Optical Sampling Technique Research Articles

Time interval measurement with linear optical sampling at the femtosecond level

High-precision time interval measurement is a fundamental technique in many advanced applications, including time and distance metrology, particle physics, and ultra-precision machining. However, many of these applications are confined by the imprecise time interval measurement of electrical signals, restricting the performance of the ultimate system to a few picoseconds, which limits ultrahigh precision applications. Here, we demonstrate an optical means for the time interval measurement of electrical signals that can successfully achieve femtosecond (fs) level precision. The setup is established using the optical frequency comb (OFC) based linear optical sampling (LOS) technique to realize timescale-stretched measurement. We achieve a measurement precision of 82 fs for a single LOS scan measurement and 3.05 fs for the 100-times average with post-processing, which is three orders of magnitude higher than the results of older electrical methods. The high-precision time interval measurement of electrical signals can substantially improve precision measurement technologies.

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.

Direct bandwidth measurement of multimode waveguides based on an optical sampling technique.

We demonstrate direct bandwidth measurement of 11-cm-long multimode polymer waveguides based on an optical sampling technique for the first time, to our knowledge. The pulse shape can be recovered after transmission due to the advantages such as high bandwidth and high refresh rate of optical sampling technology. A reduction in averaged bandwidth (bandwidth-length product) from 241 GHz (27 GHz·m) to 180 GHz (20 GHz·m) of straight waveguides is observed when using mode scramblers to fully stimulate the higher-order modes. The effects such as bending and crossing structure of the waveguides on the bandwidth are also investigated. The proposed method is effective for measuring the bandwidth and dispersion of meter- and centimeter-long waveguides, fibers, and optical devices.

Fast optical sampling by electronic repetition-rate tuning using a single mode-locked laser diode.

This paper demonstrates optical sampling by electronic repetition-rate tuning (OSBERT): a single-laser optical sampling technique capable of fast scan rates and customisable scan ranges. The method has no moving parts and is based on the electronic modulation of the repetition rate of a passively mode-locked laser diode, simply by varying the reverse bias applied directly to the saturable absorber section of the laser. Varying the repetition rate in a system built as a highly imbalanced interferometer results in pairs of (pump, probe) pulses with successive increasing delay. The resulting scan range is proportional to the magnitude of the repetition rate modulation and is scaled by the chosen length of the imbalance. As a first proof of concept, we apply the method to distance measurement, where the displacement of a target across 13.0 mm was detected with ∼0.1 mm standard deviation from an equivalent free-space distance of 36 m and at a real-time scan rate of 1 kHz. The customizable scan range and competitive scan rate of the method paves the way for single ultrafast semiconductor laser diodes to be deployed as fast, low-cost, and compact optical sampling systems in metrology, biomedical microscopy, and sensing applications.

Sub-ps resolution clock-offset measurement over a 114 km fiber link using linear optical sampling.

We demonstrate a sub-ps resolution clock-offset measurement based on linear optical sampling technique via a 114 km fiber link by transferring a dual optical frequency comb. The time deviation between two distance clocks is 110 fs at 1 s and 22 fs at 100 s averaging, and the standard deviation of the measured clock offset is 237 fs. This sub-ps level of clock offset measurement should benefit many time synchronization applications via long fiber links.

Generalized Linear Optical Sampling Technique Realized by Using Non-Pulse Electro-Optic Frequency Comb Sampling Source

We propose a novel generalized linear optical sampling (GLOS) technique realized by using an electro-optic frequency comb (EOFC) as the sampling signal. GLOS technique is demonstrated as a bandwidth compression process in frequency domain instead of gating effect in time domain. An EOFC without the limitation to be ultra-short pulse serves as sampling signal is pre-measured. In experiments, the waveforms are sampled by an EOFC with agile repetition rates and bandwidths. After a demodulation process with pre-measured information, the original signal under test in both intensity and phase fields can be recovered. The results obtained from the proposed method are consistent with those from traditional linear optical sampling technique. Besides, with a high average mode power, EOFC-based GLOS technique realizes more than 10 dB SNR improvement and has ability to detect weak signal with a power of -47.3 dBm. Our demonstration opens the way for cost-effective comb sources to be used in optical sampling fields.

