Underwater wireless coherent optical communication system and method

Abstract

The present invention discloses an underwater wireless coherent optical communication system and method, the system comprises a transmitting end and a receiving end, wherein the transmitting end comprises a coherent modulation module, an optical frequency doubling module and a beam shaping transmitting module connected in sequence, the coherent modulation module coherently modulates a digital signal to be transmitted onto an infrared optical carrier to form an infrared laser signal, the optical frequency doubling module converts the wavelength of the modulated infrared laser into a blue-green band, and the beam shaping transmitting module is used to transmit and output the converted optical signal after shaping processing; the receiving end comprises an optical signal receiving module, a coherent detection module and an information demodulation module connected in sequence, the optical signal receiving module receives an optical signal of the blue-green band transmitted underwater, the coherent detection module converts the received optical signal into an electrical signal, and the information demodulation module demodulates the electrical signal to obtain the required communication information. The present invention has the advantages of simple and compact structure, high receiving sensitivity and communication rate, and long communication distance.

Description

Technical Field

[0001] The present invention relates to the technical field of underwater wireless optical communication, and in particular to an underwater wireless coherent optical communication system and method. Background Art

[0002] There is an increasing demand for underwater communications, and underwater acoustic communication is the main solution for underwater communications. However, there are problems such as low speed and poor confidentiality, which severely limits the application of underwater acoustic communication. At present, underwater wireless optical communication usually uses blue-green visible light for communication, which has the advantages of good directionality, low latency, safety and reliability. In the future, it can be applied to various scenarios such as underwater wireless sensors and networks, marine resource exploration, and monitoring. However, due to the influence of seawater absorption and scattering, the transmission distance of blue-green visible light is short, generally only tens of meters to hundreds of meters. In addition, the effective optical communication rate is also low, only about 100Mbps.

[0003] An effective solution to the problems of optical communication transmission distance and communication rate is to apply coherent optical communication to the optical communication system to improve the optical communication receiving sensitivity, extend the optical transmission distance and increase the optical transmission rate. However, the response band of the phase modulator currently used for coherent optical modulation is mainly the long-wave band above 650nm, which cannot solve the problems of low blue-green visible light receiving sensitivity, short transmission distance and low communication rate in underwater wireless optical communication. Summary of the invention

[0004] The technical problem to be solved by the present invention is: in response to the technical problems existing in the prior art, the present invention provides an underwater wireless coherent optical communication system and method with a simple and compact structure, high receiving sensitivity and communication rate, and long communication distance, which can effectively improve the receiving sensitivity and communication rate of underwater wireless optical communication, and at the same time increase the communication distance.

[0005] In order to solve the above technical problems, the technical solution proposed by the present invention is:

[0006] An underwater wireless coherent optical communication system comprises a transmitting end, wherein the transmitting end comprises a coherent modulation module, an optical frequency doubling module and a beam shaping transmitting module which are connected in sequence, wherein the coherent modulation module is used to coherently modulate a digital signal to be transmitted onto an infrared optical carrier to form an infrared laser signal, the optical frequency doubling module is used to convert the wavelength of the modulated infrared laser signal into a blue-green band, and the beam shaping transmitting module is used to shape the converted optical signal and then transmit it for output.

[0007] Furthermore, the coherent modulation module includes a phase modulator and a laser and a microwave amplifier respectively connected to the phase modulator. The input end of the microwave amplifier is connected to the digital signal to be transmitted, and the output end outputs the amplified digital signal. The phase modulator is respectively connected to the laser emitted by the laser and the amplified digital signal output by the microwave amplifier. The digital signal to be transmitted is modulated to the phase of the laser emitted by the laser through the phase modulator, and the phase-modulated optical signal is output.

[0008] Furthermore, the optical frequency doubling module includes an optical signal processing unit and an optical wavelength conversion unit that are interconnected. The optical signal processing unit is used to power amplify and collimate the modulated optical signal output by the coherent modulation module and then output it. The optical wavelength conversion unit is used to convert the wavelength of the collimated optical signal to the blue-green band and then output it. The optical wavelength conversion unit includes a PPLN frequency doubling crystal. The PPLN frequency doubling crystal is installed in a temperature control device. The waveguide direction of the PPLN frequency doubling crystal is on the same optical axis as the incident direction of the input optical signal. The wavelength of the modulated infrared laser signal is converted to the blue-green band through the PPLN frequency doubling crystal.

[0009] Furthermore, the optical signal processing unit includes an interconnected optical fiber amplifier and an optical fiber collimator. The optical fiber amplifier receives the modulated optical signal output by the coherent modulation module for power amplification, and sends it into the optical fiber collimator through the polarization-maintaining optical fiber. The optical fiber collimator collimates the optical signal in the optical fiber and emits it in the form of free space light.

[0010] Furthermore, it also includes a receiving end for receiving and demodulating optical signals in the blue-green band transmitted underwater, the receiving end includes an optical signal receiving module, a coherent detection module and an information demodulation module connected in sequence, the optical signal receiving module is used to receive optical signals in the blue-green band transmitted underwater, the coherent detection module is used to convert the received optical signal into an electrical signal, and the information demodulation module is used to demodulate the electrical signal to obtain the required communication information.

