Underwater laser communication transmitter compatible with multiple modulation formats

Abstract

The invention discloses an underwater laser communication transmitter compatible with multiple modulation formats, which belongs to the field of communication transmitters and comprises a seed source module, a signal modulation module, a power amplification module, an LBO wavelength conversion module and a small-angle collimation transmitting module. The seed source module is a 1064nm DBR (Distributed Bragg Reflector) narrow linewidth laser; the signal modulation module comprises a modulator, and switching of OOK modulation formats, BPSK modulation formats and DPSK modulation formats is achieved by adjusting bias points; the power amplification module adopts a main oscillation power amplification structure and comprises two pre-amplification stages and two main amplification stages; the LBO wavelength conversion module is used for carrying out frequency multiplication on 1064nm light to obtain 532nm green light, and the output power is not less than 7W; and the small-angle collimation transmitting module compresses the green light and then outputs the compressed green light.

Description

Technical Field

[0001] The present invention relates to the field of underwater laser communication, and in particular to a design method for a high-power underwater laser communication transmitter compatible with multiple modulation formats. Background Art

[0002] The differences between underwater and terrestrial environments make many mature technologies on land inapplicable underwater. Underwater communications is a key technology in marine and underwater development, especially in recent years, with the construction of national "satellite-to-ground" communication links, the construction of underwater communication links is imminent.

[0003] The types of underwater information transmission are mainly divided into wired communication and wireless communication. Wired communication requires cables for connection, and its flexibility is limited due to the complex seabed environment. Wireless communication has high flexibility, with underwater acoustic communication and underwater wireless optical communication (under water wireless optical communication): the most mature is underwater acoustic communication, which has a long transmission distance but slow communication speed. UWOC has a fast transmission speed, but the attenuation of light in seawater is large, and the transmission distance is too short to meet the needs of long-distance communication. At present, although underwater laser communication has a large bandwidth and transmission rate, due to the absorption and scattering of light by seawater and the influence of seawater turbulence, the attenuation of optical signals is large and the transmission distance is limited. High-power laser communication systems can overcome this limitation by increasing the transmission power, thereby achieving longer-distance communication. Ocean turbulence can have many adverse effects on optical signals, resulting in a serious decline in communication quality. Switching between multiple modulation formats is a simple and easy-to-implement technology that can effectively improve communication quality.

[0004] In summary, the problems existing in the prior art are: (1) Although the most common underwater acoustic communication technology has a long transmission distance, its speed is too low to meet the need to obtain a large amount of data in a short time. In addition, underwater laser communication has a large attenuation and a short transmission distance, which cannot meet the needs of long-distance transmission.

[0005] (2) Existing underwater laser communication technology is limited in the transmission distance of laser in seawater and in the quality and stability of communication due to the absorption and scattering of light by seawater and the influence of ocean turbulence.

[0006] (3) Existing underwater blue-green laser communications all use blue-green light sources for direct modulation, with low output optical power and simple modulation technology, making it difficult to transmit high-quality signals over long distances in complex underwater environments.

[0007] In order to achieve long-distance, high-speed underwater data transmission in complex environments, problem (1) can be solved by using 450nm-550nm blue-green light as a light source to achieve high-speed wireless communication. Although water has the lowest absorption rate for blue-green light, the attenuation coefficient of water as a medium for optical signals is between 0.6dB / m and 6dB / m, which is still much greater than that of the atmosphere. Therefore, to increase the transmission distance, the light source transmission power must be increased. The current blue-green laser light source power is generally at the mW level and the light divergence angle is large. Therefore, it is of practical significance to realize a high-power blue-green laser light source with a small divergence angle. Problem (2) (3) can be solved by adopting a multi-modulation format switching strategy. The communication stability of the underwater laser communication link is affected by the intensity of atmospheric turbulence, and the use of a multi-modulation format switching strategy can effectively reduce the bit error rate of the communication system and enhance the reliability of the system. Summary of the Invention

[0008] In response to the problem that the semiconductor lasers used in current underwater laser communication systems have low transmission power and are unable to meet the huge losses of wireless optical links in complex underwater environments, the present invention designs a laser amplifier based on master oscillator power amplifier technology (MOPA). The laser amplifier can amplify a seed light source of tens of mW to about 30W, which is much higher than the existing underwater laser communication power, thereby improving the light source power problem that makes it difficult for existing underwater laser communication systems to achieve ultra-long-distance transmission.

