Doctor of Philosophy / Department of Physics / Brett D. DePaola / Direct frequency comb spectroscopy (DFCS) is currently the highest resolution, absolute frequency spectroscopic technique known. In general, one does DFCS by scanning the repetition rate, f[subscript]r[subscript]e[subscript]p, of a comb laser and measuring fluorescence from the excited states of the specie under study. The technique has already been successfully characterized by a theoretical model that starts with the optical Bloch equations and, with a few simplifying assumptions converts them into linear coupled iterative equations. In the present work we build on that successful model to predict the characteristics of the ion yield from photoionization by the comb laser, as a function of f[subscript]r[subscript]e[subscript]p. We show that the ion spectrum yields the same atomic structure as the fluorescence spectra, but with greater efficiency. Here, we also set up an experiment and test this theory by measuring the ion signal from direct frequency comb spectroscopy. Furthermore, instead of actively controlling the frequency comb parameters, we allow them to drift, passively measuring them and the ion signal simultaneously. The experiments were found to be in agreement with theory, and the passive comb approach was found to be functional, though not as convenient as the conventional active comb.

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Doctor of Philosophy / Department of Physics / Brian R. Washburn / Optical frequency combs produced by mode-locked fiber lasers are useful tools for high precision frequency metrology and molecular spectroscopy in a robust and portable format. We have specifically investigated erbium doped fiber mode-locked lasers that use single-walled carbon nanotubes as a saturable absorber. We have, for the first time, developed and phase- stabilized a carbon nanotube fiber laser (CNFL) frequency comb. The carbon nanotube saturable absorber, which was fabricated using an optically driven deposition method, permits a high repetition frequency (>150 MHz) since an optical nonlinearity of fibers is not used for mode-locking. The CNFL comb combined with a parabolic pulse erbium doped fiber amplifier (EDFA) has shown a compact, robust, and cost-effective supercontinuum source. The amplified pulse from the parabolic pulse EDFA was compressed with a hollow-core photonic bandgap fiber, which produced a wave-breaking-free pulse with an all-fiber set-up. The stabilized comb has demonstrated a fractional instability of 1.2 ×10[superscript]-11 at 1 sec averaging time, the reference-limited instability. We have performed optical frequency metrology with the CNFL comb and have measured an optical frequency, P(13) which is a molecular overtone transition of C2H2. The measured frequency has shown a good agreement with the known value within an uncertainty of 10 kHz. In order to extend the application of the CNFL comb such as multi-heterodyne dual comb spectroscopy, we have investigated the noise of the CNFL comb and particularly, the broad carrier envelope offset frequency (f[subscript]0) linewidth of the CNFL comb. The primary noise source is shown to be white amplitude noise on the oscillator pump laser combined with the sensitivity of the mode-locked laser to pump power fluctuations. The control bandwidth of f[subscipt]0 was limited by the response dynamics of the CNFL comb. The significant reduction of comb noise has been observed by implementing a phase-lead compensation to extend control bandwidth of the comb and by reducing the pump relative intensity noise simultaneously. Therefore the f[subscipt]0 linewidth has been narrower from 850 kHz to 220 kHz. The integrated phase noise for the f[subscipt]0 lock is 1.6 radians from 100 Hz to 102 kHz.

Phase locking of external-cavity diodes laser¡]ECDL¡^ to the stabilized optical frequency combs of a femtosecond mode-locked laser. Optical frequency combs of a femtosecond mode-locked laser can be the reference standard of dual-wavelength external-cavity diode lasers (ECDLs). Frequency stabilization of two external-cavity diode laser is also demonstrated simultaneously.Suppression of the frequency fluctuation of two ECDLs from hundreds MHz to 200 Hz is demonstrated and characterized. Meanwhile, frequency tunable continuous-wave Tera-Hertz (cw THz) wave is generated and observed by photomixing of the output of two frequency stabilized ECDLS with tunable relative frequency difference on a photoconductive antennas. In our experiment, cw THz wave is demonstrated and with tuning range from 0.200 to 1.240THz and could be attribute as frequency standard of THz.

A collinear fast ion beam laser apparatus was constructed and tested. It will be used on-line to the SLOW RI radioactive beam facility in RIKEN (Japan) and as in the present experiment for precision absolute frequency measurements of astrophysically important reference lines. In the current work we conducted absolute measurements of spectral lines of Ar ions using parallel and antiparallel geometries. To provide a reference for the laser wavelength iodine saturation spectroscopy was used. The precision of this reference was enhanced by simultaneously observing the beat node between the spectroscopy laser and the corresponding mode of a femtosecond laser frequency comb. When performing collinear and anticollinear measurements simultaneously for the laser induced fluorescence, the exact relativistic formula for the transition frequency v0 = pvcoll � vanticoll can be applied. In this geometry ion source instabilities due to pressure and anode voltage fluctuation are minimized. The procedure of fluorescence lineshapes fitting is discussed and the errors in the measurements are estimated. The result is v0 = 485, 573, 619.7 � 0.3MHz corresponding to (delta v)/v = 6 � 10?10 and is an improvement of two orders of magnitude over the NIST published value.

