INTRODUCTION:

Spectroscopy:

It is a field of study where electromagnetic light with a specific wavelength and range of wavelengths is used to analyse matter either qualitatively or quantitatively¹

The study of how matter and radiation interact is known as spectroscopy. The analysis of radiation due to its wavelength gave rise to spectroscopy. In the future, the idea was considerably enlarged to include any interaction involving radiative energy and a function the wavelength.

The fundamental tenet of spectrophotometer is that light is composed of several wavelengths, each of which corresponds to a particular frequency.

Light Amplification via Radiation Stimulated Emission

The word "laser" applies to a device that produces a powerful monochromatic light beam. Lasers work by amplification or increasing the light's intensity through a process that is called stimulated emission of radiation.

PRINCIPLE:

The basic theory behind a laser is its stimulating effect of emission, which magnifies light. In order for lasers to function, a process called stimulated emission must take place. An outside electromagnetic source, such as a discharge of electricity or light, excites a population of atoms and molecules in a laser to a higher energy level. These excited atoms or molecules can emit photons when they return to their lower energy level, which can cause more emission by additional excited atoms or molecules. Due to the fact that the existence of additional photons stimulates each photon's emission, this process is known as stimulated emission²

MECHANISM OF ACTION:

Laser Gain Medium: An element that enhances light through stimulated emission is known as a laser gain medium. It depends on the kind of laser whether it is a gas, liquid, or solid.

Pump Source: The energy from the pump source is used to stimulate the atoms or molecules in the laser gain medium to a state of greater energy, that triggers stimulated emission. Flashlamps, diodes, which or other kinds of lasers can be used as pump sources. Mirrors and lenses are a couple of optical components that can be utilised to control the laser light's direction and intensity. They are employed to collimate or focus the laser beam Population Inversion: An outside electricity the source, like a flashlamp or another laser, excites a laser gain The medium which can be a gas, liquid, or solid. The electrons in the medium are excited to higher energy levels as a result, leading to a population inversion in which more electrons are found in higher energy levels versus lower ones^3-5

TYPES OF LASERS:

Laser can be classified based on several factors such as the lasing medium, the wavelength of light emitted, methd of pumping, the type of output they produce

SOLID STATE LASERS:

Solid state lasers use a solid material as the lasing medium.

The most common solid state laser materials are neodymium-doped yttrium aluminum garnet.

The lasing medium is typically in the form of a small rod or disc That is surrounded by a flash lamp or divide array that pumps energy into the medium the solid state laser can be emit light in a variety of wavelengths depending on the glazing material used Solid state laser have several advantages over other type of lasers their compact efficient and can be produce high power output with the excellent beam quality they can also operate in a continuous wave Or pulse mode depending on the application solid state laser are more complex and expensive to manufacture than often other type of temperature control to maintain their performance.

· Semiconductor lasers

· Titanium-sapphire lasers

· Nd: YAG laser

Nd: YAG LASER:

A crystal composed of Yttrium Aluminium Garnet doped with Neodymium ions serves as the lasing medium for the Nd: YAG lasers (Neodymium-doped Yttrium Aluminium Garnet laser)^5-13, a solid-state laser.

It emits light with a wavelength of 1064nm that is near-infrared.

The four levels of energy are involved in laser action since the Nd: YAG laser is a four-level laser system.

Both continuous and pulsed operation is possible with these lasers.

INSTRUMENTATION:

Laser Nd: YAG crystal: The neodymium-doped yttrium alumina garnet crystal is the Nd: YAG laser's brains. In the crystal's lattice, neodymium ions are introduced, and when energy is introduced to the crystal, the neodymium ions change from a lower to higher level of energy.

Pumping Source: In order to pump the crystal and bring the neodymium ions up to the greater energy level, Nd: YAG lasers need an external energy source. Utilising flashing lamps or laser diodes for optical pumping is the most used pumping technique. Laser diodes offer a more effective continuous-wave pumping whereas flash lamps produce brief, strong bursts of light^6-8

Resonator Cavity: The output coupling mirror and the high reflector mirror makes up the Nd: YAG laser cavity. The high reflection mirror reflects the light from the laser inside the cavity for amplifying while allowing a little portion of it to exit through the output coupler.

Nd: YAG lasers can be used in Q-switched mode, which regulates the laser's pulse duration using a device known as a Q-switch. Energy can accumulate in the laser crystal before being released as a brief, high-power pulse thanks to the Q-switch's quick variations in cavity losses. Applications for Q-switching include laser engraving, range finding, and marking with a laser.

