Asynchronous Optical Sampling Research Articles

Tunable compact asynchronous optical sampling system using Er-doped fiber laser

Abstract We report a compact, tunable, self-starting, all-fiber laser-based asynchronous optical sampling (ASOPS) system. Two Er-doped fiber oscillators were used as the pulsed-laser source, whose repetition rate could be set at 100 MHz with a tuning range of 1.25 MHz through a fiber delay line. By employing phase-locked and temperature control loops, the repetition rate offset of the two lasers was stabilized with 7.13 × 10−11 fractional instability at an average time of 1 s. Its capabilities in the terahertz regime were demonstrated by terahertz time-domain spectroscopy, achieving a spectral bandwidth of 3 THz with a dynamic range of 30 dB. The large range of repetition rate adjustment in our ASOPS system has the potential to be a powerful tool in the terahertz regime.

Terahertz PHASR Scanner with 2 kHz, 100 picosecond Time-Domain Trace Acquisition Rate and an Extended Field-of-View Based on a Heliostat Design.

Recently, we introduced a Portable HAndheld Spectral Reflection (PHASR) Scanner to allow THz Time-Domain Spectroscopic (THz-TDS) imaging in clinical and industrial settings using a fiber-coupled and alignment-free telecentric beam steering design. The key limitations of the version 1.0 of the PHASR Scanner were its field-of-view and speed of time-domain trace acquisition. In this paper, we address these limitations by introducing a heliostat geometry for beam scanning to achieve an extended field-of-view, and by reconfiguring the Asynchronous OPtical Sampling (ASOPS) system to perform Electronically Controlled OPtical Sampling (ECOPS) measurements. The former change improved the deflection range of the beam, while also drastically reducing the coupling of the two scanning axes, the combination of which resulted in a larger than four-fold increase in the FOV area. The latter change significantly improves the acquisition speed and frequency domain performance simultaneously by improving measurement efficiency. To accomplish this, we characterized the non-linear time-axis sampling behavior of the electro-mechanical system in the ECOPS mode. We proposed methods to model and correct the non-linear time-axis distortions and tested the performance of the high-speed ECOPS trace acquisition. Therefore, here we introduce the PHASR Scanner version 2.0, which is capable of imaging a 40×27 mm2 FOV with 2000 traces per second over a 100 picosecond TDS range. This new scanner represents a significant leap towards translating the THz-TDS technology from the lab bench to the bedside for real-time clinical imaging applications.

High-resolution time-resolved spectroscopy based on hybrid asynchronous optical sampling

The capability of characterizing arbitrary and non-repetitive emission spectra with a high resolution in real-time is of great merit in various research fields. Optical frequency combs provide precise and stable frequency grids for the measurement of a single spectral line with high accuracy. Particularly, dual-comb spectroscopy enables spectral measurement with a large bandwidth spanning tens of nanometers, but it is limited to measuring absorption spectra and has to trade-off spectral resolution vs the acquisition frame rate set by the repetition rate. Here, to alleviate these restrictions, we propose and demonstrate time-resolved spectroscopy for an emission spectrum based on hybrid asynchronous optical sampling, which features a spectral resolution of 0.63 pm, a frame rate of 1 MHz, and a measurement bandwidth of 13.6 nm, simultaneously. A mode-locked fiber comb with a repetition frequency of f1 is harnessed to interrogate emission spectral features with high resolution via optical Fourier transform achieved using a time-lens. Subsequently, a soliton microcomb of repetition frequency f2s ≈ 1000f1 serving as a probe pulse implements hybrid asynchronous optical sampling, thus significantly increasing the acquisition rate by nearly 3 orders of magnitude. As a proof-of-concept demonstration, the frequency trajectory of a rapidly scanning laser with a linear chirp of 6.2 THz/s is tracked. We believe that chip-scale microcombs will make the fast and high-resolution emission spectroscopy presented here a powerful tool for widespread applications.

Integrated system of traditional THz time-domain spectroscopy and asynchronous optical sampling

Terahertz time-domain spectroscopy (THz-TDS) system, as a new means of spectral analysis and detection, plays an increasingly pivotal role in basic scientific research. However, owing to the long scanning time of the traditional THz-TDS system and the complex control of the asynchronous optical scanning (ASOPS) system, which requires frequent calibration, we combine traditional THz-TDS and ASOPS systems to form a composite system and propose an all-fiber trigger signal generation method based on the time overlapping interference signal generated by the collinear motion of two laser pulses. Finally, the time-domain and frequency-domain spectra are obtained by using two independent systems in the integrated systems. It is found that the full width at half maximum (FWHM) of the time-domain spectra and the spectral width of the frequency-domain spectra are almost the same, but the sampling speed of the ASOPS system is significantly faster than that of the traditional THz-TDS system, which conduces to the study of the transient characteristics of substances.

