Combined acousto-optic modulation method based on precise diffraction superposition

Abstract

A combined acousto-optic modulation method based on precise diffraction superposition. In the process of using a 4-F optical system to realize the coherent superposition of Bragg diffraction amplitudes of two acousto-optic modulators, one acousto-optic modulator is moved an appropriate distance from the imaging position of the other acousto-optic modulator, such that the diffraction phase shift broadening of Bragg diffraction of a focused beam is suppressed by means of momentum echo; in addition, the phase shift of high-order diffraction is adjusted, such that the high-order diffraction loss is precisely suppressed by means of a coherent cancellation effect. Further provided is a combined acousto-optic modulation method based on retroreflection interference. An optical imaging system is used to precisely retroreflect a primary diffraction output of an acousto-optic modulator, and the primary diffraction output is then input into the modulator again for secondary diffraction, such that the reuse of the acousto-optic modulator is realized, thereby achieving high diffraction efficiency or high contrast of combined acousto-optic modulation by using only a single acousto-optic modulator. The method can achieve a diffraction efficiency of 99% or above and a transmitted light single-mode suppression ratio of 30 dB or above.

Description

A combined acousto-optic modulation method based on precision diffraction superposition Technical Field

[0001] This invention belongs to the field of laser control technology, specifically involving a combination of acousto-optic modulation methods using precision diffraction superposition to improve control efficiency and bandwidth, and a combination of acousto-optic modulation methods using retro-reflection interference to improve control efficiency and bandwidth. Background Technology

[0002] From basic scientific research to industrial control, laser applications require on-demand modulation of laser output. Acousto-optic modulation (AOM) [[1] J. Thom et al, “Accurate and agile digital control of optical phase, amplitude and frequency for coherent atomic manipulation of atomic systems,” Opt. Express 21, 18712 (2013)] uses radio frequency signals to control the crystal acoustic field to achieve Bragg diffraction of incident light, thereby achieving precise adjustment of the power, phase, frequency, and propagation direction of the diffracted beam. Due to the crystal sound velocity v S Typically operating at speeds of several thousand meters per second, for beams with a width w less than a millimeter, the acousto-optic control bandwidth Δω M ≈v S / w can reach tens of megahertz, and the time τ can be adjusted. M =1 / Δω MThe speed can be as low as ten nanoseconds. Compared to the faster electro-optic modulation [[2]W.Hansel,M.Giunta,M.Fischer,M.Lezius,and R.Holzwarth,Rapid electro-optic control of the carrierenvelope-offset frequency for ultra-low noise frequency combs,2017 Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium,EFTF / IFC 2017-Proceedings,128–129(2017)], acousto-optic modulation has significant advantages such as multi-degree-of-freedom controllability, precise modulation phase and amplitude, a wide range of laser wavelength and power selection, and low instrument drive requirements. The combination of these advantages and high-speed modulation capabilities makes acousto-optic modulation devices have an almost irreplaceable and important technical position in basic research and laser technology development. However, according to research, conventional acousto-optic modulation techniques such as [1] & [3] Zhou et al, Laser frequency shift up to 5 GHz with a high-efficiency 12-pass 350-MHz acousto-optic modulator, Rev. Sci. Instrum. 91, 033201 (2020) have at least the following shortcomings:

[0003] (1) The acousto-optic diffraction efficiency is not high. With a power input of P0, the maximum diffraction efficiency of a conventional AOM, η = P1 / P0, generally does not exceed 90%. Such efficiency limits the application of AOM in fields with stringent efficiency requirements, such as high-frequency iteration [[3] Zhou et al, Laser frequency shift up to 5 GHz with a high-efficiency 12-pass 350-MHz acousto-optic modulator, Rev. Sci. Instrum. 91, 033201 (2020)], laser power synthesis [[4] S. Yu, et al, A universal programmable Gaussian boson sampler for drug discovery, Nature Computational Science 3, 839, 848 (2023)], and optical quantum information processing [[5] Arno Klenke et al, Coherent Beam Combination of Ultrafast Fiber Lasers, IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 24, NO. 5, (2018)].

[0004] (2) Corresponding to the low diffraction efficiency is the significant residual of the zero order after the AOM is turned on. Therefore, ordinary AOM cannot freely split the incident light into zero and first orders to achieve the function of a high-contrast optical beam splitter. High-speed controllable optical beam splitters have important application prospects, such as being used to achieve controllable routing of pulsed lasers, or to construct dual-path interference to control the group velocity of pulsed lasers by the beam splitting ratio [2].

[0005] (3) For ordinary acousto-optic modulation, obtaining a high diffraction efficiency η often requires a large incident light spot w, thus sacrificing the modulation bandwidth δω. M In return, to obtain a modulation bandwidth of tens of megahertz, it is common practice to focus the incident beam waist w to below 100 micrometers. However, the smaller the beam focus, the wider the corresponding wave vector broadening, which violates the Bragg condition and reduces the diffraction efficiency η.

[0006] (4) The high driving RF power of ordinary acousto-optic modulation not only limits the development of high-frequency modulation technology, but also limits the selection of acousto-optic crystals and sound field design. For example, quartz crystals with excellent optical quality and low price often require nearly 10 watts of RF driving power, and the large power consumption brings many design limitations. Acousto-optic modulation often uses crystals such as TeO2 [[6] A. Feldman, "Measurement of the Photoelastic Contents of Optical Materials", Opt. Eng. 17, 453 (1978)]. Although the driving power requirement is slightly lower, the optical insertion loss is large and the optical damage threshold is low, making it difficult to use for high-power lasers.

[0007] (5) In the combined acousto-optic modulation techniques related to this invention [[7] Chinese Patent Publication No. CN113777811B, & [8] Chinese Patent Publication No. CN113725714B], the authors proposed to achieve high diffraction efficiency and control bandwidth through precision imaging using N>1 acousto-optic modulators. However, for the configuration of N=2, the diffraction efficiency η is still difficult to exceed 95%. Although increasing the number of AOMs N can continuously improve the diffraction efficiency, the resulting problems are resource consumption and increased insertion loss.

[0008] In addition, using N>1 acousto-optic modulators to achieve the basic functions of N=1 modulators results in additional consumption of hardware resources, which is not conducive to product cost control and system miniaturization design.

