Asynchronous Optical Sampling Research Articles

Simplifying asynchronous optical sampling: an experimental approach toward industrial integration exploiting lock-in acquisition

Time-resolved optical spectroscopies are emerging as a go-to technique for non-destructive testing of nanomaterials. Inspecting the thermal and mechanical properties of a mesoscale device requires achieving delay times beyond the ns timescale in a nanoscopy setup, potentially in a vibration polluted environment. These requirements constitute a major challenge for traditional pump-probe techniques based on moving mechanical delay lines and lock-in detection. Asynchronous optical sampling (ASOPS) and electronically controlled optical sampling (ECOPS), avoiding any moving mechanical parts, are good alternatives. However, their detection scheme is based on fast-balanced photodiodes, which, as a technology, are not as widespread, not as developed, and lack the performance of lock-in based detection. In this study, we introduce what we believe is a novel approach that integrates ASOPS/ECOPS and lock-in detection methodologies, eliminating the necessity for a reference signal and streamlining the optical configuration. By leveraging the strengths of each technique, our approach enhances simplicity and efficiency. The scheme is first validated against standard approaches in the frame of a beam-depletion measurement in a sum frequency experiment. It is then tested in a paradigmatic case study to inspect the mechanics of a single gold nanodisk, with dimensions in the 100 nm range, nanopatterned on a sapphire substrate. These results widen the range of applicability of time-resolved optical techniques as a nano-metrology tool to industrial settings.

Asynchronous optical sampling of on-chip terahertz devices for real-time sensing and imaging applications.

We demonstrate that asynchronous optical sampling (ASOPS) can be used to measure the propagation of terahertz (THz) bandwidth pulses in a coplanar waveguide device with integrated photoconductive switches used for signal excitation and detection. We assess the performance of the ASOPS technique as a function of measurement duration, showing the ability to acquire full THz time-domain traces at rates up to 100 Hz. We observe a peak dynamic range of 40 dB for the shortest measurement duration of 10 ms, increasing to 88 dB with a measurement time of 500 s. Our work opens a route to real-time video-rate imaging via modalities using scanned THz waveguides, as well as real-time THz sensing of small volume analytes; we benchmark our on-chip ASOPS measurements against previously published simulations of scanning THz sensor devices, demonstrating sufficient dynamic range to underpin future video-rate THz spectroscopy measurements with these devices.

Dual-comb operation in an all-polarization-maintaining ultrafast holmium-doped fiber laser

We present a dual-comb fiber laser in the wavelength range of 2.1 μm from an all-polarization-maintaining holmium-doped fiber laser based on a reflective nonlinear amplifying loop mirror (Figure- 9) scheme. The dynamics of the laser, where net cavity dispersion is controlled by varying the length of the intra-cavity dispersion-compensating fiber (DCF), has been fully investigated. The laser is found capable of mode-locking with a net cavity dispersion value ranging from net anomalous (−0.19 ps2) to net normal (0.11 ps2). More interestingly, without any intentional design, dual-comb operation with a relatively small offset frequency, down to a few Hertz, is observed in the system. The dual-comb operation is believed to arise from direction multiplexing in the loop cavity and polarization multiplexing in the linear arm, and is likely to be a generic solution for Figure- 9 lasers. Such a dual-comb system with a characteristic small frequency difference can provide sample interval with higher precision in asynchronous optical sampling (ASOP) applications.

Solitons, bifurcation and chaotics in a bidirectional mamyshev oscillator

Bidirectional mode-locked fiber lasers are able to deliver two outputs of pulses in the clockwise (CW) and counterclockwise (CCW) directions, having important applications in dual-comb spectroscopy, asynchronous optical sampling and terahertz time-domain spectroscopy. In this paper, we have experimentally demonstrated an all-fiber Er/Yb-doped bidirectional passively mode-locked Mamyshev oscillator (MO) without an external seed. Two stable mode-locked states with remarkable different characteristics are observed by adjusting the pump power. The transition process between the two mode-locked states shows chaotic behaviors, which are further investigated with numerical simulations based on the Ginzburg-Landau equation. The bidirectional MO proposed in this work is not only a good laser source, but also a good platform to study soliton dynamics, system bifurcation, and chaos in ultrafast fiber laser systems.

