High-efficiency vertical fibre-to-polymer waveguide coupling scheme for scalable polymer photonic circuits

: Polymer photonic circuits offer a versatile platform for various applications, including communication, sensing and optical signal processing. Though polymers offer broadband, linear and nonlinear optical properties, the coupling between an optical fibre and a polymer waveguide has been a challenge. In this work, we propose and demonstrate a wafer-scale vertical coupling scheme for polymer waveguides. The scheme uses a silicon nitride grating coupler with an inverse taper to couple between an optical fibre and a SU 8 polymer waveguide. We demonstrate a maximum coupling efficiency of -3.55 dB in the C-band and -2.92 dB in the L-band with a 3-dB bandwidth of 74 and 80 nm, respectively. A detailed design and simulation, fabrication, and characterisation results are presented. The scheme demonstrates a scalable and efficient surface grating approach for polymer photonic integrated circuits.


Introduction
Polymer based photonic circuits have attracted considerable attention for a host of applications in sensing and communication. Application in sensing includes low-cost label-free fluid and biological sensing [1][2][3]. Polymer waveguides are used for chip-to-chip, intra-chip, as well as board-level optical interconnects for datacom applications [4][5][6][7]. Unlike conventional inorganic waveguide materials, polymers are relatively simple to process and incorporate functional optical components and circuits [8][9][10]. As opposed to high-index contrast waveguide platform such as SOI, sidewall scattering losses for polymer optical waveguides is considerably lower due to its low-index contrast [7,11,12]. However, the circuit footprint eventually becomes larger, which perhaps is a trade-off for loss and simplicity in the fabrication process. Polymer photonic circuits offer a versatile platform where one could tune the thermal, electrical, and optical property through doping. Tunable thermo-optic coefficients of polymers have allowed development of variable optical attenuators and tunable Bragg filters [13,14]. Similarly polymer materials can be doped with suitable chromophores to achieve high electro-optic coefficients of up to 5X higher than corresponding inorganic materials [15].
Compact and high-speed electro-optic modulators have already been demonstrated on planar hybrid polymer photonic platform [16,17]. Another major application is in the area of flexible photonics chips where the stretching feature of polymers is being explored for a host of applications such as flexible interconnects [18], substrate handles for high index materials [19,20] as well as strain sensing [21][22][23][24][25]. Lastly, a unique feature of polymers is the chemically induced optical birefringence, that can be incorporated to develop polarisation selective devices [26,27].
One of the challenges in polymer photonic circuits is achieving efficient coupling between an optical fibre and a single-mode polymer waveguides. Both in-plane, as well as out-of-plane coupling mechanisms, have been explored. The former technique, in-plane or edge coupling offers higher efficiency and broadband performance [28][29][30]. Also, due to a larger waveguide cross-section and low-index contrast, in-plane edge couplers to polymer waveguide intrinsically have higher misalignment tolerance. However, a significant drawback of this approach is a lack of scalable testing and high-volume manufacturing. Despite the advantages mentioned above, edge coupling requires consistent dicing and edge polished surface to avoid scattering loss. Due to higher modulus of elasticity, cleaving and polishing polymer waveguide is fundamentally limited. Thus out-of-plane coupling using surface diffraction grating is a scalable and suitable scheme for fibre-chip coupling. However, designing an efficient polymer grating is challenging due to the low-index contrast. Several means in the past have been suggested to address the low-index contrast such as by depositing high index contrast materials for gratings [31,32] or by using an air-suspended polymer membrane [33]. The reported efficiencies were modest with suboptimal performance. The efficiency could be increased by using thicker high-index material. This would enable higher diffractive efficiency and consequently, higher coupling performance. In addition, the coupling can further be boosted by implementing a bottom mirror to suppress substrate leakage and increase directionality that has been demonstrated in the case of SiO 2 clad SiN gratings [34][35][36].
In this work, we propose and demonstrate an efficient, scalable, and configurable grating-based coupling and polymer photonic circuit platform. The proposed approach couples light between a single-mode optical fibre and a SU8 waveguide using a combination of Silicon Nitride (SiN) grating and a spot size converter. A 3D schematic of the conceptual design is illustrated in Fig. 1. The design consists of two parts; a polymer encapsulated SiN grating for enabling coupling from a fibre to a SiN waveguide, and a hybrid polymer-SiN inverse taper to transfer the optical mode from the SiN waveguide to the single-mode polymer waveguide. The circuit is built on a silicon wafer with a silicon dioxide isolation layer. The proposed scheme uses an optimised SiN grating coupler as a generic coupling element that could be configured using a polymer waveguide to realise desired functionality. The proposal brings large volume CMOS compatible fabrication of SiN couplers with a configurable polymer waveguide platform. Using this approach, we demonstrate a maximum fibre-to-polymer waveguide coupling of 2.92 dB/coupler with a 3-dB bandwidth of 80 nm in the telecommunication L-band wavelength. To the best of our knowledge, the coupling efficiency achieved is the best reported for a grating fibre-chip coupler in a polymer waveguide platform.

