阅读说明：本技术 提高光声转换效率的空中与水下激光致声通信装置及其应用 (Aerial and underwater laser acoustic communication device for improving acoustic-optical conversion efficiency and application thereof ) 是由 赵扬 李鹏 周志权 赵占峰 陈铖 李迎春 于 2020-12-10 设计创作，主要内容包括：本发明涉及声光通信技术领域,具体的说是一种提高光声转换效率的空中与水下激光致声通信装置及其应用,包括发射机构和接收机构,其特征在于,所述发射机构中设有依次相连的DHPIM双头脉冲间隔调制模块、激光激发及扫描控制模块、脉冲CO-(2)激光器,其中脉冲CO-(2)激光器输出的光信号经光束整形机构处理后,经过空气传输使得激光信号在到达水面后,以光击穿的方式与水介质相互作用,进而把激光脉冲转化成为声波信号在水下向各个方向进行传播；所述接受机构中设有位于水下的光纤水听器,光纤水听器进行声波信号的接收；本发明与现有技术相比,能够有效降低数据丢失或被破译的概率；提高了传输容量和带宽效率,简化系统实现复杂度。(The invention relates to the technical field of acousto-optic communication, in particular to an aerial and underwater laser induced acoustic communication device for improving the acousto-optic conversion efficiency and application thereof 2 Laser in which CO is pulsed 2 After being processed by a beam shaping mechanism, an optical signal output by a laser is transmitted through air, so that the optical signal interacts with a water medium in a light breakdown mode after reaching the water surface, and then laser pulses are converted into sound wave signals which are transmitted to all directions underwater; the above-mentionedAn underwater optical fiber hydrophone is arranged in the receiving mechanism and receives sound wave signals; compared with the prior art, the method can effectively reduce the probability of data loss or decoding; the transmission capacity and the bandwidth efficiency are improved, and the system implementation complexity is simplified.)

1. The aerial and underwater laser acoustic communication device capable of improving the photoacoustic conversion efficiency comprises a transmitting mechanism and a receiving mechanism, and is characterized in that a DHPIM (distributed laser Interval) double-headed pulse interval modulation module, a laser excitation and scanning control module, a pulse CO (carbon monoxide) and a pulse CO (carbon monoxide) which are sequentially connected are arranged in the transmitting mechanism₂Laser in which CO is pulsed₂After being processed by a beam shaping mechanism, an optical signal output by a laser is transmitted through air, so that the optical signal interacts with a water medium in a light breakdown mode after reaching the water surface, and then laser pulses are converted into sound wave signals which are transmitted to all directions underwater; the receiving mechanism is internally provided with an underwater optical fiber hydrophone which receives sound wave signals; the beam shaping mechanism is used for realizing the conversion from solid beams to hollow beams and is provided with a first plano-concave lens, a second plano-concave lens, a first conical lens and a second conical lens which are coaxially arranged in sequence, wherein the concave surface of the first plano-concave lens faces to the light propagation direction, and the plane of the second plano-concave lens faces to the light propagation directionThe light propagation direction, the conical surface of first conical lens is towards the light propagation direction, the second conical lens plane is towards the light propagation direction, and the distance satisfies the following formula between each lens in the beam shaping mechanism:

wherein, ω is the beam radius after the beam expansion and collimation system composed of the first plano-concave lens and the second plano-concave lens, γ is the obscuration ratio, n is the relative refractive index between air and the conical lens, and θ is the vertex angle of the conical lens.

2. The aerial and underwater laser-induced acoustic communication device for improving photoacoustic conversion efficiency according to claim 1, wherein the beam shaping mechanism is further provided with a translation stage control mechanism and an optical guide rail, the vertex angles of the first conical lens and the second conical lens are the same, and translation stages are mounted at the bottoms of the first conical lens and the second conical lens and fixed on the optical guide rail.

3. The aerial and underwater laser induced acoustic communication device for improving photoacoustic conversion efficiency according to claim 1, wherein the receiving mechanism is provided with an optical fiber MEMS hydrophone, an acousto-electric conversion module, a preprocessing amplification filtering module and a DHPIM demodulation module, the optical fiber MEMS hydrophone receives laser acoustic signals, the acousto-electric conversion module converts the acoustic signals into electric signals, the preprocessing amplification filtering module amplifies and filters the electric signals to facilitate signal demodulation, and the DHPIM demodulation module demodulates and outputs data of pulse and empty slot position information according to a mapping relationship.

