Multi-beam optical scanning device and image forming apparatus

A multi-beam optical scanning device, comprising:

• a plurality of light sources;
• a deflection unit configured to deflect a plurality of light beams from the light sources;
• a first optical system including
• a first lens that has a diffracting surface with a power and couples the light beams from the light sources;
• a second lens that guides the light beams from the first lens to the deflection unit, said second lens being an anamorphic element having a diffracting surface with a power at least in a sub scanning direction;
• and
• a second optical system configured to guide the light beams deflected by the deflection unit to a scanning surface;
• wherein
• assuming a magnification in a main scanning direction of a whole optical system of the multi-beam optical scanning device is represented by βm0, a magnification in the sub scanning direction of the whole optical system of the multi-beam optical scanning device is represented by βs0, and a magnification in the sub scanning direction of the second optical system is represented by βs2, the following relation (4) is satisfied:
• |βm0|>|βs2|, and |βs0|>|βs2| (4), and
• a number of the light beams passing through the first lens is less than a number of the light beams passing through the second lens.

An image forming apparatus, comprising:

• a multi-beam optical scanning device;
• wherein
• the multi-beam optical scanning device includes
• a plurality of light sources;
• a deflection unit configured to deflect a plurality of light beams from the light sources;
• a first optical system configured to guide the light beams from the light sources to the deflection unit;
• and
• a second optical system configured to guide the light beams deflected by the deflection unit to a scanning surface;
• wherein
• the first optical system includes
• a first lens that couples the light beams from the light sources, and
• a second lens that guides the light beams from the first lens to the deflection unit, said second lens being an anamorphic element having power at least in a sub scanning direction,
• at least the second lens includes a diffracting surface having a power, and
• assuming
• a magnification in a main scanning direction of a whole optical system of the optical scanning device is represented by βm0,
• a magnification in a sub scanning direction of the whole optical system of the optical scanning device is represented by βs0, and
• a magnification in a sub scanning direction of the second optical system is represented by βs2, then
• the following relation is satisfied:
• |βm0|>|βs0|, and |βs0|>|βs2|, and
• assuming
• the power of the diffracting surface of the first lens in the main scanning direction is represented by P1m,
• the power of the diffracting surface of the first lens in the sub scanning direction is represented by P1s, and the power of the diffracting surface of the second lens in the sub scanning direction is represented by P2s, then
• the following relation is satisfied:
• |P2s|>|P1m|, and |P2s|>|P1s|.