Compact single ultrafast framing direct imaging device

Abstract

The invention is suitable for the field of ultrafast optical imaging, and provides a compact single ultrafast framing direct imaging device, which comprises a femtosecond laser; a frequency multiplier; the light splitting assembly is used for carrying out light splitting processing on the frequency-doubled light and the residual fundamental frequency light which is not subjected to frequency doubling; the stretcher is used for carrying out pulse broadening on the fundamental frequency light to obtain signal light; the time sequence space coding system is used for dividing the signal light after the object is irradiated into an equal-light-intensity sub-beam array in two-dimensional distribution, and adding different time delays to each sub-beam in the array to obtain an illumination signal light array with different time delays carrying object information; the delayer is used for adjusting the delay amount of the pump light, so that the pump light and the illumination signal light array are synchronized; and the optical parametric amplification imaging system is used for coupling the illumination signal light array and the delayed pump light, generating idler frequency light carrying object information and carrying out imaging recording. According to the device, single-time multi-image imaging is realized while high time-space resolution is kept.

Description

Technical Field

[0001] The present invention belongs to the technical field of ultrafast optical imaging, and particularly relates to a compact single-shot ultrafast framing direct imaging device. Background Art

[0002] With the rapid development of ultrafast optical technology, the demand for high-time-resolution imaging of transient phenomena is increasing day by day. As an important means to capture the dynamic evolution process of transient events, ultrafast framing imaging technology is widely used in the fields of physics, chemistry, biomedicine, and materials science.

[0003] However, traditional ultrafast framing imaging technology often faces problems such as complex systems, limited time resolution, or insufficient sampling frames, and usually requires multiple repeated experiments to obtain information of an object at different times, and it is impossible to achieve single-shot capture.

[0004] Although there are also ultrafast imaging technologies in the prior art that specifically achieve single-shot capture, that is, multiple pieces of information of an object at different times can be obtained with a single exposure (single shutter), there are certain defects. According to different detection means, the existing ultrafast imaging technologies that achieve single-shot capture can be divided into computational imaging and direct imaging. Among them, computational imaging mainly realizes single-shot recording through optical encoding and computational decoding, while direct imaging does not require algorithms and can directly perform single-shot recording by imaging onto a camera through optical means.

[0005] Direct imaging technology is more intuitive and reliable than computational imaging, but the existing direct imaging technology has problems such as low time resolution and complex systems. For example, the STAMP technology proposed by the University of Tokyo achieves femtosecond-level time resolution and multi-frame imaging through a complex optical system, but its device structure is complex, which limits its practical application. To solve this problem, they proposed a method based on beam splitting by diffractive optical elements and bandpass filtering to develop spectral filtering-temporal all-optical photography (SF-STAMP), which greatly simplifies the system structure, but this technology is difficult to achieve high spatial resolution, and increasing the number of frames will lead to a decrease in the spatial resolution of each sub-image, and too many sub-beams will also lead to a decrease in the light intensity of each channel, thereby affecting the imaging quality.

[0006] Therefore, developing a single-shot ultrafast framing direct imaging device with high time and space resolution and high frame rate can not only effectively simplify the system structure but also achieve multi-frame imaging while maintaining high time resolution. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide a compact single-shot ultrafast framing direct imaging device, aiming to achieve single-shot multi-frame imaging while maintaining high time and space resolution.

