Intermediate infrared ultrafast laser transmission device based on hollow-core anti-resonance optical fiber

Abstract

The invention belongs to the technical field of femtosecond laser, relates to an ultrafast laser transmission system based on a hollow-core anti-resonance optical fiber, and discloses a method for vacuum and gas filling of the hollow-core optical fiber. The system comprises a mid-infrared ultrafast laser light source, a reflector, a focusing lens, an optical fiber fixing piece, a first air chamber, a hollow-core anti-resonance optical fiber and a second air chamber. A closed, stable and dry environment is provided for the input end and the output end of the optical fiber through the optical fiber fixing piece, the air chambers and the window pieces, air in the air chambers and the hollow-core optical fiber is pumped out by pumping air in the air chambers at the two ends, and the high vacuum degree of the hollow-core optical fiber is achieved. The device has the advantages of high transmission efficiency and good quality of transmitted light beams. By applying the device, higher-power intermediate infrared ultrafast laser flexible transmission can be realized.

Description

Technical Field

[0001] The present invention relates to the field of ultrafast laser technology, and more particularly to a mid-infrared ultrafast laser transmission device based on hollow-core antiresonant optical fiber. The device can realize flexible transmission of high-power mid-infrared ultrafast laser. Background Art

[0002] The mid-infrared band includes the atmospheric transmission windows of 3-5 microns and 8-12 microns, and is an important band for mid-infrared laser long-distance detection and remote sensing applications.

[0003] Optical fiber is a medium for mid-infrared laser generation, frequency conversion, and transmission. It can improve the generation and conversion efficiency, and make the optical system more compact and convenient. The most mature quartz-based solid-core optical fiber is not suitable for the mid-infrared band because its spectral range is limited by the quartz transparent window (about 500-2000nm). Although optical fibers based on soft glass materials such as fluoride glass and chalcogenide glass have the characteristics of wide light guide window and low transmission efficiency, the preparation requirements and difficulty are relatively high compared with quartz optical fiber. Nowadays, a new type of microstructured optical fiber, antiresonant hollow-core optical fiber, uses the coherent reflection of light back and forth between the tubular glass film in the optical fiber to confine the light near the air core and transmit it along the axis. Compared with traditional optical fibers and other microstructured optical fibers, it has the advantages of large mode area, high damage threshold, low nonlinearity, adjustable dispersion, free control of the type and composition of the core gas, etc. It is very suitable as a carrier for ultrafast laser transmission. Its unique properties provide new possibilities for the transmission of ultrafast lasers.

[0004] The mid-infrared band also covers the molecular "fingerprint" region, corresponding to the characteristic absorption spectra of many important molecules, covering the strong resonance peaks of water, carbon dioxide and other substances, and has a wide range of applications in laser communications, environmental monitoring, biomedicine and other fields. However, in the field of ultrafast lasers, the absorption bands of atmospheric molecules such as water and carbon dioxide will have an adverse effect on the performance of mid-infrared ultrafast laser systems, such as pulse distortion, beam attenuation, etc., which greatly limits the application of mid-infrared ultrafast lasers. Summary of the invention

[0005] The present invention overcomes the influence of the above-mentioned atmospheric molecules on mid-infrared ultrafast laser transmission and provides a mid-infrared ultrafast laser transmission device based on hollow-core antiresonant optical fiber. The device can achieve low insertion loss, high beam quality, and high-power ten-meter-level mid-infrared ultrafast laser energy transmission.

[0006] This device uses the hollow characteristics of hollow-core optical fibers to confine the light to be transmitted inside the optical fiber while extracting the air inside the hollow-core optical fiber, thereby overcoming the influence of the atmosphere on the mid-infrared ultrafast laser. The implementation process of the present invention is to simultaneously evacuate the air chamber carrying the hollow-core optical fiber, and the laser enters the hollow-core optical fiber through the front-end end cap, propagates in the hollow-core optical fiber, and the outgoing light is transmitted to the laser pulse measurement device after passing through two polarizers, and finally the output laser is measured.

