High-speed modulation system for underwater high-power laser communication

Abstract

The invention belongs to the technical field of underwater optical communication, and particularly discloses a high-speed modulation system for underwater high-power laser communication, which comprises a laser, a Mach-Zehnder modulator, a modulation driving module and a laser transmitter, the laser is used for outputting narrow-linewidth linearly polarized laser to the Mach-Zehnder modulator, and the linearly polarized laser has high coherence and stable polarization direction; the Mach-Zehnder modulator adopts a two-arm structure and can divide a light beam into two paths; the two-arm structure is internally provided with an electro-optic phase modulator, and the electro-optic phase modulator carries out phase control by introducing an optical path difference into a voltage signal provided by a modulation driving module; and the laser modulated by the Mach-Zehnder modulator is emitted by the laser emitter and is used for underwater laser communication. According to the invention, the structural limitation that a traditional underwater optical communication system cannot consider the laser power, the modulation rate and the device adaptability at the same time is broken through, and a new technical solution is provided for constructing a long-distance, high-rate and high-reliability underwater laser communication system.

Description

Technical Field

[0001] The present invention belongs to the technical field of underwater optical communications, and in particular relates to a high-speed modulation system for underwater high-power laser communications. Background Art

[0002] Underwater Wireless Optical Communication (UWOC) is a current research hotspot in ocean exploration and communication technology. It has the advantages of high speed, low latency, and high confidentiality. It is an important technical support for future underwater sensor networking, seabed monitoring, and unmanned underwater vehicle communications.

[0003] In underwater laser communications, key determinants of link transmission performance include the transmitting laser's linewidth, output power, beam quality, and modulation rate. Laser linewidth is a key factor affecting pulse broadening during underwater laser transmission. Output power determines the energy transmitted per unit time, beam quality determines the energy's spatial focusing ability and propagation directionality, and modulation rate directly impacts the system's data throughput and response latency. These four factors work together to determine the effective distance and transmission bandwidth of the communication link, and are currently the primary technical focus for optimizing high-performance underwater laser communication systems.

[0004] The currently used underwater laser communication modulation schemes have the following key bottlenecks:

[0005] (1) Use of low-power LD: With GHz-level modulation bandwidth, it can be switched on and off quickly through direct current modulation, but its output power is limited (<100mW). The underwater transmission distance is usually only a few meters, which is only suitable for short-distance high-speed data exchange. Although multiple LDs can be combined into an array to increase the total power in theory, due to their asynchronous light emission mode, large divergence angle, and poor beam consistency, it is difficult to effectively focus them into a single high-quality far-field spot, resulting in low unit energy density, severe pulse broadening, and limited improvement in the effective communication distance.

[0006] (2) Using high-power LD direct modulation: In order to carry large current output, this type of LD usually adopts a wide waveguide structure with large parasitic capacitance and inductance, which limits its modulation bandwidth. The -3dB bandwidth is usually less than 10MHz. In addition, when using high-power LD as a pump source modulation method, the modulation rate of the laser is also affected by the fluorescence lifetime of the laser gain medium and the photon lifetime determined by the resonant cavity, which further limits the modulation rate and modulation depth, making it difficult to adapt to UWOC application requirements.

[0007] (3) Using extracavity acousto-optic modulation (AOM) technology: This technology is currently one of the few solutions that can achieve external modulation on high-power lasers. AOM devices use ultrasound to induce periodic changes in the refractive index in the crystal to form a diffraction grating, thereby changing the output light intensity. Due to the limitation of the propagation speed of ultrasound, its -3dB bandwidth in space optical communication is usually less than 10MHz. In addition, there are three major defects: (1) The carrying light power is limited by the thermal load and the material damage threshold, and the typical carrying power is 1W; (2) The diffraction efficiency is inversely proportional to the modulation frequency, and the modulation depth decreases significantly during high-speed modulation; (3) The speed of sound waves is much lower than the speed of light, and there are modulation delays and bandwidth limitations.

