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In single crystals of orthorhombic alpha-AlOOH, known also as mineral diaspore, χ(3)-nonlinear lasing by stimulated Raman scattering (SRS) and Raman-induced four-wave mixing (RFWM) is investigated. Picosecond pumping at 1.064 µm wavelength produces a broadband Stokes and anti-Stokes frequency comb with up to 25 SRS- and RFWM-generated emission lines. All observed Stokes and anti-Stokes lasing components in the visible and near-IR are identified and attributed to a single SRS-promoting vibration mode with ωSRS ≈ 445 cm−1. The first Stokes steady-state Raman gain coefficient in the visible spectral range is estimated to a value not less than 0.36 cm GW−1.

chi((3))-nonlinear optical interactions in two melilite-type stimulated Raman scattering (SRS)-active non-centrosymmetric crystals, Ca(2)MgSi(2)O(7)and Ca2Ga2SiO7, formerly known as Nd3+-laser media, are presented. Under picosecond pumping at 1.064 and 0.532 mu m cascaded and cross-cascaded effects occur in these tetragonal silicates. Besides the SRS-promoting phonon modes with energy of omega(SRS1)approximate to 908 cm(-1)and omega(SRS2)approximate to 668 cm(-1)for Ca2MgSi2O7, and omega(SRS1)approximate to 720 cm(-1)and omega(SRS2)approximate to 550 cm(-1)for Ca2Ga2SiO7, respectively, combined phonon modes are observed. For Ca(2)MgSi(2)O(7)new data in a broad wavelength range of refractive indices and their dispersion are given as well. The observed chi((3))-nonlinear properties expand the functionality of the studied silicates and foreshadow their use in self-frequency Raman laser converters (self-SRS lasers).

α-LiAlSi2O6, known as mineral spodumene, is introduced as a novel SRS-active crystal with monoclinic symmetry C2/c. Under picosecond laser excitation Raman-induced steady-state χ(3)-nonlinear generation in the visible and near-IR is observed in the crystals. All recorded Stokes and anti-Stokes high-order lasing components are identified and correspond to three SRS-promoting phonon modes with ωSRS1 ≈ 709 cm−1, ωSRS2 ≈ 357 cm−1 and ωSRS3 ≈ 1074 cm−1. On the basis of the results of a spontaneous Raman scattering study they were assigned to vibrations of the tetrahedral [SiO4] groups of the silicate chains and lattice modes of Li-O or Al-O of the crystal structure. A brief review in tabular form of so far known SRS-active natural crystals (minerals) is given as well.

The correct identification of the third-order nonlinear optical effect of stimulated Raman scattering (SRS) led in the last years to a versatile method to generate new laser wavelengths resulting from a photon-phonon-interaction. There is the possibility to down- (Stokes) or up-shifting (anti-Stokes) of the pump laser frequency. The size of the frequency shift depends on the Raman-active material and the excitability of their SRS-promoting vibration-modes. Prominent Raman crystals include BaNO3 and other nitrates, KGW and other tungstates, YVO4 and other vanadates as well as diamond. Recently, we observed SRS in the laser crystal LuAlO3 with one SRS-active phonon mode and the natural crystal Spodumene (α-LiAlSi2O6), which has three corresponding SRS-active vibration modes. Selective amplification of one particular spectral line generated through SRS is possible by placing the Raman crystal into a frequency-selective optical resonator, whose optical feedback is selective for only one Stokes- or anti-Stokes component. Raman lasers can be used in many applications, e.g. differential absorption LIDAR systems (DIAL, Light Detection and Ranging) to detect trace gases like carbon dioxide (CO2), ozone (O3) or water vapor (H2O). Various pumping schemes and resonator designs have been investigated focusing on good conversion efficiency, high spatial beam quality and high pulse energy of the output beam. The DIAL technique requires laser sources with high average output power combined with an excellent beam quality (M2 < 2). One possible solution can be found in an effect called beam-cleanup, which takes place by using Raman lasers and amplifiers.

Hexagonal Ca5(PO4)3F, known as natural crystal fluorapatite and oldest host-crystal for Ln3+-lasant ions, is presented as a Raman-active material. High-order Raman-induced χ(3)-nonlinear processes are discovered in natural crystals of fluorapatite under picosecond pumping at 1.064 μm and 0.532 μm wavelength. A multitude of Stokes and anti-Stokes components is generated in the ultraviolet, visible and near-infrared spectral region by stimulated Raman scattering (SRS) and Raman four-wave mixing (FWHM), resulting in a frequency comb with a width of 520 THz. The spectral lines are identified and attributed to the ν1(Ag) vibration mode of the tetrahedral [PO4] units which is related to a Raman shift of ωSRS ≈ 965 cm−1. The first Stokes steady-state Raman gain coefficient in the near-infrared spectral range is estimated to be >0.38 cm·GW−1. Finally, a short review of SRS-promoting vibration modes and observed χ(3)- nonlinear interactions in all known SRS-active natural crystals (minerals) is given.

