Fabrication and THz loss measurements of porous subwavelength fibers using a directional coupler method

Abstract

We report several strategies for the fabrication of porous subwavelength fibers using low density Polyethylene plastic for low-loss terahertz light transmission applications. We also characterize transmission losses of the fabricated fibers in terahertz using a novel non-destructive directional coupler method. Within this method a second fiber is translated along the length of the test fiber to probe the power attenuation of a guided mode. The method is especially suitable for measuring transmission losses through short fiber segments, a situation in which standard cutback method is especially difficult to perform. We demonstrate experimentally that introduction of porosity into a subwavelength rod fiber, further reduces its transmission loss by as much as a factor of 10. The lowest fiber loss measured in this work is 0.01 cm(-1) and it is exhibited by the 40% porous subwavelength fiber of diameter 380 microm. For comparison, the loss of a rod-in-the-air subwavelength fiber of a similar diameter was measured to be approximately 0.1 cm(-1), while the bulk loss of a PE plastic used in the fabrication of such fibers is >or= 1 cm(-1). Finally, we present theoretical studies of the optical properties of individual subwavelength fibers and a directional coupler. From these studies we conclude that coupler setup studied in this paper also acts as a low pass filter with a cutoff frequency around 0.3 THz. Considering that the spectrum of a terahertz source used in this work falls off rapidly below 0.25 THz, the reported loss measurements are, thus, the bolometer averages over the approximately 0.25 THz-0.3 THz region.

Ultra low bending loss equiangular spiral photonic crystal fibers in the terahertz regime

“…The latter often utilize a 'drill and draw' 5,6 or 'stack and fuse' 4 technology in their fabrication. Recently, very low loss operation in a PCF made from Topas R polymer has been demonstrated 5 and a new class of porous fibers have also been reported, [7][8][9] though achieving high porosity is not trivial. 9 In this letter, an Equiangular Spiral (ES) PCF design inspired by nature 10,11 in Topas R is presented for THz frequencies.…”

Direct probing of evanescent field for characterization of porous terahertz fibers

“…A directional coupler based method, where a secondary waveguide is located to and translated along the fiber being characterized, is an approach that eliminates cleaving of the fiber. 9 However, there are disadvantages in that the coupling efficiency and length of the coupler is frequency dependent, which complicates the interpretation of the results. Furthermore, a bolometer is employed as a detector, hence, fiber dispersion properties are still not accessible.…”

“…Average losses as low as 0.01 cm −1 have been reported. 9 Here, we demonstrate a technique in which we use a micromachined photoconductive probe-tip to directly sample the evanescent THz electric field in the time-domain at the vicinity of the fiber surface and hence measure simultaneously both absorption coefficient ͑␣ eff ͒ and effective refractive index ͑n eff ͒, from which group velocity and dispersion can be calculated, of the propagating modes of a porous fiber. This approach enables us to accurately explore the radial distribution of evanescent field in the air-clad of these porous fibers as a function of frequency.…”

Broadband terahertz fiber directional coupler

“…Consequently, there are now several demonstrated THz frequency waveguide concepts [2][3][4][5][6][7][8][9][10][11][12][13]. As the waveguides increase in quality, the next step is to make functional THz waveguide devices.…”

“…A THz directional coupler has been previously reported [3], where the coupling was achieved by placing two fibers in close proximity. However, both fibers were subwavelength-sized rod-in-air type fibers [2][3][4][5]7,12].…”