A mode filter, a few-mode fiber amplifier, and a multi-stage fiber amplifier system

Abstract

This invention discloses a mode filter, a few-mode fiber amplifier, and a multi-stage fiber amplifier system. The mode filter is fused to a polarization-maintaining ytterbium-doped few-mode fiber using a fiber fusion splicer. The cross-section of the mode filter includes: a core region with a core diameter smaller than that of the few-mode fiber; a cladding region surrounding the core region; two stress bars disposed on the cladding region and arranged symmetrically relative to the upper and lower axes of the core region; and Bragg gratings symmetrically distributed and etched along the edge of the core region, with the position of the Bragg gratings coinciding with the higher-order transverse mode portions transmitted within the core region. This invention uses a grating writing device to etch the Bragg grating structure around the core of the few-mode fiber, matching the target position of the grating on the fiber cross-section with the distribution of the higher-order transverse mode fields in the fiber. This results in greater loss of the higher-order modes in the fiber after passing through the grating, thus achieving the function of mode filtering.

Description

Technical Field

[0001] This invention relates to the field of laser technology, and more specifically, to a mode filter, a few-mode fiber amplifier, and a multi-stage fiber amplifier system. Background Technology

[0002] Few-mode fiber refers to optical fiber that supports transmission amplification of several higher-order modes in addition to the fundamental mode (LP01). In the field of optical communication, few-mode fiber is used for mode division multiplexing to increase channel capacity and overcome bandwidth limitations. In the field of fiber amplifiers, rare-earth-doped few-mode fiber is often used as a gain medium to amplify signals, overcoming the power limitations of small-core single-mode fiber.

[0003] US Patent No. US9595802B2, entitled "Multimode Fiber Amplifier," proposes a method for mode filtering by coiling optical fiber. The principle is to introduce loss into the fiber modes through coiling, making the loss introduced by higher-order modes greater than that of lower-order modes, thus achieving mode filtering. However, this method has three drawbacks in practical use: First, axial twisting is unavoidable during fiber coiling, introducing stress. After fixing the fiber, the torque on the fiber changes over time, leading to variations in the mode filtering effect. Second, it is sensitive to ambient temperature; mode loss changes with temperature, making it unreliable for operation in environments with varying temperatures. Third, coiling requires a certain fiber length to be effective, and for femtosecond pulse chirped amplification systems, excessively long fibers introduce unnecessary nonlinearity, leading to a degradation in laser performance.

[0004] Chinese utility model patent CN218240467U, entitled "Fiber Optic Mode Multiplexing System," proposes a method for exciting different modes in a few-mode fiber using a reconfigurable metasurface device unit. When the reconfigurable metasurface device unit is set as the filtering fundamental mode, the structure can also be used for mode filtering. However, this method uses multiple couplings from fiber to space and back to fiber, resulting in a complex structure and poor coupling stability. Furthermore, the reconfigurable metasurface device unit is expensive and has limited optical power handling capacity, making it unsuitable for high-power laser amplification.

[0005] Chinese invention patent CN103928829A, entitled "A High-Order Mode Acquisition Device Based on a Few-Mode Fiber Bragg Grating," proposes a method for reflecting different modes in a few-mode fiber using a fiber Bragg grating. By selecting the center wavelength of the light source and matching the self-coupling resonance peaks of the modes in the grating, reflection of different modes is achieved. When all high-order modes are set to be reflected by the fiber Bragg grating, the device can also be used for mode filtering. However, this method is limited by the use of narrow-linewidth light sources, and for femtosecond pulse chirped amplification systems, the pulse spectral width is typically greater than 10 nm, making this filtering method unsuitable.

[0006] Mode filtering in few-mode fiber amplifiers is an effective way to avoid intermode coupling crosstalk. However, a stable and reliable mode filtering device for few-mode fiber amplifiers suitable for femtosecond pulse chirped amplification systems still needs to be developed. Summary of the Invention

[0007] In view of the above-mentioned shortcomings of the prior art, the purpose of this invention is to provide a mode filter, a few-mode fiber amplifier, and a multi-stage fiber amplifier system to solve the technical problems of pulse quality and pulse stability caused by inter-mode coupling crosstalk in the prior art.