In this article a nonreturn-to-zero data transmission of 25 Gb/s between silicon photonic transmitter and receiver units is demonstrated, where the transmitter is based on a silicon optical Fabry–Perot modulator with a node-matched-diode geometry. This modulator type has a footprint of less than 100 μm2. On the receiver side, an integrated Ge-photo detector has been used. Mask tests according to the 100G ER4 standard have been performed and yielded error free data transmission.

We determined, for the first time, the room temperature phonon energy related to the F2g vibration mode (ωSRS(12C) ∼ 1333.2 cm–1) in a mono-crystalline single-isotope CVD 12C-diamond crystal by means of stimulated Raman scattering (SRS) spectroscopy. Picosecond one-micron excitation using a Nd3+:Y3Al5O12-laser generates a nearly two-octave spanning SRS frequency comb (∼12000 cm–1) consisting of higher-order Stokes and anti-Stokes components. The spacing of the spectral lines was found to differ by ΔωSRS ∼ 0.9 cm–1 from the comb spacing (ωSRS(natC) ∼ 1332.3 cm–1) when pumping a conventional CVD diamond crystal with a natural composition of the two stable carbon isotopes 12C (98.93%) and 13C (1.07%). (© 2016 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

This book provides a comprehensive overview of laser sources and their applications in various fields of science, industry, and technology. After an introduction to the basics of laser physics, different laser types and materials for lasers are summarized in the context of a historical survey, outlining the evolution of the laser over the past five decades. This includes, amongst other aspects, gas lasers, excimer lasers, the wide range of solid-state and semiconductor lasers, and femtosecond and other pulsed lasers where particular attention is paid to high-power sources. Subsequent chapters address related topics such as laser modulation and nonlinear frequency conversion. In closing, the enormous importance of the laser is demonstrated by highlighting its current applications in everyday life and its potential for future developments. Typical applications in advanced material processing, medicine and biophotonics as well as plasma and X-ray generation for nanoscale lithography are discussed. The book provides broad and topical coverage of laser photonics and opto-electronics, focusing on significant findings and recent advances rather than in-depth theoretical studies. Thus, it is intended not only for university students and engineers, but also for scientists and professionals applying lasers in biomedicine, material processing and everyday consumer products. Further, it represents essential reading for engineers using or developing high-power lasers for scientific or industrial applications.

Tetragonal tungstate CsLa(WO4)2:Nd3+ was found to be an attractive multifunctional simultaneously Nd3+-laser and stimulated Raman scattering (SRS)-active crystal. A multitude of Stokes and anti-Stokes components is generated in the visible and near-infrared spectral region producing frequency combs with widths of ~8600 cm−1 and ~9560 cm−1 under pumping at 0.532 07 µm and 1.064 15 µm wavelengths, respectively. All registered nonlinear emission lines are identified and attributed to a single SRS-promoting vibration ν s(WO4) mode with energy of ω SRS ~956 cm−1. Moreover, a passively Q-switched LD-pumped nanosecond self-Raman CsLa(WO4)2:Nd3+ laser is reported. An overview of Ln3+-doped tungstate self-Raman lasers is given as well.

Medical fiber optics as a part of laboratory and clinical diagnostic tools, surgical instrumentation and endoscopy should satisfy modern biomedical requirements. The flexibility, small size, bio-compatibility and feasibility to use sterilisable or disposable parts allow to apply fiber optic probes in clinical environment both ex-vivo and in-vivo. To enable spectroscopic differentiation cancer and normal tissue, we developed and applied various single and combined fiber optic probes using key spectroscopy methods such as Raman scattering, Mid IR-absorption, Diffuse NIR-reflection, and fluorescence in order to compare them and select the best combination for malignant detection of tissue in clinical environment. All four spectroscopic methods have been tested on biopsies of health and malignant tissues (colon and kidney) and bioliquids (serum, plasma and urine) of patients before and after surgery. The tiny Raman probe with 1.5 mm diameter has been developed for experimental ex-vivo tests. Further multivariate data analysis of spectroscopic data, both individual techniques and their combinations has provided a reliable cancer recognition for colon and kidney biopsies. The best synergic gain was observed of combining Mid IR-absorption and fluorescence spectroscopy (98% Sensitivity vs 63% or 88% for fluorescence or Mid IR-absorption correspondingly). Based on obtained results, both techniques were implemented within the same fiber probe to provide a simultaneous measurement of exactly the same spot at the sample surface. An innovative combi Fluo-ATR probe was designed, fabricated and tested. Diffuse NIR-reflection and fluorescence spectroscopy as the fastest measurement techniques were used for investigation of surgically removed colorectal tissue samples in a few minutes after resection. Taking into account the necessity of rapid measurement process, we developed multispectral NIR-UV probe that enables efficient excitation and collection of NIR diffuse reflectance and fluorescence spectra of the same point. Needle probe design with diameter 0.7 mm allows to penetrate in some depth of tissue and obtain most reliable spectra avoiding artefacts related to surface drying. Multivariate data analysis proved the increased sensitivity of methods combination in one dataset (94% Sensitivity vs 85% or 85% for fluorescence or NIR reflection correspondingly).