Design and simulation
Figure 2(a) shows a schematic of various sections of the circuit that needs to be designed and optimised. In this section, we present the design and optimisation of various sections.
A commercially available photosensitive SU8 2002 is chosen as a polymer waveguide core material. The waveguide structure consists of an air-clad waveguide on a SiO 2 bottom clad on the silicon handle substrate. SU8 is a versatile photosensitive polymer and has broad transparency (400-2000 nm). Depending on the resist viscosity and spin coating parameters, the core film thickness can be tuned between 1 to 3 µm. Single-mode polymer waveguide is designed and simulated using MODE [37]. Simulations are performed at 1550 nm wavelength, and a refractive index of 1.575 and 1.45 is used for SU8 and SiO 2 , respectively. Figure 2(b) shows the waveguide dispersion of a 2 µm thick SU8 polymer layer. Based on the waveguide dispersion, a core dimension of 2 × 2 µm is used as a single-mode waveguide.

Inverse taper design
The coupling between an optical fibre and the polymer waveguide is done using a SiN grating and a taper section. The grating coupler and the taper section is defined in a 400 nm thick SiN layer to operate in a single-mode regime. To spatially mode match with a single-mode fibre the grating is defined in a 12 µm wide waveguide. Coupling between the broad waveguide and the polymer waveguide is achieved using inverse taper as depicted in Fig. 2(a).
The inverse taper design was optimized using the eigenmode mode expansion (EME) solvers [37,38]. The 2D layout of the proposed design stack and its various components is illustrated in Fig. 2(c). The first stage consists of a SiN patch grating that is 12 µm wide and of length L 1 . In the second stage, the fundamental TE mode from the 12 µm wide SiN waveguide is channelled through an adiabatic two-stage section. In the first section, the 12 µm wide waveguide is tapered to 1 µm wide waveguide through an adiabatic taper, followed by a second stage, where an inverse taper is used to couple light to the overlaid polymer waveguide. The inverse tapering allows the light to leak into the single-mode SU8 waveguide present on top. The total power transmitted to the TE mode in the SU8 guide is estimated through the scattering matrix of the composite structure. The taper section is optimized in two stages. In the first optimization step, a maximum transmitted between the fundamental TE mode in patch waveguide to the corresponding 1 µm wide SiN waveguide through an adiabatic taper L 2 is achieved. In the second step, maximum transmission between the 1 µm wide SiN waveguide and the 2 µm wide SU8 through the inverse taper L 3 is optimized. Figure 3(a) shows the transmission of the first stage L 2 taper as a function of length. An L 2 of 300 µm yielding a conversion efficiency of > -0.05 dB, is fixed thereafter as the optimum intermediate taper. Figure 3(b) shows the power transmission as a function of SiN inverse taper tip width and length L 3 . Optimisation with both proprietary solvers shows an excellent agreement in optimal length and taper width. We achieve a maximum transmission loss of -0.04 dB or a transmission efficiency of 99 % for a tip width of 75 nm and L 3 of 100 µm. Even with a tip width of 100 nm, a transmission loss of -0.06 dB is achieved. Both these tip dimensions are feasible with ArF-immersion lithography. As the tip width increases to 150 nm and 200 nm, we observe an increase in the transmission loss to -0.1 and -0.2 dB/facet respectively, due to mode mismatch shows waveguide dispersion of effective mode index N eff at different widths W for SU8 film thickness of 2 µm. The inset shows the electric field intensity profile for the fundamental TE 00 mode for a core dimension of 2×2 µm. (c) shows simulated layout and cross-section along the taper length. Also depicted is the SU8 waveguide width W and height HThe cross-sectional field plot at the either end shows TE 00 mode evolution from SiN patch core to the SU8 waveguide core.
between the taper and the polymer waveguide. To achieve higher transmission efficiency tip widths <200 nm is desirable, which can be fabricated with standard DUV lithography [39][40][41]. The oscillatory fluctuation of the mode conversion loss with L 3 taper length is due to the cumulative effect of partial reflections at infinitesimal slices along the taper. These oscillations are observed to be more pronounced for higher taper widths [42]. For tip widths < 200 nm, we observe that, L 3 as short as 50 µm is sufficient to achieve transmission > -0.5 dB. Since the present design is unconstrained by footprint, we choose an L 3 of 100 µm as optimum inverse taper length.  The design offers a broadband operation with a transmission bandwidth of over 200 nm, which is essential for a wavelength scalable circuit design. The total length of the taper (L 1 + L 2 + L 3 ), for an efficiency transmission is about 420 µm. The length of this taper could be reduced by using a compact non-adiabatic taper, that could reduce the taper length by 50 % [43].
The polymer waveguide would be defined in a separate lithography step that may result in misalignment between the SiN inverse taper and the polymer waveguide. Figure 3(d) shows the effect of misalignment or offset between the taper and the polymer waveguide. We find a robust alignment tolerant transmission for tip widths of <200 nm. However, for misalignment >800 nm there is a steep increase in the transmission loss. An alignment overlay accuracy better than 800 nm is easily achievable even in a contact lithography system. The robustness in design and alignment tolerance between the SiN and the polymer layer clearly indicates the feasibility of a configurable platform where the SiN based generic coupler layout could manufacture using large volume CMOS patterning process. In contrast, the polymer circuit could be done using a simple contact lithography system as well.

