1. Field of the Invention
The invention relates to microwave, millimeter wave and submillimeter wave spectroscopy systems and components and in particular to an apparatus and method for accurately adjusting the frequency of one or both of the optical beams used in a transceiver for terahertz spectroscopy.
2. Description of the Related Art
Terahertz devices and systems generally employ electromagnetic energy between 300 GHz and 3 terahertz (3 THz), or wavelengths from 100 to 1000 microns (0.1 to 1.0 millimeters), which is also referred to as the submillimeter or far-infrared region of the electromagnetic spectrum.
One important application of terahertz systems is THz spectroscopy. Terahertz spectroscopy presents many new instrumentation and measurement applications since certain compounds and objects can be identified and characterized by a frequency-dependent absorption, dispersion, and/or reflection of terahertz signals which pass through or are reflected from the compound or object.
The generation of terahertz radiation by photomixing is a method of generating quasi-optical signals using an optical-heterodyne converter or photomixer. Typical photomixer devices include low-temperature-grown (LTG) GaAs semiconductor devices, which have been used to generate coherent radiation at frequencies up to 5 THz. The spectroscopy system typically uses two single frequency tunable lasers, such as diode lasers, to generate two optical laser beams which are directed at the surface of the photomixer. By photoconductive mixing of the two beams in the semiconductor material, a terahertz difference frequency between the two optical laser frequencies is generated. In particular, a first laser generates radiation at a first frequency and a second laser generates radiation at a second frequency. The difference frequency, equal to the difference between the first and the second laser frequencies, is swept by the user from microwave through terahertz frequencies by changing the temperature of the lasers, which coarsely changes the frequency of one or both lasers. Other types of tuning mechanisms exist, such as distributed-Bragg-reflector diode lasers with multiple electrodes, grating-loaded external cavities, etc. A terahertz transmitter includes a first photomixer that is optically coupled to the first and the second light source. A first radiative element or antenna is electrically coupled to the first photomixer. In operation, the first antenna radiates a terahertz signal generated by the first photomixer at the difference frequency. A receiver includes a second antenna positioned to receive the signal from the target radiated by the first antenna. The second antenna generates a time varying voltage proportional to the terahertz return signal. A second photomixer is electrically coupled to the second antenna and is optically coupled to the first and the second light source. The second photomixer generates a homodyne downconverted current signal in response to the time varying voltage generated by the second antenna. The downconverted signal is a measurement of the absorption or reflection of the material at each terahertz frequency. This is useful, for example, when used in conjunction with computer processing to identify unknown samples by comparing measured results to a library of reference spectra. This apparatus may also be used to characterize the frequency response characteristics of passive or active components and devices such as waveguides, filters, amplifiers, mixers, diodes, and the like designed to work at terahertz frequencies.