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Fibre crystal mid-infrared lasers As we know, a laser is an optical oscillator made out of a gas, liquid or solid with mirrors at both ends (see, e. g. , The Computer Language Company Inc. , 2007 b). To make the laser work, the material is pumped with light or electricity. This pumping has excited the electrons and has caused them to jump to more high orbits. Some electrons do spontaneously drop back to lower energy levels what releases photons (i. e.
quantum's of light). The photons do then stimulate other excited electrons, emitting additional photons with the same energy, and, hence, the same wavelength as the original ones. The light waves do build in strength as they do pass through the laser medium, and the mirrors at both ends do keep reflecting the light back and forth, creating a chain reaction and causing the laser to lase. In simple laser cavities, one mirror only has a transparent area that lets the laser beam out; in semiconductor lasers, both the mirrors can transmit a beam.
It is also known that fibre lasers are those constructed within optical fibres (e. g. , see The Computer Language Company Inc. , 2007 a). An optical fibre can be defined as a filament of optical material (for example, quartz) having the ability to guide light along its axis and launch it at the end (Irphotonics Inc. , 2006). According to one of descriptions (The Computer Language Company Inc. , 2007 a), optical fibres have used a part of the fibre to generate the laser light; in other regards, they are in principle not different from other types of laser devices. To pump a laser material, fibre lasers have usually involved diodes.
One mirror does accept the diode pump wavelength on one side of the fibre and then transmit the lased wavelength on the other one (and, through the transparent area of other mirror, out). Diode lasers consist normally of single crystals of semiconductor materials. For instance, lead-salt diode lasers are comprised of Pb Te, Pb Se, and PbS as well as various alloys of these compounds with the same materials and Sn Se, Sn Te, CdS and others (Tittel et al. , 2003). It is well known that the infrared band of the electromagnetic spectrum is very actively used in modern practice. Infrared spectroscopy and medical applications (in particular, thermometry) are only the most common examples of todays use of infrared radiation. What is characteristic of this kind of electromagnetic waves?
The infrared radiation is the region of the spectrum with wavelengths longer than visible light (400 to 710 nm) but shorter than microwaves (1 mm to 30 cm); it may be divided in such sub-regions as the near-infrared (710 nm to 2. 5? m), the mid-infrared (2. 5 to 25? m), and the far-infrared (25? m to 1 mm) (Maulini, 2006).
The near-infrared waves do interact with the matter in the same way as visible light. This range of the spectrum is of great technological importance since it is a standard for optical telecommunications and data storage. Today it is the most developed of the three above-listed sub-regions in terms of availability of laser sources and detectors. The far-infrared band is, in fact, at the boundary between light and microwaves.
Coherent far-infrared oscillators are very important for astronomy because most of the interstellar dust clouds emit in this region of the spectrum. Other applications of this radiation include biomedical imaging, absorption spectroscopy and security monitoring. But development of these applications has relatively been slow due to the lack of compact and powerful sources of coherent radiation (Maulini, 2006). Seemingly, the most significant band of this part of the spectrum is the mid-infrared one. One of reasons for this has clearly been expounded by Tittel et al. (2003): [t]he vast majority of gaseous chemical substances exhibit fundamental vibrational absorption bands in the mid-infrared spectral region (? 225?
m), and the absorption of light by these fundamental bands provides a nearly universal means for their detection. Maulini (2006) has added that since these absorption lines are very strong, concentrations in the parts-per-billion to parts-per-trillion ranges can be detected using relatively compact laser-based sensors. He has also pointed out that another interesting feature of the mid-infrared radiation are the atmospheric transmission windows between 3 - 5? m and 8 - 12? m enabling free-space optical communications, remote sensing, and thermal imaging. At last, high power lasers from the 3 - 5?
m range are also of interest in terms of developing security counter-measures. It was mentioned above that in order to pump a laser material, fibre lasers can be used, and that they are constructed within optical fibres. Concerning the infrared range, optical fibres may simply be defined as fibre optics transmitting radiation with wavelengths greater than 2 m approximately (Harrington, n. d. ).
Infrared fibre optics may be divided into three categories: glass, crystalline (such as sapphire), and hollow waveguides (such as photonic crystal fibers) (Irphotonics Inc. , 2006; Harrington, n. d. ). These categories may further be subdivided based on either the fibre material or structure or both. The first infrared fibres were made in the 1960 s from chalcogenide glasses like arsenic tri sulfide.
During the 1970 s, the interest in developing efficient and reliable infrared fibres for short-haul applications was increasing, partly in response to the emerged need for a fibre to link broadband, long wavelength radiation to remote photo detectors in military sensor applications. In addition, there was an ever-increasing need for a flexible fibre delivery system for transmitting CO 2 laser radiation in surgery; in 1975 approximately, a variety of infrared materials and fibres had been developed to meet these needs. These had included the heavy metal fluoride glass and polycrystalline fibres as well as hollow rectangular waveguides. Over the past 25 years, many novel infrared fibres have been created but only a relatively small number of them have survived (Harrington, n.
d. ). There are two infrared transmitting glass fibre systems, namely heavy metal fluoride and heavy metal germinate glass. The germinate glass fibres do generally not contain fluoride compounds; instead they contain heavy metal oxides to shift the infrared absorption edge to longer wavelengths. The advantage of germinate fibres over heavy metal fluoride glass ones is that germinate glass has a higher glass transition temperature and, hence, higher laser-damage thresholds. But the loss for the heavy metal fluoride glass fibres is lower (Harrington, n. d. ).
