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Modulation characteristics of a novel InGaAs/InAlAs multiple quantum well (MQW) microring-enhanced Mach-Zehnder modulator (MRE-MZM) is investigated in detail and its low-voltage operation with high extinction ratio is demonstrated. The MZM has a single microring resonator in one arm and is driven by the change in electrorefractive index induced by the quantum-confined Stark effect in the MQW core layer. As the MQW, a multiple five-layer asymmetric coupled quantum well (FACQW) is used to obtain a large electrorefractive index change. The driving voltage of the proposed MZM is significantly reduced owing to the enhanced phase shift in the microring resonator. High-mesa waveguide structures are grown by solid-source molecular beam epitaxy and fabricated by inductively coupled plasma etching. A directional coupler with an asymmetric branching ratio is used as an input coupler to prevent the degradation of the extinction ratio of the MZM. The extinction ratio of the fabricated MRE-MZM is approximately 27 dB. The product of the half-wave voltage and phase shifter length, Vπ·L, is 1.7 Vmm in static modulation. This value is one-quarter that of a conventional MZM with the same waveguide structure.
©2013 Optical Society of America
1. Introduction
A Mach-Zehnder modulator (MZM) is one of the key devices for ultrahigh-capacity optical communications not only because of its low-chirp modulation characteristics but also because of its suitability for spectrally efficient advanced modulation formats such as quadrature phase-shift keying (QPSK) [1]. In particular, a semiconductor MZM is compact and can easily be integrated with other semiconductor devices such as laser diodes [2]. To date, high-performance InP-based MZM and MZM-integrated tunable distributed feedback (DFB) lasers have been developed [3–5] and semiconductor differential QPSK (DQPSK) modulators have been demonstrated [6,7]. To realize a compact and low-voltage semiconductor MZM with a multiple quantum well (MQW) core layer, the MQW should have a large change in electrorefractive index Δn with a small absorption loss. A five-layer asymmetric coupled quantum well (FACQW) is one of the promising candidates for exhibiting a giant change in electrorefractive index [8]. We developed InGaAs/InAlAs FACQWs for 1.55-μm-wavelength regions [9], and the product of the half-wave voltage and phase shifter length, Vπ·L, of as small as 1.2 Vmm was successfully obtained in the FACQW MZM with a 0.5-mm-long phase shifter [10].
On the other hand, a microring resonator is one of the key devices for large-scale integrated optical cross-connect circuits using multiple-wavelength channels owing to its high functionality and compactness [11–16]. In addition, a nonlinear phase-shift effect in microring resonators [17,18] has attracted attention recently, and its applications to photonic devices, such as microring-enhanced MZMs (MRE-MZMs) and switches using free carrier injection [19–21] and the thermooptic (TO) effect [22], have been reported. Improvement in modulation linearity of a Mach–Zehnder modulator using ring resonators coupled to each arm has also been proposed [23]. A bistable microring MZ interferometer (MZI) has been proposed [24], and a microring phase shifter based on a silicon-on-insulator (SOI) using the TO effect has been demonstrated [25].
If we utilize the enhanced phase shift in the microring resonator as well as the large change in electrorefractive index in the FACQW, very low voltage MZMs and switches are expected to be realized. Recently, we have developed an InGaAs/InAlAs MQW MRE-MZM with a single microring resonator in one arm for the first time [26]. The modulator was driven by the quantum-confined Stark effect (QCSE) in the MQW, and the driving voltage was successfully reduced to one-third that of a conventional MZM with the same waveguide structure. In this device, however, the enhanced phase shift was not fully utilized because the lengths of the phase shifters in the MZM were not optimized. The extinction ratio was 17 dB and further improvement was necessary.
