An upconversion time-stretch based long-wave high-speed measurement infrared spectrometer

Abstract

This invention discloses a long-wavelength high-speed infrared spectrometer based on upconversion time stretching, comprising a femtosecond laser for generating 1μm pump pulses, a white light module, a mid-infrared optical parametric amplification module, a sample absorption module, a Martinez stretcher, a frequency upconversion module, a time stretching module, and a signal detection module. This invention combines mid-infrared optical parametric amplification based on LGS crystals, frequency upconversion process, and time stretching technology to achieve a long-wavelength infrared spectrometer with high measurement rate, high resolution, and high signal-to-noise ratio in the wavelength range of 5–12μm.

Description

Technical Field

[0001] This invention relates to the field of solid-state and fiber ultrafast laser technology, and specifically to a long-wavelength high-speed infrared spectrometer based on upconversion time stretching. Background Technology

[0002] Mid-infrared lasers contain the characteristic absorption peaks of most gaseous and liquid molecules. Theoretically, by observing the selective absorption of certain wavelengths by these molecules, it is possible to identify various materials. Therefore, research on mid-infrared spectroscopy has become a hot topic in the laser field both domestically and internationally. Mid-infrared spectroscopy offers non-destructive and label-free analytical techniques, making it widely applicable for qualitative and quantitative chemical characterization of various substances in industry, environment, and biomedicine. However, the development of mid-infrared spectrometer performance, such as sensitivity, spectral resolution, and scanning rate, has long been hampered by the limited sensitivity of narrow-bandgap detectors or the insufficient brightness of broadband light sources. While grating-based dispersive spectrometers can detect mid-infrared signals using sensor arrays, their spectral measurement rate is limited by the sensor readout rate. Fourier transform spectrometers can increase the scanning rate by employing wide-bandwidth photodetectors; mid-infrared dual-comb infrared spectroscopy has increased the scanning rate of Fourier transform infrared spectroscopy to the trillionth level of spectra per second, but due to its signal-to-noise ratio limitation, the measurement scanning rate based on this method has reached its theoretical maximum. Sweep-frequency spectroscopy, on the other hand, has a good signal-to-noise ratio because its broadband spectrum is measured by scanning laser frequencies, resulting in less noise per spectral element; its signal-to-noise ratio is superior to that of Fourier transform spectroscopy. While the scanning rate of existing mid-infrared spectrometers using frequency sweep spectroscopy is significantly improved, their spectral resolution remains limited. Infrared spectrometers combining upconversion time stretching exhibit high-speed and high-resolution broadband mid-infrared spectroscopy, making them highly promising for applications in infrared transient spectral measurements of combustion reactions such as high-throughput sequencing and reaction tracking. However, the mid-infrared wavelengths currently used for detection are limited to within 5 μm.

[0003] Therefore, the main problem with existing technologies is that high resolution, high scanning rate, and high signal-to-noise ratio are often not simultaneously achievable. At the same time, there is still a gap in the current technology for rapid spectral detection of mid-infrared wavelengths >5μm. Summary of the Invention

[0004] This invention addresses the problems existing in the prior art by providing a long-wavelength high-speed infrared spectrometer based on upconversion time stretching.

[0005] The technical solution adopted in this invention is: a long-wavelength high-speed infrared spectrometer based on upconversion time stretching, comprising a femtosecond laser for generating 1μm pump pulses, a white light module, a mid-infrared optical parametric amplification module, a sample absorption module, a Martinez stretcher, a frequency upconversion module, a time stretching module, and a signal detection module;

[0006] The white light module is used to broaden the spectrum of the pump pulse and generate the first laser pulse;

[0007] The mid-infrared optical parametric amplification module is used to amplify the first laser pulse and the pump pulse after they are combined, and then separate them to obtain the mid-infrared pulse.

[0008] The sample absorption module is used to selectively absorb the mid-infrared pulses generated by the mid-infrared optical parametric amplification module to obtain the mid-infrared pulses corresponding to the sample.

[0009] The Martinez stretcher is used to stretch the pump pulse in the time domain to generate a second laser pulse;

[0010] The frequency up-conversion module is used to perform frequency up-conversion on the combined second laser pulse and infrared absorption spectrum, and then separate them to obtain the third laser pulse.

