Ingmar Hartl

Bio: Ingmar Hartl is an academic researcher from University of Hamburg. The author has contributed to research in topics: Laser & Fiber laser. The author has an hindex of 44, co-authored 240 publications receiving 8165 citations. Previous affiliations of Ingmar Hartl include Massachusetts Institute of Technology & Helmholtz Institute Jena.

Ultrahigh-resolution optical coherence tomography using continuum generation in an air-silica microstructure optical fiber.

Abstract: We demonstrate ultrahigh-resolution optical coherence tomography (OCT) using continuum generation in an air–silica microstructure fiber as a low-coherence light source. A broadband OCT system was developed and imaging was performed with a bandwidth of 370 nm at a 1.3‐μm center wavelength. Longitudinal resolutions of 2.5 μm in air and ∼2 μm in tissue were achieved. Ultrahigh-resolution imaging in biological tissuein vivo was demonstrated.

Ultrafast fibre lasers

Abstract: Ultrafast fibre lasers are an important optical system with industrial, medical and purely scientific applications. Essential components and the operation regimes of ultrafast fibre laser systems are reviewed, as are their use in various applications.

Direct frequency comb spectroscopy in the extreme ultraviolet

Abstract: By coupling a high-power, high-repetition-rate near-infrared frequency comb to a femtosecond optical cavity, a frequency comb operating in the extreme-ultraviolet spectral range has been produced, by high harmonic generation, and provides high-resolution spectroscopy in this spectral region. Laser-based optical frequency combs, so called because they emit evenly spaced spectral lines, are used in precision spectroscopy and other applications requiring accurate measurements, such as atomic clocks. Efforts to extend this capability to shorter wavelengths in the extreme ultraviolet — which would open up exciting new applications, including searches for variation in fundamental constants — have lacked sufficient power for the purpose until now. Jun Ye and co-workers demonstrate a new approach, using a high-power, high-repetition pulsed infrared laser coupled into an optical cavity, to produce an improved extreme UV comb. In a first precision spectroscopy demonstration, they use direct frequency comb spectroscopy to determine argon and neon atomic transitions with ultra-high precision. The development of the optical frequency comb (a spectrum consisting of a series of evenly spaced lines) has revolutionized metrology and precision spectroscopy owing to its ability to provide a precise and direct link between microwave and optical frequencies1,2. A further advance in frequency comb technology is the generation of frequency combs in the extreme-ultraviolet spectral range by means of high-harmonic generation in a femtosecond enhancement cavity3,4. Until now, combs produced by this method have lacked sufficient power for applications, a drawback that has also hampered efforts to observe phase coherence of the high-repetition-rate pulse train produced by high-harmonic generation, which is an extremely nonlinear process. Here we report the generation of extreme-ultraviolet frequency combs, reaching wavelengths of 40 nanometres, by coupling a high-power near-infrared frequency comb5 to a robust femtosecond enhancement cavity. These combs are powerful enough for us to observe single-photon spectroscopy signals for both an argon transition at 82 nanometres and a neon transition at 63 nanometres, thus confirming the combs’ coherence in the extreme ultraviolet. The absolute frequency of the argon transition has been determined by direct frequency comb spectroscopy. The resolved ten-megahertz linewidth of the transition, which is limited by the temperature of the argon atoms, is unprecedented in this spectral region and places a stringent upper limit on the linewidth of individual comb teeth. Owing to the lack of continuous-wave lasers, extreme-ultraviolet frequency combs are at present the only promising route to extending ultrahigh-precision spectroscopy to the spectral region below 100 nanometres. At such wavelengths there is a wide range of applications, including the spectroscopy of electronic transitions in molecules6, experimental tests of bound-state and many-body quantum electrodynamics in singly ionized helium and neutral helium7,8,9, the development of next-generation ‘nuclear’ clocks10,11,12 and searches for variation of fundamental constants13 using the enhanced sensitivity of highly charged ions14.

Ultrafast Fiber Laser Technology

Abstract: In this paper, a review of fiber laser technology as relevant for applications in ultrafast optics is given. We discuss core enabling fiber technologies, such as fiber amplifiers, all-fiber dispersion control, and highly nonlinear and large-core fibers. We concentrate on systems built around passively mode-locked fiber lasers and fiber frequency combs, which are further amplified in large-core fiber amplifiers. Our review further encompasses coherent supercontinuum generation and techniques for absolute phase control of fiber lasers and amplifiers. Applications concerned with spectral generation in the range from the vacuum UV to the terahertz range are also described.

Photonic device fabrication in glass by use of nonlinear materials processing with a femtosecond laser oscillator

Abstract: Single-mode X couplers and three-dimensional waveguides are fabricated in transparent glasses by use of an unamplified femtosecond laser generating energies of up to 100 nJ. Changing fabrication parameters such as power and scanning speed permits creation of waveguides with a wide range of structures and refractive-index difference. Optical coherence tomography shows large refractive-index changes of up to ∼10-2 in the waveguides; these changes are consistent with guided mode analysis.

