We know that the full name of field mirror is flat field focusing mirror, also known as f-theta mirror. The reason why it is called flat field focusing lens is that after the laser passes through it, a focusing light point with uniform size and consistent focal depth will be formed in a certain laser marking format.
Why can the laser pass through the field mirror at different angles to ensure that the focus is in a certain inner plane? And not change up and down? This starts with the internal structure of the field mirror.
As shown in the figure: the inside of the field mirror is not a lens, it is a combination of multiple curvature lenses. When the laser enters the field mirror at different angles, the aberration is eliminated through the internal lens. The final emitted laser is on a plane.
The main functions of field mirror are:
(1) Improve the ability of edge beam incident on the detector;
(2) In the same main optical system, the additional field mirror will reduce the area of the detector. If the same detector area is used, the field of view can be expanded and the incident flux can be increased;
(3) The modulation disc can be placed at the position of the image plane to solve the problem that there is no place to place the modulator;
(4) The non-uniform illumination on the photosensitive surface of the detector can be homogenized;
(5) When the flat field mirror is used, the flat field image plane can be obtained;
(6) In the aspect of aberration correction, the field curvature and distortion of the system can be compensated.
The field mirror can be used to compensate the field curvature and distortion of the system. For example, if a negative field mirror is added to the image plane, the positive field curvature and orthodontic deformation can compensate the negative field curvature and distortion of the whole system without affecting other aberrations.
Function of field mirror
A lens that works near the focal plane of the objective lens and can effectively reduce the size of the detector. Accurately speaking, the field lens should work in the image plane of the objective lens. On the one hand, the object plane of the field mirror coincides with the main plane. According to the characteristics of the main plane, its magnification is 1, so it has no contribution to the magnification of the system; On the other hand, the field lens should project the aperture surface of the objective lens on the entrance pupil of the eyepiece. For a single lens, the aperture surface is the exit pupil, which can ensure that the outgoing light of the objective lens can pass through the eyepiece to the greatest extent. If used in a scanning system, the detector replaces the entry pupil of the eyepiece.
The main technical parameters of the field mirror are: working wavelength, incident pupil, scanning range and focal spot diameter. Working wavelength: it mainly depends on the wavelength of the laser. The lens of the field mirror is coated at a given laser wavelength. If the field mirror is not used within a given wavelength range, the field mirror may be burned out by the laser or the required laser transmittance is very low. Incident pupil: also known as the maximum incident spot. If a single mirror is used, the mirror is placed at the entrance pupil, and the diameter of the maximum available beam is equal to the diameter of the entrance pupil. The larger the incident pupil, the smaller the diameter of the focusing spot focused by the focusing lens. Therefore, when marking is required to be particularly fine, we will consider using a field lens with larger incident pupil, such as fine marking and galvanometer welding. Scanning range: the larger the field mirror can scan, of course, the more popular it is with users. However, if the scanning range is increased, the focus point becomes larger and the distortion also increases. In addition, if the scanning range is increased, the focal length and working distance of the field mirror should also be increased. The lengthening of working distance will inevitably lead to the loss of laser energy. In addition, the focused spot diameter is directly proportional to the focal length, which means that the scanning range is increased, the focused spot diameter is increased, the spot is not fine enough, and the laser power density decreases very fast (the power density is inversely proportional to the second power of the spot diameter), which is not conducive to processing. Therefore, users should select the most suitable field mirror according to different processing areas, or reserve several field mirrors with different scanning ranges. Focal length: it has a certain relationship with the working distance, but it is not equal to the working distance. Focus spot diameter: for the scanning system with incident laser beam diameter D, field mirror focal length f and beam quality factor Q, focus spot diameter d = 13.5qf/d (mm). Therefore, a smaller focusing spot can be obtained by using a beam expander.