A modulation method of optical signal and its transmitting system

Abstract

The present invention relates to a modulation method of an optical signal and a transmitting system thereof. The modulation method of the optical signal includes: obtaining a periodic laser pulse and information to be sent; encoding the information to be sent through a programmable integrated circuit to obtain loading information, and extracting clock information; delaying the loading information according to the clock information to make the loading information and the periodic laser pulse roughly synchronized; delaying the periodic laser pulse to make the loading information and the periodic laser pulse precisely synchronized; modulating the periodic laser pulse according to the loading information, and transmitting the modulated optical signal. The embodiment of the present invention can achieve high-precision synchronization by delaying the electrical signal and the laser pulse respectively, thereby realizing the output of a modulated signal with a higher power short pulse, thereby improving the quality of underwater communication.

Description

Technical Field

[0001] The embodiments of the present invention relate to the technical field of underwater wireless optical communication, and in particular to a modulation method of an optical signal and a transmitting system thereof. Background Art

[0002] Compared with traditional underwater acoustic, radio frequency and other communication methods, wireless laser communication has outstanding advantages such as high communication rate, low volume and power consumption, light weight and low delay. However, due to the complex underwater environment and the influence of absorption and scattering of seawater medium, the limited communication distance has always restricted the development and application of underwater wireless optical communication. Laser amplification technology can increase the underwater output power, but because the power of the continuous laser itself is very low, although the average power of the laser after amplification technology has been improved, it still cannot meet the needs of underwater long-distance communication. Pulse lasers can not only provide high pulse energy, high average power and high peak power, but also reduce the influence of absorption and scattering of particles in the seawater medium on the laser pulses in underwater laser communication during transmission, and the output response signal can still be observed under large path loss. The underwater communication transmission system with pulse laser as the transmitting light source is the development trend of underwater long-distance communication in the future.

[0003] In the related technology, in 2006, Kong et al. from the Shanghai Institute of Optics and Precision Mechanics used a continuous single-frequency laser as a seed source, and used a 4-meter-long ytterbium-doped large-mode-field double-clad optical fiber to achieve a peak power of 300 kW at a 15 ns pulse duration at 20 kHz at a wavelength of 1064 nm and a repetition frequency of 100 kHz. In 2018, the Wuhan Post and Telecommunications Science Research Institute used a semiconductor laser as a seed source to construct a two-stage optical amplification system, and obtained a pulsed laser output with a central wavelength of 1064 nm, a peak power of 10.4 kW, and a maximum average power of 21.3 W. All of the above are unmodulated periodic optical pulse outputs. To achieve communication, it is necessary to load information onto the pulsed laser. Ensuring that the information signal can be loaded onto the pulsed laser is a prerequisite for achieving high-speed and long-distance underwater communication.

[0004] Regarding the above technical solution, the inventors found that there are at least the following technical problems:

[0005] In the process of encoding and processing information, due to the response speed of the FPGA (Field Programmable Gate Array) device and the internal connection of the device, there will be a certain delay in the output of the information signal. The delay caused by these transmission processes will cause the information signal and the laser to be unable to enter the modulator synchronously. The waveform after modulation will have a difference frequency or asynchrony phenomenon, and the information cannot be fully loaded onto the pulsed laser, resulting in errors in the final transmitted data. Therefore, the information signal and the carrier signal need to be synchronized before modulation.

[0006] Therefore, it is necessary to improve one or more problems existing in the above-mentioned related technical solutions.

[0007] It should be noted that this section is intended to provide background or context for the technical solutions of the present invention stated in the claims. The description herein is not admitted to be prior art by inclusion in this section. Summary of the invention

[0008] The object of the present invention is to provide a modulation method of an optical signal and a transmission system thereof, thereby overcoming one or more problems caused by limitations and defects of the related art at least to a certain extent.

[0009] According to a first aspect of the present invention, there is provided a method for modulating an optical signal, comprising:

[0010] Obtain periodic laser pulses and information to be sent;

[0011] Encoding the information to be sent by a programmable integrated circuit to obtain loading information, and extracting clock information;

[0012] Delaying the loading information according to the clock information so as to roughly synchronize the loading information with the periodic laser pulse;

[0013] Delaying the periodic laser pulses to achieve precise synchronization between the loading information and the periodic laser pulses;

[0014] The periodic laser pulse is modulated according to the loading information, and a modulated optical signal is emitted.

[0015] In the present invention, the information to be sent is encoded by a programmable integrated circuit to obtain loading information, and clock information is extracted, including:

[0016] The information to be sent is encoded by a programmable integrated circuit using a low-density parity check code to obtain loading information.

