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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 17 — Aug. 13, 2012
  • pp: 19176–19184
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Characterization of bending loss in hollow flexible terahertz waveguides

Pallavi Doradla, Cecil S. Joseph, Jayant Kumar, and Robert H. Giles  »View Author Affiliations


Optics Express, Vol. 20, Issue 17, pp. 19176-19184 (2012)
http://dx.doi.org/10.1364/OE.20.019176


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Abstract

Attenuation characteristics of hollow, flexible, metal and metal/dielectric coated polycarbonate waveguides were investigated using an optically pumped far infrared (FIR) laser at 215 µm. The bending loss of silver coated polycarbonate waveguides were measured as a function of various bending angles, bending radii, and bore diameters. Minimal propagation losses of 1.77, 0.96 dB/m were achieved by coupling the lowest loss TE11 mode into the silver or gold coated waveguide, and HE11 mode into the silver/polystyrene coated waveguides respectively. The maximal bending loss was found to be less than 1 dB/m for waveguides of 2 to 4.1 mm bore diameters, with a 6.4 cm bend radius, and up to 150° bending angle. The investigation shows the preservation of single laser mode in smaller bore waveguides even at greater bending angles.

© 2012 OSA

1. Introduction

The Terahertz (THz) frequency regime of an electromagnetic spectrum is a rapidly developing area in source and receiver technologies with a wide range of applications in security screening and remote sensing [1

1. J. F. Federici, B. Schulkin, F. Huang, D. Gary, R. Barat, F. Oliveira, and D. Zimdars, “THz imaging and sensing for security applications—explosives, weapons and drugs,” Semicond. Sci. Technol. 20(7), S266–S280 (2005). [CrossRef]

]. The high sensitivity of THz radiation to water concentration, resulting from the low energy interaction with the low frequency molecular motions, has expanded its applications into the areas of imaging and spectroscopy [2

2. W. L. Chan, J. Deibel, and D. M. Mittleman, “Imaging with terahertz radiation,” Rep. Prog. Phys. 70(8), 1325–1379 (2007). [CrossRef]

]. Due to its non-ionizing property (unlike X-rays), THz technology has become increasingly important for biological applications. As cancerous tissues show higher hydration compared to the healthy ones (Evident from PET and MRI studies) [3

3. K. F. Ross and R. E. Gordon, “Water in malignant tissue, measured by cell refractometry and nuclear magnetic resonance,” J. Microsc. 128(1), 7–21 (1982). [CrossRef] [PubMed]

], the water detection by THz techniques can be an alternative and easy way for cancer identification. The currently available commercial THz imaging systems (TPI imaga 1000, et.al.) for cancer identification were limited to the in vivo tissue analysis of externally growing tumors. A flexible cylindrical waveguide with good mode selectivity is essential to guide the THz radiation in the field of interior in vivo medical imaging. The functionality of the imaging system can be improved by using low-loss, hollow, flexible terahertz waveguides with minimal transmission loss and specific bending characteristics.

Terahertz waveguides have been fabricated from a variety of materials [4

4. J. T. Lu, Y. C. Hsueh, Y. R. Huang, Y. J. Hwang, and C. K. Sun, “Bending loss of terahertz pipe waveguides,” Opt. Express 18(25), 26332–26338 (2010). [CrossRef] [PubMed]

6

6. K. Nielsen, H. K. Rasmussen, A. J. L. Adam, P. C. M. Planken, O. Bang, and P. U. Jepsen, “Bendable, low-loss Topas fibers for the terahertz frequency range,” Opt. Express 17(10), 8592–8601 (2009). [CrossRef] [PubMed]

] and with different cross sectional designs [7

7. C. Yeh, F. Shimabukuro, and P. H. Siegel, “Low-loss terahertz ribbon waveguides,” Appl. Opt. 44(28), 5937–5946 (2005). [CrossRef] [PubMed]

10

10. J.-T. Lu, C.-H. Lai, T.-F. Tseng, H. Chen, Y.-F. Tsai, I.-J. Chen, Y.-J. Hwang, H.-C. Chang, and C.-K. Sun, “Terahertz polarization-sensitive rectangular pipe waveguides,” Opt. Express 19(22), 21532–21539 (2011). [CrossRef] [PubMed]

]. Practical hollow core THz waveguides with reduced transmission losses below the level of 1 dB/m were reported recently [11

