Intermediate infrared super-continuum spectrum light source with high power ratio at 2.7-3m

Abstract

The invention discloses a mid-infrared super-continuum spectrum light source with a high power ratio at 2.7-3 microns. The mid-infrared super-continuum spectrum light source comprises a Cr: ZnS femtosecond laser, an As2S3 optical fiber and an erbium-doped fluoride optical fiber pumped by a 976nm laser diode, the Cr: ZnS femtosecond laser generates femtosecond pulses under the central wavelength of 2.4 microns; the femtosecond pulse is focused in the As2S3 optical fiber; when the femtosecond pulse is propagated in the As2S3 optical fiber, the spectrum is broadened; the pulse after spectrum broadening is injected into a fiber core of the erbium-doped fluoride fiber as seed light; a 976nm laser diode is used as a pumping source; the seed light obtains optical gain in the erbium-doped fluoride optical fiber and is amplified; and the amplified laser is output after residual pump light is removed through a band-pass filter. According to the invention, the problem of low ratio of 2.7-3 [mu] m power of the existing mid-infrared super-continuum spectrum is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of mid-infrared supercontinuum, and particularly to a mid-infrared supercontinuum light source with a high power ratio in the range of 2.7 - 3 μm. Background Art

[0002] Mid-infrared supercontinuum, as a revolutionary precision spectroscopy and metrology tool, has important applications in fields such as the detection of gas molecules. Among them, the mid-infrared laser in the 2.7 - 3 μm band is in the atmospheric transparent window, and at the same time covers the absorption peak of water molecules and the "fingerprint regions" of many gas molecules, virus molecules, etc. It has great application value and urgent needs in the fields of environmental monitoring, minimally invasive surgery, molecular spectroscopy, national defense, etc. Lasers in this band can be generated by mode-locked lasers, difference frequency, optical parametric oscillators, quantum cascade lasers, Kerr microcavities, etc., but the bandwidth is very limited and far from covering such a wide band as 2.7 - 3 μm. To generate broadband lasers in the 2.7 - 3 μm band, the supercontinuum generation technology is a very attractive solution. The common method is to use femtosecond lasers to pump highly nonlinear fibers such as chalcogenide fibers to obtain nonlinear spectral broadening. In 2020, the research group of Gianluca Galzerano at the Politecnico di Milano in Italy directly pumped a tapered suspended-core As 39 Se 61 fiber with a mode-locked Cr:ZnSe femtosecond laser, and a supercontinuum covering 1.4 - 4.2 μm can be generated [S.O. Leonov, et al. "Coherent mid-infrared supercontinuum generation in tapered suspended-core As 39 Se 61 fibers pumped by a few-optical-cycle Cr:ZnSe laser," Opt. Lett., 2018, 45(6): 1346 - 1349]. However, like other existing mid-infrared supercontinua, this supercontinuum has the problem of low power ratio in the 2.7 - 3 μm band. To improve the power ratio of the 2.7 - 3 μm band in the supercontinuum, the method of obtaining the supercontinuum needs to be improved. Summary of the Invention

[0003] To overcome the above-mentioned disadvantages of the prior art, the present invention proposes a mid-infrared supercontinuum light source with a high power ratio in the range of 2.7 - 3 μm. A few-cycle Cr:ZnS laser is used as the seed source. After the femtosecond pulses generated by the Cr:ZnS laser are spectrally broadened, they are injected into an erbium-doped fluoride fiber amplifier to achieve selective amplification in the 2.7 - 3 μm band, solving the problem of the low power ratio in the 2.7 - 3 μm band that commonly exists in existing mid-infrared supercontinua. The ratio can be increased from 12.6% of the traditional method to 73.2%.