Figure 1: Instrument of Nd: YAG Laser

WORKING:

This is called pump state because initially electrons will be in ground state and due to absorption of the input in the form of pump electrons will be directly excited to the E4.

Now, let us see how it functions,when we apply the pump electrons which are initially in the ground state will be excited to pump state.

But pump state has short life time there will be transition from E4 to E3 and has non-radiative decay.

Non radiative transition there is no laser production to this transition,but when an electron comes to state E3 it is meta stable state means electrons will stay in this state for comparitively larger period and mean while the number of flash will be appeared as input and that transfer electrons from groud state to E4 state and that establish the condition of population of inversion between E3 and E2 these two are lasing levels important lasing levels so,then the transition of electrons takes place from E3 to E2 naturally that is called as spontaneous emission^9-12

The spontaneously emitted photon will triger an another electron which is E3 state and this will be excitation of electron that will result in the emission of two photons and that is stimulated emission process and these 2 photos will be trigger 2 more electrons to undergo excitation and that will be cause emission of 4 photons and the processed is continued

So these way the transition is important and once these number of photos are emitted these will be reflected between reflecting surface and once sufficient intensity is reached the desired laser beam is generated out from E2 to E1.The transition takes place in the form of non radioactive decay,so these way electrons turns back to the ground state and get ready to gain energy from next flash and will be excited to E4 and gain to E3 and this way process is continued.

Figure 2: Mechanism of Nd: YAG Laser

GAS LASERS:

Gas lasers have been a particular kind of laser that produce coherent, amplified light by using a gaseous medium. Gas lasers use gases as its active medium as opposed to solid-state or semiconductor lasers, which employ solid or semiconducting materials. These lasers work by raising the energy levels of the gas molecules, atoms, or ions, then causing the emission of light as the excited particles fall back to their lower energy states.

The lasing medium, which controls the qualities as well as the characteristics of the generated light, is one of a gas laser's essential components. Depending on the laser's intended application and desired wavelength, other gases or gas mixes may be employed. Helium, neon, argon, krypton, xenon, nitrogen, carbon dioxide, and reactive gases like fluorine or chlorine are frequently used in gas lasers^13-16

With a wide range of qualities for research, industrial, medical treatments, and other applications that call for high-power and high-quality laser beams, gas lasers continue to be an essential and adaptable technology in many industries.

TYPES OF GAS LASERS:

Excimer lasers

Carbon dioxide lasers

Helium-neon lasers

Argon lasers

HELIUM-NEON LASER:

Type of laser: Gas

Number of levels involved in laser action: 4

Active system: Mixture of helium and neon gases (so the mixture of Helium and Neon gas is so prepared as that helium is taken as 85% and neon is taken as 15%.

The deexcitation of Neon gas results in to desired laser production and therefore the active centers are neon atoms

Pumping systems: Electring pumping

Active centers: neon atoms

PRINCIPLE:

The helium and neon atoms are in the ground state, they are not excited, so suitable excitation is caused by means of energetic electrons.

And these energetic electrons are produced with the help of electric discharge.

So the mixture is having greater concentration of helium thus main energetic electrons are pass through the mixture in the tube.

Helium is excited and neon remains in the ground state.

So, as the energy levels of helium is less and it is greater in proportion there are more chances to have excitation of helium.

Further these excited helium transfers its energy to neon and these process takes place in elastic position.

So, these energy transfer process result in to deexcitation of helium that is helium returns back to gas but its consequence is neon get excited to excited state as if neon atoms functions as active centers when the population inversion is restained it undergoes deexcitation and through that process the desired laserbeam having wavelength is produced^17-18

Figure 3: Principle of Helium neon laser

CONSTRUCTION:

It consist of quartz discharge tube having suitable dimensions and mixtures of helium and neons gases is filled inside it

The mixture is excited with the help of discharge electrodes which is operated with the help of power supply.

At one end consist of completely reflecting mirror and at other end it has partially reflecting mirror, thus these pair of reflecting mirrors represent optical resonator when the system is started when power supply is switched on this discharge electrodes activates and electric discharge is pass through the mixture means ionization takes place and that results in to highly energetic electrons which move across the mixture

Due to this energetic electrons initially helium is excited and that excited helium gives energy to neon and during the excitation of neon photons are emitted and these photons may under back and fourth reflections between these pair of mirrors and it produce the desired laser beam.

