Nanosecond window signal-to-noise ratio measuring device based on anti-saturable absorber and image transmission

Abstract

The invention discloses a nanosecond window signal-to-noise ratio measuring device based on an anti-saturable absorber and image transmission. The nanosecond window signal-to-noise ratio measuring device comprises a spectroscope, a first attenuation unit, a first lens, a first detector, a second attenuation unit, a reflector, a second lens, a transparent box filled with a water medium, a third lens, a second detector and a high-speed oscilloscope. Based on the anti-saturation absorption effect in the quartz box and the point-to-point image transfer effect of the lens group, the device can effectively reduce the damage of the pulse to the second detector, and ensures that the second detector is not interfered when measuring weak noise, thereby realizing the accurate measurement of the signal-to-noise ratio in a high dynamic range. In addition, the water medium has excellent recoverability, and has the characteristics of long service life and strong durability. According to the measuring device, the dynamic range of signal-to-noise ratio measurement is obviously expanded and can reach 1011 or even higher, and a reliable scheme is provided for accurate diagnosis of the signal-to-noise ratio of a high-power laser system.

Description

Technical Field

[0001] The present invention relates to parameter diagnosis of ultrashort laser pulses, and is a high-dynamic-range signal-to-noise ratio measurement device applicable to high-power nanosecond, picosecond, and femtosecond pulses. Background Art

[0002] High-power lasers are widely used in fields such as inertial confinement fusion, high-intensity γ-ray secondary radiation sources, and particle acceleration experiments. Currently, multiple petawatt laser devices have been built globally, and their focused power density can reach 10 22 W / cm 2 or higher. However, when the intensity of the laser prepulse and the pedestal noise exceed 10 12 W / cm 2 , a pre-plasma will be generated on the surface of the target material, affecting the effective action of the main pulse. Therefore, accurately measuring the signal-to-noise ratio on the nanosecond time scale, especially ensuring a high pulse contrast before the main pulse, is crucial for optimizing the laser system. Currently, signal-to-noise ratio measurement usually uses a combination of a photodiode and a high-speed oscilloscope. However, within the nanosecond time window before the main pulse, the detector needs to withstand extremely high optical power, is prone to saturation and even damage, affecting the measurement accuracy and shortening the service life. Since it is difficult for the detector to work for a long time without being damaged, existing methods have limitations in dynamic range expansion and weak noise detection.

[0003] The OMEGA EP petawatt laser device in the United States reduces the spot pointing sensitivity by using a diffuser and corrects the nonlinear effect of the detector, enabling the measurement time resolution to reach 200 ps and the dynamic range to be increased to 10 10 . However, the photodiode of this system is prone to saturation within 1 ns before the main pulse, and deconvolution technology needs to be used to correct the measurement results, and the measured optical power must be strictly controlled below 1 μJ to avoid damaging the detector (CLEO JThE117).

[0004] The Orion petawatt laser device in the United Kingdom uses an occluded photodiode and adds a spectral filter to reduce the influence of stray light in the diagnostic cavity, enabling the dynamic range of pulse contrast measurement to reach 10 10 . However, this scheme greatly shortens the service life of the photodiode and the diagnostic optical components (Journal of Physics: Conference Series 688 (2016) 012030).

[0005] Ouyang Xiaoping of the Shanghai Institute of Optics and Fine Mechanics proposed an optical amplification high-dynamic range nanosecond window signal-to-noise ratio measurement device (CN 114279579 A) in 2021. This solution enhances the noise signal by introducing an optical amplifier into the noise detection optical path. Due to the gain saturation effect, the intensity of the main pulse remains basically unchanged, thereby improving the dynamic range of signal-to-noise ratio measurement. However, this method still has certain limitations in practical applications. First, signal distortion and nonlinear effects may be introduced during the noise amplification process, affecting the authenticity and accuracy of the measurement. Summary of the Invention

[0006] The problem to be solved by the present invention is that the existing signal-to-noise ratio measurement technology based on the parallel working mode of photodetectors cannot further improve the dynamic range, and the photodetectors are vulnerable to damage by high-intensity main pulses.

[0007] The technical solution of the present invention is as follows:

[0008] A new type of high-dynamic range signal-to-noise ratio measurement device is established, which consists of a beam splitter, a first attenuation unit, a first lens, a first detector, a second attenuation unit, a mirror, a second lens, a transparent box filled with a water medium, a third lens, a second detector, and a high-speed oscilloscope.

