The laser imagesetter consists of a laser, a modulator, a light intensity converter, an optical axis drift correction lens, a beam expander, and an eight-sided rotating mirror.
Laser
It is a tube filled with helium-neon gas. One end of the tube features a total reflection mirror, while the other has a partially transmissive mirror. High-voltage discharge ionizes the gas within the tube, generating free electrons; these electrons collide with helium atoms, causing them to transition from the ground state to a metastable energy level. Metastable helium atoms collide with ground-state neon atoms, exciting the neon atoms to a high energy level through resonant energy transfer while the helium atoms return to the ground state. Neon atoms accumulate at the high energy level, creating a population inversion; they subsequently transition to a lower energy level, emitting laser light. The light reflects back and forth between the two mirrors to enhance coherence and parallelism before finally exiting through the partially transmissive mirror. Helium-neon laser tubes offer excellent beam parallelism and monochromaticity, eliminating the need for optical systems to correct chromatic aberration.
Modulator
An optical switch that controls the transmission and interruption of the beam within the optical system based on the principle of light refraction. The light modulator in the laser imagesetter utilizes an acousto-optic crystal. When the modulator is inactive, the laser beam passes directly through the crystal; when active, an RF power source causes the acousto-optic crystal to generate pressure waves. This change within the crystal causes the beam to diffract (resulting in multiple output rays). A precision light modulator can amplify the first-order diffracted ray; it is this amplified beam that exits the optical system.
Light Intensity Converter
This component adjusts the intensity of the laser beam to meet various output requirements by changing its position. It has nine operating positions: position 0 is a transparent aperture with no filtering effect, while positions 1 through 8 serve as intensity filtering settings-the higher the position number, the greater the attenuation of the light intensity.
Optical Axis Drift Correction Lens
Factors such as temperature fluctuations can cause the beam to drift off its intended path; the drift correction lens is designed to eliminate this deviation.
Beam Expander
Used to increase the diameter of the laser beam. Octagonal Rotating Mirror
This component is a pyramidal assembly featuring eight small reflecting facets that rotates clockwise at 3,000 rpm. A 45-degree mirror directs the beam emerging from the beam expander onto the facets of the rotating mirror. As each facet reaches a specific position, the beam is reflected onto the horizontal scanning plane, thereby sweeping across the film. The film advance corresponding to each facet is 0.0254 mm. A disk-shaped grating is mounted on the axis of the rotating mirror; this consists of a circular acrylic plate etched with 3,000 lines. As the mirror rotates, a computer tracks the scanning process, utilizing control signals generated by a photocell aligned with the grating lines.