Long-distance and high-precision ranging with dual-comb nonlinear asynchronous optical sampling

Precise distance metrology and measurements play an important role in many fields of scientific research and industrial manufacture. Dual-comb laser ranging combines sub-wavelength ranging precision, large non-ambiguity range, and high update rate, making it the most promising candidate in precise distance metrology and measurements. However, previous demonstrations of dual-comb ranging suffer from short working distances, limited by the decoherence of lasers in interferometric schemes or by the low sensitivity of the photodetectors in response to the sparse echo photons. Here, we propose and demonstrate time-of-flight laser ranging with dual-comb nonlinear asynchronous optical sampling and photon counting by a fractal superconducting nanowire single-photon detector, achieving ranging precision of 6.2 micrometers with an acquisition time of 100 ms and 0.9 micrometers with an acquisition time of 1 s in measuring the distance of an outdoor target approximately 298 m away.

Single-photon level ultrafast time-resolved measurement using two-color dual-comb-based asynchronous linear optical sampling

We demonstrated an ultrafast time-resolved measurement method operating at the single-photon level and employing a two-color comb-based asynchronous optical sampling (ASOPS) setup. We harnessed the two-color ASOPS photon counting approach to achieve long-term averaging of the ultralow intensity signal with a synchronized optical trigger signal, which minimizes residual timing jitter between the two combs. A pulse-width limited picosecond cross-correlation signal was successfully obtained with a power level of <1 photon/pulse. This approach enables the thorough study of ultrafast time-resolved detection of entangled photon pairs, quantum mechanical correlations in the time-frequency domain and finds wide use in optical quantum technology.

Using asynchronous optical sampling to measure timing jitter of electro-optic frequency combs

&lt;sec&gt;Electro-optic frequency combs (EOCs) are optical frequency combs constructed by phase modulation of single frequency lasers. The electro-optic modulated optical frequency combs have shown their unique advantages in many application fields due to their high repetition frequencies, high stabilities and other advantages, especially in precision measurement applications. Through accurate dispersion control, the electro-optical frequency combs can output ultra-short pulse laser sequences in the time domain, and their timing jitter characteristic is very important for precision measurement and other applications. This work presents a scheme to measure the timing jitter of the electro-optic combs directly in the time domain based on the principle of dual-comb asynchronous optical sampling method(ASOPS), which relies on temporal cross-correlation between the high repetition rate electro-optic combs and a low repetition rate passively mode-locked fiber laser. The ASOPS process allows timing jitter measurement in a magnified time scale where the timing jitter at a femtosecond level can be received and visualized by standard low speed electronics. We build a theoretical model for timing jitter measurement, conduct a numerical study to verify the model, and also construct an experimental system to characterize the period jitter of a 10-GHz electro-optic comb.&lt;/sec&gt;&lt;sec&gt;Firstly, the theoretical model for measuring timing jitter is established. In this work, the basic theory of measuring the timing jitter is discussed by analyzing the histogram directly in time domain through using the obtained ASOPS signal. Subsequently, numerical simulations are conducted to simulate the ASOPS process after establishing a sequence of Gaussian pulse train with quantum limited timing jitter. Another pulse train without timing jitter serves as a local oscillator. Through the square law optical detection after sum-frequency generation between LO and LUT, the ASOPS process can be realized and periodic jitter can be obtained directly through histogram statistical analysis. The simulation result is consistent with the theoretical result very well. Finally, an EOC system with cascaded modulators at a repetition rate of 10 GHz is designed and built, and a timing jitter measurement system is designed and built with an all-fiber configuration. The period jitter of 10-GHz EOC is measured by using a 161-MHz mode-locked fiber laser as local oscillator. Histogram analysis shows that the period jitter of the EOC is 3.86 fs.&lt;/sec&gt;&lt;sec&gt;This measurement technique does not require to use the intricate electrical phase-locked circuits or a high-speed photodetector to receive ultrashort pulses of EOC. Like the eye map analysis method commonly used in telecommunication, the histogram analysis can be used to determine the timing jitter approaching the quantum limit. This approach is easy to set up and operate, and it is anticipated to become a standard method of measuring period jitter of ultrashort pulse with high repetition frequency in a laboratory setting. It will be particularly useful for measuring timing jitters of the sources of novel high repetition rate optical frequency combs, such as micro-resonators and electro-optic frequency combs.&lt;/sec&gt;