[0011] Furthermore, the coherent detection module includes a 90° optical mixer unit, a balanced detector unit, a multiplier unit and a loop filter connected in sequence, the output end of the loop filter is connected to the input end of the local oscillator laser, the input end of the 90° optical mixer unit is respectively connected to the optical signal received by the optical signal receiving module and the local oscillator light generated by the local oscillator laser, and after 90° coherent mixing, four optical signals with relative phase differences of 0°, 180°, 90° and 270° are output, namely, two optical signals of 0° and 180°, and two optical signals of 90° and 270°. After the two 70° lights are detected, received and amplified by the two balanced detectors in the balanced detector unit, an in-phase branch and an orthogonal branch are formed to output an in-phase component and an orthogonal component respectively. The orthogonal component output by the orthogonal branch and the in-phase component output by part of the in-phase branch are multiplied by the multiplier unit to obtain a phase-locked signal. The in-phase components output by part of the in-phase branch are output to the information demodulation module for information demodulation. The phase-locked signal is filtered out by the loop filter to remove the high-frequency signal in the phase-locked signal and then fed back to the local oscillator laser.

[0012] An underwater wireless coherent optical communication method, comprising the steps of:

[0013] At the transmitting end, when a digital signal needs to be transmitted, the digital signal to be transmitted is coherently modulated onto an infrared optical carrier to form an infrared laser signal, and then the wavelength of the modulated infrared laser signal is converted to the blue-green band based on the optical nonlinear frequency doubling method, and then the converted optical signal is shaped and transmitted for output;

[0014] At the receiving end, when the blue-green band optical signal transmitted underwater is received, the homodyne coherent optical detection method is used to convert the received optical signal into an electrical signal, and then the electrical signal is demodulated to obtain the required communication information.

[0015] Furthermore, the step of coherently modulating the digital signal to be transmitted onto an infrared optical carrier to form an infrared laser signal includes:

[0016] Send the light emitted by the laser to the input end of the polarization controller through the optical fiber, adjust the polarization control so that the polarization angle is consistent with the phase modulator, and send the output signal of the polarization controller to the modulated light input end of the phase modulator through the optical fiber;

[0017] The phase modulator receives the digital signal to be transmitted and the optical signal sent through the optical fiber, modulates the digital signal to be transmitted to the phase of the laser emitted by the laser through phase modulation, and outputs the phase-modulated optical signal.

[0018] Furthermore, the step of converting the wavelength of the modulated infrared laser signal to the blue-green band based on the optical nonlinear frequency doubling method includes:

[0019] Amplifying the optical power of the phase-modulated optical signal through an optical fiber amplifier;

[0020] The optical signal after optical power amplification is collimated and then emitted as free space light;

[0021] The collimated output optical signal is incident on a PPLN frequency doubling crystal, which is installed in a temperature control device, and the waveguide direction and the incident direction of the light are on the same optical axis. The PPLN frequency doubling crystal converts the incident optical signal into an optical signal with a wavelength in the blue-green band for output.

[0022] Furthermore, the step of converting the received optical signal into an electrical signal using a homodyne coherent optical detection method comprises:

[0023] The blue-green band optical signal is received after underwater transmission, and is incident vertically on a 90° optical mixer with a local oscillator laser of the same band for coherent mixing, and four optical signals with relative phase differences of 0°, 180°, 90° and 270° are output;

[0024] The two paths of light at 0° and 180°, and the two paths of light at 90° and 270° are detected, received and amplified respectively, forming an in-phase branch and an orthogonal branch to output an in-phase component and an orthogonal component respectively;

[0025] The orthogonal component output by the orthogonal branch is multiplied by the in-phase component of the partial in-phase branch to obtain a phase detection signal, and the in-phase component output of the partial in-phase branch is demodulated to obtain the required communication information;

[0026] The phase-comparison signal is amplified, the high-frequency signal is filtered out, and then fed back to the local oscillation laser.

[0027] Compared with the prior art, the advantages of the present invention are:

[0028] 1. The present invention performs coherent modulation on the digital signal to be transmitted at the transmitting end, and then converts the wavelength of the coherently modulated infrared laser signal based on optical nonlinear frequency doubling to generate a coherently modulated blue-green laser signal. The coherent modulation method based on optical nonlinear frequency doubling is used to achieve coherent modulation of the blue-green laser, so that the blue-green laser can be coherently modulated without a blue-green band phase modulator, thereby solving the problem that the traditional coherent modulation method cannot achieve the blue-green laser band, thereby using coherent modulation to improve the receiving sensitivity, transmission distance and communication rate.

[0029] 2. The present invention uses a homodyne coherent optical detection method to implement coherent detection when receiving an optical signal in the blue-green band at the receiving end, which can effectively improve the ultimate sensitivity of the reception, thereby increasing the distance and communication rate of the coherent communication system. The electrical signal output based on the homodyne detection method is a baseband signal, which does not require secondary demodulation, and can further reduce the complexity of demodulation.