In this study, we investigate in detail the modulation characteristics the InGaAs/InAlAs FACQW MRE-MZM, in which a microring resonator is equipped in one arm and the length of the other arm is optimized to realize the maximum extinction ratio [27]. Regarding the design of the MRE-MZM, we discuss the optimization of the branching ratio of an input directional coupler and the arm length in detail. In the fabricated MRE-MZM, the driving voltage is further reduced compared with that in the previous MRE-MZM and the extinction ratio is also significantly improved by 10 dB by balancing the propagated light powers in both arms. In addition, preliminary high-speed modulation measurements are demonstrated, and the possibility of a much wider modulation bandwidth is also discussed.
2. Enhanced phase shift in microring resonator
In this section, we briefly describe the theory of the enhancement of the phase shift in a microring resonator [17,18,28]. Figure 1 shows a schematic view of a single microring resonator with a busline waveguide. Assuming that the light power losses at couplers are negligible, the effective phase ϕeff, that is, the phase of the optical electric field transmitted from Ports 1 to 3, is given by
where E1 and E3 are respectively the optical electric fields at Ports 1 and 3, ϕ is the single-pass phase in the microring, and r and K are the coupling ratio of the optical electric field and the coupling efficiency of the light power at the coupler. The symbol a is the transmission coefficient of the electric field for round-trip propagation along the microring. The symbols α and Lring are the power loss constant in the microring and the round-trip length of the microring, respectively.Figure 2 shows the effective phase shift ϕeff and the transmittance T as functions of the single-pass phase shift ϕ for K = 0.25, Lring = 480 μm, and α = 2.2 dB/mm (the round-trip light power loss αLring = 1.04 dB). In the vicinity of ϕ = 0, that is, in the on-resonance state, the marked nonlinearity and the single-pass phase shift are strongly enhanced. Here, we define the phase shift enhancement factor Fpe as
In the case of Fig. 2, Fpe is approximately 20. Using this enhancement of the phase shift in one arm of an MZI, the driving voltage of the MZM can be significantly reduced. If a microring resonator is coupled with one arm of the MZM and a change in the effective phase shift Δϕeff of π rad from ϕeff = π/2 to 3π/2 rad is used, the output light can be modulated with a small change in the single-pass phase shift Δϕ.
Fig. 2 Effective phase ϕeff and transmittance from Ports 1 to 3, T, as functions of single-pass phase ϕ for coupling efficiency K = 0.25, round-trip length of microring Lring = 480 μm, and power loss constant in microring α = 2.2 dB/mm.
Note that in the vicinity of the on-resonance state, the transmittance T is also markedly reduced owing to the propagation loss in the microring even though the enhancement of the phase shift is large. That is, there is a trade-off between the phase shift enhancement factor and the transmittance T. Therefore, this propagation loss in the microring needs to be considered, when we design the asymmetric MZM with a microring resonator in one arm.
To use the enhancement of the phase shift for the MZM, the transmission coefficient of the electric field a and the coupling coefficient K must satisfy the overcoupling condition [29]
When this condition is not satisfied, the change in the effective phase shift Δϕeff in the microring is small, and the transmitted light is not sufficiently modulated [26].3. Design and fabrication of microring Mach-Zehnder modulator
Figure 3 shows a schematic top view of the proposed InGaAs/InAlAs MQW MRE-MZM. It is composed of an MZI with a single microring resonator in one arm (Arm 1) and three directional couplers (DC1 to DC3). By applying a reverse bias voltage only to the microring and changing the effective phase ϕeff of the microring in Arm 1, the transmitted light at the output port (bar port) is modulated. The other arm (Arm 2) is longer than Arm 1 by ΔϕArm2 = 3π/2 rad in phase [27].
DC1 is an asymmetric power splitter with a branching ratio of X to 1−X. DC2 is a symmetric power combiner. The round-trip length is designed to be 480 μm to obtain a sufficiently wide margin of the single-pass phase shift ϕ for the effective phase shift Δϕeff of π rad, with considering past experimentally obtained change in refractive index. The radius of curvature of the microring is 56 μm. Considering the phase-shift-enhancement effect and the propagation loss in the microring resonator, X is designed to be 0.84 to suppress the deterioration of the extinction ratio, as discussed later.