[0007] A mid-infrared ultrafast laser transmission device based on hollow-core anti-resonant optical fiber is characterized in that it comprises: a mid-infrared ultrafast laser light source for generating mid-infrared ultrafast laser; and a plurality of dispersion compensation windows sequentially arranged along the laser transmission direction for compensating for dispersion during laser transmission;

[0008] The first focusing lens is used to focus the laser pulse, and its focal length is 1 / e of the focal spot after the laser pulse is focused. 2 The diameter at the hole is 0.64 times the inner diameter of the hollow-core fiber to achieve efficient coupling;

[0009] Hollow-core optical fiber for transmitting mid-infrared ultrafast lasers;

[0010] A collimating lens, used for collimating the laser spot outputted through the hollow-core optical fiber into parallel light;

[0011] A polarization adjustment unit, used to adjust the shape of the output pulse;

[0012] A second focusing lens is used to focus the regulated laser pulse; and

[0013] A first air chamber, used to accommodate the front end portion of the hollow core optical fiber and connected to a first gas control unit so as to control the gas pressure in the first air chamber;

[0014] The second air chamber is used to accommodate the rear end portion of the hollow core optical fiber and is connected to a second gas control unit to control the air pressure in the second air chamber; wherein,

[0015] The first air chamber and the second air chamber are both sealed air chambers, and both the light incident surface and the light exit surface are provided with optical windows for sealing the air chambers and allowing the laser to pass through;

[0016] The first gas control unit and the second gas control unit are used to evacuate the first gas chamber and the second gas chamber, thereby extracting the air in the two gas chambers and the hollow-core optical fiber to achieve a high vacuum degree of the hollow-core optical fiber.

[0017] Furthermore, the first air chamber and the second air chamber are both square cavities, with front end heads and upper end covers at the front and rear ends, which are tightly connected by screws to ensure good air tightness; a V-shaped groove is provided inside the square cavity for fixing the hollow-core optical fiber.

[0018] Furthermore, the first air chamber and the second air chamber are both supported by two translation stages, and the translation stages are adjusted to ensure that the incident pulse coincides with the air chamber axis.

[0019] Furthermore, the optical window is a CaF2 window.

[0020] Furthermore, the plurality of dispersion compensation windows include at least one Ge window and a plurality of ZnSe windows.

[0021] Furthermore, by calculating the chirp of the hollow-core optical fiber and selecting a dispersion-compensating window of appropriate thickness, the waveguide dispersion during laser transmission is effectively optimized and the transmission quality of the laser pulse is improved.

[0022] Furthermore, it also includes a pulse measurement device placed after the second focusing lens, which is composed of a spectrometer and an autocorrelator and is used to measure the time-frequency domain characteristics of the laser pulse.

[0023] Furthermore, the hollow core optical fiber has a winding diameter greater than 40 cm to reduce bending loss.

[0024] Furthermore, the focal length of the first focusing lens is 150 mm, and is used to focus the laser pulse into a hollow-core optical fiber with a core diameter of 110 μm.

[0025] Preferably, the mid-infrared ultrafast laser light source is capable of generating initial laser pulses with a central wavelength of 2780 nm, a pulse width of about 60 fs, and a repetition frequency of 30 MHz, which provides a high-quality light source foundation for subsequent laser transmission and experiments.

[0026] Preferably, the focal length of the first focusing lens is 1 / e of the focal spot after the laser pulse is focused. 2 The diameter at the point is 0.64 times the inner diameter of the hollow-core fiber; this allows the focal spot size after focusing the laser pulse to be optimally matched with the inner diameter of the hollow-core fiber, with a theoretical coupling efficiency of up to 0.98, thereby achieving efficient coupling of laser pulses.

[0027] Preferably, the dispersion compensation window includes CaF2, Ge, and ZnSe to compensate for the waveguide dispersion introduced by the hollow core optical fiber.

[0028] Preferably, the hollow-core optical fiber holes of the first air chamber and the second air chamber are slightly larger than the outer diameter of the hollow-core optical fiber, and after the hollow-core optical fiber is inserted, it is filled with ultraviolet curing glue to ensure airtightness.

[0029] Preferably, the front end of the air chamber is sealed by an optical window and a rubber O-ring through a threaded seal, and the gas control unit needs to ensure good airtightness before connection.

[0030] Preferably, the optical window, collimating lens and focusing lens are all CaF 2 Made.

[0031] Preferably, the hollow core optical fiber is about 5 m long and is wrapped around the breadboard with a diameter of 40 cm. The device can be compact and flexible to operate on a single optical platform.

[0032] Preferably, the half-wave plate and the quarter-wave plate are used together to form a polarization adjustment unit.

