Wavelength conversion method and system based on double-crystal cascade difference frequency and optical parametric amplification

Abstract

The application discloses a wavelength conversion method and system based on double-crystal cascade difference frequency and optical parametric amplification, relates to the technical field of optical communication, and comprises the following steps: obtaining first pump light and signal light, inputting the first pump light and the signal light into a first nonlinear crystal for wavelength conversion, and obtaining mid-infrared idler light; obtaining second pump light, inputting the mid-infrared idler light and the second pump light into a second nonlinear crystal for optical parametric amplification and power compensation, and obtaining amplified and compensated mid-infrared signal; and performing an inverse conversion process based on the amplified and compensated mid-infrared signal, so that the wavelength conversion and optical power amplification are integrated and cooperatively processed, and the signal recovery quality and link stability are improved.

Description

Technical Field

[0001] This invention relates to the field of optical communication technology, specifically to a wavelength conversion method and system based on dual-crystal cascaded difference frequency and optical parametric amplification. Background Technology

[0002] The mid-infrared band possesses excellent atmospheric transmission characteristics, making it highly valuable for applications in space optical communication. Compared to other bands, the mid-infrared band exhibits lower absorption and scattering losses within specific atmospheric windows and demonstrates strong resistance to scintillation caused by atmospheric turbulence. This is beneficial for improving the transmission stability and reliability of space optical communication links, providing a sound physical foundation for constructing long-distance, high-quality space optical communication systems. However, constructing mid-infrared space optical communication systems typically relies on key optoelectronic devices such as lasers, modulators, and detectors operating in this band. Although some progress has been made in related devices in recent years, overall issues remain, including insufficient device maturity, significant system integration difficulties, and high engineering application costs, thus limiting the further development and practical application of mid-infrared space optical communication systems. Therefore, there is an urgent need to find a technological path that can circumvent the bottlenecks in the development of mid-infrared direct devices. A nonlinear frequency conversion method based on quasi-phase matching technology provides a new approach for generating mid-infrared optical signals. This method fully leverages the advantages of mature near-infrared device systems and well-developed modulation and detection technologies. By converting near-infrared signals to the mid-infrared band through nonlinear wavelength transformation, it indirectly generates mid-infrared spatial light signals. This approach is expected to reduce the difficulty of system implementation and improve its engineering feasibility, thus possessing significant research value and application prospects.

[0003] Among existing nonlinear frequency conversion schemes, difference frequency generation (DFG) technology has become one of the important technical means to realize near-infrared to mid-infrared band conversion because it can better preserve the modulation information and phase characteristics of the original signal. Especially in the context of space optical communication, wavelength conversion schemes based on difference frequency generation can utilize the mature signal generation, modulation and processing links in the near-infrared band to construct mid-infrared transmitted signals. However, existing methods cannot effectively compensate the power of the converted mid-infrared signal, resulting in low transmission capacity and communication quality of mid-infrared space optical communication systems. Summary of the Invention

[0004] To address the shortcomings mentioned in the background section, the present invention aims to provide a wavelength conversion method and system based on dual-crystal cascaded difference frequency and optical parametric amplification, thereby solving the problem that the prior art cannot effectively compensate the power of the converted mid-infrared signal.

[0005] Firstly, the objective of this invention can be achieved through the following technical solution: a wavelength conversion method based on dual-crystal cascaded difference frequency and optical parametric amplification, the method comprising the following steps: The first pump light and signal light are acquired, and the first pump light and signal light are input into the first nonlinear crystal for wavelength conversion to obtain mid-infrared idler light; The second pump light is acquired, and the mid-infrared idler light and the second pump light are input into the second nonlinear crystal for optical parametric amplification and power compensation to obtain the amplified and compensated mid-infrared signal.

[0006] In conjunction with the first aspect, in some implementations of the first aspect, the method further includes: the first nonlinear crystal is selected based on the target conversion band, the pump light wavelength, the phase matching condition, and the system integration requirements.

[0007] In conjunction with the first aspect, in some implementations of the first aspect, the method further includes: the process of inputting the first pump light and signal light to the first nonlinear crystal, comprising the following steps: After the first pump light and signal light are amplified, combined and collimated in two stages, they are input into the PPLN crystal. Under quasi-phase matching conditions, the difference frequency generation process is completed, realizing the wavelength conversion of the C-band signal light to the mid-infrared band, and finally outputting the mid-infrared idler light.

