Development of a calorimetric method for making precision optical absorption measurements

Present trends toward the development and application of exceptionally high quality optical materials have made requirements on optical loss so stringent that they exceed the capabilities of existing measurement techniques. This work describes a calorimetric method for determining optical absorption in bulk materials which is over an order of magnitude more sensitive than previous methods. The large circulating optical power within a laser cavity is used to heat a small rod shaped sample of test materialplaced within the cavity. The optical absorption within the sample causes its temperature to increase until the absorbed power is balanced by heat leakage out of the rod. To minimize this leakage, the rod is thermally isolated from its surroundings. The optical loss in the sample can be calculated knowing the optical power passing through it, its temperature rise, and the cooling time constant which is determined by abruptly turning off the laser. Losses as low as 2.3 +/-0.5 dB/km at 1.064 micro have been measured with high reliability.