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Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 4 — Feb. 13, 2012
  • pp: 4280–4290

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Lossless intensity modulation in integrated photonics

Sunil Sandhu and Shanhui Fan  »View Author Affiliations


Optics Express, Vol. 20, Issue 4, pp. 4280-4290 (2012)
http://dx.doi.org/10.1364/OE.20.004280


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Abstract

We present a dynamical analysis of lossless intensity modulation in two different ring resonator geometries. In both geometries, we demonstrate modulation schemes that result in a symmetrical output with an infinite on/off ratio. The systems behave as lossless intensity modulators where the time-averaged output optical power is equal to the time-averaged input optical power.

© 2012 OSA

1. Introduction

Integrated photonics has attracted a great deal of attention in recent years because of its potential to realize faster and less power-consuming photonic devices. One key required functionality in integrated photonics is optical modulation [1

G. T. Reed, G. Mashanovich, F. Y. Gardes, and D. J. Thomson, “Silicon optical modulators,” Nat. Photonics 4, 518–526 (2010). [CrossRef]

]. For this purpose, electro-optic intensity modulators have been experimentally demonstrated in a variety of geometries such as the Mach-Zehnder interferometer [2

A. Liu, R. Jones, L. Liao, D. Samara-Rubio 1, D. Rubin, O. Cohen, R. Nicolaescu, and M. Paniccia, “A high-speed silicon optical modulator based on a metal-oxide-semiconductor capacitor,” Nature 427, 615–618 (2004). [CrossRef] [PubMed]

–7

H.-W. Chen, Y.-H. Kuo, and J. E. Bowers, “25 Gb/s hybrid silicon switch using a capacitively loaded traveling wave electrode,” Opt. Express 18, 1070–1075 (2010). [CrossRef] [PubMed]

] and resonators [8

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

–11

S. Manipatruni, K. Preston, L. Chen, and M. Lipson, “Ultra-low voltage, ultra-small mode volume silicon microring modulator,” Opt. Express 18, 18235–18242 (2010). [CrossRef] [PubMed]

]. In particular, micro-ring resonator modulators are attractive because of their potential to achieve compact, low power-consumption and high-speed modulation [1

G. T. Reed, G. Mashanovich, F. Y. Gardes, and D. J. Thomson, “Silicon optical modulators,” Nat. Photonics 4, 518–526 (2010). [CrossRef]

]. A common way of performing optical modulation in these previously studied geometries is by operating around a lossy state where the transmission through the system is near zero. For example, in systems consisting of a micro-ring coupled to a waveguide [12

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

–14

T. Ye and X. Cai, “On power consumption of silicon-microring-based optical modulators,” J. Lightwave Technol. 28, 1615–1623 (2010). [CrossRef]

], optical modulation is usually performed by operating around the critical coupling state where the ring resonator’s intrinsic loss rate is equal to its waveguide coupling rate. However, operation around such a lossy state can result in a significant loss of optical power in these modulation schemes.

In this paper, we propose an alternative mechanism that achieves lossless intensity modulation. As an illustration, we consider lossless resonant all-pass filters consisting of a waveguide side-coupled to either a single-ring resonator or coupled-ring resonators. For such a system, when we input into the waveguide a continuous-wave (CW) signal, the steady state transmission coefficient is always unity, independent of the resonance frequency or the coupling constants of the system. Nevertheless, we show that significant intensity modulation of the system output can be achieved when the system parameters such as the resonant frequencies are modulated at a rate comparable to the waveguide coupling rate. In fact, the modulation on/off ratio, defined as the ratio of the maximum to minimum output power, can be infinity. This system behaves as a lossless intensity modulator where the time-averaged output optical power is equal to the time-averaged input optical power. Thus, the peak power of the modulated output signal is in fact higher than the input CW signal peak power. We also show that in the case of a coupled-three-ring system, a clear symmetric output pulse shape can be generated by only modulating the ring resonance frequency. Examples of possible applications of our intensity modulation schemes include optical clock signal generation and optical sampling [15

Z. Pan, S. Chandel, and C. Yu, “Ultrahigh-speed optical pulse generation using a phase modulator and two stages of delayed Mach-Zehnder interferometers,” Opt. Eng. 46, 075001 (2007). [CrossRef]

, 16

C. Schmidt-Langhorst and H.-G. Weber, “Optical sampling techniques,” J. Opt. Fiber Commun. Rep. 2, 86–114 (2005). [CrossRef]

].