[0008] In conjunction with the first aspect, in some implementations of the first aspect, the method further includes: the process of amplifying, combining, and collimating the first pump light and the signal light through two stages includes the following steps: The first pump light and signal light are amplified in the first stage by a polarization-maintaining erbium-doped fiber amplifier. The amplified first pump light and signal light are then input into a polarization-maintaining ytterbium-doped fiber amplifier pumped by two laser diodes for a second stage of amplification, resulting in a two-stage amplified first pump light and signal light. The first pump light and signal light, after being amplified by two stages, are input into a wavelength division multiplexer for beam combining to obtain the combined beam. The combined beam is collimated by an optical fiber collimator, and finally the collimated beam is output.

[0009] In conjunction with the first aspect, in some implementations of the first aspect, the method further includes: the second nonlinear crystal is selected based on the mid-infrared seed light wavelength, the second pump light wavelength, the optical parametric amplification gain, thermal stability, and power carrying capacity.

[0010] In conjunction with the first aspect, in some implementations of the first aspect, the method further includes: the process of inputting the mid-infrared idler light and the second pump light into the second nonlinear crystal, comprising the following steps: After the mid-infrared idler light and the second pump light are amplified, combined and collimated in two stages, they are input into the ZGP crystal. Under the condition of satisfying the phase matching condition, the energy of the second pump light is transferred to the mid-infrared idler light, and the amplified and compensated mid-infrared signal is output, realizing the optical parametric amplification and power compensation of the mid-infrared idler light.

[0011] In conjunction with the first aspect, in some implementations of the first aspect, the method further includes: the amplified and compensated mid-infrared signal is expanded, collimated, and wavefront shaped by a lens group to optimize its free-space transmission characteristics, and finally coupled to a free-space transmitting antenna to realize the transmission of the compensated high-power mid-infrared signal.

[0012] In conjunction with the first aspect, in some implementations of the first aspect, the method further includes: injecting the amplified and compensated mid-infrared signal, together with the third pump light, into a second nonlinear crystal; and performing energy recovery on the received amplified and compensated mid-infrared signal through an inverse parametric amplification process to obtain the recovered signal. The recovered signal and the fourth pump light are injected together into the first nonlinear crystal to perform wavelength conversion on the recovered signal through an inverse difference frequency generation process, thereby obtaining the C-band recovered signal. Output the C-band recovery signal.

[0013] Secondly, in order to achieve the above objectives, the present invention discloses a wavelength conversion system based on dual-crystal cascaded difference frequency and optical parametric amplification, comprising: The conversion processing module is used to acquire the first pump light and the signal light, input the first pump light and the signal light into the first nonlinear crystal for wavelength conversion, and obtain mid-infrared idler light; The compensation output module is used to acquire the second pump light, input the mid-infrared idler light and the second pump light into the second nonlinear crystal for optical parametric amplification and power compensation, and obtain the amplified and compensated mid-infrared signal.

[0014] The beneficial effects of this invention are: This invention achieves integrated and coordinated processing of wavelength conversion and optical power amplification by organically cascading difference frequency generation and optical parametric amplification. By performing optical parametric amplification and power compensation on the converted mid-infrared signal at the transmitting end, its adaptability to medium and long-distance free space links is enhanced. Furthermore, the subsequent inverse conversion method is used to improve signal recovery quality and link stability. Attached Figure Description

[0015] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Figure 1 This is a schematic diagram of the method flow of the present invention; Figure 2 This is a schematic diagram of the conversion process of the present invention; Figure 3 This is a schematic diagram of the first-level DFG unit in an embodiment of the present invention; Figure 4 This is a schematic diagram of the second-level OPA unit in an embodiment of the present invention; Figure 5 This is a schematic diagram of optical parameter restoration according to an embodiment of the present invention; Figure 6 This is a schematic diagram of inverse wavelength conversion according to an embodiment of the present invention; Figure 7 This is a schematic diagram of the system structure of the present invention. Detailed Implementation

[0016] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0017] Example 1: like Figure 1 As shown, a wavelength conversion method based on dual-crystal cascaded difference frequency and optical parametric amplification includes the following steps: S101: Acquire the first pump light and signal light, input the first pump light and signal light into the first nonlinear crystal for wavelength conversion, and obtain mid-infrared idler light; In this embodiment, the first nonlinear crystal is selected as a PPLN crystal.

