Analysis of 3D printed photoacoustic spectrometer

3D PRINTED PHOTOACOUSTIC SPECTROMETER
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Abstract
With continued technological development, a wide range of techniques and equipment have been modified to improve their functionality and ease of use. The tunable diode laser spectroscopy is an example of such improvements that have increased the efficiency in their fields. The TDLS provides a more effective mechanism for tracing and measuring gas components in the atmosphere. Also, the technique offers a more time-efficient method as it incorporates a simple setup that can be adjusted to fit the environment in which it is being operated. Further, TDLS is developed such that it can be modified to fit the nature of the environment being tested. For instance, TDLS can be used as direct detection or quartz-enhanced photoacoustic spectrometry. Similarly, modulation techniques can be applied to make sure that the technique can work even in settings that have noise levels that can interfere with the results. The functioning of different TDLS mechanisms improves the ability of the techniques to be used in all environments that require the tracing and measurements of gas components. In so doing, the technique makes sure that the equipment can operate in all settings as desired by the user.
Tunable Diode Laser Spectroscopy
Tunable Diode Laser Spectroscopy is an innovated form of spectroscopy which more advanced and efficient compared to those that were used previously. The primary purpose of utilizing tunable diode laser is to facilitate the exploitation of the modern day techniques that make use of atoms and molecules.

According to Ilke, Bauer, and Lengden (2017, p. 2; Bolshov et al. 2015), Tunable Diode Laser Spectroscopy is a single line monochromatic which displays various advantages including the calibration that is highly stable than ever before. Users of Tunable Diode Laser Spectroscopy have been praising the techniques given that the results obtained from its use are reliable and valid compared to the traditional methods (Ilke, Bauer, and Lengden, 2017, p. 2). Change is inevitable which call for people to adopt the current methods of production to remain relevant in the industry. The second advantage of using the Tunable Diode Laser Spectroscopy is the measurement is fast and continuous which limit the interference of other gases. Preventing the interference of the gases is the main reason behind the development of Spectroscopy to Tunable Diode Laser-based.
Today, TDLS has gained popularity among people, and research indicates that it has become a proved methods that are used for gas diagnostic. The reason behind the report is the feedback from the users especially those that use it at large scale who say that the methods have been increasing convenience. According to Larcher et al. (2015, p. 84), the methods have become most appropriate when measuring gases in an environment which has high pressure, temperature, and species that have a high concentration of gases (Larcher et al. 2015, p. 83). For a long time, the tradition methods main limitation was the environmental conditions which sometimes are adverse and limit users from getting an accurate result (Bolshov et al. 2015, p. 64). The Tunable Diode Laser Spectroscopy has evolved to become what it is today, and more changes are expected shortly. When one compared the Tunable Diode Laser Spectroscopy and the methods that were used in the past it is evident that TDLS is much better and reliable.
The development of Tunable Diode Laser Spectroscopy has been systematic, and major changes have been occurring drastically (Larcher et al. 2015 87). The development takes place by adding element and figures to the algorithm that is used by the engineers. According to Pogány et al. (2015), the Tunable Diode Laser Spectroscopy is more effective when it comes to absorption, and the appropriate algorism has been put in place for data processing to ensure accuracy and accelerate diagnostic cycles which are detailed (Pogány et al., 2015, p. 258). The algorisms are made relevant using the new methods adopted from the molecular and atomic field. The paper will review the functioning of Tunable Diode Laser Spectroscopy and compare techniques such as direct detection and modulation spectroscopy.
