers and beam combining elements are used to realize dense wavelength division multiplexing in the blue-green light band;

[0007] The receiver includes: a collimator and a diffraction grating; wherein, after the light propagates 20 to 200 meters underwater, it is scattered into irregular light spots, the light spots are collected by the receiving lens and compressed in the collimator, and the diffraction grating is used to demultiplex and separate light of different wavelengths;

[0008] A digital signal processing module is used to capture the optical signal using photomultiplier and convert it into an electrical signal, which is captured by an oscilloscope and performs signal synchronization, decoding and channel equalization on a computer.

[0009] Preferably, the transmitter integrates a plurality of blue-green laser diodes LD of different wavelengths for WDM; each LD is loaded with a series of pseudo-random binary sequences generated by an arbitrary signal generator.

[0010] Preferably, the wavelength division multiplexing of the transmitter is implemented by an N×1 combiner, which is a fiber combiner or a free space optical combiner.

[0011] Preferably, when N=7, 7 different wavelengths of pigtailed laser diodes are multiplexed into a multimode optical fiber based on SBC; the multimode optical fiber is connected to a spatial light collimator to output a combined laser beam; the diameter of the combined beam is 5 mm, which is expanded to 25 mm by the collimator; the collimated beam enters the underwater channel, and after transmission in the 20-meter underwater channel, a receiving lens, a collimator and a reflective diffraction grating are used for demultiplexing; the received beam has a diameter of 30 mm, which is collected and collimated into a parallel beam of 6 mm in diameter by a collimator; a diffraction grating is used to separate all 7 wavelengths.

[0012] Preferably, the underwater channel uses a 2-meter-long and 1-meter-wide water tank to construct an optical communication transmission channel, and two reflectors placed on the short sides enable light to be transmitted underwater over a distance of 20 meters.

[0013] Preferably, the transmitter implements dense wavelength division multiplexing in the wavelength range of 490nm to 520nm within a low attenuation window in the wavelength range of 450nm to 550nm, with a minimum channel spacing of 2nm.

[0014] Preferably, when N=14, 14 end-emitting laser diodes (EEL) of different wavelengths are multiplexed based on SBC in a free-space coupled multimode optical fiber; the multimode optical fiber is connected to a spatial light collimator to output a combined laser beam; the diameter of the combined beam is 25 mm; the collimated beam enters the underwater channel, and after transmission in the underwater channel of 200 meters, a receiving lens, a collimator and a transmission diffraction grating are used for demultiplexing; the received beam has a diameter of 300 mm, which is collected and collimated into a parallel beam of 6 mm in diameter by a collimator; a transmission diffraction grating is used to separate all 14 wavelengths.

[0015] Preferably, the underwater channel uses a 50-meter-long and 5-meter-wide pool to construct an optical communication transmission channel, and two reflectors placed on the short sides enable light to be transmitted underwater for a distance of 200 meters.

[0016] Preferably, the transmitter implements dense wavelength division multiplexing in the wavelength range of 450nm to 520nm within a low attenuation window in the wavelength range of 450nm to 550nm, with a minimum channel spacing of 3nm.

[0017] The present invention has the following technical effects:

[0018] 1. Dense wavelength division multiplexing technology with good turbidity resistance in underwater optical communication. Using seven different wavelength lasers in the wavelength range of 490nm to 520nm, WDM technology with a minimum spacing of 2nm between high-density wavelength channels is realized, which improves the data transmission rate and the turbidity tolerance of the system; using 14 different wavelength lasers in the wavelength range of 450nm to 520nm, WDM technology with a minimum spacing of 3nm between high-density wavelength channels is realized, which improves the underwater transmission distance and the turbidity tolerance of the system. It can transmit stably under different water quality conditions, with a maximum data transmission rate of more than 10Gbit / s, which effectively solves the problem of decreased communication quality in traditional underwater communications when turbidity changes greatly.

[0019] 2. The spatial optical lens combination can effectively control the divergence of the combined light beam when propagating underwater, and completely collect all the light beam energy transmitted to the receiving end. By designing the collimating beam expansion device and the beam shrinking collection device, the high-quality transmission of light in the water channel is guaranteed, the additional loss caused by the loss of the water channel itself is reduced, and the overall performance of the system is enhanced.

