We investigated the dynamic response of a cascaded-ring-resonator-loaded Mach–Zehnder modulator (CRR-MZM), in which a number of cascaded ring resonators (RRs) are loaded in the interferometer as phase modulators. The analytical form is derived for the small-signal response of CRR-MZM using temporal-coupled-mode (TCM) theory, and its validity is confirmed by numerical calculations. It is revealed that the bandwidth of the CRR-MZM is maximized by setting proper delays in driving signals between neighboring RRs; the optimized delay is twice the photon lifetime of each RR. The calculated performances of CRR-MZMs are compared with those of standard modulators based on a single-ring-resonator (SRR) without interferometer, in terms of the modulation depth and bandwidth. For a given degree of the refractive index change in a waveguide, CRR-MZM can provide a larger modulation depth than a SRR-type modulator in frequency ranges exceeding 25 GHz.
© 2012 OSA
Optical modulators play a key role in optical communication systems by converting high-speed electrical signals into optical signals. The fundamental requirement for modulators is that they transmit a large amount of data per unit time . Therefore, modulators should have wide bandwidths and operate at speeds that are as high as possible. In most applications, another important metric for modulators is power consumption . It is necessary to reduce the power consumption of modulators while keeping a sufficient modulation depth. Thus, it is necessary for modulators to increase the optical response to an applied electrical signal.
The use of optical resonators has shown promise in increasing the response of modulators [2–17]. Light dwells in resonators, increasing the interaction of light with matter, and causes the enhancement of any kind of electro-optical effect [16, 17]. This approach has been actively investigated, particularly for modulators that are based on silicon [2–13]. One reason is that silicon lacks the strong electro-optical effect unlike III-V systems , and the use of resonators is therefore desired to enhance the modulation. The other reason is that the silicon waveguide is suitable for configuring the micrometer-scale resonators needed for modulators, due to the tight confinement of light in the waveguide [2–13, 19–21].
In particular, modulators that are based on a single micro ring resonator (RR) in an all-pass filter configuration have been extensively investigated [2–7]. They have achieved the lowest driving voltage and highest modulation speed among silicon modulators that use resonators [6, 7]. These modulators have sharp dips in the intensity transmission spectrum at the resonant wavelengths. A tiny change in the refractive index due to the electrical input signals causes sufficient intensity variations by shifting the resonant wavelengths; we call this type of modulator a single-ring-resonator-based spectral shifting modulator (SRR-SSM). However, one drawback is becoming increasingly noticeable in the SRR-SSM as the operating speed of silicon modulators increases; they are approaching the inherent limit in operating speed associated with the photon lifetime in the RR [6, 22–25]. For a given change in refractive index, it is necessary to increase the Q value of the RR to obtain adequate modulation. This inevitably causes a large photon lifetime in the RR, and therefore limits its modulation response at high frequencies.
CRR-MZMs can be used to mitigate this problem, in which a number of RRs are cascaded and loaded in the interferometer as phase modulators [8, 10]. For a given change in the refractive index in the waveguide, it is possible to obtain a large response by increasing the Q value of the RRs and also by increasing the number of cascaded RRs. Therefore, it is expected that CRR-MZMs provide sufficient intensity modulation while maintaining wide modulation bandwidths by using CRRs with relatively low Q, and thus short lifetime. For CRR-MZMs, while performances have been reported emphasizing the modulation enhancement and optical bandwidth , the characteristics and limitations for high-speed operations have not yet been theoretically or experimentally investigated.
In this paper, we investigated the characteristics and limitations of a CRR-MZM in high-speed operation using analytical and numerical calculations. We first derived the analytical formula of the small-signal response of CRR-MZMs by using the temporal-coupled-mode (TCM) theory [19, 26], and confirmed its validity by way of numerical calculations. Using the analytical model, we revealed that the bandwidth of the CRR-MZM is maximized by setting proper delays in the driving signals between neighboring RRs. We compared the calculated performances of the CRR-MZM with those of a standard SRR-SSM, in terms of modulation depth and bandwidth. For a given index change in waveguides, CRR-MZM can provide modulation responses that are larger than those of SRR-SSM, with a propagation loss of 10 dB/cm, and in frequency bands that are larger than 25 GHz.
