When it is useful to have a reference channel, and how it is implemented?

Which is the best probe-reference configuration and how do we normalize it to the linear absorption spectra? Basically, there is no perfect reference for the probe but there are two main approaches for selecting which configuration is best based upon certain assumptions.

Dual Probe Channel (Probe-Reference)

The second channel is used as a reference to normalize intensity for each laser pulse of pump on/ pump off pair. In this case the assumption is that the splitting ratio between sample and reference probe beams is constant. Therefore the fluctuations in the reference beam reflect the corresponding fluctuations in the sample beam not related to the excitation pulse. This method is used mainly with less stable white light.

Single Probe Channel

In case of a single probe channel the assumption is that the continuum intensity is constant for the two consequent continuum pulses. Our experience is that with the modern femtosecond lasers the second approach works very well. A single probe channel is easier to work with and is more forgiving towards the loss of light due to the sample absorption or scattering (because you effectively have twice the light to work with). Also, you don’t have to worry about how the probe light is focused into the fiber, because you don’t have to match the two probe legs in intensity and spectrum. Therefore, in most cases, we recommend our customers getting a single probe channel configuration. Of course, we can provide a dual channel (probe/reference) option as well.

See knowledge base question for the formulas we use to calculate absorption spectra in both configurations.