[0009] Current underwater laser communication systems often use OOK modulation, but their signals are easily interfered with by factors such as turbulence, and the intensity of their optical pulses decreases as the modulation rate increases, making them incapable of meeting the requirements for high-speed, long-distance optical communication. This invention designs a multi-standard, high-power underwater laser communication transmitter that combines OOK with BPSK and DPSK. BPSK and DPSK signals are more resistant to noise than OOK signals, and their modulation and demodulation methods are relatively simple. This approach improves the difficulty of achieving high-speed, long-distance transmission in existing underwater optical communication systems without increasing the complexity of the equipment to a minimum.

[0010] The high-power laser communication transmission system designed in this invention includes a seed source module, a signal modulation module, a power amplifier module, a frequency multiplier, and a coupled output system. It employs a seed source + amplifier + frequency multiplier optical structure. A Mach-Zehnder electro-optical modulator (MZM) is inserted between the seed source and the amplifier stage. External modulation is used to implement OOK / BPSK / QPSK modulation. This is then amplified by optical fiber and frequency multiplied to achieve high-power coded pulse output. Finally, green laser light is coupled into the optical fiber and output through a collimated transmission system.

[0011] The present invention is implemented as follows: a high-power underwater laser communication transmitter compatible with multiple modulation formats, comprising: Seed source module: A DBR narrow linewidth laser with a wavelength of 1064nm is used as the seed source, driven by a constant current source, with an output power of 50mW of linearly polarized light, and coupled to the output using a single-mode polarization-maintaining fiber; Signal modulation module: The pulsed electrical signal is generated by an arbitrary waveform generator (AWG) and amplified by a power amplifier (AMP) to the input of the Mach-Zehnder electro-optical modulator (MZM). Existing common intensity modulation and phase modulation can be implemented on the MZM. Depending on the MZM operating mode, the modulation method of the optical signal also varies. The three modulation formats of OOK, BPSK, and DPSK can be implemented on a single MZM.

[0012] The optical signal output by the MZM at the orthogonal operating point is OOK, which is intensity modulation, while the output at the minimum operating point is a phase-modulated BPSK signal. Since DPSK is a differential form of BPSK, adding a differential precoding function after the digital signal is transmitted can achieve the output of the DPSK signal.

[0013] Power Amplifier Module: This module uses a 1064nm DBR laser as the seed source and employs a master oscillator power amplifier (MOPA) structure. The module consists of a pre-amplifier stage connected in series with two single-mode fibers and a main amplifier stage connected in series with two double-clad fibers, both utilizing forward pumping. The seed laser is coupled into the gain fiber through a beam combiner, amplified by the pump source, and output through an optical isolator, achieving an average 1064nm laser output power of 30W.

[0014] Figure 3 The overall amplifier stage structure consists of a 1064nm seed source followed by a Mach-Zehnder electro-optic modulator (MZM). Since the absorption and emission of ytterbium-doped fiber at 976nm coincide with each other, reabsorption is very likely to occur, resulting in the generation of 1030nm laser light. Therefore, the pre-amplifier stage adopts a two-stage structure, which increases the length of the ytterbium-doped fiber in a disguised manner and reduces the single-shot gain. At the same time, in order to reduce the noise figure, the pumping mode is forward pumping.

