Arbitrary-detuning asynchronous optical sampling with amplified laser systems

Correlation between the presence of dislocations and vacancies in N-Type gallium arsenide : E. Fabre and C. Schiller. Solid State Commun.10 (1972), p. 81

A compact femtosecond dual-oscillator pump-probe setup with 48 MHz-repetition rate, relying on asynchronous optical sampling, is presented. The relative timing jitter between both lasers over the whole pump-probe delay range is of the order of or lower than 500 fs. We demonstrate that both a picosecond temporal resolution and a 48 MHz spectral resolution combined with the fast acquisition rate inherent to the asynchronous optical sampling allow performing broadband opto-acoustic imaging with a spectrum covering more than two decades from 300 MHz to 150 GHz. As an illustration, the opto-acoustic response of a supported thin film is investigated, revealing high frequency acoustic echoes close to the epicenter as well as low GHz surface acoustic waves propagating up to 40μm away from the epicenter. Semi-analytical calculations have been carried out and perfectly reproduce the dispersion of the surface acoustic waves experimentally observed.

We present the design and capabilities of a high-resolution, decade-spanning ASynchronous OPtical Sampling (ASOPS)-based TeraHertz Time-Domain Spectroscopy (THz-TDS) instrument. Our system employs dual mode-locked femtosecond Ti:Sapphire oscillators with repetition rates offset locked at 100 Hz via a Phase-Locked Loop (PLL) operating at the 60th harmonic of the ∼80 MHz oscillator repetition rates. The respective time delays of the individual laser pulses are scanned across a 12.5 ns window in a laboratory scan time of 10 ms, supporting a time delay resolution as fine as 15.6 fs. The repetition rate of the pump oscillator is synchronized to a Rb frequency standard via a PLL operating at the 12th harmonic of the oscillator repetition rate, achieving milliHertz (mHz) stability. We characterize the timing jitter of the system using an air-spaced etalon, an optical cross correlator, and the phase noise spectrum of the PLL. Spectroscopic applications of ASOPS-THz-TDS are demonstrated by measuring water vapor absorption lines from 0.55 to 3.35 THz and acetonitrile absorption lines from 0.13 to 1.39 THz in a short pathlength gas cell. With 70 min of data acquisition, a 50 dB signal-to-noise ratio is achieved. The achieved root-mean-square deviation is 14.6 MHz, with a mean deviation of 11.6 MHz, for the measured water line center frequencies as compared to the JPL molecular spectroscopy database. Further, with the same instrument and data acquisition hardware, we use the ability to control the repetition rate of the pump oscillator to enable THz frequency comb spectroscopy (THz-FCS). Here, a frequency comb with a tooth width of 5 MHz is generated and used to fully resolve the pure rotational spectrum of acetonitrile with Doppler-limited precision. The oscillator repetition rate stability achieved by our PLL lock circuits enables sub-MHz tooth width generation, if desired. This instrument provides unprecedented decade-spanning, tunable resolution, from 80 MHz down to sub-MHz, and heralds a new generation of gas-phase spectroscopic tools in the THz region.

Asynchronous optical sampling using two slightly detuned GHz-repetition rate femtosecond lasers is employed to spectrally characterize integrated THz resonators for biomolecular sensing. Temporal delay scanning at kilohertz frequencies paves the way for high-throughput analytics.

Ultra-high repetition rate (UHRR) mode-locked laser diodes (MLLD) have shown promising results for applications based on optical sampling such as asynchronous optical sampling (ASOPS), optical sampling by repetition-rate tuning (OSBERT), and optical ranging. Important metrics to consider are the repetition frequency (RF) and the RF linewidth. Here, we compare two monolithically integrated MLLDs. A quantum dot (QD) MLLD with an RF of approx. 50.1 GHz and a quantum well (QW) MLLD with an RF of approx. 51.4 GHz. The tunability of the RF is characterized by sweeping the lasers pump current, temperature, and saturable absorber (SA) reverse voltage. The QW MLLD has a tuning range of 31 MHz with an average RF linewidth of 53 kHz, while the QD MLLD has a smaller tuning range of 26 MHz with a higher average RF linewidth of 172 kHz.