[0008] To address the aforementioned problems, the first objective of this invention is to provide a mode filter that is fused with polarization-maintaining ytterbium-doped few-mode fiber using a fiber fusion splicer, wherein the cross-section of the mode filter comprises:

[0009] The core region has a core diameter smaller than the diameter of the ytterbium-doped few-mode fiber;

[0010] The cladding region surrounds the outer side of the core region; and

[0011] Two stress bars are disposed on the cladding region and arranged symmetrically with respect to the fiber core region on the upper and lower axes.

[0012] A plurality of Bragg gratings are symmetrically distributed and inscribed on the edge of the fiber core region, and the positions of the Bragg gratings coincide with the higher-order transverse mode portions transmitted in the fiber core region.

[0013] Furthermore, the Bragg grating is provided in four parts, two of which are inscribed in the vertical direction of the fiber core region along the direction of the stress bar, and the other two are inscribed in the horizontal direction of the fiber core region along the vertical direction of the stress bar.

[0014] Furthermore, the Bragg grating is a reflective grating with a reflection center wavelength of 1030-1080nm, a spectral bandwidth of 5-25nm, and a reflectivity of 50-100%.

[0015] Furthermore, the Bragg grating has a writing diameter of 2 micrometers, a length of 2 mm, and the writing position is 19 micrometers away from the center of the fiber core.

[0016] Furthermore, the core diameter of the core region is 20 micrometers, the core numerical aperture (NA) is 0.04, the diameter of the cladding region is 125 micrometers, and the cladding numerical aperture (NA) of the cladding region is 0.46.

[0017] Furthermore, the core of the fiber core region is made of pure quartz glass or mixed quartz glass doped with rare earth elements erbium, ytterbium, neodymium and holmium.

[0018] Furthermore, the mode filter is a step-index fiber or a photonic crystal fiber.

[0019] Furthermore, the mode filter is a non-polarization-maintaining few-mode fiber or a polarization-maintaining few-mode fiber with a core diameter of 10-40 micrometers and a cladding diameter of 125-400 micrometers.

[0020] A second objective of this invention is to provide a few-mode fiber amplifier based on the aforementioned mode filter, comprising: a combiner, a multimode pump, a ytterbium-doped few-mode fiber, and a mode filter, wherein:

[0021] The input end of the combiner is connected to the few-mode transmission fiber and the multimode pump, respectively. The multimode pump outputs a multimode pump light in the fundamental mode. The output end of the combiner is connected to the ytterbium-doped few-mode fiber. The combiner is used to couple the signal to be amplified in the few-mode transmission fiber with the multimode pump light in the fundamental mode and then inject it into the ytterbium-doped few-mode fiber.

[0022] A third objective of this invention is to provide a multi-stage fiber amplifier system, comprising: an all-fiber mode-locked seed source, a stretcher, a compressor, and a few-mode fiber amplifier as described above, wherein the output end of the all-fiber mode-locked seed source is connected to the signal input end of the stretcher, the incident end of the few-mode fiber amplifier is connected to the signal output end of the stretcher, the reflecting end of the few-mode fiber amplifier is connected to the input end of the compressor, and the output end of the few-mode fiber amplifier is collimated and input to the compressor.

[0023] Compared with the prior art, the present invention has the following advantages:

[0024] 1. The mode filter described in this application is particularly suitable for few-mode fiber amplifiers. A Bragg grating structure is etched around the fiber core using a grating writing device, thereby matching the target position of the Bragg grating on the fiber cross-section with the high-order transverse mode field distribution of the fiber. Since the mode filter and the ytterbium-doped few-mode fiber are fused together using a fiber fusion splicer, when a pulse is transmitted over a long distance in the few-mode fiber, coupling crosstalk between modes can occur due to the few-mode fiber's support for multiple modes, causing pulse phase changes. Especially in femtosecond pulse chirped amplification systems, this uncontrolled pulse phase change can distort or destabilize the compressed pulse. When the transmission enters the mode filter, the high-order modes in the few-mode fiber are effectively filtered out by the Bragg grating, resulting in greater loss for the high-order modes after passing through the Bragg grating, thus achieving the mode filtering function. Therefore, in fiber optic communication lines, the few-mode fiber amplifier can directly amplify the signal without going through complex processes such as photoelectric conversion, electro-optic conversion, and signal regeneration, exhibiting excellent "transparency."

