A large-aperture pulsed laser time synchronization measurement device

Abstract

A large-aperture pulsed laser time synchronization measurement device includes: a beam splitting system for splitting a seed beam into N small-aperture beams; N beam transmission systems for converting the N small-aperture beams into N large-aperture beams; at least N-1 delay adjusters for adjusting the delay of the N-1 large-aperture beams; N apertures for partially converting the N large-aperture beams into N small-aperture beams; N focusing systems for focusing the N small-aperture beams onto a target point to form a superimposed spot; a camera located at the target point for recording and monitoring the superimposed spot and transmitting the data to a computer; and a computer connected to both the delay adjusters and the camera, which determines the synchronization between beams based on the superimposed spot pattern and controls the delay adjusters to achieve time synchronization of different beams to the target. This invention has a simple structure, low cost, and can achieve precise beam synchronization measurement without significant modifications to existing devices.

Description

Technical Field

[0001] This invention belongs to the field of laser technology, specifically a large-aperture pulsed laser time synchronization measurement device. Background Technology

[0002] In the field of laser technology, especially in the field of ultrashort pulse technology, in order to obtain higher peak power, it is required to achieve temporal overlap of two or more beam pulses.

[0003] For multi-beam laser systems, the installation cannot ensure that the total equivalent optical path length of beams emitted from the same seed source is completely equal after splitting and reaching the target. This results in pulses not arriving at the target simultaneously, or two or more pulses failing to meet at the target. If this occurs, a time measurement module, such as an oscilloscope or streak camera, is typically placed at the target to observe the delay between pulses from different optical paths. A delay adjuster is then used to adjust the delay between pulses until they partially or completely overlap in time. Ultrashort pulse beams typically have pulse widths not exceeding tens of picoseconds, and often require pulse overlap with widths of picoseconds or tens of femtoseconds. Conventional measurement methods cannot resolve such high time delays. For example, the highest time resolution of high-speed oscilloscopes is no more than picoseconds, and the highest time resolution of existing streak cameras is also in the picosecond range (for near-infrared lasers). Summary of the Invention

[0004] To overcome the shortcomings of the prior art, the purpose of this invention is to propose a method for time synchronization measurement of large-aperture pulsed lasers, involving the time synchronization adjustment between beam pulses.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows:

[0006] A large-aperture pulsed laser time synchronization measurement device, characterized in that it includes:

[0007] A beam splitting system is used to split a seed beam into N small-aperture beams;

[0008] N beam transmission systems are used to convert the N small-aperture beams into N large-aperture beams.

[0009] At least N-1 delay modulators are used to adjust the delay of the N-1 large-aperture beams;

[0010] N aperture stops are used to convert the N large-aperture beams into N small-aperture beams through the aperture stops.

[0011] N focusing systems are used to converge the N small-aperture beams to the target point to form a superimposed light spot;

[0012] A camera, located at the target point, is used to record and monitor the superimposed light spot and transmit the data to a computer;

[0013] A computer is connected to the delay modulator and the camera respectively. It determines the synchronization between the beams based on the interference pattern of the superimposed light spots and controls the delay modulator to achieve synchronization of the arrival time of different beams at the target.

[0014] Furthermore, the time synchronization between the beams is determined based on the interference image of the superimposed beams: when the two pulses are not overlapping in time, the superimposed beams at the target point cannot interfere, and their image characteristics are consistent with those of a single beam image; when the two pulses are completely synchronized in time, the superimposed beams at the target point are completely coherent, and the image shows alternating bright and dark interference fringes with the highest contrast, theoretically reaching 1; when there is a certain delay between the two pulses, the superimposed beam image at the target point shows interference fringes with a certain contrast (less than 1).

[0015] Furthermore, the aperture allows a portion of the beam to pass through, becoming a small-aperture beam with better beam quality. This small-aperture beam has a large spot size at the target point, which can overcome the problem that the beams cannot coincide at the target point due to laser system pointing jitter, and has relatively regular interference fringes when the two beams are synchronized.