We study the dynamics of acoustic phonons in ultra-thin free-standing silicon membranes both experimentally and theoretically. We discuss the impact of the lifetimes of the acoustic phonons on the thermal transport properties of the membranes with thicknesses ranging from 8 nm to 1.5 μm. The phonon lifetimes are determined by measuring the dynamic variation of the reflectivity using ultra-fast pump-probe spectroscopy. This is achieved by asynchronous optical sampling (ASOPS) of two actively coupled femto-second laser oscillators. The coherent acoustic phonon lifetime is obtained from the dynamical variations of the reflectivity with a sensitivity of 10-5 and a time resolution of about 50 fs. The experimental results are compared to theoretical calculations considering both intrinsic and extrinsic relaxation scattering processes.

An overview of recent advances in measurement modalities using dual femtosecond laser configurations such as asynchronous optical sampling and frequency comb spectrometry is provided. These enable time-resolved interactions of light with matter across different parts of the electromagnetic spectrum. An enhanced energy exchange and thus signal-to-noise-ratio between a sensing system and its local environment is possible through the use of resonant structures, metamaterials, plasmonics, dielectric microspheres and other slow-light structures. Finally, new ways of interpreting measurements of relevance to nanotechnology using state space network analysis, system identification and dielectrics theory are discussed using examples from atomic force microscopy, memristive systems and nanosquids (superconducting quantum interference devices).

In this contribution, we demonstrate a free-running bidirectional Er-doped fiber dual-comb source based on asymmetric nonlinear polarization rotation (NPR) mode-locking, which for the first time realizes the repetition frequency difference (Δ<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$f_\text{rep}$</tex-math></inline-formula>) at the KHz level in directional-multiplexed Er-doped fiber dual-comb sources with fully shared cavity to the best of our knowledge. The standard deviation <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sigma$</tex-math></inline-formula> of the Δ<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$f_\text{rep}$</tex-math></inline-formula> is only 0.7 Hz over 20 min in the absence of temperature control and vibration isolation. Further, the relative timing jitter of this dual-comb pulse is simulated and characterized by asynchronous optical sampling. A visual analysis of the period jitter using eye diagrams shows a period timing jitter of 1.91 fs for the dual combs in the quantum limit. Our work provides a bidirectional NPR single-cavity dual combs, which can be used as a laser source for applications such as spectroscopy, optical sensing, and absolute distance measurement.

Terahertz (THz) waves are valuable for evaluating non-metallic materials due to their strong penetrative capabilities. Conventional THz time-domain spectroscopy (THz-TDS) systems based on asynchronous optical sampling (ASOPS) use ultrashort pulse lasers however require a mechanical delay stage, resulting in a trade-off between spectral resolution and acquisition time. Dual-comb spectroscopy (DCS) eliminates the need for delay stage, enabling broadband, high-resolution, and rapid measurements [1], and has been applied to ASOPS THz-TDS systems [2]. However, conventional DCS systems are typically large and complex, as they require precise stabilization of two independent lasers.

Single-cavity, dual-comb lasers are those specially designed mode-locked lasers that can emit more than one, asynchronous ultrashort pulse trains with stable repetition frequency difference between them. Unlike the long-studied, widely-used femtosecond lasers generating one stable pulse train, systematic investigation on them and their potential dual-comb applications only began, based on the fiber laser platform, around a decade ago, despite sporadic and limited reports of similar lasing phenomena since the beginning of the mode-locked laser studies. From a historic perspective, the birth of this novel technology is the lucky outcome of the timely collision of perpetual search for novel pulsing laser dynamics and concerted pursuit of open-minded solutions for out-of-lab dual-comb systems in the 2010s. In this review article, first, the current schemes to implement single-cavity dual optical frequency comb fiber lasers and their applications are summarized, based on the concept of multiplexed mode-locked lasers. The characteristics of reported single-cavity, dual-comb fiber lasers are discussed as well as their applications in spectroscopy, ranging, Terahertz (THz) spectroscopy, and asynchronous optical sampling (ASOPS). Finally, the more recent development of single-cavity, multi-comb lasers is presented.

