光声层析重建飞秒光丝二维横向图像仿真研究【增强内容出版】
Filament refers to a plasma channel with high laser intensity and high plasma density formed by the propagation of intense femtosecond laser pulses in a transparent medium. Several literatures have shown that the cross-section image of an optical filament at a specific z usually contains abundant structural information such as filament diameter, length, and energy distribution, which is of great significance for the visualization study of the dynamic process of filament formation. Moreover, accurate acquisition of the spatial structure and energy deposition distribution of femtosecond optical filaments are also of great significance for the development of filamentation-based atmospheric applications. Nevertheless, it is also the inherent parameter most difficult to measure directly. To solve the problem, we introduce a new medical imaging method named photoacoustic tomography (PAT) for optical filament cross-section imaging. The feasibility of reconstructing monofilament and multifilament images by photoacoustic tomography is verified theoretically. Moreover, we also study the influence of the performance parameters of the ultrasonic transducers on the optical filament image reconstruction.
We adopt a forward simulation model based on the photoacoustic wave equation to simulate the acquisition process of ultrasonic signals induced by optical filaments in air. A circular-scanning-based PAT system is considered to obtain the cross-section image of the laser filament. To simplify the problem, we assume that the initial heat source distribution of the optical filament satisfies the Gaussian distribution form, which can represent both the small high-energy core of the optical filament and its weak background energy region with a larger range. Based on experimental measurements, the initial maximum energy deposition density is assumed to be in the order of 10 mJ/cm3, and the diameter of the heat source is assumed to be in the order of 100 μm. The simulated time series of the acoustic signal is then applied to reconstruct the transverse distribution of femtosecond laser filaments with delay and sum (DAS) algorithm. Moreover, we also analyze the influence of performance parameters of ultrasonic transducers such as center frequency, bandwidth, surface size, and detection surface sensitivity on the reconstruction of filament cross-sectional images. The back-projection amplitude distribution profile along the y-axis is leveraged to compare the effect of image reconstruction.
According to the time series of ultrasound signals generated by monofilaments and multifilaments recorded at different detection distances, the frequency of monofilament and multifilament induced by femtosecond laser with multi-millijoule pulse energy is mainly concentrated within 4 MHz (Fig. 2). The signal spectrum of monofilament is single-peak structure, while the acoustic signal spectrum of multifilament is multi-peak structure (Fig. 2). The amplitude value of sound pressure signal decreases rapidly due to the attenuation of air. As the center of the optical filament deviates further from the scanning center, the cross-section image of the optical filament reconstructed by the back-projection (BP) algorithm and the DAS algorithm appears an obvious "elongated" phenomenon in the tangential direction (y-axis), which is the so-called "finite aperture effect" (Fig. 3). For monofilaments, the maximum energy amplitude decreases significantly with the increase in the center frequency of the transducer, which may be related to the filtering out of more low-frequency signals (Fig. 4). The same method is adopted to reconstruct the image of multifilament. It is found that the reconstructed multifilament image appears serious deformation with the multifilament center position deviating from the scanning center (Fig. 5). When x0=1.0 mm, the two monofilaments near the scanning origin side can still be distinguished, whereas the two monofilaments near the transducer side are fused and cannot be distinguished. Therefore, the secondary filaments around the multiple filaments are more susceptible to the "aperture effect" and the fuzzy deformation occurs. The fuzzy deformation effect will be more obvious when the distance becomes larger from the scanning center or the distance becomes smaller from the surface of the transducer. Therefore, compared with monofilament reconstruction, multi-filament image reconstruction is more affected by the "aperture effect". Especially, the blur deformation of the surrounding sub-filaments is more likely. In summary, the characteristics of the transducer have an obvious influence on the reconstruction of monofilament and multifilament cross-sectional images. A larger bandwidth of the transducer will cause a smaller surface diameter, a larger surface sensitivity parameter, and a better reconstruction quality of monofilament and multifilament images. The influence of the center frequency of the transducer on the optical fiber image reconstruction is very complicated. Therefore, it is necessary to select the transducer with the appropriate center frequency combined with the spectrum analysis of the acoustic signal in the actual measurement.