Figures 4(a) and 4(b) show schematic cross-sectional views of the waveguide and the directional coupler, respectively. The waveguide structure is the same as that of the device in [26]. The waveguide consists of a core layer with a 12-period In0.53Ga0.47As/In0.52Al0.48As FACQW, 50 nm In0.52Al0.24Ga0.24As separated confinement heterostructure (SCH) layers, and p/n-doped InP cladding layers. The large field-induced phase shift in the microring is obtained owing to the QCSE in the multiple FACQW. The total thickness of the core layer is approximately 300 nm. All layers are lattice-matched to the InP substrate. The waveguide is buried with benzocyclobutene (BCB).
The equivalent refractive index neq of the waveguide is 3.260, and the relative index difference between the waveguide and the BCB (n = 1.543) is very large. We employed a directional coupler with a shallow gap [30] for the coupling region, as shown in Fig. 4(b). The coupling efficiency K can easily be controlled by changing the width of the gap wg and the depth of the gap dg.
The designed device parameters are summarized in Table 1. The power loss constant α is evaluated considering the propagation loss in the cladding layers [30]. To model the change in electrorefractive index in the core layer, the experimental values are used, which is discussed in Sect. 4.1.
Table 1. Device parameters for MRE-MZM
Figures 5(a) and 5(b) respectively show the calculated output port spectrum responses of the designed MRE-MZM at various reverse bias voltages and the modulation characteristic at the wavelength of 1550.1 nm, assuming γ = 0 and α = 2.2 dB/mm. If the operation wavelength is set at 1550.1 nm, the power of the transmitted light is modulated and is maximum at the applied reverse voltage Va of 0 V (ON state) and minimum at Va = 3.5 V (OFF state), as shown in Fig. 5(b). The applied reverse voltages VR of 0 and 3.5 V correspond to ϕeff values of 0.89π/2 and 3.8π/2 rad, respectively. This means that ϕeff for the ON state for the MRE-MZM does not match 3π/2 rad, although the OFF state occurs at ϕeff = π/2. In this case, the phase-shift-enhancement effect is markedly reduced. Here, we define the effective phase shift enhancement factor for the MRE-MZM, Fpe(eff), as
where ϕON and ϕOFF are respectively the single-pass phases for the ON and OFF states, as shown in Fig. 6. In the proposed MRE-MZ, Fpe(eff) is calculated to be approximately 6.0.Fig. 5 (a) Calculated output port spectrum responses at various reverse bias voltages. (b) Calculated theoretical modulation characteristic of MRE-MZM with device parameters in Table 1 at wavelength of 1550.1 nm.
When we use the microring resonator in one arm of the MZM, the light power in the MZI arms becomes asymmetric because the propagation loss in the arm with the microring (Arm 1) is larger than that in the arm without it (Arm 2), as mentioned in Sect. 2. This imbalance between the light powers in the arms leads to the degradation of the extinction ratio. To prevent this, we use the asymmetric splitter (DC1).
Figure 7 shows the calculated dependence of the extinction ratio and Fpe(eff) for the MRE-MZM on the branching ratio of DC1. At X = 0.92, the light power in the OFF state in one arm becomes the same as that in the other arm, and the maximum extinction ratio can be obtained. Note that the value of X for the maximum extinction ratio is not equal to that for the maximum Fpe(eff). Therefore, in the proposed MRE-MZM, X is set to 0.84 to obtain a sufficiently high extinction ratio of 20 dB. In this case, Fpe(eff) can be expected to be approximately 4. The optimized value for X depends on the round-trip loss in the microring resonator. As a general tendency, the smaller the round-trip loss becomes, the smaller the optimized branching ratio X for becomes. For example, the optimized X value for the highest extinction ratio for the power loss constant α of 1.81 dB/mm is 0.85.
Fig. 7 Calculated dependence of extinction ratio and effective phase-shift-enhancement factor of proposed MRE-MZM on branching ratio of input coupler.