2. Analytical modeling
In this section, we derive the analytical formula of the response of CRR-MZM for small-signal modulation. First, we start with a standard single RR without interferometer, SRR-SSM, as shown in Fig. 1(a) . We used TCM theory to describe the system [19, 24–26].Fig. 1(b). R, vg, and αdB are the radius, group velocity and attenuation constant of the ring waveguide, respectively.
Throughout this paper, we consider only the refractive index modulation of the RR, in which only the refractive index is modulated according to the electrical signal that is uniformly applied to each RR. The attenuation constant and the coupling coefficient are always constant. One may assume that the group index of the waveguide is also modulated with the refractive index, and that this may affect the behavior of the RRs through Eq. (3). However, we neglected the change of the group index, as was done in other analyses [23–25]. This is reasonable because in RR-based modulators, the modulation occurs due to the interference in the coupling region, which is governed not by the group index but by the refractive index. Thus, in our analysis, ωr is a time-dependent function ωr(t). When a voltage is applied to the ring resonator, the resonant frequency ωr is modulated at a frequency of Ω around its static resonant frequency ω0 with an amplitude of δω.Equation (5) is a linear ordinary differential equation. The solution of Eq. (5) is analytically given in integral form as follows:Eq. (7), we apply:Eq. (9), the integral in Eq. (7) is calculated for a sufficiently large t, as follows:Equation (10) expressed the frequency response of RR for small signal modulation with an arbitrary choice of the parameter sets of τ, μ, Δω, and Ω. Note that μ, Δω, δω, and Ω are all normalized by τ in the above equation, with the exception of exponential terms. The right hand side of Eq. (10) contains three terms. The first term is the static solution of RR, while the second and third terms are frequency components with angular frequencies of ω ± Ω, respectively. These two components are created as side-band components around ω due to the modulation of Ω. Because of the small signal approximation, the side bands of ω ± 2Ω, ω ± 3Ω, ..., which have higher orders of δω, do not appear in Eq. (10). We introduced three functions, F(0)(μ2τ, Δωτ), F+(μ2τ, Δωτ, Ωτ), and F-(μ2τ, Δωτ, Ωτ), which correspond to the three terms, as in the second line in Eq. (10).
The response of SRR-SSM has already been reported in other works [22–25]. We present it for comparison purposes with the results for CRR-MZM. In Eq. (10), we reduced the variables by substituting μ2τ = 1 and Δωτ = 1. The former indicates a condition of the critical coupling, i.e., τc = τl . The latter condition concerns the detuning of the angular frequency between the input light and the resonance of RR. With Δωτ = 1, the output in a static solution from the RR is equal to one half of the input power. By substituting these formulas in Eq. (10), for S0 = 1, we obtained
Next, we derive the response of CRR-MZM. Figure 2 schematically shows the approach that we used to analyze CRR. At the first left-most RR, continuous light with ω is inputted, and three frequency components are created, as indicated by black bold arrows in Fig. 2 and Eq. (10). We can consider these three components as the input to the second RR, and as continuous waves with frequencies ω and ω ± Ω, respectively. Therefore, Eq. (10) can be re-applied to each of these three components to calculate their respective outputs. At the second RR, the inputs with a frequency of ω ± Ω may create frequency components of ω and ω ± 2Ω, respectively. However, these four components are all for second-order terms about δω,, and are therefore neglected in small-signal analysis. Consequently, the second RR also outputs continuous lights with frequencies of ω and ω ± Ω. By making this speculation regarding the following RRs, it is noted that even for CRR with N > 2, the frequency components with ω and ω ± Ω are enough to be considered for our small signal analysis; the input and output of the j-th RR both have three frequency components of ω and ω ± Ω. As shown in Fig. 2, we defined the amplitude of the outputs from the j-th RR with frequencies ω, ω + Ω, and ω − Ω as aj(0), aj+, and aj-, respectively. Next, we derive a recurrence formula for aj(0), aj+, and aj-. The red bold arrows correspond to the dependencies of aj(0), aj+ and aj- on those in the previous step. Obviously, aj-1+ and aj-1- are at least with the first order of δω, and therefore, they only affect aj-1+ and aj-1-, and aj(0) is therefore determined only by aj-1(0). The red arrows in Fig. 2 indicate the remaining dependencies which we have to consider. The coefficients are determined by Eq. (10). As a result, we obtainedFig. 1(c), we assumed a constant time delay of d between all pairs of neighboring RRs, which adds a phase factor of e ± iΩ(j-1)d to αj ± as in Eq. (12a). In addition, CRR-MZM uses each RR as a phase modulator. The phase change is most enhanced at the resonant frequencies of the RRs, and we therefore set the detuning as Δω = 0 [8, 10]. Note that the above recurrence formulas automatically confine the analysis to within the first order of δω and the frequency components of ω and ω ± Ω.Fig. 1(c), CRRs are loaded as phase modulators on each arm of the Mach-Zehner interferometer. The two CRRs are operated by the signals having opposite polarities. When the static phase difference of π/2 is given between two arms, the output Sout from the CRR-MZM in Fig. 1(c) becomes