11. B. Bowden, J. A. Harrington, and O. Mitrofanov, “Silver/polystyrene-coated hollow glass waveguides for the transmission of terahertz radiation,” Opt. Lett. 32(20), 2945–2947 (2007). [CrossRef] [PubMed]

15

15. C.-H. Lai, J.-Y. Lu, and H.-C. Chang, “Adding metallic layers outside terahertz antiresonant reflecting waveguides: the influence on loss spectra,” J. Opt. Soc. Am. B 28(9), 2200–2206 (2011). [CrossRef]

]. Most of the reported terahertz waveguides were either rigid or not quite flexible at larger bore diameters greater than 5 mm [11

11. B. Bowden, J. A. Harrington, and O. Mitrofanov, “Silver/polystyrene-coated hollow glass waveguides for the transmission of terahertz radiation,” Opt. Lett. 32(20), 2945–2947 (2007). [CrossRef] [PubMed]

]. On the other hand, the bendable terahertz waveguides were neither cylindrical [7

7. C. Yeh, F. Shimabukuro, and P. H. Siegel, “Low-loss terahertz ribbon waveguides,” Appl. Opt. 44(28), 5937–5946 (2005). [CrossRef] [PubMed]

,10

10. J.-T. Lu, C.-H. Lai, T.-F. Tseng, H. Chen, Y.-F. Tsai, I.-J. Chen, Y.-J. Hwang, H.-C. Chang, and C.-K. Sun, “Terahertz polarization-sensitive rectangular pipe waveguides,” Opt. Express 19(22), 21532–21539 (2011). [CrossRef] [PubMed]

] nor provide low-loss (below 2 dB/m) [16

16. T. Ito, M. Miyagi, H. Minamide, H. Ito, and Y. Matsuura, “Flexible terahertz fiber optics with low bend-induced losses,” J. Opt. Soc. Am. B 24(5), 1230–1235 (2007). [CrossRef]

,17

17. M. S. Vitiello, J.-H. Xu, F. Beltram, A. Tredicucci, O. Mitrofanov, J. A. Harrington, H. E. Beere, and D. A. Ritchie, “Guiding a terahertz quantum cascade laser into a flexible silver-coated waveguide,” J. Appl. Phys. 110, 063112 (2011). [CrossRef]

]. However the flexible cylindrical waveguides shown in the literature that provide low loss (1.3 dB/m) [12

12. Y. Matsuura and E. Takeda, “Hollow optical fibers loaded with an inner dielectric film for terahertz broadband spectroscopy,” J. Opt. Soc. Am. B 25(12), 1949–1954 (2008). [CrossRef]

] do not meet the small diameter requirement, as the fabrication technique is not feasible for bore diameters less than 3 mm. Here we report the transmission and bending characteristics of low loss (less than 2 dB/m), hollow, flexible, cylindrical terahertz waveguides with an inner metal, and metal/dielectric coatings that are small enough in diameter for endoscopic applications (2 mm bore diameter). These cylindrical hollow THz waveguides for medical imaging were fabricated using flexible polycarbonate (PC) tubing, through an extrusion process that provides a smooth inner surface. For this investigation silver (Ag) and gold (Au) were chosen to be the germane metals, due to their high reflectivity values at 214.6 µm wavelength. A dielectric layer is added to the flexible metal coated waveguides to improve reflectivity within specific wavelength ranges. Polystyrene (PS) was chosen to be the dielectric, due to its low extinction coefficient, that enhances the transmission through the waveguide [18

18. B. Bowden, J. A. Harrington, and O. Mitrofanov, “Low-loss modes in hollow metallic terahertz waveguides with dielectric coatings,” Appl. Phys. Lett. 93(18), 181104 (2008). [CrossRef]

].

