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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 6 — Mar. 25, 2013
  • pp: 7033–7040
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Wavelength independent multimode interference coupler

A. Maese-Novo, R. Halir, S. Romero-García, D. Pérez-Galacho, L. Zavargo-Peche, A. Ortega-Moñux, I. Molina-Fernández, J. G. Wangüemert-Pérez, and P. Cheben  »View Author Affiliations


Optics Express, Vol. 21, Issue 6, pp. 7033-7040 (2013)
http://dx.doi.org/10.1364/OE.21.007033


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Abstract

We propose an ultra-broadband multimode interference (MMI) coupler with a wavelength range exceeding the O, E, S, C, L and U optical communication bands. For the first time, the dispersion property of the MMI section is engineered using a subwavelength grating structure to mitigate wavelength dependence of the device. We present a 2 × 2 MMI design with a bandwidth of 450nm, an almost fivefold enhancement compared to conventional designs, maintaining insertion loss, power imbalance and MMI phase deviation below 1dB, 0.6dB and 3°, respectively. The design is performed using an in-house tool based on the 2D Fourier Eigenmode Expansion Method (F-EEM) and verified with a 3D Finite Difference Time Domain (FDTD) simulator.

© 2013 OSA

1. Introduction

In this paper we propose, for the first time, a design procedure for virtually wavelength independent MMI couplers. We demonstrate a 2 × 2 MMI design with a bandwidth of 450nm exceeding the wavelength range of O, E, S, C, L and U bands (1260nm – 1675nm). Furthermore, the device length is about one half compared to a conventional 2 × 2 MMI. This unprecedented performance is achieved by simultaneously engineering the dispersion of multiple guided modes in the multimode region of the coupler, which is composed of a specifically designed subwavelength grating.

Subwavelength gratings (SWGs) are periodic or aperiodic structures in which diffraction effects are supressed by using a grating pitch (Λ) substantially smaller than the wavelength (Λ ≪ λ). SWGs have been employed in planar waveguides for fibre-chip surface grating couplers [11

11. R. Halir, P. Cheben, J. H. Schmid, R. Ma, D. Bedard, S. Janz, D.-X. Xu, A. Densmore, J. Lapointe, and I. Molina-Fernández, “Continuously apodized fiber-to-chip surface grating coupler with refractive index engineered subwavelength structure,” Opt. Lett. 35, 3243–3245 (2010). [CrossRef]

], microphotonic waveguides [12

12. P. J. Bock, P. Cheben, J. H. Schmid, J. Lapointe, A. Delâge, S. Janz, G. C. Aers, D.-X. Xu, A. Densmore, and T. J. Hall, “Subwavelength grating periodic structures in silicon-on-insulator: a new type of microphotonic waveguide,” Opt. Express 18, 20251–20262 (2010). [CrossRef] [PubMed]

], lenses [13

13. U. Levy, M. Abashin, K. Ikeda, A. Krishnamoorthy, J. Cunningham, and Y. Fainman, “Inhomogenous dielectric metamaterials with space-variant polarizability,” Phys. Rev. Lett. 98, 243901 (2007). [CrossRef] [PubMed]

], waveguides crossings [14

14. P. J. Bock, P. Cheben, J. H. Schmid, J. Lapointe, A. Delâge, D.-X. Xu, S. Janz, A. Densmore, and T. J. Hall, “Subwavelength grating crossings for silicon wire waveguides,” Opt. Express 18, 16146–16155 (2010). [CrossRef] [PubMed]

], fibre-chip edge couplers [15

15. P. Cheben, D.-X. Xu, S. Janz, and A. Densmore, “Subwavelength waveguide grating for mode conversion and light coupling in integrated optics,” Opt. Express 14, 4695–4702 (2006). [CrossRef] [PubMed]

, 16

16. P. Cheben, P. Bock, J. Schmid, J. Lapointe, S. Janz, D. Xu, A. Densmore, A. Delâge, B. Lamontagne, and T. Hall, “Refractive index engineering with subwavelength gratings for efficient microphotonic couplers and planar waveguide multiplexers,” Opt. Lett. 35, 2526–2528 (2010). [CrossRef] [PubMed]

