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We demonstrate a compact (30 μm long) broadband bilayer inverse taper edge-coupler for silicon photonics with a 1.7 dB coupling loss from a commercially available focused fiber with 5 μm mode diameter. We compare the performance of our bilayer taper with a conventional SOI inverse taper coupler and show that our bilayer coupler achieves between 1.5 dB to 2 dB improvement in the coupling efficiencies for both the TE and TM polarizations. The dimensions and fabrication steps for our bilayer taper are simple and compatible with standard foundry processes.
© 2016 Optical Society of America
1. Introduction
Efficient coupling of light from optical fibers to silicon (Si) photonic circuits is an important consideration when designing such circuits. The absence of a good source on Si platform requires light to be delivered to the Si waveguides through single mode optical fibers (SMFs) from an external source [1]. However, a substantial mismatch exists between the mode-sizes and mode-indices of a typical SMF and a single mode Si waveguide. Such mismatches lead to inefficient coupling of light to the Si optical circuits.
Several techniques have been proposed for efficient coupling of light to Si waveguides on a silicon–on-insulator (SOI) platform. Diffraction gratings [2–8] and edge-couplers [9–13] are the more popular techniques which are widely used for fiber to Si chip coupling. Diffraction gratings are generally narrow band and require an out-of-plane mechanism to hold the fiber at a particular angle to efficiently couple light into the Si waveguides. On the other hand, edge coupling, through a tapered waveguide, can produce a high coupling efficiency over a broad band of source wavelengths, in addition this method does not require an out of plane coupling mechanism. An adiabatically tapered waveguide in which both the width and height change, improves the mode matching between the fiber and the Si waveguide, thus improving the coupling of the input light to the Si waveguides [14–16]. This method, however, suffers from some drawbacks such as complex fabrication steps and large real estate on the chip.
A somewhat different approach from the above is the inverse taper coupler where the input Si waveguide cross-section is narrowed toward the edge of the chip, so that the mode is pushed to cut-off, as a result the mode size becomes larger and more loosely guided. Through careful design it is possible to better match the mode size of the Si waveguide with the SMF mode, resulting in better coupling efficiency. In the literature, there are several reports on inverse tapers of various designs aimed at increasing the coupling efficiency between the SMF and Si waveguide. First, Shoji et al. and then Lipson et al. proposed and demonstrated the idea of an inverse taper edge-coupler with coupling efficiencies between 3 dB to 6 dB for a taper length of ~40 μm [11,12]. Mcnab et al. used the same inverse taper coupler, optimized to couple light to the fundamental TE mode of a photonic crystal waveguide, with a coupling efficiency of ~2 dB [17]. Several groups have proposed and demonstrated efficient edge couplers with additional modification of the cladding geometry of the inverse taper [10,18–21]. Bakir et al. demonstrated less than 1 dB coupling loss from a fiber with a 3 μm MFD for their 200 μm long inverse taper arrangement which also shows a polarization insensitivity [22]. In their work Wood et al. presented a 7.5 μm long taper with cantilever beam and undercut structure on SOI with coupling efficiencies of less than 1 dB from a specialized 1.5 μm MFD fiber [23]. Lipson et al. also reported a 100 μm long simple inverse taper coupler with a coupling loss of 0.7 dB per facet [13], where they used a standard 5 μm MFD fiber for coupling. For the readers’ convenience, these results are tabulated in section 5 of this manuscript in Table 1. We should also add that there are other reports of efficient edge-couplers with more complicated structures, using smaller MFD fiber [21].
