Application note
Laser Photoacoustic Measurement, OPO laser and PA201
Introduction

Results

An example of low ppb level gas measurement using extremely sensitive cantilever enhanced photoacoustic detector PA201
combined with a compact tunable mid-infrared optical parametric oscillator (OPO) laser source was performed. Target molecule in the demonstration was methane whose detection limit
was determined to be 4 ppb using 1 second observation time.

Laser wavelength was scanned from 3236.45 nm to 3295.95
nm in 0.1 nm steps. Data was integrated 0.957 second time ateach step. The resulting spectrum of 10 ppm of methane isshown in Figure 3.

Measurement Setup
Measurement setup included a Gasera PA201 photoacoustic
detector with an optical cantilever microphone in 95 mm long
cylindrical cell and a very compact mid-infrared range tunable
OPO laser source from Cobolt AB. Collimated laser beam wasdirected through the photoacoustic cell to a power meter. Laser power was modulated using a mechanical tuning fork
chopper at 135 Hz frequency.
Table I. Laser parameters.
PowerRepetition rate
Pulse width
Pulse energy
WavelengthLaser linewidth
Beam diameter

100104103237 – 3296
1.31.6mW
kHz
nsµJnmnm
mm

Figure 3. Measured spectrum of methane and simulated
HITRAN spectrum for a comparison.
The background signal and the noise level at different
wavelengths was measured by setting dry nitrogen inside thephotacoustic cell. Due to the use of amplitude modulation, thenoise followed the background signal value (Figure 14). Thismeans that main noise source is the precision of laser power
and optical microphone (0.2 %). Detection limit was calculatedby dividing the concentration by the signal-to-noise ratio (2 xRMS).

Table II. Measurement parameters.
Cell pressure
Measurement frequency
Cell temperature
Methane concentration
Observation time
Photoacoustic cell length
Photoacoustic cell diameter
Cantilever dimensions

953
13550
100.957954
5 x 1.2 x 0.01

mbar
Hz°C
ppm
smm
mmmm

Gas was sampled using an internal pump inside thephotoacoustic detector. The cell was sealed during the measurement and sample gas pressure was set to 953 mbar.
Figure 4. Measured background signal and noise levels atdifferent wavelengths.
Table III. Calculation of the detection limit.

Figure 1. Schematic drawing of the measurement setup.

Methane concentration
Wavelength
SignalBackground signal
Methane signal
Measurement time
Noise (1 x RMS)
SNR (1 x RMS)
Det-limit (2 x RMS)

103240.3
0.331
0.024
0.307
0,951
-5
5.00 x 106140
3.3ppmnmsppb

OutlookFigure 2. Picture of the measurement setup.

The combination of cantilever enhanced photoacoustics andtunable mid-infrared OPO laser source make a perfect fit for
rapid and selective trace gas analysis. High sensitivity of thephotoacoustic detector and relatively high laser power at fundamental vibration bands of any hydrocarbon and also large
selection of other molecules guarantees the low ppb sensitivity. High selectivity is achieved by the wide tuning range of thelaser source. Both the laser source and photoacoustic cell are
already very compact and fit easily to a table top size analyzer,
but it is also possible to implement the components to a portable size gas analyzer.