], wavelength multiplexers [16

16. P. Cheben, P. Bock, J. Schmid, J. Lapointe, S. Janz, D. Xu, A. Densmore, A. Delâge, B. Lamontagne, and T. Hall, “Refractive index engineering with subwavelength gratings for efficient microphotonic couplers and planar waveguide multiplexers,” Opt. Lett. 35, 2526–2528 (2010). [CrossRef] [PubMed]

], and ultra-fast optical switches [17

17. I. Glesk, P. J. Bock, P. Cheben, J. H. Schmid, J. Lapointe, and S. Janz, “All-optical switching using nonlinear subwavelength Mach-Zehnder on silicon,” Opt. Express 19, 14031–14039 (2011). [CrossRef] [PubMed]

]. Recently, their application to dispersion engineering in integrated optics was proposed [18

18. R. Halir, A. Maese-Novo, A. Ortega-Moñux, and I. Molina-Fernández, “Dispositivo acoplador de guías de onda, y método de diseño de dicho dispositivo,” Spanish patent application serial number P201230280 (2012).

] and applied to design of colorless directional couplers [19

19. R. Halir, A. Maese-Novo, A. Ortega-Moñux, I. Molina-Fernández, J. G. Wangüemert-Pérez, P. Cheben, D.-X. Xu, J. H. Schmid, and S. Janz, “Colorless directional coupler with dispersion engineered sub-wavelength structure,” Opt. Express 12, 13470–13477 (2012). [CrossRef]

].

Our SWG-MMI structure schematics is shown in Fig. 1. The device comprises a subwavelength grating multimode section and tapered transitions between conventional interconnecting waveguides and SWG access ports. The tapered transitions have a double purpose: they widen the waveguide mode laterally, and match the effective index of the silicon-wire waveguides to the significantly lower effective index of the SWG region. The number of periods are PMMI and PT in the multimode waveguide and the tapered access ports, respectively.

Fig. 1 Schematics of the ultra-wideband 2 × 2 MMI coupler with subwavelength grating multimode section and tapered transitions between conventional interconnecting waveguides and SWG access ports.

The device was designed using our in-house 2D simulation tool based on the Fourier Eigen-mode Expansion Method (F-EEM), specifically optimized for Floquet-Bloch mode calculations [20

20. L. Zavargo-Peche, A. Ortega-Moñux, J. G. Wangüemert-Pérez, and I. Molina-Fernández, “Fourier based combined techniques to design novel sub-wavelength optical integrated devices,” Prog. Electromagn. Res. 123, 447–465 (2012). [CrossRef]

]. The nominal silicon waveguide geometry that we use is shown in Fig. 2(a), with a core thickness h = 260nm and refractive indexes nSi = 3.476 (core), nSiO2 = 1.444 (substrate) and nSU–8 = 1.58 (superstrate) at λ = 1.55μm. We have applied the Effective Index Method (EIM) to obtain a simplified 2D structure for TE (in-plane principal electric field component, Ex) polarization. As illustrated in Fig. 2(a) we model the waveguide as a slab of effective index nf in the central guiding region, surrounded by SU-8. Our EIM model includes both geometrical and material dispersion. Calculations were verified with 3D FDTD simulations.

Fig. 2 (a) Schematic view of the silicon waveguide cross-section used throughout this work. (b) Conventional 2 × 2 MMI coupler structure and parameters. (c) Insertion loss of an MMI coupler (WMMI = 6μm) as a function of device length, for different center wavelengths (λ0).

This paper is organized as follows. In section 2, we briefly discuss the working principle and the geometrical considerations for a conventional MMI coupler and the main factors limiting the bandwidth of MMI devices. In section 3 we propose a design procedure exploiting subwavelength grating waveguide dispersion engineering to substantially broaden MMI bandwidth. Simulation results, including insertion loss, power imbalance and MMI phase deviation between outputs are discussed in section 4. Finally, in section 5 we draw conclusions.

