1. Introduction

Optical transceivers for optical interconnects have been developed using Si photonics technology. Currently, simple point-to-point connections are used. However, in the near future, wavelength division multiplexing (WDM) will be introduced to increase transmission capacity and to reduce the number of optical junctions. However, the rough, i.e., approximate, use of current devices is difficult. Even for coarse WDMs, wavelengths are fixed by employing distributed feedback (DFB) lasers, and some temperature and/or wavelength control are necessary for many other device components. The key issues inhibiting their practical application are the need for simple control and for the dense integration of many channels, which, if achieved, would allow a small footprint. To date, ring resonators that act as both modulators and wavelength multi-/demulti-plexers (MUX/DMUXs) [1] as well as combinations of on-chip grating and electro-absorption modulators [2] or Mach–Zehnder (MZ) modulators [3] have been reported as WDM transmitters in Si photonics. The microring-type transmitter appears to have a simple configuration; however, it requires another MUX to prepare the multi-wavelength light in a bus waveguide. Therefore, the channel wavelengths for the MUX and each microring have to be tuned to source wavelengths. The combination of a grating and modulators usually results in a large footprint, in particular, with MZ modulators of several millimeters in length.

In recent years, we have developed a compact Si MZ modulator, in which p/n-diode-loaded lattice-shifted photonic crystal waveguides (LSPCWs) generating wideband low-dispersion slow light are used as efficient phase shifters. We reported successful 25-Gbps operation [4], a wide working spectrum over 15 nm, and athermal operation for the temperature range of 19°C to 124°C [5]. We have also developed a triangular-shaped coupled-microring MUX/DMUX, which have some important advantages, compared with conventional circular and racetrack microrings, as explained later [6]. In this study, we fabricated a compact WDM transmitter comprising these two types of devices on a chip, as shown in Fig. 1, which allows a small footprint and simple wavelength control. We demonstrated this transmitter’s modulation and MUX characteristics at 25 Gbps/ch as well as its hitless wavelength tuning, which will be necessary for the flexible operation of channels. We have reported its concept with preliminary experimental results in [7], while presenting its full details including some improved characteristics in this paper.

 

Fig. 1 WDX transmitter. (a) schematic. (b) Top view of fabricated device (L = 200 μm).

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2. Design and fabrication

The device was fabricated on an 8-inch silicon-on-insulator wafer (210-nm-thick Si and 2-μm-thick silica BOX) through a Si photonics CMOS-compatible process with a minimum feature size of 180 nm. All components were formed by processing the Si layer and were covered with 2-μm-thick SiO₂ cladding. Ideally, for future use, a DFB laser array should be mounted as a light source. However, in this study, light from an external laser source was used and coupled into the spot size converters (SSCs) by a lensed fiber. Each SSC was connected to the LSPCW modulator through a Si wire waveguide (400 nm width). Modulated light was multiplexed to the bus waveguide though the coupled-microring MUX. Thanks to the compact components, the total footprint of the device including the three channels was as small as 2.0 × 0.7 mm².

The details of the modulator are described in [4]. The symmetric MZ interferometer comprised Si wire waveguides, a 1 × 2 multi-mode-interference (MMI) coupler with an excess loss less than 0.3 dB [8], and a LSPCW phase shifter. The 200-μm- or 300-μm-long LSPCW comprised a triangular lattice of holes (hole diameter 2r = 200 nm, lattice constant a = 400 nm) with a center line defect and the third rows of holes from the line defect shifted by s₃ = 95 nm. This generates a slow light for the transverse electric (TE) polarization with a group index of n_g ∼20, which provides a 2.3- to 2.5-fold enhancement in modulation efficiency, when compared with conventional rib-type modulators, even when considering the weaker modal confinement of the LSPCW. As aforementioned, the bandwidth of slow light is wider than 15 nm, which makes fine wavelength control unnecessary. A liner p/n junction (doping concentration of 4.8 × 10¹⁷ cm⁻³ in both the p and n regions) was formed along the center line of the line defect. The p⁺ and n⁺ regions were also formed 2 μm apart from the junction and connected to Al electrodes and RF pads via the holes. By applying RF signals with a reverse bias through the RF probe, the modulation was performed in a carrier-depletion mode. To adjust the phase, a 74-μm-long TiN heater (resistance R = 370 Ω, rated power P_MAX = 148 mW) was placed above each LSPCW and operated using DC pads and probe. The 2π phase shift was obtained using a small heating power of 16 mW because of the slow light effect.

