Project #: 22-061 | Year 2 of 2
Eric Larson, David Baldwin, Brandon La Lone, Matt Staska
Special Technologies Laboratory
We have built and tested a new approach to dual comb laser spectroscopy applied to gas detection in the laboratory. The dual comb system demonstrates the ability to project a dual comb laser source through a hydrogen cyanide (HCN) gas cell absorption line while maintaining stability and sensitivity. The dual combs were generated using an alternating wavelength shift and comb spacing scheme. This method used a single laser with alternating delayed combs, combined to generate the dual combs.
There are two distinct systems in the dual comb laser: optical and electronic.
The optical system starts with the ORION narrow linewidth laser. The laser is modulated with an electro-optic modulator (EOM). This modulator has 10 GHz of bandwidth to cover the width of the absorption line. The modulator requires the polarized light input from the laser, so the laser was ordered with a polarization maintaining output to match the modulator.
From the modulator, the light is split into two paths. The first path passes through the 5-µs delay line of fiber and then is reflected through the delay line to the splitter by a Faraday mirror. The second path is the prompt path. Light on this path passes through the splitter and is reflected into the splitter by a second Faraday mirror without a delay line. The combined prompt and delayed paths overlap in time on their return through the splitter/combiner.
The electronics system has been integrated in a single circuit on a PCB. The circuit provides bias control for the EOM and the dual switched comb oscillators for the EOM. The oscillators were chosen to map the absorption line with 575-MHz increments. There are two oscillators, one at 576 MHz and the other at 574 MHz. The comb switch alternates between the two comb frequencies at a 10-kHz rate. The 10 kHz rate for the stitching oscillator (labelled W3 in Figure 1) was chosen to match the 10-µs delay time of the roundtrip through the optical delay line.
A dual comb laser has been developed using a single laser, single EOM, and a delay loop, all controlled by a custom electronics drive unit. Control electronics developed for the system shift the laser central wavelength in phase with the EOM modulation switching to produce dual combs when delayed and combined. Refinements to improve wavelength range and system performance are possible but not yet fully explored.
The dual comb laser system developed in this project provides a significantly simplified means of building dual comb laser systems for a variety of applications. The system has fewer lasers, fewer electronic components such as optical modulators, and simpler electronic components such as frequency sources than laser comb sources in laboratory-based systems. These simplifications make the source amenable to reduced size, weight, and power incarnations that will enable field use, use in unmanned platforms, and use in portable handheld applications. We anticipate applying these innovations to sources for follow-on work in fieldable high resolution gas absorption sensor systems and in a fieldable vibrometry application.
This work was done by Mission Support and Test Services, LLC, under Contract No. DE-NA0003624 with the U.S. Department of Energy. DOE/NV/03624–1659.