Laser lens, spherical aberration

The ratio F/d is the F number of the lens. For F numbers much less than unity, spherical aberration precludes reaching the ultimate diffraction-limited spot size. Therefore a practical limit for the minimum spot size obtainable is approximately the wavelength of the light. Commonly this is expressed as the statement that laser light may be focused to a spot with dimensions equal to its wavelength. 

The use of a laser beam expander as a spatial filter has also been found to be satisfactory 42). The beam expander consists of an interchangeable negative input lens and a positive output lens. Both the input and output lenses are designed for minimum spherical aberration. The expansion power may be varied by using a different input lens (Fig. 23.) The laser beam... 

A laser lens for electrons has spherical aberration, which has a negligible effect on estimate (6), and also chromatic aberration, which is a more serious matter. For the particular numerical example we are discussing here, the electrons would have to be monochromatic within something on the order of 0.01%. 

Based on the fact that the lens is embryologically derived from the same ectodermal source as comeal and conjunctival epithelium, Sivak et al. (1992) used an automated scanning laser system to monitor spherical aberration and transmission of the lens in culture. They believed that this method of measuring lens damage compared favorably with standard Draize. scores. 

For both one- and two-photon processes, the irra-diance is greatest at the beam waist. Consequently, precise control of the spot size is required for accurate micromachining. A poorly controlled spot size can result in larger features widths and increased sloping of the side walls. The smallest obtainable spot size determines the minimum feature size, which is limited to half the wavelength of the hght used. Under the conditions where a perfect lens with no spherical aberration is used to focus a laser beam that is collimated, the minimum diameter of the focused laser beam is given by... 

Fig. 7.7 (a) Configuration of laser TEMSi mode with minimum intensity on optical axis with a copropagating focused atomic beam (b) distribution of the atomic beam in the focal plane of an atom lens based on the TEMqi laser mode field. The solid curve corresponds to the aberration-free case, the dashed curve shows the distribution with chromatic aberration, and the dashed and dotted curves represent the distribution with spherical aberration allowed for. The distribution caiculated with diffusion aberration coincides with that for the aberration-free case. The minimum waist of the laser beam is po = A. (Fiom Balykin and Letokhov 1987.)... 

The focuser for the laser disk system must have the smallest aberration since the lens is used in the diffraction limit. Another important point is the working distance of the lens. In the laser disk application some space must be considered between the objective lens and the focal point because we have to have a clearance of about 2 mm and must also consider the disk thickness. Therefore, we need a lens with a large diameter ( 5 mm) as well as a large NA ( 0.5). First, the microscope objective was considered and several types of microlenses have been developed, such as the mold plastic aspheric lens and the GI lens with a spherical surface. 

A recent approach is the use of anti-Stokes Raman spectroscopy [10]. The advantage of this anti-Stokes measurement is that it does not restrict the Raman excitation wavelength as does FT-Raman. The disadvantage of the anti-Stokes method is the extremely weak signal obtained. In order to surmount this problem, a new instrumental system was devised which increases the density of the anti-Stokes lines by using a chromatic aberration focal lens with a highly sensitive spherical integration cell and finally with a laser power reference system. 

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