WIDELY-TUNABLE LASER-SIDEBAND 171 lz SOURCE FOR SPECTROSCOPY & LO APPLICATIONS

There has been a recent resurgence of interest in spectroscopy in the THz. The development of "short-pulse" THz sources, and the measurements made with these sources, have pointed to a number of possible applications for THz imaging and spectroscopy in biology, medicine, and semiconductor device packaging characterization. While the "short-pulse" sources offer wide-range, low resolution, THz coverage, they have a number of drawbacks for measurement applications. The work described here represents an alternative that avoids many of these drawbacks. The work outlined in this paper marries the recent developments in highreliability, compact THz lasers," 2 with recent advances in Schottky-based sideband generators, ' 4 to yield a reliable, potentially milliwatt-level, tunable THz source. While this work has been focussed on spectroscopy applications, the results can also be applied to provide tunable THz local oscillators. Preliminary results & modeling for the tuning range and fixed-wavelength operating range will be presented. This work is supported by NIST/Gaithersburg under contract number 50-DKNI3-0-90072, and the US Army National Ground Intelligence Center, DAHC90-96-C-0010. Introduction The approach taken in the present work, to generate tunable THz, has been in use for a number of years. 5 In this method, a fixed frequency laser and a tunable millimeter-wave (MMW) source are both coupled into a high-frequency Schottky diode and the resulting sidebands are re-radiated out of said Schottky diode. Thus the resulting sideband radiation can be tuned by simply tuning the MMW source. As there will typically be quite a large amount of non-sideband radiation, at the laser frequency, also present in the reradiated beam, a method is often employed to "filter" the output — yielding only sideband radiation (often both upper and lower sidebands, though one will typically be noticeably weaker than the other). Very recently, significant advances over the previous state-of-the-art output power achievable from an SBG have been made. These advances include use of newly available THz waveguicles/embedding structures to improve the SBG-mixer-mode (we refer to as FM mode) conversion loss from 31 dB to 25 dB at 1.6 Tliz, and more significantly a new mode of operation for the SBG where a conversion loss of 14 dB has been demonstrated at 1.6 THz. In this mode of operation, SBG-phase-modulation-mode (PM), the MMW radiation is used to directly modulate the THz impedance of the SBG element/block thus modulating its reflectivity. This avoids the loss associated with coupling THz currents through the Schottky diode whose parasitic parameters