Table 1. Comparison of the Bilayer Taper performance with other Inverse Taper designs in literature
Aside from the coupling efficiency there are two other important criteria determining the performance of an edge-coupler inverse taper – the length of the taper and the MFD of the coupling fiber. For an integrated optical circuit, the size of an optical component, i.e. the space occupied by a component on the chip, is an important consideration. In other words, it is always desirable to fabricate a device that is more compact; ideally, without scarifying performance. The other parameter affecting the coupling is the MFD of the coupling fiber which essentially causes the mode-size mismatch between the fiber and Si waveguide. Commercially available SMFs, such as those used in optical communication systems have a MFD of ~10.4 μm while the lensed-facet SMFs, commonly used for fiber to chip coupling, have a MFD of ~5 μm. Finally, the simplicity of fabrication and compatibility with the commercial foundry fabrication rules are other considerations which should be kept in mind when designing a Si photonic device.
In this paper we report on the design, fabrication, and experimental demonstration of a compact (30 μm long) bilayer, inverse taper, edge-coupler with a coupling loss of 1.7 dB. The input fiber had an MFD of 5 μm, from which light is coupled to a single mode Si waveguide of 500 nm width and 220 nm height. In order to compare and contrast the performance of our bilayer inverse taper with an existing comparable design in literature, we have also fabricated (using the same facilities) a conventional inverse taper as it was presented in [12].
The rest of the paper is organized as follows: In section 2 we discuss the design and the geometry of the bilayer inverse taper. Section 3 discusses the optimization and the resultant inverse taper parameters. Section 4 presents the fabrication steps and the organization of the fabricated chip. Finally, section 5 discusses the experiment and the results for the coupling efficiency of the bilayer inverse taper presented in the paper.
2. Design of the bilayer inverse taper
The basic idea behind an inverse taper coupler is to gradually reduce the waveguide cross-section to a certain point such that the guided mode is pushed toward cutoff. Under this condition, two things happen: (1) the mode size gradually increases and (2) the mode effective index (neff) approaches that of the cladding. As the result of the increase in mode size, there is a better spatial overlap between the modes of the input fiber and the narrow waveguide. As a consequence of the change in effective mode index there is a better match between the mode indices of the input fiber and the waveguide. Both of these phenomena help the light to better couple from the input fiber to the waveguide.
To illustrate the operation of the bilayer taper we start by considering the change in effective indices of a Si waveguide on SOI wafer for the TE and TM modes. The variation of the effective indices for two different waveguide heights (150 nm and 220 nm) are plotted in Fig. 1. There are two important observations to be made in regard to Fig. 1. First, as the width of the Si waveguide decrease all the effective mode indices approach towards the SiO2 index of 1.44. At the same time, the effective index of a typical fiber mode is also close to the SiO2 index; hence, the mismatch between the effective indices of the fiber and Si waveguide can be dramatically reduced. Second, as evident in Fig. 1, for the 220 nm tall Si waveguides there is a fair amount of birefringence (difference in neff for TE and TM modes); whereas, the birefringence is reduced when the width is less than a 100 nm. The large birefringence results in a significant difference between the coupling efficiencies for the TE and TM modes of the inverse taper. On the other hand, for a Si-waveguide with the height of 150 nm the birefringence is substantially reduced for widths less than 180 nm. This means that for waveguides with transverse cross section of 150 nm by 150 nm, the birefringence is negligible while at the same time the effective mode index of the Si waveguide matches closely that of the input fiber. Therefore, a tapered Si waveguide, starting with a 150 nm height and 150 nm width can lead to an enhanced coupling of both the TE and TM polarization components from the coupling fiber. Moreover, the choice of 150 nm by 150 nm is convenient, since it is compatible with the commercial foundry fabrication rules.
Fig. 1 Effective mode index as a function of Si waveguide width for a waveguide height of 220 nm (dashed line) and 150 nm (solid line) with an oxide over-layer on the SOI wafer.