4. The airborne and underwater laser-induced acoustic communication device for improving photoacoustic conversion efficiency as claimed in claim 1, wherein the DHPIM modulation module in the launching mechanism reads and buffers data in RAM (random access memory), and then buffers each buffer numberCarrying out modulation according to the data; said pulse CO₂The laser is controlled by the output signal of the modulation code to realize the emission of laser pulse; the laser excitation and scanning control module realizes the triggering of pulse signals; and the beam shaping mechanism converts the solid laser beam into an annular hollow laser beam through a conical lens.

5. A method for using the aerial and underwater laser induced acoustic communication device for improving the photoacoustic conversion efficiency according to any one of claims 1 to 4, wherein the transmitting mechanism converts signal data into a pulse sequence with DHPIM modulation characteristics through the DHPIM modulation module, the modulated pulse sequence is sent to the laser driving circuit to control the laser to transmit a specific sequence of laser pulses, the output laser pulses are converted into annular hollow laser beams by the beam shaping mechanism, the annular laser beams are transmitted through air to enable laser energy to interact with an aqueous medium in an optical breakdown mode after reaching the water surface, the laser pulses are converted into acoustic signals which are transmitted to all directions under water, the acoustic signals are received through the fiber optic hydrophone at any position under water, and the acoustic signals are converted into electric signals through acoustoelectric conversion, the data is transmitted to the input end of the DHPIM demodulation module through amplification and filtering, the DHPIM demodulation module extracts pulse and empty time slot position information in the digital sequence signal and converts the pulse and empty time slot position information into a data set through a corresponding mapping relation, signal transmission from the air to the underwater is achieved, information is transmitted, and therefore underwater acoustic communication is achieved.

6. The method for using the air and underwater laser acoustic communication device for improving the photoacoustic conversion efficiency according to claim 5, wherein the method for using the air and underwater laser acoustic communication device for improving the photoacoustic conversion efficiency comprises the following steps:

step 1: the DHPIM modulation module completes the string of the binary sequence and converts the binary sequence into decimal, the binary sequence is converted into decimal, modulated data are cached, the pulse width of a head time slot of each modulated data is judged by the corresponding decimal by a double-head pulse interval modulation mode, different pulse widths are generated by counting, information time slot number and protection time slot number are generated by counting according to the modulated data, all cached data are modulated, the binary sequence is converted into a time slot sequence to be output, and therefore the modulation of signals is completed;

step 2: the modulation signal output by the DHPIM modulation module is loaded to the pulse CO through the driving circuit₂The laser driving module outputs pulse laser signals of different time slot types;

and step 3: the beam shaping mechanism converts the solid laser beam into an annular hollow laser beam through a conical lens;

and 4, step 4: the laser pulse reaches the water surface through an atmospheric channel, and interacts with a water medium in a light breakdown mode, so that the laser pulse is converted into a sound wave signal to be transmitted in all directions underwater;

and 5: the sound wave signal is transmitted through the underwater sound channel, the hydrophone receives the sound wave signal, converts the sound wave signal into an electric signal, and outputs the electric signal to the DHPIM demodulation module after passing through the filtering and amplifying circuit;

step 6: the DHPIM demodulation module demodulates the received sequence signal, enters an effective signal detection state after detecting the pulse signal, calculates the number of information time slots and protection time slots according to the received initial pulse time slot, maps the information into an original information source and stores the information, and outputs binary information after performing serial conversion on demodulated data.

7. The method for using the airborne and underwater laser-induced acoustic communication device for improving photoacoustic conversion efficiency as claimed in claim 6, wherein in step 1, DHPIM modulation uses two kinds of start pulses with different pulse widths, and then 1-2 protection time slots and the number of modulation information time slots are added, wherein the modulation information depends on the decimal code corresponding to the modulation data, S_kAs the DHPIM modulated symbol, k is the decimal number corresponding to the modulated symbol, d_kIs the number of information time slots in the information frame, M is the modulation order, and the symbol S_kFrom the head slot and d_kA number of empty slots, wherein:

the head slot is composed of (alpha +1) slots (alpha is a positive integer), and H is used respectively₁And H₂To indicate. H₁The pulse width is composed of 2/alpha time slots and (alpha/2 +1) protection time slots; h₂Is composed of alpha time slots and 1 protecting time slot, when k is less than 2^M-1When S is present_kThe head slot is H₁Otherwise is H₂；

