Optical framing camera and method for measuring laser spot changes within a single pulse

Abstract

The present invention belongs to the field of nonlinear optics, and discloses an optical framing camera and a method for measuring the change of laser spot in a single pulse. Two laser beams are used, one laser beam is used as pump light, and the other laser beam is used as detection light. After the detection light beam passes through a converging step reflector array, it overlaps on the material at the material position, and the pump light is converged on the material by the imaging system. The converging step reflector array is composed of multiple reflection surfaces, and each reflection surface is a plane mirror. The reflection angle of each reflection surface is different, but eventually each beam of reflected light will overlap at the same position, and there is a certain height difference between the steps of each reflection surface. Through the converging step reflector array, a beam of detection light is divided into multiple detection light pulse sequences with time delay differences, so that the law of spatial distribution change of the pump spot in a pulse can be obtained, thereby realizing ultrafast imaging, and the imaging speed can reach fs level.

Description

Technical Field

[0001] The invention relates to a nonlinear optical physical mechanism for studying materials and a single-pulse ultrafast imaging device, and belongs to the field of nonlinear optics. Background Art

[0002] The birth of transient optical imaging technology can be traced back to the 1870s, when American photographer E. Muybridge pioneered the technique by capturing horses in motion. More than a century later, scientists have been able to achieve imaging of femtosecond laser-induced lattice vibration waves in a single exposure. This gives single-exposure ultrafast imaging technology an irreplaceable advantage in non-repeatable measurements such as optical giant waves, irreversible structural dynamics in chemical reactions, and shock wave generation in inertial confinement nuclear fusion.

[0003] When ultrafast imaging of some irreversible dynamic phenomena is performed, the traditional pump-probe technology obviously cannot meet the measurement requirements, because it needs to use a mobile platform to achieve the time delay of two pulses of light, and it takes multiple pulses to repeat to obtain the spot image of the pump light under different time delays. However, this is not a true measurement of the image that changes over time within a pulse. In addition, traditional ultrafast electronic imaging technology also has the problem of multiple measurements and time jitter. Therefore, single pulse imaging technology is needed to solve the above problems. Summary of the invention

[0004] The invention solves the problem of measuring ultrafast time resolution and space resolution imaging of a single laser pulse, and proposes an optical framing camera, comprising: an incident detection laser beam, a beam splitter, a converging step reflector array, an incident pump laser beam, a sample holder, an imaging system, a compensating step reflector array, and a CCD; the detection light emitted by the incident detection laser beam passes through the beam splitter, and is reflected to the beam splitter by the converging step reflector array, and then the detection light is reflected by the rear surface of the beam splitter to a sample to be measured at the sample holder; at the same time, the pump light emitted by the incident pump laser beam is also incident on the sample to be measured, and the detection light after passing through the sample to be measured is converged by the imaging system, and then imaged in the CCD after being reflected by the compensating step reflector array; the converging step reflector array and the compensating step reflector array are both composed of a plurality of reflection surfaces, and each reflection surface is a plane mirror, and the reflection angle of each reflection surface is different, but each beam of reflected light finally overlaps at the same position, and there is a certain height difference between the steps of each reflection surface.

[0005] The working principle of the above technical solution is: the light emitted by the incident detection laser beam passes through the beam splitter and is reflected by the converging step reflector array, and then a beam of detection light is divided into multiple detection light pulse sequences with time delay differences. The detection light and the pump light overlap at the position of the sample to be measured, and at the same time, the pump light acts on any position within the range of the detection light. After the pump light acts on the sample to be measured, the pump light has different effects on the sample to be measured at different times within one pulse, so that the spot distribution of the pump light at different times is recorded by the detection light. After the detection light sequence passes through the sample to be measured, it passes through the imaging system and the compensating step reflector array and is finally imaged in the CCD.