Repetition frequency tunability and stability of BH InAs/InP QD and InGaAsP/InP QW two-section mode-locked laser diodes.

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.

Impact of Laser Intensity Noise on Dual-Comb Absolute Ranging Precision.

Noise in mode-locked lasers has been a central issue for dual-comb metrological applications. In this work, we investigate the laser intensity noise on dual-comb absolute ranging precision. Two different dual-comb schemes based on linear optical sampling (LOS) and nonlinear asynchronous optical sampling (ASOPS) have been constructed. In the LOS scheme, the ranging precision deteriorates with the increase in laser relative intensity noise (RIN). This effect can be corrected by implementing a balanced photo-detection (BPD). In the ASOPS scheme, the experiment shows that the conversion from laser RIN to dual-comb ranging precision is negligible, making a balanced detection unnecessary for ranging precision improvement. The different manners of RIN’s impact on absolute ranging precision are attributed to the distinct cross-correlation signal patterns and the underlying time-of-flight (TOF) extraction algorithms.

LT-GaAs Thin film antenna based on the asynchronous optical sampling system

Using a photoconductive antenna to generate terahertz (THz) waves is a common method, and the material of the antenna is usually low-temperature gallium arsenide (LT-GaAs). A LT-GaAs epitaxial wafer with four layers structure is prepared in this paper, and then the LT-GaAs thin film is obtained by etching the aluminium arsenide (AlAs) with dilute hydrochloric acid and transferring it to the silicon wafer to make a thin film photoconductive antenna. The performance of the antenna is evaluated using an asynchronous optical sampling (AOS) terahertz time-domain spectroscopy (THz-TDS) system with variable repetition frequency. The system uses the spectral coherence method of two lasers to obtain trigger signals and generates THz signals with high signal-to-noise ratio. This also confirms that the system has the advantages of high spectral resolution and fast detection speed. It has incomparable advantages over the traditional THz-TDS system in the environment requiring rapid measurement.

Fast and high-resolution spectroscopy based on asynchronous optical sampling.

Dispersive time stretch has made many ultrafast applications possible owing to its high frame rate, as compared to conventional spectroscopies. By further introducing a converging time lens, this spectroscopy can resolve arbitrary emission spectra within the aperture. However, a spectral resolution of tens of picometers hinders its high-precision application. There are two limitations: the temporal aperture of the acquired signal and the actual acquisition bandwidth. To overcome these restrictions, two approaches were developed. First, a large-aperture time lens, with higher-order dispersion compensation, is used to overcome the fundamental limit of the time-bandwidth product. Second, asynchronous optical sampling, based on two frequency combs, overcomes the technical limit of the acquisition bandwidth. As a result, in this study, time-stretch spectroscopy achieved a 1-pm spectral resolution, 24-nm observation bandwidth, and 1-kHz frame rate. Moreover, it was used to observe some spectral dynamics of the random lasing process and devices with narrow spectral widths. This scheme provides essential improvement for time-stretch spectroscopy to achieve high precision.

Variable repetition frequency asynchronous optical sampling method without a feedback loop

The ultrafast pump–probe measurement represents a key technique to study fs–ps dynamics. The asynchronous optical sampling (ASOPS) method realizes fast and long time-range measurement with high time resolution using different repetition frequency pump–probe light pulses. The frequency difference Δf is an important parameter, as it dictates the measurement time and time resolution. However, usual ASOPS measurements require a complex and precise stabilizer to control Δf or it is difficult to change Δf. In this study, we use two free-running titanium/sapphire pulse lasers to develop a variable repetition frequency ASOPS (VRF-ASOPS) method without a stabilizer or feedback loop, where we can easily alter Δf by changing the cavity length of the probe light laser. To detect the coincidences of the pump–probe light pulses, we cause the instantaneous reflectivity change in a 100 nm platinum film by irradiating the pump light and observe it by the probe light. We use this signal as the trigger signal to directly determine Δf, which enables us to average and convert the measured responses without a stabilizer or feedback loop. Using this VRF-ASOPS system, we obtain pulse echo signals and 100 GHz Brillouin oscillations, which are equivalent to those measured by the mechanical delay line method, confirming the validity of our developed method.