The DHPIM frame structure starts with a rectangular pulse at the nth frame, and the starting time T is T_nDuration τ, wherein τ is (1+ h)_n)αT_sA/2, wherein h_nE {0,1}, n represents the nth frame, the pulse of DHPIM is represented by equation (2):

wherein A is the pulse amplitude, T_sFor protecting the time slot, rect is a mathematical expression of a rectangular function, as shown in equation (3):

the nth frame start time is defined as:wherein T is₀Time of start pulse when n is 0, d_k∈{0,1,...(2^M-1-1) } represents the decimal value represented by the information slot of the kth frame;

average symbol length of DHPIM signal is head pulse sequence length and d_kHalf of the maximum sum is:the average width of the head pulse isThenWherein p (0) and p (1) respectively represent the probability of sending "0" and "1";

at peak power P_tUnder the condition of (2), the mistiming gap rate of the DHPIM modulation is expressed as:

where g is the channel transmission attenuation, σ is the standard deviation, and the Q function is defined as:

in addition, the error performance of DHPIM modulation is measured by the packet error rate, when a slot error is detected, the whole packet is considered to be in error, and the packet error rate is defined as follows:

wherein, P_seFor the mistiming slot rate, N is the number of bits in a packet, L_aveThe average time slot number contained in one symbol is M is a modulation order;

the error performance of the modulation mode is related to the decision threshold lambda of the threshold detector, the optimal detection threshold enables the system error rate to reach the minimum lambda value, and the derivation and the order of the formula (4) are carried outAvailable DHPIM modulation at peak power of P_tThe following optimal detection thresholds:

in general, the relation between the average transmission power and the error performance needs to be considered, and the relation between the DHPIM modulation peak power and the average power is as follows:

under the same average transmit power P, the optimal detection threshold λ' for DHPIM modulation is:

The technical field is as follows:

the invention relates to the technical field of acousto-optic communication, in particular to an aerial and underwater laser acoustic communication device which can greatly reduce system energy consumption, improve communication distance, guarantee communication information safety and confidentiality and improve opto-acoustic conversion efficiency and application thereof.

Background art:

with the increasing change of resource environment, the ocean gradually becomes the key point of the strategic development of the country. The detection of the underwater targets of the ocean is increasingly prominent in future war and national economic production, for example, real-time information exchange, communication, command control and the like are carried out in the sea warfare, so the detection under the ocean environment is very important. The photoacoustic detection technology combines the advantages of laser and sound wave, and laser is focused in seawater to excite the sound wave through an onboard or shipborne high-power laser. The high-energy pulse CO2 laser can improve the photoacoustic conversion efficiency to a certain extent, reduce energy loss, and the high-sensitivity hydrophone can accept long-distance transmission sound waves so as to improve the communication distance. In addition, the light beam is shaped into a ring, so that the influence of water mist on the laser during cross-medium transmission can be reduced, the penetration of the laser in the atmosphere is improved, the energy loss is reduced, and the long-distance excitation is realized. The sound source level of the sound wave excited by the photoacoustic effect is high, the sound wave pulse is narrow, the frequency spectrum range is wide, and the photoacoustic effect can be well applied to the marine environment and realize accurate detection.

The invention content is as follows:

the invention provides an aerial and underwater laser acoustic communication device for improving the photoacoustic conversion efficiency and application thereof, aiming at the problems of large signal loss, poor safety, short communication distance, great difficulty and the like of the traditional aerial and ocean communication.

The invention is achieved by the following measures:

