Spectral-domain intermodal interference and the effect of a low-resolution spectrometer

Spectral-domain intermodal interference is analysed theoretically at the output of a few-mode optical fibre alone and at the output of the optical fibre in a tandem configuration with a Michelson interferometer. The theoretical analysis is performed under general measurement conditions when a broadband source and a low-resolution spectrometer of a Gaussian response function are considered and when the first- and second-order intermodal dispersion effects in the optical fibre are taken into account. The theoretical analysis is performed for two different examples of dispersion curves of a two-mode optical fibre and the effect of the limiting factors is specified.     

Interferometric studies on the influence of temperature on the optical and dispersion parameters of GRIN optical fibre

Opto-thermal device attached to automate Fizeau interferometer is used to investigate the influence of temperature on opto-thermal properties of multimode graded-index (GRIN) optical fibre in the range from 27 to 54 °C. The effect of temperature on the refractive index profile of fibre is studied. The optical parameters and the opto-thermal coefficient of this fibre are determined. Also the variation of oscillation and dispersion energies, zero dispersion wavelengths, coupling efficiency, normalised frequency, number of propagation modes with the temperature and the material dispersion with the wavelength at different temperatures are calculated. Microinterferograms are given for illustration.     

Investigation on spectral-domain optical coherence tomography using a tungsten halogen lamp as light source

The objective of this research was to investigate a new detection method using a thermal light source in a spectral domain optical coherence tomography (SDOCT) operation to improve both image acquisition rate and axial resolution economically. A tungsten halogen lamp was chosen as the illumination source and a Michelson fiber interferometer was employed to generate the interference pattern of an object collected by a spectrometer; the inner structure of the object was obtained from the interference pattern by a Fourier transform. An experiment was performed by measuring a thin film coated on a glass base with the system, and the experimental results indicated that the structure of the thin film could be clearly observed as expected. It was theoretically stated and experimentally verified that the technique was feasible and enables imaging to realize the reconstructions of internal structure of engineering and biological object.     

Direct measurement of group dispersion of optical components using white-light spectral interferometry

We present a simple white-light spectral interferometric technique employing a low-resolution spectrometer for a direct measurement of the group dispersion of optical components over a wide wavelength range. The technique utilizes an unbalanced Mach-Zehnder interferometer with a component under test inserted in one arm and the other arm with adjustable path length. We record a series of spectral interferograms to measure the equalization wavelength as a function of the path length difference. We measure the absolute group refractive index as a function of wavelength for a quartz crystal of known thickness and the relative one for optical fiber. In the latter case we use a microscope objective in front and a lens behind the fiber and subtract their group dispersion, which is measured by a technique of tandem interferometry including also a Michelson interferometer.     

Propagation of bright femtosecond pulses in a nonlinear optical fibre with the third- and fourth-order dispersions

We solve the generalized nonlinear Schr\"{o}dinger equation describing the propagation of femtosecond pulses in a nonlinear optical fibre with higher-order dispersions by using the direct approach to perturbation for bright solitons, and discuss the combined effects of the third- and fourth-order dispersions on velocity, temporal intensity distribution and peak intensity of femtosecond pulses. It is noticeable that the combined effects of the third- and fourth-order dispersions on an initial propagated soliton can partially compensate each other, which seems to be significant for the stability controlling of soliton propagation features.     

Optical-frequency dependent modal birefringence and polarization mode dispersion of birefringent single-mode optical fibers over a broadband optical wavelength range

Optical-frequency dependent polarization mode dispersion as well as modal birefringence of three kinds of specially designed hollow-induced birefringent single-mode fiber are measured over an optical frequency range between 345 and 410 THz by optical frequency-domain interferometry.     

Investigation of dispersion characteristic in MI- and MII-type single mode optical fibers

Dispersion characteristic of MI and MII type single mode optical fibers is analytically investigated. For this purpose modal analysis of these fibers to obtain possible wave vectors for given system parameters are done. Then using numerical evaluation of the presented analytical relations, chromatic and waveguide dispersions are calculated. The effects of geometrical and optical parameters of the fibers on dispersion characteristics are investigated. In this analysis, we show that with increase of Δ (optical parameter) for MI structure the slope of dispersion curve is decreased and the case is reversed for MII structure. Also, with rising of Q (geometric parameter) for MI structure the slope of dispersion curve is decreased and the situation is reversed for MII structure. Finally, we show that with boosting of R₂ for MII structure the slope of dispersion is increased. As a final result, our simulations show that small values for optical parameters are better in MII structure for multi-channel optical communications. In MI structure to obtain small dispersion slope, Q can be increased that is easy for fabrication in practice. Finally, Q and R₂ are suitable parameters for control of dispersion in the proposed structures.     

Spectral polarization-encoding of broadband laser pulses by optical rotatory dispersion and its applications in spectral manipulation

We propose a kind of spectral polarization-encoding(SPE) for broadband light pulses, which is realized by inducing optical rotatory dispersion(ORD), and decoded by compensating ORD. Combining with polarization-sensitive devices, SPE can not only work to control polarization-dependent transmission for central wavelength or bandwidth-tunable filtering, but also can be used for broadband regenerative or multi-pass amplification with a polarization-dependent gain medium to improve output bandwidth. SPE is entirely passive thus very simple to be designed and aligned. By using an ORD crystal with a good transmission beyond 3-μm mid-infrared region, e.g., Ag Ga S_2, SPE promises to be applied for the wavelength tuning lasers in mid-infrared region, where the tunning devices are rather under developed compared with those in visible and near-infrared region.     