[0008] (4) Using longitudinal electro-optic modulation technology: This type of modulator is often used in high-power space laser communications or laser radar systems. It uses electro-optic crystals such as KDP (potassium dihydrogen phosphate) to adjust the refractive index by applying a high-voltage electric field along the crystal axis to achieve laser intensity modulation. This solution is suitable for large-aperture, high-power laser beams and has strong light-carrying capacity. However, longitudinal electro-optic modulators usually require the application of a high voltage of several thousand volts, and the modulation capacitance is large and the response time is slow, resulting in a modulation rate that is usually only in the kHz range.

[0009] (5) Using transverse electro-optical modulation technology: Figure 5 The following graph shows the half-wave voltage versus wavelength for a Thorlabs lateral electro-optic modulator. The data is based on a sample phase modulator fabricated from a high-electro-optic coefficient LiNbO3 (LN) crystal. As shown in the figure, while Vπ (half-wave voltage) is relatively low (approximately tens to hundreds of volts) in the blue or green wavelength bands, enabling high-speed electro-optic modulation, two key engineering challenges remain: 1. In short-wavelength, high-power laser applications, the half-wave voltage remains high, making high-voltage, high-speed drive circuits complex to implement, difficult to integrate, and energy-intensive. 2. Under watt-level continuous laser power, even with the use of MgO-doped LiNbO3 crystals to enhance resistance to photodamage, the risk of photorefractive instability and long-term instability persists, impacting device lifetime and modulation stability.

[0010] In summary, the current underwater laser communication field lacks a system solution that can both carry high-power lasers and achieve modulation rates in the hundreds of megahertz range. Traditional modulation paths are difficult to implement in engineering applications due to limited modulation bandwidth, either due to the ultrasonic propagation rate or due to excessively high drive voltages. This is especially true under short-wavelength, high-power conditions, where component selection and system stability present significant challenges. Current underwater communication links generally struggle to balance communication distance and transmission rate, and the industry lacks a truly groundbreaking modulation structure. Summary of the Invention

[0011] The purpose of the present invention is to provide a high-speed modulation system for underwater high-power laser communication, which breaks through the structural limitations of traditional underwater communication systems that cannot take into account all three aspects of laser power, modulation rate and device adaptability, and provides a new technical solution for building a long-distance, high-speed and high-reliability underwater laser communication system.

[0012] In order to achieve the above-mentioned purpose, the technical solution of the present invention is: a high-speed modulation system for underwater high-power laser communication, comprising a laser, a Mach-Zehnder modulator, a modulation drive module and a laser transmitter; the laser is used to output narrow-linewidth linearly polarized laser to the Mach-Zehnder modulator, and the linearly polarized laser has high coherence and stable polarization direction; the Mach-Zehnder modulator adopts a two-arm structure, which can split the light beam into two paths; both arm structures have built-in electro-optical phase modulators, and the electro-optical phase modulator is provided with a voltage signal by the modulation drive module to introduce an optical path difference for phase control; the laser modulated by the Mach-Zehnder modulator is emitted through the laser transmitter for underwater laser communication.

[0013] Furthermore, the Mach-Zehnder modulator includes a first non-polarizing beam splitter, a second non-polarizing beam splitter, a first electro-optical phase modulator, a second electro-optical phase modulator, a first reflector and a second reflector. The linearly polarized laser output by the laser is incident on the first non-polarizing beam splitter. The first non-polarizing beam splitter splits the incident laser into two beams, which enter two optical path arms respectively. One optical path arm is composed of the first reflector and the second non-polarizing beam splitter, and the other optical path arm is composed of the second reflector and the second non-polarizing beam splitter; the first reflector and the second reflector are used to reflect the laser emitted by the first non-polarizing beam splitter to the second non-polarizing beam splitter; the second non-polarizing beam splitter is used to recombine the two beams of light that have been phase-modulated and generate interference; the first electro-optical phase modulator and the second electro-optical phase modulator are respectively arranged in the two optical path arms, and the modulation driving module is used to apply a voltage to the first electro-optical phase modulator and the second electro-optical phase modulator to introduce an optical path difference, so as to realize the control of the optical phase by the modulation signal.

[0014] Furthermore, the electro-optical phase modulator is provided with a differential voltage signal by the modulation driving module to introduce an optical path difference to perform phase control.

[0015] Furthermore, the first electro-optical phase modulator and the second electro-optical phase modulator are lithium niobate crystals.