the aerial and underwater laser acoustic communication device capable of improving the photoacoustic conversion efficiency comprises a transmitting mechanism and a receiving mechanism, and is characterized in that a DHPIM (distributed laser Interval) double-headed pulse interval modulation module, a laser excitation and scanning control device which are sequentially connected are arranged in the transmitting mechanismModule, pulse CO₂Laser in which CO is pulsed₂After being processed by a beam shaping mechanism, an optical signal output by a laser is transmitted through air, so that the optical signal interacts with a water medium in a light breakdown mode after reaching the water surface, and then laser pulses are converted into sound wave signals which are transmitted to all directions underwater; the receiving mechanism is internally provided with an underwater optical fiber hydrophone which receives sound wave signals; the light beam shaping mechanism is used for realizing the conversion from a solid light beam to a hollow light beam, and is provided with a first plano-concave lens, a second plano-concave lens, a first conical lens and a second conical lens which are coaxially arranged in sequence, wherein the concave surface of the first plano-concave lens faces the light propagation direction, the plane of the second plano-concave lens faces the light propagation direction, the conical surface of the first conical lens faces the light propagation direction, the plane of the second conical lens faces the light propagation direction, and the distance between the lenses in the light beam shaping mechanism meets the following formula:

wherein, ω is₀The radius of a beam expanded by a beam expanding and collimating system consisting of a first plano-concave lens and a second plano-concave lens is gamma, the obscuration ratio is gamma, the relative refractive index between air and the conical lens is n, and the angle theta is the vertex angle of the conical lens.

The beam shaping mechanism is also provided with a translation stage control mechanism and an optical guide rail, further, the vertex angles of the first conical lens and the second conical lens are the same, the bottom parts of the first conical lens and the second conical lens are provided with translation stages, and the translation stages are fixed on the optical guide rail.

The receiving mechanism is provided with the optical fiber MEMS hydrophone, the sound-electricity conversion module, the preprocessing amplification filtering module and the DHPIM demodulation module, the optical fiber MEMS hydrophone receives laser sound signals, the sound-electricity conversion module converts the sound signals into electric signals, the preprocessing amplification filtering module amplifies and filters the electric signals to conveniently demodulate the signals, and the DHPIM demodulation module demodulates and outputs data of pulse and empty time slot position information according to a mapping relation.

The DHPIM modulation module in the launching mechanism reads and caches data in an RAM (random access memory), and then modulates each cached data; said pulse CO₂The laser is controlled by the output signal of the modulation code to realize the emission of laser pulse; the laser excitation and scanning control module realizes the triggering of pulse signals; and the beam shaping mechanism converts the solid laser beam into an annular hollow laser beam through a conical lens.

The invention is also provided with an industrial personal computer, the output end of the industrial personal computer for controlling the emission frequency and energy of the pulse signal passes through the DHPIM modulation module and the laser excitation scanning control module and the pulse CO₂The input ends of the lasers are connected, and the pulse CO is₂The output end of the laser passes through the scanning mirror and the vibrating mirror system and then is converted into an annular hollow laser beam by the beam shaping mechanism, the laser beam sequentially passes through an air channel and a hydroacoustic channel and then establishes wireless communication with the hydrophone, the output end of the hydrophone passes through the acoustoelectric conversion module and the preprocessing amplification filtering module and then is connected with the input end of the DHPIM demodulation module, and the output end of the DHPIM demodulation module is connected with the industrial personal computer.

The laser in the invention is high-energy pulse CO₂The pulse repetition frequency of the laser is 1-20Hz, the pulse energy under multimode is 250mJ, the average power is 5W, and the pulse width is 50-150 ns. The hydrophone is an optical fiber MEMS hydrophone, the linear frequency bandwidth is 1-10 kHz, and the sensitivity level is-210 dB (the reference value is 1V/mu Pa).

The invention also provides a using method of the aerial and underwater laser acoustic communication device for improving the photoacoustic conversion efficiency, which is characterized in that a transmitting mechanism converts signal data into a pulse sequence with DHPIM modulation characteristics through a DHPIM modulation module, the modulated pulse sequence is sent to a laser driving circuit to control a laser to transmit a laser pulse with a specific sequence, the output laser pulse is converted into an annular hollow laser beam through a beam shaping mechanism, the annular laser beam is transmitted through air to enable laser energy to interact with a water medium in a light breakdown mode after reaching the water surface, the laser pulse is converted into an acoustic wave signal which is transmitted to all directions under water, the acoustic wave signal is received through an optical fiber hydrophone at any position under water, the acoustic wave signal is converted into an electric signal through acoustoelectric conversion and then is sent to the input end of the DHPIM demodulation module through amplification and filtering, the DHPIM demodulation module extracts pulse and empty time slot position information in the digital sequence signal and converts the pulse and empty time slot position information into a data set through a corresponding mapping relation, so that signal transmission from air to underwater is realized, information is transmitted, and underwater acoustic communication is realized.

