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White light is typically considered incoherent; however, the recently popular supercontinuum laser, also known as white laser, spans the visible spectrum and features high laser intensity and good coherence, challenging this traditional limitation. The white laser has a wide range of applications, including multi-channel confocal microscopy, color holography, and white light interferometric surface topography. Although white lasers have been proposed and developed extensively in terms of technology, specific analyses of their optical wave properties—especially spatial coherence—are still lacking. Since many applications impose certain requirements on the spatial coherence of white light, the lack of research into the spatial coherence of white lasers has, to some extent, limited their practical use. This paper presents a detailed experimental study and analysis of the wavefront intensity, polarization characteristics, and spatial coherence of the high-intensity ultra-flat spectrum white laser that was independently developed by our research group in 2023. The laser is generated by broadening the spectrum of a high-intensity Ti:sapphire femtosecond laser through second- and third-order nonlinear effects. A bandpass filter is used to extract eight components from the white laser, with a central wavelength range from 405 nm to 700 nm and a bandwidth of 10 nm for each component. By measuring the performance of these eight quasi-monochromatic lasers, the characteristics of the white laser of the entire visible spectrum can be evaluated. The CCD imaging of the collimated quasi-monochromatic laser spots reveals that their wavefront intensities exhibit a quasi-Gaussian distribution with uniform beam profiles. Polarization measurements by using polarizers at various angles show that the white laser is linearly polarized. A Young’s double-slit interferometer (YDSI) is used to measure the interference fringe contrast of the eight quasi-monochromatic beams to assess their spatial coherence. The experimental results show that the average interference fringe contrast of the entire visible spectrum is 0.77, and the difference between different wavelengths is very small.This indicates that the white laser has excellent spatial coherence in the visible range. The eight quasi-monochromatic lasers in the visible spectrum all exhibit quasi-Gaussian wavefront intensity distributions, linear polarization, and high spatial coherence. This indicates that the white laser inherits the excellent properties of the Ti:sapphire laser. All of these data provide valuable guidance for the application of white lasers in color holography, white light interferometric surface tomography, microscopic imaging, and other fields that require white light with a certain degree of coherence. -
Keywords:
- white laser /
- spatial coherence /
- interference fringe visibility
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图 1 对白激光空间相干性等激光特性的分析流程
Figure 1. Analysis scheme of laser characteristics such as spatial coherence of white laser.
图 2 白激光的产生原理和基本信息 (a) 产生飞秒白激光的光路图, 该激光通过高强度钛宝石飞秒脉冲激光束经过熔融石英-CPPLN二阶与三阶非线性协同频率转换模块产生; (b)经过熔融石英平板后的光谱测量结果; (c) 经过熔融石英平板后的激光光斑; (d) 白激光的测量光谱, 展现出超平坦的光谱特性, 其中彩色带表示可见光范围, 其放大图显示在插图中; (e) 系统最终输出的白激光的光斑
Figure 2. Basic information of the white laser: (a) Schematic diagram of a homemade femtosecond white laser created by sending an intense Ti:sapphire femtosecond pulse laser beam through a fused silica-CPPLN 2 nd-NL and 3 rd-NL synergistic nonlinear frequency conversion module (b) The measured spectrum of the light after the fused silica plate. (c) The spot of the laser after the fused silica plate. (d) The measured spectrum of the white laser shows the ultra-flat spectral profile. The color band represents the visible range, whose enlarged view is shown in the inset. (e) The spot of the white laser beam.
图 3 波前强度分析, 即白激光通过中心波长分别为 (a) 405 nm, (b) 450 nm, (c) 500 nm, (d) 532 nm, (e) 580 nm, (f) 600 nm, (g) 635 nm, (h) 700 nm, 带宽为 10 nm 的滤波片后, 不同光谱成分的波前强度(左)、线强度分布(中)和光谱分布(右)
Figure 3. Wavefront analysis. Wavefront (left), line profile (center), and spectral profile (right) of the white laser at different spectral components in the visible band when it passes through 10 nm bandwidth filters centering at the wavelength of (a) 405 nm, (b) 450 nm, (c) 500 nm, (d) 532 nm, (e) 580 nm, (f) 600 nm, (g) 635 nm, and (h) 700 nm
图 4 杨氏双缝实验的实验结果 (a)杨氏双缝实验的光路图, 其中F为带有不同滤波片的可旋转支架, CE为光束准直扩束系统; (b)—(i) 八种波长的干涉条纹和强度线轮廓图, 其中(b) λ = 405 nm, $ \upsilon $ = 0.86; (c) λ = 450 nm, $ \upsilon $ = 0.86; (d) λ = 500 nm, $ \upsilon $ = 0.75; (e) λ = 532 nm, $ \upsilon $ = 0.82; (f) λ = 580 nm, $ \upsilon $ = 0.77; (g) λ = 600 nm, $ \upsilon $ = 0.62; (h) λ = 635 nm, $ \upsilon $ = 0.80; (i) λ = 700 nm, $ \upsilon $ = 0.67
Figure 4. Experimental results in the Young’s double-slit experiment: (a) The architecture of Young’s double-slit experiment, where F represents rotatable holder embedded with different filters, CE represents collimated beam expanding system; (b)—(i) the interference fringes and intensity line profile of the eight wavelengths, where (b) λ = 405 nm, $ \upsilon $ = 0.86; (c) λ = 450 nm, $ \upsilon $ = 0.86; (d) λ = 500 nm, $ \upsilon $ = 0.75; (e) λ = 532 nm, $ \upsilon $ = 0.82; (f) λ = 580 nm, $ \upsilon $ = 0.77; (g) λ = 600 nm, $ \upsilon $ = 0.62; (h) λ = 635 nm, $ \upsilon $ = 0.80; (i) λ = 700 nm, $ \upsilon $ = 0.67.
表 1 从白激光中提取光波所使用的滤波片的中心波长和带宽
Table 1. Central wavelength and bandwidth of the filter used to extract the light wave from the white laser.
Wavelength/nm 405 450 500 532 580 600 635 700 Bandwidth/nm 10 10 10 10 10 10 10 10 
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表 2 各准单色光经过滤波片后功率随偏振片角度的变化
Table 2. Power variation with polarizer angle for each quasi-monochromatic light after filtering.
$ \lambda /{\mathrm{n}}{\mathrm{m}} $ 405 450 500 532 580 600 635 700 $ {I}_{{\mathrm{m}}{\mathrm{a}}{\mathrm{x}}} $/$ {\mathrm{\mu }}{\mathrm{W}} $ 43.7 42.5 43.5 40.1 45.7 46.8 45.0 46.2 $ {\theta }_{{\mathrm{m}}{\mathrm{a}}{\mathrm{x}}} $/(°) 0 0 1 0 1 0 0 0 $ {I}_{{\mathrm{m}}{\mathrm{i}}{\mathrm{n}}} $/$ {\mathrm{\mu }}{\mathrm{W}} $ 0.2 0.2 0.1 0.1 0.2 0.2 0.2 0.2 $ {\theta }_{{\mathrm{m}}{\mathrm{i}}{\mathrm{n}}} $/(°) 90 90 89 90 89 90 90 90
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