Mid-infrared optical frequency comb

Abstract

The invention provides a mid-infrared optical frequency comb. The mid-infrared optical frequency comb comprises a near-infrared optical frequency comb seed source of a polarization-maintaining all-fiber structure; the all-fiber nonlinear amplification and pulse compression module is connected with the near-infrared optical frequency comb seed source through a polarization maintaining fiber and is used for amplifying the power of the seed light and compressing the pulse width after generating a linear chirped pulse; the all-fiber nonlinear spectrum broadening module is connected with the all-fiber nonlinear amplification and pulse compression module through a polarization maintaining fiber and is used for broadening a laser spectrum; the difference frequency module is used for receiving the laser pulse processed by the all-fiber nonlinear spectrum broadening module and generating an intermediate infrared laser pulse through intra-pulse self-difference frequency; wherein the seed source, the all-fiber nonlinear amplification and pulse compression module and the all-fiber nonlinear spectrum broadening module are connected through a polarization maintaining fiber to form an all-fiber integrated structure. By adopting the scheme, the anti-interference capability of the system can be enhanced, miniaturization and high operation reliability are realized, the debugging process is simplified, and the difference frequency efficiency is improved.

Description

Technical Field

[0001] This invention relates to the field of ultrafast laser technology, and more specifically, to a mid-infrared optical frequency comb. Background Technology

[0002] The mid-infrared band is known as the spectroscopic fingerprint region of molecules because most molecules exhibit strong absorption peaks in this area. Extending optical frequency combs to this band can overcome the limitations of traditional detection accuracy, enabling high-precision molecular analysis. This has significant application prospects in molecular spectroscopy detection, medical diagnostics, and security and defense fields, and is a key research direction for ultrafast laser technology.

[0003] Currently, mid-infrared optical frequency combs are mainly based on near-infrared optical combs, which extend the wavelength through optical difference frequency. Near-infrared optical combs are mostly non-all-fiber structures, and some rely on spatial elements to achieve mode-locked output. The difference frequency is mainly based on inter-pulse difference frequency, which requires the construction of a basic optical path to provide high-power narrow-pulse-width pump light and broadband signal light, and relies on manual adjustment of spatial elements to ensure phase matching of the two beams.

[0004] Research has revealed significant shortcomings in existing solutions: First, the inter-pulse difference frequency requires manual adjustment of the time synchronization and spatial collinearity of the two beams, and repeated adjustments still fail to achieve the ideal state, resulting in low difference frequency efficiency. Second, the near-infrared optical comb lacks integrated design, relying on spatial components to compress pulse width and couple pump light, leading to a complex system structure and cumbersome debugging. Third, the large number of spatial components results in a large system size, difficulty in integration, sensitivity to environmental disturbances, weak anti-interference capabilities, and poor stability, requiring harsh conditions such as constant temperature and vibration protection, making it unsuitable for diverse application scenarios. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to provide a mid-infrared optical frequency comb that can enhance the system's anti-interference capability, achieve miniaturization and high operational reliability, simplify the debugging process, and improve differential frequency efficiency.

[0006] This application provides a mid-infrared optical frequency comb, the mid-infrared optical frequency comb comprising: Near-infrared optical frequency comb seed source with polarization-maintaining all-fiber structure; The all-fiber nonlinear amplification and pulse compression module, which is connected to the near-infrared optical frequency comb seed source via polarization-maintaining fiber, is used to amplify the seed light and generate linear chirped pulses before compressing the pulse width. An all-fiber nonlinear spectral broadening module, connected to the all-fiber nonlinear amplification and pulse compression module via polarization-maintaining fiber, is used to broaden the laser spectrum; The difference frequency module is used to receive the laser pulse processed by the all-fiber nonlinear spectral broadening module and generate mid-infrared laser pulses through intra-pulse self-difference frequency; The near-infrared optical frequency comb seed source, the all-fiber nonlinear amplification and pulse compression module, and the all-fiber nonlinear spectral broadening module are connected by polarization-maintaining fiber to form an all-fiber integrated structure.

[0007] Optionally, the all-fiber nonlinear amplification and pulse compression module includes multiple pump sources, multiple polarization-maintaining fiber wavelength division multiplexers, multiple polarization-maintaining gain fibers, and a first polarization-maintaining single-mode fiber for pulse compression. The multiple pump sources are coupled to the multiple polarization-maintaining gain fibers through corresponding polarization-maintaining fiber wavelength division multiplexers. The plurality of pump sources consists of three, the plurality of polarization-maintaining fiber wavelength division multiplexers consists of three, and the plurality of polarization-maintaining gain fibers consists of two.

[0008] Optionally, all three pump sources are semiconductor continuous lasers with an output center wavelength of 976 nm; The three pump sources are coupled to the input or both ends of the two polarization-maintaining gain fibers through corresponding polarization-maintaining fiber wavelength division multiplexers. The first pump source is coupled to the input of the first polarization-maintaining gain fiber through the first polarization-maintaining fiber wavelength division multiplexer, the second pump source is coupled to the input of the second polarization-maintaining gain fiber through the second polarization-maintaining fiber wavelength division multiplexer, and the third pump source is coupled to the output of the second polarization-maintaining gain fiber through the third polarization-maintaining fiber wavelength division multiplexer.

[0009] Optionally, the polarization-maintaining gain fiber is a polarization-maintaining erbium-doped gain fiber, used for power amplification of near-infrared laser pulses; The first polarization-maintaining single-mode fiber is used to compress the pulse width of the linear chirped pulse generated after amplification by the polarization-maintaining gain fiber.

[0010] Optionally, the all-fiber nonlinear spectral broadening module is a polarization-maintaining high nonlinear fiber used to broaden the near-infrared laser spectrum after pulse width compression.

