University of Rochester

Gerard A Mourou

Steven L Williamson

Janis A Valdmanis

Theodore Sizer, Ii

John A Nees

Wayne Harvey Knox

Theodore B Norris

Maurice Pessot

Kevin E Meyer

Todd A Jackson

Georg F Albrecht

A system for time-resolving ultra-short electrical waveforms of up to a few hundred gigahertz bandwidth is presented. The system utilizes a fast electro-optic modulator capable of subpicosecond responsivity. A CW (continuous wave) laser is used to probe the change in birefringence resulting in the modulator due to an induced electric field. The rapid change in the transmitted optical signal due to an equally rapid changing electric field (picosecond pulse) is then detected and temporally dispersed using a picosecond resolution streak camera. The modulator operates in a region close to minimum transmission where the average optical power is below the damage threshold for the photocathode of the streak camera and where small electrical signals can significantly modulate the transmitted beam. The system can be used in either sampling mode where the modulation and subsequent detection are repeated and the data accumulated at repetition rates as high as 100 MHz or in single shot mode.

Electro-electron oscilloscope

An ultrafast traveling wave optical modulator capable of functioning at frequencies greater than 100 GHz having an optical waveguide parallel to a transmission line. The optical waveguide is in a substrate of electro-optic material (GaAs with GaAlAs layers forming the optical waveguide). The transmission line is a pair of coplanar electrodes on the substrate. A superstrate having an effective dielectric constant substantially equal to the square of the index of refraction of the substrate (a GaAs body in which the electric field on the line is substantially confined) eliminates the mismatch in velocity of propagation of the traveling electrical and optical signals thereby increasing the response time of the modulator so that it can function when the electrical modulating signal on the line exceeds 100 GHz in bandwidth.

Ultrafast optical modulator

A method for transmitting high energy subpicosecond pulses through single-mode optical fiber without stimulating nonlinear effects as are caused by self-phase modulation or Raman generation, which method increases the average power handling capacity of the fiber. The optical pulses, which may be modulated to carry data, as by pulse code modulation, are increased in temporal width before launching into the fiber. The output pulses from the fiber are compressed. Since the nonlinear effects are related to the peak power of the pulses, these effects are avoided while increasing the average power and allowing the use of the available bandwidth of the fiber thereby enabling greater data transmission rates.

Method for optical pulse transmission through optical fibers which

A measurement system using electro-optic sampling and operative in the time domain characterizes devices over a bandwidth extending to upper microwave frequencies (e.g., 100 GHz). The device under test is mounted to or integrated on a substrate of electro-optic semiconductor material and is connected to transmisison and lines on the substrate. Sampling signals are electro-optically generated and propagate along the lines toward and away from the device under test, using a laser pulse beam incident on the substrate. The signals are electrically sampled by a laser pulse sampling beam which is responsive to the change in refractive index due to the signal at locations equidistant from the generation position. The waveform resulting from electro-optic sampling near the device under test corresponds to the sum of the signal incident upon the device and the signal reflected therefrom. The waveform resulting from electro-optic sampling at the location away from the device corresponds to the incident waveform. The waveforms are processed by subtracting the waveform corresponding to the incident signal from the waveform corresponding to the sum of the incident and reflected signal and Fourier transformed to derive the scattering parameters. The laser pulses are very short (in the picosecond or sub-picosecond range) and the transmission lines are uniform in their attenuation and dispersion characteristics, thereby enabling measurements to be made to extremely high microwave frequencies in the time domain and converted by Fourier transform to the frequency domain to determine the scattering parameters characterizing devices operative at such high frequencies.

Electro-optic measurement (network analysis) system

Electrical signals are measured (analyzed and displayed) with subpicosecond resolution by electro-optic sampling of the signal in an electro-optic crystal, the index of which changes in response to the electric field produced by the signal, in accordance with the Pockels effect. The crystal is disposed adjacent to and in the fringe field of a line on a substrate, which may be part of an integrated circuit, for measuring signals propagating along the line during the operation of the circuit. A beam of short optical (laser) sampling pulses in the picosecond range is focused preferably close to the surface of the crystal and perpendicular to the optical axis of the crystal. The optical pulses transmitted through the crystal are processed to provide a display affording a measurement of the electrical signal.

