This paper reviews the experimental data that have been reported since 1980 on long-wavelength (λ1µm) optical reflectometry and discusses the different approaches being used. The principles of both the optical time domain reflectometry (OTDR) and the optical frequency domain reflectometry (OFDR) are presented. The experimental data of 35 technical papers mostly on single-mode OTDR which includes optical source wavelength, launching power, length and type of the test fiber, types of the optical source and receiver, etc, are discussed and classified into three categories as: (1) moderate-distance OTDR using a semiconductor laser diode (2) long-distance OTDR using a Nd: YAG laser and (3) coherent OTDR. The principle of the heterodyne-OTDR and its expected advantages over conventional direct-detection-OTDR are also discussed.

The single fundamental transverse mode condition, both in three-layer DH, and asymmetrical five-layer separate confinement heterostructure (SCH) laser diodes operating at 1.55 µm wavelength, has been investigated based on the effective refractive index. The simple derived formula can be applied to obtain the effective refractive index of the dominant mode in DFB laser diodes to determine the corrugation period corresponding to the designed wavelength.