An optical element of an embodiment includes an optical element made of a material transparent to light, the optical element including: a back surface facing the front surface; and a connection surface. The front surface includes a recessed surface in a region facing the connection surface. The recessed surface has a point closest to the connection surface as a closest point, and has a first singular point other than the closest point.

A small form factor optical head defines a single light pathway. The optical head includes a housing, a lens positioned on or in the housing, and a light source positioned in the housing to emit an excitation light. A mirror is positioned in the housing to reflect the excitation light to the lens and to receive emission light back through the lens, and a beamsplitter positioned in the housing directs the excitation light to the lens and directs the emission light to a detector that receives emission light after passing back through the lens and the beam splitter. The lens, the mirror, and the beam splitter define a single light pathway for the excitation light and emission light. A method for optical analysis is also disclosed.

An optical device provided with an optical parts position adjusting and fixing device for adjusting and fixing the position of the optical parts moving in an optical axis direction, wherein the position adjusting and fixing device is provided with a long slotted hole extended in the optical axis direction formed on a side surface of the optical parts; a fixing means which is mounted on an object to be fixed of the optical parts and which is movable along the long slotted hole; a non-penetrating screw hole formed in the fixing means and which is divided by a slit; and a screw which is screwed into the screw hole and spreads the slit when screwed into the screw hole so as to press the outer surface of the fixing means against the inner surface of the long slotted hole.