Figure 4: Instrumentation of Helium Neon laser

WORKING:

The working of these laser is explained by energy labelled diagram so, we have these energy and helium and neon separately the senergy levels can be considered.

These are the energy levels for helium F1, F2, F3 out of which F1 is ground state F2 and F3 represents excited state now for neon, E1 is ground state^19-20

The rest all E2 to E6 represents excited states.now, when the system is of that means the laser is not started the helium and neon atoms remain in the corresponding gaseous state.so, when we apply pumping that is when electron discharge is pass through the mixture initially the helium get excited that is this is excitation of helium through electron collision and these electrons are produced through electric discharge.

After that due to transfer of energy from excited helium to ground state mean neon the neon gets excited these inelastic collision results into energy transfer so the electrons in the state F3 shifts to E 6 and that forms F 2 will shift to E4 so this energy transfer process results in the excitation of neon from E1 To either E for or E-6 after that the excitation from E-6 to E5 results into radiation having wavelength 3.39 So this is not in the visible range whereas if there is dexitation to E42 E 3 it will give off radiation of wavelength 1.15

SO both these radiations are not in The visible rains however from E-6 to E3 the transition gives out radiation of wavelength 6328 So this radiation has red colour so these two lashing levels are very important E-6 to E3 when suitable population inversion is established between these two energy levels the spontaneously emitted photons will produce will causeway number of stimulated emissions and as discussed earlier these number of photons get reflected to between the pair of mirrors and finally intense laser beam is emitted out of the mirror which is partially reflected. The electron from state E3 will undergo the excitation to E-2 through spontaneous emission and from E 2 to E 1 the deexcitation takes place through collision with walls Of the tube so in order to have this particular de excitation the tube is made narrow so that there will be suitable de excitation so these may neon returns back to ground state in order to gain energy from excited helium this is the working of particular laser system in this laser the role of the helium is to have excitation of neon whereas actual laser production is done by de excitation of neon and therefore neon is considered as active centres.

Figure 5: Mechanism of Helium Neon Laser.

LIQUID BASED LASERS:

A liquid is used as the gain a medium, or the substance that amplifies light in a laser, to create liquid-based lasers. Rare-earth ions or organic dyes are frequently used as the gain medium in liquid-based lasers.

A liquid-based laser works by employing a source of outside energy, like a flash light or another laser, to excite the gain medium. The photons that the gain medium emits when activated are amplified by being reflected upon moving between two mirrors. Using one of the mirrors, a cohesive beam of light is produced as a result.

DYE LASERS:

As the gain medium, organic dyes are used in dye lasers, a particular kind of laser. An external light source—typically a high-intensity flash lamp or another laser—excites the dye after it has been dissolved in a solvent.

By entering a higher energy level as a result of this excitation, the dye molecules emit a laser-like burst of light as they recover to their ground state.

Because dyes and the solvents in which they are soaked are extremely adjustable, dye lasers may quickly change the output wavelength.

They can thus be used for a variety of tasks, such as spectral analysis, microscopy, and material processing.

INSTRUMENTATION OF DYE LASER:

The dye solution is kept in a container called a dye cell, which serves as the gaining medium. To allow for the passage of laser light, the dye cell is often composed of quartz or glass.

Pump source:

The dye molecules are excited by a pump source, which also induces the population inversion necessary for laser activity. Flash lamps, laser diodes, or other lasers are all acceptable pump sources.

The gain medium is contained within an optical cavity, which also serves as the laser's feedback system. In order to improve the laser emission, the cavity is often made up of two mirrors, one of which is partially reflecting.

Dye lasers are tunable, which means that the laser light's wavelength can be changed throughout a range of values. The laser light is dispersed into its component wavelengths using a tuning device, which can be a prism or diffraction grating, to do this.

Electronics for controlling the dye laser's operation are needed, including a power source for the pump's power the source, a control unit with the tuning process mechanism, and an interface for a computer to collect data.

LASER ABSORPTION SPECTROSCOPY:

Based on a gas species' capacity to absorb light at particular wavelengths, a technique known as laser absorption spectroscopy can be used to determine the quantity of that gas species in a sample.

The law of Beer-Lambert, that states that the amount of light absorbed by a gas sample is proportional to the amount of the gas and the pathlength of the light through the sample, serves as the foundation for the theory underlying laser absorption spectroscopy.

