Frequency-adjustable light source capable of covering vacuum ultraviolet band based on frequency conversion system

Abstract

The invention discloses a frequency-variable light source capable of covering a vacuum ultraviolet band based on a frequency conversion system, and belongs to the technical field of laser optics and nonlinear optics. The device comprises an infrared or visible light wave band laser, an optical frequency conversion device, an amplitude and pulse adjusting device, a frequency selection resonant cavity, a higher harmonic module and a light source output coupling module. A frequency converter and a wavelength selection resonant cavity are added behind a laser source, so that the original single-frequency light is converted into other wavelengths through frequency conversion, then the light is converted into an ultraviolet band through a higher harmonic method, and a light source with any adjustable frequency in a vacuum ultraviolet band range is realized. By means of a method of adopting a nonlinear crystal, an inflatable photonic crystal fiber and a higher harmonic module, broadband spectrum frequency conversion operation is implemented, and a vacuum ultraviolet wave band covered after final frequency conversion reaches the range of 100-300 nm. By adding an amplitude and pulse width adjusting device behind the wavelength selection resonant cavity, the pulse width adjustment of the generated laser can be realized.

Description

Technical Field

[0001] The present invention belongs to the technical fields of laser optics and nonlinear optics, and relates to a frequency-tunable light source capable of covering the vacuum ultraviolet band based on a frequency conversion system, which is applicable to the generation of a frequency-tunable laser light source in the vacuum ultraviolet band. Background Art

[0002] There are various definitions of vacuum ultraviolet light sources. A relatively broad definition range is below 300 nm, which can all be called the vacuum ultraviolet band. There is also a narrower definition referring to the range of the band from 100 nm to 300 nm. Vacuum ultraviolet means an ultraviolet spectral light source that is extremely easily absorbed under atmospheric conditions.

[0003] The light sources in this band have special uses. Regarding the known transition energy levels of organic chemical bonds currently, most of the bands of the excitation pump light sources required are in the 100 nm - 300 nm band. Specifically, the ultraviolet-visible spectrum of organic compounds is the result of the transition of outer valence electrons in their molecules. The valence electrons of organic compounds are divided into three types according to the types of chemical bonds in the molecules: σ electrons forming single bonds, π electrons of unsaturated bonds, and unbonded n electrons on heteroatoms. These three types of electrons are combined to form four transition forms, and in ascending order of transition energy: n→π * , whose absorption wavelength > 200 nm, π→π * , whose absorption wavelength is 200 nm - 400 nm, n→σ * , whose absorption wavelength is 150 nm - 250 nm, σ→σ * , whose absorption wavelength < 200 nm. On the other hand, for the nuclear transitions of many substances, their wavelengths are also in the vacuum ultraviolet band. For example, the nuclear transition of the isotope Th229 of thorium element is at 148.485 nm, and the nuclear transition wavelength of NO2 molecules is at 148.3 nm, etc.

[0004] Precisely due to this special band of vacuum ultraviolet, the uses of vacuum ultraviolet light sources are of great significance. For example, in the field of biomedicine, it can be used to detect the optical response of organic substances. By irradiating organic substance samples with light sources of different wavelengths and detecting their fluorescence responses. This can be used for the detection of unknown organic substances or the detection of chemical bonds of newly synthesized organic substances. It can be used in multiple scenarios such as medical treatment and security inspection; in terms of nuclear transitions, since the degree of interference of atomic nuclei by external electromagnetic fields is smaller than that of atoms, it is considered a research direction for the preparation of higher-precision optical clocks. And a light source in the vacuum ultraviolet band with stable frequency and extremely narrow linewidth is the basis for providing a pump light source for nuclear transitions and becomes the key to the next-generation high-precision optical clock time reference.

[0005] At present, for light sources in this wavelength band, due to the lack of a direct excitation substance, there is no laser directly generating in the vacuum ultraviolet wavelength band. Since lasers in the mid-infrared or visible wavelength bands are very mature, the excitation substances in this wavelength band, such as those doped with erbium, ytterbium, etc., have an excitation band in the near-infrared. Therefore, currently, mid-infrared lasers are used to achieve this through non-linear optical frequency conversion means, especially by methods such as four-wave mixing, frequency doubling, or high-order harmonics.

