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Polymer waveguide made by dry film process is demonstrated for silicon photonics chip packaging. With 8 μm × 11.5 μm core waveguide, little penalty is observed up to 25 Gbps before or after the light propagate through a 10-km long single-mode fiber (SMF). Coupling loss to SMF is 0.24 dB and 1.31 dB at the polymer waveguide input and output ends, respectively. Alignment tolerance for 0.5 dB loss increase is +/− 1.0 μm along both vertical and horizontal directions for the coupling from the polymer waveguide to SMF. The dry-film polymer waveguide demonstrates promising performance for silicon photonics chip packaging used in next generation optical multi-chip module.
© 2014 Optical Society of America
1. Introduction
Polymer waveguide integrated optoelectronic modules have gained considerable interests for high performance computing systems [1–3]. Optical I/O composed of optoelectronic devices, such as laser and photodiode, are integrated with CMOS ASIC or CPU chip and fiber connectors on a waveguide integrated organic carrier, forming an optical multi-chip module (MCM). Optical MCMs developed and reported so far are based on the use of multimode media where light is emitted from a vertical-cavity surface-emitting laser and received by a photodiode via multimode waveguides and fibers. In our previous studies, optical MCM has been demonstrated by integrating optical I/Os operating up to 20 Gbps per channel on a dry-film, multimode polymer waveguide integrated organic carrier [4, 5]. The advantages of dry film method include: no wet development involved, more productive and reliable for waveguides embedded on a printed circuit board, large scale working area, and relatively easier to maintain a uniform thickness of a cladding layer in the vicinity of core area [6, 7].
However, transmission distance through multimode media becomes shorter at higher data rate, which may not be fully sufficient for large data centers. On the other hand, the future progression is tend to be intra-chip interconnects that light signal is generated and processed within silicon photonics chips. Heterogeneous photonics–electronics circuits such as wavelength-division multiplexer, on-chip laser source and modulator, and photo detector are integrated on silicon-on-insulator (SOI) where all of these run under single-mode operations. For expanding the propagation distance and functionality of existing multimode optical MCM techniques by exploiting the benefits of silicon photonics and SOI devices, more advanced systems, optical link equipped with single-mode optical MCMs, are regarded by deploying single-mode waveguides and single-mode fibers [8, 9].
Due to the large dimension difference, the optical performance of such systems highly depends on effectively light coupling from a silicon photonics chip to one another via single-mode waveguides and fibers. Particularly when silicon photonics chips are bonded on an organic substrate, single-mode waveguides suitable for being a low-cost packaging carrier facilitates the realization of single-mode optical MCM. In this paper, a dry-film polymer waveguide has been designed and evaluated by both DC and high speed characterizations. DC measurements include near field pattern (NFP), far field pattern (FFP), coupling loss, and alignment tolerance. High-speed performance is shown by eye patterns and jitter values.
2. Characteristics of the dry-film polymer waveguide
The polymer waveguide reported in this paper is designed for single-mode operation under 1.31 μm wavelength fabricated by dry film method, which is a standard process for polymer materials laminated on an organic substrate with circuits. The polymer materials are Epoxy based compositions which benefit to small unevenness of the dry film thickness and easy for varnish processes on waveguide upper cladding [7]. These improve silicon photonics chips flip-chip bonded on the substrate. In order to realize lower fabrication threshold, minimize butt coupling loss, and increase alignment tolerance to SMF, the core size is chosen to be close to modal field of SMF for 1.31 μm. Furthermore, the cladding material is adjusted to fit the numerical aperture of the waveguide to 0.13. The fabricated polymer waveguide has 24 channels, 250 μm core pitch, and 3 cm in length. Figure 1 shows (a) optical loss spectrum of the waveguide core material, (b) the core, and (c) waveguide cross section dimensions. According to criterion for single-mode guidance, the core size has to be smaller than 7.7 μm, whereas the polymer waveguide works partially as a multimode medium.
Fig. 1 (a) Material spectrum of the fabricated polymer waveguide core. (b) Core dimensions with applied coordinate systems (c) Waveguide cross section.
In the study of modal properties, the waveguide is launched by a 2-m long SMF. The NFP and FFP at the output end of the waveguide are measured by HAMAMATSU A6501 and A6502, respectively. A distributed feedback (DFB) laser source operating at 1.31 μm wavelength is used. Index matching fluid (Norland Index Matching Liquid 150) is added between the polymer waveguide and the SMF in order to reduce Fresnel reflection loss. Figure 2 shows two output profiles with normalized intensity, where the range at 1/e2 is plotted in white curves and the corresponding modal diameters are summarized in Table 1. On the other hand, angular intensity distributions obtained from FFPs are shown in Fig. 3 and the numerical results are summarized in Table 2.
Table 1. Modal Diameter at 1/e2 of Normalized Intensity
Fig. 3 Angular intensity distribution at output end along x and y-direction of (a) SMF and (b) the polymer waveguide.