[0020] 3. Use diffraction gratings to achieve 2-3nm channel spacing splitting in dense wavelength division multiplexing. By accurately calculating the geometry and diffraction characteristics of the grating, accurate wavelength separation and control are achieved, so that optical signals of different wavelengths can be effectively separated within a very small interval. This grating design not only greatly reduces the complexity and cost of the equipment, but also improves the accuracy of splitting and the overall efficiency of the system. In practical applications, this technology can effectively reduce crosstalk between wavelengths under high turbidity and different water quality conditions, maintain high data transmission rates, and greatly enhance the adaptability and stability of underwater communication systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0022] Figure 1 Schematic diagram of blue-green light dense wavelength division multiplexing (WDM) underwater optical communication (UWOC) system; where λ1, λ2, λ3, ... λ n After multiple wavelength channels are combined, they enter the underwater communication channel for transmission, and at the receiving end, the communication channel is demultiplexed after grating splitting;

[0023] Figure 2The schematic diagram of the 7-channel blue-green light dense WDM-UWOC system structure is shown in Figure 1. Among them, a is the optical transmission module, b is the underwater channel, c is the optical receiving module, and d is the digital signal processing module.

[0024] Figure 3 Schematic diagram of the principle of combining N×1 free-space beams in wavelength division multiplexing of beams of different wavelengths;

[0025] Figure 4 Schematic diagram of wavelength division multiplexing of light beams of different wavelengths, where a represents the beam combining principle of 7×1 optical fiber beams, b and c represent the cross-sectional light field distribution before and after beam combining, respectively;

[0026] Figure 5 Schematic diagram of the principle of grating demultiplexing and example results; a is a schematic diagram of the light splitting principle of transmission and reflection gratings, b is the shape of the seven-channel demultiplexed light on the receiving screen in the case of grating light splitting, and c is the seven-channel light on the receiving screen when there is a cylindrical lens;

[0027] Figure 6 Schematic diagram of the specific implementation principle of different parts; a is the beam expansion and collimation device at the transmitting end; b is the beam reduction device at the collecting end; c is the schematic diagram of the grating splitting principle; d is the signal receiving optical path after grating splitting;

[0028] Figure 7 The figure is the parameter measurement of the transmitting device; a is the measured spectrum of 7 different wavelength LDs; b is the measured frequency response of 494nm, 508nm and 520nm wavelength division multiplexing channels; c is the UI and PI curves of 494nm, 506nm and 517nm lasers; d is the optimized DC and RF voltages for the lowest bit error rate of 494nm laser at a transmission rate of 1Gbit / s; e is the relationship between the injection current and wavelength offset of 494nm, 508nm and 517nm LDs;

[0029] Figure 8 The changes in attenuation coefficients for different water qualities and the corresponding communication test results;

[0030] Fig. 9 It is a communication crosstalk experiment of adjacent WDM channels; among them, a and b are the communication test results when 7 channels are turned on at the same time and one channel is turned on alone under the HD-FEC threshold condition. DETAILED DESCRIPTION

[0031] 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.

[0032] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0033] Embodiment 1:

[0034] like Figure 1 As shown, an embodiment of the present invention provides a dense wavelength division multiplexing underwater wireless optical communication system including: a transmitter, an underwater channel, a receiver and a digital signal processing (DSP) module; wherein the transmitter is used to achieve dense wavelength division multiplexing performance in the wavelength range of 450nm to 520nm within a low attenuation "window" in the wavelength range of 450nm to 550nm; the transmitter integrates 14 blue-green laser diodes (Laser diodes, LD) of different wavelengths for WDM; each LD is loaded with a series of pseudo-random binary sequences (Pseudo-random binary sequence, PRBS) generated by an arbitrary signal generator (Arbitrary Waveform Generator, AWG); the underwater channel uses a 50-meter-long and 5-meter-wide water tank to construct an optical communication transmission channel. Two reflectors placed on the short sides enable light to be transmitted underwater for a distance of 200 meters. The receiver contains a collimator and a diffraction grating. After propagating 200 meters underwater, the light is scattered into an irregular light spot, which is collected by the receiving lens and compressed in the collimator. The diffraction grating is used to demultiplex and separate light of different wavelengths. The DSP module uses a photomultiplier tube (PMT, Hamamatsu-H14447, 1Gbps bandwidth) to capture the optical signal and convert it into an electrical signal. Then, the electrical signal is captured by an oscilloscope (Tektronix, MSO58B), and signal synchronization, decoding, and channel equalization are performed on the computer.

