Justinas Pupeikis

Ultrafast solid-state glass lasers are frequently mode-locked using a semiconductor saturable absorber mirror (SESAM), which can provide reliable laser self-start and stable mode-locked operation. However, state-of-the-art GaAs-based SESAMs for the 1.55 µm wavelength range require highly strained InGaAs quantum well absorbers with a lattice-mismatch of ≥ 2% relative to the GaAs substrate, which leads to defect formation, reduced damage threshold, and limited design freedom. Here, we present the first fully strain-free SESAMs for solid-state glass lasers at 1.55 µm wavelength, which overcome these constraints. Our devices are grown on InP with a very low residual lattice-mismatch of < 0.1% and incorporate an iron-doped InGaAs bulk absorber. This enables continuous tunability of the SESAM modulation depth and precise control over the ultrafast SESAM recovery time – an unprecedented degree of design freedom. Furthermore, the SESAM structure combines an anti-resonant design with a highly reflective InAlAs/InGaAlAs bottom DBR and a TiO ₂ /SiO ₂ top DBR, which results in record-low non-saturable losses for InP-based SESAMs of 0.6% for a modulation depth of 0.6%. With this approach, we demonstrate the first stable continuous-wave mode-locked operation of a solid-state Er,Yb:glass laser using an InP-based SESAM. We achieve 224 fs pulse duration at a maximum output power of 102.5 mW and a 79.1 MHz repetition rate, in combination with excellent noise properties of 0.005% integrated RIN over [100 Hz, 1 MHz] and 3 fs integrated timing jitter over [1 kHz, 1 MHz].

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 a spatially multiplexed dual-comb mode-locked Er:Yb:glass solid-state oscillator at 500 MHz. The laser outputs two coherent pulse trains (~40 mW, 230 fs), enabling high- resolution dual-comb spectroscopy without any stabilization.

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.

Amplifier-based pump-probe systems, while versatile, often suffer from complexity and low measurement speeds, especially when probing samples requiring 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 3.4e–7. The results demonstrate the system's potential to advance the field of ultrafast spectroscopy.

Amplifier-based pump-probe systems, while versatile, often suffer from complexity and low measurement speeds, especially when probing samples requiring 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 3.4e–7. The results demonstrate the system's potential to advance the field of ultrafast spectroscopy.

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.