[0030] 3. The present invention further realizes phase locking by using a Costas phase-locked loop at the receiving end, so that the phase-locked loop has low requirements on the line width of the laser. After multiplying the in-phase and quadrature electrical signals, the phase error signal can be directly obtained. After filtering by a low-pass filter, the signal can be used as the phase-locked control signal of the local oscillator laser. The phase-locked loop can eliminate the mutual crosstalk between the data signal and the phase-locked signal, effectively reducing the phase-locked residual error. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a schematic diagram of the structure of the transmitter in the underwater wireless coherent optical communication system of this embodiment.

[0032] Figure 2 It is a schematic diagram of the specific structure of the transmitting end in a specific cited embodiment of the present invention.

[0033] Figure 3 It is a schematic diagram of the structure of the receiving end in the underwater wireless coherent optical communication system of this embodiment.

[0034] Figure 4 It is a schematic diagram of the specific structure of the receiving end in a specific reference embodiment of the present invention.

[0035] Legend: 1. Transmitter; 101. Coherent modulation module; 111. Phase modulator; 112. Laser; 113. Microwave amplifier; 102. Optical frequency doubling module; 103. Beam shaping transmission module; 2. Receiver; 201. Optical signal receiving module; 202. Coherent detection module; 221. 90° optical mixer unit; 222. Balanced detector unit; 223. Multiplier unit; 224. Loop filter; 203. Information demodulation module. DETAILED DESCRIPTION

[0036] The present invention is further described below in conjunction with the accompanying drawings and specific preferred embodiments, but the protection scope of the present invention is not limited thereby.

[0037] like Figure 1 , 2 As shown, the underwater wireless coherent optical communication system of this embodiment includes:

[0038] The transmitting end 1 is used to convert the digital signal to be transmitted into the blue-green band for transmission, and includes a coherent modulation module 101, an optical frequency doubling module 102 and a beam shaping transmitting module 103 connected in sequence. The coherent modulation module 101 is used to coherently modulate the digital signal to be transmitted onto an infrared optical carrier to form an infrared laser signal. The optical frequency doubling module 102 is used to convert the wavelength of the modulated infrared laser signal into the blue-green band. The beam shaping transmitting module 103 is used to shape the converted optical signal and then transmit it for output;

[0039] The receiving end 2 is used to receive the blue-green band optical signal transmitted underwater and demodulate it, and includes an optical signal receiving module 201, a coherent detection module 202 and an information demodulation module 203 connected in sequence. The optical signal receiving module 201 is used to receive the blue-green band optical signal transmitted underwater, the coherent detection module 202 is used to convert the received optical signal into an electrical signal, and the information demodulation module 203 is used to demodulate the electrical signal to obtain the required communication information.

[0040] In this embodiment, a coherent modulation module 101, an optical frequency doubling module 102 and a beam shaping transmission module 103 are arranged at the transmitting end 1, and the digital signal to be transmitted is first coherently modulated, and then the wavelength of the coherently modulated infrared laser signal is converted based on the optical nonlinear frequency doubling method to generate a coherently modulated blue-green laser signal, and the coherent modulation of the blue-green laser is realized based on the coherent modulation method of optical nonlinear frequency doubling, so that the blue-green laser can be coherently modulated without a blue-green band phase modulator, so that the coherent modulation can be used to improve the receiving sensitivity, transmission distance and communication rate. Further, in this embodiment, after receiving the optical signal in the blue-green band, the homodyne coherent optical detection method is used to realize coherent detection at the receiving end 2, which can effectively improve the limit sensitivity of the reception, thereby improving the distance and communication rate of the coherent communication system, and the electrical signal output based on the homodyne detection method is the baseband signal, which does not require secondary demodulation, and can further reduce the complexity of demodulation.

[0041] like Figure 2 As shown, the coherent modulation module 101 in this embodiment specifically includes a phase modulator 111 and a laser 112 and a microwave amplifier 113 respectively connected to the phase modulator 111. The input end of the microwave amplifier 113 is connected to the digital signal to be transmitted, and the output end outputs the amplified digital signal. The phase modulator 111 is respectively connected to the laser emitted by the laser 112 and the amplified digital signal output by the microwave amplifier 113. The digital signal to be transmitted is modulated to the phase of the laser emitted by the laser 112 through the phase modulator 111, and the phase-modulated optical signal is output.

[0042] In this embodiment, the digital signal to be transmitted is specifically generated by a pseudo-binary sequence generator and then amplified by a microwave amplifier 113. The amplified signal is sent to a phase modulator 111. Specifically, a 1064nm laser with high wavelength stability is used as a carrier of a phase-carrying modulation coding signal, that is, a 1064nm laser is generated by a laser 112 (preferably a distributed feedback laser) and sent to the phase modulator 111 as a carrier. The modulation method can specifically adopt phase shift keying modulation, such as a binary phase shift keying modulation method, by changing the phase sinusoidal input electric field E in Modulation includes phase inversion. out This allows binary information to be modulated onto the laser phase.