[0035] Furthermore, the wavelength division multiplexing of the transmitter in the example is realized by a 14×1 combiner, such as Figure 3 As shown. The 14×1 beam combiner is manufactured based on free-space optical beam combining technology. 14 end-emitting semiconductor laser diodes form a beam after spatial light shaping and beam combining. The output beam is coupled into the core of a 300μm output fiber, which can ensure the propagation of the combined beam.

[0036] Furthermore, the spectra of 14 different wavelength laser diodes (LDs) have peak wavelengths of 451.7nm, 464nm, 467.2nm, 470.1nm, 478.2nm, 483.1nm, 486.5nm, 491.9nm, 494nm, 496.2nm, 506.1nm, 508.2nm, 517.1nm and 520.5nm, corresponding to 452nm, 464nm, 467nm, 570nm, 478nm, 483nm, 486nm, 492nm, 494nm, 496nm, 506nm, 508nm, 517nm and 520nm of 14 WDM channels respectively. The minimum WDM channel spacing is 2nm. In the wavelength range of 450nm to 520nm, when the water quality changes from clear water to near-ocean water, these 14 dense WDMs representing wavelengths of 450nm to 520nm provide sufficient channels.

[0037] Embodiment 2:

[0038] like Figure 2 As shown, an embodiment of the present invention provides a dense wavelength division multiplexing underwater wireless optical communication system including: a transmitter, an underwater channel, a receiver and a digital signal processing (DSP) module; wherein the transmitter is used to achieve dense wavelength division multiplexing performance in the wavelength range of 490nm to 520nm within a low attenuation "window" in the wavelength range of 450nm to 550nm; the transmitter integrates 7 blue-green laser diodes (Laser diodes, LD) of different wavelengths for WDM; each LD is loaded with a series of pseudo-random binary sequences (Pseudo-random binary sequence, PRBS) generated by an arbitrary signal generator (Arbitrary Waveform Generator, AWG); the underwater channel uses a 2-meter-long and 1-meter-wide water tank to construct an optical communication transmission channel. Two reflectors placed on the short sides enable light to be transmitted underwater for a distance of 20 meters. The receiver contains a collimator and a diffraction grating. After propagating 20 meters underwater, the light is scattered into an irregular light spot, which is collected by the receiving lens and compressed in the collimator. The diffraction grating is used to demultiplex and separate light of different wavelengths. The DSP module uses a photomultiplier tube (PMT, Hamamatsu-H14447, 1Gbps bandwidth) to capture the optical signal and convert it into an electrical signal. Then, the electrical signal is captured by an oscilloscope (Tektronix, MSO58B), and signal synchronization, decoding, and channel equalization are performed on the computer.

[0039] Furthermore, the wavelength division multiplexing of the transmitter in the example is realized by a 7×1 fiber combiner, such as Figure 4As shown in a. The 7×1 fiber combiner is manufactured based on the Tapered-Fiberbundle (TFB) technology. Seven single-mode optical fibers (core: 3μm, coating: 125μm) are inserted into the capillary, then tapered and fused with the output fiber. The gap between the input fiber and the capillary is usually closed under the tapered treatment and the coverage of the output multimode fiber core. The output fiber core is 200μm, which can ensure the propagation of the combined light beam. The beam combining process can be explained by the superposition principle of light intensity, which is described as follows: Assuming that the coherence length of each beam of light is shorter than the transmission distance, the intensity I of the output light field can be considered as the superposition of the intensity of each input light field, which can be expressed as:

[0040]

[0041] Among them, I m and E m are the intensity and light field of the mth input beam respectively. Then, the mth Gaussian beam is m = 0 can be described as:

[0042]

[0043] Among them, A m is the amplitude of the m-th order Gaussian beam, w m is the beam waist radius, x m and m is the center coordinate of the mth order Gaussian beam, and w is the angular frequency of the light. If the beams are input together and have the same light field, then the amplitude A m and angular frequency w m can be considered the same. The transmission simulation before and after the light field combination is as follows Figure 4 As shown in b and c.

