Laser pulses of few a nanoseconds’ duration are focused by an appropriate converging lens system, leading to breakdown of the medium (combustible gases), resulting in the formation of intense plasma. Plasma thus induced can be used to initiate the combustion of combustible air-fuel mixtures in a spark ignition engine provided the energy of the plasma spark is high enough. Laser ignition has several advantages over the conventional spark ignition system, especially in case of lean air-fuel mixture. In this study, laser ignition of compressed natural gas was investigated in a constant volume combustion chamber (CVCC) as well as in a single-cylinder engine. Flame kernel visualizations for different pulse energy of natural gas-air mixtures were carried out in the CVCC. The images of the development of early flame kernel stages and its growth with time were recorded by shadowgraphy technique. The effect of laser pulse energy on the engine combustion, performance, and emissions was investigated using different air-fuel mixtures. Increased peak cylinder pressure, higher rate of heat release, faster combustion, and increased combustion stability were observed for higher laser pulse energies. The effect of laser pulse energy on the engine-out emissions was also investigated in this study.
Laser ignition can yield certain advantages compared to conventional sparkplug ignition. Among other already frequently
discussed reasons due to: i) option for sequential or multipoint ignition which can contribute to more reliable ignition
in direct injection engines; ii) ignition of leaner mixtures at higher compression being most relevant for gas engines. A
satisfying solution to the above mentioned requirements is the longitudinally diode-pumped passively Q-switched
Cr4+:YAG/Nd 3+:YAG laser capable of emitting ∼1-ns-pulses of at least 20 mJ . This type of solid-state laser (SSL) confectioned
in an engine-compatible form can be called a laser sparkplug. Early versions of this concept comprised a high-power
diode pump laser (quasi-cw power <500 W @ ∼500 μs duration) which were placed remote from the engine to
avoid detrimental influences of temperature, vibrations, pollution etc. In this case only the SSL is exposed to the elevated
temperature in the vicinity of the cylinder walls (<100°C). Recently, technical and cost-oriented considerations allow a
change of concept from fiber-based remote pumping via edge emitter arrays to the use of newly developed so-called
power VCSELs with two-dimensional stacking. Collimation to form a round pump beam thereby becomes much easier.
Their temperature resistance allows lower-cost direct mounting although thereby a wavelength shift is induced. The Q-switched
SSL in the sparkplug also faces temperature dependent phenomena like reduction of pulse energy and efficiency,
a change of pulse timing and beam profile which will be discussed in the paper.
A novel type of all-optical pressure sensor has been developed. In this context, a vertical-cavity surface-emitting
laser (VCSEL) has been modified in its design to provide simultaneous light emission from both facets. One
beam serves as measuring signal while the other establishes a reference; and both paths lie on the same optical
axis. The VCSELs are based on active InGaAs quantum wells for laser output close to 960 nm wavelength where
the GaAs substrate is transparent. From both top and bottom facet, single-polarization and single-mode beams
are observed, having a power ratio of 1:2 to 1:4. In this paper we give insight into this new sensing application
for VCSELs, describe the laser fabrication and the static operation characteristics as well as the noise properties
which have paramount importance for high performance of the sensor.
With regard to the sensor application in acoustics, the focus of the noise measurements is put on the low-frequency,
i.e. kHz, regime. While laser diode noise performance is readily available for the MHz to GHz
frequency range, only very limited data exists in the Hz to kHz domain. The relative intensity noise of both
beams is measured and compared and the mutual correlation properties are investigated. The frequency noise
is quantified.
Optical access to combustion chambers via windows is desirable for combustion diagnostics as well as for laser ignition. By nature, combustion deposits can form on the inner surface of the light-transmitting window, leading to malfunction. We investigated whether a Nd:YAG ignition laser could cope with combustion-chamber deposits by means of ablation. In a 1.8-kW four-stroke internal combustion engine an optical window was installed to couple in the laser light. Ignition was carried out by a spark plug. Due to inherent high fuel and oil consumption, a deposit layer would form on the substrate within some tens of minutes. Elementary analysis showed carbonaceous as well as inorganic compounds gradually reducing light transmission. With cyclic 5-ns laser shots through the window, the pass-through stayed essentially free of deposits provided the energy fluence was around 10 mJ/mm2. Microanalysis showed evidence of the soundness of the principle. In addition, even single shots with a higher flux were enough to remove a relatively thick layer of deposits at once. Thus an optical window in an internal combustion engine can in principle be kept transmissive by the action of a compact solid-state laser.