An epitaxial wafer was grown by solid-source molecular beam epitaxy (MBE). To fabricate a directional coupler with a shallow gap, we adopted a two-step etching technique. First, using an electron-beam lithography technique, a gap was formed by inductively coupled plasma reactive ion etching (ICP-RIE) with a Br-based gas. Next, using a photolithography technique, high-mesa waveguides were formed by ICP-RIE. Figures 8(a) and 8(b) show microscopic top views of the fabricated MRE-MZM. Wide grooves with a width of 7 μm were formed on both sides of each waveguide. Finally Au lumped electrodes with bonding pads were formed on the waveguides of the microring by vacuum evaporation. The width of the Au electrode on the ring waveguide was 3 μm.
4. Modulation characteristics
4.1. Static modulation characteristics
The measured bar-port spectrum responses for transverse electric (TE)-polarized light at various reverse bias voltages are shown in Figs. 9(a) and 9(b). The “Transmittance” includes both the insertion loss of the device, input and output waveguides including spot size converters and fiber coupling losses at facets. The insertion loss of the device is estimated to be approximately 3.7 dB, as discussed in Sect. 4.2. The dashed lines indicate the operating wavelength of 1550.1 nm. The free spectral range (FSR) is 1.34 nm, which is comparable to the designed value. The effective index neff was evaluated to be 3.74. A marked shift of the resonant wavelength of the microring resonator was observed.
Fig. 9 Measured output port spectrum responses at various reverse bias voltages (a) from 0 to 10 V and (b) from 0 to 4 V.
Figure 10 shows the evaluated change in the refractive index in the core layer of the microring waveguide Δncore at the resonant wavelengths of 1548.5, 1549.9, and 1551.3 nm as a function of the applied dc reverse bias voltage Va, considering the optical confinement factor (0.527) and the filling factor p of the FACQW in the core layer (p = 0.574). The change in the refractive index in the FACQW at V = −10 V is evaluated to be approximately 2.0 × 10−3. This result shows that the change in the refractive index in the FACQW is almost constant in this wavelength region, which is consistent with the discussion in [9]. The change in the refractive index is mainly caused by the QCSE in the FACQW core layer.
Fig. 10 Evaluated changes in refractive index in core layer of microring waveguide Δncore at various wavelengths in the vicinity of 1550 nm as functions of applied dc reverse voltage.
Figure 11 shows the static modulation characteristics of the proposed MRE-MZM at a wavelength of 1550.1 nm. The measured result agrees well with the result shown in Fig. 5(b). The modulation characteristics of a conventional MZM and the previously reported MRE-MZM [26] with the same waveguide structure are also plotted for comparison. The phase shifter length L of the conventional MZM is 1000 μm. The previously reported MRE-MZM has the same configuration as that of the proposed MRE-MZM; however, ΔϕArm2 and the branching ratio X are 0 rad and 0.6 respectively. The half-wave voltage of the proposed MRE-MZM, Vπ, is approximately 3.5 V. Because the voltage is only applied to the microring resonator, the phase shifter length L of the proposed MRE-MZM is equal to Lring = 480 μm. Therefore, the product of half-wave voltage and phase shifter length, Vπ·L ( = Vπ·Lring), is equal to 1.7 Vmm. The extinction ratio is approximately 27.5 dB. On the other hand, the Vπ values of the conventional MZM and the previously reported MRE-MZM are approximately 6.2 and 4.2 V, respectively. They correspond to Vπ·L valued of 6.2 and 2.0 Vmm, respectively. These results show that the driving voltage of the proposed MRE-MZM is successfully reduced to one-quarter that of the conventional MZM owing to the phase-shift- enhancement effect in the microring. Compared with those of the previously reported MRE-MZM [26], the driving voltage of the proposed MRE-MZ is reduced by 15% and the extinction ratio is improved by 10 dB owing to the asymmetric branching ratio of DC1. In addition, the extinction ratio is also improved by approximately 7 dB compared with that of the last-reported MRE-MZM [27] (from 20.0 to 27.1 dB). This is because the branching ratio of the fabricated input coupler was larger than that of the previous device in [27], leading to the improvement of the balance between light powers in both arms.