The details of the coupled-microring MUX have been described in [6]. The triangular shape comprised 5-μm-long straight waveguides and 120° bends of 2 μm radius. This shape has three advantages. The first one is the easy control of the inter-ring coupling. The gap between straight waveguides at the directional coupling becomes wider than that between circular rings, which makes fabrication easier. The second one is the small thermal crosstalk between rings when each ring is thermally tuned to compensate for fabrication errors. We assigned a heater on one side of each triangle so that two heaters of coupled rings are located with the largest distance. Racetrack rings may achieve the directional coupling similar to the triangular oneʼs, but suffer from larger thermal crosstalk unless each ring is sufficiently large. The third one is a large free spectral range (FSR) given by a small ring, which is allowed for the low thermal crosstalk. To achieve a small ring, we particularly employed the small bend radius and suppressed each bend loss to 0.1 dB by introducing a 20-nm offset at the connection between the straight and bend waveguides. Figure 2 shows example through and drop spectra measured for the TE polarization without heating. The FSR was 19 nm at λ = 1500‒1560 nm for the round-trip length of 27.6 μm. A TiN heater (R = 70 Ω, P_MAX = 17 mW) was placed above each ring and operated using other DC pads. Considering the temperature profile in the waveguide during heating [6], which was calculated using the finite element method, and the thermo-optic coefficient of Si (1.86 × 10⁻⁴ RIU/K), the wavelength shift for the change in the equivalent modal index of the waveguide, dλ/dn_eq (570 nm/RIU), at the rated power was estimated to be 18.4 nm, which covers almost the entire range of the FSR. We designed a Butterworth transfer function having a box-like spectral response at the passband. We set the full width at half maximum Δλ_FWHM = 2.4 nm so that 25-Gbps signals could pass through with a sufficient margin. Targeting the TE polarization at λ = 1550 nm, the optimum coupling efficiencies for such a transfer function were obtained by setting a gap between the bus waveguide and microring at 204 nm and the gap between two microrings at 347 nm.

 

Fig. 2 Through and drop spectra measured for triangular-shaped microring without thermal tuning.

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For hitless operation, the bus waveguide was divided into the MUX branch and the bypass branch, similarly to that reported in [9]. We did not employ the nanomechanical switch in the paper but simpler MZ thermo-optic switches. The symmetric MZ switch comprised two newly-designed 2 × 2 MMI couplers, as shown in Fig. 3(a). The detailed design was established when a −3 dB coupler was optimized by finite-difference time-domain simulation for λ ∼1550 nm. In particular, the asymmetric taper of each input and output waveguide is effective for lowering the loss and maintaining a branching ratio. Using a multi-stage device including 15 MMIs connected in series (Fig. 3(b)), the excess loss at each MMI was measured to be 0.37 dB (3.37 dB including the essential 3-dB loss) at λ = 1550 nm. The branching ratio in the single device was −3.1 ± 0.2 dB in the range of 1550 ± 20 nm. This was sufficient for the switching operation in this study. Such a switch was operated by TiN heater (R = 500 Ω, P_MAX = 72 mW) through additional DC pads.

 

Fig. 3 2 × 2 MMI coupler. (a) Design. (b) Fabricated multi-stage device. (c) Excess loss evaluated using the device in (b). (d) Branching ratio measured for single device.

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3. Measurement

First, we measured the transmission spectra of the modulators (L = 300 μm in this section) through monitor ports, as shown in Fig. 4(a). Continuous-wave (CW) laser light was controlled to TE-polarization and coupled to the SSC through a lensed fiber. Light passing through the modulator was measured through the corresponding monitor port, another SSC and lensed fiber. The corresponding MUX was heated so that the drop spectrum did not overlap with the transmission spectrum. Since the same design parameters were employed for the three modulators, a similar passband was observed at approximately 1530 nm to 1540 nm, as defined by the band-edge of the LSPCW and light line of the silica cladding. The spectrum was expected to be flatter but actually exhibited a gradual change of several dB. Our calculation using finite-difference time-domain (FDTD) method indicated that slow light mode changes its profile in the transmission band and the absorption loss due to the p⁺ and n⁺ dopings increases on the short wavelength side while the coupling loss at the junction between the LSPCW and wire waveguides increases on the long wavelength side. Suppression of these losses and flattening the spectrum will be the important issues for practical use. A dip also appeared along the long-wavelength side only for modulator #1. This might have been caused by some disordering in the LSPCWs, which produced a large phase mismatch between the two arms of the MZ, particularly near the band-edge showing a rapid increase in the group index of slow light.

 

Fig. 4 Observed spectrum. (a) slow light bandwidth of LSPCW. MUX spectra (b) without thermal tuning and (c) with thermal tuning. The number in the figure corresponds to the total heating power.

