Search

After a brief introduction on the principles of operation of a laser, the operating regimes for the lasers of interest to dermatologist are first considered. Laser–tissue interaction and laser-selective photothermolysis are then discussed. Applications of selective photothermolysis to the treatment of vascular disorders and for tattoo removal are lastly considered.

Mankind has always been interested in studying the movement of ever faster objects upon using ever shorter light pulses. As a first example of a study of macroscopic samples, mention will be made about the study of a horse, while galloping, as obtained by the photographer Eadweard Muybridge around the end of years ‘800. He was in fact able to obtain a complete recording of the horse movement upon using an ingenious system of several cameras, placed along the horse racing, equipped with shutters with a record opening time of about 2 millisecond (1 ms=10–3 s). About 50 years later (1930) the introduction of flashlamps allowed the achievement of light pulses with duration of about one microsecond (1 μs=10–6 s). Upon using flashlamps in a repetitive mode, it was then possible to record the movement of much smaller and hence much faster objects (e.g. a bullet while crossing an apple or the fast movement of a golf player while hitting the golf ball). It was, however, only after the invention of the laser (T.H. Maiman, 1960) that people became able to generate much shorter light pulses. From initial pulse durations (middle of years ‘60) of about 10 picosecond (1 ps= 10–12 s), one could then generate (during years ’90) laser light pulses of a few femtoseconds (1 fs=10–15 s) down the present values of about 70 attoseconds (1 as=10–18 s). During all these years after laser invention, the research group at the Physics Department of Politecnico di Milano, leaded by the author of the present paper, has always been at the forefront of the international research to generate and utilize laser pulses of ever shorter duration. After the introduction of the first laser at the Politecnico di Milano at the beginning of 1963 (more than 50 years ago!), the research group was able to obtain light pulses of about 5 picoseconnds in 1969, a record value for that time. The same group then obtained record values of light pulses of about 4,5 femtoseconds, at the end of years ’90, and of only 130 attoseconds around the middle of years 2000. Ultrashort laser pulses have been used, throughout all these years, by a large number of international groups to investigate the ultrafast dynamic behavior of molecules, molecular complexes, and solid state nanoscopic materials (such as quantum wells and quantum dots). As a representative example, some discussion will be presented about a recent work made by the research group of Politecnico about the retinal contained in the rodospin protein, a basic element for human vision.

The paper reviews our research efforts in femtosecond laser inscription of optical waveguides inside active Er:Yb-doped glass substrates and in the development of compact waveguide lasers. By optimising the fabrication parameters we obtain waveguides with coupling losses to standard telecom fibers as low as 0.1 dB/facet and propagation losses lower than 0.4 dB/cm. The waveguides provide net gain, with a peak value of 7 dB, and laser action in the whole telecommunications C-band (1530–1565 nm). Single-frequency operation with output power up to 50 mW and passive mode-locking are demonstrated, making the systems useful as compact telecom transmitters. Our results show that femtosecond laser writing can produce high-quality photonic devices, with performance comparable to or better than that obtainable with conventional waveguide fabrication techniques.

In this paper we present recent results obtained on the generation of ultra-high repetition rate optical pulse trains at 1.5 μm in asynchronously frequency-modulated Er–Yb : glass lasers. Nearly transform-limited pulse trains at either single or dual carrier wavelength with repetition rates up to ∼20 GHz are generated by means of an external fiber Bragg grating. These pulse trains are of great potential interest for soliton-based optical transmission systems.

Recent advances in ultrafast laser technology have led to the generation of light pulses comprising few optical cycles employing different compression techniques. In particular, two techniques have been developed, which allow addressing the issues of high peak power or frequency tunability in a wide spectral range, namely: the hollow-fiber compression technique and the optical parametric amplification. The paper analyzes the general scheme of pulse compression and reports on the most interesting results obtained using the above-mentioned techniques. The combination of spectral broadening in a gas-filled hollow fiber with ultrabroad-band dispersion control, has led to the generation of pulses with duration of /spl sim/5 fs with peak powers up to 0.11 TW. Using optical parametric amplifiers with different configurations sub-15-fs laser pulses have been generated tunable in the visible and in the near-infrared.

A detailed experimental analysis of the amplitude noise in high-repetition-rate picosecond pulse trains generated by spectral filtering of a frequency-modulated Er-Yb:glass laser is reported. Two distinct sources of noise are identified, and stabilization techniques for noise suppression are proposed and experimentally demonstrated. Intensity noise suppression of ∼20 dB, corresponding to less than 2% residual amplitude fluctuations of the pulse train, has been achieved at repetition rates of 2.5 GHz and 5 GHz with pulse durations of ∼50 ps.

