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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 17, Iss. 25 — Dec. 7, 2009
  • pp: 22571–22577
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Efficient fiber coupler for vertical silicon slot waveguides

Haishan Sun, Antao Chen, Attila Szep, and Larry R. Dalton  »View Author Affiliations


Optics Express, Vol. 17, Issue 25, pp. 22571-22577 (2009)
http://dx.doi.org/10.1364/OE.17.022571


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Abstract

A mode size converter for efficient fiber coupling to silicon slot waveguides was proposed and demonstrated. It consists of two inverted lateral tapers that extend from the two strips of the silicon slot waveguide, and an overlaid low index waveguide with expanded mode size. Parameters including taper length and taper tip width were optimized with computer simulations. Samples were fabricated with a combined electron beam and photolithography process on a silicon-on-insulator wafer. The measured coupling loss to a standard single mode optical fiber was reduced by 8 dB for TE mode and 5.2 dB for TM mode with the converter.

© 2009 OSA

1. Introduction

Silicon photonics based on the silicon-on-insulator (SOI) technology is a promising platform for the future development of integrated photonics. With SOI waveguides, it is possible to integrate different microphotonic components such as waveguides, lasers, modulators and detectors on a same silicon substrate with microelectronic circuits using CMOS (complementary metal oxide semiconductor) compatible processes. The CMOS compatibility will enable some critical applications in semiconductor industry, such as chip-scale optical interconnects or all-optical signal processing. Because of high refractive index between silicon and silicon dioxide (SiO2), silicon nanophotonic waveguides can have a cross section of several hundred nanometers and a bend radius of a few micrometers, which make high density photonic integration possible.

However, the intrinsic physical properties of silicon make it difficult to use silicon for some active optical devices, including light emission and high speed modulation. The slot waveguide configuration has emerged recently as a possible solution to these problems [1

1. V. R. Almeida, Q. Xu, C. A. Barrios, and M. Lipson, “Guiding and confining light in void nanostructure,” Opt. Lett. 29(11), 1209–1211 ( 2004). [CrossRef] [PubMed]

]. In this waveguide structure, a high percentage of light is confined in a sub-wavelength slot region with low refractive index and sandwiched by two strips (vertical slot waveguide) or slabs (horizontal slot waveguide) of a high refractive index material such as silicon. By introducing active optical materials like nanocrystal [2

2. L. Pavesi, L. Dal Negro, C. Mazzoleni, G. Franzò, and F. Priolo, “Optical gain in silicon nanocrystals,” Nature 408(6811), 440–444 ( 2000). [CrossRef] [PubMed]

], erbium doped silicon dioxide [3

3. C. A. Barrios and M. Lipson, “Electrically driven silicon resonant light emitting device based on slot-waveguide,” Opt. Express 13(25), 10092–10101 ( 2005). [CrossRef] [PubMed]

], or nonlinear optical materials [4

4. T. Baehr-Jones, M. Hochberg, G. Wang, R. Lawson, Y. Liao, P. A. Sullivan, L. Dalton, A. K. Y. Jen, and A. Scherer, “Optical modulation and detection in slotted Silicon waveguides,” Opt. Express 13(14), 5216–5226 ( 2005). [CrossRef] [PubMed]

] into the slot region, hybrid silicon lasers, amplifiers, detectors and modulators can be constructed. Kerr nonlinearity in the incorporated materials is amplified due to much higher optical power density in the slot than inside a conventional strip or ridge waveguide, making it possible to realize ultra-fast all-optical computation at the power level of typical telecom laser diodes [5

5. C. Koos, P. Vorreau, T. Vallaitis, P. Dumon, W. Bogaerts, R. Baets, B. Esembeson, I. Biaggio, T. Michinobu, F. Diederich, W. Freude, and J. Leuthold, “All-optical high-speed signal processing with silicon-organic hybrid slot waveguides,” Nat. Photonics 3(4), 216–219 ( 2009). [CrossRef]

]. Strong optical confinement in the low refractive index medium also makes the slot waveguide an attractive technology for optical sensors [6

6. C. A. Barrios, M. J. Bañuls, V. González-Pedro, K. B. Gylfason, B. Sánchez, A. Griol, A. Maquieira, H. Sohlström, M. Holgado, and R. Casquel, “Label-free optical biosensing with slot-waveguides,” Opt. Lett. 33(7), 708–710 ( 2008). [CrossRef] [PubMed]

-7

7. H. Sun, A. Chen, and L. R. Dalton, “Enhanced evanescent confinement in multiple-slot waveguides and its application in biochemical sensing,” IEEE Photon. J. 1(1), 48–57 ( 2009). [CrossRef]

].

