High-Harmonic Generation (HHG) is a highly non-linear frequency up conversion process, mostly studied from a classical point of view. Recently, independent theoretical investigations about the quantum nature of HHG predicted several, non-classical effects in the high-harmonic radiation. In addition to the fundamental interest in understanding the physics behind HHG, a better understanding of the quantum nature of this process could potentially have a broad impact on the rapidly developing field of quantum technologies. It is in this context that present here our experimental photon statistics investigations showing the quantum nature of the HHG process.
A new approach for direct third-harmonic generation is the generation inside a stack of dielectric layers. At present, our highest conversion efficiency achieved is 3.5%. This contribution provides an overview of the design process, production, measurement results, and their agreement with simulation results. To create the frequency tripling mirror designs, we use a combination of a Monte Carlo algorithm and a Meep-based algorithm to solve Maxwell's equations. Mandatory for the production of the mirrors is a very precise knowledge of the dispersion data of the materials used. For this purpose, the dispersion data of the coating materials are re-fitted using in-situ transmission data of a BBM after each coating run. In combination with various measures to maintain a stable refractive index of the used Hf_xAl_yO, high coating thickness accuracies are achieved in this way. Finally, experimental measurements and simulation results are compared using the post-fitted dispersion and layer thickness data.
Mamyshev oscillators are at the forefront of parameter scaling of ultrafast fiber oscillators. This talk focuses on the prospects of Mamyshev oscillators in the spectral 2 µm region based on thulium-doped fibers. Recent studies have reported pulse energies up to 15 nJ at a duration of 140 fs by a large normal dispersive cavity. Moreover, higher order soliton dynamics hold promise to scale the peak power into the MW-class for ultrafast fiber oscillators in the short wavelength mid-infrared.
Gain dynamics play a crucial role in the design of laser oscillators and amplifiers. In order to expand
the understanding of gain dynamics into the spectral 2 μm region, this study presents the experimental characterization of transfer functions in thulium-doped fiber amplifiers associated with the 3F4 → 3H6 transition that governs emission in this spectral range. The transfer functions for both the signal and pump are thoroughly investigated, providing detailed information on their magnitude and phase. We find a damped high-pass and a low-pass filtering behaviour for the respective signal and pump transfer function of thulium-ions in silica glass.
We report on our recent results of the numeric evaluation of the gain evolution inside a thulium-doped fiber Mamyshev oscillator in the spectral 2 μm region, comparing the core pumping scheme with the double-clad pumping scheme in order to further optimize the output parameters in the experimental setup. By comparing the two pumping schemes, we find spectral gain channeling around 1950 nm for the double-clad pumping scheme owing to a three orders of magnitude lower pump intensity and an order of magnitude higher doping concentration. The found gain distribution is highly effective to suppress amplified spontaneous emission at the maximum emission cross section of thulium ions in silica glass and thus enables operation beyond the water absorption lines. Furthermore based on the gain evolution model, also a novel broadening mechanism inside the gain fiber of Mamyshev oscillators is numerically evaluated. The pulse evolution is determined by the interplay between the anomalous dispersion and self-phase modulation inside the gain fiber and allows to self-compress the pulse, while simultaneously monotonic spectral broadening arises during the amplification. This novel pulse evolution inside a Mamyshev oscillator shows nearly transform limited high peak power pulses with a pulse duration below 100 fs directly at the output without any additional compression stage.
Recently, Mamyshev oscillators (MO) have attained a lot of attention, due to their generation of mode-locked pulses with outstanding output parameters in terms of output energy, spectral bandwidth and pulse duration. We present a MO with output pulse energies in the range of 0.5µJ, an optical spectrum ranging from 1010nm to 1060nm and an externally compressed autocorrelation duration of less than 100fs. This MO completely consists of commercially available standard step-index fibers. In order to handle the high pulse energies, we apply a few-mode gain fiber with a core-diameter of 20µm in the second arm of the oscillator.