Large single crystals of the tetragonal (space group I4̅2d) guanidinium zinc sulfate, [C(NH2)3]2Zn(SO4)2, were grown and the crystal structure was redetermined. Vibrational spectra obtained by Raman and IR investigations are presented, together with an assignment of the observed vibration modes. Precise refractive index data in the wavelength range from 0.365 to 1.083 μm are given. [C(NH2)3]2Zn(SO4)2 is introduced as a new stimulated Raman scattering (SRS)-active crystal with three SRS-active vibration modes with ω SRS1 ≈ 985 cm–1, ω SRS2 ≈ 3390 cm–1 and ω SRS3 ≈ 1015 cm–1. They allow for the generation of broadband nonlinear emission via χ (3)- (SRS and Raman-induced four wave mixing, RFWM) as well as cascaded χ (2)↔χ (3)- processes.

A study on the identification of the work piece being laser machined is conducted with the goal of developing a strategy to automatically adjust the processing parameters. The identification technique is based on the spectroscopic analysis of the plasma emission generated during laser ablation. Radiation emitted by the plasma is characteristic of the ablated material and thus can be used as a spectral “fingerprint” for identification purposes. Linear correlation and artificial neural networks are implemented as classification algorithms. Both methods present efficient identification of samples with recognizably different emission spectra. Neural networks outperform linear correlation for the identification of work pieces with very similar spectra. Additionally, the influence of processing and acquisition conditions on the performance of both algorithms is investigated. Finally, a strategy for customized processing based on the identification of each studied work piece is proposed. Linear correlation is used for general identification. Meanwhile, artificial neural networks are solely employed to identify materials for which classification performance of linear correlation decreases. Subsequently, machining parameters are adjusted according to the identified work piece.

When the first laser was realized about sixty years ago, it was considered a solution looking for its problem and only a few people were aware of the application potential offered by this new type of light source. Today, laser technology has become indispensable in science, industry and everyday life. Besides material processing and medicine, presented in the preceding chapters, lasers play a key role in optical communication, sensing and metrology. Scientific applications are primarily related to interferometric, holographic and spectroscopic techniques, the investigation of nonlinear optical phenomena as well as light detection and ranging (lidar) experiments. The unique properties of the laser, particularly its high coherence, monochromaticity and ability to reach extremely high power densities, have been paving the way for major advances in the fields of biology, chemistry, physics and astronomy over the past decades.

In contrast to light emitted by light bulbs, gas discharge lamps or LEDs, lasers are characterized by low divergence, narrow linewidth, high intensity and the possibility for generating short pulses. The following chapter provides an overview of the fundamentals required for the understanding of lasers.

Laser properties such as frequency, power, beam profile, pointing direction and polarization generally show variations which can have a disturbing effect on many applications. For instance, fluctuations in the frequency or wavelength of a helium–neon laser employed for interferometric length metrology lead to a limitation in the measurement accuracy. Instabilities in the energy and beam profile of pulsed high-power lasers applied for material processing diminish the precision of the workpiece, e.g. the diameter or shape of a drilled hole.

Lasers are used in various medical applications both in therapy and diagnosis. Common diagnostic methods including optical tomography, spectroscopy and Doppler velocity measurements largely correspond to the methods which are also employed in other fields such as biophotonics and sensing. Depending on the application domain, different mechanisms of action of the laser radiation are exploited.

Single crystals or glasses that are doped with light emitting atoms are used as the gain medium in solid-state lasers. In crystals, the atoms exist in the form of ions. Aside from conventional rods and slabs with lengths of several cm, technologies based on disks or fibers with thicknesses, respectively diameters of 10 to 100 µm become more and more prevalent. Most solid-state lasers are based on transition metal ions such as Ti, Cr and Co or rare-earths such as Nd, Ho, Er, Tm or Yb. For the latter group, laser operation often relies on transitions within partially filled inner electron shells that are effectively screened by the outer shells and thus only weakly influenced by the crystal field. As a result, narrowband transitions in the visible to near-infrared spectral region occur. Besides, broadband levels exist which are addressed for realizing tunable lasers.

