>Supercontinuum Laser Applications
>Infrared Microscopy Technology Using High Power Supercontinuum Source
外光源，或基于同步辐射的复杂光源。 Traditional infrared (IR) microscopes are based on incoherent thermal infrared light sources with extremely low brightness or complex light sources based on synchrotron radiation. The spatial resolution of an infrared microscope based on a non-coherent heat source is determined by the limiting hole placed in front of the light source. To increase the spatial resolution, the aperture must be reduced, so that the amount of light available for imaging will be greatly reduced and the lighting time will be extended. Therefore, a more intense and spatially coherent light source is needed, such as a synchrotron radiation source, but this light source is expensive and complicated to use. The broader method for increasing spatial resolution is based on attenuated total internal reflection technology, but this technology is only suitable for solid samples and not for imaging of biological materials and cellular materials embedded in liquids. To this end, this paper introduces a high-resolution non-contact infrared microscope using an infrared supercontinuum (IR SC) light source.
Figure 1 (a) is a device diagram of an IR microscope; Figure 1 (b) is a supercontinuum spectrum output from a ZBLAN fiber, the red line represents the pump wavelength of the laser, and the green line represents the zero dispersion wavelength of the fiber.
The specific principle is as follows: The SC output from the optical fiber is collimated with an aspherical ZnSe lens (focal length f = 6mm, numerical aperture NA = 0.25), and then aligned with two aspheric ZnSe lenses the same as the previous lens Focusing and refocusing, the focal point between the two lenses determines the resolution of the microscope. The sample under study is sandwiched between two 2mm-thick CaF2 windows with a sample thickness of ~ 25 μm. The sample is placed in focus, and its position is controlled by a motorized XYZ stage with an accuracy of 1 μm. The light transmitted through the sample is focused on the entrance slit of the monochromator through a ZnSe lens with a focal length of 15 cm. The monochromator has a spectral resolution of 2 nm and avoids second-order artifacts through a 3000 nm long-pass filter. A PbSe detector is used to detect the signal, and the sample is subjected to a two-dimensional raster scan at a single wavelength to obtain the relative transmission at that wavelength.
First, the chemical characteristics of pure olive oil were studied. Assemble the sample without spacers to avoid saturation of the absorbance. The wavelength-dependent absorbance was measured by scanning a monochromator, and the logarithmic ratio of the obtained spectrum to the spectrum without samples was obtained. Figure 2 compares this absorption spectrum with the FTIR absorption spectrum of a 4 μm-thick oil sample, where the blue / red lines represent the infrared absorption spectrum of a 4 μm-thick water and oil sample, and the dotted lines represent oils made by scanning the grating and using an IR SC light source Absorbance spectrum. The absorption line shown corresponds to CH-stretch in oil with good consistency.
Secondly, the samples containing oil / water mixtures were used to study the spatial resolution and chemical properties. Figure 3 (a) is a picture of a sample oil / water mixture made with an optical microscope. You can see that there are three areas: a circular structure in the middle, one above, and one below. Based on their composition, it is impossible to distinguish these areas from each other. Figures 3 (b) and (c) are IR images of the samples measured at 3.05 μm and 3.50 μm, respectively. The two wavelengths correspond to high absorption wavelengths in water and oil. The blue part of the image indicates high absorption and the red part indicates low absorption. The image size is 300 μm × 375 μm, and each pixel is 5 μm × 5 μm. Figure 3 (b) shows the water absorption. The high absorption in the upper part of the picture can be clearly seen, indicating the presence of water in this area. Figure 3 (c) shows the oil absorption. You can see the high absorption in the lower part of the picture, indicating that it is oil.
In ordinary IR microscopes, the spatial resolution is determined by the aperture combined with the objective lens. But in our device, no aperture is used, so the spatial resolution is completely determined by the spot size of the focused light. The theoretical study of the 1 / e2 spot size is shown in the right part of Figure 4. The calculation result is determined by using the ABCD matrix and the propagation law to simulate the transmission of the Gaussian mode output from the fiber. The wavelength-dependent mode field of the ZBLAN fiber is used as the starting beam, which can be found by using a finite element software package that includes the fiber geometry and the wavelength-dependent refractive index of the ZBLAN (see the illustration in Figure 4). In addition, by using the wavelength-dependent refractive index to consider the chromatic aberration of the objective lens, the chromatic aberration can be avoided by using a reflective optics. Calculations show that the minimum beam radius and focal length are wavelength dependent. Therefore, in theory, the fiber mode diameter is an important parameter for spatial resolution. Selecting an optical fiber with a higher numerical aperture can obtain better spatial resolution in an infrared microscope.
The left part of FIG. 4 is the spot size of three different wavelengths measured by the knife edge method. The data shows the wavelength-dependent changes in the focal length and minimum spot size predicted by the model. At 2.8 μm and 3.8 μm, the minimum measured beam radii are 16 μm and 12 μm, respectively. This is consistent with the calculation results. Even if the minimum beam radii for different wavelengths do not coincide, the combined beam radius at the focal point still has a minimum value of about 17 μm. This can also be determined by considering the oil / air and water / air interfaces in FIG. 3 as blades. In this way, we obtained the resolution of the water absorption image and oil absorption image at 35 μm and 25 μm, respectively. The resolution is slightly lower than the IR SC beam waist limit, probably due to the small displacement of the sample relative to the focal point of the microscope. The ability to focus fiber-based SC sources to a small area while maintaining their high brightness and wide spectrum is critical, so that faster raster scanning can be achieved in high spatial resolution measurements.
In short, the high-brightness wide-spectrum infrared supercontinuum light source based on optically pumped ZBLAN fiber is very suitable for high-resolution infrared microscopes. With the development of high-power fiber lasers and new infrared fiber materials, infrared microscopy based on supercontinuum light sources will be widely used in visible, coherent and non-coherent Raman microscopes.
From <S. Dupont, C. Petersen, J. Thgersen, C. Agger, O. Bang, SRKeiding. IR microscopy utilizing intense supercontinuum light source. Optics Express, 2012.>