2. Photon dynamics in a modulated system

The conventional way of describing optical intensity modulation is by imagining a device whose steady state transmission spectrum T varies as a function of some parameter x [Fig. 1(a)]. For example, in the simple case of a single-ring modulator [12

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

–14

T. Ye and X. Cai, “On power consumption of silicon-microring-based optical modulators,” J. Lightwave Technol. 28, 1615–1623 (2010). [CrossRef]

] shown in Fig. 2(a), x can either be the ring’s resonance frequency, its radiative loss rate or its waveguide coupling rate. At some operating frequency ω of the system, the steady state transmission spectrum has a value of Tmax for some x = x1 and a value of Tmin for some x = x2. Modulating x between x1 and x2 [Fig. 1(b)] at some frequency Ω then results in the intensity modulation of an input optical beam between the Tmax state and the Tmin state [Fig. 1(c)] at the same frequency Ω. If we return back to our example of the single-ring modulator [Fig. 2(a)], the modulation of x here can be carried out such that the single-ring system is modulated between (i) the critical coupling state where T = Tmin = 0, and (ii) away from the critical coupling state where T = Tmax ≈ 1.

Fig. 1 Conventional way of describing intensity modulation which is only valid in the adiabatic regime: (a) transmission T of system as a function of some system parameter x, (b) modulation performed on x as a function of time, (c) resultant modulation of the system transmission T as a function of time.
Fig. 2 Analysis of a single-ring system: (a) shows the schematic of the system where a(t) is the ring modal amplitude, ωo is the ring resonance frequency, Sin(t) [Sout(t)] are the incoming [outgoing] waveguide modal amplitude and γcoup(t) is the waveguide coupling rate. (b) and (c) show the system output power at t≫ 1 γo for a modulated coupling rate γcoup(t) = [0.069 + 0.025sin(Ωt)]Ω and γcoup(t) = [6.43 + 2.92sin(Ωt)]Ω, respectively. In both (b) and (c), ωo = 2π(193THz), Sin = exp(jωot) and Ω = 2π(20GHz). Circles in (b) show the output power using the approximation of Eq. (5).

It is important to realize that the schematic in Fig. 1 in fact is generally not an accurate description of the modulation process [14

T. Ye and X. Cai, “On power consumption of silicon-microring-based optical modulators,” J. Lightwave Technol. 28, 1615–1623 (2010). [CrossRef]

,17

W. D. Sacher and J. K. S. Poon, “Dynamics of microring resonator modulators,” Opt. Express 16, 15741–15753 (2008). [CrossRef] [PubMed]

]. In particular, this description implicitly assumes that the system responds instantaneously to any variation of the control parameter. However, such an instantaneous response is only valid in the adiabatic regime, when the modulation rate is far below the frequency scale of every important dynamic process of the system. A more accurate description of the modulation process requires the system dynamics to be taken into account [14

T. Ye and X. Cai, “On power consumption of silicon-microring-based optical modulators,” J. Lightwave Technol. 28, 1615–1623 (2010). [CrossRef]

, 17

W. D. Sacher and J. K. S. Poon, “Dynamics of microring resonator modulators,” Opt. Express 16, 15741–15753 (2008). [CrossRef] [PubMed]

]. In the following two sections, we study the dynamics in two types of lossless resonant all-pass filters: (i) a single lossless ring resonator coupled to a waveguide, and (ii) a lossless coupled-three-ring resonator system coupled to a waveguide. We show that in both ring systems, when we input into the waveguide a CW signal at the system resonance frequency, a symmetric modulated output with infinite on/off ratio can be achieved by modulating some parameter in the system. Both systems behave as lossless intensity modulators where the time-averaged output optical power is equal to the time-averaged input optical power.

3. Single-ring system

We first consider the system shown in Fig. 2(a), consisting of a single ring coupled to a waveguide. The system can be described by the following coupled-mode theory (CMT) equations which have been previously shown to accurately describe the propagation of light in resonator systems [18

M. F. Yanik and S. Fan, “Stopping light all optically,” Phys. Rev. Lett. 92, 083901 (2004). [CrossRef] [PubMed]

]:
da(t) dt=j ωoa(t)− [ γ coup(t)+ γ loss]a(t)+j 2 γ coup(t) S in(t) S out(t)= S in(t)+j 2 γ coup(t)a(t).
(1)
Equation (1) describes the dynamics of the amplitude a(t) of a ring resonator with the modal profile normalized such that |a(t)|2 gives the energy in the mode. γloss is the ring resonator’s amplitude-radiative loss rate, ωo is the resonance frequency of the ring, and Sin(t) [Sout(t)] denotes the amplitude of the incoming [outgoing] wave in the waveguide with |Sin(t)|2 and |Sout(t)|2 giving the power in the waveguide mode. γcoup(t) is the time-dependent waveguide-ring amplitude coupling rate, related to the waveguide-ring power coupling ratio 1 – |exp (−γcoupL/v)|2, where L = circumference of the ring and v = speed of light in the ring [12

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

].