Compared with the standard silicon strip waveguide, mode conversion of the slot waveguide poses some unique challenges. The main peak of the mode in the slot region is as narrow as several tens of nanometers. The shape of the fundamental mode of a slot waveguide does not have the smooth Gaussion-shaped single-peak distribution of a single mode optical fiber. Large mode size mismatch leads to high coupling loss in and out of the silicon slot waveguides, which might prevent the practical applications of this technology. For optical modulators with electro-optical materials in the slot, the two silicon strips need to be electrically isolated. Two types of fiber coupling techniques have been applied in conventional silicon waveguides: inverted tapers for butt coupling [8

8. T. Shoji, T. Tsuchizawa, T. Watanabe, K. Yamada, and H. Morita, “Low loss mode size converter from 0.3 μm square Si wire waveguides to singlemode fibres,” Electron. Lett. 38(25), 1669–1670 ( 2002). [CrossRef]

10

10. S. McNab, N. Moll, and Y. Vlasov, “Ultra-low loss photonic integrated circuit with membrane-type photonic crystal waveguides,” Opt. Express 11(22), 2927–2939 ( 2003). [CrossRef] [PubMed]

] and diffraction gratings for out-of-plane coupling [11

11. L. Zimmermann, T. Tekin, H. Schroeder, P. Dumon, and W. Bogaerts, “How to bring Nanphotonics to application – Silicon Phontonics packaging,” IEEE LEOS Newsletter 22, 4–14 ( 2008).

-12

12. B. Analui, D. Guckenberger, D. Kucharski, and A. Narasimha, “A fully integrated 20-Gb/s optoelectronic transceiver implemented in a standard 0.13-μm CMOS SOI technology,” IEEE J. Solid-state Circuits 41, 2945–2955 ( 2006). [CrossRef]

]. Compared with grating couplers, taper couplers have the advantages of being insensitive to optical wavelength, low polarization dependence and possibility of passive fiber alignment with V-groove structure. Similar fiber couplers have been proposed for horizontal slot waveguides [13

13. J. V. Galan, P. Sanchis, J. Blasco, A. Martinez, and J. Marti, “High efficiency fiber coupling to silicon sandwiched slot waveguides,” Opt. Commun. 281(20), 5173–5176 ( 2008). [CrossRef]

-14

14. J. V. Galan, P. Sanchis, J. Blasco, A. Martinez, J. Marti, J. M. Fedeli, E. Jordana, P. Gautier, and M. Perrin, “Silicon sandwiched slot waveguide grating couplers,” Electron. Lett. 45(5), 262–264 ( 2009). [CrossRef]

]. But up to now, no fiber coupling techniques for vertical slot waveguides have been reported. Several designs of efficient strip to slot waveguide couplers have been proposed and demonstrated [15

15. Z. Wang, N. Zhu, Y. Tang, L. Wosinski, D. Dai, and S. He, “Ultracompact low-loss coupler between strip and slot waveguides,” Opt. Lett. 34(10), 1498–1500 ( 2009). [CrossRef] [PubMed]

17

17. A. Saynatjoki, L. Karvonen, A. Khanna, T. Alasaarela, A. Tervonen, and S. Honkanen, “Silicon slot waveguides for nonlinear optics,” Proc. SPIE 7212, 72120T–1-9 (2009).

], but they don’t directly address the fiber coupling issue and an additional fiber coupler for the strip waveguide may be required. This letter presents design, fabrication and measurements of an efficient mode size converter for fiber coupling with vertical silicon slot waveguides.