[0025] 2. The few-mode fiber filter based on a Bragg grating in this invention is small in size, simple in structure, highly integrated, and insensitive to environmental changes. On the one hand, devices based on fiber Bragg gratings have high stability, which is more conducive to the operation of fiber amplifiers in scenarios with large environmental temperature variations; on the other hand, its all-fiber structure avoids the use of complex coupling devices, which is very beneficial to the integration and maintenance of amplifiers, reducing manufacturing and maintenance costs. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the structure of the pattern filter in an embodiment of the present invention;

[0027] Figure 2 This is a schematic diagram of the structure of a few-mode fiber amplifier in a femtosecond pulse chirped amplification system according to an embodiment of the present invention;

[0028] Figure 3 This is a cross-sectional schematic diagram of the pattern filter in an embodiment of the present invention;

[0029] Figure 4 This is a schematic diagram comparing the output modes of the few-mode fiber without mode filtering and the output modes of the few-mode fiber after mode filtering in an embodiment of the present invention.

[0030] Explanation of reference numerals in the attached figures:

[0031] 1-Small mode fiber; 11-Core; 12-Cladding; 2-Bracket grating. Detailed Implementation

[0032] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. 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.

[0033] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can also refer to the internal connection of two components; and they can refer to a wireless connection or a wired connection. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0034] Furthermore, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0035] Due to the small core size of optical fibers and their relatively concentrated energy, pulses travel long distances within the fiber. Nonlinear effects become the primary factor hindering performance improvement in fiber femtosecond lasers. The accumulation of significant nonlinear effects leads to pulse splitting and distortion in the compressed pulse. Therefore, traditional techniques struggle to directly achieve high average power and high peak power femtosecond laser output within optical fibers. Thanks to the invention of chirped pulse amplification (CPA), femtosecond fiber laser technology has rapidly advanced.

[0036] Chirped pulse amplification (CPA) technology mainly consists of four parts: a laser oscillator (seed source), a pulse stretcher, a laser amplifier, and a pulse compressor. Its basic principle is as follows: before amplification, the seed laser pulse is stretched in the time domain to the order of hundreds of picoseconds or nanoseconds by a dispersive device. The stretched pulse then has its power increased in the laser amplifier. Finally, the pulse compressor compensates for the dispersion introduced by the previous stage, compressing the pulse to the femtosecond level. The purpose of pulse stretching is to reduce the intensity of the laser pulse during amplification, ensuring that the peak power is below the damage threshold of system components. This avoids damage to the amplifier's optical components due to excessive power from ultrashort pulses after amplification, and also weakens or overcomes various nonlinear effects that may be caused by high-intensity lasers during amplification, allowing the pulse energy within the fiber to be increased by several orders of magnitude. Especially in high-average-power femtosecond fiber lasers, CPA technology has enabled the output of femtosecond pulses with an average power of hundreds of watts and pulse energy in the microjoule range, with peak power reaching the order of tens of megawatts, all within a single fiber.

[0037] Chirped amplification systems for infrared femtosecond pulses typically use ytterbium-doped few-mode fibers for pulse amplification. The power and pulse energy requirements of femtosecond fiber lasers in applications are constantly increasing. Due to the nonlinear effects of the fiber, the mode area of ​​the fiber also needs to increase with the increase in power and energy. When the mode area of ​​the fiber increases to a certain extent and no longer meets the single-mode condition, the fiber supports the transmission and amplification of several modes; at this point, it is classified as a few-mode fiber.

[0038] Mode multiplexing based on few-mode fiber utilizes the limited number of orthogonal modes in the few-mode fiber as independent channels to load information, thus multiplying the system's transmission capacity. Simultaneously, few-mode fiber has a relatively large mode field area and high nonlinear tolerance, effectively mitigating the adverse effects of nonlinearity on the system.

[0039] In mode multiplexing systems based on few-mode fibers, obtaining higher-order modes is crucial. Existing techniques for acquiring higher-order modes mainly include long-period fiber grating (LPFG) technology, free-space optics technology, and waveguide-based directional coupling (DC) technology. However, free-space optics technology has a relatively complex structure, and LPFG technology requires specific devices to induce periodic topographic changes in the fiber, which can easily damage the fiber. While waveguide-based DC technology can achieve lower insertion loss, efficient coupling with the fiber link remains a challenge. Therefore, designing a simple and efficient structural device to acquire higher-order modes has become one of the key research focuses in mode multiplexing technology.

[0040] Generally, the single-mode condition of optical fiber is usually determined by the V value, and its calculation formula is as follows:

[0041]

[0042] When V > 2.405, the optical fiber begins to support higher-order mode transmission.