In the case of a CW input Sin(t) = exp(jωt) and static coupling rate γcoup(t) = γcoup, the transmission spectrum of the single-ring system is:
T(ω)= S out S in= ω− ωo+j( γ coup− γ loss) ω− ωo−j( γ coup+ γ loss).
If we further assume the system is lossless (i.e. γloss = 0), the power transmission coefficient of the system is |T(ω)| = 1 for all values of the coupling rate γcoup and ring resonant frequency ωo. Thus, the conventional description of intensity modulation in Fig. 1, which neglects the system dynamics, predicts that for the lossless ring system in Fig. 2, modulating any parameter at any modulation frequency will not result in the intensity modulation of an input optical beam.

We next examine the dynamical behavior of such a lossless ring system in the case of some time-dependent coupling rate γcoup(t) and CW input Sin(t) = exp(jωo t) operating at the ring resonance frequency ωo. From Eq. (1) we can derive the following analytical form of the system output:
S out(t)= [ 1+B(t)]exp(j ωot),
(2)
B(t)=j 2 γ coup(t)A(t), A(t)=jexp [ − ∫0t γ coup( t′)d t′] ∫0t 2 γ coup(τ)exp [ ∫0τ γ coup( t′)d t′]dτ,
(3)
where the resonator amplitude a(t) = A(t) exp(jωo t). The output Sout(t) in Eq. (2) can be described as having a carrier frequency ωo and an envelope 1 + B(t). The envelope results from the interference between a direct pathway of unity amplitude and an indirect pathway ring resonance assisted amplitude B(t). The expression for B(t) in Eq. (3) consists of integrals which contains memory effects as discussed in Ref. [17

W. D. Sacher and J. K. S. Poon, “Dynamics of microring resonator modulators,” Opt. Express 16, 15741–15753 (2008). [CrossRef] [PubMed]

]. In the discussion below, we will demonstrate that these memory effects, which are significant only when the modulation is in the non-adiabatic regime, can give rise to lossless intensity modulation.

In the following examples, we specialize to a sinusoidal modulation of the waveguide coupling rate at a modulation frequency Ω = 2π(20GHz):
γ coup(t)= γo+Δγsin(Ωt)
(4)
where γo is the mean coupling rate amplitude and Δγ is the modulation amplitude. We numerically solve the single-ring system CMT equations [Eq. (1)] for the output Sout(t). Figure 2(b) and 2(c) show the output power solutions at t≫ 1 γo for the cases (γo = 0.069Ω, Δγ = 0.025Ω) and (γo = 6.43Ω, Δγ = 2.92Ω), respectively. In both of these examples, the output power is modulated between a maximum amplitude state and a zero amplitude state (i.e. infinite on/off ratio) with a modulation frequency equivalent to the coupling rate modulation frequency Ω = 2π(20GHz). Qualitatively, the maximum amplitude state in Fig. 2(b) and (c) occurs when there is constructive interference between the direct pathway amplitude and the resonance assisted indirect pathway amplitude in Eq. (2), while the zero amplitude state occurs when there is destructive interference between the pathways. In general, for any mean coupling rate amplitude γo ≪ ωo in Eq. (4), an infinite modulation on/off ratio can be achieved by an appropriate choice of the modulation amplitude Δγ.

We also see that a symmetrical output envelope is obtained in the weak coupling rate regime [Fig. 2(b)] where γo, Δγ ≪ Ω and Δγ < γo in Eq. (4). In this weak coupling rate regime, assuming a sinusoidal modulation of the coupling rate [Eq. (4)], the indirect pathway amplitude B(t) in Eq. (3) at t≫ 1 γo can be approximated as:
B(t)≈2 γ coup(t) γo [ ( Δγ 4 γo)2−1].
(5)
The circles in Fig. 2(b) shows a plot of the output power [Eq. (2)] using the approximation in Eq. (5). We see that there is excellent agreement with the unapproximated form (solid line) using Eq. (2) and (3). In Eq. (5), the resonator amplitude a(t) within the original B(t) expression [Eq. (3)] has been approximated by a constant. This constant energy within the resonator results in the modulation of the output envelope [Eq. (2)] being only driven by the γ coup(t) term in Eq. (5). Hence, the output envelope is symmetrical in the weak coupling regime. Equation (5) also shows that for any mean coupling rate γo in this weak coupling regime, an infinite on/off ratio can be achieved by using a modulation amplitude Δγ ≈ 0.73γo.

