Keywords: Tunable diode-laser spectrometers; Overtone bands; Overtone absorption spectroscopy; Molecular spectroscopy;
Line and band widths, shapes and shifts; Modulation broadening; VIS-IR spectrometers; Auxiliary equipment and techniques; Infrared spectra.
double heterostructure diode lasers (DLs) have become the cheapest
monochromatic sources in the field of the atomic and molecular
Commercially available AlGaAs and InGaAlP DLs
emissions can be easily tuned and scanned around most of the
ro-vibrational overtone absorptions of molecules like
HCN, HF, H2O,
etc.. Unfortunately these transition lines are weak and therefore
noise-reduction techniques must be used.
The frequency modulation (FM) technique
can be applied to DLs by playing with their injection current.
When the frequency of the modulation is chosen much lower than the resonance
line-width, the FM spectroscopy is usually called wavelength modulation (WM)
Overtone absorption resonances have been successfully observed by
using AlGaAs diode lasers with WM spectroscopy and harmonic detection
techniques. The WM technique, applied to coherent sources like
DLs, permits to reach good sensitivities per unit of optical
path-length even for very weak lines such as the oxygen electric
dipole forbidden ones. Since a diode laser apparatus is much cheaper
than either the dye laser based one and the high resolution Fourier
transform spectrometer, in practice it enables any research
laboratory to do high resolution spectroscopy.
WM spectroscopy techniques are currently
adopted for pressure broadening and shifting measurements. The experimental
apparatus consists of a single mode both transversely and longitudinally diode laser.
The current is driven by a stabilized low-noise current generator, which
permits also the scan of the emission wavelength by mixing to the
driving current an attenuated low frequency (~1 Hz) sawtooth signal.
The DL is temperature regulated within 0.002 K by a Peltier junction
driven by a high stability temperature controller.
The diode laser emission wavelength shows a strong and linear temperature
dependence (~0.2 nm/K): one of the major requirements, when using these sources
for spectroscopy, is the very good temperature stabilization.
The mode hops are the major DL drawback in free-running mode, which is the
simplest and cheapest way to operate. The current dependence for small variations
can be considered linear too, with a dependence of about 0.01 nm/mA. The measurement
cell is a multipass Herriott type one, 50 cm long and 7 cm diameter.
The total optical path length is 30 m. A confocal 5 cm Fabry-Perót
interferometer is adopted to mark the frequency scan and to check the
goodness of the DL emission. A 350 mm focal length monochromator is employed
for the rough wavelength reading.
For line-shift measurements, a cell filled with a gas at fixed pressure
is adopted as a reference. In case of oxygen, an open path is adopted,
taking into account the atmospheric oxygen partial pressure.
Silicon photodiodes collect the transmitted signals.
For the phase detection a sinusoidal modulation at a frequency of 10 kHz
is added to the DL injection current. The transmitted power is collected
by the photodiodes and sent to the lock-in amplifiers in order
to extract the second harmonic signals. The resulting second derivative
of the absorption feature has a very good signal to noise ratio and a
flat baseline, as it can be seen in the following figures.
knowledge of the pressure induced broadening and shifting
coefficients can be important in the atmospheric analysis, especially
for constructing spectroscopical maps of the planets. Moreover, the
knowledge of these parameters is important also to better understand
the intermolecular interactions.
of absorption spectrum: water vapor
Second derivative of the water
vapor absorption spectrum at ~823 nm (~13145 cm-1). The
measurement has been done at room temperature with a water vapor
pressure of 20 Torr, through an optical path length of 5 m and by
using a 2.5 ms time constant. The line positions agree with
what listed by HITRAN database within 0.01 cm-1: 12144.795,
12144.862, 12144.915, 12145.279 e 12145.444 cm-1
resolving power of the spectroscopic apparatus potentially permits to
discriminate different gases in a complex atmosphere, with response
times of the order of 10−100 milliseconds. Sensitivity of tens of
p.p.m. per meter of path has been obtained with molecular oxygen, as
well as with water vapor, ammonia and acetylene. As an example of
the laser diode spectrometer resolving power, the following picture
shows two close ammonia absorption lines whose position is known for
absorptions at 788 nm
Second derivative of two
ammonia absorption lines at 788.8 nm (~12673 cm-1). The
measurement has been done at room temperature with 18 Torr of
ammonia, through an optical path length of 5 m and by using a time
constant of 12.5 ms. Only one line position is well known (12673.72
cm-1). The other differs from this by 0.072 cm-1.
The last example
concerns the WMS of 91 Torr of carbon dioxide at 782 nm. The linewidths
are larger than the Voigt profile would justify at this pressure; this is
due to the large modulation amplitude
needed by the very low absorption cross sections (~5 x 10-26