[0043] For example, Nufern's PLMA-YDF-10 / 125 ytterbium-doped fiber has characteristic parameters NA = 0.075 and r = 5.5 μm. When the center wavelength is 1030 nm, the calculated V value is 2.516. Under these conditions, this fiber supports transmission and amplification in LP01 and LP11 modes, and is no longer a single-mode fiber.

[0044] When pulses are transmitted over long distances in few-mode fibers, crosstalk between modes can occur because the fiber itself supports multiple modes, causing changes in the pulse phase. Especially in femtosecond pulse chirped amplification systems, this uncontrolled pulse phase change can distort or destabilize the compressed pulse. On the one hand, few-mode fibers need to be used in femtosecond pulse chirped amplification systems to increase the mode field area; on the other hand, the pulse amplified by the few-mode fiber amplifier needs to avoid intermode coupling crosstalk.

[0045] Therefore, few-mode fiber amplifiers need to be combined with effective mode filtering methods or mode filtering devices to avoid inter-mode coupling crosstalk in order to ensure the pulse quality and pulse stability of femtosecond pulse chirped amplification systems.

[0046] To resolve the above technical issues, please refer to [link / reference]. Figure 1 As shown, this embodiment of the invention provides a mode filter, which is fused with a polarization-maintaining ytterbium-doped few-mode fiber using an optical fiber fusion splicer. The cross-section of the mode filter includes a core region, a cladding region, two stress bars, and a Bragg grating 2. The core diameter of the core region is smaller than the diameter of the ytterbium-doped few-mode fiber. The cladding region surrounds the outside of the core region. The two stress bars are disposed on the cladding region and are arranged symmetrically with respect to the upper and lower axes of the core region. Multiple Bragg gratings 2 are symmetrically distributed and inscribed on the edge of the core region, and the positions of the Bragg gratings 2 coincide with the higher-order transverse modes transmitted in the core of the core region.

[0047] Since the position of Bragg grating 2 coincides with the position of the higher-order transverse modes transmitted in the core of the few-mode fiber, the low-loss fundamental mode transmitted in the core 11 of the few-mode fiber 1 passes through the Bragg grating region, while the high-loss higher-order transverse modes transmitted in the core 11 of the few-mode fiber 1 pass through the Bragg grating region.

[0048] The mode filter in this embodiment of the invention is particularly suitable for few-mode fiber amplifiers. The Bragg grating 2 is written around the core of the few-mode fiber using a grating writing device. The target position of the Bragg grating 2 on the cross-section of the fiber is matched with the high-order transverse mode field distribution of the fiber, so that the high-order modes in the fiber have a large loss after passing through the Bragg grating 2, thereby realizing the function of mode filtering.

[0049] It is necessary to further explain that Bragg gratings are now mainly divided into two categories: fiber Bragg gratings (FBGs) and volume Bragg gratings (VBGs). A fiber Bragg grating is a grating with a periodically distributed spatial phase formed within the fiber core. Its function is essentially to form a narrow-band (transmission or reflection) filter or mirror within the fiber core. This characteristic can be used to manufacture many fiber optic devices with unique performance characteristics. In this embodiment of the invention, the Bragg grating 2 is a fiber Bragg grating.

[0050] Please see Figure 3 As shown, in a preferred embodiment of the present invention, four Bragg gratings are provided, two of which are inscribed in the vertical direction of the fiber core region along the stress bar direction, and the other two are inscribed in the horizontal direction of the fiber core region along the vertical direction of the stress bar.

[0051] Specifically, the Bragg grating 2 in this embodiment of the invention is a reflective grating with a reflection center wavelength of 1030-1080 nm, a spectral bandwidth of 5-25 nm, and a reflectivity of 50-100%. Preferably, in this embodiment, the center wavelength of the Bragg grating 2 is selected as 1030 nm, the spectral bandwidth is selected as 20 nm, and the reflectivity is selected as 99%.

[0052] Specifically, in this embodiment of the invention, the Bragg grating 2 has a writing diameter of 2 micrometers, a length of 2 mm, and a writing position 19 micrometers away from the center of the fiber core.

