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We demonstrate ultrasmall demultiplexers based on photolithographic photonic crystals. The footprint of the demultiplexers is 110 μm2 per channel. Our in-plane demultiplexers are clad with silica, which makes them stable and easy to integrate with other silicon photonic devices. We describe two types of demultiplexers with spacings of 136 and 267 GHz between channels for application to dense wavelength division multiplexing. Integrated titanium nitride heaters allow us to precisely control the channel wavelength. We report a 2.5 Gbps transmittance experiment with sufficiently small crosstalk and discuss ways of achieving even lower crosstalk between channels.
© 2017 Optical Society of America
1. Introduction
To date, wavelength division multiplexing (WDM) communication has been used in long-haul optical communication because it supports large-capacity data transmission. Silica arrayed waveguide gratings (AWGs) have been used in WDM systems because they provide us with a precise multi/demultiplexing ability. The capacity of silica AWGs has reached 400 channels with a 25 GHz spacing [1], and the footprint of the device is now about 100 cm2, which is the maximum available wafer size [1–3]. On the other hand, the ever-increasing data traffic in datacenters and access networks means that WDM must also be installed in those systems. For these applications, we must fabricate small and inexpensive multi/demultiplexers (MUX/DeMUX). Recently, silicon AWGs have been developed based on silicon photonics technologies, and they have achieved a small footprint of less than 1 cm2 [4–10] due to the high refractive index of the material. Silicon AWGs have realized a 512 channel-25 GHz spacing capacity [10]. In addition to the small footprint, these devices are CMOS compatible, which may reduce the fabrication cost significantly. These properties allow us to use MUX/DeMUXs based on silicon photonics technologies for short-haul data transmission.
To realize even smaller MUX/DeMUXs, those based on silicon photonic crystals (PhCs) are being developed. PhC technology allows light to be confined even more tightly than with conventional silicon photonic devices, and therefore, MUX/DeMUXs can be fabricated with an even smaller footprint [11–23]. Using silicon PhCs, a DeMUX has been devised with a 32 channel-100 GHz spacing capacity whose footprint is 4050 μm2. This corresponds to 100 μm2 per channel [22]. However, there are still challenges to be met if we are to use this device in a practical system. Demultiplexed lights are often emitted in a direction away from the PhC slab and the system needs additional bulky optics. Although in-plane type DeMUXs have been demonstrated, they have only a few channels and a large channel spacing [16–19]. It is necessary to fabricate an in-plane PhC DeMUX with high capacity. In addition, these devices are usually fabricated with a costly electron-beam lithography process and an air-bridge structure, which reduces the CMOS compatibility.
Here, we demonstrate an eight-channel in-plane DeMUX with a spacing of 267 GHz and also a 16-channel device with a 136 GHz spacing. The DeMUX is based on a PhC nanocavity whose resonant wavelengths are shifted slightly by tuning the lattice constant of the PhCs. The structure is fabricated with photolithography and clad with silica. The photolithographic fabrication makes these DeMUXs highly compatible with other CMOS devices. Silica cladding protects the PhC structure of the DeMUXs, and allows us to integrate heaters on the top of the PhCs. The heaters enable us to precisely tune the channel wavelengths.
The paper is organized as follows. Section 2 describes the design. Section 3 shows the spectral properties of the DeMUXs. Section 4 presents the eye patterns and crosstalk of the DeMUXs and Section 5 concludes the paper.
2. Configuration of DeMUX devices based on PhCs
A schematic illustration of our DeMUX is shown in Fig. 1(a). PhC and input/output nanowires are fabricated on the same 210-nm-thick silicon slab. The DeMUX consists of PhC nanocavities with different resonant wavelengths. A signal is received at an input nanowire and it propagates in an input PhC line defect waveguide denoted as W1.05 (105% of the original width). We call this a bus waveguide. Different nanocavities with different resonances are side-coupled at the bus waveguide to drop a signal to a drop waveguide. To obtain a different resonant wavelength at each nanocavity, the x-axis air-hole distance a (note: a is not a lattice constant but the distance between holes along the x-axis) of the PhC is discretely chirped along the x-axis direction [14,18,20,22]. We fabricated a DeMUX with eight channels. The eight-channel and 16-channel DeMUX has 1-nm and 0.5-nm different a values at each cavity, namely an = a1 – 1 nm × (n – 1), for the nth cavity. In the eight-channel DeMUX, a1 is 420 nm and a8 is 413 nm.
Fig. 1 (a) Schematic illustration of an eight-channel DeMUX. The hole diameter is 269 nm and the slab thickness is 210 nm. WM nanocavities are created by shifting the PhC hole positions 9, 6, and 3 nm. An eight-channel DeMUX has a step lattice constants of 1 nm. (b) Resonant wavelength of WM nanocavities, calculated with three-dimensional FDTD simulation. (c) Cross-section of DeMUX. Silicon PhCs have a 2000-nm-thick silica cladding. Titanium nitride heaters are embedded in the silica cladding and connected with aluminum wires.
