Electro-optic Sampling System Research Articles

Subpicosecond imaging system based on electrooptic effect

This work presents an ultrafast, interferometric electrooptic sampling system that uses a two-dimensional detector array to image the electric field present on a device. Spatial and temporal resolution are comparable to conventional "point" electrooptic sampling systems, <5 /spl mu/m and <1 ps, respectively. Voltage sensitivity is expected to be 270 mV and may achieve less than 4 mV with sensor cooling and a more effective electrooptic material. A coplanar silicon structure with 10-/spl mu/m feature size would have a field sensitivity of 27 kV/m and 400 V/m, respectively. This compares favorably with the reported sensitivity of 10/sup 5/ V/m for prior imagers and 10 V/m for point samplers. Applications for an E-field "imager" include characterization of field distributions in planar passive microwave devices, multiport analog and digital devices, and studying device and materials physics.

Electro-optic sampling system for the testing of high-speed integrated circuits using a free running solid-state laser

This paper deals with the indirect electro-optic sampling technique for the low-invasive detection of periodical voltage waveforms on lines in high-speed integrated circuits. The system introduced here is based on a passive mode-coupled Ti:Sapphire-Laser as light source for generating optical pulses in the subpicosecond regime. Therefore, we have to synchronize the resulting electric measurement signal and its external trigger onto the pulse repetition rate of this free running solid-state laser. The multi user function of the laser system forces us to transmit the pulses via a single-mode fiber into the measurement setup. For that purpose we developed a special optical arrangement to minimize the widening of the pulses in the time domain. The system's high-temporal resolution of nearly 10 ps in combination with its high-voltage sensitivity of about 800 /spl mu/V//spl radic/(Hz) is demonstrated by measurements of an integrated microwave frequency divider.

Optoelectronic generation and detection of single-flux-quantum pulses

We report on the direct observation of a single-flux-quantum (SFQ) pulse. The response of a metal–semiconductor–metal photodiode to a femtosecond laser pulse was used to switch Josephson junctions and to generate an SFQ voltage pulse on a superconducting microstrip line. The detailed shape of the pulse was measured optoelectronically, using a cryogenic electro-optic sampling system. The measured SFQ pulse had a width of 3.2 ps, an amplitude of 0.67 mV, and a total pulse content of 2.1±0.2 mV×ps, corresponding to the quantum of magnetic flux h/2e. With larger excitation, multiple SFQ pulses were observed. Numerical simulations are shown to be qualitatively similar to our experimental results.

An optoelectronic testing system of rapid, single-flux quantum circuits

We have generated picosecond voltage pulses on a superconducting microstrip line by using a metal-semiconductor-metal photodiode as an optoelectronic switch. These pulses are fed into a two-Josephson-junction pulse shaper to generate single-flux quantum (SFQ) pulses. Using a reflective electro-optic sampling system, SFQ pulses are directly observed for the first time. This important demonstration of nonintrusively detecting electrical signals from superconducting microstrip lines at the level of rapid, single-flux quantum (RSFQ) circuits opens up a new way to test such circuits, on issues such as design verification, jitter, and failure-mode testing. Further, we propose a variable-rate optoelectronic clock for testing the functional speed of RSFQ logic circuits, with an adjustable clock rate up to 38 Gb/s.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

System for direct measurement of the step response of electronic devices on the picosecond time scale

We have built a system capable of measuring the step response of III-V electronic devices on the picosecond time scale, with no alteration in device design or epitaxy. To switch on the device under test (DUT), we have designed and fabricated a new type of photoconductor, the recessed-ohmic photoconductor, which swings 0.45 V with a 2-ps rise time and maintains constant output voltage for 100 ps. This switch is monolithically integrated with the DUT. To measure the output current of the DUT, we have built a Ti:sapphire-laser-based pump-probe direct electro-optic sampling system that has a minimum detectable voltage of 70 microV/ radicalHz and a measurement bandwidth of 750 GHz. The overall system, comprised of the recessed ohmic photoconductor and the electro-optic sampling system, can be used to measure the step response of III-V electronic devices on the picosecond time scale.