The most common devices for the spectral dispersion of light are prism and grating spectrometers as well as diverse types of interferometers such as Fabry-Perot, Fizeau or Michelson interferometers.

Atoms can produce a multitude of emission lines in the visible spectral region, as shown for the hydrogen atom in Fig. 1.5, where electron transitions to the quantum state n = 2 (Balmer series) involve the emission of radiation at wavelengths 410, 434, 486 and 656 nm, amongst others. However, laser operation in hydrogen atoms is prevented by the formation of stable H2 molecules under normal conditions. Hence, dissociation of the molecules would be required during the gas discharge to produce a hydrogen gas. The generation of visible light is possible by using the noble gas neon which exists in monatomic form under standard conditions. Excitation of neon by collisions with helium atoms forms the basis for the realization of the helium–neon laser. Aside from laser emission in the visible spectral range, the He–Ne laser can also produce ultraviolet light, yet laser operation in this region is hindered by the fact that the lower laser level is the ground state in this case, thus impeding population inversion. UV lasers hence mainly rely on transitions in ions and molecules. The same holds true for the infrared spectral range, as the photon energies related to infrared transitions are much smaller than the pump energies required for populating the upper laser level in atoms. Higher quantum efficiency for generating infrared radiation is obtained in molecules which possess light-emitting states close to the ground state.

Laser emission in the infrared spectral region can be generated with molecular gas lasers by exploiting transitions between vibrational and rotational energy levels. Purely rotational transitions without changing the vibrational state of the molecule involves the emission of photons with low energy, corresponding to long-wavelength radiation in the spectral range from 25 µm to 1 mm. This leads to the far-infrared lasers (Sect. 6.1). The energy differences between vibrational-rotational levels of the same electronic state are larger, hence shorter wavelengths ranging from 5 to 30 µm are emitted from lasers based on such transitions. The CO2 laser operating at wavelengths around 10 µm represents the most important member of this group of lasers and will be comprehensively discussed in Sect. 6.2. Electronic transitions in molecules result in visible and ultraviolet emission. Here, the nitrogen (Sect. 7.1) and excimer lasers (Sect. 7.2) are of particular relevance.

The transmission of laser light through fibers is essential in the fields of telecommunication, electrical engineering, material processing and medicine. This chapter discusses the basic principles of fiber coupling and light propagation in optical waveguides. It also presents different fiber materials as well as recent developments in terms of photonic crystal fibers.

The emission of commercial solid-state lasers is shifted by Raman lasers to selected wavelengths suitable for trace gas detection. High power Raman lasers based on Ba(NO3)2, diamond and silicon detect CO2, O3 and H2O. Raman lasers base on the physical effect of stimulated Raman scattering. Two new Raman crystals (Spodumene and LuAlO3) are investigated and their SRS-spectra is shown.

Light is an electromagnetic wave which can be mathematically described by Maxwell’s equations. Based on this set of differential equations, the wave equation is derived for the electric and magnetic field.

In general, multiple longitudinal and transverse electromagnetic modes simultaneously oscillate in a laser resonator. Hence, lasers emit a range of frequencies or wavelengths, determined by the linewidth of the laser transition, e.g. 1.5 GHz (2 pm) for the He-Ne laser at 633 nm or several THz (hundreds of nm) for the titanium-sapphire laser around 800 nm. The number of oscillating modes, and thus, the emission range of a laser can be reduced by the use of frequency-selective elements that are inserted into the laser resonator, ultimately enabling single longitudinal and single transverse mode operation.

In many laser applications, it is necessary to modulate or deflect the laser beam. Apart from mechanical devices, acousto-optic and electro-optic elements are mainly employed for this purpose. In particular, acousto-optic and electro-optic modulators, beam deflectors, Pockels and Kerr cells are widely applied in laser physics.

Pulsed laser output in the ultraviolet spectral range is generated by transitions between electronic levels in molecules.

