Alignment Jig for the Precise Measurement of THz Radiation

This device can be used by optometrists to measure aberrations in lenses, and by head-mount display manufacturers.

An alignment jig (see figure) places a THz horn and power detector at the proper locations with respect to the focal points of a conic reflector in order to couple total power of the THz source radiating out of its horn into the power detector for precise measurement of its power. A visible laser beam locates focal points of the conic reflector. Measuring total diverging power from a THz point source is not an easy task. THz radiation has a wavelength range of between 0.1 and 1 mm. The power levels range from a few tens of nW to 100 mW. These power levels are low, and low temperatures (in the range of –173 °C) are typically used to house the THz power source. Because of the small target, the power emitter and the power detectors must be located in exact positions in order to fully capture the radiated energy. At these low powers, there are three common commercial power meters: a bolometer detector, a Golay Cell, and a Keating Meter. These three power meters have specific power ranges where they excel, and they must be calibrated at their overlapped power ranges. Because of the low THz power being measured, conical reflectors are used to send all of the radiated power to the detectors. These reflectors focus the energy of the THz source, and the detectors are placed at a convergent focal point to capture the radiated THz power.

One cannot place the detectors just by approximating by eye. THz waves are submillimeter, and require precise placement. The method proposed here is to use a visible low-power red laser (630 nm) with a 1-mm beam diameter (see Figure 1). The laser is beamed through a 3× beam expander to obtain a circular beam, then through a lens in order to focus the beam onto a TI DLP micro-mirror at an angle. This mirror bounces the laser light onto an ellipsoidal mirror that will focus it into a point. That is the point where the THz source is to be placed.