Scanning terahertz heterodyne imaging systems are now at an early stage of development. They were recently conceived as means of probing biological specimens and samples of materials to obtain information complementary to that obtainable from imaging systems that utilize other parts of the electromagnetic spectrum (e.g., visible light or x rays). Emerging applications for scanning terahertz heterodyne imaging systems include studies of terahertz contrast mechanisms in biological samples, pump probe excitation of phonon modes in liquids and solids, studies of effects of terahertz irradiance on functions and forms of living cells, and studies of spectral signatures indicative of binding and structures of protein molecules.
Scanning terahertz heterodyne imaging systems using continuous-wave (CW) radiation offer the wide dynamic ranges and high signal-to-noise ratios characteristic of narrow-band high-spectral-resolution systems. As such, they also invite comparison with other terahertz imaging systems that utilize short-pulse time-domain spectroscopy (TDS), which is extremely powerful as a diagnostic technique but typically involves limitations in dynamic range and spectral resolution. One especially notable result of these differences is that in wet tissues, terahertz TDS systems are limited to penetration depths of the order of microns, while terahertz heterodyne systems can reach depths of the order of millimeters. Because the capabilities afforded by the terahertz heterodyne concept are partly complementary to those afforded by the terahertz short-pulse TDS concept, imaging systems based on these concepts could be used as complements to each other to obtain more information than could be obtained by use of either system alone.
In a basic scanning terahertz heterodyne imaging system, (see Figure 1) two far-infrared lasers generate beams denoted the local-oscillator (LO) and signal that differ in frequency by an amount, denoted the intermediate frequency (IF), chosen to suit the application. The LO beam is sent directly to a mixer as one of two inputs. The signal beam is focused to a spot on or in the specimen. After transmission through or reflection from the specimen, the beams are focused to a spot on a terahertz mixer, which extracts the IF outputs. The specimen is mounted on a translation stage, by means of which the focal spot is scanned across the specimen to build up an image.
The performance of the basic scanning terahertz heterodyne imaging system is limited by a number of factors, the most prominent one being frequency instability of the lasers. The figure depicts a more complex prototype system that incorporates an IF stabilization subsystem that increases the achievable frequency stability and dynamic range. This system utilizes two mixers denoted the reference and signal mixers, and the signal from each laser is split into two beams denoted the reference and signal beams. One of the lasers is slightly detuned so that their frequencies differ by an IF between 1 and 3 MHz. The IF outputs of the two mixers are equal in frequency; however, they differ in amplitude and phase because of the loss and phase shift suffered by the signal beam that passes through the specimen and impinges on the signal mixer.
The IF output of the signal mixer becomes one of two inputs to a third mixer that is part of the IF stabilization subsystem. In a fourth mixer that is also part of the IF stabilization subsystem, the IF output of the reference mixer is mixed with a stable 14.6-MHz oscillator signal, and the resulting signal becomes the other input to the third mixer. The output of the third mixer and thus the output of the IF stabilization subsystem is a signal that has a stable frequency of 14.6 MHz but exhibits variations in amplitude and phase according to the loss and phase shift of the signal beam passing through the specimen. An improved system with and IF of 24 GHz has now been completed with a dynamic range of 100 dB (Figure 2), 100 pixels/second, and penetration of 25 mm.
This work was done by Peter Siegel and Robert Dengler of Caltech for NASA's Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Electronics/Computers category.
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Refer to NPO-40474, volume and number of this NASA Tech Briefs issue, and the page number.
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Scanning Terahertz Heterodyne Imaging Systems
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