Flat Optics With Designer Metasurfaces

Abstract: Metamaterials are artificially fabricated materials that allow for the control of light and acoustic waves in a manner that is not possible in nature. This Review covers the recent developments in the study of so-called metasurfaces, which offer the possibility of controlling light with ultrathin, planar optical components. Conventional optical components such as lenses, waveplates and holograms rely on light propagation over distances much larger than the wavelength to shape wavefronts. In this way substantial changes of the amplitude, phase or polarization of light waves are gradually accumulated along the optical path. This Review focuses on recent developments on flat, ultrathin optical components dubbed 'metasurfaces' that produce abrupt changes over the scale of the free-space wavelength in the phase, amplitude and/or polarization of a light beam. Metasurfaces are generally created by assembling arrays of miniature, anisotropic light scatterers (that is, resonators such as optical antennas). The spacing between antennas and their dimensions are much smaller than the wavelength. As a result the metasurfaces, on account of Huygens principle, are able to mould optical wavefronts into arbitrary shapes with subwavelength resolution by introducing spatial variations in the optical response of the light scatterers. Such gradient metasurfaces go beyond the well-established technology of frequency selective surfaces made of periodic structures and are extending to new spectral regions the functionalities of conventional microwave and millimetre-wave transmit-arrays and reflect-arrays. Metasurfaces can also be created by using ultrathin films of materials with large optical losses. By using the controllable abrupt phase shifts associated with reflection or transmission of light waves at the interface between lossy materials, such metasurfaces operate like optically thin cavities that strongly modify the light spectrum. Technology opportunities in various spectral regions and their potential advantages in replacing existing optical components are discussed.

Photonic crystal fibers

Abstract: Photonic crystal fibers guide light by corralling it within a periodic array of microscopic air holes that run along the entire fiber length Largely through their ability to overcome the limitations of conventional fiber optics—for example, by permitting low-loss guidance of light in a hollow core—these fibers are proving to have a multitude of important technological and scientific applications spanning many disciplines The result has been a renaissance of interest in optical fibers and their uses

Femtosecond laser micromachining in transparent materials

Abstract: Femtosecond laser micromachining can be used either to remove materials or to change a material's properties, and can be applied to both absorptive and transparent substances. Over the past decade, this technique has been used in a broad range of applications, from waveguide fabrication to cell ablation. This review describes the physical mechanisms and the main experimental parameters involved in the femtosecond laser micromachining of transparent materials, and important emerging applications of the technology. Interactions between laser and matter are fascinating and have found a wide range of applications. This article gives an overview of the fundamental physical mechanisms in the processing of transparent materials using ultrafast lasers, as well as important emerging applications of the technology.

Optical coherence tomography - principles and applications

Abstract: There have been three basic approaches to optical tomography since the early 1980s: diffraction tomography, diffuse optical tomography and optical coherence tomography (OCT). Optical techniques are of particular importance in the medical field, because these techniques promise to be safe and cheap and, in addition, offer a therapeutic potential. Advances in OCT technology have made it possible to apply OCT in a wide variety of applications but medical applications are still dominating. Specific advantages of OCT are its high depth and transversal resolution, the fact, that its depth resolution is decoupled from transverse resolution, high probing depth in scattering media, contact-free and non-invasive operation, and the possibility to create various function dependent image contrasting methods. This report presents the principles of OCT and the state of important OCT applications. OCT synthesises cross-sectional images from a series of laterally adjacent depth-scans. At present OCT is used in three different fields of optical imaging, in macroscopic imaging of structures which can be seen by the naked eye or using weak magnifications, in microscopic imaging using magnifications up to the classical limit of microscopic resolution and in endoscopic imaging, using low and medium magnification. First, OCT techniques, like the reflectometry technique and the dual beam technique were based on time-domain low coherence interferometry depth-scans. Later, Fourier-domain techniques have been developed and led to new imaging schemes. Recently developed parallel OCT schemes eliminate the need for lateral scanning and, therefore, dramatically increase the imaging rate. These schemes use CCD cameras and CMOS detector arrays as photodetectors. Video-rate three-dimensional OCT pictures have been obtained. Modifying interference microscopy techniques has led to high-resolution optical coherence microscopy that achieved sub-micrometre resolution. This report is concluded with a short presentation of important OCT applications. Ophthalmology is, due to the transparent ocular structures, still the main field of OCT application. The first commercial instrument too has been introduced for ophthalmic diagnostics (Carl Zeiss Meditec AG). Advances in using near-infrared light, however, opened the path for OCT imaging in strongly scattering tissues. Today, optical in vivo biopsy is one of the most challenging fields of OCT application. High resolution, high penetration depth, and its potential for functional imaging attribute to OCT an optical biopsy quality, which can be used to assess tissue and cell function and morphology in situ. OCT can already clarify the relevant architectural tissue morphology. For many diseases, however, including cancer in its early stages, higher resolution is necessary. New broad-bandwidth light sources, like photonic crystal fibres and superfluorescent fibre sources, and new contrasting techniques, give access to new sample properties and unmatched sensitivity and resolution.

Photonic-Crystal Fibers

Abstract: The history, fabrication, theory, numerical modeling, optical properties, guidance mechanisms, and applications of photonic-crystal fibers are reviewed