[0015] The signal light from the DBR laser first passes through two stages of ytterbium-doped polarization-maintaining fiber (PM-YSF-6 / 125), pumped by a 700mW, 976nm pump source. The laser output from a 50 / 50 beam splitter is then combined via a wavelength division multiplexer (WDM) into the ytterbium-doped polarization-maintaining fiber. An isolator prevents reverse light from propagating backward and damaging the preceding stage. The output is terminated with a fiber filter to remove spontaneous emission (ASE) generated during fiber amplification and the 1030nm laser light generated by reabsorption. After two stages of pre-amplification, a 200mW signal light is generated.

[0016] After two stages of single-mode pre-amplification, the signal light enters the main amplifier stage, which consists of the following parts: (1) Pump source: The first-stage amplification system uses a single-mode semiconductor laser with a maximum output power of 10 W, an operating wavelength of 915 nm, a core / cladding diameter of the output fiber of 105 / 125 μm, and a numerical aperture of 0.22. The second-stage amplification system uses two multi-mode semiconductor lasers with a maximum output power of 20 W, an operating wavelength of 915 nm, a core / cladding diameter of the output fiber of 105 / 125 μm, and a numerical aperture of 0.22.

[0017] (2) Combiner: The first stage of amplification uses a (1+1)×1 combiner, with a maximum power handling capacity of 30W per arm at the pump end. The core / cladding diameter of the pump end pigtail is 105 / 125µm, with a numerical aperture of 0.22. The core / cladding diameters of the signal input and output pigtails are 10 / 125µm. The second stage of amplification uses a (2+1)×1 combiner, with a core / cladding diameter of 20 / 125µm at the signal output end.

[0018] (3) Double-clad ytterbium-doped fiber: The inner cladding of the double-clad fiber used for primary amplification is octagonal, the core diameter / inner cladding diameter is 10 / 125µm, and the core numerical aperture / inner cladding numerical aperture is 0.08 / 0.46. The inner cladding of the double-clad fiber used for secondary amplification is also octagonal, the core / inner cladding diameter is 20 / 125µm, and the core numerical aperture / inner cladding numerical aperture is 0.08 / 0.46.

[0019] (4) Isolator: The optical isolator used in the first stage of amplification has an operating wavelength of 1064nm and a maximum power handling capacity of 15W. The optical isolator used in the second stage of amplification has an operating wavelength of 1064nm and a maximum power handling capacity of 50W.

[0020] LBO wavelength conversion module: uses the nonlinearity of LBO crystal to achieve frequency doubling function and output 532nm high-power laser; LBO wavelength conversion module such as Figure 4 As shown, it primarily consists of a collimating lens, a focusing lens, and a nonlinear optical element (LBO). The incident beam diameter of the collimating lens is 7 mm, and the collimating lens aperture is 20 mm. The focusing lens aperture is 10 mm. The LBO is a rectangular parallelepiped with dimensions of 25 mm × 3 mm × 2 mm, and a light-entering surface of 3 mm × 2 mm. The beam propagation length in the LBO is 12.5 mm, and the incident light wavelength is 1064 nm. By adjusting the LBO crystal angle and maintaining its temperature at 148–160°C, 30 W of 1064 nm infrared light can be converted into 532 nm green light with a conversion efficiency exceeding 25%, resulting in an output power of 7 W at 532 nm green light.

[0021] Small angle collimation emission module: Small angle collimation emission module such as Figure 5As shown, after passing through the LBO wavelength conversion module, the 1064nm signal light is converted to 532nm signal light. This light is then coupled into a multimode fiber via a fiber coupler. A harmonic separator separates the green light from the fundamental wave. The fiber coupler then couples the signal into a multimode fiber, outputting a nearly parallel wireless optical beam.

[0022] By using master oscillator power amplifier (MOPA) technology, the power of the seed light source is amplified from 50 milliwatts to 30 watts, significantly improving the laser output power, overcoming the high attenuation characteristics of seawater on optical signals, and providing conditions for achieving ultra-long-distance underwater communication.

[0023] Flexible switching among three modulation formats, OOK, BPSK, and DPSK, is achieved through a single Mach-Zehnder electro-optical modulator (MZM). Combined with the high noise immunity of BPSK / DPSK, it effectively reduces turbulence interference and bit error rate, and improves communication stability and reliability in complex seawater environments.

