We demonstrate coupling from tapered optical fibers to 450 nm by 250 nm silicon strip waveguides using compact cantilever couplers. The couplers consist of silicon inverse width tapers embedded within silicon dioxide cantilevers. Finite difference time domain simulations are used to design the length of the silicon inverse width taper to as short as 6.5 μm for a cantilever width of 2 μm. Modeling of various strip waveguide taper profiles shows reduced coupling losses for a quadratic taper profile. Infrared measurements of fabricated devices demonstrate average coupling losses of 0.62 dB per connection for the quasi-TE mode and 0.50 dB per connection for the quasi-TM mode across the optical telecommunications C band. In the wavelength range from 1477 nm to 1580 nm, coupling losses for both polarizations are less than 1 dB per connection. The compact, broadband, and low-loss coupling scheme enables direct access to photonic integrated circuits on an entire chip surface without the need for dicing or cleaving the chip.
©2011 Optical Society of America
Silicon photonics is a promising approach for chip-scale integrated optics . A single-mode silicon strip waveguide designed for operation in the infrared has a typical submicron cross-section of 450 nm × 250 nm. Highly confined optical modes allow for high density integration and waveguide bends with micrometer scale radii of curvature. The high confinement, however, also produces major challenges when attempting to efficiently couple light between silicon strip waveguides and optical fibers. Mode conversion from a single-mode fiber, with mode field diameter equal to 10 micrometers, results in a coupling loss that is greater than 20 dB. Current methods designed to achieve efficient fiber-to-chip coupling generally involve edge coupling using inverse width tapered waveguides or surface coupling using grating couplers. Inverse width tapers enable low loss and broadband edge coupling [2–14] but require dicing or cleaving the chip. Coupling losses less than 0.25 dB per connection have been achieved using 200 to 300 μm long linear tapers and an index-matched SiOx cladding . Alternatively, grating couplers enable light coupling via the surface of the chip without the need for cleaving. They require, however, a tradeoff between bandwidth and efficiency [15–21]. Coupling losses as low as 1.6 dB per connection within a 3 dB bandwidth of 80 nm have recently been reported . In our prior work, we have demonstrated both fiber-to-chip and chip-to-chip light coupling employing cantilever couplers with inverse width silicon tapers [22, 23]. Cantilever couplers enable broadband and low loss light coupling to photonic integrated circuits on an entire chip surface without the need for dicing or cleaving the chip.
In this paper, we demonstrate reduced loss and more compact cantilever couplers. Measurements of fabricated devices demonstrate average coupling losses of 0.62 dB per connection for the quasi-TE mode and 0.50 dB per connection for the quasi-TM mode across the optical telecommunications C band (1530 nm – 1565 nm) using a silicon inverse width taper length of only 6.5 μm. To the best of our knowledge, this is the shortest fiber-to-strip waveguide coupler employing inverse width silicon tapers reported in the literature. In the wavelength range from 1477 nm to 1580 nm, coupling losses for both polarizations are measured to be less than 1 dB per connection.
The paper is organized as follows. Section two describes the design of the compact and low loss cantilever couplers. Section three conveys the fabrication details. Section four discusses the experimental setup and measurement results. Finally, concluding remarks are given in section five.
A diagram of a cantilever coupler is shown in Fig. 1 . The cantilever coupler consists of a silicon inverse width taper core embedded within a patterned oxide thin film bilayer cladding. The bilayer cladding consists of plasma enhanced chemical vapor deposition (PECVD) silicon dioxide and buried oxide (BOX). The length of the cantilever is denoted L1. The length of the silicon inverse width taper core is denoted L2. A cross-sectional schematic of the cantilever coupler is shown in Fig. 2 . The top bump on top of the cantilever coupler is a consequence of thin film deposition on top of the silicon core topography. Robust and efficient mode conversion between a butt coupled tapered optical fiber and the silicon strip waveguide is enabled by both the silicon inverse width taper and the localization provided by the oxide cladding which functions as a micro-fiber. The approach exploits the bandwidth and efficiency of inverse width tapers, avoids dicing or cleaving the chip, and allows light to be coupled anywhere on a chip surface.