Using Eq. (15), we consider the optimized configuration of CRR-MZM to maximize its response at a given operating frequency. First, it is reasonable to design each RR so that Ωτ << 1, to obtain a sufficiently large modulator bandwidth for a given Ω. In this case, the response of each RR may be small, as was observed in Eq. (15) for N = 1. Therefore, we should cascade RRs to accumulate the phase change and to obtain a sufficient modulator response. The important thing here is to choose a proper delay d to minimize the effect of the summation in Eq. (15). Because we chose τ so that Ωτ << 1, then θ ≈Ωτ in Eq. (15). Consequently, if the delay d = 2τ, the summation in Eq. (15) becomes N regardless of the frequency Ω. In this case, the impact of cascading RR on the response becomes a simple multiplication with a factor of N. This choice of delay is considered to be an optimized value required for CRR-MZMs to minimize the distortion in the frequency response. This result is reasonable because the group delay of each RR is also equal to 2τ.
Figure 3 shows the calculated frequency responses of SRR-SSM and CRR-MZM, which were obtained using Eqs. (11) and (15). The response was defined as the amplitude of the sinusoidal term in |Sout|2 divided by δωτ. SRR-SSM and SRR-MZM (CRR-MZM for N = 1) have a similar dependence on Ωτ. For large Ωτ, both decrease at a rate of −10 dB/decade, as shown by the black and green curves in Fig. 3. With respect to 10-CRR-MZMs, their responses are 10 times larger than that of SRR-MZM for the smallest Ωτ, regardless of the delay d, as shown by the blue and red curves in Fig. 3. However, the bandwidth of 10-CRR-MZM is much smaller than that of SRR-MZM with d = 0, as shown by the blue curve. As expected, the bandwidth is much improved for 10-CRR-MZM with d = 2τ, and is comparable to that of SRR-MZM. If we compare 10-CRR-MZM with d = 2τ to standard SRR-SSM, the former shows a much larger response at small Ωτ with a smaller modulation bandwidth, as shown in Fig. 3. In the next section, we compared both configurations with respect to response at low frequencies and the modulation bandwidth, using a wide range of parameter sets, including the propagation loss of the waveguide and τ and N.
3. Numerical calculations
In this section, we show the calculated results of the response of CRR-MZM with a wide variety of parameter sets. The results are compared with those of standard SRR-SSMs.
3.1 Numerical method
First, we performed numerical calculations to confirm the validity of the analysis in the previous section. The configuration is summarized for SRR in Fig. 4 . We used a slowly-varying-envelope approximation, in which we defined the lightwave propagating along the ring waveguides as U(t, z) = u(t, z) ei(ωt-kz). u(t, z) is a slowly varying function when compared with the angular frequency of light ω, and satisfies the following partial differential equation :
To solve Eq. (16) and (17) numerically, we discretized time by the round-trip time τ0 = (2πR ng)/c as t = tm = mτ0. Note that in these calculations, we neglected variations of u(t) in the RR that were faster than τ0. As in the analysis in the previous section, we also neglected the modulation of the group index in the waveguide. With these assumptions from Eq. (16),
Figure 5 shows the response of CRR-MZMs calculated using both analytical and numerical methods. The results coincided with each other when the resonance was sharp with a relatively small 1-y2 (5%), which confirms the validity of the analysis in the previous section. The responses were slightly different when the resonance was broad with a relatively large 1-y2 (40%). This deviation at a large 1-y2 occurred because the TCM theory is only valid for the resonators which weakly couple to the outside . Although the analysis in the previous section is useful to gain an insight into the dynamics in CRR-MZMs, it should be carefully applied to obtain quantitative results, especially for relatively low-Q RRs. In the rest of paper, we show the results calculated using the numerical model to include low-Q RRs. In fact, 2τ, which was calculated using Eqs. (3) and (6), is equal to the group delay of RR at Δω = 0, for only high-Q RRs. A more rigorous expression of the group delay of RR was given in previous works . For the numerical calculations in this paper, we used the delay d that was calculated by this rigorous form, and inserted it in signals between two adjacent RRs.
