Asynchronous Optical Sampling Research Articles

Ultra-fast terahertz time domain spectroscopy system

Ultra-fast terahertz time-domain spectroscopy(THz-TDS) system operates based on the principle of high-speed asynchronous optical sampling. The system uses two femtosecond oscillators whose repetition frequencies can vary around 1 GHz, and uses a high-bandwidth feedback circuit to control the repetition frequencies. There is a difference of Δf in the repetition frequency between two femtosecond oscillators. One femtosecond oscillator has a repetition frequency of 1G+Δf Hz, which is the pump pulse; the other has a repetition frequency of 1G Hz, which provides the time difference between the pump pulse and the probe pulse, and the time delay varies periodically with the scanning cycle. The period can be given by 1/Δf. This system abandons the mechanical delay line necessary for the traditional THz-TDS system and uses double-photon detector to generate trigger signal. When the frequency difference between the two femtosecond laser oscillators is 1 kHz, a THz spectrum can be detected in only 1 ms, and a THz signal with a dynamic range of 21 dB and a spectral resolution of 5 GHz can be obtained in 10.3 s. Hence it can be characterized by fast detection speed and high resolution. It has incomparable advantages over the traditional THz-TDS system in the application environment that requires rapid measurement.

Large-scale absolute distance measurement with dual free-running all-polarization-maintaining femtosecond fiber lasers

We demonstrate a robust femtosecond LIDAR setup by using two free-running environmentally stable all-polarization-maintaining nonlinear amplified loop mirror mode-locked fiber lasers. Based on the asynchronous optical sampling method, a ranging accuracy of ±2 μm within 65 m has been achieved, as tested in an 80-m-long underground optical tunnel. Through the Kalman filter in real-time data processing, the measurement accuracy can be maintained at a 200 Hz update rate. This setup provides a practical tool for various large-scale industrial and astronomical ranging applications.

Characterization of longitudinal acoustic phonons in InGaAsP multiple quantum wells by asynchronous optical sampling

We performed time-resolved reflectivity measurements and characterized the time-varying oscillations of longitudinal acoustic phonons in InGaAsP multiple quantum wells (MQWs) by asynchronous optical sampling with Er-doped fiber lasers. Oscillations of longitudinal acoustic phonons ranging from a few tens to a few hundred gigahertz were observed, which were induced by forward and backward Raman scattering in Brillouin and first zone-folded acoustic phonon modes. To investigate the dynamics of each phonon mode for various pump-probe time delays, we analyzed the experimental data with a continuous wavelet transformation (CWT) and a subsequent modified inverse CWT. For high-frequency oscillations, we observed the oscillatory profile with beats and identified the contribution of each phonon mode. For low-frequency oscillations, a series of dissipations of phonon wave packets from each QW to the InP substrate was deduced from the analyzed data. These results suggest that our analysis method is useful for investigating the dynamics of acoustic phonons.

Asynchronous optical sampling data-acquisition trigger-signal derived from pulse coherence coincidence.

We propose a data acquisition trigger signal, suitable for free-space, asynchronous optical sampling (ASOPS) systems, based on Laser Pulse Coherence (LPC). At 1 GHz laser pulse repetition frequencies and higher, this method becomes an attractive alternative to established techniques, as it lacks the necessity of high peak pulse intensity, maximising the total laser power available for measurements. Additional advantages are the large unamplified signal generated, ease of initial alignment, and greatly reduced beam pointing stability errors. Employing a LPC trigger source, we realise a sub 30 fs trigger timing error for our ASOPS system.

Localized thinning for strain concentration in suspended germanium membranes and optical method for precise thickness measurement

We deposited Ge layers on (001) Si substrates by molecular beam epitaxy and used them to fabricate suspended membranes with high uniaxial tensile strain. We demonstrate a CMOS-compatible fabrication strategy to increase strain concentration and to eliminate the Ge buffer layer near the Ge/Si hetero-interface deposited at low temperature. This is achieved by a two-steps patterning and selective etching process. First, a bridge and neck shape is patterned in the Ge membrane, then the neck is thinned from both top and bottom sides. Uniaxial tensile strain values higher than 3% were measured by Raman scattering in a Ge membrane of 76 nm thickness. For the challenging thickness measurement on micrometer-size membranes suspended far away from the substrate a characterization method based on pump-and-probe reflectivity measurements was applied, using an asynchronous optical sampling technique.

Self-triggered Asynchronous Optical Sampling Terahertz Spectroscopy using a Bidirectional Mode-locked Fiber Laser

We report a self-triggered asynchronous optical sampling terahertz spectroscopy system based on a single bidirectional mode-locked fiber laser and plasmonics-enhanced photoconductive nanoantennas. The fiber laser generates two optical mutually coherent pulse trains with a stable repetition rate difference, enabling time-domain terahertz spectroscopy without using any mechanical delay line, stabilization electronics, or external trigger. The resolved terahertz spectra over a 0.1–2 THz frequency range and a 30-second measurement time show more than a 70-dB dynamic range, revealing water absorption lines matching the HITRAN database, through a light-weight and compact spectroscopy setup.