Electrooptic sampling evaluation of 1.5 μm metal-semiconductor-metal photodiodes by soliton compressed semiconductor laser pulses

An electrooptic sampling system using soliton compressed pulses from a gain switched laser diode was assembled and used to characterize an InGaAs metal-semiconductor-metal photodetector at the important optical fiber wavelength of 1.55 microns. The measured rise time of the photodiode pulse was 6.5 ps. The pump and probe pulses had a FWHM of 0.6 ps as measured by autocorrelation. The sampling system rise time was evaluated to be 2.0 ps, yielding the estimated intrinsic photodiode rise time of 2.3 ps. By introducing a differential detection scheme and optical filtering, a shot noise limited sensitivity of 0.6 mV/Sqrt[Hz] was achieved. These results represent the fastest rise time, and most sensitive semiconductor laser based, electrooptic sampling system reported to date. >

Comparison of the picosecond characteristics of silicon and silicon-on-sapphire metal-semiconductor-metal photodiodes

The picosecond characteristics of silicon-based metal-semiconductor-metal (MSM) diodes with submicrometer finger spacing and width were studied. Diodes made on both bulk silicon and silicon-on-sapphire (SOS) substrates were measured by a subpicosecond electro-optic sampling system. The response of bulk-silicon MSM diodes was strongly dependent on the wavelength of the excitation light because of the change in penetration depth. The response of SOS diodes, on the other hand, had a weak dependence on wavelength since the thickness of the silicon layer limits the depth of photogenerated carriers. The response of a 200 nm SOS diode has a full-width at half-maximum of 4.5 and 5.7 ps with blue- and red-light excitations. The external quantum efficiency of SOS diodes was also determined at several selected wavelengths.

Generation of subpicosecond electrical pulses by nonuniform illumination of GaAs transmission-line gaps

We present a detailed study of subpicosecond pulse generation by nonuniform illumination of transmission-line gaps on semiinsulating GaAs. The dependence of such pulses on bias voltage, light intensity, and wavelength was examined in detail with the aid of a subpicosecond electrooptic sampling system. A complete spatial mapping of the excitation area indicates that the pulse generation is due to the depletion of the electrical field in the illuminated section of the gap. A comparison of uniform and nonuniform gap excitation schemes pinpointed the physical differences between the two processes of electrical-transient generation. Picosecond pulses were also generated by nonuniform illumination of a photoconductive gap placed in series with a coplanar waveguide for the first time and were found to contain balanced, odd modes only. >

Picosecond electrical characterization of x-ray microchannel-plate detectors used in diagnosing inertial confinement fusion experiments

The picosecond electrical pulse-propagation characteristics of microchannel-plate detectors, used in diagnosing laser-driven inertial-confinement-fusion experiments, were measured with an electro-optic sampling system. We determined propagation velocity and signal distortion of the microchannel-plate microstrip, as well as its characteristic impedance and the substrate relative dielectric permittivity. These parameters are critical for proper calibration of the detectors and serve as a guide for their improved designs.

Elimination of phase noise of pulsed laser diodes in electrooptic sampling systems

The multimode operation of pulsed laser diode sources is converted into excess noise in electrooptic probing systems for electrical waveforms and prevents measurements with shot-noise-limited sensitivity. The transfer function of the sampling system is determined and analyzed in terms of the wavelength and electric field dependence. It is shown that by proper orientation of the polarizing components within the optical beam path, points of operation can by found which have both a vanishing wavelength dependence of the transfer function and maximum sensitivity to the electric fields to be measured. Excess noise sources due to wavelength fluctuations of the laser are thus eliminated, and shot-noise-limited voltage resolution is obtained. The system has been optimized to yield a voltage sensitivity of 2.3 mV/ square root Hz for a longitudinal LiNbO/sub 3/ probe crystal. >

Nanoscale metal–semiconductor–metal photodetectors with subpicosecond response time fabricated using electron beam lithography

Metal–semiconductor–metal photodetectors (MSM PDs) with finger spacing and width as small as 25 nm have been fabricated using high-resolution electron beam lithography. Measurements using an electro-optic sampling system show that the fastest detector has a full width at half-maximum response time of 0.87 ps and a 3 dB bandwidth of 510 GHz. Monte Carlo simulation of detector response time is studied and compared with experimental data. Finally, scaling rules for high-speed MSM PDs are proposed.

Bend-induced even and odd modes in picosecond electrical transients propagated on a coplanar waveguide

A picosecond time separation between the two sides of a single electrical transient propagated on a bent coplanar waveguide has been observed using a subpicosecond electro-optic sampling system. The separation corresponds to the geometrical path difference introduced by a 90° coplanar waveguide bend. Contrary to the straight waveguide where the transient propagates in the odd mode only, the bend forced the signal to emerge as a superposition of both the odd and even waveguide modes. Introduction of proper local grounding in the bend area, by wirebonding the ground lines on both sides of the transmission line, suppressed the unwanted even mode while preserving the standard odd-mode propagation.