Table2. Numerical Aperture at 5% of Normalized Intensity
When light propagates from SMF to the polymer waveguide, power coupling efficiency can be expressed as ratio of the output to input power, and can be written as
where Ef and Ew are the guided modes of SMF and the polymer waveguide, respectively. Although the waveguide may not be completely operated under single-mode, outer modes are weakly guided that the coupling efficiency η is estimated using the square root of the measured intensity profiles shown in Fig. 2. By using Eq. (1), the power coupling coefficient is calculated to be 0.94, which corresponds to a 0.25 dB loss.The propagation loss of the polymer waveguide, includes inherent absorption loss and scattering loss, is evaluated by cut-back method. Figure 4 shows the relation between waveguide length (x, in cm) and propagation loss (y, in dB) is y = 0.44x + 0.25. Compared to the bulk material loss (0.42 dB/cm) shown in Fig. 1(a), the 0.02 dB increasing results from light scattered by core/cladding boundary. The small scattering effect also can be observed at the FFP results, that the angle distributions shown in Fig. 3(b) are slightly increased. For coupling loss measurement, light from the DFB laser is launched through a SMF. First a 1-m long step-index multimode fiber (MMF) with 100 μm core diameter is used for collecting all the light at the output end of the waveguide. After subtracting the material loss, the coupling loss at the waveguide input side is 0.24 dB. This is close to the value estimated by Eq. (1) and the cut-back results. Next, the MMF is replaced by another SMF at the waveguide output end, and the measured coupling loss at the output end is 1.31 dB. The larger coupling loss at the output end results from combination of mismatch of modal fields (larger waveguide core to smaller SMF core) and enlarged modal angular distribution caused by light scattering within the core area [10]. The measurement results of coupling loss are summarized in Table 3.
Table 3. Coupling Losses Measurement Results
Alignment tolerance of coupling from the polymer waveguide to SMF is characterized. The SMF is moved at the output end of the waveguide along x and y directions. The measurement results are shown in Fig. 5. The measured alignment tolerance for 0.5 dB loss increase is +/− 1.0 μm in both directions. Although the modal diameter of waveguide is larger in y-direction, the tolerance is still sensitive to displacement under single-mode operation.
3. High-speed transmission performance
When modulated light propagates from a silicon photonics chip to one another via polymer waveguide and single mode fiber in an optical link, it is necessary to ensure signal integrity particular for long distance propagation. As described in Sec. 2, although the larger core size benefits to minimize coupling loss, it is estimated that 16 guided modes exist in the polymer waveguide. How multiple modes influence on power penalty under various data rate is examed by high-speed transmission test. In this test, pseudorandom binary sequence signal is fed into a modulator by a pattern generator (SHF 12102A) at 12.5, 16, 20, and 25 Gbps with experiment setup shown in Fig. 6. Transmission performance of a 10-km long SMF (with 0.324 dB/km propagation loss) is characterized to evaluate degradation of high-speed signal once the polymer waveguide is inserted before or after it. Figure 7 shows optical eye diagrams for different transmission paths with corresponding data rate. The extinction ratios of the input optical signal are around 5 dB at 12.5, 16, and 20 Gbps and 4 dB at 25 Gbps. A 2-m SMF back-to-back optical diagram is shown in Fig. 7(a) as a reference point. No distortion is observed after the polymer waveguide is inserted (see Figs. 7(b)–7(d)).
Fig. 7 Optical eye diagrams with corresponding data rate with the following connections: (a) 2-m SMF back-to-back (b) 10-km SMF back-to-back (c) 2-m SMF, waveguide, 10-km SMF (d) 10-km SMF, waveguide, 2-m SMF.
Figure 8 shows the eye opening of unit interval (UI) of corresponding horizontal bathtub curves where the eye opening extrapolated to a 10−12 bit error ratio are listed. Compared to the 2-m SMF back-to-back test, the maximum difference is only 0.07 UI which occurs at (2-m SMF, waveguide, 10-km SMF) connection with 25 Gbps data rate. Compared to the 10-km SMF back-to-back connection, results for (2-m SMF, waveguide, 10-km SMF) and (10-km SMF, waveguide, 2-m SMF) connections show less than 0.03 UI differences after the polymer waveguide is inserted to the link. The numerical results are very close when connections of 2-m and 10-km SMF are exchanged. This implies even the coupling loss at the waveguide output end is more than 1 dB and extra 3.24 dB propagation loss is added, the modal profile at waveguide output is still clear that modal dispersion is not spread after a 10-km distance propagation.
Fig. 8 Optical eye openings at 10−12 bit error ratio with the following connections: (a) 2-m SMF back-to-back (b) 10-km SMF back-to-back (c) 2-m SMF, waveguide, 10-km SMF (d) 10-km SMF, waveguide, 2-m SMF.
4. Conclusion
Dry-film polymer waveguide designed for single-mode use is demonstrated. Larger waveguide core size is selected in order to realize lower fabrication threshold, minimize butt coupling loss, and increase alignment tolerance to SMF for 1.31 um. The coupling loss of a (2-m SMF, waveguide, 2-m SMF) connection is 0.24 dB at the input end and 1.31 dB at the output end. The larger loss of coupling from waveguide to SMF results from the larger waveguide core size and inner-core scattering which makes light emission angle increased. The +/− 1.0 μm alignment tolerance is allowed in both x and y-directions for 0.5 dB loss increase.
The polymer waveguide shows promising performance in high-speed characterization despite larger core size. Both eye opening and jitter of 10-km transmission through SMF are not influenced by the addition of the polymer waveguide. We conclude that polymer waveguides made by dry-film process promise to realize single-mode optical MCM with silicon photonics chip packaging.
Acknowledgments
The authors would like to thank Toru Nakashiba, Naoyuki Kondo, Shinji Hashimoto, Takehiro Deguchi, and Shingo Maeda from Panasonic Inc. for their support on sample fabrication.
References and links
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