[0008] To solve the above technical problems, the present invention is implemented as follows. A compact single-shot ultrafast framing direct imaging device includes: A femtosecond laser for generating femtosecond laser pulses; A frequency doubler for doubling the femtosecond laser pulses to obtain frequency-doubled light and the remaining non-doubled fundamental light, where the frequency-doubled light is used as pump light; A beam splitting assembly for splitting the frequency-doubled light and the fundamental light; A stretcher for stretching the fundamental light pulses to obtain signal light that can cover the entire ultrafast event time window; the signal light is used to irradiate the object to be imaged; A time-sequential spatial encoding system for splitting the detection light after irradiating the object into an array of equal-intensity sub-beams with a two-dimensional distribution and adding different time delays to each sub-beam in the array to obtain an array of illumination signal lights with different time delays carrying object information; A delay device for adjusting the delay amount of the pump light so that the pump light is synchronized with the array of illumination signal lights; An optical parametric amplification imaging system for coupling the illumination signal light and the pump light after adjusting the delay to generate idler light carrying object information and imaging and recording the idler light.

[0009] The compact single-shot ultrafast framing direct imaging device provided by the present invention can achieve single-shot multi-frame ultrafast imaging of transient processes, specifically by combining a time-sequential spatial encoding system and an optical parametric amplification imaging system. Among them, the time-sequential spatial encoding system can split the detection light after irradiating the object into an array of equal-intensity sub-beams with a two-dimensional distribution and add different time delays to each sub-beam in the array to obtain illumination signal light carrying object information. The various parameter indicators of imaging are independent of each other. The imaging frequency depends on the time interval introduced by the time-sequential spatial encoding system, the time resolution and spatial resolution depend on the pump pulse width and spatial bandwidth of the optical parametric amplification imaging system, and the number of frames depends on the number of beam splittings of the time-sequential spatial encoding system. Therefore, it simultaneously has high imaging frequency, time resolution, spatial resolution, and number of frames. And the time-sequential spatial encoding system has a simple and compact structure. This compact design not only reduces the complexity and cost of the system, improves the stability and reliability of the system, but also significantly increases the number of imaging frames, and is applicable to various scenarios such as laboratories and industrial sites. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is the optical structure diagram of the compact single-shot ultrafast framing direct imaging device provided by the present invention; Figure 2 is Figure 1 the optical structure diagram of the time-sequential encoding system in Figure 3 isFigure 2 Structural schematic diagram of the stepped mirror in Figure 4A , Figure 4B is Figure 3 Delay schematic diagram of the stepped mirror in Figure 5 is Figure 1 Optical structure diagram of the optical parametric amplification imaging system in Specific implementation manner

[0010] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0011] Referring to Figure 1 , the compact single-shot ultrafast framing direct imaging device provided by the present invention includes a femtosecond laser 1, a frequency doubler 2, a beam splitting component 3, a stretcher 4, a time-sequential spatial encoding system 5, a delay device 6, and an optical parametric amplification imaging system 7.

[0012] Among them, the femtosecond laser 1 is used to generate femtosecond laser pulses. As an example, a titanium-sapphire femtosecond laser can be used to output femtosecond laser pulses with a pulse width of 25 fs, a pulse energy of 3.5 mJ, a central wavelength of 800 nm, and a corresponding spectral width of 60 nm.

[0013] The frequency doubler 2 is used to double the frequency of a part of the femtosecond laser pulses to obtain second harmonic generation light and the remaining fundamental frequency light that has not been frequency doubled. There are no restrictions on the material selection. For example, a (beta-barium borate) crystal with a 29.2° cutting angle and a thickness of 0.2 mm can be selected. The femtosecond laser pulses are incident perpendicularly to the crystal surface to obtain 400-nm second harmonic generation light and the un-converted 800-nm fundamental frequency light. The beam splitting component 3 is used to split the second harmonic generation light and the fundamental frequency light. The second harmonic generation light is used as the pump light of the system subsequently. Specifically, a dichroic mirror can be used. The dichroic mirror has the function of splitting two kinds of light with different frequencies, and can reflect the second harmonic generation light and transmit the remaining fundamental frequency light that has not been frequency doubled.

