Low-Loss and Ultra-Broadband Silicon Nitride Angled MMI Polarization Splitter

We experimentally demonstrate a SiN angled-MMI based polarization splitter with nearly wavelength-independent performance over C+L bands, insertion loss ∼ 0.8 dB (1.0 dB), and crosstalk < −18 dB (< −20 dB) for TE (TM) polarization.


Introduction
The polarization dependence of photonic integrated circuits (PICs) makes a polarization-diversity approach an attractive technique for efficiently interacting with input light of arbitrary polarization. In such an approach, a polarization beam splitter (PBS) is the key element for (de-)multiplexing the two orthogonal polarization states (TE and TM), which can then be processed separately and independently. Mach-Zehnder interferometers (MZIs), multimode-interference couplers (MMIs), asymmetric directional couplers (DC) and asymmetric Y-branch structures are most commonly used as the core splitting element. Silicon-on-insulator is the most widely explored platform for implementing these designs [1,2].
For some applications, such as those requiring low loss, low nonlinear absorption, or a wide transparency window, silicon nitride (SiN) is a more appropriate material than Si. However, the low refractive index contrast (∆n ∼ 0.5) of SiN waveguides makes it more challenging to design an effective PBS. Previously-demonstrated SiN PBSs, such as refs. [3,4], use active tuning or thick SiN films incompatible with typical PIC foundry processes.
We report here a new polarization splitter that employs a SiN angled multimode interference coupler (A-MMI). This type of structure has been demonstrated for wavelength-division multiplexing [5], and a recent paper proposed an A-MMI design for a Si PBS [6]. Here, we design and fabricate an A-MMI PBS in 450 nm-thick SiN. We measure low insertion loss (IL) and low crosstalk (XT) with little variation across the C and L bands. The input and output waveguides (width W 1 = 900 nm) support fundamental TE and TM modes. As the initial step of the design process, we place the MMI segment (width W 2 = 1.25 µm) at an angle of θ 1 = 5 • with respect to the input waveguide. This shallow angle minimizes radiation leakage. We then perform a 3D finitedifference time domain (FDTD) simulation of this A-MMI segment over a long distance with no output ports, noting the positions of peaks and nulls. We then place the TE-port (P 1 ) where the TE mode has a peak and the TM mode has a null (L 1 = 21 µm). Finally, the optimum angle θ 2 of the TE port with respect to the MMI axis is calculated in successive simulations of the A-MMI structure with output port P 1 . θ 2 ≈ 8 • minimizes IL and XT between the output ports. To transfer the TM mode propagating in the A-MMI region to the output port P 2 , we use an adiabatic taper of length L 2 = 50 µm. Simulation results of this final design are shown in Fig. 1(b) and 1(c). The simulation shows nearly wavelength-independent transmission characteristics over the wavelength range 1525 nm≤ λ ≤1625 nm, with XT< −15 dB.

Experimental Results and Discussion
We fabricate the designed A-MMI in 450-nm thick low-pressure chemical vapor deposited (LPCVD) SiN on 2.5 µm thick thermal SiO 2 . We define the pattern using electron-beam lithography (EBL) and inductively-coupled plasma reactive ion etching. Finally, we deposit 2.5 µm plasma-enhanced chemical vapor-deposited SiO 2 top cladding. Fig. 2(a) shows an optical microscope image of the fabricated PBSs along with a straight reference waveguide (PBS #1-5 correspond to L 1 = 20 µm, 20.5 µm, 21 µm, 21.5 µm, and 22 µm). Devices are characterized by coupling lensed fibers to inverse-tapered edge couplers. The measured transmission characteristics (normalized w.r.t. the reference waveguide) at output ports P 1 and P 2 for both TE and TM inputs ( Fig. 2(b), L 1 = 21 µm) show nearly wavelength-independent performance over the 1525 nm to 1625 nm span. The relative IL in this device is ∼0.8 dB (1.0 dB) for TE (TM) input, and XT is < −18 dB (< −20 dB). Fig. 2(c) shows the IL and XT measured for five different devices (PBS#1 − 5), each with an offset ∆L = 0.5 µm from the design L 1 = 21 µm. Error bars represent the fluctuations across the wavelength range. This data suggests some fabrication tolerance in the design; even for an offset of 500 nm from the optimized design, a much larger variation than could be expected in most processes, XT remains below -10 dB and IL does not significantly change.

Conclusion
We have designed and demonstrated an A-MMI based PBS in 450-nm thick LPCVD SiN with a device footprint of 80 µm×13 µm. The device design is straightforward and adaptable, and fabrication requires only a single-layer process. Although we pattern our SiN PBS using EBL, feature sizes are compatible with deep-UV lithography.
The measured PBS has nearly wavelength-independent XT < −18 dB (< −20 dB) and IL ∼0.8 dB (1.0 dB) for TE (TM) polarization over the measured wavelength range of 1525 nm≤ λ ≤1625 nm, and measured device variations suggest some tolerance to fabrication variations. Altogether, this device is a good candidate for a PIC foundry-compatible, SiN PBS.