In this paper we present a comprehensive review of the work done by our group on diode-pumped bulk erbium–ytterbium microlasers. Starting from an analysis of the optical properties of Er–Yb doped phosphate glasses, addressed to the modeling of the active material and to the design and optimization of laser cavities, different operating regimes and peculiar properties of these novel laser devices are described and discussed, including multi-longitudinal mode and single-frequency operation, frequency tuning and stabilization, intensity stabilization, Q-switching, mode-locking and frequency-modulation operation. Due to the high performance in terms of emission characteristics and potential low cost, these microlasers are attractive for a large number of applications and, in particular, for optical communications at 1530–1560 nm wavelengths, metrology, and telemetry at an eye-safe wavelength.

Powerful techniques for spectral broadening and ultrabroad-band dispersion control, which allow compression of high-energy femtosecond pulses to a duration of a few optical cycles, are analyzed. Spectral broadening in a gas-filled hollow fiber and compression by chirped mirrors with high-energy 20-fs input pulses are presented. Using 1-mJ seed pulses we have demonstrated the generation of 0.5-mJ 5-fs pulses at 0.8-/spl mu/m and 1-kHz repetition rate. General design criteria to scale the compression technique toward the terawatt level are presented.

Based on a symmetry analysis of recent results of femtosecond pump-probe optical-absorption experiments on KBr $F$ centers, we present a twofold theoretical analysis. The first part is concerned with a density-matrix calculation of the experimentally observed beating pattern of two modes of ${A}_{1g}$ symmetry in the sample transmission. This allows us to assign the observed frequencies to the breathing modes of the surrounding octahedron of cations in the electronic ground and excited state. The stiffening of the ${A}_{1g}$ mode in the unrelaxed excited state is consistent with simple arguments based on a lattice of point charges. In the second part, the observed ultrafast loss of initial orientation of the excited $p$ state is analyzed using a real-time integration of the wave function in the Hilbert space spanned by the vibrations of lower symmetries ${(E}_{1g}$ and ${T}_{2g})$ and the three degenerate $p$ states. The resulting reorientation of the $p$ state can be regarded as the transient analogue of the Jahn-Teller effect, and it turns out that the total wave function loses its initial orientation on a very fast time scale. This phenomenon is interpreted as a loss of coherence, so that the breathing mode oscillation becomes observable only if the pulse length is of the order of the so-defined coherence time of ${\ensuremath{\tau}}_{c}=12$ fs.

The efficiency of laser-induced electron emission from ferroelectric (FE) cathodes has been measured by illumination with a Ti:sapphire laser at a pulse length of 140 fs and is compared to earlier measurements at 40 ps and 5 ns pulse lengths. Two modes of emission are observed: (i) a “normal steady-state” emission in the presence of a constant extraction field and (ii) a transient mode after a polarization switching of the FE cathode by a high-voltage pulse. The latter mode leads to “self-emission” of high electron currents with kinetic electron energies of several kilovolts. The physical interpretation and the potential applications of laser-induced electron emission from FE cathodes are briefly pointed out.

This paper presents a brief review of the modern laser technologies that enable to generate light pulses lasting “a few optical cycles” and tunable over large spectral ranges, thus allowing to follow the evolution of extraordinarily fast dynamical processes. In particular, we will describe the results obtained from the studies of the primary events of two important processes in nature involving the absorption of light, namely vision and photosynthesis. These events last for extremely short times and trigger the corresponding overall processes. The study of these events is important both for fundamental reasons and for the design of artificial molecular systems of high efficiency to be used for new devices in the field of artificial vision and of photovoltaics.

The design and the use of positive branch unstable resonators with variable reflectivity mirrors are described in the case of two different transverse flow CO, lasers: a TEA source for LIDAR Doppler applications and a high average power DC excited source for industrial applications. In the first case, Ge and ZnSe mirrors with Gaussian and super-Gaussian reflectivity profiles have been employed. In the second one, ZnSe mirrors with Gaussian and super-Gaussian reflectivity profiles have been employed as well as partial hard-edge reflectivity mirrors with and without phase compensation. Beams with 40 mm diameter and full-angle divergence of about 1 mrad were typically obtained in both cases. The TEA source was operating in single longitudinal mode with 1J per pulse energy, while 1 kW output power was obtained from the industrial source.

