An optical apparatus includes a structured light generation unit, a conversion lens module, a collimating lens and a casing. The structured light generation unit outputs a structured light. The light beams from the structured light generation unit are expanded by the conversion lens module. The expanded light beams are collimated by the collimating lens. After the light beams pass through the conversion lens module and the collimating lens, the light beams are projected to a projection surface. Consequently, a structured light pattern is formed on the projection surface. All conversion lenses of the conversion lens module have negative optical power. Consequently, the area of the structured light pattern on the projection surface is wider. Moreover, the structured light generation unit and the conversion lens module can be accommodated within the casing having a smaller thickness.