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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 16 — Jul. 30, 2012
  • pp: 18248–18253
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An ultracompact multimode interference wavelength splitter employing asymmetrical multi-section structures

Chen Yao, Heinz-Gunter Bach, Ruiyong Zhang, Gan Zhou, Jung Han Choi, Chenhui Jiang, and Reinhard Kunkel  »View Author Affiliations


Optics Express, Vol. 20, Issue 16, pp. 18248-18253 (2012)
http://dx.doi.org/10.1364/OE.20.018248


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Abstract

By using the film mode matching method, a novel design for asymmetrical multi-section 1.55/1.31 μm wavelength splitter based on multimode interference has been proposed and simulated, which can be effectively applied to wavelength multiplexer, self-biased photodiode, and other optical devices. Compared with the conventional wavelength splitter design, the length of the novel structure has been reduced to at least 1/5, showing better performance. The presented structure is also adequate for splitting other wavelengths and more tolerable fabrications.

© 2012 OSA

1. Introduction

Optical wavelength splitters are significant components in modern optical communication, which can increase the number of channels and the information capacity of optical fibers. Moreover, they can be applied to many devices such as demultiplexers [1

1. Y. Shi, S. Anand, and S. He, “A polarization-insensitive 1310/1550-nm demultiplexer based on sandwiched multimode interference waveguides,” IEEE Photon. Technol. Lett. 19(22), 1789–1791 (2007). [CrossRef]

, 2

2. W.-Y. Chiu, J.-W. Shi, Y.-S. Wu, F.-H. Huang, W. Lin, and Y. –J. Chan, “The Monolithic Integration of a wavelength-demultiplexer with evanescently coupled uni-traveling-carrier photodiodes,” IEEE Photon. Technol. Lett. 18(11), 1246–1248 (2006).

] and self-biased photodiodes [3

3. H.-G. Bach, R. Kunkel, G. G. Mekonnen, D. Pech, T. Rosin, D. Schmidt, T. Gaertner, and R. Zhang, “Integration potential of waveguide-integrated photodiodes: self-powered photodetectors and sub-THz pin-Antennas,” in Optical Fiber Communication Conference (OFC, 2008), OMK1.

, 4

4. H.-G. Bach, “Monolithically integrated optoelectronic subassembly,” US Patent 2009/0202197 A1.

] to reduce the dimension greatly and improve their performance significantly.

Several approaches have been proposed to realize the wavelength splitter, such as Y branch devices [5

5. N. Goto and G. L. Yip, “Y-branch wavelength multi-demultiplexer for λ=1.30 μm and 1.55 μm,” Electron. Lett. 26(2), 102–103 (1990). [CrossRef]

], Mach-Zehnder interferometers [6

6. A. Tervonen, P. Poyhonen, S. Honkanen, and M. Tahkokorpi, “A guided-wave Mach-Zehnder interferometer structure for wavelength multiplexing,” IEEE Photon. Technol. Lett. 3(6), 516–518 (1991). [CrossRef]

] and multimode interference (MMI) devices [7

7. J. Xiao, X. Liu, and X. Sun, “Design of an ultracompact MMI wavelength demultiplexer in slot waveguide structures,” Opt. Express 15(13), 8300–8308 (2007). [CrossRef] [PubMed]

]. Among them, MMI devices based on the self-imaging [8

8. L. B. Soldano and E. C. M. Pennings, “Optical multi-mode interference devices based on self-imaging: Principles and applications,” J. Lightwave Technol. 13(4), 615–627 (1995). [CrossRef]

] are good candidates due to their excellent advantages, such as ease of fabrication, low excess loss, compact size, and large fabrication tolerances [1

1. Y. Shi, S. Anand, and S. He, “A polarization-insensitive 1310/1550-nm demultiplexer based on sandwiched multimode interference waveguides,” IEEE Photon. Technol. Lett. 19(22), 1789–1791 (2007). [CrossRef]

].

However, when utilized as wavelength splitters, MMI based splitters still have the problem in their extensive length [6

6. A. Tervonen, P. Poyhonen, S. Honkanen, and M. Tahkokorpi, “A guided-wave Mach-Zehnder interferometer structure for wavelength multiplexing,” IEEE Photon. Technol. Lett. 3(6), 516–518 (1991). [CrossRef]

] because the length of devices needs to match several odd or even times of beat lengths of both wavelengths. This situation is aggravated in the InP/InGaAsP material system owing to small refractive index difference. In this work, a multi-section MMI 1.55/1.31 μm wavelength splitter concept has been demonstrated and simulated. The results show excellent characteristics and size reduction of at least 80% compared to the length of the common splitter design. In addition, the novel concept is also suitable to design splitters operating in other wavelengths and imply more tolerable fabrications.

