Laser spectroscopy with heterodyne-based detection (2020-2024). Preludium Bis 1 from the National Science Centre (NCN).
Laser spectroscopy with heterodyne-based detection (2020-2024). Preludium Bis 1 from the National Science Centre (NCN).
Laser spectroscopy is a highly sensitive technology crucial for environmental monitoring, medical diagnostics, and industrial process control. While modern systems can detect trace gases down to parts-per-billion levels, significant challenges remain, such as standoff sensing or detecting molecules that lack strong absorption lines in the infrared region (e.g. hydrogen).
This project was designed to address these limitations by integrating optical heterodyne detection (OHD) into laser-based gas sensing. By mixing a weak optical signal with a strong "local oscillator" light wave, OHD down-converts optical frequencies (ca. 200 THz) into easily processable radio frequencies (RF). This approach not only amplifies weak signals but also grants direct access to the phase of the optical wave. The project successfully verified the hypothesis that these unique properties can unlock entirely new sensing configurations and improve the overall performance of laser spectroscopy.
Through rigorous experimental verification, the project yielded three major achievements in the field of gas sensing:
We developed and tested a novel endoscope-like gas probe utilizing anti-resonant hollow-core fibers (AR-HCF) in a reflective configuration. Traditionally, taking absorption measurements in this geometry is practically impossible due to strong interference fringes caused by multiple light reflections at the fiber joints. By implementing heterodyne detection, we successfully separated the useful spectroscopic signal from unwanted reflections in the RF domain. This breakthrough enables high-sensitivity gas detection (demonstrated with methane) using reflective fiber probes that were previously inaccessible to standard methods.
We demonstrated an innovative approach to detecting non-absorbing gases (like hydrogen) by monitoring oxygen concentration as a proxy. Utilizing Differential Optical Dispersion Spectroscopy (DODiS) in the visible range (761 nm) within a Mach-Zehnder interferometer setup, we used heterodyne phase-shift detection to precisely track the dilution of ambient oxygen. If a leak introduces a foreign gas into the sample, it displaces the oxygen, generating a distinct differential dispersion signal. This significantly expands the applicability of laser spectroscopy to detect gases that lack distinct IR absorption lines.
In collaboration with Gerard Wysocki from Princeton University, the project explored the use of mid-infrared quantum cascade laser frequency combs (QCL-FC) for broadband photothermal spectroscopy (PTS). By combining the multi-heterodyne detection technique with a near-infrared probe laser, we demonstrated the ability to simultaneously measure multiple absorption lines across a broad spectral range (tested on nitrous oxide). This represents a major step toward highly sensitive, multi-component gas detection based on photothermal effects.
The knowledge gained from this project has paved the way for a new generation of robust, high-sensitivity gas sensors. The research was supported by the PRELUDIUM BIS grant and culminated in a highly-rated, award-winning PhD dissertation by Dr. Grzegorz Gomółka, who is now continuing his research in chip-based terahertz frequency comb spectrometers.
The core findings of this project have been published in scientific journals:
Reflective AR-HCF probe:
G. Gomółka ... M. Nikodem, "Dual-Pass Hollow-Core Fiber Gas Spectroscopy Using a Reflective Configuration With Heterodyne-Based Signal Detection", Journal of Lightwave Technology, vol. 41, pp. 6094-6101 (2023).
Oxygen sensing::
G. Gomółka, .... M. Nikodem, "Wavelength modulation spectroscopy of oxygen inside anti-resonant hollow-core fiber-based gas cell", Optics and Laser Technology, vol. 170, pp. 110323 (2024).
DODiS for leak detection:
G. Gomółka and M. Nikodem, "Gas leak detection by measuring dilution of ambient air with differential optical dispersion spectroscopy of oxygen", Optics Express, vol. 32, pp. 48847–48857 (2024).
Broadband QCL-FC photothermal spectroscopy:
B. Huang, G. Gomółka, ... G. Wysocki, "Broadband photothermal spectroscopy with a mid-infrared quantum cascade laser frequency comb", Optics Express, vol. 33, pp. 2126-2137 (2025).