The principle of a Mamyshev oscillator depends on alternating spectral filtering between sections of spectral broadening by self-phase modulation. In the 2 µm wavelength range, this concept faces the difficulty that standard fibers are anomalous dispersive which limits the possible pulse energy to the pJ-regime without proper dispersion management. We applied ultra-high numerical aperture fibers with normal dispersion in order to achieve up-chirped pulses in an anomalous dispersive Thulium-doped gain fiber. With that design, we achieved mode-locked pulses with energies of 6.4 nJ and a compressed autocorrelation duration of 195 fs at a repetition rate of 16 MHz.
We report gamma radiation influence on an active Er3+ doped fiber amplifier. Hydrogenation of active fibers under special condition allowed for a radiation hardness increase by an order of magnitude. Stability and longevity of hydrogenation effects are investigated.
We report the first fully monolithic, polarized, fiber amplifier based on resonantly pumped Holmium doped optical fibers at wavelength longer than 2050 nm. With commercially available fibers, it was possible to achieve Watt level output powers at 2088 nm.
A novel optical concept is introduced with standard components for highly efficient coherent beam combining a system of (N x N) beams. In a proof-of-principle experiment a well-defined setup with microlens arrays (MLAs) is used to create a beam matrix of 5 x 5 beams. For the combination step the same setup is employed, and the created 25 beams are combined. A combination efficiency above 90% is achieved. Furthermore, the concept allows for dynamic beam combination, i.e., the resulting number of beams and corresponding positions can be controlled by the absolute phases of the array of input beams. A proof-of-principle experiment shows excellent agreement with the model.
We present the characteristics of a high-energy ultrafast Yb-fiber laser system, based on a Mamyshev oscillator and a subsequently arranged fiber amplifier stage. The Mamyshev oscillator emits pulses at a repetition rate of 11 MHz and pulse energies of 31 nJ. These pulses are spectrally filtered and amplified in a Yb-doped fiber up to 1 μJ pulse energy which could be temporally dechirped to less than 50 fs autocorrelation duration. We discuss the scaling as well as limiting options related to pulse energies and duration.
We present a turn-key pulsed polarized Holmium doped fiber source at 2088 nm. In our experimental setup we are utilizing an acousto-optic modulator as a mode-locker or Q-switch. The linear cavity is comprised of a fiber Bragg grating (FBG) pair with central wavelength of 2088 nm in a polarization maintaining passive fiber. The active fiber is polarization maintaining, doped with Holmium, and pumped with an in-house made Thulium doped fiber laser with an emission wavelength of 1940 nm and a maximum output power of more than 10 W. In q-switch operation it was possible to generate stable nanosecond pulses between 5 and 250 ns at repetition frequencies between 10 and 250 kHz. Shaping of the AOM driving signal allows for some modification of output pulse shape and its length. The maximum output energy was measured to be 0.5 µJ. In mode-locked operation the laser emitted pulses at 27.35 MHz repetition rate which corresponds to the optical cavity roundtrip time. The calculated band-width limited pulse width was approx. 13 ps (sech2) and the pulse energy was 0.5 nJ. In both operation regimes the signal quality was very high with optical signal-to-noise ratio greater than 50 dB without amplified spontaneous emission (ASE). Similarly, the polarization extinction ratio (PER) was greater than 20 dB. The oscillator design is fully monolithic and polarization-maintaining. Its compactness, turn-key operation, and all-fiber design makes it a very promising platform for nonlinear frequency conversion.
The generation of sub-10 ps pulses around a wavelength of 2 μm with pulse energy at millijoule-level in a compact CPA-free amplifier chain is presented. This laser source covers a broad range of pulse repetition frequencies from 1 to 100 kHz with a pulse peak power from 136 to 17MW, respectively. We used highly doped Ho:YLF crystals to achieve an overall amplification factor of almost 52 dB. A characterization of these crystals regarding upconversion losses and attainable small-signal gain supports this work.
