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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 16 — Aug. 1, 2011
  • pp: 14892–14902
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Parallel-coupled dual racetrack silicon micro-resonators for quadrature amplitude modulation

Ryan A. Integlia, Lianghong Yin, Duo Ding, David Z. Pan, Douglas M. Gill, and Wei Jiang  »View Author Affiliations


Optics Express, Vol. 19, Issue 16, pp. 14892-14902 (2011)
http://dx.doi.org/10.1364/OE.19.014892


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Abstract

A parallel-coupled dual racetrack silicon micro-resonator structure is proposed and analyzed for M-ary quadrature amplitude modulation. The over-coupled, critically coupled, and under-coupled scenarios are systematically studied. Simulations indicate that only the over-coupled structures can generate arbitrary M-ary quadrature signals. Analytic study shows that the large dynamic range of amplitude and phase of a modulated over-coupled structure stems from the strong cross-coupling between two resonators, which can be understood through a delicate balance between the direct sum and the “interaction” terms. Potential asymmetries in the coupling constants and quality factors of the resonators are systematically studied. Compensations for these asymmetries by phase adjustment are shown feasible.

© 2011 OSA

1. Introduction

Advanced optical modulation formats could offer significant advantages for optical communications [1

1. P. J. Winzer and R. J. Essiambre, “Advanced optical modulation formats,” Proc. IEEE 94(5), 952–985 (2006). [CrossRef]

,2

2. G. P. Agrawal, Fiber-Optic Communication Systems (John Wiley & Sons, 1997).

]. For example, quadrature phase-shift keying provides higher spectral efficiency, better tolerance to fiber nonlinearity and chromatic dispersion, and enhanced receiver sensitivity compared to on-off keying. Traditional lithium niobate (LiNbO3) modulators can be used for such modulation. However, LiNbO3 modulators are relatively large in size. For a general M-ary modulation format that requires a large number of optical modulator components along with their driving signal circuitries, the overall size of the entire modulator is rather cumbersome. Recent breakthroughs in silicon photonics [3

3. R. Soref, “The past, present, and future of silicon photonics,” IEEE J. Sel. Top. Quantum Electron. 12(6), 1678–1687 (2006). [CrossRef]

,4

4. B. Jalali and S. Fathpour, “Silicon photonics,” J. Lightwave Technol. 24(12), 4600–4615 (2006). [CrossRef]

], particularly silicon based optical modulators [5

5. A. S. Liu, R. Jones, L. Liao, D. Samara-Rubio, D. Rubin, O. Cohen, R. Nicolaescu, and M. Paniccia, “A high-speed silicon optical modulator based on a metal-oxide-semiconductor capacitor,” Nature 427(6975), 615–618 (2004). [CrossRef] [PubMed]

,6

6. Q. F. Xu, B. Schmidt, S. Pradhan, and M. Lipson, “Micrometre-scale silicon electro-optic modulator,” Nature 435(7040), 325–327 (2005). [CrossRef] [PubMed]

], have fundamentally changed the landscape of modulator technology. Notably, micro-resonator based silicon modulators [6

6. Q. F. Xu, B. Schmidt, S. Pradhan, and M. Lipson, “Micrometre-scale silicon electro-optic modulator,” Nature 435(7040), 325–327 (2005). [CrossRef] [PubMed]

10

10. D. M. Gill, S. S. Patel, M. Rasras, K. Y. Tu, A. E. White, Y. K. Chen, A. Pomerene, D. Carothers, R. L. Kamocsai, C. M. Hill, and J. Beattie, “CMOS-compatible si-ring-assisted Mach-Zehnder interferometer with internal bandwidth equalization,” IEEE J. Sel. Top. Quantum Electron. 16(1), 45–52 (2010). [CrossRef]

] constitute an ideal candidate for optical modulation due to their compact size, low power consumption, and ease of monolithic integration with driving circuitries on the same silicon chip. Most research on silicon microring modulators employed intensity modulation in binary formats. Recently, microring resonator based modulators for differential binary phase-shift-keying and differential quadrature phase-shift keying (QPSK) have been proposed, and satisfactory performances have been predicted [11

11. L. Zhang, J. Y. Yang, M. Song, Y. Li, B. Zhang, R. G. Beausoleil, and A. E. Willner, “Microring-based modulation and demodulation of DPSK signal,” Opt. Express 15(18), 11564–11569 (2007). [CrossRef] [PubMed]

,12

12. L. Zhang, J. Y. Yang, Y. C. Li, M. P. Song, R. G. Beausoleil, and A. E. Willner, “Monolithic modulator and demodulator of differential quadrature phase-shift keying signals based on silicon microrings,” Opt. Lett. 33(13), 1428–1430 (2008). [CrossRef] [PubMed]

]. Another work employed the anti-crossing between paired amplitude and phase resonators and demonstrated enhanced sensitivity to the input drive signal [13

13. W. M. J. Green, M. J. Rooks, L. Sekaric, and Y. A. Vlasov, “Optical modulation using anti-crossing between paired amplitude and phase resonators,” Opt. Express 15(25), 17264–17272 (2007). [CrossRef] [PubMed]

]. A high-Q microring quadrature modulator incorporating dual 2 × 2 Mach-Zehnder interferometers has also been recently proposed with beneficial performance [14

14. W. D. Sacher and J. K. S. Poon, “Microring quadrature modulators,” Opt. Lett. 34(24), 3878–3880 (2009). [CrossRef] [PubMed]

].