Components of Tunable Diode Laser Spectroscopy
The main function of the Tunable Diode Laser Spectroscopy is to maintain a record of gases that are in the atmosphere. For example, ammonia (NH3) is a hazardous gas if left to sip without control (Moruzzi, 2018 12). Therefore, it is the role of the user even those that operate the heavy commercial machine to understand its emission and how to control it from becoming harmful to the people in that environment. The components of Tunable Diode Laser Spectroscopy include Tunable Diode Laser light source, optically form of absorption medium, transmitting optics, receiving optics and detectors. The detectors are very important when detecting gases and other components and properties. The properties are separated given the users of Tunable Diode Laser Spectroscopy a platform to monitors the gas they need such as the HCl gas. In other cases, the Tunable Diode Laser Spectroscopy may have a gas analyzer which is used in many cases to detect gases (Long et al. 2014 490). The main gases detected by the Spectroscopy include the O2, HCL, HF, CO2, CO, NH3, H2O, NO, CH4, H2S, N2O, HCN among others.
How to operate Tunable Diode Laser Spectroscopy
The users should understand how to use the Tunable Diode Laser Spectroscopy to ensure reliable data. Therefore it is crucial to understand all the requirement and measures to put into consideration before starting the process. Some of the major facts are that the lasers are made up of small crystals (Kireev et al. 2018 9). The crystals include the As, GA, SB, and P. the understanding of the crystals is the first step toward a worth its operation. The second factor is that the laser center wavelength depends highly on the size as well as the composition of the crystals. The composition of the crystal is the measurement needed to operate the Tunable Diode Laser Spectroscopy. The third aspect is that the wavelength can be changed. In a case where the wavelength changes, if the shift is over a narrow range then the current should be changed. On the other hand, if it is over a wide range, then the user should change the laser operating temperature.
The temperature of the laser and all the operations of the Tunable Diode Laser Spectroscopy methods have very important factors. The ability of the Tunable Diode Laser Spectroscopy to change the temperature by changing the electric current allows for the scanning of the entire absorption features. The features send a message about the gasses in the atmospheres and understanding the measures to take to manage them and prevent them from reaching a point of harm. Another important factor of scanning the entire features is to eliminate the dust. Dust is a limiting factor due to its density and the fact that it contains a lot of impurities. When the users understand all the above information about Tunable Diode Laser Spectroscopy methods, it allows them to use it appropriately to yield accurate information. The gases are differentiated and filtered through the abortion density that they have which is different for different gases.
Application of Tunable Diode Laser Spectroscopy
Today, many people are filing cases regarding social amenities against companies which emit poisonous gases into the atmosphere. People have started complaining due to the adverse effects we have been facing due to global warming (Bauer et al., 2014, p. 4797). Therefore, companies are forced to emulate various methods of monitoring and managing the amount they release to the atmosphere. Furthermore, the agencies that monitor the gases the emitted by companies also prefer the newly adopted Tunable Diode Laser Spectroscopy to ensure efficiency in their jobs. According to Pedone et al. (2014), many companies have preferred to use Tunable Diode Laser Spectroscopy for the tracing and measuring of gases in the atmosphere. In the beginning, the technique was using mid-infrared lasers to address the absorption lines. However, there reached a point when the beneficiaries needed a mechanism which would capture cases of cooled detectors which manifested difficulties when people tried to deploy them in other applications (Li et al. 2016 390) Therefore, the adoption of Tunable Diode Laser Spectroscopy was to ensure efficiency in different conditions and situation of measurement.
Later, the evaluation had begun because people needed a strategy which would eliminate all the problem brought by the mid-infrared. Therefore the adopted more competitive applications such as the near-IR distributed-feedback which eliminated the problem of first-overtone absorption lines (Fetzner et al. 2018 675). However, there was still another problem which was the weaker absorption strength the two target gases with the same magnitude in that spectrum region. However, the utilization of Tunable Diode Laser Spectroscopy has been able to eliminate the various limitations associated with the other measures previously utilized in the field. The detectors and laser that were the development of the communication and were compatible with the single-mode optical fibre used in many industries. The fibre compatibility with the detectors and the laser to enable for the multiple-sensing points to be exploited appropriately.
