Real-time FPGA data collection of pulsed-laser cavity ringdown signals

Abstract

This paper presents results from a pulsed-laser cavity ring-down spectrometer with novel field programable gate array real-time data collection. We show both theoretically and experimentally that the data extraction can be achieved from a single cavity ringdown event, and that the absorbance can be determined without the need to fit the ringdown time explicitly. This methodology could potentially provide data acquisition rate up to 1 MHz, with the accuracy and precision comparable to nonlinear least squares fitting algorithms.

The discrete Fourier transform algorithm for determining decay constants—Implementation using a field programmable gate array

“…The evaluation board's standard input path includes conversion of a single-ended signal to a differential signal using a radio frequency (RF) transformer that is designed to work with alternating signals that, as stated in the AD9255 data sheet, "can saturate at frequencies below a few megahertz." 15 The effect of this is attenuation of low frequencies, or high-pass filtering, and our initial attempts to use the standard input path resulted in ringdown signal distortion similar to that reported by Spence et al 12 Fortunately, the evaluation board contained a secondary input path to the AD9255 with an operational amplifier (ADL5562, Analog Devices, Inc.) capable of converting DC-coupled, low frequency, single-ended signals to the requisite differential signals. Unfortunately, the majority of the alternate input path is unpopulated, and the components that were installed assumed an AC-coupled signal.…”

“…Additional components used to interface with these boards, as well as the modifications we made to the evaluation boards, are discussed in Secs. II A-II C. To our knowledge, there is only one report of using a FPGA for processing CRDS data, 12 in which an Altera FPGA evaluation board with an ADC daughter card is used to compute the decay constant. In that work, they used a modification of the discrete sum/frequency component algorithm and used a lookup table to determine the time constant from the ratio of two frequency components.…”

“…High-speed FieldProgrammable Gate Arrays (FPGAs) provide the possibility of putting a customized, highly efficient processor module immediately after the ADC. 12 Few practitioners of CRDS have applied this approach despite the fact that the ADC and FPGA integrated circuits (ICs) can be relatively inexpensive, presumably because of a perception that building the proper support circuitry around them can be time-intensive and requires a high level of skill in circuit design. Purchasing commercial high-speed data acquisition modules with (or even without) onboard FPGAs can be costly (several thousand dollars).…”

Optical injection unlocking for cavity ringdown spectroscopy

“…These issues increase the complexity in the fitting algorithm, the current pulsing timing, and the frequency shifting circuit. Using optoelectronic locking to the cavity, such as the Pound-Drever-Hall method [10], requires additional circuit complexity to turn off the electronic locking during the ringdown and then relock the laser to the cavity [8,11,12].In this Letter, we present a novel technique to move the ringdown laser off resonance in less than a microsecond, while not interfering with cavity locking, by injecting a second, pulsed diode laser into the main laser. This method does not change the current of the locked laser, allowing it to return to cavity resonance after the disrupting pulse is over.…”

“…These issues increase the complexity in the fitting algorithm, the current pulsing timing, and the frequency shifting circuit. Using optoelectronic locking to the cavity, such as the Pound-Drever-Hall method [10], requires additional circuit complexity to turn off the electronic locking during the ringdown and then relock the laser to the cavity [8,11,12].…”