OSA's Digital Library

Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 27 — Dec. 19, 2011
  • pp: 26883–26890
« Show journal navigation

Terahertz pipe-waveguide-based directional couplers

Jen-Tang Lu, Chih-Hsien Lai, Tzu-Fang Tseng, Hua Chen, Yuan-Fu Tsai, Yuh-Jing Hwang, Hung-chun Chang, and Chi-Kuang Sun  »View Author Affiliations


Optics Express, Vol. 19, Issue 27, pp. 26883-26890 (2011)
http://dx.doi.org/10.1364/OE.19.026883


View Full Text Article

Acrobat PDF (1033 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

We experimentally demonstrate a terahertz (THz) leaky mode directional coupler for future THz applications. The proposed directional coupler comprises two square pipe waveguides. The coupling efficiency is investigated for different frequencies, polarizations, and core sizes. Rectangular pipe-waveguide-based directional couplers and the issue of insertion loss are also discussed. It is found that the THz directional coupler works most efficiently in the minimal-attenuation wavelength regime.

© 2011 OSA

1. Introduction

The development of terahertz (THz) fibers and waveguides has progressed significantly during the past decade [1

1. G. Gallot, S. P. Jamison, R. W. McGowan, and D. Grischkowsky, “Terahertz waveguides,” J. Opt. Soc. Am. B 17(5), 851–863 (2000). [CrossRef]

10

10. C.-H. Lai, B. You, J.-Y. Lu, T.-A. Liu, J.-L. Peng, C.-K. Sun, and H.-C. Chang, “Modal characteristics of antiresonant reflecting pipe waveguides for terahertz waveguiding,” Opt. Express 18(1), 309–322 (2010). [CrossRef] [PubMed]

]. For future THz communication and imaging applications, it is crucial to develop directional couplers for their capability of power switching and controls. In the THz regime, directional couplers based on dielectric subwavelength fibers [11

11. J.-Y. Lu, C.-C. Kuo, C.-M. Chiu, H.-W. Chen, Y.-J. Hwang, C.-L. Pan, and C.-K. Sun, “THz interferometric imaging using subwavelength plastic fiber based THz endoscopes,” Opt. Express 16(4), 2494–2501 (2008). [CrossRef] [PubMed]

15

15. T.-F. Tseng, C.-H. Lai, J.-T. Lu, Y.-R. Huang, H. Chen, Y.-F. Tsai, Y.-J. Hwang, and C.-K. Sun, “THz Sub-wavelength fiber-based directional coupler with a high power transfer ratio and a low insertion loss,” Appl. Phys. Lett. (submitted to).

] and quartz circuits [16

16. J. L. Hesler and A. W. Lichtenberger, “THz Waveguide Couplers Using Quartz Micromachining,” in Proceedings of 21st International Symposium on Space Terahertz Technology (Oxford, UK, 2010), pp. 358–359.

] have been experimentally demonstrated. Since THz fields are guided outside the subwavelength fibers, the directional coupler based on subwavelength fibers can efficiently couple THz fields from the input fiber to the coupled fiber with a short overlap length [12

12. H.-W. Chen, C.-M. Chiu, J.-L. Kuo, P.-J. Chiang, H.-C. Chang, and C.-K. Sun, “Subwavelength dielectric-fiber-based terahertz coupler,” J. Lightwave Technol. 27(11), 1489–1495 (2009). [CrossRef]

]. However, subwavelength fibers are vulnerable to environmental disturbance and suffer high bending loss. The THz directional coupler based on quartz circuits exhibits broadband (330GHz – 500 GHz) functionality but suffer relatively high loss [16

16. J. L. Hesler and A. W. Lichtenberger, “THz Waveguide Couplers Using Quartz Micromachining,” in Proceedings of 21st International Symposium on Space Terahertz Technology (Oxford, UK, 2010), pp. 358–359.

].

Recently, we proposed dielectric pipe waveguides for low-loss THz waveguiding [9

9. C.-H. Lai, Y.-C. Hsueh, H.-W. Chen, Y.-J. Huang, H.-C. Chang, and C.-K. Sun, “Low-index terahertz pipe waveguides,” Opt. Lett. 34(21), 3457–3459 (2009). [CrossRef] [PubMed]

,10

10. C.-H. Lai, B. You, J.-Y. Lu, T.-A. Liu, J.-L. Peng, C.-K. Sun, and H.-C. Chang, “Modal characteristics of antiresonant reflecting pipe waveguides for terahertz waveguiding,” Opt. Express 18(1), 309–322 (2010). [CrossRef] [PubMed]