[0043] like Figure 2 As shown, in this embodiment, the optical frequency doubling module 102 specifically includes an optical signal processing unit 121 and an optical wavelength conversion unit 122 which are interconnected. The optical signal processing unit 121 is used to amplify the power of the modulated optical signal output by the coherent modulation module 101 and output it after collimation processing. The optical wavelength conversion unit 122 is used to convert the wavelength of the collimated optical signal to the blue-green band before outputting it. The optical signal processing unit 121 specifically includes an optical fiber amplifier 1211 and an optical fiber collimator 1212 which are interconnected. The optical fiber amplifier 1211 accesses the modulated optical signal output by the coherent modulation module 101 to amplify its power, and sends the optical signal to the optical fiber collimator 1212 through the polarization-maintaining optical fiber. The optical fiber collimator 1212 collimates the optical signal in the optical fiber and then emits it in the form of free space light. The optical fiber amplifier 1211 may specifically use an ytterbium-doped optical fiber amplifier to amplify the modulated laser to increase the optical power at the transmitting end, and then convert the high-power laser at the output port of the ytterbium-doped optical fiber amplifier pigtail into collimated light output through the optical fiber collimator 1212. The output high-power, modulated laser is collimated to facilitate long-distance underwater transmission, further increasing the distance of underwater optical communication.

[0044] In this embodiment, the optical wavelength conversion unit 122 includes a PPLN frequency doubling crystal, which is installed in a temperature control device. The waveguide direction of the PPLN frequency doubling crystal is on the same optical axis as the incident direction of the input optical signal. The wavelength of the modulated infrared laser signal is converted to the blue-green band by the PPLN frequency doubling crystal. For example, the input high-power 1064nm laser can be converted into a high-power 532nm laser by using the PPLN crystal. In this example, by using the frequency doubling crystal to convert the wavelength of the coherently modulated infrared laser to generate the coherently modulated blue-green laser, the coherent modulation of the blue-green laser can be achieved without a blue-green band phase modulator, which effectively solves the problem that the traditional phase modulation cannot be applied to the blue-green band.

[0045] like Figure 3As shown, in this embodiment, the optical signal receiving module 201 is specifically an optical receiving antenna. The receiving end 2 uses the optical receiving antenna to collect the divergent and weak signal light, and the local oscillator laser generates the local oscillator light. The frequency ω2 of the laser is kept the same as the signal light frequency ω1. The signal light collected by the optical receiving antenna and the local oscillator light generated by the local oscillator laser are input into the coherent detection module for coherent detection. The coherent detection module 202 specifically includes a 90° optical mixer unit 221, a balanced detector unit 222, a multiplier unit 223 and a loop filter 224 connected in sequence. The output end of the loop filter 224 is connected to the input end of the local oscillator laser. The input end of the 90° optical mixer 221 is respectively connected to the optical signal received by the optical signal receiving module 201 and the local oscillator light generated by the local oscillator laser. After 90° coherent mixing, four optical signals with relative phase differences of 0°, 180°, 90° and 270° are output. The two paths of light at 0° and 180°, and the two paths of light at 90° and 270° are detected, received and amplified by two balanced detectors in the balanced detector unit 222, forming an in-phase branch and an orthogonal branch to output an in-phase component and an orthogonal component respectively. The orthogonal component output by the orthogonal branch and the in-phase component output by part of the in-phase branch are multiplied by the multiplier unit 223 to obtain a phase detection signal. The in-phase component output by part of the in-phase branch is output to the information demodulation module 203 for information demodulation. The demodulator is used to demodulate the electrical signal of the in-phase branch to obtain a modulation coding signal. The phase detection signal is fed back to the local oscillator laser after the high-frequency signal in the phase detection signal is filtered out by the loop filter 224. The noise bandwidth can be effectively reduced by selecting appropriate loop filter parameters. The above-mentioned balanced detector unit 222 can specifically adopt a balanced detector based on a photomultiplier tube (PMT).

[0046] like Figure 4 As shown, in a specific application embodiment of the present invention, the coherent detection module 202 at the receiving end 2 realizes coherent light detection based on the zero-difference coherent light detection of the Costas phase-locked loop, and uses a 90° optical mixer to coherently mix the signal light and the local oscillator light so that the relative phase differences of the four output ports are 0°, 90°, 180°, and 270° respectively, and uses two balanced detectors to detect and receive the two light signals of 0° and 180° and the two light signals of 90° and 270° output by the 90° optical mixer respectively, and generates two co-directional and orthogonal branches to convert the optical signal into an electrical signal, wherein the DC components are equal, and two coherent signals are obtained after each subtraction, and the relative phase difference of the two signals is 90°, and the orthogonal branch signal and part of the co-directional branch signal are multiplied, and then amplified and filtered by the loop filter 224 to obtain a signal for driving the local oscillator laser piezoelectric ceramic, and the signal contains a phase difference signal between the local oscillator light and the signal light, and is used to achieve phase locking of the local oscillator light and the signal light. In the case of phase locking, the signal output by the same-direction branch can be used to obtain the transmitted information through a demodulator.