2. Experiment

Metal films were deposited inside the rigid or flexible tubes using an electro-less liquid phase chemical deposition (ELPCD) process. The inner surface of the polycarbonate tubing was coated with Ag using the ELPCD process in an open-loop configuration, whereas the deposition of Au involved in a closed-loop [19

19. K. Iwai, Y. W. Shi, M. Miyagi, and Y. Matsuura, “Improved coating method for uniform polymer layer in infrared hollow fiber,” Opt. Laser Technol. 39(8), 1528–1531 (2007). [CrossRef]

] ELPCD process. Maximum transmission through the THz waveguide can be achieved by depositing a metal film with a thickness much greater than its skin depth at the desired wavelength. The duration of the deposition process was about 2 to 4 minutes to obtain a several micron thick layer of Ag or Au, which is much smaller than the time reported for a micron thick metal coatings in the literature. The polystyrene coating was obtained by sending 5 to 8 ml of 16 to 20 wt% polystyrene/ cyclohexane solution through the metal coated waveguide with a maximum flow rate of 40 ml/min [20

20. P. Doradla, C. S. Joseph, J. Kumar, and R. H. Giles, “Propagation Loss Optimization in Metal/Dielectric Coated Hollow Flexible Teradhertz Waveguides,” Proc. SPIE 8261, 82601P1–82610P10 (2012).

]. The thickness of the polymer coating increases linearly with time, and as example, two SEM images taken at different coating times are shown in the insets of Fig. 1
Fig. 1 Schematic of experimental layout for the measurement of total loss in bent metal coated waveguides. Insets show straight loss measurement and cross sectional SEM images of Ag/PS waveguides at different PS coating times.
. The desired 26.8 µm thick uniform PS film was achieved by coating the PC tube for 10 minutes using the closed-loop ELPCD process.

The source used for this experiment was a CO2 optically pumped far-infrared (FIR) gas laser operating at 1.39 THz. The laser line used was 214.6 µm, horizontally polarized transition in CH2F2, pumped by the 9R34 transition of the CO2 laser. Near the laser face, the measured terahertz laser output power was 370 mW. The beam waist of the 1.4 THz laser was measured as 2.36 mm. Maximal transmission in cylindrical THz waveguides can be achieved by coupling the lowest loss TE11 mode into the metal coated waveguide and HE11 mode into the metal/dielectric coated waveguides with maximum mode coupling efficiency. An optical system was designed in such way that the ratio of the laser beam waist and bore radii, ω0/r, as 0.77, 0.64 mm to couple 90.3%, 98.1% of the lowest order TE11, HE11 mode into the metal, and metal/dielectric coated hollow waveguides respectively [16

16. T. Ito, M. Miyagi, H. Minamide, H. Ito, and Y. Matsuura, “Flexible terahertz fiber optics with low bend-induced losses,” J. Opt. Soc. Am. B 24(5), 1230–1235 (2007). [CrossRef]

,21

21. R. K. Nubling and J. A. Harrington, “Launch conditions and mode coupling in hollow glass waveguides,” Opt. Eng. 37(9), 2454–2458 (1998). [CrossRef]

].

The set-up for measuring bending loss is shown in Fig. 1. The bending loss of the waveguide depends on the bending angle, θ, and the bend radius, R. To facilitate the bending radius measurement, an aluminum disc was used to bend the flexible waveguide as shown in the experimental setup. R can be varied by changing the aluminum disc radius. In this investigation three discs of different radii 4.2 cm, 5.1 cm, and 6.4 cm were used to bend the silver coated waveguides of inner diameters 4.1 mm, 3.2 mm, and 2 mm up to 150°. The power coupled into the waveguide was measured to be 3 mW.

3. Results and discussion

3.1 Propagation loss

The total transmission loss of a waveguide, αtot, is the sum of propagation and bending losses,
αtot=αpqL+αθLR
(1)
where αpq represents the propagation loss and αθ is the bending loss of the waveguide in dB/m. L represents the length of the waveguide, and LR is the bend length.

The attenuation coefficient, αpq, for TEpq modes of hollow metal terahertz waveguides is given by [16

16. T. Ito, M. Miyagi, H. Minamide, H. Ito, and Y. Matsuura, “Flexible terahertz fiber optics with low bend-induced losses,” J. Opt. Soc. Am. B 24(5), 1230–1235 (2007). [CrossRef]

],
α(TEpq)=10u4u2p2nn2+κ2(1k02r3+p2u4r)
(2)
where r represents the bore radius of the waveguide, ñ = n – iκ is the complex refractive index of the metal, p is the mode index, k0 = 2π / λ0 is the wave number, and u is the phase constant representing qth zero of the Bessel function Jp(x).