2. Design and simulations

Optical modes of the waveguide structures were analyzed with a three dimensional (3D) mode solver based on the film mode matching method (FMM) suitable for waveguide structures of high index contrast and subwavelength geometry (FIMMWAVE, Photon Design). Two silicon strips of the silicon slot waveguide are 220 nm thick and 270 nm wide, and the width of the slot is 100 nm [Fig. 1(b)]. Refractive indices of silicon and SiO2 are 3.48 and 1.46, respectively, at the telecom wavelength of 1550 nm. We used SU8 (MicroChem) polymer as the cladding material covering the silicon slot waveguide and the core material of the low-index waveguide, which has a refractive index of 1.565. Confinement factors in the slot region of the silicon slot waveguide are calculated to be 34.6% for the transverse electric polarization (TE) and 2.7% for the transverse magnetic polarization (TM), while the confinement factors inside the silicon strips are 55.0% for TE and 21.1% for TM.

Mode mismatch losses at the waveguide interfaces were calculated with a 3D optical propagation tool using the eigenmode expansion method (EME) (FIMMPROP, Photon Design). The propagation simulation results of EME method are verified with the overlap integral of optical fields of the waveguide modes obtained from FMM mode solver. Dimensions of the SU8 polymer waveguide are 2 × 2 μm. The fundamental modes of optical fibers were estimated with Gaussian modes. Mode mismatch losses of the optical fiber with the silicon slot waveguide and the polymer waveguide are plotted as functions of the mode field diameter (MFD) of the fiber [Fig. 2(a)
Fig. 2 Simulation results of the mode size converter. (a) Mode mismatch loss from an optical fiber to the silicon slot waveguide and polymer waveguide vs. MFD of the fiber. (b) Mode mismatch loss at the taper tips vs. taper tip width. (c) Conversion efficiency along the taper section vs. taper length.
]. A minimum mode mismatch loss of 0.5 dB from the fiber to the polymer waveguide for both polarizations occurs at fiber MFD of 1.875 μm. This level of MFD is possible with a lensed/tapered fiber [18

18. T. Alder, A. Stohr, R. Heinzelmann, and D. Jager, “High-efficiency fiber-to-chip coupling using low-loss tapered single-mode fiber,” IEEE Photon. Technol. Lett. 12(8), 1016–1018 ( 2000). [CrossRef]

]. The minimum mode mismatch loss between the silicon slot waveguide and fiber, however, requires the fiber to have a MFD smaller than 1 μm (0.8 dB for TE and MFD ≈0.6 μm, and 0.6 dB for TM and MFD ≈ 0.9 μm), which is difficult to be realized with a tapered fiber. For a MFD of 10 μm, which is typical for a cleaved standard single mode fiber, the mode mismatch loss is 4.8 dB at TE and 4.9 dB at TM for the polymer waveguide at both polarizations, and it is 13.3 dB at TE and 10.4 dB at TM for the silicon slot waveguide. Additionally, lower refractive index of polymer than that of silicon also reduces reflection loss at the waveguide end face (0.18 dB than 0.36 (TM) to 0.52 dB (TE)). Thus with an ideal mode size converter, we could expect to reduce the coupling loss of the silicon slot waveguide to an optical fiber with MFD of 10 μm at most by 8.8 dB for TE and 5.8 dB for TM per coupling.

For straight tapers (the center of the tip is on the center line of the silicon strip) of 30 nm tip width, we have simulated conversion efficiency along the taper section as a function of taper length with the EME method [Fig. 2(c)]. The adiabatic limit, or the minimum length for lossless conversion, is approximately 50 μm for both TE and TM polarizations with optimal conversion efficiency of over 98% (conversion loss < 0.1 dB). Here propagation loss due to material absorption and surface scattering was not included, because it is normally very small over the relatively short length of the taper.

3. Fabrication and measurements

To characterize the transmission property of the samples, 1550 nm light from a fiber laser source, after passing through a polarization controller, was coupled to one end of the waveguides with a standard cleaved optical fiber (Corning SMF-28). The output from the other end of the waveguides was lens (magnification/numerical aperture = 20/0.5) coupled on a photo-detector (HP 81521B). A polarizer was located behind the lens to select TE or TM light. Polarization state of the input light was set by adjusting the polarization controller to maximize the output intensity of the predetermined polarization. Alignment between the fiber and polymer waveguides was achieved with a computer closed-loop alignment system and piezoelectric actuators.

