Polymer AWG multiplexer resists temperature shifts
Norbert Keil and others at The Henrich-Hertz-Institut für Nachrichtentechnnik (Berlin) have designed an all-polymer arrayed waveguide grating (AWG) wavelength router that exhibits good optical performance and athermal operation.1 They demonstrated that the temperature dependence of the wavelength can be controlled by adjusting the thermal coefficient of expansion of the substrate to match the change in refractive index with temperature (the thermo-optic coefficient) of the waveguide.
Polymer optical devices may be cheaper to make than silica-based waveguide devices. Also, because polymeric AWG devices have a thermo-optic coefficient about 10 times larger than silica, they can be temperature-tuned over a wider spectral range. A polymeric AWG could be integrated with polymer optical switches to create an add/drop filter that doesn't require as much power as a comparable silica device.
The polymer's sensitivity to temperature is a double-edged sword, however: to maintain the desired channel wavelengths, the device's temperature must be actively controlled, typically by a heard or Peltier cooler—which requires constant power. To eliminate the need for constant temperature control, researchers have experimented with several methods of making athermal devices, but these either require moving parts or require that a polymer half-waveplate be inserted in the arrayed waveguides to compensate for polarization dependence.
Keil and coworkers managed to create an all-polymer AWG that is both athermal and polarization independent. They matched the positive coefficient of thermal expansion of the substrate and the negative thermo-optic coefficient of the waveguide material.
The group made 8 x 8 AWG wavelength routers from polymer waveguides on polymer substrates, with wavelength spacing of 200 GHz around 1.55 µm. They tailored the substrate material to match the thermo-optic coefficient of the waveguide. The successful substrate had a coefficient of thermal expansion of 80 ppm/K, and showed a temperature-dependent wavelength shift of only ±0.05 nm over the temperature range from 25°C to 65°C. This is, in fact, less than a typical silica AWG.
The device exhibited a crosstalk (less than -30 dB) equivalent to that of a standard silica AWG, but had better polarization dependence (less than 0.02 nm shift). The insertion loss varied between 5.8 dB for the center port and 7.5 dB for the edge ports. For more information contact Norbert Keil at email@example.com
- N. Keil et al., Electron. Lett. 37(9), 26, 579 (April 2001).