Benjamin Willenberg

We demonstrate common repetition rate sweeping in a single-cavity dual-comb laser for improving the resolution of spectroscopy measurements. As a proof of concept, we show spectroscopy of low-pressure acetylene, achieving an effective resolution of 50 MHz within an acquisition time of 0.8 s and a normalized signal-to-noise ratio (SNR) of 20 dB H z while using an Yb:CaF ₂ laser operating at a 1-GHz repetition rate. The high repetition rate of the laser enables phase tracking and access to a broad aliasing-free optical bandwidth exceeding 4 THz. During interleaving measurements, the spectroscopy comb is actively controlled while the second comb passively tracks the first. This ensures long-term stability of radio-frequency (RF) comb line positions and maintains a one-to-one correspondence between RF and optical comb line numbers following an initial calibration against a reference database. Spectral interleaving is realized by simultaneously sweeping the repetition rate of both combs via piezoelectric cavity length modulation. This system allows for dual-comb interleaving through control of a single degree of freedom, or frequency-stabilized interleaving with only two degrees of freedom. This provides an effective compromise between resolution, simplicity, and acquisition speed, thereby showcasing a new approach for the implementation of single-cavity dual-comb spectroscopy.

We report, to the best of our knowledge, the first demonstration of dual-comb operation from a femtosecond optical parametric oscillator (OPO) at a 1-GHz pulse repetition rate. The singly-resonant OPO is fundamentally synchronously pumped by a high-power low-noise Yb:CaF₂ diode-pumped solid-state laser, enabling a compact system design, high parametric gain, and stable uniform pulse trains for both the signal and idler outputs. Dual-comb generation is realized in a spatially multiplexed single-cavity configuration for both the OPO and the pump laser. The center wavelength is tunable from 1415 nm to 1645 nm (signal) and 2960 nm to 4085 nm (idler) with average powers up to 650 mW and 200 mW per comb, respectively. The source delivers on average an instantaneous bandwidth of 2.5 THz, a power-per-combline up to 270 µW in the short-wave infrared and 75 µW in the mid-wave infrared is available. We demonstrate the potential of this source for fast dual-comb spectroscopy by detecting ambient methane (∼2 ppm) at 1645.5 nm over a 41-m path length, achieving a normalized spectral signal-to-noise ratio of 42.0 dB Hz^1/2. This measurement was performed without any active stabilization.

Hyperspectral LiDAR (HSL) enables the simultaneous acquisition of the surface geometry and spectral signatures of remote natural targets, making it valuable for various applications such as material probing, automated point cloud segmentation, and vegetation health monitoring. We present a first proof-of-concept study of a broadband dual-comb HSL system based on a 1 GHz dual-comb supercontinuum (SC). The SC spans from 820 to 1300 nm, generated via coherent spectral broadening of a free-running single-cavity dual-comb oscillator at 1053 nm in a single nonlinear photonic crystal fiber. The HSL system achieves a sub-µm ranging precision on a non-cooperative target at an update rate of 670 Hz. The shot-noise limited electronic dual-comb interferograms furthermore encode the spectral information of the target reflection across the SC bandwidth. This allows the capture of precise 3D point clouds with spectral signatures, unlocking new possibilities for spectrum-based material classification.

We demonstrate high-resolution gas spectroscopy of acetylene using a single-cavity, dual-comb Yb:CaF₂ laser operating at 1 GHz. Leveraging spectral interleaving, the system combines a fully controlled probing comb with a free-running spectrometer comb.

Amplifier-based pump-probe systems, while versatile, often suffer from complexity and low measurement speeds, especially when probing samples require low excitation fluences. To address these limitations, we introduce a pump-probe system that leverages a 60-MHz single-cavity dual-comb oscillator and an ultra-low noise supercontinuum. The setup can operate in equivalent time sampling or in programmable optical delay generation modes. We employ this system to study the wavelength-dependent excited-state dynamics of the non-fullerene electron acceptor Y6, a compound of interest in solar cell development, with excitation fluences as low as 1 nJ/cm², well below the onset of nonlinear exciton annihilation effects. Our measurements reach a shot-noise limited sensitivity in differential transmission of 3.4·10^–7. The results demonstrate the system’s potential to advance the field of ultrafast spectroscopy.

Vertical emitting optically pumped semiconductor laser technology in the GaSb material system, operating in the short-wave infrared (SWIR) regime, has made significant advancements recently. This paper reviews the key achievements leading to the first demonstration of a passively modelocked optically pumped thin-disk semiconductor laser, where both the saturable absorber and the gain quantum wells are integrated into a single semiconductor chip, known as the Modelocked Integrated eXternal-cavity Surface Emitting Laser (MIXSEL). This GaSb-based MIXSEL operates at a center wavelength of 2 μm, supporting both single and dual-comb operations, with an average output power of 30 to 50 mW, pulse repetition rates of approximately 4 GHz, and picosecond pulse durations. It enables initial proof-of-principle dual-comb spectroscopy measurements. For this, we optimized continuous wave (cw) Vertical External Cavity Surface Emitting Laser (VECSEL) operation at 2 μm without an intracavity heatspreader, enhanced group delay dispersion (GDD) compensation, and introduced an additional pump mirror integration. Compared to previous results, we achieved a significant performance increase with pump-DBR 2-μm VECSEL with an average output power of 6 W, an optical pump efficiency of 30% and a reduced thermal resistance of 1.9 K/W. Additionally, the better GDD compensation improved modelocking at 2 μm with a SESAM (Semiconductor Saturable Absorber Mirror), producing near-transform-limited femtosecond pulses with a duration of 331 fs, an average power of 30 mW at a pulse repetition rate of 2.77 GHz. Successful integration of the saturable absorber within the MIXSEL chip required matching of the cavity mode sizes on both the SESAM and the VECSEL chip. This paper details the optimization processes and resulting performance enhancements that mark a significant milestone in the development of GaSb-based thin disk laser technology.