Analytical Methods in TDLS and Molecular Mechanisms
Before addressing the analytical methods in Tunable Diode Laser Spectroscopy, it is important to address some of the factors about gases that are in the atmosphere. The gases are the reason for the existences of all the living organism including human beings. The gaseous exchange for all the organism includes the inhaling of oxygen and other gases as well as exhaling others such as water vapor and carbon dioxide (Qu et al. 2016 3754). The gases are produced through the energy-producing mechanisms in the body of living organisms. The processes in the body or an organism happen too fast, and the body has to take it in and out at a given percentage. Despite the mechanism, there are other gases which are light but are present as traces in the atmosphere (Cui et al. 2018). The light gases are produced through a various process in the body include information transfer, the biochemical reaction among other sources. The gases and another chemical should be excreted from the body to prevent accumulation the same should be monitored in the atmosphere through Tunable Diode Laser Spectroscopy to prevent them from hurting people (Burd et al. 2015, p. 7). Through Tunable Diode Laser Spectroscopy users can understand when the environment has a disease-causing organism.
The cooling technique which replaced the mid-infrared showed a positive effect used to simplify the molecular spectra. The Tunable Diode Laser Spectroscopy proved to be effective in providing high efficiency in optical absorption. With that in mind, the cooling cells used were combined with the Tunable Diode Laser Spectroscopy (Mirov et al. 2015, p. 295). Mostly many specialists choose to use the liquid nitrogen to attain a temperature of 115k to allow absorption of other gaseous molecules. The cooling effect is very crucial in this case and allows for the drop in the density if the spectral line number which can investigate CHF3 and CH4. The factors which are crucial to the process for as improving the single-line resolution using a factor of 5 was used (Elia et al., 2009, p. 9620). By enhancing the factor to 2.5 ensured that the molecules that were detectable are detected in traces and the concentrated form using the Tunable Diode Laser Spectroscopy. The technique was applicable mainly for the element that was attainable through temperature lower than those of liquid nitrogen.
Advantages of Tunable Diode Laser Spectroscopy
There exists a variety of positive impacts that can be associated with the use of TDLS. Mainly, the advantages are based according to the ability and the capability of the methods of gases diagnostics. One of the advantages is the fact that the method gets real-time process feedback (Li et al., 2014, p. 667). Getting the feedback on time helps in understanding when there is a problem which needs fixing. Timely feedback also guides the people in determining the next step on time or increase the measures taken over a specific time. The second advantage of Tunable Diode Laser Spectroscopy is the fact that the in-situ measurement is integrated across the paths that are used by the Spectroscopy (Xu et al. 2016, p. 3). When the gases are passed through the paths, the give the user a chance to choose the gas that they need measuring. Last but not least, the Tunable Diode Laser Spectroscopy has multi-path grid allay. According to Li et al. (2014, p. 668), the multi-path grid allay has the following elements including stratified application and lack of gas bias when the gases are being transported.
Disadvantages of Tunable Diode Laser Spectroscopy
The Tunable Diode Laser Spectroscopy has several advantages which can make the users choose another method of gases diagnostic. The limitations include the problem when aligning the optics. The Tunable Diode Laser Spectroscopy involves a series of heating and cooling when separating the gases. According to Li et al. (2014), the heating and cooling lead to deformation in ducting which result in misalignment. Misalignment can lead to the unwanted outcome or misleading results from the diagnosis. The second limitation is the heavy dust loading application (Pedone et al. 2014 82). Heavy dust loading application is the component because it must have the ability to transmit light as required through the path. Without heavy dust loading application then the multiplexed signals allow for the strongest signal to be passed through the paths. The evolution of Tunable Diode Laser Spectroscopy has been continuous, and through research and development, people can find a way of eradication the disadvantage over time.
