A cascaded fiber-based ultraviolet supercontinuum generation device

Abstract

The present invention discloses a device for generating ultraviolet supercontinuum spectrum based on cascaded optical fibers, comprising a laser light source, a coupling adjustment unit, an aspheric lens, a transition optical fiber, a cascaded photonic crystal fiber sample, a multimode optical fiber, and a spectrometer. The initial laser pulse generated by the laser light source is adjusted by the coupling adjustment unit and then enters the aspheric lens. The aspheric lens focuses the laser pulse and then transmits it to the transition optical fiber. The laser pulse then passes through the cascaded photonic crystal fiber sample and the multimode optical fiber and enters the spectrometer, generating an ultraviolet supercontinuum spectrum. The device can generate a supercontinuum spectrum with high conversion efficiency, high spectral flatness, and stable output power. By optimizing the number and length of the cascaded photonic crystal fiber, it is possible to generate a supercontinuum spectrum between 320 nm and 2400 nm, and in particular, a flat ultraviolet supercontinuum spectrum between 350 nm and 1030 nm.

Description

Technical Field

[0001] This invention relates to the fields of optics and laser optoelectronics, and in particular to an ultraviolet supercontinuum generation device based on cascaded optical fibers. Background Technology

[0002] Supercontinuum technology, with its high input energy and large spectral broadening capability, has significant application value in optical measurement, optical pulse compression, and biomedical imaging. However, this technology still faces significant challenges in stable output, spectral broadening extending to the ultraviolet band, and energy conversion. The traditional method of coupling spatial light into solid-core photonic crystal fibers is prone to instability during the coupling process and subsequent use, which could potentially damage the equipment. Furthermore, achieving spectral flatness in the ultraviolet band and supercontinuum spectral broadening below 350 nm are pressing issues that need to be addressed in fields such as scanning electron microscopy imaging. Summary of the Invention

[0003] To address the aforementioned problems, this invention provides an ultraviolet supercontinuum generation device based on cascaded optical fibers, aiming to solve the technical problems in the prior art, such as the difficulty in extending the supercontinuum to below 350 nm.

[0004] An ultraviolet supercontinuum generation device based on cascaded optical fibers includes, in sequence, a laser source, a coupling adjustment unit, an aspherical lens, a transition fiber, a cascaded photonic crystal fiber sample, a multimode fiber, and a spectrometer.

[0005] The first end of the transition fiber forms a first optical window, which is opposite to the aspherical lens.

[0006] The second end of the transition fiber and the first end of the cascaded photonic crystal fiber sample are fused together, and the second end of the cascaded photonic crystal fiber sample forms a second optical window, which faces the first end of the multimode fiber.

[0007] The initial laser pulse generated by the laser source is adjusted by the coupling adjustment unit and then enters the aspherical lens. The aspherical lens focuses the laser pulse and feeds it to the transition fiber. After that, the laser pulse enters the spectrometer through the cascaded photonic crystal fiber sample and multimode fiber, generating ultraviolet supercontinuum spectrum.

[0008] Furthermore, the coupling adjustment unit includes a first reflector and a second reflector arranged opposite to each other. The first reflector reflects the initial laser pulse generated by the laser source to the second reflector, and the second reflector reflects the laser pulse to the aspherical lens.

[0009] Furthermore, the transition fiber comprises a single-mode fiber and a high numerical aperture fiber fused together; a first optical window is located at one end of the single-mode fiber, and the first end of the high numerical aperture fiber and the cascaded photonic crystal fiber sample are fused together.

[0010] Furthermore, the cascaded photonic crystal fiber sample is composed of multiple solid-core quartz-based photonic crystal fibers cascaded and fused together.

[0011] Furthermore, the length of each solid-core quartz-based photonic crystal fiber is adjusted within the range of 3cm to 200cm.

[0012] Furthermore, the diameter of each solid-core quartz-based photonic crystal fiber is 0.1μm-10μm.

[0013] Furthermore, the core size of single-mode optical fiber is 1μm-10μm, and the applicable wavelength is 320nm-2200nm.

[0014] Furthermore, the numerical aperture of high numerical aperture optical fibers is 0.2-0.5, and the core size is 1μm-5μm.

[0015] Furthermore, the aspherical lens is made of silicon dioxide and coated with an anti-reflective coating in the 1050nm–1620nm wavelength range.