2. Principle and device designs

The cross section of the proposed waveguide is composed of multi-layer staggered InGaAsP and InP layers with 1045 nm deep etching depth in the ridge waveguide as shown in Fig. 1(a)
Fig. 1 (a) Cross section of the ridge waveguide (b) Conventional MMI wavelength splitter (c) Simulation results of field distribution of the conventional MMI coupler at 1.55 μm.
. The waveguide structure studied here has been successfully applied to other devices, like 90° hybrid [9

9. R.-Y. Zhang, K. Janiak, H.-G. Bach, R. Kunkel, A. Seeger, S. Schubert, and M. Schell, “Performance of InP-based 90 °-hybrids QPSK receivers within C-Band,” in International Conference on Indium Phosphide and Related Materials (IPRM, 2011).

], and high speed photodiode [3

3. H.-G. Bach, R. Kunkel, G. G. Mekonnen, D. Pech, T. Rosin, D. Schmidt, T. Gaertner, and R. Zhang, “Integration potential of waveguide-integrated photodiodes: self-powered photodetectors and sub-THz pin-Antennas,” in Optical Fiber Communication Conference (OFC, 2008), OMK1.

, 10

10. A. Beling, J. C. Campbell, H.-G. Bach, G. G. Mekonnen, and D. Schmidt, “Parallel-fed traveling wave photodetector for >100-GHz Applications,” J. Lightwave Technol. 26(1), 16–20 (2008). [CrossRef]

, 11

11. A. Beling, H.-G. Bach, G. G. Mekonnen, R. Kunkel, and D. Schmidt, “High-speed miniaturized photodiode and parallel-fed traveling-wave photodetectors based on InP,” IEEE J. Sel. Top. Quantum Electron. 13(1), 15–21 (2007). [CrossRef]

]. Three Q1.06 layers on the bottom of the waveguide are utilized as a mode field convertor when the waveguide is integrated with other devices. Detailed description about the function of each layer can be found in [11

11. A. Beling, H.-G. Bach, G. G. Mekonnen, R. Kunkel, and D. Schmidt, “High-speed miniaturized photodiode and parallel-fed traveling-wave photodetectors based on InP,” IEEE J. Sel. Top. Quantum Electron. 13(1), 15–21 (2007). [CrossRef]

]. According to the MMI theory [1

1. Y. Shi, S. Anand, and S. He, “A polarization-insensitive 1310/1550-nm demultiplexer based on sandwiched multimode interference waveguides,” IEEE Photon. Technol. Lett. 19(22), 1789–1791 (2007). [CrossRef]

], in order to divide two different wavelengths, for example, 1.31 μm and 1.55 μm, the length and width of the MMI should be designed to satisfy:
LMMI=nLπ(1.31)=(n+1)Lπ(1.55)
(1)
where n is an integer, and Lπ is the beat length of the lowest two modes guided by the MMI waveguide. For a 9 μm wide MMI waveguide, the corresponding length of MMI will be much longer than 5000 μm due to low refractive index difference in the InP/InGaAsP material system. Obviously such device occupies too large space of InP wafer and requires extensive care in the fabrication tolerances.

Figure 1(c) shows simulation results of the field distribution of conventional MMI coupler at 1.55 μm. According to the MMI theory [8

8. L. B. Soldano and E. C. M. Pennings, “Optical multi-mode interference devices based on self-imaging: Principles and applications,” J. Lightwave Technol. 13(4), 615–627 (1995). [CrossRef]

], the optical fields in the MMI waveguide alternately and periodically moves and are redistributed as plotted in Fig. 1(c) due to phase of guided modes changing. Therefore some certain lengths must exist where most of the power distributes in the opposite side to self-imaging point as shown in the “red circle region” in Fig. 2(a)
Fig. 2 (a) An asymmetrical multi-section MMI wavelength splitter with section indications: (1) divider MMI, (2) collector MMI, (3) sub-collector MMI; simulation results of field distribution of asymmetrical multi-section MMI wavelength splitter: (b) at 1.55 μm, (c) at 1.31 μm without sub-collection MMI, (d) at 1.31 μm with sub-collection MMI.
. Therefore one can design a novel wavelength splitter with appropriate width and length to maintain 1.55 μm wavelength focusing to the self-imaging point at cross output while most power of 1.31 μm light is propagating to bar port side.