On the other hand, strong coupling to the waveguide results in an asymmetrical output envelope [Fig. 2(c)]. For our sinusoidal modulation of the coupling rate in Eq. (4), the resonator amplitude a(t) within the B(t) expression [Eq. (3)] generally oscillates with the same periodicity as the coupling rate. However, in the strong coupling regime, the ratio of the variance to the mean value of |a(t)| is significant. Hence, the modulation of the output envelope is driven by the product of a γ coup(t) term and a non-constant resonator amplitude term in Eq. (3). In general, within a modulation cycle of the coupling rate, there is a time delay between the maximum points and between the minimum points of both these driving terms. Consequently, the output envelope is asymmetrical in the strong coupling regime.

We also emphasize that our above discussion of lossless optical modulation in either the weak coupling regime [Fig. 2(b)] or the strong coupling regime [Fig. 2(c)] is different as compared to the modulation schemes studied in Ref. [12

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

–14

T. Ye and X. Cai, “On power consumption of silicon-microring-based optical modulators,” J. Lightwave Technol. 28, 1615–1623 (2010). [CrossRef]

]. In particular, the modulation schemes in Ref. [12

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

–14

T. Ye and X. Cai, “On power consumption of silicon-microring-based optical modulators,” J. Lightwave Technol. 28, 1615–1623 (2010). [CrossRef]

] involve operation around the critical-coupling state which can result in a significant loss of optical power.

One common way of implementing the coupling modulation scheme in Fig. 2(a) is using either a composite interferometer [Fig. 3] or a simple directional coupler as outlined in Ref. [12

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

]. However, such an implementation can result in a longer device length scale, and also higher power consumption [1

G. T. Reed, G. Mashanovich, F. Y. Gardes, and D. J. Thomson, “Silicon optical modulators,” Nat. Photonics 4, 518–526 (2010). [CrossRef]

, 14

T. Ye and X. Cai, “On power consumption of silicon-microring-based optical modulators,” J. Lightwave Technol. 28, 1615–1623 (2010). [CrossRef]

].

Fig. 3 Example implementation of waveguide coupling rate modulation in a single-ring system using a composite interferometer (CI) [12

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

]. The CI consists of a Mach-Zehnder interferometer (MZI) sandwiched between two 3dB couplers. The MZI is driven in a push-pull configuration with modulated propagation phases ±Δθ(t) that modulate the waveguide coupling rate.

4. Coupled-three-ring system

To overcome the length scale and power consumption issues associated with the structure shown in Fig. 3, we next introduce a modulation scheme based on coupled-ring resonators, where the system’s effective waveguide coupling rate and, hence, output power can be modulated by modulating the resonance frequencies of a pair of resonators. In addition, we show that the resulting modulated output envelope of the system can be symmetrical with an infinite on/off ratio. Our system (Fig. 4) consists of a pair of side ring resonators with modal amplitudes p(t) and q(t), coupled to a central ring resonator with modal amplitude a(t). The coupling rate between each side ring and the central ring is κ, and the side rings are not directly coupled to each other. The central ring has a static resonance fequency ωo while the two side rings have dynamic resonance frequencies ωo + Δ(t) and ωo – Δ(t), respectively. The central ring is coupled to a waveguide and this central-ring-waveguide part of the system has the same geometry as the single-ring system discussed in Section 3. The coupled-three-ring system can be described by the following CMT equations:
da(t) dt=j ωoa(t)+jκ[p(t)+q(t)]−( γ coup+ γ loss)a(t)+j 2 γ coup S in(t) dp(t) dt=j[ ωo+Δ(t)]p(t)+jκa(t)− γ lossp(t) dq(t) dt=j[ ωo−Δ(t)]q(t)+jκa(t)− γ lossq(t) S out(t)= S in(t)+j 2 γ coupa(t).
(6)

Fig. 4 Schematic of the coupled-three-ring system where a(t), p(t) and q(t) are the rings’ modal amplitudes, Sin(t) [Sout(t)] is the incoming [outgoing] waveguide modal amplitude, κ is the inter-ring coupling rate, γcoup is the waveguide coupling rate, ωo is the central ring resonance frequency, and Δ(t) is the side ring detuning.

In the case of a CW input Sin(t) = exp(jωt) and a static side ring resonance frequency detuning Δ(t) = Δ, the transmission through the system is:
T(ω) = S out S in= ω− ωo+y+j( γ coup− γ loss) ω− ωo+y−j( γ coup+ γ loss) y = 2 κ2(ω− ωo−j γ loss) Δ2− ( ω− ωo−j γ loss)2.
If we further assume the system is lossless (i.e. γloss = 0), the absolute transmission of the system is |T(ω)| = 1 for all values of the detuning Δ. On the other hand, the spectra of energy stored in each of the three resonators in Fig. 4 varies with Δ.