[0016] The beneficial technical effects of this invention are: it generates supercontinuum spectra with high conversion efficiency, high spectral flatness, and stable output power; by optimizing the number of stages and length of cascaded photonic crystal fibers, it can achieve the generation of supercontinuum spectra from 320nm to 2400nm, especially the generation of flat ultraviolet supercontinuum spectra from 350nm to 1030nm, taking into account both scientific research and practical applications. The experimental device is scientifically sound, cost-controllable, and easy to maintain. Furthermore, the device has the advantages of being compact, desktop-style, and highly stable. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the experimental optical path of an ultraviolet supercontinuum generation device based on cascaded optical fibers according to the present invention, and a corresponding scanning electron microscope image of a solid-core quartz-based photonic crystal fiber.

[0018] Figure 2 The image shows a 320nm-2400nm supercontinuum spectrum generated by a specific embodiment of an ultraviolet supercontinuum generation device based on cascaded optical fibers according to the present invention.

[0019] Among them, 1-laser source; 2-first reflecting mirror; 3-second reflecting mirror; 4-aspheric lens; 5-single-mode fiber; 6-high numerical aperture fiber; 7-first-stage solid-core quartz-based photonic crystal fiber; 8-second-stage solid-core quartz-based photonic crystal fiber; 9-third-stage solid-core quartz-based photonic crystal fiber; 10-fourth-stage solid-core quartz-based photonic crystal fiber; 11-fifth-stage solid-core quartz-based photonic crystal fiber; 12-multimode fiber; 13-spectrometer. Detailed Implementation

[0020] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0022] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but this is not intended to limit the scope of the invention.

[0023] See Figure 1 The present invention provides an ultraviolet supercontinuum generation device based on cascaded optical fibers, which includes, in sequence, a laser source (1), a coupling adjustment unit, an aspherical lens (4), a transition fiber, a cascaded photonic crystal fiber sample, a multimode fiber (12), and a spectrometer (13).

[0024] The first end of the transition fiber forms a first optical window, which is opposite to the aspherical lens (4);

[0025] The second end of the transition fiber and the first end of the cascaded photonic crystal fiber sample are fused together. The second end of the cascaded photonic crystal fiber sample forms a second optical window, which faces the first end of the multimode fiber (12).

[0026] The initial laser pulse generated by the laser source (1) is adjusted by the coupling adjustment unit and then enters the aspherical lens (4). The aspherical lens (4) focuses the laser pulse and feeds it to the transition fiber. After that, the laser pulse enters the spectrometer (13) through the cascaded photonic crystal fiber sample and multimode fiber (12) to generate ultraviolet supercontinuum spectrum.

[0027] Specifically, the laser source (1) is the Superk COMPACT laser source.

[0028] The parameters of the laser source (1) are: a center wavelength of 1064 nm, a pulse width of approximately 1 ns, and a repetition frequency of 20 kHz. A nanosecond laser source with a repetition frequency of 20 kHz is used to generate a supercontinuum ultraviolet light source through the dispersive wave radiation effect. Specifically, the pulse width of the nanosecond laser pulse is from 1 ns to 100 ns.

[0029] The transition fiber includes a first end facing the aspherical lens (4) and a second end. The first end is provided with a first optical window that has been cut by a ceramic sheet. The second end is fused to the first end of the cascaded photonic crystal fiber sample by a fusion splicer. The second end of the cascaded photonic crystal fiber sample is cut by a ceramic sheet to obtain a second optical window.

[0030] Furthermore, the coupling adjustment unit includes a first reflector (2) and a second reflector (3) arranged opposite to each other. The first reflector (2) reflects the initial laser pulse generated by the laser source (1) to the second reflector (3), and the second reflector (3) reflects the laser pulse to the aspherical lens (4).

[0031] The combination of the first reflector (2) and the second reflector (3) can achieve spatial optical coupling. The first reflector (2) is configured to reflect nanosecond-level pulsed lasers, and the second reflector (3) receives the pulsed lasers from the first reflector (1) and reflects them through the aspherical lens (4).