The proposed asymmetrical multi-section MMI wavelength splitter is schematically shown in Fig. 2(a). From the input side, the first multimode waveguide is named after divider MMI (1) as indicated in Fig. 2(a). It is utilized to divide two lights with 1.55 μm and 1.31 μm wavelengths. The one with 1.55 μm is guided to the cross output, and most power of the one with 1.31 μm is guided to the bar output side. Then a narrower multimode waveguide is named after collection MMI (2). It serves as another MMI coupler and is located at the bar output side to collect efficiently 1.31 μm power. In order to improve the output power, another narrower multimode waveguide which is named after sub-collection MMI (3) follows to former multimode waveguide (2).

3. Results and discussion

Firstly, 9 μm wide multimode waveguide is chosen to validate both compact dimension and suitable length for splitting two wavelengths. As shown in Fig. 1(c), the first self-imaging point is located between 690um and 720um. Then the length of divider MMI can be obtained exactly as shown in Fig. 3
Fig. 3 The normalized power output of 1.55 μm and 1.31 μm varies with Divider MMI length.
. 707 μm is the optimal length to obtain maximum optical power at 1.55 μm. As illustrated in Fig. 2(c), only one 5 μm wide collection MMI is implemented to collect 1.31 μm at the first time with the motivation of shortening total device length and leaving 2 μm gap as depicted in Fig. 1(c) to avoid coupling loss. Unfortunately, no matter how long the collection MMI is selected, about 40% of 1.31 μm optical power is lost. A limited improvement can be realized using longer divider MMI (712.5 μm) at expense of increasing insertion loss of 1.55 μm by about 0.4 dB as shown in Fig. 3 and 4
Fig. 4 The normalized power output of 1.31 μm (707 μm and 712.5 μm for divider MMI) varies with collection MMI length. The Inset shows the field distribution of the collection MMI at output position.
. Because the input power is not concentrated on one single point as shown in Fig. 2(c) and 2(d), the power at the output of collection-MMI can’t form a perfect self-imaging point, leading relative high power loss of 1.31 μm due to mode mismatching between broader collection MMI and narrow output waveguide as shown in the inset of Fig. 4. Thus an additional narrower multimode waveguide, namely, 4 μm wide sub-collection MMI is employed to achieve a better coupling. Actually, the whole component exhibits nearly 10% higher power for 1.31 μm with a sub-collection MMI integrated as depicted in Fig. 3 and Fig. 4.

The performances of the wavelength splitter are characterized by the extinction ratio (ER) and the insertion loss (IL), which are defined as
ER=10log(Pd/Pu)
(2)
IL=10log(Pd/Pi)
(3)
where Pd, and Pu denote the intensities in the desired and undesirable output waveguides at 1.31 μm, respectively, the intensities in the bar and cross waveguides at 1.55 μm, and Pi is the input optical power. Extinction ratios of this proposed design are 22.4 dB at 1.55 μm and 12 dB at 1.31 μm, insertion losses are −0.62 dB at 1.55 μm and −1.46 dB at 1.31 μm respectively. The total length of the whole multi-section MMI device including three sections is only 963.5 μm as shown in Fig. 2(d).

For the conventional wavelength splitter structure, the optimum length is verified to exceed 5000 μm according to the simulation which is not shown here. The insertion loss is larger than 2 dB for both wavelengths when the device length is 5000 μm. So it makes no sense to optimize and calculate the exact length of conventional design. Therefore, the doubtless superiority of the novel design presented is not only its compact dimension whose length has been reduced to at least 1/5, but also lower insertion loss.

The fabrication tolerance of collection MMI width has been calculated as shown in Fig. 5
Fig. 5 Extinction ratio and insertion loss of 1.31 μm as functions of the width of collection MMI. Both performances have been normalized by choosing the values of 5 μm as 0 dB.
. With a requirement of 1dB, the fabrication tolerance of the collection MMI width is about ± 0.5 μm. The tolerances of the other sections widths are similar, which satisfy the common fabrication requirement [12

12. R. Kunkel, H.-G. Bach, D. Hoffmann, G. G. Mekonnen, R. Zhang, D. Schmidt, and M. Schell, “Athermal InP-based 90°-hybrid Rx OEICs with pin-PDs >60 GHz for coherent DP-QPSK photoreceivers,” in International Conference on Indium Phosphide and Related Materials (IPRM, 2010).