A potent method for examining how light interacts with matter is laser absorption spectroscopy. It offers useful details regarding the make-up, framework, and characteristics of many materials. Numerous essential components are commonly present in laser absorption spectroscopy apparatus. These fundamental components are typically present in such setups:

INSTRUMENTATION:

The main light source used in laser absorption spectroscopy is a laser. It produces a coherent monochromatic beam of light at a particular wavelength. The spectral range that is preferred and the sample's absorption properties influence the laser that is selected.

Optics:

The laser beam is controlled using a variety of optical components. These consist of filters, beam splitters, mirrors, and lenses. The laser beam is collimated, focused, directed at the sample, and collected light that has been reflected or absorbed using optics.

The substance under investigation is kept in the sample chamber. The sample and laser beam can interact with each other thanks to its design. Typically, quartz or borosilicate glass, which are transparent materials that work with the laser's wavelength, is used to construct the chamber.

Detector:

After light interacts with a sample, a detector gauges how much of it is transmitted or absorbed. The wavelength range and the level of sensitivity needed for the tests will determine the detector to use. Photodiodes, photomultiplier tubes (PMTs), and charge-coupled devices (CCDs) are examples of commonly used detectors.

Signal analysis and processing:

Spectral data is extracted from the detector output by processing. In order to do this, the detector signal may need to be amplified, filtered, and digitalized. After the signal has been processed, it is examined using a variety of methods, including Fourier transform spectroscopy and wavelength modulation spectroscopy, to provide absorption spectra or other pertinent data.

Measurement accuracy depends on calibration, which is essential. In order to create a curve of calibration or calibration factors, it is necessary to use sample references with known absorption properties. These calibrations make it possible to analyse the sample's absorbance characteristics quantitatively.

Data acquisition and control:

To operate the laser, capture the detector signal, and carry out real-time data processing, a computing device or data gathering system is frequently used. It makes it possible to automatically collect data, analyse it, and display the absorption spectra.

Tunable Diode Laser Absorption Spectroscopy:

The fundamental idea behind TDLAS is to use a laser beam to pass through a sample containing the target gas and then look for light that has been absorbed at a certain wavelength. A diode laser is frequently used in TDLAS because it can be configured to emit light at various wavelengths within a constrained range.

INSTRUMENTATION:

Gas analysis and trace gas detection are both accomplished using the TDLAS (tunable diode laser absorption spectroscopy) technology. A tunable diode laser is used as the light source, and the laser light's absorption by the gas sample is measured to determine the composition and concentration of the gas. The following elements are commonly included in the instrumentation of TDLAS:

The TDLAS system's primary element is a tunable diode laser. It is a semiconductor laser that can be adjusted to emit light at particular wavelengths. To cover the target gas's absorption lines, the laser needs to have a small linewidth and a large tuning range.

Laser control system: The diode laser's current and temperature are precisely controlled by the laser control system, ensuring consistent and exact wavelength tuning. To keep the laser at the appropriate operating parameters, it often contains laser current and temperature controllers.

Gas cell:

The gas to be analysed is delivered into the sample chamber known as the gas cell. In order to effectively absorb the laser light, it is built to offer a lengthy optical path length. It is common practise to use excellent windows that are visible for the laser wavelength while building gas cells.

Photodetector:

The photodetector measures the laser light's intensity after it interacts with the gas sample. Depending on the application, it can be a photodiode, a tube with a photomultiplier (PMT), or another appropriate detector. To extract the absorption signal, the detector's output is typically amplified and processed.

Signal processing and data analysis are used to extract the absorption signal from the electrical signal from the photodetector. In order to improve signal-to-noise ratio, a variety of approaches, including lock-in amplification and wavelength modulation spectroscopy (WMS), may be used. The data is then examined to ascertain the gas concentration using spectroscopic models or calibration curves.

Data acquisition and control:

The TDLAS system can be managed and run by a computer or specialised control unit. The laser wavelength may be adjusted, measurement settings can be managed, and measured data can be collected and stored for further study.

Calibration system:

A calibration system is utilised to create a connection between the gas concentration and the measured absorption signal. Known target gas concentrations are introduced to the gas cell in this method, and the related absorption signals are subsequently measured. During the actual gas analysis, the quantification process uses the calibration data.

Detectors used in Laser Tunable Diode Laser Absorption Spectroscopy:

Photodiode detector, photomultiplier tube, Indium gallium arsenide detector.

