Arrayed waveguide grating (AWG)

An arrayed waveguide grating is a diffraction grating rebuilt from waveguides. Input light diverges in a first free-propagation region (star coupler) and distributes across an array of tens to hundreds of waveguides whose lengths increase by a fixed increment $\Delta L$ path to path. That increment imposes a wavelength-dependent linear phase tilt across the array, exactly what a grating's grooves do. When the array refocuses the light in the second star coupler, each wavelength converges on a laterally shifted focal spot, and output waveguides positioned along the focal line each collect one channel.

The grating equation's role is played by $n_{eff}\,\Delta L = m\lambda_c$: the order $m$ (typically tens) sets the free spectral range, and the design wavelength $\lambda_c$ centers the passband comb. Like any grating, the response repeats each FSR. This cyclic property is deliberately exploited in "colorless" N×N AWG routers where wavelength $k$ entering port $j$ exits a port determined by arithmetic, the basis of elegant wavelength-routed architectures.

As a component, a DWDM AWG demultiplexes 40–96 channels at 100/50 GHz spacing in one passive element whose loss (2–5 dB typical) is nearly independent of channel count. That property beats cascaded thin-film filters as counts grow. Key specs: insertion loss and its uniformity across ports, adjacent-channel crosstalk (−25 dB class typical), passband shape (Gaussian versus flat-top, trading loss for filter margin), and polarization-dependent wavelength shift.

Platform matters. Silica-on-silicon PLCs dominate deployed AWGs: low loss, low index contrast (large dies), and stable, though still temperature-dependent (~11 pm/°C), hence athermal packages with mechanical compensation or heaters. SOI AWGs shrink dramatically but fight phase errors from nanometer-scale width sensitivity (correctable, with effort); InP AWGs integrate monolithically with lasers and detectors in transmitter/receiver PICs.