[0016] Furthermore, the step of determining the synchronization between beams based on the superimposed beam pattern specifically involves:

[0017] The highest contrast criterion method: The delay regulator of either of the two beams is adjusted in one direction, and the spot patterns of the two beams are monitored in real time by a camera. After interference fringes appear, the fringe contrast is calculated. When the fringe contrast reaches the maximum, it indicates that the two beams have achieved time synchronization.

[0018] Critical contrast estimation method: Adjust the delay modulator of either of the two beams in one direction. Monitor the spot patterns of the two beams in real time with a camera. When interference fringes begin to appear, record the first position of the delay modulator. Continue adjusting in the original direction. The contrast of the interference fringes will go through a process from weak to strong and then back to weak. When the interference fringes disappear, record the second position of the delay modulator. Calculate the midpoint between the first and second positions of the delay modulator. Adjust the delay modulator to this position, and the two beams will be synchronized in time.

[0019] Compared with the prior art, the beneficial effects of the present invention are:

[0020] 1) This method utilizes a simple aperture-limiting technique to select a portion of the beam to overlap at the target point, overcoming the problem of beam misalignment at the target point caused by laser system pointing jitter. Furthermore, the small-aperture beam has relatively good quality, and it is easy to obtain relatively regular interference fringes when the two beams are synchronized. This scheme is simple in structure and easy to implement, requiring no large-scale modification of existing equipment, and is particularly suitable for measuring the time synchronization between multiple pulsed laser beams.

[0021] 2) An aperture stop is used to select a portion of the large-aperture beam to pass through. A camera is used to monitor the superposition of the light spots of any two beams at the target point. The synchronization between the beams is judged by the superposition pattern of the light spots. Combined with the delay adjustment mechanism in the optical path, precise synchronization of the arrival time of different beams at the target can be achieved.

[0022] 3) This invention utilizes an aperture to select a portion of the beam to be superimposed at the target point, overcoming the problem that the beams cannot overlap at the target point due to the pointing jitter of the laser system. Furthermore, the quality of the small-aperture beam is relatively good, and it is easy to obtain more regular interference fringes when the two beams are synchronized.

[0023] 4) This invention characterizes the time delay characteristics between pulses through the spatial characteristics of the light spot. It has a simple structure, low cost, and can achieve precise synchronous measurement between beams without making major modifications to existing devices. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the large-aperture pulsed laser time synchronization measurement device of the present invention.

[0025] 1-Same-source small-aperture beam; 2-Beam splitting system; 3-Splitting small beam; 4-Beam transmission system; 5-Delay adjuster; 6-Large-aperture beam; 7-Aperture stop; 8-Limited-aperture small beam; 9-Focusing system; 10-Converging beam; 11-Target point; 12-Camera; 301-First splitting small beam; 302-Second splitting small beam; 3N-Nth splitting small beam; 1001-First converging beam; 1002-First converging beam; 10N-Nth converging beam. Detailed Implementation

[0026] The present invention will be further described below with reference to embodiments and accompanying drawings, but this should not be construed as limiting the scope of protection of the present invention.

[0027] Example:

[0028] Figure 1This is a schematic diagram of the large-aperture pulsed laser time synchronization measurement device of the present invention. As shown in the figure, the large-aperture pulsed laser time synchronization measurement method of the present invention consists of a seed small-aperture beam 1, a beam splitting system 2, a beam transmission system 4, an aperture 7, a focusing system 9, and a camera 12. The seed small beam 1 is a pulsed laser beam. After being split by the beam splitting system 2, the seed small beam 1 is divided into N beams 3, labeled as 301, 302, ..., 3N. Each beam 3N passes through the beam transmission system 4, the aperture 7, and the focusing system 9 in sequence, and then converges to a target point 11, which falls within the photosensitive surface of the camera 12. There are N beam transmission systems 4, of which at least N-1 contain at least one delay adjuster 5. The beam transmission system 4 converts the split small beams 3 into a large-aperture beam 6. The outer dimension of the aperture 7 can cover the large-aperture beam 6, and the aperture 7 allows part of the beam to pass through, becoming a limited-aperture small beam 8. The focusing system 9 converges the light beam into a converging beam 10, which converges at a focal point and can be guided to the target point 11 by the focusing system 9. The camera 12 can record the light spot of the converging beam 10 at the focal point individually, or it can record an image of N light spots superimposed on each other.