[0032] The aspherical lens (4) is configured to focus and adjust the beam diameter of the laser pulse. Specifically, the aspherical lens (4) is made of silicon dioxide and coated with an anti-reflection coating in the 1050nm–1620nm wavelength range. The lens focal length can be 7.5mm, which depends on the size of the focal spot after the initial laser pulse is focused and the mode field area of ​​the fiber waveguide. The aspherical lens is placed on the displacement stage mounting base of the displacement stage extension platform.

[0033] Furthermore, the transition fiber comprises a single-mode fiber (5) and a high numerical aperture fiber (6) fused together; a first optical window is located at one end of the single-mode fiber (5), and the first end of the high numerical aperture fiber (6) and the cascaded photonic crystal fiber sample are fused together.

[0034] Specifically, the transition fiber is formed by fusion splicing the same or different types of single-mode fiber with high numerical aperture fiber.

[0035] The laser source (1) generates an initial nanosecond laser pulse, which is focused by an aspherical lens (4) and spatially coupled into a single-mode fiber (5) placed in the V-groove of the fiber clamp on the displacement stage. Compared with spatially coupled directly into a solid-core photonic crystal fiber, the former has an order-of-magnitude improvement in the stable working time of the system. This is because there is an air hole structure around the core of the photonic crystal fiber, which, while providing high nonlinear gain and tunable dispersion, also increases the difficulty of spatially coupled optically and the instability of the system. This makes spatially coupled optically into the photonic crystal fiber not only high in loss but also easy to damage. However, spatially coupled optically into a single-mode fiber does not have the above effects, which greatly improves the coupling efficiency and system stability. This scheme significantly improves the coupling efficiency and system stability of spatially coupled optically into a solid-core photonic crystal fiber by sequentially fusion splicing single-mode fiber (5), high numerical aperture fiber (6), and solid-core photonic crystal fiber. Mode field matching is performed by high numerical aperture fiber to further reduce the splicing loss caused by the large difference in mode field area between single-mode fiber and solid-core photonic crystal fiber.

[0036] Furthermore, the core size of the single-mode fiber (5) is 1μm-10μm, and the applicable wavelength is 320nm-2200nm. The single-mode fiber can be Thorlabs' SM300, S405-XP, SM450, S630-HP, SM800G80, 1060XP, HI1060-J9, 1310BHP, SM1950, etc., or Lubang Mall's C405-PC-1, C460-PC-1, C630-PC-1, C780-PC-1, C980-PC-1, CSM28E-PC-1, etc.

[0037] Specifically, the single-mode fiber (5) is Thorlabs' quartz-based single-mode fiber 1060xp.

[0038] Furthermore, the numerical aperture of the high numerical aperture fiber (6) is 0.2-0.5, and the core size is 1μm-5μm.

[0039] Specifically, the high numerical aperture fiber (6) can be of the Thorlabs model UHNA1, UHNA3, UHNA4, UHNA7, etc. Furthermore, the cascaded photonic crystal fiber sample is formed by cascading and splicing multiple stages of solid-core silica-based photonic crystal fibers.

[0040] When a nanosecond laser pulse is coupled into a cascaded photonic crystal fiber, spectral broadening begins due to modulation instability. Through soliton generation and self-frequency shift, the spectrum gradually extends to longer wavelengths. Under phase-matching conditions, the soliton transfers energy to a linear wave of a specific frequency, efficiently achieving dispersive wave radiation. By continuously shrinking multiple cascaded photonic crystal fiber segments while maintaining the highest possible duty cycle in a solid-core silica-based photonic crystal fiber cascade, the phase-matching band of the dispersive wave extends further and further to shorter wavelengths, thus generating dispersive waves in shorter wavelength bands. This method can achieve the generation of a flat supercontinuum spectrum from 350 nm to 1030 nm, and the output laser pulse is coupled into a spectrometer for spectral measurements.

[0041] Specifically, the cascaded photonic crystal fiber sample is supported by two three-dimensional displacement stages to ensure that the incident laser pulse passes through the fiber along the optical axis. The three-dimensional displacement stages need to be equipped with an extension platform and fiber clamps. The cascaded photonic crystal fiber sample is fixed in the V-groove of the fiber clamp, with the fiber axis coinciding with the aspherical lens and the center of the optical path.

[0042] Furthermore, the length of each solid-core quartz-based photonic crystal fiber is adjusted within the range of 3cm to 200cm.

[0043] Furthermore, the diameter of each solid-core quartz-based photonic crystal fiber is 0.1μm-10μm.

