Development of a fiber-optic laser delivery system capable of delivering 213 and 266 nm pulsed Nd:YAG laser radiation for tissue ablation in a fluid environment

Ultraviolet (UV) lasers have the capability to precisely remove tissue via ablation; however, due to strong absorption of the applicable portion the UV spectrum, their surgical use is currently limited to extraocular applications at the air/tissue boundary. Here we report the development and characterization of a fiber-optic laser delivery system capable of outputting high-fluence UV laser pulses to internal tissue surfaces. The system has been developed with a view to intraocular surgical applications and has been demonstrated to ablate ocular tissue at the fluid/tissue boundary. The fifth (213?nm) and fourth(266?nm) harmonics of a Nd:YAG laser were launched into optical fibers using a hollow glass taper to concentrate the beam. Standard and modified silica/silica optical fibers were used, all commercially available. The available energy and fluence as a function of optical fiber length was evaluated and maximized. The maximum fluence available to ablate tissue was affected by the wavelength dependence of the fiber transmission; this maximum fluence was greater for 266?nm pulses (8.4?J/cm2) than for 213?nm pulses (1.4?J/cm2). The type of silica/silica optical fiber used did not affect the transmission efficiency of 266?nm pulses, but transmission of 213?nm pulses was significantly greater through modified silica/silica optical fiber. The optical fiber transmission efficiency of 213?nm pulses decreased as a function of number of pulses transmitted, whereas the transmission efficiency of 266?nm radiation was unchanged. Single pulses have been used to ablate fresh porcine ocular tissue. In summary, we report a method for delivering the fifth (213?nm) and fourth (266?nm) harmonics of a Nd:YAG laser to the surface of immersed tissue, the reliability and stability of the system has been characterized, and proof of concept via tissue ablation of porcine ocular tissue demonstrates the potential for the intraocular surgical application of this technique.     

Effect of pulsed dye-laser wavelength stabilization on spectral overlap in atmospheric NO fluorescence studies

We introduce an inexpensive application of a Fabry-Perot etalon to control long-term UV-laser line drift in atmospheric NO laser-induced fluorescence (LIF) measurements by monitoring the visible fundamental of a pulsed dye laser. A linear image sensor captures the interference pattern, and the dye grating can be adjusted to maintain a fixed wavelength through an interface with labview software. Results indicate that the laser wavelength can be fixed to an accuracy of +/-0.0001 nm in the dye fundamental and +/-0.00003 nm in the UV beam. Hence the average error in the LIF signal owing to fluctuations in spectral overlap between the laser and the NO absorption transition decreases from ~5 to ~0.05%, which results in improved measurement accuracy.     

Two-photon nitric oxide laser-induced fluorescence measurements in a diesel engine

A two-photon nitric oxide (NO) laser-induced fluorescence (LIF) technique was developed and applied to study in-cylinder diesel combustion. The technique prevents many problems associated with in-cylinder, single-photon NO planar-laser-induced fluorescence measurements, including fluorescence interference from the Schumann-Runge bands of hot O2, absorption of a UV excitation beam by in-cylinder gases, and difficulty in rejecting scattered laser light while simultaneously attempting to maximize fluorescence signal collection. Verification that the signal resulted from NO was provided by tuning of the laser to a vibrational off-resonance wavelength that showed near-zero signal levels, which resulted from either fluorescence or interference at in-cylinder pressures of as much as 20 bar. The two-photon NO LIF signal showed good qualitative agreement with NO exhaust-gas measurements obtained over a wide range of engine loads.     

High-sensitivity instrument for measuring atmospheric NO2

We report on the development of a high-sensitivity detection system for measuring atmospheric NO2 using a laser-induced fluorescence (LIF) technique around 440 nm. A tunable broad-band optical parametric oscillator laser pumped by the third harmonic of a Nd:YAG laser is used as a fluorescence excitation source. The laser wavelength is tuned at peak and bottom wavelengths around 440 nm alternatively, and the difference signal at the two wavelengths is used to extract the NO2 concentration. This procedure can give a good selectivity for NO2 and avoid interferences of fluorescent or particulate species other than NO2 in the sample air. The NO2 instrument developed has a sensitivity of 30 pptv in 10 s and S/N = 2. The practical performance of the detection system is tested in the suburban area for 24 h. The intercomparisons between the LIF instrument and a photofragmentation chemiluminescence (PF-CL) instrument have been performed under laboratory conditions. The correlation between the two instruments is measured up to 1000 pptv. A good linear relationship between the LIF measurements and the PF-CL measurements is obtained.     