We propose a novel parallel-coupled dual racetrack micro-resonator structure, illustrated in Fig. 1(a)
Fig. 1 Parallel-coupled dual racetrack resonators. (a) schematic of the structure, and typical spectra for an over-coupled structure: (b) output intensity and (c) phase in radians.
, for phase-shift keying and M-ary quadrature amplitude modulation (QAM). Two identical racetrack resonators are symmetrically side-coupled in parallel to a through waveguide in the center. The modulator can be fabricated on a silicon-on-insulator (SOI) wafer. The carriers can be injected or depleted from the racetrack resonators using a pin diode [15

15. L. L. Gu, W. Jiang, X. N. Chen, L. Wang, and R. T. Chen, “High speed silicon photonic crystal waveguide modulator for low voltage operation,” Appl. Phys. Lett. 90(7), 071105 (2007). [CrossRef]

] or metal-oxide-semiconductor capacitor [16

16. X. N. Chen, Y. S. Chen, Y. Zhao, W. Jiang, and R. T. Chen, “Capacitor-embedded 0.54 pJ/bit silicon-slot photonic crystal waveguide modulator,” Opt. Lett. 34(5), 602–604 (2009). [CrossRef] [PubMed]

] embedded in a silicon waveguide. The plasma dispersion effect [17

17. R. A. Soref and B. R. Bennett, “Electrooptical effects in silicon,” IEEE J. Quantum Electron. 23(1), 123–129 (1987). [CrossRef]

] of the injected carriers causes a change of refractive index, Δn 1, Δn 3 , in each racetrack resonator, which modifies the cross-coupled resonances of the two racetrack resonators. By carefully choosing the voltage signals applied to each resonator, the amplitude and phase of output optical signal can be controlled to generate arbitrary M-ary quadrature signals.

A distinctive feature of the proposed structure is that the coherent cross-coupling between the two racetrack resonators mediated by the center waveguide drastically modifies the amplitude/phase characteristics of resonance. This enables M-ary quadrature signal generations including quadrature phase shift keying (QPSK). The outcome of the cross-coupling of the resonances is fairly complex. However, our analysis shows that it can be understood through the direct sum and coherent “interaction” of the optical characteristics of two individual resonators as presented in Sec. 2.4. The structure of this paper is organized as follows. First the cross-coupling between the racetrack resonators is analyzed and the output transfer function of the proposed structure is presented. The critical coupling condition is obtained. Systematic studies of the over-coupled, critically coupled, and under-coupled scenarios for the parallel-coupled racetrack resonator structure indicate that strong over-coupling case is desired for arbitrary M-ary quadrature signal generation. The interaction between the resonances of two racetracks is analyzed, and its critical role in M-ary quadrature signal generation is presented. The effects of asymmetries in the coupling strengths and quality factors of resonators are systematically studied, and phase compensations for such asymmetries are presented. Lastly, the electrical aspects of the proposed modulators are briefly discussed, followed by a conclusion.

2. Principles of parallel-coupled racetrack resonators

2.1 Cross-Coupling Analysis and Output Transfer Function

The coupling between the two racetrack resonators and the through-waveguide in Fig. 1(a) can be described by multi-waveguide coupling theory [18

18. V. A. Mashkov and H. Temkin, “Propagation of eigenmodes and transfer amplitudes in optical waveguide structures,” IEEE J. Quantum Electron. 34(10), 2036–2047 (1998). [CrossRef]

20

20. C.-M. Kim and Y.-J. Im, “Switching operations of three-waveguide optical switches,” IEEE J. Sel. Top. Quantum Electron. 6(1), 170–174 (2000). [CrossRef]

]. Assume the fields in three identical single-mode waveguides have slowly varying envelopes un(z)
En(x,y,z)=Mn(x,y)exp(iβz)un(z),n=1,2,3,
(1)
where M n(x,y) is the lateral mode profile, β is the propagation constant along the waveguide axis z for an isolated waveguide. For the parallel coupled racetrack resonator structure in Fig. 1, the input fields and output fields of the coupling segments are given by
En(in)(x,y,0)=Mn(x,y)un(0)Mn(x,y)an,En(out)(x,y,L)=Mn(x,y)exp(iβL)un(L)Mn(x,y)bn,
(2)
where an and bn are the normalized input and output complex amplitudes, respectively. The solution of the coupled mode equations yields [20

20. C.-M. Kim and Y.-J. Im, “Switching operations of three-waveguide optical switches,” IEEE J. Sel. Top. Quantum Electron. 6(1), 170–174 (2000). [CrossRef]

]
[b1b2b3]=exp(iβL)[c1+1/2c2c11/2c22c1c2c11/2c2c1+1/2][a1a2a3],
(3)
where

c1=12cos(2κL),c2=12isin(2κL).

The strength of the cross-coupling between the two racetrack resonators mediated by the through waveguide is given by |c 1−1/2|. In addition, light propagation along a racetrack gives rise to the following relations
a1=η1exp(iθ1)b1,a3=η3exp(iθ3)b3,
(4)
where the amplitude attenuation along a racetrack is given by ηn<1, and the phase shift is given by θn. Assuming a unity input amplitude a 2 = 1, the output amplitude b2 can be solved from Eqs. (3) and (4)
Eout=b2=eiφ[(1/2c1)(Δu1+Δu3)+2c1Δu1Δu3](1/2c1)(Δu1+Δu3)+Δu1Δu3,
(5)
where ϕ = βL, and

Δun1eiφ+iθnηn1,n=1,3,
(6)

Because of the symmetry of the structure shown in Fig. 1(a), the output amplitude, Eq. (5), only involves terms symmetric with respect to an interchange of Δu 1 and Δu 3. As such, the symmetry of the structure can be utilized to help simplify the understanding of the device principles, as noted in the study of other devices [21

21. W. Jiang and R. T. Chen, “Multichannel optical add-drop processes in symmetrical waveguide-resonator systems,” Phys. Rev. Lett. 91(21), 213901 (2003). [CrossRef] [PubMed]

]. Detailed analysis of a modulated symmetric dual racetrack resonator structure will be given in the following sections.

2.2 Critical Coupling Condition and Vanishing Amplitude for a Modulated Over-Coupled Structure

The critical coupling condition [22

22. A. Yariv, “Critical coupling and its control in optical waveguide-ring resonator systems,” IEEE Photon. Technol. Lett. 14(4), 483–485 (2002). [CrossRef]

] can be obtained by setting b2 = 0 in Eq. (5). For symmetric parallel-coupled racetracks without modulation (Δu1=Δu3), one readily shows that the critical coupling condition for such a parallel-coupled dual racetrack structure is given by

η1=2c1=cos2κL.
(7)

The asymmetric cases will be discussed in a later section.