[0029] The working principle of this device is explained below.

[0030] Let's take a multi-pulse laser as an example. The seed beam is split into N beams by a beam splitting system. Each beam passes sequentially through a beam transmission system, an aperture, and a focusing system before converging at the target. After a single pulse is emitted from the seed beam, the design ensures that any two beams arrive at the target at the same time. However, due to insufficient installation precision, beam jitter, etc., the total equivalent optical path from the seed source to the target can deviate significantly, causing the pulses to fail to arrive at the target simultaneously, or for two or more pulses to fail to meet at the target. In such cases, a time measurement module, such as an oscilloscope or streak camera, is typically installed at the target to observe the delay between pulses from different optical paths. A delay adjuster is then used to adjust the delay until the pulses partially or completely overlap in time. For ultrashort pulses, the pulse width is generally no more than tens of picoseconds, and often requires overlapping pulses with widths of picoseconds or tens of femtoseconds. Conventional measurement methods cannot distinguish such high time delays. For example, the highest time resolution of time measurement devices such as high-speed oscilloscopes and streak cameras is generally no more than the picosecond level.

[0031] In the field of petawatt laser technology, due to limitations in the damage threshold of compression gratings and the nonlinear effects of amplification systems, achieving high target power generally requires a large-aperture beam, typically exceeding 300 mm or even reaching the meter level, in order to achieve high target power. Large-aperture beams require large-aperture optical components that are difficult to fabricate, and the entire optical system exhibits significant wavefront aberrations, leading to beam focusing difficulties and reduced peak power density at the target surface. Adaptive optics are typically used to correct these wavefront aberrations. Petawatt lasers are generally used to obtain extremely high peak power densities at the focal point for high-energy-density physics experiments, typically exceeding 10^6 kilowatts. 18 W / cm 2 The focal spot size is close to the diffraction limit. Taking a beam aperture of 350mm × 350mm as an example, the divergence angle corresponding to the focal spot diffraction limit is:

[0032] 1dL=2λ / D (1)

[0033] Where DL represents the diffraction limit, D represents the beam aperture, and λ represents the laser wavelength. Taking a high-energy petawatt laser system based on neodymium glass as the amplification medium as an example, its beam center wavelength is 1.053 μm, corresponding to a diffraction limit of approximately 6 μrad. However, large-aperture laser systems, due to their complex optical paths, inevitably experience beam pointing instability caused by environmental vibrations and mirror instability, typically with a value of 6 μrad (RMS) and a corresponding PV value exceeding 18 μrad. This means that when observing the focal spots of two beams at the target point, the maximum PV value of the angular deviation between the two beams exceeds 36 μrad. For two beams with spot sizes close to the diffraction limit, there is a certain probability that they will not overlap completely, or only partially overlap. If the spot size far exceeds the diffraction limit, it means that the beam quality is poor, and a regular, easily identifiable interference pattern cannot be formed on the focal plane. In this case, there is a certain degree of uncertainty in observing the far-field coherence pattern of two beams at the target point. To ensure that the two beams overlap on the target surface, the focal spot size needs to be increased while maintaining beam quality. As can be seen from equation (1), the diffraction limit is inversely proportional to the beam aperture. Therefore, the focal spot size can be increased by reducing the beam aperture. For example, by using an aperture to cut off a 100mm×100mm beam from a large-aperture beam, according to equation (1), the diffraction limit corresponding to the same wavelength increases to 3 times the original value, i.e., 21μrad. Under the same beam pointing control level, the probability that the two beam spots cannot be superimposed will be greatly reduced.

[0034] Compared to full-aperture beams to the target, using an aperture to cut off a portion of the beam within a large-aperture beam generally results in better beam quality, which is also more conducive to obtaining a more obvious interference pattern on the focal plane.

[0035] Next, we will explain the relationship between the interference pattern superimposed on the focal plane and the time delay between pulses.