Trace detection of atmospheric NO2 by laser-induced fluorescence using a GaN diode laser and a diode-pumped YAG laser

We report on the development of a highly sensitive detection system for measuring atmospheric NO(2) by means of a laser-induced fluorescence (LIF) technique at 473 nm using a diode-pumped Nd:YAG laser. A GaN-based laser diode emitting at 410 nm is also used as an alternative fluorescence-excitation source. For laboratory calibrations, standard NO(2) gas is diluted with synthetic air and is introduced into a fluorescence-detection cell. The NO(2) LIF signal is detected by a photomultiplier tube and processed by a photon-counting method. The minimum detectable limits of the NO(2) instrument developed have been estimated to be 0.14 ppbv and 0.39 ppbv (parts per billion, 10(-9), by volume) in 60 s integration time (signal-to-noise ratio of 2) for 473 and 410 nm excitation systems, respectively. Practical performance of the instrument has been demonstrated by the 24 hour continuous measurements of ambient NO(2) in a suburban area.     

Steady-state and transient ultraviolet resonance Raman spectrometer for the 193-270 nm spectral region

We describe a state-of-the-art tunable ultraviolet (UV) Raman spectrometer for the 193-270 nm spectral region. This instrument allows for steady-state and transient UV Raman measurements. We utilize a 5 kHz Ti-sapphire continuously tunable laser (approximately 20 ns pulse width) between 193 nm and 240 nm for steady-state measurements. For transient Raman measurements we utilize one Coherent Infinity YAG laser to generate nanosecond infrared (IR) pump laser pulses to generate a temperature jump (T-jump) and a second Coherent Infinity YAG laser that is frequency tripled and Raman shifted into the deep UV (204 nm) for transient UV Raman excitation. Numerous other UV excitation frequencies can be utilized for selective excitation of chromophoric groups for transient Raman measurements. We constructed a subtractive dispersion double monochromator to minimize stray light. We utilize a new charge-coupled device (CCD) camera that responds efficiently to UV light, as opposed to the previous CCD and photodiode detectors, which required intensifiers for detecting UV light. For the T-jump measurements we use a second camera to simultaneously acquire the Raman spectra of the water stretching bands (2500-4000 cm(-1)) whose band-shape and frequency report the sample temperature.     

Laser ablation of FOX-7: proposed mechanism of decomposition

A novel high-energy explosive material, FOX-7 (1,1-diamino-2,2-dinitroethylene), was studied using a combination of laser-induced breakdown spectroscopy (LIBS) and selected ion flow tube mass spectrometry (SIFT-MS). The LIBS technique uses short laser pulses (an ArF excimer laser) as the energy source to convert small quantities of a sample into plasma and to induce the emission of its molecular fragments or atoms. SIFT-MS is a novel method for absolute quantification based on chemical ionization using three reagent ions, with the ability to determine concentrations of trace gases and vapors of volatile organic compounds in real time. SIFT-MS was used to study the release of NO, NO(2), HCN, HONO, HCHO, CH(3)CH(2)OH, and C(2)H(2) after laser ablation of the explosive compound FOX-7 in solid crystalline form. The radiation emitted after excitation was analyzed using a time-resolved UV-vis spectrometer with an ICCD detector. The electronic bands of CN (388 nm), OH (308.4 nm), and NO (237.1 nm) radicals and the atomic lines of C, N, and H were identified.     

Strategies for laser-induced fluorescence detection of nitric oxide in high-pressure flames. I. A-X(0,0) excitation

Three different high-pressure flame measurement strategies for NO laser-induced fluorescence (LIF) with A-X(0,0) excitation have been studied previously with computational simulations and experiments in flames up to 15 bars. Interference from O2 LIF is a significant problem in lean flames for NO LIF measurements, and pressure broadening and quenching lead to increased interference with increased pressure. We investigate the NO LIF signal strength, interference by hot molecular oxygen, and temperature dependence of the three previous schemes and for two newly chosen excitation schemes with wavelength-resolved LIF measurements in premixed methane and air flames at pressures between 1 and 60 bars and a range of fuel/air ratios. In slightly lean flames with an equivalence ratio of 0.83 at 60 bars, the contribution of O2 LIF to the NO LIF signal varies between 8% and 29% for the previous schemes. The O2 interference is best suppressed with excitation at 226.03 nm.     

Fiber-optic laser-induced fluorescence probe for the detection of environmental pollutants

Laser-induced fluorescence (LIF) spectroscopy in combination with fiber optics is shown to be a powerful tool for qualitative and quantitative diagnostics of environmental pollutants in water and soil. Timeintegrated data accumulation of the LIF signals in early and late time windows with respect to the excitation pulse simplifies the method so that it becomes attractive for practical applications. Results from field measurements are reported, as oil contaminations under a gas station and in an industrial sewer system are investigated. A KrF-excimer laser and a hydrogen Raman shifter can be applied for multiwavelength excitation. This allows a discrimination between benzene, toluene, xylene, and ethylbenzene aromatics and polycyclic aromatic hydrocarbon molecules in the samples under investigation. For a rough theoretical approach, a computer simulation is developed to describe the experimental results.     

