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Optical Demonstration of a Compliant Polymer Interface between Standard Fibers and Nanophotonic Waveguides
Tymon Barwicz1, Yoichi Taira2, Shotaro Takenobu3, Nicolas Boyer4, Alexander Janta-Polczynski4, Yan
Thibodeau4, Swetha Kamlapurkar1, Sebastian Engelmann1, Hidetoshi Numata2, Robert L. Bruce1, Simon Laflamme4, Paul Fortier4, and Yurii A. Vlasov1
1IBM T.J. Watson Research Center, 1101 Kitchawan Rd., Yorktown Heights, NY 10598 USA 2IBM Research – Tokyo, 7-7 Shin-Kawasaki, Saiwai-ku, Kawasaki, Kanagawa, 212-0032 Japan
3Asahi Glass Co., AGC Electronics, Technol. Gen. Div. 1150 Hazawa-cho, Kanagawa-ku, Yokohama, 221-8755 Japan 4IBM Bromont, 23 Boul. de l’Aeroport, Bromont, QC J2L 1A3 Canada
Abstract: We demonstrate a peak transmission efficiency of -2.4dB between a standard optical fiber and a nanophotonic waveguide using a mechanically compliant polymer interface. We find a 1.5dB penalty over a 100nm bandwidth and all polarizations. OCIS codes: (130.3120) Integrated optics devices; (250.5300) Photonic integrated circuits
1. Introduction
The deployment of silicon photonics is hindered by the difficulty of single-mode optical inputs and outputs (I/Os) that are low-cost and easily scalable in both manufacturing volume and number of optical I/Os per photonic chip. To disruptively reduce cost and improve scalability, we seek to enable standard microelectronic packaging equipment for photonic packaging. This is analogous to photonic chip fabrication already leveraging standard microelectronics wafer fabrication equipment.
To address this photonic packaging challenge, we have recently proposed a compliant polymer interface between standard single-mode optical fibers and nanophotonic waveguides [1]. The approach is illustrated in Fig.1. A flexible polymer ribbon with integrated waveguides is lithographically defined and assembled to a modified ferrule to enable a standard optical interface to an optical fiber connector. A 12x1 mechanical transfer (MT) interface is shown here but others could be used as well. This compliant interface is then picked and placed onto a photonic chip using standard microelectronic packaging tools. Self-alignment structures on the ferrule, the polymer ribbon, and the photonic chip are employed to warrant the 1-2 um alignment requirements of single-mode photonics despite the much larger 10 um alignment uncertainty of high-throughput microelectronic assembly tools. Initial performance data of the mechanical self-alignment was reported in [2] where self-alignment to 1-2 um from 10 um initial misalignment was experimentally demonstrated.
The optical path is presented in Fig. 2. Butt coupling is used between fibers and the polymer waveguides at the optical fiber connector interface and adiabatic coupling is used between polymer and nanophotonic waveguides at the chip to ribbon interface. Part of the spot size conversion from fiber to nanophotonic waveguide is performed on the polymer ribbon through an adiabatic transition between Fig. 2 (b) and (c). This eases routing and chip fabrication
Ferrule lid
Polymer ribbon
Ferrule
Nanophotonic die
Coupling region
Flexible extension
Standard fiber connector interface
Nanophotonic die
a b
Ferrule lid
Polymer ribbon
Ferrule
Nanophotonic die
Coupling region
Flexible extension
Standard fiber connector interface
Nanophotonic die
a b
Fig.1. Compliant polymer interface concept in exploded view (a) and as assembled to a photonic chip (b).
requirements on the polymer to nanophotonic waveguide transition. In addition to compatibility with standard microelectronic assembly tooling, the compliant polymer interface
shows two major advantages with respect to known approaches. First, its compliant aspect mechanically decouples the chip optical interface from cycling strains in the package, which is expected to significantly improve long-term reliability when compared to solutions employing rigid interfaces or direct fiber to chip [3-4]. Second, its optical bandwidth can be very large. Here we report a 1.5 dB penalty over a 100 nm bandwidth. We have calculated in [1] a fundamental design-related penalty limit of 0.1 dB over a 200 nm bandwidth. This compares favorably to diffractive couplers that are inherently wavelength sensitive. For instance, the 1 dB bandwidth of a two-polarization vertical coupler is generally around 30 nm [3].