[0009] Summary of the Invention

[0010] The purpose of this invention is to provide an ultra-high efficiency combined acousto-optic modulation method based on precision diffraction superposition to suppress higher-order diffraction losses, thereby achieving an ultra-high diffraction efficiency of over 99% and a transmission light single-mode suppression ratio of over 30dB.

[0011] The present invention provides a combined acousto-optic modulation method based on precise diffraction superposition, which first constructs an encoding module capable of synchronizing radio frequency signals, and a precisely adjustable coherent combined acousto-optic modulation system; wherein:

[0012] The synchronous radio frequency signal encoding module provides a phase-stable, arbitrarily programmable radio frequency signal, which is amplified by a signal amplifier and input into a coherent combined acousto-optic modulation system to drive the acousto-optic modulator AOM in the system to work.

[0013] The coherent combined acousto-optic modulation system consists of two identical acousto-optic modulators (AOMs) with similar geometric dimensions and a dual-lens 4-F optical imaging system. The acousto-optic modulator converts radio frequency signals into sound waves with corresponding frequency, intensity, and phase, generating acousto-optic diffraction on the incident pulsed laser. The dual-lens 4-F optical imaging system consists of two asymmetric lenses with a focal length of F. The diffraction output of the first (i.e., the preceding) acousto-optic modulator AOM1 is precisely imaged with a magnification of M=1 onto the second (i.e., the following) acousto-optic modulator AOM2, which propagates in the same direction as the sound wave, forming interference from two diffractions.

[0014] In the process of achieving coherent superposition of the Bragg diffraction amplitudes of two acousto-optic modulators (AOMs) using a dual-lens 4-F optical imaging system, by moving the second acousto-optic modulator (AOM2) out of the imaging position of the first acousto-optic modulator (AOM1) by an appropriate distance, the diffraction phase shift broadening of the focused beam Bragg diffraction is suppressed by momentum echo. Simultaneously, the higher-order diffraction phase shift is adjusted to precisely suppress higher-order diffraction losses through coherent destructive effects, thereby achieving a diffraction efficiency of over 99% and a single-mode suppression ratio of over 30 dB. The specific steps are as follows:

[0015] (1) Let the length of the AOM acousto-optic crystal be L; use the first acousto-optic modulator (AOM1) to modulate a beam whose central wave vector is located at A laser with a transverse wave vector broadening of Δk is subjected to single-frequency acousto-optic modulation (where the wave vector of the normally incident beam is denoted as k0, and the wave vector of the modulated acoustic field is denoted as k). s The center of the output wave vector corresponding to the m-th order Bragg diffraction is located at... The wave vector broadening is Δk = π / w, where w is the waist of the Gaussian light beam.

[0016] (2) Using a dual-lens 4-F optical imaging system, the sound field of the first acousto-optic modulator (AOM1) crystal and the center of the beam interaction are used as the object plane to accurately image it onto the center of the second acousto-optic modulator AOM2; the sound field direction of the second acousto-optic modulator AOM2 is the same as the image direction of the sound field of the first acousto-optic modulator AOM1.

[0017] (3) Drive AOM1 and AOM2 with weak radio frequency signals respectively, and finely adjust the incident light angle so that the diffraction efficiency η of AOM1 and AOM2 at m=1 order is less than 50% and nearly equal; for commercial AOMs designed and optimized at 100% radio frequency drive, m≠0, and the total diffraction loss of the first order is at the level of 1%.

[0018] (4) Using AOM2 to perform secondary diffraction on all orders of diffracted light emitted from AOM1, the driving phase of AOM2 is changed to make the m=1st order diffraction brightest. The position δL of AOM2 is finely adjusted back and forth along the optical axis of the dual-lens 4-F optical imaging system. It is found that the diffraction intensities of m=-1 and 2 are... The spatial periodic oscillation, therefore δL in δL n =nδL h +δL off At this point, local minimization of diffraction loss for orders m = -1 and 2 can be achieved. Here, n is the oscillation period index, n = 0, 1, 2…, δL off This is the bias, and its value depends on the sound field length, frequency, and other sound field distribution details of the acousto-optic modulator (AOM).

[0019] (5) By jointly optimizing the drive power of AOM1 and AOM2 through program control, in This obtains the globally optimal driver configuration. Here [...] is the rounding operator. It is the average refractive index of the acousto-optic crystal. It is the optimal distance for momentum echo compensation by broadening the Δk diffraction phase under the two-mode approximation, neglecting higher-order diffraction losses. The coefficient ξ≈0.2 is related to the sound field distribution profile; when the position of AOM2 is taken as δL opt At this time, due to momentum echo compensation, the requirements of the combined acousto-optic modulation on the Bragg condition are reduced, and the zero-order residual is suppressed. On the other hand, this distance ensures the coherent destructive cancellation of higher-order diffraction caused by AOM1 and AOM2. At δL opt At a distance, a diffraction efficiency η of m = +1 order can be achieved. opt The target is a single-mode suppression ratio greater than 30dB with a suppression ratio of >99% and m=0 level.

[0020] (6) Further fine-tuning the driving sound field frequency ω S =v S k S The above step (5) can be used to... bias δL in off The efficiency η is reduced to zero while simultaneously achieving high-order diffraction suppression and momentum echo optimization, resulting in higher diffraction efficiency η. opt .

[0021] (7) In the above steps (4) and (5), the output is formed by stable multi-path interference and has a definite phase relationship; the optimal output of the diffraction order m=1 is determined by the phase difference between the driving radio frequency signals of the first acousto-optic modulator AOM1 and the second acousto-optic modulator AOM2 and the offset distance δL of the 4-F optical imaging system; by changing the common driving phase of the first acousto-optic modulator AOM1 and the second acousto-optic modulator AOM2 Simultaneously maintain phase difference By keeping the phase constant, precise phase control of the diffracted light can be achieved; simultaneously, by changing the driving intensity of the first acousto-optic modulator AOM1 and the second acousto-optic modulator AOM2, the diffraction efficiency η can be increased from zero to η. opt Fine-tuning enables high-contrast adjustable beam splitter and optical routing functions.

