Mid-infrared optical dual-comb generation and spectroscopy with one unstabilized semiconductor laser

Project Information

ONE-MIX

Grant agreement ID: 787097

DOI

10.3030/787097

Project closed

EC signature date 25 April 2018

Start date 1 January 2019

End date 31 October 2024

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 3 210 000,00

EU contribution

€ 3 210 000,00

Coordinated by

EIDGENOESSISCHE TECHNISCHE HOCHSCHULE ZUERICH
Switzerland

Periodic Reporting for period 4 - ONE-MIX (Mid-infrared optical dual-comb generation and spectroscopy with one unstabilized semiconductor laser)

Reporting period: 2023-07-01 to 2024-10-31

Summary of the context and overall objectives of the project

ONE-MIX proposes to develop single-source dual-comb lasers for applications in the short-wave infrared (SWIR) and mid-infrared (mid-IR) spectral region potentially enabling many new applications in science and industry, such as environment, safety, pharma, and health. We leverage our know-how in near-infrared MIXSELs (Modelocked Integrated eXternal-cavity Surface Emitting Lasers) with III-V GaSb (gallium-antimonide) semiconductor epitaxy operated out of the FIRST lab at ETH Zurich. The MIXSEL is an optically pumped surface emitting semiconductor laser that has the gain and the saturable absorber integrated into one semiconductor chip which forms one end mirror of a linear straight laser cavity. We use polarization duplexing to obtain two optical frequency combs with an adjustable pulse repetition rate difference for dual-comb applications allowing for fast, accurate, and sensitive measurements.

Dual-comb applications are currently limited by the cost, complexity, and size of conventional optical comb systems, typically based on two modelocked lasers with four active stabilization loops. The single-source dual-comb MIXSEL, however, substantially reduces the complexity of existing systems to a single compact free-running laser with no requirement for an additional stabilization loop for most applications. In comparison to other competing new approaches such as quantum cascade lasers or micro resonator combs, the free-running dual-comb MIXSEL provides substantially more power per comb line with low linewidth and noise, and is ideally suited for a 1 to 5 GHz comb spacing, which is optimal for many molecular spectroscopy and lidar applications and previously demonstrated in the near-infrared regime [1-3].

There is a convenient spectral window between 2.0 and 2.6 µm which is free of strong water absorption lines, allowing for spectroscopic monitoring of atmospheric trace gases, remote sensing, laser ranging, and free-space communication. Initially we will focus our MIXSEL comb development for a center wavelength around 2 µm. The first proof-of-principle spectroscopy demonstration will focus on carbon dioxide CO2 which been recognized as the main anthropogenic contributor to climate change. Moreover, mid-IR sources of femtosecond pulses with high average power are especially useful to drive nonlinear processes, e.g. long-wave infrared supercontinuum generation, difference frequency generation, and optical parametric amplification. This for example enables efficient frequency conversion to the important molecular fingerprint spectral region.

[1] S. M. Link, D. J. H. C. Maas, D. Waldburger, U. Keller, “Dual-comb spectroscopy of water-vapor with a free-running semiconductor disk laser” Science, vol. 356, pp. 1164-1168, 2017
[2] J. Nürnberg, C. G. E. Alfieri, Z. Chen, D. Waldurger, N. Picque, U. Keller, “An unstabilized femtosecond semiconductor laser for dual-comb spectroscopy of acetylene” Optics Express, vol. 27, No. 3, pp. 3190-3199, 2019
[3] J. Nürnberg, B. Willenberg, C. R. Phillips, U. Keller, “Dual-comb ranging with frequency combs from single cavity free-running laser oscillators” Optics Express, vol. 29, No. 16, pp. 24910-24918, 2021

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Semiconductor saturable absorber mirrors (SESAMs) are widely used for modelocking of various ultrafast lasers. The growing interest for SESAM-modelocked lasers in the SWIR and mid-IR regime requires precise characterization of SESAM parameters. We have newly established two SESAM characterization setups for a wavelength range of 1.9 – 3 µm to precisely measure both nonlinear reflectivity and time-resolved recovery dynamics [1]. For the nonlinear reflectivity measurement, a high accuracy (<0.04%) over a wide fluence range (0.1 – 1500 µJ/cm2) has been achieved. Using the two setups, we have fully characterized different GaSb-SESAMs at an operation wavelength of around 2 µm and 2.4 µm fabricated in the FIRST lab at ETH Zurich [1,2].

We have developed our own first optically pumped InGaSb-based vertical external cavity surface emitting laser (InGaSb VECSEL) at a center wavelength of 2 µm [3]. The VECSEL is a first step towards a MIXSEL, for which the SESAM will be integrated into the same chip. To date intracavity heatspreaders were required for good average output power which however have many trade-offs especially for passive modelocking. With our new design we have demonstrated record high average cw output power from an optically pumped InGaSb VECSEL using no intracavity heatspreader. We obtained an average output power improvement from ≈11 mW to 810 mW without an intracavity heatspreader using a backside-cooled non-resonant VECSEL chip optimized for modelocking.

In addition we have introduced and demonstrated an optical characterization for a wavelength range of 1.9 to 3 µm to precisely measure wavelength-dependent gain saturation and spectral gain of VECSELs [3]. Gain characteristics are measured as a function of wavelength, fluence, pump power and temperature. The accuracy demonstrated in this case is <0.015% in gain saturation and <0.1% in spectral gain around 2–µm. This accuracy is maintained over more than 3 orders of magnitude. Such an accuracy is much harder to achieve in the long-wavelength regime.

For the first time we have developed a 2.4-µm type-I InGaSb/GaSb quantum well-based SESAM, demonstrating low non-saturable loss (0.8%) and ultrafast recovery time (1.9 ps). By incorporating this SESAM in a 250-MHz Cr:ZnS laser cavity, we demonstrate fundamental mode-locking at 2.37 µm with 0.8 W average power and 80-fs pulse duration. This corresponds to a peak power of 39 kW which is the highest so far for any saturable absorber mode-locked Cr:ZnS(e) oscillator [2]

[1] J. Heidrich, M. Gaulke, B. O. Alaydin, M. Golling, A. Barh, U. Keller, “Full optical SESAM characterization methods in the 1.9 to 3-µm wavelength regime” Optics Express, vol. 29, No. 5, pp. 6647-6656, 2021
[2] A. Barh, J. Heidrich, B. O. Alaydin, M. Gaulke, M. Golling, C. R. Phillips, U. Keller “Watt-level and sub-100-fs self-starting modelocking Cr:ZnS oscillator enabled by GaSb-SESAMs” Optics Express, vol. 29, No. 4, pp. 5934-5946, 2021
[3] M. Gaulke, J. Heidrich, B. O. Alaydin, M. Golling, A. Barh, U. Keller, “High average output power from a backside cooled 2-µm InGaSb VECSEL with full gain characterization”, submitted

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

First demonstration of dual-comb modelocking of optically pumped semiconductor lasers in the short-wave infrared (SWIR) and mid-infrared (mid-IR) regime with first application demonstrations.

Image of cw VECSEL structure

Data of cw VECSEL at 2 µm, power slope

Lab image without PhD student - with blue light

PhD student Marco Gaulke working in the lab

PhD student Marco Gaulke working in the lab

PhD student Marco Gaulke working in the lab

PhD student Marco Gaulke working in the lab - with blue light