[0017] In the present invention, the information to be sent is encoded by a programmable integrated circuit to obtain loading information, and clock information is extracted, including:

[0018] The clock information output by the D flip-flop is extracted.

[0019] In the present invention, the periodic laser pulse is delayed so that the loading information and the periodic laser pulse are precisely synchronized, including:

[0020] The periodic laser pulse is delayed by changing the transmission distance of the periodic laser pulse before modulation.

[0021] In the present invention, the loading information is delayed according to the clock information so that the loading information and the periodic laser pulse are roughly synchronized, including:

[0022] The time difference between the loading information and the periodic laser pulse is measured, and the time range for delaying the loading information is between the time difference plus 1 ns and the time difference.

[0023] In the present invention, the periodic laser pulse is delayed so that the loading information and the periodic laser pulse are precisely synchronized, including:

[0024] The time for delaying the periodic laser pulse is the difference between the time for delaying the loading information and the time difference.

[0025] The present invention also includes: energy amplification of the modulated optical signal by stimulated radiation.

[0026] The present invention also includes: transmitting the modulated optical signal after frequency doubling.

[0027] The present invention also includes: emitting the modulated optical signal underwater and transmitting it to the detector, and after the detector receives the optical signal, demodulating and decoding the optical signal to obtain the information.

[0028] According to a second aspect of the present invention, a transmission system based on any one of the above optical signal modulation methods comprises:

[0029] A pulse light source, used to obtain periodic laser pulses;

[0030] A programmable integrated circuit, used for receiving information to be sent and encoding it to obtain loading information, and extracting clock information;

[0031] An electrical delay module, used for delaying the loading information according to the clock information, so as to achieve coarse synchronization between the loading information and the periodic laser pulse;

[0032] An optical delay module, used for delaying the periodic laser pulses, so as to achieve precise synchronization between the loading information and the periodic laser pulses;

[0033] The modulation module is used to modulate the periodic laser pulse according to the loading information and emit the modulated optical signal.

[0034] The technical solution provided by the present invention may include the following beneficial effects:

[0035] In the present invention, through the above method and system, the electrical signal and the laser pulse are delayed respectively to achieve high-precision synchronization, thereby realizing the output of a higher-power short-pulse modulated signal and improving the quality of underwater communication. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The accompanying drawings herein are incorporated into the specification and constitute a part of the specification, illustrate embodiments consistent with the present disclosure, and together with the specification are used to explain the principles of the present disclosure. Obviously, the accompanying drawings described below are only some embodiments of the present disclosure, and for ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without creative work.

[0037] Figure 1 A schematic flow chart of a method for modulating an optical signal in an exemplary embodiment of the present invention is shown;

[0038] Figure 2 A schematic diagram showing an association of a modulation method of an optical signal in an exemplary embodiment of the present invention;

[0039] Figure 3 A schematic diagram of signal synchronization in an exemplary embodiment of the present invention is shown;

[0040] Figure 4 A schematic diagram showing the results of a pulse synchronization experiment in an exemplary embodiment of the present invention is shown. DETAILED DESCRIPTION

[0041] Example embodiments will now be described more fully with reference to the accompanying drawings. However, example embodiments can be implemented in many forms and should not be construed as limited to the examples set forth herein; rather, these embodiments are provided so that the present invention will be more comprehensive and complete and fully convey the concepts of the example embodiments to those skilled in the art. The described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0042] In addition, the accompanying drawings are only schematic illustrations of embodiments of the present invention and are not necessarily drawn to scale. The same reference numerals in the drawings represent the same or similar parts, and their repeated descriptions will be omitted. Some of the block diagrams shown in the accompanying drawings are functional entities and do not necessarily correspond to physically or logically independent entities.

[0043] In this exemplary embodiment, a method for modulating an optical signal is first provided. Figure 1 As shown, the following steps are included:

[0044] Step S101: Acquire periodic laser pulses and information to be sent;

[0045] Step S102: Encoding the information to be sent by a programmable integrated circuit to obtain loading information, and extracting clock information;

[0046] Step S103: delaying the loading information according to the clock information so as to roughly synchronize the loading information with the periodic laser pulse;

[0047] Step S104: delaying the periodic laser pulse to achieve precise synchronization between the loaded information and the periodic laser pulse;

[0048] Step S105: modulate the periodic laser pulse according to the loading information, and emit the modulated optical signal.