[0044] Figure 5The demultiplexing (De-Mutiplexing, DeMUX) process of 7 communication channels with different wavelengths is demonstrated. In the actual case, the combined laser beam after multiplexing is output through a spatial optical collimator. The diameter of the combined beam is 5 mm, which is expanded to 25 mm by the collimator. Then, the collimated beam enters the underwater channel. The purpose of beam expansion is to reduce the divergence angle of light when it propagates in water. After transmission in the 20-meter underwater channel, the receiving lens, collimator and diffraction grating are used for demultiplexing. No obvious wavelength separation is observed in the 20-meter underwater channel, proving that the multiplexed beam remains in a combined state after propagating in 20 meters of water. The received beam has a diameter of 30 mm, which is collected and collimated into a parallel beam with a diameter of 6 mm by a collimator. Then, a diffraction grating (12.7 mm × 12.7 mm × 6 mm, 1200 lines / mm) is used to separate all 7 wavelengths. It is necessary to set a reasonable angle β′ between the two channel beams and the diffraction screen receiving distance L to achieve the optical spatial separation of 2 nm channel spacing of blue-green light, such as Figure 3 The simulation software Zemax is used in this example to simulate the light separation process. The simulation results show that the seven wavelengths are clearly separated in theory. According to the simulation results, in order to create a 2nm light interval at a distance of 1 meter from the diffraction screen, theoretical calculations show that the light should be irradiated on the grating at an angle of -51.2°. The simulation results are shown in Figure 3 The actual spectroscopic results obtained using this calculation parameter are shown in Figure 3 The combined light can be clearly divided into 7 beams. It is worth noting that because the light is irradiated on the grating aperture at an inclined angle of -51.2°, the light on the diffraction screen has a long strip shape, as shown in c and d. Figure 3 A cylindrical lens is used after the grating to further compress the beam to concentrate the energy density. The seven beams on the diffraction screen are clearly separated, and the spot of each wavelength is approximately a rectangular spot of 35 mm × 55 mm, as shown in Figure 3 This beam size is sufficient for PMT (PhotomultiplierTube) to receive optical signals for communication.

[0045] Figure 6 yes Figure 5 Schematic diagram of the implementation of optical paths in different areas of the communication system. Figure 6Figure a is the principle diagram of beam expansion. First, a collimator (Thorlabs-F220FC-532, f=10.9mm, NA=0.25) is used to spatially collimate the seven combined beams to form a collimated beam with an output spot diameter of 5mm and a divergence angle of 0.24rad. This beam cannot meet far-field propagation requirements, so a 5x magnification collimator is designed to further expand the spot size. It is a Galilean telescope system consisting of a positive lens with a focal length of 125mm and a negative lens with a focal length of -25mm. The diameters of the two lenses are 25.4mm and 12.5mm, respectively. The distance between the two lenses is 100mm. The magnification is M=-f p / f n =5, the diameter of the output light spot is about 25 mm. Figure 6 Figure b shows the beam reduction device at the collection end of the system. The device also uses a Galilean collimation structure to collect the divergent light spot after propagation at 20 meters underwater. It includes a convex lens and a concave lens with focal lengths of 100mm and –12.5mm respectively. The diameter of the convex lens is 50mm to ensure that all light is collected at the receiver, while the diameter of the concave lens is 25mm. Figure 6 The c is the grating spectroscopic principle behind the collecting device. The grating spectroscopic method in the present invention utilizes the high-order diffraction equation and geometric relationship of the grating. The formula of the grating equation is:

[0046] d(sinα±sinβ)=mλ

[0047] Among them, d is the grating constant, α is the angle between the incident light and the normal, β is the angle between the diffracted light and the normal, the positive or negative value of β depends on the positional relationship between the diffraction angle and the normal, m is the diffraction order of the grating, and λ is the wavelength of the incident light. Secondly, after the synthetic light is diffracted by the grating, light of different wavelengths is separated at different angles and projected onto the receiving screen. Use the following geometric equation to describe this phenomenon:

[0048] Δ=2Ltan[(β2-β1) / 2]

