Optical delay lines (ODLs) introduce a controlled time delay into an optical signal path. They achieve this by manipulating the path length that the light travels within the device compared to a straight-through path. The amount of time delay is directly proportional to the difference in path length and the speed of light in the material used. Here's a breakdown of the two main ODL types:

1. Free-space ODLs

l These ODLs utilize air as the light propagation medium. Light travels a longer path by reflecting off strategically placed mirrors.

l The key component is a set of mirrors positioned at a specific distance from each other. The light beam travels from the source, reflects off the first mirror, travels back towards the source, reflects off the second mirror, and then continues on its intended path.

l By adjusting the distance between the mirrors, the total distance the light travels within the ODL can be controlled. This, in turn, controls the time delay experienced by the signal.

l Free-space ODLs often use retroreflectors (mirrors that reflect light back in the same direction it came from) to minimize the footprint of the device.

l However, for very long delays, free-space ODLs can become quite large and require precise alignment of the mirrors to maintain efficient light coupling.

2. Fiber-optic ODLs

l These ODLs rely on optical fibers as the light propagation medium. The light travels through a spool of fiber with a specific length, creating the time delay.

l Compared to free-space ODLs, fiber-optic versions offer a more compact design. Light travels through the core of the fiber optic cable, experiencing multiple internal reflections as it bounces off the core-cladding interface. The total length of the fiber determines the time delay.

l Some fiber-optic ODLs employ techniques like coiling the fiber to further increase the path length within a limited space. This coiling introduces additional bends in the fiber, causing the light to travel a longer distance compared to a straight fiber.

However, coiling can also introduce some signal loss due to increased modal dispersion (spreading of light pulses due to different travel paths within the fiber).