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 µs, 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.

Wavelength-selective switches (WSSs) and wavelength cross-connects (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).1–4 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-electro-mechanical-system (MEMS) switches are employed as the space switches.7–9 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.12–14 Previously, a 1 × 2 WSS with 16 wavelength channels using fold-back 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 resonators11 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 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.17–21 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.

Figure 1(a) shows the architecture of an N × N WXC with M wavelength channels. The WXC has N input and N output ports, N wavelength demultiplexers and N multiplexers, and M arrays of N × N space switches, where M is the number of wavelengths. The wavelengths are first separated by demultiplexers and routed to the corresponding space switch dedicated to each wavelength. The switched wavelengths are combined by multiplexers of the output ports. Figure 1(b) depicts our experimental prototype of the 8 × 8 WXC with 8 wavelength channels (N = 8, M = 8). It has 16 grating couplers (8 input ports and 8 output ports), 16 echelle gratings (8 demultiplexers and 8 multiplexers), and 8 arrays of 8 × 8 silicon photonic MEMS switches. The waveguides are highlighted with different colors to show the routing path of each wavelength.

FIG. 1.

(a) Schematic of the wavelength cross-connect architecture. (b) Illustration of the silicon photonic wavelength cross-connect with 8 input ports (Ix), 8 output ports (Oy), and 8 channel wavelengths (λw). The waveguides are highlighted with different colors for different channel wavelengths.

FIG. 1.

(a) Schematic of the wavelength cross-connect architecture. (b) Illustration of the silicon photonic wavelength cross-connect with 8 input ports (Ix), 8 output ports (Oy), and 8 channel wavelengths (λw). The waveguides are highlighted with different colors for different channel wavelengths.

Close modal

We designed the echelle gratings to separate 8 wavelength channels (λ1, λ2, …, λ8) 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 (8 − w) × (x − 1) + (w − 1) × (8 − x) crossings between the demultiplexer and the switch. Similarly, λw routed to output port Oy passes through (8 − w) × (y − 1) + (w − 1) × (8 − y) 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 /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.

FIG. 2.

Simulated center wavelength vs multimode region length of MMI crossings. The MMI crossings are designed to have asymmetric multimode region lengths for different wavelengths in the horizontal and vertical paths.

FIG. 2.

Simulated center wavelength vs multimode region length of MMI crossings. The MMI crossings are designed to have asymmetric multimode region lengths for different wavelengths in the horizontal and vertical paths.

Close modal

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.20,21 The small footprint of the switch unit cell (110 µm × 110 µm) enables high density integration of large-scale switches. The MEMS-actuated vertical adiabatic coupler switch offers broadband operation covering L, S, and C communication bands, simple digital control, and high energy efficiency without static power consumption, which are favorable for large-scale switches and WXCs. A total of 512 switch cells (8 arrays of 8 × 8 switches) were integrated in our WXC. Unlike the MMI crossings with asymmetric lengths in the routing paths, symmetric MMI crossings were used in the switch arrays as each 8 × 8 array operates with a designated wavelength.

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.

FIG. 3.

(a) Micrograph of the fabricated silicon photonic WXC. (b) Micrograph of an echelle grating. The inset shows the SEM image of the DBRs at the reflecting facet. (c) SEM image of the silicon photonic MEMS switch unit cell.

FIG. 3.

(a) Micrograph of the fabricated silicon photonic WXC. (b) Micrograph of an echelle grating. The inset shows the SEM image of the DBRs at the reflecting facet. (c) SEM image of the silicon photonic MEMS switch unit cell.

Close modal

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 the same chip. Figure 4 shows the measured transmission spectra of the echelle grating normalized to the reference from a back-to-back grating coupler pair. The on-chip loss of the central channel is 2.1 dB with a crosstalk of −20 dB. The loss variation over the channels is 1.3 dB. The measured peak wavelengths of the WDM channels are 1493 nm, 1503 nm, 1513 nm, 1524 nm, 1534 nm, 1544 nm, 1554 nm, and 1564 nm, which are well matched to the designed wavelengths with only 3–4 nm shift.

FIG. 4.

Measured spectrum of the echelle grating to separate/combine 8 wavelength channels.

FIG. 4.

Measured spectrum of the echelle grating to separate/combine 8 wavelength channels.

