Upconversion of millimeter waves to visible waves: inexpensive focal plane array MMW imaging and ultra-fast wireless communication

Abstract

It is well known that one of the major bottlenecks of MMW and Terahertz system technology is the lack of inexpensive but sensitive and fast detectors. One way of overcoming this problem is the use of commercial neon indicator plasma lamps or glow discharge detectors (GDD) as MMW and THz detectors. Prices are on the order of about half a dollar per lamp. In electronic mode of detection, the incident MMW or THz wave electric field gives rise to an increase in free electron energy above that provided by DC bias, and thus increases ionization collision rate with neutral Ne atoms and increases current. NEP on the order of 10-10 W/sqrt Hz is easily obtainable, with rise time on the order of a microsecond. Noise is characterized by electron temperature. However, recently we have reported an optical means of detection which is manifested as an increase in light intensity emitted by the lamps caused by the incident MMW/THz wave. The detected signal is thus upconverted to the visible, and is measured using optical detectors such as avalanche photodiodes or CCD or CMOS cameras focused on the GDD to measure change in emitted light intensity. This gives rise to measured improvement in NEP by about 2 orders of magnitude and in speed by at least 3 orders of magnitude. The detection stems from increase in free electron recombination rate with positive Ne ions, especially in the cathode region, according to the increase in ionization collisions caused by the MMW/THz electric field. There, positive ions bombard the cathode and cause secondary emission of free electrons. There are thus high concentrations of positive Ne ions and free electrons in the cathode vicinity, giving rise to recombination rate increase and, thus, light intensity increase. The recombination rate increase there derives primarily from the role of positive Ne ions which also govern the secondary electron emission rate from the cathode. Consequently, noise temperature in upconversion derives primarily from the positive ion temperature, which is several orders of magnitude less than that of free electrons because of the much heavier mass of the positive ions, thus reducing NEP. Response speed in electronic detection is limited by parasitic capacitance and inductance deriving from the electrodes. However, in optical detection or upconversion, we do not use detection current in the GDD. The input is the MMW/THz wave. The output is the visible wave deriving from the change in GDD emitted light intensity. Unlike detection current, both waves propagate at the speed of light. Indeed, we report detected modulation frequencies on the order of 3 GHZ for 100 GHz wireless communication. This is limited not by the GDD but rather by external components such as optical detectors and amplifiers. We report also imaging experiment results with GDD focal plane arrays of 16x16 detectors and various techniques for increasing the number of focal plane array detectors. Super resolution techniques permit surpassing the diffraction resolution limit.