Comparison with other Techniques
Modulation Spectroscopy
The use of modulation spectroscopy in detecting and measuring the concentration of trace gases tends to differ from TDLS in a variety of aspects. TDLS makes use of laser absorption spectrometry and tunable diode lasers. Modulation spectroscopy, on the other hand, utilizes the fact that mechanical noise tends to decrease as the frequency increases (MacDonald, Brandis, and Cruden, 2018, 4067). As a result, the signal to noise ratio is enhanced through the encoding and detection of the absorption signal when under high frequency in low noise settings. Modulation spectroscopy is often achieved through the use of various techniques. The most commonly used are the frequency modulation spectroscopy (FMS) and wavelength modulation spectroscopy (WMS). Both techniques facilitate the achievement of the desired modulation so that the detection process can be carried out as desired by the (Sane et al., 2014, p. 11). Often, modulation techniques are applied to achieve a higher level of sensitivity and facilitate the detection process. Modulated spectroscopy provides the advantage of making it possible to measure the difference signal that is directly related to the species concentration. Also, the technique provides for the shifting of a signal to a region of higher frequency, which makes it possible to achieve a larger signal-to-noise ratio to increase the sensitivity.
Frequency Modulation Spectroscopy (FMS)
FMS has utilized I settings where one seeks to obtain high levels of sensitivity using a simple structure. Tunable diode lasers can be used to develop a simple structure since they facilitate the generation of a narrow as well as tunable output that can be modulated as desired. In a typical FMS setting, the wavelength of a continuous-wave laser is modulated at a certain frequency (Zhang et al., 2014, p. 1981). As the wavelength in the center is scanned across the atomic alteration, the wavelength modulation is converted into amplitude modulation, which leads to the creation of modulation in the photosensitive absorption of a sample at a similar frequency. On shifting the signal to a higher frequency, the FMS is usually able to eliminate the usual shortcomings associated with absorption measurements such as the variation of laser-intensity that usually rise at direct detection and fall at 1/f, hence the name; 1/f noise. According to Fu et al. (2018, p. 134), on utilizing FMS, the sensitivity of absorptions can reach the section per million (ppm) level. For instance, the detection of H2S, measurement of yttrium absorption, and methane detection with a 1ppb precision have been done at the ppm level.
Further improvements of the signal can also be achieved by exploiting the experimental geometrics that tends to cancel certain sources of noise or some aspects of the signal that are not desired. In other situations, on giving a reasonable level of attention to the electronics and optics, it becomes possible to suppress systematic errors, and high levels of accuracy can be achieved using the FMS technique (Argence et al., 2015, p. 456). The use of FMS is essential in situations such as when determining the specific center of a line of absorption. In most applications, the center should be located in not more than 0.1% of the line width. There are various measures through which FMS can be achieved. For instance the mechanical dithering of the mirror (i.e., from 1.5 to 3.5 kHz), modulation of the injection current into the diode, or through the use of external phase modulators.
The FM Setup
In the figure above, the FM spectrometer utilized for the radical detection of shock waves is illustrated. The major features of the FM system include:
A high frequency of modulation balancing at 1.0 GHz
A high modulation index stretching up to M = 2.0
High optical power of up to 15 mW laser power.
An accurate phase alteration achieved by combining two polarizer structures with phase shifters.

In understanding the functioning and advantages of the FMS, the analogy to the AM/FM radio can be used. In a setting where conventional absorption spectroscopy is used, scanning the laser frequency across a conversion as amplitude modulated allows for the measurement of a minor adjustment in the concentration of the transmitted light (Zhang et al., 2014, p. 1982). In some settings measuring minor changes in high laser intensities, which makes the signal noisy as it occurs in the AM radio. FMS functions by promptly fluctuating the light frequency around the central carrier frequency. As a result, absorption appears as alterations in the frequency component, which can be electronically secluded. The signal produced in the process is significantly insensitive to any modifications of the laser intensity. Also, it has a larger signal to noise ratio since it is found in the FM radio, which is often less noisy. (EOM = electro-optic modulator).

The probe laser releases an exceptionally narrow bandwidth (less than 10MHz) continuous wave radiation that can be represented as a sole carrier frequency, ω0. The probe ray is then controlled using an electro-optic point moderator such that the resulting effect is recorded as an alteration in some optical power into sidebands that have even spaces around the carrier units of the modulation frequency that is applied, ωm (Sur et al., 2015, p. 106). The sideband intensities, on the other hand, are determined by the utilized modulator voltage. They alternate positions in a specified system, which causes an FM electric field enveloping. A square-law is used in identifying the envelope square as seen in photo-receivers where a stage-moderated beam can only produce a DC signal that relates to the bean intensity.