We utilize a novel medical imaging method named PAT to reconstruct cross-section images of femtosecond laser filament formed in an air medium. The results show that the acoustic signal induced by a single filament has a single-peak structure, while that induced by a multifilament has a multi-peak structure. The performance parameters of the transducer have an obvious influence on the reconstruction results. A larger bandwidth of the transducer will lead to a smaller surface diameter, a larger surface sensitivity coefficient, and a better reconstruction effect of energy deposition distribution of optical filament. Compared with monofilament, the reconstruction of the multifilament image is more susceptible to the "finite aperture effect". Our study can provide some theoretical support for the experimental measurement of the spatial deposited energy distribution of femtosecond laser filament transmission under real atmospheric conditions.
1 引言
高功率的飞秒激光脉冲在大气中传输时,由于克尔自聚焦、衍射和等离子体散焦效应之间的动态平衡,可以形成长度很长的等离子体通道,即光丝[1]。飞秒激光成丝包含丰富的物理过程,并在远距离大气探测、引导放电、产生有机物和诱导水汽凝结等诸多大气研究领域中展现出潜在的应用价值,逐渐成为非线性光学与大气科学交叉研究的前沿热点问题之一[2-6]。
空气中形成的等离子体光丝,其重要特性之一就是它能够几乎在瞬间沉积相当一部分脉冲能量到传输介质[7]。由于光场电离和空气分子的非共振转动拉曼激发等作用是造成光丝能量沉积的主要物理过程[7],沉积能量的分布与光丝内的光场强度和自由电子密度紧密相关。因此,通过获取光丝沉积能量空间分布,可以间接了解自由电子的分布和光丝的空间结构,这对于成丝动力学过程的“可视化”研究具有重要意义[8]。同时,这部分沉积的脉冲能量,最终以热量的形式被介质吸收,能够激发形成压力脉冲波以及更长时间尺度的热传导过程,相关物理过程也被认为是诱导产生气流扰动、冰晶繁生和清理光学通道等现象的关键[9-11]。然而,由于光丝内部强度极高,使得目前对于光丝能量沉积密度空间分布的直接测量还非常困难。现有诊断光丝的方法大多是基于成丝过程中伴随产生的光、声和热等信号间接实现的[12]。其中,最为常用的是利用微麦克风测量声信号强度来间接估算能量沉积效率[7,13]。该方法实施简单,测量过程中只需要沿着光丝传输方向移动高频微麦克风,获取不同位置的声信号强度,然后根据热能与声压之间的关系
光声层析(PAT)是一种新兴的非侵入式医学成像技术,它利用光声效应对介质(如生物组织)中沉积的光能量分布进行成像[15]。利用PAT技术重建单丝横截面图像的可行性,在理论和实验上都已经得到了充分验证[14,16-17],并且还逐步向着三维重构的趋势发展[18],是一种非常有发展前景的定量获取光丝结构信息的探测方式。在PAT实验测量系统中,超声换能器是接收光声信号的关键元件,对最终重建图像的质量有重要的影响。实际实验中所用的换能器通常具有有限的尺寸,一般在3~10 mm之间,并且换能器的带宽也是有限的[19]。Xu等[20]通过推导,发现换能器的有限孔径和带宽对PAT成像的切向分辨率有明显的影响。另外,在实际测量中,所用换能器的中心频率和表面灵敏度也是有限的,接收的声信号还会存在各种噪声。这些因素综合起来对光丝图像重建的影响如何,非常值得进一步深入研究[21]。
为此,本文通过综合考虑换能器的中心频率、带宽、尺寸和灵敏度等性能参数,理论模拟了利用特定环阵式PAT系统探测单丝和多丝声信号的过程,并验证了利用延迟叠加算法重建出光丝能量沉积分布横截面图像的可行性。该研究结果可为进一步重建光丝沉积能量三维空间分布奠定理论基础,对诸多基于光丝的大气应用研究具有较好的参考价值。
2 研究方法
PAT成像大致包括两个过程:前向过程和后向过程。前向过程指的是高功率短脉冲激光照射吸收体过程中,由于热弹性膨胀激发出宽带超声波信号,并被超声换能器接收的过程。后向过程是指利用一定图像重建算法通过求解逆问题,根据收集到的光声信号,重建目标物光学吸收能量分布或者初始声压分布的过程[15]。
2.1 光丝诱导超声信号前向仿真模型