[0011] The time stretching module is used to stretch the third laser pulse in the time domain to obtain the fourth laser pulse;

[0012] The signal detection module is used to detect the fourth laser pulse.

[0013] Furthermore, the white light module includes an aperture, a plano-convex lens, a movable YAG crystal, and a first long-pass filter arranged in sequence.

[0014] Furthermore, the mid-infrared optical parametric amplification module includes a first dichroic mirror, a first LGS crystal, a second dichroic mirror, and a germanium plate arranged sequentially; the first laser pulse and the pump pulse are combined through the first dichroic mirror, amplified sequentially by the first LGS crystal to generate an infrared pulse, and separated by the second dichroic mirror and the germanium plate to obtain a mid-infrared pulse.

[0015] Furthermore, the sample used in the sample absorption module is a 0.05 mm polystyrene film.

[0016] Furthermore, the Martinez stretcher includes a first dielectric film mirror, a diffraction grating, and a strip mirror and a concave mirror respectively disposed on both sides of the diffraction grating.

[0017] Furthermore, the frequency upconversion module includes a third dichroic mirror, a second LGS crystal, a fourth dichroic mirror, and a second long-pass filter arranged sequentially; the mid-infrared pulse and the second laser pulse from the sample absorption module are combined by the third dichroic mirror, upconverted by the second LGS crystal, and separated by the fourth dichroic mirror and the long-pass filter to obtain the third laser pulse.

[0018] Furthermore, the time stretching module includes a dispersion-shifted fiber (DSF).

[0019] Furthermore, both the mid-infrared optical parametric amplification module and the frequency up-conversion module also include a time delay device, which is used to control the optical path.

[0020] Furthermore, both the first LGS crystal and the second LGS crystal are 8mm thick; after the first laser pulse and the pump pulse are combined, they pass through plano-convex lenses with focal lengths of 250mm and 150mm before entering the first LGS crystal; after the infrared absorption spectrum and the second laser pulse are combined, they pass through a plano-convex lens with a focal length of 100mm before entering the second LGS crystal.

[0021] Furthermore, the YAG crystal has a length of 10mm, the plano-convex lens has a focal length of 150mm, and the pump spectrum is broadened to 1500nm after passing through the white light module.

[0022] The beneficial effects of this invention are:

[0023] (1) This invention combines mid-infrared optical parametric amplification based on LGS crystal, frequency upconversion process and stretching technology to realize a mid-infrared spectrometer based on upconversion time stretching with high speed measurement rate, high resolution and high signal-to-noise ratio in the wavelength range of 5 to 12 μm.

[0024] (2) The present invention can achieve tunable mid-infrared pulses in the wavelength range of 5 to 12 μm by adjusting the phase matching angle of the LGS crystal, and has high average power;

[0025] (3) The present invention uses a Martinez stretcher to stretch the pump light in the time domain, making its pulse width much larger than that of the mid-infrared pulse width. In the frequency up-conversion module, it performs a difference frequency process with the mid-infrared pulse in the form of a single frequency, thereby realizing a nonlinear spectral mapping with high fidelity and high frequency. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the structure of the long-wavelength high-speed infrared spectrometer based on upconversion time stretching of the present invention.

[0027] Figure 2The images show the mid-infrared upconversion spectra measured in this embodiment of the invention, as well as the time-domain spectra obtained after time stretching using different optical fibers. a) is the mid-infrared upconversion spectrum measured by a near-infrared spectrometer; b) is the infrared spectrum obtained after time stretching of continuously measured upconversion signals through a 25km dispersion-shifted optical fiber, with a 10μs phase difference between each two signals; c) are the time-stretched upconversion mid-infrared spectra measured under the same incident signal intensity using dispersion-shifted fiber (curve A), single-mode fiber (curve B), and dispersion-compensated fiber (curve C), respectively.

[0028] Figure 3 The first image shows the pulse spectrum curve of the pump light after passing through the Martinez stretcher in an embodiment of the present invention; a is the pulse curve of the femtosecond pump light after passing through the Martinez stretcher; b is the spectral curve of the pump light after passing through the Martinez stretcher.

