Differential mode delay (DMD), chromatic dispersion, and modal dispersion measurement techniques for a multimode optical fiber based on optical frequency domain reflectometry are presented. We have used the principle of a conventional OFDR with a tunable external cavity laser and a Mach-Zehnder interferometer. We have compared our measurement results with those obtained using a traditional time-domain measurement method.
A noble measurement method by using a homodyne interferometer and Hilbert transform has been proposed for characterizing frequency sweeping light sources used in traditional optical frequency domain reflectometer (OFDR) and optical frequency domain imaging (OFDI). A Michelson interferometer with a tunable laser generates a sinusoidal beating signal. A phase of measured beating signal as a function of time is approximately proportional to optical frequency of the swept light source during frequency tuning and can be obtained by the Hilbert transformation. Thus, optical frequency chirp can be determined by a simple equation related with the phase of the beating signal from the interferometer. We have demonstrated the effectiveness and the simplicity of our proposed method by testing a temperature-tuned frequency sweeping DFB-LD and a commercial external cavity tunable laser source as practical examples. In the case of DFB-LD, the frequency sweep becomes more linear while the amount of frequency sweep saturates as the amplitude of the control voltage applied to a TEC driver increases, and the frequency-tuning rate increases as the repetition rate decreases. We also found that a commercial frequency-sweeping laser has a feed back control to adjust its frequency-sweeping rate such that the tuning rate oscillates around an intended value as a function of time. We have demonstrated the possibility of using a self-homodyne interferometer as a powerful tool for characterizing frequency sweeping laser sources. We expect this method will be useful for improving the performance of many optical frequency domain measurement techniques such as OFDR, FD-OCT or OFDI.
Optical fibers are composed of the core and the cladding that are covered by the polymer coating to protect them from subsequent handling damage. Sometimes, this polymer coating, however, should be removed to fabricate optical devices involving the optical fiber such as Bragg gratings, optical couplers, optical sensors and optical connectors. In general, the mechanical stripper is used to remove this polymer coating. In this case, the mechanical stripper may cause mechanical defects on the surface of the optical fiber and also, mechanical defects make the optical fiber weak. We have researched relationship between these mechanical defects and the residual stress gradient in the optical fiber. We have made a mechanical defect on the surface of a single mode fiber with a mechanical blade and measured the residual stress distribution along the axial direction of the optical fiber. From this research, we have observed that at the position with the mechanical defect, the residual stress in the core was converted to the compressive residual stress (about 15MPa) and the residual stress in the cladding was converted to the tensile residual stress (about 7MPa). We have demonstrated that the mechanical defect on the fiber surface can cause the gradient of the residual stress distributions in the optical fiber and also, measurement of the residual stress distribution in the optical fiber can be used as a tool to find out the mechanical defects on the optical fiber.
Novel optical fiber torsion sensor based on LPG pair was proposed and the effect of the twist angle on the torsion was characterized. The transmission of the fringes at peak wavelengths was changed by the twist of the optical fiber with the LPG pair and it is attributed to the polarization dependence upon the twisting of the fiber. The variation of the transmission depends on the input polarization state, which is affected by the applied torsion.
Refractive index change in the core of optical fibers by CO2 laser irradiation was measured by using a long period fiber grating (LPG) pair. Effect of drawing force applied to the optical fibers during drawing process on the refractive index change upon the CO2 laser irradiation was investigated. The refractive index was found to decrease linearly with the drawing force and it was due to the relaxation of the residual stress. Effect of the CO2 laser output power on the residual stress relaxation and the fiber elongation was also studied.