One way to picture the described situation involves noting that the carrier, as well as low-frequency sideband, are able to produce a beat frequency at ωm on the detector. However, the higher frequency sideband can also be paired with the carrier to produce a ωm frequency signal of the reverse phase. Eventually, the beat signals cancel perfectly provided the sidebands do not possess any differences. However, if the sidebands’ symmetry is interfered with, an amplitude modulated field envelope is created at the applied modulation frequency (Mei and Svanberg, 2015, p. 2239). In so doing, it becomes possible for the detector to DC signals and an oscillating segment at the modulation frequency, ωm. The beat frequency can be secluded electronically and taken through demodulation to form a zero-background as well as a spectroscopic technique that possesses high sensitivity and can be used for measuring the absorption and dispersion.
Wavelength Modulation Spectroscopy (WMS)
In most aspects, WMS is similar to direct absorption spectroscopy. However, the laser’s wavelength of the laser is controlled by using a prompt sinusoid as frequency f. The interaction that occurs between the promptly modulated wavelength and a non-linear feature of absorption creates harmonic elements in the signal produced by the detector that can be secluded using lock-in amplifiers (Goldenstein et al., 2015, p. 352). The second harmonic (2f) is utilized since its signal strongly depends on spectral constraints and gas aspects, which allows it to be compared to spectral replications to understand gas properties. In addition to a high sensitivity to gas properties, WMS-2f possesses a variety of benefits that make it more preferable that direct absorption in specific sensing applications. As stated by Qu et al. (2015, p. 16496), the 2f signal is also more sensitive to curvatures and spectral shapes than absolute levels of absorption, which is useful for specific high-density spectra, especially those affected by broadband emissions or absorptions. Also, using a lock-in amplifier plays the role of discarding the noise that falls external to the lock-in passband such as electronic noise and laser intensity.
For sources of laser that have synchronous tuning of the intensity and wavelength, the intensity modulation is often the strongest element of the first harmonic signal. As a result, it can be utilized to normalize the signal of the 2f against distresses to the laser intensity by window fouling, laser drift, and beam scattering or steering (Cai and Kaminski, 2014, p. 154106). The WMS attribute is mostly useful in harsh settings. Benefits associated with WMS tend to increase its preference in difficult absorption or harsh measurement settings. However, there is usually the need to calibrate the measurements on-site to allow for the acquisition of an absolute concentration and information on temperature (Goldenstein et al., 2014, p. 721). As a result, the technique becomes less desirable in real practice situations as the aspect increases undesirable complexity to the measurement process. In most cases, the WMS technique is utilized in environments where the conditions are known. However, it requires the isolation of the absorption feature and a provision for the complete tuning of the laser across the absorption feature so that the non-absorbing area on both sides can be utilized for extracting laser intensity for signal normalization. The process can be difficult in actual practice, which tends to compromise gas mixtures that contain intrusive species such as water and elevated pressure that can expand the feature beyond the laser scanning range.
Principles of TDLS with WMS
The development of TDLS with WMS was mainly motivated by the fact that it can allow the achievement of greater sensitivity than TDLS with direct detection. A higher secondary frequency and a sinusoidal modulation of the current on the laser drive increase the sensitivity in WMS (McGettrick, 2007, p. 41). In cases where the DFB drive current is differentiated, the laser signal amplitude and its optimal frequency output are then modulated (AM and FM respectively). The combined AM/FM effects produced by the laser drive sinusoidal modulation should be considered. It is essential to note that the FM produced in direct modulation lags the AM at this stage. The diagram below shows how TDLS with WMS is implemented.