As a direct check of the CMT model [Eq. (6)], we simulate a coupled-three-ring system by solving Maxwells equations using the finite-difference time-domain (FDTD) method [19

A. Taflove and S. C. Hagness, Computational Electrodynamics: The Finite-Difference Time-Domain Method , 3rd ed. (Artech House, 2005).

]. For the FDTD simulations, the straight waveguide in Fig. 4 is chosen to have a width of 0.127μm, such that the waveguide supports only a single mode in the 1.55μm wavelength range. Each ring resonator waveguide has the same width as the straight waveguide, and a ring radius of 2μm (measured from the center of the ring to its outer circumference). The center-to-center separation between the central ring and the straight waveguide is 2.417μm while the center-to-center separation between the central ring and each side ring is 4.653μm. The center-to-center separation between the side rings is 6.581μm. The straight waveguide and side rings have a refractive index of 3.5, while the central ring has a refractive index of 3.500491. This results in all three rings having an identical resonance frequency ωo = 2π(193THz) when the side ring detuning is Δ = 0. The inter-ring coupling rate between the central ring and each side ring is κ = 2π(17.9GHz), the waveguide coupling rate is γcoup = 2π(18.7GHz), and each ring has a very low amplitude-radiative loss rate of γloss = 2π(38.6MHz). The circles in Fig. 5 show the FDTD simulation results for the energy spectra |a(ω)|2 within the central ring at three different side ring detunings. Also shown in Fig. 5 are the spectras (solid lines) from the CMT model of the system [Eq. (6)] with identical values of the system parameters as in the FDTD simulations. Both the analytical CMT plots and FDTD simulation results show excellent agreement.

Fig. 5 Plots of the normalized spectra of energy |a(ω)|2 of the central ring resonator in the coupled-three-ring system (Fig. 4) from both FDTD simulations (circles) and the CMT model (solid line). The three spectras are at different side ring detunings Δ.

We next briefly comment on the spectras at the three different side ring detunings in Fig. 5: at zero detuning [Fig. 5(a)], the energy in the central ring is zero at its resonance frequency ωo, and hence the system at this resonance frequency is at a dark state that is completely decoupled from the waveguide. When the side ring detuning Δ is non-zero [Fig. 5(b) and 5(c)], the spectrum of the energy in the central ring has a peak centered at its resonance frequency. In addition, the width of this peak increases as Δ is increased. This behavior is similar to varying the waveguide coupling rate in a single-ring system [Section 3] [12

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

]. Namely, changing the waveguide coupling rate in the single-ring system also results in a variation of the resonator amplitude spectra width, while the peak center of the spectra stays fixed at the resonance frequency. This analogy suggests that varying the side ring detuning in Fig. 5 is similar to varying the effective waveguide coupling rate of the coupled-three-ring system.

The steady state analysis that was just presented motivates us to consider the possibility of modulating the system output by modulating the side ring detuning around the dark state. We next present a dynamical analysis of such a modulation process in a lossless coupled-three-ring system.

The modulation scheme we use in the following discussion involves a push-pull configuration where there is a π phase diferrence between the detunings Δ(t) of the side rings. This push-pull configuration can be shown to result in zero chirp in the output Sout(t) [Eq. (6)] for an input Sin(t) operating at the resonance frequency ω = ωo. We note that a chirpless output is also a characteristic of a waveguide-coupling modulated single-ring system [Eq. (2)]. We also specialize to a Ω = 2π(20GHz) sinusoidal Δ(t) modulation:
Δ(t)=δωsin(Ωt)
(7)
where δω is the resonance frequency modulation amplitude.

We numerically simulate the modulation process using the FDTD method. The FDTD simulation setup is identical to the setup for obtaining the central ring spectras in Fig. 5. We therefore operate in a non-adiabatic regime where the side ring detuning modulation rate Ω is comparable to the system coupling rates γcoup and κ. Figure 6 shows the FDTD result (circles) of the system output power at t ≫ 1/γcoup for the case of a CW input Sin(t) = exp(jωo t) operating at the resonance frequency ωo = 2π(193THz) of the central ring, and a side ring modulation amplitude δω = 2π(27.65GHz) in Eq. (7). We emphasize that the time-averaged output optical power in Fig. 6 is equal to the time-averaged input optical power. In addition, the modulated output waveform is symmetrical with an infinite on/off ratio, and an output modulation frequency of 40GHz that is twice the side ring modulation frequency. Also shown in Fig. 6 is the result (solid line) from numerically solving the system’s CMT equations [Eq. (6)]. The CMT simulation has identical values of the system paramaters as in the FDTD simulation, except for a slight adjustment of the side ring detuning modulation amplitude to δω = 2π(26.3GHz) in order to fit the FDTD results. We also note that the phase of Sout(t) in Eq. (6) is zero at all times during the modulation.

