Optoelectronic characterization of transmission lines and waveguides by terahertz time-domain spectroscopy

Abstract

Abstract-An overview is presented of an experimental search for an ultrawide-band transmission channel with low dispersion and loss. Such a terahertz (THz) interconnect will soon be required by the insatiable demand for higher speed devices and wider bandwidth communication. Starting with the early optoelectronic generation and detection of single-mode, subpicosecond electrical pulses on coplanar transmission lines, their complete characterization by THz time-domain spectroscopy (THz-TDS) is described. The consequent discovery of phase-coherent Cherenkov radiation in the form of an electromagnetic shock wave from these propagating electrical pulses is discussed together with its dominant role in the large measured propagation loss of these pulses. Various techniques to reduce this radiation are presented. The importance of dielectric materials characterization is explained and illustrated by THz-TDS measurements of high substrates. Newly obtained THz waveguide results are presented and compared to the performance of coplanar transmission lines.

Direct optoelectronic generation and detection of sub-ps-electrical pulses on sub-mm-coaxial transmission lines

“…These relatively short pulsewidths are comparable to those generated on coplanar transmission lines. 3 However, the pulse attenuation with propagation is much less than for coplanar lines and is comparable to that a) Permanent address: …”

“…The transient electric fields are detected by driving the antenna with the sampling laser pulses, as a function of the relative time delay between the excitation and sampling pulses. [1][2][3][4][5] The measured propagated pulses through the 15 mm long, 44 mm long, and 105 mm long coaxial transmission lines are shown in Figs. 2(a)-2(c).…”

“…Consequently, a THz interconnect capable of propagating sub-ps-pulses with minimal loss and little distortion has been realized. 3 Here, in the continuing search for the ideal THz interconnect, we report the first optoelectronic generation of subps-THz pulses launched directly into commercial sub-mmdiameter coaxial transmission lines and the optoelectronic measurement of the propagated pulses. These THz pulses were then used to characterize the frequency response of the coaxial lines by THz-TDS from 0.05 to 1 THz, the broadest bandwidth and highest frequency characterization of coaxial transmission lines to date.…”

“…1,2 Single-mode coupling and propagation were achieved for these waveguides, and thereby demonstrated much larger bandwidths with 1/10 the loss of lithographically defined coplanar transmission lines. 3 Although these waveguides are quite useful for narrow band or THz time-domain spectroscopy (THz-TDs) applications, they all have very high group velocity dispersion (GVD), which make them incapable of sub-ps-pulse propagation.…”

“…[1][2][3] This excessive pulse broadening would not occur for the transverse electromagnetic (TEM) modes of a two-wire transmission line, a coaxial transmission line, or a metal parallel-plate waveguide (PPWG), that do not have a cutoff frequency. The group and phase velocities of these TEM modes are determined solely by the surrounding dielectric.…”