[0006] The wavelength of the vacuum ultraviolet light source generated by high-order harmonics can be very low, reaching below 50 nm. The problem is that it can only generate a light source at a specific, i.e., single wavelength, and cannot achieve wide-band coverage. Although four-wave mixing can use a tunable laser as the pump source to generate a tunable light source, due to the phase matching limitations of four-wave mixing and frequency doubling, the expanded wavelength band is relatively small, generally around 200 nm, and it is still impossible to generate a laser around 100 nm. Currently, although there are designs for tunable-wavelength light sources in the vacuum ultraviolet wavelength band, they are all achieved based on frequency-converted lasers and four-wave mixing or multiple frequency-conversion methods. The final output wavelength band coverage of this kind of light source is narrow and cannot reach the 100 nm wavelength band.

[0007] Therefore, how to achieve the generation of a continuously tunable vacuum ultraviolet light source in the range of 100 nm to 300 nm is a problem that needs to be solved and is also a research content with great application prospects. Summary of the Invention

[0008] To solve the problem that the wavelength of the light source in the current vacuum ultraviolet wavelength band cannot be adjusted and cannot fully cover the vacuum ultraviolet wavelength band, i.e., the range of 100 nm to 300 nm, the purpose of the present invention is to provide a frequency-convertible light source that can cover the vacuum ultraviolet wavelength band based on a frequency conversion system. By combining an optical frequency conversion device and a frequency selection resonant cavity, the wavelength of the light source can be adjusted; by combining an optical frequency conversion device and a high-order harmonic module, the coverage of the vacuum ultraviolet wide wavelength band can be achieved. The present invention can solve the problem that the wavelength of the light source in the current vacuum ultraviolet wavelength band cannot be adjusted and cannot fully cover the vacuum ultraviolet wavelength band, i.e., the range of 100 nm to 300 nm.

[0009] The purpose of the present invention is achieved through the following technical solutions;

[0010] The disclosed frequency-convertible light source that can cover the vacuum ultraviolet wavelength band based on a frequency conversion system of the present invention includes an infrared or visible wavelength band laser, an optical frequency conversion device, an amplitude and pulse adjustment device, a frequency selection resonant cavity, a high-order harmonic module, and a light source output coupling module.

[0011] The laser generated by an infrared or visible light band laser is frequency broadened through an optical frequency conversion device, turning a single-frequency light source into a continuous spectrum that contains lasers of multiple frequencies. Since the optical power and pulse width of the laser will change after frequency conversion, an amplitude and pulse adjustment device is added after the optical frequency conversion device to compensate for the amplitude of the laser and compress the pulse width of the laser. The laser broadened by the optical frequency conversion device is sent into a frequency selection resonant cavity, and the selection of the laser frequency is achieved by adjusting the cavity length of the resonant cavity, realizing the adjustment of the final output laser wavelength. The laser with the required band, amplitude, and pulse width after adjustment is sent into a high-order harmonic module to generate high-order harmonics of the laser, converting the frequency of the laser from the mid-infrared and visible light bands to the vacuum ultraviolet band, achieving coverage of the 100 - 300 nm vacuum ultraviolet wide band. The output coupling module is used to efficiently distinguish and select the laser with the final required frequency for subsequent detection and applications.

[0012] Furthermore, an infrared or visible light band laser is used to provide a basic laser light source, with a band range of 700 - 1100 nm.

[0013] Furthermore, the optical frequency conversion device is used to change the laser frequency output by the laser, and it is required that the spectrum after frequency conversion is a continuous spectrum, and the wavelength coverage range should include the range between 900 nm and 2100 nm. The optical frequency conversion device is a device that can broaden the laser spectrum over a large range, including nonlinear optical crystals and gas-filled photonic crystal fibers.

[0014] Furthermore, the amplitude and pulse adjustment device is used to improve the optical parameters of the light source coming out of the frequency selection resonant cavity. For pulsed light and optical frequency combs, amplitude increase and pulse compression processing are carried out. The methods include using fiber amplifiers and gain media to amplify the laser amplitude; using chirped mirrors, multiple pairs of parallel grating mirrors, and multiple pairs of prism pairs for pulse compression processing. For continuous lasers, the function of amplifying the optical power is achieved by adding an optical power amplifier or gain media in the amplitude and pulse adjustment device.