[0049] It should be understood that the programmable integrated circuit can be specifically an FPGA device. FPGA emerged as a semi-custom circuit in the field of application-specific integrated circuits, which not only solves the shortcomings of custom circuits, but also overcomes the shortcomings of the limited number of gate circuits of the original programmable device. Periodic laser pulses are also periodic pulsed lasers. Periodic laser pulses use a 1064nm short-pulse laser with high peak power as the emitting light source for periodic pulse output. The pulse laser emits a pulsed laser and simultaneously outputs an electrical signal. The electrical signal enters the FPGA as an external clock, completes signal encoding in the FPGA, and delays the electrical signal to achieve coarse synchronization. As for the periodic optical pulse to be loaded with information, it is delayed through an optical fiber delay line pair to achieve fine synchronization. After ensuring that the two signals can be synchronously input to the optical modulator, the optical modulator is used to load the information onto the 1064nm pulse laser source, and the output power of the pulsed optical signal is amplified by optical amplification technology. Finally, the laser pulse signal is frequency-multiplied to obtain a 532nm green laser for underwater communication, achieving high-power pulse laser output.

[0050] It is also necessary to understand that the pulse laser emits a pulse laser, and an optical coupling device is used to split the pulse light into two paths. One optical pulse extracts the clock information and enters the FPGA as an external clock, and the other is a periodic optical pulse that needs to be loaded with information. The pulse light is finely delayed through the optical fiber delay line to achieve synchronization with the output signal of the FPGA. For the synchronized loaded information and periodic laser pulses, the principle of the acousto-optic effect is used to modulate the optical signal intensity and load the transmitted information onto the laser carrier.

[0051] It is also important to understand that the laser light source uses pulsed laser, which can not only provide greater instantaneous power, but also reduce the influence of absorption and scattering during water transmission, reduce the attenuation loss of optical signals in underwater transmission, and thus increase the underwater communication distance; the dual-mode synchronization technology design can achieve precise synchronization of modulated electrical signals and optical signals, the coarse delay range can include a pulse cycle, and the fine delay accuracy can reach 1ps, which improves the communication quality.

[0052] Through the above optical signal modulation method, the electrical signal and the laser pulse are delayed respectively to achieve high-precision synchronization, thereby realizing the output of a higher-power short-pulse modulation signal, thereby improving the quality of underwater communication.

[0053] Next, we will refer to Figures 1 to 4 Each step of the optical signal modulation method in this exemplary embodiment is described in more detail.

[0054] In one embodiment, reference Figure 2 As shown, step S102 may further include the following steps:

[0055] The information to be transmitted is encoded by a programmable integrated circuit using a low-density parity check code to obtain the loading information. It should be understood that the information sequence is encoded using quasi-cyclic LDPC (Low-density Parity-check) coding to obtain a set of code types that can control the error of the information sequence. Improve the anti-interference ability of digital signals. Then use the scrambling code to disrupt the input binary sequence according to a certain rule to reduce the occurrence of consecutive zeros, thereby reducing inter-code interference and jitter, and facilitating the clock extraction at the receiving end.

[0056] In one embodiment, reference Figure 2 As shown, step S102 may further include the following steps:

[0057] Extract the clock information output by the D flip-flop. It should be understood that the D flip-flop is an information storage device with memory function and two stable states. It is the most basic logic unit that constitutes a variety of sequential circuits and an important unit circuit in digital logic circuits. Therefore, the D flip-flop is widely used in digital systems and computers. The flip-flop has two stable states, namely "0" and "1". Under the action of certain external signals, it can flip from one stable state to another.

[0058] It is also necessary to understand that the programmable integrated circuit uses the principle that there is a delay in the input and output of the D flip-flop, and by adding a combinational logic circuit, the clock f clk When the timing logic changes, the clock output f clk_out The state is flipped, the output of the sequential logic part is used as input, and then flipped again through the combinational logic, so that each time the clock changes, there will be a short pulse change in the clock output.

[0059] f clk_out =2*f clk (1)

[0060] By continuously looping the whole process, the effect of multiple frequency multiplication can be achieved. The maximum clock that the FPGA coarse delay module can run is 1000MHz, and the delay accuracy of coarse synchronization can reach 1ns.

[0061] In one embodiment, reference Figure 2 As shown, step S104 may further include the following steps:

[0062] The periodic laser pulse is delayed by changing the transmission distance before modulation. It should be understood that the resolution of the coarse synchronization is further subdivided to achieve higher resolution synchronization. The light delay is achieved by changing the light transmission delay path to achieve different time delays. The rate of light signal transmission in quartz fiber is:

[0063]

[0064] In formula (2), n is the refractive index of the optical fiber core; c is the propagation speed of light waves in space. The delay caused by the transmission of an optical signal through an optical fiber with a length of d can be expressed by formula (3):

[0065]

[0066] Therefore, the delay accuracy of a periodic laser pulse with a central wavelength of 1064nm and a pulse width of 10ps can reach 1ps.