We present a numerical approach for the accurate simulation of the complex propagation dynamics of ultrashort optical pulses in nonlinear waveguides, especially valid for few-cycle pulses. The propagation models are derived for the analytical signal, which includes the real optical field, exempt from the commonly adopted slowly varying envelope approximation. As technical basis for the representation of the medium dispersion we use rational Pad´e approximants instead of commonly employed high-order polynomial expansions. The implementation of the propagation equation is based on the Runge-Kutta in the interaction picture method. In addition, our modular approach easily allows to incorporate a Raman response and dispersion in the nonlinear term. As exemplary use-cases we illustrate our numerical approach for the simulation of a few-cycle pulse at various center frequencies for an exemplary photonic crystal fiber and demonstrate the collision of a soliton and two different dispersive waves mediated by their group-velocity event horizon.
We present the application of the dispersion scan (d-scan) technique in order to fully characterize intensity and phase of ultrafast pulses compressed by a grating-prism (grism) compressor.
Ultrafast pulses are generated in an Yb-doped fiber oscillator, mode-locked by non-linear polarization evolution (NPE) and operating at a central wavelength of 1030 nm with a repetition rate of 50 MHz. These output pulses are stretched in a 150 m long fiber to roughly 50 ps and then amplified in an Yb-doped fiber to an average power of 150 mW. A grating-prism combination is used to compensate the second and third order dispersion that was applied to the pulse by the fiber material.
For the optimal compression of the pulse, information on its spectral and temporal phase is required. Therefore, the dispersion scan is applied. In this technique, second harmonic spectra of the pulse are generated and recorded for different amounts of dispersion compensation applied to the pulse by the grism compressor. Depending on the chirp of the pulse, the maximum of the generated second harmonic spectra shifts, leading to a characteristic trace.
The amount of dispersion is changed by varying the separation of the prisms within the compressor, while the relation of second and third order dispersion is kept almost constant. The separation is changed by a stepper motor in order to enable a fully automated recording of the spectra.
The spectral and temporal course and phase of the pulse are then retrieved from the d-scan trace by a genetic computer algorithm.
Controlled few-cycle light waveforms find numerous applications in attosecond science, most notably the production of isolated attosecond pulses in the XUV spectral region for studying ultrafast electronic processes in matter. Scaling up the pulse energy of few-cycle pulses could extend the scope of applications to even higher intensity processes, such as the generation of attosecond pulses with extreme brightness from relativistic plasma mirrors. Hollow-fiber compressors are widely used to produce few-cycle pulses with excellent spatiotemporal quality, whereby octave-spanning broadened spectra can be temporally compressed to near-single-cycle duration. In order to scale up the peak power of hollow-fiber compressors, the effective length and area mode of the fiber has to be increased proportionally, thereby requiring the use of longer waveguides with larger apertures. Thanks to an innovative design utilizing stretched flexible capillaries, we show that a stretched hollow-fiber compressor can generate pulses of TW peak power, the duration of which can be continuously tuned from the input seed laser pulse duration down to almost a single cycle (3.5fs at 750nm central wavelength) simply by increasing the gas pressure at the fiber end. The pulses are characterized online using an integrated d-scan device directly under vacuum. While the pulse duration and chirp are tuned, all other pulse characteristics, such as energy, pointing stability and focal distribution remain the same on target. This unique device makes it possible to explore the generation of high-energy attosecond XUV pulses from plasma mirrors using controllable relativistic-intensity light waveforms at 1kHz.
We present CPA-free linear amplification of 6:3 ps pulses in Ho:YLF crystals up to 100 μJ pulse energy at 10 kHz repetition rate. The seed pulses at a wavelength of 2:05 μm are provided by a Ho-based all-fiber system consisting of a soliton oscillator and a subsequent pre-amplifier followed by a free-space AOM as pulse-picker. Considering the achieved pulse peak power at MW-level, this system is a powerful tool for efficient pumping of parametric amplifiers addressing the highly demanded mid-IR spectral region. In detailed numerical simulations we verified our experimental results and discuss scaling options for pulse duration and energy.