[0022] The method of this invention has extremely high diffraction efficiency (η). opt With a rejection ratio of >99% and a zero-order suppression ratio (single-mode coupling suppression ratio α>30dB), it has extremely high operating bandwidth and precise phase modulation capability. It can replace traditional technologies such as piezoelectric, electro-optic, and traditional acousto-optic modulation in the following technical fields, update combined acousto-optic modulation technology [6,7], and improve the speed, efficiency, accuracy, and intensity damage threshold of laser modulation:

[0023] (1) Pulsed optical routing technology: realizes time-division multiplexing (TDM) of pulsed laser [7] and quantum light source [4].

[0024] (2) Pulse optical path coherent accumulation technology: coherently combine multiple laser beams to form a higher power laser [5].

[0025] (3) Adjust the group velocity of the pulsed laser by the beam splitting ratio of the interferometer to achieve carrier-envelope phase adjustment of the high-power optical comb [2].

[0026] (4) Through the iterative application of efficient acousto-optic modulation, the optical frequency can be moved over a wide range of tens of GHz [3].

[0027] Furthermore, this invention is not limited to the coherent combination of two AOMs; a multi-lens 4-F optical imaging system can be used to achieve the coherent combination of multiple AOMs. Assuming the number of sub-AOMs in the combined acousto-optic modulation system is N, the driving power required for each sub-AOM is reduced to a level typically required for [the latter]. Therefore, this invention can significantly reduce the RF drive power requirements for acousto-optic modulation, thereby expanding the range of acousto-optic modulation crystals. For example, it can facilitate the development of acousto-optic modulation systems with low insertion loss and high damage threshold using low acousto-optic materials such as quartz crystals, as well as the development of high-frequency modulation, deep ultraviolet wavelength laser modulation, and other technologies. Furthermore, in the N-AOM system, by finely adjusting the corresponding δL displacement of the acousto-optic modulator (AOM) in each imaging system, the diffraction efficiency can be further improved.

[0028] Another objective of this invention is to provide a combined acousto-optic modulation method based on retro-reflection interference, which has high diffraction efficiency, low driving power, and low cost.

[0029] The combined acousto-optic modulation method based on retro-reflection interference provided by this invention uses an optical imaging system to accurately reflect the primary diffraction output of an acousto-optic modulator (AOM), and then inputs it back into the modulator for secondary diffraction, thereby realizing the multiplexing of the acousto-optic modulator (AOM) and achieving high diffraction efficiency or high contrast using only a single acousto-optic modulator (AOM) to achieve combined acousto-optic modulation. Based on the optical path characteristics of the retro-reflection optical system, analogous to the Sagac interferometer and the Michelson interferometer [[9] M. Born and E. Wolf, Principles of Optics (Cambridge University Press) (2019)], the optical imaging system of this invention can be divided into two configurations: Sagac type and Michelson type (see Figures 6 and 7). Both configurations achieve efficient and low-power composite acousto-optic modulation using CN113777811B, CN113725714B and the aforementioned single acousto-optic modulator.

[0030] For the Saniac type, the specific steps for combined acousto-optic modulation are as follows:

[0031] (1) An acousto-optic modulator (AOM) performs a single-frequency acousto-optic modulation on the laser; let the length of the acousto-optic crystal of the acousto-optic modulator (AOM) be L, and use the acousto-optic modulator (AOM) to modulate a laser beam whose central wave vector is located at... A laser with a transverse wave vector broadening of Δk is subjected to single-frequency acousto-optic modulation to obtain the output wave vector corresponding to the m-order Bragg diffraction. The center of this wave vector is located at... w is the waist of the Gaussian beam; k0 is the wave vector of the normally incident beam, k s To modulate the sound field wave vector; let the sound wave driving frequency be f. S The angular frequency is ω S =2πf S .

[0032] (2) Secondary diffraction is formed by the acousto-optic modulator (AOM); the multi-level diffraction output of the acousto-optic modulator (AOM) is collimated by lens l1 with focal length f1, and after a propagation distance of approximately f1+f2, it is focused by lens l2 with focal length f2 onto the reflecting mirror M. s All the reflected diffraction orders pass in reverse order through lens l. 2, After l1, it re-enters the acousto-optic modulator (AOM) to form a secondary diffraction.

[0033] (3) Adjust the distance between lenses l1 and l2 to coherently add the diffraction effects; record the output end of the acousto-optic modulator (AOM) and the reflector M. s Spacing is L D From step (2), we can know that L D ≈2(f1+f2). By adjusting the distance between lenses l1 and l2, L is made...D =πc / ω S Here, c ≈ 3 × 10 8 meters per second is the speed of light. The L... D The value is chosen to ensure that the phase difference between the first and second acousto-optic modulator (AOM) diffractions is equal. Achieve coherent addition of diffraction effects.

[0034] (4) Fine-tune the incident light angle; drive the acousto-optic modulator (AOM) with a weak radio frequency signal and fine-tune the incident light angle so that the diffraction efficiency η of the m=1 order of single diffraction is less than 50%, and the diffraction spots of the m=0 and 1 orders are nearly equal; for the acousto-optic modulator (AOM) designed and optimized at 100% radio frequency drive, the total diffraction loss of the m≠0 and 1 orders is in the 1% range.

[0035] (5) Minimize local diffraction loss through fine-tuning; separate the return light and incident light using a polarization beam splitter, observe the composite acousto-optic modulated output using a digital camera, and calculate the diffraction efficiency η of the m=1 order in real time; change the RF drive intensity (A0) to maximize the m=1 order diffraction efficiency; fine-tune the distance L1 = f1 + δL between the acousto-optic modulator (AOM) and lens l1, so that the diffraction intensity of the m=-1 and 2nd orders has... Spatial periodic oscillation, therefore δL in δL n =nδL h +δL off Achieve local minimization of diffraction loss for orders m = -1 and 2; where n is the oscillation period index, n = 0, 1, 2, ..., δL off For bias;

[0036] (6) High diffraction efficiency and high single-mode suppression ratio are achieved through fine-tuning; specifically, the distance L1 = f1 + δL between the acousto-optic modulator (AOM) and lens l1 is fine-tuned. At this point, the globally optimal diffraction drive configuration is obtained, achieving a diffraction efficiency η of order m = +1. opt >99%, targets with a single-mode suppression ratio greater than 30dB at level m=0. Here [...] is the rounding symbol. It is the average refractive index of the acousto-optic crystal. This is the optimal distance for momentum echo compensation by broadening the Δk diffraction phase under the two-mode approximation, neglecting higher-order diffraction losses. The coefficient ξ is related to the sound field distribution profile. When the secondary diffraction position is δL... opt At that time, due to momentum echo compensation, the requirements of the combined acousto-optic modulation on the Bragg condition are reduced, and the zero-order residual is suppressed; on the other hand, this distance ensures that the higher-order diffraction caused by the two acousto-optic modulations coherently cancels each other out.