2. MMI couplers: working principle and bandwidth limitations

MMI couplers are based on the self-imaging principle [1

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

], where N-fold images (copies) of the field at the input plane are formed at the output plane after propagation in a multimode waveguide of width WMMI. Figure 2(b) shows the geometry of a conventional 2 × 2 MMI coupler. A fundamental mode in one of the input ports excites a set of modes ϕm in the multimode section (m = 0,...,M – 1), with mode propagation constants βm = 2πneff,m/λ, where neff,m is the effective index of the m-th mode. For perfect (error-free) imaging, it is required that propagation constants follow a parabolic relation [1

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

]:
Δβm=β0βm=m(m+2)π3Lπ,
(1)
where Lπ = π/(β0β1) is the beat length between the two lowest order modes. Lπ can be expressed, in terms of its wavelength dependence, as
Lπ(λ)=λ2(neff,0(λ)neff,1(λ)),
(2)
which can be approximated as :
Lπ(λ)4nfWMMI23λ,
(3)
where nf is the equivalent index of the multimode region resulting from the effective index method [1

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

].

As an example, let us consider a 2 × 2 MMI coupler based on paired interference, as it generally yields shorter and more tolerant devices [10

10. P. A. Besse, M. Bachmann, H. Melchior, L. B. Soldano, and M. K. Smit, “Optical bandwidth and fabrication tolerances of multimode interference couplers,” J. Lightwave Technol. 12, 1004–1009 (1994). [CrossRef]

], so that the length of the multimode region is LMMI = Lπ/2. In a paired MMI, access waveguides are located at x = ±WMMI/3 and separated sufficiently for negligible coupling (< −40dB). This condition is fulfilled with a center-to-center separation S = 2μm, which yields an MMI section width WMMI = 6μm [see Fig. 2(b)].

Imaging quality of an MMI coupler primarily depends on two parameters. First, the propagation distance (device length, LMMI) where images are formed is proportional to Lπ, which is intrinsically wavelength dependent (Eq. 2). This length is set at the center wavelength (λ0), e.g. LMMI = Lπ(λ0)/2 in the example above. Second, deviations from the optimal parabolic relation Δβm between the mode propagation constants (Eq. 1) increase for higher order modes [21

21. Z. Huang, R. Scarmozzino, and R. Osgood, “A new design approach to large input/output number multimode interference couplers and its application to low-crosstalk WDM routers,” IEEE Photon. Technol. Lett. 10, 1292–1294 (1998). [CrossRef]

]. To partially compensate this effect, the access ports width (Wa) is generally designed to excite only a limited number of lower order modes [22

22. M. T. Hill, X. J. M. Leijtens, G. D. Khoe, and M. K. Smit, “Optimizing imbalance and loss in 2×2 3-dB multimode interference couplers via access waveguide width,” J. Lightwave Technol. 21, 2305–2313 (2003). [CrossRef]

].

3. Bandwidth widening of MMI couplers using dispersion engineering with subwavelength gratings

We propose a new type of MMI couplers in which the multimode section comprises a subwavelength grating structure (Fig. 1) to effectively flatten the wavelength dependence of Lπ. In our SWG-MMI design, the calculations are carried out using Floquet-Bloch modes of the periodic structure. Note that since the SWG is non-diffractive, the field evolution of the Floquet-Bloch modes strongly resembles that of conventional waveguide modes, so that the behaviour of the MMI can still be described by the conventional waveguide mode expressions given by Eqs. (1) and (2).

