Abstract
Due to their advantages of having a wide bandwidth, low cost, and being easy to obtain, traditional photodetectors (PDs) are being widely applied in measurements of transient signals. The spatial inhomogeneity of such PD temporal responses was measured directly to account for the PD spatial effect of decay rate due to poor alignment in continuous wave cavity ringdown spectroscopy (CW-CRDS) experiments. Based on the measurements of three PDs (i.e., model 1611 (Newport), model 1811 (Newport), and model PDA10CF-EC (Thorlabs)), all the temporal responses followed a tendency of declining first and then rising, and steady platforms existed for the last two PDs. Moreover, as we expected, the closer the PD center was, the faster the response. On the other hand, the initial shut-off amplitude generally reached a larger value for a faster temporal response. As a result, the spatial effect can strongly influence the spectral line shape and value, which will introduce more errors into the precise measurements of spectral parameters using the CRDS technique if this effect is not considered. The defined effective detection area (EDA) of the PDs, which was close to the active area given by manufacturers, was the key parameter that should be paid more attention by researchers. Therefore, the PD should be aligned perfectly to make sure that the EDA covers the laser spot completely.
Highlights
Molecular spectroscopy techniques, which have advantages in terms of sensitivity, resolution, and linearity, are crucial in field applications and fundamental physical measurements
Multiple types of detectors [40], such as photon multiplier tubes (PMT) and avalanche PDs, have been provided commercially and used to detect weak light with sub-ns response times, which makes them perfectly matched for use in Cavity ringdown spectroscopy (CRDS)
For reliable and stable trace gas detection with continuous wave cavity ringdown spectroscopy (CW-CRDS), one should make sure that the laser beam is aimed at the effective detection area (EDA) of the PD and that the spot size is focused properly to match the size of this area
Summary
Molecular spectroscopy techniques, which have advantages in terms of sensitivity, resolution, and linearity, are crucial in field applications and fundamental physical measurements. The laser bandwidth exceeds the cavity mode spacing, thereby giving rise to spectral overlap between the probe laser and several cavity resonances Such a coupling between the broadband laser and the cavity causes a multimode excitation and results in a multi-exponential decay, which means the signal-to-noise ratio (SNR) and frequency resolution of the spectral signal are quite low. Multiple types of detectors [40], such as photon multiplier tubes (PMT) and avalanche PDs, have been provided commercially and used to detect weak light with sub-ns response times, which makes them perfectly matched for use in CRDS Besides these types of photodetectors, traditional PDs are good options for temporal measurements due to their advantages of have a wide response bandwidth, being low cost, and readily available. For reliable and stable trace gas detection with CW-CRDS, one should make sure that the laser beam is aimed at the EDA of the PD and that the spot size is focused properly to match the size of this area
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