Dual-comb spectroscopy has become a topic of growing interest in recent years due to the advantages it offers in terms of frequency resolution, accuracy, acquisition speed, and signal-to-noise ratio, with respect to other existing spectroscopic techniques. In addition, its characteristic of mapping the optical frequencies into radio-frequency ranges opens up the possibility of using non-demanding digitizers.
In this paper, we show that a low-cost software deﬁned radio platform can be used as a receiver to obtain such signals accurately using a dual-comb spectrometer based on gain-switched semiconductor lasers.
We compare its performance with that of a real-time digital oscilloscope, ﬁnding similar results for both digitizers. We measure an absorption line of a H13C14N cell and obtain that for an integration time of 1 s, the deviation obtained between the experimental data and the Voigt proﬁle ﬁtted to these data is around 0.97% using the low-cost digitizer while it is around 0.84% when using the high-end digitizer.
The use of both technologies, semiconductor lasers and low-cost software deﬁned radio platforms, can pave the way towards the development of cost-efﬁcient dual-comb spectrometers.
Back in January of this year we posted about PhD student Lucas Riobó's work that about about using an RTL-SDR to create a low cost optical "high-speed real-time heterodyne interferometer". In that work he used an RTL-SDR as a data acquisition tool for an optoelectronic front end sensor (opto = visual light). This allowed him to translate optical data into an RF signal, which could be received by the RTL-SDR, and then easily processed in a PC.
In this work, a general architecture for the implementation of software-defined optoelectronic systems (SDOs) is described. This concept harnesses the flexibility of software-defined hardware (SDH) to implement optoelectronic systems which can be configured to adapt to multiple high speed optical engineering applications. As an application example, a software-defined optical interferometer (SDOI) using the LimeSDR platform is built. The system is tested by performing high speed optical detection of laser-induced photoacoustic signals in a concentrated dye solution. Using software modifications only, conventional single carrier and also multicarrier heterodyne techniques with space and frequency diversity are performed.
A main difference with the other article described in this post, is that we could also use the transmission path of the LimeSDR to perform many modulation waveforms of the electromagnetic fields which will interfere, to provide a noticeable performance improvement in single-shot interferometric measurements.
In the experiment a laser is fiber optically coupled to an eletro-optic phase modulator, which modulates a 400 MHz FM signal onto the light. The light is then passed into a Carbon monoxide absorption cell with a photodiode used to take the spectroscopic measurements. The signal from the photodiode is passed into a LNA and then into the Airspy where the signal can then be processed on the PC.
The paper is very technical, but describes the setup, and how they characterized and calibrated the Airspy for their measurements. They conclude with the following:
A successful demonstration of a commercially available software defined radio as a lock-in amplifier was performed. For this purpose, the tuner front end and back end were characterized. The sensitivity and non-linearity of the receiver circuit was measured and analyzed. Acquisition of a CO spectral line was demonstrated using FM-spectroscopy with a repetition rate of 1 kHz. This proves the usability of an off-the-shelf SDR as a cheap but powerful lock-in amplifier by adding PLL driven frequency generators. The drawback of the arbitrary initial phase of the used phase locked loops can be either solved by software or hardware measures.
Thanks to PhD student Lucas Riobó of the University of Buenos Aires, Argentina for submitting his very interesting work on creating a "High-speed real-time heterodyne interferometer" with a low cost RTL-SDR dongle. This is a new application for the RTL-SDR that we have not yet seen.
Interferometers are tools that combine two separate electromagnetic waves (e.g. radio or light) and analyze the interference pattern created by their combination. One usage for example is creating a radio telescope interferometer using multiple small radio dishes. The result is that you can get the same resolution as a much larger dish without the cost of needing to build a huge dish. This has been done before with RTL-SDR's and Pulsar detection.
The paper and concept is fairly complex for someone without a background in optical science, but basically it seems that Lucas has created an optical interferometer that interfaces with an RTL-SDR dongle via a wideband optoelectronic front-end. This allows the optical data to be translated into an RF signal which can then easily be analysed with the low cost RTL-SDR. A system like this reduces costs and allows for much easier data acquisition and processing on the PC. He writes:
As you may know, optical Interferometry is a family of techniques in which the superposition of electromagnetic waves (in the optical range of the spectrum), cause the phenomenon of interference in order to extract information. In this work, we implement an optical heterodyne interferometer. This interferometer, the waves (laser beams) that superpose have a frequency shift f0 between them. When the beams interfere, the intensity from the combination of the beams (interferogram) is a sinusoid signal at a frequency f0 (i.e. a carrier signal). In this work, one of the beams reflects over a sample that has a mechanical deformation. Therefore, this information is encoded in the phase of the carrier signal.
We applied the RTL-SDR dongle to demodulate the carrier signal to extract the phase information. Instead of using an antenna, we put a photodiode with a transimpedance amplifier (TIA). Thus, since the signal obtained from the photodiode and the TIA is proportional to the interferogram, the phase/frequency recovery techniques are the same as those used in telecommunications systems (i.e. we can use many demodulation algorithms developed by the community).