Optical delay lines are devices used to introduce a known delay into an optical signal. This delay is achieved by forcing the light to travel a longer path than it would normally take. The basic principle behind optical delay lines is to control the length of the optical path.

There are two primary methods to achieve this:

• Fiber-based delay lines:These use long lengths of optical fiber to increase the path length. The longer the fiber, the greater the delay introduced.
• Mirror-based delay lines:These use a series of mirrors arranged in a specific configuration to reflect the light back and forth, effectively increasing the path length. The number of reflections and the distance between the mirrors determine the delay.

In both cases, the delay introduced is directly proportional to the length of the optical path. This means that by carefully controlling the length of the fiber or the mirror configuration, we can precisely adjust the delay introduced into the optical signal.