HANCOCK, N.H. IBM photonics researchers have come up with a promising solution to the problem of coupling light fibers to optical ICs, in a move that may help realize CMOS-based optoelectronics. The breakthrough employs the same photonic crystal techniques typically used in photonic circuit design to create highly efficient coupling between standard fiber optics and on-chip photonic waveguides.
The researchers claim a record low loss (24 dB/cm) for signal coupling between an optical fiber and a CMOS photonic waveguide. The results were reported in the journal Optics Express (Nov. 3).
The structures were built in a silicon-on-insulator layer that is used in CMOS-SOI processes. The SOI layer, composed of silicon dioxide, has a high index of refraction, which is helpful in building photonic waveguides. By etching a regular pattern of holes in the SOI layer, optical regions that totally reject the vibrational modes of light at a specific wavelength are created. Using standard lithographic techniques, it is possible to build planar optical circuits by using the hole patterns as optically insulating walls.
The wavelength of the light being used to carry a signal dictates the size of the structures. For standard optical-communications wavelengths, 100-nanometer-diameter holes spaced 500 nm apart are required. The width of the waveguide must be precisely defined, which is where the problem arises: It is physically difficult to go from the diameter of an optical fiber down to the width of the waveguide without creating unacceptable signal losses. In addition, the high index of refraction in the SOI layer, while helpful for photonic-circuit design, represents a large mismatch with the index of the optical fiber, creating further losses.
The IBM group developed a two-stage approach to the problem. First, the fiber is coupled to a conventional silicon waveguide using a spot-conversion technique. The silicon waveguide is then coupled to the photonic SOI waveguide using a technique based on varying the width of the input waveguide.
The first component of the solution uses a silicon waveguide that is narrowed to a point at the edge of the chip where it meets the optical fiber. The reason for that geometry is the ability to sidestep the size mismatch problem by using a near-field optical effect to couple the optical signal in the fiber to the much smaller silicon waveguide. The evanescent near-field accurately reproduces the signal in the small waveguide without the signal's being perturbed by the difference in size.
To make the approach work, the waveguide had to be encased in a polymer with a high index of refraction; otherwise, the Si substrate would drain the signal power.
The researchers considered the usual methods that have been used to couple conventional silicon waveguides to photonic waveguides but rejected them because of fabrication complications. The problem at this stage is the velocity difference between the two media: Since optical propagation occurs far more slowly in photonic waveguides than in pure silicon, simply connecting the two produces a noisy transition.
The conventional solution has been to introduce photonic waveguiding gradually by varying the size and spacing of the holes at the beginning of the photonic waveguide so that the light has a region in which to decelerate. But it is difficult to get the pattern right.
Numerical simulations showed that the simpler approach, of just joining the two waveguides, would work if the silicon waveguide was narrowed in diameter. The simulations also suggested an ideal width.
Fabrication of the structures on a standard 200-mm wafer fab at IBM's T.J. Watson center did result in efficient coupling, yielding the record low-loss figure.