Dynamic range limitations of non-coherent continous-wave THz photomixing systems with broadband detectors

Abstract

The dynamic range of non-coherent continuous-wave (CW) THz photomixing (PM) systems with broadband detectors can be significantly limited by various parasitic effects. Specifically, we examine the generation of parasitic (i) THz and (ii) IR radiation, and (iii) higher harmonics in CW THz PM emitters. (i) The parasitic broadband THz radiation, spanning from 100 to 250 GHz with a total output power of 20 nW, results from not perfectly clean laser spectra. As a result, for a frequency-flat Golay cell detector, the PM-system dynamic range is limited to 32.8 dB at 500 GHz, 26.7 dB at 1 THz, and 8.5 dB at 2.3 THz. In the case of detectors with a frequency-declining responsivity, the dynamic range can drop by ∼10 dB more. (ii) The IR radiation leaking from a PM emitter (≈20 μW) is sensitive to the PM emitter bias, which results in its modulation with an amplitude of about 1.3 μW, when a standard PM-emitter bias modulation is applied. The detected IR radiation could be confused for the THz signal. (iii) Parasitic generation of higher harmonics in PM systems can also limit the system’s dynamic range or create spectral artifacts. However, we show that the harmonics are low at least at ∼1 THz and above. Specifically, they are less than 400 pW for fundamental frequencies above 750 GHz, which is more than 43 dB below the power of the fundamental harmonic. The above-stated values were obtained for a commonly-used PIN-diode photomixer mounted on a Si lens and 1.5 μm distributed-feedback lasers. In general, suppression of these parasitic signals is crucial for non-coherent CW THz PM systems.