[0014]

[0015] ​The stretcher 4 is used to broaden the fundamental frequency light pulse to obtain a probe light that can cover the time window of ultrafast events; the probe light is used to irradiate the object to be imaged. The stretcher 4 can be a commonly used pulse disperser, which broadens the femtosecond laser pulse through its dispersion characteristics. For example, taking the above 25 fs laser pulse as an example, it can be broadened into a chirped laser pulse with a central wavelength of 790 nm and a pulse width of 100 ps, which can cover the time window of ultrafast events. Of course, in specific implementation, the femtosecond laser pulse can also be broadened into any required pulse width according to actual needs.

[0016] The time-sequential spatial encoding system 5 is used to divide the probe light after irradiating the object into an array of sub-beams with equal light intensity distributed two-dimensionally, and attach different time delays to each sub-beam in the array to obtain an array of illumination signal lights with different time delays carrying object information.

[0017] Further referring to Figure 2 , the time-sequential spatial encoding system 5 may include a diffractive optical element 51 and a time delay array 52. The diffractive optical element 51 is used to divide the probe light after irradiating the object into an array of sub-beams with equal light intensity distributed two-dimensionally, and the time delay array 52 is used to attach different time delays to each sub-beam in the array to obtain an illumination signal light carrying object information.

[0018] The diffractive optical element 51 can specifically be a Dammann grating or a microlens array. The Dammann grating is a phase-type binary optical element that can efficiently convert the incident monochromatic light into an equally spaced dot matrix with uniform light intensity in the far field of Fourier transform, avoiding the non-uniform light intensity distribution caused by single-hole diffraction of a general grating and having a high diffraction efficiency. Taking order Dammann grating as an example, the amplitude transmittance function of the Dammann grating can be expanded into a Fourier series as .

[0019] In the formula, represents the grating coefficient, is the Dirac function, d is the period of the grating, and are the two-dimensional coordinates on the grating plane. The amplitude transmittance function describes the modulation of light by the grating in the and directions.

[0020] Therefore, after passing through the Dammann grating, the light field is divided into an array of sub-beams with equal light intensity in the far field. Taking a two-dimensional order Dammann grating as an example, the signal light is divided into 49 sub-beams with equal light intensity arranged two-dimensionally in space after passing through the Dammann grating.

[0021] As an example, as Figure 2 shown, the time delay array 52 includes a first stepped mirror 521 and a second stepped mirror 522 arranged in sequence. The first stepped mirror 521 is used to add a delay to each sub-beam in the array in a first direction, and the second stepped mirror 522 is used to add a delay to each sub-beam in the array in a second direction, where the first direction and the second direction are orthogonal. For example, the first direction is the horizontal direction and the second direction is the vertical direction, or the first direction is the vertical direction and the second direction is the horizontal direction.

[0022] Figure 3 The structures of the first stepped mirror 521 and the second stepped mirror 522 are further shown. Both the first stepped mirror 521 and the second stepped mirror 522 are composed of glasses with two different refractive indices, and the steps of the two glasses are engaged with each other to form an overall cuboid shape. The materials of these two glasses are different, which is equivalent to having different refractive indices, and each step has a different thickness combination. After passing through different steps, the sub-beams are added with different time delays due to the optical path difference.

[0023] For example, referring to Figure 4A and Figure 4B , the stepped mirror I is composed of and glass, and the stepped mirror II is composed of and glass. The group refractive index of the glass n g2 is 1.50669, The group refractive index of the glass n g1 is 1.50092, The group refractive index of the glass n g3 is 1.54170, so the group refractive index difference of the stepped mirror I .

[0024] Similarly, the group refractive index difference of the stepped mirror II can be obtained. The length of each step of the stepped mirror I is 4.036 mm, the length of each step of the stepped mirror II is 4.600 mm, and the width between each step of the stepped mirror is 3.7 mm. The time delay of the stepped mirror I can be obtained, and the time delay of the stepped mirror II , where c is the speed of light.

[0025] Furthermore, the time-sequential space encoding system 5 may further include: a first lens 53, located between the diffractive optical element 51 and the time delay array 52, for collimating the sub-beam array output by the diffractive optical element 51 and then making it incident on the time delay array 52 in parallel.

