First demonstration of a temporal coherence analysis through a parametric interferometer

We analyze the temporal coherence of an optical infrared radiation in the visible domain by using a Mach-Zehnder interferometer and a wavelength conversion stage in each arm. We exploit a sum frequency generation process in bulk PPLN crystal to convert the infrared radiation at 1.55 μm into 0.63 μm before the interferometric mixing. The applicability of the Wiener-Kintchine theorem through up-conversion processes is here demonstrated by direct comparisons among visible and infrared measurements.     

Development of One-dimensional Fiber-Optic Radiation Sensor for Measuring Dose Distributions of High Energy Photon Beams

The purpose of this study was to develop a method for measuring the one-dimensional dose distribution of a high-energy photon beam using a miniaturized high-resolution fiber-optic radiation sensor array. The measurements were made by thin plastic optical fibers with organic scintillating fiber sensor probes that emit the visible wavelength of light. The scintillating light is guided to a silicon photodiode array by plastic optical fibers in order to convert light output to an electrical signal. The one-dimensional spatial dependence of photon beam is measured by a one-dimensional fiberoptic sensor array in a poly(methyl methacrylate) (PMMA) phantom. It is shown that this fiber-optic radiation sensor has better spatial resolution than a conventional ionization chamber and much less time is required to measure one-dimensional dose distribution in the high radiation fields. The real-time and the high spatial resolution measurements due to the small detector volume make this system suitable for dosimetry in radiation therapy.     

Light-induced ejection of calcium atoms from polymer surfaces

Laser-induced fluorescence (LIF) of calcium atoms at room temperature has been observed in a polydimethylsiloxane (PDMS) coated cell when the walls are illuminated with non resonant visible light. Ca atomic density in the gas phase, monitored by the LIF, is much higher than normal room-temperature vapour pressure of calcium. In past years photon-stimulated desorption (PSD) was observed for several alkali metals that adsorbed to solid films of PDMS polymers. High yields of photo-desorbed atoms (and molecules in the case of sodium) can be induced, at room temperature and below, by weak intensity radiation. The desorption is characterised by a frequency threshold, whereas any power threshold is undetectable. The calcium photo-ejection is characterised both by a frequency threshold (about 18 500 cm^-1) and by an observable power threshold (whose value becomes lower when the photo-ejecting light wavelength decreases).     

Paper un-printing: using lasers to remove toner-print in order to reuse office paper

In this article, lasers in the ultraviolet, visible and infrared light spectra working with pulse widths in the nanosecond range are applied to a range of toner-paper combinations to determine their ability to remove toner. If the laser energy fluence can be chosen to stay below the ablation threshold of paper at the same time that it surpasses that of toner, paper could be cleaned and re-used instead of being recycled or disposed into a landfill. This could significantly reduce the environmental impact of paper production and use. Although there are a variety of paper conservation studies which have investigated the effects of laser radiation on blank and soiled paper, none has previously explored toner-print removal from paper by laser ablation. Colour analysis under the L ^∗ a ^∗ b ^∗ colour space and SEM examination of the outcome indicate that it is possible to remove toner from paper without damaging and discolouring the substrate. Best results are obtained when employing visible radiation at a wavelength of 532 nm working with a pulse width of 4 ns and energy fluences under 1.6 J/cm². This means that it is technically feasible to remove toner-print for paper re-use.     

Simultaneous generation of violet,blue, and green lasers using Nd:YAl3(BO3)4 channel waveguides under pumping at 815 nm

Violet, blue, and green lasers were simultaneously generated by nonlinear processes using ultrafast laser inscribed neodymium‐doped yttrium aluminum borate (Nd:YAl₃(BO₃)₄ or Nd:YAB) channel waveguides under pumping at 815 nm. These visible lasers were generated by the frequency doubling, self‐frequency summing, and self‐frequency doubling processes based on a 1062 nm laser radiation that corresponded to the ⁴F_3/2 → ⁴I_11/2 transition of Nd³⁺ ions. Further, the wavelength tunability for the violet and blue lasers was achieved by simply tuning the pump wavelength within the ⁴I_9/2 → ⁴F_5/2 transition. The results obtained indicate that Nd:YAB waveguides are promising candidates for efficient compact visible laser sources. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical and magnetic properties of iron garnet films with high defect density

We have used spectroscopic and magneto-optic methods to study the effect of defects on the transmission spectrum, thickness, refractive index, and period of the striated domain structure in thin iron garnet films exposed to fast neutron fluences of 4.5·10¹⁷ and 10¹⁹ cm^–2. We observed both a decrease in transmission and a smearing of the interference maxima. The decrease in the quality of the interference fringes was observed in the visible spectrum for these samples. It was due to increased diffuse scattering of light by nonuniformities whose dimensions were comparable to the wavelength of the scattered light. It was determined that radiation defect clusters had no influence on the film thickness, but they did significantly alter the wavelength dependence of the refractive index and increased the period of domain structure striations.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 45–48, August, 1988.We wish to thank A. G. Shishkov for discussions of the experimental results.     