[0053] As will be understood by those skilled in the art, grating writing is a technique that uses a pulsed light source and gratings to write microstructures on the surface of an optical fiber. Grating writing can create three-dimensional structures on the surface of an optical fiber, including various folds, pores, voids, and irregularities, to alter the optical properties of the fiber, such as refractive index, insulation thickness, surface refractive index, and extinction coefficient.

[0054] Currently, the commonly used methods for fabricating fiber gratings mainly include the two-beam interferometry method and the phase mask method. Among them, the phase mask method is the most common method for fabricating fiber gratings. The phase mask used is a one-dimensional periodic structure etched onto the surface of a silicon wafer by electron beam lithography or holography. In this way, after the photosensitive fiber is placed close to the phase mask, the interference fringes generated by the near-field diffraction of the phase mask periodically modulate the effective refractive index of the fiber, thus forming the fiber grating.

[0055] As a preferred embodiment of the present invention, the Bragg grating 2 is inscribed inside the fiber core using an ultraviolet laser through a phase mask, or directly inscribed inside the fiber core using a femtosecond laser.

[0056] Specifically, in the embodiments of the present invention, the core diameter of the core region is 20 micrometers, the core numerical aperture NA is 0.04, the diameter of the cladding region is 125 micrometers, and the cladding numerical aperture NA of the cladding region is 0.46.

[0057] Because optical signal transmission differs from electrical signal transmission, for electrical signals, as long as the output of the amplifier is connected to the transmission cable, the signal can be transmitted normally. However, for optical communication, some of the light incident on the end face of the optical fiber cannot enter the optical fiber, and the light that enters the end face of the optical fiber may not be able to be transmitted in the optical fiber. Only light that meets a certain specific condition can undergo total internal reflection in the optical fiber and be transmitted.

[0058] Therefore, regardless of the angle range, only light rays entering the optical fiber along a specific light cone angle can be transmitted normally. The sine value of this angle θ is called the optical fiber numerical aperture NA (Numerical Apeture). It is one of the important optical parameters of optical fiber, and it is dimensionless and has no unit.

[0059] The formula for calculating the numerical aperture (NA) of an optical fiber is as follows:

[0060] NA = n*Sinθ

[0061] Where n is the refractive index of the medium, and θ is the angle between the ray and the fiber axis, usually called the half angle.

[0062] As can be seen from the formula, only light rays with a numerical aperture NA ≦ nsinθ can be coupled into the optical fiber for transmission. The numerical aperture NA represents the optical fiber's ability to receive light.

[0063] A larger numerical aperture (NA) means a stronger light-receiving capability of the optical fiber. From the perspective of increasing the optical power entering the fiber, a larger NA is better, because a larger numerical aperture is advantageous for fiber optic connections. However, when the NA is too large, the mode distortion of the fiber increases, which affects the fiber's bandwidth. Therefore, there are certain requirements for the numerical aperture of the optical fiber in optical fiber communication systems.

[0064] Please see Figure 1 , 2 As shown, this embodiment of the invention also provides a few-mode fiber amplifier 103, which includes a combiner, a multimode pump, a ytterbium-doped few-mode fiber, and a mode filter, wherein:

[0065] The input end of the combiner is connected to the few-mode transmission fiber and the multimode pump, respectively. The multimode pump outputs the fundamental mode multimode pump light. The output end of the combiner is connected to the ytterbium-doped few-mode fiber. The combiner is used to couple the signal to be amplified in the few-mode transmission fiber with the fundamental mode multimode pump light and then inject it into the ytterbium-doped few-mode fiber.

[0066] In this embodiment, the combiner is a (1+1)x1 polarization-maintaining single-mode optical signal / pump combiner. The signal fiber of the combiner uses PM980 polarization-maintaining single-mode fiber, and the pump fiber of the combiner uses multimode fiber with a core diameter of 105 micrometers and an NA of 0.22.

[0067] The multimode pump is a 10W, 976nm wavelength-locked multimode fiber output semiconductor laser. The multimode pump output fiber uses multimode fiber with a core diameter of 105 micrometers. The pulse signal passes through a combiner and enters the 5.5-micrometer diameter core of the combiner output fiber, while the pump light passes through the combiner and enters the 125-micrometer diameter cladding of the combiner output fiber.

[0068] The output fiber of the combiner (i.e., the output end of the combiner) is fused with the polarization-maintaining ytterbium-doped few-mode fiber using a fiber fusion splicer. The core diameter of the polarization-maintaining ytterbium-doped few-mode fiber is 20 micrometers, the core NA is 0.04, the cladding diameter is 125 micrometers, and the cladding NA is 0.46. Therefore, this fiber does not meet the single-mode requirements of fiber and can support the transmission and amplification of LP01 and LP11 modes.