Close modal

The switching characteristic of the WXC was investigated by addressing MEMS actuators in the space switch cells. Figure 5(a) shows the transfer curve of a representative switch cell. The transfer characteristic exhibits a digital switching behavior with an ON voltage (VON) of ∼40 V and an OFF voltage (VOFF) of ∼27 V. The extinction ratio over 40 dB was achieved by the MEMS switching mechanism. Figure 5(b) shows the distribution of VON measured from all 512 switch cells. The average VON was measured to be 39.6 V with the standard deviation of 2.3 V. Although there exist small variations of the switching voltages, the bistable switching characteristic of the MEMS switch enables simple digital control without customized 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.

FIG. 5.

(a) Measured switching transmission vs bias voltage. (b) Switching voltage (VON) distribution.

FIG. 5.

(a) Measured switching transmission vs bias voltage. (b) Switching voltage (VON) distribution.

Close modal
FIG. 6.

Switching speed measurement.

FIG. 6.

Switching speed measurement.

Close modal

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 (λ8) in the plot are the measured I1-O1, I2-O2, …, and I8-O8 transmissions when the switching cells (1, 1), (2, 2), …, and (8, 8) of the λ8 switch array are on, respectively. For each channel wavelength, signals of different paths (In-On) passed through the different input and output echelle gratings. The measured spectra show slight peak wavelength shifts within 2 nm and confirm the echelle gratings with a good uniformity. The measured crosstalks for the central channels are better than −40 dB, which are currently limited by the noise floor of our photodetector. It is expected that the worst-case crosstalk is still lower than −30 dB when all 8 channels are utilized. Note that the curved noise floors are due to the normalization to the grating coupler reference.

FIG. 7.

In-On (n = 1, 2, …, 8) transmission spectra measurement of the fabricated WXC device.

FIG. 7.

In-On (n = 1, 2, …, 8) transmission spectra measurement of the fabricated WXC device.

Close modal

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 polarization-independent WCX without polarization splitters/rotators still requires the development of polarization-independent wavelength (de)multiplexers.

FIG. 8.

(a) Measured on-chip loss vs number of crossings in the path. (b) Loss distributions of each wavelength channel.

FIG. 8.

(a) Measured on-chip loss vs number of crossings in the path. (b) Loss distributions of each wavelength channel.

Close modal

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.

This work was supported in part by the Advanced Research Projects Agency-Energy (ARPA-E) ENLITENED program (Grant No. DE-AR0000849); National Science Foundation (NSF) Center of Integrated Access Networks (CIAN) (Grant No. EEC-0812072) and the Partnership for Innovation–Technology Transition (PFI-TT) program (Grant No. 1827633); the Google Faculty Research Award; the UC Berkeley Bakar Fellow program; and the GIST Research Institute (GRI) grant. The authors would like to thank the staff of Marvell Nanofabrication Lab at Berkeley for their assistance in postprocessing.

The authors declare no competing financial interests.