Difference frequency generation in proustite from 11 to 23 μm

Tunable infrared radiation was produced in the spectral range 11 μm to 23 μm by nonlinear mixing of two ruby-pumped dye lasers in proustite. A peak power of about 3 watts was observed at 19 μm. A 7^o crystal angle was sufficient to achieve phase matching over the entire wavelength range. Ordinary wave absorption measurements in proustite correlate well with the observed difference frequency output, once allowance has been made for the convergence of the dye laser beams. This research was supported by the Energy Research Development Agency under contract E29-2-3597 and by the Office of Naval Research contract No. N00014-76-C-0431.     

A time-resolved study of the u-v radiation produced in the initial time period of a pulsed argon discharge

The time development of the integral intensity of ultra-violet radiation from the positive column of a pulsed discharge was studied and the results compared with those for visible light. The measurements were made at various pressures in the range 0·3–4·8 torr. We found that at a pressure about 4 torr the maxima of intensity of the u-v and the visible light occurred at nearly the same time after the initiation of the discharge. On the other hand, at a pressures of about 0·5 torr the positions of the maxima were different.     

Effect of atmospheric turbulence on propagation of ultraviolet radiation

We investigate the effect of atmospheric optical turbulence on ultraviolet (UV) radiation with a wavelength of 253.7 nm. The normalized irradiance variance (scintillation index) was measured using a UV scintillometer with a path length of 185 m. The dependence of the UV scintillations on the atmospheric turbulence structure parameter and inner scale was determined through simultaneous measurements of these quantities made with a visible laser scintillometer. The dependence of the UV scintillation index and its probability density function on receiver aperture size was also measured. It was found that the scintillation predicted by currently available models which take into account the effects of inner scale, saturation and aperture averaging was in good agreement with the measurements made under various conditions in weak turbulence.     

Widely tunable diffraction limited 1000 mW external cavity diode laser in Littman/Metcalf configuration for cavity ring-down spectroscopy

We report on recent progress on external cavity diode lasers (ECDL) using a new concept of a Littman/Metcalf configuration. Within this concept one facet of the diode laser chip is used for coupling to a high quality Littman/Metcalf resonator whereas the other side of the diode laser chip emits the output beam. The alignment of the external resonator is independent from the alignment of the output beam and there is no need for any compromise in the alignment. This results in an improved behavior of the external resonator with the benefit of a drastic increase in power and single mode tuning.We investigated this light source for high resolution spectroscopy in the field of cw-cavity ring-down spectroscopy (CRDS). The monitoring of environmental and medical gases from vehicles or human breath requires a suitable radiation source in the mid-infrared (MIR) between 3 and 5 μm that is frequency stable and can be widely tuned. Since this wavelength cannot be reached via direct emitting room temperature semiconductor lasers, additional techniques like difference frequency generation (DFG) are essential. Tunable difference frequency generation relies on high power, small linewidth, fast tunable, robust laser diode sources with excellent beam quality.With our new compact, alignment-insensitive and robust ECDL concept, we achieved an output power of 1000 mW and an almost Gaussian shaped beam quality (M²<1.2). The coupling efficiency for optical waveguides as well as single mode fibers exceeds 70%. The wavelength is widely tunable within the tuning range of 20 nm via remote control. This laser system operates longitudinally in single mode with a mode-hop free tuning range of more than 150 GHz without current compensation and a side-mode-suppression better than 50 dB. This concept is currently realized within the wavelength regime between 750 and 1080 nm.Our high powered Littman/Metcalf laser system was part of a MIR-light source which utilizes DFG in periodically poled lithium niobate (PPLN) crystals. At the wavelength of 3.3 μm we were able to achieve a high-resolution absorption spectrum of water with four different isotoplogues of H₂O components. This application clearly demonstrates the suitability of this laser for high-precision measurements. PACS 07.57.Ty; 42.55.Px; 42.62.Fi     

Compact all-solid-state continuous-wave single-frequency UV source with frequency stabilization for laser cooling of Be<Superscript>+</Superscript> ions