The new rod-imaging unstable resonator configuration that we have recently introduced is theoretically analysed and experimentally characterized. A variable reflectivity mirror of super-Gaussian profile has been used as output coupler. Using a pulsed Nd:YAG laser head, which with a traditional stable resonator generates beams of 260 W, an average output power larger than 150 W with a beam divergence of only a few times the diffraction limit has been obtained. It is shown how an intracavity spatial filter can further reduce the beam divergence.

Quasi-continuous wave laser operation of Cr 4+ -doped Y 2 SiO 5 at room temperature was demonstrated for the first time. When pumping with a Nd:YLF laser at 1047 nm, laser operation at 1304 nm with a threshold of 1.5 W was achieved. Room temperature laser operation by pumping with a long pulse flash lamp pumped Nd:YAG laser was also obtained.

The output characteristics of a continuous-wave lamp-pumped Nd:YAG laser using a rod of 6.4 mm diameter and stable resonators with large fundamental mode volume have been systematically investigated by testing various resonators. It is shown that, independently of the resonator, thermal focusing and birefringence set an intrinsic limitation to the fundamental mode volume, such that the maximum usable TEM00 spot size in the rod is about 1 mm, even if in principle the rod cross-section could sustain a much larger mode. A qualitative theoretical model explains the experimental data. A maximum output power of about 40 W was observed in a nearly diffractionlimited beam.

A simplified model of the cw operated erbium-ytterbium laser, based on a rate equation analysis, is presented. The model takes into account the spatial variations of both the pump beam and the cavity field, and generally requires numerical solutions. Approximate analytical solutions for the threshold power and the slope efficiency have also been derived. A few numerical plots have been generated and may be used as guidelines to design the laser. Following the design criteria outlined, a miniaturized Er:Yb:phosphate glass laser for optical communications has been constructed and optimized.

Results on the bleaching decay observed in low-band gap thiophene-based polymers by femtosecond pump and probe experiments are presented. The laser used for excitation and probe is a newly developed cavity dumped femtosecond dye laser. The excited state kinetics of these materials presents a fast biexponential time behavior that may be related to the formation and decay of self-trapped excitons. >

In a first part of the talk a discussion will be presented about how the laser was conceived and then realized. In the second part, some early developments in laser science and technology, which have paved the way to all nowadays applications, will be reviewed.

This new Fifth Edition of Principles of Lasers incorporates corrections to the previous edition. The text’s essential mission remains the same: to provide a wide-ranging yet unified description of laser behavior, physics, technology, and current applications. Dr. Svelto emphasizes the physical rather than the mathematical aspects of lasers, and presents the subject in the simplest terms compatible with a correct physical understanding. Praise for earlier editions: "Professor Svelto is himself a longtime laser pioneer and his text shows the breadth of his broad acquaintance with all aspects of the field … Anyone mastering the contents of this book will be well prepared to understand advanced treatises and research papers in laser science and technology." (Arthur L. Schawlow, 1981 Nobel Laureate in Physics) "Already well established as a self-contained introduction to the physics and technology of lasers … Professor Svelto’s book, in this lucid translation by David Hanna, can be strongly recommended for self-study or teaching at the final-year undergraduate or first-year post-graduate levels." (Physics Bulletin) "A thorough understanding of this book in conjunction with one of the existing volumes on laser safety will go a long way in providing the health physicist with the understanding he needs … Highly recommended." (Health Physics) "Introduces laser science and technology with the accessibility appropriate for the nonspecialist and the enthusiasm of the pioneer." (Laser Focus) "A very good introduction to laser theory and practice … aimed at upper-level undergraduate students. It is logically organized and easy to read … Most of the basic mathematical framework needed to understand this evolving field is presented. Every chapter contains a good set of problems, answers to some of which are given in the back." (Sci-Tech News) Orazio Svelto is Professor of Quantum Electronics at the Polytechnic Institute of Milan and Director of the Quantum Electronics Center of the Italian National Research Council. His research has covered a wide range of activity in the field of laser physics and quantum electronics, starting from the very beginning of these disciplines. This activity includes ultrashort-pulse generation and applications, development of laser resonators and mode-selection techniques, laser applications in biology and medicine, and development of solid-state lasers. Professor Svelto is the author of more than 150 scientific papers and his researches have been the subject of more than 50 invited papers and international conferences. He has served as a program chair of the IX International Quantum Electronics Conference (1976), as a chair of the European program committee for CLEO ’85 and CLEO ’90, and he was general co-chair for the first CLEO-Europe Conference (1994). He is an elected member of the Italian "Accademia dei XL" and a Fellow of the IEEE.

