Quantum cascadelaserspectroscopy

Learn more about different phenomena associated with light waves and their behaviour under different conditions with the team of expert mentors at BYJU’S.

Quantum cascadelaserapplications

White light is primarily composed of light of different wavelengths (colours) viz. violet, indigo, blue, green, yellow, and red with red having the highest wavelength while violet having the lowest wavelength.

Block has used its decades-long experience with infrared spectroscopy and system-level packaging to develop complete systems that are capable of detecting and identifying substances. Even though the heart of the Block systems is the QCL, thermal stabilization, elimination of environmental interferences, algorithmic analysis and telecommunications-type of packaging expertise have all contributed to the development of the laser-based systems offered by Block today.

One of the key breakthroughs in the development of QCLs has been the availability of Molecular Beam Epitaxy (MBE) machines, which have enabled the growth of thin layers with dramatic precision and control. These machines are now considered mature and reliable and the QCL manufacturers have been using them routinely. Furthermore, over the past few years, unique coatings technologies have been developed and perfected, allowing the deposition of such coatings on the facets of the lasers with impressive optical properties and operational reliability. QCLs today are used in both pulsed and continuous wave (CW) modes of operation, each one offering specific advantages depending on the particular application.

QCLs are semiconductor devices, which operate differently from conventional semiconductor lasers. In general, a semiconductor material absorbs photons when excited electrons move from the valence band into the conduction band (leaving positive "holes"). In reverse, photons are emitted when electrons drop into the valence band and eliminate the "holes". In conventional semiconductor lasers, these actions occur in the "active region", which is typically a two-layer structure of different semiconductor materials, forming a p-n junction.

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This operation of the Block spectrometers is often called "pre-dispersive spectroscopy", since light is split into the various wavelengths (i.e. each laser pulse corresponds to a different wavelength) prior to its interaction with the target substance. Such operation offers very narrow spectral linewidths, as narrow as 0.1-0.5 cm-1, which enable high resolution spectral measurements, especially for gases or substances that have spectral features too close to each other.

"Quantum Leap: Spectrometry equipment suppliers are starting to take advantage of the quantum cascade laser," N. Anscombe, Electro Optics [pdf]

Qcl laserprice

Dispersion is defined as the separation of white light into different colours when the light is passed through the prism. The scattering of light depends on the wavelength of the light. Therefore, it can be said that the degrees of deviation is dependent on the wavelengths. The deviation in the path of the light is inversely proportional to the wavelength.

Specially designed cladding layers around the active region constrain the generated photons, which are forced to bounce between two specially coated facets that act as the traditional mirrors of a laser cavity. The wavelength of the emitted light is determined by the "energy gap" between the valence and conduction bands of the semiconductor material, which is predetermined and it is controlled by the optical properties of the semiconductor material. Therefore, the wavelengths can only be changed by selecting different materials, a process that is not easy in general.

Qcl laserreview

In contrast to the above described conventional semiconductor lasers, the QCLs rely on the emission of photons only within the conduction band. Electrons "cascade" from higher to lower energy levels of quantum wells within the band and as they do so, photons are emitted. This process generates wavelengths that are now controlled only by the thickness of the quantum-well layers. Therefore, the emitted wavelengths can be designed to fall within essentially any region of the mid- and far-infrared spectrum by controlling the thickness of the layers, rather than the specific optical properties of the semiconductor materials.

Block then utilizes its unique laser capabilities as the basis for broad spectral analyzers for a variety of gas, liquid, and surface detection applications. In these systems each laser pulse is monitored by built-in, unique, high-speed detectors and electronics, which analyze it after its interaction with the substances under investigation and the algorithms convert the measurements into infrared spectra. Built-in libraries analyze these spectra, account for background interferences and provide identification of the substance. The QCLs operate typically in the 200 kHz range and all data processing is performed in real time, enabling sub-second measurements.

Quantum cascadelaserworking principle

In optics, a prism is defined as a transparent solid body that has three rectangular lateral surfaces and two triangular faces that are inclined at an angle. The angle at which the surfaces are inclined is known as the angle of the prism. It is also known as refracting angle. The other important term in the prism is the angle of deviation of a prism. It is defined as the angle between the incident ray and the emergent ray. Following is the formula used for calculating the angle of deviation:

Qcl lasercost

Block is using pulsed Quantum Cascade Lasers in a grating-based, External Cavity configuration under various designs. Block’s integrated laser systems offer continuous tuning anywhere between 5.4 - 12.8 µm, the widest tuning range in a compact fully integrated system commercially available today.

"Quantum Cascade Lasers: Young technology is the next big thing in molecular spectroscopy," M. Jacoby, Chemical & Engineering News [external link]

"PHOTONICS APPLIED: MEDICAL DIAGNOSTICS: Early cancer diagnosis – the next-best thing to a cure," G. Overton, OptoIQ [external link]

When white light is made to pass through a prism, a spectrum of seven colours is formed which shows that white light is a mixture of seven different colours. Prism only acts as a medium for the separation of the seven colours. When light falls on the glass prism, refraction takes place. Since the wavelength of different components of light is different and the frequency is constant, each component gets deviated by a different angle due to the difference in velocity in the glass medium. The red colour having the maximum wavelength deviates the least and forms the upper part of the spectrum whereas violet having the least wavelength deviates the most.

Red light suffers the least amount of deviation and violet the most. Since all the wavelengths suffer different angles of deviation, when white light passes from one optical medium to another, different colours of the light split, and this phenomenon of splitting of light into its components as result refraction is called dispersion.

Block Engineering's laser-based products utilize next generation quantum cascade lasers (QCLs) and infrared absorption spectroscopy. QCLs were invented and first fabricated at Bell Laboratories in the late '70s by Federico Capasso, Jerome Faist and their colleagues, but their wide use for applications outside the laboratory is much more recent.