Improved stability against electrical dc bias drift has been demonstrated in LiNbO3 electro-optic modulators by replacing the commonly used SiO2 buffer layer with indium tin oxide (ITO), a transparent conductor. The long term drift of the modulators having an ITO buffer layer with a sheet resistivity of ∼20 Ω/⧠ is less than 0.3% in 8 h. The mechanism of the dc drift phenomenon is discussed using an electrical equivalent circuit model of the modulator.
A planar Rowland spectrometer was fabricated and characterized as a wavelength demultiplexer for multimode fiber-optic applications. The spectrometer consisted of a planar multimode glass waveguide with two curved end faces and a cylindrical concave grating attached to one of the end faces. Semiconductor lasers with wavelengths between 0.825 and 0.845 microm were used for the measurements. Cross-talk isolation between two adjacent fibers with center-to-center separation of 175 microm (100 A in wavelength difference) was measured to be 18 dB. The device's performance was limited by grating diffraction efficiency, optical aberration , waveguide dispersion, and waveguide losses.
The characteristics of an optical fiber external resonator in conjunction with (GaAI)As stripe geometry lasers are described. We have observed a 6-10% reduction in the threshold current and have obtained 150 ps pulses at gigahertz repetition rates. The fiber resonator has also been used to quench self-pulsations in a (GaAI)As injection laser. In order to explain many of our results we have used a model that uses the conventional semiconductor rate equations modified by the addition of saturable electron traps and the effects of the external cavity. Our results predict many of the self-locking effects observed in injection lasers operating in an external cavity. Furthermore, the degree of self-locking will be a strong function of the external cavity length and the density of saturable absorbers.
We report an analysis of the phenomenon of TE to TM mode conversion in LiNbO3 waveguides. The process is a consequence of the photorefractivity associated with the tensor character of the photovoltaic effect. We predict the existence of a nonlinear phase shift during the energy transfer process as well as a threshold behavior in the dynamics of energy exchange.
The operation of injection lasers with an external optical cavity has become an important subject because it can lead to the generation of short optical pulses at Ghz rates. Potential applications for such pulses are optic logic circuits and the phase locking of microwave semiconductor sources. Conventional external cavities, composed of lenses and mirrors, are bulky and difficult to align. In this talk we report the characteristics of a novel external cavity configuration which is mechanically stable and simple to construct. The external cavity consists of a piece of multimode graded index fiber with both ends cleaved. One end of the fiber is coated with a thin layer of Au to form a reflector. A spherical lens (typically 80-100 µ in radius) is formed on the other end of the fiber. By aligning the lase(1) to the lens end of the fiber we have obtained laser oscillation in the composite cavity whose threshold was typically 6-10% less than without the fiber resonator. The light output vs. current is shown in Figure 1.
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