A new unstable resonator configurations, called rod-imaging resonator, which uses a graded reflectivity output mirror of supergaussian profile has been developed. This resonator allows one to control rod thermal lensing for generating high power and low divergence beams. Output beams with divergence about twenty times loser than obtained with a traditional multimode stable resonator and average power in excess of 200 W have been obtained from a pulsed Nd:YAG laser.

We report on a new resonator configuration for a cavity dumped hybridly mode-locked dye laser which allows the generation of extremely stable pulses as short as 60 fs. This is a demonstration of the superior performance of low misalignment sensitivity resonators compared to traditional nearly confocal configurations.

We report on intensity noise suppression in a diode-pumped, single-frequency erbium bulk-glass laser codoped with ytterbium. Using an optoelectronic feedback circuit, we acheived a 30-dB reduction of the relaxation oscillation peak, at 160-kHz frequency, to a relative intensity noise of −114 dB/Hz. A useful output power of 15 mW at 1533-nm wavelength was obtained.

A bulk erbium:ytterbium:phosphate glass laser has been actively mode locked at the third-order harmonic by a bulk lithium niobate phase modulator. Light pulses of 9.6-ps duration, 2.5-GHz repetition rate, and 3-mW average power at 1530-nm wavelength have been obtained.

The purpose of this chapter is to specialize some of the results and considerations of the previous chapter to the somewhat more complicated case of molecules and semiconductors. Particular emphasis will be given to semiconductors, either in bulk or quantum well form, since they play an increasingly important role as laser media.

In previous chapters, we have discussed several features of the components that make up a laser. These are the laser medium itself, whose interaction with an e.m. wave was considered in Chaps. 2 and 3, the passive optical resonator (Chap. 5) and the pumping system (Chap. 6). In this chapter we will make use of results from these earlier chapters to develop the theoretical background required to describe the continuous wave, c.w., laser behavior. The case of transient laser behavior will be considered in the next chapter. The theory developed here uses the so-called rate-equation approximation and the laser equations are derived on the basis of a simple notion that there should be a balance between the total atoms undergoing a transition and total number of photons which are being created or annihilated (1, 2). This theory has the advantage of providing a rather simple and intuitive picture of laser behavior. Furthermore, it gives sufficiently accurate results for most practical purposes. For a more refined treatment one should use either the semiclassical approach (in which the matter is quantized while the e.m. radiation is treated classically, i.e., through Maxwell’s equations) or the full quantum electrodynamics approach (in which both matter and radiation are quantized). We refer the reader elsewhere for these more advanced treatments (3).

In this chapter, we will consider a few cases where the pump rate and/or cavity losses are time dependent. We will also consider situations in which a nonlinear optical element, such as a saturable absorber, is inserted in the laser cavity, where the non-linearity leads to the laser departing from stable cw operation. For these various cases we are thus dealing with transient laser behavior. The transient cases to be considered can be separated into two categories: (i) Cases, such as relaxation oscillations, Q-switching, gain switching and cavity dumping, where, ideally, a single mode laser is involved and which can be described by a rate equation treatment. (ii) Cases where many modes are involved, e.g. mode-locking, and for which a different treatment needs to be considered. This requires a description in terms of either the fields of all oscillating modes (frequency domain description) or in terms of a self-consistent circulating pulse within the cavity (time domain description).

In this chapter, the most important types of lasers involving high density active media are considered, namely solid-state, dye and semiconductor lasers. The chapter concentrates on those examples that are in widest use and whose characteristics are representative of a whole class of lasers. The main emphasis here is on stressing the physical behavior of the laser and relating this behavior to the general concepts developed in the previous chapters. Some engineering details are also provided with the main aim again of helping to provide for a better physical insight into the behavior of the particular laser. To complete the picture, some data relating to laser performances (e.g., oscillating wavelength(s), output power or energy, wavelength tunability, etc.) are also included to help providing some indication of the laser’s applicability. For each laser, after some introductory comments, the following items are generally covered: (1) Relevant energy levels; (2) excitation mechanisms; (3) characteristics of the laser transition(s); (4) engineering details of the laser structure(s); (5) characteristics of the output beam; (6) applications.

