Robin Bocquet

Detection and quantification of multiple molecular species in mainstream cigarette smoke by continuous-wave terahertz spectroscopy

Continuous-wave terahertz spectroscopy by photomixing is applied to the analysis of mainstream cigarette smoke. Using the wide tunability of the source, spectral signatures of hydrogen cyanide (HCN), carbon monoxide (CO), formaldehyde (H2CO), and water (H2O) have been observed from 500 to 2400 GHz. The fine spectral purity allows direct concentration measurement from the pure rotational transitions of HCN and CO. The quantification of the measurement was validated by the means of a calibration gas containing CO. The potential of this technique for trace gas detection is demonstrated with an estimated detection limit of HCN equal to 9 parts in 10(6).

Terahertz frequency difference from vertically integrated low-temperature-grown GaAs photodetector

We report on the development of a photoconductive detector based on low-temperature-grown GaAs which is vertically integrated with terahertz spiral antennas. A non steady-state velocity overshoot effect was expected in the photoresponse with a responsivity of 0.04 A/W at a bias voltage of 8 V. Photomixing experiments using two optical 0.8 μm beating lasers show a 3 dB bandwith of 700 GHz with a radiation power at terahertz frequency of 0.5 μW under 2×30 mW optical pumping.

THz photomixing synthesizer based on a fiber frequency comb.

A frequency doubled erbium doped modelocked fiber frequency comb is used to implement a THz photomixing synthesizer. The useful THz linewidth is in order of 150 kHz and has been assessed along with the frequency accuracy by spectroscopic measurements demonstrating a relative accuracy of 10(-8) at frequencies around 1 THz. The THz synthesizer is used to implement a THz spectrometer to study the rotational absorption spectrum of carbonyl sulfide (OCS). Measurement of the principal transitions between 813 GHz and 1283 GHz allowed the properties of the THz spectrometer to be compared with competing techniques, and demonstrates the potential of the THz photomixing synthesizer as an alternative means to explore the THz domain.

Inscription of long-period gratings in pure silica and Germano-silicate fiber cores by femtosecond laser irradiation

Long-period gratings (LPGs) have been inscribed by femtosecond laser radiation into fiber optics with Ge-doped and pure silica cores. Grating parameters of 16-dB rejection and 0.3-dB insertion loss are obtained for a Ge-doped fiber core. Numerical calculation of the transmission characteristics yielded a good correlation between the measured and synthetic data showing that an induced refractive index change up to 3/spl times/10/sup -4/ can be achieved. Inscription of an LPG in pure silica fiber core is demonstrated and thought to be first achieved in this fiber type. Greater laser intensity is required in pure silica compared with Ge-doped cores to produce a refractive index change of similar magnitude.

Recent Developments of an Opto-Electronic THz Spectrometer for High-Resolution Spectroscopy

A review is provided of sources and detectors that can be employed in the THz range before the description of an opto-electronic source of monochromatic THz radiation. The realized spectrometer has been applied to gas phase spectroscopy. Air-broadening coefficients of HCN are determined and the insensitivity of this technique to aerosols is demonstrated by the analysis of cigarette smoke. A multiple pass sample cell has been used to obtain a sensitivity improvement allowing transitions of the volatile organic compounds to be observed. A solution to the frequency metrology is presented and promises to yield accurate molecular line center measurements.

Anomalous dispersion measurement in terahertz frequency region by photomixing

Photomixing and coherent homodyne detection have been used to measure the absorption and dispersion of dielectric media. The signal to noise ratio was sufficient to allow the dispersion and absorption characteristics of a polytetrafluoroethylen (Teflon) sample to be extracted up to a frequency of 1.6THz. Due to the relatively narrow spectral linewidth of terahertz source, estimated to be in order of 5MHz, the anomalous dispersion in the vicinity of a rotational transition of carbonyl sulfide is clearly observed, with a refractive index variation of about 10−4 being measured at frequencies within 40MHz of the transition.

Gas filter correlation instrument for air monitoring at submillimeter wavelengths

A gas filter correlation (GFC) instrument for air monitoring at submillimeter wavelengths has been developed. We used a high-power terahertz radiation source in combination with a specific gas filter cell to obtain a highly selective instrument to differentiate species present in an unknown mixture. This approach provides a new method for survey measurements in the spectrum from 100 to 1000 GHz, in which many molecular rotational lines appear. Basic operational considerations and preliminary experiments with hydrogen sulfide are described.

Versatile Sub-THz Spectrometer for Trace Gas Analysis

A THz sensor system based on a commercially available multiplier chain is applied to the analysis of trace gases. Frequency modulation of the source is used to achieve the high sensitivity required. Particular attention is paid to the line broadening effect caused by the modulation as this can hamper the quantification of the various chemical species being monitored. A numerical model using a Voigt absorption profile along with a single calibration step is used to provide quantitative information. The sensitivity obtained by this prototype instrument permits the analysis of various samples extracted during the industrial sintering of iron ore as part of the manufacturing of steel.

Spectroscopy of the formaldehyde isotopomer H213CO in the microwave to terahertz region

The pure rotational spectrum of H213CO in its ground vibrational state has been investigated extensively between 8 and 1900 GHz using samples of natural isotopic abundance as well as samples enriched in 13C. The accurate new line frequencies were subjected to a combined fit with previously published data, resulting in distinctly improved spectroscopic constants, which include several octic and one decic centrifugal distortion constants. Fits employing Watsons S and A reductions will be compared. The following rotational constants have been obtained: AS=281993.0397 (31), BS=37809.106966 (213), CS=33215.941417 (207) and AA=281993.0148 (32), BA=37811.090040 (219), CA=33213.976745 (207) MHz, respectively.

Long path length cw-THz spectrometer using a multipass cell

Multipass cells have been extensively used in the infrared. Their use at THz frequencies is minimal due to the difficulty of coupling the highly divergent THz beams. The divergence of a cw-THz photomixing source was characterized and included in a beam propagation calculation to match the THz beam to a multipass cell. Path lengths up to 20 m have been realized. The spectra of carbonyl sulfide (OCS), vinyl chloride (CH2CHCl) and dimethyl sulfoxide (DMSO), (CH3)2SO are used to demonstrate the improved sensitivity of the spectrometer.