Research On Quasi-optical Technology In Millimeter Wave Space Beam Power Combining

Compared with microwave,millimeter-wave and submillimeter wave have characteristics of shorter wavelength,wider frequency band,thinner beam with the same aperture and higher resolution and so on,which have a large number of applications in communications,radar,medical and radio astronomy and other fields.In recent years,with the rapid development of short millimeter wave and terahertz technologies,the demand for a signal source with high efficiency,stability and large output power has greatly increased.At present,the signal source commonly used in engineering is divided into two categories:one is composed of electric vacuum devices and the other is based on solid-state power devices.Semiconductor solid-state devices with unique features of small size,low supply voltage and stable performance compared to vacuum devices,in the millimeter-wave system has been more widely used.However,the output power of a single solid-state device in the millimeter-wave band is very low,which is difficult to meet the needs of practical engineering applications.In order to meet the system's high-power needs,people usually use power combining technology combined with multiple solid-state devices to obtain high-power output.Traditional power combining circuit structure,such as splitters,couplers in form of microstrip,etc.,because of its high transmission loss which leads to the low combining efficiency in the millimeter wave band,is not applicable.To this end,people have studied and implemented a variety of new power combining structures,such as spatial power combining and quasi-optical power combining.These new power combining technologies have more advantages than the traditional technologies,mainly in the low loss and high efficiency.In this paper the quasi-optical technique is studied in detail and the beam power combining technique is proposed from the perspective of beam propagation in space.It is different from the power combining techniques introduced earlier and the most important feature is that multiple coherent beams propagated in space are combined into one beam,and then the power of the combined beam is superimposed of the multiple incident beams.Several new beam power combining structures are proposed and the principle of beam combining is described.Then the process of beam combining is simulated and the design and processing of some structures for experimental verification are carried out.The main contributions of this paper include:1.A new type of profiled horn with contour constituted of three kinds of curves is presented and designed.The results from simulation and test show that its radiation pattern has advantages of a good circular symmetry,low sidelobes,low cross-polarization and high Gaussian coupling efficiency,which is equivalent to corrugated horn.It can be regarded as an efficient Gaussian beam launcher instead of corrugated horn,which is relatively difficult and costly to fabricate in short millimeter wave or submillimeter wave band.2.A novel beam combining structure based on spatial beamguides is proposed,which uses beamguides system consisting of ellipsoidal mirrors and parabolic mirrors.The incident Gaussian beam radiated by four equal amplitudes and the same polarized source horns are transformed into a combining beam that is propagated in the same direction and is close to each other.It is converted into a Gaussian beam by a waveform conversion lens or a reflective array antenna to achieve beam transformation.For these two waveform transformations,the simulation results both show that the beam combining efficiency is about 70%,and the loss is mainly due to the beam-conversion,and beam transform efficiency is not high enough,and the transformed beam and the receiving antenna are not completely matched.The power combining structure is designed,processed and test.There is a gap between the actual measurement results and the simulation results.The main reason is that the amplitude and phase consistency of the four source horn ports are not fully realized.In addition there is an error in the spatial position of each mirror,which may cause power leakage in the beam transformation.3.A new beam combining structure based on a quasi-optical beam splitter is proposed.It uses a mirror or a lens to realize Gaussian beam transformation,and makes the spatial orthogonal polarization beams with the same beam parameters focus at the same position of the quasi-optical beam splitter.One beam at the beam splitter is totally transmitted while another beam is totally reflected.The transmitted wave and the reflected wave propagate along the same path to realize beam combining.This structure is simple and easy to expand for any integer n beams.If the system uses the ellipsoidal reflector to realize the beam transformation,the loss will be very low.At this time,the simulation results show that the system combining efficiency is above97%.Experiment has been done and the results also verify the good beam combining performance.4.A beam combining structure based on quasi-optical sum-and-difference(?/?)network is proposed.From the sum operation principle of the sum-and-difference network,two incident beams are combined by a?/?network to realize beam combining of 2 to 1,then beam combining of 4 to 1 can be realized by cascading?/?.The simulation results show that the combining efficiency is more than 95%with the one?/?for 2 to 1 beam combining,and it can be over 93%with the two-levels?/?network for 4 to 1 beam combining.This structure is also easily to expand for achieving 2n beams to one beam.5.A new type of refractive-diffractive lens with a diffractive zone on the back surface and the quadric surface in front is proposed and designed.The simulation results show that it has some features with good focusing characteristic,spatial resolution up to 0.5°and field of view up to?10~0.Moreover the measured results and simulations are well matched.It has been applied in sub-millimeter imaging systems as a focused imaging lens.