[0029] Figure 4 The images show the upconversion infrared spectra of mid-infrared pulses with different center wavelengths before and after passing through a 0.05 mm polystyrene film; the center wavelength of a is 8.3 μm, the center wavelength of b is 8.9 μm, and the center wavelength of c is 9.78 μm; d is the time-stretched upconversion mid-infrared spectrum corresponding to a, e is the time-stretched upconversion mid-infrared spectrum corresponding to b, and f is the time-stretched upconversion mid-infrared spectrum corresponding to c; g is the time-stretched mid-infrared spectrum corresponding to a after an average of 200 cycles; h is the time-stretched mid-infrared spectrum corresponding to b after an average of 200 cycles; and i is the time-stretched mid-infrared spectrum corresponding to c after an average of 200 cycles.

[0030] Figure 5 Comparison between the time-stretched upconversion infrared transmission spectrum and the Fourier transform transmission spectrum of polystyrene film.

[0031] In the diagram: 1-Femtosecond laser, 2-White light module, 3-Mid-infrared optical parametric amplification module, 4-Sample absorption module, 5-Martinez stretcher, 6-Frequency upconversion module, 7-Time stretching module, 8-Signal detection module;

[0032] HWP - Half-wave plate, TFP - Thin-film polarizer, LGS - LiGaS2 crystal, LPF - Long-pass filter, L - Lens, HR - 1000~1080nm dielectric film mirror, M - Silver mirror, G - Grating, CM - Concave mirror, LM - Long strip mirror, DM1 - Dichroic mirror (HT@1135~1600nm, HR@1030nm); DM2 - Dichroic mirror (HT@5~11μm, HR@1030nm), BD - Baffle, DSF - Dispersion-shifted fiber, PD - Photodetector. Detailed Implementation

[0033] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0034] like Figure 1 As shown, a long-wavelength high-speed infrared spectrometer based on upconversion time stretching includes a femtosecond laser 1 for generating 1μm pump pulses, a white light module 2, a mid-infrared optical parametric amplification module 3, a sample absorption module 4, a Martinez stretcher 5, a frequency upconversion module 6, a time stretching module 7, and a signal detection module 8.

[0035] White light module 2 is used to broaden the spectrum of the pump pulse through self-phase modulation to generate the first laser pulse. White light module 2 includes an aperture, a plano-convex lens L, a movable YAG crystal, and a first long-pass filter LPF arranged sequentially. A movable platform is positioned under the YAG crystal, allowing the distance between the YAG crystal and the plano-convex lens L to be adjusted. Spectrum broadening is achieved through self-phase modulation of the YAG crystal, which has a length of 10 mm, while the plano-convex lens L has a focal length of 150 mm. The power and spot size of the 1 μm laser focused into the crystal are controlled by changing the size of the aperture. By adjusting the distance between the YAG crystal and the plano-convex lens, the lens focal point is positioned on the rear surface of the YAG crystal, thus broadening the spectrum to approximately 1500 nm. The pump pulse generated by femtosecond laser 1 enters white light module 2 after passing through the second dielectric film reflector HR. The pump pulse generated by femtosecond laser 1 has a frequency of 200 kHz, a power of 20 W, and an energy of 100 μJ.

[0036] The mid-infrared optical parametric amplification module 3 is used to amplify the first laser pulse and the pump pulse after beam combining, and simultaneously generate a mid-infrared pulse. The pump light and the first laser pulse are then separated to obtain the desired mid-infrared pulse. The mid-infrared optical parametric amplification module 3 includes a first dichroic mirror DM1, a first LGS crystal, a second dichroic mirror DM2, and a germanium plate Ge arranged sequentially. The first laser pulse and the pump pulse are combined by the first dichroic mirror (HT@1135~1600nm, HR@1030nm), amplified sequentially by the first LGS crystal to generate an infrared pulse, and then separated by the second dichroic mirror and the germanium plate to obtain the mid-infrared pulse.

