1. Field of the Invention
The present invention relates to a light beam scanning apparatus for recording or reading an image or other information with a light beam according to a synchronizing signal which is generated from a synchronizing light beam.
2. Description of the Related Art
There have been proposed various apparatus for scanning a medium with a laser beam to record an image or the like on or read an image or the like from the medium. For recording an image or the like on a medium highly accurately, it is necessary to turn on and off a laser beam accurately at the position where the medium is scanned by the laser beam.
One known mechanism for generating a synchronizing signal to control the times to turn on and off a laser beam is illustrated in FIG. 8 of the accompanying drawings. According to the illustrated mechanism, a scanning laser beam for scanning a medium is guided by a scanning optical system, and a synchronizing laser beam S is also guided by the same scanning optical system toward a reference grating 2. When the synchronizing laser beam S passes through the reference grating 2, it is converted to a pulsed synchronizing laser beam S which is introduced into a cylindrical light guide rod 4. The pulsed synchronizing laser beam S enters from an entrance side of the cylindrical light guide rod 4 and is diffused by a diffusion surface that faces the entrance side of the cylindrical light guide rod 4. The diffused light is reflected by an inner surface of the cylindrical light guide rod 4 toward a pair of photodiodes 6a, 6b mounted respectively on opposite ends of the cylindrical light guide rod 4. In response to the applied diffused light, the photodiodes 6a, 6b generate a synchronizing signal.
While the light guide rod 4 is cylindrical in shape, each of the photodiodes 6a, 6b is normally of a square shape. As a result, a light loss is caused where the photodiodes 6a, 6b are joined to the cylindrical light guide rod 4 due to different shapes of the cylindrical light guide rod 4 and the square photodiodes 6a, 6b. Furthermore, the light energy suffers an appreciable loss because of the reflection by the inner surface of the cylindrical light guide rod 4.
As shown in FIGS. 9 and 10 of the accompanying drawings, one solution has been to use a prismatic light guide rod 8 whose transverse cross-sectional shape is progressively greater or smaller in the longitudinal direction. A photodiode 10 is mounted on a longitudinal end of the prismatic light guide rod 8 which has a larger cross-sectional shape. For details, reference should be made to Japanese laid-open utility model publication No. 3-20314.
As shown in FIGS. 9 and 10, a synchronizing laser beam S introduced through a reference grating 2 into the light guide rod 8 from an entrance side thereof is diffused by a diffusion surface 14 which faces the entrance side. The diffused light is then reflected by an inner surface of the light guide rod 8 progressively toward the end of the larger cross-sectional shape, where the light is applied to the photodiode 10, which then generates a synchronizing signal. With the arrangement shown in FIGS. 9 and 10, the shape of the end of the light guide rod 8 and the shape of the photodiode 10 can be brought into conformity with each other, and any light energy loss owing to the reflection by the inner surface of the light guide rod 8 is held to a minimum. Therefore, it is possible for the light guide rod 8 to guide the synchronizing laser beam S efficiently.
However, if the temperature of the scanning apparatus varies or the scanning optical system suffers a positional displacement, then the synchronizing laser beam S may possibly be applied to the reference grating 2 at a displaced position. For example, as shown in FIG. 10, when the synchronizing laser beam S is displaced from a normal position indicated by the solid line to a position indicated by the dotted line S', the synchronizing laser beam S fails to fall on the diffusion surface 14, and mostly passes through the light guide rod 8. Consequently, the amount of light that reaches the photodiode 10 is greatly reduced, making the photodiode 10 unable to generate an accurate synchronizing signal. Increasing the width of the diffusion surface 14 would not solve the above problem, but rather would result in an increased energy loss because the synchronizing laser beam S diffused by the diffusion surface 14 is reflected by the inner surface of the light guide rod 8 and applied at an increased rate back to the diffusion surface 14 before reaching the photodiode 10 on the end of the light guide rod 8.