[0024] LBO crystal frequency doubling technology is used to efficiently convert 1064nm infrared light into 532nm green light (conversion efficiency > 25%), with an output power of 7W. Combined with a small-angle collimated emission system, the beam divergence angle is compressed, the optical signal concentration is enhanced, and the effective transmission distance is further extended.

[0025] A multi-stage fiber amplification structure (pre-amplification + main amplification) and forward pumping design are used to reduce noise figure and reabsorption effects; isolators and filters are used to suppress reverse light and spontaneous emission (ASE), improving signal purity.

[0026] It comprehensively solves the problems of insufficient light source power, single modulation format, weak anti-interference ability and other problems in the existing technology, and realizes high-speed, high-quality and long-distance underwater laser communication in complex seawater turbulence environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a design principle diagram of a high-power underwater laser communication transmitter compatible with multiple modulation formats according to the present invention; Figure 2 This is a schematic diagram of the principle of the signal modulation module MZM modulator of the present invention to realize multiple modulation formats.

[0028] Figure 3 This is a schematic diagram of the signal amplification principle of the power amplifier module of the present invention based on the MOPA structure.

[0029] Figure 4 This is a schematic diagram of the LBO wavelength conversion module of the present invention; Figure 5 This is a schematic diagram of the principle of the small-angle collimated emission module of the present invention. DETAILED DESCRIPTION

[0030] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments.

[0031] In response to the problems of insufficient light source power, single modulation format and weak anti-interference ability in the prior art, the present invention provides a design method for a high-power underwater laser communication transmitter that is compatible with multiple modulation formats. The present invention is described in detail below with reference to the accompanying drawings.

[0032] An embodiment of the present invention provides a high-power underwater laser communication transmitter compatible with multiple modulation formats. It uses a 1064nm DBR semiconductor laser as a seed source for signal modulation and amplification. It then uses an LBO crystal to frequency-double the 1064nm signal light into 532nm green light, thereby achieving high-power green light output.

[0033] like Figure 1 As shown, an embodiment of the present invention provides a high-power underwater laser communication transmitter compatible with multiple modulation formats, including a seed source module, a signal modulation module, a power amplification module, an LBO wavelength conversion module and a small-angle collimation transmission module.

[0034] In the seed source module, the seed source is a narrow-pulse DBR laser driven by a constant current source and with an output power of 50 mW. The seed source module couples 50 mW of linearly polarized light through a single-mode polarization-maintaining fiber and outputs it to a Mach-Zehnder modulator (MZM). In the signal modulation module, an arbitrary waveform generator (AWG) generates a pulsed electrical signal, which is then amplified by a power amplifier (AMP) to the modulator input, driving the Mach-Zehnder modulator (MZM). The MZM can operate in three modes, or bias points, depending on the applied bias voltage: minimum bias, quadrature bias, and maximum bias. A bias controller is used to adjust the bias voltage applied to each arm of the MZM, enabling operation at the quadrature bias point, at which point OOK modulation is possible. A bias controller is also used to adjust the bias voltage applied to each arm of the MZM, enabling operation at the minimum bias point, at which point BPSK and DPSK modulation are possible. The modulated linearly polarized light is then coupled to the power amplifier module via a single-mode polarization-maintaining fiber.

[0035] In the power amplifier module, the module adopts a master oscillator power amplifier (MOPA) structure, including two pre-amplifier stages and two main amplifier stages.

[0036] Pre-amplification stage: The first stage uses ytterbium-doped polarization-maintaining fiber (PM-YSF-6 / 125) and a 976nm, 700mW semiconductor laser as the pump source. The pump light and signal light are combined via a wavelength division multiplexer (WDM). The second stage has the same structure as the first stage, but the output power is increased to 200mW when the two stages are connected in series.

