Continuous-wave Emitter Research Articles

Terahertz transmission-mode scanning-probe near-field optical microscopy based on a flexible step-index sapphire fiber

Terahertz scanning-probe near-field optical microscope (THz-SNOM), operating in transmission mode and relying on a flexible sapphire fiber, was developed. For this aim, a 300-μm-diameter step-index sapphire optical fiber with flat ends, high surface, and volume quality was fabricated using the edge-defined film-fed growth technique. For the lowest-order guided mode of such fiber, numerical analysis revealed small-to-moderate THz-wave propagation loss, as well as strongly subwavelength guided mode confinement in a fiber core thanks to a high refractive index of sapphire in the THz range. Experimental setup realizing the THz-SNOM principles was assembled based on thus fabricated fiber, as a probe, a backward-wave oscillator, as a continuous-wave emitter with the output wavelength of λ = 1.2 mm, and a Golay cell, as a THz beam power detector. By combining numerical analysis and experimental study, a subwavelength resolution of our THz-SNOM setup was demonstrated. The resolution depends on orientation of the guided mode polarization toward the image plane, but always remains strongly subwavelengths and overcomes the Abbe diffraction limit. For certain orientations, it can be as low as 0.23λ. In our opinion, thanks to the subwavelength resolution, high-energy efficiency, and flexibility of design, the developed sapphire-fiber-based THz-SNOM holds strong potential in different branches of THz science and technology.

Reader Coverage Analysis for Multi-Carrier Passive UHF RFID Systems

Applications of passive radio frequency identification (RFID) systems have gained considerable attentions in recent years. Because a passive tag must obtain its operating power from a continuous wave transmitted from a reader in a conventional RFID system, reader coverage is limited. Thus, expanding reader coverage is a current goal in RFID research. In this work, passive tags are provided with additional operating power via continuous waves in multiple frequency bands. In an interrogation region, continuous wave emitters, which provide additional operating power to passive tags, are deployed according to the base station configuration in a cellular phone system. Because transmission power of continuous wave emitters must consider the reader command demodulation constraint and minimum operating power required by a tag, transmission power of continuous wave emitters must be chosen carefully. A method for analyzing reader coverage in multi-carrier passive UHF RFID systems is derived in this work. Assuming all tags are uniformly distributed in an interrogation region, the optimal continuous wave emitter transmission power that achieves the largest reader coverage can be analytically determined. Simulation results verify that continuous wave emitters with suitable transmission power expand reader coverage in a multi-carrier passive UHF RFID system. Additionally, adjusting reader power in the forward (reader-to-tag) link duration can loosen the reader command demodulation constraint and thereby further expand reader coverage.

A Frequency Diverse Gen2 RFID System with Isolated Continuous Wave Emitters

The “Gen2” specification for UHF passive RFID systems released by EPCglobal has become an intense research interest. A Gen2 tag derives its power from the RF wave emitted by a Gen2 RFID reader and responds its modulated backscatter signals to the reader. Due to the large propagation loss, the accessible range of a Gen2 tag is hence limited. Moreover, the readability of a Gen2 tag is often influenced by the multipath fading problem. In order to mitigate the problems, a multi-carrier UHF passive RFID system utilizing the frequency diverse backscatter ability of a Gen2 tag is proposed in a prior work. In this work, a thorough analysis of the system is given. Especially, the appropriate powers of the transceiver and CWEs are derived to make the system more feasible in practical applications.