[0011] Optionally, the difference frequency module includes a second polarization-maintaining single-mode fiber for dispersion adjustment, a polarization-maintaining fiber collimator, an achromatic half-wave plate, a first off-axis parabolic mirror, a nonlinear crystal, a second off-axis parabolic mirror, and a filter.

[0012] Optionally, the filter is a mid-infrared bandpass filter, used to filter out the near-infrared laser remaining after the difference frequency and retain the mid-infrared laser; The achromatic half-wave plate is used to adjust the polarization direction of the near-infrared laser pulse to match the polarization direction of the nonlinear crystal.

[0013] Optionally, the nonlinear crystal is one of periodically polarized lithium niobate, oriented patterned gallium phosphide, gallium selenide, or zinc germanium phosphide crystal, used to convert near-infrared laser pulses into mid-infrared laser pulses by intra-pulse self-difference frequency conversion. The first off-axis parabolic reflector is used to focus the polarized near-infrared laser pulse into the nonlinear crystal, and the second off-axis parabolic reflector is used to collimate and output the mid-infrared laser pulse generated in the nonlinear crystal.

[0014] Optionally, the near-infrared optical frequency comb seed source is a polarization-maintaining all-fiber structure based on saturable absorber mode-locking, nonlinear polarization rotation mode-locking, or nonlinear amplifying ring mirror mode-locking, outputting near-infrared laser pulses with femtosecond pulse widths. The repetition frequency of the mid-infrared optical frequency comb is locked to the repetition frequency of the near-infrared optical frequency comb seed source, and the repetition frequency changes synchronously with the repetition frequency of the seed source.

[0015] Optionally, the output wavelength of the mid-infrared optical frequency comb is tunable, achieved by adjusting the polarization period of the nonlinear crystal and matching the length of the second polarization-maintaining single-mode fiber.

[0016] The technical solution provided in this application includes, but is not limited to, the following beneficial effects: The polarization-maintaining all-fiber near-infrared optical frequency comb seed source adopts a polarization-maintaining all-fiber structure. On the one hand, it can ensure the stability of the polarization state of the output near-infrared laser throughout the entire process, avoiding the impact of polarization shift on the optical signal processing quality of subsequent modules, and providing a stable initial signal foundation for the entire system. On the other hand, the all-fiber structure does not rely on space optical components, reducing the interference of environmental vibration and temperature changes on the seed source, improving the seed source's own resistance to environmental disturbances, and thus ensuring the consistency and reliability of the input signals of all subsequent modules.

[0017] The all-fiber nonlinear amplification and pulse compression module is directly connected to the seed source via polarization-maintaining fiber, maintaining all-fiber transmission characteristics throughout the process. This eliminates the need for manual adjustment of the spatial optical path, simplifying system connection and debugging. Its core functions are: first, to amplify the seed light, providing sufficient optical energy for subsequent spectral broadening and difference frequency modulation processes, meeting the power requirements for mid-infrared laser generation; and second, to obtain narrow-pulse laser by generating linearly chirped pulses and compressing the pulse width, creating favorable conditions for subsequent nonlinear spectral broadening. Simultaneously, the all-fiber design continuously maintains stable laser polarization, preventing polarization disturbances during power amplification and pulse width adjustment, ensuring that the output optical parameters meet the input requirements of the next-level module.

[0018] The all-fiber nonlinear spectral broadening module connects to the amplification and compression module in an all-fiber configuration, maintaining the system's all-fiber characteristics and avoiding optical losses and parameter fluctuations introduced by spatial components. By broadening the laser spectrum, it provides the difference frequency module with a wide-spectrum near-infrared laser—a key prerequisite for generating multi-wavelength mid-infrared lasers via intra-pulse self-difference frequency generation. This ensures that the difference frequency process covers a wider range of mid-infrared bands, improving the application adaptability of the mid-infrared optical frequency comb. Simultaneously, the all-fiber structure and polarization-maintaining design further solidify the laser's polarization stability and beam quality, providing high-quality input light for the efficient operation of the difference frequency module.

[0019] The difference frequency module generates mid-infrared laser through an intra-pulse self-differential frequency mechanism. Unlike existing technologies, it eliminates the need to adjust the time synchronization and spatial collinearity of two independent laser beams (pump light and signal light), simplifying the difference frequency operation process in principle, significantly reducing debugging difficulty, and effectively improving difference frequency efficiency. It can directly receive laser pulses processed by the spectral broadening module without the need for additional optical signal adjustment components. While ensuring stable output of mid-infrared laser, it further aligns with the integrated design concept of the system, meeting the core requirements of mid-infrared laser for subsequent molecular spectroscopy detection, medical diagnosis, and other scenarios.

[0020] The seed source, all-fiber nonlinear amplification and pulse compression module, and all-fiber nonlinear spectral broadening module provided in this application form an all-fiber integrated structure connected by polarization-maintaining fiber. Combined with the intra-pulse self-differential frequency design of the difference frequency module, the entire system achieves synergistic advantages: On the one hand, the all-fiber integration significantly reduces the use of spatial components, shrinks the system size, and reduces complexity, while significantly improving the ability to resist environmental disturbances and avoiding the impact of temperature and vibration on system stability; on the other hand, through polarization-maintaining transmission and functional adaptation, each module ensures the stability of optical signal parameters throughout the entire process from the seed source to the mid-infrared output. This not only solves the problems of low difference frequency efficiency and cumbersome debugging in the prior art, but also realizes the miniaturization and high reliability of the system, fully meeting the needs of diverse application scenarios.