[0037] (7) Further, through fine-tuning, the optimization of high-order diffraction suppression and momentum echo compensation is achieved; specifically, by finely adjusting the driving sound field frequency ω S =v S k S And repeat steps (1)-(5), and the steps in step (6) can be completed. bias δL in off The efficiency η is reduced to zero while simultaneously optimizing high-order diffraction suppression and momentum echo compensation, resulting in higher diffraction efficiency η. opt .

[0038] (8) Furthermore, the relative phase of the incident beam after two diffractions by the acousto-optic modulator (AOM) Stable. By changing the drive phase of the acousto-optic modulator (AOM). Precise phase control of the m=1st order diffracted light under combined acousto-optic modulation can be achieved; simultaneously, by changing the driving intensity A0 of the acousto-optic modulator (AOM), the m=1st order diffraction efficiency η can be increased from zero to η. opt Fine-tuning enables high-contrast adjustable beam splitter and optical routing functions.

[0039] Furthermore,

[0040] For the Michelson type, the combined acousto-optic modulation method specifically involves changing the position of the reflector in the Sanignac type to the back focal point of the collimating lens l1 (see Figure 3), adjusting the reflector angle so that the outputs of each stage of the first diffraction can return along the original path, and removing subsequent optical elements. Let the new reflector be M. m Then the acousto-optic modulator (AOM) - lens l1 - reflector M m This constitutes a Michelson-type combined acousto-optic modulation system. The characteristic of this configuration is that the diffraction order m' of the second acousto-optic diffraction corresponds to the first diffraction order m with the relationship m' = 1 - m. Therefore, the first-order diffraction phase evolves at twice the driving radio frequency. Therefore, the diffraction efficiency exhibits periodic oscillations.

[0041] Based on the Michelson combined acousto-optic modulation configuration, by synchronizing the driving radio frequency signal and the mode-locked pulse laser output, the repetition frequency of f can be reduced to f. rep =4f S The mode-locked pulsed laser of / (2n+1) achieves efficient switching between m=0 order transmission and m=1 order diffraction, thereby simultaneously realizing the f' of the incident laser in both transmission and diffraction optical paths. rep =f rep / 2 frequency division, the single-mode coupling suppression ratio of adjacent pulses can be as high as 30dB or more; n is an integer.

[0042] The efficient combined acousto-optic modulation method based on retro-reflection interferometry of this invention reduces the RF signal power of the acousto-optic modulator (AOM) in the constructed coherent combined acousto-optic modulation system to the power required by conventional acousto-optic modulation techniques. This alleviates power consumption pressure and expands the selection and design space for acousto-optic crystals; moreover, by finely adjusting the corresponding δL displacement of the acousto-optic modulator (AOM) in each imaging system, the diffraction efficiency is further improved.

[0043] For the Saniac type, a diffraction efficiency exceeding 99% and a single-mode suppression ratio exceeding 30 dB can be achieved, enabling on-demand laser routing. For the Michelson type, a single-mode contrast exceeding 30 dB can be achieved, enabling synchronous frequency division of high-repetition-rate pulsed lasers. This invention combines the advantages of acousto-optic modulation technology in high diffraction efficiency and low driving power, while reducing costs and facilitating the development of miniaturized devices. This invention has broad application prospects in precision optical manipulation fields such as pulsed laser modulation, optical and quantum information processing, and laser coherent beam splitting / combining. Attached Figure Description

[0044] Figure 1 is a schematic diagram of the dual AOM coherent combined acousto-optic modulation system of the present invention.

[0045] Figure 2 shows the diffraction results of the dual AOM coherent combination system under different driving conditions.

[0046] Figure 3 shows the variation of the m=1 order diffraction efficiency of the dual AOM coherent combination system when driven by different RF intensities, where the horizontal axis is the ratio of the actual RF drive intensity to the optimized drive intensity.

[0047] Figure 4 shows the changes in the maximum diffraction efficiency and the corresponding 0th-order suppression ratio of the dual-AOM coherent combination system and the single-AOM system when the driving frequency is changed.

[0048] Figure 5 shows the relationship between the position offset δL of AOM2 and the diffraction efficiency of m = -1, 0, 1, and 2 orders in a coherent combined acousto-optic modulation system with two AOMs, given by numerical simulation. Higher-order diffraction (m = -1, 2 orders) exhibits periodic oscillations, with oscillation periods...

[0049] Figure 6 is a schematic diagram of the combined acousto-optic modulation system of the present invention in the form of a Saniac.

[0050] Figure 7 is a schematic diagram of the combined acousto-optic modulation system of the present invention in Michelson type.

[0051] Figure 8 is a schematic diagram of the transformation from a Saniac-type mirror to a Michelson-type mirror.

[0052] Figure 9 shows the diffraction effect of the Sagnac interferometer configuration compared to a conventional single AOM.

[0053] Figure 10 shows the experimental results of frequency division of mode-locked pulsed laser using the Michelson interferometer configuration. Among them, (a) is the time-domain plot of the input pulsed laser, and (b) is the time-domain plot of the pulsed laser after synchronous frequency division using the Michelson interferometer configuration. Detailed Implementation

[0054] This invention utilizes a precision diffraction superposition method to achieve an ultra-high acousto-optic diffraction efficiency far exceeding that of a traditional single AOM. Its core lies in the process of precision diffraction coherent superposition, where AOM2 is moved an appropriate distance from the imaging position of AOM1. This suppresses the diffraction phase shift broadening of the focused beam Bragg diffraction through momentum echo, while simultaneously adjusting higher-order diffraction phase shifts to precisely suppress higher-order diffraction losses through coherent destructive effects. This results in a first-order diffraction efficiency exceeding 99% and a 0th-order single-mode suppression ratio exceeding 30 dB.

