Silicon photonic wavelength cross-connect with integrated MEMS switching

We report on monolithically integrated wavelength cross-connects (WXCs) on an enhanced silicon photonic platform with integrated micro-electro-mechanical-system (MEMS) actuators. An 8 × 8 WXC with 8 wavelength channels comprising 16 echelle gratings and 512 silicon photonic MEMS switches is integrated on a 9.7 mm × 6.7 mm silicon chip. The WXC inherits the fundamental advantages of silicon photonic MEMS space switches, including low loss, broad optical bandwidth, large fabrication tolerance, and simple digital control. The WXC exhibits a low crosstalk of − 30 dB, a submicrosecond switching time of 0.7 μ s, and on-chip optical insertion losses varying from 8.8 dB to 16.4 dB. To our knowledge, it is the largest channel capacity (64 channels = 8 ports × 8 wavelengths) integrated WXC ever reported.


I. INTRODUCTION
Wavelength-selective switches (WSSs) and wavelength crossconnects (WXCs) are widely used to route individual wavelengths in wavelength-division-multiplexing (WDM) networks. They also allow adding or dropping of signals in reconfigurable optical add/drop multiplexers (ROADMs). [1][2][3][4] WXCs can be implemented by deploying WSSs based on the route-and-select architecture or combining wavelength (de)multiplexers and space switches (SSs). For the former approach, WSSs are realized with free-space optics utilizing liquid crystal on silicon (LCOS) spatial light modulators (SLMs) or digital micromirror devices (DMDs). 5 The latter approach is based on integrated planar lightwave circuits (PLCs) technology. 6 Hybrid approaches have been also reported in which arrayed waveguide gratings (AWGs) are implemented on PLCs as the wavelength (de)multiplexers and SLMs or 3D micro-electromechanical-system (MEMS) switches are employed as the space switches. [7][8][9] As the port counts and the number of wavelength channels scale up, the size, weight, and cost of WXCs grow rapidly. Traditionally, the WXCs and ROADMs are deployed in core and metropolitan networks. Recently, there has been increasing interest in using WXCs to facilitate the scaling of data center networks. 10,11 Fully integrated WXCs with reduced size and cost are desirable for data center applications. Furthermore, faster switching time enables dynamic provisioning of wavelengths for flow control.
Silicon photonics is a promising platform to integrate many optical components on a small chip at low cost using advanced complementary metal-oxide-semiconductor (CMOS) foundries. [12][13][14] Previously, a 1 × 2 WSS with 16 wavelength channels using foldback AWGs, 15 a 4 × 8 WXC with 12 wavelength channels using AWGs and ring resonators, 16 and an 8 × 4 WXC with 2 wavelength channels using feedback-controlled ring resonators 11 have been demonstrated. However, the ring resonators and phase shifters used in these WXCs are prone to fabrication variations and ambient temperature changes. Thermo-optic tuning was used to compensate for these variations, leading to a complex control scheme with high APL Photonics ARTICLE scitation.org/journal/app power consumption. The scalability of the WXC was also limited by the optical loss and crosstalk. Silicon photonic MEMS is the hybrid of silicon photonics and MEMS technologies. [17][18][19][20][21] MEMS enables physical movement of silicon waveguides, which provides effective switching of broadband optical signals with unprecedented loss, crosstalk, and extinction ratio. Using MEMS-actuated vertical adiabatic couplers, we have demonstrated 64 × 64 and 240 × 240 silicon photonic switches. 20,21 The advantages of silicon photonic MEMS are also favorable for large-scale WXCs.
In this paper, we will describe the design, fabrication, and characterization of a fully integrated WXC with echelle grating multiplexers/demultiplexers and silicon photonic MEMS space switches. As a proof of concept, we have experimentally demonstrated an 8 × 8 WXC with 8 wavelength channels. We measured on-chip optical insertion losses of 8.8-16.4 dB, a switching time of 0.7 μs, and crosstalk below −30 dB. The WXC has simple digital control and does not have static power consumption. We designed the echelle gratings to separate 8 wavelength channels (λ 1 , λ 2 , . . ., λ 8 ) from 1490 nm to 1560 nm with a channel spacing of 10 nm based on the Rowland configuration. 22 The designed echelle gratings have a Rowland radius of 120 μm, a grating period of 4.4 μm, and an output waveguide spacing of 3.3 μm. The input and output waveguides are 2-μm-wide. The second order distributed Bragg reflectors (DBRs) with a period of 400 nm and a duty cycle of 50% were used at the reflection facets of the gratings to enhance the performance. 23 The simulated reflectivity of the DBR facet is 91%, vs 20% reflectivity of the etched Si facet. In the layout shown in Fig. 1(b), each signal path experiences different number of crossings, depending on the wavelength and the input and output ports. Wavelength λw from input Ix passes through (