]. The pipe waveguides consist of a circular air core region and a thin dielectric cladding. THz fields are well-guided in the air core region and suffer low attenuation. It was found that the guiding principle of the pipe waveguides is anti-resonant reflection with a leaky mode nature [9

9. C.-H. Lai, Y.-C. Hsueh, H.-W. Chen, Y.-J. Huang, H.-C. Chang, and C.-K. Sun, “Low-index terahertz pipe waveguides,” Opt. Lett. 34(21), 3457–3459 (2009). [CrossRef] [PubMed]

,10

10. C.-H. Lai, B. You, J.-Y. Lu, T.-A. Liu, J.-L. Peng, C.-K. Sun, and H.-C. Chang, “Modal characteristics of antiresonant reflecting pipe waveguides for terahertz waveguiding,” Opt. Express 18(1), 309–322 (2010). [CrossRef] [PubMed]

,17

17. M. A. Duguay, Y. Kokubun, T. L. Koch, and L. Pfeiffer, “Antiresonant reflecting optical waveguides in SiO2–Si multilayer structures,” Appl. Phys. Lett. 49(1), 13–15 (1986). [CrossRef]

]. With commercial Teflon pipes, we previously demonstrated that attenuation constants lower than 0.001 cm−1 can be achieved [9

9. C.-H. Lai, Y.-C. Hsueh, H.-W. Chen, Y.-J. Huang, H.-C. Chang, and C.-K. Sun, “Low-index terahertz pipe waveguides,” Opt. Lett. 34(21), 3457–3459 (2009). [CrossRef] [PubMed]

] and that the pipe waveguides possess magnificent flexibility and suffer low bending loss [18

18. 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]

]. Furthermore, a recent study indicated that thin-wall silica circular pipe waveguides hold a high passband width and suffer low group velocity dispersion at the anti-resonant regime [19

19. E. Nguema, D. Férachou, G. Humbert, J.-L. Auguste, and J.-M. Blondy, “Broadband terahertz transmission within the air channel of thin-wall pipe,” Opt. Lett. 36(10), 1782–1784 (2011). [CrossRef] [PubMed]

].

For future THz applications, in this paper we develop a pipe-waveguide-based directional coupler. Since the contact area between two circular pipe waveguides is limited and the THz fields cannot be efficiently transferred between two waveguides, we constructed the directional coupler with two square pipe waveguides [20

20. 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]

] instead of circular ones. Different from previously reported THz directional coupler, the pipe-waveguide-based directional coupler is with a leaky mode nature. Even though leaky mode directional couplers had been discussed in the optical regime [21

21. M. Mann, U. Trutschel, C. Wächter, L. Leine, and F. Lederer, “Directional coupler based on an antiresonant reflecting optical waveguide,” Opt. Lett. 16(11), 805–807 (1991). [CrossRef] [PubMed]

23

23. Y. Park, D. Suh, Y. Sohn, H.-S. Chung, M. C. Paek, and M. Y. Park, “Leaky Mode Directional Coupler for Prism Coupling Interconnection,” IEEE Photon. Technol. Lett. 18(1), 34–36 (2006). [CrossRef]

], few of them were proposed in the THz range. Recently, THz directional couplers based on two slab antiresonant reflecting hollow waveguides (ARRHWs) were theoretically discussed [24

24. C.-H. Lai, C.-K. Sun, and H.-C. Chang, “Terahertz antiresonant-reflecting-hollow-waveguide-based directional coupler operating at antiresonant frequencies,” Opt. Lett. 36(18), 3590–3592 (2011). [CrossRef] [PubMed]

], and it was shown that the coupling length can be significantly reduced at the antiresonant frequencies. Since the slab type ARRHWs are the one-dimensional version of the pipe waveguides, the model proposed in [24

24. C.-H. Lai, C.-K. Sun, and H.-C. Chang, “Terahertz antiresonant-reflecting-hollow-waveguide-based directional coupler operating at antiresonant frequencies,” Opt. Lett. 36(18), 3590–3592 (2011). [CrossRef] [PubMed]

] can be treated as a simplified one for the studied directional coupler.

In this paper, the square pipe-waveguide-based THz directional couplers are experimentally investigated for different frequencies, polarizations, and air-core sizes. We further introduce the rectangular directional couplers for comparison. The issue of insertion loss is also discussed for different coupler shapes and frequencies. Because of the guiding mechanism of the pipe waveguides, it is found that the directional coupler works most efficiently in our interested minimal-attenuation wavelength regime.