[0006] Since the converging step reflector array is distributed in a step-like shape, after the detection light passes through each step, different optical path differences will be generated between the transmitted light beams, so that time delay will be generated between each light beam. The reflected light beam of each reflector surface is directly reflected in parallel to the position of the sample to be tested at a certain angle, and the reflected light beams of all reflector units overlap at the position of the sample to be tested. After the pump light acts on the sample to be tested, since the particle population at different positions of the sample to be tested is related to the excitation light intensity and changes continuously with time, if the light intensity distribution at different positions in the pump light beam is different, a light spot image corresponding to the pump light will be generated in the corresponding detection light spot. At the same time, since there are steps between each reflector unit, the influence of the reflected detection light at different times is different. The change in the intensity of the pump light area in the detection beam of the sample to be tested can be used to know the distribution of the pump light intensity at different times. By analyzing the distribution of the pump light intensity at different times, the change of the pump light spot in a single pump light pulse can be obtained. The law of spatial distribution change of the pump light spot within a pulse can be obtained, thereby realizing ultrafast imaging with an imaging speed of up to fs level.

[0007] A better solution is that the pump light and the detection light beams overlap at the sample to be tested, and the spot size of the pump light at the sample to be tested is smaller than the spot size of the detection light.

[0008] A further solution is that any reflective surface on the compensating step reflector array is complementary to the angle of a corresponding reflective surface on the converging step reflector array, and the overall shape composed of the reflective surfaces on the compensating step reflector array is complementary to the overall shape composed of the reflective surfaces on the converging step reflector array.

[0009] Preferably, the overall shape of the reflection surfaces on the compensating step reflector array is a convex surface, and the overall shape of the reflection surfaces on the converging step reflector array is a concave surface.

[0010] A further solution is that the height of the protrusions of each reflective surface on the compensating step reflector array is equal to the depth of the depressions of the corresponding reflective surfaces on the converging step reflector array.

[0011] The steps for measuring using the above scheme are:

[0012] 1) No sample to be tested is placed on the sample holder, and the CCD is used to receive the light spot reflected by the converging step reflector array; the light spot received by the CCD is the detection light background spot;

[0013] 2) Place the sample to be tested on the sample holder, and use CCD to receive the light spot after being reflected by the converging step reflector array and passing through the sample to be tested. The light spot received by CCD is the spatially distributed spot of the pump light spot;

[0014] 3) Process the two light spots obtained above to obtain the required image of the light spot change of the pump light within one pulse.

[0015] It is worth noting that there is no order between the steps (1) and (2), and the same effect can be achieved by exchanging them. Alternatively, a beam splitter is used to split the light path into two identical light paths, one of which is used to place the sample to be tested, and the other is not used to place the sample to be tested. The two light spots are imaged simultaneously on a CCD.

[0016] In the above technical solution, the detection optical path can be a lens imaging system or a 4f imaging system. In the lens imaging system, the sample to be tested is placed on the object plane of the converging lens, and the CCD is placed on the image plane of the converging lens. For the 4f imaging system, the sample to be tested is placed on the object plane of the system, and the CCD is placed on the image plane of the system.

[0017] In the above technical solution, in step (1), when no sample to be tested is placed, the light spot of the detection light received by the CCD is the background light spot.

[0018] In the above technical solution, the step (2) is to place the sample to be tested in the detection light path, and the CCD receives the spot of the detection light, which contains the spatial distribution information of the pump light spot;

[0019] In the above technical solution, the processing in step 3) includes subtracting the background image in step 1) from the spot image in step 2), and then subtracting the previous pulse image from the next pulse image in the order of the delay time of the pulse sequence, and finally obtaining the distribution of the spot of the pulse light at different times in a single pulse.

[0020] In the above technical solution, the stepped reflector array in step (1) is a concave-like structure, and there is a certain height difference between the steps of each reflective surface. The light reflected by each reflective surface can overlap the reflected light beam sequence at the position of the sample to be tested.

[0021] In the above technical solution, in the novel optical framing camera device in step (1), the imaging system behind the sample to be tested is an imaging lens, the sample to be tested is placed on the object plane of the imaging system, and the CCD is placed on the image plane of the imaging system.