[0026] Furthermore, the time-sequential space encoding system 5 may further include: a second lens 54, located after the time delay array 52, for focusing the illumination signal light onto the optical parametric amplification imaging system 7.

[0027] Continue to refer to Figure 1 , the optical delay device 6 is used to adjust the delay amount of the pump light so that the pump light is synchronized with the illumination signal light array. The overlapping part of the picosecond signal photon pulse sequence in time is much larger than the pump light pulse width, and it is easy to achieve the time synchronization of the pump light with each beam of signal photon pulses simultaneously.

[0028] The optical parametric amplification imaging system 7 is used to couple the illumination signal light array and the pump light after adjusting the delay, generate the idler light carrying the object information, and image and record the idler light.

[0029] Figure 5 The structure of the optical parametric amplification imaging system 7 is further shown, including an optical parametric amplifier 71, a filter 72, a third lens 73, and a CCD camera 74.

[0030] Among them, the optical parametric amplifier 71 is used to perform non-linear optical parametric coupling and amplification on the illumination signal light and the pump light. The amplified light beam includes the idler light carrying the object information, the illumination signal light, and the pump light. Specifically, an optical parametric crystal with a 29.2° cutting angle and a thickness of 0.5 mm can be selected. The illumination signal light array and the pump light are incident on the surface of the optical parametric crystal in a collinear matching manner at normal incidence, and 49 idler photon beams with a central wavelength of 810 nm are generated through optical parametric conversion.

[0031] The filter 72 is located after the optical parametric amplifier 71 and is used to filter the light beam coupled and amplified by the optical parametric amplifier 71, allowing only the idler light to pass through. After filtering by the filter, the idler photon beam is separated from the illumination signal light and the pump light.

[0032] The third lens 73 is used to image the idler light passing through the filter 72.

[0033] The CCD camera 74 is used to record the image formed by the third lens 73.

[0034] Since each detected photon beam has a different time delay, after the optical parametric amplification effect, the pump light is equivalent to a short shutter, and each idler photon beam carrying information of the object at different moments is sampled. Therefore, the information of 49 different moments of the transient event can be captured on a CCD camera 74 in a single exposure.

[0035] It should also be noted that if some reflection and refraction elements are added in this device only to change the propagation direction of light, it still belongs to the content scope of this patent. For example, in Figure 1 , the single-shot ultrafast framing direct imaging device further includes a mirror 8, which is located on the reflection optical path of the dichroic mirror and is used to reflect the second harmonic light to the retarder 6.

[0036] In summary, the compact single-shot ultrafast framing direct imaging device provided by the present invention has the following advantages: 1. It is realized by combining a time-sequential spatial encoding system and an optical parametric amplification imaging system. Among them, the time-sequential spatial encoding system can divide the detected light after irradiating the object into an array of sub-beams with equal light intensity in a two-dimensional distribution, and attach different time delays to each sub-beam in the array to obtain the illumination signal light carrying object information. Each parameter index of imaging is independent of each other. The imaging frequency depends on the time interval introduced by the time-sequential spatial encoding system, the time resolution and spatial resolution depend on the pump pulse width and spatial bandwidth of the optical parametric amplification imaging system, and the number of frames depends on the number of beam splittings of the time-sequential spatial encoding system. Therefore, it simultaneously has high imaging frequency, time resolution, spatial resolution and number of frames.

[0037] 2. Through the time-sequential spatial encoding system, this compact design not only reduces the complexity and cost of the system, improves the stability and reliability of the system, but also significantly increases the number of frames of imaging, and is applicable to various scenarios such as laboratories and industrial sites. 3. The introduction of the nonlinear optical parametric amplification technology also provides the imaging system with high gain and wavelength conversion capabilities. The high-gain characteristic can significantly amplify weak light signals, so the system has high sensitivity. The wavelength conversion characteristic enables the detection wavelength of imaging to be extended to mid-infrared and other bands, which can meet the requirements of different application scenarios for different imaging wavelengths.