[0033] The working process is as follows:

[0034] The laser light source outputs the initial laser pulse, which passes through the dispersion compensation window, the first focusing lens and the first gas chamber CaF 2 The window focuses and couples into the hollow-core fiber. The front end of the hollow-core fiber passes through the hollow-core fiber hole of the first air chamber, and the rear end of the hollow-core fiber passes through the hollow-core fiber hole of the second air chamber. 2 The output of the window plate is collimated into parallel light by the first collimating lens, and then the shape of the output pulse is adjusted by the half-wave plate and the quarter-wave plate, and then coupled to the pulse measurement device for measurement by the second focusing lens. When the laser system is running, the vacuum pump needs to be connected to the gas chamber quick connector at the same time, and the first and second gas chambers need to be evacuated at the same time, so that high-energy, ten-meter-level ultrafast laser pulses can be transmitted flexibly.

[0035] In summary, the advantages of the present invention are summarized as follows:

[0036] The ultrafast laser pulse transmission device based on hollow-core antiresonant optical fiber of the present invention has the remarkable characteristics of adjustable output energy, excellent coherence and high and stable output power.

[0037] By performing a vacuum treatment on the hollow-core optical fiber, the present invention greatly improves the transmission efficiency and output beam quality of the mid-infrared ultrafast laser.

[0038] In addition, the device of the present invention has wide adaptability and is compatible with any wide-spectrum mid-infrared ultrafast light source. Through pulse compression and dispersion compensation technology, low-loss femtosecond laser transmission is achieved.

[0039] In terms of scientific research and practical application, the present invention takes into account both exploration and practicality. The design of the experimental device is scientific and reasonable, the cost is effectively controlled, and the maintenance is simple, which greatly reduces the threshold for use. At the same time, the device also has the outstanding advantages of miniaturization and flexible transmission, opening up new possibilities for the application of laser technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 Schematic diagram of the optical path of an ultrafast laser transmission device based on hollow-core antiresonant optical fiber according to an embodiment of the present invention.

[0041] Figure 2 Schematic diagram of the structural packaging of the air chamber of the hollow-core anti-resonant optical fiber ultrafast laser transmission device in an embodiment of the present invention.

[0042] Figure 3 It is the spectrum diagram and autocorrelation diagram of the ultrafast mid-infrared laser with a central wavelength of 2.78 μm transmitted in the embodiment of the present invention, wherein a is the autocorrelation diagram of the mid-infrared laser output, b is the spectrum diagram of the mid-infrared laser output, c is the autocorrelation diagram of the mid-infrared laser output transmitted through a 5 m hollow-core anti-resonant optical fiber, and d is the spectrum diagram of the mid-infrared laser transmitted through a 5 m hollow-core anti-resonant optical fiber.

[0043] Reference numerals:

[0044] Self-developed mid-infrared ultrafast laser light source-1; Ge window-2; first ZnSe window-3; second ZnSe window-4; third ZnSe window-5; fourth ZnSe window-6; fifth ZnSe window-7; first focusing lens-8; first CaF 2 Window-9; first air chamber-10; first gas control unit-11; hollow core optical fiber-12; second gas control unit-13; second air chamber-14; second CaF 2 Window-15; collimating lens-16; quarter wave plate-17; second half wave plate-18; second focusing lens-19; pulse measuring device-20; front end sealing head-21; optical fiber fixing V groove-22; upper end sealing cover-23; square cavity-24. DETAILED DESCRIPTION

[0045] The present invention is further described below by way of examples and drawings, but the protection scope of the present invention shall not be limited thereto.

[0046] Figure 1 A mid-infrared ultrafast laser transmission device based on hollow core anti-resonant fiber includes: 1. a self-developed mid-infrared ultrafast laser light source; 2. a Ge window; 3. a first ZnSe window; 4. a second ZnSe window; 5. a third ZnSe window; 6. a fourth ZnSe window; 7. a fifth ZnSe window; 8. a first focusing lens; 9. a first CaF 2 Window; 10. First air chamber; 11. Hollow core optical fiber; 12. Second air chamber; 13. Second CaF 2 Window; 14. Collimating lens; 15. Quarter wave plate; 16. Half wave plate; 17. Second focusing lens; 18. Pulse measurement device

[0047] The self-developed mid-infrared ultrafast laser light source 1 generates an initial laser pulse, and the laser light source parameters are: 2780nm central wavelength, about 60fs pulse width, and 30MHz repetition frequency.