[0004] A mid-infrared supercontinuum light source with a high power ratio in the range of 2.7 - 3 μm according to the present invention includes a Cr:ZnS femtosecond laser, a section of As 2 S 3 fiber and a section of erbium-doped fluoride fiber pumped by a 976 nm laser diode; the Cr:ZnS femtosecond laser generates femtosecond pulses at a central wavelength of 2.4 μm; the femtosecond pulses are focused into the As 2 S 3 fiber through an aspherical lens; when the femtosecond pulses propagate in the As 2 S 3 fiber, the spectrum is broadened through self-phase modulation; the output end of the As 2 S 3 fiber is butt-coupled with the end face of the erbium-doped fluoride fiber; the pulses after spectral broadening are injected into the core of the erbium-doped fluoride fiber as seed light; the 976 nm laser diode serves as the pump source and is focused on the cladding of the erbium-doped fluoride fiber through reflection by a total reflector and a dichroic mirror and then focused by a CaF 2 lens; the 976 nm pump light is absorbed by the erbium-doped fluoride fiber to excite erbium ions from the ground state 4 I 15 / 2 to a high energy level 4 I 11 / 2 ; the seed light excites 4 I 11 / 2 → 4 I 13 / 2 energy level transitions in the erbium-doped fluoride fiber to obtain optical gain and be amplified; the amplified laser is filtered by a band-pass filter with a wavelength range of 1.9 - 6 μm to remove the residual pump light and then output.

[0005] Further, the model of the 976 nm laser diode is KepuLin DS3 - 51512.

[0006] Further, the full width at half maximum of the femtosecond pulses is 190 nm, the repetition frequency is 173 MHz, the pulse width is 37 fs, and the output average power is greater than 400 mW.

[0007] Further, the As 2 S3 The fiber length is 0.74 m, the cladding diameter is 125 μm, the core diameter is 5 μm, the numerical aperture is 0.3, and the loss at 2.4 μm is 0.2 dB / m.

[0008] Further, the length of the erbium-doped fluoride fiber is 3.8 m, the doping concentration is 70000 ppm, the cladding diameter is 250 μm, and the core diameter is 15 μm.

[0009] Further, the focal length f of the aspherical lens 1 = 6 mm.

[0010] Further, the focal length f of the CaF 2 lens 2 = 20 mm.

[0011] Further, the reflectivity of the total reflection mirror for 976 nm light is 99.5%.

[0012] Further, the reflectivity of the dichroic mirror for 976 nm light is 85.2%, and the transmittance for light from 2.2 μm to 4.3 μm is about 75%.

[0013] Further, the band-pass filter uses WG91050-C9 of Thorlabs brand.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0015] A mid-infrared supercontinuum light source with a high power ratio in the range of 2.7 - 3 μm is provided. The mid-infrared supercontinuum obtained by broadening-amplifying can increase the power ratio in the range of 2.7 - 3 μm from 12.6% of the traditional single nonlinear broadening method to 73.2%. In addition, the broadening-amplifying system of the present invention can flexibly control the supercontinuum peak through the selection of the gain medium, and has very good spectral shaping ability, providing an effective solution to the problem of spectral enhancement of the low-intensity part of the supercontinuum. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments.

[0017] Figure 1 It is a data diagram of the overall structure of a mid-infrared supercontinuum light source with a high power ratio in the range of 2.7 - 3 μm and the pulses emitted by a Cr:ZnS femtosecond laser according to an embodiment of the present invention. Among them, (a) is a system structure diagram; (b) is an output spectrum and pulse sequence diagram of the Cr:ZnS femtosecond laser; (c) is an autocorrelation curve diagram of the output pulses of the Cr:ZnS femtosecond laser.

[0018] Figure 2 This is a spectral broadening diagram obtained after conducting a spectral broadening experiment on a mid-infrared supercontinuum light source with a high power ratio in the range of 2.7 - 3μm according to an embodiment of the present invention.

[0019] Figure 3 This is a data diagram of the amplification characteristics of optical pulses of a mid-infrared supercontinuum light source with a high power ratio in the range of 2.7 - 3μm according to an embodiment of the present invention under different pump powers and seed powers. Among them, (a) is a comparison diagram of output spectra under different pump powers; (b) is a power comparison diagram of the full band and the range of 2.7 - 3μm under different pump powers; (c) is a comparison diagram of output spectra under different seed pulse energies; (d) is a power comparison diagram of the full band and the range of 2.7 - 3μm under different seed powers.

[0020] Figure 4 This is a data diagram for evaluating the stability of a mid-infrared supercontinuum light source with a high power ratio in the range of 2.7 - 3μm according to an embodiment of the present invention. Among them, (a) is a power stability data diagram; (b) is a diagram of the output pulse sequence; (c) is a radio frequency spectrum diagram of the amplifier; (d) is a comparison diagram of phase noise data of the Cr:ZnS laser and the amplifier.