Martin Höhl, Daphne DeTemple, Stefan Lyutenski, Georg Leuteritz, Arthur Varkentin, Heike Andrea Schmitt, Thomas Lenarz, Bernhard Roth, Merve Meinhardt-Wollweber, Uwe Morgner
Optical techniques are effective tools for diagnostic applications in medicine and are particularly attractive for the noninvasive analysis of biological tissues and fluids in vivo. Noninvasive examinations of substances via a fiber optic probe need to consider the optical properties of biological tissues obstructing the optical path. This applies to the analysis of the human perilymph, which is located behind the round window membrane. The composition of this inner ear liquid is directly correlated to inner ear hearing loss. In this work, experimental methods for studying the optical properties of the human round window membrane ex vivo are presented. For the first time, a comprehensive investigation of this tissue is performed, including optical transmission, forward scattering, and Raman scattering. The results obtained suggest the application of visible wavelengths (>400 nm) for investigating the perilymph behind the round window membrane in future.
The present contribution is addressed to an improved method to fabricate dielectric dispersive compensating mirrors (CMs) with an increased laser induced damage threshold (LIDT) by the use of ternary composite layers. Taking advantage of a novel in-situ phase monitor system, it is possible to control the sensitive deposition process more precisely. The study is initiated by a design synthesis, to achieve optimum reflection and GDD values for a conventional high low stack (HL)n. Afterwards the field intensity is analyzed, and layers affected by highest electric field intensities are exchanged by ternary composites of TaxSiyOz. Both designs have similar target specifications whereby one design is using ternary composites and the other one is distinguished by a (HL)n. The first layers of the stack are switched applying in-situ optical broad band monitoring in conjunction with a forward re-optimization algorithm, which also manipulates the layers remaining for deposition at each switching event. To accomplish the demanded GDD-spectra, the last layers are controlled by a novel in-situ white light interferometer operating in the infrared spectral range. Finally the CMs are measured in a 10.000 on 1 procedure according to ISO 21254 applying pulses with a duration of 130 fs at a central wavelength of 775 nm to determine the laser induced damage threshold.
Resonant enhancement of Raman signals is a useful method to increase sensitivity in samples with low concentration such as biological tissue. The investigation of resonance profiles shows the optimal excitation wavelength and yields valuable information about the molecules themselves. However careful characterization and calibration of all experimental parameters affecting quantum yield is required in order to achieve comparability of the single spectra recorded. We present an experimental technique for measuring the resonance profiles of different amino acids. The absorption lines of these molecules are located in the ultraviolet (UV) wavelength range. One limitation for broadband measurement of resonance profiles is the limited availability of Raman filters in certain regions of the UV for blocking the Rayleigh scattered light. Here, a wavelength range from 244.8 nm to 266.0 nm was chosen. The profiles reveal the optimal wavelength for recording the Raman spectra of amino acids in aqueous solutions in this range. This study provides the basis for measurements on more complex molecules such as proteins in the human perilymph. The composition of this liquid in the inner ear is essential for hearing and cannot be analyzed non-invasively so far. The long term aim is to implement this technique as a fiber based endoscope for non-invasive measurements during surgeries (e. g. cochlear implants) making it available as a diagnostic tool for physicians. This project is embedded in the interdisciplinary cluster of excellence "Hearing for all" (H4A).
In the presented work a fast frequency domain measurement system to determine group delay (GD) and group delay dispersion (GDD) of optical coatings is proposed. The measurements are performed in situ directly on moving substrates during the thin film coating process. The method is based on a Michelson interferometer, which is equipped with a high power broad band light source and a fast spectrometer. Especially for the production of chirped mirrors it is advantageous to obtain group delay and group delay dispersion data of the last layers. This additional information allows for online corrections of coating errors to enhance the precision of complex interference filters for short pulse applications.