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Figure 4(b) displays the transmission spectra measured through the WDM port. Since we also employed the same design parameters for the coupled microrings, similar spectra were observed without thermal tuning, although only the spectrum from modulator #1 was slightly red-shifted and deformed. The spectral sprit of the passband was caused by a slight detuning of the resonance between the two microrings. It can also occur severely when the coupling constant between the rings and that between the ring and bus-waveguide do not satisfy the Butterworth condition; a larger split was actually observed in [7] because of this reason. In this study, we used the Butterworth condition correctly so that we could compensate for the detuning and set different MUX passbands by tuning each microring, as shown in Fig. 4(c). After tuning, the MUXs exhibited box-like spectra with a ∼2 nm passband and 3 nm wavelength separation. In this process, we consumed maximally 22 mW heating power in total because of the relatively wide spectral shift particularly for MUX #1. If we employed slightly different design for each MUX so that the drop spectrum appeared at each target wavelength, the power consumption would be reduced drastically. Just for compensating the spectral detuning, the power consumption was less than 1 mW [6]. Figure 5 summarizes the tuned MUX spectra in the slow light band. We measured each spectrum one by one and estimated the crosstalk between adjacent channels from their overlapping to be from −13 dB to −17 dB, which was degraded by a relatively large roll-off of the Butterworth response. If we employed a Chebyshev response with a smaller roll-off [6], the crosstalk would be reduced to less than −20 dB.

 

Fig. 5 Thermal-tuned MUX spectra and observed 25 Gbps eye patterns.

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The insertion loss of the modulator ranged from 7‒9 dB, depending on the operating wavelength. The loss of the MUX at the passband ranged from 3‒6 dB without the thermal tuning, but was reduced to ∼1 dB by the tuning. The loss of the switch after the thermal tuning was less than 1 dB. Thus the total on-chip loss from the input waveguide to the WDM waveguide was 9‒11 dB. Since the lowest loss we have measured independently for the modulator was 5 dB, the total loss could be reduced to 7 dB if the spectral flatness of the modulator is improved in future studies. On this condition, the modulators were driven by 25-Gbps non-return-to-zero 2³¹ − 1 bit pseudo-random-bit-sequence signals from a pulse pattern generator (PPG), Anritsu MP1800A. We set V_pp = 3.5 V and V_DC = −1.9 V on the display of the PPG when no termination impedance was connected to the modulator, meaning that the actual voltages were higher. Modulated light was multiplexed to the bus waveguide through the MUX and eye patterns were observed through the WDM port, as shown in Fig. 5, to exhibit clear open eyes with an extinction ratio greater than 3 dB. In this measurement, we set the modulation loss to be 6−7 dB to compensate for the small phase shift in the phase shifter (L = 300 μm) with the linear p/n junction. This is the reason that S/N of the eye pattern was degraded from those in [7], where L was set at 200 μm. One might expect that the phase shift is enhanced by increasing L. However, we estimated theoretically that the velocity mismatch between slow light and RF signals becomes severe for L ≥ 300 μm with a group index of 20 for slow light and modulation speed of 25 Gbps. Therefore, L = 200 μm was better in the present experiment. To improve this situation further, the interleaved p/n junction [4] is effective for increasing the phase shift, although it constrains the modulation speed beyond 25 Gbps due to increased junction capacitance C and RC time constant.

Finally, let us present the hitless operation. Now, we consider a situation where passbands of MUX #2 and #3 are fixed at λ_c and λ_b, respectively, and that of MUX #1 is tuned from λ_a→λ_b→λ_c→λ_d (Fig. 6(a)). When MUX #1 overlaps with #2 and #3, the signals from #2 and #3 are blocked because they are dropped from the bus waveguide to the monitor port through MUX #1. To avoid this situation, MZ switch #1 was operated during the wavelength tuning to bypass MUX #1. Figures 6(b) and (c) display the observed MUX spectra and the 25-Gbps eye patterns at the WDM port, respectively. Figures 6(d) and (e) show the switching characteristics of SW #1 at λ_c and the corresponding change in eye patterns, respectively. For example, when signals at λ_c from Modulator #2 were multiplexed to the bus waveguide and MUX #1 was tuned to λ_c, the signals were dropped and outputted from monitor port #1. So MUX #1 was disconnected from the bus waveguide using SW #1, then the eye pattern from Monitor #1 completely disappeared (D in Fig. 6(e)). During the tuning of MUX #1 and switching at SW #1, the eye pattern from the WDM port was maintained to be clearly opened. Similarly, for signals passing through MUX #3, such hitless operation was observed.

 

Fig. 6 Hitless wavelength tuning. (a) Schematic operation. (b) Measured MUX spectra and tuning of MUX #1. (c) 25 Gbps eye patterns. (d) Switching characteristics of SW #1. (e) Eye patterns generated by Modulator #2 and observed at Monitor #1 and the WDM port.

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4. Summary

We demonstrated a successful operation of a three-channel compact and hitless wavelength tunable WDM transmitter. The footprint was reduced to 2.0 × 0.7 mm² by employing LSPCW MZ modulators assisted by the slow light effect and triangular-shaped coupled-microring MUXs. The MUXs’ thermal tuning was only conducted so that box-like spectra could be formed at the target passbands. Open eyes at 25 Gbps were observed through the WDM port. Hitless wavelength tuning was also achieved by bypassing a MUX under tuning using the low-loss switch integrated into the bus waveguide.