Properties of the electromagnetic waves propagating through a helix-loaded waveguide are investigated, including the important influence of the outer conducting wall on the dispersion properties. A closed algebraic dispersion relation for the eigenfrequency ω and the axial wave number k is obtained for arbitrary azimuthal harmonic number. It is shown that in the limiting case, where the outer conducting wall approaches close to the helix, this dispersion relation is reduced to three distinctive modes. These are the transverse electric mode, the transverse magnetic mode, and the helix mode, which can be further simplified to straight lines in the (ω, k) parameter space. Numerical investigation of the dispersion relation is also presented.
The effect of broadening the bandwidth with a dielectric load in a cylindrical gyrotron is investigated for a hollow electron beam. The linear dispersion relation for the azimuthally symmetric, transverse electric (TE) modes is obtained by the method of the wave impedance matching. It is found that the TE perturbations exhibit three unstable modes characterized by their phase velocities vph : one fast wave, the long wavelength mode (LWM, vph ≳c), and two slow waves, the intermediate (IWM, c≳vph ≳cε−1/2) and the short (SWM, vph <cε−1/2) wavelength modes. The optimum conditions for the wide band operation are obtained individually for each mode. Although the bandwidth in excess of 40% is possible for the slow waves (IWM, SWM) at a small axial momentum spread, it decreases rapidly as the spread increases. On the other hand, the LWM yields approximately 10% of the bandwidth insensitive to the spread. It is also shown that for a small spread (<5%), the slow wave (IWM) is preferable for the wide band operation, whereas for a large spread (≳5%) the fast wave (LWM) is desirable.
An experimental acousto-optic tunable filter that has a narrow spectral bandwidth (0.2 nm at 1550 nm) and a fast (10-micros) tuning capability with a continuous tuning range of approximately 50 nm is described. The tunable filter consists of an acousto-optic beam deflector with a diffraction grating whose grating vector is transverse to the direction of light propagation.
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