[0049] Among them, Δ is the distance between the center points of two adjacent light spots, β1 and β2 are the diffraction angles of adjacent wavelength channels, and L is the distance from the grating to the receiving screen. It can be deduced that sinβ2–sinβ1 is equal to (mΔλ) / d, where Δλ is the wavelength difference between adjacent channels, m is the diffraction order, and d is the grating constant. To completely separate the light spots of adjacent channels, the center spacing of adjacent channels or the distance between the grating and the receiving screen can be increased, and the latter will increase the spot size of each channel accordingly. Therefore, the best way to separate adjacent channels is to select an appropriate Δ. According to the above equation, when the grating constant d cannot be further reduced, a higher grating diffraction order m can effectively increase the center distance Δ. For this purpose, the present invention uses the higher-order diffraction of the grating equation to achieve the effect of separating adjacent channels. This is an ingenious and simple design of the present invention to solve the demultiplexing process of the underwater partial multiplexing system. Figure 6 d shows the verification process of the crosstalk between adjacent channels in an embodiment of the present invention, by measuring the ratio of the received optical power (ROP) during normal wavelength division multiplexing (WDM) operation to the noise level when the channel signal is turned off and the other six channels are operating normally (expressed as 10log(ROP / NOISE)) to measure the crosstalk (ICI) between adjacent channels.

[0050] Figure 7 The a shows the spectrum of seven laser diodes (LD) with different wavelengths, with peak wavelengths of 491.9nm, 494nm, 496.2nm, 506.1nm, 508.2nm, 517.1nm and 520.5nm, corresponding to 492nm, 494nm, 496nm, 506nm, 508nm, 517nm and 520nm of seven WDM channels respectively. The minimum channel spacing of WDM is 2nm. Within the wavelength range of 490nm to 520nm, when the water quality changes from clear water to near ocean water, the attenuation changes of these wavelengths are relatively small. These seven dense WDMs representing wavelengths of 490nm to 520nm provide sufficient channels. Figure 7 Figure b shows three typical responses of 494nm, 508nm and 520nm channels. For the -20dB response, the cutoff frequency of all LDs is up to about 1GHz. Figure 7Figure c shows the voltage-current (UI) and optical power-current (PI) measurement curves of three typical lasers (494nm, 508nm and 517nm). An OOK (On-OffKeying) modulated pseudo-random binary sequence (PRBS) signal is loaded into an arbitrary waveform generator (AWG, RIGOL-DG70000) to generate a radio frequency (RF) signal. The RF signal is superimposed on the direct current (DC) through a bias circuit (Mini-Circuits, ZFBT-4R2GW+), and the generated RF+DC signal is directly injected into the LD. The RF and DC values ​​are optimized to obtain the lowest bit error rate (BER), so that each LD works in the best working state. Figure 7 As shown in Figure d, the 494nm LD has the lowest BER at 1Gbit / s data rate when operating in the 6-6.5V DC range and RF is 3Vpp. As the injection current of the blue-green LD is modulated, the wavelength will drift. Figure 7 Figure 4 illustrates the wavelength shift of the 494nm, 508nm, and 517nm LDs. When the injection current increases from 60mA to 200mA, a wavelength shift of 1-1.5nm occurs, which corresponds to the optimal RF+DC values ​​of these LDs.

[0051] In order to verify the turbidity tolerance of the WDM communication system, the communication performance was measured when the turbidity changed from clean water to ocean water. Figure 8 a to c show the communication results of Shimizu. Figure 8 d to f show the results of Ocean Water 1. Figure 8 From g to j, the results of Ocean Water 2 are shown. Figure 8 As shown in a, d, and g of Figure 2, the attenuation of each wavelength increases significantly from clean water to ocean water 2, resulting in a decrease in the maximum data rate of each wavelength. Figure 8 In the data in b, e, and h, the PMT collected ten PRBS packets at different data rates and modulation formats for data analysis. Each packet contains 16383 OOK symbols. Using 3.8×10 -3 The hard decision forward error correction threshold of PMT determines the maximum data rate of each channel. However, due to the inherent Gaussian noise of PMT, each channel does not show the optimal data rate, so the equalization algorithm is used to alleviate this noise. The result of unequalization is as follows Figure 8 The corresponding recursive least squares (RLS) equalization results are shown in b, e and h. Figure 8 Compare Figure 8As can be seen from the insets at the bottom corners of e and f, the noise signal is filtered out after RLS equalization, resulting in an improvement in the data rate of each channel. For example, the eye diagram inset of the maximum data rate at 517nm shows a clear open eye diagram at around 2Gbit / s.