In this chapter, the most important types of lasers involving low density active media are considered, namely gas, chemical and free electron lasers. Some considerations on X-ray lasers involving highly ionized plasmas will also be presented. The main emphasis, again, is to stress the physical behavior of the laser and to relate this behavior to the general concepts developed in the previous chapters. Some engineering details are also presented with the main intention again of providing for a better physical insight into the behavior of the particular laser. To complete the picture, some data relating to laser performances (e.g., oscillation wavelength(s), output power or energy, wavelength tunability, etc.) are also included to help provide some indication of the laser’s field of application. For each laser, after some introductory comments, the following items are generally covered: (1) Relevant energy levels; (2) excitation mechanisms; (3) characteristics of the laser transition(s); (4) engineering details relating to the laser structure(s); (5) characteristics of the output beam; (6) applications.

Continuous-wave mode-locking operation of a bulk Er:Yb:phosphate glass laser, pumped by either a Ti:sapphire or an InGaAs diode laser, is reported for what is to our knowledge the first time. Pulses with a 90-ps duration, a 100-MHz repetition rate, and as much as 7-mW average power have been obtained.

We have developed a continuous-wave, lamp-pumped, intracavity frequency-doubled laser that provides as much as 13.5 W of power in the green, linearly polarized TEM00 output, and very low misalignment sensitivity. These results have been obtained with a Nd:YLF rod of large cross section, a lithium triborate frequency-doubling crystal, and a novel resonator design. We demonstrate that the problem of power fluctuations and chaos in this class of lasers (the green problem) can effectively be overcome and that fluctuations of less than ±1% on a millisecond time scale can be obtained.

A single-frequency, diode-pumped, Er-Yb:glass microchip laser at a 1530-nm wavelength has been designed and operated. An output power of greater than 25 mW, a linewidth narrower than 1 kHz, and a slope efficiency of 22% have been obtained.

An output coupler with radially variable reflectivity, based on a Fabry-Perot interferometer made of two suitable spherical mirrors, is proposed. Application to a pulsed Nd:YAG laser with an unstable resonator has produced diffraction-limited output beams with energy up to 300 mJ.

Analysis of dispersion produced by a lambda/4 multilayer dielectric mirror as a function of its construction parameters is presented. The amount of dispersion can be estimated from a careful inspection of the measured reflectivity curve. In some cases, which seem to be of practical relevance, the value of dispersion is such as to prevent pulses shorter than ~50 fsec from being produced in the mode-locked laser cavity.

We report on a source of ultrabroadband self-phase-stabilized near-IR pulses by difference-frequency generation of a hollow-fiber broadened supercontinuum followed by two-stage optical parametric amplification. We demonstrate energies up to 200 microJ with 15 fs pulse width, making this source suited as a driver for attosecond pulse generation.

In this review we report on recent advances in laser technology, which have contributed to the fast development of attosecond science. In particular we will concentrate on two experimental methods for the generation of high-peak-power, few-optical-cycle laser pulses with controlled electric field, which are crucial for the generation of isolated attosecond pulses. The first method is the hollow-fiber compression technique, introduced in 1996 and now routinely used in several laboratories. So far, isolated attosecond pulses have been generated by using few-cycle pulses produced by such compression technique, in combination with active stabilization of the carrier-envelope phase. More recently, few-cycle pulses tunable in the infrared region have been generated by optical parametric amplification with passive stabilization of the carrier-envelope phase. Such parametric sources represent excellent drivers for the generation of harmonic radiation with an extended cutoff, and offer the possibility to extend attosecond science towards the soft-X rays region. Finally, we will briefly discuss the basic elements of attosecond metrology.

We report on the first demonstration of passive mode-locked operation of a diode-pumped Yb:KYF4 crystal. Transform-limited pulses with duration of 170 fs, average power of 60 mW, and repetition rate of 55 MHz are obtained.

Passive mode locking of a diode-pumped Yb3+:KYF4 laser is demonstrated using a semiconductor saturable absorber mirror. A high-stability, transform-limited pulse train. with a repetition rate of 57 MHz is generated. Solitonlike pulses with maximum average output power of 250 mW, minimum pulse duration of 170 fs, and rms time jitter of 360 fs were obtained. (C) 2008 Optical Society of America.

A systematic theoretical and experimental investigation of unstable resonators with variable reflectivity mirrors of a super-Gaussian profile is presented. The validity of a geometrical optics formalism for an algebraic mode calculation is discussed in comparison to the diffraction theory. A new method for the calculation of the output energy by means of a closed-form relationship that accounts for the effects of the transverse mode profile on gain saturation is presented. Various dielectric super-Gaussian mirrors have been fabricated, characterized, and tested in a pulsed Nd:YAG laser. Super-Gaussian resonators can provide even higher output energies than those of traditional unstable resonators and with better beam-focusing properties. >