Graphene-coated surface of a multimode optical fiber with embedded microstructures can saturable absorber

Abstract

The application discloses a graphene-coated surface saturable absorber device of a multimode optical fiber with an internal microstructure, which adopts a mixed structure of graphene and a graded-index multimode optical fiber. The saturable absorber device comprises a first single-mode optical fiber, a mixed structure based on a multimode optical fiber, and a second single-mode optical fiber. The mixed structure based on the multimode optical fiber is composed of a tapered graded-index multimode optical fiber with an internal microstructure and a single-layer graphene film. The graded-index multimode optical fiber is first subjected to a micro-tapering treatment, then a microstructure is inscribed in the core inside the taper waist, then the graphene film is coated on the surface of the taper waist of the optical fiber, and finally the single-mode optical fibers are fused at both ends of the multimode optical fiber. The saturable absorber combines the nonlinear multimode interference effect in the multimode optical fiber and the saturable absorption characteristics of graphene, can perform secondary compression on the pulse width of a high-energy pulse, and thus obtains an ultrashort pulse. The application has the advantages of a solid structure, strong stability, a high damage threshold and the like, and provides an effective technical means for obtaining a stable ultrashort pulse mode locking.

Description

Technical Field

[0001] This invention relates to the fields of fiber lasers and nonlinear optics, and more specifically to a saturable absorber device with graphene coated on the surface of a multimode fiber with an embedded microstructure. Technical Background

[0002] Saturable absorbers are key components in passively mode-locked lasers for generating ultrashort pulses. Saturable absorbers with large modulation depths can support high pulse energies and facilitate the generation of ultrafast solitons, playing a crucial role in research on multi-soliton pulse dynamics. Currently, two commonly used saturable absorbers have been reported. The first type is based on the inherent saturable absorption properties of the material itself, including semiconductor saturable absorber prisms, two-dimensional nanomaterials such as single carbon nanotubes, topological insulators, graphene, transition metal dichalcogenides, and black scale. Among these, semiconductor saturable absorber prisms are the earliest and most mature technology; however, their application is limited by high cost, low damage threshold, and narrow operating bandwidth. The second type utilizes the nonlinear or birefringence effects of optical elements, such as nonlinear deflection rotation techniques and nonlinear amplifying ring mirrors; however, such devices exhibit poor stability in practical applications.

[0003] Compared to traditional saturable absorber prisms and single-carbon nanotubes, graphene exhibits a high damage threshold, fast recovery time, and stable performance. Furthermore, as a special semiconductor with zero bandgap, graphene absorbs light across almost all wavelengths, making it an ultrawideband, ultrafast saturable absorber. A common method is to deposit graphene onto side-polished D-shaped or tapered optical fibers, where light interacts with the graphene in the form of evanescent waves to achieve mode-locking. Since the theory of "single-mode-multimode-single-mode hybrid structures" as saturable absorbers was proposed, all-fiber mode-locked lasers based on the multimode interference effect in graded-index multimode fibers have attracted extensive research. Graded-index multimode fibers provide a wealth of novel and complex nonlinear phenomena for the generation of ultrashort pulses. However, precise control of the multimode fiber length is required in practical applications, which undoubtedly increases production difficulty and cost.

[0004] This invention proposes a saturable absorber based on the combined effect of nonlinear multimode interference and graphene. This is the first time that two saturable absorption mechanisms have been combined, making full use of the advantages of graphene in saturable absorption, while eliminating the defect of requiring precise control of the multimode fiber length in nonlinear multimode interference. It can be used in passive mode-locked lasers to obtain stable ultrashort pulses. Summary of the Invention

[0005] In view of the shortcomings of existing technologies, the purpose of this invention is to provide a saturable absorber with graphene coated on the surface of a multimode fiber with an embedded microstructure, and to apply it to a passively mode-locked fiber laser to obtain stable ultrashort pulse output. Its main advantages are a robust and compact structure, stable performance, and low manufacturing cost.

[0006] According to the stated purpose, the technical measures adopted by this invention to solve the technical problem are as follows: a hybrid structure combining graphene and graded-index multimode fiber. The saturable absorber device includes a first single-mode fiber, a hybrid structure based on the multimode fiber, and a second single-mode fiber. The hybrid structure based on the multimode fiber consists of a tapered graded-index multimode fiber with an embedded microstructure and a few-layer graphene film. The graded-index multimode fiber is first micro-tapered, then a microstructure is etched inside the fiber core at the tapered waist, then a graphene film is coated onto the surface of the tapered waist, and finally, single-mode fibers are fused at both ends of the multimode fiber. This saturable absorber combines the nonlinear multimode interference effect in multimode fiber with the saturable absorption characteristics of graphene, enabling secondary compression of the pulse width of high-energy pulses, thereby obtaining ultrashort pulses.

[0007] Preferably, the hybrid structure based on multimode fiber includes a section of tapered multimode fiber with microstructures inscribed on it and a single layer of graphene, wherein the single layer of graphene covers the surface of the tapered multimode fiber at the waist position.

[0008] Preferably, the tapered multimode fiber is made by tapering a graded-index multimode fiber using a fiber tapering machine, and its length is 27cm and the diameter at the waist of the tapered fiber is 60μm.

[0009] Preferably, the built-in microstructure is inscribed inside the core of the tapered multimode fiber at the waist of the fiber using femtosecond micromachining technology, and the microstructure is a cube shape with a side length of 10 μm.

[0010] Preferably, the graphene is commercially available single-layer graphene and is coated on the surface of a tapered multimode optical fiber inscribed with microstructure positions.

