K.J. Weingarten

Subpicosecond laser timing stabilization

The 0.25 Hz-25 kHz pulse timing fluctuations of a Nd:YAG CW-mode-locked laser are reduced from 20.6 ps to less than 0.30 ps rms by an electronic phase-lock loop. Stabilizer design considerations include the laser phase control characteristics, phase detector additive noise, and spurious detection of the laser amplitude noise by the loop phase detector. Applications include synchronization of multiple picosecond pulsed lasers, and synchronization of lasers and electronic signal sources in picosecond physical measurements. >

Reduction of timing fluctuations in a mode-locked Nd:YAG laser by electronic feedback

The timing fluctuations of a mode-locked Nd:YAG laser are reduced by electronic feedback. Timing fluctuations at rates of 50 to 250 Hz are reduced by more than 20 dB, the total timing fluctuations are reduced from 2.9 to 0.9 psec rms, and long-term drift is reduced to 0.5 psec/min. Applications include time-resolved probing experiments and synchronization of lasers.

Internal Microwave Propagation and Distortion Characteristics of Travelling-Wave Amplifiers Studied by Electro-Optic Sampling

The internal signal propagation and saturation characteristics of two monolithic microwave travelling-wave amplifiers (TWA) are measured by electro-optic sampling. Gate and drainline responses are compared with theory and simulation, leading to revisions in the FET models. Drain voltage frequency dependence and harmonic current propagation together lead to more complex saturation behavior than is discussed in the literature.

Microwave Measurements of GaAs Integrated Circuits Using Electrooptic Sampling

We describe the electrooptic sampling system at Stanford with emphasis on the requirements for microwave measurements. Results presented include internal-node measurements of 20 GHz distributed amplifiers, propagation delays in GaAs frequency dividers clocked to 18 GHz, and VSWR on IC transmission lines to 40 GHz.

Picosecond Sampling of GaAs Integrated Circuits

Gallium Arsenide (GaAs) microwave integrated circuits (IC’s) are now being developed for operation in the millimeter-wave range, while GaAs digital IC’s have demonstrated ring-oscillator propagation delays of 5–10 ps, with gate delays of 50–100 ps for larger scale circuits. Digital IC’s are currently tested only by indirect techniques (multi-stage propagation delay or cycle times), while microwave circuits are tested by external scattering parameter measurement; if the circuit does not perform to expectations, the cause is not easily identified. Electrooptic sampling was initially developed to measure the response of photoconductors and photodetectors faster than the time resolution of sampling oscilloscopes and used an external electrooptic modulator connected to the device under test [1,2]. The system developed at Stanford uses the GaAs IC substrate as the electrooptic material, permitting detailed internal-node circuit evaluation with picosecond time resolution.

Picosecond Electrooptic Sampling and Harmonic Mixing in GaAs

Electro-optic sampling is a powerful technique for exploiting the capabilities of modern mode-locked laser systems to make high speed electronic measurements. Using ultrashort light pulses to probe the electric fields of microstrip transmission lines deposited on LiNbO3 and LiTaO3, VALDMANIS et al. [1,2] demonstrated an electro-optic sampling system capable of resolving picosecond and subpicosecond rise-time photoconductive switches. KOLNER et al. [4,5] utilized a similar system to characterize photodiodes exhibiting bandwidths of 100 GHz. In both cases, a hybrid connection between the device under test and the electro-optic transmission line was required. VALDMANIS et al. [6] and MEYER and MOUROU [7] have shown that by placing an electro-optic crystal in contact with the circuit under test, picosecond waveforms could be measured without a hybrid connection. Although these techniques have demonstrated impressive results, they potentially compromise the true device response by reactive loading of the transmission line systems. This occurs due to 1) the fundamental mode mismatch between similar transmission lines on different dielectrics, 2) parasitic reactances associated with the bonding wires between the two transmission line systems or 3) capacitive loading of a transmission line by close proximity to the sampling crystal.

Electro-Optic Sampler For Gallium Arsenide Integrated Circuits

We report a new technique for directly sampling electrical waveforms in GaAs integrated circuits with picosecond time resolution. This noninvasive sampling system provides a powerful new tool for the design and diagnosis of GaAs integrated circuits. The technique is based on the intrinsic electro-optic effect in GaAs and utilizes a mode-locked and compressed Nd:YAG laser to electro-optically sample the fringing fields of microstrip transmission lines in GaAs integrated circuits. The frequency doubled output of the laser can be used to excite on-chip photodetectors which serve as test signal generators, or the circuits can be driven by external signal generators. We also demonstrate how the sampling system can be operated as a harmonic mixer for evaluation of pulse-to-pulse timing jitter in mode-locked lasers.

GaAs Integrated Circuit Testing Using Electrooptic Sampling

The principles of electrooptic sampling for high-speed testing of GaAs IC, its capabilities as both a time-domain sampling oscilloscope and a frequency-domain network analyzer, and recent measurements results are described. Applications of this system include measurements of internal-node switching signals and propagation delays in digital circuits with picosecond time resolution, small-signal and large-signal analysis of microwave circuits, and measurement of the one-port S-parameters on IC transmission lines to millimeter-wave frequencies. A method to measure two-port S-parameters using the optical probe is described. This technique defines an on-chip reference plane, reducing measurement errors and eliminating the calibration standards and routines required with conventional network analyzers.

Characterization of GaAs integrated circuits by direct electro-optic sampling

With the recent demonstration of direct electro-optic sampling of GaAs circuits, a new method to characterize high-speed monolithic microwave and digital circuits exists. This technique uses picosecond pulses from a laser to non-invasively probe voltage waveforms at points internal to monolithic circuits with a measurement bandwidth in excess of 50 GHz. This paper presents measurements of a GaAs MESFET traveling wave amplifier and an 8-bit multiplexer/demultiplexer.