This invention relates to a phase shifter for electromagnetic traveling waves and especially to an optically variable phase shifter for traveling waves of microwave, millimeter and submillimeter wavelengths.
Conventional phase shifters are electrically variable, that is, the control signal applied to the phase-shifter is electrical. The most common phase shifters use ferrite, diodes or PIN semiconductors as an interaction material to induce phase shifting.
The operation of the ferrite phase shifter is dependent upon the interaction between a slab of ferrite material and a magnetic biasing field for its phase shifting effect. However, the relatively high attenuation of microwave signals by ferrite material at millimeter wavelengths has precluded this method of phase shifting in this frequency range.
The diode phase shifters employ one or more diodes mounted inside a waveguide. The diodes are responsive to a D.C. bias voltage applied across the diode electrodes. The field produced by the bias voltage induces a change in the electrical characteristics of the diode, which in turn affects the microwave impedance at various points within the waveguide. The change in impedance causes a change in phase shift in a microwave signal transmitted through the waveguide. At millimeter wavelength frequencies, the internal dimensions of the waveguide are relatively small so that accurate positioning of a diode is a problem. Also, the attenuation of a microwave signal by a variable reactance diode increases with increasing frequency.
A PIN semiconductor phase shifter is a slab of variable-conductivity semiconductor material in contact with substantially the entire surface area of one of the internal narrow-dimensioned waveguide walls. The microwave conductivity of the semiconductive slab is responsive to the polarity of a D.C. bias voltage applied across the slab electrodes. The polarity of the applied bias voltage changes the conductivity of the slab and causes the phase-determining broad wall dimension of the waveguide to electrically change.
These conventional phase shifters require the application of an electrical signal by either inductive coupling, such as by coils, to the ferrite, or by wiring to the diode or PIN semiconductor. Such applications require structures and circuitry, some of which must be attached to the interaction material and which may cause spurious interference and insertion loss to the phase shifting performance. The circuitry typically includes isolation networks to prevent such interference. The structures and circuitry are costly and may be inconvenient for specific applications where space is limited.
The response time, that is, the time for the traveling wave to shift in phase in response to the electrical signal applied to the phase shifter, is slow for conventional phase shifters because the response time is dependent on the medium which conducts the electrical signal. The response time for the PIN semiconductor phase shifter is further dependent on the traversal of electron-hole pairs across its entire intrinsic region.