]. Therefore the novel structure can be fabricated easily by just one step photolithography following the normal process. Besides, the wavelength splitters for other wavelengths can be developed conveniently by the same method, which shows the suitability and compatibility.

4. Conclusion

This paper proposed the novel ultra compact asymmetrical multi-section MMI wavelength splitter. In order to overcome the huge size and high insertion losses of the conventional MMI splitter, film mode matching method was utilized. Simulation results showed the insertion losses of −0.62dB and −1.46dB at 1.55 μm and 1.31μm, respectively. The size of the proposed splitter was 963.5μm, which was regarded as 80% reduction in size compared to conventional MMI splitter. It should be noted that this design methodology also applies to other wavelength MMI structures. We believe that the proposed structure is a remarkably attractive one with respect to low cost and ultra compact optical devices, such as wavelength demultiplexers, self biased photodiode, and so on.

Acknowledgments

The authors gratefully thank Prof. Jian Wu from Beijing University of Posts and Telecommunications for his continuous encouragement and help. This work was financed partly by the 100GET program of the German Federal Ministry of Education and Research, the EC’s 100GET/CELTIC project, and EC’s project MIRTHE within the FP7 program.

References and links

1.

Y. Shi, S. Anand, and S. He, “A polarization-insensitive 1310/1550-nm demultiplexer based on sandwiched multimode interference waveguides,” IEEE Photon. Technol. Lett. 19(22), 1789–1791 (2007). [CrossRef]

2.

W.-Y. Chiu, J.-W. Shi, Y.-S. Wu, F.-H. Huang, W. Lin, and Y. –J. Chan, “The Monolithic Integration of a wavelength-demultiplexer with evanescently coupled uni-traveling-carrier photodiodes,” IEEE Photon. Technol. Lett. 18(11), 1246–1248 (2006).

3.

H.-G. Bach, R. Kunkel, G. G. Mekonnen, D. Pech, T. Rosin, D. Schmidt, T. Gaertner, and R. Zhang, “Integration potential of waveguide-integrated photodiodes: self-powered photodetectors and sub-THz pin-Antennas,” in Optical Fiber Communication Conference (OFC, 2008), OMK1.

4.

H.-G. Bach, “Monolithically integrated optoelectronic subassembly,” US Patent 2009/0202197 A1.

5.

N. Goto and G. L. Yip, “Y-branch wavelength multi-demultiplexer for λ=1.30 μm and 1.55 μm,” Electron. Lett. 26(2), 102–103 (1990). [CrossRef]

6.

A. Tervonen, P. Poyhonen, S. Honkanen, and M. Tahkokorpi, “A guided-wave Mach-Zehnder interferometer structure for wavelength multiplexing,” IEEE Photon. Technol. Lett. 3(6), 516–518 (1991). [CrossRef]

7.

J. Xiao, X. Liu, and X. Sun, “Design of an ultracompact MMI wavelength demultiplexer in slot waveguide structures,” Opt. Express 15(13), 8300–8308 (2007). [CrossRef] [PubMed]

8.

L. B. Soldano and E. C. M. Pennings, “Optical multi-mode interference devices based on self-imaging: Principles and applications,” J. Lightwave Technol. 13(4), 615–627 (1995). [CrossRef]

9.

R.-Y. Zhang, K. Janiak, H.-G. Bach, R. Kunkel, A. Seeger, S. Schubert, and M. Schell, “Performance of InP-based 90 °-hybrids QPSK receivers within C-Band,” in International Conference on Indium Phosphide and Related Materials (IPRM, 2011).

10.

A. Beling, J. C. Campbell, H.-G. Bach, G. G. Mekonnen, and D. Schmidt, “Parallel-fed traveling wave photodetector for >100-GHz Applications,” J. Lightwave Technol. 26(1), 16–20 (2008). [CrossRef]

11.

A. Beling, H.-G. Bach, G. G. Mekonnen, R. Kunkel, and D. Schmidt, “High-speed miniaturized photodiode and parallel-fed traveling-wave photodetectors based on InP,” IEEE J. Sel. Top. Quantum Electron. 13(1), 15–21 (2007). [CrossRef]

12.