[0055] Example 1

[0056] Taking the coherent dual-AOM combination system shown in Figure 1 as an example, an experiment was conducted. In the experiment, the two AOMs used as basic units were of the same type, with an optimized frequency of 80MHz and a sound velocity of 4260m / s. Continuous light (λ=780nm) was incident on AOM1 under the Bragg condition, and the outgoing light was precisely imaged onto AOM2 by a 4-F optical imaging system composed of two achromatic lenses (F=100mm) for secondary diffraction. It was necessary to ensure that the sound field direction of AOM2 was the same as the image direction of the sound field of AOM1. The driving phase of AOM2 was changed to make the first-order diffraction brightest. The position of AOM2 was finely adjusted back and forth along the optical axis of the 4-F imaging system to minimize the local higher-order diffraction loss. The final diffraction intensity distribution was collected using a CCD camera and fed back to the optimization program to optimize the AOM. 1,2 With the driving power, excellent diffraction effect is finally obtained, as shown in Figure 2(c).

[0057] Figure 2 shows the output results of the above demonstration experimental setup under three different driving conditions: (a) the state when neither AOM is driven, in which case no diffraction occurs; (b) the state when only one AOM is driven to reach diffraction maxima, at which point the first-order diffraction efficiency is approximately 90%, and higher-order diffraction spots are clearly visible; (c) the state when both AOMs are driven simultaneously under optimal driving conditions, at which point the first-order diffraction efficiency is >99%, and higher-order diffraction spots are almost invisible, while the 0th-order spot is also greatly suppressed. As shown in Figure 4, the 0th-order suppression ratio can reach approximately 22 dB in free space, and after single-mode ray mode selection, the 0th-order suppression ratio can be further increased to over 30 dB.

[0058] Figure 3 shows the trend of first-order diffraction efficiency when the driving intensity of the two AOMs is adjusted simultaneously without changing the phase difference between them. This experimental result demonstrates that the present invention can achieve fine control of the first-order diffraction efficiency of a dual-AOM coherent combination system by changing the AOM driving intensity.

[0059] Figure 4 shows the maximum first-order diffraction efficiency (solid line) and the corresponding zero-order suppression ratio (dashed line) in free space for single-AOM diffraction and dual-AOM coherent combination diffraction at other driving frequencies. As the driving frequency shifts further away from the optimized frequency (80MHz), both the maximum first-order diffraction efficiency and the zero-order suppression ratio decrease. However, within a driving range of 80±40MHz, the dual-AOM coherent combination diffraction consistently outperforms the single-AOM diffraction. These experimental results demonstrate that the dual-AOM coherent combination system not only possesses superior first-order diffraction efficiency and zero-order suppression ratio compared to single-AOM, but also boasts a larger driving frequency bandwidth.

[0060] Figure 5 illustrates the effect of the fine-tuning distance δL on different diffraction orders, based on numerical simulation. The horizontal axis represents the fine-tuning distance δL, and the vertical axis represents the diffraction efficiency. The large trend in efficiency between the 1st and 0th orders is due to the diffraction phase shift broadening caused by the Bragg diffraction of the focused beam. The optimal distance for momentum echo compensation is achieved when δL satisfies this requirement. When ξ≈0.2, the first-order diffraction efficiency reaches its peak. The oscillation trend within a small range is influenced by higher-order diffraction. As δL continuously changes, the coherence conditions of higher-order diffraction in the second diffraction change periodically, and the higher-order diffraction also oscillates periodically, with a period of . The numerical simulation results show that the fine-tuning distance of the second AOM in the dual AOM coherent system is the result of the combined effect of the momentum echo compensation distance and the position corresponding to the lowest point of the higher-order oscillation.

[0061] Example 2

[0062] This embodiment utilizes precise retroreflection to achieve efficient combined acousto-optic modulation using only a single acousto-optic modulator (AOM). Its core lies in using an imaging system to precisely retroreflect the primary diffraction output of the AOM, then inputting it back into the same AOM for secondary diffraction, thus achieving multiplexing of the AOM. Based on the characteristics of the retroreflection optical system, it can be divided into two types: the Sanignac configuration and the Michelson configuration.

[0063] Figure 6 shows a schematic diagram of a Saniac-type combined acousto-optic modulation system. A system was built according to the schematic diagram for demonstration experiments. In the experiment, the optimized frequency of the acousto-optic modulator (AOM) was 200MHz, and the speed of sound in the crystal was approximately 4260m / s. The collimated continuous light (λ = 780nm) was focused by an achromatic lens l0 (focal length f0 = 100mm) and incident on the acousto-optic modulator (AOM) under the Bragg condition. The first diffraction output was collimated by an achromatic lens l1 with a focal length f1 = 100mm, and after propagating for a distance of approximately f1 + f2, it was focused onto mirror M by a plano-convex lens l2 with a focal length f2 = 250mm. s Adjust the angle of the reflecting mirror so that the diffraction orders m = ..., -1, 0, 1, 2, ... are reflected back along paths ..., 2, 1, 0, -1, ...; all the reflected diffraction orders pass back through lens l in reverse order. 2,1 Then, it re-enters (AOM) to form secondary diffraction; the AOM is driven to m=0, and the first-order diffraction spots are nearly identical. The lens l2 and the mirror M are optimized. s The distance between the system and lens l1 is adjusted to make the m=1st order diffraction spot in the final outgoing light the brightest; the position of AOM is finely adjusted back and forth along the optical axis of the system to minimize the local higher-order diffraction loss; the final diffraction intensity distribution is collected by a CCD camera and fed back to the optimization program to optimize the driving power of AOM, and finally excellent diffraction effect is obtained, as shown in Figure 9(c).

