An apparatus includes a heat exchanger with a body having a passage through the body. The passage defines apertures on multiple sides of the body, and the passage is configured to allow optical signals to pass through the body. One or more tapered edges are at least partially around one or more of the apertures, and each tapered edge is configured to reflect optical radiation inward into the passage. One or more absorptive surfaces are within the passage, and the one or more absorptive surfaces configured to absorb the reflected optical radiation. The heat exchanger is configured to convert the absorbed optical radiation into heat, and the body further includes one or more cooling channels configured to receive coolant that absorbs the heat.