1. Introduction

EO modulators are vital components in optical networks and on-chip optical interconnects [1, 2]. Among the many EO materials, EO polymers feature large EO coefficient (r₃₃), ultrafast response time and very low dispersion, which promise low-power consumption and ultra-high speed operation [3]. Traditional waveguide modulators consist of a bottom cladding layer, an EO polymer core and a top cladding layer, all sandwiched between top and bottom electrodes [4, 5]. However, the refractive index contrast between the core (n~1.6) and the claddings (n ~1.5) is low, requiring the cladding layers to be thick enough to avoid absorption loss from the metal electrodes. The resulting large inter-electrode gap leads to a π-voltage-length product V_πL of typically 5-10 V·cm in MZI modulators [4, 5]. In addition, compact ring resonators play a crucial role in photonic integrated platforms [6]. EO modulators in the ring structure permit EO activity enhancement and facilitate wavelength selective operations, but traditional EO polymer waveguides suffer from a large bending radius around hundreds of microns [7, 8]. Therefore, there is a strong motivation to consider novel waveguide architectures leading to a low V_πL in MZI and enabling a more compact ring resonator.

Recently, several waveguide structures have been developed in order to shorten the inter-electrode gap and to reduce the V_πL, such as Si slot waveguides and plasmonic waveguides [2, 9, 10]. In both of these cases, the driving voltage can drop off across a 100-200 nm wide slot filled with EO polymer, and the light can also be confined in the slot region due to the high refractive index contrast or plasmonic effect. By using the specially designed EO polymer with an EO coefficient of 180-230 pm/V, sub-voltage driving modulators with short device lengths have been demonstrated in the MZI waveguides [9, 10]. The V_πL can also be decreased by using the slow-light effect in a photonic crystal [11]. (See Appendix Table 1 of the detailed comparison)

Table 1. Comparison of MZI modulators

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In this work, we design and investigate slot waveguide modulators on Si-on-insulator (SOI) substrates. Our devices are fabricated by using the thin TiO₂ and polymer films simply being obtained by layered depositions. The designed structure not only confines the optical mode tightly in the sub-wavelength region but also effectively enhances the electric-field for poling and driving the modulator. To satisfy various applications, we present two kinds of EO modulators based on this waveguide, i.e. MZI and ring resonator modulators. The MZI shows a V_πL of 0.66 V·cm and a 3-dB bandwidth of 31 GHz. Meanwhile, the ring resonator modulator has an EO resonant tunability of 0.065 nm/V and a bandwidth close to the theoretical limit.

2. Plate-slot waveguide design

The cross-section of the designed slot waveguide modulator is shown in Fig. 1(a), where the slot consists of the plates of Si, EO polymer, and TiO₂ on a SOI wafer. The thicknesses of the EO polymer and TiO₂ are both 200 nm, and the width of the waveguide is 2 μm. The sub-micrometer thick EO polymer and TiO₂ layers can be obtained by spin-coating and RF-sputtering technique, respectively. We used commercialized SOI substrate having a 100 nm-thick device layer and a 5 μm-box layer. To avoid any absorption loss from the top Au electrode, a 1.5μm-thick SiO₂ buffer was inserted between the SOI and the top electrode. Using refractive indices of TiO₂ (2.30), Si (3.47), and EO polymer (1.65), the mode calculation by Rsoft indicated that the TM₀₀ mode was highly concentrated within the EO polymer as shown in Fig. 1(b). Subsequently, the calculations gave the confinement factor (Γ) of the optical field distributed in the EO polymer to be 58% relative to the total modal intensity. We found that the TM₀₀ mode overlaps little with the lateral boundaries of the TiO₂ ridge, so that the propagation loss should be insensitive to the sidewall roughness.

 

Fig. 1 (a) Cross-section of the designed plate-slot waveguide modulator, (b) the calculated quasi-TM₀₀ mode distribution, indicating a highly concentrated optical field within the EO polymer, and (c) calculated electric-field distribution in the vertical-direction.

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The modulator was driven by using a strip line electrode, where the Au top electrode was patterned as the traveling-wave-electrode on the SiO₂ cladding [12]. The bottom electrode was SOI having a resistivity of 1 / Ω‧mm, which is terminated to the Al electrode with a 4 μm-distance. The electric-field in the modulator was simulated using COMSOL, and the normalized field intensity distribution in the vertical direction is shown in Fig. 1(c). Because of the large contrasts of the dielectric constants (ε) of the EO polymer (ε~3) to TiO₂ (ε~30) and Si (ε~11), an obvious electric-field concentration in the slot can be observed. In this way, both optical and electric fields are well overlapped in the slot, so that the crucial EO modulation can be expected in the proposed device.