Supervised machine learning for automatic classification of in vivo scald and contact\xa0burn injuries using the terahertz Portable Handheld Spectral Reflection (PHASR) Scanner

We present an automatic classification strategy for early and accurate assessment of burn injuries using terahertz (THz) time-domain spectroscopic imaging. Burn injuries of different severity grades, representing superficial partial-thickness (SPT), deep partial-thickness (DPT), and full-thickness (FT) wounds, were created by a standardized porcine scald model. THz spectroscopic imaging was performed using our new fiber-coupled Portable HAndheld Spectral Reflection Scanner, incorporating a telecentric beam steering configuration and an f-theta scanning lens. ASynchronous Optical Sampling in a dual-fiber-laser THz spectrometer with 100 MHz repetition rate enabled high-speed spectroscopic measurements. Given twenty-four different samples composed of ten scald and ten contact burns and four healthy samples, supervised machine learning algorithms using THz-TDS spectra achieved areas under the receiver operating characteristic curves of 0.88, 0.93, and 0.93 when differentiating between SPT, DPT, and FT burns, respectively, as determined by independent histological assessments. These results show the potential utility of our new broadband THz PHASR Scanner for early and accurate triage of burn injuries.

Femtosecond multidimensional spectroscopy with multiple repetition-frequency-stabilized lasers: tutorial

Time-resolved multidimensional spectroscopy is an advanced spectroscopic technique that can be used to study the molecular structure and dynamics of chromophores in condensed phases by probing multiple resonances of chemical and biophysical systems. To achieve such a multidimensional measurement, the nonlinear optical response of materials should be measured for varying time delays between multiple optical pulses. Inevitably, the data acquisition time dramatically increases with the number of dimensions and the length of each time-delay scan. Therefore, technical breakthroughs toward improved data acquisition rates and time resolutions have long been sought for more versatile and extensive use of coherent multidimensional spectroscopy. Here, we present a tutorial description of the concepts and methods of coherent multidimensional spectroscopy with multiple repetition-frequency-stabilized lasers. Pulse trains from two lasers with slightly different repetition frequencies enable an asynchronous optical sampling (ASOPS), i.e., an automatic scan of the time delay between the pulses. By combining mechanical delay lines for interferometric pulse-pair generation and signal field detection with an incoherent ASOPS scheme for probing slow population dynamics of chromophores in condensed phases, we show that the dynamic range of time-delay scan and data acquisition speed for coherent multidimensional spectroscopy could be dramatically enhanced. In this tutorial paper, we summarize the current developments in this and related research areas, and provide perspectives on all optically controlled multidimensional spectroscopy with multiple synchronized lasers.

High speed time-of-flight displacement measurement based on dual-comb electronically controlled optical sampling.

We demonstrate a direct time-of-flight approach that utilizes dual-comb electronically controlled optical sampling (ECOPS) to measure small displacements. ECOPS is enabled by electrically controlling the repetition rate of one laser via an intracavity electric-optical modulator (EOM). The acquisition rate is set by the EOM modulation frequency, which is much higher than commonly used asynchronous optical sampling (ASOPS). In a proof-of-principle experiment, an 80-kHz acquisition rate is obtained with a pair of ∼105 MHz repetition rate Er-fiber lasers. At an average time of 30 ms, a measurement precision evaluated with Allan deviation reaches 26.1 nm for a 40-µm static displacement. In a dynamic measurement, a 500-Hz sinusoidal vibration with 15 µm amplitude has also been identified. The high-precision and high-speed displacement measurement technique can be potentially used in 3D surface profilometry of microelectronic step-structures and real-time monitoring of high frequency mechanical vibrations, etc.

Delay-Spectral Focusing Dual-Comb Coherent Raman Spectroscopy for Rapid Detection in the High-Wavenumber Region