During the last few years, ultra-short laser pulses have proven their potential for application in medical tissue
treatment in many ways. In hard tissue ablation, their aptitude for material ablation with negligible collateral
damage provides many advantages. Especially teeth representing an anatomically and physiologically very
special region with less blood circulation and lower healing rates than other tissues require most careful treatment.
Hence, overheating of the pulp and induction of microcracks are some of the most problematic issues in dental preparation.
Up till now it was shown by many authors that the application of picosecond or femtosecond pulses allows to
perform ablation with very low damaging potential also fitting to the physiological requirements indicated. Beside
the short interaction time with the irradiated matter, scanning of the ultra-short pulse trains turned out to be
crucial for ablating cavities of the required quality. One main reason for this can be seen in the fact that during
scanning the time period between two subsequent pulses incident on the same spot is so much extended that no
heat accumulation effects occur and each pulse can be treated as a first one with respect to its local impact.
Extension of this advantageous technique to biocompatible materials, i.e. in this case dental restoration materials
and titanium plasma-sprayed implants, is just a matter of consequence. Recently published results on composites
fit well with earlier data on dental hard tissue. In case of plaque which has to be removed from implants, it turns
out that removal of at least the calcified version is harder than tissue removal. Therefore, besides ultra-short lasers,
also Diode and Neodymium lasers, in cw and pulsed modes, have been studied with respect to plaque removal and sterilization. The temperature increase during laser exposure has been experimentally evaluated in parallel.
In dental hard tissue ablation, ultra-short laser pulses have proven sufficiently their potential for material ablation with
negligible collateral damage providing many advantages. The absence of micro-cracks and the possibility to avoid overheating
of the pulp during dental cavity preparation may be among the most important issues, the latter opening up an
avenue for potential painless treatment. Beside the evident short interaction time of laser radiation with the irradiated
tissue, scanning of the ultra-short pulse trains turned out to be crucial for ablating cavities of required quality and shape.
Additionally, long-pulsed laser systems have demonstrated successfully their suitability for decontamination purposes. In
this paper, an overview of different indications for laser application in dental therapies in both pulse regimes is presented.
A special focus is set on the decontamination of dental implants in periimplantitis therapy. Having employed
commercially available long pulse systems for dental applications and ultra-short 330 fs pulses, we present first results
for temperature development and corresponding ablation thresholds for dental implants, as in the future more gentle
implant cleaning by ultra-short laser pulses could become of interest.
In the past decades, many efforts have been made to replace mechanical tools in oral applications by various
laser systems. The reasons therefore are manifold: i) Friction causes high temperatures damaging adjacent tissue.
ii) Smear layers and rough surfaces are produced. iii) Size and shape of traditional tools are often unsuitable for
geometrically complicated incisions and for minimum invasive treatment. iv) Mechanical damage of the remaining
tissue occurs. v) Online diagnosis for feedback is not available.
Different laser systems in the µs and sub-&mrg;s-pulse regime, among them Erbium lasers, have been tested in the
hope to overcome the mentioned drawbacks and, to some extent, they represent the current state of the art with
respect to commercial and hence practical application. In the present work the applicability of scanned ultrashort
pulse lasers (USPLs) for biological hard tissue as well as dental restoration material removal was tested. It
is shown that cavities with features superior to mechanically treated or Erbium laser ablated cavities can be generated
if appropriate scan algorithms and optimum laser parameters are matched. Smooth cavity rims, no microcracks,
melting or carbonisation and precise geometry are the advantages of scanned USLP ablation. For bone
treatment better healing conditions are expected as the natural structure remains unaffected by the preparation
procedure.