Research on THz On-Chip System Based on Asynchronous Optical Sampling

Research on THz On-Chip System Based on Asynchronous Optical Sampling

Asynchronous Optical Sampling for Ultrafast Pump‐Probe Spectroscopy (Invited)

Asynchronous Optical Sampling for Ultrafast Pump‐Probe Spectroscopy (Invited)

Open-Air Testing of Dual-Comb Time-of-Flight Measurement.

We configured a long-distance ranging apparatus to test the principle of dual-comb time-of-flight measurement using ultrashort lasers. Emphasis was given to the evaluation of open-air performance quantitatively in terms of the measurement resolution and stability. The test results revealed that our dual-comb asynchronous optical pulse sampling permits micrometer-resolved ranging with a repeatability of 2.05 μm over a 648 m distance in dry weather conditions. Further atmospheric effects were evaluated in three different weather conditions with corresponding Allan deviations. Finally, the capability of simultaneous determination of multiple targets was verified with the potential of advanced industrial applications, such as manufacturing, surveying, metrology, and geodesy.

High capacity NP-Complete problems solver based on dual-comb asynchronous optical sampling

Photonic schemes to solve the Nondeterministic Polynomial-Complete (NP–C) problems could offer an interesting alternative to such computationally intensive tasks. Previous studies had shown that they can be solved by measuring the propagation delays through an optical oracle that corresponds to the NP-C problem. However, to tackle a problem with a large instance, the previous schemes face significant challenges to achieve a large measurement range, fast speed and high resolution, simultaneously. In this work, we propose an optical solver based on asynchronous optical sampling driven by a simple fiber dual-comb laser. It is experimentally demonstrated that it can realize picosecond resolution and tens of nanosecond range with a fiber oracle network consisting of 15 couplers and sections of fibers that represents a typical Traveling Salesman problem. With a width of the pulse interferogram of less than 1.5 ps, i.e. an optical path resolution of ∼0.3 mm in fiber, the output from an oracle with 58 likely paths is measured accurately, from which the Hamilton path is easily identified in ∼0.7 ms. It is expected that further optimizing the parameters of the laser as well as the oracle would enable the solving larger scale NP-C problems.

Optimizing pump-probe reflectivity measurements of ultrafast photoacoustics with modulated asynchronous optical sampling.

Time-resolved optical pump-probe experiments enable the study of complex light-matter interactions on ultrafast timescales, provided that they reach sufficient sensitivity. For instance, with pump-induced ultrafast photoacoustics, probing the typically small changes in optical properties requires a high signal-to-noise ratio. Asynchronous optical sampling (ASOPS), using two separate pulsed lasers at slightly different repetition rates, can be effective at removing noise by averaging many rapidly acquired traces. However, the pump-probe delay scan with ASOPS is always as long as the pump pulse interval, which is inefficient if the delay-time range of interest is shorter. Here, we demonstrate two modified ASOPS schemes that optimize measurement efficiency by only scanning the range of interest. The modification based on frequency modulated ASOPS (MASOPS) is most efficient, especially in the presence of low-frequency flicker noise. We provide a proof-of-concept measurement of ultrafast photoacoustics in which we use MASOPS to scan a time delay of 1/20 of the pump pulse interval. The resulting noise floor is 20 times lower compared to conventional ASOPS, allowing for 20 times faster measurements. Furthermore, we show that by taking experimental noise characteristics into account, more traditional pump-probe methods can also be optimized.

