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The high complexity of presently developed photonic integration technologies on InP necessitates a thorough knowledge on the prominence of reflections in guiding layers as they can profoundly influence the circuit performance. These can originate at spatially localized refractive index discontinuities in the guide and can have several potential sources ranging from the growth-or process-induced defects and component side-walls to such key elements for photonic integration as the butt-joints between active and passive components and the tapered regions for spot-size conversion. The work described in this paper precisely addresses this issue using an upgraded high precision reflectometer in two ways. First, by monitoring and precisely locating reflections in the guiding layer and second, by extracting relevant information on the device performance through a study of reflections and their spectral properties.The high precision reflectometer employed here is basically a Michelson interferometer equipped with two different low-coherence light probes of central wavelength -1.3 or -1.55 m and a spectral half-width of about -55 nm. One arm of the interferometer is coupled to the device under test through a lensed single mode fiber while the other to a movable reference mirror which scans the optical path to detect refractive index discontinuities less than lo4 (dynamic range of -80 dB) in the device. Additionally, the up-graded facilities to record the transmitted probe light and the edge-electroluminescence from the device under carrier injection, to be described here in detail, permit to investigate the optical and opto-electronic quality of the guide cavity and further help to locate precisely .the unknown reflection. Indeed, a localized reflection in a guiding layer automatically induces a pair of resonant sub-cavities (one with respect to each facet) whose cavity lengths can be extracted by canying out FFT (fast Fourier transform) analysis on the spectra recorded at a high resolution.After briefly describing the basic operation principle of this instrument, the example of a growth-induced defect in a deep-ridge InGaAsPAnP waveguide (see fip. 1) is considered first to illustrate the methodology (experimental procedure and data analysis) to spatially localize reflections from transmitted spectral data. Later on, some relevant examples of key elements in photonic circuits are considered to demonstrate how reflection monitoring and the associated methodology can be efficiently employed to optimize the geometry of butt-joints for minimum reflections and also the design of bent guides [ 11 or MMI couplers [Z]. For example, in the case of bent guides reflections have been monitored as a function of the radius of curvature &), while in MMI couplers they are localized and further identified (spurious or back reflections) as a function of MMI length both in splitter and combiner modes (see fig. 2). Subsequently, as back reflections are highly undesirable, Gottesman et. al., have recently proposed a novel design for h4MI s to ...