Acoustic Detection Module for QEPAS

Click to Enlarge
Schematic Showing Module Interior

Features

• Includes Custom Quartz Tuning Fork, Microresonator (µR) Tubes, and Preamplifier with High Gain
• Low Resonance Frequency
• Q Factor at Atmospheric Pressure: >12000 (Typ.)
• Small Gas Sample Volume: 7 cm³
• Ø1/2" Uncoated BaF₂ Wedged Windows for 200 nm - 11 µm

The Acoustic Detection Module is a photoacoustic cell designed to be easily built into a complete quartz-enhanced photoacoustic spectroscopy (QEPAS) system. The ADM01 module has an inlet and outlet allowing a gas sample to be pumped into the airtight chamber, which contains microresonator tubes and a custom quartz tuning fork (QTF). When an external laser source passes through the wedged windows, the tubes enhance the acoustic signal generated by the relaxation of the excited gas particles. This acoustic signal is transduced by the custom QTF and then amplified by the integrated high-gain preamplifier, enabling excellent signal-to-noise ratio. For more information on this technique, see the QEPAS tab.

Gas Interface
Each ADM01 module is equipped with two gas connection ports. If not operated in a flow cell configuration, the second gas port could instead be used to connect a pressure gauge or other similar device to the cell. The gas ports are equipped with standard Hylok stainless steel tube fittings for 6 mm tubing. The tube fittings can be disconnected for access to the G1/8 straight female thread connector.

The Ø1/2" wedged windows for 200 nm - 11 µm are interchangeable to support additional wavelengths, and the microresonator tubes can be changed for measuring a different gas matrix than N₂. Please contact us for assistance and details.

Mounting
As shown in the schematic to the right, four through holes are provided for securing the ADM01 module directly to metric or imperial breadboards. The bottom of the module also features an 8-32 (M4) tap for mounting to a PY005(/M) 5-Axis Stage on a PY005A1(/M) Slotted Mounting Plate. First mount the 5-axis stage to the slotted mounting plate and then mount the ADM01 module on top of the stage. Lastly, mount the entire assembly to the breadboard using the slots in the PY005A1 plate. The module also features several Ø2.1 mm (0.08") holes to serve as rotation locks. A single DIN915 screw fastened to the 6-32 taps of the 5-axis stage will lock the rotational position of the module when the smooth head of the DIN915 screw embeds into one of the four Ø2.1 mm holes.

Alignment
A laser source can be aligned to the ADM01 module by either beam steering via two mirrors or mounting the module to a multi-axis stage such as the PY005(/M) 5-axis stage. To prevent photothermal effects, the laser beam must not touch the walls of the tubes, so the beam diameter must be less than the inner diameter of 1.6 mm. A power sensor may be used before and after the module to assess the power lost and ensure the beam has not contacted the walls of the tubes. A lens might also be used to focus the laser light at the plane of the QTF to optimize signal strength.

Click to Enlarge
QEPAS Diagram

Photoacoustic spectroscopy is a technique based on the photoacoustic effect that is able to accurately detect trace gas concentrations for a wide variety of applications. Similar to laser absorption spectroscopy, a laser beam is sent through a gastight chamber to excite the target gas molecules. However, instead of detecting the absorption lines with an optical detector, the pressure wave generated by the relaxation of those molecules is detected by a transducer.

In the case of quartz-enhanced photoacoustic spectroscopy (QEPAS), the transducer is a sharply resonant QTF. The high-Q acoustic resonance enables the detection of weak excitation within small volumes, bypassing the acoustic resonance restrictions of conventional methods. When the laser source is modulated with a sine wave, the induced pressure (sound) wave will have double the frequency as that of the light modulation; therefore, the laser source modulation must be at half of the resonance frequency of the quartz tuning fork (QTF). The resulting amplitude from the QTF is directly proportional to the concentration of trace gas in the sample.

The QTF also has good environmental noise immunity due to its being an acoustic quadrupole, since the primary vibrational modes require the prongs to move away from each other to be piezoelectrically active. Sound from external sources has a longer wavelength than the prong separation and will cause the prongs to move in the same direction, resulting in no piezoelectric response.

For more information on the science of QEPAS, please see the citation below.

ADM01 Acoustic Detection Module
The ADM01 module is designed for on-axis QEPAS, which uses two tubes on either side of the QTF as an acoustic microresonator which the light must pass through, as shown in the diagram to the right. Each tube is ~λ/4 with a small gap between them for the QTF, where λ is the wavelength of sound in air at the resonance frequency of the tuning fork. The acoustic resonator increases the effective interaction length between the generated sound and the QTF, allowing the QTF to have greater sensitivity to the near-field photoacoustic wave.

P. Patimisco, A. Sampaolo, L. Dong, F. K. Tittel, and V. Spagnolo, "Recent advances in quartz enhanced photoacoustic sensing," Appl. Phys. Rev. 5, 011106 (2018).