[0018] In this scheme, the first nonlinear crystal is not limited to PPLN crystals. Depending on the wavelength of the first pump light, the wavelength of the signal light, the mid-infrared output wavelength of the target, the phase matching method, the nonlinear conversion efficiency, the optical damage threshold, and the system integration conditions, the first nonlinear crystal can also be a PPLT crystal, PPKTP crystal, OP-GaAs crystal, AgGaS2 crystal, AgGaSe2 crystal, GaSe crystal, LiGaS2 crystal, or LiGaSe2 crystal, etc., which are nonlinear crystals capable of generating difference frequencies. For different crystals, parameters such as crystal polarization period, cutting angle, operating temperature, and incident light polarization state need to be adjusted accordingly based on their refractive index dispersion characteristics, transmission band, and phase matching conditions.

[0019] PPLT crystals possess good thermal stability and resistance to photorefractive indexes, but their effective nonlinear coefficient and engineering application maturity differ somewhat from those of PPLN crystals. PPKTP crystals exhibit a high optical damage threshold and good mechanical properties, but their achievable polarization period, effective operating length, and mid-infrared transmission range limit the wavelength conversion capabilities of this scheme. OP-GaAs crystals have a large nonlinear coefficient and a wide mid-infrared transmission range, but their oriented patterned structure is difficult to fabricate, resulting in relatively high manufacturing costs and difficulties in obtaining large-size, high-quality crystals. AgGaS2, AgGaSe2, and GaSe crystals can all be used for mid-infrared difference frequency conversion. Among them, AgGaS2 and AgGaSe2 crystals are relatively limited in terms of thermal performance, optical damage threshold, and high power handling capacity, while GaSe crystals are prone to cleavage due to their layered structure, making cutting, polishing, coating, and long-term assembly difficult. LiGaS2 and LiGaSe2 crystals have a wide mid-infrared transmission range and some potential for high-power applications, but their material preparation, processing technology, and engineering application maturity are relatively limited. Existing research has shown that periodically polarized lithium niobate can achieve high-efficiency difference-frequency conversion using a quasi-phase-matching method, while different materials have different applicable ranges in terms of transmission band, phase-matching method, and engineering processing performance.

[0020] Compared to the aforementioned candidate crystals, this embodiment preferably uses MgO:PPLN crystal, i.e., magnesium oxide-doped periodically polarized lithium niobate crystal. PPLN crystals can achieve quasi-phase matching by setting periodically reversed ferroelectric domains, and the target conversion wavelength can be precisely controlled by adjusting the polarization period and operating temperature, making them suitable for converting near-infrared communication signals to the mid-infrared band at approximately 3.6 μm. The use of MgO doping also improves the lithium niobate crystal's resistance to photorefractive damage and high-power operating stability, while retaining its high nonlinear conversion capability.

[0021] Specifically, in this embodiment, the preferred length of the MgO:PPLN crystal is 50 mm, the preferred MgO doping concentration is 5 mol%, the preferred polarization period is 30 μm, and the preferred crystal operating temperature is controlled at approximately 53.02 °C. A first pump light with a wavelength of 1083 nm and a C-band signal light with a center wavelength of 1550 nm are amplified, combined, and collimated before being input into the MgO:PPLN crystal. Under quasi-phase matching conditions, difference-frequency interaction occurs, generating mid-infrared idler light with a wavelength of approximately 3.6 μm, specifically approximately 3594 nm. By using a longer crystal length, the effective nonlinear interaction distance of the input beam in the crystal can be increased; by setting the appropriate polarization period and operating temperature, quasi-phase matching can be maintained between the first pump light, the signal light, and the mid-infrared idler light, thereby improving the difference-frequency conversion efficiency and output stability.

[0022] Considering the 1083nm first pump light, 1550nm C-band signal light, and approximately 3.6μm mid-infrared target output wavelength used in this scheme, as well as the crystal's nonlinear conversion efficiency, quasi-phase-matching control capability, resistance to optical damage, processing maturity, and system integration difficulty, the MgO:PPLN crystal exhibits good overall adaptability. Therefore, in this embodiment, the MgO:PPLN crystal is ultimately preferred as the first nonlinear crystal.

[0023] The preferred PPLN crystal is MgO:PPLN crystal, i.e., magnesium oxide-doped periodically polarized lithium niobate crystal. The MgO:PPLN crystal achieves quasi-phase matching for nonlinear optical processes by incorporating periodically reversed ferroelectric domain structures within the lithium niobate crystal. Compared to undoped PPLN crystals, MgO doping improves the crystal's resistance to optical damage and operational stability, making it more suitable for difference-frequency wavelength conversion under higher power pump conditions.