After insertion losses of each waveguide is measured, the sample was cleaved again to remove the mode size converters at the input side to measure insertion losses when fiber is directly coupled to the silicon slot waveguides. The same lens out-coupling was used for both measurements. Insertion losses of waveguides with and without input converters are presented in Fig. 4
Fig. 4 Measured insertion loss of silicon slot waveguides with and without input taper couplers. (a) is for TE and (b) is for TM polarization.
. The slope and y-axis intercept of linear fitting of insertion loss as function of waveguide length gives the waveguide propagation loss and total coupling loss (of both input and output ends), respectively. The propagation loss of the silicon slot waveguide is 14.3 dB/cm for TE and 7.8 dB/cm for TM. Insertion loss contributed by a single converter is about 4.6 dB for both TE and TM, whereas fiber coupling loss to the silicon slot waveguide is 12.6 dB for TE and 9.9 dB for TM. The power gain is 8 dB for TE and 5.2 dB for TM by using the mode size converter, which is in good agreement with the simulations. Here we have neglected the loss from the 90° bend, which is much smaller than the coupling loss. We also assumed that the lens could capture all the output light from the waveguides, i.e. output coupling is lossless. Such assumptions make the above figures of coupling losses to be more conservative, and don’t affect the power gain results. Although the data presented in this paper is based on slot waveguide of 100nm slot and 270 nm ridges, slot waveguide of different slot width (from 100 to 130 nm) and silicon ridge width (from 220 to 270 nm) were also fabricated and tested and their results showed the same level of improvement in fiber coupling within measurement error. The results suggest that coupling design is not sensitive to slot waveguide geometries as long as the taper provides a smooth mode transition.

4. Conclusion

We have developed a mode size converter for low loss coupling of silicon slot waveguides to optical fibers. Simulations based on FMM and EME algorithms at the 1550 nm wavelength suggest that a tapers width smaller than 30 nm and taper length larger than 50 μm could confine the total conversion loss below 0.2 dB. To accommodate lower resolution lithography process, 100 nm or wider taper tip could be used if the two taper tips have larger spacing. Samples with optimized design were fabricated on a SOI wafer, and computer simulation results were experimentally confirmed. This work provides a practical and efficient solution for fiber coupling of future devices based on vertical silicon slot waveguides.

Acknowledgements

This work is supported by NSF Center on Materials and Devices for Information Technology Research (CMDITR), Grant Number DMR-0120967, and Air Force Office of Scientific Research, Grant Number FA9550-08-0101. This work was performed in part at the Cornell Nanoscale Facility, a member of the National Nanotechnology Infrastructure Network, which is supported by the National Science Foundation.

References and links

1.

V. R. Almeida, Q. Xu, C. A. Barrios, and M. Lipson, “Guiding and confining light in void nanostructure,” Opt. Lett. 29(11), 1209–1211 ( 2004). [CrossRef] [PubMed]

2.

L. Pavesi, L. Dal Negro, C. Mazzoleni, G. Franzò, and F. Priolo, “Optical gain in silicon nanocrystals,” Nature 408(6811), 440–444 ( 2000). [CrossRef] [PubMed]

3.

C. A. Barrios and M. Lipson, “Electrically driven silicon resonant light emitting device based on slot-waveguide,” Opt. Express 13(25), 10092–10101 ( 2005). [CrossRef] [PubMed]

4.

T. Baehr-Jones, M. Hochberg, G. Wang, R. Lawson, Y. Liao, P. A. Sullivan, L. Dalton, A. K. Y. Jen, and A. Scherer, “Optical modulation and detection in slotted Silicon waveguides,” Opt. Express 13(14), 5216–5226 ( 2005). [CrossRef] [PubMed]

5.

C. Koos, P. Vorreau, T. Vallaitis, P. Dumon, W. Bogaerts, R. Baets, B. Esembeson, I. Biaggio, T. Michinobu, F. Diederich, W. Freude, and J. Leuthold, “All-optical high-speed signal processing with silicon-organic hybrid slot waveguides,” Nat. Photonics 3(4), 216–219 ( 2009). [CrossRef]

6.

C. A. Barrios, M. J. Bañuls, V. González-Pedro, K. B. Gylfason, B. Sánchez, A. Griol, A. Maquieira, H. Sohlström, M. Holgado, and R. Casquel, “Label-free optical biosensing with slot-waveguides,” Opt. Lett. 33(7), 708–710 ( 2008). [CrossRef] [PubMed]

7.

H. Sun, A. Chen, and L. R. Dalton, “Enhanced evanescent confinement in multiple-slot waveguides and its application in biochemical sensing,” IEEE Photon. J. 1(1), 48–57 ( 2009). [CrossRef]

8.

