Single-modulator, direct frequency comb spectroscopy via serrodyne modulation.

Traditional electro-optic frequency comb spectrometers rely upon the use of an acousto-optic modulator (AOM) to provide a differential frequency shift between probe and local oscillator (LO) legs of the interferometer. Here we show that these modulators can be replaced by an electro-optic phase modulator (EOM) which is driven by a sawtooth waveform to induce serrodyne modulation. This approach enables direct frequency comb spectroscopy to be performed with a single dual-drive Mach-Zehnder modulator (DD-MZM), allowing for lower differential phase noise. Further, this method allows for simpler production of integrated photonic comb spectrometers on the chip scale.

High dynamic range electro-optic dual-comb interrogation of optomechanical sensors.

An interleaved, chirped electro-optic dual comb system is demonstrated for rapid, high dynamic range measurements of cavity optomechanical sensors. This approach allows for the cavity displacements to be interrogated at measurement times as fast as 10 µs over ranges far larger than can be achieved with alternative methods. While the performance of this novel, to the best of our knowledge, readout approach is evaluated with an optomechanical accelerometer, this method has a wide range of applications including temperature, pressure, and humidity sensing as well as acoustics and molecular spectroscopy.

Electromagnetically induced transparency in vacuum and buffer gas potassium cells probed via electro-optic frequency combs.

Electromagnetically induced transparency (EIT) in K39 and K41 was probed using electro-optic frequency combs generated by applying chirped waveforms to a phase modulator. The carrier tone of the frequency comb served as the pump beam and induced the necessary optical cycling. Comb tooth spacings as narrow as 20 kHz were used to probe potassium in both buffer gas and evacuated cells at elevated temperatures. Atomic absorption features as narrow as 33(5) kHz were observed, allowing for the K39 lower-state hyperfine splitting to be optically measured with a fit uncertainty of 2 kHz. Due to the ultranarrow width of the EIT features, long-lived optical free induction decays were also observed which allowed for background-free detection.

Precise methane absorption measurements in the 1.64 µm spectral region for the MERLIN mission.

In this article we describe a high-precision laboratory measurement targeting the R(6) manifold of the 2ν3 band of 12CH4. Accurate physical models of this absorption spectrum will be required by the Franco-German, Methane Remote Sensing LIDAR (MERLIN) space mission for retrievals of atmospheric methane. The analysis uses the Hartmann-Tran profile for modeling line shape and also includes line-mixing effects. To this end, six high-resolution and high signal-to-noise absorption spectra of air-broadened methane were recorded using a frequency-stabilized cavity ring-down spectroscopy apparatus. Sample conditions corresponded to room temperature and spanned total sample pressures of 40 hPa - 1013 hPa with methane molar fractions between 1 µmol mol-1 and 12 µmol mol-1. All spectroscopic model parameters were simultaneously adjusted in a multispectrum nonlinear least-squares fit to the six measured spectra. Comparison of the fitted model to the measured spectra reveals the ability to calculate the room-temperature, methane absorption coefficient to better than 0.1% at the on-line position of the MERLIN mission. This is the first time that such fidelity has been reached in modeling methane absorption in the investigated spectral region, fulfilling the accuracy requirements of the MERLIN mission. We also found excellent agreement when comparing the present results with measurements obtained over different pressure conditions and using other laboratory techniques. Finally, we also evaluated the impact of these new spectral parameters on atmospheric transmissions spectra calculations.

Ultra-sensitive cavity ring-down spectroscopy in the mid-infrared spectral region.

We describe an ultra-sensitive cavity ring-down spectrometer which operates in the mid-infrared spectral region near 4.5 µm. With this instrument a noise-equivalent absorption coefficient of 2.6×10-11 cm-1 Hz-1/2 was demonstrated with less than 150 nW of optical power incident on the photodetector. Quantum noise was observed in the individual ring-down decay events, leading to quantum-noise-limited short-time performance. We believe that this spectrometer's combination of high sensitivity and robustness make it well suited for measurements of ultra-trace gas species as well as applications in optics and fundamental physics.

