Microwave guiding along double parallel lines of femtosecond-laser-generated plasma filament has been demonstrated over a distance of about 8 cm in air, corresponding to a maximum microwave signal intensity enhancement more than sixfold the free-space propagation. It is shown that the operating frequency and the line electric width influence the propagation coefficient of microwaves propagating along this transmission line. Based on channeling microwaves along this line and by measuring and comparing the propagated microwave signals, the basic parameters of laser-generated plasma filament, namely, its electron density and conductivity, are obtained.

A microwave waveguide that consists of a set of laser plasma filaments produced in air by the propagation of femtosecond laser pulses is investigated according to the hollow conducting waveguide concept. The conductivity, skin depth of the electromagnetic waves in this plasma waveguide and the energy required to excite such a waveguide are calculated for different possible configurations. A hollow conducting plasma waveguide is shown to support guided modes of electromagnetic radiation from millimetre to centimetre wavelength range. Our calculations show that, under the concept of conducting waveguide, it is more suitable to use the TE01 mode rather than TE11 to achieve an extended attenuation length. The attenuation length of the low-loss mode TE01 is shown to be dependent on the geometry of the plasma waveguides, the operating frequency and the plasma effective electron density. The effect of the plasma wall density spread on TE01 propagation is evaluated. Using the hollow conducting plasma waveguide operating in TE01 mode, an enhancement of microwave transmission over both free space propagation and dielectric plasma waveguide is obtained.

A proposed model for a circular bunch of plasma filaments used as a virtual waveguide for microwave radiation is presented. The virtual circular guiding system is shown to support guided modes of electromagnetic radiation in the millimetre and sub-millimetre wavelength range. Transmission is due to total internal reflection of the wave on the less dense plasma cladding. The energy loss of microwave radiation propagating in such a virtual waveguide is calculated. Optimum spatial configurations of circular plasma filament waveguides are proposed. Plasma filament waveguides are shown to enhance microwave transmission relative to free space propagation of microwave radiation over limited distances.

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