Optical heterodyne measurement of pulsed lasers: Toward high-precision pulsed spectroscopy

Dual color, frequency, pulse duration and shape agile laser system for particle spectroscopy and manipulation

A dual color, frequency and pulse duration agile laser system, capable of delivering laser pulses in arbitrary temporal profiles with ∼1 ns to ∼1 µs pulse duration, chirping rates of ∼27 MHz/ns with an achievable chirping range of several GHz across the pulse duration, and with energies ranging from a few nJ to hundreds of mJ per pulse has been developed. The flexibility and capability of this laser system provide a wide range of laser parameters that can be exploited to optimize operational conditions in various experiments ranging from laser diagnostics to spectroscopy and optical manipulation of matter. The developed system is successfully demonstrated to obtain coherent Rayleigh-Brillouin scattering (CRBS) in both single and dual color configuration, in an effort to expand the non-intrusive accessibility towards lower pressure regime for neutral gas and plasma diagnostics.

Chirp-compensated pulsed titanium-sapphire laser system for precision spectroscopy

Active frequency-chirp control for a narrowband pulsed titanium-sapphire laser oscillator-amplifier laser system is demonstrated using an intra-cavity electro-optic modulator, resulting in improved spectral resolution and stability. Beat-note measurements of its output to a continuous-wave laser locked to an optical frequency comb yields an Allan deviation of 5×10^-11 (at 10 s). Correction of residual chirp from a comparison with the fourth-harmonic upconverted pulsed output to a molecular H₂ two-photon resonance delivers a value for the uncertainty contribution due to frequency chirp below the 1.5×10^-10 level.

Precision measurement of the fundamental vibrational frequencies of tritium-bearing hydrogen molecules: T2, DT, HT

High-resolution coherent Raman spectroscopic measurements of all three tritium-containing molecular hydrogen isotopologues T2, DT and HT were performed to determine the ground electronic state fundamental Q-branch (v = 0 → 1, ΔJ = 0) transition frequencies at accuracies of 0.0005 cm-1. An over hundred-fold improvement in accuracy over previous experiments allows the comparison with the latest ab initio calculations in the framework of non-adiabatic perturbation theory including nonrelativisitic, relativisitic and QED contributions. Excellent agreement is found between experiment and theory, thus providing a verification of the validity of the NAPT-framework for these tritiated species. While the transition frequencies were corrected for ac-Stark shifts, the contributions of non-resonant background as well as quantum interference effects between resonant features in the nonlinear spectroscopy were quantitatively investigated, also leading to corrections to the transition frequencies. Methods of saturated CARS with the observation of Lamb dips, as well as the use of continuous-wave radiation for the Stokes frequency were explored, that might pave the way for future higher-accuracy CARS measurements.

Relativistic and QED Effects in the Fundamental Vibration of T_{2}

The hydrogen molecule has become a test ground for quantum electrodynamical calculations in molecules. Expanding beyond studies on stable hydrogenic species to the heavier radioactive tritium-bearing molecules, we report on a measurement of the fundamental T_{2} vibrational splitting (v=0→1) for J=0-5 rotational levels. Precision frequency metrology is performed with high-resolution coherent anti-Stokes Raman spectroscopy at an experimental uncertainty of 10-12 MHz, where sub-Doppler saturation features are exploited for the strongest transition. The achieved accuracy corresponds to a 50-fold improvement over a previous measurement, and it allows for the extraction of relativistic and QED contributions to T_{2} transition energies.

High-precision laser spectroscopy of the CO A(1)Π - X(1)Σ(+) (2,0), (3,0), and (4,0) bands

High-precision two-photon Doppler-free frequency measurements have been performed on the CO A(1)Π - X(1)Σ(+) fourth-positive system (2,0), (3,0), and (4,0) bands. Absolute frequencies of forty-three transitions, for rotational quantum numbers up to J = 5, have been determined at an accuracy of 1.6 × 10(-3) cm(-1), using advanced techniques of two-color 2  +  1' resonance-enhanced multi-photon ionization, Sagnac interferometry, frequency-chirp analysis on the laser pulses, and correction for AC-Stark shifts. The accurate transition frequencies of the CO A(1)Π - X(1)Σ(+) system are of relevance for comparison with astronomical data in the search for possible drifts of fundamental constants in the early universe. The present accuracies in laboratory wavelengths of Δλ/λ = 2 × 10(-8) may be considered exact for the purpose of such comparisons.