The novelty of this work is represented by a comprehensive compilation of various experimental results intended
to assess the performance of USPLs. In this context, various pulse durations in the picosecond and femtosecond
regime were applied to dental and bone tissue as well as dental restoration materials which is considered to be
indispensable for a complete assessment. Parameters like ablation rates describing the efficiency of the ablation
process, and ablation thresholds were determined - some of them for the first time - and compared to the corresponding
Erbium values. The morphology of the tissue surfaces remaining after laser preparation was investigated
and the surface roughness was evaluateded. Selective ablation was stressed and the temperature impact induced
by USPLs was analyzed. Due to the limited space only a selection of results can be presented.
Within the last years, modern ultra-short pulse lasers have successfully proven their potential for application in medical tissue treatment in many respects. In dentistry, overheating of the pulp and induction of micro cracks are usually among the most problematic issues which can be solved in this way. An additional benefit can be seen in the possibility of plasma emission spectroscopy as a means of feedback. Up till now it was shown by many authors that the application of picosecond or femtosecond pulses allows to perform ablation with very low damaging potential also fitting to the special physiological requirements. Beside the short interaction time with the irradiated biological matter, lateral scanning of ultra-short pulses following optimized algorithms turned out to be crucial for ablating cavities with the required quality and size, a finding which we also believe to be valid for dental restoration materials. Additionally, out of practical reasons, scanning is necessary to treat larger volumes than just the focal spots typically having dimensions on the order of more than 1 mm3, thereby allowing to realize an "optical drill".
Due to the demands of the market to increase efficiencies and power densities of gas engines, existing ignition schemes are gradually reaching their limits. These limitations initially triggered the development of laser ignition as an effective alternative, first only for gas engines and now for a much wider range of internal combustion engines revealing a number of immediate advantages like no electrode erosion or flame kernel quenching. Furthermore and most noteworthy, already the very first engine tests about 5 years ago had resulted in a drastic reduction of NOx emissions. Within this broad range investigation, laser plasmas were generated by ns Nd-laser pulses and characterized by emission and Schlieren diagnostic methods. High-pressure chamber experiments with lean hydrogen-methane-air mixtures were successfully performed and allowed the determination of essential parameters like minimum pulse energies at different ignition pressures and temperatures as well as at variable fuel air compositions. Multipoint ignition was studied for different ignition point locations. In this way, relevant parameters were acquired allowing to estimate future laser ignition systems. Finally, a prototype diode-pumped passively Q-switched Nd:YAG laser was tested successfully at a gasoline engine allowing to monitor the essential operation characteristics. It is expected that laser ignition involving such novel solid-state lasers will allow much lower maintenance efforts.
In dental hard tissue ablation, ultra-short laser pulses have proven sufficiently their potential for material ablation with negligible collateral damage providing many advantages. The absence of microcracks and the possibility to avoid overheating of the pulp during dental cavity preparation may be among the most important issues, the latter opening up an avenue for potential painless treatment. Beside the evident short interaction time of laser radiation with the irradiated tissue, scanning of the ultra-short pulse trains turned out to be crucial for ablating cavities of required quality and shape. Additionally, such a technique allows to treat larger areas like the ones dentists are used to work with, i.e. ~ 1 mm Ø.In this paper, an overview of different scanning methods together with the algorithms used and an assessment of their applicability is presented. A variety of pulse durations from ~100 fs up to several ps has been used by numerous authors over the last approximately ten years. Having employed 330 fs pulses, we present the corresponding ablation thresholds for dental hard tissue (enamel, dentine; human and bovine), for a number of dental restoration materials, as well as for different types of bovine bone. Dental implants frequently have to be cleaned from plaque being deposited around their necks in areas where the gums have already retreated. A preliminary investigation is presented on the applicability of ultra-short pulses with mentioned duration for the gentle cleaning of titanium implants focusing on the preservation of the special plasma-sprayed biocompatible implant surface.