The straight losses for the hollow flexible 50 - 60 cm long silver, and gold coated THz waveguides of different bore sizes were measured at 214.6 µm. The total transmission loss of a waveguide was found experimentally using Beer-Lambert’s law as follows,
α=10Llog(PinPout)dBm
(3)
where L represents the length of the waveguide, and P is the power. A cutback technique was used to accurately determine the absorption coefficient for each straight waveguide. The loss measurement for the entire length of the waveguide determines the value of Pout. The loss in the residual short length waveguide that was left after the cut, determines the value of Pin. The length of the waveguide is the difference between the long and short length of waveguide. The attenuation coefficients were calculated with cutback method by using waveguides of six different lengths from 10 to 60 cm. The transmission losses increase linearly with the waveguide length. The linear fit to the data allows an estimation of total transmission loss and experimental coupling efficiencies of the waveguide. The theoretical and experimental attenuation coefficients in cylindrical metal coated waveguides were obtained from Eq. (2) and (3) respectively. Figure 2
Fig. 2 Experimental and theoretical attenuation coefficients as a function of bore diameter for silver, gold, and silver/polystyrene coated terahertz waveguides.
shows the theoretical and experimental attenuation coefficients of both silver and gold coated waveguides as a function of bore diameter, d. The theoretical and experimental results were found to be in good agreement. The experimental propagation losses of the waveguides were generally higher than the theoretical losses. The predicted trend of decreasing loss with increasing bore diameter, and the convergence of the attenuation coefficient difference between silver and gold coated waveguides at larger bore diameters were clearly visible from Fig. 2. The better agreement between experimental and theoretical losses in case of gold coated waveguides can be due to the greater uniformity resulted from closed loop ELPCD process. Propagation losses of 1.77 dB/m, and 2 dB/m were achieved with 4 mm bore diameter Ag, Au coated terahertz waveguides respectively. Twelve tested 2 - 4.1 mm bore diameter silver, gold coated waveguides had experimental coupling efficiencies between 74% and 79%.

Maximum transmission in metal/dielectric coated THz waveguides was achieved by coupling the lowest loss HE11 mode. The theoretical attenuation coefficient for hollow metal/dielectric coated THz waveguides is given by [22

22. M. Miyagi and S. Kawakami, “Design Theory of Dielectric-Coated Circular Metallic Waveguides for Infrared Transmission,” J. Lightwave Technol. 2(2), 116–126 (1984). [CrossRef]

],
α(HEpq)=10u2(n0k0)2r3[nn2+κ2]12(1+nd2nd21)2
(4)
where n – iκ represents the complex refractive index of metal, u is the qth zero of Jp-1(x). Here ñd is the complex refractive index of the dielectric, which is assumed to be lossless. Both the theoretical and experimental attenuation coefficients for silver/polystyrene coated 40 – 50 cm long terahertz waveguides were plotted as a function of bore diameter d, as shown in Fig. 2. The maximum measurement error for each individual data point calculated using the standard deviations in the power measurement is less than 0.2 dB. The predicted trend of decreased loss with increasing bore diameter, and the abatement in the difference of Ag, and Ag/PS coated waveguides loss at larger bore diameters was clearly visible from Fig. 2. Also, the Ag/PS coated waveguides show higher transmission compared to Ag coated waveguides for all bore diameters. The discrepancy between theoretical and experimental attenuation coefficients can be due to the edge coupling effect, roughness of metal or non-uniformity of the dielectric coating. Also, the theoretical attenuation coefficients were calculated by assuming a lossless dielectric (κd = 0). Due to the nonzero absorption coefficient of polystyrene, with complex refractive index of 1.58 – i3.58x10−3, the theoretical attenuation coefficients in Fig. 2 should be at a higher level. In this investigation, the propagation loss of less than 1 dB/m was achieved in 4 mm diameter silver/polystyrene coated terahertz waveguides. Five out of the six tested 2 – 4.1 mm bore diameter Ag/PS coated waveguides had experimental coupling efficiencies between 84% and 89%. The outlier has a coupling efficiency of 77%, due to the non uniformity caused in 29.5 µm thick polystyrene coating. Maximum coupling efficiency of 89% was achieved in 26.2 µm thick PS coated waveguide.

