Michael J. Thorpe

Quantum Coherence between Two Atoms beyond Q=10{sup 15}

We place two atoms in quantum superposition states and observe coherent phase evolution for 3.4×10(15) cycles. Correlation signals from the two atoms yield information about their relative phase even after the probe radiation has decohered. This technique allowed a frequency comparison of two (27)Al(+) ions with fractional uncertainty 3.7(-0.8)(+1.0)×10(-16)/√[τ/s]. Two measures of the Q factor are reported: The Q factor derived from quantum coherence is 3.4(-1.1)(+2.4)×10(16), and the spectroscopic Q factor for a Ramsey time of 3 s is 6.7×10(15). We demonstrate a method to detect the individual quantum states of two Al(+) ions in a Mg(+)-Al(+)-Al(+) linear ion chain without spatially resolving the ions.

Broadband Cavity Ringdown Spectroscopy for Sensitive and Rapid Molecular Detection

We demonstrate highly efficient cavity ringdown spectroscopy in which a broad-bandwidth optical frequency comb is coherently coupled to a high-finesse optical cavity that acts as the sample chamber. 125,000 optical comb components, each coupled into a specific longitudinal cavity mode, undergo ringdown decays when the cavity input is shut off. Sensitive intracavity absorption information is simultaneously available across 100 nanometers in the visible and near-infrared spectral regions. Real-time, quantitative measurements were made of the trace presence, the transition strengths and linewidths, and the population redistributions due to collisions and the temperature changes for molecules such as C2H2, O2, H2O, and NH3.

Phase-stabilized, 1.5 W frequency comb at 2.8–4.8 μm

We present a high-power optical-parametric-oscillator (OPO) based frequency comb in the mid-IR wavelength region. The system employs periodically poled lithium niobate and is singly resonant for the signal. It is synchronously pumped by a 10 W femtosecond Yb:fiber laser centered at 1.07 microm. The idler (signal) wavelength can be continuously tuned from 2.8 to 4.8 microm (1.76 to 1.37 microm) with a simultaneous bandwidth as high as 0.3 microm and a maximum average idler output power of 1.50 W. We also demonstrate the performance of the stabilized comb by recording the heterodyne beat with a narrow-linewidth diode laser. This OPO is an ideal source for frequency comb spectroscopy in the mid-IR.

Cavity-enhanced direct frequency comb spectroscopy

Cavity-enhanced direct frequency comb spectroscopy combines broad spectral bandwidth, high spectral resolution, precise frequency calibration, and ultrahigh detection sensitivity, all in one experimental platform based on an optical frequency comb interacting with a high-finesse optical cavity. Precise control of the optical frequency comb allows highly efficient, coherent coupling of individual comb components with corresponding resonant modes of the high-finesse cavity. The long cavity lifetime dramatically enhances the effective interaction between the light field and intracavity matter, increasing the sensitivity for measurement of optical losses by a factor that is on the order of the cavity finesse. The use of low-dispersion mirrors permits almost the entire spectral bandwidth of the frequency comb to be employed for detection, covering a range of ∼ 10% of the actual optical frequency. The light transmitted from the cavity is spectrally resolved to provide a multitude of detection channels with spectral resolutions ranging from several gigahertz to hundreds of kilohertz. In this review we will discuss the principle of cavity-enhanced direct frequency comb spectroscopy and the various implementations of such systems. In particular, we discuss several types of UV, optical, and IR frequency comb sources and optical cavity designs that can be used for specific spectroscopic applications. We present several cavity-comb coupling methods to take advantage of the broad spectral bandwidth and narrow spectral components of a frequency comb. Finally, we present a series of experimental measurements on trace gas detections, human breath analysis, and characterization of cold molecular beams. These results demonstrate clearly that the wide bandwidth and ultrasensitive nature of the femtosecond enhancement cavity enables powerful real-time detection and identification of many molecular species in a massively parallel fashion.

