The photonic spectrum analyzer replaces the usually used electronic local oscillator by a photonic one. Therefore, two laser signals that differ in frequency by the desired local oscillator frequency illuminate a photoconductive material, modulating its resistance at the difference frequency, i.e. the local oscillator frequency. At the same time, the signal under test is received by an antenna attached to the photoconductive material that converts the signal to a bias. Combined with the conductance modulation by the lasers, the photoconductive material generates an intermediate frequency corresponding to the difference of local oscillator and signal frequency. By tuning the frequency of the lasers, the intermediate frequency can be chosen smaller than 1 GHz where inexpensive and effective post detection electronics exist commercially. As the signal under test and the laser beat note (i.e. the photonic local oscillator) are not phase locked, phase information cannot be recovered but the spectral power at the IF frequency can be measured in order to obtain the spectral characteristics of the source under test.
We have improved and thoroughly characterized three photonic spectrum analyzer systems and demonstrated its applications in several use cases: an inexpensive photonic spectrum analyzer that is driven by two state-of-the-art distributed feed back lasers. Its frequency coverage is solely determined by the tuning range of the lasers. The responsivity, however, rolls off towards higher frequencies. If the device under test is powerful enough, this system can work at several THz. The system is directly compatible with commercial continuous-wave homodyne THz systems with little extra effort. The main advantage of this system is the low cost. Two versions of a high performance system using optical frequency combs: The systems achieve Hz-level resolution, beyond the goal of the project. The first high performance system uses an inexpensive electro-optic continuous-wave comb system for the photonic local oscillator. The second system uses a more expensive frequency comb that offers an unprecedented tuning range. We have investigated transitions via dielectric waveguides to ground-source-ground probes in order to enable on-chip measurements of electronic THz circuits.
The scientific and technical outcomes of the project are:
-Three prototypes of free space photonic spectrum analyzers developed
-Hz-level spectral resolution achieved
-frequency coverage mainly determined by the tuning range of the laser subsystem
-GSG transitions demonstrated enabling on-chip measurements
-Several case studies carried out to demonstrate the versatility of the photonic spectrum analyzer concept
-Dissemination through one journal publication, two peer-reviewed conference proceedings
-Several presentations at conferences and potential partners for commercialization
-Two further journal publications on the high performance systems are in progress, one of them is already submitted