R. Kunkel, H.-G. Bach, D. Hoffmann, G. G. Mekonnen, R. Zhang, D. Schmidt, and M. Schell, “Athermal InP-based 90°-hybrid Rx OEICs with pin-PDs >60 GHz for coherent DP-QPSK photoreceivers,” in International Conference on Indium Phosphide and Related Materials (IPRM, 2010).

OCIS Codes
(230.0250) Optical devices : Optoelectronics
(230.3120) Optical devices : Integrated optics devices
(230.5170) Optical devices : Photodiodes
(230.7390) Optical devices : Waveguides, planar

ToC Category:
Optical Devices

History
Original Manuscript: May 7, 2012
Revised Manuscript: June 24, 2012
Manuscript Accepted: July 5, 2012
Published: July 25, 2012

Citation
Chen Yao, Heinz-Gunter Bach, Ruiyong Zhang, Gan Zhou, Jung Han Choi, Chenhui Jiang, and Reinhard Kunkel, "An ultracompact multimode interference wavelength splitter employing asymmetrical multi-section structures," Opt. Express 20, 18248-18253 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-16-18248


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References

  1. Y. Shi, S. Anand, and S. He, “A polarization-insensitive 1310/1550-nm demultiplexer based on sandwiched multimode interference waveguides,” IEEE Photon. Technol. Lett.19(22), 1789–1791 (2007). [CrossRef]
  2. W.-Y. Chiu, J.-W. Shi, Y.-S. Wu, F.-H. Huang, W. Lin, and Y. –J. Chan, “The Monolithic Integration of a wavelength-demultiplexer with evanescently coupled uni-traveling-carrier photodiodes,” IEEE Photon. Technol. Lett.18(11), 1246–1248 (2006).
  3. H.-G. Bach, R. Kunkel, G. G. Mekonnen, D. Pech, T. Rosin, D. Schmidt, T. Gaertner, and R. Zhang, “Integration potential of waveguide-integrated photodiodes: self-powered photodetectors and sub-THz pin-Antennas,” in Optical Fiber Communication Conference (OFC, 2008), OMK1.
  4. H.-G. Bach, “Monolithically integrated optoelectronic subassembly,” US Patent 2009/0202197 A1.
  5. N. Goto and G. L. Yip, “Y-branch wavelength multi-demultiplexer for λ=1.30 μm and 1.55 μm,” Electron. Lett.26(2), 102–103 (1990). [CrossRef]
  6. A. Tervonen, P. Poyhonen, S. Honkanen, and M. Tahkokorpi, “A guided-wave Mach-Zehnder interferometer structure for wavelength multiplexing,” IEEE Photon. Technol. Lett.3(6), 516–518 (1991). [CrossRef]
  7. J. Xiao, X. Liu, and X. Sun, “Design of an ultracompact MMI wavelength demultiplexer in slot waveguide structures,” Opt. Express15(13), 8300–8308 (2007). [CrossRef] [PubMed]
  8. L. B. Soldano and E. C. M. Pennings, “Optical multi-mode interference devices based on self-imaging: Principles and applications,” J. Lightwave Technol.13(4), 615–627 (1995). [CrossRef]
  9. R.-Y. Zhang, K. Janiak, H.-G. Bach, R. Kunkel, A. Seeger, S. Schubert, and M. Schell, “Performance of InP-based 90 °-hybrids QPSK receivers within C-Band,” in International Conference on Indium Phosphide and Related Materials (IPRM, 2011).
  10. A. Beling, J. C. Campbell, H.-G. Bach, G. G. Mekonnen, and D. Schmidt, “Parallel-fed traveling wave photodetector for >100-GHz Applications,” J. Lightwave Technol.26(1), 16–20 (2008). [CrossRef]
  11. A. Beling, H.-G. Bach, G. G. Mekonnen, R. Kunkel, and D. Schmidt, “High-speed miniaturized photodiode and parallel-fed traveling-wave photodetectors based on InP,” IEEE J. Sel. Top. Quantum Electron.13(1), 15–21 (2007). [CrossRef]
  12. R. Kunkel, H.-G. Bach, D. Hoffmann, G. G. Mekonnen, R. Zhang, D. Schmidt, and M. Schell, “Athermal InP-based 90°-hybrid Rx OEICs with pin-PDs >60 GHz for coherent DP-QPSK photoreceivers,” in International Conference on Indium Phosphide and Related Materials (IPRM, 2010).

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