A THz cavity consisting of a basic semiconfocal Fabry–Perot resonator. The cavity consists of a wire-grid polarizer that acts as both the input and output coupling mirror, and a spherical mirror used to refocus the beam and minimize diffraction loss. In gray is the resonantly enhanced THz beam, with the confocal length of the beam equal to the focal length of the cavity and the beam waist indicated by . The metal wires of the grid and the E-field of the beam are aligned parallel to each other, creating maximally reflecting conditions. A 50:50 beam splitter is employed to redirect the output-coupling beam to the detector.
A THz cavity consisting of an “off-axis semiconfocal” Fabry–Perot resonator. This cavity is, in principle, the same as that shown in Fig. 1, with the significant difference that the output beam is directed away from the input beam by an off-axis parabolic mirror. A curved off-axis mirror refocuses the beam and minimizes diffraction losses compared to a parallel plate resonator. Two separate wire-grid polarizers, each positioned at the focal length of the mirror (, maintaining confocal conditions) are now used to couple the beam in to and out of the cavity, which circumvents the need for a beam splitter. In the view shown, both the E-field of the beam and the wires of the polarizers are aligned along the z-axis, perpendicular to the plane in which the arms are rotated. The angle of incidence of the beam on the surface of the parabolic mirror is indicated by .
(Color online) Frequency spectrum for a semiconfocal cavity arrangement such as seen in Fig. 1, recorded in AM mode. The critically coupled fundamental modes are labeled M, and several higher order can also be seen. The width of the modes is 2.2 MHz, which corresponds to a measured = 1.4 × 105, approximately one quarter of the theoretical limit of Q = 5.6 × . Standing waves between the various optics (etalons) are also prominent in the spectrum.
(Color online) Frequency spectrum for an off-axis cavity arrangement such as that seen in Fig. 2, recorded in AM mode. Separation of the input and output beams significantly simplifies the spectrum, with both the higher order modes and etalon effects filtered out of the spectrum, leaving only the fundamental modes (labeled M) on an otherwise flat baseline. The width of the modes is 2.0 MHz, corresponding to Q = 1.5 × 105. This is close to the theoretical limit of Q = 2.2 × 105.
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