Electro-optic Sampling System Research Articles

Time Domain Simulated Characterization of the Coplanar Waveguide in an On-Chip System for Millimeter Waveform Metrology

We investigate the time domain characterization of a coplanar waveguide (CPW) based on an on-chip electro-optic sampling (EOS) system for millimeter waveform metrology. The CPW is fabricated on a thin layer of low-temperature gallium arsenide (LT-GaAs), and the substrate material is GaAs. A femtosecond laser generates and detects ultrashort pulses on the CPW. The forward propagating pulses are simulated using a simplified current source for the femtosecond laser at different positions on the CPW for the first time. Then, the influences of the CPW geometry parameters on the measured pulses are discussed. The varying slot width has larger influences on the amplitude of millimeter wave pulses than the center conductor width and the pumping gap. Finally, in the frequency range of 10 GHz to 500 GHz, the transfer functions calculated by the time domain pulses are in good agreement with the transfer functions calculated by the frequency domain ports. The above results are important for improving the measurement precision of the millimeter waveform on the CPW for millimeter waveform metrology.

Phase Diversity Electro-optic Sampling: A new approach to single-shot terahertz waveform recording

Recording electric field evolution in single-shot with THz bandwidth is needed in science including spectroscopy, plasmas, biology, chemistry, Free-Electron Lasers, accelerators, and material inspection. However, the potential application range depends on the possibility to achieve sub-picosecond resolution over a long time window, which is a largely open problem for single-shot techniques. To solve this problem, we present a new conceptual approach for the so-called spectral decoding technique, where a chirped laser pulse interacts with a THz signal in a Pockels crystal, and is analyzed using a grating optical spectrum analyzer. By borrowing mathematical concepts from photonic time stretch theory and radio-frequency communication, we deduce a novel dual-output electro-optic sampling system, for which the input THz signal can be numerically retrieved—with unprecedented resolution—using the so-called phase diversity technique. We show numerically and experimentally that this approach enables the recording of THz waveforms in single-shot over much longer durations and/or higher bandwidth than previous spectral decoding techniques. We present and test the proposed DEOS (Diversity Electro-Optic Sampling) design for recording 1.5 THz bandwidth THz pulses, over 20 ps duration, in single-shot. Then we demonstrate the potential of DEOS in accelerator physics by recording, in two successive shots, the shape of 200 fs RMS relativistic electron bunches at European X-FEL, over 10 ps recording windows. The designs presented here can be used directly for accelerator diagnostics, characterization of THz sources, and single-shot Time-Domain Spectroscopy.

The coplanar waveguide with gallium arsenide substrate for the electro-optic sampling system with a bandwidth over 110 GHz

The coplanar waveguide with gallium arsenide substrate for the electro-optic sampling system with a bandwidth over 110 GHz

Open-loop electrooptic sampling for real-time analysis and near-field imaging of ultrafast electronic devices

In this paper, we present an open-loop electrooptic sampling system for real-time characterization and near-field scanning of ultrafast electronic devices. The system capabilities such as picosecond time resolution as well as 210 GHz of measurement bandwidth are verified with measurement of a CMOS nonlinear transmission line and an ultimate bandwidth of 230 GHz has been achieved with a post-process algorithm. The noise of the system is quantified and imaging over a broad range of frequencies for an on-chip antenna is demonstrated.

Stabilization of a high-order harmonic generation seeded extreme ultraviolet free electron laser by time-synchronization control with electro-optic sampling

