Unabsorbed infrared laser light that has passed though plastic parts to be welded with a low absorption TTIr process is recirculated back to the low absorption weld interface for reabsorption in the process. A beam of infrared laser light is directed at the plastic parts to be welded, a transmissive first part and an absorptive (or partially absorptive) second part. The infrared laser light impinges the transmissive part and first transits through the transmissive part to be welded to a weld interface at the junction of the two parts. At the weld interface, either the infrared laser light is partially absorbed by an additive infrared absorber, the infrared laser light is partially absorbed by the absorptive part, or both. The portion of the infrared laser light that is not absorbed continues through the absorptive part and exits the far side. This infrared laser light is then redirected back to the weld interface. On the second pass (and any subsequent passes thereafter), more infrared laser light is absorbed in the partially absorbing medium (the additive infrared absorber, the absorptive part, or both). In an aspect, the parts are tubular parts with the transmissive part coaxially surrounding the absorptive part. The infrared laser light is redirected with a cylindrical mirror that coaxially surrounds the tubular parts. In an aspect, the parts include a tubular part and a fitting and the infrared laser is redirected with a spherical mirror that surrounds the parts.