Asynchronous Optical Sampling for Laser-Based Vector Network Analysis on Coplanar Waveguides

We employ asynchronous optical sampling to realize a laser-based vector network analyzer for the measurement of complex electrical reflection coefficients. This sampling method utilizes two separate femtosecond lasers for the generation and detection of ultrashort voltage pulses on a waveguide structure and does not require any optical delay lines. The scheme allows us to increase the measurement epoch, enabling a frequency resolution of 76 MHz while simultaneously covering a broad frequency range up to approximately 500GHz. Our analysis comprises a direct comparison with measurements from a conventional synchronous optical sampling technique. We obtain a very good agreement between the two measurement methods. However, asynchronous sampling reveals a four times higher frequency resolution and shows a reduced residual systematic error. In addition, we investigate different laser stabilization methods utilizing laser-pulse-cross-correlation measurements. Our analysis reveals a temporal jitter of better than 46 fs with the possibility to use one free-running laser oscillator.

Coherent generation and detection of acoustic phonons in topological nanocavities

Inspired by concepts developed for fermionic systems in the framework of condensed matter physics, topology and topological states are recently being explored also in bosonic systems. Recently, some of these concepts have been successfully applied to acoustic phonons in nanoscale multilayered systems. The reported demonstration of confined topological phononic modes was based on Raman scattering spectroscopy [M. Esmann et al., Phys. Rev. B 97, 155422 (2018)], yet the resolution did not suffice to determine lifetimes and to identify other acoustic modes in the system. Here, we use time-resolved pump-probe measurements using an asynchronous optical sampling (ASOPS) technique to overcome these resolution limitations. By means of one-dimensional GaAs/AlAs distributed Bragg reflectors (DBRs) used as building blocks, we engineer high frequency (∼200 GHz) topological acoustic interface states. We are able to clearly distinguish confined topological states from stationary band edge modes. The generation/detection scheme reflects the symmetry of the modes directly through the selection rules, evidencing the topological nature of the measured confined state. These experiments enable a new tool in the study of the more complex topology-driven phonon dynamics such as phonon nonlinearities and optomechanical systems with simultaneous confinement of light and sound.

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.

Recent advances in long-range high-resolution optical reflectometry (Invited)

In this invited talk, three advanced methods to improve both the spatial resolution and the measurement range are presented which paves the way for new horizons in high-end applications. The first one is to develop an optical source with ultra-highly-linear frequency sweeping rate, and use it for optical frequency domain reflectometry (OFDR). The second one is to use linear optical sampling (LOS) technique to measure ultra-short pulse used for optical time domain reflectometry (OTDR) and adopt dispersion compensation scheme. The third one is to use LOS technique for characterizing the ultra-wide linearly chirped signals for pulse compression.

Implementation of electrochemical, optical and denuder-based sensors and sampling techniques on UAV for volcanic gas measurements: examples from Masaya, Turrialba and Stromboli volcanoes