The subwavelength grating in the multimode region is designed so that the beat length of its two lowest order Floquet-Bloch modes is approximately constant with wavelength. The SWG geometry provides two degrees of freedom to control waveguide dispersion: grating pitch (Λ) and duty cycle (DC = a/Λ). The range of values for pitch and duty cycle are limited by technological and practical considerations. Specifically, we constrain our geometry to a duty cycle of 50%, since this choice maximizes the minimum feature size for a given pitch. Furthermore, the device needs to be designed to operate below the Bragg condition over the entire bandwidth to avoid bandgap opening, so that Λ < λmin/(2neff), where neff is the effective index of the fundamental Floquet-Bloch mode of the periodic structure at the minimum operational wavelength λmin. From elementary effective permitivity theory [23

23. S. M. Rytov, “Electromagnetic properties of a finely stratified medium,” Sov. Phys. JETP 2, 466–475 (1956).

] for the electric field parallel to the grating lines, it follows: neff(nSi2DC+nSU82(1DC))1/22.72. This yields an upper bound for the pitch of Λ ≃ 230nm at the wavelength λmin = 1.25μm. We then calculate the beat length, Lπ, as a function of pitch and wavelength. The results are shown in the contour map of Fig. 3(a). It should be pointed out that such a systematic calculation can be performed efficiently in a comparatively short time using our in-house simulation tool [20

20. L. Zavargo-Peche, A. Ortega-Moñux, J. G. Wangüemert-Pérez, and I. Molina-Fernández, “Fourier based combined techniques to design novel sub-wavelength optical integrated devices,” Prog. Electromagn. Res. 123, 447–465 (2012). [CrossRef]

]. Albeit larger values of pitch are desirable to ease device fabrication, this implies a rapidly varying coupling length as the Bragg resonance is approached [white region in Fig. 3(a)]. On the other hand, pitch values below 170nm results in narrower Lπ bandwidths and indeed also smaller minimum feature sizes. Therefore, from Fig. 3(a) an optimal region is identified near Λ = 190nm. As shown in Fig. 3(b), Lπ wavelength dependence is mitigated for the SWG-MMI with a sub-wavelength grating pitch of Λ = 190nm and a duty cycle of 50%. The wavelength dependence of the beat length of a conventional MMI device with same width WMMI = 6μm is shown for comparison. These results were confirmed with 3D simulations carried out with MEEP [24].

Fig. 3 (a) Beat length of an SWG-MMI coupler as a function of grating pitch and wavelength, for DC = 50% and WMMI = 6μm. (b) Coupling length of the optimized design as a function of wavelength, compared to a conventional MMI. (c) Modal phase error (MPEm) for the first 5 modes of the SWG-MMI composite section for a device length of LMMI = 23.18μm.

For Λ = 190nm, the wavelength averaged beat length is Lπ = 46.5μm, so that the number of periods for a paired 2 × 2 MMI is given by PMMI = (Lπ/2)/Λ, i.e. PMMI = 122 periods. This results in a length of the multimode section 23.18μm, which almost coincides with one half of the wavelength-averaged beat length. Note that a conventional MMI of the same width is about twice as long (LMMI = 48.2μm). The shortening of the SWG-based device is attributed to the lower equivalent index nf in the multimode region [see Eq. (3)] compared to a conventional MMI device.

4. Evaluation of MMI broadband performance

We evaluate the bandwidth performance of the wavelength independent 2 × 2 MMI, in terms of insertion loss, IL = −10log(|s31|2 + |s41|2), power imbalance, PI = |10log(|s31|2/|s41|2)|, and MMI phase deviation from the nominal 90° phase difference between the MMI outputs, PD = |∠ (s31/s41)| − 90°, where ∠ denotes the phase of a complex number. Insertion loss of the SWG-MMI design is compared in Fig. 4(a) with a conventional MMI coupler with similar dimensions: WMMI = 6μm, Wa = 1.2μm, S = 2μm, LT = 10μm and LMMI = 48.2μm at λ = 1.47μm. Power imbalance and MMI phase deviations are shown in Fig. 4(b) and Fig. 4(c), respectively. The SWG-MMI shows a significant bandwidth enhancement in all these parameters: insertion loss and power imbalance are less than 1dB and 0.6dB, respectively, and the MMI phase deviation is less than 3° within a wavelength range of 450nm. This corresponds to an almost fivefold bandwidth enhancement compared to conventional MMI devices. These results have been verified with RSoft FullWAVE [27], a 3D FDTD simulator. As in the 2D case, material dispersion was included in the 3D calculations. Note that due to the inherent approximations of the 2D model device dimensions must be re-optimized in the 3D case. This was done by keeping the 50% duty-cycle in the SWG, and iteratively adjusting the pitch of the SWG and the length of the MMI region around the values obtained with the 2D model. According to this 3D FDTD design, the SWG pitch is Λ = 198nm, and the number of periods of the MMI region and the access tapers are PMMI = 94 and PT = 51, respectively. It is noticed from the results shown in Fig. 4(a–c) that our 2D design approach accurately describes the performance of the device, and gives a good initial point for the final design with reduced computational effort.