2. Experimental setup

We fabricated the square pipe waveguides by sticking four polyethylene (PE) strips [20

20. 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]

]. The general directional coupler setup is shown in Fig. 1(a)
Fig. 1 (a) General setup for the pipe-waveguide-based directional couplers. (b) Modified experimental setup for the directional couplers.
. THz waves emitted from a Gunn oscillator module, which was tunable between 324 GHz and 420 GHz, were directly coupled into a square pipe waveguide. We made two pipe waveguides, the cladding thickness of which was 1mm, touch together to establish the directional coupler. However, we noticed that it is hard to make two pipe waveguides contact perfectly and that there were some inevitable air gaps between two waveguides, which led to inconsistent experimental errors and poorer coupling efficiency. To overcome this problem, we modified the experimental setup (shown in Fig. 1(b)). We replaced the two 1-mm-thick PE strips between two waveguides with a 2-mm-thick strip. We named this 2-mm-thick strip “coupling cladding.” This modified setup worked as if two square pipe waveguides contact perfectly. In the modified setup, THz waves were first coupled into a 100-cm-long square pipe waveguides (input waveguide), which eliminated high order modes and made only the fundamental mode exist [20

20. 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]

]. Then we butt-coupled the directional coupler with the input waveguide and measured the output power with a Golay cell. The coupling efficiency (C) was defined as:

C=PB/(PA+PB)
(1)

3. Results and discussion

Before measuring the performance of the directional coupler, we measured the attenuation spectra of the square pipe waveguides first. Figure 2
Fig. 2 The attenuation spectra of PE square pipe waveguides for t=1mm (blue circles) and t=2mm (red squares). The core sizes were kept the same (S=6mm).
shows the attenuation spectra of two PE square pipe waveguides with a cladding thickness (t) of 1mm and 2mm, respectively. The side length (S) of air core was kept the same (6mm). We recently demonstrated that, like the circular ones, the guiding principle of the square pipe waveguides is the anti-resonant reflection mechanism [9

9. C.-H. Lai, Y.-C. Hsueh, H.-W. Chen, Y.-J. Huang, H.-C. Chang, and C.-K. Sun, “Low-index terahertz pipe waveguides,” Opt. Lett. 34(21), 3457–3459 (2009). [CrossRef] [PubMed]

,10

10. C.-H. Lai, B. You, J.-Y. Lu, T.-A. Liu, J.-L. Peng, C.-K. Sun, and H.-C. Chang, “Modal characteristics of antiresonant reflecting pipe waveguides for terahertz waveguiding,” Opt. Express 18(1), 309–322 (2010). [CrossRef] [PubMed]

,20

20. 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]

]. As a Fabry-Perot resonator, at the resonant frequencies, the cladding is nearly transparent and THz waves penetrate through the cladding outside the pipe waveguides. On the other hand, at the anti-resonant frequency regime, THz fields are strongly confined in the air-core region and suffer low attenuation. From Fig. 2, we found that 376 GHz is the resonant frequency and the anti-resonant frequency for the pipe waveguide with t = 2mm and t = 1mm, respectively. That is to say, if the operating frequency is 376 GHz, THz waves are well-confined in the core region and suffer low attenuation in a pipe waveguide with t = 1mm, but as we make two square pipe waveguides with t = 1mm touch together, THz waves can be efficiently transferred between two pipe waveguides, because the cladding between them, the thickness of which is 2mm, becomes nearly transparent at 376GHz. As a result, the directional coupler works most efficiently in our interested minimal-attenuation wavelength regime.

Figure 3
Fig. 3 Measurement of the THz directional coupler for (a) TM polarization and (b) TE polarization to the coupling cladding. The directional coupler was with S=6mm and t=1mm. The red double-dashed arrow indicates the direction of polarization. The blue circles, red squares, and black triangles represent data obtained at 376, 368, and 360 GHz, respectively.
demonstrates the coupling characteristics of a pipe-waveguide-based directional coupler with S=6mm and t=1mm. It is found that the directional coupler works most efficiently at the resonant frequency of the coupling cladding, namely 376GHz, while as the operating frequency is away from 376 GHz, it takes longer overlap length to reach high coupling efficiency. This phenomenon is well matched with the simulation results shown in [24

24. C.-H. Lai, C.-K. Sun, and H.-C. Chang, “Terahertz antiresonant-reflecting-hollow-waveguide-based directional coupler operating at antiresonant frequencies,” Opt. Lett. 36(18), 3590–3592 (2011). [CrossRef] [PubMed]

]. From Fig. 3(a), it is observed that the coupling efficiency of the directional coupler operated at 376GHz reaches over 90% as the overlap length is 25cm, while the directional coupler operated at 368GHz should be with a longer overlap length (about 47.5 cm) to reach over 90% coupling efficiency. It is expected that the overlap length should be even longer for the directional coupler operated at 360GHz to reach 90% coupling efficiency. This particular coupling phenomenon has been explained by [24