Abstract. Volcanoes are a natural source of several reactive gases (e.g., sulfur and halogen containing species) and nonreactive gases (e.g., carbon dioxide) to the atmosphere. The relative abundance of carbon and sulfur in volcanic gas as well as the total sulfur dioxide emission rate from a volcanic vent are established parameters in current volcano-monitoring strategies, and they oftentimes allow insights into subsurface processes. However, chemical reactions involving halogens are thought to have local to regional impact on the atmospheric chemistry around passively degassing volcanoes. In this study we demonstrate the successful deployment of a multirotor UAV (quadcopter) system with custom-made lightweight payloads for the compositional analysis and gas flux estimation of volcanic plumes. The various applications and their potential are presented and discussed in example studies at three volcanoes encompassing flight heights of 450 to 3300 m and various states of volcanic activity. Field applications were performed at Stromboli volcano (Italy), Turrialba volcano (Costa Rica) and Masaya volcano (Nicaragua). Two in situ gas-measuring systems adapted for autonomous airborne measurements, based on electrochemical and optical detection principles, as well as an airborne sampling unit, are introduced. We show volcanic gas composition results including abundances of CO2, SO2 and halogen species. The new instrumental setups were compared with established instruments during ground-based measurements at Masaya volcano, which resulted in CO2 ∕ SO2 ratios of 3.6 ± 0.4. For total SO2 flux estimations a small differential optical absorption spectroscopy (DOAS) system measured SO2 column amounts on transversal flights below the plume at Turrialba volcano, giving 1776 ± 1108 T d−1 and 1616 ± 1007 T d−1 of SO2 during two traverses. At Stromboli volcano, elevated CO2 ∕ SO2 ratios were observed at spatial and temporal proximity to explosions by airborne in situ measurements. Reactive bromine to sulfur ratios of 0.19 × 10−4 to 9.8 × 10−4 were measured in situ in the plume of Stromboli volcano, downwind of the vent.

Fiber optics frequency comb enabled linear optical sampling with operation wavelength range extension.

Although the linear optical sampling (LOS) technique is powerful enough to characterize various advanced modulation formats with high symbol rates, the central wavelength of a pulsed local oscillator (LO) needs to be carefully set according to that of the signal under test, due to the coherent mixing operation. Here, we experimentally demonstrate wideband LOS enabled by a fiber optics frequency comb (FOFC). Meanwhile, when the broadband FOFC acts as the pulsed LO, we propose a scheme to mitigate the enhanced sampling error arising in the non-ideal response of a balanced photodetector. Finally, precise characterizations of arbitrary 128Gbps PDM-QPSK wavelength channels from 1550 to 1570nm are successfully achieved, when a 101.3MHz frequency spaced comb with a 3dB spectral power ripple of 20nm is used.

Linear optical sampling technique for simultaneously characterizing WDM signals with a single receiving channel.

Wavelength-division multiplexing (WDM) schemes have greatly increased optical transmission capacity, especially when using massively parallel optical frequency combs (OFCs) as carriers. In this paper, we show that linear optical sampling (LOS) is a kind of multiheterodyne process and it can be performed with arbitrary-shaped periodic waveforms. Meanwhile, it can be used for characterizing WDM signals with only a single receiving channel. We successfully demonstrate an observation of 40 WDM channels with 1 THz total bandwidth and total 800 Gbit/s data rate by only using one sampling channel. We show that the adoption of the concept of multiheterodyne detection can improve the performance of LOS technique, and can also greatly simplify multi-channel WDM monitoring systems.

Ultrahigh Resolution Optical Reflectometry Based on Linear Optical Sampling Technique With Digital Dispersion Compensation

We demonstrate an ultrahigh-resolution optical time domain reflectometry (OTDR) system by using a mode-locked laser as the pulse source and a linear optical sampling technique to detect the reflected signals. Taking advantage of the ultrashort input pulse, the large detection-bandwidth, as well as the low timing jitter of linear optical sampling system, a sub-mm spatial resolution is achieved. As the pulse-width is broadened with the increase of distance due to chromatic dispersion and large bandwidth of the ultrashort pulse, by adopting digital chromatic dispersion compensation, we achieved a spatial resolution of $340\,\mu {\text{m}}$ when measuring the reflector at 10 km. This technique helps OTDR find new foreground in long-range and ultrahigh-resolution distributed applications such as remote optical identification device detection for diagnosing passive optical network links, or precisely detecting fault positions in aircrafts.