Versatile Femtosecond Laser Synchronization for Multiple-Timescale Transient Infrared Spectroscopy.

Several ways to electronically synchronize different types of amplified femtosecond laser systems are presented based on a single freely programmable electronics hardware: arbitrary-detuning asynchronous optical sampling (ADASOPS), as well as actively locking two femtosecond laser oscillators, albeit not necessarily to the same round-trip frequency. They allow us to rapidly probe a very wide range of timescales, from picoseconds to potentially seconds, in a single transient absorption experiment without the need to move any delay stage. Experiments become possible that address a largely unexplored aspect of many photochemical reactions, in particular in the context of photo-catalysis as well as photoactive proteins, where an initial femtosecond trigger very often initiates a long-lasting cascade of follow-up processes. The approach is very versatile and allows us to synchronize very different lasers, such as a Ti:Sa amplifier and a 100 kHz Yb-laser system. The jitter of the synchronization, and therewith the time-resolution in the transient experiment, lies in the range from 1 to 3 ps, depending on the method. For illustration, transient IR measurements of the excited state solvation and decay of a metal carbonyl complex as well as the full reaction cycle of bacteriorhodopsin are shown. The pros and cons of the various methods are discussed, with regard to the scientific question one might want to address, and also with regard to the laser systems that might be already existent in a laser lab.

A two-color dual-comb system for time-resolved measurements of ultrafast magnetization dynamics using triggerless asynchronous optical sampling.

We report on an Er-doped fiber (EDF)-laser-based dual-comb system that allows us to perform triggerless asynchronous optical sampling pump-probe measurements of ultrafast demagnetization and spin precession in magnetic materials. Because the oscillation frequencies of the two frequency-comb light sources are highly stabilized, the pulse-to-pulse timing jitter is sufficiently suppressed, and data accumulation without any trigger signals is possible. To effectively induce spin precession in ferromagnetic thin films, the spectral bandwidth of the output of one of the EDF frequency comb sources is broadened by a highly nonlinear fiber and then amplified at a wavelength of about 1030nm by a Yb-doped fiber amplifier. The output of the other frequency comb source is converted to about 775nm by second harmonic generation. We used this system to observe ultrafast demagnetization and spin precession dynamics on the picosecond and nanosecond time scales in a permalloy thin film. This time-domain spectroscopy system is promising for the rapid characterization of spin-wave generation and propagation dynamics in magnetic materials.

Parallel imaging with phonon microscopy using a multi-core fibre bundle detection

In this paper, we show a proof-of-concept method to parallelise phonon microscopy measurements for cell elasticity imaging by demonstrating a 3-fold increase in acquisition speed which is limited by current acquisition hardware. Phonon microscopy is based on time-resolved Brillouin scattering, which uses a pump–probe method with asynchronous optical sampling (ASOPS) to generate and detect coherent phonons. This enables access to the cell elasticity via the Brillouin frequency with sub-optical axial resolution. Although systems based on ASOPS are typically faster compared to the ones built with a mechanical delay line, they are still very slow to study real time changes at the cellular level. Additionally, the biocompatibility is reduced due to long light exposure and scanning time. Using a multi-core fibre bundle rather than a single channel for detection, we acquire 6 channels simultaneously allowing us to speed-up measurements, and open a way to scale-up this method.