[0047] This embodiment uses a Costas phase-locked loop at the receiving end 2 to achieve phase locking, so that the phase-locked loop has low requirements on the line width of the laser. The phase error signal can be directly obtained by multiplying the in-phase and quadrature electrical signals. The signal can be used as the phase-locked control signal of the local oscillator laser after filtering by a low-pass filter. The phase-locked loop can effectively eliminate the mutual crosstalk between the data signal and the phase-locked signal, so that the phase-locked residual error is low.

[0048] The steps of implementing the underwater wireless coherent optical communication method using the above system in this embodiment include:

[0049] S01. At the transmitting end 1, when a digital signal needs to be transmitted, the digital signal to be transmitted is coherently modulated onto an infrared optical carrier to form an infrared laser signal, and then the wavelength of the modulated infrared laser signal is converted to the blue-green band based on an optical nonlinear frequency doubling method, and then the converted optical signal is shaped and transmitted for output;

[0050] S02. At the receiving end 2, when a blue-green band optical signal transmitted underwater is received, a homodyne coherent optical detection method is used to convert the received optical signal into an electrical signal, and then the electrical signal is demodulated to obtain the required communication information.

[0051] In this embodiment, the step of coherently modulating the digital signal to be transmitted onto the infrared optical carrier to form an infrared laser signal in step S01 includes:

[0052] S101. Send the laser light to the input end of the polarization controller through the optical fiber, adjust the polarization control so that the polarization angle is consistent with the phase modulator, and send the output signal of the polarization controller to the modulated light input end of the phase modulator 111 through the optical fiber;

[0053] S102. The phase modulator 111 receives the digital signal to be transmitted and the optical signal sent through the optical fiber, modulates the digital signal to be transmitted to the phase of the laser emitted by the laser through phase modulation, and outputs the phase-modulated optical signal.

[0054] In a specific application embodiment, when a digital signal needs to be sent, the light emitted by the 1064nm laser is first sent to the input end of the polarization controller through an optical fiber, the polarization control is adjusted so that the polarization angle is consistent with that of the phase modulator, and the output end of the polarization controller is sent to the modulated light input end of the phase modulator 111 through the optical fiber; the optical input end of the phase modulator 111 is connected to the output end of the polarization controller through an optical fiber, and the high-speed digital signal to be transmitted is sent to the signal input end of the microwave amplifier 112 through a coaxial cable, amplified by 20 to 30 dB by the microwave amplifier 112, and the output amplified signal is sent to the modulated signal input port of the phase modulator 111 through a coaxial cable, so as to modulate the digital signal to the phase of the 1064nm laser.

[0055] In step S01 of this embodiment, the step of converting the wavelength of the modulated infrared laser signal to the blue-green band based on the optical nonlinear frequency doubling method includes:

[0056] S111. The phase modulated optical signal is amplified by an optical fiber amplifier;

[0057] S112. After the optical power is amplified, the optical signal is collimated and then emitted as free space light;

[0058] S113. The collimated output optical signal is incident on a PPLN frequency doubling crystal, which is installed in a temperature control device, and the waveguide direction and the incident direction of the light are on the same optical axis. The PPLN frequency doubling crystal converts the incident optical signal into an optical signal with a wavelength in the blue-green band for output.

[0059] In this embodiment, the step S02 of converting the received optical signal into an electrical signal using a homodyne coherent optical detection method includes:

[0060] S201. Receive the blue-green band optical signal after underwater transmission, and vertically incident on the 90° optical mixer with the local oscillator laser of the same band for coherent mixing, and output four optical signals with relative phase differences of 0°, 180°, 90° and 270°;

[0061] S202. The 0° and 180° two-way light and the 90° and 270° two-way light are detected, received and amplified to form an in-phase branch and an orthogonal branch to output the in-phase component and the orthogonal component respectively;

[0062] S203. The orthogonal component output by the orthogonal branch is multiplied by the in-phase component of the partial in-phase branch to obtain a phase detection signal, and the in-phase component output of the partial in-phase branch is demodulated to obtain the required communication information;

[0063] S204. Amplify the phase-locked signal, filter out the high-frequency signal, and then feed it back to the local oscillator laser.

[0064] The present invention is further described below by taking the method of the present invention as an example to realize underwater wireless coherent optical communication in a specific application embodiment. Figure 2 , 4As shown, this embodiment constructs an underwater wireless coherent optical communication system, including a 1064nm single longitudinal mode laser with high wavelength stability, a pseudo-binary sequence generator, a polarization controller, a 1064nm band phase modulator, a microwave amplifier, an ytterbium-doped fiber amplifier, a fiber collimator, a temperature controller, a heating and fixing device and a PPLN crystal that responds to 1064nm, an optical receiving antenna, a 532nm single longitudinal mode laser with high wavelength stability, a 90° optical mixer, two balanced detectors based on photomultiplier tubes (PMTs), a demodulator, a multiplier and a loop filter. After the 1064nm laser is modulated by the infrared band phase modulator, its phase carries the transmission information. The modulated infrared light enters the PPLN crystal through the fiber amplifier and the fiber collimator to achieve frequency doubling to generate a high-power 532nm laser whose phase carries the transmission information.