[0021] In the figure: 1. Cr:ZnS femtosecond laser; 2. Aspherical lens; 3. As 2 S 3 Optical fiber; 4. Erbium-doped fluoride optical fiber; 5. CaF 2 Lens; 6. Total reflection mirror; 7. Dichroic mirror; 8. 976nm laser diode; 9. Band-pass filter. Detailed implementation manners

[0022] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0023] In the description of the present invention, it should be noted that unless otherwise clearly defined and limited, the term "butt coupling" should be understood in a broad sense. For example, it can be mechanical butt coupling of end face to end face, or coupling through fiber fusion; it can be direct connection coupling, or coupling through an intermediate medium. For those of ordinary skill in the art, the specific meaning of the above terms in the present invention can be understood according to specific situations. In addition, the technical features involved in different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0024] Such as Figure 1As shown, an embodiment of the present invention provides a mid-infrared supercontinuum light source with a high power ratio at 2.7 - 3μm, which includes a Cr:ZnS femtosecond laser 1, a section of As 2 S 3 fiber 3, and a section of erbium-doped fluoride fiber 4 pumped by a 976nm laser diode 8; the Cr:ZnS femtosecond laser 1 generates femtosecond pulses at a central wavelength of 2.4μm; the femtosecond pulses first pass through a total reflection mirror M1 and a total reflection mirror M2 in sequence, and then are focused into the As 2 S 3 fiber 3 through an aspheric lens 2; when the femtosecond pulses propagate in the As 2 S 3 fiber 3, the spectrum is broadened through self-phase modulation; the output end of the As 2 S 3 fiber 3 is butt-coupled with the end face of the erbium-doped fluoride fiber 4, and the pulse after spectral broadening is injected into the core of the erbium-doped fluoride fiber 4 as seed light; the 976nm laser diode 8 serves as a pump source, and after being reflected by a total reflection mirror 6 and a dichroic mirror 7, it is focused by a CaF 2 lens 5 to achieve cladding pumping of the erbium-doped fluoride fiber 4; the 976nm pump light is absorbed by the erbium-doped fluoride fiber 4, and the erbium ions are excited from the ground state 4 I 15 / 2 to a high energy level 4 I 11 / 2 ; the seed light excites 4 I 11 / 2 → 4 I 13 / 2 energy level transition in the erbium-doped fluoride fiber 4 to obtain optical gain and be amplified; the amplified laser is filtered by a band-pass filter 9 with a wavelength range of 1.9 - 6μm to remove the residual pump light and then output. The model of the 976nm laser diode 8 is KepuLin DS3-51512. The full width at half maximum of the femtosecond pulses is 190nm, the repetition frequency is 173MHz, the pulse width is 37fs, and the output average power exceeds 400mW. The length of the As 2 S 3 fiber 3 is 0.74m, the cladding diameter is 125μm, the core diameter is 5μm, the numerical aperture is 0.3, and the loss at 2.4μm is 0.2dB / m. The length of the erbium-doped fluoride fiber 4 is 3.8m, the doping concentration is 70000ppm, the cladding diameter is 250μm, and the core diameter is 15μm. The focal length f 1 of the aspheric lens 2 2 is 6mm. The focal length f 2= 20 mm. The reflectivity of the total reflection mirror 6 for 976 nm light is 99.5%. The reflectivity of the dichroic mirror 7 for 976 nm light is 85.2%, and the transmittance for light from 2.2 μm to 4.3 μm is approximately 75%. The band - pass filter 9 uses the WG91050 - C9 of Thorlabs brand.

[0025] Using the above - mentioned system for experiments, when the femtosecond pulses generated by the Cr:ZnS laser are transmitted in the As 2 S 3 optical fiber 3, due to self - phase modulation, the spectrum will broaden. In the experiment, a Fourier transform infrared spectrometer (FTIR, Arcoptix FTIR L1 - 120 - 4TE, used to measure the spectrum above 2 μm) and an InGaAs detector array spectrometer (OceanHood NIRPro, used to measure the spectrum below 2 μm) are used to record the frequency - domain broadening results (the output beam of the As 2 S 3 optical fiber 3 is directly collimated by a lens, and the output light does not pass through the erbium - doped fluoride fiber 4). By changing the input pulse energy, the variation of the spectral broadening effect with the incident femtosecond pulse energy is studied. As Figure 2 shown, as the incident pulse energy increases, the nonlinear effect gradually enhances, resulting in more significant spectral broadening. The initial bandwidth of the spectrum directly output by the Cr:ZnS laser at - 20 dB is 1080 nm; under the action of the nonlinear effect, when the pulse energy reaches 0.84 nJ (corresponding to a peak power of 22.64 kW and an average power of 144.93 mW), the spectral bandwidth expands to 1750 nm (1890 nm - 3640 nm). At this time, the optical power in the 2.7 - 3 μm band only accounts for 12.6% of the total power.

