A Manual Adjustable Fiber Delay Line (MADL) operates on a simple principle: it introduces a controlled delay in an optical signal by varying the length of the optical fiber through which the signal travels.  

1. Fiber Spool: The core component is a spool of optical fiber, typically single-mode or multi-mode, wound around a drum or cylinder.  

2. Mechanical Adjustment: A mechanical mechanism, such as a knob or lever, allows the user to manually adjust the position of the fiber spool.

3. Fiber Length Variation: As the spool is rotated, the effective length of the fiber through which the light travels increases or decreases.  

Delay Introduction: A longer fiber path results in a longer propagation time for the light signal, thereby introducing a delay. Conversely, a shorter fiber path reduces the delay.