24. C.-H. Lai, C.-K. Sun, and H.-C. Chang, “Terahertz antiresonant-reflecting-hollow-waveguide-based directional coupler operating at antiresonant frequencies,” Opt. Lett. 36(18), 3590–3592 (2011). [CrossRef] [PubMed]

] and is attributed to a three-mode beating behavior. Here, we provide an alternative but intuitive explanation in the following. As stated previously, the coupling cladding is nearly transparent at resonant frequencies, but it becomes less transparent as operating frequencies are away from the resonant frequencies. As a result, at the resonant frequencies of the coupling cladding, THz waves can easily penetrate the coupling cladding, leading to higher coupling efficiency.

Figure 3(a) and Fig. 3(b) demonstrate the coupling performance of the directional coupler for different polarizations: TM- and TE-polarization to the coupling cladding, respectively. It is found that the directional coupler is polarization-sensitive. As THz waves are TM polarized to the coupling cladding (shown in Fig. 3(a)), higher coupling efficiency can be obtained. As we can see from the Fig. 3, at 376GHz, the coupling efficiency reaches 90.5% for TM polarization but only 42.8% for TE polarization as the overlap length is 25cm. Since the transmittivity of TM waves is higher than that of TE waves, THz waves with TM-polarization characteristics would more likely penetrate through the coupling cladding into another waveguide, thus lead to higher coupling efficiency.

We further investigated the dependence of coupling efficiency on air-core sizes. Figure 4(a)
Fig. 4 (a) Measurement of the THz directional couplers for S=6mm (blue circles), S = 8mm (red squares) and S=10mm (black triangles). The cladding thickness was 1mm and the operating frequency was 376GHz. THz waves were TM-polarized to the coupling cladding. (b) The relation between air core size and coupling length.
shows the coupling efficiency of the directional couplers with S=6mm, S=8mm, and S=10mm. The operating frequency was 376GHz and THz waves were TM-polarized to the coupling cladding. We found that the directional coupler with smaller air-core sizes works more efficiently. This phenomenon can be explained by ray optics: As THz waves are guided in the pipe waveguides, THz waves would bounce back and forth between claddings. The smaller the core size is, the more frequent bounces occur at the coupling cladding, leading to higher coupling efficiency. Furthermore, the pipe waveguides with a smaller core size suffer higher attenuation [9

9. C.-H. Lai, Y.-C. Hsueh, H.-W. Chen, Y.-J. Huang, H.-C. Chang, and C.-K. Sun, “Low-index terahertz pipe waveguides,” Opt. Lett. 34(21), 3457–3459 (2009). [CrossRef] [PubMed]

,10

10. C.-H. Lai, B. You, J.-Y. Lu, T.-A. Liu, J.-L. Peng, C.-K. Sun, and H.-C. Chang, “Modal characteristics of antiresonant reflecting pipe waveguides for terahertz waveguiding,” Opt. Express 18(1), 309–322 (2010). [CrossRef] [PubMed]

], which results from the fact that weaker reflection (higher transmission) occurs at the air-cladding interfaces for the pipe waveguides with a smaller core size, leading to higher coupling efficiency.

We then modified the square directional couplers into rectangular ones. The air core size was 10mmx6mm and the cladding thickness was 1mm. We measured the coupling efficiency of the longer-side-coupled (LSC) and shorter-side-coupled (SSC) rectangular directional coupler (shown in Fig. 5
Fig. 5 Measurement of the longer-side-coupled (blue circles) and shorter-side coupled (red squares) THz directional coupler. The air core size was 10mmx6mm and the cladding thickness was 1mm. The operating frequency was 376GHz. Polarization of THz waves was TM polarization to the coupling cladding.
). The operating frequency was 376GHz and THz waves were TM-polarized to the coupling cladding. It is observed that the LSC directional coupler worked more efficiently than the SSC directional coupler. From the viewpoint of ray optics, this phenomenon is reasonable. As two rectangular waveguides are coupled with the longer cladding, there are more bounces happening at the coupling cladding due to the rectangular structure. Also, it is logical that higher coupling efficiency can be obtained as the contact area between two waveguides increases. The LSC directional coupler reaches about 95% coupling efficiency as the overlap length is 20cm, while the SSC directional coupler should be 50-cm-long to reach such high coupling efficiency.