Optimizing three-frequency Na, Fe, and He lidars for measurements of wind, temperature, and species density and the vertical fluxes of heat and constituents

The measurement accuracies of three-frequency resonance fluorescence Doppler lidars are limited by photon noise and uncertainties in the laser frequency and line width. We analyze the performance of Na, Fe, and He lidars using a new technique, which incorporates precise information about the absorption spectrum of the species and the pulse spectrum of the lasers. We derive the measurement errors associated with photon noise, laser frequency errors, and laser line width errors. Optimizing the lidar design, based upon the measurement requirements, can improve system performance by reducing the required integration times, enabling measurements to be made in less time or at higher altitudes where the densities and signal levels are smaller. The optimum frequency shift for observing heat and constituent transport velocities is 689 MHz (580 MHz) at night (day) for Na lidars and 774 MHz (597 MHz) for Fe lidars. The optimum frequency shift for observing winds, temperature, and He densities is 3.66 GHz (3.16 GHz) at night (day) for He lidars.

Single-shot coherent Rayleigh-Brillouin scattering using a chirped optical lattice

We present a method for obtaining coherent Rayleigh-Brillouin scattering (CRBS) spectra on timescales of hundreds of nanoseconds using rapidly chirped, pulsed, optical lattices. This enables us to transfer the spectral profile to a temporal profile which can be easily recorded on a single shot of an oscilloscope. These spectra are demonstrated to have sufficient signal-to-noise ratio to study CRBS models over a wide range of gas densities.

Injection-seeded optical parametric amplifier for generating chirped nanosecond pulses

We constructed an optical parametric amplifier with BiBO crystals, which was injection seeded by a phase-modulated cw beam in the 1,040-1,070 nm region. Two-stage pre-amplification by Yb-doped fibers were implemented for stable injection to the OPA. The frequency chirp in the OPA pulse was actively controlled by adjusting the RF wave for the phase modulation and its synchronization to the OPA firing. Down/up chirps with up to 500 MHz shift were demonstrated. The output pulse energy was ~40 mJ, which is sufficient for future application of frequency conversion and coherent population transfer.

Mid-altitude wind measurements with mobile Rayleigh Doppler lidar incorporating system-level optical frequency control method

A mobile Rayleigh Doppler lidar based on double-edge technique is developed for mid-altitude wind observation. To reduce the systematic error, a system-level optical frequency control method is proposed and demonstrated. The emission of the seed laser at 1064 nm is used to synchronize the FPI in the optical frequency domain. A servo loop stabilizing the frequency of the seed laser is formed by measuring the absolute frequency of the second harmonic against an iodine absorption line. And, the third harmonic is used for Rayleigh lidar detection. The frequency stability is 1.6 MHz at 1064 nm over 2 minutes. A locking accuracy of 0.3 MHz at 1064 nm is realized. In comparison experiments, wind profiles from the lidar, radiosonde and European Center for Medium range Weather Forecast (ECMWF) analysis show good agreement from 8 km to 25 km. Wind observation over two months is carried out in Urumqi (42.1°N, 87.1°E), northwest of China, demonstrating the stability and robustness of the system. For the first time, quasi-zero wind layer and dynamic evolution of high-altitude tropospheric jet are observed based on Rayleigh Doppler lidar in Asia.

Chirp-corrected, nanosecond Ti:sapphire laser with 6 MHz linewidth for spectroscopy of antiprotonic helium

A nanosecond titanium sapphire laser with spectral linewidth Gamma(pl)~6 MHz and pulse energy of 50-100 mJ was demonstrated by using an intracavity electro-optic modulator to correct the frequency chirp in the output beam. The laser was referenced against a femtosecond frequency comb and used to measure the 6s-8s (F=4) two-photon transition frequency of Cs with a precision of 1.4 parts in 10(9).

A narrow-band injection-seeded pulsed titanium:sapphire oscillator-amplifier system with on-line chirp analysis for high-resolution spectroscopy

A narrow-band tunable injection-seeded pulsed titanium:sapphire laser system has been developed for application in high-resolution spectroscopic studies at the fundamental wavelengths in the near infrared as well as in the ultraviolet, deep ultraviolet, and extreme ultraviolet after upconversion. Special focus is on the quantitative assessment of the frequency characteristics of the oscillator-amplifier system on a pulse-to-pulse basis. Frequency offsets between continuous-wave seed light and the pulsed output are measured as well as linear chirps attributed mainly to mode pulling effects in the oscillator cavity. Operational conditions of the laser are found in which these offset and chirp effects are minimal. Absolute frequency calibration at the megahertz level of accuracy is demonstrated on various atomic and molecular resonance lines.

Determination of the antiproton-to-electron mass ratio by precision laser spectroscopy of pHe+

A femtosecond optical frequency comb and continuous-wave pulse-amplified laser were used to measure 12 transition frequencies of antiprotonic helium to fractional precisions of (9-16)x10(-9). One of these is between two states having microsecond-scale lifetimes hitherto unaccessible to our precision laser spectroscopy method. Comparisons with three-body QED calculations yielded an antiproton-to-electron mass ratio of Mp/me=1836.152674(5).

