QCL Quantum Cascade Laser news
The ETH Zurich research team has developed a novel technique for creating high-power frequency combs using a dual-waveguide quantum cascade laser (QCL), which is a major and important step for the field of integrated photonics. It describes a method for extracting light from ring-shaped lasers with an efficiency that is two orders of magnitude higher than previously achievable. The researchers have brought a very stable but traditionally “dim” laser state into the realm of useful, high-brightness applications for spectroscopy and telecommunications by reaching output levels of up to 120 mW.
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The Power Barrier in Ring Lasers
The goal of creating compact, brilliant frequency comb sources in the terahertz and mid-infrared (mid-IR) bands has been studied by scientists for many years. As “optical rulers,” frequency combs produce uniformly spaced spectral lines that are crucial for molecular sensing and high-precision metrology. Although self-starting combs were initially demonstrated by Fabry–Perot Quantum Cascade Lasers, ring QCLs have recently drawn attention because of their capacity to generate remarkably stable “quantum walk combs.”
The extremely quick gain recovery time present in quantum cascade lasers is used by these quantum walk combs (QWCs). By suppressing amplitude fluctuations and creating characteristics that scientists refer to as “liquid,” this speed enables the complicated Ginzburg-Landau equation to accurately predict the light. But the closed-loop cavity design that gives these rings their stability also serves as a trap. Outcoupling efficiency has been infamously low, frequently limiting total extracted power to just hundreds of microwatts because the light is contained within the ring to minimize backscattering.
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A Two-Waveguide Approach
The ETH Zurich team, under the direction of Jerome Faist and Alessio Cargioli, used a complex structural redesign to overcome this issue. They homogeneously combined a “racetrack” (RC) quantum cascade laser with a passive InGaAs waveguide to emit light instead of depending on the ring itself.
Importantly, metal organic vapour phase epitaxy (MOVPE) is used to regrow this passive waveguide on top of the active area plane. The extraction structure can be completely constructed independently of the laser’s geometry with this “top-layer” architecture. The light is directed to an extraction facet by the passive U-shaped waveguide after evanescently coupling with it as it moves through the active racetrack.
This method has two benefits: it creates a dedicated channel for high-power light output while maintaining the ring’s clean, stable quantum walk state. Additionally, the researchers greatly decreased electrical complexity and power dissipation by employing passive waveguides instead of active bus sections, opening the door for more effective “lab-on-a-chip” systems.
Ballistic Expansion Observation
The researchers used time-resolved spectrum measurements to demonstrate that their high-power device functioned as a real quantum walk comb. They noticed a phenomenon called ballistic expansion, in which the application of radio-frequency (RF) modulation causes the laser’s spectrum to expand quickly and linearly.
In their experiments, this expansion stabilized in about 700 nanoseconds and reached a bandwidth of 30 cm⁻¹, which is consistent with the theoretical requirements for quantum walk dynamics in synthetic space. The researchers further confirmed that the light was acting in accordance with the predicted laws of quantum walks by observing “Bloch oscillations” when the modulation was slightly detuned from the roundtrip frequency of the cavity.
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Waveguide Width’s Effect
Testing various passive extraction waveguide widths was a crucial component of the research. The researchers discovered that the stability of the resulting comb and the dispersion of the light were directly influenced by the size of this top layer:
- It was discovered that narrow waveguides (~3 µm) were more dependable for producing a stable, adjustable comb. Only one “TM00” mode was supported, which improved mode matching with the active region and produced a smooth power output.
- Wide Waveguides (~9 µm): These added complexity even though they were intended to lower group velocity dispersion (GVD) and possibly increase the comb’s bandwidth. Depending on the current bias, the light leaked between various modes (such TM20 or TM00) because to the wide guides supporting numerous modes, causing power oscillations.
At a heatsink temperature of 253 K, the researchers managed to attain a peak output of 120 mW in spite of these oscillations in the larger designs. They also demonstrated that the double waveguide laser can function at room temperature, which makes it competitive with more intricate active-outcoupler designs—possibly more significant for commercial usage.
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Uses and Prospects for the Future
Numerous opportunities for the mid-infrared spectral range are made possible by the ability to monolithically integrate arbitrary passive elements, such as these waveguides, on the same chip as a high-power laser. Because many compounds exhibit distinctive absorption patterns at these wavelengths, mid-IR light is frequently referred to as the “fingerprint region.”
Their high-power QWCs will have a “significant potential impact” on a number of sectors, according to the ETH Zurich team:
- Environmental Monitoring: The enhanced brightness allows for more sensitive detection of atmospheric trace gases.
- Biomedical Diagnostics: Non-invasive medical testing using frequency comb spectroscopy.
- Telecommunications and Ranging: Making use of the high modulation bandwidth (which, in comparable systems, can surpass 10 GHz) to measure distance and transport data quickly.
The stability is still hampered by the “back-reflection” of light from the extraction facet, but anti-reflection coatings can help. Future plans call for scaling this universal waveguide coupling technology to even higher wavelengths, which could allow for a single integrated platform that covers the whole 3–15 µm spectral range.
