C OMPACT W AVELENGTH S ELECTIVE C ROSSBAR S WITCH W ITH C ASCADED F IRST O RDER M ICRO - RING R ESONATORS

We demonstrate a compact 4 × 4 wavelength selective switch with 50 % fewer electrical pads as compared with our previous generation. We report loss and crosstalk for different paths of the switch. We measure median loss of 5.32 dB and worst-case crosstalk of -35 dB. The microring resonators tune by more than one free spectral range, which is an improvement over our previous generation of switches. This switch can support 8 channels at 400 GHz spacing. We conclude that it is not possible to drive both microring resonators with the same voltage and separate control is required because of fabrication variation of the current technology.

In this paper, we present a 4x4 switch with L = 2 (2 cascaded first order MRR at every crosspoint) switch that can tune over one full FSR with a more compact footprint. We also show full free spectral tuning of the MRR. MRR spectra are very sensitive to gap between waveguides. For the case of higher order ring resonators specific relationship between multiple coupling coefficients must be met for a flattop spectra. An easier approach to achieve a given 3 dB bandwidth and at the same time a higher out of band rejection is to cascade the drop transmission spectra of multiple first-order ring resonators. This results in a higher power penalty as compared to second order ring resonator, but the ease of design makes this one of the choices for the filter our switch.
4x4 switch with L = 2 with second-order cascaded first-order rings has up to 64 signal pads and the area of the switch scales as 2LN 2 . The area of this switch can be reduced by half if both of the ring resonator heaters are connected together. This places stringent constraints on acceptable resonant wavelength variation caused by the fabrication process. In this work, we measure a switch fabricated in a 220 nm Si Photonics foundry and report the standard deviation of loss, continuous wave (CW) crosstalk, resonant wavelength at zero bias, and full width at half maximum (FWHM).