[0009] The characteristics of this signal-to-noise ratio measurement device are as follows: (1) Based on the interaction between laser and matter, due to the strong energy of the main pulse signal, plasma is generated after interacting with the medium in water, resulting in a significant attenuation of its energy, thus effectively protecting the detector from damage by high-intensity main pulses. The extremely weak noise signal has almost no effect on its propagation because its energy is not sufficient to generate plasma, and the energy change can be ignored. At the same time, the fluid effect of the water medium makes the measurement device have the advantages of strong recoverability, no need to be replaced following the laser firing times, and good durability. (2) Based on the point-to-point image transfer system composed of the second lens and the third lens, combined with the nonlinear effects, deflection, scattering, and absorption occurring in water, the energy coupled into the detector after the main pulse passes through the transparent box filled with the water medium is significantly reduced, avoiding the direct action of high-intensity light beams on the second detector, further protecting the detector and improving the reliability of the system.

[0010] The positional relationship of the above components is as follows: The measured pulse is split into two beams of light by the beam splitter. One transmitted beam first enters the first attenuation unit, then passes through the first lens and enters the first detector; the other reflected beam enters the second attenuation unit, then passes through the second lens, is focused in the transparent box filled with the water medium, then passes through the third lens, and finally enters the second detector. The first detector and the second detector transmit the received signals to the high-speed oscilloscope together.

[0011] The beam splitter mentioned above is a beam splitter with a transmittance of 0.1% and a reflectance of 99.9%.

[0012] The first attenuation unit and the second attenuation unit mentioned above are absorption-type attenuation filters.

[0013] The mirror mentioned above is a mirror with a reflectance of ≥95%.

[0014] The first detector and the second detector mentioned above are fast-response phototubes, diodes and other detection devices based on the photoelectric effect.

[0015] The first lens, the second lens and the third lens mentioned above are plano-convex lenses or biconvex lenses.

[0016] The transparent box filled with the water medium is made of quartz, K9 glass or sapphire crystal.

[0017] The working steps of the present invention are as follows:

[0018] 1) The high-power measured pulse enters the measurement device through the beam splitter.

[0019] 2) The transmitted light first enters the first attenuation unit, then passes through the first lens and enters the first detector to detect the intensity of the main pulse.

[0020] 3) The reflected light enters the second attenuation unit, then is focused by the second lens. The focused light beam passes through the transparent box filled with the water medium, then passes through the third lens, and finally enters the second detector for detecting weak noise.

[0021] 4) The first detector and the second detector transmit the received signals to the high-speed oscilloscope together. Combining with the calibrated attenuation ratio in the attenuation unit, the intensities of the subsequent noise and the main pulse are inverted to obtain the signal-to-noise ratio of the laser pulse.

[0022] The beneficial technical effects of the present invention:

[0023] (1) The present invention utilizes the interaction between light and matter to effectively protect the detector, thereby expanding the high dynamic range of the measurement device. Since the energy of the main pulse is relatively high, a nonlinear effect occurs in the water medium, resulting in a significant attenuation of the energy, thereby reducing the light intensity entering the detector and avoiding damage to the detector by high-power laser. The noise signal has a relatively low energy and arrives before the main pulse in the time window, and cannot trigger a significant nonlinear effect, and its energy remains basically unchanged, ensuring the effective transmission of the noise signal. In addition, the flowing water medium has excellent recoverability, long life and high durability, ensuring the stability and reliability of the long-term operation of the measurement device.

[0024] (2) Through the focusing of the second lens, the present invention utilizes the non-linear deflection mechanism of intense laser at the focal point, combined with the point-to-point image transfer of the third lens, to further weaken the energy of the main pulse entering the detector and reduce the direct impact of the high-intensity beam on the detector. This design not only avoids damaging the detector but also ensures its ability to effectively detect weaker noise signals, thereby significantly enhancing the dynamic range of SNR measurement. Brief Description of the Drawings

[0025] Figure 1 Fig. is a schematic structural diagram of Embodiment 1 of the nanosecond window SNR measurement device based on an anti-saturable absorber and image transfer according to the present invention.

[0026] Figure 2 Fig. is an energy comparison diagram measured after the main pulse passes through the transparent box filled with water medium in Embodiment 1 of the nanosecond window SNR measurement device based on an anti-saturable absorber and image transfer according to the present invention. Detailed Embodiments

[0027] The present invention will be further described below in conjunction with embodiments and the drawings, but the protection scope of the present invention should not be limited thereby.