Fig. 6 Plot showing the 40GHz modulated output power for the coupled-three-ring system (Fig. 4) at t ≫ 1/γcoup from both CMT (solid line) and FDTD (circles) simulations. The side ring resonance frequency detuning Δ(t) is modulated at a frequency 20GHz and amplitude δω = 2π(27.65GHz), while the other parameters are as follows: κ = 2π(17.9GHz), γcoup = 2π(18.7GHz), ωo = 2π(193THz), and Sin(t) = exp(jωot).

We next provide a qualitative explanation of the modulated output waveform in Fig. 6 based on the CMT model [Eq. (6)] of the system. Similar to the dynamics of the single-ring system discussed earlier, the output amplitude Sout(t) of the coupled-three-ring system (Fig. 4) is the interference between a direct-path amplitude and an indirect-path amplitude, where the latter amplitude is now a coupled-three-ring resonance assisted indirect-path amplitude. Starting from any maximum output point in Fig. 6, the modulated output power trajectory in half a modulation period consists of the following three characteristic states whose electric field plots are shown in Fig. 7: (a) a maximum output power state, (b) a dark state and (c) a zero output power state. The maximum output power state [Fig. 7(a)] occurs when the direct-path amplitude interferes constructively with the indirect-path amplitude, while the zero output power state [Fig. 7(c)] occurs when there is destructive interference between the two pathways. In between this maximum and zero output power states is the dark state shown in Fig. 7(b) where the central ring amplitude a(t) is zero. At this dark state, the central ring is completely decoupled from the waveguide, and the system ouput consists of only the direct-path amplitude [Sout(t) = Sin(t)].

Fig. 7 Coupled-three-ring system electric field plots from FDTD simulations around the (a) maximum output power state, (b) dark state, and (c) zero output power state in Fig. 6. The electric field is polarized normal to the page.

We also note that the output envelope in Fig. 6 has a modulation rate that is double the side ring modulation rate Ω [Eq. (7)]: within one modulation period 2π/Ω of the side ring detuning Δ(t), the coupled-three-ring system states at times t = t1 and t = t2 = t1 +π/Ω are identical up to a flip in the sign of the detunings in both side resonators. Consequently, the system output has identical values at both times t1 and t2 within a modulation period, resulting in two identical output pulses for every one modulation cycle of the side ring detuning. This frequency doubling can be avoided by using a modulation Δ(t) in Eq. (6) that is always positive, for example.

5. Conclusion

In this paper, we presented a dynamical analysis of lossless modulation in two different resonator geometries: a single-ring system and a coupled-three-ring system. In both geometries, we demonstrated modulation schemes that result in a symmetrical output with an infinite on/off ratio. Both systems behave as lossless intensity modulators where the time-averaged output optical power is equal to the time-averaged input optical power. Although we only considered ring resonators with negligible intrinsic loss, the addition of intrinsic loss to the resonators results in little loss of optical power during the modulation process, as long as the resonator intrinsic loss rate is much smaller than the coupling rates of the system. For example, for a ring resonator with intrinsic loss rate of γloss = 0.65GHz [20

Q. Reed, P. Dong, and M. Lipson, “Breaking the delay-bandwidth limit in a photonic structure,” Nat. Phys. 3, 406–410 (2007). [CrossRef]

], both systems can be designed to have a time-averaged output optical power that is > 90% of the time-averaged input optical power.

In the coupled-three-ring system, lossless output modulation was achieved by performing push-pull modulation of the side ring detuning Δ(t) around the dark state of the system. In our numerical simulation example (Fig. 6), modulation of Δ(t) requires a fractional refractive index tuning of ∼ 10−4 which can be implemented using free carrier injection/depletion in silicon [20

Q. Reed, P. Dong, and M. Lipson, “Breaking the delay-bandwidth limit in a photonic structure,” Nat. Phys. 3, 406–410 (2007). [CrossRef]

–22

Q. Xu, “Silicon dual-ring modulator,” Opt. Express 17, 20783–20793 (2009). [CrossRef] [PubMed]

]. Ref. [22

Q. Xu, “Silicon dual-ring modulator,” Opt. Express 17, 20783–20793 (2009). [CrossRef] [PubMed]

] also includes an example implementation for modulating the two side rings in parallel within a simple integrated circuit that allows for fast modulation of the side ring detuning.

Acknowledgments

The simulations were performed at the Pittsburgh Bigben Supercomputing Center and at the San Diego Trestles Supercomputer Center.

