Optical Delay Line Stages – How Do They Work?

Deep in the hallowed halls of the Paris observatory in the late 17^th Century, a paradigm shift was underway. The speed of light conjectured by luminaries such as Descartes and Aristotle as infinite and instantaneous was empirically showing delays. The revelatory observation of eclipses of the Jovian Moon, Io, had provided a basis for what would become the first practical measurement of the speed of light using an optical delay from a variable path length. Today, very precise path length control in optical delay lines are critical to techniques in transient absorption spectroscopy (using pump-probe), Fourier Transform Infrared Spectroscopy (FTIR), and many other applications.

Why Minute Time Increments are Critical
In time-resolved spectroscopy, dynamic processes in materials and chemical compounds can be studied precisely. With the help of ultra-short pulse lasers, changes can be made visible, with time scales down to the attosecond range. The calculation below shows that a 10nm step in a free space delay line adds a delay of ~.07 femtoseconds (delay in solid materials, such as optical fibers, depends on their optical properties). With this in mind, it becomes clear that in order to control a delay line in the Femtosecond and Attosecond range, nanopositioning capabilities are essential.

Optical Delay Line Basics – Distance and Time
In the below diagram, we show the basic operating principle of a free space optical delay line. Here, the path is shown at Position X (green path) and again after a small linear translation to Position X’ (purple path). Note the green path overlaps a portion of the purple path. Let’s use this simple diagram to calculate optical path length difference and corresponding optical delay: