1. Field of the Invention
The present invention relates to a laser beam image recording apparatus for recording an area-modulated image on a recording medium with a semiconductor laser beam.
2. Description of the Related Art
In the art of image recording, there has been employed a laser beam image recording apparatus for driving a laser beam optical system with a processed digital image signal to record an area-modulated image on a recording medium. The recording medium with the image recorded thereon is supplied to an image developing machine, which develops the recorded image from a latent image representation into a visible image representation.
The laser beam optical system has a laser beam source which may be a coherent laser beam source such as a single transverse mode semiconductor laser, an optical fiber laser, etc., or an incoherent laser beam source such as an optical fiber coupled laser, a semiconductor laser array, etc. capable of producing a high laser beam output power. More attention has been drawn to semiconductor lasers than gas lasers because the semiconductor lasers offer many advantages as they are small in size, light in weight, high in efficiency, and long in service life. Laser beam image recording apparatus incorporating semiconductor lasers have been developed in the art.
When a laser beam emitted by a coherent laser beam source is focused on a recording medium, the focused spot of the laser beam has a Gaussian intensity distribution.
When a laser beam is emitted by an optical fiber coupled laser, since the laser beam is guided by an optical fiber to a position near a recording medium, the focused spot of the laser beam on the recording medium is a circular spot with a substantially uniform intensity distribution.
Laser beam image recording apparatus usually generate a two-dimensional image on a recording medium by scanning the recording medium in a main scanning direction with a laser beam while the recording medium is being fed in an auxiliary scanning direction normal to the main scanning direction. Because the optical energy of the laser beam applied as a circular spot to the recording medium is integrated in the main scanning direction, the laser beam spot on the recording medium has an intensity distribution that is close to a Gaussian intensity distribution in the auxiliary scanning direction.
A laser beam emitted by a semiconductor laser array is focused on a recording medium with a noded linear intensity configuration along the array of semiconductor lasers.
If the integrated intensity of a laser beam on a recording medium has a Gaussian distribution along the auxiliary scanning direction as shown in FIG. 13 of the accompanying drawings, then when the laser beam varies in intensity or the focused laser beam spot positionally deviates from the recording medium, the integrated intensity varies as indicated by a characteristic curve A or B, so that the coloring range determined by the coloring threshold of the recording medium changes to a coloring range "a" or "b". The change in the coloring range results in an irregularity of the image density. If an image having a straight edge in the main scanning direction is recorded by the laser beam, then the image density irregularity makes the edge of the image staggered in the auxiliary scanning direction. Furthermore, if a recording medium has sensitivity irregularities or an image developing machine has image developing ability irregularities, then since these irregularities tend to change the coloring threshold of the recording medium, an image on the recording medium suffers image density irregularities. If the focused spot of a laser beam on a recording medium has a noded linear intensity configuration, then an image recorded on the recording medium also suffers image density irregularities due to the noded linear intensity configuration.
According to a radiation source for a printer disclosed in Japanese patent publication No. 8-20621, a telecentric optical system is disposed between a laser diode array and a recording medium for focusing a near field pattern of a laser beam emitted from the laser diode array on the recording medium thereby to keep the minimum level of a noded linear intensity distribution higher than the coloring threshold of the recording medium. With the disclosed radiation source, however, the output power level of the laser diode array has to be higher than the coloring threshold, resulting in a very poor laser beam utilization efficiency. Because the laser diode array is required to produce a high output power level, the laser diode array suffers a relatively short service life, and needs to be replaced frequently.