[0052] The crosstalk communication test results of adjacent channels are as follows: Fig. 9 As shown in a and b of . It can be seen that under the HD-FEC threshold condition, the communication data rate of each channel remains almost unchanged. For example, under the HD-FEC threshold condition, the data rate of the 494nm channel is 1.8Gbit / s when there is adjacent channel communication working and when there is no adjacent channel communication working. Fig. 9 The comparison of the eye opening in the illustrations a and b further verifies that the channel works the same in both cases and the effect of crosstalk can be ignored.

[0053] The cost-effective, water-turbidity-resistant dense wavelength division multiplexing (WDM) system in the blue-green light band of the embodiment of the present invention can achieve a minimum blue-green light channel spacing of 2nm, which is more than an order of magnitude smaller than the previously reported WDM UWOC channel spacing. Using seven different wavelength lasers of 492nm, 494nm, 496nm, 506nm, 508nm, 517nm and 520nm, they are multiplexed into a single beam of light through SBC (Spectralbeam combination) technology, and then reliably transmitted in a 20-meter underwater channel. Diffraction gratings are used at the receiving end for dense wavelength demultiplexing. These seven wavelength channels can be successfully separated and detected. For different water turbidity levels, the total communication rate of this communication link can exceed 10Gbit / s at different turbidity levels from clear water to ocean water.

[0054] The embodiment of the present invention can stably implement high-speed UWOC communication solutions in both clear water and ocean water through dense blue-green WDM. At the transmitter end, a blue-green laser diode with low-cost direct modulation is designed. The beam combining and collimation of dense wavelength combination light beams are realized in a 20-meter underwater channel from 490nm to 520nm. At the receiver end, a beam collection lens combination is used to completely collect the collimated light beam after propagating 20 meters underwater, and demultiplexing with a minimum channel spacing of 2nm is achieved through a grating. Under different water turbidity conditions, the total communication data rate of the present invention can exceed 10Gbit / s; at the same time, there is no crosstalk between channels.

[0055] The embodiment of the present invention can simultaneously ensure high-quality communication for blue light in ocean water and high-quality communication for green light in coastal water. -3Under the communication (HD-FEC, Hard-Decision Forward-Error-Correction) decision threshold, the total data transmission rate exceeds 10Gbit / s. The WDM UWOC achieves a wavelength channel spacing of 2nm in the blue-green band and simultaneous multiplexing of 7 wavelength channels. In the actual application of UWOC, these densely multiplexed wavelength channels can effectively tolerate the changes in turbidity of ocean water and nearshore water, and provide reliable and high-speed underwater communications.

[0056] The cost-effective, water-turbidity-resistant dense wavelength division multiplexing (WDM) system in the blue-green light band of the embodiment of the present invention can achieve a minimum blue-green light channel spacing of 3nm, which is more than an order of magnitude smaller than the previously reported WDM UWOC channel spacing. Using 14 different wavelengths of lasers at 452nm, 464nm, 467nm, 570nm, 478nm, 483nm, 486nm, 492nm, 494nm, 496nm, 506nm, 508nm, 517nm and 520nm, they are multiplexed into a single beam of light through SBC (Spectralbeam combination) technology, and then reliably transmitted in a 200-meter underwater channel. A projected diffraction grating is used at the receiving end for dense wavelength demultiplexing. These 14 wavelength channels can be successfully separated and detected. For different water turbidity levels, the total communication rate of this communication link can exceed 20Gbit / s at different turbidity levels from clear water to ocean water.

[0057] The embodiment of the present invention can stably implement high-speed UWOC communication solutions in both clear water and ocean water through dense blue-green WDM. At the transmitter end, a blue-green laser diode with low-cost direct modulation is designed. The beam combining and collimation of dense wavelength combination light beams are realized in a 200-meter underwater channel from 450nm to 520nm. At the receiver end, a beam collection lens combination is used to completely collect the collimated light beam after propagating 200 meters underwater, and demultiplexing with a minimum channel spacing of 3nm is achieved through a grating. Under different water turbidity conditions, the total communication data rate of the present invention can exceed 20Gbit / s; at the same time, there is no crosstalk between channels.

