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  1. 3 Acta Physica Polonica A

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  1. 3 2008

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  1. 3 Starecki T.

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Influence of External Acoustic Noise οn the Operation of an Open Photoacoustic Helmholtz Cell

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Starecki T.
Acta Physica Polonica A
|
2008
|
vol. 114
|
issue 6A
A-199-A-204
EN
One of the most important problems in the design of open photoacoustic cells is appropriate attenuation of the external acoustic noise, because in the case of weak photoacoustic signals even relatively low level of such an acoustic noise can severely limit sensitivity of the setup. It is important that the mentioned filtering is not limited to the frequencies close to the light modulation frequency, but performed in the possibly widest band width, because although it is usually considered that setup sensitivity depends directly on the parameters of the final narrowband filtering and/or synchronous detection, it should be noticed that it can be also seriously affected by saturation of the signal amplifiers being overdriven by the signal components resulting from the mentioned acoustic noise. The paper describes how influence of the external acoustic noise on the operation of an open photoacoustic Helmholtz cell depends on mechanical dimensions of the cell. The results show that appropriate selection of the dimensions results in noticeable improvement of the external acoustic noise attenuation.
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Windowless Open Photoacoustic Helmholtz Cell

100%
Starecki T.
Acta Physica Polonica A
|
2008
|
vol. 114
|
issue 6A
A-211-A-216
EN
A phenomenon which is usually considered as the main disadvantage of photoacoustic Helmholtz cells is photoacoustic background signal resulting from the light absorption by the cell windows. Although there are methods which reduce this background signal, e.g. light wavelength modulation or time domain filtering of the photoacoustic response from the sample, the methods are limited to particular applications, and none of them would completely remove the mentioned background signal. The paper presents design of a windowless open photoacoustic Helmholtz cell, and preliminary results of frequency response measurements of such a cell. Removal of the windows eliminated the main source of the photoacoustic background signal. Additionally, the proposed design allows for unrestricted or forced flow of the fluid filling the cell. As a result, the cell can be easily adapted for continuous, real-time photoacoustic measurements. Under proper selection of the cell dimensions, Q-factors of such open photoacoustic Helmholtz resonators are comparable to the Q-factors of corresponding closed Helmholtz cells.
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Optimization of the Duty Factor οf Semiconductor Light Sources Used in Photoacoustics

100%
Starecki T.
Acta Physica Polonica A
|
2008
|
vol. 114
|
issue 6A
A-205-A-210
EN
There are two most common approaches to the light intensity modulation that can be found in the papers describing photoacoustic experiments. In the first one, photoacoustic signal is produced by short light pulses of relatively high power, obtained usually from pulse mode lasers. In the other, the light intensity is square or sine modulated with the duty factor of approximately 50%. However, in the case of semiconductor light sources, like LED or laser diodes, it should be taken into account that duty factor of the modulation signal can be (at least in some range) exchanged with the optical output power. The paper presents discussion of the problem, including simplified theoretical analysis and experimental results. The analysis shows that in some cases, if the duty factor of a LED diode is optimized toward maximum amplitude of the induced photoacoustic signal, the resulting signal gain can be greater than 50%. Optimal duty factor depends on characteristics of the particular light source, but at least in the case of LED diodes it will be usually less than the typically used 50%.
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