Figure 12 shows the static modulation characteristics of the proposed MRE-MZM at various wavelengths in the vicinity of 1550 nm. Although the extinction ratio slightly depends on the operation wavelength, the operation voltage remains unchanged because the change in the refractive index in the multiple FACQW is almost constant in this narrow wavelength range, as shown in Fig. 10.
Fig. 12 Static modulation characteristics of microring MZ modulator at various wavelengths around 1550.0 nm.
Here we discuss the dependence of the dip depth on the applied reverse bias Va. Being different from the dip depths in Fig. 5(a), the experimental dip depths in Fig. 9 show the dependence on Va. The reason is considered to be that the round-trip loss in the microring increases with the increase in Va by the QCSE in the MQW core layer even though the round-trip loss is assumed to be constant in Fig. 5(a). Figures 13(a) and 13(b) show the output port spectral responses under various Va, assuming that the round-trip loss increases with the increase in Va, as shown in the caption in Figs. 13(a) and 13(b). In the calculation, it is also assumed that K and X are 0.22 and 0.86, respectively, that are the experimental values, as discussed in Sect. 4.2, and the difference in phase between Arm 1 and Arm 2, ΔϕArm2, is 2.8π/2 rad that is determined by comparing the theoretical and experimental results. Although the dip depths are not equal to those of the experimental results, the tendency of the change in the dip depth is successfully reproduced. This result shows that the dependence of the dip depth on Va is owing to the change in the round-trip loss. Therefore, we should consider the change in the round-trip loss in the microring to obtain a high extinction ratio in the asymmetric MRE-MZM
Fig. 13 Calculated output port spectral responses at various applied reverse bias Va (a) from 0 to 10 V and (b) from 0 to 4 V.
4.2. Evaluation of device parameters
To evaluate the coupling efficiency K between the microring and the busline and the propagation loss α in the microring, we measured the spectrum response of a single microring resonator with two buslines of the same structure with the proposed MRE-MZM [26]. The experimentally evaluated parameters are summarized in the right-hand column in Table 1. The values of K and α are evaluated to be 0.22 and 1.6 dB/mm, respectively, and they satisfy the overcoupling condition [Eq. (5)]. The measured coupling efficiency is very close to the designed value. This indicates that the gap in the coupling region is fabricated almost as designed. The propagation loss in the microring resonator and the waveguides in the MZ interferometer are estimated to be approximately 3 and 0.7 dB using the experimentally-obtained propagation loss of 1.6 dB/mm, respectively. Therefore, the insertion loss of the device is estimated to be approximately 3.7 dB. The branching ratio X is evaluated to be 0.86 using these values of K and α on the assumption that the coupling length of DC1, lDC1, is the same as the designed value. The evaluated round-trip loss includes the coupling loss in the directional coupler.
4.3. Dynamic modulation characteristics
As a preliminary experiment, the dynamic modulation characteristics of the MRE-MZM were measured. Figure 14(a) shows the dynamic modulation driven by a sinusoidal reverse voltage with a repetition rate of 3.5 GHz and an amplitude of 4.0 V. The dependence of the electro-optic (EO) response on modulation frequency is shown in Fig. 14(b). Although the modulation characteristics were only measured from 3.0 to 4.5 GHz for our measurement system, the modulation frequency is much higher than that of a device driven by the TO effect. Therefore it was confirmed that the MRE-MZM is driven not by the TO effect but also by the change in refractive index caused by the QCSE in the FACQW core layer.
Fig. 14 (a) Dynamic modulation driven by sinusoidal reverse voltage with repetition rate of 3.5 GHz and amplitude of 4.0 V. (b) Dependence of electro-optic (EO) response on modulation frequency.