3. Modulator fabrication and measurement

The fabricating steps of the modulators are illustrated in Fig. 2(a). Firstly, a 300-nm thick Al was deposited onto the SOI as the bottom electrode by using thermal evaporation and lift-off techniques. Subsequently, a layer of the EO polymer was deposited by spin-coating, and then baked at 120°C for 48 hours to form a 200 nm thick film. The side-chain EO polymer shown in Fig. 2(b) has a high Tg and an excellent thermal stability [13]. A 200-nm-thick TiO₂ layer was RF sputtered (ELIONIX, EIS-220) onto the EO polymer with a sputtering speed of 5 nm/min. During the deposition the substrate was maintained lower than 30°C to avoid any thermal damage to the polymer layer. The MZI and ring resonator waveguides were patterned onto the TiO₂ layer by conventional photolithography techniques and electron beam lithography, respectively. Then, the TiO₂ and EO polymer were etched (SAMCO, RIE-400iPB) to generate slot waveguides by CHF₃ and O₂ gases, respectively. After coating a 1.5 μm-thick sol-gel SiO₂, the top Au electrodes were manufactured by the electrolytic plating technique [13]. A 10 nm-thick gold seed layer with a 50 nm-thick titanium adhesion layer was thermally evaporated and then patterned. After deposition of the 3 μm-thick and 40 μm-wide electrode, resist and seed layers were removed by etching. By following this procedure, we prepared the MZI and ring modulators. For the MZI modulator, light entering the 2μm-wide waveguide was adiabatically split into two arms by using a 3-dB Y-branch. The two phase shifter arms had an identical length of 8 mm, and one of the arms was driven by a 6 mm-long Au traveling electrode. The resonator modulator consists of a ring with a radius of 50 μm and a bus waveguide with a gap of 300 nm to the ring (Fig. 2(c)). The radius of the ring modulator may be reduced further if using the higher refractive index material such as Si to replace the TiO₂. The 300-nm thick Al electrode surrounding 2/3 of the ring was deposited. The top Au electrode overlaps with the ring resonator in order to drive the modulator. Finally, the EO polymer was poled by applying a DC field of 80 V/μm across the device near to the Tg of the EO polymer (164°C). The device was rapidly cooled to room temperature to lock in the molecular orientation, and the DC field was then removed.

 

Fig. 2 (a) Fabricating process of the plate-slot waveguide modulators, (b) EO polymer structure, and (c) top-view SEM image of the ring resonator (before coating sol-gel SiO₂).

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The modulation of the fabricated devices was measured by an end-fire coupling system. Light from a tunable laser (TLS-510 Santec) was coupled into the waveguide by using a polarization-maintaining lensed-fiber. A polarizer was used between the laser and the fiber to ensure the input laser as TM polarization. The output light from the waveguide was collected by another lensed-fiber and then detected by a photo-detector (PDA10CS, Thorlabs) for the DC measurements or an optical spectrum analyzer (AQ6370, Yokogawa Electric) for the RF measurements.

4. Results and discussions

4.1 Low-frequency modulation tests

Figure 3 shows the modulation curve when a 1 kHz triangular waveform voltage was applied to the MZI modulator. From the relationship between the applied voltage and output optical power, a V_π of 1.1V was measured with an electrode length L of 6 mm, corresponding to a figure of merit V_πL of 0.66 V·cm. The in-device EO coefficient r₃₃ was calculated and found to be 96 pm/V using the equation $r_{33}=\frac{\lambda d}{neff3V\pi L\Gamma }$, where λ is the working wavelength, d the inter-electrode distance, and n_eff the effective refractive index. The propagation loss of the device was 0.5 dB/mm measured at a wavelength of 1.55 μm by the cut-back method. The coupling loss between the lensed fiber and the modulator is relatively high, because of the mode mismatch between each other. The coupling efficiency can be improved by using a mode transformer based on the quasi-vertical taper in future work [14].

 

Fig. 3 Transfer function at 1 kHz for the MZI modulator, where the V_π is extracted in the over-modulated optical signals.

