Optical delay line: The key to time-resolved measurement

Optical delay line: The key to time-resolved measurement
In order to obtain an accurate method for generating reliable delays in any time-resolved spectroscopy or dynamic experiments, several factors at the delay line level must be considered to reduce or eliminate errors related to the linear level. In any time-resolved spectroscopy and dynamics experiments, one of the most crucial components is the optical delay line. A typical optical delay line consists of a rear reflector or folding mirror on a translation stage (Figure 1). When selecting the translation stage, certain parameters on the stage and the driver or controller should be considered, as they can affect data analysis and interpretation. The key motion control parameters that affect time-resolved measurements include total delay, minimum incremental motion (MIM), repeatability, accuracy, and mechanical error.

The first parameter that needs to be considered at the linear level is the total delay (T) – the time required for light to propagate to the backward-reflection optical device and form the return path. This is directly related to the travel range (L) of the linear stage: T = 2*L/c, where c is the speed of light in a vacuum. The next most important parameter is the delay resolution (Δτ), which is related to the MIM of the translation level and is calculated using the formula Δτ = 2*MIM/c.
It is crucial to distinguish between MIM and the resolution of the motion system because they represent two distinct concepts. MIM refers to the smallest incremental movement that the device can consistently and reliably transmit, thus it represents a system capability; on the other hand, resolution (display or encoder resolution) is the smallest value that the controller can display or the smallest incremental value of the encoder, referring to the design feature.
Another stage parameter that is equally important as MIM is stage repeatability, which refers to the system’s ability to reach the commanded position after multiple attempts . In typical time-resolved measurements, the linear stage scans within a certain distance (corresponding to a specific time delay) and records some signals of the target sample as a function of time delay. Based on the signal intensity of the sample and the expected signal-to-noise ratio, the average value of multiple scans is a commonly used method in time-resolved measurements. Through this procedure, it is crucial for the linear stage to have high repeatability.