The frequency modulation size used on the laser is characterized by the modulation index (m), which is the FM component amplitude divided by the HWHM line-width of the absorption profile of interest. In the presence of the interrogated gas, the laser FM component interacts with the absorption profile to produce an amplitude modulation signal. The line-shape of the absorption profile is not retrieved directly McGettrick, 2007, p. 42). Instead, the harmonic signal accompanies the corresponding derivative of the fundamental absorption line-shape to an extent determined by the index of modulation. The polarity of the harmonic signals produced by the LIA depends on the phase of detection associated with the input. At small m values, the harmonic signal is closely identical to its derivative, although it can deviate increasingly as m increased. The figure below shows the relationship between the absorption profile and its 1st and 2nd derivatives.

In the absence of gas, the FM produced by sinusoidal modulation does not contribute to the signal. Instead, the LIA signal is an effect of the AM. When gas is present, AM contribution possesses a secondary impact of distorting the derivative signals produced by the FM and absorption profile interaction. The total AM effect is referred to as the Residual Amplitude Modulation and is minimized because it is considered an altering side effect McGettrick, 2007, p. 43). TDLS with WMS is used in inferring gas concentration with a one-point calibration that considers all scaling factors involved in the system. In 1st harmonic detection, the highest amplitude directly relates to gas concentration while in 1st harmonic, the highest amplitude and line-center slope can relate to the gas concentration. With perfect normalization schemes, TDLS with WMS can be accurate, calibration stable, and reliable.
Challenges Associated with TDLS with WMS
TDLS with WMS tends to be accurate in cases where the operating conditions do not change. On analyzing the modulation index of 1st and 2nd harmonic line shapes, it is clear that the modulation index significantly affects their signal amplitudes McGettrick, 2007, p. 46). A section of the system scaling factor obtained from the calibration process tends to rely heavily on the crucial parameters of the modulation index; δυ and γ. Although δυ can be regulated in the short-term, it can be affected by minor drifts over long periods of time especially due to the degradation of the laser and/or a position change of the non-linear frequency of the laser vs. current components related to the absorption profile McGettrick, 2007, p. 46). As a result, δυ would require recalibration at fixed intervals because the parameter must be accurately identified.
Gas temperature and pressure affect the absorption profile line-width, which causes the need to obtain its accurate measurements in environments where the gas characteristics can change so one can account for variations in operating conditions in the normalization process McGettrick, 2007, p. 47). Similarly, the 1st and 2nd harmonic signal amplitudes are highly dependent on the line-width such that any errors in the measurement can affect the gas concentration measurement. Also, errors in deriving other parameters can corrupt the system scaling elements as well as the calibration. Further, a gas temperature modification significantly impacts the line-strength of the absorption profile, which worsens the problem.
Applications of TDLS with WMS: Temperature and Pressure
An experiment to examine the severity of the limitations of TDLS with WMS revealed that the effects of temperature and pressure in the technique should be accounted for during normalization to improve its accuracy McGettrick, 2007, p. 48). The experiment showed that increasing pressure causes a corresponding increase in the absorption profile line-width and a decrease in modulation index, both of which are primary causes of changes in the amplitude as shown in the figure below.

Similarly, increasing gas temperature from 800°C to 950°C caused a decrease in the line-strength of the absorption profile as shown in the figure below. As a result, the experiment showed that gas temperature variations need to be accounted for to maintain the accuracy of measurements in TDLS with WMS (McGettrick, 2007, p. 49).

TDLS with Direct Detection
Direct absorption spectroscopy incorporates the simplest execution of laser-based absorption methods. In direct detection, a tunable laser beam is sent through a sample of gas, and the transmitted intensity is measured with a detector (Ono et al., 2014, p. 6490). In a situation where the frequency of the light is near a molecular or atomic transition, the light is often absorbed, which decreases the transmitted intensity. The concentration of the species used to absorb can also be computed from the relative modification of the intensity as proposed by the law of Lambert-Beer (Prokhorov, Kluge, and Janssen, 2016, p. 3931). Although the technique is simple, one its main shortcoming is that its sensitivity level is restricted by the low-frequency noise that exists in the signal, which is in most cases initiated by mechanical instabilities and laser intensity noise among other external variations. According to Chen et al. (2016, p. 100701), the defined situation is in most cases, termed as the 1/f noise, especially because its power range scales disproportionately with the opposite frequency. The influence possessed by the 1/f noise can be significantly reduced by changing the detection greater frequencies through the use of modulation methods.
