Arrayed waveguide grating can have one or both slab waveguides with relatively sharply folded optical paths and a mirror that provides the folding of the path. The folded optical paths through the slab waveguides can result in a more compact geometry of the waveguides through the device as well as smaller slab waveguides such that the device can be formed with a significantly smaller overall footprint. Also, arrayed waveguide gratings that cooperate with pivotable mirrors can adjust light passage through the waveguide in response to temperature changes to provide for thermally compensated operation of the device. Thus, very compact planar lightwave circuits filters are described that provide thermally compensated operation.