3.2 Bending loss

The bending loss in hollow flexible THz waveguides depends on the bend radius (R), angle of bending (θ), and bore radius (r). In the mid-IR region it has been shown that, this additional bending loss varies as r3/R [23

23. M. Miyagi and S. Kawakami, “Losses and phase constant changes caused by bends in the general class of hollow waveguides for the infrared,” Appl. Opt. 20(24), 4221–4226 (1981). [CrossRef] [PubMed]

]. The total loss for bent terahertz waveguides at 214.6 µm wavelength is shown in Fig. 3
Fig. 3 Total attenuation coefficient as a function of bending angle for the silver coated waveguides of a) 3 mm inner diameter at various bending radii, b) different bore diameters with 6.4 cm bend radius.
. All the measurements were taken using 50 to 60 cm long silver coated waveguides with the laser polarization perpendicular to the plane of bending (s-pol). If the bending loss, αθ, is proportional to r3/R then,
αtot=αpqL+αθLR=αpqL+(Cr3R)(Rθ)=αpqL+Cr3θ
(5)
which means, if αtot is plotted as a function of θ at different bending radii for silver coated waveguides of fixed bore diameter the slope must be constant. Figure 3(a) shows the total attenuation coefficient, αtot, for a 3 mm bore diameter silver coated waveguide as a function of bending angle, θ, at different bending radii 4.2 cm, 5.1 cm, 6.4 cm. It is observed that the total loss increases rapidly with increasing bending angle or by decreasing bend radius. The Y-intercept of Fig. 3(a) gives the straight loss, which is the total transmission loss, measured when the bending angle was 0°. A bending loss of 0.8 dB/m was measured in 2 mm silver coated waveguides with a bend radius of 6.4 cm. From Fig. 3(a), it is evident that the slope, C, is invariant for the silver coated waveguides bent at different bending radii. As the slope of the bending loss curve is independent of the bend radius, which confirms the result predicted by Eq. (5) that, the bending attenuation coefficient αθ varies inversely with bend radius R.

Also, as the propagation loss, αpq, is proportional to 1/r3,

αtot/r3=(αpqL+αθLR)/r3=((C1r3)L+(Cr3R)Rθ)/r3=C1(Lr6)+C(θ)
(6)

Figure 3(b) shows the total attenuation coefficient per r3, αtot/r3, of the silver coated waveguide with 6.4 cm bend radius as a function of bending angle for several bore diameter tubes namely 2 mm, 3.2 mm, and 4.1 mm. It shows the direct dependence of total loss on bending angle. Also it is observed that smaller bore diameters lead to a rapid increase in the attenuation coefficient αtot/r3, as predicted by the presence of 1/r6 term in Eq. (6). Further, the maximal bending loss was determined to be less than 1 dB/m for all silver coated waveguides with bore diameters greater than 1 mm. As the slope of bending loss curve in Fig. 3(b), C, is invariant for the waveguides of different bore diameters, it is evident that the bending loss varies as the cube of the bore radius as predicted from theory. The bending losses in 2 – 4.1 mm bore 40 cm long Ag/PS coated waveguides were measured up to 200 bending angle by using an aluminum disc of 6.4 cm diameter. Due to the great difficulty involved in bending a 30 µm thick PS coated polycarbonate waveguide, bending losses are hard to measure at greater bending angles without suppressing the uniformity of the PS layer. The maximum bending loss measured in 2 – 4.1 mm bore diameter Ag/PS coated waveguides with 200 bending angle and 6.4 cm bend radius was 0.3 dB/m.

3.3 Modal characteristics

In principle, metal waveguides do not preserve a single mode launched into the waveguide while bending. The existence of higher order modes were observed in the output beam profiles of 4mm diameter waveguide as it was bent. Even at greater bending angles, as compared to 4 mm waveguide, guides with 3 mm bore diameter showed less mode mixing. The smallest bore waveguides exhibit the least amount of mode mixing, as would be predicted bythe general dependence of αpq on the mode parameter u in Eq. (2). That is, the loss for higher order modes is relatively greater in small bore waveguides compared to large bore sizes. This effect can be seen qualitatively in the mode profile data in Fig. 5 for three different bore diameters of 4.1 mm, 3.2 mm and 2 mm.