By comparing the LSC rectangular (with a 10mmx6mm air core) and the square (S=10mm) directional coupler, we found that the ratio of the waveguide longer to shorter sides affects coupling length. The coupling claddings of these two kinds of directional coupler are with the same width, but the coupling length of the rectangular directional coupler (20 cm) is significantly shorter than that of the square directional coupler (47.5 cm). We can infer that if the width of coupling side is fixed, the LSC rectangular directional coupler with a shorter horizontal side works more efficiently. This inference can be also attributed to the fact that there are more bounces happening at the coupling cladding for the LSC directional coupler with shorter horizontal sides. From the ray optics viewpoint, the bounce number happening at the coupling cladding would be inverse proportional to the length of horizontal side of the LSC rectangular directional coupler. However, if the bounce number is the only one factor to affect coupling length, the coupling length of the square directional coupler with S=10mm would be 20cm (coupling length of the LSC directional coupler) ÷ 6mm (the length of the horizontal side of the LSC directional coupler) x 10mm (the length of the horizontal side of the square directional coupler)≒ 33.3cm, which is smaller than the experimental result (47.5 cm). As a result, bounce number is not the only one factor to influence coupling length and other issue should be taken into account. As we discussed in the case of the square directional coupler, stronger reflection (weaker transmission) occurs at the coupling cladding for the directional coupler with a bigger air core size, leading to lower coupling efficiency. Since the square directional coupler with S = 10mm possesses a bigger air core region than the LSC directional coupler, it is reasonable that the experimental coupling length of the square directional coupler is longer than the coupling length predicted by the bounce number method.

As we can see from Fig. 1, in the directional coupler region, we replaced the two 1-mm-thick strips between two waveguides (Fig. 1(a)) with a 2-mm-thick strip (Fig. 1(b)), which makes the directional coupler become an integral and compact device. To measure the loss of the directional coupler, in Fig. 1(b), we treat PO and PA + PB as the input power and the total output power, respectively. The insertion loss (L) of the direction couplers can be calculated as:

L=10×log(POPA+PB)(indB)
(2)

From Table 1, we found that the insertion loss is nearly independent of the coupler shapes. The primary cause is that there is compensation between guiding loss and overlap length. The directional couplers which work more efficiently (namely, reach 50% coupling efficiency with shorter overlap length) are made of the pipe waveguides with higher attenuation. For example, even though the smaller square directional couplers work more efficiently than the bigger one, the pipe waveguides with a smaller air-core also suffer higher guiding loss [9

9. C.-H. Lai, Y.-C. Hsueh, H.-W. Chen, Y.-J. Huang, H.-C. Chang, and C.-K. Sun, “Low-index terahertz pipe waveguides,” Opt. Lett. 34(21), 3457–3459 (2009). [CrossRef] [PubMed]

,20

20. 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]

]. Therefore, there is an insignificant relation between insertion loss and the size of the square directional couplers (within the butt-coupling loss range). The same phenomena happen in the case of rectangular directional couplers: in order to make THz waves TM-polarized to the coupling plane, for the longer-side-coupled rectangular couplers, the polarization of THz waves is parallel to the shorter axis of the rectangular waveguides, while for the shorter-side-coupled couplers, the polarization is parallel to the longer axis of the rectangular waveguides. At the same time, THz waves suffer lower attenuation if polarization is parallel to the longer axis of rectangular waveguides [20

20. 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]

]. Consequently, to obtain the same coupling efficiency, even though the longer-side-coupled coupler works with shorter overlap length than the shorter-side-coupled coupler, there is little difference in the insertion loss between them because of the compensation between guiding loss and overlap length.

We further investigated the relation between insertion loss and operating frequency. From Table 2

Table 2. Insertion loss for square (S = 6 mm) pipe-waveguide-based 3 dB directional couplers for different frequencies

table-icon
View This Table
| View All Tables
, it is observed that the insertion loss increases as the operating frequency is away from 376GHz. Although we can find from Fig. 2 that the square pipe waveguide with S = 6mm and t = 1mm suffers about the same attenuation for 376GHz, 368GHz, and 360GHz, the pipe-waveguide-based directional coupler operated at 376GHz work most efficiently. As a result, to obtain the same coupling efficiency, the overlap length should be longer if the operating frequency is away from 376GHz, leading to higher insertion loss.

4. Conclusion

We propose a leaky mode THz directional coupler for future THz applications. We construct the directional coupler with two square pipe waveguides so that the contact area between two waveguides can be significantly increased, leading to higher coupling efficiency. Due to the anti-resonant reflecting guiding mechanism, the proposed directional coupler works most efficiently in the minimal-attenuation wavelength regime. The directional coupler is polarization-sensitive and higher coupling efficiency can be obtained if guided waves are TM-polarized to the cladding between two waveguides. Moreover, the directional coupler with smaller air-core sizes works more efficiently. We further introduce rectangular pipe-waveguide-based directional couplers and the longer-side-coupled rectangular directional coupler is preferred. It is also found that the insertion loss is independent of the coupler shapes, while the insertion loss increases as the operating frequency is away from the resonant frequency of the coupling cladding. We believe that the pipe-waveguide-based directional coupler is promising for many THz applications, including THz communications, sensing, and imaging.