[0044] Specifically, the cascaded photonic crystal fiber sample consists of a first-stage solid-core quartz-based photonic crystal fiber (7), a second-stage solid-core quartz-based photonic crystal fiber (8), a third-stage solid-core quartz-based photonic crystal fiber (9), a fourth-stage solid-core quartz-based photonic crystal fiber (10), and a fifth-stage solid-core quartz-based photonic crystal fiber (11) which are sequentially fused together.

[0045] Specifically, the lengths of the first-stage solid quartz-based photonic crystal fiber (7), the second-stage solid quartz-based photonic crystal fiber (8), the third-stage solid quartz-based photonic crystal fiber (9), the fourth-stage solid quartz-based photonic crystal fiber, and the fifth-stage solid quartz-based photonic crystal fiber are 100cm, 30cm, 30cm, 30cm, and 30cm, respectively.

[0046] Specifically, the first-stage solid-core silica-based photonic crystal fiber (7) has a core of 4.35 μm and a duty cycle of 0.90; the second-stage solid-core silica-based photonic crystal fiber (8) has a core of 3.35 μm and a duty cycle of 0.80; the third-stage solid-core silica-based photonic crystal fiber (9) has a core of 2.84 μm and a duty cycle of 0.85; the fourth-stage solid-core silica-based photonic crystal fiber (10) has a core of 1.90 μm and a duty cycle of 0.85; and the fifth-stage solid-core silica-based photonic crystal fiber (11) has a core of 1.45 μm and a duty cycle of 0.95. All the above-mentioned solid-core silica-based photonic crystal fibers are made of fused silica.

[0047] Specifically, the multimode fiber (12) is used to connect the Optical Spectrum Analyzer-aq6374 spectrometer (350nm-1750nm filters, YOKOGAWA), or dedicated jumpers for corresponding spectrometers such as MAYA2000pro (300nm-1100nm filters, Ocean Optics) and NIR Quest (800nm-2500nm filters, Ocean Optics).

[0048] Specifically, the spectrometer (13) can be a commercial spectrometer, such as the Optical Spectrum Analyzer-aq6374 spectrometer, MAYA2000pro spectrometer and NIR Quest spectrometer mentioned above.

[0049] Preferably, the present invention uses a Fujikura 80S+ fusion splicer when splicing optical fibers.

[0050] Specific Implementation: The Superk COMPACT laser source outputs an initial laser pulse with a center wavelength of 1064nm, a pulse width of approximately 1ns, and a repetition frequency of 20kHz. This pulse is then coupled and adjusted by a coupling unit before reaching an aspherical lens. The aspherical lens focuses the laser pulse, and its focal length is determined based on the focal spot size and the inner diameter of the single-mode fiber. Specifically, the receiving surface of the CCD camera is placed at the focal point, and the size of the light spot displayed by the CCD camera is observed. The focal length of the focusing lens is then changed to alter the size of the focal spot, with a diameter of approximately 9.3μm, which is the classical value for the single-mode fiber mode field diameter, being optimal. In this embodiment, the focal length of the aspherical lens is 7.5mm. The initial laser pulse is then focused by the aspherical lens onto a 255cm long five-stage cascaded photonic crystal fiber sample. The spatial position of the five-stage cascaded photonic crystal fiber sample is adjusted using a three-dimensional displacement stage, efficiently coupling the incident laser pulse into the cascaded photonic crystal fiber sample to maximize the laser pulse output energy and produce a uniform circular output spot. The pulse at the fiber output end is then subjected to spectral measurement using a spectrometer.

[0051] This invention overcomes the technical problems of poor stability and difficulty in extending the supercontinuum spectrum below 350 nm in existing systems, and establishes a compact, tabletop, highly stable ultraviolet supercontinuum laser generation device based on cascaded solid-core silica-based photonic crystal fibers. Using this device, a spectrally flat supercontinuum source with a minimum ultraviolet wavelength extension to 320 nm can be generated, such as... Figure 2 As shown.

