Daniel T. Cassidy

Atmospheric pressure monitoring of trace gases using tunable diode lasers

High-sensitivity measurements of infrared absorptions due to isolated lines broadened by air at atmospheric pressure are reported. Tunable diode lasers and harmonic techniques were employed to detect absorptions as small as 0.01% over path lengths up to 250 m through the open air. The limiting noise source is caused by a base line signal which varies with the optical alignment. The reported sensitivity is not a fundamental limit. Techniques for further increasing sensitivity are discussed.

Technique for measurement of the gain spectra of semiconductor diode lasers

A simple technique for determining the gain spectra of semiconductor lasers from measurements of the emission spectra of the laser is presented. The technique is insensitive to the response function of the device used to determine the emission spectra of the laser. Accurate estimates of the gain can be obtained from data which have been convolved with an instrument response function of ≲0.5 A FWHM for a cavity free spectral range of 2.5 A. Two applications of the gain data obtained by the technique are presented.

Harmonic detection with tunable diode lasers —two-tone modulation

The minimum absorption detectable by a tunable diode laser spectrometer utilising harmonic techniques is often limited by interference fringes generated by scattered light. The sensitivity of the spectrometer to absorption can be increased by applying a jitter modulation. In this paper, the theory of harmonic response for single- and two-tone modulation over optical fringes and Lorentzian absorption lines is developed and compared to experimental measurements. A simple analytic expression for the two-tone harmonic line shape is derived. This expression provides a physical understanding of the effects of the second modulation, and a means to unravel the effects of the second modulation on the linewidth and line shape. For a specific choice of the jitter frequency and phase, it is possible to simultaneously minimise the fringe signal and increase the harmonic absorption signal. The results of this investigation are applicable to trace gas detection using tunable diode lasers, and to other areas of spectroscopy and magnetic resonance where harmonic techniques are used.

Wavelength-dependent transmission of monomode optical fiber tapers.

The transmission of light as a function of wavelength through biconical tapers formed in sections of monomode and low-mode number optical fibers is considered. It is found that in the monomode propagation regime the transmission varies sinusoidally with wavelength with a 50% peak loss and a 50-nm period, for tapers typical of those used in the manufacture of fused-biconical directional couplers. The observed dependence of transmission on wavelength can be understood using a coupled-mode formalism and leads to new insights into the operation of monomode fused-biconical directional couplers.

Trace gas detection with short-external-cavity InGaAsP diode laser transmitter modules operating at 1.58 μm

Detection of CO(2) and CO at 1.58 microm (6322 cm(-1)) using an InGaAsP diode laser and mode control is described.Mode control is a technique whereby a short cavity, external to the laser, is used to force the laser to operate in a single mode. By monitoring the voltage across the terminals of the laser and using electronic feedback it is possible to optimize continually the external cavity so that the laser operates reliably in the mode of choice for scans >3.5 cm(-1). This technique offers the possibility of high-sensitivity detection over a region of ~30 cm(-1) with continuous coverage in overlapping sections of ~3 cm(-1) with conventional (and hence inexpensive) multimode diode lasers.

Detection of oxygen using short external cavity GaAs semiconductor diode lasers

High sensitivity detection of O(2) in the near infrared with short external cavity (SXC) AlGaAs semiconductor diode lasers is reported. The mode control provided by the SXC enhances the performance of these diode lasers by allowing a number of single laser modes (up to ten) to be individually selected and continuously scanned over extended frequency intervals. At a constant laser heat sink temperature, this provides nearly complete spectral coverage of up to approximately 40 cm(-1) for overlapping modes. When using second harmonic detection techniques, these SXC controlled lasers were found to provide high sensitivity detection of O(2) at atmospheric pressure. A minimum detectable absorbance of approximately 4 x 10(-6) has been achieved with a SNR of 1. The stability of these laser systems has also allowed the continuous monitoring of an absorption signal over extended time intervals. Absorbances of 1 x 10(-2) have been continuously monitored for durations of up to 15 h with an rms uncertainty of +/-6 x 10(-5). These results are reproducible for measurements made on all the laser modes that could be selected with the SXC.

High-sensitivity detection of trace gases using sweep integration and tunable diode lasers.

A tunable diode laser spectrometer capable of high-sensitivity detection of trace gases over a wide range of pressures is described. Infrared absorptions of 10(-3)% over path lengths of up to 200 m have been observed in low-pressure gases. In preliminary measurements at atmospheric pressure we have detected differential absorptions of 10(-3)% over path lengths of a few meters. Data are acquired by rapidly sweeping the laser wavelength over ~0.4 cm(-1) and summing the results of many sweeps using a signal averager. To increase sensitivity and allow efficient use of the digital signal averager, an analog circuit is used to null the intensity modulations which arise from wavelength-modulating the diode laser. This sweep integration technique allows direct determination of line shapes and linewidths from the data and improves the repeatability of the recorded spectra.

1.4-μm InGaAsP-InP strained multiple-quantum-well laser for broad-wavelength tunability

InGaAsP-InP strained multiple-quantum-well (MQW) lasers for extended wavelength tunability in external cavity operation were designed, fabricated, and tested. The active layer was a strain compensated structure consisting of three 3.2/spl plusmn/0.3 nm and three 6.4/spl plusmn/0.3 nm 1.0% compressive strained wells and five 10.3/spl plusmn/0.3 nm 0.45% tensile strained barrier layers. A 2-/spl mu/m-wide ridge waveguide laser of length 250 /spl mu/m, when used in a grating external cavity and with no coatings to alter the reflectivity of the facets, was observed to operate over a range >110 nm. The lasers were designed for applications in trace gas and liquid detection with the goal to maximize the tunable range when operated in external cavities and with no facet coatings.

Linewidth measurements of tunable diode lasers using heterodyne and etalon techniques

Measurements of the linewidths of Pb-salt diode lasers operating in the 8- and 9-microm region are reported. The linewidths of the 9-microm lasers were determined by conventional heterodyne techniques, while for the 8-microm lasers a new technique based on a Fabry-Perot etalon was used. The new technique avoids the complexity and limited wavelength range of the heterodyne measurements and can be used for any tunable laser. The linewidths observed varied from 0.6- to >500-MHz FWHM. The linewidth was found to vary dramatically from device to device, to depend strongly on junction temperature and injection current, and to be correlated with vibrations caused by operation of a closed-cycle refrigerator.

Imaging of stresses in GaAs diode lasers using polarization-resolved photoluminescence

Images of stress distributions in GaAs-based diode lasers have been obtained using scanned polarization-resolved photoluminescence. A stress resolution of about 10/sup 7 /dyn/cm/sup 2 /and a spatial resolution of about 1 mu m have been obtained. The experimental technique is described, and measured stress distributions due to ridge structures, metallization, applied force, and bonding are presented. >