[0016] Furthermore, the interfered light is outputted from two output ports of the second non-polarization beam splitter respectively, and the intensity distribution of the output light varies with the phase difference introduced by the first electro-optical phase modulator and the second electro-optical phase modulator.

[0017] Furthermore, one of the output ports serves as a main output port, and the other port serves as a monitoring port or a power absorption port.

[0018] Furthermore, the modulation signal of the modulation driving module comes from a signal source, and after processing, drives two electro-optical phase modulators to achieve phase modulation, and obtains intensity modulation after interference through the second non-polarization beam splitter.

[0019] Furthermore, the laser adopts an external cavity semiconductor surface emitting laser based on VECSEL, and a birefringent filter and a frequency doubling crystal are introduced into the external cavity semiconductor surface emitting laser.

[0020] Furthermore, the Mach-Zehnder modulator also includes a first polarization beam splitter. The linearly polarized laser output by the laser is incident on the first polarization beam splitter. The first polarization beam splitter is used to sort the polarization state of the input laser and select it as the linear polarization direction required by the system; the laser after the polarization direction is selected by the first polarization beam splitter will be incident on the first non-polarization beam splitter.

[0021] Furthermore, the Mach-Zehnder modulator includes a quarter-wave plate, a second polarization beam splitter, a third polarization beam splitter, a first electro-optical phase modulator, a second electro-optical phase modulator, a first reflector and a second reflector; the narrow linewidth linearly polarized laser output by the laser is incident on the quarter-wave plate, and the quarter-wave plate converts the linearly polarized laser into a circularly polarized laser; the circularly polarized laser is incident on the second polarization beam splitter, and the second polarization beam splitter splits the incident laser into two beams, which enter two optical path arms respectively, one optical path arm is composed of the first reflector and the third polarization beam splitter, and the other optical path arm is composed of the second reflector and The optical path difference is introduced by applying a voltage to the first electro-optical phase modulator and the second electro-optical phase modulator through the modulation driving module, thereby realizing the control of the optical phase by the modulation signal; a linear polarizer or an analyzer is arranged at the output end of the third polarization beam splitter to realize intensity modulated output.

[0022] The working principle of this technical solution is as follows: the laser outputs high-coherence linearly polarized laser light, which serves as communication carrier light and enters the Mach-Zehnder modulator; the two arms of the Mach-Zehnder modulator are equipped with built-in electro-optical phase modulators, and the modulation drive module provides a differential voltage signal for high-speed phase control; the modulation signal comes from the signal source, and after processing, it drives the electro-optical phase modulator to achieve intensity modulation; the modulated laser is emitted by the laser transmitter to realize underwater laser communication.

[0023] The beneficial effects of this technical solution are:

[0024] 1. Optimized integrated light source design fully matches modulator operating requirements: This invention utilizes a VECSEL-based external-cavity semiconductor laser structure and incorporates a birefringent filter within the cavity to achieve single longitudinal mode locking, linewidth compression, and linearly polarized output of the output laser. This light source boasts a high coherence length and high polarization purity, perfectly meeting the stringent light source requirements of the Mach-Zehnder modulator, significantly improving interference contrast and modulation depth, and laying the foundation for enhanced modulation system performance.

[0025] 2. Improve frequency doubling efficiency and obtain short-wavelength high-power laser output: The spectral stability and polarization consistency of the fundamental frequency light effectively improve the phase matching efficiency of the frequency doubling crystal, which significantly increases the power density of the blue or green light output by the nonlinear frequency doubling, just covering the optical window of the ocean and inland waters. At the same time, the narrow linewidth light source can significantly reduce the broadening of the laser pulse in the underwater link, thereby increasing the effective propagation distance of the underwater communication link.

[0026] 3. Resolving the structural contradiction between modulation crystal selection and high-power laser coupling: In high-power, short-wavelength modulation, traditional longitudinal modulation crystals such as KDP (potassium dihydrogen phosphate) have high light-carrying capacity, but their half-wave voltage is as high as kilovolts, making them difficult to drive. LN crystals, while capable of high speed, have limited light-carrying capacity. This invention utilizes an integrated solution of LN crystals, dual-arm beam splitting, and dual-arm differential drive. This reduces the light-carrying density of a single LN crystal arm, avoids the risk of photorefractive inversion, and effectively halves the half-wave voltage, enabling the system to achieve high-speed, stable modulation with a controllable drive voltage.

