QCL Quantum Cascade Laser Enables Quantum Walk Combs
News about QCL The ETH Zurich research team created high-power frequency combs using a dual-waveguide quantum cascade laser (QCL), a notable advance in integrated photonics. An efficiency two orders of magnitude higher than before is achieved while extracting light from ring-shaped lasers. The researchers reached 120 mW output from a stable but “dim” laser state, making it appropriate for high-brightness spectroscopy and telecommunications.
The Ring Laser Power Barrier For years, scientists have sought to create compact, bright frequency comb sources in the terahertz and mid-IR regions. Frequency combs provide regularly spaced spectral lines for molecular sensing and high-precision metrology as "optical rulers." Since Fabry–Perot Quantum Cascade Lasers first demonstrated self-starting combs, ring QCLs have garnered attention for their ability to build stable “quantum walk combs.”
Quantum walk combs exploit quantum cascade lasers' fast gain recovery time. This speed suppresses amplitude variations and creates "liquid" qualities, allowing the difficult Ginzburg-Landau equation to effectively forecast light. However, the closed-loop cavity design that stabilizes these rings traps them. Because light is enclosed in the ring to reduce backscattering, outcoupling efficiency is notoriously low, limiting extracted power to hundreds of microwatts.
A Two-Waveguide Method The ETH Zurich team, led by Jerome Faist and Alessio Cargioli, solved this problem with a complicated structural redesign. Instead of using the ring, they homogeneously integrated a “racetrack” (RC) quantum cascade laser with a passive InGaAs waveguide to emit light.
Metal organic vapour phase epitaxy (MOVPE) generates this passive waveguide on the active area plane. With this “top-layer” architecture, the extraction structure can be built independently of the laser geometry. The passive U-shaped waveguide evanescently couples light to an extraction facet as it travels across the active racetrack.
This strategy maintains the ring's clean, stable quantum walk state and generates a dedicated channel for high-power light output. With passive waveguides instead of active bus sections, the researchers reduced electrical complexity and power dissipation, enabling better “lab-on-a-chip” systems.
Ballistic Expansion Watch The researchers proved their high-power gadget was a quantum walk comb using time-resolved spectrum measurements. They observed ballistic expansion, where radio-frequency (RF) modulation rapidly and linearly expands the laser's spectrum.
The experiments showed that the expansion stabilized in 700 nanoseconds and reached a bandwidth of 30 cm⁻¹, meeting theoretical conditions for quantum walk dynamics in synthetic space. The researchers saw “Bloch oscillations” when the modulation was slightly detuned from the cavity's roundtrip frequency, confirming that the light was following quantum walk principles.
Effect of Waveguide Width Experimental passive extraction waveguide widths were essential to the investigation. The researchers found that the size of this top layer directly affected the stability of the comb and light dispersion:
Narrow waveguides (~3 µm) were found to be more reliable for creating stable, adjustable combs. One “TM00” mode was provided, which enhanced mode matching with the active zone and smoothed power output. Wide waveguides (~9 µm) were introduced to reduce group velocity dispersion (GVD) and possibly enhance comb bandwidth, but they also increased complexity. Due to the wide guides supporting several modes, light leaked between modes like TM20 or TM00 depending on the current bias, generating power oscillations. The researchers achieved 120 mW peak output at 253 K heatsink temperature despite these oscillations in the larger designs. They also showed that the double waveguide laser can operate at room temperature, making it competitive with more complex active-outcoupler designs and perhaps more useful for commercial use.
Future uses and prospects The ability to monolithically integrate passive devices like these waveguides on a chip with a high-power laser opens up several mid-infrared spectral range potential. Mid-IR light is called the “fingerprint region” because many substances absorb differently at these wavelengths. News about QCL The ETH Zurich research team created high-power frequency combs using a dual-waveguide quantum cascade laser (QCL), a notable advance in integrated photonics. An efficiency two orders of magnitude higher than before is achieved while extracting light from ring-shaped lasers. The researchers reached 120 mW output from a stable but “dim” laser state, making it appropriate for high-brightness spectroscopy and telecommunications.