Multiplexed sub-Doppler spectroscopy with an optical frequency comb.

An optical frequency comb generated with an electro-optic phase modulator and a chirped radiofrequency waveform is used to perform pump-probe spectroscopy on the D1 and D2 transitions of atomic potassium at 770.1 nm and 766.7 nm, respectively. With a comb tooth spacing of 200 kHz and an optical bandwidth of 2 GHz the hyperfine transitions can be simultaneously observed. Interferograms are recorded in as little as 5 µs (a timescale corresponding to the inverse of the comb tooth spacing). Importantly, the sub-Doppler features can be measured as long as the laser carrier frequency lies within the Doppler profile, thus removing the need for slow scanning or a priori knowledge of the frequencies of the sub-Doppler features. Sub-Doppler optical frequency comb spectroscopy has the potential to dramatically reduce acquisition times and allow for rapid and accurate assignment of complex molecular and atomic spectra which are presently intractable.

Multiheterodyne spectroscopy with optical frequency combs generated from a continuous-wave laser.

Dual-drive Mach-Zehnder modulators were utilized to produce power-leveled optical frequency combs (OFCs) from a continuous-wave laser. The resulting OFCs contained up to 50 unique frequency components and spanned more than 200 GHz. Simple changes to the modulation frequency allowed for agile control of the comb spacing. These OFCs were then utilized for broadband, multiheterodyne measurements of CO2 using both a multipass cell and an optical cavity. This technique allows for robust measurements of trace gas species and alleviates much of the cost and complexity associated with the use of femtosecond OFCs produced with mode-locked pulsed lasers.

Comb-linked, cavity ring-down spectroscopy for measurements of molecular transition frequencies at the kHz-level.

We present a low uncertainty measurement technique for determining molecular transition frequencies. This approach is complementary to sub-Doppler saturation spectroscopies and is expected to enable new frequency measurements for a wide variety of molecular species with uncertainties at the kHz-level. The technique involves measurements of Doppler broadened lines using cavity ring-down spectroscopy whereby the probe laser is actively locked to the ring-down cavity and the spectrum frequencies are linked directly to an optical frequency comb that is referenced to an atomic frequency standard. As a demonstration we have measured the transition frequency of the (30012) ← (00001) P14e line of CO2 near 1.57 µm with a combined standard uncertainty of ~9 kHz. This technique exhibits exceptional promise for measurements of transition frequencies and pressure shifting parameters of many weak absorbers, and indicates the potential for substantially improved measurements when compared to those obtained with conventional spectroscopic methods.

Frequency-stabilized cavity ring-down spectroscopy measurements of line mixing and collision-induced absorption in the O2 A-band.

Frequency-stabilized cavity ring-down spectroscopy measurements were performed in the P-branch of the O(2) A-band [b(1)Σ(g) (+) ← X (3)Σ(g) (-)(0,0)] near atmospheric pressure. Line mixing parameters and collision-induced absorption were quantified and reported. These measurements show qualitative differences with those taken at relatively high pressure (2 MPa-20 MPa). We also assess the implications of these measurements on atmospheric retrievals.

Spectroscopic measurement of the vapour pressure of ice.

We present a laser absorption technique to measure the saturation vapour pressure of hexagonal ice. This method is referenced to the triple-point state of water and uses frequency-stabilized cavity ring-down spectroscopy to probe four rotation-vibration transitions of at wavenumbers near 7180 cm(-1). Laser measurements are made at the output of a temperature-regulated standard humidity generator, which contains ice. The dynamic range of the technique is extended by measuring the relative intensities of three weak/strong transition pairs at fixed ice temperature and humidity concentration. Our results agree with a widely used thermodynamically derived ice vapour pressure correlation over the temperature range 0°C to -70°C to within 0.35 per cent.

Measurement of H2O broadening of O2 A-band transitions and implications for atmospheric remote sensing.