[0015] Furthermore, the frequency selection resonant cavity is used to select the laser of the required band from the frequency-broadened light source coming out of the optical frequency conversion device through the selection of the frequency selection resonant cavity.

[0016] Furthermore, the high-order harmonic module is used to change the light source from the infrared band to the ultraviolet band, and it is an important module for generating vacuum ultraviolet band lasers. As the order of the high-order harmonics increases, the generated laser wavelength becomes smaller, and it can finally meet the requirement that the output spectral range covers the 100 - 300 nm vacuum ultraviolet band. The methods for the high-order harmonic module to generate high-order harmonics include gas high-order harmonics, solid high-order harmonics, and plasma flow high-order harmonics.

[0017] Further, the light source output coupling system is used to distinguish and output lasers of multiple wavelengths generated by high - order harmonics, remove the interference of other bands, and only retain the ultraviolet light of the required order; the output coupling methods include using Brewster plates, diffraction grating mirrors, machining small holes behind the output mirror, and non - collinear output coupling methods.

[0018] Beneficial effects:

[0019] 1. The tunable light source based on the frequency conversion system disclosed in the present invention, which can cover the vacuum ultraviolet band, can convert the light of the original single frequency to other wavelengths by adding a frequency conversion device and a wavelength - selective resonator behind the laser source, and then convert it to the ultraviolet band by the method of high - order harmonics, realizing a light source with adjustable frequency within the vacuum ultraviolet band.

[0020] 2. The tunable light source based on the frequency conversion system disclosed in the present invention, through the method of using a nonlinear crystal, an air - filled photonic crystal fiber, and a high - order harmonic module to implement wide - spectrum frequency conversion operation, can achieve that the finally frequency - converted vacuum ultraviolet band covers the range of 100 - 300 nm. Moreover, for other high - order harmonics, it can also generate light of specific wavelengths in the spatial light band, and the covered wavelengths of the light source are wide.

[0021] 3. The tunable light source based on the frequency conversion system disclosed in the present invention can adjust the pulse width of the generated laser by adding an amplitude and pulse width adjustment device behind the wavelength - selective resonator. Description of the drawings

[0022] Figure 1 is the schematic flow structure diagram of the tunable light source based on the frequency conversion system disclosed in the present invention;

[0023] Figure 2 is the schematic diagram of the tunable light source based on the frequency conversion system disclosed in the embodiment of the present invention;

[0024] Figure 3 is another schematic diagram of output coupling integration of the present invention;

[0025] Figure 4 is the schematic diagram of high - order harmonics involved in the present invention;

[0026] Among them, 1 - infrared or visible light band laser, 2 - optical frequency conversion device, 3 - amplitude and pulse adjustment device, 4 - frequency - selective resonator, 5 - high - order harmonic module, 6 - light source output coupling module. Detailed implementation manners

[0027] To better illustrate the purpose and advantages of the present invention, the following further explains the content of the invention in combination with the accompanying drawings and examples. The purpose of the present invention is to solve the problem that the wavelength of the light source in the vacuum ultraviolet band cannot be adjusted at present and cannot fully cover the vacuum ultraviolet band, that is, the range of 100 nm to 300 nm. To make the purpose, technical solution and advantages of the present invention clearer, the following further elaborates on the present invention in detail in combination with specific module flow diagrams, specific embodiments, and with reference to the accompanying drawings.

[0028] As Figure 1 , 2 shown, the tunable frequency light source based on the frequency conversion system disclosed in this embodiment that can cover the vacuum ultraviolet band is composed of an infrared or visible light band laser 1, an optical frequency conversion device 2, an amplitude and pulse adjustment device 3, a frequency selection resonant cavity 4, a high-order harmonic module 5, and a light source output coupling module 6.

[0029] The arrows between modules represent the propagation of "light". It is specifically stated here that Figure 1 the "light" in the transmission process between adjacent modules can be carried out by means of spatial light (such as shown in the specific embodiment), or can also be completely connected by optical fiber. That is, the propagation mode of light will not limit the final output result of the present invention.