For modulated racetracks, the phase shift θn in each ring will be a linear function of the refractive index changes, Δnn, due to carrier injection or depletion in the respective racetrack resonator. Therefore the output amplitude b 2 depends on Δnn through the phase shift terms. To understand the modulation characteristics, it is helpful to rewrite the output amplitude in the following form

b2=eiφ[1+(2c1+1)(1/2c1)(1/Δu1+1/Δu3)+1].
(8)

As c 1 is a real number, for a modulated symmetric (η 1 = η 3) dual-racetrack structure, the output amplitude can vanish only if Δu1=Δu3*. Indeed, one can show that even if the critical coupling condition is not satisfied in absence of modulation, the modulated amplitude can still vanish under the following modulation condition
φ+θ1=2m1πΔθ,andφ+θ3=2m3π+Δθ,
(9a)
cosΔθ=η1[1+c1(1/η121)c1+1/2],
(9b)
where m 1 and m 3 are two integers. For real nonzero Δθ, this requires
η1>2c1=cos2κL,
(10)
which corresponds to over-coupling in comparison to Eq. (8). The spectra of an over-coupled dual racetrack structure (without modulation) are illustrated in Fig. 1(b) and 1(c).

2.3 Arbitrary M-ary Quadrature Signal Generation Capability

For intensity and phase modulation, the refractive index of the silicon waveguides in each racetrack is varied on the order of 0.001. Such an amount of Δn can be achieved with carrier concentration changes ΔN e, ΔN h~3 × 1017cm−3 according to the well-known plasma dispersion relation reported in [17

17. R. A. Soref and B. R. Bennett, “Electrooptical effects in silicon,” IEEE J. Quantum Electron. 23(1), 123–129 (1987). [CrossRef]

].

Figure 2
Fig. 2 Intensity (a) and phase (b) variations under refractive index modulation for the parallel-coupled dual racetrack resonators. r 1 = r 3 = 3μm, L = 3μm, η 1 = η 3 = 0.994, c 1 = 0.4243. The intensity vanishes at two points (Δn 1n 3) = ( ± 3.5 × 10−4, ∓3.5 × 10−4). The color code for the phase is in radians in (b).
depicts the simulated intensity and phase variations as a function of refractive index variations Δn1 and Δn3 at the resonant wavelength for an over-coupled structure. The structure parameters are r 1 = r 3 = 3μm, L = 3μm, η 1 = η 3 = 0.994, c 1 = 0.4243. Note that compact silicon racetrack resonators have been systematically characterized recently [23

23. M. Soltani, S. Yegnanarayanan, Q. Li, and A. Adibi, “systematic engineering of waveguide-resonator coupling for silicon microring/microdisk/racetrack resonators: theory and experiment,” IEEE J. Quantum Electron. 46(8), 1158–1169 (2010). [CrossRef]

]. It was shown that the coupling strength and quality factor can be varied over large ranges by changing the Si pedestal layer thickness of the rib waveguide and modifying the gap width between the waveguide and the resonator. The parameters used here are in accordance with the ranges given Ref [23

23. M. Soltani, S. Yegnanarayanan, Q. Li, and A. Adibi, “systematic engineering of waveguide-resonator coupling for silicon microring/microdisk/racetrack resonators: theory and experiment,” IEEE J. Quantum Electron. 46(8), 1158–1169 (2010). [CrossRef]

]. Evidently, the intensity vanishes at two points (Δn 1n 3) = ( ± 3.5 × 10−4, ∓3.5 × 10−4), in accordance with the analytic results given in Eq. (9b). In all phase plots starting from Fig. 2, the overall constant phase factor ei ϕ in b 2 is omitted to better illustrate the symmetry of the modulated output. On a side note, if η 1 and η 3 decrease simultaneously (η 1 = η 3), the two “eyes” on the diagonal of Fig. 2(a) widen and the phase contours in Fig. 2(b) expand accordingly.

To visualize the complex amplitude, E outn 1n 3), for M-ary signal generations, the ensemble of complex b2 values for all values of Δn1and Δn3 are mapped onto the complex plane of the normalized output electric field. Each blue point in Fig. 3(a)
Fig. 3 Mapping of the normalized complex output field amplitude E out on the complex plane for refractive index Δn 1, Δn 3 varying in the range of −0.002 ~0.002. (a)-(c) for parallel-coupled dual racetrack resonators; (d)-(f) for two uncoupled racetrack resonators in series. Evidently, only case (a) is suitable for arbitrary M-ary quadrature signal generation. Constellations for QPSK (brown circles) and 16-QAM (red squares) modulation formats are illustrated in (a).
gives the amplitude and phase of the output signal for a particular pair of Δn 1, Δn 3 values in the aforementioned range. Evidently, the ensemble of blue points covers most part of the unit circle (the symbol space), therefore, allowing for the access of a wide range of amplitude and phase values. A close examination of Fig. 2 indicates that the intensity and phase varies widely in the second and fourth quadrants where Δn 1 and Δn 3 have opposite signs, which corresponds to a push-pull configuration. In contrast, the intensity and phase are much less sensitive to Δn 1 and Δn 3 when they have the same sign. Indeed, our simulations indicate that the push-pull configuration is usually responsible for over 80% of coverage on the complex E plane. Hence a push-pull modulation configuration is preferred for such a parallel-coupled dual-racetrack structure.

2.4 The Cross-Coupling of Two Racetrack Resonances: Direct Sum and “Interaction”

It should be noted that the broad coverage inside the unit circle observed in Fig. 3(a) is a signature of the strong cross-coupling between the two racetrack resonators mediated by the center waveguide. To illustrate this point, the simulated typical coverage of a critically coupled case and an under-coupled case is shown in Fig. 3(b) and 3(c), respectively, for parallel-coupled dual racetrack resonators. In addition, the simulated typical coverage for two uncoupled racetrack resonator in series is plotted in Fig. 3(d)3(f). None of the cases illustrated in Fig. 3(b)3(f) has adequate coverage for arbitrary M-ary quadrature signal generation.