Reduction of Moving Target Time-of-Flight Measurement Uncertainty in Femtosecond Laser Ranging by Singular Spectrum Analysis Based Filtering

Femtosecond laser ranging has drawn great interest in recent years, particularly based on an asynchronous optical sampling implementation where a pair of femtosecond lasers are used. High precision absolute ranging either relies on tightly-phase-locked optical frequency combs (a dual-comb setup) or multiple averaging of the measurements from two free-running femtosecond lasers. The former technique is too complicated for practical applications, while the latter technique does not apply to moving targets. In this report, we propose a new route to utilizing a powerful singular spectrum analysis (SSA) filtering method to improve femtosecond laser ranging precision for moving targets with acceleration. The SSA method is capable of separating complex patterns in signals without a priori knowledge of the dynamical model. Here, we utilize the basic SSA filter to extract the target trajectory in the presence of measurement noise both in numerical simulation and in the absolute ranging experiment based on a pair of free-running femtosecond lasers. The experimentally-achieved absolute ranging uncertainty of a moving target is well below 110 nm at a 200-Hz update rate by applying the basic SSA filter. This method paves the way to the practical applications of femtosecond absolute ranging for dynamic objects.

Timing Jitter of the Dual-Comb Mode-Locked Laser: A Quantum Origin and the Ultimate Effect on High-Speed Time- and Frequency-Domain Metrology

We study on the timing jitter characteristics of a passively mode-locked dual-wavelength dual-comb Er-fiber laser that generates two femtosecond optical frequency combs with an offset repetition rate in a shared cavity. The relative timing jitter is characterized directly in the time domain with tens of attoseconds precision by utilizing the intrinsic asynchronous optical sampling process between the two laser beams. This sensitive measurement reveals an amplified spontaneous emission (ASE) noise dominated random walk of the relative pulse timing, which has not been observed before in a dual-comb laser. The quantum-limited relative timing jitter behavior shows that the common-mode noise suppression based on a shared cavity does not apply to quantum noise. We further conduct a time-of-flight ranging experiment and a Fourier-transform spectroscopy simulation based on this dual-comb laser, showing that the quantum-limited relative timing jitter sets an ultimate limit on the reachable performance in time- and frequency-domain dual-comb applications particularly when the high acquisition rate is desired.

Temporal radio-frequency spectrum analyzer, based on asynchronous optical sampling assisted temporal convolution.

We propose and experimentally demonstrated an all-optical radio-frequency (RF) spectrum analyzer, based on asynchronous optical sampling (ASOPS) assisted temporal convolution. The RF spectrum is mapped onto the time axis with the help of the temporal convolution system. In combination with the bandwidth compression capability of the ASOPS scheme, up to 28-GHz RF spectrum can be directly read out by an acquisition system with bandwidth as low as 20 MHz. The experimental results demonstrated about 100-MHz resolution and 28-GHz observation bandwidth. The resolution can be improved by increasing the amount of temporal dispersion or optical spectral bandwidth, and the bandwidth can be further extended by compensating the higher-order dispersion, although it is currently mainly limited by that of the electro-optic modulator. The frame rate is flexibly tunable by changing the repetition rate difference between the two mode-locked fiber lasers. Moreover, nearly 25-dB dynamic range indicates this system has a promising application prospect.

Asynchronous sampling terahertz time‐domain spectroscopy using semiconductor lasers

Asynchronous optical sampling terahertz time-domain spectroscopy using semiconductor laser-based ultra-short pulse sources is demonstrated. A pair of hybridly mode-locked edge-emitting external cavity lasers operating at a wavelength of 830 nm is used. Terahertz traces with spectral components over a range of 250 GHz are achieved.

Ultrafast time-stretch microscopy based on dual-comb asynchronous optical sampling.

The ultrafast time-stretch microscopy based on a single-pixel detector has become a hotspot of the research, owing to its high sensitivity compared to those pixel sensors. However, gigahertz or tens of gigahertz acquisition bandwidth is required for this scheme, resulting in great expense for the whole imaging system and hindering its wide applications. In this Letter, a dual-comb asynchronous optical sampling is applied for the conventional time-stretch microscopy, whose ultrafast temporal axis is magnified by 9200 times. The acquisition bandwidth requirement is thus greatly relaxed, and 320kHz bandwidth successfully resolves 2.3μm spatial resolution with tens of kilohertz frame rate.