[0065] At the transmitting end 1, the digital signal to be transmitted is generated by a pseudo-binary sequence generator, and then amplified by a microwave amplifier 112, and the digital signal to be transmitted is converted into a modulation signal V required by a phase modulator through digital signal processing. in The laser generates a 1064nm laser with high wavelength stability, which is used as a carrier that carries the phase modulation coding signal (the laser is preferably a distributed feedback laser, which is easy to achieve stable single-mode operation); the phase modulator is used to modulate the modulation coding signal to the phase of the 1064nm laser. The signal modulation adopts a binary phase shift keying modulation scheme to convert the phase sinusoidally changing input electric field E in Modulation includes phase inversion. out To achieve the modulation of binary information onto the laser phase; the ytterbium-doped fiber amplifier is used to amplify the modulated 1064nm laser power to increase the optical power at the transmitting end; the fiber collimator is used to convert the high-power laser at the output port of the ytterbium-doped fiber amplifier pigtail into collimated light output so that it can be focused into the nonlinear crystal; the PPLN crystal is used to convert the input high-power 1064nm laser into a high-power 532nm laser. In order to achieve the best optical conversion efficiency, the temperature of the PPLN and the degree of convergence of the input light need to be optimized; the beam shaper is used to collimate the output high-power, modulated 532nm laser for long-distance underwater transmission.

[0066] At the receiving end 2, an optical receiving antenna is used to collect the divergent and weak signal light; a local oscillator laser is used to generate local oscillator light, and the frequency w2 of the laser is kept the same as the signal light frequency w1 to achieve homodyne detection; a 90° optical mixer is used to coherently mix the signal light and the local oscillator light, so that the relative phase differences of the four output ports are 0°, 90°, 180°, and 270° respectively; a balanced detector is used to convert the 0° and 180° two-way light and the 90° and 270° two-way light signals output by the 90° optical mixer into telecommunication signals. The two signals have the same DC components, and two coherent signals are obtained by subtracting each other, and the relative phase difference of the two signals is 90°; the multiplier is used to multiply the signals of the in-phase channel and the orthogonal channel to obtain an intermediate frequency signal, which contains a phase difference signal between the local oscillator light and the signal light; the loop filter is used to filter out the intermediate frequency component and high-frequency noise in the intermediate frequency signal to obtain a phase difference signal between the local oscillator light and the signal light, and selecting appropriate loop filter parameters can reduce the noise bandwidth; the demodulator is used to demodulate the electrical signal of the same-direction branch to obtain a modulated coded signal.

[0067] Based on the above underwater wireless coherent optical communication system, the detailed steps of implementing underwater wireless coherent optical communication in this embodiment are as follows:

[0068] Step 1: Underwater optical signal transmission and coherent modulation

[0069] Step 1.1: The light emitted by the 1064nm laser is sent to the input end of the polarization controller through the optical fiber, and the polarization control is adjusted so that the polarization angle is consistent with that of the phase modulator. The output end of the polarization controller is sent to the modulated light input end of the phase modulator through the optical fiber;

[0070] Step 1.2: The optical input end of the phase modulator is connected to the output end of the polarization controller through an optical fiber. The high-speed digital signal to be transmitted is sent to the signal input end of the microwave amplifier through a coaxial cable, amplified by 20 to 30 dB by the microwave amplifier, and the output amplified signal is sent to the modulation signal input port of the phase modulator through a coaxial cable to modulate the digital signal to the phase of the 1064nm laser.

[0071] Step 2: Optical frequency doubling

[0072] Step 2.1: The phase-modulated optical signal is sent to the Yb-doped fiber amplifier through the polarization-maintaining fiber for optical power amplification, and then sent to the fiber collimator through the polarization-maintaining fiber. The fiber collimator collimates the optical signal in the fiber and emits it as free-space light;

[0073] Step 2.2: The collimated 1064nm optical signal is incident on the PPLN crystal. The PPLN crystal is installed in a heating and fixing device, and the waveguide direction and the incident direction of the light are on the same optical axis. To achieve a higher conversion efficiency, the PPLN crystal should reach the specific temperature required for wavelength conversion under the control of the temperature controller. After passing through the PPLN crystal, the 1064nm optical signal will be converted into a 532nm green light signal output. At this time, the green light phase still carries the digital signal that needs to be transmitted.

[0074] Step 3: Underwater optical signal reception and coherent light detection

[0075] Step 3.1: After underwater transmission, the 532nm signal light is converged and collimated by the optical receiving antenna, and is vertically incident on the 90° optical mixer with the local oscillator laser of the same wavelength of 532nm for coherent mixing, and outputs four optical signals with relative phase differences of 0°, 180°, 90° and 270°. The two optical signals of 0° and 180° and the two optical signals of 90° and 270° are detected, received and amplified by two balanced detectors respectively, forming an in-phase branch and an orthogonal branch;

[0076] Step 3.2: The electrical signal of the orthogonal branch is multiplied by the electrical signal of part of the in-phase branch through a multiplier to obtain a phase detection signal. The phase detection signal is amplified and then filtered out by a loop filter to remove the high-frequency signal. The signal output by the filter, i.e., the phase difference between the local oscillator light and the signal light, is driven by a driving circuit to drive the piezoelectric ceramic on the local oscillator laser. At the same time, the frequency of the local oscillator laser is adjusted to quickly bring the frequencies of the signal light and the local oscillator laser closer, thereby achieving stable frequency and phase locking between the local oscillator light and the signal light.

