This is a study of the potential of IBS for the production of high-quality ITO layers for optical applications with additional electrical requirements. The electrical properties of ITO layers combined with underlying optical interference layers and their optical properties and surface roughness are presented.
The laser systems currently used in ophthalmology either have some pulse length dependent side effects or are very expensive due to their complexity. Therefore, a newly developed approach using picosecond laser sources is investigated. These lasers combine the advantages of the low price of currently used short-pulse laser sources with the cold material ablation possibilities of high-end femtosecond sources. The surgeries intended are laser iridotomy, capsulotomy/post-cataract treatment and selective laser-trabeculoplasty (SLT). They are demonstrated on post mortem porcine eyes. The result is a more precise, less frayed tissue ablation with picosecond pulses in comparison to nanosecond pulses. The pulse energy could be reduced to (50 20) µJ per pulse instead of 1mJ to 10mJ per pulse, which is currently applied. The study of shock waves and cavitation bubbles revealed a huge difference in pressure between picosecond pulses (0:25MPa at 50 µJ) and nanosecond pulses (37MPa at 5 mJ). Therefore, the risk of collateral damage leading to potential additional clinical patterns and adverse effects could be significantly reduced.
We demonstrate sub-100 ps pulses with a Yb3+:YAG microchip laser passively Q-switched by a Cr4+:YAG saturable absorber. By introducing a subcavity, the laser threshold and the saturation energy are decreased which helps to prevent damage and to vary the effective emission and absorption cross sections. Pulse widths of 84 ps, repetition rates of 3.3 kHz and pulse energies of 32 μJ are achieved. This allows direct micromaterial processing e.g. for ophthalmic surgeries. To the best of our knowledge, this is the first sub-100 ps Yb3+:YAG/Cr4+:YAG microchip laser. A new approximation is used to calculate the rate equations for multiple longitudinal modes and to determine the threshold for single-longitudinal-mode operation.
We report on a passively mode-locked, diode-pumped femtosecond laser that provides ~200fs, 1053nm optical solitonlike pulses with < 8 orders of magnitude (OM) temporal contrast. The average output power of the compact, air-cooled laser is ~450mW at 77MHz pulse repetition rate. The spectral bandwidth (FWHM) is around 5-6nm, corresponding to transform limited pulses. For synchronization purposes the pulse repetition rate can be fine tuned to and continuously kept at a desired value employing a controller circuit, a motorized translation stage and a piezo actuator. The center wavelength of the pulses is tunable and can be precisely set to 1053nm by adjusting the laser pump power. The selfstarting laser is mode-locked using a semiconductor saturable absorber mirror (SESAM). The SESAM parameters are optimized for producing fs pulses with high temporal contrast. The effects of an intra-cavity fused silica plate inserted near Brewster angle and used for fine wavelength tuning and for polarization selection on the temporal contrast of the emitted pulses are discussed. The high temporal contrast pulses from this laser oscillator are well suited for seeding very high gain, multiple Joule type, Nd:glass, chirped pulse amplification systems routinely used in high intensity laser interaction experiments. The automated high dynamic range autocorrelator (HDR-AC) capable of recording the autocorrelation trace over 8 OM is described. The HDR-AC is based on a BBO crystal for second harmonic generation and a PMT for high sensitivity detection. A lock-in amplifier increases the detectable signal range by ~2 OM.
We present a passively Q-switched Nd3+:YAG/Cr4+:YAG laser with subsequent nonlinear pulse compression. This miniature laser combines both the high pulse energy of several tens of micro Joules and the short pulse duration of <20 ps without any amplification. It is therefore readily usable for many ultrafast applications including micro machining and medical applications. With these parameters, the laser shows a new level of compactness in comparison to other sub-100 picosecond laser sources. We utilize a Nd3+:YAG and a Cr4+:YAG crystal in a flat-Brewster, Brewsterflat configuration, respectively, positioned closely and the Brewster faces adjacent to each other. Pumped with approx. 6W from a fiber-coupled, 808 nm laser diode, the miniature, passively Q-switched laser oscillator delivers a pulse energy of 54 μJ with a pulse width of 339 ps and a repetition rate of 8.5 kHz. These pulses are subsequently coupled into a 20 μm core dia. large mode area PM fiber. Following a fiber propagation of 1.65 m the pulses are spectrally broadened by about a factor 40 due to self-phase modulation. Thereafter the nearly linearly chirped laser pulses are compressed by a chirped volume Bragg grating (CVBG). The optimized laser output pulses have a pulse width of 11.8 ps and a pulse energy of 20 μJ. We measured the polarized (PER<20dB) beam quality to be close to the diffraction limit with an M2≈1.5. A 13 hours continuous, stable laser operation has indicated a good long term stability and reliability.