A fully coherent free electron laser (FEL) seeded with a higher-order harmonic (HH) pulse from high-order harmonic generation (HHG) is successfully operated for a sufficiently prolonged time in pilot user experiments by using a timing drift feedback. For HHG-seeded FELs, the seeding laser pulses have to be synchronized with electron bunches. Despite seeded FELs being non-chaotic light sources in principle, external laser-seeded FELs are often unstable in practice because of a timing jitter and a drift between the seeding laser pulses and the accelerated electron bunches. Accordingly, we constructed a relative arrival-timing monitor based on non-invasive electro-optic sampling (EOS). The EOS monitor made uninterrupted shot-to-shot monitoring possible even during the seeded FEL operation. The EOS system was then used for arrival-timing feedback with an adjustability of 100 fs for continual operation of the HHG-seeded FEL. Using the EOS-based beam drift controlling system, the HHG-seeded FEL was operated over half a day with an effective hit rate of 20%–30%. The output pulse energy was $20~{\rm\mu}\text{J}$ at the 61.2 nm wavelength. Towards seeded FELs in the water window region, we investigated our upgrade plan to seed high-power FELs with HH photon energy of 30–100 eV and lase at shorter wavelengths of up to 2 nm through high-gain harmonic generation (HGHG) at the energy-upgraded SPring-8 Compact SASE Source (SCSS) accelerator. We studied a benefit as well as the feasibility of the next HHG-seeded FEL machine with single-stage HGHG with tunability of a lasing wavelength.

Diminishing relative jitter in electrooptic sampling of active mm-wave and THz circuits

In this work a novel approach in synchronization of electrooptic sampling systems for the ultra-broadband characterization of active mm-wave and THz devices is presented. The relative time jitter between sampled circuit and probing electrooptic head is eliminated by using a femtosecond laser system both as the generator of CW driving the device under test as well as the impulsively probing element. Previous ultra-broadband approaches were applicable to passive components driven by THz impulses, only. The presented system is more generally applicable to active mm-wave and THz components driven by conventional CW electronic sources. Broadband analysis on silicon nonlinear transmission line elements up to a frequency of 300 GHz is presented in order to illustrate the capabilities of the concept.

Traceable Waveform Calibration With a Covariance-Based Uncertainty Analysis

We describe a method for calibrating the voltage that a step-like pulse generator produces at a load at every time point in the measured waveform. The calibration includes an equivalent-circuit model of the generator that can be used to determine how the generator behaves when it is connected to arbitrary loads. The generator is calibrated with an equivalent-time sampling oscilloscope and is traceable to fundamental physics via the electro-optic sampling system at the National Institute of Standards and Technology. The calibration includes a covariance-based uncertainty analysis that provides the uncertainty at each time in the waveform vector and the correlations between the uncertainties at the different times. From the calibrated waveform vector and its covariance matrix, we calculate pulse parameters and their uncertainties. We compare our method with a more traditional parameter-based uncertainty analysis.

N+GaAs subpicosecond photodetector irradiated by fast neutrons

We report experimental results of the characterization of an N+GaAs photodetector irradiated by fast neutrons with flux up to doses of 4×1017 n/cm2 using a constructed electro-optic sampling system and illumination with 80-fs-wide laser pulses. The investigated N+GaAs sample was compared with the same non-irradiated sample. The device shows a response time of 680 fs full width at half maximum (648 GHz, 3-dB bandwidth) with a voltage amplitude of 3.1 mV. Changes in the shape of the electrical signal for different beam power excitations and bias voltages have been demonstrated. Using X-ray diffraction and diffuse scattering analyses, we have observed a decrease of the lattice constant and an almost three times decreased radius of nanoclusters; the density dislocations increased by over four times after neutron irradiation.

Design and evaluation of a THz time domain imaging system using standard optical design software