In light of the above considerations, we propose an inverse taper edge-coupler as shown Fig. 2. The taper begins with a tip of 150 nm by 150 nm cross section and then widens out in the lateral (horizontal) direction to match the 500 nm width of a standard single mode Si waveguide. A second layer taper is then introduced at the end of the first taper section, L1Taper (see Fig. 2 (b)), which also increases the taper height by an additional 70 nm, making the total height equal to 220 nm at the beginning of the second taper section, L2Taper. Finally, by the end of the L2Taper section, the waveguide has cross-section dimensions of 220 nm by 500 nm for its height and width, respectively; which matches the dimensions of a single mode Si waveguide on SOI wafer.
Fig. 2 Schematics of the proposed bilayer taper. (a) A 3D artistic impression. (b) A top view of the taper structure with different important design parameters. The green portion indicates the partially etched silicon with 150 nm height, while the orange portion indicates the full 220 nm height silicon section. TOx and BOx denotes the top oxide cladding and the bottom oxide layer of an SOI chip.
3. Numerical analysis
We used a 3D commercial full wave simulator based on the finite difference time domain (FDTD) method [24] to optimize our proposed taper design. We used a conformal variant 1 mesh settings with accuracy level 3 and a PML boundary condition in the simulations. Our design goal is to achieve high coupling efficiencies for both the TE and TM polarizations while keeping the taper dimension as small as possible. We used a fiber mode source in the full width half max (FWHM) wavelength span of 1500 nm to 1600 nm in our simulation with a 5 μm MFD to match the scenario of coupling light from a focused fiber to single mode Si waveguide. We chose the total length of the taper (LTot) to be 30 μm, which is 10 microns shorter than the one proposed in [12]. We optimized the wTrans (see Fig. 2) and the ratio of L1Taper to the total length (LTot = L1Taper + L2Taper), in order to maximize the coupling from a fiber tip with 5 μm MFD to a single mode Si waveguide. Here the parameter wTrans is the transitional width of the bottom layer of the taper where the tip of the top layer starts as shown by the start of the orange colored layer in Fig. 2(b). The optimized value of L1Taper /LTot was found to be 0.7 when wTrans is equal to 350 nm.
Figure 3 shows the calculated coupling losses for the 30 μm long bilayer inverse taper for both TE and TM inputs. As the figure indicates, the coupling losses for TM mode at 1550 nm is 1.98 dB, whereas for the TE mode the loss is 1.01 dB. These performance characteristics make the proposed bilayer inverse taper a highly desirable edge coupler device. As far as the overall device dimensions are concerned we should add that increasing the total length by 10 μm (i.e. LTot = 40 μm) reduces the TE coupling losses to 0.97 dB while decreasing the total length by 10 μm (i.e. LTot = 20 μm) increases the TE coupling losses to 1.4 dB for operation at 1550 nm wavelength. The TM coupling efficiencies follow the same trend as the TE coupling efficiency with the change of the total taper length. Since the improvement of coupling efficiency doesn’t scale at the same ratio as increasing the length we settled with the choice of 30 μm as the total length of the bilayer inverse taper.
4. Fabrication of the bilayer inverse taper
Fabrication of the bilayer inverse taper involves a two-step Si etch process on an SOI wafer– a partial Si etch of 70 nm etch-depth and a full etch of 220 nm. The process steps are shown in Figs. 4(a) and 4(b). First, tungsten markers are fabricated on a piece of SOI wafer using e-beam lithography (Vistec EBPG 5000 + ). Then, using ZEP 520 positive resist, an opening was patterned in order to perform the partial etch (70 nm etch-depth). The partial etch of the Si layer was carried out using an Oxford Instruments Plasma Pro Estrelas100 DRIE System. In the next step, a negative resist (HSQ) was spun on the wafer and patterned to performed the full etch (220 nm) portion of the taper. Markers were used to properly align the layers and the full etched was carried out using the DRIE system. During this step, the 500 nm wide Si waveguide which is attached to the end of the taper structure (also 500 nm wide) were also fabricated. The last step was the deposition of a 2 μm thick oxide (SiO2) layer all over the sample which acts as the top cladding for the Si waveguides. This deposition was done using a plasma enhanced chemical vapor deposition system (Oxford Instruments PlasmaLab System 100 PECVD). All the fabrication steps were carried out using the cleanroom facilities of the Toronto Nano Fabrication Center (TNFC) at the University of Toronto.