We present laboratory measurements of H(2)O-broadened (16)O(2) A-band (b(1)Σ(g)(+) ← X(3)Σ(g)(-)(0,0)) absorption spectra acquired with a laser-based photoacoustic spectroscopy method. This absorption band is widely used in a variety of high-precision atmospheric remote sensing applications. We report H(2)O broadening parameters for six of the strongest transitions in this band, and we show that these measured values are nominally 1.5-2 times greater than the corresponding air-broadening parameters. Simulations of atmospheric transmission spectra in the O(2) A-band that incorporate our measured H(2)O broadening parameters indicate that H(2)O present at concentrations typically found in the Earth's atmosphere can influence the column-integrated transmission relative to the dry air case. Further, because of spatial and seasonal variations in humidity, failure to account for the enhanced H(2)O pressure broadening effects can lead to concomitant biases in atmospheric O(2) A-band retrievals of quantities such as surface pressure and path length in greenhouse gas retrievals.

Pound-Drever-Hall-locked, frequency-stabilized cavity ring-down spectrometer.

We describe a high sensitivity and high spectral resolution laser absorption spectrometer based upon the frequency-stabilized cavity ring-down spectroscopy (FS-CRDS) technique. We used the Pound-Drever-Hall (PDH) method to lock the probe laser to the high-finesse ring-down cavity. We show that the concomitant narrowing of the probe laser line width leads to dramatically increased ring-down event acquisition rates (up to 14.3 kHz), improved spectrum signal-to-noise ratios for weak O(2) absorption spectra at λ = 687 nm and substantial increase in spectrum acquisition rates compared to implementations of FS-CRDS that do not incorporate high-bandwidth locking techniques. The minimum detectable absorption coefficient and the noise-equivalent absorption coefficient for the spectrometer are about 2×10(-10) cm(-1) and 7.5×10(-11) cm(-1)Hz(-1/2), respectively.

Standard photoacoustic spectrometer: model and validation using O2 A-band spectra.

We model and measure the absolute response of an intensity-modulated photoacoustic spectrometer comprising a 10 cm long resonator and having a Q-factor of approximately 30. We present a detailed theoretical analysis of the system and predict its response as a function of gas properties, resonance frequency, and sample energy transfer relaxation rates. We use a low-power continuous wave laser to probe O(2) A-band absorption transitions using atmospheric, humidified air as the sample gas to calibrate the system. This approach provides a convenient and well-characterized method for calibrating the absolute response of the system provided that water-vapor-mediated relaxation effects are properly taken into account. We show that for photoacoustic spectroscopy (PAS) of the O(2) A-band, the maximum conversion efficiency of absorbed photon energy to acoustic energy is approximately 40% and is limited by finite collision-induced relaxation rates between the two lowest-lying excited electronic states of O(2). PAS also shows great potential for high-resolution line shape measurements: calculated and experimental values for the PAS system response differ by about 1%.

Wavelength-modulation laser hygrometer for ultrasensitive detection of water vapor in semiconductor gases.

Water vapor is measured by use of a near-infrared diode laser and wavelength-modulation absorption spectroscopy. Humidity levels as low as 5 nmol/mol [1 nmol/mol = 1 ppb (1 ppb equals 1 part in 10(9))] of water vapor in air are measured with a sensitivity of better than 0.2 nmol/mol (3varsigma). The sensitivity, linearity, and stability of the technique are determined in experiments conducted at the National Institute of Standards and Technology, Gaithersburg, Maryland, by use of the low frost-point humidity generator over the range from 5 nmol/mol to 2.5 mumol/mol of water vapor in air. The pressure-broadening coefficients for water broadened by helium [0.0199(6) cm(-1) atm(-1) HWHM] and by hydrogen chloride [0.268(6) cm(-1) atm(-1) HWHM] are reported for the water line at 1392.5 nm.

Pulsed, single-mode cavity ringdown spectroscopy.

