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Vertical optical coupling is demonstrated in a multimode interferometer structure fabricated using silicon-on-insulator (SOI) material. These CMOS-compatible couplers are suitable to be used to transfer optical power between stacked waveguide layers of a three-dimensional photonic circuit. Coupling between these layers can be restricted to certain regions by selectively fabricating a silicon channel between them, resulting in an isolated multimode waveguide section. Standard photolithography and etching techniques were used to fabricate proof-of-concept devices consisting of a channel waveguide coupling into a silicon waveguide that is vertically multimode. An optical coupling ratio of 93±4% between the upper and lower waveguide regions of the device was achieved with a coupler length of 241 µm.
©2011 Optical Society of America
1. Introduction
The proliferation of silicon-on-insulator (SOI) based structures for optoelectronics has offered significant reduction in the footprint of integrated optical components due to waveguides in SOI having high-refractive index contrast, resulting in devices that are more compact than those fabricated using other lower-contrast material systems such as silica or III-V semiconductors [1]. Yet the number of individual optical components that can be packed onto a standard-sized chip is still limited by optical cross-talk [2]. Vertical integration promises to enable highly-dense photonic integrated circuits through the stacking of multiple waveguide layers. Vertically-integrated couplers that transfer optical signals between each of these layers constitute the basis of three-dimensional photonic circuits.
A patterned oxygen ion implantation process has been used to demonstrate vertical coupling between multiple buried SOI waveguide layers [3]. Alternatively, we have recently proposed vertically-integrated couplers that can be fabricated using a wafer bonding-based process [4]. This latter technique allows for the integration of photonics and electronics onto each device layer, since doping and metallization steps can be applied to individual layers before they are stacked. It should also improve manufacturing yields for the overall three-dimensional device as each layer can be inspected before stacking takes place.
Optical coupling between vertically-integrated silicon layers can be achieved simply by stacking waveguide structures (e.g. rib waveguide) while separating them with a thin intermediate layer of insulating material, using either an oxide or a nitride. However, unless the thickness of this separating layer is selectively tapered, optical cross-talk between the stacked layers will occur wherever the waveguides in each layer intersect. Avoiding intersection in areas where coupling is undesirable would negate the benefits of vertical integration and hence we have proposed an alternative coupling structure in [5]. Optical coupling can be restricted by selective fabrication of a silicon channel in between the stacked waveguides. The oxide surrounding this intermediate silicon channel may be made sufficiently thick in order to optically isolate each waveguide layer in the areas where coupling is undesired. The resulting coupler structure is multimode along the vertical plane, with its operating principle being the same as a conventional multimode interferometer (MMI) device. We have successively demonstrated vertical coupling in such a structure, fabricated using standard photolithographic and etching techniques.
2. Device operating principle and simulation results
Our proposed vertically-integrated 2×2 multimode interferometer (MMI) coupler is shown in Fig. 1 a ). Light coupled into the tall multimode section of the coupler results in the excitation of all of its optical modes. The subsequent beating between these modes results in the splitting of the input optical field, with the amount of optical power coupled into each output waveguide dependant on the length of the multimode coupler section. This effect is based on the self-imaging behavior of MMIs, and is described in [6]. Complete power transfer between each layer can be achieved if the coupler section is designed with the appropriate length. It should be noted that our silicon-based coupler is similar to the polymer-based design demonstrated in [7].
Fig. 1 a) Proposed vertically-integrated 2×2 MMI coupler in SOI b) Optical power coupled between the lower and upper waveguides of a vertically-integrated MMI coupler with H = 2 µm, W = 2 µm and HMMI = 5 µm.