[0027] 4. Achieve the unity of high modulation rate and high modulation depth: The modulation structure based on the Mach-Zehnder interference principle has a sub-nanosecond response time. In the structure of the present invention, a modulation rate of ≥100 MHz can be achieved. At the same time, relying on a high-coherence light source and a high interference contrast design, the modulation depth can reach more than 90%, meeting the dual requirements of the medium- and high-speed underwater communication system for rate and power dynamic range.

[0028] 5. The driving voltage is greatly reduced, and the system bandwidth potential is significantly improved. Through the dual-arm symmetrical modulation structure, the system equivalent half-wave voltage is reduced to half of the traditional structure, and the typical Vπ is controlled in the range of 20-50V, which significantly reduces the difficulty of driving circuit design and is conducive to the integration of commercial circuits such as high-speed push-pull amplifiers, achieving system bandwidth exceeding 100 MHz, and has the potential to expand to higher speeds.

[0029] 6. Modular Structural Design, Excellent Engineering Adaptability: The system achieves compact coupling between the light source, frequency multiplication, and modulation structure, providing excellent power and beam matching capabilities. The overall compact structure and clear modules make it easy to package and integrate, making it suitable for deployment in underwater node communication devices, sensor platforms, unmanned underwater vehicles, and other scenarios, with broad engineering application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a schematic diagram of a high-speed modulation system for underwater high-power laser communication according to the present invention;

[0031] Figure 2 Schematic diagram of the structure of the laser;

[0032] Figure 3 for Figure 1 A schematic diagram of the structure of a Mach-Zehnder modulator;

[0033] Figure 4 for Figure 2 Another structural diagram of the Mach-Zehnder modulator;

[0034] Figure 5 The figure is a trend diagram of the half-wave voltage of the transverse electro-optic modulator in the background technology changing with the wavelength. DETAILED DESCRIPTION

[0035] The following is further described in detail through specific implementation methods:

[0036] The figure marks in the drawings of the specification include: laser 1, Mach-Zehnder modulator 2, optical antenna 3, modulation drive module 4, signal source 5, heat sink 6, gain chip 7, birefringent filter 8, folding output mirror 9, rear end reflector 10, frequency doubling crystal 11, first polarization beam splitter 12, first non-polarization beam splitter 13, first electro-optical phase modulator 14, second electro-optical phase modulator 15, second reflector 16, first reflector 17, second non-polarization beam splitter 18, quarter wave plate 19, second polarization beam splitter 20, third polarization beam splitter 21.

[0037] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0038] The overall concept is as follows: Currently, it's difficult to find a suitable modulation structure for high-speed modulation of high-power, short-wavelength lasers. While KDP crystals can withstand high power, their Vπ (half-wave voltage, over a thousand volts) is too high, resulting in slow modulation speeds and making them unusable. LN (lithium niobate) crystals offer slightly faster modulation speeds, but they suffer from two critical issues: 1. The half-wave voltage is still high, reaching tens to hundreds of volts, making the drive circuits difficult to design. 2. At high laser powers (above 10 watts), they are prone to burning out, and even with MgO doping, they are not very stable.

[0039] Therefore, it is not enough to simply select a crystal, but a system integration innovation is required. There are three key points:

[0040] LN crystal must be used because it is the only one that can modulate fast enough;

[0041] A dual-arm structure must be used to split the light equally into two paths, so that the power of each LN crystal is only half, which can be tolerated;

[0042] It must be differential drive, so that the phase difference is doubled and the equivalent half-wave voltage is halved, making the circuit easier to make;

[0043] Moreover, MZI interferometric modulation requires linearly polarized lasers with high coherence length, and ordinary lasers are simply not suitable.

[0044] The solution utilizes a VECSEL as the light source. By introducing a birefringent filter, the output laser is tuned to a single longitudinal mode (long coherence length) and linearly polarized laser. These characteristics are perfectly suited to the MZI modulator. This system is a highly ingenious collaborative design encompassing the entire "light source - modulator - crystal - circuit" chain, rather than a single breakthrough.