[0024] In this embodiment, the length of the MgO:PPLN crystal is preferably 50 mm, the MgO doping concentration is preferably 5 mol%, and the polarization period is preferably 30 μm. The crystal uses a quasi-phase-matching method for difference frequency generation, and its optimal operating temperature is preferably 53.02 °C. In actual use, the crystal temperature can be adjusted and stabilized using a temperature control device to compensate for changes in ambient temperature or thermal effects caused by pump light, ensuring that the difference frequency process continuously meets the quasi-phase-matching conditions.

[0025] During the difference-frequency conversion process, the C-band signal light with a center wavelength of 1550 nm and the first pump light with a wavelength of 1083 nm are amplified, combined, and collimated before being input into the MgO:PPLN crystal. Under quasi-phase-matched conditions with a polarization period of approximately 30 μm and a crystal temperature of approximately 53.02 °C, the first pump light and the signal light undergo difference-frequency interaction in the MgO:PPLN crystal, generating mid-infrared idler light with a wavelength of approximately 3.6 μm, specifically with an output wavelength of approximately 3594 nm.

[0026] The above wavelength relationship satisfies the frequency conservation relationship in the difference frequency generation process, that is, the pump light frequency is equal to the sum of the signal light frequency and the idler light frequency. Based on the first pump light of 1083nm and the signal light of 1550nm, the obtained idler light wavelength is approximately 3594nm, which corresponds to the approximately 3.6μm mid-infrared output wavelength set in this embodiment.

[0027] By employing a 50mm long MgO:PPLN crystal, the effective nonlinear interaction distance between the pump light and the signal light within the crystal can be increased. By using 5mol% MgO doping, the crystal's resistance to photorefractive damage under high input optical power conditions can be improved. By setting a polarization period of approximately 30μm and controlling the operating temperature at approximately 53.02℃, the difference frequency process between the 1083nm pump light and the 1550nm signal light can satisfy the quasi-phase matching condition, thereby improving the conversion efficiency and output stability of the mid-infrared idler light.

[0028] like Figure 2 As shown, the process of inputting the first pump light and signal light into the first nonlinear crystal includes the following steps: After the first pump light and signal light are amplified, combined and collimated in two stages, they are input into the PPLN crystal. Under quasi-phase matching conditions, the difference frequency generation process is completed, realizing the wavelength conversion of the C-band signal light to the mid-infrared band, and finally outputting the mid-infrared idler light.

[0029] The process of amplifying, combining, and collimating the first pump light and signal light through two stages includes the following steps: The first pump light and signal light are amplified in the first stage by a polarization-maintaining erbium-doped fiber amplifier. The amplified first pump light and signal light are then input into a polarization-maintaining ytterbium-doped fiber amplifier pumped by two laser diodes for a second stage of amplification, resulting in a two-stage amplified first pump light and signal light. The first pump light and signal light, after being amplified by two stages, are input into a wavelength division multiplexer for beam combining to obtain the combined beam. The combined beam is collimated by an optical fiber collimator, and finally the collimated beam is output.

[0030] Specifically, in this embodiment: like Figure 3 As shown, in the DFG unit, the C-band signal source outputs signal light with a center wavelength of 1550 nm. This signal light first undergoes a first-stage amplification via a polarization-maintaining erbium-doped fiber amplifier (PM-EDFA) to increase the input signal power. Subsequently, it is isolated by an optical isolator (ISO) to prevent back reflection from affecting the preceding light source and amplifier. Afterward, the signal light is input for a second-stage amplification via a polarization-maintaining ytterbium-doped fiber amplifier (PM-Yb) pumped by two laser diodes (LDs) to further meet the input signal power requirements of the difference frequency conversion process.

[0031] Simultaneously, the first pump light with a wavelength of 1083 nm is amplified in two stages by a polarization-maintaining ytterbium-doped fiber amplifier (PM-YDFA) and a PM-Yb amplifier, and then input together with the 1550 nm signal light amplified in two stages into a wavelength-division multiplexer (WDM) for beam combining. The combined beam is then collimated by a fiber collimator (Col) and injected into the PPLN crystal. Under quasi-phase-matched conditions, the difference frequency generation process is completed, thereby realizing the wavelength conversion from the C-band to the mid-infrared band.

[0032] S102: Acquire the second pump light, input the mid-infrared idler light and the second pump light into the second nonlinear crystal for optical parametric amplification and power compensation, and obtain the amplified and compensated mid-infrared signal.

[0033] In this embodiment, the second nonlinear crystal is selected as a ZGP crystal.