Figure 1(b) summarizes the resonant wavelength as a function of a calculated in the three-dimensional finite difference time domain (FDTD). It shows that the resonant wavelength exhibits a linear relationship. In addition, the calculation showed that a width-modulated (WM) nanocavity supports an intrinsic Q of 6.1 × 104 even it is clad with silica where the slab thickness, hole diameter and lattice constant are 210, 269 and 420 nm, respectively. The Q value is higher than that obtained with an L3 nanocavity (~103) that is widely used for applications. Indeed, the potential of the WM nanocavity is much higher. Our previous research realized a loaded Q of 105 experimentally by using a silica clad WM nanocavity [24]. A target loaded Q of 104 is needed to achieve dense WDM to suppress channel crosstalk when the channel spacing is 100 GHz [22]. The use of a PhC nanocavity with a high intrinsic Q is needed if we would like to achieve a reasonably high Q and high transmittance simultaneously.
Figure 1(c) shows the design of our device. The silicon structure is clad with silica, and includes an integrated titanium nitride heater to tune the resonant wavelength of the fabricated cavity. The silica cladding not only allows the DeMUX to have high mechanical stability and resistance to dust, but it enables heaters to be integrated on the top of the silicon layer.
3. Characteristics of fabricated DeMUX
Figure 2(a) is a scanning electron microscope (SEM) image of the fabricated DeMUX. The input light couples to a silicon nanowire via a spot size converter (SSC) and then enters the PhC DeMUX. The transmitted light is coupled to an optical fiber via an SSC. This system shows the in-plane operation, which is advantageous for high-density integration. Figure 2(b) is an optical microscope image of the entire eight-channel DeMUX with heaters. We can apply voltage via aluminum pads on the top surface, and the titanium nitride heaters will tune the cavity resonances.
Fig. 2 (a) SEM image of a fabricated DeMUX. The silica cladding is removed for the SEM observation. (b) Optical microscope image of an eight-channel DeMUX. (c) Heater tunability of the eight-channel DeMUX. (d), (e) Transmission spectra of eight- and 16-channel DeMUXs, with channel spacings of 267 and 136 GHz, respectively.
The transmission spectra of the eight-channel device are shown in Fig. 2(d), where the average spacing between the channels is 267 GHz. The figure shows that clear DeMUX operation is possible. We also fabricated a device with a 136 GHz spacing with 16 channels, as shown in Fig. 2(e). The wavelength spacings in those two devices are sufficiently dense to support WDM technology. This is achieved thanks to the high loaded Q of 4.6 × 104 of the WM nanocavities. This Q value is sufficient to support 4 GHz operation. Our DeMUX has a 25.6 dB loss, including 0.2 dB at the silicon nanowires, 1.8 dB at the lenses used for coupling to spot size converters (SSCs), 2.8 dB at the SSCs, 8.1 dB at the coupling between a WM nanocavity and input/output PhC W1.05 waveguides, and 12.7 dB at the interfaces between the silicon nanowires and PhC W1.05 waveguides. The coupling loss between a WM nanocavity and input/output PhC W1.05 waveguides agrees well with the loss of 5.9 dB calculated with an intrinsic Q of 6.1 × 104 and a loaded Q of 4.6 × 104. There is still a plenty of room left for the optimization of the structures to reduce the losses of 17.5 dB from the silicon nanowires, the lenses, the SSCs and the interfaces. The 8.1 dB coupling loss between a WM nanocavity and the PhC waveguides could be reduced by optimizing the coupling Q. Another important aspect of such a device is tunability. Figure 2(c) shows the result for heater tuning, where we successfully tuned the resonance so that it was larger than the channel spacing.
We would like to emphasize that previous devices [16,19] were fabricated with an air-bridge structure, whereas our device is clad with silica. In addition, we achieved dense channel spacing and in-plane operation thanks to the high-Q of the silica-clad WM nanocavity. Table 1 compares the performance of our device with that of previously reported devices.
Table 1. Supplementary Reports in OSA Journals.
4. Demonstration of DeMUX operation
4. 1 Signal transmission demonstration and crosstalk measurement
To demonstrate the operation of the DeMUX, we performed a signal transmittance experiment and measured the resulting eye diagrams and crosstalk. The setup is shown in Fig. 3(a). For the eye-diagram measurement, the input was modulated with a non-return-to-zero pseudo-random bit sequence (PRBS) signal of 210 − 1 and the channel 1 output was recorded with an optical sampling oscilloscope. The eye diagrams for 1 and 2.5 Gbps are shown in Figs. 3(b) and 3(c), respectively. The extinction ratio (ER) and signal-to-noise ratio (SNR) at 1 Gbps are 10.7 and 10.3 dB, respectively. The ER and SNR at 2.5 Gbps are 14.1 and 10.9 dB, respectively. The insets show back-to-back reference eye diagrams that we obtained with the same transmittance power. Clear eye opening is observed and there is no significant signal degradation. Although the demonstration is conducted at a speed of 2.5 Gbps, it is only limited by the bandwidth of our instruments, and 10 Gbps operation should be possible with our DeMUX having Qs of 4.6 × 104.