[0048] The focal length of the first focusing lens depends on the size of the focal spot after the laser pulse is focused and the inner diameter of the hollow core waveguide.2 When the diameter at is about 0.64 times the inner diameter of the hollow-core capillary fiber, the theoretical coupling efficiency of the fundamental mode is about 0.98.

[0049] The focusing lens and the collimating lens are CaF 2 Made.

[0050] The first air chamber 10 and the second air chamber 14 are both supported by two translation stages to ensure that the incident pulse passes through the axis. The hollow-core optical fiber 12 is fixed in the V-groove 22 in the first air chamber 10 and the second air chamber 14, and the front axis of the hollow-core optical fiber 12 coincides with the axis of the first air chamber 10; the rear axis of the hollow-core optical fiber 12 coincides with the axis of the second air chamber 14. The first air chamber 10 and the second air chamber 14 are sealed by the first optical window 9 and the second optical window 15 at the front and back, respectively. The first air chamber 10 and the second air chamber 14 are respectively connected to the first gas control unit 11 and the second gas control unit 14, which are used for the filling, exhausting and air pressure control of the gas in the first air chamber 10 and the second air chamber 14.

[0051] The hollow core optical fiber 11 needs to be wound with a larger diameter (winding diameter of more than 60 cm).

[0052] The first focusing lens 8 is used to focus the laser pulse, and the focal length thereof is determined by the numerical relationship between the inner diameter of the hollow core optical fiber 11 and the size of the light spot at the focus. Specifically, the size of the light spot at the focus is detected by a CCD camera, and the focal spot is 1 / e smaller by selecting lenses with different focal lengths. 2 The diameter is close to or equal to 0.64 of the inner diameter of the first-stage hollow-core optical fiber 11, and the focal length of the lens is optimal at this time. For this embodiment, the focal length of the first focusing lens 8 is 150mm. The initial laser pulse is focused into the hollow-core optical fiber 11 with a core diameter of 110μm through the first focusing lens 8. By repeatedly adjusting the translation stage, the axis of the first air chamber coincides with the axis of the initial laser pulse, so that the laser pulse is efficiently coupled into the hollow-core optical fiber 11 after being focused by the first lens 8, and at the same time, the energy of the output pulse is the highest and the spot mode is the fundamental mode. The pulse at the output end of the optical fiber is focused by the second focusing lens into the pulse measurement device for pulse time-frequency domain measurement. The focal length and lens material of the second focusing lens need to be selected according to actual conditions. In this embodiment, the second focusing lens is CaF with a focal length of 6mm. 2 lens.

[0053] The hollow core optical fiber 11 will introduce waveguide dispersion, so it is necessary to calculate the chirp amount of the hollow core optical fiber and select a dispersion compensation window of appropriate thickness to optimize dispersion compensation.

[0054] Figure 2 , a schematic diagram of the structural packaging of the air chamber of a mid-infrared ultrafast laser transmission device based on a hollow-core antiresonant fiber: 21. Front end cap; 22. Fiber fixing V-groove; 23. Upper end cap; 24. Square cavity.

[0055] The front end sealing head 21 and the upper end sealing cover 23 for installing the window sheet are tightly connected to the square cavity 24 by screws. When connecting, the screws need to be tightened to ensure good air tightness and avoid thread slippage.

[0056] Among them, the optical fiber fixing V-groove 22 is used to fix the hollow-core optical fiber. The displacement stage is used to adjust the spatial position of the hollow-core optical fiber while ensuring the stability of the hollow-core optical fiber and reducing the heat impact of high-energy lasers. A specific embodiment is as follows: the front end cap 21 is connected to the square cavity 24 for sealing and fixing the optical window; the cover 24 is connected to the square cavity 24 for sealing; the V-groove is fixed in the square cavity with screws to fix the hollow-core optical fiber. First, the hollow-core optical fiber is inserted through the small hole behind the square cavity and placed on the V-groove. The optical fiber is glued with vacuum ultraviolet curing glue, and the displacement stage is adjusted so that the optical fiber port is located at the focus. Finally, the front end seal 81 and the upper cover plate 82 are connected and installed in sequence, and the small hole behind the square cavity is sealed with shadowless glue to ensure that there is no air leakage.

