We demonstrate the high sensitivity of gas sensing using a novel air-guiding photonic bandgap fiber. The bandgap fiber is spliced to a standard single-mode fiber at the input end for easy coupling and filled with gas through the other end placed in a vacuum chamber. The technique is applied to characterize absorption lines of acetylene and hydrogen cyanide employing a tunable laser as light source. Measurements with a LED are also performed for comparison. Detection of weakly absorbing gases such as methane and ammonia is explored.

We report on saturated absorption in a hollow-core photonic band-gap fiber filled with 12C2H2 molecules. We find that slow molecules provide a major contribution to the signal in the limit of low optical power and low pressure where the signal deviates significantly from the usual Lorentzian line shape. In particular, we observe a linewidth reduction of about 3 times as compared to the transit-time limited linewidth.

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Third-order optical nonlinear properties of four thienyleneethynylenes and two thienylenevinylenes containing from two to five thiophene units were measured. Third-order nonlinear susceptibilities x (~) ( -3w;o,o,o) at the fundamental wavelength of 1064 nm were measured by third harmonic generation (THG) for thin films of the oligomers in poly(methy1 methacrylate) (PMMA) using the Maker fringe technique. The data were reduced to the equivalent molecular second hyperpolarizabilities y ( -3w;w,o,w). Theoretical calculations of the static (zero-frequency) ( y ) values were performed at the semiempirical Austin model one/finite field (AM1/ FF) and intermediate neglect of differential overlap/single and double configuration interaction (INDO/SDCI) sum-over-states (SOS) levels and at the nonempirical valence effective Hamiltonian (VEH)-SOS level. The evolution of y as a function of the torsion angle between the thiophene rings was investigated via the AM1/ FF method. With the INDO-SOS technique, the frequency dependence of y( -3o;o,o,w) was also analyzed. For the ethynylenes, the experimental THG values of y lie between 63 x and 2300 x esu and increase with the number of heterocycle units of the oligomers. These values are affected by three-photon resonance. The resonant THG y-values of the ethynylenes lie within 10% of the corresponding values of the vinylenes. After a simple correction for three-photon resonance, the dependence of y-values on the number of n-electrons is fitted to power laws and compared with the theoretical calculations. The theoretical static y-values indicate a higher response in the vinylene derivatives than in the corresponding ethynylene derivatives. However, the experimental THG results provide the reverse trend: this can be explained by a stronger resonance enhancement of the THG y-values in the ethynylene compounds, as is demonstrated by the results of the frequency-dependent calculations.

An international comparison of 12712 He-Ne stabilized lasers, used as length standards, has been made. Five laboratories from Sweden, Denmark (two laboratories), Finland and Germany were represented. In addition, two lasers from the Bureau International des Poids et Mesures took part in order to relate the results to earlier comparisons. The reproducibility of the frequency mean of the four components d, e, f and g in the transition R(127)ll-5 in iodine 127 for the different lasers was found to be 15 kHz, taken as the standard deviation of the distribution of frequency among the group of ten lasers. Using laser BIPM4 as reference the frequency average of the group of lasers was +3,6 kHz, making this a good reference laser for the current ensemble. Measurements of frequency dependence on modulation amplitude, iodine pressure and, for some lasers, intracavity power are reported. The relative frequency partition of the four components d, e, f and g of each laser was also determined and shown to exhibit characteristics which are new for this type of stabilized laser.

We report a study of the correlation between three optical methods for characterizing surface roughness: a laboratory scatterometer measuring the bi-directional reflection distribution function (BRDF instrument), a simple commercial scatterometer (rBRDF instrument), and a confocal optical profiler. For each instrument, the effective range of spatial surface wavelengths is determined, and the common bandwidth used when comparing the evaluated roughness parameters. The compared roughness parameters are: the root-mean-square (RMS) profile deviation (Rq), the RMS profile slope (Rdq), and the variance of the scattering angle distribution (Aq). The twenty-two investigated samples were manufactured with several methods in order to obtain a suitable diversity of roughness patterns. Our study shows a one-to-one correlation of both the Rq and the Rdq roughness values when obtained with the BRDF and the confocal instruments, if the common bandwidth is applied. Likewise, a correlation is observed when determining the Aq value with the BRDF and the rBRDF instruments. Furthermore, we show that it is possible to determine the Rq value from the Aq value, by applying a simple transfer function derived from the instrument comparisons. The presented method is validated for surfaces with predominantly 1D roughness, i.e. consisting of parallel grooves of various periods, and a reflectance similar to stainless steel. The Rq values are predicted with an accuracy of 38% at the 95% confidence interval.

Near infrared reflectance spectroscopy (NIRS) was evaluated for determining botanical composition of preformed mixtures containing 'Regal' iadino clover (Trifollum repens L.) and 'Kentucky 31' Iowendophyte tall fescue (Festuca arundinacea Schreb.) as well as the botanical composition of samples harvested from grazing paddocks. Both species were grown in separate plots, harvested at monthly intervals, and hand-mixed. This provided combinations from 100% grass to 100% legume, resulting in a total of 60 samples that were used for NIRS calibration. The NIRS calibration equations for percentage tall fescue and white clover resulted in standard errors of calibration of 2.98 and 2.80%, respectively. Synthetic mixtures composed of material from grazing paddocks were used for validation of the equations. Species composition was predicted with excellent precision and accuracy showing biases of-1.64 and 1.80%, and standard errors of prediction (SEP) of 3.16 and 3.08% for the tall fescue and white clover equations, respectively. High R~ was also obtained when predicting the percentage of both component species. The NIRS prediction of pasture samples, whose actual composition was determined by hand separation, yielded higher SEP and bias. This additional variation (compared to the prediction of synthetic mixtures) was assumed to be made up of error associated with the hand-separation procedure and the variation of the prediction samples not present in the calibration set. These data indicate that with the proper selection of calibration samples, wavelengths, and data transformation, NIRS can be used to determine the botanical composition of grass-legume mixed samples.

Conjugated polymers such as polyacetylenes and polythiophenes have spurred considerable research activity for several years because of their potential applications in, for example, photonic devices''"] and highly conducting organic thin films.14] In order to optimize the electrical and optical properties of such materials, control of both the microscopic and the macroscopic solid-state structures is important, since they collectively define the electronic structure. A clear demonstration of the relationship between structural regularity and improved electrical and optical properties is found in Naarmann's defect-free polya~ety1ene.l'~ Recently, enhanced electrical conductivity in regio-regular poly(3-alkylthiophenes) has also been demon~trated["~I ( Fig. 1).In the present communication the effect of regio-regularity in polythiophenes on the electronic contribution to the third order nonlinear optical susceptibility, ~(~) (-3w;w,w,o), is addressed. The experimental strategy has been to measure the nonlinear susceptibility x (~) of regio-regular poly(3-dodecylthiophene) (RR-PDDT) thin films by third harmonic generation (THG) and compare it with similar THG measurements on regio-irregular (RIR-PDDT i.e.