Transition from single-mode to multimode operation of an injection-seeded pulsed optical parametric oscillator

Optical-heterodyne measurements are made on ~842-nm signal output of an injection-seeded optical parametric oscillator (OPO) based on periodically poled KTiOPO4 pumped at 532 nm by long (~27-ns) pulses from a Nd:YAG laser. At low pump energies (</= 2.5 times the free-running threshold), the narrowband tunable OPO output is single-longitudinal-mode (SLM) and frequency chirp can be <10 MHz, much less than the transform-limited optical bandwidth (~17.5 MHz). We explore the transition from SLM operation to multimode operation as pump energy or phase mismatch are increased, causing unseeded cavity modes to build up later in the pulse.

Pulsed injection-seeded optical parametric oscillator with low frequency chirp for high-resolution spectroscopy

An injection-seeded optical parametric oscillator (OPO), based on periodically poled KTiOPO4 and pumped by a frequency-doubled, nanosecond-pulsed Nd:YAG laser, generates continuously tunable, single-longitudinal-mode, pulsed output at approximately 842 nm for high-resolution spectroscopy. Optical-heterodyne measurements show that the OPO frequency chirp increases linearly with detuning from the free-running (unseeded) OPO frequency and can be maintained as low as 10 MHz. Other factors affecting chirp are identified.

Calibration of a computer-controlled precision wavemeter for use with pulsed lasers

The design of a pulsed wavemeter to monitor the high-precision tuning of pulsed (as well as cw) laser sources is presented. This device is developed from a combination of silver-coated Fabry-Perot etalons with various plate spacings. These etalons provide stepwise refinement of the wavelength to be measured. The wavemeter is controlled by a computer through a CAMAC interface, which measures the absolute wavelength in the visible with an accuracy of 2 parts in 10(8). The time required for data acquisition and computation to measure the refined wavelength with a single 2-MHz CPU is less than 100 ms. We describe the calibration of the instrument over the wavelength range 400-850 nm. We obtain the required calibration lines by locking lasers on hyperfine transitions of iodine, uranium, rubidium, and cesium. Methods to reduce the number of calibration lines required for calibration of the system are described. The expected wavelength-dependent phase shift of the silver coatings is compared with that measured for the etalon following calibration. The differences are larger than expected because of either optical aberations or the use of centroids to measure the fringe position.

Generation of vacuum ultraviolet radiation for precision laser spectroscopy

We report on the generation of tunable nanosecond pulsed VUV radiation near 120 nm using difference-frequency mixing in H(2). Our scheme uses two dye lasers, one fixed at 606 nm and the other tunable in the red. These convenient wavelengths simplify the metrology needed for accurate VUV laser spectroscopy. Efficient VUV generation is attained with modest Nd:YAG pump laser energies (approximately 160 mJ at 532 nm), making the scheme attractive even when narrow bandwidths are not essential.

Chirp-free nanosecond laser amplifier for precision spectroscopy

We describe a simple method for greatly reducing optical phase perturbations in a nanosecond pulsed dye amplifier. The laser dye mix is tailored to produce a susceptibility near zero at the operating wavelength. Frequency shifts are reduced to less than 3 MHz, and frequency chirping to less than 10 MHz, without significant loss of amplified power. This technique has been used to improve the accuracy of precision far-UV wavelength measurements in H(2) to ~7 parts in 10(9).

How accurate is optical second-harmonic generation?

We have checked whether the frequency ratio between fundamental and second-harmonic optical frequencies is exactly 1:2 by measuring the frequency difference between the second harmonics of two diode lasers and comparing it with the frequency difference of the fundamentals. We have improved the 30-year-old experimental limit on the accuracy of the 1:2 frequency ratio by 7 orders of magnitude for frequency-dependent shifts to 2.3 parts in 10(13) for a KNbO(3) crystal.

Compact, high-gain pulsed dye amplifier for weak cw sources

A compact two-cuvette dye amplifier capable of pulse amplifying weak cw visible sources by factors in excess of 10(9) has been demonstrated. Seeded with a 300-µW single-frequency He-Ne laser and without the need for a Faraday isolator, the preamplifier yields >40-µJ pulses of 4.5-ns duration and 1.4× transform-limited linewidths around 140 MHz. Subsequent power amplification yields 4-ns FWHM, 2-mJ pulses with excellent pulse-to-pulse stability and linewidths around 170 MHz. Scaling of the operational envelope and extension to pulse amplification of cw single-mode tunable diode lasers is discussed.