[0036] When the two pulses do not overlap completely, they arrive at the camera target surface sequentially, making interference superposition impossible. When the two pulses arrive at the camera target surface completely synchronously, and the difference in pulse shape is small, the two pulses will interfere, resulting in the highest relative fringe contrast. When there is a certain delay between the two pulses, and the delay is less than the pulse width, the two pulses will arrive at the camera target surface simultaneously for part of the time, while the independent characteristics of each beam are recorded by the camera at other times. Since the camera exposure time is generally no more than a few microseconds, the duration of the image recorded in the same frame will generally completely record the images of both pulses arriving at the camera, regardless of whether the two beams coherently superimpose or arrive independently, all times are recorded in one image. Therefore, the images recorded by the camera can be divided into three types: one is where the two beams do not overlap at all, and their image characteristics are consistent with the pattern of a single beam arriving at the camera; the second is completely coherent, exhibiting alternating bright and dark interference fringes, with a theoretical fringe contrast of 1; the third mode is a combination of the two, equivalent to the superposition of the first and second patterns with different weights. By observing which mode the fringes belong to, and combining this with the delay adjuster to determine the pulse delay, the specific methods can be divided into the following two types:

[0037] The first method is the highest contrast criterion. The delay regulator of one of the two beams is adjusted unidirectionally, and the spot patterns of the two beams are monitored in real time by a camera. Once interference fringes appear, the fringe contrast is calculated. When the fringe contrast reaches its maximum, it indicates that the two beams have achieved time synchronization.

[0038] The second method is the critical contrast estimation method. The delay modulator of one of the two beams is adjusted unidirectionally. The spot patterns of the two beams are monitored in real time by a camera. When interference fringes begin to appear, the first position of the delay modulator is recorded. Adjusting along the original direction, the contrast of the interference fringes will go through a process from weak to strong and then back to weak. When the interference fringes disappear, the second position of the delay modulator is recorded. The midpoint between the first and second positions of the delay modulator is calculated. Adjusting the delay modulator to this position will achieve time synchronization between the two beams.

Claims

1. A large-aperture pulsed laser time synchronization measurement device, characterized in that, include: A beam splitting system is used to split a seed beam into N small-aperture beams; N beam transmission systems are used to convert the N small-aperture beams into N large-aperture beams. At least N-1 delay modulators are used to adjust the delay of the N-1 large-aperture beams; N aperture stops are used to convert the N large-aperture beams into N small-aperture beams through the aperture stops. N focusing systems are used to converge the N small-aperture beams to the target point to form a superimposed light spot; A camera, located at the target point, is used to record and monitor the superimposed light spot and transmit the data to a computer; A computer, connected to the delay modulator and the camera respectively, determines the synchronization between the beams based on the interference pattern of the superimposed light spots, and controls the delay modulator to achieve synchronization of the arrival time of different beams at the target; that is, any two beams arrive at the target at the same time. The aperture allows a portion of the beam to pass through, thus becoming a small-aperture beam. This small-aperture beam has good beam quality, which enables regular interference fringes when the two beams are synchronized. The small-aperture beam forms a large superimposed spot at the target point, which can solve the problem of beams not coinciding at the target point due to laser system pointing jitter.

2. The large-aperture pulsed laser time synchronization measurement device according to claim 1, characterized in that, The method of determining the synchronization between beams based on the superposition pattern of light spots specifically involves: The highest contrast criterion method: The delay regulator of either of the two beams is adjusted in one direction, and the spot patterns of the two beams are monitored in real time by a camera. After interference fringes appear, the fringe contrast is calculated. When the fringe contrast reaches the maximum, it indicates that the two beams have achieved time synchronization. Critical contrast estimation method: Adjust the delay modulator of either of the two beams in one direction. Monitor the spot patterns of the two beams in real time with a camera. When interference fringes begin to appear, record the first position of the delay modulator. Continue adjusting in the original direction. The contrast of the interference fringes will go through a process from weak to strong and then back to weak. When the interference fringes disappear, record the second position of the delay modulator. Calculate the midpoint between the first and second positions of the delay modulator. Adjust the delay modulator to this position, and the two beams will be synchronized in time.

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