[0022] In the above technical solution, the step widths of the converging step reflector array and the compensating step reflector array are determined by the image size in the imaging system, the spot size of the detection light, and the pixel size of the CCD. The algorithm is: the step width of the converging step reflector array = N × CCD pixel size / imaging system magnification, N is the number of pixels occupied by each plane mirror in the converging step reflector array; the step height of the converging step reflector array = delay time resolution × speed of light / 2.

[0023] In the above technical solution, the acquisition of the spot containing the pump light spot information in step (2) requires that the pump light and the overlapping detection light sequence beams coincide with each other. In order to fully display the spot distribution of the pump light, the spot size of the pump light is smaller than the spot size of the detection light.

[0024] Through this scheme, the spatial distribution change of a single pump light spot at different times within a pulse can be obtained. By establishing a quasi-concave reflective step mirror, a large-size parallel light spot can be overlapped on the sample to be tested at a certain position, and the pump light is introduced into the optical path and acts on the sample to be tested. Considering the delay time between the strong pump light and the pulse detection light sequence generated by the reflective step mirror, the change of a single pulse pump light spot over time is finally obtained according to the change of the detection light spot over time, thereby realizing the change of the pump light spot at different times, that is, completing the realization of ultrafast all-optical framing technology.

[0025] Through the above technical solution, a new idea is used to realize the measurement of the laser spot changes at different times within a pulse. Compared with other measurement technologies, it has the following advantages:

[0026] 1. The optical path is very simple. Only an angle step reflector, lens and CCD are needed to measure the spot time and spatial resolution of the laser in one pulse.

[0027] 2. No expensive CCD is required to achieve ps or even fs-level imaging speed;

[0028] 3. The measurement is very convenient. There is no need to move the sample to be measured. Only one laser pulse is needed to obtain the spot change of the pump light within a pulse, that is, to obtain the pump light within a single pulse;

[0029] 4. This method does not require a high-precision moving platform in the traditional optical path to achieve time delay, and only requires a time-delay step reflector to achieve time delay;

[0030] 5. This method achieves high spatial resolution while greatly improving the temporal resolution of the measurement compared to current gated framing cameras (tens of picoseconds). BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art are briefly introduced below.

[0032] Figure 1 It is a structural schematic diagram of an optical framing camera;

[0033] Among them: 1. Incident detection laser beam, 2. Beam splitter, 3. Converging step mirror array, 4. Incident pump laser beam, 5. Sample holder, 6. Imaging system, 7. Compensating step mirror array, 8. CCD.

[0034] Figure 2 This is the background spot diagram of the detection light at different times within a pulse when there is no pump light.

[0035] Figure 3 This is the background spot diagram of the detection light at different times within a pulse when the pump light energy is strong.

[0036] Figure 4 Schematic diagram of the structure of a converging stepped reflector array in one embodiment.

[0037] Figure 5 Schematic diagram of the structure of a compensation stepped reflector array in one embodiment. DETAILED DESCRIPTION

[0038] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the accompanying drawings in the embodiments of the present invention.

[0039] like Figure 1 As shown, an optical framing camera comprises, in sequence along the optical axis direction: an incident detection laser beam 1, a beam splitter 2, a converging step reflector array 3, an incident pump laser beam 4, a sample holder 5, an imaging system 6, a compensating step reflector array 7, and a CCD 8; the detection light emitted by the incident detection laser beam passes through the beam splitter, and is reflected to the beam splitter by the converging step reflector array, and then the detection light is reflected by the rear surface of the beam splitter to the sample to be tested at the sample holder; at the same time, the pump light emitted by the incident pump laser beam is also incident on the sample to be tested, and the detection light after passing through the sample to be tested is converged by the imaging system, and then imaged in the CCD after being reflected by the compensating step reflector array; the converging step reflector array and the compensating step reflector array are both composed of a plurality of reflection surfaces, and each reflection surface is a plane mirror, and the reflection angle of each reflection surface is different, but eventually each beam of reflected light overlaps at the same position, and there is a certain height difference between the steps of each reflection surface.