T. Shoji, T. Tsuchizawa, T. Watanabe, K. Yamada, and H. Morita, “Low loss mode size converter from 0.3 μm square Si wire waveguides to singlemode fibres,” Electron. Lett. 38(25), 1669–1670 ( 2002). [CrossRef]

9.

V. R. Almeida, R. R. Panepucci, and M. Lipson, “Nanotaper for compact mode conversion,” Opt. Lett. 28(15), 1302–1304 ( 2003). [CrossRef] [PubMed]

10.

S. McNab, N. Moll, and Y. Vlasov, “Ultra-low loss photonic integrated circuit with membrane-type photonic crystal waveguides,” Opt. Express 11(22), 2927–2939 ( 2003). [CrossRef] [PubMed]

11.

L. Zimmermann, T. Tekin, H. Schroeder, P. Dumon, and W. Bogaerts, “How to bring Nanphotonics to application – Silicon Phontonics packaging,” IEEE LEOS Newsletter 22, 4–14 ( 2008).

12.

B. Analui, D. Guckenberger, D. Kucharski, and A. Narasimha, “A fully integrated 20-Gb/s optoelectronic transceiver implemented in a standard 0.13-μm CMOS SOI technology,” IEEE J. Solid-state Circuits 41, 2945–2955 ( 2006). [CrossRef]

13.

J. V. Galan, P. Sanchis, J. Blasco, A. Martinez, and J. Marti, “High efficiency fiber coupling to silicon sandwiched slot waveguides,” Opt. Commun. 281(20), 5173–5176 ( 2008). [CrossRef]

14.

J. V. Galan, P. Sanchis, J. Blasco, A. Martinez, J. Marti, J. M. Fedeli, E. Jordana, P. Gautier, and M. Perrin, “Silicon sandwiched slot waveguide grating couplers,” Electron. Lett. 45(5), 262–264 ( 2009). [CrossRef]

15.

Z. Wang, N. Zhu, Y. Tang, L. Wosinski, D. Dai, and S. He, “Ultracompact low-loss coupler between strip and slot waveguides,” Opt. Lett. 34(10), 1498–1500 ( 2009). [CrossRef] [PubMed]

16.

N.-N. Feng, R. Sun, L. C. Kimerling, and J. Michel, “Lossless strip-to-slot waveguide transformer,” Opt. Lett. 32(10), 1250–1252 ( 2007). [CrossRef] [PubMed]

17.

A. Saynatjoki, L. Karvonen, A. Khanna, T. Alasaarela, A. Tervonen, and S. Honkanen, “Silicon slot waveguides for nonlinear optics,” Proc. SPIE 7212, 72120T–1-9 (2009).

18.

T. Alder, A. Stohr, R. Heinzelmann, and D. Jager, “High-efficiency fiber-to-chip coupling using low-loss tapered single-mode fiber,” IEEE Photon. Technol. Lett. 12(8), 1016–1018 ( 2000). [CrossRef]

OCIS Codes
(130.0130) Integrated optics : Integrated optics
(230.7370) Optical devices : Waveguides
(350.4238) Other areas of optics : Nanophotonics and photonic crystals

ToC Category:
Integrated Optics

History
Original Manuscript: September 22, 2009
Revised Manuscript: November 16, 2009
Manuscript Accepted: November 17, 2009
Published: November 24, 2009

Citation
Haishan Sun, Antao Chen, Attila Szep, and Larry R. Dalton, "Efficient fiber coupler for vertical silicon slot waveguides," Opt. Express 17, 22571-22577 (2009)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-25-22571