[0037] Main amplifier stage: The first stage uses a 915nm, 10W single-mode pump source, which is injected into a double-clad ytterbium-doped fiber (core / inner cladding diameter 10 / 125μm) through a (1+1)×1 combiner. The second stage uses two 915nm, 20W multimode pump sources, which are injected into a double-clad ytterbium-doped fiber (core / inner cladding diameter 20 / 125μm) through a (2+1)×1 combiner (312).

[0038] After the signal light output by the signal modulation module is amplified twice by the pre-amplifier stage and the main amplifier stage, the 1064nm laser output power reaches 30W, and the isolator ensures that the optical path is unidirectionally output to the LBO wavelength conversion module.

[0039] In the LBO wavelength conversion module, the high-power optical signal output by the power amplifier module is first input into the wavelength conversion module's collimating lens, which has a diameter of 20mm. The collimating lens then shapes the 1064nm optical signal into parallel light. This light is then focused onto the LBO crystal through a focusing lens with a focal length of 105mm. By adjusting the LBO crystal angle and maintaining its temperature between 148°C and 160°C, 30W of 1064nm infrared light can be converted into 532nm green light with a conversion efficiency exceeding 25%, resulting in an output power of 7W of 532nm green light.

[0040] In the low-angle collimated transmitter module, 532nm green light is coupled to a fiber collimator via a multimode fiber. The collimator uses an aspheric lens assembly to compress the beam divergence to less than 2mrad, outputting a near-parallel optical signal that is transmitted through a waterproof optical window to the underwater communication link.

[0041] The entire system can ultimately achieve underwater high-power green light output with an output wavelength of 532nm and an optical power greater than 7W.

Claims

1. A multi-modulation format compatible underwater laser communication transmitter, characterized in that: include: Seed source module, signal modulation module, power amplification module, LBO wavelength conversion module and small angle collimation emission module; The seed source module is a 1064nm DBR narrow linewidth laser; The signal modulation module includes a modulator, which realizes switching of OOK, BPSK, and DPSK modulation formats by adjusting the bias point; The power amplification module adopts a main oscillation power amplification structure, which includes two pre-amplification stages and two main amplification stages; The LBO wavelength conversion module frequency-doubles 1064nm light into 532nm green light with an output power of no less than 7W; The small-angle collimated emission module compresses the green light and then outputs it.

2. The multi-modulation format compatible underwater laser communication transmitter according to claim 1, characterized in that: In the signal modulation module, the MZM implements OOK modulation at the orthogonal bias point; the MZM implements BPSK modulation at the minimum bias point, and generates a DPSK signal through differential precoding.

3. The multi-modulation format compatible underwater laser communication transmitter according to claim 1, characterized in that: In the power amplifier module, the pre-amplification stage is composed of two stages of single-mode ytterbium-doped optical fibers connected in series; the main amplification stage includes a first-stage amplification and a second-stage amplification.

4. The multi-modulation format compatible underwater laser communication transmitter according to claim 1, characterized in that: The LBO wavelength conversion module includes a collimating lens, a focusing lens and an LBO crystal; the crystal temperature is maintained at 148-160°C, and the frequency doubling conversion efficiency is greater than 25%.

5. The multi-modulation format compatible underwater laser communication transmitter according to claim 4, characterized in that: The small-angle collimation emission module adopts an aspheric lens group, and the beam divergence angle is compressed to 1.5-2mrad.

6. The multi-modulation format compatible underwater laser communication transmitter according to claim 5, characterized in that: An isolator and a filter are provided in the optical path of the main amplifier stage, which can suppress reverse light and spontaneous radiation noise.

7. The multi-modulation format compatible underwater laser communication transmitter according to claim 3, characterized in that: The first-stage amplification includes a 915nm / 10W single-mode pump injected into the core of a 10 / 125μm double-clad optical fiber via a (1+1)×1 combiner.

8. The multi-modulation format compatible underwater laser communication transmitter according to claim 5, characterized in that: The secondary amplification includes two 915nm / 20W multimode pumps injected into the core of a 20 / 125μm double-clad optical fiber via a (2+1)×1 combiner.

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