Cavity-enhanced optical frequency comb spectroscopy: application to human breath analysis

Broad-bandwidth, high-spectral-resolution optical detection of human breath has identified multiple important biomarkers correlated with specific diseases and metabolic processes. This optical-frequency-comb-based breath analysis system comes with excellent performance in all criteria: high detection sensitivity, ability to identify and distinguish a large number of analytes, and simultaneous, real-time information processing. We demonstrate a minimum detectable absorption of 8 x 10(-10)cm(-1), a spectral resolution of 800 MHz, and 200 nm of spectral coverage from 1.5 to 1.7 microm where strong and unique molecular fingerprints exist for many biomarkers. We present a series of breath measurements including stable isotope ratios of CO(2), breath concentrations of CO, and the presence of trace concentrations of NH(3) in high concentrations of H(2)O.

Human breath analysis via cavity-enhanced optical frequency comb spectroscopy

Broad-bandwidth, high-spectral-resolution optical detection of human breath has identified multiple important biomarkers correlated with specific diseases and metabolic processes. This optical-frequency-comb-based breath analysis system comes with excellent performance in all criteria: high detection sensitivity, ability to identify and distinguish a large number of analytes, and simultaneous, real-time information processing. We demonstrate a minimum detectable absorption of 8 x 10(-10)cm(-1), a spectral resolution of 800 MHz, and 200 nm of spectral coverage from 1.5 to 1.7 microm where strong and unique molecular fingerprints exist for many biomarkers. We present a series of breath measurements including stable isotope ratios of CO(2), breath concentrations of CO, and the presence of trace concentrations of NH(3) in high concentrations of H(2)O.

Cavity-Enhanced Direct Frequency Comb Spectroscopy: Technology and Applications

Cavity-enhanced direct frequency comb spectroscopy combines broad bandwidth, high spectral resolution, and ultrahigh detection sensitivity in one experimental platform based on an optical frequency comb efficiently coupled to a high-finesse cavity. The effective interaction length between light and matter is increased by the cavity, massively enhancing the sensitivity for measurement of optical losses. Individual comb components act as independent detection channels across a broad spectral window, providing rapid parallel processing. In this review we discuss the principles, the technology, and the first applications that demonstrate the enormous potential of this spectroscopic method. In particular, we describe various frequency comb sources, techniques for efficient coupling between comb and cavity, and detection schemes that utilize the techniques high-resolution, wide-bandwidth, and fast data-acquisition capabilities. We discuss a range of applications, including breath analysis for medical diagnosis, trace-impurity detection in specialty gases, and characterization of a supersonic jet of cold molecules.

Ultralow phase noise microwave generation with an Er:fiber-based optical frequency divider

We present an optical frequency divider based on a 200 MHz repetition rate Er:fiber mode-locked laser that, when locked to a stable optical frequency reference, generates microwave signals with absolute phase noise that is equal to or better than cryogenic microwave oscillators. At 1 Hz offset from a 10 GHz carrier, the phase noise is below -100 dBc/Hz, limited by the optical reference. For offset frequencies >10 kHz, the phase noise is shot noise limited at -145 dBc/Hz. An analysis of the contribution of the residual noise from the Er:fiber optical frequency divider is also presented.

Cavity-ringdown molecular spectroscopy based on an optical frequency comb at 1.45-1.65 μm

Optical frequency comb-based cavity-ringdown spectroscopy has recently enabled high-sensitivity absorption detection of molecules over a broad spectral range. We demonstrate an improved system based on a mode-locked erbium-doped fiber laser source centered at 1.5 microm, resulting in a spectrometer that is inexpensive, simple, and robust. It provides a very large spectral bandwidth (1.45-1.65 microm) for investigation of a wide variety of molecular absorptions. Strong molecular absorptions at 1.5 mum allow for detection at sensitivities approaching the 1 part in 10(9) volume level. We provide a detailed description of our spectrometer and present measurements of the rovibrational spectra for CO, NH3, and C2H2 with an absorption sensitivity of 2 x 10(-8) cm(-1)Hz(-1/2) per detection channel.

Direct frequency comb spectroscopy

We summarize recent developments in direct frequency-comb spectroscopy that allowed high-resolution, broad-bandwidth measurements of multiple atomic and molecular resonances using only a phase-stabilized femtosecond laser, opening the way for merging precision spectroscopy with coherent control.