Patent ID: 7538945

Claim:
An optical module comprising: an optical system using a diffraction optical element for bending an optical axis of an incident light beam with a specific wavelength at about 90°, wherein the diffraction optical element is a transmission-type diffraction grating, a number N of grooves in the transmission-type diffraction grating is represented by the formula: N =±(cos α+ sin α)/( m λ) in which ± is the same sign as that of m when an mth-order diffracted light beam is used in a condition that the light beam with the specific wavelength λ is incident at an incident angle α on the transmission-type diffraction grating wherein a substrate of the transmission-type diffraction grating is mounted at an angle in a range of α±5° with respect to the incident light beam, wherein: the substrate of the transmission-type diffraction grating is mounted at an angle in the range of α+Δθ with respect to the incident light beam, where Δθ represents a shift in the transmission-type diffraction grating mounting angle; a light beam diffracted by the transmission-type diffraction grating is converged by a lens with a focal length f so as to be incident on a photodetector having a photodetecting surface with a width W along a direction of the incident light beam; and that is satisfied the relation: tan |Δθ|≦ W /(2 f ).