To determine the coupler length required for complete power transfer, we must first find the beat-length of the MMI using Eq. (1):
where β 0,1 are the propagation constants of the two lowest-order modes of the multimode section, ng is the refractive index of the guide layer, He is the effective modal height of the coupler section and λ is the input wavelength [6]. For a high-refractive index contrast material such as SOI, this effective height can be approximated by the physical height of the coupler section, HMMI, due to its strong optical confinement and hence small evanescent field.The relationship between the MMI coupler length and its beat length is given by Eq. (2):
with the input optical field being directly imaged when p is even and mirrored when p is odd. Therefore, the shortest coupler length required to achieve complete power transfer between stacked waveguides is 3Lπ. If p is a half-integer (p = 1/2, 3/2, …), the input optical field becomes a two-fold image, with each fold having equal power. This coupler length may be chosen for instance when an optical power splitter is desired.The beam propagation method (BPM) can be used to accurately determine the coupler length required for complete power transfer without approximating the effective height of the optical field. Simulation results for the percentage of the total optical power that would be coupled into the upper and lower output channel waveguides as a function of propagation length in the MMI coupler section are shown in Fig. 1 b), and were obtained using RSoft’s BeamPROP simulation software [8]. These results are for the fabricated device with H = 2 µm, W = 2 µm and HMMI = 5 µm and predict that optical power launched into the lower waveguide will be completely transferred to the upper waveguide after propagating a length of 236 µm in the MMI coupler section. As expected, the power splitting ratio is 3 dB at half this coupler length. This simulation was performed for a TE-polarized input at a wavelength of 1550 nm, with the following refractive indices: nSi = 3.48, nSiO2 = 1.44 and nc = 1. It was also found that the optimal coupler length required for complete power transfer decreases to 224 µm when a TM-polarized input is used. Note that the sum of the two curves in Fig. 1 b) is not always 100%, as some of the optical power may be in the 1 µm silicon channel in between. The overall dimensions of 2 µm × 5 µm for this MMI coupler resulted in the propagation, and beating, of six modes, as determined by their individual convergence using an imaginary distance-BPM mode solver. Additional simulation results, including device tolerance to fabrication errors, can be found in [5].
It is important to note that the input/output channel waveguides of this device are multimode themselves, as their dimensions were limited by the contact photolithography process used for this work. Since single-mode operation is desired for most waveguide-based devices, the proposed coupler was fabricated with in-line tapered mode converters placed ahead of the multimode section. These converters linearly tapered the slab regions of a single-mode rib waveguide, etched to a depth of 0.5 µm, from a width of 15 µm to the 2 µm width of the input channel waveguide, and are similar to those presented in [9]. Simulations reveal that the length of this tapered converter must be longer than 150 µm in order to excite only the fundamental mode of the input channel waveguide, but the device was fabricated with a more conservative length of 500 µm. This tapered mode converter is illustrated in the fabrication process described in Fig. 2 .
The necessity of these mode converters can be eliminated if the dimensions of the input/output waveguides are reduced to those of a typical SOI photonic wire (200-300 nm). These waveguides exhibit single-mode propagation without the need for surrounding slab regions. While these dimensions would also reduce the required MMI coupler length to only tens of microns, the alignment tolerance of ±1 µm quoted for most commercially-available wafer bonding apparatus is greater than the width of the waveguides themselves. Photonic wire-based couplers would require advanced wafer bonding techniques with greater alignment precision, such as the bonding procedure utilizing interlocking structures demonstrated in [10].
3. Device fabrication
The fabrication process for the proof-of-concept MMI coupler used to demonstrate vertical optical coupling is illustrated in Fig. 2. This process was designed to avoid the necessity of wafer bonding for this initial demonstration of vertical coupling. The starting SOI wafer consisted of a 4.95±0.05 µm-thick device layer and a 0.39 µm-thick buried oxide layer. The silicon above the input waveguide was first chemically etched to a depth of 3 µm using tetramethyl ammonium hydroxide (TMAH), with the length of the MMI coupler section protected by a 0.05 µm-thick thermal oxide mask that had been patterned using contact photolithography (PL) and etched with buffered hydrofluoric acid (BHF). This mask was removed in BHF, and then a 2 µm-wide waveguide was patterned into a 0.2 µm-thick thermally-grown oxide mask. The silicon was etched to a depth of 0.5 µm with inductively coupled plasma-reactive ion etching (ICP-RIE) using a modified Bosch process, forming the input rib waveguide. The waveguide photoresist mask was removed in a Piranha bath, followed by additional PL to pattern the tapered mode converter. The remaining silicon was etched to the buried oxide using ICP-RIE to complete the sidewalls of the MMI coupler section and the input channel waveguide. The photoresist mask was removed in a Piranha bath, and the oxide mask etched in BHF, to reveal the final device structure.
Electron microscopy (SEM) images of the resulting device are shown in Fig. 3 . Processing irregularities associated with patterning the waveguide over an etched ledge at the transition from the lower input channel waveguide into the MMI coupler section are apparent in Fig. 3 b). The end of the input channel is flared, while the beginning of the MMI coupler is narrower than the designed width of 2 µm. This results in additional insertion loss but should not significantly affect the coupling ratio as this tapering is over a short propagation distance. Portions of unetched silicon (likely due to underexposure during photolithography) should not affect propagation considering the strong optical confinement of the channel waveguide.
Fig. 3 SEM images of a) Tapered mode converters leading into MMI couplers of varying length b) Transition from the lower input waveguide into the MMI coupler section.
