C.V. Bennett

UPCONVERSION TIME MICROSCOPE DEMONSTRATING 103 MAGNIFICATION OF FEMTOSECOND WAVEFORMS

We present the operational principles and results of a temporal imaging system, configured as a time microscope, that achieves 103 x magnification of waveforms with 300-fs resolution and a 5.7-ps field of view. The quadratic-phase time-lens element is realized by upconversion of the dispersed input waveform with a linearly chirped 5-THz bandwidth pump. The system allows expansion of ultrafast optical waveforms to a time scale that is directly accessible with slower conventional technology, in real time, on a single-shot basis.

Principles of parametric temporal imaging. I. System configurations

The recently developed process of temporal imaging expands or compresses time waveforms while preserving the shapes of their envelope profiles. A key element in a temporal imaging system is a time lens which imparts a quadratic phase modulation to the waveform being imaged. Several methods, such as electrooptic modulation, can be used to produce the phase modulation. In this paper, we concentrate on the parametric mixing of a signal waveform with a linearly chirped optical pump as the time lens mechanism. We analyze all single-lens system configurations including sum- and difference-frequency mixing schemes with positive and negative group velocity dispersions using temporal ray diagrams as an aid in understanding their operation.

TEMPORAL MAGNIFICATION AND REVERSAL OF 100 GB/S OPTICAL DATA WITH AN UP-CONVERSION TIME MICROSCOPE

We have developed an up‐conversion time microscope capable of expanding ultrafast optical wave forms to a time scale accessible to ordinary sampling oscilloscopes. In this system, a 100 Gb/s optical word is magnified (slowed down) to a rate of 8.55 Gb/s with a time lens placed between two dispersive delay lines. The time lens is a nonlinear crystal which mixes the dispersed data with a linearly chirped pump pulse thus imparting a linear frequency sweep to the unconverted wave form. A second dispersive delay line completes the arrangement and forms the temporal analog of a single lens spatial imaging system resulting in a time reversed wave form with a magnification M=−11.7.

Aberrations in temporal imaging

Recent advances in temporal imaging allow construction of systems that can expand or compress arbitrary waveforms in time, while maintaining the shape of their envelope profile with subpicosecond resolution. The process is analogous to imaging in space, with the quadratic spectral phase introduced by narrow-band dispersion performing the time-domain role of paraxial diffraction and quadratic temporal phase modulation acting as a time lens. Higher order phase terms in the dispersive networks and the time lens modulation introduce aberrations into the system. The effect each aberration has on the final temporal image varies depending on the system configuration and where the source is located in the system. A theoretical and experimental study of aberration effects is presented.

AM and high-harmonic FM laser mode locking

We demonstrate a new technique of active mode locking that combines amplitude-modulated (AM) mode locking at the cavity fundamental repetition rate with frequency-modulated (FM) mode locking at a high harmonic. This method combines the advantages of pulse shortening by high-harmonic mode locking while preserving the higher peak powers available at the fundamental repetition rate. We demonstrate this technique using a Nd:YAG laser that is simultaneously AM mode locked at 80 MHz and FM mode locked at the 22nd harmonic (1.76 GHz). Pulses as short as 16 ps with a peak power of 6.25 kW were measured.

Phase noise reduction and photoelectron acceleration in a high-Q RF gun

The phase noise and jitter characteristics of the laser and RF systems of a high-gradient X-band photoinjector have been measured experimentally. The laser oscillator is a self-mode-locked titanium:sapphire system operating at the 108th subharmonic of the RF gun. The X-band signal is produced from the laser by a phase-locked dielectric resonance oscillator and amplified by a pulsed TWT and klystron. A comparison between the klystron and TWT amplifier phase noise and the fields excited in the RF gun demonstrates the filtering effect of the high-Q structure, thus indicating that the RF gun can be used as a master oscillator and could be energized by either an RF oscillator, such as a magnetron, or a compact source, such as a cross-field amplifier. In particular, the RF gun can play the role of a pulsed RF clock to synchronize the photocathode laser system; direct drive of a synchronously mode-locked AlGaAs quantum well laser has been achieved using the X-band gun RF fields. This novel, gigahertz repetition rate, laser system is being developed to replace the more conventional femtosecond Ti:Al/sub 2/O/sub 3/ system. Some advantages include pumping this laser with a stabilized current source instead of a costly, low-efficiency pump laser. Finally, dark current measurements and initial photoelectron measurements are reported.

Bounce-by-Bounce Cavity Ring-Down Spectroscopy : Femtosecond Temporal Imaging**

A time microscope (100× magnification) allows light pulses exiting an optical cavity to be viewed one at a time. A linearly chirped Gaussian pulse is mixed in a nonlinear crystal with the dispersed input waveform; the up-converted light is sent onto an output dispersive network. The resulting temporal image is recorded both with a streak camera and with a spectrometer.

Subpicosecond single-shot waveform measurement using temporal imaging

The recording of single-transient phenomena with a very large time-bandwidth product is a particularly challenging task. While many techniques have been developed to record ultrafast optical signals with sub 100 fs detail they often have practical limitations on the total amount of time that can be recorded or rely on sampling of a repetitive waveform. A waveform manipulation technique known as temporal imaging is being developed to overcome some of these limitations. Temporal imaging can expand an arbitrary optical waveform in time while maintaining the shape of the unknown envelope profile, thus allowing it to be recorded by a slower technology with a resolution improved by the magnification of the imaging system. We present here the first single-shot temporal images produced with an upconversion temporal imaging system and recorded with a streak camera.

Space-time duality and temporal imaging

The equations describing the diffraction of beams in space and the dispersion of pulses in dielectrics can be made to appear virtually identical. This suggests that a temporal equivalent to a space lens in combination with dispersion should yield a system for imaging waveforms in the time domain. This paper describes the principles of such an imaging system and suggests some applications for time scale transformation.

Compton backscattering focused x-ray source for advanced biomedical applications

At ultrahigh intensities, where the normalized vector potential of the laser wave exceeds unity, the electron axial velocity modulation due to radiation pressure yields nonlinear Compton backscattered spectra. For applications requiring a narrow Doppler upshifted linewidth, such as the gamma-gamma collider or focused x-ray generation, this can pose a serious problem. It is shown that temporal laser pulse shaping using spectral filtering at the Fourier plane of a chirped pulse laser amplifier can alleviate this problem, and that this technique can be scaled to the required multi-TW range. Compton backscattered spectra are derived in three cases: hyperbolic secant, hybrid pulses (hyperbolic secant trnasient and flat-top), and square optical pulses similar to those experimentally obtained by Weiner et al. It is found that the optimum laser pulse shapes correspond to square pulses, yielding a high contrast ratio between the main spectral line and the transient lines. The corresponding spectral filter function is also determined, and its practical implementation in a chirped pulse laser amplifier is addressed.