E. A. Ballik

Sensitivity limits of a tunable diode laser spectrometer, with application to the detection of NO 2 at the 100-ppt level

A laser absorption spectrometer is described which uses a tunable diode laser and a 1-m multipass White cell to detect NO(2) in air with a sensitivity of better than 100 ppt. The modulation techniques employed to achieve this sensitivity are described in detail, and the noise mechanisms, which currently limit the detectable absorption coefficients to greater, similar 10(-7) m(-1), are examined.

High sensitivity pollution detection employing tunable diode lasers

A laser absorption spectrometer is described which employs a wavelength-tunable Pb(1-x) Sn(x) Se diode in conjunction with a multipass White cell and which is capable of measuring SO(2) concentrations in the low ppb range. We describe in some detail the modulation techniques used in signal detection which enable us to measure absorption coefficients as low as 10(-7) m(-1). In addition, calibration of the instrumentation using small sample cells is described, and the question of interference from unwanted molecular species is discussed. The instrumentation allows the measurement, basically at the same time, of a large number of other atmospheric gases which are of significance in pollution studies. For example, the present diode operates over 1050-1150 cm(-1) and can measure SO(2), O(3), N(2)O, CO(2), H(2)O, NH(3), and PAN. The addition of a second diode to the system will allow most gases of any atmospheric importance to be monitored. In general, these gases have much stronger ir absorption bands than SO(2) and hence can be detected at concentrations much less than 1 ppb.

High sensitivity point monitoring of atmospheric gases employing tunable diode lasers

A laser absorption spectrometer is described which employs a wavelength tunable diode laser in conjunction with a multipass White cell. The instrument is sensitive to absorption coefficients as low as 3 x 10(-8) m(-1) and is designed for use as a portable pollution monitor. The performance of the instrument is illustrated by describing the measurement of three gases commonly found in the atmosphere: SO(2) and N(2)O with sensitivities of 1 ppb and NH(3) with a sensitivity of better than 0.1 ppb. The linearity of response and calibration of the instrument are also discussed in some detail.

High sensitivity point monitoring of ozone, and high resolution spectroscopy of the ν 3 band of ozone using a tunable semiconductor diode laser

Accurate measurements of the wavenumbers of more than 100 lines in the nu(3) band of ozone have been made using a tunable semiconductor diode laser and a temperature-stabilized germanium etalon. The accuracy for the line centers is estimated to be within 3-10 MHz and compares well with the accuracy attained using more complicated heterodyne techniques. The relevance of these results to ozone monitoring instruments based on CO(2) lasers is discussed. The tunability of the diode laser is also exploited to monitor ambient concentrations of ozone in the presence of interfering molecular species. A detection sensitivity of ~0.5 ppb of ozone is demonstrated.

A vibrational‐bath model for the dynamics of SF6 absorption near 10.4 μm as a function of wavelength and absorbed energy

A ’’vibrational‐bath’’ model is employed to determine the characteristics of the absorption of pulsed CO2 laser radiation by SF6 at the 10.4‐μm band of CO2. This model allows calculation of the absorption of any CO2 P line, other than that of the incident pulse, both during and after the pulse transmission and over a wide range of pulse intensities. Infrared double‐resonance experiments, using both short and long pump pulses, demonstrate the appropriateness of the model in predicting the SF6 absorption behavior and the range of SF6 pressures and input pulse durations where such a model applies.

Reduction of the pumping efficiency in CO2 lasers at high discharge energy

It has frequently been observed that, beyond a certain point, the gain obtained in TE CO2 lasers fails to increase with increasing discharge input energy. This saturation effect could be caused by problems of a secondary nature such as the onset of discharge deterioration, or the effects of increasing gas temperature. On the other hand, an intrinsic property of the discharge may be responsible for this limitation on the pumping efficiency. In this paper, evidence is presented in favor of the latter possibility. Measurements of small−signal gain have been carried out in He : CO2 and He : N2 : CO2 gas mixtures at moderately low pressure in a transversely excited resistor−pin amplifier. It is shown that this system permits the clear separation of the region of discharge deterioration from the onset of gain saturation. Furthermore, auxiliary measurements are undertaken which permit corrections to be made for the effects of increasing gas temperature. The net result is that the discharge pumping effectiveness ...

4.3‐μm cascade CO2 laser

An in‐cavity hot CO2 cell is used to obtain 4.3‐μm lasing from a cascade process in a conventional CO2 laser. The 4.3‐μm laser operates in a Q‐switched mode, is line tunable, and has average output powers greater than 100 mW.

Point Monitoring of Ambient Concentrations of Atmospheric Gases Using Tunable Lasers

A laser absorption spectrometer (LAS) is described which employs a wavelength tunable Pb 1-xSnxSe laser diode in conjunction with a multi-pass White cell. Absorption coefficients as low as 3 x 10-3 m-1 can be measured as a result of using a laser wavelength modulation technique to suppress noise in the detection system. This constitutes an improvement of several orders of magnitude in sensitivity compared with earlier reported applications of the direct absorption technique employing tunable diode lasers. The performance of the LAS is illustrated by describing the measurement of three gases commonly found in the atmosphere: SO2 and N2 0 with sensitivities of 1 ppb, and NH3 with a sensitivity of better than 0.1 ppb. The linearity of re-sponse and calibration of the instrument are also discussed in some detail.

12-μm NH 3 laser pumped by a sequence CO 2 laser

A sequence CO2 laser is used to pump NH3 optically to produce laser action in the 12-μm region. The pump-frequency offsets for the 9.4-μm P(7) and P(17) sequence lines are measured to be 86 and 142 MHz, respectively. These small offsets enable efficient laser operation to be obtained at low pumping powers.

4.3-μm TE CO 2 laser

We describe a pulsed 4.3-μm CO2 laser that employs conventional TE CO2 discharges. The laser is efficient, line tunable over two different 4.3-μm bands, and potentially scalable to high pressures and output powers.We describe a pulsed 4.3-..mu..m CO/sub 2/ laser that employs conventional TE CO/sub 2/ discharges. The laser is efficient, line tunable over two different 4.3-..mu..m bands, and potentially scalable to high pressures and output powers.