The cross-coupling present in the parallel coupled racetrack resonators helps only the over-coupling case to achieve sufficient coverage over all four quadrants inside the unit circle. It can be shown that such a behavior stems from a delicate balance between the direct sum term Δu 1 + Δu 3 and the “interaction” term Δu 1Δu 3 on both the numerator and denominator in Eq. (5). Based on their definitions Δun=(1eiφ+iθnηn)/eiφ+iθnηn, Δun can be regarded as the normalized change of the field amplitude after one round-trip propagation in a racetrack. Here the initial field amplitude is unity, and the amplitude change is normalized by the final field amplitude eiφ+iθnηn. For a racetrack without modulation (Δn 1 = Δn 3 = 0), Δui is small (on the order of 1−η 1) near resonance, and Δu 1 and Δu 3 are in phase. Therefore, we find
|Δu1Δu3|<<|(1/2c1)(Δu1+Δu3)|
because 1−η 1<2(1/2−c 1) according to the strong coupling condition. The dominance of the direct sum term in Eq. (5) yields an output amplitude close to −1. With sufficient modulation in a push-pull configuration, Δun can gain large imaginary parts (Im(Δun)~Δθn, up to ± 0.09 at Δnn = 0.001) with opposite signs whereas their real parts remain small. Therefore, the product term exceeds the sum by a large margin, |Δu 1Δu 3|>>|Δu 1 + Δu 3| such that (1/2c1)(Δu1+Δu3) and Δu1Δu3 in Eq. (5) become comparable. Now the output amplitude can take virtually any value. Particularly, the two terms in the numerator can exactly cancel each other so that the output amplitude vanishes. Hence the large dynamic range of |(1/2c1)(Δu1+Δu3)/Δu1Δu3| in the over-coupling case causes the output amplitude given by Eq. (5) to vary widely, traversing a large fraction of the area in the unit circle. Thus the output amplitude and phase have a large dynamic range. In contrast, for an under-coupling case, it is straightforward to show that, in general,

|(1/2c1)(Δu1+Δu3)|<|Δu1Δu3|.

The dominance of the “interaction” term limits the accessible area in the unit circle.

3. Asymmetry Effect in Parallel-Coupled Dual Racetrack Resonators

As two racetrack resonators are involved in this structure, their asymmetry due to fabrication imperfections can be a major concern for practical applications. Note that the relatively long straight segments of racetracks ensure that the cross-coupling between the two resonators is insensitive to small misalignment between the left and right racetracks. As two racetracks can be patterned in one e-beam lithography process with a typical positioning accuracy of 20nm or better, the misalignment is estimated less than 1% for a coupling length L>2μm. Optical path differences between the two racetracks can usually be compensated by a proper DC bias or by additional thermo-optic heaters [24

24. M. H. Khan, H. Shen, Y. Xuan, L. Zhao, S. J. Xiao, D. E. Leaird, A. M. Weiner, and M. H. Qi, “Ultrabroad-bandwidth arbitrary radiofrequency waveform generation with a silicon photonic chip-based spectral shaper,” Nat. Photonics 4(2), 117–122 (2010). [CrossRef]

,25

25. W. A. Zortman, D. C. Trotter, and M. R. Watts, “Silicon photonics manufacturing,” Opt. Express 18(23), 23598–23607 (2010). [CrossRef] [PubMed]

]. However, the asymmetries in quality factors and coupling ratios cannot be directly compensated as easily. Therefore, their impacts on the device performance must be evaluated.

3.1 Asymmetric Coupling

For three parallel waveguides with asymmetric coupling constants, the coupled mode equations can be written as
ddz[u1(z)u2(z)u3(z)]=i[0κ120κ120κ230κ230][u1(z)u2(z)u3(z)],
(11)
where the coupling constants between waveguide pairs (1,2) and (2,3) are κ 12 and κ 23, respectively. To solve such a set of differential equation, ddz[um]=i[κmn][un], the coupling matrix is decomposed into the following form [κmn]=XΛX+, where Λ is a diagonal matrix whose diagonal elements are the eigenvalues of the matrix [κmn], the columns of X are the eigenvectors of [κmn], and XX + = I. The original equation can then be integrated according to

[um(z)]=exp(i[κmn]z)[un(0)]=Xexp(iΛz)X+[un(0)].
(12)

Thus the solution of Eq. (11) is given by
[u1(z)u2(z)u3(z)]=[cos(κ+z)ρ12+ρ32isin(κ+z)ρ1cos(κ+z)ρ1ρ3ρ1ρ3isin(κ+z)ρ1cos(κ+z)isin(κ+z)ρ3cos(κ+z)ρ1ρ3ρ1ρ3isin(κ+z)ρ3cos(κ+z)ρ32+ρ12][u1(0)u2(0)u3(0)],
(13)
where κ+=κ122+κ232, ρ1=κ12/κ+, and ρ3=κ23/κ+. In a symmetric case, κ+=2κ12=2κ23=2κ, ρ1=ρ3=1/2, Eq. (13) returns to Eq. (3). The output amplitude b 2 can be solved in a procedure similar to that given for the symmetric case. After lengthy calculations, the final result is surprisingly simple
b2=eiφ[1+1+cos(κ+L)[1cos(κ+L)](ρ12/Δu1+ρ32/Δu3)+1],
(14)
where Δun are defined the same way as in the symmetric case. Comparing Eq. (14) and Eq. (8), it is evident that all asymmetry effects can be effectively factored into the term

ρ12/Δu1+ρ32/Δu3=κ122κ+2eiφ+iθ1η11eiφ+iθ1η1+κ232κ+2eiφ+iθ3η31eiφ+iθ3η3.
(15)

As a consequence, for reasonable asymmetries in the coupling constants and resonator quality factors, there exists a pair of phases Δθ 1 and Δθ 3 such that the output amplitude b 2 vanishes. The required phase variations are plotted against the asymmetric coupling ratio, κ2312, in Fig. 4(a)
Fig. 4 Effect of asymmetric coupling constants. (a) Required phase compensation in each racetrack for up to 50% asymmetry in the coupling ratios. The characteristics of the asymmetric dual racetrack structure for the worst case scenario (κ2312 = 1.5) are illustrated in (b)-(d). (b) Output spectrum without modulation; (c) Intensity variation with index modulation; (d) Mapping of the output field on the complex plane. All parameters are the same as those used in Fig. 2 except κ 23/κ 12 is varied.
for up to 50% asymmetry. As Δθ 1 and Δθ 3 generally have opposite signs, we plot Δθ 1 and −Δθ 3 to better illustrate the deviation from symmetry. Note that Δθ 1 = −Δθ 3 is required for b 2 = 0 in a symmetric structure (κ 23/κ 12 = 1), according to Eq. (9). The difference between Δθ 1 and −Δθ 3 becomes larger as the asymmetry increases.