The silicon inverse width taper may be viewed as a tapered impedance transformer. Analytical solutions of the transmission properties of tapered impedance transformers are based on the theory of small reflections . The total reflection from a continuously tapered structure is estimated by integrating small reflections at infinitesimal slices along the waveguide taper. An analytical solution for the cantilever coupler would neglect width-dependent propagation velocity and the effect of the oxide micro-fiber. Therefore, three-dimensional finite difference time domain (FDTD) simulations are conducted to model the coupling loss. The simulations account for reflection loss and mode conversion loss at the fiber/coupler interface, geometry-dependent propagation velocity, and the localization provided by the oxide cladding. The effect of sidewall surface roughness on all interfaces is neglected. The presence of the top bump on top of the cantilever is also neglected in the simulations by setting the height of the bump to zero.
The simulated coupling loss as a function of the length, L2, of a quadratic silicon inverse width taper is shown in Fig. 3 . The length of the cantilever, L1, is designed to be 1 μm greater than L2 in order to provide etch isolation during the fabrication process. The silicon inverse width taper begins with a taper width, wt,start, equal to 60 nm and ends with a taper width, wt,end, equal to 450 nm. The height of the silicon inverse width taper is fixed at ht equal to 250 nm throughout the taper. The width of the coupler, wc, is set to 2 μm, the height of the bottom cladding of the cantilever, ho, is set to 1 μm, and the height of the top cladding, hp, is set to 1.1 μm. The width of the air gap, wg, is set to 5 μm. The diameter of the tapered optical fiber is set to 1.5 μm. The input wavelength is set to 1.55 μm. After an initial regime of poor coupling for short taper lengths, the coupling loss oscillates near an optimal coupling value as the taper length increases. The oscillations of the coupling loss with taper length for lengths greater than 6 μm are due to the combined effect of partial reflections at infinitesimal slices along the tapered waveguide . The locations of the local minima and maxima shift to longer taper lengths as the cantilever width increases.
In order to minimize both the coupler footprint and loss, we choose the device length that falls in the first null of the quasi-TE mode coupling loss curve in Fig. 3. In particular, the quadratic-width taper exhibits a loss minimum equal to 0.29 dB per connection for the quasi-TE mode and 0.37 dB per connection for the quasi-TM mode at the taper length, L2, equal to 6.5 μm. The total cantilever length, L1, is therefore designed to be 7.5 μm.
The simulated coupling loss as a function of the cantilever width, wc, is shown in Fig. 4 for a cantilever length, L2, equal to 6.5 μm. Coupling losses are minimized when the cantilever width, wc, is approximately the same as the diameter of the tapered optical fiber (1.5 μm). Beginning with cantilever widths of 0.5 μm, coupling losses initially decrease as the cantilever width increases to approximately 0.75 μm due to decreasing reflections at the fiber/coupler interface. The coupling loss then remains relatively low for cantilever widths less than approximately 2.3 μm. As the cantilever width expands beyond 2.3 μm, the oxide cladding provides less localization, resulting in increased coupling losses. We choose a cantilever width, wc, greater than 1.5 μm to avoid the bump on top of the cantilever that manifests during fabrication.
The simulated quasi-TE mode coupling loss using the profile of the inverse width taper as a parameter is shown in Fig. 5 for L2 equal to 6.5 μm and wc equal to 2 μm. The waveguide width for each taper profile is shown in Fig. 5(a) as a function of longitudinal coordinate along the direction of propagation (normalized to length L2). Coupling loss versus wavelength is shown in Fig. 5(b). The quadratic taper exhibits the lowest coupling loss , and weakest wavelength dependence.
The nominal design values for the cantilever coupler therefore features a 2 μm wide cantilever surrounding a 6.5 μm long silicon inverse width quadratic taper. Optical mode profiles from FDTD simulations using the nominal design values are shown in Fig. 6 to demonstrate the evolution of the x-y cross-section optical field within the cantilever coupler. Simulations of both the quasi-TE and quasi-TM modes for our nominal coupler design show coupling losses less than 0.5 dB per connection across a 120 nm band from 1460 nm to 1580 nm wavelength.
The cantilever couplers are fabricated on a silicon-on-insulator (SOI) wafer. Cross-sectional diagrams of the fabrication process are given in Fig. 7 . The thickness of the silicon device layer is 250 nm and the thickness of the BOX is 1 μm.