For the water vapour DIAL “WALES” the wavelength regions around 935 nm, 942 nm and 944 nm have been identified as the most suitable wavelength ranges. These wavelengths can be obtained using opticalparametric-oscillators (OPOs), stimulated Raman shifters and the Ti-Sapphire laser but none of these systems could deliver all the needed parameters like beam quality, efficiency, pulse length and energy yet. Also these systems are comparably big and heavy making them less suitable for a satellite based application. A fourth possibility to achieve these wavelength ranges is to shift the quasi-3-level laser lines (938 nm and 946 nm) of the Nd:YAG laser by replacing aluminium and yttrium by other rare earth elements. Changes of the host lattice characteristics lead to a shift of the upper and lower laser levels. These modified crystals are summarized under the name of "Mixed Garnet" crystals. Only the Mixed Garnet lasers can be pumped directly with diode laser and use a direct approach to generate the required laser pulses without frequency conversion. Therefore no additional non-linear crystals or special pump lasers are needed and a higher electric to optical efficiency is expected as well as single frequency operation using spectral tuning elements like etalons. In a first phase such mixed garnet crystals had been grown and characterised. The outcome was the selection of the gadolinium-scandium garnet for the most suitable laser crystal. During a second phase the complete laser system with output energy about 18 mJ in single 20 ns pulses and up to 8 mJ in free running mode with a combined pulse width of 250 μs at 942 nm have been demonstrated. The results of the first laser operation and the achieved performance parameter are reported.

The discovery of chi((3))-nonlinear lasing by stimulated Raman scattering (SRS) in natural crystals of zircon, ZrSiO4, is reported. All recorded Stokes and anti-Stokes Raman-induced lasing components are identified and attributed to the two occurring SRS-promoting phonon modes with omega(SRS1) approximate to 1008 cm(-1) and omega(SRS2) approximate to 356 cm(-1), which are related to the anti-symmetric stretching vibration B-1g(nu(3)) and the bending mode E-g(nu(4)), respectively, of the crystal with symmetry D-4h(19) = I4(1)/amd. The study indicates the high potential of ZrSiO4 as an effective SRS crystal. [GRAPHICS] Crystals of ZrSiO4, as used for sample preparation, together with a sample for SRS measurement. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

In single crystals of orthorhombic YAlO3, widely known as a host-matrix for Ln3+-lasant ions, many-phonon stimulated Raman scattering interactions as well as different manifestations of cascaded and cross-cascaded nonlinear χ(3)χ(3) processes are initiated by picosecond laser pulses. The scientific and applicative potential of YAlO3 crystals is considerably expanded by the demonstration of its SRS properties. In particular, the studies revealed the manifestation of eight χ(3)-active vibrational modes. The corresponding Stokes and anti-Stokes lines have been assigned and the steady-state Raman gain coefficients related to the strongest phonon mode have been estimated. In addition, a short review presents the stimulated emission channels of its Ln3+-ions together with some χ(3)-nonlinear laser properties of crystals belonging to the binary Y2O3-Al2O3 system.

In single crystals of the beryllium silicate Be2SiO4 with trigonal symmetry , known also as the mineral phenakite, χ(3)-nonlinear lasing by stimulated Raman scattering (SRS) is investigated. All observed Stokes and anti-Stokes lasing components are identified and ascribed to a single SRS-promoting vibration mode with ωSRS ≈876 cm−1. With picosecond single-wavelength pumping at one micrometer the generation of an octave-spanning Stokes and anti-Stokes comb is observed.

Active remote sensing represents a promising technology to accurately measure the atmospheric concentration of the climate-relevant trace gases carbon dioxide and methane. For this application, frequency stabilized laser sources with high pulse energy emitting at specific absorption lines around 1.6 μm wavelength are required. While Raman frequency conversion of high power solid-state laser systems provides sources suitable for CO2 detection, compact resonantly pumped Er:YAG lasers are demonstrated to be feasible for CH4 absorption measurements.

Laser-induced breakdown spectroscopy (LIBS) and two classification methods, i.e. linear correlation and artificial neural networks (ANN), are used to monitor P1, P2 and P3 scribing steps of Copper-Indium-Diselenide (CIS) solar cells. Narrow channels featuring complete removal of desired layers with minimum damage on the underlying film are expected to enhance efficiency of solar cells. The monitoring technique is intended to determine that enough material has been removed to reach the desired layer based on the analysis of plasma emission acquired during multiple pass laser scribing. When successful selective scribing is achieved, a high degree of similarity between test and reference spectra has to be identified by classification methods in order to stop the scribing procedure and avoid damaging the bottom layer. Performance of linear correlation and artificial neural networks is compared and evaluated for two spectral bandwidths. By using experimentally determined combinations of classifier and analyzed spectral band for each step, classification performance achieves errors of 7, 1 and 4% for steps P1, P2 and P3, respectively. The feasibility of using plasma emission for the supervision of processing steps of solar cell manufacturing is demonstrated. This method has the potential to be implemented as an online monitoring procedure assisting the production of solar cells.