Nanoscale ultrafast metal-semiconductor-metal photodetectors

Summary form only given. MSM PDs (metal-semiconductor-metal photodetectors) with finger spacing and width as small as 25 nm have been fabricated on bulk GaAs, LT (low-temperature-grown)-GaAs, and bulk Si, using a custom-built electron-beam lithography system. Measurements using an electrooptic sampling system with a 100-fs pulse laser showed that the shortest full width at half maximum impulse response and the highest 3-dB bandwidth of these nanoscale MSM PDs are, respectively, 0.87 ps and 480 GHz for MSM PDs on LT-GaAs, 1.5 ps and 300 GHz for bulk GaAs, and 10.7 ps and 40 GHz for bulk Si. Monte Carlo simulation was used to understand the impulse response of the MSM PDs and to explore the ultimate speed limitation of transit-time-limited MSM PDs on GaAs and Si. Comparison with experimental data showed that the long tail in responses of the MSM PDs on bulk GaAs and Si was primarily due to slow-moving holes instead of photoexcited carriers deep inside the semiconductor. >

Time-domain characterization of bent coplanar waveguides

A comprehensive experimental study of the propagation of picosecond steplike transients on bent coplanar waveguides (CPWs) on GaAs substrates is presented. Bent CPWs with different degrees of bend-smoothing were investigated with the aid of a subpicosecond electrooptic sampling system. The observations indicated that the physical length of the bent CPW had to be taken into account in order to evaluate the signal propagation velocity correctly. Smoothing of the bends significantly improved the HF performance of the lines. The results demonstrate that transients with a bandwidth in excess of 100 GHz suffer only limited signal distortion when they are propagated over substantial distances on bent CPWs. >

Picosecond characterization of bent coplanar waveguides

Picosecond electrical pulse propagation on Au coplanar waveguides fabricated on semi-insulating GaAs substrates has been analyzed. Propagation speed and signal distortion between the straight and bent transmission lines of different geometries were measured and compared with the aid of an electrooptic sampling system. The results indicate that the bent coplanar waveguides are capable of linking electronic devices operating in the sub-THz frequency regime. It is also found that smoothing of the bends can considerably improve high-frequency performance of these lines. >

Noise suppression by a novel probe beam polarization modulation in electro-optic sampling

A new scheme of noise suppression in electro-optic sampling system by the probe beam polarization modulation is proposed, its operation principle is analyzed, and the performance is demonstrated experimentally. The method has the advantages that narrow-band detection can be used instead of modulating the device under test with doubled sensitivity compared with conventional methods.

External electro-optic integrated circuit probing

Abstract An external electro-optic measurement system with subpicosecond resolution has been developed. This electro-optic sampling system is designed to operate as a non-contact probe of voltages in electrical devices and circuits with modified wafer-level test equipment and no special circuit preparation. Measurements demonstrate the system's ability to probe continuous and pulsed signals on microwave integrated circuits on arbitrary substrates with single-micron spatial resolution. We also discuss the application of external electro-optic sampling to various aspects of time-domain circuit studies, including the generation of short electrical test pulses using novel photoconductive techniques and the propagation of pulses on interconnects.

A mixer based electro-optic sampling system for submillivolt signal detection

We report on a new technique which allows electro-optic sampling to be performed in a frequency range which is above the 1/f noise regime of the pump-probe laser, while at the same time utilizing the high-sensitivity, low-noise properties of an audio-frequency lock-in amplifier. This new approach to electro-optic signal extraction has resulted in an unprecedented improvement of two orders of magnitude in signal-to-noise capability. The new technique is compared to previous, conventional approaches.

Subpicosecond sampling using a noncontact electro-optic probe

This paper discusses a theoretical as well as experimental study of the performance of electro-optic sampling using a noncontact electro-optic probe, i.e., external electro-optic sampling. Sensitivity, temporal resolution, and the capacitive loading effect of this system are calculated by analysis of a static electric field coupled to an electro-optic crystal placed in close proximity to transmission lines such as microstrip lines and coplanar strips. Full-wave analysis is also applied to investigate effect of the electro-optic crystal on the transient property of high-speed electrical signals. Based on these analytical considerations, we have developed an external electro-optic sampling system using precise probe-positioning technology, which improves the measurement reproducibility. A temporal resolution of 0.5 ps and a spatial resolution of 1 μm are confirmed with this system.

Generation of picosecond electrical pulses by a pulse-forming optoelectronic device

Full-wave analysis demonstrates that a picosecond electrical pulse can be generated by a pulse-forming optoelectronic device, composed of a photoconductor and an impedance mismatch structure in a coplanar stripline. An electro-optic sampling system is used to measure the response of the pulse-forming optoelectronic device, fabricated on a semi-insulating GaAs substrate. Electrical pulses with 1.3 ps duration (full width at half maximum) are achieved using the device.