[0040] In the framing experiment, each reflective surface of the converging step reflector array 3 is processed to 4mm×4mm, and 3 reflective units are processed in each row, with a total of 3 rows processed. There is a 1mm interval between each row and each column, so that 9 reflective units can be obtained at a time. After passing through the 4f imaging optical path, the energy of 9 detection lights can be obtained. The step height between each reflector surface is determined according to the measurement requirements, which can achieve a time resolution of femtoseconds to picoseconds. The specific calculation formula is 20*h / 3 picoseconds, and the unit of height h is millimeters. Different from ordinary step reflector arrays, such as Figure 4 As shown, in the converging step reflector array, in addition to a certain time delay between each reflection unit, the reflection angle of each reflection unit is different, and finally a concave mirror-like structure is formed, but each reflection unit is a plane mirror. The light reflected by the converging step reflector array will overlap at a position, and the overlapping position can be regarded as the focus of the converging step reflector array. A sample to be tested with an excited state or a carrier with a long lifetime is placed at this position. Since the reflected light is parallel incident, the size of the 9 spots is ensured to be approximately 4mm×4mm. The pump light acts on the sample to be tested, and the size of the pump light spot is smaller than the detection spot size, and the time delay between the pump light pulse and the detection light is adjusted. It should be noted that the visible light reflected by the converging step reflector array cannot be directly imaged on the CCD after passing through the sample to be tested, because all the detection lights overlap at the position of the observed pump light spot. Therefore, it is necessary to design the following Figure 5 The structure of the compensating step reflector array shown is complementary to that of the converging step reflector array as much as possible. After reflection by different reflection surfaces in the compensating step reflector array, the originally overlapping images can be separated and finally imaged on the CCD. After image processing, an image of the change in the detection light caused by the change in the spatial distribution of a single pump pulse spot in the sample area to be tested at different times can be obtained. The change in the spatial distribution of the detection light spot after passing through the sample to be tested is the change caused by the spatial energy distribution of the pump light, so that the spatial distribution image of the single pulse of pump light at different times can be obtained. Two CCDs can be used for simultaneous detection, one to monitor the spatial distribution of the detection light, and the other to monitor the spatial distribution of the detection light after the pump light acts, so as to improve the measurement accuracy of the time and spatial resolution of the single pulse spot.

[0041] In this embodiment, the laser emits a frequency-doubled signal of 532 nm with a pulse width of 21 ps, and the CCD is ADIMEC-4020m. The sample to be tested is zinc selenide (ZnSe), which is a two-photon absorption optical nonlinearity at 532 nm, the step height of the converging step reflector array is 300 μm, and the time resolution is 2 ps.

[0042] The specific detection steps are as follows: 1) No sample to be tested is placed on the sample rack, and CCD is used to receive the light spot after being reflected by the converging step reflector array; the light spot received by CCD is the detection light background spot; 2) The sample to be tested is placed on the sample rack, and CCD is used to receive the light spot after being reflected by the converging step reflector array and then passing through the sample to be tested, and the light spot received by CCD is the light spot with the spatial distribution information of the pump light spot; 3) The two light spots obtained above are processed to obtain the required image of the change of the pump light spot within one pulse.

[0043] The specific process of theoretical calculation of ZnSe optical nonlinearity is as follows:

[0044] Since the probe light energy is very weak compared to the pump light, the two-photon absorption of the pump light becomes the only way to generate free carriers, and its rate can be expressed as:

[0045]

[0046] I in the formula e is the intensity of the pump light at the sample to be tested, τ r is the free carrier lifetime. The change in the absorption coefficient caused by free carriers is proportional to the excess carrier concentration and can be expressed as:

[0047] Δα f =σ α ΔN(t) (2)

[0048] Where σ α is the free carrier absorption cross section. Considering the combination of bound electron effect and free carrier joint effect, using thin sample and slow amplitude approximation, the propagation formula of probe light and pump light inside ZnSe crystal is:

[0049]

[0050]