Asynchronous Upconversion Sampling of Frequency Modulated Combs

AbstractIn recent years, optical frequency combs with a frequency modulated output have attracted considerable attention. While first observed decades ago in externally modulated lasers, a whole class of semiconductor lasers has since been identified to spontaneously enter such a regime of self frequency modulation. However, the temporal properties of these sources could so far only be measured indirectly via phase‐resolved frequency domain measurements. In this work, an asynchronous optical sampling technique which operates directly in time domain is demonstrated. On the basis of a mid‐infrared quantum cascade laser frequency comb, both its instantaneous intensity and optical frequency are measured. The results confirm a quasi‐constant intensity output alongside a close to linear frequency chirp, in accordance with phase‐resolved spectral measurements. Contrary to previous works, stable phase‐locking is achieved in a regime of positive intracavity dispersion resulting in an inversion of the observed frequency modulation.

An Investigation of All Fiber Free-Running Dual-Comb Spectroscopy.

A dual-comb spectroscopy (DCS) system uses two phase-locked optical frequency combs with a slight difference in the repetition frequency. The spectrum can be sampled in the optical frequency (OF) domain and reproduces the characteristics in the radio frequency (RF) domain through asynchronous optical sampling. Therefore, the DCS system shows great advantages in achieving precision spectral measurement. During application, the question of how to reserve the mutual coherence between the two combs is the key issue affecting the application of the DCS system. This paper focuses on a software algorithm used to realize the mutual coherence of the two combs. Therefore, a pair of free-running large anomalous dispersion fiber combs, with a center wavelength of approximately 1064 nm, was used. After the signal process, the absorption spectra of multiple species were simultaneously obtained (simulated using the reflective spectra of narrow-bandwidth fiber Bragg gratings, abbreviated as FBG). The signal-to-noise ratio (SNR) could reach 13.97 dB (25) during the 100 ms sampling time. In this study, the feasibility of the system was first verified through the simulation system; then, a principal demonstration experiment was successfully executed. The whole system was connected by the optical fiber without additional phase-locking equipment, showing promise as a potential solution for the low-cost and practical application of DCS systems.

High-Speed THz Time-of-Flight Imaging with Reflective Optics

In this study, we develop a 3D THz time-of-flight (TOF) imaging technique by using reflective optics to preserve the high-frequency components from a THz antenna. We use an Fe:InGaAs/InAlAs emitter containing relatively high-frequency components. THz-TOF imaging with asynchronous optical sampling (ASOPS) enables the rapid scanning of 100 Hz/scan with a time delay span of 100 ps. We characterize the transverse resolution using knife edge tests for a focal length of 5; the Rayleigh resolution has been measured at 1.0 mm at the focal plane. Conversely, the longitudinal resolution is determined by the temporal pulse width, confirmed with various gap structures enclosed by a quartz substrate. The phase analysis reveals that reflected waves from the top interface exhibit a phase shift when the gap is filled by high-indexed materials such as water but shows in-phase behavior when it is filled with air and low-indexed material. Our imaging tool was effective for inspecting the packaged chip with high lateral and longitudinal resolution. Importantly, the phase information in 2D and 3D images is shown to be a powerful tool in identifying the defect-in particular, delamination in the chip-which tends to be detrimental to the packaged chip's stability.

Terahertz Time-of-Flight Ranging with Adaptive Clock Asynchronous Optical Sampling.

We propose and implement a terahertz time-of-flight ranging system based on adaptive clock asynchronous optical sampling, where the timing jitter is corrected in real time to recover the depth information in the acquired interferograms after compensating for laser instabilities using electronic signal processing. Consequently, the involved measurement uncertainties caused by the timing jitter during the terahertz sampling process and the noise intensity of the terahertz electric field have been reduced by the utilization of the adaptive clock. The achieved uncertainty range is about 2.5 μm at a 5 cm distance after averaging the acquisition time of 1876 ms 5000 times, showing a significant improvement compared with the asynchronous optical sampling using a constant clock. The implemented terahertz ranging system only uses free-running mode-locked lasers without any phase-locked electronics, and this favors simple and robust operations for subsequent applications that extend beyond the laboratory conditions.

Single-cavity dual-comb fiber lasers and their applications

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.