II. ARCHITECTURE AND DESIGN
crossings between the demultiplexer and the switch. Similarly, λw routed to output port Oy passes through crossings between the switch and the multiplexer. The total number of crossings of the 8 × 8 WXC varies from 0 to 98. Therefore, it is essential to reduce the crossing loss. We employed multimode interference (MMI) crossings with a simulated insertion loss of 0.011 dB/crossing, which had been also used in our previous silicon photonic switches. 20 It should be pointed out that the horizontal and vertical waveguides of the crossing here have different wavelengths, which is unique to WXC. To minimize the crossing losses for both wavelengths, we designed the crossing with different multimode region lengths for the horizontal and vertical waveguides. Figure 2 shows the calculated center wavelengths of MMI crossings vs multimode region lengths. It varies linearly with a slope of dλ/dL = −4.88 nm/100 nm. Based on this calculation, the MMI crossings were optimized to have insertion losses as low as 0.01 dB for all wavelength channels.
For the space switches, we utilized 8 blocks of 8 × 8 silicon photonic MEMS switches. It has been shown that silicon photonic MEMS switches are highly scalable, owing to their low insertion loss and low crosstalk. 20

III. FABRICATION AND CHARACTERIZATION
The designed WXCs were fabricated in a commercial 200-mm CMOS foundry at TSI semiconductors. The 8 × 8 WXCs with 8 wavelength channels are monolithically integrated on a 220-nm silicon-on-insulator (SOI) chip with an area of 9.7 mm × 6.7 mm. The detailed process steps have been described in our previous switch paper. 20 The microscopic die photograph of the fabricated device and the close-up view of the echelle grating are shown in Fig. 3, together with the scanning electron micrographs (SEMs) of the DBR reflectors in the echelle grating and the switch unit cell.
TE-polarized light was coupled from a linear fiber array to the fabricated device through grating couplers. The coupling loss was measured to be 4.5 dB/facet at the peak wavelength. Although grating couplers with limited optical bandwidths were used here to simplify fabrication and testing, they can be replaced by broadband couplers such as edge couplers. We first measured the spectral response of the echelle grating from a standalone test structure on bias for individual switching cells. From the temporal response measurement, submicrosecond switching was confirmed as in our previous silicon photonic MEMS switches. The ON and OFF switching times were measured to be 0.7 μs and 0.3 μs, respectively, as shown in Fig. 6.
The full transmission spectra through the echelle gratings, the MMI crossings, and the MEMS switches were measured between the input ports and the output ports. Figure 7 shows the representative transmission spectra from the input port In to the output port On when the corresponding switching cell (n, n) of all switch arrays is turned on (n = 1, 2, . . ., 8). For example, the red curves (λ 8  The insertion loss of the WXC depends on the number of crossings and the length of routing waveguides of the selected input/output (Ix and Oy) and wavelength (λw). To characterize the insertion loss, we measured the transmissions of all possible configurations and plotted as a function of the number of crossings in Fig. 8(a). The measured on-chip losses of the device vary from 8.8 dB to 16.4 dB. The loss increases with the number of crossings at a rate of 0.06 dB/crossing. This loss is higher than the simulation (0.01 dB/crossing), most likely due to the additional propagation loss of the routing waveguides between crossings and fabrication imperfection. In addition, the small wavelength shifts in echelle grating (de)multiplexers may also contribute to the extra crossing loss. The loss distributions of each wavelength are plotted in Fig. 8(b). Due to the routing scheme of the device, the center wavelength has the least variation in the number of crossings (42-56). Consequently, its optical loss has a tight distribution around 10 dB. We anticipate the overall on-chip losses and their variations to be reduced by improving the fabrication process. Alternatively, we can employ two-layer waveguides to eliminate all crossings in the routing area. 24,25 In addition, the silicon photonic MEMS switches based on two-layer waveguides can be designed for polarization-agnostic operation. 25 However, the complete demonstration of polarizationindependent WCX without polarization splitters/rotators still requires the development of polarization-independent wavelength (de)multiplexers.

IV. CONCLUSION
We have experimentally demonstrated 8 × 8 silicon photonic wavelength cross connects with 8 wavelength channels. 16 echelle gratings and 512 silicon photonic MEMS switches, along with 1568 MMI crossings, have been successfully integrated on a 9.7 mm × 6.7 mm silicon chip. To our knowledge, it is the largest channel capacity (64 channels = 8 ports × 8 wavelengths) silicon photonic WXC ever reported. The fabricated device exhibits low crosstalk (<−30 dB) and submicrosecond switching time (0.7 μs). The on-chip insertion losses are measured to vary from 8.8 dB to 16.4 dB. The loss uniformity can be further improved by employing two-layer routing waveguides.