[0045] The advantages of doing this are: 1. The modulated pulse obtained by single-frequency laser has smaller broadening effect in underwater links (low dispersion effect); 2. The actual modulation rate can reach 100MHz or even higher; 3. Vπ is compressed to tens of volts and can be driven by ordinary amplifiers; 4. The light-bearing pressure of each crystal is reduced and it is not easy to burn; 5. The system can be used for underwater communications, supports both blue light and green light, and can transmit farther and modulate faster.

[0046] Example 1

[0047] Basically as attached Figure 1Figure 1 shows a high-speed modulation system for underwater high-power laser communication, comprising a laser 1, a Mach-Zehnder modulator 2, a modulation driver module 4, and a laser transmitter. Laser 1 outputs narrow-linewidth linearly polarized laser light to Mach-Zehnder modulator 2. The linearly polarized laser light exhibits high coherence and stable polarization. Mach-Zehnder modulator 2 employs a two-arm structure, capable of splitting the light beam into two paths. Both arms incorporate an electro-optical phase modulator. The modulation driver module 4 provides a voltage signal to the electro-optical phase modulator, which introduces an optical path difference for phase control. To improve modulation efficiency, a differential voltage signal drive scheme is employed. The modulation signal from the modulation driver module 4 comes from a signal source 5. After processing, it drives the two electro-optical phase modulators, achieving phase modulation. Interference through a second non-polarizing beam splitter 18 results in intensity modulation. The laser light modulated by the Mach-Zehnder modulator 2 is emitted by a laser transmitter for underwater laser communication. The laser transmitter employs an optical antenna 3.

[0048] like Figure 3As shown, the Mach-Zehnder modulator 2 includes a first polarization beam splitter 12, a first non-polarization beam splitter 13, a second non-polarization beam splitter 18, a first electro-optical phase modulator 14, a second electro-optical phase modulator 15, a first reflector 17, and a second reflector 16. The linearly polarized laser light output by the laser 1 is incident on the first polarization beam splitter 12. The first polarization beam splitter 12 is used to sort the polarization state of the input laser light and select it as the linear polarization direction required by the system. The laser light after the polarization direction is selected by the first polarization beam splitter 12 is incident on the first non-polarization beam splitter 13. If the laser 1 used itself outputs stable linearly polarized light and its polarization direction is consistent with the system design requirements, the first polarization beam splitter 12 can be omitted. The first non-polarization beam splitter 13 splits the incident laser light into two beams, which enter two optical path arms respectively. One optical path arm is composed of the first reflector 17 and the second non-polarization beam splitter 18, and the other optical path arm is composed of the second reflector 16 and the second non-polarization beam splitter 18. The first reflector 17 and the second reflector 16 are used to reflect the laser light emitted by the first non-polarizing beam splitter 13 to the second non-polarizing beam splitter 18; the second non-polarizing beam splitter 18 is used to recombine the two phase-modulated light beams and generate interference. The first electro-optical phase modulator 14 and the second electro-optical phase modulator 15 are lithium niobate crystals (such as MgO:LiNbO3). The first electro-optical phase modulator 14 and the second electro-optical phase modulator 15 are respectively arranged in two optical path arms. The modulation drive module 4 applies a differential voltage to the first electro-optical phase modulator 14 and the second electro-optical phase modulator 15 to introduce an optical path difference, thereby realizing the control of the optical phase by the modulation signal. The interfered light is output from the two output ports of the second non-polarizing beam splitter 18 respectively. The intensity distribution of the output light varies periodically with the phase difference introduced by the first electro-optical phase modulator 14 and the second electro-optical phase modulator 15. One of the output ports serves as the main output port, and the other port serves as a monitoring port or power absorption port.

[0049] This structure, based on the Mach-Zehnder interference principle, creates interference fringes at the output port by applying a controllable phase delay to two beams of equal intensity. Because the refractive index of the lithium niobate crystal changes with the driving voltage, the relative phase between the two beams dynamically shifts. This phase difference is converted into a change in the light intensity distribution at the output ports, achieving the conversion from electrical signal modulation to light intensity variation.

[0050] For example, when the two arms are in phase (a phase difference of 0), the interference output light is primarily concentrated at one port (e.g., the vertical port). When the phase difference is π (anti-phase interference), the output light is completely diverted to the other port (e.g., the lateral port). In actual use, by setting appropriate static bias and AC modulation signals, the output light intensity can be made to vary linearly with the input electrical signal, achieving high-speed, continuous intensity-modulated output.