Dual-Frequency Comb Transient Absorption: Broad Dynamic Range Measurement of Femtosecond to Nanosecond Relaxation Processes.

We experimentally demonstrate a dual-frequency comb-based transient absorption (DFC-TA) technique, which has a 12 fs time resolution and an ultrafast scan rate. Here, the fast scan rate is achieved by employing asynchronous optical sampling (ASOPS), which utilizes two independent mode-locked lasers with a slightly detuned repetition rates. The ASOPS approach is advantageous because photodegradation damage of optical sample during TA measurements can be minimized by a gated sampling. We show that the vibrational and electronic population relaxations of near-IR dye molecules in solution that occur in the time range from femtoseconds to nanoseconds can be resolved even with a single time scan measurement. The phase coherent nature of our dual-frequency comb lasers is shown to be the key for successful coherent averaging with femtosecond time resolution preserved over many data acquisitions. We anticipate that the present DFC-TA method without using any pump-probe time delay devices could be of use in developing ultrafast TA-based microscopy and time-resolved coherent multidimensional spectroscopy.

Dual repetition-rate mode-locked Yb: YAG ceramic laser

In recent years, dual repetition-rate mode-locked lasers with slightly different pulse repetition rates, as newly developed ultrafast lasers, have attracted great interest and shown their applications in ultrafast dual-comb spectroscopy, asynchronous optical sampling without mechanical movement, etc. The traditional dual-comb system composed of a pair of independent optical frequency combs with slightly detuned comb spacing is still considered expensive, complex and fragile. It is imperative to develop practical and compact dual-comb devices. Dual repetition-rate ultrafast lasers generating asynchronous ultrafast pulses directly from a single cavity can be a promising alternative to the current dual-laser-based comb source. A dual-comb setup based on single laser has the advantages of compact structure, low cost and intrinsic mutual coherence. This technique paves the way for developing the compact, robust and environmental-immune dual-comb systems. In this paper we develop an alternative dual repetition-rate mode-locked Yb:YAG ceramic laser that emits a pair of pulses with spatially separated beams from a single cavity by using a semiconductor saturable absorber mirror and a dual-path pump configuration. In our experiment, a high quality transparent Yb:YAG ceramic prepared by non-aqueous taper-casting method is selected as the gain medium, which is pumped by a 940 nm laser diode. A dual-path pump configuration consisting of a pair of polarization beam splitters and a pair of half-wave plates is designed, in which total pump power from a laser diode is divided equally for pumping the two separate laser beams. When the total absorbed pump power is 5.6 W, dual repetition-rate continuous mode-locked laser operation is achieved under the gain-loss balanced cavity condition. The pulse repetition rates of Pulse1 and Pulse2 are 448.918 MHz and 448.923 MHz, respectively. The difference between repetition rates is 5 kHz mainly caused by the different optical path lengths in the cavity. Under an absorbed pump power of 7 W, the maximum total output power extracted from this laser reaches 170 mW, i.e., 89 mW for Pulse1 and 81 mW for Pulse2. The two mode-locked pulses have nearly identical spectral shapes centered at 1029.6 nm and 1029.8 nm, respectively. The spectral bandwidths for Pulse1 and Pulse2 are 1 nm and 1.16 nm, respectively. The corresponding pulse durations are 2.8 ps and 2.6 ps for the Pulse1 and Pulse2 respectively. Our scheme integrates the advantages of self-starting operation, high repetition-rate, suppression of gain competition. These results indicate that dual-path pump configuration is feasible for dual-repetition-rate mode-locked lasers. These co-generated, dual repetition-rate pulses from one laser cavity possess similar laser characteristics and can be operated independently by dual-path pump configuration. This laser has potential advantages of compact, cost-effective and high-stability for single-cavity-based dual-comb applications in dual-comb spectroscopy, distance ranging, etc.

Multiscale control and rapid scanning of time delays ranging from picosecond to millisecond.

Femtosecond amplifiers seeded by two independent femtosecond oscillators normally produce amplified pulse pairs with a timing jitter equal to the oscillator period, which is typically around 12 ns for Titanium:Sapphire lasers. Combining Arbitrary-Detuning Asynchronous Optical Sampling (AD-ASOPS) with an appropriate selection of amplified pulses, we demonstrate that the time-delay distribution can be narrowed down to a 25-ps time window, allowing to produce spectral interference fringes for each amplified pulse pair. Subsequent AD-ASOPS determination of the actual time delay with subpicosecond accuracy allows to tailor the delay distribution with an electronic control all the way to the repetition period of the amplifiers. We thus demonstrate rapid scanning of the time delays up to nearly 1 ms with a sub-picosecond accuracy, which makes this method an ideal tool for multiscale pump-probe spectroscopy.

High-speed asynchronous optical sampling based on GHz Yb:KYW oscillators.