[0037] In the mid-infrared optical parametric amplification module 3, the first LGS crystal is 8mm long, and mid-infrared pulses are generated by amplification within the first LGS crystal. Before incident, the first laser pulse and the pump pulse are focused by plano-convex lenses with focal lengths of 250mm and 150mm, respectively, so that the spot size incident on the front surface of the crystal is 1mm, thereby controlling the peak intensity of the incident pulse at the front surface of the crystal to be between 20 and 40 GW / cm². 2 To prevent damage to the crystal, a time delay device is also included to control the pump optical path, ensuring that the first laser pulse and the pump pulse coincide in the time domain. The first LGS crystal has dimensions of 5x5x8 mm. 3Theta = 51.2°, Phi = 0°, corresponding to type I phase matching. After the LGS crystal, the generated mid-infrared pulse is separated from the first laser pulse and pump pulse by passing the dichroic mirror 2 (HT@5~11μm, HR@1030nm) and germanium plate in sequence.

[0038] The sample absorption module 4 is used to selectively absorb the mid-infrared pulses generated by the mid-infrared optical parametric amplification module 3 to obtain the infrared absorption spectrum corresponding to the sample. The sample is a polystyrene film with a thickness of 0.05 mm, through which characteristic absorption is performed to generate mid-infrared pulses of a specific wavelength.

[0039] The Martinez stretcher 5 is used to temporally stretch the pump pulse output from the femtosecond laser 1 to generate a second laser pulse. The Martinez stretcher 5 includes a first dielectric reflective film HR, a diffraction grating G, and a strip mirror LM and a concave mirror CM respectively disposed on either side of the diffraction grating. The focal length of the concave mirror is 250 mm. By introducing positive dispersion through the Martinez stretcher 5, the femtosecond pump pulse is temporally stretched to a pulse width of hundreds of picoseconds. The femtosecond pump light is incident on the grating G at an angle of 61.2°. The total diffraction efficiency of the grating G is 80%, and the distance between the grating and the concave mirror is 180 mm to ensure that an appropriate amount of positive dispersion is introduced to stretch the femtosecond pump light to hundreds of picoseconds, thus obtaining the second laser pulse. The stretched pulse is reflected back to the first dielectric reflective film HR, and then enters the frequency up-conversion module 6 after passing through the third dielectric reflective film HR.

[0040] The frequency upconversion module 6 is used to perform frequency upconversion on the combined second laser pulse and infrared absorption spectrum, and then separate them to obtain the third laser pulse. The frequency upconversion module 6 includes a third dichroic mirror DM2, a second LGS crystal, a fourth dichroic mirror DM1, and a second long-pass filter LPF arranged sequentially. The mid-infrared pulse and the second laser pulse from the sample absorption module 4 are combined by the third dichroic mirror (HT@3~12μm, HR@1030nm), undergo frequency upconversion by the second LGS crystal, and then separated by the fourth dichroic mirror and the long-pass filter to obtain the third laser pulse. The second LGS crystal is 8mm long, and the frequency upconversion process is performed within it to generate the third laser pulse. Before incident, both the second laser pulse and the mid-infrared pulse are focused by a 100mm plano-convex lens, controlling the spot size incident on the front surface of the crystal to be 150μm. A time delay device is used to control the optical path of the second laser pulse to ensure that the second laser pulse and the mid-infrared pulse coincide in the time domain. The LGS crystal has dimensions of 5x5x8 mm. 3Theta = 37.1°, Phi = 90°, corresponding to type II phase matching. An 8mm crystal length ensures sufficiently high conversion efficiency. After passing through the second LGS crystal, the generated third laser pulse is separated from the second laser pulse and mid-infrared pulse by a fourth dichroic mirror DM1 (HT@1135~1600nm, HR@1030nm) and a second long-pass filter (1050nm). The frequency up-conversion module 6 includes a time delay device comprising two opposing mirrors.

[0041] The time stretching module 7 is used to stretch the third laser pulse in the time domain to obtain the fourth laser pulse. The time stretching module 7 uses a dispersion-shifting fiber with a length of 25km to separate the different wavelength components of the third laser pulse in time, which ensures the resolution of mid-infrared spectral detection.

[0042] The signal detection module 8 is used to detect the fourth laser pulse. The signal detection module 8 includes a photodetector PD and an oscilloscope.