Commercial picosecond sources have found widespread applications. Typical system parameters are pulse widths below 20 ps, repetition rates between 0.1 and 2 MHz, and microjoule level pulse energies. Most systems are based on short pulse mode-locked oscillators, regenerative amplifiers, and pockel cells as active beam switches. In contrast, we present a completely passive system, consisting of a passively Q-switched microchip laser, a single-stage amplifier, and a pulse compressor. The Q-switched microchip laser has a 50-μm-long Nd:YVO4 gain material optically bonded to a 4.6-mm-thick undoped YVO4 crystal. It delivers pulse widths of 40 ps and repetition rates of 0.2 to 1.4 MHz at a wavelength of 1.064 μm. The pulse energy is a few nanojoule. These 40-ps pulses are spectrally broadened in a standard single-mode fiber and then compressed in a 24-mm-long chirped Bragg grating to as low as 3.3 ps. The repetition rate can be tuned from ∼0.2 to 1.4 MHz by changing the pump power, while the pulse width and the pulse energy from the microchip laser are unchanged. The spectral broadening in the fiber is observed throughout the pulse repetition rate, supporting sub-10-ps pulses. Finally, the pulses are amplified in a single-stage Nd:YVO4 amplifier up to the microjoule level (up to 4 μJ pulse energy). As a result, the system delivers sub-10-ps pulses at a microjoule level with about 1 MHz repetition rate, and thus fulfills the requirements for ps-micromachining. It does not contain any active switching elements and can be integrated in a very compact setup.
Commercial picosecond sources have found widespread applications. Typical system parameters are pulse widths below 20 ps, repetition rates between 0.1 to 2 MHz, and micro Joule level pulse energies. Most systems are based on short pulse modelocked oscillators, regenerative amplifiers, and pockel cells as active beam switches. In contrast we present a completely passive system, consisting of a passively Q-switched microchip laser, a single-stage amplifier, and a pulse compressor. The Q-switched microchip laser has a 50 μm long Nd:YVO4-gain material optically bonded to a 4.6 mm thick undoped YVO4-crystal. It delivers pulse widths of 40 ps and repetition rates of 0.2 – 1.4 MHz at a wavelength of 1.064 μm. The pulse energy is a few nJ. These 40-ps pulses are spectrally broadened in a standard single mode fibre and then compressed in a 24 mm long chirped Bragg grating to as low as 3.3 ps. The repetition rate can be tuned from app. 0.2 to 1.4 MHz by changing the pump power while the pulse width and the pulse energy from the microchip laser are unchanged. The spectral broadening in the fibre is observed throughout the pulse repetition rate, supporting sub-10- ps pulses. Finally, the pulses are amplified in a single-stage Nd:YVO4-amplifier up to the microjoule level (up to 4 μJ pulse energy). As a result the system delivers sub-10-ps pulses at a microjoule level with about 1 MHz repetition rate, and thus fulfills the requirements for ps-micromachining. It does not contain any active switching elements and can be integrated in a very compact setup.
