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Abstract
An integrated 8 x 8 wavelength router based on the micro-ring resonators using 2 x 2 multi-interference (MMI) crossing is demonstrated on silicon-on-insulator (SOI) technology, which is manufactured with microelectronics equipment. Experimental results show a free spectral range (FSR) about ~37 nm, an on/off contrast larger than 20 dB, an imbalance among the channels less than 2 dB, a crosstalk of channels smaller than −10 dB, a spacing between close channels about 3.6 ± 0.7 nm and an output efficiency of every channel smaller than 20 dB.
© 2017 Optical Society of America
1. Introduction
With the development of the information technology, copper on-chip electrical interconnect is difficult to satisfy the future requirements of larger bandwidth, smaller latency and lower power [1–8]. Integrated photonic links are being adopted as reliable and attractive alternative to traditional metallic interconnects. They hold promise to higher rate data transfer with minimal power dissipation for photonic network on chip (NoC). The optical routers are the building blocks of photonic NoCs, which constitute the core of the photonic NoCs and provide an optical link between input and output ports by routing signals based on their wavelengths multiplexing [8–14].
In this manuscript, an 8 x 8 wavelength router is introduced, which works as a wavelength router of routing signals between input and output ports. SOI technology is used to fabricate the router, which is a promising platform for the convergence of microelectronics and photonics [2]. This will be helpful for a similar design of an optical interconnect chip.
2. 8x8 router
An 8x8 router is shown in Fig. 1, which is composed of 8 initiator and target ports, 28 building blocks constituting by 56 mirco-ring resonators with 8 different radii for 8 resonant wavelengths. Initiator and target ports are 8 resonant sources (initiator ports: I1… I8) and corresponding detectors (target ports: T1… T8). The data is sent optically through the router from each initiator to one or more targets by selecting a specific wavelength. The optical path of a data between initiator Ii and target Tj traveled in the network depends on an optical resonant wavelength. The router has eight columns for eight resonances of λ1, λ2, λ3, λ4, λ5, λ6, λ7, and λ8 as shown in Fig. 1, respectively. In which, the building blocks on the same column have same resonances due to same radius.
The zooming picture of Fig. 1 is a basic building block of the router, which work in a similar way with classical electronic switch from a functional point of view as shown in Fig. 2 [6, 15]. The optical resonant wavelength is dropped in a straight direction in Fig. 2(a) and the non-resonant signal propagates in a diagonal direction [see Fig. 2(b)]. This can realize the key functionality of choosing and redirecting a signal based on its wavelength.
The Table 1 shows resonant wavelengths for each initiator port and corresponding target port of an 8x8 router. For example, if a light with 8 signals is injected at I6, these eight signals will output on different target ports (T1-T8) based on the signal wavelengths 6. For example, if a light is injected at I6 as shown in the red box of Table 1, the output wavelength on the eight target ports of T1, T2, T3, T4, T5, T6, T7, and T8 will be λ2, λ1, λ3, λ8, λ4, λ7, λ5, and λ6, respectively.
Table 1. The optical path between initiator Ii and target Tj
Note, in Table 1, there exists one non-resonant wavelength corresponding with a throughput path for a continuous light source that propagates in a diagonal direction at each traveled path for each initiator. The wavelength in the black box in Table 1 is non-resonant wavelength for each initiator. For example, a non-wavelength is λ3 in an optical path I6 to T3. Moreover, one can see that the Table 1 is symmetrical based on the black box, which mean that a return optical path is the same with a transmission optical path.
3. Basic building block
The basic building block of the router is as shown in the zooming picture of Fig. 1. A 2 by 2 MMI is introduced into the basic building block as a waveguide crossing of the two rings, because a conventional crossing has a relatively large insertion loss and crosstalk at a crossing junction due to a wavefront expansion, particularly in high-index-contrast waveguide platform [16,17]. Based on the research about a MMI crossing [17], an optimized 2 by 2 MMI design is developed as shown in Fig. 3(a). In which, the width W, the length L of multimode section, the spacing ec1 and ec2 of input and output waveguides relative to the axis of symmetry has be adjusted to reduce the diffraction losses. Figure 3(b) shows the field distribution in an optimized design (W = 2.0 µm, ec1 = 0.36 µm, ec2 = 0.36 µm, L = 8.6 µm) by a Finite-Difference Time- Domain (FDTD) method. The insertion loss of the device is about 93% (−0.1 dB).
Fig. 3 (a) Schematic view of a 2 by 2 MMI. (b) The simulation of the light propagation in a 2 by 2 MMI by a FDTD method [17].
A good balance of a building block is desired for a router, which mean that the output of light injected at the input1 and input2 port should be similar in the zooming picture in Fig. 1. The measurement outputs for two different inputs are shown in Fig. 4 for a basic building block with a radius of 2.5 µm. One can see there is a good agreement for the outputs of input1 and input2, which means there is a good balance of the basic building block with a MMI crossing. The FSR of a building block is about 37 nm and the contrast (out-of-band signal rejection) is more than 20 dB.
