Using PXI and LabVIEW for Photoacoustic Solids Testing

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"Thanks to the flexibility of the NI PXI platform and LabVIEW software, we implemented a system for photoacoustic spectroscopy, which usually consists of several individual instruments and specialized software, as a unique hardware-software system."

- Aleksandar Popović, UNO-LUX NS d.o.o.

The Challenge:
Combining several individual instruments for photoacoustic spectroscopy with the added benefit of software analysis.

The Solution:
Using NI PXI and NI LabVIEW to create an instrument for simultaneous transmissive and reflective modulated photoacoustic solids spectroscopy.

Aleksandar Popović - UNO-LUX NS d.o.o.

In the 1880s, using a hearing trumpet, Alexander Graham Bell determined that sound produced by a metal membrane illuminated by a beam of sun modulated with intensity. In 1976, Allan Rosencwaig gave a theoretical interpretation of the photoacoustic effect in solids as a result of the photothermal effect. When any substance is exposed to the impact of the widest range of electromagnetic radiation wavelengths or the effect of beam particles (such as electrons, protons, neutrons, or ions), the substance absorbs part of the excitation energy. After absorption, many processes occur in which part of the absorbed energy is converted into heat. The photothermal effect creates heat due to the interaction of substances with electromagnetic radiation or beam particle excitation.

The photothermal effect produces phenomena such as

  • Temperature change on a sample surface due to heat propagation
  • Pressure change in the sample and its nearby environment (also known as the photoacoustic effect)
  • An optical refractive index gradient appearance in the sample and its nearby environment (thermal lens effect due to density changes)
  • Sample surface vibration due to volume changes and acoustic wave propagation

Photoacoustic and photothermal experimental techniques are a group of methods used to measure a sample’s optical absorption and thermoelastic characteristics. The basis of photothermal spectroscopy is the change in a sample’s thermal state resulting from radiation absorption. The light absorbed (not lost by emission) results in heat. The heat raises the temperature, thereby influencing the sample’s thermodynamic properties. Temperature, pressure, and/or density changes that occur due to optical absorption are ultimately the basis for photoacoustic and photothermal measurements.

System Setup

Classic photoacoustic spectroscopy systems (see Figure 1) consist of a light source, chopper or driver, photoacoustic cell, microphone, amplifier, lock-in amplifier, and PC. A lock-in amplifier (also known as a phase-sensitive detector) is a type of amplifier that can extract a signal with a known carrier wave from an extremely noisy environment (signal-to-noise ratio can be -60 dB or less). 

The Lockin amplifier community example provides virtual instrumentation for precisely measuring AC signals that are partially or completely buried in noise. The start-up kit extends the power of LabVIEW to measure signals that could not be measured before, such as very small signals buried 100 dB deep in noise.

A lock-in amplifier precisely measures AC signals. It removes the noise by performing a Fourier transform on the input signal at the frequency and phase carried by the reference signal. A lock-in amplifier acts as a narrow bandpass filter (as narrow as 1 mHz) around the reference signal frequency (ωR). The reference signal is fed into an internal reference synthesizer that extracts frequency and phase information from the reference signal and generates a pure sine wave. A lowpass filter that follows the mixer ideally rejects everything but the DC component, which is proportional to the amplitude of the signal component at the ωR and the cosine of its phase relative to the phase of the internal reference (A*cos(ω)). This quantity is referred to as “X.” If we apply the same process (mixing and filtering) using a pure cosine wave at ωR, quantity “Y” is generated. Y is proportional to the amplitude of the signal component times the sine of the phase shift (A*sin(ω)). From these two quantities, we can determine input signal amplitude and relative phase.

We can use photoacoustic modulated spectroscopy to accurately determine the optical, thermoelastic, electronic, and other physical properties of various inhomogeneous layers, such as optoelectronic devices, optical fibers, and biological tissues. The combined measurement requires either different sample preparations and long measurement times, or simultaneous detection. A typical approach to simultaneous measurements uses an appropriate number of synchronous detectors and complex signal processing with expensive electronics. To overcome these disadvantages, we created an instrument based on the NI PXI platform and LabVIEW software. This results in multiple detections using much less hardware and offers time-rate savings, automation, higher measurement reliability, and higher instrument sensitivity.

An NI PXI-4462 acquisition module performs identically to typical desktop lock-in amplifiers, and due to its 24-bit resolution, a preamplifier is not necessary, and a microphone signal can directly connect to the card input. Typical photoacoustic spectroscopy systems give amplitude and phase from lock-in amplifier output as a result, but with this system, we can watch the signals, their logging to file, and analysis in the time domain. System flexibility helps us realize various typical and user-defined analyses. With small software modifications, we can create a 3-channel lock-in amplifier. The system we created consists of a light source, an LED driver, a photoacoustic cell, a microphone, and a PC/PXI system (see Figure 2).



Thanks to the flexibility of the NI PXI platform and LabVIEW software, we implemented a system for photoacoustic spectroscopy, which usually consists of several individual instruments and specialized software, as a unique hardware-software system (see Figure 3). 

The system we created gives us a unique PC-based device for control, acquisition, logging, and data analysis. It offers automated experiment conduction by frequency ramp generation and online signal, amplitude, and phase monitoring, as well as XY graphics formation. We can easily extend the system with new functions and analysis.

Besides monitoring lock-in amplifier signals, we can display time-domain and online signals on XY graphs, adjust acquisition parameters, manually change LED control parameters, define ramp parameters for automatic LED control, save and load signals, and set lock-in PLL (phase-locked loop) and filter parameters.

Author Information:
Aleksandar Popović
UNO-LUX NS d.o.o.
Generala Milutina Vlajica 36
11147 Belgrade
Tel: +381112511122

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