Receiver, detection system, and detection method

A receiver comprising:

• a divider configured to divide an optical signal transmitted for detecting a detection target on a traveling path over a plurality of paths, including a first path, a second path, a third path, a fourth path, and a fifth path;
• a detector configured to detect a received light intensity of the optical signal at a first position of the first path where the received light intensity becomes maximum, and at more than three positions out of a second position of the second path where the received light intensity increases when an optical axis of the optical signal is shifted in an upper right direction, a third position of the third path where the received light intensity increases when the optical axis of the optical signal is shifted in a lower right direction, a fourth position of the fourth path where the received light intensity increases when the optical axis is shifted in an upper left direction, and a fifth position of a fifth path where the received light intensity increases when the optical axis is shifted in a lower left direction;
• a processor;
• and
• a memory storing instructions executable by the processor to:
• determine presence or absence of an optical axis shift and calculate an amount of the optical axis shift based on a change in a received light intensity ratio between the received light intensity of the optical signal at the first position and the received light intensity at the second, third, fourth, or fifth position.

A detection method comprising:

• dividing an optical signal over a plurality of paths including a first path and a second path;
• receiving the optical signal at a first position of the first path where a received light intensity increases when an optical axis of the optical signal is shifted and at a second position of the second path where the received light intensity decreases when the optical axis is shifted;
• calculating, by a processor, a received light intensity ratio based on the received light intensity at the first position and the received light intensity at the second position;
• and
• determining, by the processor, presence or absence of an optical axis shift based on a change in the received light intensity ratio over time.