Figure 4(a) shows that although it is not easy to directly compensate the asymmetric coupling constants themselves, asymmetric phase shifts (through different DC biases applied to the two resonators) can be introduced to recover the low intensity states (b 2~0). The un-modulated output spectrum for the worst case (κ 23/κ 12 = 1.5) is illustrated in Fig. 4(b) and shows no anomaly. However, the intensity variation upon refractive index modulation shows obvious distortion from the symmetric case. Nonetheless, two features remain: (1) there are two points with relatively small index changes ( ± 2.2 × 10−4, ∓5.4 × 10−4) where the intensity vanishes; (2) the intensity varies significantly in the push-pull configuration and much less otherwise. The coverage on the complex E plane is slightly enhanced, although a small hole exists at a large amplitude value, which may limit the maximum accessible amplitude to 0.78 for a generic M-ary modulation format.

3.2 Asymmetric Quality Factors

The effects of asymmetric quality factors are illustrated in Fig. 5
Fig. 5 Effect of asymmetric quality factors. (a) Required phase compensation in each racetrack for asymmetry in the quality factors. The characteristics of the asymmetric dual racetrack structure for the worst case scenario (Q 3 = 0.5Q 1) are illustrated in (b)-(d). (b) Output spectrum; (c) Intensity variation with index modulations; (d) Mapping of the output field on the complex plane. All parameters are the same as those used in Fig. 2 except η 3 is varied to yield different Q 3.
. The required phase shifts, Δθ 1 and −Δθ 3, for vanishing b 2, are plotted against the ratio of the quality factors in Fig. 5(a). The unloaded quality factor Q 1 is fixed at its original value ~2.5 × 104.

Note that Δθ 1 = −Δθ 3 for the case of Q 3/Q 1 = 1 in accordance to the symmetric case. The un-modulated output spectrum for the worst case (Q 3/Q 1 = 0.5) is illustrated in Fig. 5(b). A small yet noticeable spike appears at the resonance due to the asymmetric quality factors of the two racetrack resonators. The modulated intensity variation upon refractive index modulation depicted in Fig. 5(c) shows less severe distortion compared to the distortion observed in the Fig. 4(c). Again, two features remain: (1) there are two points with relatively small index changes ( ± 4.4 × 10−4, ∓4.1 × 10−4) where the intensity vanishes; (2) the intensity varies significantly in the push-pull configuration and much less otherwise. The coverage on the complex E plane slightly deteriorates. There exists a small hole, which may limit the maximum accessible amplitude to 0.74 for a generic M-ary modulation format.

Note that the evolution from symmetry to the worst case asymmetry is gradual. For example, the two “eyes” in Fig. 4(c) gradually narrow as the asymmetry in the coupling constant worsens. Also, the centers of the “eyes” rotate clockwise around the origin (Δn 1 = Δn 3 = 0). As the asymmetry in the quality factors worsens, the “eye” centers do not narrow or rotate substantially although there are some deformations.

Overall, the asymmetry analysis presented above show that substantial asymmetries in coupling constants and quality factors of the two racetrack resonators can be compensated by refractive index changes on the order of 4 × 10−4, which can be readily provided with a low-power heater or a small change of the DC bias. Fundamentally, such compensations are possible because all these asymmetries enter the output amplitude, Eq. (14), through the term given in Eq. (15). For structures with asymmetric η’s or Q’s, asymmetric phase shifts can restore the value of the term given in Eq. (15) to a corresponding symmetric structure. Specifically, to achieve vanishing output intensity under modulation, a structure with 50% asymmetry in the coupling constant requires (Δn 1n 3) = ( ± 2.2 × 10−4, ∓5.4 × 10−4) whereas a symmetric structure requires (Δn 1n 3) = ( ± 3.5 × 10−4, ∓3.5 × 10−4). The difference between |Δn 1| and |Δn 3| in the asymmetric case is used to restore Eq. (15) to the value of the symmetric case such that b2 = 0.

4. Discussion

In general, an encoder is needed to convert an original M-ary digital signal into the driving signal for the modulator. Consider the case of a QPSK signal with four symbols shown in Fig. 3(a). The encoder will have a two-bit input and two output ports. Each output port has four output voltage levels. The design of such an encoder and its supporting circuitries has been well studied in the state-of-the-art high-speed data conversion systems [26

26. B. Razavi, Principles of Data Conversion System Design (Wiley-IEEE Press, 1994).

] and CMOS VLSI [27

27. N. H. E. Weste and D. M. Harris, CMOS VLSI Design: A Circuit and Systems Perspective (Addison-Wesley, 2010).

]. Under the given specifications (resolution, signal-to-noise ratio, bandwidth, driving power, etc.), this encoder can be easily architected and implemented as a high-speed digital-to-analog data converter, which can be fabricated economically using the silicon-on-insulator technology together with the dual racetrack resonator modulator. Note that a conventional nested Mach-Zehnder QPSK [1

1. P. J. Winzer and R. J. Essiambre, “Advanced optical modulation formats,” Proc. IEEE 94(5), 952–985 (2006). [CrossRef]

] modulator needs two output voltage levels for each port. The additional voltage levels required for the proposed QPSK modulator will somewhat increase the size of the driving circuitry. However, electronic devices such as transistors are generally significantly smaller than photonic devices. Therefore, the enlargement of the driving circuitry is usually negligible compared to the significant space saving offered by changing from a bulky nested Mach-Zehnder modulator to the proposed dual racetrack resonators.