[0069] The multimode pump has an absorption coefficient of 16dB / m, a usable length of 0.5m, and a coil diameter of 8cm, wound twice. A pulse signal with an average power of 1mW can be amplified to 1W after passing through the few-mode fiber amplifier 103.

[0070] Specifically, in this embodiment of the invention, the fiber core 11 of the fiber core region is pure quartz glass or mixed quartz glass doped with rare earth elements erbium, ytterbium, neodymium and holmium.

[0071] Optical fiber actually refers to a medium consisting of a core made of transparent material and a cladding made of a material with a slightly lower refractive index than the core. The light signal incident on the core is reflected by the cladding interface, allowing the light signal to propagate through the core.

[0072] Few-mode fiber is an optical fiber that supports transmission amplification of several higher-order modes in addition to the fundamental mode (LP01). In the field of optical communication, few-mode fiber is used for mode division multiplexing to increase channel capacity and overcome bandwidth limitations.

[0073] In multimode fiber, the core diameter is 15µm-50µm, roughly the thickness of a human hair. Single-mode fiber, on the other hand, has a core diameter of 8µm-10µm. The core is surrounded by a glass cladding with a lower refractive index to keep light within the core. This is followed by a thin plastic jacket to protect the glass cladding. Fiber optic cables are typically bundled together and protected by an outer shell. The core is usually a small, double-layered concentric cylinder made of quartz glass; it is brittle and easily broken, thus requiring an external protective layer.

[0074] Preferably, the mode filter in the embodiments of the present invention is a step-index fiber or a photonic crystal fiber.

[0075] Step-index fiber is a type of fiber with a step-index refractive index distribution. The core refractive index is higher than the cladding refractive index, allowing input light to propagate through repeated total internal reflections at the core-cladding interface. The core of this fiber has a uniform refractive index, while the cladding has a slightly lower refractive index. The refractive index from the core to the cladding is abrupt, with only one step, hence the name step-index multimode fiber, or simply step-index fiber, or abrupt-index fiber.

[0076] Photonic crystal fiber (PCF), also known as micro-structured fiber (MSF), has a complex refractive index distribution in its cross-section and usually contains pores arranged in different ways. The size of these pores is roughly on the same order of magnitude as the wavelength of light and runs through the entire length of the device. Light waves can be confined to the low-refractive-index fiber core region for propagation.

[0077] Photonic crystal fibers possess many unique properties. For example, they can support only one mode transmission over a wide bandwidth; the arrangement of pores in the cladding region can greatly affect mode properties; and asymmetrically arranged pores can also produce a large birefringence effect, making it possible to design high-performance polarization devices.

[0078] Preferably, the mode filter in this embodiment of the invention is a non-polarization-maintaining few-mode fiber or a polarization-maintaining few-mode fiber with a core diameter of 10-40 micrometers and a cladding diameter of 125-400 micrometers.

[0079] Please see Figure 2 As shown, other embodiments of the present invention also provide a multi-stage fiber amplifier system, the multi-stage fiber amplifier system comprising an all-fiber mode-locked seed source 101, a stretcher 102, a compressor 104, and a few-mode fiber amplifier 103, wherein:

[0080] The output end of the all-fiber mode-locked seed source 101 is connected to the signal input end of the stretcher 102, the incident end of the few-mode fiber amplifier 103 is connected to the signal output end of the stretcher 102, the reflection end of the few-mode fiber amplifier 103 is connected to the input end of the compressor 104, and the output end of the few-mode fiber amplifier 103 is collimated and input into the compressor 104.

[0081] In this embodiment, the all-fiber mode-locked seed source 101 is a passive mode-locked laser with a center wavelength of 1030nm, a spectral width of 15nm, an average power of 1mW, a repetition frequency of 40MHz, and a pulse width of 5ps.

[0082] It should be noted that mode-locked fiber lasers are mainly divided into two types from a technical perspective: active mode-locking and passive mode-locking. Active mode-locked fiber lasers typically employ modulation devices within the cavity, which introduces additional cavity losses. Since modulation devices are mostly non-fiber components, their introduction makes it difficult to achieve all-fiber integration, thus hindering the all-fiber development of this technology. At the same time, active mode-locking is easily affected by external environmental factors such as temperature changes, mechanical vibrations, supermode noise, and polarization state fluctuations within the resonant cavity. This requires many complex technologies to improve system stability, resulting in a significant increase in system complexity and laser cost.