The use method of the aerial and underwater laser acoustic communication device for improving the photoacoustic conversion efficiency comprises the following steps:

step 1: the DHPIM modulation module completes the string of the binary sequence and converts the binary sequence into decimal, the binary sequence is converted into decimal, modulated data are cached, the pulse width of a head time slot of each modulated data is judged by the corresponding decimal by a double-head pulse interval modulation mode, different pulse widths are generated by counting, information time slot number and protection time slot number are generated by counting according to the modulated data, all cached data are modulated, the binary sequence is converted into a time slot sequence to be output, and therefore the modulation of signals is completed;

step 2: the modulation signal output by the DHPIM modulation module is loaded to the pulse CO through the driving circuit₂The laser driving module outputs pulse laser signals of different time slot types;

and step 3: the beam shaping mechanism converts the solid laser beam into an annular hollow laser beam through a conical lens;

and 4, step 4: the laser pulse reaches the water surface through an atmospheric channel, and interacts with a water medium in a light breakdown mode, so that the laser pulse is converted into a sound wave signal to be transmitted in all directions underwater;

and 5: the sound wave signal is transmitted through the underwater sound channel, the hydrophone receives the sound wave signal, converts the sound wave signal into an electric signal, and outputs the electric signal to the DHPIM demodulation module after passing through the filtering and amplifying circuit;

step 6: the DHPIM demodulation module demodulates the received sequence signal, enters an effective signal detection state after detecting the pulse signal, calculates the number of information time slots and protection time slots according to the received initial pulse time slot, maps the information into an original information source and stores the information, and outputs binary information after performing serial conversion on demodulated data.

In step 1 of the present invention, DHPIM modulation uses two kinds of initial pulses with different pulse widths, and then adds 1-2 guard time slots and modulation information time slot numbers, wherein the modulation information depends on decimal code corresponding to modulation data, S_kAs the DHPIM modulated symbol, k is the decimal number corresponding to the modulated symbol, d_kIs the number of information time slots in the information frame, M is the modulation order, and the symbol S_kFrom the head slot and d_kA number of empty slots, wherein:

the head slot is composed of (alpha +1) slots (alpha is a positive integer), and H is used respectively₁And H₂To indicate. H₁The pulse width is composed of 2/alpha time slots and (alpha/2 +1) protection time slots; h₂Is composed of alpha time slots and 1 protecting time slot, when k is less than 2^M-1When S is present_kThe head slot is H₁Otherwise is H₂；

The DHPIM frame structure starts with a rectangular pulse at the nth frame, and the starting time T is T_nDuration τ, wherein τ is (1+ h)_n)αT_sA/2, wherein h_nE {0,1}, n represents the nth frame, the pulse of DHPIM is represented by equation (2):

wherein A is the pulse amplitude, T_sFor protecting the time slot, rect is a mathematical expression of a rectangular function, as shown in equation (3):

the nth frame start time is defined as:wherein T is₀Time of start pulse when n is 0, d_k∈{0,1,...(2^M-1-1) } represents the decimal value represented by the information slot of the kth frame;

average symbol length of DHPIM signal is head pulse sequence length and d_kHalf of the maximum sum is:the average width of the head pulse isThenWherein p (0) and p (1) respectively represent the probability of sending "0" and "1";

at peak power P_tUnder the condition of (2), the mistiming gap rate of the DHPIM modulation is expressed as:

where g is the channel transmission attenuation, σ is the standard deviation, and the Q function is defined as:

in addition, the error performance of DHPIM modulation is measured by the packet error rate, when a slot error is detected, the whole packet is considered to be in error, and the packet error rate is defined as follows:

wherein, P_seIs a mistiming gap rate, N isNumber of bits in a packet, L_aveThe average time slot number contained in one symbol is M is a modulation order;

the error performance of the modulation mode is related to the decision threshold lambda of the threshold detector, the optimal detection threshold enables the system error rate to reach the minimum lambda value, and the derivation and the order of the formula (4) are carried outAvailable DHPIM modulation at peak power of P_tThe following optimal detection thresholds:

in general, the relation between the average transmission power and the error performance needs to be considered, and the relation between the DHPIM modulation peak power and the average power is as follows:

under the same average transmit power P, the optimal detection threshold λ' for DHPIM modulation is:

in step 1 of the present invention, the DHPIM modulation specifically comprises the following steps: firstly, writing 4bit/symbol data to be transmitted into RAM in sequence, stopping writing when RAM is full, at this time, utilizing reading control signal to read several 4bit data from RAM and store them into buffer memory, successively modulating data, according to the code character unique to said modulation and last pulse added as stop bit the modulation completed time can be clearly defined, then reading control signal to read a group of superframe data again and placing it into buffer memory, continuously making the above-mentioned process to complete data buffer memory and modulation clock control, finally completing all data modulation.