Driving voltages and power consumption are important issues for silicon modulators used in optical interconnects [28

28. W. Jiang, L. Gu, X. Chen, and R. T. Chen, “Photonic crystal waveguide modulators for silicon photonics: device physics and some recent progress,” Solid-State Electron. 51(10), 1278–1286 (2007). [CrossRef]

,29

29. D. A. B. Miller, “Device requirements for optical interconnects to silicon chips,” Proc. IEEE 97(7), 1166–1185 (2009). [CrossRef]

]. For a nested Mach-Zehnder QPSK modulator which is biased across the minimum point of the transfer curve, a lower driving voltage and lower RF power consumption can be achieved at the expense of a lower maximum output intensity (which entails a trade-off with the detector sensitivity or the input laser power). For the proposed parallel-coupled dual racetrack modulator, a similar power reduction scheme is possible. For simplicity, we consider silicon racetrack resonators with embedded MOS capacitors, whose index change is approximately linearly dependent on the voltage. As illustrated in Fig. 6(a)
Fig. 6 Output intensity as a function of the driving power for parallel-coupled dual racetrack modulators with varying degrees of asymmetry. (a) For various κ2312 values and (b) For various Q 3 /Q 1 values. The output intensity is normalized by the input intensity. The driving power is normalized by the power level corresponding to the case that each racetrack is driven to Δn = 0.001.
, the driving power can be significantly lower at lower output intensity. Asymmetries of the coupling constants and quality factors could entail extra power penalty but the power remains reasonable. According to Fig. 6, if the asymmetry is large, electrical power penalty is significantly lower when the modulator operates at a lower output intensity level. Therefore, for a modulator that happens to have a large asymmetry due to imperfection in fabrication, the balance of the power trade-off may tip towards enhancing the detector sensitivity.

5. Conclusion

In summary, we have proposed and analyzed a parallel-coupled dual racetrack micro-resonator modulator for arbitrary M-ary quadrature signal generation. The critical coupling condition is obtained for such a structure. The intensity and phase modulations are obtained by varying the refractive indices of the silicon waveguides in the two parallel-coupled resonators. It is shown that a push-pull configuration effectively modulates the intensity and phase. The coverage of the complex plane of the output field E out is systematically studied for over-coupling, critical-coupling, and under-coupling scenarios, and is compared to the corresponding scenarios of two uncoupled racetrack-resonators in series. It is found that only the over-coupling scenario of a parallel-coupled dual racetrack resonator structure results in adequate coverage for arbitrary M-ary quadrature signal generation. The interaction between the parallel-coupled racetrack resonators is key to the coverage of the complex E plane. In an over-coupled dual racetrack structure, a delicate balance is achieved between the direct sum and the interaction of the two racetrack resonances, which results in a large dynamic range of the output amplitude and phase. Particularly, the modulated intensity can reach zero in a push-pull configuration although the intensity of the un-modulated over-coupled racetrack resonators do not vanish at any wavelength. The effects of asymmetries in the coupling constants and quality factors are systematically studied. Despite the distortion of the intensity and phase mapping, small refractive index changes, which can be readily obtained with a reasonable thermal or electrical bias, can be used to compensate the asymmetry. The coverage of the complex E plane remains sufficient despite asymmetries.

Acknowledgments

The authors are grateful to Zhong Shi, Qianfan Xu, Lin Zhang, Rene-Jean Essiambre, and Ying Qian for helpful discussions. This work is supported in part by AFOSR Grants No. FA9550-10-C-0049 and No. FA9550-08-1-0394 (G. Pomrenke). R. A. I. acknowledges the partial support of a NSF IGERT Traineeship.

References and links

1.

P. J. Winzer and R. J. Essiambre, “Advanced optical modulation formats,” Proc. IEEE 94(5), 952–985 (2006). [CrossRef]

2.

G. P. Agrawal, Fiber-Optic Communication Systems (John Wiley & Sons, 1997).

3.

R. Soref, “The past, present, and future of silicon photonics,” IEEE J. Sel. Top. Quantum Electron. 12(6), 1678–1687 (2006). [CrossRef]

4.

B. Jalali and S. Fathpour, “Silicon photonics,” J. Lightwave Technol. 24(12), 4600–4615 (2006). [CrossRef]

5.

A. S. Liu, R. Jones, L. Liao, D. Samara-Rubio, D. Rubin, O. Cohen, R. Nicolaescu, and M. Paniccia, “A high-speed silicon optical modulator based on a metal-oxide-semiconductor capacitor,” Nature 427(6975), 615–618 (2004). [CrossRef] [PubMed]

6.

Q. F. Xu, B. Schmidt, S. Pradhan, and M. Lipson, “Micrometre-scale silicon electro-optic modulator,” Nature 435(7040), 325–327 (2005). [CrossRef] [PubMed]

7.

C. Li, L. J. Zhou, and A. W. Poon, “Silicon microring carrier-injection-based modulators/switches with tunable extinction ratios and OR-logic switching by using waveguide cross-coupling,” Opt. Express 15(8), 5069–5076 (2007). [CrossRef] [PubMed]

8.

Y. C. Li, L. Zhang, M. P. Song, B. Zhang, J. Y. Yang, R. G. Beausoleil, A. E. Willner, and P. D. Dapkus, “Coupled-ring-resonator-based silicon modulator for enhanced performance,” Opt. Express 16(17), 13342–13348 (2008). [CrossRef] [PubMed]

9.

D. M. Gill, M. Rasras, K. Y. Tu, Y. K. Chen, A. E. White, S. S. Patel, D. Carothers, A. Pomerene, R. Kamocsai, C. Hill, and J. Beattie, “Internal Bandwidth Equalization in a CMOS-Compatible Si-Ring Modulator,” IEEE Photon. Technol. Lett. 21(4), 200–202 (2009). [CrossRef]

10.

D. M. Gill, S. S. Patel, M. Rasras, K. Y. Tu, A. E. White, Y. K. Chen, A. Pomerene, D. Carothers, R. L. Kamocsai, C. M. Hill, and J. Beattie, “CMOS-compatible si-ring-assisted Mach-Zehnder interferometer with internal bandwidth equalization,” IEEE J. Sel. Top. Quantum Electron. 16(1), 45–52 (2010). [CrossRef]

11.