[0083] Therefore, the passive mode-locked fiber laser used in this embodiment of the invention has attracted widespread attention at home and abroad due to its advantages such as simple structure, stable performance and easy integration, and has been increasingly widely used in many fields such as communication, medicine, processing, sensing and detection.

[0084] Passive mode-locked fiber lasers are mainly achieved through mechanisms such as nonlinear optical ring mirrors, nonlinear polarization rotation, and semiconductor saturable absorber mirrors (SESAMs). Among these, passive mode-locking technology based on SESAMs has many advantages, including flexible design, system stability, and self-starting. Furthermore, the key parameters such as modulation depth, recovery time, and saturation flux can be flexibly controlled during the fabrication of SESAMs, and they can be integrated into the fiber optic end cap as needed, facilitating all-fiber integration. Therefore, this type of passive mode-locked fiber laser has attracted widespread attention in practical applications.

[0085] In addition, the output end of the all-fiber mode-locked seed source 101 uses PM980 polarization-maintaining single-mode fiber, with a core diameter of 5.5 micrometers, a numerical aperture of 0.12, a cladding diameter of 125 micrometers, and a coating diameter of 245 micrometers.

[0086] In this embodiment, the stretcher 102 uses PM980 polarization-maintaining single-mode fiber. The output pulse of the all-fiber mode-locked seed source 101 is stretched by the stretcher 102, which consists of a fiber circulator and a chirped grating. The pulse after passing through the stretcher 102 maintains the input spectral shape, with a center wavelength of 1030 nm, a spectral width of 15 nm, and is stretched to 500 ps. The stretched pulse then enters the few-mode fiber amplifier 103.

[0087] In this embodiment, the few-mode fiber amplifier 103 consists of a combiner, a multimode pump, a ytterbium-doped few-mode fiber, and a mode filter.

[0088] The combiner is a (1+1)x1 polarization-maintaining single-mode optical signal / pump combiner. The signal fiber of the combiner uses PM980 polarization-maintaining single-mode fiber, and the pump fiber of the combiner uses multimode fiber with a core diameter of 105 micrometers and an NA of 0.22.

[0089] The multimode pump is a 10W, 976nm wavelength-locked multimode fiber output semiconductor laser. The pump output fiber uses multimode fiber with a core diameter of 105 micrometers. The pulse signal passes through a combiner and enters the 5.5-micrometer core of the combiner output fiber, while the pump light passes through the combiner and enters the 125-micrometer cladding of the combiner output fiber.

[0090] The output fiber of the combiner is fused to the polarization-maintaining ytterbium-doped few-mode fiber using a fiber fusion splicer. The polarization-maintaining ytterbium-doped few-mode fiber has a core diameter of 20 micrometers, a core NA of 0.04, a cladding diameter of 125 micrometers, and a cladding NA of 0.46. This fiber does not meet the single-mode requirements of optical fibers and can support the transmission and amplification of LP01 and LP11 modes.

[0091] The multimode pump has an absorption coefficient of 16dB / m, a working length of 0.5m, and a coil diameter of 8cm. The pulse signal with an average power of 1mW is amplified to 1W after passing through the few-mode fiber amplifier 103.

[0092] The mode filter is located behind the ytterbium-doped few-mode fiber. The mode filter and the ytterbium-doped few-mode fiber are fused together using a fiber fusion splicer. The mode filter is made of passive polarization-maintaining few-mode fiber with a core diameter of 20 micrometers, a core NA of 0.04, a cladding diameter of 125 micrometers, and a cladding NA of 0.46, which perfectly matches the dimensions of the polarization-maintaining ytterbium-doped few-mode fiber.

[0093] Please see Figure 3 As shown, the mode filter contains four fiber Bragg gratings, which are reflective gratings with a center wavelength of 1030 nm, a spectral bandwidth of 20 nm, and a reflectivity of 99%. The gratings are 2 micrometers in diameter, 2 mm in length, and located 19 micrometers from the center of the fiber core. Two gratings are etched along the stress bar direction, and two are etched perpendicular to the stress bar direction.