Rapid multiplex coherent anti-Stokes Raman scattering (CARS) spectroscopy in the high-wavenumber (HW) region shows great advantages in real-time dynamic process visualizations, clinical diagnosis, abundant microplastic assessment, etc. Fourier transform CARS (FT-CARS) improves the acquisition speed of multiplex CARS to the order of tens of kHz, yet typical approaches to utilize the intrinsic coherence of ultrabroadband pulses notably impede the attainable Raman vibrations in the HW region. Here, a novel delay-spectral focusing dual-comb (DC) CARS scheme is proposed for rapid HW Raman detection based on two fiber combs with 100 MHz repetition rates. By combining the particular advantages of DC asynchronous optical sampling and spectral focusing instantaneous single molecular vibration excitation, the Raman spectrum directly maps to DC relative delay, releasing the coherence constraint of excitation sources. Thus, an Er comb and a Yb comb, as pump and Stokes excitation pulses, respectively, flexibly match the Raman vibration in the HW region. Further, a rapid delay focusing method with intracavity electro-optic modulation is applied to actively control the DC relative delay scanning in the Raman shift region of interest. With these efforts, the spectral acquisition rate is improved more than 1000-fold up to 40 000 spectra/s, while keeping spectral resolution (∼10 cm–1) and the signal-to-noise ratio (∼260) stable along with active acquisition rate tuning. Combined with a microfluidic device, high-speed measurement in the HW region of 15 μm microbeads in flow is demonstrated by the system, reaching a high throughput of around 2150 events/s and more than 99% classification consistency. The prospect of rapid acquiring multiplex CARS spectra without the sacrifice of spectral resolution or the need for the coherence of the pulse sources offers huge potential in rapid monitoring circumstances, such as flow cytometry and microspectroscopic imaging in biomedicine.

Time-resolved spectroscopy of thioflavin T solutions: Asynchronous optical sampling method with two frequency-upconverted mode-locked lasers

We carried out transient absorption spectroscopy of thioflavin T (ThT) molecules in various solvents employing an asynchronous optical sampling (ASOPS) scheme with dual synchronized and frequency up-converted mode-lock lasers in the near UV (NUV) spectral region. We developed a pair of synchronized femtosecond lasers with tunable center wavelengths ranging from 380 to 430nm and spectral bandwidths of 30nm. As a proof-of-principle experiment, we measured interferometrically detected time and frequency-resolved pump-probe signals of ThT in various solvents to study the twisted intramolecular charge transfer process of photo-excited ThT molecules. Both single-color NUV-NUV and two-color NUV-near IR (NIR) pump-probe measurements reveal that the vibronic coupling strengths of two vibrational modes with frequencies of 214 and 526cm-1 in the excited state of ThT are reduced when ThT is dissolved in a chlorine-containing solvent, e.g., chloroform. We confirm theoretically that these vibrational modes have relatively high electric dipole moments in the excited state. As a result, the intramolecular charge transfer process of ThT in chloroform, which is driven by the solvation process of surrounding polar solvent molecules, could occur less efficiently, which results in an increase in the fluorescence quantum yield. Here, we demonstrate that the NUV-NUV and NUV-NIR ASOPS-transient absorption could be useful techniques for studying ultrafast photochemical reactions in condensed phases.

Modelling two-laser asynchronous optical sampling using a single 2-section semiconductor mode-locked laser diode.

We present a theoretical overview and a proposed methodology which demonstrates SLASOPS (single laser asynchronous optical sampling) as a single-laser alternative to the conventional two-laser ASOPS technique. We propose the optical and electronic setup in which SLASOPS may be achieved experimentally with a single 2-section mode-locked laser diode as the pulsed-laser source and simulate how asynchronous optical sampling is generated and detected theoretically. We highlight the technique's ability to provide customizable scan ranges, scan rates and scan resolutions through variation of the imbalance in the interferometer arms and by tuning the repetition rate of the pulsed-laser source, which we present as optical cross-correlations between pulse pairs. We incorporate jitter into the system mathematically to assess the limitations on resolving both intensity and interferometric cross-correlation traces and to investigate the effects of averaging such traces in real-time. Analysis is then carried out on cross-correlation trace amplitude, width, and temporal positioning in order to discuss the technique's ability for deployment in typical optical sampling applications. In particular we note SLASOPS' ability to conduct asynchronous optical sampling using only a single laser, halving both the expense and technical requirements, doing so at megahertz scan rates, and within a spatial precision of just a few microns.

Experimental realization and characterization of a two–color dual–comb system for practical large–scale absolute distance measurements

In this paper, a two–color dual–comb system has been realized and characterized for the specific applications of the practical large–scale absolute distance measurements, especially for the investigations of asynchronous optical sampling (ASOPS) with a slight difference of repetition rates. The fundamental 1550 nm wavelength is emitting from the Er–fiber femtosecond frequency combs with a repetition rate of around 200 MHz and carrier-envelope offset frequency of 20 MHz, and the other wavelength of 775 nm is generated by the second harmonic generation crystals. The optical characteristics are investigated in detail, and the stability of frequency parameters are also evaluated for long–term time. The preliminary experimental results of ASOPS show that this two-color dual-comb system has the potential to be applied for further diversified research.