[0067] In one embodiment, reference Figure 3 As shown, step S103 may further include the following steps:

[0068] The time difference between the loading information and the periodic laser pulse is measured, and the time range for delaying the loading information is between the time difference plus 1ns and the time difference. It should be understood that the difference Δt between the two signals is first measured using an oscilloscope, and the signal of the loading information is delayed by t1 during coarse synchronization. Since the accuracy of coarse synchronization can only reach nanosecond (ns) accuracy, it is necessary to ensure that the delay amount t1 is in the range of Δt+1ns>t1>Δt.

[0069] In one embodiment, reference Figure 3 As shown, step S104 may further include the following steps:

[0070] The time for delaying the periodic laser pulse is the difference between the time for delaying the loaded information and the time difference. It should be understood that the optical signal is finely adjusted using the optical fiber delay line to delay the optical signal by t2, t2 = t1-Δt, and the accuracy of the optical fiber delay line reaches picosecond (ps) accuracy.

[0071] In one embodiment, reference Figure 2As shown, the optical signal modulation method may further include the following steps:

[0072] Step S106: amplifying the energy of the modulated optical signal by stimulated radiation.

[0073] It is important to understand that stimulated emission is the phenomenon in which a luminescent atom in an excited state emits photons when it transitions to a lower energy state or ground state under the action of an external radiation field. At this time, the energy of the external radiation must be exactly the energy difference between the two energy levels of the atom. The frequency, phase, propagation direction, and polarization state of the photons emitted by stimulated emission are exactly the same as those of the external photons. Stimulated emission is a necessary condition for the generation of lasers. Based on the principle of stimulated emission of lasers, amplification is achieved by converting the energy of pump light into the energy of signal light. Pump light is a process in which light is used to raise (or "pump") electrons from lower energy levels in atoms or molecules to higher energy levels.

[0074] In one embodiment, reference Figure 2 As shown, the optical signal modulation method may further include the following steps:

[0075] Step S107: transmitting the modulated optical signal after frequency doubling.

[0076] It should be understood that by utilizing the nonlinear effect of the crystal, a certain wavelength signal carrying information can be processed and transferred to another wavelength. Specifically, a 1064nm laser can be frequency-doubled to obtain a 532nm high-power pulse modulation signal output.

[0077] In one embodiment, reference Figure 2 As shown, the optical signal modulation method may further include the following steps:

[0078] Step S108: The modulated optical signal is emitted underwater and transmitted to the detector. After the detector receives the optical signal, the optical signal is demodulated and decoded to obtain information.

[0079] It should be understood that the modulated optical signal can be frequency-doubled to obtain a 532nm high-power pulse laser to transmit information through the entire seawater channel. The detector receives the signal, then demodulates and decodes the signal to obtain the final information.

[0080] Furthermore, in an embodiment of the present invention, a transmission system based on the optical signal modulation method in any of the above embodiments is provided, referring to Figure 2 As shown, including:

[0081] A pulse light source, used to obtain periodic laser pulses;

[0082] A programmable integrated circuit, used for receiving information to be sent and encoding it to obtain loading information, and extracting clock information;

[0083] An electrical delay module, used for delaying the loading information according to the clock information, so as to achieve coarse synchronization between the loading information and the periodic laser pulse;

[0084] An optical delay module is used to delay the periodic laser pulse so as to achieve precise synchronization between the loading information and the periodic laser pulse;

[0085] The modulation module is used to modulate the periodic laser pulse according to the loading information and emit the modulated optical signal.

[0086] It should be understood that the programmable integrated circuit can be an FPGA, and the electrical delay module can be integrated in the programmable integrated circuit. The electrical delay module can extract clock information from the periodic laser pulse through a D flip-flop and input it into the FPGA as an external clock. The optical delay module can specifically be an optical fiber delay line, and specifically can be an optical fiber delay line of different lengths or different paths. The modulation module can be an acousto-optic modulator, which loads the information to be transmitted onto the optical carrier by intensity modulating the optical signal.

[0087] It should also be understood that the electrical delay module and the optical delay module can together form a pulse delay synchronization module, the electrical delay module delays the electrical signal to achieve rough synchronization, and then delays the pulse light through the optical fiber delay line to achieve fine synchronization, so as to achieve synchronization between the pulse light and the output signal of the FPGA.