[0026] As Figure 3 shown, after ensuring that there is no parasitic laser component in the amplified spectrum, the present invention studies the amplification characteristics of pulses under different pump powers and seed powers. Figure 3 (a) shows the optical pulse amplification effects corresponding to different pump powers at a seed average power of 45.83 mW (corresponding to a pulse energy of 0.26 nJ). As the pump power increases, the power in the 2.7 - 3 μm band increases significantly: when the pump power is 1.2 W, the maximum gain coefficient of 69.3 is measured at 2.8 μm, and the spectrum broadens to more than 4 μm at - 20 dB intensity (relative to the intensity at 2.4 μm).

[0027] Figure 3(b) presents the variation of the total - band power and the 2.7 - 3μm - band power with the pump power at a seed power of 45.83 mW. Both show a linear growth trend, with slope efficiencies of 7.45% and 7.97% respectively. When the pump power reaches 1.2 W, the power in the 2.7 - 3μm band is 101.16 mW, accounting for 73.2% of the total - band power, corresponding to a maximum gain coefficient of 27.57. There is no sign of gain saturation in the figure, indicating that the system still has further amplification potential. However, further increasing the pump power will induce parasitic lasing, limiting the improvement of the amplification effect.

[0028] Figure 3 (c) shows the amplification effect of optical pulses at different seed - pulse energies with a pump power of 0.8 W. It can be observed that the larger the seed - pulse energy, the more significant the spectral broadening. As the spectral width increases, the intensity at 2.8μm decreases because a wider spectrum means more frequency components competing for gain. However, for the 2.7 - 3μm band, the power still increases with the increase of the seed - pulse energy. Figure 4 (d) shows the variation of the total - band power and the 2.7 - 3μm - band power at different seed powers with a pump power of 0.8 W. When the seed power is 68.75 mW, the output power of the ∼2.8μm pulse reaches a maximum of 74.85 mW (accounting for 60.51% of the total - band power), and the gain coefficient is 8.98. While when the seed power is 34.38 mW, the output power is 60.84 mW (accounting for 69.77% of the total - band power), and at this time the gain coefficient is the highest, reaching 24.33. The slow growth of the power in the 2.7 - 3μm band and the gain - saturation phenomenon indicate that increasing the pump power can further enhance the amplification effect.

[0029] To evaluate the power stability of the experimental setup, the output power was monitored for 14 minutes at a sampling frequency of approximately 16.6 Hz under the conditions of a pump power of 0.4 W and a seed power of 45.83 mW. As Figure 4 (a) shows, the normalized root - mean - square deviation is 1.3%, and the small fluctuations in the average power are mainly due to the instability of the Cr:ZnS laser. Figure 4 (a) The inset shows the beam profile of the output pulse, indicating that the laser operates in the fundamental mode. Figure 4 (b) shows the output - pulse train measured by a fast HgCdTe photodetector (PVM - 10.6 - 1x1 - BNC - NW - 102, bandwidth 1 GHz, operating wavelength range 2 - 12μm, from Vigo Systems). The pulse repetition frequency is 172.4 MHz, and there is no obvious modulation. Figure 4(c) The radio frequency spectrum of the amplifier measured by a signal analyzer (Keysight N9020A), with a resolution bandwidth (RBW) of 3 MHz. The signal-to-noise ratio at the fundamental frequency repetition rate is 38 dB, and the DC component is negligible, confirming that parasitic effects are effectively suppressed. Figure 4 (d) Shows the phase noise of the fundamental frequency repetition rate components of the Cr:ZnS laser and the erbium-doped fluoride fiber amplifier measured by the signal analyzer. The phase noise of both is very close, indicating that the fiber amplifier hardly introduces additional phase noise.