[0028] Please refer to Figure 1 , Figure 1 which is a schematic structural diagram of Embodiment 1 of the present invention. The structure of the device includes: a beam splitter 1, a first attenuation unit 2, a first lens 3, a first detector 4, a second attenuation unit 5, a mirror 6, a second lens 7, a transparent box 8 filled with water medium, a third lens 9, a second detector 10, and a high-speed oscilloscope 11. The pulse to be measured is split by the beam splitter 1 in sequence. The transmitted light enters the first detector 4 after passing through the first attenuation unit 2 and the first lens 3 in sequence; the reflected light passes through the second attenuation unit 5, the mirror 6, the second lens 7, and the transparent box 8 filled with water medium, and then enters the second detector 10 through the third lens 9. The signals received by the first detector 4 and the second detector 10 are transmitted to the high-speed oscilloscope 11 for analysis. In this optical path, the first attenuation unit 2 and the second attenuation unit 5 are absorption-type attenuation sheets, the size of the transparent box 8 filled with water medium is 30mm×20mm×40mm, the material of the transparent box is quartz, the focal lengths of the first lens 3 and the second lens 7 are both 25mm, the focal length of the third lens 9 is 50mm, the first detector 4 and the second detector 10 are fast-response phototubes, and the reflectivity of the mirror 6 is ≥95%.

[0029] Working steps:

[0030] Taking the incident pulse parameters of energy 4mJ, pulse width 1ps, beam aperture 10mm, and power density 5×10 9 W / cm 2 as an example. The specific working steps of the device are as follows:

[0031] 1) Spectral splitting processing. The measured pulse is first split into two parts, the transmitted light and the reflected light, by a beam splitter. The transmittance of the beam splitter is 0.1%, and the reflectance is 99.9%. The transmitted light is used for the measurement of the main pulse signal, and the reflected light is used for the measurement of the noise signal. The two optical signals are transmitted through independent optical paths to ensure dynamic separation and reduce the direct damage of the high-power main pulse to the detector.

[0032] 2) Acquisition of the main pulse signal of the transmitted light. The transmitted light with a power density of 5×10 6 W / cm 2 passes through the first attenuation unit (calibration magnification is T1) and the first lens in sequence and is focused and then incident on the first detector. The first detector converts the main pulse optical signal into an electrical signal and transmits it to a high-speed oscilloscope for obtaining the signal intensity of the main pulse. Assuming that the output voltage of the detector corresponding to the main pulse is V1, combined with the calibration magnification T1 of the attenuation unit, the actual power density of the main pulse can be inversely calculated.

[0033] 3) Acquisition of the noise signal of the reflected light. The reflected light with a power density of 5×10 9 W / cm 2 passes through the second attenuation unit (calibration magnification is T2) and the mirror in sequence and then is transmitted to the point-to-point image transfer system of the lens group composed of the second lens and the third lens. A transparent box filled with a water medium is placed at the focal point of this system. The anti-saturation absorption effect of the water medium is used to significantly attenuate the energy of the main pulse, and at the same time, the image transfer effect of the lens group is used to adjust the propagation direction of the main pulse to protect the second detector. As Figure 2 shown, the energy comparison diagram before and after passing through the transparent box filled with the water medium measured in this embodiment. The light beam attenuated by the transparent box filled with the water medium is incident on the second detector. The second detector converts the noise signal into an electrical signal and transmits it to the high-speed oscilloscope to obtain the intensity of the weak noise signal. Assuming that the output voltage of the detector corresponding to the noise signal is V2, combined with the calibration magnification T2 of the attenuation unit, the actual power density of the noise signal can be inversely calculated.

