US Patent 7586661 Optical scanning device and image forming apparatus

A coupling optical system that includes a coupling lens, a linear-image forming lens, and an aperture couples light beams emitted from a light source to form a light flux. A deflector scans the light flux in a main scanning direction to form an image as a scanning line on a surface to be scanned through first and second scanning lenses. While an image is being formed on an image plane, the image plane is moved in a direction perpendicular to the main scanning direction. The linear-image forming lens has a diffractive optical surface capable of collecting light in a sub-scanning direction. The temperature dependence dλ/dT of wavelength of the light source satisfies dλ/dT<0.15.