A compact setup for generation, frequency stabilization, and precision tuning of UV laser radiation at 313 nm was developed. The source is based on frequency quintupling of a C-band telecom laser at 1565 nm, amplified in a fiber amplifier. The maximum output power of the source at 313 nm is 100 mW. An additional feature of the source is the high-power output at the fundamental and the intermediate second- and third-harmonic wavelengths. The source was tested by demonstration of laser cooling of Be⁺ ions in an ion-trap apparatus. The output of the source at the third-harmonic wavelength (522 nm) was used for stabilization of the laser frequency to molecular iodine transitions. Sub-Doppler spectroscopy and frequency measurements of hyperfine transitions in molecular iodine were carried out in the range relevant for the Be⁺ laser cooling application.     

Optical properties of MnAl films

The optical properties of MnAl films of different compositions, deposited at different substrate temperatures and in the thickness range 25 to 90 nm, are reported. The reflectance and transmittance are measured in the wavelength range 320–900 nm for near normal incidence of light. These measurements are used to calculate the optical constants namely, refractive index (n) and extinction coefficient (k). The wavelength and thickness dependence of these optical constants are reported. The optical measurements for films with higher Mn content and grown at higher substrate temperature reveal that n and k values are constant over a wide wavelength range (500–900 nm) showing high reflectance in the visible and near infrared region.     

Calibration of mid-infrared transmission measurements for hydrocarbon detection and propane concentration measurements in harsh environments by using a fiber optical sensor

The local fuel concentration at the ignition position is a crucial parameter, especially for spark ignited engines. Hydrocarbon concentration can be determined by infrared transmission measurements because of the attenuation of infrared radiation in the 2940 cm⁻¹ frequency range due to the excitation of fundamental CH-stretch vibrations of the molecules. In-cylinder measurements can be done by modified spark plugs with integrated absorption paths. A tungsten halide lamp was used in the experiments as an infrared light source. There is a non-linear relation between measured transmissions and hydrocarbon densities. So far, infrared transmission measurements have to be calibrated on absolute density values by using various hydrocarbon test gases with different concentrations. Furthermore in-cylinder transmission measurements can be calibrated at homogeneous engine operation conditions. In this case, a well mixed air/fuel composition is expected at the end of the compression stroke. Both time-consuming calibration processes lead to a new calibration procedure for band integrated transmission measurements which is presented in this study. The non-linear correlations between band integrated transmissions and absorber densities were calculated by using the absorption cross section as a spectral characteristic of absorbing molecules and the filter transmission as the spectral influence of the experimental setup. The differences between measurement and calculation can be corrected by a single measurement using a test gas with known hydrocarbon concentration. The calibration procedure was verified on temperature and pressure influences by measurements using a heatable optical cell at pressures up to 1800 kPa and within a temperature range from 298 to 473 K. Finally, the calibration procedure was adapted from cell measurements to an infrared fiber optical sensor which was used for in-cylinder measurements. An adapter for the spark plug bore enables a parallel use of a flame-ionization-detector and the optical sensor. Simultaneous propane concentration measurements were made in a motored engine with both measurement systems.     

Laser plasma monitored by silicon carbide detectors

The excellent physical and chemical properties and the radiation hardness of silicon carbide (SiC) render this material particularly suitable for the realization of radiation detectors. In this paper we describe the main properties of SiC and the processes needed to realize good performance detectors. To this purpose, we made SiC Schottky diodes that were electrical characterized by using different techniques. In order to test the radiation hardness, the diodes were irradiated with different ion beams and the analysis of the electrical measurements allowed to identify the defects responsible of the device degradation. These detectors have been used to monitor the multi-MeV ions of the plasma emitted by irradiation of various targets with 300-ps laser at high intensity (10¹⁶?W/cm²). These measurements highlighted that the use of SiC detectors enhances the sensitivity to ions detection due to the cutting of the visible and soft ultraviolet radiation emitted from plasma. The small rise time and the proportionality to ion energy evidence that these detectors are a powerful tool for the characterization of ion generated by high-intensity pulsed laser.     

