The ultrafast laser industry has been built upon the scientific research market with the assumption that certain factors such as electrical power, cooling, and vibration isolation can be taken for granted. As ultrafast lasers stand ready to break into the industrial laser market, these and many other assumptions must be re-examined. This paper presents a discussion of the many non-trivial engineering issues that must be addressed in order to bring a highly sophisticated laser system into the harsh industrial environment. These practical considerations include performance, reliability, and cost of ownership. The foundation of the discussion is based upon lessons drawn from successful existing industrial laser systems such as CO2 and Nd:YAG. Relevant results from this study will then be applied to the particular case of ultrafast lasers with the goal of examining how current scientific lasers compare with industrial requirements and highlighting some technologies which can help bridge this gap.
We describe a compact Kerr lens modelocked Ti:Sapphire laser operating at 700nm and 800nm using Gires-Tournois interferometric mirrors & broadband negative dispersive mirrors for dispersion compensation. Average powers of more than 1W have been achieved with a pulse lengths of <100 femtoseconds using 5W of pump light.
Ultrashort-pulse lasers are, in many ways, the ideal light source for diverse applications. A particularly appealing ultrashort-pulse laser technology is diode-pumped erbium-fiber13.
Fiber-based systems easily offer milliwatt-level femtosecond pulses with electrical-to-optical conversion efficiencies of several percent, while laboratory demonstrations have pushed these
limits to watt-level4 average powers and pJ pulse energies5; electrical-to-optical conversion efficiencies in the 10's of percent range should be possible. However, despite their appeal,
these and other ultrashort-pulse laser systems offer only limited selections of output wavelengths. Nonlinear optical frequency conversion is often used to extend the useful wavelength range
of available laser sources. However, in many cases, inadequate properties of conventional nonlinear materials have prevented the application of frequency conversion to low power sources
such as diode-pumped fiber lasers. Recent advances in nonlinear materials, particularly that of the quasi-phasematching (QPM) technique, have enabled the generation of an increasing
range of wavelengths with improved efficiencies and with lower power laser sources. Most of the previous work on QPM frequency conversion has focused on continuous-wave (OW) or nanosecond-pulse lasers, while the work described in this summary focuses on the ultrashortpulse regime.
We show a fiber delivery system capable of delivering lOOfs pulse trains at 76MHz with an average energy of more than 30mW. The pulses are pre-chirped with a negative dispersion grating expander and recompressed in passing through a 3m single-mode polarization preserving transport fiber and an additional bulk material element. The upper power limit is given by the onset of selfphase-
modulation (SPM) driven bandwidth reduction in the fiber.
A number of compact, diode-pumped sources of femtosecond optical pulses have been demonstrated by combining fiber and electric-field-poled lithium niobate technologies. Use of chirped-period-poled lithium niobate (CPPLN) for simultaneous pulse compression and second-harmonic generation has led to the demonstration of
robust high-power and high-energy fiber-amplifier based chirped pulse amplification (CPA) systems. Use of parametric gain in PPLN has led to the demonstration of a new class of compact diode-pumped femtosecond microamplifiers, which can achieve up to 80 dB of single-pass gain with arbitrarily broad, optically engineerable bandwidth and can provide microjoule output-pulse energies.
Tightly focused femtosecond laser pulses can be nonlinearly absorbed inside transparent materials, creating a highly excited electron-ion plasma. These conditions exist only in a small volume at the laser focus. This tight confinement and extreme conditions lead to an explosive expansion -- a microexplosion. In solid materials, a microexplosion can result in permanent structural changes. We find that the damage produced by femtosecond pulses in this way is surprisingly small, with only a 200-nm diameter. Material left at the center of the microexplosion is either amorphous and less dense or entirely absent. The threshold for breakdown and structural change is nearly independent of material. Time- resolved measurements of microexplosions in water allow us to observe the dynamics of the explosive expansion. The structural changes in solids resulting from microexplosions allow for three-dimensional data storage and internal microstructuring of transparent solids.
Application of ultrafast lasers to materials synthesis and processing is rapidly developing in directions of industrial relevance. Before full value can be extracted from such technology however, an operational understanding of their advantages and disadvantages needs to occur. Important issues regarding such applications are discussed in this paper in relation to fundamental aspects of energy absorption, lattice response, threshold damage production, and ablation plume development. These phenomena relate to the practical use of ultrafast lasers in micromachining and thin film deposition and reflect the physical differences to be found between long pulse and short pulse effects in materials. Understanding of these physical processes is enhanced through the use of practical computer models for the electronic and thermodynamic response of a material and the hydrodynamic and electrodynamic expansion of ablation plumes in terms of ion species and energies. Preliminary results on thin film deposition of boron nitride as a function of substrate temperature and ablation ionics is presented as an example of the unique possibilities provided by ultrafast lasers in the area of thin film synthesis and growth processing. Films are analyzed by spectroscopic ellipsometry for optical properties, ion beam analysis for stoichiometry, infrared absorption for structural properties, and atomic force microscopy for surface properties.
