In the paper, we report experimental results of the effect of UV-light exposure on the profile of thermal poling induced
second-order optical nonlinearity (SON) in twin-hole optical fibers. Before UV-exposure, uniform thermal poling at
320°C and 3.5 kV along a 5-7 cm long section of twin-hole fiber produced a uniform SON of 0.263 pm/V in the fiber
core. A focused light beam from a frequency-doubled Ar+ laser operating at 244 nm was used to locally erase the
induced nonlinearity in the fibers to create a periodic structure for quasi-phase matching applications. The UV-exposed
fibers were then observed under a second-harmonic microscope to characterize the distribution profile of any residual
nonlinearity after exposure. Effects of scanning speeds of the laser beam on the length of nonlinearity-erased section and
amplitude of the residual nonlinearity were investigated. It was found that the required UV-light fluence to fully erase
the induced nonlinearity is only ~0.5% of that typically required for fabricating fiber Bragg gratings.
Stable second-order nonlinearity (SON) was created in Pyrex borosilicate glass by the temperature/electric field thermal
poling method. The distribution and amplitude of the induced nonlinearity were characterized with second harmonic
microscopy. It was found that the SON was located in a narrow layer around 1.9 μm under the anode surface. An
effective d33 as high as 0.24 pm/V was obtained; a value comparable to that obtained in fused silica samples. The
migration of different mobile alkali ions during the poling process was characterized with energy dispersive x-ray
spectrometry in conjunction with scanning electron microscopy (SEM). It was found that Na was depleted from a region
about 3.3 μm beneath the anode surface, while K was first depleted from the immediate region under the anode, and then
accumulated in the Na-depleted region with its peak at ~1.8 μm beneath the anode. SEM observation of the cross-section
of the poled glass region, after it had been etched in diluted hydrofluoric acid for several minutes, revealed an etched
trench, ~1.8 μm under the anode edge and ~0.3 μm in width; while in post-annealed samples, no such etched trench
could be observed. A frozen-in space-charge field due to charge migration is believed to be responsible for the creation
of the SON and the altered etching rate in the poled region.
A fiber loop mirror constructed from two polarization- maintaining dispersion-shifted fibers with different zero- dispersion wavelengths is theoretically investigated. Our results show that this fiber loop can be used for both four- wave mixing without optical filtering and all-optical switching.
Compact and stable multiwavelength lasers (MWL) are attractive for wavelength division multiplexing (WDM) system. Recently, multiwavelength Er3+-doped fiber lasers have been reported by several groups. In this paper, we proposed and demonstrate a novel fiber MWL by inserting a double-pass Mach-Zehnder (M-Z) interferometer as a comb filter in the fiber laser cavity. The two output ports of the fiber M-Z device are spliced with a piece of Er3+-doped fiber in between to provide the optical gain. One of the input ports is connected with a WDM fiber loop which totally reflects light at 1550 nm and transmits light at 980 nm, and the other input port is connected with a partially reflected fiber loop as the output reflector to the laser cavity. 10 lines lasing simultaneously are obtained when the Er3+-doped fiber is immersed into liquid nitrogen. By controlling the optical path difference between the two arms in the M-Z device, the lasing peak wavelengths and their spacings can be adjusted to meet the ITU recommendation for WDM applications.
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