Short-pulse laser systems have found entry into industrial micro material fabrication processes on a large scale during the past ten years. In the same way the demand of simple, compact and cost-efficient seed sources has grown. The physical parameters needed for short-pulse laser processing range between a few femtoseconds to some ten picoseconds at repetition rates of up to 1 MHz. Up to now these laser systems are based on high repetition rate oscillators and regenerative amplifiers. These systems are rather complex and expensive. In contrast a Q-switched microchip laser in combination with a single pass amplifier permits a much simpler approach. In the following we present a 50 μm Nd3+:YVO4 microchip laser that is passively Q-switched by a semiconductor saturable absorber mirror. To overcome handling problems of the small crystal dimensions the 50 μm 3 at.-% doped Nd3+:YVO4 crystal is optically bonded to an undoped YVO4 crystal of a length of about 500 μm. The system provides pulse widths around 26 ps at a repetition rate of up to 0.9 MHz. The average output power is 15 mW at a wavelength of 1,064 nm, at an energy of 17 nJ. We will discuss the prospects and limits in terms of pulse width, repetition rate, output power, and system stability. The experimental data are compared to theoretical calculations.
A new, synchronously pumped picosecond OPO for CARS microscopy is presented. It is based on non-critically
phasematched interaction in LBO pumped by a frequency-doubled modelocked Nd:Vanadat laser at 532 nm.
Within the parametric process a tuneable pair of two different wavelengths in the NIR range is generated (Signal <680
...990 nm, Idler 1150...>2450 nm). In this system they are extracted from the cavity at the same mirror and therefore
propagating collinear at the same beam path. Due to the mechanism of their generation there is no jitter between Signal
and Idler. Though the wavelengths are different the GVD is negligible for this picosecond pulse duration. As a result the
two pulse trains are spatially and temporally perfectly matched.
The pulses generated are close to transform limit with about 5-6 ps pulse duration, excellent beam quality (M2 < 1,1) and
high pointing stability. The output power for Signal and Idler is about 1 W each @ 4 W pump power. The tuning
mechanism is split into two parts - temperature tuning for rough variations and fast angular BRF tuning for the fine
adjustment of the output wavelength.
The perfect spatial and temporal overlap make the described OPO an ideal and nearly hands-free laser source for CARS
microscopy with a tuneable energy difference 1,400 ... >10,000 cm-1. The absolute wavelength range is resulting in high
penetration depth and low photo damage of the analyzed samples.
Finally some CARS-images are presented and the latest results and methods for further sensitivity enhancements are
shown.
Continuum Generation (CG) in optical waveguides has been recently attracting widespread interest in fields requiring large spectral bandwidth such as metrology and Optical Coherence Tomography (OCT). Real time and in-vivo tissue imaging with cell resolution (Δz<1μm) is rapidly becoming the ultimate frontier of several OCT medical applications. CG wavelength and bandwidth are the pertinent criteria to obtain ultra high imaging resolution. The axial resolution in tissues is inversely proportional to the bandwidth whereas the central wavelength is chosen according to the minimum absorption of water and hemoglobin. Therefore optimal candidates for OCT low coherence sources1 are continua around 1μm as this is the zero group velocity dispersion wavelength of water.
In this work we demonstrate for the first time a low-noise continuum at very low powers in high index planar waveguides pumped at 1.04 μm. Bandwidths in excess of 150 nm at -3dB are generated with launching energies <1nJ/pulse in a ~2μm2 single mode ridge waveguides pumped in the normal dispersion regime. Self-Phase Modulation (SPM) had proven to be the only nonlinear process responsible for the CG. The polarization of the generated continua is highly preserved. Great flexibility in engineering waveguide dispersion, mode matching and optical functionality on chip is demonstrated by the planar approach.
This paper reports preliminary results from the development and application of a two-dimensional MEMS endoscopic scanner for OCT imaging. A 1 mm diameter mirror provides high aperture over large scan angle and can scan at rates of hundreds of Hz in both axes. The mirror is integrated with focusing optics and a fiber-optic collimator into a package of ~5 mm diameter. Using a broadband femtosecond laser light source, ultrahigh axial image resolution of < 5 um in tissue is achieved at 1.06 um center wavelength. Ultrahigh resolution cross-sectional and three-dimensional OCT imaging is demonstrated with the endoscope with ~12 um transverse resolution and < 5 um axial resolution.
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.