[0064] Figure 7 shows a schematic diagram of a Michelson-type combined acousto-optic modulation system. A system was built based on this diagram for demonstration experiments. In the experiment, the incident light was a picosecond pulsed mode-locked laser (λ = 795 nm), and the pulse repetition frequency f... rep =80MHz; therefore, the driving frequency f of the acousto-optic modulator (AOM) is... s =100MHz, the velocity of sound in the crystal is approximately 4260m / s; the collimated pulsed mode-locked laser is focused by an achromatic lens l0 (focal length f0 = 100mm) and then incident on an acousto-optic modulator (AOM) under Bragg conditions; the first diffraction output is collimated by an achromatic lens l1 with a focal length f1 = 100mm, and the reflecting mirror M, located at the rear focal plane of lens l1, is adjusted. m The diffracted light of each order is re-injected into the AOM along the original optical path to form secondary diffraction; the acousto-optic modulator (AOM) is driven to m=0, and the first-order diffracted light spots are nearly identical. The phase of the acousto-optic modulator (AOM) driving radio frequency is changed to synchronize it with the pulse output of the mode-locked laser; a high-speed photodetector is used to monitor one channel in the output to optimize the driving intensity of the AOM and further suppress the intensity of adjacent pulses to improve its contrast.

[0065] Figure 8 is a schematic diagram of the transformation from a Saignac-type adjustable reflector to a Michelson-type system, which is a schematic diagram of the switching method for the two types of combined acousto-optic modulation systems shown in Figures 6 and 7. It illustrates the switching method for two types of combined acousto-optic modulation systems, combining the Saignac-type combined acousto-optic modulation system shown in Figure 6 and the Michelson-type combined acousto-optic modulation system shown in Figure 7. Specifically, a Michelson-type reflector (M...) is inserted into the illustrated system. m This optical path is equivalent to that in Figure 7, forming a Michelson-type combined acousto-optic modulation. However, if the Michelson-type reflector (M...) is removed... m ), and add a Sanigneac-type reflector (M s This forms a Saniac-type combined acousto-optic modulation equivalent to that shown in Figure 6. A system can be built according to the schematic diagram for demonstration experiments.

[0066] Figure 9 shows a comparison of the diffraction results of the Sagnac-type combined acousto-optic modulation technology and the conventional acousto-optic modulation technology: (a) is the light spot incident on the Sagnac-type combined acousto-optic modulation device and the conventional acousto-optic modulation device; (b) is the optimal diffraction result of the conventional acousto-optic modulation device, which can be obtained after one diffraction of the Sagnac-type combined acousto-optic modulation device, at which time the first-order diffraction efficiency is about 90%; (c) is the optimized diffraction result of the Sagnac-type combined acousto-optic modulation device, where the brightness of the 0th order and higher-order diffractions is significantly weaker, and the first-order diffraction efficiency is about 98% (which can be further optimized), and the 0th order suppression ratio in free space is about 20dB. After single-mode fiber selection, the 0th order suppression ratio will be further improved to more than 30dB.

[0067] Figure 10 shows the time-domain plots of the incident light and the output light from one of the channels in the Michelson-type combined acousto-optic modulation device. (a) is the time-domain plot of the incident pulsed mode-locked laser with a repetition frequency of 80 MHz; (b) is the time-domain plot of the pulsed light output from one of the channels, where the repetition frequency of the output laser drops to 40 MHz, and almost no pulse residue is visible between two adjacent pulses. At this point, the contrast is approximately 23 dB (which can be further optimized).

[0068] References:

[0069] [1] J.Thom et al, "Accurate and agile digital control of optical phase, amplitude and frequency for coherent atomic manipulation of atomic systems," Opt.Express 21,18712(2013).

[0070] [2]W.Hansel,M.Giunta,M.Fischer,M.Lezius,and R.Holzwarth,Rapid electro-optic control of the carrierenvelope-offset frequency for ultra-low noise frequency combs,2017Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium,EFTF / IFC 2017-Proceedings,128–129(2017).

[0071] [3]Zhou et al,Laser frequency shift up to5GHz with a high-efficiency 12-pass 350-MHz acousto-optic modulator,Rev.Sci.Instrum.91,033201(2020).

[0072] [4]S.Yu,et al,A universal programmable Gaussian boson sampler for drug discovery,Nature Computational Science 3,839,848(2023).

[0073] [5]Arno Klenke et al,Coherent Beam Combination of Ultrafast Fiber Lasers,IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS,VOL.24,NO.5,(2018).

[0074] [6]A.Feldman,“Measurement of the Photoelastic Contents of Optical Materials,”Opt.Eng.17,453(1978).

[0075] [7] Wu Saijun, Ma Yudi, Liu Ruijuan, Qiu Liyang, Invention Patent: High-bandwidth composite acousto-optic modulation method based on multiple 4F imaging, Patent No.: CN113777811B.

[0076] [8] Wu Saijun, Liu Ruijuan, Ma Yudi, Qiu Liyang, Invention Patent: Laser Pulse Repetition Rate Ultra-High Speed ​​Frequency Division Method Based on Dual-Path Acousto-Optical Interference, Patent No.: CN113725714B.

[0077] [9] M. Born and E. Wolf, Principles of Optics (Cambridge University Press) (2019).