[0021] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, preferred embodiments are described below in detail with reference to the accompanying drawings. Attached Figure Description

[0022] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1A schematic diagram of a mid-infrared optical frequency comb provided in an embodiment of the present invention is shown; Figure 2 A schematic diagram of the structure of the second type of mid-infrared optical frequency comb provided in an embodiment of the present invention is shown; Figure 3 The following is a temporal intensity distribution diagram of the near-infrared laser pulse compressed by the first polarization-maintaining single-mode fiber, provided in an embodiment of the present invention. Figure 4 A schematic diagram of the structure of the third type of mid-infrared optical frequency comb provided in an embodiment of the present invention is shown; Figure 5 A schematic diagram of the structure of the fourth mid-infrared optical frequency comb provided in an embodiment of the present invention is shown; Figure 6 The output spectral distribution of the mid-infrared optical frequency comb provided in an embodiment of the present invention is shown.

[0024] Figure labels: 1-Near-infrared optical frequency comb seed source; 2-All-fiber nonlinear amplification and pulse compression module; 3-All-fiber nonlinear spectral broadening module; 4-Difference frequency module; 21-First pump source; 22-First polarization-maintaining fiber wavelength division multiplexer; 23-First polarization-maintaining erbium-doped gain fiber; 24-Second pump source; 25-Second polarization-maintaining fiber wavelength division multiplexer; 26-Second polarization-maintaining erbium-doped gain fiber; 27-Third pump source; 28-Third polarization-maintaining fiber wavelength division multiplexer; 29-First polarization-maintaining single-mode fiber; 31-Polarization-maintaining high nonlinear fiber; 41-Second polarization-maintaining single-mode fiber; 42-Polarization-maintaining fiber collimator; 43-Achromatic half-wave plate; 44-First off-axis parabolic mirror; 45-Nonlinear crystal; 46-Second off-axis parabolic mirror; 47-Filter. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, 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. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0026] To facilitate understanding of this application, the following is combined with... Figure 1The schematic diagram of a mid-infrared optical frequency comb provided in the embodiment of the present invention is shown below, and the content described herein will be described in detail for the embodiments of this application.

[0027] See Figure 1 As shown, Figure 1 This diagram illustrates a mid-infrared optical frequency comb according to an embodiment of the present invention, wherein the mid-infrared optical frequency comb comprises: Near-infrared optical frequency comb seed source with polarization-maintaining all-fiber structure 1.

[0028] Specifically, the near-infrared optical frequency comb seed source 1 adopts a polarization-maintaining all-fiber structure design, with three modes of mode locking: mode locking based on a saturable absorber, mode locking based on nonlinear polarization rotation, and mode locking based on a nonlinear amplifying ring mirror. All three modes of mode locking can ensure the polarization stability of the laser output from the seed source, adapting to the polarization requirements of an all-fiber system.

[0029] From the perspective of output characteristics, the laser pulse power output by the near-infrared optical frequency comb seed source 1 is on the order of milliwatts, and the pulse width is on the order of femtoseconds. This low power and narrow pulse width characteristic is the basis for subsequent efficient amplification and broad spectrum broadening. It can avoid the limitation of spectral broadening caused by the initial pulse being too wide, and can also prevent nonlinear distortion in the amplification process caused by the initial power being too high.

[0030] The all-fiber nonlinear amplification and pulse compression module 2, which is connected to the near-infrared optical frequency comb seed source 1 via a polarization-maintaining fiber, is used to amplify the seed light and generate linear chirped pulses before compressing the pulse width.

[0031] Specifically, the all-fiber nonlinear amplification and pulse compression module 2 adopts an all-fiber structure design. All internal optical components and connections to external modules are achieved through polarization-maintaining fibers, eliminating the need for spatial optical paths and maintaining the stability of the laser polarization state throughout the entire process. Its functionality is realized in three stages.

[0032] The first stage is power amplification. The all-fiber nonlinear amplification and pulse compression module 2 adopts a staged amplification design with dual-gain fibers and three pump sources. The second stage is linear chirp generation. By controlling the lengths of the two polarization-maintaining gain fibers and the power of the three pump sources, the nonlinear effects within the gain fibers and the synergistic effect of the gain spectrum are utilized to generate a parabolic linear chirp in the amplified laser pulse, creating conditions for subsequent pulse width compression.

[0033] The third stage is pulse width compression, achieved using polarization-maintaining single-mode fiber. The dispersion characteristics of polarization-maintaining single-mode fiber are opposite to those of linearly chirped pulses, which can cancel the linear chirp of the pulse and compress the pulse width to below 100 fs. At the same time, power loss is controlled to ensure that the laser still maintains high power after compression, meeting the subsequent spectral broadening requirements.

[0034] The all-fiber nonlinear spectral broadening module 3, which is connected to the all-fiber nonlinear amplification and pulse compression module via polarization-maintaining fiber, is used to broaden the laser spectrum.

[0035] Specifically, the core component of this all-fiber nonlinear spectral broadening module 3 is a polarization-maintaining high nonlinear fiber 31, without any other spatial optical components, which can maintain the laser polarization state and beam quality. When the narrow-pulse-width high-power laser output from the all-fiber nonlinear amplification and pulse compression module 2 is injected into the polarization-maintaining high nonlinear fiber 31, a strong self-phase modulation effect is generated within the fiber.

[0036] This effect causes the phase of different time components of the laser pulse to change nonlinearly with light intensity, thereby broadening the spectral bandwidth. The bandwidth of the initial near-infrared laser spectrum is significantly increased after broadening, providing a basis for the subsequent generation of a mid-infrared laser with a wide tuning range by the difference frequency module 4, ensuring that the mid-infrared band can cover more application scenarios.

[0037] The difference frequency module 4 is used to receive the laser pulse processed by the all-fiber nonlinear spectral broadening module and generate a mid-infrared laser pulse through intra-pulse self-difference frequency.