In step 6 of the present invention, the DHPIM demodulation specifically comprises: when a first pulse signal is detected, the system enters a demodulation state, firstly, demodulated data are stored in a buffer, then effective information is continuously detected, when a stop bit is detected, the detection of a plurality of data information is finished, at the moment, the buffer stores all the data, superframe demodulation is finished, the data can be stored in an RAM under an input effective clock, a next clock starts new empty time slot detection and data demodulation, finally, all pulse signals are converted into the data information, and the demodulated data of each superframe are stored in the RAM.

Compared with the prior art, the invention has the following beneficial effects: (1) the laser outputs a pulse modulation signal which changes randomly, and the signal is in an irregular state, so that the probability of data loss or decoding can be effectively reduced. (2) The DHPIM modulation and demodulation technology is adopted, so that the transmission capacity and the bandwidth efficiency are improved, symbol synchronization is not needed, and the complexity of system realization is simplified to a certain extent. In addition, the power utilization rate can be further improved by adopting a higher modulation order, and the method is an efficient modulation mode and has great advantages in the application of laser underwater acoustic communication. (3) Using pulsed CO₂When the laser is used, the photoacoustic conversion efficiency can reach about 10 percent, and compared with Nd: the optical-acoustic conversion efficiency of the YAG solid laser is greatly improved by about 3 percent. In addition, the optical fiber MEMS hydrophone has good low-frequency response sensitivity, low main frequency and narrow bandwidth of received acoustic signals, and the communication distance is greatly improved. (4) The beam shaping mechanism converts the solid laser beam into the annular hollow laser beam through the conical lens, and can reduce the influence of water mist on the laser beam to a certain extent, thereby reducing energy loss. (5) In the underwater channel, the hydrophone can be placed at any position in the underwater detectable range, and the underwater detection device has the characteristics of flexibility. (6) Aiming at the characteristics of an air-underwater communication environment, the communication safety between two communication parties is ensured, the communication quality of the two communication parties is improved, and the communication difficulty existing under the environment interference is overcome.

Description of the drawings:

FIG. 1 is a schematic diagram of the system structure of the present invention.

FIG. 2 is a flow chart of the method of the present invention.

Fig. 3 is a block diagram of a dual-headed pulse interval modulation module according to the present invention.

Fig. 4 is a block diagram of a DHPIM demodulation module of the present invention.

FIG. 5 is a schematic structural diagram of a light beam transformation mechanism according to the present invention.

Reference numerals: pulsed CO₂The laser device comprises a laser 1, a first plano-concave lens 2, a second plano-concave lens 3, a first conical lens 4, a second conical lens 5, a translation stage control mechanism 6 and an optical guide rail 7.

The specific implementation mode is as follows:

the following further describes the embodiments of the present invention with reference to the drawings, but the present invention is not limited thereto.

FIG. 1 shows an aerial and underwater laser-induced acoustic communication device for improving the efficiency of photoacoustic conversion, which comprises a transmitting mechanism and a receiving mechanism, wherein the transmitting mechanism comprises a double-head pulse interval modulation (DHPIM) module and a pulse CO₂The receiving mechanism comprises an optical fiber MEMS hydrophone, an acousto-electric conversion module, a preprocessing amplification filtering module and a DHPIM demodulation module;

the DHPIM modulation module reads and caches data in a Random Access Memory (RAM), and then modulates each cached data;

said pulse CO₂The laser is controlled by the output signal of the modulation code to realize the emission of laser pulse;

the laser excitation and scanning control module realizes the triggering of pulse signals;

the beam shaping mechanism converts the solid laser beam into an annular hollow laser beam through a conical lens;

the optical fiber MEMS hydrophone completes the receiving of laser acoustic signals;

the acoustic-electric conversion module converts an acoustic signal into an electric signal;

the preprocessing amplifying and filtering module is used for amplifying and filtering the electric signals, so that the signals can be conveniently demodulated;

the DHPIM demodulation module demodulates and outputs data to the pulse and empty time slot position information according to the mapping relation;

the transmitting mechanism and the receiving mechanism are communicated by combining laser excitation ultrasonic waves in the air and underwater sound-generating waves.

