A R EVIEW OF D EVICES F OR I NTEGRATED S ILICON P HOTONIC S WITCHES

In this paper, we review devices used in silicon photonic switches. Devices in switches are divided into active and passive devices. Active devices consist of microring resonator, contra directional couplers, mach zhender switches. Passive devices consist of waveguide crossings and arrayed waveguide gratings. We also list the state of the art in devices in a comparison table.

Tuning time is on the order of µs.Thermal undercut of the ring resonator is reported in [38]. This arrangement has a lower power consumption than other MRRs. A comparison of silicon photonic heaters is provided in [39]. Adiabatic designs reduce overlap between optical mode and the heater and reduce optical loss [40].
The heater vias are the bottleneck in these designs and burnout when we pass a lot of current. We show that tuning times for MRR are on the order of 10s of µs as compared to 1 µs as tuning across many wavelengths is required [18]. This tuning time is much smaller as compared with time required for arbitration in a software defined network (SDN) (1s).
Optical loss is much higher for electo-optic tuning. There are multiple ways to tune MRR with electro optic tuning. We consider only carrier-injection tuning [41] as it is relevant for switches. In carrier-injection tuning we pass current which changes the refractive index of the waveguide. This leads to a change in resonant wavelength of the MRR.
We review active and passive devices used in switches in the next two sections.

Active Devices
We review active devices used in switches in this section. Active devices have thermal heaters or carrier injection phase shifters.

MRR
Microring resonator (MRR) is a device with a circular waveguide coupled to a straight waveguide or waveguides used for switching or filtering a particular wavelength from a WDM channel. MRRs can be either first order, higher order coupled ring resonators [42,43] or series coupled [44]. A flat top filter shape with high out of band rejection is required for lower crosstalk in a switch. Higher order rings have these features and are sensitive to fabrication variation: optical waveguide shape, coupling gap and thickness uniformity of the wafer. A review of MRRs is reported in [45]. Theoretical approach for coupled resonator optical waveguides (CROW) is reported in [42]. Figure 1: (a) second order MRR with thermal tuning and low innsertion loss [46], (b) contra directional coupler [47], (c) MZI [48] Ring resonators in silicon photonics process have a footprint of 10µm × 10µm for FSR greater than 20nm. Switching speed of 1µs are reported by thermal tuning [40] and 1ns by electro optic tuning [49]. Thermal tuning tunes farther than electro-optic tuning with minimal optical loss. We reported a 15µs rise time to tune across the multiple WDM channels of the MRR in [18]. In most papers, 1/e switching time is reported but for wavelength selective switching, tuning across whole FSR is required which can be more than 30 nm for 8 channel at 400 GHz spacing. MRRs in InP and SiN platform have higher footprint and power consumption due to their size and lower index contrast.

Contra directional couplers
Contra directional couplers (CDC) are grating filters that can be designed for a very wide filter shape. The filtered wavelengths propagate in opposite direction to the input direction and rest of the wavelengths pass to the through port. These filters can be used as a broadband filter to drop multiple WDM channel and have infinite free spectral range. Switches with very high port counts can be designed with these devices. Apodized contra directional couplers with thermal tuning are reported in [47,50]. In apodization, the strength of the grating follows a gaussian curve. Apodization can produce flat top filter shapes with higher out of band rejection. A tunable MRR-contra directional switch is reported in [51]. Contra directional couplers with heaters are also reported in [52].

MZI
Mach zhender interferometer (MZI) is a device with two directional couplers and a waveguide section between them. These switches have a higher footprint than (MRR) and can be tuned with carrier -injection phase shifters or thermal tuning. A low power MZI with thermal tuning is reported in [48]. A fast nanosecond MZ switch is reported in [53]. Low power thermal tuning with adiabatic bends and integrated thermal tuners is reported in [54]. Many switching networks on chip use broadband directional couplers or multi mode interferometers (MMI). These switches are used to build bigger integrated fiber switches that switch all wavelength division multiplexing (WDM) channels from a fiber.

Passive elements
In this section, we provide an overview of passive elements.

AWGR
Arrayed waveguide grating router (AWGR) consists of input and output waveguides, two free-propagation slab regions, and arrayed waveguides where each neighboring waveguide has a constant path length difference. Each input port of the AWGR can communicate with different output port with specific WDM wavelengths based on the wavelength routing function. Since AWGR is an interferometric device, the phase errors cause a degraded crosstalk. Silicon nitride (SiN) AWGR is superior to silicon AWGR in mitigating the degraded crosstalk since the variation of the average effective index of the arrayed waveguides is lower. The state-of-the-art SiN AWGR can have the adjacent and non-adjacent channel crosstalk as low as -39 dB and -33.5 dB, respectively [55].

Waveguide crossing
Waveguide crossing is a device used for crossing optical waveguides [57]. Optical waveguide width is higher at the junction to focus the mode at the intersection of the two waveguides to minimize optical scatering. Waveguide crossings with loss of less than 0.02 dB and crosstalk less than -37 dB are reported for SOI platform in [56]. This device is 9µm × 9µm These waveguide crossings use optimization algorithms to change the layout of the crossing. Straight waveguide crossing incur a larger optical loss. Other notable crossings are reported in [58]. Silicon and silicon nitride trilayer waveguide crossings are reported in [59]. These gratings have greater size as compared with SOI waveguide crossings but enable more complex photonic integrated circuits.

Discussion
The following table summarizes the best devices and we bold their best metrics. We report state of the art devices in integrated silicon photonic switches. The devices from the table can be easily reproduced and fabricated in a silicon photonic foundry.