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This research was funded by the National Science Centre, Poland, grant number 2019/35/O/ST7/04176
Optical signal amplification in the spectral range 1650 to 1750 nm for optical spectroscopy (2019 – 2023). Opus 16 from the National Science Centre (NCN)
The 1650–1750 nm spectral region is highly valuable for both trace gas spectroscopy and the expansion of optical telecommunications. However, research and practical applications in this band have historically been limited by the lack of effective optical amplifiers. This project was established to bridge that technological gap. Our primary goal was to design, develop, and characterize novel fiber-based optical amplifiers capable of delivering high gain, broad spectral coverage, and substantial output power. Furthermore, we aimed to investigate exactly how these new amplifiers could be integrated into laser-based gas detection systems to significantly enhance their sensitivity and performance in the >1650 nm wavelength regime.
The project successfully met all its primary objectives, yielding several record-breaking results and uncovering new phenomena in amplified spectroscopy:
Through international collaboration and iterative design optimization, we constructed a bismuth-doped fiber power amplifier. By utilizing a bidirectional pumping scheme, we achieved an output power exceeding 334 mW at 1651 nm and nearly 200 mW at 1687 nm.
We integrated our novel amplifiers into methane detection setups to test their practical impact:
Photoacoustic & Photothermal Spectroscopy: Because photoacoustic and photothermal signals scale directly with optical power, our BDFA dramatically improved system sensitivity. Using a Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS) setup paired with our amplifier, we achieved an extraordinary detection limit of ~11 parts per billion (ppb) for methane, a highly challenging feat in the near-infrared band.
WMS Background: We also tested Wavelength Modulation Spectroscopy (WMS). Interestingly, we discovered and documented that combining optical amplifiers with WMS introduces non-linear background signal not previously reported in literature, and we proposed methodologies to mitigate this issue.
Capitalizing on the broad emission characteristics of bismuth, we developed a continuous-wave fiber laser with a record-breaking tuning range. By strategically combining erbium-doped and bismuth-doped active fibers in a single cavity, we achieved continuous, seamless tuning across a span of over 260 nm (from 1.55 to 1.8 µm).
The project had a profound impact on the academic development of our team. Several aspects of this research were carried out by students, and three resulting theses (by Grzegorz Gomółka, Monika Krajewska, and Przemysław Chmielowski) were awarded top prizes in the prestigious Professor Adam Smoliński National Competition for the best diplomas in optoelectronics.
The technologies developed here hold massive potential not just for spectroscopy, but for extending optical telecommunication windows beyond 1610 nm. Currently, our team is continuing this trajectory, focusing our ongoing research on optical amplification in the >1700 nm band utilizing thulium-doped fibers.
The core findings and technological milestones of this project have been published in following papers:
M. Zatorska, G. Gomółka, and M. Nikodem, "Near-infrared quartz-enhanced photoacoustic spectroscopy system for ppb-level methane detection," Optics Continuum 2, 266-273 (2023). (Highlighted as "Editor's Pick")
P. Chmielowski and M. Nikodem, "Widely tunable continuous-wave fiber laser in the 1.55-1.8 µm wavelength region," Optics Express 30, 42300-42307 (2022).
G. Gomolka, M. Krajewska, A. Khegai, S. Alyshev, A. Lobanov, S. Firstov, D. Pysz, G. Stepniewski, R. Buczynski, M. Klimczak, and M. Nikodem, "Heterodyne photothermal spectroscopy of methane near 1651 nm inside hollow-core fiber using a bismuth-doped fiber amplifier," Applied Optics 60, C84-C91 (2021).
G. Gomółka, M. Krajewska, M. Kaleta, A. Khegai, S. Alyshev, A. Lobanov, S. Firstov, M. Nikodem, "Operation of a single-frequency bismuth-doped fiber power amplifier near 1.65 µm," Photonics 7, 128 (2020).
G. Gomolka, A. M. Khegai, S. V. Alyshev, A. S. Lobanov, S. V. Firstov, and M. Nikodem, "Characterization of a single-frequency bismuth-doped fiber power amplifier with a continuous wave and modulated seed source at 1687 nm," Applied Optics 59, 1558-1563 (2020).
M. Nikodem, A. Khegai, and S. Firstov, "Single-frequency bismuth-doped fiber power amplifier at 1651 nm," Laser Physics Letters 16, 115102 (2019).
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This research was funded by the National Science Centre, Poland, grant number 2018/29/B/ST7/01730