Acknowledgments

References and links

1.

G. Gallot, S. P. Jamison, R. W. McGowan, and D. Grischkowsky, “Terahertz waveguides,” J. Opt. Soc. Am. B 17(5), 851–863 (2000). [CrossRef]

2.

K. Wang and D. M. Mittleman, “Metal wires for terahertz wave guiding,” Nature 432(7015), 376–379 (2004). [CrossRef] [PubMed]

3.

T.-I. Jeon, J. Zhang, and D. Grischkowsky, “THz Sommerfeld wave propagation on a single metal wire,” Appl. Phys. Lett. 86(16), 161904 (2005). [CrossRef]

4.

L.-J. Chen, H.-W. Chen, T.-F. Kao, J.-Y. Lu, and C.-K. Sun, “Low-loss subwavelength plastic fiber for terahertz waveguiding,” Opt. Lett. 31(3), 308–310 (2006). [CrossRef] [PubMed]

5.

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]

6.

A. Hassani, A. Dupuis, and M. Skorobogatiy, “Porous polymer fibers for low-loss Terahertz guiding,” Opt. Express 16(9), 6340–6351 (2008). [CrossRef] [PubMed]

7.

S. Atakaramians, S. Afshar V, B. M. Fischer, D. Abbott, and T. M. Monro, “Porous fibers: a novel approach to low loss THz waveguides,” Opt. Express 16(12), 8845–8854 (2008). [CrossRef] [PubMed]

8.

K. Nielsen, H. K. Rasmussen, A. J. Adam, P. C. 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]

9.

C.-H. Lai, Y.-C. Hsueh, H.-W. Chen, Y.-J. Huang, H.-C. Chang, and C.-K. Sun, “Low-index terahertz pipe waveguides,” Opt. Lett. 34(21), 3457–3459 (2009). [CrossRef] [PubMed]

10.

C.-H. Lai, B. You, J.-Y. Lu, T.-A. Liu, J.-L. Peng, C.-K. Sun, and H.-C. Chang, “Modal characteristics of antiresonant reflecting pipe waveguides for terahertz waveguiding,” Opt. Express 18(1), 309–322 (2010). [CrossRef] [PubMed]

11.

J.-Y. Lu, C.-C. Kuo, C.-M. Chiu, H.-W. Chen, Y.-J. Hwang, C.-L. Pan, and C.-K. Sun, “THz interferometric imaging using subwavelength plastic fiber based THz endoscopes,” Opt. Express 16(4), 2494–2501 (2008). [CrossRef] [PubMed]

12.

H.-W. Chen, C.-M. Chiu, J.-L. Kuo, P.-J. Chiang, H.-C. Chang, and C.-K. Sun, “Subwavelength dielectric-fiber-based terahertz coupler,” J. Lightwave Technol. 27(11), 1489–1495 (2009). [CrossRef]

13.

A. Dupuis, J.-F. Allard, D. Morris, K. Stoeffler, C. Dubois, and M. Skorobogatiy, “Fabrication and THz loss measurements of porous subwavelength fibers using a directional coupler method,” Opt. Express 17(10), 8012–8028 (2009). [CrossRef] [PubMed]

14.

Y.-W. Huang, T.-F. Tseng, C.-C. Kuo, Y.-J. Hwang, and C.-K. Sun, “Fiber-based swept-source terahertz radar,” Opt. Lett. 35(9), 1344–1346 (2010). [CrossRef] [PubMed]

15.

T.-F. Tseng, C.-H. Lai, J.-T. Lu, Y.-R. Huang, H. Chen, Y.-F. Tsai, Y.-J. Hwang, and C.-K. Sun, “THz Sub-wavelength fiber-based directional coupler with a high power transfer ratio and a low insertion loss,” Appl. Phys. Lett. (submitted to).

16.

J. L. Hesler and A. W. Lichtenberger, “THz Waveguide Couplers Using Quartz Micromachining,” in Proceedings of 21st International Symposium on Space Terahertz Technology (Oxford, UK, 2010), pp. 358–359.

17.

M. A. Duguay, Y. Kokubun, T. L. Koch, and L. Pfeiffer, “Antiresonant reflecting optical waveguides in SiO2–Si multilayer structures,” Appl. Phys. Lett. 49(1), 13–15 (1986). [CrossRef]

18.

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]

19.