Laser ignition of internal combustion engines reveals a number of advantages. High-pressure chamber experiments with lean hydrogen-methane-air mixtures were successfully performed and monitored by optical Schlieren diagnostics. Multipoint ignition was tested for 2 and 3 ignition points with different separations. In this way, relevant ignition parameters were acquired allowing estimate future laser ignition systems. Transportation of high intensity 6-ns Nd:YAG laser pulses via photonic bandgap fibers with hollow core was investigated. Evacuation of the core for the first time allowed to increase the peak intensity of the propagating pulses far beyond the breakdown limit of silica yielding 600 μJ fiber output with single mode characteristics.
A Nd:YAG laser was employed to ignite methane- and hydrogen-air mixtures to investigate relevant parameters of laser ignition. The lean side ignition limit of methane was found to be at air/fuel-equivalence ratios (λ) of 2.4 applying a laser pulse energy of 50 mJ. It has to be mentioned, however, that above λ = 2.2 only slowest combustions causing weak pressure rises could be observed. Successful ignitions of hydrogen-air mixtures were achieved up to λ = 8 but it was not possible to examine the lean side limit due to weakest pressure rises far below detection limits for λ >8. Despite much lower values of minimum ignition energy for reported hydrogen-air mixtures in the literature, the minimum laser pulse energies examined for ignition are of the same magnitude as for ignition of rich methane-air mixtures lying around 5 mJ. Minimum pulse energy needed for ignition was decreasing with increasing pressure for hydrogen-air mixtures showing the same trend as in case of methane. The ignition delay time for hydrogen at λ = 2.0 could be observed as ~7 ms being 40 times shorter compared to methane at the same air/fuel ratio. Unfavorable transmission losses of laser energy were observed for methane/air mixtures below λ = 2.1 demanding optimized focusing optics and temporal pulse shaping for future laser ignition systems.
Laser-induced ignition of methane-air mixtures of varied composition was investigated experimentally using nano-second pulses generated by Q-switched Nd:YAG lasers (wavelength 1064 nm, 532 nm and 355 nm) at initial pressures up to 4 MPa. The minimum focal spot diameter was found to be about 20 μm for effective ignition, independent of the laser wavelength, indicating that small impurity particles provide the seeds for laser plasma generation. The minimum laser pulse energy needed for ignition ranged from 2-15 mJ decreasing reciprocally with initial pressure and with fuel equivalence ratio Φ in a mixing of Φ=0.91 to Φ=0.56. Corresponding threshold intensities ranged from 1010 to 1011 W/cm2. In this way, evidence for a non-resonant breakdown mechanism was established. Optical in-situ diagnosis of water vapor concentration covering the whole timespan of the combustion process in a stationary high pressure vessel with four optical windows was performed involving linear absorption measurements over the entire spectral absorption linewidth by rapidly tuned diode laser radiation at 2.55 μm. Additionally, planar laser-induced fluorescence was measured in a time-resolving fashion yielding 3D determination of the OH concentrations during the process. To the knowledge of the authors, these are the first results on laser-induced ignition under laboratory conditions well above atmospheric pressure being relevant for several technical combustion systems.
Trends of development of ultra-short pulse solid-state lasers go towards new (infrared) wavelengths and shortest pulses, or towards high-average powers in the subpicosecond regime generated with high efficiency by diode-pumping. Avenues of pulse quality improvement are carrier-envelope phase control and amplitude shaping. Applications may be focused on shortest pulse durations, broadest spectra or high-average powers allowing innovative approaches.
An high-efficiency optical filter composed of counter-propagating wavelength converter in ZnTe, is demonstrated. The device has a extremely narrow bandwidth 0.5 angstrom, corresponding to pump intensity 1.2MW/cm2 and length of crystal 1cm, is tunable over several nanometers in bandwidth using combination of length of device and pump intensity. The presented scheme is also a candidate of wavelength-dependant switcher.
We present a review of our work on mirror-dispersion- controlled (MDC) Kerr-lens mode-locked (KLM) Cr:LiSGaF and Cr:LiSAF lasers, aimed at studying nonlinear phenomena in the 15-fs regime. Such effects as pulse self-frequency shift a side-band generation due to high-order dispersion (HOD), are looked at in more detail. These phenomena take place in any crystalline solitary mode-locked oscillator, and represent important limitations towards achieving ultimately short pulse durations.
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