Sources Based on Mixing of Optical Sources

It has also long been recognized that difference frequency mixing of 'optical' sources could be used to produce Submillimeter radiation [1]. Although the concept is straightforward, practical implementations depend upon the frequency stability and calibration of the lasers sources as well as upon the efficiency of the mixing process.

One of the earliest spectroscopically successful implementation is shown in

shows a nice example of the observation of the electronic quadrupole hyperfine structure of DI.

Tunable laser sources in the visible and near infrared have also been used for difference frequency mixing sources. These have typically used photoconductive mixers in which radiating Terahertz antennas have been integrated with the semiconductor switch [2]. Since these sources operate at two to three orders of magnitude higher frequency than the Terahertz radiation produced, a number of frequency control and measurement schemes have evolved.

Figure: Survey Scan of SO₂ shows an example of a spectrum taken with system in which two dye lasers were mixed in a photoconductive switch. Because this work was focused on measurements of pressure broadening, it was possible to calibrate the difference frequencies by the observation of the optical fringes produces by the scanning dye laser.

The system in Figure: Solid State THz Source [6].is particularly interesting in that it provides a novel scheme for absolute frequency measurement. In this system, two cw diode lasers at 850 nm are locked to different orders of a FP cavity whose free spectral range is 3 GHz, while a third is offset locked to one of the cavity locked lasers via a tunable 3 - 6 GHz microwave oscillator. By knowledge of the free spectral range of the cavity, the difference in mode order between the two fixed locked lasers, and measurement of the microwave offset, absolute frequency calibration of 10^-7 has been achieved.

'Size' as measured in wavelengths is an important limiting factor in 'electronic' approaches to THz technology. However, the 'dimensionality' of the device also plays an important role. Most harmonic generators, solid state sources, and frequency converters are '0-dimensional' devices, small in all three dimensions. An important example of a '1-dimensional' tube is the backward wave oscillator (BWO) discussed below. Much of its success can be traced to its macroscopic (in terms of wavelengths) length along the direction of the electron beam. Figure: Traveling-Wave Photomixer System shows an example of a solid state mixer that takes advantage of this concept. Not only does its distributed interaction region allow the use of higher laser pump powers without burn out, also because the circuit elements are distributed, capacitance limitations on bandwidth are reduced [5].

Photomixers are closely related to frequency multiplers except that they convert the frequency difference between two infrared laser into the SMM/THz [2]. Operationally, they are typically more broadband than frequency multiplier sources, but at a given frequency produce somewhat less power. Because their frequency is determined by the difference between two much larger frequencies frequencies they typically have somewhat less spectral purity and stability and have frequencies that are more difficult to calibrate. However, their characteristic are particularly well suited to the spectroscopy of the broader resonances found in solids.

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