The present contribution is concentrated on an improved method to manufacture dielectric dispersion compensating mirrors in the ultra violet (UV) range by applying a novel online phase monitoring device. This newly developed measurement tool monitors the group delay (GD) and group delay dispersion (GDD) of the electromagnetic field in situ during the deposition of the layer system. Broad band monitoring of the phase enhances the accuracy in the near infrared spectral range (NIR), significantly. In this study, the correlation of the GDD in the NIR and in the UV spectral range is investigated. A design synthesis is introduced to achieve optimum reflection and GDD target values in the UV and NIR. This requires a similar behavior of both bands according to deposition errors, to guarantee switching off the UV GDD target band proper, while monitoring the GDD in the NIR spectral range. The synthesis results in a design, characterized by a GDD of -100fs2±20fs2 between 330nm and 360nm in the UV and by -450fs2±10fs2 within 820nm to 870nm in the NIR. The fabricated sample, applying an ion beam sputtering process, consists of a 9μm layer stack of Hafnium oxide and Silicon dioxide. The first layers of the stack are switched and controlled by a conventional in situ spectrometric broad band monitoring in conjunction with a forward re-optimization algorithm, which also manipulates the layers remaining for deposition at each switching event. To accomplish the demanded GDD-spectra, the last layers are controlled by the novel in situ GDD monitor.
Jan Matyschok, Thomas Binhammer, Tino Lang, Oliver Prochnow, Stefan Rausch, Piotr Rudawski, Anne Harth, Miguel Miranda, Chen Guo, Eleonora Lorek, Johan Mauritsson, Cord Arnold, Anne L'Huillier, Uwe Morgner
A compact, high-repetition rate optical parametric chirped pulse amplifier system emitting CEP-stable, few-cycle pulses
with 10 μJ of pulse energy is reported for the purpose of high-order harmonic generation. The system is seeded from a
commercially available, CEP-stabilized Ti:sapphire oscillator, delivering an octave-spanning spectrum from 600-1200 nm. The oscillator output serves on the one hand as broadband signal for the parametric amplification process and
on the other hand as narrowband seed for an Ytterbium-based fiber preamplifier with subsequent main amplifiers and
frequency doubling. Broadband parametric amplification up to 17 μJ at 200 kHz repetition rate was achieved in two
5 mm BBO crystals using non-collinear phase matching in the Poynting-vector-walk-off geometry. Efficient pulse
compression down to 6.3 fs is achieved with chirped mirrors leading to a peak power exceeding 800 MW. We observed
after warm-up time a stability of < 0.5 % rms over 100 min. Drifts of the CE-phase in the parametric amplifier part could
be compensated by a slow feedback to the set point of the oscillator phase lock. The CEP stability was measured to be
better than 80 mrad over 15 min (3 ms integration time).
The experimentally observed output spectra and energies could be well reproduced by simulations of the parametric
amplification process based on a (2+1)-dimensional nonlinear propagation code, providing important insight for future
repetition rate scaling of OPCPA systems. The system is well-suited for attosecond science experiments which benefit
from the high repetition rate. First results for high-order harmonic generation in argon will be presented.
Optical coherence tomography (OCT) is a non-invasive in vivo biomedical imaging modality capable of three-dimensional
visualization of tissue morphology permitting imaging at high speed and sensitivity. Coherent Anti-
Stokes Raman Scattering (CARS) is a nonlinear spectroscopic technique which provides molecular information
due to a four wave mixing process. In order to extend the performance of OCT towards detecting the molecular
fingerprint of biological samples a combined CARS/OCT setup has been developed that employs only a single
ultrashort pulse Ti:Sapphire laser which enables high axial resolution OCT and simultaneously combined with
a spectral shaper a CARS setup. During first measurements the same area of a sample was imaged twice,
applying OCT and CARS consecutively. OCT was used to perform three-dimensional morphological screening.
Due to CARS additional chemical information could be gained for two dimensions. The spectrum was modified
computer controlled to match the requirements for the generation of a CARS signal whereas for OCT the
unmodified spectrum was applied. Fluids such as dimethylsulfoxide (DMSO) and PBS were compared in a
cuvette to demonstrate the functionality of the multimodal setup. As a biological sample a 100 m thickcr oss
section through a human optic nerve surrounded by sclera was investigated.