[0051] This structure employs a dual-drive MZI (Dual-Drive MZI) symmetrical modulation scheme, employing electro-optical phase modulators in each arm and applying equal-amplitude, opposite-direction modulation signals. This structure not only achieves an equivalent phase difference multiplication effect but also significantly reduces the system's equivalent half-wave voltage (Vπ), improving modulation depth and response bandwidth.

[0052] More importantly, because the laser power is evenly distributed across two optical paths, the laser power borne by a single arm is significantly reduced, allowing the use of crystal materials with high electro-optical response coefficients but low light-carrying capacity (such as magnesium-doped lithium niobate MgO:LiNbO3). This type of material has a higher electro-optical coefficient and can reduce the half-wave voltage to tens of volts, facilitating the use of low-voltage, high-speed electronic drive solutions, significantly increasing the operating frequency of the modulation system, allowing it to operate at speeds exceeding 100MHz or even higher, making it particularly suitable for underwater laser communication systems with high power and high-speed requirements.

[0053] The laser 1 adopts an external cavity semiconductor surface emitting laser 1 based on VECSEL, and a birefringent filter 8 and a frequency doubling crystal 11 are introduced into the external cavity semiconductor surface emitting laser 1. Figure 2 As shown, the specific structure of laser 1 includes a pump source, a heat sink 6, a gain chip 7, a rear reflector 10, a folding output mirror 9, a birefringent filter 8, and a frequency-doubling crystal 11. The gain chip 7 is mounted on the heat sink 6. The pump source is used to emit pump light and transmit it to the gain chip 7. The gain chip 7 is used to absorb the energy of the pump light and generate stimulated radiation at the wavelength of the pump light. The folding output mirror 9 is used to transmit the stimulated radiation emitted by the gain chip 7 to the rear reflector 10. The birefringent filter 8 is arranged between the gain chip 7 and the folding output mirror 9, and the frequency-doubling crystal 11 is arranged between the folding output mirror 9 and the rear reflector 10. The birefringent filter 8 selectively feeds back the VECSEL output spectrum, effectively compressing the laser linewidth, improving the frequency-doubling efficiency, and locking the linear polarization direction, providing a high-quality light source for subsequent modulation devices. The frequency-doubling crystal 11 uses a nonlinear frequency-doubling crystal (such as LBO or BBO) to convert the fundamental frequency light into a blue-green laser (such as 490nm) with minimal underwater transmission loss. Its output power can reach 10W, and its high coherence and spectral line compression characteristics improve the frequency-doubling efficiency. The rear-end reflector 10 is used to reflect the fundamental and frequency-doubled light; the folded output mirror 9 is used to highly reflect the fundamental light and highly transmit the frequency-doubled light.

[0054] The specific implementation process is as follows:

[0055] Laser 1 outputs highly coherent linearly polarized laser light as communication carrier light, which enters Mach-Zehnder modulator 2. Mach-Zehnder modulator 2 has built-in electro-optical phase modulators in both arms, and modulation drive module 4 provides differential voltage signals for high-speed phase control. The modulation signal comes from signal source 5, which drives the electro-optical phase modulator after processing to achieve intensity modulation. The modulated laser is emitted through optical antenna 3 to achieve underwater laser communication.

[0056] Example 2

[0057] A high-speed modulation system for underwater high-power laser communication. The difference between this embodiment and the first embodiment lies in the structure of the Mach-Zehnder modulator: Figure 4 As shown, the Mach-Zehnder modulator includes a quarter-wave plate 19, a second polarization beam splitter 20, a third polarization beam splitter 21, a first electro-optical phase modulator 14, a second electro-optical phase modulator 15, a first reflector 17, and a second reflector 16. The narrow-linewidth linearly polarized laser light output by the laser 1 is incident on the quarter-wave plate 19, which converts the linearly polarized laser light into circularly polarized laser light. The circularly polarized laser light is incident on the second polarization beam splitter 20, which splits the incident laser light into two beams, which enter two optical path arms respectively. One optical path arm consists of the first reflector 17 and the third polarization beam splitter 21, and the other optical path arm consists of the second reflector 16 and the third polarization beam splitter 21. The first reflector 17 and the second reflector 16 are used to reflect the laser light emitted from the second polarization beam splitter 20 to the third polarization beam splitter 21. The third polarization beam splitter 21 is used to recombine the two phase-modulated light beams and generate a new polarization state containing modulation information. The first electro-optical phase modulator 14 and the second electro-optical phase modulator 15 are respectively arranged in the two optical path arms. The modulation driving module 4 applies a differential voltage to the first electro-optical phase modulator 14 and the second electro-optical phase modulator 15 to introduce an optical path difference, thereby realizing the control of the optical phase by the modulation signal.