[0030] To better illustrate the specific functions of each module, specific embodiments will be used for detailed description. In the embodiment, as Figure 2 shown in the schematic diagram, the present invention discloses a tunable frequency light source based on the frequency conversion system that can cover the vacuum ultraviolet band, which is used to generate a light source in the range of the vacuum ultraviolet light source (100 - 300 nm), and to realize a wide-band tunable frequency vacuum ultraviolet light source.

[0031] In Figure 2 the specific devices are Figure 1 representatives of the process structure diagram, and are not limited to the types of devices. Figure 2 The femtosecond laser in

[0032] represents the infrared or visible light band laser 1; the gas-filled cavity and the hollow photonic crystal fiber represent the optical frequency conversion device 2; the fiber amplifier and the pulse compression grating pair represent the amplitude and pulse adjustment device 3; the piezoelectric ceramic sheet, software control, four infrared wide-band high reflectivity mirrors M1 - M4, and the photodetector represent the frequency selection resonant cavity 4; the jet nozzle injecting xenon gas flow to generate high-order harmonics represents the high-order harmonic module 5; the Brewster plate represents the output coupling module 6.

[0033] In an embodiment, the optical frequency conversion module 2 refers to a frequency conversion system composed of an inflated cavity and a hollow photonic crystal fiber. In the experiment, gas will be filled and exhausted through the gas valve of the inflated cavity. By controlling the type and pressure of the gas, different optical nonlinear results can be achieved. Finally, the optical wavelength output from the output end of the hollow photonic crystal fiber will be broadened to the range of λ2-λ3, where the relationship between the wavelengths is λ2 < λ1 < λ3. Commonly used nonlinear gases such as xenon (Xe), helium (He), argon (Ar), hydrogen (H2), etc. filled can all achieve broadband frequency conversion. For example, based on this experiment of the inflated photonic crystal fiber, when a femtosecond pulsed laser with a wavelength of 805nm (λ1) is selected and hydrogen at 1 atmosphere is filled, the final wavelength can be broadened to a continuous spectrum in the range of 120nm - 1200nm (λ2 - λ3). Finally, at the output end of the hollow photonic crystal fiber, a lens (which should meet the requirements of the incident light wavelength) is used to collimate the output light and enter the amplitude and pulse adjustment device 3.

[0034] In an embodiment, the amplitude and pulse adjustment device 3 uses an optical fiber amplifier to amplify the laser energy. In fact, other gain media or optical amplification means can be used to amplify the optical signal. This step is to make the laser energy pass through two-stage amplification by the amplitude and pulse adjustment device 3 and the frequency selection resonant cavity 4, and finally meet the optical energy requirements for exciting the high harmonic module 5. The single-pulse energy of the laser during high harmonics is in the range of 100 μJ - 10 mJ. The specific amplification factor should be determined according to the laser used; in addition, in the embodiment, a parallel grating pair is used to compress the optical pulse of the laser. There are various ways to adjust the laser pulse, and a prism pair or chirped mirrors can be used to achieve it. Pulse compression is beneficial to the final output efficiency of high harmonics. According to experimental experience, it is best to compress the laser pulse to within 200 fs.

[0035] In an embodiment, the frequency selection resonant cavity 4 shows a butterfly cavity composed of four infrared broadband high-reflectivity mirrors. The function of the frequency selection resonant cavity is to perform frequency selection on the broadband light output from module 3. By controlling the expansion and contraction of the piezoelectric ceramic sheet controlled by software on the computer side, the cavity length can be controlled, and thus the selection of a specific wavelength band ν4 can be achieved. Light of other wavelengths will attenuate due to phase mismatch and multiple reflections in the frequency selection resonant cavity 4 and will not be output to the high harmonic module 5 finally. The role of the frequency selection resonant cavity 4 is to determine the frequency and achieve secondary amplification (the amplification factor is two orders of magnitude). The frequency selection resonant cavity can also be designed in the form of 2 mirrors, 6 mirrors, 8 mirrors, etc., as long as the above purposes can be achieved. Finally, the single wavelength ν4 output from the frequency selection resonant cavity 4 will enter the vacuum cavity and enter the high harmonic module 5.