Strategies for laser-induced fluorescence detection of nitric oxide in high-pressure flames. II. A-X(0,1) excitation

A-X(0,1) excitation is a promising new approach for NO laser-induced fluorescence (LIF) diagnostics at elevated pressures and temperatures. We present what to our knowledge are the first detailed spectroscopic investigations within this excitation band using wavelength-resolved LIF measurements in premixed methane/air flames at pressures between 1 and 60 bar and a range of fuel/air ratios. Interference from O2 LIF is a significant problem in lean flames for NO LIF measurements, and pressure broadening and quenching lead to increased interference with increased pressure. Three different excitation schemes are identified that maximize NO/O2 LIF signal ratios, thereby minimizing the O2 interference. The NO LIF signal strength, interference by hot molecular oxygen, and temperature dependence of the three schemes are investigated.     

Strategies for laser-induced fluorescence detection of nitric oxide in high-pressure flames. III. Comparison of A-X excitation schemes

Laser-induced fluorescence (LIF) has proven a reliable technique for nitric oxide (NO) diagnostics in practical combustion systems. However, a wide variety of different excitation and detection strategies are proposed in the literature without giving clear guidelines of which strategies to use for a particular diagnostic situation. We give a brief review of the high-pressure NO LIF diagnostics literature and compare strategies for exciting selected transitions in the A-X(0, 0), (0, 1), and (0, 2) bands using a different detection bandpass. The strategies are compared in terms of NO LIF signal strength, attenuation of laser and signal light in the hot combustion gases, signal selectivity against LIF interference from O2 and CO2, and temperature and pressure sensitivity of the LIF signal. The discussion is based on spectroscopic measurements in laminar premixed methane-air flames at pressures between 1 and 60 bars and on NO and O2 LIF spectral simulations.     

Bichromatic, phase compensating interferometer based on prism pair compressors

We present a bichromatic prism pair interferometer (BPPI) for controlling the delay between laser pulses of two different frequencies propagating collinearly in a single beam. The BPPI is especially useful when working with ultrafast laser pulses because it intrinsically allows for independent control over the second-order dispersion experienced by the differently colored pulses. We use this control to demonstrate successful precompensation for blue (lambda approximately 390 nm) and UV (lambda approximately 260 nm) pulses that pass through 2.2 cm of dispersive material after the interferometer. The BPPI is extremely flexible and works with all frequencies from the UV to the near-infrared. We demonstrate this by describing measurements made with BPPIs configured for three different combinations of central frequencies.     

Fiber-optic probes with improved excitation and collection efficiency for deep-UV Raman and resonance Raman spectroscopy

The ability of ultraviolet resonance Raman spectroscopy (UVRRS) to determine structural, environmental, and analytical information concerning low-concentration aqueous biomolecules makes it a powerful bioanalytical and biophysical technique. Unfortunately, its utility has been limited by experimental requirements that preclude in situ or in vivo studies in most cases. We have developed the first high-performance fiber-optic probes suitable for long-term use in pulsed UVRRS applications in the deep- UV (DUV, 205-250 nm). The probes incorporate recently developed improved ultraviolet (IUV) fibers that do not exhibit the rapid solarization and throughput decay that previously hampered the use of optical fibers for delivering pulsed, DUV light. A novel 90 degrees mirrored collection geometry is used to overcome the inner-filtering effects that plague flush-probe geometries. The IUV fibers are characterized with respect to their efficacy at transmitting pulsed, DUV laser light, and prototype probes are used to obtain pulsed UVRRS data of aromatic amino acids, proteins, and hormones at low concentrations with 205-240-nm pulsed excitation. Efficient probe geometries and fabrication methods are presented. The performance of the probes in examining resonance-enhanced Raman signals from absorbing chromophores is investigated, and the optimal excitation wavelength is shown to be significantly red-shifted from the maximum of the resonance Raman enhancement profile. Generally applicable procedures for determining optimal experimental conditions are also introduced.     

Improved hollow-core photonic crystal fiber design for delivery of nanosecond pulses in laser micromachining applications

We report the delivery of high-energy nanosecond pulses (approximately 65 ns pulse width) from a high-repetition-rate (up to 100 kHz) Q-switched Nd:YAG laser through the fundamental mode of a hollow-core photonic crystal fiber (HC-PCF) at 1064 nm. The guided mode in the HC-PCF has a low overlap with the glass, allowing delivery of pulses with energies above those attainable with other fibers. Energies greater than 0.5 mJ were delivered in a single spatial mode through the hollow-core fiber, providing the pulse energy and high beam quality required for micromachining of metals. Practical micromachining of a metal sheet by fiber delivery has been demonstrated.     