4. Conclusion

In conclusion, the propagation losses of Ag, Au, and Ag/PS coated waveguides of several bore diameters were investigated at 214.6 µm wavelength. Total losses as low as 1.77 dB/m and 2 dB/m were achieved in 4 mm Ag, Au coated waveguides respectively by coupling the TE11 mode. The propagating modes transitioned from TE11 dominant to HE11 dominant as the film thickness increased from 0 to 26.8 µm. Propagation losses as low as 0.96 dB/m were achieved in a 4 mm metal/dielectric coated waveguide with 26 µm PS film thickness. Straight Ag/PS coated terahertz waveguides of bore sizes up to 15λ showed the preservation of single laser mode. The bending loss of silver coated waveguides for several bore diameters were characterized as a function of bending angle, and bend radii. The total loss in a bent waveguide increases rapidly with increasing bending angle or decreasing bend radius. The maximum bending loss was determined to be as low as 0.8 dB/m for bore diameters greater than 1 mm. Even when the bending radius was reduced from 6.4 cm to 4.2 cm, the increment in the bending loss was found to be less than 2 dB/m. It was observed that the bending loss varies proportionally to the cube of the bore diameter and inversely with bending radius. At larger bending angles, output beam profiles of 4 mm bore waveguides showed higher mode mixing, whereas the smallest bore 2 mm waveguides exhibited the least amount of mode mixing. Our investigation indicates that the small-bore silver, silver/polystyrene coated flexible waveguides preserves the single laser mode in addition to possessing a low bending loss. The development of these waveguides can lead to increased applications in THz sensing, communication and biomedical imaging applications.

References and links

1.

J. F. Federici, B. Schulkin, F. Huang, D. Gary, R. Barat, F. Oliveira, and D. Zimdars, “THz imaging and sensing for security applications—explosives, weapons and drugs,” Semicond. Sci. Technol. 20(7), S266–S280 (2005). [CrossRef]

2.

W. L. Chan, J. Deibel, and D. M. Mittleman, “Imaging with terahertz radiation,” Rep. Prog. Phys. 70(8), 1325–1379 (2007). [CrossRef]

3.

K. F. Ross and R. E. Gordon, “Water in malignant tissue, measured by cell refractometry and nuclear magnetic resonance,” J. Microsc. 128(1), 7–21 (1982). [CrossRef] [PubMed]

4.

J. T. Lu, Y. C. Hsueh, Y. R. Huang, Y. J. Hwang, and C. K. Sun, “Bending loss of terahertz pipe waveguides,” Opt. Express 18(25), 26332–26338 (2010). [CrossRef] [PubMed]

5.

R. George and J. A. Harrington, “Infrared transmissive, hollow plastic waveguides with inner Ag-AgI coatings,” Appl. Opt. 44(30), 6449–6455 (2005). [CrossRef] [PubMed]

6.

K. Nielsen, H. K. Rasmussen, A. J. L. Adam, P. C. M. Planken, O. Bang, and P. U. Jepsen, “Bendable, low-loss Topas fibers for the terahertz frequency range,” Opt. Express 17(10), 8592–8601 (2009). [CrossRef] [PubMed]

7.

C. Yeh, F. Shimabukuro, and P. H. Siegel, “Low-loss terahertz ribbon waveguides,” Appl. Opt. 44(28), 5937–5946 (2005). [CrossRef] [PubMed]

8.

J. A. Harrington, R. George, P. Pedersen, and E. Mueller, “Hollow polycarbonate waveguides with inner Cu coatings for delivery of terahertz radiation,” Opt. Express 12(21), 5263–5268 (2004). [CrossRef] [PubMed]

9.

O. Mitrofanov, R. James, F. Aníbal Fernández, T. K. Mavrogordatos, and J. A. Harrington, “Reducing transmission losses in hollow THz waveguides,” THz. Technol. 1(1), 2159547 (2011).

10.

J.-T. Lu, C.-H. Lai, T.-F. Tseng, H. Chen, Y.-F. Tsai, I.-J. Chen, Y.-J. Hwang, H.-C. Chang, and C.-K. Sun, “Terahertz polarization-sensitive rectangular pipe waveguides,” Opt. Express 19(22), 21532–21539 (2011). [CrossRef] [PubMed]

11.

B. Bowden, J. A. Harrington, and O. Mitrofanov, “Silver/polystyrene-coated hollow glass waveguides for the transmission of terahertz radiation,” Opt. Lett. 32(20), 2945–2947 (2007). [CrossRef] [PubMed]

12.

Y. Matsuura and E. Takeda, “Hollow optical fibers loaded with an inner dielectric film for terahertz broadband spectroscopy,” J. Opt. Soc. Am. B 25(12), 1949–1954 (2008). [CrossRef]

13.