Coherent Nonlinear Spectroscopy with Multiple Mode-Locked Lasers.

Coherent multidimensional spectroscopy has been widely used to study the structure and dynamics of chemical and biological systems. Each ultrashort pulse from a single mode-locked laser is split into multiple pulses by beam splitters. Their arrival times at a given molecular sample are controlled with mechanical time-delay generators for time-resolved measurements of molecular responses. Such nonlinear vibrational, electronic, or vibrational-electronic spectroscopy can now be carried out with multiple mode-locked lasers with highly stabilized repetition and sometimes carrier-envelope-offset frequencies. By precisely controlling the repetition frequencies of multiple mode-locked lasers, one can achieve automatic delay time scanning, known as asynchronous optical sampling, to investigate various relaxation processes associated with photochemical or photobiological phenomena at one sweep in time. In this Perspective, the current developments and applications of multiple mode-locked laser-based techniques to time-resolved nonlinear spectroscopy of chromophores in condensed phases are discussed. The author's perspective on this approach is also presented.

Lifting restrictions on coherence loss when characterizing non-transparent hypersonic phononic crystals

Hypersonic phononic bandgap structures confine acoustic vibrations whose wavelength is commensurate with that of light, and have been studied using either time- or frequency-domain optical spectroscopy. Pulsed pump-probe lasers are the preferred instruments for characterizing periodic multilayer stacks from common vacuum deposition techniques, but the detection mechanism requires the injected sound wave to maintain coherence during propagation. Beyond acoustic Bragg mirrors, frequency-domain studies using a tandem Fabry–Perot interferometer (TFPI) find dispersions of two- and three-dimensional phononic crystals (PnCs) even for highly disordered samples, but with the caveat that PnCs must be transparent. Here, we demonstrate a hybrid technique for overcoming the limitations that time- and frequency-domain approaches exhibit separately. Accordingly, we inject coherent phonons into a non-transparent PnC using a pulsed laser and acquire the acoustic transmission spectrum on a TFPI, where pumped appear alongside spontaneously excited (i.e. incoherent) phonons. Choosing a metallic Bragg mirror for illustration, we determine the bandgap and compare with conventional time-domain spectroscopy, finding resolution of the hybrid approach to match that of a state-of-the-art asynchronous optical sampling setup. Thus, the hybrid pump–probe technique retains key performance features of the established one and going forward will likely be preferred for disordered samples.

Widely stretchable soliton crystals in a passively mode-locked fiber laser.

We present the first direct demonstration of a new type of stable and extremely elastic soliton crystals, the bond length and bond strength of which can be individually controlled in a wide range. The stretching and compressing can be repeated many times, conserving the overall structure by incorporating a highly asymmetric tunable Mach-Zehnder interferometer into a specially designed passively mode-locked fiber laser. The temporal structure and dynamics of the generated soliton crystals were measured using an asynchronous optical sampling system with picosecond resolution. We demonstrated that a stable and robust soliton crystal can be formed by two types of primitive structures: single dissipative solitons and (or) pairs each consisting of a dissipative soliton and a pulse with a lower amplitude. Continuous stretching and compression of the soliton crystal by an extraordinarily high factor of more than 30 has been demonstrated, the smallest recorded separation between the pulses being as low as 5 ps, corresponding to an effective repetition frequency of 200 GHz. Collective pulse dynamics, including soliton crystal cracking and transformation of crystals comprising high/low-amplitude pulse pairs to the crystals of similar pulses, has been observed experimentally.

Discrimination of nano-objects via cluster analysis techniques applied to time-resolved thermo-acoustic microscopy

Time-effective, unsupervised clustering techniques are exploited to discriminate nanometric metal disks patterned on a dielectric substrate. The discrimination relies on cluster analysis applied to time-resolved optical traces obtained from thermo-acoustic microscopy based on asynchronous optical sampling. The analysis aims to recognize similarities among nanopatterned disks and to cluster them accordingly. Each cluster is characterized by a fingerprint time-resolved trace, synthesizing the common features of the thermo-acoustics response of the composing elements. The protocol is robust and widely applicable, not relying on any specific knowledge of the physical mechanisms involved. The present route constitutes an alternative diagnostic tool for on-chip non-destructive testing of individual nano-objects.