Multichannel Optical Detector for Time-Resolved Spectroscopy*

As part of our work towards time-resolved measurements of solvated electron concentrations at the plasma/water interface, we are developing a multi-channel, fast photodiode detector. Our measurement is based on the work of Rumbach, et al. (2015), who measured solvated electron concentration using total internal reflection absorption spectroscopy (TIRAS).  We extend their technique using a pulsed supercontinuum laser (FYLA Iceblink) to probe the absorption spectrum of solvated electrons at the plasma-water interface in a stroboscopic manner. The supercontinuum pulses have a width of ~100 ps and a repetition rate of 1 MHz.  We disperse the light using a transmission grating and collect the relevant portion of the spectrum on a multi-element photodiode array (OSI A5C-38). The resulting current pulses are quite small and require an amplifier to be effectively measured. The bandwidth of our amplifier must be able to respond to the input pulses while also detecting intensity modulation in the kHz frequency range (plasma pulse rate). We chose a two-stage amplification scheme to provide sufficient gain and bandwidth for each channel. The first stage is a low-noise high-speed trans-impedance amplifier using the LT6268-10. This preamplifier provides a particular challenge in its design as the bandwidth of the pulse train signal spans six orders of magnitude. This results in a lower limit on maximum gain compared to trans-impedance amplifiers in other use cases. The second stage is a lock-in amplifier at the modulation frequency. We present the particulars of the design and preliminary results.