[0034] The second nonlinear crystal in this scheme is not limited to ZGP crystals. Depending on the pump light wavelength, mid-infrared signal light wavelength, target output band, nonlinear conversion efficiency, optical damage threshold, thermal stability, and engineering processing conditions, the second nonlinear crystal can also be AgGaS2 crystal, AgGaSe2 crystal, GaSe crystal, OP-GaAs crystal, or CdSiP2 crystal, or other nonlinear crystals capable of achieving mid-infrared parametric amplification.

[0035] Specifically, AgGaS2 crystals have a wide mid-infrared transmission range, but their nonlinear conversion capability, thermal performance, and high power handling capacity are relatively limited; AgGaSe2 crystals are suitable for mid-infrared frequency conversion in longer wavelength bands, but their mechanical properties, thermal stability, and resistance to optical damage are relatively insufficient; GaSe crystals have a high nonlinear coefficient and a wide transmission range, but due to their layered structure, they are prone to cleavage, making processing, coating, and long-term assembly difficult; OP-GaAs crystals have a high nonlinear coefficient and a wide wavelength tuning range, but their orientation patterned structure fabrication process is complex, requiring high processing precision and crystal quality; CdSiP2 crystals have strong nonlinear conversion capability, but their crystal growth, processing technology, and engineering application maturity are relatively limited.

[0036] Compared to the aforementioned crystals, ZGP crystals, or ZnGeP2 crystals, possess higher second-order nonlinear coefficients, a wider mid-infrared transmission range, better thermal stability, and stronger power handling capacity, making them suitable for mid-infrared parametric amplification under approximately 2μm pump conditions. ZGP crystals employ a birefringence angle phase-matching method, where phase-matching conditions can be met by adjusting the crystal cut angle, incident angle, and incident light polarization state, eliminating the need for periodic polarization structures and reducing the difficulty of crystal manufacturing and system integration.

[0037] In this embodiment, the approximately 3.6μm mid-infrared signal light obtained after the first-stage wavelength conversion is used as the seed light input to the ZGP crystal. Simultaneously, the approximately 2.05μm second pump light is combined with the seed light, collimated, and injected into the ZGP crystal. Under phase-matching conditions, the energy of the second pump light is transferred to the mid-infrared seed light, thereby achieving optical parametric amplification and power compensation of the 3.6μm mid-infrared signal light. The length of the ZGP crystal can be set to 10–20 mm, preferably 20 mm in engineering implementation. The crystal can operate at room temperature, and the operating temperature can be controlled between 20–25°C according to pump power, phase-matching status, and thermal stability requirements.

[0038] Considering the pump light wavelength, signal light wavelength, optical parametric gain, power compensation capability, thermal stability, crystal processing conditions, and system engineering feasibility of this scheme, the ZGP crystal has better overall adaptability compared to other optional mid-infrared nonlinear crystals. Therefore, this embodiment ultimately selects the ZGP crystal as the second nonlinear crystal.

[0039] Among them, ZnGeP2 crystal is preferred for ZGP crystal. ZGP crystal is a nonlinear crystal suitable for optical parametric amplification in the mid-infrared band. It has a high second-order nonlinear coefficient, a wide mid-infrared transmission range, good thermal stability, and strong power carrying capacity, making it suitable for optical parametric amplification and power compensation of mid-infrared signals.

[0040] The crystal length of the ZGP crystal can be set to 10–20 mm, preferably 20 mm. The crystal adopts a birefringence angle phase matching method, that is, by adjusting the crystal cutting angle, incident angle, and polarization state of pump light and signal light, the phase matching condition between pump light, signal light and idler light is met, thereby realizing a high-efficiency optical parametric amplification process.

[0041] In this embodiment, the 3.6μm mid-infrared signal light output after the first-stage PPLN crystal differential frequency conversion is used as the seed light input to the ZGP crystal. Simultaneously, a second pump light with a wavelength of 2.05μm is input to the ZGP crystal, and under phase-matching conditions, the pump light energy is transferred to the 3.6μm mid-infrared signal light, achieving optical parametric amplification and power compensation of the mid-infrared signal light. After the ZGP crystal's action, the output light includes the amplified 3.6μm mid-infrared signal light, and simultaneously generates a corresponding mid-infrared idler light.