3.2 Calculated performance comparison between SRR-SSM and CRR-MZM
We numerically calculated the DC response ΔPDC = ΔP(0) and the 3-dB bandwidth of SRR-SSMs and CRR-MZMs. Table 1 summarizes the conditions of the calculations. As shown in Table 1, we directly assigned values to some of the parameters, whereas the others were calculated from those values indicated in the table. We used the same conditions of the frequency detuning for SRR-SSM and CRR-MZM as mentioned in sections 2.1 and 2.2, respectively. We determined the values of ng, R, and αdB by making reference to other studies on silicon-based modulators [2–10].
Figure 6 shows the calculated results for SRR-SSM, and CRR-MZMs with αdB = 0 and different values of τ and N. The left axis is the response of the modulators to a DC input, which was analytically calculated by Eqs. (17) and (18) using static solutions. The right axis indicates the 3-dB bandwidth, f3dB, at which the response ΔP(Ω) becomes half of the DC response ΔPDC. Figure 6(a) shows general dependencies in RR-based modulators, regardless of the structures of the modulators in which the DC responses increase with the photon lifetime of the RR, whereas the 3-dB bandwidths decrease. The slopes of the DC response are constant for an increasing photon lifetime, and are almost the same among different structures. For a constant lifetime, the differences between the red, green, and blue solid curves are about 6-dB, corresponding to N = 1, 4, and 16 for CRR-MZMs. This means that an increase of the response by N-times is obtained for CRR-MZMs with N-CRRs. However, in this case, the 3-dB bandwidths decrease with increasing N, as shown by the dashed curves. To compare the results on the same basis, we drew parametric plots between the DC response and the 3-dB bandwidth with the parameter of the photon lifetime, as shown in Fig. 6(b). As clearly shown in the graph, CRR-MZMs have a greater response with an increase in N for a constant 3-dB bandwidth, which confirms our predictions for CRR-MZMs which were mentioned in the introduction of this paper. Without propagation loss of the RRs, these gains are preserved at any 3-dB bandwidth. In other words, they are independent of the photon lifetime of the RRs.
In reality, RRs are always accompanied by a certain amount of propagation loss due to side-wall scattering and radiation in the curved waveguides. To investigate the impact of the loss on the design of CRR-MZMs, we calculated the performances of CRR-MZMs with a propagation loss of 10 dB/cm assigned to the RRs. In this case, we neglected loss of the bus waveguide because such loss is usually much smaller than those in RRs. In RRs, light propagates effectively long waveguides due to the resonance and those waveguides are supposed to be lossy due to dopants and electrode. By contrast, the bus waveguide is passive and non-resonant. Therefore, the bus waveguide will give only minor impact on the modulation characteristics of the CRR-MZM in terms of loss when necessary and sufficient spaces are given between neighboring RRs. As such space, about 20 μm would be sufficient for RRs with several-micron radius .