光丝诱导超声信号前向仿真模型的建立,需要结合光丝诱导产生声信号所经历物理过程的时间尺度考虑。在飞秒激光脉冲通过传输介质后,先激发电子密度的相干振荡过程(~100 ps),形成等离子体尾迹;产生的等离子体及其吸收的部分脉冲能量在等离子体复合之后,最终转换成气体的热能(~10 ns);随后,为达到机械平衡,会产生压力脉冲波,激发形成声信号(~1 μs);在压力平衡后,发生热传导过程,可持续至1 ms以上量级[7,13,22]。由此可见,飞秒光丝激发形成声信号的过程,本质上是一个热致声过程。近似地,光丝诱导产生的初始压力与光丝沉积的热量成正比[13]。因此,通过重建光丝诱导初始声压强度分布,可以间接得到相应的沉积能量分布[23]。根据对相关物理过程的时间尺度的分析结果,在考虑热扩散,忽略介质黏性的情况下,前向过程可以用下列方程组[24]进行描述:
式中:p1和T1分别为气压和温度的扰动量;H为初始热源分布;ρ0为介质密度;vs为绝热声速;
在忽略热扩散、介质的黏度,以及沿z方向激光辐射强度衰减的情况下,联立
式中:
式中:H0表示单位体积内激光脉冲沉积的最大能量;w0为光丝热源分布高斯半径;r为径向坐标位置。空气介质中初始气压分布与沉积脉冲能量之间近似满足如下关系[16]:
将
式中:
式中:
在后续的仿真中,主要的参数取值为vs=330 m/s,cp=1.002 kJ/(kg·K),cv=0.717 kJ/(kg·K),β=1/273.15 ℃-1。H0和w0的取值在后文中说明。另外,由于空气介质的黏性,超声信号在空气中传输时会发生明显的衰减。为了考虑声压信号的衰减作用,文献[24]中提出了一种等效方法,即将热源半径w0等效为
2.2 图像重建方法
光声图像重建是一个典型的逆问题,即由光声信号p计算出介质电磁吸收分布A的问题。在诸多的典型PAT实验系统中,2D环形阵列扫描可以覆盖较为完整的目标视角,常用于吸收体断层面的成像,其示意图如
式中:
在DAS算法中,为了综合考虑换能器的中心频率、带宽、尺寸和灵敏度等性能参数,如
式中:
图 1. 环阵PAT仿真模型设置。(a)环形阵列探测示意图;(b)换能器表面探测单元划分示意图
Fig. 1. Simulation model setup of ring array PAT system. (a) Schematic diagram of ring array detection; (b) schematic diagram of division of transducer surface detection unit
作为对比,本文还采用传统的反投影(BP)算法对光丝图像进行重建。所用到的BP算法有两种:一种是理想点换能器模型(记为BPoint);另一种是无限大平面换能器模型(记为BPinf)。BPinf算法的反投影线是一组与探测器平面平行的直线,相应的反投影值[19]为
式中:
3 分析与讨论
3.1 光丝声信号特征
实验测量结果表明,由Ti:Sapphire飞秒激光器(805 nm,55 fs,10 Hz)产生的入射能量Ein约为2~3 mJ的单脉冲在空气介质中传输成丝时,形成的单丝内部能量沉积密度峰值H0=(50±10)mJ/cm3,光丝能量沉积分布半径w0=(60±10)μm[29]。如果在光束传输路径上增加掩膜板,相应的能量沉积密度变为H0=(15±5)mJ/cm3,光丝能量沉积分布半径w0=(250±50)μm。因此,本文假设由初始单丝沉积的能量分布满足
图 2. 飞秒光丝光声仿真结果。(a)初始单丝沉积能量分布横截面;(b)不同接收位置处的声压信号;(c)声压信号的频谱分布;(d)初始多丝沉积能量分布横截面;(e)不同接收位置处的声压信号;(f)声压信号的频谱分布
Fig. 2. Photoacoustic simulation results of femtosecond laser filament. (a) Initial cross-sectional energy deposition distribution of single filament; (b) acoustic pressure signals recorded at different positions; (c) frequency spectrum of acoustic pressure signals; (d) initial cross-sectional energy deposition distribution of multiple filaments; (e) acoustic pressure signals recorded at different positions; (f) frequency spectrum of acoustic pressure signals
图
3.2 单丝图像重建
在仿真得到单丝和多丝产生的声信号以后,就可以利用BP和DAS两种图像重建算法对光丝横向沉积能量分布图像进行重建。具体实施过程中,先重建得到初始气压分布,然后利用
图 3. 光丝中心分别位于x轴上不同位置时单丝沉积能量分布横截面图像重建结果。(a)x0=0 mm;(b)x0=1.0 mm;(c)x0=2.0 mm
Fig. 3. Reconstructed cross-sectional energy deposition distribution of single filament when centers of filament are located at different positions on x-axis. (a) x0=0 mm; (b) x0=1.0 mm; (c) x0=2.0 mm