[0042] ZGP crystals can operate at room temperature. In engineering applications, they can be set to a temperature control of 20–25°C. Passive cooling or active temperature control methods can also be used depending on the phase matching angle, pump power, and thermal stability requirements to ensure the stability of the crystal during optical parametric amplification. By setting appropriate crystal length, pump power, incident angle, and phase matching conditions, the parametric gain efficiency of the ZGP crystal can be improved, thereby enhancing the power of the output mid-infrared signal and its free-space transmission capability.

[0043] The process of inputting mid-infrared idler light and second pump light into a second nonlinear crystal includes the following steps: After the mid-infrared idler light and the second pump light are amplified, combined and collimated in two stages, they are input into the ZGP crystal. Under the condition of satisfying the phase matching condition, the energy of the second pump light is transferred to the mid-infrared idler light, and the amplified and compensated mid-infrared signal is output, realizing the optical parametric amplification and power compensation of the mid-infrared idler light.

[0044] The amplified and compensated mid-infrared signal is expanded, collimated, and wavefront shaped by a lens group to optimize its free-space transmission characteristics, and finally coupled to the free-space transmitting antenna to realize the transmission of the compensated high-power mid-infrared signal.

[0045] Specifically, such as Figure 4As shown, after separation by filter F1, the mid-infrared idler light output from the first stage is used as seed light input to the second-stage optical parametric amplification unit. Simultaneously, the independent pump laser outputs pump light with a wavelength of 2.05 μm. This pump light is sequentially amplified by two stages of polarization-maintaining thulium-doped fiber amplifiers (PM-TDFAs) to meet the pump power requirements for high-efficiency optical parametric amplification using the ZGP crystal.

[0046] The amplified 2.05 μm pump light output is then combined with the mid-infrared seed light from the first stage via WDM. The combined beam is then collimated by Col and injected into the ZGP crystal. Under phase-matching conditions, the energy of the pump light is transferred to the seed light, thereby achieving optical parametric amplification of the mid-infrared signal.

[0047] The mid-infrared beam, amplified by the OPA process, is then expanded, collimated, and wavefront shaped by a lens group to optimize its free-space transmission characteristics. Finally, it is coupled to the free-space transmitting antenna to achieve the transmission of high-power mid-infrared signals.

[0048] The amplified and compensated mid-infrared signal is injected into the second nonlinear crystal along with the third pump light. Energy recovery of the received amplified and compensated mid-infrared signal is then performed through an inverse parametric amplification process to obtain the recovered signal. The recovered signal and the fourth pump light are injected together into the first nonlinear crystal to perform wavelength conversion on the recovered signal through an inverse difference frequency generation process, thereby obtaining the C-band recovered signal. Output the C-band recovery signal.

[0049] Specifically, such as Figure 5 As shown, the mid-infrared signal, after transmission through the free-space atmospheric channel, is first received by the receiving antenna and then focused by lens L3 to improve the coupling efficiency of subsequent optical paths and meet the incident conditions required for nonlinear interaction. To achieve high-quality recovery of the received signal, the pump light with a wavelength of 2.05 μm is amplified in two stages and then collimated by Col to obtain the beam quality and power level required for subsequent parametric interaction. Subsequently, the pump light passes through a half-wave plate (HWP) to adjust its polarization state, ensuring that it meets the phase-matching condition with the received mid-infrared signal light in the crystal, thereby improving the efficiency of nonlinear interaction.

[0050] The polarization-adjusted pump light is reflected by plane mirror M3 into the subsequent optical path, and finally spatially combined and focused with the received mid-infrared signal light by plano-concave dielectric mirror M5, so that the two are injected into the ZGP crystal in a matched spatial mode. Under the condition of satisfying the phase matching condition, the received signal light and the pump light undergo parametric interaction in the ZGP crystal, realizing optical parametric recovery or gain compensation of the mid-infrared signal, providing power and quality assurance for subsequent reverse wavelength conversion and reconstruction of the original signal.

[0051] Furthermore, by coordinating the configuration of lens L3, collimator Col, half-wave plate HWP, plane mirror M3, and plano-concave dielectric mirror M5, precise matching of the received signal light and pump light in terms of propagation direction, polarization state, spatial mode, and focusing position can be achieved. This improves the parametric efficiency in the ZGP crystal, reduces the adverse effects of link transmission loss and noise accumulation on system performance, and thus helps to improve the signal recovery quality at the receiver and the overall communication performance of the system.

[0052] like Figure 6 As shown, after parametric recovery is completed in the ZGP crystal, the output beam contains the target light component used for subsequent inverse wavelength conversion. To further reconstruct the original communication signal, the output beam is filtered and split, and the target light participating in the inverse conversion is selected and introduced into the inverse difference frequency conversion unit corresponding to the PPLN crystal. At the same time, the local continuous light source outputs auxiliary pump light for the inverse difference frequency process. After two stages of amplification, polarization control, and collimation, the auxiliary pump light is injected into the PPLN crystal together with the target light.