Fig. 4 (a) Insertion loss, (b) power imbalance and (c) MMI phase deviation as a function of wavelength for the SWG-MMI and conventional MMI design. (d) Light propagation (electric field component Ex) in the SWG-MMI device, for wavelength λ = 1.26μm, λ = 1.47μm and λ = 1.675μm.

Simulated field propagation (electric field conponent Ex) in the SWG-MMI device is shown in Fig. 4(d), for wavelengths λ = 1.26μm (O band), λ = 1.47μm (S band) and λ = 1.675μm (U band). As expected, in all cases the field pattern propagated along the MMI is almost invariant. Thus, images are formed properly at approximately the same z-position, confirming that the wavelength dependence of the beat length, Lπ, has been minimized. The SWG-based MMI thus covers most of the bands presently used in optical communications (O, E, S, C, L and U) in the wavelength range from 1260nm to 1675nm.

5. Conclusions

With proper device design, MMI bandwidth is solely limited by the wavelength dependence of the beat length Lπ. We have shown that by engineering the dispersion properties of a multimode region using subwavelength gratings, MMI wavelength dependence can be mitigated. Our subwavelength grating dispersion engineered MMI device exhibits substantially wavelength independent beat length and consequently an ultra-broadband response, covering most of the bands used for communications in the wavelength range of 1260nm–1675nm. The device has calculated insertion loss, power imbalance and MMI phase deviations of less than 1dB, 0.6dB and 3° in a 450nm bandwidth, that is an almost fivefold bandwidth enhancement compared to conventional MMI designs. Furthermore, the length of the device is reduced to one half compared to a conventional design. The design has been performed using our in-house 2D simulation tool and verified with a commercial 3D FDTD software. The proposed method of planar waveguide dispersion engineering with subwavelength gratings opens promising prospects for designing new types of MMI couplers and other broadband integrated optical devices.

Acknowledgments

This work was supported by the Spanish Ministerio de Ciencia (project TEC2009-10152), the Andalusian Regional Ministry of Science, Innovation and Business (project P07-TIC-02946), and the European Mirthe project ( FP7-2010-257980).

References and links

1.

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

2.

D. Kim, A. Barkai, R. Jones, N. Elek, H. Nguyen, and A. Liu, “Silicon-on-insulator eight-channel optical multiplexer based on a cascade of asymmetric Mach-Zehnder interferometers,” Opt. Lett. 33, 530–532 (2008). [CrossRef] [PubMed]

3.

D. Kwong, Y. Zhang, A. Hosseini, Y. Liu, and R. T. Chen, “1×12 even fanout using multimode interference optical beam splitter on silicon nanomembrane,” Electron. Lett. 46, 1281–1283 (2010). [CrossRef]

4.

M. Bachmann, P. A. Besse, and H. Melchior, “Overlapping-image multimode interference couplers with a reduced number of self-images for uniform and nonuniform power splitting,” Appl. Opt. 34, 6898–6910 (1995). [CrossRef] [PubMed]

5.

R. Halir, G. Roelkens, A. Ortega-Moñux, and J. G. Wangüemert-Pérez, “High-performance 90° hybrid based on a silicon-on-insulator multimode interference coupler,” Opt. Lett. 36, 178–180 (2011). [CrossRef] [PubMed]

6.