[0011] Preferably, the two single-mode optical fibers are 50cm long, have an outer cladding diameter of 125μm, and a core diameter of 8μm.

[0012] The present invention has the following beneficial effects:

[0013] 1. The saturable absorber of the present invention uses inexpensive multimode optical fiber, single-mode optical fiber, and graphene, which not only has low manufacturing cost, but also simple, convenient and quick manufacturing steps.

[0014] 2. The multimode fiber of the saturable absorber of the present invention introduces a microstructure, which eliminates the need for precise control of the length of the multimode fiber when using the traditional "single-mode-multimode-single-mode structure" as a saturable absorber, and reduces the manufacturing difficulty.

[0015] 3. The saturable absorber of the present invention combines the saturable absorption characteristics of graphene with the nonlinear multimode interference effect of multimode fiber. When applied to a passive mode-locked laser, it can achieve secondary compression of the pulse to obtain a stable ultrashort pulse. Attached Figure Description

[0016] To more intuitively illustrate the embodiments or technical solutions of the present invention, the present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0017] Figure 1 This is a schematic diagram of the fiber laser system of the present invention.

[0018] Figure 2 This is a schematic diagram of the structure of the saturable absorber of the present invention.

[0019] In the figure, 1. Semiconductor pump source, 2. Wavelength division multiplexer, 3. Gain fiber, 4. Isolator, 5. Coupler, 6. Polarization controller, 7. Saturable absorber, 8. First single-mode fiber, 9. Graphene film, 10. Microstructure, 11. Tapered multimode fiber, 12. Second single-mode fiber. Detailed Implementation

[0020] To enhance understanding of the present invention, it will be further described below with reference to the accompanying drawings and embodiments:

[0021] A schematic diagram of the fiber laser system of the present invention is shown below. Figure 1 As shown, the semiconductor pump source 1 is connected to the input end of the wavelength division multiplexer 2. The wavelength division multiplexer 2, the gain fiber 3, the isolator 4, the coupler 5, the polarization controller 6, and the saturable absorber 7 are connected in sequence to form a ring, forming a ring laser resonant cavity.

[0022] A schematic diagram of the saturable absorber structure provided by this invention is shown below. Figure 2 As shown, the saturable absorber structure 7 is composed of a first single-mode fiber 8, a graphene film 9, a microstructure 10, a tapered multimode fiber 11, and a second single-mode fiber 12.

[0023] The specific preparation process of the saturable absorber structure is as follows:

[0024] (1) Tapering operation is performed on graded refractive index multimode fiber using an optical fiber tapering machine to obtain tapered multimode fiber 11.

[0025] (2) A cubic microstructure was inscribed inside the waist of the tapered multimode fiber 11 using femtosecond micromachining technology.

[0026] (3) The treated single-layer graphene is coated on the tapered waist surface of the tapered multimode fiber with built-in microstructure.

[0027] (4) By fusing one end of the tapered multimode fiber 11 with the first single-mode fiber and connecting the other end with the second single-mode fiber, a saturable absorber based on graphene coating on the surface of the multimode fiber with built-in microstructure can be obtained.

[0028] Combination Figure 2 This section introduces the specific working principle of saturable absorbers: Based on the theoretical explanation of nonlinear multimode interference, the nonlinear effects (such as self-phase modulation and cross-phase modulation) in graded-index multimode fibers cause changes in the fiber's equivalent refractive index, resulting in a difference in the self-focusing length at high peak power compared to low peak power. When the length of the graded-index multimode fiber is precisely controlled to a certain length, i.e., an integer multiple of the coherence length, the beam has the lowest transmittance from the multimode fiber to the single-mode fiber during low-power linear propagation. However, for high-power signals, the light will remain and couple to the single-mode fiber core. Therefore, power-determined transmission leads to more efficient coupling of high-intensity signals from the multimode fiber to the single-mode fiber. The built-in microstructure proposed in this invention affects the mode field distribution in multimode fiber, thereby eliminating the need for precise control of the length of graded-index multimode fiber by traditional saturable absorbers and simplifying the manufacturing process. Secondly, the higher-order modes excited by the microstructure interact with the graphene on the fiber surface in the form of evanescent waves. The saturable absorption characteristics of the graphene film further absorb the low-power portion of the pulse signal, and finally output a stable ultrashort pulse.

[0029] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A saturable absorber device with graphene coated on the surface of a multimode optical fiber with an embedded microstructure, used in a passively mode-locked fiber laser, characterized in that: It includes a first single-mode fiber, a hybrid structure based on multimode fiber, and a second single-mode fiber; the hybrid structure based on multimode fiber includes a section of tapered multimode fiber with microstructures inscribed on it and a single layer of graphene, wherein the single layer of graphene covers the surface of the tapered multimode fiber at the waist position. The built-in microstructure is inscribed inside the core of the tapered multimode fiber at the waist of the fiber using femtosecond micromachining technology. The microstructure is a cube shape with a side length of 10 μm. The graphene used is commercially available single-layer graphene, which is coated on the surface of a tapered multimode optical fiber inscribed with microstructure locations.

2. The saturable absorber device with graphene coated on the surface of a multimode optical fiber with an embedded microstructure as described in claim 1, characterized in that: The tapered multimode fiber is made by tapering graded-index multimode fiber using a fiber tapering machine. It has a length of 27cm and a diameter of 60μm at the waist.

3. A saturable absorber device with graphene coated on the surface of a multimode optical fiber with an embedded microstructure as described in claim 1 or 2, characterized in that: The two single-mode optical fibers are 50cm long, with an outer cladding diameter of 125μm and a core diameter of 8μm.

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