[0051] I in the formula p Represents the intensity of the detection light. The background spot and the spot diagram containing the pump spot information are shown in Figure 2 When only the detection light acts on the sample to be tested, the nonlinear effect of the material is very weak and basically does not change. When the surrounding detection light passes through the sample to be tested, it is linearly transmitted without any change. Figure 2The linear spot after the laser spot passes through the material is the background spot. When the pump light acts on the sample to be tested, the stronger pump light will excite the sample to be tested to produce nonlinearity. The step reflector on the sample to be tested produces a detection spot sequence, and the time delay between the detection light and the pump light is properly adjusted. The nonlinear mechanism of ZnSe is a transient absorption process of two-photon absorption, so there is a rapid absorption process within the pulse width time, which is manifested in the spot image as the light intensity at this location is weaker than the linear spot. Figure 3 This is the result of subtracting the spot pattern containing pump light information from the background spot pattern. The stronger the red box is, the weaker the pump light is at that time. Figure 3 Each spot image is the shape of the pump spot at a certain moment in the pulse. In this way, the spatial and temporal resolved changes of the pump spot in a single pulse can be obtained simultaneously by detecting the spot sequence and the nonlinear sample to be tested.

[0052] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined in this application may be implemented in other embodiments without departing from the spirit or scope of the present invention.

Claims

1. An optical framing camera, characterized in that include: Incident detection laser beam, beam splitter, converging step mirror array, incident pump laser beam, sample holder, imaging system, compensating step mirror array, CCD; The detection light emitted by the incident detection laser beam passes through the beam splitter, is reflected to the beam splitter by the converging step reflector array, and then is reflected by the rear surface of the beam splitter to the sample to be tested at the sample holder; At the same time, the pump light emitted by the incident pump laser beam is also incident on the sample to be tested. The detection light after passing through the sample to be tested is converged by the imaging system, and then reflected by the compensation step mirror array and imaged in the CCD; The converging step reflector array and the compensating step reflector array are both composed of a plurality of reflective surfaces, and each reflective surface is a plane mirror. The reflection angle of each reflective surface is different, but eventually each beam of reflected light overlaps at the same position, and there is a height difference between each reflective surface; The sample to be tested is placed on the object plane of the imaging system, and the CCD is placed on the image plane of the imaging system; The step widths of the converging step reflector array and the compensating step reflector array are determined by the image size in the imaging system, the spot size of the detection light, and the pixel size of the CCD. The algorithm is: step width of the converging step reflector array = N × CCD pixel size / imaging system magnification, N is the number of pixels occupied by each plane mirror in the converging step reflector array; step height of the converging step reflector array = delay time resolution × light speed / 2; Any reflecting surface on the compensating step reflector array is complementary to a corresponding reflecting surface on the converging step reflector array, and the overall shape composed of the reflecting surfaces on the compensating step reflector array is complementary to the overall shape composed of the reflecting surfaces on the converging step reflector array.

2. The optical framing camera according to claim 1, characterized in that: At the sample to be measured, the pump light and the detection light beams overlap, and at the sample to be measured, the spot size of the pump light is smaller than the spot size of the detection light.

3. The optical framing camera according to claim 1, characterized in that: The overall shape of the reflection surfaces on the compensating step reflector array is a convex surface, and the overall shape of the reflection surfaces on the converging step reflector array is a concave surface.

4. The optical framing camera according to claim 3, characterized in that: The height of the protrusions of each reflecting surface on the compensating step reflecting mirror array is equal to the depth of the depressions of the corresponding reflecting surfaces on the converging step reflecting mirror array.

5. A method for measuring the change of laser spot within a single pulse, characterized in that: The detection is performed using the optical framing camera described in any one of claims 1 to 4, wherein the detection steps are as follows: 1) No sample to be tested is placed on the sample holder, and the CCD is used to receive the light spot reflected by the converging step reflector array; the light spot received by the CCD is the detection light background spot; 2) Place the sample to be tested on the sample holder, and use CCD to receive the light spot after being reflected by the converging step reflector array and passing through the sample to be tested. The light spot received by CCD is the light spot with the spatial distribution of the pump light spot; the light spot with the spatial distribution information of the pump light spot 3) Processing the background light spot and pump light spot obtained above to obtain the required pump light spot change image within one pulse.

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