[0057] like Figure 3 As shown, after vacuuming, Figure 3 (d) Compared to Figure 3 (b) The water absorption line is significantly weakened and the energy is significantly increased by 30%. The pulse is then compressed back to 70fs through a dispersion compensation window, achieving high-efficiency mid-infrared ultrashort pulse transmission.

Claims

1. A mid-infrared ultrafast laser transmission device based on hollow-core anti-resonant optical fiber, characterized in that: include: A mid-infrared ultrafast laser light source (1), used for generating mid-infrared ultrafast laser; and a plurality of dispersion compensation windows arranged in sequence along the laser transmission direction, for compensating for the dispersion during the laser transmission process; A first focusing lens (8) is used to focus the laser pulse, and its focal length is such that the diameter of the focal spot at 1 / e2 after the laser pulse is focused is 0.64 times the inner diameter of the hollow-core optical fiber; A hollow core optical fiber (11) for transmitting mid-infrared ultrafast laser light; A collimating lens (16) for collimating the laser spot outputted through the hollow core optical fiber (11) into parallel light; A polarization adjustment unit, used to adjust the shape of the output pulse; A second focusing lens, used for focusing the regulated laser pulse; as well as, A first air chamber (10) is used to accommodate the front end portion of the hollow core optical fiber (11) and is connected to a first gas control unit so as to control the gas pressure in the first air chamber; The second air chamber (14) is used to accommodate the rear end portion of the hollow core optical fiber and is connected to a second gas control unit so as to control the gas pressure in the second air chamber; wherein, The first air chamber (10) and the second air chamber (14) are both sealed air chambers, and both the light incident surface and the light exit surface are provided with optical windows for sealing the air chambers and allowing laser light to pass through; The first gas control unit and the second gas control unit are used to evacuate the first gas chamber (10) and the second gas chamber (14), thereby extracting air from the two gas chambers and the hollow-core optical fiber, thereby achieving a high vacuum degree of the hollow-core optical fiber.

2. The mid-infrared ultrafast laser transmission device based on hollow-core anti-resonant fiber according to claim 1, characterized in that: The first air chamber (10) and the second air chamber (14) are both square cavities, with front and rear ends provided with front end caps and upper end caps, which are tightly connected by screws to ensure good air tightness; a V-shaped groove is provided inside the square cavity for fixing the hollow core optical fiber.

3. The mid-infrared ultrafast laser transmission device based on hollow-core anti-resonant fiber according to claim 1 or 2, characterized in that: The first air chamber (10) and the second air chamber (14) are both supported by two translation stages, and the translation stages are adjusted to ensure that the incident pulse coincides with the air chamber axis.

4. The mid-infrared ultrafast laser transmission device based on hollow-core anti-resonant fiber according to claim 1 or 2, characterized in that: The optical window is a CaF2 window.

5. The mid-infrared ultrafast laser transmission device based on hollow-core anti-resonant fiber according to claim 1, characterized in that: The plurality of dispersion compensation windows include at least one Ge window and a plurality of ZnSe windows.

6. The mid-infrared ultrafast laser transmission device based on hollow-core anti-resonant fiber according to claim 5, characterized in that: The thickness of the dispersion compensating window is selected through calculation to optimize the compensation for the waveguide dispersion introduced by the hollow core optical fiber.

7. The mid-infrared ultrafast laser transmission device based on hollow-core anti-resonant fiber according to claims 1-6, characterized in that: It also comprises a pulse measuring device (18) arranged behind the second focusing lens, which is composed of a spectrometer and an autocorrelator and is used to measure the time-frequency domain characteristics of the laser pulse.

8. The mid-infrared ultrafast laser transmission device based on hollow-core anti-resonant fiber according to claim 1, characterized in that: The hollow core optical fiber (11) has a surrounding diameter greater than 60 cm to reduce bending loss.

9. The mid-infrared ultrafast laser transmission device based on hollow-core anti-resonant fiber according to claim 1, characterized in that: The focal length of the first focusing lens (8) is 150 mm, and is used to focus the laser pulse into a hollow-core optical fiber with a core diameter of 110 μm.

10. The mid-infrared ultrafast laser transmission device based on hollow-core anti-resonant fiber according to claim 1, characterized in that: The polarization adjustment unit comprises a quarter wave plate (17) and a half wave plate (18).

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