[0053] Under quasi-phase-matching conditions, the target light and the auxiliary pump light undergo inverse difference-frequency interaction in the PPLN crystal, thereby reconverting the mid-infrared signal transmitted in free space back to the C-band and recovering the near-infrared signal carrying the original modulation information. Because the PPLN crystal possesses good nonlinear frequency conversion characteristics and quasi-phase-matching control capabilities, it can maintain the phase characteristics and modulation information of the original signal while ensuring conversion efficiency, providing favorable conditions for subsequent signal demodulation.

[0054] The C-band recovered signal output from the PPLN crystal can be further filtered by filter F2 to remove out-of-band stray light and residual pump light, thereby improving the purity of the output signal. Subsequently, the recovered C-band signal enters the subsequent coherent receiving and demodulation unit, where the original information is extracted through photoelectric detection, digital signal processing, and data recovery. This completes the entire process of the mid-infrared signal reconstruction at the receiving end, from ZGP parametric recovery to PPLN inverse difference frequency conversion, and finally to C-band signal reconstruction.

[0055] Furthermore, by employing a reverse cascaded structure combining ZGP crystals and PPLN crystals at the receiver, effective recovery and high-quality reconstruction of mid-infrared signals transmitted in free space can be achieved. On one hand, the parametric processes in the ZGP crystal can compensate for or recover the energy of the received signal, mitigating the adverse effects of link transmission losses. On the other hand, the inverse difference frequency conversion process in the PPLN crystal can map the recovered mid-infrared signal back to the C-band with high fidelity, facilitating signal detection using mature near-infrared receiving devices and demodulation techniques. This structure not only improves the overall receiving sensitivity of the system but also enhances the stability and engineering feasibility of the mid-infrared space optical communication link.

[0056] Example 2: To achieve the above objective, such as Figure 7 As shown, based on Embodiment 1, this invention discloses a wavelength conversion system based on dual-crystal cascaded difference frequency and optical parametric amplification, comprising: The conversion processing module 11 is used to acquire the first pump light and the signal light, input the first pump light and the signal light into the first nonlinear crystal for wavelength conversion, and obtain mid-infrared idler light; The compensation output module 12 is used to acquire the second pump light, input the mid-infrared idler light and the second pump light into the second nonlinear crystal for optical parametric amplification and power compensation, and obtain the amplified and compensated mid-infrared signal.

[0057] Based on the same inventive concept, this invention also provides a computer device, comprising: one or more processors, and a memory for storing one or more computer programs; the programs include program instructions, and the processor executes the program instructions stored in the memory. The processor may be a Central Processing Unit (CPU), or other general-purpose processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. It is the computing and control core of the terminal, used to implement one or more instructions, specifically for loading and executing one or more instructions stored in a computer storage medium to implement the above-described method.

[0058] It should be further explained that, based on the same inventive concept, the present invention also provides a computer storage medium storing a computer program, which, when executed by a processor, performs the above-described method. This storage medium can be any combination of one or more computer-readable media. The computer-readable medium can be a computer-readable signal medium or a computer-readable storage medium. The computer-readable storage medium can be, for example, but not limited to, an electrical, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any combination thereof. More specific examples of computer-readable storage media (a non-exhaustive list) include: an electrical connection having one or more wires, a portable computer disk, a hard disk, random access memory (RAM), read-only memory (ROM), erasable programmable read-only memory (EPROM or flash memory), optical fiber, portable compact disk read-only memory (CD-ROM), optical storage device, magnetic storage device, or any suitable combination thereof. In the present invention, the computer-readable storage medium can be any tangible medium containing or storing a program that can be used by or in conjunction with an instruction execution system, apparatus, or device.

[0059] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of this disclosure. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0060] The foregoing has shown and described the basic principles, main features, and advantages of this disclosure. Those skilled in the art should understand that this disclosure is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of this disclosure. Various changes and modifications can be made to this disclosure without departing from its spirit and scope, and all such changes and modifications fall within the scope of this disclosure as claimed.