[0030] In the embodiment of the present invention, 37 fs pulses (center wavelength 2.4 μm, repetition rate 173 MHz) generated by a Cr:ZnS laser are used to achieve spectral broadening in As 2 S 3 Optical fiber 3. When the input pulse energy is 0.84 nJ, the spectral range covers 1890 nm to 3640 nm, and the output average power is 120 mW, where the power in the 2.7 - 3 μm band is 15.1 mW, accounting for 12.6% of the total power. The broadened laser is used as a seed source and injected into an erbium-doped fluoride fiber amplifier for amplification. Under the conditions of a pump power of 1.2 W and a seed power of 45.83 mW, the power in the 2.7 - 3 μm band reaches 101.16 mW, accounting for 73.2% of the total power, which is significantly higher than the power ratio of the 2.7 - 3 μm band after simple spectral broadening (12.6%). Compared with the traditional single broadening system, the broadening-amplification system of the embodiment of the present invention can also flexibly control the supercontinuum peak through the selection of the gain medium, has very good spectral shaping ability, and provides an effective solution to the problem of spectral enhancement of the low-intensity part of the supercontinuum.

[0031] As described above, it is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.

Claims

1. A mid-infrared supercontinuum light source with high power ratio at 2.7-3 μm, characterized in that: The invention comprises a Cr:ZnS femtosecond laser, a section of As2S3 optical fiber and a section of erbium-doped fluoride optical fiber pumped by a 976nm laser diode; the Cr:ZnS femtosecond laser generates femtosecond pulses at a central wavelength of 2.4μm; the femtosecond pulses are focused into the As2S3 optical fiber through an aspheric lens; when the femtosecond pulses propagate in the As2S3 optical fiber, the spectrum is broadened through self-phase modulation; the output end of the As2S3 optical fiber is butt-coupled with the end face of the erbium-doped fluoride optical fiber, and the pulses after spectrum broadening are injected into the core of the erbium-doped fluoride optical fiber as seed light; the 976nm laser diode is used as a pump source, and after being reflected by a total reflection mirror and a dichroic mirror, it is focused by a CaF2 lens to realize cladding pumping of the erbium-doped fluoride optical fiber; the 976nm pump light is absorbed by the erbium-doped fluoride optical fiber, and erbium ions are transferred from the ground state to the erbium-doped fluoride optical fiber. 4 I 15 / 2 Stimulate to the highest energy level 4 I 11 / 2 ; Seed light excitation in erbium-doped fluoride fiber 4 I 11 / 2 → 4 I 13 / 2 The energy level transition obtains optical gain and is amplified; the amplified laser is filtered by a 1.9-6μm bandpass filter to remove residual pump light before output.

2. A mid-infrared supercontinuum light source with a high power ratio at 2.7-3 μm according to claim 1, characterized in that: The model of the 976nm laser diode is Keplin DS3-51512.

3. A mid-infrared supercontinuum light source with high power ratio at 2.7-3 μm according to claim 1, characterized in that: The half-width of the femtosecond pulse is 190nm, the repetition frequency is 173MHz, the pulse width is 37fs, and the output average power exceeds 400mW.

4. A mid-infrared supercontinuum light source with high power ratio at 2.7-3 μm according to claim 1, characterized in that: The As2S3 optical fiber has a length of 0.74 m, a cladding diameter of 125 μm, a core diameter of 5 μm, a numerical aperture of 0.3, and a loss of 0.2 dB / m at 2.4 μm.

5. The mid-infrared supercontinuum light source with high power ratio at 2.7-3 μm according to claim 1, characterized in that: The erbium-doped fluoride optical fiber has a length of 3.8 m, a doping concentration of 70,000 ppm, a cladding diameter of 250 μm, and a core diameter of 15 μm.

6. A mid-infrared supercontinuum light source with high power ratio at 2.7-3 μm according to claim 1, characterized in that: The focal length of the aspherical lens is f1 = 6 mm.

7. A mid-infrared supercontinuum light source with high power ratio at 2.7-3 μm according to claim 1, characterized in that: The focal length of the CaF2 lens is f2 = 20 mm.

8. The mid-infrared supercontinuum light source with high power ratio at 2.7-3 μm according to claim 1, characterized in that: The dichroic mirror has a reflectivity of 85.2% for 976 nm light and a transmittance of about 75% for 2.2 μm to 4.3 μm light.

9. The mid-infrared supercontinuum light source with high power ratio at 2.7-3 μm according to claim 1, characterized in that: The bandpass filter is WG91050-C9 of the Thorlabs brand.

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