In this work, we theoretically and numerically investigate nonclassical phonon states created in the collective vibration of a Raman medium by the generation of a dual-pumped Raman optical frequency comb in an optical cavity. This frequency comb is generated by cascaded Raman scattering driven by two phase-locked pump lasers that are separated in frequency by three times the Raman phonon frequency. We characterize the variety of conditioned phonon states that are created when the number of photons in all optical frequency modes except the pump modes are measured. Almost all of these conditioned phonon states are extremely well approximated as three-phonon-squeezed states or Schrödinger-cat states, depending on the outcomes of the photon number measurements. We show how the combinations of first-, second-, and third-order Raman scattering that correspond to each set of measured photon numbers determine the fidelity of the conditioned phonon state with model three-phonon-squeezed states and Schrödinger-cat states. All of the conditioned phonon states demonstrate preferential growth of the phonon mode along three directions in phase space. That is, there are three preferred phase values that the phonon state takes on as a result of Raman scattering. We show that the combination of Raman processes that produces a given set of measured photon numbers always produces phonons in multiples of three. In the quantum number-state representation, these multiples of three are responsible for the threefold phase-space symmetry seen in the conditioned phonon states. With a semiclassical model, we show how this three-phase preference can also be understood in light of phase correlations that are known to spontaneously arise in single-pumped Raman frequency combs. Additionally, our semiclassical model predicts that the optical modes also grow preferentially along three phases, suggesting that the dual-pumped Raman optical frequency comb is partially phase-stabilized.

Extreme ultraviolet (XUV) frequency combs exhibit promise for enabling high-precision spectroscopic measurements of myriad chemical species for the first time. Coherent XUV radiation can be generated through high harmonic generation (HHG) in femtosecond enhancement cavities. HHG efficiency is limited by nonlinear phase shifts induced by residual intracavity plasma. The goal of this work is to gain insight regarding plasma dynamics in order to allay the detrimental effects of plasma interactions. Our approach is to conduct simulations of cavity pump-probe experiments by probing with higher-order transverse modes. We propose methods for estimating spatial plasma profiles, gas jet velocities, and the plasma recombination coefficient based on measurements of plasma-induced phase shifts. Beam distortion due to plasma interaction is analyzed and used as another reference for plasma dynamics.

Dual-comb spectroscopy (DCS) has widespread applications. It has become a more prominent spectroscopic tool because it has broad spectral coverage with high frequency resolution. We demonstrate the broadband and high resolution of DCS to probe transient events, showing the rst use of DCS of laser-induced plasmas (LIPs). Our measurements span absorption features 7 THz wide, simultaneously detecting Rb D2, K D1 and D2 absorption lines with the ability to resolve the isotope ratios in the Rb D2 line. This technique is more broadband and faster than tunable laser absorption spectroscopy because it eliminates the requirement to scan across transitions. Additionally, DCS makes higher resolution measurements than laser-induced break- down spectroscopy. Our ultimate goal is to use DCS as a technique to ascertain the chemical composition of unknown samples. Our rst demonstration of this technique illustrates that DCS makes broadband, high-resolution measurements with the ability to measure isotope ratios, which is necessary for determining sample composition.

In this project laser absorption spectroscopy was performed on a flame in a Fabry-Pérot cavity, using an optical frequency comb. Optical frequency comb spectroscopy is a technique that allows broadband ultra-sensitive detection of molecular species in gas phase. Optical frequency combs are generated by femtosecond mode-locked lasers, which generate short pulses and whose spectrum consists of a comb of sharp laser lines covering a broad spectral range. Doing spectroscopy with optical frequency combs can hence be compared to measurements with thousand of synchronised continuous wave lasers simultaneously, which enables broadband sensitive measurements in short acquisition times. A Vernier spectrometer uses the filtering ability of the cavity to allow sequential transmission of parts of the frequency comb spectrum. Its technical simplicity and robustness make it a good candidate for measuring in turbulent environments. The aim of the project was to implement continuous-filtering Vernier spectroscopy in a setup for measuring absorption spectra in air and in a flame. This was done by using an Er:fiber femtosecond laser emitting in the near-infrared wavelength range and a Fabry-Pérot cavity containing the flame. The cavity, which consists of two highly reflective mirrors, lets the light of the comb interact with the molecules in the flame for each of the many round-trips it perform; thus increasing the sensitivity to absorption. An active locking mechanism was implemented to stabilize the coupling of the optical frequency comb to the cavity. The locking allowed multiple measurements to be averaged which reduced noise. A galvanometer scanner was added to the system which was used to measure a broad part of the comb spectrum. Hot water absorption lines were detected in the swept comb spectrum and a candidate absorption peak for OH absorption was recorded. The spectrometer today has opportunities for improvements. A frequency calibration should be implemented which is essential for making estimates of reactant/product concentrations in combustion processes.