Year: 2009
Type: dissertation
Abstract: Higher-order-mode fibers generate today a great interest and find various applications in new photonic devices. The knowledge of the intermodal dispersion characteristics of higher-order-mode fibers is of interest for dispersion compensation, fiber refractive-index profile characterization, and a variety of in-fiber devices. In this context it is necessary to identify the individual transverse modes and to precisely characterize the dispersion properties of these modes. On the other hand, light beams, possessing special polarization structure, have recently attracted a great deal of interest. Several fiber techniques have been developed to generate such beams while employing higher-order modes of respective shapes. For example, radially or azimuthally polarized doughnut beams have found applications in various fields. They have a field similar to TM01 or TE01 vector mode in the LP11 mode group of a circular fiber. Therefore, these doughnut beams can be considered to be generated by an optical fiber. It is shown in the present work that in practice a TM01 or TE01 mode can not propagate in step-index fibers which support LP01 and LP11 modes. The reason for this is the anisotropy of the fiber and the degeneracy of the modes in the LP11 mode group. However, these doughnut beams can be generated at the end facet of the fiber by selective excitation of LP11 modes in the fiber under adequate pressure and twist. Unfortunately, since this mode selection process involves several fiber modes propagating at different phase velocities, the transverse intensity distribution will vary strongly along the fiber. However, in many applications, a stable transverse beam pattern is required. In order to excite a single higher-order mode in optical fibers, a full characterization of the intermodal dispersion and mode profiles as well as the polarization state of these modes is essential. This thesis focuses on a novel time-domain low-coherence interferometry technique for characterizing the intermodal dispersion and mode profiles of any type of higher-order-mode optical fibers. The measurement can be done without any prior knowledge of the fiber properties. The interferometry uses an erbium-doped fiber amplifier (EDFA) as a low-coherence broadband light source. In the fiber arm the light is coupled into the tested few-mode fiber. In the reference arm, a mirror is controlled to obtain the same optical path as each of fiber modes. A phase-shifting algorithm is used to interpret the interference fringes obtained on a camera at the interferometer’s output. The mode intensity profile and the amplitude of the mode are obtained in real time. Depending on the features of the tested fiber, an interferometer of Michelson or Mach-Zehnder type is used. One of the fibers under test is a few-mode all-glass fiber with special refractive-index profile. The Michelson low-coherence interferometer is used to characterize the intermodal dispersion and the mode profiles of the fiber. The measured intermodal dispersion of the LP01, LP11, LP21 and LP02 modes agrees very well with the results of our numerical simulations. It is shown that the four modes in the LP11 mode group of the tested fiber have a larger group-index difference than those of step-index fibers. This is indeed the advantage of the fiber design. The group-index difference of the degenerate HE21 modes in the LP11 mode group is used to determine the ellipticity of the fiber core. The estimated ellipticity is experimentally confirmed by the measurement of the group-index difference of the fundamental LP01 modes using a frequency-domain low-coherence interferometry technique. Additionally, the Mach-Zehnder low-coherence interferometer is used to characterize the chromatic dispersion of the fundamental modes and the intermodal dispersion of a few-mode solid-core photonic crystal fiber. The group-index difference between the LP01 and LP11 modes agrees very well with the results of the numerical simulations. The absolute chromatic dispersion of the LP01 mode is estimated without sweeping the wavelength. The measurement resolution is discussed.
Author: Yuzhao Ma
Language: English
Subfield: Atomic and Molecular Physics, and Optics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W3167138002