The working principle of the invention is as follows: the DHPIM modulation module converts signal data into a pulse sequence with DHPIM modulation characteristics, the modulated pulse sequence is sent to a laser driving circuit, a laser can be controlled to emit laser pulses with a specific sequence, the output laser pulses pass through a scanning mirror and a vibrating mirror system and then are converted into annular hollow laser beams by a beam shaping mechanism, the annular laser beams are transmitted through air, so that laser energy reaches the water surface and then interacts with a water medium in a light breakdown mode, the laser pulses are converted into sound wave signals which are transmitted to all directions under water, the sound wave signals are received through an optical fiber hydrophone at any position under water, the sound wave signals are converted into electric signals through sound and electricity and then are sent to the input end of the DHPIM demodulation module through simple amplification and filtering, and the demodulation system extracts pulse and empty slot time slot position information in the digital sequence signals and converts the pulse and empty slot position information into a data set through a corresponding mapping relation, therefore, signal transmission from the air to the underwater is realized, and further information transmission is carried out, so that underwater acoustic communication is realized.

Fig. 2 shows a method for using the aerial and underwater laser acoustic communication device for improving the photoacoustic conversion efficiency, which comprises the following steps:

step (1): the DHPIM modulation module completes the string of the binary sequence and converts the binary sequence into decimal, the binary sequence is converted into decimal, modulated data are cached, the pulse width of a head time slot of each modulated data is judged by the corresponding decimal by a double-head pulse interval modulation mode, different pulse widths are generated by counting, information time slot number and protection time slot number are generated by counting according to the modulated data, all cached data are modulated, the binary sequence is converted into a time slot sequence to be output, and therefore the modulation of signals is completed;

step (2): the modulation signal output by the DHPIM modulation module is loaded to the pulse CO through the driving circuit₂A laser driving module for outputting pulse laser signals with different time slot types；

And (3): the beam shaping mechanism converts the solid laser beam into an annular hollow laser beam through a conical lens;

and (4): the laser pulse reaches the water surface through an atmospheric channel, and interacts with a water medium in a light breakdown mode, so that the laser pulse is converted into a sound wave signal to be transmitted in all directions underwater;

and (5): the sound wave signal is transmitted through the underwater sound channel, the hydrophone receives the sound wave signal, converts the sound wave signal into an electric signal, and outputs the electric signal to the DHPIM demodulation module after passing through the filtering and amplifying circuit;

and (6): the DHPIM demodulation module demodulates the received sequence signal, enters an effective signal detection state after detecting the pulse signal, calculates the number of information time slots and protection time slots according to the received initial pulse time slot, maps the information into an original information source and stores the information, and outputs binary information after performing serial conversion on demodulated data.

In step (1), DHPIM modulation uses two starting pulses of different pulse widths, followed by the addition of 1-2 guard slots and the number of modulation information slots, where the modulation information depends on the decimal code corresponding to the modulation data. S_kAs the DHPIM modulated symbol, k is the decimal number corresponding to the modulated symbol, d_kIs the number of information time slots in the information frame, M is the modulation order, and the symbol S_kFrom the head slot and d_kA number of empty slots, wherein:

the head slot is composed of (alpha +1) slots (alpha is a positive integer), and H is used respectively₁And H₂To indicate. H₁The pulse width is composed of 2/alpha time slots and (alpha/2 +1) protection time slots; h₂Consists of a pulse width of alpha time slots and 1 protection time slot. When k is less than 2^M-1When S is present_kThe head slot is H₁Otherwise is H₂。

DHPIM frame structureStarting with a rectangular pulse in the nth frame with a start time T equal to T_nDuration τ, wherein τ is (1+ h)_n)αT_sA/2, wherein h_nE {0,1}, n represents the nth frame. The pulse of DHPIM can be represented by equation (2):

wherein A is the pulse amplitude, T_sFor protecting the time slot, rect is a mathematical expression of a rectangular function, as shown in equation (3):

the nth frame start time is defined as:wherein T is₀Time of start pulse when n is 0, d_k∈{0,1,...(2^M-1-1) } denotes the decimal value represented by the information slot of the k-th frame.