J. Anthony, R. Leonhardt, S. G. Leon-Saval, and A. Argyros, “THz propagation in kagome hollow-core microstructured fibers,” Opt. Express 19(19), 18470–18478 (2011). [CrossRef] [PubMed]

14.

A. Dupuis, K. Stoeffler, B. Ung, C. Dubois, and M. Skorobogatiy, “Transmission measurements of hollow-core THz Bragg fibers,” J. Opt. Soc. Am. B 28(4), 896–907 (2011). [CrossRef]

15.

C.-H. Lai, J.-Y. Lu, and H.-C. Chang, “Adding metallic layers outside terahertz antiresonant reflecting waveguides: the influence on loss spectra,” J. Opt. Soc. Am. B 28(9), 2200–2206 (2011). [CrossRef]

16.

T. Ito, M. Miyagi, H. Minamide, H. Ito, and Y. Matsuura, “Flexible terahertz fiber optics with low bend-induced losses,” J. Opt. Soc. Am. B 24(5), 1230–1235 (2007). [CrossRef]

17.

M. S. Vitiello, J.-H. Xu, F. Beltram, A. Tredicucci, O. Mitrofanov, J. A. Harrington, H. E. Beere, and D. A. Ritchie, “Guiding a terahertz quantum cascade laser into a flexible silver-coated waveguide,” J. Appl. Phys. 110, 063112 (2011). [CrossRef]

18.

B. Bowden, J. A. Harrington, and O. Mitrofanov, “Low-loss modes in hollow metallic terahertz waveguides with dielectric coatings,” Appl. Phys. Lett. 93(18), 181104 (2008). [CrossRef]

19.

K. Iwai, Y. W. Shi, M. Miyagi, and Y. Matsuura, “Improved coating method for uniform polymer layer in infrared hollow fiber,” Opt. Laser Technol. 39(8), 1528–1531 (2007). [CrossRef]

20.

P. Doradla, C. S. Joseph, J. Kumar, and R. H. Giles, “Propagation Loss Optimization in Metal/Dielectric Coated Hollow Flexible Teradhertz Waveguides,” Proc. SPIE 8261, 82601P1–82610P10 (2012).

21.

R. K. Nubling and J. A. Harrington, “Launch conditions and mode coupling in hollow glass waveguides,” Opt. Eng. 37(9), 2454–2458 (1998). [CrossRef]

22.

M. Miyagi and S. Kawakami, “Design Theory of Dielectric-Coated Circular Metallic Waveguides for Infrared Transmission,” J. Lightwave Technol. 2(2), 116–126 (1984). [CrossRef]

23.

M. Miyagi and S. Kawakami, “Losses and phase constant changes caused by bends in the general class of hollow waveguides for the infrared,” Appl. Opt. 20(24), 4221–4226 (1981). [CrossRef] [PubMed]

OCIS Codes
(230.7370) Optical devices : Waveguides
(040.2235) Detectors : Far infrared or terahertz
(110.6795) Imaging systems : Terahertz imaging

ToC Category:
Optical Devices

History
Original Manuscript: June 4, 2012
Revised Manuscript: July 16, 2012
Manuscript Accepted: July 31, 2012
Published: August 6, 2012

Citation
Pallavi Doradla, Cecil S. Joseph, Jayant Kumar, and Robert H. Giles, "Characterization of bending loss in hollow flexible terahertz waveguides," Opt. Express 20, 19176-19184 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-17-19176


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References

  1. J. F. Federici, B. Schulkin, F. Huang, D. Gary, R. Barat, F. Oliveira, and D. Zimdars, “THz imaging and sensing for security applications—explosives, weapons and drugs,” Semicond. Sci. Technol.20(7), S266–S280 (2005). [CrossRef]
  2. W. L. Chan, J. Deibel, and D. M. Mittleman, “Imaging with terahertz radiation,” Rep. Prog. Phys.70(8), 1325–1379 (2007). [CrossRef]
  3. K. F. Ross and R. E. Gordon, “Water in malignant tissue, measured by cell refractometry and nuclear magnetic resonance,” J. Microsc.128(1), 7–21 (1982). [CrossRef] [PubMed]
  4. J. T. Lu, Y. C. Hsueh, Y. R. Huang, Y. J. Hwang, and C. K. Sun, “Bending loss of terahertz pipe waveguides,” Opt. Express18(25), 26332–26338 (2010). [CrossRef] [PubMed]
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