In this paper, we report on the first optical demonstration of fiber to nanophotonic waveguide coupling through a compliant polymer interface. We find ~1.5dB excess loss to our predictions [1] and identify its main components.
2. Results and discussion
The type of assembly studied here is shown in Fig. 3 (a). A ferrule was not assembled to the polymer ribbon in this work. Instead, a standard MTP connector was aligned to the edge of the polymer ribbon as shown in Fig. 3(b). Hence, the current data includes all sources of loss of the compliant interface concept except for misalignment at assembly between ferrule and polymer ribbon. This additional loss is expected to be under 0.5 dB. The nanophotonic die employed includes loopback waveguides connecting adjacent optical channels. The full roundtrip loss was divided by two to provide the loss between the fiber connector and the full-width Si waveguide on the die.
A typical loss spectrum is shown in Fig. 3 (c) and example loss distributions through the 12 channels of the interface are shown in Fig. 4 (a). The on-chip polarization could be explicitly identified in spectra through comparison with channel responses including polarization sensitive structures. The polarization could not be identified with certitude in single-wavelength measurements. However, in Fig. 4(a), we expect the polarization related to the minimum transmission (referred to as “Min polarization”) to be TM. Sample B in Fig. 4(a) shows a clear trend in loss across the channels. This assembly was cross-sectioned near the chip edge and through the center of the adiabatic coupler. The measured adhesive height separating the polymer waveguide to the chip is reported in Fig. 4(b). The impact of adhesive height and assembly misalignment on loss is presented in Fig. 4(c)-(f). We notice
Fig. 2. Optical path in compliant interface. (a) Top view of assembly with lid removed for clarity. The interface has 12 parallel optical ports. (b) - (e) are waveguide cross-sections at locations indicated in (a).
a b
Nanophotonic die
Polymer ribbon
Wavelength (um)
Loss from fiber to silicon (dB)
1.47 1.49 1.51 1.53 1.55 1.57‐4.5
‐4
‐3.5
‐3
‐2.5
‐2
TE
TM
c
Automated assembly
Active alignment
a b
Nanophotonic die
Polymer ribbon
Wavelength (um)
Loss from fiber to silicon (dB)
1.47 1.49 1.51 1.53 1.55 1.57‐4.5
‐4
‐3.5
‐3
‐2.5
‐2
TE
TM
c
Automated assembly
Active alignment
Fig. 3. Optical data presented here was acquired without ferrule on the polymer ribbon. The assembly is shown in (a). It was probed with a standard MTP connector as shown in (b). A typical transmission spectrum is shown in (c).
that a thin adhesive is desired at the adiabatic coupler but a thick adhesive is desired near the abrupt chip and taper edge transitions to prevent loss by scattering.
Comparing the data of Fig. 4(a)-(b) to the computations of Fig. 4(c)-(f), we conclude that the main source of excess loss is consistent with chip edge and taper edge scattering. The spectral and polarization dependence of Fig. 3(c) is also consistent with edge scattering. This scattering is negligible when investigated at the polymer design point and the initially expected ~1-um adhesive height. In our current assembly process, however, the gap near the taper edge is much smaller than 1 um. In addition, even at 1-um adhesive height the chip edge scattering is not negligible at the worst case point within fabrication tolerances. The impact of edge scattering was missed in our initial analysis but its loss contribution can be eliminated by design and by modifications to our assembly process. We have the ability to further selectively increase the adhesive gap near the chip edge to eliminate chip edge scattering and we can include in design a smooth taper bend out to prevent taper edge scattering.
Another source of excess loss is larger than anticipated polymer propagation loss. Our tolerance analysis employed 0.4dB/cm loss as reported in polymer waveguides employing the same material system but lower index contrast and stronger mode overlap with the waveguide core [5]. In the current samples, we estimate the propagation loss to average just above ~1 dB/cm at 1550nm. The propagation path through the polymer ribbon is of about ~1 cm.
3. Conclusion
We have presented the first optical demonstration of fiber to nanophotonic waveguide coupling through a compliant polymer interface and identified the main causes and solutions to the ~1.5dB excess loss observed.
