1. Field of the Invention
This invention relates to a method of coalescing laser beams, and more particularly to a method of coalescing two or more laser beams on an object to get a multiplied power of the laser beams on the object. The "coalescing" means mingling or doubling of two or more laser beams on an object onto which the laser beams are to impinge in the doubled or multiplied power.
In the present invention, the two or more laser beams are first merged into a single laser beam in which the laser beams are substantially aligned with each other or one another to make a doubled or multiplied laser beam, in which however the laser beams are usually not perfectly merged into a single laser beam. In this specification, however, the laser beams thus merged or roughly mixed together are called "merged laser beams" or "a single merged laser beam" because they are substantially "merged". In a more precise sense, on the other hand, these laser beams first merged into a substantial single laser beam are not perfectly aligned with each other or one another but are out of perfect parallelism and out of perfect coincidence or alignment. In this sense, the laser beams thus merged are then "collimated" in a precise sense so that they are directed to a common focal point through a converging lens. Of course, the collimated laser beams after "merged" are so close to each other or one another that they advance through the substantially the same part of the same optical elements such as lenses and mirrors.
2. Description of the Prior Art
Laser beams are widely used for optical communication, measurement, processing such as laser welding, laser drilling, and the like. When two or more laser beams are used, it is often required to make them coalesced to get a multiplied power or a laser beam of wider wavelength coverage.
For example, in printing scanners, when a color image original is read out by use of a He-Ne laser beam, an Ar.sup.+ laser beam and a He-Cd laser beam and a monochromatic image is reproduced on a photosensitive material by use of the electric signal obtained by the read-out, it is necessary to converge three laser beams to a single point on the color image original for conducting the read-out. Also, in a radiation image recording and reproducing system as disclosed, for example, in U.S. Pat. Nos. 4,258,264 and 4,346,295, and Japanese Unexamined Patent Publication No. 56(1981)-11395, a stimulable phosphor sheet carrying a radiation image stored therein is scanned with a laser beam which causes the stimulable phosphor sheet to emit light in proportion to the radiation energy stored. The emitted light is detected and converted into an electric signal, and a photosensitive material is scanned with a laser beam modulated based on the electric signal by a light modulator, thereby reproducing an image in the photosensitive material. In this case, in order to increase the read-out speed and the reproducing speed, it is necessary to increase the scanning speed of the laser beam. However, if only the scanning speed of the laser beam is increased with the power thereof maintained on the same level, the intensity of the laser beam per unit area of the surface of the stimulable phosphor sheet or a film scanned thereby inevitably becomes low. Therefore, it is necessary to increase the scanning speed and, at the same time, to increase the power of the laser beam. In order to increase the power of the laser beam, the output power of the laser beam source should be increased. However, when the output power of the laser beam source is increased, the size of the laser beam source inevitably becomes large, and the problem that the laser beam source cannot be incorporated in the radiation image recording and reproducing system or the problem that the radiation image recording and reproducing system must be made large arises. One approach to elimination of the problem is to use two laser beam sources and to coalesce the laser beams emitted thereby by use of an optical merging means such as a polarization beam splitter or a diffraction grating, thereby multiplying the laser beam power. This method is based on the finding that, in the case where the same level of laser beam power should be obtained, it is possible to make the size of the laser beam emitting system smaller when two laser beam sources are used and the laser beams emitted thereby are merged than when a single laser beam source is used. However, also in this method, it is required to make the two laser beams align with each other. The same thing is required also in laser processing apparatuses such as laser welding apparatuses.
However, the step of making two or more very thin laser beams align with each other is not always easy to conduct, or requires a complicated mechanism, or requires much time to adjust the mechanism.