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.

Figure 1. The Alignment Jig for measurement of THz power employs an ellipsoidal mirror with a THz horn at one focal point and the power meter at the second focal point.
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.

Figure 2. (a) Quantification of Optical Aberration as intorduced by an arbitrary lens is manifested by amplitude of its Zernicke coefficients specified in the order of (n= 0, 1, 2, 3, etc.: m=–n, . . . ,0, . . . , n if n even ; –n, . . . , n if n odd). (b) First 10 individual Zernicke functions are plotted.
The initial focal point (laser source) is marked in 3D space as if the TI micro-mirror wasn’t there. This virtual focal point is where the detector is to be placed. Since a circular beam is used, it is easier to locate the focal points by watching for the bright red spot that signals beam convergence.

Once the two focal points are found, and the energy source and energy detectors are in place, it is necessary to check calibration. Array of circular patterns can be beamed from the DLP chip to evaluate Zernike’s refraction aberrations in real time (see Figure 2). In addition, various diagnostic patterns can be beamed from the DLP chip in order to measure aberrations associated with field variation. For example, a spot diagram can be beamed off of the DLP in order to analyze point spread.

This method is useful for the semiconductor industry to evaluate surface metrology of thin transparent optics, clinical optometry to measure lens aberration, telescopes and astronomical receivers to align mirrors covering optics and radiation sources, and head-mount displays to evaluate beam splitters.

This work was done by Hamid H. Javadi of Caltech for NASA’s Jet Propulsion Laboratory.


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Alignment Jig for Precise Measurement of THz Radiation

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This article first appeared in the June, 2009 issue of NASA Tech Briefs Magazine.

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