Recently, great effort has been devoted to waveguide lasers, because of their inherent simplicity with respect to
fiber lasers. Actually, due to their compactness, such lasers are expected to achieve a higher temporal coherence,
making them attracting for fiber optical reflectometry, distribute sensing, and range finding applications. Furthermore,
the availablity of fast saturable absorbers based on carbon nanotubes allows for a cheap and reliable
implementation of the passive mode-locking technique with the potential for generating high repetition rate pulse
trains. Such lasers will provide low-noise and inexpensive pulsed sources for applications in optical communications,
optically sampled analog-to-digital converters, and spectral line-by-line pulse shaping. We report here on
advanced waveguide lasers, operating both in continuous wave and pulsed regimes, based on active waveguides
fabricated by femtosecond laser writing in a phosphate glass substrate. A single longitudinal mode waveguide
laser providing more than 50 mW with 21% slope efficiency was demonstrated. Furthermore, by combining a high
gain waveguide and an innovated fiber-pigtailed saturable absorber based on carbon nanotubes, a mode-locked
ring laser providing transform limited 1.6-ps pulses was also demonstrated.
The fabrication of telecom active devices, such as waveguide amplifiers and lasers, with femtosecond laser pulses is of great industrial interest due to the simplicity, low cost and 3D capabilities of this technology with respect to the standard ones. In this work we will present the various improvements that brought us to demonstrate net gain and the first waveguide laser fabricated with femtosecond laser pulses on an erbium-ytterbium-doped phosphate glass. The first results have been obtained with an amplified, low repetition rate (1 kHz), Ti:Sapphire system. The target of matching the mode field of the fabricated waveguides to that of standard telecom fibers pushed us to develop a novel astigmatic focusing of the writing beam to overcome the asymmetry of the waveguide transverse profile intrinsic in the transversal writing geometry. Despite the circularization of the transverse profile, the high coupling losses allowed only for internal gain in an all-fiber coupling configuration. The best results have been obtained with a very compact, unamplified, diode-pumped Yb:glass laser, with a higher repetition rate (166/505 kHz) and lower energy. In this case, the waveguides exhibited almost perfect mode matching with a telecom fiber allowing coupling losses as low as 0.18 dB and propagation losses of 0.5 dB/cm. Such figures enabled net gain when pumping with 980-nm laser diodes and laser action by terminating the waveguide with two fiber Bragg gratings. These results pave the way to a transfer of femtosecond waveguide writing into the industrial arena for the realization of practical telecom components.
A diode-pumped Yb:glass femtosecond laser oscillator with electro-optical cavity-dumping has been applied for nonlinear laser-scanning microscopy and processing of biomaterials. The high-energy pulses delivered by this source in combination with its unique parameters proved very efficient for micro-processing of biomaterials.
The fabrication of telecom active devices, such as waveguide amplifiers and lasers, with femtosecond laser pulses is of great industrial interest due to the simplicity, low cost and 3D capabilities of this technology with respect to the standard ones. In this work we will present the various improvements that brought us to demonstrate net gain and the first waveguide laser fabricated with femtosecond laser pulses on an erbium-ytterbium-doped phosphate glass. The first results have been obtained with an amplified, low repetition rate (1 kHz), Ti:Sapphire system. The target of matching the mode field of the fabricated waveguides to that of standard telecom fibers pushed us to develop a novel astigmatic focusing of the writing beam to overcome the asymmetry of the waveguide transverse profile intrinsic in the transversal writing geometry. Despite the circularization of the transverse profile, the high coupling losses allowed only for internal gain in an all-fiber coupling configuration. The best results have been obtained with a very compact, unamplified, diode-pumped Yb:glass laser, with a higher repetition rate (166/505 kHz) and lower energy. In this case, the waveguides exhibited almost perfect mode matching with a telecom fiber allowing coupling losses as low as 0.18 dB and propagation losses of 0.5 dB/cm. Such figures enabled net gain when pumping with 980-nm laser diodes and laser action by terminating the waveguide with two fiber Bragg gratings. These results pave the way to a transfer of femtosecond waveguide writing into the industrial arena for the realization of practical telecom components.
We report on a diode pumped tunable Yb:glass femtosecond laser oscillator with electro-optical cavity dumping. Pulses with energies exceeding 400 nJ and peak powers of above 1MW were generated at repetition frequencies as high as 200kHz. We discuss two issues, the possibility of enhanced stability and spiking suppression by implementation of an active feedback technology. This laser forms a compact light source for various scientific and industrial applications like micromachining.