A terahertz (THz) time domain imaging system is analyzed and optimized with standard optical design software (ZEMAX). Special requirements to the illumination optics and imaging optics are presented. In the optimized system, off-axis parabolic mirrors and lenses are combined. The system has a numerical aperture of 0.4 and is diffraction limited for field points up to 4 mm and wavelengths down to 750 microm. ZEONEX is used as the lens material. Higher aspherical coefficients are used for correction of spherical aberration and reduction of lens thickness. The lenses were manufactured by ultraprecision machining. For optimization of the system, ray tracing and wave-optical methods were combined. We show how the ZEMAX Gaussian beam analysis tool can be used to evaluate illumination optics. The resolution of the THz system was tested with a wire and a slit target, line gratings of different period, and a Siemens star. The behavior of the temporal line spread function can be modeled with the polychromatic coherent line spread function feature in ZEMAX. The spectral and temporal resolutions of the line gratings are compared with the respective modulation transfer function of ZEMAX. For maximum resolution, the system has to be diffraction limited down to the smallest wavelength of the spectrum of the THz pulse. Then, the resolution on time domain analysis of the pulse maximum can be estimated with the spectral resolution of the center of gravity wavelength. The system resolution near the optical axis on time domain analysis of the pulse maximum is 1 line pair/mm with an intensity contrast of 0.22. The Siemens star is used for estimation of the resolution of the whole system. An eight channel electro-optic sampling system was used for detection. The resolution on time domain analysis of the pulse maximum of all eight channels could be determined with the Siemens star to be 0.7 line pairs/mm.

Two-port vectorial terahertz electro-optic sampling system

We present a fiber-based two-port terahertz electro-optic (EO) sampling system at 1.55μm wavelength, including an ultrafast In0.53Ga0.47As photoconductive switch and a freely positionable prismatic EO probe. Frequency components are extended up to 2THz and the dynamic range is larger than 40dB, regardless of the direction of the electromagnetic wave propagating in the waveguide, thanks to the two-port system. The symmetrical two-port pigtailed EO probe allows to determine the direction of propagation of the guided picosecond electromagnetic pulses.

Calibration of sampling oscilloscopes with high-speed photodiodes

We calibrate the magnitude and phase response of equivalent-time sampling oscilloscopes to 110 GHz. We use a photodiode that has been calibrated with our electrooptic sampling system as a reference input pulse source to the sampling oscilloscope. We account for the impedance of the oscilloscope and the reference photodiode and correct for electrical reflections and distortions due to impedance mismatch. We also correct for time-base imperfections such as drift, time-base distortion, and jitter. We have performed a rigorous uncertainty analysis, which includes a Monte Carlo simulation of time-domain error sources combined with error sources from the deconvolution of the photodiode pulse, from the mismatch correction, and from the jitter correction

Fiber-based probe for electrooptic sampling

A fiber-based electrooptic probe that can be incorporated into existing electrooptic sampling systems is presented. The probe is made by attaching an electrooptic crystal to the end of a standard optical fiber and thinned to a thickness of 1.6 mum using selective wet etching. Measurements of the electric field over a coplanar waveguide structure verify the probe's ability to sense electric fields. The probe can be accurately placed a few micrometers over a test point by monitoring the power reflected back into the probe off of a conductor on a circuit under test

Covariance-based uncertainty analysis of the NIST electrooptic sampling system

We develop a covariance matrix describing the uncertainty of mismatch-corrected measurements performed on the National Institute of Standards and Technology's electrooptic sampling system. This formulation offers a general way of describing the uncertainties of the measurement system in both the temporal and frequency domains. We illustrate the utility of the approach with several examples, including determining the uncertainty in the temporal voltage generated by the photodiode

Calibrated 200-GHz waveform measurement

We develop a method for mismatch-correcting temporal waveforms measured with a high-speed electrooptic sampling system to 200 GHz. The new calibration determines a complete equivalent-circuit model describing the source in both the time and frequency domains with uncertainties, and accounts for all impedances and multiple reflections in the measurement system.

High-Speed Large-Area Inverted InGaAs Thin-Film Metal–Semiconductor–Metal Photodetectors

Inverted metal-semiconductor-metal (I-MSM) photodetectors, which are thin-film MSMs with the growth substrate removed and fingers on the bottom of the device (to eliminate finger shadowing to enhance responsivity), are reported herein for high-speed high-efficiency large-area photodetectors. Reported herein are the highest speed vertically addressed large-area (40-/spl mu/m diameter) photodetectors reported to date, which operate with a responsivity of 0.16 A/W and a full-width half-maximum of less than 5 ps. Materials, fabrication processes, heterogeneous integration, and characterization of I-MSM photodetectors are presented in this paper, as measured using a fiber-based electrooptic sampling system. These large-area photodetectors are ideal for vertically addressed high-speed optical links which need alignment-tolerant packaging for cost sensitive applications.