[0058] The beam emitted by laser 1 is first converted into circularly polarized light by a quarter-wave plate 19. It then enters a second polarization beam splitter 20, where it is split into two orthogonal linearly polarized components, which are then directed into the two optical path arms of the interference structure. Each arm is equipped with an electro-optical phase modulator (divided into a first electro-optical phase modulator 14 and a second electro-optical phase modulator 15). A controllable optical path difference is introduced by applying a differential modulation voltage.

[0059] After the two modulated beams are recombined at the third polarization beam splitter 21, the polarization state of the output beam changes periodically with the phase difference. If output directly, a polarization-modulated signal is generated. If a linear polarizer (analyzer) is configured at the output end, the polarization change is converted into a change in light intensity, achieving an intensity-modulated output.

[0060] This structure shares the same interferometric modulation principle as Example 1 and also employs a two-arm symmetrical modulation scheme, which can achieve equivalent phase difference multiplication, reduce half-wave voltage, and increase modulation depth. However, its output signal is a polarization state change and requires use with an analyzer or polarization-sensitive receiver module. It is only applicable in certain experimental or polarization-maintaining environments.

[0061] It should be noted that, in this document, relational terms such as first and second, etc., are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "comprises," "comprising," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that includes a list of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such process, method, article, or apparatus.

[0062] The above is only an embodiment of the present invention. Common knowledge such as the known specific structures and characteristics in the scheme is not described in detail here. Ordinary technicians in the field are aware of all common technical knowledge in the technical field of the invention before the application date or priority date, can obtain all existing technologies in the field, and have the ability to apply conventional experimental means before that date. Ordinary technicians in the field can improve and implement this scheme in combination with their own abilities under the inspiration given by this application. Some typical known structures or known methods should not become obstacles for ordinary technicians in the field to implement this application. It should be pointed out that for those skilled in the art, without departing from the structure of the present invention, several variations and improvements can be made, which should also be regarded as the scope of protection of the present invention. These will not affect the effect of the implementation of the present invention and the practicality of the patent. The scope of protection required by this application shall be based on the content of its claims, and the specific implementation methods and other records in the specification can be used to interpret the content of the claims.

Claims

1. A high-speed modulation system for underwater high-power laser communication, characterized by: The invention comprises a laser (1), a Mach-Zehnder modulator (2), a modulation drive module (4) and a laser transmitter; the laser (1) is used to output narrow-linewidth linearly polarized laser light to the Mach-Zehnder modulator (2), and the linearly polarized laser light has high coherence and a stable polarization direction; the Mach-Zehnder modulator (2) adopts a two-arm structure and can split the light beam into two paths; the two-arm structures are both equipped with an electro-optical phase modulator, and the electro-optical phase modulator is provided with a voltage signal by the modulation drive module (4) to introduce an optical path difference for phase control; the laser light modulated by the Mach-Zehnder modulator (2) is emitted through the laser transmitter and is used for underwater laser communication.