紫外/红外双波长荧光防伪纤维的制备及性能

通过熔融纺丝的方法成功制备了紫外/红外双波长荧光防伪纤维。分别采用X射线衍射、差示扫描量热分析、荧光光谱、纤维强伸度、扫描电镜等方法对制备的荧光纤维进行表征,探讨了荧光粉含量对纤维的力学性能、结晶性能和荧光性能的影响。结果表明,该纤维能在254 nm紫外光的激发下发出红色荧光,也能在980 nm红外光的激发下发射出绿色荧光,具有双重防伪效果。随着荧光粉含量的增加,双波长荧光防伪纤维的荧光强度不断增强,但荧光粉的团聚情况加剧。此外,随着荧光粉含量的增加,纤维的拉伸强度与断裂伸长率不断减小,但是当荧光粉含量≤10%时,对纤维拉伸强度的影响很小。     

Tunable excitation source for coherent Raman spectroscopy based on a single fiber laser

We demonstrate a wavelength tunable optical excitation source for coherent Raman scattering (CRS) spectroscopy based on a single femtosecond fiber laser. Electrically controlled wavelength tuning of Stokes optical pulses was achieved with soliton self frequency shift in an optical fiber, and linear frequency chirping was applied to both the pump and the Stokes waves to significantly improve the spectral resolution. The coherent anti-Stokes Raman scattering (CARS) spectrum of cyclohexane was measured and vibrational resonant Raman peaks separated by 70?cm(-1) were clearly resolved. Single laser-based tunable excitation may greatly simplify CRS measurements and extend the practicality of CRS microscopy.     

Use of hollow core fibers, fiber lasers, and photonic crystal fibers for spark delivery and laser ignition in gases

The fiber-optic delivery of sparks in gases is challenging as the output beam must be refocused to high intensity (approximately 200 GW/cm(2) for nanosecond pulses). Analysis suggests the use of coated hollow core fibers, fiber lasers, and photonic crystal fibers (PCFs). We study the effects of launch conditions and bending for 2 m long coated hollow fibers and find an optimum launch f# of approximately 55 allowing spark formation with approximately 98% reliability for bends up to a radius of curvature of 1.5 m in atmospheric pressure air. Spark formation using the output of a pulsed fiber laser is described, and delivery of 0.55 mJ pulses through PCFs is shown.     

High peak-power single-frequency pulses using multiple stage, large core phosphate fibers and preshaped pulses

We have developed a monolithic high power pulsed fiber laser in a master oscillator power amplifier (MOPA) configuration, which is capable of reaching 0.38 mJ pulse energy and 128 kW peak power for 3 ns pulses at ~1550 nm while maintaining transform-limited linewidth. The fiber laser pulse seed was achieved by directly modulating a CW single-frequency fiber laser using an electro-optic modulator. We used an arbitrary waveform generator to preshape the fiber laser pulses before amplification to avoid pulse steepening and dynamic gain saturation. Single-mode, polarization maintaining highly Er/Yb codoped large core phosphate fibers were used in the power amplifier stages to scale the transform-limited fiber laser pulses, avoiding any nonlinearities.     

Tunable kHz deep ultraviolet (193-210 nm) laser for Raman application

The performance characteristics of a kilohertz solid-state laser source for ultraviolet Raman spectroscopy are described. Deep ultraviolet (UV) excitation in the 193-210 nm region is provided by mixing of the fundamental and third harmonics of a Ti-sapphire laser, which is pumped by the second harmonic of a Q-Switched Nd-YLF laser. The combination of tunability, narrow linewidth, high average power, good stability, and kilohertz repetition rate makes this laser suitable for deep UV resonance Raman applications. The short pulse duration (approximately 20 ns) permits nanosecond time resolution in pump-probe applications. The low peak power and high data rate provide artifact-free spectra with a high signal-to-noise ratio. UV Raman cross-section and Raman excitation profiles are reported for gaseous O2 (relative to N), aqueous ClO4-, tyrosine, phenylalanine, tryptophan, histidine, and hemoglobin excited between 193 nm and 210 nm to illustrate laser performance.     

Ultrasensitive dual-beam absorption and gain spectroscopy: applications for near-infrared and visible diode laser sensors

A dual-beam detection strategy with automatic balancing is described for ultrasensitive spectroscopy. Absorbances of 2 × 10(-7) Hz(-?) in free-space configurations and 5 × 10(-6) Hz(-?) in fiber-coupled configurations are demonstrated. With the dual-beam technique, atmospherically broadened absorption transitions may be resolved with InGaAsP, AlGaAs, and AlGaInP single-longitudinal-mode diode lasers. Applications to trace measurements of NO(2), O(2), and H(2)O are described by the use of simple, inexpensive laser and detector systems. Small signal gain measurements on optically pumped I(2) with a sensitivity of 10(-5) are also reported.