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Compact ring modulators are ideal for large-scale integration to form single-chip optical switch arrays and wavelength-division multiplexing modules. Ring modulators are also attractive for use as miniaturized electric field sensors from DC to GHz frequencies [15]. In addition, the performance in the previous Si-organic ring modulators is still unsatisfied (Appendix Table 2). Encouraged by the high r₃₃ in the MZI modulator, it was believed to be worthwhile to characterize the ring resonator modulators having the same slot waveguide structure. Figure 4(a) shows the measured transmission spectrum of the fabricated ring resonator by scanning the wavelength of the laser. From the measured spectrum, the ring has a Q factor of 1.30 × 10⁴ and a free spectral range of 3.8 nm. The EO resonant tunability of the device was determined by measuring the transmission as a function of wavelength with the device biased. DC bias voltages from −2 V to + 2 V were generated from a digital function generator and loaded into the device. Figure 4(b) shows the change in the transmission spectra of one resonance of around 1599.67 nm by applying voltages of −2, 0 and 2 V. The high-resolution spectra were fitted by a Lorentz function based on the measured points. As can be observed in the spectral change, the extinction ratio varies less than 1.5 dB and the Q factor remains almost constant. A linear regression of the peak shifts shows an EO tunability as 0.065 nm/V (Fig. 4(b) inset). This tunability is higher than the previously reported all-organic / Si-organic ring modulators [16, 17] and comparable to the Si plasma-dispersion ring modulators [18]. Based on the wavelength shift against the voltage, we obtained an in-device r₃₃ of 88 pm/V for the ring modulator [19].

Table 2. Comparison of ring resonator modulators

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Fig. 4 (a) Transmission spectrum of the ring resonator without bias voltage, and (b) the fitted high resolution spectra of one resonant peak at 1599.67 nm shift with a range of bias voltages. The shift of the resonance peak is linearly fitted with the bias voltages (inset).

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4.2 High frequency response of the modulation

The RF responses of the MZI and ring modulators are characterized by using the sideband measurement technique. When the modulators are driven by a high frequency RF signal, two sidebands appear in the transmission spectrum, equally spaced around the main peak (insets of Fig. 5) [20]. The power of the main peak and first sideband is proportional to the square of the zero-order and first-order Bessel function of the first kind as a function of phase modulation index (η) [11, 20]. Based on the ratio of the main peak power and sideband power, the η can be extracted by P = J₀/J₁ = [J₀(η)/J₁(η)]²≈(2/η)². The RF signal provided by a signal generator (HMC-T2270, Hittite Microwave Corp.) was applied onto the electrode via a picoprobe (40A-GSG-250-DS, GGB Industries Inc.). The electrode was terminated with an external 50 Ω load. The output light from the modulators was directly connected to an optical spectrum analyzer. Figure 5 show the change of the normalized η of the MZI and ring modulator as a function of signal frequency. It can be observed that the η undergoes 3-dB attenuation at 31 GHz for the MZI and at 13 GHz for the ring.

 

Fig. 5 Normalized modulation indices (η) as a function of frequency of the MZI and ring modulators. Insets show the measured optical transmission spectra of the MZI and ring modulators.

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According to the theoretical calculations, the RC time constant of the EO polymer modulator can reach 400 GHz [12], so it does not affect the device bandwidth. In addition, the EO response time of the EO polymer is in the femtosecond range. Therefore, the limitations of the high frequency response may primarily originate from the RF power propagation loss and the impedance miss-matching between the microwave guides and external electrical connectors. On the other hand, the 3-dB bandwidth (f_3dB) of a ring modulator can be determined by the RC time and the photon lifetime as expressed by $\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{${\left(f_{3\text{dB}}\right)}^2$}\right.\approx {\left(2\pi \tau \right)}^2+{\left(2\pi RC\right)}^2$, where τ = λQ/(2πc) is the cavity photon lifetime (c is the light speed in vacuum and Q is the quality factor), R the contact resistance and C the device capacitance. Since the electrode of the ring modulator is much shorter than that of the MZI modulator, the RF propagation loss can be omitted. The f_3dB is mainly determined by the cavity photon lifetime required to build up and release the energy from the ring resonator. From the obtained Q factor, the f_3dB was calculated to be approximately 14 GHz, which is relatively close to the measured result. The equation above also predicts that the f_3dB can be improved further with a reduced Q factor by shrinking the ring resonator or using more narrow waveguides.

5. Conclusions

We have successfully demonstrated plate-slot waveguide modulators on SOI wafers. The fabricated slot waveguide cannot only effectively concentrate both the optical and electric fields within the sub-wavelength region, but can also offer a low propagation loss and an excellent poling efficiency. The MZI modulator exhibits a relatively low V_π and high modulation bandwidth. The compact ring modulator clearly displays an EO tunability of 0.065 nm/V and a 3dB-bandwidth of 13 GHz. We believe that the modulators are able to be easily integrated with other photonic and electronic devices on one chip and anticipate further development of Si-organic hybrid integrated photonics systems based upon the proposed plate-slot waveguides.