[0088] It is also necessary to understand that the transmission system may also include an optical amplification module, a frequency doubling module, etc. The optical amplification module, based on the principle of stimulated radiation of lasers, achieves amplification by converting the energy of pump light into the energy of signal light. The frequency doubling module uses the nonlinear effect of the crystal to process a certain wavelength signal carrying information and reprint it to another wavelength. The 1064nm laser is frequency doubling to obtain a 532nm high-power pulse modulated signal output. The 532nm high-power pulse laser obtained by frequency doubling transmits information through the entire seawater channel. The detector receives the signal, then demodulates and decodes the signal to obtain the final information.

[0089] According to the above-mentioned embodiments, the present invention further provides the following specific embodiments. Figure 4 According to the proposed design concept, after the system debugging experiment, all technical indicators of the scheme have reached the design requirements, the output pulse waveform performance is stable, the system has strong anti-interference performance and high reliability. For the actual effect of the pulse delay module, please refer to Figure 4 (a~f), Figure 4The wider line is the waveform of the loading information signal, and the narrower line is the pulse signal waveform. Only when the rising edge of the pulse signal and the loading information signal are output synchronously can the information be loaded accurately. (a) The initial time difference between the electrical signal and the optical signal is about 6ns, (b) The initial time difference between the electrical signal and the optical signal is about 1ns, and (c) The initial time difference between the electrical signal and the optical signal is about -1ns. After coarse synchronization, the difference between the two signals is controlled to be 1ns, and then the optical fiber delay line is used for fine delay, and the delay amounts are 400ps, 490ps, and 500ps respectively. Ensure that the error between the two signals can be lower than the ps level.

[0090] It should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise" and the like in the above description indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the embodiments of the present invention and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the embodiments of the present invention.

[0091] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of the embodiments of the present invention, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0092] In the embodiments of the present invention, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0093] In the embodiments of the present invention, unless otherwise clearly specified and limited, a first feature being "above" or "below" a second feature may include the first and second features being in direct contact, or may include the first and second features being in contact not directly but through another feature between them. Moreover, a first feature being "above", "above" and "above" a second feature includes the first feature being directly above and obliquely above the second feature, or simply indicates that the first feature is higher in level than the second feature. A first feature being "below", "below" and "below" a second feature includes the first feature being directly below and obliquely below the second feature, or simply indicates that the first feature is lower in level than the second feature.

[0094] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine different embodiments or examples described in this specification.

[0095] Those skilled in the art will readily appreciate other embodiments of the present invention after considering the specification and practicing the invention disclosed herein. This application is intended to cover any variations, uses or adaptations of the present invention, which follow the general principles of the present invention and include common knowledge or customary techniques in the art that are not disclosed by the present invention. The specification and examples are intended to be exemplary only, and the true scope and spirit of the present invention are indicated by the appended claims.

Claims

1. A method for modulating an optical signal, characterized in that: include: Obtain periodic laser pulses and information to be sent; Encoding the information to be sent by a programmable integrated circuit to obtain loading information, and extracting clock information; Delaying the loading information according to the clock information so that the loading information and the periodic laser pulse are roughly synchronized; wherein the time difference between the loading information and the periodic laser pulse is measured, and the time range for delaying the loading information is between the time difference plus 1ns and the time difference; Delaying the periodic laser pulse so that the loading information and the periodic laser pulse are precisely synchronized; wherein the time for delaying the periodic laser pulse is the difference between the time for delaying the loading information and the time difference; The periodic laser pulse is modulated according to the loading information, and a modulated optical signal is emitted.

2. The modulation method according to claim 1, characterized in that: The information to be sent is encoded by a programmable integrated circuit to obtain loading information, and clock information is extracted, including: The information to be sent is encoded by a programmable integrated circuit using a low-density parity check code to obtain loading information.

3. The modulation method according to claim 1, characterized in that: The information to be sent is encoded by a programmable integrated circuit to obtain loading information, and clock information is extracted, including: The clock information output by the D flip-flop is extracted.

4. The modulation method according to claim 1, characterized in that: Delaying the periodic laser pulses to achieve precise synchronization between the loading information and the periodic laser pulses, comprising: The periodic laser pulse is delayed by changing the transmission distance of the periodic laser pulse before modulation.

5. The modulation method according to claim 1, characterized in that: Also includes: The modulated optical signal is energy amplified by stimulated emission of radiation.

6. The modulation method according to claim 1, characterized in that: Also includes: The modulated optical signal is transmitted after being frequency-doubled.

7. The modulation method according to any one of claims 1 to 6, characterized in that: Also includes: The modulated optical signal is emitted underwater and transmitted to the detector. After the detector receives the optical signal, the optical signal is demodulated and decoded to obtain the information.

Images