Femtosecond lasers enable materials processing of most any material with extremely high precision and negligible shock or thermal loading to the surrounding area. Applications ranging from drilling
teeth to cutting explosives to making high-aspect ratio cuts in metals with no heat-affected zone are made possible by this technology. For material removal at reasonable rates, we developed a fully computer-controlled 15-Watt average power, 100-fs laser machining system.
The spectroscopic and photochemical properties of several photosensitive compounds are compared using conventional single-photon excitation (SPE) and simultaneous two-photon excitation (TPE). TPE is achieved using a mode-locked titanium:sapphire laser, the near infrared output of which allows direct promotion of non-resonant TPE. Excitation spectra and excited state properties of both type I and type II photodynamic therapy (PDT) agents are examined. In general, while SPE and TPE selection rules may be somewhat different, the excited state photochemical properties are equivalent for both modes of excitation. In vitro promotion of a two-photon photodynamic effect is demonstrated using bacterial and human breast cancer models. These results suggest that use of TPE may be beneficial for PDT, since the technique allows replacement of visible or ultraviolet excitation with non- damaging near infrared light. Further, a comparison of possible excitation sources for TPE indicates that the titanium:sapphire laser is exceptionally well suited for non- linear excitation of PDT agents in biological systems due to its extremely short pulse width and high repetition rate; these features combine to effect efficient PDT activation with minimal potential for non-specific biological damage.
Photodisruptive lasers working at non absorbing wavelengths can be delivered through the ocular medium and focused to a surgical target to produce optical breakdown. When multiple pulses are used, non-invasive surgical procedures, such as posterior capsulotomy, can be performed. The clinical use of photodisruptive lasers is limited however, due to the large volumes of tissue affected by pulses from commercially available Nd:YAG lasers, which operate in the nanosecond pulse duration range. Photodisruptive lasers with pulse durations in the sub-picosecond or femtosecond range have much lower energy thresholds and secondary shock waves, leading to more localized surgical effects. Due to their limited collateral tissue damage, ultrafast lasers can be used to perform high precision noninvasive intraocular applications, such as corneal and glaucoma surgery.
The use of fluorescent probes is a powerful technique for the study of living specimens. Unfortunately, living tissues are vulnerable to photodamage from the excitation illumination and they make poor optical specimens due to their light-scattering nature. Multiphoton (two or more photon) excitation imaging offers significant advantages compared to laser-scanning confocal fluorescence microscopy for fluorescence microscopy of live specimens: considerable reduction in total sample fluorophore excitation and hence less photodamage, increased depth penetration due to increased tolerance for scattering, and increased detection sensitivity as more signal photons can be used for imaging. These advantages become more significant if 3D or 4D (multifocal plane, time-lapse) imaging is undertaken. In addition, multiphoton excitation imaging allows UV excited probes such as DAPI or INDO I or endogenous fluorophores such as NAD(P)H and serotonin to be imaged without UV excitation. We, and others, have been evaluating the potential of multi-photon excitation imaging for biological microscopy and have found all of the aforementioned advantages particularly significant for laser-scanning fluorescence imaging of developing embryos; a summary of currently pursued developmental biology applications will be presented. The current status of all-solid-state ultrafast lasers as excitation sources will also be reviewed since these lasers offer tremendous potential for affordable, reliable, 'turnkey' multiphoton imaging systems. The combination of demonstrated applications, simple ultrafast laser sources, and affordable commercial systems may promote a revolution in the study of embryogenesis with the light microscope.
This contribution discusses some biological applications of ultrashort laser pulses. Some examples are given of recently developed techniques that exploit the special features offered by ultrashort laser pulses: real time two-photon microscopy with multipoint excitation, fluorescence lifetime measurement by double pulse saturation excitation and pH-sensing by multiphoton quantum control.
Subpicosecond laser ultrasonic techniques have been used for nearly a decade to study the physical and chemical properties of thin films and multilayers. Recent advances in computational methods, optical design and laser technology have made it possible to develop further related techniques for thin film process metrology. In this paper we describe the principles and applications of picosecond ultrasonics, with an emphasis on in-line characterization of metal and dielectric films used in ULSI chip manufacturing. Topics to be discussed include measurements of simple metal films ranging from 50 A to 2 microns thick, and opaque multilayers consisting of up to six sequentially deposited metal films.
Recently several THz sampling detection systems have been used to characterize the temporal and spatial distribution of free- space broadband, pulsed electromagnetic radiation (THz beams). Free-space sampling systems use electro-optic or magneto-optic sensors and a femtosecond laser system, to provide diffraction-limited spatial resolution, picosecond temporal resolution, and DC-THz spectral bandwidth. In this paper, we review recent progress and preliminary applications of free- space electro-optic and magneto-optic sensors.
We have proposed and realized a structure of insulator-gap photoconductive switches by using an atomic force microscope (AFM). The insulator-gap structure prevents discharge in a photoconductive gap, to realize strong electric field in photo-absorbing region. We have made photoconductive switches with a gap of 43 nm and 100 nm. We also made multiple gap structures to reduce dark current. Ultrafast response for transmission modes have been estimated by the electro-optic (EO) sampling which can measure vector components of electric field. The radiation modes from the photoconductive switches with antenna structures have been measured by a Fourier transform polarizing interferometer.