A. Ortega-Moñux, L. Zavargo-Peche, A. Maese-Novo, I. Molina-Fernández, R. Halir, J. G. Wangüemert-Pérez, P. Cheben, and J. H. Schmid, “High-performance multimode interference coupler in silicon waveguides with subwavelenght structures,” IEEE Photon. Technol. Lett. 23, 1406–1408 (2011). [CrossRef]

7.

B. M. A. Rahman, N. Somasiri, C. Themistos, and K. T. V. Grattan, “Design of optical polarization splitters in a single-section deeply etched MMI waveguide,” Appl. Phys. B 73, 613–618 (2001). [CrossRef]

8.

D. S. Levy, R. Scarmozzino, and R. M. Osgood Jr., “Length reduction of tapered N × N devices,” IEEE Photon. Technol. Lett. 10, 830–832 (1998). [CrossRef]

9.

I. Molina-Fernández, A. Ortega-Moñux, and J. G. Wangüemert-Pérez, “Improving multimode interference couplers performance through index profile engineering,” J. Lightwave Technol. 27, 1307–1314 (2009). [CrossRef]

10.

P. A. Besse, M. Bachmann, H. Melchior, L. B. Soldano, and M. K. Smit, “Optical bandwidth and fabrication tolerances of multimode interference couplers,” J. Lightwave Technol. 12, 1004–1009 (1994). [CrossRef]

11.

R. Halir, P. Cheben, J. H. Schmid, R. Ma, D. Bedard, S. Janz, D.-X. Xu, A. Densmore, J. Lapointe, and I. Molina-Fernández, “Continuously apodized fiber-to-chip surface grating coupler with refractive index engineered subwavelength structure,” Opt. Lett. 35, 3243–3245 (2010). [CrossRef]

12.

P. J. Bock, P. Cheben, J. H. Schmid, J. Lapointe, A. Delâge, S. Janz, G. C. Aers, D.-X. Xu, A. Densmore, and T. J. Hall, “Subwavelength grating periodic structures in silicon-on-insulator: a new type of microphotonic waveguide,” Opt. Express 18, 20251–20262 (2010). [CrossRef] [PubMed]

13.

U. Levy, M. Abashin, K. Ikeda, A. Krishnamoorthy, J. Cunningham, and Y. Fainman, “Inhomogenous dielectric metamaterials with space-variant polarizability,” Phys. Rev. Lett. 98, 243901 (2007). [CrossRef] [PubMed]

14.

P. J. Bock, P. Cheben, J. H. Schmid, J. Lapointe, A. Delâge, D.-X. Xu, S. Janz, A. Densmore, and T. J. Hall, “Subwavelength grating crossings for silicon wire waveguides,” Opt. Express 18, 16146–16155 (2010). [CrossRef] [PubMed]

15.

P. Cheben, D.-X. Xu, S. Janz, and A. Densmore, “Subwavelength waveguide grating for mode conversion and light coupling in integrated optics,” Opt. Express 14, 4695–4702 (2006). [CrossRef] [PubMed]

16.

P. Cheben, P. Bock, J. Schmid, J. Lapointe, S. Janz, D. Xu, A. Densmore, A. Delâge, B. Lamontagne, and T. Hall, “Refractive index engineering with subwavelength gratings for efficient microphotonic couplers and planar waveguide multiplexers,” Opt. Lett. 35, 2526–2528 (2010). [CrossRef] [PubMed]

17.

I. Glesk, P. J. Bock, P. Cheben, J. H. Schmid, J. Lapointe, and S. Janz, “All-optical switching using nonlinear subwavelength Mach-Zehnder on silicon,” Opt. Express 19, 14031–14039 (2011). [CrossRef] [PubMed]

18.

R. Halir, A. Maese-Novo, A. Ortega-Moñux, and I. Molina-Fernández, “Dispositivo acoplador de guías de onda, y método de diseño de dicho dispositivo,” Spanish patent application serial number P201230280 (2012).

19.

R. Halir, A. Maese-Novo, A. Ortega-Moñux, I. Molina-Fernández, J. G. Wangüemert-Pérez, P. Cheben, D.-X. Xu, J. H. Schmid, and S. Janz, “Colorless directional coupler with dispersion engineered sub-wavelength structure,” Opt. Express 12, 13470–13477 (2012). [CrossRef]

20.