Claims

1. A combined acousto-optic modulation method based on precision diffraction superposition, characterized in that, First, a coding module of synchronous radio frequency signal and a coherent combination acousto-optic modulation system with precise adjustment are constructed; wherein: The coding module of synchronous radio frequency signal provides phase-stable and arbitrarily programmable radio frequency signal, which is amplified by a signal amplifier and then input into the coherent combination acousto-optic modulation system to drive the acousto-optic modulator (AOM) in the system to work; The coherent combination acousto-optic modulation system is composed of two same type acousto-optic modulators (AOM) with close geometric size and a double-lens 4-F optical imaging system; wherein, the acousto-optic modulator converts the radio frequency signal into sound waves with corresponding frequency, intensity and phase, and produces acousto-optic diffraction on the incident pulsed laser; the double-lens 4-F optical imaging system is composed of two aplanatic lenses with focal length F; the diffraction output of the first acousto-optic modulator (AOM1) is precisely imaged to the second acousto-optic modulator (AOM2) with sound wave co-propagation with magnification M=1, forming interference of twice diffraction; In the process of realizing the amplitude coherent superposition of Bragg diffraction of two acousto-optic modulators (AOM) by using the double-lens 4-F optical imaging system, the second acousto-optic modulator (AOM2) is moved out of the imaging position of the first acousto-optic modulator (AOM1) by a proper distance to suppress the diffraction phase shift spread of the focused beam Bragg diffraction in the form of momentum echo, and the high-order diffraction phase shift is adjusted to precisely suppress the loss of high-order diffraction by coherent destructive effect, so as to obtain diffraction efficiency of more than 99% and single-mode suppression ratio of transmitted light of more than 30dB; the specific steps are as follows: (1) The length of the acousto-optic crystal of the acousto-optic modulator (AOM) is L; a first acousto-optic modulator (AOM1) is used to modulate a light beam with a central wave vector located at The laser with transverse wave vector spread of Δk is single-frequency acousto-optically modulated to obtain the output wave vector center of m-order Bragg diffraction corresponding to Ak = π / w, w is the waist of the Gaussian light; k0 is the wave vector of the normally incident light beam, k s is the wave vector of the modulated sound field. (2) Using the double-lens 4-F optical imaging system, the first acousto-optic modulator (AOM1) crystal sound field and the light beam interaction center are imaged to the center of the second acousto-optic modulator (AOM2); the sound field direction of the second acousto-optic modulator (AOM2) is kept the same as the image direction of the first acousto-optic modulator (AOM1); (3) The first acousto-optic modulator (AOM1) and the second acousto-optic modulator (AOM2) are driven by weak radio frequency signals respectively, the incident light angle is adjusted, so that the m=1 order diffraction efficiency of the first acousto-optic modulator (AOM1) and the second acousto-optic modulator (AOM2) is less than 50% and close to equal; for the light modulator (AOM) designed and optimized at 100% radio frequency drive, the sum of the diffraction losses of m≠0, 1 order is in the level of 1%; (4) The second acousto-optic modulator (AOM2) performs secondary diffraction on all levels of diffraction light emitted by the first acousto-optic modulator (AOM1), the driving phase of the second acousto-optic modulator (AOM2) is changed to make the m=1 order diffraction the brightest, the position δL of the second acousto-optic modulator (AOM2) is adjusted forward and backward along the optical axis of the double-lens 4-F optical imaging system, and the m=-1, 2 order diffraction is made the brightest, and the diffraction efficiency of the m=1 order diffraction is adjusted to 99% or more. The intensity of the radiation has of the spatial period of oscillation, so that δL is in the range δL n = nεL h + δL off achieves a local minimum of the m = -1, 2 order diffraction loss; where n is the oscillation period number, n = 0, 1, 2,..., δL off is the offset; (5) by program control jointly optimizing the driving power of the first acousto-optic modulator (AOM1) and the second acousto-optic modulator (AOM2), in obtaining a globally optimal drive configuration, where [...] is the rounding symbol, is the average refractive index of the acousto-optic crystal, is the optimal distance for Δk phase broadening to compensate momentum blips in two-mode approximation, ignoring high-order diffraction loss, the coefficient ξ is related to the acoustic field profile; when the position of the second acousto-optic modulator (AOM2) is set as δL opt , the requirement of the combined acousto-optic modulation to the Bragg condition is reduced due to momentum blip compensation, and the zero-order residual is suppressed; on the other hand, this distance ensures the coherence cancellation of high-order diffraction caused by the first acousto-optic modulator (AOM1) and the second acousto-optic modulator (AOM2); at the δL opt distance, the m = + 1 order diffraction efficiency η opt > 99%, and the m = 0 order single-mode suppression ratio is greater than 30 dB.

2. The combination acousto-optic modulation method based on precision diffraction superposition according to claim 1, characterized in that, Further fine-tuning the driving sound field frequency ω S =v S k S In step (5) bias δL in off The efficiency η is reduced to zero while simultaneously optimizing high-order diffraction suppression and momentum echo compensation, resulting in higher diffraction efficiency η. opt .

3. The combined acousto-optic modulation method based on precision diffraction superposition according to claim 1, characterized in that, In the process of steps (4) and (5), the output is formed by stable multi-path interference with a determined phase relationship; the optimal output of the diffraction order m=1 is determined by the phase difference corresponding to the driving radio frequency signals of the first acousto-optic modulator (AOM1) and the second acousto-optic modulator (AOM2) and the offset distance δL of the 4-F optical imaging system; the common driving phase of the first acousto-optic modulator (AOM1) and the second acousto-optic modulator (AOM2) is changed While keeping the phase difference unchanged, the precise phase control of the diffracted light can be realized; at the same time, by changing the driving intensity of the first acousto-optic modulator (AOM1) and the second acousto-optic modulator (AOM2), the diffraction efficiency η can be finely adjusted from zero to η opt , thereby realizing the functions of high-contrast adjustable beam splitter and optical routing.

4. The ultra-high efficiency combined acousto-optic modulation method based on precision diffraction superposition according to any one of claims 1-3, characterized in that, In the constructed coherent combination acousto-optic modulation system, the number of acousto-optic modulators (AOMs) is N, the number of corresponding 4-F optical imaging systems is N, N≥2, and the RF signal power required to drive a single acousto-optic modulator (AOM) is reduced to The power consumption pressure is relieved, the acousto-optic crystal selection and design space are expanded, and the diffraction efficiency is further improved through the corresponding δL displacement fine adjustment of the acousto-optic modulator (AOM) in each imaging system.