We discuss the use of single-mode cavity ringdown spectroscopy with pulsed lasers for quantitative gas density and line strength measurements. The single-mode approach to cavity ringdown spectroscopy gives single exponential decay signals without mode beating, which allows measurements with uncertainties near the shot-noise limit. The technique is demonstrated with a 10-cm-long ringdown cavity and a pulsed, frequency-stabilized optical parametric oscillator as the light source. A noise-equivalent absorption coefficient of 5 x 10(-10) cm(-1) Hz(-1/2) is demonstrated, and the relative standard deviation in the ringdown time (sigma(tau)/tau) extracted from a fit to an individual ringdown curve is found to be the same as that for an ensemble of hundreds of independent measurements. Repeated measurement of a line strength is shown to have a standard deviation <0.3%. The effects of normally distributed noise on quantities measured using cavity ringdown spectroscopy are discussed, formulas for the relative standard deviation in the ringdown time are given in the shot- and technical-noise limits, and the noise-equivalent absorption coefficient in these limits are compared for pulsed and continuous-wave light sources.

Laser bandwidth effects in quantitative cavity ring-down spectroscopy.

We have investigated the effects of laser bandwidth on quantitative cavity ring-down spectroscopy using the (r)R transitions of the b(ν = 0)?X(ν = 0) band of molecular oxygen. It is found that failure to account properly for the laser bandwidth leads to systematic errors in the number densities determined from measured ring-down signals. When the frequency-integrated expression for the ring-down signal is fitted and measured laser line shapes are used, excellent agreement between measured and predicted number densities is found.

Far-field scattering of an axisymmetric laser beam of arbitrary profile by an on-axis spherical particle.

Experimental laser beam profiles often deviate somewhat from the ideal Gaussian shape of the TEM(00) laser mode. In order to take these deviations into account when calculating light scattering, we propose a method for approximating the beam shape coefficients in the partial wave expansion of an experimental laser beam. We then compute scattering by a single dielectric spherical particle placed on the beam's axis using this method and compare our results to laboratory data. Our model calculations fit the laboratory data well.

Far-field scattering of a non-Gaussian off-axis axisymmetric laser beam by a spherical particle.

Experimental laser beam profiles often deviate somewhat from the ideal Gaussian shape of the axisymmetric TEM(00) laser mode. To take these deviations into account when calculating light scattering of an off-axis beam by a spherical particle, we use our phase-modeling method to approximate the beam-shape coefficients in the partial wave expansion of an experimental laser beam. We then use these beam-shape coefficients to compute the near-forward direction scattering of the off-axis beam by the particle. Our results are compared with laboratory data, and we give a physical interpretation of the various features observed in the angular scattering patterns.

Forward scattering of a Gaussian beam by a nonabsorbing sphere.

The forward scattering of a Gaussian laser beam by a spherical particle located along the beam axis is analyzed with the generalized Lorenz-Mie theory (GLMT) and with diffraction theory. Forwardscattering and near-forward-scattering profiles from electrodynamically levitated droplets, 51.6 µm in diameter, are also presented and compared with GLMT-based predictions. The total intensity in the forward direction, formed by the superposition of the incident and the scattered fields, is found to correlate with the particle-extinction cross section, the particle diameter, and the beam width. Based on comparison with the GLMT, the diffraction solution is accurate when beam widths that are approximately greater than or equal to the particle diameter are considered and when large particles that have an extinction efficiency near the asymptotic value of 2 are considered. However, diffraction fails to describe the forward intensity for more tightly focused beams. The experimental observations, which are in good agreement with GLMT-based predictions, reveal that the total intensity profile about the forward direction is quite sensitive to particle axial position within a Gaussian beam. These finite beam effects are significant when the ratio of the beam to the particle diameter is less than approximately 5:1. For larger beam-to-particle-diameter ratios, the total field in the forward direction is dominated by the incident beam.

Observation of sudden temperature jumps in optically levitated microdroplets due to morphology-dependent input resonances.

During the slow evaporation of an optically levitated microdroplet of a glycerol-water mixture (3:1) (approximately 12.44 µm in radius) several morphology-dependent input resonances have been observed in its Raman spectrum. These resonances yield sudden temperature jumps of approximately 10 °C in the microdroplet as evidenced by sudden shifts in the output (Raman) resonance spectra. The latter effects could be explained by a simple energy balance calculation and the dependence of droplet refractive index and density on temperature.