The Ring Laser Power Barrier For years, scientists have sought to create compact, bright frequency comb sources in the terahertz and mid-IR regions. Frequency combs provide regularly spaced spectral lines for molecular sensing and high-precision metrology as "optical rulers." Since Fabry–Perot Quantum Cascade Lasers first demonstrated self-starting combs, ring QCLs have garnered attention for their ability to build stable “quantum walk combs.”
Quantum walk combs exploit quantum cascade lasers' fast gain recovery time. This speed suppresses amplitude variations and creates "liquid" qualities, allowing the difficult Ginzburg-Landau equation to effectively forecast light. However, the closed-loop cavity design that stabilizes these rings traps them. Because light is enclosed in the ring to reduce backscattering, outcoupling efficiency is notoriously low, limiting extracted power to hundreds of microwatts.
A Two-Waveguide Method The ETH Zurich team, led by Jerome Faist and Alessio Cargioli, solved this problem with a complicated structural redesign. Instead of using the ring, they homogeneously integrated a “racetrack” (RC) quantum cascade laser with a passive InGaAs waveguide to emit light.
Metal organic vapour phase epitaxy (MOVPE) generates this passive waveguide on the active area plane. With this “top-layer” architecture, the extraction structure can be built independently of the laser geometry. The passive U-shaped waveguide evanescently couples light to an extraction facet as it travels across the active racetrack.
This strategy maintains the ring's clean, stable quantum walk state and generates a dedicated channel for high-power light output. With passive waveguides instead of active bus sections, the researchers reduced electrical complexity and power dissipation, enabling better “lab-on-a-chip” systems.
Ballistic Expansion Watch The researchers proved their high-power gadget was a quantum walk comb using time-resolved spectrum measurements. They observed ballistic expansion, where radio-frequency (RF) modulation rapidly and linearly expands the laser's spectrum.
The experiments showed that the expansion stabilized in 700 nanoseconds and reached a bandwidth of 30 cm⁻¹, meeting theoretical conditions for quantum walk dynamics in synthetic space. The researchers saw “Bloch oscillations” when the modulation was slightly detuned from the cavity's roundtrip frequency, confirming that the light was following quantum walk principles.
Effect of Waveguide Width Experimental passive extraction waveguide widths were essential to the investigation. The researchers found that the size of this top layer directly affected the stability of the comb and light dispersion:
Narrow waveguides (~3 µm) were found to be more reliable for creating stable, adjustable combs. One “TM00” mode was provided, which enhanced mode matching with the active zone and smoothed power output. Wide waveguides (~9 µm) were introduced to reduce group velocity dispersion (GVD) and possibly enhance comb bandwidth, but they also increased complexity. Due to the wide guides supporting several modes, light leaked between modes like TM20 or TM00 depending on the current bias, generating power oscillations. The researchers achieved 120 mW peak output at 253 K heatsink temperature despite these oscillations in the larger designs. They also showed that the double waveguide laser can operate at room temperature, making it competitive with more complex active-outcoupler designs and perhaps more useful for commercial use.
Future uses and prospects The ability to monolithically integrate passive devices like these waveguides on a chip with a high-power laser opens up several mid-infrared spectral range potential. Mid-IR light is called the “fingerprint region” because many substances absorb differently at these wavelengths.
The ETH Zurich team expects their high-power QWCs to have a “significant potential impact” on several sectors:
Environmental Monitoring: Brighter light detects air trace gasses better. Medical Diagnostics: Non-invasive frequency comb spectroscopy testing. Telecommunications and Ranging: Measure distance and send data swiftly using the high modulation bandwidth (which can exceed 10 GHz in comparable systems). The “back-reflection” of light from the extraction facet still hinders stability, however anti-reflection coatings can aid. Scaling this universal waveguide coupling method to higher wavelengths could enable a single integrated platform covering the 3-15 µm spectral range in the future. The ETH Zurich team expects their high-power QWCs to have a “significant potential impact” on several sectors:
Environmental Monitoring: Brighter light detects air trace gasses better. Medical Diagnostics: Non-invasive frequency comb spectroscopy testing. Telecommunications and RangingNews about QCL The ETH Zurich research team created high-power frequency combs using a dual-waveguide quantum cascade laser (QCL), a notable advance in integrated photonics. An efficiency two orders of magnitude higher than before is achieved while extracting light from ring-shaped lasers. The researchers reached 120 mW output from a stable but “dim” laser state, making it appropriate for high-brightness spectroscopy and telecommunications.
