Photonic Spectrum Analyzer for Wireless Signals in the THz Range

Abstract

We present an ultra-broadband and inexpensive photonic spectrum analyzer (PSA) for wireless signals with a frequency coverage from the microwave range till deep into the terahertz range. The difference frequency of two continuous-wave laser diodes works as the local oscillator frequency and a photoconductive antenna downconverts a signal under test with the aid of the optical local oscillator. With this approach we achieve a frequency coverage from less than 25 GHz to more than 1.25 THz, mostly limited by the tuning range of the lasers. No component of our spectrum analyzer needs to be interchanged in order to achieve the full tuning range, which makes our spectrum analyzer a fraction of the cost of an electronic spectrum analyzer that requires several extension modules for covering a similar frequency range. The system offers a minimum resolution bandwidth of 1.2 MHz at a displayed average noise level (DANL) as low as -113.8 dBm/Hz at 100 GHz or as low as -88.2 dBm/Hz at 1050 GHz.

Phase noise of an electro-optical phase modulator based electro-optical comb

“…The photomixer (either a photoconductor in the case of a photonic spectrum analyzer [15,21] or a p-i-n-diode as used in this paper) absorbs the laser signal resulting in a DC and AC charge carrier modulation, 𝜎, that follows the optical beat note and acts as local oscillator,…”

“…The simplest implementation of this are two free-running continuous-wave (CW) lasers. As these are independent and just thermally stabilized they showcase a typical linewidth in the tens of kHz to the low MHz range [15] and thus large phase noise. The phase noise can be significantly reduced by referencing these two lasers either to themselves, to a high finesse cavity or to a frequency standard [16,17].…”

A Photonic Spectrum Analyzer for Waveguide-Coupled Sources in the Terahertz Range

“…1. The PSA mixes the signal under test (SUT) with the beat signal of the two lasers within the PCA [4]. This results in an intermediate frequency signal whose spectral power at or around the laser beat frequency is obtained after filtering and squaring.…”

“…Sweeping the beat signal frequency and simultaneously measuring the IF signal creates a frequency spectrum of the SUT. The difference to the PSA version in [4] is the PCA. The logarithmic-periodic antenna and the silicon lens are replaced by a broadband planar end-fire Vivaldi antenna (VA) [8], similar to the one shown in [9], coupled to a dielectric waveguide [9], [10].…”

“…With a photonically generated local oscillator (LO) and a photoconductive antenna (PCA) employed as a mixer, we have shown that a single setup is able to measure the full range from below 100 GHz to 1 THz at the example of a free-space photonic spectrum analyzer (PSA) [4] and a freespace photonic vector network analyzer [5]. The LO was gen-This work was funded by European Research Council (ERC) starting grant "Pho-T-Lyze", grant agreement number 713780 and by the ERC Proof-of-Concept grant "PhoSTer THz", grant agreement number 101057162.…”