Absolute distance measurement based on the dual-comb system is a promising tool for the large-scale measurement in the industrial field. There are two combs with slightly different repetition rates in the dual-comb system, and the distance information could be obtained by the asynchronous optical sampling (ASOPS). However, the measured range could be limited within the pulse-to-pulse interval range by the non-ambiguity range (NAR) without the extension. This paper proposes a method to extend the unambiguous range by changing the repetition range. Firstly, the method of dual-comb ranging and the principle of extending the non-ambiguity range were introduced. Secondly, the two implementations of changing single repetition frequency and switching repetition frequency were compared through simulation. Then, according to the simulated results, the effective conditions and theoretical maximum range of extended ranging were clarified. Finally, a long absolute distance measurement was designed and the ranging results of switching repetition frequency were compared with the conventional incremental interferometer, which proves that this method is reliable for extending the non-ambiguous range.

A high time resolution in the picosecond range is required for the time-domain investigation of phonon dynamics in crystalline systems. Following a recently developed scheme in the visible spectrum, this resolution can be achieved by a method called asynchronous optical x-ray sampling (ASOXS). A pulsed femtosecond laser with high repetition rate is synchronized to the electron bunches in a storage ring. A slight frequency detuning changes the mutual delay continuously, resulting in a time-domain x-ray sampling of the laser-excited system. At the synchrotron radiation source ANKA a machine mode with low momentum compaction factor αc is available, which delivers ultra-short x-ray pulses in the picosecond range.

We investigate an optical system for the absolute distance measurement of objects with rough surfaces, using the principle of asynchronous optical sampling (ASOPS). A telescope system collects the scattered laser light that returns from the rough surface and directs it into a collimator with high precision. The contrastive absolute distance measurement experiments are carried out using a plane mirror and objects with rough surfaces. The residual error is kept within 10 μm within a 2-ms sampling time for the rough surfaces. Our results indicate that this ASOPS method is a promising candidate for metrological applications in some industrial sites.

Recent advances on Ytterbium ultrafast lasers have led to compact and versatile systems with high average output power, potentially high pulse energy and easily tunable repetition rate. Such lasers are well suited for time-resolved pump-probe experiments, a field which involves a wide range of techniques spanning from X-rays diffraction imaging and spectroscopy to asynchronous optical sampling (ASOPS) [1, 2]. In many cases, the laser has to be synchronized to a temporal reference to interact with other physical events. To facilitate the pump-probe experiment, one wants to keep as low as possible the residual timing jitter between the laser and the reference.

Coherent control of phonon modes by use of multipulse setups has been studied in various semiconductor systems ranging from bulk materials to multiple-quantum-well super­ lattice structures 1-6 as well as in other solid-state systems 7.8 and for molecular vibrationsY The control of first- and second-order back-folded acoustic modes in a superlattice has been demonstrated. lo In contrast to previous work, we present selective excitation of phonon modes within a single triplet of zone-folded modes where the frequency spacing is only a few tens of GHz. The evolution of the modes is followed throughout different control pulse delays to map out the continuous tuning of their amplitude ratios. The principle is based on the use of a second pump or control pulse for phonon generation with a defined phase with respect to the first excitation. The coherent superposition leads to a modification of the phonon amplitudes within the triplet structure of the first branch of zone-folded acoustic phonons. This results in an enhancement or suppression of the phonon oscillations. The manipulation and selective excitation of the phonon modes within a zone-folded triplet are possible by taking advantage of the high signal-to-noise ratio and the fast data acquisition rate of a high-speed asynchronous optical sampling (ASOPS) system. II The demonstrated principle could be extended to prepare well-defined phonon states for phonon spectroscopy and phonon amplification schemes 12 and to study phonon­ phonon interactions.