Characterization of millimeter-wave antenna using photonic measurement techniques

This article describes an electric-field measurement system that is based on electro-optic sampling (EOS). This system is able to map near- and far-field patterns of millimeter waves radiated from antennas by scanning the electric-field sensor probe. With the bandwidth set to more than 150 GHz, it can detect an electric field of less than 1.0 V/m at 60 GHz. This electric field is about one-tenth the field limit detectable by conventional microwave EOS systems. A comparison of measured and calculated electric fields in both the frequency- and time-domains demonstrates the accuracy and usefulness of the system. © 2004 Wiley Periodicals, Inc. Int J RF and Microwave CAE 14: 290–297, 2004.

Picosecond response of gallium-nitride metal–semiconductor–metal photodetectors

Metal–semiconductor–metal ultraviolet photodiodes fabricated on GaN were tested in the picosecond regime with an electro-optic sampling system. A device with a feature size of 1 μm showed a response with 1.4 ps rise time and 3.5 ps full width at half maximum. The derived electron velocity, 1.43×107 cm/s, is in good agreement with independent photoexcitation measurements. A slower impulse response was observed in a device with smaller feature size of 0.5 μm.

Ultrafast optics establishes metrological standards in high-frequency electronics

A measurement system is reported that allows for the complete characterization of high-speed oscilloscopes. The measurements are based on ultrafast optical methods, which are used to generate picosecond electrical pulses with photoconductive switches as test signals. The properties of the test pulses are determined by the electro-optic sampling method, which provides traceability to the unit of length and, hence, to the unit of time. The photoconductive switches combined with the electro-optic sampling system represent a primary standard in electrical pulse metrology. With the well-characterized pulses from this standard, the complex transfer function, the impulse and step response, as well as the intrinsic risetime of high-speed oscilloscopes are determined. Risetimes of 6–7 ps can be measured with an expanded uncertainty of 1.2 ps.

Electro-optic sampling system with a single-crystal 4-N,N-dimethylamino-4′-N′-methyl-4-stilbazolium tosylate sensor

We present an electro-optic (EO) sampling system based on an organic ionic salt crystal, 4-N,N-dimethylamino-4′-N′-methyl-4-stilbazolium tosylate (DAST) transducer. Compared to LiTaO3, the DAST lower dielectric permittivity and much higher electro-optic coefficient dramatically improveme electric-field coupling into the EO crystal, which results in a much better signal-to-noise ratio of the sampling system. Submillivolt signals can be easily measured with the DAST sensor. Time resolution of the DAST-based EO system is the same as that of the LiTaO3-based sampler.

Phase-locked-loop-based delay-line-free picosecond electro-optic sampling system

A delay-line-free, high-speed electro-optic sampling (EOS) system is proposed by employing a delay-time-controlled ultrafast laser diode as the optical probe. Versatile optoelectronic delay-time controllers (ODTCs) based on modified voltage-controlled phase-locked-loop phase-shifting technologies are designed for the laser. The integration of the ODTC circuit and the pulsed laser diode has replaced the traditional optomechanical delay-line module used in the conventional EOS system. This design essentially prevents sampling distortion from misalignment of the probe beam, and overcomes the difficulty in sampling free-running high-speed transients. The maximum tuning range, error, scanning speed, tuning responsivity, and resolution of the ODTC are 3.9π (700°), <5% deviation, 25–2405 ns/s, 0.557 ps/mV, and ∼1 ps, respectively. Free-running wave forms from the analog, digital, and pulsed microwave signals are sampled and compared with those measured by the commercial apparatus.