Crystalline infrared fibres are a good alternative to glass ones since most non-oxide crystalline materials can transmit longer wavelength radiation than infrared glasses and, in the case of sapphire, exhibit also some superior physical properties. The main disadvantage is that crystalline fibres are quite difficult to produce. There are two types of crystalline fibres: single-crystal and polycrystalline. The main difference between the polycrystalline and glass fibres is that the polycrystalline ones plastically deform well. This plastic deformation leads to increased loss as a result of increased scattering from separated grain boundaries. Hence, the fibres should not be bent beyond their yield point; too much bending can lead to permanent damage and a high loss region in the fibre.
At the same time, sapphire, the most popular single-crystal fibre, is an insoluble, uni-axial crystal with a melting point over 2000 oC, extremely hard and robust. It is an almost ideal infrared fibre candidate for any applications related to wavelengths less than 3. 2 m (Harrington, n. d. ). As to hollow waveguides, they are, in general, an impressive alternative to usual solid-core infrared fibres for laser power delivery due to advantage of their air core. Hollow waveguides not only enjoy the advantage of high laser power thresholds but also low insertion loss, no end reflection, ruggedness, and small beam divergence. They may be divided into two categories: (1) those whose inner core materials have refractive indices greater than 1 (leaky guides), and (2) those whose inner wall material has a refractive index less than 1 (attenuated total reflectance guides).
Leaky (or n> 1) guides have metallic and dielectric films deposited on the inside of metallic, plastic, or glass tubing. Attenuated total reflectance guides are those made from dielectric materials with refractive indices less than one; therefore, the n< 1 guides are fiber-like in that the core index (n>> 1) is greater than the clad one. Hollow sapphire fibres operating at 10. 6 m (n = 0. 67) are a good example of this class of hollow guide (Harrington, n. d. ). Hollow-core waveguides are nowadays one of the best alternatives for power delivery in infrared laser surgery and industrial laser delivery systems. However, a loss on bending is their considerable disadvantage (Harrington, n.
d. ). Other promising materials are now in the focus when developing the infrared fibre laser realm. For example, Bahram Jalali, a Professor of electrical engineering at the University of California, Los Angeles (UCLA), and Oral Boyraz, a postdoctoral student at UCLA, had recently used silicon to emit pulses of photons at 1675 nm. They found they could work with the atomic vibrations of the silicon to produce new wavelengths of light.
They pumped the material with a mode-locked fibre laser operating around 1540 nm. The vibrations in the atoms scatter the light at another frequency. Atomic vibrations scatter more of the pump light into the new wavelength, Jalali said. It is essentially a feedback process. The researchers found a sudden increase in emission of 1675 -nm photons when the pump power reached 9 W, indicating that lasing took place. The next step will seemingly be building a device with a cascading laser cavity where the feedback process produces a series of increasing frequencies, causing the emission to hop to longer and longer wavelengths.
That will bring the laser emissions into the mid-infrared, namely between 2 and 10 m, where ordinary semiconductor lasers cannot reach. It is a fundamentally new device, Professor said. This is a range where there are a lot of critical applications, but there are no practical lasers that operate in the mid-infrared. Such devices could improve detectors for biological molecules, almost all of which have fundamental vibrational frequencies in the mid-infrared; current detection schemes rely on harmonics of those frequencies in the near-infrared. The military could use the lasers to blind the detectors of heat-seeking missiles (Savage, 2005). It may be concluded that during the past 25 years of the development of infrared fibres there is a great deal of fundamental research designed to produce fibres with optical and mechanical properties close to that of classical silica.
It can today be seen that we are still far from the perfection but some productive infrared fibres have certainly been emerged. As a class, they can be used to address some of the needs for fibres able to transmit greater than 2 m. Yet we are still limited with the current infrared fibre technology by high loss and low strength. Nevertheless, more applications are being found for infrared fibres as users become aware of their limitations and, what is even more important, how to design around their properties (Harrington, n.
d. ). References Harrington, J. A. (n. d. ). A review of infrared fibers. Retrieved December 29, 2006, from web Irphotonics Inc. (2006).
Definitions. Retrieved February 10, 2007, from web Maulini, R. (2006). Broadly tunable mid-infrared quantum cascade lasers for spectroscopic applications. DSc thesis, Universit?
e de Neuch? atel, Neuch? atel. Retrieved from web Savage, N. (2005). Deep into the infrared. SPIE The International Society for Optical Engineering.
Retrieved March 5, 2007, from web The Computer Language Company Inc. (2007 a). Definition of: fiber laser. Retrieved March 6, 2007, from web The Computer Language Company Inc. (2007 b). Definition of: laser.
Retrieved March 3, 2007, from web Tittel, F. K. , Richter, D. , & Fried, A. (2003). Mid-infrared laser applications in spectroscopy. In I. T. Sorokina & K.
L. Vodopyanov (Eds. ), Topics in Applied Physics (pp. 445 - 516). Berlin / Heidelberg: Springer Verlag.
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Research essay sample on Fibre Crystal Mid Infrared Lasers