[0077] Step 3.3: In the case of phase locking, the electrical signal output by some in-phase branches is demodulated by the demodulator to obtain the transmitted digital signal.

[0078] Compared with the intensity modulation / direct detection scheme widely used in traditional underwater wireless optical communication systems, the above-mentioned coherent optical communication scheme of the present invention can effectively improve the sensitivity of the receiver by 20dB, as well as improve the transmission distance and communication rate of the system.

[0079] The above is only a preferred embodiment of the present invention, and does not limit the present invention in any form. Although the present invention has been disclosed as a preferred embodiment, it is not intended to limit the present invention. Therefore, any simple modification, equivalent change and modification made to the above embodiment according to the technical essence of the present invention without departing from the content of the technical solution of the present invention shall fall within the scope of protection of the technical solution of the present invention.

Claims

1. An underwater wireless coherent optical communication system, comprising a transmitting end (1), characterized in that: The transmitting end (1) comprises a coherent modulation module (101), an optical frequency doubling module (102) and a beam shaping transmitting module (103) which are connected in sequence, wherein the coherent modulation module (101) is used to coherently modulate a digital signal to be transmitted onto an infrared optical carrier to form an infrared laser signal, the optical frequency doubling module (102) is used to convert the wavelength of the modulated infrared laser signal into a blue-green band, and the beam shaping transmitting module (103) is used to shape the converted optical signal and then transmit it for output; At the transmitting end, when a digital signal needs to be transmitted, the digital signal to be transmitted is coherently modulated onto an infrared optical carrier to form an infrared laser signal, and then the wavelength of the modulated infrared laser signal is converted to the blue-green band based on the optical nonlinear frequency doubling method, and then the converted optical signal is shaped and transmitted for output; The method of converting the wavelength of the modulated infrared laser signal to the blue-green band based on the optical nonlinear frequency doubling method comprises: Amplifying the optical power of the phase-modulated optical signal through an optical fiber amplifier; After the optical power is amplified, the optical signal is collimated and then emitted as free-space light; The collimated output optical signal is incident on the PPLN frequency doubling crystal, which is installed in the temperature control device, and the waveguide direction and the incident direction of the light are on the same optical axis. The PPLN frequency doubling crystal converts the incident optical signal into an optical signal with a wavelength in the blue-green band for output; At the receiving end, when the blue-green band optical signal transmitted underwater is received, the received optical signal is converted into an electrical signal, and then the electrical signal is demodulated to obtain the required communication information.

2. The underwater wireless coherent optical communication system according to claim 1, characterized in that: The coherent modulation module (101) comprises a phase modulator (111), and a laser (112) and a microwave amplifier (113) respectively connected to the phase modulator (111); the input end of the microwave amplifier (113) is connected to a digital signal to be transmitted, and the output end outputs an amplified digital signal; the phase modulator (111) is respectively connected to the laser light emitted by the laser (112) and the amplified digital signal output by the microwave amplifier (113); the digital signal to be transmitted is modulated by the phase modulator (111) onto the phase of the laser light emitted by the laser (112), and a phase-modulated optical signal is output.

3. The underwater wireless coherent optical communication system according to claim 2, characterized in that: The optical frequency doubling module (102) comprises an optical signal processing unit (121) and an optical wavelength conversion unit (122) which are connected to each other. The optical signal processing unit (121) is used to amplify the power of the modulated optical signal output by the coherent modulation module (101) and perform collimation processing before outputting the signal. The optical wavelength conversion unit (122) is used to convert the wavelength of the collimated optical signal to a blue-green band before outputting the signal. The optical wavelength conversion unit (122) comprises a periodically poled lithium niobate PPLN frequency doubling crystal. The PPLN frequency doubling crystal is installed in a temperature control device. The waveguide direction of the PPLN frequency doubling crystal and the incident direction of the input optical signal are on the same optical axis. The wavelength of the modulated infrared laser is converted to the blue-green band through the PPLN frequency doubling crystal.

4. The underwater wireless coherent optical communication system according to claim 3, characterized in that: The optical signal processing unit (121) comprises an optical fiber amplifier (1211) and an optical fiber collimator (1212) which are connected to each other. The optical fiber amplifier (1211) receives the modulated optical signal output by the coherent modulation module (101) for power amplification and transmits the modulated optical signal to the optical fiber collimator (1212) through a polarization-maintaining optical fiber. The optical fiber collimator (1212) collimates the optical signal in the optical fiber and then emits the optical signal in the form of free space light.