L. Zavargo-Peche, A. Ortega-Moñux, J. G. Wangüemert-Pérez, and I. Molina-Fernández, “Fourier based combined techniques to design novel sub-wavelength optical integrated devices,” Prog. Electromagn. Res. 123, 447–465 (2012). [CrossRef]

21.

Z. Huang, R. Scarmozzino, and R. Osgood, “A new design approach to large input/output number multimode interference couplers and its application to low-crosstalk WDM routers,” IEEE Photon. Technol. Lett. 10, 1292–1294 (1998). [CrossRef]

22.

M. T. Hill, X. J. M. Leijtens, G. D. Khoe, and M. K. Smit, “Optimizing imbalance and loss in 2×2 3-dB multimode interference couplers via access waveguide width,” J. Lightwave Technol. 21, 2305–2313 (2003). [CrossRef]

23.

S. M. Rytov, “Electromagnetic properties of a finely stratified medium,” Sov. Phys. JETP 2, 466–475 (1956).

24.

http://ab-initio.mit.edu/meep/.

25.

R. Ulrich and T. Kamiya, “Resolution of self-images in planar optical waveguides,” J. Opt. Soc. Am. 68, 583–592 (1978). [CrossRef]

26.

D. Pérez-Galacho, R. Halir, L. F. Zavargo-Peche, J. G. Wangüemert-Pérez, A. Ortega-Moñux, I. Molina-Fernández, and P. Cheben, “Adiabatic transitions for sub-wavelength grating waveguides,” European Conference on Integrated Optics (ECIO), April 16–18 2012, Sitges (Spain), paper 71.

27.

http://www.rsoftdesign.com.

OCIS Codes
(130.3120) Integrated optics : Integrated optics devices
(230.7370) Optical devices : Waveguides
(260.2030) Physical optics : Dispersion
(160.1245) Materials : Artificially engineered materials
(260.2065) Physical optics : Effective medium theory
(050.6624) Diffraction and gratings : Subwavelength structures

ToC Category:
Integrated Optics

History
Original Manuscript: October 26, 2012
Revised Manuscript: December 28, 2012
Manuscript Accepted: December 31, 2012
Published: March 13, 2013

Citation
A. Maese-Novo, R. Halir, S. Romero-García, D. Pérez-Galacho, L. Zavargo-Peche, A. Ortega-Moñux, I. Molina-Fernández, J. G. Wangüemert-Pérez, and P. Cheben, "Wavelength independent multimode interference coupler," Opt. Express 21, 7033-7040 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-6-7033


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References

  1. L. B. Soldano and E. C. M. Pennings, “Optical multi-mode interference devices based on self-imaging: Principles and applications,” J. Lightwave Technol.13, 615–627 (1995). [CrossRef]
  2. D. Kim, A. Barkai, R. Jones, N. Elek, H. Nguyen, and A. Liu, “Silicon-on-insulator eight-channel optical multiplexer based on a cascade of asymmetric Mach-Zehnder interferometers,” Opt. Lett.33, 530–532 (2008). [CrossRef] [PubMed]
  3. D. Kwong, Y. Zhang, A. Hosseini, Y. Liu, and R. T. Chen, “1×12 even fanout using multimode interference optical beam splitter on silicon nanomembrane,” Electron. Lett.46, 1281–1283 (2010). [CrossRef]
  4. M. Bachmann, P. A. Besse, and H. Melchior, “Overlapping-image multimode interference couplers with a reduced number of self-images for uniform and nonuniform power splitting,” Appl. Opt.34, 6898–6910 (1995). [CrossRef] [PubMed]
  5. R. Halir, G. Roelkens, A. Ortega-Moñux, and J. G. Wangüemert-Pérez, “High-performance 90° hybrid based on a silicon-on-insulator multimode interference coupler,” Opt. Lett.36, 178–180 (2011). [CrossRef] [PubMed]
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