Waveguide interconnection enables configurable lightwave circuits
Silica-based planar lightwave circuit (PLC) technology is attractive because it provides highly integrated optical devices with excellent stability. PLCs are now key components of optical devices used in commercial photonic networks. PLC technology can be used to make optical devices on which various types of optical elements are densely integrated, such as power dividers, interferometers, and phase shifters.1 As PLCs become more complex and larger scale, several issues arise, including a longer time-to-market and higher non-recurring engineering costs. In the semiconductor industry, similar issues were overcome by developing programmable logic components such as the field-programmable gate array (FPGA). The designer can change the functions of these components as required after the manufacturing process. There is also a similar demand for configurable optical circuits.
We have investigated the use of direct waveguide writing to add certain parts of optical circuits to a PLC and to interconnect optical paths between optical elements in a chip post-manufacture (Figure 1). PLCs are made using a wafer process similar to that used for large-scale integration in silicon microchips. This process consists of forming a circuit pattern by employing photolithography, etching, and deposition. After making PLCs with a generic circuit layout that includes various optical elements, we interconnect these elements with a view to realizing certain functions. To achieve such goals, we use a femtosecond laser to write waveguides in PLCs.2,3 A focused near-IR femtosecond pulse increases the refractive index directly inside a glass material,4 and this enables us to write planar (2D) and non-planar (3D) circuits.5 However, it is more difficult to write a waveguide in a PLC than in commonly used bulk glass. This is because the glass of a PLC has a thin laminar structure consisting of different weak glasses doped with several materials. As such, it is susceptible to laser damage.
To accomplish direct writing in PLCs, we employed a new writing method that uses a lens with a low numerical aperture. This approach causes no optical damage and achieves a symmetrical rectangular core. The core size and shape can be controlled more precisely than with the conventional method and without any complex optics. Figure 2 shows the refractive index profile of the written waveguides measured with a commercially available refracted near-field profilometer. The written waveguide exhibited a refractive index increase of about 4 × 10-3. The propagation loss was low at 0.34dB/cm. This shows that the multiple scanning method is a useful way of writing low-loss waveguides in PLCs.
We then investigated waveguide interconnection in the PLC with the written waveguide. To connect a PLC waveguide with low loss, the mode field diameter of the written waveguide was adjusted to that of the waveguide in the PLC. Figure 3(a) shows the waveguide layout we used in this work. Before interconnection, the two waveguide ends were separated by 2000μm. A directly written waveguide with a 3D configuration in the PLC sample passed over the other waveguide. Images (b) and (c) in Figure 3 show the connection and crossing points of a directly written waveguide with a 3D configuration. These images indicate that the position and angle between the PLC waveguide and the written waveguide are well controlled, and also that the waveguide is well written over the PLC waveguide. We investigated cross-sectional views of the 3D written waveguides in detail by slicing the PLC as shown in Figure 4. The written waveguide passed over the PLC waveguide without touching it. The connection was achieved with a very low loss of 1.3dB.
We have demonstrated optical interconnection between optical circuits within a PLC chip by writing a waveguide directly with a femtosecond laser. This result is very attractive because it means that the circuit layout of a PLC can be changed after manufacture. The written waveguides can also be eliminated by high-temperature treatment, which in future should enable programmable optical circuits, similar to FPGAs.
