Abstract
We carried out an experimental comparison study of the two most established optoelectronic emitters for continuous-wave (cw) terahertz generation: a uni-traveling-carrier photodiode (UTC-PD) and a pin-photodiode (PIN-PD). Both diodes are commercially available and feature a similar package (fiber-pigtailed housings with a hyper-hemispherical silicon lens). We measured the terahertz output as a function of optical illumination power and bias voltage from 50 GHz up to 1 THz, using a precisely calibrated terahertz power detector. We found that both emitters were comparable in their spectral power under the operating conditions specified by the manufacturers. While the PIN-PD turned out to be more robust against varying operating parameters, the UTC-PD showed no saturation of the emitted terahertz power even for 50 mW optical input power. In addition, we compared the terahertz transmission and infrared (IR) blocking ratio of four different filter materials. These filters are a prerequisite for correct measurements of the absolute terahertz power with thermal detectors.
Highlights
Within the last decade, terahertz technologies attracted more and more interest due to an evergrowing number of industrial, “real-world” applications
We present a detailed comparison of the emitted terahertz power as a function of frequency, optical excitation power, and bias voltage for the PIN-PD and the uni-traveling-carrier photodiode (UTC-PD) emitter
For the PIN-PD, we find that an optical power higher than 30 mW is beneficial for achieving a high terahertz output, while a bias voltage of − 1.5 V seems adequate
Summary
Terahertz technologies attracted more and more interest due to an evergrowing number of industrial, “real-world” applications. A photodiode can efficiently convert an optical signal into an electrical one by so-callled “photomixing.” To this end, a pair of single-mode lasers generates an optical beat with frequency fbeat = |fLaser1 – fLaser2|. Choosing two lasers with emission wavelengths of 1540 nm (194.67 THz) and 1547.95 nm (193.67 THz), respectively, results in an optical beat with fbeat = 1 THz. When fed into a photodiode, the two-color signal induces a photocurrent that is modulated at the beat frequency. When tunable lasers are used as optical sources, the difference frequency of the beat note can be changed over a broad spectral range, which translates directly into widely tunable terahertz emission
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