[0094] Please see Figure 3 , 4 As shown in the figure, the power concentration region of the grating does not overlap with that of the LP01 fundamental mode, so the fundamental mode can pass through the grating region with no loss or very low loss. The grating overlaps well with the main power concentration region of the LP11 higher-order modes, so the higher-order modes are reflected by the grating and cannot pass through the grating region or pass through the grating region with very high loss. Therefore, only the LP01 fundamental mode passes through the grating region, achieving the function of mode filtering.

[0095] After passing through the mode filter, the pulse signal enters the spatial optical transmission through the optical fiber and enters the compressor 104. The compressor 104 is composed of a volume grating, in which the pulse signal is transmitted in space and pulse compression is achieved. The compressed pulse is about 300 fs.

[0096] Please see Figure 4 As shown, mode filters can effectively filter out higher-order modes in few-mode fibers. Figure 4 The image shows the far-field beam distribution with and without a mode filter. The beam quality also differs significantly, being 1.22 without a mode filter and 1.01 with a mode filter.

[0097] While the present invention has been disclosed above, its scope of protection is not limited thereto. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of this disclosure, and all such changes and modifications will fall within the scope of protection of this invention.

Claims

1. A mode filter, fused with ytterbium-doped few-mode fiber using a fiber optic fusion splicer, characterized in that, The cross-section of the pattern filter includes: The core region has a core diameter smaller than the diameter of the ytterbium-doped few-mode fiber; The cladding region surrounds the outer side of the core region; Two stress bars are disposed on the cladding region and arranged symmetrically about the upper and lower axes relative to the core region; and A plurality of Bragg gratings are symmetrically distributed and inscribed on the edge of the fiber core region, and the positions of the Bragg gratings coincide with the higher-order transverse mode portions transmitted in the fiber core region.

2. The pattern filter according to claim 1, characterized in that, The Bragg grating is provided in four parts, two of which are inscribed in the vertical direction of the fiber core region along the direction of the stress bar, and the other two are inscribed in the horizontal direction of the fiber core region along the vertical direction of the stress bar.

3. The pattern filter according to claim 1, characterized in that, The Bragg grating is a reflective grating with a reflection center wavelength of 1030-1080nm, a spectral bandwidth of 5-25nm, and a reflectivity of 50-100%.

4. The pattern filter according to claim 1, characterized in that, The Bragg grating has a writing diameter of 2 micrometers, a length of 2 mm, and the writing position is 19 micrometers away from the center of the fiber core.

5. The pattern filter according to claim 1, characterized in that, The core region has a core diameter of 20 micrometers and a core numerical aperture (NA) of 0.

04. The cladding region has a diameter of 125 micrometers and a cladding numerical aperture (NA) of 0.

46.

6. The pattern filter according to claim 1, characterized in that, The fiber core of the fiber core region is made of pure quartz glass or mixed quartz glass doped with rare earth elements erbium, ytterbium, neodymium and holmium.

7. The pattern filter according to any one of claims 1-6, characterized in that, The mode filter is a step-index fiber or a photonic crystal fiber.

8. The pattern filter according to any one of claims 1-6, characterized in that, The mode filter is a non-polarization-maintaining few-mode fiber or a polarization-maintaining few-mode fiber with a core diameter of 10-40 micrometers and a cladding diameter of 125-400 micrometers.

9. A few-mode fiber amplifier based on the mode filter of claim 1, characterized in that, include: Bundle, multimode pump, ytterbium-doped few-mode fiber, and mode filter, among which: The input end of the combiner is connected to the few-mode transmission fiber and the multimode pump, respectively. The multimode pump outputs a multimode pump light in the fundamental mode. The output end of the combiner is connected to the ytterbium-doped few-mode fiber. The combiner is used to couple the signal to be amplified in the few-mode transmission fiber with the multimode pump light in the fundamental mode and then inject it into the ytterbium-doped few-mode fiber.

10. A multi-stage fiber optic amplifier system, characterized in that, include: The system comprises an all-fiber mode-locked seed source, a stretcher, a compressor, and a few-mode fiber amplifier as described in claim 9, wherein the output end of the all-fiber mode-locked seed source is connected to the signal input end of the stretcher, the incident end of the few-mode fiber amplifier is connected to the signal output end of the stretcher, the reflecting end of the few-mode fiber amplifier is connected to the input end of the compressor, and the output end of the few-mode fiber amplifier is collimated and input to the compressor.

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