[0036] In an embodiment, the high-order harmonic module 5 is located inside the vacuum chamber. That is, during the experiment, it is required to ensure that the experiment is carried out in a vacuum environment. By passing the high-order harmonic gas (xenon Xe) through the laser vertically at a certain rate through the gas nozzle, the high-order harmonic effect of the laser will occur, and the output wavelength is λ4 / n, where n represents the order of the high-order harmonic (the high-order harmonic only has odd orders). And because the high-order harmonic cannot achieve 100% conversion efficiency, the output light is a mixed light containing multiple wavelengths and is input to the final light source output coupling module 6.

[0037] In an embodiment, the light source output coupling module 6 uses the form of a Brewster plate to complete the separation of the final required wavelength ν5. The Brewster plate needs to be placed at a certain angle with the incident laser, and this angle depends on the Brewster angle of the final output light wavelength we need. Since the Brewster angles of lights with different wavelengths are different, only the light with the wavelength ν5 is reflected according to the adjustment of the angle of the plate, and the reflected light is used as the final output light.

[0038] In this patent, the order of some of the 6 modules can be adjusted or combined. For example, the amplitude and pulse adjustment device 3 can operate on the light after the frequency selection resonant cavity 4; at the same time, the three modules of the frequency selection resonant cavity 4, the high-order harmonic module 5, and the light source output coupling module 6 in the embodiment can also be completely integrated together and placed in the vacuum chamber together (this is also a commonly used method in current experiments), as Figure 3 shown.

[0039] Adding the optical frequency conversion module 2 can realize a tunable light source in the 100nm - 300nm band; as Figure 4 shown, the high-order harmonic means that the initial input light frequency (the light frequency ω and the light wavelength λ satisfy ωλ = c, where c refers to the light speed in the same medium) is ω0, and the output light frequency after the high-order harmonic is ω = (2n + 1)ω0, where n refers to the order of the harmonic. Due to the problem of phase matching, the harmonic only has odd-order harmonics. As the order of the high-order harmonic increases, although the wavelength will decrease and change towards the ultraviolet band, at the same time, the optical power (amplitude) of its output light will also decay. According to different decay degrees, the high-order harmonics are divided into a plateau region and a decay region. In order to improve the optical power of the high-order harmonic output light as much as possible, the final output wavelength should be within the plateau region of the high-order harmonic.

[0040] In order to achieve a wavelength range after high-order harmonics that can cover the interval of 100 nm to 300 nm, by taking 100 nm as the lower edge and 300 nm as the upper edge for reverse calculation, and multiplying by odd multiples, we know that if the input wavelength of the initial high-order harmonic module 5 can cover the range of 900 nm to 2100 nm, the wavelength conversion of the 5th to 7th harmonics can be achieved, as shown in Table 1. Finally, the vacuum ultraviolet interval with a wavelength coverage of 100 nm to 300 nm is achieved. Moreover, according to experimental experience, the 7th order of high-order harmonics of xenon is still in the plateau region and meets the requirements.

[0041] Table 1. Odd harmonic multiples of frequency

[0042] Lower wavelength edge (nm) Upper wavelength edge (nm) Odd-order harmonic 100 300 1st 300 900 3rd 500 1500 5th 700 2100 7th 900 2700 9th

[0043] The specific embodiments described above further elaborate on the purpose, technical solutions, and beneficial effects of the present invention. It should be understood that the above are only specific embodiments of the present invention and are not used to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. Based on the frequency conversion system, it can cover the frequency-adjustable light source in the vacuum ultraviolet band, characterized by: It comprises an infrared or visible light band laser (1), an optical frequency conversion device (2), an amplitude and pulse adjustment device (3), a frequency-selective resonant cavity (4), a high-order harmonic module (5) and a light source output coupling module (6); The laser light generated by the infrared or visible light band laser (1) is frequency-broadened by an optical frequency conversion device (2), so that the single-frequency light source is converted into a continuous spectrum, and the continuous spectrum contains laser light of multiple frequencies; since the optical power and pulse width of the laser light will change after the frequency conversion, an amplitude and pulse adjustment device (3) is added after the optical frequency conversion device (2) to compensate the amplitude of the laser light and compress the pulse width of the laser light; The laser light broadened by the optical frequency conversion device (2) is sent to the frequency selection resonant cavity (4), and the laser frequency is selected according to the adjustment of the cavity length of the resonant cavity, so as to adjust the wavelength of the final output laser light; the laser light with the required wavelength band, amplitude and pulse width after adjustment is sent to the high-order harmonic module (5) to generate laser high-order harmonics, so that the frequency of the laser light is converted from the mid-infrared and visible light wavelength bands to the vacuum ultraviolet wavelength band, so as to achieve coverage of the vacuum ultraviolet wide wavelength band of 100 to 300 nm; the laser light with the final required frequency is efficiently distinguished and selected by using the output coupling module (6) for subsequent detection and application.