5. A combined acousto-optic modulation method based on retro-reflective interference, characterized in that, The application utilizes an optical imaging system to precisely reflect the first diffraction output of an acousto-optic modulator (AOM) and input the modulator again for second diffraction, realizes multiplexing of the acousto-optic modulator (AOM), and achieves high diffraction efficiency or high contrast of combined acousto-optic modulation by using only a single acousto-optic modulator (AOM). The optical imaging system is divided into two configurations: Sagnac type and Michelson type. For the Sagnac type, the specific steps of the combined acousto-optic modulation are as follows: (1) The laser is modulated by an acousto-optic modulator (AOM) once in a single frequency. The length of the acousto-optic crystal of the acousto-optic modulator (AOM) is L. A beam of light with a central wave vector located at k0is modulated by the acousto-optic modulator (AOM). The laser with transverse wave vector spread of Δk is single-frequency acousto-optically modulated to obtain an output wave vector corresponding to m-order Bragg diffraction, and the center of the wave vector is located at Δk = π / w, where w is the waist of the Gaussian beam; k0 is the wave vector of the normally incident beam, and k s To modulate the sound field wave vector; let the sound wave driving frequency be f. S The angular frequency is ω S =2πf S ; (2) the second diffraction is formed by an acousto-optic modulator (AOM); the multi-stage diffraction output of the acousto-optic modulator (AOM) is collimated by a lens l1 with a focal length of f1, and after propagating a distance of about f1+f2, is focused by a lens l2 with a focal length of f2 to a mirror M s ; all the reflected diffraction orders pass through the lenses l 2, , l1 in reverse order, and then enter the acousto-optic modulator (AOM) again to form the second diffraction; (3) Adjust the distance between the lenses l1, l2 to make the diffraction effect coherently added; record the exit end of the acousto-optic modulator (AOM) and the mirror M s The distance is L D According to step (2), L D ≈2(f1+f2); by adjusting the distance between the lenses l1, l2, so that L D =πc / ω S ; here c≈3×10 8 m / s is the speed of light, and the value of L D ensures that the phase difference of the first and second acousto-optic modulators (AOM) diffraction is Realize coherent addition of diffraction effects; (4) Fine-tune the incident light angle; drive the acousto-optic modulator (AOM) with a weak radio frequency signal to fine-tune the incident light angle, so that the m=1 order diffraction efficiency of single diffraction is less than 50%, and the m=0, 1 order diffraction spots are close to being identical; for the design optimization of the acousto-optic modulator (AOM) driven at 100% radio frequency, the total diffraction loss of m≠0, 1 order is in the level of 1%; (5) By fine-tuning, the diffraction loss is locally minimized; the back-reflected light and the incident light are separated by means of a polarization beam splitter, the composite acousto-optic modulation exit is observed by using a digital camera, and the diffraction efficiency η of m = 1 is calculated in real time; the radio frequency driving strength (A0) is changed to maximize the diffraction efficiency of m = 1; the distance L1 = f1+ δL between the acousto-optic modulator (AOM) and the lens l1 is fine-tuned, and the m = -1, 2 order diffraction intensities have of the spatial period oscillation, δL in δL n = nδL h + δL off achieves the local minimization of the m = -1, 2 order diffraction loss; wherein n is the oscillation period number, n = 0, 1, 2…, δL off is the bias; (6) By fine adjustment, high diffraction efficiency and high single-mode suppression ratio are realized; specifically, the interval L1 of the fine adjustment acousto-optic modulator (AOM) and the lens l1 = f1 + δL, wherein f1 is the focal length of the lens l1, and δL is the fine adjustment interval of the AOM and the lens l1. The diffraction drive configuration is obtained globally optimal, and m = +1 level diffraction efficiency η is realized opt > 99%, the target of m = 0 level single mode suppression ratio is greater than 30dB; here [...] is the integral symbol, is the average refractive index of the acousto-optic crystal, is the optimal distance for Δk diffraction phase broadening to achieve momentum echo compensation in two-mode approximation, ignoring the higher order diffraction loss, the coefficient ξ is related to the sound field distribution profile; when the two diffraction positions are taken as δL opt When the two diffraction positions are taken as δL , the requirement of the Bragg condition for the combined acoustooptic modulation is reduced due to the momentum echo compensation, and the zero-order residual is suppressed; on the other hand, this distance Ensure that the high-order diffraction of twice acousto-optic modulation is coherent and cancelled.

6. The combined acousto-optic modulation method of claim 5, wherein, Further by fine tuning, the optimization of high-order diffraction suppression and momentum kick compensation is achieved; specifically, by fine tuning the driving acoustic field frequency ω S = v S k S , and repeating steps (1)-(5), the bias δL in step (6) off is reduced to zero, while achieving the optimization of high-order diffraction suppression and momentum kick compensation, obtaining a higher diffraction efficiency η opt .

7. The combined acousto-optic modulation method of claim 5, wherein, The relative phase of the two successive diffractions of the incident light beam by the acousto-optic modulator (AOM) Stabilization; by changing the driving phase of the acousto-optic modulator (AOM) Realize the precise phase control of the m = 1 order diffracted light under the combined acousto-optic modulation; at the same time, by changing the driving strength A0 of the acousto-optic modulator (AOM), the m = 1 order diffraction efficiency η is realized from zero to η opt Fine adjustment, realize high contrast adjustable beam splitter and optical routing function.

8. The combined acousto-optic modulation method according to any one of claims 5 to 7, characterized in that, Adjust the position of the mirror in the reflection optical system to be Michelson type, specifically, place the mirror at the back focal point of the collimating lens l1, adjust the angle of the mirror so that the output of each level of the first diffraction can return the original way, and remove the subsequent optical elements; record the adjusted mirror as M m , then the acousto-optic modulator (AOM) - lens l1 - mirror M m Michelson type combined acousto-optic modulation system is formed; this configuration corresponds to the diffraction order m' of the second acousto-optic diffraction and the diffraction order m of the first diffraction with the relationship m' = 1-m; therefore, the first-order diffraction phase evolves at twice the frequency of the driving radio frequency, , so that the diffraction efficiency oscillates periodically 9. The combined acousto-optic modulation method of claim 8, wherein, By synchronizing the driving radio frequency signal and the mode-locked pulsed laser output, the mode-locked pulsed laser with a repetition frequency of f rep =4f S / (2n+1) can realize efficient switching between m=0 order transmission and m=1 order diffraction, and further realize f rep =f rep / 2 frequency division in the transmission and diffraction optical paths, and the single-mode coupling suppression ratio of adjacent pulses is as high as 30dB or more; n is an integer.

10. The combined acousto-optic modulation method of claim 8, wherein, The optical imaging system is switched between Sagnac type and Michelson type in the following way: a Michelson configuration mirror M m is inserted in the optical imaging system, which forms a Michelson type combined acousto-optic modulation; and if the Michelson configuration mirror M m is removed and a Sagnac type mirror M s is added, a Sagnac type combined acousto-optic modulation is formed.

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