A low-cost scheme of high-speed asynchronous optical sampling based on Yb:KYW oscillators is reported. Two GHz diode-pumped oscillators with a slight pulse repetition rate offset serve as pump and probe source, respectively. The temporal resolution of this system is limited to 500 fs mainly by the pulse duration of the oscillators and also by relative timing jitter between the oscillators. A near-shot-noise noise floor around 10-6 (∆R/R) is obtained within a data acquisition time of a few seconds. The performance of the system is demonstrated by measurements of coherent acoustic phonons in a semiconductor sample that resembles a semiconductor saturable absorber mirror or an optically pumped semiconductor chip.

Device for acoustic pulse echo experiments in the subterahertz range

We present experimental results on the light-to-sound transduction by a superlattice embedded in an optical microcavity, using picosecond ultrasonics. Experiments on the ability to generate and detect phonons at 0.3 and 0.6 THz were performed at low temperature. Finally, we demonstrate that such a device can detect the acoustic burst that it has generated and which has propagated through the substrate, been reflected on the backside of the substrate, and has returned to the device. We study the efficiency of this detection by varying independently the laser wavelength of the pump and probe, by means of asynchronous optical sampling.

Quantum-limited timing jitter characterization of mode-locked lasers by asynchronous optical sampling.

We demonstrate a novel time domain timing jitter characterization method for ultra-low noise mode-locked lasers. An asynchronous optical sampling (ASOPS) technique is employed, allowing timing jitter statistics on a magnified timescale. As a result, sub femtosecond period jitter of an optical pulse train can be readily accessible to slow detectors and electronics (~100 MHz). The concept is applied to determine the quantum-limited timing jitter for a passively mode-locked Er-fiber laser. Period jitter histogram is acquired following an eye diagram analysis routinely used in electronics. The identified diffusion constant for pulse timing agrees well with analytical solution of perturbed master equation.

Ultrafast time-domain spectroscopy system using 10 GHz asynchronous optical sampling with 100 kHz scan rate.

An ultrafast time-domain spectroscopy system employing asynchronous optical sampling at a repetition rate of 10 GHz is presented. Two ultra-compact Ti:sapphire femtosecond ring lasers allow to achieve scan rates as high as 100 kHz for a 100 ps long time window and a time-delay resolution of 100 fs. The feasibility of this high-speed ASOPS system is evaluated by performing THz time domain spectroscopy on molecular gases where signal-to-noise ratios exceeding 30 dB for averaging times in the millisecond range have been obtained. In order to demonstrate the benefits of this system for ultrafast pump-probe spectroscopy we demonstrate the high-sensitivity detection of coherent acoustic phonons with dephasing times in the range of the 100 ps time window.

Two-Dimensional Phononic Crystals: Disorder Matters.

The design and fabrication of phononic crystals (PnCs) hold the key to control the propagation of heat and sound at the nanoscale. However, there is a lack of experimental studies addressing the impact of order/disorder on the phononic properties of PnCs. Here, we present a comparative investigation of the influence of disorder on the hypersonic and thermal properties of two-dimensional PnCs. PnCs of ordered and disordered lattices are fabricated of circular holes with equal filling fractions in free-standing Si membranes. Ultrafast pump and probe spectroscopy (asynchronous optical sampling) and Raman thermometry based on a novel two-laser approach are used to study the phononic properties in the gigahertz (GHz) and terahertz (THz) regime, respectively. Finite element method simulations of the phonon dispersion relation and three-dimensional displacement fields furthermore enable the unique identification of the different hypersonic vibrations. The increase of surface roughness and the introduction of short-range disorder are shown to modify the phonon dispersion and phonon coherence in the hypersonic (GHz) range without affecting the room-temperature thermal conductivity. On the basis of these findings, we suggest a criteria for predicting phonon coherence as a function of roughness and disorder.

Terahertz cyclotron resonance spectroscopy of an AlGaN/GaN heterostructure using a high-field pulsed magnet and an asynchronous optical sampling technique

The effective mass, sheet carrier concentration, and mobility of electrons within a two-dimensional electron gas in an AlGaN/GaN heterostructure were determined using a laboratory-based terahertz cyclotron resonance spectrometer. The ability to perform terahertz cyclotron resonance spectroscopy with magnetic fields of up to 31 T was enabled by combining a high-field pulsed magnet with a modified asynchronous optical sampling terahertz detection scheme. This scheme allowed around 100 transmitted terahertz waveforms to be recorded over the 14 ms magnetic field pulse duration. The sheet density and mobility were measured to be 8.0 × 1012 cm−2 and 9000 cm2 V−1 s−1 at 77 K. The in-plane electron effective mass at the band edge was determined to be 0.228 ± 0.002m0.