Photodiode Detector:

To detect the attenuated light after it has reacted with the material, photodiode detectors are frequently employed in tunable diode laser absorption spectroscopy (TDLAS). In TDLAS, photodiode detectors are used as follows:

Principle:

Fundamentally, photodiodes are semiconductors that transform photons into an electrical current. The photoelectric effect is the basis for how they work. Light causes electron-hole pairs to form when it hits the photosensitive region of the photodiode. According to the intensity of the incident light, a photocurrent develops.

Detection of absorbed light:

TDLAS uses a tunable diode laser to emit light at a precise wavelength that matches the gas's absorption band in order to detect absorbed light. When light at a given wavelength is transmitted into a sample gas, it passes through the gas and, if the gas absorbs the light, the light intensity is reduced.

Photodiode Positioning:

After the light has gone through the sample and attenuated, it is directed towards the photodiode. It serves as a detector, transforming the light that is still there onto an electrical signal.

Signal processing and amplification:

The photodiode normally produces relatively little electrical current. As a result, it is frequently required to boost the signal using the proper electronic equipment. The signal is then further processed, often using data analysis and analog-to-digital conversion methods.

Calibration and Concentration Determination:

The gas absorption can be estimated by measuring the light intensity before and after the sample. On the basis of existing curves for calibration or known absorption coefficients, this data is then utilised to compute the amount of the target gas in the sample.

TDLAS frequently use photodiode detectors because of their sensitivity, quick response times, and affordability. They are useful for various gas detection applications and are simple to integrate into TDLAS systems, particularly when intermediate to excellent sensitivity is required.

INDIUM GALLIUM ARSENIDE DETECTOR:

Near-infrared (NIR) applications frequently use photodetectors of the Indium Gallium Arsenide (In GaAs) type. It refers to a semiconductor device that's capable of sensing light with wavelengths between about 0.9 and 1.7 micrometres (m).

Applications in spectroscopy, telecommuting, environmental monitoring, including industrial process control are particularly well suited for IN GaAs detectors. They are appropriate for low-light situations due to their strong NIR sensitivity and ability to detect light with little noise.

The main benefit of choosing in GaAs as the detector's material is that it is capable of absorbing photons within the near-infrared spectrum thanks to its high bandgap energy. The ratio of indium (In) to gallium (Ga) in IN GaAs can be changed, allowing the detector's spectral reactivity to be tuned within the NIR range.

In GaAs detectors, unlike some other types of detectors like mercury cadmium telluride (MCT) detectors, may work at room temperature, negating the need for cooling systems. They are now more practical and economical for a variety of applications.

In GaAs detectors are useful instruments in many areas of research, technology, and industry because they offer a flexible and effective solutions of the detection and measurement of light within the near-infrared range.

CASE STUDY:

Detection of microbial growth in aseptic food products using non invasive tunable diode laser absorption spectroscopy.

The tunable diode laser absorption spectroscopy apparatus tracks variations in carbon dioxide brought on by microbial growth using laser light.

When food was contaminated at 100CFU/ml, TDLAS was able to identify the growth of Lacto coccus lactis in 20hours and Staphylococcus pasteuri in 55hours.

In diagnosing the microbiological contamination by common spoilage bacteria in commercially sterilised dairy beverages, the TDLAS technology was trustworthy within certain bounds.

APPLICATIONS OF LASER ABSORPTION SPECTROSCOPY:

Chemical identification: By analysing the individual absorption spectra of the molecules or chemicals present in a sample, laser absorption spectroscopy may be utilised to identify their presence.

Quantification:

Using laser absorption spectroscopy, one can determine the concentration of a specific chemical in a sample by calculating the amount of light that sample absorbs.

Temperature measurement:

By examining the absorption spectra of particular molecules that are temperature-sensitive, laser absorption spectroscopy may be employed as well to determine the temperature of a sample.

Gas phase studies:

Laser absorption spectroscopy is frequently used to investigate the behaviour of gases, such as ambient gas absorption, gas diffusion in porous materials, or gas interactions with surfaces.

Environmental monitoring: Measurement of the concentration of contaminants in the air or water is one application of laser absorption spectroscopy, which is a useful technique for environmental monitoring.

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Received on 02.04.2024 Revised on 26.10.2024

Accepted on 28.01.2025 Published on 03.03.2025

Available online from March 07, 2025

Asian J. Res. Pharm. Sci. 2025; 15(1):27-34.

DOI: 10.52711/2231-5659.2025.00005

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