[0038] Specifically, the difference frequency module 4 is the core unit for converting near-infrared laser into mid-infrared laser. The input signal it receives is the broadband near-infrared laser output from the all-fiber nonlinear spectral broadening module 3, and wavelength conversion is achieved using an intra-pulse self-differential frequency mechanism. Intra-pulse self-differential frequency utilizes the interaction of different spectral components within the same laser pulse to generate mid-infrared laser, eliminating the need to control the synchronization and overlap of two independent pulses, thus improving system stability and difference frequency efficiency.

[0039] During the difference frequency process, the high-frequency and low-frequency components of the near-infrared laser interact within the nonlinear crystal to generate the mid-infrared laser. The repetition frequency of the mid-infrared laser is directly inherited from the near-infrared optical frequency comb seed source 1, eliminating the need for an additional locking mechanism, simplifying the system structure, and ensuring the stability of the repetition frequency to meet the requirements of precision applications.

[0040] The near-infrared optical frequency comb seed source, the all-fiber nonlinear amplification and pulse compression module, and the all-fiber nonlinear spectral broadening module are connected by polarization-maintaining fiber to form an all-fiber integrated structure.

[0041] Specifically, the optical components between the three modules and within each module are connected via polarization-maintaining optical fibers. The polarization-maintaining fiber connectors are of a specific type to reduce insertion loss and reflected light interference, and to control power loss during laser transmission.

[0042] The all-fiber integrated structure significantly enhances the system's resistance to environmental disturbances. Under certain temperature ranges and vibration conditions, the output power fluctuations and polarization state changes are minimal, making it suitable for practical applications outside the laboratory. Simultaneously, this structure makes the system compact and lightweight, facilitating integration into a small metal housing, achieving miniaturization and weight reduction, enabling portable applications, and simplifying the installation and commissioning process, thus shortening commissioning time.

[0043] In an optional implementation, see Figure 2 As shown, Figure 2 The diagram shows a second mid-infrared optical frequency comb provided in an embodiment of the present invention. The all-fiber nonlinear amplification and pulse compression module 2 includes multiple pump sources, multiple polarization-maintaining fiber wavelength division multiplexers, multiple polarization-maintaining gain fibers, and a first polarization-maintaining single-mode fiber 29 for pulse compression. The multiple pump sources are coupled to the multiple polarization-maintaining gain fibers through corresponding polarization-maintaining fiber wavelength division multiplexers.

[0044] Specifically, the implementation scheme clarifies the component composition and connection logic of the all-fiber nonlinear amplification and pulse compression module 2. The number of multiple pump source wavelength division multiplexers and polarization-maintaining gain fibers is configured as three pump sources, three polarization-maintaining fiber wavelength division multiplexers, and two polarization-maintaining gain fibers. This configuration can balance power amplification efficiency and nonlinear distortion suppression.

[0045] Each pump source forms an independent unit with its corresponding wavelength division multiplexer and gain fiber, ensuring precise injection of pump light into the target gain fiber and avoiding mutual interference. The polarization-maintaining fiber wavelength division multiplexer can efficiently couple pump light and signal light, reducing signal light insertion loss and ensuring minimal energy loss. The dispersion parameters of the first polarization-maintaining single-mode fiber 29 are matched with the linearly chirped pulse. By controlling the length, complete chirp cancellation is achieved. After compression, the pulse profile is close to Gaussian with no obvious sidelobes, ensuring stable subsequent spectral broadening.

[0046] The plurality of pump sources consists of three, the plurality of polarization-maintaining fiber wavelength division multiplexers consists of three, and the plurality of polarization-maintaining gain fibers consists of two.

[0047] Specifically, the configuration of three pump sources, three wavelength division multiplexers, and two polarization-maintaining gain fibers is the experimentally verified optimal solution, achieving a balance between power stability and cost. The staged amplification design distributes the total amplification gain across two stages, with the gain of each stage controlled within a reasonable range to avoid nonlinear effects caused by high gain in a single stage.

[0048] The three pump sources have clearly defined functions: the first pump source 21, in conjunction with the first polarization-maintaining gain fiber 23, performs initial amplification; the second pump source 24 and the third pump source 27, in conjunction with the second polarization-maintaining gain fiber 26, perform secondary amplification. This bidirectional pumping design improves pump efficiency, ensures uniform temperature distribution within the gain fiber, and prevents localized overheating damage. Both polarization-maintaining gain fibers are erbium-doped gain fibers with identical parameters, adapted to the absorption characteristics of 976nm pump light, ensuring that pump light energy is fully converted into signal light power. The small core diameter design increases signal light power density and enhances stimulated emission.

[0049] More specifically, the first amplification stage consists of a first pump source 21, a first polarization-maintaining fiber wavelength division multiplexer 22, and a first polarization-maintaining gain fiber 23. The pump light from the first pump source 21 is coupled with the seed light through the wavelength division multiplexer and then injected into the first polarization-maintaining gain fiber 23, amplifying the seed light power from the milliwatt level to the tens of milliwatt level. The second amplification stage consists of a second pump source 24, a second polarization-maintaining fiber wavelength division multiplexer 25, a second polarization-maintaining gain fiber 26, a third pump source 27, and a third polarization-maintaining fiber wavelength division multiplexer 28. The second and third pump sources inject pump light from both ends of the second polarization-maintaining gain fiber 26, forming bidirectional pumping, further amplifying the laser power to the hundreds of milliwatt level, avoiding the gain unevenness caused by single-end pumping.

[0050] In one alternative implementation, all three pump sources are semiconductor continuous lasers with an output center wavelength of 976 nm.

[0051] Specifically, the three pump sources (first pump source 21, second pump source 24, and third pump source 27) are semiconductor continuous-wave lasers with a fixed output center wavelength of 976 nm, determined based on the gain characteristics of the polarization-maintaining gain fiber. The polarization-maintaining gain fiber used in the all-fiber nonlinear amplification and pulse compression module 2 is erbium-doped gain fiber. Erbium ions have a strong absorption peak in the 976 nm band, which can efficiently absorb pump light and convert it into signal light energy, with an energy conversion efficiency higher than other pump wavelengths.