Architecture
In this section, we describe the architecture of the switch . Fig 1 (a) shows a N × N switch with M wavelengths at each input port. We need only L = 2, i.e two MRR per crosspoint for near optimal latency [13]. Fig 1 (b) Tx corresponds to transmitter and Rx corresponds to receiver. Fig 1 (c) shows switch unit cell. Fig 1 (d) shows layout of a 4 × 4 (L = 2) switch. Fig 1 (e) shows micro graph of 4 × 4 (L = 2) switch. The MRR resonances are tuned using thermal tuning. We built the switch using MRR from AIM photonics process design kit (PDK).
During data transmission, time is divided into timeslots and at the start of every timeslot a centralized arbiter performs scheduling of traffic and wavelength assignment. More information about assignment and arbitration can be found in [4]. At the start of every timeslot the arbitration algorithm generates a traffic matrix. Each entry in the traffic matrix corresponds to number of wavelength channels required between input and output ports.  In this section, we report experimental results of the switch . Fig 2 (a) shows the measured transfer spectra comparison of one and two cascaded first-order ring resonators. The out-of-band rejection 400 GHz away from resonance wavelength of ring resonator increases from -21 dB to -40 dB as we change the ring filter from one to two cascaded filters. The 3-dB bandwidth of a single ring resonator is 64 GHz, from which we expect a 3-dB bandwidth of 38 GHz from the cascaded two-ring filters. This is the expected 3-dB bandwidth of the cascaded filters. The curve in red shows the measured cascaded filter response from one of the unit cells in the switch. However, as shown in the red curved in Fig 2  (a), the measured cascaded filter response from one of the unit cells in the switch has a 3-dB bandwidth of 40 GHz. This is due to the fact that the heaters of the two ring resonators are tied together. Thus, individual control of resonators is not possible and any misalignment between the resonators changes the filter shape. The extinction on the through port is -17 dB .   Fig 2(b) shows the tuning process of the cascaded first order ring filter. In this measurement, an increasing voltage is applied to one cascaded ring from a unit cell of the 4x4 switch. The ring tunes by 27.06 nm (Free Spectral Range (FSR) = 25.6 nm) with a tuning efficiency of 0.37 nm/mW (for both rings). This ring tunes by more than the FSR and thus can be used for selecting all WDM channels present in the system. The MRR can support 8 wavelength division multiplexed (WDM) channels at 400 GHz spacing. We automated the measurements with functions from Lumos, an instrument control library in python [14]. We use MRR from foundry process design kit (PDK) in this paper. We advise the reader to use MRR reported in [15] for a lower off resonance loss.
The low loss and full FSR tuning make this MRR an ideal candidate for opto-electronic switches. Tuning curve and Heater IV and RV are reported in Fig 2 (c) and (d). The tuning curve demonstrates full FSR tuning. The resonance wavelength is 1536.9 nm and the histogram of resonant wavelengths measured on different filters on the die is given in Fig 4. Heater IV in Fig 2 (d) shows that the current changes from 6 mA to 20 mA as the voltage changes from 0 to 3.7 mA. The resistance changes from 80 Ω to 180 Ω.
There are in total 32 signal pads and 16 ground pads with a total footprint of 1.6 × 1.55 mm 2 . The signals were routed with a 1 µm/mA rule to avoid burnout due to electro migration. The layout was done with Cadence Virtuoso and waveguide crossings were designed using Phoenix Optodesigner. Traces of 70 µm and 20 µm are used to connect to the ground pads. Due to the low measured resistance of the ring resonators, traces with twice the width or connecting all grounds to a ground plane on a different electrical routing layer might be a better choice for future switches. Pad sizes are 60 µm × 60 µm with a pitch of 160 µm. A pitch of < 135 µm is unsuitable for flip-chip bonding to organic carriers and we choose this pitch even though we did not flip chip this chip in this work. All pads were filled with square vias, as larger number of vias connecting different pad layers prevent pad peel off problem. The waveguide dimension used in routing is 220 nm × 400 nm and we use 5 µm radius bends for routing all waveguides. Foundry-specified waveguide loss is 2 dB/cm and edge coupler loss is 2.7 dB/facet. A 100 µm trench is provided at the chip edge for ease of optical coupling. Waveguide crossings are designed with particle swarm optimization and have a reported average loss of 0.028 dB and worst-case crosstalk of 37 dB [16]. The designed footprint of the crossing is 17 µm x 17 µm due to 4 µm linear tapers used to taper the waveguide from 400 nm to 500 nm. Spirals with simultaneous tapered width from 400 nm to 150 nm and radius from 5 µm to 0.2 µm are used for waveguide termination. These spirals have a 20-dB reflection simulated with Lumerical 3D Finite Difference Time Domain package at 1550 nm .   Fig 3 (a) shows transmission (dB) vs. paths in the 4x4 switch. Blue dots correspond to measured data points at different wavelengths and red corresponds to median of the points for each path . Fig 3 (b) shows a median loss of 5.32 dB. Median is used instead of mean due to skewed distribution of the data. The outliers in Fig 3 (a) and (b) correspond to cascaded filters with a large difference of resonant wavelengths. Continuous wave (CW) off loss is measured at 1546.8 nm . Fig 3 (c) and (d), shows the crosstalk measurement. We set a MRR on a chosen path ring to 1546.8 nm and we measure the crosstalk power at other ports.   One can see that the median loss of chip 1 is 3.23 dB which is smaller than 8.53 dB of chip 2. The distribution of path losses of chip 2 is skewed towards lower values. Fig. 5 (b) shows the FWHM of chip1 and chip 2. The median FWHM of chip 1 is 0.435 nm which is smaller than 0.54 nm of chip 2. The IQR of chip 1 is 0.56 nm which is much greater than 0.29 nm of chip 2. This shows that there is a smaller variation in FWHM values. Fig. 5 (c) shows V1 and V2 applied to two rings on a test structure cascaded in the same arrangement as the rings on the switch. These voltages are optimized for maximum peak transmission of a given channel wavelength. Red line shows V 1 = V 2 . The dissimilarity of the drive voltages could be due to two factors, variation in heater resistances or variation in the fabrication process (thickness of the chip). As the rings are placed right next to each other it is unlikely that thickness can be a factor. Different doping of resistors or other fabrication variation that can change dimensions of the resistors can change the effective index of the ring resonator. This could be the reason for different resonant wavelength and heater resistances.

Conclusion
We report a compact switch with same drive for two cascaded MRRs. We report a median path loss of 5.32 dB. The MRRs in the switch can tune across an entire FSR which is an improvement over our previous switches. We conclude that due to fabrication variation of current technology, we cannot drive cascaded MRRs with the same drive voltage and separate control is required.