Acoustic Detection Module for QEPAS

Overview
Specs
QEPAS
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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.

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 Factor^a	>12000 (Typ.)
Volume of Sample Chamber	7 cm³
Microresonator Tubes (2)	Inner Diameter: 1.6 mm Length, Each: 12.4 mm
Wedged Windows^b(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.

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).

Posted Comments:

Ryan Sun (posted 2020-05-27 04:25:02.667)

Dear Thorlabs, I am trying to lower the pressure inside the ADM01 to 1Torr. I want to know if the ADM01's window can withstand a relative pressure of almost 1atm and if it can be repaired if it breaks. I completed the test from 1.5bar to 0.5bar pressure, but the experiment was stopped because I was not sure whether the ADM01 could work normally at a pressure of 0.5bar or less. Please explain the exact limit of the window installed in ADM01. Thank you :)

wskopalik (posted 2020-06-04 06:54:09.0)

This is a response from Wolfgang at Thorlabs. Thank you for your inquiry! The windows in the ADM01 can withstand a pressure of 1 Torr on the inside. We have tested the ADM01 successfully with low pressures in this range. There would however need to be continuous gas flow and pumping at such low pressures to keep the pressure low. In the case of static measurements at low pressures, the pressure would rise quickly, i.e. you would draw air from the outside. In case of a damage, the windows can easily be exchanged. The windows WW00530 are used in the ADM01 which are available in our catalog. I will contact you directly to provide further assistance.

yazhou wang (posted 2020-04-23 14:01:50.773)

Hello, It mentioned that: '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).' I feel confused that why the induced pressure (sound) wave will have double the frequency as that of the light modulation. Cloud you please give some comments on this? Thanks!

dpossin (posted 2020-05-04 04:22:19.0)

Dear Customer, Thank you for your feedback. Well, for photoacoustic purposes the frequency of the incident laser light is modulated. Therefore the absorption line of the gas inside the QUEPAS cell is hit twice within one full modulation wave. One time in the first half wave when the laser wavelenght rises and a second time in the second half wave when the laser wavelenght drops again. The vibronic movement of the gas leads to sound waves which have double the modulation frequency of those of the incident laser light. To make sure, that the resonance frequency of the quartz tuning fork (QTF) is met, the modulation of the incident laser must be half the resonance frequency of the QTF. Because the bandwith of the QTF is quite narrow it is crucial that the resonance freqency is hit. I am reaching out to you for further discussion.

Mincheol Jeon (posted 2020-02-02 19:48:57.607)

Hello Thorlabs, in sepcs, its maximum gas pressure is 1.5bar. But how about in vacuum? Is it possible to measure gas with adm01 even in vacuum? Thank you!

dpossin (posted 2020-02-19 08:16:39.0)

Dear customer, Thank you for your feedback. There are many things to care of when you like to evacuate the ADM01 chamber. As an example I will enlarge on two different topics: Leakage: How good the vacuum can be depends very much on the leakage rate and the power of the pump. Leakage rate is given by the seals of the ADM01 cell itself and also on the fact if and how much the inner components are outgasing. As we did not tested the ADM01 for that purpose we can not say if vacuum can be applied there. However the Entrance and output windows withstand the ambient pressure. Limit of detection (LOD): The limit of detection also depends on the pressure as for lower gas pressure or even vacuum, there won´t be ambient gas any more which transmits the vibrational movements onto the tuning fork. As an result, most probably the LOD will decrease.

Ryan Sun (posted 2020-01-13 03:50:37.59)

Dear Thorlabs, Could you explain to me what is the current's range(ampere) required by the power supply of ADM01?

nreusch (posted 2020-01-14 02:04:48.0)

This is a response from Nicola at Thorlabs. Thank you for contacting us. The current consumption is about 20-30 mA.

Mincheol Jeon (posted 2019-12-06 03:21:37.25)

Dear Thorlabs, could you please inform the drawing or size of the QTF?(ex. width, length of each tine and length between two tines...) Thank you!

nreusch (posted 2019-12-09 02:30:37.0)

This is a response from Nicola at Thorlabs. Thank you for your feedback. You can download the AutoCAD file by clicking on the red document icon next to the item number and then choosing the Auto CAD PDF document or any other file format that you are interested in. Please contact your local Tech Support team if any information is missing.

Mincheol Jeon (posted 2019-11-11 04:56:58.69)

I want to know what kind of the electrical modulation(sine) IN for QTF characterization.(voltage modulation? or current modulation..) And I want to know how much modulation input I have to put in. Best regards, Mincheol.

dpossin (posted 2019-11-12 03:21:26.0)

Hello Mincheol, Thank you for your feedback. To characterize the QTF an sine signal between 3 and 50 mV (peak to peak) shall be applied. The actual voltage level depends on the pressure inside the measuring cell. To find the resonance frequency of the QTF one should start with a frequency slightly below 12455 Hz and slowly increase the frequency until resonant vibration is found. Please note that the applied voltage into the MODULATION IN is reduced by an factor of 256 via an voltage divider before it reaches the QTF.

Mincheol Jeon (posted 2019-11-11 04:53:01.51)

I want to know what kind of the electrical modulation(sine) IN for QTF characterization.(voltage modulation? or current modulation..) Best regards, Mincheol.

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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Acoustic Detection Module for QEPAS

Features