E. Nguema, D. Férachou, G. Humbert, J.-L. Auguste, and J.-M. Blondy, “Broadband terahertz transmission within the air channel of thin-wall pipe,” Opt. Lett. 36(10), 1782–1784 (2011). [CrossRef] [PubMed]

20.

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]

21.

M. Mann, U. Trutschel, C. Wächter, L. Leine, and F. Lederer, “Directional coupler based on an antiresonant reflecting optical waveguide,” Opt. Lett. 16(11), 805–807 (1991). [CrossRef] [PubMed]

22.

M. Shamonin, M. Lohmeyer, and P. Hertel, “Directional coupler based on radiatively coupled waveguides,” Appl. Opt. 36(3), 635–641 (1997). [CrossRef] [PubMed]

23.

Y. Park, D. Suh, Y. Sohn, H.-S. Chung, M. C. Paek, and M. Y. Park, “Leaky Mode Directional Coupler for Prism Coupling Interconnection,” IEEE Photon. Technol. Lett. 18(1), 34–36 (2006). [CrossRef]

24.

C.-H. Lai, C.-K. Sun, and H.-C. Chang, “Terahertz antiresonant-reflecting-hollow-waveguide-based directional coupler operating at antiresonant frequencies,” Opt. Lett. 36(18), 3590–3592 (2011). [CrossRef] [PubMed]

25.

J.-T. Lu, C.-H. Lai, Y.-R. Huang, Y.-C. Hsueh, Y.-J. Huang, H.-C. Chang, and C.-K. Sun, “Investigation on Mode Coupling and Bending Loss Characteristics of Terahertz Air-Core Pipe Waveguides,” in Conference on Lasers and Electro-Optics, OSA Technical Digest (CD) (Optical Society of America, 2010), paper JWA118. http://www.opticsinfobase.org/abstract.cfm?URI=CLEO-2010-JWA118.

OCIS Codes
(060.1810) Fiber optics and optical communications : Buffers, couplers, routers, switches, and multiplexers
(130.2790) Integrated optics : Guided waves
(230.5440) Optical devices : Polarization-selective devices
(230.7370) Optical devices : Waveguides

ToC Category:
Integrated Optics

History
Original Manuscript: October 6, 2011
Revised Manuscript: November 21, 2011
Manuscript Accepted: December 6, 2011
Published: December 16, 2011

Citation
Jen-Tang Lu, Chih-Hsien Lai, Tzu-Fang Tseng, Hua Chen, Yuan-Fu Tsai, Yuh-Jing Hwang, Hung-chun Chang, and Chi-Kuang Sun, "Terahertz pipe-waveguide-based directional couplers," Opt. Express 19, 26883-26890 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-27-26883