Direct detection has often been utilized to make measurements based on absorption of gas properties in harsh settings due to its accuracy, simplicity, and the ability to make complete measurements. For absorption by small molecules at low to moderate pressures, the wavelength that is scanned different from direct absorption spectroscopy is in most cases utilized (Klein, Witzel, and Ebert, 2014, p. 21505). In the figure below, a distinctive scanned-wavelength direct-absorption measurement using a tunable diode laser is shown. The diode laser injection current is tuned with a repetitive ramp waveform. The setting often has the impact of inclining the laser wavelength and the intensity (Bolshov, Kuritsyn, and Romanovskii, 2015, p. 51). In a case where the nominal wavelength of the laser tends to relate to a spectral absorption feature for a type of element in the investigated sample of gas, then the laser is tuned across the feature to create a signal that is similar to the one shown in the diagram (Liu et al., 2015, p. 1376). The non-absorbing areas of the inclining intensity signal are fit with a baseline which is utilized to normalize for any modifications in the incident laser intensity and provides for the computation of the absolute absorbance using the Beer-Lambert relation.

Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS)
The QEPAS provides an alternative technique to the photoacoustic detection of tracing gases with the use of a quartz tuning fork taking the place of an acoustic transducer to facilitate the detection of frail photoacoustic excitation as well as providing for the use of exceptionally minor volumes (Wang, Geng, and Ren, 2017, p. 1837). In such a measure, it becomes possible to avoid any limitations caused on the gas cell by circumstances of the acoustic resonance. A quartz crystal is an ordinary candidate for the mechanism since it is a low cost and well as low loss piezoelectric material. High quartz crystals can often be used as a way of setting the standards frequency in clocks, smartphones, and watches. Similarly, quartz tuning forks that have a resonant frequency of around 215 or approximately 32,768 Hz can be suitable for use in the process (Ma et al., 2017, p. 31107). Also, quartz tuning forks can have a Q of approximately 100,000 or more when compressed in vacuum and 10,000 under standard atmospheric pressure. Therefore, the resulting time used for energy buildup at atmospheric pressure is approximated at 320 ms.
Acoustically, quartz tuning forks are a quadruples that tend to create a good environmental immunity to noise. It is essential to note that the width of the quartz tuning fork resonance under standard pressure is approximately 4Hz (Liu et al., 2016, p. 214). So only the components of the frequency in the narrow spectral band can create efficient excitation of the quartz tuning fork vibrations. Waves of sound in the air having approximately 32 kHz may possess an estimated acoustic wavelength of one centimeter and is therefore created by external acoustic sources. Often, such waves can apply energy in a similar direction on both quartzes tuning fork prongs situated at approximately 1 mm distance (Zheng et al., 2016, p. 11103). As a result, the sound waves do not stimulate the piezoelectrically dynamic mode where both prongs head towards opposite directions and produce a zero electrical response.
Therefore, there only exists one way of making the quartz tuning forks resonate through the photoacoustic impact to create sound waves from an origin situated between two quartz tuning fork prongs. The most ordinary way of achieving the described condition involves the laser beam excitation so that it can pass through the space created between the prongs such that it does not touch them (Wu et al., 2017, p. 15331). Generating a photoacoustic wave incorporates the transfer of energy from internal to translational little freedom degrees (Rück, Bierl, and Matysik, 2018, p. 2465). QEPAS sensors have been utilized in detecting a variety of organic and inorganic trace gases with the use of laser sources in a range of wavelength that covers the visible, ultraviolet, mid-infrared, and near infrared. The figure below shows a sketch of a QEPAS sensor.

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