[0058] The embodiment of the present invention can simultaneously ensure high-quality communication for blue light in ocean water and high-quality communication for green light in coastal water. -3Under the communication (HD-FEC, Hard-Decision Forward-Error-Correction) decision threshold, the total data transmission rate exceeds 20Gbit / s. The WDM UWOC achieves 3nm wavelength channel spacing in the blue-green band and simultaneous multiplexing of 14 wavelength channels. In the practical application of UWOC, these densely multiplexed wavelength channels can effectively tolerate the changes in turbidity of ocean water and nearshore water, and provide reliable and high-speed underwater communications.

[0059] The embodiments described above are only descriptions of the preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A dense wavelength division multiplexing underwater wireless optical communication system, characterized in that: include: Transmitter, receiver and digital signal processing module; wherein: The transmitter includes wavelengths λ1, λ2, λ3, ... λ n Multiple directly modulated semiconductor lasers and beam combining elements are used to realize dense wavelength division multiplexing in the blue-green light band; The receiver includes: a collimator and a diffraction grating; wherein, after the light propagates 20 to 200 meters underwater, it is scattered into irregular light spots, the light spots are collected by the receiving lens and compressed in the collimator, and the diffraction grating is used to demultiplex and separate light of different wavelengths; A digital signal processing module is used to capture the optical signal using photomultiplier and convert it into an electrical signal, which is captured by an oscilloscope and performs signal synchronization, decoding and channel equalization on a computer.

2. The dense wavelength division multiplexing underwater wireless optical communication system according to claim 1, characterized in that: The transmitter integrates multiple blue-green laser diodes LD with different wavelengths for WDM; each LD is loaded with a series of pseudo-random binary sequences generated by an arbitrary signal generator.

3. The dense wavelength division multiplexing underwater wireless optical communication system according to claim 2, characterized in that: The wavelength division multiplexing of the transmitter is realized by an N×1 combiner, which is a fiber combiner or a free space optical combiner.

4. The dense wavelength division multiplexing underwater wireless optical communication system according to claim 3, characterized in that: When N=7, seven different wavelengths of pigtailed laser diodes are multiplexed into a multimode optical fiber based on SBC; the multimode optical fiber is connected to a spatial optical collimator to output a combined laser beam; the diameter of the combined beam is 5 mm, which is expanded to 25 mm by the collimator; the collimated beam enters the underwater channel, and after being transmitted in the 20-meter underwater channel, a receiving lens, a collimator and a reflective diffraction grating are used for demultiplexing; the received beam has a diameter of 30 mm, which is collected and collimated into a parallel beam of 6 mm in diameter by a collimator; a diffraction grating is used to separate all seven wavelengths.

5. The dense wavelength division multiplexing underwater wireless optical communication system according to claim 4, characterized in that: The underwater channel uses a 2-meter-long and 1-meter-wide water tank to construct an optical communication transmission channel. Two reflectors placed on the short sides enable light to be transmitted underwater over a distance of 20 meters.

6. The dense wavelength division multiplexing underwater wireless optical communication system according to claim 5, characterized in that: The transmitter achieves dense wavelength division multiplexing in the wavelength range of 490nm to 520nm within the low attenuation window of 450nm to 550nm, with a minimum channel spacing of 2nm.

7. The dense wavelength division multiplexing underwater wireless optical communication system according to claim 3, characterized in that: When N=14, 14 different wavelengths of end-emitting laser diodes (EEL) are multiplexed in a free-space coupled multimode optical fiber based on SBC; the multimode optical fiber is connected to a spatial optical collimator to output a combined laser beam; the diameter of the combined beam is 25 mm; the collimated beam enters the underwater channel, and after being transmitted in the 200-meter underwater channel, a receiving lens, a collimator and a transmission diffraction grating are used for demultiplexing; the received beam has a diameter of 300 mm, which is collected and collimated into a parallel beam of 6 mm in diameter by a collimator; a transmission diffraction grating is used to separate all 14 wavelengths.

8. The dense wavelength division multiplexing underwater wireless optical communication system according to claim 7, characterized in that: The underwater channel uses a 50-meter-long and 5-meter-wide pool of water to construct an optical communication transmission channel. Two reflectors placed on the short sides enable light to be transmitted underwater over a distance of 200 meters.

9. The dense wavelength division multiplexing underwater wireless optical communication system according to claim 8, characterized in that: The transmitter achieves dense wavelength division multiplexing in the wavelength range of 450nm to 520nm within the low attenuation window of 450nm to 550nm, with a minimum channel spacing of 3nm.

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