[0052] Semiconductor continuous lasers are small in size, lightweight, low in power consumption, and long in life. They do not require complex cooling systems and can operate stably with only natural air cooling, making them suitable for the miniaturization requirements of all-fiber systems. The output power of the three pump sources is adjustable, allowing for flexible control of the signal light power according to actual needs. They exhibit high output wavelength stability with minimal center wavelength drift within a certain temperature range, ensuring that the pump light always matches the absorption peak of the erbium-doped gain fiber and preventing a decrease in amplification efficiency.

[0053] The three pump sources are coupled to the input or both ends of the two polarization-maintaining gain fibers through corresponding polarization-maintaining fiber wavelength division multiplexers. Specifically, the first pump source 21 is coupled to the input of the first polarization-maintaining gain fiber 23 through the first polarization-maintaining fiber wavelength division multiplexer 22, the second pump source 24 is coupled to the input of the second polarization-maintaining gain fiber 26 through the second polarization-maintaining fiber wavelength division multiplexer 25, and the third pump source 27 is coupled to the output of the second polarization-maintaining gain fiber 26 through the third polarization-maintaining fiber wavelength division multiplexer 28.

[0054] Specifically, this coupling method is designed for efficient pumping and uniform gain. The first polarization-maintaining gain fiber 23 is single-ended pumped. Because the pump energy required in the first amplification stage is low, the single-ended coupling structure is simple, which can reduce the number of components and improve stability. The pump light and the signal light are coupled together through a wavelength division multiplexer and injected into the input end of the gain fiber without mutual interference.

[0055] The second polarization-maintaining gain fiber 26 employs bidirectional pumping to address the gain unevenness caused by pump light attenuation during single-end pumping. Bidirectional pumping ensures uniform pump light distribution within the gain fiber, minimizing gain differences and preventing pulse distortion. The pump lights from the second and third pump sources are coupled to both ends of the gain fiber via corresponding wavelength division multiplexers. These multiplexers provide reverse isolation, preventing pump light interference with subsequent modules and ensuring stable amplification of the signal light under uniform pumping, forming an ideal parabolic pulse.

[0056] In one optional implementation, the polarization-maintaining gain fiber is a polarization-maintaining erbium-doped gain fiber used to amplify the power of near-infrared laser pulses; the first polarization-maintaining single-mode fiber 29 is used to compress the pulse width of the linear chirped pulses generated after amplification by the polarization-maintaining gain fiber.

[0057] Specifically, the gain band of the polarization-maintaining erbium-doped gain fiber (specifically, the first polarization-maintaining erbium-doped gain fiber 23 and the second polarization-maintaining erbium-doped gain fiber 26) is highly matched with the near-infrared band of the signal light, and its excellent polarization-maintaining characteristics ensure the stability of the polarization state during signal light amplification. Its core diameter is small, the numerical aperture is appropriate, and the erbium ion doping concentration is reasonable, ensuring sufficient gain while avoiding concentration quenching effects and improving pump light utilization.

[0058] After the signal light is injected, it undergoes stimulated emission with high-energy erbium ions, amplifying the power and maintaining a high polarization extinction ratio to meet the polarization requirements of subsequent modules. The first polarization-maintaining single-mode fiber 29 operates based on the dispersion compensation principle, which can counteract the time-domain broadening caused by linear chirp, compressing the pulse width to below 100 fs, increasing the pulse peak power, providing sufficient peak power for subsequent spectral broadening, and exciting significant nonlinear effects.

[0059] See Figure 3 As shown, Figure 3The diagram illustrates the temporal intensity distribution of a near-infrared laser pulse compressed by the first polarization-maintaining single-mode fiber, as provided in this embodiment of the invention. The vertical axis is labeled "Intensity (au)," representing the intensity of the laser pulse in arbitrary units (au); the horizontal axis is labeled "Time (fs)," representing time in femtoseconds (fs). This diagram visually presents the temporal waveform of the compressed near-infrared laser pulse. 70 fs refers to the actual pulse width of the near-infrared laser pulse after compression by the first polarization-maintaining single-mode fiber, i.e., the duration of the laser pulse in the time dimension is 70 femtoseconds. This verifies the pulse width compression effect of the first polarization-maintaining single-mode fiber in the all-fiber nonlinear amplification and pulse compression module, ensuring that the output narrow pulse width meets the input requirements of the subsequent all-fiber nonlinear spectral broadening module.

[0060] In an optional implementation, see Figure 4 As shown, Figure 4 The diagram shows a third type of mid-infrared optical frequency comb provided in an embodiment of the present invention. The all-fiber nonlinear spectral broadening module 3 is a polarization-maintaining high nonlinear fiber 31, used to broaden the near-infrared laser spectrum after pulse width compression.

[0061] Specifically, the high nonlinearity of polarization-maintaining high nonlinear fiber 31 stems from its small core diameter and special materials, enabling the excitation of strong nonlinear optical effects at relatively low peak power. Its small core diameter, moderate length, and high nonlinear coefficient contribute to this. The small core diameter increases signal power density, while the moderate length balances broadening effect and power loss. The high nonlinear coefficient efficiently excites self-phase modulation effects, which is the main mechanism for spectral broadening.

[0062] When a narrow-pulse-width, high-power laser is injected, the strong self-phase modulation effect causes nonlinear phase shifts in different time components of the pulse, which are converted into frequency shifts, thus broadening the spectral bandwidth. The degree of spectral broadening can be controlled by the input laser power; the higher the power, the more significant the broadening. Simultaneously, the polarization-maintaining characteristics of the high-nonlinear fiber 31 ensure the stability of the laser polarization state, guaranteeing a stable polarization input for the subsequent difference frequency module 4 and improving difference frequency efficiency.