1.
E. B.
Basch
,
R.
Egorov
,
S.
Gringeri
, and
S.
Elby
, “
Architectural tradeoffs for reconfigurable dense wavelength-division multiplexing systems
,”
IEEE J. Sel. Top. Quantum Electron.
12
,
615
626
(
2006
).
2.
M.
Sharma
,
P.
Hansen
,
B.
Nayar
, and
P.
Wigley
, “
Next-generation ROADM technologies and architecture
,” in
Optical Metro Networks and Short-Haul Systems IV
(
International Society for Optics and Photonics
,
2012
), Vol. 8283, p.
828309
.
3.
B. C.
Collings
, “
Advanced ROADM technologies and architectures
,” in
Optical Fiber Communications Conference and Exhibition
(
Optical Society of America
,
2015
), paper Tu3D.3.
4.
H.
Yang
,
B.
Robertson
,
P.
Wilkinson
, and
D.
Chu
, “
Low-cost CDC ROADM architecture based on stacked wavelength selective switches
,”
J. Opt. Commun. Networking
9
,
375
384
(
2017
).
5.
T. A.
Strasser
and
J. L.
Wagener
, “
Wavelength-selective switches for ROADM applications
,”
IEEE J. Sel. Top. Quantum Electron.
16
,
1150
1157
(
2010
).
6.
T.
Goh
 et al, “
Port scalable PLC-based wavelength selective switch with low extension loss for multi-degree ROADM/WXC
,” in
OFC/NFOEC 2008–2008 Conference on Optical Fiber Communication/National Fiber Optic Engineers Conference
(
Optical Society of America
,
2008
), OWC6.
7.
R.
Ryf
 et al, “
Scalable wavelength-selective crossconnect switch based on MEMS and planar waveguides
,” in
Proceedings 27th European Conference on Optical Communication (Cat. No.01TH8551)
(
IEEE
,
2001
), Vol. 6, pp.
76
77
.
8.
S.
Yuan
 et al, “
Fully integrated NxN MEMS wavelength selective switch with 100% colorless add-drop ports
,” in
OFC/NFOEC 2008–2008 Conference on Optical Fiber Communication/National Fiber Optic Engineers Conference
(
Optical Society of America
,
2008
), OWC2.
9.
K.
Suzuki
,
K.
Seno
, and
Y.
Ikuma
, “
Application of waveguide/free-space optics hybrid to ROADM device
,”
J. Lightwave Technol.
35
,
596
606
(
2017
).
10.
Q.
Cheng
,
M.
Bahadori
,
M.
Glick
, and
K.
Bergman
, “
Scalable space-and-wavelength selective switch architecture using microring resonators
,” in
Conference on Lasers and Electro-Optics
(
Optical Society of America
,
2019
), paper STh1N.4.
11.
A. S. P.
Khope
 et al, “
Multi-wavelength selective crossbar switch
,”
Opt. Express
27
,
5203
5216
(
2019
).
12.
A.
Rickman
, “
The commercialization of silicon photonics
,”
Nat. Photonics
8
,
579
582
(
2014
).
13.
A. E.
Lim
 et al, “
Review of silicon photonics foundry efforts
,”
IEEE J. Sel. Top. Quantum Electron.
20
,
405
416
(
2014
).
14.
D.
Thomson
 et al, “
Roadmap on silicon photonics
,”
J. Opt.
18
,
073003
(
2016
).
15.
F.
Nakamura
 et al, “
Integrated silicon photonic wavelength-selective switch using wavefront control waveguides
,”
Opt. Express
26
,
13573
13589
(
2018
).
16.
F.
Testa
 et al, “
Integrated reconfigurable silicon photonics switch matrix in IRIS project: Technological achievements and experimental results
,”
J. Lightwave Technol.
37
,
345
355
(
2019
).
17.
M.-C. M.
Lee
and
M. C.
Wu
, “
Tunable coupling regimes of silicon microdisk resonators using MEMS actuators
,”
Opt. Express
14
,
4703
(
2006
).
18.
Y.
Akihama
and
K.
Hane
, “
Single and multiple optical switches that use freestanding silicon nanowire waveguide couplers
,”
Light: Sci. Appl.
1
,
e16
(
2012
).
19.
S.
Han
,
T. J.
Seok
,
N.
Quack
,
B.-W.
Yoo
, and
M. C.
Wu
, “
Large-scale silicon photonic switches with movable directional couplers
,”
Optica
2
,
370
(
2015
).
20.
T. J.
Seok
,
N.
Quack
,
S.
Han
,
R. S.
Muller
, and
M. C.
Wu
, “
Large-scale broadband digital silicon photonic switches with vertical adiabatic couplers
,”
Optica
3
,
64
(
2016
).
21.
T. J.
Seok
,
K.
Kwon
,
J.
Henriksson
,
J.
Luo
, and
M. C.
Wu
, “
Wafer-scale silicon photonic switches beyond die size limit
,”
Optica
6
,
490
494
(
2019
).
22.
J.
Brouckaert
,
W.
Bogaerts
,
P.
Dumon
,
D. V.
Thourhout
, and
R.
Baets
, “
Planar concave grating demultiplexer fabricated on a nanophotonic silicon-on-insulator platform
,”
J. Lightwave Technol.
25
,
1269
1275
(
2007
).
23.
J.
Brouckaert
 et al, “
Planar concave grating demultiplexer with high reflective Bragg reflector facets
,”
IEEE Photonics Technol. Lett.
20
,
309
311
(
2008
).
24.
W. D.
Sacher
 et al, “
Tri-layer silicon nitride-on-silicon photonic platform for ultra-low-loss crossings and interlayer transitions
,”
Opt. Express
25
,
30862
30875
(
2017
).
25.
S.
Han
 et al, “
Large-scale polarization-insensitive silicon photonic MEMS switches
,”
J. Lightwave Technol.
36
,
1824
1830
(
2018
).