Efficient VUV sensitization of Eu3+ emission by Tb3+ in potassium rare‐earth double phosphate

The luminescence properties of K₃Tb(PO₄)₂ activated by Eu³⁺ were studied at excitation over the 120–300 nm wavelength range. It is demonstrated that Tb³⁺ ions, exhibiting a strong absorption band in the vacuum‐ultraviolet (VUV), can provide efficient sensitisation of Eu³⁺ emission in this wave length range, giving rise to intense red luminescence at 150 nm excitation. A proof is given for the concept of VUV sensitisation enabling the engineering of luminescence materials with enhanced conversion efficiency of VUV radiation into visible light. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A laser optical method for the determination of tropospheric OH concentrations

A long-path absorption procedure for the determination of tropospheric OH concentrations is described in detail. Initial measurements using this method were carried out in Frankfurt a. M. By including in the evaluation the visible radiation of a frequencydoubled dye laser used in the measurements, it was possible to mathematically reduce the signal fluctuations caused by the scattering and density fluctuations of tropospheric air. The resulting detection limit was 3×10⁶ OH per cm³. SO₂ concentrations which had to be simultaneously determined in order to eliminate an interference effect, could be detected in the range of 1–40 ppb on the absorption path.     

ʌ-type doubling and spin-rotation splitting of NO,measured in simultaneous one- and two-photon laser spectroscopy

Doppler-free two-photon excitation of NO A²Σ⁺ by visible dye laser radiation is recorded simultaneously with single-photon absorption of frequency-doubled UV laser light. The measurement of the frequency difference between corresponding lines in the one- and two-photon spectrum provides a new method for a direct optical determination of Λ-type doubling in the X²Π ground state. In addition, the spin-rotation splitting of the A²Σ⁺ excited state is resolved in the Doppler-free two-photon spectrum.     

All-solid-state continuous wave intracavity frequency-tripled Nd:YVO4-LiB3O4 blue laser using double-resonant approach

In the paper, a new way of intracavity frequency-tripled all-solid-state laser to generate continuous wave blue coherent radiation is firstly demonstrated. High-efficiency of continuous wave third harmonic generation using the approach of double-resonance at fundamental frequency and second harmonic is developed by insertion of one wedge prism for the phase control. The maximum output power at the wavelength of 447 nm, which was generated with two long LiB₃O₄ crystals by noncritical phase matching, is about 1.15 W with a beam quality factor of M² of 1.05. From the experimental results, the generation of continuous wave blue light using this way with higher conversion efficiency can be achieved.     

Rapid shifted excitation Raman difference spectroscopy with a distributed feedback diode laser emitting at 785 nm

A distributed feedback (DFB) laser diode emitting at 785 nm was tested and applied as a light source for shifted excitation Raman difference spectroscopy (SERDS). Due to the physical properties of the laser diode, it was possible to shift the emission wavelength by 8 cm^-1 (0.5 nm) required for our SERDS measurements by simply changing the injection current. The internal grating ensured single mode operation at both wavelength with the frequency stability of ±0.06 cm^-1 (0.004 nm) required for high resolution Raman spectroscopic applications. The shifted spectra were used for calculating enhanced Raman spectra being obscured by a strong scattering background. A 16 dB (≈38 fold) improvement of the signal-to-background noise S̄/σ_B was demonstrated using blackboard chalk as a sample. The tunable DFB laser is a versatile excitation source for SERDS, which could be used in any dispersive Raman system to subtract fluorescence contributions and scattering background. PACS 82.80.Gk; 42.55.-f; 42.64.Fi     

Radiometric characterization of radiochromic gel material applied for UVA radiation dosimetry

Diverse kinds of chemical products are useful for UVA dosimetry due to chemical incitation properties of ultraviolet radiation. It was recognized and approved that radiochromic gel materials have a specific behavior under UVA radiation exposure. The relationship between the UVA dose and the variation of visible light absorbance was found to follow a second-order function with good accuracy. Specific metrological properties of the gel material for radiometric UVA dosimetry application were studied at irradiation levels simulate deferent natural solar UVA radiation levels. The behavior of the gel absorbance properties, detector linearity, induced reaction and temporal and radiometric stability at various dose rate levels were investigated. The relationship between UVA dose and changes in optical absorbance is found independent of the UVA exposure rate including the solar UVA maximum level which could be detected at sea level. The studied gel material absorbs more than 90% of UVA radiation in 1 cm of the gel thickness. Gel detector linearity was investigated over a UVA range (20-84 W/m²) which exceeds the natural solar UVA radiation level and the linearity factor was found to be close to 1 with statistical differences ≤ 2%. The UVA induced dark reaction in the gel material continues for almost 4 min after stopping irradiation; which must be considered during measurements. The detector has good temporal stability even under high UVA exposure; hence the gel absorption in the UVA spectrum region is stable. Measured deviation was less than 1%, and relative variation of gel optical absorbance in UVA spectrum is about 35 times lower than that occur in visible region.