5. The underwater wireless coherent optical communication system according to any one of claims 1 to 4, characterized in that: The invention also comprises a receiving end (2) for receiving and demodulating an optical signal in the blue-green band transmitted underwater, wherein the receiving end (2) comprises an optical signal receiving module (201), a coherent detection module (202) and an information demodulation module (203) connected in sequence, wherein the optical signal receiving module (201) is used to receive an optical signal in the blue-green band transmitted underwater, the coherent detection module (202) is used to convert the received optical signal into an electrical signal, and the information demodulation module (203) is used to demodulate the electrical signal to obtain the required communication information.

6. The underwater wireless coherent optical communication system according to claim 5, characterized in that: The coherent detection module (202) comprises a 90° optical mixer unit (221), a balanced detector unit (222), a multiplier unit (223) and a loop filter (224) which are connected in sequence, the output end of the loop filter (224) being connected to the input end of a local oscillator laser, the input end of the 90° optical mixer unit (221) being respectively connected to the optical signal received by the optical signal receiving module (201) and the local oscillator light generated by the local oscillator laser, and after 90° coherent mixing, four optical signals with relative phase differences of 0°, 180°, 90° and 270° are output, the two optical signals of 0° and 180° being respectively connected to the optical signal received by the optical signal receiving module (201) and the local oscillator light generated by the local oscillator laser. The two paths of light, 90° and 270°, are detected, received and amplified by two balanced detectors in the balanced detector unit (222) to form an in-phase branch and an orthogonal branch to output an in-phase component and an orthogonal component respectively. The orthogonal component output by the orthogonal branch and the in-phase component output by part of the in-phase branch are multiplied by the multiplier unit (223) to obtain a phase detection signal. The in-phase components output by part of the in-phase branch are output to the information demodulation module (203) for information demodulation. The phase detection signal is filtered by the loop filter (224) to remove the high-frequency signal in the phase detection signal and then fed back to the local oscillation laser.

7. An underwater wireless coherent optical communication method, characterized in that the steps include: At the transmitting end, when a digital signal needs to be transmitted, the digital signal to be transmitted is coherently modulated onto an infrared optical carrier to form an infrared laser signal, and then the wavelength of the modulated infrared laser signal is converted to a blue-green band based on an optical nonlinear frequency doubling method, and then the converted optical signal is shaped and then transmitted and output; the wavelength conversion of the modulated infrared laser signal to the blue-green band based on the optical nonlinear frequency doubling method includes: Amplifying the optical power of the phase-modulated optical signal through an optical fiber amplifier; After the optical power is amplified, the optical signal is collimated and then emitted as free-space light; The collimated output optical signal is incident on the PPLN frequency doubling crystal, which is installed in the temperature control device, and the waveguide direction and the incident direction of the light are on the same optical axis. The PPLN frequency doubling crystal converts the incident optical signal into an optical signal with a wavelength in the blue-green band for output; At the receiving end, when the blue-green band optical signal transmitted underwater is received, the homodyne coherent optical detection method is used to convert the received optical signal into an electrical signal, and then the electrical signal is demodulated to obtain the required communication information.

8. The underwater wireless coherent optical communication method according to claim 7, characterized in that: The step of coherently modulating the digital signal to be transmitted onto an infrared optical carrier to form an infrared laser signal comprises: Sending the light emitted by the laser to the input end of the polarization controller through the optical fiber, adjusting the polarization control so that the polarization angle is consistent with that of the phase modulator, and sending the output signal of the polarization controller to the modulated light input end of the phase modulator (111) through the optical fiber; The phase modulator (111) receives the digital signal to be transmitted and the optical signal sent through the optical fiber, modulates the digital signal to be transmitted to the phase of the laser emitted by the laser through phase modulation, and outputs the phase-modulated optical signal.

9. The underwater wireless coherent optical communication method according to claim 7, characterized in that: The step of converting the wavelength of the modulated infrared laser signal to the blue-green band based on the optical nonlinear frequency doubling method comprises: Amplifying the optical power of the phase-modulated optical signal through an optical fiber amplifier; After the optical power is amplified, the optical signal is collimated and then emitted as free-space light; The collimated output optical signal is incident on a PPLN frequency doubling crystal, which is installed in a temperature control device, and the waveguide direction and the incident direction of the light are on the same optical axis. The PPLN frequency doubling crystal converts the incident optical signal into an optical signal with a wavelength in the blue-green band for output.

10. The underwater wireless coherent optical communication method according to any one of claims 7 to 9, characterized in that: The step of converting the received optical signal into an electrical signal by adopting the homodyne coherent optical detection method comprises: The blue-green band optical signal is received after underwater transmission, and is incident vertically on a 90° optical mixer with a local oscillator laser of the same band for coherent mixing, and four optical signals with relative phase differences of 0°, 180°, 90° and 270° are output; The two paths of light at 0° and 180°, and the two paths of light at 90° and 270° are detected, received and amplified respectively, forming an in-phase branch and an orthogonal branch to output an in-phase component and an orthogonal component respectively; The orthogonal component output by the orthogonal branch is multiplied by the in-phase component of the partial in-phase branch to obtain a phase detection signal, and the in-phase component output of the partial in-phase branch is demodulated to obtain the required communication information; The phase-comparison signal is amplified, the high-frequency signal is filtered out, and then fed back to the local oscillation laser.

Images