As a result, the DC responses showed much different behaviors from those without losses, whereas the dependences of the 3-dB bandwidth on the photon lifetime were almost the same as those without losses. Unlike the loss-less case, the DC responses showed decreases with relatively large photon lifetime values for CRR-MZMs, as shown by the red, green, and blue solid curves in Fig. 7(a) . This dependence is caused by the enhancement of the loss in RRs. When the lifetime increases towards the critical coupling, both the loss and phase shift are enhanced. The transmitted power from RRs exponentially decreases with this enhancement factor and the number of RRs, whereas the phase-change only linearly increases with those parameters. The balance between them determines the overall dependencies of the DC response of RRs and the maximum achievable response at a specific photon lifetime. Note that the photon lifetime which gives the maximum DC response decreases with N, as shown in Fig. 7(a). Figure 7(b) is the parametric plot of the photon lifetime, and corresponds to Fig. 6(b). For a 3-dB bandwidth smaller than 25 GHz, CRR-MZMs have a relatively smaller DC response when compared with the SRR-SSM, regardless of N, and the strategy of cascading RRs does not work. This is due to the large photon lifetime and the enhancement of loss in RRs. However, this dependence is not the case for a large 3-dB bandwidth with a relatively small photon lifetime. For a 3-dB bandwidth larger than 25 GHz, the modulation of CRR-MZM accumulates with an increase of N, and CRR-MZMs provide larger responses than those provided by SRR-SSM, as shown in Fig. 7(b). Therefore, cascading RRs are still more effective than SRR-SSM for obtaining larger responses at high frequencies, even with the propagation losses.
Throughout this paper, we assumed that a constant amplitude of the resonant-frequency shifts was induced in each RR by refractive index modulation. This situation implied that the electrical signals applied to the RRs had constant voltage amplitudes. Such a voltage amplitude is limited in most applications. In particular, when the driver circuits are based on CMOS technologies, a voltage amplitude in the sub-volt range is required. With this constraint, CRR-MZMs have a degree of freedom in their design that increases the modulation response for high-operating frequencies by cascading RRs with a relatively small photon lifetime. This approach may be considered to be a compromise because the power consumption would increase with the number of RRs driven. Even so, this approach is useful, given that the standard SRR-SSM would fail to provide sufficient modulation of light at high-frequencies due to its limited configuration. In addition, note that as long as y is less than 1, the presence of the RRs ensures that there is some enhancement of the phase modulation in the CRR-MZM with a factor of τ/τ0, when compared with the standard non-resonant Mach-Zehnder modulator .
Finally, we calculated the output waveforms from SRR-SSM and CRR-MZM for a 50-Gb/s pseudorandom binary sequence (PRBS) of 27-1. In the calculation, we used the same procedure as that described in section 3.1. The use of Eqs. (16) to (18) is not limited within small signal analysis and is valid for large signal modulation. Therefore, we simply applied the input signals of PRBS to those equations and the procedure to calculate eye diagrams. We set the rise and fall time to be 5.9 ps for the transition between 0 and 1 state with the thresholds of 10 and 90%, respectively. This rise/fall time was large enough for the round-trip time of 0.67 ps. By using Fig. 7(a), we determined the photon lifetimes of the RRs used in the two device configurations, respectively, such that both devices had the same 3-dB bandwidth of about 40 GHz. With this constant 3-dB bandwidth, it is expected that both devices equally show sufficient qualities of eye diagrams for 50-Gb/s operations. Thus, it is possible to evaluate the modulation depths of the two devices and compare them for 50-Gb/s operations. Choice of the smaller 3-dB bandwidth and the longer photon lifetimes would cause inadequate qualities of eye diagrams for 50-Gb/s operations. Figures 8(a) and (b) show the resulting eye diagrams, with the parameters used in the calculation shown in the graphs. By cascading 4-RRs with a relatively small photon lifetime, the CRR-MZM provided an eye opening that was wider than that of the SRR-SSM.
We investigated the high-speed modulation characteristics of CRR-MZMs using analytical and numerical calculations. We derived an analytical formula for the small-signal response of CRR-MZM based on TCM theory, for which we numerically confirmed the validity. The analysis indicated that CRR-MZMs had a maximum operating bandwidth when proper delays were set in the applied electrical signals between neighboring RRs. The optimized delay was equal to twice the photon lifetime of each RR.
We determined the performances of both CRR-MZMs and standard SRR-SSM for a wide variety of parameters, including the photon lifetime of RRs, the number of RRs cascaded, and the propagation loss of the RR. We compared the results between those two device configurations in terms of the modulation depth and bandwidth. For a given magnitude of the waveguide index change, CRR-MZM can provide a larger modulation response than an SRR-type modulator in a frequency range greater than 25 GHz. The results showed the advantage of CRR-MZM over SRR-SSM in designs aimed at providing a larger modulation response at high-operating frequencies.
The authors would like to thank Tatsuya Usuki for the helpful discussions and suggestions that he made. The authors also acknowledge valuable assistance from Tsuyoshi Yamamoto.
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