Grating design
The SiN grating is optimised using 2D simulations using a finite-difference-time-domain (FDTD) solver from [37]. A cross-section schematic of the SU8 cladded SiN grating coupler is depicted in Fig. 4(a). The grating in the SiN is designed to phase match with the fundamental propagating TE-mode of the SiN waveguide. For efficient coupling, we use a bottom reflector to improve directionality. According to the first-order Bragg phase-matching condition, the grating period Λ can be expressed as: Where N g eff is the effective grating index, n c is the cladding index, and θ is the angle of incidence.  1.575, 1.44, 2.015, 3.53, and 3.46, respectively. A Gaussian source of 10.4 µm mode field diameter (MFD) is used as an illuminating source above the SU8 cladding). All the grating parameters such as Λ, θ, d c , and buried oxide thickness (BOX) are optimised to achieve maximum coupling to the SiN waveguide. For a simple SiN fabrication process, a single full 400 nm etch for both grating and waveguide is considered. Grating coupling efficiency is initially optimised with a just an oxide bottom clad layer on a Si substrate. Figure 4(b) compares the impact on coupling efficiency of the polymer clad SiN grating, with and without a the bottom reflector. For the case of no reflector, peak coupling efficiency at C-band is observed to be -4.3 dB with a 1-dB bandwidth of 56 nm at a period of 1.05 µm and 9 • inclination. The coupling efficiency can be further improved by suppressing substrate leakage by incorporating a bottom reflector beneath the BOX layer, such as a distributed Bragg reflector (DBR). The DBR stack consists of 2 cascaded layers of a − Si/SiO 2 , with a quarter-wave thickness of 110/270 nm. At an optimal BOX thickness, constructive interference occurs between the substrate reflected and the upper diffracted light from the DBR, which results in a higher directionality. With a 2-pair of reflector stack, the coupling efficiency is improved to -2.14 dB with a 1 and 3 dB bandwidths of 64 nm and 114 nm respectively. Since improvement in the coupling efficiency is achieved by phase-matching, the reflected and diffracted light from the grating, the BOX thickness is critical. Figure 4(c) depicts the effect of BOX thickness on the coupling efficiency. The optimal BOX thickness for the gratings is found to be 2.22 µm. For a BOX thickness variation of ±80 nm, we observe the coupling penalty to be only -0.8 dB. The effect of grating period, fibre inclination angles and duty-cycles is plotted in Figs. 4(d)-(f) respectively. Based on the optimisation, optimal grating parameters is chosen for device fabrication.