References and links

1.

G. T. Reed, G. Mashanovich, F. Y. Gardes, and D. J. Thomson, “Silicon optical modulators,” Nat. Photonics 4, 518–526 (2010). [CrossRef]

2.

A. Liu, R. Jones, L. Liao, D. Samara-Rubio 1, D. Rubin, O. Cohen, R. Nicolaescu, and M. Paniccia, “A high-speed silicon optical modulator based on a metal-oxide-semiconductor capacitor,” Nature 427, 615–618 (2004). [CrossRef] [PubMed]

3.

L. Liao, D. Samara-Rubio, M. Morse, A. Liu, D. Hodge, D. Rubin, U. Keil, and T. Franck, “High speed silicon Mach-Zehnder modulator,” Opt. Express 13, 3129–3135 (2005). [CrossRef] [PubMed]

4.

L. Liao, A. Liu, J. Basak, H. Nguyen, M. Paniccia, D. Rubin, Y. Chetrit, R. Cohen, and N. Izhaky, “40 Gbit/s silicon optical modulator for highspeed applications,” Electron. Lett. 43, 1196–1197 (2007). [CrossRef]

5.

A. Liu, L. Liao, D. Rubin, H. Nguyen, B. Ciftcioglu, Y. Chetrit, N. Izhaky, and M. Paniccia, “High-speed optical modulation based on carrier depletion in a silicon waveguide,” Opt. Express 15, 660–668 (2007). [CrossRef] [PubMed]

6.

X. 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, 602–604 (2009). [CrossRef] [PubMed]

7.

H.-W. Chen, Y.-H. Kuo, and J. E. Bowers, “25 Gb/s hybrid silicon switch using a capacitively loaded traveling wave electrode,” Opt. Express 18, 1070–1075 (2010). [CrossRef] [PubMed]

8.

Q. Xu, B. Schmidt, S. Pradhan, and M. Lipson, “Micrometre-scale silicon electro-optic modulator,” Nature 435, 325–327 (2005). [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, 200–202 (2009). [CrossRef]

10.

T. Tanabe, K. Nishiguchi, E. Kuramochi, and M. Notomi, “Low power and fast electro-optic silicon modulator with lateral p-i-n embedded photonic crystal nanocavity,” Opt. Express 17, 22505–22513 (2009). [CrossRef]

11.

S. Manipatruni, K. Preston, L. Chen, and M. Lipson, “Ultra-low voltage, ultra-small mode volume silicon microring modulator,” Opt. Express 18, 18235–18242 (2010). [CrossRef] [PubMed]

12.

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

13.

W. D. Sacher and J. K. S. Poon, “Characteristics of microring resonators with waveguide-resonator coupling modulation,” J. Lightwave Technol. 27, 3800–3811 (2009). [CrossRef]

14.

T. Ye and X. Cai, “On power consumption of silicon-microring-based optical modulators,” J. Lightwave Technol. 28, 1615–1623 (2010). [CrossRef]

15.

Z. Pan, S. Chandel, and C. Yu, “Ultrahigh-speed optical pulse generation using a phase modulator and two stages of delayed Mach-Zehnder interferometers,” Opt. Eng. 46, 075001 (2007). [CrossRef]

16.

C. Schmidt-Langhorst and H.-G. Weber, “Optical sampling techniques,” J. Opt. Fiber Commun. Rep. 2, 86–114 (2005). [CrossRef]

17.

W. D. Sacher and J. K. S. Poon, “Dynamics of microring resonator modulators,” Opt. Express 16, 15741–15753 (2008). [CrossRef] [PubMed]

18.

M. F. Yanik and S. Fan, “Stopping light all optically,” Phys. Rev. Lett. 92, 083901 (2004). [CrossRef] [PubMed]

19.

A. Taflove and S. C. Hagness, Computational Electrodynamics: The Finite-Difference Time-Domain Method , 3rd ed. (Artech House, 2005).

20.

Q. Reed, P. Dong, and M. Lipson, “Breaking the delay-bandwidth limit in a photonic structure,” Nat. Phys. 3, 406–410 (2007). [CrossRef]

21.

M. Lipson, “Guiding, modulating, and emitting light on silicon-challenges and opportunities,” J. Lightwave Technol. 23, 4222 – 4238 (2005). [CrossRef]

22.