进一步利用DAS算法,综合分析了换能器的扫描半径Dr、换能器数量N、换能器的中心频率fc、换能器带宽W、换能器探头直径L和表面灵敏度系数σ对单丝重建结果的影响,结果如
图 4. 不同换能器相关因素对单丝沉积能量分布重建结果的影响。(a)扫描半径Dr;(b)换能器数量N;(c)换能器中心频率fc;(d)换能器带宽W;(e)换能器直径L;(f)换能器表面灵敏度σ
Fig. 4. Influence of different transducer related factors on reconstruction of energy deposition distribution of single filament. (a) Scanning radius Dr; (b) number of transducers N; (c) center frequency fc; (d) transducer bandwidth W; (e) transducer diameter L; (f) transducer surface sensitivity σ
表 1. 不同参数下重建图像切向分辨率变化值
Table 1. Changes in tangential resolution of reconstructed image under different parameters
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3.3 多丝图像重建
利用BP和DAS两种算法对多丝图像进行重建,结果如
图 5. 光丝中心分别位于x轴上不同位置时多丝沉积能量分布横截面图像重建结果。(a)x0=0 mm;(b)x0=1.0 mm;(c)x0=2.0 mm
Fig. 5. Reconstructed cross-sectional energy deposition distribution of multiple filaments when centers of filaments are located at different positions on x-axis. (a) x0=0 mm; (b) x0=1.0 mm; (c) x0=2.0 mm
从
同样地,进一步利用DAS算法,研究了扫描半径Dr、换能器数量N、换能器的中心频率fc、换能器带宽W、换能器探头直径L和表面灵敏度系数σ等参数对多丝图像重建结果的影响,结果如
图 6. 不同换能器相关因素对多丝沉积能量分布重建结果的影响。(a)扫描半径Dr;(b)换能器数量N;(c)换能器中心频率fc;(d)换能器带宽W;(e)换能器直径L;(f)换能器表面灵敏度σ
Fig. 6. Influence of different transducer related factors on reconstruction of energy deposition distribution of multiple filaments. (a) Scanning radius Dr; (b) number of transducers N; (c) center frequency fc; (d) transducer bandwidth W; (e) transducer diameter L; (f) transducer surface sensitivity σ
4 结论
基于热传导方程和波动方程构成的光声信号前向仿真模型,分别研究了飞秒激光在大气中传输形成单丝和多丝结构诱导产生声信号的特征,并利用BP算法和DAS算法对单丝和多丝沉积能量横截面分布图像进行了重建。初步验证了利用环阵PAT重建单丝和多丝沉积能量横向分布图像的可行性,并得到以下结论:
1)在考虑声信号衰减的情况下,单丝诱导产生的声压信号为单峰结构,而多丝声信号为多峰结构;随着传播距离的增大,声信号高频部分衰减明显,单丝峰值频率(约0.8 MHz)要比多丝的最大峰值频率(约2.0 MHz)小;
2)PAT法具备重建多丝图像的能力,但是相比于单丝重建,多丝图像重建受“孔径效应”的影响更大,尤其是周围副丝更容易出现模糊变形;
3)换能器特性对单丝和多丝横截面图像的重建有明显影响,换能器的带宽越大、表面直径越小和表面灵敏度参数越大,越有利于提高单丝和多丝图像重建质量。换能器中心频率对光丝图像重建的影响较为复杂,实际测量时需要结合声信号频谱分析选择具有合适中心频率的换能器。结合本文研究结果来看,对数毫焦飞秒脉冲产生的单丝和多丝,当最大沉积脉冲能量密度在数十毫焦每立方厘米量级时,选择中心频率为2 MHz的超声换能器测量效果较好。
值得注意的是,DAS算法本质上也是一种BP算法,而BP方法是一种解析计算方法,虽然计算速度快,但缺乏有效获取成像区域定量信息的能力。在本文研究中,虽然DAS算法能够较好地重建出单丝和多丝的沉积能量横向分布轮廓信息,并且也具有一定的抗噪能力,但是最后重建出的光丝沉积能量最大幅度值与初始值存在较大差异。这种结果的影响因素是多方面的,主要与超声信号在空气中剧烈衰减,以及换能器的中心频率和带宽有限相关。后续可以使用基于模型的重建算法来综合考虑传输介质特性、声速衰减补偿和换能器特性等影响因素,从而更好地估计出光丝沉积能量分布。另外,开展相关的测量实验也将是下一步的工作重点。
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