Fig. 4 (a) Top view schematics of the fabrication steps for the bilayer inverse taper including the metal alignment markers on the chip. (b) Fabrication steps of the bilayer inverse taper along the cross-sectional plane. The projection of only the front facet of the cross section at the plane is shown to avoid confusion.
4.1 Sample organization
In addition to the bilayer inverse taper, on the same chip, we also fabricated inverse taper as described in [12] next to the bilayer inverse taper. This was done in order to compare the performance of our taper with that of a conventional inverse taper fabricated under same conditions. In this manuscript, we refer to the inverse taper of [12] as the “reference inverse taper.” Fig. 5 shows a schematic of the chip layout, where each combined group of bilayer and “reference inverse taper” is designated by letters M1, M2, and M3. Figure 5 also shows a set of standard Si waveguide (220 nm height by 500 nm width) designated as . These Si waveguides run the entire length of the chip, from edge-to-edge, and are used to measure the propagation losses associated with our Si waveguides.
Fig. 5 Schematic diagram of the sample with three measurement groups of waveguides (M1, M2 and M3). Each measurement group has a “reference inverse taper” (top) and a bilayer inverse taper (bottom) Loffset is the offset distance between each measurement group. The sample was cleaved at the distance Lcleave from the left edge of the wafer. The right edge was cleaved abruptly, so all the waveguides terminated at the edge. M′ at the top is the group of facet cleaved waveguides for propagation loss measurement.
One of the major challenges in the measurement process is the cleaving of the chip at a proper position with high enough precision so that the edge of the cleaved chip and the taper facets coincide. Such accurate cleaving is necessary for optimum coupling, but is very difficult to achieve even with high precision cleaving instruments. Laser assisted cleaving [25], deep trench cleaving [26] are different techniques to achieve high precision cleaving. In order to confirm the coupling efficiency of the bilayer taper we fabricated several experimental sets of waveguides where the start position of the waveguides was staggered in 200 μm increments. After cleaving we were able to use the waveguide groups (i.e. M1, M2 etc.) on the sample where the waveguides were best positioned relative to the cleave facet. In the resultant cleaved sample, the taper tips of the M1 waveguide set (Fig. 5) fell closest to the cleaved edge of the sample at a distance (Lcleave) of 30.76 μm measured by scanning electron microscopy (SEM). The light beam from the fiber has to traverse this distance Lcleave partially through both the top and bottom oxide layer to reach the front facet of the tip of the taper before coupling. With such setup of waveguide groups staggered in position, we can construct equations of loss calculation for the waveguides in each of the measurement sets and extract the value of power loss (Pox) due to propagation through the offset distance in the SiO2 layer between the cleaved edge of the chip and the taper tip. The knowledge of Pox is necessary to calculate the absolute coupling efficiency of the bilayer inverse taper. After cleaving, the waveguide set M′ (above M1 in Fig. 5), which was initially connected to a bilayer and “reference inverse taper” prior to cleaving, terminates at the left and right edges running through the full length of the chip. The power measurement from the waveguide set M′ facilitates the Fabry Perot measurement of the propagation loss through the waveguides fabricated on the chip.
5. Experiment
We have characterized the performance of our bilayer inverse taper under two scenarios: a) Free space coupling, which allows us to carefully control the polarization of the input optical field, and, b) Using a focused SMF, with 5 μm MFD, which is a more common approach in industry. Figures 6(a) and 6(b) show schematics of the two aforementioned scenarios.
Fig. 6 Measurement setups for, (a) a free space coupling using objective input lens, (b) a focused single mode fiber input with 5 μm MFD.