Q. Xu, “Silicon dual-ring modulator,” Opt. Express 17, 20783–20793 (2009). [CrossRef] [PubMed]

OCIS Codes
(140.4780) Lasers and laser optics : Optical resonators
(230.0230) Optical devices : Optical devices
(350.4238) Other areas of optics : Nanophotonics and photonic crystals

ToC Category:
Integrated Optics

History
Original Manuscript: November 4, 2011
Revised Manuscript: January 20, 2012
Manuscript Accepted: January 30, 2012
Published: February 7, 2012

Citation
Sunil Sandhu and Shanhui Fan, "Lossless intensity modulation in integrated photonics," Opt. Express 20, 4280-4290 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-4-4280


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References

  1. G. T. Reed, G. Mashanovich, F. Y. Gardes, and D. J. Thomson, “Silicon optical modulators,” Nat. Photonics4, 518–526 (2010). [CrossRef]
  2. A. 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,” Nature427, 615–618 (2004). [CrossRef] [PubMed]
  3. L. Liao, D. Samara-Rubio, M. Morse, A. Liu, D. Hodge, D. Rubin, U. Keil, and T. Franck, “High speed silicon Mach-Zehnder modulator,” Opt. Express13, 3129–3135 (2005). [CrossRef] [PubMed]
  4. L. Liao, A. Liu, J. Basak, H. Nguyen, M. Paniccia, D. Rubin, Y. Chetrit, R. Cohen, and N. Izhaky, “40 Gbit/s silicon optical modulator for highspeed applications,” Electron. Lett.43, 1196–1197 (2007). [CrossRef]
  5. A. Liu, L. Liao, D. Rubin, H. Nguyen, B. Ciftcioglu, Y. Chetrit, N. Izhaky, and M. Paniccia, “High-speed optical modulation based on carrier depletion in a silicon waveguide,” Opt. Express15, 660–668 (2007). [CrossRef] [PubMed]
  6. X. 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, 602–604 (2009). [CrossRef] [PubMed]
  7. H.-W. Chen, Y.-H. Kuo, and J. E. Bowers, “25 Gb/s hybrid silicon switch using a capacitively loaded traveling wave electrode,” Opt. Express18, 1070–1075 (2010). [CrossRef] [PubMed]
  8. Q. Xu, B. Schmidt, S. Pradhan, and M. Lipson, “Micrometre-scale silicon electro-optic modulator,” Nature435, 325–327 (2005). [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, 200–202 (2009). [CrossRef]
  10. T. Tanabe, K. Nishiguchi, E. Kuramochi, and M. Notomi, “Low power and fast electro-optic silicon modulator with lateral p-i-n embedded photonic crystal nanocavity,” Opt. Express17, 22505–22513 (2009). [CrossRef]
  11. S. Manipatruni, K. Preston, L. Chen, and M. Lipson, “Ultra-low voltage, ultra-small mode volume silicon microring modulator,” Opt. Express18, 18235–18242 (2010). [CrossRef] [PubMed]
  12. A. Yariv, “Critical coupling and its control in optical waveguide-ring resonator systems,” IEEE Photon. Technol. Lett.14, 483–485 (2002). [CrossRef]
  13. W. D. Sacher and J. K. S. Poon, “Characteristics of microring resonators with waveguide-resonator coupling modulation,” J. Lightwave Technol.27, 3800–3811 (2009). [CrossRef]
  14. T. Ye and X. Cai, “On power consumption of silicon-microring-based optical modulators,” J. Lightwave Technol.28, 1615–1623 (2010). [CrossRef]
  15. Z. Pan, S. Chandel, and C. Yu, “Ultrahigh-speed optical pulse generation using a phase modulator and two stages of delayed Mach-Zehnder interferometers,” Opt. Eng.46, 075001 (2007). [CrossRef]
  16. C. Schmidt-Langhorst and H.-G. Weber, “Optical sampling techniques,” J. Opt. Fiber Commun. Rep.2, 86–114 (2005). [CrossRef]
  17. W. D. Sacher and J. K. S. Poon, “Dynamics of microring resonator modulators,” Opt. Express16, 15741–15753 (2008). [CrossRef] [PubMed]
  18. M. F. Yanik and S. Fan, “Stopping light all optically,” Phys. Rev. Lett.92, 083901 (2004). [CrossRef] [PubMed]
  19. A. Taflove and S. C. Hagness, Computational Electrodynamics: The Finite-Difference Time-Domain Method, 3rd ed. (Artech House, 2005).
  20. Q. Reed, P. Dong, and M. Lipson, “Breaking the delay-bandwidth limit in a photonic structure,” Nat. Phys.3, 406–410 (2007). [CrossRef]
  21. M. Lipson, “Guiding, modulating, and emitting light on silicon-challenges and opportunities,” J. Lightwave Technol.23, 4222 – 4238 (2005). [CrossRef]
  22. Q. Xu, “Silicon dual-ring modulator,” Opt. Express17, 20783–20793 (2009). [CrossRef] [PubMed]

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