Sort:  Author  |  Year  |  Journal  |  Reset  

References

  1. G. Gallot, S. P. Jamison, R. W. McGowan, and D. Grischkowsky, “Terahertz waveguides,” J. Opt. Soc. Am. B17(5), 851–863 (2000). [CrossRef]
  2. K. Wang and D. M. Mittleman, “Metal wires for terahertz wave guiding,” Nature432(7015), 376–379 (2004). [CrossRef] [PubMed]
  3. T.-I. Jeon, J. Zhang, and D. Grischkowsky, “THz Sommerfeld wave propagation on a single metal wire,” Appl. Phys. Lett.86(16), 161904 (2005). [CrossRef]
  4. L.-J. Chen, H.-W. Chen, T.-F. Kao, J.-Y. Lu, and C.-K. Sun, “Low-loss subwavelength plastic fiber for terahertz waveguiding,” Opt. Lett.31(3), 308–310 (2006). [CrossRef] [PubMed]
  5. 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]
  6. A. Hassani, A. Dupuis, and M. Skorobogatiy, “Porous polymer fibers for low-loss Terahertz guiding,” Opt. Express16(9), 6340–6351 (2008). [CrossRef] [PubMed]
  7. S. Atakaramians, S. Afshar V, B. M. Fischer, D. Abbott, and T. M. Monro, “Porous fibers: a novel approach to low loss THz waveguides,” Opt. Express16(12), 8845–8854 (2008). [CrossRef] [PubMed]
  8. K. Nielsen, H. K. Rasmussen, A. J. Adam, P. C. Planken, O. Bang, and P. U. Jepsen, “Bendable, low-loss Topas fibers for the terahertz frequency range,” Opt. Express17(10), 8592–8601 (2009). [CrossRef] [PubMed]
  9. C.-H. Lai, Y.-C. Hsueh, H.-W. Chen, Y.-J. Huang, H.-C. Chang, and C.-K. Sun, “Low-index terahertz pipe waveguides,” Opt. Lett.34(21), 3457–3459 (2009). [CrossRef] [PubMed]
  10. C.-H. Lai, B. You, J.-Y. Lu, T.-A. Liu, J.-L. Peng, C.-K. Sun, and H.-C. Chang, “Modal characteristics of antiresonant reflecting pipe waveguides for terahertz waveguiding,” Opt. Express18(1), 309–322 (2010). [CrossRef] [PubMed]
  11. J.-Y. Lu, C.-C. Kuo, C.-M. Chiu, H.-W. Chen, Y.-J. Hwang, C.-L. Pan, and C.-K. Sun, “THz interferometric imaging using subwavelength plastic fiber based THz endoscopes,” Opt. Express16(4), 2494–2501 (2008). [CrossRef] [PubMed]
  12. H.-W. Chen, C.-M. Chiu, J.-L. Kuo, P.-J. Chiang, H.-C. Chang, and C.-K. Sun, “Subwavelength dielectric-fiber-based terahertz coupler,” J. Lightwave Technol.27(11), 1489–1495 (2009). [CrossRef]
  13. A. Dupuis, J.-F. Allard, D. Morris, K. Stoeffler, C. Dubois, and M. Skorobogatiy, “Fabrication and THz loss measurements of porous subwavelength fibers using a directional coupler method,” Opt. Express17(10), 8012–8028 (2009). [CrossRef] [PubMed]
  14. Y.-W. Huang, T.-F. Tseng, C.-C. Kuo, Y.-J. Hwang, and C.-K. Sun, “Fiber-based swept-source terahertz radar,” Opt. Lett.35(9), 1344–1346 (2010). [CrossRef] [PubMed]
  15. T.-F. Tseng, C.-H. Lai, J.-T. Lu, Y.-R. Huang, H. Chen, Y.-F. Tsai, Y.-J. Hwang, and C.-K. Sun, “THz Sub-wavelength fiber-based directional coupler with a high power transfer ratio and a low insertion loss,” Appl. Phys. Lett. (submitted to).
  16. J. L. Hesler and A. W. Lichtenberger, “THz Waveguide Couplers Using Quartz Micromachining,” in Proceedings of 21st International Symposium on Space Terahertz Technology (Oxford, UK, 2010), pp. 358–359.
  17. M. A. Duguay, Y. Kokubun, T. L. Koch, and L. Pfeiffer, “Antiresonant reflecting optical waveguides in SiO2–Si multilayer structures,” Appl. Phys. Lett.49(1), 13–15 (1986). [CrossRef]
  18. 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]
  19. E. Nguema, D. Férachou, G. Humbert, J.-L. Auguste, and J.-M. Blondy, “Broadband terahertz transmission within the air channel of thin-wall pipe,” Opt. Lett.36(10), 1782–1784 (2011). [CrossRef] [PubMed]
  20. 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. Express19(22), 21532–21539 (2011). [CrossRef] [PubMed]
  21. M. Mann, U. Trutschel, C. Wächter, L. Leine, and F. Lederer, “Directional coupler based on an antiresonant reflecting optical waveguide,” Opt. Lett.16(11), 805–807 (1991). [CrossRef] [PubMed]
  22. M. Shamonin, M. Lohmeyer, and P. Hertel, “Directional coupler based on radiatively coupled waveguides,” Appl. Opt.36(3), 635–641 (1997). [CrossRef] [PubMed]
  23. Y. Park, D. Suh, Y. Sohn, H.-S. Chung, M. C. Paek, and M. Y. Park, “Leaky Mode Directional Coupler for Prism Coupling Interconnection,” IEEE Photon. Technol. Lett.18(1), 34–36 (2006). [CrossRef]
  24. C.-H. Lai, C.-K. Sun, and H.-C. Chang, “Terahertz antiresonant-reflecting-hollow-waveguide-based directional coupler operating at antiresonant frequencies,” Opt. Lett.36(18), 3590–3592 (2011). [CrossRef] [PubMed]
  25. J.-T. Lu, C.-H. Lai, Y.-R. Huang, Y.-C. Hsueh, Y.-J. Huang, H.-C. Chang, and C.-K. Sun, “Investigation on Mode Coupling and Bending Loss Characteristics of Terahertz Air-Core Pipe Waveguides,” in Conference on Lasers and Electro-Optics, OSA Technical Digest (CD) (Optical Society of America, 2010), paper JWA118. http://www.opticsinfobase.org/abstract.cfm?URI=CLEO-2010-JWA118 .

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.

Figures

Fig. 1 Fig. 2 Fig. 3
 
Fig. 4 Fig. 5
 

« Previous Article  |  Next Article »

OSA is a member of CrossRef.

CrossCheck Deposited