[0043] Figure 2 The figures show the measured mid-infrared upconversion spectra and their time-domain spectra obtained after time stretching through different optical fibers. As can be seen from the figures, when time-stretched, curve B shows significantly less time-domain broadening compared to curve A; while curve C, due to excessive loss, exhibits a significantly reduced signal-to-noise ratio.

[0044] Figure 3 The images show the pulse and spectral shapes of the pump light after passing through the Martinez stretcher. a) shows the pulse shape of the femtosecond pump light after passing through the Martinez stretcher, with a full width at half maximum (FWHM) of approximately 122 ps; b) shows the spectral shape of the pump light after passing through the Martinez stretcher.

[0045] Figure 4 The figures show the upconversion infrared spectra of mid-infrared pulses with different center wavelengths before and after passing through a 0.05 mm polystyrene film. As can be seen from the figures, the upconversion infrared spectra of mid-infrared pulses at different center wavelengths after passing through the characteristic absorption of the sample are compared with those obtained using time stretching. Figure 1 This demonstrates that the long-wave mid-infrared time-stretched upconversion spectrometer is reliable.

[0046] Figure 5 To compare the time-stretched upconversion infrared transmission spectrum (TFTIR) of a polystyrene film with its Fourier transform transmission spectrum, dashed lines A, B, and C represent the TFTIR transmission spectra obtained after absorption by the polystyrene film from mid-infrared pulses at center wavelengths of 8.3 μm, 8.9 μm, and 9.78 μm, respectively; the solid line represents the Fourier transform transmission spectrum of the polystyrene film. It can be seen that the experimentally obtained transmission spectrum and the Fourier transform transmission spectrum are in high agreement.

[0047] This invention comprises a femtosecond laser, a white light module, a mid-infrared optical parametric amplification module, a sample absorption module, a Martinez stretcher, a frequency upconversion module, a time stretching module, and a signal detection module. The femtosecond laser generates a pump pulse; the white light module broadens the pump pulse spectrum to obtain a first laser pulse; the mid-infrared optical parametric amplification module generates a long-wavelength mid-infrared pulse; the sample absorption module performs characteristic absorption on certain wavelengths of the mid-infrared pulse to obtain the infrared absorption spectrum of the sample; the Martinez stretcher broadens the pump pulse to obtain a second laser pulse; the frequency upconversion module performs frequency difference analysis on the second laser pulse and the mid-infrared pulse to obtain a third laser pulse; the time stretching module stretches the third laser pulse in the time domain, appropriately separating its different wavelength components in the time domain to obtain a fourth laser pulse; and the signal detection module detects the fourth laser pulse. This invention achieves the detection of long-wavelength mid-infrared upconversion time-stretched spectra through a mid-infrared optical parametric amplification process and a frequency upconversion process based on LiGaS2 crystal, while ensuring a certain measurement rate.

[0048] A broadband seed pulse is provided by white light generated through self-phase modulation in a YAG crystal using a femtosecond pump source. A long-wavelength mid-infrared femtosecond pulse is then generated using a mid-infrared optical parametric amplification system based on an LGS crystal. The wavelength can be tuned within the range of 5–12 μm by adjusting the phase matching angle of the crystal. Positive dispersion provided by a Martinez stretcher broadens the femtosecond pump pulse in the time domain to hundreds of picoseconds, making it much wider than the mid-infrared pulse width, thus ensuring high-fidelity spectral mapping during frequency upconversion. The generated signal light is time-stretched using a 25 km long dispersion-shifting fiber, allowing for appropriate temporal separation of its different wavelength components. The upconverted signal spectrum can be obtained by detecting its time-domain waveform using a photodetector and oscilloscope. The spectral resolution of this time-stretched upconversion infrared spectroscopy can be adjusted by controlling the length of the dispersion-shifting fiber. Due to the advantages of time-stretching technology in terms of signal-to-noise ratio, this upconversion time-stretched infrared spectrometer achieves high-speed scanning in the long-wavelength mid-infrared band. By employing LGS crystals with a wide transmission spectrum for mid-infrared optical parametric amplification and frequency upconversion, the study of time-stretched upconversion infrared spectroscopy has been extended to the long-wavelength direction, which is of great significance for applications such as molecular detection and biomedicine.