2. The high-speed modulation system for underwater high-power laser communication according to claim 1, characterized in that: The Mach-Zehnder modulator (2) comprises a first non-polarizing beam splitter (13), a second non-polarizing beam splitter (18), a first electro-optical phase modulator (14), a second electro-optical phase modulator (15), a first reflector (17) and a second reflector (16); the linearly polarized laser light output by the laser (1) is incident on the first non-polarizing beam splitter (13); the first non-polarizing beam splitter (13) splits the incident laser light into two beams, which respectively enter two optical path arms, one optical path arm being composed of the first reflector (17) and the second non-polarizing beam splitter (18); and the other optical path arm being composed of the second reflector (16) and the second The invention is composed of a non-polarization beam splitter (18); a first reflector (17) and a second reflector (16) are used to reflect the laser emitted by the first non-polarization beam splitter (13) to the second non-polarization beam splitter (18); the second non-polarization beam splitter (18) is used to re-combine the two phase-modulated light beams and generate interference; the first electro-optical phase modulator (14) and the second electro-optical phase modulator (15) are respectively arranged in two optical path arms, and an optical path difference is introduced by applying a voltage to the first electro-optical phase modulator (14) and the second electro-optical phase modulator (15) through the modulation drive module (4), thereby realizing the control of the optical phase by the modulation signal.

3. The high-speed modulation system for underwater high-power laser communication according to claim 1, characterized in that: The electro-optical phase modulator is provided with a differential voltage signal by the modulation drive module (4) to introduce an optical path difference for phase control.

4. The high-speed modulation system for underwater high-power laser communication according to claim 2, characterized in that: The first electro-optical phase modulator (14) and the second electro-optical phase modulator (15) are lithium niobate crystals.

5. The high-speed modulation system for underwater high-power laser communication according to claim 2, characterized in that: The interfered light is output from two output ports of the second non-polarization beam splitter (18) respectively, and the intensity distribution of the output light varies with the phase difference introduced by the first electro-optical phase modulator (14) and the second electro-optical phase modulator (15).

6. The high-speed modulation system for underwater high-power laser communication according to claim 5, characterized in that: One of the output ports is used as the main output port, and the other port is used as the monitoring port or power absorption port.

7. The high-speed modulation system for underwater high-power laser communication according to claim 2, characterized in that: The modulation signal of the modulation drive module (4) comes from a signal source (5), and after being processed, drives two electro-optical phase modulators to achieve phase modulation, and obtains intensity modulation after interference through a second non-polarization beam splitter (18).

8. The high-speed modulation system for underwater high-power laser communication according to claim 1, characterized in that: The laser (1) adopts an external cavity semiconductor surface emitting laser (1) based on VECSEL, and a birefringent filter (8) and a frequency doubling crystal (11) are introduced into the external cavity semiconductor surface emitting laser (1).

9. The high-speed modulation system for underwater high-power laser communication according to claim 2, characterized in that: The Mach-Zehnder modulator (2) further includes a first polarization beam splitter (12). The narrow-linewidth linearly polarized laser light output by the laser (1) is incident on the first polarization beam splitter (12). The first polarization beam splitter (12) is used to sort out the polarization state of the input laser light and select it as the linear polarization direction required by the system. The laser light after the polarization direction is selected by the first polarization beam splitter (12) is incident on the first non-polarization beam splitter (13).

10. The high-speed modulation system for underwater high-power laser communication according to claim 1, characterized in that: The Mach-Zehnder modulator (2) comprises a quarter-wave plate (19), a second polarization beam splitter (20), a third polarization beam splitter (21), a first electro-optical phase modulator (14), a second electro-optical phase modulator (15), a first reflector (17) and a second reflector (16); the narrow linewidth linear polarization laser output by the laser (1) is incident on the quarter-wave plate (19), and the quarter-wave plate (19) converts the linear polarization laser into a circular polarization laser; the circular polarization laser is incident on the second polarization beam splitter (20), and the second polarization beam splitter (20) splits the incident laser into two beams, which enter two optical path arms respectively, one optical path arm is composed of the first reflector (17) and the third polarization beam splitter (21), and the other optical path arm is composed of the second reflector (17). The invention is composed of a first reflector (17) and a second reflector (16) for reflecting the laser emitted by the second polarization beam splitter (20) to the third polarization beam splitter (21); the third polarization beam splitter (21) is used to re-combine the two phase-modulated light beams and generate a new polarization state containing modulation information; the first electro-optical phase modulator (14) and the second electro-optical phase modulator (15) are respectively arranged in two optical path arms, and an optical path difference is introduced by applying a voltage to the first electro-optical phase modulator (14) and the second electro-optical phase modulator (15) through the modulation drive module (4), so as to realize the control of the optical phase by the modulation signal; and a linear polarizer or an analyzer is arranged at the output end of the third polarization beam splitter (21) to realize intensity modulated output.

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