How do Optical Delay Lines Work?

Formerly thought to be unlimited and instantaneous by intellectuals like Descartes and Aristotle, the speed of light was empirically shown to include delays, leading to a paradigm change in Paris in the late 17th century.

 

The first practical measurement of the speed of light was made utilizing an optical delay from a changing route length by viewing eclipses of Io, the Jovian Moon.

 

Modern methods like Fourier Transform Infrared (FTIR) and transient absorption spectroscopy (TAS) depend on exceedingly accurate path length control in optical delay lines (using pump-probe).

 

The Value of Insignificant Time Increments

With the use of time-resolved spectroscopy, dynamic processes in materials and chemical compounds can be carefully examined. Ultra-short pulse lasers with timeframes in the attosecond region can be used to observe these changes.

 

The math below shows that a free space delay line adds a delay of approximately.07 femtoseconds for every 10 nm step. This delay depends on the optical characteristics of the material in solid materials like optical fibers.

 

Hence, the ability to operate a delay line in the femtosecond and attosecond range requires nanopositioning skills.

 

Distance and Time Basics of Optical Delay Lines

The basic workings of a free space optical delay line are shown in the diagram below.

 

The path follows a slight linear translation to Position X's position and is represented there (green path) (purple path). It should be noticed that a portion of the purple trail is covered by the green path. 

 

Calculating the difference in optical route length and any accompanying optical delay can be done using the diagram below:

  1. X=A+B+C (green path length) (green path length)

 

  1. X'= A+ A'+ B+ C'+ C (purple path length)

 

Path length difference is also determined by:

 

  1. A' + C' = A'+C' - X'-X=A+A'+B+C'+C - (A+B+C)=A-A+B-B+C-C (Yet, the incremental motion step is A'=C')

 

As a result of this equation,

 

  1. X’-X=2A’

 

The optical delay in the time domain is given by:

 

  1. t= 2A’/c (where c is the speed of light) (where c is the speed of light)

 

Simply put, because this determines the optical delay in terms of temporal delay resolution, it serves as an example of the significance of Minimal Incremental Motion (MIM) capability in the chosen linear translation stage.

 

MIM, or Minimum Interval Motion, is a defined motion term that refers to the shortest consistent and reliable step that may be made with the stage.