Dual-comb spectroscopy (DCS) enables high-resolution measurements at high speeds without the trade-off between resolution and update rate inherent to mechanical delay scanning. However, high complexity and limited sensitivity remain significant challenges for DCS systems. We address these via a wavelength-tunable dual-comb optical parametric oscillator (OPO) combined with an up-conversion detection method. The OPO is tunable from 1300-1670 nm (signal) and 2700-5000 nm (idler). Spatial multiplexing in both the laser and OPO cavities creates a near-common path arrangement, enabling comb-line-resolved measurements in free-running operation. The narrow instantaneous bandwidth results in high power per comb-line up to 160 μW in the mid-infrared. Through intra-cavity up-conversion based on cross-comb spectroscopy, we leverage these power levels while overcoming the sensitivity limitations of direct mid-infrared detection. This approach yields a high signal-to-noise ratio (50.2 dB Hz^1/2) and high dual-comb figure of merit (3.5 × 10⁸ Hz^1/2). This scheme enabled detecting ambient methane over a 3-meter path length in millisecond time scale.

We investigate terahertz time-domain spectroscopy using a low-noise dual-frequency-comb laser based on a single spatially multiplexed laser cavity. The laser cavity includes a reflective biprism, which enables generation of a pair of modelocked output pulse trains with slightly different repetition rates and highly correlated noise characteristics. These two pulse trains are used to generate the THz waves and detect them by equivalent time sampling. The laser is based on Yb:CALGO, operates at a nominal repetition rate of 1.18 GHz, and produces 110 mW per comb with 77 fs pulses around 1057 nm. We perform THz measurements with Fe-doped photoconductive antennas, operating these devices with gigahertz 1 µm lasers for the first time, to our knowledge, and obtain THz signal currents approximately as strong as those from reference measurements at 1.55 µm and 80 MHz. We investigate the influence of the laser’s timing noise properties on THz measurements, showing that the laser’s timing jitter is quantitatively explained by power-dependent shifts in center wavelength. We demonstrate reduction in noise by simple stabilization of the pump power and show up to 20 dB suppression in noise by the combination of shared pumping and shared cavity architecture. The laser’s ultra-low-noise properties enable averaging of the THz waveform for repetition rate differences from 1 kHz to 22 kHz, resulting in a dynamic range of 55 dB when operating at 1 kHz and averaging for 2 s. We show that the obtained dynamic range is competitive and can be well explained by accounting for the measured optical delay range, integration time, as well as the measurement bandwidth dependence of the noise from transimpedance amplification. These results will help enable a new approach to high-resolution THz-TDS enabled by low-noise gigahertz dual-comb lasers.

Dual-comb microscopy enables high-speed and high-precision optical sampling by simultaneously extracting both amplitude and phase information from the interference signals with frequency division multiplexing. In this Letter, we introduce a spatiotemporal encoding approach for dual-comb microscopy that overcomes previous limitations such as mechanical scanning, low sampling efficiency, and system complexity. By employing free-space angular-chirp-enhanced delay (FACED) and a low-noise single-cavity dual-comb laser, we achieve scan-less 3D imaging with nanometer precision and a 3D distance-imaging rate of 330 Hz, restricted only by the repetition rate difference of the dual-comb laser. Specifically, the FACED unit linearly arranges the laser beam into an array. A grating subsequently disperses this array transversely into lines, facilitating ultrafast spectroscopic applications that are 1–2 orders of magnitude quicker than traditional dual-comb methods. This spatiotemporal encoding also eases the stringent conditions on various dual-comb laser parameters, such as repetition rates, coherence, and stability. Through carefully designed experiments, we demonstrate that our scan-less system can measure 3D profiles of microfabricated structures at a rate of 7 million pixels per second. Our method significantly enhances measurement speed while maintaining high precision, using a compact light source. This advancement has the potential for broad applications, including phase imaging, surface topography, distance ranging, and spectroscopy.

Dual-comb lasers are a new class of ultrafast lasers that enable fast, accurate and sensitive measurements without any mechanical delay lines. Here, we demonstrate a 2-µm laser called MIXSEL (Modelocked Integrated eXternal-cavity Surface Emitting Laser), based on an optically pumped passively modelocked semiconductor thin disk laser. Using III-V semiconductor molecular beam epitaxy, we achieve a center wavelength in the shortwave infrared (SWIR) range by integrating InGaSb quantum well gain and saturable absorber layers onto a highly reflective mirror. The cavity setup consists of a linear straight configuration with the semiconductor MIXSEL chip at one end and an output coupler a few centimeters away, resulting in an optical comb spacing between 1 and 10 GHz. This gigahertz pulse repetition rate is ideal for ambient pressure gas spectroscopy and dual-comb measurements without requiring additional stabilization. In single-comb operation, we generate 1.5-ps pulses with an average output power of 28 mW, a pulse repetition rate of 4 GHz at a center wavelength of 2.035 µm. For dual-comb operation, we spatially multiplex the cavity using an inverted bisprism operated in transmission, achieving an adjustable pulse repetition rate difference estimated up to 4.4 MHz. The resulting heterodyne beat reveals a low-noise down-converted microwave frequency comb, facilitating coherent averaging.