[0063] In an optional implementation, see Figure 5 As shown, Figure 5 The diagram shows a fourth type of mid-infrared optical frequency comb provided in an embodiment of the present invention. The difference frequency module 4 includes a second polarization-maintaining single-mode fiber 41 for dispersion adjustment, a polarization-maintaining fiber collimating lens 42, an achromatic half-wave plate 43, a first off-axis parabolic mirror 44, a nonlinear crystal 45, a second off-axis parabolic mirror 46, and a filter 47.

[0064] Specifically, the components of the difference frequency module 4 are designed according to the process of dispersion adjustment, collimation, polarization matching, focusing, difference frequency collimation, and filtering. Each component has a clear function, and together they ensure high efficiency and stability of the difference frequency. The second polarization-maintaining single-mode fiber 41 is used for dispersion adjustment to compensate for the time shift of spectral components during laser transmission, so that all components of the broadband laser reach the nonlinear crystal 45 synchronously, maximizing the difference frequency efficiency.

[0065] The polarization-maintaining fiber collimator 42 converts fiber-coupled laser light into parallel light, adapting to the operational requirements of subsequent components. It offers high collimation accuracy, a small divergence angle, and maintains the polarization state. The achromatic half-wave plate 43 is used for polarization matching. By rotating it, the laser polarization direction is adjusted to align with the polarization direction of the nonlinear crystal 45, satisfying the phase-matching condition, improving difference-frequency conversion efficiency, and covering the near-infrared band in the operating wavelength range, ensuring uniform polarization adjustment across a wide spectrum of laser light.

[0066] The first off-axis parabolic reflector 44 focuses parallel light onto the center of the nonlinear crystal 45, increasing the power density within the crystal and exciting a strong difference-frequency effect. The off-axis design prevents reflected light from interfering with the pre-amplifier module. The nonlinear crystal 45 is the core difference-frequency reactant element, and various types are available to adapt to different mid-infrared bands. The second off-axis parabolic reflector 46 collimates the mid-infrared diverging light into parallel light, ensuring good beam quality and facilitating subsequent filtering and applications. The filter 47 is used for filtering and purification, filtering out near-infrared laser light that has not participated in the reaction while retaining mid-infrared laser light, ensuring high purity of the output laser.

[0067] In an optional implementation, the filter is a mid-infrared bandpass filter used to filter out the near-infrared laser remaining after the difference frequency and retain the mid-infrared laser.

[0068] Specifically, the passband range of the mid-infrared bandpass filter precisely covers the output wavelength of the mid-infrared laser. The center wavelength of the passband can be adjusted according to the type of nonlinear crystal 45 to ensure efficient transmission of the mid-infrared laser. The full width at half maximum (FWHM) of the passband can accommodate wavelength changes caused by tuning, while suppressing stray light outside the passband.

[0069] It has strong suppression capability for near-infrared lasers and extremely low transmittance, which can minimize the interference of near-infrared stray light on mid-infrared laser applications and ensure the accuracy of detection and other applications. The filter has stable optical performance, with small drift of the passband center wavelength and small change in transmittance within a certain temperature range. The physical structure adopts a multi-layer dielectric film design, and the substrate material has high transmittance and good mechanical strength in the mid-infrared band, which facilitates fixed installation within the module.

[0070] The achromatic half-wave plate is used to adjust the polarization direction of the near-infrared laser pulse to match the polarization direction of the nonlinear crystal.

[0071] Specifically, the polarization matching function of the achromatic half-wave plate 43 is crucial for the difference frequency to meet the phase matching condition. If the laser polarization direction does not match the crystal polarization direction, the difference frequency efficiency will drop significantly or even fail to generate mid-infrared laser. Its achromatic characteristics ensure stable phase delay over a wide near-infrared spectral range, unifying the polarization direction of all wavelength components and avoiding fluctuations in difference frequency efficiency.

[0072] In practical adjustments, rotating the half-wave plate allows for precise alignment of the laser polarization direction with the crystal polarization direction, resulting in high adjustment accuracy and improving the difference frequency efficiency to a reasonable range. The half-wave plate's aperture is larger than the incident beam diameter, preventing laser edge cutting, ensuring good optical uniformity, reducing laser wavefront distortion, and utilizing high mechanical strength and strong anti-interference capabilities. Furthermore, the adjustment accuracy shows no significant decrease over long-term use.

[0073] In an optional implementation, the nonlinear crystal 45 is one of periodically polarized lithium niobate, oriented patterned gallium phosphide, gallium selenide, or zinc germanium phosphide crystals, used to convert near-infrared laser pulses into mid-infrared laser pulses via intra-pulse self-difference frequency conversion.

[0074] Specifically, all four nonlinear crystals possess excellent nonlinear optical properties and phase matching capabilities, making them suitable for intra-pulse differential frequency requirements. Each crystal is adapted to different mid-infrared wavelength ranges, allowing for selection as needed. Periodically polarized lithium niobate crystals have a wide adjustable polarization period and high differential frequency conversion efficiency, making them suitable for low-to-medium power differential frequency scenarios, but they have a lower threshold for resistance to optical damage.

[0075] Patterned gallium phosphide crystals have a high resistance to optical damage threshold, can withstand higher power density lasers, and have high transmittance in specific mid-infrared bands, making them suitable for high-power applications. Gallium selenide crystals have extremely high nonlinear coefficients and the highest difference-frequency conversion efficiency. They exhibit stable optical performance in specific bands, but have low mechanical strength and are susceptible to moisture, requiring a dry environment for use.

