Bruce A. Richman

Applications of cavity ring-down spectroscopy to high precision isotope ratio measurement of 13C/12C in carbon dioxide

Recent measurements of carbon isotopes in carbon dioxide using near-infrared, diode-laser-based cavity ring-down spectroscopy (CRDS) are presented. The CRDS system achieved good precision, often better than 0.2‰, for 4% CO2 concentrations, and also achieved 0.15–0.25‰ precision in a 78 min measurement time with cryotrap-based pre-concentration of ambient CO2 concentrations (360 ppmv). These results were obtained with a CRDS system possessing a data rate of 40 ring-downs per second and a loss measurement of 4.0u2009×u200910−11 cm−1 Hz−1/2. Subsequently, the measurement time has been reduced to under 10 min. This standard of performance would enable a variety of high concentration (3–10%) isotopic measurements, such as medical human breath analysis or animal breath experiments. The extension of this ring-down to the 2 μm region would enable isotopic analysis at ambient concentrations, which, combined with the small size, robust design, and potential for frequent measurements at a remote site, make CRDS technology attractive for remote atmospheric measurement applications.

A review of the Stanford Mark III infrared FEL program

Abstract The performance of the Mark III infrared FEL with a new microwave gun will be reviewed. Operation of the accelerator is now close to design values. The Mark III has provided over 2000 hours of laser time to experiments in FEL physics, materials science and medical physics. Highlights of the experimental program will be presented and the new facility at Duke will be described.

A high quality permanent-magnet wiggler for the Rocketdyne/Stanford infrared free electron laser

Abstract A high quality, variable gap, variable taper, permanent-magnet wiggler has been built for infrared free electron laser (FEL) experiments to be performed at the Stanford Photon Research Laboratory. The design and characterization procedure used to assemble the wiggler is discussed. A simulated annealing code was used to minimize field errors arising from variations in the individual magnets. The computed electron trajectories associated with the measured magnetic fields are presented for a range of different operating points of the wiggler. These plots indicate a very high quality field over a large range of different wiggler operating regimes. Resultant trajectory wander over the 2 m long wiggler for a 40 MeV electron at a wiggler gap corresponding to 3.3 kG was calculated to be less than 25 μm. The ability to control trajectory wander and optical phase slip using the simulated annealing code suggests future extensions to extremely long wigglers.

First operation of the Rocketdyne/Stanford free electron laser

Abstract A near infrared free electron laser (FEL) has been built and installed by Rocketdyne in the Stanford Photon Research Laboratory. The Rocketdyne/Stanford FEL utilizes a very high quality, 2 m long, permanent magnet planar wiggler whose gap may be continuously tuned, and magnetic field axially tapered by varying the gap at one end relative to the other. The laser is operated with an e-beam supplied by the Stanford Mark-III accelerator. A stable resonator with a broadband, dielectric coated element permits transmissive outcoupling over the 2.7–3.7 μm wavelength range. Results from initial operation of this laser are presented.

Temporal characterization of the Stanford mid-IR FEL micropulses by “FROG”

Abstract We present the results of frequency resolved optical gating (FROG) measurements on the Stanford mid-IR FEL. FROG is a recently developed method to acquire complete and uniquely invertible amplitude and phase temporal dependence of optical pulses. Unambiguous phase and amplitude profiles are recovered from the data. Experimental examples of near-transform-limit, chirped, and atmospheric absorption perturbed pulses are discussed.

Short wavelength (5.36–1.85 μm) nonlinear spectroscopy of coupled InGaAs/AlAs intersubband quantum wells

We report short wavelength second‐harmonic generation (SHG) spectroscopy of asymmetric coupled In0.6Ga0.4As/AlAs quantum wells (QWs). The QW is designed to show maximum second‐order nonlinear susceptibility χ(2) for SHG of 4 and 2 μm wavelengths by single and double resonance effects, respectively. SHG spectroscopy across the midinfrared is measured using both a CO2 and a free electron laser as pumps. The χ(2) of the QW is extracted from interference of the second‐harmonic fields from the QW and GaAs substrate, determined by the azimuthal dependence of the SHG power. We measure χ(2) of the QW for harmonic wavelengths between 5.36 and 1.85 μm. This is the shortest wavelength SHG to date by any QW intersubband interaction. Good agreement of experiment with theory for the dispersion of χ(2) for both singly and doubly resonant conversion is observed throughout the midinfrared.

FEL center user diagnostics and control

Abstract In the past year, the Stanford Picosecond FEL Center has produced more than two thousand hours of beam time dedicated to user experiments. To assure reliable beam delivery and to maximize productivity of our users we have developed a sophisticated system of diagnostics and control. An integrated display is now available in all experimental areas which provides continuously updated measurements of beam spectrum, micropulse duration, power, position, and pointing — all of which may be saved to document beam conditions during an experiment. The beam is actively wavelength and amplitude stabilized to better than 0.01% and 2%, respectively. Direct wavelength control is available to users in every experimental area, allowing changes of wavelength as large as a few percent. Larger wavelength shifts, and adjustments in macropulse or micropulse width or timing, are readily available with operator assistance.

THE DETERMINATION OF AN ELECTRON BEAM'S LONGITUDINAL PHASE SPACE DISTRIBUTION THROUGH THE USE OF PHASE-ENERGY MEASUREMENTS

Abstract We have been able to measure the longitudinal phase space distribution of the Stanford Superconducting Accelerators (SCA) electron beam by applying tomographic techniques to energy spectra taken as a function of the relative phase between the beam and the accelerating field. The temporal profile of the beam obtained by projecting the distribution onto the time axis is compared with that obtained from interferometric transition radiation measurements.

Initial results from the free-electron-laser master oscillator/power amplifier experiment

Abstract We describe the first master oscillator/power amplifier experiment in which both the master oscillator and power amplifier are free-electron-laser devices driven by time-sharing an electron beam from a single radiofrequency linear accelerator. The optimized, small-signal gain spectrum realized in the untapered power amplifier is presented. Up to 60% gain was observed at 3 μ with an estimated peak current of 35 A. Additional Q -switched experiments are also discussed.

FEL cavity length measurement with an external laser

Abstract The nature of linac-driven free electron laser (FEL) operation requires that the length of the laser optical resonator be matched to the repetition rate of the accelerator in order for the returning optical pulse to overlap the newly arriving electron bunch. The range of cavity lengths over which oscillation will occur is limited in typical FEL designs to a few tens of microns, while the total resonator length is often many meters. The task of correctly setting the length of a new FEL resonator is therefore quite difficult, particularly if the cavity geometry contains more than a single line segment. We have demonstrated that such an absolute length measurement with micron accuracy over many meters in any cavity configuration is possible by establishing resonances in the cavity with an external laser of known repetition frequency. This technique was successfully applied in the development of two new FELs at the Stanford FEL Center. We will discuss the advantages of this method over conventional measurement schemes.