This paper describes two approaches to measuring transmission losses in optical fibers by the thermal-rise technique. Rapid and convenient loss determinations can be made on short (~10-cm) lengths of fibers. Values of loss as low as ~10-7 cm-1 (0.043 dB/km) can be measured with transmitted light power of ~100mW.
R. L. Cohen, K. W. West, P. D. Lazay, and J. Simpson, "Loss Measurements in Optical Fibers. 2: Bolometric Measuring Instrumentation," Appl. Opt. 13, 2522-2524 (1974)