[0076] Phosphorus-germanium-zinc crystals cover a wide mid-infrared band, enabling far-mid-infrared laser output and filling the gaps in other crystal bands, making them suitable for far-mid-infrared applications. The four crystals have essentially the same size parameters, facilitating fixed installation within modules. Their high surface finish on both the incident and exit surfaces reduces laser reflection loss and improves difference frequency efficiency.

[0077] The first off-axis parabolic reflector 44 is used to focus the polarized near-infrared laser pulse into the nonlinear crystal 45, and the second off-axis parabolic reflector 46 is used to collimate and output the mid-infrared laser pulse generated in the nonlinear crystal 45.

[0078] Specifically, both off-axis parabolic mirrors employ an off-axis design, eliminating spherical and chromatic aberrations to ensure waveform distortion-free laser focusing and collimation, thus guaranteeing difference frequency efficiency and mid-infrared laser quality. The focal length of the first off-axis parabolic mirror 44 is determined based on the crystal size and beam diameter, resulting in a small focused spot diameter and high power density, which can excite a strong difference frequency effect. The off-axis design also prevents reflected light from interfering with the pre-amplifier module.

[0079] The second off-axis parabolic reflector 46 has a focal length matched with the first reflector, enabling it to collimate the diverging mid-infrared laser into parallel light. After collimation, the small divergence angle and beam diameter are adapted to the filter's aperture, ensuring that the laser completely passes through the filter, facilitating subsequent transmission and applications. Both reflectors are coated with a mid-infrared high-reflectivity film to reduce reflection loss and improve system efficiency. The substrate material has high processing precision and good surface accuracy, ensuring focusing and collimation accuracy.

[0080] In one optional implementation, the near-infrared optical frequency comb seed source is a polarization-maintaining all-fiber structure based on saturable absorber mode-locking, nonlinear polarization rotation mode-locking, or nonlinear amplifying ring mirror mode-locking, outputting near-infrared laser pulses with femtosecond pulse widths.

[0081] Specifically, all three mode-locking methods are based on a polarization-maintaining all-fiber structure, ensuring high stability, high integration, and narrow pulse width output of the seed source. The seed source based on saturable absorber mode-locking inserts a saturable absorber into the polarization-maintaining fiber loop, generating femtosecond pulses through gating. This results in a low start-up threshold, high pulse stability, and ease of integration.

[0082] Seed sources based on nonlinear polarization rotation mode-locking utilize the nonlinear polarization evolution effect within the fiber for mode-locking, eliminating the need for additional absorbers and exhibiting high all-fiber integration, strong anti-interference capability, and long lifetime. Seed sources based on nonlinear amplifying ring mirror mode-locking achieve higher power seed light output and high pulse energy stability through nonlinear phase shift difference mode-locking in the ring amplification optical path.

[0083] The seed source output pulse widths of the three mode-locking methods are all in the femtosecond range, with the same repetition frequency. The power meets the requirements of subsequent amplification, and all maintain a high polarization extinction ratio, providing a basis for polarization matching of subsequent modules.

[0084] The repetition frequency of the mid-infrared optical frequency comb is locked to the repetition frequency of the near-infrared optical frequency comb seed source, and the repetition frequency changes synchronously with the repetition frequency of the seed source.

[0085] Specifically, the repetition frequency of the mid-infrared optical frequency comb is consistent with that of the near-infrared optical frequency comb seed source 1, determined by the intra-pulse self-difference frequency physical mechanism. This eliminates the need for an additional locking circuit, simplifying the system structure and reducing costs. The repetition frequency of the near-infrared optical frequency comb seed source 1 is precisely locked via a built-in locking circuit, thus ensuring high accuracy of the mid-infrared laser repetition frequency and meeting the requirements for precision measurement.

[0086] When adjusting the repetition frequency of the mid-infrared laser, only the repetition frequency of the seed source 1 of the near-infrared optical frequency comb needs to be adjusted, and the mid-infrared laser will change synchronously. The power fluctuation during the adjustment process is small, ensuring system stability, avoiding the problem of matching the repetition frequency of dual pulses in traditional differential frequency systems, and improving ease of use.

[0087] In an optional implementation, the output wavelength of the mid-infrared optical frequency comb is tunable by adjusting the polarization period of the nonlinear crystal 45 and matching the length of the second polarization-maintaining single-mode fiber 41.

[0088] Specifically, wavelength tuning is achieved through the coordinated adjustment of the polarization period of the nonlinear crystal and the length of the second polarization-maintaining single-mode fiber, ensuring that the phase matching condition is always met during tuning and maintaining high difference-frequency efficiency. The polarization period of the nonlinear crystal 45 directly determines the difference-frequency phase-matching wavelength; the longer the polarization period, the longer the mid-infrared wavelength, and vice versa. Wavelength tuning can be achieved by changing the crystal or by electronic control.

[0089] The length adjustment of the second polarization-maintaining single-mode fiber 41 is used for dispersion matching. When the crystal polarization period changes, the near-infrared laser spectral components and dispersion requirements for the difference frequency change accordingly. Adjusting the fiber length can compensate for the dispersion change, ensuring that all spectral components of the laser arrive at the crystal synchronously, thus satisfying phase matching. This cooperative tuning method achieves stable tuning over a wide range of mid-infrared wavelengths, covering various crystal-compatible bands. During tuning, power fluctuations are small and the repetition frequency is stable, meeting different application requirements.

[0090] See Figure 6 As shown, Figure 6 The diagram shows the output spectral distribution of the mid-infrared optical frequency comb provided in this embodiment of the invention. The vertical axis represents the relative intensity of the laser, and the horizontal axis is labeled "wavelength (nm)," representing the laser wavelength in nanometers (nm). This diagram visually presents the spectral distribution of the mid-infrared laser, verifying the output characteristics of the difference frequency module generating mid-infrared laser through intra-pulse self-difference frequency, as well as the effectiveness of the wavelength tuning function of the mid-infrared optical frequency comb, ensuring that the mid-infrared laser can meet the needs of scenarios such as molecular spectroscopy detection.