The modulation bandwidth of the MRE-MZM, f3dB, is determined by both the RC time constant of the driving circuit and the photon lifetime in the microring resonator τp, as expressed by [31]
where c is the velocity of light in vacuum and Q is the quality factor of the microring resonator. Q is given by [32] where Qloss and Qcouple are the quality factors related to the propagation loss and the coupling loss at the coupler, respectively. Figure 15(a) shows the dependence of the 3 dB bandwidth determined by τp on K, calculated using Eqs. (7)-(10). The effective phase shift enhancement factor Fpe(eff) is also plotted in the figure. There is a trade-off between the bandwidth and the phase shift enhancement factor. When K is less than 0.25, the device parameters do not satisfy the overcoupling condition in the case of α = 2.2 dB/mm. For the designed MRE-MZM (K = 0.25), a bandwidth of 12 GHz is expected if the RC time constant is sufficiently small. Figure 15 (b) shows the bandwidths of the microrings with and Lring = 280 and 480 μm as functions of Fpe(eff). The round-trip length of 280 μm is the lower limit of the round-trip length determined by the overcoupling condition in Eq. (5) when the power loss constant α is assumed to be 2.2 dB/mm. When the round-trip length is reduced to 280 μm, the modulation bandwidth increases. That is, Fpe(eff) is significantly improved for the case of Lring = 280 μm compared with that for the case of Lring = 480 μm even when the Q factors of the microrings are the same in both cases, because the effective phase shift for the ON state in Fig. 6, ϕeff (ϕON), becomes close to 3π/2. This means that in the case of Lring = 280 μm, the Q factor is reduced for the same value of Fpe(eff), leading to the larger bandwidth.Fig. 15 (a) Dependence of the 3-dB bandwidth of proposed MRE-MZM (Lring = 480 μm) determined by photon life time τp on coupling efficiency K. (b) 3-dB bandwidth of MRE-MZM with Lring = 480 and 280 μm as functions of effective phase-shift-enhancement factor Fpe(eff).
Actually, the device structure including the lumped electrodes is not optimized for high-speed operation and the bandwidth is dominated by the RC time constant. In this device, the capacitance of the electrodes including a contact pad for the microring resonator is evaluated to be approximately 6.0 pF. Assuming a load resistance is 50 Ω, the modulation bandwidth determined by the RC constant, f3dB = (2πRC)−1, is calculated to be approximately 5.3 GHz, which is consistent with the modulation result in Fig. 15(b).
5. Conclusions
We have investigated the novel InGaAs/InAlAs MQW MRE-MZM in detail and its low-voltage operation with high extinction ratio. The MZM has a single microring resonator with a round-trip length of 480 μm and is driven by the change in the refractive index induced by the QCSE in the FACQW core layer. High-mesa waveguide structures were grown by solid-source molecular beam epitaxy and fabricated by inductively coupled plasma etching. A directional coupler with an asymmetric branching ratio (0.84:0.16) was used as an input coupler to prevent the degradation of the extinction ratio of the MZM. The driving voltage of the proposed MZM is significantly reduced owing to the phase-shift-enhancement effect in the microring resonator. The product of the half-wave voltage and phase shifter length, Vπ·L, is 1.7 Vmm in static modulation. This value is one-quarter that of a conventional MZM with the same waveguide structure. The extinction ratio of the fabricated MRE-MZM is approximately 27 dB. The preliminary high-frequency modulation measurement shows that the MRE-MZM is driven by the change in refractive index, and high-speed modulation of more than 10 GHz is expected if the total capacitance of the electrodes is reduced.
Acknowledgements
The authors express sincere thanks to Dr. T. Kawanishi and Dr. A. Kanno of National Institute of Information and Communications Technology (NICT). This work was partly supported by SCOPE, Ministry of Internal Affairs and Communications, and a Grant-in-Aid for Scientific Research B (No. 24360025) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
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