[0038] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A compact single-shot ultrafast framing direct imaging device, characterized in that: include: A femtosecond laser for generating femtosecond laser pulses; A frequency doubler, used for frequency doubling the femtosecond laser pulse to obtain frequency-doubled light and remaining un-frequency-doubled fundamental frequency light, wherein the frequency-doubled light is used as pump light; A light splitting component, used for performing light splitting processing on the frequency-doubled light and the fundamental frequency light; A stretcher, used for stretching the pulse of the fundamental frequency light to obtain a signal light that can cover the entire ultrafast event time window; the signal light is used to illuminate the object to be imaged; A temporal spatial coding system is used to divide the detection light after irradiating the object into a two-dimensionally distributed array of equal-intensity sub-beams, and add different time delays to each sub-beam in the array to obtain an array of illumination signal lights with different time delays carrying object information; A delay device, used for adjusting the delay amount of the pump light so as to synchronize the pump light with the illumination signal light array; The optical parametric amplification imaging system is used to couple the illumination signal light array and the pump light after adjusting the delay, generate idler light carrying object information, and record the idler light by imaging.

2. The single-shot ultrafast framing direct imaging device according to claim 1, characterized in that: The light splitting component is a dichroic mirror, which is used to reflect the doubled frequency light and transmit the remaining undoubled fundamental frequency light.

3. The single-shot ultrafast framing direct imaging device according to claim 2, characterized in that: The single-shot ultrafast framing direct imaging device further comprises: A reflector is located on the reflection light path of the dichroic mirror and is used to transmit the frequency-doubled light to the delay device.

4. The single-shot ultrafast framing direct imaging device according to claim 1, characterized in that: The temporal space coding system comprises: A diffractive optical element, used for dividing the detection light after irradiating the object into a sub-beam array with equal light intensity and two-dimensional distribution; The time delay array is used to add different time delays to each sub-beam in the array to obtain an illumination signal light array with different time delays that carries object information.

5. The single-shot ultrafast framing direct imaging device according to claim 4, characterized in that: The diffractive optical element is a Dammann grating.

6. The single-shot ultrafast framing direct imaging device according to claim 4, characterized in that: The time delay array comprises a first step mirror and a second step mirror placed in sequence; The first step mirror is used to add a delay to each sub-beam in the array in a first direction, and the second step mirror is used to add a delay to each sub-beam in the array in a second direction; The first direction and the second direction are orthogonal.

7. The single-shot ultrafast framing direct imaging device according to claim 6, characterized in that: The first step mirror and the second step mirror are both made of glass with two different refractive indices, and the steps of the two glasses are interlocked with each other and are in a rectangular shape as a whole.

8. The single-shot ultrafast framing direct imaging device according to claim 4, characterized in that: The temporal space coding system also includes: The first lens is located between the diffractive optical element and the time delay array, and is used for collimating the sub-beam array output by the diffractive optical element and then parallelly incident on the time delay array.

9. The single-shot ultrafast framing direct imaging device according to claim 4, characterized in that: The temporal space coding system also includes: The second lens is located after the time delay array and is used to focus the illumination signal light onto the optical parametric amplification imaging system.

10. The single-shot ultrafast framing direct imaging device according to claim 1, characterized in that: The optical parametric amplification imaging system also includes: An optical parametric amplifier, used for performing nonlinear optical parametric coupling amplification on the illumination signal light and the pump light, wherein the amplified light beam includes idler light carrying object information; An optical filter, located after the optical parametric amplifier, for filtering the light beam coupled and amplified by the optical parametric amplifier to allow only the idler light to pass; A third lens, used for imaging the idler light; The CCD camera is used to record the image formed by the third lens.

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