Fig. 4 Normalized output power on the drop port of the basic block when light input on the input1 or input2 ports for TE polarization mode.
The router works by selecting the optical wavelength using 8 different resonant wavelength through different radius of micro-ring resonators. In order to accurately design the radius of micro-ring resonator for 8 different resonant wavelength in the router, three basic building blocks with the radius (2.5 μm, 2.51 μm and 2.52 μm) are evaluated. In our design, the width of bus and ring waveguide is 380 nm and 450 nm to improve the coupling of bus and ring waveguides, respectively [18–20]. Figure 5 plots the measured results of the basic building blocks with three radius, which show that a 10 nm increase of radius shifts the resonant wavelength about 4 nm.
Fig. 5 Normalized output on the drop port of the three basic building blocks with the radius of 2.5 µm, 2.51 µm and 2.52 µm for TE polarization mode [6].
4. Results and discussion
An 8x8 router of Fig. 1 is fabricated, in which, the 8 different radius is designed to increase by 0.01µm from 2.50 µm to 2.57µm. The MMI crossing is designed as W = 2.0 µm, ec1 = 0.36 µm, ec2 = 0.36 µm and L = 8.6 µm. The width of bus and ring waveguide is 380 nm and 450 nm, respectively. The 8x8 λ-router is fabricated with microelectronics equipment. An experimental set-up is designed, in which, the sample is mounted on a XYZ translational stage (precision alignment). Light from a laser source is coupled to a polarization maintaining fiber and directed through a polarization controller, which can obtain TE or TM like polarization. Then, the light is coupled into the sample by a tapered fiber. After passing through the sample, the light is collected by a tapered fiber and transmitted to an optical spectrum analyzer. The input and output tapered fibers are mounted on XYZ micrometer stages for precision alignment with respect to the sample. A linear infrared camera with a microscope objective is used to observe the light diffracted at the side wall of the waveguides during the measurement.
The measured transmission spectrum of the router with light injected at initiator I6 is shown in Fig. 6 for TE polarization mode. Three groups are observed, which means a good repeatability of the router.
Fig. 6 Normalized output on 8 targets of the router with light injected at initiator I6 for TE polarization mode.
The second group in Fig. 6 is zoomed as shown in Fig. 7. Figure 7 reveals an equal space along the wavelength range for 7 peaks. The eighth peak (red line in the plot) misses because we use a continuous light source in the experiment, one of the target ports have to be throughput port, on which, there is no resonant wavelength. In this case, a path I6-T3 corresponds to the throughput path for a light injected at initiator I6 according to Table.1. In Fig. 7, one can see that the imbalance among the 8 channels (the output power difference at other targets) is lower than 2 dB. The wavelength interval of two close channels is about 3.6 ± 0.7 nm. The output efficiency of every channel is smaller than 20 dB.
Fig. 7 Close up of the second group of Fig. 6 for the normalized output on T1-T8 with light injected at initiator I6 for TE polarization mode.
The crosstalk on each target port is summarized as shown in Table 2, which is calculated by the output difference of the destination and another target port. Because the path of I6-T3 is a throughput path for light injected at initiator I6, there is no resonance on a target port of T3. One can find from Table 2 that the crosstalk is usually smaller than −10 dB. The exceptional case of crosstalk between T4 and T2 is caused by the overlap of two resonant peaks as shown in Fig. 7. This can be avoided if using the discrete source.
Table 2. Experimental crosstalk for the signal injected at initiator I6
Similar measurement is also performed for light injected at other seven initiators. The similar results are obtained with the results of light injected at I6. The measured resonant wavelengths for different initiator Ii and target Tj of the router are shown in Table 3, which exits about ≤1 nm difference of a same resonance for different initiators due to fabrication imperfection.
Table 3. The resonant wavelengths (nm) for different initiator Ii and target Tj (-means no resonance)
5. Summary
In this paper, a design, fabrication and experimental characterization of an 8 x 8 wavelength router is presented with a footprint of 0.37x0.26 mm2, which constitutes of 56 ring resonators connected using a MMI crossing. The imbalance between the 8 channels is lower than 2 dB. The crosstalk of channels is smaller than −10 dB. The wavelength interval between close channels is about 3.6 ± 0.7 nm. The output efficiency of every channel is smaller than 20 dB. The total losses for one channel is about −4 dB. Next step, we will focus the reduction of the resonator loss and crossing loss, study a wavelength router using a racetrack resonator to have a better control of the coupling, which will bring better balance between different channels.
Funding
This work is supported by the STREP European program “WADIMOS”, the National Natural Science Foundation of China (11574328), National Key Scientific Instrument and Equipment Development Projects of China (2014YQ090709), and Beijing Jiaotong University Basic Scientific Research Foundation (2016RC046).
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