Fig. 3 (a) Setup for measuring eye diagrams and crosstalk. TLD: tunable laser diode (Santec TSL-510, linewidth of 200 kHz). EO: electro-optical modulator. EDFA: erbium-doped fiber amplifier. BPF: band-pass filter. VOA: variable optical attenuator. PPG: pulse pattern generator (Keysight 81134A, 3.35-GHz bandwidth). OSO: optical sampling oscilloscope (Agilent 86103A, 2.85-GHz bandwidth). (b), (c) Eye diagrams of the output at channel 1 with PRBS of 210 – 1 at 1 Gbps and 2.5 Gbps, respectively. Insets are reference eye diagrams when the DeMUX was replaced with another VOA with an attenuation the same as the transmittance of the channel 1 resonance. (d) Measured crosstalk with 1-GHz square pulse. Output signals in the same row were measured when the input signal had the same center wavelength. Output signals in the same columns were measured at the same output channels.
Next, we measured the crosstalk between the channels. The result is shown in Fig. 3(d). A square pulse with a 1 GHz repetition rate was injected as a signal. When we changed the wavelength, we observed outputs at different channels. Figure 3(d) shows the result for one of our devices with eight channels, where we obtained clear DeMUX operation.
However, a closer observation reveals the presence of crosstalk, particularly when we set the input signal wavelength at channels 6, 7 and 8. Next, we discuss crosstalk reduction.
4.2 Crosstalk reduction
The crosstalk is mainly due to the lower resonant peaks in higher channels (shorter wavelength) as shown in Fig. 2(d). Figures 4(a) and 4(b) show the transmission spectra calculated with two-dimensional FDTD. The insets show magnified illustrations of one of the channels in the DeMUX. When we applied our fabricated parameters, we obtained the calculated transmittance spectra shown in Fig. 4(a), which agree well with the experimental results. When we shift the output W1.05 waveguide by three columns to the right, we obtain the output spectra shown in Fig. 4(b). While the peak transmittance decreases in Fig. 4(a) as the channel number increases, the peak transmittance becomes higher and remains flat in Fig. 4(b).
Fig. 4 Transmission spectra calculated with two-dimensional FDTD simulation. Output W1.05 waveguides are placed in the same position as in the fabricated structure (a) and are shifted three rows to the right (b). In the two-dimensional FDTD simulation, we set the effective refractive index of silicon, neff, at 2.81. The diameter is specified at 300 nm to tune the resonances as in the experiment. The loaded Q of the optimized structure is 2.0 × 104.
A detailed analysis also revealed the influence of the modes in the W0.98 waveguides used as the barriers of the WM nanocavity. Figure 5(a) shows the spectra of channels 1 and 8 shown in Fig. 2(d). The peak at 1563 nm in channel 1 is due to the band-edge mode of the W0.98 waveguide, and this is well reproduced in the two-dimensional FDTD calculation as shown in Fig. 5(b). The coupling of the band-edge mode to a different channel is problematic, but this has also been solved by three-column shift optimization.
Fig. 5 (a) Transmission spectra of channels 1 (red) and 8 (dark blue) of the fabricated DeMUX. (b) and (c) are the calculated transmission spectra of channels 1 and 8 for the original structure and the three-column shift optimized structure, respectively. The inset shows the |Ey| profile of the band-edge mode, corresponding to the peak indicated with the arrow.
Three-column shift optimization has prevented the band-edge mode from coupling with the output W1.05 waveguide. Figure 5(c) shows the spectra of channels 1 and 8 shown in Fig. 4(b) with the optimized structure. The peak of the band-edge mode does not appear in the spectra, and the inset shows that the band-edge mode does not couple to the output W1.05 waveguide. This is because the interface between the W0.98 and the WM nanocavity shifts the phase of the band-edge mode by π/2, and destructive interference occurs at the location of the output W1.05 waveguide. In other words, by placing the output waveguides carefully at the point where the band-edge mode of W0.98 exhibits a destructive interference, we can avoid any large crosstalk; and this has been achieved by shifting the output W1.05 waveguide three rows from its original position.
5. Summary
We have demonstrated ultrasmall in-plane DeMUXs with spacings of 136 and 267 GHz, using photolithographic PhC nanocavities. The devices are clad with silica and have full compatibility with other silicon photonic devices. Although the PhCs are clad with silica, the nanocavities have Qs of higher than 104 due to the WM-nanocavity design thus allowing us to realize dense WDM DeMUXs. We showed that the channels of the DeMUXs can be precisely tuned with heaters and described a 2.5 Gbps signal transmittance experiment. The footprint of our device is extremely small at 110 μm2 per channel.
Funding
Part of this work was supported by the Strategic Information and Communications R&D Promotion Programme (SCOPE), from the Ministry of Internal Affairs and Communications, Japan.
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