Measurement of electrical signals with subpicosecond resolution

A dye laser system wherein the dye medium is pumped by frequency doubled mode locked laser pulses from a Nd:YAG laser and also mode locked by colliding pulses in an anti-resonant ring having a saturable absorber.

Dye laser

Electrical signals are measured (analyzed and displayed) with subpicosecond resolution by electro-optic sampling of the signal in an electro-optic crystal, the index of which changes in response to the electric field produced by the signal, in accordance with the Pockels effect. The crystal is disposed adjacent to a transmission line along which the signals propagate the line may be a coplanar wave guide having a plurality of parallel strips of conductive material on the surface of the crystal. The crystal may be disposed adjacent to and in the fringe field of a line on a substrate, which may be part of an integrated circuit, for measuring signals propagating along the line during the operation of the circuit. A beam of short optical (laser) sampling pulses in the picosecond range is focused so that the region where the beam is confocal is disposed where the field is parallel in the crystal. The confocal region (where the optical wavefront is planar is preferrably close to the surface of the crystal and perpendicular to the optical axis of the crystal. The optical pulses transmitted through the crystal are processed to provide a display affording a measurement of the electrical signal.

Subpicosecond pulses less than 70 femtoseconds in duration are obtained with a synchronously pumped dye laser using a mixture of laser dye and a fast recovery saturable absorber which passes in a jet between folding mirrors in a laser cavity. The ratio of absorber to dye in the mixture is selected to compensate for dispersion effects in the laser medium which tend to limit the spectrum of the laser pulses and increase their duration.

Dye laser medium for subpicosecond laser pulse generation

The temporal shape of optical pulses is measured over a wide dynamic range, for example, 10 orders of magnitude, by passing an optical signal corresponding to the autocorrelation function of the optical pulses through a variable attenuation filter, the position of which is a function of the attenuation. By plotting the attenuation of the filter in terms of the position thereof, against the duration of the temporal overlap of the pulses in a mixing crystal which produces the optical signal corresponding to the autocorrelation function, the temporal shape of the pulses is displayed.

Optical pulse correlation measurement

Electrical signals are measured (analyzed and displayed) with picosecond resolution by the electrooptic sampling of the signal being analyzed in a traveling wave Pockels cell. Sampling pulses, from an optical pulse generator such as a colliding pulse mode-locked laser, of subpicosecond duration are transmitted through the cell as polarized light and translated into a difference output corresponding to the difference in amplitude between the transmitted and rejected components of the polarized light. The signals, synchronous with the optical sampling pulses, are generated to propagate along the cell in a direction transverse to the transmission of the optical sampling pulses and in variably delayed relationship therewith. A separate beam of the optical pulses is desirably chopped and used to activate a photoconductive device which produces the signals. The difference output is processed, preferably by a lock-in amplifier and signal averager; the lock-in amplifier being synchronized with the chopping of the launched pulses, and displayed on a time base synchronous with the variable delay of the pulses. Accordingly, the signal is displayed on an expanded time scale for measurement and other analysis. The response of photodetectors, photoconductive switches and other ultrafast light activated devices can be determined, when these devices are used as the source of the signals being analyzed and displayed.

Measurement of electrical signals with picosecond resolution

Electrical signals are measured (analyzed and displayed) with picosecond resolution and sensitivity in the microvolt (less than 100 microvolts) range by electron-optically sampling the signal. Sampling electron bursts are produced in response to a train of subpicosecond optical pulses. A beam of these electron bursts samples successive portions of the signal as it is transmitted as a travelling wave along deflection plates which act as a transmission line. The bursts are deflected in accordance with the amplitude of the successive portions of the signal and translated into spots of light, as on a phosphor screen. The deflection is significantly less than the diameter of the spot. The deviation of the spot with respect to the position thereof in the absence of the signal on the deflection plates is translated into a difference output. The signal to be analyzed is generated, synchronously with the optical pulses, to propagate along the deflection plates in variably delayed relationship therewith. The signal is optically-induced using a separate beam of the optical pulses which is desirably chopped into optical pulses. The difference output is processed, preferably by a lock-in amplifier and signal averager; the lock-in amplifier being synchronized with the chopping of the beam into the optical pulses, and displayed on a time base synchronous with the variable delay of the optical pulses. Accordingly, the signal is displayed on an expanded time scale for measurement and other analysis.

