Boxcar averagers (sometimes also known with the name of “gated integrators”) are instruments used to measure signals stemming from low-duty-cycle experiments.
The information contained in low-duty-cycle pulsed signals is concentrated within a short time duration; thus, the idea behind the boxcar averager is to record the signal only during the time window where the information is present while ignoring everything outside of it – i.e. only noise.
For a comprehensive analysis of the boxcar averager and its working principles, take a look at our Principles of Boxcar Averaging white paper and related video.
The Zurich Instruments UHF-BOX Boxcar Averager (upgrade option to the UHFLI 600 MHz Lock-in Amplifier) equips the user with two independent digital boxcar averager units, configurable either with the LabOne user interface or the APIs. The purpose of this blog post is to provide practical guidance for setting up the units, optimizing your measurements, and extracting the maximum benefit from pulsed signal detection, utilizing all the advantages of digital signal processing.
Introduction
In optics and photonics applications, a widespread example of low-duty-cycle experiments is Pump-probe spectroscopy (sketch in Figure 1). This technique consists of exciting the sample with a first laser pulse, the pump, thereby promoting it to some excited state. The dynamics of this excited state can then be investigated by the interaction with a second pulse, the probe; by measuring the relative pump-induced intensity variations on the probe pulses (often denoted as ΔA/A, where A represents the intensity of the probe pulses) as a function of the time delay between the pump and the probe, one can then reconstruct the time-dependent dynamics of the excited state. One of the most important advantages of this technique lies in the fact that the time resolution is limited only by the duration of the laser pulses.
For this reason, pump-probe spectroscopy has nowadays become extensively used to study ultrafast phenomena, with many different experimental variations originating from it – e.g. Stimulated Raman Scattering (SRS) and Terahertz Time-domain spectroscopy.
However, due to the high nonlinearity of the investigated processes, the magnitude of the signals from these experiments turns out to be exceptionally small (relative variations ΔA/A down to 10-6 or smaller). Therefore, proper signal analysis is key to achieving high signal-to-noise ratios (SNR) and ensuring proper extraction of the information from the measurements.
To maximize time resolution, many experimental instances of pump-probe spectroscopy make use of ultrashort laser pulses, running at repetition rates ranging from a few kHz to 10s of MHz and measured by fast photodetectors. Such a configuration leads to signals whose duty cycle is extremely low, e.g. 100 fs pulses running at 100 kHz repetition rate detected with a 100 MHz photodetector (i.e. 10 ns rise time) result in a duty cycle of about 0.1% - the duty cycle being defined as PW*100/T, where PW is the pulse width and T is the inverse of the repetition rate.
Consequently, in these cases, boxcar averaging becomes the better measurement approach leading to the best signal-to-noise ratio.