Claims

1. A wavelength conversion method based on dual-crystal cascaded difference frequency and optical parametric amplification, characterized in that, The method includes the following steps: The first pump light and signal light are acquired, and the first pump light and signal light are input into the first nonlinear crystal for wavelength conversion to obtain mid-infrared idler light; The second pump light is acquired, and the mid-infrared idler light and the second pump light are input into the second nonlinear crystal for optical parametric amplification and power compensation to obtain the amplified and compensated mid-infrared signal.

2. The wavelength conversion method based on dual-crystal cascaded difference frequency and optical parametric amplification according to claim 1, characterized in that, The first nonlinear crystal is selected based on the target conversion band, pump light wavelength, phase matching conditions, and system integration requirements.

3. The wavelength conversion method based on dual-crystal cascaded difference frequency and optical parametric amplification according to claim 1, characterized in that, The process of inputting the first pump light and signal light into the first nonlinear crystal includes the following steps: After the first pump light and signal light are amplified, combined and collimated in two stages, they are input into the PPLN crystal. Under quasi-phase matching conditions, the difference frequency generation process is completed, realizing the wavelength conversion of the C-band signal light to the mid-infrared band, and finally outputting the mid-infrared idler light.

4. The wavelength conversion method based on dual-crystal cascaded difference frequency and optical parametric amplification according to claim 3, characterized in that, The process of amplifying, combining, and collimating the first pump light and the signal light through two stages includes the following steps: The first pump light and signal light are amplified in the first stage by a polarization-maintaining erbium-doped fiber amplifier. The amplified first pump light and signal light are then input into a polarization-maintaining ytterbium-doped fiber amplifier pumped by two laser diodes for a second stage of amplification, resulting in a two-stage amplified first pump light and signal light. The first pump light and signal light, after being amplified by two stages, are input into a wavelength division multiplexer for beam combining to obtain the combined beam. The combined beam is collimated by an optical fiber collimator, and finally the collimated beam is output.

5. The wavelength conversion method based on dual-crystal cascaded difference frequency and optical parametric amplification according to claim 1, characterized in that, The second nonlinear crystal is selected based on the mid-infrared seed light wavelength, the second pump light wavelength, the optical parametric amplification gain, thermal stability, and power carrying capacity.

6. The wavelength conversion method based on dual-crystal cascaded difference frequency and optical parametric amplification according to claim 1, characterized in that, The process of inputting mid-infrared idler light and second pump light into the second nonlinear crystal includes the following steps: After the mid-infrared idler light and the second pump light are amplified, combined and collimated in two stages, they are input into the ZGP crystal. Under the condition of satisfying the phase matching condition, the energy of the second pump light is transferred to the mid-infrared idler light, and the amplified and compensated mid-infrared signal is output, realizing the optical parametric amplification and power compensation of the mid-infrared idler light.

7. The wavelength conversion method based on dual-crystal cascaded difference frequency and optical parametric amplification according to claim 6, characterized in that, The amplified and compensated mid-infrared signal is expanded, collimated, and wavefront shaped by a lens group to optimize its free-space transmission characteristics, and finally coupled to a free-space transmitting antenna to realize the transmission of the compensated high-power mid-infrared signal.

8. The wavelength conversion method based on dual-crystal cascaded difference frequency and optical parametric amplification according to claim 7, characterized in that, The amplified and compensated mid-infrared signal is injected into the second nonlinear crystal along with the third pump light. Energy recovery of the received amplified and compensated mid-infrared signal is then performed through an inverse parametric amplification process to obtain the recovered signal. The recovered signal and the fourth pump light are injected together into the first nonlinear crystal to perform wavelength conversion on the recovered signal through an inverse difference frequency generation process, thereby obtaining the C-band recovered signal. Output the C-band recovery signal.

9. A wavelength conversion system based on dual-crystal cascaded difference frequency and optical parametric amplification, employing the wavelength conversion method based on dual-crystal cascaded difference frequency and optical parametric amplification as described in any one of claims 1 to 8, characterized in that, include: The conversion processing module is used to acquire the first pump light and the signal light, input the first pump light and the signal light into the first nonlinear crystal for wavelength conversion, and obtain mid-infrared idler light; The compensation output module is used to acquire the second pump light, input the mid-infrared idler light and the second pump light into the second nonlinear crystal for optical parametric amplification and power compensation, and obtain the amplified and compensated mid-infrared signal.

10. A terminal device, comprising a memory, a processor, and a computer program stored in the memory and capable of running on the processor, characterized in that, The memory stores a computer program that can run on the processor. When the processor loads and executes the computer program, it employs the wavelength conversion method based on dual-crystal cascaded difference frequency and optical parametric amplification as described in any one of claims 1 to 8.

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