4. References
[1] T. Barwicz and Y. Taira, "Low-cost interfacing of fibers to nanophotonic waveguides: design for fabrication and assembly tolerances," IEEE Photonics Journal 6, 6600818, (2014).
[2] T. Barwicz et al, “Assembly of mechanically compliant interfaces between optical fibers and nanophotonic chips,” in proc. of 2014 ECTC (IEEE, New York, 2014), pp. 179-185.
[3] A. Mekis et al. “A Grating-Coupler-Enabled CMOS Photonics Platform,” IEEE JSTQE 17, 597-608 (2011). [4] A. Khilo et al., "Efficient Planar Fiber-to-Chip Coupler based on Two-Stage Adiabatic Evolution," Optics Express 18, 15790-15806 (2010). [5] S. Takenobu and Y. Kaida, "Single-Mode Polymer Optical Interconnects for Si Photonics with Heat Resistant and Low Loss at
1310/1550nm," in ECOC, OSA Technical Digest (online) (Optical Society of America, 2012), paper P2.20.
a
b
c
Adhesive height in coupler (um)
Adiabatic transition loss (dB)
0 0.5 1 1.5 2‐3
‐2
‐1
0
Misalignment (um)
Adiabatic transition loss (dB)
0 1 2 3 4‐3
‐2
‐1
0
Adhesive height (um)
Chip edge loss (dB)
0 0.6 1.2 1.8 2.4
‐1.6
‐1.2
‐0.8
‐0.4
0
TE designTM design
Worst TEWorst TM
Adhesive height (um)
Taper edge loss (dB)
0 0.6 1.2 1.8 2.4
‐1.6
‐1.2
‐0.8
‐0.4
0
TE designTM design
Worst TEWorst TM
TETM
TETM
e
fd
Half roundtrip loss (dB)
Channel roundtrip1‐2 3‐4 5‐6 7‐8 9‐10 11‐12
‐5
‐4.5
‐4
‐3.5
‐3
‐2.5
‐2
Min polarization
Max polarization
Sample B
Sample A
Adhesive gap
(um)
Channel roundtrip1‐2 3‐4 5‐6 7‐8 9‐10 11‐120
0.2
0.4
0.6
0.8
1
1.2
Sample B, adhesive height near chip edge
Sample B, adhesive heightin adiabatic coupler
Current samples
Current samples
a
b
c
Adhesive height in coupler (um)
Adiabatic transition loss (dB)
0 0.5 1 1.5 2‐3
‐2
‐1
0
Misalignment (um)
Adiabatic transition loss (dB)
0 1 2 3 4‐3
‐2
‐1
0
Adhesive height (um)
Chip edge loss (dB)
0 0.6 1.2 1.8 2.4
‐1.6
‐1.2
‐0.8
‐0.4
0
TE designTM design
Worst TEWorst TM
Adhesive height (um)
Taper edge loss (dB)
0 0.6 1.2 1.8 2.4
‐1.6
‐1.2
‐0.8
‐0.4
0
TE designTM design
Worst TEWorst TM
TETM
TETM
e
fd
Half roundtrip loss (dB)
Channel roundtrip1‐2 3‐4 5‐6 7‐8 9‐10 11‐12
‐5
‐4.5
‐4
‐3.5
‐3
‐2.5
‐2
Min polarization
Max polarization
Sample B
Sample A
Adhesive gap
(um)
Channel roundtrip1‐2 3‐4 5‐6 7‐8 9‐10 11‐120
0.2
0.4
0.6
0.8
1
1.2
Sample B, adhesive height near chip edge
Sample B, adhesive heightin adiabatic coupler
Current samples
Current samples
Fig. 4. (a) Loss distribution at 1550 nm among the 12 channels of two exemplar samples. (b) Measurement of adhesive height at two cross-sections in sample B. (c)-(f) Computational results. The loss from scattering at the abrupt chip edge transition is shown in (c) and at the abrupt start of the Si taper in (d). ‘Worst’ indicates the maximum loss within fabrication and assembly tolerances.
The sensitivity of the adiabatic coupling transition to adhesive height and misalignment is shown in (e) and (f). The longer the adiabatic transition the more tolerant it is to assembly and fabrication offsets. Here, we used a conservative 1750 um length.