[0091] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures. In addition, the terms "first", "second", "third", etc. are used only to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0092] Finally, it should be noted that the above-described embodiments are merely specific implementations of the present invention, used to illustrate the technical solutions of the present invention, and not to limit it. The scope of protection of the present invention is not limited thereto. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that any person skilled in the art can still modify or easily conceive of changes to the technical solutions described in the foregoing embodiments within the scope of the technology disclosed in the present invention, or make equivalent substitutions for some of the technical features; and these modifications, changes, or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention. All should be covered within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A mid-infrared optical frequency comb, characterized in that, The mid-infrared optical frequency comb includes: Near-infrared optical frequency comb seed source with polarization-maintaining all-fiber structure; The all-fiber nonlinear amplification and pulse compression module, which is connected to the near-infrared optical frequency comb seed source via polarization-maintaining fiber, is used to amplify the seed light and generate linear chirped pulses before compressing the pulse width. An all-fiber nonlinear spectral broadening module, connected to the all-fiber nonlinear amplification and pulse compression module via polarization-maintaining fiber, is used to broaden the laser spectrum; The difference frequency module is used to receive the laser pulse processed by the all-fiber nonlinear spectral broadening module and generate mid-infrared laser pulses through intra-pulse self-difference frequency; The near-infrared optical frequency comb seed source, the all-fiber nonlinear amplification and pulse compression module, and the all-fiber nonlinear spectral broadening module are connected by polarization-maintaining fiber to form an all-fiber integrated structure.

2. The mid-infrared optical frequency comb as described in claim 1, characterized in that, The all-fiber nonlinear amplification and pulse compression module includes multiple pump sources, multiple polarization-maintaining fiber wavelength division multiplexers, multiple polarization-maintaining gain fibers, and a first polarization-maintaining single-mode fiber for pulse compression. The multiple pump sources are coupled to the multiple polarization-maintaining gain fibers through corresponding polarization-maintaining fiber wavelength division multiplexers. The plurality of pump sources consists of three, the plurality of polarization-maintaining fiber wavelength division multiplexers consists of three, and the plurality of polarization-maintaining gain fibers consists of two.

3. The mid-infrared optical frequency comb as described in claim 2, characterized in that, All three pump sources are semiconductor continuous lasers with an output center wavelength of 976nm. The three pump sources are coupled to the input or both ends of the two polarization-maintaining gain fibers through corresponding polarization-maintaining fiber wavelength division multiplexers. The first pump source is coupled to the input of the first polarization-maintaining gain fiber through the first polarization-maintaining fiber wavelength division multiplexer, the second pump source is coupled to the input of the second polarization-maintaining gain fiber through the second polarization-maintaining fiber wavelength division multiplexer, and the third pump source is coupled to the output of the second polarization-maintaining gain fiber through the third polarization-maintaining fiber wavelength division multiplexer.

4. The mid-infrared optical frequency comb as described in claim 2, characterized in that, The polarization-maintaining gain fiber is a polarization-maintaining erbium-doped gain fiber, used for power amplification of near-infrared laser pulses; The first polarization-maintaining single-mode fiber is used to compress the pulse width of the linear chirped pulse generated after amplification by the polarization-maintaining gain fiber.

5. The mid-infrared optical frequency comb as described in claim 1, characterized in that, The all-fiber nonlinear spectral broadening module is a polarization-maintaining high nonlinear fiber used to broaden the near-infrared laser spectrum after pulse width compression.

6. The mid-infrared optical frequency comb as described in claim 1, characterized in that, The difference frequency module includes a second polarization-maintaining single-mode fiber for dispersion adjustment, a polarization-maintaining fiber collimator, an achromatic half-wave plate, a first off-axis parabolic mirror, a nonlinear crystal, a second off-axis parabolic mirror, and a filter.

7. The mid-infrared optical frequency comb as described in claim 6, characterized in that, The filter is a mid-infrared bandpass filter, used to filter out the near-infrared laser remaining after the difference frequency and retain the mid-infrared laser; The achromatic half-wave plate is used to adjust the polarization direction of the near-infrared laser pulse to match the polarization direction of the nonlinear crystal.

8. The mid-infrared optical frequency comb as described in claim 6, characterized in that, The nonlinear crystal is one of periodically polarized lithium niobate, oriented patterned gallium phosphide, gallium selenide, or zinc germanium phosphide crystal, used to convert near-infrared laser pulses into mid-infrared laser pulses by intra-pulse self-difference frequency conversion. The first off-axis parabolic reflector is used to focus the polarized near-infrared laser pulse into the nonlinear crystal, and the second off-axis parabolic reflector is used to collimate and output the mid-infrared laser pulse generated in the nonlinear crystal.

9. The mid-infrared optical frequency comb as described in claim 1, characterized in that, The near-infrared optical frequency comb seed source is a polarization-maintaining all-fiber structure based on saturable absorber mode-locking, nonlinear polarization rotation mode-locking, or nonlinear amplifying ring mirror mode-locking, and outputs near-infrared laser pulses with femtosecond pulse widths. The repetition frequency of the mid-infrared optical frequency comb is locked to the repetition frequency of the near-infrared optical frequency comb seed source, and the repetition frequency changes synchronously with the repetition frequency of the seed source.

10. The mid-infrared optical frequency comb as described in claim 6, characterized in that, The output wavelength of the mid-infrared optical frequency comb is tunable. Wavelength tuning is achieved by adjusting the polarization period of the nonlinear crystal and matching the length of the second polarization-maintaining single-mode fiber.

Images