Experimental realization of a novel dispersion- compensating optical fiber amplifier for simultaneous compensation of positive dispersion and losses

Erbium-doped dispersion-compensating optical fiber (EDCF) has been theoretically simulated and experimentally fabricated using Modified Chemical Vapor Deposition (MCVD) for optimum operation at 5.0km. It is optimized for both gain as well as negative dispersion. The erbium has been doped into the cladding region while the core of the optical fiber is chosen to be narrow so as to have a high negative dispersion. Measured gain of 3.1 dB at 200 m using 100 mW pumping power (980 nm wavelength) at 1550 nm has been obtained and the gain of 32 dB at 5.0 km using same pumping scheme has been predicted. The chromatic dispersion of this EDCF has been also measured to be –43.5 ps/km-nm at 1550 nm and thus, providing the dispersion of –217.5 ps/nm at 5 km. The bend-induced losses are found to be negligible. We are the first to report the experimental realization of EDCF.     

Dispersion error of a beam splitter cube in white-light spectral interferometry

We revealed that the phase function of a thin-film structure measured by a white-light spectral interferometric technique depends on the path length difference adjusted in a Michelson interferometer. This phenomenon is due to a dispersion error of a beam splitter cube, the effective thickness of which varies with the adjusted path length difference. A technique for eliminating the effect in measurement of the phase function is described. In a first step, the Michelson interferometer with same metallic mirrors is used to measure the effective thickness of the beam splitter cube as a function of the path length difference. In a second step, one of the mirrors of the interferometer is replaced by a thin-film structure and its phase function is measured for the same path length differences as those adjusted in the first step. In both steps, the phase is retrieved from the recorded spectral interferograms by using a windowed Fourier transform applied in the wavelength domain.     

Determination of thickness and optical dispersion property of gold film using spectroscopy of surface plasmon in frequency domain

We propose to use wavelength modulation approach, i.e., the spectroscopy of surface plasmon in frequency domain to characterize the optical dispersion property of gold film. Using this method, we determine the dispersion relationship of gold film in a wavelength range from 537.12 nm to 905.52 nm, and our results accord well with the reported results by other authors. This method is particularly suited for studying the optical dispersion properties of thin metal films, because a series of dielectric constants over a wide spectral range can be determined simultaneously via only a single scan of the incident angle, thereby avoiding the repeated measurements required when using angular modulation approach.     

The determination of the thickness and the optical dispersion property of gold film using spectroscopy of a surface plasmon in the frequency domain

We propose to use wavelength modulation approach,i.e.,the spectroscopy of a surface plasmon in the frequency domain,to characterize the optical dispersion property of gold film.Using this method,we determine the dispersion relationship of gold film in a wavelength range from 537.12 nm to 905.52 nm,and our results accord well with the reported results by other authors.This method is particularly suited for studying the optical dispersion properties of thin metal films,because a series of dielectric constants over a wide spectral range can be determined simultaneously via only a single scan of the incident angle,thereby avoiding the repeated measurements required when using the angular modulation approach.     

Refractive-index dispersion measurement of bulk optical materials using a fiber raman laser widely tunable in the visible and near-infrared

We propose a simple, highly sensitive fiber-optic autocollimation method for refractive-index dispersion measurement of solid-state and liquid bulk optical materials using a double-pass fiber Raman laser with Littrow-prism-tuned emission. The optical fiber is a key element of the scheme and serves simultaneously as a point laser source for the test, as a highly sensitive point receiver (or spatial filter) of the autocollimation backreflectance signal and as a medium for nonlinear frequency conversion and generation of a broadband continuum spectrum. When the Raman medium is a graded-index multimode fiber with powerful pumping (over 100 kW) using the second harmonic of a Q-switched Nd:YAG laser (λ_p=532nm), we obtain widely tunable (0.54-1.01 μm) generation in both the visible and near-IR ranges. The results obtained in the refractive-index dispersion measurements are fitted to the Sellmeier dispersion equation and the standard deviation of the experimental data from the analytical curve does not exceed 5x10^-5.     

Measurements of nonlinear optical refraction and absorption in an amino-triazole push–pull derivative by a vectorial self-diffraction method

We present a method for measuring the magnitude and sign of the nonlinear refractive and absorptive coefficients in isotropic media based on vectorial two-wave mixing. The technique consists of the measurement of the self-diffracted signals as a function of the angle between the linear polarizations of two input pulses. The technique was used to investigate the nonlinear optical response of three amino-triazole derivatives in solution, using nanosecond pulses at 532 nm. For these samples the results show that nonlinear refraction is isotropic, and that nonlinear absorption is present. Input–output experiments show nonlinear absorption consisting of a combination of saturation and induced absorption. A three-level model for the nonlinear absorption was used to explain the experimental results.