L. Zhang, J. Y. Yang, M. Song, Y. Li, B. Zhang, R. G. Beausoleil, and A. E. Willner, “Microring-based modulation and demodulation of DPSK signal,” Opt. Express 15(18), 11564–11569 (2007). [CrossRef] [PubMed]

12.

L. Zhang, J. Y. Yang, Y. C. Li, M. P. Song, R. G. Beausoleil, and A. E. Willner, “Monolithic modulator and demodulator of differential quadrature phase-shift keying signals based on silicon microrings,” Opt. Lett. 33(13), 1428–1430 (2008). [CrossRef] [PubMed]

13.

W. M. J. Green, M. J. Rooks, L. Sekaric, and Y. A. Vlasov, “Optical modulation using anti-crossing between paired amplitude and phase resonators,” Opt. Express 15(25), 17264–17272 (2007). [CrossRef] [PubMed]

14.

W. D. Sacher and J. K. S. Poon, “Microring quadrature modulators,” Opt. Lett. 34(24), 3878–3880 (2009). [CrossRef] [PubMed]

15.

L. L. Gu, W. Jiang, X. N. Chen, L. Wang, and R. T. Chen, “High speed silicon photonic crystal waveguide modulator for low voltage operation,” Appl. Phys. Lett. 90(7), 071105 (2007). [CrossRef]

16.

X. N. Chen, Y. S. Chen, Y. Zhao, W. Jiang, and R. T. Chen, “Capacitor-embedded 0.54 pJ/bit silicon-slot photonic crystal waveguide modulator,” Opt. Lett. 34(5), 602–604 (2009). [CrossRef] [PubMed]

17.

R. A. Soref and B. R. Bennett, “Electrooptical effects in silicon,” IEEE J. Quantum Electron. 23(1), 123–129 (1987). [CrossRef]

18.

V. A. Mashkov and H. Temkin, “Propagation of eigenmodes and transfer amplitudes in optical waveguide structures,” IEEE J. Quantum Electron. 34(10), 2036–2047 (1998). [CrossRef]

19.

A. Hardy and W. Streifer, “Coupled modes of multiwaveguide systems and phased arrays,” J. Lightwave Technol. 4(1), 90–99 (1986). [CrossRef]

20.

C.-M. Kim and Y.-J. Im, “Switching operations of three-waveguide optical switches,” IEEE J. Sel. Top. Quantum Electron. 6(1), 170–174 (2000). [CrossRef]

21.

W. Jiang and R. T. Chen, “Multichannel optical add-drop processes in symmetrical waveguide-resonator systems,” Phys. Rev. Lett. 91(21), 213901 (2003). [CrossRef] [PubMed]

22.

A. Yariv, “Critical coupling and its control in optical waveguide-ring resonator systems,” IEEE Photon. Technol. Lett. 14(4), 483–485 (2002). [CrossRef]

23.

M. Soltani, S. Yegnanarayanan, Q. Li, and A. Adibi, “systematic engineering of waveguide-resonator coupling for silicon microring/microdisk/racetrack resonators: theory and experiment,” IEEE J. Quantum Electron. 46(8), 1158–1169 (2010). [CrossRef]

24.

M. H. Khan, H. Shen, Y. Xuan, L. Zhao, S. J. Xiao, D. E. Leaird, A. M. Weiner, and M. H. Qi, “Ultrabroad-bandwidth arbitrary radiofrequency waveform generation with a silicon photonic chip-based spectral shaper,” Nat. Photonics 4(2), 117–122 (2010). [CrossRef]

25.

W. A. Zortman, D. C. Trotter, and M. R. Watts, “Silicon photonics manufacturing,” Opt. Express 18(23), 23598–23607 (2010). [CrossRef] [PubMed]

26.

B. Razavi, Principles of Data Conversion System Design (Wiley-IEEE Press, 1994).

27.

N. H. E. Weste and D. M. Harris, CMOS VLSI Design: A Circuit and Systems Perspective (Addison-Wesley, 2010).

28.

W. Jiang, L. Gu, X. Chen, and R. T. Chen, “Photonic crystal waveguide modulators for silicon photonics: device physics and some recent progress,” Solid-State Electron. 51(10), 1278–1286 (2007). [CrossRef]

29.

D. A. B. Miller, “Device requirements for optical interconnects to silicon chips,” Proc. IEEE 97(7), 1166–1185 (2009). [CrossRef]

OCIS Codes
(130.3120) Integrated optics : Integrated optics devices
(230.4110) Optical devices : Modulators
(230.5750) Optical devices : Resonators

ToC Category:
Integrated Optics

History
Original Manuscript: March 28, 2011
Revised Manuscript: June 14, 2011
Manuscript Accepted: June 15, 2011
Published: July 19, 2011

Citation
Ryan A. Integlia, Lianghong Yin, Duo Ding, David Z. Pan, Douglas M. Gill, and Wei Jiang, "Parallel-coupled dual racetrack silicon micro-resonators for quadrature amplitude modulation," Opt. Express 19, 14892-14902 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-16-14892