The Ring Laser Power Barrier For years, scientists have sought to create compact, bright frequency comb sources in the terahertz and mid-IR regions. Frequency combs provide regularly spaced spectral lines for molecular sensing and high-precision metrology as "optical rulers." Since Fabry–Perot Quantum Cascade Lasers first demonstrated self-starting combs, ring QCLs have garnered attention for their ability to build stable “quantum walk combs.”
Quantum walk combs exploit quantum cascade lasers' fast gain recovery time. This speed suppresses amplitude variations and creates "liquid" qualities, allowing the difficult Ginzburg-Landau equation to effectively forecast light. However, the closed-loop cavity design that stabilizes these rings traps them. Because light is enclosed in the ring to reduce backscattering, outcoupling efficiency is notoriously low, limiting extracted power to hundreds of microwatts.
A Two-Waveguide Method The ETH Zurich team, led by Jerome Faist and Alessio Cargioli, solved this problem with a complicated structural redesign. Instead of using the ring, they homogeneously integrated a “racetrack” (RC) quantum cascade laser with a passive InGaAs waveguide to emit light.
Metal organic vapour phase epitaxy (MOVPE) generates this passive waveguide on the active area plane. With this “top-layer” architecture, the extraction structure can be built independently of the laser geometry. The passive U-shaped waveguide evanescently couples light to an extraction facet as it travels across the active racetrack.
This strategy maintains the ring's clean, stable quantum walk state and generates a dedicated channel for high-power light output. With passive waveguides instead of active bus sections, the researchers reduced electrical complexity and power dissipation, enabling better “lab-on-a-chip” systems.
Ballistic Expansion Watch The researchers proved their high-power gadget was a quantum walk comb using time-resolved spectrum measurements. They observed ballistic expansion, where radio-frequency (RF) modulation rapidly and linearly expands the laser's spectrum.
The experiments showed that the expansion stabilized in 700 nanoseconds and reached a bandwidth of 30 cm⁻¹, meeting theoretical conditions for quantum walk dynamics in synthetic space. The researchers saw “Bloch oscillations” when the modulation was slightly detuned from the cavity's roundtrip frequency, confirming that the light was following quantum walk principles.
Effect of Waveguide Width Experimental passive extraction waveguide widths were essential to the investigation. The researchers found that the size of this top layer directly affected the stability of the comb and light dispersion:
Narrow waveguides (~3 µm) were found to be more reliable for creating stable, adjustable combs. One “TM00” mode was provided, which enhanced mode matching with the active zone and smoothed power output. Wide waveguides (~9 µm) were introduced to reduce group velocity dispersion (GVD) and possibly enhance comb bandwidth, but they also increased complexity. Due to the wide guides supporting several modes, light leaked between modes like TM20 or TM00 depending on the current bias, generating power oscillations. The researchers achieved 120 mW peak output at 253 K heatsink temperature despite these oscillations in the larger designs. They also showed that the double waveguide laser can operate at room temperature, making it competitive with more complex active-outcoupler designs and perhaps more useful for commercial use.
Future uses and prospects The ability to monolithically integrate passive devices like these waveguides on a chip with a high-power laser opens up several mid-infrared spectral range potential. Mid-IR light is called the “fingerprint region” because many substances absorb differently at these wavelengths.
The ETH Zurich team expects their high-power QWCs to have a “significant potential impact” on several sectors:
Environmental Monitoring: Brighter light detects air trace gasses better. Medical Diagnostics: Non-invasive frequency comb spectroscopy testing. Telecommunications and Ranging: Measure distance and send data swiftly using the high modulation bandwidth (which can exceed 10 GHz in comparable systems). The “back-reflection” of light from the extraction facet still hinders stability, however anti-reflection coatings can aid. Scaling this universal waveguide coupling method to higher wavelengths could enable a single integrated platform covering the 3-15 µm spectral range in the future.: Measure distance and send data swiftly using the high modulation bandwidth (which can exceed 10 GHz in comparable systems). The “back-reflection” of light from the extraction facet still hinders stability, however anti-reflection coatings can aid. Scaling this universal waveguide coupling method to higher wavelengths could enable a single integrated platform covering the 3-15 µm spectral range in the future.