[0052] In summary, the advantages of this invention are as follows:

[0053] This invention generates a supercontinuum spectrum with high conversion efficiency, high spectral flatness, and stable output power based on solid-core quartz-based photonic crystal fiber. By fusion splicing single-mode fiber with a high numerical aperture to form a transition fiber, the stability of spatially coupled solid-core photonic crystal fiber is improved. By optimizing the number of stages and length of cascaded photonic crystal fibers, supercontinuum spectrum generation in the 320nm-2400nm range can be achieved, especially flat ultraviolet supercontinuum spectrum in the 350nm-1030nm range. It takes into account both scientific research and practical applications. The experimental device is scientifically sound, cost-effective, and easy to maintain. Furthermore, the device has the advantages of being compact, desktop-style, and highly stable.

[0054] The above are merely preferred embodiments of the present invention and are not intended to limit the implementation methods and protection scope of the present invention. Those skilled in the art should recognize that any equivalent substitutions and obvious changes made based on the description and illustrations of the present invention should be included within the protection scope of the present invention.

Claims

1. A device for generating ultraviolet supercontinuum based on cascaded optical fibers, characterized in that, The components include, in sequence, a laser source, a coupling adjustment unit, an aspherical lens, a transition fiber, a cascaded photonic crystal fiber sample, a multimode fiber, and a spectrometer. The transition optical fiber forms a first optical window at its first end, and the first optical window is opposite to the aspherical lens; The second end of the transition fiber and the first end of the cascaded photonic crystal fiber sample are fused together, and the second end of the cascaded photonic crystal fiber sample forms a second optical window, which faces the first end of the multimode fiber. The initial laser pulse generated by the laser source is adjusted by the coupling adjustment unit and then enters the aspherical lens. The aspherical lens focuses the laser pulse and feeds it to the transition fiber. After that, the laser pulse passes through the cascaded photonic crystal fiber sample and multimode fiber and enters the spectrometer to generate ultraviolet supercontinuum spectrum. The coupling adjustment unit includes a first reflector and a second reflector disposed opposite to each other. The first reflector reflects the initial laser pulse generated by the laser source to the second reflector, and the second reflector reflects the laser pulse to the aspherical lens. The cascaded photonic crystal fiber sample comprises a first-stage solid-core quartz-based photonic crystal fiber, a second-stage solid-core quartz-based photonic crystal fiber, a third-stage solid-core quartz-based photonic crystal fiber, a fourth-stage solid-core quartz-based photonic crystal fiber, and a fifth-stage solid-core quartz-based photonic crystal fiber, which are sequentially fused together. The lengths of the first-stage solid quartz-based photonic crystal fiber, the second-stage solid quartz-based photonic crystal fiber, the third-stage solid quartz-based photonic crystal fiber, the fourth-stage solid quartz-based photonic crystal fiber, and the fifth-stage solid quartz-based photonic crystal fiber are 100cm, 30cm, 30cm, 30cm, and 30cm, respectively.

2. The ultraviolet supercontinuum generation device based on cascaded optical fibers as described in claim 1, characterized in that, The transition fiber comprises a single-mode fiber and a high numerical aperture fiber fused together; the first optical window is located at one end of the single-mode fiber, and the high numerical aperture fiber and the first end of the cascaded photonic crystal fiber sample are fused together.

3. The ultraviolet supercontinuum generation device based on cascaded optical fibers as described in claim 1, characterized in that, The cascaded photonic crystal fiber sample is formed by cascading and fusing multiple levels of solid-core quartz-based photonic crystal fibers.

4. The ultraviolet supercontinuum generation device based on cascaded optical fibers as described in claim 3, characterized in that, The length of the solid-core quartz-based photonic crystal fiber in each stage is adjusted within the range of 3cm to 200cm.

5. The ultraviolet supercontinuum generation device based on cascaded optical fibers as described in claim 3, characterized in that, The diameter of each solid-core quartz-based photonic crystal fiber is 0.1μm-10μm.

6. The ultraviolet supercontinuum generation device based on cascaded optical fibers as described in claim 2, characterized in that, The single-mode fiber has a core size of 1μm-10μm and is suitable for wavelengths of 320nm-2200nm.

7. The ultraviolet supercontinuum generation device based on cascaded optical fibers as described in claim 2, characterized in that, The high numerical aperture fiber has a numerical aperture of 0.2-0.5 and a core size of 1μm-5μm.

8. The ultraviolet supercontinuum generation device based on cascaded optical fibers as described in claim 1, characterized in that, The aspherical lens is made of silicon dioxide and coated with an anti-reflective film in the 1050nm–1620nm wavelength band.

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