The output facet of the MMI couplers was etched simultaneously with its sidewalls. The remainder of the device substrate was removed using a dicing saw whose diamond resin blade was positioned 15 µm from the etched facet. To ensure a perfectly smooth facet for the input rib waveguide, the sample was mechanically thinned and then cleaved along a crystallographic plane. Cleaving could not be used for the output facet as it would be impossible to have consistent control of the MMI coupler length. We have also proposed another process more suitable for industrial manufacturing that relies on wafer bonding and planarization techniques and is presented in [5].
4. Experimental results
Vertical coupling of light between the lower and upper 2 µm square channel regions of the fabricated MMI coupler was quantified using an infrared imaging system. Light from an IR diode laser at 1550 nm was launched into the input rib waveguide using a tapered optical fiber with a spot size of 2.5 µm. The excited fundamental mode of the rib waveguide was then gradually tapered over 500 µm into the lower 2 µm square input channel waveguide of the device, which then excited the modes of the MMI coupler. The light emerging from the output facet of the MMI coupler was then focused using a 10× objective lens and imaged using a 12-bit IR camera with an image acquisition area of 320 × 256 pixels.
The coupling ratio was defined as: Pc = PUpper / (PUpper+PLower), where PUpper and PLower are the optical powers measured in the upper and lower 2 µm square channel regions of the MMI coupler respectively. These measurements were made by integrating the image over each region with the National Instruments Vision software used for image acquisition. A background image taken with the laser turned off was subtracted from each acquired image before performing this integration.
Example output optical field images from MMI couplers of varying length, along with their measured coupling ratios, are provided in Fig. 4 . These images have been cropped to 100 × 100 pixels and were acquired with a TE-polarized input. Also shown are the locations of the integration boxes used to determine their coupling ratios, which remained fixed for each coupler length measured.
The vertical placement of the MMI coupler facet relative to the center of the image acquisition area can change due to non-uniform substrate thickness or by a slight tilt in the horizontal translation of the sample stage. The optical fiber was therefore lowered to illuminate the silicon substrate below to reveal the horizontal plane of the buried oxide layer before each measurement. Its location was then centered in the image acquisition area by vertically translating the focusing objective lens before realigning the input fiber to optimize transmission through the device. This procedure ensured consistent placement of the image integration boxes for each measurement.
The small amount of coupling observed even without a MMI coupler section present can be attributed to the evanescent tail of the optical field and blooming from the objective lens. This image represents the input optical field of the 2 µm channel waveguide leading into the MMI coupler. This optical field is then approximately divided in half after propagating in a 108 µm-long MMI coupler. A maximum coupling ratio of 93±4% was measured after propagating 241 µm, which is slightly longer than the expected 236 µm. The quoted coupling ratio is the average of five measurements taken for each coupler length, with its uncertainty found with the standard deviation of these five trials. For each measurement, the input fiber was realigned and the objective lens refocused. Most importantly, the sample was vertically repositioned as the measured coupling ratio was strongly dependent on the location of the image relative to the integration boxes. The measurement error was typically 3-4% and can be attributed in the main to variations in the focus and vertical position of each acquired image.
The coupling ratios measured for the varying fabricated coupler lengths are plotted in Fig. 5 for both the TE and TM input polarization states, as set by polarization paddles placed in-line with the tapered input fiber. For clarity, error bars are only shown for the TE-polarized results, but are of similar magnitude for the TM polarization. At the optimal coupler length of 241 µm determined with a TE input polarization, the coupling ratio decreased to 88±4% when a TM-polarized input was used. Using a tunable diode laser, the coupling ratio measured for the same length was found to vary by 5% over a wavelength range of 1530 to 1570 nm. This is in agreement with the simulated wavelength sensitivity of 4%, and within the 2% error of these measurements. The error is smaller for these measurements since the position of the sample remained fixed.
5. Conclusion
We have successfully demonstrated optical coupling in a vertically-integrated MMI coupler structure, with measured efficiencies up to 93±4%. These devices are suitable for coupling light between the stacked waveguide layers of a 3D photonic integrated circuit. Previously proposed designs require a thin insulating layer in between the stacked layers to achieve efficient coupling over a reasonable length, at the expense of optical cross-talk wherever the stacked waveguides overlap. Conversely, these MMI couplers optically isolate each layer with a thick separating oxide, while selectively coupling between the stacked waveguides using an intermediate channel of silicon of relatively short length. This proof-of-concept coupling demonstration can be extended to commercially-suitable applications using a wafer bonding process and work is ongoing to fabricate couplers with two stacked waveguide layers.
References and links
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