Novel optical components enable the generation of the shortest pulses and broadest spectra from Kerr-Lens mode-locked laser oscillators without extracavity spectral broadening, namely 20-fs-pulses from Cr4+:YAG around 1.5μm, 14-fs-pulses from Cr:forsterite around 1.3μm, 5-fs-pulses from Ti:sapphire around 0.8μm, and 10-fs-Pulses from Cr3+:LiCAF around 0.8μm. Key components are well adapted phase correcting mirrors ("double-chirped mirrors") which allow for high reflectivity and dispersion compensation in bandwidths up to one octave. In parallel to the development of new broadband light sources based on femtosecond technology micron resolution imaging with Optical Coherence Tomography using theses sources has been achieved. The availability of the high resolution OCT technology for future clinical applications will depend on the development of low cost, compact sources of ultrabroad bandwidth light. Especially Cr3+:LiCAF is a very promising material for femtosecond laser sources as compact replacements for Ti:sapphire oscillators because of its low quantum defect, a broadband emission range around 800 nm, and an absorption band in a spectral range where high-brightness laser diodes are available.
We discuss the dependence of nonlinear optical signals on the carrier-envelope offset phase, i.e., the phase between the rapidly oscillating carrier wave and the electric field envelope of a few-cycle pulse. Our results in the regime on non-perturbative resonant nonlinear optics are compared with the know off-resonant perturbative regime. In particular, we introduce camouflage third-harmonic generation, which shows up as a peak at twice the carrier-envelope offset frequency in the radio-frequency spectra. Corresponding experiments on ZnO with intense 5 fs optical pulses are discussed. We speculate on the perspectives of using such effects for determining the carrier-envelope offset phase itself.
Using state of the art laser technology, third generation ophthalmologic optical coherence tomography (OCT) has been developed which enables ultrahigh resolution, non-invasive in vivo imaging of retinal morphology with an unprecedented axial resolution of 3 micrometers . This represents a quantum leap in performance over the 10-15 micrometers resolution currently available in ophthalmic OCT systems and, to our knowledge, is the highest resolution in vivo ophthalmologic imaging achieved to date. This resolution enables optical biopsy, i.e. the in vivo visualization of intraretinal architectural morphology which had previously only been possible with histopathology. Image processing and segmentation techniques are demonstrated for automatic identification and quantification of retinal morphology. Ultrahigh resolution ophthalmic OCT has the potential to enhance the sensitivity and specificity for early diagnosis of several ocular diseases, e.g. glaucoma, which requires precise imaging and measurement of retinal nerve fiber layer thickness, as well as improve monitoring of disease progression and efficacy of therapy.
The longitudinal resolution of optical coherence tomography (OCT) is currently limited by the optical bandwidth of the light source, typically a superluminescent diodes, to approximately 10-15 micrometers . This resolution is insufficient to identify individual cells or to assess subcellular structures such as nuclei or mitotic figures. The ability to perform subcellular imaging with OCT could greatly enhance the detection of early neoplastic changes and improve early cancer diagnosis or the imaging of developing biological morphology. Higher resolution OCT would also improve specificity of diagnosis for several ocular diseases, such as glaucoma, which require precise, detailed imaging and measurement of retinal nerve fiber layer thickness. State of the art Kerr-lens mode-locked Ti:Al2O3 lasers using double chirped dispersion compensating mirrors can generate pulse durations of < 7 fs and bandwidths of 200 nm or more at 800 nm center wavelength. These pulse durations and bandwidths can be used for OCT, resulting in longitudinal resolutions of less than 2 micrometers . The use of such broad bandwidths also enables the extraction of localized, wavelength dependent absorption and scattering tissue characteristic by detecting the full interferometric fringe signa and using Fourier signal processing. In this paper we demonstrate an ultra-high-subcellular level resolution, spectroscopic OCT system based on a mode-locked Ti:Al2O3 laser. In vivo imaging of development biology specimens as well as preliminary in vivo spectroscopic OCT result are demonstrated.
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