KEYWORDS: Optical coherence tomography, In vivo imaging, Image resolution, Imaging systems, Femtosecond phenomena, Neodymium glass lasers, Light sources, Single mode fibers, Skin, Real time imaging
We demonstrate compact ultrahigh resolution OCT systems for in vivo studies, with broadband light sources based on a commercially available Nd:Glass femtosecond laser and nonlinear fiber continuum generation. In vivo OCT images of hamster cheek pouch and human skin acquired at 4 frames per second and with 5.5 μm axial resolution are presented. These systems are robust, compact and portable.
Ultrahigh resolution OCT imaging is demonstrated using compact broadband light sources based on a commercially available Nd:Glass femtosecond laser with nonlinear fiber continuum generation. A tapered single mode fiber is used to generate broadband light centered at 1300 nm. Broadband light near 1064 nm can also be generated using a high numerical aperture single mode germanium doped fiber. These light sources enable ultrahigh resolution OCT imaging with 5-6 μm axial resolution at both 1064 nm and 1300 nm.
Femtosecond and picosecond pulses can find many applications if they can be produced with laser sources that are not only powerful and efficient but also compact and reliable. In continuous wave operation, diode pumping of solid-state lasers has allowed for a rapid progress towards powerful, compact and reliable sources, while the often used technique of Kerr lens modelocking for pulsed operation tends to be in conflict with requirements for diode-pumpable high power designs. Passive modelocking with semiconductor saturable absorber mirrors solves this problem as it relaxes the restrictions on the cavity design. We report on our recent achievements in this field. In particular we present a novel semiconductor device for dispersion compensation and various improved diode-pumped passively modelocked lasers. Also we show which laser parameters determine the stability of a passively modelocked lasers against Q-switching instabilities.
A diode-pumped Nd:fluorophosphate regenerative amplifier was developed. Chirped seed pulses were amplified from between 54 (mu) J at 100 Hz to 5 (mu) J at 10 kHz and compressed to 850 femtoseconds. The effects of continuous-wave pumping on the pulse energy at high repetition rates are also presented.
`Real-world' applications of femtosecond pulses require laser sources that are reliable, compact, and easy-to-use. Diode-pumped lasers are one key step in this direction, and as a next key step we have developed and demonstrated a simple technique using semiconductor saturable absorbers to passively start and stabilize mode-locked lasers. Because the saturable absorber stabilizes soliton modelocking, we achieve self-starting modelocking over a wide cavity stability range, in contrast to KLM, which tends to require critical cavity alignment and is usually not self-starting. We discuss different saturable absorber designs. An A-FPSA is used in a diode-pumped Nd:glass laser (130 fs, 100 mW avg output power) and a Cr:LiSAF laser (45 fs, 80 mW output). A thin saturable absorber design provides self-starting mode- locking over a wavelength range of 30 nm, and a low-loss design supports a record mode- locked output power of 120 mW from a Cr:LiSAF laser. A dispersive saturable absorber mirror design combines both negative dispersion compensation and saturable absorption within one semiconductor device and produces a compact mode-locked Cr:LiSAF laser (160 fs, 25 mW) without the need of prisms for dispersion compensation. Finally, we present an optimized diode-pumped cavity layout for Cr:LiSAF which allows for higher output powers (> 1 W cw).
We have successfully demonstrated that an appropriately designed semiconductor saturable absorber device, the antiresonant Fabry-Perot saturable absorber, can reliably start and sustain stable mode locking of solid state lasers such as Nd:YAG, Nd:YLF, Nd:Glass, Cr:LiSAF, and Ti:sapphire lasers. Especially for solid state lasers with long upper-state lifetimes, previous attempts to produce self-starting passive mode locking with saturable absorbers was always accompanied by self-Q-switching. We derive criteria that characterize the dynamic behavior of solid state lasers in the important regimes of Q-switching, mode-locked Q-switching, and continuous-wave mode locking in the picosecond and femtosecond range for the pulsewidth. We demonstrate that semiconductor absorbers can be designed to predetermine the dynamic behavior of a laser for a given solid state laser material and present an experimental verification. This allows for the development and design of robust, compact pico- and femtosecond solid state laser sources for scientific and industrial applications.
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