Modal decomposition technique for multimode fibers

We propose a new solution for modal decomposition in multimode fibers, based on a spectral and spatial imaging technique. The appearance of spurious modes in the spectral and spatial processing of the images at the output of the fiber under test when it has more than two modes is demonstrated theoretically. The new method, which allows us to identify spurious modes, is more accurate, simpler, and faster than previously reported methods. For demonstration, measurements in a standard step-index multimode fiber and a small-core microstructured fiber are carried out successfully.     

Launching light from semiconductor lasers into plane-ended multimode optical fibers

A systematic and detailed study of launching light from semiconductor lasers into plane-ended multimode optical fibers has been carried out--we believe for the first time. Three different semiconductor lasers and four multimode fibers having numerical apertures in the 0.16-0.40 range were used. Simple theoretical models developed for the launching efficiency eta give good agreement with experimental results. We show how erroneous results can be obtained when considering only the stimulated emission of the lasers in calculating eta. The dependence of eta on axial, lateral, and angular misalignments is also investigated and explained qualitatively with ray optics.     

Generation of linearly polarized transition radiation X-ray beam

We have successfully produced almost linearly polarized X-ray beam by extracting Transition Radiation (TR) X-rays through a rectangular slit 3 mrad long and 0.2 mrad wide placed downstream of a 7.5-μm thick Kapton foil stack radiator bombarded with a 1-GeV electron beam. Our calculation predicted that the linearity of TR photon was 94% for both a vertical and a horizontal slit. The measured results for the vertical slit agreed with the calculation, and the linearity of obtained TR X-ray beam was more than 90%. For the horizontal slit, the measured results differed from our expectations due to a prolonged electron beam profile. In the polarization measurement of TR photon we used the Bragg reflection on a lithium fluoride crystal [LiF(200)] which acted as both a polarimeter and a spectrometer.     

Finite-element modal analysis of large-core multimode optical fibers

Plastic optical fibers that are a typical large-core multimode optical fiber support a great number of modes compared with conventional silica-glass multimode optical fibers. So far the WKB method hasbeen used for most of the modal analyses of these fibers because of a great number of guided modes. We describe the accurate eigenmodal analysis of large-core multimode optical fibers with the finite-element method (FEM) and compute the propagation constants of all LP modes. In addition, the FEM has a strong advantage for arbitrary core profiles whereas the WKB method is not suitable fornonmonotonic profiles. To demonstrate the advantage, we apply the FEM to the fiber having sinusoidal fluctuations.     

Nondestructive inspection of explosive materials using linearly polarized two-colored photon beam

A nondestructive inspection method for screening explosive materials that are hidden in passenger vehicles, trucks, and cargo containers with radiation shielding was presented. The method was examined experimentally using linearly polarized two-colored photon beam. A sample object was irradiated with the photon beam, followed by an emission of gamma-rays in nuclear resonance fluorescence. The gamma-rays from oxygen and nitrogen emitted through nuclear resonance fluorescence were measured using high-purity germanium detectors. We were able to evaluate the element concentration ratio.     

Free-space optical relay for the interconnection of multimode fibers

We present results from a system that shows that multimode fibers can be used for both the input and the output of a free-space optical system. The system consists of plastic microlenses integrated with plastic optomechanical components that are suitable for low-cost fabrication and assembly. Such a system opens up opportunities to construct large repeaters and switches for multigigabit ethernet applications by integration with two-dimensional arrays of optoelectronic devices. We demonstrate a 2.5-Gbit/s transmission rate by using commercial vertical-cavity surface-emitting lasers coupled to 62.5-mum core fibers. We consider the design constraints and the capabilities of custom optical modules suitable for mass production.     

One-step measurement of optical fields in multimode circular fibers

We propose to determine the optical field in multimode circular fibers by using a one-step method that measures the Wigner distribution function of a section of the field in the fiber. This method allows an estimation not only of the power carried by each mode but also of the relative phases of different modes in the fiber. An additional measurement with the same setup can even determine the propagation constants of different modes. An example is provided, and the connection of this method of field recovery to the coupling coefficient between fibers and light sources is also discussed.     