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References

  1. V. R. Almeida, Q. Xu, C. A. Barrios, and M. Lipson, “Guiding and confining light in void nanostructure,” Opt. Lett. 29(11), 1209–1211 (2004). [CrossRef] [PubMed]
  2. L. Pavesi, L. Dal Negro, C. Mazzoleni, G. Franzò, and F. Priolo, “Optical gain in silicon nanocrystals,” Nature 408(6811), 440–444 (2000). [CrossRef] [PubMed]
  3. C. A. Barrios and M. Lipson, “Electrically driven silicon resonant light emitting device based on slot-waveguide,” Opt. Express 13(25), 10092–10101 (2005). [CrossRef] [PubMed]
  4. T. Baehr-Jones, M. Hochberg, G. Wang, R. Lawson, Y. Liao, P. A. Sullivan, L. Dalton, A. K. Y. Jen, and A. Scherer, “Optical modulation and detection in slotted Silicon waveguides,” Opt. Express 13(14), 5216–5226 (2005). [CrossRef] [PubMed]
  5. C. Koos, P. Vorreau, T. Vallaitis, P. Dumon, W. Bogaerts, R. Baets, B. Esembeson, I. Biaggio, T. Michinobu, F. Diederich, W. Freude, and J. Leuthold, “All-optical high-speed signal processing with silicon-organic hybrid slot waveguides,” Nat. Photonics 3(4), 216–219 (2009). [CrossRef]
  6. C. A. Barrios, M. J. Bañuls, V. González-Pedro, K. B. Gylfason, B. Sánchez, A. Griol, A. Maquieira, H. Sohlström, M. Holgado, and R. Casquel, “Label-free optical biosensing with slot-waveguides,” Opt. Lett. 33(7), 708–710 (2008). [CrossRef] [PubMed]
  7. H. Sun, A. Chen, and L. R. Dalton, “Enhanced evanescent confinement in multiple-slot waveguides and its application in biochemical sensing,” IEEE Photon. J. 1(1), 48–57 (2009). [CrossRef]
  8. T. Shoji, T. Tsuchizawa, T. Watanabe, K. Yamada, and H. Morita, “Low loss mode size converter from 0.3 μm square Si wire waveguides to singlemode fibres,” Electron. Lett. 38(25), 1669–1670 (2002). [CrossRef]
  9. V. R. Almeida, R. R. Panepucci, and M. Lipson, “Nanotaper for compact mode conversion,” Opt. Lett. 28(15), 1302–1304 (2003). [CrossRef] [PubMed]
  10. S. McNab, N. Moll, and Y. Vlasov, “Ultra-low loss photonic integrated circuit with membrane-type photonic crystal waveguides,” Opt. Express 11(22), 2927–2939 (2003). [CrossRef] [PubMed]
  11. L. Zimmermann, T. Tekin, H. Schroeder, P. Dumon, and W. Bogaerts, “How to bring Nanphotonics to application – Silicon Phontonics packaging,” IEEE LEOS Newsletter 22, 4–14 (2008).
  12. B. Analui, D. Guckenberger, D. Kucharski, and A. Narasimha, “A fully integrated 20-Gb/s optoelectronic transceiver implemented in a standard 0.13-μm CMOS SOI technology,” IEEE J. Solid-state Circuits 41, 2945–2955 (2006). [CrossRef]
  13. J. V. Galan, P. Sanchis, J. Blasco, A. Martinez, and J. Marti, “High efficiency fiber coupling to silicon sandwiched slot waveguides,” Opt. Commun. 281(20), 5173–5176 (2008). [CrossRef]
  14. J. V. Galan, P. Sanchis, J. Blasco, A. Martinez, J. Marti, J. M. Fedeli, E. Jordana, P. Gautier, and M. Perrin, “Silicon sandwiched slot waveguide grating couplers,” Electron. Lett. 45(5), 262–264 (2009). [CrossRef]
  15. Z. Wang, N. Zhu, Y. Tang, L. Wosinski, D. Dai, and S. He, “Ultracompact low-loss coupler between strip and slot waveguides,” Opt. Lett. 34(10), 1498–1500 (2009). [CrossRef] [PubMed]
  16. N.-N. Feng, R. Sun, L. C. Kimerling, and J. Michel, “Lossless strip-to-slot waveguide transformer,” Opt. Lett. 32(10), 1250–1252 (2007). [CrossRef] [PubMed]
  17. A. Saynatjoki, L. Karvonen, A. Khanna, T. Alasaarela, A. Tervonen, and S. Honkanen, “Silicon slot waveguides for nonlinear optics,” Proc. SPIE 7212, 72120T–1-9 (2009).
  18. T. Alder, A. Stohr, R. Heinzelmann, and D. Jager, “High-efficiency fiber-to-chip coupling using low-loss tapered single-mode fiber,” IEEE Photon. Technol. Lett. 12(8), 1016–1018 (2000). [CrossRef]

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