[0034] 4) Signal intensity inversion and initial signal-to-noise ratio calculation. The voltage value V1 of the main pulse and the voltage value V2 of the noise signal are obtained separately through the high-speed oscilloscope. Combined with the calibration magnifications T1 and T2 of the first attenuation unit and the second attenuation unit, the calculation formula for the signal-to-noise ratio is:

[0035] SNR=(T1×V1) / (T2×V2)

[0036] For example, when the main pulse power density is 5×10 9 W / cm 2 , and the noise signal power density is 5×10-2W / cm 2, when the output voltages of the corresponding detectors are V1 = 600 mV and V2 = 0.6 μV respectively, and the calibration magnification factors are T1 = 100000 and T2 = 10, the signal-to-noise ratio is:

[0037] SNR = (100000 × 600 mV) / (10 × 0.6 μV) = 10 10

[0038] At this time, the dynamic range of the initial signal-to-noise ratio is 10 10 。

[0039] 5) Dynamic range extension. Through the design of the reflection optical path and the quartz cell, the main pulse is significantly attenuated in the quartz cell, reducing the impact of the high-power main pulse on the second detector. This allows the calibration magnification factor T2 of the second attenuation unit to be reduced to T3 (for example, from 10 to 1), while ensuring that the voltage value V3 of the noise signal is close to V2, and the main pulse voltage value remains V1. The signal-to-noise ratio calculation formula becomes:

[0040] SNR = (T1 × V1) / (T3 × V3)

[0041] Assume that the calibration magnification factor of the second attenuation unit is reduced by 10 times, that is, from T2 = 10 to T3 = 1, then the dynamic range of the signal-to-noise ratio is extended to:

[0042] SNR = (100000 × 600 mV) / (1 × 0.6 μV) = 10 11

[0043] By reducing the attenuation magnification factor to extend the dynamic range, the signal-to-noise ratio is increased from the initial 10 10 to 10 11 or even higher, while effectively protecting the second detector and extending its service life.

[0044] In summary, a nanosecond window signal-to-noise ratio measurement device based on reverse saturable absorber and image transfer proposed by the present invention can effectively protect the photodetector from being damaged by the main pulse, thereby providing the feasibility of high dynamic range extension of the measurement device. Theoretically, the dynamic range can reach 10 11 , and has the advantages of strong recoverability and good durability.

Claims

1. A nanosecond window signal-to-noise ratio measurement device based on an anti-saturable absorber and image transfer, characterized in that: The device comprises a spectroscope (1), a first attenuation unit (2), a first lens (3), a first detector (4), a second attenuation unit (5), a reflector (6), a second lens (7), a transparent box containing an aqueous medium (8), a third lens (9), a second detector (10) and a high-speed oscilloscope (11); The pulse to be measured is divided into two paths through a spectroscope (1), one path of transmitted light passes through a first attenuation unit (2) and a first lens (3) in sequence and then enters a first detector (4), which is used for intensity detection of the main pulse; the other path of reflected light passes through a second attenuation unit (5), a reflector (6) and a second lens (7) in sequence, is focused in a transparent box (8) filled with water, and then passes through a third lens (9) and enters a second detector (10), which is used for intensity detection of noise; the first detector (4) and the second detector (10) transmit the received signals together to a high-speed oscilloscope (11).

2. The nanosecond window signal-to-noise ratio measurement device based on anti-saturable absorber and image transfer according to claim 1, characterized in that: The transparent box (8) containing the water medium is used to attenuate the energy of the main pulse signal, ensuring the normal operation of the second detector (10), thereby achieving the expansion of the high dynamic range of the measuring device.

3. The nanosecond window signal-to-noise ratio measurement device based on anti-saturable absorber and image transfer according to claim 1, characterized in that: The transmittance of the beam splitter (1) is 0.1% and the reflectivity is 99.9%.

4. The nanosecond window signal-to-noise ratio measurement device based on anti-saturable absorber and image transfer according to claim 1, characterized in that: The first attenuation unit (2) and the second attenuation unit (5) are absorption-type attenuation sheets.

5. The nanosecond window signal-to-noise ratio measurement device based on anti-saturable absorber and image transfer according to claim 1, characterized in that: The reflectivity of the reflector (6) is ≥95%.

6. The nanosecond window signal-to-noise ratio measurement device based on anti-saturable absorber and image transfer according to claim 1, characterized in that: The first detector (4) and the second detector (10) are detection devices based on the photoelectric effect, and include fast-response photoelectric tubes or diodes.

7. The nanosecond window signal-to-noise ratio measurement device based on anti-saturable absorber and image transfer according to claim 1, characterized in that: The first lens (3), the second lens (7) and the third lens (9) are plano-convex lenses or bi-convex lenses.

8. The nanosecond window signal-to-noise ratio measurement device based on anti-saturable absorber and image transfer according to claim 1, characterized in that: The transparent box (8) containing the water medium is made of quartz, K9 glass or sapphire crystal.

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