Claims

1. A long-wavelength high-speed infrared spectrometer based on upconversion time stretching, characterized in that, It includes a femtosecond laser (1) for generating 1μm pump pulses, a white light module (2), a mid-infrared optical parametric amplification module (3), a sample absorption module (4), a Martinez stretcher (5), a frequency upconversion module (6), a time stretching module (7), and a signal detection module (8). The white light module (2) is used to broaden the spectrum of the pump pulse and generate the first laser pulse; The mid-infrared optical parametric amplification module (3) is used to amplify the combined beam of the first laser pulse and the pump pulse, and then separate them to obtain a mid-infrared pulse. The mid-infrared optical parametric amplification module (3) includes a first dichroic mirror, a first LGS crystal, a second dichroic mirror and a germanium plate arranged in sequence. The first laser pulse and the pump pulse are combined through the first dichroic mirror, and then amplified by the first LGS crystal to generate an infrared pulse. After being separated by the second dichroic mirror and the germanium plate, a mid-infrared pulse is obtained. The first LGS crystal is a LiGaS2 crystal with a length of 8 mm, theta=51.2°, and Phi=0°. The sample absorption module (4) is used to selectively absorb the mid-infrared pulse generated by the mid-infrared optical parametric amplification module (3) to obtain the mid-infrared pulse corresponding to the sample; The Martinez stretcher (5) is used to stretch the pump pulse in the time domain to a hundred picoseconds to generate a second laser pulse; the Martinez stretcher (5) includes a first dielectric film mirror, a diffraction grating, and a strip mirror and a concave mirror respectively disposed on both sides of the diffraction grating; The frequency upconversion module (6) is used to perform frequency upconversion on the second laser pulse and the infrared absorption spectrum after beam combining, and then separate them to obtain the third laser pulse; the frequency upconversion module (6) is used to perform frequency difference on the second laser pulse and the mid-infrared pulse to obtain the third laser pulse; the frequency upconversion module (6) includes a third dichroic mirror, a second LGS crystal, a fourth dichroic mirror and a second long-pass filter arranged in sequence; the mid-infrared pulse and the second laser pulse from the sample absorption module (4) are combined by the third dichroic mirror, the second LGS crystal is used for frequency upconversion, and the third laser pulse is obtained after separation by the fourth dichroic mirror and the second long-pass filter; wherein, the second LGS crystal is a LiGaS2 crystal with a length of 8 mm, theta=37.1°, and Phi=90°; The time stretching module (7) is used to stretch the third laser pulse in the time domain to obtain the fourth laser pulse; the time stretching module (7) includes a dispersion-shifting fiber (DSF); The signal detection module (8) is used to detect the fourth laser pulse.

2. The long-wavelength high-speed infrared spectrometer based on upconversion time stretching according to claim 1, characterized in that, The white light module (2) includes an aperture, a plano-convex lens, a movable YAG crystal, and a first long-pass filter arranged in sequence.

3. The long-wavelength high-speed infrared spectrometer based on upconversion time stretching according to claim 1, characterized in that, The sample used in the sample absorption module (4) is a 0.05 mm polystyrene film.

4. The long-wavelength high-speed infrared spectrometer based on upconversion time stretching according to claim 1, characterized in that, The mid-infrared optical parametric amplification module (3) and the frequency up-conversion module (6) both include a time delay device, which is used to control the optical path.

5. The long-wavelength high-speed infrared spectrometer based on upconversion time stretching according to claim 1, characterized in that, After the first laser pulse and pump pulse are combined, they pass through plano-convex lenses with focal lengths of 250 mm and 150 mm before entering the first LGS crystal; after the infrared absorption spectrum and the second laser pulse are combined, they pass through a plano-convex lens with a focal length of 100 mm before entering the second LGS crystal.

6. A long-wavelength high-speed infrared spectrometer based on upconversion time stretching according to claim 2, characterized in that, The YAG crystal has a length of 10 mm and a focal length of 150 mm. After passing through the white light module (2), the spectrum is broadened to 1500 nm.

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