Launching light from semiconductor lasers into multimode optical fibers having hemispherical ends and taper-with-hemisphere ends

The plane ends (PE) of various multimode optical fibers have been transformed into (a) hemispherical ends (HE) and (b) taper-with-hemisphere ends (TE) by using a microtorch and also a CO2 laser for HE only. We confirmed that the launching efficiency eta from semiconductor lasers into multimode fibers is much greater than TE and HE compared with PE, with TE being the most efficient. A rigorous comparison between these three launching arrangements is reported here for the first time together with detailed analysis of similar published work. The effect of various misalignments is also investigated and compared for the three fiber terminations.     

Use of analyte-modulated modal power distribution in multimode optical fibers for simultaneous single-wavelength evanescent-wave refractometry and spectrometry

A new method is described for the simultaneous determination of absorbance and refractive index of a sample medium. The method is based on measurement of the analyte-modulated modal power distribution (MPD) in a multimode waveguide. In turn, the MPD is quantified by the far-field spatial pattern and intensity of light, i.e., the Fraunhofer diffraction pattern (registered on a CCD camera), that emerges from a multimode optical fiber. Operationally, light that is sent down the fiber interacts with the surrounding analyte-containing medium by means of the evanescent wave at the fiber boundary. The light flux in the propagating beam and the internal reflection angles within the fiber are both affected by optical absorption connected with the analyte and by the refractive index of the analyte-containing medium. In turn, these angles are reflected in the angular divergence of the beam as it leaves the fiber. As a result, the Fraunhofer diffraction pattern of that beam yields two parameters that can, together, be used to deduce refractive index and absorbance. This MPD based detection offers important advantages over traditional evanescent-wave detection strategies which rely on recording only the total transmitted optical power or its lost fraction. First, simultaneous determination of sample refractive index and absorbance is possible at a single probe wavelength. Second, the sensitivity of refractometric and absorption measurements can be controlled simply, either by adjusting the distance between the end face of the fiber and the CCD detector or by monitoring selected modal groups at the fiber output. As a demonstration of these capabilities, several weakly absorbing solutions were examined, with refractive indices in the range from 1.3330 to 1.4553 and with absorption coefficients in the range 0-16 cm-1. The new detection strategy is likely to be important in applications in which sample coloration varies and when it is necessary to compensate for variations in the refractive index of a sample.     

Design and performance of thin cylindrical diffusers created in Ge-doped multimode optical fibers

Cylindrical fiber diffusers have become common tools for various medical therapies. However, their large outer diameters and short lengths restrict their clinical application in some newly developed light therapies. Here, a 250-microm outer-diameter diffuser with an active length that exceeds 5 cm is presented. Diffusers are created in photosensitive optical fibers with outer cladding diameters of 140 microm by use of a structured beam from an excimer laser. Predetermined emission profiles can be achieved. Photometric characteristics, including longitudinal, polar, and azimuthal emission diagrams, were determined by use of a goniometer to assess the light-emission performance of the diffuser. Longitudinal isotropy of better than +/- 10% was achieved. Polar and azimuthal emissions were within +/- 15% of those of an ideal linear Lambertian emitter. Polar uniformity could be improved by an insignificant increase in the outer diameter by use of a diffusing recoating compound. The residual leakage of light at the distal end of the diffuser can be as little as 1%. Other physical parameters tested include minimal bending radius after recoating (< 5 mm) and maximum power handling (> 1.0W cm(-1)). All materials employed were biocompatible.     

Kramers-Kronig analysis of molecular evanescent-wave absorption spectra obtained by multimode step-index optical fibers

Spectral distortions that arise in evanescent-wave absorption spectra obtained with multimode step-index optical fibers are analyzed both theoretically and experimentally. Theoretical analysis is performed by the application of Kramers-Kronig relations to the real and the imaginary parts of the complex refractive index of an absorbing external medium. It is demonstrated that even when the extinction coefficient of the external medium is small, anomalous dispersion of that medium in the vicinity of an absorption band must be considered. Deviations from Beer's law, band distortions, and shifts in peak position are quantified theoretically as a function of the refractive index and the extinction coefficient of the external medium; the effect of bandwidth for both Lorentzian and Gaussian bands is also evaluated. Numerical simulations are performed for two types of sensing sections in commonly used plastic-clad silica optical fibers. These sensors include an unclad fiber in contact with a lower-index absorbing liquid and a fiber with the original cladding modified with an absorbing species. The numerical results compare favorably with those found experimentally with these types of sensing sections.     