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References

  1. P. J. Winzer and R. J. Essiambre, “Advanced optical modulation formats,” Proc. IEEE 94(5), 952–985 (2006). [CrossRef]
  2. G. P. Agrawal, Fiber-Optic Communication Systems (John Wiley & Sons, 1997).
  3. R. Soref, “The past, present, and future of silicon photonics,” IEEE J. Sel. Top. Quantum Electron. 12(6), 1678–1687 (2006). [CrossRef]
  4. B. Jalali and S. Fathpour, “Silicon photonics,” J. Lightwave Technol. 24(12), 4600–4615 (2006). [CrossRef]
  5. A. S. Liu, R. Jones, L. Liao, D. Samara-Rubio, D. Rubin, O. Cohen, R. Nicolaescu, and M. Paniccia, “A high-speed silicon optical modulator based on a metal-oxide-semiconductor capacitor,” Nature 427(6975), 615–618 (2004). [CrossRef] [PubMed]
  6. Q. F. Xu, B. Schmidt, S. Pradhan, and M. Lipson, “Micrometre-scale silicon electro-optic modulator,” Nature 435(7040), 325–327 (2005). [CrossRef] [PubMed]
  7. C. Li, L. J. Zhou, and A. W. Poon, “Silicon microring carrier-injection-based modulators/switches with tunable extinction ratios and OR-logic switching by using waveguide cross-coupling,” Opt. Express 15(8), 5069–5076 (2007). [CrossRef] [PubMed]
  8. Y. C. Li, L. Zhang, M. P. Song, B. Zhang, J. Y. Yang, R. G. Beausoleil, A. E. Willner, and P. D. Dapkus, “Coupled-ring-resonator-based silicon modulator for enhanced performance,” Opt. Express 16(17), 13342–13348 (2008). [CrossRef] [PubMed]
  9. D. M. Gill, M. Rasras, K. Y. Tu, Y. K. Chen, A. E. White, S. S. Patel, D. Carothers, A. Pomerene, R. Kamocsai, C. Hill, and J. Beattie, “Internal Bandwidth Equalization in a CMOS-Compatible Si-Ring Modulator,” IEEE Photon. Technol. Lett. 21(4), 200–202 (2009). [CrossRef]
  10. D. M. Gill, S. S. Patel, M. Rasras, K. Y. Tu, A. E. White, Y. K. Chen, A. Pomerene, D. Carothers, R. L. Kamocsai, C. M. Hill, and J. Beattie, “CMOS-compatible si-ring-assisted Mach-Zehnder interferometer with internal bandwidth equalization,” IEEE J. Sel. Top. Quantum Electron. 16(1), 45–52 (2010). [CrossRef]
  11. L. Zhang, J. Y. Yang, M. Song, Y. Li, B. Zhang, R. G. Beausoleil, and A. E. Willner, “Microring-based modulation and demodulation of DPSK signal,” Opt. Express 15(18), 11564–11569 (2007). [CrossRef] [PubMed]
  12. L. Zhang, J. Y. Yang, Y. C. Li, M. P. Song, R. G. Beausoleil, and A. E. Willner, “Monolithic modulator and demodulator of differential quadrature phase-shift keying signals based on silicon microrings,” Opt. Lett. 33(13), 1428–1430 (2008). [CrossRef] [PubMed]
  13. W. M. J. Green, M. J. Rooks, L. Sekaric, and Y. A. Vlasov, “Optical modulation using anti-crossing between paired amplitude and phase resonators,” Opt. Express 15(25), 17264–17272 (2007). [CrossRef] [PubMed]
  14. W. D. Sacher and J. K. S. Poon, “Microring quadrature modulators,” Opt. Lett. 34(24), 3878–3880 (2009). [CrossRef] [PubMed]
  15. L. L. Gu, W. Jiang, X. N. Chen, L. Wang, and R. T. Chen, “High speed silicon photonic crystal waveguide modulator for low voltage operation,” Appl. Phys. Lett. 90(7), 071105 (2007). [CrossRef]
  16. X. N. Chen, Y. S. Chen, Y. Zhao, W. Jiang, and R. T. Chen, “Capacitor-embedded 0.54 pJ/bit silicon-slot photonic crystal waveguide modulator,” Opt. Lett. 34(5), 602–604 (2009). [CrossRef] [PubMed]
  17. R. A. Soref and B. R. Bennett, “Electrooptical effects in silicon,” IEEE J. Quantum Electron. 23(1), 123–129 (1987). [CrossRef]
  18. V. A. Mashkov and H. Temkin, “Propagation of eigenmodes and transfer amplitudes in optical waveguide structures,” IEEE J. Quantum Electron. 34(10), 2036–2047 (1998). [CrossRef]
  19. A. Hardy and W. Streifer, “Coupled modes of multiwaveguide systems and phased arrays,” J. Lightwave Technol. 4(1), 90–99 (1986). [CrossRef]
  20. C.-M. Kim and Y.-J. Im, “Switching operations of three-waveguide optical switches,” IEEE J. Sel. Top. Quantum Electron. 6(1), 170–174 (2000). [CrossRef]
  21. W. Jiang and R. T. Chen, “Multichannel optical add-drop processes in symmetrical waveguide-resonator systems,” Phys. Rev. Lett. 91(21), 213901 (2003). [CrossRef] [PubMed]
  22. A. Yariv, “Critical coupling and its control in optical waveguide-ring resonator systems,” IEEE Photon. Technol. Lett. 14(4), 483–485 (2002). [CrossRef]
  23. M. Soltani, S. Yegnanarayanan, Q. Li, and A. Adibi, “systematic engineering of waveguide-resonator coupling for silicon microring/microdisk/racetrack resonators: theory and experiment,” IEEE J. Quantum Electron. 46(8), 1158–1169 (2010). [CrossRef]
  24. M. H. Khan, H. Shen, Y. Xuan, L. Zhao, S. J. Xiao, D. E. Leaird, A. M. Weiner, and M. H. Qi, “Ultrabroad-bandwidth arbitrary radiofrequency waveform generation with a silicon photonic chip-based spectral shaper,” Nat. Photonics 4(2), 117–122 (2010). [CrossRef]
  25. W. A. Zortman, D. C. Trotter, and M. R. Watts, “Silicon photonics manufacturing,” Opt. Express 18(23), 23598–23607 (2010). [CrossRef] [PubMed]
  26. B. Razavi, Principles of Data Conversion System Design (Wiley-IEEE Press, 1994).
  27. N. H. E. Weste and D. M. Harris, CMOS VLSI Design: A Circuit and Systems Perspective (Addison-Wesley, 2010).
  28. W. Jiang, L. Gu, X. Chen, and R. T. Chen, “Photonic crystal waveguide modulators for silicon photonics: device physics and some recent progress,” Solid-State Electron. 51(10), 1278–1286 (2007). [CrossRef]
  29. D. A. B. Miller, “Device requirements for optical interconnects to silicon chips,” Proc. IEEE 97(7), 1166–1185 (2009). [CrossRef]

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