For the TM mode, using the free space coupling scheme and a Thorlabs broadband diode source in telecom wavelengths (C-band), we measured an output power of 68.90 μW for our inverse bilayer taper as compared to an output power of 47.52 μW for the “reference inverse taper,” at the detector. This represents a 1.61 dB improvement in coupling efficiency for our bilayer taper. Using the same setup, for the TE mode, we measured an improvement of 1.91 dB in coupling efficiency for our bilayer taper as compared to the “reference inverse taper”. We note that for the setup shown in Fig. 6(a), the absolute power coupling efficiency depends on the magnification power of the input objective lens. Therefore, a comparative study, such as the one carried above, is a more suitable approach in characterizing the performance of the bilayer inverse taper coupler as opposed to characterizing the absolute loss number when using the free space coupling set up with an objective lens.
We now return to the second measurement setup shown in Fig. 6(b); i.e. the focused SMF excitation. Continuing with the comparative measurements, we have observed an improvement of approximately 1.48 dB in the coupling efficiency for our bilayer inverse taper as compared to the “reference inverse taper”. Using an Agilent optical spectrum analyzer (OSA), we have also compared the spectra of our bilayer inverse taper with that of the “reference inverse taper.” Results are shown in Fig. 7 obtained using the setup in Fig. 6(b). The comparison shows a consistent improvement of the coupling efficiency for the bilayer inverse taper over the “reference inverse taper,” over a wavelength window of 50 nm. Here, our measurement is limited by the bandwidth of the available source. We expect the bilayer inverse taper coupler to demonstrate a broader bandwidth similar to the calculated spectrum shown in Fig. 3.
Fig. 7 Measured spectral responses of the bilayer inverse taper and the reference inverse taper. Light was coupled from a non-polarization maintaining lensed fiber of 5 μm mode field diameter.
Using the setup in Fig. 6(b) we can also ascertain the absolute coupling efficiency – or the coupling loss – associated with our bilayer taper for the fiber coupling case. To do so, we needed to measure and compensate for: propagation losses in the Si waveguide section, the reflection and diffraction losses (Pox) associated with propagating through the distance Lcleave in the SiO2 (see Figs. 5 and 2(a)) between the chip edge and taper facet, and, the Si waveguide out coupling losses. At the excitation wavelength of 1550 nm, these losses were measured to be 2.4 dB, 3.21 dB, and 2.03 dB, respectively. Using these results we estimated the coupling loss of our bilayer inverse taper at the excitation wavelength of 1550 nm is 1.7 dB – when light is fed to the taper using a SMF with 5 μm MFD.
Table 1, below, summarizes and compares the results for our proposed bilayer inverse taper with other inverse taper edge-couplers in the literature. A survey of the results shows that for the given MFD (5 μm in our case), our proposed bilayer taper is the shortest edge-coupler with insertion loss of only 1.7 dB, while keeping the fabrication and device geometry simple (for example, no need for complex undercut cantilever as in [21]). We should also add that in the case of our bilayer inverse taper, the partial etch-depth of 70 nm (i.e. 150 nm Si height) was chosen so that the design is compatible with the standard partial etch-depth and design rules allowed by the commercial foundries.
6. Conclusion
We have presented the design, fabrication, and experimental demonstration of a compact and low loss bilayer inverse taper edge-coupler for Si photonics circuits. The proposed bilayer taper shows enhancements in free space to waveguide coupling efficiencies, for both the TE and TM modes, of more than 1.5 dB as compared to the conventional inverse taper coupler of [13]. Also the bilayer inverse taper demonstrates a similar enhancement in coupling efficiencies for the case of fiber to Si waveguide coupling: i.e. the coupling loss is approximately 1.7 dB when a focused SMF (with 5 μm MFD) is used to couple the light into the Si waveguide. The proposed edge-coupler is simple to fabricate, it is only 30 μm long, and the device geometry and dimensions conform to the standard design rules for Si foundries.
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