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Acoustic Detection Module for QEPAS

  • Designed for Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS)
  • Acoustic Microresonator Tubes and Matched Preamplifier for Optimal SNR
  • Custom Quartz Tuning Fork with 0.8 mm Prong Separation
  • Low Resonance Frequency
  • High Q Factor


Acoustic Detection Module for QEPAS

Zoomed-In View Through Window


Microresonator Tubes

See QEPAS Tab for More Details

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Acoustic Detection Module, Cutaway View
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Schematic Showing Module Interior


  • 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.

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.

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.

Electrical Connections
Color Purpose Comment
Red +12 V Recommended Voltage, up to 24 V Maximum
Black GND -
Blue -12 V Recommended Voltage, up to 24 V Maximum
White MODULATION IN Electrical Modulation (sine) IN for QTF Characterization
Green SIGNAL OUT Amplified QTF Signal OUT
Linear up to 1.8 V, 50 Ω Termination
ADM01 Specifications
Resonance Frequency f0 a 12455 Hz (Typ.)
Q Factora >12000 (Typ.)
Volume of Sample Chamber 7 cm3
Microresonator Tubes (2) Inner Diameter: 1.6 mm
Length, Each: 12.4 mm
Wedged Windows(2) WW00530
Uncoated BaF2, 200 nm - 11 µm
Gas Connectors G1/8 Straight for 6 mm Tubing
Recommended Gas Flow <200 sccm
Maximum Gas Pressure 1.5 bar
  • At Atmospheric Pressure
  • Wedged windows can be exchanged to accommodate a different excitation wavelength. Contact us for more information.
Acoustic Detection Module Diagram
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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). 

Posted Comments:
Jean-Michel Melkonian  (posted 2019-09-20 08:41:50.633)
Dear Thorlabs, could you please update the specs of ADM01 to confirm whether it can be pumped down to near vacuum, because in relative pressure this results in -1 bar. Thank you.
MKiess  (posted 2019-10-14 06:08:23.0)
This is a response from Michael at Thorlabs. Thank you for the Feedback. We stated 1.5bar maximum pressure since that was up to now the maximum we tested with QEPAS measurements and are confident that it works. In terms of vacuum the lower limit depends on two factors: the tightness of the ADM01 and the pump that is used to evacuate. The strongest pump we have in our labortory brought us to the lower limit. The pressure sensor we have can measured 1Torr. Therefore, it would be worth considering changing the pressure specifications as follows: 1mbar to 1.5 bar (absolute pressure), i.e. from 1 Torr to 1125 Torr.
Ryan Sun  (posted 2019-09-09 16:37:17.883)
I wonder if the Maximum Gas Pressure in the specifications is absolute or relative. And what is the available temperature range of the module? Please explain these things, Best Regards. Ryan
dpossin  (posted 2019-09-10 09:10:56.0)
Hello Ryan, Thank you for your feedback. In general, the epoxy adhesive limits the mechanical use of the ADM module. The specified temperature range of this adhesive is from -40°C up to +90°C. Further the noise of the integrated pre amplifier shows higher noise with respect to increasing temperature. The temperature range for this part is -40°C up to 125°C. The specified pressure is given in absolute value.

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ADM01 Support Documentation
ADM01Acoustic Detection Module for QEPAS, Hylok Fittings, BaF2 Windows
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