Device fabrication and characterization
Device fabrication was carried on a standard Si wafer. A two-layer Bragg reflector of 270 nm SiO 2 and 110 nm thick a − Si is deposited using Plasma Enhanced Chemical Vapour Deposition (PECVD). This is followed by depositing 2.22 µm thick SiO 2 and then a 400 nm thick SiN layer also using the PECVD process. Following the layer deposition grating and waveguide patterning is performed using e-beam lithography. Gratings of different periods and duty-cycles along with inverse tapers are patterned using a negative tone resist (MaN-2403). The inverse taper tip width is fixed at 100 nm for all devices. The SiN layer was then fully etched using inductively coupled plasma reactive ion etching (ICP-RIE) with SF6 chemistry. Subsequently, a 2 µm thick SU8 2002 photosensitive polymer is spin-coated on this etched pattern. Figure 5 depicts the detailed process flow for the fabricated devices.
The gratings are connected by pattering the SU8 layer. The photosensitive polymer could be either patterned using direct-laser writer or optical projection/contact lithography. In this work, we use a direct-write scheme to demonstrate a configurable polymer photonic circuit. The direct-write process is optimised to achieve low-loss smooth SU8 waveguides [44,45]. The test structures were fabricated with straight polymer waveguides. Following the writing process, the waveguides are developed and baked at 65 • C and 95 • C for one and four minutes, respectively. Subsequently, the sample was thermally cured at 175 • C for 15 minutes to remove the solvents. Figure 6(a) depicts test structures fabricated using the direct-writing process. Figures 6(b) and  The fabricated devices were characterized using a tunable laser (Keysight 81690A) and a photodetector (Keysight 81636B). The characterization summary is shown in Fig. 7. Here, Figs. 7(a)-(b) and Figs. 7(c)-(d) show the effect of the grating period and fibre inclination angle on the fibre-to-SU8 coupling efficiency in the C-band and L-band, respectively. We measure a maximum coupling efficiency of -3.55 dB/coupler (at 1560 nm) with 74 nm of 3-dB bandwidth in the C-band. While in the L-band, we measure a maximum efficiency of -2.92 dB/coupler (at 1603 nm) with a 3-dB bandwidth of close to 80 nm. The maximum efficiency is achieved with a grating period, duty cycle and fibre angle combination of 1.05 µm, 50 %, 8 • and 1.04 µm, 45 % and 5 • for C-band and L-band couplers, respectively. Table 1 summarizes the type and performance metric of various surface grating schemes demonstrated for polymer waveguide coupling. It may be pointed out that although we have used electron beam and direct-write lithography, the design allows flexibility in determining minimum feature dimensions that is compatible with scalable, high-volume optical lithography typically employed in foundries. The proposed device can serve as a suitable template for developing on-chip PCB interconnects [6] as well as a substrate handle for flexible polymer circuits [19].

Conclusion
We demonstrate a low-loss coupling scheme between a single-mode optical fibre and a singlemode polymer waveguide using SiN grating coupler. A detailed design, simulation, fabrication, and characterisation of the proposed scheme is presented. The grating couplers in SiN offers a flexible platform for building polymer circuits. Using the proposed approach, we demonstrate a maximum coupling efficiency of -3.55 and -2.92 dB to a single-mode SU8 waveguide in C and L band region, respectively. The proposed configurable platform offers flexibility is pattering the polymer waveguide without any constrains on the light-chip coupler. A generic predefined SiN coupler could be mass-fabricated and used as a platform for polymer circuit fabrication. The results hold promise for realising a scalable and efficient surface grating approach for polymer photonic integrated circuits.

Funding. Ministry of Human Resource Development; Ministry of Electronics and Information technology; Mission
on Nano Science and Technology.