Average symbol length of DHPIM signal is head pulse sequence length and d_kHalf of the maximum sum is:the average width of the head pulse isThenWhere p (0) and p (1) represent the probability of transmitting "0" and "1", respectively.

At peak power P_tUnder the condition of (2), the mistiming gap rate of the DHPIM modulation can be expressed as:

where g is the channel transmission attenuation, σ is the standard deviation, and the Q function is defined as:

in addition, the error performance of DHPIM modulation is measured by the packet error rate, when a slot error is detected, the whole packet is considered to be in error, and the packet error rate is defined as follows:

wherein, P_seFor the mistiming slot rate, N is the number of bits in a packet, L_aveM is the average number of slots contained in a symbol, and M is the modulation order.

The modulation mode error performance is related to the threshold detector decision threshold lambda. The optimal detection threshold is the lambda value which can make the system error rate reach the minimum, and the derivation is carried out on the formula (4) and the order is carried outAvailable DHPIM modulation at peak power of P_tThe following optimal detection thresholds:

generally, the relation between the average transmission power and the error performance needs to be considered. DHPIM modulated peak power versus average power as follows:

under the same average transmit power P, the optimal detection threshold λ' for DHPIM modulation is:

as shown in fig. 3, the execution flow of the signal modulation module in the present invention is as follows: firstly, writing 4bit/symbol data to be transmitted into the RAM in sequence, and stopping writing when the RAM is full. At this time, a plurality of 4-bit data are read from the RAM by using a reading control signal and stored in the buffer, data are modulated in sequence, the modulation completion time can be determined according to the code word characteristic unique to the modulation and the last pulse added as a stop bit, then the reading control signal reads a group of superframe data again and puts the superframe data into the buffer, the process is continued, the clock control of data buffering and modulation is completed, and finally all data are modulated.

As shown in fig. 4, the execution flow of the signal demodulation module in the present invention is as follows: when the first pulse signal is detected, a demodulation state is entered. Firstly storing the demodulated data in a buffer, then continuously detecting valid information, when a stop bit is detected, indicating that the detection of the data information is finished, at the moment, storing all the data in the buffer, and finishing the superframe demodulation. The data can be stored in the RAM under the input effective clock, the next clock starts new empty time slot detection and data demodulation, all pulse signals are converted into data information finally, and the demodulated data of each superframe is stored in the RAM.

Fig. 5 shows a beam shaping structure arrangement: involving pulsing CO₂The device comprises a laser 1, a beam expanding and collimating system consisting of a first plano-concave lens 2 and a second plano-concave lens 3, a first conical lens 4, a second conical lens 5, a translation table control mechanism 6 and an optical guide rail 7; the vertex angles of the first conical lens 4 and the second conical lens 5 are the same, and the bottom parts of the two lenses are provided with translation stages which are fixed on the optical guide rail.

The specific implementation process is as follows: CO2₂The laser 1 is placed on an optical guide rail, and a beam expanding and collimating system composed of a first plano-concave lens 2 (concave surface facing to light propagation direction) and a second plano-concave lens 3 (plane facing to light propagation direction) are coaxially placed along the laser propagation direction in sequence, a first conical lens 4 (conical surface facing to light propagation direction), and a second conical lens 5 (plane facing to light propagation direction)) Thereby realizing the conversion from the solid beam to the hollow beam.

The distance between the lenses of the beam shaping device is required to satisfy the following conditions:

wherein, ω is the radius of the beam after being expanded by the beam expanding collimation system composed of the plano-concave lenses 2 and 3, γ is the obscuration ratio, n is the relative refractive index between air and the conical lens, and θ is the vertex angle of the conical lens.

In the present invention, the emitting mechanism uses pulsed CO₂The laser outputs and transmits annular laser beams through beam shaping of the conical lens, and the receiving system uses the optical fiber MEMS hydrophone, so that the energy loss is greatly reduced, and the communication distance is greatly improved; in addition, the two communication parties carry out information interaction according to the laser signals with certain frequency and characteristics, so that the safety and the confidentiality of the information of the two communication parties can be guaranteed, the communication under the open channel environment is realized, and the method has certain practical application value.