Modal analysis of M-type-dielectric-profile optical fibers in the weakly guiding approximation

We study the applicability of the weakly guiding approximation (WGA) to the modal analysis of an M-type optical fiber in which a ring-shaped core lies between two uniform cladding layers. Besides being dependent on the refractive indices, the accuracy of the approximation is shown to be substantially affected by the transverse dimensions of the core. The accuracy is characterized by calculating an overlap integral between the exact and WGA-approximated modal fields. Fibers that have an inner cladding similar to the outer cladding, or similar to vacuum, are considered in detail. The feasibility of the WGA in determining the fiber parameters for single-mode guidance is also discussed.     

Measurement of launching efficiency into multimode optical fibersusing semiconductor laser diodes

A detailed and systematic study has been carried out of the launching efficiency of light, from semiconductor laser diodes into graded-index multimode optical fiber waveguides. Four different terminations of the fibers were investigated: plane-ends, hemispherical ends, taper-with-hemisphere ends, and roof-top (RT) ends. Besides launching efficiency, the parameters of interest were the various misalignments, such as axial, lateral, and angular, between laser and fiber, and the effect of these to produce a reduction in the launching efficiency. It is shown that RT fibers increase the launching efficiency considerably but rotational optimization of the fiber about its axis is essential. It is noted that this is a considerable disadvantage as most of the other techniques considered do not require the optimization     

Bandwidth estimation for multimode optical fibers using the frequency correlation function of speckle patterns

In this paper we present a new method for estimating the bandwidth of multimode optical fibers based on the frequency correlation function of the speckle patterns generated by the interference of fiber modes. This technique, which does not require a pulse or signal generator, can be utilized to estimate the bandwidth of a multimode fiber using a relatively short length of fiber. By applying this method to a test fiber we obtained a bandwidth of approximately 36 MHz km which is in relatively good agreement with the approximately 44-MHz x km bandwidth measured by a conventional pulsed technique.     

Processing photofluorographic images by means of decomposition into empirical modes

It is shown that the efficiency in processing photofluorographic images depends directly on the noise suppression level. It is proposed that a method based on decomposition into empirical modes should be used for noise suppression in photofluorographic images. A noise suppression algorithm based on threshold processing of the frequency components of a photofluorographic image is developed. A quantitative estimate of the level of noise suppression is presented.     

Modal analysis of the self-imaging phenomenon in optical fibers with an annular core

We investigate the occurrence of self-images, or Talbot images, in a spatially multimode field that propagates along an optical fiber whose core has an annular-shaped cross section. By use of full-vectorial modal analysis, we study the effect of the transverse fiber dimensions on the self-imaging properties. According to our analysis, good self-images can be expected when the fiber core is thin and the modes are far from their cutoffs. However, as the core diameter is made larger to increase the number of modes available in the imaging, the general self-imaging properties tend to deteriorate.     

Coupling of gaussian Schell-model beams into single-mode optical fibers

We develop analytic equations that describe the mean and normalized variance of the coupling efficiency of gaussian Schell-model beams into single-mode optical fibers. Numerical methods and computer simulations are used to evaluate the accuracy of the various approximations used in this analysis, and, with some insight, empirical compensation is made for the identified shortcomings. The simulations make use of both speckled and nonspeckled beams by employing two different Monte Carlo methods to generate randomly drawn optical fields. While the analytic approximations break down in certain cases, the use of empirical compensation demonstrated accuracies of better than 5% for the mean coupling efficiency in all cases, and generally better than 40% for the coupling efficiency variance. By optimizing the compensation for particular beam characteristics, even higher accuracies can be achieved.