Sub-THz-range linearly chirped signals characterized using linear optical sampling technique to enable sub-millimeter resolution for optical sensing applications

Pulse compression technique is a particularly competitive method that enables both high spatial resolution and dynamic range in coherent radar and distributed fiber sensing systems. Up to now, the frequency bandwidths of most pulse compression techniques are restricted to tens of GHz. In this paper, we propose an all-optic sub-THz-range linearly chirped optical source and a large-bandwidth detection system to characterize it. Taking advantage of the chromatic dispersion effect, ultrashort optical pulses are stretched to be ~10 ns linearly chirped pulses with sub-THz range, which yields a large time-bandwidth product of 4500, a high compression ratio of 4167 and a chirp rate of 45 GHz/ns. The generated waveform is characterized with high precision thanks to the large detection bandwidth of linear optical sampling technique. A spatial resolution of 120 μm and an extinction ratio of 20.4 dB is demonstrated by using this technique, which paves the way for ultra-high spatial resolution and long range sensing applications such as LIDAR and optical reflectometry.

Loop-Assisted Coherent Matched Detector for Parallel Time-Frequency Sampling

Optical sampling is performed mostly in either time or frequency domain. In the former approach, sampling points are temporally taken by photo-mixing of received signals with locally generated ultrashort pulses. In the latter approach, the received signal is spectrally sliced and decomposed, which are taken as sampling points in the frequency domain. In this paper, as an alternative way, we propose and investigate an optical sampling technique called optical time-frequency-domain sampling, in which sampling points are taken in the time-frequency domain in an optoelectronic manner, by means of optoelectronic coherent matched detectors. Using this technique, it is possible to demultiplex and detect subchannels of ultrawideband multi-carrier signals at e.g. 100, 400 Gb/s, 1 Tb/s or higher, without relying on either ultrashort pulses or high-resolution optical filters necessary in the conventional time- or frequency-domain sampling approaches. Moreover, in this paper, we propose and demonstrate a loop-assisted coherent matched detector that simply and practically extends the time-frequency sampling to parallel sampling configuration just using a single coherent matched detector. Provided are some experimental proof focusing on demultiplexing and detection of orthogonal time-frequency-domain multipelxed (OTFDM) signals, a sort of ultrawideband multicarrier signals. All subchannels of 160-Gb/s QPSK-OTFDM signals are simultaneously detected and measured with the loop-assisted coherent matched detector employing a single set of low-speed 10-Gbaud photo-receivers.

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.

Characterization of re-grown floc size and structure: effect of mixing conditions during floc growth, breakage and re-growth process

The impact of mixing speed in three stages-before breakage, during breakage, and after breakage-on re-grown floc properties was investigated by using a non-intrusive optical sampling and digital image analysis technique, respectively. And then, on the basis of different influence extent of mixing speed during each stage on size and structure of re-grown flocs, coagulation performance with varying mixing speed was analyzed. The results indicated that the broken flocs could not re-grow to the size before breakage in all cases. Furthermore, increasing mixing intensity contributed to the re-formation of smaller flocs with higher degree of compactness. For slow mixing before breakage, an increase in mixing speed had less influence on re-grown floc properties due to the same breakage strength during breakage, resulting in inconspicuous variation of coagulation efficiency. For rapid mixing during breakage, larger mixing speed markedly decreased the coagulation efficiency. This could be attributed that mixing speed during breakage generated greater influence on re-grown floc size. However, as slow mixing after breakage was elevated, the coagulation efficiency presented significant rise, indicating that slow mixing after breakage had more influence on re-grown floc structure upon re-structuring and re-arrangement mechanism.

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.

In situ probing of mode-locked vertical-external-cavity-surface-emitting lasers.

We utilize an asynchronous optical sampling technique to study the gain dynamics of vertical-external-cavity-surface-emitting lasers (VECSELs) under mode-locked operation. This allows for an in situ characterization of the gain depletion and recovery over nanoseconds with femtosecond-scale resolution. Our method allows for a more direct study of intracavity gain dynamics than traditional pump/probe measurements. We observe a rapid depletion of the gain on the timescale of the intracavity pulse. Afterward, a rapid recovery over a few picoseconds due to intraband scattering and carrier heating takes place, followed by a long recovery attributed to the continuous supply of carriers by the pump laser.