Electron-optical wide band signal measurement system

Picosecond switching of electric current in response to optical signals is obtained by conversion of the optical signal, such as an optical pulse, into a photoelectron burst (a photoelectronic signal) which is a faithful temporal replica of the optical signal. Electron optics increase the energy of the electrons of the photoelectronic signal which is imaged so as to illuminate essentially the entire gap formed between electrodes on a body of semiconductor material. The photoelectrons are absorbed in the semiconductor material to create throughout the gap a degenerate layer. The gap geometry and the image formed by the optical signal on a photocathode, which provides the photoelectronic signal, are such that space charge effects do not distort the photoelectronic signal and a temporal replica of the optical signal illuminates the entire gap. The gap geometry affords broad bandwidth operation. Due to the gain in the system, the high photoelectron energy obtainable after electron acceleration permits the use of large band gap semiconductor materials which have high dielectric strength and are not prone to thermal breakdown effects. By deflecting the photoelectrons across a plurality of side-by-side gaps on the semiconductor, extremely high speed demultiplexing of extremely high frequency optical signals (in picosecond samples) can be obtained.

Photoelectron switching in semiconductors in the picosecond time domain

A laser system generates high energy ultra-short pulses using a dye cell amplifier driven by ultra-short pulses from a dye laser and pumped by pump pulses from a laser amplifier. The laser amplifier and dye laser are synchronously driven and pumped by the same laser such that the signal pulse from the dye laser arrives at the dye cell amplifier immediately upon the completion of population inversion therein in response to the pump pulse. Efficient high-power, ultra-short laser pulses are obtained from the dye amplifier, since amplified spontaneous emission (ASE) is avoided. Doubler crystals are used to provide the pump pulses to the dye laser from the common driver laser and to provide pump pulses from the laser amplifier to the dye cell amplifier at the proper wave length for the materials used therein. This system is tunable by selecting appropriate dyes for the dye laser and dye cell amplifier and/or tuning elements within the dye laser cavity.

Laser system using organic dye amplifier

An inexpensive, simple and highly accurate sweep drive circuit for streak cameras generates a ramp voltage for the deflection plates of an image converter tube of a streak camera. A solid state switch is used in a manner which eliminates the need for a pulsed multi-kilovolt bias voltage and the use of cryogenics. High voltage direct current in the multi-kilovolt range is applied to a charged circuit which may include a high voltage capacitor or use the capacitance presented by the deflection plates of the tube. The switch is laser activated and becomes photo-conducting. The charge in the capacitor passes through a charging resistor which controls the sweep rate to the deflection plates. After the activating laser pulse, the switch returns rapidly to a nonconducting state, during the recombination time of the switch material. The photo-electron beam is swept linearly over a substantial portion of the recombination time from off the image forming phosphor screen to off screen on the other side thereof. A resistor connected to the deflection plates provides a time constant long compared to the transient event lifetime, which may be the fluorescence decay time of the system under study, and the beam remains off the phosphor screen for a very large time compared to the fluorescence decay time. In a second configuration when the deflection plates are used as the charged source, the laser-activated switch is connected between a deflection plate and a point of reference potential (ground) through the charging resistor.

Sweep drive circuit for a streak camera

A body of semiconductor material is biased with multi-kilovolt voltage to establish an electric field approaching the dielectric breakdown field for the semiconductor material. Low level optical energy, such as a laser pulse in the nano-joule range produces free carriers in the semiconductor which multiply in the presence of the electric field to produce avalanche conduction through the semiconductor body thereby switching the multi-kilovolt voltage in precise timed (picosecond) relationship with the application of the optical energy and with high switching or turn on sensitivity.

Growing community of inventors

Rochester, NY, United States of America

Gerard A Mourou