In the first step, the silicon waveguide core is defined in HSQ resist using electron-beam lithography and inductively coupled plasma reactive ion etching (ICP-RIE) using Cl2 chemistry. Back-to-back silicon inverse width tapers are joined by straight silicon strip waveguides with width equal to 450 nm and height equal to 250 nm. Next, approximately 1.1 μm of SiO2 cladding is deposited on top of the silicon core using PECVD. To define the cantilevers, a Ti/Ni etch mask is evaporated and patterned over the top cladding using focused ion beam (FIB) milling. A subsequent reactive ion etch using SF6 chemistry is used to release the cantilevers from the substrate by anisotropically etching the SiO2 and isotropically etching the Si substrate. The metal etch mask is removed by wet chemical etching. In order to ease butt coupling via tapered optical fiber, a 12 x 45 μm2 pit is etched in front of the cantilever at the same time that the cantilever is released from the substrate. A scanning electron microscope (SEM) image of a fabricated cantilever coupler is shown in Fig. 8 .
4. Measurement and experimental results
Coupling and propagation losses of fabricated devices are measured in the laboratory using the setup shown in Fig. 9 . An infrared continuous-wave laser source is first connected to a polarization controller which outputs linearly polarized quasi-TE or quasi-TM light with cross-polarization rejection ratio of more than 17 dB. Two tapered optical fibers with tip diameters of ~1.5 μm are fabricated and mounted on 6-axis positioning stages . The chip consists of back-to-back cantilever couplers interconnected by 450 nm × 250 nm silicon strip waveguides of varying lengths. Light from the output fiber is collected by a photodetector and measured by a power meter.
The waveguides interconnecting the back-to-back cantilever couplers are fabricated with lengths between 250 μm and 2 mm, excluding the lengths of the input and output cantilever couplers. The three longest waveguides each include four 30 μm radius 90° bends to ensure that light is guided in the silicon waveguide core rather than in a slab mode. At 1550 nm wavelength, the measured insertion loss of back-to-back couplers as a function of interconnecting waveguide length is shown in Fig. 10 . The coupling loss per pair of couplers is determined from the y-intercept of a linear fit. The waveguide propagation loss is determined from the slope.
The measured coupling loss is 0.956 dB per pair of couplers (0.48 dB per connection) for the quasi-TE mode and 0.789 dB per pair of couplers (0.39 dB per connection) for the quasi-TM mode at 1550 nm. The experimental results are in good agreement with the simulation results. We attribute the larger coupling loss measured in the laboratory to sidewall scattering, misalignment, and differences in dimensions between the fabricated devices and the nominal design. The measured propagation loss is 9.5 dB/cm for the quasi-TE mode and 11 dB/cm for the quasi-TM mode. We attribute the propagation loss to primarily the sidewall surface roughness of the silicon waveguide core.
The measured coupling loss versus wavelength is shown in Fig. 11 . Measured coupling losses are less than 1 dB per connection from 1477 to 1580 nm. The difference in coupling losses between the two polarizations is less than 0.75 dB across the measured band with a mean value of 0.18 dB.
In summary, we demonstrate coupling from tapered optical fibers to 450 nm × 250 nm silicon strip waveguides using compact and low loss cantilever couplers. FDTD simulations are used to design the length of the embedded silicon inverse width taper to as short as 6.5 μm for a cantilever width of 2 μm. Simulations comparing various tapered waveguide profiles show that the taper profile can impact the coupling loss by as much as 2 dB per connection. The quadratic taper exhibits the lowest coupling loss and weakest wavelength dependence. Measurements of fabricated cantilever couplers demonstrate coupling losses that are less than 1 dB per connection across the wavelength range from 1477 to 1580 nm. Future work involves reducing the size of the 12 × 45 μm2 pit that enables butt coupling to the cantilever coupler via a tapered optical fiber. We estimate that the length of the pit could be reduced to 20 μm without significantly increasing the challenge of fiber butt coupling, resulting in a 12 × 27.5 μm2 footprint. Reduction in footprint enables dense integration of the cantilever couplers for use in future silicon photonic integrated circuits. In addition, transitioning from the current cantilever fabrication process that involves FIB milling to an all-photolithography process would allow for high volume production of the compact cantilever couplers.
This work was supported by the National Science Foundation (NSF).
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