2. According to claim 1, the frequency-adjustable light source based on the frequency conversion system can cover the vacuum ultraviolet band, characterized in that: The six components mentioned above are connected by optical paths using spatial light or by optical fibers.

3. According to claim 1, the frequency-adjustable light source based on the frequency conversion system can cover the vacuum ultraviolet band, characterized in that: The infrared or visible light band laser (1) is used to provide a basic laser light source, and the wavelength range is 700-1100nm.

4. According to claim 1, the frequency-adjustable light source based on the frequency conversion system can cover the vacuum ultraviolet band, characterized in that: The optical frequency conversion device (2) is used to change the laser frequency output by the laser (1), and the spectrum after frequency conversion is required to be a continuous spectrum, and the wavelength coverage range should include between 900nm and 2100nm; the optical frequency conversion device (2) is a device that widens the laser spectrum over a large range, including nonlinear optical crystals and gas-filled photonic crystal fibers.

5. According to claim 1, the frequency-adjustable light source based on the frequency conversion system can cover the vacuum ultraviolet band, characterized in that: The amplitude and pulse adjustment device (3) is used to improve the optical parameters of the light source from the frequency-selective resonant cavity (3); for the pulse light and the optical frequency comb, the amplitude is increased and the pulse is compressed, and the method includes using an optical fiber amplifier and a gain medium to amplify the laser amplitude; using a chirped mirror, multiple pairs of parallel grating mirrors, and multiple pairs of prisms to compress the pulse; For a continuous laser, the optical power amplification function is achieved by adding an optical power amplifier or a gain medium to the amplitude and pulse adjustment device (3).

6. According to claim 1, the frequency-adjustable light source based on the frequency conversion system that can cover the vacuum ultraviolet band is characterized by: The frequency-selective resonant cavity (4) is used to retain laser light of a required wavelength band through the selection of the frequency-selective resonant cavity for the frequency-broadened light source output from the optical frequency conversion device (2).

7. According to claim 1, the frequency-adjustable light source based on the frequency conversion system that can cover the vacuum ultraviolet band is characterized by: The high-order harmonic module (5) is used to change the light source from the infrared band to the ultraviolet band, and is an important module for generating vacuum ultraviolet band lasers; as the high-order harmonic order increases, the wavelength of the generated laser becomes smaller, and the output spectrum range can eventually meet the vacuum ultraviolet band of 100 to 300 nm; the method for the high-order harmonic module (5) to realize high-order harmonics includes gas high-order harmonics, solid high-order harmonics, and plasma flow high-order harmonics.

8. According to claim 1, the frequency-adjustable light source based on the frequency conversion system that can cover the vacuum ultraviolet band is characterized by: The light source output coupling system (6) is used to distinguish and output lasers of multiple wavelengths generated by the high-order harmonics (5), remove interference from other wavelength bands, and retain only ultraviolet light of the required order; the output coupling method includes using a Brewster plate, a diffraction grating mirror, processing a small hole behind the output reflector, and a non-collinear output coupling method.

9. According to claim 1, the frequency-adjustable light source based on the frequency conversion system that can cover the vacuum ultraviolet band is characterized by: The frequency light source is changed by combining an optical frequency conversion device (2) and a frequency selection resonant cavity (4) to achieve frequency conversion of a wide spectrum wavelength; the optical frequency conversion device (2) and the high-order harmonic module (5) are used to convert the mid-infrared and visible light band lasers to the vacuum ultraviolet band.

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